HandshakeSYN

HandshakeSYN is a term used in computer networking to refer to the initial synchronization packet in the TCP connection establishment process. It denotes the first step of the TCP three-way handshake, wherein a client sends a segment with the SYN (synchronize) flag set to initiate a connection with a server.

Mechanics: The client sets the initial sequence number and may advertise options such as maximum segment size, window scale, and selective acknowledgment. The server responds with a SYN-ACK if it is willing to establish the connection, acknowledging the received ISN and providing its own ISN and options. The client completes the handshake with an ACK, after which data transfer may begin.

Impact and usage: In network monitoring and security contexts, a handshakeSYN event can be observed as a connection attempt. Excessive handshakeSYN packets without completing the handshake may indicate a SYN flood attack, mitigated by SYN cookies, rate limiting, or firewall rules.

Context and limitations: While every TCP connection begins with a handshakeSYN, the term is not a formal protocol name; it is used descriptively to refer to the initial SYN packet. TLS handshakes or application-layer handshakes are independent of the TCP handshake.

See also: TCP three-way handshake, SYN flood, SYN cookies, TCP options, initial sequence number.