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Versions: (draft-chan-pcn-encoding-comparison) 00 01 02 03 04 05 06 07 08 09 RFC 6627

PCN                                                              K. Chan
Internet-Draft                                       Huawei Technologies
Intended status: Informational                            G. Karagiannis
Expires: April 29, 2010                             University of Twente
                                                            T. Moncaster
                                                             BT Research
                                                                M. Menth
                                                  University of Wurzburg
                                                              P. Eardley
                                                              B. Briscoe
                                                             BT Research
                                                        October 26, 2009


            Pre-Congestion Notification Encoding Comparison
                 draft-ietf-pcn-encoding-comparison-01

Status of this Memo

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   Please review these documents carefully, as they describe your rights
   and restrictions with respect to this document.

Abstract

   A number of mechanisms have been proposed to support differential
   Qualiy of Service for packets in the Internet.  DiffServ is an
   example of such a mechanism.  However, the level of assurance that
   can be provided with DiffServ without substantial over-provisioning
   is limited.  Pre-Congestion Notification (PCN) uses path congestion
   information across a PCN region to enable per-flow admission control
   to provide the required service guarantees for the admitted traffic.
   While admission control will protect the QoS under normal operating
   conditions, an additional flow termination mechanism is necessary to
   cope with extreme events (e.g. route changes due to link or node
   failure).

   In order to allow the PCN mechanisms to work it is necessary for IP
   packets to be able to carry the pre-congestion information to the PCN
   egress nodes.  This document collects the lessons learned as we
   explore the different ways in which this information can be encoded
   into IP packets.  This document does not choose the encoding but
   provide information on trade offs with the encoding choices,
   providing guidance based on different criteria.  This document
   provides a historical trace of the consideration on different
   encoding alternatives for Pre-Congestion Notification.

























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Table of Contents

   1.  Introduction . . . . . . . . . . . . . . . . . . . . . . . . .  5
   2.  Encoding Environment . . . . . . . . . . . . . . . . . . . . .  5
   3.  Encoding Requirements  . . . . . . . . . . . . . . . . . . . .  6
     3.1.  Encoding States  . . . . . . . . . . . . . . . . . . . . .  6
     3.2.  Encoding and Operating Environment . . . . . . . . . . . .  7
       3.2.1.  PCN Capable (Non PCN Capable) Packet Encoding State  .  8
       3.2.2.  Nonce Encoding State . . . . . . . . . . . . . . . . .  9
       3.2.3.  Non-PCN Traffic Entering PCN Domain  . . . . . . . . .  9
       3.2.4.  PCN Traffic Leaving PCN Domain . . . . . . . . . . . . 10
       3.2.5.  PCN Encoding for Both Edge to Edge and End to End
               Deployment . . . . . . . . . . . . . . . . . . . . . . 11
       3.2.6.  PCN Encoding and Alternate ECN Semantics . . . . . . . 11
       3.2.7.  PCN Encoding and Tunnels . . . . . . . . . . . . . . . 12
     3.3.  Encoding Selection Criteria  . . . . . . . . . . . . . . . 13
   4.  Encoding Options . . . . . . . . . . . . . . . . . . . . . . . 14
     4.1.  Encoding Using ECN and DSCP Fields . . . . . . . . . . . . 15
       4.1.1.  The Use of '01' and '10' Encoding for PCN  . . . . . . 16
       4.1.2.  The Use of '11' Encoding for PCN . . . . . . . . . . . 16
       4.1.3.  The Use of '00' Encoding for PCN . . . . . . . . . . . 16
       4.1.4.  Benefits of Using DSCP and ECN Fields  . . . . . . . . 17
       4.1.5.  Drawbacks of Using DSCP and ECN Fields . . . . . . . . 17
       4.1.6.  Comparing DSCP and ECN Fields Encoding Options . . . . 17
       4.1.7.  Concerns on Alternate Semantics for the ECN Field  . . 18
       4.1.8.  Encoding Choice Considerations . . . . . . . . . . . . 20
     4.2.  Encoding Using DSCP Field  . . . . . . . . . . . . . . . . 21
       4.2.1.  Benefits of Using DSCP Field . . . . . . . . . . . . . 21
       4.2.2.  Drawbacks of Using DSCP Field  . . . . . . . . . . . . 22
       4.2.3.  Comparing DSCP Field Encoding Options  . . . . . . . . 23
   5.  Encoding Recommendations . . . . . . . . . . . . . . . . . . . 23
   6.  Security Implications  . . . . . . . . . . . . . . . . . . . . 24
   7.  IANA Considerations  . . . . . . . . . . . . . . . . . . . . . 25
   8.  Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 25
   Appendix A.   Encoding Using ECN Field . . . . . . . . . . . . . . 25
   Appendix A.1. Benefits of Using ECN Field  . . . . . . . . . . . . 26
   Appendix A.2. Drawbacks of Using ECN Field . . . . . . . . . . . . 27
   Appendix A.3. Concerns on Alternate Semantics for the ECN Field  . 27
   Appendix A.4. Encoding Choice Considerations . . . . . . . . . . . 30
   Appendix B.   Out-of-Band Channel as Encoding Transport  . . . . . 30
   Appendix B.1. Benefits of Using Out-Of-Band Channel  . . . . . . . 31
   Appendix B.2. Drawbacks of Using Out-Of-Band Channel . . . . . . . 31
   Appendix C.   Current PCN Detection, Marking and Transport
                 Mechanisms . . . . . . . . . . . . . . . . . . . . . 31
   Appendix C.1. Detection, Marking and Transport Mechanisms in
                 CL-PHB . . . . . . . . . . . . . . . . . . . . . . . 31
   Appendix C.2. Detection, Marking and Transport Mechanisms in
                 Three State Marking  . . . . . . . . . . . . . . . . 31



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   Appendix C.3. Detection, Marking and Transport Mechanisms in
                 Single Marking . . . . . . . . . . . . . . . . . . . 32
   Appendix C.4. Detection, Marking and Transport Mechanisms in
                 Load Control Marking . . . . . . . . . . . . . . . . 32
   9.  Informative References . . . . . . . . . . . . . . . . . . . . 32
   Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 35













































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1.  Introduction

   This document provides an historical account on the examination of
   the different ways to encode pre-congestion notification (PCN)
   [RFC5559] information in IP packets for transporting the information
   from the PCN ingress nodes, through the PCN interior nodes, to the
   PCN egress nodes.  Documenting the examination results to indicate
   the reasoning behind the approach in selection of PCN encoding in IP
   packets, up to the time that the encoding specifications described in
   [I-D.ietf-pcn-baseline-encoding] has been selected.  Providing an
   account of reasoning and lessons learned from the different encoding
   choices.

   The most important factor affecting the choice of encoding is the
   environment the encoding needs to operate in.  The environment with
   the limited number of available bits to represent the encoding place
   the most stringent restrictions on the encoding choices.

   This document first describes the different environments and their
   constraints on the encoding choices.

   This document then discuss the PCN information that is required to be
   transported.

   Lastly, this document indicates possible ways the required PCN
   information may be encoded and carried in the different environment
   with their constraints.


2.  Encoding Environment

   The operating environment PCN encoding may operate in are:

   1.  Segregated Domain using DiffServ.  Within a single domain with
       PCN traffic segregated from all other traffic using DiffServ Per
       Hop Behavior (DS PHB).  Using DS PHB to separate PCN traffic from
       all other traffic allows the use of the ECN field for PCN
       purpose.  This is the operating environment PCN is mainly focused
       on.  Hence the majority of this document's content is on this
       operating environment.  Variations will also be discussed.  For
       example, using DS PHB to separate PCN and ECN traffic, and use of
       ECN field to distinguish PCN/non-PCN and ECN/non-ECN traffic.
       Another example at one extreme is the use of variations of DSCPs
       to support different PCN encoding options, while not using the
       ECN field for PCN purposes at all.

   2.  Single Domain without PCN and ECN distinctions.  Within a single
       Domain without using a PCN DS PHB.  In this operating



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       environment, PCN and ECN traffic will be treated by the same DS
       PHB, and will need to handle the considerations for alternate
       semantics for ECN field indicated by RFC 4774 [RFC4774].

