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Network Working Group                                       T. MoncasterRequest for Comments: 5696                                    B. BriscoeCategory: Standards Track                                             BT                                                                M. Menth                                                 University of Wuerzburg                                                           November 2009Baseline Encoding and Transport of Pre-Congestion InformationAbstract   The objective of the Pre-Congestion Notification (PCN) architecture   is to protect the quality of service (QoS) of inelastic flows within   a Diffserv domain.  It achieves this by marking packets belonging to   PCN-flows when the rate of traffic exceeds certain configured   thresholds on links in the domain.  These marks can then be evaluated   to determine how close the domain is to being congested.  This   document specifies how such marks are encoded into the IP header by   redefining the Explicit Congestion Notification (ECN) codepoints   within such domains.  The baseline encoding described here provides   only two PCN encoding states: Not-marked and PCN-marked.  Future   extensions to this encoding may be needed in order to provide more   than one level of marking severity.Status of This Memo   This document specifies an Internet standards track protocol for the   Internet community, and requests discussion and suggestions for   improvements.  Please refer to the current edition of the "Internet   Official Protocol Standards" (STD 1) for the standardization state   and status of this protocol.  Distribution of this memo is unlimited.Copyright Notice   Copyright (c) 2009 IETF Trust and the persons identified as the   document authors.  All rights reserved.   This document is subject toBCP 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   to this document.  Code Components extracted from this document must   include Simplified BSD License text as described in Section 4.e of   the Trust Legal Provisions and are provided without warranty as   described in the BSD License.Moncaster, et al.           Standards Track                     [Page 1]

RFC 5696                 Baseline PCN Encoding             November 2009   This document may contain material from IETF Documents or IETF   Contributions published or made publicly available before November   10, 2008.  The person(s) controlling the copyright in some of this   material may not have granted the IETF Trust the right to allow   modifications of such material outside the IETF Standards Process.   Without obtaining an adequate license from the person(s) controlling   the copyright in such materials, this document may not be modified   outside the IETF Standards Process, and derivative works of it may   not be created outside the IETF Standards Process, except to format   it for publication as an RFC or to translate it into languages other   than English.Table of Contents1.  Introduction . . . . . . . . . . . . . . . . . . . . . . . . .32.  Requirements Notation  . . . . . . . . . . . . . . . . . . . .33.  Terminology and Abbreviations  . . . . . . . . . . . . . . . .33.1.  Terminology  . . . . . . . . . . . . . . . . . . . . . . .33.2.  List of Abbreviations  . . . . . . . . . . . . . . . . . .44.  Encoding Two PCN States in IP  . . . . . . . . . . . . . . . .44.1.  Marking Packets  . . . . . . . . . . . . . . . . . . . . .54.2.  Valid and Invalid Codepoint Transitions  . . . . . . . . .64.3.  Rationale for Encoding . . . . . . . . . . . . . . . . . .74.4.  PCN-Compatible Diffserv Codepoints . . . . . . . . . . . .74.4.1.  Co-Existence of PCN and Not-PCN Traffic  . . . . . . .85.  Rules for Experimental Encoding Schemes  . . . . . . . . . . .86.  Backward Compatibility . . . . . . . . . . . . . . . . . . . .97.  Security Considerations  . . . . . . . . . . . . . . . . . . .98.  Conclusions  . . . . . . . . . . . . . . . . . . . . . . . . .109.  Acknowledgements . . . . . . . . . . . . . . . . . . . . . . .1010. References . . . . . . . . . . . . . . . . . . . . . . . . . .1010.1. Normative References . . . . . . . . . . . . . . . . . . .1010.2. Informative References . . . . . . . . . . . . . . . . . .10Appendix A.  PCN Deployment Considerations (Informative) . . . . .11A.1.  Choice of Suitable DSCPs . . . . . . . . . . . . . . . . .11A.2.  Rationale for Using ECT(0) for Not-Marked  . . . . . . . .12Appendix B.  Co-Existence of PCN and ECN (Informative) . . . . . .13Moncaster, et al.           Standards Track                     [Page 2]

