draft-ietf-pcn-baseline-encoding-02.txt   draft-ietf-pcn-baseline-encoding-03.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: August 14, 2009 BT & UCL Expires: October 9, 2009 BT & UCL
M. Menth M. Menth
University of Wuerzburg University of Wuerzburg
February 10, 2009 April 7, 2009
Baseline Encoding and Transport of Pre-Congestion Information Baseline Encoding and Transport of Pre-Congestion Information
draft-ietf-pcn-baseline-encoding-02 draft-ietf-pcn-baseline-encoding-03
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. provisions of BCP 78 and BCP 79. This document may contain material
from IETF Documents or IETF Contributions published or made publicly
available before November 10, 2008. The person(s) controlling the
copyright in some of this material may not have granted the IETF
Trust the right to allow modifications of such material outside the
IETF Standards Process. Without obtaining an adequate license from
the person(s) controlling the copyright in such materials, this
document may not be modified outside the IETF Standards Process, and
derivative works of it may not be created outside the IETF Standards
Process, except to format it for publication as an RFC or to
translate it into languages other than English.
Internet-Drafts are working documents of the Internet Engineering Internet-Drafts are working documents of the Internet Engineering
Task Force (IETF), its areas, and its working groups. Note that Task Force (IETF), its areas, and its working groups. Note that
other groups may also distribute working documents as Internet- other groups may also distribute working documents as Internet-
Drafts. Drafts.
Internet-Drafts are draft documents valid for a maximum of six months Internet-Drafts are draft documents valid for a maximum of six months
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.
The list of Internet-Draft Shadow Directories can be accessed at The list of Internet-Draft Shadow Directories can be accessed at
http://www.ietf.org/shadow.html. http://www.ietf.org/shadow.html.
This Internet-Draft will expire on August 14, 2009. This Internet-Draft will expire on October 9, 2009.
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 Provisions Relating to IETF Documents in effect on the date of
(http://trustee.ietf.org/license-info) in effect on the date of publication of this document (http://trustee.ietf.org/license-info).
publication of this document. Please review these documents Please review these documents carefully, as they describe your rights
carefully, as they describe your rights and restrictions with respect and restrictions with respect to this document.
to this document.
Abstract Abstract
Pre-congestion notification (PCN) provides information to support The objective of Pre-Congestion Notification (PCN) is to protect the
admission control and flow termination in order to protect the quality of service (QoS) of inelastic flows within a Diffserv domain.
Quality of Service of inelastic flows. It does this by marking The overall rate of the PCN-traffic is metered on every link in the
packets when traffic load on a link is approaching or has exceeded a PCN-domain, and PCN-packets are appropriately marked when certain
threshold below the physical link rate. This document specifies how configured rates are exceeded. The level of marking allows the
such marks are to be encoded into the IP header. The baseline boundary nodes to make decisions about whether t o admit or block a
encoding described here provides for only two PCN encoding states. new flow request, and (in abnormal circumstances) whether to
It is designed to be easily extended to provide more encoding states terminate some of the existing flows, thereby protecting the QoS of
but such schemes will be described in other documents. previously admitted flows. This document specifies how such marks
are to be encoded into the IP header by re-using the ECN codepoints
within this controlled domain. The baseline encoding described here
provides for only two PCN encoding states, unmarked and marked.
