of layering in the previous section has perhaps given the impression that
the Internet is a carefully organized and highly intertwined structure.
This is certainly true in the sense that all of the network entities (end
systems, routers, and bridges) use a common set of protocols, enabling
the entities to communicate with each other. However, from a topological
perspective, to many people the Internet seems to be growing in a chaotic
manner, with new sections, branches, and wings popping up in random places
on a daily basis. Indeed, unlike the protocols, the Internet's topology
can grow and evolve without approval from a central authority. Let us now
try to get a grip on the seemingly nebulous Internet topology.
As we mentioned
at the beginning of this chapter, the topology of the Internet is loosely
hierarchical. Roughly speaking, from bottom-to-top the hierarchy consists
of end systems (PCs, workstations, and so on) connected to local Internet
service providers (ISPs). The local ISPs are in turn connected to regional
ISPs, which are in turn connected to national and international ISPs. The
national and international ISPs are connected together at the highest tier
in the hierarchy. New tiers and branches can be added just as a new piece
of Lego can be attached to an existing Lego construction.
In this section
we describe the topology of the Internet in the United States as of 2000.
Let's begin at the top of the hierarchy and work our way down. Residing
at the very top of the hierarchy are the national ISPs, which are called
national service providers (NSPs). The NSPs form independent
backbone networks that span North America (and typically extend abroad
as well). Just as there are multiple long-distance telephone companies
in the United States, there are multiple NSPs that compete with each other
for traffic and customers. The existing NSPs include internetMCI, SprintLink,
PSINet, UUNet Technologies, and AGIS. The NSPs typically have high-bandwidth
transmission links, with bandwidths ranging from 1.5 Mbps to 622 Mbps and
higher. Each NSP also has numerous hubs that interconnect its links
and at which regional ISPs can tap into the NSP.
The NSPs themselves
must be interconnected to each other. To see this, suppose one regional
ISP, say MidWestnet, is connected to the MCI NSP and another regional ISP,
say EastCoastnet, is connected to Sprint's NSP. How can traffic be sent
from MidWestnet to EastCoastnet? The solution is to introduce switching
centers, called network access points (NAPs), which interconnect
the NSPs, thereby allowing each regional ISP to pass traffic to any other
regional ISP. To keep us all confused, some of the NAPs are not referred
to as NAPs but instead as MAEs (metropolitan area exchanges). In the United
States, many of the NAPs are run by RBOCs (regional Bell operating companies);
for example, PacBell has a NAP in San Francisco and Ameritech has a NAP
in Chicago. For a list of major NSPs (those connected into at least three
MAPs/MAE's), see [Haynal
1999]. In addition to connecting to each other at NAPs, NSPs can connect
to each other through so-called private peering points; see Figure 1.26.
For a discussion of NAPs as well as private peering among NSPs, see [Huston
Internet structure: Network of networks
NAPs relay and switch tremendous volumes of Internet traffic, they are
typically in themselves complex high-speed switching networks concentrated
in a small geographical area (for example, a single building). Often the
NAPs use high-speed ATM switching technology in the heart of the NAP, with
IP riding on top of ATM. Figure 1.27 illustrates PacBell's San Francisco
NAP. The details of Figure 1.27 are unimportant for us now; it is worthwhile
to note, however, that the NSP hubs can themselves be complex data networks.
The PacBell NAP architecture (courtesy of the Pacific Bell Web site)
Running an NSP
is not cheap. In June 1996, the cost of leasing 45 Mbps fiber optics from
coast to coast, as well as the additional hardware required, was approximately
$150,000 per month. And the fees that an NSP pays the NAPs to connect to
the NAPs can exceed $300,000 annually. NSPs and NAPs also have significant
capital costs in equipment for high-speed networking. An NSP earns money
by charging a monthly fee to the regional ISPs that connect to it. The
fee that an NSP charges to a regional ISP typically depends on the bandwidth
of the connection between the regional ISP and the NSP; clearly a 1.5 Mbps
connection would be charged less than a 45 Mbps connection. Once the fixed-bandwidth
connection is in place, the regional ISP can pump and receive as much data
as it pleases, up to the bandwidth of the connection, at no additional
cost. If an NSP has significant revenues from the regional ISPs that connect
to it, it may be able to cover the high capital and monthly costs of setting
up and maintaining an NSP. For a discussion of the current practice of
financial settlement among interconnected network providers, see [Huston
A regional ISP
is also a complex network, consisting of routers and transmission links
with rates ranging from 64 Kbps upward. A regional ISP typically taps into
an NSP (at an NSP hub), but it can also tap directly into a NAP, in which
case the regional ISP pays a monthly fee to a NAP instead of to an NSP.
A regional ISP can also tap into the Internet backbone at two or more distinct
points (for example, at an NSP hub or at a NAP). How does a regional ISP
cover its costs? To answer this question, let's jump to the bottom of the
gain access to the Internet by connecting to a local ISP. Universities
and corporations can act as local ISPs, but backbone service providers
can also serve as a local ISP. Many local ISPs are small "mom and pop"
companies, however. A popular Web site known simply as "The List" contains
links to nearly 8,000 local, regional, and backbone ISPs [List
1999]. The local ISPs tap into one of the regional ISPs in its region.
Analogous to the fee structure between the regional ISP and the NSP, the
local ISP pays a monthly fee to its regional ISP that depends on the bandwidth
of the connection. Finally, the local ISP charges its customers (typically)
a flat, monthly fee for Internet access: the higher the transmission rate
of the connection, the higher the monthly fee.
this section by mentioning that any one of us can become a local ISP as
soon as we have an Internet connection. All we need to do is purchase the
necessary equipment (for example, router and modem pool) that is needed
to allow other users to connect to our so-called point of presence. Thus,
new tiers and branches can be added to the Internet topology just as a
new piece of Lego can be attached to an existing Lego construction.