Transport Layer

Transport Layer

Transport Layer provides end-to-end communication between applications.

Transport Layer Protocols

TCP (Transmission Control Protocol)

TCP is connection-oriented, reliable protocol.

Features:

• Connection-oriented
• Reliable delivery
• Flow control
• Congestion control
• Full-duplex
• Byte-stream

TCP Header Fields:

• Source port (16 bits)
• Destination port (16 bits)
• Sequence number (32 bits)
• Acknowledgment number (32 bits)
• Flags (SYN, ACK, FIN, etc.)
• Window size (16 bits)
• Checksum (16 bits)

UDP (User Datagram Protocol)

UDP is connectionless, unreliable protocol.

Features:

• Connectionless
• Unreliable delivery
• No flow control
• No congestion control
• Low overhead
• Message-oriented

UDP Header Fields:

• Source port (16 bits)
• Destination port (16 bits)
• Length (16 bits)
• Checksum (16 bits)

TCP Connection Management

Three-Way Handshake

Establishing Connection:

1. SYN: Client sends SYN (seq = x)
2. SYN-ACK: Server sends SYN-ACK (seq = y, ack = x+1)
3. ACK: Client sends ACK (seq = x+1, ack = y+1)

Connection Established

Four-Way Handshake

Terminating Connection:

1. FIN: Client sends FIN
2. ACK: Server sends ACK
3. FIN: Server sends FIN
4. ACK: Client sends ACK

Connection Terminated

Flow Control

Flow Control prevents sender from overwhelming receiver.

Sliding Window Protocol

Window Size: Number of bytes receiver can accept.

Mechanism:

• Receiver advertises window size
• Sender sends up to window size
• Receiver acknowledges received data
• Window slides as data acknowledged

Example:

• Window size = 4
• Send bytes 1-4
• ACK 1, window slides
• Send bytes 5-8

Congestion Control

Congestion Control prevents network overload.

TCP Congestion Control Algorithms

1. Slow Start

Initial Phase:

• Start with cwnd = 1 MSS
• Double cwnd every RTT
• Until threshold or loss

2. Congestion Avoidance

After Threshold:

• Increase cwnd by 1 MSS per RTT
• Linear growth

3. Fast Retransmit

On Duplicate ACKs:

• Retransmit immediately
• Don't wait for timeout

4. Fast Recovery

After Fast Retransmit:

• Reduce cwnd
• Enter congestion avoidance

TCP States

States:

• CLOSED
• LISTEN
• SYN-SENT
• SYN-RECEIVED
• ESTABLISHED
• FIN-WAIT-1
• FIN-WAIT-2
• CLOSE-WAIT
• CLOSING
• TIME-WAIT
• LAST-ACK

Ports

Port identifies application/service.

Well-Known Ports (0-1023):

• HTTP: 80
• HTTPS: 443
• FTP: 21
• SSH: 22
• SMTP: 25
• DNS: 53

Registered Ports (1024-49151):

Dynamic Ports (49152-65535):

GATE CS Important Points

1. TCP vs UDP: Connection-oriented vs connectionless
2. Three-Way Handshake: Connection establishment
3. Flow Control: Sliding window protocol
4. Congestion Control: Slow start, congestion avoidance
5. TCP States: State transitions

Practice Tips

1. TCP Handshake: Understand three-way and four-way
2. Flow Control: Practice sliding window problems
3. Congestion Control: Understand algorithms
4. Port Numbers: Know common ports
5. Previous Year Questions: Solve GATE transport layer questions