draft-ietf-pcn-baseline-encoding-04.txt   draft-ietf-pcn-baseline-encoding-05.txt 
Congestion and Pre Congestion T. Moncaster Congestion and Pre Congestion T. Moncaster
Internet-Draft BT Internet-Draft B. Briscoe
Intended status: Standards Track B. Briscoe Intended status: Standards Track BT
Expires: November 20, 2009 BT & UCL Expires: February 21, 2010 M. Menth
M. Menth
University of Wuerzburg University of Wuerzburg
May 19, 2009 August 20, 2009
Baseline Encoding and Transport of Pre-Congestion Information Baseline Encoding and Transport of Pre-Congestion Information
draft-ietf-pcn-baseline-encoding-04 draft-ietf-pcn-baseline-encoding-05
Status of This Memo Status of This Memo
This Internet-Draft is submitted to IETF in full conformance with the This Internet-Draft is submitted to IETF in full conformance with the
provisions of BCP 78 and BCP 79. This document may contain material provisions of BCP 78 and BCP 79. This document may contain material
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available before November 10, 2008. The person(s) controlling the available before November 10, 2008. The person(s) controlling the
copyright in some of this material may not have granted the IETF copyright in some of this material may not have granted the IETF
Trust the right to allow modifications of such material outside the Trust the right to allow modifications of such material outside the
IETF Standards Process. Without obtaining an adequate license from IETF Standards Process. Without obtaining an adequate license from
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and may be updated, replaced, or obsoleted by other documents at any and may be updated, replaced, or obsoleted by other documents at any
time. It is inappropriate to use Internet-Drafts as reference time. It is inappropriate to use Internet-Drafts as reference
material or to cite them other than as "work in progress." material or to cite them other than as "work in progress."
The list of current Internet-Drafts can be accessed at The list of current Internet-Drafts can be accessed at
http://www.ietf.org/ietf/1id-abstracts.txt. http://www.ietf.org/ietf/1id-abstracts.txt.
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This Internet-Draft will expire on November 20, 2009. This Internet-Draft will expire on February 21, 2010.
Copyright Notice Copyright Notice
Copyright (c) 2009 IETF Trust and the persons identified as the Copyright (c) 2009 IETF Trust and the persons identified as the
document authors. All rights reserved. document authors. All rights reserved.
This document is subject to BCP 78 and the IETF Trust's Legal This document is subject to BCP 78 and the IETF Trust's Legal
Provisions Relating to IETF Documents in effect on the date of Provisions Relating to IETF Documents in effect on the date of
publication of this document (http://trustee.ietf.org/license-info). publication of this document (http://trustee.ietf.org/license-info).
Please review these documents carefully, as they describe your rights Please review these documents carefully, as they describe your rights
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previously admitted flows. This document specifies how such marks previously admitted flows. This document specifies how such marks
are to be encoded into the IP header by re-using the Explicit are to be encoded into the IP header by re-using the Explicit
Congestion Notification (ECN) codepoints within this controlled Congestion Notification (ECN) codepoints within this controlled
domain. The baseline encoding described here provides for only two domain. The baseline encoding described here provides for only two
PCN encoding states, Not-marked and PCN-marked. PCN encoding states, Not-marked and PCN-marked.
Table of Contents Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3
2. Requirements notation . . . . . . . . . . . . . . . . . . . . 5 2. Requirements notation . . . . . . . . . . . . . . . . . . . . 5
3. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 5 3. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 6
4. Encoding two PCN States in IP . . . . . . . . . . . . . . . . 6 4. Encoding two PCN States in IP . . . . . . . . . . . . . . . . 6
4.1. Valid and Invalid Codepoint Transitions . . . . . . . . . 6 4.1. Valid and Invalid Codepoint Transitions . . . . . . . . . 7
4.2. Rationale for Encoding . . . . . . . . . . . . . . . . . . 7 4.2. Rationale for Encoding . . . . . . . . . . . . . . . . . . 8
4.3. PCN-Compatible Diffserv Codepoints . . . . . . . . . . . . 8 4.3. PCN-Compatible Diffserv Codepoints . . . . . . . . . . . . 8
4.3.1. Co-existence of PCN and not-PCN traffic . . . . . . . 8 4.3.1. Co-existence of PCN and not-PCN traffic . . . . . . . 9
5. Rules for Experimental Encoding Schemes . . . . . . . . . . . 8 5. Rules for Experimental Encoding Schemes . . . . . . . . . . . 9
6. Backwards Compatibility . . . . . . . . . . . . . . . . . . . 9 6. Backwards Compatibility . . . . . . . . . . . . . . . . . . . 9
7. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 9 7. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 10
8. Security Considerations . . . . . . . . . . . . . . . . . . . 9 8. Security Considerations . . . . . . . . . . . . . . . . . . . 10
9. Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . 10 9. Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . 10
10. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 10 10. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 10
11. Comments Solicited . . . . . . . . . . . . . . . . . . . . . . 10 11. Comments Solicited . . . . . . . . . . . . . . . . . . . . . . 11
12. References . . . . . . . . . . . . . . . . . . . . . . . . . . 10 12. References . . . . . . . . . . . . . . . . . . . . . . . . . . 11
12.1. Normative References . . . . . . . . . . . . . . . . . . . 10 12.1. Normative References . . . . . . . . . . . . . . . . . . . 11
12.2. Informative References . . . . . . . . . . . . . . . . . . 11 12.2. Informative References . . . . . . . . . . . . . . . . . . 11
Appendix A. PCN Deployment Considerations (Informational) . . . . 11 Appendix A. PCN Deployment Considerations (Informational) . . . . 12
A.1. Choice of Suitable DSCPs . . . . . . . . . . . . . . . . . 11 A.1. Choice of Suitable DSCPs . . . . . . . . . . . . . . . . . 12
A.2. Rationale for Using ECT(0) for Not-marked . . . . . . . . 12 A.2. Rationale for Using ECT(0) for Not-marked . . . . . . . . 13
1. Introduction 1. Introduction
The objective of Pre-Congestion Notification (PCN) is to protect the The objective of Pre-Congestion Notification (PCN) [RFC5559] is to
quality of service (QoS) of inelastic flows within a Diffserv domain, protect the quality of service (QoS) of inelastic flows within a
in a simple, scalable and robust fashion. The overall rate of the Diffserv domain, in a simple, scalable and robust fashion. The
PCN-traffic is metered on every link in the PCN-domain, and PCN- overall rate of the PCN-traffic is metered on every link in the PCN-
packets are appropriately marked when certain configured rates are domain, and PCN-packets are appropriately marked when certain
exceeded. These configured rates are below the rate of the link thus configured rates are exceeded. These configured rates are below the
providing notification before any congestion occurs (hence "pre- rate of the link thus providing notification before any congestion
congestion notification"). The level of marking allows the boundary occurs (hence "pre-congestion notification"). The level of marking
nodes to make decisions about whether to admit or block a new flow allows the boundary nodes to make decisions about whether to admit or
request, and (in abnormal circumstances) whether to terminate some of block a new flow request, and (in abnormal circumstances) whether to
the existing flows, thereby protecting the QoS of previously admitted terminate some of the existing flows, thereby protecting the QoS of
flows. previously admitted flows.
This document specifies how these PCN marks are encoded into the IP This document specifies how these PCN marks are encoded into the IP
header by re-using the bits of the Explicit Congestion Notification header by re-using the bits of the Explicit Congestion Notification
(ECN) field [RFC3168]. It also describes how packets are identified (ECN) field [RFC3168]. It also describes how packets are identified
as belonging to a PCN flow. Some deployment models require two PCN as belonging to a PCN flow. Some deployment models require two PCN
encoding states, others require more. The baseline encoding encoding states, others require more. The baseline encoding
described here only provides for two PCN encoding states. However described here only provides for two PCN encoding states. However
the encoding can be easily extended to provide more states. Rules the encoding can be easily extended to provide more states. Rules
for such extensions are given in Section 5. for such extensions are given in Section 5.
Changes from previous drafts (to be removed by the RFC Editor): Changes from previous drafts (to be removed by the RFC Editor):
From -04 to -05:
Clarified throughout that the PCN WG is not requesting a specific
DSCP for PCN. Rather we are recommending a set of DSCPs that
might be suitable. Appendix A.1 has been re-written to reflect
this. References to maintaining a list of PCN-compatible DSCPs
have also been removed.
Last sentence of Section 6 altered.
Several spelling corrections.
References updated throughout.
From -03 to -04: From -03 to -04:
Major WGLC comments addressed: Major WGLC comments addressed:
* Added Section 4.3.1 to clarify why we need the not-PCN * Added Section 4.3.1 to clarify why we need the not-PCN
codepoint. codepoint.
* Stated that the PCN WG will maintain a list of PCN-compatible * Stated that the PCN WG will maintain a list of PCN-compatible
DSCPs. This should help avoid inter-operability issues. DSCPs. This should help avoid inter-operability issues.
skipping to change at page 4, line 19 skipping to change at page 4, line 34
* Security considerations re-written. * Security considerations re-written.
* Appendixes re-written to improve clarity. * Appendixes re-written to improve clarity.
* Numerous minor nits and language changes throughout. * Numerous minor nits and language changes throughout.
Extensive other minor changes throughout. Extensive other minor changes throughout.
