draft-ietf-pcn-3-in-1-encoding-06.txt   draft-ietf-pcn-3-in-1-encoding-07.txt 
Congestion and Pre-Congestion B. Briscoe Congestion and Pre-Congestion B. Briscoe
Notification BT Notification BT
Internet-Draft T. Moncaster Internet-Draft T. Moncaster
Obsoletes: 5696 (if approved) Moncaster Internet Consulting Obsoletes: 5696 (if approved) Moncaster Internet Consulting
Intended status: Standards Track M. Menth Intended status: Standards Track M. Menth
Expires: January 11, 2012 University of Tuebingen Expires: January 31, 2012 University of Tuebingen
July 10, 2011 July 30, 2011
Encoding 3 PCN-States in the IP header using a single DSCP Encoding 3 PCN-States in the IP header using a single DSCP
draft-ietf-pcn-3-in-1-encoding-06 draft-ietf-pcn-3-in-1-encoding-07
Abstract Abstract
The objective of Pre-Congestion Notification (PCN) is to protect the The objective of Pre-Congestion Notification (PCN) is to protect the
quality of service (QoS) of inelastic flows within a Diffserv domain. quality of service (QoS) of inelastic flows within a Diffserv domain.
The overall rate of the PCN-traffic is metered on every link in the The overall rate of the PCN-traffic is metered on every link in the
PCN domain, and PCN-packets are appropriately marked when certain PCN domain, and PCN-packets are appropriately marked when certain
configured rates are exceeded. Egress nodes pass information about configured rates are exceeded. Egress nodes pass information about
these PCN-marks to decision points which then decide whether to admit these PCN-marks to decision points which then decide whether to admit
or block new flow requests or to terminate some already-admitted or block new flow requests or to terminate some already-admitted
skipping to change at page 1, line 48 skipping to change at page 1, line 48
Internet-Drafts are working documents of the Internet Engineering Internet-Drafts are working documents of the Internet Engineering
Task Force (IETF). Note that other groups may also distribute Task Force (IETF). Note that other groups may also distribute
working documents as Internet-Drafts. The list of current Internet- working documents as Internet-Drafts. The list of current Internet-
Drafts is at http://datatracker.ietf.org/drafts/current/. Drafts is at http://datatracker.ietf.org/drafts/current/.
Internet-Drafts are draft documents valid for a maximum of six months Internet-Drafts are draft documents valid for a maximum of six months
and may be updated, replaced, or obsoleted by other documents at any and may be updated, replaced, or obsoleted by other documents at any
time. It is inappropriate to use Internet-Drafts as reference time. It is inappropriate to use Internet-Drafts as reference
material or to cite them other than as "work in progress." material or to cite them other than as "work in progress."
This Internet-Draft will expire on January 11, 2012. This Internet-Draft will expire on January 31, 2012.
Copyright Notice Copyright Notice
Copyright (c) 2011 IETF Trust and the persons identified as the Copyright (c) 2011 IETF Trust and the persons identified as the
document authors. All rights reserved. document authors. All rights reserved.
This document is subject to BCP 78 and the IETF Trust's Legal This document is subject to BCP 78 and the IETF Trust's Legal
Provisions Relating to IETF Documents Provisions Relating to IETF Documents
(http://trustee.ietf.org/license-info) in effect on the date of (http://trustee.ietf.org/license-info) in effect on the date of
publication of this document. Please review these documents publication of this document. Please review these documents
skipping to change at page 3, line 12 skipping to change at page 3, line 12
the Trust Legal Provisions and are provided without warranty as the Trust Legal Provisions and are provided without warranty as
described in the Simplified BSD License. described in the Simplified BSD License.
