INTERNET-DRAFT Philippe Jacquet
IETF MANET Working Group Paul Muhlethaler
Expiration: 02 September 2001 Amir Qayyum
Anis Laouiti
Laurent Viennot
Thomas Clausen
INRIA Rocquencourt, France
2 March 2001
Optimized Link State Routing Protocol
draft-ietf-manet-olsr-04.txt
Status of this Memo
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Abstract
This document describes the Optimized Link State Routing (OLSR)
protocol for mobile ad hoc networks. The protocol is an
optimization of the pure link state algorithm tailored to the
requirements of a mobile wireless LAN. The key concept used in the
protocol is that of multipoint relays (MPRs) [1] & [2]. MPRs are
selected nodes which forward broadcast messages during the
flooding process. This technique substantially reduces the message
overhead as compared to pure flooding mechanism where every node
retransmits each message when it receives the first copy of the
packet. In OLSR, information flooded in the network "through"
these MPRs is also "about" the MPRs. Thus a
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second optimization is achieved by minimizing the "contents" of
the control messages flooded in the network. Hence, as contrary to
the classic link state algorithm, only a small subset of links
with the neighbor nodes are declared instead of all the
links. This information is then used by the OLSR protocol for
route calculation. As a consequence hereof, the routes contain
only the MPRs as intermediate nodes from a Source to a
Destination. OLSR provides optimal routes (in terms of number of
hops). The protocol is particularly suitable for large and dense
networks as the technique of MPRs works well in this context.
Contents
Status of This Memo 1
Abstract 1
1. Introduction 3
2. Changes 3
3. OLSR terminology 4
4. Applicability Section 5
4.1 Networking Context 5
4.2 Protocol Characteristics and Mechanisms 6
5. Protocol Overview 8
6. Multipoint relays 9
7. Protocol functioning 10
7.1 Protocol and port number 10
7.2 Packet format 10
7.2.1 Packet Header 12
7.2.2 Message Header 12
7.2.3 Packet Processing 13
7.3 Neighbor sensing 15
7.3.1 Neighbor sensing information base 15
7.3.2 HELLO message broadcast 16
7.3.3 HELLO message processing 19
7.4 Multipoint relay selection 21
7.5 Multipoint relay information declaration 22
7.5.1 Topology information base 22
7.5.2 TC message broadcast 23
7.5.3 TC message processing 24
7.6 Routing table calculation 26
7.7.Link layer notification 27
8. Packet Forwarding 28
8.1 Data packet forwarding 28
8.2 Control message forwarding 28
9. Proposed values for constants 29
10. Sequence numbers 29
11. References 30
12. Authors' addresses 31
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1. Introduction
This Optimized Link State Routing protocol inherits the concept of
forwarding and relaying from HIPERLAN (a MAC layer protocol) which
is standardized by ETSI [3]. The OLSR protocol is developed in the
IPANEMA project (part of Euclid program) and in the PRIMA project
(part of RNRT program).
This protocol is developed for mobile ad hoc networks. It operates
as a table driven and proactive protocol and exchanges topology
information with other nodes of the network at regular intervals.
The nodes which are selected as a multipoint relay by some
neighbor nodes announce this information periodically in their
control messages. The protocol uses the MPRs to facilitate
efficient flooding of control messages in the network. In route
calculation, the MPRs are used to form the route from a given node
to any destination in the network.
MPRs are selected by a node among its one hop neighbors with
"symmetric", i.e. bi-directional, link. Therefore, selecting the
route through MPRs automatically avoids the problems associated
with data packet transfer on uni-directional links (such as the
problem of not getting link-layer acknowledgments for the data
packets at each hop)
The OLSR protocol is developed to work independently from other
protocols. But it can be adapted to operate with a protocol (like
IMEP [4]) which could provide common functionalities such as
neighbor sensing, multipoint relaying, security authentication,
etc.
2. Changes
Major changes from version 03 to version 04
- Finalized the generic packet/message format to
include features for scope-limited (diameter-bound)
flooding of messages and to handle duplicate messages.
- Editorial changes towards language consistency.
Major changes from version 02 to version 03
- Introduction of assigned port number for use with OLSR.
- The packet format now uses "message length" rather than an
offset to the next message.
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- Optional section describing how link-layer notifications
can be utilized included.
Major changes from version 01 to version 02
- Introduction of a unified packet format for encapsulation of
all messages being exchanged between nodes. This also serves
to facilitate extensions in future versions of the protocol
(i.e. introduction of new protocol messages) without breaking
backwards compatibility.
- Removal of "Power Conservation" from this draft. Power
Conservation may be considered as an extension to the basic
routing capabilities, and the information is therefore moved
to draft-ietf-manet-olsr-extensions-00.txt.
3. OLSR Terminology
The keywords "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 [9].
The OLSR protocol uses the following terminology, in addition to
the terms defined in [5].
connection
A communication channel or medium *on the same physical
interface*, over which the nodes can communicate with each
other.
holding time
The lifetime associated with an entry in any table. An entry is
kept in the table for a period of time, equal to its holding
time. If the entry is not refreshed during this period, it is
removed from the table when the holding time expires.
multipoint relay (MPR)
A node which is selected by its one-hop neighbor, node X, to
"re-transmit" all the broadcast messages that it receives from
X, provided that the same message is not already received, and
the time to live field of the message is greater than zero.
