Congestion and Pre Congestion T. Moncaster
Internet-Draft B. Briscoe
Intended status: Standards Track BT
Expires: February 21, 2010 M. Menth
University of Wuerzburg
August 20, 2009
Baseline Encoding and Transport of Pre-Congestion Information
draft-ietf-pcn-baseline-encoding-05
Status of This Memo
This Internet-Draft is submitted to IETF in full conformance with the
provisions of BCP 78 and BCP 79. This document may contain material
from IETF Documents or IETF Contributions published or made publicly
available before November 10, 2008. The person(s) controlling the
copyright in some of this material may not have granted the IETF
Trust the right to allow modifications of such material outside the
IETF Standards Process. Without obtaining an adequate license from
the person(s) controlling the copyright in such materials, this
document may not be modified outside the IETF Standards Process, and
derivative works of it may not be created outside the IETF Standards
Process, except to format it for publication as an RFC or to
translate it into languages other than English.
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 February 21, 2010.
Copyright Notice
Copyright (c) 2009 IETF Trust and the persons identified as the
document authors. All rights reserved.
Moncaster, et al. Expires February 21, 2010 [Page 1]
Internet-Draft Baseline PCN Encoding August 2009
This document is subject to BCP 78 and the IETF Trust's Legal
Provisions Relating to IETF Documents in effect on the date of
publication of this document (http://trustee.ietf.org/license-info).
Please review these documents carefully, as they describe your rights
and restrictions with respect to this document.
Abstract
The objective of Pre-Congestion Notification (PCN) is to protect the
quality of service (QoS) of inelastic flows within a Diffserv domain.
The overall rate of the PCN-traffic is metered on every link in the
PCN-domain, and PCN-packets are appropriately marked when certain
configured rates are exceeded. The level of marking allows the
boundary nodes to make decisions about whether to admit or block a
new flow request, and (in abnormal circumstances) whether to
terminate some of the existing flows, thereby protecting the QoS of
previously admitted flows. This document specifies how such marks
are to be encoded into the IP header by re-using the Explicit
Congestion Notification (ECN) codepoints within this controlled
domain. The baseline encoding described here provides for only two
PCN encoding states, Not-marked and PCN-marked.
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3
2. Requirements notation . . . . . . . . . . . . . . . . . . . . 5
3. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 6
4. Encoding two PCN States in IP . . . . . . . . . . . . . . . . 6
4.1. Valid and Invalid Codepoint Transitions . . . . . . . . . 7
4.2. Rationale for Encoding . . . . . . . . . . . . . . . . . . 8
4.3. PCN-Compatible Diffserv Codepoints . . . . . . . . . . . . 8
4.3.1. Co-existence of PCN and not-PCN traffic . . . . . . . 9
5. Rules for Experimental Encoding Schemes . . . . . . . . . . . 9
6. Backwards Compatibility . . . . . . . . . . . . . . . . . . . 9
7. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 10
8. Security Considerations . . . . . . . . . . . . . . . . . . . 10
9. Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . 10
10. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 10
11. Comments Solicited . . . . . . . . . . . . . . . . . . . . . . 11
12. References . . . . . . . . . . . . . . . . . . . . . . . . . . 11
12.1. Normative References . . . . . . . . . . . . . . . . . . . 11
12.2. Informative References . . . . . . . . . . . . . . . . . . 11
Appendix A. PCN Deployment Considerations (Informational) . . . . 12
A.1. Choice of Suitable DSCPs . . . . . . . . . . . . . . . . . 12
A.2. Rationale for Using ECT(0) for Not-marked . . . . . . . . 13
Moncaster, et al. Expires February 21, 2010 [Page 2]
Internet-Draft Baseline PCN Encoding August 2009
1. Introduction
The objective of Pre-Congestion Notification (PCN) [RFC5559] is to
protect the quality of service (QoS) of inelastic flows within a
Diffserv domain, in a simple, scalable and robust fashion. The
overall rate of the PCN-traffic is metered on every link in the PCN-
domain, and PCN-packets are appropriately marked when certain
configured rates are exceeded. These configured rates are below the
rate of the link thus providing notification before any congestion
occurs (hence "pre-congestion notification"). The level of marking
allows the boundary nodes to make decisions about whether to admit or
block a new flow request, and (in abnormal circumstances) whether to
terminate some of the existing flows, thereby protecting the QoS of
previously admitted flows.
