TCP/IP Protocols:

The TCP/IP protocol suite establishes the technical foundation of the Internet. The development of the TCP/IP started as DOD projects. Now, most protocols in the suite are developed by the Internet Engineering Task Force (IETF) under the Internet Architecture Board (IAB), an organization initially sponsored by the US government and now an open and autonomous organization. The IAB provides coordination for the R&D underlying the TCP/IP protocols and guides the evolution of the Internet. The TCP/IP protocols are well documented in the Request for Comments (RFC), which are drafted, discussed, circulated and approved by the IETF committees. All documents are open and free and can be found online in the IETF site listed in the reference. TCP/IP architecture does not exactly match the OSI model. Unfortunately, there is no universal agreement regarding how to describe TCP/IP with a layered model. It is generally agreed that TCP/IP has fewer levels (from three to five layers) than the seven layers of the OSI model. In this article, we force TCP/IP protocols into the OSI 7 layers structure for comparison purposes. The TCP/IP suite’s core functions are addressing and routing (IP/IPv6 in the networking layer) and transportation control (TCP, UDP in the transport layer).

IP - Internet Protocol

Addressing of network components is a critical issue for information routing and transmission in network communications. Each technology has its own convention for transmitting messages between two machines within the same network. On a LAN, messages are sent between machines by supplying the six bytes unique identifiers (the “MAC” address). In an SNA network, every machine has Logical Units with their own network addresses. DECnet, app letalk, and Novell IPX all have a scheme for assigning numbers to each local network and to each workstation attached to the network. On top of these local or vendor-specific network addresses, IP assigns a unique number to every network device in the world, which is called an IP address. This IP address is a four bytes value in IPv4 that, by convention, is expressed by converting each byte into a decimal number (0 to 255) and separating the bytes with a period. In IPv6, the IP address has been increased to 16 bytes. Details of the IP and IPv6 protocols are presented in separate documents.

TCP - Transmission Control Protocol

TCP provides a reliable stream delivery and virtual connection service to applications through the use of sequenced acknowledgment with retransmission of packets when necessary. TCP provides stream data transfer, transportation reliability, efficient flow control, full-duplex operation, and multiplexing. Check the TCP section for more details. In the following TCP/IP protocol stack table, we list all the protocols according to their functions in mapping to the OSI 7 layers network communication reference model. However, the TCP/IP architecture does not follow the OSI model closely, for example, most TCP/IP applications directly run on top of the transport layer protocols, TCP and UDP, without the presentation and session layers in between.

 

TCP/IP Four Layers Architecture Model:

TCP/IP architecture does not exactly follow the OSI model. Unfortunately, there is no universal agreement regarding how to describe TCP/IP with a layered model. It is generally agreed that TCP/IP has fewer levels (from three to five layers) than the seven layers of the OSI model. We adopt a four layers model for the TCP/IP architecture. TCP/IP architecture omits some features found under the OSI model, combines the features of some adjacent OSI layers and splits other layers apart.

The 4-layer structure of TCP/IP is built as information is passed down from applications to the physical network layer. When data is sent, each layer treats all of the information it receives from the upper layer as data, adds control information (header) to the front of that data and then passes it to the lower layer. When data is received, the opposite procedure takes place as each layer processes and removes its header before passing the data to the upper layer. The TCP/IP 4-layer model and the key functions of each layer is described below:

Application Layer

The Application Layer in TCP/IP groups the functions of OSI Application, Presentation Layer and Session Layer. Therefore any process above the transport layer is called an Application in the TCP/IP architecture. In TCP/IP socket and port are used to describe the path over which applications communicate. Most application-level protocols are associated with one or more port numbers.

Transport Layer

In TCP/IP architecture, there are two Transport Layer protocols. The Transmission Control Protocol (TCP) guarantees information transmission. The User Datagram Protocol (UDP) transports datagram without end-to-end reliability checking. Both protocols are useful for different applications.

Network Layer

The Internet Protocol (IP) is the primary protocol in the TCP/IP Network Layer. All upper and lower layer communications must travel through IP as they are passed through the TCP/IP protocol stack. In addition, there are many supporting protocols in the Network Layer, such as ICMP, to facilitate and manage the routing process.

Network Access Layer

In the TCP/IP architecture, the Data Link Layer and Physical Layer are normally grouped together to become the Network Access layer. TCP/IP makes use of existing Data Link and Physical Layer standards rather than defining its own. Many RFCs describe how IP utilizes and interfaces with the existing data link protocols such as Ethernet, Token Ring, FDDI, HSSI, and ATM. The physical layer, which defines the hardware communication properties, is not often directly interfaced with the TCP/IP protocols in the network layer and above.