TCP and UDP performance measurements in presence of ... - CiteSeerX

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TCP and UDP performance measurements in presence of Fast Handovers in an IPv6-based mobility environment T. Melia, R. Schmitz, T. Bohnert NEC Europe Ltd., Network Laboratories Heidelberg

melia schmitz @netlab.nec.de




Abstract Within the framework of the IST project Moby Dick an IPv6-based, mobility-enabled and security aware architecture, including Quality of Service mechanism, is defined and implemented. This paper, based on results originated in the mobility evaluation of the project, analyses in detail the effects of Fast Handovers on the performance of TCP connections and real-time UDP data streams. Based on our optimised Linux kernel implementation, we present quantitative results on the benefits of the Fast Handover support for Mobile IPv6 i) in an integrated architecture comprising end-to-end Quality of Service and appropriate AAA mechanisms and ii) independent of the deployed access technology. The analysis for Wireless LAN (i.e., IEEE 802.11b) intra-technology handover shows handover interruption times below 1ms and interruptionless Ethernet to/from Wireless LAN inter-technology handover. Finally, our results show the impact of the delay introduced by Quality of Service and AAA on the overall handover completion time. Index Terms Mobile IPv6, Fast Handover, TCP performance, UDP real-time performance, handover delay

I. I NTRODUCTION The availability of portable, powerful computing and communication devices provides the basis for future pervasive and ubiquitous networks and offers a variety of services to roaming mobile users. This vision creates the demand for a mobility enabled and security aware architecture, including Quality of Service (QoS) and supporting several kinds of access technologies (like Wireless LAN 802.1 or Bluetooth), as defined and implemented within the Moby Dick [1] project in a fully IPv6-based network. Following the IETF draft Fast Handovers for Mobile IPv6 [2], an enhanced Fast Handover (FHO) stack has been designed and implemented as an extension to Mobile IPv6 [3]. The provisioning of fast handovers (i.e., small handover latency/interruption time and small or zero packet loss) in next generation IP networks is an important research challenge, since roaming mobile users demand uninterrupted services, as experienced in today’s cellular mobile phone system. Therefore, the detailed analysis of transport layer protocol performance, in presence of fast handovers in Mobile IPv6, is essential for the deployment of future packet switched networks. The minimization of handover latency and packet loss aims on avoiding noticable communication disruption in real-time UDP data streams and prevent performance degradation due to mobility in TCP connections. Our optimised linux kernel implementation, evaluated within this paper, achieves an average interruption time below one millisecond for intra-technology handover (i.e., WLAN - WLAN) and interruptionless inter-technology (i.e., WLAN - Ethernet) handover. We show by a detailed analysis of real-time UDP traffic and TCP data transfer, that the performance does not suffer in case of the implemented Mobile IPv6 fast handover. The overall handover time (i.e., including preparation phase, completion time, AAA context transfer and QoS message exchange), is about 8ms for intra-technology and 26ms for inter-technology handover, assuming ideal QoS and AAA entities. The design and implementation of real QoS and AAA modules influences the overall handover time significantly, as discussed in the analysis and in the conclusion. The section provides a general background on Mobile IPv6, Fast Handover and TCP. Section III presents related work. Section IV illustrates the Moby Dick and Fast Handover architecture.

