Internet Engineering Task Force M. Menth
Internet-Draft F. Lehrieder
Expires: January 8, 2009 University of Wuerzburg
July 7, 2008
Marking Converter for Excess-Marked Traffic
draft-menth-pcn-marking-converter-00
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Abstract
This document proposes an algorithm that converts packet markings of
a stream that was excess-marked based on a lower-rate into packet
markings that correspond to a stream that was excess-marked based on
a higher-rate. It may be applied in the PCN context to convert
marked admissible-rate-overload into marked supportable-rate-
overload. This allows to perform marked flow termination when
packets are excess-marked based on the admissible rate only.
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3
2. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 4
3. Algorithm for Conversion of AS-Markings into ET-Markings . . . 5
4. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 7
5. Security Considerations . . . . . . . . . . . . . . . . . . . 8
6. Conclusion . . . . . . . . . . . . . . . . . . . . . . . . . . 9
7. References . . . . . . . . . . . . . . . . . . . . . . . . . . 10
7.1. Normative References . . . . . . . . . . . . . . . . . . . 10
7.2. Informative References . . . . . . . . . . . . . . . . . . 10
7.3. Other References . . . . . . . . . . . . . . . . . . . . . 10
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 11
Intellectual Property and Copyright Statements . . . . . . . . . . 12
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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 [I-D.ietf-pcn-architecture]. This is achieved by marking
packets on interior nodes according to some metering function
implemented at each node. Links are associated with an admissible
and a supportable rate threshold (AR, SR).
o When the PCN traffic rate on a link exceeds the AR of that link,
the link is AR-pre-congested and the PCN rate above AR is AR-
overload.
o When the PCN traffic rate on a link exceeds the SR of that link,
the link is SR-pre-congested and the PCN rate above SR is SR-
overload.
Excess marking is a mechanism marking packets exceeding a certain
reference rate. If applied with AR or SR as reference rate on a link
of the PCN domain, excess marking marks the AR- or SR-overload. We
call the marks based on AR admission-stop (AS) marking and the marks
based on SR excess-traffic (ET) marking. Admission control requires
AS-marking while flow termination requires ET-marking. Having two
different markers is desirable to perform admission control and flow
termination based on direct feedback from the network, but it
increases hardware and encoding complexity.
The single-marking draft [I-D.charny-pcn-single-marking] proposes one
method to perform measured rate termination based on AR-overload. It
requires that SR=u*AR on all links within the PCN domain.
In this document we present a conversion algorithm that converts AS-
markings of a packet stream into ET-markings. Admission control can
be performed based on the original AS-markings and flow termination
can be performed based on the converted ET-markings. To that end,
any flow termination method working with SR-overload can be applied
([Menth08-PCN-Comparison], Section 7).
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 RFC 2119 [RFC2119].
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2. Terminology
Most of the terminology used in this document is defined in
[I-D.ietf-pcn-architecture]. The following additional terms are
defined.
o Admissible rate (AR) - PCN lower rate
o Supportable rate (SR) - PCN upper rate
o AR-overload - PCN traffic rate above AR
o SR-overload - PCN traffic rate above SR
o Excess marking - metering and marking mechanism marking all
packets exceeding a reference rate (excess rate marking in
[I-D.eardley-pcn-marking-behaviour])
o Admission-stop (AS) marking - marking based on AR as reference
rate
o Excess-traffic (ET) marking - marking based on SR as reference
rate
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3. Algorithm for Conversion of AS-Markings into ET-Markings
The conversion algorithm is applied by the egress node on an ingress-
egress aggregate basis. It is called for each packet arrival and
either converts an existing AS-mark into an ET-mark or clears it.
The algorithm is based on a token bucket (TB) with size S, fill state
F, and threshold T. It differs from conventional TB implementations
as it does not have a constant fill rate R. Its operation is
explained in Algorithm 1.
