Internet DRAFT - draft-ietf-pcn-3-state-encoding
draft-ietf-pcn-3-state-encoding
Congestion and Pre Congestion T. Moncaster
Internet-Draft University of Cambridge
Intended status: Historic B. Briscoe
Expires: September 13, 2012 BT
M. Menth
University of Tuebingen
March 12, 2012
A PCN encoding using 2 DSCPs to provide 3 or more states
draft-ietf-pcn-3-state-encoding-02
Abstract
Pre-congestion notification (PCN) is a mechanism designed to protect
the Quality of Service of inelastic flows within a controlled domain.
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 experimental encoding scheme specifies how three encoding states
can be carried in the IP header using a combination of two DSCPs and
the ECN bits. The Basic scheme only allows for three encoding
states. The Full scheme provides 6 states, enough for limited end-
to-end support for ECN as well.
Status
Since its original publication, the baseline encoding (RFC5696) on
which this document depends has become obsolete. The PCN working
Group has chosen to publish this as a historical document to preserve
the details of the encoding and to allow it to be cited in other
documents.
Status of this Memo
This Internet-Draft is submitted in full conformance with the
provisions of BCP 78 and BCP 79.
Internet-Drafts are working documents of the Internet Engineering
Task Force (IETF). Note that other groups may also distribute
working documents as Internet-Drafts. The list of current Internet-
Drafts is at http://datatracker.ietf.org/drafts/current/.
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."
This Internet-Draft will expire on September 13, 2012.
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Copyright Notice
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 4
1.1. Changes from Previous Drafts (to be removed by the RFC
Editor) . . . . . . . . . . . . . . . . . . . . . . . . . 5
2. Requirements notation . . . . . . . . . . . . . . . . . . . . 5
3. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 6
4. The Requirement for Three PCN Encoding States . . . . . . . . 6
5. Adding Limited End-to-End ECN Support to PCN . . . . . . . . . 7
6. Encoding Three PCN States in IP . . . . . . . . . . . . . . . 7
6.1. Basic Three State Encoding . . . . . . . . . . . . . . . . 8
6.2. Full Three State Encoding . . . . . . . . . . . . . . . . 8
6.3. Common Diffserv Per-Hop Behaviour . . . . . . . . . . . . 9
6.4. Valid and invalid codepoint transitions at
PCN-ingress-nodes . . . . . . . . . . . . . . . . . . . . 9
6.5. Valid and invalid codepoint transitions at
PCN-interior-nodes . . . . . . . . . . . . . . . . . . . . 10
6.6. Forwarding traffic out of the PCN-domain . . . . . . . . . 10
7. PCN-domain support for the PCN extension encoding . . . . . . 11
7.1. End-to-End transport behaviour compliant with the PCN
extension encoding . . . . . . . . . . . . . . . . . . . . 11
8. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 12
9. Security Considerations . . . . . . . . . . . . . . . . . . . 12
10. Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . 12
11. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 12
12. Comments Solicited . . . . . . . . . . . . . . . . . . . . . . 13
13. References . . . . . . . . . . . . . . . . . . . . . . . . . . 13
13.1. Normative References . . . . . . . . . . . . . . . . . . . 13
13.2. Informative References . . . . . . . . . . . . . . . . . . 13
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 14
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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.
The baseline encoding described in [RFC5696] provides for deployment
scenarios that only require two PCN encoding states. This document
describes an experimental extension to the base-encoding in the IP
header that adds two capabilities:
o the addition of a third PCN encoding state in the IP header
o preservation of the end-to-end semantics of the ECN field even
though PCN uses the field within a PCN-region that interrupts the
end-to-end path
The second of these capabilities is optional and the reasons for
doing it are discussed in Section 5.
As in the baseline encoding, this extension encoding re-uses the ECN
bits within the IP header within a controlled PCN-domain. This
extension requires the use of two DSCPs as described later in this
document. This experimental scheme is one of three that are being
proposed within the PCN working group. The aim is to allow
implementors to decide which scheme is most suitable for possible
future standardisation.
Following the publication of new rules relating to the tunnelling of
ECN marks [RFC6040], the PCN workign group decided to obsolete
[RFC5696] in favour of the 3-in-1 encoding
[I-D.ietf-pcn-3-in-1-encoding]. A side-effect of this decision was
to make the encoding described in this document obsolete. However
the PCN working group feels it is useful to have a formal historical
record of this encoding. This ensures details of the encoding are
not lost and also allows it to be cited in other documents.
