IETF MANET Working Group Mario Gerla
INTERNET-DRAFT Xiaoyan Hong
Expiration: December 17, 2002 Li Ma
University of California, Los Angeles
Guangyu Pei
Rockwell Scientific Company
June 17, 2002
Landmark Routing Protocol (LANMAR) for Large Scale Ad Hoc Networks
<draft-ietf-manet-lanmar-04.txt>
Status of This Memo
This document is an Internet-Draft and is subject to all provisions
of Section 10 of RFC2026.
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This Internet-Draft is a submission to the IETF Mobile Ad Hoc
Networks (MANET) Working Group. Comments on this draft may be sent
to the Working Group at manet@itd.nrl.navy.mil, or may be sent
directly to the authors.
Abstract
The Landmark Routing Protocol (LANMAR) utilizes the concept of
landmark for scalable routing in large, mobile ad hoc networks.
It relies on the notion of group mobility: i.e., a logical group
(for example a team of coworkers at a convention) moves in a
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coordinated fashion. The existence of such logical group can be
efficiently reflected in the addressing scheme. It assumes that
an IP like address is used consisting of a group ID (or subnet ID)
and a host ID, i.e. <Group ID, Host ID>. A landmark is dynamically
elected in each group. The route to a landmark is propagated
throughout the network using a Distance Vector mechanism.
Separately, each node in the network uses a scoped routing
algorithm (e.g., FSR) to learn about routes within a given (max
number of hops) scope. To route a packet to a destination outside
its scope, a node will direct the packet to the landmark
corresponding to the group ID of such destination. Once the packet
approaches the landmark, it will typically be routed directly to
the destination. A solution to nodes outside of the scope of their
landmark (i.e., drifters) is also addressed in the draft. Thus,
by summarizing in the corresponding landmarks the routing
information of remote groups of nodes and by using the truncated
local routing table, LANMAR dramatically reduces routing table size
and routing update overhead in large networks. The dynamic
election of landmarks enables LANMAR to cope with mobile
environments. LANMAR is well suited to provide an efficient and
scalable routing solution in large, mobile, ad hoc environments in
which group behavior applies and high mobility renders traditional
routing schemes inefficient.
Contents
Status of This Memo . . . . . . . . . . . . . . . . . . . . . . . 1
Abstract . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 3
2. Changes . . . . . . . . . . . . . . . . . . . . . . . . . . . 4
3. Terminology . . . . . . . . . . . . . . . . . . . . . . . . 5
3.1. General Terms . . . . . . . . . . . . . . . . . . . . . 5
3.2. Specification Language . . . . . . . . . . . . . . . . . 6
4. Protocol Applicability . . . . . . . . . . . . . . . . . . . . 6
4.1. Networking Context . . . . . . . . . . . . . . . . . . . 6
4.2. Protocol Characteristics and Mechanisms . . . . . . . . 7
5. Protocol Overview . . . . . . . . . . . . . . . . . . . . . . 9
5.1. Protocol Descriptions . . . . . . . . . . . . . . . . . 9
5.2. Landmark Election . . . . . . . . . . . . . . . . . . . 9
5.3. Drifters . . . . . . . . . . . . . . . . . . . . . . . . 10
6. Protocol Specifications . . . . . . . . . . . . . . . . . . . 11
6.1. Data Structures . . . . . . . . . . . . . . . . . . . 11
6.1.1 Landmark Status tuple . . . . . . . . . . . . . . 11
6.1.2 Landmark Distance Vector . . . . . . . . . . . . 11
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6.1.3 Drifter List . . . . . . . . . . . . . . . . . . 11
6.1.4 Neighbor List . . . . . . . . . . . . . . . . . . 12
6.2. LANMAR Update Message Format . . . . . . . . . . . . 12
6.2.1 Description of the fields . . . . . . . . . . . . 13
6.2.2 Propagation of LANMAR Update Messages . . . . . . 13
6.3. Processing Landmark Updates . . . . . . . . . . . . . . 14
6.3.1 Originating a Landmark in a Subnet . . . . . . . 14
6.3.2 Receiving Landmark Updates . . . . . . . . . . . 14
6.3.3 Winner Competition . . . . . . . . . . . . . . . 15
6.4. Processing Drifter Updates . . . . . . . . . . . . . . . 15
6.4.1 Originating a Drifter Entry . . . . . . . . . . . 15
6.4.2 Receiving Drifter Updates . . . . . . . . . . . . 16
6.4.3 Removing a Drifter Entry . . . . . . . . . . . . 16
6.5. Processing Neighbor List . . . . . . . . . . . . . . . . 16
6.5.1 Update Neighbor List . . . . . . . . . . . . . . 16
6.5.2 Operations Regarding to Lost Neighbors . . . . . 16
7. Data Packet Forwarding . . . . . . . . . . . . . . . . . . . . 16
8. Discussion about Storage and Processing Overhead for Drifters 17
9. Scoped Routing Operations . . . . . . . . . . . . . . . . . . 17
9.1. Fisheye State Routing Protocol . . . . . . . . . . . . . 17
9.2. Destination-Sequenced Distance Vector Routing Protocol . 18
9.3. Optimized Link State Routing Protocol . . . . . . . . . 18
10. Implementation Status . . . . . . . . . . . . . . . . . . . . 18
Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . . . 18
References . . . . . . . . . . . . . . . . . . . . . . . . . . . 18
Chair's Address . . . . . . . . . . . . . . . . . . . . . . . . . 19
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 20
1. Introduction
This document describes the Landmark Routing Protocol (LANMAR) [1,2]
developed by the Wireless Adaptive Mobility (WAM) Laboratory [4] at
Computer Science Department, University of California, Los Angeles.
