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From: ccci!tcs@uunet.UU.NET (Terry Slattery)
Newsgroups: comp.dcom.sys.cisco
Subject: Re: minimum maximum transfer unit.
Message-ID: <34877@boulder.Colorado.EDU>
Date: 8 May 91 17:42:35 GMT
Sender: daemon@boulder.Colorado.EDU
Lines: 28

The MTU of 576 whas chosen because it is the largest datagram which can be
transported over the old Arpanet/Milnet PSNs (packet switching node, aka
IMP) as a single unit.  When the internet was dying due to congestion in the
mid-to-late 80's, dropping the MTU to this value for foreign networks ment
that your data had a *much* greater chance of making it across the internet
and avoided adding to the congestion if it didn't.

If you used larger packets, they would be fragmented by the gateway (called
routers these days) which connected to the internet PSN.  If the source was
on an ethernet and the destination was across the internet, the gateway
would create three fragments.  The PSNs could drop any one of the three, and
often did when congestion was high.  The chances of one of your fragments
not making it across the internet was reasonably high, which in turn caused
the entire packet to be retransmitted (as required by IP).  This added to
the already high congestion.

Using 576 byte packets avoided fragmentation and therefore prevented the
retransmission of data which had successfully made it to the destination.

The BSD "subnetsarelocal" hack was from the recognition that subnets must be
contiguous, therefore the internet could not be used to tie two subnets of
the same network together.  Since the data didn't have to traverse the
internet, the MTU of the outgoing interface could be used if the source and
destination networks were the same (ignoring subnetting information.)  This
reduces the packet processing overhead on the hosts and makes more efficient
utilization of the network.

	-tcs