INTERNET-DRAFT Thomas Clausen
IETF MANET Working Group Philippe Jacquet
Expiration: 10 June 2003 Anis Laouiti
Pascale Minet
Paul Muhlethaler
Amir Qayyum
Laurent Viennot
INRIA Rocquencourt, France
10 December 2002
Optimized Link State Routing Protocol
draft-ietf-manet-olsr-07.txt
Status of this Memo
This document is a submission by the Mobile Ad Hoc Networking Working
Group of the Internet Engineering Task Force (IETF). Comments should
be submitted to the manet@itd.nrl.navy.mil mailing list.
Distribution of this memo is unlimited.
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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 classical link state algorithm tailored to the requirements of
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INTERNET-DRAFT Optimized Link State Routing 10 December 2002
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 a
classical flooding mechanism, where every node retransmits each
message when it receives the first copy of the message. In OLSR, link
state information is generated only by nodes elected as MPRs. Thus, a
second optimization is achieved by minimizing the number of control
messages flooded in the network. As a third optimization, an MPR node
may chose to report only links between itself and its MPR selectors.
Hence, as contrary to the classic link state algorithm, partial link
state information is distributed in the network. This information is
then used by the OLSR protocol for route calculation. 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.
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 4
1.1. Changes . . . . . . . . . . . . . . . . . . . . . . . . . 5
1.2. OLSR Terminology . . . . . . . . . . . . . . . . . . . . . 6
1.3. Applicability Section . . . . . . . . . . . . . . . . . . 8
1.4. Protocol Overview . . . . . . . . . . . . . . . . . . . . 8
1.5. Multipoint Relays . . . . . . . . . . . . . . . . . . . . 9
2. Protocol Functioning . . . . . . . . . . . . . . . . . . . . 10
2.1. Packet Format and Forwarding . . . . . . . . . . . . . . . 10
2.1.1. Protocol and Port Number . . . . . . . . . . . . . . . . 11
2.1.2. Multiple Interfaces and Main Address . . . . . . . . . . 11
2.1.3. Packet Format . . . . . . . . . . . . . . . . . . . . . 12
2.1.3.1. Packet Header . . . . . . . . . . . . . . . . . . . . 12
2.1.3.2. Message Header . . . . . . . . . . . . . . . . . . . . 13
2.1.4. Packet Processing and Message Flooding . . . . . . . . . 14
2.1.5. Message Emission and Jitter . . . . . . . . . . . . . . 16
2.2. Neighbor Sensing . . . . . . . . . . . . . . . . . . . . . 17
2.2.1. HELLO Message Format . . . . . . . . . . . . . . . . . . 18
2.2.2. Neighbor Sensing Information Base . . . . . . . . . . . 21
2.2.2.1. Neighbor Set . . . . . . . . . . . . . . . . . . . . . 21
2.2.2.2. 2-hop Neighbor Set . . . . . . . . . . . . . . . . . . 21
2.2.2.3. MPR Set . . . . . . . . . . . . . . . . . . . . . . . 22
2.2.2.4. MPR Selector Set . . . . . . . . . . . . . . . . . . . 22
2.2.3. HELLO Message Generation . . . . . . . . . . . . . . . . 22
2.2.4. HELLO Message Processing . . . . . . . . . . . . . . . . 23
2.2.5. Multipoint Relay Selection . . . . . . . . . . . . . . . 27
2.2.6. Neighborhood and 2-hop Neighborhood Changes . . . . . . 30
2.2.7. Advanced Neighbor Sensing Functioning . . . . . . . . . 30
2.2.7.1. Link Hysteresis . . . . . . . . . . . . . . . . . . . 32
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2.2.7.2. Optional Link layer notification . . . . . . . . . . . 33
2.3. Topology Discovery . . . . . . . . . . . . . . . . . . . . 34
2.3.1. TC Message Format . . . . . . . . . . . . . . . . . . . 34
2.3.2. Topology Information Base . . . . . . . . . . . . . . . 35
2.3.3. Advertized Neighbor Set . . . . . . . . . . . . . . . . 35
2.3.4. TC Message Generation . . . . . . . . . . . . . . . . . 36
2.3.5. TC Message Processing . . . . . . . . . . . . . . . . . 37
2.3.6. Multiple Interface Association . . . . . . . . . . . . . 39
2.3.6.1. Multiple Interface Association Information Base . . . 39
2.3.6.2. MID Message Format . . . . . . . . . . . . . . . . . . 39
2.3.6.3. MID Message Generation . . . . . . . . . . . . . . . . 40
2.3.6.4. MID Message Processing . . . . . . . . . . . . . . . . 40
2.3.7. Associated Networks and Hosts . . . . . . . . . . . . . 42
2.3.7.1. HNA Message Format . . . . . . . . . . . . . . . . . . 42
2.3.7.2. Host and Network Association Information Base . . . . 43
2.3.7.3. HNA Message Generation . . . . . . . . . . . . . . . . 43
2.3.7.4. HNA Message Processing . . . . . . . . . . . . . . . . 43
2.3.8. Routing Table Calculation . . . . . . . . . . . . . . . 45
2.3.9. Advanced Topology Discovery Functioning . . . . . . . . 48
2.3.9.1. Reaction to Link Failure with a MPR Selector . . . . . 48
2.3.9.2. Advanced Fast Re-routing Mechanism . . . . . . . . . . 48
2.3.9.2.1. FRR Message Format . . . . . . . . . . . . . . . . . 49
2.3.9.2.2. FRR Message Generation . . . . . . . . . . . . . . . 50
2.3.9.2.3. FRR Message Processing . . . . . . . . . . . . . . . 50
3. Node Configuration . . . . . . . . . . . . . . . . . . . . . 51
3.1. Address Assignment . . . . . . . . . . . . . . . . . . . . 51
3.2. Routing Configuration . . . . . . . . . . . . . . . . . . 51
3.3. Data Packet Forwarding . . . . . . . . . . . . . . . . . . 51
4. IPv6 Considerations . . . . . . . . . . . . . . . . . . . . 51
5. Security Considerations . . . . . . . . . . . . . . . . . . 52
5.1. Confidentiality . . . . . . . . . . . . . . . . . . . . . 52
5.2. Integrity . . . . . . . . . . . . . . . . . . . . . . . . 52
6. Proposed Values for the Constants . . . . . . . . . . . . . 53
7. Sequence Numbers . . . . . . . . . . . . . . . . . . . . . . 54
8. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . 55
9. Authors' Addresses . . . . . . . . . . . . . . . . . . . . . 55
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1. Introduction
The Optimized Link State Routing Protocol (OLSR) is developed for
mobile ad hoc networks. It operates as a table driven, proactive pro-
tocol. I.e., exchanges topology information with other nodes of the
network regularly. Each node selects a set of its neighbor nodes as
"multipoint relays" (MPR). In OLSR, only nodes, selected as such
MPRs, are responsible for forwarding control traffic, intended for
diffusion into the entire network. I.e. MPRs provide an efficient
mechanism for flooding control traffic by reducing the number of
transmissions required.
Nodes, selected as MPRs, also have a special responsibility when
declaring link state information in the network. Indeed, the only
requirement for OLSR to provide routes to all destinations is that
MPR nodes declare link-state information for links between them self
and their MPR selectors. Additional available link-state information
may be utilized e.g. for redundancy.
These nodes which have been selected as a multipoint relay by some
neighbor nodes announce this information periodically in their con-
trol messages. Thereby, a node announces to the network, that it has
reachability to the nodes which have selected it as MPR. In route
calculation, the MPRs are used to form the route from a given node to
any destination in the network. Furthermore, the protocol uses the
MPRs to facilitate efficient flooding of control messages in the net-
work.
A node selects MPRs from among its one hop neighbors with "symmetri-
cal", i.e. bi-directional, linkages. Therefore, selecting the route
through MPRs automatically avoids the problems associated with data
packet transfer over uni-directional links (such as the problem of
not getting link-layer acknowledgments for data packets at each hop)
OLSR is developed to work independently from other protocols. Like-
wise, OLSR makes no assumptions about the underlying link-layer.
OLSR inherits the concept of forwarding and relaying from HIPERLAN (a
MAC layer protocol) which is standardized by ETSI [3]. The protocol
is developed in the IPANEMA project (part of the Euclid program) and
in the PRIMA project (part of the RNRT program).
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1.1. Changes
Major changes from version 06 to version 07
- Introduction of relay willingness in hellos
- Introduction of redundant link set in TC messages
- Introduction of packet sequence number in order to clarify
link management
Major changes from version 05 to version 06
- Clarification of the usage of link layer notification and fast
re-routing
- Clarification of MPR selection
- Clarification of multiple interfaces
Major changes from version 04 to version 05
- Introduction of support for multiple interfaces
- Introduction of support for associated hosts and networks.
