Congestion and Pre Congestion T. Moncaster
Internet-Draft BT
Intended status: Standards Track B. Briscoe
Expires: November 16, 2008 BT & UCL
M. Menth
University of Wuerzburg
May 15, 2008
Encoding and Transport of (Pre-)Congestion Information from within a
DiffServ Domain to the Egress
draft-moncaster-pcn-baseline-encoding-00
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Abstract
Pre-congestion notification (PCN) is a mechanism designed 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
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such marks are to be encoded into the IP header. The baseline
encoding described here provides for two PCN encoding states.
Status
This memo is posted as an Internet-Draft with an intent to eventually
progress to standards track.
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3
2. Requirements notation . . . . . . . . . . . . . . . . . . . . 3
3. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 3
4. Encoding Two PCN States in IP . . . . . . . . . . . . . . . . 3
4.1. Rationale for Encoding . . . . . . . . . . . . . . . . . . 4
4.2. PCN-Enabled DiffServ Codepoints . . . . . . . . . . . . . 5
4.3. Valid and Invalid Encoding Transitions at a PCN Node . . . 5
5. Backwards Compatability . . . . . . . . . . . . . . . . . . . 5
6. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 6
7. Security Considerations . . . . . . . . . . . . . . . . . . . 6
8. Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . 6
9. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 7
10. Comments Solicited . . . . . . . . . . . . . . . . . . . . . . 7
11. References . . . . . . . . . . . . . . . . . . . . . . . . . . 7
11.1. Normative References . . . . . . . . . . . . . . . . . . . 7
11.2. Informative References . . . . . . . . . . . . . . . . . . 7
Appendix A. Tunnelling Constraints . . . . . . . . . . . . . . . 8
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 9
Intellectual Property and Copyright Statements . . . . . . . . . . 10
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1. Introduction
Pre-congestion notification is a mechanism designed to help protect
the Quality of Service of inelastic flows. It does this by measuring
the pre-congestion level on the path used by that flow. The pre-
congestion level at each node is indicated by marking packets when
traffic load is approaching or has exceeded a threshold below the
physical link rate. [PCN-arch] describes how PCN marking can be used
to assure the quality of service of inelastic flows within a single
DiffServ domain. This document specifies how those PCN marks are
encoded into the IP header. It also describes how packets are
identified as belonging to a PCN flow. The baseline encoding
described here provides for two PCN encoding states.
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-Capable codepoints - collective term for all the NM and PM
codepoints.
o PCN enabled Diffserv codepoint - a Diffserv codepoint for which
PCN has been enabled on a particular machine.
In addition the document uses the terminology described in
[PCN-arch].
4. Encoding Two PCN States in IP
The PCN encoding states are defined using a combination of the DSCP
field and ECN field in 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). The following table defines how to encode these states in IP:
+--------+--------------+-------------+-------------+---------+
| DSCP | Not-ECT (00) | ECT(0) (10) | ECT(1) (01) | CE (11) |
+--------+--------------+-------------+-------------+---------+
| DSCP n | Not-PCN | NM | NM | PM |
+--------+--------------+-------------+-------------+---------+
Where DSCP n is a PCN-enabled DiffServ codepoint (see Section 4.2)
Table 1: Encoding PCN in IP
The following rules apply to all PCN traffic:
o PCN traffic MUST be marked with a DiffServ codepoint that
indicates PCN is enabled. 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 capable (Not-PCN) but which shares the
same DiffServ codepoint as PCN capable traffic MUST have the ECN
field set to 00.
o Any packet that is PCN capable and Not Marked (NM) MUST have the
ECN field set to one of the two ECT codepoints 10 or 01.
o Any packet that is PCN capable and has been PCN-marked by an
interior node MUST have the ECN field set to 11.
4.1. Rationale for Encoding
The exact choice of encoding was dictated by the constraints imposed
by existing IETF RFCs. 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 more
detail. The main effect of this constraint was that any PCN marking
has to use the ECN field set to 11 (CE codepoint). An additional
constraint was 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.
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The encoding scheme 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.
4.2. PCN-Enabled 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-Enabled 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 a codepoint other than Not-PCN.
Enabling PCN marking behaviour disables any other marking behaviour
(e.g. enabling PCN also disables the default ECN marking behaviour
introduced in [RFC3168]). The scheduling behaviour used for a packet
does not change whether PCN is enabled for a DSCP or not and whatever
the setting of the ECN field.
4.3. Valid and Invalid Encoding Transitions at a PCN Node
PCN edge node behaviour compliant with the PCN baseline encoding:
o Any packets with the ECN field already marked as CE or ECT
arriving at a PCN ingress node SHOULD be dropped or alternatively
MAY be tunnelled through the PCN region. They MUST NOT be
admitted to the PCN region directly.
o On leaving the PCN region the ECN bits MUST be set to 00 (Not
ECT).
PCN interior node behaviour compliant with the PCN baseline encoding:
o PCN Interior nodes MUST NOT change Not-PCN to another codepoint
and they MUST NOT change a PCN-Capable codepoint to Not-PCN.
o PCN interior nodes that are in a pre-congestion state above the
configured level MUST set the PM codepoint by changing the ECN
bits to 11.
o PM MUST NOT be changed to NM.
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
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techniques to allow the co-existence of default ECN and alternative
ECN semantics. The alternative semantics specified here are
compliant with this BCP:
o they use a DSCP to allow routers to distinguish that traffic uses
the alternate ECN semantics;
o these semantics are defined for use within a controlled domain;
o ECN marked traffic is blocked from entering the PCN domain
directly (though it might be tunnelled through the domain).
6. IANA Considerations
This document makes no request to IANA. It does however suggest a
change to the default ([RFC3168]) behaviour for the ECN field for the
Voice-Admit [voice-admit] DSCP.
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. The PCN working group has initially been chartered to
only consider a PCN region 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 regions and use PCN edge-nodes
back-to back at borders. Then any one domain's security against its
neighbours would be described as part of the edge-node behaviour
document as above. There is only one proposal on the table to extend
PCN across multiple domains without PCN edge nodes back-to-back at
borders [re-PCN]. it is believed that the encoding described here
would not be incompatible 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-
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existence of non-PCN traffic within the same DSCP. 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, Michael Menth, Philip
Eardley, Bob Briscoe and others. Full details of the alternative
schemes that were considered for adoption can be found in the sister
document [pcn-enc-compare].
10. Comments Solicited
Comments and questions are encouraged and very welcome. They can be
addressed to the IETF Transport Area working group mailing list
<tsvwg@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.
11.2. Informative References
[PCN-arch]
Eardley, P., "Pre-Congestion Notification Architecture",
draft-ietf-pcn-architecture-03 (work in progress),
February 2008.
[PCN-charter]
"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.
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[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.
[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",
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
to exist. The limited functionality mode sets the outer header to
Not ECT, regardless of the value of the inner header. 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 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.
This was because the security community had decided the available
bandwidth of the covert channel offered by ECN was too low to be a
significant threat. 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.
There is a further issue involving tunnelling. In RFC3168, 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 field that have already been marked may have that mark
concealed further into the region. This is undesirable for many PCN
schemes and thus standard IP in IP tunnels SHOULD NOT be used within
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a PCN region.
Authors' Addresses
Toby Moncaster
BT
B54/70, Adastral Park
Martlesham Heath
Ipswich IP5 3RE
UK
Phone: +44 1473 648734
Email: toby.moncaster@bt.com
URI: http://www.cs.ucl.ac.uk/staff/B.Briscoe/
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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