Not sure I follow. PMTU relies on ICMP to function, but ICMP is not a pmtu aware client. There is no icmp mss/mtu. Likewise, no udp/IP mss. Only TCP does pmtu.
Regards -----Original Message----- From: Bit Gossip [mailto:bit.gos...@chello.nl] Sent: Friday, October 09, 2009 1:46 AM To: Harry Reynolds Cc: juniper-nsp Subject: RE: [j-nsp] path-mtu-discovery Harry, Experts, that makes a lot of sense: mss is an indirect indication of the mtu But what about non TCP traffic, ICMP for example, 'path-mtu-discovery' doesn't seem to work. Following setup shows that when jr4 pings with a packet bigger than the MTU of the router in the middle and with the 'do-not-fragment' it receives from the router the 'ICMP unreachabel - packet too big' At this point it should learn the mtu=1500 for the path to the host and if I do a ping to the host with same big packet but not 'do-not-fragment' it should fragment the packet. It doesn't seem to do it. Thanks, bit. jr4 - ge-1/3/0 ip=192.168.34.4 - mtu 1600 router - mtu 1500 host - ip=192.168.23.2 - mtu 1500 l...@jr4> show configuration system internet-options path-mtu-discovery; l...@jr4> ping source 192.168.34.4 192.168.23.2 do-not-fragment count 1 size 1473 PING 192.168.23.2 (192.168.23.2): 1473 data bytes 36 bytes from 192.168.34.3: frag needed and DF set (MTU 1500) Vr HL TOS Len ID Flg off TTL Pro cks Src Dst 4 5 00 05dd 7225 2 0000 3f 01 09a4 192.168.34.4 192.168.23.2 --- 192.168.23.2 ping statistics --- 1 packets transmitted, 0 packets received, 100% packet loss l...@jr4> ping source 192.168.34.4 192.168.23.2 count 1 size 1473 PING 192.168.23.2 (192.168.23.2): 1473 data bytes On Thu, 2009-10-08 at 09:40 -0700, Harry Reynolds wrote: > You are correct. Its called mss (max segment size) in the show system > connections, and it does not include tcp/ip OH. Note that we have some tcp > noop and time-stamp options, so the tcp header tends to be > 20 bytes. > > When pmtu is off you tend to see a mss of ~ 530 for off net, else it's the > smallest value discovered in that direction. IIRC, bgp pmtu is off by default > and has to be enabled. The default internet PMTU (internet-options > path-mtu-discovery) is enabled by default. Docs indicated otherwise, and doc > pr 75430 was opened to get that corrected. > > Also, once we discover a reduced pmtu we do not ramp back up. The connection > has to be cleared for ~7 minutes to completely age out previous pmtu state, > and then we can discover a larger pmtu if things have changed. > > > HTHs > > > > -----Original Message----- > From: juniper-nsp-boun...@puck.nether.net > [mailto:juniper-nsp-boun...@puck.nether.net] On Behalf Of Bit Gossip > Sent: Thursday, October 08, 2009 5:08 AM > To: juniper-nsp > Subject: [j-nsp] path-mtu-discovery > > Experts, > I guess that the effect of this command is to maintain a cache of all the > active connection and for each of them assign the discovered value of the max > mtu allowed accross the path. > At least the output of 'show system connections inet extensive' > doesn't show any trace of PMTU; > Anyidea of where I can find this information? > > BTW: on a linux box I found that with the command: > 'ip route show table cache' > > _______________________________________________ > juniper-nsp mailing list juniper-nsp@puck.nether.net > https://puck.nether.net/mailman/listinfo/juniper-nsp _______________________________________________ juniper-nsp mailing list juniper-nsp@puck.nether.net https://puck.nether.net/mailman/listinfo/juniper-nsp