Network Working Group P. Natarajan Internet-Draft Cisco Systems Intended status: Standards Track P. Amer Expires: August 22, 2009 E. Yilmaz University of Delaware R. Stewart The Resource Group J. Iyengar Franklin & Marshall College February 18, 2009 Non-Renegable Selective Acknowledgements (NR-SACKs) for SCTP draft-natarajan-tsvwg-sctp-nrsack-03.txt Status of this Memo This Internet-Draft is submitted to IETF in full conformance with the provisions of BCP 78 and BCP 79. 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 August 22, 2009. Copyright Notice 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 (http://trustee.ietf.org/license-info) in effect on the date of publication of this document. Please review these documents carefully, as they describe your rights and restrictions with respect Natarajan, et al. Expires August 22, 2009 [Page 1] Internet-Draft NR-SACKs for SCTP February 2009 to this document. Abstract Stream Control Transmission Protocol (SCTP) [RFC4960] specifies Selective Acknowledgements (SACKs) to allow a transport layer data receiver to acknowledge DATA chunks which arrive out-of-order. In SCTP, SACK information is advisory because the data receiver is permitted to renege; that is, later discard a DATA chunk which previously has been SACKed. Since delivery of a SACKed out-of-order DATA chunk is not guaranteed, a copy of this DATA chunk MUST be kept in the data sender's retransmission queue until this DATA chunk is cumulatively acked. This document specifies Non-Renegable Selective Acknowledgements (NR- SACKs), an extension to SCTP's acknowledgment mechanism. NR-SACKs enable a data receiver to explicitly acknowledge out-of-order DATA chunks that have been delivered to the receiving application. (Recall that, in SCTP, out-of-order data sometimes can be delivered.) NR-SACKs also enable a data receiver to indicate any out-of-order DATA chunks on which the receiver guarantees never to renege. As opposed to SACKed DATA chunks, a sender can consider NR-SACKed DATA chunks as never requiring retransmission, thus freeing space in the data sender's retransmission queue sooner. Natarajan, et al. Expires August 22, 2009 [Page 2] Internet-Draft NR-SACKs for SCTP February 2009 Table of Contents 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 4 2. Conventions . . . . . . . . . . . . . . . . . . . . . . . . . 5 3. Negotiation . . . . . . . . . . . . . . . . . . . . . . . . . 6 4. The New Chunk Type: Non-Renegable SACK (NR-SACK) . . . . . . . 6 5. An Illustrative Example . . . . . . . . . . . . . . . . . . . 11 6. Procedures . . . . . . . . . . . . . . . . . . . . . . . . . . 15 6.1. Sending an NR-SACK chunk . . . . . . . . . . . . . . . . . 15 6.2. Receiving an NR-SACK Chunk . . . . . . . . . . . . . . . . 17 7. Security Considerations . . . . . . . . . . . . . . . . . . . 18 8. IANA considerations . . . . . . . . . . . . . . . . . . . . . 18 9. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 18 10. References . . . . . . . . . . . . . . . . . . . . . . . . . . 19 10.1. Normative References . . . . . . . . . . . . . . . . . . . 19 10.2. Informative References . . . . . . . . . . . . . . . . . . 19 Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 19 Natarajan, et al. Expires August 22, 2009 [Page 3] Internet-Draft NR-SACKs for SCTP February 2009 1. Introduction In providing end-to-end reliable transport layer data transfer, SCTP specifies Cumulative Acknowledgements (ACKs), Selective ACKs (SACKs), and Duplicate Selective ACKs (D-SACKs). These three types of acks are carried in the following fields of the SACK chunk, respectively: Cumulative TSN Ack, Gap Ack Block, and Duplicate TSN. In this document, we refer to the Cumulative TSN Ack as the "cum-ack", the selective Gap Ack Blocks as "gap-acks", and the Duplicate TSN selective acks as "dup-TSN reports". Gap-acks acknowledge DATA chunks that arrive out-of-order to a transport layer data receiver. A gap-ack in SCTP is advisory, in that, while it notifies a data sender about the reception of indicated DATA chunks, the data receiver is permitted to later discard DATA chunks that it previously had gap-acked. Discarding a previously gap-acked DATA chunk is known as "reneging." Because of the