Varying needs introduce varying complexity

• Protocols address a wide variety of problems and needs. They employ many mechanisms, in combination:
  • sequence numbers,
  • adaptive timeouts,
  • negative acknowledgments,
  • piggybacking,
  • pipelining,
  • sliding-windows, and
  • fragmentation.
• Multiple techniques often exist in a single protocol, with increased complexity.
• Layered protocols encourage a distinct header for each layer and (ideally) an independence between layers.
• Much common work (e.g. checksums and sequence numbers) is implemented in multiple layers.
• Protocols attempting "optimal" delivery of their data will assess current conditions of:
  • bandwidth and its effect on timing,
  • frame capacities as dictated by the lower layer,
  • expected and observed error rates,
  • expected and observed round-trip times,
  • level of activity (on shared media),
  • priority of data to be delivered, and
  • the importance of reliable delivery.