Encoding 3 PCN-States in the IP header using a single DSCP
draft-ietf-pcn-3-in-1-encoding-08
The information below is for an old version of the document.
| Document | Type |
This is an older version of an Internet-Draft that was ultimately published as RFC 6660.
|
|
|---|---|---|---|
| Authors | Bob Briscoe , Toby Moncaster , Michael Menth | ||
| Last updated | 2012-02-28 (Latest revision 2011-08-18) | ||
| Replaces | draft-briscoe-pcn-3-in-1-encoding | ||
| RFC stream | Internet Engineering Task Force (IETF) | ||
| Formats | |||
| Reviews | |||
| Additional resources | Mailing list discussion | ||
| Stream | WG state | WG Document | |
| Document shepherd | (None) | ||
| IESG | IESG state | Became RFC 6660 (Proposed Standard) | |
| Consensus boilerplate | Unknown | ||
| Telechat date |
(None)
Needs a YES. Needs 10 more YES or NO OBJECTION positions to pass. |
||
| Responsible AD | David Harrington | ||
| IESG note | |||
| Send notices to | pcn-chairs@tools.ietf.org, draft-ietf-pcn-3-in-1-encoding@tools.ietf.org |
draft-ietf-pcn-3-in-1-encoding-08
Congestion and Pre-Congestion B. Briscoe
Notification BT
Internet-Draft T. Moncaster
Obsoletes: 5696 (if approved) Moncaster Internet Consulting
Intended status: Standards Track M. Menth
Expires: February 19, 2012 University of Tuebingen
August 18, 2011
Encoding 3 PCN-States in the IP header using a single DSCP
draft-ietf-pcn-3-in-1-encoding-08
Abstract
The objective of Pre-Congestion Notification (PCN) is to protect the
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
PCN domain, and PCN-packets are appropriately marked when certain
configured rates are exceeded. Egress nodes pass information about
these PCN-marks to decision points which then decide whether to admit
or block new flow requests or to terminate some already-admitted
flows during serious pre-congestion.
This document specifies how PCN-marks are to be encoded into the IP
header by re-using the Explicit Congestion Notification (ECN)
codepoints within a PCN-domain. This encoding provides for up to
three different PCN marking states using a single DSCP: not-marked
(NM), threshold-marked (ThM) and excess-traffic-marked (ETM). Hence,
it is called the 3-in-1 PCN encoding. This document obsoletes
RFC5696.
Status of This Memo
This Internet-Draft is submitted in full conformance with the
provisions of BCP 78 and BCP 79.
Internet-Drafts are working documents of the Internet Engineering
Task Force (IETF). Note that other groups may also distribute
working documents as Internet-Drafts. The list of current Internet-
Drafts is at http://datatracker.ietf.org/drafts/current/.
Internet-Drafts are draft documents valid for a maximum of six months
and may be updated, replaced, or obsoleted by other documents at any
time. It is inappropriate to use Internet-Drafts as reference
material or to cite them other than as "work in progress."
This Internet-Draft will expire on February 19, 2012.
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Copyright Notice
Copyright (c) 2011 IETF Trust and the persons identified as the
document authors. All rights reserved.
This document is subject to BCP 78 and the IETF Trust's Legal
Provisions Relating to IETF Documents
(http://trustee.ietf.org/license-info) in effect on the date of
publication of this document. Please review these documents
carefully, as they describe your rights and restrictions with respect
to this document. Code Components extracted from this document must
include Simplified BSD License text as described in Section 4.e of
the Trust Legal Provisions and are provided without warranty as
described in the Simplified BSD License.