Table of Contents Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 4 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 4
1.1. Requirements Language . . . . . . . . . . . . . . . . . . 5 1.1. Requirements Language . . . . . . . . . . . . . . . . . . 5
1.2. Changes in This Version (to be removed by RFC Editor) . . 5 1.2. Changes in This Version (to be removed by RFC Editor) . . 5
2. Definitions and Abbreviations . . . . . . . . . . . . . . . . 7 2. Definitions and Abbreviations . . . . . . . . . . . . . . . . 7
2.1. Terminology . . . . . . . . . . . . . . . . . . . . . . . 7 2.1. Terminology . . . . . . . . . . . . . . . . . . . . . . . 7
2.2. List of Abbreviations . . . . . . . . . . . . . . . . . . 7 2.2. List of Abbreviations . . . . . . . . . . . . . . . . . . 8
3. Definition of 3-in-1 PCN Encoding . . . . . . . . . . . . . . 8 3. Definition of 3-in-1 PCN Encoding . . . . . . . . . . . . . . 8
4. Requirements for and Applicability of 3-in-1 PCN Encoding . . 9 4. Requirements for and Applicability of 3-in-1 PCN Encoding . . 9
4.1. PCN Requirements . . . . . . . . . . . . . . . . . . . . . 9 4.1. PCN Requirements . . . . . . . . . . . . . . . . . . . . . 9
4.2. Requirements Imposed by Tunnelling . . . . . . . . . . . . 9 4.2. Requirements Imposed by Tunnelling . . . . . . . . . . . . 10
4.3. Applicability of 3-in-1 PCN Encoding . . . . . . . . . . . 10 4.3. Applicability of 3-in-1 PCN Encoding . . . . . . . . . . . 10
5. Behaviour of a PCN-node to Comply with the 3-in-1 PCN 5. Behaviour of a PCN-node to Comply with the 3-in-1 PCN
Encoding . . . . . . . . . . . . . . . . . . . . . . . . . . . 10 Encoding . . . . . . . . . . . . . . . . . . . . . . . . . . . 11
5.1. PCN-ingress Node Behaviour . . . . . . . . . . . . . . . . 10 5.1. PCN-ingress Node Behaviour . . . . . . . . . . . . . . . . 11
5.2. PCN-interior Node Behaviour . . . . . . . . . . . . . . . 11 5.2. PCN-interior Node Behaviour . . . . . . . . . . . . . . . 11
5.2.1. Behaviour Common to all PCN-interior Nodes . . . . . . 11 5.2.1. Behaviour Common to all PCN-interior Nodes . . . . . . 11
5.2.2. Behaviour of PCN-interior Nodes Using Two 5.2.2. Behaviour of PCN-interior Nodes Using Two
PCN-markings . . . . . . . . . . . . . . . . . . . . . 11 PCN-markings . . . . . . . . . . . . . . . . . . . . . 12
5.2.3. Behaviour of PCN-interior Nodes Using One 5.2.3. Behaviour of PCN-interior Nodes Using One
PCN-marking . . . . . . . . . . . . . . . . . . . . . 11 PCN-marking . . . . . . . . . . . . . . . . . . . . . 12
5.3. Behaviour of PCN-egress Nodes . . . . . . . . . . . . . . 12 5.3. Behaviour of PCN-egress Nodes . . . . . . . . . . . . . . 13
6. Backward Compatibility . . . . . . . . . . . . . . . . . . . . 13 6. Backward Compatibility . . . . . . . . . . . . . . . . . . . . 13
6.1. Backward Compatibility with ECN . . . . . . . . . . . . . 13 6.1. Backward Compatibility with ECN . . . . . . . . . . . . . 13
6.2. Backward Compatibility with the Baseline Encoding . . . . 13 6.2. Backward Compatibility with the Baseline Encoding . . . . 14
7. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 13 7. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 14
8. Security Considerations . . . . . . . . . . . . . . . . . . . 14 8. Security Considerations . . . . . . . . . . . . . . . . . . . 14
9. Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . 14 9. Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . 15
10. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 14 10. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 15
11. Comments Solicited . . . . . . . . . . . . . . . . . . . . . . 15 11. Comments Solicited . . . . . . . . . . . . . . . . . . . . . . 15
12. References . . . . . . . . . . . . . . . . . . . . . . . . . . 15 12. References . . . . . . . . . . . . . . . . . . . . . . . . . . 15
12.1. Normative References . . . . . . . . . . . . . . . . . . . 15 12.1. Normative References . . . . . . . . . . . . . . . . . . . 15
12.2. Informative References . . . . . . . . . . . . . . . . . . 15 12.2. Informative References . . . . . . . . . . . . . . . . . . 16
Appendix A. Choice of Suitable DSCPs . . . . . . . . . . . . . . 16 Appendix A. Choice of Suitable DSCPs . . . . . . . . . . . . . . 17
Appendix B. Co-existence of ECN and PCN . . . . . . . . . . . . . 18 Appendix B. Co-existence of ECN and PCN . . . . . . . . . . . . . 18
Appendix C. Example Mapping between Encoding of PCN-Marks in Appendix C. Example Mapping between Encoding of PCN-Marks in
IP and in MPLS Shim Headers . . . . . . . . . . . . . 20 IP and in MPLS Shim Headers . . . . . . . . . . . . . 20
Appendix D. Rationale for different behaviours for single Appendix D. Rationale for Discrepancy Between the Schemes
marking schemes . . . . . . . . . . . . . . . . . . . 21 using One PCN-Marking . . . . . . . . . . . . . . . . 22
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 21 Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 22
1. Introduction 1. Introduction
The objective of Pre-Congestion Notification (PCN) [RFC5559] is to The objective of Pre-Congestion Notification (PCN) [RFC5559] is to
protect the quality of service (QoS) of inelastic flows within a protect the quality of service (QoS) of inelastic flows within a
Diffserv domain, in a simple, scalable, and robust fashion. Two Diffserv domain, in a simple, scalable, and robust fashion. Two
mechanisms are used: admission control, to decide whether to admit or mechanisms are used: admission control, to decide whether to admit or
block a new flow request, and flow termination to terminate some block a new flow request, and flow termination to terminate some
existing flows during serious pre-congestion. To achieve this, the existing flows during serious pre-congestion. To achieve this, the
overall rate of PCN-traffic is metered on every link in the domain, overall rate of PCN-traffic is metered on every link in the domain,
skipping to change at page 5, line 20 skipping to change at page 5, line 20
mapping between IP and MPLS. mapping between IP and MPLS.
