Improving Performance of Transmission Control Protocol for Mobile Networks

Transcription

1 Improving Performance of Transmission Control Protocol for Mobile Networks Dulal Kar, Swetha Pandala, and Ajay Katangur Department of Computing Sciences, Texas A&M University-Corpus Christi, Corpus Christi, Texas, USA Abstract - Advances in wireless communications have enabled access to the Internet for mobile users from anywhere, anytime using wireless networks. However, the underlying traditional Transmission Control Protocol (TCP), which is tuned for wired networks, involves mechanisms that often lead to many problems for wireless networks including frequent disconnections of mobile hosts and low throughput due to high bit errors. Recent research to improve the performance of TCP in wireless networks has led the development of Mobile Transmission Control Protocol (M- TCP) as an extension of TCP for mobile networks, which can support multimedia services over high bandwidth links. Similarly, the Snoop protocol has been designed to improve the performance of TCP over networks that have both wired and wireless links. M-TCP and Snoop protocols are some of the improvements made earlier to the TCP to improve its performance over wireless networks. In this work, we propose an enhanced Mobile Transmission Control protocol that exhibits improved performance of TCP when a mobile node uses Internet services. Our experimental results suggest improved performance of the approach over existing M-TCP. Keywords: Transmission control protocol, wireless networks, mobile TCP. 1 Introduction In recent years, the rapid increase of mobile computing devices such as personal digital assistants (PDA), personal computers, and laptops has driven a revolutionary change in the computing world. Internet services have grown rapidly and millions of people are using these services in their day to day life. The proliferation of mobile computing devices with improved processing capabilities allows mobile users to connect to the global Internet. The impact of this phenomenal growth has changed the modality of communication and increased its challenges. The most common issues are management of the wireless communication and poor performance of the existing protocols over wireless networks. These problems often act as the major obstacles for the largescale deployment of wireless technologies. The Internet uses the Transmission Control Protocol (TCP) as the transport layer protocol for communications over wired, fixed node networks. TCP is a connection-oriented protocol that provides a reliable service over the Internet. Many applications and services that make use of application layer protocols such as HTTP, FTP, SMTP, and telnet use TCP for communication over the Internet. Besides reliability, TCP provides network congestion control service that uses some control mechanisms to avoid network congestion. These mechanisms control the flow of traffic and keep the data flow below a certain rate that would cause the congestion [1]. However, the TCP congestion control protocol can cause performance degradation in mobile networks. In a mobile computing environment, there is a combination of wired networks and wireless networks as shown in Figure 1. Wireless networks are prone to frequent disconnections because of high bit error rates and the frequent hand-offs. Since the traditional TCP protocol is designed for wired hosts, any delay in ACK (acknowledgement) for a transmitted segment of data is normally viewed as congestion. To recover from such congestion, TCP defensively slows down the rate of transmission of segments over the network by invoking a congestion control algorithm [1]. A congestion control algorithm reduces the sender s window (meaning the sender TCP reduces the rate at which segments are sent) which in turn reduces the load to the network to relieve congestion. However, in mobile networks the delay in ACK or no ACK from the receiver may not be due to congestion, but due to the momentarily unreachable or out-of-range location of the mobile receiving/sending node in the wireless network or due to frequent transmission errors suffered by the wireless link. This can result in a significant reduction in the throughput in an active connection [5]. Sender Wired Mobile Station Wireless Figure 1. Mobile Networks Mobile The connection-oriented service of TCP leads to several shortcomings. The major shortcomings with the TCP in wireless networks are waste of available capacity during slowstart process and corruption of segments due to high bit