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multipoint relay selector (MPR selector, MS)
A node which has selected its one-hop neighbor, node X, as its
multipoint relay, will be called a multipoint relay selector
of node X.
node
A MANET router which implements this Optimized Link State
Routing protocol.
symmetric link
A bi-directional *link* between two neighbor nodes, i.e. node X
and node Y where both can hear each other.
4. Applicability Section
This section dictates the characteristics of the OLSR protocol as
specified in the Applicability Statement draft [6].
4.1. Networking Context
OLSR is well suited to large and dense mobile networks, as the
optimi- zation achieved using the MPRs works well in this
context. The larger and more dense a network, the more
optimization can be achieved as compared to the normal link state
algorithm. OLSR uses hop-by-hop routing, i.e. each node uses its
local information to route packets.
OLSR is well suited for networks, where the traffic is random and
sporadic between "several" nodes rather than being almost
exclusively between a small specific set of nodes. The performance
of the protocol, compared to a reactive protocol, is even better
if these [source, destination] pairs change with time [8]. Such
changes may initiate substantial traffic (Query flooding) in case
of reactive protocol, but nothing in OLSR, as the routes are
maintained for each known destination all the time.
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4.2. Protocol Characteristics and Mechanisms
* Does the protocol provide shortest path routes ?
Yes.
* Does the protocol provide support for unidirectional links? (if
so, how?)
No. However, the use of uni-directional links may easily be
enabled through optional extensions to the protocol.
* Does the protocol require the use of tunneling? (if so, how?)
No.
* Does the protocol require using some form of source routing? (if
so, how?)
No.
* Does the protocol require the use of periodic messaging? (if so,
how?)
Yes. Periodically, each node in the network sends a message
containing the addresses of the neighbors which have selected
that node as a MPR. This information enables other nodes to
build routes to that node through the MPRs.
* Does the protocol require the use of reliable or sequenced packet
delivery? (if so, how?)
No.
* Does the protocol provide support for routing through a multi-
technology routing fabric? (if so, how?)
No. However, provisions for multiple interfaces may easily be
enabled through extensions to the protocol.
* Does the protocol provide support for multiple hosts per router?
(if so, how?)
Yes. The hosts are added to the MPR selector set of the node
(router), which will then announce that the hosts can be
reached through that node.
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* Does the protocol support the IP addressing architecture? (if so,
how?)
Yes. Nodes are assigned and addressed by regular IP-addresses.
* Does the protocol require link or neighbor status sensing (if so,
how?)
Yes. The protocol requires link status sensing. This service is
provided by sending/receiving periodic HELLO messages to/from
one hop neighbors.
* Does the protocol depend on a central entity? (if so, how?)
No.
* Does the protocol function reactively? (if so, how?)
No.
* Does the protocol function proactively? (if so, how?)
Yes. Each node periodically sends information about its MPR
selectors, which enables the nodes to construct routes to these
MPR selectors through the node.
* Does the protocol provide loop-free routing? (if so, how?)
Yes. As the protocol uses a link state algorithm, routing is
loop-free when in a stable state.
* Does the protocol provide for sleep period operation? (if so, how?)
No. However, provisions for sleep-operation may easily be
enabled through extensions to the protocol.
* Does the protocol provide some form of security? (if so, how?)
No.
* Does the protocol provide support for utilizing multi-channel,
link-layer technologies? (if so, how?)
Yes. OLSR makes no assumptions on the underlying link-layer
other, than that local broadcast must be available.
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5. Protocol Overview
OLSR is a proactive routing protocol for mobile ad hoc networks.
The protocol inherits the stability of a link state algorithm and
has the advantage of having routes immediately available when
needed due to its proactive nature. OLSR is an optimization over
the pure link state protocol, tailored for mobile ad hoc networks.
Firstly, it reduces the size of the control messages: rather than
declaring all links, a node declares only a subset of links with
its neighbors, namely the links to those nodes which are its MPR
selectors (see section 6 on MPRs). Secondly, OLSR minimizes
flooding of control traffic by using only selected nodes, called
MPRs, to diffuse its messages. This technique significantly
reduces the number of retransmissions in a flooding or broadcast
procedure.
OLSR MAY optimize the reactivity to topological changes by
reducing the time interval for periodic control message
transmission. Furthermore, as OLSR keeps the routes for all
destinations in the network, the protocol is beneficial for
traffic patterns where a large subset of nodes are communicating
with another large subset of nodes, and where the
[source,destination] pairs are changing over time. The protocol is
particularly suited for large and dense networks, as the
optimization done using the MPRs works well in this context. The
larger and more dense a network, the more optimization can be
achieved as compared to the normal link state algorithm.
OLSR is designed to work in a completely distributed manner and
does thus not depend on any central entity. The protocol does NOT
REQUIRE reliable transmission for control messages: each node
sends control messages periodically, and can therefore sustain an
occasional loss of some such messages. Such losses occur frequent
in radio networks due to collisions or other transmission problems.
Also, OLSR does NOT REQUIRE sequenced delivery of messages. Each
control message contains a sequence number which is incremented
for each message. Thus the recipient of a control message can
easily identify which information is newer - even if messages have
been re-ordered while in transmission.
Furthermore, OLSR provides support for protocol extensions such as
sleep mode operation, multicast-routing etc. Such extensions may be
introduced as additions to the protocol without breaking backwards
compatibility with earlier versions.
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OLSR performs hop by hop routing, i.e. each node uses its most
recent local information to route a packet. Hence for OLSR to be
able to route packets, the frequency of control messages should be
tuned to the speed of the mobile nodes such that their movements
can be tracked by their neighborhood.
