I went and ran the ping test to see how SLES8 vs SLES9 would behave given the tracepath differences I've found. Some background to just refresh:
Hipersocket interface is set to 64 K on the CHPID. There is 8K of overhead so the MTU is really 57344, and that is what the interface sets itself to at IPL time. I have tried hard coding MTU and letting the interface discover MTU by itself, the results are the same. --------------- SNIP------------- In your case, you should succeed on the second try and the MTU would immediately drop to 1492. So, something else is going on with the tracepath program. Without a packet trace of what is actually going on, as seen by the network and the program, it is difficult to figure out what is really occurring. In this case the problem appears to be in the host from which the trace route is issued, SLES9. There are two questions: Is the MTU really 1492 and why is tracepath acting as it is? The first is of the most concern. I would recommend using a ping with a payload size of 57344 and the do-not-fragment option enabled. If this does not work, then remove the do-not-fragment option and see if it succeeds ----------- SNIP ------------- The following is length and I apologize for that but I think it's relevant because of the differences between SLES8 and SLES9 in the ping response when packet sizes are set. Results of ping testing with packet sizes specified - SLES 8 system. One byte shy of MTU. Works as expected nokomis:~ # ping -c 5 -M do -s 57315 192.168.252.1 PING 192.168.252.1 (192.168.252.1) from 192.168.252.16 : 57315(57343) bytes of data. 57323 bytes from 192.168.252.1: icmp_seq=1 ttl=64 time=46.7 ms 57323 bytes from 192.168.252.1: icmp_seq=2 ttl=64 time=44.6 ms 57323 bytes from 192.168.252.1: icmp_seq=3 ttl=64 time=1.13 ms 57323 bytes from 192.168.252.1: icmp_seq=4 ttl=64 time=0.906 ms 57323 bytes from 192.168.252.1: icmp_seq=5 ttl=64 time=1.93 ms --- 192.168.252.1 ping statistics --- 5 packets transmitted, 5 received, 0% loss, time 4036ms rtt min/avg/max/mdev = 0.906/19.075/46.751/21.754 ms Packet size = MTU - works as expected. nokomis:~ # ping -c 5 -M do -s 57316 192.168.252.1 PING 192.168.252.1 (192.168.252.1) from 192.168.252.16 : 57316(57344) bytes of data. 57324 bytes from 192.168.252.1: icmp_seq=1 ttl=64 time=2.77 ms 57324 bytes from 192.168.252.1: icmp_seq=2 ttl=64 time=3.87 ms 57324 bytes from 192.168.252.1: icmp_seq=3 ttl=64 time=0.991 ms 57324 bytes from 192.168.252.1: icmp_seq=4 ttl=64 time=0.807 ms 57324 bytes from 192.168.252.1: icmp_seq=5 ttl=64 time=17.7 ms Exceed packet size, Failure reported - works as expected. --- 192.168.252.1 ping statistics --- 5 packets transmitted, 5 received, 0% loss, time 4043ms rtt min/avg/max/mdev = 0.807/5.235/17.737/6.354 ms nokomis:~ # ping -c 5 -M do -s 57317 192.168.252.1 PING 192.168.252.1 (192.168.252.1) from 192.168.252.16 : 57317(57345) bytes of data. ping: local error: Message too long, mtu=57344 ping: local error: Message too long, mtu=57344 ping: local error: Message too long, mtu=57344 ping: local