Connection establishment refers to the process of setting up a connection between two or more devices or systems so that they can communicate with each other. Typically, in networking, it pertains to the initiation phase before data transmission starts. The most well-known example of this is the TCP three-way handshake.
Here’s a step-by-step breakdown of connection establishment, particularly using the TCP three-way handshake as an illustration:
SYN (Synchronize sequence numbers):
- The initiating device (often termed as the “client”) sends a TCP segment with the SYN flag set to the receiving device (often termed as the “server”). This indicates a request to establish a connection. Along with the SYN flag, a sequence number (usually random) is sent which marks the beginning sequence number for data that the client will transmit.
SYN-ACK (Synchronize-Acknowledge):
- Upon receiving the SYN request, the server responds with a TCP segment that has both the SYN and ACK flags set. The acknowledge number (in the ACK field) is set to one more than the received sequence number from the client. Simultaneously, the server also sends its own sequence number to the client to establish bi-directional communication.
ACK (Acknowledge):
- The client receives the SYN-ACK from the server. It then sends an ACK back to the server to acknowledge the server’s initial sequence number. The sequence number here is the next expected byte, and the ACK number is one more than the received sequence number from the server’s SYN.
Once these three steps are completed, the connection is established, and both the client and server can begin transmitting data. This whole process ensures a reliable initiation of a connection, verifying that both devices are ready to communicate and are aware of the starting sequence numbers for data transmission.