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Re: shim6-proto-07 review

On 3-jan-2007, at 17:20, Wesley Eddy wrote:


I'm all in favor of helping TCP to "do the right thing", but I think
this proposal (and specifically that suggested text) misses the mark.


If someone cares to suggest something different...


If this direction were to be pursued, I'd recommend not making any
distinction between fast and slow interfaces, but rather specifying
one behavior that is followed in all cases.
Let me explain why I make a big deal about the speed difference between the primary and the backup interface. When designing a multihomed network, the speed differences between the different possible configurations after different kinds of outages is an important concern. What I've seen in practice is that a backup that is 10% or less of the bandwidth of the original bandwidth used before the failure, is completely useless because congestion is immediate, persistent and fatal. I always recommend a minimum of 25% backup capacity. So far so good for multihomed _sites_ where these decisions are made explicitly. But shim6 can also easily work on a single host where the speed difference between the fastest interface (100 - 1000 Mbps) and the slowest (9.6 - 56 kbps) is easily four orders of magnitude. Ignoring this will create problems.
We have actually discussed and worked on a solution to this in TCPM that 
is intended to work with shim6, among other protocols (including at
least mip4, mip6, and hip):
http://tools.ietf.org/wg/tcpm/draft-schuetz-tcpm-tcp-rlci-00.txt


Good, let's put in a reference.
I think this type of general solution is far preferable architecturally than asking shim6 to differentiate between path/interface properties and 
transport protocols and take special actions.
If the information is available, it doesn't make sense to ignore it. I think it's entirely reasonable for TCP or other transport protocols to have different behaviors for path changes where the speed difference is 1:2 vs where the speed difference is 1:1000.