Performance Evaluations of PRR-CFDAMA for TCP Traffic ... - CiteSeerX

Performance Evaluations of PRR-CFDAMA for TCP Traffic over Geosynchronous Satellite Links Yuheng Li, Zhifeng Jiang and Victor C.M. Leung Department of Electrical and Computer Engineering The University of British Columbia Vancouver, BC, Canada V6T 1Z4 Abstract - In satellite-based Internet access, improvements in TCP protocol and a suitable media access control (MAC) scheme are two key factors in maximizing system throughput. While a number of techniques to enhance TCP performance over satellite links have been proposed, this paper focuses on the influence of satellite MAC protocol on TCP performance. We present a PreReturn Reservation Combined Free/Demand Assignment Multiple Access (PRR-CFDAMA) protocol over an MFTDMA/TDM satellite link, and analyze its performance with Poisson and empirical Internet traffic by simulations. Results show that PRR-CFDAMA can provide higher throughput and shorter delay for TCP traffic. We also present simulation results for FTP over TCP and show that by tuning the TCP parameters, we can improve the system performance noticeably.

I.

INTRODUCTION

The Transmission Control Protocol (TCP) is a transport layer protocol that provides end-to-end reliable data delivery for Internet applications, such as World Wide Web access using HTTP (Hypertext Transfer Protocol), file transfer using FTP (File Transfer Protocol), remote terminal access via Telnet, and e-mail using SMTP (Simple Mail Transfer Protocol), etc. Over the past decades, TCP/IP have worked well over the Internet consisting mainly of terrestrial networks. With the explosive growth of bandwidth requirements for Internet service on a global basis, and considering that satellites are capable of providing high capacity data communication links over wide coverage areas, increasing attention is given to the performance of TCP over satellite networks. TCP was originally designed for terrestrial networks, and its throughput performance suffers over satellite networks due to the inherently very long propagation delay. Many recent papers have proposed extensions to TCP to improve its throughput over satellite channels. These extensions include large initial windows [1, 2], selective acknowledgment (SACK) [3], byte counting [4], and fast retransmission/fast recovery [5, 6]. Simulations and practical implementations have proven that these extensions improved TCP performance noticeable over end-to-end connections with large delaybandwidth products where packets could be lost due to factors other than congestion. On the other hand, characteristics of satellite networks due to their designs, including the physical layer, medium access control (MAC) and data link layer protocols, also have a large impact on TCP performance. As TCP traffic becomes

dominant in satellite networks, more attention should be given to the optimization of satellite network design with respect to TCP performance. For this purpose, we present a novel MAC protocol for satellite channels, and illustrate its superior performance by simulations. We also show how TCP parameters can be fine-tuned for our MAC protocol. The organization of this paper is as follows. Section 2 gives some background on satellite communications and reviews performance issues of TCP over satellite links. Section 3 presents our novel pre-return reservation combined free/demand assignment multiple access (PRR-CFDAMA) protocol for MF-TDMA/TDM satellite channels, taking into account of the physical layer and satellite on-board processing. Simulation results from OPNET models are presented in section 4 to compare the performance of demand assignment multiple access (DAMA), combined free/demand assignment multiple access (CFDAMA) and PRR-CFDAMA. In this section, we also present simulation results comparing the performance of DAMA, CFDAMA and PRR-CFDAMA for FTP over TCP, and investigate the tuning of TCP parameters to improve system performance. Section 5 concludes the paper. II.

PERFORMANCE ISSUES OF TCP OVER SATELLITE LINKS

2.1 Satellite Communications Generally speaking, satellite networks offer three advantages over terrestrial point-to-point networks. First, satellites provide wireless communication coverage over wide areas. Several GEO satellites can provide global coverage seamlessly. Communication costs is not distance sensitive, making satellite systems ideal to provide network access to sparsely populated or hard-to-reach areas, as well as economically disadvantaged areas that have not been able to invest in a terrestrial network. Second, satellite downlinks are inherently broadcast channels that can efficiently deliver information to a large group of receivers. Thirdly, satellite uplinks can be efficiently shared by a large number of stations by means of DAMA. This is particularly effective for bursty data traffic such as found in the Internet. On the other hand, GEO satellite channels have certain disadvantages. The propagation delay is very long - the round trip delay between the earth and a GEO satellite is about 280ms. Consequently, the delay-bandwidth product is very large. The delay-bandwidth product is a measure of the link capacity in terms of the maximum amount of data that can be

