Congestion and Pre Congestion T. Moncaster
Internet-Draft BT
Intended status: Standards Track B. Briscoe
Expires: October 9, 2009 BT & UCL
M. Menth
University of Wuerzburg
April 7, 2009
Baseline Encoding and Transport of Pre-Congestion Information
draft-ietf-pcn-baseline-encoding-03
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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 t o 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 ECN codepoints
within this controlled domain. The baseline encoding described here
provides for only two PCN encoding states, unmarked and marked.
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3
2. Requirements notation . . . . . . . . . . . . . . . . . . . . 5
3. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 5
4. Encoding two PCN States in IP . . . . . . . . . . . . . . . . 5
4.1. Valid and Invalid Codepoint Transitions . . . . . . . . . 6
4.2. Rationale for Encoding . . . . . . . . . . . . . . . . . . 7
4.3. PCN-Compatible DiffServ Codepoints . . . . . . . . . . . . 8
5. Rules for Experimental Encoding Schemes . . . . . . . . . . . 8
6. Backwards Compatibility . . . . . . . . . . . . . . . . . . . 8
7. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 9
8. Security Considerations . . . . . . . . . . . . . . . . . . . 9
9. Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . 9
10. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 9
11. Comments Solicited . . . . . . . . . . . . . . . . . . . . . . 10
12. References . . . . . . . . . . . . . . . . . . . . . . . . . . 10
12.1. Normative References . . . . . . . . . . . . . . . . . . . 10
12.2. Informative References . . . . . . . . . . . . . . . . . . 10
Appendix A. PCN Deployment Considerations . . . . . . . . . . . . 11
A.1. Choice of Suitable DSCPs . . . . . . . . . . . . . . . . . 11
A.2. Rationale for Using ECT(0) for Not Marked . . . . . . . . 11
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1. Introduction
The objective of Pre-Congestion Notification (PCN) is to protect the
quality of service (QoS) of inelastic flows within a Diffserv domain,
in a simple, scalable and robust fashion. 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. These configured rates are below the rate of the link thus
providing notification before any congestion occurs (hence "pre-
congestion notification"). 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 these PCN marks are encoded into the IP
header by re-using the bits of the ECN field. 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. However the encoding can be easily extended to provide more
states and rules for such extensions are given in this document.
Changes from previous drafts (to be removed by the RFC Editor):
From -02 to -03:
Extensive changes to address comments made by Gorry Fairhurst
including:
* Abstract re-written.
* Clarified throughout that this re-uses the ECN bits in the IP
header.
* Re-arranged order of terminology section for clarity.
* Table 2 replaced with new table and text.
* Security considerations re-written.
* Appendixes re-written to improve clarity.
* Numerous minor nits and language changes throughout.
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Extensive other minor changes throughout.
From -01 to -02:
Removed Appendix A and replaced with reference to
[I-D.ietf-tsvwg-ecn-tunnel]
Moved Appendix B into main body of text.
Changed Appendix C to give deployment advice.
Minor changes throughout including checking consistency of
capitalisation of defined terms.
Clarified that LU was deliberately excluded from encoding.
From -00 to -01:
Added section on restrictions for extension encoding schemes.
Included table in Appendix showing encoding transitions at
different PCN nodes.
Checked for consistency of terminology.
Minor language changes for clarity.
Changes from previous filename
Filename changed from draft-moncaster-pcn-baseline-encoding.
Terminology changed for clarity (PCN-compatible DSCP and PCN-
enabled packet).
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 draft-moncaster -01 to -02:
Minor changes throughout including tightening up language to
remain consistent with the PCN Architecture terminology
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From draft-moncaster -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.
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 PCN-compatible Diffserv codepoint - a Diffserv codepoint for which
the ECN field is used to carry PCN markings rather than [RFC3168]
markings.
o PCN-marked - codepoint indicating packets that have been marked at
a PCN-interior-node using some PCN marking behaviour
[I-D.ietf-pcn-marking-behaviour]. Abbreviated to PM.
o Not-marked - codepoint indicating packets that are PCN-capable,
but are not PCN-marked. Abbreviated to NM.
o PCN-enabled codepoints - collective term for all NM and PM
codepoints. By definition, packets carrying such codepoints are
PCN-packets.
o not-PCN - packets that are not PCN-enabled.
In addition, the document uses the terminology defined in
[I-D.ietf-pcn-architecture].