1.1. Requirements Language 1.1. Requirements Language
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].
1.2. Changes in This Version (to be removed by RFC Editor) 1.2. Changes in This Version (to be removed by RFC Editor)
From draft-ietf-pcn-3-in-1-encoding-06 to -07:
* Clarified that each operator not the IETF chooses which DSCP(s)
are PCN-compatible, and made it unambiguous that only PCN-nodes
recognise that PCN-compatible DSCPs enable the 3-in-1 encoding.
* Removed statements about the PCN working group, given RFCs are
meant to survive beyond the life of a w-g.
* Corrected the final para of "Rationale for Different Behaviours
in Schemes with Only One Marking"
From draft-ietf-pcn-3-in-1-encoding-05 to -06: From draft-ietf-pcn-3-in-1-encoding-05 to -06:
* Draft re-written to obsolete baseline encoding [RFC5696]. * Draft re-written to obsolete baseline encoding [RFC5696].
* New section defining utilising this encoding for single * New section defining utilising this encoding for only one PCN-
marking. Added an appendix explaining an apparent Marking. Added an appendix explaining an apparent
inconsistency relating to single marking. inconsistency within this section.
* Moved (and updated) informative appendixes from [RFC5696] to * Moved (and updated) informative appendixes from [RFC5696] to
this document. Original Appendix C was omitted as it is now this document. Original Appendix C was omitted as it is now
redundant. redundant.
* Significant re-structuring of document. * Significant re-structuring of document.
From draft-ietf-pcn-3-in-1-encoding-04 to -05: From draft-ietf-pcn-3-in-1-encoding-04 to -05:
* Draft moved to standards track as per working group * Draft moved to standards track as per working group
skipping to change at page 7, line 18 skipping to change at page 7, line 27
The terms PCN-domain, PCN-node, PCN-interior-node, PCN-ingress-node, The terms PCN-domain, PCN-node, PCN-interior-node, PCN-ingress-node,
PCN-egress-node, PCN-boundary-node, PCN-traffic, PCN-packets and PCN- PCN-egress-node, PCN-boundary-node, PCN-traffic, PCN-packets and PCN-
marking are used as defined in [RFC5559]. The following additional marking are used as defined in [RFC5559]. The following additional
terms are defined in this document: terms are defined in this document:
PCN encoding: mapping of PCN marking states to specific codepoints PCN encoding: mapping of PCN marking states to specific codepoints
in the packet header. in the packet header.
PCN-compatible Diffserv codepoint: a Diffserv codepoint indicating PCN-compatible Diffserv codepoint: a Diffserv codepoint indicating
packets for which the ECN field is used to carry PCN-markings packets for which the ECN field carries PCN-markings rather than
rather than [RFC3168] markings (see Appendix A). [RFC3168] markings. Note that an operator configures PCN-nodes to
recognise PCN-compatible DSCPs, whereas the same DSCP has no PCN-
specific meaning to a node outside the PCN domain.
Threshold-marked codepoint: a codepoint that indicates packets that Threshold-marked codepoint: a codepoint that indicates packets that
have been marked at a PCN-interior-node as a result of an have been marked at a PCN-interior-node as a result of an
indication from the threshold-metering function [RFC5670]. indication from the threshold-metering function [RFC5670].
Abbreviated to ThM. Abbreviated to ThM.
Excess-traffic-marked codepoint: a codepoint that indicates packets Excess-traffic-marked codepoint: a codepoint that indicates packets
that have been marked at a PCN-interior-node as a result of an that have been marked at a PCN-interior-node as a result of an
indication from the excess-traffic-metering function [RFC5670]. indication from the excess-traffic-metering function [RFC5670].
Abbreviated to ETM. Abbreviated to ETM.
skipping to change at page 8, line 35 skipping to change at page 9, line 5
| | Codepoint in ECN field of IP header | | | Codepoint in ECN field of IP header |
| DSCP | <RFC3168 codepoint name> | | DSCP | <RFC3168 codepoint name> |
| +--------------+-------------+-------------+---------+ | +--------------+-------------+-------------+---------+
| | 00 <Not-ECT> | 10 <ECT(0)> | 01 <ECT(1)> | 11 <CE> | | | 00 <Not-ECT> | 10 <ECT(0)> | 01 <ECT(1)> | 11 <CE> |
+--------+--------------+-------------+-------------+---------+ +--------+--------------+-------------+-------------+---------+
| DSCP n | Not-PCN | NM | ThM | ETM | | DSCP n | Not-PCN | NM | ThM | ETM |
+--------+--------------+-------------+-------------+---------+ +--------+--------------+-------------+-------------+---------+
Figure 1: 3-in-1 PCN Encoding Figure 1: 3-in-1 PCN Encoding
As mentioned above, the 3-in-1 PCN encoding is an extension of the A PCN-node (i.e. a node within a PCN-domain) will be configured to
baseline encoding [RFC5696]. Like the baseline encoding it uses a recognise certain DSCPs as PCN-compatible. Appendix A discusses the
combination of a PCN-compatible DSCP (DSCP n in Figure 1) and the ECN choice of suitable DSCPs. In Figure 1 'DSCP n' indicates such a PCN-
field for the encoding of PCN-marks. Appendix A discusses the choice compatible DSCP. Within the PCN-domain, any packet carrying a PCN-
of suitable DSCPs. The PCN-marks have the following meaning. compatible DSCP is a PCN-packet as defined in [RFC5559].
