The TCP/IP Guide - TCP Connection Establishment Process: The "Three-Way Handshake"

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TCP is also set up to handle the situation where both devices perform an active OPEN instead of one doing a passive OPEN. This may occur if two clients are trying to reach each other instead of a client and a server. It is uncommon, however, and only happens under certain circumstances.Simultaneous connection establishment can also only happen if a well-known port is used as the source port for one of the devices.

In this case, the steps are different for both devices. Each client will perform an active OPEN and then proceed through both the SYN-SENT and SYN-RECEIVED states until their SYNs are acknowledged. This means there isn't a “three-way handshake” any more as shown in Table 152. Instead, it is like two simultaneous “two-way handshakes”. Each client sends a SYN, receives the other's SYN and ACKs it, and then waits for its own ACK. The transaction, simplified, is described in Table 153 and Figure 212.

**Table 153: TCP Simultaneous Open Connection Establishment Procedure**
Client A			Client B
Start State	Action	Move To State	Start State	Action	Move To State
*CLOSED*	Client A Step #1 Transmit: Client A performs an active OPEN, creating a TCB and sending a SYN message to the server.	*SYN-SENT*	*CLOSED*	Client B Step #1 Transmit: Client B performs an active OPEN, creating a TCB and sending a SYN to the server.	*SYN-SENT*
*SYN-SENT*	Client B Step #1 Receive and Step #2 Transmit: Client A receives Client B's SYN and sends it an ACK. It is still waiting for an ACK to its own SYN.	*SYN-RECEIVED*	*SYN-SENT*	Client A Step #1 Receive and Step #2 Transmit: Client B receives Client A's SYN and sends it an ACK. It is still waiting for an ACK to its own SYN.	*SYN-RECEIVED*
*SYN-RECEIVED*	Client A Step #2 Receive: Client A receives the ACK from Client B for its SYN and is done with connection establishment.	*ESTABLISHED*	*SYN-RECEIVED*	Client B Step #2 Receive: Client B receives the ACK from Client A for its SYN and is done with connection establishment.	*ESTABLISHED*

**Figure 212: TCP Simultaneous Open Connection Establishment Procedure**
This diagram shows what happens when two devices try to open a connection to each other at the same time. In this case instead of a three-way handshake, each sends a *SYN* and receives an *ACK*. They each follow the same sequence of states, which differs from both sequences in the normal three-way handshake.

To keep the table size down, I have shown the activities performed by the two devices occurring simultaneously (in the same row). In “real life” the actions don’t need to occur at exactly the same time, and probably won’t. All that has to happen for the simultaneous procedure to be followed is that each device receives a SYN before getting an ACK for their own SYN, as Figure 212 shows.