Internet Protocols - CCIE Routing and Switching Exam Certification Guide - page 266

Foundation Topics

TCP/IP Protocol Architecture
TCP/IP protocols are the protocols that you use to run the Internet. Documentation for these
protocols is in the form of Request For Comments (RFCs). These documents are published and
reviewed by the Internet Engineering Task Force (IETF) working groups. IP RFC 791 (published
in September 1981) describes version 4 of the protocol. The next version, IPv6, is described in
RFC 2460 (published in 1998).

The TCP/IP protocols were created before and after the OSI reference model was published.
There is no TCP/IP model, but the architecture can be compared. Figure 6-1 shows the
comparison between the OSI model and the TCP/IP architecture.

TCP/IP Model Compared to the OSI Model

Figure 6-1

OSI Model TCP/IP Architecture TCP/IP Protocols

Application

SNMP , Telnet,
FTP, TFTP, NTP,
NFS, SMTP

Presentation Application

Session

Transport Transport TCP, UDP

IP, OSPF,
RIP, ICMP
Use of lower
layer protocol
standards

Network Internet

Data Link

Network Interface

Physical

TCP/IP's architecture does not use the presentation and session layers. The application layer
protocols use the transport layer services directly. The OSI transport layer provides connection-
oriented service; in TCP/IP, this service is provided by TCP. TCP/IP also provides connectionless
service in the transport layer with UDP. These protocols are discussed later in the chapter.

The Internet layer of TCP/IP corresponds to the network layer of the OSI model. Although
OSI network-layer protocols provide connection-oriented (Connection-Mode Network Service
(CMNS), X.25) or Connectionless Network Service (CLNS), IP provides only connectionless
network service. The routing protocols are network layer protocols with an IP protocol number.
One exception is the Border Gateway Protocol (BGP), which uses a TCP port number. Another
is Intermediate System-to-Intermediate System (IS-IS), which resides over the data-link layer.
Routing protocols are discussed in Chapter 7, “Static Routing and Distance Vector Routing
Protocols,” Chapter 8, “IP Link-State Routing Protocols,” and Chapter 9, “Border Gateway
Protocol.”