Quality of service for mobile ad hoc networks

Transcription

1 Research Collection Master Thesis Quality of service for mobile ad hoc networks Author(s): Stuedi, Patrick Publication Date: 2003 Permanent Link: Rights / License: In Copyright - Non-Commercial Use Permitted This page was generated automatically upon download from the ETH Zurich Research Collection. For more information please consult the Terms of use. ETH Library

2 Quality of Service for Mobile Ad Hoc Networks Diploma Thesis of Patrick Stüdi Assistant: Jianbo Xue Supervisor: Prof. Dr. Gustavo Alonso March 2003

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4 Abstract The fast adaptation of IP-based communications for mobile and hand-held devices equipped with wireless interfaces is creating a new challenge for Quality of Service (QoS) provision. Due the error-prone nature of wireless links and the high mobility of mobile devices, traditional Internet QoS protocols like RSVP cannot be easily migrated to the wireless environment. This is specially true for Mobile Ad Hoc Networks (MANETs) where every node moves arbitrarily causing the multi-hop network topology to change randomly and at unpredictable times. In this thesis a new framework coping with the specific issues of MANETs is proposed. The framework includes a QoS signaling protocol and flexible resource allocation and adaptation mechanisms. In order to prove its efficiency the system is implemented and simulated using the ns-2 network simulator. Keywords: MANET, QoS, In-band Signaling, Adaptation, Resource Reservation, ASAP

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6 Contents 1 Introduction Problem Statement Mobile Ad Hoc Networks Quality of Service Outline QoS Models for MANETs QoS Models IntServ DiffServ IntServ over DiffServ Quality of Service in Ac Hoc Networks Special Issues and Difficulties in MANETS Drawbacks of the different QoS Models Conclusion Protocol Design Issues Towards developing a QoS Framework for MANETs QoS from a Layered Perspective QoS-Signaling and Routing Interaction QoS-Signaling: Design Issues In-band versus Out-of-band Signaling Reservation Mechanism: One-pass versus Two-pass Soft-state versus Hard-state Local Repair QoS Adaptation Application Requirements Adaptation Strategies Monitoring Interval and Soft-state Timer

7 vi CONTENTS 3.6 Conclusion Existing Technologies RSVP RSVP Extensions FQMM INSIGNIA Some further Approaches imaq INORA ASAP Framework Concepts Soft/Hard Reservation Soft-State Management Adaptive QoS Monitoring Signaling System QoS Table Message Types Setup Procedure QoS Monitoring Implementing ASAP using IPv IPv6 in a Nutshell IPv6 Header Format Using IPv6 for ASAP Signaling Ad Hoc Extensions for ASAP Problems of ASAP in MANETs Flow Restoration Reverse Path Problem Lost Hard-Reservation Messages Extensions QoS Option Field for Soft-Reservation Message Local Repair Dynamic Virtual Path Adaptation Conclusion

8 CONTENTS vii 7 Implementation Overview NS Nodes Packets Agents Implementation Requirements Main Building Blocks QoS Management Unit Overview Internal Structure Message Serialization/Deserialization Reservation Processing Adaptation Control Unit Application QoS Request Unit Some further Components Queuing Measurements Logging Node Interface Conclusion Simulation and Analysis Overview Simulation Framework Flow Objects Measurements A Remark in Advance Evaluation Local Repair Adaptation QoS Performance Reservation Efficiency Signaling Overhead Conclusion Summary and Outlook 53

9 viii CONTENTS 9.1 Summary Future Work Bibliography 56

10 Chapter 1 Introduction 1.1 Problem Statement The introduction of real-time audio, video and data services into wireless networks presents a number of technical obstacles to overcome. Traditional internet QoS protocols like RSVP cannot be easily migrated to the wireless environment due to the error-prone nature of wireless links and the high mobility of mobile devices. This is specially true for Mobile Ad Hoc Networks (MANETs) where every node moves arbitrarily causing the multi-hop network topology to change randomly and at unpredictable times. In this thesis an existing QoS framework is extended to be suitable for MANETs. In order to prove its correctness and efficiency the system is implemented and simulated using the ns-2 network simulator. 1.2 Mobile Ad Hoc Networks A mobile ad hoc network is a concept that has received attention in scientific research since the 1970s. A clear picture of what exactly is meant by an ad hoc network is difficult to pinpoint. In today s literature the term is used in many different ways. The Internet Engineering Task Force (IETF), the body responsible for guiding the evolution of the Internet, provides the definition as given below [23]: A mobile ad hoc network (MANET) is an autonomous system of mobile routers (and associated hosts) connected by wireless links. The routers are free to move randomly and organize themselves arbitrarily; thus, the network s wireless topology may change rapidly and unpredictably. Such a network may operate in a stand-alone fashion, or may be connected to the larger Internet MANETs are useful in many applications because they do not need any infrastructure support. Collaborative computing and communications in smaller areas (building organizations, conferences, etc.) can be set up using MANETS. Communications in battlefields and disaster recovery areas are further examples of application environments. With the evolution of Multimedia Technology, Quality of Service in MANETs became an area of great interest. Besides the problems that exist for QoS in wire-based networks, MANETS impose new constraints. This is due the dynamic behaviour and the limited resources of such networks.