"in flight" (data that has been transmitted but not yet acknowledged) at any time. For a TCP connection, when the sending window size equals to the delay-bandwidth product the channel capacity will be fully utilized. This problem is not unique to satellite networks, as advanced optical fiber networks with ultra-wide bandwidth also have very high delay-bandwidth products. 2.2 TCP Performance Issues TCP employs acknowledgment, retransmission and congestion control mechanisms to provide reliable data delivery as fast as possible without overwhelming the network. Although these mechanisms work efficiently in terrestrial networks, they may impair TCP performance over satellite networks, specifically due to the following issues. 2.2.1 Datagram Fragmentation The TCP/IP protocol suite has a layered architecture. Application data are encapsulated first by a TCP segment, then by an IP datagram, then by a MAC frame, and finally transmitted. Clearly, a large segment size reduces the overhead and gives a higher throughput. On the other hand, if the link bit error rate is not negligible, a large segment size could increase the chance that a segment is corrupted by errors and thus requires retransmission. Thus the segment size should be selected based on the tradeoffs between overhead and retransmissions. 2.2.2 Maximum Window Size Each side of a TCP connection has a buffer for incoming data. The amount of buffer space available is called the window size. The sending window buffers data that has been sent but not yet acknowledged. The receiver advertises its current window size to the sender so that the sender may determine how much data the receiver is willing to accept at any given time. This sets a limit to the sending window size. Since it takes at least one round trip time (RTT) for the data in the sending window to be acknowledged, the maximum throughput is limited by the following formula [8]: Throughput = sending window size/RTT. For the default TCP window size of 64Kbytes and RTT=0.56 s over a GEO satellite network, the maximum throughput for the given windows size is 0.936Mbps. Therefore, with standard TCP implementation, the throughput of TCP connections over satellite links is limited to 1Mbps. 2.2.3 Slow Start When a host begins sending data on a TCP connection the host has no knowledge of the current state of the network between itself and the receiver. In order to avoid transmitting inappropriately large bursts of traffic, the sender uses the slow start algorithm to control the amount of outstanding data being injected into the network at the beginning of a transfer or after repairing a loss. The slow start algorithm gradually increases the amount of unacknowledged data the TCP sender injects into the network. It uses the congestion window (CWND) to control this amount. The default value of the initial CWND is 1 segment. For each acknowledgment (ACK)

received during slow start, CWND is increased by 1 segment. TCP cannot inject more than CWND segments of unacknowledged data into the network. For a given RTT, the time that the slow start algorithm uses to reach a given window size of W segments on a network is [9]: slow start time = RTT*log2W Thus the slow start algorithm may take a long time to reach a given window size. Over a data link in which a fixed bandwidth is assigned to the TCP connection, much of this bandwidth is wasted during slow start phase. Based on the above, we see that in addition to previously proposed extensions of TCP to mitigate the impact of satellite networks, minimizing the RTT (or end-to-end delay over the satellite network) is the most important. The focus of this paper is to present a novel MAC protocol for satellite networks that minimizes end-to-end delay while maintaining a high channel throughput. III. PRE-RETURN RESERVATION COMBINED FREE/DEMAND ASSIGNMENT MULTIPLE ACCESS (PRR-CFDAMA) In satellite systems, it is important to select an efficient MAC protocol that gives high channel utilization, due to the high cost of the communication resource. Generally, there are four main classes of MAC protocols: fixed assignment, random access, DAMA and hybrid access. With fixed assignment, users can get the minimum end-to-end delay, but it provides poor channel utilization and is not suitable for bursty data traffic. With random access, it can get reasonable end-to-end delay only at lower throughput. DAMA can give higher channel throughput, but it provides poorer end-to-end delay for packet communications because of link establishment. Hybrid multiple access schemes are combinations of aforementioned schemes. The purpose is to reduce the transmission delay and increase the channel throughput. By combining random access and DAMA, hybrid access can get the lower delay of Aloha and higher throughput of DAMA. Of the hybrid access schemes, CFDAMA shows good performance in channel throughput and end-to-end delay [10, 11]. It is also a practical upgrade that is compatible to existing DAMA systems. With increasing Internet traffic transported over satellite networks, it is important to consider the characteristics of such traffic in the MAC protocol design. TCP employs an acknowledgement mechanism to provide reliable data transfer. TCP traffic has two noticeable characteristics. First, ACKs not only affect reliability but also influence the data transmission efficiency. If a TCP sender does not receive an ACK for a given segment within a certain amount of time, the timeout mechanism will trigger a retransmission of the segment, thus reducing the utilization of the satellite channel. Second, traffic volumes are highly asymmetrical. For example, in a typical HTTP transaction, a small request message from the client to the server results in a much larger response from the server to the client.