The concept of landmark routing was first introduced in wired area
networks [6]. The original scheme required predefined multi-level
hierarchical addressing. The hierarchical address of each node
reflects its position within the hierarchy and helps to find a route
to it. Each node knows the routes to all the nodes within its
hierarchical partition. Moreover, each node knows the routes to
various landmarks at different hierarchical levels. Packet
forwarding is consistent with the landmark hierarchy and the path
is gradually refined from the top level hierarchy to lower levels
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as a packet approaches its destination.
LANMAR borrows the concept of landmark and extends it to the
wireless ad hoc environment. LANMAR scheme does not require
predefined hierarchical address, but it uses the notion of landmarks
to keep track of logical subnets in which the members have a
commonality of interests and are likely to move as a group
(e.g., brigade in the battlefield, a group of students from same
class and a team of co-workers at a convention). Each such
logical group has an elected landmark. For each group,
the underlying scoped routing algorithm will provide accurate
routing information for nodes within scope. The routing
update packets are restricted only within the scope. The
routing information to remote nodes (nodes outside the node's
scope) is summarized by the corresponding landmarks. Thus,
the LANMAR scheme largely reduces the routing table size and
the routing update traffic overhead. It greatly improves
scalability.
In addition, in order to recover from landmark failures,
a landmark node is elected in each subnet. Landmark election
provides a flexible way for the LANMAR protocol to cope with a
dynamic and mobile network. The protocol also provides a solution
for nodes that are outside the scopes of the landmarks of
their logical groups (drifters). Extra storage, processing and
line overhead will be incurred for landmark election and drifter
bookkeeping. However, the design of the algorithms provides
solutions without compromising scalability. For example, the
routing overhead for handling drifters is typically small if the
fraction of drifting nodes is small. More analysis is given in
Section 8.
The LANMAR runs on top of a proactive routing protocol. It
requires that the underlying routing protocol support the scoped
subnetworking. Fisheye State Routing Protocol (FSR) [7,8] is
such a protocol that supports LANMAR. In FSR, the link state
protocol is used within the scope. The scope technique
can also be applied to a distance vector type protocol,
such as DSDV [3], in which the hop distance can be used to
limit the scope of routing message updating. The main advantage of
LANMAR is that the routing table includes only the nodes within the
scope and the landmark nodes. This feature greatly improves
scalability by reducing routing table size and update traffic O/H.
Thus the Landmark Routing Protocol provides an efficient and
scalable routing solution for a mobile, ad hoc environment while
keeping line and storage overhead (O/H) low. Moreover, the
election provides a much needed recovery from landmark failures.
2. Changes
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Major changes from version 03 to version 04:
- Removed "neighbor landmark flag" field from neighbor list.
Clarified the operations when a neighbor is lost.
- Clarified the processing of landmark update messages,
especially, the operations when an infinite distance metric
occurs. Operation regarding to an infinite distance metric is
also added in data forwarding.
- A separate section describing the operation before sending a
landmark update message is added.
- Reported current implementation status.
- Editorial changes.
Major changes from version 02 to version 03:
- A drifter sequence number is used in drifter list to indicate
each new occurrence of a drifter.
- Processing of lost neighbors is added.
- A separate section describing the modifications made to various
proactive protocols. Operations of these protocols will then
only perform within a certain hop distances.
