Congestion and Pre Congestion T. Moncaster
Internet-Draft BT
Intended status: Standards Track B. Briscoe
Expires: April 3, 2009 BT & UCL
M. Menth
University of Wuerzburg
September 30, 2008
Baseline Encoding and Transport of Pre-Congestion Information
draft-ietf-pcn-baseline-encoding-00
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Abstract
Pre-congestion notification (PCN) provides information to support
admission control and flow termination in order to protect the
Quality of Service of inelastic flows. It does this by marking
packets when traffic load on a link is approaching or has exceeded a
threshold below the physical link rate. This document specifies how
such marks are to be encoded into the IP header. The baseline
encoding described here provides for only two PCN encoding states.
It is designed to be easily extensible to provide more encoding
states but such schemes will be described in other documents.
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3
2. Requirements notation . . . . . . . . . . . . . . . . . . . . 4
3. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 4
4. Encoding two PCN States in IP . . . . . . . . . . . . . . . . 4
4.1. Rationale for Encoding . . . . . . . . . . . . . . . . . . 5
4.2. PCN-Compatible DiffServ Codepoints . . . . . . . . . . . . 6
5. Backwards Compatability . . . . . . . . . . . . . . . . . . . 6
6. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 6
7. Security Considerations . . . . . . . . . . . . . . . . . . . 6
8. Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . 7
9. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 7
10. Comments Solicited . . . . . . . . . . . . . . . . . . . . . . 7
11. References . . . . . . . . . . . . . . . . . . . . . . . . . . 7
11.1. Normative References . . . . . . . . . . . . . . . . . . . 7
11.2. Informative References . . . . . . . . . . . . . . . . . . 8
Appendix A. Tunnelling Constraints . . . . . . . . . . . . . . . 9
Appendix B. PCN Node Behvaiours . . . . . . . . . . . . . . . . . 9
B.1. Valid and Invalid Encoding Transitions at a PCN Node . . . 10
Appendix C. Deployment Scenarios for PCN Using Baseline
Encoding . . . . . . . . . . . . . . . . . . . . . . 10
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 11
Intellectual Property and Copyright Statements . . . . . . . . . . 12
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1. Introduction
Pre-congestion notification (PCN) provides information to support
admission control and flow termination in order to protect the
quality of service (QoS) of inelastic flows. This is achieved by
marking packets according to the level of pre-congestion at nodes
within a PCN-domain. These markings are evaluated by the egress
nodes of the PCN-domain. [PCN-arch] describes how PCN packet
markings can be used to assure the QoS of inelastic flows within a
single DiffServ domain.
This document specifies how these PCN marks are encoded into the IP
header. It also describes how packets are identified as belonging to
a PCN flow. Some deployment models require two PCN encoding states,
others require more. The baseline encoding described here only
provides for two PCN encoding states. An extension of the baseline
encoding described in [PCN-3-enc-state] provides for three PCN
encoding states. Other extensions have also been suggested all of
which can build on the baseline encoding.
Changes from previous drafts (to be removed by the RFC Editor):
From -02 to -03:
Minor changes throughout.
Modified meaning of ECT(1) state to EXP.
Moved text relevant to behaviour of nodes into appendix for later
transfer to new document on edge behaviours
From -01 to -02:
Minor changes throughout including tightening up language to
remain consistent with the PCN Architecture terminology
From -00 to -01:
Change of title from "Encoding and Transport of (Pre-)Congestion
Information from within a DiffServ Domain to the Egress"
Extensive changes to Introduction and abstract.
Added a section on the implications of re-using a DSCP.
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Added appendix listing possible operator scenarios for using this
baseline encoding.
Minor changes throughout.
2. Requirements notation
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in [RFC2119].
3. Terminology
The following terms are used in this document:
o Not-PCN - packets that are not PCN capable.
o PCN-marked - codepoint indicating packets that have been marked at
a PCN-interior-node using some PCN marking behaviour. Also PM.
o Not-Marked - codepoint indicating packets that are PCN capable but
are not PCN-marked. Also NM.
o PCN-enabled codepoints - collective term for all the NM and PM
codepoints.
o PCN-compatible Diffserv codepoint - a Diffserv codepoint for which
the ECN field is used to carry PCN markings rather thatn [RFC3168]
markings.
