draft-ietf-pcn-baseline-encoding-03.txt   draft-ietf-pcn-baseline-encoding-04.txt 
Congestion and Pre Congestion T. Moncaster Congestion and Pre Congestion T. Moncaster
Internet-Draft BT Internet-Draft BT
Intended status: Standards Track B. Briscoe Intended status: Standards Track B. Briscoe
Expires: October 9, 2009 BT & UCL Expires: November 20, 2009 BT & UCL
M. Menth M. Menth
University of Wuerzburg University of Wuerzburg
April 7, 2009 May 19, 2009
Baseline Encoding and Transport of Pre-Congestion Information Baseline Encoding and Transport of Pre-Congestion Information
draft-ietf-pcn-baseline-encoding-03 draft-ietf-pcn-baseline-encoding-04
Status of This Memo Status of This Memo
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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
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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
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Please review these documents carefully, as they describe your rights Please review these documents carefully, as they describe your rights
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The objective of Pre-Congestion Notification (PCN) is to protect the The objective of Pre-Congestion Notification (PCN) is to protect the
quality of service (QoS) of inelastic flows within a Diffserv domain. quality of service (QoS) of inelastic flows within a Diffserv domain.
The overall rate of the PCN-traffic is metered on every link in the The overall rate of the PCN-traffic is metered on every link in the
PCN-domain, and PCN-packets are appropriately marked when certain PCN-domain, and PCN-packets are appropriately marked when certain
configured rates are exceeded. The level of marking allows the configured rates are exceeded. The level of marking allows the
boundary nodes to make decisions about whether t o admit or block a boundary nodes to make decisions about whether t o admit or block a
new flow request, and (in abnormal circumstances) whether to new flow request, and (in abnormal circumstances) whether to
terminate some of the existing flows, thereby protecting the QoS of terminate some of the existing flows, thereby protecting the QoS of
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 ECN codepoints are to be encoded into the IP header by re-using the Explicit
within this controlled domain. The baseline encoding described here Congestion Notification (ECN) codepoints within this controlled
provides for only two PCN encoding states, unmarked and marked. domain. The baseline encoding described here provides for only two
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 . . . . . . . . . . . . . . . . . . . . . . . . . 5
4. Encoding two PCN States in IP . . . . . . . . . . . . . . . . 5 4. Encoding two PCN States in IP . . . . . . . . . . . . . . . . 6
4.1. Valid and Invalid Codepoint Transitions . . . . . . . . . 6 4.1. Valid and Invalid Codepoint Transitions . . . . . . . . . 6
4.2. Rationale for Encoding . . . . . . . . . . . . . . . . . . 7 4.2. Rationale for Encoding . . . . . . . . . . . . . . . . . . 7
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
5. Rules for Experimental Encoding Schemes . . . . . . . . . . . 8 5. Rules for Experimental Encoding Schemes . . . . . . . . . . . 8
6. Backwards Compatibility . . . . . . . . . . . . . . . . . . . 8 6. Backwards Compatibility . . . . . . . . . . . . . . . . . . . 9
7. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 9 7. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 9
8. Security Considerations . . . . . . . . . . . . . . . . . . . 9 8. Security Considerations . . . . . . . . . . . . . . . . . . . 9
9. Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . 9 9. Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . 10
10. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 9 10. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 10
11. Comments Solicited . . . . . . . . . . . . . . . . . . . . . . 10 11. Comments Solicited . . . . . . . . . . . . . . . . . . . . . . 10
12. References . . . . . . . . . . . . . . . . . . . . . . . . . . 10 12. References . . . . . . . . . . . . . . . . . . . . . . . . . . 10
12.1. Normative References . . . . . . . . . . . . . . . . . . . 10 12.1. Normative References . . . . . . . . . . . . . . . . . . . 10
12.2. Informative References . . . . . . . . . . . . . . . . . . 10 12.2. Informative References . . . . . . . . . . . . . . . . . . 11
Appendix A. PCN Deployment Considerations . . . . . . . . . . . . 11 Appendix A. PCN Deployment Considerations (Informational) . . . . 11
A.1. Choice of Suitable DSCPs . . . . . . . . . . . . . . . . . 11 A.1. Choice of Suitable DSCPs . . . . . . . . . . . . . . . . . 11
