PERFORMANCE ANALYSIS OF SNOOP TCP WITH FREEZING AGENT OVER CDMA2000 NETWORKS

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1 PERFORMANCE ANALYSIS OF SNOOP TCP WITH FREEZING AGENT OVER CDMA2 NETWORKS Sang-Hee Lee +, Hong-gu Ahn +, Jae-Sung Lim +, Seung-Hwan Kwak ++, Sung Kim ++ The Graduate School of Information and Communication, Ajou University + Tel: , Fax: dreami, piriboy, Network R&D center, SK Telecom ++ Tel: , Fax: kwaksh, Abstract - The standard TCP which is originally devised to perform congestion control mechanism does not work well in wireless networks since the wireless links produce significant non-congestion related packet losses due to fading and handoff. In this paper, to improve the TCP performance over cdma 2 wireless networks, we propose an architecture with a wireless TCP server and a wireless TCP mechanism that applies a freezing with Snoop protocol. Through computer simulations carried by NS-2 network simulator, it is shown that the snoop function yields significant improvement under the proposed system architecture and the freezing mechanism prevents timeout which causes an unnecessary congestion control. 1. Introduction Technology advances in wireless communications have created new opportunities for wireless data communication. In the wireless network, the transmission quality is characterized by bursty errors with high bit error rate (BER) unlike randomly occurring errors with low BER in wired network. Hence the design of a transmission protocol for a network with wireless links must take into account additional packet losses in wireless networks. Almost all multimedia services that require reliable transmissions employ the TCP as transport layer protocol. Therefore in wireless networks, the use of TCP is necessary to support various multimedia services. But, the standard TCP such as TCP-Reno is not a suitable mechanism for the wireless networks. Because the standard TCP was developed for wired networks, it performs the congestion control mechanisms when packet losses happen successively. Accordingly it results in the unnecessary congestion control and the performance degradation in wireless environments. Many TCP mechanisms have been researched and proposed to prevent such performance degradation[1-6]. One of these mechanisms is the snoop TCP[2] which is proposed to implement on a BS(base station). So it requires modifying the working system such as the cdma2 network. In this paper, we introduce a new server called WTCP server(wireless TCP server) into the cdma2 network and propose a modified snoop TCP mechanism which is based on a freeze scheme. For the simulations, we build the cdma2 network with the WTCP server using the NSsimulator. The rest of this paper is organized as follows. In the next section we review the most important modifications of TCP to improve TCP performance in wireless environment. In section 3, we present the proposed system model and the proposed TCP scheme. Then we evaluate system performance by computer simulations in Section 4. Finally, concluding remarks are given in Section Related Works In this section, we discuss some protocols that have been proposed to improve the performance of TCP over wireless networks. The TCP mechanisms could be classified by several ways, but we divide them into two classes; TCP- Aware and TCP-Unaware approaches. TCP-Aware approaches such as ELN(Explicit Loss Notification)[3] need to modify TCP in FH(Fixed Host) or MH(Mobile Host). The ELN defines special option called ELN bit in TCP ACKs. When a packet is dropped on wireless networks, a cumulative acknowledgement due to the lost packet is marked to identify what the loss occurred in wireless link. The TCP-Unaware approaches use new software module or mechanism to distinguish packet loss in wireless link from that in wired link. These approaches do not modify TCP in FH and MH and it just change the BS. Typical examples are Snoop and Indirect-TCP[4]. The Indiret-TCP protocol is implemented in the BS. The protocol splits a TCP connection between FH and MH into two separate connections and hides TCP from the wireless link by using a protocol optimized for wireless link. However, the Indirect-TCP can not maintain end-to-end TCP semantics, and it must have large buffers to keep a whole packet which are from the sender. The Snoop TCP uses a module which is introduced in the BS. FH and MH operate on the standard TCP. When FH sends a packet, the BS stores it in the Snoop buffer and forwards to the MH. The buffered packet is for local

