draft-ietf-pcn-baseline-encoding-07.txt   rfc5696.txt 
Congestion and Pre Congestion T. Moncaster Network Working Group T. Moncaster
Internet-Draft B. Briscoe Request for Comments: 5696 B. Briscoe
Intended status: Standards Track BT Category: Standards Track BT
Expires: March 29, 2010 M. Menth M. Menth
University of Wuerzburg University of Wuerzburg
September 25, 2009 November 2009
Baseline Encoding and Transport of Pre-Congestion Information Baseline Encoding and Transport of Pre-Congestion Information
draft-ietf-pcn-baseline-encoding-07
Status of This Memo
This Internet-Draft is submitted to IETF in full conformance with the
provisions of BCP 78 and BCP 79. This document may contain material
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material or to cite them other than as "work in progress."
The list of current Internet-Drafts can be accessed at The objective of the Pre-Congestion Notification (PCN) architecture
http://www.ietf.org/ietf/1id-abstracts.txt. is to protect the quality of service (QoS) of inelastic flows within
a Diffserv domain. It achieves this by marking packets belonging to
PCN-flows when the rate of traffic exceeds certain configured
thresholds on links in the domain. These marks can then be evaluated
to determine how close the domain is to being congested. This
document specifies how such marks are encoded into the IP header by
redefining the Explicit Congestion Notification (ECN) codepoints
within such domains. The baseline encoding described here provides
only two PCN encoding states: Not-marked and PCN-marked. Future
extensions to this encoding may be needed in order to provide more
than one level of marking severity.
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improvements. Please refer to the current edition of the "Internet
Official Protocol Standards" (STD 1) for the standardization state
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Abstract RFC 5696 Baseline PCN Encoding November 2009
The objective of the pre-congestion notification (PCN) architecture This document may contain material from IETF Documents or IETF
is to protect the QoS of inelastic flows within a Diffserv domain. Contributions published or made publicly available before November
It achieves this by marking packets belonging to PCN-flows when the 10, 2008. The person(s) controlling the copyright in some of this
rate of traffic exceeds certain configured thresholds on links in the material may not have granted the IETF Trust the right to allow
domain. These marks can then be evaluated to determine how close the modifications of such material outside the IETF Standards Process.
domain is to being congested. This document specifies how such marks Without obtaining an adequate license from the person(s) controlling
are encoded into the IP header by redefining the Explicit Congestion the copyright in such materials, this document may not be modified
Notification (ECN) codepoints within such domains. The baseline outside the IETF Standards Process, and derivative works of it may
encoding described here provides only two PCN encoding states: not- not be created outside the IETF Standards Process, except to format
marked and PCN-marked. Future extensions to this encoding may be it for publication as an RFC or to translate it into languages other
needed in order to provide more than one level of marking severity. than English.
Table of Contents Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3
2. Requirements notation . . . . . . . . . . . . . . . . . . . . 6 2. Requirements Notation . . . . . . . . . . . . . . . . . . . . 3
3. Terminology and Abbreviations . . . . . . . . . . . . . . . . 6 3. Terminology and Abbreviations . . . . . . . . . . . . . . . . 3
3.1. List of Abbreviations . . . . . . . . . . . . . . . . . . 7 3.1. Terminology . . . . . . . . . . . . . . . . . . . . . . . 3
4. Encoding two PCN States in IP . . . . . . . . . . . . . . . . 8 3.2. List of Abbreviations . . . . . . . . . . . . . . . . . . 4
4.1. Marking Packets . . . . . . . . . . . . . . . . . . . . . 9 4. Encoding Two PCN States in IP . . . . . . . . . . . . . . . . 4
4.2. Valid and Invalid Codepoint Transitions . . . . . . . . . 9 4.1. Marking Packets . . . . . . . . . . . . . . . . . . . . . 5
4.3. Rationale for Encoding . . . . . . . . . . . . . . . . . . 10 4.2. Valid and Invalid Codepoint Transitions . . . . . . . . . 6
4.4. PCN-Compatible Diffserv Codepoints . . . . . . . . . . . . 10 4.3. Rationale for Encoding . . . . . . . . . . . . . . . . . . 7
4.4.1. Co-existence of PCN and not-PCN traffic . . . . . . . 11 4.4. PCN-Compatible Diffserv Codepoints . . . . . . . . . . . . 7
5. Rules for Experimental Encoding Schemes . . . . . . . . . . . 11 4.4.1. Co-Existence of PCN and Not-PCN Traffic . . . . . . . 8
6. Backwards Compatibility . . . . . . . . . . . . . . . . . . . 12 5. Rules for Experimental Encoding Schemes . . . . . . . . . . . 8
7. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 12 6. Backward Compatibility . . . . . . . . . . . . . . . . . . . . 9
