NEMO Working Group P. Thubert
Internet-Draft Cisco
Expires: octobre 30, 2004 N. Montavont
LSIIT - ULP
May 2004
Nested Nemo Tree Discovery
draft-thubert-tree-discovery-00.txt
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Copyright Notice
Copyright (C) The Internet Society (2004). All Rights Reserved.
Abstract
The purpose of this paper is to describe a minimum set of features
that extends the Nemo basic support in order to avoid loops in the
nested Nemo case. As a result, Mobile Routers assemble into a tree
that can be optimized based on various metrics.
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 3
2. Terms and Abbreviations . . . . . . . . . . . . . . . . . . 3
3. Motivations . . . . . . . . . . . . . . . . . . . . . . . . 4
3.1 Multihomed nested mobile network . . . . . . . . . . . . . . 4
3.2 Loops in nested Nemo . . . . . . . . . . . . . . . . . . . . 5
4. Router Advertisement extensions . . . . . . . . . . . . . . 6
4.1 Router Advertisement message . . . . . . . . . . . . . . . . 6
4.2 Tree Information Option . . . . . . . . . . . . . . . . . . 6
5. Tree Discovery . . . . . . . . . . . . . . . . . . . . . . . 8
5.1 tree selection . . . . . . . . . . . . . . . . . . . . . . . 9
5.2 Tree Hop Timer . . . . . . . . . . . . . . . . . . . . . . . 10
5.3 Collisions and stability . . . . . . . . . . . . . . . . . . 11
5.3.1 Hold down . . . . . . . . . . . . . . . . . . . . . . . . . 11
5.3.2 Instability . . . . . . . . . . . . . . . . . . . . . . . . 11
5.3.3 Collision . . . . . . . . . . . . . . . . . . . . . . . . . 12
6. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . 12
References . . . . . . . . . . . . . . . . . . . . . . . . . 13
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . 14
Intellectual Property and Copyright Statements . . . . . . . 15
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1. Introduction
As per Nemo Basic support, a Mobile Router autoconfigures a single
Care of Address to register to its Home Agent and terminate its MR-HA
tunnel. That CoA is the MR's point of attachment to the nested Nemo.
Consequently, if loops are avoided, the nested Nemo assumes the shape
of a tree. The nodes of the tree are Mobile Routers, the root is
either a fixed or a Mobile Router, called in the latter case the root
Mobile Router in NEMO terminology [6]. The leaves are mobile or fixed
hosts, called Local Fixed Nodes, Local Mobile Nodes and Visiting
Mobile Nodes in the NEMO terminology.
This paper provides (1) a minimum extension to IPv6 Neighbour
Discovery Router Advertisements in order to ensure that Mobile
Routers attaching to one another actually avoid loops and end up
forming a tree, and (2) the minimum common part of all MR algorithms
that is required to ensure that whatever their specific decisions,
loops between MRs will be avoided.
The method is based on an autonomous decision by each MR with no
global state convergence such as a MANET proactive routing protocol.
In fact, MRs may make different decisions from a same input, based on
their own configuration and their own algorithms.
In order to build trees of Mobile Routers, we propose an extension to
the ICMP Router Advertisement (RA) message, the Tree Information
Option (TIO). The RA/TIO allows MRs to advertise the tree they belong
to, and to select and move to the best location within the available
trees. MRs propagate the TIO down the tree, updating some metrics
such as the tree depth, leaving alone root information such as the
tree identifier, and sending the result in RAs over the ingress
interfaces.
2. Terms and Abbreviations
This document assumes that the reader is familiar with Mobile IPv6 as
defined in [3] and with the concept of Mobile Router defined in the
Nemo terminology document [6].
For the needs of this paper, the following new definitions are
introduced:
Nemo Clusterhead: The root of a tree of mobile routers. When the tree
of Mobile Routers is attached to the infrastructure, the fixed
Access Router may act as cluster head if it supports the Tree
Information Option described in this document. If it does not,
then the Clusterhead coincides with the root MR in NEMO
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terminology. A clusterhead is elected even when the tree is not
attached to the infrastructuture. A stand-alone MR is a
Clusterhead.
Floating Tree: A Nested Nemo which Clusterhead is a MR that is not
attached to an AR.
Grounded Tree: A Nested Nemo whose Clusterhead is attached to the
infrastructure. In other words, the clusterhead is either a fixed
router that supports RA/TIO or is a MR which attachment router is
a fixed router that does not support RA/TIO.
