49
The name space of the DNS is organized hierarchically. Top-level domains
(country specific domains such as .de and .uk and generic domains such as .edu,
.gov) are rare. They are approved by the Internet Corporation for Assigned Names
and Numbers (ICANN) and administered by top-level domain administrators. These
administrators in turn grant sub-domains to commercial organizations, academic
institutions, government ministries, etc., which in turn receive the authority to grant
further sub-domains in their zone. Internet Domain names are written with the most
local domain label first and the top-domain label last. The labels of the domain names are separated by a period. The organizations responsible for a domain or a subdomain propagate the information on their domain names and the corresponding IP
addresses on authoritative DNS servers. An institution will only be granted a second-level domain if it will operate a name server for this domain according to Internet standards (Comer, 2006: 439). The DNS servers are geographically distributed in the Internet. These servers use specialized protocols to cooperate in mapping
domain names to Internet addresses for all domain names in the public Internet.
Computers attached to the Internet access a DNS server to translate a URL40 or an
E-mail address into an IP address needed for forwarding the data.
3.4.4 Routers and routing
The information on the location of a specific network host in the Internet, given by
the IP address, is evaluated by specialized computers, called routers. Routers are
situated at network nodes and interconnection points where the networks making up
the Internet interconnect. These routers have the function of forwarding data packages across network boundaries according to rules specified in so-called routing protocols. When more than one possible path to the final destination is available, routing refers to the process of deciding on a specific route.41 It is necessary to explain
the process of routing in more detail here, because the routing functions performed
by the different ISPs active in the Internet are the central differentiating characteristics between ISPs to be discussed in the subsequent chapters. The following description of routing differentiates between routing within autonomous systems,
where traffic is directed according to the IP address and routing between autonomous systems, where traffic is forwarded using the AS number.
Routing within autonomous systems
Package-forwarding within autonomous systems follows the principle of “next-hop
routing.” A router does not know the complete path to the final destination, but only
the next router along the way to the final destination. A router evaluates the destination IP address of any incoming package and sends the package along to the next
40 URL stands for Uniform Resource Locator, a technical term for web-address.
41 More recently the term “forwarding” has become more common than “routing” (Comer,
2006: 93).
50
router on its way to its final destination according to information given in its routing
table. A routing table pairs IP addresses (destinations in the network) and routing
paths (next router in line to the destination) (Comer, 2006: 97).
The information listed in the routing tables of neighboring routers obviously
needs to be coordinated, so that the “next-hop” approach leads to packages reaching
their destinations without inefficient detours. This coordination can be achieved by
network administrators inserting the information listed in routing tables manually.
However, because the Internet is dynamic with many hosts being connected and
disconnected at any given time, automated systems are considered more reliable and
stable compared to manual systems. Such automated systems are based on routing
protocols by which routers dynamically learn new routing paths from their neighboring routers. Examples for such protocols are the Routing Information Protocol and
the Open Shortest Path First Protocol. Using these protocols, routers advertise their
current network capacity and the connections they can establish to neighboring routers and receive analogous information. By this exchange of information, routers
acquire a complete picture of network topology within their region and fill their
routing tables accordingly.
The criteria which determine which route a router should add to its routing table
are specified by the routing protocols. Distance-vector routing protocols, such as the
Routing Information Protocol, use a hop-count to measure the distance to the final
destination, whereby one hop corresponds to one router passed along the way.42
Routers communicating via this protocol advertise which destinations can be
reached via their network and the number of hops (the number of routers passed) to
the final destination via this particular route. The choice of the path a datagram is to
take can then be based on the fixed assumption of shortest path. A router will add
that route to its routing table which minimizes the number of hops to the destination.
The router increases the hop-count with which the route is propagated to other routers by one before adding this route to its own routing table.
Link-state routing protocols, such as the Open Shortest Path First Protocol, allow
basing the routing decision on more complex criteria. In addition to sending information on the distance to a particular destination, routers that communicate via this
protocol also send information on the state of the connection to the destination. This
protocol therefore allows for more complex routing, such as routing according to the
type of service (for instance higher quality routes are reserved for real-time applications) or routing in order to balance traffic load (Comer, 2006: 94).43
42 Because traversing networks can take different amounts of time, depending on the technology
of the network, the protocol allows for routers to set hop-counts artificially high for connections via slow networks (Comer, 2006: 275).
43 Even though Internet technology, as compared to the technology of PSTN networks, is characterized by the fact that most of the intelligent software is located at the edge of the network, whereas the internal switches generally execute only simple functions, the routing
protocols can be used for more refined steering of the Internet traffic. The features of the existing routing protocols, if taken advantage of, can even be used to realize some of the functi-
51
To keep the information stored in routing tables to a minimum, routers initially
evaluate only the network prefix of the destination IP address. For each possible
network prefix, the routing table stores information on the routing path. In this system “the amount of information a router needs to keep is proportional to the number
of networks in the internet, not the number of computers” (Comer, 2006: 35).44 Only
when the home network of the destination host is reached, is the host part of the IP
address evaluated. The final router in the routing path is connected to the home
network of the destination host. It will map a physical address to the IP address of
the host and deliver the IP packet directly to the destination host.45 A further simplification of routing tables is that routers, in general, do not explicitly list all network
prefixes. Rather, the largest part of the destinations reachable in the Internet will be
summed into a default route applicable to all networks for which no explicit routing
information is listed in the routing table (Comer, 2006: 100). This default route will
point to a router which has a more complete routing table.
