TCP/IP Tutorial and Technical Overview - IBM Redbooks

the transmitted packets and shape transmitted traffic to the requirements of the
Intserv services in use.
The RSVP end-to-end signaling messages are exchanged between the hosts in
the intranets. Thus, the RSVP/Intserv resource reservation is accomplished
outside the Diffserv region.
The edge routers act as admission control agents of the Diffserv network. They
process the signaling messages from the hosts in both intranets and apply
admission control. Admission control is based on resources available within the
Diffserv region. It is also defined in a company's policy for the intranets.
Because the border routers in our sample (R2 and R3) are not aware of RSVP,
they act as pure Diffserv routers. These routers control and submit packets
based on the specified DSCP and the agreement for the host's aggregate traffic
control.
The Diffserv network or region supports aggregate traffic control and is assumed
to be incapable of MF classification. Therefore, any RSVP messages will pass
transparently with negligible performance impact through a Diffserv network
region.
The next aspect to be considered is the mapping of the Intserv service type and
the set of quantitative parameters, known as flowspec. Because Diffserv
networks use PHB or a set of PHBs, a mapping of the Intserv flowspec has to be
defined accordingly. The mapping value is a bit combination in the DSCP.
However, this mapping has to viewed under bandwidth management
considerations for the Diffserv network.
The DSCP value has been made known to all routers in the Diffserv network.
The question arises of how the DSCP will be propagated to these routers.
There are two choices:
► DSCPs can be marked at the entrance of the Diffserv region (at the boundary
routers). In this case, they can be also re-marked at the exit of the Diffserv
region (at the other boundary router).
► DSCP marking can occur in a host or in a router of the intranet. In this case,
the appropriate mapping needs to be communicated to the marking device.
This can be provided by RSVP.
The following sequence shows how an application obtains end-to-end QoS
support:
1. HostA, attached to an intranet, requests a service from hostB, attached to
another intranet. Both intranets are connected by a Diffserv network (see
Figure 8-20 on page 320).
Chapter 8. Quality of service 321