DiffServ Architecture: Impact of scheduling on QoS Introduction: With the rapid growth of the Internet, customers are demanding multimedia applications such as telephony and video on demand, to be available on the Internet. The greatest challenge facing the providers of these services is the provisioning of Quality of Service (QoS). The best effort service provided by the IP layer does not provide any QoS guarantees. The elaborate reliability features provided by TCP at the expense of a greater end to end delay are inappropriate for multimedia communication. The key requirements of multimedia traffic are low end to end delay and bounded jitter. There have been various architectures, proposed during the 1990 s, which have promised to meet these requirements. Integrated Services(IntServ) architecture proposed during the mid 90 s advocates the reservation of resources in advance [RFC 1633]. Each flow is supposed to reserve the needed resources before the actual transmission of information. All the routers are required to be IntServ compliant by having information about each and every flow and make separate decision for each of them. Differentiating each and every flow according to its needed resources ensures strict QoS guarantees. However, the reason IntServ has not been accepted in the Internet is its scalability problem. Typically more than 250,000 flows pass through Internet core routers and storing information about each of them is simply impractical. It was felt that it would be efficient to transfer the processing requirement from the core of the network to the edges. Differentiated Services (DiffServ) architecture proposes the differentiation of traffic on the basis of its class [RFC 2474]. Each class is composed of an aggregation of similar flows. Therefore, whereas there may be more than 250,000 flows passing through the Internet routers, they would be differentiated into 5-10 classes. This architecture is scalable but has several problems before it can be actually implemented. Service can be easily stolen in this architecture by marking packets to be of a high priority class. It also fails to meet the varying delay constraints imposed by various multimedia applications under heavy network load since there can be jitter within a traffic of certain class. However, this architecture provides a basis which could be used to provide QoS guarantees to specific applications. DiffServ Architecture: In this architecture, traffic is aggregated into classes at the edge routers and each class receives certain per hop behavior (PHB) at the core routers. The three main classes proposed in this architecture are the Expedited Forwarding (EF) class, Assured Forwarding (AF) class and the Best Effort class. The EF class is assured a minimum departure rate which is equal to at least its arrival rate. This class is suitable for applications which require very low jitter and no packet loss, such as voice. On the other hand, AF class is not guaranteed any strict bandwidth allocation but it is assured a priority in packet dropping over the Best Effort class. The Best Effort class is not provided any guarantee. The PHB of the core routers depends on the class to which the traffic belongs. The key to achieving a certain PHB is the use of appropriate scheduling mechanisms combined with buffer management schemes. This ensures that the router is able to give differential treatment to the traffic in terms of bandwidth allocation and
packet dropping. Therefore, these PHB at all routers can translate into end to end QoS guarantees. However, the extent of these guarantees is still being actively researched since it is very important for the future growth of Internet. This project is aimed at studying the various scheduling schemes available for DiffServ architecture. The two QoS parameters which are analyzed are jitter and packet loss. With the sophisticated compression schemes employed nowadays, packet loss can have significant impact on the QoS. It is due to the fact that with the help of compression, one packet contains a lot of information, the loss of which results in losing significant amount of information. Moreover, sometimes subsequent packets/frames are encoded with respect to previous packets/frames and therefore one packet drop can have impact on the subsequent packets as well. Jitter, which is the variation in the end to end delay, is another very important QoS parameter. Jitter is mainly produced due to the variable queuing delay encountered at the routers. With a fixed end to end delay, multimedia applications such as video streaming can employ a buffer and with the help of some initial buffering can start non-stop play out. However, if jitter is present and is unbounded we can not design a suitable buffering mechanism which could ensure that we are able to play non-stop videos. Therefore, a bounded jitter is essential in the provisioning of QoS. Simulation: The network simulator, OPNET IT-GURU [5] has been used for simulation purposes. It has a QoS module which allows three different scheduling schemes, 1)First Come First Serve (FCFS), 2)Priority Queuing (PQ) and 3) Weighted Fair Queuing (WFQ). Network Model:
The network model consists of two routers having three kinds of traffic sources, FTP traffic, VoIP traffic and Video Conferencing traffic. The link connecting the two routers is the bottleneck in the communication. The capacity of this link is 1.54 Mbps whereas all the other links have a capacity of 10Mbps. The Video Source and VoIP source are acting as background source traffics for VoIP and Video Conferencing. Their respective sinks are present and their statistics are not monitored. They have been included to ensure that in the presence of separate queues for each traffic type, the original traffic gets mixed with the traffic of similar type. This would ensure that the queuing delay for each packet at each hop is variable. The various parameters for each application are as follows: FTP Command Inter Request File Size Type of Service Mix(Get/Total) Time(sec) 50% Exponential(10) Constant(50000) Best Effort(0) Video Conferencing Frame Inter arrival Frame Size Type of Service Time 15 frames/sec 128X120 pixels Streaming Multimedia(4) VoIP Silence Length Talk Spurt Encode Type of Service Length Scheme Exponential (0.85) Exponential(0.325) GSM silence Interactive Voice(6) Scheduling Schemes: There are various scheduling schemes, each having its own advantages and disadvantages. In this report, simulations study of FCFS and PQ has been conducted. The results are as follows: FCFS: It is the simplest scheduling scheme to implement. All the packets leave in the same order in which they arrive. It is quite obvious that we can not provide any differential treatment to any of the packets. There is no guarantee on the number of packets dropped for each traffic flow and the delay encountered by packets at each hop. It is also unfair in the sense that flows sending the traffic at higher rates get most of the buffer and bandwidth share. On the other hand, flows of lesser bit rates are adversely affected. They get lesser buffer and bandwidth share and a greater packet loss. Therefore, well behaved sources working over TCP would be greatly affected because of congestion control mechanisms employed by TCP. All the above assertions are verified by the simulations. The graphs below show that the loss is the greatest for FTP which is the most conservative flow. Similarly, it is the least for VoIP which has an almost Constant Bit Rate traffic which is higher than the other sources.