11 2 Chapter 1. Introduction 1.3 Quality of Service QoS is usually defined as a set of service requirements that needs to be met by the network while transporting a packet stream from a source to its destination. The network needs are governed by the service requirements of end user applications. The network is expected to guarantee a set of measurable prespecified service attributes to the users in terms of end-to-end performance, such as delay, bandwidth, probability of packet loss, delay variance (jitter), etc. Power consumption is another QoS attribute which is more specific to MANETs. In the literature, the research on QoS support in MANETs spans over all the layers in the network: ffl QoS models specify an architecture in which some kinds of services could be provided. It is the system goal that has to be implemented. ffl QoS Adaptation hides all environment-related features from awareness of the multimedia-application above and provides an interface for applications to interact with QoS control. ffl Above the network layer QoS signaling acts as a control center in QoS support. The functionality of QoS signaling is determined by the QoS model. ffl QoS routing is part of the network layer and searches for a path with enough resources but does not reserve resources. ffl QoS MAC protocols are essential components in QoS for MANETs. QoS supporting components at upper layers, such as QoS signaling or QoS routing assume the existence of a MAC protocol, which solves the problems of medium contention, supports reliable communication, and provides resource reservation. This document does not treat QoS MAC and QoS routing any further and instead focuses on upper layers like QoS models and signaling. 1.4 Outline After analysing existing QoS models with respect to the dynamic behaviour of ad hoc networks in chapter 2 this document proceeds presenting some of the fundamental design issues to be considered when developing a QoS framework for MANETs. Chapter 4 focuses on some existing approaches classifying each one according to the design issues identified in chapter 3. ASAP is a QoS framework developed at the University of Stuttgart and was originally designed to support QoS in last-hop-wireless networks and is proposed in chapter 5. The ad hoc extensions to ASAP, the implementation for ns-2 and its simulation are discussed in chapter 6, 7 and 8. Finally this document concludes with a summary and gives an outlook on potential further work.

12 Chapter 2 QoS Models for MANETs 2.1 QoS Models Today s Internet applies best effort (BE) IP forwarding. The network attempts to deliver all traffic as soon as possible within the limits of its abilities, but without guarantees related to throughput, delay or packet loss. It is left up to the end systems to cope with network transport impairments. Although best effort will remain adequate for most applications, QoS support is required to satisfy the growing need for multimedia over IP, like video streaming or IP telephony. The existing QoS models can be classified into two types according to their fundamental operation; the Integrated Services (IntServ) framework provides explicit reservations end-to-end and the Differentiated Services (DiffServ) architecture offers hop-by-hop differentiated treatment of packets IntServ The IntServ[7][15] model merges the advantages of two different paradigms: datagram networks and circuit switched networks. It can provide a circuit-switched service in packetswitched networks. The Resource Reservation Protocol (RSVP) was designed as the primary signaling protocol to setup and maintain the virtual connection. RSVP is also used to propagate the attributes of the data flow and to request resources along the path. Routers finally apply corresponding resource management schemes to support QoS specifications of the connection. Based on these mechanisms, IntServ provides quantitative QoS for every flow DiffServ DiffServ[5][26][10] was designed to overcome the difficulty of implementing and deploying IntServ and RSVP in the Internet backbone and differs in the kind of service it provides. While IntServ provides per-flow guarantees, Differentiated Services (DiffServ) follows the philosophy of mapping multiple flows into a few service levels. At the boundary of the network, traffic entering a network is classified, conditioned and assigned to different behaviour aggregates by marking a special DS (Differentiated Services) field in the IP packet header (TOS field in IPv4 or CLASS field in IPv6). Within the core of the network, packets are forwarded according to the per-hop behaviour (PHB) associated with the DSCP (Differentiated Service Code Point). This eliminates the need to keep any flow state information elsewhere in the network.

