On 12/25/2011 12:58 PM, Henrik Nordström wrote: > sön 2011-12-25 klockan 03:09 +0100 skrev Fritz Elfert: > >> Studying the source of libssh2 (current git), I noticed, that while >> libssh2 supports sending keepalive messages as well as replying to >> keepalives sent from the server, it apparently does *not* act on the >> responses from the server even when want_reply is set while invoking >> libssh2_keepalive_send(). > > The keep-alive implementation in libssh2 is very rudimentary, and not > without problems. Currently it helps keeping an idle connection alive, > but not good for detecting communication issues, and may even cause > additional communication issues if EAGAIN is seen when sending the > keep-alive message.
EAGAIN is silently ignored in libssh2_keepalive_send(). Isn't that correct? > >> So am I missing something here? >> Or: How am I (as a user of libssh2) supposed to find out, if the server >> has properly responded to the keepalive that I just sent? > > From what I can tell you can't. > > There really should be timestamps recording time of last sent, last sent > with want_reply set and last received packet, and keepalive adjusted to > monitor these both for determining if keep-alive is needed and for > detecting session timeout. Technically just one of the last sent > timestamps is sufficient, but to keep the logics of which time to track > for keep-alive isolated within keepalive it's easier if both are > recorded in the session. > I see. Well if you take a look at openssh, they simply maintain a counter (server_alive_timeouts) which gets incremented when sending a keepalive and reset to 0 whenever a SSH_MSG_REQUEST_SUCCESS or SSH_MSG_REQUEST_FAILURE is received. Before sending the keepalive request, they check this counter against their configurable server_alive_count_max. Also: They *always* set want_reply to 1 - Wouldn't make much sense otherwise. From that, my understanding is that the current implementation of libssh2 (regrding maintaining a single timestamp of last sent) is sufficient. The only thing that's missing is the timeouts-counter. Of course - since want_reply is optional in libssh2 - the counter should only be incremented if want_reply is set. > It's not really important to track if the received (or sent) packets are > in reply to keep-alive or not. It's sufficient to note that there is > traffic. Agreed. Cheers -Fritz > > I don't know why ticket #183 was closed (Daniel?). Does not look > outdated to me. May even work to some degree. But needs to be improved > slightly to handle want_reply == 0 without timing out the session, and > do not solve a couple of other keep-alive issues. > > Regards > Henrik > > _______________________________________________ > libssh2-devel http://cool.haxx.se/cgi-bin/mailman/listinfo/libssh2-devel _______________________________________________ libssh2-devel http://cool.haxx.se/cgi-bin/mailman/listinfo/libssh2-devel