RFC 5696                 Baseline PCN Encoding             November 20091.  Introduction   The objective of the Pre-Congestion Notification (PCN) architecture   [RFC5559] is to protect the quality of service (QoS) of inelastic   flows within a Diffserv domain in a simple, scalable, and robust   fashion.  The overall rate of PCN-traffic is metered on every link in   the PCN-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 before any congestion   occurs (hence "Pre-Congestion Notification").  The level of marking   allows the boundary nodes to make decisions about whether to admit or   block a new flow request, and (in abnormal circumstances) whether to   terminate some of the existing flows, thereby protecting the QoS of   previously admitted flows.   This document specifies how these PCN-marks are encoded into the IP   header by reusing the bits of the Explicit Congestion Notification   (ECN) field [RFC3168].  It also describes how packets are identified   as belonging to a PCN-flow.  Some deployment models require two PCN   encoding states, others require more.  The baseline encoding   described here only provides for two PCN encoding states.  However,   the encoding can be easily extended to provide more states.  Rules   for such extensions are given inSection 5.2.  Requirements Notation   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].3.  Terminology and Abbreviations3.1.  Terminology   The terms PCN-capable, PCN-domain, PCN-node, PCN-interior-node, PCN-   ingress-node, PCN-egress-node, PCN-boundary-node, PCN-traffic, PCN-   packets and PCN-marking are used as defined in [RFC5559].  The   following additional terms are defined in this document:   o  PCN-compatible Diffserv codepoint - a Diffserv codepoint      indicating packets for which the ECN field is used to carry PCN-      markings rather than [RFC3168] markings.   o  PCN-marked codepoint - a codepoint that indicates packets that      have been marked at a PCN-interior-node using some PCN-marking      behaviour [RFC5670].  Abbreviated to PM.Moncaster, et al.           Standards Track                     [Page 3]

RFC 5696                 Baseline PCN Encoding             November 2009   o  Not-marked codepoint - a codepoint that indicates packets that are      PCN-capable but that are not PCN-marked.  Abbreviated to NM.   o  not-PCN codepoint - a codepoint that indicates packets that are      not PCN-capable.3.2.  List of Abbreviations   The following abbreviations are used in this document:   o  AF = Assured Forwarding [RFC2597]   o  CE = Congestion Experienced [RFC3168]   o  CS = Class Selector [RFC2474]   o  DSCP = Diffserv codepoint   o  ECN = Explicit Congestion Notification [RFC3168]   o  ECT = ECN Capable Transport [RFC3168]   o  EF = Expedited Forwarding [RFC3246]   o  EXP = Experimental   o  NM = Not-marked   o  PCN = Pre-Congestion Notification   o  PM = PCN-marked4.  Encoding Two PCN States in IP   The PCN encoding states are defined using a combination of the DSCP   and ECN fields within the IP header.  The baseline PCN encoding   closely follows the semantics of ECN [RFC3168].  It allows the   encoding of two PCN states: Not-marked and PCN-marked.  It also   allows for traffic that is not PCN-capable to be marked as such (not-   PCN).  Given the scarcity of codepoints within the IP header, the   baseline encoding leaves one codepoint free for experimental use.   The following table defines how to encode these states in IP:Moncaster, et al.           Standards Track                     [Page 4]