From -01 to -02: From -01 to -02:
Removed Appendix A and replaced with reference to [ECN-tunnel] Removed Appendix A and replaced with reference to
[I-D.ietf-tsvwg-ecn-tunnel]
Moved Appendix B into main body of text. Moved Appendix B into main body of text.
Changed Appendix C to give deployment advice. Changed Appendix C to give deployment advice.
Minor changes throughout including checking consistency of Minor changes throughout including checking consistency of
capitalisation of defined terms. capitalisation of defined terms.
Clarified that LU was deliberately excluded from encoding. Clarified that LU was deliberately excluded from encoding.
skipping to change at page 5, line 13 skipping to change at page 5, line 29
Minor changes throughout. Minor changes throughout.
Modified meaning of ECT(1) state to EXP. Modified meaning of ECT(1) state to EXP.
Moved text relevant to behaviour of nodes into appendix for later Moved text relevant to behaviour of nodes into appendix for later
transfer to new document on edge behaviours. transfer to new document on edge behaviours.
From draft-moncaster -01 to -02: From draft-moncaster -01 to -02:
Minor changes throughout including tightening up language to Minor changes throughout including tightening up language to
remain consistent with the PCN Architecture terminology remain consistent with the PCN Architecture terminology.
From draft-moncaster -00 to -01: From draft-moncaster -00 to -01:
Change of title from "Encoding and Transport of (Pre-)Congestion Change of title from "Encoding and Transport of (Pre-)Congestion
Information from within a Diffserv Domain to the Egress" Information from within a Diffserv Domain to the Egress"
Extensive changes to Introduction and abstract. Extensive changes to Introduction and abstract.
Added a section on the implications of re-using a DSCP. Added a section on the implications of re-using a DSCP.
skipping to change at page 5, line 45 skipping to change at page 6, line 15
3. Terminology 3. Terminology
The following terms are used in this document: The following terms are used in this document:
o PCN-compatible Diffserv codepoint - a Diffserv codepoint for which o PCN-compatible Diffserv codepoint - a Diffserv codepoint for which
the ECN field is used to carry PCN markings rather than [RFC3168] the ECN field is used to carry PCN markings rather than [RFC3168]
markings. markings.
o PCN-marked - codepoint indicating packets that have been marked at o PCN-marked - codepoint indicating packets that have been marked at
a PCN-interior-node using some PCN marking behaviour a PCN-interior-node using some PCN marking behaviour
[PCN-metering-marking]. Abbreviated to PM. [I-D.ietf-pcn-marking-behaviour]. Abbreviated to PM.
o Not-marked - codepoint indicating packets that are PCN-capable, o Not-marked - codepoint indicating packets that are PCN-capable,
but are not PCN-marked. Abbreviated to NM. but are not PCN-marked. Abbreviated to NM.
o PCN-enabled codepoints - collective term for all NM and PM o PCN-enabled codepoints - collective term for all NM and PM
codepoints. By definition, packets carrying such codepoints are codepoints. By definition, packets carrying such codepoints are
PCN-packets. PCN-packets.
o not-PCN - packets that are not PCN-enabled. o not-PCN - packets that are not PCN-enabled.
In addition, the document uses the terminology defined in In addition, the document uses the terminology defined in [RFC5559].
[PCN-architecture].
4. Encoding two PCN States in IP 4. Encoding two PCN States in IP
The PCN encoding states are defined using a combination of the DSCP The PCN encoding states are defined using a combination of the DSCP
and ECN fields within the IP header. The baseline PCN encoding and ECN fields within the IP header. The baseline PCN encoding
closely follows the semantics of ECN [RFC3168]. It allows the closely follows the semantics of ECN [RFC3168]. It allows the
encoding of two PCN states: Not-marked and PCN-marked. It also encoding of two PCN states: Not-marked and PCN-marked. It also
allows for traffic that is not PCN-capable to be marked as such (not- allows for traffic that is not PCN-capable to be marked as such (not-
PCN). Given the scarcity of codepoints within the IP header the PCN). Given the scarcity of codepoints within the IP header the
baseline encoding leaves one codepoint free for experimental use. baseline encoding leaves one codepoint free for experimental use.
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+---------------+-------------+-------------+-------------+---------+ +---------------+-------------+-------------+-------------+---------+
| ECN codepoint | Not-ECT | ECT(0) (10) | ECT(1) (01) | CE (11) | | ECN codepoint | Not-ECT | ECT(0) (10) | ECT(1) (01) | CE (11) |
| | (00) | | | | | | (00) | | | |
+---------------+-------------+-------------+-------------+---------+ +---------------+-------------+-------------+-------------+---------+
| DSCP n | not-PCN | NM | EXP | PM | | DSCP n | not-PCN | NM | EXP | PM |
+---------------+-------------+-------------+-------------+---------+ +---------------+-------------+-------------+-------------+---------+
Where DSCP n is a PCN-compatible Diffserv codepoint (see Section 4.3) Where DSCP n is a PCN-compatible Diffserv codepoint (see Section 4.3)
and EXP means available for Experimental use. N.B. we deliberately and EXP means available for Experimental use. N.B. we deliberately
reserve this codepoint for experimental use only (and not local use) reserve this codepoint for experimental use only (and not local use)
to prevent future compatability issues. to prevent future compatibility issues.
