A Network of Networks
We saw earlier that end systems (PCs, smartphones, Web servers, mail servers, and so on) connect into the internet via an access ISP. The access ISP can provide either wired or wireless connectivity, using an array of access technologies including DSL, cable, FTTH, Wi-Fi, and cellular. Note that the access ISP does not have to be a telco or a cable company; instead it can be, for example, a university (providing internet access to students, staff, and faculty), or a company (providing access for its employees). But connecting end users and content provides into an access ISP is only a small piece of solving the puzzle of connecting billions of end systems that make up the internet. To complete this puzzle, the access ISPs themselves must be interconnected. This is done by creating a network of networks – understanding this phrase is the key to understanding the internet.
Over the years, the network of networks that forms the internet has evolved into a very complex structure. Much of this evolution is driven by economic and national policy, rather than by performance considerations. In order to understand today’s internet network structure, let’s incrementally build a series of network structures, with each new structure being a better approximation of the complex internet that we have today. Recall that the overarching goal is to interconnect the access ISPs so that all end systems can send packets to each other. One naïve approach would be to have each access ISP directly connect with every other access ISP. Such a mesh design is, of course, much too costly for the access ISPs, as it would require each access ISP to have a separate communication link to each of the hundreds of thousands of other access ISPs all over the world.
Our first network structure , Network Structure 1, interconnects all of the access ISPs with a single global transit ISP. Our (imaginary) global transit ISP is a network of routers and communication links that not only spans the globe, but also has at least one router near each of the hundreds of thousands of access ISPs. Of course, it would be very costly for the global ISP to build such an extensive network. To be profitable, it would naturally charge each of the access ISPs for connectivity, with the pricing reflecting (but not necessarily directly proportional to ) the amount of traffic an access ISP exchanges with the global ISP. Since the access ISP pays the global transit ISP, the access ISP is said to be a customer and the global transit ISP is said to be a provider.
Now if some company builds and operates a global transit ISP that is profitable, then it is natural for other companies to build their own global transit ISPs and compete with the original transit ISP. This leads to Network Structure 2, which consists of hundreds of thousands of ISPs and multiple global transit ISPs. The access ISPs certainly prefer Network Structure 2 over Network Structure 1 since they can now choose among the global transit provider as a function of their pricing and services. Note, however, that the global transit ISPs themselves must interconnect: Otherwise access ISPs connected to one of the global transit providers would not be able to communicate with access ISPs connected to the other global transit providers.
Network Structure 2, just described, is a two –tier hierarchy with global transit providers residing at the top tier and access ISPs at the bottom tier. This assumes that global transit ISPs are not only capable of getting close to each and every access ISP, but also find it economically desirable to do so. In reality, although some ISPs do have impressive global coverage and do directly connect with many access ISPs, no ISP has presence in each and every city in the world. Instead, in any given region, there may be a regional ISP to which the access ISPs in the region connect. Each regional ISP then connects to tier-1 ISPs. Tier-1 ISPs are similar to our (imaginary) global transit ISP; but tier-1 ISPs, which actually do exist ,do not have a presence in every city in the world. There are approximately a dozen tier-1 ISPs, including Level 3 Communications, AT &T , Sprint, and NTT. Interestingly, no group, officially sanctions tier-1 status; as the saying goes – if you have to ask if you’re a member of a group, you’re probably not.
Returning to this network of networks, not only are there multiple competing tier-1 ISP, there may be multiple competing regional ISPs in a region. In such a hierarchy, each access ISP pays the regional ISP to which it connects, and each regional ISP pays the tier-1 ISP to which it connects. (An access ISP can also connect directly to a tier-1 ISP, in which case it pays the tier-1 ISP). Thus, there is customer-provider relationship at each level of the hierarchy. To further complicate matters, in some regions, there may be a larger regional ISP (possibly spanning an entire country) to which the smaller regional iSPs in that region connect; the larger the regional ISP then connects to a tier-1 ISP. For example, in China, there are access ISPs in each city, which connect to provincial ISPs, which in turn connect to national ISPs, which finally connect to tier-1 ISPs. We refer to this multi-tier hierarchy, which is still only a crude approximation of today’s internet, as Network Structure 3.
