> What I meant was that there were cases when a receiver could not > tell weather no data was coming or communication was interrupted. > Once connection is established, a route is available between a > server and a client. Let's say this route is broken for some > reasons, i.e. someone unplugged a cable or a firewall started > dropping or rejecting between these server and client, a sender > may not notice as soon as it happens but at least, a sender knows > a massages was not delivered right. On the other hand, receiver > side does not have any idea that a message delivery failure has > happened at all or for a while unless using heartbeat messages > in upper layer. KEEP_ALIVE option seems to be implementation > dependent such that you cannot assure TCP connection availability > for every minute.
The whole point of TCP (vs IP alone, or UDP) is to establish reliable end-to-end communication over unreliable underlying links. If a packet is corrupted or lost, it gets resent. If a route goes down, and an alternate is available, TCP will -- eventually -- find it and recover. If the last (or only) route goes down, TCP will in principle wait indefinitely for a route to become available, whether by reestablishment of the original or provision of an alternative. So you are correct that a receiver can't tell the difference between a loss of connectivity and the sender having crashed, however the situation is entirely symmetric: the sender can't tell the difference either. It all gets sorted out when communication is reestablished; at that point traffic will resume (if the link had been down) or the uncrashed end will get a connection reset (if its peer had crashed). The practice of sending keep-alive packets simply converts a temporary (thus potentially recoverable) communication loss into what amounts to an unrecoverable crash of whichever end gets impatient first. _______________________________________________ freebsd-hackers@freebsd.org mailing list http://lists.freebsd.org/mailman/listinfo/freebsd-hackers To unsubscribe, send any mail to "freebsd-hackers-unsubscr...@freebsd.org"
