ADSM-L

Re: Why does ADSM use 536-byte data packets?

1995-11-20 16:44:58
Subject: Re: Why does ADSM use 536-byte data packets?
From: Mitch Sukalski <syw AT ECMWF DOT INT>
Date: Mon, 20 Nov 1995 21:44:58 +0000
Andy-

Not knowing how your clients and server are connected and addressed, I'd
suggest two possibilities. If you've already looked at these issues, please
feel free to ignore this information.

The first is that your network interface MTU (maximum transmission unit) might
be set too low. The default for ethernet should be 1500 bytes (or more
depending upon whether or not it includes the the data link and IP headers).
Under AIX, you can check this via "netstat -i".

If that isn't your problem (and it most likely isn't), then I would suspect
your TCP's default maximum segment size (MSS). Besides negotiating window size,
TCP will also negotiate the largest packet, or segment, that will be accepted
by the receiver. Due to a traditional lack of end-to-end MTU detection and a
great desire to avoid IP packet fragmentation at routers, most systems will
attempt to be "nice" to the network by advertising a small receive MSS when
they believe their partner is located far away across the distant Internet
(e.g., by seeing if their partner is on the same IP network or subnetwork as
themselves). In olden days when ARPAnet ruled the world, being nice meant using
an IP datagram length no larger than 576 octets, or setting your TCP MSS to
about 512 octets. You can see if this is the problem by capturing the TCP
handshake (look for the synchronize flag), and checking for the MSS option
(from both TCPs).

If that is your problem and the offending partner is running AIX, you can
change tcp_mssdflt via no, or if you're using subnets of a common IP network
you should make sure that no's subnetsarelocal is true (1). On other systems
there may be a TCP/IP configuration option, or (aacck!) it may be hard-coded
(in which case you may need to readdress to convince that machine that it is
local to the client/server)!

Cheers,

Mitch

On Nov 20,  2:38pm, Andy Raibeck wrote:

> Subject: Why does ADSM use 536-byte data packets?
> Using a Sniffer, we've noticed that ADSM sends out packets, typically 598 or
> 604 bytes in size. When we look at the packet in greater detail, the actual
> data portion is 536 bytes, with the remainder being protocol headers (LLC,
> DLC, IP, SNAP, etc.). This strikes me as a major bottleneck; that if I could
> send a smaller number of larger packets, I could improve performance by quite
> a bit (potentially). Oh, BTW I'm using TCP/IP for connectivity to an ADSM/MVS
> server.
>
> I use TCPBUFFSIZE 32 and TCPWINDOWSIZE 24, which are the only tuning options
> available in ADSM itself (for TCP/IP). IBM Level 2 tells me that the sizes
> must be set somewhere in the network or network card.
>
> Has anyone noticed this, and if so, can anything be done about it? I've seen
> this for NetWare and AIX. I haven't sniffed my other client types yet, but
> I believe I'd probably find something similar.
>
> Any help would be appreciated.
>
> Thanks,
>
> Andy Raibeck
> Connecticut Mutual
> 203-987-3521
>-- End of excerpt from Andy Raibeck



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