- Introduction of support for advanced neighbor sensing through
hysteresis.
- Modularity between neighbor sensing and topology discovery
emphasized.
Major changes from version 03 to version 04
- Finalized the generic packet/message format to include fea-
tures 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 Conser-
vation may be considered as an extension to the basic routing
capabilities.
1.2. 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 [5]. The OLSR
protocol uses the following terminology:
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 one.
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.
interface
A network device participating in the MANET (usually a wire-
less device). A node may have several interfaces, each inter-
face assigned an unique IP address.
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link
A link is a pair of interfaces (from two different nodes) sus-
ceptible to hear one another (i.e. one may be able to receive
traffic from the other). A node is said to have a link to
another node when one of its interface has a link to one of
the interfaces of the other node.
neighbor interface
An interface I is a neighbor interface of an interface J if J
can hear I (i.e. it is possible to send traffic from I to J).
sender interface
The sender interface of a control message is the neighbor
interface over which the message has been transmitted.
receiver interface
The receiver interface of a control message is the (local)
interface, over which a control message has been received.
neighbor node
A node X is a neighbor node of node Y if node Y can hear node
X (i.e. one of X interfaces is a neighbor interface of some
interface of Y).
2-hop interface
An interface heard by a neighbor.
symmetric link
A bi-directional link between two interfaces, i.e. interface I
and interface J where both can hear each other.
symmetric neighborhood
The symmetric neighborhood of any node X is the set of nodes
which have at least one symmetric link to X.
symmetric 2-hop neighborhood
The symmetric 2-hop neighborhood of X is the set of nodes
which don't have a symmetric link to X but have a symmetric
link to the symmetric neighborhood of X.
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main address
The main address of a node, which will be used in OLSR control
traffic as the "originator address" of all messages emitted by
this node. It is the address of one of its interfaces.
1.3. Applicability Section
OLSR is a proactive routing protocol for mobile ad-hoc networks
(MANETs). It is well suited to large and dense mobile networks, as
the optimization 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 classic 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 proto-
col, compared to a reactive protocol, is even better if these
[source, destination] pairs change over time [4]. Such changes may
initiate substantial traffic (query flooding) in case of a reactive
protocol, but no additional traffic in OLSR, as the routes are main-
tained for each known destination all the time.
1.4. 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 classical link
state protocol, tailored for mobile ad hoc networks.
OLSR minimizes the overhead from flooding of control traffic by using
only selected nodes, called MPRs, to retransmit control messages.
This technique significantly reduces the number of retransmissions
required to flood a message to all nodes in the network. Secondly,
OLSR requires only partial link state to be flooded in order to pro-
vide optimal routes. The minimal set of link state information
required is, that all nodes, selected as MPRs, declare the links to
their MPR selectors. Additional topological information, if present,
may be utilized e.g. for redundancy purposes.
OLSR MAY optimize the reactivity to topological changes by reducing
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the maximum time interval for periodic control message transmission.
Furthermore, as OLSR continuously maintains routes to all destina-
tions 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 chang-
ing over time. The protocol is particularly suited for large and
dense networks, as the optimization done using MPRs works well in
this context. The larger and more dense a network, the more optimiza-
tion can be achieved as compared to the classic 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 of control messages: each node sends control
messages periodically, and can therefore sustain an occasional loss
of some such messages. Such losses occur frequently in radio networks
due to collisions or other transmission problems.
Also, OLSR does not require sequenced delivery of messages. Each con-
trol message contains a sequence number which is incremented for each
message. Thus the recipient of a control message can, if required,
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.
OLSR does not require any changes to the format of IP packets. Thus
any existing IP stack can be used as is: the protocol only interacts
with routing table management.
1.5. 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
symmetric 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 broad-
cast messages received from node N.
Each node selects its MPR set among its one hop symmetric neighbors.
This set is selected such that it covers (in terms of radio range)
all nodes that are two hops away. The MPR set of N, denoted as
MPR(N), is then an arbitrary subset of the symmetric neighborhood of
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N which satisfies the following condition: every node in the symmet-
ric 2-hop neighborhood of N must have a symmetric link towards
MPR(N). The smaller the MPR set is, the more optimal is the routing
protocol. [2] gives an analysis and example of MPR selection algo-
rithms.
Each node maintains information about the set of neighbors that have
selected it as MPR. This set is called the "Multipoint Relay Selector
set" (MPR selector set) of a node. A node obtains this information
from periodic HELLO messages received from the neighbors.
A broadcast message, intended to be diffused in the whole network,
coming from any of the MPR selectors of node N is assumed to be
retransmitted by node N. 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.
2. 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 and the algorithms (e.g. for packet handling and
routing table calculation).
The "Packet Forwarding" and "Neighbor Sensing" mechanisms may be seen
as a transfer layer for the routing protocol. This provides nodes
with information about their neighbors and offers an optimized mecha-
nism for flooding messages through the concept of MPRs. It completely
relies on the exchange of HELLO messages.
The "Topology Discovery" mechanism relies on the "Packet Forwarding"
and "Neighbor Sensing" mechanisms. A node uses neighbor and MPR
information from the "Neighbor Sensing" mechanism in diffusing local
topology information to the larger network. Topology information is
diffused through the "Packet Forwarding" mechanism, and relies on TC,
MID and HNA messages. Resulting from the "Topology Discovery" mecha-
nism is information which allows routing table calculation.
The key notion for these mechanisms is the MPR relationship.
2.1. Packet Format and Forwarding
OLSR communicates using a 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. Also, this
provides an easy way of piggybacking different "types" of information
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into a single transmission, and thus for a given implementation to
optimize towards utilizing the maximal frame-size, provided by the
network. These packets are embedded in UDP datagrams for transmission
over the network. The present draft is presented with IPv4 addresses.
Considerations regarding IPv6 are given in section 4.
Each packet 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.
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 transmitting a message to the
neighborhood of a node is just a special case of flooding. When
flooding any control message, duplicate retransmissions will be elim-
inated locally (i.e. each node maintains a duplicate table to prevent
transmitting the same message twice) and minimized in the entire net-
work through the usage of MPRs as described in later sections.
Furthermore, a node can examine the header of a message to obtain
information on the distance (in terms of number of hops) to the orig-
inator of the message. This feature may be useful in situations
where, e.g., the time information from a received control messages
stored in a node depends on the distance to the originator.
2.1.1. Protocol and Port Number
Packets in OLSR are communicated using UDP. Port 698 has been
assigned by IANA for exclusive usage by the OLSR protocol.
2.1.2. Multiple Interfaces and Main Address
A node may have several wireless interfaces, each of them having a
distinct IP address. OLSR supports such nodes with multiple inter-
faces. For this reason, each node SHOULD choose one of its interface
addresses as its "main address". It is of no importance which address
is chosen, however a node SHOULD always use the same address as its
main address. For example, the smallest interface address may be cho-
sen as the main interface. The main address MUST be used in OLSR con-
trol traffic as the "originator address" of all messages emitted by a
node.
A node must transmit and retransmit all control messages on all
interfaces. The source address in the IP header must contain the IP-
address of the interface where the message is transmitted. This
address will be denoted the "sender interface address".
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2.1.3. Packet Format
The basic layout of any packet in OLSR is as follows (omitting the IP
and UDP headers):
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 | Packet Sequence Number |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Message Type | Reserved | Message Size |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Originator Address |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Time To Live | Hop Count | Message Sequence Number |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
: MESSAGE :
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Message Type | Reserved | Message Size |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Originator Address |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Time To Live | Hop Count | Message Sequence Number |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
: MESSAGE :
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
: :
(etc)
2.1.3.1. Packet Header
Packet Length
The length (in bytes) of the packet
Packet Sequence Number
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The Packet Sequence Number (PSN) MUST be incremented at each
any new OLSR packet transmitted.
The sender information for a packet is obtainable from the IP header.
2.1.3.2. Message Header
Message Type
This field indicates which type of message is to be found in
the "MESSAGE" partition. Message types in the range of 0-127
are reserved for messages in this draft and in extension
drafts.
Reserved
MUST be set to "00000000" to be in compliance with this ver-
sion of the draft.
Message Size
This field gives the size of this message, counted in bytes
and 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 - until the end of the
packet).
Originator Address
This field contains the main address of the node, which has
originally generated this message. This field SHOULD NOT be
confused with the source address from the IP header, which is
changed each time to the address of the intermediate interface
which is re-transmitting this message. The Originator Address
field MUST *NEVER* be changed in retransmissions.
Time To Live
This field contains the maximum number of hops a message will
be transmitted. Before a message is retransmitted, the Time To
Live MUST be decremented by 1. When a node receives a message
with a Time To Live equal to 0 or 1, the message MUST NOT be
retransmitted under any circumstances. Normally, a node would
not receive a message with a TTL of zero.
Thus, by setting this field, the originator of a message can
limit the flooding radius.
