Congestion and Pre Congestion                               T. Moncaster
Internet-Draft                                                        BT
Intended status: Standards Track                              B. Briscoe
Expires: April 17, August 14, 2009                                        BT & UCL
                                                                M. Menth
                                                 University of Wuerzburg
                                                        October 14, 2008
                                                       February 10, 2009

     Baseline Encoding and Transport of Pre-Congestion Information
                  draft-ietf-pcn-baseline-encoding-01
                  draft-ietf-pcn-baseline-encoding-02

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Abstract

   Pre-congestion notification (PCN) provides information to support
   admission control and flow termination in order to protect the
   Quality of Service of inelastic flows.  It does this by marking
   packets when traffic load on a link is approaching or has exceeded a
   threshold below the physical link rate.  This document specifies how
   such marks are to be encoded into the IP header.  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.

Table of Contents

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

1.  Introduction

   Pre-congestion notification (PCN) provides information to support
   admission control and flow termination in order 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
   within a PCN-domain.  These markings are evaluated by the egress
   nodes of the PCN-domain. [pcn-arch] describes how PCN packet markings
   can be used to assure the QoS of inelastic flows within a single
   DiffServ domain.

   This document specifies how these PCN marks are encoded into the IP
   header.  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 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 to ensure
   backward compatibility any alternative encoding schemes that claim
   compliance with PCN standards MUST extend this baseline scheme.

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

   From -01 to -02:

      Removed Appendix A and replaced with reference to [ecn-tunneling]

      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 - packets that are not PCN-enabled.

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

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

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

   o  PCN-compatible Diffserv codepoint - a Diffserv codepoint for which
      the ECN field is used to carry PCN markings rather than [RFC3168]
      markings.

   In addition the document uses the terminology defined in [pcn-arch].

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- (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   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)
    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].  Guidelines for mixing traffic-types within a PCN-
      domain are given in [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.

4.1.  Rationale for Encoding

   The exact choice of encoding was dictated by following table sets out the constraints imposed
   by existing IETF RFCs, in particular [RFC3168] valid and [RFC4774].  One of
   the tightest constraints was the need invalid codepoint
   transitions at PCN-nodes for any PCN encoding to survive
   being tunnelled through either an IP in IP tunnel or an IPSec Tunnel.
   Appendix A explains this in 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 is being tunneled then only the 11
   codepoint gets copied into the inner header upon decapsulation.  An
   additional constraint is the need to minimise baseline encoding.  Extension
   encodings may have different rules regarding the use validity of DiffServ
   codepoints as the
   transitions.  Note that this table assumes there is a limited supply of standards track codepoints
   remaining.  Section 4.2 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 functional
   separation between 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.

   The encoding scheme above seems to meet all these constraints PCN-boundary-node and
   ends up looking very similar to ECN.  This is perhaps a PCN-interior-node such
   that PCN-boundary-nodes do not surprising
   given the similarity in architectural intent between PCN and ECN.

4.2.  PCN-Compatible DiffServ Codepoints

   Equipment complying with perform packet metering or marking
   functions.  PCN-nodes MUST follow the baseline PCN encoding transition rules set
   out in this table (e.g. they 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 a DSCP switches NOT set invalid codepoints on PCN marking behaviour for
   packets
   with that DSCP, but they forward).  This table only if those packets also have their ECN field
   set applies to indicate a codepoint other than not-PCN.

   Enabling PCN-packets.

   +-----------+-------------+-----------------+-----------------------+
   | PCN marking behaviour disables any node  |  Codepoint  | Valid codepoint | Invalid codepoint out |
   |   type    |      in     |        out      |                       |
   +-----------+-------------+-----------------+-----------------------+
   |  ingress  |     Any     | NM (or Not-PCN) |          PM           |
   | interior  |     NM      |     NM or PM    |     Not-PCN or EXP    |
   | interior  |     EXP +   |     EXP or PM   |        Not-PCN        |
   | interior  |   Not-PCN   |      Not-PCN    |  Any other marking behaviour
   (e.g. enabling PCN disables codepoint  |
   | interior  |     PM      |        PM       |  Any other codepoint  |
   |  egress   |     Any     |        00       | 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 default ECN marking behaviour
   introduced in [RFC3168]).  All traffic scheduling egress node knows that other marks may be safely
      exposed outside the PCN-domain (e.g. [PCN-3-enc-state]).

            Table 2: Valid and conditioning
   behaviours are discussed in [pcn-marking-behaviour].

