IP ... - IEEE Xplore

REFERENCES [1]

[2]

[3]

[4]

[5]

[6]

[7]

[8]

[9]

[10]

[11]

[12]

[13]

[14]

[15]

Bar-Shalom, Y., and Li, X. R. Multitarget-Multisensor Tracking: Principles and Techniques. Storrs, CT: YBS Publishing, 1995. Blackman, S., and Popoli, R. Design and Analysis of Modern Tracking Systems. Boston: Artech House, 1999. Blair, W. D., and Brandt-Pierce, M. Statistical description of monopulse parameters for tracking Rayleigh targets. IEEE Transactions on Aerospace and Electronic Systems, 34, 2 (Apr. 1998), 597—611. Blair, W. D. Practical aspects of multisensor tracking. In Y. Bar-Shalom and W. D. Blair (Eds.), Multitarget-Multisensor Tracking: Applications and Advances Vol. III, Boston: Artech-House, 2000. Chang, K. C., Saha, R. K., and Bar-Shalom, Y. On optimal track-to-track fusion. IEEE Transactions on Aerospace and Electronics Systems, 33, 4 (Oct. 1997), 1271—1275. Chen, H., Kirubarajan, T., and Bar-Shalom, Y. Performance limits of track-to-track fusion versus centralized estimation: Theory and application. IEEE Transactions on Aerospace and Electronics Systems, 39, 2 (Apr. 2003), 386—400. Chong, C. Y., Chang, K. C., and Mori, S. Distributed tracking in distributed sensor networks. In Proceedings of the American Control Conference, Seattle, WA, 1986. Cover, T., and Thomas, J. Elements of Information Theory. New York: Wiley, 1991. Drummond, O. Track and tracklet fusion using data from distributed sensors. In Proceedings of the Workshop on Estimation, Tracking and Data Fusion: A Tribute to Yaakov Bar-Shalom, Monterey, CA, May 2001. Duda, R. O., Hart, P. E., and Stork, D. G. Pattern Classification (2nd ed.). New York: Wiley, 2001. Li, X. R., and Zhang, K. Optimality and efficiency of optimal distributed fusion. In Proceedings of the Workshop on Estimation, Tracking and Data Fusion: A Tribute to Yaakov Bar-Shalom, Monterey, CA, May 2001. Li, X. R. Reduction of communication in distributed fusion. Presented at the 5th Annual ONR/GTRI Peer Review, San Diego, CA, May 2003. Peebles, P. Digital Communication Systems. Englewood Cliffs, NJ: Prentice-Hall, 1987. Ruan, Y., Willett, P., and Marrs, A. Fusion of quantized measurements via particle filtering. In Proceedings of the 2003 Aerospace Conference, Big Sky, MT, Mar. 2003. Willett, P., Blair, W. D., and Bar-Shalom, Y. On the correlation between horizontal and vertical monopulse measurements. IEEE Transactions on Aerospace and Electronic Systems, 39, 2 (Apr. 2003), 533—549.

Impact of Van Jacobson Header Compression on TCP/IP Throughput Performance over Lossy Space Channels

The impact of Van Jacobson header compression (VJHC) on the throughput performance of Transmission Control Protocol/Internet Protocol (TCP/IP) over lossy space channels is studied in an experimental manner using a test-bed. The experimental results show that VJHC benefits the transmission at bit error rates (BERs) around 10¡6 or less, but also results in performance degradation in an environment with higher BER.

I. INTRODUCTION The original header compression scheme proposed for the Transmission Control Protocol/Internet Protocol (TCP/IP) was Van Jacobson header compression (VJHC) [1] developed to perform on a hop-by-hop basis at the link layer. The main goal behind TCP/IP’s VJHC is to improve line efficiency for a serial link. Following the transmission of the first uncompressed TCP/IP header, only the encoded difference to the preceding header, is transmitted in following headers. Uncompressed headers are recovered by applying the differences contained in the newly received compressed header to the preceding header. However, if a segment is lost or corrupted, an invalid uncompressed header will be created when the incorrect changes are applied to the stored compression state and detected by the TCP checksum. Following this, all packets arriving after the lost or corrupted packet will be decompressed improperly, causing them to be discarded at the receiver. During this period, the TCP/IP sender will not receive acknowledgment, thus the self-clocking of the transmission is broken. As a result, the sender is forced into a timeout. The TCP/IP sender eventually retransmits the original corrupted or lost packet uncompressed to recover synchronization for compression. Synchronization recovery is costly, especially when operating with a large bandwidth-delay product (BDP) environment, as it causes go-back-n retransmission behavior [2]. The go-back-n behavior dictates that the invalid packets, up to one BDP’s worth, are lost, when a single packet is lost or corrupted. Following the implementation Manuscript received April 20, 2004; revised September 23, 2004; released for publication November 4, 2004. IEEE Log No. T-AES/41/2/849037. Refereeing of this contribution was handled by T. F. Roome.

