ip fragmentation follies


Michael Greenwald (Greenwald@SCRC-STONY-BROOK.ARPA)
Sun, 29 Dec 85 14:45 EST


Yeah, it's been noticed. I thought Dave had even commented on it in his
"implementation notes", but I can't find my copies, so I didn't check it
up. I noticed this in multics (it hadn't actually happened, but if you
remember I was trying to decide when you'd rather have *large* ip
packets, and when you'd rather restrict ip packets to the max network
size. IP reassembly was cheaper than TCP reassembly. (fewer packet
headers to process.) I thought the only drawback was that in case of a
lost fragment, you had to retransmit the entire packet, but when I
thought about it, I made the same realization that you did.) My
(minimal) solution is mentioned below.

My multics code had foo$retransmit_packet, foo$forward_packet, and
foo$send_packet for each datagram protocol named "foo". (IP, UDP, GGP,
and ICMP, as far as I can remember.) Not only did it keep the same ID,
but it eliminated a certain category of error checking and checksum
generation, where possible.



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