8. Security Considerations . . . . . . . . . . . . . . . . . . . 12 7. Security Considerations . . . . . . . . . . . . . . . . . . . 9
9. Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . 13 8. Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . 10
10. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 13 9. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 10
11. Comments Solicited . . . . . . . . . . . . . . . . . . . . . . 13 10. References . . . . . . . . . . . . . . . . . . . . . . . . . . 10
12. References . . . . . . . . . . . . . . . . . . . . . . . . . . 14 10.1. Normative References . . . . . . . . . . . . . . . . . . . 10
12.1. Normative References . . . . . . . . . . . . . . . . . . . 14 10.2. Informative References . . . . . . . . . . . . . . . . . . 10
12.2. Informative References . . . . . . . . . . . . . . . . . . 14 Appendix A. PCN Deployment Considerations (Informative) . . . . . 11
Appendix A. PCN Deployment Considerations (Informational) . . . . 15 A.1. Choice of Suitable DSCPs . . . . . . . . . . . . . . . . . 11
A.1. Choice of Suitable DSCPs . . . . . . . . . . . . . . . . . 15 A.2. Rationale for Using ECT(0) for Not-Marked . . . . . . . . 12
A.2. Rationale for Using ECT(0) for Not-marked . . . . . . . . 16 Appendix B. Co-Existence of PCN and ECN (Informative) . . . . . . 13
Appendix B. Co-existence of PCN and ECN (Informational) . . . . . 17
RFC 5696 Baseline PCN Encoding November 2009
1. Introduction 1. Introduction
The objective of the Pre-Congestion Notification (PCN) Architecture The objective of the Pre-Congestion Notification (PCN) architecture
[RFC5559] is to protect the quality of service (QoS) of inelastic [RFC5559] is to protect the quality of service (QoS) of inelastic
flows within a Diffserv domain, in a simple, scalable and robust flows within a Diffserv domain in a simple, scalable, and robust
fashion. The overall rate of the PCN-traffic is metered on every fashion. The overall rate of PCN-traffic is metered on every link in
link in the PCN-domain, and PCN-packets are appropriately marked when the PCN-domain, and PCN-packets are appropriately marked when certain
certain configured rates are exceeded. These configured rates are configured rates are exceeded. These configured rates are below the
below the rate of the link thus providing notification before any rate of the link, thus providing notification before any congestion
congestion occurs (hence "pre-congestion notification"). The level occurs (hence "Pre-Congestion Notification"). The level of marking
of marking allows the boundary nodes to make decisions about whether allows the boundary nodes to make decisions about whether to admit or
to admit or block a new flow request, and (in abnormal circumstances) block a new flow request, and (in abnormal circumstances) whether to
whether to terminate some of the existing flows, thereby protecting terminate some of the existing flows, thereby protecting the QoS of
the QoS of previously admitted flows. previously admitted flows.
This document specifies how these PCN-marks are encoded into the IP This document specifies how these PCN-marks are encoded into the IP
header by re-using the bits of the Explicit Congestion Notification header by reusing 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): 2. Requirements Notation
From -06 to -07:
Changes made following IESG review comments.
Changed Section 4 and added Appendix B to clarify the correct
behaviour when handling packets that already have values other
than not-ECT in their ECN field.
Added paragraph to end of Section 6 clarifying that a PCN-domain
has "hard" edges.
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 Eardley.
From -04 to -05:
Clarified throughout that the PCN WG is not requesting a specific
DSCP for PCN. Rather we are recommending a set of DSCPs that
might be suitable. Appendix A.1 has been re-written to reflect
this. References to maintaining a list of PCN-compatible DSCPs
have also been removed.
Last sentence of Section 6 altered.
Several spelling corrections.
References updated throughout.
From -03 to -04:
Major WGLC comments addressed:
* Added Section 4.4.1 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:
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:
Removed Appendix A and replaced with reference to
[I-D.ietf-tsvwg-ecn-tunnel]
Moved Appendix B into main body of text.
Changed Appendix C to give deployment advice.
Minor changes throughout including checking consistency of
capitalisation of defined terms.
Clarified that LU was deliberately excluded from encoding.
From -00 to -01:
Added section on restrictions for extension encoding schemes.
Included table in Appendix showing encoding transitions at
different PCN nodes.
Checked for consistency of terminology.
Minor language changes for clarity.
Changes from previous filename
Filename changed from draft-moncaster-pcn-baseline-encoding.
Terminology changed for clarity (PCN-compatible DSCP and PCN-
enabled packet).
Minor changes throughout.
Modified meaning of ECT(1) state to EXP.
Moved text relevant to behaviour of nodes into appendix for later
transfer to new document on edge behaviours.
