tcp socket problem

[jm.carp]

I'm writing a tcp client that grabs data from a server at 32hz. But
the connection drops exactly one minute after it's opened. I can get
data from the server fine for the first 60s, and then the connection
goes dead. What's going on?

[Dan Stromberg]

On Fri, 25 Jul 2008 16:09:13 -0700, jm.carp wrote:

I'm writing a tcp client that grabs data from a server at 32hz. But the
connection drops exactly one minute after it's opened. I can get data
from the server fine for the first 60s, and then the connection goes
dead. What's going on?

What does "goes dead" mean in this case? Is python giving a traceback?
Is some I/O blocking?

I'd probably use a system call tracer to track down something like that,
unless there's a useful traceback. It may be useful in the client, or
the server, or both.

http://stromberg.dnsalias.org/~strom...-with-syscall-
tracers.html

Also, be careful to check that you aren't reading two chunks when you
think you're getting one or something. That could cause some I/O to
block in a hard-to-track-down way. IOW, TCP feels free to chop your
packets into multiple packets, or to aggregate multiple packets into a
smaller number of them - whatever it wants, pretty much. This sometimes
helps performance or reliability, but it complicates writing socket
programs at times. Put another way, a single send might show up as two
recv's, for example.

Oh, and using a sniffer may help too. I like wireshark or tshark (GUI
and text interfaces of the same underlying sniffer), but there are many
available. Old hands seem to prefer tcpdump, though I'm clear on why.
If you fire up a sniffer against your network communication, it should be
possible to see, for example, the last packet transferred, which system
it was to, and which system it was from. You may not know which host is
having the problem without a detailed knowledge of the protocol, but
since you're coding the program, you may have that. :) This may show
packet splitting/aggregation too, but keep in mind that a sniffer only
shows you the data stream at a particular point in the transfer - you
could see different things when running a sniffer on the client, on the
server, or even on a subnet in between the two. Oh, and many networks
are switched now, so most sniffers don't do that well on subnets in
between, but they frequently still work well on the client and server
themselves.

HTH

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