Date: Sun, 22 Mar 2009 23:52:53 -0400 From: Yoshihiro Ota <ota@j.email.ne.jp> To: Robert Watson <rwatson@FreeBSD.org> Cc: freebsd-hackers@freebsd.org Subject: Re: 2 uni-directional TCP connection good? Message-ID: <20090322235253.432874dd.ota@j.email.ne.jp> In-Reply-To: <alpine.BSF.2.00.0903201321570.48549@fledge.watson.org> References: <20090320045319.04484fc5.ota@j.email.ne.jp> <alpine.BSF.2.00.0903201321570.48549@fledge.watson.org>
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On Fri, 20 Mar 2009 13:24:09 +0000 (GMT) Robert Watson <rwatson@FreeBSD.org> wrote: > > On Fri, 20 Mar 2009, Yoshihiro Ota wrote: > > > 1. With TCP connections, only sender side can detect some communication > > issues passively if happened. By using two connections, you lost that > > ability by your self. I agree on this one. > > Could you expand a bit on this point? While the connection creation process > (usually) asymmetric, once the connection is built it's essentially the same > state machine on both sides of the connection, and socket semantics with > respect to the state machine are effectively identical. Application on both > sides should be able to detect disconnect, monitor connection state using > TCP_INFO, etc. 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. Thanks, Hiro
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