[Info-vax] TCPIP performance for VMS

[JF Mezei]

jbriggs444 at gmail.com wrote:

> However there's the interesting possibility that the PPPoE
> encapsulation is reducing the available MTU 

It isn't a possibility, it is a fact. The PPPoE kludge reduces MTU to
1492 (PPPoE header takes 8 btes minimum, and if you use MLPPP (multipe
links bonded into one), the headers are larger.

> clear or  ignored.  A MAC using a MTU less than 1500 could be getting
> its packets through intact while the DSL modem is forced to fragment
> full sized 1500 byte frames being emitted by the VMS box. 

It wouldn't be the DSL modem, it would bethe router (semantics...). The
modem always fragments the PPPoE kludge into ancient ATM packets (53
byte packet, 48 byte payload).

> If the
> speed testing site is detecting IP fragments, it could be falsely
> concluding that PMTUD is not supported by the client

The www.speedguide.net:8080 looks at the TCP options. And it says that
VMS doesn't have MTU discovery enabled. But the Mac has it. Both go
though the same router/link.

MTU discovery SHOULD tell VMS that to reach beyond that router, it needs
to lower packets to 1492.


> when the truth
> might be that it is not supported by the DSL modem.

If the Mac can discover that the link is 1492, then I would have
expected VMS to also support this.  The TCPIP Stack is young enough to
have the pmtu_enabled flag in its sysconfig. And the documentation even
mentions to turn off mtu discovery to increase performance !.

I am somewhat surprised that I would be the first one to notice/complain
about this. Shirley, I am not the only one running VMS over a DSL link.