For a specific example of a protocol structure see How Does TCP/IP Work? (local copy) in the TCP/IP Tutorial.
TCP/IP is based on the "catenet model". (This is described in more detail in IEN 48.) This model assumes that there are a large numberof independent networks connected together by gateways. The user should be able to access computers or other resources on any of these networks. Datagrams will often pass through a dozen different networks before getting to their final destination. The routing needed to accomplish this should be completely invisible to the user. As far as the user is concerned, all he needs to know in order to access another system is an "Internet address". (Source)
TCP/IP Headers
TCP/IP: Message path 2
TCP/IP: Message path 3
TCP/IP Tutorial and Technical Overview: an authoritative (but some dated) discussion of internet protocols and architectures.