- Editorial changes.
Major changes from version 01 to version 02:
- Update of Status of This Memo.
Major changes from version 00 to version 01:
- A destination sequence number for each landmark is used to
ensure loop-free updates for a particular landmark.
- Landmark updates are propagated in separate messages, instead of
being piggybacked on local routing updates. This modification
decouples landmark routing from the underlying proactive routing
protocol.
3. Terminology
3.1. General Terms
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This section defines terminology used in LANMAR.
node
A MANET router that implements Landmark Routing Protocol.
neighbor
Nodes that are within the radio transmission range.
scope
A network area that is centered at each node and bounded
by a certain maximum hop distances.
host protocol
Also known as local routing protocol, i.e., a proactive
protocol that works together with the Landmark Routing
Protocol, but only operates within the scope of each node.
underlying protocol
This term is used interchangeably with host protocol.
scoped routing protocol
A routing protocol that only exchanges routing information
up to a certain hop distance (scope).
subnet
Logical groups of nodes that present similar motion behavior.
group
This term is used interchangeably with subnet.
3.2. Specification Language
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 RFC 2119 [5].
4. Protocol Applicability
4.1. Networking Context
LANMAR is best suited for large scale mobile ad hoc wireless
networks. The landmark scheme on top of a scoped routing
algorithm has large advantages in reducing routing update packet
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size and keeping reasonably accurate routes to remote nodes.
It achieves high data packet delivery ratio. Moreover, the fact
that the route error is blurred by distance obviously reduces the
sensitivity to network size.
LANMAR is also suited for high mobility ad hoc wireless networks.
This is because in a mobile environment, a change on a link far
away from the source does not necessarily cause a change in the
routing table at the source since all the information about
remote nodes is summarized by landmarks.
4.2. Protocol Characteristics and Mechanisms
* Does the protocol provide support for unidirectional links?(if so,
how?)
No.
* Does the protocol require the use of tunneling? (if so, how?)
No.
* Does the protocol require using some form of source routing? (if
so, how?)
No.
* Does the protocol require the use of periodic messaging? (if so,
how?)
Yes. The LANMAR periodically broadcasts landmark information
to its neighbors.
* Does the protocol require the use of reliable or sequenced packet
delivery? (if so, how?)
No. As the packets are sent periodically, they need not be
sent reliably.
* Does the protocol provide support for routing through a multi-
technology routing fabric? (if so, how?)
Yes. It is assumed that each node's network interface is
assigned a unique IP address.
* Does the protocol provide support for multiple hosts per router?
(if so, how?)
Yes. The router that has multiple hosts can use network ID of
these hosts as the address to participate LANMAR.
* Does the protocol support the IP addressing architecture? (if so,
how?)
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Yes. Each node is assumed to have a unique IP address (or
set of unique IP addresses in the case of multiple interfaces).
The LANMAR references all nodes/interfaces by their IP address.
This version of the LANMAR also supports IP network addressing
(network prefixes) for routers that provide access to a
network of non-router hosts.
* Does the protocol require link or neighbor status sensing (if so,
how?)
No.
* Does the protocol have dependence on a central entity? (if so,
how?)
No.
* Does the protocol function reactively? (if so, how?)
No.
* Does the protocol function proactively? (if so, how?)
Yes. The LANMAR proactively maintains landmark information
at each node.
* Does the protocol provide loop-free routing? (if so, how?)
Yes. For in-scope destinations, the protocol uses routing
paths learned from the host protocol. If the host protocol
provides loop-free routing, e.g., FSR and DSDV, so does LANMAR.
For out-scope destinations, only routes to landmarks are used.
Because these routes are DSDV, it is loop free. When a packet
approaches the destination, in-scope routes are used again.
* Does the protocol provide for sleep period operation?(if so, how?)
Yes. However, this requires TDMA MAC layer support. The
router can be scheduled to sleep during idle periods.
* Does the protocol provide some form of security? (if so, how?)
Yes. When a node broadcasts routing update message, only
entries of in-scope nodes and landmarks are included. This
will prevent other remote nodes from being heard.
* Does the protocol provide support for utilizing multi-channel,
link-layer technologies? (if so, how?)
Yes. In fact, the multi-channel can be used to separate
routing messages from user data packets.
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5. Protocol Overview
5.1. Protocol Descriptions
As mentioned in Section 1, the landmark concept we adopt here uses
the notion of logical subnets in which the members have a
commonality of interests and are likely to move as a group.
