Congestion Control and Source allocation in Computer Network, Slides of Computer Networks

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Program Name :B.Tech

Semester :ECE 6th Sem

Course Name: Computer Network

Course Code:EC 602

Facilitator Name: Megha Dewan

2 Chapter 6

Module:-

Congestion Control and Resource

Allocation

https://book.systemsapproach.org/c

ongestion.html

Chapter Outline

 Issues in Resource Allocation  Queuing Disciplines  TCP Congestion Control  Congestion Avoidance Mechanism  Quality of Service

Congestion Control and Resource Allocation  Resources  Bandwidth of the links  Buffers at the routers and switches  Packets contend at a router for the use of a link, with each contending packet placed in a queue waiting for its turn to be transmitted over the link

Congestion Control and Resource Allocation  Congestion control and Resource Allocation  Two sides of the same coin  If the network takes active role in allocating resources  The congestion may be avoided  No need for congestion control

Congestion Control and Resource Allocation  Allocating resources with any precision is difficult  Resources are distributed throughout the network  On the other hand, we can always let the sources send as much data as they want  Then recover from the congestion when it occurs  Easier approach but it can be disruptive because many packets may be discarded by the network before congestions can be controlled

Issues in Resource Allocation

 Network Model  Packet Switched Network  (^) We consider resource allocation in a packet-switched network (or internet) consisting of multiple links and switches (or routers).  (^) In such an environment, a given source may have more than enough capacity on the immediate outgoing link to send a packet, but somewhere in the middle of a network, its packets encounter a link that is being used by many different traffic sources

Issues in Resource Allocation

 Network Model  Packet Switched Network A potential bottleneck router.

Issues in Resource Allocation

 Network Model  Connectionless Flows Multiple flows passing through a set of routers

Issues in Resource Allocation

 Network Model  Connectionless Flows  (^) One of the powers of the flow abstraction is that flows can be defined at different granularities. For example, a flow can be host-to-host (i.e., have the same source/destination host addresses) or process-to-process (i.e., have the same source/destination host/port pairs).  (^) In the latter case, a flow is essentially the same as a channel, as we have been using that term throughout this book. The reason we introduce a new term is that a flow is visible to the routers inside the network, whereas a channel is an end-to-end abstraction.

Issues in Resource Allocation

 Network Model  Connectionless Flows  Soft state represents a middle ground between a purely connectionless network that maintains no state at the routers and a purely connection-oriented network that maintains hard state at the routers.  In general, the correct operation of the network does not depend on soft state being present (each packet is still routed correctly without regard to this state), but when a packet happens to belong to a flow for which the router is currently maintaining soft state, then the router is better able to handle the packet.

Issues in Resource Allocation

 Taxonomy  Router-centric versus Host-centric  (^) In a router-centric design, each router takes responsibility for deciding when packets are forwarded and selecting which packets are to dropped, as well as for informing the hosts that are generating the network traffic how many packets they are allowed to send.  In a host-centric design, the end hosts observe the network conditions (e.g., how many packets they are successfully getting through the network) and adjust their behavior accordingly.  (^) Note that these two groups are not mutually exclusive.

Issues in Resource Allocation

 Taxonomy  Window-based versus Rate-based  Window advertisement is used within the network to reserve buffer space.  Control sender’s behavior using a rate, how many bit per second the receiver or network is able to absorb.

Issues in Resource Allocation

 Evaluation Criteria  Effective Resource Allocation  (^) A good starting point for evaluating the effectiveness of a resource allocation scheme is to consider the two principal metrics of networking: throughput and delay.  (^) Clearly, we want as much throughput and as little delay as possible.  (^) Unfortunately, these goals are often somewhat at odds with each other.  (^) One sure way for a resource allocation algorithm to increase throughput is to allow as many packets into the network as possible, so as to drive the utilization of all the links up to 100%.  (^) We would do this to avoid the possibility of a link becoming idle because an idle link necessarily hurts throughput.  (^) The problem with this strategy is that increasing the number of packets in the network also increases the length of the queues at each router. Longer queues, in turn, mean packets are delayed longer in the network