Figure 3.4 illustrates the logic of next-hop routing. The routing table for router R
specifies that all incoming packages with network prefixes 20.0.0.0 or 30.0.0.0 can
be delivered directly because router R has a direct connection to both of these networks. For packages destined to networks 10.0.0.0 or 40.0.0.0 the routing table lists
the IP address of the next-hop router, to which R has a direct connection.
Routing between autonomous systems
In the early Internet, routing between networks was very similar to the way routing
now functions within an autonomous system. When the Internet grew to dimensions
of many thousands of interconnected networks, the routing tables, however, did not
scale well to this large number of possible destinations. Autonomous systems were
invented as a means of combining networks for the purposes of routing. A group of
networks is subsumed using one autonomous systems number such that routing
tables need routing information for only this AS number in order to be able to reach
all of the included networks.
For routing between autonomous systems, one or more routers within the autonomous system collect the information on the networks reachable within the AS and
propagate this information (or a subset of the reachable networks) to the rest of the
Internet. The protocol by which these routers communicate with the corresponding
routers of the other autonomous systems is the Border Gateway Protocol (BGP).46
The information propagated via the BGP protocol is not comparable to the routing
information exchanged by routers within autonomous systems. BGP only propagates
the fact that a particular network can be reached via the router. When two or more
ons that are to be made possible by the IPv6 protocol, for instance the differential treatment
of particular traffic.
44 Comer uses the term “internet,” without the capital I, to refer to autonomous systems.
45 The protocol that maps physical addresses to IP addresses is the Address Resolution Protocol
(Comer, 2006: 57ff.).
46 The standard common today is the fourth version of the BGP protocol, BGP-4 (Comer, 2006:
254).
52
routers advertise the reachability of this network, a router cannot use this information in selecting an optimal route according to criteria such as number of hops or the
quality of the connection (Comer, 2006: 264). Halabi (2000: 101) states correctly,
that: “ …the primary difference between intra-AS and inter-AS routing is that intra-
AS routing is usually optimized in accordance with the required technical demands,
while inter-AS usually reflects political and business relationships between the networks and companies involved.”
Figure 3.4: Next-hop routing
Source: Comer, 2006: 99
Consistency in routing
The routing information propagated in the Internet needs to be consistent and complete in order to avoid routing loops and data loss. The functioning of the Internet
therefore hinges to a great extent on the premise that the routes advertised by routers
are valid. In the early Internet, a so-called core system of central routers, which kept
complete information on all destinations in the Internet, guaranteed this consistency.
All routers that were not part of this core system could direct a default route towards
Q R S
N etwork
1 0.0.0.0.
Network
20.0.0.0
Network
30.0.0.0.
N etwork
4 0.0.0.0.
20.0.0.5 30.0.0.6 40.0.0.7
10.0. 0.5 20.0.0.6 30.0. 0.7
TO REACH HO STS
ON NETWOR K
ROUTE TO
THIS AD RESS
20.0.0.0 DELIVER DIRECTLY
30.0.0.0 DELIVER DERECTLY
10.0.0.0 20.0.0.5
40.0.0.0 30.0.0.7
(a) An exa mple internet with 4 networks and 3 routers, and
(b) the routing table in R.
(a)
(b)
53
a core router and rely on this router to have the information on how to reach all valid
destinations in the Internet. The core system also discarded any data packages with
no valid destination and thereby guaranteed that this traffic would not block valuable
network capacity.47
Within autonomous systems the consistency of routes is checked in much the
same way as with the core system explained above (central routers have information
on all possible destinations within the autonomous system). Between autonomous
systems, ISPs check that propagated routes are valid by comparing reachability
advertisements of other ISPs with information listed by so-called routing registries.
These registries contain information on which ISPs have been allocated which IP
address blocks (Comer, 2006: 266). Of course the data in these routing registries
need to be valid and up-to-date. When the NSFNET was privatized, the NSF awarded a contract for a Routing arbiter project, which was to coordinate the exchange of
routing information between the independent commercial operators. Merit networks
was awarded this contract and has since played a leading role in Internet routing.
Today Merit manages the Routing Assets Database, one of the most popular routing
registries used by network operators around the world to register their routes and to
send queries on routing problems.48 However, many other routing registries exist in
parallel such that there is no central authority among the routing registries. Routing
problems can and do occur, when routing registries need time for recognizing and
repairing inconsistencies.
