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Versions: 00 draft-ietf-pcn-psdm-encoding

Congestion and Pre Congestion                                   M. Menth
Internet-Draft                                   University of Wuerzburg
Intended status: Experimental                                 J. Babiarz
Expires: January 7, 2008                                 Nortel Networks
                                                            T. Moncaster
                                                                      BT
                                                              B. Briscoe
                                                                BT & UCL
                                                            July 7, 2008


          PCN Encoding for Packet-Specific Dual Marking (PSDM)
                    draft-menth-pcn-psdm-encoding-00

Status of this Memo

   By submitting this Internet-Draft, each author represents that any
   applicable patent or other IPR claims of which he or she is aware
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   Internet-Drafts are working documents of the Internet Engineering
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   This Internet-Draft will expire on December 25, 2008.

Copyright Notice

   Copyright (C) The IETF Trust (2008).

Abstract

   This document proposes how PCN marks can be encoded into the IP
   header.  The presented encoding reuses the ECN field of the Voice-
   Admit DSCP in a single PCN domain.  The encoding of unmarked PCN



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   packets indicates whether they are subject to either excess- or
   exhaustive-marking.  This is useful, e.g., when data and probe
   packets require different marking mechanisms.

Status

   This memo is posted as an Internet-Draft with an intent to eventually
   be published as an experimental RFC.


Table of Contents

   1.  Introduction . . . . . . . . . . . . . . . . . . . . . . . . .  3
     1.1.  Requirements notation  . . . . . . . . . . . . . . . . . .  3
   2.  Terminology  . . . . . . . . . . . . . . . . . . . . . . . . .  4
   3.  Encoding for Packet-Specific Dual Marking  . . . . . . . . . .  4
     3.1.  Proposed Encoding and Expected Node Behavior . . . . . . .  4
       3.1.1.  PCN Codepoints . . . . . . . . . . . . . . . . . . . .  5
       3.1.2.  Codepoint Handling by PCN Ingress Nodes  . . . . . . .  5
       3.1.3.  Codepoint Handling by PCN Interfaces . . . . . . . . .  5
       3.1.4.  Codepoint Handling by PCN Egress Nodes . . . . . . . .  5
     3.2.  Reasons for the Proposed Encoding  . . . . . . . . . . . .  6
       3.2.1.  Problems with DSCPs  . . . . . . . . . . . . . . . . .  6
       3.2.2.  Problems with Tunneling  . . . . . . . . . . . . . . .  6
       3.2.3.  Problems with the ECN Field  . . . . . . . . . . . . .  7
     3.3.  Handling of ECN Traffic  . . . . . . . . . . . . . . . . .  7
   4.  IANA Considerations  . . . . . . . . . . . . . . . . . . . . .  8
   5.  Security Considerations  . . . . . . . . . . . . . . . . . . .  8
   6.  Conclusions  . . . . . . . . . . . . . . . . . . . . . . . . .  8
   7.  Comments Solicited . . . . . . . . . . . . . . . . . . . . . .  8
   8.  References . . . . . . . . . . . . . . . . . . . . . . . . . .  8
     8.1.  Normative References . . . . . . . . . . . . . . . . . . .  8
     8.2.  Informative References . . . . . . . . . . . . . . . . . .  8
   Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . .  9
   Intellectual Property and Copyright Statements . . . . . . . . . . 11
















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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 [PCN-arch].  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 exhaustive marking marks all PCN
   packets on a link when the PCN traffic rate exceeds a reference rate
   [PCN-marking-behaviour].  These marks are monitored by the egress
   nodes of the PCN domain.

   This document proposes how PCN marks can be encoded into the IP
   header.  The presented encoding reuses the ECN field of the Voice-
   Admit DSCP in a single PCN domain.  The encoding of unmarked PCN
   packets indicates whether they are subject to either excess- or
   exhaustive-marking.  Therefore, we call this proposal encoding for
   packet-specific dual marking (PSDM).

   PSDM supports exhaustive marking and excess marking as long as
   individual packets are subject to only one of them.  It can be
   applied in networks implementing

   o  only AC based on exhaustive marking (reference rate = admissible
      rate),

   o  only FT based on excess marking (reference rate = supportable
      rate),

   o  both AC and FT based on excess marking (reference rate =
      admissible rate)

   o  Probe-based AC based on exhaustive marking (reference rate =
      admissible rate) and FT based on excess marking (reference rate =
      supportable rate).

   Although the motivation for this encoding scheme is to exhaustive-
   mark probe packets and to excess-mark data packets, routers do not
   need to differentiate explicitly between probe and data packets since
   packets are a priori marked with an appropriate codepoint indicating
   the marking mechanism applying to them.

1.1.  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].