RFC 5696                 Baseline PCN Encoding             November 2009   +---------------+-------------+-------------+-------------+---------+   | ECN codepoint |   Not-ECT   | ECT(0) (10) | ECT(1) (01) | CE (11) |   |               |     (00)    |             |             |         |   +---------------+-------------+-------------+-------------+---------+   |     DSCP n    |   not-PCN   |      NM     |     EXP     |    PM   |   +---------------+-------------+-------------+-------------+---------+                        Table 1: Encoding PCN in IP   In the table above, DSCP n is a PCN-compatible Diffserv codepoint   (seeSection 4.4) and EXP means available for Experimental use.  N.B.   we deliberately reserve this codepoint for experimental use only (and   not local use) to prevent future compatibility issues.   The following rules apply to all PCN-traffic:   o  PCN-traffic MUST be marked with a PCN-compatible Diffserv      codepoint.  To conserve DSCPs, Diffserv codepoints SHOULD be      chosen that are already defined for use with admission-controlled      traffic.Appendix A.1 gives guidance to implementors on suitable      DSCPs.  Guidelines for mixing traffic types within a PCN-domain      are given in [RFC5670].   o  Any packet arriving at the PCN-ingress-node that shares a PCN-      compatible DSCP and is not a PCN-packet MUST be marked as not-PCN      within the PCN-domain.   o  If a packet arrives at the PCN-ingress-node with its ECN field      already set to a value other than not-ECT, then appropriate action      MUST be taken to meet the requirements of [RFC3168].  The simplest      appropriate action is to just drop such packets.  However, this is      a drastic action that an operator may feel is undesirable.Appendix B provides more information and summarises other      alternative actions that might be taken.4.1.  Marking Packets   [RFC5670] states that any encoding scheme document must specify the   required action to take if one of the marking algorithms indicates   that a packet needs to be marked.  For the baseline encoding scheme,   the required action is simply as follows:   o  If a marking algorithm indicates the need to mark a PCN-packet,      then that packet MUST have its PCN codepoint set to 11, PCN-      marked.Moncaster, et al.           Standards Track                     [Page 5]

RFC 5696                 Baseline PCN Encoding             November 20094.2.  Valid and Invalid Codepoint Transitions   A PCN-ingress-node MUST set the Not-marked (10) codepoint on any   arriving packet that belongs to a PCN-flow.  It MUST set the not-PCN   (00) codepoint on all other packets sharing a PCN-compatible Diffserv   codepoint.   The only valid codepoint transitions within a PCN-interior-node are   from NM to PM (which should occur if either meter indicates a need to   PCN-mark a packet [RFC5670]) and from EXP to PM.  PCN-nodes that only   implement the baseline encoding MUST be able to PCN-mark packets that   arrive with the EXP codepoint.  This should ease the design of   experimental schemes that want to allow partial deployment of   experimental nodes alongside nodes that only implement the baseline   encoding.  The following table gives the full set of valid and   invalid codepoint transitions.                    +-------------------------------------------------+                    |                  Codepoint Out                  |     +--------------+-------------+-----------+-----------+-----------+     | Codepoint in | not-PCN(00) |   NM(10)  |  EXP(01)  |   PM(11)  |     +--------------+-------------+-----------+-----------+-----------+     |  not-PCN(00) |    Valid    | Not valid | Not valid | Not valid |     +--------------+-------------+-----------+-----------+-----------+     |       NM(10) |  Not valid  |   Valid   | Not valid |   Valid   |     +--------------+-------------+-----------+-----------+-----------+     |     EXP(01)* |  Not valid  | Not valid |   Valid   |   Valid   |     +--------------+-------------+-----------+-----------+-----------+     |       PM(11) |  Not valid  | Not valid | Not valid |   Valid   |     +--------------+-------------+-----------+-----------+-----------+        * This MAY cause an alarm to be raised at a management layer.          See paragraph above for an explanation of this transition.          Table 2: Valid and Invalid Codepoint Transitions for                       PCN-Packets at PCN-Interior-Nodes   The codepoint transition constraints given here apply only to the   baseline encoding scheme.  Constraints on codepoint transitions for   future experimental schemes are discussed inSection 5.   A PCN-egress-node SHOULD set the not-PCN (00) codepoint on all   packets it forwards out of the PCN-domain.  The only exception to   this is if the PCN-egress-node is certain that revealing other   codepoints outside the PCN-domain won't contravene the guidance given   in [RFC4774].  For instance, if the PCN-ingress-node has explicitly   informed the PCN-egress-node that this flow is ECN-capable, then it   might be safe to expose other codepoints.Moncaster, et al.           Standards Track                     [Page 6]