5.  Rules Invalid Codepoint Transitions for Experimental Encoding Schemes

   Any experimental encoding scheme MUST follow these rules to ensure
   backward compatibility
                        PCN-packets at PCN-nodes

   If a pcn-interior-node compliant with this baseline scheme:

   o encoding receives
   a

4.1.  Rationale for Encoding

   The 00 codepoint exact choice of encoding was dictated by the constraints imposed
   by existing IETF RFCs, in particular [RFC3168], [RFC4301] and
   [RFC4774].  One of the ECN field MUST mean not-PCN.

   o 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] 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 MUST mean PCN-marked (though
      this doesn't exclude other codepoints from carrying field.  If the same
      meaning).

   o  Once set packet is being
   tunneled then only the 11 codepoint in gets copied into the ECN field MUST NOT be changed inner header
   upon decapsulation.  An additional constraint is the need to
      any other codepoint.

6.  Backwards Compatibility

   BCP 124 [RFC4774] gives guidelines for specifying alternative
   semantics for minimise
   the ECN field.  It sets out use of DiffServ codepoints as there is a limited supply of
   standards track codepoints remaining.  Section 4.2 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 be
   taken into consideration.  It also suggests various techniques set 10 as the NM state.  These included similarity
   to
   allow ECN, presence of tunnels within the co-existence domain, leakage into and out
   of default ECN PCN-domain and alternative ECN semantics. incremental deployment.

   The baseline encoding specified 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 this document defines PCN-
   compatible architectural intent between PCN and ECN.

4.2.  PCN-Compatible DiffServ codepoints as no longer supporting the default
   ECN semantics.  As such this document is compatible Codepoints

   Equipment complying with BCP 124.  It
   should be noted that this the baseline PCN encoding blocks end-to-end ECN
   except where mechanisms are put in place MUST allow PCN to tunnel such traffic
   across the PCN-domain.

7.  IANA Considerations
   be enabled for certain Diffserv codepoints.  This document makes no request to IANA.

8.  Security Considerations

   Packets claim entitlement to be PCN marked by carrying a PCN-
   Compatible DSCP and a PCN-Enabled ECN codepoint.  This encoding
   document is intended to stand independently of defines
   the architecture used
   to determine whether specific packets are authorised to be term "PCN-compatible Diffserv codepoint" for such a DSCP.
   Enabling PCN
   marked, which will be described in for a future separate document DSCP switches on PCN
   edge-node marking behaviour (see Appendix B).

   The PCN working group has initially been chartered to for packets
   with that DSCP, but only consider a
   PCN-domain if those packets also have their ECN field
   set to be entirely under the control of one operator, or indicate a set
   of operators who trust each codepoint other [PCN-charter].  However there is a
   requirement to keep inter-domain scenarios in mind when defining the than Not-PCN.

   Enabling 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 marking behaviour disables any one domain's security against its neighbours would
   be described as part of the proposed edge-node other marking behaviour document.

   One proposal on the table allows one to extend PCN across multiple
   domains without PCN-boundary-nodes back-to-back at borders [re-PCN].
   It is believed that the encoding described here would be compatible
   with the security framework described there.

9.  Conclusions

   This document defines the baseline
   (e.g. enabling PCN encoding utilising a
   combination of a PCN-enabled DSCP and disables the default ECN field marking behaviour
   introduced in the IP header. [RFC3168]).  All traffic scheduling and conditioning
   behaviours are discussed in [pcn-marking-behaviour].  This baseline encoding allows ensures
   compliance with the existence of two PCN encoding
   states, not-Marked and PCN-Marked.  It also allows BCP guidance set out in [RFC4774].

5.  Rules for the co-
   existence of competing traffic within the same DSCP so long as that
   traffic doesn't require end-to-end ECN support.  The Experimental Encoding Schemes

   Any experimental encoding scheme
   is conformant MUST follow these rules to ensure
   backward compatibility with [RFC4774].

10.  Acknowledgements

   This document builds extensively on work done this baseline scheme:

   o  The 00 codepoint in the PCN working
   group by Kwok Ho Chan, Georgios Karagiannis, Philip Eardley, Anna
   Charny, Joe Babiarz and others.  Thanks to Ruediger Geib for
   providing detailed comments on ECN field MUST mean Not-PCN.

   o  The 11 codepoint in the ECN field MUST mean PCN-marked (though
      this document.

11.  Comments Solicited

   Comments and questions are encouraged and very welcome.  They can be
   addressed to doesn't exclude other codepoints from carrying the IETF congestion and pre-congestion working group
   mailing list <pcn@ietf.org>, and/or to same
      meaning).

   o  Once set the authors.

12.  References

12.1.  Normative References

   [RFC2119]                Bradner, S., "Key words for use 11 codepoint in RFCs the ECN field MUST NOT be changed to
                            Indicate Requirement Levels",
      any other codepoint.

   o  Any experimental scheme MUST include details of all valid and
      invalid codepoint transitions at any PCN nodes.

6.  Backwards Compatibility

   BCP 14,
                            RFC 2119, March 1997. 124 [RFC4774]                Floyd, S., "Specifying Alternate Semantics gives guidelines for specifying alternative
   semantics for the Explicit Congestion Notification
                            (ECN) Field", BCP 124, RFC 4774,
                            November 2006.

