Date: Sat, 02 Dec 2000 16:09:51 -0500 From: "Louis A. Mamakos" <louie@TransSys.COM> To: "Mike E. Matsnev" <mike@po.cs.msu.su> Cc: freebsd-net@FreeBSD.ORG Subject: Re: TCP sockets connecting to tmeselves Message-ID: <200012022109.eB2L9p580245@whizzo.transsys.com> In-Reply-To: Your message of "Sat, 02 Dec 2000 23:11:39 %2B0300." <20001202231139.A4050@haali.cs.msu.ru> References: <20001202231139.A4050@haali.cs.msu.ru>
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The TCP protocol spec specifically allows for simultaneous opens. That is, both connection endpoints can start the TCP state machine in SYN-SENT state and neither starting in LISTEN state. The behavior that you're seeing is an effect of that. 20 years ago, there were a number of "ECHO" hosts on the ARPANET at the time; all these things did was swap the IP source and destination addresses, and send the packet back at you. I was a hand way to test (then new) TCP protocol implementations without needing another host. > Also what rfc specifies this strange behaviour? I looked through rfc793 and didnt > find any info about "symmetric opens". I also tried tcpdump -i lo0, and saw only > 2 packets exchanged: one SYN and one ACK, no SYN+ACK was sent. So the SYN was sent, and an ACK returned for the SYN. Since both "ends" are the same, that's all you need. As far as the TCP stack is concerned, it's SYN has been acked, and the connection is open. louie To Unsubscribe: send mail to majordomo@FreeBSD.org with "unsubscribe freebsd-net" in the body of the message
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