TCP Connection Management

Before exchanging data, sender and receiver process handshake to:

agree to establish connection (each knowing the other is willing to establish connection).
agree on connection parameters (e.g., starting sequence #).
decides buffer size for flow control.

2-way handshake problem

The server may receive extraneous connection request due to delay.
This holds server resources wastefully, not knowing what happended on the client side.
This can be used as DDos attack by invoking many hanging connections.

Client chooses initial sequence number x and sends TCP SYN(synchronization) message to the client.
- This doesn’t involve ACK yet.
Server also chooses initial sequence number y and sends TCP SYNACK message, acknowledging client’s SYN.
- Server doesn’t commit to connection yet.
Client receives SYNACK(x) which indicates the server is alive, and sends ACK for server’s SYNACK.
- This segment can contain client-to-server data.
Server receives ACK(y) which indicates the client is alive.
Then, connection establishment is agreed.

Client and server each close their side of connection.
- each sends TCP segement with FIN bit = 1.
Receiver responds to received FIN with ACK.
- on receiving FIN, ACK can be combined with own FIN to agree on closing the connection.
Simultaneous FIN exchanges can be handled.
This means that client has to send “close request” and receive ACK for reliable termination.