In addition the document uses the terminology defined in [PCN-arch].
4. Encoding two PCN States in IP
The PCN encoding states are defined using a combination of the DSCP
and ECN fields within the IP header. The baseline PCN encoding
closely follows the semantics of ECN [RFC3168]. It allows the
encoding of two PCN states: Not-Marked and PCN-Marked. It also
allows for traffic that is not PCN capable to be marked as such (not-
PCN). Given the scarcity of codepoints within the IP header the
baseline encoding leaves one codepoint free for experimental use.
The following table defines how to encode these states in IP:
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+---------------------+------------+-----------+-----------+--------+
| DSCP \ RFC3168 ECN | not-ECT | ECT(0) | ECT(1) | CE |
| codepoint | (00) | (10) | (01) | (11) |
+---------------------+------------+-----------+-----------+--------+
| DSCP n | not-PCN | NM | EXP | PM |
+---------------------+------------+-----------+-----------+--------+
Where DSCP n is a PCN-enabled DiffServ codepoint (see Section 4.2)
and EXP means available for Experimental use.
Table 1: Encoding PCN in IP
The following rules apply to all PCN traffic:
o PCN traffic MUST be marked with a PCN-compatible DiffServ
Codepoint. That is a DiffServ codepoint that indicates that PCN
could be enabled by setting the appropriate value in the ECN
field. To conserve DSCPs, DiffServ Codepoints SHOULD be chosen
that are already defined for use with admission controlled
traffic, such as the Voice-Admit codepoint defined in
[voice-admit].
o Any packet that is not PCN-enabled (not-PCN) but which shares the
same DiffServ codepoint as PCN-enabled traffic MUST have the ECN
field set to 00.
4.1. Rationale for Encoding
The exact choice of encoding was dictated by the constraints imposed
by existing IETF RFCs, in particular [RFC3168] and [RFC4774]. Full
details are contained in [pcn-enc-compare]. One of the tightest
constraints was the need for any PCN encoding to survive being
tunnelled through either an IP in IP tunnel or an IPSec Tunnel.
Appendix A explains this in detail. The main effect of this
constraint is that any PCN marking has to use the ECN field set to 11
(CE codepoint). If the packet is being tunneled then only the CE
codepoint gets copied into the inner header upon decapsulation. An
additional constraint is the need to minimise the use of DiffServ
codepoints as these are in increasingly short supply. Section 4.2
explains how we have minimised this still further by reusing pre-
existing Diffserv codepoint(s) such that non-PCN traffic can still be
distinguished from PCN traffic.
The encoding scheme (Table 1) that best addresses the above
constraints ends up looking very similar to ECN. This is perhaps not
surprising given the similarity in architectural intent between PCN
and ECN.
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4.2. PCN-Compatible DiffServ Codepoints
Equipment complying with the baseline PCN encoding MUST allow PCN to
be enabled for a certain Diffserv codepoint or codepoints. This
document defines the term "PCN-Compatible Diffserv Codepoint" for
such a DSCP. Enabling PCN for a DSCP switches on PCN marking
behaviour for packets with that DSCP, but only if those packets also
have their ECN field set to indicate a codepoint other than not-PCN.
Enabling PCN marking behaviour disables any other marking behaviour
(e.g. enabling PCN disables the default ECN marking behaviour
introduced in [RFC3168]). The scheduling behaviour is discussed in
[pcn-marking-behaviour].
5. Backwards Compatability
BCP 124 [RFC4774] gives guidelines for specifying alternative
semantics for the ECN field. It sets out a number of factors that
must be taken into consideration. It also suggests various
techniques to allow the co-existence of default ECN and alternative
ECN semantics. The baseline encoding specified in this document
defines PCN-compatible DiffServ Codepoints as no longer supporting
the default ECN semantics. As such this document is compatible with
BCP 124.
6. IANA Considerations
This document makes no request to IANA.
7. Security Considerations
Packets claim entitlement to be PCN marked by carrying a PCN-enabled
DSCP and a PCN-Capable ECN codepoint. This encoding document is
intended to stand independently of the architecture used to determine
whether specific packets are authorised to be PCN marked, which will
be described in a future separate document on PCN edge-node behaviour
(see Appendix B).
