Dynamic Quality of Service on IP Networks

Tippyarat Tansupasiri and Kanchana Kanchanasut

Computer Science and Information Management ProgramAsian Institute of TechnologyPathumthani, 12120, Thailand{fon, kk}@cs.ait.ac.th

Abstract. In this paper, we propose a dynamic QoS, D-QoS, modelwhich allows the QoS requirements be reconfigured dynamically. A priv-ilege user can request a network interruption to guarantee its own smoothtraffic flow at the expense of possibly interruptions or blockages of othertraffic flows. Based on the concept of active IP network, an interruptioncan be triggered by sending an active packet requesting for an interrup-tion to the network. Under normal circumstances, our model providesDiffServ based on CBQ and, in case of an interruption, priority queue isemployed. The two queuing mechanisms operate alternately in responseto the programs sent via active packets. A prototype of the model hasbeen successfully implemented where the two operation modes have beenshown to operate satisfactorily. Details of the design and implementationof D-QoS and its evaluation are discussed in this paper.

1 Introduction

Since the introduction of time sensitive applications on the Internet, the needfor quality of service (QoS) has been raised and focused on. Among the proposedQoS models, one which can scale well and is suitable as the core network QoSmodel is Differentiated Services (DiffServ) model [1], initiated by the IETF’sDifferentiated Services Charter [2]. It provides a simple and coarse way to offerdifferentiated classes of service on flow aggregates, where the complexity onpacket classification marking, policing and shaping functions are needed only atnetwork boundaries. Packets are classified into predefined classes based on thevalues, called codepoints, specified in their Differentiated Services (DS) fields ofthe IP headers [3]. Different traffic classes may receive different packet forwardingtreatments on DiffServ routers.

In the case of internetworking among a number of network domains with dif-ferent service policies, any adjacent domains should establish the service agree-ments, known as Service Level Agreement (SLA), to provide the same level ofservice for a particular codepoint. However, DiffServ model itself operates onthe static predefined SLAs in which human intervention for reconfiguration isneeded when there are changes on the predefined QoS. As Internet traffic couldbe randomly generated at any time and possibly from anywhere, these recon-figurations are definitely necessary. For example, one may wish to send bulky

telemedicine traffic which requires high precision of data transmission over thenet during an emergency situation.

This paper proposes a dynamic QoS system based on the concept of activenetwork. This concept was introduced in 1994 by Defense Advanced ResearchProjects (DARPA) research community as a future direction of networking sys-tem [4]. It replaces normal packets with active packets containing small programsand possibly data. Network nodes are substituted by active nodes capable of per-forming user- or application-specific functions on packets passing through them.For the IP networks, Murphy [5] has proposed an active IP network that allowsthe IP network to perform customized computations on user data, as additionalprocessing to the traditional IP forwarding mechanism. His work has demon-strated how to construct an active IP node, and how to seamlessly integratean active IP node into IP network. Hence, both normal IP nodes and active IPnodes can cooperate and coexist in a single IP network.

As the feasibility of the active network implementation on IP networks hasbeen shown, a model that provides dynamic QoS based on this concept can alsobe constructed. With our proposed QoS model, Dynamic Quality of Service (D-QoS), the IP network could be reconfigured without human intervention. It aimsto provide dynamic QoS upon receiving the interruption requests from autho-rized users to get highest possible network bandwidth. However, our prototypeimplementation does not fully implement an active IP node, but only emulatesthe active IP node with restricted features related to providing dynamic QoS.

Details of the D-QoS model and its implementation are explained in Section 2.Section 3 provides the experimental results from several tests based on differentnetwork scenarios. Related works to this paper are discussed in Section 4. Finally,the last section presents the discussion and conclusion.

2 Dynamic Quality of Service (D-QoS)

Unlike DiffServ in which a predefined portion of the bandwidth is shared withineach traffic class, the proposed model is a reconfigurable system that allowsmulti-level interruptions upon the requests of prioritized traffic flows. We referto the prioritized flow as a super user flow which has all the privilege over otherflows, in the same manner as an operating system super user. Default QoS setup of the system is configured as DiffServ service. A super user can send arequest for a specific interruption level prior to the actual flow transmission.Upon receiving a request, the interruptions are generated on all the nodes alongthe communication path and the normal DiffServ is then suspended where thenetwork output queuing mechanism on each node is automatically reconfigured.This allows high priority telemedicine traffic to flow through with the highestpossible bandwidth at the expense of other lower priority traffic. Normal DiffServwill resume after the prioritized flow has completed. D-QoS allows multi-level ofinterruptions where a number of super users may request for the interruptionsat the same time. All interruption requests are kept in a stack structure whereprevious QoS configuration is resumed after an interruption has finished. The D-

QoS also requires that the active IP network consists of a contiguous set of activeIP nodes, which is not the case in [5], otherwise, unexpected result may occursince the normal IP nodes do not provide the requested QoS to the prioritizedflow.