   3.  Use of Tunnels with PCN.  There have been much discussions with
       the use of IP and MPLS Tunnels with ECN in
       [I-D.ietf-tsvwg-ecn-tunnel] and RFC 5129 [RFC5129]) respectively.
       The encodings examined by this document will take the tunneling
       limitations into consideration.  In particular, the ECN with
       Tunnel [I-D.ietf-tsvwg-ecn-tunnel] proposes to make the tunneling
       of ECN (and hence tunneling of PCN) simpler and more efficient,
       unifying to use one tunneling rule for ECN (and PCN) packets for
       all IP-in-IP tunneling (ECN with IPSec tunneling is specified by
       RFC 4301 [RFC4301] and ECN with MPLS tunneling is specified by
       RFC 5129 [RFC5129]).  The use of [I-D.ietf-tsvwg-ecn-tunnel] will
       help PCN on making the encoding choices.  This approach is being
       taken by [I-D.ietf-pcn-baseline-encoding].

   4.  Multiple Domains using PCN.  Currently this operating environment
       is out of scope for the PCN Working Group, hence this will not be
       discussed here.  But one of the goals of the standardized PCN
       encoding is to use the same encoding in extended environments
       without modification of the standard.  We believe the
       considerations we expressed for single domain can be extended for
       multiple domains when we use DS PHB to facilitate the deployment
       of PCN.


3.  Encoding Requirements

   The internal PCN encoding requirements are based on the functionality
   of PCN [RFC5559], and possibly how the PCN Marking Algorithms achieve
   the functionality.  There may be external requirements depending on
   the environment in which PCN operates, for example co-existence with
   ECN as indicated by RFC 4774 [RFC4774], see Section 2.  These are
   discussed secondary to the internal PCN encoding requirements because
   we have limited the PCN operational environment in the PCN WG's first
   phase charter.  But we also need to take into consideration of the
   encoding standard should not need to be modified for PCN to work in
   both current charter's environment and when current charter's
   environment is expanded, for example, to multi-domain and end-to-end,
   see Section 2.

3.1.  Encoding States

   Currently, there are a number of proposals for Pre-Congestion
   Detection Algorithms.  The authors of the different PCN Algorithm
   documents have agreed to use the notion of Encoding States to



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   represent the information each algorithm wants to export, and hence
   to be carried from the interior nodes to the edge nodes for flow
   admission control and flow termination decisions.  These Encoding
   States form the fundamental functional requirements for the encoding
   choices.

   Please notice the number of "Encoding State" can be different from
   the number of encoding bit patterns.  For example more than two
   "Encoding States" may be carried by two encoding bit pattern when the
   multiple "Encoding States" can be modulated/ multiplexed over some
   time domains.

   For simplicity purpose, we indicate the main required encoding states
   for PCN capable packets:

   o  Not-Marked (NM), for indication of No Pre-Congestion Indication.

   o  Admission Marked (AM), for indication of Flow Admission
      Information.

   o  Termination Marked (TM), for indication of Flow Termination
      Information.

   o  Affected Marked (AfM), for indication of ECMP Information.

   o  Not-PCN, for indication of packets that are not PCN-enabled.

   A total of five main required encoding states for PCN capable
   packets.

   There are also encoding states that may be required, depending on the
   environment assumptions made, these encoding states are described in
   the following sub sections together with their environmental
   considerations.

3.2.  Encoding and Operating Environment

   Currently the PCN Working Group Charter indicates that the operating
   environment being a single domain.  In order to support
   extensibility, it is necessary to specify a consistent encoding used
   for the currently considered operating environments, e.g., a single
   PCN domain, multiple PCN domains, and PCN encoded packet reaching the
   IP end-point, see Section 2.

   In this section, we first discuss the operating environment's affect
   on the encoding states.  We then investigate the effect of the
   operating environment on the encoding options.




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3.2.1.  PCN Capable (Non PCN Capable) Packet Encoding State

   This section describes the PCN Capable packet encoding state, which
   is used to identify the PCN Capable from Non PCN Capable packets.
   This encoding allows the PCN nodes to provide the PCN treatments to
   only the PCN Capable packets.

   The PCN Working Group considers that the PCN traffic will be
   identified by the DSCP codepoint it carries.  But the precise meaning
   of this is not entirely clear.  There is a question whether:

   1.  a DSCP is meant to only represent a scheduling behaviour and
       (pre-)congestion marking behaviour is an optional addition that
       needs to be turned on or off within each existing DSCP (as for
       RFC 3168 [RFC3168] ECN), or

   2.  we redefine the meaning of the DSCP field to represent a
       combination of scheduling and marking behaviour.

   If the first approach is used, for certain PHBs (e.g.  EF [RFC3246])
   PCN marking would need the congestion marking behaviour turned on by
   the setting of another field (e.g. the ECN field).  Then there would
   be a need to further distinguish PCN from Not-PCN packets, both using
   the same DSCP.  Requiring a PCN Capable/Non PCN Capable Encoding
   State represented by a bit pattern using bits outside of the DSCP
   field.  Notice for this approach, we indicate Non PCN Capable bit
   pattern because the use of the other PCN encoding bit patterns can
   indicate PCN Capable.  This approach may be used if we want to
   conserve on DSCP code points by using the same DSCP code point for
   both PCN Capable and Non PCN Capable packets.

   In the second approach, for each scheduling behaviour needing to be
   combined with PCN marking, a new paired DSCP would need to be
   defined.  Then both DSCPs would map to the same scheduling behaviour
   but one will and one will not receive PCN treatment.  For this
   approach, the DSCP provides the indication of PCN Capable packets.

   Hence the decision of taking approach one or approach two will
   indicate if Non PCN Capable Packet Encoding State will be necessary.

   When the Non PCN Capable Packet Encoding State is needed, we use the
   encoding of Not-PCN Capable (Not-PCN) to represent this state.  The
   use of the other required PCN Encoding States will indicate this is a
   PCN Capable Packet.

   In this document when we discuss the encoding options of using both
   the DSCP field and the ECN field to represent the PCN encoding
   states, we assume the use of the second approach above.  This allows



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   the separation between PCN Capable and Non PCN Capable packets be
   totally taken care of by the use of the DiffServ field, leaving the
   ECN field totally available for the other required PCN encoding
   states' usage.

   We believe the use of the second approach is a good choice because we
   do not envision that PCN will be used for many different scheduling
   behaviours.  Furthermore, the use of the second approach allow PCN to
   use the natural ability of DiffServ for PCN packets be handled
   separately, with the PCN domain viewed as a separate forwarding
   domain within routers that can handle multiple forwarding behaviors.
   This use of DiffServ also ease the adaptation of multi-domain and
   end-to-end PCN in the future, using inter-domain DiffServ agreements.

   The proposed new DSCP for capacity-admitted traffic
   [I-D.ietf-tsvwg-admitted-realtime-dscp] seems like it could turn on
   PCN marking with EF scheduling (approach 2).  In this document an
   early version of PCN is given as an example of schemes that might
   need to use the new voice-admit codepoint.  The proposed new DSCP for
   capacity-admitted traffic [I-D.ietf-tsvwg-admitted-realtime-dscp] may
   be used with PCN for the combined use of DSCP and PCN scenario.

3.2.2.  Nonce Encoding State

   The ECN nonce RFC 3540 [RFC3540] for end-to-end ECN is used to
   protect the sender from cheating by the receiver and/or by other down
   stream nodes.  PCN may or may not need a mechanism like the ECN
   nonce.  However single bit nonce schemes such as the ECN nonce
   require in-order, reliable data delivery to function correctly.  As
   PCN operates at the IP layer, in-order delivery cannot be guaranteed.
   If PCN needs a nonce functionality, it may need to think beyond the
   current ECN nonce mechanism.  And this is beyond the scope of this
   document.

   Currently, the PCN work we are doing assumes the trust relationships
   between all the functional entities are already established.  If this
   assumption is not true, then the trust relationships will need to be
   addressed, but may or may not involve the need of additional encoding
   states or the use of the ECN nonce mechanism.

3.2.3.  Non-PCN Traffic Entering PCN Domain

   One of the operating environmental concerns is the accidental
   handling of Non PCN packets by PCN nodes.  The Non PCN packets may
   be:

   o  Non ECN capable packets.




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   o  ECN capable packets.

   With concerns on the impacts of such non PCN packets on:

   o  the processing of PCN packets.

   o  the result of PCN processing on the non PCN packets.

   We first look at the impacts of PCN processing on the non PCN packet
   with original ECN bits:

   o  '00': This indicates the original packet is non ECN capable.  The
      best action is to drop this packet when congestion is experienced.
      The changing of '00' to any other bit patterns will turn such
      packet into an ECN capable packet for any down stream nodes.

   o  '01': This indicates the original packet is ECN capable.  For ECN,
      the only valid change is to change this packet to '11' when the
      offered load needs to be reduced.  Changing this packet to any
      other bit pattern may affect down stream ECN nodes.

   o  '10': This indicates the original packet is ECN capable.  This
      packet have the same concerns as the '01' packets.

   o  '11': This indicates the original packet is ECN capable.  This
      encoding is used to indicate congestion and there is trust for the
      sender to reduce the sending rate when the '11' encoding is
      received by ECN end points.