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1.1. Changes from Previous Drafts (to be removed by the RFC Editor)
From draft-ietf-pcn-3-state-encoding-01 to 02:
o Changed the document from teh experimental to the historic track
o Added notes to the Introduciton and Abstract explaining the change
to historical
o Updated refs
From draft-ietf-pcn-3-state-encoding-00 to 01:
o Removed text implying the two DSCPs have different priority and
added Section 6.3 specifying they must both have the same PHB.
o Made IANA considerations text more precise.
o Changed variable names for DSCP 1 & DSCP 2 to DSCP n & DSCP m to
be consistent with baseline encoding.
o Updated refs
From draft-moncaster-pcn-3-state-encoding-01 to
draft-ietf-pcn-3-state-encoding-00:
o Changed to WG draft. Title changed from "A three state extended
PCN encoding scheme"
o Imposed new structure on document. This structure is intended to
be followed by all extensions to the baseline PCN encoding scheme.
o Extensive changes throughout to ensure consistency with the
baseline PCN encoding scheme.
From draft-moncaster-pcn-3-state-encoding-00 to 01:
o Checked terminology for consistency with [RFC5696]
o Minor editorial changes.
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
Most of the terminology used in this document is defined either in
[RFC5559] or in [RFC5696]. The following additional terms are
defined in this document:
o PCN-flow - a flow covered by a reservation but which hasn't
signalled that it requires end-to-end ECN support.
o PCN-enabled-ECN-flow - a flow covered by reservation and for which
the end-to-end transport has explicitly negotiated ECN support
from the PCN-boundary-nodes.
o Not-marked (xxx), where xxx represents a standard ECN codepoint -
packets that are PCN capable but carry no PCN mark. Abbreviated
as NM(xxx). The (xxx) represents the ECN codepoint that the
packet arrived with at the PCN-ingress-node e.g. NM(CE)
represents a PCN capable packet that has no PCN marking but which
arrived with the ECN bits set to congestion experienced.
4. The Requirement for Three PCN Encoding States
The PCN Marking Behaviours document [RFC5670] describes proposed PCN
schemes that require traffic to be metered and marked using both
Threshold and Excess Traffic schemes. In order to achieve this it is
necessary to allow for three PCN encoding states. The constraints
imposed by the way tunnels process the ECN field severely limit how
to encode these states as explained in [RFC5696] and [RFC6040]. The
obvious way to provide one more encoding state than the base encoding
is through the use of an additional PCN-compatible DiffServ
codepoint.
One aim of this document is to allow for experiments to show whether
such schemes are better than those that only employ two PCN encoding
states. As such, the additional DSCP will be taken from the EXP/LU
pools defined in [RFC2474]. If the experiments demonstrate that PCN
schemes employing three encoding states are significantly better than
those only employing two, then at a later date IANA might be asked to
assign a new PCN enabled DSCP from pool 1. Note that there are other
experimental encoding schemes being considered which only use one
DSCP but require either alternative tunnel semantics
([I-D.ietf-pcn-3-in-1-encoding]) or additional signalling
([I-D.ietf-pcn-psdm-encoding])in order to work.
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5. Adding Limited End-to-End ECN Support to PCN
There are a number of use-cases where explicit preservation of end-
to-end ECN semantics might be needed across a PCN domain. One of the
use-cases suggests that the end-nodes might be running rate-adaptive
codecs that would respond to ECN marks by reducing their transmission
rate. If the sending transport sets the ECT codepoint, the setting
of the ECN field as it arrives at the PCN ingress node will need to
be re-instated as it leaves the PCN egress node.
If a PCN region is starting to suffer pre-congestion then it may make
sense to expose marks generated within the PCN region by forwarding
CE marks from the PCN egress to such a rate-adaptive endpoint. They
would be in addition to any CE marks generated elsewhere on the end-
to-end path. This would allow the endpoints to reduce the traffic
rate. This will in turn help to alleviate the pre-congestion,
potentially averting any need for call blocking or termination.
However, the 'leaking' of CE marks out of the PCN region is
potentially dangerous and could violate [RFC4774] if the end hosts
don't understand ECN (see section 18.1.4 of [RFC3168]).
Therefore, a PCN region can only support end-to-end ECN if the PCN-
boundary-nodes are sure that the end-to-end transport is ECN-capable.