13 4 Chapter 2. QoS Models for MANETs IntServ over DiffServ This model provides a reservation-based QoS architecture with feedback signaling. It uses RSVP to signal resource needs but uses DiffServ as the technology to do the actual resource sharing among flows. 2.2 Quality of Service in Ac Hoc Networks This section discusses unique issues and difficulties for supporting QoS in a MANET environment and ends up showing the major drawbacks of each of the two QoS architectures described above with respect to these characteristics Special Issues and Difficulties in MANETS MANETs differ from the traditional wired Internet infrastructures. The differences introduce difficulties for achieving Quality of Service in such networks. The following list itemizes some of the problems: ffl Dynamic topologies: Nodes are free to move arbitrarily; thus, the network topology - which is typically multihop - may change randomly and rapidly at unpredictable times, and may consist of both bidirectional and unidirectional links. ffl Bandwidth-constrained, variable capacity links: Wireless links will continue to have significantly lower capacity than their hardwired counterparts. In addition, the realized throughput of wireless communications - after accounting for the effects of multiple access, fading, noise, and interference conditions, etc.- is often much less than a radio s maximum transmission rate. One effect of the relatively low to moderate link capacities is that congestion is typically the norm rather than the exception, i.e. aggregate application demand will likely approach or exceed network capacity frequently. As the mobile network is often simply an extension of the fixed network infrastructure, mobile ad hoc users will demand similar services. These demands will continue to increase as multimedia computing and collaborative networking applications rise. ffl Energy-constrained operation: Some or all of the nodes in a MANET may rely on batteries or other exhaustible means for their energy. For these nodes, the most important system design criteria for optimization may be energy conservation Drawbacks of the different QoS Models IntServ in MANETS IntServ has the following salient shortcomings in MANET environments: ffl Scalability: IntServ provides per-flow granularity, so the amount of state information increases proportionally with the number of flows. This results in a storage and processing overhead on routers, which is the well-known scalability problem of IntServ. The scalability problem is less likely to occur in current MANETs considering the small number of flows, the limited size of the network and the bandwidth of the wireless links. On the other hand, as the quality of wireless technology increases rapidly, high speed and large size MANETs may be a matter of fact some day. Though one could argue that whenever large high-performance MANETs will be developed in future, processing and storing capabilities will increase as well.

14 2.3 Conclusion 5 ffl Signaling: Signaling protocols generally contain three phases: connection establishment, connection maintenance and connection teardown. In highly dynamic networks such as MANETs this is no promising approach since routes may change very fast and the adaptation process of protocols using a complex handshaking mechanism would just be too slow. Furthermore the signaling overhead while maintaining the connection is a potential problem as well. DiffServ in MANETS The main drawbacks of a DiffServ approach in MANETs are listed below: ffl Soft QoS guarantees: DiffServ uses a relative-priority scheme to map the quality of service requirements to a service level. This aggregation results in a more scalable but also in more approximate service to user flow. ffl SLA (Service Level Agreement): DiffServ is based on the concept of SLA s. In the Internet an SLA is a kind of contract between a customer and its Internet Service Provider (ISP) that specifies the forwarding service the customer should receive. The Administration of a DiffServ domain must assure that sufficient resources are provisioned to support the SLA s committed by the domain [2]. Moreover, the DiffServ boundary nodes are required to monitor the arriving traffic for each service class and to perform traffic classification and conditioning to enforce the negotiated SLA s. Generally speaking if someone acquires QoS parameters and he pays for such parameters then of course there must be some entity which will assures them. In a completely ad hoc topology where there is no concept of service provider and client and where there are only clients it would be quite difficult to innovate QoS, since there is no obligation from somebody to somebody else what makes QoS almost infeasible. ffl Ambiguous core network: The benefit of DiffServ is that traffic classification and conditioning only has to be done at the boundary nodes[26]. This makes quality of service provisioning much easier in the core of the network. In MANETs though there is no clear definition of what is the core network because every node is a potential sender, receiver and router. This drawback would again take us back to the IntServ model where several separate flow states are maintained. 2.3 Conclusion The merit and limits of both IntServ and DiffServ are reflected in the trade-off between scalability and level of QoS performance assurance. Neither a pure IntServ nor a pure DiffServ model is suitable for Ad Hoc Networks. In order to make use of advantages of both models and avoid the disadvantages, a combination of DiffServ and IntServ as described in section could be an interesting approach.

15 6 Chapter 2. QoS Models for MANETs

16 Chapter 3 Protocol Design Issues 3.1 Towards developing a QoS Framework for MANETs In the last chapter it was shown that MANETs propose different requirements to quality of service infrastructures than wired networks do. Neither a pure IntServ nor a DiffServ approach is satisfying. The following sections identify some of the fundamental issues to be considered when designing a quality of service infrastructure for MANETs and helps classifying existing protocols later on. 3.2 QoS from a Layered Perspective A network s ability to provide a specific QoS depends upon the inherent properties of the network itself which span over all the layers in the network. The physical layer should take care of changes in transmission quality, for example by adaptively increasing or decreasing the transmission power. Similarly, the link layer should react to the changes in link error rate, let s say by including the use of automatic repeat-request technique. QoS-Routing and QoS-Signaling operate at the network layer in order to search for routes with sufficient resources or to allocate and release bandwidth respectively. Finally, QoS-adaptation hides all the environmentrelated features from the awareness of multimedia QoS Adaptation Network Layer QoS Signaling Link Layer QoS Routing QoS MAC applications. Moreover it provides an interface for applications to submit their requirements and is responsible to dynamically react to QoS changes for a certain flow, according to these requirements. This document does not treat MAC layer or physical layer issues any further, but instead concentrates on the issue of end-to-end QoS control over IP, meaning QoS-Signaling in particular. 3.3 QoS-Signaling and Routing Interaction In order to improve the performance of the QoS framework in a dynamic environment, QoS-signaling and routing can be coupled. There are three scales of coupling [15][17] which are described as follows:

