Recently I wrote about TCP Vegas, an interesting transport protocol that came close to being used as a replacement for Reno. Challenges come up when one tries to get a "better" protocol out to replace the existing one. A new transport protocol will thrive most when it offers something that existing protocols don't provide.
Implicit in a transport protocol's design is the way it fulfills an application's requirements. For example, TCP allows a file to be transferred in its entirety, and with the guarantee that it will arrive in the same form it is sent. Multimedia streaming transport protocols are built to best accommodate the needs of a multimedia application.
A transport-layer protocol also needs to account for the underlying architecture. TCP's assumption is that loss is primarily due to packet loss in queues at the network layer. Wireless transport protocols, such as ATP and Hop-by-Hop, are built to maximize throughput in a wireless network, where loss is due primarily to the hidden- and exposed-node problems. The key to a successful transport protocol is in how well it meets the needs of a prevalent network, and how well it improves the application at the user space.
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