IVOA Recommendation: VOEvent Transport Protocol Version 2.0

The IVOA VOEvent Recommendation defines a means of describing transient celestial events but, purposely, remains silent on the topic of how those descriptions should be transmitted. This document formalizes a TCP-based protocol for VOEvent transportation that has been in use by members of the VOEvent community for several years and discusses the topology of the event distribution network. It is intended to act as a reference for the production of compliant protocol implementations.