Each logical subnet has one node serving as a landmark of that
subnet. The protocol requires that the landmark of each subnet have
the knowledge of all the members in its group. The LANMAR protocol
also uses a routing scope at each node. The size of the scope is a
parameter measured in hop distance. It is chosen in such a way that
if a node is at the center of a subnet, the scope will cover the
majority of the subnet members. If the shape of a subnet is likely
to be a cycle, the center node's scope will cover all the members of
the subnet. If this center node is elected as a landmark, it
fulfills the requirement of the protocol. The elected landmark
uses a destination sequence number to ensure its routing entry
update loop-free. The landmarks are propagated in a
distance vector mechanism. Each node maintains a distance vector
for landmarks of all the subnets. The size of the landmark distance
vector equals to the number of logical subnets and thus landmark
nodes. If a landmark does not locate at the center, there will be
some members drifting off its scope. The landmark will keep a
distance vector for drifters of its group. The distance vectors
for landmarks and drifters are exchanged among neighbors in
periodical routing update packets.
The LANMAR relies on an underlying proactive protocol with the
ability of providing routing within the scope. In this local scoped
routing, each node broadcasts routing information periodically to
its immediate neighbors. In each update packet, the node includes
all the routing table entries within the scope centered at it.
5.2. Landmark Election
Dynamic election/re-election of landmark node is essential for
LANMAR to work in a wireless mobile environment. Basically, each
node tracks other nodes of its group in its scope and computes
weight, e.g. the number of the nodes it has found. At the
beginning of the LANMAR, no landmark exists. Protocol LANMAR
only uses the host protocol functionality. As host routing
computation progresses, one of the nodes will learn (from
the host protocol's routing table) that more than a certain number
of group members (say, T) are in its scope. It then proclaims
itself as a landmark for this group and adds itself to the landmark
distance vector. Landmarks broadcast the election weights to
the neighbors jointly with the landmark distance vector update
packets.
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When more than one node declares itself as a landmark in the same
group, as the landmark information floods out, each node will
perform a winner competition procedure. Only one landmark for each
group will survive and it will be elected. To avoid flapping
between landmarks (very possible in a mobile situation), we use
hysteresis in the replacement of an existing landmark. I.e., the
old Landmark is replaced by the new one only if its weight is, say
less than 1/2 of the weight of the current election winner. Once
ousted, the old leader needs the full weight superiority to be
reinstated.
This procedure is carried out periodically in the background (low
overhead, anyway). At steady state, a landmark propagates its
presence to all other nodes like a sink in DSDV. It is extremely
simple and it converges (by definition). In a mobile environment,
an elected landmark may eventually lose its role. The role shifting
is a frequent event. In a transient period, there exist several
landmarks in a single group. The transient period may be actually
the norm at high mobility. This transient behavior can be
drastically reduced by using hysteresis.
When a landmark dies, its neighbors will detect the silence after
a given timeout. The neighbors of the same group will then take
the responsibilities as landmarks and broadcast new landmark
information. A new round of landmark election then floods over
the entire network.
5.3. Drifters
Typically, all members in a logical subnet are within the scope of
the landmark, thus the landmark has a route to all members. It may
happen, however, that some of the members drift off the scope,
for example, a tank in a battalion may become stranded or lost.
To keep track of such drifters, i.e., to make the route to them
known to the landmark, the following modification to the routing
table exchange is necessary. Each node, say i, on the shortest
path between a landmark L and a drifter l associated with that
landmark keeps a distance vector entry to l. Note that if l is
within the scope of i, this entry is already included in the
routing table of node i. When i transmits its distance vector to
neighbor, say j, then j will retain the entry to member l only if
d(j,l) is smaller than the scope or d(j,L) is smaller than d(i,L).
The latter condition occurs if j is on the shortest path from i
(and therefore from l) to L. This way, a path is maintained from
the landmark to each of its members, including drifters. The
procedure starts from l, at the time when a node finds it becomes
a drifter. It informs the landmark hop by hop about its presence.
The occurrences of drifters are dynamic in a mobile network. In
order to timely remove the staled drifter information, the time
when a node hears a drifter is recorded. A node monitors whether
it becomes a drifter periodically and refreshes its occurrence
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along the path towards the landmark.
6. Protocol Specifications
This section discusses the operation of LANMAR routing protocol.
The sending and receiving of landmark updates are in the proactive
nature. The routing packets are processed separately from
ordinary data packets.