FTP Video Conferencing VoIP Packet Loss: The difference between the Average Traffic Sent and the Average Traffic Received Another important issue in FCFS is the buffer size. We can reduce packet loss by increasing the buffer size. However, increasing the buffer size would result in a greater variation in the end to end delay since the packets originally being dropped are being stored at the end of the buffer, thus incurring a larger end to end delay. IP traffic dropped as a function of Max Queue Size Voice Traffic Received as a function of Max Queue Size Voice Packet delay variation 1) Voice Traffic Received as a function of Jitter Graph 1 summarizes the above issue by showing the trade of between the traffic received and the jitter. A lower jitter would mean a lower traffic received which is equivalent to a higher packet loss.
PQ: The above simulation for FCFS shows that it is unfair and can not provide differential treatment to the higher priority traffic. PQ can be implemented by having a separate buffer for each of the traffic. The queue having a higher priority is given preference in the scheduling decision. We only move to the lower priority queues if the higher priority queues are empty. The advantage of PQ is that QoS guarantees can be provided for the higher priority queues. However, the QoS given to the lower priority queues is very much dependent on the traffic characteristics of the higher priority queues. If there is a steady inflow of traffic in the higher priority queues, lower priority queues can be starved. For the simulation purpose, the Type of Service(ToS) field in the IP header is used to assign priority to a traffic source. As mentioned above, the highest priority is given to Voice traffic which requires strict QoS guarantees. The next priority is given to Video Conferencing traffic while the FTP traffic belongs to the Best Effort class. Video Conferencing VoIP FTP The above graphs compare the Traffic Received for the three traffic types in the two scenarios. The blue curves how the traffic received in case of PQ and the red one is for FCFS. The highest priority traffic i.e. VoIP, has an improvement in the received traffic. In fact by assigning it the highest priority we have eliminated its packets losses. The results for the second priority traffic class, Video Conferencing, show that the packet losses for this traffic class, has increased with the use of PQ. This is due to the fact that it only got an opportunity when the queue for VoIP traffic was empty. Since, there are packet losses in the second priority class, it is natural that the best effort class would not have got any service. This is shown by the traffic received for FTP traffic. It has been completely starved and no traffic was received. Implementing PQ greatly reduces jitter for higher priority queues. It is due to the fact that a packet only has to wait behind the packets of the same traffic type. In FCFS, since there is only one queue, a packet has to wait behind a greater number of packets belonging to different traffic types. This greatly increases the variability in the delay incurred at each hop. The graphs below compare the jitter for Voice and Video Conferencing Traffic in the two scenarios.
VoIP jitter : FCFS PQ Video Conferencing jitter: FCFS PQ The worst case jitter for VoIP in case of FCFS was approximately 2.0 seconds while in PQ it is almost 0.0000005 seconds. Similarly, the worst case jitter for Video Conferencing in case of FCFS is 1 second while in PQ it is 0.004 seconds (Note:There is a difference in the scale for FCFS and PQ graphs) Conclusion: The above simulations show that in order to provide any kind of differentiation, we have to use some scheme other than FCFS. The use of PQ has shown an improvement in packet loss and jitter for the highest priority class. However, we can not provide similar guarantees to the lower priority queues. This makes PQ suitable for servicing Expedited Forwarding class which requires strict guarantees. However, there is a possibility of starvation of traffic belonging to Assured Forwarding and Best Effort classes. Future Work: Despite its advantages, PQ can lead to starvation and is thus not fair. The next scheduling scheme which would be considered is the Weighted Fair Queuing(WFQ). A comparison of WFQ with PQ and FCFS would be done through simulation. Moreover, there are other scheduling schemes, which have been proposed to provide QoS guarantees in a DiffServ. Schemes such as [1] have proposed the use of combination of PQ and WFQ. Apart from these simple and traditional schemes, there have been
proposals such as [2,3,4], which have advocated for schemes such as Weighted Earliest Due Date (WEDD), Adaptive Priority (AP), Worst Cast Fair Waited Fair Queuing (WF 2 Q) respectively. All the above mentioned schemes would be reviewed in the final report. References: [1] F. Tobagi, W. Noureddine, B. Chen, A. Markopoulou, C. Fraleigh, M. Karam, JM Pulido, J. Kimura, "Service Differentiation in the Internet to Support Multimedia Traffic", in Springer Verlag LNCS, Vol. 2170, September 2001. [2] Bodamer, S.*: A new scheduling mechanism to provide relative differentiation for real-time IP traffic, Proceedings of IEEE GLOBECOM 2000, San Francisco, 2000, S. 646-650. [3] N.Natchimuthu, J.Khan, Adaptive Traffic Management Algorithm for IP-Based DiffServ WAN, ATNAC 2003. [4] Rosario Giuseppe Garroppo, Stefano Giordano, Saverio Niccolini, Franco Russo: DiffServ Aggregation Strategies of Real Time Services in a WFQ+ Schedulers Network, IWDC 2001: 481-491 [5] OPNET Technologies, Inc. http://www.opnet.com/