Network Working Group B. Adamson Internet-Draft Naval Research Laboratory Obsoletes: 3940 (if approved) C. Bormann Intended status: Standards Track Universitaet Bremen TZI Expires: October 26, 2009 M. Handley University College London J. Macker Naval Research Laboratory April 24, 2009 NACK-Oriented Reliable Multicast Protocol draft-ietf-rmt-pi-norm-revised-11 Status of this Memo This Internet-Draft is submitted to IETF in full conformance with the provisions of BCP 78 and BCP 79. This document may contain material from IETF Documents or IETF Contributions published or made publicly available before November 10, 2008. The person(s) controlling the copyright in some of this material may not have granted the IETF Trust the right to allow modifications of such material outside the IETF Standards Process. Without obtaining an adequate license from the person(s) controlling the copyright in such materials, this document may not be modified outside the IETF Standards Process, and derivative works of it may not be created outside the IETF Standards Process, except to format it for publication as an RFC or to translate it into languages other than English. Internet-Drafts are working documents of the Internet Engineering Task Force (IETF), its areas, and its working groups. Note that other groups may also distribute working documents as Internet- Drafts. Internet-Drafts are draft documents valid for a maximum of six months and may be updated, replaced, or obsoleted by other documents at any time. It is inappropriate to use Internet-Drafts as reference material or to cite them other than as "work in progress." The list of current Internet-Drafts can be accessed at http://www.ietf.org/ietf/1id-abstracts.txt. The list of Internet-Draft Shadow Directories can be accessed at http://www.ietf.org/shadow.html. This Internet-Draft will expire on October 26, 2009. Copyright Notice Adamson, et al. Expires October 26, 2009 [Page 1] Internet-Draft NORM Protocol April 2009 Copyright (c) 2009 IETF Trust and the persons identified as the document authors. All rights reserved. This document is subject to BCP 78 and the IETF Trust's Legal Provisions Relating to IETF Documents in effect on the date of publication of this document (http://trustee.ietf.org/license-info). Please review these documents carefully, as they describe your rights and restrictions with respect to this document. Abstract This document describes the messages and procedures of the Negative- ACKnowledgment (NACK) Oriented Reliable Multicast (NORM) Protocol. This protocol is designed to provide end-to-end reliable transport of bulk data objects or streams over generic IP multicast routing and forwarding services. NORM uses a selective, negative acknowledgment mechanism for transport reliability and offers additional protocol mechanisms to allow for operation with minimal a priori coordination among senders and receivers. A congestion control scheme is specified to allow the NORM protocol to fairly share available network bandwidth with other transport protocols such as Transmission Control Protocol (TCP). It is capable of operating with both reciprocal multicast routing among senders and receivers and with asymmetric connectivity (possibly a unicast return path) between the senders and receivers. The protocol offers a number of features to allow different types of applications or possibly other higher level transport protocols to utilize its service in different ways. The protocol leverages the use of FEC-based repair and other IETF Reliable Multicast Transport (RMT) building blocks in its design. This document obsoletes RFC 3940. Adamson, et al. Expires October 26, 2009 [Page 2] Internet-Draft NORM Protocol April 2009 Table of Contents 1. Introduction and Applicability . . . . . . . . . . . . . . . . 5 1.1. Requirements Language . . . . . . . . . . . . . . . . . . 6 1.2. NORM Data Delivery Service Model . . . . . . . . . . . . . 6 1.3. NORM Scalability . . . . . . . . . . . . . . . . . . . . . 8 1.4. Environmental Requirements and Considerations . . . . . . 9 2. Architecture Definition . . . . . . . . . . . . . . . . . . . 9 2.1. Protocol Operation Overview . . . . . . . . . . . . . . . 11 2.2. Protocol Building Blocks . . . . . . . . . . . . . . . . . 13 2.3. Design Tradeoffs . . . . . . . . . . . . . . . . . . . . . 13 3. Conformance Statement . . . . . . . . . . . . . . . . . . . . 14 4. Message Formats . . . . . . . . . . . . . . . . . . . . . . . 16 4.1. NORM Common Message Header and Extensions . . . . . . . . 16 4.2. Sender Messages . . . . . . . . . . . . . . . . . . . . . 19 4.2.1. NORM_DATA Message . . . . . . . . . . . . . . . . . . 19 4.2.2. NORM_INFO Message . . . . . . . . . . . . . . . . . . 29 4.2.3. NORM_CMD Messages . . . . . . . . . . . . . . . . . . 30 4.3. Receiver Messages . . . . . . . . . . . . . . . . . . . . 48 4.3.1. NORM_NACK Message . . . . . . . . . . . . . . . . . . 48 4.3.2. NORM_ACK Message . . . . . . . . . . . . . . . . . . . 54 4.4. General Purpose Messages . . . . . . . . . . . . . . . . . 56 4.4.1. NORM_REPORT Message . . . . . . . . . . . . . . . . . 56 5. Detailed Protocol Operation . . . . . . . . . . . . . . . . . 56 5.1. Sender Initialization and Transmission . . . . . . . . . . 58 5.1.1. Object Segmentation Algorithm . . . . . . . . . . . . 59 5.2. Receiver Initialization and Reception . . . . . . . . . . 60 5.3. Receiver NACK Procedure . . . . . . . . . . . . . . . . . 60 5.4. Sender NACK Processing and Response . . . . . . . . . . . 62 5.4.1. Sender Repair State Aggregation . . . . . . . . . . . 63 5.4.2. Sender FEC Repair Transmission Strategy . . . . . . . 64 5.4.3. Sender NORM_CMD(SQUELCH) Generation . . . . . . . . . 65 5.4.4. Sender NORM_CMD(REPAIR_ADV) Generation . . . . . . . . 65 5.5. Additional Protocol Mechanisms . . . . . . . . . . . . . . 66 5.5.1. Greatest Round-trip Time Collection . . . . . . . . . 66 5.5.2. NORM Congestion Control Operation . . . . . . . . . . 67 5.5.3. NORM Positive Acknowledgment Procedure . . . . . . . . 75 5.5.4. Group Size Estimate . . . . . . . . . . . . . . . . . 77 6. Security Considerations . . . . . . . . . . . . . . . . . . . 77 6.1. Baseline Secure NORM Operation . . . . . . . . . . . . . . 79 6.1.1. IPsec Approach . . . . . . . . . . . . . . . . . . . . 79 6.1.2. IPsec Requirements . . . . . . . . . . . . . . . . . . 82 7. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 83 7.1. Explicit IANA Assignment Guidelines . . . . . . . . . . . 83 7.1.1. NORM Header Extension Types . . . . . . . . . . . . . 83 7.1.2. NORM Stream Control Codes . . . . . . . . . . . . . . 84 7.1.3. NORM_CMD Message Sub-types . . . . . . . . . . . . . . 85 8. Suggested Use . . . . . . . . . . . . . . . . . . . . . . . . 86 Adamson, et al. Expires October 26, 2009 [Page 3] Internet-Draft NORM Protocol April 2009 9. Changes from RFC3940 . . . . . . . . . . . . . . . . . . . . . 86 10. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 87 11. References . . . . . . . . . . . . . . . . . . . . . . . . . . 87 11.1. Normative References . . . . . . . . . . . . . . . . . . . 87 11.2. Informative References . . . . . . . . . . . . . . . . . . 88 Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 90 Adamson, et al. Expires October 26, 2009 [Page 4] Internet-Draft NORM Protocol April 2009 1. Introduction and Applicability The Negative-acknowledgment (NACK) Oriented Reliable Multicast (NORM) protocol is designed to provide reliable transport of data from one or more sender(s) to a group of receivers over an IP multicast network. The primary design goals of NORM are to provide efficient, scalable, and robust bulk data (e.g., computer files, transmission of persistent data) transfer across possibly heterogeneous IP networks and topologies. The NORM protocol design provides support for distributed multicast session participation with minimal coordination among senders and receivers. NORM allows senders and receivers to dynamically join and leave multicast sessions at will with minimal overhead for control information and timing synchronization among participants. To accommodate this capability, NORM protocol message headers contain some common information allowing receivers to easily synchronize to senders throughout the lifetime of a reliable multicast session. NORM is designed to be self-adapting to a wide range of dynamic network conditions with little or no pre- configuration. The protocol is purposely designed to be tolerant of inaccurate timing estimations or lossy conditions that may occur in many networks including mobile and wireless. The protocol is also designed to exhibit convergence and efficient operation even in situations of heavy packet loss and large queuing or transmission delays. This document obsoletes the Experimental RFC 3940 specification. This document is a product of the IETF RMT working group and follows the guidelines provided in the Author Guidelines for Reliable Multicast Transport (RMT) Building Blocks and Protocol Instantiation documents [RFC3269]. Statement of Intent This memo contains the definitions necessary to fully specify a Reliable Multicast Transport protocol in accordance with the criteria of IETF Criteria for Evaluating Reliable Multicast Transport and Application Protocols [RFC2357]. The NORM specification described in this document was previously published in the "Experimental Category" [RFC3940]. It was the stated intent of the RMT working group to re- submit this specifications as an IETF Proposed Standard in due course. This Proposed Standard specification is thus based on RFC 3940 and has been updated according to accumulated experience and growing protocol maturity since the publication of RFC 3940. Said experience applies both to this specification itself and to congestion control strategies related to the use of this specification. The differences between RFC 3940 and this document are listed in Section 9. Adamson, et al. Expires October 26, 2009 [Page 5] Internet-Draft NORM Protocol April 2009 1.1. Requirements Language The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this document are to be interpreted as described in RFC 2119 [RFC2119]. 1.2. NORM Data Delivery Service Model A NORM protocol instance (NormSession) is defined within the context of participants communicating connectionless (e.g., Internet Protocol (IP) or User Datagram Protocol (UDP)) packets over a network using pre-determined addresses and host port numbers. Generally, the participants exchange packets using an IP multicast group address, but unicast transport may also be established or applied as an adjunct to multicast delivery. In the case of multicast, the participating NormNodes will communicate using a common IP multicast group address and port number that has been chosen via means outside the context of the given NormSession. Other IETF data format and protocol standards exist that may be applied to describe and convey the required a priori information for a specific NormSession (e.g., Session Description Protocol (SDP) [RFC4566], Session Announcement Protocol (SAP) [RFC2974], etc.). The NORM protocol design is principally driven by the assumption of a single sender transmitting bulk data content to a group of receivers. However, the protocol MAY operate with multiple senders within the context of a single NormSession. In initial implementations of this protocol, it is anticipated that multiple senders will transmit independent of one another and receivers will maintain state as necessary for each sender. However, in future versions of NORM, it is possible that some aspects of protocol operation (e.g., round-trip time collection) may provide for alternate modes allowing more efficient performance for applications requiring multiple senders. NORM provides for three types of bulk data content objects (NormObjects) to be reliably transported. These types include: 1. static computer memory data content ("NORM_OBJECT_DATA" type), 2. computer storage files ("NORM_OBJECT_FILE" type), and 3. non-finite streams of continuous data content ("NORM_OBJECT_STREAM" type). The distinction between "NORM_OBJECT_DATA" and "NORM_OBJECT_FILE" is simply to provide a hint to receivers in NormSessions serving multiple types of content as to what type of storage should be allocated for received content (i.e., memory or file storage). Other than that distinction, the two are identical, providing for reliable transport of finite (but potentially very large) units of content. Adamson, et al. Expires October 26, 2009 [Page 6] Internet-Draft NORM Protocol April 2009 These static data and file services are anticipated to be useful for multicast-based cache applications with the ability to reliably provide transmission of large quantities of static data. Other types of static data/file delivery services might make use of these transport object types, too. The use of the "NORM_OBJECT_STREAM" type is at the application's discretion and could be used to carry static data or file content also. The NORM reliable stream service opens up additional possibilities such as serialized reliable messaging or other unbounded, perhaps dynamically produced content. The "NORM_OBJECT_STREAM" provides for reliable transport analogous to that of the Transmission Control Protocol (TCP), although NORM receivers will be able to begin receiving stream content at any point in time. The applicability of this feature will depend upon the application. The NORM protocol also allows for a small amount of out-of-band data (sent as "NORM_INFO" messages) to be attached to the data content objects transmitted by the sender. This readily-available out-of- band data allows multicast receivers to quickly and efficiently determine the nature of the corresponding data, file, or stream bulk content being transmitted. This allows application-level control of the receiver node's participation in the current transport activity. This also allows the protocol to be flexible with minimal pre- coordination among senders and receivers. The "NORM_INFO" content is designed to be atomic in that its size MUST fit into the payload portion of a single NORM message. NORM does NOT provide for global or application-level identification of data content within in its message headers. Note the "NORM_INFO" out-of-band data mechanism could be leveraged by the application for this purpose if desired, or identification could alternatively be embedded within the data content. NORM does identify transmitted content (NormObjects) with transport identifiers that are applicable only while the sender is transmitting and/or repairing the given object. These transport data content identifiers (NormTransportIds) are assigned in a monotonically increasing fashion by each NORM sender during the course of a NormSession. Each sender maintains its NormTransportId assignments independently so that individual NormObjects may be uniquely identified during transport with the concatenation of the sender session-unique identifier (NormNodeId) and the assigned NormTransportId. The NormTransportIds are assigned from a large, but fixed, numeric space in increasing order and may be reassigned during long-lived sessions. The NORM protocol provides mechanisms so that the sender application may terminate transmission of data content and inform the group of this in an efficient manner. Other similar protocol control mechanisms (e.g., session termination, receiver synchronization, etc.) are specified so that reliable multicast application variants may construct different, complete bulk Adamson, et al. Expires October 26, 2009 [Page 7] Internet-Draft NORM Protocol April 2009 transfer communication models to meet their goals. To summarize, the NORM protocol provides reliable transport of different types of data content (including potentially mixed types). The senders enqueue and transmit bulk content in the form of static data or files and/or non-finite, ongoing stream types. NORM senders provide for repair transmission of data and/or FEC content in response to NACK messages received from the receiver group. Mechanisms for out-of-band information and other transport control mechanisms are specified for use by applications to form complete reliable multicast solutions for different purposes. 1.3. NORM Scalability Group communication scalability requirements lead to adaptation of negative acknowledgment (NACK) based protocol schemes when feedback for reliability is required [RmComparison]. NORM is a protocol centered around the use of selective NACKs to request repairs of missing data. NORM provides for the use of packet-level forward error correction (FEC) techniques for efficient multicast repair and optional proactive transmission robustness [RFC3453]. FEC-based repair can be used to greatly reduce the quantity of reliable multicast repair requests and repair transmissions [MdpToolkit] in a NACK-oriented protocol. The principal factor in NORM scalability is the volume of feedback traffic generated by the receiver set to facilitate reliability and congestion control. NORM uses probabilistic suppression of redundant feedback based on exponentially distributed random backoff timers. The performance of this type of suppression relative to other techniques is described in [McastFeedback]. NORM dynamically measures the group's round-trip timing status to set its suppression and other protocol timers. This allows NORM to scale well while maintaining reliable data delivery transport with low latency relative to the network topology over which it is operating. Feedback messages can be either multicast to the group at large or sent via unicast routing to the sender. In the case of unicast feedback, the sender relays the feedback state to the group to facilitate feedback suppression. In typical Internet environments, it is expected that the NORM protocol will readily scale to group sizes on the order of tens of thousands of receivers. A study of the quantity of feedback for this type of protocol is described in [NormFeedback]. NORM is able to operate with a smaller amount of feedback than a single TCP connection, even with relatively large numbers of receivers. Thus, depending upon the network topology, it is possible that NORM may scale to larger group sizes. With respect to computer resource usage, the NORM protocol does NOT require that state be kept on all receivers in the group. NORM senders maintain Adamson, et al. Expires October 26, 2009 [Page 8] Internet-Draft NORM Protocol April 2009 state only for receivers providing explicit congestion control feedback. However, NORM receivers must maintain state for each active sender. This may constrain the number of simultaneous senders in some uses of NORM. 1.4. Environmental Requirements and Considerations All of the environmental requirements and considerations that apply to the Multicast NACK Building Block [RFC5401], FEC Building Block [RFC5052], and TCP-Friendly Multicast Congestion Control (TFMCC) Building Block [RFC4654] also apply to the NORM protocol. The NORM protocol SHALL be capable of operating in an end-to-end fashion with no assistance from intermediate systems beyond basic IP multicast group management, routing, and forwarding services. While the techniques utilized in NORM are principally applicable to flat, end-to-end IP multicast topologies, they could also be applied in the sub-levels of hierarchical (e.g., tree-based) multicast distribution if so desired. NORM can make use of reciprocal (among senders and receivers) multicast communication under the Any-Source Multicast (ASM) model defined in Host Extensions for IP Multicasting [RFC1112], but SHALL also be capable of scalable operation in asymmetric topologies such as Source-Specific Multicast (SSM) [RFC4607] where there may only be unicast routing service from the receivers to the sender(s). NORM is compatible with IPv4 and IPv6. Additionally, NORM may be used with networks employing Network Address Translation (NAT) providing the NAT device supports IP multicast and/or can cache UDP traffic source port numbers for remapping feedback traffic from receivers to the sender(s). 2. Architecture Definition A NormSession is comprised of participants (NormNodes) acting as senders and/or receivers. NORM senders transmit data content in the form of NormObjects to the session destination address and the NORM receivers attempt to reliably receive the transmitted content using negative acknowledgments to request repair. Each NormNode within a NormSession is assumed to have a preselected unique 32-bit identifier (NormNodeId). NormNodes MUST have uniquely assigned identifiers within a single NormSession to distinguish between possible multiple senders and to distinguish feedback information from different receivers. There are two reserved NormNodeId values. A value of "0x00000000" is considered an invalid NormNodeId value and a value of "0xffffffff" is a "wild card" NormNodeId. While the protocol does not preclude multiple sender nodes concurrently transmitting within Adamson, et al. Expires October 26, 2009 [Page 9] Internet-Draft NORM Protocol April 2009 the context of a single NORM session (i.e., many- to-many operation), any type of interactive coordination among NORM senders is assumed to be controlled by the application or higher protocol layer. There are some optional mechanisms specified in this document that can be leveraged for such application layer coordination. As previously noted, NORM allows for reliable transmission of three different basic types of data content. The first type is "NORM_OBJECT_DATA", which is used for static, persistent blocks of data content maintained in the sender's application memory storage. The second type is "NORM_OBJECT_FILE", which corresponds to data stored in the sender's non-volatile file system. The "NORM_OBJECT_DATA" and "NORM_OBJECT_FILE" types both represent NormObjects of finite but potentially very large size. The third type of data content is "NORM_OBJECT_STREAM", which corresponds to an ongoing transmission of undefined length. This is analogous to the reliable stream service provide by TCP for unicast data transport. The format of the stream content is application-defined and may be byte or message oriented. The NORM protocol provides for "flushing" of the stream to expedite delivery or possibly enforce application message boundaries. NORM protocol implementations may offer either (or both) in-order delivery of the stream data to the receive application or out-of-order (more immediate) delivery of received segments of the stream to the receiver application. In either case, NORM sender and receiver implementations provide buffering to facilitate repair of the stream as it is transported. All NormObjects are logically segmented into FEC coding blocks and symbols for transmission by the sender. In NORM, an FEC encoding symbol directly corresponds to the payload of "NORM_DATA" messages or "segment". Note that when systematic FEC codes are used, the payload of "NORM_DATA" messages sent for the first portion of a FEC encoding block are source symbols (actual segments of original user data), while the remaining symbols for the block consist of parity symbols generated by FEC encoding. These parity symbols are generally sent in response to repair requests, but some number may be sent proactively at the end each encoding block to increase the robustness of transmission. When non-systematic FEC codes are used, all symbols sent consist of FEC encoding parity content. In this case, the receiver must receive a sufficient number of symbols to reconstruct (via FEC decoding) the original user data for the given block. Transmitted NormObjects are temporarily yet uniquely identified within the NormSession context using the given sender's NormNodeId, NormInstanceId, and a temporary NormObjectTransportId. Depending upon the implementation, individual NORM senders may manage their NormInstanceIds independently, or a common NormInstanceId may be agreed upon for all participating nodes within a session if needed as Adamson, et al. Expires October 26, 2009 [Page 10] Internet-Draft NORM Protocol April 2009 a session identifier. NORM NormObjectTransportId data content identifiers are sender-assigned and applicable and valid only during a NormObject's actual transport (i.e., for as long as the sender is transmitting and providing repair of the indicated NormObject). For a long-lived session, the NormObjectTransportId field can wrap and previously-used identifiers may be re-used. Note that globally unique identification of transported data content is not provided by NORM and, if required, must be managed by the NORM application. The individual segments or symbols of the NormObject are further identified with FEC payload identifiers which include coding block and symbol identifiers. These are discussed in detail later in this document. 