possibility of reneging in SCTP, any gap-acked DATA chunk MUST NOT be removed from the data sender's retransmission queue until the DATA chunk is later cum-acked. Situations exist when a data receiver knows that reneging on a particular out-of-order DATA chunk will never take place, such as (but not limited to) after an out-of-order DATA chunk is delivered to the receiving application. This document describes an extension to SCTP to allow for Non-Renegable Selective Acknowledgments (NR-SACKs.) A new NR-SACK chunk type is described that allows this extension to be implemented. The NR-SACK chunk is an extension of the existing SACK chunk. Several fields are identical, including the Cumulative TSN Ack, the Advertised Receiver Window Credit (a_rwnd), Gap Ack Blocks, and Duplicate TSNs. These fields have the same semantics as described in [RFC4960]. NR-SACKs extend SACKs by also identifying out-of-order DATA chunks that a receiver either: (1) has delivered to its receiving application, or (2) takes full responsibility to eventually deliver to its receiving application. These out-of-order DATA chunks are "non-renegable." Non-Renegable data are reported in the NR Gap Ack Block field of the NR-SACK chunk as described in Section 4.1. We refer to non-renegable selective acknowledgements as "nr-gap-acks." When an out-of-order DATA chunk is nr-gap-acked, the data sender no longer needs to keep that particular DATA chunk in its retransmission queue, thus allowing the data sender to free up its buffer space sooner than if the DATA chunk were only gap-acked. NR-SACKs are expected to improve throughput, particularly in multi-streamed SCTP Natarajan, et al. Expires August 22, 2009 [Page 4] Internet-Draft NR-SACKs for SCTP February 2009 associations where out-of-order data often can be delivered, at the trade-off of requiring additional processing of acknowledgement information at both the data receiver and data sender [Natarajan]. An SCTP message is encapsulated within a single DATA chunk or within multiple DATA chunks in case of fragmentation. In this document without loss of generality, each application message maps to a single transport layer DATA chunk, and delivering a DATA chunk to a receiving application means delivering the message carried within the DATA chunk to a receiving application. SCTP divides an end-to-end association into independent logical data streams (a.k.a. multistreaming.) A DATA chunk that arrives in- sequence within a stream can be delivered to the receiving application even if the DATA chunk is out-of-order relative to the association's overall flow of data. These out-of-order DATA chunks are "deliverable." By definition, a DATA chunk marked for unordered delivery also is "deliverable" to the receiving application immediately upon reception, regardless of its position within the overall flow of data. With current SACKs in SCTP, it is not possible for a data receiver to inform a data sender if or when a particular out-of-order "deliverable" DATA chunk has been "delivered" to the receiving application. Thus the data sender MUST keep a copy of every gap- acked out-of-order DATA chunk(s) in the data sender's retransmission queue until the DATA chunk is cum-acked. This use of the data sender's retransmission queue is wasteful. Given the receiving application has received the data, the data sender has no reason to keep this data in its retransmission queue. Yet, the sending transport layer keeps the data because no mechanism currently exists to indicate which out-of-order DATA chunks have been delivered. (Note: once a DATA chunk is delivered to the receiving application, it is impossible for the data receiver to renege that DATA chunk.) If NR-SACKs are used, the data receiver MAY include the TSN of a delivered out-of-order DATA chunk in an NR-SACK to inform the data sender that the delivery has occurred, allowing the data sender to remove the copy of the delivered DATA chunk from the data sender's retransmission queue even before the DATA chunk is cum-acked. 