Table of Contents Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3
2. Requirements notation . . . . . . . . . . . . . . . . . . . . 4 2. Requirements notation . . . . . . . . . . . . . . . . . . . . 5
3. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 4 3. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 5
4. Encoding two PCN States in IP . . . . . . . . . . . . . . . . 5 4. Encoding two PCN States in IP . . . . . . . . . . . . . . . . 5
4.1. Rationale for Encoding . . . . . . . . . . . . . . . . . . 6 4.1. Valid and Invalid Codepoint Transitions . . . . . . . . . 6
4.2. PCN-Compatible DiffServ Codepoints . . . . . . . . . . . . 7 4.2. Rationale for Encoding . . . . . . . . . . . . . . . . . . 7
5. Rules for Experimental Encoding Schemes . . . . . . . . . . . 7 4.3. PCN-Compatible DiffServ Codepoints . . . . . . . . . . . . 8
5. Rules for Experimental Encoding Schemes . . . . . . . . . . . 8
6. Backwards Compatibility . . . . . . . . . . . . . . . . . . . 8 6. Backwards Compatibility . . . . . . . . . . . . . . . . . . . 8
7. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 8 7. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 9
8. Security Considerations . . . . . . . . . . . . . . . . . . . 8 8. Security Considerations . . . . . . . . . . . . . . . . . . . 9
9. Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . 8 9. Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . 9
10. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 9 10. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 9
11. Comments Solicited . . . . . . . . . . . . . . . . . . . . . . 9 11. Comments Solicited . . . . . . . . . . . . . . . . . . . . . . 10
12. References . . . . . . . . . . . . . . . . . . . . . . . . . . 9 12. References . . . . . . . . . . . . . . . . . . . . . . . . . . 10
12.1. Normative References . . . . . . . . . . . . . . . . . . . 9 12.1. Normative References . . . . . . . . . . . . . . . . . . . 10
12.2. Informative References . . . . . . . . . . . . . . . . . . 9 12.2. Informative References . . . . . . . . . . . . . . . . . . 10
Appendix A. PCN Deployment Considerations . . . . . . . . . . . . 10 Appendix A. PCN Deployment Considerations . . . . . . . . . . . . 11
A.1. Choice of Suitable DSCPs . . . . . . . . . . . . . . . . . 10 A.1. Choice of Suitable DSCPs . . . . . . . . . . . . . . . . . 11
A.2. Rationale for Using ECT(0) for Not Marked . . . . . . . . 10 A.2. Rationale for Using ECT(0) for Not Marked . . . . . . . . 11
1. Introduction 1. Introduction
Pre-congestion notification (PCN) provides information to support The objective of Pre-Congestion Notification (PCN) is to protect the
admission control and flow termination in order to protect the quality of service (QoS) of inelastic flows within a Diffserv domain,
quality of service (QoS) of inelastic flows. This is achieved by in a simple, scalable and robust fashion. The overall rate of the
marking packets according to the level of pre-congestion at nodes PCN-traffic is metered on every link in the PCN-domain, and PCN-
within a PCN-domain. These markings are evaluated by the egress packets are appropriately marked when certain configured rates are
nodes of the PCN-domain. [pcn-arch] describes how PCN packet markings exceeded. These configured rates are below the rate of the link thus
can be used to assure the QoS of inelastic flows within a single providing notification before any congestion occurs (hence "pre-
DiffServ domain. congestion notification"). The level of marking allows the boundary
nodes to make decisions about whether to admit or block a new flow
request, and (in abnormal circumstances) whether to terminate some of
the existing flows, thereby protecting the QoS of 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. It also describes how packets are identified as belonging to header by re-using the bits of the ECN field. It also describes how
a PCN flow. Some deployment models require two PCN encoding states, packets are identified as belonging to a PCN flow. Some deployment
others require more. The baseline encoding described here only models require two PCN encoding states, others require more. The
provides for two PCN encoding states. An extension of the baseline baseline encoding described here only provides for two PCN encoding
encoding described in [PCN-3-enc-state] provides for three PCN states. However the encoding can be easily extended to provide more
encoding states. Other extensions have also been suggested all of states and rules for such extensions are given in this document.
which can build on the baseline encoding. In order to ensure
backward compatibility any alternative encoding schemes that claim
compliance with PCN standards MUST extend this baseline scheme.
Changes from previous drafts (to be removed by the RFC Editor): Changes from previous drafts (to be removed by the RFC Editor):
From -02 to -03:
Extensive changes to address comments made by Gorry Fairhurst
including:
* Abstract re-written.
* Clarified throughout that this re-uses the ECN bits in the IP
header.
* Re-arranged order of terminology section for clarity.
* Table 2 replaced with new table and text.
* Security considerations re-written.
* Appendixes re-written to improve clarity.