OLSR does NOT REQUIRE any changes to the format of IP packets. Thus
any existing IP stack can be used as it is: the protocol only
interacts with routing table management.
6. Multipoint Relays
The idea of multipoint relays is to minimize the overhead of
flooding messages in the network by reducing duplicate
retransmissions in the same region. Each node in the network
selects a set of nodes in its neighborhood which may retransmit
its messages. This set of selected neighbor nodes is called the
"Multipoint Relay" (MPR) set of that node. The neighbors of node N
which are *NOT* in its MPR set, receive and process broadcast
messages but do not retransmit broadcast messages received from
node N.
Each node selects its MPR set among its one hop neighbors. This
set is selected such that it covers (in terms of radio range) all
nodes that are two hops away. The neighborhood of any node N can
be defined as the set of nodes which have a symmetric link to
N. The 2-hop neighborhood of N can be defined as the set of nodes
which don't have a symmetric link to N but have a symmetric link
to the neighborhood of N. The MPR set of N, denoted as MPR(N), is
then an arbitrary subset of the neighborhood of N which satisfies
the following condition: every node in the 2-hop neighborhood of N
must have a symmetric link toward MPR(N). The smaller the MPR set
is, the more optimal is the routing protocol. [2] gives an
analysis and example about MPR selection algorithms.
Each node maintains information about a set of its neighbors. This
is the set of neighbors, called the "Multipoint Relay Selector
set" (MPR selector set), which have selected the node as a MPR. A
node obtains this information from the periodic HELLO messages
received from the neighbors. A broadcast message, intended to be
diffused in the whole network, coming from these MPR selector
neighbor nodes is assumed to be retransmitted by the node. This
set can change over time (i.e. when a node selects another
MPR-set) and is indicated by the selector nodes in their HELLO
messages. Each node has a specific "Multipoint relay Selector
Sequence Number" (MSSN) associated with this set. Whenever its MPR
selector set is updated, the node also increments its MSSN.
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OLSR relies on selection of MPRs, and calculates routes through
these nodes. I.e. MPR nodes are selected as intermediate nodes in
the path between a source and a destination. To enable this, each
node in the network periodically broadcast the information
describing which neighbors have selected it as a MPR. Upon
receipt of this "MPR Selector" information, each node calculates
or updates the route to each known destination. So principally,
the route is a sequence of hops through the MPRs from source to
the destination.
MPRs are selected among the one hop neighbors with "symmetric"
i.e. bi-directional link. Therefore, selecting the route through
MPRs automatically avoids the problems associated with data packet
transfer on uni-directional links such as the problem of not
getting an acknowledgment for the data packets at each hop.
7. Protocol Functioning
This section describes the details of the protocol functioning.
This includes descriptions of the format and contents of the
packets being exchanged by routers, the algorithms (e.g. for
packet handling and routing table calculation) and suggested data
structures internally in each router.
7.1. Protocol and Port Number
Packages in OLSR are communicated using UDP. Port 698 has been
assigned by IANA for exclusive usage by the OLSR protocol.
7.2. Packet Format
OLSR communicates using an unified packet format for all data
related to the protocol. The purpose of this is to facilitate
extensibility of the protocol without breaking backwards
compatibility as well as to provide an easy way of piggybacking
different "types" of information into a single transmission. These
packets are embedded in UDP datagrams for transmission over the
network. The present draft uses IPv4 addresses. Support for IPv6
will be included in a future draft.
Each package encapsulates one or more messages. The messages share
a common header format, which enables nodes to correctly accept
and (if applicable) retransmit messages of an unknown type.
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Messages can be flooded onto the entire network, or flooding can
be limited to nodes within a diameter (in terms of number of hops)
from the originator of the message. Thus, broadcasting a message
to a nodes neighborhood is just a special case of flooding. When
flooding any control message, duplicate retransmissions will be
eliminated locally (i.e. each node maintains a duplicate table to
prevent transmitting the same message twice) and minimized in the
entire network through the usage of MPRs as described in section 5
and 6.
Furthermore, a node can examine the header of a message to obtain
information on the distance (in terms of number of hops) to the
originator of the message. This feature may be useful in
situations where, e.g., the time information from a recieved
control messages is stored in a node depends on the distance to
the originator.
The basic layout of any packet in OLSR will be as follows:
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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Packet Length | Reserved for future use |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Originator Address |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Message Size | Time To Live | Hop Count |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Message Type | Message Sequence Number |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
: MESSAGE :
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Originator Address |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Message Size | Time To Live | Hop Count |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Message Type | Message Sequence Number |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
: MESSAGE :
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
: :
(etc)
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7.2.1. Packet Header
Packet Length
The length (in bytes) of the packet
Reserved for future use
MUST be set to '0000000000000000' to be in compliance with this
version of the draft.
The sender information for a packet is obtainable from the UDP
header.
7.2.2. Message Header
Originator Address
This field contains the address of the node, which has
originally generated this message. This field SHOULD NOT be
confused with the source address from the UDP header, which is
changed each time to the address of the intermediate node which
is "re-transmitting" this message. The Originator Address field
MUST *NEVER* be changed in the retransmissions.
Message Sequence Number
While generating the TC message, the "originator" node will
assign a unique identification number to each message. This
number is inserted into the Sequence Number field of the
message. The sequence number is increased by 1 (one) for each
message originating from the node - "wrap-arounds" are handled
as described in section 10.
Message Size
This field gives the size of this message, measured from the
beginning of the "Message Type" field and until the beginning
of the next "Message Type" field (or - if there are no
following messages - the end of the packet).