Table 1: Encoding PCN in IP Table 1: Encoding PCN in IP
The following rules apply to all PCN traffic: The following rules apply to all PCN traffic:
o PCN-traffic MUST be marked with a PCN-compatible Diffserv o PCN-traffic MUST be marked with a PCN-compatible Diffserv
Codepoint. To conserve DSCPs, Diffserv Codepoints SHOULD be Codepoint. To conserve DSCPs, Diffserv Codepoints SHOULD be
chosen that are already defined for use with admission controlled chosen that are already defined for use with admission controlled
traffic, such as the Voice-Admit codepoint defined in traffic. Appendix A.1 gives guidance to implementiors on suitable
[Voice-Admit]. Guidelines for mixing traffic-types within a PCN- DSCPs. Guidelines for mixing traffic-types within a PCN-domain
domain are given in [PCN-metering-marking]. are given in [I-D.ietf-pcn-marking-behaviour].
o Any packet that is not-PCN but which shares the same Diffserv o Any packet that is not-PCN but which shares the same Diffserv
codepoint as PCN-enabled traffic MUST have the ECN field of its codepoint as PCN-enabled traffic MUST have the ECN field of its
outermost IP header equal to 00. outermost IP header equal to 00.
4.1. Valid and Invalid Codepoint Transitions 4.1. Valid and Invalid Codepoint Transitions
A PCN-ingress-node MUST set the Not-marked (10) codepoint on any A PCN-ingress-node MUST set the Not-marked (10) codepoint on any
arriving packet that belongs to a PCN-flow. It MUST set the not-PCN arriving packet that belongs to a PCN-flow. It MUST set the not-PCN
(00) codepoint on all other packets sharing a PCN-compatible Diffserv (00) codepoint on all other packets sharing a PCN-compatible Diffserv
codepoint. codepoint.
The only valid codepoint transitions within a PCN-interior-node are The only valid codepoint transitions within a PCN-interior-node are
from NM to PM (which should occur if either meter indicates a need to from NM to PM (which should occur if either meter indicates a need to
PCN-mark a packet [PCN-metering-marking]) and from EXP to PM (which PCN-mark a packet [I-D.ietf-pcn-marking-behaviour]) and from EXP to
MAY be allowed by some future experimental extensions). The PM (which MAY be allowed by some future experimental extensions).
following table gives the full set of valid and invalid codepoint The following table gives the full set of valid and invalid codepoint
transitions. transitions.
+-------------------------------------------------+ +-------------------------------------------------+
| Codepoint Out | | Codepoint Out |
+--------------+-------------+-----------+-----------+-----------+ +--------------+-------------+-----------+-----------+-----------+
| Codepoint in | not-PCN(00) | NM(10) | EXP(01) | PM(11) | | Codepoint in | not-PCN(00) | NM(10) | EXP(01) | PM(11) |
+--------------+-------------+-----------+-----------+-----------+ +--------------+-------------+-----------+-----------+-----------+
| not-PCN(00) | Valid | Not valid | Not valid | Not valid | | not-PCN(00) | Valid | Not valid | Not valid | Not valid |
+--------------+-------------+-----------+-----------+-----------+ +--------------+-------------+-----------+-----------+-----------+
| NM(10) | Not valid | Valid | Not valid | Valid | | NM(10) | Not valid | Valid | Not valid | Valid |
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this is if the PCN-egress-node is certain that revealing other this is if the PCN-egress-node is certain that revealing other
codepoints outside the PCN-domain won't contravene the guidance given codepoints outside the PCN-domain won't contravene the guidance given
in [RFC4774]. in [RFC4774].