From draft-moncaster -01 to -02:
Minor changes throughout including tightening up language to
remain consistent with the PCN Architecture terminology.
From draft-moncaster -00 to -01:
Change of title from "Encoding and Transport of (Pre-)Congestion
Information from within a Diffserv Domain to the Egress"
Extensive changes to Introduction and abstract.
Added a section on the implications of re-using a DSCP.
Added appendix listing possible operator scenarios for using this
baseline encoding.
Minor changes throughout.
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 and Abbreviations 3. Terminology and Abbreviations
The following terms are defined in this document: 3.1. Terminology
o PCN-compatible Diffserv codepoint - a Diffserv codepoint for which The terms PCN-capable, PCN-domain, PCN-node, PCN-interior-node, PCN-
the ECN field is used to carry PCN markings rather than [RFC3168] ingress-node, PCN-egress-node, PCN-boundary-node, PCN-traffic, PCN-
markings. packets and PCN-marking are used as defined in [RFC5559]. The
following additional terms are defined in this document:
o PCN-marked - codepoint indicating packets that have been marked at o PCN-compatible Diffserv codepoint - a Diffserv codepoint
a PCN-interior-node using some PCN marking behaviour indicating packets for which the ECN field is used to carry PCN-
[I-D.ietf-pcn-marking-behaviour]. Abbreviated to PM. markings rather than [RFC3168] markings.
o Not-marked - codepoint indicating packets that are PCN-capable, o PCN-marked codepoint - a codepoint that indicates packets that
but are not PCN-marked. Abbreviated to NM. have been marked at a PCN-interior-node using some PCN-marking
behaviour [RFC5670]. Abbreviated to PM.
o PCN-enabled codepoints - collective term for all NM and PM RFC 5696 Baseline PCN Encoding November 2009
codepoints. By definition, packets carrying such codepoints are
PCN-packets.
o not-PCN - packets that are not PCN-enabled. o Not-marked codepoint - a codepoint that indicates packets that are
PCN-capable but that are not PCN-marked. Abbreviated to NM.
In addition, the document uses the terminology defined in [RFC5559]. o not-PCN codepoint - a codepoint that indicates packets that are
not PCN-capable.
3.1. List of Abbreviations 3.2. List of Abbreviations
The following abbreviations are used in this document: The following abbreviations are used in this document:
o AF = Assured Forwarding [RFC2597] o AF = Assured Forwarding [RFC2597]
o CE = Congestion Experienced [RFC3168] o CE = Congestion Experienced [RFC3168]
o CS = Class Selector [RFC2474] o CS = Class Selector [RFC2474]
o DSCP = Diffserv codepoint o DSCP = Diffserv codepoint
skipping to change at page 8, line 5 skipping to change at page 4, line 39
o EF = Expedited Forwarding [RFC3246] o EF = Expedited Forwarding [RFC3246]
o EXP = Experimental o EXP = Experimental
o NM = Not-marked o NM = Not-marked
o PCN = Pre-Congestion Notification o PCN = Pre-Congestion Notification
o PM = PCN-marked 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:
RFC 5696 Baseline PCN Encoding November 2009
+---------------+-------------+-------------+-------------+---------+ +---------------+-------------+-------------+-------------+---------+
| 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 4.4)
and EXP means available for Experimental use. N.B. we deliberately
reserve this codepoint for experimental use only (and not local use)
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: In the table above, DSCP n is a PCN-compatible Diffserv codepoint
(see Section 4.4) and EXP means available for Experimental use. N.B.
we deliberately reserve this codepoint for experimental use only (and
not local use) to prevent future compatibility issues.
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 implementors 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 [RFC5670].
o Any packet arriving at the PCN-ingress that shares a PCN- o Any packet arriving at the PCN-ingress-node that shares a PCN-
compatible DSCP and is not-PCN MUST be marked as not-PCN within compatible DSCP and is not a PCN-packet MUST be marked as not-PCN
the PCN-domain. within the PCN-domain.
o If a packet arrives at the PCN-ingress with its ECN field already o If a packet arrives at the PCN-ingress-node with its ECN field
set to a value other than not-ECT, then appropriate action MUST be already set to a value other than not-ECT, then appropriate action
taken to meet the requirements of [RFC3168]. The simplest MUST be taken to meet the requirements of [RFC3168]. The simplest
appropriate action is to just drop such packets. However this is appropriate action is to just drop such packets. However, this is
a drastic action that an operator may feel is undesirable. a drastic action that an operator may feel is undesirable.
Appendix B provides more information and summarises other Appendix B provides more information and summarises other
alternative actions that might be taken. alternative actions that might be taken.
