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Keith - 4/29/2014 8:23:58 AM
   
Modify VTAM to use PMTUD=TCPVALE
My office is wants to modify VTAM to use PMTUD=TCPVALE. Anyone have reccomendations, trials, tribulations to share? I have read the manuals but can't find much in forums. Would love to hear if this has been found useful by anyone else?



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Keith - 4/29/2014 10:16:37 AM
   
RE:Modify VTAM to use PMTUD=TCPVALE
Spelling error. I meant to write PMTUD=TCPVALUE

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SamReynolds - 5/19/2014 12:09:33 PM
   
RE:Modify VTAM to use PMTUD=TCPVALE
I can't provide you with the user perspective, but I will provide the developer's perspective. We implemented path MTU discovery for EE based on requests from several customers that wanted that capability. For the most part, it appears to me that most installations leave PMTUD set to the default value of TCPVALUE (and therefore, take advantage of PMTUD unless they have disabled it for TCP/IP in general).

The caveat is that PMTUD does not perform optimally in networks with firewalls that block ICMP responses (since those responses are how PMTUD learns of the need to adjust the MTU size). If the MTU size is too large, and ICMP responses are blocked, we will eventually notice that large packets are getting dropped and will slam the MTU size to a minimal value for 20 minutes, after which we will raise it back to the original value. (And the process will repeat assuming we are traversing the same path/firewalls.)

Note that the incompatibility with firewalls that block ICMP responses is not EE-specific, but is instead a limitation of the PMTUD algorithm. Therefore, TCP/IP connections will be similarly affected.

Since the PMTUD start option is dynamically modifiable, you can easily enable it, and then disable it if you suspect that you are experiencing performance issues related to it.

You can use the detailed form of the DISPLAY EE command (DISPLAY ID=<ee pu>,LIST=DETAIL) to see what the MTU size is for each EE port on the connection. That same output will also show the packet transmission statistics, so monitoring that can provide an indication as to whether packet loss is occurring (due to MTU size issues or other issues).

Lastly, if you disable PMTUD, but suspect that you are having MTU size issues, you can specify the maximum MTU value for an EE connection on the switched PU definition.

Regards,
Sam Reynolds
z/OS Communications Server

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Keith - 5/19/2014 12:27:41 PM
   
RE:Modify VTAM to use PMTUD=TCPVALE
Thank you for the wonderfully thorough post. I have learned that our external firewalls will block ICMP traffic, therefore we will not attempt PMTUD externally. But we are going to implement for our internal traffic and see how it does.

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