The PCN working group has initially been chartered to only consider a
PCN-domain to be entirely under the control of one operator, or a set
of operators who trust each other [PCN-charter]. However there is a
requirement to keep inter-domain scenarios in mind when defining the
PCN encoding. One way to extend to multiple domains would be to
concatenate PCN-domains and use PCN-boundary-nodes back to back at
borders. Then any one domain's security against its neighbours would
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be described as part of the edge-node behaviour document as above.
One proposal on the table allows one to extend PCN across multiple
domains without PCN-boundary-nodes back-to-back at borders [re-PCN].
It is believed that the encoding described here would be compatible
with the security framework described there.
8. Conclusions
This document defines the baseline PCN encoding utilising a
combination of a PCN-enabled DSCP and the ECN field in the IP header.
This baseline encoding allows the existence of two PCN encoding
states, not-Marked and PCN-Marked. It also allows for the co-
existence of traffic that is not PCN-capable within the same DSCP so
long as theat traffic doesn't require end-to-end ECN support. The
encoding scheme is conformant with [RFC4774].
9. Acknowledgements
This document builds extensively on work done in the PCN working
group by Kwok Ho Chan, Georgios Karagiannis, Philip Eardley and
others. Full details of the alternative schemes that were considered
for adoption can be found in the document [pcn-enc-compare]. Thanks
to Ruediger Geib for providing detailed comments on this document.
10. Comments Solicited
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.
11. References
11.1. Normative References
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
[RFC4774] Floyd, S., "Specifying Alternate Semantics for the
Explicit Congestion Notification (ECN) Field", BCP 124,
RFC 4774, November 2006.
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11.2. Informative References
[PCN-3-enc-state]
Moncaster, T., Briscoe, B., and M. Menth, "A three state
extended PCN encoding scheme",
draft-moncaster-pcn-3-state-encoding-00 (work in
progress), June 2008.
[PCN-arch]
Eardley, P., "Pre-Congestion Notification Architecture",
draft-ietf-pcn-architecture-03 (work in progress),
February 2008.
[PCN-charter]
IETF, "IETF Charter for Congestion and Pre-Congestion
Notification Working Group".
[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.
[ecn-tunnelling]
Briscoe, B., "Layered Encapsulation of Congestion
Notification", draft-briscoe-tsvwg-ecn-tunnel-01 (work in
progress), July 2008.
[pcn-enc-compare]
Chan, K., Karagiannis, G., Moncaster, T., Menth, M.,
Eardley, P., and B. Briscoe, "Pre-Congestion Notification
Encoding Comparison",
draft-chan-pcn-encoding-comparison-03 (work in progress),
February 2008.
[pcn-marking-behaviour]
Eardley, P., "Marking behaviour of PCN-nodes",
draft-eardley-pcn-marking-behaviour-01 (work in progress),
June 2008.
[re-PCN] Briscoe, B., "Emulating Border Flow Policing using Re-ECN
on Bulk Data", draft-briscoe-re-pcn-border-cheat-00 (work
in progress), July 2007.
[voice-admit]
Baker, F., Polk, J., and M. Dolly, "DSCPs for Capacity-
Admitted Traffic",
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draft-ietf-tsvwg-admitted-realtime-dscp-04 (work in
progress), February 2008.
Appendix A. Tunnelling Constraints
The rules that govern the behaviour of the ECN field for IP-in-IP
tunnels were defined in [RFC3168]. This allowed for two tunnel
modes. The limited functionality mode sets the outer header to not-
ECT, regardless of the value of the inner header, in other words
disabling ECN within the tunnel. The full functionality mode copies
the inner ECN field into the outer header if the inner header is not-
ECT or either of the 2 ECT codepoints. If the inner header is CE
then the outer header is set to ECT(0). On decapsulation, if the CE
codepoint is set on the outer header then this is copied into the
inner header. Otherwise the inner header is left unchanged. The
stated reason for blocking CE from being copied to the outer header
was to prevent this from being used as a covert channel through IPSec
tunnels.
