Preface

A large class of routing protocols for MANETs, namely, reactive protocols, employs some form of caching to reduce the no. of route discoveries .the simplest form of caching is based on associating a timeout with each cache entry.

Such timer based cache scheme can increase the protocol efficiency. However the timeout is not well tuned, a sever performance degradation arises as entries are removed either too early or too late from the cache .

In this paper, we address the problem designing a proactive protocol scheme that does not rely on any timer-based mechanism. This scheme guarantees that valid cached routes are never removed while stale routes are removed aggressively .This proactive caching scheme has been embedded in the Zone Routing Protocol (ZRP) framework.

1: INTRODUCTION

The term MANET (Mobile Adhoc Network)refers to a multihop packet based wireless network composed of a set of mobile nodes that can communicate and move at the same time , without using any kind of fixed wired infrastructure. MNET are actually self organizing and adaptive networks that can be formed and deformed on-the-fly without the need of any centralized administration.

As for other packet data networks, one –to-one communication in a MANET is achieved by unicast routing each single packet. Routing in MANET is challenging due to the constraints existing on the transmission bandwidth battery power and CPU time and the requirement to cope with the frequent topological changes resulting from the mobility of the nodes. Nodes of a MANET cooperate in the task of routing packets to destination nodes since each node of the network is able to communicate only with those nodes located within its transmission radius R, while the source and destination nodes can be located at a distance much higher than R.

A first attempt to cope with the mobility is to use the specific techniques aimed to tailoring the conventional routing protocols to the mobile environment while preserving their nature. For this reason the protocol designed around such techniques are referred to as table-driven or proactive protocols. To guarantee that routing tables are up to date and reflect the actual network topology, nodes running a protocol continuously exchange route updates and re calculate paths to all possible destinations. The main advantage of the proactive protocols is that a route is immediately available when is needed for data transmissions. However if

The user traffic is not generated, and then resources are wasted due the proactive route update mechanism.

A different approach in the design of the routing protocol is to calculate a path only when it is necessary for data transmission. These types of protocols are called as the reactive protocols or on-demand routing protocols. A reactive protocol is characterized by a path discovery procedure and a maintenance procedure. Path discovery is based on a query –reply cycle that adopts flooding of queries. The destination is eventually reached by the query an at least one reply is generated. Path discovery procedure is called when there is a need for data transmission and the source does not the path to the destination. Discovered paths are maintained by the route maintance procedure until they are no longer in used.

The main advantage of the reactive protocol is that f data traffic is not generated by nodes, and then routing activity is totally absent. The main drawback is the network-wide path discovery required to obtain routing information. Since discovery is based on flooding, such a procedure is very costly. The main strategy to reduce the cost is to defer path discovery as much as possible so that non-optimal but available routes are preferred to the effort of finding the current best path. The natural solution is route caching.

Proactive and reactive approaches are merged in hybrid protocols that aim to combine he advantages of both approaches. The Zone routing protocol (ZRP) is a well example of a hybrid protocol. ZRP is based on the notion of the zone. Each node n is the center of a zone with radius of k hopes, denoted Zk(n).nodes at a distance greater than or equal to k from n belong to Zk(n). A reactive protocol is used by n to reach node outsides its zone .Practically k is set to value much smaller then the network diameter to get a fast convergence of proactive

Component of zone routing protocol. Again caching is useful in ZRP to reduce the path discoveries.

The simplest form of caching is based on timeouts associated with the cach entries. When an entry is cached a timer starts, when the timeout elapses, the entry is removed from the cache. Each time the entry is used, the timer restarts. Therefore the effectiveness of such a scheme depends upon the timeout value associated with a cached route. If the timeout is well tuned the protocol performance is increase; otherwise that will create a problem of removing the cache entry too early or too late from the cache.

So in this seminar report I propose a cache scheme which is based on the notion of a caching-zone whose center is a node called as the cach leader. The leader is responsible for advertising routes, detected during some route discovery, inside its caching zone.

