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

Harald Tveit Alvestrand wrote:

> --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.

Many do, but then ICMP is filtered with reckless abandon on the Internet 
(big I), breaking the feedback loop for a perfectly good PMTU enabled 
stack. MTU must still hand-managed *way* more than it should be.

- Mark

> 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.



- Mark

>
>                    Harald
>
>
>
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