TCP Connection Establishment

16 Jun, 2022
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.
Acknowledgement 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 acknowledgement 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.
Acknowledgement 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 acknowledgement 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). 
GATE question – GATE IT 2008 | Question 67
Related next article – TCP Connection Termination 
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