The leader must grantee the correctness of the advertised routes. The leader monitors each advertised route proactively and it sends the control messages inside its caching zone as soon as a route becomes stale. In this report we present an implementation of this ZRP that is ZRP plus a proactive caching scheme and is known as C-ZRP.

2: An overview of the Zone Routing Protocol

ZRP is a hybrid routing protocol which aims to combine the advantages of both the proactive and the reactive approaches. Such a protocol is based on the notion of a routing zone or simply known as a zone. A zone Zk(n)

With radius k is define for each node n as the set of nodes at a distance at most k hopes from n Zk (n) = {i: H (n, i) k from the source node, ZRP invokes the IERP component to calculate an interzone path. An inter path from S to D is formed by a sequence of nodes B0 =S, B1, B2, B3, B4………….Bm, Bm+1 =D, in which node Bi+1is a node within Bi’s routing zone. The path is calculated on demand using a form of selective flooding that exploits the underlying zone structure generated by IARP.

Specifically, flooding is based on sending query packets only to the peripheral nodes (also called border nodes) , using a special kind of multicast transmission dubbed bordercast. When a node receives the query packet for a target node D, it can either reply to the source- if D is a member of its routing zone - or bordercast the query packet to its peripheral nodes. Eventually, the query packet reaches a node having D as a member of its zone so that a reply control packet is generated and send back to the source. A route to D can be accumulated in the query packet during forwarding or - to reduce the query packet length - stored temporarily at nodes and accumulated in the reply control packet during the reply phase.

In ZRP, zones heavily overlap due to the lack of coordination among nodes. As a result, a node can be a member as well as a border node of many zones. Thus, the basic query mechanism can perform even worse than in standard flooding since a node can forward the same query packet many times, while the same zone can be queried more than once.

ZRP provides several solutions to deal with this problem of stopping and controlling redundant query threads, as well as to prevent the sending of redundant queries. Loop back termination (LT) is a mechanism which is able to detect if a query thread returns to a routing zone that it previously queried and to discard the thread accordingly. The Query detection scheme (QD1 and QD2) detect and discard a query thread I it queries a zone already queried by other threads. The QD2 mechanism extends this capability also to nodes that are in the transmission range of the query transmitting node. Early termination extends the capacity t terminate threads also to nodes internal to a zone. Finally selective bordercast prevents thread overlap by using a modified IARP that provides network topology information for an extended zone with a radius of 2k.

A routing pat only contains the border nodes that have to be traversed . Forwarding along border nodes is table driven since the distance between border nodes is k. Route maintenance is responsible for maintaing interzone path.

3: The C-ZRP PRORTOCOL

In this section , we first give an overview of the proactive caching scheme and of the C-ZRP behavior. Then the implementation of the protocol.

1. Overview of the proactive caching scheme The proposed caching scheme is based on the notion of caching zone, cache leader, and active path. The caching zone with radius k* for a cache leader n is defined as the set o0f nodes at a distance at most k* hops from n an active path is created as a result of the discover phase and it is composed of a set of nodes, hereafter referred to as active nodes forming a path from source node S to a destination node D. cache leader nodes are a subset of the active nodes. The key consideration is to avoid the possibility that nodes can cache route information autonomously. Therefore a cache leader n is only the node which is authorized to advertise route information inside its caching zone which is written into caches. On receiving the advertising message, anode proactively maintains a path to n so that it can be used as the next hope node to any of the advertised routes. A cache leader is responsible for the validity of the advertised routes. Thus it monitors such routes and it forces each node in its caching zone to remove a route as soon as it becomes stale. So, the deletion policy is proactive

2. Implementation of the C-ZRP In the following we provide a description of a possible implementation of C-ZRP. For simplicity the implementation assumes the following aspects as on the next page:

A. All active nodes act as cache leader nodes and vice versa. B. Only paths to active nodes are advertised as external routes. C. Caches are managed using explicit injection / deletion message. D. k = k*. When a node S executes a route request for a node D, an interzone path from S to D is identified. A node Bi belonging to an interzone path is an active node for caching scheme (an example of interzone path between S and D formed by nodes b, e, p and t is shown in figure.) Thus, an interzone path is an active path. An interzone path is stored according to a distributed next hop fashion, where next-hop node is an active node. Bi stores Bi +1, as the next hop active node for all the downstream nodes from Bi+2 to Bm+1 and Bi-1 is the next hop active node for all the upstream nodes from B0 to Bi-2. These two active nodes will be referred to as companion nodes (as an example the companion nodes of node B with respect to the interzone path from S to D).

