Metering and marking behaviour of
PCN-nodesBTAdastral Park, Martlesham HeathIpswichIP5 3REUKphilip.eardley@bt.com
Transport Area
PCN Working GroupSampleDraftThe objective of Pre-Congestion Notification (PCN) is to protect the
quality of service (QoS) of inelastic flows within a Diffserv domain, in
a simple, scalable and robust fashion. This document specifies the two
metering and marking behaviours of PCN-nodes. Threshold-metering and
-marking marks all PCN-packets if the PCN traffic rate is greater than a
configured rate ("PCN-threshold-rate"). Excess-traffic-metering and
-marking marks a proportion of PCN-packets, such that the amount marked
equals the traffic rate in excess of a configured rate
("PCN-excess-rate"). The level of marking allows PCN-boundary-nodes to
make decisions about whether to admit or terminate PCN-flows.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.The objective of Pre-Congestion Notification (PCN) is to protect the
quality of service (QoS) of inelastic flows within a Diffserv domain, in
a simple, scalable and robust fashion. Two mechanisms are used:
admission control, to decide whether to admit or block a new flow
request, and (in abnormal circumstances) flow termination to decide
whether to terminate some of the existing flows. To achieve this, the
overall rate of PCN traffic is metered on every link in the domain, and
PCN packets are appropriately marked when certain configured rates are
exceeded. These configured rates are below the rate of the link thus
providing notification to boundary nodes about overloads before any
congestion occurs (hence "pre-congestion notification"). The level of
marking allows boundary nodes to make decisions about whether to admit
or terminate.This document standardises the two metering and marking behaviours of
PCN-nodes. Their aim is to enable PCN-nodes to give an "early warning"
of potential congestion before there is any significant build-up of
PCN-packets in their queues. In summary, their objectives are:threshold-metering and -marking: its objective is to mark all
PCN-packets (with a "threshold-mark") whenever the rate of
PCN-packets is greater than its configured rate
("PCN-threshold-rate");excess traffic marking: whenever the rate of PCN-packets is
greater than its configured rate ("PCN-excess-rate"), its objective
is to mark PCN-packets (with an "excess-traffic-mark") at a rate
equal to the difference between the bit rate of PCN-packets and the
PCN-excess-rate. defines a broadly RED-like default
congestion marking behaviour, but allows alternatives to be defined;
this document defines such an alternative.Section 2 below specifies the functions involved, which in outline
(see Figure 1) are:Behaviour aggregate (BA) classification: decide whether an
incoming packet is a PCN-packet or not.Dropping (optional): drop packets if the link is overloaded.Threshold-meter: determine whether the rate of PCN-packets is
greater than its configured PCN-threshold-rate. The meter operates
on all PCN-packets on the link, and not on individual flows.Excess-traffic-meter: measure by how much the rate of PCN-packets
is greater than its configured PCN-excess-rate. The meter operates
on all PCN-packets on the link, and not on individual flows.PCN-mark: actually mark the PCN-packets, if the meter functions
indicate to do so.Appendix A gives an example of algorithms that fulfil the
specification of Section 2, and Appendix B provides some explanations of
and comments on Section 2. Both the Appendices are informative.In addition to the terminology defined in and , the following terms are defined:Competing-non-PCN-packet: a non PCN-packet that shares a link
with PCN-packets and competes with them for its forwarding
bandwidth. Competing-non-PCN-packets MUST NOT be PCN-marked (ie
only PCN-packets can be PCN-marked). Note: In general it is not
advised to have any competing-non-PCN-traffic.Metered-packet: a packet that is metered by the metering
functions specified in Sections 2.3 and 2.4. A PCN-packet MUST be
treated as a metered-packet (with the minor exception noted below
in Section 2.4). A competing-non-PCN-packet MAY be treated as a
metered-packet.This section specifies the two PCN-metering and -marking behaviours.