2 retransmission when packet losses in wireless links occur. The Snoop module monitors an acknowledgement from the MH and retransmits the packet when duplicated ACKs arrive. This is the way that the BS hides the packet loss from the FH by not propagating the duplicated ACKs, thereby prevents unnecessary invocations of the congestion control mechanisms at the FH[2]. Although the Snoop satisfies the end-to-end semantics and it does not modify the FH and MH, it needs some modification of the BS. Furthermore, it can make some problems in cdma 2 system, which has the link layer recovery protocol called RLP(Radio Protocol) and packet scheduling schemes implemented at link layer. 3. Snoop TCP with freezing agent approach. These protocols aim at preventing packet losses by disconnection of wireless links. In the M-TCP, an intermediate host, called SH(Superior Host), takes care of the disconnection. When the SH gets ACKs from an MH, it saves the ACK of the last byte, in order to prevent loss of outstanding packets. If the MH is disconnected from nowhere, then the SH stops getting the ACKs and assumes that the MH has been temporarily disconnected and sends the ACKs of the last byte that it saved previously. The Freeze-TCP does not require any intermediary and any change on the sender side. Instead of intermediate node, the receiver handles the task of identifying an imminent disconnection due to potential handoff, fading signal strength, or any other problems. Both of them use freezing function when the wireless link disconnection is expected. FH WTCP Server Snoop TCP with Freezing Agent PDSN PPP RNC RLP Fig. 1 Proposed System Model BS BTS MH PPP RLP A simplified architecture of a cdma2 wireless network is shown in Fig.1. The BS is connected to a node called the PDSN(Packet Data Service Node) and it performs CDMA specific functions such as soft handoffs, encryption, power control etc. It also performs link layer retransmission using RLP. The PDSN terminates PPP with the MH and forward PPP/ packet to the BS[7]. The BS fragments the packet, which receives from PDSN, into a number of radio frames and then performs transmission and local retransmission of these radio frames using RLP protocol. It also schedules the radio frames, which is received from the PDSN, on the wireless link using a scheduling algorithm. The MH receives the radio frames and if it discovers loss of radio frames, it requests local retransmission using the RLP protocol. However the RLP can not recover wireless packet loss perfectly even through it performs retransmissions repeatedly. Therefore proper TCP mechanisms for wireless networks are needed. The proposed WTCP server is located behind the PDSN. A new packet arrives from the FH, and the WTCP server adds it to its buffer and passes the packet onto the MH. The WTCP server also monitors all ACKs sent from the MH. When a packet loss is detected, the WTCP server performs local retransmission of the loss packet to the MH. This action of packet retransmission is the same as the Snoop module except performing at TCP layer. The WTCP server can maintain the connections with several PDSNs and control packets for each PDSN. Hence, it has advantages with respect of cost and implementation. The proposed scheme does not need the modification of current BSs. The M-TCP[5] and the Freeze TCP[6] use the freeze IF (packet arrived from FH) Forward packet; IF (NEW packet) IF (In sequence) Cache packet; Mark as Congestion Loss; Reset Rexmit counter; IF (ACK arrived from MH) IF (NEW ACK) Free buffer; Update RTT; Forward ACK to FH; IF (Duplicated ACK) IF (first duplicated ACK) Send lost packet to MH; Discard; Discard; IF (WTCP server's RTO is expired) Send Freeze ACK to FH; Retransmit lost packet to MH; Fig. 2 Operation of the freezing agent in WTCP server The proposed TCP mechanism in this paper, however, takes up freezing to avoid the slow start activation in the sender side. When the WTCP server reaches the local retransmission timeout threshold, it sends an ACK, which sets the sequence number by the previous ACK number

3 and the received window size to zero 1. If the TCP sender receives the ACK with zero window, the TCP sender becomes aware of the fact that the receiver can not accept packets anymore. Consequently TCP sender stops sending packets temporarily, and waits for arrival of a new ACK with available received window number. During this waiting time, the TCP sender s retransmission timeout timer stops, thereupon the congestion window size does not decrease. The Snoop TCP mechanism with freezing function obtains outstanding benefit. Especially, the MH is in the burst error environments and RTTs change frequently. The operation of freezing agent on the WTCP server is shown in Fig Performance Analysis In order to evaluate the performance of the proposed scheme, we carried out the computer simulations using the network simulator NS-2[8]. FH WTCP server Fig. 3 Ns-2 Simulator PDSN The Fig. 3 shows the simulation network model. The FH is a sender and the MH is a receiver. The node 2 is the WTCP server, and it uses Snoop mechanism which caches packets from the FH and monitors ACKs from the MH. The proposed TCP mechanism also works in the WTCP server. For the cdma2 network, we assume that the PDSN makes a random delay of packet scheduling from 2ms to 26ms and performs 2-3 RLP mechanism[9] between the PDSN and the MH. We also assume that the error in wireless link does not happen in reverse link, whereas forward link yields frame errors from 1% to 1%. We consider only one connection between the FH and the MH. The performance parameters are average transmission rate and congestion window size. 1 Although freeze ACK uses the duplicated sequence number with previous one, the congestion avoidance mechanism is not activated, because the standard TCP uses three duplicated ACK scheme. MH Table 1. Simulation Parameters Parameters Value TCP Version Application FTP TCP/ Segment Size 15Bytes Wired Speed 26kbps Wired Delay 5/1/3ms Wireless Speed 9.6/144kbps Wireless Delay 26ms Max cwnd size Unlimited Doppler frequency.1, 1. RLP Version 2-3 RLP Error Model - Uniform for Random error - 2-state Markov model for Burst error Simulator NS-2.1b8a Simulation Time 6(s) The Fig. 4 and 5 show the transmission rate in burst error environments. The simulation results mean what the Snoop protocol shows better performance than the TCP- Reno, when the frame error rate is lower than 5%. However, the Snoop protocol produces the similar performance with the TCP-Reno when the wireless link speed is low such as 9.6kbps. It means that as the bandwidth becomes narrow and the FH s maximum window size is reduced. That is, the Snoop protocol makes benefits by means of reducing the number of congestion control, and it gets to use the channel bandwidth maximally. In the Fig. 4, the offered bandwidth is too narrow, so the Snoop protocol can not improve the performance. The local retransmission of the Snoop even leads to increasing frame error rate because of a long RTT[9]. From the Fig. 5, we can see that the transmission rate is improved. It is due to a decrease in congestion window by the local retransmission of WTCP server. Finally, the Snoop protocol achieves the performance improvement to make the best of the given bandwidth. The Fig. 6 shows the transmission rate in random error environments. The Snoop based WTCP server gets better performance than the TCP-Reno. Especially, the Snoop maintains high transmission rate even if FER becomes high, whereas the transmission rate of the TCP-Reno is decreased. It is caused by that the Snoop protocol performs local retransmission only when two duplicated ACKs received or the timeout timer is expired while the standard TCP performs the retransmission when three duplicated ACKs arrived in the sender side. If the Snoop holds the lost packet, the performance is not influenced by the wired link delay. The Fig 7 (a) and (b) show the variation of congestion window size of the proposed Snoop TCP with freezing agent under the burst error environment. The X-axis points the simulation time from to 6 seconds and the Y-axis displays the congestion window size. In accordance with these graphs, the proposed scheme activates the slow start only once when it start a packet transmission while the Snoop without freezing agent leads to the nine times of the slow start. Because we do not limit the maximum size of