Not-PCN: indicates a non-PCN-packet, i.e., a packet that is not PCN-nodes MUST interpret the ECN field of a PCN-packet using the
subject to PCN metering and marking. 3-in-1 PCN encoding, rather than [RFC3168]. This does not change the
behaviour for any packet with a DSCP that is not PCN-compatible, or
for any node outside a PCN-domain. In all such cases the 3-in-1
encoding is not applicable and so by default the node will interpret
the ECN field using [RFC3168].
When using the 3-in-1 encoding, the codepoints of the ECN field have
the following meanings:
Not-PCN: indicates a non-PCN-packet, i.e., a packet that uses a PCN-
compatible DSCP but is not subject to PCN metering and marking.
NM: Not-marked. Indicates a PCN-packet that has not yet been marked NM: Not-marked. Indicates a PCN-packet that has not yet been marked
by any PCN marker. by any PCN marker.
ThM: Threshold-marked. Indicates a PCN-packet that has been marked ThM: Threshold-marked. Indicates a PCN-packet that has been marked
by a threshold-marker [RFC5670]. by a threshold-marker [RFC5670].
ETM: Excess-traffic-marked. Indicates a PCN-packet that has been ETM: Excess-traffic-marked. Indicates a PCN-packet that has been
marked by an excess-traffic-marker [RFC5670]. marked by an excess-traffic-marker [RFC5670].
skipping to change at page 9, line 31 skipping to change at page 10, line 8
[I-D.ietf-pcn-cl-edge-behaviour]. In the latter case, three PCN [I-D.ietf-pcn-cl-edge-behaviour]. In the latter case, three PCN
marking states are needed: not-marked (NM) to indicate not-marked marking states are needed: not-marked (NM) to indicate not-marked
packets, threshold-marked (ThM) to indicate packets marked by the packets, threshold-marked (ThM) to indicate packets marked by the
threshold-marker, and excess-traffic-marked (ETM) to indicate packets threshold-marker, and excess-traffic-marked (ETM) to indicate packets
marked by the excess-traffic-marker [RFC5670]. Threshold-marking and marked by the excess-traffic-marker [RFC5670]. Threshold-marking and
excess-traffic-marking are configured to start marking packets at excess-traffic-marking are configured to start marking packets at
different load conditions, so one marking behaviour indicates more different load conditions, so one marking behaviour indicates more
severe pre-congestion than the other. Therefore, a fourth PCN severe pre-congestion than the other. Therefore, a fourth PCN
marking state indicating that a packet is marked by both markers is marking state indicating that a packet is marked by both markers is
not needed. However a fourth codepoint is required to indicate not needed. However a fourth codepoint is required to indicate
packets that do not use PCN (the not-PCN codepoint). packets that use a PCN-compatible DSCP but do not use PCN-marking
(the not-PCN codepoint).
In all current PCN edge behaviors that use two marking behaviours In all current PCN edge behaviors that use two marking behaviours
[RFC5559], [I-D.ietf-pcn-cl-edge-behaviour], excess-traffic-marking [RFC5559], [I-D.ietf-pcn-cl-edge-behaviour], excess-traffic-marking
is configured with a larger reference rate than threshold-marking. is configured with a larger reference rate than threshold-marking.
We take this as a rule and define excess-traffic-marked as a more We take this as a rule and define excess-traffic-marked as a more
severe PCN-mark than threshold-marked. severe PCN-mark than threshold-marked.
4.2. Requirements Imposed by Tunnelling 4.2. Requirements Imposed by Tunnelling
[RFC6040] defines rules for the encapsulation and decapsulation of [RFC6040] defines rules for the encapsulation and decapsulation of
skipping to change at page 10, line 21 skipping to change at page 10, line 47
also be used with only one marking behaviour, in which case one of also be used with only one marking behaviour, in which case one of
the codepoints MUST NOT be used throughout the PCN-domain (see the codepoints MUST NOT be used throughout the PCN-domain (see
Section 5.2.3). Section 5.2.3).
For the full 3-in-1 encoding to apply, any tunnel endpoints (IP-in-IP For the full 3-in-1 encoding to apply, any tunnel endpoints (IP-in-IP
and IPsec) within the PCN-domain MUST comply with the ECN and IPsec) within the PCN-domain MUST comply with the ECN
encapsulation and decapsulation rules set out in [RFC6040] (see encapsulation and decapsulation rules set out in [RFC6040] (see
Section 4.2). There is one exception to this rule outlined next. Section 4.2). There is one exception to this rule outlined next.