A.2. Rationale for Using ECT(0) for Not Marked . . . . . . . . 11 A.2. Rationale for Using ECT(0) for Not-marked . . . . . . . . 12
1. Introduction 1. Introduction
The objective of Pre-Congestion Notification (PCN) is to protect the The objective of Pre-Congestion Notification (PCN) is to protect the
quality of service (QoS) of inelastic flows within a Diffserv domain, quality of service (QoS) of inelastic flows within a Diffserv domain,
in a simple, scalable and robust fashion. The overall rate of the in a simple, scalable and robust fashion. The overall rate of the
PCN-traffic is metered on every link in the PCN-domain, and PCN- PCN-traffic is metered on every link in the PCN-domain, and PCN-
packets are appropriately marked when certain configured rates are packets are appropriately marked when certain configured rates are
exceeded. These configured rates are below the rate of the link thus exceeded. These configured rates are below the rate of the link thus
providing notification before any congestion occurs (hence "pre- providing notification before any congestion occurs (hence "pre-
congestion notification"). The level of marking allows the boundary congestion notification"). The level of marking allows the boundary
nodes to make decisions about whether to admit or block a new flow nodes to make decisions about whether to admit or block a new flow
request, and (in abnormal circumstances) whether to terminate some of request, and (in abnormal circumstances) whether to terminate some of
the existing flows, thereby protecting the QoS of previously admitted the existing flows, thereby protecting the QoS of previously admitted
flows. 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 ECN field. It also describes how header by re-using the bits of the Explicit Congestion Notification
packets are identified as belonging to a PCN flow. Some deployment (ECN) field [RFC3168]. It also describes how packets are identified
models require two PCN encoding states, others require more. The as belonging to a PCN flow. Some deployment models require two PCN
baseline encoding described here only provides for two PCN encoding encoding states, others require more. The baseline encoding
states. However the encoding can be easily extended to provide more described here only provides for two PCN encoding states. However
states and rules for such extensions are given in this document. the encoding can be easily extended to provide more states. Rules
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 -03 to -04:
Major WGLC comments addressed:
* Added Section 4.3.1 to clarify why we need the not-PCN
codepoint.
* Stated that the PCN WG will maintain a list of PCN-compatible
DSCPs. This should help avoid inter-operability issues.
Also addressed a number of WGLC nits.
From -02 to -03: From -02 to -03:
Extensive changes to address comments made by Gorry Fairhurst Extensive changes to address comments made by Gorry Fairhurst
including: including:
* Abstract re-written. * Abstract re-written.
* Clarified throughout that this re-uses the ECN bits in the IP * Clarified throughout that this re-uses the ECN bits in the IP
header. header.
skipping to change at page 4, line 9 skipping to change at page 4, line 19
* 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 Removed Appendix A and replaced with reference to [ECN-tunnel]
[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 4 skipping to change at page 5, line 14
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.
Added appendix listing possible operator scenarios for using this Added appendix listing possible operator scenarios for using this
baseline encoding. baseline encoding.
Minor changes throughout. Minor changes throughout.
skipping to change at page 5, line 34 skipping to change at page 5, line 45
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
[I-D.ietf-pcn-marking-behaviour]. Abbreviated to PM. [PCN-metering-marking]. 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
[I-D.ietf-pcn-architecture]. [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.
The following table defines how to encode these states in IP: The following table defines how to encode these states in IP:
+---------------+-------------+-------------+-------------+---------+ +---------------+-------------+-------------+-------------+---------+
| 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 compatability 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, such as the Voice-Admit codepoint defined in
[I-D.ietf-tsvwg-admitted-realtime-dscp]. Guidelines for mixing [Voice-Admit]. Guidelines for mixing traffic-types within a PCN-
traffic-types within a PCN-domain are given in domain are given in [PCN-metering-marking].