error: Message too long, mtu=57344 ping: local error: Message too long, mtu=57344 As can be seen, under SLES8, it eventually reports I've exceeded the MTU size. Under SLES9, the behaviour is different: Deliberately exceeded Buffer size - Warning as expected. abinodji:~ # ping -c 5 -M do -s 57320 192.168.252.1 PING 192.168.252.1 (192.168.252.1) 57320(57348) bytes of data. >From 192.168.252.22 icmp_seq=1 Frag needed and DF set (mtu = 57344) >From 192.168.252.22 icmp_seq=1 Frag needed and DF set (mtu = 57344) >From 192.168.252.22 icmp_seq=1 Frag needed and DF set (mtu = 57344) >From 192.168.252.22 icmp_seq=1 Frag needed and DF set (mtu = 57344) >From 192.168.252.22 icmp_seq=1 Frag needed and DF set (mtu = 57344) --- 192.168.252.1 ping statistics --- 0 packets transmitted, 0 received, +5 errors Made sure the resulting packet would not be OVER the MTU. Works ok. abinodji:~ # ping -c 5 -M do -s 57310 192.168.252.1 PING 192.168.252.1 (192.168.252.1) 57310(57338) bytes of data. 57318 bytes from 192.168.252.1: icmp_seq=1 ttl=64 time=0.907 ms 57318 bytes from 192.168.252.1: icmp_seq=2 ttl=64 time=1.08 ms 57318 bytes from 192.168.252.1: icmp_seq=3 ttl=64 time=0.891 ms 57318 bytes from 192.168.252.1: icmp_seq=4 ttl=64 time=1.51 ms 57318 bytes from 192.168.252.1: icmp_seq=5 ttl=64 time=1.96 ms --- 192.168.252.1 ping statistics --- 5 packets transmitted, 5 received, 0% packet loss, time 4633ms rtt min/avg/max/mdev = 0.891/1.271/1.961/0.411 ms 2nd deliberately too large request throws same error. Expected. abinodji:~ # ping -c 5 -M do -s 57318 192.168.252.1 PING 192.168.252.1 (192.168.252.1) 57318(57346) bytes of data. >From 192.168.252.22 icmp_seq=1 Frag needed and DF set (mtu = 57344) >From 192.168.252.22 icmp_seq=1 Frag needed and DF set (mtu = 57344) >From 192.168.252.22 icmp_seq=1 Frag needed and DF set (mtu = 57344) >From 192.168.252.22 icmp_seq=1 Frag needed and DF set (mtu = 57344) >From 192.168.252.22 icmp_seq=1 Frag needed and DF set (mtu = 57344) --- 192.168.252.1 ping statistics --- 0 packets transmitted, 0 received, +5 errors Set a packet size that results in a packet equalling the MTU and it fails, reporting the MTU is now 32000! abinodji:~ # ping -c 5 -M do -s 57316 192.168.252.1 PING 192.168.252.1 (192.168.252.1) 57316(57344) bytes of data. >From 192.168.252.22 icmp_seq=1 Frag needed and DF set (mtu = 32000) >From 192.168.252.22 icmp_seq=1 Frag needed and DF set (mtu = 32000) >From 192.168.252.22 icmp_seq=1 Frag needed and DF set (mtu = 32000) >From 192.168.252.22 icmp_seq=1 Frag needed and DF set (mtu = 32000) >From 192.168.252.22 icmp_seq=1 Frag needed and DF set (mtu = 32000) --- 192.168.252.1 ping statistics --- 0 packets transmitted, 0 received, +5 errors Set a packet size that results in a packet that is two bytes smaller than the MTU specified on the CHPID, and the ifconfig command still indicates is 57344 MTU returned as 32000 abinodji:~ # ping -c 5 -M do -s 57314 192.168.252.1 PING 192.168.252.1 (192.168.252.1) 57314(57342) bytes of data. >From 192.168.252.22 icmp_seq=1 Frag needed and