4.1. Marking Packets 4.1. Marking Packets
[I-D.ietf-pcn-marking-behaviour] states that any encoding scheme [RFC5670] states that any encoding scheme document must specify the
document must specify the required action to take if one of the required action to take if one of the marking algorithms indicates
marking algorithms indicates that a packet needs to be marked. For that a packet needs to be marked. For the baseline encoding scheme,
the baseline encoding scheme the required action is simply as the required action is simply as follows:
follows:
o If a marking algorithm indicates the need to mark a PCN-packet 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- then that packet MUST have its PCN codepoint set to 11, PCN-
marked. marked.
RFC 5696 Baseline PCN Encoding November 2009
4.2. Valid and Invalid Codepoint Transitions 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 [RFC5670]) and from EXP to PM. PCN-nodes that only
PM. PCN-nodes that only implement the baseline encoding MUST be able implement the baseline encoding MUST be able to PCN-mark packets that
to PCN mark packets that arrive with the EXP codepoint. This should arrive with the EXP codepoint. This should ease the design of
ease the design of experimental schemes that want to allow partial experimental schemes that want to allow partial deployment of
deployment of experimental nodes alongside nodes that only implement experimental nodes alongside nodes that only implement the baseline
the baseline encoding. The following table gives the full set of encoding. The following table gives the full set of valid and
valid and invalid codepoint transitions. 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 MAY cause an alarm to be raised at a management layer. * This MAY cause an alarm to be raised at a management layer.
See paragraph above for an explanation of this transition. See paragraph above for an explanation of this transition.
Table 2: Valid and Invalid Codepoint Transitions for PCN-packets Table 2: Valid and Invalid Codepoint Transitions for
at PCN-interior-nodes PCN-Packets at PCN-Interior-Nodes
The codepoint transition constraints given here apply only to the The codepoint transition constraints given here apply only to the
baseline encoding scheme. Constraints on codepoint transitions for baseline encoding scheme. Constraints on codepoint transitions for
future experimental schemes are discussed in Section 5. 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]. For instance if the PCN-ingress-node has explicitly in [RFC4774]. For instance, if the PCN-ingress-node has explicitly
informed the PCN-egress-node that this flow is ECN-capable then it informed the PCN-egress-node that this flow is ECN-capable, then it
might be safe to expose other codepoints. might be safe to expose other codepoints.
RFC 5696 Baseline PCN Encoding November 2009
4.3. 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. [ECN-TUN] 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 guaranteed to be copied down into the that is guaranteed to be copied down into the forwarded 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 because there is a use of Diffserv codepoints because there is a limited supply of
limited supply of standards track codepoints remaining. Section 4.4 Standards Track codepoints remaining. Section 4.4 explains how we
explains how we have minimised this still further by reusing pre- have minimised this still further by reusing pre-existing Diffserv
existing 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. from PCN-traffic.
There are a number of factors that were considered before choosing to There are a number of factors that were considered before choosing to
set 10 as the NM state instead of 01. These included similarity to set 10 as the NM state instead of 01. These included similarity to
ECN, presence of tunnels within the domain, leakage into and out of ECN, presence of tunnels within the domain, leakage into and out of
PCN-domain and incremental deployment (see Appendix A.2). the 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.
4.4. 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 replaces the default ECN marking marking behaviour (e.g., enabling PCN replaces the default ECN
behaviour introduced in [RFC3168]) with the PCN metering and marking marking behaviour introduced in [RFC3168]) with the PCN-metering and
behaviours described in [I-D.ietf-pcn-marking-behaviour]). This -marking behaviours described in [RFC5670]). This ensures compliance
ensures compliance with the BCP guidance set out in [RFC4774]. with the Best Current Practice (BCP) guidance set out in [RFC4774].
The PCN working group has chosen not to define a single DSCP for use
with PCN for several reasons. Firstly, the PCN mechanism is
applicable to a variety of different traffic classes. Secondly,
Standards Track DSCPs are in increasingly short supply. Thirdly, PCN
is not a scheduling behaviour -- rather, it should be seen as being
RFC 5696 Baseline PCN Encoding November 2009
The PCN Working Group has chosen not to define a single DSCP for use
with PCN for several reasons. Firstly the PCN mechanism is
applicable to a variety of different traffic classes. Secondly
standards track DSCPs are in increasingly short supply. Thirdly PCN
is not a scheduling behaviour - rather it should be seen as being
essentially a marking behaviour similar to ECN but intended for essentially a marking behaviour similar to ECN but intended for
inelastic traffic. More details are given in the informational inelastic traffic. More details are given in the informational
Appendix A.1. Appendix A.1.