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Hop Count
This field contains the number of hops a message has attained.
Before a message is retransmitted, the Hop Count MUST be
incremented by 1.
Initially, this is set to '0' by the originator of the mes-
sage.
Message Sequence Number
While generating a 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 orig-
inating from the node - "wrap-arounds" are handled as
described in a later section. Message sequence numbers are
used to ensure that a given message is not retransmitted more
than once by any node.
2.1.4. Packet Processing and Message Flooding
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 fate of the message. A node may
receive the same message several times. Thus, to avoid re-processing
of some messages which were already received and processed, each node
maintains a Duplicate table. In this table, the node records informa-
tion about the most recently received messages where duplicate pro-
cessing of a message is to be avoided. For such a message, 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.
In a node, the set of Duplicate Tuples are denoted the "Duplicate
set".
In this section, the term "Originator Address" will be used for the
main address of the node which sent the message. The term "Sender
Interface Address" will be used for the sender address (given in the
IP header of the packet containing the message) of the interface
which sent the message.
Thus, upon receiving a basic packet, a node performs the following
tasks for each encapsulated message:
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1 If the time to live of the message is less than or equal to
'0' (zero), the message MUST silently be dropped.
2 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.
3 Otherwise, if the node implements the Message Type of the mes-
sage, the message MUST be processed according to the specifi-
cations for the message type.
4 Otherwise, if the node does not implement the Message Type of
the message the message SHOULD be processed according to the
following algorithm:
4.1 If the sender interface address of the message is not
detected to be in the symmetric neighborhood of the node,
the message MUST silently be dropped.
4.2 If the sender interface address of the message is
detected to be in the symmetric neighborhood of the node,
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.
4.3 If the sender interface address is an interface address
of a MPR selector of this node and if the time to live of
the message is greater than '1', the message MUST be for-
warded according to the following:
4.3.1
The TTL of the message is reduced by one.
4.3.2
The hop-count of the message is increased by one
4.3.3
The message is broadcasted on all interfaces
(Notice: The remaining fields of the message header
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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 classical flooding of a message type is
required (e.g. to transmit control information over unidirectional
links), the algorithm which specifies how such messages should be
handled will simply rebroadcast the message, regardless of MPRs.
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 REQUIRED
message types for OLSR are:
- HELLO-messages, performing the task of neighbor sensing.
- TC-messages, performing the task of topology declaration
(advertisement of link states)
- MID-messages, performing the task of multiple interface decla-
ration.
- HNA-messages, performing the task of associated host and/or
network declaration.
- FRR-messages, performing the task of initiating fast re-rout-
ing in case of link failure.
Extensions may for example be messages enabling power conservation /
sleep mode, multicast routing, support for unidirectional links,
auto-configuration/address assignment etc.
2.1.5. Message Emission and Jitter
As a basic implementation requirement, synchronization of control
messages SHOULD be avoided. As a consequence, periodic OLSR messages
SHOULD be emitted such that they avoid synchronization.
Emission of control traffic from neighboring nodes may, for various
reasons (mainly timer interactions with packet processing), become
synchronized such that several neighbor nodes attempt to transmit
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control traffic simultaneously. Depending on the nature of the under-
lying link-layer, this may or may not lead to collisions and hence
message loss - possibly loss of several subsequent messages of the
same type.
To avoid such synchronizations, the following simple strategy for
emitting control messages is proposed. A node MAY add an amount of
jitter to the interval at which messages are generated. The jitter
must be a random value for each message generated. Thus, for a node
utilizing jitter:
Actual message interval = MESSAGE_INTERVAL - jitter
Where jitter is a value, randomly selected from the interval
[0,MAXJITTER] and MESSAGE_INTERVAL is the value of the message inter-
val specified for the message being emitted (e.g. HELLO_INTERVAL for
HELLO messages, TC_INTERVAL for TC-messages etc.).
Jitter MAY also be introduced when forwarding messages. The follow-
ing simple strategy may be adopted: when a message is to be forwarded
by a node, it should be kept in the node during a short period of
time :
Keep message period = jitter
Where jitter is a random value in [0,MAXJITTER].
Notice that when the node sends a control message, the opportunity to
piggyback other messages (before their keeping period is expired) may
be taken to reduce the number of packet transmissions.
It should be noticed that the present draft imposes a minimal rate of
control message emission. However a node MAY send control messages at
a higher rate (e.g. for better reacting to high mobility). Tuning the
rate of control traffic to the actual conditions under which the pro-
tocol is to operate is an implementation issue.
2.2. Neighbor Sensing
Each node should detect the neighbor interfaces with which it has a
direct and symmetric link. Uncertainties over radio propagation may
make some links unidirectional. Consequently, all links MUST be
checked in both directions in order to be considered valid.
To perform neighbor detection, each node broadcasts HELLO messages,
containing information about heard neighbor interfaces and their link
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status. The link status may either be "symmetric", "heard" (asymmet-
ric), "MPR" or "lost". "Symmetric" indicates, that the link has been
verified to be bi-directional, i.e. it is possible to transmit data
in both directions. "Heard" indicates that the node can hear HELLO
messages from a neighbor interface, but it is not confirmed that this
neighbor interface is also able to receive messages from the node.
MPR indicates that the sender has selected the node as MPR. A status
of MPR further implies that the link is symmetric. "Lost" indicates
that the link with this neighbor interface is now lost.
HELLO messages are broadcast to all one-hop neighbors and are emitted
on each MANET interface of the node. They are *not relayed* to fur-
ther nodes.
More precisely, a HELLO message contains for each interface I:
- a list of neighbor interface addresses, having a symmetric
link to interface I;
- a list of neighbor interface addresses, which are "heard" by
interface I (for historical reasons, the term "asymmetric" is
used for "heard");
- a list of neighbor interface addresses of neighbors selected
as MPRs, AND having a symmetric link to interface I;
- a list of neighbor interface addresses, which have been lost.
These lists are computed as described in the HELLO message generation
section.
2.2.1. HELLO Message Format
To accommodate the above requirements, as well as to accommodate for
future extensions, an approach similar to the overall packet format
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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Reserved | Willingness | # Interfaces |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Interface Address |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
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| Interface Address |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
: . . . :
: :
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Link Type | Interface # | Link Message Size |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Neighbor Interface Address |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Neighbor Interface Address |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
: . . . :
: :
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Link Type | Interface # | Link Message Size |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Neighbor Interface Address |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Neighbor Interface Address |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
: :
: :
(etc)
This is sent as the data-portion of the general packet format
described in the Packet Format and Forwarding section, with the "Mes-
sage Type" set to HELLO_MESSAGE and the TTL field set to 1 (one).
Reserved
This field is reserved for future usage, and MUST be set to
"0000000000000000" for compliance with this draft.
Willingness
This field indicates the "willingness" of the sender to act as
a multipoint relay. The willingness parameter is an integer
between 0 and 7. The value 0 is for nodes which should never
forward packets to other destinations (e.g. because their
power supply is critical). The higher the willingness of a
node, the more likely is the node to be selected as MPR by its
neighbors. The value 7 is reserved for nodes which should
always be selected as multipoint relay (e.g. when the node
belongs to a pre-defined backbone). An OLSR router in normal
operation SHOULD have willingness equal to 3.
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# Interfaces
This field indicates the number of additional MANET interfaces
(in addition to the main interface) possessed by the node. If
the node has only one interface, this number is 0.
Interface Address
This field indicates the addresses of additional MANET inter-
faces. The first one will be referenced as interface address
number 1, the second as interface address number 2, and so on.
The main address of the node (whose address is given in the
originator field of the message header) will be referenced as
interface address number 0.
Link Type
This field specifies the type of the link between the inter-
face of the sender, as identified by the interface number, and
the following list of neighbor interfaces. As a minimum, the
following four link types are REQUIRED by OLSR:
- ASYM_LINK, indicating that the links are asymmetric (i.e.
the neighbor interface is "heard").
- SYM_LINK, indicating that the links are symmetric.
- MPR_LINK, indicating, that the links are symmetric AND
that the neighbors have been selected as MPR by the
sender.
- LOST_LINK, indicating that the links have been lost.
Interface #
This field indicates the number of the interface to which the
following list of neighbor interfaces corresponds. Number 0
indicates the main interface (whose address is the main
address).
Link Message Size
The size of the link message, counted in bytes and 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).
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Neighbor Interface Address
An address of a neighbor interface.
Neighbor sensing is performed using HELLO message exchange, updating
the neighbor sensing information base in each node.
2.2.2. Neighbor Sensing Information Base
The neighbor sensing information base stores information about neigh-
bor interfaces, 2-hop neighbors, MPRs and MPR selectors.
