The suite of networking protocols in use on the Internet is familiarly known by the acronyms that represent the two core protocols in the suite: the Transmission Control Protocol (TCP) and the Internet Protocol (IP). Below is a brief summary of how the TCP/IP suite operates.
At the top of the TCP/IP stack is the Application Services Layer. This layer, typically implemented as software, is where the protocols that directly support the applications reside. For example, the Simple Mail Transfer Protocol (SMTP) and the Post Office Protocol version 3 (POP3) define how email is exchanged. The Simple Network Management Protocol (SNMP) defines how network management information is relayed. The HyperText Transfer Protocol (HTTP) defines how elements of the World Wide Web (e.g., pages, scripts, images) are transferred from Web server to browser. There are several other services present in this layer as well.
If the Application Services Layer is going to be effective, it must trust that things sent across the network arrive with no errors. Since virtually every application requires this, rather than implement that capability into each Application Services protocol, we define a new layer focused on this capability. This Layer is called the Transport Layer. In the TCP/IP suite, there are four protocols at this layer. The User Datagram Protocol (UDP) is focused primarily on message integrity and application addressing. It is ideal for transaction-oriented applications like the Domain Name System (DNS). The Real-time Transport Protocol (RTP) is used in conjunction with UDP and adds timing or sequencing information. It is useful for real-time voice and video transmission. The Transmission Control Protocol (TCP) not only verifies message integrity and provides for application addressing, it also recovers from any communication failures (e.g., bit errors, lost messages, incorrectly ordered messages) and supports a flow control function. The Secure Socket Layer (SSL), also known as Transport Layer Security (TLS), adds encryption and authentication to TCP and is useful for ecommerce applications.
If the Transport Layer is going to do its job, it requires the ability to route packets of information between two systems in an arbitrarily large and arbitrarily complex interconnection of networks. Rather than build this into each Transport Layer protocol, each of these protocols depends on an underlying layer, called the Internet Layer, to provide this capability. The protocol that performs this task is called the Internet Protocol (IP) and we are currently using version 4 (IPv4).
For IP to do its job, each device must have the ability to transmit and receive information. This is the job of the Network Interface Layer. In the TCP/IP suite, the Network Interface Layer can be virtually anything, including frame relay, ATM, Ethernet, Wi-Fi, WiMAX, MPLS, a dial-up POTS line, ISDN, SONET, DS-1 or DS-3 circuits, and EV-DO, to name a few.
TCP/IP vs. OSI
The Internet protocol suite consists of four protocol layers that approximately correspond to the OSI Reference Model, as depicted on the visual.
TCP/IP’s Network Interface Layer defines the interface between the host system and the network medium. It is specific to the network implementation and roughly corresponds to the combined OSI Reference Model Physical Layer and Data Link Layer; some OSI Network Layer functionality could also be included.
The Internet Layer defines communications between hosts on networks. It provides the path to link these networks into a single internetwork. Using IP and a number of adjunct protocols, the Internet Layer can efficiently route packets across the internetwork. It corresponds to the OSI Network Layer.
The OSI Transport Layer provides reliable end-to-end communication between hosts, while the OSI Session Layer provides end-to-end communication between two communicating processes within the hosts. TCP provides a reliable, assured, connection-oriented service between two hosts, and UDP provides an unreliable, connectionless service between two hosts. TCP and UDP offer some Session Layer functionality since they provide addressing for higher layer applications.
TCP/IP’s Application Services Layer provides the end-user window into the network as well as useful functions for the user. It is functionally similar to OSI Layers 5–7 (i.e., Session, Presentation, Application) and offers such applications as e-mail, file transfer, remote terminal access, and access to the World Wide Web (WWW).