Supervisor: Kaisa Sere
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3.7 DiscussionEvery protocol tries to reduce control message overhead and therefore provide better scalability. In AODV an expanding ring search is specified in order to reduce control message complexity. ZRP limits the propagation of control messages by partitioning the network into zones from which certain messages cannot propagate. As we saw in OLSR, it reduces flooding of messages by making sure that only certain nodes can forward these, effectively reducing the message complexity. In the discussion of proactive versus reactive protocols, the winner is determined by the main goal one wants to achieve with the network. If we know that delay sensitive traffic like IP telephony is going to be used then the proactive type will be the winner as these reduce delays due to the fact that routes are always available. On the other hand if, as much as possible of the available data rate has to be preserved for applications like file sharing for instance then reactive protocols should be used. In fact, there is also a proposition for so-called Active Networks (AN) [Tschudin+00], in which no single protocol is used. Instead AN allows a programmable architecture to be tailored for certain needs of traffic types (i.e. quality of service, topologies and mobility patterns). So actually, many protocols may be run in parallel (distributed as control packets) allowing the most suitable to be chosen. 4TCP FOR WIRELESS NETWORKSDifferent TCP implementations for wireless ad hoc and cell based single hop networks have been proposed. This has to be done because it has been shown that ordinary TCP does not work well in ad hoc networks. Results of [Bakre+95] indicate a 62% drop in throughput when stationary nodes are compared with mobile nodes, roaming between non-overlapping cells, using ordinary TCP. The ad hoc network is connected to static Internet by one or more fixed neighbors (base station) over a wireless interface. The static node also connects to the Internet via a wired interface. The ad hoc network can either be a network of the Internet or subnet of a network. This allows the ad hoc subnets internal structure to be hidden from the rest of the Internet, and the nodes are accessed by the rest of the Internet only through a static redirection node (home agent in Mobile IP, see 4.1). The home agent knows to which Internet router (foreign agent) the mobile node is currently connected. The redirection node can be located anywhere on the Internet. This allows the ad hoc nodes to have non-uniform addresses, (addresses with different network identifiers) in the view of the rest of the Internet. 4.1 Mobile IPIn [Lei+97] Lei and Perkins report on their system for providing Internet access to mobile nodes. It based on a modified version of Mobile IP in which the foreign agent’s coverage is extended to the ad hoc network. Basic Mobile IP provides nomadic access for one-hop wireless subnet members by having a foreign agent connected to the subnet, accepting registrations from visiting nodes. The so-called home agent on the mobile node’s home network is also informed when registration occurs and can therefore forward messages intended for the mobile node via the foreign agent. This operation is known as binding and it sets up a tunnel between the agents and the mobile node can access the rest of the Internet through the foreign agent that will be the default router for the node. The proposed ad hoc version of Mobile IP is used by a mobile node to locate foreign agents on the Internet by issuing router discovery messages, which is answered with an agent advertisement by the foreign agent. Since the foreign agent and ad hoc nodes might not be in direct contact, the Mobile IP was paired with a special version of the Routing Information Protocol (RIP), to relay foreign agent advertisement messages. In this scheme RIP and Mobile IP has access to the kernels routing tables and can insert routing entries associated with both protocols, for instance, a nexthop towards the foreign agent. 4.2 Connecting to the Internet – TCP-IIn order to provide TCP/IP successfully for ad hoc nodes, modifications to the transport layer has to be made. A technique for handling TCP connections called Indirect TCP (I-TCP) [Bakre+95] exists. I-TCP solves the problem of non-terminated TCP connections caused by roaming between fixed cells. In this scheme, so-called Mobile Support Routers (MSR) was placed in every cell. Mobile nodes connect to the MSR which in turn is connected to the Internet, forming a TCP connection with not only two end points but also an intermediate point. The retransmission timers are tuned to take the unreliability of radio links into account by supporting notifications of link breakage to the MSR. The cell’s MSR is responsible for managing all the mobile node’s TCP connections, visiting that cell. When the mobile node moves from one cell to another, the sockets connecting the mobile node and Internet node are migrated to a new MSR in the new cell. This is enough to keep the TCP connection alive. When a mobile host wants to connect to a fixed host on the Internet, it will contact the MSR which will in turn complete the connection on behalf of the mobile node and thereafter continue to relay the stream. The operation of moving the sockets from one MSR to another is a complex task and is done in the following way: TCP segments that are in transit are buffered by the sender for retransmission if they become lost during roaming. Next, the socket state is freezed and migrated to another MSR (the old and new MSRs communicate directly), which restarts the connection. In order to enable static Internet nodes to find mobile ad hoc nodes, Mobile IP must be used. A logical choice would be to make the MSR a foreign agent. It must also be noted that TCP-I does not work in multi-hop ad hoc networks, but the principal is inspiring for future development. In order for TCP-I to work in ad hoc networks it must be modified for multihop support like TCP-BuS, which is discussed in the following section. Download 298.62 Kb. Share with your friends:
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