Congestion and Pre Congestion                               T. Moncaster
Internet-Draft                                                        BT
Intended status: Standards Track                              B. Briscoe
Expires: August 14, October 9, 2009                                        BT & UCL
                                                                M. Menth
                                                 University of Wuerzburg
                                                       February 10,
                                                           April 7, 2009

     Baseline Encoding and Transport of Pre-Congestion Information
                  draft-ietf-pcn-baseline-encoding-02
                  draft-ietf-pcn-baseline-encoding-03

Status of This Memo

   This Internet-Draft is submitted to IETF in full conformance with the
   provisions of BCP 78 and BCP 79.  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.

   Internet-Drafts are working documents of the Internet Engineering
   Task Force (IETF), its areas, and its working groups.  Note that
   other groups may also distribute working documents as Internet-
   Drafts.

   Internet-Drafts are draft documents valid for a maximum of six months
   and may be updated, replaced, or obsoleted by other documents at any
   time.  It is inappropriate to use Internet-Drafts as reference
   material or to cite them other than as "work in progress."

   The list of current Internet-Drafts can be accessed at
   http://www.ietf.org/ietf/1id-abstracts.txt.

   The list of Internet-Draft Shadow Directories can be accessed at
   http://www.ietf.org/shadow.html.

   This Internet-Draft will expire on August 14, October 9, 2009.

Copyright Notice

   Copyright (c) 2009 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

   This document is subject to BCP 78 and the IETF Trust's Legal
   Provisions Relating to IETF Documents
   (http://trustee.ietf.org/license-info) in effect on the date of
   publication of this document. document (http://trustee.ietf.org/license-info).
   Please review these documents carefully, as they describe your rights
   and restrictions with respect to this document.

Abstract

   Pre-congestion notification

   The objective of Pre-Congestion Notification (PCN) provides information to support
   admission control and flow termination in order is to protect the
   Quality
   quality of Service service (QoS) of inelastic flows.  It does this by marking
   packets when traffic load on flows within a link Diffserv domain.
   The overall rate of the PCN-traffic is approaching metered on every link in the
   PCN-domain, and PCN-packets are appropriately marked when certain
   configured rates are exceeded.  The level of marking allows the
   boundary nodes to make decisions about whether t o admit or has exceeded block a
   threshold below
   new flow request, and (in abnormal circumstances) whether to
   terminate some of the physical link rate. existing flows, thereby protecting the QoS of
   previously admitted flows.  This document specifies how such marks
   are to be encoded into the IP header. header by re-using the ECN codepoints
   within this controlled domain.  The baseline encoding described here
   provides for only two PCN encoding states.
   It is designed to be easily extended to provide more encoding states
   but such schemes will be described in other documents. states, unmarked and marked.

Table of Contents

   1.  Introduction . . . . . . . . . . . . . . . . . . . . . . . . .  3
   2.  Requirements notation  . . . . . . . . . . . . . . . . . . . .  4  5
   3.  Terminology  . . . . . . . . . . . . . . . . . . . . . . . . .  4  5
   4.  Encoding two PCN States in IP  . . . . . . . . . . . . . . . .  5
     4.1.  Valid and Invalid Codepoint Transitions  . . . . . . . . .  6
     4.2.  Rationale for Encoding . . . . . . . . . . . . . . . . . .  6
     4.2.  7
     4.3.  PCN-Compatible DiffServ Codepoints . . . . . . . . . . . .  7  8
   5.  Rules for Experimental Encoding Schemes  . . . . . . . . . . .  7  8
   6.  Backwards Compatibility  . . . . . . . . . . . . . . . . . . .  8
   7.  IANA Considerations  . . . . . . . . . . . . . . . . . . . . .  8  9
   8.  Security Considerations  . . . . . . . . . . . . . . . . . . .  8  9
   9.  Conclusions  . . . . . . . . . . . . . . . . . . . . . . . . .  8  9
   10. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . .  9
   11. Comments Solicited . . . . . . . . . . . . . . . . . . . . . .  9 10
   12. References . . . . . . . . . . . . . . . . . . . . . . . . . .  9 10
     12.1. Normative References . . . . . . . . . . . . . . . . . . .  9 10
     12.2. Informative References . . . . . . . . . . . . . . . . . .  9 10
   Appendix A.  PCN Deployment Considerations . . . . . . . . . . . . 10 11
     A.1.  Choice of Suitable DSCPs . . . . . . . . . . . . . . . . . 10 11
     A.2.  Rationale for Using ECT(0) for Not Marked  . . . . . . . . 10 11

