1.3 • THE NETWORK CORE 27
hand in each and every router, or does the Internet use a more automated procedure?
This issue will be studied in depth in Chapter 4. But to whet your appetite here,
we’ll note now that the Internet has a number of special routing protocols that are
used to automatically set the forwarding tables. A routing protocol may, for exam-
ple, determine the shortest path from each router to each destination and use the
shortest path results to configure the forwarding tables in the routers.
How would you actually like to see the end-to-end route that packets take in the
Internet? We now invite you to get your hands dirty by interacting with the Trace-
route program. Simply visit the site www.traceroute.org, choose a source in a particu-
lar country, and trace the route from that source to your computer. (For a discussion of
Traceroute, see Section 1.4.)
1.3.2 Circuit Switching
There are two fundamental approaches to moving data through a network of links
and switches: circuit switching and packet switching. Having covered packet-
switched networks in the previous subsection, we now turn our attention to circuit-
switched networks.
In circuit-switched networks, the resources needed along a path (buffers, link
transmission rate) to provide for communication between the end systems are
reserved for the duration of the communication session between the end systems. In
packet-switched networks, these resources are not reserved; a session’s messages
use the resources on demand, and as a consequence, may have to wait (that is,
queue) for access to a communication link. As a simple analogy, consider two
restaurants, one that requires reservations and another that neither requires reserva-
tions nor accepts them. For the restaurant that requires reservations, we have to go
through the hassle of calling before we leave home. But when we arrive at the
restaurant we can, in principle, immediately be seated and order our meal. For the
restaurant that does not require reservations, we don’t need to bother to reserve a
table. But when we arrive at the restaurant, we may have to wait for a table before
we can be seated.
Traditional telephone networks are examples of circuit-switched networks.
Consider what happens when one person wants to send information (voice or fac-
simile) to another over a telephone network. Before the sender can send the infor-
mation, the network must establish a connection between the sender and the
receiver. This is a bona fide connection for which the switches on the path
between the sender and receiver maintain connection state for that connection. In
the jargon of telephony, this connection is called a circuit. When the network
establishes the circuit, it also reserves a constant transmission rate in the net-
work’s links (representing a fraction of each link’s transmission capacity) for the
duration of the connection. Since a given transmission rate has been reserved for
this sender-to-receiver connection, the sender can transfer the data to the receiver
at the guaranteed constant rate.
hand in each and every router, or does the Internet use a more automated procedure?
This issue will be studied in depth in Chapter 4. But to whet your appetite here,
we’ll note now that the Internet has a number of special routing protocols that are
used to automatically set the forwarding tables. A routing protocol may, for exam-
ple, determine the shortest path from each router to each destination and use the
shortest path results to configure the forwarding tables in the routers.
How would you actually like to see the end-to-end route that packets take in the
Internet? We now invite you to get your hands dirty by interacting with the Trace-
route program. Simply visit the site www.traceroute.org, choose a source in a particu-
lar country, and trace the route from that source to your computer. (For a discussion of
Traceroute, see Section 1.4.)
1.3.2 Circuit Switching
There are two fundamental approaches to moving data through a network of links
and switches: circuit switching and packet switching. Having covered packet-
switched networks in the previous subsection, we now turn our attention to circuit-
switched networks.
In circuit-switched networks, the resources needed along a path (buffers, link
transmission rate) to provide for communication between the end systems are
reserved for the duration of the communication session between the end systems. In
packet-switched networks, these resources are not reserved; a session’s messages
use the resources on demand, and as a consequence, may have to wait (that is,
queue) for access to a communication link. As a simple analogy, consider two
restaurants, one that requires reservations and another that neither requires reserva-
tions nor accepts them. For the restaurant that requires reservations, we have to go
through the hassle of calling before we leave home. But when we arrive at the
restaurant we can, in principle, immediately be seated and order our meal. For the
restaurant that does not require reservations, we don’t need to bother to reserve a
table. But when we arrive at the restaurant, we may have to wait for a table before
we can be seated.
Traditional telephone networks are examples of circuit-switched networks.
Consider what happens when one person wants to send information (voice or fac-
simile) to another over a telephone network. Before the sender can send the infor-
mation, the network must establish a connection between the sender and the
receiver. This is a bona fide connection for which the switches on the path
between the sender and receiver maintain connection state for that connection. In
the jargon of telephony, this connection is called a circuit. When the network
establishes the circuit, it also reserves a constant transmission rate in the net-
work’s links (representing a fraction of each link’s transmission capacity) for the
duration of the connection. Since a given transmission rate has been reserved for
this sender-to-receiver connection, the sender can transfer the data to the receiver
at the guaranteed constant rate.
