3 Way Handshake | TCP Connection

Transmission Control Protocol-

 

Before you go through this article, make sure that you have gone through the previous article on TCP in Networking.

 

We have discussed-

• TCP establishes an end to end connection between the sender and receiver.
• This connection is established before exchanging the data.
• TCP connection is reliable and ensures in order delivery.

 

Also Read- TCP Header

 

In this article, we will discuss how a TCP connection is established.

 

3 Way Handshake-

 

Three Way Handshake is a process used for establishing a TCP connection.

 

Consider-

• Client wants to establish a connection with the server.
• Before Three Way Handshake, both client and server are in closed state.

 

TCP Handshake involves the following steps in establishing the connection-

 

Step-01: SYN-

 

For establishing a connection,

• Client sends a request segment to the server.
• Request segment consists only of TCP Header with an empty payload.
• Then, it waits for a reply segment from the server.

 

 

Request segment contains the following information in TCP header-

1. Initial sequence number
2. SYN bit set to 1
3. Maximum segment size
4. Receiving window size

 

1. Initial Sequence Number-

 

• Client sends the initial sequence number to the server.
• It is contained in the sequence number field.
• It is a randomly chosen 32 bit value.

 

2. SYN Bit Set To 1-

 

Client sets SYN bit to 1 which indicates the server-

• This segment contains the initial sequence number used by the client.
• It has been sent for synchronizing the sequence numbers.

 

Also Read- TCP Sequence Number

 

3. Maximum Segment Size (MSS)-

 

• Client sends its MSS to the server.
• It dictates the size of the largest data chunk that client can send and receive from the server.
• It is contained in the Options field.

 

4. Receiving Window Size-

 

• Client sends its receiving window size to the server.
• It dictates the limit of unacknowledged data that can be sent to the client.
• It is contained in the window size field.

 

Step-02: SYN + ACK-

 

After receiving the request segment,

• Server responds to the client by sending the reply segment.
• It informs the client of the parameters at the server side.

 

 

Reply segment contains the following information in TCP header-

1. Initial sequence number
2. SYN bit set to 1
3. Maximum segment size
4. Receiving window size
5. Acknowledgment number
6. ACK bit set to 1

 

1. Initial Sequence Number-

 

• Server sends the initial sequence number to the client.
• It is contained in the sequence number field.
• It is a randomly chosen 32 bit value.

 

2. SYN Bit Set To 1-

 

Server sets SYN bit to 1 which indicates the client-

• This segment contains the initial sequence number used by the server.
• It has been sent for synchronizing the sequence numbers.

 

3. Maximum Segment Size (MSS)-

 

• Server sends its MSS to the client.
• It dictates the size of the largest data chunk that server can send and receive from the client.
• It is contained in the Options field.

 

4. Receiving Window Size-

 

• Server sends its receiving window size to the client.
• It dictates the limit of unacknowledged data that can be sent to the server.
• It is contained in the window size field.

 

5. Acknowledgement Number-

 

• Server sends the initial sequence number incremented by 1 as an acknowledgement number.
• It dictates the sequence number of the next data byte that server expects to receive from the client.

 

6. ACK Bit Set To 1-

 

• Server sets ACK bit to 1.
• It indicates the client that the acknowledgement number field in the current segment is valid.

 

Step-03: ACK-

 

After receiving the reply segment,

• Client acknowledges the response of server.
• It acknowledges the server by sending a pure acknowledgement.

 

 

With these, a Full Duplex connection is established.

 

Important Points-

 

Point-01:

 

In step-01 and step-02-

• The connection parameters are established for the first side.
• They are acknowledged by the second side.

 

In step-02 and step-03-

• The connection parameters are established for the second side.
• They are acknowledged by the first side.

 

Point-02:

 

Connection establishment phase consume 1 sequence number of both the sides.

 

• Request segment consumes 1 sequence number of the requester.
• Reply segment consumes 1 sequence number of the respondent.
• Pure acknowledgements do not consume any sequence number.

 

Point-03:

 

Pure acknowledgement for the reply segment is not necessary.

 

This is because-

• If client sends the data packet immediately, then it will be considered as an acknowledgement.
• It means that in the first two steps only, the full duplex connection is established.

 

Point-04:

 

For all the segments except the request segment, ACK bit is always set to 1.

 

This is because-

• For the request segment, acknowledgement number field will always be invalid.
• For all other segments, acknowledgement number field will always be valid.

 

Point-05:

 

Certain parameters are negotiated during connection establishment.

 

The negotiation can be on setting the values of following parameters-

1. Window size
2. Maximum segment size
3. Timer values

 

Point-06:

 

In any TCP segment,

• If SYN bit = 1 and ACK bit = 0, then it must be the request segment.
• If SYN bit = 1 and ACK bit = 1, then it must be the reply segment.
• If SYN bit = 0 and ACK bit = 1, then it can be the pure ACK or segment meant for data transfer.
• If SYN bit = 0 and ACK bit = 0, then this combination is not possible.

 

Point-07:

 

• The combination SYN bit = 0 and ACK bit = 0 is not possible.
• It is because SYN bit = 0 signifies it is not the request segment and reply segment.
• For all other segments, ACK bit is always set to 1.

 

Point-08:

 

• Consider sender sends the segments of size greater than MSS of receiver.
• Then, they are first fragmented first at the receiver side.
• It causes an extra overhead.

 

Point-09:

 

• There is no dedicated field for sending MSS in TCP header.
• This is because MSS has to informed only once.
• So, if dedicated field would be present, then sending it each time would not be required.
• For this reason, MSS is informed once using Options field.

 

Also Read- TCP Connection Termination

 

PRACTICE PROBLEMS BASED ON THREE WAY HANDSHAKE-

 

Problem-01:

 

SYN = 0 and ACK = 1 indicates-

1. Open connection packet
2. Open connection ACK
3. Data packet
4. ACK packet

 

Solution-

 

Options (C) and (D) are correct.

 

Problem-02:

 

An acknowledgement by TCP sender guarantees-

1. Data has been delivered to the application
2. Data has been received by TCP module
3. Data has been received by application interface
4. None of the above

 

Solution-

 

Option (A) is correct.

 

To gain better understanding about Three Way Handshake,

Watch this Video Lecture

 

Next Article- TCP Retransmission

 

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