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. - : send the line "unsubscribe linux-net" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html