   With the assumption of using DSCP to separate PCN capable and non PCN
   capable packets, we have to realize the non PCN packets that are
   receiving PCN processing somehow are using the PCN DSCP.  It may be
   beneficial to assume the responsibility of what packets are allowed
   to use the PCN DSCP inside the PCN domain rests with the PCN ingress
   node.  Making such assumption will also allow the support of multiple
   PCN domains and eventually the support of end to end PCN.  With the
   PCN encoding choice and PCN processing being friendly to non PCN
   packets, inside the PCN domain a second line of defense has to be
   supported, after the use of the correct PCN DSCP.

   We defer the investigation of the impact of non PCN packets on PCN
   processing to the sections that describe the encoding choices.

3.2.4.  PCN Traffic Leaving PCN Domain

   There may be two kinds of packets leaving the PCN domain
   unintentionally, valid PCN packets and non PCN packets that received
   PCN processing.  Non PCN packets that did not receive PCN treatment



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   are considered never entered the PCN domain.

   The first line of defense is still the use of the PCN DSCP, only
   packets using the PCN DSCP will receive PCN treatment.  Hence any PCN
   packets leaving the PCN domain will have the PCN DSCP.  Since the PCN
   DSCP is unique, the only danger is for down stream domains to remark
   the PCN DSCP to the best effort DSCP and the PCN packets being treat
   as ECN packets.

3.2.5.  PCN Encoding for Both Edge to Edge and End to End Deployment

   It is the goal of the PCN Working Group to define a standard for PCN
   encoding to allow the encoding be used first in the edge to edge and
   then in the multi-domain and end to end deployment scenarios without
   the need to change the standard.  This section explores this
   environmental consideration by indicating the requirement this
   consideration will place on the PCN encoding selection.

3.2.6.  PCN Encoding and Alternate ECN Semantics

   In order to emphasize the PCN encoding and alternate ECN semantics
   the following effects are discussed:

   o  ECN packets leaked into the PCN domain and processed by PCN
      interior node.

   o  PCN packets leaked into the PCN domain and processed by PCN
      interior node.

   o  ECN packets leaked out of the PCN domain into the ECN domain and
      processed by ECN router and ECN end-points.

   o  PCN packets leaked out of the PCN domain into the ECN domain and
      processed by ECN router and ECN end-points.

   [RFC4774] have also required one to give consideration to what harm
   might be caused by the leaking of PCN traffic into a non-PCN domain.
   The following discussion focuses on each ECN codepoint and shows what
   harm, if any, would be caused when that codepoint leaks out from the
   PCN domain:

   o  '00': The leak should be safe in all circumstances.  RFC 3168
      compliant routers will believe such packets to be not-ECN capable
      and as such will drop them if the router is congested.  This
      codepoint may be suitable for different use by PCN.

   o  '01' and '10': RFC 3168 compliant routers will believe these
      packets are from an ECN capable flow.  If the routers are



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      congested they will mark these packets '11' (CE) instead of
      dropping them.  If the endpoints are not ECN capable then this is
      not good for congestion control.  The use of '01' and '10' by PCN
      can be a potential issue.  To be completely safe, it would be best
      to avoid giving any PCN semantics to these codepoints.

   o  '11': If the packet was already part of an ECN capable flow then
      receivers will believe this was an indication of congestion on the
      path.  They will thus inform their source of this and the source
      will perform a congestion response.  This codepoint may be
      suitable for different use by PCN, the degree of suitability may
      depend on the exact PCN encoding and the metering and marking
      algorithm using the encoding.

   More detailed consideration of these points are provided in the
   sections describing the encoding options.

3.2.7.  PCN Encoding and Tunnels

   Tunneling techniques are used in many parts of today's Internet.  It
   is considered tunneling could start and end at any location, e.g.
   outside the PCN domain, at the edges or inside the PCN domain.  The
   use of tunneling techniques post additional restrictions in PCN
   encoding choices.  There has been much work in considering the use of
   ECN field with tunnels, as indicated by [I-D.ietf-tsvwg-ecn-tunnel].

   The additional restriction placed on the ECN field by tunneling rules
   can be summarized as follows:

   1.  RFC 3168 [RFC3168] indicates the ECN bits of the outer IP header
       may be set to Not-ECT at tunnel ingress.  And at tunnel egress,
       the ECN bits of the inner IP header are unchanged by the tunnel
       mechanism.  This will apply packet dropping and not packet
       marking when congestion is encountered during tunnel.

   2.  RFC 3168 [RFC3168] also indicates only the Not-ECT and ECT code
       point of the ECN bits may be copied from the inner IP header to
       the outer IP header at tunnel ingress and only the CE code point
       may be copied back to the inner IP header at tunnel egress.  This
       limits the use of only the CE code point during tunnel.

   3.  RFC 4301 [RFC4301] allows the complete ECN field be copied from
       the inner IP header to the outer header at ingress but only allow
       the CE code point be copied from outer IP header to the inner IP
       header at egress.  Less restrictive than RFC 3168 [RFC3168] but
       still limits the use of the ECN field by PCN functionalities.





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   4.  Requires the use of '00' code point for represent Not-ECT for ECN
       (Not-PCN) for PCN.  When transfering ECN code points between the
       tunnel's outside header and inside header, have strick ordering
       of code point transistions no change from '00' (Not-ECT); '10'
       (ECT(0)) to '11' (CE); '01' (ECT(1)) to '11' (CE); no change from
       '11' (CE).  These code point transfer restrictions prohibit the
       use of '00' encoding for PCN specific marking purpose, and
       restrict the use of '11' code point for representing the most
       server pre-congestion marking.

   These additional restrictions limit the encoding choices for PCN.
   [I-D.ietf-tsvwg-ecn-tunnel] proposes to make the tunneling of ECN
   (and hence tunneling of PCN) simpler and more efficient, unifying to
   use one tunneling rule for ECN (and PCN) packets for all IP-in-IP
   tunneling (ECN with IPSec tunneling is specified by RFC 4301
   [RFC4301] and ECN with MPLS tunneling is specified by RFC 5129
   [RFC5129]).  The use of [I-D.ietf-tsvwg-ecn-tunnel] will help PCN on
   making the encoding choices.  This approach is being taken by
   [I-D.ietf-pcn-baseline-encoding].

3.3.  Encoding Selection Criteria

   Two possible locations within the IP header have been identified as
   suitable for encoding PCN.  These are the 2 bit ECN field whose
   default meaning is defined in RFC 3168 [RFC3168] and the 6 bit DSCP
   field defined in RFC 2474 [RFC2474] and RFC 2475 [RFC2475].  It is
   already accepted that PCN traffic will be distinguished according to
   which DSCP codepoint it carries.  The implications of this decision
   were discussed in section 2.2.1 above.  The current assumption is
   that PCN will need to be specified as the marking behaviour through
   definition of a new PCN DSCP.

   There are a number of other potential issues that might affect the
   exact choice of encoding to be used.  The key ones are:

   1.  The support of the required encoding states to satisfy the
       functional requirement of PCN.  These required encoding states
       may need two, or three, or four encoding code points to
       represent.

   2.  Compliance with RFC 4774 [RFC4774] if the ECN field is to be re-
       used for PCN encoding.

   3.  Compliance with the requirements for specifying DSCPs and DSCP
       per-hop-behaviour groups [RFC2474].

   4.  Any PCN marking has to carry the '11' codepoint in the ECN field
       since this is the only codepoint that is guaranteed to be copied



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       down into the tunneling inner header upon decapsulation.  This
       criterion is related to the constraints that any PCN encoding
       needs to survive being tunnelled through either an IP in IP
       tunnel or an IPsec Tunnel, see Section 3.2.7.

   5.  Co-existence of PCN and not-PCN traffic: It is important to note
       that the scarcity of pool 1 DSCPs coupled with the fact that PCN
       is envisaged as a marking behaviour that could be applied to a
       number of different DSCPs makes it essential that we provide a
       not-PCN state.  Because PCN re-defines the meaning of the ECN
       field for such DSCPs it is important to allow an operator to
       still use the DSCP for traffic that isn't PCN-enabled.  This is
       achieved by providing a not-PCN state within the encoding scheme.

   Each of these are examined in further details in encoding option
   sections describing their usage.

   With the above discussion, in addition to the criteria indicated so
   far, we should give higher preference to encoding options that:

   o  Minimize problems if there are packet leakage by the PCN domain.

   o  Is safest for wider deployment of PCN, when the current chartered
      environment restriction is relaxed.


