The Internet Still Hierarchical After All These Years

Although the Internet has grown away from the single-backbone architecture of the ARPANET,described in Chapter 1, it retains a certain hierarchical structure. At the lowest level, Internet subscribers connect to an Internet service provider (ISP). In many cases, that ISP is one of many small providers in the local geographic area (called local ISPs). For example, there are presently almost 200 ISPs in Colorado's 303 area code. These local ISPs in turn are the customers of larger ISPs that cover an entire geographic region such as a state or a group of adjacent states. These larger ISPs are called regional service providers. Examples in Colorado are CSD Internet and Colorado Supernet. The regional service providers, in turn, connect to large ISPs with high-speed (DS-3 or OS-3 or better) backbones spanning a national or global area. These largest providers are the network service providers and include companies such as MCI/WorldCom (UUNET), SprintNet, Cable & Wireless, Concentric Network, and PSINet. More commonly, these various providers are referred to as Tier III, Tier II, and Tier I providers, respectively.

Figure 2-6 shows how these different types of ISPs are interrelated. In each case, a subscriber—whether an end user or a lower-level service provider—connects to a higher-level service provider at that ISP's Point of Presence (POP). A POP is just a nearby router to which the subscriber can connect via dialup or a dedicated local loop. At the highest level, the network service providers interconnect via network access points (NAPs). A NAP is a LAN or switch—typically Ethernet, FDDI, or ATM—across which different providers can exchange routes and data traffic.

Figure 2-6 ISP/NAP Hierarchy

Figure 2-6 ISP/NAP Hierarchy

Cdn Hierarchy Isp

As Table 2-1 shows, some NAPs are known by names such as Commercial Internet Exchange (CIX), Federal Internet Exchange (FIX), and Metropolitan Area Exchange (MAE—originally called Metropolitan Area Ethernets, a creation of Metropolitan Fiber Systems, Inc.). CIX, FIX, and MAE-East were early experiments to connect backbones; based on the experience gained from these connection points, the National Science Foundation implemented the first four NAPs in 1994 as part of the decommissioning of the NSFnet.

Table 2-1 Weil-Known Network Access Points in the United States



Maintained By

New York NAP*

Pennsauken, New Jersey


Chicago NAP*

Chicago, Illinois

Ameritech and Bellcore

San Francisco NAP*

San Francisco, California

Pacific Bell

Big East NAP

Bohemia, New York

ICS Network Systems


San Jose, California



Washington, DC



Los Angeles, California


continues continues

Table 2-1 Well-Known Network Access Points in the United States (Continued)



Maintained By


Houston, Texas



Dallas, Texas



New York City, New York



Chicago, Illinois



College Park, Maryland

University of Maryland


Moffett Field, California

NASA Ames Research Center


Santa Clara, California


Digital PAIX

Palo Alto, California

Digital Equipment Corporation

*One of the original four NSF NAPs

*One of the original four NSF NAPs

In addition to the major NAPs shown in Table 2-1, where the NSPs come together, there are many smaller NAPs. These usually interconnect smaller regional providers. Examples of regional NAPs are Seattle Internet eXchange (SIX) and the New Mexico network access point.

In conjunction with the formation of the NAPs, the NSF funded the Routing Arbiter (RA) project. One of the duties of the RA is to promote Internet stability and manageability. To this end, the RA proposed a database (the RADB, or Routing Arbiter Database) of routes (topology) and policies (preferred paths) from the service providers. The database is maintained at NAPs on a route server, a UNIX workstation or server running BGP. Rather than peering with every other router at the NAP, each provider's router peers with only the route server. Routes and policies are communicated to the server, which uses a sophisticated database language called RIPE-181 to process and maintain the information. The appropriate routes are then passed to the other routers.

Although the route server speaks BGP and processes routes, it does not perform packet forwarding. Instead, its updates inform routers of the best next-hop router that is directly reachable across the NAP You are already familiar with this concept from the discussion in Chapter 1 of EGP third-party neighbors. By making one-to-many peering feasible rather than many-to-many peering, route servers increase the stability, manageability, and throughput of traffic through the NAPs.