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2.  Terminology

   Most of the terminology used in this document is defined in
   [PCN-arch].  The following additional terms are defined in this
   document:

   o  Exhaustive marking - generalization of threshold and ramp marking

   o  PCN-capable flow - a flow subject to PCN-based admission control
      and or flow termination

   o  PCN-enabled DSCP - DSCP indicating within a PCN domain that
      packets possibly belong to a PCN-capable flow

   o  PCN-capable ECN codepoint (PCN codepoint) - DSCP set to a PCN-
      enabled DSCP and ECN field set to a codepoint indicating that a
      packet belongs to a PCN-capable flow (not-ExM, not-EhM, or M,
      explained below)

   o  PCN packet - a packet belonging to a PCN capable flow within a PCN
      domain, must have a PCN-enabled DSCP and a PCN-capable ECN
      codepoint

   o  not-PCN capable (not-PCN) - new ECN codepoint for packets of non-
      PCN-capable flows when a PCN-enabled DSCP is set

   o  not-excess-marked (not-ExM) - new ECN codepoint for unmarked PCN
      packets that are subject to excess marking

   o  not-exhaustive-marked (not-EhM) - new ECN codepoint for unmarked
      PCN packets that are subject to exhaustive marking

   o  marked (M) - new ECN codepoint for marked PCN packets regardless
      whether they were subject to excess or exhaustive marking.


3.  Encoding for Packet-Specific Dual Marking

   In this section the encoding for packet-specific single marking
   (PSDM) is presented and the reasons for the proposed design are
   outlined.

3.1.  Proposed Encoding and Expected Node Behavior

   The encoding reuses the Voice-Admit DSCP [voice-admit] as a PCN-
   enabled DSCP to indicate packets of PCN-capable flows within a PCN
   domain.  So far, this is the only DSCP considered for that use, but
   this encoding scheme is easily extensible towards multiple PCN-



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   enabled DSCPs.

3.1.1.  PCN Codepoints

   The ECN field of packets with a PCN-enabled DSCP is interpreted
   within a PCN domain as PCN codepoint while it is interpreted as ECN
   codepoint outside PCN domains.  Four new PCN codepoints are defined
   in Table 1.

          +------------------+---------+---------+---------+----+
          |       DSCP       |    00   |    10   |    01   | 11 |
          +------------------+---------+---------+---------+----+
          | PCN-enabled DSCP | not-PCN | not-ExM | Not-EhM |  M |
          +------------------+---------+---------+---------+----+

           Table 1: Mapping of PCN codepoints into the ECN field

3.1.2.  Codepoint Handling by PCN Ingress Nodes

   When packets belonging to PCN flows arrive at the ingress router of
   the PCN domain, the ingress router first drops all CE-marked packets.
   Then, it sets the DSCP of the remaining PCN packets to an PCN-enabled
   DSCP and re-marks the ECN field of all PCN packets that are subject
   to exhaustive marking to not-EhM (e.g. probe packets), and all PCN
   packets that are subject to excess marking to not-ExM (e.g. data
   packets).  If packets with a PCN-enabled DSCP arrive that belong to
   non-PCN flows, the PCN ingress node re-marks their ECN field to not-
   PCN.

3.1.3.  Codepoint Handling by PCN Interfaces

   If the meter for excess marking of a PCN node indicates that a PCN
   packet should be marked, its ECN field is set to marked (M) only if
   it was not-ExM before.  If the meter for exhaustive marking of a PCN
   node indicates that a PCN packet should be marked, its ECN field is
   set to marked (M) only if it was not-EhM before.

3.1.4.  Codepoint Handling by PCN Egress Nodes

   If the egress node of a PCN domain receives a marked PCN packet, it
   infers somehow whether the packet was not-ExM or not-EhM by the PCN
   ingress node to interpret the marking.  This can be done as probe
   packets must be distinguishable from PCN data packets.  The egress
   node resets the ECN field of all packets with PCN-enabled DSCPs to
   not-ECT.  This breaks the ECN capability for all flows with PCN-
   enabled DSCPs, regardless whether they are PCN-capable or not.
   Appropriate tunnelling across a PCN domain can preserve the ECN
   marking of packets with PCN-enabled DSCPs and the ECN-capability of



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   their flows (see Section 3.3).

3.2.  Reasons for the Proposed Encoding

3.2.1.  Problems with DSCPs

   DSCPs are a scarce resource in the IP header such that at most one
   should be used for PCN.  To avoid the requirement for a new DSCP, the
   Voice-Admit DSCP is reused.  To differentiate pure Voice-Admit
   traffic from PCN traffic within a PCN domain, pure Voice-Admit
   traffic has its ECN field set to not-PCN within a PCN domain.  The
   encoding should be extensible towards different data plane priorities
   for PCN traffic in PCN domains which requires different PCN-enabled
   DSCPs, one for each priority level.

3.2.2.  Problems with Tunneling

   The encoding scheme must cope with tunnelling within PCN domains.
   However, various tunnelling schemes limit the persistence of ECN
   marks in the top-most IP header to a different degree.  Two IP-in-IP
   tunnelling modes are defined in [RFC3168] and a third one in
   [RFC4301] for IP-in-IPsec tunnels.

