Congestion and Pre-Congestion B. Briscoe
Notification BT
Internet-Draft T. Moncaster
Intended status: Experimental Moncaster Internet Consulting
Expires: July 15, 2011 M. Menth
University of Tuebingen
January 11, 2011
Encoding 3 PCN-States in the IP header using a single DSCP
draft-ietf-pcn-3-in-1-encoding-04
Abstract
The objective of Pre-Congestion Notification (PCN) is to protect the
quality of service (QoS) of inelastic flows within a Diffserv domain.
On every link in the PCN domain, the overall rate of the PCN-traffic
is metered, and PCN-packets are appropriately marked when certain
configured rates are exceeded. Egress nodes provide decision points
with information about the PCN-marks of PCN-packets which allows them
to take decisions about whether to admit or block a new flow request,
and 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 builds on the baseline
encoding of RFC5696 and provides for 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.
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
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This Internet-Draft will expire on July 15, 2011.
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Copyright Notice
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3
1.1. Changes in This Version (to be removed by RFC Editor) . . 4
2. Requirements Language . . . . . . . . . . . . . . . . . . . . 5
2.1. Terminology . . . . . . . . . . . . . . . . . . . . . . . 5
3. Requirements for and Applicability of 3-in-1 PCN Encoding . . 5
3.1. PCN Requirements . . . . . . . . . . . . . . . . . . . . . 5
3.2. Requirements Imposed by Baseline Encoding . . . . . . . . 6
3.3. Applicability of 3-in-1 PCN Encoding . . . . . . . . . . . 7
4. Definition of 3-in-1 PCN Encoding . . . . . . . . . . . . . . 7
5. Behaviour of a PCN Node Compliant with the 3-in-1 PCN
Encoding . . . . . . . . . . . . . . . . . . . . . . . . . . . 8
6. Backward Compatibility . . . . . . . . . . . . . . . . . . . . 8
6.1. Backward Compatibility with Pre-existing PCN
Implementations . . . . . . . . . . . . . . . . . . . . . 8
6.2. Recommendations for the Use of PCN Encoding Schemes . . . 9
6.2.1. Use of Both Excess-Traffic-Marking and
Threshold-Marking . . . . . . . . . . . . . . . . . . 9
6.2.2. Unique Use of Excess-Traffic-Marking . . . . . . . . . 9
6.2.3. Unique Use of Threshold-Marking . . . . . . . . . . . 10
7. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 10
8. Security Considerations . . . . . . . . . . . . . . . . . . . 10
9. Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . 10
10. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 10
11. Comments Solicited . . . . . . . . . . . . . . . . . . . . . . 11
12. References . . . . . . . . . . . . . . . . . . . . . . . . . . 11
12.1. Normative References . . . . . . . . . . . . . . . . . . . 11
12.2. Informative References . . . . . . . . . . . . . . . . . . 12
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 12
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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
configured with 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 provide information about the PCN-marks to
decision points which take decisions about flow admission and
termination on this basis [I-D.ietf-pcn-cl-edge-behaviour],
[I-D.ietf-pcn-sm-edge-behaviour].
The baseline encoding defined in [RFC5696] describes how two PCN
marking states (Not-marked and PCN-Marked) can be encoded using a
single Diffserv codepoint. It also provides an experimental
codepoint (EXP), along with guidelines for use of that codepoint. To
support the application of two different marking algorithms in a PCN-
domain, for example as required in [I-D.ietf-pcn-cl-edge-behaviour],
three PCN marking states are needed. This document describes an
extension to the baseline encoding that uses the EXP codepoint to
provide a third PCN marking state in the IP header, still using a
single Diffserv codepoint. This encoding scheme is called "3-in-1
PCN encoding".
This document only concerns the PCN wire protocol encoding for all IP
headers, whether IPv4 or IPv6. It makes no changes or
recommendations concerning algorithms for congestion marking or
congestion response. Other documents define the PCN wire protocol
for other header types. For example, the MPLS encoding is defined in
[RFC5129]. Appendix A provides an informative example for a mapping
between the encodings in IP and in MPLS.
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1.1. Changes in This Version (to be removed by RFC Editor)
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.
* 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.
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* Introduction altered to include new template description of
PCN.
* References updated.
* Terminology brought into line with [RFC5670].
* Minor corrections.
2. 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].
2.1. Terminology
General PCN-related terminology is defined in the PCN architecture
[RFC5559], and terminology specific to packet encoding is defined in
the PCN baseline encoding [RFC5696]. Additional terminology is
defined below.
PCN encoding: mapping of PCN marking states to specific codepoints
in the packet header.
3. Requirements for and Applicability of 3-in-1 PCN Encoding
3.1. 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
schemes: threshold-marking and excess-traffic-marking [RFC5670].
