TCP/IP Tutorial and Technical Overview - IBM Redbooks

Figure 8-18 shows the traffic conditioning in an interior node.
Packets
Figure 8-18 DS interior component
Traffic classifying and prioritized routing is done in every interior component of a
DS domain. After a data packet has crossed a domain, it reaches the boundary
router of the next domain and possibly gets remarked to cross this domain with
the requested QoS.
Source domains
The IETF DS working group defines a source domain as the domain that
contains one or more nodes that originate the traffic that receives a particular
service. Traffic sources and intermediate nodes within a source domain can
perform traffic classification and conditioning functions. The traffic that is sent
from a source domain can be marked by the traffic sources directly or by
intermediate nodes before leaving the source domain.
In this context, it is important to understand that the first PHB marking of the data
packets is not done by the sending application itself. Applications do not notice
the availability of Differentiated Services in a network. Therefore, applications
using DS networks must not be rewritten to support DS. This is an important
difference from Integrated Services, where most applications support the RSVP
protocol directly when some code changes are necessary.
The first PHB marking of packets that are sent from an application is done in the
source host or the first router the packet passes. The packets are identified with
their IP address and source port. For example, a client has an SLA with a service
provider that guarantees a higher priority for the packets sent by an audio
application. The audio application sends the data packets through a specific port
and can be recognized in multi-field classifiers. This classifier type recognizes
the IP address and port number of a packet and can distinguish the packets from
different applications. If the host contains a traffic conditioner with an MF
classifier, the IP packet can be marked with the appropriate PHB value and
consequently receives the QoSs that are requested by the client. If the host does
not contain a traffic conditioner, the initial marking of the packets is done by the
318 TCP/IP Tutorial and Technical Overview
Interior Router
DS Byte Classifier Queue Management Scheduler