Mobile Access Router: A Mobile Router that provides Access Router
services to other MRs.
Attachment Router: The Router that is selected as Access Router by a
Mobile Router, making it its parent in the nested NEMO tree.
Propagation: The action by a Mobile Router that consists in receiving
a RA/TIO from its Attachment Router, recomputing a few specific
fields, removing unknown suboption, and appending the resulting
TIO to RAs sent over the ingress interfaces.
3. Motivations
3.1 Multihomed nested mobile network
A nested mobile network that is made of multiple MRs having a direct
connection to the Internet is said to be multihomed. Multihoming in
Nemo offers useful properties to MNNs. Reference [8] lists potential
multihomed configurations for Nemo and explains the different
problems and advantages that some configurations may introduce.
Multihoming offers three main abilities to the Nemo: it allows route
recovery on failure, redundancy and load-sharing between MRs (or
between MRs'interfaces). However, for the moment there are no
requirements nor protocol defining how to use several egress
interfaces inside a Nemo.
In a nested Nemo, the hierarchy of MRs increases the complexity of
the route and/or router selection for MNNs. Each level of a Nemo
implies the usage of a new tunnel between the MR and its home agent.
Thus if a MNN connects to a sub-Nemo which is also a sub-Nemo,
packets from the MNN will be encapsulated three times.
When the Nemo where the MN is connected to is multihomed, the MN may
have the choice between several AR to be its default router.
Reference [9] introduces new options in Router Advertisement to allow
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any node on a link to choose between several routers. This option
mainly consists of a 2-bits flag that indicates the preference of the
router (low, medium or high). Furthermore, the same flag can be set
in the Route Information option indicating the preference of a
specific prefix. Therefore, any node can determine its best default
router(s) according to a given destination and its best router for
default, which will be used by default.
However this preference is only useful in a flat topology; It gives a
way to the node to choose between different access routers
advertising prefixes on the node link. But if the node is inside a
hierarchical topology (some access routers are not at the same level)
the node can not learn the level of each access router.
One of the usage of the new option introduced in this document is to
distribute information on the hierarchy of MRs. This information can
be distributed to ARs, MRs and MNNs as well in order to allow better
route selection and to increase the knowledge of the Nemo topology on
each node.
3.2 Loops in nested Nemo
When several MRs attach to each other to form a nested Nemo, loops
can be created if they are not explicitely avoided. In the simplest
case, when egress and ingress interfaces of an MR are all wireless, a
mobile router may be listening to Router Advertisement from its own
ingress interface, creating a confliction problem. In the general
case, arbitrary attachment of MRs will form graphs that are not
exempt of loops. For instance: Assume a nested Nemo where MR1 is
connected to the infrastructure, and MR3 is attached to MR2. Say that
MR2 can hear both MR3 and MR1 over its wireless egress interface. If
MR2 select MR1, the connectivity to the infrastruture is provided for
all. But if MR2 selects MR3, MR2 and MR3 end up forming a loop and
are disconnected from their Home Agents.
With Nemo basic support, a MR uses a single primary CareOf to attach
to the nested structure. As a result, if loops are avoided, the
nested NEMO end up forming a tree. It is beneficial to be able to
form that tree in an optimum fashion for a given set of metrics such
as tree depth.
The shape of a nested Nemo may change rapidly due to MRs movement. It
is thus impractical to expect each MR to be able to maintain states
about the whole tree structure in a link state fashion. On the
contrary, it is also beneficial to allow each MR to make its own
independent selection based on a minimum information about its
immediate neighbors, in order to reestablish the tree quickly upon
erratic movements.
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Each MR should be able to make its own attachment router selection
based on its own condition (eg battery level), its own set of
constraints that may not apply to other MRs in the tree, and in
general its own algorithm. As a result, the standardization effort
should concentrate on a common minimum set of rules that must be
common to all MRs in order to prevent routing loops in the nested
NEMO while leaving MRs independent in their AR selection algorithms.
4. Router Advertisement extensions
New extensions of Router Advertisement are proposed to distribute the
knowledge of the MR hierarchy inside a nested Nemo. These extensions
are defined in different options/sub-options: a flag bit from the
reserved flag field of Router Advertisement message is used to
indicate whether the sending router is a MR or not; a new option is
defined to transport minimum information on the tree to avoid loops
generation;
4.1 Router Advertisement message
We propose to use a reserved flag of the Router Advertisement message
to inform whether the sending router is a MR or not.