2 errors. Disconnections are common in mobile networks which result in loss of segments. Another problem is serial timeouts which result from continuous retransmission of segments at the time of connection loss [2]-[8],[13][14]. Various modifications have been proposed to overcome the difficulties of using TCP for mobile networks. In this work, we propose a new algorithm called Enhanced Mobile Transmission Protocol (EM-TCP) which is a significant enhancement of the Mobile TCP proposed earlier by Brown [5]. This protocol retains all the salient and necessary features of TCP related to flow control. It uses a split connection approach to implement the protocol. The EM-TCP improves the performance of the TCP connections when a mobile node uses the services across the Internet. In the following, in Section 2, we review existing solutions for improvement of TCP over wireless networks. In Section 3, we qualitatively analyze the Snoop and M-TCP protocols, identify their drawbacks, and provide possible ways to improve TCP s performance further. In Section 4, we discuss the network model used for simulation study of our proposed EM-TCP protocol. Finally, in Section 5 we provide and discuss our simulation results. Section 6 concludes the paper with some future research directions. 2 Related Works Several mechanisms have been proposed to improve the performance of TCP over the wireless network. Different properties between the wired network and the wireless network are considered by these mechanisms. For example, the Snoop Protocol is a TCP protocol designed to improve the performance of TCP over networks that have both wired and wireless links. This protocol deals with the problem of segment loss due to network congestion [3]. The Snoop protocol works by deploying a Snoop agent at the base station, performing retransmissions of lost segments based on duplicate ACKs (as duplicate ACKs are strong indicators of lost segments), and locally estimating last-hop round-trip times [4]. The end-to-end semantics of the transport layer connection is maintained in the Snoop protocol [2] [3]. The packets passed across the wired-wireless link are buffered at the base station. The buffered packets are used to retransmit unacknowledged packets and reduce the number of timeouts by suppressing the duplicate ACKs. When an ACK is received from the mobile host, Snoop distinguishes it as genuine, spurious, or duplicate and performs the appropriate action. Snoop avoids timeouts and maintains a larger value of TCP s congestion window, thus resulting in better throughputs [14]. It can improve TCP performance quite well in wireless links but has a problem; when there are no duplicate ACKs, the Snoop protocol cannot notice the segment loss until the local retransmission timer is expired [6] as it cannot identify the disconnection due to handoff. M-TCP (Mobile-TCP), an enhanced protocol over Snoop, provides better performance in cases of frequent disconnections, changing bandwidths, and low bit wireless links [8]. The mechanism for M-TCP essentially splits TCP into two protocol blocks: Mobile TCP (M-TCP) and Supervisory TCP (SH-TCP). Figure 2 shows how TCP is split. At the sender s side, TCP protocol remains unchanged and at the supervisory host (SH), it uses a modified TCP called SH-TCP to communicate with the sender. The M-TCP is used for communication between a mobile host (MH) and the supervisory host. Fixed TCP SH-TCP Supervisory (SH) M-TCP The end-to-end semantics are maintained in the M-TCP approach. Upon receiving a segment from the fixed host, the SH-TCP does not immediately send any ACK of the segment to the fixed host (wired network) unless the segment is transmitted to the mobile host (wireless network) and the segment is acknowledged by the mobile host [5]. When M- TCP does not receive an ACK from the mobile host for a transmitted segment, it causes the sender (fixed host) to be in the persistent state where the sender sets its window size to zero. In this state the sender does not suffer from time-out and does not slowly reduce its window size. When the mobile host reconnects, the supervisory host sends a greeting segment to the fixed host with an ACK for the previously sent segment of data. Once the greeting segment is received by the fixed host, it sets its congestion window size to its previous window size. There are certain drawbacks with the M-TCP protocol. When a segment is lost, it is retransmitted by the fixed host in the wired network, which is retransmitted back by the supervisory host to the wireless network. Although this process of retransmission of lost segments is effective when the mobile host moves temporarily out of the range but this model does not help in improving throughput when loss of segments occurs due to bit errors. 3 Enhancing Mobile TCP M-TCP Figure 1. Splitting a TCP connection [5]. Mobile In the previous section, we indicate some shortcomings of the Snoop and M-TCP protocols in the context of wireless networks. In the following, we show how further improvement of throughput over M-TCP and Snoop is possible. We assume the same split-connection network model as used in the Snoop