c 2005 IEEE 0018-9251/05/$17.00 ° CORRESPONDENCE

681

Fig. 1. File transfer time versus BER for symmetric channel.

of VJHC, several header compression schemes have been proposed for Internet protocols [3—8]. But these schemes have not been widely deployed yet due to various reasons. VJHC is still widely used in today’s Internet. As the Internet has been dramatically changing the world, the government and the space industry will consider expanding the Internet into the space environment using the Internet-type protocols. There has been some work done in studying the performance of VJHC over long delay and high bit error rate (BER) communication channels such as in space and wireless environments. However, most studies have been done based on only the definitions and methodology of the scheme [9—11]. There are hardly any experimental results seen in support of the discussions on the impact of TCP/IP’s VJHC in lossy communication channels, particularly for low bit-rate wireless and satellite links. The work presented here studies the impact of VJHC on TCP/IP throughput for low bit-rate GEO-hop satellite channels in an experimental manner using the space-to-ground link simulation (SGLS) test-bed [12, 13]. The VJHC for TCP/IP packets is implemented with the point-to-point protocol (PPP) which comes with the Red-Hat Linux 7.3 operating system. The experiments are conducted by running file transfer tests using the actual protocol stack, FTP/TCP/IP/PPP, via the SGLS test-bed. For a detailed description of the SGLS test-bed and experimental methodologies, see [12]—[14]. The discussion based on quantitative numbers from the experimental work is expected to be useful in developing new header compression schemes and developing adaptive resource allocation protocols for particular applications. It is well known that space communication and mobile communication environments show many similarities when observed from the perspective of network protocols. Therefore, the results of this study are also expected to be useful in mobile and similar environments. 682

II.

EXPERIMENT RESULTS AND DISCUSSION

This section discusses the experiment results of TCP/IP with VJHC enabled and disabled at different BERs for both symmetric and asymmetric channel rates. A delay of 120 ms is chosen to simulate the transmission delay of a GEO-satellite communication link. This will also adequately emulate communications between a ground station and a low-Earth orbiting satellite where the communications link is routed through a GEO relay satellite. Space channels are frequently operated in both symmetric (equal return and forward data rates) and asymmetric (unequal return and forward data rates with the return usually much higher than the forward) modes, both symmetric and asymmetric channels are simulated using the channel rates of 115,200 bit/s: 115,200 bit/s and 115,200 bit/s: 2400 bit/s, respectively. Here, the first data rate is the return data rate and the second number is the forward data rate. Three BERs, 0, 10¡6 , and 10¡5 , are chosen to simulate error-free, a moderate error rate and a maximum acceptable error rate on the channel. Although the BERs of 10¡6 and 10¡5 are considered as a high error rate for wired ground Internet channels, it is not unusual for wireless and space channels. The BERs of 10¡6 and 10¡5 are within the minimum communication specifications for the National Aeronautics and Space Administration’s Space Network [15]. A sample size of 16 runs is chosen for each file transfer. The performance is analyzed by plotting the averaged transfer time of the 16 runs for a 1 Mbyte file and by conducting the statistical t-test mean comparisons [16, 17]. A. Performance Comparison for Symmetric Channel Fig. 1 plots the averaged file transfer time of TCP/IP versus BER with VJHC enabled and disabled

IEEE TRANSACTIONS ON AEROSPACE AND ELECTRONIC SYSTEMS VOL. 41, NO. 2 APRIL 2005

Fig. 2. TSG comparison for symmetric channel with BER = 0.

Fig. 3. Segment of TSG comparison for symmetric channel with BER = 0.

for symmetric channel rate. To show a clearer plot of file transfer time versus BER, we use BER = ¡7 to represent the BER value of log 10 = 0, i.e., the error-free channel. The same notation is followed for all plots here. Tests with an error-free channel are expected to predict the protocol performance over what can be taken to be an error-free satellite link. In practice, when the combined convolutional and Reed-Solomon forward coding is used as an inner-code/outer-code configuration, the actual space channel tends to approach this level of performance. From the plot, we can observe that the transmission CORRESPONDENCE

with VJHC enabled takes much less time than that with VJHC disabled for the lower BERs of 0 and 10¡6 . However, for a very high BER around 10¡5 , the file transmission time with VJHC disabled is significantly less than the time taken with it enabled. The t-test results show that the time differences of all three pairs at the tested BERs are statistically significant at a 95% confidence level. Therefore, it can be concluded that for a symmetric channel, TCP/IP with VJHC enabled shows a much higher throughput than TCP/IP with VJHC disabled when 683

Fig. 4. TPG comparison for symmetric channel with BER = 0.

