Relay-Version: version B 2.10 5/3/83; site utzoo.UUCP
Path: utzoo!utgpu!water!watmath!clyde!rutgers!ames!ucbcad!ucbvax!CCV.BBN.COM!hinden
From: hinden@CCV.BBN.COM.UUCP
Newsgroups: comp.protocols.tcp-ip
Subject: Re: Datagram sizes
Message-ID: <8705292200.AA01640@ucbvax.Berkeley.EDU>
Date: Fri, 29-May-87 18:03:11 EDT
Article-I.D.: ucbvax.8705292200.AA01640
Posted: Fri May 29 18:03:11 1987
Date-Received: Sat, 30-May-87 11:20:47 EDT
References: <8705291824.AA01387@phun.riacs.edu>
Sender: daemon@ucbvax.BERKELEY.EDU
Distribution: world
Organization: The ARPA Internet
Lines: 39


I am not sure that it is always the best strategy to avoid IP
fragmentation.  There are some cases where it might provide better
service.

I think that in cases of networks where the MTU are large and not too
divergent, avoiding fragmentation by using the minimum MTU is good.
A good example is a 1500 byte Ethernet connected to the 1006 byte
Arpanet.  Using 1006 MTU for the TCP segments probably provides the best
service.

In the case where one network has a large MTU and the other network
has a small MTU it may be better to send one large packet over the
first network and have it fragmented.  For example, suppose you had
10,000 bytes of data to send using TCP from an Arpanet host to a host
on the other side of Satnet (256 MTU).  If you assumed an MTU for the
connection of 256 it would require 47 datagrams (each w/ 20 bytes IP
header, 20 bytes TCP header, 216 bytes of data) to send the data.  If
you use an MTU of 1006, it would require 11 datagrams (each w/ 20 IP,
20 TCP, 996 data) over the Arpanet and 55 fragments (11 * 5) over
Satnet (first with 20 IP, 20 TCP, 216 data, next three with 20 IP,
236 data, last with 20 IP, 72 data).  I would think that using the
1006 MTU would provide better service.  It reduces the number of
datagrams into the Arpanet by a factor of four and adds 8 datagrams
into Satnet.  Assuming the worst case TCP, it would eliminate 36 TCP
ACK's on the return trip.

If you take the more complicated case of Ethernet to Arpanet to
Satnet, then it is, of course, more complicated.  Using 1006 for an
MTU would reduce the number of datagrams that the Ethernet-Arpanet
gateway would have to send into the Arpanet.  This gateway is
probably the biggest bottleneck in the path (10M bps in, 50K bps
out).  Using an MTU of 256 probably causes this gateway to block or
drop lots of packets.  Using an MTU of 1500 gives one the worst of
both worlds.

Isn't this fun.  No simple solutions.

Bob