17 8 Chapter 3. Protocol Design Issues The de-coupled option refers to the fact that QoS and routing mechanisms operate independently of each other and the QoS implementation is not dependent on a particular routing protocol. Changes in the network topology have to be handled by actively monitoring the network (for example by sending periodic monitoring messages). In a loosely coupled approach QoS-Signaling and routing interact with each other. Interaction can be understood as bi-directional. Some routing protocols allow upper layers for installation of upcall procedures to be called whenever a route changes. This might significantly decrease the reaction time of the QoS-Signaling to restore a certain reservation for a flow rerouted. On the other hand QoS-Signaling could provide feedback information to the routing layer regarding the route chosen and ask the routing protocol for alternate routes if the route provided doesn t satisfy the QoS requirements. Another example is to let signaling query the forwarding table directly. Pre-allocation would be an appliance for such an approach. Despite of these benefits any kind of interaction between QoS-signaling and routing may lead to a solution which is dependant on the specific routing protocol. This shortcoming can be minimized by designing a generic interface to access the routing layer and to develop adapters for a concrete routing protocol implementing this interface. In a closely coupled approach, the same signaling mechanism is used to propagate the routing and QoS information, what mostly refers a unique QoS-routing protocol. QoS-routing tries to search for routes to a given destination with respect to the QoS requirements. Having a such strong coupling between QoS control and Routing does of course lead to very fast flow restoration but also has some major drawbacks. First the solution is dependent on the routing protocol used. This is currently not suitable because routing in MANETs still underlies heavy research and there is no one routing protocol to be used in future. Second the route computation with this strategy may take too long[4] or be too complex. The next few sections discuss QoS-Signaling as well as different strategies for bandwidth allocation and adaptation. 3.4 QoS-Signaling: Design Issues QoS-Signaling is used to reserve and release resources, set up, tear down, and renegotiate flows in the networks. There are a few issues to be considered when designing a QoSsignaling protocol (especially in MANETs) as to how the control information is carried along with data and how the flow path is established In-band versus Out-of-band Signaling The term in-band signaling refers to the fact that any control information is piggybacked into the packet header. Hence in-band signaling systems could be more efficient for wireless networks in case of route adaptation. The signaling path will always follow the data path, even in cases when the route has changed because of a failure of an intermediate node. This leads to very fast flow state restoration times. Out-of-band signaling on the other hand uses explicit control packets. This approach can be characterized as heavyweight because extra information is carried in the network and consumes more network bandwidth. Moreover in out-of-band signaling systems, signaling packets must have higher priority than data packets in order to achieve on-time notification. This can lead to a complex system where performance will degrade substantially. The benefit of this approach is that it is more scalable since the control messages don t rely on the transmission of data packets. Furthermore the supported services can be rich and powerful.

18 3.4 QoS-Signaling: Design Issues Reservation Mechanism: One-pass versus Two-pass If the resource reservation is done by a two-pass mechanism, then in the first pass (sender initiated) end-to-end information (like bandwidth availability) of the specific connection is gathered. This allows for detection of any bottleneck nodes within the path. The actual reservation is made in a second pass (receiver initiated) considering the available QoS of the connection. Suppose a path comprises three nodes A, B and C with corresponding bandwidth 300K, 200K and 300K. In a first pass node B would be detected as a bottleneck due to its available bandwidth of 200K, which is the minimum on this path. An actual resource reservation in a second pass right after will not try to request more than this 200K because it cannot be provided anyway. Two-pass based reservations avoid wasting of resources but their drawback is their latency. The need for two control messages to be sent slows down the reservation process. This could be critical in a highly dynamic environment where paths have to be re-established frequently. In addition two-pass mechanisms assume bi-directional routes between the nodes. That means if A can reach B, it must be given that B can reach A as well. This assumption is not automatically true in ad hoc networks for several reasons. First routing is not guaranteed to by symmetric, second a route may change between the first and the second pass and last, wireless links often do not show symmetric behaviour at all. Reservation schemes based on a one-pass mechanism fix these shortcomings of the twopass approach using only one control message to do the actual reservation, regardless of any end-to-end information. This makes the mechanism very stable and flexible in reaction time but results in a potential bandwidth waste. Given the scenario above and assuming that there a is a bandwidth request of 300K, nodes A and C would be able to satisfy the request but node B would not. As a consequence it may happen that for a short period of time 300K of bandwidth are allocated on these two nodes but never used Soft-state versus Hard-state As to the method of keeping resource reservation, two states can be defined: Soft and Hard state. For hard state, once the reservation is established, the reserved resource and the reservation record is always kept. This happens until an explicit release message is sent. Soft state reservation has a lifetime. When a reservation is established, a timer is triggered. Within a certain time period, specific signaling or traffic will update the soft state reservation and reset the timer. Otherwise, when the timer times out without receiving any refreshing messages, the soft state reservation will be released. Hard state based reservation is simple and efficient. It needs no signaling to keep the reservation alive, and no timing processing has to be done either. Only the release message is mandatory. In a mobile ad hoc network, soft state reservation is the more suitable approach. The wireless connection is unstable, and apt to be broken. Once a mobile node has lost its connection with the network, it might not have a chance to send any signaling for hard state release. So the reserved resource would permanently be kept unused. Soft state management may easily solve this problem because the reservation state just times out Local Repair Local Repair is a mechanism which a signaling protocol should employ in order to achieve fast and efficient flow restoration in case of route changes. Local means that signaling is kept within a very small area, end-nodes must not be involved. Consider the following scenario: In a network with four nodes A, B, C and D node A sends stream data to node D

