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Versions: 00 01 draft-ietf-pcn-3-state-encoding

Congestion and Pre Congestion                               T. Moncaster
Internet-Draft                                                        BT
Intended status: Experimental                                 B. Briscoe
Expires: September 10, 2009                                     BT & UCL
                                                                M. Menth
                                                 University of Wuerzburg
                                                           March 9, 2009


               A three state extended PCN encoding scheme
                draft-moncaster-pcn-3-state-encoding-01

Status of this Memo

   This Internet-Draft is submitted to IETF in full conformance with the
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Copyright Notice

   Copyright (c) 2009 IETF Trust and the persons identified as the



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   document authors.  All rights reserved.

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

   Pre-congestion notification (PCN) is a mechanism designed 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 baseline encoding
   specified how two encoding states could be encoded into the IP
   header.  This document specified an extension to the baseline
   encoding that enables three encoding states to be carried in the IP
   header as well as enabling limited support for end-to-end ECN.

Status (to be removed by RFC Editor)

   This memo is posted as an Internet-Draft with an intent to eventually
   be published as an experimental RFC.  The PCN Working Group will be
   asked to adopt this memo as a Working Group document describing one
   of several possible experimental PCN encoding schemes.  The intention
   is that the title of this document will change to avoid confusion
   with the three state marking scheme.

Changes from previous drafts

   From 00 to 01:

   o  Checked terminology for consistency with
      [I-D.ietf-pcn-baseline-encoding]

   o  Minor editorial changes.















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Table of Contents

   1.  Introduction . . . . . . . . . . . . . . . . . . . . . . . . .  4
   2.  Requirements notation  . . . . . . . . . . . . . . . . . . . .  4
   3.  Terminology  . . . . . . . . . . . . . . . . . . . . . . . . .  4
   4.  The Requirement for Three PCN Encoding States  . . . . . . . .  5
   5.  Adding Limited End-to-End ECN Support to PCN . . . . . . . . .  5
   6.  Encoding Three PCN States in IP  . . . . . . . . . . . . . . .  6
     6.1.  Basic Three State Encoding . . . . . . . . . . . . . . . .  6
     6.2.  Full Three State Encoding  . . . . . . . . . . . . . . . .  7
       6.2.1.  Forwarding Traffic Out of the PCN-domain . . . . . . .  7
   7.  PCN domain support for the PCN extension encoding  . . . . . .  8
     7.1.  End-to-End transport behaviour compliant with the PCN
           extension encoding . . . . . . . . . . . . . . . . . . . .  8
     7.2.  PCN-boundary-node behaviour compliant with the PCN
           extension encoding . . . . . . . . . . . . . . . . . . . .  9
       7.2.1.  Behaviour for packets belonging to a PCN-flow  . . . .  9
       7.2.2.  Behaviour for packets belonging to a
               PCN-enabled-ECN-flow . . . . . . . . . . . . . . . . .  9
     7.3.  PCN-interior-node behaviour compliant with the PCN
           extension encoding . . . . . . . . . . . . . . . . . . . . 10
     7.4.  Behaviour of any PCN node compliant with the PCN
           extension encoding . . . . . . . . . . . . . . . . . . . . 10
   8.  IANA Considerations  . . . . . . . . . . . . . . . . . . . . . 10
   9.  Security Considerations  . . . . . . . . . . . . . . . . . . . 10
   10. Conclusions  . . . . . . . . . . . . . . . . . . . . . . . . . 11
   11. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 11
   12. Comments Solicited . . . . . . . . . . . . . . . . . . . . . . 11
   13. References . . . . . . . . . . . . . . . . . . . . . . . . . . 11
     13.1. Normative References . . . . . . . . . . . . . . . . . . . 11
     13.2. Informative References . . . . . . . . . . . . . . . . . . 11
   Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 12



















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1.  Introduction

   Pre-congestion notification provides information to support admission
   control and flow termination at the boundary nodes of a Diffserv
   region in order to protect the quality of service (QoS) of inelastic
   flows [I-D.ietf-pcn-architecture].  This is achieved by marking
   packets on interior nodes according to some metering function
   implemented at each node.  Excess traffic marking marks PCN packets
   that exceed a certain reference rate on a link while threshold
   marking marks all PCN packets on a link when the PCN traffic rate
   exceeds a higher reference rate.  These marks are monitored by the
   egress nodes of the PCN-domain.