3.1 PRR-CFDAMA Protocol Mechanism We present an ACK-based pre-return reservation combined free/demand assignment multiple access (PRRCFDAMA) protocol that enhances CFDAMA by taking into account of Internet traffic characteristics, specifically those of TCP traffic. It works with MF-TDMA/TDM physical layer to provide a reliable data delivery service for interactive Internet applications. PRR-CFDAMA enhances the pre-assignment strategy in CFDAMA by dividing the packets coming from the terminals into two kinds: data packets and control packets. The control packets include ACK packets. While all data and control packets are sent by free assigned or reserved slots, during low and medium traffic, a data packet that is successfully transmitted to the recipient terminal embeds a return slot assignment for the recipient terminal using the free slot assignment mechanism. This means that when a terminal receives a data packet, it may also receive a free slot assignment to enable it to transmit the ACK in the next frame rather than waiting a long time to transmit a reservation request and receive a slot assignment. This strategy works specifically for the case where the host is connected directly to the recipient terminal, as is the case in most subscriber premises installations, and should not be used where the recipient terminal is a gateway that provides interconnection with the Internet. For CFDAMA, the slots that remain after assignments have been made for all outstanding requests are freely assigned to all the terminals in a round robin manner. Free slots assigned to the terminals with no data to transmit will therefore be wasted. Considering the need for an immediate return slot for ACK packet as discussed above, we modify the free assignment strategy in PRR-CFDAMA as follows. After honoring the reservation requests, a terminal that has just received a data packet on the downlink has a higher priority to receive a free slot assignment in the next uplink super frame, and then all remaining slots are assigned to all the terminals in a round robin fashion. This enhancement ensures that the slots are used more efficiently and the sender receives acknowledgement more quickly. Considering the asymmetrical ratio between data and control data is about 150 [2], the ACK-based PRR-CFDAMA is fair for all the terminals. Simulation results show that PRRCFDAMA can get good delay performance for short to medium messages. The performance improvement depends on the population of terminals and the channel capacity. 3.2 Physical Layer and Satellite On-board Processing 3.2.1 Physical Layer Selection Technological advancements have enabled satellites to provide higher transmit power by employing larger antennas and more powerful amplifiers onboard. On the other hand, the transmit power of earth terminals, particularly VSATs, are limited by the small dish antennas they employ. Based on these considerations, we assume uplink transmissions using MF-TDMA and downlink transmissions using TDM over the satellite channel. By this means, terminals can use a lower power to transmit data in a given frequency band and receive

all downlink broadcast messages easily from the satellite. It simplifies the satellite onboard processing and is more economical for earth terminals to change their uplink transmit frequency only rather than both uplink and downlink frequencies when changing channel. 3.2.2 On Board Scheduler Satellites can be classified as having bent-pipe transponders or on board processing (OBP) transponders. For new generation satellites with OBP capability, a centralized scheduler can be located in the satellite so that it takes a requesting terminal only one round-trip delay plus the scheduler queuing/processing time to receive the reply to its reservation. For the bent-pipe satellite the scheduler would be located in an earth station. In this case it takes the requesting terminal two round-trip delays plus the scheduler queue delay to receive a reservation. For our performance evaluations, we consider that a satellite with OBP is used. We consider that the uplink is divided in time into MFTDMA super-frames, each consisting of M TDMA frames. Each TDMA frame consists of K overhead mini-slots and N data slots. The total number of mini-slots in each super-frame is such that every terminal has its own fixed mini-slot in each super-frame for requesting reserved data slots. Each request indicates the number of control and data packets. Generally, the length of a packet is limited so that it fits into a data slot. All data slots in a super-frame are assigned based on requests sent in the mini-slots in a super-frame one round trip time earlier. The OBP firstly assigns slots to the requests from earth terminals, then to the recipient terminals by analyzing the destinations of the data packets being broadcast over the downlink, and finally to all terminals using the prioritized round robin method described before. The assignment information is queued onboard the satellite. The maximum length of the queue equals to the total available uplink data slots (N×M slots) in a super frame. It honors terminals’ requests on a first-in-first-served basis. If the total number of requested slots is greater than that available, we assume for simplicity that the unfilled requests are discarded. The data slot assignments are broadcast to the terminals in the corresponding downlink overhead slots. IV.

