The TCP/IP Application Layer: This pertains to communications services to applications and acts as a go-between for the network and the application. It also handles presentation and administering communication sessions. It encompasses the Application Layer, Presentation Layer and Session Layer of the OSI reference model.
- Examples: HTTP, POP3, and SNMP. The TCP/IP
Transport Layer: This layer addresses multiple functions, including selection of protocols, error recovery and flow control. This layer can also support retransmission, i.e., error recovery and may use flow control to curtail unneeded congestion by sending data at a rate the network can handle, or depending on the choice of protocols, it may bypass this function. Multiplexing of received data for various flows to applications on the same host is also performed. Reordering of the incoming data stream when packets come in out of order is also handled. This is associated with the Transport Layer of the OSI reference model.
- Examples: TCP and UPD, which are called Transport Layer, or Layer 4 protocols. TCP supports connection-oriented service while UDP offers connectionless support in the Transport Layer.
The TCP/IP Internetwork Layer: Also known as the Internet Layer, defines end-to-end delivery of packets and defines logical addressing to accomplish this. It also defines the process of routing and how routes are processed; and how to break down a packet into smaller packets to work with media with smaller maximum transmission unit sizes. It corresponds with the Network Layer of the OSI reference model. However, while the OSI network-layer protocols offer connection-based (Connection-Mode Network Service (CMNS), X.25) or Connectionless Network Service (CLNS), IP only provides connectionless network service. The routing protocols are network layer protocols assigned with an IP protocol number. Border Gateway Protocol (BGP) is different in that it uses a TCP port number; and Intermediate System-to-Intermediate System (IS-IS) presides over the data-link layer.
The TCP/IP Network Interface Layer: This handles the physical characteristics of the transmission medium, and transfers data across one particular link or medium. It specifies delivery across an individual link as well as the physical layer specifications. It encompasses the Data Link Layer and Physical Layer of the OSI reference model.
- Examples include: Ethernet and Frame Relay.