It may not be possible to upgrade every pre-RFC6040 tunnel endpoint It may not be possible to upgrade every pre-RFC6040 tunnel endpoint
within a PCN-domain. In such cirsumstances a limited version of the within a PCN-domain. In such circumstances a limited version of the
3-in-1 encoding can still be used but only under the following 3-in-1 encoding can still be used but only under the following
stringent condition. If any pre-RFC6040 tunnel endpoint exists stringent condition. If any pre-RFC6040 tunnel endpoint exists
within a PCN-domain then every PCN-node in the PCN-domain MUST be within a PCN-domain then every PCN-node in the PCN-domain MUST be
configured so that it never sets the ThM codepoint. The behaviour of configured so that it never sets the ThM codepoint. The behaviour of
PCN-interior nodes in this case is defined in Section 5.2.3.1. In PCN-interior nodes in this case is defined in Section 5.2.3.1, which
all other situations where legacy tunnel endpoints might be present describes the rules for using only the Excess Traffic marking
within the PCN domain, the 3-in-1 encoding is not applicable. function. In all other situations where legacy tunnel endpoints
might be present within the PCN domain, the 3-in-1 encoding is not
applicable.
5. Behaviour of a PCN-node to Comply with the 3-in-1 PCN Encoding 5. Behaviour of a PCN-node to Comply with the 3-in-1 PCN Encoding
As mentioned in Section 4.3 above, all PCN-nodes MUST comply with As mentioned in Section 4.3 above, all PCN-nodes MUST comply with
[RFC6040]. [RFC6040].
5.1. PCN-ingress Node Behaviour 5.1. PCN-ingress Node Behaviour
PCN-traffic MUST be marked with a PCN-compatible Diffserv codepoint. PCN-traffic MUST be marked with a PCN-compatible Diffserv codepoint.
To conserve DSCPs, Diffserv codepoints SHOULD be chosen that are To conserve DSCPs, Diffserv codepoints SHOULD be chosen that are
skipping to change at page 12, line 47 skipping to change at page 13, line 26
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. packets it forwards out of the PCN-domain.
The only exception to this is if the PCN-egress-node is certain that The only exception to this is if the PCN-egress-node is certain that
revealing other codepoints outside the PCN-domain won't contravene revealing other codepoints outside the PCN-domain won't contravene
the guidance given in [RFC4774]. For instance, if the PCN-ingress- the guidance given in [RFC4774]. For instance, if the PCN-ingress-
node has explicitly informed the PCN-egress-node that this flow is node has explicitly informed the PCN-egress-node that this flow is
ECN-capable, then it might be safe to expose other codepoints. ECN-capable, then it might be safe to expose other codepoints.
Appendix B gives details of how such schemes might work, but such Appendix B gives details of how such schemes might work, but such
schemes are currently experimental. schemes are currently only tentative ideas.
If the PCN-domain is configured to use only excess-traffic marking, If the PCN-domain is configured to use only excess-traffic marking,
the PCN-egress node MUST treat ThM as ETM and if only threshold- the PCN-egress node MUST treat ThM as ETM and if only threshold-
marking is used it should treat ETM as ThM. However it SHOULD raise marking is used it should treat ETM as ThM. However it SHOULD raise
a management alarm in either instance since this means there is some a management alarm in either instance since this means there is some
misconfiguration in the PCN-domain. misconfiguration in the PCN-domain.
6. Backward Compatibility 6. Backward Compatibility
6.1. Backward Compatibility with ECN 6.1. Backward Compatibility with ECN
skipping to change at page 17, line 5 skipping to change at page 17, line 33
RFC 5696, November 2009. RFC 5696, November 2009.
[RFC5865] Baker, F., Polk, J., and M. Dolly, "A Differentiated [RFC5865] Baker, F., Polk, J., and M. Dolly, "A Differentiated
Services Code Point (DSCP) for Capacity-Admitted Traffic", Services Code Point (DSCP) for Capacity-Admitted Traffic",
RFC 5865, May 2010. RFC 5865, May 2010.
Appendix A. Choice of Suitable DSCPs Appendix A. Choice of Suitable DSCPs
This appendix is informative, not normative. This appendix is informative, not normative.