The NAPs and the RA project proved that the competing network service providers could cooperate to provide manageable connectivity and stability to the Internet. As a result, the NSF ceased funding of the route servers and NAPs on January 1, 1997, and turned the operations over to the commercial interests. Although publicly funded Internet research continues with such projects as Internet2, GigaPOPs, and the very high-speed Backbone Network Service (vBNS), the present Internet can be considered a commercial operation.

A result of the transition to commercial control of the Internet is that the topology of the modern Internet is far from the tidy picture drawn by the preceding paragraphs. The largest service providers, driven by financial, competitive, and policy interests, generally choose to peer directly rather than peer through route servers. The peering also takes place at many levels, rather than just at the top level shown in Figure 2-6.

When two or more service providers agree to share routes across a NAP, either directly or through a route server, they enter into a peering agreement. A peering agreement may be established directly between two providers (a bilateral peering agreement) or between a group of similar-sized providers (a multilateral peering agreement, or MLPA). Traffic patterns play a major role in determining the financial nature of the agreement. If the traffic between the peering partners is reasonably balanced in both directions, money usually does not exchange hands. The peering is equitable for the two partners. However, if the traffic is heavier in one direction than in the other across the peering point, as is the case when a small provider peers with a larger provider, the small provider usually must pay for the peering privilege. The rationale here is that the small provider benefits more from the peering than the larger provider.

Another factor muddling the Internet picture is the location of peering points. NAPs in which many providers come together, such as the ones listed in Table 2-1, are public peering sites. In addition to these public sites, service providers have created hundreds of smaller NAPs at sites where they find themselves co-located with other service providers. The peering agreements at such sites are usually private agreements between two or a few providers. Private peering is encouraged because it helps relieve congestion at the national NAPs, adds to route diversity, and can decrease delay for some traffic.

Another fact hinting that real life is not as tidy as Figure 2-6 suggests is that many national and regional service providers also sell local Internet access, in direct competition with the local ISPs. The "starting point" of the route traces in Example 2-7, for example, is a dial-in POP belonging to Concentric Network—a backbone provider. Regional service providers also frequently have a presence at the backbone NAPs. They might connect to one or more network service providers across the NAP, or they might connect to other regional service providers across the NAP, bypassing any network service provider.

The route traces in Example 2-7 show a little of the Internet backbone structure. Both traces originated from a Concentric Network POP in Denver. In the first trace, the packets traverse Concentric Network's backbone to MAE-East, where they connect to the BBN Planet backbone (lines 3 and 4). The packets traverse BBN Planet's backbone to a Tier II NAP

shared by BBN and US West in Minneapolis (lines 10 and 11) and then are passed to the US West destination.

Example 2-7 Route Traces from a Concentric Network POP in Denver

--- traceroute to (, 30 hops max, 18 byte packets

10 11 12 13 174 ms

207.86.24,29} 192*41♦177.2} 4,0,2.18) 4*0*24$.254} ( (

us -do -wash rBaeeast2,bbnplanet.,net 225 m 232 ms 222 ms 223 ms 235 ms 239 ms h1-0,rainneapoai*crl,bbnplanet,net 258 m hi -0.uswest-mn.bbnplanet,net 260 m 249 ms 258 ms

--- traceroute to (, 30 hops max, 18 byte packets

1 2 161

10 11 12

20 21 152 ms rt001e0102.den -ADDR.ARPA 144.228,207.73} 144*232,5.62} 192.157«$$.1$} 190 ms 207.88.^ 178 206 ms si -gwi8 -clU «5 -1-0-T3. 210 as 216 ms 211 ms 225 ms 236 ms sl-rmp1-pen-4-0-0.sprintlink*net 228 ms 263 m f 1 -1.160 - 6 ^ Rest on. 13. ans. net 264 ms 286 ms 283 ms 292 ms 309 ms 313 ms 306ms 305 ms 285 ms

The packets in the second trace take a pretty thorough tour of the United States before arriving at their destination, a few miles from their origination. First, they follow Concentric's backbone through a router in California (line 4) and then to the Chicago NAP, where they connect to the Sprint backbone (line 6). The packets are routed to the New York NAP in Pennsauken, New Jersey, where they are passed to the ANS backbone (lines 11 and 12). They then visit routers in Reston, Houston, St. Louis, and Chicago (again), and finally arrive back in Denver.

Like the packets in the last trace, we have taken a rather lengthy and circuitous route to get back to the topic at hand, CIDR.

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