Why is MTU an issue? (RE: Please read - proposed WG termination)

--On 1. september 2005 11:19 -0700 "H.K. Jerry Chu" <[email protected]> 
wrote:

> What's more hairy to the OS stack is the per-destination MTU and different
> MTU for multicast than for unicast inherited in IPoIB CM.

I don't quite understand why this should be an issue.
At the moment, TCP/IPv4 stacks have 3 options:

- Set MTU to 576 bytes (not b* likely)
- Set MTU to physical interface MTU and depend on fragmentation in the 
network to save their bacon in the case of smaller MTUs (*curse* PPPoE)
- Set initial MTU to physical interface MTU, set DF bit, and support Path 
MTU discovery

I *think* all "modern" IP stacks take the last choice. Which means that 
they have built-in support for variable MTU already.
Setting the initial MTU for the connection depending on info from the 
underlying infrastructure retrieved before connection startup may be a bit 
hairy. But there SHOULD be quite a bit of "variable MTU" support there 
already.

                    Harald

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