CONGESTION CONTROL and RESOURCE ALLOCATION

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

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

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

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

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

B. Taxonomy 1.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

B. Taxonomy 2.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 system end 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

B. Taxonomy 3.Window-based versus Rate-based –Window-based system receiver advertises a window to the sender (window advertisement) –Rate-based system how many bit per second the receiver or the network is able to absorb Ex.: multimedia streaming application

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

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

Fair Resource Allocation –Fair means equal ? –Raj Jain’s fairness index : Flow throughput = (x 1,x 2,…, x n ) in bps

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!

Queuing Disciplines 1.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

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

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

2. Fair Queuing If F i 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/n th of the link bandwidth

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

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