1.1.3 Internet Protocol Suite

To communicate using the Internet system, a host must implement the layered set of protocols comprising the Internet protocol suite. A host typically must implement at least one protocol from each layer.

The protocol layers used in the Internet architecture are as follows [INTRO:4]:

Application Layer

The application layer is the top layer of the Internet protocol suite. The Internet suite does not further subdivide the application layer, although some of the Internet application layer protocols do contain some internal sub-layering. The application layer of the Internet suite essentially combines the functions of the top two layers -- Presentation and Application -- of the OSI reference model.

We distinguish two categories of application layer protocols: user protocols that provide service directly to users, and support protocols that provide common system functions. Requirements for user and support protocols will be found in the companion RFC [INTRO:1].

The most common Internet user protocols are:

• Telnet (remote login)
• FTP (file transfer)
• SMTP (electronic mail delivery)

There are a number of other standardized user protocols [INTRO:4] and many private user protocols.

Support protocols, used for host name mapping, booting, and management, include SNMP, BOOTP, RARP, and the Domain Name System (DNS) protocols.

Transport Layer

The transport layer provides end-to-end communication services for applications. There are two primary transport layer protocols at present:

• Transmission Control Protocol (TCP)
• User Datagram Protocol (UDP)

TCP is a reliable connection-oriented transport service that provides end-to-end reliability, resequencing, and flow control. UDP is a connectionless ("datagram") transport service.

Other transport protocols have been developed by the research community, and the set of official Internet transport protocols may be expanded in the future.

Transport layer protocols are discussed in Chapter 4.

Internet Layer

All Internet transport protocols use the Internet Protocol (IP) to carry data from source host to destination host. IP is a connectionless or datagram internetwork service, providing no end-to-end delivery guarantees. Thus, IP datagrams may arrive at the destination host damaged, duplicated, out of order, or not at all. The layers above IP are responsible for reliable delivery service when it is required. The IP protocol includes provision for addressing, type-of-service specification, fragmentation and reassembly, and security information.

The datagram or connectionless nature of the IP protocol is a fundamental and characteristic feature of the Internet architecture. Internet IP was the model for the OSI Connectionless Network Protocol [INTRO:12].

ICMP is a control protocol that is considered to be an integral part of IP, although it is architecturally layered upon IP, i.e., it uses IP to carry its data end- to-end just as a transport protocol like TCP or UDP does. ICMP provides error reporting, congestion reporting, and first-hop gateway redirection.

IGMP is an Internet layer protocol used for establishing dynamic host groups for IP multicasting.

The Internet layer protocols IP, ICMP, and IGMP are discussed in Chapter 3.

Link Layer

To communicate on its directly-connected network, a host must implement the communication protocol used to interface to that network. We call this a link layer or media-access layer protocol.

There is a wide variety of link layer protocols, corresponding to the many different types of networks. See Chapter 2.