TCP/IP Tutorial and Technical Overview - IBM Redbooks

Figure 8-17 shows an example of remarking data packets that travel trough two
different domains.
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Interior Router Boundary Router Interior Router
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Classifying Remarking
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Figure 8-17 Remarking data packets
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If a data packet travels through multiple domains, the DS field can be remarked
at every boundary component to guarantee the QoS that was contracted in the
SLA. The SLA contains the details of the Traffic Conditioning Agreement (TCA)
that specifies classifier rules and temporal properties of a traffic stream. The TCA
contains information about how the metering, marking, discarding, and shaping
of packets must be done in the traffic conditioner to fulfill the SLA. The TCA
information must be available in all boundary components of a DS network to
guarantee that packets passing through different DS domains receive the same
service in each domain.
DS interior components
The interior components of a DS domain select the forwarding behavior for
packets based on their DS field. The interior component is usually a router that
contains a traffic prioritization algorithm. Because the value of the DS field
normally does not change inside of a DS domain, all interior routers must use the
same traffic forwarding policies to comply with the QoS agreement. Data packets
with different PHB values in the DS field receive different QoSs according to the
QoS definitions for this PHB. Because all interior routers in a domain use the
same policy functions for incoming traffic, the traffic conditioning inside of an
interior node is done only by a packet classifier. It selects packets based on their
PHB value or other IP header fields and forwards the packets to the queue
management and scheduling instance of the node.
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Chapter 8. Quality of service 317