Most cryptographic protocols are designed for a synchronous communication model, i.e., there is a known upper bound on message transmission delays. That means, the time period between the point at which a protocol message is sent and the point at which the message is delivered is smaller than this bound. Additionally, often the assumption is made that the computation proceeds in synchronized rounds and that the parties are connected by a complete network of private (i.e. untappable and authenticated) point-to-point channels.
There is an importand distinction between fully synchronous and partially synchronous communication model with respect to coverage and the resulting adverserial power. However, a detailed discussion of such issues is beyond the scope of this manual. The reader is referred to the famous textbook Introduction to Reliable and Secure Distributed Programming for an introduction and discussion on that topic.