Network Protocols

a protocol will comprise different types of messages, known as protocol data units(PDUs)^[A unit of data specified in a protocol of a given layer in the OSI model.] Each PDU will have a particular syntax formatted as textual information or as binary data: - Textual PDUs have a syntax and grammar that describes their format(Examples: HTTP/1.1, SMTP, SIP, Jabber) - Binary PDUs similarly have rules describing their format(Examples: TCP/IP, RTP)

PDUs define what messages are legal to send.

protocol Semantics

• who can send PDUs, and when they can be sent
• roles for the host(e.g. client & server, peer-to-peer)
• What are the entities that communicate and how they are named
• error handling

States

shown using a state-transition diagram - states indicate stages of protocol operation - Transitions occur in response to PDUs, and may result in other PDUs being sent