DF set (mtu = 32000) >From 192.168.252.22 icmp_seq=1 Frag needed and DF set (mtu = 32000) >From 192.168.252.22 icmp_seq=1 Frag needed and DF set (mtu = 32000) >From 192.168.252.22 icmp_seq=1 Frag needed and DF set (mtu = 32000) >From 192.168.252.22 icmp_seq=1 Frag needed and DF set (mtu = 32000) --- 192.168.252.1 ping statistics --- 0 packets transmitted, 0 received, +5 errors Again, same result even smaller packet. abinodji:~ # ping -c 5 -M do -s 57312 192.168.252.1 PING 192.168.252.1 (192.168.252.1) 57312(57340) bytes of data. >From 192.168.252.22 icmp_seq=1 Frag needed and DF set (mtu = 32000) >From 192.168.252.22 icmp_seq=1 Frag needed and DF set (mtu = 32000) >From 192.168.252.22 icmp_seq=1 Frag needed and DF set (mtu = 32000) >From 192.168.252.22 icmp_seq=1 Frag needed and DF set (mtu = 32000) >From 192.168.252.22 icmp_seq=1 Frag needed and DF set (mtu = 32000) --- 192.168.252.1 ping statistics --- 0 packets transmitted, 0 received, +5 errors 31000 packet size works. No complaint abinodji:~ # ping -c 5 -M do -s 31000 192.168.252.1 PING 192.168.252.1 (192.168.252.1) 31000(31028) bytes of data. 31008 bytes from 192.168.252.1: icmp_seq=1 ttl=64 time=1.49 ms 31008 bytes from 192.168.252.1: icmp_seq=2 ttl=64 time=4.88 ms 31008 bytes from 192.168.252.1: icmp_seq=3 ttl=64 time=0.821 ms 31008 bytes from 192.168.252.1: icmp_seq=4 ttl=64 time=0.926 ms 31008 bytes from 192.168.252.1: icmp_seq=5 ttl=64 time=0.620 ms --- 192.168.252.1 ping statistics --- 5 packets transmitted, 5 received, 0% packet loss, time 4046ms rtt min/avg/max/mdev = 0.620/1.747/4.881/1.593 ms Deliberately exceeding the aforementioned 32000 MTU by 27 bytes works! abinodji:~ # ping -c 5 -M do -s 31999 192.168.252.1 PING 192.168.252.1 (192.168.252.1) 31999(32027) bytes of data. 32007 bytes from 192.168.252.1: icmp_seq=1 ttl=64 time=0.723 ms 32007 bytes from 192.168.252.1: icmp_seq=2 ttl=64 time=0.581 ms 32007 bytes from 192.168.252.1: icmp_seq=3 ttl=64 time=0.984 ms 32007 bytes from 192.168.252.1: icmp_seq=4 ttl=64 time=0.731 ms 32007 bytes from 192.168.252.1: icmp_seq=5 ttl=64 time=1.79 ms --- 192.168.252.1 ping statistics --- 5 packets transmitted, 5 received, 0% packet loss, time 4035ms rtt min/avg/max/mdev = 0.651/0.955/1.308/0.263 ms Increase more. abinodji:~ # ping -c 5 -M do -s 33000 192.168.252.1 PING 192.168.252.1 (192.168.252.1) 33000(33028) bytes of data. 33008 bytes from 192.168.252.1: icmp_seq=1 ttl=64 time=2.30 ms 33008 bytes from 192.168.252.1: icmp_seq=2 ttl=64 time=0.703 ms 33008 bytes from 192.168.252.1: icmp_seq=3 ttl=64 time=10.9 ms 33008 bytes from 192.168.252.1: icmp_seq=4 ttl=64 time=0.788 ms 33008 bytes from 192.168.252.1: icmp_seq=5 ttl=64 time=1.25 ms --- 192.168.252.1 ping statistics --- 5 packets transmitted, 5 received, 0% packet loss, time 5232ms rtt min/avg/max/mdev = 0.703/3.207/10.980/3.928 ms increase more still. abinodji:~ # ping -c 5 -M do -s 56000 192.168.252.1 PING 192.168.252.1 (192.168.252.1) 56000(56028) bytes of data. 56008 bytes from 192.168.252.1: icmp_seq=1 ttl=64 time=1.02 ms 56008 bytes