This document specifies how these PCN marks are encoded into the IP
header by re-using the bits of the Explicit Congestion Notification
(ECN) field [RFC3168]. It also describes how packets are identified
as belonging to a PCN flow. Some deployment models require two PCN
encoding states, others require more. The baseline encoding
described here only provides for two PCN encoding states. However
the encoding can be easily extended to provide more states. Rules
for such extensions are given in Section 5.
Changes from previous drafts (to be removed by the RFC Editor):
From -04 to -05:
Clarified throughout that the PCN WG is not requesting a specific
DSCP for PCN. Rather we are recommending a set of DSCPs that
might be suitable. Appendix A.1 has been re-written to reflect
this. References to maintaining a list of PCN-compatible DSCPs
have also been removed.
Last sentence of Section 6 altered.
Several spelling corrections.
References updated throughout.
From -03 to -04:
Major WGLC comments addressed:
* Added Section 4.3.1 to clarify why we need the not-PCN
codepoint.
Moncaster, et al. Expires February 21, 2010 [Page 3]
Internet-Draft Baseline PCN Encoding August 2009
* Stated that the PCN WG will maintain a list of PCN-compatible
DSCPs. This should help avoid inter-operability issues.
Also addressed a number of WGLC nits.
From -02 to -03:
Extensive changes to address comments made by Gorry Fairhurst
including:
* Abstract re-written.
* Clarified throughout that this re-uses the ECN bits in the IP
header.
* Re-arranged order of terminology section for clarity.
* Table 2 replaced with new table and text.
* Security considerations re-written.
* Appendixes re-written to improve clarity.
* Numerous minor nits and language changes throughout.
Extensive other minor changes throughout.
From -01 to -02:
Removed Appendix A and replaced with reference to
[I-D.ietf-tsvwg-ecn-tunnel]
Moved Appendix B into main body of text.
Changed Appendix C to give deployment advice.
Minor changes throughout including checking consistency of
capitalisation of defined terms.
Clarified that LU was deliberately excluded from encoding.
From -00 to -01:
Added section on restrictions for extension encoding schemes.
Moncaster, et al. Expires February 21, 2010 [Page 4]
Internet-Draft Baseline PCN Encoding August 2009
Included table in Appendix showing encoding transitions at
different PCN nodes.
Checked for consistency of terminology.
Minor language changes for clarity.
Changes from previous filename
Filename changed from draft-moncaster-pcn-baseline-encoding.
Terminology changed for clarity (PCN-compatible DSCP and PCN-
enabled packet).
Minor changes throughout.
Modified meaning of ECT(1) state to EXP.
Moved text relevant to behaviour of nodes into appendix for later
transfer to new document on edge behaviours.
From draft-moncaster -01 to -02:
Minor changes throughout including tightening up language to
remain consistent with the PCN Architecture terminology.
From draft-moncaster -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].
Moncaster, et al. Expires February 21, 2010 [Page 5]
Internet-Draft Baseline PCN Encoding August 2009
3. Terminology
The following terms are used in this document:
o PCN-compatible Diffserv codepoint - a Diffserv codepoint for which
the ECN field is used to carry PCN markings rather than [RFC3168]
markings.
o PCN-marked - codepoint indicating packets that have been marked at
a PCN-interior-node using some PCN marking behaviour
[I-D.ietf-pcn-marking-behaviour]. Abbreviated to PM.
o Not-marked - codepoint indicating packets that are PCN-capable,
but are not PCN-marked. Abbreviated to NM.
o PCN-enabled codepoints - collective term for all NM and PM
codepoints. By definition, packets carrying such codepoints are
PCN-packets.
o not-PCN - packets that are not PCN-enabled.
In addition, the document uses the terminology defined in [RFC5559].
4. Encoding two PCN States in IP
The PCN encoding states are defined using a combination of the DSCP
and ECN fields within 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). Given the scarcity of codepoints within the IP header the
baseline encoding leaves one codepoint free for experimental use.
The following table defines how to encode these states in IP:
+---------------+-------------+-------------+-------------+---------+
| ECN codepoint | Not-ECT | ECT(0) (10) | ECT(1) (01) | CE (11) |
| | (00) | | | |
+---------------+-------------+-------------+-------------+---------+
| DSCP n | not-PCN | NM | EXP | PM |
+---------------+-------------+-------------+-------------+---------+
Where DSCP n is a PCN-compatible Diffserv codepoint (see Section 4.3)
and EXP means available for Experimental use. N.B. we deliberately
reserve this codepoint for experimental use only (and not local use)
to prevent future compatibility issues.
