Congestion and Pre-Congestion B. Briscoe
Notification T. Moncaster
Internet-Draft BT
Intended status: Experimental March 08, 2010
Expires: September 9, 2010
PCN 3-State Encoding Extension in a single DSCP
draft-ietf-pcn-3-in-1-encoding-02
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. The level of marking allows the
boundary nodes to make decisions about whether to admit or block a
new flow request, and (in abnormal circumstances) whether to
terminate some of the existing flows, thereby protecting the QoS of
previously admitted flows. This document specifies how such marks
are to be encoded into the IP header by re-using the Explicit
Congestion Notification (ECN) codepoints within this controlled
domain. This encoding builds on the baseline encoding and provides
for three PCN encoding states: Not-marked, Threshold-marked and
Excess-traffic-marked.
Status of this Memo
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This Internet-Draft will expire on September 9, 2010.
Copyright Notice
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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 (in abnormal circumstances) flow
termination to decide whether to terminate some of the existing
flows. 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 congestion occurs (hence
"pre-congestion notification").
The level of marking allows boundary nodes to make decisions about
whether to admit or terminate. This is achieved by marking packets
on interior nodes according to some metering function implemented at
each node. Threshold-traffic-marking marks all PCN packets once they
exceed the threshold-traffic-rate on a link while Excess-traffic-
marking marks only those PCN packets that exceed the excess-traffic-
rate, which is higher than the threshold-traffic-rate [RFC5670].
These marks are monitored by the egress nodes of the PCN domain.
To fully support these two types of marking, three encoding states
are needed. The baseline encoding described in [RFC5696] provides
for deployment scenarios that only require two PCN encoding states
using a single Diffserv codepoint. This document describes an
experimental extension to the baseline-encoding that adds a third PCN
encoding state in the IP header, still using a single Diffserv
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codepoint. For brevity it will be called the 3-in-1 PCN Encoding.
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]. Note that [RFC5696] requires
the PCN Working Group to maintain a list of all DSCPs used for PCN
experiments.
1.1. Changes in This Version (to be removed by RFC Editor)
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
[I-D.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.
* 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 RFC 2119 [RFC2119].
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3. The Requirement for Three PCN Encoding States
The PCN architecture [RFC5559] describes proposed PCN schemes that
expect traffic to be metered and marked using both Threshold and
Excess Traffic schemes. In order to achieve this it is necessary to
allow for three PCN encoding states: one as a Not Marked (NM) state
and the other two to distinguish these two levels of marking severity
[RFC5670]. The way tunnels processed the ECN field before
[I-D.ietf-tsvwg-ecn-tunnel] severely limited how to encode these
states.
The two bit ECN field seems to offer four possible encoding states,
but one (00) is set aside for traffic controlled by transports that
do not understand PCN marking [RFC5696], so it would be irregular and
risky to use it as a PCN encoding state. Of the three remaining ECN
codepoints, only one (11) can be introduced by a congested node
within a tunnel and still survive the decapsulation behaviour of a
tunnel egress not updated to comply with [I-D.ietf-tsvwg-ecn-tunnel].
The two remaining codepoints are (10) and (01). But if a node within
the tunnel used either of these two remaining codepoints to try to
mark packets with a second severity level, a tunnel not updated to
comply with [I-D.ietf-tsvwg-ecn-tunnel] would remove this marking on
decapsulation. The ECN field was constrained to two marking states
in this way irrespective of which earlier ECN tunnelling
specification the tunnel complied with, whether regular IP in IP
tunnelling [RFC3168] or IPsec tunnelling [RFC4301].
One way to provide another encoding state that survives tunnelling is
to use a second Diffserv codepoint [I-D.ietf-pcn-3-state-encoding].
Instead, to avoid wasting scarce Diffserv codepoints, a network
operator can require tunnels in a PCN region to comply with
[I-D.ietf-tsvwg-ecn-tunnel], thus removing the constraints imposed by
earlier tunnelling specifications.