2. Conventions 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 [RFC2119]. Natarajan, et al. Expires August 22, 2009 [Page 5] Internet-Draft NR-SACKs for SCTP February 2009 3. Negotiation Before sending/receiving NR-SACKs, both peer endpoints MUST agree on using NR-SACKs. This agreement MUST be negotiated during association establishment. NR-SACK is an extension to the core SCTP, and SCTP extensions that an endpoint supports are reported to the peer endpoint in Supported Extensions Parameter during association establishment (see Section 4.2.7 of [RFC5061].) The Supported Extensions Parameter consists of a list of non-standard Chunk Types that are supported by the sender. An endpoint supporting the NR-SACK extension MUST list the NR-SACK chunk in the Supported Extensions Parameter carried in the INIT or INIT-ACK chunk, depending on whether the endpoint initiates or responds to the initiation of the association. If the NR-SACK chunk type ID is listed in the Chunk Types List of the Supported Extensions Parameter, then the receiving endpoint MUST assume that the NR-SACK chunk is supported by the sending endpoint. Both endpoints MUST support NR-SACKs for either endpoint to send an NR-SACK. If an endpoint which tries to establish an association with a remote endpoint does not list NR-SACK in the Supported Extensions Parameter carried in INIT chunk, then both endpoints of the association MUST NOT use NR-SACKs. After association establishment, an endpoint MUST NOT renegotiate the use of NR-SACKs. Once both endpoints indicate during association establishment that they support the NR-SACK extension, each endpoint SHOULD acknowledge received DATA chunks with NR-SACK chunks, and not SACK chunks. That is, throughout an SCTP association, both endpoints SHOULD send either SACK chunks or NR-SACK chunks, but not a mixture of the two. 4. The New Chunk Type: Non-Renegable SACK (NR-SACK) Table 1 illustrates a new chunk type that will be used to transfer NR-SACK information. Chunk Type Chunk Name -------------------------------------------------------------- 0x10 Non-Renegable Selective Acknowledgment (NR-SACK) Table 1: NR-SACK Chunk As the NR-SACK chunk replaces the SACK chunk, many of the SACK chunk fields are preserved in the NR-SACK chunk. These fields preserved in the NR-SACK chunk have the same semantics with the corresponding SACK chunk fields, as defined in [RFC4960], Section 3.3.4. For Natarajan, et al. Expires August 22, 2009 [Page 6] Internet-Draft NR-SACKs for SCTP February 2009 completeness, we describe all fields of the NR-SACK chunk, including those that are identical in the SACK chunk. All arithmetic operations discussed in this document are based on "Serial Number Arithmetic" as described in Section 1.6 of [RFC4960]. Similar to the SACK chunk, the NR-SACK chunk is sent to a peer endpoint to (1) acknowledge DATA chunks received in-order, (2) acknowledge DATA chunks received out-of-order, and (3) indicate DATA chunks received more than once (i.e., duplicate.) In addition, the NR-SACK chunk also informs the peer endpoint of non-renegable out-of- order DATA chunks. Natarajan, et al. Expires August 22, 2009 [Page 7] Internet-Draft NR-SACKs for SCTP February 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 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Type = 0x10 | Chunk Flags | Chunk Length | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Cumulative TSN Ack | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Advertised Receiver Window Credit (a_rwnd) | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Number of Gap Ack Blocks = N |Number of NR Gap Ack Blocks = M| +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Number of Duplicate TSNs = X | Reserved | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Gap Ack Block #1 Start | Gap Ack Block #1 End | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ / / \ ... \ / / +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Gap Ack Block #N Start | Gap Ack Block #N End | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | NR Gap Ack Block #1 Start | NR Gap Ack Block #1 End | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ / / \ ... \ / / +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | NR Gap Ack Block #M Start | NR Gap Ack Block #M End | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Duplicate TSN 1 | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ / / \ ... \ / / +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Duplicate TSN X | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ Type: 8 bits This field holds the IANA defined chunk type for NR-SACK chunk. The suggested value of this field for IANA is 0x10. Chunk Flags: 8 bits Format of this field is RRRRRRAR where R stands for Reserved and A stands for A bit. R bits are currently not used; it is recommended a Natarajan, et al. Expires August 22, 2009 [Page 8] Internet-Draft NR-SACKs for SCTP February 2009 sender set all R bits to zero on transmit, and a receiver ignore the R bits. A bit: 1 bit The (A)ll bit, if set to '1', indicates that all of the out-of-order data blocks acknowledged in this NR-SACK chunk are non-renegable. If (A)ll bit is set to '1', then Number of Gap Ack Blocks (N) MUST be set to '0' (for example, see CASE-3 in Section 5). If an endpoint never reneges, it SHOULD set the A bit in all of its NR-SACKs. Chunk Length: 16 bits (unsigned integer) [Same as SACK chunk] This value represents the size of the chunk in bytes including the Chunk Type, Chunk Flags, Chunk Length, and Chunk Value fields. Cumulative TSN Ack: 32 bits (unsigned integer) [Same as SACK chunk] The value of the Cumulative TSN Ack is the last TSN received before a break in the sequence of received TSNs occurs. The next TSN value following the Cumulative TSN Ack has not yet been received at the endpoint sending the NR-SACK. Advertised Receiver Window Credit (a_rwnd): 32 bits (unsigned integer) [Same as SACK chunk] Indicates the updated receive buffer space in bytes of the sender of this NR-SACK, see Section 6.2.1 of [RFC4960] for details. Number of Gap Ack Blocks (N): 16 bits (unsigned integer) [Same as SACK chunk] Indicates the number of Gap Ack Blocks included in this NR-SACK. If the (A)ll bit is set to one, meaning that all out-of-order data blocks are non-renegable, then the Number of NR Gap Ack Blocks MUST be set to '0'. Number of NR Gap Ack Blocks (M): 16 bits (unsigned integer) Indicates the number of Non-Renegable Gap Ack Blocks included in this NR-SACK. Number of Duplicate TSNs (X): 16 bits [Same as SACK chunk] Contains the number of duplicate TSNs the endpoint has received. Each duplicate TSN is listed following the NR Gap Ack Block list. Reserved : 16 bits Natarajan, et al. Expires August 22, 2009 [Page 9] Internet-Draft NR-SACKs for SCTP February 2009 Currently not used. It is recommended a sender set all bits to zero on transmit, and a receiver ignore this field. Gap Ack Blocks: [Same as SACK chunk] The NR-SACK contains zero or more Gap Ack Blocks. The Gap Ack Blocks in an NR-SACK have the same semantics as in the existing SACK chunk. Each Gap Ack Block acknowledges a subsequence of TSNs received following a break in the sequence of received TSNs. By definition, all TSNs acknowledged by Gap Ack Blocks are greater than the value of the Cumulative TSN Ack. Gap Ack Blocks are repeated for each Gap Ack Block up to 'N' defined in the Number of Gap Ack Blocks field. All DATA chunks with TSNs greater than or equal to (Cumulative TSN Ack + Gap Ack Block Start) and less than or equal to (Cumulative TSN Ack + Gap Ack Block End) of each Gap Ack Block are assumed to have been received correctly. Gap Ack Block Start: 16 bits (unsigned integer) [Same as SACK chunk] Indicates the Start offset TSN for this Gap Ack Block. This number is set relative to the cumulative TSN number defined in Cumulative TSN Ack field. To calculate the actual start TSN number, the Cumulative TSN Ack is added to this offset number. The calculated TSN identifies the first TSN in this Gap Ack Block that has been received. Gap Ack Block End: 16 bits (unsigned integer) [Same as SACK chunk] Indicates the End offset TSN for this Gap Ack Block. This number is set relative to the cumulative TSN number defined in the Cumulative TSN Ack field. To calculate the actual TSN number, the Cumulative TSN Ack is added to this offset number. The calculated TSN identifies the TSN of the last DATA chunk received in this Gap Ack Block. N(on)R(enegable) Gap Ack Blocks: The NR-SACK contains zero or more NR Gap Ack Blocks. Each NR Gap Ack Block acknowledges a continuous subsequence of non-renegable out-of- order DATA chunks. Each sequence