The descriptions are functional and are not intended to restrict the
implementation. The informative Appendices supplement this section.A PCN-node MUST classify a packet as a PCN-packet if the value of
its DSCP and ECN fields correspond to a PCN-enabled codepoint, as
defined in the encoding scheme applicable to the PCN-domain. Otherwise
the packet MUST NOT be classified as a PCN-packet.A PCN-node MUST classify a packet as a competing-non-PCN-packet if
it is not a PCN-packet and it competes with PCN-packets for its
forwarding bandwidth on a link.Note: if the PCN-node's queue overflows then naturally packets are
dropped. This section describes additional action.On all links in the PCN-domain, dropping MAY be done by:metering all metered-packets to determine if the rate of
metered-traffic is greater than its scheduling rate (ie determine
if any packets are out-of-profile).if the rate of metered-traffic is too high, then drop
metered-packets.If the PCN-node drops PCN-packets then:PCN-packets that arrive at the PCN-node already
excess-traffic-marked SHOULD be preferentially dropped;the PCN-node's excess-traffic-meter SHOULD NOT meter the
PCN-packets that it drops.A PCN-node MUST implement a threshold-meter that has behaviour
functionally equivalent to the following.The meter acts like a token bucket, which is sized in bits and has
a configured bit rate, termed PCN-threshold-rate. The amount of tokens
in the token bucket is termed Ttm. Tokens are added at the
PCN-threshold-rate, to a maximum value BStm. Tokens are removed equal
to the size in bits of the metered-packet, to a minimum Ttm=0.
(Explanation of abbreviations: T is short for Tokens, BS for bucket
size, and tm for threshold-meter.)The token bucket has a configured intermediate depth, termed
threshold. If Ttm < threshold, then the meter indicates to the
marking function that the packet is to be threshold-marked; otherwise
it does not.A PCN-packet SHOULD NOT be metered (by this excess-traffic-meter
function) in the following two cases:If the packet is already excess-traffic-marked on arrival at
the PCN-node;If this PCN-node drops the packet.Otherwise the PCN-packet MUST be treated as a metered-packet, that
is it is metered by the excess-traffic-meter.A PCN-node MUST implement an excess-traffic-meter that has
behaviour functionally equivalent to the following.The meter acts like a token bucket, which is sized in bits and has
a configured bit rate, termed PCN-excess-rate. The amount of tokens in
the token bucket is termed Tetm. Tokens are added at the
PCN-excess-rate, to a maximum value BSetm. Tokens are removed equal to
the size in bits of the metered-packet, to a minimum Tetm=0. If the
token bucket is empty (Tetm = 0), then the meter indicates to the
marking function that the packet is to be excess-traffic-marked.
(Explanation of abbreviations: T is short for Tokens, BS for bucket
size, and etm for excess-traffic-meter.)In addition to the above, if the token bucket is within an MTU of
being empty, then the meter SHOULD indicate to the marking function
that the packet is to be excess-traffic-marked; MTU means the maximum
size of PCN-packets on the link ("packet size independent
marking").Otherwise the meter MUST NOT indicate marking.A PCN-packet MUST be marked to reflect the metering results by
setting its encoding state appropriately, as specified by the specific
encoding scheme that applies in the PCN-domain. A consistent choice of
encoding scheme MUST be made throughout a PCN-domain.A PCN-node MUST NOT:PCN-mark a packet that is not a PCN-packet;change a non PCN-packet into a PCN-packet;change a PCN-packet into a non PCN-packet.This document makes no request of IANA.Note to RFC Editor: this section may be removed on publication as an
RFC.Security considerations are discussed in detail in .This document is the result of extensive collaboration within the PCN
WG. Amongst the most active contributors to the development of the ideas
specified in this document have been Jozef Babiarz, Bob Briscoe, Kwok-Ho
Chan, Anna Charny, Philip Eardley, Georgios Karagannis, Michael Menth,
Toby Moncaster, Daisuke Satoh, and Joy Zhang. Appendix A is based on
text from Michael Menth.This document is a development of . Its authors are therefore
contributors to this document: Bob Briscoe, Philip Eardley, Dave
Songhurst, Francois Le Faucheur, Anna Charny, Vassilis Liatsos, Jozef
Babiarz, Kwok-Ho Chan, Stephen Dudley, Georgios Karagiannis, Attila
Bader, Lars Westberg.Thanks to those who've made comments on this draft: Michael Menth,
Joe Babiarz, Fred Baker, Bob Briscoe, Ken Carlberg, Anna Charny,
Ruediger Geib, Wei Gengyu, Fortune Huang, Christian Hublet, Ingemar
Johansson, Georgios Karagiannis, Toby Moncaster, Dimitri Papadimitriou,
Daisuke Satoh. Michael Menth, Joe Babiarz, Anna Charny reviewed a