That way the PCN-egress-nodes can ensure that they only expose CE
marks to those receivers that will correctly interpret them as a
notification of congestion. The end-points may indicate they are
ECN-capable through some higher-layer signalling process that sets up
their reservation with the PCN boundary nodes. The exact process of
negotiation is beyond the scope of this document but is likely to
involve explicit two way signalling between the end-host and the PCN-
domain.
In the absence of such signalling the default behaviour of the PCN
egress node will be to clear the ECN field to 00 as in the baseline
PCN encoding [RFC5696].
6. Encoding Three PCN States in IP
The three state PCN encoding scheme is based closely on that defined
in [RFC5696] so that there will be no compatibility issues if a PCN-
domain changes from using the baseline encoding scheme to the
experimental scheme described here. There are two versions of the
scheme. The basic three state scheme allows for carrying both
Threshold-marked (ThM) and Excess-traffic-marked (ETM) traffic. The
full scheme additionally allows end-to-end ECN to be carried across
the PCN-domain.
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6.1. Basic Three State Encoding
Table 1 below shows how to encode the three PCN states in IP.
+--------+--------------+-------------+-------------+---------+
| DSCP | Not-ECT (00) | ECT(0) (10) | ECT(1) (01) | CE (11) |
+--------+--------------+-------------+-------------+---------+
| DSCP n | Not-PCN | NM | CU | ThM |
| DSCP m | Not-PCN | CU | CU | ETM |
+--------+--------------+-------------+-------------+---------+
(where DSCP n is a PCN-compatible DiffServ codepoint (see [RFC5696])
and DSCP m is a PCN-compatible DSCP from the EXP/LU pools as defined
in [RFC2474])
Table 1: Encoding three PCN states in IP
6.2. Full Three State Encoding
Table 2 shows how to additionally carry the end-to-end ECN state in
the IP header.
+--------+--------------+-------------+-------------+---------+
| DSCP | Not-ECT (00) | ECT(0) (10) | ECT(1) (01) | CE (11) |
+--------+--------------+-------------+-------------+---------+
| DSCP n | Not-PCN | NM(Not-ECT) | NM(CE) | ThM |
| DSCP m | Not-PCN | NM(ECT(0)) | NM(ECT(1)) | ETM |
+--------+--------------+-------------+-------------+---------+
(where DSCP n is a PCN-compatible DiffServ codepoint (see [RFC5696])
and DSCP m is a PCN-compatible DSCP from the EXP/LU pools as defined
in [RFC2474])
Table 2: Encoding three PCN states in IP
The four different Not-marked (NM) states allow for the addition of
limited end-to-end ECN support as explained in the previous section.
WARNING: In order to comply with this encoding all the nodes within
the PCN-domain MUST be configured with this encoding scheme.
However there may be operators who choose not to be fully
compliant with the scheme. If an operator chooses to leave some
PCN-interior-nodes that only support two marking states (the
baseline encoding [RFC5696]), then they must be aware of the
following: Ideally such nodes would be configured to indicate pre-
congestion or congestion using the ETM state since this would
ensure they could notify worst-case congestion, however this is
not possible since it requires the packets to be re-marked to DSCP
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m (hence altering the baseline encoding). This means that such
nodes will only be able to indicate ThM traffic.
6.3. Common Diffserv Per-Hop Behaviour
Packets carrying Diffserv codepoint 'DSCP n' or 'DSCP m' MUST all be
treated with the same Diffserv PHB [RFC2474]. The choice of PHB is
discussed in [RFC5559] and [RFC5696].
Two DSCPs are merely used to provide sufficient PCN encoding states,
there is no need or intention to provide different scheduling or drop
preference for each row in the table of PCN codepoints.
Specifically:
o Both DSCPs MUST be served in the same queue to prevent reordering
within an application flow.
o Both DSCPs MUST be assigned the same drop preference. Note that
[RFC5670] already provides for preferential drop of excess-rate-
marked packets, so assigning additional drop preference at the
coarser granularity of each DSCP would be incorrect.
6.4. Valid and invalid codepoint transitions at PCN-ingress-nodes
A PCN-ingress-node operating the Basic version of the 3-State
Encoding scheme MUST set the Not-marked codepoint on any arriving
packet that belongs to a PCN-flow. It MUST set the not-PCN codepoint
on any other packet.