Fig. 5. TSG comparison for symmetric channel with BER = 10¡5 .

the BER is not very high. With the increase in BER, both cases show significant decrease in throughput. When BER is about 10¡5 , the performance with VJHC disabled exceeds that with VJHC enabled. This can be understood in the way that with VJHC enabled, the loss of packets due to high BER in poor channels results in loss of synchronization and difficulty in recovery and consequently degrades the throughput performance [9, 11]. Now let’s look at the comparison pair with the error-free channel in Fig. 1 using various performance 684

measures. Fig. 2 compares the time sequence graphs (TSGs) of two scenarios, one with VJHC enabled and the other with VJHC disabled, for an error-free symmetric channel. There is no retransmission observed in either case because of the error-free channel. An error-free channel represents the ideal condition in which header compression works. From the plot, it is observed that the file transfer time with VJHC enabled is much shorter. VJHC compresses the packets’ headers, minimizing the time taken to send 1 Mbyte files. VJHC provides greater throughput


Fig. 6. Consecutive retransmission scenarios with VJHC enabled at BER = 10¡5 .

Fig. 7. Consecutive retransmission scenarios with VJHC disabled at BER = 10¡5 .

for file transfers over channels with zero or low BER. Fig. 3 shows a segment of Fig. 2. The segment shows the file transmission in a steady state for both scenarios. Each packet and/or packet cluster is transmitted adaptively along with an acknowledgment (ACK) received from the receiver. Each returning packet updates both the ACK and the end of the window. The transmission is definitely not limited by the window size since the window is never fully filled. The transmission rate is limited by the serial, low-rate CORRESPONDENCE

PPP link and the long link delay of 120 ms. It can also be observed that it takes less time to acknowledge each packet in the case of VJHC enabled compared with VJHC disabled. This is reasonable because enabling VJHC compresses the size of a packet, reducing the packet transmission time and thus makes packets acknowledged earlier by the receiver. Fig. 4 compares the averaged throughput graphs (TPGs) for transmissions with error-free and a symmetric channel rate. When VJHC is enabled, the average throughput varies through the complete 685

Fig. 8. TPG comparison for symmetric channel with BER = 10¡5 .

Fig. 9. File transfer time versus BER for asymmetric channel.

span of the packet transmission. At the onset of transmission when the channel is empty, there is a sudden rise in throughput because of the large number of packets dumped by the sender. As the transmission enters into steady state, the average throughput stabilizes to around 18800 bytes/s. In comparison, the average throughput for VJHC disabled is nearly consistent throughout the transmission and on the whole is around 8000 bytes/s lower than the throughput with VJHC enabled. A comparison pair at BER = 10¡5 is shown in Fig. 5 which illustrates a comparison of the TSGs for VJHC enabled and disabled. In Fig. 1, it was concluded that the TCP/IP performance with VJHC disabled exceeds that with VJHC enabled. This can be verified by their TSG plots in which the total file 686

transfer time with VJHC enabled is slightly longer than the time with VJHC disabled. It can also be seen that the TSG with VJHC disabled increases more linearly as compared with its counterpart. As explained, in the case of VJHC disabled, only the corrupted or lost packet is retransmitted. The avoidance of unnecessary retransmissions saves considerable time when the transmission channel has a very high error rate. Comparing the TSGs with error-free in Fig. 2, a severe degradation of the transmission performance is observed for both configurations with BER = 10¡5 . The degradation is caused by frequent retransmissions due to the introduction of a much higher error rate. These frequent retransmissions are indicated by many overlapped Rs and Ss. An R represents a packet


Fig. 10. TSG comparison for asymmetric channel with BER = 0.