3 and M-TCP protocols. Let us consider the following notations: RT fs : roundtrip time from the fixed host to the supervisory (or intermediate) host, RT sm : roundtrip time from the supervisory host to the mobile host, RT fm : roundtrip time from the fixed host to the mobile host, and RTO : TCP retransmission timeout at the fixed host. Typically, a mobile host is in close proximity of the supervisory host and therefore, the propagation delay is very small. On the contrary, the propagation delay over a longhauled internet path is very large. Also, a wireless link bandwidth is much greater than the effective bandwidth of a long-hauled internet path. As a result, for most situations, RT sm will be significantly smaller than RT fs. This fact justifies the reason of having an intermediate, supervisory, or proxy TCP host as the endpoint of the wired part of the path that can buffer and retransmit the segments on behalf of the fixed host to improve throughput. The Berkeley Snoop module running on the supervisory host inspects the header of all TCP data and ACK segments as well as buffers copies of all data segments. It also forwards data and acknowledgement segments in both directions. When Snoop detects a duplicate acknowledgement (originating from the mobile host) which is an indication of lost segments, it checks its buffer to retrieve the lost segments, if any, and then retransmits them over the wireless link to the mobile host. By doing local retransmissions, it saves at least RT fs amount of time for each case of successful detection. However, to detect each case of lost segments, the Snoop module needs to wait at least (RT fm /2 + RT sm /2) amount of time. In order to detect a duplicate ACK, it has to allow the fixed host to timeout and to retransmit data at least once, which takes RT fm /2 time (assuming symmetric path bandwidth) to reach the mobile host. It takes another RT sm /2 to receive the duplicate ACK from mobile host. In addition, it also causes of shrinking of the congestion window since the fixed host is made aware of loss of segments, which in turn causes the fixed host to slow down its data transmission. Evidently we observe some opportunity to improve the throughput by devising a protocol that can reduce such wait time of (RT fm /2 + RT sm /2). Accordingly, we take this opportunity to incorporate techniques in our proposed EM-TCP protocol to improve TCP s performance. Snoop maintains a roundtrip timer and retransmits unacknowledged segments accordingly to the mobile host. In order to prevent the fixed host invoking congestion control, Snoop also maintains a persist-timer of 200 ms whose expiration triggers retransmission of some data from the supervisory host itself. It is reported that Snoop performs well in high BER environments as well as for bursty loss of 2-6 packets. However, Snoop performs poorly when the wireless link experiences frequent and lengthy disconnections. One of our goals in the proposed EM-TCP protocol is to address such issues using similar ideas as found in M-TCP. Like Snoop, M-TCP uses the split TCP network model. However, the design of M-TCP does not emphasize how to improve TCP s throughput under high bit errors in wireless environment. Instead, it is designed to improve TCP s performance in a situation a mobile host experiences long or frequent disconnections. Frequent disconnections can cause serial timeouts at the TCP sender, thus in turn triggering its exponential back-off action on its retransmission timer for the retransmission. M-TCP chokes the TCP sender when the mobile host is disconnected and allows the sender transmits at full speed by manipulating the TCP sender s window. The manipulation of the TCP sender s window by M-TCP is done by controlling acknowledgements transmitted to the sender in a specific way such as sending ACKs with the receiver s window size to 0. To handle disconnections, M-TCP uses a modified version of TCP on the mobile host that sends a reconnection ACK when the mobile host regains its connection. As reported in [5], M-TCP performs very well under environments of disconnections and low bandwidths. However, the performance of M-TCP in environments of high bit errors is not as good as Snoop. Another drawback with the M-TCP protocol is that, when a packet is lost, it has to be retransmitted by the TCP sender, thus incurring extra time since the lost segment has to be transmitted by the TCP sender, but not by the supervisory host, which incurs an RTT (round-trip time) including the time to retransmit and to receive ACK by the fixed host. In our proposed protocol, we handle this situation more efficiently. Also, the retransmit and persist timers can be controlled more effectively to increase the throughput, which we incorporate in our EM-TCP. We propose an Enhanced M-TCP protocol that provides solutions that addresses the drawbacks found in Snoop and M- TCP protocols. The drawback with the M-TCP protocol is that, when a packet is lost, it is retransmitted by wired networks. In case of bit errors and data loss in wireless network we do not have to force the wired network to retransmit just because there is no ACK from the mobile host. We could just as well try to retransmit the lost segment multiple times from the supervisory host so that even if one copy of the segment is in error another might reach the mobile host. The M-TCP model does not consider much about the data lost during the transmission in a wireless network. If there is no ACK, it makes the sender to retransmit the packet again. Our enhanced mobile transmission protocol improves the performance of TCP throughput over the wireless Internet

