TCP

Overview

• TCP stands for Transmission Control Protocol.
• It is a connection-oriented protocol in the Transport Layer of the OSI and TCP/IP models.
• TCP provides reliable, ordered, and error-checked delivery of data between applications running on hosts in a network.
• It is one of the core protocols of the Internet Protocol Suite (IP).

Key Features of TCP

• Connection-Oriented:

  • Before transmitting data, a connection must be established between the sender and receiver.
  • This is done using a process known as the three-way handshake.

• Reliable Communication:

  • TCP ensures data delivery by using acknowledgements (ACKs) and retransmissions if data packets are lost.

• Ordered Delivery:

  • Data is delivered to the receiving application in the same order in which it was sent.

• Error Detection and Recovery:

  • TCP uses a checksum to detect errors.
  • If errors are detected, TCP retransmits the affected data.

• Full-Duplex Communication:

  • TCP allows both sender and receiver to transmit data simultaneously.

TCP Header Structure

A TCP segment consists of a header and data. The TCP header is 20 to 60 bytes in size.

Field	Size (Bits)	Description
Source Port	16	Port number of the sender
Destination Port	16	Port number of the receiver
Sequence Number	32	Order of the first byte of data in the segment
Acknowledgement Number	32	Next expected byte from the sender
Data Offset	4	Size of the TCP header (in 32-bit words)
Reserved	3	Reserved for future use (always 0)
Flags	9	Control flags (e.g., SYN, ACK, FIN)
Window Size	16	Size of the receive window
Checksum	16	Error-checking value
Urgent Pointer	16	Points to urgent data (if URG flag is set)
Options	Variable	Optional parameters (e.g., MSS, Window Scaling)
Data	Variable	The actual application data

Flags in TCP Header:

• SYN: Initiates a connection.
• ACK: Acknowledges receipt of data.
• FIN: Gracefully terminates a connection.
• RST: Resets an existing connection.
• PSH: Requests immediate delivery of data to the application.
• URG: Indicates urgent data.

TCP Three-Way Handshake

The three-way handshake is used to establish a reliable connection between two devices.

1. SYN (Synchronize):

   • The client sends a SYN packet to the server to initiate a connection.
   • This packet contains a sequence number.

2. SYN-ACK (Synchronize + Acknowledge):

   • The server responds with a SYN-ACK packet to acknowledge the client’s request.
   • The packet contains the server's sequence number and an acknowledgment number.

3. ACK (Acknowledge):

   • The client sends an ACK packet to confirm the connection.

Diagram:

Client         Server
  | SYN      ->     |
  |          <-  SYN-ACK |
  | ACK      ->     |
Connection Established!

TCP Connection Termination

To close a TCP connection, a four-step process is used:

1. FIN Sent by Client: The client sends a FIN packet to indicate it has finished sending data.
2. ACK Sent by Server: The server acknowledges the FIN.
3. FIN Sent by Server: The server sends its own FIN to indicate it has finished sending data.
4. ACK Sent by Client: The client acknowledges the server's FIN.

Diagram:

Client           Server
  | FIN     ->      |
  |         <-   ACK |
  |         <-   FIN |
  | ACK     ->      |
Connection Closed!

Reliability in TCP

TCP ensures reliability through:

• Acknowledgements (ACKs): The receiver acknowledges the data it successfully receives.
• Retransmissions: If an acknowledgement is not received within a timeout period, the sender retransmits the data.
• Checksum: Used to detect errors in transmitted data.
• Sliding Window Protocol: Controls the amount of data that can be sent before receiving an ACK.

Flow Control in TCP

TCP prevents the sender from overwhelming the receiver using flow control mechanisms:

• Sliding Window Protocol:

  • The sender keeps track of a "window" of data that can be sent without receiving further ACKs.
  • The receiver communicates the window size in its acknowledgements, ensuring that it never receives more data than it can process.

• Dynamic Adjustment:

  • TCP adjusts the window size dynamically based on the receiver’s advertised capacity.
  • If the receiver’s buffer fills up, it can reduce the window size to zero, pausing data transmission until it is ready to accept more data.

• Stop-and-Wait Mechanism:

  • In situations where fine-grained flow control is necessary, TCP can send one packet at a time, waiting for an acknowledgement before sending the next.

Congestion Control in TCP

Congestion occurs when the network becomes overloaded. TCP uses these algorithms:

• Slow Start:

  • The sender starts with a small congestion window (cwnd) and increases it exponentially until packet loss occurs or a threshold is reached.

• Congestion Avoidance:

  • After reaching a threshold, TCP increases the window size linearly to avoid overwhelming the network.

• Fast Retransmit:

  • Upon receiving three duplicate ACKs, the sender retransmits the lost packet immediately without waiting for a timeout.

• Fast Recovery:

  • Instead of resetting to slow start, TCP reduces the congestion window size to half and enters congestion avoidance mode to recover from congestion quickly.

• TCP Tahoe vs. TCP Reno:

  • TCP Tahoe: Resets the congestion window to 1 and restarts slow start after detecting packet loss.
  • TCP Reno: Optimizes congestion recovery with fast retransmit and fast recovery, improving performance in certain scenarios.

Advantages of TCP

• Reliable data delivery.
• Connection-oriented communication.
• Ordered data transmission.
• Handles flow and congestion control.

Disadvantages of TCP

• Higher overhead due to acknowledgements and error-checking mechanisms.
• Slower compared to connectionless protocols like UDP.
• Inefficient for real-time applications where latency is critical.

Use Cases of TCP

TCP is used where reliability is crucial. Examples:

• Web Browsing: HTTP/HTTPS uses TCP.
• File Transfers: FTP and SFTP rely on TCP.
• Email: SMTP, POP3, and IMAP use TCP.
• Remote Access: SSH and Telnet use TCP.
• Streaming Services: Some non-time-critical video and audio streaming services use TCP.

Comparison: TCP vs. UDP

Feature	TCP	UDP
Connection Type	Connection-oriented	Connectionless
Reliability	Reliable (ACKs, retransmissions)	Unreliable
Data Order	Ordered	Unordered
Error Checking	Yes (Checksum)	Yes (Checksum)
Speed	Slower	Faster
Use Case	File transfers, web browsing	VoIP, video streaming