2.1. Protocol Operation Overview A NORM sender primarily generates messages of type "NORM_DATA". These messages carry original data segments or FEC symbols and repair segments/symbols for the bulk data/file or stream NormObjects being transferred. By default, redundant FEC symbols are sent only in response to receiver repair requests (NACKs) and thus normally little or no additional transmission overhead is imposed due to FEC encoding. However, the NORM implementation MAY be optionally configured to proactively transmit some amount of redundant FEC symbols along with the original content to potentially enhance performance (e.g., improved delay) at the cost of additional transmission overhead. This option may be sensible for certain network conditions and can allow for robust, asymmetric multicast (e.g., unidirectional routing, satellite, cable) [FecHybrid] with reduced receiver feedback, or, in some cases, no feedback. A sender message of type "NORM_INFO" is also defined and is used to carry OPTIONAL out-of-band context information for a given transport object. A single "NORM_INFO" message can be associated with a NormObject. Because of its atomic nature, missing "NORM_INFO" messages can be NACKed and repaired with a slightly lower delay process than NORM's general FEC-encoded data content. "NORM_INFO" may serve special purposes for some bulk transfer, reliable multicast applications where receivers join the group mid-stream and need to ascertain contextual information on the current content being transmitted. The NACK process for "NORM_INFO" will be described later. When the "NORM_INFO" message type is used, its transmission should precede transmission of any "NORM_DATA" message for the associated NormObject. The sender also generates messages of type "NORM_CMD" to assist in certain protocol operations such as congestion control, end-of- transmission flushing, round trip time estimation, receiver synchronization, and optional positive acknowledgment requests or Adamson, et al. Expires October 26, 2009 [Page 11] Internet-Draft NORM Protocol April 2009 application defined commands. The transmission of "NORM_CMD" messages from the sender is accomplished by one of three different procedures. These procedures are: single, best effort unreliable transmission of the command; repeated redundant transmissions of the command; and positively-acknowledged commands. The transmission technique used for a given command depends upon the function of the command. Several core commands are defined for basic protocol operation. Additionally, implementations MAY wish to consider providing the OPTIONAL application-defined commands that can take advantage of the transmission methodologies available for commands. This allows for application-level session management mechanisms that can make use of information available to the underlying NORM protocol engine (e.g., round-trip timing, transmission rate, etc.). A notable distinction between "NORM_DATA" message and some "NORM_CMD" message transmissions is that typically a receiver will need to allocate resources to manage reliable reception when "NORM_DATA" messages are received. However some "NORM_CMD" messages may be completely atomic and no specific state may need to be kept. Thus, for session management or other purposes it is possible that even participants acting principally as data receivers MAY transmit "NORM_CMD" messages. However, it is RECOMMENDED that this is not done within the context of the NORM multicast session unless congestion control is addressed. For example, many receiver nodes transmitting "NORM_CMD" messages simultaneously can cause congestion for the destination(s). All sender transmissions are subject to rate control governed by a peak transmission rate set for each participant by the application. This can be used to limit the quantity of multicast data transmitted by the group. When NORM's congestion control algorithm is enabled the rate for senders is automatically adjusted. In some networks, it may be desirable to establish minimum and maximum bounds for the rate adjustment depending upon the application even when dynamic congestion control is enabled. However, in the case of the general Internet, congestion control policy SHALL be observed that is compatible with coexistent TCP flows. NORM receivers generate messages of type "NORM_NACK" or "NORM_ACK" in response to transmissions of data and commands from a sender. The "NORM_NACK" messages are generated to request repair of detected data transmission losses. Receivers generally detect losses by tracking the sequence of transmission from a sender. Sequencing information is embedded in the transmitted data packets and end-of-transmission commands from the sender. "NORM_ACK" messages are generated in response to certain commands transmitted by the sender. In the general (and most scalable) protocol mode, "NORM_ACK" messages are sent only in response to congestion control commands from the sender. The feedback volume of these congestion control "NORM_ACK" messages Adamson, et al. Expires October 26, 2009 [Page 12] Internet-Draft NORM Protocol April 2009 is controlled using the same timer-based probabilistic suppression techniques as for "NORM_NACK" messages to avoid feedback implosion. In order to meet potential application requirements for positive acknowledgment from receivers, other "NORM_ACK" messages are defined and available for use. 2.2. Protocol Building Blocks The operation of the NORM protocol is based primarily upon the concepts presented in the Multicast NACK Building Block [RFC5401] document. This includes the basic NORM architecture and the data transmission, repair, and feedback strategies discussed in that document. The reliable multicast building block approach, as described in Reliable Multicast Transport Building Blocks for One-to- Many Bulk-Data Transfer [RFC3048], is applied in creating the full NORM protocol instantiation. NORM also makes use of the parity-based encoding techniques for repair messaging and optional transmission robustness as described in The Use of Forward Error Correction (FEC) in Reliable Multicast [RFC3453]. NORM uses the FEC Payload ID as specified by the FEC Building Block document [RFC5052]. Additionally, for congestion control, this document fully specifies a baseline congestion control mechanism (NORM-CC) based on the TCP- Friendly Multicast Congestion Control (TFMCC) scheme[TfmccPaper], [RFC4654]. 2.3. Design Tradeoffs While the various features of NORM are designed to provide some measure of general purpose utility, it is important to emphasize the understanding that "no one size fits all" in the reliable multicast transport arena. There are numerous engineering trade-offs involved in reliable multicast transport design and this requires an increased awareness of application and network architecture considerations. Performance requirements affecting design can include: group size, heterogeneity (e.g., capacity and/or delay), asymmetric delivery, data ordering, delivery delay, group dynamics, mobility, congestion control, and transport across low capacity connections. NORM contains various parameters to accommodate many of these differing requirements. The NORM protocol and its mechanisms MAY be applied in multicast applications outside of bulk data transfer, but there is an assumed model of bulk transfer transport service that drives the trade-offs that determine the scalability and performance described in this document. The ability of NORM to provide reliable data delivery is also governed by any buffer constraints of the sender and receiver applications. NORM protocol implementations SHOULD be designed to operate with the greatest efficiency and robustness possible within Adamson, et al. Expires October 26, 2009 [Page 13] Internet-Draft NORM Protocol April 2009 application-defined buffer constraints. Buffer requirements for reliability, as always, are a function of the delay-bandwidth product of the network topology. NORM performs best when allowed more buffering resources than typical point-to-point transport protocols. This is because NORM feedback suppression is based upon randomly- delayed transmissions from the receiver set, rather than immediately transmitted feedback. There are definitive trade-offs between buffer utilization, group size scalability, and efficiency of performance. Large buffer sizes allow the NORM protocol to perform most efficiently in large delay-bandwidth topologies and allow for longer feedback suppression backoff timeouts. This yields improved group size scalability. NORM can operate with reduced buffering but at a cost of decreased efficiency (lower relative goodput) and reduced group size scalability. 3. Conformance Statement This RMT Protocol Instantiation document, in conjunction with the Multicast Negative-Acknowledgment (NACK) [RFC5401] and Forward Error Correction (FEC) [RFC5052] Building Blocks, completely specifies a working reliable multicast transport protocol that conforms to the requirements described in RFC 2357. This document specifies the following message types and mechanisms which are REQUIRED in complying NORM protocol implementations: +------------------------+------------------------------------------+ | Message Type | Purpose | +------------------------+------------------------------------------+ | "NORM_DATA" | Sender message for application data | | | transmission. Implementations must | | | support at least one of the | | | "NORM_OBJECT_DATA", "NORM_OBJECT_FILE", | | | or "NORM_OBJECT_STREAM" delivery | | | services. The use of the NORM FEC | | | Object Transmission Information header | | | extension is OPTIONAL with "NORM_DATA" | | | messages. | | "NORM_CMD(FLUSH)" | Sender command to excite receivers for | | | repair requests in lieu of ongoing | | | "NORM_DATA" transmissions. Note the use | | | of the "NORM_CMD(FLUSH)" for positive | | | acknowledgment of data receipt is | | | OPTIONAL. | | "NORM_CMD(SQUELCH)" | Sender command to advertise its current | | | valid repair window in response to | | | invalid requests for repair. | Adamson, et al. Expires October 26, 2009 [Page 14] Internet-Draft NORM Protocol April 2009 | "NORM_CMD(REPAIR_ADV)" | Sender command to advertise current | | | repair (and congestion control state) to | | | group when unicast feedback messages are | | | detected. Used to control/suppress | | | excessive receiver feedback in | | | asymmetric multicast topologies. | | "NORM_CMD(CC)" | Sender command used in collection of | | | round trip timing and congestion control | | | status from group (this may be OPTIONAL | | | if alternative congestion control | | | mechanism and round trip timing | | | collection is used). | | "NORM_NACK" | Receiver message used to request repair | | | of missing transmitted content. | | "NORM_ACK" | Receiver message used to proactively | | | provide feedback for congestion control | | | purposes. Also used with the OPTIONAL | | | NORM Positive Acknowledgment Process. | +------------------------+------------------------------------------+ This document also describes the following message types and associated mechanisms which are OPTIONAL for complying NORM protocol implementations: +-------------------------+-----------------------------------------+ | Message Type | Purpose | +-------------------------+-----------------------------------------+ | "NORM_INFO" | Sender message for providing ancillary | | | context information associated with | | | NORM transport objects. The use of the | | | NORM FEC Object Transmission | | | Information header extension is | | | OPTIONAL with "NORM_INFO" messages. | | "NORM_CMD(EOT)" | Sender command to indicate it has | | | reached end-of-transmission and will no | | | longer respond to repair requests. | | "NORM_CMD(ACK_REQ)" | Sender command to support | | | application-defined, positively | | | acknowledged commands sent outside of | | | the context of the bulk data content | | | being transmitted. The NORM Positive | | | Acknowledgment Procedure associated | | | with this message type is OPTIONAL. | | "NORM_CMD(APPLICATION)" | Sender command containing | | | application-defined commands sent | | | outside of the context of the bulk data | | | content being transmitted. | Adamson, et al. Expires October 26, 2009 [Page 15] Internet-Draft NORM Protocol April 2009 | "NORM_REPORT" | Optional message type reserved for | | | experimental implementations of the | | | NORM protocol. | +-------------------------+-----------------------------------------+ 4. Message Formats As mentioned in Section 2.1, there are two primary classes of NORM messages: sender messages and receiver messages. "NORM_CMD", "NORM_INFO", and "NORM_DATA" message types are generated by senders of data content, and "NORM_NACK" and "NORM_ACK" messages generated by receivers within a NormSession. Sender messages SHOULD be governed by congestion control for Internet use. For session management or other purposes, receivers may wish to employ "NORM_CMD" message transmissions. The principal rationale for distinguishing sender and receiver messages is that receivers will typically need to allocate resources to support reliable reception from sender(s) and NORM sender messages are subject to congestion control. NORM receivers MAY employ the "NORM_CMD" message type for application-defined purposes but it is RECOMMENDED that congestion control and feedback implosion issues be addressed. Additionally, an auxiliary message type of "NORM_REPORT" is also provided for experimental purposes. This section describes the message formats used by the NORM protocol. These messages and their fields are referenced in the detailed functional description of the NORM protocol given in Section 5. Individual NORM messages are designed to be compatible with the MTU limitations of encapsulating Internet protocols including IPv4, IPv6, and UDP. The current NORM protocol specification assumes UDP encapsulation and leverages the transport features of UDP. The NORM messages are independent of network addresses and can be used in IPv4 and IPv6 networks. 4.1. NORM Common Message Header and Extensions There are some common message fields contained in all NORM message types. Additionally, a header extension mechanism is defined to expand the functionality of the NORM protocol without revision to this document. All NORM protocol messages begin with a common header with information fields as follows: 0 1 2 3 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ |version| type | hdr_len | sequence | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | source_id | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ Adamson, et al. Expires October 26, 2009 [Page 16] Internet-Draft NORM Protocol April 2009 NORM Common Message Header Format The "version" field is a 4-bit value indicating the protocol version number. NORM implementations SHOULD ignore received messages with version numbers different from their own. This number is intended to indicate and distinguish upgrades of the protocol which may be non- interoperable. The NORM version number for this specification is 1. The message "type" field is a 4-bit value indicating the NORM protocol message type. These types are defined as follows: +---------------+-------+ | Message | Value | +---------------+-------+ | "NORM_INFO" | 1 | | "NORM_DATA" | 2 | | "NORM_CMD" | 3 | | "NORM_NACK" | 4 | | "NORM_ACK" | 5 | | "NORM_REPORT" | 6 | +---------------+-------+ The 8-bit "hdr_len" field indicates the number of 32-bit words that comprise the given message's header portion. This is used to facilitate header extensions that may be applied. The presence of header extensions are implied when the "hdr_len" value is greater than the base value for the given message "type". The "sequence" field is a 16-bit value that is set by the message originator. The "sequence" field serves two separate purposes, depending upon the message type: 1. NORM senders MUST set the "sequence" field of sender messages ("NORM_INFO", "NORM_DATA", and "NORM_CMD") so that receivers can monitor the "sequence" value to maintain an estimate of packet loss that can be used for congestion control purposes (See Section 5.5.2 for a detailed description of NORM Congestion Control operation). A monotonically-increasing sequence number space MUST be maintained to mark NORM sender messages in this way. Note that this "sequence" number is explicitly NOT used in NORM as part of its reliability procedures. The NORM object and FEC payload identifiers are used to detect missing content for reliable transfer purposes. 2. NORM receivers SHOULD set the "sequence" field to support protection from message replay attacks of "NORM_NACK" or "NORM_NACK" messages. Note that, depending upon configuration, NORM feedback messages may be sent to the session multicast address or unicast address[es] of the active NORM sender[s]. Thus, a separate, monotonically-increasing sequence number space Adamson, et al. Expires October 26, 2009 [Page 17] Internet-Draft NORM Protocol April 2009 MUST be maintained for each destination address to which the NORM receiver is transmitting feedback messages. Note that these two separate purposes necessitate the maintenance of separate sequence spaces to support the functions described here. And, in the case of NORM receivers, additional sequence spaces are needed when feedback messages are sent to the sender unicast address[es] instead of the session address. The "source_id" field is a 32-bit value that uniquely identifies the node that sent the message within the context of a single NormSession. This value is termed the NORM node identifier (NormNodeId) and unique NormNodeId identifiers MUST be assigned within a single NormSession. In some cases, use of the host IP address or a hash of it can suffice, but alternative methodologies for assignment and potential collision resolution of node identifiers within a multicast session SHOULD be considered. For example, the techniques for managing the 32-bit "synchronization source" (SSRC) identifiers defined in the Real-Time Protocol (RTP) specification [RFC3550] are applicable for use with NORM node identifiers. In most deployments of the NORM protocol to date, the NormNodeId assignments are administratively configured. NORM Header Extensions When header extensions are applied, they follow the message type's base header and precede any payload portion. There are two formats for header extensions, both of which begin with an 8-bit "het" (header extension type) field. One format is provided for variable- length extensions with "het" values in the range from 0 through 127. The other format is for fixed length (one 32-bit word) extensions with "het" values in the range from 128 through 255. These formats are given here: 0 1 2 3 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | het <=127 | hel | | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | | Header Extension Content | | ... | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ NORM Variable Length Header Extension Format Adamson, et al. Expires October 26, 2009 [Page 18] Internet-Draft NORM Protocol April 2009 0 1 2 3 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | het >=128 | reserved | Header Extension Content | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ NORM Fixed Length (32-bit) Header Extension Format The "Header Extension Content" portion of the header extension is defined for each extension type. Some header extensions are defined within this document for NORM baseline FEC and congestion control operations. 4.2. Sender Messages NORM sender messages include the "NORM_DATA" type, the "NORM_INFO" type, and the "NORM_CMD" type. "NORM_DATA" and "NORM_INFO" messages contain application data content while "NORM_CMD" messages are used for various protocol control functions. 4.2.1. NORM_DATA Message The "NORM_DATA" message is expected to be the predominant type transmitted by NORM senders. These messages are used to encapsulate segmented data content for objects of type "NORM_OBJECT_DATA", "NORM_OBJECT_FILE", and "NORM_OBJECT_STREAM". "NORM_DATA" messages may contain original or FEC-encoded application data content. The format of "NORM_DATA" messages is comprised of three logical portions: 1) a fixed-format "NORM_DATA" header portion, 2) a FEC Payload ID portion with a format dependent upon the FEC encoding used, and 3) a payload portion containing source or encoded application data content. Note for objects of type "NORM_OBJECT_STREAM", the payload portion contains additional fields used to appropriately recover stream content. NORM implementations MAY also extend the "NORM_DATA" header to include a FEC Object Transmission Information (EXT_FTI) header extension. This allows NORM receivers to automatically allocate resources and properly perform FEC decoding without the need for pre-configuration or out- of-band information. Adamson, et al. Expires October 26, 2009 [Page 19] Internet-Draft NORM Protocol April 2009 0 1 2 3 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ |version| type=2| hdr_len | sequence | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | source_id | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | instance_id | grtt |backoff| gsize | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | flags | fec_id | object_transport_id | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | fec_payload_id | | ... | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | header_extensions (if applicable) | | ... | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | payload_len* | payload_msg_start* | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | payload_offset* | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | payload_data* | | ... | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ NORM_DATA Message Format *IMPORTANT NOTE: The "payload_len", "payload_msg_start" and "payload_offset" fields are present ONLY for objects of type "NORM_OBJECT_STREAM". These fields, as with the entire payload, are subject to any FEC encoding used. Thus, when systematic FEC codes are used, these values may be directly interpreted for packets containing source symbols only while packets containing FEC parity content require decoding before these fields can be interpreted. The "version", "type", "hdr_len", "sequence", and "source_id" fields form the NORM Common Message Header as described in Section 4.1. The value of the "NORM_DATA" "type" field is 2. The "NORM_DATA" base "hdr_len" value is 4 (i.e. 4 32-bit words) plus the size of the "fec_payload_id" field. The "fec_payload_id" field size depends upon the FEC encoding type referenced by the "fec_id" field. For example, when small block, systematic codes are used, a "fec_id" value of 129 is indicated and the size of the "fec_payload_id" is two 32-bit words. In this case the "NORM_DATA" base "hdr_len" value is 6. The cumulative size of any header extensions applied is added into the "hdr_len" field. The "instance_id" field contains a value generated by the sender to Adamson, et al. Expires October 26, 2009 [Page 20] Internet-Draft NORM Protocol April 2009 uniquely identify its current instance of participation in the NormSession. This allows receivers to detect when senders have perhaps left and rejoined a session in progress. When a sender (identified by its "source_id") is detected to have a new "instance_id", the NORM receivers SHOULD drop their previous state on the sender and begin reception anew, or at least treat this "instance" as a new, separate sender. The "grtt" field contains a non-linear quantized representation of the sender's current estimate of group round-trip time (GRTT) (this is also referred to as "R_max" in [TfmccPaper]). This value is used to control timing of the NACK repair process and other aspects of protocol operation as described in this document. Normally, the advertised "grtt" value will correspond to what the sender has measured based on feedback from the group, but, at low transmission rates, the advertised "grtt" SHALL be set to "MAX(grttMeasured, NormSegmentSize/senderRate)" where the "NormSegmentSize" is sender's segment size in bytes and the "senderRate" is the sender's current transmission rate in bytes per second. The algorithm for encoding and decoding this field is described in the Multicast NACK Building Block [RFC5401]. The "backoff" field value is used by receivers to determine the maximum backoff timer value used in the timer-based NORM NACK feedback suppression. This 4-bit field supports values from 0-15 which is multiplied by the sender GRTT to determine the maximum backoff timeout. The "backoff" field informs the receivers of the sender's backoff factor parameter "Ksender". Recommended values and their use are described in the NORM receiver NACK procedure description in Section 5.3. The "gsize" field contains a representation of the sender's current estimate of group size. This 4-bit field can roughly represent values from ten to 500 million where the most significant bit value of 0 or 1 represents a mantissa of 1 or 5, respectively and the three least significant bits incremented by one represent a base 10 exponent (order of magnitude). For examples, a field value of "0x0" represents 1.0e+01 (10), a value of "0x8" represents 5.0e+01 (50), a value of "0x1" represents 1.0e+02 (100), and a value of "0xf" represents 5.0e+08. For NORM feedback suppression purposes, the group size does not need to be represented with a high degree of precision. The group size may even be estimated somewhat conservatively (i.e., overestimated) to maintain low levels of feedback traffic. A default group size estimate of 10,000 ("gsize" = 0x3) is recommended for general purpose reliable multicast applications using the NORM protocol. The "flags" field contains a number of different binary flags Adamson, et al. Expires October 26, 2009 [Page 21] Internet-Draft NORM Protocol April 2009 providing information and hints regarding how the receiver should handle the identified object. Defined flags in this field include: +------------------------+-------+----------------------------------+ | Flag | Value | Purpose | +------------------------+-------+----------------------------------+ | "NORM_FLAG_REPAIR" | 0x01 | Indicates message is a repair | | | | transmission | | "NORM_FLAG_EXPLICIT" | 0x02 | Indicates a repair segment | | | | intended to meet a specific | | | | receiver erasure, as compared to | | | | parity segments provided by the | | | | sender for general purpose (with | | | | respect to an FEC coding block) | | | | erasure filling. | | "NORM_FLAG_INFO" | 0x04 | Indicates availability of | | | | "NORM_INFO" for object. | | "NORM_FLAG_UNRELIABLE" | 0x08 | Indicates that repair | | | | transmissions for the specified | | | | object will be unavailable | | | | (One-shot, best effort | | | | transmission). | | "NORM_FLAG_FILE" | 0x10 | Indicates object is file-based | | | | data (hint to use disk storage | | | | for reception). | | "NORM_FLAG_STREAM" | 0x20 | Indicates object is of type | | | | "NORM_OBJECT_STREAM". | +------------------------+-------+----------------------------------+ "NORM_FLAG_REPAIR" is set when the associated message is a repair transmission. This information can be used by receivers to help observe a join policy where it is desired that newly joining receivers only begin participating in the NACK process upon receipt of new (non-repair) data content. "NORM_FLAG_EXPLICIT" is used to mark repair messages sent when the data sender has exhausted its ability to provide "fresh" (not previously transmitted) parity segments as repair. This flag could possibly be used by intermediate systems implementing functionality to control sub-casting of repair content to different legs of a reliable multicast topology with disparate repair needs. "NORM_FLAG_INFO" is set only when optional "NORM_INFO" content is actually available for the associated object. Thus, receivers will NACK for retransmission of "NORM_INFO" only when it is available for a given object. "NORM_FLAG_UNRELIABLE" is set when the sender wishes to transmit an object with only "best effort" delivery and will not supply repair transmissions for the object. NORM receivers SHOULD NOT execute repair requests for objects marked with the "NORM_FLAG_UNRELIABLE" flag. Note that receivers may inadvertently request repair of such objects when all segments (or Adamson, et al. Expires October 26, 2009 [Page 22] Internet-Draft NORM Protocol April 2009 info content) for those objects are not received (i.e., a gap in the "object_transport_id" sequence is noted). In this case, the sender should invoke the "NORM_CMD(SQUELCH)" process as described in Section 4.2.3. "NORM_FLAG_FILE" can be set as a hint from the sender that the associated object should be stored in non-volatile storage. "NORM_FLAG_STREAM" is set when the identified object is of type "NORM_OBJECT_STREAM". The presence of "NORM_FLAG_STREAM" overrides that of "NORM_FLAG_FILE" with respect to interpretation of object size and the format of "NORM_DATA" messages. The "fec_id" field corresponds to the FEC Encoding Identifier described in the FEC Building Block document [RFC5052]. The "fec_id" value implies the format of the "fec_payload_id" field and, coupled with FEC Object Transmission Information, the procedures to decode FEC encoded content. Small block, systematic codes ("fec_id" = 129) are expected to be used for most NORM purposes and the "NORM_OBJECT_STREAM" requires systematic FEC codes for most efficient performance. The "object_transport_id" field is a monotonically and incrementally increasing value assigned by the sender to NormObjects being transmitted. Transmissions and repair requests related to that object use the same "object_transport_id" value. For sessions of very long or indefinite duration, the "object_transport_id" field may be repeated, but it is presumed that the 16-bit field size provides an adequate enough sequence space to avoid object confusion amongst receivers and sources (i.e., receivers SHOULD re-synchronize with a server when receiving object sequence identifiers sufficiently out- of-range with the current state kept for a given source). During the course of its transmission within a NORM session, an object is uniquely identified by the concatenation of the sender "source_id" and the given "object_transport_id". Note that "NORM_INFO" messages associated with the identified object carry the same "object_transport_id" value. The "fec_payload_id" identifies the attached "NORM_DATA" "payload" content. The size and format of the "fec_payload_id" field depends upon the FEC type indicated by the "fec_id" field. These formats are given in the descriptions of specific FEC schemes such as those described in the FEC Basic Schemes [RFC5445] specification or in other FEC Schemes. As an example, the format of the "fec_payload_id" format for Small Block, Systematic codes ("fec_id" = 129) from theFEC Basic Schemes [RFC5445] specification is given here: Adamson, et al. Expires October 26, 2009 [Page 23] Internet-Draft NORM Protocol April 2009 0 1 2 3 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | source_block_number | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | source_block_len | encoding_symbol_id | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ Example: FEC Payload Id Format for 'fec_id' = 129 In this example FEC payload identifier, the "source_block_number", "source_block_len", and "encoding_symbol_id" fields correspond to the "Source Block Number", "Source Block Length, and "Encoding Symbol ID" fields of the FEC Payload ID format for Small Block Systematic FEC Schemes identified by a "fec_id" value of 129 as specified by the FEC Basic Schemes [RFC5445] specification. The "source_block_number" identifies the coding block's relative position with a NormObject. Note that, for NormObjects of type "NORM_OBJECT_STREAM", the "source_block_number" may wrap for very long lived sessions. The "source_block_len" indicates the number of user data segments in the identified coding block. Given the "source_block_len" information of how many symbols of application data are contained in the block, the receiver can determine whether the attached segment is data or parity content and treat it appropriately. Some applications may dynamically "shorten" code blocks when the pending information content is not predictable (e.g. real-time message streams). In that case, the "source_block_len" value given for an "encoding_symbol_id" that contains FEC parity content SHALL take precedence over the "source_block_len" value provided for any packets containing source symbols. Also, the "source_block_len" value given for an ordinally higher "encoding_symbol_id" SHALL take precedence over the "source_block_len" given for prior encoding symbols. The reason for this is that the sender may only know the maximum source block length at the time is transmitting source symbols, but then subsequently "shorten" the code and then provide that last source symbol and/or encoding symbols with FEC parity content. The "encoding_symbol_id" identifies which specific symbol (segment) within the coding block the attached payload conveys. Depending upon the value of the "encoding_symbol_id" and the associated "source_block_len" parameters for the block, the symbol (segment) referenced may be a user data or an FEC parity segment. For systematic codes, encoding symbols numbered less than the "source_block_len" contain original application data while segments greater than or equal to "source_block_len" contain parity symbols calculated for the block. The concatenation of "object_transport_id::fec_payload_id" can be viewed as a unique transport protocol data unit identifier for the attached segment with respect to the NORM sender's instance within a session. Adamson, et al. Expires October 26, 2009 [Page 24] Internet-Draft NORM Protocol April 2009 Additional FEC Object Transmission Information (FTI) (as described in the FEC Building Block [RFC5052]) is required to properly receive and decode NORM transport objects. This information MAY be provided as out-of-band session information. However, in some cases, it may be useful for the sender to include this information "in-band" to facilitate receiver operation with minimal pre-configuration. For this purpose, the NORM FEC Object Transmission Information Header Extension (EXT_FTI) is defined. This header extension MAY be applied to "NORM_DATA" and "NORM_INFO" messages to provide this necessary information. The format of the EXT_FTI consists of two parts, a general part that contains the size of the associated transport object and a portion that depends upon the FEC scheme being used. The "fec_id" field in "NORM_DATA" and "NORM_INFO" messages identifies the FEC scheme. The format of the EXT_FTI general part is given here. 0 1 2 3 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | het = 64 | hel = 4 | object_size (msb) | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | object_size (lsb) | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | FEC Scheme specific content ... | EXT_FTI Header Extension General Portion Format The header extension type "het" field value for the EXT_FTI header extension is 64. The header extension length "hel" value depends upon the format of the FTI for encoding type identified by the "fec_id" field. The 48-bit "object_size" field indicates the total length of the object (in bytes) for the static object types of "NORM_OBJECT_FILE" and "NORM_OBJECT_DATA". This information is used by receivers to determine storage requirements and/or allocate storage for the received object. Receivers with insufficient storage capability may wish to forego reliable reception (i.e., not NACK for) of the indicated object. In the case of objects of type "NORM_OBJECT_STREAM", the "object_size" field is used by the sender to advertise the size of its stream buffer to the receiver group. In turn, the receivers SHOULD use this information to allocate a stream buffer for reception of corresponding size. As noted, the format of the extension depends upon the FEC code in use, but in general it SHOULD contain any required details on the FEC code in use (e.g., FEC Instance ID, etc.). As an example, the format of the EXT_FTI for small block systematic codes ("fec_id" = 129) is Adamson, et al. Expires October 26, 2009 [Page 25] Internet-Draft NORM Protocol April 2009 given here: 0 1 2 3 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | het = 64 | hel = 4 | object_size (msb) | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | object_size (lsb) | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | fec_instance_id | segment_size | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | fec_max_block_len | fec_num_parity | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ Example: EXT_FTI Header Extension Format for 'fec_id' = 129 In this example (for "fec_id" = 129), the "hel" field value is 4. The size of the EXT_FTI header extension may be different for other FEC schemes. The 48-bit "object_size" serves the purpose described previously. The "fec_instance_id" corresponds to the "FEC Instance ID" described in the FEC Building Block [RFC5052]. In this case, the "fec_instance_id" is a value corresponding to the particular type of Small Block Systematic Code being used (e.g., Reed-Solomon GF(2^8), Reed-Solomon GF(2^16), etc). The standardized assignment of FEC Instance ID values is described in RFC 5052. The "segment_size" field indicates the sender's current setting for maximum message payload content (in bytes). This allows receivers to allocate appropriate buffering resources and to determine other information in order to properly process received data messaging. Typically, FEC parity symbol segments will be of this size. The "fec_max_block_len" indicates the current maximum number of user data segments per FEC coding block to be used by the sender during the session. This allows receivers to allocate appropriate buffer space for buffering blocks transmitted by the sender. The "fec_num_parity" corresponds to the "maximum number of encoding symbols that can be generated for any source block" as described in for FEC Object Transmission Information for Small Block Systematic Codes in the FEC Building Block [RFC5052]. For example, Reed-Solomon codes may be arbitrarily shortened to create different code variations for a given block length. In the case of Reed-Solomon (GF(2^8) and GF(2^16)) codes, this value indicates the maximum number of parity segments available from the sender for the coding blocks. This field MAY be interpreted differently for other systematic codes Adamson, et al. Expires October 26, 2009 [Page 26] Internet-Draft NORM Protocol April 2009 as they are defined. The payload portion of "NORM_DATA" messages includes source data or FEC encoded application content. The content of this payload depends upon the FEC scheme being employed, and support for streaming using the "NORM_OBJECT_STREAM" type, when applicable, necessitates some additional content in the payload. The "payload_len", "payload_msg_start", and "payload_offset" fields are present ONLY for transport objects of type "NORM_OBJECT_STREAM". These fields allow senders to arbitrarily vary the size of "NORM_DATA" payload segments for streams. This allows applications to flush transmitted streams as needed to meet unique streaming requirements. For objects of types "NORM_OBJECT_FILE" and "NORM_OBJECT_DATA", these fields are unnecessary since the receiver can calculate the payload length and offset information from the "fec_payload_id" using the REQUIRED block partitioning algorithm described in the FEC Building Block [RFC5052]. When systematic FEC codes (e.g., "fec_id" = 129) are used, the "payload_len", "payload_msg_start", and "payload_offset" fields contain actual payload_data length, message start index (or stream control code), and byte offset values for the associated application stream data segment (the remainder of the "payload_data" field content) for those "NORM_DATA" messages containing source data symbols. In "NORM_DATA" messages that contain FEC parity content, these fields do not contain values that can be directly interpreted, but instead are values computed from FEC encoding the "payload_len", "payload_msg_start", and "payload_offset" fields for the source data segments of the corresponding coding block. The actual "payload_msg_start", "payload_len" and "payload_offset" values of missing data content can be determined upon decoding a FEC coding block. Note that these fields do NOT contribute to the value of the "NORM_DATA" "hdr_len" field. These fields are present only when the "flags" portion of the "NORM_DATA" message indicate the transport object is of type "NORM_OBJECT_STREAM". The "payload_len" value, when non-zero, indicates the length (in bytes) of the source content contained in the associated "payload_data" field. However, when the "payload_len" value is equal to ZERO, this indicates that the "payload_msg_start" field should be alternatively interpreted as a "stream_control_code". The only "stream_control_code" value defined is "NORM_STREAM_END = 0". The "NORM_STREAM_END" code indicates that the sender is terminating transmission of stream content at the corresponding position in the stream and the receiver should not expect content (or NACK for any content) following that position in the stream. It is expected that additional specifications may extend the functionality of the NORM stream transport mode by defining additional stream control codes. Adamson, et al. Expires October 26, 2009 [Page 27] Internet-Draft NORM Protocol April 2009 These control codes are delivered to the recipient application reliably, in-order with respect to the streamed application data content. The "payload_msg_start" field serves one of two exclusive purposes. When the "payload_len" value is non-zero, the "payload_msg_start" field, when also set to a non-zero value, indicates that the associated "payload_data" content contains an application-defined message boundary (start-of-message). When such a message boundary is indicated, the first byte of an application-defined message, with respect to the "payload_data" field, will be found at an offset of "payload_msg_start - 1" bytes. Thus, if a "NORM_DATA" payload for a "NORM_OBJECT_STREAM" contains the start of an application message at the first byte of the "payload_data" field, the value of the "payload_msg_start" field will be '1'. NORM implementations SHOULD provide sender stream applications with a capability to mark message boundaries in this manner. Similarly, the NORM receiver implementation SHOULD enable the application to recover such message boundary information. This enables NORM receivers to "synchronize" reliable reception of transmitted message stream content in a meaningful way (i.e., meaningful to the application) at any time, whether joining a session already in progress, or departing the session and returning. Note that if the value of the "payload_msg_start" field is ZERO, no message boundary is present. The "payload_msg_start" value will always be less than or equal to the "payload_len" value except for the special case of "payload_len = 0", that indicates the "payload_msg_start" field should instead be interpreted as a "stream_control_code" The "payload_offset" field indicates the relative byte position (from the sender stream transmission start) of the source content contained in the "payload_data" field. Note that for long-lived streams, the "payload_offset" field may wrap. The "payload_data" field contains the original application source or parity content for the symbol identified by the "fec_payload_id". The length of this field SHALL be limited to a maximum of the sender's NormSegmentSize bytes as given in the FTI for the object. Note the length of this field for messages containing parity content will always be of length NormSegmentSize. When encoding data segments of varying sizes, the FEC encoder SHALL assume ZERO value padding for data segments with length less than the NormSegmentSize. It is RECOMMENDED that a sender's NormSegmentSize generally be constant for the duration of a given sender's term of participation in the session, but may possibly vary on a per-object basis. The NormSegmentSize is expected to be configurable by the sender application prior to session participation as needed for network topology maximum transmission unit (MTU) considerations. For IPv6, Adamson, et al. Expires October 26, 2009 [Page 28] Internet-Draft NORM Protocol April 2009 MTU discovery may be possibly leveraged at session startup to perform this configuration. The "payload_data" content may be delivered directly to the application for source symbols (when systematic FEC encoding is used) or upon decoding of the FEC block. For "NORM_OBJECT_FILE" and "NORM_OBJECT_STREAM" objects, the data segment length and offset can be calculated using the block partitioning algorithm described in the FEC Building Block [RFC5052]. For "NORM_OBJECT_STREAM" objects, the length and offset is obtained from the segment's corresponding embedded "payload_len" and "payload_offset" fields. 4.2.2. NORM_INFO Message The "NORM_INFO" message is used to convey OPTIONAL, application- defined, out-of-band context information for transmitted NormObjects. An example "NORM_INFO" use for bulk file transfer is to place MIME type information for the associated file, data, or stream object into the "NORM_INFO" payload. Receivers may use the "NORM_INFO" content to make a decision as whether to participate in reliable reception of the associated object. Each NormObject can have an independent unit of "NORM_INFO" associated with it. "NORM_DATA" messages contain a flag to indicate the availability of "NORM_INFO" for a given NormObject. NORM receivers may NACK for retransmission of "NORM_INFO" when they have not received it for a given NormObject. The size of the "NORM_INFO" content is limited to that of a single NormSegmentSize for the given sender. This atomic nature allows the "NORM_INFO" to be rapidly and efficiently repaired within the NORM reliable transmission process. When "NORM_INFO" content is available for a NormObject, the NORM_FLAG_INFO flag SHALL be set in "NORM_DATA" messages for the corresponding "object_transport_id" and the "NORM_INFO" message shall be transmitted as the first message for the NormObject. Adamson, et al. Expires October 26, 2009 [Page 29] Internet-Draft NORM Protocol April 2009 0 1 2 3 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ |version| type=1| hdr_len | sequence | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | source_id | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | instance_id | grtt |backoff| gsize | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | flags | fec_id | object_transport_id | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | header_extensions (if applicable) | | ... | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | payload_data | | ... | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ NORM_INFO Message Format The "version", "type", "hdr_len", "sequence", and "source_id" fields form the NORM Common Message Header as described in Section 4.1. The value of "hdr_len" field when no header extensions are present is 4. The "instance_id", "grtt", "backoff", "gsize", "flags", "fec_id", and "object_transport_id" fields carry the same information and serve the same purpose as with "NORM_DATA" messages. These values allow the receiver to prepare appropriate buffering, etc, for further transmissions from the sender when "NORM_INFO" is the first message received. As with "NORM_DATA" messages, the NORM FTI Header Extension (EXT_FTI) may be optionally applied to "NORM_INFO" messages. To conserve protocol overhead, some NORM implementations may wish to apply the EXT_FTI when used to "NORM_INFO" messages only and not to "NORM_DATA" messages. The "NORM_INFO" "payload_data" field contains sender application- defined content which can be used by receiver applications for various purposes as described above. 4.2.3. NORM_CMD Messages "NORM_CMD" messages are transmitted by senders to perform a number of different protocol functions. This includes functions such as round- trip timing collection, congestion control functions, synchronization of sender/receiver repair "windows", and notification of sender status. A core set of "NORM_CMD" messages is enumerated. Adamson, et al. Expires October 26, 2009 [Page 30] Internet-Draft NORM Protocol April 2009 Additionally, a range of command types remain available for potential application-specific use. Some "NORM_CMD" types may have dynamic content attached. Any attached content will be limited to maximum length of the sender NormSegmentSize to retain the atomic nature of commands. All "NORM_CMD" messages begin with a common set of fields, after the usual NORM message common header. The standard "NORM_CMD" fields are: 0 1 2 3 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ |version| type=3| hdr_len | sequence | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | source_id | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | instance_id | grtt |backoff| gsize | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | flavor | | +-+-+-+-+-+-+-+-+ NORM_CMD Content + | ... | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ NORM_CMD Standard Fields The "version", "type", "hdr_len", "sequence", and "source_id" fields form the NORM Common Message Header as described in Section 4.1. The value of the "hdr_len" field for "NORM_CMD" messages without header extensions present depends upon the "flavor" field. The "instance_id", "grtt", "backoff", and "gsize" fields provide the same information and serve the same purpose as with "NORM_DATA" and "NORM_INFO" messages. The "flavor" field indicates the type of command to follow. The remainder of the "NORM_CMD" message is dependent upon the command type ("flavor"). NORM command flavors include: +-------------------------+--------+--------------------------------+ | Command | Flavor | Purpose | +-------------------------+--------+--------------------------------+ | "NORM_CMD(FLUSH)" | 1 | Used to indicate sender | | | | temporary end-of-transmission. | | | | (Assists in robustly | | | | initiating outstanding repair | | | | requests from receivers). May | | | | also be optionally used to | | | | collect positive | | | | acknowledgment of reliable | | | | reception from subset of | | | | receivers. | Adamson, et al. Expires October 26, 2009 [Page 31] Internet-Draft NORM Protocol April 2009 | "NORM_CMD(EOT)" | 2 | Used to indicate sender | | | | permanent end-of-transmission. | | "NORM_CMD(SQUELCH)" | 3 | Used to advertise sender's | | | | current repair window in | | | | response to out-of-range NACKs | | | | from receivers. | | "NORM_CMD(CC)" | 4 | Used for GRTT measurement and | | | | collection of congestion | | | | control feedback. | | "NORM_CMD(REPAIR_ADV)" | 5 | Used to advertise sender's | | | | aggregated repair/feedback | | | | state for suppression of | | | | unicast feedback from | | | | receivers. | | "NORM_CMD(ACK_REQ)" | 6 | Used to request | | | | application-defined positive | | | | acknowledgment from a list of | | | | receivers (OPTIONAL). | | "NORM_CMD(APPLICATION)" | 7 | Used for application-defined | | | | purposes which may need to | | | | temporarily preempt data | | | | transmission (OPTIONAL). | +-------------------------+--------+--------------------------------+ 4.2.3.1. NORM_CMD(FLUSH) Message The "NORM_CMD(FLUSH)" command is sent when the sender reaches the end of all data content and pending repairs it has queued for transmission. This may indicate a temporary or permanent end of data transmission, but the sender is still willing to respond to repair requests. This command is repeated once per "2*GRTT" to excite the receiver set for any outstanding repair requests up to and including the transmission point indicated within the "NORM_CMD(FLUSH)" message. The number of repeats is equal to "NORM_ROBUST_FACTOR" unless a list of receivers from which explicit positive acknowledgment is expected ("acking_node_list") is given. In that case, the "acking_node_list" is updated as acknowledgments are received and the "NORM_CMD(FLUSH)" is repeated according to the mechanism described in Section 5.5.3. The greater the "NORM_ROBUST_FACTOR", the greater the probability that all applicable receivers will be excited for acknowledgment or repair requests (NACKs) AND that the corresponding NACKs are delivered to the sender. A default value of "NORM_ROBUST_FACTOR" equal to 20 is RECOMMENDED. If a "NORM_NACK" message interrupts the flush process, the sender SHALL re-initiate the flush process after any resulting repair transmissions are completed. Note that receivers also employ a timeout mechanism to self-initiate Adamson, et al. Expires October 26, 2009 [Page 32] Internet-Draft NORM Protocol April 2009 NACKing (if there are outstanding repair needs) when no messages of any type are received from a sender. This inactivity timeout is related to the "NORM_CMD(FLUSH)" and "NORM_ROBUST_FACTOR" and is specified in Section 5.3. Receivers SHALL self-initiate the NACK repair process when the inactivity timeout has expired for a specific sender and the receiver has pending repairs needed from that sender. With a sufficiently large "NORM_ROBUST_FACTOR" value, data content is delivered with a high assurance of reliability. The penalty of a large "NORM_ROBUST_FACTOR" value is the potential transmission of excess "NORM_CMD(FLUSH)" messages and a longer inactivity timeout for receivers to self-initiate a terminal NACK process. For finite-size transport objects such as "NORM_OBJECT_DATA" and "NORM_OBJECT_FILE", the flush process (if there are no further pending objects) occurs at the end of these objects. Thus, FEC repair information is always available for repairs in response to repair requests elicited by the flush command. However, for "NORM_OBJECT_STREAM", the flush may occur at any time, including in the middle of an FEC coding block if systematic FEC codes are employed. In this case, the sender will not yet be able to provide FEC parity content for the concurrent coding block and will be limited to explicitly repairing the stream with source data content for that block. Applications that anticipate frequent flushing of stream content SHOULD be judicious in the selection of the FEC coding block size (i.e., do not use a very large coding block size if frequent flushing occurs). For example, a reliable multicast application transmitting an on-going series of intermittent, relatively small messages will need to trade-off using the "NORM_OBJECT_DATA" paradigm versus the "NORM_OBJECT_STREAM" paradigm with an appropriate FEC coding block size. This is analogous to application trade-offs for other transport protocols such as the selection of different TCP modes of operation such as "no delay", etc. Adamson, et al. Expires October 26, 2009 [Page 33] Internet-Draft NORM Protocol April 2009 0 1 2 3 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ |version| type=3| hdr_len | sequence | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | source_id | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | instance_id | grtt |backoff| gsize | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | flavor = 1 | fec_id | object_transport_id | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | fec_payload_id | | ... | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | acking_node_list (if applicable) | | ... | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ NORM_CMD(FLUSH) Message Format The "version", "type", "hdr_len", "sequence", and "source_id" fields form the NORM Common Message Header as described in Section 4.1. In addition to the NORM common message header and standard "NORM_CMD" fields, the "NORM_CMD(FLUSH)" message contains fields to identify the current status and logical transmit position of the sender. The "fec_id" field indicates the FEC type used for the flushing "object_transport_id" and implies the size and format of the "fec_payload_id" field. Note the "hdr_len" value for the "NORM_CMD(FLUSH)" message is 4 plus the size of the "fec_payload_id" field when no header extensions are present. The "object_transport_id" and "fec_payload_id" fields indicate the sender's current logical "transmit position". These fields are interpreted in the same manner as in the "NORM_DATA" message type. Upon receipt of the "NORM_CMD(FLUSH)", receivers are expected to check their completion state THROUGH (including) this transmission position. If receivers have outstanding repair needs in this range, they SHALL initiate the NORM NACK Repair Process as described in Section 5.3. If receivers have no outstanding repair needs, no response to the "NORM_CMD(FLUSH)" is generated. For "NORM_OBJECT_STREAM" objects using systematic FEC codes, receivers MUST request "explicit-only" repair of the identified "source_block_number" if the given "encoding_symbol_id" is less than the "source_block_len". This condition indicates the sender has not yet completed encoding the corresponding FEC block and parity content is not yet available. An "explicit-only" repair request consists of Adamson, et al. Expires October 26, 2009 [Page 34] Internet-Draft NORM Protocol April 2009 NACK content for the applicable "source_block_number" which does not include any requests for parity-based repair. This allows NORM sender applications to "flush" an ongoing stream of transmission when needed, even if in the middle of an FEC block. Once the sender resumes stream transmission and passes the end of the pending coding block, subsequent NACKs from receivers SHALL request parity-based repair as usual. Note that the use of a systematic FEC code is assumed here. It should also be noted that a sender has the option of arbitrarily shortening a given code block when such an application "flush" occurs. In this case, the receiver will request explicit repair, but the sender MAY provide FEC-based repair (parity segments) in response. These parity segments MUST contain the corrected "source_block_len" for the shortened block and that "source_block_len" associated with segments containing parity content SHALL override the previously advertised "source_block_len". Similarly, the "source_block_len" associated with the highest ordinal "encoding_symbol_id" shall take precedence over prior symbols when a difference (e.g., due to code shortening at the sender) occurs. Normal receiver NACK initiation and construction is discussed in detail in Section 5.3. The OPTIONAL "acking_node_list" field contains a list of NormNodeIds for receivers from which the sender is requesting explicit positive acknowledgment of reception up through the transmission point identified by the "object_transport_id" and "fec_payload_id" fields. The length of the list can be inferred from the length of the received "NORM_CMD(FLUSH)" message. When the "acking_node_list" is present, the lightweight positive acknowledgment process described in Section 5.5.3 SHALL be observed. 