[Info-vax] TCPIP performance for VMS

[jbriggs444 at gmail.com]

On Apr 22, 7:28 pm, JF Mezei <jfmezei.spam... at vaxination.ca> wrote:
> jbriggs... 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
>
> Thewww.speedguide.net:8080looks 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.

PMTUD isn't implemented in the TCP options.  The DF bit is in the IP
header.
(semantics :-).

If you're still using Cisco router gear, there is a tweak you can try.

In interface configuration mode

        ip tcp adjust-mss 1436   ! (or other selected value)

This causes the router to reach into the TCP SYN and SYN+ACK packets
and adjust the MSS advertised by the client to the server and the
server to the client.  The upshot is that the two ends wind up sending
smaller packets.