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 4
1.1. Requirements Language . . . . . . . . . . . . . . . . . . 5
1.2. Changes in This Version (to be removed by RFC Editor) . . 5
2. Definitions and Abbreviations . . . . . . . . . . . . . . . . 7
2.1. Terminology . . . . . . . . . . . . . . . . . . . . . . . 7
2.2. List of Abbreviations . . . . . . . . . . . . . . . . . . 8
3. Definition of 3-in-1 PCN Encoding . . . . . . . . . . . . . . 8
4. Requirements for and Applicability of 3-in-1 PCN Encoding . . 9
4.1. PCN Requirements . . . . . . . . . . . . . . . . . . . . . 9
4.2. Requirements Imposed by Tunnelling . . . . . . . . . . . . 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
Encoding . . . . . . . . . . . . . . . . . . . . . . . . . . . 11
5.1. PCN-ingress Node Behaviour . . . . . . . . . . . . . . . . 11
5.2. PCN-interior Node Behaviour . . . . . . . . . . . . . . . 11
5.2.1. Behaviour Common to all PCN-interior Nodes . . . . . . 11
5.2.2. Behaviour of PCN-interior Nodes Using Two
PCN-markings . . . . . . . . . . . . . . . . . . . . . 12
5.2.3. Behaviour of PCN-interior Nodes Using One
PCN-marking . . . . . . . . . . . . . . . . . . . . . 12
5.3. PCN-egress Node Behaviour . . . . . . . . . . . . . . . . 13
6. Backward Compatibility . . . . . . . . . . . . . . . . . . . . 13
6.1. Backward Compatibility with ECN . . . . . . . . . . . . . 13
6.2. Backward Compatibility with the RFC5696 Encoding . . . . . 14
7. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 14
8. Security Considerations . . . . . . . . . . . . . . . . . . . 14
9. Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . 15
10. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 15
11. Comments Solicited . . . . . . . . . . . . . . . . . . . . . . 15
12. References . . . . . . . . . . . . . . . . . . . . . . . . . . 15
12.1. Normative References . . . . . . . . . . . . . . . . . . . 15
12.2. Informative References . . . . . . . . . . . . . . . . . . 16
Appendix A. Choice of Suitable DSCPs . . . . . . . . . . . . . . 18
Appendix B. Co-existence of ECN and PCN . . . . . . . . . . . . . 19
Appendix C. Example Mapping between Encoding of PCN-Marks in
IP and in MPLS Shim Headers . . . . . . . . . . . . . 21
Appendix D. Rationale for Difference Between the Schemes
using One PCN-Marking . . . . . . . . . . . . . . . . 22
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1. Introduction
The objective of Pre-Congestion Notification (PCN) [RFC5559] is to
protect the quality of service (QoS) of inelastic flows within a
Diffserv domain, in a simple, scalable, and robust fashion. Two
mechanisms are used: admission control, to decide whether to admit or
block a new flow request, and flow termination to terminate some
existing flows during serious pre-congestion. To achieve this, the
overall rate of PCN-traffic is metered on every link in the domain,
and PCN-packets are appropriately marked when certain configured
rates are exceeded. These configured rates are below the rate of the
link thus providing notification to boundary nodes about overloads
before any real congestion occurs (hence "pre-congestion
notification").
[RFC5670] provides for two metering and marking functions that are
generally configured with different reference rates. Threshold-
marking marks all PCN packets once their traffic rate on a link
exceeds the configured reference rate (PCN-threshold-rate). Excess-
traffic-marking marks only those PCN packets that exceed the
configured reference rate (PCN-excess-rate). The PCN-excess-rate is
typically larger than the PCN-threshold-rate [RFC5559]. Egress nodes
monitor the PCN-marks of received PCN-packets and pass information
about these PCN-marks to decision points which then decide whether to
admit new flows or terminate existing flows
[I-D.ietf-pcn-cl-edge-behaviour], [I-D.ietf-pcn-sm-edge-behaviour].
The encoding defined in [RFC5696] described how two PCN marking
states (Not-marked and PCN-Marked) could be encoded into the IP
header using a single Diffserv codepoint. It defined 01 as an
experimental codepoint (EXP), along with guidelines for its use. Two
PCN marking states are sufficient for the Single Marking edge
behaviour [I-D.ietf-pcn-sm-edge-behaviour]. However, PCN-domains
utilising the controlled load edge behaviour
[I-D.ietf-pcn-cl-edge-behaviour] require three PCN marking states.
This document extends the RFC5696 encoding by redefining the
experimental codepoint as a third PCN marking state in the IP header,
still using a single Diffserv codepoint. This encoding scheme is
therefore called the "3-in-1 PCN encoding". It obsoletes the
[RFC5696] encoding, which provides only a sub-set of the same
capabilities.
The full version of this encoding requires any tunnel endpoint within
the PCN-domain to support the normal tunnelling rules defined in
[RFC6040]. There is one limited exception to this constraint where
the PCN-domain only uses the excess-traffic-marking behaviour and
where the threshold-marking behaviour is deactivated. This is
discussed in Section 5.2.3.1.
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This document only concerns the PCN wire protocol encoding for IP
headers, whether IPv4 or IPv6. It makes no changes or
recommendations concerning algorithms for congestion marking or
congestion response. Other documents will define the PCN wire
protocol for other header types. Appendix C discusses a possible
mapping between IP and MPLS.
1.1. Requirements Language
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in [RFC2119].
1.2. Changes in This Version (to be removed by RFC Editor)
From draft-ietf-pcn-3-in-1-encoding-07 to -08: Editorial corrections
and clarifications.
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:
* Draft re-written to obsolete baseline encoding [RFC5696].
* New section defining utilising this encoding for only one PCN-
Marking. Added an appendix explaining an apparent
inconsistency within this section.
* Moved (and updated) informative appendixes from [RFC5696] to
this document. Original Appendix C was omitted as it is now
redundant.