* Numerous minor nits and language changes throughout.
Extensive other minor changes throughout.
From -01 to -02: From -01 to -02:
Removed Appendix A and replaced with reference to [ecn-tunneling] 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 4, line 50 skipping to change at page 5, line 28
2. Requirements notation 2. Requirements notation
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in [RFC2119]. document are to be interpreted as described in [RFC2119].
3. Terminology 3. Terminology
The following terms are used in this document: The following terms are used in this document:
o Not-PCN - packets that are not PCN-enabled. o PCN-compatible Diffserv codepoint - a Diffserv codepoint for which
the ECN field is used to carry PCN markings rather than [RFC3168]
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-marking-behaviour]. Also PM. [I-D.ietf-pcn-marking-behaviour]. Abbreviated to PM.
o Not-marked - codepoint indicating packets that are PCN-capable but o Not-marked - codepoint indicating packets that are PCN-capable,
are not PCN-marked. Also NM. but are not PCN-marked. Abbreviated to NM.
o PCN-enabled codepoints - collective term for all the 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 PCN-compatible Diffserv codepoint - a Diffserv codepoint for which o not-PCN - packets that are not PCN-enabled.
the ECN field is used to carry PCN markings rather than [RFC3168]
markings.
In addition the document uses the terminology defined in [pcn-arch]. In addition, the document uses the terminology defined in
[I-D.ietf-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 4.2) 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 any possible 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
[voice-admit]. Guidelines for mixing traffic-types within a PCN- [I-D.ietf-tsvwg-admitted-realtime-dscp]. Guidelines for mixing
domain are given in [pcn-marking-behaviour]. traffic-types within a PCN-domain are given in
[I-D.ietf-pcn-marking-behaviour].
o Any packet that is not PCN-enabled (Not-PCN) but which shares the o Any packet that is not-PCN but which shares the same DiffServ
same DiffServ codepoint as PCN-enabled traffic MUST have the ECN codepoint as PCN-enabled traffic MUST have the ECN field equal to
field equal to 00. 00.
The following table sets out the valid and invalid codepoint 4.1. Valid and Invalid Codepoint Transitions
transitions at PCN-nodes for this baseline encoding. Extension
encodings may have different rules regarding the validity of the
transitions. Note that this table assumes there is a functional
separation between a PCN-boundary-node and a PCN-interior-node such
that PCN-boundary-nodes do not perform packet metering or marking
functions. PCN-nodes MUST follow the encoding transition rules set
out in this table (e.g. they MUST NOT set invalid codepoints on
packets they forward). This table only applies to PCN-packets.
+-----------+-------------+-----------------+-----------------------+ A PCN-ingress-node MUST set the Not-marked (10) codepoint on any
| PCN node | Codepoint | Valid codepoint | Invalid codepoint out | arriving packet that belongs to a PCN-flow. It MUST set the not-PCN
| type | in | out | | (00) codepoint on all other packets.
+-----------+-------------+-----------------+-----------------------+
| ingress | Any | NM (or Not-PCN) | PM | A PCN-interior-node MUST observe the rules for valid and invalid
| interior | NM | NM or PM | Not-PCN or EXP | codepoint transitions as set out in the following table. The precise
| interior | EXP + | EXP or PM | Not-PCN | rules governing which valid transition to use are set out in
| interior | Not-PCN | Not-PCN | Any other codepoint | [I-D.ietf-pcn-marking-behaviour]
| interior | PM | PM | Any other codepoint | +-------------------------------------------------+
| egress | Any | 00 | Any other codepoint * | | Codepoint Out |
+-----------+-------------+-----------------+-----------------------+ +--------------+-------------+-----------+-----------+-----------+
+ This SHOULD cause an alarm to be raised at a higher layer. The | Codepoint in | not-PCN(00) | NM(10) | EXP(01) | PM(11) |
packet MUST be treated as if it were NM. +--------------+-------------+-----------+-----------+-----------+
* Except where the egress node knows that other marks may be safely | not-PCN(00) | Valid | Not valid | Not valid | Not valid |
exposed outside the PCN-domain (e.g. [PCN-3-enc-state]). +--------------+-------------+-----------+-----------+-----------+
| NM(10) | Not valid | Valid | Not valid | Valid |
+--------------+-------------+-----------+-----------+-----------+
| EXP(01)* | Not valid | Not valid | Valid | Valid |
+--------------+-------------+-----------+-----------+-----------+
| PM(11) | Not valid | Not valid | Not valid | Valid |
+--------------+-------------+-----------+-----------+-----------+
* This SHOULD cause an alarm to be raised at a higher layer. The
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-nodes PCN-packets at PCN-interior-nodes
If a pcn-interior-node compliant with this baseline encoding receives A PCN-egress-node SHOULD set the not-PCN (00) codepoint on all
a packets it forwards out of the PCN-domain. The only exception to
this is if the PCN-egress-node is certain that revealing other
codepoints outside the PCN-domain won't contravene the guidance given
in [RFC4774].