[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 equal to codepoint as PCN-enabled traffic MUST have the ECN field of its
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. (00) codepoint on all other packets sharing a PCN-compatible Diffserv
codepoint.
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
PCN-mark a packet [PCN-metering-marking]) and from EXP to PM (which
MAY be allowed by some future experimental extensions). The
following table gives the full set of valid and invalid codepoint
transitions.
A PCN-interior-node MUST observe the rules for valid and invalid
codepoint transitions as set out in the following table. The precise
rules governing which valid transition to use are set out in
[I-D.ietf-pcn-marking-behaviour]
+-------------------------------------------------+ +-------------------------------------------------+
| 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 |
+--------------+-------------+-----------+-----------+-----------+ +--------------+-------------+-----------+-----------+-----------+
| EXP(01)* | Not valid | Not valid | Valid | Valid | | EXP(01)* | Not valid | Not valid | Valid | Valid* |
+--------------+-------------+-----------+-----------+-----------+ +--------------+-------------+-----------+-----------+-----------+
| PM(11) | Not valid | Not valid | Not valid | Valid | | PM(11) | Not valid | Not valid | Not valid | Valid |
+--------------+-------------+-----------+-----------+-----------+ +--------------+-------------+-----------+-----------+-----------+
* This SHOULD cause an alarm to be raised at a higher layer. The * This SHOULD cause an alarm to be raised at a higher layer. The
packet MUST be treated as if it carried the NM codepoint. packet MUST be treated as if it carried the NM codepoint.
Table 2: Valid and Invalid Codepoint Transitions for Table 2: Valid and Invalid Codepoint Transitions for
PCN-packets at PCN-interior-nodes PCN-packets at PCN-interior-nodes
A PCN-egress-node SHOULD set the not-PCN (00) codepoint on all A PCN-egress-node SHOULD set the not-PCN (00) codepoint on all
skipping to change at page 7, line 35 skipping to change at page 7, line 44
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. [I-D.ietf-tsvwg-ecn-tunnel] explains this in or an IPsec Tunnel. [ECN-tunnel] explains this in more detail. The
more detail. The main effect of this constraint is that any PCN main effect of this constraint is that any PCN marking has to carry
marking has to carry the 11 codepoint in the ECN field since this is the 11 codepoint in the ECN field since this is the only codepoint
the only codepoint that is guaranteeed to be copied down into the that is guaranteeed to be copied down into the inner header upon
inner header upon decapsulation. An additional constraint is the decapsulation. An additional constraint is the need to minimise the
need to minimise the use of DiffServ codepoints as there is a limited use of Diffserv codepoints because there is a limited supply of
supply of standards track codepoints remaining. Section 4.3 explains standards track codepoints remaining. Section 4.3 explains how we
how we have minimised this still further by reusing pre-existing have minimised this still further by reusing pre-existing Diffserv
Diffserv codepoint(s) such that non-PCN traffic can still be codepoint(s) such that non-PCN traffic can still be distinguished
distinguished from PCN traffic. There are a number of factors that from PCN traffic. There are a number of factors that were considered
were considered before deciding to set 10 as the NM state. These before choosing to set 10 as the NM state instead of 01. These
included similarity to ECN, presence of tunnels within the domain, included similarity to ECN, presence of tunnels within the domain,
leakage into and out of PCN-domain and incremental deployment (see leakage into and out of PCN-domain and incremental deployment (see
Appendix A.2). 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. To be the term "PCN-compatible Diffserv codepoint" for such a DSCP and the
clear, any packets with such a DSCP will be PCN enabled only if they PCN working group will compile a list of such DSCPs. To be clear,
also have their ECN field set to indicate a codepoint other than not- any packets with such a DSCP will be PCN enabled only if they are
PCN. within a PCN-domain and have their ECN field set to indicate a
codepoint other than not-PCN.
