On Friday, 01/29/2016 at 04:28 GMT, "Donald J." <[email protected]>
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
[email protected]
IBM Endicott
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