4.1. 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-tunneling] explains this in more detail. or an IPSec Tunnel. [I-D.ietf-tsvwg-ecn-tunnel] explains this in
The main effect of this constraint is that any PCN marking has to more detail. The main effect of this constraint is that any PCN
carry the 11 codepoint in the ECN field. If the packet is being marking has to carry the 11 codepoint in the ECN field since this is
tunneled then only the 11 codepoint gets copied into the inner header the only codepoint that is guaranteeed to be copied down into the
upon decapsulation. An additional constraint is the need to minimise inner header upon decapsulation. An additional constraint is the
the use of DiffServ codepoints as there is a limited supply of need to minimise the use of DiffServ codepoints as there is a limited
standards track codepoints remaining. Section 4.2 explains how we supply of standards track codepoints remaining. Section 4.3 explains
have minimised this still further by reusing pre-existing Diffserv how we have minimised this still further by reusing pre-existing
codepoint(s) such that non-PCN traffic can still be distinguished 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 deciding to set 10 as the NM state. These included similarity were considered before deciding to set 10 as the NM state. These
to ECN, presence of tunnels within the domain, leakage into and out included similarity to ECN, presence of tunnels within the domain,
of PCN-domain and incremental deployment. leakage into and out of PCN-domain and incremental deployment (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.
4.2. 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. the term "PCN-compatible Diffserv codepoint" for such a DSCP. To be
Enabling PCN for a DSCP switches on PCN marking behaviour for packets clear, any packets with such a DSCP will be PCN enabled only if they
with that DSCP, but only if those packets also have their ECN field also have their ECN field set to indicate a codepoint other than not-
set to indicate a codepoint other than Not-PCN. PCN.
Enabling PCN marking behaviour disables any other marking behaviour Enabling PCN marking behaviour disables any other marking behaviour
(e.g. enabling PCN disables the default ECN marking behaviour (e.g. enabling PCN disables the default ECN marking behaviour
introduced in [RFC3168]). All traffic scheduling and conditioning introduced in [RFC3168]). All traffic scheduling and conditioning
behaviours are discussed in [pcn-marking-behaviour]. 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].
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 MUST mean Not-PCN. 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.
Therefore an ingress node wishing to disable PCN marking for a
packet within a PCN-compatible DiffServ Codepoint MUST set the ECN
field to 00.
o The 11 codepoint in the ECN field MUST mean PCN-marked (though o The 11 codepoint in the ECN field SHALL indicate PCN-marked
this doesn't exclude other codepoints from carrying the same (though this does not exclude the 01 Experimental codepoint from
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
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
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 except where mechanisms are put in place to tunnel such
traffic across the PCN-domain. traffic across the PCN-domain.
7. IANA Considerations 7. IANA Considerations
This document makes no request to IANA. This document makes no request to IANA.