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

12.2.  Informative References

   [PCN-3-enc-state]        Moncaster, T., Briscoe, B., and M. Menth, "A
                            three state extended PCN encoding scheme",
                            draft-moncaster-pcn-3-state-encoding-00
                            (work in progress), June 2008.

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

   [RFC3168]                Ramakrishnan, K., Floyd, S., and D. Black,
                            "The Addition ECN field.  It sets out a number of Explicit Congestion
                            Notification (ECN) factors to IP", RFC 3168,
                            September 2001.

   [RFC4301]                Kent, S. and K. Seo, "Security Architecture
                            for be
   taken into consideration.  It also suggests various techniques to
   allow the Internet Protocol", RFC 4301,
                            December 2005.

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

   [pcn-marking-behaviour]  Eardley, P., "Marking behaviour co-existence of PCN-
                            nodes", draft-ietf-pcn-marking-behaviour-00
                            (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., default ECN and M. Dolly, "DSCPs
                            for Capacity-Admitted Traffic",
                            draft-ietf-tsvwg-admitted-realtime-dscp-04
                            (work in progress), February 2008.

Appendix A.  Tunnelling Constraints alternative ECN semantics.
   The rules that govern the behaviour of baseline encoding specified in this document defines PCN-
   compatible DiffServ codepoints as no longer supporting the default
   ECN field for IP-in-IP
   tunnels were defined 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 [RFC3168].  This allowed for two place to tunnel
   modes.  The limited functionality mode sets such
   traffic across the outer header PCN-domain.

7.  IANA Considerations

   This document makes no request to not-
   ECT, regardless of the value IANA.

8.  Security Considerations

   Packets claim entitlement to be PCN marked by carrying a PCN-
   Compatible DSCP and a PCN-Enabled ECN codepoint.  This encoding
   document is intended to stand independently of the inner header, architecture used
   to determine whether specific packets are authorised to be PCN
   marked, which will be described in other words
   disabling ECN within the tunnel. a future separate document on PCN
   edge-node behaviour.

   The full functionality mode copies
   the inner ECN field into the outer header if PCN working group has initially been chartered to only consider a
   PCN-domain to be entirely under the inner header is not-
   ECT control of one operator, or either a set
   of the 2 ECT codepoints.  If the inner header is CE
   then the outer header operators who trust each other [PCN-charter].  However there is set a
   requirement to ECT(0).  On decapsulation, if the CE
   codepoint is set on the outer header then this is copied into the
   inner header.  Otherwise keep inter-domain scenarios in mind when defining the inner header is left unchanged.  The
   stated reason for blocking CE from being copied
   PCN encoding.  One way to the outer header
   was extend to prevent this from being used 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 a covert channel through IPSec
   tunnels.

   The IPSec protocol [RFC4301] changed part of the ECN tunnelling rule to allow
   IPSec tunnels proposed edge-node behaviour document.

   One proposal on the table allows one to simply copy extend PCN across multiple
   domains without PCN-boundary-nodes back-to-back at borders [re-PCN].
   It is believed that the inner header into encoding described here would be compatible
   with the outer header.
   On decapsulation security framework described there.

9.  Conclusions

   This document defines the outer header is discarded baseline PCN encoding utilising a
   combination of a PCN-enabled DSCP and the ECN field is
   only copied down if it is set to CE.

   Because of in the IP header.
   This baseline encoding allows the possible existence of tunnels, only CE (11) can be
   used as a two PCN marking as it is encoding
   states, not-Marked and PCN-Marked.  It also allows for the only mark that will always survive
   decapsulation.  However there is a need for caution with all
   tunneling co-
   existence of competing traffic within the PCN-domain.  RFC3168 full functionality IP in IP
   tunnels are expected to set the ECN field to ECT(0) if the inner same DSCP so long as that
   traffic doesn't require end-to-end ECN
   field support.  The encoding scheme
   is set to CE.  This leads to the possibility that some packets
   within the PCN-domain that have already been marked may have that
   mark concealed further into the domain. conformant with [RFC4774].

10.  Acknowledgements

   This is undesirable for many
   PCN schemes and thus document builds extensively on work done in the PCN working
   group needs to decide whether by Kwok Ho Chan, Georgios Karagiannis, Philip Eardley, Anna
   Charny, Joe Babiarz and others.  Thanks to
   advise against the use of full functionality RFC3168 IP in IP tunnels
   within a PCN-domain Ruediger Geib for
   providing detailed comments on this document.

11.  Comments Solicited

   Comments and questions are encouraged and very welcome.  They can be
   addressed to support the ongoing work within the Transport
   Area IETF congestion and pre-congestion working group
   mailing list <pcn@ietf.org>, and/or to rationalise the behaviour of IP in IP tunnels authors.