of TSNs in an NR Gap Ack Block MUST be a subsequence of one of the Gap Ack Blocks except for the special case when 'A' bit is set to 1 (for example, see CASE-3 in Section 5.) Note that there can be more than one NR Gap Ack Block per Gap Ack Block. If a TSN is nr-gap-acked in any NR-SACK chunk, then all subsequent NR-SACKs gap-acking that TSN SHOULD also nr-gap-ack that TSN. Natarajan, et al. Expires August 22, 2009 [Page 10] Internet-Draft NR-SACKs for SCTP February 2009 NR Gap Ack Blocks are repeated for each NR Gap Ack Block up to 'M' defined in the Number of NR Gap Ack Blocks field. All DATA chunks with TSNs greater than or equal to (Cumulative TSN Ack + NR Gap Ack Block Start) and less than or equal to (Cumulative TSN Ack + NR Gap Ack Block End) of each NR Gap Ack Block are assumed to be Non- Renegable. NR Gap Ack Block Start: 16 bits (unsigned integer) Indicates the Start offset TSN for this NR Gap Ack Block. This number is set relative to the cumulative TSN number defined in Cumulative TSN Ack field. To calculate the actual TSN number, the Cumulative TSN Ack is added to this offset number. The calculated TSN identifies the first TSN in this NR Gap Ack Block that has been received. NR Gap Ack Block End: 16 bits (unsigned integer) Indicates the End offset TSN for this NR Gap Ack Block. This number is set relative to the cumulative TSN number defined in Cumulative TSN Ack field. To calculate the actual TSN number, the Cumulative TSN Ack is added to this offset number. The calculated TSN identifies the TSN of the last DATA chunk received in this NR Gap Ack Block. Duplicate TSN: 32 bits (unsigned integer) [Same as SACK chunk] Contain the TSNs received in duplicate since the last NR-SACK was sent. They are repeated for each duplicate TSN up to 'X' defined in Number of Duplicate TSNs field. Each duplicate TSN is listed in this field as many times as the TSN was received since the previous NR-SACK was sent. For example, if a receiver were to get the TSN 19 three times, the receiver would list 19 twice in the outbound NR-SACK. After sending the NR-SACK if the receiver received yet one more TSN 19, the receiver would list 19 as a duplicate once in the next outgoing NR- SACK. 5. An Illustrative Example Assume the following DATA chunks have arrived at the receiver. Natarajan, et al. Expires August 22, 2009 [Page 11] Internet-Draft NR-SACKs for SCTP February 2009 -------------------------------- | TSN=16| SID=2 | SSN=N/A| U=1 | -------------------------------- | TSN=15| SID=1 | SSN= 4 | U=0 | -------------------------------- | TSN=14| SID=0 | SSN= 4 | U=0 | -------------------------------- | TSN=13| SID=2 | SSN=N/A| U=1 | -------------------------------- | | -------------------------------- | TSN=11| SID=0 | SSN= 3 | U=0 | ------------------------------- | | -------------------------------- | | -------------------------------- | TSN=8 | SID=2 | SSN=N/A| U=1 | -------------------------------- | TSN=7 | SID=1 | SSN= 2 | U=0 | -------------------------------- | TSN=6 | SID=1 | SSN= 1 | U=0 | -------------------------------- | TSN=5 | SID=0 | SSN= 1 | U=0 | -------------------------------- | | -------------------------------- | TSN=3 | SID=1 | SSN= 0 | U=0 | -------------------------------- | TSN=2 | SID=0 | SSN= 0 | U=0 | -------------------------------- The above figure shows the list of DATA chunks at the receiver. TSN denotes the transmission sequence number of the DATA chunk, SID denotes the SCTP stream id to which the DATA chunk belongs, SSN denotes the sequence number of the DATA chunk within its stream, and U bit denotes whether the DATA chunk requires ordered or unordered delivery [RFC4960]. This data can be viewed as three separate streams as follows (assume each stream begins with SSN=0.) Note that in this example, the application uses stream 2 for unordered data transfer. By definition, SSN fields of unordered DATA chunks are ignored. Stream-0: Natarajan, et al. Expires August 22, 2009 [Page 12] Internet-Draft NR-SACKs for SCTP February 2009 SSN: 0 1 2 3 4 TSN: | 2 | 5 | | 11 | 14 | U-Bit: | 0 | 0 | | 0 | 0 | Stream-1: SSN: 0 1 2 3 4 TSN: | 3 | 6 | 7 | | 15 | U-Bit: | 0 | 0 | 0 | | 0 | Stream-2: SSN: N/A N/A N/A TSN: | 8 | 13 | 16 | U-Bit: | 1 | 1 | 1 | The