Coordination Strategies: Communications Infrastructure

• UDP or IP connectionless
  The connectionless protocols are optimized for best-effort, low-overhead, low-latency communication. They are best-suited for LANs, but work on WANS with reduced reliability (since, if the packet is dropped anywhere along the path, it's gone, and WANs introduce more opportunities for packet loss due to the longer routes). The reliable protocols (such as TCP) are built on top of these protocols. Broadcast-based protocols (as distinguished from broadcast/fill and directed multicase protocols) are also implemented using these protocols.
• TCP connection-oriented
  These are by far the most-used underlying communications protocols for agents; they incorporate reliable delivery (all packets are guaranteed to arrive in order and none will be dropped) with characteristics making them suitable on both LANs and WANs. They suffer from increased setup overhead and variable latency (since a dropped packet forces all following packets to wait while it is retransmitted---otherwise, packets would arrive out of order; this latter characteristic makes TCP unsuitable for, e.g., digital audio, which can easily substitute a few milliseconds of silence for an isolated dropped packet, but which is not nearly so forgiving of the long pause required to handle a timeout and retransmission.

  Commonly-used versions of reliable transport include:

  • Store-and-forward (mail). This is a high-latency approach which requires careful interaction with users' mailboxes, but can go places the connected Internet cannot (e.g., through firewalls, along UUCP hops, and so forth).
  • Real-time (telnet, web). This is a lower-latency, more-flexible transport which requires the connected Internet to function. Most of the popular protocols are implemented using this transport.
• Multicast
  Directed multicast is a compromise form between a true reliable point-to-point connection, as in TCP, and an unreliable broadcast medium. It's useful for bootstrapping (finding other potential agents from a short list) and also for high-bandwidth transmission of the same information to a small number of other agents, such as is often done (to clients, not agents) on the MBONE.

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