19 10 Chapter 3. Protocol Design Issues using a route over node C. It is assumed that accurate quality of service is already provided along this path. Now at a certain time node C starts moving and finally gets out of node A s transmission range, routing then finds a new path from A to D over node B. Local Repair is now in charge to detect the route change and to restore the best possible quality of service on the new path as well as to delete the old reservation. The goal is to re-establish reservation as quickly as possible and at the same time keep the signaling overhead low. There are several approaches to do this, which can roughly be classified into Pro-Active and Re-Active. Pro-Active Pre-Active local repair mechanisms try to reserve the required quality of service for a given flow in advance, that means before the old path is broken and a new one is established. This can be done by either trying to guess route changes in advance, using node movement tracking or transmission quality measurements, or by excessive resource allocation. In the latter case signaling just reserves resources on every possible path, eventually through inspecting the routing forwarding table. Both strategies do have the shortcoming that they potentially waste bandwidth due to their over-reservation, excessive allocation in particular. Re-Active Re-active signaling protocols do not reserve any resources in advance, instead they try to react to route changes as fast as possible. The easiest way to achieve this goal is to be triggered by the routing layer whenever a route changes. Another possible solution is to detect route changes for a certain flow by periodically sending monitoring messages. It is assumed that the performance of such an approach hardly depends on the interval by which monitoring messages are sent. 3.5 QoS Adaptation In contrast to a wired network, the QoS situation in a MANET may change rapidly and dramatically all the time due to wireless link characteristics or mobility. For example even if the signaling provides a very fast local repair mechanisms it is not guaranteed that after a path breaks, the same quality of service can be granted on the new path. Under certain circumstances it may happen that an active flow is rerouted to a bottleneck node which causes the end-to-end bandwidth of the flow to decrease. On the other hand, the available bandwidth will increase if less traffic is in the network or if any application releases its reserved bandwidth. So applications can t rely on the QoS investigation done during session establishment. To solve the problem the QoS framework has to actively monitor the network dynamics and adapt flows in response to observed changes based on some adaptation strategy. In the following sections a few concepts are discussed which might be helpful designing the adaptation part of a QoS framework Application Requirements As mentioned above, QoS-adaptation provides an interface for applications to submit their requirements. Some applications are capable to expand their QoS profile, so that instead of being a single value indicating the level of service needed by an application, it becomes a range of service levels in which the application can operate, together with the current reserved value within that range. This provides the network flexibility so that reservations can be maintained as network conditions change rather than forcing the network to make

20 3.5 QoS Adaptation 11 a binary "admit/fail" decision for each flow. Applications request QoS by specifying the minimum level of service they are willing to accept and the maximum level of service they are able to utilize, and then adapt to the specified point within this range that the network commits to provide, which may change with time. Changes in allocation have to be signaled to the application, which adapts its behaviour to match to what is available. The following three adaptation strategies are based on the concept of bandwidth ranges Adaptation Strategies The adaptation strategy decides how and when QoS of a certain flow has to be investigated and determines how to react to QoS changes. See [12] for details. Greedy Strategy The Greedy adaptation strategy is the simplest possible. Regardless of any end-to-end information every node tries to allocate the bandwidth requested. Suppose a scenario with 4 nodes A, B, C and D with the corresponding bandwidth resources of 200K, 300K, 200K and 300K. Furthermore assume a bandwidth request for the path A, B, C, D of within a range of 200K minimum and 300K maximum, also written as [200K,300K]. Using a greedy adaptation strategy node B allocates 300K although node A is only able to grant 200K of bandwidth. Nodes C and D act the same. If for some reason the available bandwidth on node C increases, according to the greedy adaptation strategy node C immediately allocates additional 100K to have 300K reserved in total. This is done even though node A still is a bottleneck and the end-to-end bandwidth still is 200K. The idea is that it might be quite hard to reach maximum reservation in one pass, so bandwidth is increased stepwise. Maybe at some point node A as well gains further 100K of bandwidth and the whole end-to-end reservation will be 300K which finally can be adapted by the application. Bottleneck driven Strategy In a bottleneck driven strategy each node would only try to allocate as much bandwidth as has been allocated already by previous hops for a certain flow, except the first node in the path of course which always tries to allocate maximum bandwidth. This avoids temporarily bandwidth waste but on the other hand makes it very difficult to increase endto-end bandwidth for a flow. In the scenario discussed above each node would only reserve 200K of bandwidth in a first step, which is the same as the end-to-end bandwidth for this flow. But imagine the available bandwidth of node A and B would toggle between 200K and 300K. If they never reach 300K at the same time end-to-end bandwidth will never be increased. Fair Strategy As the number of application flows competing for resources increases, rather than simply refusing to admit new flows or pre-empting existing flows, a fair adaptation strategy attempts to adjust the allocation for each flow, so that all flows can be accommodated. The strategy attempts to give each flow the minimum bandwidth requested, plus a fraction which is proportional to the requested bandwidth range. Suppose a scenario of a few nodes where each of them provides 300K of bandwidth. Assume a QoS request for a range of 150K to 230K bandwidth. There is no problem for the network to provide maximum bandwidth for this flow. But as soon as a second request arrives, for say 100K to 120K, using the same path or just a part of it, not even the minimum bandwidth could be granted. In case