Message Type
This field indicates which type of message are to be found in
the "MESSAGE" partition. Message types in the range of 0-127 are
reserved for messages in this draft and in
draft-ietf-manet-olsr-extensions-00.txt.
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Time To Live
This field contains the maximum number of hops a message will
be retransmitted. Before a message is transmitted, the Time To
Live MUST be decremented by 1. When a node receives a message
with a Time To Live equal to 0, the message MUST NOT be
retransmitted under any circumstances.
Thus, by setting this field, the originator of a message can
limit the flooding radius.
Hop Count
This field will contain the number of hops a message has
attained. Before a message is (re-) transmitted, the Hop Count
MUST be incremented by 1.
Initially, this is set to '0' by the originator of the message.
Reserved
This field is reserved for future usage, and MUST be set to
'00000000' for compliance with this draft.
7.2.3. Packet Processing
Upon receiving a basic packet, the protocol parser examines each
of the "message headers". Based on the value of the "Message Type"
field, the parser can determine the faith of the message. A node
may receive the same message in several packets. This can happen
only if the message is retransmitted by two nodes in the receivers
neighborhood, i.e. the "Time To Live" and the "Hop Count" fields
in the message satisfies the following condition:
Time To Live + Hop Count > 1
Thus, to avoid re-processing of a message which was already
received and processed, each node maintains a Duplicate table. In
this table, the node records information about the most recently
received messages where the above condition holds. For each
message, satisfying the above condition, a node records a
"Duplicate Tuple" (D_addr, D_seq_num, D_time), where D_addr is the
originator address of the message, D_seq_num is the message
sequence number of the message and D_time specifies the time at
which a tuple expires and *MUST* be removed.
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In a node, the set of Duplicate Tuples are denoted the "Duplicate
set".
Thus, upon receiving a basic packet, a node performs the following
tasks for each encapsulated message:
1. If there exists a tuple in the duplicate set, where:
D_addr == Originator Address, AND
D_seq_num == Message Sequence Number
then the message has already been completely processed
and MUST silently be ignored.
2. Otherwise, if the Message Type of the message is known to
the node, the message MUST be processed according to the
specifications of such message type.
3. Otherwise, If the Message Type of the message is not known
to the node, the message MUST be processed according to the
following algorithm:
3.1 If the sender of the message is not in the MPR selector
set of the node, the message MUST silently be dropped.
3.2 If the time to live of the message is less than or equal
to '0' (zero), the message MUST silently be dropped.
3.3 Otherwise, if the sender of the message is an MPR
selector of this node and if the time to live of the
message is greater than '0' (zero), the message MUST be
forwarded according to the following algorithm:
3.3.1 The time to live of the message is reduced by one.
3.3.2 The hop-count of the message is increased by one
3.3.3 An entry in the duplicate set is recorded with:
D_addr = originator address
D_seq_num = Message Sequence Number
D_time = current time + D_HOLD_TIME.
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3.3.4 The message is retransmitted (Notice: The
remaining fields of the message header SHOULD
be left unmodified.)
Notice: known message types are *not* forwarded "blindly" by this
algorithm. Forwarding (and setting the correct message header in
the forwarded, known, message) is the responsibility of the
algorithm specifying how the message is to be handled. This
enables, e.g., a message type to be specified such that the
message can be modified while in transit (e.g. to reflect the
route the message has taken). Further, it enables that the
optimization through the MPRs can be bypassed: if for some reason
pure flooding of a message type is required (e.g. to transmit
control information over unidirectional links), the algorithm
specifying handling of these messages will simply rebroadcast
the message, regardless of MPR selectors.
Finally, notice that a message, which is to be broadcast in the
neighborhood, but not flooded into the entire network, (e.g. a
HELLO-message) is simply specified by setting the time to live to
'0' (zero), and that no duplicate entries are recorded for such
messages.
By defining a set of message types, which MUST be recognized by all
implementations of OLSR, it will be possible to extend the protocol
through introduction of additional message types, while still be
able to maintain compatibility with older implementations. The two
REQUIRED message types for OLSR are:
- HELLO-messages, performing the task of neighbor sensing.
- TC-messages, performing the task of MPR information declaration.
Extensions may e.g. be PC-messages for enabling power conservation
/ sleep mode, multicast routing, gateway announcements,
auto-configuration/address assignment etc.
7.3. Neighbor sensing
7.3.1. Neighbor sensing information base
7.3.1.1 Neighbor information
A node maintains information (obtained from the HELLO messages)
about its one hop neighbors, the status of the link with these
neighbors, a list of 2-hop neighbors that these one hop neighbors
give access to, and an associated holding time.
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Thus, for each neighbor, a node records a "Neighbor Tuple"
(N_addr, N_status, N_time) where N_addr is the address of the
neighbor, N_status designates the status of the link with that
neighbor (MPR, symmetric, heard) and N_time specifies the time at
which this record expires and *MUST* be removed.
Likewise, a node records a set of "2-hop tuples" (N_addr,
N_2hop_addr, N_time), describing symmetric or MPR links between
its neighbors and the 2-hop neighborhood. N_addr is the address of
a neighbor, N_2hop_addr is the address of a 2-hop neighbor and
N_time specifies the time at which a tuple expires and *MUST*
be removed.
In a node, the set of Neighbor Tuples are denoted the "Neighbor
Set" and the set of 2-hop tuples are denoted the "2-hop neighbor
set".