RFC 5696                 Baseline PCN Encoding             November 20094.3.  Rationale for Encoding   The exact choice of encoding was dictated by the constraints imposed   by existing IETF RFCs, in particular [RFC3168], [RFC4301], and   [RFC4774].  One of the tightest constraints was the need for any PCN   encoding to survive being tunnelled through either an IP-in-IP tunnel   or an IPsec Tunnel.  [ECN-TUN] explains this in more detail.  The   main effect of this constraint is that any PCN-marking has to carry   the 11 codepoint in the ECN field since this is the only codepoint   that is guaranteed to be copied down into the forwarded header upon   decapsulation.  An additional constraint is the need to minimise the   use of Diffserv codepoints because there is a limited supply of   Standards Track codepoints remaining.Section 4.4 explains how we   have minimised this still further by reusing pre-existing Diffserv   codepoint(s) such that non-PCN-traffic can still be distinguished   from PCN-traffic.   There are a number of factors that were considered before choosing to   set 10 as the NM state instead of 01.  These included similarity to   ECN, presence of tunnels within the domain, leakage into and out of   the PCN-domain, and incremental deployment (seeAppendix A.2).   The encoding scheme above seems to meet all these constraints and   ends up looking very similar to ECN.  This is perhaps not surprising   given the similarity in architectural intent between PCN and ECN.4.4.  PCN-Compatible Diffserv Codepoints   Equipment complying with the baseline PCN encoding MUST allow PCN to   be enabled for certain Diffserv codepoints.  This document defines   the term "PCN-compatible Diffserv codepoint" for such a DSCP.  To be   clear, any packets with such a DSCP will be PCN-enabled only if they   are within a PCN-domain and have their ECN field set to indicate a   codepoint other than not-PCN.   Enabling PCN-marking behaviour for a specific DSCP disables any other   marking behaviour (e.g., enabling PCN replaces the default ECN   marking behaviour introduced in [RFC3168]) with the PCN-metering and   -marking behaviours described in [RFC5670]).  This ensures compliance   with the Best Current Practice (BCP) guidance set out in [RFC4774].   The PCN working group has chosen not to define a single DSCP for use   with PCN for several reasons.  Firstly, the PCN mechanism is   applicable to a variety of different traffic classes.  Secondly,   Standards Track DSCPs are in increasingly short supply.  Thirdly, PCN   is not a scheduling behaviour -- rather, it should be seen as beingMoncaster, et al.           Standards Track                     [Page 7]

RFC 5696                 Baseline PCN Encoding             November 2009   essentially a marking behaviour similar to ECN but intended for   inelastic traffic.  More details are given in the informationalAppendix A.1.4.4.1.  Co-Existence of PCN and Not-PCN Traffic   The scarcity of pool 1 DSCPs, coupled with the fact that PCN is   envisaged as a marking behaviour that could be applied to a number of   different DSCPs, makes it essential that we provide a not-PCN state.   As stated above (and expanded inAppendix A.1), the aim is for PCN to   re-use existing DSCPs.  Because PCN redefines the meaning of the ECN   field for such DSCPs, it is important to allow an operator to still   use the DSCP for non-PCN-traffic.  This is achieved by providing a   not-PCN state within the encoding scheme.Section 3.5 of [RFC5559]   discusses how competing-non-PCN-traffic should be handled.5.  Rules for Experimental Encoding Schemes   Any experimental encoding scheme MUST follow these rules to ensure   backward compatibility with this baseline scheme:   o  All PCN-interior-nodes within a PCN-domain MUST interpret the 00      codepoint in the ECN field as not-PCN and MUST NOT change it to      another value.  Therefore, a PCN-ingress-node wishing to disable      PCN-marking for a packet with a PCN-compatible Diffserv codepoint      MUST set the ECN field to 00.   o  The 11 codepoint in the ECN field MUST indicate that the packet      has been PCN-marked as the result of one or both of the meters      indicating a need to PCN-mark a packet [RFC5670].  The      experimental scheme MUST define which meter(s) trigger this      marking.   o  The 01 Experimental codepoint in the ECN field MAY mean PCN-marked      or it MAY carry some other meaning.  However, any experimental      scheme MUST define its meaning in the context of that experiment.   o  If both the 01 and 11 codepoints are being used to indicate PCN-      marked, then the 11 codepoint MUST be taken to be the more severe      marking and the choice of which meter sets which mark MUST be      defined.   o  Once set, the 11 codepoint in the ECN field MUST NOT be changed to      any other codepoint.   o  Any experimental scheme MUST include details of all valid and      invalid codepoint transitions at any PCN-nodes.Moncaster, et al.           Standards Track                     [Page 8]

