Re: IPSEC transport mode w/2.2.x kernels and large packets

Alan Cox (alan@lxorguk.ukuu.org.uk)
Sat, 7 Aug 1999 19:26:06 +0100 (BST)


> > The IPv4 minimum MTU is 68 bytes. Its probably safe to throw away reports
> > for < 128 bytes (NetROM).
>
> BTW do you know that rfc1122 prohibits advertising MSS < 536. 8)

As I read it it says you must be prepared to accept 536 byte MSS whatever
you argued. It doesn't say you cant ask for a lower MSS.

> Fragmentation is less expensive than pure TCP in such cases.

Not on radio links. Trust me. Linux acquired the per route mss originally
to handle amateur radio links. Some of these folks got 40% throughput
increases when TCP SACK went in, thats how grotty the links are.

> Actually, if we rejected IP frames of size > 576, we also would
> be right. Nobody is allowed to send them to use without our permission.
> But we are not in children garden yet.
>
> Nobody takes seriously the fact that IP is supposed to work over links
> with mtu 68. We just fragment in this case.

68 no, 128 yes.

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