3.4.5 Quality of service differentiation in Internet routing software
In the beginning of the commercial Internet, Internet access services were generally
provided over narrowband local network infrastructure. The bandwidth offered by
narrowband communication lines was sufficient to support the standard applications
of this time, such as E-mail, file transfer, and remote login. Over time, technological
improvements and investments into local telecommunications infrastructure have
increased the available bandwidth in the local infrastructure substantially. With the
advent of broadband infrastructure, more bandwidth-intensive Internet applications
have become available, such as online-gaming, video-on-demand, and voice-over-IP
(VoIP).
The transition to broadband in Internet access services has also had an impact on
the demands on network management and routing functions in Internet backbone
services. Historically, routing was programmed as a best-effort service. Packets
were treated equally in the backbone network, independent of which applications
generated the packet flow. With increased use of bandwidth-intensive applications,
47 On the core system of the original Internet see Comer, 2006: 238ff.
48 Furthermore, Merit conducts projects that are to heighten the accuracy of information in
routing registries (Blunk and Karir, 2005).
53
a core router and rely on this router to have the information on how to reach all valid
destinations in the Internet. The core system also discarded any data packages with
no valid destination and thereby guaranteed that this traffic would not block valuable
network capacity.47
Within autonomous systems the consistency of routes is checked in much the
same way as with the core system explained above (central routers have information
on all possible destinations within the autonomous system). Between autonomous
systems, ISPs check that propagated routes are valid by comparing reachability
advertisements of other ISPs with information listed by so-called routing registries.
These registries contain information on which ISPs have been allocated which IP
address blocks (Comer, 2006: 266). Of course the data in these routing registries
need to be valid and up-to-date. When the NSFNET was privatized, the NSF awarded a contract for a Routing arbiter project, which was to coordinate the exchange of
routing information between the independent commercial operators. Merit networks
was awarded this contract and has since played a leading role in Internet routing.
Today Merit manages the Routing Assets Database, one of the most popular routing
registries used by network operators around the world to register their routes and to
send queries on routing problems.48 However, any other routing registries exist in
parallel such that there is no central authority among the routing registries. Routing
problems can and do occur, when routing registries need time for recognizing and
repairing inconsistencies.
3.4.5 Quality of service differentiation in Internet routing software
In the beginning of the commercial Internet, Internet access services were generally
provided over narrowband local network infrastructure. The bandwidth offered by
narrowband communication lines was sufficient to support the standard applications
of this ti e, such as E-mail, file transfer, and remote login. Over time, technological
improvements and investments into local telecommunications infrastructure have
increased the available bandwidth in the local infrastructure substantially. With the
advent of broadband infrastructure, more bandwidth-intensive Internet applications
have become available, such as online-gaming, video-on-demand, and voice-over-IP
(VoIP).
The transition to broadband in Internet access services has also had an impact on
the demands on network management and routing functions in Internet backbone
services. Historically, routing was programmed as a best-effort service. Packets
were treated equally in the backbone network, independent of which applications
generated the packet flow. With increased use of bandwidth-intensive applications,
7 On core syst m of the original Internet see Comer, 2006: 238ff.
48 Fur hermore, Merit conducts p ojects that are to heighten the accuracy of information in
routing registries (Blunk and Karir, 2005).
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References
Zusammenfassung
Die Konvergenz der Netztechnologien, die dem Internet, der Telekommunikation und dem Kabelfernsehen zu Grunde liegen, wird die Regulierung dieser Märkte grundlegend verändern. In den sogenannten Next Generation Networks werden auch Sprache und Fernsehinhalte über die IP-Technologie des Internets transportiert. Mit den Methoden der angewandten Mikroökonomie untersucht die vorliegende Arbeit, ob eine ex-ante sektorspezifische Regulierung auf den Märkten für Internetdienste wettbewerbsökonomisch begründet ist. Im Mittelpunkt der Analyse stehen die Größen- und Verbundvorteile, die beim Aufbau von Netzinfrastrukturen entstehen, sowie die Netzexternalitäten, die im Internet eine bedeutende Rolle spielen. Die Autorin kommt zu dem Ergebnis, dass in den Kernmärkten der Internet Service Provider keine monopolistischen Engpassbereiche vorliegen, welche eine sektor-spezifische Regulierung notwendig machen würden. Der funktionsfähige Wettbewerb zwischen den ISP setzt jedoch regulierten, diskriminierungsfreien Zugang zu den verbleibenden monopolistischen Engpassbereichen im vorgelagerten Markt für lokale Netzinfrastruktur voraus. Die Untersuchung zeigt den notwendigen Regulierungsumfang in der Internet-Peripherie auf und vergleicht diesen mit der aktuellen Regulierungspraxis auf den Telekommunikationsmärkten in den Vereinigten Staaten und in Europa. Sie richtet sich sowohl an die Praxis (Netzbetreiber, Regulierer und Kartellämter) als auch an die Wissenschaft.