1.  Introduction

   Pre-congestion notification

   The objective of Pre-Congestion Notification (PCN) provides information to support
   admission control and flow termination in order is to protect the
   quality of service (QoS) of inelastic flows.  This is achieved by
   marking packets according to the level of pre-congestion at nodes flows within a PCN-domain. Diffserv domain,
   in a simple, scalable and robust fashion.  The overall rate of the
   PCN-traffic is metered on every link in the PCN-domain, and PCN-
   packets are appropriately marked when certain configured rates are
   exceeded.  These markings configured rates are evaluated by below the egress
   nodes rate of the PCN-domain. [pcn-arch] describes how PCN packet markings
   can be used 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 assure terminate some of
   the existing flows, thereby protecting the QoS of inelastic flows within a single
   DiffServ domain. previously admitted
   flows.

   This document specifies how these PCN marks are encoded into the IP
   header.
   header by re-using the bits of the ECN field.  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.  An extension of  However the baseline
   encoding described in [PCN-3-enc-state] provides for three PCN encoding states.  Other extensions have also been suggested all of
   which can build on the baseline encoding.  In order be easily extended to ensure
   backward compatibility any alternative encoding schemes that claim
   compliance with PCN standards MUST extend provide more
   states and rules for such extensions are given in this baseline scheme. document.

   Changes from previous drafts (to be removed by the RFC Editor):

   From -02 to -03:

      Extensive changes to address comments made by Gorry Fairhurst
      including:

      *  Abstract re-written.

      *  Clarified throughout that this re-uses the ECN bits in the IP
         header.

      *  Re-arranged order of terminology section for clarity.

      *  Table 2 replaced with new table and text.

      *  Security considerations re-written.

      *  Appendixes re-written to improve clarity.

      *  Numerous minor nits and language changes throughout.

      Extensive other minor changes throughout.

   From -01 to -02:

      Removed Appendix A and replaced with reference to [ecn-tunneling]
      [I-D.ietf-tsvwg-ecn-tunnel]

      Moved Appendix B into main body of text.

      Changed Appendix C to give deployment advice.

      Minor changes throughout including checking consistency of
      capitalisation of defined terms.

      Clarified that LU was deliberately excluded from encoding.

   From -00 to -01:

      Added section on restrictions for extension encoding schemes.

      Included table in Appendix showing encoding transitions at
      different PCN nodes.

      Checked for consistency of terminology.

      Minor language changes for clarity.

   Changes from previous filename

      Filename changed from draft-moncaster-pcn-baseline-encoding.

      Terminology changed for clarity (PCN-compatible DSCP and PCN-
      enabled packet).

      Minor changes throughout.

      Modified meaning of ECT(1) state to EXP.

      Moved text relevant to behaviour of nodes into appendix for later
      transfer to new document on edge behaviours.

   From draft-moncaster -01 to -02:

      Minor changes throughout including tightening up language to
      remain consistent with the PCN Architecture terminology
   From draft-moncaster -00 to -01:

      Change of title from "Encoding and Transport of (Pre-)Congestion
      Information from within a DiffServ Domain to the Egress"

      Extensive changes to Introduction and abstract.

      Added a section on the implications of re-using a DSCP.

      Added appendix listing possible operator scenarios for using this
      baseline encoding.

      Minor changes throughout.

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

   The following terms are used in this document:

   o  Not-PCN  PCN-compatible Diffserv codepoint - packets that are not PCN-enabled. a Diffserv codepoint for which
      the ECN field is used to carry PCN markings rather than [RFC3168]
      markings.

   o  PCN-marked - codepoint indicating packets that have been marked at
      a PCN-interior-node using some PCN marking behaviour
      [pcn-marking-behaviour].  Also
      [I-D.ietf-pcn-marking-behaviour].  Abbreviated to PM.

   o  Not-marked - codepoint indicating packets that are PCN-capable PCN-capable,
      but are not PCN-marked.  Also  Abbreviated to NM.

   o  PCN-enabled codepoints - collective term for all the NM and PM
      codepoints.  By definition definition, packets carrying such codepoints are
      PCN-packets.

   o  PCN-compatible Diffserv codepoint  not-PCN - a Diffserv codepoint for which
      the ECN field is used to carry PCN markings rather than [RFC3168]
      markings. packets that are not PCN-enabled.

   In addition addition, the document uses the terminology defined in [pcn-arch].
   [I-D.ietf-pcn-architecture].