preliminary version of the prior individual internet draft.Updates to take account of last call comments as follows:renamed from "marking" to "metering and marking" (throughout) -
the former was intended as shorthand for the latter, but this was
found confusingadded 'common capsule' summary of PCN to Introduction and
removed extraneous materialreplaced the term 'traffic conditioning' by 'dropping'
(throughout) - since the former has a wider meaning than just
dropping.discussion of the case with baseline encoding where there are
two PCN states - this is now done just once - in Section B.2.added in Section B.5 "The PCN-threshold-rate is configured at
less than the rate allocated to the PCN-traffic class" and in B.6
"The PCN-excess-rate is configured at less than (or possibly equal
to) the rate allocated to the PCN-traffic class".configuring the PCN-excess-rate at greater than (or possibly
equal to) the PCN-threshold-rate - this is now in one place, as
advice is B5 & B6.SB.1: "voice-admit" corrected with references to I-D
ietf-tsvwg-admitted-realtime-dscp and RFC5127."CL/SM edge behaviour" altered to the less obscure "controlled
load edge behaviour" and a reference added.S2.3, 2.4 & Appendix A: altered some of the abbreviations,
for better consistency with approach of RFC2698. eg
TBthreshold.fill => Ttm.the ACKs section improvedother minor corrections and clarificationsUpdates as follows:added notes (end of S1.1 & 2.5) to clarify what
"excess-traffic-marked" means when there is only one encoding for
PCN-markingadded explanations for in Section B.4 and B.6 about why various
things are SHOULD or SHOULD NOT rather than MUST or MUST NOT.Deleted a couple of paragraphs about encoding states, as they
are relevant to encoding documents rather than this document.Updates as follows:corrected the term 'not PCN-marked' to 'not-marked'
(throughout)re-phrased the definition of competing-non-PCN-packetscorrected the definition of metered-packetdelete most of Section 2.5 (marking function). The material
deleted belongs as part of ; other encoding
schemes would need to include similar material.deleted Appendix C (it was only a temporary archive of material
concerning per domain behaviour and PCN-boundary-node
operation)clarifications throughoutmade all references InformativeFirst version of WG draft, derived from
draft-eardley-pcn-marking-behaviour-01, with the following
changes:Removed material concerning per domain behaviour and
PCN-boundary-node operation (temporarily archived to Appendix
C)Removed mention of downgrading as an option for per-hop traffic
conditioning. In fact, downgrading is no longer allowed because S
2.6 now says "A PCN-node MUST NOT ...change a PCN-packet into a
non PCN-packet".Traffic conditioning is now a MAY. Since in general flow
termination (not traffic conditioning) is PCN's method for
handling problems of too much traffic.Metered-packets: competing-non-PCN-packets now MAY be metered.
Since it is recommended that the operator doesn't allow any
competing-non-PCN-traffic, and (if there is) there are potentially
other ways of coping.No changes (outside traffic conditioning & metering of
competing-non-PCN-traffic) to the Normative sections of the
draft.Appendix B.1 added about competing-non-PCN-traffic. Recommended
that there is no such traffic, but guidance given if there is.MenthNote: This Appendix is informative, not normative. It is an example
of algorithms that implement Section 2 and is based on and .There is no attempt to optimise the algorithms. It implements the
metering and marking functions together. It is assumed that three
encoding states are available (one for threshold-marked, one for
excess-traffic-marked and one for not PCN-marked). It is assumed that
all metered-packets are PCN-packets and that the link is never
overloaded.A token bucket with the following parameters:PCN-threshold-rate: token rate of token bucket
(bits/second)BStm: depth of token bucket (bits)threshold: marking threshold of token bucket (bits)lastUpdate: time the token bucket was last updated
(seconds)Ttm: amount of tokens in token bucket (bits)A PCN-packet has the following parameters:packet_size: the size of the PCN-packet (bits)packet_mark: the PCN encoding state of the packetIn addition there are the parameters:now: the current time (seconds)The following steps are performed when a PCN-packet arrives on a
link:Ttm = min(BStm, Ttm + (now - lastUpdate) * PCN-threshold-rate);
// add tokens to token bucketTtm = max(0, Ttm - packet_size); // remove tokens from token
bucketif ((Ttm < threshold) AND (packet_mark !=
excess-traffic-marked)) then packet_mark = threshold-marked; // do
threshold marking, but don't re-mark packets that are already