4.2.3.2. NORM_CMD(EOT) Message The "NORM_CMD(EOT)" command is sent when the sender reaches permanent end-of-transmission with respect to the NormSession and will not respond to further repair requests. This allows receivers to gracefully reach closure of operation with this sender (without requiring any timeout) and free any resources that are no longer needed. The "NORM_CMD(EOT)" command SHOULD be sent with the same robust mechanism as used for "NORM_CMD(FLUSH)" commands to provide a high assurance of reception by the receiver set. Adamson, et al. Expires October 26, 2009 [Page 35] Internet-Draft NORM Protocol April 2009 0 1 2 3 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ |version| type=3| hdr_len | sequence | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | source_id | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | instance_id | grtt |backoff| gsize | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | flavor = 2 | reserved | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ NORM_CMD(EOT) Message Format The value of the "hdr_len" field for "NORM_CMD(EOT)" messages without header extensions present is 4. The "reserved" field is reserved for future use and MUST be set to an all ZERO value. Receivers MUST ignore the "reserved" field. 4.2.3.3. NORM_CMD(SQUELCH) Message The "NORM_CMD(SQUELCH)" command is transmitted in response to outdated or invalid "NORM_NACK" content received by the sender. Invalid "NORM_NACK" content consists of repair requests for NormObjects for which the sender is unable or unwilling to provide repair. This includes repair requests for outdated objects, aborted objects, or those objects which the sender previously transmitted marked with the "NORM_FLAG_UNRELIABLE" flag. This command indicates to receivers what content is available for repair, thus serving as a description of the sender's current "repair window". Receivers SHALL NOT generate repair requests for content identified as invalid by a "NORM_CMD(SQUELCH)". The "NORM_CMD(SQUELCH)" command is sent once per "2*GRTT" at the most. The "NORM_CMD(SQUELCH)" advertises the current "repair window" of the sender by identifying the earliest (lowest) transmission point for which it will provide repair, along with an encoded list of objects from that point forward that are no longer valid for repair. This mechanism allows the sender application to cancel or abort transmission and/or repair of specific previously enqueued objects. The list also contains the identifiers for any objects within the repair window that were sent with the "NORM_FLAG_UNRELIABLE" flag set. In normal conditions, it is expected the "NORM_CMD(SQUELCH)" will be needed infrequently, and generally only to provide a reference repair window for receivers who have fallen "out-of-sync" with the sender due to extremely poor network conditions. The starting point of the invalid NormObject list begins with the Adamson, et al. Expires October 26, 2009 [Page 36] Internet-Draft NORM Protocol April 2009 lowest invalid NormTransportId greater than the current "repair window" start from the invalid NACK(s) that prompted the generation of the squelch. The length of the list is limited by the sender's NormSegmentSize. This allows the receivers to learn the status of the sender's applicable object repair window with minimal transmission of "NORM_CMD(SQUELCH)" commands. The format of the "NORM_CMD(SQUELCH)" message is: 0 1 2 3 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ |version| type=3| hdr_len | sequence | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | source_id | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | instance_id | grtt |backoff| gsize | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | flavor = 3 | fec_id | object_transport_id | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | fec_payload_id | | ... | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | invalid_object_list | | ... | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ NORM_CMD(SQUELCH) Message Format In addition to the NORM common message header and standard "NORM_CMD" fields, the "NORM_CMD(SQUELCH)" message contains fields to identify the earliest logical transmit position of the sender's current repair window and an "invalid_object_list" beginning with the index of the logically earliest invalid repair request from the offending NACK message which initiated the "NORM_CMD(SQUELCH)" transmission. The value of the "hdr_len" field when no extensions are present is 4 plus the size of the "fec_payload_id" field that is dependent upon the FEC scheme identified by the "fec_id" field. The "object_transport_id" and "fec_payload_id" fields are concatenated to indicate the beginning of the sender's current repair window (i.e., the logically earliest point in its transmission history for which the sender can provide repair). The "fec_id" field implies the size and format of the "fec_payload_id" field. This serves as an advertisement of a "synchronization" point for receivers to request repair. Note, that while an "encoding_symbol_id" may be included in the "fec_payload_id" field, the sender's repair window SHOULD be aligned on FEC coding block boundaries and thus the "encoding_symbol_id" SHOULD be zero. Adamson, et al. Expires October 26, 2009 [Page 37] Internet-Draft NORM Protocol April 2009 The "invalid_object_list" is a list of 16-bit NormTransportIds that, although they are within the range of the sender's current repair window, are no longer available for repair from the sender. For example, a sender application may dequeue an out-of-date object even though it is still within the repair window. The total size of the "invalid_object_list" content is can be determined from the packet's payload length and is limited to a maximum of the NormSegmentSize of the sender. Thus, for very large repair windows, it is possible that a single "NORM_CMD(SQUELCH)" message may not be capable of listing the entire set of invalid objects in the repair window. In this case, the sender SHALL ensure that the list begins with a NormObjectId that is greater than or equal to the lowest ordinal invalid NormObjectId from the NACK message(s) that prompted the "NORM_CMD(SQUELCH)" generation. The NormObjectIds in the "invalid_object_list" MUST be ordinally greater than the "object_transport_id" marking the beginning of the sender's repair window. This insures convergence of the squelch process, even if multiple invalid NACK/ squelch iterations are required. This explicit description of invalid content within the sender's current window allows the sender application (most notably for discrete object transport) to arbitrarily invalidate (i.e., dequeue) portions of enqueued content (e.g., certain objects) for which it no longer wishes to provide reliable transport. 4.2.3.4. NORM_CMD(CC) Message The "NORM_CMD(CC)" messages contains fields to enable sender-to- receiver group greatest round-trip time (GRTT) measurement and to excite the group for congestion control feedback. A baseline NORM congestion control scheme (NORM-CC), based on the TCP-Friendly Multicast Congestion Control (TFMCC) scheme of RFC 4654 is fully specified in Section 5.5.2 of this document. The "NORM_CMD(CC)" message is usually transmitted as part of NORM-CC congestion control operation. A NORM header extension is defined below to be used with the "NORM_CMD(CC)" message to support NORM-CC operation. Different header extensions may be defined for the "NORM_CMD(CC)" (and/or other NORM messages as needed) to support alternative congestion control schemes in the future. If NORM is operated in a network where resources are explicitly dedicated to the NORM session and therefore congestion control operation is disabled, the "NORM_CMD(CC)" message is then used soley for GRTT measurement and may optionally be sent less frequently than with congestion control operation. Adamson, et al. Expires October 26, 2009 [Page 38] Internet-Draft NORM Protocol April 2009 0 1 2 3 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ |version| type=3| hdr_len | sequence | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | source_id | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | instance_id | grtt |backoff| gsize | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | flavor = 4 | reserved | cc_sequence | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | send_time_sec | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | send_time_usec | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | header extensions (if applicable) | | ... | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | cc_node_list (if applicable) | | ... | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ NORM_CMD(CC) Message Format The NORM common message header and standard "NORM_CMD" fields serve their usual purposes. The value of the "hdr_len" field when no header extensions are present is 6. The "reserved" field is for potential future use and MUST be set to ZERO in this version of the NORM protocol and its baseline NORM-CC congestion control scheme. It may be possible that alternative congestion control schemes may use the "NORM_CMD(CC)" message defined here and leverage the "reserved" field for scheme-specific purposes. The "cc_sequence" field is a sequence number applied by the sender. For NORM-CC operation, it is used to provide functionality equivalent to the "feedback round number" ("fb_nr") described in RFC 4654. The most recently received "cc_sequence" value is recorded by receivers and can be fed back to the sender in congestion control feedback generated by the receivers for that sender. The "cc_sequence" number can also be used in NORM implementations to assess how recently a receiver has received "NORM_CMD(CC)" probes from the sender. This can be useful instrumentation for complex or experimental multicast routing environments. The "send_time" field is a timestamp indicating the time that the "NORM_CMD(CC)" message was transmitted. This consists of a 64-bit field containing 32-bits with the time in seconds ("send_time_sec") Adamson, et al. Expires October 26, 2009 [Page 39] Internet-Draft NORM Protocol April 2009 and 32-bits with the time in microseconds ("send_time_usec") since some reference time the source maintains (usually 00:00:00, 1 January 1970). The byte ordering of the fields is "Big Endian" network order. Receivers use this timestamp adjusted by the amount of delay from the time they received the "NORM_CMD(CC)" message to the time of their response as the "grtt_response" portion of "NORM_ACK" and "NORM_NACK" messages generated. This allows the sender to evaluate round-trip times to different receivers for congestion control and other (e.g., GRTT determination) purposes. To facilitate the baseline NORM-CC scheme described in Section 5.5.2, a NORM-CC Rate header extension (EXT_RATE) is defined to inform the group of the sender's current transmission rate. This is used along with the loss detection "sequence" field of all NORM sender messages and the "NORM_CMD(CC)" GRTT collection process to support NORM-CC congestion control operation. The format of this header extension is as follows: 0 1 2 3 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | het = 128 | reserved | send_rate | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ The "send_rate" field indicates the sender's current transmission rate in bytes per second. The 16-bit "send_rate" field consists of 12 bits of mantissa in the most significant portion and 4 bits of base 10 integer exponent (E) information in the least significant portion. The 12-bit mantissa portion of the field is scaled such that a base 10 mantissa (M) floating point value of 0.0 corresponds to 0 and a value of 10.0 corresponds to 4096 in the upper 12 bits of the 16-bit "send_rate" field. Thus: send_rate = (((int)(M * 4096.0 / 10.0 + 0.5)) << 4) | E; For example, to represent a transmission rate of 256kbps (3.2e+04 bytes per second), the lower 4 bits of the 16-bit field contain a value of 0x04 to represent the exponent (E) while the upper 12 bits contain a value of 0x51f (M) as determined from the equation given above: send_rate = (((int)((3.2 * 4096.0 / 10.0) + 0.5)) << 4) | 4; = (0x51f << 4) | 0x4 = 0x51f4 To decode the "send_rate" field, the following equation can be used: value = (send_rate >> 4) * (10/4096) * power(10, (send_rate & x000f)) Note the maximum transmission rate that can be represented by this scheme is approximately 9.99e+15 bytes per second. Adamson, et al. Expires October 26, 2009 [Page 40] Internet-Draft NORM Protocol April 2009 When this extension is present, a "cc_node_list" may be attached as the payload of the "NORM_CMD(CC)" message. The presence of this header extension also implies that NORM receivers should respond according to the procedures described in Section 5.5.2. The "cc_node_list" consists of a list of NormNodeIds and their associated congestion control status. This includes the current limiting receiver (CLR) node, any potential limiting receiver (PLR) nodes that have been identified, and some number of receivers for which congestion control status is being provided, most notably including the receivers' current RTT measurement. The maximum length of the "cc_node_list" provides for at least the CLR and one other receiver, but may be configurable for more timely feedback to the group. The list length can be inferred from the length of the "NORM_CMD(CC)" message. Each item in the "cc_node_list" is in the following format: 0 1 2 3 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | cc_node_id | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | cc_flags | cc_rtt | cc_rate | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ The "cc_node_id" is the NormNodeId of the receiver which the item represents. The "cc_flags" field contains flags indicating the congestion control status of the indicated receiver. The following flags are defined: +----------------------+-------+------------------------------------+ | Flag | Value | Purpose | +----------------------+-------+------------------------------------+ | "NORM_FLAG_CC_CLR" | 0x01 | Receiver is the current limiting | | | | receiver (CLR). | | "NORM_FLAG_CC_PLR" | 0x02 | Receiver is a potential limiting | | | | receiver (PLR). | | "NORM_FLAG_CC_RTT" | 0x04 | Receiver has measured RTT with | | | | respect to sender. | | "NORM_FLAG_CC_START" | 0x08 | Sender/receiver is in "slow start" | | | | phase of congestion control | | | | operation (i.e., The receiver has | | | | not yet detected any packet loss | | | | and the "cc_rate" field is the | | | | receiver's actual measured receive | | | | rate). | Adamson, et al. Expires October 26, 2009 [Page 41] Internet-Draft NORM Protocol April 2009 | "NORM_FLAG_CC_LEAVE" | 0x10 | Receiver is imminently leaving the | | | | session and its feedback should | | | | not be considered in congestion | | | | control operation. | +----------------------+-------+------------------------------------+ The "cc_rtt" contains a quantized representation of the RTT as measured by the sender with respect to the indicated receiver. This field is valid only if the "NORM_FLAG_CC_RTT" flag is set in the "cc_flags" field. This one byte field is a quantized representation of the RTT using the algorithm described in the Multicast NACK Building Block [RFC5401]. The "cc_rate" field contains a representation of the receiver's current calculated (during steady-state congestion control operation) or twice its measured (during the slow start phase) congestion control rate. This field is encoded and decoded using the same technique as described for the "NORM_CMD(CC)" "send_rate" field. 4.2.3.5. NORM_CMD(REPAIR_ADV) Message The "NORM_CMD(REPAIR_ADV)" message is used by the sender to "advertise" its aggregated repair state from "NORM_NACK" messages accumulated during a repair cycle and/or congestion control feedback received. This message is sent only when the sender has received "NORM_NACK" and/or "NORM_ACK(CC)" (when congestion control is enabled) messages via unicast transmission instead of multicast. By relaying this information to the receiver set, suppression of feedback can be achieved even when receivers are unicasting that feedback instead of multicasting it among the group [NormFeedback]. 0 1 2 3 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ |version| type=3| hdr_len | sequence | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | source_id | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | instance_id | grtt |backoff| gsize | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | flavor = 5 | flags | reserved | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | header extensions (if applicable) | | ... | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | repair_adv_payload | | ... | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ Adamson, et al. Expires October 26, 2009 [Page 42] Internet-Draft NORM Protocol April 2009 NORM_CMD(REPAIR_ADV) Message Format The "instance_id", "grtt", "backoff", "gsize", and "flavor" fields serve the same purpose as in other "NORM_CMD" messages. The value of the "hdr_len" field when no extensions are present is 4. The "flags" field provide information on the "NORM_CMD(REPAIR_ADV)" content. There is currently one "NORM_CMD(REPAIR_ADV)" flag defined: NORM_REPAIR_ADV_FLAG_LIMIT = 0x01 This flag is set by the sender when it is unable to fit its full current repair state into a single NormSegmentSize. If this flag is set, receivers should limit their NACK response to generating NACK content only up through the maximum ordinal transmission position (objectId::fecPayloadId) included in the "repair_adv_content". When congestion control operation is enabled, a header extension may be applied to the "NORM_CMD(REPAIR_ADV)" representing the most limiting (in terms of congestion control feedback suppression) congestion control response. This allows the "NORM_CMD(REPAIR_ADV)" message to suppress receiver congestion control responses as well as NACK feedback messages. The field is defined as a header extension so that alternative congestion control schemes may be used with NORM without revision to this document. A NORM-CC Feedback Header Extension (EXT_CC) is defined to encapsulate congestion control feedback within "NORM_NACK", "NORM_ACK", and "NORM_CMD(REPAIR_ADV)" messages. If another congestion control technique (e.g., Pragmatic General Multicast Congestion Control (PGMCC) [PgmccPaper]) is used within a NORM implementation, an additional header extension MAY need to be defined encapsulate any required feedback content. The NORM-CC Feedback Header Extension format is: 0 1 2 3 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | het = 3 | hel = 3 | cc_sequence | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | cc_flags | cc_rtt | cc_loss | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | cc_rate | cc_reserved | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ The "cc_sequence" field contains the current greatest "cc_sequence" value receivers have received in "NORM_CMD(CC)" messages from the sender. This information assists the sender in congestion control operation by providing an indicator of how current ("fresh") the receiver's round-trip measurement reference time is and whether the receiver has been successfully receiving recent congestion control probes. For example, if it is apparent the receiver has not been Adamson, et al. Expires October 26, 2009 [Page 43] Internet-Draft NORM Protocol April 2009 receiving recent congestion control probes (and thus possibly other messages from the sender), the sender may choose to take congestion avoidance measures. For "NORM_CMD(REPAIR_ADV)" messages, the sender SHALL set the "cc_sequence" field value to the value set in the last "NORM_CMD(CC)" message sent. The "cc_flags" field contains bits representing the receiver's state with respect to congestion control operation. The possible values for the "cc_flags" field are those specified for the "NORM_CMD(CC)" message node list item flags. These fields are used by receivers in controlling (suppressing as necessary) their congestion control feedback. For "NORM_CMD(REPAIR_ADV)" messages, the "NORM_FLAG_CC_RTT" should be set only when all feedback messages received by the sender have the flag set. Similarly, the "NORM_FLAG_CC_CLR" or "NORM_FLAG_CC_PLR" should be set only when no feedback has been received from non-CLR or non-PLR receivers. And the "NORM_FLAG_CC_LEAVE" should be set only when all feedback messages the sender has received have this flag set. These heuristics for setting the flags in "NORM_CMD(REPAIR_ADV)" ensure the most effective suppression of receivers providing unicast feedback messages. The "cc_rtt" field SHALL be set to a default maximum value and the "NORM_FLAG_CC_RTT" flag SHALL be cleared when no receiver has yet received RTT measurement information. When a receiver has received RTT measurement information, it shall set the "cc_rtt" value accordingly and set the "NORM_FLAG_CC_RTT" flag in the "cc_flags" field. For "NORM_CMD(REPAIR_ADV)" messages, the sender SHALL set the "cc_rtt" field value to the largest non-CLR/non-PLR RTT it has measured from receivers for the current feedback round. The "cc_loss" field represents the receiver's current packet loss fraction estimate for the indicated source. The loss fraction is a value from 0.0 to 1.0 corresponding to a range of zero to 100 percent packet loss. The 16-bit "cc_loss" value is calculated by the following formula: "cc_loss" = floor(decimal_loss_fraction * 65535.0) For "NORM_CMD(REPAIR_ADV)" messages, the sender SHALL set the "cc_loss" field value to the largest non-CLR/non-PLR loss estimate it has received from receivers for the current feedback round. The "cc_rate" field represents the receivers current local congestion control rate. During "slow start", when the receiver has detected no loss, this value is set to twice the actual rate it has measured from the corresponding sender and the "NORM_FLAG_CC_START" is set in the "cc_flags' field. Otherwise, the receiver calculates a congestion Adamson, et al. Expires October 26, 2009 [Page 44] Internet-Draft NORM Protocol April 2009 control rate based on its loss measurement and RTT measurement information (even if default) for the "cc_rate" field. For "NORM_CMD(REPAIR_ADV)" messages, the sender SHALL set the "cc_loss" field value to the lowest non-CLR/non-PLR "cc_rate" report it has received from receivers for the current feedback round. The "cc_reserved" field is reserved for future NORM protocol use. Currently, senders SHALL set this field to ZERO, and receivers SHALL ignore the content of this field. The "repair_adv_payload" is in exactly the same form as the "nack_content" of "NORM_NACK" messages and can be processed by receivers for suppression purposes in the same manner, with the exception of the condition when the "NORM_REPAIR_ADV_FLAG_LIMIT" is set. 4.2.3.6. NORM_CMD(ACK_REQ) Message The "NORM_CMD(ACK_REQ)" message is used by the sender to request acknowledgment from a specified list of receivers. This message is used in providing a lightweight positive acknowledgment mechanism that is OPTIONAL for use by the reliable multicast application. A range of acknowledgment request types is provided for use at the application's discretion. Provision for application-defined, positively-acknowledged commands allows the application to automatically take advantage of transmission and round-trip timing information available to the NORM protocol. The details of the NORM positive acknowledgment process including transmission of the "NORM_CMD(ACK_REQ)" messages and the receiver response ("NORM_ACK") are described in Section 5.5.3. The format of the "NORM_CMD(ACK_REQ)" message is: 0 1 2 3 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ |version| type=3| hdr_len | sequence | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | source_id | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | instance_id | grtt |backoff| gsize | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | flavor = 6 | reserved | ack_type | ack_id | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | acking_node_list | | ... | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ NORM_CMD(ACK_REQ) Message Format Adamson, et al. Expires October 26, 2009 [Page 45] Internet-Draft NORM Protocol April 2009 The NORM common message header and standard "NORM_CMD" fields serve their usual purposes. The value of the "hdr_len" field for "NORM_CMD(ACK_REQ)" messages with no header extension present is 4. The "ack_type" field indicates the type of acknowledgment being requested and thus implies rules for how the receiver will treat this request. The following "ack_type" values are defined and are also used in "NORM_ACK" messages described later: +------------------------+--------+---------------------------------+ | ACK Type | Value | Purpose | +------------------------+--------+---------------------------------+ | "NORM_ACK_CC" | 1 | Used to identify "NORM_ACK" | | | | messages sent in response to | | | | "NORM_CMD(CC)" messages. | | "NORM_ACK_FLUSH" | 2 | Used to identify "NORM_ACK" | | | | messages sent in response to | | | | "NORM_CMD(FLUSH)" messages. | | "NORM_ACK_RESERVED" | 3-15 | Reserved for possible future | | | | NORM protocol use. | | "NORM_ACK_APPLICATION" | 16-255 | Used at application's | | | | discretion. | +------------------------+--------+---------------------------------+ The "NORM_ACK_CC" value is provided for use only in "NORM_ACKs" generated in response to the "NORM_CMD(CC)" messages used in congestion control operation. Similarly, the "NORM_ACK_FLUSH" is provided for use only in "NORM_ACKs" generated in response to applicable "NORM_CMD(FLUSH)" messages. "NORM_CMD"(ACK_REQ) messages with "ack_type" of "NORM_ACK_CC" or "NORM_ACK_FLUSH" SHALL NOT be generated by the sender. The "NORM_ACK_RESERVED" range of "ack_type" values is provided for possible future NORM protocol use. The "NORM_ACK_APPLICATION" range of "ack_type" values is provided so that NORM applications may implement application-defined, positively- acknowledged commands that are able to leverage internal transmission and round-trip timing information available to the NORM protocol implementation. The "ack_id" provides a sequenced identifier for the given "NORM_CMD(ACK_REQ)" message. This "ack_id" is returned in "NORM_ACK" messages generated by the receivers so that the sender may associate the response with its corresponding request. The "reserved" field is reserved for possible future protocol use and SHALL be set to ZERO by senders and ignored by receivers. Adamson, et al. Expires October 26, 2009 [Page 46] Internet-Draft NORM Protocol April 2009 The "acking_node_list" field contains the NormNodeIds of the current NORM receivers that are desired to provide positive acknowledge ("NORM_ACK") to this request. The packet payload length implies the length of the "acking_node_list" and its length is limited to the sender NormSegmentSize. The individual NormNodeId items are listed in network (Big Endian) byte order. If a receiver's NormNodeId is included in the "acking_node_list", it SHALL schedule transmission of a "NORM_ACK" message as described in Section 5.5.3. 