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The implementation and measurement setup are described in section V. The results of the TCP and UDP performace evaluation are explained in section VI. Finally section VII concludes the paper. II. BACKGROUND A. Mobile IPv6 Mobile IPv6 (MIPv6) provides global mobility management and enables hosts to roam between different networks, an operation commonly known as handover. MIPv6 introduces new functional entities: Mobile Node (MN): any non stationary host in the network (e.g. PDA or laptop) Home Agent (HA): retains MN’s location in the visited network Correspondent Node (CN): any host in the Internet communicating with the MN Access Router (AR): offers network connectivity and forwarding services to the MN In order to support reachability, a MN has assigned two IP addresses. The home address reflects the static IP address in the home network and remains unchanged when the device roams to foreign networks. The Care-of Address (CoA) represents the topological correct address of the visited networks and is adopted on the recepion of Router Advertisements (RA), when the MN is moving into another network (e.g. wireless cell). The CoA, as identifier of the current location, is registered via Binding Update (BU) with the Home Agent, in order to enable forwarding of data which is addressed to the home address and destined for the mobile node. B. Fast Handover Several local mobility enhancements to Mobile IP have been discussed within the IETF. Simulation studies, e.g., as given in [16] and [17], have examined the pros and cons of the various approaches. As a trade-off between network complexity and scalability the FHO scheme has been choosed for real world testing. FHO aims to improve standard Mobile IP handover performances following the make before break philosophy: before leaving the old link, the MN establishes the connection with the new access point and, unless it is not registered at the HA with its new location, the old access point forwards all data to the new CoA. This mechanism allows the MN to keep alive ongoing and real-time connections with low probability of packet loss. C. TCP TCP is a window-based, reliable transport protocol. It is able to adapt to the network conditions and, by inflating and deflating the window size, in order to control the flows. Therefore, the receiver acknowledges all the data. Regular TCP (Tahoe) assumes packet loss when a restransmission timer (RTO) expires. In such a case Slow-Start (SS) is unavoidable and SS threshold ssthresh is dropped by half reducing the cwnd and the amount of data which can be sent. To improve performance in case of single packet loss, TCP Reno introduces duplicated acknowledgements. When the sender receives a number of duplicated acknowledgements (usually three) the Fast Recovery Algorithm takes place in order to avoid SS. This algorithm retransmits the lost packet and keeps the round trip delay close to the expected average. The enhanced version New Reno improves TCP performances in case more than one packet is lost in the same window. Basically The protocol stays in Fast Recovery until all the lost data is transmitted. Finally, Selective Acknowledgement (SACK) has been introduced to allow the receiver to retransmit selectively only the lost data blocks. However, since TCP has not been designed taking into account roaming users, a packet loss is always understood as network congestion. Therefore, congestion avoidance algorithms take place even if data is lost due to mobility.

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III. R ELATED W ORK Some solutions have been proposed to improve TCP performances over Mobile Networks [4] and we can identify two main categories: link layer protocols and split-connection protocols. The Link Layers protocols ([5]) aim on hiding the wireless medium from the transport layer. Recovery of packet loss takes place at link layer without affecting the upper layers. In contrast, Split connection protocols separate the wired and the wireless connection, in order to isolate wireless and mobility related problems from the fixed network. The connection between the CN and the MN is split at the base station. Examples are the I-TCP [6] protocol, and the MTCP [7] protocol. In a comparison, the most significant difference between the two approaches is that link layer protocols maintain the end-to-end semantics of TCP, while split connections break this paradigm. The effect of MIP handoffs on the performance of TCP has been evaluated in [8] and [9], assuming interruption times in the range of seconds. We will explain and illustrate that the Fast Handover solution is able to avoid TCP performance degradation, as already shown for Mobile IPv4 in [14]. IV. M OBILITY ARCHITECTURE INCLUDING AAA AND Q O S The presented architecture aims to evolve 3rd Generation mobile and wireless infrastructure towards the Internet, in order to provide uninterrupted, interactive and distributed multimedia services to roaming mobile users. The overall approach is independent of the deployed access technology. Therefore, the testbed comprises IEEE 802.11b Wireless LAN and Ethernet as example access technologies. The integration of FHO, QoS and AAA into the architecture is presented in the following.

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Fig. 1. Moby Dick Fast handover Architecture: (a) Signalling Flow including QoS and AAA, (b) Test network setup

The Moby Dick approach extends the basic IETF FHO signalling flow, mainly by the adoption of the AAA and QoS messages, as described in figure 1(a). On the reception of a new Router Advertisement (1), the signal strength of the new access point is evaluated and in case a threshold is exceeded, the MN initiates the fast handover process with the Router Soliciation for Proxy (RtSolPr) message (2) to the oAR. The potential delay, which could be introduced in the communication with the QoS broker (A,B,C), is an uncritical factor for the handover latency performance, since the message exchange takes place during the handover preparation phase. However, this parameter affects the radio cell planning (i.e. size of overlapping radio coverage areas). Therefore, the overall handover time is linearly depending on the QoS answer time, as discussed in section VII. The AAA context is acquired locally on the oAR, transferred in the Handover Initiate message and locally relayed to the AAA attendant on the nAR. Since there is no impact of AAA and QoS delay or jitter on the handover latency, the following experiments assume idealised AAA and QoS modules with minimal processing time in the order of 1ms.