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
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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:
+---------------+-------------+-------------+-------------+---------+
| ECN codepoint | Not-ECT | ECT(0) (10) | ECT(1) (01) | CE (11) |
| | (00) | | | |
+---------------+-------------+-------------+-------------+---------+
| DSCP n | not-PCN | NM | EXP | PM |
+---------------+-------------+-------------+-------------+---------+
Where DSCP n is a PCN-compatible DiffServ codepoint (see Section 4.3)
and EXP means available for Experimental use. N.B. we deliberately
reserve this codepoint for experimental use only (and not local use)
to prevent future compatability issues.
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. 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
[I-D.ietf-tsvwg-admitted-realtime-dscp]. Guidelines for mixing
traffic-types within a PCN-domain are given in
[I-D.ietf-pcn-marking-behaviour].
o Any packet that is not-PCN but which shares the same DiffServ
codepoint as PCN-enabled traffic MUST have the ECN field equal to
00.
4.1. Valid and Invalid Codepoint Transitions
A PCN-ingress-node MUST set the Not-marked (10) codepoint on any
arriving packet that belongs to a PCN-flow. It MUST set the not-PCN
(00) codepoint on all other packets.
A PCN-interior-node MUST observe the rules for valid and invalid
codepoint transitions as set out in the following table. The precise
rules governing which valid transition to use are set out in
[I-D.ietf-pcn-marking-behaviour]
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+-------------------------------------------------+
| Codepoint Out |
+--------------+-------------+-----------+-----------+-----------+
| Codepoint in | not-PCN(00) | NM(10) | EXP(01) | PM(11) |
+--------------+-------------+-----------+-----------+-----------+
| not-PCN(00) | Valid | Not valid | Not valid | Not valid |
+--------------+-------------+-----------+-----------+-----------+
| NM(10) | Not valid | Valid | Not valid | Valid |
+--------------+-------------+-----------+-----------+-----------+
| EXP(01)* | Not valid | Not valid | Valid | Valid |
+--------------+-------------+-----------+-----------+-----------+
| PM(11) | Not valid | Not valid | Not valid | Valid |
+--------------+-------------+-----------+-----------+-----------+
* This SHOULD cause an alarm to be raised at a higher layer. The
packet MUST be treated as if it carried the NM codepoint.
Table 2: Valid and Invalid Codepoint Transitions for
PCN-packets at PCN-interior-nodes
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].
4.2. Rationale for Encoding
The exact choice of encoding was dictated by the constraints imposed
by existing IETF RFCs, in particular [RFC3168], [RFC4301] and
[RFC4774]. 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. [I-D.ietf-tsvwg-ecn-tunnel] explains this in
more detail. The main effect of this constraint is that any PCN
marking has to carry the 11 codepoint in the ECN field since this is
the only codepoint that is guaranteeed to be copied down into the
inner header upon decapsulation. An additional constraint is the
need to minimise the use of DiffServ codepoints as there is a limited
supply of standards track codepoints remaining. Section 4.3 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. There are a number of factors that
were considered before deciding to set 10 as the NM state. These
included similarity to ECN, presence of tunnels within the domain,
leakage into and out of PCN-domain and incremental deployment (see
Appendix A.2).
The encoding scheme above seems to meet all these constraints and
ends up looking very similar to ECN. This is perhaps not surprising
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given the similarity in architectural intent between PCN and ECN.
4.3. PCN-Compatible DiffServ Codepoints
Equipment complying with the baseline PCN encoding MUST allow PCN to
be enabled for certain Diffserv codepoints. This document defines
the term "PCN-compatible Diffserv codepoint" for such a DSCP. To be
clear, any packets with such a DSCP will be PCN enabled only if they
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]). All traffic scheduling and conditioning
behaviours are discussed in [I-D.ietf-pcn-marking-behaviour]. This
ensures compliance with the BCP guidance set out in [RFC4774].
5. Rules for Experimental Encoding Schemes
Any experimental encoding scheme MUST follow these rules to ensure
backward compatibility with this baseline scheme:
o The 00 codepoint in the ECN field SHALL indicate not-PCN and MUST
NOT be changed to any otehr codepoint within a PCN-domain.
Therefore an ingress node wishing to disable PCN marking for a
packet within a PCN-compatible DiffServ Codepoint MUST set the ECN
field to 00.
o The 11 codepoint in the ECN field SHALL indicate PCN-marked
(though this does not exclude the 01 Experimental codepoint from
carrying the same meaning).
o Once set, the 11 codepoint in the ECN field MUST NOT be changed to
any other codepoint.
o Any experimental scheme MUST include details of all valid and
invalid codepoint transitions at any PCN nodes.
o Any experimental scheme MUST NOT update the meaning of the 00 and
11 codepoints defined above.
