lecture 6 : the lightweight mobile routing algorithm

Mobile and Wireless Computing

Institute for Computer Science, University of Freiburg

Western Australian Interactive Virtual Environments Centre (IVEC)

Lecture 6 : The Lightweight Mobile Routing Algorithm

Lecture 6.1 : Basic ideas behind the LMR algorithm and initialization of a network

Lecture 6.2 : Route maintenance and detailed description




The Domain of the LMR Algorithm

The lightweight mobile routing (LMR) algorithm was developed for routing in highly dynamic mobile networks.

If the mobility is extremely high, the only possible routing strategy is flooding the network with a message and hoping that the message will eventually reach the destination.

All mobile routing protocols degenerate into flooding in extremely mobile environments.




The Domain of the LMR Algorithm

On the other hand, if mobility is low, shortest path based algorithms are more appropriate as shortest path computations converge before the nodes move.

The LMR algorithm is suitable in an intermediate domain when mobility is not so low that shortest path algorithms can converge, and mobility is not so high that flooding is the only possibility.




Properties of the LMR Algorithm

The LMR algorithm is loosely based on the link-reversal strategy of the Gafni-Bertsekas algorithm.

However, there are significant differences between the two algorithms. The most important difference is that the LMR algorithm works even when the network is partitioned.

Also, the LMR algorithm finds routes on-demand instead of maintaining paths to a destination from all other nodes.




Properties of the LMR Algorithm

For a given destination D, instead of maintaining routes from all source nodes to D, the algorithm guarantees route maintenance only for those sources that actually need the routes.

This property ensures that the control overhead of the algorithm is quite low.

The objective of the LMR protocol is not to find a shortest path between a source-destination pair, rather to find any path.




Network Model

The neighbours of a node are denoted by For each neighbour , there exists a link

between nodes and This link may be directed or undirected. An underlying link level protocol provides the

following services : – Each node is aware of all its neighbours – A packet transmitted by a node is received correctly

by all its neighbours regardless of the status of the link.

i iNiNj jil ,

i j

jil ,

iiN




Control Packets

The LMR protocol uses three types of control packets :

– QRY (Query) : consists of a source node ID, a destination node ID, a sequence counter and a transmitting node ID

– RPY (Reply)– FQ (Failure query)




QRY

The sequence counter in a QRY produces increasing sequence number.

The source node ID is the ID of the node which is sending the QRY.

The transmitting node ID is the ID of the node which is forwarding the QRY.

Both RPYs and FQs consist only of a destination node ID and a transmitting node ID.




Link Status Table

A node maintains a link status table entry LS(j) for each neighbour j. The status of an active link is one of the following :

Unassigned (UN), upstream (UP), downstream (DN), downstream-blocked (DN-B), unassigned-waiting (UN-W), and awaiting-broadcast (A-BR).

Links marked UN, UN-W and A-BR are undirected.

Remember that this is the link status table only with respect to one destination.




The General Case

We will discuss the protocol in terms of a single destination D and a link status table at each node for D.

Also, we will talk about a single DAG rooted at D. However, in general there are many destinations and one DAG for each destination.

Each node keeps track of each of its neighbour´s link status for each of the destinations.




Initialization Phase

In the beginning, the network is unassigned.

Only the neighbours of the destination D have routes to D. All other nodes do not have any route to D. The links for all these nodes are unassigned or undirected.

A node is said to have a route if it has at least one downstream link, though it may not have a complete route to D.




Initializing the Network

Eventually a node i which is not a neighbour of D will require a root to D and send a QRY.

This QRY starts the initialization phase and floods the network in the following sense.

Each node that receives the QRY broadcasts it to all of its neighbours only once. QRYs travel over unassigned links.




RPY

A node with a route to the destination D initiates a RPY message.

If a node receives a RPY over an unassigned link, it marks the link to the neighbour (from whom the RPY came) as downstream.

If there is no RPY for a certain period of time, the initiator of the QRY may start another QRY.




An Example of Initialization

S

QRY

Nodes that have received QRY

Nodes that have paths to D

D






S

QRY


D

RPY





S



D

RPY

RPY

RPY





S



D

RPY RPY




Initialization Completed

If the initiator of the QRY i.e., node S is in the partition of the network that contains D, eventually some neighbouring node of D will receive the QRY.

The resulting RPY messages initializes the network into a DAG rooted at D. S obtains one or more loop-free routes to D.

Even other nodes receive routes to D as a result of the initialization.

lecture 6 : the lightweight mobile routing algorithm

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