Enabling PCN marking behaviour disables any other marking behaviour Enabling PCN marking behaviour for a specific DSCP disables any other
(e.g. enabling PCN disables the default ECN marking behaviour marking behaviour (e.g. enabling PCN disables the default ECN marking
introduced in [RFC3168]). All traffic scheduling and conditioning behaviour introduced in [RFC3168]). All traffic metering and marking
behaviours are discussed in [I-D.ietf-pcn-marking-behaviour]. This behaviours are discussed in [PCN-metering-marking]. This ensures
ensures compliance with the BCP guidance set out in [RFC4774]. compliance with the BCP guidance set out in [RFC4774].
4.3.1. Co-existence of PCN and not-PCN traffic
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.
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
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.
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 The 00 codepoint in the ECN field SHALL indicate not-PCN and MUST
NOT be changed to any otehr codepoint within a PCN-domain. NOT be changed to any other codepoint within a PCN-domain.
Therefore an ingress node wishing to disable PCN marking for a Therefore an ingress node wishing to disable PCN marking for a
packet within a PCN-compatible DiffServ Codepoint MUST set the ECN packet within a PCN-compatible Diffserv Codepoint MUST set the ECN
field to 00. field to 00.
o The 11 codepoint in the ECN field SHALL indicate PCN-marked o The 11 codepoint in the ECN field SHALL indicate PCN-marked
(though this does not exclude the 01 Experimental codepoint from (though this does not exclude the 01 Experimental codepoint from
carrying the same meaning). carrying 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
skipping to change at page 9, line 4 skipping to change at page 9, line 25
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.
6. Backwards Compatibility 6. Backwards Compatibility
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 except where mechanisms are put in place to tunnel such to-end ECN unless mechanisms are put in place to tunnel such traffic
traffic across the PCN-domain. across the PCN-domain. Standard IP-in-IP or IPsec tunnels will
always copy the CE codepoint from teh outer header into the inner
header in decapsulation (unless the inner packet is not-ECT). If an
operator it is essential that any operator wishing to allow ECN to
exist end-to-end ensures there are no tunnel end-points within the
PCN-domain.
7. IANA Considerations 7. IANA Considerations
This document makes no request to IANA. This document makes no direct request to IANA. However this document
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 whether specific packets are architecture used to determine how specific packets are authorised to
authorised to be PCN-marked, which will be described in separate be PCN-marked, which will be described in separate documents on PCN-
documents on PCN edge-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 but such considerations are beyond the scope of this
document. document.
9. Conclusions 9. Conclusions
skipping to change at page 10, line 16 skipping to change at page 10, line 41
(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
[I-D.ietf-pcn-marking-behaviour] Eardley, P., "Marking [PCN-metering-marking] Eardley, P., "Metering and marking behaviour
behaviour of PCN-nodes", dra of PCN-nodes",
ft-ietf-pcn-marking- draft-ietf-pcn-marking-behaviour-03 (work in
behaviour-02 (work in progress), May 2009.
progress), March 2009.
[RFC2119] Bradner, S., "Key words for [RFC2119] Bradner, S., "Key words for use in RFCs to
use in RFCs to Indicate Indicate Requirement Levels", BCP 14,
Requirement Levels", BCP 14,
RFC 2119, March 1997. RFC 2119, March 1997.
[RFC3168] Ramakrishnan, K., Floyd, S., [RFC3168] Ramakrishnan, K., Floyd, S., and D. Black,
and D. Black, "The Addition "The Addition of Explicit Congestion
of Explicit Congestion Notification (ECN) to IP", RFC 3168,
Notification (ECN) to IP", September 2001.
RFC 3168, September 2001.
