From: "Randy.Dunlap" <rddunlap@osdl.org>
Date: Sun, 18 May 2003 17:40:24 -0700
On Fri, 16 May 2003 23:16:03 -0300 (EST) Felipe Massia Pereira <massia@ic.unicamp.br> wrote: [on LKML]
| 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?
Maybe someone else can answer this...
PMTU is stored at routing cache entries (inside of the dst_entry part
of the rtable). If routing cache entry is flushed, this information
is forgotten. Also, PMTUs are expired periodically, and this is
controlled by calling dst_set_expires() which expires the entry
at ip_rt_mtu_expires jiffies into the future.
You can use a hammer to force all PMTU information to be forgotten
by flushing entire routing cache, this is done via:
echo "0" >/proc/sys/net/ipv4/route/flush
| 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?
In 2.4.x there are 100 jiffies / second on a PC (x86 arch).
However, /proc/sys/net/ipv4/route/mtu_expires is hiding this HZ
conversion factor from us, so that the value is expressed in seconds
and not in jiffies, so 600 is 600 seconds = 10 minutes.
This is correct, the user's value is converted into jiffies and
stored into ip_rt_mtu_expires.
BUT! This only takes effect for NEW pmtu information gathered.
It does not apply to existing ones. Best idea is to flush the
routing cache after such a change.
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