Algorithm 1:
Input: token bucket parameters S, F, and T, packet size B and marking M
1: if (M == unmarked) then
2: F = min(S, F + (u - 1) * B);
3: else if (F >= T) then
4: F = F - B;
5: M = unmarked;
6: else
7: M = ET;
8: end if
The number of tokens in the bucket F indicates how many AS-marked
bytes can be re-marked to unmarked. Initially, the token bucket
should be filled. For each non-AS-marked byte, the fill state F is
incremented by u-1 tokens (cf. line 1-2). When a packet arrives AS-
marked and if the fill state F is larger than a certain threshold T,
the packet is re-marked to unmarked and the fill state of the TB is
reduced by the packet size B. Otherwise, the packet remains marked
which is then interpreted as ET-marking (cf. line 3-8).
The threshold T is used to achieve packet-size independent marking
conversion and should be set to the maximum transfer unit. A
sufficiently large TB size S is needed to tolerate short-term
variations of packet markings, i.e. a burst of S AS-marked bytes
should not be ET-marked. However, this tolerance also delays initial
re-marking. Further experimentation and performance evaluation of
this approach is required.
First simulations give a proof of concept. The conversion algorithm
works well if the rate of the controlled ingress-egress-aggregate is
large enough and if it is a rather large fraction (>10%) of the
traffic rate on the bottleneck link. If this is not the case,
packets with AS-markings occur almost random which leads to a
geometrically distributed distance between packet markings within an
ingress-egress-aggregate such that very large bursts of AS-marked
packets can occur even when the PCN rate is between AR and SR on the
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bottleneck link. This leads to wrong ET-markings. As a result,
there is some chance for overtermination
([I-D.menth-pcn-performance]) when marked flow termination for
ingress-egress-aggregates is used. This, however, is not a property
of the conversion algorithm, it's rather a property of the single
marking approach and also measured rate termination suffers from this
phenomenon. Further evaluation is required to configure the
conversion algorithm appropriately and to validate flow termination
mechanisms in combination with this converter.
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4. IANA Considerations
TBD
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5. Security Considerations
TBD
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6. Conclusion
This document describes an algorithm that converts marked AR-overload
into marked SR-overload. It makes flow termination mechanisms
requiring SR-overload applicable in networks that mark AR-overload
only. This algorithm does not solve the problem that flow
termination based on AR-overload does not work well for multipath
routing ([Menth08-PCN-Comparison], Section 8.3).
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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.
7.2. Informative References
[I-D.charny-pcn-single-marking]
Charny, A., Zhang, X., Faucheur, F., and V. Liatsos, "Pre-
Congestion Notification Using Single Marking for Admission
and Termination", draft-charny-pcn-single-marking-03
(work in progress), November 2007.
[I-D.eardley-pcn-marking-behaviour]
Eardley, P., "", draft-eardley-pcn-marking-behaviour-01
(work in progress), I-D Status active, June 2008.
[I-D.ietf-pcn-architecture]
Eardley, P., "Pre-Congestion Notification Architecture",
draft-ietf-pcn-architecture-01 (work in progress),
October 2007.
[I-D.menth-pcn-performance]
Menth, M. and F. Lehrieder, "Performance Evaluation of
PCN-Based Algorithms", draft-menth-pcn-performance-02
(work in progress), February 2008.
7.3. Other References
[Menth08-PCN-Comparison]
Menth, M., Lehrieder, F., Briscoe, B., Eardley, P.,
Moncaster, T., Babiarz, J., Chan, K., Charny, A.,
Karagiannis, G., and X. Zhang, "PCN-Based Admission
Control and Flow Termination", 2008, <http://
www3.informatik.uni-wuerzburg.de/staff/menth/Publications/
Menth08-PCN-Comparison.pdf>.
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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
Frank Lehrieder
University of Wuerzburg
Am Hubland
Wuerzburg D-97074
Germany
Phone: +49-931-888-6651
Email: lehrieder@informatik.uni-wuerzburg.de
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