On Fri, 08 May 2015 21:24:13 +0200 David du Colombier <0in...@gmail.com> wrote:
> On the loopback medium, I suppose this is the opposite issue.
> Since the TCP stack didn't fix the MSS in the incoming
> connection, the programs sent multiple small 1500 bytes
> IP packets instead of large 16384 IP packets, but I don't
> know why it leads to such a slowdown.

Looking at the first few bytes in each dir of the initial TCP
handshake (with tcpdump) I see:

        0x0000:  4500 0030 24da 0000  <= from plan9 to freebsd

        0x0000:  4500 0030 d249 4000  <= from freebsd to plan9

Looks like FreeBSD always sets the DF (don't fragment) bit
(0x40 in byte 6), while plan9 doesn't (byte 6 is 0x00).

May be plan9 should set the DF (don't fragment) bit in the IP
header and try to do path MTU discovery? Either by default or
under some ctl option.

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