* Significant re-structuring of document.
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From draft-ietf-pcn-3-in-1-encoding-04 to -05:
* Draft moved to standards track as per working group
discussions.
* Added Appendix B discussing ECN handling in the PCN-domain.
* Clarified that this document modifies [RFC5696].
From draft-ietf-pcn-3-in-1-encoding-03 to -04:
* Updated document to reflect RFC6040.
* Re-wrote introduction.
* Re-wrote section on applicability.
* Re-wrote section on choosing encoding scheme.
* Updated author details.
From draft-ietf-pcn-3-in-1-encoding-02 to -03:
* Corrected mistakes in introduction and improved overall
readability.
* Added new terminology.
* Rewrote a good part of Section 4 and 5 to achieve more clarity.
* Added appendix explaining when to use which encoding scheme and
how to encode them in MPLS shim headers.
* Added new co-author.
From draft-ietf-pcn-3-in-1-encoding-01 to -02:
* Corrected mistake in introduction, which wrongly stated that
the threshold-traffic rate is higher than the excess-traffic
rate. Other minor corrections.
* Updated acks & refs.
From draft-ietf-pcn-3-in-1-encoding-00 to -01:
* Altered the wording to make sense if
draft-ietf-tsvwg-ecn-tunnel moves to proposed standard.
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* References updated
From draft-briscoe-pcn-3-in-1-encoding-00 to
draft-ietf-pcn-3-in-1-encoding-00:
* Filename changed to draft-ietf-pcn-3-in-1-encoding.
* Introduction altered to include new template description of
PCN.
* References updated.
* Terminology brought into line with [RFC5670].
* Minor corrections.
2. Definitions and Abbreviations
2.1. Terminology
The terms PCN-domain, PCN-node, PCN-interior-node, PCN-ingress-node,
PCN-egress-node, PCN-boundary-node, PCN-traffic, PCN-packets and PCN-
marking are used as defined in [RFC5559]. The following additional
terms are defined in this document:
PCN encoding: mapping of PCN marking states to specific codepoints
in the packet header.
PCN-compatible Diffserv codepoint: a Diffserv codepoint indicating
packets for which the ECN field carries PCN-markings rather than
[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 a packet has
been threshold-marked; that is a packet that has been marked at a
PCN-interior-node as a result of an indication from the threshold-
metering function [RFC5670]. Abbreviated to ThM.
Excess-traffic-marked codepoint: a codepoint that indicates packets
that have been marked at a PCN-interior-node as a result of an
indication from the excess-traffic-metering function [RFC5670].
Abbreviated to ETM.
Not-marked codepoint: a codepoint that indicates PCN-packets that
are not PCN-marked. Abbreviated to NM.
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not-PCN codepoint: a codepoint that indicates packets that are not
PCN-packets.
2.2. List of Abbreviations
The following abbreviations are used in this document:
o AF = Assured Forwarding [RFC2597]
o CE = Congestion Experienced [RFC3168]
o CS = Class Selector [RFC2474]
o DSCP = Diffserv codepoint
o ECN = Explicit Congestion Notification [RFC3168]
o ECT = ECN Capable Transport [RFC3168]
o EF = Expedited Forwarding [RFC3246]
o ETM = Excess-traffic-marked
o EXP = Experimental
o IP = Internet protocol
o NM = Not-marked
o PCN = Pre-Congestion Notification
o ThM = Threshold-marked
3. Definition of 3-in-1 PCN Encoding
The 3-in-1 PCN encoding scheme allows for two or three PCN-marking
states to be encoded within the IP header. The full encoding is
shown in Figure 1.
+--------+----------------------------------------------------+
| | Codepoint in ECN field of IP header |
| DSCP | <RFC3168 codepoint name> |
| +--------------+-------------+-------------+---------+
| | 00 <Not-ECT> | 10 <ECT(0)> | 01 <ECT(1)> | 11 <CE> |
+--------+--------------+-------------+-------------+---------+
| DSCP n | Not-PCN | NM | ThM | ETM |
+--------+--------------+-------------+-------------+---------+
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Figure 1: 3-in-1 PCN Encoding
A PCN-node (i.e. a node within a PCN-domain) will be configured to
recognise certain DSCPs as PCN-compatible. Appendix A discusses the
choice of suitable DSCPs. In Figure 1 'DSCP n' indicates such a PCN-
compatible DSCP. Within the PCN-domain, any packet carrying a PCN-
compatible DSCP and with the ECN-field anything other than 00 (Not-
PCN) is a PCN-packet as defined in [RFC5559].
PCN-nodes MUST interpret the ECN field of a PCN-packet using the
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
by any PCN marker.
ThM: Threshold-marked. Indicates a PCN-packet that has been marked
by a threshold-marker [RFC5670].