A PCN-ingress-node operating the Full version of the 3-State Encoding
scheme MUST establish whether a packet is a member of a PCN-enabled-
ECN-flow. If it is, the PCN-ingress-node MUST set the appropriate
NM(xxx) codepoint depending on the value carried in the ECN field of
that packet. To be clear:
o A packet carrying the not-ECT codepoint in the ECN field MUST be
assigned the NM(not-ECT) codepoint
o A packet carrying the ECT(0) codepoint in the ECN field MUST be
assigned the NM(ECT(0)) codepoint
o A packet carrying the ECT(1) codepoint in the ECN field MUST be
assigned the NM(ECT(1)) codepoint
o A packet carrying the CE codepoint in the ECN field MUST be
assigned the NM(CE) codepoint
If it is not a member of such a flow then the behaviour MUST be the
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same as for the Basic version of the Encoding scheme.
6.5. Valid and invalid codepoint transitions at PCN-interior-nodes
A PCN-interior-node MUST obey the following rules:
o It MUST NOT change the not-PCN codepoint to any other codepoint.
o It MAY change any Not-marked codepoint to either the Threshold-
marked or Excess-traffic-marked codepoints.
o It MUST NOT change a Not-marked codepoint to the not-PCN
codepoint.
o A Not-marked codepoint MUST NOT be changed to any other Not-marked
codepoint.
o It MAY change the ThM codepoint to the ETM codepoint but it MUST
NOT change the ThM codepoint to any other codepoint.
o It MUST NOT change the ETM codepoint to any other codepoint.
Obviously in every case a codepoint can remain unchanged. The
precise rules governing which valid transition to use are set out in
[RFC5670]
6.6. Forwarding traffic out of the PCN-domain
As each packet exits the PCN-domain, the PCN-egress-node MUST check
whether it belongs to a PCN-enabled-ECN-flow. If it belongs to such
a flow then the following rules dictate how the ECN field should be
reset:
o A packet carrying the not-PCN codepoint MUST be given the not-ECT
codepoint.
o A packet carrying the NM(not-ECT) codepoint MUST be assigned the
not-ECT codepoint.
o A packet carrying the NM(ECT(0)) codepoint MUST be assigned the
ECT(0) codepoint.
o A packet carrying the NM(ECT(1)) codepoint MUST be assigned the
ECT(1) codepoint.
o A packet carrying the NM(CE) codepoint MUST be assigned the CE
codepoint.
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o A packet carrying the ThM codepoint MUST be assigned CE codepoint.
o A packet carrying the ETM codepoint MUST be assigned CE codepoint.
If the packet is part of a PCN-flow then it MUST be assigned the not-
ECT codepoint regardless of which PCN-codepoint it carried.
In addition all packets should have their DSCP reset to the
appropriate DSCP for the next hop. If the next hop is not another
PCN region this will not be a PCN-compatible DSCP, and by default
will be the best-efforts DSCP. Alterntively, higher layer signalling
mechanisms may allow the DSCP that packets entered the PCN-domain
with to be reinstated.
7. PCN-domain support for the PCN extension encoding
PCN traffic MUST be marked with a DiffServ codepoint that indicates
PCN is enabled. To comply with the PCN extension encoding, codepoint
'DSCP n' MUST be a PCN-compatible DSCP assigned by IANA for use with
the baseline PCN encoding [RFC5696] while 'DSCP m' can be a DSCP from
pools 2 or 3 for experimental and local use [RFC2474]. The exact
choice of DSCP may vary between PCN-domains but MUST be fixed within
each PCN-domain.
7.1. End-to-End transport behaviour compliant with the PCN extension
encoding
Transports wishing to use both PCN and end-to-end ECN MUST establish
that their path supports this combination. Support of end-to-end ECN
by PCN-boundary-nodes is OPTIONAL. Therefore transports MUST check
with both the PCN-ingress-node and PCN-egress-node for each flow.
The sending of such a request MUST NOT be taken to mean the request
has been granted. The PCN-boundary-nodes MAY choose to inform the
end-node of a successful request. The exact mechanism for such
negotiation is beyond the scope of this document. A transport that
receives no response or a negative response to a request to support
end-to-end ECN within a flow reservation MUST set the ECN field of
all subsequent packets in that flow to Not-ECT if it wishes to
guarantee that the flow will receive PCN treatment.
If a domain wishes to use the full scheme described in Table 2 all
nodes in that domain MUST be configured to understand the full
scheme.
If either of a PCN ingress-egress pair does not support end-to-end
ECN or if the end-to-end transport does not request support for end-
to-end ECN then the PCN-boundary-nodes MUST assume the packet belongs
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to a PCN-flow.