6.1. Data Structures
Each node has a unique logical identifier defined by a subnet
field and a host field. The host field is unique in the subnet and
might in fact coincide with the physical address. The logical
identifier can also be an IP address when the subnet address
logically reflects the grouping of nodes.
As LANMAR runs on top of a host routing protocol, it shares the
routing table with the host protocol. Also, LANMAR may maintain
a separate neighbor list, or share it with the host protocol.
In addition, LANMAR keeps a landmark distance vector and a drifter
list. And each node also has a landmark status tuple. In this
draft, we only describe data structures that pertain to LANMAR.
6.1.1. Landmark Status Tuple
Each node has not only a logical identifier, which basically is
its address, but also a landmark status tuple. The tuple includes
a flag which indicates whether the node is a landmark or not, a
election weight (the number of group members the node detects within
its scope) and a sequence number. When a node is elected, the
status tuple will be copied to its landmark distance vector. The
sequence number is advanced. There are three fields for a tuple:
- Landmark flag
- Number of group members in its scope
- Sequence number
6.1.2. Landmark Distance Vector
Landmark distance vector (LMDV) gives the next hop information to
all landmarks in the network. Every subnet has an entry in LMDV.
The latest route information to the landmark of each subnet is
learned when a landmark update message is received. LMDV functions
as a part of the routing table. It has the following fields:
- Landmark status tuple
- Next hop address
- Distance
6.1.3. Drifter List
The drifter list (DFDV) of LANMAR provides the next hop information
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of the drifters known to the current node. The entries are updated
with landmark update message. The latest time a drifter is heard
is recorded in DFDV. The DFDV works as a part of routing table.
It has the following fields:
- Destination drifter address
- Next hop address
- Distance
- Drifter sequence number
- Last heard time
6.1.4. Neighbor List
The neighbor list of LANMAR keeps current neighbor information for
a node. The latest time a neighbor is heard is recorded. The
neighbor list has the following fields:
- Neighbor address
- Last heard time
If the host routing protocol maintains at least the above neighbor
information, LANMAR does not need to keep this separate neighbor
list. It can share the neighbor information with the host routing
protocol.
6.2. LANMAR Update Message Format
There is only one message type of LANMAR protocol: LANMAR Update
(LMU). The messages are periodically exchanged with neighbors.
They update the landmark distance vector LMDV and the drifter
list DFDV. It is possible that LMDV or DFDV is empty, so no
entries of the empty table will be included. The processing of
the LMDV and DFDV will be describe separately. The following format
does not include the node's identifier because it can be obtained
from IP Header.
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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Landmark Flag | N_landmarks | N_drifters | Reserved |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Landmark Address 1 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Next Hop Address 1 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Distance 1 | N_members 1 | Sequence Number 1 :
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
: . . . :
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Drifter Address 1 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Next Hop Address 1 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Distance 1 | Drifter Sequence Number 1 | ... :
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+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
: . . . |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
6.2.1. Description of the fields
Landmark Flag
The landmark flag of the original sender.
N_landmarks
The number of entries of the landmark distance vector.
N_drifters
The number of entries of the drifter list.
Reserved
The bits are set to '0' and are ignored on reception.
Landmark Address 1, Next Hop Address 1, Distance 1, N_members 1
and Sequence Number 1
The first entry in the landmark distance vector.
Landmark Address 1, N_members 1 and Sequence Number 1 are the
status tuple of the destination landmark.
Next Hop Address 1 and Distance 1 is the next hop and distance
to the landmark.
These fields are repeated N_landmarks times for each entry in
landmark distance vector.
Drifter Address 1, Next Hop Address 1, Distance 1 and
Drifter Sequence Number 1
The first entry in the drifter list.
Next Hop Address 1 and Distance 1 are the next hop and distance
to the Drifter Address 1.
These fields are repeated N_drifters times for each entry in
the drifter list.
The length of the message is limited to the maximum IP packet size.
In that case, multiple packets may be required to broadcast all the
entries.
6.2.2. Propagation of LANMAR Update Messages
LANMAR update messages (LMUs) are propagated periodically. The
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frequency could be set according to mobility. Propagation jitters
must be used for each message transmission to reduce collisions.
Before sending a LMU, each node checks whether it is a drifter
and does corresponding calculations (see 6.4.1). An existing
drifter node increases its drifter sequence number by 2. If the
node is a landmark, it increases its sequence number by 2 both
to its status tuple and to its entry in LMDV. Then the node
assembles in the LMU the LMDV and DFDV. This message will be
sent after a small random time interval.