ETM: Excess-traffic-marked. Indicates a PCN-packet that has been
marked by an excess-traffic-marker [RFC5670].
4. Requirements for and Applicability of 3-in-1 PCN Encoding
4.1. PCN Requirements
In accordance with the PCN architecture [RFC5559], PCN-ingress-nodes
control packets entering a PCN-domain. Packets belonging to PCN-
controlled flows are subject to PCN-metering and -marking, and PCN-
ingress-nodes mark them as Not-marked (PCN-colouring). Any node in
the PCN-domain may perform PCN-metering and -marking and mark PCN-
packets if needed. There are two different metering and marking
behaviours: threshold-marking and excess-traffic-marking [RFC5670].
Some edge behaviors require only a single marking behaviour
[I-D.ietf-pcn-sm-edge-behaviour], others require both
[I-D.ietf-pcn-cl-edge-behaviour]. In the latter case, three PCN
marking states are needed: not-marked (NM) to indicate not-marked
packets, threshold-marked (ThM) to indicate packets marked by the
threshold-marker, and excess-traffic-marked (ETM) to indicate packets
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marked by the excess-traffic-marker [RFC5670]. Threshold-marking and
excess-traffic-marking are configured to start marking packets at
different load conditions, so one marking behaviour indicates more
severe pre-congestion than the other. Therefore, a fourth PCN
marking state indicating that a packet is marked by both markers is
not needed. However a fourth codepoint is required to indicate
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
[RFC5559], [I-D.ietf-pcn-cl-edge-behaviour], excess-traffic-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
severe PCN-mark than threshold-marked.
4.2. Requirements Imposed by Tunnelling
[RFC6040] defines rules for the encapsulation and decapsulation of
ECN markings within IP-in-IP tunnels. The publication of RFC6040
removed the tunnelling constraints that existed when the encoding of
[RFC5696] was written (see section 3.3.2 of
[I-D.ietf-pcn-encoding-comparison]).
Nonetheless, there is still a problem if there are any legacy (pre-
RFC6040) decapsulating tunnel endpoints within a PCN domain. If a
PCN node Threshold-marks the outer header of a tunnelled packet that
has a Not-marked codepoint on the inner header, a legacy decapsulator
will forward the packet as Not-marked, losing the threshold marking.
The rules on applicability in Section 4.3 below are designed to avoid
this problem.
4.3. Applicability of 3-in-1 PCN Encoding
The 3-in-1 encoding is applicable in situations where two marking
behaviours are being used in the PCN-domain. The 3-in-1 encoding can
also be used with only one marking behaviour, in which case one of
the codepoints MUST NOT be used throughout the PCN-domain (see
Section 5.2.3).
With one exception (see next paragraph), any tunnel endpoints
(IP-in-IP and IPsec) within the PCN-domain MUST comply with the ECN
encapsulation and decapsulation rules set out in [RFC6040] (see
Section 4.2).
It may not be possible to upgrade every pre-RFC6040 tunnel endpoint
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
stringent condition. If any pre-RFC6040 tunnel endpoint exists
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within a PCN-domain then every PCN-node in the PCN-domain MUST be
configured so that it never sets the ThM codepoint. PCN-interior
nodes in this case MUST solely use the Excess Traffic marking
function, as defined in Section 5.2.3.1. 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
Any tunnel endpoint implementation on a PCN-node MUST comply with
[RFC6040]. Since PCN is a new capability, this is considered a
reasonable requirement.
5.1. PCN-ingress Node Behaviour
PCN-traffic MUST be marked with a PCN-compatible Diffserv codepoint.
To conserve DSCPs, Diffserv codepoints SHOULD be chosen that are
already defined for use with admission-controlled traffic.
Appendix A gives guidance to implementors on suitable DSCPs.
Guidelines for mixing traffic types within a PCN-domain are given in
[RFC5670].
If a packet arrives at the PCN-ingress-node that shares a PCN-
compatible DSCP and is not a PCN-packet, the PCN-ingress-node MUST
mark it as not-PCN.
If a PCN-packet arrives at the PCN-ingress-node, the PCN-ingress-node
MUST change the PCN codepoint to Not-marked.
If a PCN-packet arrives at the PCN-ingress-node with its ECN field
already set to a value other than not-ECT, then appropriate action
MUST be taken to meet the requirements of [RFC4774]. The simplest
appropriate action is to just drop such packets. However, this is a
drastic action that an operator may feel is undesirable. Appendix B
provides more information and summarises other alternative actions
that might be taken.
5.2. PCN-interior Node Behaviour
5.2.1. Behaviour Common to all PCN-interior Nodes
Interior nodes MUST NOT change not-PCN to any other codepoint.
Interior nodes MUST NOT change NM to not-PCN.
Interior nodes MUST NOT change ThM to NM or not-PCN.
