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From: art@opal.acc.com (Art Berggreen)
Newsgroups: comp.dcom.sys.cisco
Subject: Re: minimum maximum transfer unit.
Message-ID: <1991May8.225230.16766@salt.acc.com>
Date: 8 May 91 22:52:30 GMT
References: <34877@boulder.Colorado.EDU>
Sender: news@salt.acc.com
Reply-To: art@opalacc.com (Art Berggreen)
Organization: Advanced Computer Communications, Santa Barbara, California
Lines: 48

Let's try to quash a little misinformation here...

In article <34877@boulder.Colorado.EDU> ccci!tcs@uunet.UU.NET (Terry Slattery) writes:
>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

Sorry, but the PSNs could handle up to 1007 (usually rounded down to 1006)
bytes in a message.  This still was not optimal for fragmenting Ethernet
sized packets.

>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.

This unfortuneatly was painfully true for a time.

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

This happens to be true because of the PSNs larger MTU.

The "magic" 576 is not the minimum sized packet an IP net has to be
able to carry (which is 68, 60 bytes max IP header + 8 bytes data).
The 576 is the minimum sized TCP SEGMENT which the underlying IP has
to be able to REASSEMBLE without TCP negotiation.  This was intended
to allow for 64 byte of header and 512 bytes of TCP data (amazingly,
the same as typical disk blocks of the time).  576 bytes is then
just a "good guess" in lieu of any other information (like the new
Path MTU Discovery RFC).

>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.

Rather that subnets were usually local collections of ethernets and
since most traffic was local to this environment, it was desireable
to use the largest usable MTU.

Again, there is no architectural reason for subnets to be contiguous,
only that available routing protocols lacked the ability to describe the
topological information (i.e. RIP and EGP).  One of OSPF's advantages
will be the ability to have noncontiguous subnets.

>	-tcs

Art