4.2. Rationale for Encoding 4.2. Rationale for Encoding
The exact choice of encoding was dictated by the constraints imposed The exact choice of encoding was dictated by the constraints imposed
by existing IETF RFCs, in particular [RFC3168], [RFC4301] and by existing IETF RFCs, in particular [RFC3168], [RFC4301] and
[RFC4774]. One of the tightest constraints was the need for any PCN [RFC4774]. One of the tightest constraints was the need for any PCN
encoding to survive being tunnelled through either an IP in IP tunnel encoding to survive being tunnelled through either an IP in IP tunnel
or an IPsec Tunnel. [ECN-tunnel] explains this in more detail. The or an IPsec Tunnel. [I-D.ietf-tsvwg-ecn-tunnel] explains this in
main effect of this constraint is that any PCN marking has to carry more detail. The main effect of this constraint is that any PCN
the 11 codepoint in the ECN field since this is the only codepoint marking has to carry the 11 codepoint in the ECN field since this is
that is guaranteeed to be copied down into the inner header upon the only codepoint that is guaranteed to be copied down into the
decapsulation. An additional constraint is the need to minimise the inner header upon decapsulation. An additional constraint is the
use of Diffserv codepoints because there is a limited supply of need to minimise the use of Diffserv codepoints because there is a
standards track codepoints remaining. Section 4.3 explains how we limited supply of standards track codepoints remaining. Section 4.3
have minimised this still further by reusing pre-existing Diffserv explains how we have minimised this still further by reusing pre-
codepoint(s) such that non-PCN traffic can still be distinguished existing Diffserv codepoint(s) such that non-PCN traffic can still be
from PCN traffic. There are a number of factors that were considered distinguished from PCN traffic. There are a number of factors that
before choosing to set 10 as the NM state instead of 01. These were considered before choosing to set 10 as the NM state instead of
included similarity to ECN, presence of tunnels within the domain, 01. These included similarity to ECN, presence of tunnels within the
leakage into and out of PCN-domain and incremental deployment (see domain, leakage into and out of PCN-domain and incremental deployment
Appendix A.2). (see Appendix A.2).
The encoding scheme above seems to meet all these constraints and The encoding scheme above seems to meet all these constraints and
ends up looking very similar to ECN. This is perhaps not surprising ends up looking very similar to ECN. This is perhaps not surprising
given the similarity in architectural intent between PCN and ECN. given the similarity in architectural intent between PCN and ECN.
4.3. PCN-Compatible Diffserv Codepoints 4.3. PCN-Compatible Diffserv Codepoints
Equipment complying with the baseline PCN encoding MUST allow PCN to Equipment complying with the baseline PCN encoding MUST allow PCN to
be enabled for certain Diffserv codepoints. This document defines be enabled for certain Diffserv codepoints. This document defines
the term "PCN-compatible Diffserv codepoint" for such a DSCP and the the term "PCN-compatible Diffserv codepoint" for such a DSCP. To be
PCN working group will compile a list of such DSCPs. To be clear, clear, any packets with such a DSCP will be PCN enabled only if they
any packets with such a DSCP will be PCN enabled only if they are are within a PCN-domain and have their ECN field set to indicate a
within a PCN-domain and have their ECN field set to indicate a
codepoint other than not-PCN. codepoint other than not-PCN.
Enabling PCN marking behaviour for a specific DSCP disables any other Enabling PCN marking behaviour for a specific DSCP disables any other
marking behaviour (e.g. enabling PCN disables the default ECN marking marking behaviour (e.g. enabling PCN disables the default ECN marking
behaviour introduced in [RFC3168]). All traffic metering and marking behaviour introduced in [RFC3168]). All traffic metering and marking
behaviours are discussed in [PCN-metering-marking]. This ensures behaviours are discussed in [I-D.ietf-pcn-marking-behaviour]. This
compliance with the BCP guidance set out in [RFC4774]. ensures compliance with the BCP guidance set out in [RFC4774].
The PCN Working Group has chosen not to define a single DSCP for use
with PCN for several reasons. Firstly the PCN mechanism is
applicable to a variety of different traffic classes. Secondly
standards track DSCPs are in increasingly short supply. Thirdly PCN
should be seen as being essentially a marking behaviour similar to
ECN but intended for inelastic traffic. More details are given in
the informational appendix Appendix A.1.
4.3.1. Co-existence of PCN and not-PCN traffic 4.3.1. Co-existence of PCN and not-PCN traffic
The scarcity of pool 1 DSCPs coupled with the fact that PCN is 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 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. different DSCPs makes it essential that we provide a not-PCN state.
As stated above (and expanded in Appendix A.1) the aim is for PCN to As stated above (and expanded in Appendix A.1) the aim is for PCN to
re-use existing DSCPs. Because PCN re-defines the meaning of the ECN re-use existing DSCPs. Because PCN re-defines the meaning of the ECN
field for such DSCPs it is important to allow an operator to still 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 use the DSCP for traffic that isn't PCN-enabled. This is achieved by
providing a not-PCN state within the encoding scheme. providing a not-PCN state within the encoding scheme.
