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21_Unit3_Topic05_TCP.md

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title Unit 3: Connection-Oriented Transport: TCP
id unit3-topic5
tags
unit3
transport-layer
tcp
reliability
flow-control
handshake
aliases
TCP

Unit 3, Topic 5: Connection-Oriented Transport: TCP

We now arrive at the Internet's workhorse transport protocol: TCP (Transmission Control Protocol). TCP provides a reliable, connection-oriented service to application processes. It implements the principles of reliable data transfer (RDT) we just studied, but in a pipelined fashion, and adds two other critical services: flow control and congestion control.

1. The TCP Segment Structure

A TCP segment consists of a header and a data payload. The header is at least 20 bytes long and contains crucial control information.

 0                   15 16                  31
+--------------------+--------------------+
|    Source Port     |  Destination Port  |
+-----------------------------------------+
|            Sequence Number              |
+-----------------------------------------+
|          Acknowledgment Number          |
+----+----+----------+--------------------+
| Hdr| Rsvd|U|A|P|R|S|F |   Receive Window   |
| Len|     |R|C|S|S|Y|I |                    |
|    |     |G|K|H|T|N|N |                    |
+----+----+----------+--------------------+
|   TCP Checksum     |  Urgent Data Ptr   |
+--------------------+--------------------+
|        Options (if any) ...             |
+-----------------------------------------+
|            Application Data             |
+-----------------------------------------+

Key Header Fields

  • Source Port & Destination Port: Used for multiplexing and demultiplexing.
  • Sequence Number: This is a byte-stream number. It is the sequence number of the first byte of data in the current segment. TCP views the data as an unstructured, ordered stream of bytes.
  • Acknowledgment Number: The sequence number of the next byte the sender is expecting from the other side. TCP uses cumulative acknowledgements, meaning an ACK for byte y implies all bytes up to y-1 have been received correctly.
  • Header Length: Specifies the length of the TCP header in 32-bit words (needed because the Options field is variable).
  • Flags (6 bits):
    • ACK: Indicates that the Acknowledgment Number field is valid.
    • SYN: Used in the initial three-way handshake to establish a connection.
    • FIN: Used to terminate a connection.
    • RST, PSH, URG: Other flags for reset, push, and urgent data.
  • Receive Window: Used for flow control. It tells the sender how much buffer space the receiver has available.
  • Checksum: Used for error detection, calculated over the header and data.

2. TCP Reliable Data Transfer

TCP provides reliability by implementing a pipelined RDT protocol based on the principles from our last topic.

  • Timeout Mechanism: TCP uses a single retransmission timer for the oldest unacknowledged segment. If the timer expires, that segment is retransmitted.
  • Fast Retransmit (An Important Optimization): Waiting for a timeout can be slow. If a sender receives three duplicate ACKs for the same sequence number, it takes this as a strong indication that the packet after that acknowledged packet was lost. The sender then performs a fast retransmit, re-sending the lost packet immediately without waiting for its timer to expire.

3. TCP Flow Control

  • Purpose: To prevent the sender from sending data faster than the receiver can process it, thereby overflowing the receiver's buffer. This is about protecting the receiver, not the network.
  • How it works: The receiver advertises its available buffer space in the Receive Window field of every segment it sends back. The sender must ensure that the amount of unacknowledged ("in-flight") data is always less than the last advertised receive window.

4. TCP Connection Management

Before data can be exchanged, a connection must be established using the famous Three-Way Handshake.

      Client                                   Server
        |                                        |
        |  1. SYN (seq=x) ----------------------> |
        |                                        |
        |  <---------------------- 2. SYN-ACK (seq=y, ack=x+1)
        |                                        |
        |  3. ACK (seq=x+1, ack=y+1) -----------> |
        |                                        |
      Connection                               Data
     Established                             Transfer
  1. Client -> Server (SYN): The client sends a special segment with the SYN flag set and a random initial sequence number x.
  2. Server -> Client (SYN-ACK): The server receives the SYN, allocates buffers for the new connection, and sends back a segment with both the SYN and ACK flags set. It chooses its own random initial sequence number y and acknowledges the client's SYN by setting the acknowledgment number to x+1.
  3. Client -> Server (ACK): The client receives the SYN-ACK, allocates its own buffers, and sends a final segment back to the server, acknowledging the server's SYN by setting the acknowledgment number to y+1.

At this point, the connection is established, and application data can begin to flow.

Connection Teardown: Closing a connection is also a formal process, typically involving a four-way exchange using the FIN flag, where each side independently signals that it is done sending data.

Exam Focus

  • 2-Mark Questions:
    • What is the difference between a TCP sequence number and an acknowledgment number?
    • What is the purpose of the Receive Window field in the TCP header? (Flow control).
    • List the three steps of the TCP three-way handshake.
  • 10-Mark Question:
    • "Draw the TCP segment structure and explain the purpose of the sequence number, acknowledgment number, and the SYN/ACK flags. Then, describe the three-way handshake process for establishing a TCP connection."
      • Tip: A clear diagram of the header is essential. For the handshake, a diagram showing the three steps with their seq and ack numbers is critical. Explain what each step achieves (e.g., the SYN-ACK from the server both acknowledges the client's request and proposes its own sequence number).
  • Distinction Question:
    • "Differentiate between Flow Control and Congestion Control." (Flow control protects the receiver from being overwhelmed. Congestion control protects the network from being overwhelmed. We will study congestion control next.)