Congestion and Pre Congestion T. Moncaster
Internet-Draft BT
Intended status: Standards Track B. Briscoe
Expires: November 20, 2009 BT & UCL
M. Menth
University of Wuerzburg
May 19, 2009
Baseline Encoding and Transport of Pre-Congestion Information
draft-ietf-pcn-baseline-encoding-04
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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 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. The baseline encoding described here provides for only two
PCN encoding states, Not-marked and PCN-marked.
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3
2. Requirements notation . . . . . . . . . . . . . . . . . . . . 5
3. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 5
4. Encoding two PCN States in IP . . . . . . . . . . . . . . . . 6
4.1. Valid and Invalid Codepoint Transitions . . . . . . . . . 6
4.2. Rationale for Encoding . . . . . . . . . . . . . . . . . . 7
4.3. PCN-Compatible Diffserv Codepoints . . . . . . . . . . . . 8
4.3.1. Co-existence of PCN and not-PCN traffic . . . . . . . 8
5. Rules for Experimental Encoding Schemes . . . . . . . . . . . 8
6. Backwards Compatibility . . . . . . . . . . . . . . . . . . . 9
7. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 9
8. Security Considerations . . . . . . . . . . . . . . . . . . . 9
9. Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . 10
10. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 10
11. Comments Solicited . . . . . . . . . . . . . . . . . . . . . . 10
12. References . . . . . . . . . . . . . . . . . . . . . . . . . . 10
12.1. Normative References . . . . . . . . . . . . . . . . . . . 10
12.2. Informative References . . . . . . . . . . . . . . . . . . 11
Appendix A. PCN Deployment Considerations (Informational) . . . . 11
A.1. Choice of Suitable DSCPs . . . . . . . . . . . . . . . . . 11
A.2. Rationale for Using ECT(0) for Not-marked . . . . . . . . 12
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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 Explicit Congestion Notification
(ECN) field [RFC3168]. 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. Rules
for such extensions are given in Section 5.
Changes from previous drafts (to be removed by the RFC Editor):
From -03 to -04:
Major WGLC comments addressed:
* Added Section 4.3.1 to clarify why we need the not-PCN
codepoint.
* Stated that the PCN WG will maintain a list of PCN-compatible
DSCPs. This should help avoid inter-operability issues.
Also addressed a number of WGLC nits.
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.
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* 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.
Extensive other minor changes throughout.
From -01 to -02:
Removed Appendix A and replaced with reference to [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.
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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
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
[PCN-metering-marking]. 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.
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o not-PCN - packets that are not PCN-enabled.
In addition, the document uses the terminology defined in
[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
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
[Voice-Admit]. Guidelines for mixing traffic-types within a PCN-
domain are given in [PCN-metering-marking].
o Any packet that is not-PCN but which shares the same Diffserv
codepoint as PCN-enabled traffic MUST have the ECN field of its
outermost IP header 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 sharing a PCN-compatible Diffserv
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codepoint.
The only valid codepoint transitions within a PCN-interior-node are
from NM to PM (which should occur if either meter indicates a need to
PCN-mark a packet [PCN-metering-marking]) and from EXP to PM (which
MAY be allowed by some future experimental extensions). The
following table gives the full set of valid and invalid codepoint
transitions.
+-------------------------------------------------+
| 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. [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 because 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
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codepoint(s) such that non-PCN traffic can still be distinguished
from PCN traffic. There are a number of factors that were considered
before choosing to set 10 as the NM state instead of 01. 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
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 and the
PCN working group will compile a list of such DSCPs. To be clear,
any packets with such a DSCP will be PCN enabled only if they are
within a PCN-domain and have their ECN field set to indicate a
codepoint other than not-PCN.
Enabling PCN marking behaviour for a specific DSCP disables any other
marking behaviour (e.g. enabling PCN disables the default ECN marking
behaviour introduced in [RFC3168]). All traffic metering and marking
behaviours are discussed in [PCN-metering-marking]. This ensures
compliance with the BCP guidance set out in [RFC4774].
4.3.1. Co-existence of PCN and not-PCN traffic
The scarcity of pool 1 DSCPs coupled with the fact that PCN is
envisaged as a marking behaviour that could be applied to a number of
different DSCPs makes it essential that we provide a not-PCN state.
As stated above (and expanded in Appendix A.1) the aim is for PCN to
re-use existing DSCPs. Because PCN re-defines the meaning of the ECN
field for such DSCPs it is important to allow an operator to still
use the DSCP for traffic that isn't PCN-enabled. This is achieved by
providing a not-PCN state within the encoding scheme.
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 other 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.
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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-
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 unless mechanisms are put in place to tunnel such traffic
across the PCN-domain. Standard IP-in-IP or IPsec tunnels will
always copy the CE codepoint from teh outer header into the inner
header in decapsulation (unless the inner packet is not-ECT). If an
operator it is essential that any operator wishing to allow ECN to
exist end-to-end ensures there are no tunnel end-points within the
PCN-domain.
7. IANA Considerations
This document makes no direct request to IANA. However this document
allows for a set of Diffserv Codepoints to be assigned different ECN
semantics within a controlled domain as described in [RFC4774]. A
list of such DSCPs will be maintained by the PCN working group.
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 how specific packets are authorised to
be PCN-marked, which will be described in separate documents on PCN-
boundary-node behaviour.
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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.
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
[PCN-metering-marking] Eardley, P., "Metering and marking behaviour
of PCN-nodes",
draft-ietf-pcn-marking-behaviour-03 (work in
progress), May 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,
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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
[ECN-tunnel] Briscoe, B., "Tunnelling of Explicit
Congestion Notification",
draft-ietf-tsvwg-ecn-tunnel-02 (work in
progress), March 2009.
[PCN-architecture] Eardley, P., "Pre-Congestion Notification
(PCN) Architecture",
draft-ietf-pcn-architecture-11 (work in
progress), April 2009.
[RFC3540] Spring, N., Wetherall, D., and D. Ely,
"Robust Explicit Congestion Notification
(ECN) Signaling with Nonces", RFC 3540,
June 2003.
[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.
[Voice-Admit] Baker, F., Polk, J., and M. Dolly, "DSCP for
Capacity-Admitted Traffic",
draft-ietf-tsvwg-admitted-realtime-dscp-05
(work in progress), November 2008.
Appendix A. PCN Deployment Considerations (Informational)
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
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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 [Voice-Admit] defines for use with admission control,
such as the three video classes CS4, CS3 and AF4 and the Admitted
Telephony Class. The PCN working group will maintain a list of PCN-
compatible Diffserv Codepoints.
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: because of the lack of take-up
of the ECN nonce [RFC3540], leakage of ECT(1) is less likely to
occur so might be considered safer.
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
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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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