18.8 Simultaneous Open

It is possible, although improbable, for two applications to both perform an active open to each other at the same time. Each end must transmit a SYN, and the SYNs must pass each other on the network. It also requires each end to have a local port number that is well known to the other end. This is called a simultaneous open.

For example, one application on host A could have a local port of 7777 and perform an active open to port 8888 on host B. The application on host B would have a local port of 8888 and perform an active open to port 7777 on host A.

This is not the same as connecting a Telnet client on host A to the Telnet server on host B, at the same time that a Telnet client on host B is connecting to the Telnet server on host A. In this Telnet scenario, both Telnet servers perform passive opens, not active opens, and the Telnet clients assign themselves an ephemeral port number, not a port number that is well known to the other Telnet server.

TCP was purposely designed to handle simultaneous opens and the rule is that only one connection results from this, not two connections. (Other protocol suites, notably the OSI transport layer, create two connections in this scenario, not one.)

When a simultaneous open occurs the state transitions differ from those shown in Figure 18.13. Both ends send a SYN at about the same time, entering the SYN_SENT state. When each end receives the SYN, the state changes to SYN_RCVD (Figure 18.12), and each end resends the SYN and acknowledges the received SYN. When each end receives the SYN plus the ACK, the state changes to ESTABLISHED. These state changes are summarized in Figure 18.17.

Figure 18.17. Segments exchanged during simultaneous open.

A simultaneous open requires the exchange of four segments, one more than the normal three-way handshake. Also notice that we don't call either end a client or a server, because both ends act as client and server.

An Example

It is possible, though hard, to generate a simultaneous open. The two ends must be started at about the same time, so that the SYNs cross each other. Having a long round-trip time between the two ends helps, to let the SYNs cross. To do this we'll execute one end on our host bsdi, and the other end on the host vangogh.cs.berkeley.edu. Since there is a dialup SLIP link between them, the round-trip time should be long enough (a few hundred milliseconds ) to let the SYNs cross.

One end ( bsdi ) assigns itself a local port of 8888 (the -b command-line option) and performs an active open to port 7777 on the other host:

 bsdi %  sock -v -b8888 vangogh.cs.berkeley.edu 7777  connected on 140.252.13.35.8888 to 128.32.130.2.7777     TCP_MAXSEG = 512  hello, world   we type this line  and hi there  this line was typed on other end  connection closed by peer  this is output when FIN received

The other end is started at about the same time, assigns itself a local port of 7777, and performs an active open to port 8888:

 vangogh %  sock -v -b7777 bsdi.tuc.noao.edu 8888  connected on 128.32.130.2.7777 to 140.252.13.35.8888     TCP_MAXSEG = 512     hello, world  this is typed on the other end   and hi there   we type this line   ^D   and then type our EOF character

We specify the -v flag to our sock program to verify the IP address and port numbers on each end of the connection. This flag also prints the MSS used by each end of the connection. We also type in one line on each end, which is sent to the other end and printed, to verify that both ends are indeed talking to each other.

Figure 18.18 shows the exchange of segments across the connection. (We have deleted some new TCP options that appear in the original SYN from vangogh, a 4.4BSD system. We describe these newer options in Section 18.10.) Notice the two SYNs (lines 1 and 2) followed by the two SYNs with ACKs (lines 3 and 4). These perform the simultaneous open.

Figure 18.18. Exchange of segments during simultaneous open.

Line 5 shows the input line "hello, world" going from bsdi to vangogh, with the acknowledgment in line 6. Lines 7 and 8 correspond to the line "and hi there" going in the other direction. Lines 9 “12 show the normal connection termination.

Many Berkeley-derived implementations do not support the simultaneous open correctly. On these systems, if you can get the SYNs to cross, you end up with an infinite exchange of segments, each with a SYN and an ACK, in each direction. The transition from the SYN_SENT state to the SYN_RCVD state in Figure 18.12 is not always tested in many implementations.