NR-SACK to acknowledge the above data MUST be constructed as follows for each of the three cases described below (the a_rwnd is arbitrarily set to 4000): CASE-1: Minimal Data Receiver Responsibility - no out-of-order deliverable data yet delivered None of the deliverable out-of-order DATA chunks have been delivered and the receiver of the above data does not take responsibility for any of the received out-of-order DATA chunks. The receiver reserves the right to renege on any or all of the out-of-order DATA chunks. +-----------------------------+-----------------------------+ | Type = 0x10 | 000000|A=0|0 | Chunk Length = 32 | +-----------------------------+-----------------------------+ | Cumulative TSN Ack = 3 | +-----------------------------+-----------------------------+ | a_rwnd = 4000 | +-----------------------------+-----------------------------+ | Num of Gap Ack Blocks = 3 |Num of NR Gap Ack Blocks = 0 | +-----------------------------+-----------------------------+ | Num of Duplicates = 0 | 0x00 | +-----------------------------+-----------------------------+ | Gap Ack Block #1 Start = 2 | Gap Ack Block #1 End = 5 | +-----------------------------+-----------------------------+ | Gap Ack Block #2 Start = 8 | Gap Ack Block #2 End = 8 | +-----------------------------+-----------------------------+ | Gap Ack Block #3 Start = 10 | Gap Ack Block #3 End = 13 | +-----------------------------+-----------------------------+ CASE-2: Minimal Data Receiver Responsibility - all out-of-order Natarajan, et al. Expires August 22, 2009 [Page 13] Internet-Draft NR-SACKs for SCTP February 2009 deliverable data delivered In this case, the NR-SACK chunk is being sent after the data receiver has delivered all deliverable out-of-order DATA chunks to its receiving application. The receiver reserves the right to renege on all undelivered out-of-order DATA chunks. +------------------------------+------------------------------+ | Type = 0x10 | 000000|A=0|0 | Chunk Length = 44 | +------------------------------+------------------------------+ | Cumulative TSN Ack = 3 | +------------------------------+------------------------------+ | a_rwnd = 4000 | +------------------------------+------------------------------+ | Num of Gap Ack Block = 3 | Num of NR Gap Ack Block = 3 | +------------------------------+------------------------------+ | Num of Duplicates = 0 | 0x00 | +------------------------------+------------------------------+ | Gap Ack Block #1 Start = 2 | Gap Ack Block #1 End = 5 | +------------------------------+------------------------------+ | Gap Ack Block #2 Start = 8 | Gap Ack Block #2 End = 8 | +------------------------------+------------------------------+ | Gap Ack Block #3 Start = 10 | Gap Ack Block #3 End = 13 | +------------------------------+------------------------------+ |NR Gap Ack Block #1 Start = 2 | NR Gap Ack Block #1 End = 5 | +------------------------------+------------------------------+ |NR Gap Ack Block #2 Start = 10| NR Gap Ack Block #2 End = 10 | +------------------------------+------------------------------+ |NR Gap Ack Block #3 Start = 13| NR Gap Ack Block #3 End = 13 | +------------------------------+------------------------------+ CASE-3: Maximal Data Receiver Responsibility In this special case, all out-of-order data blocks acknowledged are non-renegable. This case would occur when the data receiver is programmed never to renege, and takes responsibility to deliver all DATA chunks that arrive out-of-order. In this case the (A)ll bit is set to '1' indicating all out-of-order data blocks are nr-gap-acked. Only when A = 1 is a sequence of TSNs in an NR Gap Ack Block not a subsequence of one of the Gap Ack Blocks. Natarajan, et al. Expires August 22, 2009 [Page 14] Internet-Draft NR-SACKs for SCTP February 2009 +--------------------------------+-------------------------------+ | Type = 0x10 | 000000|A=1|0 | Chunk Length = 32 | +--------------------------------+-------------------------------+ | Cumulative TSN Ack = 3 | +--------------------------------+-------------------------------+ | a_rwnd = 4000 | +--------------------------------+-------------------------------+ | Num of Gap Ack Blocks = 0 | Num of NR Gap Ack Blocks = 3 | +--------------------------------+-------------------------------+ | Num of Duplicates = 0 | 0x00 | +--------------------------------+-------------------------------+ | NR Gap Ack Block #1 Start = 2 | NR Gap Ack Block #1 End = 5 | +--------------------------------+-------------------------------+ | NR Gap Ack Block #2 Start = 8 | NR Gap Ack Block #2 End = 8 | +--------------------------------+-------------------------------+ | NR Gap Ack Block #3 Start = 10 | NR Gap Ack Block #3 End = 13 | +--------------------------------+-------------------------------+ 6. Procedures The procedures regarding when to send an NR-SACK chunk are identical to the procedures regarding when to send a SACK chunk, as outlined in Section 6.2 of [RFC4960]. 6.1. Sending an NR-SACK chunk All of the NR-SACK chunk fields identical to the SACK chunk MUST be formed as described in Section 6.2 of [RFC4960]. It is up to the data receiver whether or not to take responsibility for delivery of each out-of-order DATA chunk. An out-of-order DATA chunk that has already been delivered, or that the receiver takes responsibility to deliver (i.e., guarantees not to renege) is Non Renegable(NR), and SHOULD be included in an NR Gap Ack Block field of the outgoing NR-SACK. Consider three cases: CASE-1: Data receiver takes no responsibility for delivery of any out-of-order DATA chunks CASE-2: Data receiver takes responsibility for all out-of-order DATA chunks that are "deliverable" (i.e., DATA chunks in-sequence within the stream they belong to, or DATA chunks whose (U)nordered bit is set to '1') Natarajan, et al. Expires August 22, 2009 [Page 15] Internet-Draft NR-SACKs for SCTP February 2009 CASE-3: Data receiver takes responsibility for delivery of all out- of-order DATA chunks, whether deliverable or not deliverable The data receiver SHOULD follow the procedures outlined below for building the NR-SACK. CASE-1: 1A) Identify the TSNs received out-of-order. 1B) For these out-of-order TSNs, identify the Gap Ack Blocks. Fill the Number of Gap Ack Blocks (N) field, Gap Ack Block #i Start, and Gap Ack Block #i End where i goes from 1 to N. 1C) Set the (A)ll bit to '0'. 1D) Set the Number of NR Gap Acks (M) field to '0'. CASE-2: 2A) Identify the TSNs received out-of-order. 2B) For these out-of-order TSNs, identify the Gap Ack Blocks. Fill the Number of Gap Ack Blocks (N) field, Gap Ack Block #i Start, and Gap Ack Block #i End where i goes from 1 to N. 2C) For the received out-of-order TSNs, check the (U)nordered bit of each TSN. Tag unordered TSNs as NR. 2D) For each stream, identify the TSNs which are out-of-order relative to the association's entire flow of data, but in-sequence within that stream. Tag those in-sequence TSNs as NR. 2E) For those TSNs tagged as NR, identify the NR Blocks. Fill the Number of NR Gap Ack Blocks(M) field, NR Gap Ack Block #i Start, and NR Gap Ack Block #i End where i goes from 1 to M. CASE-3: 3A) Identify the TSNs received out-of-order. All of these TSNs SHOULD be nr-gap-acked. Natarajan, et al. Expires August 22, 2009 [Page 16] Internet-Draft NR-SACKs for SCTP February 2009 3B) For these out-of-order TSNs, identify the NR Gap Ack Blocks. According to that information, fill the Number of NR Gap Ack Blocks (M) field, NR Gap Ack Block #i Start, and NR Gap Ack Block #i End where i goes from 1 to M. 3C) Set the (A)ll bit to '1'. 3D) Set the Number of Gap Acks (N) field to '0'. RFC4960 states the SCTP endpoint MUST report as many Gap Ack Blocks as can fit in a single SACK chunk limited by the current path MTU. An NR-SACK chunk having many Gap Ack Blocks and NR Gap Ack Blocks may also exceed the chunk size limited by the current path MTU. The following procedure SHOULD be followed to fill Gap Ack Blocks, NR Gap Ack Blocks and Duplicate TSN fields of the NR-SACK chunk: 1) Fill as many Gap Ack Blocks as possible starting from Cumulative TSN Ack value. 2) If space remains, fill as many NR Gap Ack Blocks as possible starting from Cumulative TSN Ack value. 3) If space remains, fill as many NR Gap Ack Blocks as possible starting from Cumulative TSN Ack value. Note: For CASE-1, no NR Gap Ack Blocks are reported and only the 1st and 3rd steps of the above procedure SHOULD be followed. For CASE-2, all steps of the above procedure SHOULD be followed. For CASE-3 when the A bit is set and all Gap Ack Blocks are the same as, and reported as NR Gap Ack Blocks, only the 2nd and 3rd steps SHOULD be followed. 