Re: Slow Vid and MTU's

Ernest Smothers (
Thu, 30 Oct 1997 12:33:41 -0500

ike relucio wrote:
> 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)

Actually, RFC 1122 defines MTU as the largest packet that can be
and defines a packet as the message data plus the 20 byte IP header. For
CU-SeeMe, the message data is a UDP packet, which includes an 8 byte
So, an MTU of 576 would allow you to send 548 bytes of data and 28 bytes

> 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.

No, it is not, at least not on NT 4.0 or SCO Unix, BSD Unix or Linux.
Under NT,
if you specify a size of 1472, then the total packet size will be 1492
of the 20 byte IP header added to the ICMP packet. For the Unix variants
size does not include the ICMP 8 byte header, so 1472 would result in a
packet size of 1500 (1472 + 8 + 20). Since RFC 1122 specifies an MTU of
bytes for IEEE 802.3 hosts, servers and routers, and an MTU of 1500 for
hosts, servers and routers, your ping errors with size set to 1473
(total packet
size of of 1493 for NT, 1501 for Unix) were most likely due to hitting
an 802.3
host with NT ping, or an Ethernet host with Unix ping.

> 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.

Changing MTU can definitely change performance. Whether it improves or
gets worse
is subject to several factors. See my separate message today about this.

Randy Smothers