Re: Slow Vid and MTU's

ike relucio (ike_relucio@cyberdude.com)
Wed, 29 Oct 1997 21:56:39 +0800


MTU is the largest packet that can be sent without having to be fragmented
by tcp/ip. You can use the "ping" command with the -l switch to specify a
size to send and the -f switch to tell tcp/ip not to fragment the data. If
you start with a -l of 576 and start increasing the value, you will
eventually reach a size where you will receive an error because the data is
too big to be sent without being fragmented. That maximum size is the MTU
plus the header data which may be 28 or 34 bytes (I think, the header size
may be different depending if you use ip header compression, maybe someone
else may know these values)

Since the MTU setting exists in each PC/Server in the network, the least MTU
setting will prevail when sending packets.

For instance, between your pc and your isp's server you may me able to send
a max of 1472 bytes without fragmentation but if the server on the next hop
after your isp can only handle 576 bytes then i think it gets propagated
back to your pc and you will only be able to send a maximum of 576 to your
isp's server

P.S. The value of 1472 that I used above is the value i got pinging a couple
of sites on the net. At 1473 i get an error. Remember the 1472 is MTU +
header.

So it could be that increasing the MTU might improve performance since the
cpu doesn't have to spend time breaking up the data into smaller packets.

-----Original Message-----
From: Brian Dowtin <dowtin@ncat.edu>
To: CU-SEEME-L@cornell.edu <CU-SEEME-L@cornell.edu>
Date: Wednesday, October 29, 1997 3:05 PM
Subject: Slow Vid and MTU's

>
>I haven't seen this implicated in ROLF's and others slow video problem
>but I've just learned about it and it seems to have made a difference
>real or percieved.
>
>Apparently Micro$oft has a bad default setting in their TCP/IP config for
>Windows 95.
>
>Its the MTU setting - its set too large in most Win95 machines.
>
>The default setting is 1032 or something
>and a more optimal one is 576.
>
>MTU = Max Transmission Unit
>Has to do with packets and nice TCP stuff.
>
>To 'fix' it you can go into the registry and tweak the settings,
>or use a program to set it for ya.
>
>There are at least two. You can find them by going to
>
>http://www.download.com and searching for 'MTU'
>
>one is freeware one is shareware. The shareware one gives an excellent
>explanaition of whats going on and why the fix helps.
>
>
>-- then again not sure if this affects UDP at all, so may have no
>effect..have to look that up... but just in case it does there's the
>info...
>
>
>Hope it Helps!
>'Brian Dowtin
>--> http://www.eng.ncat.edu/~dowtin
>