Interior nodes MUST NOT change ETM to any other codepoint.
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5.2.2. Behaviour of PCN-interior Nodes Using Two PCN-markings
If the threshold-meter function indicates a need to mark a packet,
the PCN-interior node MUST change NM to ThM.
If the excess-traffic-meter function indicates a need to mark a
packet:
o the PCN-interior node MUST change NM to ETM;
o the PCN-interior node MUST change ThM to ETM.
If both the threshold meter and the excess-traffic meter indicate the
need to mark a packet, the Excess-traffic-marking rules MUST take
precedence.
5.2.3. Behaviour of PCN-interior Nodes Using One PCN-marking
Some PCN edge behaviours require only one PCN-marking within the PCN-
domain. The Single Marking edge behaviour
[I-D.ietf-pcn-sm-edge-behaviour] requires PCN-interior nodes to mark
packets using the excess-traffic-meter function [RFC5670]. It is
possible that future schemes may require only the threshold-meter
function. Appendix D explains the rationale for the behaviours
defined in this section.
5.2.3.1. Marking Using only the Excess-traffic-meter Function
The threshold-traffic-meter function SHOULD be disabled and MUST NOT
trigger any packet marking.
The PCN-interior node SHOULD raise a management alarm if it receives
a ThM packet, but the frequency of such alarms SHOULD be limited.
If the excess-traffic-meter function indicates a need to mark the
packet:
o the PCN-interior node MUST change NM to ETM;
o the PCN-interior node MUST change ThM to ETM. It SHOULD also
raise an alarm as above.
5.2.3.2. Marking using only the Threshold-meter Function
The excess-traffic-meter function SHOULD be disabled and MUST NOT
trigger any packet marking.
The PCN-interior node SHOULD raise a management alarm if it receives
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an ETM packet, but the frequency of such alarms SHOULD be limited.
If the threshold-meter function indicates a need to mark the packet:
o the PCN-interior node MUST change NM to ThM;
o the PCN-interior node MUST NOT change ETM to any other codepoint.
It SHOULD raise an alarm as above if it encounters an ETM packet.
5.3. PCN-egress Node Behaviour
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 this is if the PCN-egress-node is certain that
revealing other codepoints outside the PCN-domain won't contravene
the guidance given in [RFC4774]. For instance, if the PCN-ingress-
node has explicitly informed the PCN-egress-node that this flow is
ECN-capable, then it might be safe to expose other codepoints.
Appendix B gives details of how such schemes might work, but such
schemes are currently only tentative ideas.
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-
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
misconfiguration in the PCN-domain.
6. Backward Compatibility
6.1. Backward Compatibility with ECN
BCP 124 [RFC4774] gives guidelines for specifying alternative
semantics for the ECN field. It sets out a number of factors to be
taken into consideration. It also suggests various techniques to
allow the co-existence of default ECN and alternative ECN semantics.
The encoding specified in this document uses one of those techniques;
it defines PCN-compatible Diffserv codepoints as no longer supporting
the default ECN semantics within a PCN domain. As such, this
document is compatible with BCP 124.
On its own, the 3-in-1 encoding cannot support both ECN marking end-
to-end (e2e) and PCN-marking within a PCN-domain. Appendix B
discusses possible ways to do this, e.g. by carrying e2e ECN across a
PCN-domain within the inner header of an IP-in-IP tunnel. Although
Appendix B recommends various approaches over others, it is merely
informative and all such schemes are beyond the normative scope of
this document.
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In any PCN deployment, traffic can only enter the PCN-domain through
PCN-ingress-nodes and leave through PCN-egress-nodes. PCN-ingress-
nodes ensure that any packets entering the PCN-domain have the ECN
field in their outermost IP header set to the appropriate codepoint.
PCN-egress-nodes then guarantee that the ECN field of any packet
leaving the PCN-domain has appropriate ECN semantics. This prevents
unintended leakage of ECN marks into or out of the PCN-domain, and
thus reduces backward-compatibility issues.
6.2. Backward Compatibility with the RFC5696 Encoding
A PCN node implemented to use the obsoleted RFC5696 encoding could
conceivably have been configured so that the Threshold-meter function
marked what is now defined as the ETM codepoint in the 3-in-1
encoding. However, there is no known deployment of such an
implementation and no reason to believe that such an implementation
would ever have been built. Therefore, it seems safe to ignore this
issue.
7. IANA Considerations
This memo includes no request to IANA.
Note to RFC Editor: this section may be removed on publication as an
RFC.
8. Security Considerations
PCN-marking only carries a meaning within the confines of a PCN-
domain. This encoding document is intended to stand independently of
the architecture used to determine how specific packets are
authorised to be PCN-marked, which will be described in separate
documents on PCN-boundary-node behaviour.