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V. I MPLEMENTATION AND MEASUREMENT SETUP The fast handover mechanism is implemented as Linux kernel module in the kernel 2.4.16. Beyond the FHO functionality, the module manages the kernel-userspace interfaces to the AAA and QoS attendants on the AR. The test-network, as illustrated in figure 1(b). The system incorporates movement detection implementation (i.e., via WLAN signal strength measurement and evaluation) to support real MN movement. However, the presented measurement results base on manually determined signal strength modification, in order to provide automated, precise, uniform and comparable results. This implementation deploys the 802.11 ad-hoc mode, because of hardware and software restrictions in the current WLAN infrastructure mode (e.g., artificial handover latency increase due to physical attach to all available cells). An additional filtering mechanism is added to the WLAN driver in the ad-hoc mode, which extracts router advertisements for signal quality evaluation and movement detection. To simulate individual traffic and to evaluate the system under varying conditions, we developed a fully IPv6 capable traffic generator. VI. E XPERIMENTAL RESULTS This section presents the experimental evaluation results of the TCP and UDP performance in presence of Fast Handover. Intentionally, the comparison to standard Mobile IP is omitted, because MIP never intended the provisioning of Fast Handovers. A simulative comparison of different handover management approaches is provided in [15], [16] and [17]. The measurement of handover latency during the intra-technology (i.e., WLAN-WLAN) handover is performed via time stamps, added directly into the FHO module implementation. The average handover latency for intra-technology handover is 0.23ms, while in case of intertechnology handover, there is no interruption. The availability of two independent interfaces allows the set up of a new connection via the new device, while it is still possible to communicate via the previous technology. This process is explained in detail in the following section (see figure 2). Furthermore, the overall handover completion time depends on several parameters, such as round trip time (rtt) and load in the fixed network, processing time of QoS and AAA components and access technology. Our measurements present 8ms and 26ms overall handover time for intra- and inter-technology handover, respectively. The only parameter affecting these results within the considered scenario, is the deployed access technology, since we assume idealised QoS and AAA and the fixed network is not loaded with other traffic. The time consuption of AAA is minimal, since it is local processing (i.e., oAR and nAR), while the processing and rtt of QoS is below 1ms. The increase of this value in a real world scenario is explained in the conclusion. For the detailed UDP and TCP analysis, the traffic generator creates a data stream with an inter-packet delay of 5ms and 500 Byte packet size. These values are beyond real applications, but the results are transferable, since the high load within this scenario place even more demands on the system. A. UDP real-time traffic and Fast Handover The figures 2(a) and 2(b) represent the user experience in the test environment for intra- and inter-technology handover: there is no noticeable interruption in a real-time audio and real-time video data stream during a Fast Handover. Both figures show the handover process in presence of a UDP data stream observed on the MN as receiver. The bicasting mechanism fills the gap between packets to the old and to the new CoA. Finally, the BU updates the location information at the HA and CN and, consequently, packets are destined to the new CoA.

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Figure 2(a) illustrates the interruption between the packets to the old CoA and the encapsulated datagrams, whereas figure 2(b) shows the presence of duplicated packets. These packets only appear in case of inter-technology FHO, due to the management of the interfaces, as explained above. The conducted experiments show, that duplicated packets appear in average for 20ms. To conclude, the user does not experience noticeable impact on the performance - neither by the short interruption of intra-technology, nor by the duplicate packets of the inter-technology handover. B. TCP data traffic and Fast Handover In the following we present how the implemented solution can help to avoid TCP performance degradation due to handover latency. 380000

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Fig. 3. TCP connection in presence of WLAN-WLAN handover, observed by the sender (CN): (a)WLAN-WLAN handover, (b)Ethernet-WLAN handover

In figure 3(a) the WLAN-WLAN handover procedure during a TCP connection is depicted from the perspective of the sender (i.e., CN). Between label 1 and label 2, the connection to the MN is disrupted, and therefore, packet 2 is lost. This single packet loss represents the worst case of the measurements, since only in 2% of the performed measurements one packet was lost, and packet loss of more than one packet never appeared. Label 3 is the acknowledgement for packet 1, which contains a destination option header, updating the CN about the new MN’s location. Afterwards, packets are sent to the new CoA. The acknowledgements 4,5,6 are duplicated and trigger the fast recovery algorithm, thus, packet 2 is retransmitted. The sender exits fast recovery phase by acknowledging all the lost data in 7. Figure 3(b) illustrates an Ethernet-WLAN inter-technology handover during a TCP connection, again captured by the sender (i.e., CN). There is a small increase of the inter-packet delay at 3.55ms, which does not originate because of handover interruption, but due to a delayed ACK from the MN caused by layer3 reconfiguration and duplicate packet handling processing time

