1.3.3 A Network of Networks (p.58~)

• The access ISP can provide either wired or wireless connectivity, using an array of access technologies including DSL, cable, FTTH, Wi-Fi, and cellular.
• 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).
• Network Structure 1, interconnects all of the access ISPs with a single global transit ISP.

• 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.
• 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

• Note that the tier-1 ISPs do not pay anyone as they are at the top of the hierarchy

• We refer to this multi-tier hierarchy, which is still only a crude approximation of today’s Internet, as 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.
• Any ISP (except for tier-1 ISPs) may choose to multi-home, that is, to connect to two or more provider 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.
• To reduce these costs, a pair of nearby ISPs at the same level of the hierarchy can peer,
• tier-1 ISPs also peer with one another, settlement-free.
• 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 today’s Internet. Network Structure 5, illustrated in Figure 1.15