8. IANA Considerations
This document asks IANA to assign one DiffServ codepoint from Pool 2
or Pool 3 (for experimental/local use)[RFC2474]. Should this
experimental PCN scheme prove sufficiently successful then IANA will
be requested in a later document to assign a dedicated DiffServ
codepoint from pool 1 for standards use and the experimental
codepoint will be returned to its IANA pool.
9. Security Considerations
The security concerns relating to this extended PCN encoding are
essentially the same as those in [RFC5696].
This extension coding gives end-to-end support for the ECN nonce
[RFC3540], which is intended to protect the sender against the
receiver or against network elements concealing a congestion
experienced marking or a lost packet. PCN-based reservations
combined with end-to-end ECN are intended for partially inelastic
traffic using rate-adaptive codecs. Therefore the end-to-end
transport is unlikely to be TCP, but at this time the nonce has only
been defined for TCP transports.
10. Conclusions
This document describes an extended encoding scheme for PCN that
provides for three encoding states as well as optional support for
end-to-end ECN. The encoding scheme builds on the baseline encoding
described in [RFC5696]. Using this encoding scheme it is possible
for operators to conduct experiments to check whether the addition of
an extra encoding state will significantly improve the performance of
PCN. It will also allow experiments to determine whether there is a
need for end-to-end ECN support within the PCN-domain (as against
end-to-end ECN support through the use of IP-in-IP tunnelling or by
downgrading the traffic to a lower service class).
11. Acknowledgements
This document builds extensively on work done in the PCN working
group by Kwok Ho Chan, Georgios Karagiannis, Philip Eardley, Joe
Babiarz and others. Full details of alternative schemes that were
considered for adoption can be found in the document
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[I-D.ietf-pcn-encoding-comparison].
12. Comments Solicited
(Section to be removed by RFC_Editor) 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.
13. References
13.1. 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.
[RFC5670] Eardley, P., "Metering and Marking Behaviour of PCN-
Nodes", RFC 5670, November 2009.
[RFC5696] Moncaster, T., Briscoe, B., and M. Menth, "Baseline
Encoding and Transport of Pre-Congestion Information",
RFC 5696, November 2009.
13.2. Informative References
[I-D.ietf-pcn-3-in-1-encoding]
Briscoe, B., Moncaster, T., and M. Menth, "Encoding 3 PCN-
States in the IP header using a single DSCP",
draft-ietf-pcn-3-in-1-encoding-09 (work in progress),
March 2012.
[I-D.ietf-pcn-encoding-comparison]
Karagiannis, G., Chan, K., Moncaster, T., Menth, M.,
Eardley, P., and B. Briscoe, "Overview of Pre-Congestion
Notification Encoding",
draft-ietf-pcn-encoding-comparison-09 (work in progress),
March 2012.
[I-D.ietf-pcn-psdm-encoding]
Menth, M., Babiarz, J., Moncaster, T., and B. Briscoe,
"PCN Encoding for Packet-Specific Dual Marking (PSDM
Encoding)", draft-ietf-pcn-psdm-encoding-01 (work in
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progress), March 2010.
[RFC2474] Nichols, K., Blake, S., Baker, F., and D. Black,
"Definition of the Differentiated Services Field (DS
Field) in the IPv4 and IPv6 Headers", RFC 2474,
December 1998.
[RFC3168] Ramakrishnan, K., Floyd, S., and D. Black, "The Addition
of Explicit Congestion Notification (ECN) to IP",
RFC 3168, September 2001.
[RFC3540] Spring, N., Wetherall, D., and D. Ely, "Robust Explicit
Congestion Notification (ECN) Signaling with Nonces",
RFC 3540, June 2003.
[RFC5559] Eardley, P., "Pre-Congestion Notification (PCN)
Architecture", RFC 5559, June 2009.
[RFC6040] Briscoe, B., "Tunnelling of Explicit Congestion
Notification", RFC 6040, November 2010.
Authors' Addresses
Toby Moncaster
University of Cambridge
Computer Laboratory
JJ Thomson Avenue
Cambridge CB3 0FD
UK
Phone: +44 1223 763654
Email: toby@moncaster.com
Bob Briscoe
BT
B54/77, Adastral Park
Martlesham Heath
Ipswich IP5 3RE
UK
Phone: +44 1473 645196
Email: bob.briscoe@bt.com
URI: http://www.bobbriscoe.net
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Michael Menth
University of Tuebingen
Department of Computer Science
Sand 13
Tuebingen D-72076
Germany
Phone: +49 07071 29 70505
Email: menth@informatik.uni-tuebingen.de
URI: http://www.kn.inf.uni-tuebingen.de
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