8.2 Reliable Data Transfer Protocols

Reliable data transfer protocols ensure accurate, sequential delivery of data across networks. They use sequence numbers, acknowledgments, timeouts, and sliding windows to handle packet loss, duplication, and misordering, addressing the unreliable nature of IP.

Stop-and-Wait, Go-Back-N, and Selective Repeat are key protocols with varying efficiency. Each has strengths and weaknesses, balancing simplicity, bandwidth usage, and performance based on network conditions and application needs.

Reliable Data Transfer Protocols

Importance of reliable data transfer

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• Guarantees data delivered accurately and sequentially
  • Essential for error-sensitive applications (file transfer, email)
• Addresses packet loss, duplication, and misordering
  • Network layer (IP) offers unreliable delivery
• Implemented in transport layer, often using TCP (Transmission Control Protocol)

Principles of transfer protocols

• Sequence numbers identify and sequence packets
  • Detects lost, duplicated, or out-of-order packets
• Acknowledgments (ACKs) verify packet receipt
  • Can acknowledge multiple packets (cumulative ACKs)
• Timeouts trigger packet retransmission
  • Sender retransmits if ACK not received before timeout
• Sliding window enables concurrent packet transmission
  • Improves throughput via pipelining

Stop-and-Wait protocol operation

• Sends one packet, waits for ACK before next transmission
• Limitations reduce performance
  • Idle time waiting for ACKs wastes bandwidth
  • Network latency constrains throughput
  • Inefficiently utilizes available bandwidth
• Round-trip time (RTT) dictates transmission speed
  • Maximum throughput: 1/RTT packets per second

Go-Back-N vs Selective Repeat

• Go-Back-N (GBN) pipelines unacknowledged packets
  • Receiver accepts sequential packets, sends cumulative ACKs
  • Discards subsequent packets until missing one retransmitted
  • Improves Stop-and-Wait throughput and bandwidth utilization
• Selective Repeat (SR) maintains separate sender/receiver windows
  • Receiver accepts out-of-order packets, sends individual ACKs
  • Sender retransmits only lost or corrupted packets
  • Outperforms GBN in high-latency or lossy networks

Comparison of transfer protocols

• Stop-and-Wait: simplest but inefficient
  • Suitable for low-latency, reliable networks (local area networks)
• Go-Back-N: more efficient than Stop-and-Wait
  • Less complex than Selective Repeat
  • May waste bandwidth on unnecessary retransmissions (noisy wireless networks)
• Selective Repeat: optimizes bandwidth usage
  • More complex implementation than Go-Back-N
  • Requires larger sender/receiver buffers for out-of-order packets (high-speed networks)
• Protocol selection based on network traits, application needs, implementation complexity (error rate, latency, throughput requirements)