RFC 5696                 Baseline PCN Encoding             November 20096.  Backward CompatibilityBCP 124 [RFC4774] gives guidelines for specifying alternative   semantics for the ECN field.  It sets out a number of factors to be   taken into consideration.  It also suggests various techniques to   allow the co-existence of default ECN and alternative ECN semantics.   The baseline encoding specified in this document defines PCN-   compatible Diffserv codepoints as no longer supporting the default   ECN semantics.  As such, this document is compatible withBCP 124.   On its own, this baseline encoding cannot support both ECN marking   end-to-end (e2e) and PCN-marking within a PCN-domain.  It is possible   to do this by carrying e2e ECN across a PCN-domain within the inner   header of an IP-in-IP tunnel, or by using a richer encoding such as   the proposed experimental scheme in [PCN-ENC].   In any PCN deployment, traffic can only enter the PCN-domain through   PCN-ingress-nodes and leave through PCN-egress-nodes.  PCN-ingress-   nodes ensure that any packets entering the PCN-domain have the ECN   field in their outermost IP header set to the appropriate PCN   codepoint.  PCN-egress-nodes then guarantee that the ECN field of any   packet leaving the PCN-domain has the correct ECN semantics.  This   prevents unintended leakage of ECN marks into or out of the PCN-   domain, and thus reduces backward-compatibility issues.7.  Security Considerations   PCN-marking only carries a meaning within the confines of a PCN-   domain.  This encoding document is intended to stand independently of   the architecture used to determine how specific packets are   authorised to be PCN-marked, which will be described in separate   documents on PCN-boundary-node behaviour.   This document assumes the PCN-domain to be entirely under the control   of a single operator, or a set of operators who trust each other.   However, future extensions to PCN might include inter-domain versions   where trust cannot be assumed between domains.  If such schemes are   proposed, they must ensure that they can operate securely despite the   lack of trust.  However, such considerations are beyond the scope of   this document.   One potential security concern is the injection of spurious PCN-marks   into the PCN-domain.  However, these can only enter the domain if a   PCN-ingress-node is misconfigured.  The precise impact of any such   misconfiguration will depend on which of the proposed PCN-boundary-   node behaviour schemes is used, but in general spurious marks will   lead to admitting fewer flows into the domain or potentially   terminating too many flows.  In either case, good management shouldMoncaster, et al.           Standards Track                     [Page 9]

RFC 5696                 Baseline PCN Encoding             November 2009   be able to quickly spot the problem since the overall utilisation of   the domain will rapidly fall.8.  Conclusions   This document defines the baseline PCN encoding, utilising a   combination of a PCN-compatible DSCP and the ECN field in the IP   header.  This baseline encoding allows the existence of two PCN   encoding states: Not-marked and PCN-marked.  It also allows for the   co-existence of competing traffic within the same DSCP, so long as   that traffic does not require ECN support within the PCN-domain.  The   encoding scheme is conformant with [RFC4774].  The working group has   chosen not to define a single DSCP for use with PCN.  The rationale   for this decision along with advice relating to the choice of   suitable DSCPs can be found inAppendix A.1.9.  Acknowledgements   This document builds extensively on work done in the PCN working   group by Kwok Ho Chan, Georgios Karagiannis, Philip Eardley, Anna   Charny, Joe Babiarz, and others.  Thanks to Ruediger Geib and Gorry   Fairhurst for providing detailed comments on this document.10.  References10.1.  Normative References   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate              Requirement Levels",BCP 14,RFC 2119, March 1997.   [RFC3168]  Ramakrishnan, K., Floyd, S., and D. Black, "The Addition              of Explicit Congestion Notification (ECN) to IP",RFC 3168, September 2001.   [RFC4774]  Floyd, S., "Specifying Alternate Semantics for the              Explicit Congestion Notification (ECN) Field",BCP 124,RFC 4774, November 2006.   [RFC5670]  Eardley, P., Ed., "Metering and Marking Behaviour of PCN-              Nodes",RFC 5670, November 2009.Moncaster, et al.           Standards Track                    [Page 10]

