Congestion and Pre-Congestion                                 B. Briscoe
Notification                                                          BT
Internet-Draft                                          October 27, 2008
Intended status: Experimental
Expires: April 30, 2009

            PCN 3-State Encoding Extension in a single DSCP

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   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 document specifies an
   extension to the two-state PCN baseline encoding that enables three
   encoding states to be carried in the IP header without using more
   than one Diffserv codepoint.  It presupposes a standards action has
   removed the limit of two encoding states in current tunnelling

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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 [I-D.ietf-pcn-marking-behaviour].
   These marks are monitored by the egress nodes of the PCN domain.

   To fully support these two types of marking, three encoding states
   are needed: one encoding for packets that are not marked plus two
   encodings for the two types of marking.  The baseline encoding
   described in [I-D.ietf-pcn-baseline-encoding] provides for deployment
   scenarios that only require two PCN encoding states using a single
   Diffserv codepoint.  This document describes an experimental
   extension to the baseline-encoding that adds a third PCN encoding
   state in the IP header, still using a single Diffserv codepoint.  For
   brevity it will be called the 3-in-1 PCN Encoding.

   If more than three states are required, e.g. to support end-to-end
   ECN as well as edge-to-edge PCN [I-D.sarker-pcn-ecn-pcn-usecases],
   end-to-end ECN would have to be encapsulated in the inner header of a
   tunnel through the PCN region, as outlined in

   General PCN-related terminology is defined in the PCN architecture
   [I-D.ietf-pcn-architecture], and terminology specific to packet
   encoding is defined in the PCN baseline encoding

2.  Requirements Language

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   document are to be interpreted as described in RFC 2119 [RFC2119].

3.  The Requirement for Three PCN Encoding States

   The PCN architecture [I-D.ietf-pcn-architecture] describes proposed
   PCN schemes that expect 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: one as a Not Marked

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   (NM) state and the other two to distinguish these two levels of
   marking severity.  The way tunnels process the ECN field severely
   limits how to encode these states.

   The two bit ECN field seems to offer four possible encoding states,
   but one (00) is set aside for traffic controlled by transports that
   do not understand PCN marking, so it would be irregular to use it as
   a PCN encoding state.  Of the three remaining ECN codepoints, only
   one (11) can be introduced by a congested node within a tunnel and
   still survive the decapsulation behaviour of a tunnel egress as
   currently standardised.  The two remaining codepoints are (10) and
   (01).  But if a node within the tunnel used either of these two
   remaining codepoints to try to mark packets with a second severity
   level, this marking would be removed on decapsulation.  The ECN field
   is constrained to two marking states in this way irrespective of
   whether regular IP in IP tunnelling [RFC3168] or IPsec tunnelling
   [RFC4301] is used.

   One way to provide another encoding state that survives tunnelling is
   to use a second Diffserv codepoint
   [I-D.moncaster-pcn-3-state-encoding].  Instead, to avoid wasting
   scarce Diffserv codepoints, we could modify standard tunnels in the
   PCN region to remove the constraints imposed by standard tunnelling.

   Therefore this document presupposes tunnels in the PCN region comply
   with the newly proposed Comprehensive Decapsulation Rules defined in
   Appendix C of [I-D.ietf-tsvwg-ecn-tunnel].  Then the constraints of
   standard tunnels no longer apply so this document can define a
   3-state encoding for PCN within one Diffserv codepoint.

   Note that [I-D.ietf-tsvwg-ecn-tunnel] only records the Comprehensive
   Decapsulation Rules in an appendix, solely to allow us to discuss
   whether they should be brought on to the standards track.  So, if
   they are not, the offending tunnelling constraints might not be
   removed.  However, [I-D.ietf-tsvwg-ecn-tunnel] carefully establishes
   that the constraints imposed by standard tunnelling are actually
   unnecessary; they are merely the result of an unfortunate sequence of
   historical events.  Then the analysis in Appendix C of
   [I-D.ietf-tsvwg-ecn-tunnel] shows that the proposed new rules would
   not introduce any compatibility issues; they only affect one
   combination of inner and outer header which has never occured under
   any legal transitions of any IETF tunnelling schemes.  Therefore it
   is a reasonable working assumption for the purposes of this document
   that tunnelling constraints will one day be removed.

