Congestion and Pre Congestion T. Moncaster
Internet-Draft BT
Intended status: Standards Track B. Briscoe
Expires: December 25, 2008 BT & UCL
M. Menth
University of Wuerzburg
June 23, 2008
Baseline Encoding and Transport of Pre-Congestion Information
draft-moncaster-pcn-baseline-encoding-01
Status of this Memo
By submitting this Internet-Draft, each author represents that any
applicable patent or other IPR claims of which he or she is aware
have been or will be disclosed, and any of which he or she becomes
aware will be disclosed, in accordance with Section 6 of BCP 79.
Internet-Drafts are working documents of the Internet Engineering
Task Force (IETF), its areas, and its working groups. Note that
other groups may also distribute working documents as Internet-
Drafts.
Internet-Drafts are draft documents valid for a maximum of six months
and may be updated, replaced, or obsoleted by other documents at any
time. It is inappropriate to use Internet-Drafts as reference
material or to cite them other than as "work in progress."
The list of current Internet-Drafts can be accessed at
http://www.ietf.org/ietf/1id-abstracts.txt.
The list of Internet-Draft Shadow Directories can be accessed at
http://www.ietf.org/shadow.html.
This Internet-Draft will expire on December 25, 2008.
Copyright Notice
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
rate threshold below the physical link rate. This document specifies
Moncaster, et al. Expires December 25, 2008 [Page 1]
Internet-Draft Baseline PCN Encoding June 2008
how such marks are to be encoded into the IP header. The baseline
encoding described here provides for only two PCN encoding states.
Another document describes an extended encoding scheme that allows
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 . . . . . . . . . . . . . . . . . . . . . . . . . 3
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 . . . . 5
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 . . . . . . . . . . . . . . . . . . 8
Appendix A. Tunnelling Constraints . . . . . . . . . . . . . . . 9
Appendix B. Deployment Scenarios for PCN Using Baseline
Encoding . . . . . . . . . . . . . . . . . . . . . . 10
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 11
Intellectual Property and Copyright Statements . . . . . . . . . . 12
Moncaster, et al. Expires December 25, 2008 [Page 2]
Internet-Draft Baseline PCN Encoding June 2008
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 -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:
Moncaster, et al. Expires December 25, 2008 [Page 3]
Internet-Draft Baseline PCN Encoding June 2008
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
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.
Moncaster, et al. Expires December 25, 2008 [Page 4]
Internet-Draft Baseline PCN Encoding June 2008
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 was 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 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.
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 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.
4.2.1. 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
Moncaster, et al. Expires December 25, 2008 [Page 5]
Internet-Draft Baseline PCN Encoding June 2008
which the semantics according to [RFC3168] apply. This is done by
using a DSCP to indicate that the ECN field is to be interpreted in
the PCN context instead of the ECN context by PCN-enabled nodes.
Non-PCN-enabled forwarding nodes outside or inside the PCN domain
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 that the PCN encoding of packets is not falsely
interpreted by forwarding nodes as ECN encoding, 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 was
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 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-domain. They MUST NOT be
admitted to the PCN-domain directly.
o On leaving the PCN-domain 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
Moncaster, et al. Expires December 25, 2008 [Page 6]
Internet-Draft Baseline PCN Encoding June 2008
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 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-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 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 edge nodes back-to-
back at borders [re-PCN]. It is believed that the encoding described
Moncaster, et al. Expires December 25, 2008 [Page 7]
Internet-Draft Baseline PCN Encoding June 2008
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-
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, 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 comments on this document.
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-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.
Moncaster, et al. Expires December 25, 2008 [Page 8]
Internet-Draft Baseline PCN Encoding 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
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 apparent 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.
Moncaster, et al. Expires December 25, 2008 [Page 9]
Internet-Draft Baseline PCN Encoding June 2008
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.
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 domain. This is undesirable for many PCN
schemes and thus standard IP in IP tunnels SHOULD NOT be used within
a PCN-domain.
Appendix B. Deployment Scenarios for PCN Using Baseline Encoding
We illustrate the use of PCN baseline encoding for different PCN
deployment scenarios and explain also a case for which baseline
encoding is not applicable. {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 may be used
as the only PCN metering and marking algorithm. As a
consequence, the packet marks M 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 may be
used as the only PCN metering and marking algorithm. As a
consequence, the packet marks M are interpreted as excess-traffic
(ET) marks. The flow termination algorithm is 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 packet marks 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
Moncaster, et al. Expires December 25, 2008 [Page 10]
Internet-Draft Baseline PCN Encoding June 2008
are believed to work better with small ingress-egress aggregates.
Then two different markings are needed that cannot be recorded 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
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
Moncaster, et al. Expires December 25, 2008 [Page 11]
Internet-Draft Baseline PCN Encoding June 2008
Full Copyright Statement
Copyright (C) The IETF Trust (2008).
This document is subject to the rights, licenses and restrictions
contained in BCP 78, and except as set forth therein, the authors
retain all their rights.
This document and the information contained herein are provided on an
"AS IS" basis and THE CONTRIBUTOR, THE ORGANIZATION HE/SHE REPRESENTS
OR IS SPONSORED BY (IF ANY), THE INTERNET SOCIETY, THE IETF TRUST AND
THE INTERNET ENGINEERING TASK FORCE DISCLAIM ALL WARRANTIES, EXPRESS
OR IMPLIED, INCLUDING BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF
THE INFORMATION HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED
WARRANTIES OF MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE.
Intellectual Property
The IETF takes no position regarding the validity or scope of any
Intellectual Property Rights or other rights that might be claimed to
pertain to the implementation or use of the technology described in
this document or the extent to which any license under such rights
might or might not be available; nor does it represent that it has
made any independent effort to identify any such rights. Information
on the procedures with respect to rights in RFC documents can be
found in BCP 78 and BCP 79.
Copies of IPR disclosures made to the IETF Secretariat and any
assurances of licenses to be made available, or the result of an
attempt made to obtain a general license or permission for the use of
such proprietary rights by implementers or users of this
specification can be obtained from the IETF on-line IPR repository at
http://www.ietf.org/ipr.
The IETF invites any interested party to bring to its attention any
copyrights, patents or patent applications, or other proprietary
rights that may cover technology that may be required to implement
this standard. Please address the information to the IETF at
ietf-ipr@ietf.org.
Acknowledgments
Funding for the RFC Editor function is provided by the IETF
Administrative Support Activity (IASA). This document was produced
using xml2rfc v1.32 (of http://xml.resource.org/) from a source in
RFC-2629 XML format.
Moncaster, et al. Expires December 25, 2008 [Page 12]