What is a request-response pair called?

For example in HTTP you send a request and receive a response; is there a noun that describes that request-response pair?

I'd thought of "dialog" or "conversation" however those imply multiple request-response pairs, whereas I'm looking for a word that indicates exactly one.

The reason for asking is that I need to name an object which encapsulates the details of both, and RequestResponse seems fairly lame.