excess-traffic-markedlastUpdate = nowA token bucket with the following parameters:TBexcess.PCN-excess-rate: token rate of token bucket
(bits/second)BSetm: depth of TB in token bucket (bits)TBexcess.lastUpdate: time the token bucket was last updated
(seconds)Tetm: amount of tokens in token bucket (bits)A PCN-packet has the following parameters:packet_size: the size of the PCN-packet (bits)packet_mark: the PCN encoding state of the packetIn addition there are the parameters:now: the current time (seconds)MTU: the maximum transfer unit of the link (or the known
maximum size of PCN-packets on the link) (bits)The following steps are performed when a PCN-packet arrives on a
link:Tetm = min(BSetm, Tetm + (now - TBexcess.lastUpdate) *
TBexcess.PCN-excess-rate); // add tokens to token bucketif (packet_mark != excess-traffic-marked) then Tetm = max(0,
Tetm - packet_size); // remove tokens from token bucket, but do
not meter packets that are already excess-traffic-markedif (Tetm < MTU) then packet_mark = excess-traffic-marked; //
do (packet size independent) excess traffic markinglastUpdate = nowNote: This Appendix is informative, not normative. It comments on
Section 2.In general it is not advised to have any competing-non-PCN-traffic,
essentially because the unpredictable amount of
competing-non-PCN-traffic makes the PCN mechanisms less accurate and
so reduces PCN's ability to protect the QoS of admitted PCN-flows
. But if there is
competing-non-PCN-traffic, then there needs to be:a mechanism to limit it, for example:limit the rate at which competing-non-PCN-traffic can be
forwarded on each link in the PCN-domain. One method for
achieving this is to queue competing-non-PCN-packets
separately from PCN-packets, and to limit the scheduling rate
of the former. Another method is to drop
competing-non-PCN-packets in excess of some rate.police competing-non-PCN-traffic at the PCN-ingress-nodes.
For example, as in the Diffserv architecture - although its
static traffic conditioning agreements risk a focused overload
of traffic from several PCN-ingress-nodes on one link.it is known by design that the level of
competing-non-PCN-traffic is always very small - perhaps it
consists of operator control messages only.In general PCN's mechanisms should take account of
competing-non-PCN-traffic, in order to improve the accuracy of the
decision about whether to admit (or terminate) a PCN-flow. For
example:competing-non-PCN-traffic contributes to the PCN meters (ie
competing-non-PCN-packets are treated as metered-packets).each PCN-node reduces, on its links, the PCN-threshold-rate
and PCN-excess-rate, in order to allow 'headroom' for the
competing-non-PCN-traffic; also limit the maximum forwarding
rate of competing-non-PCN-traffic to be less than the
'headroom'. In this case competing-non-PCN-packets are not
treated as metered-packets.It is left up to the operator to decide on appropriate
action. Dropping is discussed further in Section B.4.One specific example of competing-non-PCN-traffic occurs if the
PCN-compatible Diffserv codepoint is one of those that ) defines as
suitable for use with admission control, and there is such non
PCN-traffic in the PCN-domain. A similar example could occur for
Diffserv codepoints of the Real-Time Treatment Aggregate ). In such cases PCN-traffic and
competing-non-PCN-traffic are distinguished by different values of the
ECN field .Another example would occur if there is more than one
PCN-compatible Diffserv codepoint in a PCN-domain. For instance,
suppose there are two PCN-BAs treated at different priorities. Then as
far as the lower priority PCN-BA is concerned, the higher priority
PCN-traffic needs to be treated as competing-non-PCN-traffic.It may be known, eg by the design of the network topology, that
some links can never be pre-congested (even in unusual circumstances,
eg after the failure of some links). There is then no need to deploy
PCN behaviour on those links.The meters can be implemented on the ingoing or outgoing interface
of a PCN-node. It may be that existing hardware can support only one
meter per ingoing interface and one per outgoing interface. Then for
instance threshold-metering could be run on all the ingoing interfaces
and excess-traffic-metering on all the outgoing interfaces; note that
the same choice must be made for all the links in a PCN-domain to
ensure that the two metering behaviours are applied exactly once for
all the links.The baseline encoding specifies only two
encoding states (PCN-marked and not-marked). In this case,
"excess-traffic-marked" means a packet that is PCN-marked as a result
of the excess-traffic-meter function, and "threshold-marked" means a
packet that is PCN-marked as a result of the threshold-meter function.
As far as terminology is concerned, this interpretation is consistent
with that defined in .