The PCN working group chose not to define a single DSCP for use with A single DSCP has not been defined for use with PCN for several
PCN for several reasons. Firstly, the PCN mechanism is applicable to reasons. Firstly, the PCN mechanism is applicable to a variety of
a variety of different traffic classes. Secondly, Standards Track different traffic classes. Secondly, Standards Track DSCPs are in
DSCPs are in increasingly short supply. Thirdly, PCN is not a increasingly short supply. Thirdly, PCN is not a scheduling
scheduling behaviour -- rather, it should be seen as being a marking behaviour -- rather, it should be seen as being a marking behaviour
behaviour similar to ECN but intended for inelastic traffic. The similar to ECN but intended for inelastic traffic. The choice of
choice of which DSCP is most suitable for a given PCN-domain is which DSCP is most suitable for a given PCN-domain is dependent on
dependent on the nature of the traffic entering that domain and the the nature of the traffic entering that domain and the link rates of
link rates of all the links making up that domain. In PCN-domains all the links making up that domain. In PCN-domains with sufficient
with sufficient aggregation, the appropriate DSCPs would currently be aggregation, the appropriate DSCPs would currently be those for the
those for the Real-Time Treatment Aggregate [RFC5127]. The PCN Real-Time Treatment Aggregate [RFC5127]. It is suggested that
working group suggests using admission control for the following admission control could be used for the following service classes
service classes (defined in [RFC4594]): (defined in [RFC4594] unless otherwise stated):
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)
o the VOICE-ADMIT codepoint defined in [RFC5865].
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. PCN can also be applied to the VOICE- applied to signalling traffic.
ADMIT codepoint defined in [RFC5865].
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
skipping to change at page 18, line 19 skipping to change at page 18, line 49
The PCN encoding described in this document re-uses the bits of the The PCN encoding described in this document re-uses the bits of the
ECN field in the IP header. Consequently, this disables ECN within ECN field in the IP header. Consequently, this disables ECN within
the PCN domain. Appendix B of [RFC5696] (obsoleted) included advice the PCN domain. Appendix B of [RFC5696] (obsoleted) included advice
on handling ECN traffic within a PCN-domain. This appendix on handling ECN traffic within a PCN-domain. This appendix
reiterates and clarifies that advice. reiterates and clarifies that advice.
For the purposes of this appendix we define two forms of traffic that For the purposes of this appendix we define two forms of traffic that
might arrive at a PCN-ingress node. These are Admission-controlled might arrive at a PCN-ingress node. These are Admission-controlled
traffic and Non-admission-controlled traffic. traffic and Non-admission-controlled traffic.
Admission-controlled traffic will be remarked to a PCN-compatible Admission-controlled traffic will be re-marked to a PCN-compatible
DSCP by the PCN-ingress node. Two mechanisms can be used to identify DSCP by the PCN-ingress node. Two mechanisms can be used to identify
such traffic: such traffic:
a. flow signalling associates a filterspec with a need for admission a. flow signalling associates a filterspec with a need for admission
control (e.g. through RSVP or some equivalent message, e.g. from control (e.g. through RSVP or some equivalent message, e.g. from
a SIP server to the ingress), and the PCN-ingress remarks traffic a SIP server to the ingress), and the PCN-ingress re-marks
matching that filterspec to a PCN-compatible DSCP, as its chosen traffic matching that filterspec to a PCN-compatible DSCP, as its
admission control mechanism. chosen admission control mechanism.
b. Traffic arrives with a DSCP that implies it requires admission b. Traffic arrives with a DSCP that implies it requires admission
control such as VOICE-ADMIT [RFC5865] or Interactive Real-Time, control such as VOICE-ADMIT [RFC5865] or Interactive Real-Time,
Broadcast TV when used for video on demand, and Multimedia Broadcast TV when used for video on demand, and Multimedia
Conferencing [RFC4594][RFC5865]. Conferencing [RFC4594][RFC5865] (see Appendix A).
All other traffic can be thought of as Non-admission-controlled (and All other traffic can be thought of as Non-admission-controlled (and
therefore outside the scope of PCN). However such traffic may still therefore outside the scope of PCN). However such traffic may still
need to share the same DSCP as the Admission-controlled traffic. need to share the same DSCP as the Admission-controlled traffic.
This may be due to policy (for instance if it is high priority voice This may be due to policy (for instance if it is high priority voice
traffic), or may be because there is a shortage of local DSCPs. traffic), or may be because there is a shortage of local DSCPs.
ECN [RFC3168] is an end-to-end congestion notification mechanism. As ECN [RFC3168] is an end-to-end congestion notification mechanism. As
such it is possible that some traffic entering the PCN-domain may such it is possible that some traffic entering the PCN-domain may
also be ECN capable. also be ECN capable.
Unless specified otherwise, for any of the cases in the list below, Unless specified otherwise, for any of the cases in the list below,
an IP-in-IP tunnel can be used to preserve ECN markings across the an IP-in-IP tunnel can be used to preserve ECN markings across the
PCN domain. However the tunnelling action should be applied wholly PCN domain. The tunnelling action should be applied wholly outside
outside the PCN-domain as illustrated in the following figure: the PCN-domain as illustrated in the following figure:
, . . . . . PCN-domain . . . . . . , . . . . . PCN-domain . . . . . .
. ,--------. ,--------. . . ,--------. ,--------. .
. _| PCN- |___________________| PCN- |_ . . _| PCN- |___________________| PCN- |_ .
. / | ingress | | egress | \ . . / | ingress | | egress | \ .
.| '---------' '--------' |. .| '---------' '--------' |.
| . . . . . . . . . . . . . . .| | . . . . . . . . . . . . . . .|
,--------. ,--------. ,--------. ,--------.