   The limited-functionality option in [RFC3168] requires that the ECN
   codepoint in the outer header is set to not-ECT such that ECN is
   disabled for all tunnel routers, i.e., they drop packets instead of
   mark them in case of congestion.  The tunnel egress just decapsulates
   the packet and leaves the ECN codepoints of the inner packet header
   unchanged.

   o  This mode protects the inner IP header from being PCN-marked upon
      decapsulation.  It can be used to tunnel ECN marks across PCN
      domains such that PCN marking is applied to the outer header
      without affecting the inner header.

   o  This mode is not useful to tunnel PCN traffic with PCN-enabled
      DSCP and PCN-capable PCN-codepoints within PCN domain because the
      ECN marking information from the outer ECN fields is lost upon
      decapsulation.

   The full-functionality option in [RFC3168] requires that the ECN
   codepoint in the outer header is copied from the inner header unless
   the inner header codepoint is CE.  In this case, the outer header
   codepoint is set to ECT(0).  This choice has been made to disable the
   ECN fields of the outer header as a covert channel.  Upon
   decapsulation, the ECN codepoint of the inner header remains
   unchanged unless the outer header ECN codepoint is CE.  In this case,
   the inner header codepoint is also set to CE.  This preserves outer



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   header information if it is CE.  However, the fact that CE marks of
   the inner header are not visible in the outer header may be a problem
   for excess marking as it takes already marked traffic into account
   and for some required packet drop policies.

   Tunnelling with IPSec copies the inner header ECN bits to the outer
   header ECN bits RFC4301, Sect. 5.1.2.1 [RFC4301] upon encapsulation.
   Upon decapsulation, CE-marks of the outer header are copied into the
   inner header, the other marks are ignored.  With this tunnelling
   mode, CE marks of the inner header become visible to all meters,
   markers, and droppers for tunnelled traffic.  In addition, limited
   information from the outer header is propagated into the inner
   header.  Therefore, only IPSec tunnels should be used inside PCN
   domains when ECN bits are reused for PCN encoding.  Another
   consequence is that CE is the only codepoint that can be used to
   indicate a marked packet beyond tunnelling.

3.2.3.  Problems with the ECN Field

   The guidelines in [RFC4774] describe how the ECN bits can be reused
   while being compatible with [RFC3168].  A CE mark of a packet must
   never be changed to another ECN codepoint.  Furthermore, a not-ECT
   mark of a packet must never be changed to one of the ECN-capable
   codepoints ECT(0), ECT(1), or CE.  Care must be taken that this rule
   is enforced when PCN packets leave the PCN domain.  As a consequence,
   all CE-marked Voice-Admit packets must be dropped before entering a
   PCN domain and the ECN field of all Voice-Admit packets must be set
   to not-ECT when leaving a PCN domain.

3.3.  Handling of ECN Traffic

   ECN is intended to control elastic traffic as TCP reacts to ECN
   marks.  Inelastic real-time traffic is mostly not transmitted over
   TCP such that this application of ECN is not appropriate.  However,
   there are plans to reuse ECN signals for rate adapatation
   [ecn-pcn-usecases].  Therefore, two different options might be
   useful.

   o  preserve ECN marks from outside a PCN domain, i.e.  CE-marked
      packets should not be dropped.  To handle this case, ECN packets
      should be tunnelled through a PCN domain such that the ECN marking
      is hidden from the PCN control and PCN marking is applied only to
      the outer header.

   o  add PCN markings to the ECN field if applications wish to receive
      the PCN markings for whatever purpose.  In that case IPSec tunnels
      should be used for tunnelling.  This, however, must be done only
      if end systems are ECN capable and signal that they wish to



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      receive this additional PCN marking information.  If this is
      useful, the required signalling needs to be defined.

   Both options are an independent of the way how PCN marks are encoded.
   Therefore, they are not in the scope of this document.


4.  IANA Considerations

   This document makes no request to IANA.  It does however suggest a
   change to the ([RFC3168]) behaviour for the ECN field for the Voice-
   Admit [voice-admit] DSCP within a PCN domain.


5.  Security Considerations

   {ToDo}


6.  Conclusions

   This document describes an encoding scheme with the following
   benefits: {ToDo}


7.  Comments Solicited

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


8.  References

8.1.  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.

8.2.  Informative References

   [PCN-arch]
              Eardley, P., "Pre-Congestion Notification Architecture",
              draft-ietf-pcn-architecture-03 (work in progress),



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              February 2008.

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

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

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

   [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-01 (work in progress),
              May 2008.

   [voice-admit]
              Baker, F., Polk, J., and M. Dolly, "DSCPs for Capacity-
              Admitted Traffic",
              draft-ietf-tsvwg-admitted-realtime-dscp-04 (work in
              progress), February 2008.


Authors' Addresses

   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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   Jozef Babiarz
   Nortel Networks
   3500 Carling Avenue
   Ottawa  K2H 8E9
   Canada

   Phone: +1-613-763-6098
   Email: babiarz@nortel.com


   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




















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