Note that a deployment needs to make a consistent choice throughout
the PCN-domain whether PCN-marked is interpreted as
excess-traffic-marked or threshold-marked.Note that even if there are only two encoding states, it is still
required that both the meters are implemented, in order to ease
compatibility between equipment, and to remove a configuration option
and associated complexity. Hardware with limited availability of token
buckets could be configured to run only one of the meters, but it must
be possible to enable either meter. Although in the scenario with two
encoding states indications from one of the meters are ignored by the
marking function, they may be logged or acted upon in some other way,
for example by the management system or an explicit signalling
protocol; such considerations are out of scope of this document.Configuration of PCN-nodes will define what values of the DSCP and
ECN fields indicate a PCN-packet in a particular PCN-domain.Configuration will also define what values of the DSCP and ECN
fields indicate a competing-non-PCN-packet in a particular
PCN-domain.The objective of the dropping function is to minimise the queueing
delay suffered by metered-traffic at a PCN-node, since PCN-traffic
(and perhaps competing-non-PCN-traffic) is expected to be inelastic
traffic generated by real time applications. In practice it would be
defined as exceeding a specific traffic profile, typically based on a
token bucket.If there is no competing-non-PCN-traffic, then it is not expected
that the dropping function is needed, since PCN's flow admission and
termination mechanisms limit the amount of PCN-traffic. Even so, it
still might be implemented as a back stop against misconfiguration of
the PCN-domain, for instance.If there is competing-non-PCN-traffic, then the details of the
dropping function will depend on how the router's implementation
handles the two sorts of traffic (the discussion here is based on that
in ):a common queue for PCN-traffic and competing-non-PCN-traffic,
and a traffic conditioner for the competing-non-PCN-traffic;
orseparate queues. In this case the amount of
competing-non-PCN-traffic can be limited by limiting the rate at
which the scheduler (for the competing-non-PCN-traffic) forwards
packets.Note that only dropping of packets is allowed. Downgrading of
packets to a lower priority BA is not allowed (see B.7), since it
would lead to packet mis-ordering. Shaping ("the process of delaying
packets" ) is not suitable if the
traffic comes from real time applications.In general it is reasonable for competing-non-PCN-traffic to get
harsher treatment than PCN-traffic (ie competing-non-PCN-packets are
preferentially dropped), because PCN's flow admission and termination
mechanisms are stronger than the mechanisms that are likely to be
applied to the competing-non-PCN-traffic. The PCN mechanisms also mean
that a dropper should not be needed for the PCN-traffic.Preferential dropping of excess-traffic-marked packets: Section 2.3
specifies: "If the PCN-node drops PCN-packets then ... PCN-packets
that arrive at the PCN-node already excess-traffic-marked SHOULD be
preferentially dropped". In brief, the reason is that, with the
"controlled load" edge behaviour this avoids
over-termination in the event of multiple bottlenecks in the
PCN-domain . A fuller
explanation is as follows. The optimal dropping behaviour depends on
the particular edge behaviour . A single
dropping behaviour is defined, as it is simpler to standardise,
implement and operate. The standardised dropping behaviour is at least
adequate for all edge behaviours (and good for some), whereas others
are not (for example with tail dropping far too much traffic may be
terminated with the "controlled load" edge behaviour, in the event of
multiple bottlenecks in the PCN-domain ). The dropping behaviour is
defined as a ‘SHOULD’, rather than a ‘MUST’,
in recognition that other dropping behaviour may be preferred in
particular circumstances, for example: (1) with the "marked flow"
termination edge behaviour, preferential dropping of unmarked packets
may be better ; (2) tail dropping may make
PCN marking behaviour easier to implement on current routers.Exactly what "preferentially dropped" means is left to the
implementation. It is also left to the implementation what to do if
there are no excess-traffic-marked PCN-packets available at a
particular instant.Section 2.2 also specifies: "the PCN-node's excess-traffic-meter
SHOULD NOT meter the PCN-packets that it drops." This avoids
over-termination . Effectively it means
that the dropping function (if present) should be done before the
meter functions - which is natural.The description is in terms of a ‘token bucket with
threshold’ (which views as a virtual
queue). However the description is not intended to standardise
implementation.The PCN-threshold-rate is configured at less than the rate
allocated to the PCN-traffic class. Also, the PCN-threshold-rate is
less than, or possibly equal to, the PCN-excess-rate.Section 2.3 defines: "If Ttm < threshold, then the meter
indicates to the marking function that the packet is to be
threshold-marked; otherwise it does not." Note that a PCN-packet is