RFC 5696                 Baseline PCN Encoding             November 200910.2.  Informative References   [ECN-TUN]  Briscoe, B., "Tunnelling of Explicit Congestion              Notification", Work in Progress, July 2009.   [PCN-ENC]  Moncaster, T., Briscoe, B., and M. Menth, "A PCN encoding              using 2 DSCPs to provide 3 or more states", Work              in Progress, April 2009.   [RFC2474]  Nichols, K., Blake, S., Baker, F., and D. Black,              "Definition of the Differentiated Services Field (DS              Field) in the IPv4 and IPv6 Headers",RFC 2474,              December 1998.   [RFC2597]  Heinanen, J., Baker, F., Weiss, W., and J. Wroclawski,              "Assured Forwarding PHB Group",RFC 2597, June 1999.   [RFC3246]  Davie, B., Charny, A., Bennet, J., Benson, K., Le Boudec,              J., Courtney, W., Davari, S., Firoiu, V., and D.              Stiliadis, "An Expedited Forwarding PHB (Per-Hop              Behavior)",RFC 3246, March 2002.   [RFC3540]  Spring, N., Wetherall, D., and D. Ely, "Robust Explicit              Congestion Notification (ECN) Signaling with Nonces",RFC 3540, June 2003.   [RFC4301]  Kent, S. and K. Seo, "Security Architecture for the              Internet Protocol",RFC 4301, December 2005.   [RFC4594]  Babiarz, J., Chan, K., and F. Baker, "Configuration              Guidelines for DiffServ Service Classes",RFC 4594,              August 2006.   [RFC5127]  Chan, K., Babiarz, J., and F. Baker, "Aggregation of              DiffServ Service Classes",RFC 5127, February 2008.   [RFC5559]  Eardley, P., "Pre-Congestion Notification (PCN)              Architecture",RFC 5559, June 2009.Moncaster, et al.           Standards Track                    [Page 11]

RFC 5696                 Baseline PCN Encoding             November 2009Appendix A.  PCN Deployment Considerations (Informative)A.1.  Choice of Suitable DSCPs   The PCN working group chose not to define a single DSCP for use with   PCN for several reasons.  Firstly, the PCN mechanism is applicable to   a variety of different traffic classes.  Secondly, Standards Track   DSCPs are in increasingly short supply.  Thirdly, PCN is not a   scheduling behaviour -- rather, it should be seen as being a marking   behaviour similar to ECN but intended for inelastic traffic.  The   choice of which DSCP is most suitable for a given PCN-domain is   dependent on the nature of the traffic entering that domain and the   link rates of all the links making up that domain.  In PCN-domains   with sufficient aggregation, the appropriate DSCPs would currently be   those for the Real-Time Treatment Aggregate [RFC5127].  The PCN   working group suggests using admission control for the following   service classes (defined in [RFC4594]):   o  Telephony (EF)   o  Real-time interactive (CS4)   o  Broadcast Video (CS3)   o  Multimedia Conferencing (AF4)   CS5 is excluded from this list since PCN is not expected to be   applied to signalling traffic.   PCN-marking is intended to provide a scalable admission-control   mechanism for traffic with a high degree of statistical multiplexing.   PCN-marking would therefore be appropriate to apply to traffic in the   above classes, but only within a PCN-domain containing sufficiently   aggregated traffic.  In such cases, the above service classes may   well all be subject to a single forwarding treatment (treatment   aggregate [RFC5127]).  However, this does not imply all such IP   traffic would necessarily be identified by one DSCP -- each service   class might keep a distinct DSCP within the highly aggregated region   [RFC5127].   Additional service classes may be defined for which admission control   is appropriate, whether through some future standards action or   through local use by certain operators, e.g., the Multimedia   Streaming service class (AF3).  This document does not preclude the   use of PCN in more cases than those listed above.   Note: The above discussion is informative not normative, as operators   are ultimately free to decide whether to use admission control forMoncaster, et al.           Standards Track                    [Page 12]