4.2.3.7. NORM_CMD(APPLICATION) Message This command allows the NORM application to robustly transmit application-defined commands. The command message preempts any ongoing data transmission and is repeated up to "NORM_ROBUST_FACTOR" times at a rate of once per "2*GRTT". This rate of repetition allows the application to observe any response (if that is the application's purpose for the command) before it is repeated. Possible responses may include initiation of data transmission, other "NORM_CMD(APPLICATION)" messages, or even application-defined, positively-acknowledge commands from other NormSession participants. The transmission of these commands will preempt data transmission when they are scheduled and may be multiplexed with ongoing data transmission. This type of robustly transmitted command allows NORM applications to define a complete set of session control mechanisms with less state than the transfer of FEC encoded reliable content requires while taking advantage of NORM transmission and round-trip timing information. 0 1 2 3 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ |version| type=3| hdr_len | sequence | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | source_id | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | instance_id | grtt |backoff| gsize | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | flavor = 7 | reserved | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Application-Defined Content | | ... | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ NORM_CMD(APPLICATION) Message Format The NORM common message header and "NORM_CMD" fields are interpreted as previously described. The value of the "NORM_CMD(APPLICATION)" "hdr_len" field when no header extensions are present is 4. Adamson, et al. Expires October 26, 2009 [Page 47] Internet-Draft NORM Protocol April 2009 The "Application-Defined Content" area contains information in a format at the discretion of the application. The size of this payload SHALL be limited to a maximum of the sender's NormSegmentSize setting. Upon reception, the NORM protocol implementation SHALL deliver the content to the receiver application. Note that any detection of duplicate reception of a "NORM_CMD(APPLICATION)" message is the responsibility of the application. 4.3. Receiver Messages The NORM message types generated by participating receivers consist of the "NORM_NACK" and "NORM_ACK" message types. "NORM_NACK" messages are sent to request repair of missing data content from sender transmission and "NORM_ACK" messages are generated in response to certain sender commands including "NORM_CMD(CC)" and "NORM_CMD(ACK_REQ)". 4.3.1. NORM_NACK Message The principal purpose of "NORM_NACK" messages is for receivers to request repair of sender content via selective, negative acknowledgment upon detection of incomplete data. "NORM_NACK" messages will be transmitted according to the rules of "NORM_NACK" generation and suppression described in Section 5.3. "NORM_NACK" messages also contain additional fields to provide feedback to the sender(s) for purposes of round-trip timing collection and congestion control. The payload of "NORM_NACK" messages contains one or more repair requests for different objects or portions of those objects. The "NORM_NACK" message format is as follows: Adamson, et al. Expires October 26, 2009 [Page 48] Internet-Draft NORM Protocol April 2009 0 1 2 3 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ |version| type=4| hdr_len | sequence | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | source_id | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | server_id | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | instance_id | reserved | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | grtt_response_sec | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | grtt_response_usec | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | header extensions (if applicable) | | ... | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | nack_payload | | ... | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ NORM_NACK Message Format The NORM common message header fields serve their usual purposes. The value of the "hdr_len" field for "NORM_NACK" messages without header extensions present is 6. The "server_id" field identifies the NORM sender to which the "NORM_NACK" message is destined. The "instance_id" field contains the current session identifier given by the sender identified by the "server_id" field in its sender messages. The sender SHOULD ignore feedback messages which contain an invalid "instance_id" value. The "grtt_response" fields contain an adjusted version of the timestamp from the most recently received "NORM_CMD(CC)" message for the indicated NORM sender. The format of the "grtt_response" is the same as the "send_time" field of the "NORM_CMD(CC)". The "grtt_response" value is relative to the "send_time" the source provided with a corresponding "NORM_CMD(CC)" command. The receiver adjusts the source's "NORM_CMD(CC)" "send_time" timestamp by adding the time delta from when the receiver received the "NORM_CMD(CC)" to when the "NORM_NACK" is transmitted in response to calculate the value in the "grtt_response" field. This is the "receive_to_response_delta" value used in the following formula: grtt_response = NORM_CMD(CC) send_time + receive_to_response_delta Adamson, et al. Expires October 26, 2009 [Page 49] Internet-Draft NORM Protocol April 2009 The receiver SHALL set the "grtt_response" to a ZERO value, to indicate that it has not yet received a "NORM_CMD(CC)" message from the indicated sender and that the sender should ignore the "grtt_response" in this message. For NORM-CC operation, the NORM-CC Feedback Header Extension, as described in the "NORM_CMD(REPAIR_ADV}" message description, is added to "NORM_NACK" messages to provide feedback on the receivers current state with respect to congestion control operation. Note that alternative header extensions for congestion control feedback may be defined for alternative congestion control schemes for NORM use in the future. The "reserved" field is for potential future NORM use and SHALL be set to ZERO for this version of the protocol. The "nack_payload" of the "NORM_NACK" message specifies the repair needs of the receiver with respect to the NORM sender indicated by the "server_id" field. The receiver constructs repair requests based on the "NORM_DATA" and/or "NORM_INFO" segments it requires from the sender in order to complete reliable reception up to the sender's transmission position at the moment the receiver initiates the NACK Procedure as described in Section 5.3. A single NORM Repair Request consists of a list of items, ranges, and/or FEC coding block erasure counts for needed "NORM_DATA" and/or "NORM_INFO" content. Multiple repair requests may be concatenated within the "nack_payload" field of a "NORM_NACK" message. Note that a single NORM Repair Request can possibly include multiple "items", "ranges", or "erasure_counts". In turn, the "nack_payload" field MAY contain multiple repair requests. A single NORM Repair Request has the following format: 0 1 2 3 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | form | flags | length | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | repair_request_items | | ... | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ NORM Repair Request Format The "form" field indicates the type of repair request items given in the "repair_request_items" list. Possible values for the "form" field include: Adamson, et al. Expires October 26, 2009 [Page 50] Internet-Draft NORM Protocol April 2009 +----------------------+-------+ | Form | Value | +----------------------+-------+ | "NORM_NACK_ITEMS" | 1 | | "NORM_NACK_RANGES" | 2 | | "NORM_NACK_ERASURES" | 3 | +----------------------+-------+ A "form" value of "NORM_NACK_ITEMS" indicates each repair request item in the "repair_request_items" list is to be treated as an individual request. A value of "NORM_NACK_RANGES" indicates that the "repair_request_items" list consists of pairs of repair request items that correspond to inclusive ranges of repair needs. And the "NORM_NACK_ERASURES" "form" indicates that the repair request items are to be treated individually and that the "encoding_symbol_id" portion of the "fec_payload_id" field of the repair request item (see below) is to be interpreted as an erasure count for the FEC coding block identified by the repair request item's "source_block_number". The "flags" field is currently used to indicate the level of data content for which the repair request items apply (i.e., an individual segment, entire FEC coding block, or entire transport object). Possible flag values include: +---------------------+-------+-------------------------------------+ | Flag | Value | Purpose | +---------------------+-------+-------------------------------------+ | "NORM_NACK_SEGMENT" | 0x01 | Indicates the listed segment(s) or | | | | range of segments are required as | | | | repair. | | "NORM_NACK_BLOCK" | 0x02 | Indicates the listed block(s) or | | | | range of blocks in entirety are | | | | required as repair. | | "NORM_NACK_INFO" | 0x04 | Indicates that "NORM_INFO" is | | | | required as repair for the listed | | | | object(s). | | "NORM_NACK_OBJECT" | 0x08 | Indicates the listed object(s) or | | | | range of objects in entirety are | | | | required as repair. | +---------------------+-------+-------------------------------------+ When the "NORM_NACK_SEGMENT" flag is set, the "object_transport_id" and "fec_payload_id" fields are used to determine which sets or ranges of individual "NORM_DATA" segments are needed to repair content at the receiver. When the "NORM_NACK_BLOCK" flag is set, this indicates the receiver is completely missing the indicated coding block(s) and requires transmissions sufficient to repair the indicated block(s) in their entirety. When the "NORM_NACK_INFO" flag Adamson, et al. Expires October 26, 2009 [Page 51] Internet-Draft NORM Protocol April 2009 is set, this indicates the receiver is missing the "NORM_INFO" segment for the indicated "object_transport_id". Note the "NORM_NACK_INFO" may be set in combination with the "NORM_NACK_BLOCK" or "NORM_NACK_SEGMENT" flags, or may be set alone. When the "NORM_NACK_OBJECT" flag is set, this indicates the receiver is missing the entire NormTransportObject referenced by the "object_transport_id". This also implicitly requests any available "NORM_INFO" for the NormObject, if applicable. The "fec_payload_id" field is ignored when the flag "NORM_NACK_OBJECT" is set. The "length" field value is the length in bytes of the "repair_request_items" field. The "repair_request_items" field consists of a list of individual or range pairs of transport data unit identifiers in the following format. 0 1 2 3 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | fec_id | reserved | object_transport_id | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | fec_payload_id | | ... | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ NORM Repair Request Item Format The "fec_id" indicates the FEC type and can be used to determine the format of the "fec_payload_id" field. The "reserved" field is kept for possible future use and SHALL be set to a ZERO value and ignored by NORM nodes processing NACK content. The "object_transport_id" corresponds to the NormObject for which repair is being requested and the "fec_payload_id" identifies the specific FEC coding block and/or segment being requested. When the "NORM_NACK_OBJECT" flag is set, the value of the "fec_payload_id" field is ignored. When the "NORM_NACK_BLOCK" flag is set, only the FEC code block identifier portion of the "fec_payload_id" is to be interpreted. The format of the "fec_payload_id" field depends upon the "fec_id" field value. When the receiver's repair needs dictate that different forms (mixed ranges and/or individual items) or types (mixed specific segments and/or blocks or objects in entirety) are required to complete reliable transmission, multiple NORM Repair Requests with different "form" and or "flags" values can be concatenated within a single Adamson, et al. Expires October 26, 2009 [Page 52] Internet-Draft NORM Protocol April 2009 "NORM_NACK" message. Additionally, NORM receivers SHALL construct "NORM_NACK" messages with their repair requests in ordinal order with respect to "object_transport_id" and "fec_payload_id" values. The "nack_payload" size SHALL NOT exceed the NormSegmentSize for the sender to which the "NORM_NACK" is destined. NORM_NACK Content Examples: In these examples, a small block, systematic FEC code ("fec_id" = 129) is assumed with a user data block length of 32 segments. In Example 1, a list of individual "NORM_NACK_ITEMS" repair requests is given. In Example 2, a list of "NORM_NACK_RANGES" requests AND a single "NORM_NACK_ITEMS" request are concatenated to illustrate the possible content of a "NORM_NACK" message. Note that FEC coding block erasure counts could also be provided in each case. However, the erasure counts are not really necessary since the sender can easily determine the erasure count while processing the NACK content. However, the erasure count option may be useful for operation with other FEC codes or for intermediate system purposes. Example 1: "NORM_NACK" "nack_payload" for: Object 12, Coding Block 3, Segments 2,5,and 8 0 1 2 3 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | form = 1 | flags = 0x01 | length = 36 | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | fec_id = 129 | reserved | object_transport_id = 12 | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | source_block_number = 3 | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | source_block_length = 32 | encoding_symbol_id = 2 | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | fec_id = 129 | reserved | object_transport_id = 12 | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | source_block_number = 3 | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | source_block_length = 32 | encoding_symbol_id = 5 | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | fec_id = 129 | reserved | object_transport_id = 12 | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | source_block_number = 3 | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | source_block_length = 32 | encoding_symbol_id = 8 | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ Adamson, et al. Expires October 26, 2009 [Page 53] Internet-Draft NORM Protocol April 2009 Example 2: "NORM_NACK" "nack_payload" for: Object 18, Coding Block 6, Segments 5, 6, 7, 8, 9, 10; and Object 19 "NORM_INFO" and Coding Block 1, segment 3 0 1 2 3 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | form = 2 | flags = 0x01 | length = 24 | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | fec_id = 129 | reserved | object_transport_id = 18 | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | source_block_number = 6 | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | source_block_length = 32 | encoding_symbol_id = 5 | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | fec_id = 129 | reserved | object_transport_id = 18 | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | source_block_number = 6 | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | source_block_length = 32 | encoding_symbol_id = 10 | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | form = 1 | flags = 0x05 | length = 12 | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | fec_id = 129 | reserved | object_transport_id = 19 | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | source_block_number = 1 | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | source_block_length = 32 | encoding_symbol_id = 3 | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 4.3.2. NORM_ACK Message The "NORM_ACK" message is intended to be used primarily as part of NORM congestion control operation and round-trip timing measurement. As mentioned in the "NORM_CMD(ACK_REQ)" message description, the acknowledgment type "NORM_ACK_CC" is provided for this purpose. The generation of "NORM_ACK(CC)" messages for round-trip timing estimation and congestion-control operation is described in Section 5.5.1 and Section 5.5.2, respectively. However, some multicast applications may benefit from some limited form of positive acknowledgment for certain functions. A simple, scalable positive acknowledgment scheme is defined in Section 5.5.3 that can be leveraged by protocol implementations when appropriate. The "NORM_CMD(FLUSH)" may be used for OPTIONAL collection of positive acknowledgment of reliable reception to a certain "watermark" transmission point from specific receivers using this mechanism. The "NORM_ACK" type "NORM_ACK_FLUSH" is provided for this purpose and the format of the "nack_payload" for this acknowledgment type is given below. Beyond that, a range of application-defined "ack_type" values Adamson, et al. Expires October 26, 2009 [Page 54] Internet-Draft NORM Protocol April 2009 is provided for use at the NORM application's discretion. Implementations making use of application-defined positive acknowledgments may also make use the "nack_payload" as needed, observing the constraint that the "nack_payload" field size be limited to a maximum of the NormSegmentSize for the sender to which the "NORM_ACK" is destined. 0 1 2 3 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ |version| type=5| hdr_len | sequence | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | source_id | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | server_id | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | instance_id | ack_type | ack_id | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | grtt_response_sec | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | grtt_response_usec | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | header extensions (if applicable) | | ... | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | ack_payload (if applicable) | | ... | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ NORM_ACK Message Format The NORM common message header fields serve their usual purposes. The value of the "hdr_len" field when no header extensions are present is 6. The "server_id", "instance_id", and "grtt_response" fields serve the same purpose as the corresponding fields in "NORM_NACK" messages. And header extensions may be applied to support congestion control feedback or other functions in the same manner. The "ack_type" field indicates the nature of the "NORM_ACK" message. This directly corresponds to the "ack_type" field of the "NORM_CMD(ACK_REQ)" message to which this acknowledgment applies. The "ack_id" field serves as a sequence number so that the sender can verify that a "NORM_ACK" message received actually applies to a current acknowledgment request. The "ack_id" field is not used in the case of the "NORM_ACK_CC" and "NORM_ACK_FLUSH" acknowledgment types. Adamson, et al. Expires October 26, 2009 [Page 55] Internet-Draft NORM Protocol April 2009 The "ack_payload" format is a function of the "ack_type". The "NORM_ACK_CC" message has no attached content. Only the "NORM_ACK" header applies. In the case of "NORM_ACK_FLUSH", a specific "ack_payload" format is defined: 0 1 2 3 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | fec_id | reserved | object_transport_id | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | fec_payload_id | | ... | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ The "object_transport_id" and "fec_payload_id" are used by the receiver to acknowledge applicable "NORM_CMD(FLUSH)" messages transmitted by the sender identified by the "server_id" field. The "ack_payload" of "NORM_ACK" messages for application-defined "ack_type" values is specific to the application but is limited in size to a maximum the NormSegmentSize of the sender referenced by the "server_id". 4.4. General Purpose Messages Some additional message formats are defined for general purpose in NORM multicast sessions whether the participant is acting as a sender and/or receiver within the group. 4.4.1. NORM_REPORT Message This is an OPTIONAL message generated by NORM participants. This message may be used for periodic performance reports from receivers in experimental NORM implementations. The format of this message is currently undefined. Experimental NORM implementations may define "NORM_REPORT" formats as needed for test purposes. These report messages SHOULD be disabled for interoperability testing between different compliant NORM implementations. 5. Detailed Protocol Operation This section describes the detailed interactions of senders and receivers participating in a NORM session. A simple synopsis of protocol operation is given here: 1. The sender periodically transmits "NORM_CMD(CC)" messages as needed to initialize and collect round-trip timing and congestion control feedback from the receiver set. Adamson, et al. Expires October 26, 2009 [Page 56] Internet-Draft NORM Protocol April 2009 2. The sender transmits an ordinal set of NormObjects segmented in the form of "NORM_DATA" messages labeled with NormTransportIds and logically identified with FEC encoding block numbers and symbol identifiers. "NORM_INFO" messages may optionally precede the transmission of data content for NORM transport objects. 3. As receivers detect missing content from the sender, they initiate repair requests with "NORM_NACK" messages. Note the receivers track the sender's most recent objectId::fecPayloadId transmit position and NACK ONLY for content ordinally prior to that transmit position. The receivers schedule random backoff timeouts before generating "NORM_NACK" messages and wait an appropriate amount of time before repeating the "NORM_NACK" if their repair request is not satisfied. 4. The sender aggregates repair requests from the receivers and logically "rewinds" its transmit position to send appropriate repair messages. The sender sends repairs for the earliest ordinal transmit position first and maintains this ordinal repair transmission sequence. FEC parity content not previously transmitted for the applicable FEC coding block is used for repair transmissions to the greatest extent possible. If the sender exhausts its available FEC parity content on multiple repair cycles for the same coding block, it resorts to an explicit repair strategy (possibly using parity content) to complete repairs. (The use of explicit repair is expected to be an exception in general protocol operation, but the possibility does exist for extreme conditions). The sender immediately assumes transmission of new content once it has sent pending repairs. 5. The sender transmits "NORM_CMD(FLUSH)" messages when it reaches the end of enqueued transmit content and pending repairs. Receivers respond to the "NORM_CMD(FLUSH)" messages with "NORM_NACK" transmissions (following the same suppression backoff timeout strategy as for data) if they require further repair. 6. The sender transmissions are subject to rate control limits determined by congestion control mechanisms. In the baseline NORM-CC operation, each sender in a NormSession maintains its own independent congestion control state. Receivers provide congestion control feedback in "NORM_NACK" and "NORM_ACK" messages. "NORM_ACK" feedback for congestion control purposes is governed using a suppression mechanism similar to that for "NORM_NACK" messages. While this overall concept is relatively simple, there are details to each of these aspects that need to be addressed for successful, efficient, robust, and scalable NORM protocol operation. Adamson, et al. Expires October 26, 2009 [Page 57] Internet-Draft NORM Protocol April 2009 5.1. Sender Initialization and Transmission Upon startup, the NORM sender immediately begins sending "NORM_CMD(CC)" messages to collect round trip timing and other information from the potential group. If NORM-CC congestion control operation is enabled, the NORM-CC Rate header extension MUST be included in these messages. Congestion control operation SHALL be observed at all times when not operating using dedicated resources, like in the general Internet. Even if congestion control operation is disabled at the sender, it may be desirable to use the "NORM_CMD(CC)" messaging to collect feedback from the group using the baseline NORM-CC feedback mechanisms. This proactive feedback collection can be used to establish a GRTT estimate prior to data transmission and potential NACK operation. In some cases, applications may wish for the sender to also proceed with data transmission immediately. In other cases, the sender may wish to defer data transmission until it has received some feedback or request from the receiver set indicating that receivers are indeed present. Note, in some applications (e.g., web push), this indication may come out-of-band with respect to the multicast session via other means. As noted, the periodic transmission of "NORM_CMD(CC)" messages may precede actual data transmission in order to have an initial GRTT estimate. With inclusion of the OPTIONAL NORM FEC Object Transmission Information Header Extension (EXT_FTI), the NORM protocol sender message headers can contain all information necessary to prepare receivers for subsequent reliable reception. This includes FEC coding parameters, the sender NormSegmentSize, and other information. If this header extension is not used, it is presumed that receivers have received the FEC Object Transmission Information via other means. Additionally, applications may leverage the use of "NORM_INFO" messages associated with the session data objects in the session to provide application-specific context information for the session and data being transmitted. These mechanisms allow for operation with minimal pre-coordination among the senders and receivers. The NORM sender begins segmenting application-enqueued data into "NORM_DATA" segments and transmitting it to the group. For objects of type "NORM_OBJECT_DATA" and "NORM_OBJECT_FILE", the segmentation algorithm described in FEC Building Block [RFC5052] is RECOMMENDED. For objects of type "NORM_OBJECT_STREAM", segmentation will typically be into uniform FEC coding block sizes, with individual segment sizes controlled by the application. In most cases, the application and NORM implementation SHOULD strive to produce full-sized ("NormSegmentSize") segments when possible. The rate of transmission Adamson, et al. Expires October 26, 2009 [Page 58] Internet-Draft NORM Protocol April 2009 is controlled via congestion control mechanisms or is a fixed rate if desired for closed network operations. The receivers participating in the multicast group provide feedback to the sender as needed. When the sender reaches the end of data it has enqueued for transmission or any pending repairs, it transmits a series of "NORM_CMD(FLUSH)" messages at a rate of one per "2*GRTT". Receivers may respond to these "NORM_CMD(FLUSH)" messages with additional repair requests. A protocol parameter ""NORM_ROBUST_FACTOR"" determines the number of flush messages sent. If receivers request repair, the repair is provided and flushing occurs again at the end of repair transmission. The sender may attach an OPTIONAL "acking_node_list" to "NORM_CMD(FLUSH)" containing the NormNodeIds for receivers from which it expects explicit positive acknowledgment of reception. The "NORM_CMD(FLUSH)" message may be also used for this optional function any time prior to the end of data enqueued for transmission with the "NORM_CMD(FLUSH)" messages multiplexed with ongoing data transmissions. The OPTIONAL NORM positive acknowledgment procedure is described in Section 5.5.3. 