[Info-vax] TCPIP performance for VMS

[johnwallace4 at yahoo.co.uk]

On Apr 22, 8:51 am, JF Mezei <jfmezei.spam... at vaxination.ca> wrote:
> Found out that my old MAC can sustain outbound FTP transfers at roughly
> 80KB/s (640kbps on an 800kbps ATM/PPPoE link (ADSL).
>
> The Alpha VMS host, sitting next to the mac at the ethernet levl only
> does 49 KB/s ( 392 kbps) sending data to the same FTP host on the internet.
>
> going towww.speedguide.net:8080gives me some of the TCP parameters.
> (you can curlhttp://www.speedguide.net:8080on VMS and then copy the
> url in the results (a redirect done with evil javascript) and paste it
> in a browser to see the results).
>
> while pmtu_enable is set to 1 on the VMS host, speedguide reporrs MTU
> discovery is set to off.
>
> Setting ipmtu to 1492 (ADSL MTUs) with ifconfig make no difference to
> the FTP performance on the VMS host.
>
> I tried changing tcp_rexmit_interval_min to different values, and it
> just seemed to decrease performance when the manual says it should
> increase it for high latency links.
>
> The TCPIP stack also doesn't seem to support timespamps in the TCTP
> header. Anyone know if this can be enabled ? (this is used by modern
> stacks to configure their congestion management).
>
> Has anyone ever dealt with this ?
>
> Is the answer: Migrate to Multinet ?

I can't  comment on TCP/IP Services but am happy to comment on
domestic TCP stuff in general, based on broadband tinkering since DSL
became affordable in the UK.

My favourite tweaks site is www.dslreports.com/tweaks - you will get
the same info as Speedguide provide, plus more, including important
things like retries. There's yet more (including graphs) if you hit
"more detail". You can post a URL if you wish, but be aware it will
expose your public IP address. They also have a nice Speed page, at
www.dslreports.com/speed. It's all PC/Windows oriented but the
principles remain the same.

When you do a speed test, you really need to run it for a reasonable
length of time. Many consumer-oriented speed tests don't run for long
enough to overcome any oddness introduced by the differing (and
occasionally not very compatible) behaviours of different
implementations of TCP Slow Start (dslreports again has a mention on
their Speed page). My favourite speedtest is www.numion.com/maxspeed
(I like the graphs); their other pseudo-browsing test is neat too
though not relevant here. Will they work from VMS?

"while pmtu_enable is set to 1 on the VMS host, speedguide reporrs MTU
discovery is set to off."

PMTU Discovery uses particular flavours of ICMP ("ping") packets. The
usual reason for sites reporting it's disabled when you've seemingly
enabled it is that a firewall is eating the relevant ICMP packets so
the other end never sees them. Do you see ICMP enabled when you test
from your Mac? What's different about its connectivity ("sitting next
to" means what, in IP terms)?

"I tried changing tcp_rexmit_interval_min to different values, and it
just seemed to decrease performance when the manual says it should
increase it for high latency links."

1) Why change it?
2) Did performance decrease when you decreased the parameter, or when
you increased the parameter (or both)?

What's the TCP/IP Services equivalent of RWIN? This one controls how
much unacknowledged traffic can be in flight at any given time
(closely related to the bandwidth*delay product). Bigger is better,
within reason, in particular if high latencies are involved. Too big
means, in the event of errors, too much stuff needs to be
retransmitted. Where's your ftp site? What's the round trip time?
http://www.dslreports.com/faq/tweaks/5._RWIN

"Setting ipmtu to 1492 (ADSL MTUs) with ifconfig make no difference to
the FTP performance on the VMS host. "

1492 made no difference from what previous MTU? Anyway, changing MTU a
little tends not to make a huge difference as long as you're in the
right region (something over 1400 for typical broadband setups).

Have you tried a DIY MTU test to the ftp site in question using pings
of various sizes with the -f flag? (Usually it's not worth the time
+effort of "optimising" your MTU to the last few bytes, but it can
highlight if something is misconfigured somewhere.

Mind you if PMTUD isn't working, who knows...

What network interface and connectivity does the Alpha have?
Presumably its LAN traffic is meeting your performance expectations,
in terms of speed and also error/retry rates?

Is there a Wireshark or similar packet sniffer on the LAN?

"timestamps ... used by modern stacks to configure their congestion
management"

Really? Got a good reference? The last time I checked (admittedly a
few years back) there was *no* meaningful end to end congestion
management in a DSL-based broadband network, and nor could there be,
not least because the likely places for congestion to occur are
actually passing PPP(oA) not pure IP and therefore your actual IP
packets are invisible to them and their IP addresses are invisible to
you. (Exceptions apply, as always, especially where ISPs are using
"traffic management"/DPI kit such as Ellacoyas - but that's another
story as I'm sure you are well aware).