   The baseline encoding described in [I-D.ietf-pcn-baseline-encoding]
   provides for deployment scenarios that only require two PCN encoding
   states.  This document describes an experimental extension to the
   base-encoding in the IP header that adds two capabilities:

   o  the encoding of a third PCN encoding state in the IP header

   o  preservation of the end-to-end semantics of the ECN field even
      though PCN uses the field within a PCN-region that interrupts the
      end-to-end path

   The second of these capabilities is optional and the reasons for
   doing it are discussed in


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

   Most of the terminology used in this document is defined either in
   [I-D.ietf-pcn-architecture] or in [I-D.ietf-pcn-baseline-encoding].
   The following additional terms are defined in this document:

   o  PCN-flow - a flow covered by a reservation but which hasn't
      signalled that it requires end-to-end ECN support.

   o  PCN-enabled-ECN-flow - a flow covered by reservation and for which
      the end-to-end transport has explicitly negotiated ECN support
      from the PCN-boundary-nodes.




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   o  Not-Marked (xxx), where xxx represents a standard ECN codepoint -
      packets that are PCN capable but carry no PCN mark.  Also NM(xxx).
      The (xxx) represents the ECN codepoint that the packet arrived
      with at the PCN-ingress-node e.g.  NM(CE) represents a PCN capable
      packet that has no PCN marking but which arrived with the ECN bits
      set to congestion experienced.


4.  The Requirement for Three PCN Encoding States

   The PCN architecture [I-D.ietf-pcn-architecture] describes proposed
   PCN schemes that require traffic to be metered and marked using both
   Threshold and Excess Traffic schemes.  In order to achieve this it is
   necessary to allow for three PCN encoding states.  The constraints
   imposed by the way tunnels process the ECN field severely limit how
   to encode these states as explained in
   [I-D.ietf-pcn-baseline-encoding] and [I-D.ietf-tsvwg-ecn-tunnel].
   The obvious way to provide one more encoding state than the base
   encoding is through the use of an additional PCN-compatible DiffServ
   codepoints.  One aim of this document is to allow for experiments to
   show whether such schemes are better than those that only employ two
   PCN encoding states.  As such, the additional DSCP will be taken from
   as the EXP/LU pools defined in [RFC2474].  If the experiments
   demonstrate that PCN schemes employing three encoding states are
   significantly better than those only employing two then at a later
   date IANA might be asked to assign a new PCN enabled DSCP from pool
   1.


5.  Adding Limited End-to-End ECN Support to PCN

   [I-D.sarker-pcn-ecn-pcn-usecases] suggests a number of use-cases
   where explicit preservation of end-to-end ECN semantics might be
   needed across a PCN domain.  One of the use-cases suggests that the
   end-nodes might be running rate-adaptive codecs that would respond to
   ECN marks by reducing their transmission rate.  If the sending
   transport sets the ECT codepoint, the setting of the ECN field as it
   arrives at the PCN ingress node will need to be re-instated as it
   leaves the PCN egress node.

   If a PCN region is starting to suffer pre-congestion then it may make
   sense to expose marks generated within the PCN region by forwarding
   CE marks from the PCN egress to such a rate-adaptive endpoint.  They
   would be in addition to any CE marks generated elsewhere on the end-
   to-end path.  This would allow the endpoints to reduce the traffic
   rate.  This will in turn help to alleviate the pre-congestion,
   potentially averting any need for call blocking or termination.
   However, the 'leaking' of CE marks out of the PCN region is



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   potentially dangerous and could violate [RFC4774] if the end hosts
   don't understand ECN (see section 18.1.4 of [RFC3168]).

   Therefore, a PCN region can only support end-to-end ECN if the PCN
   edge nodes are sure that the end-to-end transport is ECN-capable.
   That way the PCN egress nodes can ensure that they only expose CE
   marks to those receivers that will correctly interpret them as a
   notification of congestion.  The end-points may indicate they are
   ECN-capable through some higher-layer signalling process that sets up
   their reservation with the PCN boundary nodes.  The exact process of
   negotiation is beyond the scope of this document but is likely to
   involve explicit two way signalling between the end-host and the PCN-
   domain.