marked without explicit additional bias for the packet's size.The behaviour must be functionally equivalent to the description in
Section 2.3. "Functionally equivalent" means the observable 'black
box' behaviour is the same or very similar, for example if either
precisely the same set of packets is marked, or if the set is shifted
by one packet. It is intended to allow implementation freedom over
matters such as:whether tokens are added to the token bucket at regular time
intervals or only when a packet is processed.whether the new token bucket depth is calculated before or
after it is decided whether to mark the packet. The effect of this
is simply to shift the sequence of marks by one packet.when the token bucket is very nearly empty and a packet arrives
larger than Ttm, then the precise change in Ttm is up to the
implementation. For instance:set Ttm = 0 and indicate threshold-mark to the Marking
function.check whether Ttm < threshold and if it is then indicate
threshold-mark to the Marking function; then set Ttm = 0.leave Ttm unaltered and indicate threshold-mark to the
Marking function.similarly, when the token bucket is very nearly full and a
packet arrives larger than (BStm - Ttm), then the precise change
in Ttm is up to the implementation.Note that all packets, even if already marked, are metered by
the threshold-meter function (unlike the excess-traffic-meter
function), because all packets should contribute to the decision
whether there is room for a new flow.The description is in terms of a token bucket, however the
implementation is not standardised.The PCN-excess-rate is configured at less than (or possibly equal
to) the rate allocated to the PCN-traffic class. Also, the
PCN-excess-rate is greater than, or possibly equal to, the
PCN-threshold-rate.As in Section B.3, "functionally equivalent" allows some
implementation flexibility when the token bucket is very nearly empty
or very nearly full.Section 2.4 specifies: "A packet SHOULD NOT be metered (by this
excess traffic meter function) ... If the packet is already
excess-traffic-marked on arrival at the PCN-node". This avoids
over-termination (with some edge behaviours) in the event that the
PCN-traffic passes through multiple bottlenecks in the PCN-domain
. Note that an
implementation could determine whether the packet is already
excess-traffic-marked as an integral part of its BA classification
function. The behaviour is defined as a ‘SHOULD NOT’,
rather than a ‘MUST NOT’, because it may be slightly
harder to implement than a metering function that is blind to previous
packet markings.Section 2.4 specifies: "A packet SHOULD NOT be metered (by this
excess traffic meter function) ... If this PCN-node drops the packet."
This avoids over-termination . (A similar
statement could also be made for the threshold meter function, but is
irrelevant, as a link that is overloaded will already be substantially
pre-congested and hence threshold-marking all packets.) It seems
natural to perform the dropping function before the metering
functions, although for some equipment it may be harder to implement;
hence the behaviour is defined as a ‘SHOULD NOT’, rather
than a ‘MUST NOT’.Packet size independent marking is specified as a SHOULD in Section
2.4 ( "if the token bucket is within an MTU of being empty, then the
meter SHOULD indicate to the Marking function that the packet is to be
excess-traffic-marked; MTU means the maximum size of PCN-packets on
the link".) Without it, large packets are more likely to be
excess-traffic-marked than small packets and this means that, with
some edge behaviours, flows with large packets are more likely to be
terminated than flows with small packets . The behaviour is a ‘SHOULD’,
rather than a ‘MUST’, because packet size independent
marking may be slightly harder for some equipment to implement, and
the impact of not doing it is undesirable but moderate (sufficient
traffic is terminated, but flows with large packets are more likely to
be terminated).Note that BSetm is independent of BStm; Tetm is independent of Ttm
(except in that a packet changes both); and the two configured rates
(PCN-excess-rate and PCN-threshold-rate) are independent (except that
PCN-excess-rate >= PCN-threshold-rate).Section 2.5 defines: "A PCN-node MUST NOT ...change a PCN-packet
into a non PCN-packet". This means that a PCN-node is not allowed to
downgrade a PCN-packet into a lower priority Diffserv BA (eg it is not
allowed as an alternative to dropping, Section 2.2).Section 2.5 defines: "A PCN-node MUST NOT ...PCN-mark a packet that
is not a PCN-packet". This means that in the scenario where
competing-non-PCN-packets are treated as metered-packets, a meter may
indicate a packet is to be PCN-marked, but the marking function knows
it cannot be marked. It is left open to the implementation exactly
what to do in this case; one simple possibility is to mark the next
PCN-packet. Note that unless the PCN-packets are a large fraction of
all the metered-packets then the PCN mechanisms may not work well.Although the metering functions are described separately from the
marking function, they can be implemented in an integrated
fashion.