4.  Encoding Options

   There are couple of methods to carry the encoding states.  The method
   used affects the encoding options.  Hence when we describe the
   different encoding options in this section, we group them based on
   how the encoding states are carried.

   The encoding transport methods considered are:

   o  using the combination of the ECN and DSCP bits of a data packet
      header

   o  using only the ECN bits of a data packet header

   o  using only the DSCP bits of a data packet header

   We discuss the encoding options for each of the encoding transport
   methods separately in their own subsections.  For shorter reading, we
   have moved the encoding choices the working group have agreed to not
   consider (Using only ECN field, Out-of-Band Channel) sections to the
   Appendix.




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4.1.  Encoding Using ECN and DSCP Fields

   The use of both DSCP and ECN fields is following the second approach
   indicated in section 3.2.1.  This approach allows a clean traffic
   treatment separation of PCN Capable traffic and Non PCN Capable
   traffic.  This natural use of the DSCP field, to provide treatment
   differentiation of packets using different DSCP encoding, is one way
   of providing the "PCN Capable Packet" encoding state.  The using of
   this approach allows us to focus on encoding the four required PCN
   Encoding States, as indicated in section 3.1, using the two ECN bits.


 -----------------------------------------------------------------------
| ECN Bits     ||    00    |    10    |    01    |    11    ||   DSCP   |
|==============++==========+==========+==========+==========++==========|
| RFC 3168     || Not-ECT  |  ECT(0)  |  ECT(1)  |    CE    ||   NA     |
|==============++==========+==========+==========+==========++==========|
| Option 1     ||    AM    |    NM    |    NM    |    TM    ||   PCN-1  |
|--------------++----------+----------+----------+----------++----------|
| Option 2     ||    AfM   |    NM    |    NM    |   AM/TM  ||   PCN-1  |
|--------------++----------+----------+----------+----------++----------|
| Option 3     ||    NM    |    NA    |    NA    |   AM/TM  ||   PCN-1  |
|--------------++----------+----------+----------+----------++----------|
| Option 4     || Not-PCN  |    NM    |   EXP    |   AM/TM  ||   PCN-1  |
|--------------++----------+----------+----------+----------++----------|
| Option 5     ||    NM    |    NA    |    NA    |    AM    ||   PCN-1  |
|              ||          |          |          |    TM    ||   PCN-2  |
|--------------++----------+----------+----------+----------++----------|
| Option 6     ||    AM    |    NM    |    TM    |     NA   ||   PCN-1  |
 -----------------------------------------------------------------------

   Notes: NA means Not Applicable.  PCN-1, PCN-2 under the DSCP column
   denotes specific DSCPs used to indicate PCN capable packets.  AM/TM
   means the two encoding states are sharing the same encoding bit
   pattern.  NM means Not-Marked to represent Not Pre-Congested.  Not-
   PCN means that packets are not PCN enabled.

      Figure 1: Encoding of PCN Information Using DSCP and ECN Fields

   In Figure 1, we listed the fundamental options when both DSCP and ECN
   fields are used.  In Option 4 the ECN codepoints '01' and '10' could
   both be used for NM encoding or one of them could be used NM encoding
   and the other for experimental encoding.  There are couple of
   variations of the theme provided by these options.  One way of
   comparing these options is by examining the pros and cons of the
   different ways the four code points provided by the two ECN bits are
   used.  We group these discussions in the following way:




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   1.  The '01' and '10' code points.

   2.  The '11' code point.

   3.  The '00' code point.

   We discuss each of them in the following sub-sections.

4.1.1.  The Use of '01' and '10' Encoding for PCN

   There can be different degrees of usage of the '01' and '10' code
   points by PCN:

   1.  PCN Does NOT use the '01' and '10' code points, see Option 3 and
       Option 5.  This will be the safest choice.  But this choice will
       leave us with only two usable code points, unless we want to
       deploy more than one PCN DSCPs.  Even when the PCN domain does
       not use these code points, the PCN domain still have to handle
       the receiving of '01' and '10' packets at ingress.  The notion of
       safe comes in two flavors, first if there is any packets in the
       PCN domain having the '01' or '10' encoding, it is immediately
       known that these are packets in error, either they are leaked
       into the PCN domain in error or are set to '01' or '10' in error
       inside the PCN domain.  In both cases, action can be taken.  The
       second flavor of safe is if a legitimate PCN packet leaks out of
       the PCN domain, it will not have the '01' or '10' encoding and
       should not cause an ECN router to mistaken the PCN packets to be
       ECN packets.

   2.  PCN uses '01' and '10' code points in an ECN friendly manner, see
       Options 1, 2, 4, and 6.  One ECN friendly manner is to have both
       '01' and '10' to mean "PCN Capable Packet".  The determination of
       ECN friendliness depends on the use of code points beside '01'
       and '10'.  Furthermore, the use of '01' and '10' codepoints allow
       the transport of the Not-PCN encoding, see Option 4.

4.1.2.  The Use of '11' Encoding for PCN

   Not using the '11' code point for PCN will be a safe choice form the
   ECN semantic point of view, see Option 6.  However, this will reduce
   the possible numger of encoding codepoints to three.  The encoding
   codepoint '11' is used in Options 1, 2, 3, 4, 5.

4.1.3.  The Use of '00' Encoding for PCN

   The '00' codepoint are used by ECN to indicate Not ECN enabled.  A
   safe use of '00' codepoint by PCN will be to indicat Not PCN enabled,
   as in Option 4.  The other usage may have problem in some of the



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   environments.

4.1.4.  Benefits of Using DSCP and ECN Fields

   A major feature of using both DSCP and ECN fields is the ability to
   use the inherent nature of DiffServ for traffic class separation to
   allow PCN treatment be applied to PCN traffic, without concerns of
   applying PCN treatment to none PCN traffic and vise versa.  This
   feature frees this approach for PCN encoding from some of the
   concerns raised by RFC 4774 [RFC4774].  This feature will also keep
   none PCN Capable traffic out of the PCN treatment mechanisms,
   allowing the PCN treatment mechanisms focus on their respective PCN
   tasks.

   This approach also leaves the ECN field available totally for PCN
   encoding states purposes.  Removing the need to carry the Not-PCN
   Encoding in the ECN field.

4.1.5.  Drawbacks of Using DSCP and ECN Fields

   The use of both DSCP and ECN fields will require the setting aside of
   one (or possibly two) DSCP for use by PCN.  This may add complexity
   to the PCN encoding standardization effort.

4.1.6.  Comparing DSCP and ECN Fields Encoding Options

   Here we discuss the differences between the different encoding
   options when both DSCP and ECN fields are used.  There are many
   encoding options, we have provided the ones we think are favorable in
   Figure 1.

   When DSCP is used to differentiate between PCN capable and Not-PCN
   capable traffic, the encoding of "Not-PCN" in the ECN field is not
   required.  This is the motivation for Option 1 in Figure 1, where the
   encoding "00" for "Not-ECT" is being used for "AM" (Admission
   Marking) encoding state.  The encodings "01" and "10" for "ECT(1)"
   and "ECT(0)" supports the required encoding states for "Not Pre-
   Congested Marking" (PCN), and reserving them for any "Nonce Marking"
   if necessary.  With the possible additional encoding of "PCN(A)" and
   "PCN(T)" in place of "ECT(1)" and "ECT(0)" for indicating percentage
   of Admission Marked traffic and percentage of Termination Marked
   traffic when the algorithm benefits from such additional information.

   Option 2 in Figure 1 uses the "00" encoding for "AfM".  With '01' and
   '10' encoding the same as for Option 1, requiring the use of "11"
   encoding for both "AM" (Admission Mark) and "TM" (Termination Mark)
   states or requiring the allocation of a DSCP for encoding the "TM"
   state.



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   Option 4 is the only option that can fulfill both criteria 4 and 5,
   listed in Section 3.3.

4.1.7.  Concerns on Alternate Semantics for the ECN Field

   Section 2 of RFC 4774 [RFC4774] raised couple of concerns for usage
   of alternate semantics for the ECN field.  We try to address each of
   the concerns in this section.

   1.  Section 3.1 of RFC 4774 [RFC4774] discusses Concern 1: "How
       routers know which ECN semantics to use with which packets."
       This use of DSCP and ECN for encoding PCN states address this by
       following the recommendation of RFC 4774 [RFC4774] on using a
       diffserv codepoint to identify the packets using the alternate
       ECN semantics.  This diffserv codepoint may possibly be a new
       diffserv codepoint to minimize the possible confusion between
       using the old per hop behavior of the codepoint and the using of
       the alternate ECN semantics per hop behavior of the codepoint.

