1.3 • THE NETWORK CORE 33
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 ISPs, there may be multiple competing regional ISPs in a region. In such a hierar-
chy, 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. Note that the tier-1 ISPs do not pay anyone
as they are at the top 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 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 [Tian 2012]. 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 hier-
archy, 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 telecommunica-
tions provider to directly connect 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
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 ISPs, there may be multiple competing regional ISPs in a region. In such a hierar-
chy, 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. Note that the tier-1 ISPs do not pay anyone
as they are at the top 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 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 [Tian 2012]. 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 hier-
archy, 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 telecommunica-
tions provider to directly connect 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