6.3. Processing Landmark Updates
Landmark update information is a part of the LANMAR periodic
routing update message. The update information includes sender's
LMDV. Landmark update message is used for landmark election and
building paths to landmarks.
6.3.1. Originating a Landmark in a Subnet
Every time a node detects a topology change within the scope
(either a neighbor change or a topology change leaned from the
host protocol), it recalculates the number of group members
in its scope. The new number of group members is recorded in
its election weight field. If this number is greater than a
threshold T, the node qualifies as a landmark when one of the
following conditions is met.
(1) it is the only landmark for the group so far;
(2) the existing landmark has an infinite distance metric;
(3) it wins the election (see 6.3.3) when competing with the
existing landmark.
When the node becomes a landmark, it increases its sequence
number by 2. Its current landmark status tuple will be inserted
into the LMDV or the existing landmark is replaced with the new
winner. This landmark entry will be broadcast to neighbors
with the next update packet.
6.3.2. Receiving Landmark Updates
When a node receives a landmark update message, it compares its
LMDV entries with the entries in the incoming LMDV message for
each subnet. There are three situations to consider:
(1) An incoming landmark entry corresponding to a new subnet
and its distance metric is less than infinity, the entry
will be copied.
(2) An incoming entry having the same landmark as an existing
one (in node's LMDV), it will be accepted only if one of
the following conditions is met.
(a) it contains a larger sequence number and the distance
metric is less than infinity;
(b) it contains a larger sequence number and the distance
metric equals to infinity and it happens to be the
next hop in the already existing entry;
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INTERNET-DRAFT Landmark Routing Protocol June 17, 2002
(c) it has the same sequence number with the existing
entry, but a smaller distance metric.
(3) If an incoming entry contains a different landmark for
the same subnet as recorded in LMDV, only the landmark
that does not have an infinite distance and is elected
through a winner competition algorithm (see 6.3.3) is
accepted. The LMDV entry will be kept/updated according
to the outcomes of the competition.
6.3.3. Winner Competition
When more than one node declares itself as a landmark in the same
group, a simple solution is to let the node with the largest
number of group members win the election and in case of tie,
lowest ID breaks the tie. The other competing nodes defer.
However, this method is likely to cause the oscillation of
landmark roles between nodes.
To use hysteresis in replacing an existing landmark, let us assume
the competing node's number of members is M, the existing
landmark's number of members is N and a factor value S. When M is
greater than N*S, then the competing node replaces the existing
landmark. Or, when N reduces to a value smaller than a threshold T,
then it gives up the landmark role. A tie occurs when M falls
within an interval [N*1/S, N*S], then the node with a larger member
number wins the election. If a tie occurs again with equal member
number, i.e., M equals to N, it is broken using lowest ID. A tie
can always be broken using lowest ID as the address is used as ID
and it is unique.
6.4. Processing Drifter Updates
Drifter update information is a part of the LANMAR periodical
routing update message. The update information is the drifter list
(DFDV) of the sender. The computation of the DFDV at each node
includes checking the node itself to see whether it is a drifter
and recording paths to other drifters.
6.4.1. Originating a Drifter Entry
By checking the distance to the landmark of its group, each node
easily knows whether it has become a drifter. If the distance is
larger than the scope, the node will put itself into its drifter
list. This drifter information will be sent back to the landmark
hop by hop along the shortest path to it which can be learned from
the LMDV. For each drifter, only the node on its shortest path
to the landmark needs to receive its information, so before the
entry is broadcast, the drifter or the intermediate nodes
look for the next hop to drifter's landmark in their LMDVs first.
Then the next hop is included in LMU within the drifter entry.
Each drifter also maintains a drifter sequence number.
Each time a node finds itself a drifter, the sequence number
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will be increased by 2. The DFDV will be propagated with the
next update packet.
6.4.2. Receiving Drifter Updates
Upon receiving an update packet, the DFDV part is retrieved and
processed. If an entry of incoming DFDV indicates that the current
node is its next hop to the landmark, i.e., the current node is on
the drifter's shortest path to the landmark, the current node will
insert or update its drifter list. The receiving time is stamped
in the DFDV. The node sending the update packet is recorded in
DFDV as the next hop to the drifter. The reverse path to the
drifter is thus built up. The procedure ends when the landmark
receives the drifter entry. The updated DFDV will be propagated
with the next update packet.
6.4.3. Removing a Drifter Entry
Each entry in DFDV is time stamped of its last receiving time.
Every time before the DFDV is sent or routing by DFDV is needed,
the table is checked for staled entries. If such an entry is found,
it is removed.