   2.  Section 4 of RFC 4774 [RFC4774] discusses Concern 2: "How does
       the possible presence of old routers affect the performance of
       the alternate ECN connections."  With the notion of old routers
       meaning routers that performs RFC 3168 ECN processing instead of
       PCN processing.  An answer to this question is given by assuming
       that the environment using the alternate ECN semantics is
       envisioned to be within a single administrative domain, see
       Section 2, and it has the ability to ensure that all routers
       along the path understand and agree to the use of the alternate
       ECN semantics for the traffic identified by the use of a diffserv
       codepoint.  This uses option 2 indicated in section 4.2 of RFC
       4774 [RFC4774].  But incase there is a mis-configuration, the
       choice of encoding may make a difference:

       *  With encoding Option 1, the old routers will interprete:

          +  '00' encoding as Not-ECT, and will drop AM marked packets.
             The PCN edge nodes should not admit traffic that it does
             not receive, hence the PCN admission functionality should
             be OK.

          +  '01' encoding as ECT(1), which indicates ECN capable and
             can be remarked to '11' to indicate congestion experienced.
             The RFC 3168 ECN CE encoding have the same functionality as
             the PCN TM encoding, to reduce the offered traffic load.
             Hence the PCN termination functionality should be OK.

          +  '10' encoding as ECT(0).  The discussion for '01' above
             applies equally to this encoding.



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          +  '11' encoding as CE.  The old router should use this
             encoding to reduce the offered traffic load and should not
             remark this to any other ECN encoding, the same
             functionality the PCN TM encoding requires, hence should be
             OK for PCN.

          The above discussion for Option 1 applies equally for PCN
          traffic leaked out of the PCN domain and interpreted by RFC
          3168 ECN nodes.

       *  With encoding Option 2, the old routers will interpret:

          +  '00' encoding as Not-ECT, and will drop AfM marked packets.
             This may possibly affect the efficiency of the Affected
             Marking functionality.

          +  '01' encoding as ECT(1), which indicates ECN capable and
             can be remarked to '11' to indicate congestion experienced.
             The RFC 3168 ECN CE encoding have the same functionality as
             the PCN TM encoding, to reduce the offered traffic load.
             Depending on the PCN algorithm on how AM and TM share the
             same '11' encoding, this may or may not affect the
             functionality of PCN.

          +  '10' encoding as ECT(0).  The discussion for '01' above
             applies equally to this encoding.

          +  '11' encoding as CE.  The old router should use this
             encoding to reduce the offered traffic load and should not
             remark this to any other ECN encoding.  Depending on the
             PCN algorithm on how AM and TM share the same '11'
             encoding, this may or may not affect the functionality of
             PCN.

          The above discussion for Option 2 applies equally for PCN
          traffic leaked out of the PCN domain and interpreted by RFC
          3168 ECN nodes.

   3.  Concern 3: "How does the possible presence of old routers affect
       the coexistence of the alternate ECN traffic with competing
       traffic on the path."  Within the PCN domain, the PCN (alternate
       ECN) traffic is separated from the other traffic using diffserv.
       If by mis-configuration, an old routers that does not understand
       PCN handles PCN traffic, the PCN traffic will get the per hop
       behavior as the other traffic, hence not receiving the benefits
       of PCN at the old router, but will not affect the coexistence of
       the PCN and the other traffic.  If the old router uses RFC 3168
       ECN congestion treatment, then the discussion for Concern 2 above



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       applies.

   4.  Concern 4: "How well does the alternate ECN traffic perform."
       The performance of the different proposed PCN (alternate ECN)
       metering and marking algorithms are currently under study with
       their simulation and study results described by their respective
       documents.

   The environment using the alternate ECN semantics is envisioned to be
   within a single administrative domain.  With the ability to ensure
   that all routers along the path understand and agree to the use of
   the alternate ECN semantics for the traffic identified by the use of
   a Diffserv codepoint.  This uses option 2 indicated in section 4.2 of
   RFC 4774 [RFC4774].

4.1.8.  Encoding Choice Considerations

   o  If three encoding states need to be separately represented, Option
      1 is recommended.

   o  If two encoding states need to be separately represented, for
      example the marking algorithm allows the AM and TM encoding states
      be represented using the same bit pattern, Options 2 and 3 are
      recommended.  If the Not-PCN encoding state is required then
      Option 4 is recommended.

   o  If [RFC4774] concerns need to be addressed by PCN encoding, then
      Option 1 is recommended, please see section 3.1.4 for the detail
      discussion.  Options 2 and 3 may be able to address the RFC 4774
      [RFC4774] concerns, but a heavier burden is placed on the metering
      and marking algorithms to differentiate between TM and AM meaning
      of the '11' encoding when a RFC 3168 ECN router sets the '11'
      encoding.

   o  If the metering and marking algorithm requires the use of Affected
      Marking encoding state, Option 2 is recommended.  Alternatively
      one of the bit patterns of '01' or '10' may be used for the AfM
      purpose.  But using '01' or '10' bit patterns for AfM may increase
      the interference between RFC 3168 ECN and PCN encodings, please
      see section 3.1.4 for the detail discussion.

   o  If Option 1 is used and the functionality of Affected Marking
      encoding state is required, the metering and marking algorithms
      will need to provide this functionality either without the use of
      the Affected Marking encoding state, or using an additional DSCP
      to encode the Affected Marking encoding state.





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   o  If Option 4 is used and the functionality of Affected Marking
      encoding state is required, the metering and marking algorithms
      will need to provide this functionality using one of the NA
      encoding states associated with the '01' or '10' bit patterns.

4.2.  Encoding Using DSCP Field

   In this type of encoding and transport method the congestion and
   precongestion information is encoded into the 6 DSCP bits that are
   transported in the IP header of the data packets.  Four possible
   alternatives can be distinguished, as can be seen in Figure 2, with
   details provided by draft-westberg-pcn-load-control-02.txt
   [I-D.westberg-pcn-load-control].  Option 7 needs 2 additional DSCP
   values, Options 8 and 9 need three additional DSCP values and Option
   10 needs four additional DSCP values.  Note that all additional and
   experimental DSCP values are representing and are associated with the
   same PHB.  The 1st, 2nd, 3rd, and 4th DSCP values are representing
   DSCP values that are assigned by IANA as DSCP experimental values,
   see RFC 2211 [RFC2211].  Furthermore, all options listed in Figure 2
   are able to support the Not-PCN encoding state.


 -----------------------------------------------------------------------
| DSCP Bits || Original |Add DSCP 1 |Add DSCP 2 |Add DSCP 3 |Add DSCP 4 |
|===========++==========+===========+===========+===========+===========|
| Option 7  || Not-PCN  |    UM     |   AM/TM   |    NA     |    NA     |
|-----------++----------+-----------+-----------+-----------+-----------|
| Option 8  || Not-PCN  |    UM     |   AM/TM   |    AfM    |    NA     |
|-----------++----------+-----------+-----------+-----------+-----------|
| Option 9  || Not-PCN  |    UM     |     AM    |    TM     |    NA     |
|-----------++----------+-----------+-----------+-----------+-----------|
| Option 10 || Not-PCN  |    UM     |     AM    |    TM     |    AfM    |
 -----------------------------------------------------------------------

   Notes: Not-PCN means the packet is not PCN capable.  UM for Un-Marked
   meaning Not Pre-Congested

          Figure 2: Encoding of PCN Information Using DSCP Field

4.2.1.  Benefits of Using DSCP Field

   The main benefits of using the DSCP field for PCN encoding are:

   o  it is not affecting the end-to-end ECN semantics and therefore the
      issues and concerns raised in RFC 4774 [RFC4774] are not
      applicable for this encoding scheme.





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   o  it is not affected by the PCN tunneling issues discussed in
      Section 3.2.7.

   o  all 4 DSCP encoding options depicted in Figure 2 can support the
      PCN capable not congested/UnMarked (UM) indication, the admission
      control (AM) and flow termination (TM) encoding states.

   o  the experimental DSCPs are lightly standardized and therefore, the
      rules on how to apply and use them are limited.  This provides a
      high flexibility to network operators to apply and use them in
      different settings.

   o  simple packet classification, since a router needs only to read
      the DSCP field, instead of reading both DSCP and ECN fields.

   o  Option 8 and 10 support the Affected Marking (AfM) encoding, which
      according to [I-D.westberg-pcn-load-control], it has benefits if
      the PCN-domain operates ECMP routing and is not using DSCP for
      route selection.

   o  by using an additional DSCP to encode the not congested PCN state,
      all PCN-ingress-nodes can be configured to encode this state into
      all packets that are entering the PCN domain and are PCN aware.
      This will solve any PCN-egress-node misconfiguration problems,
      which can allow a AM/TM or SM encoded packet to outgo a PCN-
      domain.