2.2.2.1. Neighbor Set
For each of its interface I and for each neighbor interface NI, heard
by I, a node records a "Neighbor Tuple" (N_if_id, N_if_addr,
N_main_addr, N_willing, N_SYM_time, N_ASYM_time, N_time) where
N_if_id is the identifier number of the local interface I, N_if_addr
is the address of the neighbor interface NI, N_main_addr is the main
address of the neighbor possessing NI, N_willing is the willingness
of the neighbor possessing NI, N_SYM_time is the time until which the
link is considered symmetric, N_ASYM_time is the time until which the
neighbor interface is considered heard, and N_time specifies the time
at which this record expires and *MUST* be removed. When N_SYM_time
and N_ASYM_time are expired, the link is considered lost.
This information is used when declaring the neighbor interfaces in
the HELLO messages.
N_SYM_time and N_ASYM_time are used to decide the Link Type declared
for the neighbor interface. If N_SYM_time is not expired, the link is
declared symmetric. If N_SYM_time is expired but N_ASYM_time is not,
the link is declared heard. If both N_SYM_time and N_ASYM_time are
expired, the link is declared lost.
In a node, the set of Neighbor Tuples are denoted the "Neighbor Set".
2.2.2.2. 2-hop Neighbor Set
A node records a set of "2-hop tuples" (N_main_addr, N_if_addr,
N_2hop_addr, N_2hop_main_addr, N_time), describing symmetric or MPR
links between its neighbors and the symmetric 2-hop neighborhood.
N_main_addr and N_if_addr are the main address and an interface
address of a neighbor, N_2hop_addr is an interface address of a 2-hop
neighbor with a symmetric or MPR link to interface, N_2hop_main_addr
is the main address of the 2-hop node,and N_time specifies the time
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at which the tuple expires and *MUST* be removed.
This information is gathered from the HELLO messages received by a
node from its neighbor nodes. The N_2hop_main_addr is acquired
through the multiple interface association base.
In a node, the set of 2-hop tuples are denoted the "2-hop Neighbor
Set".
2.2.2.3. MPR Set
A node maintains a set of neighbors which are selected as MPR. Their
main addresses are listed in the so-called MPR Set. The Multipoint
relay selection section describes how MPRs are selected.
2.2.2.4. MPR Selector Set
A node maintains information (obtained from the HELLO messages) about
the neighbors which have selected this node as a MPR.
Thus, a node records an MPR-selector tuple (MS_if_addr, MS_main_addr,
MS_time). MS_if_addr is the address of an interface of a node which
has selected the node as MPR, MS_main_addr is the main address of the
MPR selector and MS_time specifies the time at which a tuple expires
and *MUST* be removed.
In a node, the set of MPR-selector tuples are denoted the "MPR Selec-
tor Set".
2.2.3. HELLO Message Generation
The lists of addresses declared in a HELLO message are computed from
the Neighbor Set as follows:
- for each tuple where:
N_SYM_time < current time (i.e. expired) AND
N_ASYM_time < current time (i.e. expired)
N_if_addr is declared with LOST_LINK Link Type for interface
N_if_id;
- for each tuple where:
N_SYM_time >= current time (not expired)
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N_if_addr is declared with MPR_LINK Link Type if N_main_addr
is in the MPR set and SYM_LINK Link Type otherwise;
- for each tuple where:
N_SYM_time < current time (expired) AND
N_ASYM_time >= current time (not expired),
N_if_addr is declared with ASYM_LINK Link Type.
The list of neighbor interfaces in a HELLO message can be partial
(e.g. due to message size limitations, imposed by the network), the
rule being the following: for each interface a neighbor interface is
heard on, its address is cited with corresponding interface id at
least once within a predetermined refreshing period, REFRESH_INTER-
VAL. To keep track of fast connectivity change a HELLO message must
be sent at least every HELLO_INTERVAL period, smaller than or equal
to REFRESH_INTERVAL.
Notice that for limiting the impact from loss of control messages, it
is desirable that a message (plus the generic package header) can fit
into a single MAC frame.
Each HELLO message generated is broadcast on each MANET interface of
the node.
2.2.4. HELLO Message Processing
In this section, the terms "Originator Address", "Sender Interface",
"Receiver Interface", and "Link Interface" are used. They are defined
bellow and illustrated in the following figure:
______________ _____________
| J1 |= -- = |I1 |
| Main -> J2 |= = |I2 <- Main |
| J3 |= -- = |I3 |
-------------- -------------
|
______________ /
| K1 |=
| Main -> K2 |=
| |
--------------
J1, J2 and J3 are the addresses of local interfaces on node J. Like-
wise, I1, I2 and I3 are addresses of local interfaces on node I and
K1 and K2 are addresses of local interfaces on node K. There are
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symmetric links between J1 and I3 and between J3 and K1.
The three nodes have selected, respectively, J2, I2 and K2 as their
"main addresses". I.e. the Originator Address of all OLSR-messages
sent by node J will have the Originator Address J2. For node I and K,
the main addresses will be I2 and K2, respectively
A HELLO message, sent by node J, is transmitted on all node J's
interfaces. Received by node I, the following naming conventions
apply:
- The term "Originator Address" is the main address of the node,
originating a message. In the example above, the Originator
Address of the HELLO is J2.
- The term "Sender Interface" for the HELLO message is the
interface over which the HELLO was transmitted. In the example
above, the Sender Interface Address is J1.
- The term "Receiver Interface" will be used for the interface
which received the HELLO message. In the example above, node
I's "Receiver Interface" for the HELLO generated by node J is
I3.
- The term "Link Interface" will be used in a HELLO message to
designate on which interface (of the sender of a HELLO mes-
sage) a neighbor is detected. In the example above, K1 is
listed in the HELLO message of J with Link Interface J3. The
"Link Interface" is calculated based on the Interface # in the
HELLO message. Notice that an interface address may be listed
several times with different Link Interfaces.
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).
Notice, that a HELLO message MUST neither be forwarded nor be
recorded in the duplicate table.
The Neighbor Set should be updated as follows:
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1 Upon receiving a HELLO message, if there exists no neighbor
tuple with
N_if_addr == Sender Interface Address
and
N_if_id == identifier of the Receiver Interface,
a new tuple is created with
N_if_addr = Sender Interface Address
N_if_id = identifier of the Receiver Interface
N_SYM_time = current time - 1 (expired)
N_time = current time + NEIGHB_HOLD_TIME
2 The tuple (existing or new) with:
N_if_addr == Sender Interface Address
and
N_if_id == identifier of the Receiver Interface,
is then modified as follows:
2.1 N_main_addr = Originator Address.
2.2 N_willing = Originator Willingness
2.4 N_time = max (N_time, current time +
NEIGHB_HOLD_TIME);
2.5 N_ASYM_time = current time + NEIGHB_HOLD_TIME;
2.6 if the node finds the Receiver Interface Address among
the addresses listed in the HELLO with:
Link Interface Address == Sender Interface Address,
then, the tuple is modified as follows:
if Link Type == LOST_LINK then
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N_SYM_time = current time - 1 (i.e. expired)
else:
N_SYM_time = current time + NEIGHB_HOLD_TIME,
N_time = current time + 2 *
NEIGHB_HOLD_TIME.
The rule for setting N_time is the following: a link loosing its sym-
metry should still be advertised during at least NEIGHB_HOLD_TIME.
This allows neighbors to detect the link breakage.
The 2-hop Neighbor Set is updated as follows:
1 for each 2-hop interface address listed in the HELLO message
with Link Type SYM_LINK or MPR_LINK, a 2-hop tuple is created
with:
N_main_addr = Originator Address;
N_if_addr = Link Interface Address corresponding
to the 2-hop interface address;
N_2hop_addr = the interface address of the 2-hop
neighbor;
N_time = current time + 2HOP_HOLD_TIME.
N_2hop_main_addr = the main address of the node,
extracted from the multiple interface association infor-
mation base; if no address is available, the interface
address N_if_addr is used.
This tuple may replace an older similar tuple with same
N_if_addr and N_2hop_addr values.
2 For each 2-hop interface address listed in the HELLO message
with Link Type LOST_LINK or ASYM_LINK, all the 2-hop tuples
where:
N_if_addr == Link Interface Address corresponding to the
2-hop interface address,
and
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N_2hop_addr == the 2-hop interface address
are deleted.
Based on the information obtained from the HELLO messages, each node
constructs its MPR selector set.
Thus, upon receiving a HELLO message, if a node finds one of its
interface addresses in a 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 there exists no MPR selector tuple with:
MS_if_addr == Link Interface Address
and
MS_main_addr == Originator Address
2 If there exists no MPR selector tuple with:
MS_if_addr == Link Interface Address
then a new tuple is created with:
MS_if_addr = Link Interface Address
3 The tuple is then modified as follows:
MS_main_addr = Originator Address,
MS_time = current time + NEIGHB_HOLD_TIME.