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 Not-marked and PCN-Marked. PCN-marked.  It also
   allows for traffic that is not PCN capable PCN-capable to be marked as such (Not- (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:

   +---------------+-------------+-------------+-------------+---------+
   | ECN codepoint |   Not-ECT   | ECT(0) (10) | ECT(1) (01) | CE (11) |
   |               |     (00)    |             |             |         |
   +---------------+-------------+-------------+-------------+---------+
   |     DSCP n    |   Not-PCN   not-PCN   |      NM     |     EXP     |    PM   |
   +---------------+-------------+-------------+-------------+---------+

   Where DSCP n is a PCN-compatible DiffServ codepoint (see Section 4.2) 4.3)
    and EXP means available for Experimental use.  N.B. we deliberately
   reserve this codepoint for experimental use only (and not local use)
                  to prevent any possible future compatability issues.

                        Table 1: Encoding PCN in IP

   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, such as the Voice-Admit codepoint defined in
      [voice-admit].
      [I-D.ietf-tsvwg-admitted-realtime-dscp].  Guidelines for mixing
      traffic-types within a PCN-
      domain PCN-domain are given in [pcn-marking-behaviour].
      [I-D.ietf-pcn-marking-behaviour].

   o  Any packet that is not PCN-enabled (Not-PCN) not-PCN but which shares the same DiffServ
      codepoint as PCN-enabled traffic MUST have the ECN field equal to
      00.

   The following table sets out the valid

4.1.  Valid and invalid codepoint
   transitions at PCN-nodes for this baseline encoding.  Extension
   encodings may have different rules regarding the validity of Invalid Codepoint Transitions

   A PCN-ingress-node MUST set the
   transitions.  Note Not-marked (10) codepoint on any
   arriving packet that this table assumes there is a functional
   separation between a PCN-boundary-node and belongs to a PCN-flow.  It MUST set the not-PCN
   (00) codepoint on all other packets.

   A PCN-interior-node such
   that PCN-boundary-nodes do not perform packet metering or marking
   functions.  PCN-nodes MUST follow observe the encoding transition rules for valid and invalid
   codepoint transitions as set out in this table (e.g. they MUST NOT set invalid codepoints on
   packets they forward).  This table only applies the following table.  The precise
   rules governing which valid transition to PCN-packets.

   +-----------+-------------+-----------------+-----------------------+
   | PCN node  |  Codepoint  | Valid codepoint | Invalid codepoint use are set out in
   [I-D.ietf-pcn-marking-behaviour]
                  +-------------------------------------------------+
                  |                  Codepoint Out                  |   type
   +--------------+-------------+-----------+-----------+-----------+
   | Codepoint in |        out not-PCN(00) |   NM(10)  |
   +-----------+-------------+-----------------+-----------------------+  EXP(01)  |  ingress   PM(11)  |     Any     | NM (or Not-PCN) |          PM
   +--------------+-------------+-----------+-----------+-----------+
   |  not-PCN(00) | interior    Valid    |     NM Not valid |     NM or PM Not valid |     Not-PCN or EXP Not valid |
   +--------------+-------------+-----------+-----------+-----------+
   | interior       NM(10) |     EXP +  Not valid  |     EXP or PM   Valid   |        Not-PCN Not valid |   Valid   | interior
   +--------------+-------------+-----------+-----------+-----------+
   |   Not-PCN     EXP(01)* |      Not-PCN  Not valid  |  Any other codepoint Not valid |   Valid   | interior   Valid   |     PM
   +--------------+-------------+-----------+-----------+-----------+
   |        PM       |  Any other codepoint       PM(11) |  Not valid  |  egress Not valid |     Any Not valid |        00   Valid   | Any other codepoint
   +--------------+-------------+-----------+-----------+-----------+
   * |
   +-----------+-------------+-----------------+-----------------------+
    + This SHOULD cause an alarm to be raised at a higher layer. The
       packet MUST be treated as if it were NM.
    * Except where the egress node knows that other marks may be safely
      exposed outside carried the PCN-domain (e.g. [PCN-3-enc-state]). NM codepoint.

            Table 2: Valid and Invalid Codepoint Transitions for
                      PCN-packets at PCN-nodes

   If a pcn-interior-node compliant with PCN-interior-nodes

   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 baseline encoding receives
   a

4.1. is if the PCN-egress-node is certain that revealing other
   codepoints outside the PCN-domain won't contravene the guidance given
   in [RFC4774].