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4.  The 3-in-1 PCN Encoding

   The 3-in-1 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 evolves from using the baseline
   encoding scheme to the experimental scheme described here.  The exact
   manner in which the PCN encoding states are carried in the IP header
   is shown in Table 1.

                    Codepoint in ECN field of IP header
                         <RFC3168 codepoint name>

      |  DSCP  | 00 <Not-ECT> | 10 <ECT(0)> | 01 <ECT(1)> | 11 <CE> |
      | DSCP n |    Not-PCN   |      NM     |     ThM     |   ETM   |

                       Table 1: 3-in-1 PCN Encoding

   In Table 1 the 3 PCN states are encoded in the ECN field ([RFC3168])
   of an IP packet with its Diffserv field ([RFC2474]) set to DSCP n,
   which is any PCN-Compatible DiffServ codepoint as defined in Section
   4.2 of the PCN baseline encoding [I-D.ietf-pcn-baseline-encoding]).
   The PCN codepoint of a packet defines its marking state as follows:

   Not-PCN:  The packet is controlled by a transport that does not
      understand PCN marking, therefore the only valid action to notify
      congestion is to drop the packet;

   NM:  Not marked.  A packet in the NM state has not (yet) had its
      marking state changed to the ThM or ETM states, but it may be
      changed to one of these states by a node experiencing congestion
      or pre-congestion;

   ThM:  Threshold marked.  Such a packet has had its marking state
      changed by the threshold marking algorithm

   ETM:  Excess traffic marking.  Such a packet has had its marking
      state changed by the excess rate marking algorithm

   Packets marked NM, ThM or ETM are termed PCN-Enabled packets because
   they are controlled by edge nodes that understand how to process PCN

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5.  Behaviour of a PCN Node Compliant with the 3-in-1 PCN Encoding

   To be compliant with the 3-in-1 PCN Encoding, an PCN interior node
   behaves as follows:

   o  It MUST NOT change Not-PCN to a PCN-Enabled codepoint and MUST NOT
      change a PCN-Enabled codepoint to Not-PCN;

   o  It MUST NOT change ThM to NM;

   o  It MUST NOT change ETM to ThM or to NM;

   In other words, a PCN interior node may increase the severity of
   packet marking but it MUST NOT decrease it, where the order of
   severity increases from NM through ThM to ETM.

6.  IANA Considerations

   This memo includes no request to IANA.

   Note to RFC Editor: this section may be removed on publication as an

7.  Security Considerations

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

8.  Conclusions

   The 3-in-1 PCN Encoding provides three states to encode PCN markings
   in the ECN field of an IP packet using just one Diffserv codepoint.
   One state is for not marked packets while the two others are for PCN
   nodes to mark packets with increasing levels of severity.  Use of the
   encoding presupposes that any tunnels in the PCN region have been
   updated to use proposed Comprehensive Decapsulation Rules because
   standard tunnel decapsulation rules unnecessarily constrain PCN

9.  Acknowledgements

   Thanks to Phil Eardley for reviewing this.

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10.  Comments Solicited

   Comments and questions are encouraged and very welcome.  They can be
   addressed to the IETF Congestion and Pre-Congestion working group
   mailing list <>, and/or to the authors.

11.  References

11.1.  Normative References

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

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

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

   [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.

11.2.  Informative References

              Eardley, P., "Pre-Congestion Notification (PCN)
              Architecture", draft-ietf-pcn-architecture-08 (work in
              progress), October 2008.

              Moncaster, T., Briscoe, B., and M. Menth, "Baseline
              Encoding and Transport of Pre-Congestion Information",
              draft-ietf-pcn-baseline-encoding-01 (work in progress),
              October 2008.

              Moncaster, T., Briscoe, B., and M. Menth, "A three state

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              extended PCN encoding scheme",
              draft-moncaster-pcn-3-state-encoding-00 (work in
              progress), June 2008.

              Sarker, Z. and I. Johansson, "Usecases and Benefits of end
              to end ECN support in PCN Domains",
              draft-sarker-pcn-ecn-pcn-usecases-01 (work in progress),
              May 2008.

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

Author's Address

   Bob Briscoe
   B54/77, Adastral Park
   Martlesham Heath
   Ipswich  IP5 3RE

   Phone: +44 1473 645196

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