Deletion of MPR selector tuples occurs in case of expiration of the
timer or in case of link breakage as described in the neighborhood
and 2-hop neighborhood changes section.
2.2.5. Multipoint Relay Selection
MPRs are used to flood control messages from that node into the net-
work while reducing the number of retransmissions that will occur in
a region. Thus, the concept of MPR is an optimization of a classical
flooding mechanism.
Each node in the network selects, independently, its own set of MPRs
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among its symmetric neighborhood. The symmetric links with MPRs are
advertised with Link Type MPR_LINK instead of SYM_LINK in HELLO mes-
sages.
The MPR set MUST be calculated by a node in such a way that it,
through the neighbors in the MPR-set, can reach all symmetric 2-hop
neighbors. (Notice that a node, a, which is a direct neighbor of
another node, b, is not also a 2-hop neighbor of node b). This means
that the union of the symmetric neighborhoods of the MPR nodes con-
tains the symmetric 2-hop neighborhood. MPR set recalculation should
occur when changes are detected in the neighborhood or in the 2-hop
neighborhood.
While it is not essential that the MPR set is minimal, it is essen-
tial that all 2-hop neighbors can be reached through the selected MPR
nodes. A node SHOULD select an MPR set such that any 2-hop neighbor
is covered by at least MPR_COVERAGE MPR nodes. With MPR_COVERAGE=1
the overhead of the protocol is kept at a minimum. In presence of
mobility and link failure, an MPR_COVERAGE=2 could provide a more
redundant connectivity, for example to support a link failure without
re-routing.
Notice that MPR_COVERAGE can be tuned locally without affecting the
consistency of the protocol. For example, a node can set MPR_COVER-
AGE=2 if it observes many changes in its neighbor information base
caused by mobility, while otherwise keeping MPR_COVERAGE=1.
By default, the MPR set can coincide with the entire symmetric neigh-
bor set. This will be the case at network initialization (and will
correspond to classic link-state routing).
The following specifies a proposed heuristic for selection of MPRs
[2] adapted for multiple interfaces support. It constructs an MPR-set
that enables it to reach any symmetrical 2-hop interface (i.e. any
interface of a 2-hop neighbor having a symmetrical link with a neigh-
bor). The following terminology will be used in describing this algo-
rithm:
N:
The set of neighbors with which there exists a symmetric link.
N2:
The set made of the main addresses of the 2-hop neighbor set
excluding (i) the nodes only reachable by members of N with
willingness zero, (ii) all the nodes in N and (iii) the node
performing the computation.
D(y):
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The degree of an one hop neighbor node y (where y is a member
of N), is defined as the number of symmetric neighbors of node
y, EXCLUDING all the members of N and EXCLUDING the node per-
forming the computation.
Poorly covered node:
A poorly covered node is a node in N2 which is covered by less
than MPR_COVERAGE nodes in N.
The proposed heuristic is as follows:
1 Start with an MPR set made of all members of N with willing-
ness equal to seven
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 cover the poorly covered
nodes in N2. The nodes are then removed from N2 for the rest
of the computation.
4 While there exist nodes in N2 which are not covered by at
least MPR_COVERAGE nodes in the MPR set:
4.1 For each node in N, calculate the reachability, i.e. the
number of nodes in N2 which are not yet covered by at
least MPR_COVERAGE nodes in the MPR set, and which are
reachable through this one hop neighbor;
4.2 Select as a MPR the node with highest willingness among
the nodes in N with non-zero reachability. In case of
multiple choice select the node which provides reachabil-
ity to the maximum number of nodes in N2. In case of
multiple nodes providing the same amount of reachability,
select the node as MPR whose D(y) is greater. Remove the
nodes from N2 which are now covered by MPR_COVERAGE nodes
in the MPR set.
5 As an optimization, process each node y in the MPR set in the
increasing order of their willingness. If all nodes in N2 are
still covered by at least MPR_COVERAGE nodes in the MPR set
excluding y and willingness of node y is smaller than seven,
then node y is removed from the MPR set.
When the MPR set has been computed, all the corresponding main
addresses are stored in the MPR Set.
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2.2.6. Neighborhood and 2-hop Neighborhood Changes
A change in the neighborhood is detected when:
- The N_SYM_time field of a neighbor tuple expires. This is con-
sidered as a neighbor loss if it was the last link with a
neighbor node (on the contrary, a link with an interface may
break while a link with another interface of the neighbor node
remains).
- The N_ASYM_time field of a neighbor tuple expires and
N_SYM_time is expired. The link is then considered lost.
- A new neighbor tuple is inserted in the Neighbor Set with a
non-expired N_SYM_time or a tuple with expired N_SYM_time is
modified so that N_SYM_time becomes non-expired. This is con-
sidered as a neighbor appearance if there was previously no
link with the corresponding neighbor node.
A change in the 2-hop neighborhood is detected when a 2-hop neighbor
tuple expires or is deleted according to the HELLO message processing
section.
The following processing should occur when changes in the neighbor-
hood or the 2-hop neighborhood are detected:
- In case of neighbor loss, all the 2-hop tuples with
N_main_addr == Main Address of the neighbor are deleted.
- In case of neighbor loss, all the MPR selector tuples with
MS_main_addr == Main Address of the neighbor are deleted
- The MPR set is re-calculated when a neighbor appearance or
loss is detected, or when a change in the 2-hop neighborhood
is detected.
- An additional HELLO message MAY be sent when the MPR set
changes.
2.2.7. Advanced Neighbor Sensing Functioning
Established links should be as reliable as possible to avoid data
packet loss. To enhance the reliability of the neighbor sensing mech-
anism, the following implementation recommendations should be consid-
ered.
Each neighbor tuple in the neighbor set SHOULD, in addition to what
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is described in previous sections, include a N_pending field, a
N_quality field, and a N_LOST_time field. N_pending is a boolean
value specifying if the link is considered pending (i.e. the link is
not considered established). N_quality is a dimensionless number
between 0 and 1 describing the quality of the link. N_LOST_time is a
timer for declaring a link as lost when an established link becomes
pending.
HELLO message generation should consider those new fields as follows:
1 if N_LOST_time is not expired, the link is advertised with a
link type of LOST_LINK;
2 otherwise, if N_LOST_time is expired and N_pending is set to
"true", the link SHOULD NOT be advertised at all;
3 otherwise, if N_LOST_time is expired and N_pending is set to
"false", the link is advertised as described in the HELLO mes-
sage generation section.
A node considers that it has a symmetric link for each neighbor tuple
where
1 N_LOST_time is expired, AND
2 N_pending is "false", AND
3 N_SYM_time is not expired.
This should be taken as definition for computing the symmetric neigh-
borhood when computing the MPR set.
Apart from the above points, what has been described previously does
not interfere with these advanced neighbor sensing fields in the
neighbor tuples. The N_quality, N_pending and N_LOST_time fields are
exclusively updated according to the present section. Advanced neigh-
bor sensing does not modify the function of any other fields in the
neighbor tuples.
This advanced functioning is described as separately as possible to
increase readability.
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2.2.7.1. Link Hysteresis
The link between a node and some of its neighbor interfaces might be
"bad", i.e. from time to time let HELLOs pass through only to fade
out immediately after. In this case, the neighbor information base
would contain a bad link for at least NEIGHB_HOLD_TIME. In absence of
link layer notification, such a bad link might affect routing badly.
To cope with such unstable links, the following hysteresis strategy
SHOULD be adopted.
For each neighbor interface NI heard by interface I, the N_quality
field of the corresponding Neighbor Tuple determines the establish-
ment of the link. The value of N_quality is compared to two thresh-
olds HYST_THRESHOLD_HIGH, HYST_THRESHOLD_LOW, fixed between 0 and 1
and such that HYST_THRESHOLD_HIGH >= HYST_THRESHOLD_LOW.
The N_pending field is set according to the following:
1 if N_quality > HYST_THRESHOLD_HIGH:
N_pending = false
N_LOST_time = current time - 1 (expired)
2 otherwise, if N_quality < HYST_THRESHOLD_LOW:
N_pending = true
N_LOST_time = min (N_time, current time +
NEIGHB_HOLD_TIME)
(the link is then considered as lost according to the
Neighborhood and 2-hop neighborhood changes section and
this may produce a neighbor loss).
3 otherwise, if HYST_THRESHOLD_LOW <= N_quality <= HYST_THRESH-
OLD_HIGH:
N_pending and N_LOST_time remain unchanged.
The condition for considering a link established is thus stricter
than the condition for loosing it.
As a basic implementation requirement, an estimation of the link
quality must be maintained and stored in the N_quality field. If some
measure of the signal/noise level on a received message is available
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(e.g. as a link layer notification), then it can be used as estima-
tion after normalization.