4 congestion window, the window size can grow until 55. The flat part means the long waiting time for receiving an ACK, when there is high frame errors such as 1% of the whole packet transmission burstly. 5. Conclusion We have proposed a wireless TCP model with a WTCP server and a new TCP mechanism to improve the TCP performance over the wireless networks. The proposed TCP mechanism is a variant of the Snoop TCP, which works on the transport layer with freezing function. The simulation results carried out over cdma2 network show that the Snoop makes better efficiency when the wireless link speed is higher than 64kbps. On the other side, the proposed scheme accomplishes an improvement of the system performance in terms of preventing unnecessary congestion control while the MH is staying in bad channel environments. Through computer simulations, the proposed mechanism maintains congestion window size higher than 8 and it would lead to better performance especially when the RTTs become suddenly long. 6. References [1] J. Liu Transport Layer Protocol for Wireless Networks, Ph.D. thesis University of South Carolina, 1999 [2] H. Balakrishna and R. H. Katz. et al., Improving TCP/ Performance over Wireless Networks, Proc. 1 st ACM Int l Conf on Mobicom, Nov [3] H. Balakrishnan and R. H. Katz, Explicit Loss Notification and Wireless Web Performance, Proc. IEEE Globecom Internet Mini-Conference, Nov [4] B. R. Badrinath, A. Bakre, I-TCP: Indirect TCP for Mobile Hosts, Proc. 15 th Int l Conference on Distributed Computing, pp , May 1995 [5] K. Brown, S. Singh, M-TCP: TCP for Mobile Cellular Netowrks, ACM CCR, vol. 27, no. 5, 1997 [6] T. Goff, J. Moronski, D. S. Phatak and V. Gupta, Freeze-TCP: A True End-to-end TCP Enhancement Mechanism for Mobile Environments, Proc. IEEE INFOCOM, pp , Mar. 2 [7] M. C. Chan and R. Ramjee, TCP/ Performance over 3G Wireless s with Rate and Delay Variation, Mobicom 22, Sep. 22 [8] S. Bajaj et al., Virtual InterNetwork Test bed: Status and Research Agenda, Tech Rep., pp , University of Southern California, July [9] TIA/EIA/IS-77, Data Services Option Standard for Wideband Spread Spectrum Digital Cellular System, Feb [1] J. Rendon et al., SNOOP TCP Performance over GPRS, Proc. IEEE VTS 53rd Vehicular Technology Conference Spring, pp , May TCP-Reno(Wired Delay = 5ms) TCP-Reno(Wired Delay = 1ms) TCP-Reno(Wired Delay = 3ms) Snoop based(wired Delay = 5ms) Fig. 4 Transmission rate in burst error environments (Wireless Speed= 9.6kbps, FdT =.1) (Wired Delay = 5ms) (Wired Delay = 1ms) (Wired Delay = 3ms) Snoop based(wired Delay = 5ms) Fig. 5 Transmission rate in burst error environments (Wireless Speed= 144kbps, FdT =.1) (Wired Delay = 5ms) (Wired Delay = 1ms) (Wired Delay = 3ms) Snoop based(wired Delay = 5ms) Fig.6 Transmission rate in random error environments (Wireless Speed= 144kbps, FdT = 1.)

5 Fig. 7(a) Variation of congestion window size in burst error environments (Snoop without freezing, Wireless Speed= 144kbps, FdT =.1, FER = 1%) Fig. 7(b) Variation of congestion window size in burst error environments (Snoop with freezing, Wireless Speed= 144kbps, FdT = 1., FER = 1%)

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