TITRE Adaptive Packet Video Streaming Over IP Networks - LaBRI

epresent user traffic. The delay can be bounded using an envelope curve calculated for each
network element. Thus, the transfer delay of a particular packet is computed from the time it enters
the network until it leaves a particular network element [125]. Furthermore, policy is done at the
edge router of the network to check weather a particular flow is conforming to TSpec or not. When
non-conforming packet arrives, they receive best-effort service. Appropriate scheduling
mechanisms, such as WFQ (Weighted Fair Queuing), are used each network element.
Intuitively, the most straightforward method for assuring service guarantees to packet video
applications is to reserve resources according to an application’s request. However, reservation
based schemes involve admission control algorithms, reservation protocols such as RSVP,
monitoring protocols to track the change in network conditions, and signaling mechanisms. These
kinds of schemes are generally complex and require extensive network support. This can lead to
network scalability and robustness problems.
3.2.4.3 Differentiated Services
The Differentiated Service Architecture (Diffserv) [126] has been motivated by the market
need for immediate deployment of a QoS solution for IP networks. In contrast to the per-flow
orientation of RSVP, Diffserv networks classifies packets into one of a small number of aggregated
classes, based on the DSCP (Diffserv Code Point) in the packet’s IP header [127]. At each Diffserv
router, packets are subjected to a PHB (Per-Hop Behavior), which is invoked by the DSCP. A PHB
is defined to provide a specific forwarding behavior at each router within the Diffserv domain.
Two PHB schemas known as EF (Expedited Forwarding) [128] and AF (Assured Forwarding)
[129] has been proposed to provide differential treatment of traffic. The EF PHB can be used to
build a low loss, low delay, low jitter, assured bandwidth, and end-to-end service through Diffserv
domains, while AF PHB group provides different levels of forwarding assurance for IP packets by
discarding more low priority packets during times of congestion than high priority packets. AF
PHB provides four IP packet forwarding classes. Within each AF class, an IP packet is assigned one
of the three different levels of drop precedence (i.e. a total of twelve code points).
Diffserv architecture defines, at a lower level, four types of data-path elements: traffic
classifiers, actions elements, meters and queuing elements as shown in Figure 3-11.
Meter
Input traffic
Classifier Marker Scheduler
Output traffic
Figure 3-11: IP differentiated service components
• The Classifier performs the classification of traffic based on multiples fields in the IP
packet such as source / destination address, source / destination port, Type of Service
(TOS) field, etc.
• The Meter measures the amount of traffic received of a particular flows. It checks
weather the traffic is conforming to the traffic specification or not. It also apply some
policy on confirming and non-conforming traffic.
56