_____| Tunnel | | Tunnel |____ _____| Tunnel | | Tunnel |____
| Ingress | - - ECN preserved inside tunnel - - | Egress | | Ingress | - - ECN preserved inside tunnel - - | Egress |
'---------' '--------' '---------' '--------'
Figure 2: Separation of tunneling and PCN actions Figure 2: Separation of tunneling and PCN actions
There are four cases for how e2e ECN traffic may wish to be treated There are three cases for how e2e ECN traffic may wish to be treated
while crossing a PCN domain: while crossing a PCN domain:
ECN capable traffic that does not require admission control and does a) Does not require admission control:
not carry a DSCP that the PCN-ingress is using for PCN-capable
traffic. This requires no action.
ECN capable traffic that does not require admission control but * Does not carry a PCN-compatible DSCP: No action required.
carries a DSCP that the PCN-ingress is using for PCN-capable
traffic. There are two options.
* The ingress maps the DSCP to a local DSCP with the same * Arrives carrying a DSCP that uses the same codepoint as a PCN-
scheduling PHB as the original DSCP, and the egress re-maps it compatible DSCP: There are two options:
to the original PCN-compatible DSCP.
* The ingress tunnels the traffic, setting not-PCN in the outer 1. The ingress maps the DSCP to a local DSCP with the same
header; note that this turns off ECN for this traffic within scheduling PHB as the original DSCP, and the egress re-maps
the PCN domain. it to the original PCN-compatible DSCP.
The first option is recommended unless the operator is short of 2. The ingress tunnels the traffic, setting not-PCN in the
local DSCPs. outer header; note that this turns off ECN for this traffic
within the PCN domain.
ECN-capable Admission-controlled traffic: There are two options. The first option is recommended unless the operator is short of
local DSCPs.
b) Requires Admission-control: There are two options.
* The PCN-ingress places this traffic in a tunnel with a PCN- * The PCN-ingress places this traffic in a tunnel with a PCN-
compatible DSCP in the outer header. The PCN-egress zeroes the compatible DSCP in the outer header. The PCN-egress zeroes the
ECN-field before decapsulation. ECN-field before decapsulation.
* The PCN-ingress drops CE-marked packets and the PCN-egress * The PCN-ingress drops CE-marked packets and the PCN-egress
zeros the ECN field of all PCN packets. zeros the ECN field of all PCN packets.
The second option is emphatically not recommended, unless perhaps The second option is emphatically not recommended, unless perhaps
as a last resort if tunnelling is not possible for some as a last resort if tunnelling is not possible for some
insurmountable reason. insurmountable reason.
ECN-capable Admission-controlled traffic where the e2e transport c) Requires Admission Control and asks to see PCN marks: NOTE this
somehow indicates that it wants to see PCN marks: NOTE this is scheme is currently only a tentative idea.
currently tentative only.
Schemes have been suggested where PCN marks may be leaked out of For real-time data generated by an adaptive codec, schemes have
the PCN-domain and used by the end hosts to modify realtime data been suggested where PCN marks may be leaked out of the PCN-domain
rates. Currently all such schemes require further study and the so that end hosts can drop to a lower data rate, thus deferring
following is for guidance only. the need for admission control. Currently such schemes require
further study and the following is for guidance only.
The PCN-ingress needs to tunnel the traffic, taking care to comply The PCN-ingress needs to tunnel the traffic as in Figure 2, taking
with [RFC6040]. In this case the PCN-egress should not zero the care to comply with [RFC6040]. In this case the PCN-egress should
ECN field, and then the [RFC6040] tunnel egress will preserve any not zero the ECN field, and then the [RFC6040] tunnel egress will
PCN-marking. Note that a PCN interior node may turn ECT(0) into preserve any PCN-marking. Note that a PCN interior node may turn
ECT(1), which would not be compatible with the (currently ECT(0) into ECT(1), which would not be compatible with the
experimental) ECN nonce [RFC3540]. (currently experimental) ECN nonce [RFC3540].
Appendix C. Example Mapping between Encoding of PCN-Marks in IP and in Appendix C. Example Mapping between Encoding of PCN-Marks in IP and in
MPLS Shim Headers MPLS Shim Headers
This appendix is informative not normative. This appendix is informative not normative.
The 6 bits of the DS field in the IP header provide for 64 The 6 bits of the DS field in the IP header provide for 64
codepoints. When encapsulating IP traffic in MPLS, it is useful to codepoints. When encapsulating IP traffic in MPLS, it is useful to
make the DS field information accessible in the MPLS header. make the DS field information accessible in the MPLS header.
However, the MPLS shim header has only a 3-bit traffic class (TC) However, the MPLS shim header has only a 3-bit traffic class (TC)
field [RFC5462] providing for 8 codepoints. The operator has the field [RFC5462] providing for 8 codepoints. The operator has the
freedom to define a site-local mapping of a subset of the 64 freedom to define a site-local mapping of the 64 codepoints of the DS
codepoints of the DS field to the 8 codepoints in the TC field. field onto the 8 codepoints in the TC field.
