draft-ietf-pcn-baseline-encoding-05.txt   draft-ietf-pcn-baseline-encoding-06.txt 
Congestion and Pre Congestion T. Moncaster Congestion and Pre Congestion T. Moncaster
Internet-Draft B. Briscoe Internet-Draft B. Briscoe
Intended status: Standards Track BT Intended status: Standards Track BT
Expires: February 21, 2010 M. Menth Expires: March 8, 2010 M. Menth
University of Wuerzburg University of Wuerzburg
August 20, 2009 September 4, 2009
Baseline Encoding and Transport of Pre-Congestion Information Baseline Encoding and Transport of Pre-Congestion Information
draft-ietf-pcn-baseline-encoding-05 draft-ietf-pcn-baseline-encoding-06
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
from IETF Documents or IETF Contributions published or made publicly from IETF Documents or IETF Contributions published or made publicly
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
skipping to change at page 1, line 44 skipping to change at page 1, line 44
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 February 21, 2010. This Internet-Draft will expire on March 8, 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
and restrictions with respect to this document. and restrictions with respect to this document.
Abstract Abstract
The objective of Pre-Congestion Notification (PCN) is to protect the The objective of the pre-congestion notification (PCN) architecture
quality of service (QoS) of inelastic flows within a Diffserv domain. is to protect the QoS of inelastic flows within a Diffserv domain.
The overall rate of the PCN-traffic is metered on every link in the It achieves this by marking packets belonging to PCN-flows when the
PCN-domain, and PCN-packets are appropriately marked when certain rate of traffic exceeds certain configured thresholds on links in the
configured rates are exceeded. The level of marking allows the domain. These marks can then be evaluated to determine how close the
boundary nodes to make decisions about whether to admit or block a domain is to being congested. This document specifies how such marks
new flow request, and (in abnormal circumstances) whether to are encoded into the IP header by redefining the Explicit Congestion
terminate some of the existing flows, thereby protecting the QoS of Notification (ECN) codepoints within such domains. The baseline
previously admitted flows. This document specifies how such marks encoding described here provides only two PCN encoding states: not-
are to be encoded into the IP header by re-using the Explicit marked and PCN-marked. Future extensions to this encoding may be
Congestion Notification (ECN) codepoints within this controlled needed in order to provide more than one level of marking severity.
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 . . . . . . . . . . . . . . . . . . . . 6
3. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 6 3. Terminology and Abbreviations . . . . . . . . . . . . . . . . 6
4. Encoding two PCN States in IP . . . . . . . . . . . . . . . . 6 3.1. List of Abbreviations . . . . . . . . . . . . . . . . . . 7
4.1. Valid and Invalid Codepoint Transitions . . . . . . . . . 7 4. Encoding two PCN States in IP . . . . . . . . . . . . . . . . 7
4.2. Rationale for Encoding . . . . . . . . . . . . . . . . . . 8 4.1. Marking Packets . . . . . . . . . . . . . . . . . . . . . 8
4.3. PCN-Compatible Diffserv Codepoints . . . . . . . . . . . . 8 4.2. Valid and Invalid Codepoint Transitions . . . . . . . . . 8
4.3.1. Co-existence of PCN and not-PCN traffic . . . . . . . 9 4.3. Rationale for Encoding . . . . . . . . . . . . . . . . . . 9
5. Rules for Experimental Encoding Schemes . . . . . . . . . . . 9 4.4. PCN-Compatible Diffserv Codepoints . . . . . . . . . . . . 10
6. Backwards Compatibility . . . . . . . . . . . . . . . . . . . 9 4.4.1. Co-existence of PCN and not-PCN traffic . . . . . . . 10
7. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 10 5. Rules for Experimental Encoding Schemes . . . . . . . . . . . 11
8. Security Considerations . . . . . . . . . . . . . . . . . . . 10 6. Backwards Compatibility . . . . . . . . . . . . . . . . . . . 11
9. Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . 10 7. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 12
10. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 10 8. Security Considerations . . . . . . . . . . . . . . . . . . . 12
11. Comments Solicited . . . . . . . . . . . . . . . . . . . . . . 11 9. Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . 12
12. References . . . . . . . . . . . . . . . . . . . . . . . . . . 11 10. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 12
12.1. Normative References . . . . . . . . . . . . . . . . . . . 11 11. Comments Solicited . . . . . . . . . . . . . . . . . . . . . . 13
12.2. Informative References . . . . . . . . . . . . . . . . . . 11 12. References . . . . . . . . . . . . . . . . . . . . . . . . . . 13
Appendix A. PCN Deployment Considerations (Informational) . . . . 12 12.1. Normative References . . . . . . . . . . . . . . . . . . . 13
A.1. Choice of Suitable DSCPs . . . . . . . . . . . . . . . . . 12 12.2. Informative References . . . . . . . . . . . . . . . . . . 13
A.2. Rationale for Using ECT(0) for Not-marked . . . . . . . . 13 Appendix A. PCN Deployment Considerations (Informational) . . . . 14
A.1. Choice of Suitable DSCPs . . . . . . . . . . . . . . . . . 14
A.2. Rationale for Using ECT(0) for Not-marked . . . . . . . . 15
1. Introduction 1. Introduction
The objective of Pre-Congestion Notification (PCN) [RFC5559] is to The objective of the Pre-Congestion Notification (PCN) Architecture
protect the quality of service (QoS) of inelastic flows within a [RFC5559] is to protect the quality of service (QoS) of inelastic
Diffserv domain, in a simple, scalable and robust fashion. The flows within a Diffserv domain, in a simple, scalable and robust
overall rate of the PCN-traffic is metered on every link in the PCN- fashion. The overall rate of the PCN-traffic is metered on every
domain, and PCN-packets are appropriately marked when certain link in the PCN-domain, and PCN-packets are appropriately marked when
configured rates are exceeded. These configured rates are below the certain configured rates are exceeded. These configured rates are
rate of the link thus providing notification before any congestion below the rate of the link thus providing notification before any
occurs (hence "pre-congestion notification"). The level of marking congestion occurs (hence "pre-congestion notification"). The level
allows the boundary nodes to make decisions about whether to admit or of marking allows the boundary nodes to make decisions about whether
block a new flow request, and (in abnormal circumstances) whether to to admit or block a new flow request, and (in abnormal circumstances)
terminate some of the existing flows, thereby protecting the QoS of whether to terminate some of the existing flows, thereby protecting
previously admitted flows. 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 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 -05 to -06:
Extensive changes to the text following IETF Last Call and Gen-ART
review comments.
Abstract updated following mailing list discussions after Gen-ART
review by Spencer Dawkins.
Added list of abbreviations
New [section 4.1] added to explain the required action when a node
indicates the need to mark a packet.
Clarified text and Table 2 in Section 4.2.
Improved explanation of rules for experimental encoding schemes in
Section 5. Removed any ambiguity about meaning of PCN-marked in
such a context. Added requirements for experimental schemes to
define which meter causes which mark.
Clarified text in Section 6 relating to support for e2e ECN.
Added text in Section 8 relating to injection of PCN-marks into
the PCN-domain.
Changed text of Appendix A.1 to reflect comments from Spencer
Dawkins and Philip Eardey.
From -04 to -05: From -04 to -05:
Clarified throughout that the PCN WG is not requesting a specific Clarified throughout that the PCN WG is not requesting a specific
DSCP for PCN. Rather we are recommending a set of DSCPs that DSCP for PCN. Rather we are recommending a set of DSCPs that
might be suitable. Appendix A.1 has been re-written to reflect might be suitable. Appendix A.1 has been re-written to reflect
this. References to maintaining a list of PCN-compatible DSCPs this. References to maintaining a list of PCN-compatible DSCPs
have also been removed. have also been removed.