4.4.1. 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 redefines 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 non-PCN-traffic. This is achieved by providing a
providing a not-PCN state within the encoding scheme. S3.5 of not-PCN state within the encoding scheme. Section 3.5 of [RFC5559]
[RFC5559] discusses how competing-non-PCN-traffic should be handled. 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 PCN-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, a PCN-ingress-node wishing to disable
marking for a packet with a PCN-compatible Diffserv Codepoint MUST PCN-marking for a packet with a PCN-compatible Diffserv codepoint
set the ECN field to 00. MUST set the ECN field to 00.
o The 11 codepoint in the ECN field MUST indicate that the packet o The 11 codepoint in the ECN field MUST indicate that the packet
has been PCN-marked as the result of one or both of the meters has been PCN-marked as the result of one or both of the meters
indicating a need to PCN-mark a packet indicating a need to PCN-mark a packet [RFC5670]. The
[I-D.ietf-pcn-marking-behaviour]. The experimental scheme MUST experimental scheme MUST define which meter(s) trigger this
define which meter(s) trigger this marking. marking.
o The 01 Experimental codepoint in the ECN field MAY mean PCN-marked o The 01 Experimental codepoint in the ECN field MAY mean PCN-marked
or it MAY carry some other meaning. However any experimental or it MAY carry some other meaning. However, any experimental
scheme MUST define its meaning in the context of that experiment. 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- 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 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 marking and the choice of which meter sets which mark MUST be
defined. 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.
6. Backwards Compatibility RFC 5696 Baseline PCN Encoding November 2009
6. Backward 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. ECN semantics. As such, this document is compatible with BCP 124.
On its own, this baseline encoding cannot support both ECN marking On its own, this baseline encoding cannot support both ECN marking
end to end and PCN marking within a PCN-domain. It is possible to do end-to-end (e2e) and PCN-marking within a PCN-domain. It is possible
this by carrying e2e ECN across a PCN domain within the inner header to do this by carrying e2e ECN across a PCN-domain within the inner
of an IP in IP tunnel, or by using a richer encoding such as the header of an IP-in-IP tunnel, or by using a richer encoding such as
proposed experimental scheme in [I-D.ietf-pcn-3-state-encoding]. the proposed experimental scheme in [PCN-ENC].
In any PCN deployment, traffic can only enter the PCN-Domain through In any PCN deployment, traffic can only enter the PCN-domain through
PCN-ingress-nodes and leave through PCN-egress-nodes. PCN-ingress- PCN-ingress-nodes and leave through PCN-egress-nodes. PCN-ingress-
nodes ensure that any packets entering the PCN- domain have the ECN- nodes ensure that any packets entering the PCN-domain have the ECN
field in their outermost IP header set to the appropriate PCN field in their outermost IP header set to the appropriate PCN
codepoint. PCN-egress-nodes then guarantee that the ECN-field of any codepoint. PCN-egress-nodes then guarantee that the ECN field of any
packet leaving the PCN-domain has the correct ECN semantics. This packet leaving the PCN-domain has the correct ECN semantics. This
prevents leakage of ECN marks into or out of the PCN-domain and thus prevents unintended leakage of ECN marks into or out of the PCN-
reduces backward compatibility issues. domain, and thus reduces backward-compatibility issues.
7. IANA Considerations
This document makes no request to IANA.
8. Security Considerations 7. 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. This encoding document is intended to stand independently of domain. This encoding document is intended to stand independently of
the architecture used to determine how specific packets are the architecture used to determine how specific packets are
authorised to be PCN-marked, which will be described in separate authorised to be PCN-marked, which will be described in separate
documents on PCN-boundary-node behaviour. 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 One potential security concern is the injection of spurious PCN-marks
into the PCN-domain. However these can only enter the domain if a into the PCN-domain. However, these can only enter the domain if a
PCN-ingress-node is misconfigured. The precise impact of any such PCN-ingress-node is misconfigured. The precise impact of any such
misconfiguration will depend on which of the proposed PCN-boundary- misconfiguration will depend on which of the proposed PCN-boundary-
node behaviour schemes is used, but in general spurious marks will node behaviour schemes is used, but in general spurious marks will
lead to admitting fewer flows into the domain or potentially lead to admitting fewer flows into the domain or potentially
terminating too many flows. In either case good management should be terminating too many flows. In either case, good management should
able to quickly spot the problem since the overall utilisation of the
domain will rapidly fall.
9. Conclusions RFC 5696 Baseline PCN Encoding November 2009
This document defines the baseline PCN encoding utilising a be able to quickly spot the problem since the overall utilisation of
combination of a PCN-enabled DSCP and the ECN field in the IP header. the domain will rapidly fall.