4 based on the end-to-end approach. The main goal in developing EM-TCP protocol is to lower the effect of high error rates on throughput by means of dynamic connection migration while keeping the benefits of M-TCP to handle disconnections efficiently. To some extent, EM-TCP attempts to achieve the benefits of both Snoop and M-TCP by merging the techniques of both protocols. It characteristically improves TCP performance over Snoop and M-TCP in mobile networks while maintaining end-to-end TCP semantics. In EM-TCP, no congestion control is performed over the wireless link. Instead, EM-TCP aggressively retransmits the same segment multiple times with progressively smaller and smaller time intervals in between two successive transmissions. This alleviates the problem of high bit errors and disconnections invoking the wired network to retransmit the packet again thereby increasing the throughput. The wired network does not have to retransmit the data lost in the wireless network due to high bit error rates and frequent disconnections unless the mobile host becomes disconnected for a long time, i.e., more than the round-trip-time. In that case, the TCP sender is set into the persist-state by sending an ACK packet with the receiving window size set to 0, much like the way M-TCP does. In the following we discuss the network model and a simulation study for evaluating the performance of the EM- TCP protocol. 4 Network Model for EM-TCP EM-TCP is based on the same network model as used in M- TCP or Snoop protocol. It allows modifying TCP on the mobile network to increase the throughput in disconnections and varying bandwidth as well as to handle high bit error rates [3][5]. Figure 3 shows the network path and elements for our simulation study of EM-TCP to evaluate its performance. unnecessary deflation of the cwnd. In this way, the available bandwidth of the network can be preserved for other TCP communications. During the data transfer the WH connects to the intermediate host (IH) through a WLAN link, the IH is connected to the wired network. Figure 4 shows the state diagram of the EM-TCP module running at the intermediate host. When a segment is received from the wired host, the intermediate host buffers the segment and then transmits it to the mobile host. The intermediate host when receives some acknowledgement from the mobile host, it checks for any duplicate ACK and accordingly only forwards non-duplicate ACKs to the wired host. It also transmits over the wireless link the same segment multiple times, progressively with shorter and shorter time intervals between two successive transmissions. These time intervals are far less than the retransmission timer at the wired sender. In the process of resending the data packets, the duplicate ACKs are rejected when received. When the retransmission timer expires then the EM-TCP is moved to a persistent state. At this state, the wired TCP sender is set to persist. Then EM- TCP waits for ACK from the mobile host. Since EM-TCP at IH only enters the persistent state when the data is not acknowledged, the mobile receiver needs to send an ACK to remove it from the persistent state. For that EM-TCP keeps sending data to the mobile host until it receives ACK. We simulate TCP, M-TCP, and EM-TCP over UDP (user datagram protocol) by opening UDP sockets for two end hosts (WH and MH) and one intermediate host (IH) [9]-[12]. The default TCP behavior is simulated using UDP to run for WH. However, IH-TCP, EM-TCP, M-TCP, and TCP on the mobile host are all modified versions of TCP (Figure 3). The simulation of EM-TCP to measure its performance includes the following features: TCP IH-TCP EM-TCP TCP Sender (WH: Wired ) IH: Intermediate Figure 3. EM-TCP network model. MH: Mobile In this study of the TCP protocol, a closed loop network is considered. The wired host (WH) is the sender who transmits data and uses the ACKs received as feedback from the mobile host (MH) to increase or decrease its congestion window size (cwnd). The ACKs received from the network are used in TCP congestion control and flow control mechanisms. Hence, in case of disconnections a signal should be sent as indication. Then TCP could react appropriately by preventing Connection Establishment For a connection originating on a fixed network, a TCP connection between the fixed host and the intermediate host is completed. Then it initiates and completes the connection between the IH and MH. The connection is initialized by calling the connect method of the Socket. Then the remote

