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CONGESTION CONTROL and RESOURCE ALLOCATION

Learn about congestion control and resource allocation processes in networks, including definitions, network models, service models, taxonomies, and evaluation criteria.

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CONGESTION CONTROL and RESOURCE ALLOCATION

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  1. CONGESTION CONTROL andRESOURCE ALLOCATION

  2. Definition • Resource Allocation : Process by which network elements try to meet the competing demands that applications have for network resources –primarily link bandwidth and buffer space in routers- • Congestion control :Efforts made by network nodes to prevent or respond to overload conditions

  3. Definition • Involve both host and network elements such as routers • In network elements, queuing disciplines can be used to control the order in which packets get transmitted and which packets get dropped • At the end host, the congestion-control mechanism paces how fast sources are allowed to send packets  flow control

  4. Issues in Resource Allocation A. Network Model • Packet-switched network Congestion in a packet-switched network Contrast with circuit-switched network, were links are reserved for certain transmission

  5. A. Network Model • Connectionless Flow • All datagrams are certainly switched independently, but it is usually the case that a stream of datagrams between a particular pair of host flows through a particular set of routers • Soft state : state of information for each flow, information that can be used to make resource allocation decisions about the packets belong to the flow • Flow explicit implicit

  6. Network Model • Service Model • Best-effort service no guarantees for packet delivery, order delivery, and the integrity of data  unreliable service • Quantitative guarantees of QoSexample : bandwidth needed for video streaming • We will use best-effort service model for the rest of discussion

  7. B. Taxonomy • Router-centric versus Host-centric • Router centric :each router makes responsibilities for deciding (forward or drop packets) as well as informs end host how many packets which is allowed to send • Host centric :end hosts observe the network conditions & adjust their behavior accordingly

  8. B. Taxonomy • Reservation-based vs Feedback-based • Reservation-based system end host asks the network for a certain amount of capacity at the time a flow is established • Feedback-based systemend hosts begin sending data without first reserving any capacity, then their sending rate according to the feedback they receive • Explicit  i.e. congested router sends a “please slow down” message to the host • Implicit  i.e. end host adjusts its sending rate accordingly to externally observable behavior of the network such as packet losses

  9. B. Taxonomy • Window-based versus Rate-based • Window-based systemreceiver advertises a window to the sender (window advertisement) • Rate-based system how many bit per second the receiver or the network is able to absorbEx.: multimedia streaming application

  10. C. Evaluation Criteria • Effective Resource Allocation • Two principal metrics of networking:throughput and delay • As much throughput and as little delay as possibleRatio : • The objective is to maximize the ratio, which is a function of how much load placed on the network

  11. Effective Resource Allocation • Ratio of throughput to delay as a function of load

  12. Fair Resource Allocation • Fair means equal ? • Raj Jain’s fairness index : Flow throughput = (x1 ,x2,…, xn) in bps

  13. Exercise Suppose a congestion-control scheme results a collection of competing flows that achieve the following throughput rates: 100 KBps, 60 KBps, 110 KBps, 95 KBps, and 150 KBps. Calculate the fairness index for this scheme!

  14. Queuing Disciplines • FIFO or FCFS • First packet that arrives at a router is the first packet to be transmitted • Combined with tail drop policy FIFO Queuing Tail drop at a FIFO Queue

  15. Queuing Disciplines • Priority queuing :a variation of basic FIFO queuing • Idea :mark each packet with a priority, usually in ToS (Type of Service) • Routers implement multiple FIFO queues, one for each priority class • The network charge more to deliver high-priority packets than low-priority packets  economic reason

  16. 2. Fair Queuing • Solve main problem in FIFO queuing : discriminate different traffic sources • Idea : maintain separate queue for each flow currently being handled by the router and services these queues in a round-robin manner

  17. 2. Fair Queuing • If Fi denotes time when router finishes transmitting packets i (called timestamps) then the next packet to transmit is always one with the lowest timestamp • 2 important things about FQ: • Link is never left idle as long as there is at least one packet in queue, known as work-conserving • If link is fully loaded & there are n flows sending data, we can’t use more than 1/nth of the link bandwidth

  18. 2. Fair Queuing • Example of fair queuing (bit-by-bit RR) in action :

  19. 2. Fair Queuing • A variation of FQ, is Weighted Fair Queuing (WFQ) • Idea: allows a weight to be assigned to each flow • The weight logically specifies how many bits to transmit, effectively controls the percentage of link’s bandwidth of the flow

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