[RFC4774] Floyd, S., "Specifying [RFC4774] Floyd, S., "Specifying Alternate Semantics
Alternate Semantics for the for the Explicit Congestion Notification
Explicit Congestion (ECN) Field", BCP 124, RFC 4774,
Notification (ECN) Field",
BCP 124, RFC 4774,
November 2006. November 2006.
12.2. Informative References 12.2. Informative References
[I-D.ietf-pcn-architecture] Eardley, P., "Pre-Congestion [ECN-tunnel] Briscoe, B., "Tunnelling of Explicit
Notification (PCN) Congestion Notification",
Architecture", draft-ietf- draft-ietf-tsvwg-ecn-tunnel-02 (work in
pcn-architecture-10 (work in
progress), March 2009. progress), March 2009.
[I-D.ietf-tsvwg-admitted-realtime-dscp] Baker, F., Polk, J., and M. [PCN-architecture] Eardley, P., "Pre-Congestion Notification
Dolly, "DSCP for Capacity- (PCN) Architecture",
Admitted Traffic", draft- draft-ietf-pcn-architecture-11 (work in
ietf-tsvwg-admitted- progress), April 2009.
realtime-dscp-05 (work in
progress), November 2008.
[I-D.ietf-tsvwg-ecn-tunnel] Briscoe, B., "Tunnelling of [RFC3540] Spring, N., Wetherall, D., and D. Ely,
Explicit Congestion "Robust Explicit Congestion Notification
Notification", draft-ietf- (ECN) Signaling with Nonces", RFC 3540,
tsvwg-ecn-tunnel-02 (work in June 2003.
progress), March 2009.
[RFC4301] Kent, S. and K. Seo, [RFC4301] Kent, S. and K. Seo, "Security Architecture
"Security Architecture for for the Internet Protocol", RFC 4301,
the Internet Protocol", December 2005.
RFC 4301, December 2005.
[RFC5127] Chan, K., Babiarz, J., and [RFC5127] Chan, K., Babiarz, J., and F. Baker,
F. Baker, "Aggregation of "Aggregation of DiffServ Service Classes",
DiffServ Service Classes",
RFC 5127, February 2008. RFC 5127, February 2008.
Appendix A. PCN Deployment Considerations [Voice-Admit] Baker, F., Polk, J., and M. Dolly, "DSCP for
Capacity-Admitted Traffic",
draft-ietf-tsvwg-admitted-realtime-dscp-05
(work in progress), November 2008.
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 dependant 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]. If the PCN domain includes lower speed links it
would also be appropriate to use the DSCPs of the other traffic would also be appropriate to use the DSCPs of the other traffic
classes that [I-D.ietf-tsvwg-admitted-realtime-dscp] defines for use classes that [Voice-Admit] defines for use with admission control,
with admission control, such as the three video classes CS4, CS3 and such as the three video classes CS4, CS3 and AF4 and the Admitted
AF4 and the Admitted Telephony Class. Telephony Class. The PCN working group will maintain a list of PCN-
compatible Diffserv Codepoints.
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: ECT(1) is slightly less likely o Leakage of traffic into PCN-domain: because of the lack of take-up
to occur so might be considered safer. of the ECN nonce [RFC3540], leakage of ECT(1) is less likely to
occur so might be considered safer.
o Leakage of traffic out of PCN-domain: Either ECT is equally unsafe o Leakage of traffic out of PCN-domain: Either ECT is equally unsafe
(since this would incorrectly indicate the traffic was ECN-capable (since this would incorrectly indicate the traffic was ECN-capable
outside the controlled PCN-domain). outside the controlled PCN-domain).
o Incremental deployment: Either codepoint is suitable providing o Incremental deployment: Either codepoint is suitable providing
that the codepoints are used consistently. that the codepoints are used consistently.
o Conceptual consistency with other schemes: ECT(0) is conceptually o Conceptual consistency with other schemes: ECT(0) is conceptually
consistent with [RFC3168]. consistent with [RFC3168].
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