7.3.1.2 MPR Selector information
A node maintains information (obtained from the HELLO messages)
about the neighbors which have selected the node as a MPR.
Thus, a node records a MPR-selector tuple (MS_addr, MS_time), for
each neighbor which has selected the node as MPR. MS_addr is the
address of a node which has selected the node as MPR, and MS_time
specifies the time at which a tuple expires and *MUST* be
removed.
In a node, the set of MPR-tuples are denoted the "MPR selector
set" A sequence number, MSSN, is associated with this
set. Whenever a tuple is added or removed to this set, the MSSN is
incremented by 1.
7.3.2. HELLO message broadcast
Each node should detect the neighbor nodes with which it has a direct
and symmetric link. The uncertainties over radio propagation may
make some links asymmetric. Consequently, all links MUST be checked
in both directions in order to be considered valid.
To accomplish this, each node broadcasts HELLO messages,
containing information about neighbors and their link status. The
link status may either be "symmetric", "heard" (asymmetric) or
"MPR". "Symmetric" indicates, that the link has been verified to
be bi-directional, i.e. it is possible to transmit data in both
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INTERNET-DRAFT Optimized Link State Routing 2 March 2001
directions. "Heard" indicates that the node can hear HELLO
messages from a neighbor, but it is not confirmed that this
neighbor is also able to receive messages from the node. "MPR"
indicates, that a node is selected by the sender as a MPR. A
status of MPR further implies that the link is symmetric.
These control messages are broadcast to all one-hop neighbors, but
are *not relayed* to further nodes. A HELLO-message contains:
- a list of addresses of neighbors, to which there exists a
symmetric link;
- a list of addresses of neighbors, which have been "heard";
- a list of neighbors, which have been selected as MPRs.
The list of neighbors in a HELLO message can be partial (e.g. due
to message size limitations, imposed by the network), the rule
being that all neighbor nodes are cited at least once within a
predetermined refreshing period (HELLO_INTERVAL).
To accommodate for the above constraints, as well as to accommodate
for future extensions, an approach similar to the overall packet
format (see section 6.1) is taken. Thus the proposed format of a
HELLO message is:
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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Link Type | Reserved | Link Message Size |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Neighbor Address |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Neighbor Address |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
: . . . :
: :
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Link Type | Reserved | Link Message Size |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Neighbor Address |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Neighbor Address |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
: :
: :
(etc)
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This is sent as the data-portion of the general packet format
described in 6.1, with the "Message Type" set to HELLO_MESSAGE and
the TTL field set to 0.
7.3.2.1. Description of the fields
MPR Sequence Number
This field indicates the sequence number corresponding to the
most recent MPR set, calculated by the sender node.
Link Message Size
The size of the link message, measured from the beginning of
the "Link Type" field and until the next "Link Type" field (or
- if there are no more link types - the end of the message).
Link Type
This field specifies the type of link the sending node has to
the following list of neighbors. As a minimum, the following
three link types are REQUIRED by OLSR:
- ASYM_LINK - indicating that the links between the sender
and the neighbors in the following list are asymmetric
(i.e. the neighbor is "heard").
- SYM_LINK - indicating that the links between the sender
and the neighbors in the following list are symmetric.
- MPR_LINK - indicating, that the nodes in the following
list have been selected by the sender as MPR.
(Notice: this implies, that the links from the sender
of the HELLO and to the nodes in the list are symmetric).
It is possible to provide additional information by specifying
additional link-types, e.g. LOST_LINK - indicating that the link
between the sender and the neighbors in the following list has
been lost. Upon processing a HELLO message, a node silently
ignores link-types, which are unknown.
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Reserved
This field is reserved for future usage, and MUST be set to
000000000000000000000000 for compliance with this draft.
Neighbor Address
The address of a neighbor.
7.3.3. HELLO message processing
Upon receiving a HELLO message, the node SHOULD update the
neighbor information corresponding to the sender node address (a
node may - e.g. for security reasons - wish to restrict updating
the neighbor-table, i.e. ignoring HELLO messages from some nodes).
In this section, the term "Originator Address" will be used for
the address of the node which sent the HELLO-message.
1. If there exists a neighbor tuple with N_addr = Originator
Address:
1.1 if for that tuple N_status == ASYM_LINK:
1.1.1 if the node finds its own address among the
addresses listed in the HELLO message (with Link
Type ASYM_LINK, SYM_LINK or MPR_LINK), it updates
the N_status of the tuple to SYM_LINK and sets
N_time = current time + NEIGHB_HOLDING_TIME.
1.1.2 otherwise, if the node does not find its own
address among the addresses listed in the HELLO
message, it sets N_time = current time +
NEIGHB_HOLDING_TIME.
1.2 otherwise, if for that tuple:
N_status == SYM_LINK OR
N_status == MPR_LINK
then:
1.2.1 if the node finds its own address among the addresses
listed in the HELLO message (with Link Type
ASYM_LINK, SYM_LINK or MPR_LINK), it sets N_time =
current time NEIGHB_HOLDING_TIME.
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2. Otherwise, a new neighbor tuple is created with:
N_addr = Originator Address
N_status with the value of SYM_LINK if the node
finds its own address (with Link Type ASYM_LINK, SYM_LINK
or MPR_LINK) among the addresses listed in the HELLO
message, and to the value of ASYM_LINK otherwise
N_time = current time + NEIGHB_HOLD_TIME
The 2-hop neighbor set is updated as follows: for each 2-hop
neighbor address listed in the HELLO message with Link Type
SYM_LINK or MPR_LINK:
1. if a 2-hop tuple exists with:
N_addr == Originator Address AND
N_2hop_address == the address of the 2-hop neighbor,
then the N_time of that tuple is set to:
N_time = current time + NEIGHB_HOLD_TIME
2. otherwise a new 2-hop tuple is created with:
N_addr = Originator Address,
N_2hop_address = the address of the 2-hop neighbor,
N_time = current time + NEIGHB_HOLD_TIME.
