23.4 Summary

As we said earlier, the keepalive feature is controversial . Protocol experts continue to debate whether it belongs in the transport layer, or should be handled entirely by the application.

It operates by sending a probe packet across a connection after the connection has been idle for 2 hours. Four different scenarios can occur: the other end is still there, the other end has crashed, the other end has crashed and rebooted, or the other end is currently unreachable. We saw each of these scenarios with an example, and saw different errors returned for the last three conditions.

In the first two examples that we looked at, had this feature not been provided, and without any application-level timer, our client would never have known that the other end had crashed, or crashed and rebooted. In the final example, however, nothing was wrong with the other end, the connection between them was temporarily down. We must be aware of this limitation when using keepalives .