To transport 3G/4G services through IP networks
and provide end-to-end QoS provisioning
for an IP packet, the Internet Engineering Task
Force (IETF) defines two models — integrated
services (IntServ) and differentiated services
(DiffServ). The IntServ model uses Resource
Reservation Protocol (RSVP) to signal and
reserve the desired QoS for each flow in the IP
network. Under IntServ, video chat has a very
strict guaranteed service providing firm bounds
on end-to-end delay and ensuring bandwidth for
the traffic. Although it is theoretically possible to
provide such QoS for each flow in the network,
practically it is very hard as every device along
the path of a packet needs to be fully aware of
RSVP and capable of delivering the required
QoS. The DiffServ model is relatively simple and
coarse as it groups the network flows based on
different classes, also called classes of service
(CoSs), and applies distinct QoS parameters for
each class. The type of service octet in IPv4
stores the 6-bit DiffServ code point in the IP
header to identify the CoS, whereas in IPv6 the
traffic class octet is used. Under the DiffServ
model, delay-sensitive video chat traffic falls
under the conversational CoS.