Why the temptation for dial users to crank back rwin/mtu?

Mon Feb 8 17:16:59 UTC 1999

> It's always been my belief that the 576 values were generated by hosts
> that didn't support Path MTU discovery and wanted to be conservative
> and avoid fragmentation. Fair enough..
....
> apparently there's some performance value in this (at least to the
....
> 
> MTU - at least this makes a little bit of sense.. If they're doing
> HTTP/1.0 stuff with parallel connections then a smaller MTU is going
> to make that parallelization latency much more effective and perceived
> performance will go up some.. it doesn't impact full document
> retrieval time though (at least not positively!).. are dial links
> really lossy enough that chopping the segment size to 1/3 is a big win
> in retransmit time or are the win95/98 stacks really braindead enough
> that they don't do pmtud so are just trying to dodge fragmentation? I
> found it really odd that [7] which I use all the time to track
> features in a myriad of shipped OS's actually has a blank entry for
> pmtud on both of those (neither yes nor no..)

This is discussed in depth in RFC1191 and W. Richard Stevens' book
"TCP/IP Illustrated Volume 1" (see page 29-31).

Aron