6. Backwards Compatibility
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 baseline encoding specified in this document defines PCN-
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compatible DiffServ codepoints as no longer supporting the default
ECN semantics. As such this document is compatible with BCP 124. It
should be noted that this baseline encoding effectively disables end-
to-end ECN except where mechanisms are put in place to tunnel such
traffic across the PCN-domain.
7. IANA Considerations
This document makes no request to IANA.
8. Security Considerations
PCN-marking only carries a meaning within the confines of a PCN-
domain. Packets wishing to be treated as belonging to a PCN-flow
must carry a PCN-Compatible DSCP and a PCN-Enabled 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 separate
documents on PCN edge-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 but such considerations are beyond the scope of this
document.
9. 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 competing traffic within the same DSCP so long as that
traffic does not require ECN support within the PCN-domain. The
encoding scheme is conformant with [RFC4774].
10. Acknowledgements
This document builds extensively on work done in the PCN working
group by Kwok Ho Chan, Georgios Karagiannis, Philip Eardley, Anna
Charny, Joe Babiarz and others. Thanks to Ruediger Geib and Gorry
Fairhurst for providing detailed comments on this document.
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11. Comments Solicited
(To be removed by the 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
[I-D.ietf-pcn-marking-behaviour] Eardley, P., "Marking
behaviour of PCN-nodes", dra
ft-ietf-pcn-marking-
behaviour-02 (work in
progress), March 2009.
[RFC2119] Bradner, S., "Key words for
use in RFCs to Indicate
Requirement Levels", BCP 14,
RFC 2119, March 1997.
[RFC3168] Ramakrishnan, K., Floyd, S.,
and D. Black, "The Addition
of Explicit Congestion
Notification (ECN) to IP",
RFC 3168, September 2001.
[RFC4774] Floyd, S., "Specifying
Alternate Semantics for the
Explicit Congestion
Notification (ECN) Field",
BCP 124, RFC 4774,
November 2006.
12.2. Informative References
[I-D.ietf-pcn-architecture] Eardley, P., "Pre-Congestion
Notification (PCN)
Architecture", draft-ietf-
pcn-architecture-10 (work in
progress), March 2009.
[I-D.ietf-tsvwg-admitted-realtime-dscp] Baker, F., Polk, J., and M.
Dolly, "DSCP for Capacity-
Admitted Traffic", draft-
ietf-tsvwg-admitted-
realtime-dscp-05 (work in
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progress), November 2008.
[I-D.ietf-tsvwg-ecn-tunnel] Briscoe, B., "Tunnelling of
Explicit Congestion
Notification", draft-ietf-
tsvwg-ecn-tunnel-02 (work in
progress), March 2009.
[RFC4301] Kent, S. and K. Seo,
"Security Architecture for
the Internet Protocol",
RFC 4301, December 2005.
[RFC5127] Chan, K., Babiarz, J., and
F. Baker, "Aggregation of
DiffServ Service Classes",
RFC 5127, February 2008.
Appendix A. PCN Deployment Considerations
A.1. Choice of Suitable DSCPs
The PCN Working Group chose not to define a single DSCP 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 should be seen
as being essentially 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 dependant 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 uniformly high link rates, the
appropriate DSCPs would currently be those for the Real Time Traffic
Class [RFC5127]. If the PCN domain includes lower speed links it
would also be appropriate to use the DSCPs of the other traffic
classes that [I-D.ietf-tsvwg-admitted-realtime-dscp] defines for use
with admission control, such as the three video classes CS4, CS3 and
AF4 and the Admitted Telephony Class.
A.2. Rationale for Using ECT(0) for Not Marked
The choice of which ECT codepoint to use for the Not Marked state was
based on the following considerations:
o [RFC3168] full functionality tunnel within the PCN-domain: Either
ECT is safe.
o Leakage of traffic into PCN-domain: ECT(1) is slightly less likely
to occur so might be considered safer.
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o Leakage of traffic out of PCN-domain: Either ECT is equally unsafe
(since this would incorrectly indicate the traffic was ECN-capable
outside the controlled PCN-domain).
o Incremental deployment: Either codepoint is suitable providing
that the codepoints are used consistently.
o Conceptual consistency with other schemes: ECT(0) is conceptually
consistent with [RFC3168].
Overall this seemed to suggest ECT(0) was most appropriate to use.
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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