6.5. Processing Neighbor List
6.5.1. Update Neighbor List
When a node receives either a landmark update or a host protocol
routing update, the neighbor list is inserted if the update comes
from a new neighbor, or the corresponding neighbor entry is
updated. Current time is recorded with the entry. The
neighbor list is also search at this time for possible lost
neighbors according to the time stamps. If such a neighbor is
found, it is removed from the list.
6.5.2. Operations Regarding to Lost Neighbors
A lost neighbor will be discovered by checking staled entries in
the neighbor list or by feedback from the MAC layer protocol.
A neighbor loss leads to searches in LMDV and DFDV. If the lost
neighbor happens to be the next hop to a landmark or a drifter,
the corresponding table entry in LMDV is marked an infinite
distance metric, while the corresponding table entry in DFDV is
removed. The infinite distance entries in LMDV will be
propagated with a sequence number increased by 1. Thus, the new
link break information will overwrite the entries at downstream
nodes. As long as the landmark propagates next new update
message with a sequence number increased by 2, new routes are
built up.
7. Data Packet Forwarding
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Data packets are relayed hop by hop. The host protocol's
routing table, drifter list and landmark distance vector are
looked up sequentially for the destination entry. If the
destination is within a node's scope, the entry can be found
directly in the routing table and the packet is forwarded to
the next hop node. Otherwise, the drifter list DFDV is
searched for the destination. If the entry is found, the
packet is forwarded using the next hop address from DFDV.
If not, the logical subnet field of the destination is retrieved
and the LMDV entry of the landmark corresponding to the
destination's logical subnet is searched. If the distance
metric is not an infinity, the data packet is then routed
towards the landmark using the next hop address from LMDV.
If all these attempts are failed, the data packet is dropped.
When the data packet is routed towards a landmark, it is not
necessary for the packet to pass through the landmark. Rather,
once the packet gets within the scope of the destination on
its way towards the landmark, it is routed to the destination
directly.
8. Discussion about Storage and Processing Overhead for Drifters
The routing storage and processing overhead introduced by the
distance vector extension to handle drifters is typically small
if the fraction of drifting nodes is small. Consider a network
with N nodes and L landmarks, and assume that a fraction F of the
members of each logical subnet have drifted. In the worst case,
the path length from landmark to drifter is the square root of N
(assuming a grid topology). Thus, sqrt(N) is the bound on the
number of extra routing entries required at the nodes along the
path to the drifter. The total number of extra routing entries is
sqrt(N)*L*(F*N/L) where N/L is the average logical group size.
Thus, the extra storage per node is F*sqrt(N). Now, let us assume
that the number of nodes in the scope = # of landmarks = logical
group size = sqrt(N). Then, the basic routing table overhead per
node (excluding drifters) is 3*sqrt(N). Thus, the extra overhead
caused by drifters is F/3. If 20% of the nodes in a group are
outside of the landmark scope, i.e., have drifted, the extra
routing O/H required to keep track of them is only 7%.
9. Scoped Routing Operations
9.1. Fisheye State Routing protocol
Fisheye State Routing (FSR) [7][8] is easy to adapt to a host
protocol. A two level Fisheye scope is used when FSR is used
as a host protocol. For nodes within the scope, the updating
is in a certain frequency. But for nodes beyond the scope,
the update frequency is reduced to zero. As a result,
each node maintains accurate routing information for in-scope
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nodes. A data packet directed to a remote destination initially
aims at the landmark of that remote group; as it gets closer
to the landmark, it may eventually switch to the accurate
route to the destination provided by FSR at some nodes within
the scope of the destination.
9.2. Destination-sequenced Distance Vector Routing protocol
Distance Vector type routing protocols use smaller routing
tables (comparing to Link State type) and generate lower
routing overhead. Destination-sequenced Distance Vector
Routing (DSDV) [3] uses destination sequenced sequence numbers
to prevent the forming of loops. The protocol can also work
together with LANMAR. The modifications include containing
only the destinations within the local scope in the periodic
routing update messages and turning off the triggered updates.
9.3. Optimized Link State Routing protocol
Optimized Link State Routing (OLSR) [9] provides the facility
for scope-limited flooding of messages. The generic message
format contains a Time To Live field, which gives the maximum
number of hops that a message will travel. Each time a message
is retransmitted, the Time To Live field is decreased by 1.
When the value of this field is reduced to zero, the massage
will not be forwarded any more.
