Two types of services have been standardized for RSVP: Controlled load,
(which offers service with QoS equal to that of the unloaded network), and
Guaranteed (with hard QoS limits).
An industry-wide effort in the Integrated Services over Specific Link Layers
(ISSLL) working group of the IETF has produced detailed specifications on
how to map IP QoS in the integrated services architecture onto most link-layer
technologies (such as slow links, Point-to-Point Protocol (PPP), Ethernet 802.3-
style LANs, and legacy technologies, such as Frame Relay and ATM).
Integrated Services and DiffServ Networks
Using differentiated services at the edge of the network in the access portion
and differentiated services in the core is a good match for end-to-end QoS.
Several intermediate QoS models are possible between the extremes of
RSVP and differentiated services, as shown in Figure 5.2. For example, so-
called RSVP aggregators can have another, aggregated RSVP as the output, or
certain classes of services can be associated with guaranteed delivery. The lat-
ter raises the intriguing possibility of a standard class of service for voice
across the Internet and private IP networks. Since voice seems to require only
a small part of the overall bandwidth compared to data, setting a standard
QoS for telephony might simplify considerably end-to-end IP network design.
Multiprotocol Label Switching
Multiprotocol Label Switching (MPLS) is a controversial protocol and an often
mentioned reason for its deployment is VoIP. Recent marketing push by ven-
dors and traditional carriers for MPLS [15], and the resulting standards activi-
ties require, therefore, some clarifications here.
On the positive side, MPLS may be useful in the internal plumbing of large
Internet service providers for traffic engineering[16], for steering traffic along
certain paths (for example, between a north and a south route crossing the
Atlantic Ocean).
On the negative side, MPLS has been pushed to market not necessarily for
the benefit of Internet users, but as a revenue enhancer for equipment vendors
and their traditional carrier customers. Facility-based Internet domain owners
may also see MPLS as a tool to discriminate against emerging VoIP services
provided over the open Internet.
This is not an advantage for users who may want to reach anyone one the
Internet.
Other MPLS issues include the following:
■■ MPLS does not provide QoS, but can only invoke DS in networks
where QoS using DS is already implemented.
Real-Time Internet Multimedia 89