6.2. Receiving an NR-SACK Chunk When an NR-SACK chunk is received, all of the NR-SACK fields identical to a SACK chunk SHOULD be processed and handled as in SACK chunk handling outlined in Section 6.2.1 of [RFC4960]. The NR Gap Ack Block Start(s) and NR Gap Ack Block End(s) are offsets relative to the cum-ack. To calculate the actual range of nr-gap- acked TSNs, the cum-ack MUST be added to the Start and End. For example, assume an incoming NR-SACK chunk's cum-ack is 12 and an NR Gap Ack Block defines the NR Gap Ack Block Start=5, and the NR Gap Ack Block End=7. This NR Gap Ack block nr-gap-acks TSNs 17 through 19(inclusive.) Natarajan, et al. Expires August 22, 2009 [Page 17] Internet-Draft NR-SACKs for SCTP February 2009 Upon reception of an NR-SACK chunk, all TSNs falling within an NR-Gap Ack Block SHOULD be removed from the data sender's retransmission queue as their delivery to the receiving application has either already occurred, or is guaranteed by the data receiver. Most of NR-SACK processing at the data sender can be implemented by using the same routines as in SACK that process cum ack and gap ack, followed by removal of nr-gap-acked DATA chunks from the retransmission queue. However, with NR-SACKs, as out-of-order DATA chunks also can be removed from the retranmission queue, gap ack processing routine cannot rely on the contents of the data sender's retransmission queue for correct processing. For example, while calculating missing reports,the gap ack processing routine cannot assume that the highest TSN transmitted is always at the tail (right edge) of the retransmission queue. 7. Security Considerations There are no security considerations addressed by this memo. 8. IANA considerations This document defines a new chunk type to transfer the NR-SACK information. Table 2 illustrates the new chunk type. The new chunk type must come from the range of chunk types where the upper two bits are zero. We recommend 0x10 but any other available code point with the upper two bits set to zero is acceptable. Chunk Type Chunk Name -------------------------------------------------------------- 0x10 Non-Renegable Selective Acknowledgment (NR-SACK) Table 2: NR-SACK Chunk 9. Acknowledgments This Internet Draft is the result of a great deal of constructive discussion with several people, notably Phillip Conrad, Nasif Ekiz, and Jonathan Leighton. 10. References Natarajan, et al. Expires August 22, 2009 [Page 18] Internet-Draft NR-SACKs for SCTP February 2009 10.1. Normative References [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate Requirement Levels", BCP 14, RFC 2119, March 1997. [RFC5061] Stewart, R., Xie, Q., Tuexen, M., Maruyama, S., and M. Kozuka, "Stream Control Transmission Protocol (SCTP) Dynamic Address Reconfiguration", RFC 5061, September 2007. [RFC4960] Stewart, R., "Stream Control Transmission Protocol", RFC 4960, September 2007. 10.2. Informative References [Natarajan] Natarajan, P., Ekiz, N., Yilmaz, E., Amer, P., Iyengar, J., and R. Stewart, "Performance of Non-Renegable Selective Acknowledgments (NR-SACKs) for SCTP", Technical Report 2008/344, Computer and Information Sciences Dept., University of Delaware , (in progress). Authors' Addresses Preethi Natarajan Cisco Systems 170 West Tasman Drive San Jose, CA 95134 USA Phone: Email: nataraja@cis.udel.edu Paul D. Amer University of Delaware Computer and Information Sciences Department Newark, DE 19716 USA Phone: Email: amer@cis.udel.edu Natarajan, et al. Expires August 22, 2009 [Page 19] Internet-Draft NR-SACKs for SCTP February 2009 Ertugrul Yilmaz University of Delaware Computer and Information Sciences Department Newark, DE 19716 USA Phone: Email: eyilmaz@cis.udel.edu Randall R. Stewart The Resource Group 1700 Pennsylvania Ave NW Suite 560 Washington, DC 20006 USA Phone: 202-595-8626 Email: rrs@lakerest.net Janardhan Iyengar Franklin & Marshall College Math and Computer Science PO Box 3003 Lancaster, PA 17604-3003 USA Phone: 717-358-4774 Email: jiyengar@fandm.edu Natarajan, et al. Expires August 22, 2009 [Page 20]