from 192.168.252.1: icmp_seq=2 ttl=64 time=0.856 ms 56008 bytes from 192.168.252.1: icmp_seq=3 ttl=64 time=1.32 ms 56008 bytes from 192.168.252.1: icmp_seq=4 ttl=64 time=1.28 ms 56008 bytes from 192.168.252.1: icmp_seq=5 ttl=64 time=0.893 ms Increase again --- 192.168.252.1 ping statistics --- 5 packets transmitted, 5 received, 0% packet loss, time 4052ms rtt min/avg/max/mdev = 1.019/4.823/18.723/6.959 ms abinodji:~ # ping -c 5 -M do -s 57300 192.168.252.1 PING 192.168.252.1 (192.168.252.1) 57300(57328) bytes of data. 57308 bytes from 192.168.252.1: icmp_seq=1 ttl=64 time=0.846 ms 57308 bytes from 192.168.252.1: icmp_seq=2 ttl=64 time=0.813 ms 57308 bytes from 192.168.252.1: icmp_seq=3 ttl=64 time=0.839 ms 57308 bytes from 192.168.252.1: icmp_seq=4 ttl=64 time=0.897 ms 57308 bytes from 192.168.252.1: icmp_seq=5 ttl=64 time=0.902 ms --- 192.168.252.1 ping statistics --- 5 packets transmitted, 5 received, 0% packet loss, time 4037ms rtt min/avg/max/mdev = 0.813/0.859/0.902/0.043 ms Increase again past max MTU - complaint as expected. abinodji:~ # ping -c 5 -M do -s 57340 192.168.252.1 PING 192.168.252.1 (192.168.252.1) 57340(57368) bytes of data. >From 192.168.252.22 icmp_seq=1 Frag needed and DF set (mtu = 57344) >From 192.168.252.22 icmp_seq=1 Frag needed and DF set (mtu = 57344) >From 192.168.252.22 icmp_seq=1 Frag needed and DF set (mtu = 57344) >From 192.168.252.22 icmp_seq=1 Frag needed and DF set (mtu = 57344) >From 192.168.252.22 icmp_seq=1 Frag needed and DF set (mtu = 57344) --- 192.168.252.1 ping statistics --- 0 packets transmitted, 0 received, +5 errors Packet size = MTU, the 32000 stuff appears. abinodji:~ # ping -c 5 -M do -s 57316 192.168.252.1 PING 192.168.252.1 (192.168.252.1) 57316(57344) bytes of data. >From 192.168.252.22 icmp_seq=1 Frag needed and DF set (mtu = 32000) >From 192.168.252.22 icmp_seq=1 Frag needed and DF set (mtu = 32000) >From 192.168.252.22 icmp_seq=1 Frag needed and DF set (mtu = 32000) >From 192.168.252.22 icmp_seq=1 Frag needed and DF set (mtu = 32000) >From 192.168.252.22 icmp_seq=1 Frag needed and DF set (mtu = 32000) --- 192.168.252.1 ping statistics --- 0 packets transmitted, 0 received, +5 errors Should be under 57344 MTU but this complains that it's now 32000 also abinodji:~ # ping -c 5 -M do -s 57315 192.168.252.1 PING 192.168.252.1 (192.168.252.1) 57315(57343) bytes of data. >From 192.168.252.22 icmp_seq=1 Frag needed and DF set (mtu = 32000) >From 192.168.252.22 icmp_seq=1 Frag needed and DF set (mtu = 32000) >From 192.168.252.22 icmp_seq=1 Frag needed and DF set (mtu = 32000) >From 192.168.252.22 icmp_seq=1 Frag needed and DF set (mtu = 32000) >From 192.168.252.22 icmp_seq=1 Frag needed and DF set (mtu = 32000) --- 192.168.252.1 ping statistics --- 0 packets transmitted, 0 received, +5 errors ---------------------------------------------------------------------- For LINUX-390 subscribe / signoff / archive access instructions, send email to [EMAIL PROTECTED] with the message: INFO LINUX-390 or visit http://www.marist.edu/htbin/wlvindex?LINUX-390
