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Computer Networks/Networks




                    Notes          10.2.3 Congestion Prevention Policies

                                   Open loop systems are designed to minimize the congestion at the place of its origin. Applying
                                   congestion prevention policies at different layers solves the  problem  in case  of open  loop
                                   systems. The policies at data link, network and transport layers that affects congestion is given
                                   below:

                                   Data  link  layer:  The  issues  such  as  retransmission  of  packets,  out  of  order  caching,
                                   acknowledgement of the received packets from destination machine and flow control  affect
                                   congestion at this layer.

                                   Network layer: Setting up of virtual channel and datagram inside the subnet, packet queueing
                                   and forwarding at router, dropping of packets at router, routing algorithms, packet lifetime
                                   management, etc are the factors affecting congestion at this layer.
                                   Transport layer:  Retransmission of packets,  out of  order caching, acknowledgement of  the
                                   received packets from  destination machine, flow control mechanism, time out packets,  etc.
                                   affect congestion at this layer.

                                   Traffic Shaping

                                   One of the main reasons for congestion is the bursty traffic. Another cause is the transmission of
                                   packets at an unpredictable rate. Hence, the traffic shaping approach includes transmission of
                                   packets at uniform rate and in more predictable rate in case of open loop method. Thus, the
                                   traffic shaping attempts to regularize the average rate of data transmission. For example, the
                                   ATM networks exploit this method to a greater extent.  To reduce congestion, the user and the
                                   subnet agree on a certain traffic pattern in a virtual circuit. Such agreements are of great importance
                                   for transfer  of  real time  audio  and video connections,  which do  not tolerate  congestion.
                                   Monitoring traffic pattern is called traffic policing.
                                   Leaky Bucket


                                   The leaky bucket algorithm finds its use in the context of network traffic shaping or rate limiting.
                                   The algorithm enables to  control the rate at which data is injected into a network and  thus
                                   handling  burstiness in  the data  rate.  A  leaky bucket  implementation and  a token  bucket
                                   implementation  are predominantly  used for traffic shaping  algorithms.  The leaky-bucket
                                   algorithm is used to control the rate at which traffic is sent to the network and shape the bursty
                                   traffic to a steady stream of traffic. Figure 10.3 shows the leaky bucket algorithm.


                                                          Figure  10.3: Leaky  Bucket  Algorithm

                                                            Incoming packets












                                                                         Outgoing packets





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