Chapter 5

Question 1

Demultiplexor - Definition and Purpose

Answer 1

• Simple Transport Protocol: Extends host-to-host delivery service to process-to-process communication.
• Multiple Processes: Protocol allows multiple application processes on each host to share the network.
• Example Protocol: User Datagram Protocol (UDP).

Question 2

Demultiplexor - Addressing and Identification

Answer 2

• Direct Identification: Processes could use OS-assigned process IDs (PIDs).
• Common Approach: Processes use an abstract locator called a port.
• Port Usage: Source sends a message to a port; destination receives from a port.

Question 3

Demultiplexor - UDP Header

Answer 3

• Port Identifier: Contains identifiers (ports) for sender and receiver.
• Port Limit: UDP port field is 16 bits long, allowing up to 64k ports.
• Host-Port Pair: Process identified by a (port, host) pair, creating the demultiplexing key for UDP.

Question 4

Demultiplexor - Learning the Port

Answer 4

• Client-Server Interaction: Client process initiates contact with server process.
• Well-Known Ports: Servers accept messages at fixed, widely published ports.
• Port Mapper Service: A single well-known port can help clients find the appropriate port for services.

Question 5

Demultiplexor - Implementation of Ports

Answer 5

• Port Abstraction: Implemented differently across systems, often as message queues.
• Message Handling: Protocol appends messages to the queue; discards if full.
• Blocking and Receiving: Process blocks if the queue is empty, waiting for messages.

Question 6

Demultiplexor - UDP Checksum

Answer 6

• Checksum Purpose: Ensures correctness of the message.
• Checksum Components:
  
  • UDP header
  • Message body
  • Pseudoheader
    
    • Protocol number
    • Source/destination IP addresses
    • UDP length field
• Checksum Verification: Helps detect if a message is misdelivered by verifying the correct endpoints.

Question 7

Definition and Properties -Reliable Byte Stream (TCP)

Answer 7

• TCP Overview: TCP provides a reliable, connection-oriented, byte-stream service.
• Applications: Useful for applications needing in-order, reliable data delivery.
• Protocol Popularity: TCP is widely used and highly tuned.
• Duplex Communication: TCP supports full-duplex, meaning two simultaneous byte streams in each direction.
• Flow Control: Mechanism to limit the data sent by the sender based on the receiver’s capacity.

Question 8

End-to-End Issues - TCP

Answer 8

• Sliding Window Algorithm: Core to TCP, adapted for the Internet’s complexity.
• Differences from Link-Level Sliding Window: Runs over the Internet, handling more complications.

Question 9

Reliable and Ordered Delivery - TCP

Answer 9

• Send and Receive Buffers: Maintains data not yet acknowledged or read by the application.
• Data Pointers: Pointers track sent, acknowledged, written, read, expected, and received bytes.
• Buffer Management: Data management in buffers for reliable delivery.

Question 10

Flow Control - TCP

Answer 10

• Receive Buffer Management: Ensures the sender doesn’t overrun the receiver’s buffer.
• Advertised Window: Limits the sender’s unacknowledged data based on receiver’s buffer.
• Window Size Calculation: Based on buffer capacity and data pointers.

Question 11

TCP Header and Segment Format

Answer 11

• TCP Header Fields: Contains source and destination ports, sequence and acknowledgment numbers, window size, and flags.
• Segment Structure: Includes data payload and various control flags for managing connections.

Question 12

TCP Connection Management

Answer 12

• Connection Setup: Three-way handshake for connection establishment.
• Connection Teardown: Multiple states for graceful connection closure, ensuring all data is transmitted and acknowledged.

Question 13

TCP Variants and Extensions

Answer 13

• Timeout Mechanism: Extensions to improve RTT measurement and handle high-speed networks.
• Window Scaling: Option to handle larger window sizes for high bandwidth-delay networks.
• Selective Acknowledgment (SACK): Allows for efficient retransmission of lost segments without retransmitting correctly received segments.

Question 14

Sliding Window Revisited

Answer 14

• TCP’s Use: Ensures reliable delivery, in-order delivery, and flow control.
• Advertised Window: Receiver controls the sender’s data transmission based on buffer availability.