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The TCP/IP Guide  9  TCP/IP Lower-Layer (Interface, Internet and Transport) Protocols (OSI Layers 2, 3 and 4)       9  TCP/IP Transport Layer Protocols            9  Transmission Control Protocol (TCP) and User Datagram Protocol (UDP)                 9  TCP/IP Transmission Control Protocol (TCP)                      9  TCP Basic Operation: Connection Establishment, Management and Termination

TCP Connection Establishment Process: The "Three-Way Handshake" 1 2 3 TCP Connection Management and Problem Handling, the Connection Reset Function, and TCP "Keepalives"

TCP Sequence Number Synchronization

Once each device has chosen its ISN, it sends this value to the other device in the Sequence Number field in its initial SYN message. The device receiving the SYN responds with an ACK message acknowledging the SYN (which may also contain its own SYN, as in step #2 of the three-way handshake). In the ACK message, the Acknowledgment Number field is set to the value of the ISN received from the other device plus one. This represents the next sequence number the device expects to receive from its peer; the ISN actually thus represents the sequence number of the last byte received (fictitious in this case, since the connection is new and nothing yet has been received). We'll see later on that this is consistent with how these two fields are used for normal data exchange.

Here's a simplified example of the three-way handshake steps showing how this is done (see Figure 213 as well). I chose small ISNs for readability but remember that they can be any 32-bit number:

1. Connection Request By Client: The client chooses an ISN for its transmissions of 4,567. It sends a SYN with the Sequence Number field set to 4,567.
   
   
2. Acknowledgment and Connection Request By Server: The server chooses an ISN for its transmissions of 12,998. It receives the client's SYN. It sends a SYN+ACK with an Acknowledgment Number field value of 4,568 (one more than the client's ISN). This message has a Sequence Number field value of 12,998.
   
   
3. Acknowledgment By Client: The client sends an ACK with the Acknowledgment Number field set to 12,999.
   

   Figure 213: TCP Sequence Number Synchronization

   This diagram illustrates the same three-way handshake connection establishment procedure introduced in Figure 211, except this time I have shown the Sequence Number and Acknowledgment Number fields in each message so you can see how they are used by each of the two devices to establish initial sequence numbers for data exchange.

   
   

With the connection now established, the client will send data whose first byte will be given sequence number 4,568. The server's first byte of data will be numbered 12,999.

TCP Connection Establishment Process: The "Three-Way Handshake" 1 2 3 TCP Connection Management and Problem Handling, the Connection Reset Function, and TCP "Keepalives"