This document assumes the PCN-domain to be entirely under the control
of a single operator, or a set of operators who trust each other.
However, future extensions to PCN might include inter-domain versions
where trust cannot be assumed between domains. If such schemes are
proposed, they must ensure that they can operate securely despite the
lack of trust. However, such considerations are beyond the scope of
this document.
One potential security concern is the injection of spurious PCN-marks
into the PCN-domain. However, these can only enter the domain if a
PCN-ingress-node is misconfigured. The precise impact of any such
misconfiguration will depend on which of the proposed PCN-boundary-
node behaviours is used, but in general spurious marks will lead to
admitting fewer flows into the domain or potentially terminating too
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many flows. In either case, good management should be able to
quickly spot the problem since the overall utilisation of the domain
will rapidly fall.
9. Conclusions
The 3-in-1 PCN encoding uses a PCN-compatible DSCP and the ECN field
to encode PCN-marks. One codepoint allows non-PCN traffic to be
carried with the same PCN-compatible DSCP and three other codepoints
support three PCN marking states with different levels of severity.
In general, the use of this PCN encoding scheme presupposes that any
tunnel endpoints within the PCN-domain comply with [RFC6040].
10. Acknowledgements
Many thanks to Phil Eardley for providing extensive feedback,
criticism and advice. Thanks also to Teco Boot, Kwok Ho Chan,
Ruediger Geib, Georgios Karaginannis and everyone else who has
commented on the document.
11. Comments Solicited
To be removed by RFC Editor: Comments and questions are 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
12.1. Normative References
[RFC2119] Bradner, S., "Key words for use
in RFCs to Indicate Requirement
Levels", BCP 14, RFC 2119,
March 1997.
[RFC2474] Nichols, K., Blake, S., Baker,
F., and D. Black, "Definition of
the Differentiated Services Field
(DS Field) in the IPv4 and IPv6
Headers", RFC 2474,
December 1998.
[RFC3168] Ramakrishnan, K., Floyd, S., and
D. Black, "The Addition of
Explicit Congestion Notification
(ECN) to IP", RFC 3168,
September 2001.
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[RFC5559] Eardley, P., "Pre-Congestion
Notification (PCN) Architecture",
RFC 5559, June 2009.
[RFC5670] Eardley, P., "Metering and
Marking Behaviour of PCN-Nodes",
RFC 5670, November 2009.
[RFC6040] Briscoe, B., "Tunnelling of
Explicit Congestion
Notification", RFC 6040,
November 2010.
12.2. Informative References
[I-D.ietf-pcn-cl-edge-behaviour] Charny, A., Huang, F.,
Karagiannis, G., Menth, M., and
T. Taylor, "PCN Boundary Node
Behaviour for the Controlled Load
(CL) Mode of Operation", draft-
ietf-pcn-cl-edge-behaviour-09
(work in progress), June 2011.
[I-D.ietf-pcn-encoding-comparison] Karagiannis, G., Chan, K.,
Moncaster, T., Menth, M.,
Eardley, P., and B. Briscoe,
"Overview of Pre-Congestion
Notification Encoding", draft-
ietf-pcn-encoding-comparison-06
(work in progress), June 2011.
[I-D.ietf-pcn-sm-edge-behaviour] Charny, A., Karagiannis, G.,
Menth, M., and T. Taylor, "PCN
Boundary Node Behaviour for the
Single Marking (SM) Mode of
Operation", draft-ietf-pcn-sm-
edge-behaviour-06 (work in
progress), June 2011.
[RFC2597] Heinanen, J., Baker, F., Weiss,
W., and J. Wroclawski, "Assured
Forwarding PHB Group", RFC 2597,
June 1999.
[RFC3246] Davie, B., Charny, A., Bennet,
J., Benson, K., Le Boudec, J.,
Courtney, W., Davari, S., Firoiu,
V., and D. Stiliadis, "An
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Expedited Forwarding PHB (Per-Hop
Behavior)", RFC 3246, March 2002.
[RFC3540] Spring, N., Wetherall, D., and D.
Ely, "Robust Explicit Congestion
Notification (ECN) Signaling with
Nonces", RFC 3540, June 2003.
[RFC4594] Babiarz, J., Chan, K., and F.
Baker, "Configuration Guidelines
for DiffServ Service Classes",
RFC 4594, August 2006.
[RFC4774] Floyd, S., "Specifying Alternate
Semantics for the Explicit
Congestion Notification (ECN)
Field", BCP 124, RFC 4774,
November 2006.
[RFC5127] Chan, K., Babiarz, J., and F.
Baker, "Aggregation of DiffServ
Service Classes", RFC 5127,
February 2008.
[RFC5129] Davie, B., Briscoe, B., and J.
