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Internet Engineering Task Force M. Menth
Internet-Draft University of Wuerzburg
Expires: January 8, 2009 J. Babiarz
Nortel
T. Moncaster
BT
July 7, 2008
End-to-end Extension for PCN Encoding
draft-menth-pcn-e2e-encoding-00
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Copyright Notice
Copyright (C) The IETF Trust (2008).
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Internet-Draft End-to-end Extension for PCN Encoding July 2008
Abstract
This document proposes an encoding of PCN marks based on the ECN
field of the Voice-Admit DSCP. It has global meaning and ECN
semantics are not applied to that field. The PCN codepoints are the
same as those for packet-specific dual marking (PSDM) within a single
PCN domain, but the general concept can also be applied to other
encodings requiring only a single PCN-enabled DSCP.
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3
2. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 4
3. Encoding Support for End-to-End PCN . . . . . . . . . . . . . 5
4. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 6
5. Security Considerations . . . . . . . . . . . . . . . . . . . 7
6. Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . 8
7. References . . . . . . . . . . . . . . . . . . . . . . . . . . 9
7.1. Normative References . . . . . . . . . . . . . . . . . . . 9
7.2. Informative References . . . . . . . . . . . . . . . . . . 9
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 10
Intellectual Property and Copyright Statements . . . . . . . . . . 11
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1. Introduction
Pre-congestion notification provides information to support admission
control and flow termination at the boundary nodes of a Diffserv
region in order to protect the quality of service (QoS) of inelastic
flows [PCN-arch]. This is achieved by marking packets on interior
nodes according to some metering function implemented at each node.
Excess traffic marking marks PCN packets that exceed a certain
reference rate on a link while exhaustive marking marks all PCN
packets on a link when the PCN traffic rate exceeds a reference rate
[PCN-marking-behaviour]. These marks are monitored by the egress
nodes of the PCN domain. This idea that the use of PCN marks is
limited to a single domain is called edge-to-edge PCN.
Packet-specific dual marking (PSDM) [PCN-pcn-encoding-PSDM] proposes
an encoding of PCN marks that reuse the ECN field of PCN-enabled DSCP
to record the marks in IP packets. However, these codepoints can be
used only within PCN domains and packets with such PCN codepoints
must not leave the PCN domain.
With end-to-end PCN the entities monitoring and evaluating the PCN
marks reside in the end systems such that the notion of PCN domains
no longer exists [Menth08-PCN-Comparison]. Here, an encoding is
required that has a global scope. In this document, we propose to
use the ECN field of the Voice-Admit DSCP for that use. As a
consequence, ECN cannot be used with Voice-Admit anymore. Note that
Voice-Admit DSCP with this modified meaning can be used both for
edge-to-edge and for end-to-end PCN.
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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2. Terminology
The terminology used in this document is defined either in [PCN-arch]
or in [PCN-pcn-encoding-PSDM].
o Edge-to-edge PCN - Admission for a PCN domain is requested
explicitly for flows. When their packets enter the PCN domain,
the ingress node sets a PCN-enabled DSCP and a PCN-capable
codepoint (ECN field). The packets are metered and possibly re-
marked by the traversed node in the PCN domain. The markings are
evaluated by the egress node of that domain, and the DSCP and ECN
field of the packet is reset before packets leave the PCN domain.
o End-to-end PCN - The source node or a proxy thereof issues PCN
packets with a PCN-enabled DSCP and PCN-capable codepoint (ECN
field). Packets are metered and possibly marked by PCN nodes on
their way. They pass ingress and egress nodes without special
treatment of their DSCP or ECN field. The destination node or a
proxy thereof evaluate the PCN markings.
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3. Encoding Support for End-to-End PCN
In this section describes the changes to [PCN-pcn-encoding-PSDM] to
support end-to-end PCN.
In [PCN-pcn-encoding-PSDM] the Voice-Admit DSCP is proposed as the
preferred PCN-enabled DSCP to implement the PSDM encoding within
single domains. PCN codepoints are interpreted as such only within
PCN domains while they are interpreted as ECN codepoints outside PCN
domains. Being conform with [RFC4774] requires special treatment of
PCN packets by ingress and egress nodes of a PCN domain.
We propose that the default ECN semantics [RFC3168] do not apply for
the Voice-Admit DSCP [voice-admit] , but alternate PCN semantics are
applied which are end-to-end PCN. This requires an update to
[voice-admit].
Then, the ECN field can be used for PCN purposes in the general
Internet. One of the benefits is that end-to-end PCN can be
supported by such a codepoint.
This extension of the scope of Voice-Admit DSCP for PCN has no impact
on the treatment of the PCN encoding for edge-to-edge PCN wich is
described in [PCN-pcn-encoding-PSDM]. o PCN packets have a PCN-
capable codepoint only within a PCN domain. o Ingress and egress
nodes need to perform special treatment of PCN packets of edge-to-
edge controlled PCN flows.
In contrast, packets from end-to-end PCN controlled flows are not
dropped by the ingress node of a PCN domain and not re-marked by the
egress node of a PCN domain. Special treatment of ECN-capable PCN
flows is not needed because PCN flows are not ECN capable.
PCN ingress and egress nodes can differentiate packets from end-to-
end and edge-to-edge PCN flows because edge-to-edge PCN flows
explicitly request admission for the PCN domain such that PCN ingress
and egress nodes keep per-flow states and know whether an incoming
packet is edge-to-edge or end-to-end PCN controlled.
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4. IANA Considerations
This document makes no request to IANA. It does however request a
change to [voice-admit], requesting that a statement to be added that
the default behaviour for the ECN semantics [RFC3168] does not apply
for the Voice-Admit [voice-admit] DSCP.
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5. Security Considerations
TBD
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6. Conclusions
The proposed encoding allows that PCN codepoint can be used in the
general Internet. This opens the possibility for end-to-end PCN.
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7. References
7.1. Normative References
[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.
7.2. Informative References
[PCN-arch]
Eardley, P., "Pre-Congestion Notification Architecture",
draft-ietf-pcn-architecture-03 (work in progress),
February 2008.
[PCN-marking-behaviour]
Eardley, P., "Marking behaviour of PCN-nodes",
draft-eardley-pcn-marking-behaviour-01 (work in progress),
June 2008.
[PCN-pcn-encoding-PSDM]
Menth, M., Babiarz, J., Moncaster, T., and B. Briscoe,
"PCN Encoding for Packet-Specific Dual Marking (PSDM)",
July 2008.
[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.
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Authors' Addresses
Michael Menth
University of Wuerzburg
Am Hubland
Wuerzburg D-97074
Germany
Phone: +49-931-888-6644
Email: menth@informatik.uni-wuerzburg.de
Jozef Z. Babiarz
Nortel
3500 Carling Avenue
Ottawa, Ont. K2H 8E9
Canada
Phone: +1-613-763-6098
Email: babiarz@nortel.com
Toby Moncaster
BT
B54/70, Adastral Park
Ipswich IP5 3RE
UK
Phone: +44 1473 648734
Email: toby.moncaster@bt.com
Menth, et al. Expires January 8, 2009 [Page 10]
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