5. Rules for Experimental Encoding Schemes 5. Rules for Experimental Encoding Schemes
Any experimental encoding scheme MUST follow these rules to ensure Any experimental encoding scheme MUST follow these rules to ensure
backward compatibility with this baseline scheme: backward compatibility with this baseline scheme:
o The 00 codepoint in the ECN field SHALL indicate not-PCN and MUST o All Interior-nodes within a PCN-domain MUST interpret the 00
NOT be changed to any other codepoint within a PCN-domain. codepoint in the ECN field as not-PCN and MUST NOT change it to
Therefore an ingress node wishing to disable PCN marking for a another value. Therefore an ingress node wishing to disable PCN
packet within a PCN-compatible Diffserv Codepoint MUST set the ECN marking for a packet within a PCN-compatible Diffserv Codepoint
field to 00. MUST set the ECN field to 00.
o The 11 codepoint in the ECN field SHALL indicate PCN-marked o The 11 codepoint in the ECN field MUST indicate PCN-marked (though
(though this does not exclude the 01 Experimental codepoint from this does not exclude the 01 Experimental codepoint from carrying
carrying the same meaning). the same meaning).
o Once set, the 11 codepoint in the ECN field MUST NOT be changed to o Once set, the 11 codepoint in the ECN field MUST NOT be changed to
any other codepoint. any other codepoint.
o Any experimental scheme MUST include details of all valid and o Any experimental scheme MUST include details of all valid and
invalid codepoint transitions at any PCN nodes. invalid codepoint transitions at any PCN nodes.
o Any experimental scheme MUST NOT update the meaning of the 00 and o Any experimental scheme MUST NOT update the meaning of the 00 and
11 codepoints defined above. 11 codepoints defined above.
skipping to change at page 9, line 30 skipping to change at page 10, line 7
BCP 124 [RFC4774] gives guidelines for specifying alternative BCP 124 [RFC4774] gives guidelines for specifying alternative
semantics for the ECN field. It sets out a number of factors to be semantics for the ECN field. It sets out a number of factors to be
taken into consideration. It also suggests various techniques to taken into consideration. It also suggests various techniques to
allow the co-existence of default ECN and alternative ECN semantics. allow the co-existence of default ECN and alternative ECN semantics.
The baseline encoding specified in this document defines PCN- The baseline encoding specified in this document defines PCN-
compatible Diffserv codepoints as no longer supporting the default compatible Diffserv codepoints as no longer supporting the default
ECN semantics. As such this document is compatible with BCP 124. It ECN semantics. As such this document is compatible with BCP 124. It
should be noted that this baseline encoding effectively disables end- should be noted that this baseline encoding effectively disables end-
to-end ECN unless mechanisms are put in place to tunnel such traffic to-end ECN unless mechanisms are put in place to tunnel such traffic
across the PCN-domain. Standard IP-in-IP or IPsec tunnels will across the PCN-domain. Standard IP-in-IP or IPsec tunnels will
always copy the CE codepoint from teh outer header into the inner always copy the CE codepoint from the outer header into the inner
header in decapsulation (unless the inner packet is not-ECT). If an header in decapsulation (unless the inner packet is not-ECT). If an
operator it is essential that any operator wishing to allow ECN to operator wishes to allow ECN to exist end-to-end they must ensure
exist end-to-end ensures there are no tunnel end-points within the there are no tunnel end-points within the PCN-domain to prevent any
PCN-domain. risk of PCN-markings being exposed to endpoints.
7. IANA Considerations 7. IANA Considerations
This document makes no direct request to IANA. However this document This document makes no direct request to IANA.
allows for a set of Diffserv Codepoints to be assigned different ECN
semantics within a controlled domain as described in [RFC4774]. A
list of such DSCPs will be maintained by the PCN working group.
8. Security Considerations 8. Security Considerations
PCN-marking only carries a meaning within the confines of a PCN- PCN-marking only carries a meaning within the confines of a PCN-
domain. Packets wishing to be treated as belonging to a PCN-flow domain. Packets wishing to be treated as belonging to a PCN-flow
must carry a PCN-compatible DSCP and a PCN-Enabled ECN codepoint. must carry a PCN-compatible DSCP and a PCN-Enabled ECN codepoint.
This encoding document is intended to stand independently of the This encoding document is intended to stand independently of the
architecture used to determine how specific packets are authorised to architecture used to determine how specific packets are authorised to
be PCN-marked, which will be described in separate documents on PCN- be PCN-marked, which will be described in separate documents on PCN-
boundary-node behaviour. boundary-node behaviour.
This document assumes the PCN-domain to be entirely under the control This document assumes the PCN-domain to be entirely under the control
of a single operator, or a set of operators who trust each other. of a single operator, or a set of operators who trust each other.
However future extensions to PCN might include inter-domain versions However future extensions to PCN might include inter-domain versions
where trust cannot be assumed between domains. If such schemes are where trust cannot be assumed between domains. If such schemes are
proposed they must ensure that they can operate securely despite the proposed they must ensure that they can operate securely despite the
lack of trust but such considerations are beyond the scope of this lack of trust. However such considerations are beyond the scope of
document. this document.
