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rfc:rfc5696

Network Working Group T. Moncaster Request for Comments: 5696 B. Briscoe Category: Standards Track BT

                                                              M. Menth
                                               University of Wuerzburg
                                                         November 2009
   Baseline Encoding and Transport of Pre-Congestion Information

Abstract

 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 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
 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 Contents

 1.  Introduction . . . . . . . . . . . . . . . . . . . . . . . . .  3
 2.  Requirements Notation  . . . . . . . . . . . . . . . . . . . .  3
 3.  Terminology and Abbreviations  . . . . . . . . . . . . . . . .  3
   3.1.  Terminology  . . . . . . . . . . . . . . . . . . . . . . .  3
   3.2.  List of Abbreviations  . . . . . . . . . . . . . . . . . .  4
 4.  Encoding Two PCN States in IP  . . . . . . . . . . . . . . . .  4
   4.1.  Marking Packets  . . . . . . . . . . . . . . . . . . . . .  5
   4.2.  Valid and Invalid Codepoint Transitions  . . . . . . . . .  6
   4.3.  Rationale for Encoding . . . . . . . . . . . . . . . . . .  7
   4.4.  PCN-Compatible Diffserv Codepoints . . . . . . . . . . . .  7
     4.4.1.  Co-Existence of PCN and Not-PCN Traffic  . . . . . . .  8
 5.  Rules for Experimental Encoding Schemes  . . . . . . . . . . .  8
 6.  Backward Compatibility . . . . . . . . . . . . . . . . . . . .  9
 7.  Security Considerations  . . . . . . . . . . . . . . . . . . .  9
 8.  Conclusions  . . . . . . . . . . . . . . . . . . . . . . . . . 10
 9.  Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 10
 10. References . . . . . . . . . . . . . . . . . . . . . . . . . . 10
   10.1. Normative References . . . . . . . . . . . . . . . . . . . 10
   10.2. Informative References . . . . . . . . . . . . . . . . . . 10
 Appendix A.  PCN Deployment Considerations (Informative) . . . . . 11
   A.1.  Choice of Suitable DSCPs . . . . . . . . . . . . . . . . . 11
   A.2.  Rationale for Using ECT(0) for Not-Marked  . . . . . . . . 12
 Appendix B.  Co-Existence of PCN and ECN (Informative) . . . . . . 13

Moncaster, et al. Standards Track [Page 2] RFC 5696 Baseline PCN Encoding November 2009

1. 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 in Section 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 Abbreviations

3.1. 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-marked

4. 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
 (see Section 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 2009

4.2. 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 in Section 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 2009

4.3. 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 (see Appendix 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 being

Moncaster, 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 informational
 Appendix 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 in Appendix 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 2009

6. Backward Compatibility

 BCP 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 with BCP 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 should

Moncaster, 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 in Appendix 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. References

10.1. 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 2009

10.2. 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 2009

Appendix 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 for

Moncaster, 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.de

Moncaster, et al. Standards Track [Page 15]

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