The ns Manual (formerly ns Notes and Documentation)1 - NM Lab at ...

9.2 ImplementationThe procedures and functions described in this section can be found in ~ns/diffserv/dsred, dsredq, dsEdge, dsCore, dsPolicy.{cc,h}.9.2.1 RED queue in DiffServ moduleA DiffServ queue (in class dsREDQueue) derived from the base class Queue is implemented in DiffServ module to providethe basic DiffServ router functionality, see dsred.{h,cc}). dsREDQueue has the following abilities:• to implement multiple physical RED queues along a single link;• to implement multiple virtual queues within a physical queue, with individual set of parameters for each virtual queue;• to determine in which physical and virtual queue a packet is enqueued according to its code point;• to determine in from which physical and virtual queue a packet is dequeued according to the scheduling scheme chosen.The class dsREDQueue consists of four physical RED queues, each containing three virtual queues. The number of physicaland virtual queues are defined in numPrec and numQueues_. Each combination of physical and virtual queue number isassociated with a code point (or a drop preference), which specifies a certain level of service.The physical queue is defined in class redQueue, which enables traffic differentiation by defining virtual queues withindependent configuration and state parameters, see dsredq.{h,cc}. For example, the length of each virtual queue iscalculated only on packets mapped to that queue. Thus, packet dropping decisions can be applied based on the state andconfiguration parameters of that virtual queues. Class redQueue is not equivalent to class REDQueue, which was alreadypresent in ns. Instead, it is a modified version of RED implementation with the notion of virtual queues and is only used byclass redQueue to realize physical queues. All user interaction with class redQueue is handled through the commandinterface of class dsREDQueue.Class dsREDQueue contains a data structure known as the Per Hop Behavior (PHB) Table In DiffServ, edge routers markpackets with code points and core routers simply respond to existing code points; both of them use PHB table to map a codepoint to a particular physical and virtual queue. The PHB Table is defined as an array with three fields:struct phbParam {int codePt_; // corresponding code pointint queue_; // physical queueint prec_; // virtual queue (drop precedence)};9.2.2 Edge and core routersThe DiffServ edge and core routers are defined in class edgeQueue and class coreQueue, which are derived fromclass dsREDQueue, see dsEdge, dsCore.{h,cc}.Packet marking is implemented in class edgeQueue. A packet is marked with a code point according to the policy specifiedbefore it is put into the corresponding physical and virtual queue. Class edgeQueue has a reference to an instance ofclass PolicyClassifier, which contains policies for packet marking.88