4.2.2.  Drawbacks of Using DSCP Field

   The main drawbacks of using the DSCP field for PCN encoding are the
   following:

      this type of encoding needs to use per PHB, in addition to the
      original DSCP and depending on the encoding option used, one, two,
      three, or four DSCP values, respectively.  These additional DSCP
      values can be taken from the DSCP values that are not defined by
      standards action, see RFC 2211 [RFC2211].  Note that all the
      additional DSCP values are representing and are associated with
      one PHB.  The value of this DSCP/PHB can either follow a standards
      action or use a value that is applied for experimental or local
      use.  It is important to note that the number of the DSCP values
      used for local or experimental use is restricted and therefore the
      number of different PHBs supported in the PCN domain will also be
      restricted.

      applying the DSCP field as PCN encoding transport within an PCN
      aware MPLS domain, see RFC 5129 [RFC5129], can be problematic due
      to the scarce packet header real-estate.



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      when the PCN-domain is operating ECMP that uses DSCP to select the
      routes, a risk of mis-ordering of packets within a flow might
      occur.  The impact of this drawback depends on the following:

      1.  the level of deployment of ECMP algorithms that use DSCP for
          route selection;

      2.  mis-ordering of packets within a flow when there is
          termination marking may be acceptable;

      3.  the possibility of configuring the ECMP algorithms that use
          DSCP for route selection in the PCN-domain that the used PCN
          aware DSCPs are belonging to the same PHB and therefore, all
          these DSCP values should be converted to one preconfigured
          DSCP value before applying it in the ECMP routing algorithm.
          Note that all the additional experimental DSCPs that are used
          within PCN are belonging to the same PHB.

4.2.3.  Comparing DSCP Field Encoding Options

   Option 7 can support the basic encoding states, i.e,.not PCN, not
   congested (UM), and the AM/TM encoding states.  Option 8 can support
   the basic encoding states supported by Option 7, but in addition it
   can support the AfM state.  Option 9 can support the following basic
   encoding states: not PCN, not congested (UM), AM and TM states.
   Option 10 can support the states supported by Option 9, but in
   addition it can support the AfM state.  Furthermore, in options 7 and
   8 the encoding sequence associated with Admission Control and Flow
   Termination is independent of each other.  In options 9 and 10 a
   packet cannot be AM encoded if it has been earlier TM encoded.  All
   options can support the Not-PCN encoding state.


5.  Encoding Recommendations

   This memo describes the selection process of the PCN encoding in IP
   packets, up to the time that the encoding specifications described in
   the [I-D.ietf-pcn-baseline-encoding] have been selected.  The
   encoding specification given in [I-D.ietf-pcn-baseline-encoding]
   needs to specify how the AM and TM encoding states are represented
   using the same bit pattern.  Furthermore, it is considered that the
   Not-PCN encoding state is required.

   Based on the above listed [I-D.ietf-pcn-baseline-encoding]
   considerations and on the provided discussions in Sections 4.1 and
   4.2, it can be deduced that only Option 4 fulfills the above listed
   [I-D.ietf-pcn-baseline-encoding] considerations and the criteria
   listed in Section 3.3.  Therefore, it is recommended that



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   [I-D.ietf-pcn-baseline-encoding] should use Option 4 to represent the
   'Baseline Encoding and Transport of Pre-Congestion Information'.


6.  Security Implications

   Packets from normal precedence and higher precedence sessions
   [ITU-MLPP] aren't distinguishable by PCN Interior Nodes.  This
   prevents an attacker specifically targeting, in the data plane,
   higher precedence packets (perhaps for DoS or for eavesdropping).
   However, PCN End Nodes can access this information to help decide
   whether to admit or terminate a flow.  The separation of network
   information provided by the Interior Nodes and the precedence
   information at the PCN End Nodes allows simpler, easier and better
   focused security enforcement.

   PCN End Nodes police packets to ensure a flow sticks within its
   agreed limit.  This is similar to the existing IntServ behaviour.
   Between them the PCN End Nodes must fully encircle the PCN-Region,
   otherwise packets could enter the PCN-Region without being subject to
   admission control, which would potentially destroy the QoS of
   existing flows.

   It is assumed that all the Interior Nodes and PCN End Nodes run PCN
   and trust each other (ie the PCN-enabled Internet Region is a
   controlled environment).  For instance a non-PCN router wouldn't be
   able to alert that it's suffering pre-congestion, which potentially
   would lead to too many calls being admitted (or too few being
   terminated).  Worse, a rogue router could perform attacks such as
   marking all packets so that no flows were admitted.

   So security requirements are focussed at specific parts of the PCN-
   Region:

      The PCN End Nodes become the trust points.  The degree of trust
      required depends on the kinds of decisions it has to make and the
      kinds of information it needs to make them.  For example when the
      PCN End Node needs to know the contents of the sessions for making
      the decisions, when the contents are highly classified, the
      security requirements for the PCN End Nodes involved will also
      need to be high.

      PCN-marking by the Interior Nodes along the packet forwarding path
      needs to be trusted, because the PCN End Nodes rely on this
      information.






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7.  IANA Considerations

   This memo includes no request to IANA.


8.  Acknowledgements

   We would like to acknowledge the members of the PCN working group for
   the discussions that generated the contents of this memo.


Appendix A.  Encoding Using ECN Field

   This section takes the approach 1 option indicated in section 2.1.1.
   Which the DSCP field only indicates the packet forwarding behavior,
   for which both PCN Capable and Non PCN Capable traffic use/share the
   same DSCP.  This approach requires the use of the Not PCN Capable
   Encoding State to be encoding using the ECN bits.  Hence this section
   describes the encoding options that uses only the ECN field (without
   the DSCP field) available in the IP header of the data packets to
   encode the PCN states.

   The use of the same DSCP for both PCN Capable and Non PCN Capable
   also opens the question of having PCN and RFC 3168 ECN traffic using
   the same DSCP.  Which increases the importance of satisfying the
   concerns indicated in RFC 4774.


 -----------------------------------------------------------------------
| ECN Bits     ||    00    |    01    |    10    |    11    ||   DSCP   |
|==============++==========+==========+==========+==========++==========|
| RFC 3168     || Not-ECT  |  ECT(1)  |  ECT(0)  |    CE    ||    NA    |
|==============++==========+==========+==========+==========++==========|
| Option 11    || Not-PCN  |    AM    |   PCN    |    TM    ||    NA    |
|--------------++----------+----------+----------+----------++----------|
| Option 12    || Not-PCN  |    PCN   |    PCN   |  AM/TM   ||    NA    |
|--------------++----------+----------+----------+----------++----------|
| Option 13    || Not-PCN  |    AfM   |    PCN   |  AM/TM   ||    NA    |
 -----------------------------------------------------------------------

           Figure 3: Encoding of PCN Information Using ECN Field

   In Figure 2, we listed the fundamental options when only the ECN
   field is used.  Like in Figure 1, there are variations of the theme
   provided by these options.  For example, when both "01" and "10"
   encoding are used for NPM in Option 5, they can be interpreted as
   PCN(A) and PCN(T) instead of just PCN.  Using the PCN(A) and PCN(T)
   variation provides the additional information of the ratio of packets



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   AM marked to packets Not AM marked, and the ratio of packets TM
   marked to packets Not TM marked.  Having these ratios being
   independent from one another.

   For Option 11, the use of '01' for AM and '10' for PCN can be swapped
   and provide the same functionality.  For Option 13, the use of '01'
   for AfM and '10' for PCN can also be swapped without change of
   functionality.

Appendix A.1.  Benefits of Using ECN Field

   The using of only the ECN field for encoding PCN encoding states
   allow more efficient use of the DSCP field, not requiring the
   allocation of PCN specific DSCP values.

   This approach also opens the question of possibly having both PCN and
   ECN traffic using the same DSCP.

   When the same treatment can be provided to both ECN and PCN traffic
   to achieve each of ECN and PCN purpose, then not having DiffServ as
   separation between ECN and PCN traffic may be a benefit.  Under such
   circumstances, having the same encoding between ECN and PCN may be
   desireable.  But this can only be true if the requirement set forth
   in RFC 4774 [RFC4774] for alternate ECN semantics can be satisfied.

