per route MTU settings
Subsystems (2)
Hello everybody.
I have done some tests with per route
mtu settings.
Here is the results on the 10.81.104.254 linux box running 3.6.9 on Slackware 14 32bit:
ip route add 10.81.105.109/32 via 10.81.104.1 mtu lock 1450 ip route flush cache
ping -M do 10.81.105.109 -c 5 -s 1450
PING 10.81.105.109 (10.81.105.109) 1450(1478) bytes of data.
Quoted 5 lines
From 10.81.104.254 icmp_seq=1 Frag needed and DF set (mtu = 1450) From 10.81.104.254 icmp_seq=1 Frag needed and DF set (mtu = 1450) From 10.81.104.254 icmp_seq=1 Frag needed and DF set (mtu = 1450) From 10.81.104.254 icmp_seq=1 Frag needed and DF set (mtu = 1450) From 10.81.104.254 icmp_seq=1 Frag needed and DF set (mtu = 1450)
Here is the results on my linux box with ip address 10.81.104.126 (the default gateway is 10.81.104.254) running linux 3.7.0 on Slackware 14 64bit
ping -M do 10.81.105.109 -c 5 -s 560
PING 10.81.105.109 (10.81.105.109) 560(588) bytes of data.
Quoted 5 lines
From 10.81.104.254 icmp_seq=1 Frag needed and DF set (mtu = 576) From 10.81.104.126 icmp_seq=2 Frag needed and DF set (mtu = 576) From 10.81.104.126 icmp_seq=2 Frag needed and DF set (mtu = 576) From 10.81.104.126 icmp_seq=2 Frag needed and DF set (mtu = 576) From 10.81.104.126 icmp_seq=2 Frag needed and DF set (mtu = 576)
When packets are generated locally (on
the 10.81.104.254 box), linux icmp message 'need to frag' report the correct mtu of next hop. Instead on the forwarded packets (those that aren't originated on the 10.81.104.254 box), I always get the 576 value on the mtu of nexthop.
Is this the expected behaviour?
Any response are welcome
TIA
Hi,
10.81.104.254 will never transmit anything with
Quoted 1 line
ping -M do 10.81.105.109 -c 5 -s 1450
because the host already knows a 1478 Byte packet won't fit the 1450 Byte route you made towards 10.81.104.1.
You do realize that 1450 is your ICMP payload, plus ICMP header (8B) and IP header (20B) and you are at 1478B, which exceeds your route's MTU.
I guess 10.81.104.1 has a 576B MTU route/interface towards 10.81.105.109, and you did the first test on 10.81.104.254, and only afterwards you tried "-s 560" from .126, so only then the .254 host realized the transport to 10.81.104.1 is actually a 576B MTU path.
You cannot do pings with 2 different packet sizes on 2 different hosts, and expect them to behave exactly the same.
Running from both hosts pings with:
-s 548
-s 549
-s 1422
-s 1423
and then analyzing the results will probably give us a better idea of what actually happens.
Regards,
Lukas
----------------------------------------
Quoted 54 lines
Date: Fri, 11 Jan 2013 17:29:28 +0100
From: [email protected]
To: [email protected]
Subject: per route MTU settingsHello everybody.
I have done some tests with per route
mtu settings.Here is the results on the 10.81.104.254
linux box running 3.6.9 on Slackware 14
32bit:ip route add 10.81.105.109/32 via 10.81.104.1 mtu lock 1450 ip route flush cache
ping -M do 10.81.105.109 -c 5 -s 1450
PING 10.81.105.109 (10.81.105.109) 1450(1478) bytes of data. From 10.81.104.254 icmp_seq=1 Frag needed and DF set (mtu = 1450) From 10.81.104.254 icmp_seq=1 Frag needed and DF set (mtu = 1450) From 10.81.104.254 icmp_seq=1 Frag needed and DF set (mtu = 1450) From 10.81.104.254 icmp_seq=1 Frag needed and DF set (mtu = 1450) From 10.81.104.254 icmp_seq=1 Frag needed and DF set (mtu = 1450)Here is the results on my linux box with
ip address 10.81.104.126 (the default
gateway is 10.81.104.254) running linux
3.7.0 on Slackware 14 64bitping -M do 10.81.105.109 -c 5 -s 560
PING 10.81.105.109 (10.81.105.109) 560(588) bytes of data. From 10.81.104.254 icmp_seq=1 Frag needed and DF set (mtu = 576) From 10.81.104.126 icmp_seq=2 Frag needed and DF set (mtu = 576) From 10.81.104.126 icmp_seq=2 Frag needed and DF set (mtu = 576) From 10.81.104.126 icmp_seq=2 Frag needed and DF set (mtu = 576) From 10.81.104.126 icmp_seq=2 Frag needed and DF set (mtu = 576)When packets are generated locally (on
the 10.81.104.254 box), linux icmp message 'need to frag' report the correct mtu of next hop. Instead on the forwarded packets (those that aren't originated on the 10.81.104.254 box), I always get the 576 value on the mtu of nexthop.
Is this the expected behaviour?Any response are welcome
TIA
--
To unsubscribe from this list: send the line "unsubscribe netdev" in the body of a message to [email protected]
More majordomo info at http://vger.kernel.org/majordomo-info.html