5 address is set by the connect method, and sends the SYN packet. The connection attempt goes through three-way handshaking process. From the connection point of view, IH is made transparent to the TCP sender in the fixed network. On connection setup, IH creates sockets with local address bonded to both TCP sender and MH's addresses. round-trip propagation delay estimated between itself and the MH. Slow-Start Mechanism The simulation uses the slow-start mechanism. The congestion window size is not reduced on timeouts because they do not occur due to congestion. Due to this, the slow-start behavior is limited to the beginning of the connection. Receive Data from WH Send Data Multiple times at EM-TCP Reject Duplicate ACK Wait for ACK Wait for ACK Retransmission Timer Expiration Wait for ACK Waiting for ACK Persistent State Timer Expires Delayed ACK Resend data Check Acknowledgements Reconnection ACK Retransmission Figure 4. State Transmission Diagram for EM-TCP. Congestion Window Settings A timer function determines when to send a window size reduction update to the TCP sender (WH). Before the sender invokes the congestion control and timer expires, IH must generate a packet and send it to the TCP sender to stop invocation of congestion control. Since IH is situated in the middle, IH can estimate RTT between WH and IH and RTT between IH and MH and hence can estimate retransmission timeout (RTO) of the TCP sender retransmission timer. It is to be noted that the wired host s RTO is based on the sender s 5 Performance Testing The performance of EM-TCP is tested against that of M-TCP protocol. Several data transfer operations at various bit error rates (BER) are performed in the simulation environment and accordingly, data transfer times are recorded for the purpose of comparison of the proposed EM-TCP protocol with the other protocol. Each simulation for a protocol, whether EM- TCP or M-TCP, is run under the same path conditions in terms of bandwidths, propagation delays, BERs and so on. The total time to transfer a data from the fixed host TCP

6 sender (WH) to mobile host (MH) is measured. Particularly, we consider a test case with the following parameters: Data size = 3 MB, Maximum segment size = 1024 bytes, Timeout for EM-TCP or M-TCP to receive ACK from MH = 0.15 ms, Retransmission timeout (Freeze timeout) at IH or SH = 1 ms, Persist Timeout at IH = 5 ms, Maximum interval between two consecutive segments transmitted = 0.05 ms, Number of hops = 3, and Wireless link: IEEE g The results are shown in Figure 5. For this particular test case, one can see that EM-TCP performs better than M-TCP when the bit errors are limited within 300 segments. However, when the bit errors exceed over 300 segments, both protocols achieve the same or similar performance. throughput and bandwidth usage in a network and adapt to dynamically changing bandwidth, frequent disconnections, and handoffs over the wireless link. The EM-TCP algorithm reduces the data transfer time, increases the throughput, and reduces retransmission attempts compared to TCP and M- TCP. The basic assumption of our simulation study of the EM-TCP protocol is that bit errors occur in a continuous stream of segments. That is, several consecutive segments are in error in a stream of segments. This may not be the case in some communication scenario where the packets may be corrupted anywhere in the data. One of the future tasks would be to test the simulation by sending the error segments randomly, and accordingly analyze and explore the new possibilities that arise from this scenario. With the help of this simulation with random distribution of error packets, it is possible to devise a protocol that can offer better performance in all situations. 7 Acknowledgement This work was supported by a grant from National Science Foundation, under grant number: CNS M-TCP EM-TCP Figure 5. M-TCP vs. EM-TCP under various error conditions. 6 Conclusions In this work, we propose a scheme to enhance TCP s performance for mobile applications, in which network services are provided over wireless networks tethered to a wired network infrastructure. Based on earlier works on Mobile TCP (M-TCP) and Snoop, we develop a enhanced M- TCP protocol to handle communications from the wired endpoint to a mobile host. As our test results demonstrate, the EM-TCP protocol improves TCP s performance in terms of 8 References [1] M. Allman and V. Paxson. TCP Congestion Control. (last visited May 23, 2011).

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