What characterizes synchronous transport protocols?

Synchronous transport protocols are defined by a specific model of communication where a request is sent from a client to a server and the client waits for a response before continuing with any further processing. This behavior is encapsulated in the notion that one thread sends a request and blocks until a reply is received.

In this model, since the client does not proceed until the server has responded, it ensures that the process is linear and predictable, which is ideal for scenarios where the timing of operations is crucial. For instance, in a synchronous call, if the client sends a message and immediately requires the result to proceed, this protocol effectively manages that flow of control.

The other choices reflect different characteristics commonly associated with asynchronous protocols or alternative programming models. For example, multiple threads processing requests simultaneously and a callback mechanism to handle replies indicate parallel processing and non-blocking behavior, which are hallmarks of asynchronous operations. Additionally, protocols that have no waiting for replies are indicative of asynchronous communication as well, further distinguishing them from the synchronous model.