This baseline encoding allows the existence of two PCN encoding
states, not-Marked and PCN-marked. It also allows for the co- 8. Conclusions
existence of competing traffic within the same DSCP so long as that
traffic does not require ECN support within the PCN-domain. The This document defines the baseline PCN encoding, utilising a
encoding scheme is conformant with [RFC4774]. The Working Group has combination of a PCN-compatible DSCP and the ECN field in the IP
header. This baseline encoding allows the existence of two PCN
encoding states: Not-marked and PCN-marked. It also allows for the
co-existence of competing traffic within the same DSCP, so long as
that traffic does not require ECN support within the PCN-domain. The
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
for this decision along with advice relating to choice of suitable for this decision along with advice relating to the choice of
DSCPs can be found in Appendix A.1. suitable DSCPs can be found in Appendix A.1.
10. Acknowledgements 9. Acknowledgements
This document builds extensively on work done in the PCN working This document builds extensively on work done in the PCN working
group by Kwok Ho Chan, Georgios Karagiannis, Philip Eardley, Anna group by Kwok Ho Chan, Georgios Karagiannis, Philip Eardley, Anna
Charny, Joe Babiarz and others. Thanks to Ruediger Geib and Gorry Charny, Joe Babiarz, and others. Thanks to Ruediger Geib and Gorry
Fairhurst for providing detailed comments on this document. Fairhurst for providing detailed comments on this document.
11. Comments Solicited 10. References
(To be removed by the RFC-Editor.) Comments and questions are 10.1. Normative References
encouraged and very welcome. They can be addressed to the IETF
congestion and pre-congestion working group mailing list
<pcn@ietf.org>, and/or to the authors.
12. References [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
12.1. Normative References Requirement Levels", BCP 14, RFC 2119, March 1997.
[I-D.ietf-pcn-marking-behaviour] Eardley, P., "Metering and marking [RFC3168] Ramakrishnan, K., Floyd, S., and D. Black, "The Addition
behaviour of PCN-nodes", of Explicit Congestion Notification (ECN) to IP",
draft-ietf-pcn-marking-behaviour-05 RFC 3168, September 2001.
(work in progress), August 2009.
[RFC2119] Bradner, S., "Key words for use in [RFC4774] Floyd, S., "Specifying Alternate Semantics for the
RFCs to Indicate Requirement Explicit Congestion Notification (ECN) Field", BCP 124,
Levels", BCP 14, RFC 2119, RFC 4774, November 2006.
March 1997.
[RFC3168] Ramakrishnan, K., Floyd, S., and D. [RFC5670] Eardley, P., Ed., "Metering and Marking Behaviour of PCN-
Black, "The Addition of Explicit Nodes", RFC 5670, November 2009.
Congestion Notification (ECN) to
IP", RFC 3168, September 2001.
[RFC4774] Floyd, S., "Specifying Alternate RFC 5696 Baseline PCN Encoding November 2009
Semantics for the Explicit
Congestion Notification (ECN)
Field", BCP 124, RFC 4774,
November 2006.
12.2. Informative References 10.2. Informative References
[I-D.ietf-pcn-3-state-encoding] Moncaster, T., Briscoe, B., and M. [ECN-TUN] Briscoe, B., "Tunnelling of Explicit Congestion
Menth, "A PCN encoding using 2 Notification", Work in Progress, July 2009.
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 [PCN-ENC] Moncaster, T., Briscoe, B., and M. Menth, "A PCN encoding
Explicit Congestion Notification", using 2 DSCPs to provide 3 or more states", Work
draft-ietf-tsvwg-ecn-tunnel-03 in Progress, April 2009.
(work in progress), July 2009.
[RFC2474] Nichols, K., Blake, S., Baker, F., [RFC2474] Nichols, K., Blake, S., Baker, F., and D. Black,
and D. Black, "Definition of the "Definition of the Differentiated Services Field (DS
Differentiated Services Field (DS Field) in the IPv4 and IPv6 Headers", RFC 2474,
Field) in the IPv4 and IPv6 December 1998.
Headers", RFC 2474, December 1998.
[RFC2597] Heinanen, J., Baker, F., Weiss, W., [RFC2597] Heinanen, J., Baker, F., Weiss, W., and J. Wroclawski,
and J. Wroclawski, "Assured "Assured Forwarding PHB Group", RFC 2597, June 1999.
Forwarding PHB Group", RFC 2597,
June 1999.
[RFC3246] Davie, B., Charny, A., Bennet, J., [RFC3246] Davie, B., Charny, A., Bennet, J., Benson, K., Le Boudec,
Benson, K., Le Boudec, J., J., Courtney, W., Davari, S., Firoiu, V., and D.
Courtney, W., Davari, S., Firoiu, Stiliadis, "An Expedited Forwarding PHB (Per-Hop
V., and D. Stiliadis, "An Expedited Behavior)", RFC 3246, March 2002.
