[Bug 173444] [socket] [patch] IPV6_USE_MIN_MTU and TCP is broken

bugzilla-noreply at freebsd.org bugzilla-noreply at freebsd.org
Thu Mar 30 18:21:09 UTC 2017


https://bugs.freebsd.org/bugzilla/show_bug.cgi?id=173444

Andrey V. Elsukov <ae at FreeBSD.org> changed:

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                 CC|                            |ae at FreeBSD.org

--- Comment #7 from Andrey V. Elsukov <ae at FreeBSD.org> ---
(In reply to marka from comment #6)
> (In reply to Hiren Panchasara from comment #5)
> My test system died years ago but I believe that it still is a problem.
> 
> It should be trivial to check.
> 
> create a IPv6 TCP socket.
> set IPV6_USE_MIN_MTU=1 using setsockopt
> connect to a data sink
> write 1400 bytes to the socket in a single operation
> 
> Examine the packets sent with tcpdump.  There should be no fragmented
> packets being sent as TCP is supposed to take into account MTU
> information.

According to RFC3542 this is what the kernel should do - do IP fragmentation as
application requested.

https://tools.ietf.org/html/rfc3542#section-11.1

"If the packet is larger than the minimum MTU and this feature has been enabled
the IP layer will fragment to the minimum MTU."

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