Congestion and Pre Congestion T. Moncaster
Internet-Draft BT
Intended status: Standards Track B. Briscoe
Expires: January 12, 2009 BT & UCL
M. Menth
University of Wuerzburg
July 11, 2008
Baseline Encoding and Transport of Pre-Congestion Information
draft-moncaster-pcn-baseline-encoding-02
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Copyright (C) The IETF Trust (2008).
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
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such marks are to be encoded into the IP header. The baseline
encoding described here provides for only two PCN encoding states.
Other documents describe extended encoding schemes that allow for
three 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 . . . . . . . . . . . . . . . . . . . . . . . . . 4
4. Encoding two PCN States in IP . . . . . . . . . . . . . . . . 4
4.1. Rationale for Encoding . . . . . . . . . . . . . . . . . . 5
4.2. PCN-Enabled DiffServ Codepoints . . . . . . . . . . . . . 5
4.2.1. Implications of re-using a DiffServ Codepoint . . . . 6
4.3. Valid and Invalid Encoding Transitions at a PCN Node . . . 6
5. Backwards Compatability . . . . . . . . . . . . . . . . . . . 7
6. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 7
7. Security Considerations . . . . . . . . . . . . . . . . . . . 7
8. Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . 8
9. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 8
10. Comments Solicited . . . . . . . . . . . . . . . . . . . . . . 8
11. References . . . . . . . . . . . . . . . . . . . . . . . . . . 8
11.1. Normative References . . . . . . . . . . . . . . . . . . . 8
11.2. Informative References . . . . . . . . . . . . . . . . . . 9
Appendix A. Tunnelling Constraints . . . . . . . . . . . . . . . 9
Appendix B. Deployment Scenarios for PCN Using Baseline
Encoding . . . . . . . . . . . . . . . . . . . . . . 10
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 11
Intellectual Property and Copyright Statements . . . . . . . . . . 13
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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 the PCN-domain. Two algorithms exist for that purpose.
Excess traffic marking marks all PCN packets exceeding a certain
reference rate on a link while threshold marking marks all PCN
packets on a link when the PCN traffic rate exceeds the reference
rate. 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 three. The baseline encoding described here only
provides for two PCN encoding states. An extended encoding described
in [PCN-3-enc-state] provides for three PCN encoding states.
Changes from previous drafts (to be removed by the RFC Editor)
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.
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].
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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 defined in [PCN-arch].
4. Encoding two PCN States in IP
The PCN encoding states are defined using the Type of Service field
of the IP header which is a combination of the DSCP field and ECN
field. 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). 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 PCN-enabled DiffServ Codepoint.
That is a DiffServ codepoint that indicates that 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].
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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 belongs to a PCN capable flow MUST have the ECN
field set to one of the two ECT codepoints 10 or 01 at the PCN-
ingress-node.
o Any packet that is PCN capable and has been PCN-marked by a PCN-
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, 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.
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 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 used for a packet
does not change whether PCN is enabled for a DSCP or not and whatever
the setting of the ECN field.
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4.2.1. Implications of re-using a DiffServ Codepoint
[RFC4774] requires that packets for which alternate ECN semantics
(PCN semantics) are used are clearly distinguished from packets to
which the default ECN semantics [RFC3168] apply. One means of doing
this is using a DSCP to indicate that the ECN field is to be
interpreted in a different manner. We have chosen to use this
approach for PCN. Non-PCN-enabled forwarding nodes treat packets
with a PCN-enabled DSCP like ECN traffic if appropriate ECN
codepoints are set in the IP header. This has several consequences.
o Care must be taken to ensure that forwarding nodes do not
interpret PCN encodings as ECN encodings, and that no harm is done
if this were to happen. To that end, appropriate marking and re-
marking is performed at the ingress and the egress of a PCN-
domain.
o The re-used DSCP should be able to serve its original purpose
which was not PCN support. This is achieved by marking the
packets of such flows with a not-PCN codepoint.
o The scheduling behaviour is coupled with the DSCP only.
Therefore, the same scheduling and buffer management rules are
applied for non-PCN-capable and PCN-capable traffic using the same
PCN-enabled DSCP.
o Once the ECN field of a packet is used for PCN encoding, it has
lost its previous information unless this information is tunnelled
through the PCN domain. Therefore, the baseline PCN encoding
disables ECN for PCN-enabled DSCPs. [PCN-3-enc-state] provides
end-to-end ECN support where this is needed.
4.3. Valid and Invalid Encoding Transitions at a PCN Node
PCN-boundary-node behaviour compliant with the PCN baseline encoding:
o Any packet with the ECN field already marked as CE or ECT arriving
at a PCN-ingress-node SHOULD be dropped or downgraded to a lower
class of service. Alternatively it MAY be tunnelled through the
PCN-domain. It MUST NOT be admitted to the PCN-domain directly.
o On leaving the PCN-domain the ECN bits of every PCN-packet 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.
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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 of NM marked packets to 11.
o The PM codepoint 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
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 PCN-domain).
o All traffic leaving the controlled domain is re-marked as not-ECT.
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-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
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against its neighbours would 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 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.
[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. The limited functionality mode sets the outer header to not-
ECT, regardless of the value of the inner header, in other words
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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 tunnels in respect to the ECN
field.
Appendix B. 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
admission-stop (AS) marks. The admission-control algorithm is
based on "admissible-rate overload".
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
excess-traffic (ET) marks. The flow termination algorithm is
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based on "supportable-rate overload".
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 may be used as the only PCN metering and marking
algorithm. As a consequence, the PM marks on the packets are
interpreted as admission-stop (AS) marks. Both the admission
control and the flow termination algorithm are based on
"admissible-rate overload".
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
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
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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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