2.2.1 Protocol Layering

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 [ARCH:7]:

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 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 [ARCH:8]. The Application Layer in the Internet protocol suite also includes some of the function relegated to the Session Layer in 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. 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 and many private user protocols.

Support protocols, used for host name mapping, booting, and management include SNMP, BOOTP, TFTP, the Domain Name System (DNS) protocol, and a variety of routing protocols.

Application Layer protocols relevant to routers are discussed in chapters 7, 8, and 9 of this memo.

Transport Layer

The Transport Layer provides end-to-end communication services. This layer is roughly equivalent to the Transport Layer in the OSI Reference Model, except that it also incorporates some of OSI's Session Layer establishment and destruction functions.

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 relevant to routers are discussed in Chapter 6.

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. 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.

The datagram or connectionless nature of IP is a fundamental and characteristic feature of the Internet architecture.

The Internet Control Message Protocol (ICMP) is a control protocol that is considered to be an integral part of IP, although it is architecturally layered upon IP - it uses IP to carry its data end-to-end. ICMP provides error reporting, congestion reporting, and first-hop router redirection.

The Internet Group Management Protocol (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 4.

Link Layer

To communicate on a directly connected network, a host must implement the communication protocol used to interface to that network. We call this a Link Layer protocol.

Some older Internet documents refer to this layer as the Network Layer, but it is not the same as the Network Layer in the OSI Reference Model.

This layer contains everything below the Internet Layer and above the Physical Layer (which is the media connectivity, normally electrical or optical, which encodes and transports messages). Its responsibility is the correct delivery of messages, among which it does not differentiate.

Protocols in this Layer are generally outside the scope of Internet standardization; the Internet (intentionally) uses existing standards whenever possible. Thus, Internet Link Layer standards usually address only address resolution and rules for transmitting IP packets over specific Link Layer protocols. Internet Link Layer standards are discussed in chapter 3.