5.1.1. Object Segmentation Algorithm NORM senders and receivers MUST use a common algorithm for logically segmenting transport data into FEC encoding blocks and symbols so that appropriate NACKs can be constructed to request repair of missing data. NORM FEC coding blocks are comprised of multi-byte symbols (segments) that are transmitted in the payload of "NORM_DATA" messages. Each "NORM_DATA" message will contain one or more source or encoding symbol(s) identified by the "fec_payload_id" field and the NormSegmentSize sender parameter defines the maximum size (in bytes) of the "payload_data" field containing the content (a "segment"). The FEC encoding type and associated parameters govern the source block size (number of source symbols per coding block, etc.). NORM senders and receivers use these FEC parameters, along with the NormSegmentSize and transport object size to compute the source block structure for transport objects. These parameters are provided in the FEC Object Transmission Information for each object. The block partitioning algorithm described in the FEC Building Block [RFC5052] is RECOMMENDED for use to compute a source block structure such that all source blocks are as close to being equal length as possible. This helps avoid the performance disadvantages of "short" FEC blocks. Note this algorithm applies only to the statically-sized "NORM_OBJECT_DATA" and "NORM_OBJECT_FILE" transport object types where the object size is fixed and predetermined. For "NORM_OBJECT_STREAM" objects, the object is segmented according to the maximum source block length given in the FEC Transmission Information, unless the FEC Payload ID indicates an alternative size for a given block. Adamson, et al. Expires October 26, 2009 [Page 59] Internet-Draft NORM Protocol April 2009 5.2. Receiver Initialization and Reception The NORM protocol is designed such that receivers may join and leave the group at will. However, some applications may be constrained such that receivers need to be members of the group prior to start of data transmission. NORM applications may use different policies to constrain the impact of new receivers joining the group in the middle of a session. For example, a useful implementation policy is for new receivers joining the group to limit or avoid repair requests for transport objects already in progress. The NORM sender implementation may wish to impose additional constraints to limit the ability of receivers to disrupt reliable multicast performance by joining, leaving, and rejoining the group often. Different receiver "join policies" may be appropriate for different applications and/or scenarios. For general purpose operation, a default policy where receivers are allowed to request repair only for coding blocks with a NormTransportId and FEC coding block number greater than or equal to the first non-repair "NORM_DATA" or "NORM_INFO" message received upon joining the group is RECOMMENDED. For objects of type "NORM_OBJECT_STREAM" it is RECOMMENDED that the join policy constrain receivers to start reliable reception at the current FEC coding block for which non-repair content is received. For typical operation, it is expected that NORM receivers will join a specified multicast group and/or listen on an specific port number for sender transmissions. As the NORM receiver receives "NORM_DATA" messages it will provide content to its application as appropriate. 5.3. Receiver NACK Procedure When the receiver detects it is missing data from a sender's NORM transmissions, it initiates its NACKing procedure. The NACKing procedure SHALL be initiated ONLY at FEC coding block boundaries, NormObject boundaries, upon receipt of a "NORM_CMD(FLUSH)" message, or upon an "inactivity" timeout when "NORM_DATA" or "NORM_INFO" transmissions are no longer received from a previously active sender. The RECOMMENDED value of such an inactivity timeout is: T_inactivity = NORM_ROBUST_FACTOR * 2 * GRTTSender where the ""GRTTsender"" value corresponds to the GRTT estimate advertised in the "grtt" field of NORM sender messages. A minimum ""T_inactivity"" value of 1 second is RECOMMENDED. The NORM receiver SHOULD reset this inactivity timer and repeat NACK initiation upon timeout for up to "NORM_ROBUST_FACTOR" times or more depending upon the application's need for persistence by its receivers. It is also important that receivers rescale the ""T_inactivity"" timeout as the sender's advertised GRTT changes. Adamson, et al. Expires October 26, 2009 [Page 60] Internet-Draft NORM Protocol April 2009 The NACKing procedure begins with a random backoff timeout. The duration of the backoff timeout is chosen using the "RandomBackoff" algorithm described in the Multicast NACK Building Block [RFC5401] using ("Ksender*GRTTsender") for the "maxTime" parameter and the sender advertised group size ("GSIZEsender") as the "groupSize" parameter. NORM senders provide values for "GRTTsender", "Ksender" and "GSIZEsender" via the "grtt", "backoff", and "gsize" fields of transmitted messages. The "GRTTsender" value is determined by the sender based on feedback it has received from the group while the "Ksender" and "GSIZEsender" values may determined by application requirements and expectations or ancillary information. The backoff factor ""Ksender"" MUST be greater than "one" to provide for effective feedback suppression. A value of "K = 4" is RECOMMENDED for the Any Source Multicast (ASM) model while a value of "K = 6" is RECOMMENDED for Single Source Multicast (SSM) operation. Thus: T_backoff = RandomBackoff(Ksender*GRTTsender, GSIZEsender) To avoid the possibility of NACK implosion in the case of sender or network failure during SSM operation, the receiver SHALL automatically suppress its NACK and immediately enter the "holdoff" period described below when "T_backoff" is greater than "(Ksender-1)*GRTTsender". Otherwise, the backoff period is entered and the receiver MUST accumulate external pending repair state from "NORM_NACK" messages and "NORM_CMD(REPAIR_ADV)" messages received. At the end of the backoff time, the receiver SHALL generate a "NORM_NACK" message only if the following conditions are met: 1. The sender's current transmit position (in terms of objectId:: fecPayloadId) exceeds the earliest repair position of the receiver. 2. The repair state accumulated from "NORM_NACK" and "NORM_CMD(REPAIR_ADV)" messages do not equal or supersede the receiver's repair needs up to the sender transmission position at the time the NACK procedure (backoff timeout) was initiated. If these conditions are met, the receiver immediately generates a "NORM_NACK" message when the backoff timeout expires. Otherwise, the receiver's NACK is considered to be "suppressed" and the message is not sent. At this time, the receiver begins a "holdoff" period during which it constrains itself to not re-initiate the NACKing process. The purpose of this timeout is to allow the sender worst- case time to respond to the repair needs before the receiver requests repair again. The value of this "holdoff" timeout ("T_rcvrHoldoff") as described in [RFC5401] is: T_rcvrHoldoff =(Ksender+2)*GRTTsender Adamson, et al. Expires October 26, 2009 [Page 61] Internet-Draft NORM Protocol April 2009 The "NORM_NACK" message contains repair request content beginning with lowest ordinal repair position of the receiver up through the coding block prior to the most recently heard ordinal transmission position for the sender. If the size of the "NORM_NACK" content exceeds the sender's NormSegmentSize, the NACK content is truncated so that the receiver only generates a single "NORM_NACK" message per NACK cycle for a given sender. In summary, a single NACK message is generated containing the receiver's lowest ordinal repair needs. For each partially-received FEC coding block requiring repair, the receiver SHALL, on its FIRST repair attempt for the block, request the parity portion of the FEC coding block beginning with the lowest ordinal parity "encoding_symbol_id" (i.e., "encoding_symbol_id" = "source_block_len") and request the number of FEC symbols corresponding to its data segment erasure count for the block. On subsequent repair cycles for the same coding block, the receiver SHALL request only those repair symbols from the first set it has not yet received up to the remaining erasure count for that applicable coding block. Note that the sender may have provided other different, additional parity segments for other receivers that could also be used to satisfy the local receiver's erasure-filling needs. In the case where the erasure count for a partially-received FEC coding block exceeds the maximum number of parity symbols available from the sender for the block (as indicated by the "NORM_DATA" "fec_num_parity" field), the receiver SHALL request all available parity segments plus the ordinally highest missing data segments required to satisfy its total erasure needs for the block. The goal of this strategy is for the overall receiver set to request a lowest common denominator set of repair symbols for a given FEC coding block. This allows the sender to construct the most efficient repair transmission segment set and enables effective NACK suppression among the receivers even with uncorrelated packet loss. This approach also requires no synchronization among the receiver set in their repair requests for the sender. For FEC coding blocks or NormObjects missed in their entirety, the NORM receiver constructs repair requests with "NORM_NACK_BLOCK" or "NORM_NACK_OBJECT" flags set as appropriate. The request for retransmission of "NORM_INFO" is accomplished by setting the "NORM_NACK_INFO" flag in a corresponding repair request. 5.4. Sender NACK Processing and Response The principle goal of the sender is to make forward progress in the transmission of data its application has enqueued. However, the sender must occasionally "rewind" its logical transmission point to satisfy the repair needs of receivers who have NACKed. Aggregation of multiple NACKs is used to determine an optimal repair strategy Adamson, et al. Expires October 26, 2009 [Page 62] Internet-Draft NORM Protocol April 2009 when a NACK event occurs. Since receivers initiate the NACK process on coding block or object boundaries, there is some loose degree of synchronization of the repair process even when receivers experience uncorrelated data loss. 5.4.1. Sender Repair State Aggregation When a sender is in its normal state of transmitting new data and receives a NACK, it begins a procedure to accumulate NACK repair state from "NORM_NACK" messages before beginning repair transmissions. Note that this period of aggregating repair state does NOT interfere with its ongoing transmission of new data. As described in [RFC5401], the period of time during which the sender aggregates "NORM_NACK" messages is equal to: T_sndrAggregate = (Ksender+1)*GRTT where ""Ksender"" is the same backoff scaling value used by the receivers, and "GRTT" is the sender's current estimate of the group's greatest round-trip time. Note that for NORM unicast sessions the ""T_sndrAggregate"" time can be set to ZERO since there is only one receiver. Similarly, the ""Ksender"" value should be set to ZERO for NORM unicast sessions to minimize repair latency. When this period ends, the sender "rewinds" by incorporating the accumulated repair state into its pending transmission state and begins transmitting repair messages. After pending repair transmissions are completed, the sender continues with new transmissions of any enqueued data. Also, at this point in time, the sender begins a "holdoff" timeout during which time the sender constrains itself from initiating a new repair aggregation cycle, even if "NORM_NACK" messages arrive. As described in [RFC5401], the value of this sender "holdoff" period is: T_sndrHoldoff = (1*GRTT) If additional "NORM_NACK" messages are received during this sender "holdoff" period, the sender will immediately incorporate these late- arriving messages into its pending transmission state ONLY if the NACK content is ordinally greater than the sender's current transmission position. This "holdoff" time allows worst case time for the sender to propagate its current transmission sequence position to the group, thus avoiding redundant repair transmissions. After the holdoff timeout expires, a new NACK accumulation period can be begun (upon arrival of a NACK) in concert with the pending repair and new data transmission. Recall that receivers are not to initiate the NACK repair process until the sender's logical transmission position exceeds the lowest ordinal position of their repair needs. With the new NACK aggregation period, the sender repeats the same Adamson, et al. Expires October 26, 2009 [Page 63] Internet-Draft NORM Protocol April 2009 process of incorporating accumulated repair state into its transmission plan and subsequently "rewinding" to transmit the lowest ordinal repair data when the aggregation period expires. Again, this is conducted in concert with ongoing new data and/or pending repair transmissions. 5.4.2. Sender FEC Repair Transmission Strategy The NORM sender should leverage transmission of FEC parity content for repair to the greatest extent possible. Recall that the receivers use a strategy to request a lowest common denominator of explicit repair (including parity content) in the formation of their "NORM_NACK" messages. Before falling back to explicitly satisfying different receivers' repair needs, the sender can make use of the general erasure-filling capability of FEC-generated parity segments. The sender can determine the maximum erasure filling needs for individual FEC coding blocks from the "NORM_NACK" messages received during the repair aggregation period. Then, if the sender has a sufficient number (less than or equal to the maximum erasure count) of previously unsent parity segments available for the applicable coding blocks, the sender can transmit these in lieu of the specific packets the receiver set has requested. Only after exhausting its supply of "fresh" (unsent) parity segments for a given coding block should the sender resort to explicit transmission of the receiver set's repair needs. In general, if a sufficiently powerful FEC code is used, the need for explicit repair will be an exception, and the fulfillment of reliable multicast can be accomplished quite efficiently. However, the ability to resort to explicit repair allows the protocol to be reliable under even very extreme circumstances. "NORM_DATA" messages sent as repair transmissions SHALL be flagged with the "NORM_FLAG_REPAIR" flag. This allows receivers to obey any policies that limit new receivers from joining the reliable transmission when only repair transmissions have been received. Additionally, the sender SHOULD additionally flag "NORM_DATA" transmissions sent as explicit repair with the "NORM_FLAG_EXPLICIT" flag. Although NORM end system receivers do not make use of the "NORM_FLAG_EXPLICIT" flag, this message transmission status could be leveraged by intermediate systems wishing to "assist" NORM protocol performance. If such systems are properly positioned with respect to reciprocal reverse-path multicast routing, they need to sub-cast only a sufficient count of non-explicit parity repairs to satisfy a multicast routing sub-tree's erasure filling needs for a given FEC coding block. When the sender has resorted to explicit repair, then the intermediate systems should sub-cast all of the explicit repair Adamson, et al. Expires October 26, 2009 [Page 64] Internet-Draft NORM Protocol April 2009 packets to those portions of the routing tree still requiring repair for a given coding block. Note the intermediate systems will be required to conduct repair state accumulation for sub-routes in a manner similar to the sender's repair state accumulation in order to have sufficient information to perform the sub-casting. Additionally, the intermediate systems could perform additional "NORM_NACK" suppression/aggregation as it conducts this repair state accumulation for NORM repair cycles. The detail of this type of operation are beyond the scope of this document, but this information is provided for possible future consideration. 5.4.3. Sender NORM_CMD(SQUELCH) Generation If the sender receives a "NORM_NACK" message for repair of data it is no longer supporting, the sender generates a "NORM_CMD(SQUELCH)" message to advertise its repair window and squelch any receivers from additional NACKing of invalid data. The transmission rate of "NORM_CMD(SQUELCH)" messages is limited to once per "2*GRTT". The "invalid_object_list" (if applicable) of the "NORM_CMD(SQUELCH)" message SHALL begin with the lowest "object_transport_id" from the invalid "NORM_NACK" messages received since the last "NORM_CMD(SQUELCH)" transmission. Lower ordinal invalid "object_transport_ids" should be included only while the "NORM_CMD(SQUELCH)" payload is less than the sender's NormSegmentSize parameter. 5.4.4. Sender NORM_CMD(REPAIR_ADV) Generation When a NORM sender receives "NORM_NACK" messages from receivers via unicast transmission, it uses "NORM_CMD(REPAIR_ADV)" messages to advertise its accumulated repair state to the receiver set since the receiver set is not directly sharing their repair needs via multicast communication. A NORM sender implementation MAY use a separate port number from the NormSession port number as the source port for its transmissions. Thus NORM receivers can direct any unicast feedback messages to this sender port number that is distinct from the NORM session (or destination) port number. Then, the NORM sender implementation can discriminate unicast feedback messages from multicast feedback messages when there is a mix of multicast and unicast feedback receivers. The "NORM_CMD(REPAIR_ADV)" message is multicast to the receiver set by the sender. The payload portion of this message has content in the same format as the "NORM_NACK" receiver message payload. Receivers are then able to perform feedback suppression in the same manner as with "NORM_NACK" messages directly received from other receivers. Note the sender does not merely retransmit NACK content it receives, but instead transmits a representation of its aggregated repair state. The transmission of "NORM_CMD(REPAIR_ADV)" messages are subject to the sender transmit Adamson, et al. Expires October 26, 2009 [Page 65] Internet-Draft NORM Protocol April 2009 rate limit and NormSegmentSize limitation. When the "NORM_CMD(REPAIR_ADV)" message is of maximum size, receivers SHALL consider the maximum ordinal transmission position value embedded in the message as the senders current transmission position and implicitly suppress requests for ordinally higher repair. For congestion control operation, the sender may also need to provide information so that dynamic congestion control feedback can be suppressed as needed among receivers. This document specifies the NORM-CC Feedback Header Extension that is applied for baseline NORM-CC operation. If other congestion control mechanisms are used within a NORM implementation, other header extensions may be defined. Whatever content format is used for this purpose should ensure that maximum possible suppression state is conveyed to the receiver set. 5.5. Additional Protocol Mechanisms In addition to the principal function of data content transmission and repair, there are some other protocol mechanisms that help NORM to adapt to network conditions and play fairly with other coexistent protocols. 5.5.1. Greatest Round-trip Time Collection For NORM receivers to appropriately scale backoff timeouts and the senders to use proper corresponding timeouts, the participants must agree on a common timeout basis. Each NORM sender monitors the round-trip time of active receivers and determines the group greatest round-trip time (GRTT). The sender advertises this GRTT estimate in every message it transmits so that receivers have this value available for scaling their timers. To measure the current GRTT, the sender periodically sends "NORM_CMD(CC)" messages that contain a locally generated timestamp. Receivers are expected to record this timestamp along with the time the "NORM_CMD(CC)" message is received. Then, when the receivers generate feedback messages to the sender, an adjusted version of the sender timestamp is embedded in the feedback message ("NORM_NACK" or "NORM_ACK"). The adjustment adds the amount of time the receiver held the timestamp before generating its response. Upon receipt of this adjusted timestamp, the sender is able to calculate the round-trip time to that receiver. The round-trip time for each receiver is fed into an algorithm that weights and smoothes the values for a conservative estimate of the GRTT. The algorithm and methodology are described in the Multicast NACK Building Block [RFC5401] in the section entitled "One-to-Many Sender GRTT Measurement". A conservative estimate helps guarantee feedback suppression at a small cost in overall protocol repair delay. The sender's current estimate of GRTT is advertised in the "grtt" field found in all NORM sender messages. The advertised GRTT Adamson, et al. Expires October 26, 2009 [Page 66] Internet-Draft NORM Protocol April 2009 is also limited to a minimum of the nominal inter-packet transmission time given the sender's current transmission rate and system clock granularity. The reason for this additional limit is to keep the receiver somewhat event-driven by making sure the sender has had adequate time to generate any response to repair requests from receivers given transmit rate limitations due to congestion control or configuration. When the NORM-CC Rate header extension is present in "NORM_CMD(CC)" messages, the receivers respond to "NORM_CMD(CC)" messages as described in Section 5.5.2, "NORM Congestion Control Operation". The "NORM_CMD(CC)" messages are periodically generated by the sender as described for congestion control operation. This provides for proactive, but controlled, feedback from the group in the form of "NORM_ACK" messages. This provides for GRTT feedback even if no "NORM_NACK" messages are being sent. If operating without congestion control in a closed network, the "NORM_CMD(CC)" messages may be sent periodically without the NORM-CC Rate header extension. In this case, receivers will only provide GRTT measurement feedback when "NORM_NACK" messages are generated since no "NORM_ACK" messages are generated. In this case, the "NORM_CMD(CC)" messages may be sent less frequently, perhaps as little as once per minute, to conserve network capacity. Note that the NORM-CC Rate header extension may also be used to proactively solicit RTT feedback from the receiver group per congestion control operation even though the sender may not be conducting congestion control rate adjustment. NORM operation without congestion control should be considered only in closed networks. 5.5.2. NORM Congestion Control Operation This section describes baseline congestion control operation for the NORM protocol (NORM-CC). The supporting NORM message formats and approach described here are an adaptation of the equation-based TCP- Friendly Multicast Congestion Control (TFMCC) approach[RFC4654]. This congestion control scheme is REQUIRED for operation within the general Internet unless the NORM implementation is adapted to use another IETF-sanctioned reliable multicast congestion control mechanism. With this TFMCC-based approach, the transmissions of NORM senders are controlled in a rate-based manner as opposed to window- based congestion control algorithms as in TCP. However, it is possible that the NORM protocol message set may alternatively be used to support a window-based multicast congestion control scheme such as PGMCC. The details of that alternative may be described separately or in a future revision of this document. In either case (rate-based TFMCC or window-based PGMCC), successful control of sender transmission depends upon collection of sender-to-receiver packet loss estimates and RTTs to identify the congestion control bottleneck Adamson, et al. Expires October 26, 2009 [Page 67] Internet-Draft NORM Protocol April 2009 path(s) within the multicast topology and adjust the sender rate accordingly. The receiver with loss and RTT estimates that correspond to the lowest resulting calculated transmission rate is identified as the "current limiting receiver" (CLR). In the case of a tie (where candidate CLRs are within 10% of the same calculated rate), the receiver with the largest RTT value SHOULD be designated as the CLR. As described in [TcpModel], a steady-state sender transmission rate, to be "friendly" with competing TCP flows can be calculated as: S Rsender = ---------------------------------------------------------- tRTT*(sqrt((2/3)*p) + 12*sqrt((3/8)*p) * p * (1 + 32*(p^2))) where "S" = nominal transmitted packet size. (In NORM, the "nominal" packet size can be determined by the sender as an exponentially weighted moving average (EWMA) of transmitted packet sizes to account for variable message sizes). "tRTT" = RTT estimate of the current "current limiting receiver" (CLR). "p" = loss event fraction of the CLR. To support congestion control feedback collection and operation, the NORM sender periodically transmits "NORM_CMD(CC)" command messages. "NORM_CMD(CC)" messages are multiplexed with NORM data and repair transmissions and serve several purposes: 1. Stimulate explicit feedback from the general receiver set to collect congestion control information. 