Internet protocols

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CHAPTER

3 0

Internet Protocols
Background
The Internet protocols are the world’s most popular open-system (nonproprietary) protocol suite because they can be used to communicate across any set of interconnected networks and are equally well suited for LAN and WAN communications. The Internet protocols consist of a suite of communication protocols, of which the two best known are theTransmission Control Protocol (TCP) and the Internet Protocol (IP). The Internet protocol suite not only includes lower-layer protocols (such as TCP and IP), but it also specifies common applications such as electronic mail, terminal emulation, and file transfer. This chapter provides a broad introduction to specifications that comprise the Internet protocols. Discussions include IP addressing and keyupper-layer protocols used in the Internet. Specific routing protocols are addressed individually later in this document. Internet protocols were first developed in the mid-1970s, when the Defense Advanced Research Projects Agency (DARPA) became interested in establishing a packet-switched network that would facilitate communication between dissimilar computer systems at research institutions.With the goal of heterogeneous connectivity in mind, DARPA funded research by Stanford University and Bolt, Beranek, and Newman (BBN). The result of this development effort was the Internet protocol suite, completed in the late 1970s. TCP/IP later was included with Berkeley Software Distribution (BSD) UNIX and has since become the foundation on which the Internet and the World Wide Web (WWW) arebased. Documentation of the Internet protocols (including new or revised protocols) and policies are specified in technical reports called Request For Comments (RFCs), which are published and then reviewed and analyzed by the Internet community. Protocol refinements are published in the new RFCs. To illustrate the scope of the Internet protocols, Figure 30-1 maps many of the protocols of the Internetprotocol suite and their corresponding OSI layers. This chapter addresses the basic elements and operations of these and other key Internet protocols.

Internet Protocols 30-1

Internet Protocol (IP)

Figure 30-1

Internet protocols span the complete range of OSI model layers.

OSI Reference Model

Internet Protocol Suite

Application

NFS

Presentation

FTP, Telnet, SMTP,SNMP

XDR

Session

RPC

Transport

TCP, UDP

Network

Routing Protocols

IP ARP, RARP

ICMP

Link Not Specified
ith2801

Physical

Internet Protocol (IP)
The Internet Protocol (IP) is a network-layer (Layer 3) protocol that contains addressing information and some control information that enables packets to be routed. IP is documented in RFC 791 and is the primarynetwork-layer protocol in the Internet protocol suite. Along with the Transmission Control Protocol (TCP), IP represents the heart of the Internet protocols. IP has two primary responsibilities: providing connectionless, best-effort delivery of datagrams through an internetwork; and providing fragmentation and reassembly of datagrams to support data links with different maximum-transmission unit (MTU)sizes.

IP Packet Format
An IP packet contains several types of information, as illustrated in Figure 30-2.

30-2

Internetworking Technology Overview, June 1999

IP Packet Format

Figure 30-2

Fourteen fields comprise an IP packet.
32 bits

Version

IHL

Type-of-service

Total length

Identification

Flags

Fragment offset

Time-to-live

Protocol

Header checksumSource address

Destination address

Options (+ padding)

The following discussion describes the IP packet fields illustrated in Figure 30-2:

• • • • • •

Version—Indicates the version of IP currently used. IP Header Length (IHL)—Indicates the datagram header length in 32-bit words. Type-of-Service—Specifies how an upper-layer protocol would like a current datagram to be handled, and...
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