Therefore this document presupposes tunnels in the PCN region comply
with the newly proposed decapsulation rules defined in
[I-D.ietf-tsvwg-ecn-tunnel]. Then the constraints of standard
tunnels no longer apply so this document can define a 3-state
encoding for PCN within one Diffserv codepoint.
4. The 3-in-1 PCN Encoding
The 3-in-1 PCN Encoding scheme is based closely on the baseline
encoding defined in [RFC5696] so that there will be no compatibility
issues if a PCN-domain evolves from using the baseline encoding
scheme to the experimental scheme described here. The exact manner
in which the PCN encoding states are carried in the IP header is
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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 |
+--------+--------------+-------------+-------------+---------+
Figure 1: 3-in-1 PCN Encoding
In Figure 1 the 3 PCN states are encoded in the ECN field [RFC3168]
of an IP packet with its Diffserv field [RFC2474] set to DSCP n,
which is any PCN-Compatible DiffServ codepoint as defined in Section
4.2 of the PCN baseline encoding [RFC5696]). The PCN codepoint of a
packet defines its marking state as follows:
Not-PCN: The packet is controlled by a transport that does not
understand PCN marking, therefore the only valid action to notify
congestion is to drop the packet;
NM: Not marked. A packet in the NM state has not (yet) had its
marking state changed to the ThM or ETM states, but it may be
changed to one of these states by a node experiencing congestion
or pre-congestion;
ThM: Threshold-marked. Such a packet has had its marking state
changed by the threshold-meter function [RFC5670];
ETM: Excess-traffic-marked. Such a packet has had its marking state
changed by the excess-traffic-meter function [RFC5670].
Packets marked NM, ThM or ETM are termed PCN-packets. Their entry
into the pcn-domain is controlled by edge nodes that understand how
to process PCN markings [RFC5559].
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 Except where explicitly stated otherwise, it MUST comply with the
baseline encoding specified in [RFC5696]
o It MUST change NM to ThM if the threshold-meter function indicates
to mark the packet.
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o It MUST change NM or ThM to ETM if the excess-traffic-meter
function indicates to mark the packet.
o It MUST NOT change Not-PCN to a PCN-Enabled codepoint and MUST NOT
change a PCN-Enabled codepoint 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 may increase the severity of
packet marking but it MUST NOT decrease it, where the order of
severity increases from NM through ThM to ETM.
6. IANA Considerations
This memo includes no request to IANA.
Note to RFC Editor: this section may be removed on publication as an
RFC.
7. Security Considerations
The security concerns relating to this extended PCN encoding are the
same as those in [RFC5696].
8. Conclusions
The 3-in-1 PCN Encoding provides three states to encode PCN markings
in the ECN field of an IP packet using just one Diffserv codepoint.
One state is for not marked packets while the two others are for PCN
nodes to mark packets with increasing levels of severity. Use of
this encoding presupposes that any tunnels in the PCN region have
been updated to comply with [I-D.ietf-tsvwg-ecn-tunnel].
9. Acknowledgements
Thanks to Phil Eardley, Teco Boot, Kwok Ho Chan and Michael Menth for
reviewing this.
10. Comments Solicited
To be removed by RFC Editor: Comments and questions are encouraged
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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.
11. References
11.1. Normative References
[I-D.ietf-tsvwg-ecn-tunnel]
Briscoe, B., "Tunnelling of Explicit Congestion
Notification", draft-ietf-tsvwg-ecn-tunnel-08 (work in
progress), March 2010.
[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.
[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.
11.2. Informative References
[I-D.ietf-pcn-3-state-encoding]
Briscoe, B., Moncaster, T., and M. Menth, "A PCN encoding
using 2 DSCPs to provide 3 or more states",
draft-ietf-pcn-3-state-encoding-01 (work in progress),
February 2010.
[RFC4301] Kent, S. and K. Seo, "Security Architecture for the
Internet Protocol", RFC 4301, December 2005.
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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
BT
c/o B54/70, Adastral Park
Martlesham Heath
Ipswich IP5 3RE
UK
Phone: +44 1206 332805
Email: toby.moncaster@bt.com
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