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Type | Code | Checksum |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Cur Hop Limit |M|O|H|N|Reservd| Router Lifetime |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Reachable Time |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Retrans Timer |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Options ...
+-+-+-+-+-+-+-+-+-+-+-+-
Nemo enabled router (N)
The Nemo enabled router (N) bit is set when the sending router is a
Mobile Router.
4.2 Tree Information Option
The following option regroups the minimum information that allows a
MR to discover a tree and select its point of attachment while
avoiding loop generation. It can also be used by MNNs to select their
best default router. If the default router of a non-MR sends Router
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Advertisements with a tree discovery option, the non-MR MUST set the
N flag of its own Router Advertisement to 0 and copy the Tree
Discovery Option in its own Router Advertisement.
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Type | Length |G|H| Reserved |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Preference | BootTimeRandom |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| TreeDepth | TreePref. | TreeDelay |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| PathDigest |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
+ +
| TreeID |
+ +
| |
+ +
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| sub-option(s)...
+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 2. Tree Information Option
Type 8-bit unsigned integer set to 10 by the Clusterhead.
Length 8-bit unsigned integer set to 4. The length of the option
(including the type and length fields) in units of 8 octets.
Grounded (G) The Grounded (G) flag is set when the Clusterhead is
attached to a fixed network infrastructure (such as the Internet).
Home (H) The Home (H) flag is set when the Clusterhead is attached to
its home network.
Reserved 16-bit unsigned integer set to 0 by the Clusterhead.
Preference The administrative preference of that (mobile) Access
Router. Default is 0. 255 is the highest possible preference. Set
by each MR at propagation time.
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BootTimeRandom A random value computed at boot time and recomputed in
case of a duplication with another AR. The concatenation of the
Preference and the BootTimeRandom is a 32-bit extended preference
that is used to resolve collisions. It is set by each MR at
propagation time.
TreeDepth 8-bit unsigned integer. The tree depth of the clusterhead
is 0 if it is a fixed router and 1 if it is a Mobile Router. The
tree Depth of a tree Node is the depth of its attachment router as
received in a TIO, incremented by one. All nodes in the tree
advertise their tree depth in the Tree Information Options that
they append to the RA messages over their ingress interfaces as
part of the propagation process.
TreePreference 8-bit unsigned integer set by the Clusterhead to its
preference and unchanged at propagation. Default is 0 (lowest
preference).
TreeDelay 16-bit unsigned integer set by the Clusterhead indicating
the delay before changing the tree configuration, in milliseconds.
A default value is 128ms. It is expected to be an order of
magnitude smaller than the RA-interval so if the clusterhead has a
sub-second RA-interval, the Tree delay may be shorter than 100ms.
It is also expected to be an order of magnitude longer than the
typical propagation delay inside the nested Nemo.
PathDigest 32-bit unsigned integer CRC, updated by each MR. This is
the result of a CRC-32c computation on a bit string obtained by
appending the received value and the MR Care of Address.
Clusterheads use a 'previous value' of zeroes to initially set the
PathDigest.
TreeID 128-bit unsigned integer which uniquely identify a tree set by
the Clusterhead. The global IPv6 home address of the Clusterhead
can be used.
5. Tree Discovery
Here follows a set of rules and definitions that MUST be followed be
all Mobile Routers:
1. An MR that is not attached to an AR is the Nemo Clusterhead of
its own, floating tree.
2. An MR that is attached to an AR that does not support TIO, is the
clusterhead of its own, grounded tree.
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3. A router sending a RA without TIO is considered a grounded AR at
depth 0. Thus, a MR that attaches to an AR that does not include
a TIO in its RA becomes a Clusterhead of depth 1.
4. The Nemo ClusterHead of a tree exposes the tree in the RA/TIO and
MRs propagate the TIO down the tree with the RAs that they
forward over their ingress links.
5. An MR that is already part of a tree MAY move at any time and
with no delay in order to get closer to the clusterhead of its
current tree - i.e. in order to reduce its own tree depth. But an
MR MUST NOT move down the tree that it is attached to. MRs MUST
ignore RAs that are received from other routers located deeper or
at the same depth within the same tree.
6. An MR may move from its current tree into any different tree at
any time and whatever the depth its reaches in the new tree, but
it may have to wait for a Tree Hop timer to elapse in order to do
so. The MR will join that other tree if it is more preferable for
reasons of connectivity, configured preference, size, security,
bandwidth, tree depth, or whatever metrics the MR cares to use.
