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Scott Klement wrote:

The Cisco VPN client may be smart enough to ignore keepalives.

I'd say that in that case, the word "smart" ought to be in quotation marks. Smart as in equus asinius.

One thing I turned up was the possibility of an in-band alternative keepalive, involving periodic sending of IAC NOPs. (But can a TN5250 server in a keyboard lock state handle IAC NOPs without being driven insane?)

In the proprietary protocols used by our products, instead of depending on keepalive probes (which I'd never heard of at the time, and which wouldn't have been as effective in any event), we solved the "orphaned child-server" problem with what I call a "heartbeat": the protocol is required to have a NOP request defined, and the client is required, when idle, to send a NOP every 30 seconds. If the child-server is waiting for requests, and doesn't get at least a heartbeat within 2 minutes of entering the "wait for requests" state, it assumes that the client to which it is connected has abended, locked-up, or gone insane, and it quietly "starves to death."

--
JHHL

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