To build a network that more closely resembles today’s internet, we must add points of presence (PoPs), multi-homing, peering, and internet exchange points (IXPs) to the hierarchical Network Structure 3. PoPs exist in all levels of the hierarchy, except for the bottom (access ISP) level. A PoP is simply a group of one or more routers (at the same location) in the provider’s network where customer ISPs can connect into the provider ISP. For a customer network to connect to a provider’s PoP, it can lease a high-speed link from a third-party telecommunications provider to directly connect to one of its routers to a router at the PoP. Any ISP (except for tier-1 ISPs) may choose to multi-home, that is, to connect to two or more provider ISPs. So, for example, an access ISP may multi-home with two regional ISPs, or it may multi-home with two regional ISPs and also with a tier-1 ISP. Similarly, a regional ISP may multi-home with multiple tier-1 ISPs. When an ISP multi-homes, it can continue to send and receive packets into the internet even if one of its providers has a failure.
As we just learned, customer ISPs pay their provider ISPs to obtain global Internal interconnectivity. The amount that a customer ISP pays a provider ISP reflects the amount of traffic it exchanges with the provider. To reduce these costs, a pair of nearby ISPs at the same level of the hierarchy can peer, that is, they can directly connect their networks together so that all of the traffic between them passes over the direct connection rather than through upstream intermediaries. When two ISPs peer, it is typically settlement-free, that is, neither ISP pays the other. As noted earlier, tier-1 ISPs also peer with one another, settlement-free. Along the same lines, a third-party company can create an Internet Exchange Point (IXP) (typically in a stand-alone building with its own switches), which is a meeting point where multiple ISPs can peer together. There are roughly 300 IXPs in the internet today [Augustin 2009]. We refer to this ecosystem- consisting of access ISPs, regional ISPs, tier-1 ISPs, PoPs, multi-homing, peering, and IXPs – as Network Structure 4.
We now finally arrive at Network Structure 5, which describes the Internet of 2012. Network Structure 5, illustrated in Figure 1.15, builds on top of Network Structure 4 by adding content provider networks. Google is currently one of the leading examples of such a content provider network. As of this writing, it is estimated that Google has 30 to 50 data centres distributed across North America, Europe, Asia, South America, and Australia. Some of these data centres house over one hundred thousand servers, while other data centres are smaller, housing only hundreds of servers. The Google data centres are all interconnected via Google’s private TCP/IP network, which spans the entire globe but is nevertheless separate from the public internet. Importantly, the Google private network only carries traffic to/from Google servers. As shown in fig.1.15, the Google private network attempts to “bypass” the upper tiers of the Internet by peering (settlement free) with lower-tier ISPs, either by directly connecting with them or by connecting with them at IXPs. However, because many access ISPs can still only be reached by transiting thorough tier-1 networks, the Google network also connects to tier-1 ISPs, and pays those ISPs for the traffic it exchanges with them. By creating its own network, a content provider not only reduces its payments to upper-tier ISPs, but also has greater control of how its services are ultimately delivered to end users.
In summary, today’s internet –a network of networks – is complex, consisting of dozen or so tier-1 ISPs and hundreds of thousands of lower-tier ISPs. The ISPs are diverse in their coverage, with some spanning multiple continents and oceans, and other limited to narrow geographic regions. The lower-tier ISPs connect to the higher-tier ISPs, and the higher-tier ISPs interconnect with one another. Users and content providers are customers of lower-tier ISPs, and lower-tier ISPs are customers of higher-tier ISPs. In recent years, major content providers have also created their own networks and connect directly into lower-tier ISPs where possible.