Some edge behaviors require only a single marking scheme
[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
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 scheme indicates more
severe pre-congestion than the other. Therefore, a fourth PCN
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marking state indicating that a packet is marked by both markers is
not needed. However a fourth codepoint is required to indicate
packets that are not PCN-capable (the not-PCN codepoint).
In all current PCN edge behaviors that use two marking schemes
[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.
3.2. Requirements Imposed by Baseline Encoding
The baseline encoding scheme [RFC5696] was defined so that it could
be extended to accommodate an additional marking state. It provides
rules to embed the encoding of two PCN states in the IP header.
Figure 1 shows the structure of the former type-of-service field. It
contains the 6-bit Differentiated Services (DS) field that holds the
DS codepoint (DSCP) [RFC2474] and the 2-bit ECN field [RFC3168].
0 1 2 3 4 5 6 7
+-----+-----+-----+-----+-----+-----+-----+-----+
| DS FIELD | ECN FIELD |
+-----+-----+-----+-----+-----+-----+-----+-----+
Figure 1: Structure of the former type-of-service field in IP
Baseline encoding defines that the DSCP must be set to a PCN-
compatible DSCP n and the ECN-field [RFC3168] indicates the specific
PCN-mark. Baseline encoding offers four possible encoding states
within a single DSCP with the following restrictions.
o Codepoint `00' (not-ECT) is used to indicate non-PCN traffic as
"not-PCN". This allows both PCN and non-PCN traffic to use the
same DSCP.
o Codepoint `10' (ECT(0)) is used to indicate Not-marked PCN
traffic.
o Codepoint `11' (CE) is used to indicate the most severe PCN-mark.
o Codepoint `01' (ECT(1)) is available for experimental use and may
be re-used by other PCN encodings such as the presently defined
3-in-1 PCN encoding (subject to the rules defined in [RFC5696]).
[RFC6040] defines rules for the encapsulation and decapsulation of
ECN markings within IP-in-IP tunnels. This RFC removes some of the
constraints that existed when [RFC5696] was written. Happily the
rules for use of the EXP codepoint are fully compatible with
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[RFC6040]. In particular, the relative severity of each marking is
the same: CE (PM) is more severe than ECT(1) (EXP) is more severe
than ECT(0) (NM). This is discussed in more detail in both the
baseline encoding document [RFC5696] and in
[I-D.ietf-pcn-encoding-comparison].
3.3. Applicability of 3-in-1 PCN Encoding
The 3-in-1 encoding is applicable in situations where two marking
schemes are being used in the PCN-domain. In some circumstances it
can also be used in PCN-domains with only a single marking scheme in
use. Further guidance on choosing an encoding scheme can be found in
Section 6.2. All nodes within the PCN-domain MUST be fully compliant
with the ECN encapsulation rules set out in [RFC6040]. As such the
encoding is not applicable in situations where legacy tunnels might
exist.
4. Definition of 3-in-1 PCN Encoding
The 3-in-1 PCN encoding scheme is an extension of the baseline
encoding scheme defined in [RFC5696]. The PCN requirements and the
extension rules for baseline encoding presented in the previous
section determine how PCN encoding states are carried in the IP
headers. This is shown in Figure 2.
+--------+----------------------------------------------------+
| | 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 |
+--------+--------------+-------------+-------------+---------+
Figure 2: 3-in-1 PCN Encoding
Like baseline encoding, 3-in-1 PCN encoding also uses a PCN
compatible DSCP n and the ECN field for the encoding of PCN-marks.
The PCN-marks have the following meaning.
Not-PCN: indicates a non-PCN-packet, i.e., a packet that 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.
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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].
5. Behaviour of a PCN Node Compliant with the 3-in-1 PCN Encoding
To be compliant with the 3-in-1 PCN Encoding, an PCN interior node
behaves as follows:
o It MUST change NM to ThM if the threshold-meter function indicates
a need to mark the packet;
o It MUST change NM or ThM to ETM if the excess-traffic-meter
function indicates a need to mark the packet;
o It MUST NOT change not-PCN to NM, ThM, or ETM;
o It MUST NOT change a NM, ThM, or ETM to not-PCN;
o It MUST NOT change ThM to NM;
o It MUST NOT change ETM to ThM or to NM;
In other words, a PCN interior node MUST NOT mark PCN-packets into
non-PCN packets and vice-versa, and it may increase the severity of
the PCN-mark of a PCN-packet, but it MUST NOT decrease it.
6. Backward Compatibility
Discussion of backward compatibility between PCN encoding schemes and
previous uses of the ECN field is given in Section 6 of [RFC5696].
6.1. Backward Compatibility with Pre-existing PCN Implementations
This encoding complies with the rules for extending the baseline PCN
encoding schemes in Section 5 of [RFC5696].