If no signal/noise information is available from the link layer, an
algorithm such as the following can be utilized. The algorithm is
parameterized by a scaling parameter HYST_SCALING which is a number
fixed between 0 and 1. For each neighbor interface NI heard by inter-
face I, the first time NI is heard by I, N_quality is set to
HYST_SCALING (N_pending is set to true and N_LOST_time to current
time - 1).
A tuple is updated according to two rules. Every time an OLSR packet
emitted by NI is received by I, the stability rule is applied:
N_quality = (1-HYST_SCALING)*N_quality + HYST_SCALING.
When an OLSR packet emitted by NI is lost by I, the instability
rule is applied:
N_quality = (1-HYST_SCALING)*N_quality.
The loss of OLSR packet is detected by tracking the missing Packet
Sequence Numbers on a per interface basis and by long period of
silence. If no HELLO message has been received for a HELLO_INTERVAL
period, a loss of HELLO message is detected.
2.2.7.2. Optional 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
information base and the MPR selector set.
Thus, upon receiving a link-layer notification that the link between
a node and a neighbor interface is broken, the following actions are
taken with respect to neighbor sensing:
1 if the link to a neighboring symmetric or asymmetric interface
is broken, the corresponding neighbor tuple is modified:
N_LOST_time and N_time are set to current time +
NEIGHB_HOLD_TIME.
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2 this is considered as a link loss and the appropriate process-
ing described in the neighborhood and 2-hop neighborhood
changes section should be performed.
2.3. Topology Discovery
The Neighbor Sensing part of the protocol basically offers to each
node a list of neighbors with which it can communicate directly and
an optimized flooding mechanism through MPRs. Based on this mecha-
nism, topology information is disseminated through the network. The
present section describes what part of the information given by the
Neighbor Sensing is disseminated and how it is used to construct
routes.
Routes are constructed through advertised links and links with neigh-
bors. A node must at least disseminate its MPR-selector set, in order
to provide sufficient information to enable routing. If a node has
multiple interfaces, it must also disseminate the list of its inter-
face addresses.
2.3.1. TC Message Format
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 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Advertized neighbor Main Address |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Advertized neighbor Main Address |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| ... |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
This is sent as the data-portion of the general message format 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 net-
work.
MPR Selector Sequence Number (MSSN)
A sequence number is associated with the advertized neighbor
set. Every time a node detects a change in its advertized
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neighbor 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 mes-
sage, it can decide on the basis of this MPR Selector Sequence
Number, whether or not the received information about the MPR
selectors of the originator node is more recent than what it
already has.
Advertized Neighbor Main Address
This field contains the main address of a neighbor node. All
main 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 have not been inserted into the TC-
message, more TC messages will be generated until the entire
MPR selector set has been sent. Extra main addresses of neigh-
bor nodes may be included, if redundancy is desired.
Reserved
This field is reserved for future usage, and MUST be set to
"0000000000000000" for compliance with this draft.
2.3.2. Topology Information Base
Each node in the network maintains topological information about the
network. This information is acquired from TC-messages and is 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 main
address of a node, which may be reached in one hop from the node with
the main address T_last. Typically, T_last is a MPR of T_dest. 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".
2.3.3. Advertized Neighbor Set
A TC message is sent by a node in the network to declare a set of
links, called the advertized neighbor set, which MUST include at
least the links to all nodes of its MPR Selector set, i.e., the
neighbors which have selected the sender node as a MPR.
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The sequence number (MSSN) associated with the advertized neighbor
set is also sent with the list. The MSSN number MUST be incremented
when links are removed from the advertized neighbor set; the MSSN
number SHOULD be incremented when links are added to the advertized
neighbor set.
In order to provide redundancy to the topology information base, the
advertized neighbor set of a node can contain links to neighbor nodes
which are not in the MPR selector set of the node. The advertized
neighbor set can be the whole neighbor set of the node. In this case
the nodes receiving the TC message will get the knowledge of all the
adjacent links of the sender node. The advertized neighbor set can be
built according to the following rule based on a local parameter
called TC_REDUNDANCY.
1 if the TC_REDUNDANCY parameter of the node is zero, then its
advertized neighbor set is limited to the MPR selector set.
2 If the TC_REDUNDANCY parameter of the node is one, then its
advertized neighbor set is the union of its MPR set and its
MPR selector set.
3 If the TC_REDUNDANCY parameter of the node is two, then its
advertized neighbor set is its neighbor set.
A node with willingness equal to zero SHOULD have TC_REDUNDANCY also
equal to zero.
2.3.4. TC Message Generation
In order to build the topology information base needed, each node,
which has been selected as MPR, broadcasts Topology Control (TC) mes-
sages. TC messages are flooded to all nodes in the network and take
advantage of MPRs. MPRs enable a better scalability in the distribu-
tion of topology information [1].
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 the advertized neighbor set of a node MUST be
complete within a certain refreshing period (TC_INTERVAL). The infor-
mation diffused in the network by these TC messages will help each
node calculate its routing table.
A node which has an empty advertized neighbor set may not generate
any TC message. Indeed, when its advertized neighbor set becomes
empty, it SHOULD still send (empty) TC-messages during TOP_HOLD_TIME
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to invalidate the previous TC-messages. It MAY then stop sending TC-
messages until some node is inserted in its advertized neighbor set.
A node MAY transmit additional TC-messages to increase its reactive-
ness 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_INTER-
VAL.
2.3.5. 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 infor-
mation that is exchanged through TC messages, according to the fol-
lowing algorithm:
1 If the sender interface (NB: not originator) of this message
is not in the symmetric neighborhood of this node, the message
is discarded.
2 A tuple is inserted into the duplicate table to prevent it
from 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).
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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 advertized neighbor main address received in
the TC message:
5.1 If there exist some tuple in the topology set where:
T_dest == advertized neighbor main address, AND
T_last == originator address,
then the holding time of that tuple is set to:
T_time = current time + TOP_HOLD_TIME.
5.2 Otherwise, a new tuple is recorded in the topology set
where:
T_dest = advertized neighbor main address,
T_last = originator address,
T_seq = MSSN,
T_time = current time + TOP_HOLD_TIME.
6 If the sender address is an interface address of a MPR selec-
tor of this node and if the time to live of the message is
greater than '1', the message MUST be forwarded according to
the following:
6.1 The TTL of the message is reduced by one.
6.2 The hop-count of the message is increased by one
6.3 The message is broadcasted on all interfaces (Notice: The
remaining fields of the message header SHOULD be left
unmodified.)
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2.3.6. Multiple Interface Association
Each node in the network maintains interface information about the
other nodes in the network. This information acquired from MID-mes-
sages, emitted by nodes with multiple interfaces participating in the
MANET, and is used for routing table calculations.
2.3.6.1. Multiple Interface Association Information Base
For each destination in the network, "Interface association Tuples"
(I_if_addr, I_main_addr, I_time) are recorded. I_if_addr is an inter-
face address of a node, I_main_addr is the main address of this node.
I_time specifies the time at which this tuple expires and *MUST* be
removed.
In a node, the set of Interface association Tuples is denoted the
"Interface association Set".
2.3.6.2. MID Message Format
The proposed format of a MID 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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Interface Address |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Interface Address |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| ... |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
This is sent as the data-portion of the general message format with
the "Message Type" set to MID_MESSAGE. The time to live SHOULD be
set to 255 (maximum value) to diffuse the message into the entire
network.
Interface Address
This field contains the address of an interface of the node
other than its main address (already indicated in the origina-
tor address). All interface addresses other than the main
address of the Originator node are put in the MID message. If
the maximum allowed message size (as imposed by the network)
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is reached while there are still interface addresses which
have not been inserted into the MID-message, more MID messages
are generated until the entire interface addresses set has
been sent.
2.3.6.3. MID Message Generation
In order to build the interface association information base, each
node with multiple interfaces broadcast Multiple Interface Declara-
tion (MID) messages. MID messages are flooded to all nodes in the
network and take advantage of MPRs.
A MID message is sent by a node in the network to declare its multi-
ple interfaces (if any). I.e., the MID message contains the list of
interface addresses which are associated to its main address. The
list of addresses can be partial in each TC message (e.g. due to mes-
sage size limitations, imposed by the network), but parsing of all
MID messages describing a nodes interface set MUST be complete within
a certain refreshing period (MID_INTERVAL). The information diffused
in the network by these MID messages will help each node to calculate
its routing table. A node which has only a single interface address
participating in the MANET (i.e. running OLSR) and this address is
its main address, MUST NOT generate any MID message.
A node with more interfaces, where only one is participating in the
MANET and running OLSR (e.g. a node is connected to a wired network
as well as to a MANET) MUST NOT generate any MID messages.
2.3.6.4. MID Message Processing
MID 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 multiple interface association set are recorded
with the information that is exchanged through MID messages, follow-
ing the following algorithm:
1 If the sender (NB: not originator) of this message is not in
the symmetric neighborhood of this node, the message is dis-
carded.