4.2.  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-tunneling]  [I-D.ietf-tsvwg-ecn-tunnel] 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.  If the packet field since this is being
   tunneled then only
   the 11 only codepoint gets that is guaranteeed to be copied down into the
   inner header upon decapsulation.  An additional constraint is the
   need to minimise the use of DiffServ codepoints as there is a limited
   supply of standards track codepoints remaining.  Section 4.2 4.3 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 deciding to set 10 as the NM state.  These
   included similarity to ECN, presence of tunnels within the domain,
   leakage into and out of PCN-domain and incremental deployment. 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.

4.2.

4.3.  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.
   Enabling PCN for  To be
   clear, any packets with such a DSCP switches on will be PCN marking behaviour for packets
   with that DSCP, but enabled only if those packets they
   also have their ECN field set to indicate a codepoint other than Not-PCN. not-
   PCN.

   Enabling PCN marking behaviour disables any other marking behaviour
   (e.g. enabling PCN disables the default ECN marking behaviour
   introduced in [RFC3168]).  All traffic scheduling and conditioning
   behaviours are discussed in [pcn-marking-behaviour]. [I-D.ietf-pcn-marking-behaviour].  This
   ensures compliance with the BCP guidance set out in [RFC4774].

5.  Rules for Experimental Encoding Schemes

   Any experimental encoding scheme MUST follow these rules to ensure
   backward compatibility with this baseline scheme:

   o  The 00 codepoint in the ECN field SHALL indicate not-PCN and MUST mean Not-PCN.
      NOT be changed to any otehr codepoint within a PCN-domain.
      Therefore an ingress node wishing to disable PCN marking for a
      packet within a PCN-compatible DiffServ Codepoint MUST set the ECN
      field to 00.

   o  The 11 codepoint in the ECN field MUST mean SHALL indicate PCN-marked
      (though this doesn't does not exclude other codepoints the 01 Experimental codepoint from
      carrying the same meaning).

   o  Once set 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.

   o  Any experimental scheme MUST NOT update the meaning of the 00 and
      11 codepoints defined above.

6.  Backwards 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.  It
   should be noted that this baseline encoding effectively disables end-
   to-end ECN except where mechanisms are put in place to tunnel such
   traffic across the PCN-domain.

7.  IANA Considerations

   This document makes no request to IANA.

8.  Security Considerations

   PCN-marking only carries a meaning within the confines of a PCN-
   domain.  Packets claim entitlement wishing to be PCN marked by carrying treated as belonging to a PCN-
   Compatible PCN-flow
   must carry a PCN-Compatible DSCP and a PCN-Enabled ECN codepoint.
   This encoding document is intended to stand independently of the
   architecture used to determine whether specific packets are
   authorised to be PCN
   marked, PCN-marked, which will be described in a future separate document
   documents on PCN edge-node behaviour.

   The PCN working group has initially been chartered to only consider a

   This document assumes the PCN-domain to be entirely under the control
   of one a single operator, or a set of operators who trust each other [PCN-charter]. other.
   However there is a
   requirement to keep inter-domain scenarios in mind when defining the
   PCN encoding.  One way to extend to multiple domains would be to
   concatenate PCN-domains and use PCN-boundary-nodes back to back at
   borders.  Then any one domain's security against its neighbours would
   be described as part of the proposed edge-node behaviour document.

   One proposal on the table allows one future extensions to extend PCN across multiple
   domains without PCN-boundary-nodes back-to-back at borders [re-PCN].
   It is believed 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 encoding described here would be compatible
   with
   lack of trust but such considerations are beyond the security framework described there. scope of this
   document.

9.  Conclusions

   This document defines the baseline PCN encoding utilising a
   combination of a PCN-enabled 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. PCN-marked.  It also allows for the co-
   existence of competing traffic within the same DSCP so long as that
   traffic doesn't does not require end-to-end ECN support. support within the PCN-domain.  The
   encoding scheme is conformant with [RFC4774].

10.  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.

11.  Comments Solicited

   (To be removed by the RFC-Editor.)  Comments and questions are
   encouraged and very welcome.  They can be addressed to the IETF
   congestion and pre-congestion working group mailing list
   <pcn@ietf.org>, and/or to the authors.

12.  References

12.1.  Normative References

   [I-D.ietf-pcn-marking-behaviour]         Eardley, P., "Marking
                                            behaviour of PCN-nodes", dra
                                            ft-ietf-pcn-marking-
                                            behaviour-02 (work in
                                            progress), March 2009.

   [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.