All routing information concerning node belonging to an interzone path is advertised inside the caching zone of each member of the path, which thus acts as cache leader for those information. Such routes are then maintained proactively by the IARP. If a new node joins Bi’s zone it acquires by means of IARP, all previously advertised routing information by Bi. Since a node may belong to more than one overlapping zone, it can acquire more than a single path to the same destination.

When a new node, say Bi+1 leaves Bi’s routing zone, not all the routing information gathered during the route request/reply is lost. Roughly, speaking two active paths from S to Bi-1 and from Bi+1 to D are still up. Hence, all the routing information concerning these sub-paths is still valid. However, nodes B0 … Bi-1 (Bi-1 … Bm+1) notify the nodes inside their own zones using a delete control message that the destination the Bi-1 … Bm+1 (B0 … Bi) are no longer reachable.

We remark that both delete and inject messages can be piggy backed on messages regularly exchanged by the proactive routing protocol. However, the use of explicit messages provides a faster reaction to topological changes.

3: Data structures Each node X uses the following local data structures:

• Internal zone routing Table (IZT). An entry of IZT is a triple (d, n, #h), where d is the destination node, n is the next hop node (located in the X’s transmission range) and #h is the path cost in no. of hops.

• External zone routing table (EZT): A row of EZT is a triple (d, n, #z), where d is the destination node, n is the next hop active node (n belongs to the X’s transmission range) and #z is the path cost from X to d given as the no. of active nodes that have to be traversed. For example, in Fig.1 node b sets node c as next-hop active node for p with cost two (node c and p).

• Interzone path table (IZP): An interzone path corresponds to an entry in X’s IZP table provided that X is an active node and (X(S, D). In this case let the path id be ID and X=Bi. The entry is the triple (ID, Bi-1, Bi+1).

• Reachable nodes (RN) list. This is a sequence of pairs (d, #z), where d is an active node belonging to an interzone path and #z is the cost of the path from X expressed as number of active nodes that must be transversed to reach d. A node X advertises RN to nodes belonging to Zk(X). RN includes the projection of EZT along the first and third components. For example, node b of Fig.1 will include the pairs (p, 2), (t, 3) and (D, 4) in RN.

• Unreachable nodes (UN) set. This set of nodes is used to advertise the destinations that become unreachable.

The following consistency relations are always guaranteed: (d, n, -)( EZT ((n,-,-) (IZT (ID,a,b) (IZP((a,-,-), (b,-,-) (IZT An example of EZT, IZT and IZP data structures is given in Fig.1

4: IERP and inter zone path management: When a node S has a new message m t send to a node D, it first checks if either IZT or EZT have an entry for D. if this is in the case , it invokes the IERP for calculating a new path and a new route discovery triggered as in fig 2. Interzone path creation: A single inter zone path from S to D is created during a route / request cycle by allowing only the destination D to send a single for a given request. The path is tagged with a unique identifier Id, for example, obtained by using increasing sequence numbers generated by the requesting node. When S triggers a new route discovery for a node D, it bordercast a query message to all its border nodes. The message contains the identifier ID and a route accumulation vector AV [0] =S .let M be the number of active nodes (not including S and D.).

1. When a border node X! =D receives a query message, if the message s received for the first time and the redundant query filter rules are satisfied: a: it adds its own identification into the accumulation vector as an Example, if the node X corrospond to node Bj in the interzone path, then AV[j]=X.

b: if D belongs to X's routing zone then the latter unicast the Query message to D. otherwise , it executes a bordercast. 2. When the destination node D receives a query message with an identifier ID for the first time: a: It stores the tupple (AV[i],AV[M],M+1-i),for 0