TCP/IP Tutorial and Technical Overview - IBM Redbooks

8.1 Why QoS?
In the Internet and intranets of today, bandwidth is an important subject. More
and more people are using the Internet for private and business purposes. The
amount of data that is being transmitted through the Internet is increasing
exponentially. Multimedia applications, such as IP telephony and
videoconferencing systems, need a lot more bandwidth than the applications that
were used in the early years of the Internet. While traditional Internet
applications, such as WWW, FTP, or Telnet, cannot tolerate packet loss but are
less sensitive to variable delays, most real-time applications show just the
opposite behavior, meaning they can compensate for a reasonable amount of
packet loss but are usually very critical toward high variable delays.
This means that without any bandwidth control, the quality of these real-time
streams depends on the bandwidth that is currently available. Low or unstable
bandwidth causes bad quality in real-time transmissions by leading to, for
example, dropouts and hangs. Even the quality of a transmission using the
Real-Time Protocol (RTP) depends on the utilization of the underlying IP delivery
service.
Therefore, certain concepts are necessary to guarantee a specific quality of
service (QoS) for real-time applications on the Internet. A QoS can be described
as a set of parameters that describe the quality (for example, bandwidth, buffer
usage, priority, and CPU usage) of a specific stream of data. The basic IP
protocol stack provides only one QoS, which is called best-effort. Packets are
transmitted from point to point without any guarantee for a special bandwidth or
minimum time delay. With the best-effort traffic model, Internet requests are
handled with the first come, first serve strategy. This means that all requests
have the same priority and are handled one after the other. There is no possibility
to make bandwidth reservations for specific connections or to raise the priority for
special requests. Therefore, new strategies were developed to provide
predictable services for the Internet.
Today, there are two main rudiments to bring QoS to the Internet and IP-based
internetworks: Integrated Services and Differentiated Services.
Integrated Services
Integrated Services bring enhancements to the IP network model to support
real-time transmissions and guaranteed bandwidth for specific flows. In this
case, we define a flow as a distinguishable stream of related datagrams from a
unique sender to a unique receiver that results from a single user activity and
requires the same QoS.
288 TCP/IP Tutorial and Technical Overview