PERFORMANCE EVALUATIONS

4.1 Simulations with Poisson and On-off traffic Models The performance evaluations are by means of computer modeling and simulations in the OPNET environment. We model the DAMA, CFDAMA and PRR-CFDAMA protocols to compare their performance under Poisson and empirical Internet traffic. In order to simulate the ACK-based and interactive mechanism, we assume that when a recipient receives a data packet it has to send a return answer packet. Figs. 1, 2 show PRR-CFDAMA performances versus channel utilization with 200 and 400 earth terminals. Data packet inter-arrival times conform either to the Poisson distribution or on-off distribution for Internet traffic. We assume that a

of getting free slots decreases, so the delay increases. (4) The delay performance of PRR-CFDAMA with Poisson traffic is better than that with highly correlated on-off distribution. This is because the terminal with continuous load has to depend more on reservation slots rather than prereserving or free assigned slots. 4.2 Simulations with FTP over TCP

UDP

Fig. 1.

Delay performance with 200 terminals

super frame consists of 1600 slots, the duration of a slot is 0.18msec and the overhead is 1%. With the empirical on-off model for Internet traffic, the on-size is 17,932 bytes and offtime is 1313 seconds [13]. It can be seen that compared with CFDAMA, PRR-CFDAMA performs better at medium to high throughput, particularly with a larger number of terminals. A more detailed examination of the results reveals the following.

Fig. 2.

Delay performance with 400 terminals

(1) At lower channel utilization, every terminal has a good chance to get free slot assignments. The effect of wasted free slots that has been assigned to terminals with no data to send is insignificant. So the average delay for all terminals is small and the difference between CFDAMA and PRR-CFDAMA is small as well. (2) As channel utilization increases, the probability of getting free slots decreases for every terminal. The effect of wasted slots becomes more significant. With PRR-CFDAMA, recipient terminals have a higher priority to get a pre-reserved free slot assignment. That means that unused slots are allocated intentionally to where they are needed rather than freely assigned to terminals with no data to transmit. Eventually, as channel throughput approaches 1.0, most of the packets get through via demand assignment and the delay of either CFDAMA or PRR-CFDAMA is the same as DAMA. The probability of obtaining free-assigned slots is proportional to the number of earth terminals. (3) With increasing number of earth terminals, the chance

FTP

FTP

Tpal

Tpal

TCP

UDP

RSVP

TCP

RSVP

IP encap

IP encap IP

PRR-CFDAMA

PRR-CFDAMA

1Mbps

Fig. 3.

IP 1Mbps

Model of TCP/IP over PRR-CFDAMA

For a more realistic application scenario, we enhance the OPNET model to run FTP sessions over standard TCP/IP layered on top of the above-mentioned MAC protocols to compare their performance. Fig. 3 shows the system configuration. The transmission speed is 1 Mbps over the satellite uplink. The TCP parameters are set as Maximum segment size 536 bytes, Receive Buffer 65536 bytes and Slow-Start initial count (MSS) 1. The size of the file transferred in a FTP session is 50,000 bytes. We evaluate the system performance in terms of throughput from server to client, throughput from client to server and transmission time. For the fixed access scheme, the end-to-end delay is 0.28 seconds at lower load. In fact, it will increase with higher traffic load due to the bursty traffic. Therefore, we only consider the case where the channel throughput is smaller than 0.8 for fixed access. For DAMA, CFDAMA and PRRCFDAMA, the end-to-end delay increases gradually as the satellite channel throughput increases. We run the FTP session over different satellite channel throughput values and record their results. The results are presented in Figs. 4, 5. Compared with DAMA, we can see that CFDAMA and PRRCFDAMA make a great improvement. Also, it is evident that PRR-CFDAMA gives a better performance than CFDAMA.

