Chapter 24. Congestion Control and Quality of Service part 2
Post on 22-Feb-2016
DESCRIPTIONChapter 24. Congestion Control and Quality of Service part 2. 23.5 Quality of Service 23.6 Techniques to Improve QoS 23.7 Integrated Services 23.8 Differentiated Services. Quality of Service (QoS). Flow Characteristics: Reliability : - PowerPoint PPT Presentation
Chapter 24. Congestion Control and Quality of Service part 2
24-1Chapter 24. Congestion Control and Quality of Servicepart 223.5 Quality of Service23.6 Techniques to Improve QoS23.7 Integrated Services23.8 Differentiated Services24-2Quality of Service (QoS)Flow Characteristics:Reliability: needed by flow, Lack of reliability means losing a packet or acknowledgment, which entails retransmission.Delay: applications can tolerate delay in different degrees.Jitter: the variation in delay for packets belonging to the same flowHigh jitter means the difference between delays is large; low jitter means the variation is small. Bandwidth: Different applications need different bandwidths.
Flow Classes:Based on the characteristics, we can classify flows into groups, with each group having similar levels of characteristics24-3QoS Techniquesfour common techniques that can be used to improve the quality of service :Scheduling: A good scheduling technique treats the different flows in a fair and appropriate manner.Traffic shaping: Leaky bucket, token bucketResource reservationAdmission control: accept or reject a flow based on predefined parameters called flow specification
1. SchedulingFIFO queuing: packets wait in a buffer (queue) until the (router or switch) is ready to process them. If the average arrival rate is higher than the average processing rate, the queue will fill up and new packets will be discarded.
Priority Queuing:Packets are first assigned to priority class. Each priority class has its own queueThe packets in the highest-priority queue are processed firstStarvation may occurs
24-51. Scheduling (cont..)Weighted Fair Queuing
The queues are weighted based on the priority of the queuesThe system processes packets in each queue in a round-robin fashion with the number of packets selected from each queue based on the weight
24-62. Traffic ShapingTraffic shaping is a mechanism to control the amount and the rate of the traffic sent to the network. Two techniques can shape traffic: leaky bucket and token bucket.
First technique :Leaky Bucket algorithm shapes bursty traffic into fixed-rate traffic by averaging the data rate. It may drop the packets if the bucket is full.
24-72. Traffic Shaping (cont)
simple Leaky Bucket Implementation:A FIFO queue holds the packets. If the traffic consists of fixed-size packets the process removes a fixed number of packets from the queue at each tick of the clock. If the traffic consists of variable-length packets, the fixed output rate must be based on the number of bytes or bits.
Algorithm for variable-length packets:Initialize a counter to n at the tick of the clockIf n is greater than the size of the packet, send packet and decrement the counter by the packet size. Repeat this step until n is smaller than the packet sizeReset the counter and go to step 1
24-82. Traffic Shaping (cont)Second technique: Token BucketThe token bucket allows bursty traffic at a regulated maximum rate.The bucket holds tokens. To transmit a packet, we use one token.Allows the output rate to vary.Generate a token every r time unitsFor an arriving packet enqueueWhile buffer not empty and there are tokens send a packet and discard a token
2. Traffic Shaping (cont)
24-9arrivalqueueToken bucketsentp1 (5)-0-p2 (2)p13-p3 (1)p26-5=1p14-2-1=1p3,p246Token bucket example:parameters:MaxTokens=6 (3 token/time)Combining Token Bucket and Leaky Bucket:The two techniques can be combined to credit an idle host and at the same time regulate the traffic. The leaky bucket is applied after the token bucket the rate of the leaky bucket needs to be higher than the rate of tokens dropped in the bucket.24-10Integrated Services (IntServ)Integrated Services is a flow-based QoS model designed for IPSignaling:implement a flow-based model over a connectionless protocol Resource Reservation Protocol (RSVP)Flow specification:Rspec (resource specification) defines the resource that the flow needs to reserve (buffer, bandwidth, etc.)Tspec (traffic specification) defines the traffic characterization of the flowAdmission: a router decides to admit or deny the flow specification based on the previous commitments of the router and the current availability of the resource.Service classes: guaranteed service and controlled-load serviceGuaranteed service class: guaranteed minimum end-to-end delayControlled-load service class: accept some delays, but is sensitive to an overloaded network and to the danger of losing packets
24-11RSVPIn IntServ, the resource reservation is for a flow, a kind of virtual circuit network out of the IPRSVP is a signaling protocol to help IP create a flow and consequently make a resource reservationRSVP is a signaling system designed for multicastingReceiver-based reservationRSVP message several types of messages : Path and Resv24-12RSVP MessagesResv Messages : Make a resource reservation from each receiver to sender
Path message: from sender to all receivers. Recall that the receivers in a flow make the reservation in RSVP.
the resources are not reserved for each receiver in a flow; the reservation is merged.
Rc3 requests a 2-Mbps bandwidth and Rc2 requests a 1-Mbps bandwidth. Router R3 needs to make a bandwidth reservation, merges the twoRequests and reserve is made for 2 Mbps so it can handle both requests. The same situation is true for R2. 24-13
24-14Reservation StylesWhen there is more than one flow the router needs to make a reservation to accommodate all of them. RSVP defines three types of reservation stylesWild card filter style: a single reservation for all sendersFixed filter style: a distinct reservation for each flowShared explicit style: a single reservation which can be shared by a set of flow
Reservation information (state): soft state Reservation information stored in every node for a flow needs to be refreshed periodically. hard state used in other virtual-circuit protocols such as ATM or Frame Relay, where the information about the flow is maintained until it is erased. Default interval for refreshing is currently 30 s. Problems with Integrated ServicesScalabilityThe Integrated Services model requires that each router keep information for each flow, the Internet is growing every day, this is a serious problem.
2. Service-Type LimitationThe Integrated Services model provides only two types of services guaranteed and control-load. Those opposing this model argue that applications may need more than these two types of services.24-1524-16Differentiated Service (Diffserv)Differentiated Services is a class-based QoS model designed for IP.Diffserv handles the shortcomings of IntServ
1. The main processing was moved from the core of the network to the edge of the network. This solves the scalability problem, where routers do not have to store information about flows applications, or hosts, define the type of service need by each send a packet.
2. The per-flow service is changed to per-class service. The router routes the packet based on the class of service defined in the packet, not the flow. This solves the service-type limitation problem.
Differentiated Service (Diffserv)In Diffserv, each packet contains a field called the DS field. The value of DS field is set at the boundary of the network by the host or the first router designated as the boundary router.Ds filed replace the existing TOS (type of service) field in IPv4 or theclass field in IPv6DS field contains two subfields:DSCP (DS Code Point) is a 6-bit field that define per-hop behavior (PHB)CU (currently unused) is 2-bit
The Diffserv capable node (router) uses the DSCP 6 bits as an index to table defining the packet-handling mechanism for the current packet being processed.
24-18Per-hop Behavior (PHB)Diffserv defines three PHBs
DE PHB (default PHB) is the same as best-effort delivery
EF PHB (expedited forwarding PHB) provides the following services:Low loss, low latency, ensured bandwidth
AF PHB (assured forwarding PHB) delivers the packet with a high assurance as long as the class traffic does not exceed the traffic profile of the node24-19Traffic ConditionerMeter checks to see if the incoming flow matches the negotiated traffic profileMarker can re-mark a packet with best-effort delivery or down-mark a packet based on the meter information; no up-markShaper use the meter information to reshape the traffic if not compliant with the negotiated profile. Dropper, like a shaper with no buffer, discard packets if the flow severely violates the profile