Maximum size IP packets


James Van Bokkelen (jbvb@vax.ftp.com)
Fri, 22 Apr 88 00:15:36 EDT


Our PC/DOS TCP/IP will only assemble packets smaller than the MTU of
the attached networks. We decided that anything fancier than that
would waste too much memory for essentially no benefit. It helps
that the smallest MTU any of our interfaces use is about 1K bytes,
well above the IP requirement of 576.

Mobygrams fail miserably in the face of *any* packet loss, because none
of the IPs I know anything about allow retransmissions with the same IP
identification value. All the fragments of a particular retransmission
of a mobygram must survive the net before it can be re-assembled. In PCs
in particular, you can't get below a certain level of packet loss due to
cheap network interfaces, so you're stuck.

NFS is the only conspicuous user of mobygrams that I know of, and I think
that Van Jacobsen and Mike Karels have demonstrated that Sun's choice of
fragmented UDP mobygrams, unchecksummed, was a costly way to achieve
high throughput.

James VanBokkelen
FTP Software Inc.



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