9. Conclusions 9. Conclusions
This document defines the baseline PCN encoding utilising a This document defines the baseline PCN encoding utilising a
combination of a PCN-enabled DSCP and the ECN field in the IP header. combination of a PCN-enabled DSCP and the ECN field in the IP header.
This baseline encoding allows the existence of two PCN encoding This baseline encoding allows the existence of two PCN encoding
states, not-Marked and PCN-marked. It also allows for the co- states, not-Marked and PCN-marked. It also allows for the co-
existence of competing traffic within the same DSCP so long as that existence of competing traffic within the same DSCP so long as that
traffic does not require ECN support within the PCN-domain. The traffic does not require ECN support within the PCN-domain. The
encoding scheme is conformant with [RFC4774]. encoding scheme is conformant with [RFC4774]. The Working Group has
chosen not to define a single DSCP for use with PCN. The rationale
for this decision along with advice relating to choice of suitable
DSCPs can be found in Appendix A.1.
10. Acknowledgements 10. Acknowledgements
This document builds extensively on work done in the PCN working This document builds extensively on work done in the PCN working
group by Kwok Ho Chan, Georgios Karagiannis, Philip Eardley, Anna group by Kwok Ho Chan, Georgios Karagiannis, Philip Eardley, Anna
Charny, Joe Babiarz and others. Thanks to Ruediger Geib and Gorry Charny, Joe Babiarz and others. Thanks to Ruediger Geib and Gorry
Fairhurst for providing detailed comments on this document. Fairhurst for providing detailed comments on this document.
11. Comments Solicited 11. Comments Solicited
(To be removed by the RFC-Editor.) Comments and questions are (To be removed by the RFC-Editor.) Comments and questions are
encouraged and very welcome. They can be addressed to the IETF encouraged and very welcome. They can be addressed to the IETF
congestion and pre-congestion working group mailing list congestion and pre-congestion working group mailing list
<pcn@ietf.org>, and/or to the authors. <pcn@ietf.org>, and/or to the authors.
12. References 12. References
12.1. Normative References 12.1. Normative References
[PCN-metering-marking] Eardley, P., "Metering and marking behaviour [I-D.ietf-pcn-marking-behaviour] Eardley, P., "Metering and marking
of PCN-nodes", behaviour of PCN-nodes",
draft-ietf-pcn-marking-behaviour-03 (work in draft-ietf-pcn-marking-behaviour-05
progress), May 2009. (work in progress), August 2009.
[RFC2119] Bradner, S., "Key words for use in RFCs to [RFC2119] Bradner, S., "Key words for use in
Indicate Requirement Levels", BCP 14, RFCs to Indicate Requirement
RFC 2119, March 1997. Levels", BCP 14, RFC 2119,
March 1997.
[RFC3168] Ramakrishnan, K., Floyd, S., and D. Black, [RFC3168] Ramakrishnan, K., Floyd, S., and D.
"The Addition of Explicit Congestion Black, "The Addition of Explicit
Notification (ECN) to IP", RFC 3168, Congestion Notification (ECN) to
September 2001. IP", RFC 3168, September 2001.
[RFC4774] Floyd, S., "Specifying Alternate Semantics [RFC4774] Floyd, S., "Specifying Alternate
for the Explicit Congestion Notification Semantics for the Explicit
(ECN) Field", BCP 124, RFC 4774, Congestion Notification (ECN)
Field", BCP 124, RFC 4774,
November 2006. November 2006.
12.2. Informative References 12.2. Informative References
[ECN-tunnel] Briscoe, B., "Tunnelling of Explicit [I-D.ietf-tsvwg-ecn-tunnel] Briscoe, B., "Tunnelling of
Congestion Notification", Explicit Congestion Notification",
draft-ietf-tsvwg-ecn-tunnel-02 (work in draft-ietf-tsvwg-ecn-tunnel-03
progress), March 2009. (work in progress), July 2009.
[PCN-architecture] Eardley, P., "Pre-Congestion Notification
(PCN) Architecture",
draft-ietf-pcn-architecture-11 (work in
progress), April 2009.
[RFC3540] Spring, N., Wetherall, D., and D. Ely, [RFC3540] Spring, N., Wetherall, D., and D.
"Robust Explicit Congestion Notification Ely, "Robust Explicit Congestion
(ECN) Signaling with Nonces", RFC 3540, Notification (ECN) Signaling with
June 2003. Nonces", RFC 3540, June 2003.
[RFC4301] Kent, S. and K. Seo, "Security Architecture [RFC4301] Kent, S. and K. Seo, "Security
for the Internet Protocol", RFC 4301, Architecture for the Internet
December 2005. Protocol", RFC 4301, December 2005.
[RFC5127] Chan, K., Babiarz, J., and F. Baker, [RFC5127] Chan, K., Babiarz, J., and F.