2.1 An Implementation of D-QoS

A prototype of D-QoS consisting of three D-QoS nodes has been implemented todemonstrate the functioning of the D-QoS model, as shown in Fig. 1. Each D-QoSnode is a Pentium 100 MHz PC running FreeBSD. It emulates an active IP nodewith restricted primitives for the queuing manipulation related to interruptionhandling, which are primitives for making and removing an interruption. Activepackets carrying programs are transmitted through an opened UDP socket wheretheir contents are examined and the appropriate actions are performed. Theseprogram packets contain the information used to identify a particular super userflow and a required interruption level as the parameters for each primitive. Aspecific flow is identified by the source and destination addresses, source anddestination port numbers and protocol. Data packets are represented as normalIP packets carrying the flow payload.

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Fig. 1. The D-QoS Prototype

Upon receiving an interruption request packet, the program carried in thepacket’s content is executed. This program periodically forwards the requestpacket to the next neighbor towards the destination until a confirmation messageis received on time out. Hence, only one program packet is required to trigger allthe nodes along the path. At the end of the flow transmission, another programpacket is transmitted to remove the interruptions and recover the nodes back tothe previous state according to the information in the stacks.

Since traditional FreeBSD provides only FIFO, other queuing mechanisms forour QoS implementation are presented by Alternate Queuing (ALTQ) which wasintroduced by Cho [6]. In normal circumstances, the system provides DiffServservice with Assured Forwarding (AF) and Expedited Forwarding (EF) as spec-ified in RFC 2597 [7] and RFC 3246 [8] respectively. DiffServ implementationoperates on a hierarchical structure of Class-Based Queuing (CBQ) proposedby Floyd and Jacobson [9]. CBQ is a hierarchical structure of classes used torepresent the bandwidth partitioning and sharing among classes. Each class has

its own queue with a predefined bandwidth partition. In this case, the CBQstructure for DiffServ consists of four AF classes and one EF class, as illustratedin Fig. 2. Each of the four AF classes applies an extended version of RandomEarly Detection with In and Out (RIO) to provide three drop precedence levels.An AF class may receive excess bandwidth if available up to 100% of the totalbandwidth, while the EF class has a limitation on its maximum bandwidth ac-quired at 75% of the total bandwidth. As EF class requires low-loss, low-delayand low-jitter, this class is configured to have higher priority. Unknown traffic isclassified into AF3 Class.

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Fig. 2. Class-Based Queuing (CBQ) Structure for DiffServ Implementation on D-QoSPrototype

When a node receives an interruption request, this CBQ structure for Diff-Serv is automatically substituted by the Priority Queue (PQ). The requestedinterruption level is examined, if that level is not available, a lower priority isgiven to that requesting flow. Our PQ offers 16 interruption levels ranging from0-15. When changing the queuing mechanism, the queue content of the previousmechanism is discarded. The super user flow is given the specified priority level,while other traffic flows are treated as having lowest priority level (priority 0).Fig. 3(a) demonstrates the network output queuing structure of CBQ for Diff-Serv. The queuing structure of PQ in Fig. 3(b) illustrates a particular case whenthere are two interruptions by super user A, with the highest priority level, andsuper user B, with lower priority level, at the same time.

3 Experimental Results

Our focus is mainly on the case where the available bandwidth is not largeenough for the total bandwidth requirements of flows. Hence, the two Ethernetlinks connecting the D-QoS nodes have been configured to have different speeds,100 Mbps and 10 Mbps. Each traffic flow was generated, by a traffic generatorprogram called Iperf [10], as a unidirectional UDP flow from node A to node C.The middle node, Node B, acted as the bottleneck where all flows competed forthe bandwidth. ALTQ was enabled on node B’s interface connecting to node Cto provide appropriate queuing mechanisms on traffic passing through it.