Table 1: Encoding PCN in IP
Moncaster, et al. Expires February 21, 2010 [Page 6]
Internet-Draft Baseline PCN Encoding August 2009
The following rules apply to all PCN traffic:
o PCN-traffic MUST be marked with a PCN-compatible Diffserv
Codepoint. To conserve DSCPs, Diffserv Codepoints SHOULD be
chosen that are already defined for use with admission controlled
traffic. Appendix A.1 gives guidance to implementiors on suitable
DSCPs. Guidelines for mixing traffic-types within a PCN-domain
are given in [I-D.ietf-pcn-marking-behaviour].
o Any packet that is not-PCN but which shares the same Diffserv
codepoint as PCN-enabled traffic MUST have the ECN field of its
outermost IP header equal to 00.
4.1. Valid and Invalid Codepoint Transitions
A PCN-ingress-node MUST set the Not-marked (10) codepoint on any
arriving packet that belongs to a PCN-flow. It MUST set the not-PCN
(00) codepoint on all other packets sharing a PCN-compatible Diffserv
codepoint.
The only valid codepoint transitions within a PCN-interior-node are
from NM to PM (which should occur if either meter indicates a need to
PCN-mark a packet [I-D.ietf-pcn-marking-behaviour]) and from EXP to
PM (which MAY be allowed by some future experimental extensions).
The following table gives the full set of valid and invalid codepoint
transitions.
+-------------------------------------------------+
| Codepoint Out |
+--------------+-------------+-----------+-----------+-----------+
| Codepoint in | not-PCN(00) | NM(10) | EXP(01) | PM(11) |
+--------------+-------------+-----------+-----------+-----------+
| not-PCN(00) | Valid | Not valid | Not valid | Not valid |
+--------------+-------------+-----------+-----------+-----------+
| NM(10) | Not valid | Valid | Not valid | Valid |
+--------------+-------------+-----------+-----------+-----------+
| EXP(01)* | Not valid | Not valid | Valid | Valid* |
+--------------+-------------+-----------+-----------+-----------+
| PM(11) | Not valid | Not valid | Not valid | Valid |
+--------------+-------------+-----------+-----------+-----------+
* This SHOULD cause an alarm to be raised at a higher layer. The
packet MUST be treated as if it carried the NM codepoint.
Table 2: Valid and Invalid Codepoint Transitions for
PCN-packets at PCN-interior-nodes
A PCN-egress-node SHOULD set the not-PCN (00) codepoint on all
packets it forwards out of the PCN-domain. The only exception to
Moncaster, et al. Expires February 21, 2010 [Page 7]
Internet-Draft Baseline PCN Encoding August 2009
this is if the PCN-egress-node is certain that revealing other
codepoints outside the PCN-domain won't contravene the guidance given
in [RFC4774].
4.2. Rationale for Encoding
The exact choice of encoding was dictated by the constraints imposed
by existing IETF RFCs, in particular [RFC3168], [RFC4301] and
[RFC4774]. 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. [I-D.ietf-tsvwg-ecn-tunnel] explains this in
more detail. The main effect of this constraint is that any PCN
marking has to carry the 11 codepoint in the ECN field since this is
the only codepoint that is guaranteed to be copied down into the
inner header upon decapsulation. An additional constraint is the
need to minimise the use of Diffserv codepoints because there is a
limited supply of standards track codepoints remaining. Section 4.3
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. There are a number of factors that
were considered before choosing to set 10 as the NM state instead of
01. These included similarity to ECN, presence of tunnels within the
domain, leakage into and out of PCN-domain and incremental deployment
(see Appendix A.2).
The encoding scheme above seems to meet all these constraints and
ends up looking very similar to ECN. This is perhaps not surprising
given the similarity in architectural intent between PCN and ECN.
4.3. PCN-Compatible Diffserv Codepoints
Equipment complying with the baseline PCN encoding MUST allow PCN to
be enabled for certain Diffserv codepoints. This document defines
the term "PCN-compatible Diffserv codepoint" for such a DSCP. To be
clear, any packets with such a DSCP will be PCN enabled only if they
are within a PCN-domain and have their ECN field set to indicate a
codepoint other than not-PCN.
Enabling PCN marking behaviour for a specific DSCP disables any other
marking behaviour (e.g. enabling PCN disables the default ECN marking
behaviour introduced in [RFC3168]). All traffic metering and marking
behaviours are discussed in [I-D.ietf-pcn-marking-behaviour]. This
ensures compliance with the BCP guidance set out in [RFC4774].