   If the same treatment can be applied to both ECN and PCN traffic,
   then:

   o  The first issue of RFC 4774 [RFC4774]: "How routers know which ECN
      semantics to use with which packets." may be solved because there
      are no difference in the treatments of ECN and PCN packets, hence
      they can use the same semanics.

   o  The second and third issues of RFC 4774 [RFC4774]: "How does the
      possible presence of old routers affect the performance of the
      alternate ECN connections." and "How does the possible presence of
      old routers affect the coexistence of the alternate ECN traffic
      with competing traffic on the path." are also solved because there
      are no difference in the treatment of ECN and PCN packets.

   o  The forth issue of RFC 4774 [RFC4774]: "How well does the
      alternate ECN traffic perform." are dependent on the algorithm
      used, and should be provided by the respective algorithm document,
      and not in the scope of this document.







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Appendix A.2.  Drawbacks of Using ECN Field

   Notice this group of encoding options does not use DiffServ code
   points for PCN encoding.  With this group of encoding options, the
   required states of "PCN Capable Transport"/"None PCN Capable
   Transport" must be encoded using the ECN field.  Leaving less
   encoding real estate to carry the remaining required PCN encoding
   states.  Another drawback is without the protection/separation
   capability provided by DiffServ, it is typically harder to satisfy
   the requirement set forth in RFC 4774 [RFC4774] for alternate ECN
   semantics.

Appendix A.3.  Concerns on Alternate Semantics for the ECN Field

   Section 2 of RFC 4774 [RFC4774] raised couple of concerns for usage
   of alternate semantics for the ECN field.  We try to address each of
   the concerns in this section.

   1.  Section 3.1 of RFC 4774 [RFC4774] discusses Concern 1: "How
       routers know which ECN semantics to use with which packets."
       When this group of PCN encodings are used without the use of
       DSCP, routers can not distinguished PCN encoded packets from RFC
       3168 ECN encoded packets.  Hence there needs to be some kind of
       differentiation between PCN and RFC 3168 ECN packets, may be
       using PCN for real-time traffic types (with specific DSCP) and
       ECN for elastic traffic (with specific DSCP).  And only
       distinguishing PCN Capable and Non-PCN Capable packets in real-
       time traffic.  Only distinguishing ECT and Not-ECT packets in
       elastic traffic.  But not having PCN and ECN traffic together.

   2.  Section 4 of RFC 4774 [RFC4774] discusses Concern 2: "How does
       the possible presence of old routers affect the performance of
       the alternate ECN connections."  With the notion of old routers
       meaning routers that performs RFC 3168 ECN processing instead of
       PCN processing, or drop packets instead of encoding the
       congestion information.  The easy answer is the environment using
       the alternate ECN semantics is envisioned to be within a single
       administrative domain.  With the ability to ensure that all
       routers along the path understand and agree to the use of the
       alternate ECN semantics for the traffic identified to be PCN
       Capable.  This uses option 2 indicated in section 4.2 of RFC 4774
       [RFC4774].  But incase there is mis-configuration, the choice of
       encoding may make a difference:

       *  With encoding Option 11, the old routers will interpret:

          +  '00' encoding as Not-ECT, and will drop Not-PCN marked
             packets when congestion is detected.  With '00' the



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             encoding for Not-PCN, requiring the same functionality as
             Not-ECT, the presence of old routers will not affect the
             performance of PCN functionality.

          +  '01' encoding as ECT(1), which indicates ECN capable and
             can be remarked to '11' to indicate congestion experienced.
             For Option 3, the old router can possibly remark AM to TM.
             This puts a burden on the metering and marking algorithms
             to treat TM encoded packets to indicate stop admission.
             This may or may not be acceptable, depending on the
             algorithm.

          +  '10' encoding as ECT(0), which indicates ECN capable and
             can be remarked to '11' to indicate congestion experienced.
             The RFC 3168 ECN CE encoding have the same functionality as
             the PCN TM encoding, to reduce the offered traffic load.
             Hence the PCN termination functionality should be OK.

          +  '11' encoding as CE.  The old router should use this
             encoding to reduce the offered traffic load and should not
             remark this to any other ECN encoding, the same
             functionality the PCN TM encoding requires, hence should be
             OK for PCN.

          The above discussion for Option 11 applies equally for PCN
          traffic leaked out of the PCN domain and interpreted by RFC
          3168 ECN nodes.

       *  With encoding Option 12, the old routers will interprete:

          +  '00' encoding as Not-ECT, and will drop Not-PCN marked
             packets when congestion is detected.  With '00' the
             encoding for Not-PCN, requiring the same functionality as
             Not-ECT, the presence of old routers will not affect the
             performance of PCN functionality.

          +  '01' encoding as ECT(1), which indicates ECN capable and
             can be remarked to '11' to indicate congestion experienced.
             The RFC 3168 ECN CE encoding have the same functionality as
             the PCN TM encoding, to reduce the offered traffic load.
             Depending on the PCN algorithm on how AM and TM share the
             same '11' encoding, this may or may not affect the
             functionality of PCN.

          +  '10' encoding as ECT(0).  The discussion for '01' above
             applies equally to this encoding.





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          +  '11' encoding as CE.  The old router should use this
             encoding to reduce the offered traffic load and should not
             remark this to any other ECN encoding.  Depending on the
             PCN algorithm on how AM and TM share the same '11'
             encoding, this may or may not affect the functionality of
             PCN.

          The above discussion for Option 12 applies equally for PCN
          traffic leaked out of the PCN domain and interpreted by RFC
          3168 ECN nodes.

       *  With encoding Option 13, the old routers will interprete:

          +  '00' encoding as Not-ECT, and will drop Not-PCN marked
             packets when congestion is detected.  With '00' the
             encoding for Not-PCN, requiring the same functionality as
             Not-ECT, the presence of old routers will not affect the
             performance of PCN functionality.

          +  '01' encoding as ECT(1), which indicates ECN capable and
             can be remarked to '11' to indicate congestion experienced.
             For Option 6, the old router can possibly remark AfM to TM.
             This may or may not be acceptable, depending on the
             algorithm's Affected Marking functionality.

          +  '10' encoding as ECT(1), which indicates ECN capable and
             can be remarked to '11' to indicate congestion experienced.
             The RFC 3168 ECN CE encoding have the same functionality as
             the PCN TM encoding, to reduce the offered traffic load.
             Depending on the PCN algorithm on how AM and TM share the
             same '11' encoding, this may or may not affect the
             functionality of PCN.

          +  '11' encoding as CE.  The old router should use this
             encoding to reduce the offered traffic load and should not
             remark this to any other ECN encoding.  Depending on the
             PCN algorithm on how AM and TM share the same '11'
             encoding, this may or may not affect the functionality of
             PCN.

          The above discussion for Option 13 applies equally for PCN
          traffic leaked out of the PCN domain and interpreted by RFC
          3168 ECN nodes.

   3.  Concern 3: "How does the possible presence of old routers affect
       the coexistence of the alternate ECN traffic with competing
       traffic on the path."  If RFC 3168 ECN and PCN traffic are to be
       treated within a single DiffServ PHB, because with these encoding



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       there is no way to differentiate between the ECN packets from the
       PCN traffic, the metering and marking algorithm used must be
       totally friendly between ECN and PCN traffic, else they will
       affect each other in possibly non-acceptable ways.  These
       encoding will work OK with traffic besides ECN because of the use
       of 'Not-PCN' encoding.

   4.  Concern 4: "How well does the alternate ECN traffic perform."
       The performance of the different proposed PCN (alternate ECN)
       metering and marking algorithms are currently under study with
       their simulation and study results described by their respective
       documents.

Appendix A.4.  Encoding Choice Considerations

   o  If three encoding states need to be separately represented, Option
      11 is recommended.

   o  If the marking algorithm allows the AM and TM encoding states be
      represented using the same bit pattern, Option 12 is recommended.

   o  If the marking algorithm requires the use of Affected Marking
      encoding state, Option 13 is recommended.  For Option 13,
      alternative NPM bit patterns ('01' or '10') may be used for the
      AfM purpose.


Appendix B.  Out-of-Band Channel as Encoding Transport

   In this type of encoding and transport method the congestion and
   precongestion information can be encoded using the IPFIX protocol RFC
   3955 [18], that is normally used to carry flow-based IP traffic
   measurements from an observation point to a collecting point.  Note
   that this encoding scheme is denoted in this document as "IPFIX
   channel".  An observation point is a location in a network where IP
   packets can be observed and measured.  A collecting point can be a
   process or a node that receives flow records from one or more
   observation points.  In the PCN case, each PCN-interior-node will be
   an IPFIX observation point and the PCN-egress-node will be the IPFIX
   collecting point.

