The design space for network architectures can be conveniently described as a 3-tuple of . Application requirements can range from reliability and small message inter-arrival delay to communications secrecy. Protocol elements include acknowledgements and error-correcting codes, timers and a variety of cryptographic transformations. Network conditions include delay, delay variance, loss rates, bit error rates (BERs), topology, and available bandwidths. For any given triple, and in particular for a choice of application and requirements, there are assumptions about operating conditions made, and protocol elements selected to meet the application requirements under these conditions.