Section 2: The IP QoS Architecture

The Internet Engineering Task Force (IETF) has proposed many service models and mechanisms to meet the requirements of QoS. Prominent  among them are the Integrated Services/RSVP model, the Differentiated Services model, MPLS, Traffic Engineering and Constraint Based Routing. The focus here is on the two architectures developed by the IETF, the Integrated Services architecture referred to as Int-Serv and Differentiated Services architecture called as Diff-Serv.

2.1 Integrated Services:-
Int-Serv was defined in Request for Comments [RFC1633], which proposed the Resource Reservation Protocol (RSVP) ,as a working protocol for signaling in the Int-Serv architecture.Int-Serv is implemented at the edge of enterprise networks where user flows can be managed at the desktop user level. This protocol assumes that resources are reserved for every flow requiring QoS at every router hop in the path between receiver and transmitter, using end-to-end signaling and must maintain a per flow soft state at every router in the network .
Rather than only concentrating on the  real time QoS it is essential to control bandwidth sharing of a particular link among different traffic classes in the from of dividing the traffic into a few administrative classes and assigning to each a minimum percentage of the link bandwidth under conditions of overload, while allowing  the unused bandwidth to be available at other times.The above explanation is called Controlled Link-Sharing in a nutshell.

The Int-Serv model for IP QoS architecture defines three classes of service:

Using a method similar to ATM's Switched Virtual Circuits, Int-Serv uses RSVP between senders and receivers for per-flow signaling. RSVP messages traverse the network to request/reserve resources. Routers along the path, which includes the core ones should also maintain 'soft-state'.
It would be apt to explain to the reader as to what a 'soft-state' is ; a 'soft-state' is a temporary state governed by the periodic expiration of resource reservations, so that no explicit teardown request is required. Soft-States are refreshed by periodic RSVP messages.