Based on the information obtained from the HELLO messages, each
node construct its MPR selector set.
Thus, upon receiving a HELLO message, if a node finds its own
address in the address list with a link type of "MPR", it MUST
update the MPR selector set to contain updated information about
the sender of the HELLO message:
1. If a MPR selector tuple exists with:
MS_addr == Originator Address
then the expiration time of that tuple is set to:
MS_time = current time + NEIGHB_HOLD_TIME.
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2. Otherwise, a new MPR selector tuple is created with:
MS_addr = Originator Address
MS_time = current time + NEIGHB_HOLD_TIME
2.1 MSSN is incremented by one to indicate that the MPR
selector table has been changed.
If link layer information describing connectivity to neighboring
nodes is available (i.e. loss of connectivity such as through
absence of an acknowledgment), this MAY be used in addition to
the information from the HELLO-messages to maintain the neighbor
table and the MPR selector table as described in section 7.7.
7.4. Multipoint relay selection
Each node in the network selects independently its own set of
MPRs. MPRs are used to flood control messages from that node into
the network while reducing the number of retransmissions that will
occur in a region. Thus, the concept of MPRs is an optimization
of a pure flooding mechanism.
The MPR set must be calculated by a node in a way such that it,
through the neighbors in the MPR-set, can reach all 2-hop
neighbors. This means that the union of the neighbor sets the MPR
nodes contains the entire 2-hop neighbor set. While it is not
essential that the MPR set is minimal, it is essential that all
2-hop neighbors can be reached through the selected MPR nodes. The
smaller a MPR-set, however, the more optimizations are achieved.
By default, the MPR set can coincide with the entire neighbor
set. This will be the case at network initialization.
The following specifies a proposed heuristic for selection of MPRs
[2]. The following terminology will be used in describing this
algorithm:
N: The net of neighbors with which there exists a
symmetric link.
N2: The set of 2-hop neighbors. This set does not contain
any one hop neighbors.
D(y): Degree of one hop neighbor node y (where y is a member
of N), is defined as the number of symmetric one hop
neighbors of node y, EXCLUDING the node performing the
computation and all its direct neighbors.
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The proposed heuristic is as follows:
1. Start with an empty MPR set
2. Calculate D(y), where y is a member of N, for all nodes
in N.
3. Select as MPRs those nodes in N which provide the
"only path" to some nodes in N2
4. While there exist nodes in N2 which are not covered by MPR:
4.1 For each node in N, calculate the number of nodes in
N2 which are not yet covered by MPR and are
reachable through this one hop neighbor;
4.2 Select as a MPR that node of N which reaches the
maximum number of uncovered nodes in N2. In case of a
tie, select that node as MPR whose D(y) is greater.
5. As an optimization, process each node y in MPR.
If MPR\{y} still covers all nodes in N2, Y SHOULD be
removed from the MPR set.
After selecting the MPRs among the neighbors, the link status of
the corresponding one hop neighbors is changed from SYM_LINK to
MPR_LINK in the neighbor table. MPR_Seq_Num value in the Neighbor
table is also incremented by one.
The MPR set is re-calculated when:
- a change in the neighborhood is detected, i.e. either a
symmetric link with a neighbor is failed, or a new neighbor
with a symmetric link is added; or
- a change is detected in the 2-hop neighborhood such that
a symmetric link is either detected or broken between a
2-hop neighbor and a neighbor.
7.5. Multipoint relay information declaration
7.5.1 Topology information base
Each node in the network maintains topological information about
the network. This information is acquired from TC-messages and
used for routing table calculations.
Thus, for each destination in the network, a "Topology Tuple"
(T_dest, T_last, T_seq, T_time) is recorded. T_dest is the address
of a node, which may be reached in one hop from the node with the
address T_last. T_seq is a sequence number, and T_time specifies
the time at which this tuple expires and *MUST* be removed.
In a node, the set of Topology Tuples are denoted the "Topology
Set".
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7.5.2. TC Message Broadcast
In order to build the topology information base needed, each node,
which has been selected as MPR, broadcasts Topology Control (TC)
messages. TC messages are flooded to all nodes in the network and
take advantage of MPRs. MPRs enable a better scalability in the
distribution of topology information [1].
A TC message is sent by a node in the network to declare its MPR
Selector set. I.e., the TC message contains the list of neighbors
which have selected the sender node as a MPR. The sequence number
(MSSN) associated with this MPR selector set is also sent with the
list. The list of addresses can be partial in each TC message
(e.g. due to message size limitations, imposed by the network),
but parsing of all TC messages describing a nodes MPR selector set
MUST be complete within a certain refreshing period
(TC_INTERVAL). The information diffused in the network by these TC
messages will help each node to calculate its routing table. A
node which has an empty MPR selector set, i.e. nobody has selected
it as a MPR, MUST NOT generate any TC message.
A node MAY transmit additional TC-messages to increase its
reactiveness to link failures. I.e. when a change to the MPR
selector set is detected and this change can be attributed to a
link failure, a TC-message SHOULD be transmitted after a shorter
interval than TC_INTERVAL.
