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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.