8. Security Considerations 8. Security Considerations
Packets claim entitlement to be PCN marked by carrying a PCN- PCN-marking only carries a meaning within the confines of a PCN-
Compatible DSCP and a PCN-Enabled ECN codepoint. This encoding domain. Packets wishing to be treated as belonging to a PCN-flow
document is intended to stand independently of the architecture used must carry a PCN-Compatible DSCP and a PCN-Enabled ECN codepoint.
to determine whether specific packets are authorised to be PCN This encoding document is intended to stand independently of the
marked, which will be described in a future separate document on PCN architecture used to determine whether specific packets are
edge-node behaviour. authorised to be PCN-marked, which will be described in separate
documents on PCN edge-node behaviour.
The PCN working group has initially been chartered to only consider a
PCN-domain to be entirely under the control of one operator, or a set
of operators who trust each other [PCN-charter]. However there is a
requirement to keep inter-domain scenarios in mind when defining the
PCN encoding. One way to extend to multiple domains would be to
concatenate PCN-domains and use PCN-boundary-nodes back to back at
borders. Then any one domain's security against its neighbours would
be described as part of the proposed edge-node behaviour document.
One proposal on the table allows one to extend PCN across multiple This document assumes the PCN-domain to be entirely under the control
domains without PCN-boundary-nodes back-to-back at borders [re-PCN]. of a single operator, or a set of operators who trust each other.
It is believed that the encoding described here would be compatible However future extensions to PCN might include inter-domain versions
with the security framework described there. where trust cannot be assumed between domains. If such schemes are
proposed they must ensure that they can operate securely despite the
lack of trust but such considerations are beyond the scope of 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 doesn't require end-to-end ECN support. The encoding scheme traffic does not require ECN support within the PCN-domain. The
is conformant with [RFC4774]. encoding scheme is conformant with [RFC4774].
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 for Charny, Joe Babiarz and others. Thanks to Ruediger Geib and Gorry
providing detailed comments on this document. Fairhurst for providing detailed comments on this document.
11. Comments Solicited 11. Comments Solicited
Comments and questions are encouraged and very welcome. They can be (To be removed by the RFC-Editor.) Comments and questions are
addressed to the IETF congestion and pre-congestion working group encouraged and very welcome. They can be addressed to the IETF
mailing list <pcn@ietf.org>, and/or to the authors. congestion and pre-congestion working group mailing list
<pcn@ietf.org>, and/or to the authors.
12. References 12. References
12.1. Normative References 12.1. Normative References
[RFC2119] Bradner, S., "Key words for use in RFCs to [I-D.ietf-pcn-marking-behaviour] Eardley, P., "Marking
Indicate Requirement Levels", BCP 14, behaviour of PCN-nodes", dra
ft-ietf-pcn-marking-
behaviour-02 (work in
progress), March 2009.
[RFC2119] Bradner, S., "Key words for
use in RFCs to Indicate
Requirement Levels", BCP 14,
RFC 2119, March 1997. RFC 2119, March 1997.
[RFC4774] Floyd, S., "Specifying Alternate Semantics [RFC3168] Ramakrishnan, K., Floyd, S.,
for the Explicit Congestion Notification and D. Black, "The Addition
(ECN) Field", BCP 124, RFC 4774, of Explicit Congestion
Notification (ECN) to IP",
RFC 3168, September 2001.
[RFC4774] Floyd, S., "Specifying
Alternate Semantics for the
Explicit Congestion
Notification (ECN) Field",
BCP 124, RFC 4774,
November 2006. November 2006.
12.2. Informative References 12.2. Informative References
[PCN-3-enc-state] Moncaster, T., Briscoe, B., and M. Menth, "A [I-D.ietf-pcn-architecture] Eardley, P., "Pre-Congestion
three state extended PCN encoding scheme", Notification (PCN)
draft-moncaster-pcn-3-state-encoding-00 Architecture", draft-ietf-
(work in progress), June 2008. pcn-architecture-10 (work in
progress), March 2009.
[PCN-charter] IETF, "IETF Charter for Congestion and Pre- [I-D.ietf-tsvwg-admitted-realtime-dscp] Baker, F., Polk, J., and M.
Congestion Notification Working Group". Dolly, "DSCP for Capacity-
Admitted Traffic", draft-
ietf-tsvwg-admitted-
realtime-dscp-05 (work in
progress), November 2008.