The proposed format of a TC message is
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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| MSSN | Reserved |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Multipoint Relay Selector Address |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Multipoint Relay Selector Address |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| ... |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
This is sent as the data-portion of the general message format
described in 6.1, with the "Message Type" set to TC_MESSAGE. The
time to live SHOULD be set to 255 (maximum value) to diffuse the
message into the entire network.
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INTERNET-DRAFT Optimized Link State Routing 2 March 2001
7.5.2.1. Description of the fields
MPR Selector Sequence Number (MSSN)
A sequence number is associated with the MPR selector
set. Every time a node detects a change in its MPR selector
set, it increments this sequence number. This number is sent in
this MSSN field of the TC message to keep track of the most
recent information. When a node receives a TC message, it can
decide on the basis of this MPR Sequence Number, whether or not
the received information about the MPR selectors of the
originator node is more recent than what it already has.
Multipoint Relay Selector Address (MPR-S)
This field contains the address of a node, which has selected
the Originator node (of the TC message) as a MPR. All
addresses of the MPR selectors of the Originator node are put
in the TC message. If the maximum allowed message size (as
imposed by the network) is reached while there are still MPR
selector addresses which which have not been inserted into the
TC-message, more TC messages will be generated until the entire
MPR selector set has been sent.
Reserved
This field is reserved for future usage, and MUST be set to
'0000000000000000' for compliance with this draft.
7.5.3. TC Message Processing
TC messages are broadcasted and retransmitted by the MPRs in order
to diffuse the messages in the entire network.
In this section, the term "originator" is used to designate the
node from which the message originally originated, while the term
"sender" is used to designate the node from which the message was
received (i.e. the "last hop" of the message).
The tuples in the topology set are recorded with the topology
information that is exchanged through TC messages, following the
following algorithm:
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INTERNET-DRAFT Optimized Link State Routing 2 March 2001
1. If the sender (NB: not originator) of this message is not in
the neighbor set of this node, the message is discarded.
2. A tuple is inserted into the duplicate table to prevent
being processed again with:
D_addr = originator address
D_seq_num = Message Sequence
D_time = current time + D_HOLD_TIME.
3. If there exist some tuple in the topology set where:
T_last == originator address AND
T_seq > MSSN,
then no further processing of this TC message is performed
and the message is silently discarded (case: message
received out of order).
4. All tuples in the topology set where:
T_last == originator address AND
T_seq < MSSN
are removed from the topology set.
5. For each of the MPR selector address received in the TC
message:
5.1 If there exist some tuple in the topology set where:
T_dest == MPR selector address, AND
T_last == originator address,
then the holding time of that tuple is set to:
T_time = current time + TOP_HOLD_TIME.
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5.2 Otherwise, a new tuple is recorded in the topology set
where:
T_dest = MPR selector address,
T_last = originator address,
T_seq = MSSN,
T_time = current time + TOP_HOLD_TIME.
7.6. Routing table calculation
Each node maintains a routing table which allows it to route the
messages for the other destinations in the network. The routing
table is based on the information contained in the neighbor set
and the topology set. Therefore, if any of these tables is
changed, the routing table is re-calculated to update the route
information about each destination in the network. The route
entries are recorded in the routing table in the following format:
1. R_dest R_next R_dist
2. R_dest R_next R_dist
3. ,, ,, ,,
Each entry in the table consists of R_dest, R_next and R_dist,
which specifies that the node identified by R_dest is estimated to
be R_dist hops away from the local node, and that the one hop
neighbor node with address R_next is the next hop node in the route
to R_dest. Entries are recorded in the table for each destination
in the network for which the route is known. All the destinations
for which the route is broken or partially known are not entered in
the table.
This routing table is updated when a change is detected in
the neighbor set, or the topology set. The update of this routing
information does not generate or trigger any messages to be
transmitted, neither in the network, nor in the one-hop
neighborhood.
To construct the routing table of node X, a shortest path
algorithm is run on the directed graph containing the arcs X -> Y
where Y is any one hop neighbor of X (with Link Type SYM_LINK or
MPR_LINK) and the arcs U -> V where there exists an entry in the
topology set with V as T_dest and U as T_last.
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The following procedure is given as an example to calculate (or
re-calculate) the routing table :
1. All the entries from the routing table are removed.
2. The new routing entries are added starting with the one
hop neighbors (h=1) as the destination nodes. For each neighbor
entry in the neighbor table, whose Link Type is SYM_LINK or
MPR_LINK, a new routing entry is recorded in the routing table
where R_dest and R_next are both set to the address of the
neighbor and R_dist is set to 1.
3. The new route entries for the destination nodes h+1 hops
away are recorded in the routing table. The following procedure
is executed for each value of h, starting with h=1 and
incrementing it by 1 each time. The execution will stop if no
new entry is recorded in an iteration.
3.1 For each topology entry in the topology table, if its
T_dest does not correspond to R_dest of any route entry
in the routing table AND its T_last corresponds to R_dest
of a route entry whose R_dist is equal to h, then a new
route entry is recorded in the routing table where :
- R_dest is set to T_dest;
- R_next is set to R_next of the route entry whose
R_dest is equal to T_last; and
- R_dist is set to h+1.
7.7 Link layer notification
OLSR is designed not to impose or expect any specific information
from the link layer. However, if information from the link-layer
is available, a node MAY use this as described in this section.
If link layer information describing connectivity to neighboring
nodes is available (i.e. loss of connectivity such as through
absence of a link layer acknowledgment), this information is
used in addition to the information from the HELLO-messages to
maintain the neighbor set and the MPR selector set.
