BT

Adastral Park, Martlesham Heath Ipswich IP5 3RE UK philip.eardley@bt.com

Transport Area PCN Working Group Sample Draft 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. 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.

|Threshold|-------+ | | Meter | | | +---------+ V +----------+ +- - - - -+ | +------+ | BA | | | | | | Marked Packet =>|Classifier|==>| Dropper |==?===============>|Marker|==> Packet Stream | | | | | | | Stream +----------+ +- - - - -+ | +------+ | +---------+ ^ | | Excess | | +->| Traffic |-------+ | Meter | Result +---------+ ]]> Figure 1: Schematic of functions for PCN-metering and -marking

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 confusing added 'common capsule' summary of PCN to Introduction and removed extraneous material replaced 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 improved other minor corrections and clarifications

Updates 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-marking added 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-packets corrected the definition of metered-packet delete 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 throughout made all references Informative

First 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.

Note: 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 packet In 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 bucket Ttm = max(0, Ttm - packet_size); // remove tokens from token bucket if ((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-marked lastUpdate = now

A 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 packet In 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 bucket if (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-marked if (Tetm < MTU) then packet_mark = excess-traffic-marked; // do (packet size independent) excess traffic marking lastUpdate = now

Note: 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; or separate 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.