21 12 Chapter 3. Protocol Design Issues of a fair strategy the first flow would be adjusted to run with its minimum of 150K, what would allow the second flow to run with its minimum as well. The remaining bandwidth of 50K ( ) could be shared between both flows according to their bandwidth range, which would result in a final reservation of 190K for the first flow and 110K for the second. The major drawback of a fair adaptation strategy is its signaling overhead due to adjusting active flows. However it would be interesting to test such an approach within a simulation environment Monitoring Interval and Soft-state Timer As mentioned in section 3.5.2, an adaptation Strategy not only determines how to react on QoS changes, but also decides when and how frequently the available QoS for a fixed flow has to be investigated. If QoS investigation is done by periodically sending monitoring messages the time interval by which these messages are sent is an important factor and should be dependent on the network condition. For example if node mobility is high in a MANET or the network is unstable, then more frequent monitoring is needed in order to adapt to bandwidth fluctuations. On the other hand the monitoring interval time should also be dependant on the actual bandwidth of the flow. The aim is to keep the amount of data constant which potentially could be sent in case of increased bandwidth availability or which could be lost in case of bandwidth degradation. It does not make sense for a flow running with 100K to monitor the network as frequently as a flow running width 300K does. But when bandwidth ranges are used then the actual bandwidth of a flow is conditioned by the adaptation process itself which on his part reacts to QoS changes. To make it even worse let s focus on another parameter mentioned in section In a soft-state based framework the question arises of how large to choose the timeout interval. Actually the size of the timeout interval should be a direct function of the monitoring messages because they update the soft state. The problem is just that the soft state timeout is usually managed by each node within a path while the monitoring interval is specified by end nodes only. 3.6 Conclusion During this chapter several design issues concerning QoS-Signaling and QoS-adaptation were discussed in more detail. These concepts are not distinct from each other, often they can be combined. As in the example of soft state and hard state, one could use explicit reservation release messages in a soft-state based framework even though it is a hard-state concept. However these concepts should facilitate classifying existing technologies as it is done in the next chapter.

22 Chapter 4 Existing Technologies This chapter describes some of the currently existing QoS technologies. Based on the concepts identified in the last chapter assets and drawbacks of each approach are pinpointed. 4.1 RSVP As mentioned in chapter 2, RSVP [25][19] is a typical IntServ protocol for the fixed IP networks environment. It was designed to enable senders, receivers and routers of communication sessions to communicate with each other in order to set up the necessary router states to support the quality of service requested by the application. A communication session is identified by the combination of destination address, transport-layer protocol type, and destination port number. RSVP is a classic two-pass protocol using out-of-band signaling. The messages used are the Path message, which originates from the traffic sender, and the Resv message[6], which originates from the traffic receivers. The primary roles of the Path message are first to install reverse routing state in each router along the path, and second to provide receivers with information about the characteristics of the sender traffic and end-to-end path so that they can make appropriate reservation requests. Resv messages finally carry reservation requests to the routers along the distribution tree between receivers and senders. RSVP state is "soft-state", after a certain expire time, the state of the path and the reserved resource is released. Periodical issuing of Path or Resv messages are necessary to keep the reservation alive. Additional signaling information allows the soft state timeout to adapt to the refresh period. Furthermore RSVP provides a routing triggered local repair [8] mechanism to overcome the need for a very fast refresh rate in order to react to route changes. There are many shortcomings of RSVP when used in MANETs: ffl The two-pass reservation model employed by RSVP is not suitable for MANETs, specially in case of local repair. ffl RSVP is based on a fixed QoS level approach. As a consequence no mechanism for a fast adaptation to QoS changes can be provided. To solve this problem reservation requests should specify ranges of values instead. ffl Due to its out-of-band approach, RSVP produces a significant signaling overhead. This may be of importance if the refresh rate high because the message size is not negligible in RSVP. A high refresh rate might occur when no route-changenotification service from the routing layer is available. This causes local repair to fail.