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Fig. 3. (a) Queuing Structure for DiffServ in D-QoS (b) Queuing Structure for Multi-level Interruption in D-QoS

The experiments aimed to measure the transmission quality of a particulartelesurgery flow, as an example of telemedicine flows, which needs high precisionof data transmission and also has high bandwidth requirement. Similar to thetransmission of other time sensitive streams where the transmission rate is pre-ferred over reliability, the flow was transmitted over UDP rather than TCP. Theflow was presented as a constant rate UDP stream of size 6 Mbps which con-sumed 60% of the 10 Mbps link. It has been found that when generating a trafficflow with 6 Mbps bandwidth, the bandwidth achievement result may be up to6.3 Mbps as there are overheads from packet headers. The maximum bandwidththat could be achieved from the 10 Mbps link is 9.6 Mbps. Each experimentalresult is provided as a bandwidth achievement graph.

To validate the correctness of our prototype implementation, two experi-ments were conducted with two different settings, the traditional best-effort IPnetwork and the DiffServ service in D-QoS. In both experiments, two traffic flowsrepresenting the real-time traffic and the default Internet traffic were first gen-erated, with the same size of 6 Mbps for 40 seconds. After the first 10 seconds,the telesurgery flow was generated and lasted for 20 seconds.

From Fig. 4(a) which is the result obtained from the traditional best-effort IPnetwork, during the first 10 seconds, the real-time traffic and the default Internettraffic flows acquired equal bandwidth of about 4.7 Mbps in average. In the next20 seconds where three traffic flows were generated, total link bandwidth wasequally shared among them, each flow gained 3.3 Mbps in average.

Experimental result from DiffServ service based on the CBQ structure inFig. 2 is shown in Fig. 4(b). The real-time traffic flow was classified into EFclass with higher priority compared to the default Internet traffic flow placedin AF3 class. During the first 10 seconds, the real-time traffic flow gained therequired bandwidth of 6.2 Mbps while the default Internet traffic flow got 3.3Mbps bandwidth. Then, in the period where three traffic flows were generated,75% of link bandwidth or 7.5 Mbps assigned to EF class was shared by both the

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Fig. 4. (a) Result from Best-Effort IP Network (b) Result from DiffServ Service

telesurgery and the real-time traffic flows, while the default Internet traffic flowobtained the rest available bandwidth of 2.3 Mbps. The packet loss percentagesexperienced by each flow in this period were 42% for the telesurgery traffic flow,44% for the real-time traffic flow and 63% for the default Internet traffic flow.In the last 10 seconds of the experiment, there was a period where the real-timetraffic flow obtained about 7.2 Mbps which was higher than the generated rateand nearly reached the maximum bandwidth limitation for EF class. The reasonis because there were some packets left in the buffer queue during the congestiontime and they were forwarded out when the bandwidth became available.

3.1 Single Interruption

Fig. 5 demonstrates the bandwidth achievement results in the case where therewas an interruption of a super user flow. Two experiments were conducted withdifferent sizes of telesurgery flows, 6 Mbps and 10 Mbps. Other two flows weregenerated in the same manner as the previous experiments. Fig. 5(a) shows theresult when the telesurgery flow was 6 Mbps and Fig. 5(b) is the case whenit was 10 Mbps. The results in the first 10 seconds were based on DiffServservice and yielded the same result as that shown in Fig. 4(b). Then, at the timewhen the telesurgery traffic flow was generated, an interruption on the outputqueuing mechanism was created and normal DiffServ service with CBQ wasautomatically substituted by PQ providing two priority levels. The telesurgeryflow was treated as the higher priority flow, other flows were classified into thelowest priority class. When the telesurgery flow completed, normal DiffServ onCBQ was resumed.

During the interruption period, the telesurgery traffic flow acquired 6.3 Mbpswith no packet losses. This is obviously different from the case of DiffServ servicewhere it experienced 42% of packet loss percentage. The rest of the availablelink bandwidth was shared by both the real-time and the default traffic flowswith equal bandwidth of 1.6 Mbps with packet loss percentages of 62.3% and59.8% respectively. The graph on the last 10 seconds shows the result when the

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Fig. 5. (a) Result from Single Interruption of Telesurgery Flow of Size 6 Mbps (b)Result from Single Interruption of Telesurgery Flow of Size 10 Mbps

interruption of the telesurgery flow was completed and normal DiffServ resumed,which was the same as in the first 10 seconds.