Fig. 4.

Server to client throughput

ACKNOWLEDGMENTS This work was supported by a grant from the Canadian Institute for Telecommunications Research under the NCE Program of the Canadian Government, and jointly by the Canadian Natural Sciences and Engineering Research Council and the Canadian Space Agency under grant CSAPJ22323298. Yuheng Li’s visit to the University of British Columbia was supported by the Education Committee of China. REFERENCES Fig. 5.

Transmission time vs. throughput

4.2.2 Some Improvements Because of the inherent high delay of satellite channel, we can make some further improvements by properly tuning the TCP parameters, which default values have been chosen based on their suitability for terrestrial networks. In particular, we investigate the system performance by adjusting the value of the initial congestion window (ICW). For the given end-to-end delay values of 0.28 and 0.5 seconds, Figs. 6, 7 show the positive effect of increasing ICW. It means that a large ICW can mitigate the problems introduced by the high end-to-end delays of satellite links. We can see that when the end-to-end delay is 0.5 seconds and ICW is set as 16 MSS (maximum segment size), we can get the same performance as that when end-to-end delay is 0.28 seconds.

Fig. 6.

Server to client throughput vs. ICW Ⅴ. CONCLUSIONS

PRR-CFDAMA combines CFDAMA and ACK-based prereserved free slot assignments to provide a high channel throughput and short end-to-end delay for TCP applications that are prevalent in Internet traffic. As we can see from the simulation results of a FTP transfer over the satellite channel, although the delay performance of a hybrid MAC protocol is not as good as that of fixed assignment access, considering the fixed assignment wastes a lot of precious bandwidth for bursty packet communications, PRR-CFDAMA gives a better trade-off between the transmission delay and satellite channel utilization. Also, by tuning the TCP parameters, such as increasing the initial congestion window, we can improve the system performance noticeably.

[1] M. Allman, S. Floyd and C. Partridge, “Increasing TCP's initial window”, RFC 2414, Sep. 1998. [2] M. Allman and C. Hayes, “An evaluation of TCP with large initial windows”, ACM Computer Communication Review, 28 (3), July 1998. [3] M. Mathis and J. Ahdavi, “TCP selective acknowledgement options”, RFC 2018, Oct. 1996. [4] M. Allman, “TCP byte counting refinements”, ACM Computer Communication Review, 29 (3), July 1999. [5] V. Jacobson, R. Braden and D. Borman, “TCP extensions for high performance”, RFC 1323, May 1992. [6] M. Allman, P. Dawkins, D. Glover and J. Griner, “Ongoing TCP research related to satellites”, RFC 2760, Feb. 2000. [7] N. Ghani and S. Dixit, “TCP/IP enhancements for satellite networks”, IEEE Communication Magazine, pp. 64-72, July 1999. [8] J. Postel, “Transmission control protocol”, RFC793, Sep. 1981. [9] M. Allman, "Improving TCP performance over satellite channels", master’s thesis, June 1997. [10] T. Le-Ngoc and I.J. Mohammed, “Performance analysis of CFDAMA-PB protocol for packet satellite communications”, IEEE Trans. Commun., vol. 46, pp. 1206-1213, Sep. 1998. [11] T. Le-Ngoc and I.J. Mohammed, “Combined free/demand assignment multiple access (CFDAMA) protocols for packet satellite communication”, Proc. IEEE ICUPC, pp. 824-828, 1993. [12] W.M. Shvodian, “Multiple priority distribution round robin MAC Protocol for satellite ATM”, Proc. IEEE MILCOM, pp. 258-262, June 1998. [13] B.A. Mah, “An empirical model of HTTP network traffic”, Proc. IEEE INFOCOM, pp. 592 –600, 1997.

Fig. 7.

Transmission time vs. ICW