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Re: [RRG] Are we solving the wrong problem?



On 19 feb 2008, at 19:38, Elwyn Davies wrote:

Since then I have been struggling with the issue of how to deal with sub-flows where the traffic is carried over multiple paths through the network.

You are contemplating a shim6/(real) sctp (i.e. not the 'interim' solution which choses 1 of n paths) solution using multiple addresses.

I was thinking about an IP layer solution which could identify the sub-flows in a useful but low cost and non-revealing way without much involvement from the sending host, but would not rquire multiple addesses at the transport level.

My best thought on this so far was for each router through which the traffic passes to take a suitable field for the packet (part of the flow label field in IPv6 would seem a good idea), hash the value with some suitable fixed but randomized value that is characteristic of the router and write it back into the packet. When the packet arrives at the destination this field would contain the signature of the path that the packet had taken. Assuming deterministic hash functions and a fixed initial value related to the exit path from the source domain that is also passed along to the destination, the 'ack' could return the values which would enable the source to determine which paths were working best.

By some strange coincidence I've been giving this problem some thought as well recently. But I was looking to attack the problem from the other direction: let a host write a value in the packet header that routers can use in their path selection process. For IPv6, an obvious candidate for this would be the flow label, another would be the diffserv bits. Even if DSCPs are used for other applications, it's likely that a few can be found that hosts can use to tell routers that the packet in question should take a different path than the default.

The sender would then correlate the ACKs that come back with the packets that were sent and the paths that they were sent over and maintain congestion state separately for each path.

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