[RFC5129] describes how ECN markings in the IP header can also be [RFC5129] describes how ECN markings in the IP header can also be
mapped to codepoints in the MPLS TC field. Appendix A of [RFC5129] mapped to codepoints in the MPLS TC field. Appendix A of [RFC5129]
gives an informative description of how to support PCN in MPLS by gives an informative description of how to support PCN in MPLS by
extending the way MPLS supports ECN. But [RFC5129] was written while extending the way MPLS supports ECN. But [RFC5129] was written while
PCN specifications were in early draft stages. The following PCN specifications were in early draft stages. The following
provides a clearer example of a mapping between PCN in IP and in MPLS provides a clearer example of a mapping between PCN in IP and in MPLS
using the PCN terminology and concepts that have since been using the PCN terminology and concepts that have since been
specified. specified.
To support PCN in a MPLS domain, codepoints for all used PCN-marks To support PCN in a MPLS domain, a PCN-compatible DSCP ('DSCP n')
need to be provided in the TC field. That means, when e.g. only needs codepoints to be provided in the TC field for all the PCN-marks
excess-traffic-marking is used for PCN purposes, the operator needs used. That means, when for instance only excess-traffic-marking is
to define a site-local mapping to codepoints in the MPLS TC field for used for PCN purposes, the operator needs to define a site-local
IP headers with: mapping to two codepoints in the MPLS TC field for IP headers with:
o DSCP n and ECT(0) o DSCP n and ECT(0)
o DSCP n and CE o DSCP n and CE
If both excess-traffic-marking and threshold-marking are used, the If both excess-traffic-marking and threshold-marking are used, the
operator needs to define a site-local mapping to codepoints in the operator needs to define a site-local mapping to codepoints in the
MPLS TC field for IP headers with all three of the 3-in-1 codepoints: MPLS TC field for IP headers with all three of the 3-in-1 codepoints:
o DSCP n and ECT(0) o DSCP n and ECT(0)
skipping to change at page 21, line 26 skipping to change at page 21, line 49
o DSCP n and CE o DSCP n and CE
In either case, if the operator wishes to support the same Diffserv In either case, if the operator wishes to support the same Diffserv
PHB but without PCN marking, it will also be necessary to define a PHB but without PCN marking, it will also be necessary to define a
site-local mapping to an MPLS TC codepoint for IP headers marked site-local mapping to an MPLS TC codepoint for IP headers marked
with: with:
o DSCP n and Not-ECT o DSCP n and Not-ECT
Appendix D. Rationale for different behaviours for single marking Clearly, given so few TC codepoints are available, it may be
schemes necessary to compromise by merging together some capabilities.
Readers may notice an apparent discrepancy between the two single Appendix D. Rationale for Discrepancy Between the Schemes using One
marking behaviours in Section 5.2.3.1 and Section 5.2.3.2. For the PCN-Marking
excess-traffic only marking an unexpected ThM marked packet is
remarked as ETM. For the threshold only marking, an unexpected ETM
marked packet is simply ignored (apart from an optional management
alarm).
There are two reasons for having these seemingly contradictory Readers may notice an apparent discrepancy between the two behaviours
requirements. Firstly these behaviours conform with the expected in Section 5.2.3.1 and Section 5.2.3.2. With only excess-traffic
behaviour where both metering functions are being used for marking-- marking enabled, an unexpected ThM packet can be re-marked to ETM.
ETM is always a more severe marking than ThM and so should never be However, with only threshold marking, an unexpected ETM packet cannot
re-marked. Secondly the threshold-metering behaviour in [RFC5670] be re-marked to ThM.
uses the current marking state of the arriving packet to determine
what action to take. Consequently, in the ETM only marking it would This apparent inconsistency is deliberate, for two reasons:
be potentially unsafe to allow ThM packets to propagate forward in
the network as they may adversely affect the threshold-metering o If only one type of marking function is meant to be used
function. throughout the PCN-domain but the other type unexpectedly appears
on some packets, it is safest to assume that some link is trying
to signal that it is pre-congested, but that it is somehow using
the wrong signal. This only needs to be corrected if the
behaviour of other nodes depends on the marking a packet arrives
with. In [RFC5670], the excess-traffic-metering behaviour depends
on the markings on arriving packets, whereas threshold-metering
does not. Therefore, if ThM should not be present, it seems safe
to allow it to be re-marked to ETM, but if ETM should not be
present there is no need to re-mark it to ThM.
o The behaviour with only threshold marking keeps to the rule that
ETM is more severe and must never be changed to ThM even though
ETM is not a valid marking in this case. Otherwise
implementations would have to allow operators to configure an
exception to this rule, which would not be safe practice.
Authors' Addresses Authors' Addresses
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
 End of changes. 44 change blocks. 
115 lines changed or deleted 152 lines changed or added

This html diff was produced by rfcdiff 1.41. The latest version is available from http://tools.ietf.org/tools/rfcdiff/