7. If a MR has selected a new attachment router but has not moved
yet (because it is waiting for Tree Hop timer to elapse), the MR
is unstable and refrains from sending RATIOs.
8. When an MR joins a tree, moves within its tree, or when it
receives a modified TIO from its current access router, the MR
sends an unsolicited Router Advertisement message on all its
mobile networks (i.e. all its ingress interfaces). The RA
contains a TIO that propagates the new tree information. At the
same time, the MR MAY send a Binding Update to its home agent or
a local proxy of some sort, because the tree it is attached to
has changed. If the MR fails to reach its HA, it MAY attempt to
roll back the movement or to retry the HA discovery procedure.
9. This allows the new higher parts of the tree to take place first
eventually dragging their sub-tree with them, and allowing
stepped sub-tree reconfigurations, limiting relative movements.
5.1 tree selection
The tree selection is implementation and algorithm dependent. In
order to limit erratic movements, and all metrics being equal, MRs
SHOULD stick to their previous selection. Also, MRs SHOULD provide a
mean to filter out candidate ARs whose availability is detected as
fluctuating, at least when more stable choices are available. For
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instance, the MR MAY place the failed AR in a Hold Down mode that
ensures that the AR will not be reused for a given period of time.
The known trees are associated with the AR that advertises them and
kept in an ordered list, per order of preference, by extending the
Default Router List. DRL entries are extended to store the
information received from the last TIO, and a timer ID -and related
data- to delay the reaction to a better RA message, the tree hop
timer, as described in the next section.
When connection to a fixed network is not possible or preferable for
security or other reasons, scattered trees should aggregate as much
as possible into larger trees in order to allow inner connectivity.
How to balance these trees is implementation dependent, and MAY use a
specific visitor-counter suboption in the TIO.
5.2 Tree Hop Timer
The tree Hop timer serves 2 purposes:
Delay the reattachment of a subtree that has been forced to
detach. This allows to make sure that when a subtree has detached,
the RATIO that is initiated by the new clusterhead has spread down
the subtree so that two different trees have formed.
Limit RA/TIO storms when two trees collide. The idea is that
between the nodes from tree A that wish to move to tree B, those
that see the highest place in tree B will move first and advertise
their new locations before other nodes from tree A actually move.
A new tree is discovered upon a router advertisement message with or
without a RA/TIO. The MR joins the tree by selecting the source of
the RA message as its access router (default gateway) and propagating
the TIO accordingly.
When a new tree is discovered, the candidate AR that advertises the
new tree is placed in a held up state for the duration of a Tree Hop
timer. If the new AR is more preferable than the current one, the MR
expects to jump and becomes unstable.
A MR that is unstable may discover other ARs from the same new tree
during the instability phase. It needs to start a new Tree Hop timer
for all these. The first timer that elapses for a given new tree
clears them all for that tree, allowing the MR to jump to the highest
position available in the new tree.
The duration of the Tree Hop timer depends on the tree delay of the
new tree and on the depth of AR that triggers it:
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(AR's depth + random) * AR's tree_delay (where 0 <= random < 1). It
is randomized in order to limit collisions and synchronizations.
5.3 Collisions and stability
Attachment routers in the DRL may or may not be usable for roaming
depending on runtime conditions. The following states are defined:
Current This AR is used for roaming
Candidate This AR can be used for roaming. Typically, an initial
held-up period is over but the candidate is not preferred over the
current AR.
Held-Up This AR can not be used till tree hop timer elapses. This
does not occur for a fixed AR that does not send a TIO since the
tree delay is null in that case..
Held-Down This AR can not be used till hold down timer elapses. At
the end of the hold-down period, the router is removed from the
DRL, and will be reinserted if it appears again with a RA.
Collision This AR can not be used till its next Router Advertisement
5.3.1 Hold down
When a router is 'removed' from the Default Router List, it is
actually held down for a hold down timer period, in order to prevent
flapping. This happens when an AR disappears (upon expiration timer),
and when an AR is tried but can not reach the HA (upon expiration of
another AR, or upon tree hop for that AR).
An Attachment Router that is held down is not considered for the
purpose of roaming. When the hold down timer elapses, the AR is
removed from the DRL.
5.3.2 Instability
A MR is instable when a Held-Up AR is placed before the current AR.
Instability is transient (In the order of tree hop timers). When a MR
is instable, it MUST NOT send RAs with TIO. This avoids loops when MR
A wishes to attach to MR B and MR B wishes to attach to MR A. Unless
RA cross (which is a short window of time), a MR receives TIO from
stable ARs, which do not plan to attach to itself, so the MR can
safely attach to them.