2. Communicate state to the receiver set on the sender's current congestion control status including details of the CLR. 3. Initiate rapid (immediate) feedback from the CLR in order to closely track the dynamics of congestion control for that current worst path in the group multicast topology. The format of the "NORM_CMD(CC)" message is described in Section 4.2.3 of this document. The "NORM_CMD(CC)" message contains information to allow measurement of RTTs, to inform the group of the congestion control CLR, and to provide feedback of individual RTT measurements to the receivers in the group. The "NORM_CMD(CC)" also provides for exciting feedback from OPTIONAL "potential limiting receiver" (PLR) nodes that may be determined administratively or possibly algorithmically based on congestion control feedback. PLR nodes are receivers that have been identified to have potential for Adamson, et al. Expires October 26, 2009 [Page 68] Internet-Draft NORM Protocol April 2009 (perhaps soon) becoming the CLR and thus immediate, up-to-date feedback is beneficial for congestion control performance. The PLR list may be populated with a small number of receivers the sender identifies as approaching the CLR loss and delay conditions based on feedback from the group. 5.5.2.1. NORM_CMD(CC) Transmission The "NORM_CMD(CC)" message is transmitted periodically by the sender along with its normal data transmission. Note that the repeated transmission of "NORM_CMD(CC)" messages may be initiated some time before transmission of user data content at session startup. This may be done to collect some estimation of the current state of the multicast topology with respect to group and individual RTT and congestion control state. A "NORM_CMD(CC)" message is immediately transmitted at sender startup. The interval of subsequent "NORM_CMD(CC)" message transmission is determined as follows: 1. By default, the interval is set according to the current sender GRTT estimate. A startup GRTT of 0.5 seconds is recommended when no feedback has yet been received from the group. 2. Until a CLR has been identified (based on previous receiver feedback) or when no data transmission is pending, the "NORM_CMD(CC)" interval is doubled up from its current interval to a maximum of once per 30 seconds. This results in a low duty cycle for "NORM_CMD(CC)" probing when no CLR is identified or there is no pending data to transmit. 3. When a CLR has been identified (based on receiver feedback) and data transmission is pending, the probing interval is set to the RTT between the sender and the CLR ("RTT_clr"). 4. Additionally, when the data transmission rate is low with respect to the "RTT_clr" interval used for probing, the implementation should ensure that no more than one "NORM_CMD(CC)" message is sent per "NORM_DATA" message when there is data pending transmission. This ensures that the transmission of this control message is not done to the exclusion of user data transmission. The "NORM_CMD(CC)" "cc_sequence" field is incremented with each transmission of a "NORM_CMD(CC)" command. The greatest "cc_sequence" recently received by receivers is included in their feedback to the sender. This allows the sender to determine the age of feedback to assist in congestion avoidance. The NORM-CC Rate Header Extension is applied to the "NORM_CMD(CC)" message and the sender advertises its current transmission rate in the "send_rate" field. The rate information is used by receivers to Adamson, et al. Expires October 26, 2009 [Page 69] Internet-Draft NORM Protocol April 2009 initialize loss estimation during congestion control startup or restart. The "cc_node_list" contains a list of entries identifying receivers and their current congestion control state (status "flags", "rtt" and "loss" estimates). The list may be empty if the sender has not yet received any feedback from the group. If the sender has received feedback, the list will minimally contain an entry identifying the CLR. A "NORM_FLAG_CC_CLR" flag value is provided for the "cc_flags" field to identify the CLR entry. It is RECOMMENDED that the CLR entry be the first in the list for implementation efficiency. Additional entries in the list are used to provide sender-measured individual RTT estimates to receivers in the group. The number of additional entries in this list is dependent upon the percentage of control traffic the sender application is willing to send with respect to user data message transmissions. More entries in the list may allow the sender to be more responsive to congestion control dynamics. The length of the list may be dynamically determined according to the current transmission rate and scheduling of "NORM_CMD(CC)" messages. The maximum length of the list corresponds to the sender's NormSegmentSize parameter for the session. The inclusion of additional entries in the list based on receiver feedback are prioritized with following rules: 1. Receivers that have not yet been provided a RTT measurement get first priority. Of these, those with the greatest loss fraction receive precedence for list inclusion. 2. Secondly, receivers that have previously been provided a RTT measurement are included with receivers yielding the lowest calculated congestion rate getting precedence. There are "cc_flag" values in addition to "NORM_FLAG_CC_CLR" that are used for other congestion control functions. The "NORM_FLAG_CC_PLR" flag value is used to mark additional receivers from that the sender would like to have immediate, non-suppressed feedback. These may be receivers that the sender algorithmically identified as potential future CLRs or that have been pre-configured as potential congestion control points in the network. The "NORM_FLAG_CC_RTT" indicates the validity of the "cc_rtt" field for the associated receiver node. Normally, this flag will be set since the receivers in the list will typically be receivers from which the sender has received feedback. However, in the case that the NORM sender has been pre-configured with a set of PLR nodes, feedback from those receivers may not yet have been collected and thus the "cc_rtt" field does not contain a valid value when this flag is not set. Similarly, a value of ZERO for the "cc_rate" field here should be treated as an invalid value and be ignored for the purposes of feedback suppression, etc. Adamson, et al. Expires October 26, 2009 [Page 70] Internet-Draft NORM Protocol April 2009 5.5.2.2. NORM_CMD(CC) Feedback Response Receivers explicitly respond to "NORM_CMD(CC)" messages in the form of a "NORM_ACK(RTT)" message. The goal of the congestion control feedback is to determine the receivers with the lowest congestion control rates. Receivers that are marked as CLR or PLR nodes in the "NORM_CMD(CC)" "cc_node_list" immediately provide feedback in the form of a "NORM_ACK" to this message. When a "NORM_CMD(CC)" is received, non-CLR or non-PLR nodes initiate random feedback backoff timeouts similar to that used when the receiver initiates a repair cycle (see Section 5.3) in response to detection of data loss. The backoff timeout for the congestion control response is generated as follows: T_backoff = RandomBackoff(K*GRTTsender, GSIZEsender) The ""RandomBackoff()"" algorithm provides a truncated exponentially distributed random number and is described in the Multicast NACK Building Block [RFC5401]. The same backoff factor "K = Ksender" MAY be used as with "NORM_NACK" suppression. However, in cases where the application purposefully specifies a very small "Ksender" backoff factor to minimize the NACK repair process latency (trading off group size scalability), it is RECOMMENDED that a larger backoff factor for congestion control feedback is maintained, since there may often be a larger volume of congestion control feedback than NACKs in many cases and some congestion control feedback latency may be tolerable where reliable delivery latency is not. As previously noted, a backoff factor value of "K = 4" is generally recommended for ASM operation and "K = 6" for SSM operation. A receiver SHALL cancel the backoff timeout and thus its pending transmission of a "NORM_ACK(RTT)" message under the following conditions: 1. The receiver generates another feedback message ("NORM_NACK" or other "NORM_ACK") before the congestion control feedback timeout expires (these messages will convey the current congestion control feedback information), 2. A "NORM_CMD(CC)" or other receiver feedback with an ordinally greater "cc_sequence" field value is received before the congestion control feedback timeout expires (this is similar to the TFMCC feedback round number), 3. When the "T_backoff" is greater than "1*GRTTsender". This prevents NACK implosion in the event of sender or network failure, 4. "Suppressing" congestion control feedback is heard from another receiver (in a "NORM_ACK" or "NORM_NACK") or via a "NORM_CMD(REPAIR_ADV)" message from the sender. The local receiver's feedback is "suppressed" if the rate of the competing feedback ("Rfb") is sufficiently close to or less than the local receiver's calculated rate ("Rcalc"). The local receiver's Adamson, et al. Expires October 26, 2009 [Page 71] Internet-Draft NORM Protocol April 2009 feedback is canceled when "Rcalc > (0.9 * Rfb)". Also note receivers that have not yet received an RTT measurement from the sender are suppressed only by other receivers that have not yet measured RTT. Additionally, receivers whose RTT estimate has aged considerably (i.e., they haven't been included in the "NORM_CMD(CC)" "cc_node_list" in a long time) may wish to compete as a receiver with no prior RTT measurement after some long term expiration period. When the backoff timer expires, the receiver SHALL generate a "NORM_ACK(RTT)" message to provide feedback to the sender and group. This message may be multicast to the group for most effective suppression in ASM topologies or unicast to the sender depending upon how the NORM protocol is deployed and configured. Whenever any feedback is generated (including this "NORM_ACK(RTT)" message), receivers include an adjusted version of the sender timestamp from the most recently received "NORM_CMD(CC)" message and the "cc_sequence" value from that command in the applicable "NORM_ACK" or "NORM_NACK" message fields. For NORM-CC operation, any generated feedback message SHALL also contain the NORM-CC Feedback header extension. The receiver provides its current "cc_rate" estimate, "cc_loss" estimate, "cc_rtt" if known, and any applicable "cc_flags" via this header extension. During slow start (when the receiver has not yet detected loss from the sender), the receiver uses a value equal to two times its measured rate from the sender in the "cc_rate" field. For steady- state congestion control operation, the receiver "cc_rate" value is from the equation-based value using its current loss event estimate and sender<->receiver RTT information. (The GRTT is used when the receiver has not yet measured its individual RTT). The "cc_loss" field value reflects the receiver's current loss event estimate with respect to the sender in question. When the receiver has a valid individual RTT measurement, it SHALL include this value in the "cc_rtt" field. The "NORM_FLAG_CC_RTT" MUST be set when the "cc_rtt" field is valid. After a congestion control feedback message is generated or when the feedback is suppressed, a non-CLR receiver begins a "holdoff" timeout period during which it will restrain itself from providing congestion control feedback, even if "NORM_CMD(CC)" messages are received from the sender (unless the receive becomes marked as a CLR or PLR node). The value of this holdoff timeout ("T_ccHoldoff") period is: T_ccHoldoff = (K*GRTT) Adamson, et al. Expires October 26, 2009 [Page 72] Internet-Draft NORM Protocol April 2009 Thus, non-CLR receivers are constrained to providing explicit congestion control feedback once per "K*GRTT" intervals. Note, however, that as the session progresses, different receivers will be responding to different "NORM_CMD(CC)" messages and there will be relatively continuous feedback of congestion control information while the sender is active. 5.5.2.3. Congestion Control Rate Adjustment During steady-state operation, the sender will directly adjust its transmission rate to the rate indicated by the feedback from its currently selected CLR. As noted in [TfmccPaper], the estimation of parameters (loss and RTT) for the CLR will generally constrain the rate changes possible within acceptable bounds. For rate increases, the sender SHALL observe a maximum rate of increase of one packet per RTT at all times during steady-state operation. The sender processes congestion control feedback from the receivers and selects the CLR based on the lowest rate receiver. Receiver rates are either determined directly from the slow start "cc_rate" provided by the receiver in the NORM-CC Feedback header extension or by performing the equation-based calculation using individual RTT and loss estimates ("cc_loss") as feedback is received. The sender can calculate a current RTT for a receiver ("RTT_rcvrNew") using the "grtt_response" timestamp included in feedback messages. When the "cc_rtt" value in a response is not valid, the sender simply uses this "RTT_rcvrNew" value as the receiver's current RTT ("RTT_rcvr"). For non-CLR and non-PLR receivers, the sender can use the "cc_rtt" value provided in the NORM-CC Feedback header extension as the receiver's previous RTT measurement ("RTT_rcvrPrev") to smooth according to: RTT_rcvr = 0.5 * RTT_rcvrPrev + 0.5 * RTT_rcvrNew For CLR receivers where feedback is received more regularly, the sender SHOULD maintain a more smoothed RTT estimate upon new feedback from the CLR where: RTT_clr = 0.9 * RTT_clr + 0.1 * RTT_clrNew ""RTT_clrNew"" is the new RTT calculated from the timestamp in the feedback message received from the CLR. The "RTT_clr" is initialized to "RTT_clrNew" on the first feedback message received. Note that the same procedure is observed by the sender for PLR receivers and that if a PLR is "promoted" to CLR status, the smoothed estimate can be continued. There are some additional periods besides steady-state operation that need to be considered in NORM-CC operation. These periods are: Adamson, et al. Expires October 26, 2009 [Page 73] Internet-Draft NORM Protocol April 2009 1. during session startup, 2. when no feedback is received from the CLR, and 3. when the sender has a break in data transmission. During session startup, the congestion control operation SHALL observe a "slow start" procedure to quickly approach its fair bandwidth share. An initial sender startup rate is assumed where: Rinitial = MIN(NormSegmentSize / GRTT, NormSegmentSize) bytes/second. The rate is increased only when feedback is received from the receiver set. The "slow start" phase proceeds until any receiver provides feedback indicating that loss has occurred. Rate increase during slow start is applied as: Rnew = Rrecv_min where "Rrecv_min" is the minimum reported receiver rate in the "cc_rate" field of congestion control feedback messages received from the group. Note that during slow start, receivers use two times their measured rate from the sender in the "cc_rate" field of their feedback. Rate increase adjustment is limited to once per GRTT during slow start. If the CLR or any receiver intends to leave the group, it will set the "NORM_FLAG_CC_LEAVE" in its congestion control feedback message as an indication that the sender should not select it as the CLR. When the CLR changes to a lower rate receiver, the sender should immediately adjust to the new lower rate. The sender is limited to increasing its rate at one additional packet per RTT towards any new, higher CLR rate. The sender should also track the age of the feedback it has received from the CLR by comparing its current "cc_sequence" value ("Seq_sender") to the last "cc_sequence" value received from the CLR ("Seq_clr"). As the age of the CLR feedback increases with no new feedback, the sender SHALL begin reducing its rate once per "RTT_clr" as a congestion avoidance measure. The following algorithm is used to determine the decrease in sender rate (Rsender bytes/sec) as the CLR feedback, unexpectedly, excessively ages: Age = Seq_sender - Seq_clr; if (Age > 4) Rsender = Rsender * 0.5; This rate reduction is limited to the lower bound on NORM transmission rate. After "NORM_ROBUST_FACTOR" consecutive "NORM_CMD(CC)" rounds without any feedback from the CLR, the sender SHOULD assume the CLR has left the group and pick the receiver with the next lowest rate as the new CLR. Note this assumes that the sender does not have explicit knowledge that the CLR intentionally left the group. If no receiver feedback is received, the sender MAY Adamson, et al. Expires October 26, 2009 [Page 74] Internet-Draft NORM Protocol April 2009 wish to withhold further transmissions of "NORM_DATA" segments and maintain "NORM_CMD(CC)" transmissions only until feedback is detected. After such a CLR timeout, the sender will be transmitting with a minimal rate and should return to slow start as described here for a break in data transmission. When the sender has a break in its data transmission, it can continue to probe the group with "NORM_CMD(CC)" messages to maintain RTT collection from the group. This will enable the sender to quickly determine an appropriate CLR upon data transmission restart. However, the sender should exponentially reduce its target rate to be used for transmission restart as time since the break elapses. The target rate SHOULD be recalculated once per "RTT_clr" as: Rsender = Rsender * 0.5; If the minimum NORM rate is reached, the sender should set the "NORM_FLAG_START" flag in its "NORM_CMD(CC)" messages upon restart and the group should observer slow start congestion control procedures until any receiver experiences a new loss event. 5.5.3. NORM Positive Acknowledgment Procedure NORM provides options for the source application to request positive acknowledgment (ACK) of "NORM_CMD(FLUSH)" and "NORM_CMD(ACK_REQ)" messages from members of the group. There are some specific acknowledgment requests defined for the NORM protocol and a range of acknowledgment request types that are left to be defined by the application. One predefined acknowledgment type is the "NORM_ACK_FLUSH" type. This acknowledgment is used to determine if receivers have achieved completion of reliable reception up through a specific logical transmission point with respect to the sender's sequence of transmission. The "NORM_ACK_FLUSH" acknowledgment may be used to assist in application flow control when the sender has information on a portion of the receiver set. Another predefined acknowledgment type is "NORM_ACK(CC)", which is used to explicitly provide congestion control feedback in response to "NORM_CMD(CC)" messages transmitted by the sender for NORM-CC operation. Note the "NORM_ACK(CC)" response does NOT follow the positive acknowledgment procedure described here. The "NORM_CMD(ACK_REQ)" and "NORM_ACK" messages contain an "ack_type" field to identify the type of acknowledgment requested and provided. A range of "ack_type" values is provided for application-defined use. While the application is responsible for initiating the acknowledgment request and interprets application-defined "ack_type" values, the acknowledgment procedure SHOULD be conducted within the protocol implementation to take advantage of timing and transmission scheduling information available to the NORM transport. Adamson, et al. Expires October 26, 2009 [Page 75] Internet-Draft NORM Protocol April 2009 The NORM positive acknowledgment procedure uses polling by the sender to query the receiver group for response. Note this polling procedure is not intended to scale to very large receiver groups, but could be used in large group setting to query a critical subset of the group. Either the "NORM_CMD(ACK_REQ)", or when applicable, the "NORM_CMD(FLUSH)" message is used for polling and contains a list of NormNodeIds for receivers that should respond to the command. The list of receivers providing acknowledgment is determined by the source application with a priori knowledge of participating nodes or via some other application-level mechanism. The ACK process is initiated by the sender that generates "NORM_CMD(FLUSH)" or "NORM_CMD(ACK_REQ)" messages in periodic rounds. For "NORM_ACK_FLUSH" requests, the "NORM_CMD(FLUSH)" contain a "object_transport_id" and "fec_payload_id" denoting the watermark transmission point for which acknowledgment is requested. This watermark transmission point is echoed in the corresponding fields of the "NORM_ACK(FLUSH)" message sent by the receiver in response. "NORM_CMD(ACK_REQ)" messages contain an "ack_id" field which is similarly echoed in response so that the sender may match the response to the appropriate request. In response to the "NORM_CMD(ACK_REQ)", the listed receivers randomly spread "NORM_ACK" messages uniformly in time over a window of (1*GRTT). These "NORM_ACK" messages are typically unicast to the sender. (Note that "NORM_ACK(CC)" messages SHALL be multicast or unicast in the same manner as "NORM_NACK" messages). The ACK process is self-limiting and avoids ACK implosion in that: 1. Only a single "NORM_CMD(ACK_REQ)" message is generated once per (2*GRTT), and, 2. The size of the "acking_node_list" of NormNodeIds from which acknowledgment is requested is limited to a maximum of the sender NormSegmentSize setting per round of the positive acknowledgment process. Because the size of the included list is limited to the sender's NormSegmentSize setting, multiple "NORM_CMD(ACK_REQ)" rounds may be required to achieve responses from all receivers specified. The content of the attached NormNodeId list will be dynamically updated as this process progresses and "NORM_ACK" responses are received from the specified receiver set. As the sender receives valid responses (i.e., matching watermark point or "ack_id") from receivers, it SHALL eliminate those receivers from the subsequent "NORM_CMD(ACK_REQ)" message "acking_node_list" and add in any pending receiver NormNodeIds while keeping within the NormSegmentSize limitation of the list size. Each receiver is queried a maximum number of times Adamson, et al. Expires October 26, 2009 [Page 76] Internet-Draft NORM Protocol April 2009 ("NORM_ROBUST_FACTOR", by default). Receivers not responding within this number of repeated requests are removed from the payload list to make room for other potential receivers pending acknowledgment. The transmission of the "NORM_CMD(ACK_REQ)" is repeated until no further responses are required or until the repeat threshold is exceeded for all pending receivers. The transmission of "NORM_CMD(ACK_REQ)" or "NORM_CMD(FLUSH)" messages to conduct the positive acknowledgment process is multiplexed with ongoing sender data transmissions. However, the "NORM_CMD(FLUSH)" positive acknowledgment process may be interrupted in response to negative acknowledgment repair requests (NACKs) received from receivers during the acknowledgment period. The "NORM_CMD(FLUSH)" positive acknowledgment process is restarted for receivers pending acknowledgment once any the repairs have been transmitted. In the case of "NORM_CMD(FLUSH)" commands with an attached "acking_node_list", receivers will not ACK until they have received complete transmission of all data up to and including the given watermark transmission point. All receivers SHALL interpret the watermark point provided in the request NACK for repairs if needed as for "NORM_CMD(FLUSH)" commands with no attached "acking_node_list". 5.5.4. Group Size Estimate NORM sender messages contain a "gsize" field that is a representation of the group size and is used in scaling random backoff timer ranges. The use of the group size estimate within the NORM protocol does not require a precise estimation and works reasonably well if the estimate is within an order of magnitude of the actual group size. By default, the NORM sender group size estimate may be administratively configured. Also, given the expected scalability of the NORM protocol for general use, a default value of 10,000 is RECOMMENDED for use as the group size estimate. It is possible that group size may be algorithmically approximated from the volume of congestion control feedback messages which follow the exponentially weighted random backoff. However, the specification of such an algorithm is currently beyond the scope of this document. 6. Security Considerations The same security considerations that apply to the Multicast NACK [RFC5401], TFMCC [RFC4654], and FEC [RFC5052] Building Blocks also apply to the NORM protocol. In addition to the vulnerabilities that any IP and IP multicast protocol implementation may be generally subject to, the NACK-based feedback of NORM may be exploited by Adamson, et al. Expires October 26, 2009 [Page 77] Internet-Draft NORM Protocol April 2009 replay attacks which force the NORM sender to unnecessarily transmit repair information. This MAY be addressed by network layer IP security implementations that guard against this potential security exploitation or alternatively with a security mechanism that uses the "EXT_AUTH" header extension for similar purposes. Such security mechanisms SHOULD be deployed and used when available. The NORM protocol is compatible with the use of IP security (IPsec) [RFC4301] and the IPsec Encapsulating Security Payload (ESP) protocol or Authentication Header (AF) extension can be used to secure IP packets transmitted by NORM participants. A baseline approach to secure NORM operation using IPsec is described below. Compliant implementations of this specification are REQUIRED to be compatible with IPsec usage as described in Section 6.1. Additionally, the "EXT_AUTH" header extension (HET = 1) is defined for use by security mechanisms to provide an alternative form of authentication and/or encryption of NORM messages. The format of this header extension and its processing is outside the scope of this document and is to be communicated out-of-band as part of the session description. It is possible that an EXT_AUTH implementation of MAY also provide for encryption of NORM message payloads as well as authentication. The use of this approach as compared to IPsec can allow for header compression techniques to be applied jointly to IP and NORM protocol headers. In cases where security analysis deems that encryption of NORM protocol header content is beneficial or necessary, the aforementioned use of IPsec ESP may be more appropriate. If EXT_AUTH is present, whatever packet authentication checks that can be performed immediately upon reception of the packet MUST be performed before accepting the packet and performing any congestion control-related action on it. Some packet authentication schemes impose a delay of several seconds between when a packet is received and when the packet can be fully authenticated. Any congestion control related action that is appropriate MUST NOT be postponed by any such full packet authentication. Consideration MUST also be given to the potential for replay-attacks that would transplant authenticated packets from one NORM session to another to disrupt service. To avoid this potential, unique keys SHOULD be assigned on a per-session basis or NORM sender nodes SHOULD be configured to use unique "instance_id" identifiers that are managed as part of the security association for the sessions. It should be noted that NORM implementations can use the "sequence" field from the NORM Common Message Header to detect replay attacks. This can be accomplished if the NORM sender maintains state on receivers which are NACKing. A cache of such receiver state can be used to provide protection against NACK replay attacks. NORM Adamson, et al. Expires October 26, 2009 [Page 78] Internet-Draft NORM Protocol April 2009 receivers MUST also maintain similar state for protection against possible replay of other receiver messages in ASM operation as well. For example, a receiver could be suppressed from providing NACK or congestion control feedback by replay of certain receiver messages. For these reasons, authentication of NORM messages (e.g., via IPsec) SHOULD be applied for protection against similar attacks that use fabricated messages. Also, encryption of messages to provide confidentiality of application data and protect privacy of users MAY also be applied using IPsec or similar mechanisms. When applicable security measures are used, automated key management mechanisms such as those described in the Group Domain of Interpretation (GDOI) [RFC3547], Multimedia Internet KEYing (MIKEY) [RFC3830] or Group Secure Association Key Management Protocol (GSAKMP) [RFC4535] specifications SHOULD be applied. It is also important to note that while NORM does leverage FEC-based repair for scalability, this alone does not guarantee integrity of received data. Application-level integrity-checking of received data content is highly RECOMMENDED. 