The PCN Working Group has chosen not to define a single DSCP for use
with PCN for several reasons. Firstly the PCN mechanism is
applicable to a variety of different traffic classes. Secondly
standards track DSCPs are in increasingly short supply. Thirdly PCN
Moncaster, et al. Expires February 21, 2010 [Page 8]
Internet-Draft Baseline PCN Encoding August 2009
should be seen as being essentially a marking behaviour similar to
ECN but intended for inelastic traffic. More details are given in
the informational appendix Appendix A.1.
4.3.1. Co-existence of PCN and not-PCN traffic
The scarcity of pool 1 DSCPs coupled with the fact that PCN is
envisaged as a marking behaviour that could be applied to a number of
different DSCPs makes it essential that we provide a not-PCN state.
As stated above (and expanded in Appendix A.1) the aim is for PCN to
re-use existing DSCPs. Because PCN re-defines the meaning of the ECN
field for such DSCPs it is important to allow an operator to still
use the DSCP for traffic that isn't PCN-enabled. This is achieved by
providing a not-PCN state within the encoding scheme.
5. Rules for Experimental Encoding Schemes
Any experimental encoding scheme MUST follow these rules to ensure
backward compatibility with this baseline scheme:
o All Interior-nodes within a PCN-domain MUST interpret the 00
codepoint in the ECN field as not-PCN and MUST NOT change it to
another value. Therefore an ingress node wishing to disable PCN
marking for a packet within a PCN-compatible Diffserv Codepoint
MUST set the ECN field to 00.
o The 11 codepoint in the ECN field MUST indicate PCN-marked (though
this does not exclude the 01 Experimental codepoint from carrying
the same meaning).
o Once set, the 11 codepoint in the ECN field MUST NOT be changed to
any other codepoint.
o Any experimental scheme MUST include details of all valid and
invalid codepoint transitions at any PCN nodes.
o Any experimental scheme MUST NOT update the meaning of the 00 and
11 codepoints defined above.
6. Backwards Compatibility
BCP 124 [RFC4774] gives guidelines for specifying alternative
semantics for the ECN field. It sets out a number of factors to be
taken into consideration. It also suggests various techniques to
allow the co-existence of default ECN and alternative ECN semantics.
The baseline encoding specified in this document defines PCN-
compatible Diffserv codepoints as no longer supporting the default
ECN semantics. As such this document is compatible with BCP 124. It
Moncaster, et al. Expires February 21, 2010 [Page 9]
Internet-Draft Baseline PCN Encoding August 2009
should be noted that this baseline encoding effectively disables end-
to-end ECN unless mechanisms are put in place to tunnel such traffic
across the PCN-domain. Standard IP-in-IP or IPsec tunnels will
always copy the CE codepoint from the outer header into the inner
header in decapsulation (unless the inner packet is not-ECT). If an
operator wishes to allow ECN to exist end-to-end they must ensure
there are no tunnel end-points within the PCN-domain to prevent any
risk of PCN-markings being exposed to endpoints.
7. IANA Considerations
This document makes no direct request to IANA.
8. Security Considerations
PCN-marking only carries a meaning within the confines of a PCN-
domain. Packets wishing to be treated as belonging to a PCN-flow
must carry a PCN-compatible DSCP and a PCN-Enabled ECN codepoint.
This encoding document is intended to stand independently of the
architecture used to determine how specific packets are authorised to
be PCN-marked, which will be described in separate documents on PCN-
boundary-node behaviour.
This document assumes the PCN-domain to be entirely under the control
of a single operator, or a set of operators who trust each other.
However future extensions to PCN might include inter-domain versions
where trust cannot be assumed between domains. If such schemes are
proposed they must ensure that they can operate securely despite the
lack of trust. However such considerations are beyond the scope of
this document.
9. 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 competing traffic within the same DSCP so long as that
traffic does not require ECN support within the PCN-domain. The
encoding scheme is conformant with [RFC4774]. The Working Group has
chosen not to define a single DSCP for use with PCN. The rationale
for this decision along with advice relating to choice of suitable
DSCPs can be found in Appendix A.1.
10. Acknowledgements
This document builds extensively on work done in the PCN working
group by Kwok Ho Chan, Georgios Karagiannis, Philip Eardley, Anna
Moncaster, et al. Expires February 21, 2010 [Page 10]
Internet-Draft Baseline PCN Encoding August 2009
Charny, Joe Babiarz and others. Thanks to Ruediger Geib and Gorry
Fairhurst for providing detailed comments on this document.
