Data Networks

598 8.4 Integrated Services (IS)

rienced quality of service may vary widely from vendor to vendor and from

one traffic environment to another. There will have to be agreements made

on what constitutes acceptable service for the various classes of ATM and

how that service will be provided. Developing mechanisms to ensure quality

of service in ATM is one of the most important topics that the ATM Forum

is currently investigating.

8.4 Integrated Services (IS)

The fundamental service model of the Internet, exemplified in the best-

effort delivery service of IP, has remained essentially unchanged for over 20

years. This model has serviced legacy applications such as file transfer and

terminal access well, but routing delays and congestion mean that real-time

applications do not work so well on a best-effort Internet. The Integrated

Services (IS) model is intended to solve these problems by becoming a key

component of future Internet architecture. This new Internet architecture

includes support for both the current best-effort services and emerging real-

time services. IS is designed to optimize network resources for real-time

applications such as videoconferencing, video broadcast, and audioconfer-

encing. These applications all require guaranteed QoS in order to offer

acceptable quality. IS enables Internet traffic to be separated into legacy best-

effort traffic and real-time data flows requiring guaranteed QoS. IS defines

two service classes specifically designed for real-time traffic, as follows:

 Guaranteed service—intended for applications requiring a fixed delay

[32].

 Predictive service—intended for applications requiring prognosticate

delay [32].

IS integrates (hence the name) all of these services over a common link,

using a scheme called controlled link sharing. IS is also designed to work

equally well with multicast as well as unicast traffic. The IS model is speci-

fied by the IETF in [11].

8.4.1 Implementation model

There are four main components required in an implementation of IS;

these comprise the packet scheduler, the admission control routine, the clas-

sifier, and the reservation setup protocol. These are discussed briefly below.

 Reservation setup protocol. IS use the reservation protocol (RSVP)

for the signaling of the reservation messages. The IS instances com-

8.4 Integrated Services (IS) 599

Chapter 8

municate via RSVP to create and maintain flow-specific states in the

end-point hosts and in routers along the path of a flow. An applica-

tion that wants to send data packets in a reserved flow communicates

with the reservation instance RSVP. The RSVP protocol tries to set

up a flow reservation with the requested QoS, which will be accepted

if the application fulfilled the policy restrictions and the routers can

handle the requested QoS. RSVP advises the packet classifier and

packet scheduler in each node to process the packets for this flow ade-

quately. If the application now delivers the data packets to the classi-

fier in the first node, which has mapped this flow into a specific

service class complying to the requested QoS, the flow is recognized

with the sender IP address and is transmitted to the packet scheduler.

The packet scheduler forwards the packets, depending on their ser-

vice class, to the next router or, finally, to the receiving host. Because

RSVP is a simplex protocol, QoS reservations are made only in one

direction: from the sending node to the receiving node. If the applica-

tion in our example wants to cancel the reservation for the data flow,

it sends a message to the reservation instance, which frees the reserved

QoS resources in all routers along the path, so the resources can be

used for other flows.

 Admission control. The admission control contains the decision algo-

rithm that a router uses to determine if there are enough routing

resources to accept the requested QoS for a new flow. If there are not

enough free routing resources, accepting a new flow would impact

earlier guarantees and the new flow must be rejected. If the new flow

is accepted, the reservation instance in the router assigns the packet

classifier and the packet scheduler to reserve the requested QoS for

this flow. Admission control is invoked at each router along a reserva-

tion path to make a local accept/reject decision at the time a host

requests a real-time service. The admission control algorithm must be

consistent with the service model. Admission control should not be

confused with policy control, which is a packet-by-packet function

processed by the packet scheduler. It ensures that a host does not vio-

late its promised traffic characteristics. Nevertheless, to ensure that

QoS guarantees are honored, the admission control will be concerned

with enforcing administrative policies on resource reservations. Some

policies will be used to check the user authentication for a requested

reservation. Unauthorized reservation requests can be rejected.

Admission control will play an important role in accounting costs for

Internet resources in the future.