23 14 Chapter 4. Existing Technologies ffl As an IntServ based protocol RSVP lacks of scalability. The amount of state information increases proportionally with the number of flows what causes storage and processing overhead. Although the scalability problem may not be likely to happen in current MANETs due to the limited size of the network and the bandwidth of wireless links, one could argue that it will occur with the development of fast radio technology and potential large number of users in the near future RSVP Extensions Forced by the shortcomings of RSVP in Wireless networks, some approaches were made to enhance the signaling protocol[18]. Most of them intend to solve micro-mobility issues in infrastructure based wireless networks and do not address MANET problems directly. However MRSVP and DRSVP, two extension of RSVP to support mobility and dynamic network environments, try to overcome some of the disadvantages of RSVP mentioned above and are discussed in the following sections. MRSVP MRSVP[24] addresses mobility issues of a mobile node changing the point of attachment to the fixed network and follows a Pro-Active approach as discussed in section Two types of reservations are defined in MRSVP: active and passive reservations. Active reservation makes the resource exclusively reserved for the flow, no additional traffic is allowed to use the reserved resource. Passive reservation is different, it makes resource reservation in advance before the flow uses it. These passive resources are open to any other traffic until the flow actually needs the reservation. In order to make reservations in advance, it is necessary to specify the set of locations the mobile host may visit in future. The mobile node thus passively establishes paths with sufficient resources to a possibly large set of attachment points the mobile host eventually moves to. When the node arrives at a particular point of attachment, the path to that attachment becomes active and the path to the previous one passive, so that the data can still be delivered effectively. Even though MRSVP ensures good QoS provision in case of route change it suffers from many drawbacks when used in ad hoc networks. ffl MRSVP is designed for mobile wireless access networks not for MANETs. The concept of a mobile node and attachment points is not given in ad hoc networks where every node is a mobile having many attachment points. Adapting MRSVP to MANETs would result in a very large set of possible locations a path for a given flow ffl Many resources are reserved that may never be used. Even though they are available for other requests it requires the nodes to maintain a lot of state information regarding active and passive reservations. ffl It may be very difficult to accurately determine the set of nodes to which a certain flow eventually is routed. ffl Like RSVP it does not support any QoS adaptation, relying on the reservation in the initial phase. Neither the current QoS is monitored nor bandwidth ranges are used. ffl Considering one flow, reservation signaling has to be sent from each node in the path to all the possible neighbours. This causes a huge overhead and makes the approach almost unusable.

24 4.2 FQMM 15 DRSVP DRSVP[16] aims to overcome the shortcomings of RSVP in terms of QoS adaptation. By treating a reservation as a request for service somewhere within such a range, flexibility needed to deal with network dynamics is gained. As available resources change, the network can readjust allocations within the reservation range. If resources decrease below the level currently allocated, the network can offer a more reasonable response than simply dropping the reservation. In addition DRSVP provides a fair adaptation strategy as discussed in section The available bandwidth is divided up among admitted flows, taking into account the desired range for each flow. DRSVP definitely addresses one of the major shortcomings of RSVP, namely the adaptation process. Using bandwidth ranges is a reasonable approach to tackle the problem of QoS adaptation. But DRSVP does still not solve all the other problems of RSVP, like local repair or signaling overhead. 4.2 FQMM FQMM[14] (Flexible Quality of Service Model for Mobile Ad Hoc Networks) combines the IntServ and the DiffServ model discussed in the first chapter. Three kinds of nodes are defined, exactly as in DiffServ. An ingress node is a mobile node that sends data. Interior nodes are the nodes forwarding data for other nodes. An egress node is a destination node. The basic idea of FQQM is that it uses both the per-flow state property of IntServ and the service differentiation of DiffServ. This is achieved by preserving per-flow granularity for a small portion of traffic in the MANET, given that a large amount of the traffic belongs to per aggregate of flows, that is, per-class granularity. A traffic conditioner is placed at the ingress nodes where the traffic originates. It is responsible for re-marking or discarding packets according to the traffic profile, which describes the temporal properties of the traffic stream such as transmission rate and burst size. FQMM is an interesting attempt at proposing a QoS model for MANETs, however it suffers of major problems: ffl FQQM aims to tackle the scalability problem of IntServ. But without an explicit control on the number of services with per-flow granularity, the problem still exists. ffl Due to its DiffServ behaviour in ingress nodes, FQMM may not be able to satisfy hard QoS requirements[26]. It could be difficult to code the PHB in the DS field if the PHB includes per-flow granularity, considering the DS field is at most 8 bits without extension. ffl How to make a dynamically negotiated traffic profile is a well-known DiffServ problem (see 2.2.2) and FQMM seems not to solve it. 4.3 INSIGNIA INSIGNIA[22][21] is a signaling protocol designed explicitly for MANETs. It supports fast flow reservation, restoration and adaptation algorithms that are specifically designed to deliver adaptive real-time service. INSIGNIA implements an in-band approach by encapsulating some control signals in the IP option of every data packet (see figure 4.3), which is now called INSIGNIA option. Furthermore flow state information is kept in every node of

