The Internet Encyclopedia (Volume 3)

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786 WIDEAREA ANDMETROPOLITANAREANETWORKS

0

1 0

1 10

1 11 0

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Host_ID

Net_ID

Multicast address

Net_ID

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Host_ID

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Net_ID Host_ID

1234 8

1

6

2

4

3

bit position 1

A

B

C

D

E

class

Figure 12: IPv4 addressing format.

needed for most, meaning that a lot of address space can

be wasted. On the other hand, a single class C address ac-

commodates only 255 interfaces, which is too small for

most organizations, requiring them to have more than

1. From a routing perspective, the two-level hierarchical
   address structure means that routers need to keep track
   of over 16 million netIDs just for class C networks, as
   well as calculate paths through the Internet to each one.
   A number of schemes were developed to solve some of the
   addressing and router problems (subnet masking, class-
   less interdomain routing or CIDR), but those were not
   the only issues. Rising interest in using the Internet to
   carry voice, video, multimedia application, and commer-
   cial transaction traffic increased the demand for security
   and quality of service support, neither of which were built
   into IPv4. Consequently, the IETF began work on a new
   version, IP-ng, to handle the next generation.

IP Version 6 (IPv6)

IP version 6 (IPv6) represents that next generation of Net-

work Layer services. It extends the addressing space from

32 to 128 bits, simplifies the packet header and allows for

future expansion, and adds new capabilities to label flows

of packets (same source to a single destination), to assign

packets priority in support of QoS handling, and to pro-

vide authentication and security. Several of these features

(CIDR, DiffServ, and IPsec) were designed so they could

be added onto IPv4. In fact, such retrofitting solved IPv4

problems well enough in the late 1990s that people be-

gan to question whether a move to IPv6 was necessary.

Upgrading the large numbers of routers involved with In-

ternet traffic would be expensive, time-consuming, and

require careful coordination. Transition strategies and

mechanisms would likely be needed over a considerable

period of time. Unfortunately, retrofits cannot do much

about the size of IPv4 addresses. Sufficient growth in the

numbers and types of devices people want to connect

to or through the Internet (handheld devices, household

appliances, automobile systems, etc.) and international

pressure from countries without enough addresses will

eventually make IPv4 addressing inadequate. The only

question seems to be when.

Border Gateway Protocol (BGP)

Border gateway protocol (BGP) is the exterior routing pro-

tocol used by independent or autonomous systems (ASs)

to exchange routing information throughout the Internet.

Published in 1995 as RFC 1771, it defines procedures to

establishneighborrelationships, and to test thereachabil-

ityof neighbors and other networks. A router at the edge

of an AS uses BGP to work with adjacent (i.e., directly

connected) routers in other ASs. Only after two routers

(one in each AS) have agreed to become neighbors can

they exchange routing information or relay traffic for each

other’s AS. Unlike IRPs, which use the services of IP to

accomplish their communication, BGP uses the reliable

transport services of TCP (transmission control protocol,

running over IP). In this way, BGP can be simpler because

it depends on the error control functions of TCP, and its

messages are not limited in size by the constraints of an

IP datagram.

BGP is purposefully designed to allow an AS to con-

trol what detail of internal information is made visible

outside the AS (aggregating routes using CIDR, for ex-

ample). Typically each BGP router screens potential rout-

ing updates or reachability advertisements against a con-

figuration file that specifies what type of information it

is allowed to send to each particular neighbor. This ap-

proach promotes policy-based routing, but at the expense

of needing to calculate paths from incomplete detail about

the network topology. Thus BGP will not always choose

the optimal path across an internetwork to reach a par-

ticular destination. It does, however, allow a country or

company constituting an AS to make appropriate political

or business decisions about when and where to route its

traffic.

Questions about the scalability of BGP have been

raised in light of predictions for continued substantial

growth in Internet traffic, and particularly as more orga-

nizations consider deploying delay-sensitive applications

over the Internet (e.g., voice, video, conferencing). Intelli-

gent route control, virtual routing, and new approaches to

traffic engineering are among the options being explored

to solve performance problems before they become seri-

ous impediments to effective use of the Internet.

Multiprotocol Label Switching (MPLS)

Multiprotocol label switching (MPLS) has been designed

by the IETF to improve the performance of routed net-

works by layering a connection-oriented framework over

an IP-based internetwork. MPLS requires edge routers to

assign labels to traffic entering the network so that in-

termediate routers (called label-switching routers, LSRs)

can make forwarding decisions quickly, choosing the ap-

propriate output port according to the packet’s label and

rewriting that label (which is intended to have local