The term "compatibility" is meant in the following sense. It is
possible to operate nodes with baseline encoding [RFC5696] and 3-in-1
encoding in the same PCN domain. The nodes with baseline encoding
MUST perform excess-traffic-marking because the 11 codepoint of
3-in-1 encoding also means excess-traffic-marked. PCN-boundary-nodes
of such domains are required to interpret the full 3-in-1 encoding
and not just baseline encoding, otherwise they cannot interpret the
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01 codepoint.
Using nodes that perform only excess-traffic-marking may make sense
in networks using the CL edge behavior
[I-D.ietf-pcn-cl-edge-behaviour]. Such nodes are able to notify the
egress only about severe pre-congestion when traffic needs to be
terminated. This seems reasonable for locations that are not
expected to see any pre-congestion, but excess-traffic-marking gives
them a means to terminate traffic if unexpected overload occurs.
6.2. Recommendations for the Use of PCN Encoding Schemes
NOTE: This sub-section is informative not normative.
When deciding which PCN encoding is suitable an operator needs to
take account of how many PCN states need to be encoded. The
following table gives guidelines on which encoding to use with either
threshold-marking, excess-traffic marking or both.
+------------------------+--------------------------------+
| Used marking schemes | Recommended encoding scheme |
+------------------------+--------------------------------+
| Only threshold-marking | Baseline encoding [RFC5696] |
+------------------------+--------------------------------+
| Only excess-traffic- | Baseline encoding [RFC5696] |
| marking | or 3-in-1 PCN encoding |
+------------------------+--------------------------------+
| Threshold-marking and | 3-in-1 PCN encoding |
| excess-traffic-marking | |
+------------------------+--------------------------------+
Figure 3: Guidelines for choosing PCN encoding schemes
6.2.1. Use of Both Excess-Traffic-Marking and Threshold-Marking
If both excess-traffic-marking and threshold-marking are enabled in a
PCN-domain, 3-in-1 encoding should be used as described in this
document.
6.2.2. Unique Use of Excess-Traffic-Marking
If only excess-traffic-marking is enabled in a PCN-domain, baseline
encoding or 3-in-1 encoding may be used. They lead to the same
encoding because PCN-boundary nodes will interpret baseline "PCN-
marked (PM)" as "excess-traffic-marked (ETM)".
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6.2.3. Unique Use of Threshold-Marking
No scheme is currently proposed that solely uses threshold-marking.
If such a scheme is proposed, the choice of encoding scheme will
depend on whether nodes are compliant with [RFC6040] or not. Where
it is certain that all nodes in the PCN-domain are compliant then
either 3-in-1 encoding or baseline encoding are suitable. If legacy
tunnel decapsulators exist within the PCN-domain then baseline
encoding SHOULD be used.
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
The security concerns relating to this extended PCN encoding are the
same as those in [RFC5696]. In summary, PCN-boundary nodes are
responsible for ensuring inappropriate PCN markings do not leak into
or out of a PCN domain, and the current phase of the PCN architecture
assumes that all the nodes of a PCN-domain are entirely under the
control of a single operator, or a set of operators who trust each
other.
Given the only difference between the baseline encoding and the
present 3-in-1 encoding is the use of the 01 codepoint, no new
security issues are raised, as this codepoint was already available
for experimental use in the baseline encoding.
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.
The use of this PCN encoding scheme presupposes that any tunnels in
the PCN region have been updated to comply with [RFC6040].
10. Acknowledgements
Thanks to Phil Eardley, Teco Boot, and Kwok Ho Chan for reviewing
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this 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.
[RFC4301] Kent, S. and K. Seo, "Security Architecture for the
Internet Protocol", RFC 4301, December 2005.
[RFC5129] Davie, B., Briscoe, B., and J. Tay, "Explicit Congestion
Marking in MPLS", RFC 5129, January 2008.
[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.
[RFC5696] Moncaster, T., Briscoe, B., and M. Menth, "Baseline
Encoding and Transport of Pre-Congestion Information",
RFC 5696, November 2009.
[RFC6040] Briscoe, B., "Tunnelling of Explicit Congestion
Notification", RFC 6040, November 2010.
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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-08 (work in progress),
December 2010.
[I-D.ietf-pcn-encoding-comparison]
Chan, K., Karagiannis, G., Moncaster, T., Menth, M.,
Eardley, P., and B. Briscoe, "Pre-Congestion Notification
Encoding Comparison",
draft-ietf-pcn-encoding-comparison-03 (work in progress),
October 2010.
[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-05
(work in progress), December 2010.
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/
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Michael Menth
University of Tuebingen
Sand 13
72076 Tuebingen
Germany
Phone: +49 7071 2970505
Email: menth@informatik.uni-tuebingen.de
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