12.  References

12.1.  Normative References

   [RFC2119]                Bradner, S., "Key words for use in respect RFCs to
   the ECN field and bring them in line with the behaviour of IPSec
   tunnels [ecn-tunnelling].

Appendix B.  PCN Node Behaviours

   The following table of valid and invalid transitions, while necessary
                            Indicate Requirement Levels", BCP 14,
                            RFC 2119, March 1997.

   [RFC4774]                Floyd, S., "Specifying Alternate Semantics
                            for the correct functioning of Explicit Congestion Notification
                            (ECN) Field", BCP 124, RFC 4774,
                            November 2006.

12.2.  Informative References

   [PCN-3-enc-state]        Moncaster, T., Briscoe, B., and M. Menth, "A
                            three state extended PCN they is not strictly part of the encoding scheme.  The PCN working group needs to decide whether to
   include this in this baseline encoding or whether to transfer it to
   an alternative document.

   +-----------+-------------+-----------------+-----------------------+
   | PCN node  |  Codepoint  | Valid codepoint | Invalid codepoint out |
   |   type    | scheme",
                            draft-moncaster-pcn-3-state-encoding-00
                            (work in     |        out      |                       |
   +-----------+-------------+-----------------+-----------------------+
   |  ingress  |     Any     | NM (or Not-PCN) |           PM          |
   | interior  |      NM     |     NM or PM    |        not-PCN        |
   | interior  |   Not-PCN   |      Not-PCN    |  Any other codepoint  |
   |  egress   |     Any     |        00       | Any other codepoint * |
   +-----------+-------------+-----------------+-----------------------+
    * Except where the egress node knows that other marks may be safely
         exposed outside the PCN-domain (e.g. [PCN-3-enc-state]).

            Table 2: Valid progress), June 2008.

   [PCN-charter]            IETF, "IETF Charter for Congestion and Invalid Transitions at PCN nodes

   It is also necessary Pre-
                            Congestion Notification Working Group".

   [RFC3168]                Ramakrishnan, K., Floyd, S., and D. Black,
                            "The Addition of Explicit Congestion
                            Notification (ECN) to define a safe behaviour IP", RFC 3168,
                            September 2001.

   [RFC4301]                Kent, S. and K. Seo, "Security Architecture
                            for baseline-
   compliant nodes to follow should they unexpectedly encounter a packet
   carrying the EXP (01) codepoint.  The obvious safe 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 would be
   to treat this as if it were a NM packet but to raise an alarm at a
   higher layer to check why the packet was there.  An alternative safe
   approach is to treat it as a not-PCN packet but this might jeopardise
   partial deployment of any future experimental encoding scheme.

Appendix C.  Deployment Scenarios for PCN Using Baseline Encoding

   This appendix illustrates possible PCN deployment scenarios where the
   baseline encoding can be used 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 also explain a case 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 choice of which
   baseline encoding is not sufficient. {Note this appendix DSCP is provided most suitable for information only}.

   1.  an operator requires only admission control.  Then admission
       control the PCN-domain is triggered from PCN-packets
   dependant on the nature of the traffic entering that are threshold-marked domain and this baseline encdoding scheme suffices.

   2.  an operator requires only flow termination.  Then flow
       termination is triggered from PCN-packets the
   link rates of all the links making up that are excess-
       traffic-marked and this baseline encdoding scheme suffices.

   3.  an operator requires both admission control and flow termination. domain.  In PCN-domains
   with uniformly high link rates, the appropriate DSCPs would currently
   be those for the Real Time Traffic Class [RFC5127].  If both admission control and flow termination are triggered from
       PCN-packets that are excess-traffic-marked then this baseline
       encoding scheme suffices.

   4.  an operator requires both admission control triggered by packets the PCN
   domain includes lower speed links it would also be appropriate to use
   the DSCPs of the other traffic classes that are threshold-marked [voice-admit] defines for
   use with admission control, such as the three video classes CS4, CS3
   and AF4 and flow termination triggered by
       packets that are excess-traffic-marked.  In this case the
       baseline encoding provides insufficient encoding states Admitted Telephony Class.

A.2.  Rationale for Using ECT(0) for Not Marked

   The choice of which ECT codepoint to
       achieve this. use for the Not Marked state was
   based on the following considerations:

   o  [RFC3168] full functionality tunnel within PCN-domain: Either ECT
      is safe.

   o  Leakage of traffic into PCN-domain: ECT(1) is less often correct.

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

   o  Incremental deployment: Either ECT is suitable as long as they are
      used consistently.

   o  Conceptual consistency with other schemes: ECT(0) is conceptually
      consistent with [RFC3168].

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

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   This document was produced using xml2rfc v1.33 (of
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