The Internet Encyclopedia (Volume 3)

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SWITCHING,ROUTING,ANDSIGNALING 785

ERP

IRP

Subnet 1.1

Subnet 1.2

Subnet 1.3

IRP

Subnet 2.1

Subnet 2.2

Domain A

Domain

B

router

Figure 11: Routing within and between autonomous domains.

than selling services, as was typical in public networks.

This meant that customers could specify the technology

used to put bits on the medium rather than subscribing

only to specific services offered by providers. As advances

in optics and use of switching allowed Ethernet to cover

even greater distances, the geographic limits that distin-

guished LAN from MAN technologies began to disappear.

In fact, new providers sprang up offering Ethernet con-

nectivity from the business doorstep to other locations

across town or beyond. The great competitive question

was whether Ethernet MANs could be made as reliable

and fault-tolerant as more traditional MAN/WAN tech-

nologies built over SONET.

Resilient Packet Ring (RPR)

Resilient packet ring (RPR) is an effort begun by the IEEE

802.17 working group in late 2000 to design a high-speed

access protocol combining familiar Ethernet interfaces

with the fault-tolerance and rapid restoration capability

of ring-based MAN technologies like SONET. RPR defines

a new medium access control (MAC sublayer of OSI Layer

2) protocol that extends Ethernet framing from the LAN

into the MAN/WAN environment. As seen by the RPR

Alliance (an industry consortium designed to promote

adoption of RPR), this approach combines the cost-

effective scalability of Ethernet access interfaces with a

MAN that can be optimized for rapidly increasing vol-

umes of data traffic. Because it focuses on the MAC

sublayer, RPR is independent of the underlying Layer-1

technology, making it suitable to run over much of the

MAN infrastructure already in place.

Routing Technologies

In the OSI Reference Model, routing takes place at Layer

3, the Network Layer. Essentially routing consists of three

major functions: maintaining information about the net-

work environment, finding a path through the network

from particular sources to destinations, and forwarding

packets at each relay point. The Internet protocol (IP) is

the dominant method of interconnecting packet-switched

networks (i.e., for internetworking) at Layer 3. It provides

connectionless network services (CLNS), with no guar-

antee of delivery or packet ordering, and is widely used

today for private and public LANs, MANs, and WANs, in-

cluding the Internet. IP is primarily concerned with the

format for packets (also called datagrams), the defini-

tion and structure of addresses, a packet-forwarding al-

gorithm, and the mechanisms for exchanging information

about conditions in and control of the network.

Routing responsibility in an internetwork is divided be-

tween intradomain or interior routing protocols (IRPs)

and interdomain or exterior routing protocols (ERPs) as

shown in Figure 11. IRPs are used for internetworks that

belong to a single administrative authority, such as an en-

terprise LAN, a single service provider’s MAN, or a private

WAN. ERPs are used when routers tie together networks

belonging to multiple independent authorities, as in the

Internet. These protocols differ in how much informa-

tion is kept about the state of the network and how rout-

ing updates are performed using the mechanisms defined

by IP.

IP Version 4 (IPv4)

IP version 4 (IPv4) was defined by the Internet Engi-

neering Task Force (IETF) for the original ARPAnet and

published as (Request for Comments) RFC 791 in 1981.

It specifies that each interface capable of originating or

receiving internetwork traffic be identified by a unique

32-bit address consisting of an ordered pair containing a

network identifier (netID) and a host/interface identifier

(hostID). Three primary classes of network addresses (A,

B, and C) were designed to promote efficient routing, with

additional classes defined for special or future uses (Fig-

ure 12). Although the Internet is not centrally managed, it

was necessary to establish a single authority to assign ad-

dresses so that there would be no duplicates or conflicts.

As the Internet grew through the 1980s, a number of

limitations in the design of IPv4 became apparent. The al-

location of addresses, especially classes A and B, tended to

be wasteful. For example, a single class B address assigned

to one organization accommodates one network with over

64,000 IP interfaces—much larger than is practical or