   In the absence of such signalling the default behaviour of the PCN
   egress node will be to clear the ECN field to 00 as in the baseline
   PCN encoding [I-D.ietf-pcn-baseline-encoding].


6.  Encoding Three PCN States in IP

   The three state PCN encoding scheme is based closely on that defined
   in [I-D.ietf-pcn-baseline-encoding] so that there will be no
   compatibility issues if a PCN-domain changes from using the baseline
   encoding scheme to the experimental scheme described here.  There are
   two versions of the scheme.  The basic three state scheme allows for
   carrying both Threshold Marked (ThM) and Excess traffic MArked (ETM)
   traffic.  The full scheme additionally allows us to carry end-to-end
   ECN.

6.1.  Basic Three State Encoding

   The following table shows how to encode the three PCN states in IP.
   The authors spent some time trying to establish which way round to
   put the two marked states before settling on this.  Because it is
   envisaged that DSCP 2 will be of lower priority than DSCP 1 the
   change in marking from Threshold to Excess Traffic involves
   downgrading the traffic which seems to be consistent with the
   requirement that such changes should not be reversed.












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      +--------+--------------+-------------+-------------+---------+
      |  DSCP  | Not-ECT (00) | ECT(0) (10) | ECT(1) (01) | CE (11) |
      +--------+--------------+-------------+-------------+---------+
      | DSCP 1 |    Not-PCN   |      NM     |      CU     |   ThM   |
      | DSCP 2 |    Not-PCN   |      CU     |      CU     |   ETM   |
      +--------+--------------+-------------+-------------+---------+

         Where DSCP 1 is a PCN-compatible DiffServ codepoint (see
   [I-D.ietf-pcn-baseline-encoding]) and DSCP 2 is a PCN-compatible DSCP
               from the EXP/LU pools as defined in [RFC2474]

                 Table 1: Encoding three PCN states in IP

6.2.  Full Three State Encoding

   Table 2 shows how to additionally carry the end-to-end ECN state in
   the IP header.

      +--------+--------------+-------------+-------------+---------+
      |  DSCP  | Not-ECT (00) | ECT(0) (10) | ECT(1) (01) | CE (11) |
      +--------+--------------+-------------+-------------+---------+
      | DSCP 1 |    Not-PCN   | NM(Not-ECT) |    NM(CE)   |   ThM   |
      | DSCP 2 |    Not-PCN   |  NM(ECT(0)) |  NM(ECT(1)) |   ETM   |
      +--------+--------------+-------------+-------------+---------+

         Where DSCP 1 is a PCN-compatible DiffServ codepoint (see
   [I-D.ietf-pcn-baseline-encoding]) and DSCP 2 is a PCN-compatible DSCP
               from the EXP/LU pools as defined in [RFC2474]

                 Table 2: Encoding three PCN states in IP

   The four different Not Marked (NM) states allow for the addition of
   limited end-to-end ECN support as explained in the previous section.

Warning

6.2.1.  Forwarding Traffic Out of the PCN-domain

   As each packet exits the PCN-domain, the PCN-egress-node MUST check
   whether it belongs to a PCN-enabled-ECN-flow.  If it belongs to such
   a flow then the following table shows how the ECN field should be re-
   set.  In addition all packets should have their DSCP reset to the
   appropriate DSCP for the next hop.  If the next hop is not another
   PCN region this will not be a PCN-compatible DSCP, and by default
   will be the best-efforts DSCP.  Alterntively higher layer signalling
   mechanisms may allow the DSCP that packets entered the PCN-domain
   with to be re-instated.




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   +-------+-------------+-----------------+-----------------+---------+
   |  DSCP |      00     |        10       |        01       |    11   |
   +-------+-------------+-----------------+-----------------+---------+
   |  DSCP | Not PCN --> | NM(Not-ECT) --> |  NM(CE) --> CE  | ThM --> |
   |   1   |   Not ECT   |     not-ECT     |                 |    CE   |
   |  DSCP | Not PCN --> |  NM(ECT(0)) --> |  NM(ECT(1)) --> | ETM --> |
   |   2   |   Not ECT   |      ECT(0)     |      ECT(1)     |    CE   |
   +-------+-------------+-----------------+-----------------+---------+

     Where each cell gives the incoming PCN state and the outgoing ECN
                                  state.

