Transports in Salt are used by Channels to send messages between Masters, Minions, and the Salt CLI. Transports can be brokerless or brokered. There are two types of server / client implementations needed to implement a channel.
The publish server implements a publish / subscribe paradigm and is used by Minions to receive jobs from Masters.
The publish client subscribes to, and receives messages from a Publish Server.
The request server implements a request / reply paradigm. Every request sent by the client must receive exactly one reply.