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I've not yet looked deeply into it but i note from http://fasterdata.es.net/host-tuning/linux/ # recommended for hosts with jumbo frames enabled net.ipv4.tcp_mtu_probing=1 and If you are using Jumbo Frames, we recommend setting tcp_mtu_probing = 1 to help avoid the problem of MTU black holes. Setting it to 2 sometimes causes performance problems. otherwise i guess it should be 0. I also note http://fasterdata.es.net/host-tuning/esnet-perfsonar-tuning/ dan * Dr Daniel Traynor, Grid cluster system manager * Tel +44(0)20 7882 6560, Particle Physics,QMUL ________________________________________ From: Testbed Support for GridPP member institutes <[log in to unmask]> on behalf of John Bland <[log in to unmask]> Sent: 23 September 2015 16:57 To: [log in to unmask] Subject: Re: mtu probing on perfosnar boxes. On 23/09/15 16:54, Brian Davies wrote: > Hi , I would like to ask…. > > What do other sites have set as the value for: > > net.ipv4.tcp_mtu_probing > > in their sysctl on their perfsonar machines. > > RAL’s are set to zero ( and this seems maybe a mistake on our part? [root@hepsonar1 ~]# cat /proc/sys/net/ipv4/tcp_mtu_probing 0 Certainly not altered by us. Seems to be the default. John -- John Bland [log in to unmask] System Administrator office: 220 High Energy Physics Division tel (int): 42911 Oliver Lodge Laboratory tel (ext): +44 (0)151 794 2911 University of Liverpool http://www.liv.ac.uk/physics/hep/ "I canna change the laws of physics, Captain!"