It is also possible that the interrupting flow may solely occupy the whole linkbandwidth, as shown by the experimental result in Fig. 5(b). The telesurgeryflow was generated with bandwidth of 10 Mbps. During the interruption period,the telesurgery flow automatically obtained 9.6 Mbps which was the maximumbandwidth that could be achieved from the link while other flows got less than 0.1Mbps. However, even the telesurgery traffic flow had higher priority, its packetloss percentage was 2.5%, due to the limitation on link bandwidth, while theother two flows had 99.6% and 98.7% of packet loss. Comparing to the resultobtained in case of the DiffServ service, the interruption mechanism provided inD-QoS has shown that the telesurgery flow could be delivered with much betterquality based on the available bandwidth of links.

3.2 Multi-level Interruption

This section discusses on the experimental result obtained from the D-QoS pro-totype when there were a number of interruptions with different priority levels.There were two interruption requests from super user flow Y and Z, with thepriority of 10 and 15 respectively, on an existing flow X. All flows had the samesize of 6 Mbps. The total time of the experiment was divided into periods of10 seconds each. In the first period, there was only one flow, flow X, and thenetwork was based on DiffServ service with CBQ. This CBQ was then changedto PQ with two priority levels when an interruption of flow Y was received. FlowY, which would last for 40 seconds, was given higher priority and got the band-width of 6.2 Mbps, while flow X only got 3.4 Mbps. When another super userflow, flow Z, was generated with an interruption level 15, the previous PQ withtwo priority levels was substituted by a PQ providing three priority levels. FlowZ received 6.2 Mbps of bandwidth during its transmission of 20 seconds. Flow Ywith lower priority received 3.3 Mbps. Flow X received less than 0.1 Mbps as ithad the lowest priority. Upon completion of flow Z, the PQ resumed with onlytwo priority levels. Finally, normal DiffServ was resumed.

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Fig. 6. Result from Multi-level Interruptions

To validate that we can switch our priority settings upon each interruption,we ran an experiment with multi-level interruptions whose levels changed dy-namically during the transmission. Three flows were generated with the samesize of 6 Mbps and their priority levels within each period are shown in Table 1.

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Flow X 5 5 10 10 10 15 5 5Flow Y 10 10 5 10 15 15 15 10Flow Z - 15 15 15 15 10 10 -

Table 1. Priority Level in Different Time Periods

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Fig. 7. Result from Dynamically Priority Changing

The result, illustrated in Fig. 7, has shown that the prototype correctly re-acted to different levels of the interruption requests. The bandwidth acquiredby each flow depended on its priority at that time. The first highest priorityflow got 6.2 Mbps while the rest of the link available bandwidth, 3.3 Mbps, wasgiven to the second highest priority flow and the lowest priority flow got lessthan 0.1 Mbps. In the case where two flows had the same priority, they shared

the available bandwidth equally based on their priorities compared to anotherflow; 10 Mbps bandwidth was shared if they both had the highest priority, or theremaining bandwidth of 3.4 Mbps was shared if they both had lower priority.

4 Related Works

Dynamic QoS may also be accomplished with another technique, i.e., with theinteroperation of Integrated Service (IntServ) based on Resource ReservationProtocol (RSVP) [11] and DiffServ as presented in RFC 2998 [12] and Detti etal. [13]. The main idea of those works is to have an additional network func-tion that acts as admission control agent or bandwidth broker within DiffServnetworks that monitors the resource usage within DiffServ networks and com-municates with other IntServ nodes outside DiffServ networks. However, theinteroperation requires that DiffServ network must be able to provide supportfor the standard IntServ services and that the admission control informationmust be provided.

The idea of giving privilege to some important flows in our system is alsosimilar to the work proposed by Quadros et al. [14]. Nevertheless, their workis based on another technique where a DiffServ packet handling mechanism,called Dynamic Degradation Distribution (D3), was introduced. D3 dynamicallydistributes network resources, transmission capacity and memory, to DiffServclasses according to their loss and delay sensitivities. In case of congestion, moresensitive classes experience less performance degradation. On the other hand,in D-QoS, the flows are transmitted according to their priorities. Therefore, thesuper user flow with the highest priority receives the best network service.