11. Comments Solicited
(To be removed by the RFC-Editor.) 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.
12. References
12.1. Normative References
[I-D.ietf-pcn-marking-behaviour] Eardley, P., "Metering and marking
behaviour of PCN-nodes",
draft-ietf-pcn-marking-behaviour-05
(work in progress), August 2009.
[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.
12.2. Informative References
[I-D.ietf-tsvwg-ecn-tunnel] Briscoe, B., "Tunnelling of
Explicit Congestion Notification",
draft-ietf-tsvwg-ecn-tunnel-03
(work in progress), July 2009.
[RFC3540] Spring, N., Wetherall, D., and D.
Ely, "Robust Explicit Congestion
Notification (ECN) Signaling with
Nonces", RFC 3540, June 2003.
[RFC4301] Kent, S. and K. Seo, "Security
Moncaster, et al. Expires February 21, 2010 [Page 11]
Internet-Draft Baseline PCN Encoding August 2009
Architecture for the Internet
Protocol", RFC 4301, December 2005.
[RFC5127] Chan, K., Babiarz, J., and F.
Baker, "Aggregation of DiffServ
Service Classes", RFC 5127,
February 2008.
[RFC5559] Eardley, P., "Pre-Congestion
Notification (PCN) Architecture",
RFC 5559, June 2009.
Appendix A. PCN Deployment Considerations (Informational)
A.1. Choice of Suitable DSCPs
The PCN Working Group chose not to define a single DSCP for use with
PCN for several reasons. Firstly the PCN mechanism is applicable to
a variety of different traffic classes. Secondly standards track
DSCPs are in increasingly short supply. Thirdly PCN should be seen
as being essentially a marking behaviour similar to ECN but intended
for inelastic traffic. The choice of which DSCP is most suitable for
a given PCN-domain is dependent on the nature of the traffic entering
that domain and the link rates of all the links making up that
domain. In PCN-domains with uniformly high link rates, the
appropriate DSCPs would currently be those for the Real Time Traffic
Class [RFC5127]. To be clear the PCN Working Group recommends using
admission control for the following service classes:
o Telephony (EF)
o Real-time interactive (CS4)
o Broadcast Video (CS3)
o Multimedia Conferencing (AF4)
PCN marking is intended to provide a scalable admission control
mechanism for traffic with a high degree of statistical multiplexing.
PCN marking would therefore be appropriate to apply to traffic in the
above classes, but only within a PCN region containing highly
aggregated traffic. In such cases, the above service classes may
well all be subject to a single forwarding treatment (treatment
aggregate [RFC5127]). However, this does not imply all such IP
traffic would necessarily be identified by one DSCP - each service
class might keep a distinct DSCP within the highly aggregated region
[RFC5127].
Moncaster, et al. Expires February 21, 2010 [Page 12]
Internet-Draft Baseline PCN Encoding August 2009
Additional service classes may be defined for which admission control
is appropriate, whether through some future standards action or
through local use by certain operators, e.g. the Multimedia Streaming
service class (AF3). This document does not preclude the use of PCN
in more cases than those listed above.
NOTE: The above discussion is informative not normative, as operators
are ultimately free to decide whether to use admission control for
certain service classes and whether to use PCN as their mechanism of
choice.
A.2. Rationale for Using ECT(0) for Not-marked
The choice of which ECT codepoint to use for the Not-marked state was
based on the following considerations:
o [RFC3168] full functionality tunnel within the PCN-domain: Either
ECT is safe.
o Leakage of traffic into PCN-domain: because of the lack of take-up
of the ECN nonce [RFC3540], leakage of ECT(1) is less likely to
occur so might be considered safer.
o Leakage of traffic out of PCN-domain: Either ECT is equally unsafe
(since this would incorrectly indicate the traffic was ECN-capable
outside the controlled PCN-domain).
o Incremental deployment: Either codepoint is suitable providing
that the codepoints are used consistently.
o Conceptual consistency with other schemes: ECT(0) is conceptually
consistent with [RFC3168].
Overall this seemed to suggest ECT(0) was most appropriate to use.
Authors' Addresses
Toby Moncaster
BT
B54/70, Adastral Park
Martlesham Heath
Ipswich IP5 3RE
UK
Phone: +44 1473 648734
EMail: toby.moncaster@bt.com
Moncaster, et al. Expires February 21, 2010 [Page 13]
Internet-Draft Baseline PCN Encoding August 2009
Bob Briscoe
BT
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 February 21, 2010 [Page 14]