Last sentence of Section 6 altered. Last sentence of Section 6 altered.
Several spelling corrections. Several spelling corrections.
References updated throughout. 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 4.4.1 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.
Also addressed a number of WGLC nits. 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
skipping to change at page 6, line 5 skipping to change at page 6, line 30
baseline encoding. baseline encoding.
Minor changes throughout. Minor changes throughout.
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 and Abbreviations
The following terms are used in this document: The following terms are defined 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. [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 [RFC5559]. In addition, the document uses the terminology defined in [RFC5559].
3.1. List of Abbreviations
The following abbreviations are used in this document:
o AF = Assured Forwarding [RFC2597]
o CE = Congestion Experienced [RFC3168]
o CS = Class Selector [RFC2474]
o DSCP = Diffserv codepoint
o ECN = Explicit Congestion Notification [RFC3168]
o ECT = ECN Capable Transport [RFC3168]
o EF = Expedited Forwarding [RFC3246]
o EXP = Experimental
o NM = Not-marked
o PCN = Pre-Congestion Notification
o PM = PCN-marked
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 4.4)
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 compatibility 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. Appendix A.1 gives guidance to implementiors on suitable traffic. Appendix A.1 gives guidance to implementors on suitable
DSCPs. Guidelines for mixing traffic-types within a PCN-domain DSCPs. Guidelines for mixing traffic-types within a PCN-domain
are given in [I-D.ietf-pcn-marking-behaviour]. 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 its ECN field set to
outermost IP header equal to 00. 00.
4.1. Valid and Invalid Codepoint Transitions 4.1. Marking Packets
[I-D.ietf-pcn-marking-behaviour] states that any encoding scheme
document must specify the required action to take if one of the
marking algorithms indicates that a packet needs to be marked. For
the baseline encoding scheme the required action is simply as
follows:
o If a marking algorithm indicates the need to mark a PCN-packet
then that packet MUST have its PCN codepoint set to 11, PCN-
marked.
4.2. 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 [I-D.ietf-pcn-marking-behaviour]) and from EXP to PCN-mark a packet [I-D.ietf-pcn-marking-behaviour]) and from EXP to
PM (which MAY be allowed by some future experimental extensions). PM. PCN-nodes that only implement the baseline encoding MUST be able
The following table gives the full set of valid and invalid codepoint to PCN mark packets that arrive with the EXP codepoint. This should
transitions. ease the design of experimental schemes that want to allow partial
deployment of experimental nodes alongside nodes that only implement
the baseline encoding. The following table gives the full set of
valid and invalid codepoint 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 |
+--------------+-------------+-----------+-----------+-----------+ +--------------+-------------+-----------+-----------+-----------+
| 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 MAY cause an alarm to be raised at a management layer.
packet MUST be treated as if it carried the NM codepoint. See paragraph above for an explanation of this transition.
Table 2: Valid and Invalid Codepoint Transitions for Table 2: Valid and Invalid Codepoint Transitions for PCN-packets
PCN-packets at PCN-interior-nodes at PCN-interior-nodes
The codepoint transition constraints given here apply only to the
baseline encoding scheme. Constraints on codepoint transitions for
future experimental schemes are discussed in Section 5.
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
packets it forwards out of the PCN-domain. The only exception to packets it forwards out of the PCN-domain. The only exception to
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]. For instance if the PCN-ingress-node has explicitly
informed the PCN-egress-node that this flow is ECN-capable then it
might be safe to expose other codepoints.
