TCP Connection Establishment

• Difficulty Level : Medium
• Last Updated : 08 Sep, 2021

Prerequisite – TCP 3-Way Handshake Process 
TCP is a connection-oriented protocol and every connection-oriented protocol needs to establish a connection in order to reserve resources at both the communicating ends. 

Connection Establishment – 

1. Sender starts the process with the following: 

• Sequence number (Seq=521): contains the random initial sequence number generated at the sender side.
• Syn flag (Syn=1): request the receiver to synchronize its sequence number with the above-provided sequence number.
• Maximum segment size (MSS=1460 B): sender tells its maximum segment size, so that receiver sends datagram which won’t require any fragmentation. MSS field is present inside Option field in TCP header.
• Window size (window=14600 B): sender tells about his buffer capacity in which he has to store messages from the receiver. 
   

2. TCP is a full-duplex protocol so both sender and receiver require a window for receiving messages from one another. 

• Sequence number (Seq=2000): contains the random initial sequence number generated at the receiver side.
• Syn flag (Syn=1): request the sender to synchronize its sequence number with the above-provided sequence number.
• Maximum segment size (MSS=500 B): sender tells its maximum segment size, so that receiver sends datagram which won’t require any fragmentation. MSS field is present inside Option field in TCP header. 
  Since MSSreceiver < MSSsender, both parties agree for minimum MSS i.e., 500 B to avoid fragmentation of packets at both ends.

  Therefore, receiver can send maximum of 14600/500 = 29 packets.This is the receiver's sending window size.

• Window size (window=10000 B): receiver tells about his buffer capacity in which he has to store messages from the sender.

  Therefore, sender can send a maximum of 10000/500 = 20 packets.This is the sender's sending window size.

• Acknowledgment Number (Ack no.=522): Since sequence number 521 is received by the receiver so, it makes a request for the next sequence number with Ack no.=522 which is the next packet expected by the receiver since Syn flag consumes 1 sequence no.
• ACK flag (ACk=1): tells that the acknowledgment number field contains the next sequence expected by the receiver.

3. Sender makes the final reply for connection establishment in the following way: 

• Sequence number (Seq=522): since sequence number = 521 in 1st step and SYN flag consumes one sequence number hence, the next sequence number will be 522.
• Acknowledgment Number (Ack no.=2001): since the sender is acknowledging SYN=1 packet from the receiver with sequence number 2000 so, the next sequence number expected is 2001.
• ACK flag (ACK=1): tells that the acknowledgment number field contains the next sequence expected by the sender.

Since the connection establishment phase of TCP makes use of 3 packets, it is also known as 3-way Handshaking (SYN, SYN + ACK, ACK).