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by the TCP stack. However, this does not change the TCP state and therefore, there is no affect on TCP throughput. Summarising the TCP behaviour in presence of fast handover, no significant negative effect can be observed, since the deployed TCP new Reno with selective ACK version is able to process single packet loss effectively. VII. C ONCLUSIONS AND F UTURE W ORK The positive effects on the performance of TCP and UDP in an integrated mobility environment including AAA and QoS by our Fast Handover implementation are proven via the analysis of handover latency, packet loss, TCP congestion avoidance algorithms and UDP real time ongoing communications via measurements in a real IPv6 environment. Our optimised Fast Handover kernel implementation provides interruption times below 1ms for intra-technology handover and interruptionless inter-technology handover. Neither the small interruption time, nor the duplicated packets affect TCP or UDP performance. Beyond the handover interruption time, the overall handover completion time is an important factor. In the examined environment, the overall handover completion time is about 8ms and 26ms for intra- and inter-technology handover, respectively, assuming idealised AAA and QoS components with a QoS delay (i.e., processing and round trip time) below 1ms. On the one hand, since AAA processing takes place locally on the Access Routers, the effect can be neglected. On the other hand, the QoS messages are exchanged in the handover preparation phase and, therefore, do not influence the handover interruption time, but the overall handover completion time. Considering a real QoS implementation and a loaded network, the total handover time depends linearly on the QoS delay. Therefore, the influence of this delay on the size of the overlapping radio coverage area in combination with the user velocity will be analysed in detail in the future. ACKNOWLEDGMENT The authors would like to thank Hannes Hartenstein for his comments and suggestions that helped to improve the quality of the paper. R EFERENCES [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [14] [15] [16] [17]

IST project Moby Dick - Mobility and Differentiated Services in a Future IP Network, Project Number: IST-2000-25394, www.ist-mobydick.org G. Dommety, A. Yegin, C. Perkins, G. Tsirtsis, K. Malki, M. Khalil, Fast Handovers for Mobile IPv6, draft-ietf-mobileipfast-mipv6-03.txt, Internet Draft, work in progress, Nov 2001 D. Johnson, C. Perkins, J. Arkko, Mobility Support in IPv6, draft-ietf-mobileip-ipv6-22.txt, Internet Draft, work in Progress, May 2003 H. Balakrishnan, V.N. Padmanabhan, S. Seshan, R.H. Katz, Improving TCP performances over Mobile Networks, available http://daedalus.cs.berkeley.edu, September 2002 H. Balakrishnan, S. Seshan, R.H. Katz Improving reliable transport and handoff performance in cellular wireless networks. Wireless Networks 1, 4, 469-481. A. Bakre, B. Dadinath, I-TCP: Indirect TCP for mobile hosts, Proc. Conf. on Distributed Computing Systems, Vancouver, Canada, 1995 K. Brown, S. Singh, M-TCP: TCP for Mobile Cellular Networks, ACM Computer Communication Review, vol. 27, no. 5, 1997 Fladenmueller, De Silva, The effect of Mobile IP handoffs on the performance of TCP, Mobile Networks and Applications 4, 1999, pp 131-135 Caceres, Iftode Improving the performance of reliable transport protocols in mobile computing environments, IEEE JSAC, vol 13, no.5 June 1995, pp 850-857 Mobile IPv6 for Linux, Helsinki University of Technology (HUT), www.mipl.mediapoli.com H. Hartenstein, K. Jonas, M. Liebsch, M. Stiemerling, R. Schmitz, D. Westhoff, Performance of TCP in the Presence of Under Mobile IP Handoffs, proc. International Teletraffic Congress (ICT), Bucharest, 2001 R. Hsieh, A. Seneviratne, H. Soliman, K. El-Malki, Performance analysis of Hierarchical Mobile IPv6 with Fast-handoff over End-to-End TCP, proc. GLOBECOM, Taipei, Taiwan, 2002. X. Perez-Costa, M. Torrent-Moreno, H. Hartenstein, A Simulation Study on the Performance of Hierarchical Mobile IPv6, to appear in Proceedings of International Teletraffic Congress (ITC), 2003 M. Torrent-Moreno, X. Perez-Costa, S. Sallent-Ribes, A Performance Study of Fast Handovers for Mobile IPv6, to appear in Proceedings of IEEE Local Computer Networks (LCN), 2003