Re: TCPIP performance for VMS

On Apr 22, 7:28 pm, JF Mezei <jfmezei.spam...@xxxxxxxxxxxxx> wrote:
jbriggs...@xxxxxxxxx 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.
.

Relevant Pages

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  • Re: TCPIP performance for VMS
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