Multipath routing

Multipath routing is the routing technique of using multiple alternative paths through a network, which can yield a variety of benefits such as fault tolerance, increased bandwidth, or improved security. The multiple paths computed might be overlapped, edge-disjointed or node-disjointed with each other. Extensive research has been done on multipath routing techniques, but multipath routing is not yet widely deployed in practice.

To improve performance or fault tolerance:

CMR (Concurrent Multipath Routing) is often taken to mean simultaneous management and utilization of multiple available paths for the transmission of streams of data emanating from an application or multiple applications. In this form, each stream is assigned a separate path, uniquely to the extent supported by the number of paths available. If there are more streams than available paths, some streams will share paths. This provides better utilization of available bandwidth by creating multiple active transmission queues. It also provides a measure of fault tolerance in that, should a path fail, only the traffic assigned to that path is affected, the other paths continuing to serve their stream flows; there is also, ideally, an alternative path immediately available upon which to continue or restart the interrupted stream.

This method provides better transmission performance and fault tolerance by providing:

Shortcomings of this method are:

A more powerful form of CMR (true CMR) goes beyond merely presenting paths to applications to which they can bind. True CMR aggregates all available paths into a single, virtual path. All applications offer their packets to this virtual path, which is de-muxed at the Network Layer, the packets then being distributed to the actual paths via some method such as round-robin or weighted fair queuing. Should a link or relay node fail, thus invalidating one or more paths, succeeding packets are not directed to that (/those) path(s). The stream continues uninterrupted, transparently to the application. This method provides significant performance benefits over the former:

It is noted that true CMR can, by its nature, cause out-of-order delivery (OOOD) of packets, which is severely debilitating for standard TCP. Standard TCP, however, has been exhaustively proven to be inappropriate for use in challenged wireless environments and must, in any case, be augmented by a facility, such as a TCP gateway, that is designed to meet the challenge. One such gateway tool is , which, through its Selective Negative Acknowledgement (SNACK) capability, deals successfully with the OOOD problem.

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