RFC 5696                 Baseline PCN Encoding             November 2009   certain service classes and whether to use PCN as their mechanism of   choice.A.2.  Rationale for Using ECT(0) for Not-Marked   The choice of which ECT codepoint to use for the Not-marked state was   based on the following considerations:   o  [RFC3168] full-functionality tunnel within the PCN-domain: Either      ECT is safe.   o  Leakage of traffic into PCN-domain: Because of the lack of take-up      of the ECN nonce [RFC3540], leakage of ECT(1) is less likely to      occur and so might be considered safer.Moncaster, et al.           Standards Track                    [Page 13]

RFC 5696                 Baseline PCN Encoding             November 2009   o  Leakage of traffic out of PCN-domain: Either ECT is equally unsafe      (since this would incorrectly indicate the traffic was ECN-capable      outside the controlled PCN-domain).   o  Incremental deployment: Either codepoint is suitable, providing      that the codepoints are used consistently.   o  Conceptual consistency with other schemes: ECT(0) is conceptually      consistent with [RFC3168].   Overall, this seemed to suggest that ECT(0) was most appropriate to   use.Appendix B.  Co-Existence of PCN and ECN (Informative)   This baseline encoding scheme redefines the ECN codepoints within the   PCN-domain.  As packets with a PCN-compatible DSCP leave the PCN-   domain, their ECN field is reset to not-ECT (00).  This is a problem   for the operator if packets with a PCN-compatible DSCP arrive at the   PCN-domain with any ECN codepoint other than not-ECN.  If the ECN-   codepoint is ECT(0) (10) or ECT(1) (01), resetting the ECN field to   00 effectively turns off end-to-end ECN.  This is undesirable as it   removes the benefits of ECN, but [RFC3168] states that it is no worse   than dropping the packet.  However, if a packet was marked with CE   (11), resetting the ECN field to 00 at the PCN egress node violates   the rule that CE-marks must never be lost except as a result of   packet drop [RFC3168].   A number of options exist to overcome this issue.  The most   appropriate option will depend on the circumstances and has to be a   decision for the operator.  The definition of the action is beyond   the scope of this document, but we briefly explain the four broad   categories of solution below: tunnelling the packets, using an   extended encoding scheme, signalling to the end systems to stop using   ECN, or re-marking packets to a different DSCP.   o  Tunnelling the packets across the PCN-domain (for instance, in an      IP-in-IP tunnel from the PCN-ingress-node to the PCN-egress-node)      preserves the original ECN marking on the inner header.   o  An extended encoding scheme can be designed that preserves the      original ECN codepoints.  For instance, if the PCN-egress-node can      determine from the PCN codepoint what the original ECN codepoint      was, then it can reset the packet to that codepoint.  [PCN-ENC]      partially achieves this but is unable to recover ECN markings if      the packet is PCN-marked in the PCN-domain.Moncaster, et al.           Standards Track                    [Page 14]

RFC 5696                 Baseline PCN Encoding             November 2009   o  Explicit signalling to the end systems can indicate to the source      that ECN cannot be used on this path (because it does not support      ECN and PCN at the same time).  Dropping the packet can be thought      of as a form of silent signal to the source, as it will see any      ECT-marked packets it sends being dropped.   o  Packets that are not part of a PCN-flow but which share a PCN-      compatible DSCP can be re-marked to a different local-use DSCP at      the PCN-ingress-node with the original DSCP restored at the PCN-      egress.  This preserves the ECN codepoint on these packets but      relies on there being spare local-use DSCPs within the PCN-domain.Authors' Addresses   Toby Moncaster   BT   B54/70, Adastral Park   Martlesham Heath   Ipswich  IP5 3RE   UK   Phone: +44 7918 901170   EMail: toby.moncaster@bt.com   Bob Briscoe   BT   B54/77, Adastral Park   Martlesham Heath   Ipswich  IP5 3RE   UK   Phone: +44 1473 645196   EMail: bob.briscoe@bt.com   Michael Menth   University of Wuerzburg   Institute of Computer Science   Am Hubland   Wuerzburg  D-97074   Germany   Phone: +49 931 318 6644   EMail: menth@informatik.uni-wuerzburg.deMoncaster, et al.           Standards Track                    [Page 15]