5 Discussion and Conclusion

The proposed D-QoS model has been successfully implemented and evaluatedthrough our D-QoS prototype which offers dynamic QoS reconfiguration uponthe receiving the authorized requests. The DiffServ service with CBQ is dy-namically substituted by PQ when an interruption request is received. Availablenetwork bandwidth is always given to the privilege flow such as telemedicinetraffic flow. Telemedicine traffic flow which needs high precision of data trans-mission is transmitted with the best service the network can offer. Comparedto the static QoS configuration of DiffServ model, the super user flow can gainmore bandwidth as required based on the network bandwidth capacity. Thisalso leads to the dramatically reduction of packet loss percentages. In the casewhere the prioritized traffic flow needs the bandwidth of equal or more than thenetwork link capacity, that link is then dedicated to the flow and other traffic iscompletely discarded. However, the privilege traffic may experience some packetlosses in this case.

Additionally, in the case of multi-level interruption where super users mayrequest for interruptions at different interruption levels at the same time, the pro-totype can dynamically switch its queuing mechanisms according to the requests

and correctly services the flows according to their priorities. Upon the completionof each interruption, the previous QoS state is automatically resumed.

Even though the experiments were performed on a small D-QoS networkwith only three machines, it is sufficient to demonstrate the transmission qualityexperienced by a number of flows through a network bottleneck. More networknodes may be added to the prototype, but it would not impact the experimentalresults. However, our future work is to evaluate the D-QoS model by networksimulation software (ns-2). With ns-2, the model can be applied to more com-plicated networks with a larger number of nodes, various types of network linksand variety of network traffic patterns which is closer to the real Internet char-acteristics, where the scalability issue of D-QoS can be examined.

References

1. Blake, S., Black, D., Carlson, M., Davies, E., Wang, Z. and W. Weiss, ‘An Archi-tecture for Differentiated Services’, RFC 2475, December 1998.

2. IETF, Differentiated Services (DiffServ) Charter,http://www.ietf.org/html.charters/diffserv-charter.html

3. Nichols, K., Blake, S., Baker, F. and D. Black, ‘Definition of the DifferentiatedServices Field (DS Field) in the IPv4 and IPv6 Headers’, RFC 2474, December1998.

4. Tennenhouse, D. L., Smith, J. M., Sincoskie, W. D., Wetherall, D. J. and G. J.Minden, ‘A Survey of Active Network Research’, IEEE Communications Magazine,1997.

5. Murphy, D. M., ‘Building an Active Node on the Internet’, Master’s thesis, MIT,1997.

6. Cho, K., ‘A Framework for Alternate Queueing: Towards Traffic Management byPC-UNIX Based Routers’, Proceedings of USENIX 1998 Annual Technical Con-ference, 1998.

7. Heinanen, J., Baker, F., Weiss, W. and J. Wroclawski, ‘Assured Forwarding PHBGroup’, RFC 2597, June 1999.

8. Davie, B., Charny, A., Bennett, J. C. R., Benson, K., Le Boudec, J. Y., Courtney,W., Davari, S., Firoiu, V. and D. Stiliadis, ‘An Expedited Forwarding PHB (Per-Hop Behavior)’, RFC 3246, March 2002.

9. Floyd, S. and V. Jacobson, ‘Link-Sharing and Resource Management Models forPacket Network’, IEEE/ACM Transactions on Networking, Vol. 3 No. 4, August1995.

10. Tirumala, A., Qin, F., Dugan, J. and J. Ferguson, ‘Iperf’, May 2002,http://dast.nlanr.net/Projects/Iperf/

11. Braden, B., Zhand, L., Berson, S., Herzog, S. and S. Jamin, ‘Resource ReservationProtocol (RSVP) - Version 1 Functional Specification’, RFC 2205, September 1997.

12. Bernet, Y., Yavatkar, R., Baker, F., Zhang, L., Speer, M., Braden, R., Davie, B.,Wroclawski, J. and Felstaine E., FRC 2998, ‘A Framework for Integrated ServicesOperation over Diffserv Networks’, November 2000.

13. Detti, A., Listanti, M., Salsano, S. and L. Veltri, ‘Supporting RSVP in a Differ-entiated Service Domain: an Architectural Framework and a Scalability Analysis’,ICC’99, June 1999.

14. Quadros, G., Alves, A., Monteiro, E. and F. Boavida, ‘An Approach to SupportTraffic Classes in IP Networks’, QoFIS’2000, Berlin, Germany, September 2000.