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
Ely, "Robust Explicit Congestion Congestion Notification (ECN) Signaling with Nonces",
Notification (ECN) Signaling with 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
Architecture for the Internet Internet Protocol", RFC 4301, December 2005.
Protocol", RFC 4301, December 2005.
[RFC4594] Babiarz, J., Chan, K., and F. [RFC4594] Babiarz, J., Chan, K., and F. Baker, "Configuration
Baker, "Configuration Guidelines Guidelines for DiffServ Service Classes", RFC 4594,
for DiffServ Service Classes", August 2006.
RFC 4594, August 2006.
[RFC5127] Chan, K., Babiarz, J., and F. [RFC5127] Chan, K., Babiarz, J., and F. Baker, "Aggregation of
Baker, "Aggregation of DiffServ DiffServ Service Classes", RFC 5127, February 2008.
Service Classes", RFC 5127,
February 2008.
[RFC5559] Eardley, P., "Pre-Congestion [RFC5559] Eardley, P., "Pre-Congestion Notification (PCN)
Notification (PCN) Architecture", Architecture", RFC 5559, June 2009.
RFC 5559, June 2009.
Appendix A. PCN Deployment Considerations (Informational) RFC 5696 Baseline PCN Encoding November 2009
Appendix A. PCN Deployment Considerations (Informative)
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 is not a DSCPs are in increasingly short supply. Thirdly, PCN is not a
scheduling behaviour - rather it should be seen as being a marking scheduling behaviour -- rather, it should be seen as being a marking
behaviour similar to ECN but intended for inelastic traffic. The behaviour similar to ECN but intended for inelastic traffic. The
choice of which DSCP is most suitable for a given PCN-domain is choice of which DSCP is most suitable for a given PCN-domain is
dependent on the nature of the traffic entering that domain and the dependent on the nature of the traffic entering that domain and the
link rates of all the links making up that domain. In PCN-domains link rates of all the links making up that domain. In PCN-domains
with sufficient aggregation, the appropriate DSCPs would currently be with sufficient aggregation, the appropriate DSCPs would currently be
those for the Real Time Treatment Aggregate [RFC5127]. The PCN those for the Real-Time Treatment Aggregate [RFC5127]. The PCN
Working Group suggests using admission control for the following working group suggests using admission control for the following
service classes (defined in [RFC4594]): 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 CS5 is excluded from this list since PCN is not expected to be
applied to signalling traffic. 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-domain containing sufficiently 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
service class (AF3). This document does not preclude the use of PCN Streaming service class (AF3). This document does not preclude the
in more cases than those listed above. use of PCN in more cases than those listed above.
NOTE: The above discussion is informative not normative, as operators Note: The above discussion is informative not normative, as operators
are ultimately free to decide whether to use admission control for are ultimately free to decide whether to use admission control for
RFC 5696 Baseline PCN Encoding November 2009
certain service classes and whether to use PCN as their mechanism of certain service classes and whether to use PCN as their mechanism of
choice. choice.
A.2. Rationale for Using ECT(0) for Not-marked A.2. Rationale for Using ECT(0) for Not-Marked
The choice of which ECT codepoint to use for the Not-marked state was The choice of which ECT codepoint to use for the Not-marked state was
based on the following considerations: based on the following considerations:
o [RFC3168] full functionality tunnel within the PCN-domain: Either o [RFC3168] full-functionality tunnel within the PCN-domain: Either
ECT is safe. ECT is safe.
o Leakage of traffic into PCN-domain: because of the lack of take-up o Leakage of traffic into PCN-domain: Because of the lack of take-up
of the ECN nonce [RFC3540], leakage of ECT(1) is less likely to of the ECN nonce [RFC3540], leakage of ECT(1) is less likely to
occur so might be considered safer. occur and so might be considered safer.
RFC 5696 Baseline PCN Encoding November 2009
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].
Overall this seemed to suggest ECT(0) was most appropriate to use. Overall, this seemed to suggest that ECT(0) was most appropriate to
use.
