2.1.2(a) Router Reference Model Components

Below is a description of each of the four components as explained in RFC 1633:

a) Packet Scheduler:- The Packet Scheduler manages the forwarding of different packet streams using a set of queues and other mechanisms like timers. It should be implemented at the point where packets are queued. A part of the packet scheduler is the Estimator.
This is actually an algorithm which is used to measure properties of the outgoing traffic stream, to develop statistics that control packet scheduling and admission control.

b) Classifier:- For the purpose of traffic control, each incoming packet must be mapped into some class; all packets in the same class get the same treatment from the packet scheduler. This mapping is done by the Classifier. A class might correspond to a wide variety of flows or all flows that are attributed to a particular organization. On the other hand a class might hold only a single flow.Like in the case of backbone routers which may choose to map many flows into a few aggregated classes, while routers near the periphery there is much less aggregation, may use a separate class for each flow.

c) Admission Control:- Admission control implements the decision algorithm that a router or host uses to determine whether a new flow can be granted the requested QoS without impacting earlier guarantees. This is invoked at each node to make a local accept/reject decision, at the time when a host requests a real-time service along some path through the Internet.
Admission Control, is sometimes confused with policing or enforcement, which is a packet-by-packet function at the edge of the network to ensure that a host does not violate it's promised traffic characteristics.It is the packet scheduler which does the policing of the packets.
In addition to ensuring that QoS guarantees are met ,admission control is also cooncerned with the enforcement of administrative policies. on resource reservations.It also has an imperative role in accounting and administration.

d) Reservation Set-up Protocol:- This is necessary to create and maintain flow-specific state in the endpoint hosts and in routers along the path of flow.A section regarding this protocol which discusses about the RSVP overview, Flowspecs and Filterspecs, Reservation styles, and Soft and Hard states are given in Section 2.3.

The given figure of the IP router given above shows the components that fit in it. The IP router has a distinct demarcation. The forwarding path below the demarcation and the background code above the line. The forwarding path of the router is executed for every packet and must therefore be highly optimized. It is divided into three sections: input driver, internet forwarder and output driver.The internet forwarder interprets the internetworking protocol header appropriate to the protocol suite. The output driver implements the packet scheduler.

The background code is simply loaded into router memory and executed by a general-purpose CPU. These background routines create data structures that control the forwarding path. The routing agent implements a particular routing protocol and builds a routing database.The reservation set-up agent implements the protocol used to set up resource reservations. If admission control gives the permission for a new request, then the appropriate changes are made to the classifier and scheduler databaseto implement the desired QoS. Also every router supports an agent for network management.The management agent must be able to modify the classifier and packet scheduler databases to set up controlled link-sharing and to set admission control policies.