Re: ip_rt_min_pmtu

Hi,
Just a short note:
RFC 791 indeed set 68 for internet module MTU.

But RFC 791 also declares 576 as PMTU:
"All hosts must be prepared to accept datagrams of up to 576 octets".
and it says also:
"The number 576 is selected to allow a reasonable sized data block to
be transmitted in addition to the required header information."

It seems that there is a distinction between a module sending MTU and
hosts receiving MTU.

Regarding the historical details of why it was sent at that time  -
I don't have an idea.

Regards,
Rami Rosen
http://ramirose.wix.com/ramirosen



On Tue, Dec 4, 2012 at 11:04 PM, Christopher Schramm
[off-list ref] wrote:

Hi,

I'm looking into an interesting detail of the Linux IPv4 implementation I
stumbled upon during a University course.

In route.c there's a value ip_rt_min_pmtu, defined as 512 + 20 + 20, that
tells Linux a minimum PMTU to use, even if e. g. an ICMP message tells it to
set a smaller one.

Of course, this is not a problem in real world, but not standard-compliant,
since RFC 791 defines a minimum MTU of 68 for IPv4. So I wonder what's the
reason for the restriction.

I looked into it and found that it appeared in Linux 2.3.15 with the
following ID in route.c:

v 1.71 1999/08/20 11:05:58 davem

While it was not present in Linux 2.3.14 with:

v 1.69 1999/06/09 10:11:02 davem

I couldn't find any related discussion or patch on the LKML around that
dates, so I'm asking you for any hints to find out the reason for
implementing this lower bound.

What I've found on the LKML is a topic around February 15th, 2001, titled
"MTU and 2.4.x kernel", where Alexey Kuznetsov points out that the handling
of "DF on syn frames" is broken for MTUs smaller than 128 and "Preventing
DoSes requires to block pmtu discovery at 576 or at least 552".

Does anybody know the actual reason for the change in 2.3.15? I first
thought it's the common misinterpretation that 576 would be the lower bound
for MTUs in IPv4, but I wonder why it was put in place as a patch years
after the IPv4 implementation was already done. There seems to have been
some clear reason for it. I also wonder why it has never been removed up to
today if it's really nothing more than a mistake.

Would be great if someone could help me shed some light on this.

Regards
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