4.2. Rationale for Encoding 4.3. 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. [I-D.ietf-tsvwg-ecn-tunnel] explains this in
more detail. The main effect of this constraint is that any PCN more detail. The main effect of this constraint is that any PCN
marking has to carry the 11 codepoint in the ECN field since this is marking has to carry the 11 codepoint in the ECN field since this is
the only codepoint that is guaranteed to be copied down into the the only codepoint that is guaranteed to be copied down into the
inner header upon decapsulation. An additional constraint is the inner header upon decapsulation. An additional constraint is the
need to minimise the use of Diffserv codepoints because there is a need to minimise the use of Diffserv codepoints because there is a
limited supply of standards track codepoints remaining. Section 4.3 limited supply of standards track codepoints remaining. Section 4.4
explains how we have minimised this still further by reusing pre- explains how we have minimised this still further by reusing pre-
existing Diffserv codepoint(s) such that non-PCN traffic can still be existing Diffserv codepoint(s) such that non-PCN traffic can still be
distinguished from PCN traffic. There are a number of factors that distinguished from PCN traffic.
were considered before choosing to set 10 as the NM state instead of
01. These included similarity to ECN, presence of tunnels within the There are a number of factors that were considered before choosing to
domain, leakage into and out of PCN-domain and incremental deployment set 10 as the NM state instead of 01. These included similarity to
(see Appendix A.2). ECN, presence of tunnels within the domain, 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.
4.3. PCN-Compatible Diffserv Codepoints 4.4. 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. To be
clear, any packets with such a DSCP will be PCN enabled only if they clear, any packets with such a DSCP will be PCN enabled only if they
are within a PCN-domain and have their ECN field set to indicate a are 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 replaces the default ECN marking
behaviour introduced in [RFC3168]). All traffic metering and marking behaviour introduced in [RFC3168]) with the PCN metering and marking
behaviours are discussed in [I-D.ietf-pcn-marking-behaviour]. This behaviours described in [I-D.ietf-pcn-marking-behaviour]). This
ensures 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 The PCN Working Group has chosen not to define a single DSCP for use
with PCN for several reasons. Firstly the PCN mechanism is with PCN for several reasons. Firstly the PCN mechanism is
applicable to a variety of different traffic classes. Secondly applicable to a variety of different traffic classes. Secondly
standards track DSCPs are in increasingly short supply. Thirdly PCN standards track DSCPs are in increasingly short supply. Thirdly PCN
should be seen as being essentially a marking behaviour similar to is not a scheduling behaviour - rather it should be seen as being
ECN but intended for inelastic traffic. More details are given in essentially a marking behaviour similar to ECN but intended for
the informational appendix Appendix A.1. inelastic traffic. More details are given in the informational
Appendix A.1.
4.3.1. Co-existence of PCN and not-PCN traffic 4.4.1. 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. S3.5 of
[RFC5559] discusses how competing-non-PCN-traffic should be handled.
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 All Interior-nodes within a PCN-domain MUST interpret the 00 o All Interior-nodes within a PCN-domain MUST interpret the 00
codepoint in the ECN field as not-PCN and MUST NOT change it to codepoint in the ECN field as not-PCN and MUST NOT change it to
another value. Therefore an ingress node wishing to disable PCN another value. Therefore an ingress node wishing to disable PCN
marking for a packet within a PCN-compatible Diffserv Codepoint marking for a packet with a PCN-compatible Diffserv Codepoint MUST
MUST set the ECN field to 00. set the ECN field to 00.
o The 11 codepoint in the ECN field MUST indicate PCN-marked (though o The 11 codepoint in the ECN field MUST indicate that the packet
this does not exclude the 01 Experimental codepoint from carrying has been PCN-marked as the result of one or both of the meters
the same meaning). indicating a need to PCN-mark a packet
[I-D.ietf-pcn-marking-behaviour]. The experimental scheme MUST
define which meter(s) trigger this marking.
o The 01 Experimental codepoint in the ECN field MAY mean PCN-marked
or it MAY carry some other meaning. However any experimental
scheme MUST define its meaning in the context of that experiment.
o If both the 01 and 11 codepoints are being used to indicate PCN-
Marked then the 11 codepoint MUST be taken to be the more severe
marking and the choice of which meter sets which mark MUST be
defined.
