Subject: (IPng 6443) Re: Host Fragmentation

From: [email-masked] (Wayne Hathaway)
Date: Fri, 11 Sep 1998 09:30:05 -0700

I'm quoting Dave Borman, but the same idea has appeared three or
four times now:

> 1) Having a device that is capable of rapid fragmentation
>    doesn't mean that the destination host is capable of
>    rapid reassembly.

That might be true if the world were symmetrical, but it is not.
Instead, the world is primarily LARGE SERVERS FEEDING HUNDREDS OF
SMALL CLIENTS.

The server of course wants to send as fast as possible.  If it is
on Gigabit Ethernet, then using a large MTU clearly increases its
maximum sending rate.  (The article that started this thread
demonstrates that point nicely, as do many other test results.
In spite of numerous attempts to prove that this bumblebee can't
fly.)

A decent router should be able to fragment at wire speed no
matter what the wire, so network fragmentation shouldn't be a
bottleneck.

And since client connections are typically Ethernet or much less,
even the smallest client should have no trouble reassembling fast
enough.

That is, a server is sending much more data than a SINGLE client
is receiving.  The bottleneck is indeed sending speed and not
receiving speed.

This does not address the issue of RELIABILITY, of course, and
the need to retransmit an entire TCP segment for a single lost
fragment.  Clearly fragmentation should be avoided over
unreliable paths.  Nobody is claiming otherwise.  (And by the
way, TCP itself could help a lot with avoiding fragmentation,
simply by decreasing segment size when it sees retransmissions.
That is, don't react by sending fewer segments, react by sending
smaller ones.)

The point is that it is silly for a protocol of the future to
outlaw the use of network fragmentation simply because there are
situations in which it might be bad with current router or host
stack implementations.

    Wayne Hathaway              Internet: [email-masked]
    Chief Architect                Phone: 408-360-5503
    Alteon Networks                  FAX: 408-360-5501
    6351 San Ignacio Avenue
    San Jose, CA  95119
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