13.3 Retransmission Strategies

Retransmission strategies are crucial for error control in computer networks. They ensure reliable data transfer by allowing receivers to request resending of lost or corrupted packets, maintaining data integrity while balancing efficiency and reliability.

ARQ protocols implement these strategies using sequence numbers, acknowledgments, and timers. Different schemes like Stop-and-Wait, Go-Back-N, and Selective Repeat offer varying trade-offs between simplicity, efficiency, and reliability, catering to diverse network conditions and application needs.

Here are the expanded notes on Retransmission Strategies in Error Control and Recovery, following the provided guidelines:

Retransmission Strategies in Error Control and Recovery

Role of retransmission in error control

• Key mechanism for ensuring reliable data transfer in computer networks
  • Allows receiver to request sender to retransmit lost or corrupted packets (TCP)
  • Maintains data integrity and completeness
• Triggered when receiver detects an error or missing packet
  • Receiver sends negative acknowledgment (NACK) to sender, requesting retransmission
  • Sender resends requested packet upon receiving NACK
• Introduces additional latency and overhead in network
  • Trade-off between reliability and efficiency must be considered (VoIP vs file transfer)

Principles of ARQ protocols

• Used to implement retransmission strategies in computer networks
  • Ensure reliable data transfer by automatically requesting retransmission of lost or corrupted packets (UDP vs TCP)
• Use sequence numbers to identify and track packets
  • Sender assigns unique sequence number to each transmitted packet
  • Receiver uses sequence numbers to detect missing or out-of-order packets
• Use acknowledgments (ACKs) to confirm receipt of packets
  • Receiver sends ACK to sender upon successfully receiving packet
  • Sender uses ACKs to determine which packets have been successfully delivered
• Use timers to detect lost packets and trigger retransmissions
  • Sender starts timer when sending packet and waits for ACK
  • If no ACK received before timer expires, sender assumes packet is lost and retransmits it (timeout)

Comparison of ARQ schemes

• Stop-and-Wait ARQ
  • Sender transmits one packet at a time and waits for ACK before sending next packet
  • Simple to implement but inefficient for high-latency networks or large data transfers (satellite links)
  • Vulnerable to lost or delayed ACKs, which can cause unnecessary retransmissions
• Go-Back-N ARQ
  • Sender transmits multiple packets without waiting for individual ACKs
  • Receiver sends cumulative ACKs, acknowledging last correctly received packet
  • If packet is lost or corrupted, receiver discards all subsequent packets and sends NACK
  • Sender retransmits all packets starting from lost or corrupted packet
  • More efficient than Stop-and-Wait but can lead to unnecessary retransmissions of correctly received packets (burst errors)
• Selective Repeat ARQ
  • Sender transmits multiple packets without waiting for individual ACKs
  • Receiver sends individual ACKs for each correctly received packet
  • If packet is lost or corrupted, receiver sends NACK for that specific packet
  • Sender retransmits only lost or corrupted packet, not entire sequence
  • Most efficient ARQ scheme but requires more complex implementation and buffer management (sliding window)

Efficiency vs reliability in retransmission

• Efficiency measured by throughput and latency of data transfer
  • Higher efficiency means more data transferred in given time with lower latency (bandwidth)
• Reliability measured by accuracy and completeness of received data
  • Higher reliability means fewer errors and lost packets in received data (bit error rate)
• Trade-offs between efficiency and reliability in retransmission strategies:
  1. More frequent retransmissions improve reliability but reduce efficiency due to increased overhead and latency
  2. Less frequent retransmissions improve efficiency but may lead to lower reliability if errors or lost packets not detected and retransmitted promptly
  3. Choice of ARQ scheme affects balance between efficiency and reliability
     • Stop-and-Wait ARQ prioritizes reliability over efficiency
     • Go-Back-N ARQ provides balance between reliability and efficiency
     • Selective Repeat ARQ prioritizes efficiency while maintaining high reliability
• Optimal retransmission strategy depends on specific network conditions and application requirements
  • High-latency networks may benefit from more efficient ARQ schemes like Go-Back-N or Selective Repeat (satellite)
  • Applications with strict reliability requirements may prioritize reliability over efficiency and use Stop-and-Wait or Selective Repeat ARQ (banking)