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.
should be noted that this baseline encoding effectively disables end-
to-end ECN unless mechanisms are put in place to tunnel such traffic On its own, this baseline encoding cannot support both ECN marking
across the PCN-domain. Standard IP-in-IP or IPsec tunnels will end to end and PCN marking within a PCN-domain. It is possible to do
always copy the CE codepoint from the outer header into the inner this by carrying e2e ECN across a PCN domain within the inner header
header in decapsulation (unless the inner packet is not-ECT). If an of an IP in IP tunnel, or by using a richer encoding such as the
operator wishes to allow ECN to exist end-to-end they must ensure proposed experimental scheme in [I-D.ietf-pcn-3-state-encoding].
there are no tunnel end-points within the PCN-domain to prevent any
risk of PCN-markings being exposed to endpoints.
7. IANA Considerations 7. IANA Considerations
This document makes no direct request to IANA. This document makes no request to IANA.
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. This encoding document is intended to stand independently of
must carry a PCN-compatible DSCP and a PCN-Enabled ECN codepoint. the architecture used to determine how specific packets are
This encoding document is intended to stand independently of the authorised to be PCN-marked, which will be described in separate
architecture used to determine how specific packets are authorised to documents on PCN-boundary-node behaviour.
be PCN-marked, which will be described in separate documents on PCN-
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. However such considerations are beyond the scope of lack of trust. However such considerations are beyond the scope of
this document. this document.
One potential security concern is the injection of spurious PCN-marks
into the PCN-domain. However these can only enter the domain if a
PCN-ingress-node is misconfigured. The precise impact of any such
misconfiguration will depend on which of the proposed PCN-boundary-
node behaviour schemes is used, but in general spurious marks will
lead to admitting fewer flows into the domain or potentially
terminating too many flows. In either case good management should be
able to quickly spot the problem since the overall utilisation of the
domain will rapidly fall.
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]. The Working Group has encoding scheme is conformant with [RFC4774]. The Working Group has
chosen not to define a single DSCP for use with PCN. The rationale chosen not to define a single DSCP for use with PCN. The rationale
skipping to change at page 11, line 41 skipping to change at page 13, line 40
IP", RFC 3168, September 2001. IP", RFC 3168, September 2001.
[RFC4774] Floyd, S., "Specifying Alternate [RFC4774] Floyd, S., "Specifying Alternate
Semantics for the Explicit Semantics for the Explicit
Congestion Notification (ECN) Congestion Notification (ECN)
Field", BCP 124, RFC 4774, Field", BCP 124, RFC 4774,
November 2006. November 2006.
12.2. Informative References 12.2. Informative References
[I-D.ietf-pcn-3-state-encoding] Moncaster, T., Briscoe, B., and M.
Menth, "A PCN encoding using 2
DSCPs to provide 3 or more states",
draft-ietf-pcn-3-state-encoding-00
(work in progress), April 2009.
[I-D.ietf-tsvwg-ecn-tunnel] Briscoe, B., "Tunnelling of [I-D.ietf-tsvwg-ecn-tunnel] Briscoe, B., "Tunnelling of
Explicit Congestion Notification", Explicit Congestion Notification",
draft-ietf-tsvwg-ecn-tunnel-03 draft-ietf-tsvwg-ecn-tunnel-03
(work in progress), July 2009. (work in progress), July 2009.
[RFC2474] Nichols, K., Blake, S., Baker, F.,
and D. Black, "Definition of the
Differentiated Services Field (DS
Field) in the IPv4 and IPv6
Headers", RFC 2474, December 1998.
[RFC2597] Heinanen, J., Baker, F., Weiss, W.,
and J. Wroclawski, "Assured
Forwarding PHB Group", RFC 2597,
June 1999.
[RFC3246] Davie, B., Charny, A., Bennet, J.,
Benson, K., Le Boudec, J.,
Courtney, W., Davari, S., Firoiu,
V., and D. Stiliadis, "An Expedited
Forwarding PHB (Per-Hop Behavior)",
RFC 3246, March 2002.
