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Keep In Touch (tcp Keepalives Etc.)

By A.P. Lawrence

You can't please everyone, so you got to please yourself.. (Ricky Nelson)

I don't imagine Ricky Nelson was thinking about tcp timeouts and keepalives in the 70's, and probably isn't still, but the subject comes up fairly often for me and never fails to be annoying.

Annoying? OK, maybe confusing is the better choice. This is one of those areas where the cure can be worse than the disease and where discussions by tcp programmers can get acrid and heated. The biggest problem is that users often really don't know what they want, and if they do know, it probably shouldn't have anything to do with the tcp stack, but next thing you know somebody is changing kernel level values, and probably not getting the results they expected.

Typically it starts on one side or the other of the basic question, which is "when should a connection to another machine be dropped?". The other side has gone away, lost its mind (or at least its network connection) or maybe some router in between the two of you is misbehaving. Whatever. Packets that should be flowing between you and X are not, and you want something done about it now. It is absolutely critical that you be aware of this connection problem immediately so that you can take action to fix it. That's one side. The other side is that you got disconnected from machine X because there was some stupid momentary glitch and that's not what you want to happen: it's critical that your process remain connected so that it doesn't have to restart processing whatever data it was chewing on.

Ricky should sing the chorus now.

So what is poor tcp to do? Break the connection at the first sign of trouble or hang in there waiting for it to come back? Most TCP implementations have "keepalives": (from http://www.ietf.org/rfc/rfc1122.txt)

Implementors MAY include "keep-alives" in their TCP implementations, although this practice is not universally accepted. If keep-alives are included, the application MUST be able to turn them on or off for each TCP connection, and they MUST default to off.

Keep-alive packets MUST only be sent when no data or acknowledgement packets have been received for the connection within an interval. This interval MUST be configurable and MUST default to no less than two hours.

Click here to continue reading...

About the Author:
A.P. Lawrence provides SCO Unix and Linux consulting services. http://www.pcunix.com

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