   The PCN-interior-node will support the metering process and the flow
   records.  Note that in this case each flow record can be associated
   with the record of the congestion and pre-congestion metering
   information associated with each PHB.  The PCN-egress-node will then
   support the IPFIX collecting process, which will receive flow records
   from one or more congested and pre-congested PCN-interior-nodes.
   Using this encoding method the encoding modes/states can be



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   aggregated and transported to the egress node by using the flow
   records at regular intervals or at the moment that a congestion and
   pre-congestion situation occurs.  The used transport channel in this
   case is not the data path but a signaling protocol.

Appendix B.1.  Benefits of Using Out-Of-Band Channel

   This encoding scheme does not use the data path for encoding and
   transport, but it is able to transport the congestion and pre-
   congestion information associated with the encoding states by using a
   separate signaling channel.  Another benefit of using this encoding
   scheme is that it is not affecting the end-to-end ECN semantics and
   therefore the issues and concerns raised in RFC 4774 are not
   applicable for this encoding scheme.

Appendix B.2.  Drawbacks of Using Out-Of-Band Channel

   The "IPFIX channel" encoding mode needs a separate signaling channel
   for the transport of the congestion and precongestion information
   from the PCN-interior-nodes towards the PCN-egress-node.  The
   requirement of using an additional channel increases the complexity
   and influences negatively the performance of the PCN-interior-nodes
   since each PCN-interior-node needs to support in addition to the data
   path a separate channel.


Appendix C.  Current PCN Detection, Marking and Transport Mechanisms

   This appendix indicates the different available PCN based mechanisms
   that can be used for congestion and pre-congestion detection and
   marking used at interior nodes.  The requirements and characteristics
   of such algorithms may influence the encoding and transport of the
   PCN encoding states.

Appendix C.1.  Detection, Marking and Transport Mechanisms in CL-PHB

   Please see draft-briscoe-tsvwg-cl-phb-03.txt
   [I-D.briscoe-tsvwg-cl-phb] for details on the Controlled-Load PHB
   Algorithm.

Appendix C.2.  Detection, Marking and Transport Mechanisms in Three
               State Marking

   Please see draft-babiarz-pcn-3sm-01.txt [I-D.babiarz-pcn-3sm] for
   details on the Three State Marking Algorithm.






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Appendix C.3.  Detection, Marking and Transport Mechanisms in Single
               Marking

   Please see draft-charny-pcn-single-marking-03.txt
   [I-D.charny-pcn-single-marking] for details on the Single Marking
   Algorithm.

Appendix C.4.  Detection, Marking and Transport Mechanisms in Load
               Control Marking

   Please see draft-westberg-pcn-load-control-02.txt
   [I-D.westberg-pcn-load-control] for details on the Load Control
   Algorithm.


9.  Informative References

   [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-07 (work in progress),
              September 2009.

   [I-D.ietf-tsvwg-ecn-tunnel]
              Briscoe, B., "Tunnelling of Explicit Congestion
              Notification", draft-ietf-tsvwg-ecn-tunnel-03 (work in
              progress), July 2009.

   [I-D.babiarz-pcn-3sm]
              Babiarz, J., Liu, X., Chan, K., and M. Menth, "Three State
              PCN Marking", draft-babiarz-pcn-3sm-01 (work in progress),
              November 2007.

   [I-D.charny-pcn-single-marking]
              Charny, A., Zhang, X., Faucheur, F., and V. Liatsos, "Pre-
              Congestion Notification Using Single Marking for Admission
              and  Termination", draft-charny-pcn-single-marking-03
              (work in progress), November 2007.

   [I-D.westberg-pcn-load-control]
              Westberg, L., Bhargava, A., Bader, A., Karagiannis, G.,
              and H. Mekkes, "LC-PCN: The Load Control PCN Solution",
              draft-westberg-pcn-load-control-05 (work in progress),
              November 2008.

   [I-D.briscoe-tsvwg-cl-phb]
              Briscoe, B., "Pre-Congestion Notification marking",
              draft-briscoe-tsvwg-cl-phb-03 (work in progress),



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              October 2006.

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

   [I-D.ietf-tsvwg-mlef-concerns]
              Baker, F. and J. Polk, "MLEF Without Capacity Admission
              Does Not Satisfy MLPP Requirements",
              draft-ietf-tsvwg-mlef-concerns-00 (work in progress),
              February 2005.

   [RFC1633]  Braden, B., Clark, D., and S. Shenker, "Integrated
              Services in the Internet Architecture: an Overview",
              RFC 1633, June 1994.

   [RFC2211]  Wroclawski, J., "Specification of the Controlled-Load
              Network Element Service", RFC 2211, September 1997.

   [RFC2309]  Braden, B., Clark, D., Crowcroft, J., Davie, B., Deering,
              S., Estrin, D., Floyd, S., Jacobson, V., Minshall, G.,
              Partridge, C., Peterson, L., Ramakrishnan, K., Shenker,
              S., Wroclawski, J., and L. Zhang, "Recommendations on
              Queue Management and Congestion Avoidance in the
              Internet", RFC 2309, April 1998.

   [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.

   [RFC2475]  Blake, S., Black, D., Carlson, M., Davies, E., Wang, Z.,
              and W. Weiss, "An Architecture for Differentiated
              Services", RFC 2475, December 1998.

   [RFC2597]  Heinanen, J., Baker, F., Weiss, W., and J. Wroclawski,
              "Assured Forwarding PHB Group", RFC 2597, June 1999.

   [RFC2702]  Awduche, D., Malcolm, J., Agogbua, J., O'Dell, M., and J.
              McManus, "Requirements for Traffic Engineering Over MPLS",
              RFC 2702, September 1999.

   [RFC2998]  Bernet, Y., Ford, P., Yavatkar, R., Baker, F., Zhang, L.,
              Speer, M., Braden, R., Davie, B., Wroclawski, J., and E.
              Felstaine, "A Framework for Integrated Services Operation
              over Diffserv Networks", RFC 2998, November 2000.



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

   [RFC3246]  Davie, B., Charny, A., Bennet, J., Benson, K., Le Boudec,
              J., Courtney, W., Davari, S., Firoiu, V., and D.
              Stiliadis, "An Expedited Forwarding PHB (Per-Hop
              Behavior)", RFC 3246, March 2002.

   [RFC3247]  Charny, A., Bennet, J., Benson, K., Boudec, J., Chiu, A.,
              Courtney, W., Davari, S., Firoiu, V., Kalmanek, C., and K.
              Ramakrishnan, "Supplemental Information for the New
              Definition of the EF PHB (Expedited Forwarding Per-Hop
              Behavior)", RFC 3247, March 2002.

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

   [RFC3955]  Leinen, S., "Evaluation of Candidate Protocols for IP Flow
              Information Export (IPFIX)", RFC 3955, October 2004.

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

   [RFC4594]  Babiarz, J., Chan, K., and F. Baker, "Configuration
              Guidelines for DiffServ Service Classes", RFC 4594,
              August 2006.

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

   [RFC5129]  Davie, B., Briscoe, B., and J. Tay, "Explicit Congestion
              Marking in MPLS", RFC 5129, January 2008.

   [RFC5559]  Eardley, P., "Pre-Congestion Notification (PCN)
              Architecture", RFC 5559, June 2009.

   [DClark]   "Supporting Real-Time Applications in an Integrated
              Services Packet Network: Architecture and Mechanisms",
              Proceedings of SIGCOMM '92 at Baltimore MD, August 1992.

   [ITU-MLPP]
              "Multilevel Precedence and Pre-emption Service (MLPP)",
              ITU-T Recommendation I.255.3, 1990.

   [Reid]     "Economics and Scalability of QoS Solutions", BT



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              Technology Journal Vol 23 No 2, April 2005.


Authors' Addresses

   Kwok Ho Chan
   Huawei Technologies
   125 Nagog Park
   Acton, MA  01720
   USA

   Email: khchan@huawei.com


   Georgios Karagiannis
   University of Twente
   P.O. Box 217
   7500 AE Enschede,
   The Netherlands

   Email: g.karagiannis@ewi.utwente.nl


   Toby Moncaster
   BT Research
   B54/70, Sirius House Adastral Park Martlesham Heath
   Ipswich, Suffolk  IP5 3RE
   United Kingdom

   Email: toby.moncaster@bt.com


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

   Email: menth@informatik.uni-wuerzburg.de











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   Philip Eardley
   BT Research
   B54/77, Sirius House Adastral Park Martlesham Heath
   Ipswich, Suffolk  IP5 3RE
   United Kingdom

   Email: philip.eardley@bt.com


   Bob Briscoe
   BT Research
   B54/77, Sirius House Adastral Park Martlesham Heath
   Ipswich, Suffolk  IP5 3RE
   United Kingdom

   Email: bob.briscoe@bt.com



































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