Linux mtu_expires

Linux mtu_expires

Post by Gary Ru » Thu, 29 Aug 2002 03:28:14



Hello,
     I've been searching around trying to find out what this setting
does:
/proc/sys/net/ipv4/route/mtu_expires

I'm currently having a problem ftping a file from a Solaris box to an
IBM mainframe. However the problem does not occur in Linux.

As best I can tell the solaris machine is sending the ftp data in
large packets and this causes a problem. My guess is that somewhere in
between the two systems an ICMP Destination Unreachable/Fragmentation
Needed packet is being dropped that would allow the solaris machine's
Path MTU discovery to work and fragment. However this does not appear
to be a problem in Linux. I'm wondering if this mtu_expires option may
have anything to do with this. I haven't been able to find any good
documentation on this option.

Thanks,
Gary

 
 
 

Linux mtu_expires

Post by Gary Ru » Thu, 29 Aug 2002 07:50:30



> Hello,
>      I've been searching around trying to find out what this setting
> does:
> /proc/sys/net/ipv4/route/mtu_expires

> I'm currently having a problem ftping a file from a Solaris box to an
> IBM mainframe. However the problem does not occur in Linux.

> As best I can tell the solaris machine is sending the ftp data in
> large packets and this causes a problem. My guess is that somewhere in
> between the two systems an ICMP Destination Unreachable/Fragmentation
> Needed packet is being dropped that would allow the solaris machine's
> Path MTU discovery to work and fragment. However this does not appear
> to be a problem in Linux. I'm wondering if this mtu_expires option may
> have anything to do with this. I haven't been able to find any good
> documentation on this option.

> Thanks,
> Gary

After more digging it appears Linux's default mss is 40? If so that
would explain things.
Gary

 
 
 

1. mtu_expires variable

Hello,

I've been searching for the exact meaning of this variable through
Documentation and I've found just empty descriptions (to be filled in, in
Advanced Linux Routing HOWTO, and nothing also in
Documentation/networking/ip-sysctl.txt). I've tried also to read the
kernel source code (net/ipv4/route.c) but I could not figure out what they
mean.

I've came accross this var because we want to do some experiments in class
with Path MTU discovery.  But it happens that MTU is recorded between
experiments (and it's what we expect: that the stack does not do a PMTU
every time). BTW where is the PMTU value kept? Is MTU value recorded for
each destination or for each route?

So it would be nice if we could make the value found in a experiment to be
forgotten by the kernel so the students could execute the ping several
times. (ping -c 2 -m want ...) Is mtu_expires what we're looking for?

We tried to "echo 1 > /proc/sys/net/ipv4/route/mtu_expires" considering
that it's expressed in seconds. The usual value is 600. But I've read that
it's expressed in jiffies. Jiffies occur 100 times per sec on a PC, is it?
So the value 600 on a PC means 6 seconds?

tia,
--
Felipe

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