Multipath Routing - Multipath Routing in Wireless Networks

Multipath Routing in Wireless Networks

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:

• Simultaneous, parallel transport over multiple carriers.
• Load balancing over available assets.
• Avoidance of path discovery when re-assigning an interrupted stream.

Shortcomings of this method are:

• Some applications may be slower in offering traffic to the transport layer, thus starving paths assigned to them, causing under-utilization.
• Moving to the alternative path will incur a potentially disruptive period during which the connection is re-established.

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) paths. The stream continues uninterrupted, transparently to the application. This method provides significant performance benefits over the former:

• By continually offering packets to all paths, the paths are more fully utilized.
• No matter how many nodes (and thus paths) fail, so long as at least one path constituting the virtual path is still available, all sessions remain connected. This means that no streams need to be restarted from the beginning and no re-connection penalty is incurred.

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 SCPS-TP, which, through its Selective Negative Acknowledgement (SNACK) capability, deals successfully with the OOOD problem.

Another important benefit of true CMR, desperately needed in wireless network communications, is its support for enhanced security. Simply put, for an exchange to be compromised, multiple of the routes it traverses must be compromised. The reader is referred to the references in the “To improve network security” section for discussion on this topic.