OLSR can be one of the underlying protocol of LANMAR. The
Time To Live field is set to the scope defined in LANMAR
when a message is originated. The advantage of the combination
is the scalability to both dense and sparse network with
large number of nodes and large terrain size.
10. Implementation Status
LANMAR version 1 (according to version 3 of the draft, but
excluding the drifter operations) has been implemented in Linux
and is in use for laboratory experiments.
Acknowledgments
This work was supported in part by NSF under contract ANI-9814675,
by DARPA under contract DAAB07-97-C-D321, and by ONR under
contract N00014-01-C-0016.
References
[1] G. Pei, M. Gerla and X. Hong, LANMAR: Landmark Routing for
Large Scale Wireless Ad Hoc Networks with Group Mobility,
Gerla, Hong, Ma and Pei [Page 18]
INTERNET-DRAFT Landmark Routing Protocol June 17, 2002
Proceedings of IEEE/ACM MobiHOC 2000, Boston, MA, Aug. 2000.
[2] M. Gerla, X. Hong, G. Pei, Landmark Routing for Large Ad Hoc
Wireless Networks, Proceedings of IEEE GLOBECOM 2000,
San Francisco, CA, Nov. 2000.
[3] C.E. Perkins and P. Bhagwat, Highly Dynamic Destination-
Sequenced Distance-Vector Routing (DSDV) for Mobile Computers,
In Proceedings of ACM SIGCOMM'94, London, UK, Sep. 1994,
pp. 234-244.
[4] UCLA Wireless Adaptive Mobility (WAM) Laboratory.
http://www.cs.ucla.edu/NRL/wireless
[5] S. Bradner. Key words for use in RFCs to Indicate
Requirement Levels. RFC 2119, March 1997.
[6] P. F. Tsuchiya, The Landmark Hierarchy: a new hierarchy for
routing in very large networks, Computer Communication Review,
vol.18, no.4, Aug. 1988, pp. 35-42.
[7] G. Pei, M. Gerla, and T.-W. Chen, Fisheye State Routing:
A Routing Scheme for Ad Hoc Wireless Networks, Proceedings of
ICC 2000, New Orleans, LA, Jun. 2000.
[8] G. Pei, M. Gerla, and T.-W. Chen, Fisheye State Routing in
Mobile Ad Hoc Networks, Proceedings of Workshop on Wireless
Networks and Mobile Computing, Taipei, Taiwan, Apr. 2000.
[9] P. Jacquet, P. Muhlethaler, A. Qayyum, A. Laouiti, L. Viennot
and T. Clausen, Optimized Link State Routing Protocol, Internet
Draft, IETF MANET Working Group, draft-ietf-manet-olsr-04.txt,
Mar. 2002.
Chair's Address
The MANET Working Group can be contacted via its current chairs:
M. Scott Corson
Flarion Technologies, Inc.
Bedminster One
135 Route 202/206 South
Bedminster, NJ 07921
USA
Phone: +1 908 947-7033
Email: corson@flarion.com
Joseph Macker
Information Technology Division
Gerla, Hong, Ma and Pei [Page 19]
INTERNET-DRAFT Landmark Routing Protocol June 17, 2002
Naval Research Laboratory
Washington, DC 20375
USA
Phone: +1 202 767-2001
Email: macker@itd.nrl.navy.mil
Authors' Addresses
Questions about this document can also be directed to the authors:
Mario Gerla
3732F Boelter Hall
Computer Science Department
University of California
Los Angeles, CA 90095-1596
USA
Phone: +1 310 825-4367
Fax: +1 310 825-7578
Email: gerla@cs.ucla.edu
Xiaoyan Hong
3803F Boelter Hall
Computer Science Department
University of California
Los Angeles, CA 90095-1596
USA
Phone: +1 310 825-4623
Fax: +1 310 825-7578
Email: hxy@cs.ucla.edu
Li Ma
3803D Boelter Hall
Computer Science Department
University of California
Los Angeles, CA 90095-1596
USA
Phone: +1 310 825-1888
Fax: +1 310 825-7578
Email: mary@cs.ucla.edu
Guangyu Pei
Rockwell Scientific Company
1049 Camino Dos Rios
P.O. Box 1085
Gerla, Hong, Ma and Pei [Page 20]
INTERNET-DRAFT Landmark Routing Protocol June 17, 2002
Thousand Oaks, CA 91358-0085
USA
Phone: +1 805 373-4639
Fax: +1 805 373-4383
Email: gpei@rwsc.com
Gerla, Hong, Ma and Pei [Page 21]