Appendix B. Co-existence of PCN and ECN (Informational) Appendix B. Co-Existence of PCN and ECN (Informative)
This baseline encoding scheme redefines the ECN codepoints within the This baseline encoding scheme redefines the ECN codepoints within the
PCN-domain. As packets with a PCN-compatible DSCP leave the PCN- PCN-domain. As packets with a PCN-compatible DSCP leave the PCN-
domain, their ECN field is reset to not-ECT (00). This is a problem domain, their ECN field is reset to not-ECT (00). This is a problem
for the operator if packets with a PCN-compatible DSCP arrive at the for the operator if packets with a PCN-compatible DSCP arrive at the
PCN-domain with any ECN codepoint other than not-ECN. If the ECN- PCN-domain with any ECN codepoint other than not-ECN. If the ECN-
codepoint is ECT(0) (10) or ECT(1) (01), resetting the ECN field to codepoint is ECT(0) (10) or ECT(1) (01), resetting the ECN field to
00 effectively turns off end-to-end ECN. This is undesirable as it 00 effectively turns off end-to-end ECN. This is undesirable as it
removes the benefits of ECN but [RFC3168] states it is no worse than removes the benefits of ECN, but [RFC3168] states that it is no worse
dropping the packet. However, if a packet was marked with CE (11), than dropping the packet. However, if a packet was marked with CE
resetting the ECN field to 00 at the PCN egress node violates the (11), resetting the ECN field to 00 at the PCN egress node violates
rule that CE-marks must never be lost except as a result of packet the rule that CE-marks must never be lost except as a result of
drop [RFC3168]. packet drop [RFC3168].
A number of options exist to overcome this issue. The most A number of options exist to overcome this issue. The most
appropriate option will depend on the circumstances and has to be a appropriate option will depend on the circumstances and has to be a
decision for the operator. The definition of the action is beyond decision for the operator. The definition of the action is beyond
the scope of this document but we briefly explain the four broad the scope of this document, but we briefly explain the four broad
categories of solution below: tunnelling the packets, using an categories of solution below: tunnelling the packets, using an
extended encoding scheme, signalling to the end-systems to stop using extended encoding scheme, signalling to the end systems to stop using
ECN or re-marking packets to a different DSCP. ECN, or re-marking packets to a different DSCP.
o Tunnelling the packets across the PCN-domain (for instance in an o Tunnelling the packets across the PCN-domain (for instance, in an
IP-in-IP tunnel from the PCN-ingress-node to the PCN-egress-node) IP-in-IP tunnel from the PCN-ingress-node to the PCN-egress-node)
preserves the original ECN marking on the inner header. preserves the original ECN marking on the inner header.
o An extended encoding scheme can be designed that preserves the o An extended encoding scheme can be designed that preserves the
original ECN codepoints. For instance if the PCN-egress-node can original ECN codepoints. For instance, if the PCN-egress-node can
determine from the PCN codepoint what the original ECN codepoint determine from the PCN codepoint what the original ECN codepoint
was then it can reset the packet to that codepoint. was, then it can reset the packet to that codepoint. [PCN-ENC]
[I-D.ietf-pcn-3-state-encoding] partially achieves this but is partially achieves this but is unable to recover ECN markings if
unable to recover ECN markings if the packet is PCN-marked in the the packet is PCN-marked in the PCN-domain.
PCN-domain.
RFC 5696 Baseline PCN Encoding November 2009
o Explicit signalling to the end systems can indicate to the source o Explicit signalling to the end systems can indicate to the source
that ECN cannot be used on this path (because it does not support that ECN cannot be used on this path (because it does not support
ECN & PCN at the same time). Dropping the packet can be thought ECN and PCN at the same time). Dropping the packet can be thought
of as a form of silent signal to the source as it will see any of as a form of silent signal to the source, as it will see any
ECT-marked packets it sends being dropped. ECT-marked packets it sends being dropped.
o Packets that are not-PCN but which share a PCN-compatible DSCP can o Packets that are not part of a PCN-flow but which share a PCN-
be re-marked to a different local-use DSCP at the PCN-ingress-node compatible DSCP can be re-marked to a different local-use DSCP at
with the original DSCP restored at the PCN-egress. This preserves the PCN-ingress-node with the original DSCP restored at the PCN-
the ECN codepoint on the packets, but it relies on there being egress. This preserves the ECN codepoint on these packets but
spare local-use DSCPs within the PCN-domain. relies on there being spare local-use DSCPs within the PCN-domain.
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 7918 901170
EMail: toby.moncaster@bt.com EMail: toby.moncaster@bt.com
Bob Briscoe Bob Briscoe
BT BT
B54/77, Adastral Park B54/77, Adastral Park
Martlesham Heath Martlesham Heath
Ipswich IP5 3RE Ipswich IP5 3RE
UK UK
Phone: +44 1473 645196 Phone: +44 1473 645196
EMail: bob.briscoe@bt.com EMail: bob.briscoe@bt.com
Michael Menth Michael Menth
University of Wuerzburg University of Wuerzburg
room B206, Institute of Computer Science Institute of Computer Science
Am Hubland Am Hubland
Wuerzburg D-97074 Wuerzburg D-97074
Germany Germany
Phone: +49 931 888 6644 Phone: +49 931 318 6644
EMail: menth@informatik.uni-wuerzburg.de EMail: menth@informatik.uni-wuerzburg.de
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