Tay, "Explicit Congestion Marking
in MPLS", RFC 5129, January 2008.
[RFC5462] Andersson, L. and R. Asati,
"Multiprotocol Label Switching
(MPLS) Label Stack Entry: "EXP"
Field Renamed to "Traffic Class"
Field", RFC 5462, February 2009.
[RFC5696] Moncaster, T., Briscoe, B., and
M. Menth, "Baseline Encoding and
Transport of Pre-Congestion
Information", RFC 5696,
November 2009.
[RFC5865] Baker, F., Polk, J., and M.
Dolly, "A Differentiated Services
Code Point (DSCP) for Capacity-
Admitted Traffic", RFC 5865,
May 2010.
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Appendix A. Choice of Suitable DSCPs
This appendix is informative, not normative.
A single DSCP has not been defined 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 a marking behaviour
similar to ECN but intended for inelastic traffic. The 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 link rates of
all the links making up that domain. In PCN-domains with sufficient
aggregation, the appropriate DSCPs would currently be those for the
Real-Time Treatment Aggregate [RFC5127]. It is suggested that
admission control could be used for the following service classes
(defined in [RFC4594] unless otherwise stated):
o Telephony (EF)
o Real-time interactive (CS4)
o Broadcast Video (CS3)
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
applied to signalling traffic.
PCN-marking is intended to provide a scalable admission-control
mechanism for traffic with a high degree of statistical multiplexing.
PCN-marking would therefore be appropriate to apply to traffic in the
above classes, but only within a PCN-domain containing sufficiently
aggregated traffic. In such cases, the above service classes may
well all be subject to a single forwarding treatment (treatment
aggregate [RFC5127]). However, this does not imply all such IP
traffic would necessarily be identified by one DSCP -- each service
class might keep a distinct DSCP within the highly aggregated region
[RFC5127].
Additional service classes may be defined for which admission control
is appropriate, whether through some future standards action or
through local use by certain operators, e.g., the Multimedia
Streaming service class (AF3). This document does not preclude the
use of PCN in more cases than those listed above.
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Note: The above discussion is informative not normative, as operators
are ultimately free to decide whether to use admission control for
certain service classes and whether to use PCN as their mechanism of
choice.
Appendix B. Co-existence of ECN and PCN
This appendix is informative, not normative.
The PCN encoding described in this document re-uses the bits of the
ECN field in the IP header. Consequently, this disables ECN within
the PCN domain. Appendix B of [RFC5696] (obsoleted) included advice
on handling ECN traffic within a PCN-domain. This appendix
reiterates and clarifies that advice.
For the purposes of this appendix we define two forms of traffic that
might arrive at a PCN-ingress-node. These are admission-controlled
traffic and non-admission-controlled traffic.
Admission-controlled traffic will be re-marked to a PCN-compatible
DSCP by the PCN-ingress-node. Two mechanisms can be used to identify
such traffic:
a. Flow signalling, which associates a filterspec with a need for
admission control (e.g. through RSVP or some equivalent message,
e.g. from a SIP server to the ingress); the PCN-ingress-node re-
marks traffic matching that filterspec to a PCN-compatible DSCP.
b. Traffic arrives with a DSCP that implies it requires admission
control such as VOICE-ADMIT [RFC5865] or Real-Time Interactive,
Broadcast Video when used for video on demand, and multimedia
conferencing [RFC4594][RFC5865] (see Appendix A).
All other traffic can be thought of as non-admission-controlled (and
therefore outside the scope of PCN). However such traffic may still
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
traffic), or may be because there is a shortage of local DSCPs.
ECN [RFC3168] is an end-to-end congestion notification mechanism. As
such it is possible that some traffic entering the PCN-domain may
also be ECN capable.
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
PCN domain. The tunnelling action should be applied wholly outside
the PCN-domain as illustrated in the following figure:
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, . . . . . PCN-domain . . . . . .
. ,--------. ,--------. .
. _| PCN- |___________________| PCN- |_ .
. / | ingress | | egress | \ .
.| '---------' '--------' |.
| . . . . . . . . . . . . . . .|
,--------. ,--------.
_____| Tunnel | | Tunnel |____
| Ingress | - - ECN preserved inside tunnel - - | Egress |
'---------' '--------'
Figure 2: Separation of tunnelling and PCN actions
There are three cases for how e2e ECN traffic may wish to be treated
while crossing a PCN domain:
a) Traffic that does not require admission control:
For example, traffic that does not match flow signaling being used
for admission control. In this case the e2e ECN traffic is not
treated as PCN-traffic. There are two possible scenarios:
* Arriving traffic does not carry a PCN-compatible DSCP: no
action required.
* Arriving traffic carries a DSCP that clashes with a PCN-
compatible DSCP. There are two options:
1. The ingress maps the DSCP to a local DSCP with the same
scheduling PHB as the original DSCP, and the egress re-maps
it to the original PCN-compatible DSCP.