Subsequently, detection of a link failure through a link-layer
notification may trigger additional TC-messages to increase the
protocols reactiveness to link failures. I.e. when a change to the
MPR selector set is detected and this change can be attributed to
a link failure, a TC-message SHOULD be transmitted.
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Thus, upon receiving a link-layer notification that the link
between a node and any neighbor is broken, the following actions
are taken:
1. if the link is broken to either a symmetric or asymmetric
neighbor, the tuple for that neighbor is removed from the
neighbor set,
2. if the link is broken to a neighbor, which is selected as MPR,
the tuple for that neighbor is removed from the neighbor
set and the MPR set is recalculated,
3. if the link is broken to a neighbor, which has selected this
node as MPR, the MPR selector set is updated and a
TC message SHOULD be generated.
8. Packet forwarding
8.1. Data packet forwarding
OLSR itself does not perform packet forwarding. Rather, it
maintains the routing table in the underlying operating system,
which is assumed to be forwarding packets as specified in RFC1812.
8.2. Control message forwarding
Control messages, destined for flooding into the entire network,
SHOULD be relayed by the MPR via the following rule:
A node retransmits a message only when it is received from one
of its MPR selector AND it is not before registered in the
duplicate table AND the time to live is greater than zero.
Before retransmitting, the hop count is incremented by one and the
time to live is decremented by one.
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9. Proposed values for the constants
This section list the values for the constants used in the
description of the protocol.
HELLO_INTERVAL = 2 seconds
TC_INTERVAL = 5 seconds
NEIGHB_HOLD_TIME = 3 x HELLO_INTERVAL
TOP_HOLD_TIME = 3 x TC_INTERVAL
D_TIME = 30 seconds
HELLO_MESSAGE = 1
TC_MESSAGE = 2
ASYM_LINK = 1
SYM_LINK = 2
MPR_LINK = 3
10. Sequence Numbers
Sequence numbers are used in OLSR with the purpose of discarding
"old" information, i.e. messages received out of order. However
with a limited number of bits for representing sequence numbers,
wrap-arounds (that the sequence number is incremented from the
maximum possible value to zero) will occur. To prevent this from
interfering with the operation of the protocol, the following
MUST be observed.
The term MAXVALUE designates in the following the largest possible
value for a sequence number.
The sequence number S1 is said to be "greater than" the sequence
number S2 iff:
S1-S2 < MAXVALUE/2 OR
S1 < MAXVALUE/2 AND S2 > S1 + MAXVALUE/2
Thus when comparing two messages, it is possible - even in the
presence of wrap-around - to determine which message contains the
most recent information.
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INTERNET-DRAFT Optimized Link State Routing 28 February 2001
11. References
1. P. Jacquet, P. Minet, P. Muhlethaler, N. Rivierre. Increasing
reliability in cable free radio LANs: Low level forwarding in
HIPERLAN. Wireless Personal Communications, 1996
2. A. Qayyum, L. Viennot, A. Laouiti. Multipoint relaying: An
efficient technique for flooding in mobile wireless networks.
INRIA research report RR-3898, 2000
3. ETSI STC-RES10 Committee. Radio equipment and systems: HIPERLAN
type 1, functional specifications ETS 300-652, ETSI, June 1996
4. Corson et al. Internet MANET Encapsulation Protocol. Internet
draft, draft-ietf-manet-imep-spec-01.txt, Work in progress.
5. Perkins, C.E., Mobile Ad Hoc Networking Terminology, Internet
draft, draft-ietf-manet-term-00.txt, work in progress.
6. Corson, S., MANET Routing Protocol Applicability Statement,
Internet draft, draft-ietf-manet-appl-00.txt, Work in progress.
7. S. Bradner. Key words for use in RFCs to Indicate Requirement
Levels. Request for Comments (Best Current Practice) 2119,
Internet Engineering Task Force, March 1997.
8. Philippe Jacquet and Laurent Viennot, Overhead in Mobile Ad-hoc
Network Protocols, INRIA research report RR-3965, 2000
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12. Authors' Addresses
Amir Qayyum
Project HIPERCOM
INRIA Rocquencourt
BP 105
78153 Le Chesnay Cedex, France
Phone: +33 1 3963 5273
Email: Amir.Qayyum@inria.fr
Philippe Jacquet
Project HIPERCOM
INRIA Rocquencourt
BP 105
78153 Le Chesnay Cedex, France
Phone: +33 1 3963 5263
Email: Philippe.Jacquet@inria.fr
Anis Laouiti
Project HIPERCOM
INRIA Rocquencourt
BP 105
78153 Le Chesnay Cedex, France
Phone: +33 1 3963 508832
Email: Anis.Laouiti@inria.fr
Laurent Viennot
Project HIPERCOM
INRIA Rocquencourt
BP 105
78153 Le Chesnay Cedex, France
Phone: +33 1 3963 5225
Email: Laurent.Viennot@inria.fr
Paul Muhlethaler
Project HIPERCOM
INRIA Rocquencourt
BP 105
78153 Le Chesnay Cedex, France
Phone: +33 1 3963 5278
Email: Thomas.Clausen@inria.fr
Thomas Clausen
Project HIPERCOM
INRIA Rocquencourt
BP 105
78153 Le Chesnay Cedex, France
Phone: +33 1 3963 5133
Email: Thomas.Clausen@inria.fr
Jacquet, Muhlethaler, Qayyum et. al. Expires 2 September 2001 [Page 31]