25 16 Chapter 4. Existing Technologies Service Mode Bandwidth Indicator Payload Type Bandwidth Request RES/BE BQ/EQ MAX/MIN MAX MIN 1bit 1bit 1bit 16bit Figure 4.1: ASAP/ns Insignia Option Field a particular path. This is done in a soft-state manner as explained in section 3.4.3, that is, the flow state information is periodically refreshed by the received signaling information. In the following the basic operation of the signaling system is described with respect to INSIGNIA IP option. INSIGNIA offers a one-pass reservation (3.4.2). When a source node wants to establish a reservation to a destination node it sets the reservation (RES) mode bit in the INSIGNIA IP option service mode of a data packet and sends the packet toward the destination. The bandwidth request field allows a source to specify its maximum (MAX) and minimum (MIN) bandwidth requirements. On reception of a RES intermediate routing nodes execute admission control to accept or deny the request. When a node accepts a request, resources are committed and subsequent packets are scheduled accordingly. In contrast, if the reservation is denied, packets are treated as best effort (BE) mode packets. In the case where a RES packet is received and no resources have been allocated, the admission controller attempts to make a new reservation. This is a re-active local repair mechanism (3.4.4) and commonly occurs when flows are rerouted during the lifetime of an ongoing session due to host mobility. The bandwidth indicator field of INSIGNIA option plays an important role during reservation setup and adaptation. Reception of a setup request packet with the bandwidth indicator bit set to MAX indicates that all nodes encountered have sufficient resource to support the maximum bandwidth requested. On the other hand, a bandwidth indicator set to MIN implies that at least one of the intermediate nodes between the source and destination is a bottleneck node and the maximum bandwidth requirement may not be met. When a reservation is received at the destination node, INSIGNIA checks the reservation establishment status. The status is determined by inspecting the IP option field service mode, which should be set to RES. If the bandwidth indication is set to MAX, this implies that all nodes between a source-destination pair have successfully allocated resources to meet the QoS requirements of the source node. In contrast if the bandwidth indication is set to MIN this indicates that only the minimum bandwidth can be currently supported. As a result "partial reservations" will exist between source and bottleneck node, these resources remain reserved until explicitly released. QoS reporting message can be sent by destination nodes to inform source nodes of the ongoing status of flows. They do not have to travel on the reverse path toward a node. The INSIGNIA system supports two QoS report commands in order to provide some kind of adaptation. A scale-down command requests a source either to send with the rate specified as MINIMUM instead of MAXIMUM or to send its packets as best effort instead of MINIMUM depending on the current sending rate of the source node. This will have the effect of clearing any partial reservation. A scale up requests a source node to initiate a reservation for some MINIMUM or MAXIMUM rate, depending on the actual flow state. Although INSIGNIA presents a quite promising approach to QoS support in ad hoc networks, the system still lacks of some basic mechanisms:

26 4.4 Some further Approaches 17 Max. Reserved Min. Reserved QoS report: bandwidth indicator = MIN Node Mobility Figure 4.2: ASAP/ns Insignia Monitoring ffl The most frequently mentioned drawback of INSIGNIA in literature is its scalability problem due to the flow state information which is kept within the nodes of a certain path. This is an inherent problem of IntServ but it is doubtful whether it will be of importance for MANETs in future (see 2.2.2). ffl INSIGNIA s bandwidth usage is not efficient. The extra reservation on the path from the sending node to the bottleneck is a waste of bandwidth until an explicit release message is sent. Although this waste won t last long, topology changing of MANET will make this reservation waste propagate frequently. Furthermore releasing partial reservations using QoS reports enforces source nodes either to set the bandwidth indicator of the INSIGNIA option field to MINIMUM or to send the packets as best effort depending on the actual flow state. In both cases the opportunity to scale up is lost. ffl INSIGNIA does not provide any mechanism to dynamically change the frequency by which control signals are inserted into the data packets. This imposes a major processing overhead on the network. ffl Only two bandwidth levels to be used are offered, MINIMUM and MAXIMUM. A more fine-grained approach would be needed in order to satisfy application requirements and to fully exploit the resources available. 4.4 Some further Approaches imaq imaq[1] is a cross-layer architecture to support the transmission of multimedia data over a MANET. They use a location-based pro-active QoS-Routing. Neither hard QoS guarantees can be provided nor are any resources reserved. Because cross-layer designs and QoS- Routing are not within the scope of this document, the imaq approach is not considered any further INORA INORA[11] is a QoS support mechanism that makes use of the INSIGNIA in-band signaling and TORA routing protocol for MANETs. INORA represents a QoS-signaling approach in a loosely coupled kind of manner. The idea is based upon the property of TORA to provide multiple routes between a given source and destination. While INSIGNIA does