[RFC3540] Spring, N., Wetherall, D., and D. [RFC3540] Spring, N., Wetherall, D., and D.
Ely, "Robust Explicit Congestion Ely, "Robust Explicit Congestion
Notification (ECN) Signaling with Notification (ECN) Signaling with
Nonces", RFC 3540, June 2003. Nonces", RFC 3540, June 2003.
[RFC4301] Kent, S. and K. Seo, "Security [RFC4301] Kent, S. and K. Seo, "Security
Architecture for the Internet Architecture for the Internet
Protocol", RFC 4301, December 2005. Protocol", RFC 4301, December 2005.
[RFC4594] Babiarz, J., Chan, K., and F.
Baker, "Configuration Guidelines
for DiffServ Service Classes",
RFC 4594, August 2006.
[RFC5127] Chan, K., Babiarz, J., and F. [RFC5127] Chan, K., Babiarz, J., and F.
Baker, "Aggregation of DiffServ Baker, "Aggregation of DiffServ
Service Classes", RFC 5127, Service Classes", RFC 5127,
February 2008. February 2008.
[RFC5559] Eardley, P., "Pre-Congestion [RFC5559] Eardley, P., "Pre-Congestion
Notification (PCN) Architecture", Notification (PCN) Architecture",
RFC 5559, June 2009. RFC 5559, June 2009.
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 is not a
as being essentially a marking behaviour similar to ECN but intended scheduling behaviour - rather it should be seen as being a marking
for inelastic traffic. The choice of which DSCP is most suitable for behaviour similar to ECN but intended for inelastic traffic. The
a given PCN-domain is dependent on the nature of the traffic entering choice of which DSCP is most suitable for a given PCN-domain is
that domain and the link rates of all the links making up that dependent on the nature of the traffic entering that domain and the
domain. In PCN-domains with uniformly high link rates, the link rates of all the links making up that domain. In PCN-domains
appropriate DSCPs would currently be those for the Real Time Traffic with sufficient aggregation, the appropriate DSCPs would currently be
Class [RFC5127]. To be clear the PCN Working Group recommends using those for the Real Time Treatment Aggregate [RFC5127]. The PCN
admission control for the following service classes: Working Group suggests using admission control for the following
service classes (defined in [RFC4594]):
o Telephony (EF) o Telephony (EF)
o Real-time interactive (CS4) o Real-time interactive (CS4)
o Broadcast Video (CS3) o Broadcast Video (CS3)
o Multimedia Conferencing (AF4) o Multimedia Conferencing (AF4)
CS5 is excluded from this list since PCN is not expected to be
applied to signalling traffic.
PCN marking is intended to provide a scalable admission control PCN marking is intended to provide a scalable admission control
mechanism for traffic with a high degree of statistical multiplexing. mechanism for traffic with a high degree of statistical multiplexing.
PCN marking would therefore be appropriate to apply to traffic in the PCN marking would therefore be appropriate to apply to traffic in the
above classes, but only within a PCN region containing highly above classes, but only within a PCN-domain containing sufficiently
aggregated traffic. In such cases, the above service classes may aggregated traffic. In such cases, the above service classes may
well all be subject to a single forwarding treatment (treatment well all be subject to a single forwarding treatment (treatment
aggregate [RFC5127]). However, this does not imply all such IP aggregate [RFC5127]). However, this does not imply all such IP
traffic would necessarily be identified by one DSCP - each service traffic would necessarily be identified by one DSCP - each service
class might keep a distinct DSCP within the highly aggregated region class might keep a distinct DSCP within the highly aggregated region
[RFC5127]. [RFC5127].
Additional service classes may be defined for which admission control Additional service classes may be defined for which admission control
is appropriate, whether through some future standards action or is appropriate, whether through some future standards action or
through local use by certain operators, e.g. the Multimedia Streaming through local use by certain operators, e.g. the Multimedia Streaming
 End of changes. 45 change blocks. 
118 lines changed or deleted 244 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/