TCP Control Flags

• SYN: synchronize sequence numbers; used to open a connection
• ACK: acknowledgement field is valid
• FIN: sender has no more data; used to close a connection

Connection Establishment: Three-Way Handshake

1. Client sends SYN with its initial sequence number $x$.
2. Server replies SYN + ACK with its own sequence number $y$ and ack $x+1$.
3. Client sends ACK with ack $y+1$. Connection is established.

Connection Termination (four-way)

1. Side A sends FIN.
2. Side B ACKs it (it may still send remaining data: half-close).
3. Side B sends its own FIN when done.
4. Side A ACKs and enters TIME_WAIT.

Significance of TIME_WAIT

The side that closes first waits 2 x MSL (Maximum Segment Lifetime) before fully closing. Reasons:

• Reliable termination: if the final ACK is lost, the peer retransmits its FIN, and the waiting side can re-send the ACK.
• Avoid stale segments: lets old duplicate segments from the previous connection expire so they are not mistaken for data of a new connection using the same address/port pair.