     Table 3: Egress rules for resetting ECN field for PCN Enabled ECN
                                   Flows

   For packets belonging to a PCN-flow the ECN field MUST be reset to
   not-ECT (00) as defined in [I-D.ietf-pcn-baseline-encoding].


7.  PCN domain support for the PCN extension encoding

   PCN traffic MUST be marked with a DiffServ codepoint that indicates
   PCN is enabled.  To comply with the PCN extension encoding, this
   codepoint is either a PCN-compatible DSCP assigned by IANA for use
   with the baseline PCN encoding [I-D.ietf-pcn-baseline-encoding] or a
   DSCP from pools 2 or 3 for experimental and local use [RFC2474].  The
   exact choice of DSCP may vary between PCN-domains but MUST be fixed
   within each PCN-domain.

   All nodes within a PCN-domain MUST understand and support the three
   PCN states of the PCN extension coding.  Therefore if any PCN-node
   does not support three PCN encoding states, any node in the same PCN-
   domain MUST NOT be configured to use three PCN encoding states as
   defined here.

7.1.  End-to-End transport behaviour compliant with the PCN extension
      encoding

   Transports wishing to use both a reservation and end-to-end ECN MUST
   establish that their path supports this combination.  Support of end-
   to-end ECN by PCN boundary nodes is OPTIONAL.  Therefore transports
   MUST check with both the PCN-ingress-node and PCN-egress-node for
   each flow.  The sending of such a request MUST NOT be taken to mean
   the request has been granted.  The PCN-boundary-nodes MAY choose to
   inform the end-node of a successful request.  The exact mechanism for
   such negotiation is beyond the scope of this document.  A transport
   that receives no response or a negative response to a request to
   support end-to-end ECN within a flow reservation MUST set the ECN



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   field of all subsequent packets in that flow to Not-ECT if it wishes
   to guarantee that the flow will receive PCN treatment.

   If a domain wishes to use the full scheme described in Table 2 all
   nodes in that domain MUST be configured to understand the full
   scheme.

7.2.  PCN-boundary-node behaviour compliant with the PCN extension
      encoding

   o  If both the PCN ingress and egress nodes support end-to-end ECN,
      and the transport has successfully requested end-to-end ECN the
      flow becomes a PCN-enabled-ECN-flow.

   o  If either of a PCN ingress-egress pair does not support end-to-end
      ECN or if the end-to-end transport does not request support for
      end-to-end ECN then the PCN-boundary-nodes MUST assume the packet
      belongs to a PCN-flow.

7.2.1.  Behaviour for packets belonging to a PCN-flow

   o  If a packet belongs to a PCN-flow arrives at the PCN-ingress-node
      with its ECN field already marked as CE or ECT, it SHOULD be
      dropped.  Alternatively it MAY be downgraded to a lower (non-PCN)
      service class or MAY be tunnelled through the PCN region.  It MUST
      NOT be admitted to the PCN region directly.

   o  When a packet belonging to a PCN-flow carrying the not-ECT
      codepoint arrives at the PCN-ingress-node, the ECN field MUST be
      set to ECT(0) (10) and the DiffServ field set to DSCP 1.

   o  When a packet belonging to a PCN-flow leaves the PCN-domain
      through the PCN-egress-node, the ECN bits MUST be set to not-ECT
      (00).

7.2.2.  Behaviour for packets belonging to a PCN-enabled-ECN-flow

   o  When a packet belonging to a PCN-enabled-ECN-flow arrives at the
      PCN-ingress-node, then the ECN field and DSCP MUST be set to the
      appropriate NM(xxx) setting as shown in Table 2.

   o  When a packet belonging to a PCN-enabled-ECN-flow leaves the PCN-
      region through a PCN-egress-node, the ECN bits MUST be set
      according to Table 3 and the DSCP MUST be set to the appropriate
      DSCP for the next hop as discussed in Section 6.2.1 above.






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7.3.  PCN-interior-node behaviour compliant with the PCN extension
      encoding

   o  If a PCN interior node indicates that a packet is to be threshold
      marked then the ThM codepoint MUST be set by changing the ECN bits
      to 11 and ensuring the Diffserv field is set to DSCP1.

   o  If a PCN interior node indicates that a packet is to be excess
      traffic marked then the EM codepoint MUST be set by changing the
      ECN bits to 11 and ensuring the Diffserv field is set to DSCP2 as
      defined above.