[RFC3168] Ramakrishnan, K., Floyd, S., and D. Black, [I-D.ietf-tsvwg-ecn-tunnel] Briscoe, B., "Tunnelling of
"The Addition of Explicit Congestion Explicit Congestion
Notification (ECN) to IP", RFC 3168, Notification", draft-ietf-
September 2001. tsvwg-ecn-tunnel-02 (work in
progress), March 2009.
[RFC4301] Kent, S. and K. Seo, "Security Architecture [RFC4301] Kent, S. and K. Seo,
for the Internet Protocol", RFC 4301, "Security Architecture for
December 2005. the Internet Protocol",
RFC 4301, December 2005.
[RFC5127] Chan, K., Babiarz, J., and F. Baker, [RFC5127] Chan, K., Babiarz, J., and
"Aggregation of DiffServ Service Classes", F. Baker, "Aggregation of
DiffServ Service Classes",
RFC 5127, February 2008. RFC 5127, February 2008.
[ecn-tunneling] Briscoe, B., "Layered Encapsulation of
Congestion Notification",
draft-ietf-tsvwg-ecn-tunnel-01 (work in
progress), October 2008.
[pcn-arch] Eardley, P., "Pre-Congestion Notification
(PCN) Architecture",
draft-ietf-pcn-architecture-07 (work in
progress), September 2008.
[pcn-marking-behaviour] Eardley, P., "Marking behaviour of PCN-
nodes", draft-ietf-pcn-marking-behaviour-01
(work in progress), October 2008.
[re-PCN] Briscoe, B., "Emulating Border Flow Policing
using Re-ECN on Bulk Data",
draft-briscoe-re-pcn-border-cheat-00 (work
in progress), July 2007.
[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 Appendix A. PCN Deployment Considerations
A.1. Choice of Suitable DSCPs A.1. Choice of Suitable DSCPs
The choice of which DSCP is most suitable for the PCN-domain is The PCN Working Group chose not to define a single DSCP for use with
dependant on the nature of the traffic entering that domain and the PCN for several reasons. Firstly the PCN mechanism is applicable to
link rates of all the links making up that domain. In PCN-domains a variety of different traffic classes. Secondly standards track
with uniformly high link rates, the appropriate DSCPs would currently DSCPs are in increasingly short supply. Thirdly PCN should be seen
be those for the Real Time Traffic Class [RFC5127]. If the PCN as being essentially a marking behaviour similar to ECN but intended
domain includes lower speed links it would also be appropriate to use for inelastic traffic. The choice of which DSCP is most suitable for
the DSCPs of the other traffic classes that [voice-admit] defines for a given PCN-domain is dependant on the nature of the traffic entering
use with admission control, such as the three video classes CS4, CS3 that domain and the link rates of all the links making up that
and AF4 and the Admitted Telephony Class. domain. In PCN-domains with uniformly high link rates, the
appropriate DSCPs would currently be those for the Real Time Traffic
Class [RFC5127]. If the PCN domain includes lower speed links it
would also be appropriate to use the DSCPs of the other traffic
classes that [I-D.ietf-tsvwg-admitted-realtime-dscp] defines for use
with admission control, such as the three video classes CS4, CS3 and
AF4 and the Admitted Telephony Class.
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 PCN-domain: Either ECT o [RFC3168] full functionality tunnel within the PCN-domain: Either
is safe. ECT is safe.
o Leakage of traffic into PCN-domain: ECT(1) is less often correct. o Leakage of traffic into PCN-domain: ECT(1) is slightly less likely
to occur so might be considered safer.
o Leakage of traffic out of PCN-domainL 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 ECT is suitable as long as they are o Incremental deployment: Either codepoint is suitable providing
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].
Overall this seemed to suggest ECT(0) was most appropriate to use.
Authors' Addresses Authors' Addresses
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
 End of changes. 60 change blocks. 
212 lines changed or deleted 266 lines changed or added

This html diff was produced by rfcdiff 1.35. The latest version is available from http://tools.ietf.org/tools/rfcdiff/