6.1. Baseline Secure NORM Operation This section describes a baseline mode of secure NORM protocol operation based on application of the IPsec security protocol. This approach is documented here to provide a reference, interoperable secure mode of operation. However, additional approaches to NORM security, including other forms of IPsec application, MAY be specified in the future. For example, the use of the EXT_AUTH header extension could enable NORM-specific authentication or security encapsulation headers similar to those of IPsec to be specified and inserted into the NORM protocol message headers. This would allow header compression techniques to be applied to IP and NORM protocol headers when needed in a similar fashion to that of RTP [RFC3550] and as preserved in the specification for Secure Real Time Protocol (SRTP) [RFC3711]. The baseline approach described is applicable to NORM operation configured for SSM (or SSM-like) operation where there is a single sender and the receivers are providing unicast feedback. This form of NORM operation allows for IPsec to be used with a manageable number of security associations (SA). 6.1.1. IPsec Approach For NORM one-to-many SSM operation with unicast feedback from receivers, each node SHALL be configured with two transport mode IPsec security associations and corresponding Security Policy Adamson, et al. Expires October 26, 2009 [Page 79] Internet-Draft NORM Protocol April 2009 Database (SPD) entries. One entry will be used for sender-to-group multicast packet authentication and optionally encryption while the other entry will be used to provide security for the unicast feedback messaging from the receiver(s) to the sender. The NORM sender SHALL use an IPsec SA configured for ESP protocol [RFC4303] operation with the option for data origination authentication enabled. It is also RECOMMENDED that this IPsec ESP SA be also configured to provide confidentiality protection for IP packets containing NORM protocol messages. This is suggested to make the realization of complex replay attacks much more difficult. The encryption key for this SA SHALL be preplaced at the sender and receiver(s) prior to NORM protocol operation. Use of automated key management is RECOMMENDED as a rekey SHALL be required prior to expiration of the sequence space for the SA. This is necessary so that receivers may use the built-in IPsec replay attack protection possible for an IPsec SA with a single source (the NORM sender). Thus the receivers SHALL enable replay attack protection for this SA used to secure NORM sender traffic. An IPsec SPD entry MUST be configured to process outbound packets to the session (destination) address and UDP port number of the applicable (NormSession). The NORM receiver(s) MUST be configured with the SA and SPD entry to properly process the IPsec-secured packets from the sender. The NORM receiver(s) SHALL also use a common, second IPsec SA (common Security Parameter Index (SPI) and encryption key) configured for ESP operation with the option for data origination authentication enabled. Similar to the NORM sender, is RECOMMENDED this IPsec ESP SA be also configured to provide confidentiality protection for IP packets containing NORM protocol messages. The receivers MUST have an IPsec SPD entry configured to process outbound NORM/UDP packets directed to the NORM sender source address and port number using this second SA. As noted for NORM unicast feedback, the sender's transmission port number SHOULD be selected to be distinct from the multicast session port number to allow discrimination between unicast and multicast feedback messages when access to the IP destination address is not possible (e.g., a user-space NORM implementation). For processing of packets from receivers, the NORM sender SHALL be configured with this common, second SA (and the corresponding SPD entry needed) in order to properly process messages from the receiver. Multiple receivers using a common IPsec SA for traffic directed to the NORM sender (i.e., many-to-one) typically prevents the use of built-in IPsec replay attack protection by the NORM sender with current IPsec implementations. Thus the built-in IPsec replay attack protection for this second SA at the sender MUST be disabled unless the particular IPsec implementation manages its replay protection on Adamson, et al. Expires October 26, 2009 [Page 80] Internet-Draft NORM Protocol April 2009 a per-source basis. So, to support a fully secure mode of operation, the NORM sender implementation MUST provide replay attack protection based upon the "sequence" field of NORM protocol messages from receivers. This can be accomplished with high assurance of security, even with the limited size (16-bits) of this field, because 1. NORM receiver NACK and non-CLR ACK feedback messages are sparse. 2. The more frequent "NORM_ACK" feedback from CLR or PLR nodes are only a small set of receivers for which the sender must keep more persistent replay attack state. 3. "NORM_NACK" feedback messages that precede the sender's current repair window do not significantly impact protocol operation (generation of "NORM_CMD(SQUELCH)" is limited) and could be in fact ignored. This means the sender can prune any replay attack state for receivers that precede the current repair window. 4. "NORM_ACK" messages correspond to either a specific sender "ack_id", the sender "cc_sequence" for ACKs sent in response to "NORM_CMD(CC)", or the sender's current repair window in the case of ACKs sent in response to "NORM_CMD(FLUSH)". Thus, the sender can prune any replay attack state for receivers that precede the current applicable sequence or repair window space. Note that use of ESP confidentiality for secure NORM protocol operation makes it more difficult for adversaries to conduct any form of replay attacks. Additionally, it should be noted that a NORM sender implementation with access to the full ESP protocol header could also use the ESP sequence information to make replay attack protection even more robust, by maintaining per-source sequence state. The design of this baseline security approach for NORM intentionally places any more complex processing state or processing (e.g. replay attack protection given multiple receivers) at the NORM sender since NORM receiver implementations may need to have a more light-weight realization in many cases. This baseline approach can be used for NORM protocol sessions with multiple senders if the SA pairs described are established for each sender. For small-sized groups, it is even possible that many-to- many (ASM) IPsec configuration could be achieved where each participant uses a unique SA (with a unique SPI). This does not scale to larger group sizes given the complex set of SA and SPD entries each participant would need to maintain. It is anticipated in early deployments of this baseline approach to NORM security that key management will be conducted out-of-band with respect to NORM protocol operation. In the case of one-to-many NORM operation, it is possible that receivers may retrieve keying information from a central server as needed or otherwise conduct group key updates with a similar centralized approach. However, it Adamson, et al. Expires October 26, 2009 [Page 81] Internet-Draft NORM Protocol April 2009 may be possible with some key management schemes for rekey messages to be transmitted to the group as a message or transport object within the NORM reliable transfer session. Similarly, for group-wise communication sessions it is possible that potential group participants may request keying and/or rekeying as part of NORM communications. Additional specification is necessary to define an in-band key management scheme for NORM sessions perhaps using the mechanisms of the automated group key management specifications cited in this document. 6.1.2. IPsec Requirements In order to implement this secure mode of NORM protocol operation, the following IPsec capabilities are required. 6.1.2.1. Selectors The implementation MUST be able to use the source address, destination address, protocol (UDP), and UDP port numbers as selectors in the SPD. 6.1.2.2. Mode IPsec in transport mode MUST be supported. The use of IPsec [RFC4301] processing for secure NORM traffic MUST be configured such that unauthenticated packets are not received by the NORM protocol implementation. 6.1.2.3. Key Management An automated key management scheme for group key distribution and rekeying such as GDOI [RFC3547], GSAKMP [RFC4535], or MIKEY [RFC3830] is RECOMMENDED for use. Relatively short-lived NORM sessions MAY be able to use Manual Keying with a single, preplaced key, particularly if Extended Sequence Numbering (ESN) [RFC4303] is available in the IPsec implementation used. It should also be noted that it may be possible for key update messages (e.g., the GDOI GROUPKEY-PUSH message) to be included as part of the NORM application reliable data transmission if appropriate interfaces are available between the NORM application and the key management daemon. 6.1.2.4. Security Policy Receivers MUST accept protocol messages only from the designated, authorized sender(s). It is expected that appropriate key management will provide encryption keys only to receivers authorized to participate in a designated session. The approach outlined here allows receiver sets to be controlled on a per-sender basis. Adamson, et al. Expires October 26, 2009 [Page 82] Internet-Draft NORM Protocol April 2009 6.1.2.5. Authentication and Encryption Large NORM group sizes will necessitate some form of key management that does rely upon shared secrets. The GDOI and GSAKMP protocols mentioned here allow for certificate-based authentication. It is RECOMMENDED these certificates use IP addresses for authentication although it may alternatively possible to have authentication associated with pre-assigned NormNodeId values. However, it is likely that available group key management implementations will not be NORM-specific. 6.1.2.6. Availability The IPsec requirements profile outlined here is commonly available on many potential NORM hosts. The principal issue is that configuration and operation of IPsec typically requires privileged user authorization. Automated key management implementations are typically configured with the privileges necessary to effect system IPsec configuration needed. 7. IANA Considerations Values of NORM Header Extension Types, Stream Control Codes, and "NORM_CMD" message sub-types are subject to IANA registration. They are in the registry named "Reliable Multicast Transport (RMT) NORM Protocol Parameters" located at time of publication at: http:///www.iana.org/assignments/norm-parameters It should be also noted that reliable multicast building block components used by this specification also have their respective IANA considerations and those documents should be consulted accordingly. In particular, the FEC Building Block used by NORM does require IANA registration of the FEC codecs used. The registration instructions for FEC codecs are provided in RFC 5052. 7.1. Explicit IANA Assignment Guidelines This document introduces three namespaces that are registered for the NORM Header Extension Types, Stream Control Codes and "NORM_CMD" Message Sub-types. This section describes explicit IANA assignment guidelines for each of these. 7.1.1. NORM Header Extension Types This document defines a namespace for NORM Header Extension Types named: Adamson, et al. Expires October 26, 2009 [Page 83] Internet-Draft NORM Protocol April 2009 "ietf:rmt:norm:extension" The NORM Header Extension Type field is an 8-bit value. The values of this field identify extended header content that allows the protocol functionality to be expanded to include additional features and operating modes. The values that can be assigned within the "ietf:rmt:norm:extension" namespace are numeric indexes in the range {0, 255}, boundaries included. Values in the range {0,127} indicate variable length extended header fields while values in the range {128,255} indicate extensions of a fixed 4-byte length. This specification registers the following NORM Header Extension Types: +-------+------------+--------------------+ | Value | Name | Reference | +-------+------------+--------------------+ | 1 | "EXT_AUTH" | This specification | | 3 | "EXT_CC" | This specification | | 64 | "EXT_FTI" | This specification | | 128 | "EXT_RATE" | This specification | +-------+------------+--------------------+ Requests for assignment of additional NORM Header Extension Type values are granted on a "Specification Required" basis as defined by IANA Guidelines [RFC5226]. Any such header extension specifications MUST include a description of protocol actions to be taken when the extension type is encountered by a protocol implementation not supporting that specific option. For example, it may be possible for protocol implementations to ignore unknown header extensions in many cases. 7.1.2. NORM Stream Control Codes This document defines a namespace for NORM Stream Control Codes named: "ietf:rmt:norm:streamControlCode" NORM Stream Control Codes are 16-bit values that may be inserted within a "NORM_OBJECT_STREAM" delivery object to convey sequenced, out-of-band (with respect to the stream data) control signaling applicable to the referenced stream object. These control codes are to be delivered to the application or protocol implementation with reliable delivery, in-order with respect to the their inserted position within the stream. This specification registers the following NORM Stream Control Code: Adamson, et al. Expires October 26, 2009 [Page 84] Internet-Draft NORM Protocol April 2009 +-------+-------------------+--------------------+ | Value | Name | Reference | +-------+-------------------+--------------------+ | 0 | "NORM_STREAM_END" | This specification | +-------+-------------------+--------------------+ Additional NORM Stream Control Code value assignment requests are granted on a "Specification Required" basis as defined by IANA Guidelines [RFC5226]. The full 16-bit space outside of the value assigned in this specification are available for future assignment. Note that in addition to describing the control code's expected interpretation, such specifications MUST include a description of protocol actions to be taken when the control code is encountered by a protocol implementation not supporting that specific option. 7.1.3. NORM_CMD Message Sub-types This document defines a namespace for "NORM_CMD" Message Sub-types named: "ietf:rmt:norm:command" The "NORM_CMD" sub-type field is an 8-bit value with valid values in the range of 1-255. Note the value 0 is reserved to indicate an invalid "NORM_CMD" message sub-type. The current specification defines a number of "NORM_CMD" message sub-types that senders can use to signal the receivers in various aspects of NORM protocol operation. This specification registers the following "NORM_CMD" Message Sub-types: +-------+-------------------------+--------------------+ | Value | Name | Reference | +-------+-------------------------+--------------------+ | 0 | reserved | This specification | | 1 | "NORM_CMD(FLUSH)" | This specification | | 2 | "NORM_CMD(EOT)" | This specification | | 3 | "NORM_CMD(SQUELCH)" | This specification | | 4 | "NORM_CMD(CC)" | This specification | | 5 | "NORM_CMD(REPAIR_ADV)" | This specification | | 6 | "NORM_CMD(ACK_REQ)" | This specification | | 7 | "NORM_CMD(APPLICATION)" | This specification | +-------+-------------------------+--------------------+ Future specifications extending NORM may wish to define additional "NORM_CMD" messages to enhance protocol functionality. "NORM_CMD" message sub-type value assignment requests are granted on a "Specification Required" basis as defined by IANA Guidelines [RFC5226]. Note that in addition to describing the command sub- Adamson, et al. Expires October 26, 2009 [Page 85] Internet-Draft NORM Protocol April 2009 type's expected interpretation, specifications MUST include a description of protocol actions to be taken when the command is encountered by a protocol implementation not supporting that specific option. Note that this specification already provides for an "application- defined" "NORM_CMD" message sub-type that may be used at the discretion of individual applications using NORM for transport. These "application-defined" commands may be suitable for many application-specific purposes and do not require standards action. In any case, such additional messages SHALL be subject to the same congestion control constraints as the existing NORM sender message set. 8. Suggested Use The present NORM protocol is seen as useful tool for the reliable data transfer over generic IP multicast services. It is not the intention of the authors to suggest it is suitable for supporting all envisioned multicast reliability requirements. NORM provides a simple and flexible framework for multicast applications with a degree of concern for network traffic implosion and protocol overhead efficiency. NORM-like protocols have been successfully demonstrated within the MBone for bulk data dissemination applications, including weather satellite compressed imagery updates servicing a large group of receivers and a generic web content reliable "push" application. In addition, this framework approach has some design features making it attractive for bulk transfer in asymmetric and wireless internetwork applications. NORM is capable of successfully operating independent of network structure and in environments with high packet loss, delay, and out-of-order delivery. Hybrid proactive/reactive FEC-based repairing improve protocol performance in some multicast scenarios. A sender-only repair approach often makes additional engineering sense in asymmetric networks. NORM's unicast feedback capability may be suitable for use in asymmetric networks or in networks where only unidirectional multicast routing/delivery service exists. Asymmetric architectures supporting multicast delivery are likely to make up an important portion of the future Internet structure (e.g., DBS/cable/PSTN hybrids) and efficient, reliable bulk data transfer will be an important capability for servicing large groups of subscribed receivers. 9. Changes from RFC3940 This section lists the changes between the Experimental version of Adamson, et al. Expires October 26, 2009 [Page 86] Internet-Draft NORM Protocol April 2009 this specification, RFC 3940, and this version: 1. Removal of the "NORM_FLAG_MSG_START" for "NORM_OBJECT_STREAM", replacing it with the "payload_msg_start" field in the FEC- encoded preamble of the "NORM_OBJECT_STREAM NORM_DATA" payload, 2. Definition of IANA namespace for header extension assignment, 3. Removal of file blocking scheme description that is now specified in the FEC Building Block document [RFC5052], 4. Removal of restriction of NORM receiver feedback message rate to local NORM sender rate (This caused congestion control failures in high speed operation. The extremely low feedback rate of the NORM protocol as compared to TCP avoids any resultant impact to the network as shown in [Mdpcc]), 5. Correction of errors in some message format descriptions, and 6. Correction of inconsistency in specification of the inactivity timeout. 7. Addition of IPsec secure mode description with IPsec requirements. 8. Addition of the EXT_AUTH header extension definition. 9. Clarification of interpretation of "Source Block Length" when FEC codes are arbitrarily shortened by the sender. 10. Acknowledgments (and these are not Negative) The authors would like to thank Rick Jones, Vincent Roca, Rod Walsh, Toni Paila, Michael Luby, and Joerg Widmer for their valuable input and comments on this document. The authors would also like to thank the RMT working group chairs, Roger Kermode and Lorenzo Vicisano, for their support in development of this specification, and Sally Floyd for her early input into this document. 11. References 11.1. Normative References [RFC1112] Deering, S., "Host extensions for IP multicasting", STD 5, RFC 1112, August 1989. [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate Requirement Levels", BCP 14, RFC 2119, March 1997. [RFC4301] Kent, S. and K. Seo, "Security Architecture for the Internet Protocol", RFC 4301, December 2005. Adamson, et al. Expires October 26, 2009 [Page 87] Internet-Draft NORM Protocol April 2009 [RFC4303] Kent, S., "IP Encapsulating Security Payload (ESP)", RFC 4303, December 2005. [RFC4607] Holbrook, H. and B. Cain, "Source-Specific Multicast for IP", RFC 4607, August 2006. [RFC4654] Widmer, J. and M. Handley, "TCP-Friendly Multicast Congestion Control (TFMCC): Protocol Specification", RFC 4654, August 2006. [RFC5052] Watson, M., Luby, M., and L. Vicisano, "Forward Error Correction (FEC) Building Block", RFC 5052, August 2007. [RFC5226] Narten, T. and H. Alvestrand, "Guidelines for Writing an IANA Considerations Section in RFCs", BCP 26, RFC 5226, May 2008. [RFC5401] Adamson, B., Bormann, C., Handley, M., and J. Macker, "Multicast Negative-Acknowledgment (NACK) Building Blocks", RFC 5401, November 2008. 11.2. Informative References [FecHybrid] Gossink, D. and J. Macker, "Reliable Multicast and Integrated Parity Retransmission with Channel Estimation", IEEE Globecomm , 1998. [McastFeedback] Nonnenmacher, J. and E. Biersack, "Optimal Multicast Feedback", IEEE INFOCOM, p. 964, March/April 1998. [MdpToolkit] Macker, J. and B. Adamson, "The Multicast Dissemination Protocol (MDP) Toolkit", Proc. IEEE MILCOM , October 1999. [Mdpcc] Adamson, B. and J. Macker, "A TCP-Friendly, Rate-based Mechanism for NACK-Oriented Reliable Multicast Congestion Control", Proc. IEEE GLOBECOMM , November 2001. [NormFeedback] Adamson, B. and J. Macker, "Quantitative Prediction of NACK-Oriented Reliable Multicast (NORM) Feedback", IEEE MILCOM , October 2002. [PgmccPaper] Rizzo, L., "pgmcc: A TCP-Friendly Single-Rate Multicast Congestion Control Scheme", ACM SIGCOMM , August 2000. Adamson, et al. Expires October 26, 2009 [Page 88] Internet-Draft NORM Protocol April 2009 [RFC2357] Mankin, A., Romanov, A., Bradner, S., and V. Paxson, "IETF Criteria for Evaluating Reliable Multicast Transport and Application Protocols", RFC 2357, June 1998. [RFC2974] Handley, M., Perkins, C., and E. Whelan, "Session Announcement Protocol", RFC 2974, October 2000. [RFC3048] Whetten, B., Vicisano, L., Kermode, R., Handley, M., Floyd, S., and M. Luby, "Reliable Multicast Transport Building Blocks for One-to-Many Bulk-Data Transfer", RFC 3048, January 2001. [RFC3269] Kermode, R. and L. Vicisano, "Author Guidelines for Reliable Multicast Transport (RMT) Building Blocks and Protocol Instantiation documents", RFC 3269, April 2002. [RFC3453] Luby, M., Vicisano, L., Gemmell, J., Rizzo, L., Handley, M., and J. Crowcroft, "The Use of Forward Error Correction (FEC) in Reliable Multicast", RFC 3453, December 2002. [RFC3547] Baugher, M., Weis, B., Hardjono, T., and H. Harney, "The Group Domain of Interpretation", RFC 3547, July 2003. [RFC3550] Schulzrinne, H., Casner, S., Frederick, R., and V. Jacobson, "RTP: A Transport Protocol for Real-Time Applications", STD 64, RFC 3550, July 2003. [RFC3711] Baugher, M., McGrew, D., Naslund, M., Carrara, E., and K. Norrman, "The Secure Real-time Transport Protocol (SRTP)", RFC 3711, March 2004. [RFC3830] Arkko, J., Carrara, E., Lindholm, F., Naslund, M., and K. Norrman, "MIKEY: Multimedia Internet KEYing", RFC 3830, August 2004. [RFC3940] Adamson, B., Bormann, C., Handley, M., and J. Macker, "Negative-acknowledgment (NACK)-Oriented Reliable Multicast (NORM) Protocol", RFC 3940, November 2004. [RFC4535] Harney, H., Meth, U., Colegrove, A., and G. Gross, "GSAKMP: Group Secure Association Key Management Protocol", RFC 4535, June 2006. [RFC4566] Handley, M., Jacobson, V., and C. Perkins, "SDP: Session Description Protocol", RFC 4566, July 2006. [RFC5445] Watson, M., "Basic Forward Error Correction (FEC) Schemes", RFC 5445, March 2009. Adamson, et al. Expires October 26, 2009 [Page 89] Internet-Draft NORM Protocol April 2009 [RmComparison] Pingali, S., Towsley, D., and J. Kurose, "A Comparison of Sender-Initiated and Receiver-Initiated Reliable Multicast Protocols", Proc. INFOCOMM, San Francisco CA, October 1993. [TcpModel] Padhye, J., Firoiu, V., Towsley, D., and J. Kurose, "Modeling TCP Throughput: A Simple Model and its Empirical Validation", ACM SIGCOMM , 1998. [TfmccPaper] Widmer, J. and M. Handley, "Extending Equation-Based Congestion Control to Multicast Applications", ACM SIGCOMM , August 2001. Authors' Addresses Brian Adamson Naval Research Laboratory Washington, DC 20375 USA Email: adamson@itd.nrl.navy.mil Carsten Bormann Universitaet Bremen TZI Postfach 330440 D-28334 Bremen Germany Email: cabo@tzi.org Mark Handley University College London Gower Street London WC1E 6BT UK Email: M.Handley@cs.ucl.ac.uk Adamson, et al. Expires October 26, 2009 [Page 90] Internet-Draft NORM Protocol April 2009 Joe Macker Naval Research Laboratory Washington, DC 20375 USA Email: macker@itd.nrl.navy.mil Adamson, et al. Expires October 26, 2009 [Page 91]