7.4.  Behaviour of any PCN node compliant with the PCN extension
      encoding

   o  PCN nodes MUST NOT change not-PCN to another codepoint and they
      MUST NOT change a PCN-Capable codepoint to not-PCN.

   o  ThM MUST NOT be changed to NM.

   o  ETM MUST NOT be changed to ThM or to NM.


8.  IANA Considerations

   This document asks IANA to assign one DiffServ codepoint from Pool 2
   or Pool 3 (for experimental/local use)[RFC2474].  Should any of the
   three encoding state experimental PCN schemes prove sufficiently
   successful then, at a later date, IANA will be requested in a later
   document to assign a dedicated DiffServ codepoint from pool 1 for
   standards use.


9.  Security Considerations

   The security concerns relating to this extended PCN encoding are
   essentially the same as those in [I-D.ietf-pcn-baseline-encoding].

   This extension coding gives end-to-end support for the ECN nonce
   [RFC3540], which is intended to protect the sender against the
   receiver or against network elements concealing a congestion
   experienced marking or a lost packet.  PCN-based reservations
   combined with end-to-end ECN are intended for partially inelastic
   traffic using rate-adaptive codecs.  Therefore the end-to-end
   transport is unlikely to be TCP, but at this time the nonce has only
   been defined for TCP transports.





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10.  Conclusions

   This document describes an extended encoding scheme for PCN that
   provides for three encoding states as well as support for end-to-end
   ECN.  The encoding scheme builds on the baseline encoding described
   in [I-D.ietf-pcn-baseline-encoding].  Using this encoding scheme it
   is possible for operators to conduct experiments to check whether the
   addition of an extra encoding state will significantly improve the
   performance of PCN.  It will also allow experiments to determine
   whether there is a need for end-to-end ECN support within the PCN-
   domain (as against end-to-end ECN support through the use of IP-in-IP
   tunnelling or by downgrading the traffic to a lower service class).


11.  Acknowledgements

   This document builds extensively on work done in the PCN working
   group by Kwok Ho Chan, Georgios Karagiannis, Philip Eardley, Joe
   Babiarz and others.  Full details of alternative schemes that were
   considered for adoption can be found in the document
   [I-D.chan-pcn-encoding-comparison].


12.  Comments Solicited

   Comments and questions are encouraged and very welcome.  They can be
   addressed to the IETF Transport Area working group mailing list
   <tsvwg@ietf.org>, and/or to the authors.


13.  References

13.1.  Normative References

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119, March 1997.

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

13.2.  Informative References

   [I-D.chan-pcn-encoding-comparison]
              Chan, K., Karagiannis, G., Moncaster, T., Menth, M.,
              Eardley, P., and B. Briscoe, "Pre-Congestion Notification
              Encoding Comparison",
              draft-chan-pcn-encoding-comparison-04 (work in progress),



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              February 2008.

   [I-D.ietf-pcn-architecture]
              Eardley, P., "Pre-Congestion Notification (PCN)
              Architecture", draft-ietf-pcn-architecture-09 (work in
              progress), January 2009.

   [I-D.ietf-pcn-baseline-encoding]
              Moncaster, T., Briscoe, B., and M. Menth, "Baseline
              Encoding and Transport of Pre-Congestion Information",
              draft-ietf-pcn-baseline-encoding-02 (work in progress),
              February 2009.

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

   [I-D.sarker-pcn-ecn-pcn-usecases]
              Sarker, Z. and I. Johansson, "Usecases and Benefits of end
              to end ECN support in PCN Domains",
              draft-sarker-pcn-ecn-pcn-usecases-02 (work in progress),
              November 2008.

   [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.

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

   [RFC3540]  Spring, N., Wetherall, D., and D. Ely, "Robust Explicit
              Congestion Notification (ECN) Signaling with Nonces",
              RFC 3540, June 2003.















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Authors' Addresses

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

   Phone: +44 1473 648734
   Email: toby.moncaster@bt.com
   URI:   http://www.cs.ucl.ac.uk/staff/B.Briscoe/


   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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