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2 A tuple is inserted into the duplicate table to prevent it
from being processed again with:
D_addr = originator address
D_seq_num = Message Sequence
D_time = current time + D_HOLD_TIME.
3 For each of the interface address received in the MID message:
3.1 If there exist some tuple in the interface association
set where:
I_if_addr == interface address, AND
I_main_addr == originator address,
then the holding time of that tuple is set to:
I_time = current time + MID_HOLD_TIME.
3.2 Otherwise, a new tuple is recorded in the interface asso-
ciation set where:
I_if_addr = interface address,
I_main_addr = originator address,
I_time = current time + MID_HOLD_TIME.
4 If the sender address is an interface address of a MPR selec-
tor of this node and if the time to live of the message is
greater than '1', the message MUST be forwarded according to
the following:
4.1 The TTL of the message is reduced by one.
4.2 The hop-count of the message is increased by one
4.3 The message is broadcasted on all interfaces (Notice: The
remaining fields of the message header SHOULD be left
unmodified.)
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2.3.7. Associated Networks and Hosts
A node MAY provide access to a set of associated hosts. I.e., a node
may act as a "gateway" between the MANET and a number of associated
hosts and/or subnets, not running OLSR and thus not participating in
the MANET. Thus, a node SHOULD be able to inject routing information
describing these associated hosts or networks into MANET, as SHOULD
all nodes be capable of interpreting such information.
Notice that this is a different case from that of "multiple inter-
faces", described previously. Where, in the "Multiple Interface Asso-
ciations", all the described interfaces were participating in the
MANET through running the OLSR protocol, this section addresses the
interfaces which do not participate. This is, e.g., the case where a
node has both a wireless interface (participating in the MANET) and a
wired interface, through which a number of hosts statically connect
(to the nodes in the MANET).
To accomplish this, a node, to which there are associated hosts
and/or networks, periodically issues an Host and Network Association
(HNA) message, containing sufficient information for the recipients
to construct an appropriate routing table.
2.3.7.1. HNA Message Format
The proposed format of an HNA-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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Network Address |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Netmask |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Network Address |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Netmask |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| ... |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
This is sent as the data part of the general packet format with the
"Message Type" set to HNA and the TTL field set to 255.
It should be noticed, that the HNA-message can be considered as a
"generalized version" of the TC-message: the originator of both the
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HNA and TC messages announce "reachability" to some other host(s). In
the TC-message, no netmask is required, since all reachability is
announced on a per-host basis. In HNA-messages, announcing reachabil-
ity to an address sequence through a network- and netmask address is
typically preferred over announcing reachability to individual host
addresses.
Network Address
The network address of the associated network
Netmask
The netmask, corresponding to the network address immediately
above.
2.3.7.2. Host and Network Association Information Base
Each node maintains information concerning which nodes may act as
"gateways" to associated hosts and networks by recording "association
tuples" (GW_main_addr, NET_addr, NET_mask, GS_time), where
GW_main_addr is the main address of the gateway, NET_addr and
NET_mask specifies the network address and netmask of a network,
reachable through this gateway, and GS_time specifies the time at
which this tuple expires and hence *MUST* be removed.
The set of all association tuples in a node is called the "associa-
tion set".
2.3.7.3. HNA Message Generation
A node with associated hosts and/or networks SHOULD periodically gen-
erate a Host and Network Association (HNA) message, containing pairs
of (network address, netmask) corresponding to the connected hosts and
networks. HNA-messages SHOULD be transmitted periodically every
HNA_INTERVAL.
A node without any associated hosts and/or networks SHOULD NOT gener-
ate HNA-messages.
2.3.7.4. HNA Message Processing
In this section, the term "originator address" is used to designate
the main address of the node which originally issued the HNA-message.
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Upon receiving a HNA-message, the node performs the following:
1 An entry in the duplicate set is recorded for this message
with:
D_addr = originator address
D_seq_num = Message Sequence Number
D_time = current time + D_HOLD_TIME.
2 For each (network address, netmask) pair in the message:
2.1 if an entry in the association set already exists, where:
GW_main_addr == originator address
NET_addr == network address
NET_mask == netmask
then the holding time for that tuple is set to:
GW_time = current time + GW_HOLDING_TIME
2.2 otherwise, a new tuple is recorded with:
GW_main_addr == originator address
NET_addr == network address
NET_mask == netmask
GW_time = current time + GW_HOLDING_TIME
3 If the sender address is an interface address of a MPR selec-
tor of this node and if the time to live of the message is
greater than '1', the message MUST be forwarded according to
the following:
3.1 The TTL of the message is reduced by one.
3.2 The hop-count of the message is increased by one
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3.3 The message is broadcasted on all interfaces (Notice: The
remaining fields of the message header SHOULD be left
unmodified.)
2.3.8. Routing Table Calculation
Each node maintains a routing table which allows it to route data,
destined for the other nodes in the network. The routing table is
based on the information contained in the neighbor sensing informa-
tion base, the interface association set, the topology set and the
host and network association set. Therefore, if any of these tables
are 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 R_if_id
2. R_dest R_next R_dist R_if_id
3. ,, ,, ,,
Each entry in the table consists of R_dest, R_next, R_dist, and
R_if_id which specifies that the node identified by R_dest is esti-
mated to be R_dist hops away from the local node, and that the sym-
metric neighbor node with interface address R_next is the next hop
node in the route to R_dest, and this one hop is reachable through
the local interface
R_if_id. 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 neigh-
bor information base, the interface association set or the topology
set. More precisely, it is re-calculated in case of neighbor appear-
ance or loss, or when a topology tuple is created or removed. 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 symmetric 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 symmetric
neighbors (h=1) as the destination nodes. I.e. for each neigh-
bor tuple in the neighbor set where:
N_LOST_time < current time AND
N_pending == false AND
N_SYM_time >= current time
(there is a symmetric link to the neighbor), a new routing
entry is recorded in the routing table with:
R_dest = N_main_addr of the neighbor;
R_next = N_if_addr of the neighbor interface;
R_dist = 1;
R_if_id = N_if_id of the entry.
If N_if_addr is distinct from N_main_addr, another new routing
entry with is added, with:
R_dest = N_main_addr;
R_next = N_if_addr;
R_dist = 1;
R_if_id = N_if_id.
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 increment-
ing 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
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of a route entry whose R_dist is equal to h, then a new
route entry is recorded in the routing table (if it does
not already exist) where:
R_dest = T_dest;
R_next = R_next of the recorded route entry whose
R_dest == T_last
R_dist = h+1; and
R_if_id = R_if_id of the recorded route entry whose
R_dest == T_last.
3.2 Several topology entries may be used to select a next hop
R_next for reaching the node R_dest. When h=1, ties
should be broken such that nodes with highest willingness
and MPR selectors are preferred as next hop.
The routing table is further completed by using the multiple inter-
face association set:
1 For each entry in the multiple interface association base for
which there exists a routing entry such that:
R_dest == I_main_addr (of the multiple interface inter-
face association entry)
a route entry is created in the routing table with:
R_dest = I_if_addr (of the multiple interface associa-
tion entry)
R_next = R_next (of the recorded route entry)
R_dist = R_dist (of the recorded route entry)
R_if_id = R_if_id (of the recorded route entry).
The routing table is finally completed by using the host and network
association set:
1 For each tuple in the host and network association set, record
an entry in the routing table, with:
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R_dest = NET_addr/NET_mask
R_next = the next hop on the path from the node to
GW_main_addr
R_dist = dist to GW_main_addr
R_next and R_if_id are set to the same values as the
tuple from the routing set with R_dest == GW_main_addr.
2.3.9. Advanced Topology Discovery Functioning
Due to mobility, some links of the broadcasted topology may fail.
Additional messages may be sent to recover quickly. Ideally the load
of control messages should increase smoothly with mobility. This sec-
tion describes how this may be achieved in OLSR.
2.3.9.1. Reaction to Link Failure with a MPR Selector
Detection of a link failure between a node and one of its MPR selec-
tors through a link-layer notification may trigger additional TC-mes-
sages to increase the protocol 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, an additional TC-message MAY be
transmitted.
More precisely, if a link failure appears to be a neighbor loss with
a neighbor, which has selected this node as MPR, the MPR selector set
is updated (MSSN is thus incremented) and a TC message MAY be gener-
ated. Moreover if it appears that data traffic was flowing through
this link, a TC message SHOULD be generated. Notice, that a node may
be aware of data traffic flowing to the lost neighbor in case of a
link layer notification coming from a missing acknowledgement or when
statistics about packet forwarding are given by the IP stack.
2.3.9.2. Advanced Fast Re-routing Mechanism
When a link breaks, information stored in the neighbor sensing infor-
mation base may be used to compute an alternative route immediately
if necessary.
