Re: How to use an IP-header bit for Path MTU discovery.

[Steve Deering <[e-mail]>]

In response to John Wobus's clever new proposal, Jeff says:

>	Basically, if one is forced to think of how many rational
>	possibilities there are for the sender's segment size, one
>	can easily come up with 2: "576" and the first-hop-net MTU.
>	Coming up with a third choice is only necessary if we believe
>	that it is too far from optimal to be stuck with these two
>	choices.

Another scenario that *may* become important is FDDI-backbone-FDDI,
where the backbone has a 1500 byte MTU.  You'd rather not end up using
576 bytes as the path-MTU.

Also, the 576 rule still incurs fragmentation on those subnets whose MTU
is less than 576.  If the goal is to eliminate *all* fragmentation,
the simple "two-sizes" rule won't do it (unless you mandate intra-subnet
fragmentation and reassembly for subnets with MTU less than 576).

Steve