The IPSec protocol [RFC4301] changed the ECN tunnelling rule to allow
IPSec tunnels to simply copy the inner header into the outer header.
On decapsulation the outer header is discarded and the ECN field is
only copied down if it is set to CE.
Because of the possible existence of tunnels, only CE (11) can be
used as a PCN marking as it is the only mark that will survive
decapsulation. However there is a need for caution with all
tunneling within the PCN-domain. RFC3168 full functionality IP in IP
tunnels are expected to set the ECN field to ECT(0) if the inner ECN
field is set to CE. This leads to the possibility that some packets
within the PCN-domain that have already been marked may have that
mark concealed further into the domain. This is undesirable for many
PCN schemes and thus standard IP in IP tunnels SHOULD NOT be used
within a PCN-domain. Further work is needed within the Transport
Area to rationalise the behaviour of IP in IP tunnels in respect to
the ECN field and bring them in line with the behaviour of IPSec
tunnels [ecn-tunnelling].
Appendix B. PCN Node Behvaiours
Any packet that belongs to a PCN capable flow MUST have the ECN field
set to indicate a NM state at the PCN-ingress-node.
Any packet that is PCN capable and has been PCN-marked by a PCN-
interior-node MUST have the ECN field set to indicate a PM state.
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Any packet leaving the PCN-domain SHOULD have the ECN field reset to
00. The only exception is where the egress node knows the end-hosts
will react safely to any PCN marks they receive.
B.1. Valid and Invalid Encoding Transitions at a PCN Node
o PCN-interior-nodes MUST NOT change not-PCN to another codepoint
and they SHOULD NOT change a PCN-Capable codepoint to not-PCN
except where they need to downgrade the packet to a lower class of
service.
o PCN-interior-nodes that are in a pre-congestion state above the
configured level MUST set a PM codepoint as defined in Table 1 or
in any local/experimental scheme running within the PCN-domain.
o Packets carrying the 01 ECT(1) codepoint are for local/
experimental use only and their unexpected presence SHOULD cause
an alarm to be raised at the management level. However, to allow
for the possibility of misconfiguration they SHOULD be treated as
NM packets.
o The PM codepoint MUST NOT be changed to NM.
Appendix C. Deployment Scenarios for PCN Using Baseline Encoding
This appendix illustrates possible PCN deployment scenarios where the
baseline encoding can be used and also explain a case for which
baseline encoding is not sufficient. {Note this appendix is provided
for information only}.
1. An operator may wish to use PCN-based admission control only. To
that end, threshold marking based on admissible rates might be
used as the only PCN metering and marking algorithm. As a
consequence, the PM marks on the packets are interpreted as
meaning the ingress should stop admitting new traffic.
2. An operator may wish to use PCN-based flow termination only. To
that end, excess rate marking based on supportable rates might be
used as the only PCN metering and marking algorithm. As a
consequence, the PM marks on the packets are interpreted as
meaning the ingress shoudl start terminating appropriate flows.
3. An operator may wish to use both PCN-based admission control and
flow termination. To that end, excess rate marking based on
admissible rates might be used as the only PCN metering and
marking algorithm. The level of marks will be used to determine
when the ingress shoudl stop admitting new traffic and whether
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the ingress should terminate any flows.
4. An operator may wish to implement admission control based on
threshold marking at admissible rates and flow termination based
on excess rate marking at supportable rates because these methods
are believed to work better with small ingress-egress aggregates.
Then two different markings are needed. Such a deployment
scenario is not supported by the PCN baseline encoding.
Authors' Addresses
Toby Moncaster
BT
B54/70, Adastral Park
Martlesham Heath
Ipswich IP5 3RE
UK
Phone: +44 1473 648734
Email: toby.moncaster@bt.com
Bob Briscoe
BT & UCL
B54/77, Adastral Park
Martlesham Heath
Ipswich IP5 3RE
UK
Phone: +44 1473 645196
Email: bob.briscoe@bt.com
Michael Menth
University of Wuerzburg
room B206, Institute of Computer Science
Am Hubland
Wuerzburg D-97074
Germany
Phone: +49 931 888 6644
Email: menth@informatik.uni-wuerzburg.de
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