12.2.  Informative References

   [PCN-3-enc-state]        Moncaster, T., Briscoe, B.,

   [I-D.ietf-pcn-architecture]              Eardley, P., "Pre-Congestion
                                            Notification (PCN)
                                            Architecture", draft-ietf-
                                            pcn-architecture-10 (work in
                                            progress), March 2009.

   [I-D.ietf-tsvwg-admitted-realtime-dscp]  Baker, F., Polk, J., and M. Menth, "A
                            three state extended PCN encoding scheme",
                            draft-moncaster-pcn-3-state-encoding-00
                                            Dolly, "DSCP for Capacity-
                                            Admitted Traffic", draft-
                                            ietf-tsvwg-admitted-
                                            realtime-dscp-05 (work in
                                            progress), June November 2008.

   [PCN-charter]            IETF, "IETF Charter for Congestion and Pre-
                            Congestion Notification Working Group".

   [RFC3168]                Ramakrishnan, K., Floyd, S., and D. Black,
                            "The Addition

   [I-D.ietf-tsvwg-ecn-tunnel]              Briscoe, B., "Tunnelling of
                                            Explicit Congestion
                            Notification (ECN) to IP", RFC 3168,
                            September 2001.
                                            Notification", draft-ietf-
                                            tsvwg-ecn-tunnel-02 (work in
                                            progress), March 2009.

   [RFC4301]                                Kent, S. and K. Seo,
                                            "Security Architecture for
                                            the Internet Protocol",
                                            RFC 4301, December 2005.

   [RFC5127]                                Chan, K., Babiarz, J., and
                                            F. Baker, "Aggregation of
                                            DiffServ Service Classes",
                                            RFC 5127, February 2008.

   [ecn-tunneling]          Briscoe, B., "Layered Encapsulation of
                            Congestion Notification",
                            draft-ietf-tsvwg-ecn-tunnel-01 (work in
                            progress), October 2008.

   [pcn-arch]               Eardley, P., "Pre-Congestion Notification
                            (PCN) Architecture",
                            draft-ietf-pcn-architecture-07 (work in
                            progress), September 2008.

   [pcn-marking-behaviour]  Eardley, P., "Marking behaviour of PCN-
                            nodes", draft-ietf-pcn-marking-behaviour-01
                            (work in progress), October 2008.

   [re-PCN]                 Briscoe, B., "Emulating Border Flow Policing
                            using Re-ECN on Bulk Data",
                            draft-briscoe-re-pcn-border-cheat-00 (work
                            in progress), July 2007.

   [voice-admit]            Baker, F., Polk, J., and M. Dolly, "DSCP for
                            Capacity-Admitted Traffic",
                            draft-ietf-tsvwg-admitted-realtime-dscp-05
                            (work in progress), November 2008.

Appendix A.  PCN Deployment Considerations

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 should be seen
   as being essentially a marking behaviour similar to ECN but intended
   for inelastic traffic.  The choice of which DSCP is most suitable for the
   a given PCN-domain is dependant 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 uniformly high link rates, the
   appropriate DSCPs would currently be those for the Real Time Traffic
   Class [RFC5127].  If the PCN domain includes lower speed links it
   would also be appropriate to use the DSCPs of the other traffic
   classes that [voice-admit] [I-D.ietf-tsvwg-admitted-realtime-dscp] defines for use
   with admission control, such as the three video classes CS4, CS3 and
   AF4 and the Admitted Telephony Class.

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: ECT(1) is slightly less often correct. likely
      to occur so might be considered safer.

   o  Leakage of traffic out of PCN-domainL PCN-domain: Either ECT is equally unsafe
      (since this would incorrectly indicate the traffic was ECN capable ECN-capable
      outside the controlled PCN-domain).

   o  Incremental deployment: Either ECT codepoint is suitable as long as they 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 ECT(0) was most appropriate to use.

Authors' Addresses

   Toby Moncaster
   BT
   B54/70, Adastral Park
   Martlesham Heath
   Ipswich  IP5 3RE
   UK

   Phone: +44 1473 648734
   EMail: toby.moncaster@bt.com

   Bob Briscoe
   BT & UCL
   B54/77, Adastral Park
   Martlesham Heath
   Ipswich  IP5 3RE
   UK

   Phone: +44 1473 645196
   EMail: bob.briscoe@bt.com

   Michael Menth
   University of Wuerzburg
   room B206, Institute of Computer Science
   Am Hubland
   Wuerzburg  D-97074
   Germany

   Phone: +49 931 888 6644
   EMail: menth@informatik.uni-wuerzburg.de