A MR is instable when it is prepared to move shortly to another AR.
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This happens typically when it discovers a new and more preferred
candidate AR, for the duration of the tree hop timer. During that
time, the new candidate is placed in Held-up state. Instability may
also occur when the current AR is lost and the next best is still
held up. Instability is resolved when the tree hop timer of all the
AR (s) causing instability elapse. Such AR is changes state to
Current or Held-Down.
5.3.3 Collision
A race condition occurs if 2 MRs send RA/TIO at the same time and
wish to join each other. In order to detect the situation, MRs time
stamp the sending of RA/TIO. Any RA/TIO received within a short
media-dependant period introduces a risk. To divide the risk, A
32bits extended preference is added in the TIO. The first byte is the
AR own preference (default 0), the rest is a boot time computed
random.
A MR that decides to join an AR will do so between (AR depth) and (AR
depth + 1) times the AR tree delay. But since a MR is unstable as
soon as it receives the RA/TIO from the preferred AR, it will
restrain from sending a RA/TIO between the time it receives the RA
and the time it actually jumps. So the crossing of RA may only happen
during the propagation time between the AR and the MR, plus some
internal queuing and processing time within each machine. It is
expected that one tree delay normally covers that interval, but
ultimately it is up to the implementation and the configuration of
the AR to define the duration of risk window.
There is risk of a collision when a MR receives an RA, for an other
mobile router that is more preferable than the current AR, within the
risk window. In the face of a potential collision, the Mobile Router
with the lowest extended preference processes the RA/TIO normally,
while the router with the highest preference places the other in
collision state, does not start the tree hop timer, and does not
become instable. It is expected that next RAs between the two will
not cross anyway.
6. Acknowledgments
The authors wish to thank Marco Molteni and Patrick Wetterwald
(cisco) for their participation to this design and the review of the
document, and Massimo Villari (university of Messina), for his early
work on simulation and research on the subject. This work is also
based on prior publications, in particular HMRA [7] by Hosik Cho and
Eun-Kyoung Paik from Seoul National University and other non IETF
publications coauthored with Thierry Ernst and Thomas Noel.
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References
[1] Narten, T., Nordmark, E. and W. Simpson, "Neighbor Discovery for
IP Version 6 (IPv6)", RFC 2461, December 1998.
[2] Thomson, S. and T. Narten, "IPv6 Stateless Address
Autoconfiguration", RFC 2462, December 1998.
[3] Johnson, D., Perkins, C. and J. Arkko, "Mobility Support in
IPv6", draft-ietf-mobileip-ipv6-24 (work in progress), July
2003.
[4] Devarapalli, V., "Nemo Basic Support Protocol",
draft-ietf-nemo-basic-support-02 (work in progress), December
2003.
[5] Ernst, T., "Network Mobility Support Goals and Requirements",
draft-ietf-nemo-requirements-02 (work in progress), February
2004.
[6] Ernst, T. and H. Lach, "Network Mobility Support Terminology",
draft-ietf-nemo-terminology-01 (work in progress), February
2004.
[7] Cho, H., "Hierarchical Mobile Router Advertisement for nested
mobile networks", draft-cho-nemo-hmra-00 (work in progress),
January 2004.
[8] Ng, C-W., Charbon, J., Paik, E-Y. and T. Ernst, "Analysis of
Multihoming in Network Mobility Support",
draft-ng-nemo-multihoming-issues-03 (work in progress), February
2004.
[9] Draves, R. and R. Hinden, "Default Router Preferences and
More-Specific Routes", draft-ietf-ipv6-router-selection-03 (work
in progress), December 2003.
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Authors' Addresses
Pascal Thubert
Cisco Systems
Village d'Entreprises Green Side
400, Avenue de Roumanille
Batiment T3
Biot - Sophia Antipolis 06410
FRANCE
Phone: (33) 4 97 23 26 34
EMail: pthubert@cisco.com
Nicolas Montavont
LSIIT - Univerity Louis Pasteur
Pole API, bureau C444
Boulevard Sebastien Brant
Illkirch 67400
FRANCE
Phone: (33) 3 90 24 45 87
EMail: montavont@dpt-info.u-strasbg.fr
URI: http://www-r2.u-strasbg.fr/~montavont/
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Internet-Draft TD May 2004
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Acknowledgment
Funding for the RFC Editor function is currently provided by the
Internet Society.
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