On Friday, 01/29/2016 at 04:28 GMT, "Donald J." <dona...@4email.net> wrote: > More important is too large an MSS which can cause fragmentation. > You can test your network route from z/OS with this ping: > ping 192.168.20.54 (PMTU yes LENGTH 1472 Verbose > > 'PMTU yes' says 'do not fragment'. On my host 1472 is max value that > does not fragment. 1473 will need fragmentation. Going over a > VPN will have larger TCP/IP headers giving a smaller MSS. > For dynamic detection to work, your routers must pass the necessary > ICMP packets. By default, some do not. > > Good article here: > http://www.networkworld.com/article/2224654/cisco-subnet/mtu-size-issues.html
Even with all of that in place, you still need to get your local MTU value correct. It represents a ceiling on the MTU size PMTU discovery will try, and it sets the limit on the TCP MSS. Everyone connected to the same LAN segment needs to have the same MTU on that LAN segment. This is still very much a stone-knives-and-bearskins issue with TCP/IP. IMO, it's something IEEE should address since the mothership for the MTU is the local ethernet switch. That's where the MTU is configured ("Are we using jumbo frames or not?"). There's a smart device at the other end of the wire. There are a variety of things negotiated at a low level (duplex, speed, VEPA) and there's no practical reason "maximum frame size" can't be discovered. MTU for a particular protocol is a function of the MFS. (We're familiar with MFS from HiperSockets.) Alan Altmark Senior Managing z/VM and Linux Consultant Lab Services System z Delivery Practice IBM Systems & Technology Group ibm.com/systems/services/labservices office: 607.429.3323 mobile; 607.321.7556 alan_altm...@us.ibm.com IBM Endicott ---------------------------------------------------------------------- For LINUX-390 subscribe / signoff / archive access instructions, send email to lists...@vm.marist.edu with the message: INFO LINUX-390 or visit http://www.marist.edu/htbin/wlvindex?LINUX-390 ---------------------------------------------------------------------- For more information on Linux on System z, visit http://wiki.linuxvm.org/