If there exists a N_2hop_address listed in the 2-hop neighbor set,
and for which no route exists, a route entry is created in the rout-
ing table with:
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R_dest = N_2hop_address
R_next = R_next of the recorded route entry whose R_dest is
equal to N_main_addr of the corresponding 2-hop tuple
R_dist = 2
R_if_id = R_if_id of the recorded route entry whose R_dest is
equal to N_main_addr of the corresponding 2-hop tuple.
If an entry in the multiple interface association base records a main
address I_main_addr reporting an interface I_if_addr =
N_2hop_address, then R_dest is set to I_main_addr instead of
N_2hop_address.
If the two hop neighbor allows to reach other nodes at distance
greater than 2 according to the topology set then other entries may
be added in the routing table with same R_next for these nodes.
The routing table is finally completed using the multiple interface
association set and the host and network association set as described
in the routing table calculation section.
To allow other nodes to benefit from the alternative route, the node
MAY trigger a fast re-routing event by generating a FRR message.
2.3.9.2.1. FRR Message Format
The proposed format of a FRR 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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Next Hop Address |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Two Hop Address |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Two Hop Address |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| ... |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
This is sent as the data-portion of the general message format with
the "Message Type" set to FRR_MESSAGE. The time to live SHOULD be
set to 1, ensuring that the message is only received by one-hop
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INTERNET-DRAFT Optimized Link State Routing 10 December 2002
neighbors.
Next Hop Address
This field contains the main address of a node which is used
as next hop by the Originator for reaching all the nodes iden-
tified by the Two Hop Address fields.
Two Hop Address
This field contain the main address of a 2-hop neighbor of the
Originator of the message.
2.3.9.2.2. FRR Message Generation
When the fast re-routing mechanism allows to reach 2-hop neighbors
through a neighbor which is not recorded as MPR of them in the topol-
ogy set, the node MAY inform this next hop by generating a FRR Mes-
sage with Next Hop Address containing the main address of this next
hop and listing the main addresses of these 2-hop neighbors.
If some information allows to deduce that some data traffic flows
through the node to some of these 2-hop neighbors, such a FRR Message
SHOULD be generated. This can be the case if such a 2-hop neighbor
was previously a neighbor and a link layer notification of a missing
acknowledgment has been received or statistics about packet forward-
ing are provided by the IP stack. Otherwise, an FFR-message MAY be
generated.
2.3.9.2.3. FRR Message Processing
Upon reception of a FRR message a node performs the following:
1 If the Next Hop Address field is not the main address of the
node, the message is dropped.
2 For each Two Hop Address listed in the FRR Message for which
there exists a neighbor tuple with N_main_addr = the Two Hop
Address and for which there exists no tuple in the MPR selec-
tor set with MS_main_addr = the Two Hop Address, a new tuple
is inserted with:
MS_main_addr = the Two Hop Address
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INTERNET-DRAFT Optimized Link State Routing 10 December 2002
MS_if_addr = N_if_addr of the corresponding neighbor
tuple
MS_time = current time + 2HOP_HOLD_TIME.
If the MPR set has changed and a TC message containing the new MPR
selector set SHOULD be generated.
3. Node Configuration
3.1. Address Assignment
The nodes in the MANET network SHOULD be assigned addresses within a
defined address sequence. I.e., the nodes in the MANET SHOULD be
addressable through a network address and a netmask.
Likewise, the nodes in each associated network SHOULD be assigned
addresses from a defined address sequence, distinct from that being
used in the MANET.
3.2. Routing Configuration
Any MANET node with associated networks or hosts SHOULD
be configured such that it has routes set up to the interfaces with
associated hosts or network.
3.3. 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.
4. IPv6 Considerations
All the operations and parameters described in this document used by
OLSR for IP version 4 are the same as those used by OLSR for IP ver-
sion 6. However, to operate with IP version 6, the only required
change is to replace the IPv4 addresses with Ipv6 address.
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5. Security Considerations
Currently, OLSR does not specify any security measures. However as a
proactive routing protocol, it makes a target for various attacks.
The various possible vulnerability are discussed in this section.
5.1. Confidentiality
Being a proactive protocol, OLSR periodically diffuses topological
information. Hence, if used in an unprotected wireless network, the
network topology is revealed to anyone who listens to OLSR control
messages.
In situations where the confidentiality of the network topology is of
importance, regular cryptographic techniques can be applied to ensure
that control traffic can be read and interpreted by only those autho-
rized to do so.
5.2. Integrity
In OLSR, each node is injecting topological information into the net-
work through transmitting HELLO messages and, for some nodes, TC mes-
sages. If some nodes for some reason, malicious or malfunction,
injects invalid control traffic, network integrity may be compro-
mised.
Different such situations may occur:
1 a node generates TC messages, adverticing links to non-neigh-
bor nodes:
2 a node generates TC messages, pretending to be another node,
3 a node generate HELLO messages, adverticing non-neighbor
nodes,
4 a node generate HELLO messages, pretending to be another node.
5 a node forwards broadcast control messages unaltered, but does
not forward unicast data traffic
Authenticated signatures on control messages (for situation 2 and 4)
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INTERNET-DRAFT Optimized Link State Routing 10 December 2002
and on the individual links announced in the control messages (for
situation 1 and 3) may be used as a countermeasure. However to pre-
vent nodes from repeating old (and correctly authenticated) informa-
tion temporal information is required, allowing a node to positively
identify such delayed messages.
Signatures and other required security information may be transmitted
as a separate OLSR message type, thereby allowing that "secured" and
"unsecured" nodes can coexist in the same network, if desired.
6. Proposed Values for the Constants
This section list the values for the constants used in the descrip-
tion of the protocol.
HELLO_INTERVAL = 2 seconds
REFRESH_INTERVAL = 2 seconds
TC_INTERVAL = 5 seconds
MID_INTERVAL = TC_INTERVAL
HNA_INTERVAL = TC_INTERVAL
NEIGHB_HOLD_TIME = 3 x HELLO_INTERVAL
2HOP_HOLD_TIME = 3 x REFRESH_INTERVAL
TOP_HOLD_TIME = 3 x TC_INTERVAL
D_TIME = 30 seconds
I_TIME = 3 x MID_INTERVAL
GW_TIME = 3 x HNA_INTERVAL
HELLO_MESSAGE = 1
TC_MESSAGE = 2
MID_MESSAGE = 3
HNA_MESSAGE = 4
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FRR_MESSAGE = 5
ASYM_LINK = 1
SYM_LINK = 2
MPR_LINK = 3
LOST_LINK = 4
HYST_THRESHOLD_HIGH = 0.8
HYST_THRESHOLD_LOW = 0.3
HYST_SCALING = 0.5
MPR COVERAGE = 1
MAXJITTER = HELLO_INTERVAL / 4
7. 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 num-
ber S2 iff:
S1 > S2 AND S1 - S2 <= MAXVALUE/2 OR
S2 > S1 AND S2 - S1 > MAXVALUE/2
Thus when comparing two messages, it is possible - even in the pres-
ence of wrap-around - to determine which message contains the most
recent information.
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INTERNET-DRAFT Optimized Link State Routing 10 December 2002
8. Acknowledgments
The authors would like to thank Joseph Macker and his team
<macker@itd.nrl.navy.mil> for their valuable suggestions on the
advanced neighbor sensing mechanism.
The authors would also like to thank Christopher Dearlove
<chris.dearlove@baesystems.com> for valueable input on the MPR selec-
tion heuristics.
9. Authors' Addresses
Thomas Heide Clausen Project HIPERCOM INRIA Rocquencourt BP 105 78153
Le Chesnay Cedex, France Phone: +33 1 3963 5133 Email:
Thomas.Clausen@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
Pascale Minet Project HIPERCOM INRIA Rocquencourt BP 105 78153 Le
Chesnay Cedex, France Phone: +33 1 3963 508832 Email: Pas-
cale.Minet@inria.fr
Paul Muhlethaler Project HIPERCOM INRIA Rocquencourt BP 105 78153 Le
Chesnay Cedex, France Phone: +33 1 3963 5278 Email: Paul.Muh-
lethaler@inria.fr
Amir Qayyum Avaz Networks 5-A Constitution Avenue Islamabad, Pakistan
Phone: +92-51-2826160 Email: qayyum@avaznet.com
Laurent Viennot Project HIPERCOM INRIA Rocquencourt BP 105 78153 Le
Chesnay Cedex, France Phone: +33 1 3963 5225 Email: Laurent.Vien-
not@inria.fr
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INTERNET-DRAFT Optimized Link State Routing 10 December 2002
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2. A. Qayyum, L. Viennot, A. Laouiti. Multipoint relaying: An efficient
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4. Philippe Jacquet and Laurent Viennot, Overhead in Mobile Ad-hoc Net-
work Protocols, INRIA research report RR-3965, 2000
5. S. Bradner. Key words for use in RFCs to Indicate Requirement Lev-
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