"Aggregation of DiffServ Service Classes", Baker, "Aggregation of DiffServ
RFC 5127, February 2008. Service Classes", RFC 5127,
February 2008.
[Voice-Admit] Baker, F., Polk, J., and M. Dolly, "DSCP for [RFC5559] Eardley, P., "Pre-Congestion
Capacity-Admitted Traffic", Notification (PCN) Architecture",
draft-ietf-tsvwg-admitted-realtime-dscp-05 RFC 5559, June 2009.
(work in progress), November 2008.
Appendix A. PCN Deployment Considerations (Informational) Appendix A. PCN Deployment Considerations (Informational)
A.1. Choice of Suitable DSCPs A.1. Choice of Suitable DSCPs
The PCN Working Group chose not to define a single DSCP for use with The PCN Working Group chose not to define a single DSCP for use with
PCN for several reasons. Firstly the PCN mechanism is applicable to PCN for several reasons. Firstly the PCN mechanism is applicable to
a variety of different traffic classes. Secondly standards track a variety of different traffic classes. Secondly standards track
DSCPs are in increasingly short supply. Thirdly PCN should be seen DSCPs are in increasingly short supply. Thirdly PCN should be seen
as being essentially a marking behaviour similar to ECN but intended as being essentially a marking behaviour similar to ECN but intended
for inelastic traffic. The choice of which DSCP is most suitable for for inelastic traffic. The choice of which DSCP is most suitable for
a given PCN-domain is dependant on the nature of the traffic entering a given PCN-domain is dependent on the nature of the traffic entering
that domain and the link rates of all the links making up that that domain and the link rates of all the links making up that
domain. In PCN-domains with uniformly high link rates, the domain. In PCN-domains with uniformly high link rates, the
appropriate DSCPs would currently be those for the Real Time Traffic appropriate DSCPs would currently be those for the Real Time Traffic
Class [RFC5127]. If the PCN domain includes lower speed links it Class [RFC5127]. To be clear the PCN Working Group recommends using
would also be appropriate to use the DSCPs of the other traffic admission control for the following service classes:
classes that [Voice-Admit] defines for use with admission control,
such as the three video classes CS4, CS3 and AF4 and the Admitted o Telephony (EF)
Telephony Class. The PCN working group will maintain a list of PCN-
compatible Diffserv Codepoints. o Real-time interactive (CS4)
o Broadcast Video (CS3)
o Multimedia Conferencing (AF4)
PCN marking is intended to provide a scalable admission control
mechanism for traffic with a high degree of statistical multiplexing.
PCN marking would therefore be appropriate to apply to traffic in the
above classes, but only within a PCN region containing highly
aggregated traffic. In such cases, the above service classes may
well all be subject to a single forwarding treatment (treatment
aggregate [RFC5127]). However, this does not imply all such IP
traffic would necessarily be identified by one DSCP - each service
class might keep a distinct DSCP within the highly aggregated region
[RFC5127].
Additional service classes may be defined for which admission control
is appropriate, whether through some future standards action or
through local use by certain operators, e.g. the Multimedia Streaming
service class (AF3). This document does not preclude the use of PCN
in more cases than those listed above.
NOTE: The above discussion is informative not normative, as operators
are ultimately free to decide whether to use admission control for
certain service classes and whether to use PCN as their mechanism of
choice.
A.2. Rationale for Using ECT(0) for Not-marked A.2. Rationale for Using ECT(0) for Not-marked
The choice of which ECT codepoint to use for the Not-marked state was The choice of which ECT codepoint to use for the Not-marked state was
based on the following considerations: based on the following considerations:
o [RFC3168] full functionality tunnel within the PCN-domain: Either o [RFC3168] full functionality tunnel within the PCN-domain: Either
ECT is safe. ECT is safe.
o Leakage of traffic into PCN-domain: because of the lack of take-up o Leakage of traffic into PCN-domain: because of the lack of take-up
skipping to change at page 13, line 5 skipping to change at page 14, line 5
Toby Moncaster Toby Moncaster
BT BT
B54/70, Adastral Park B54/70, Adastral Park
Martlesham Heath Martlesham Heath
Ipswich IP5 3RE Ipswich IP5 3RE
UK UK
Phone: +44 1473 648734 Phone: +44 1473 648734
EMail: toby.moncaster@bt.com EMail: toby.moncaster@bt.com
Bob Briscoe Bob Briscoe
BT & UCL BT
B54/77, Adastral Park B54/77, Adastral Park
Martlesham Heath Martlesham Heath
Ipswich IP5 3RE Ipswich IP5 3RE
UK UK
Phone: +44 1473 645196 Phone: +44 1473 645196
EMail: bob.briscoe@bt.com EMail: bob.briscoe@bt.com
Michael Menth Michael Menth
University of Wuerzburg University of Wuerzburg
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