2. The ingress tunnels the traffic, setting not-PCN in the
outer header; note that this turns off ECN for this traffic
within the PCN domain.
The first option is recommended unless the operator is short of
local DSCPs.
b) Traffic that requires admission-control:
In this case the e2e ECN traffic is treated as PCN-traffic across
the PCN domain. There are two options.
* The PCN-ingress-node places this traffic in a tunnel with a
PCN-compatible DSCP in the outer header. The PCN-egress zeroes
the ECN-field before decapsulation.
* The PCN-ingress-node drops CE-marked packets and otherwise sets
the ECN-field to NM and sets the DCSP to a PCN-compatible DSCP.
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The PCN-egress zeroes the ECN field of all PCN packets; note
that this turns off e2e ECN.
The second option is emphatically not recommended, unless perhaps
as a last resort if tunnelling is not possible for some
insurmountable reason.
c) Traffic that requires admission control where the endpoints ask to
see PCN marks:
Note that this scheme is currently only a tentative idea.
For real-time data generated by an adaptive codec, schemes have
been suggested where PCN marks may be leaked out of the PCN-domain
so that end hosts can drop to a lower data rate, thus deferring
the need for admission control. Currently such schemes require
further study and the following is for guidance only.
The PCN-ingress-node needs to tunnel the traffic as in Figure 2,
taking care to comply with [RFC6040]. In this case the PCN-egress
does not zero the ECN field contrary to the recommendation in
Section 5.3, so that the [RFC6040] tunnel egress will preserve any
PCN-marking. Note that a PCN interior node may change the ECN-
field from 10 to 01 (NM to ThM), which would be interpreted by the
e2e ECN as a change from ECT(0) into ECT(1). This would not be
compatible with the (currently experimental) ECN nonce [RFC3540].
Appendix C. Example Mapping between Encoding of PCN-Marks in IP and in
MPLS Shim Headers
This appendix is informative not normative.
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
make the DS field information accessible in the MPLS header.
However, the MPLS shim header has only a 3-bit traffic class (TC)
field [RFC5462] providing for 8 codepoints. The operator has the
freedom to define a site-local mapping of the 64 codepoints of the DS
field onto the 8 codepoints in the TC field.
[RFC5129] describes how ECN markings in the IP header can also be
mapped to codepoints in the MPLS TC field. Appendix A of [RFC5129]
gives an informative description of how to support PCN in MPLS by
extending the way MPLS supports ECN. [RFC5129] was written while PCN
specifications were in early draft stages. The following provides a
clearer example of a mapping between PCN in IP and in MPLS using the
PCN terminology and concepts that have since been specified.
To support PCN in a MPLS domain, a PCN-compatible DSCP ('DSCP n')
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needs codepoints to be provided in the TC field for all the PCN-marks
used. That means, when for instance only excess-traffic-marking is
used for PCN purposes, the operator needs to define a site-local
mapping to two codepoints in the MPLS TC field for IP headers with:
o DSCP n and NM
o DSCP n and ETM
If both excess-traffic-marking and threshold-marking are used, 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:
o DSCP n and NM
o DSCP n and ThM
o DSCP n and ETM
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
site-local mapping to an MPLS TC codepoint for IP headers marked
with:
o DSCP n and Not-PCN
The above sets of codepoints are required for every PCN-compatible
DSCP. Clearly, given so few TC codepoints are available, it may be
necessary to compromise by merging together some capabilities.
Appendix D. Rationale for Difference Between the Schemes using One PCN-
Marking
Readers may notice a difference between the two behaviours in
Section 5.2.3.1 and Section 5.2.3.2. With only excess-traffic
marking enabled, an unexpected ThM packet can be re-marked to ETM.
However, with only Threshold-marking, an unexpected ETM packet cannot
be re-marked to ThM.
This apparent inconsistency is deliberate. The behaviour with only
threshold marking keeps to the rule of Section 5.2.1 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 and would require different code in the data
plane compared to the common behaviour.
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Authors' Addresses
Bob Briscoe
BT
B54/77, Adastral Park
Martlesham Heath
Ipswich IP5 3RE
UK
Phone: +44 1473 645196
EMail: bob.briscoe@bt.com
URI: http://bobbriscoe.net/
Toby Moncaster
Moncaster Internet Consulting
Dukes
Layer Marney
Colchester CO5 9UZ
UK
Phone: +44 7764 185416
EMail: toby@moncaster.com
URI: http://www.moncaster.com/
Michael Menth
University of Tuebingen
Sand 13
Tuebingen 72076
Germany
Phone: +49 7071 2970505
EMail: menth@informatik.uni-tuebingen.de
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