Fig. 11. Retransmission scenarios for asymmetric channel with VJHC disabled.

retransmission when VJHC is enabled and an S represents a SACK option requesting for a packet retransmission when VJHC is disabled. About 80 retransmission scenarios are occurring in each data transmission stream, which is correct according to the calculation: 1000003 £ 8 £ 10¡5 ¼ 80 errors. Fig. 6 and Fig. 7 illustrate the scenarios of two consecutive packet corruptions and resulted retransmissions for VJHC enabled and disabled of Fig. 5. In both cases, due to a very high error CORRESPONDENCE

rate, a packet corruption happened following the transmission recovery of the preceding packet corruption with only two new packets transmitted between them. The difference is that five extra packets are retransmitted for each packet corruption in the case of VJHC enabled while only the corrupted packet is retransmitted in the case of VJHC disabled. The difference can also be observed from the corruption recovery time. It takes about 3 s for TCP/IP with VJHC disabled to recover from two packet corruptions, which is much less than the time 687

Fig. 12. Segment of TSG for asymmetric channel with VJHC enabled.

Fig. 13. Segment of TSG for asymmetric channel with VJHC disabled.

for TCP with VJHC enabled, about 5.5 s. The whole file of 1 Mbyte is transmitted in the similar pattern for each configuration. This explains the conclusion that over a very lossy channel, transmission with VJHC disabled has a higher throughput than transmission with VJHC enabled. Fig. 8 shows the TPG comparison with BER = 10¡5 . With VJHC enabled, the throughput increases considerably in the beginning and then gradually stabilizes. With VJHC disabled, the throughput stabilizes much faster. The overall throughput 688

with VJHC enabled is slightly higher than that with VJHC disabled. But the transmission with VJHC disabled ends earlier as verified from the transmission time comparison in Fig. 1. In other words, although the averaged throughput with VJHC enabled is a bit higher, its file transmission time is longer. This happens because enabling VJHC results in the large number of additional retransmissions (due to the go-back-n behavior of the VJHC) that are also counted in the calculation of throughput.


Fig. 14. TPG comparison for asymmetric channel with BER = 0.

Fig. 15. ODG comparison for asymmetric channel with BER = 0.

Based on the above analysis, the impact of VJHC on TCP/IP performance in GEO-satellite symmetric channel environment can be summarized below. 1) VJHC benefits data transmission and can bring a higher throughput when BER is not very high (around 10¡6 or less). 2) As BER increases, the significant advantages of VJHC disappear. At a very high BER (around 10¡5 ), VJHC hurts the performance of TCP/IP because of the ineffectiveness of the large number of unnecessary retransmissions and difficulty of synchronization recovery due to its go-back-n behavior. CORRESPONDENCE

B. Performance Comparison for Asymmetric Channel Fig. 9 plots the file transfer time versus BER with VJHC enabled and disabled for an asymmetric channel rate. Similar to the case of symmetric channel rate, the transmission with VJHC enabled takes much less time than the transmission with VJHC disabled when BER is not very high. But when BER is increased to 10¡5 , both transmissions merge, indicating that they show about the same performance in a very lossy channel. It is possible that TCP/IP with VJHC disabled performs better than TCP/IP 689

Fig. 16. TSG comparison for asymmetric channel with BER = 10¡5 .

Fig. 17. Segment of TSG comparison for asymmetric channel with BER = 10¡5 .

with VJHC disabled when BER is extremely high such as 10¡4 . But no experimental data is available for verifying this prediction in this experiment. The t-test results show that their performance differences are statistically significant at the BERs of 0 and 10¡6 but not statistically significant at the BER of 10¡5 . Fig. 10 compares the TSGs of two error-free configurations with an asymmetrical channel rate. The file transfer time with VJHC disabled is much higher than the time with VJHC enabled. Two retransmission 690

scenarios are seen for the transmission with VJHC disabled. Fig. 11 shows the first retransmission scenario in which four consecutive packets are retransmitted. The retransmissions are unlikely to be caused by bit error corruptions since no bit error is introduced to the channel. These four packets are delayed by several seconds and are not retransmitted until the retransmission timer is fired. Most likely, the transmission has a very long queue built up somewhere in the middle which causes the delay to increase.


Fig. 18. TPG comparison for asymmetric channel with BER = 10¡5 .

Based on the above analysis for the GEO-satellite asymmetric channel, the following conclusions can be derived: TCP/IP over an asymmetric channel shows similar traits as over a symmetric channel and VJHC benefits the transmission when BER is not very high (< 10¡5 ), but brings no advantages when BER is as high as 10¡5 . Figs. 12 and 13 show a segment of the TSG for VJHC enabled and VJHC disabled. For each configuration, data are transmitted in a series of packet flights with each flight containing about the same number of packets. A lull is observed between every two successive flights. When the sender has already sent out an entire flight, channel capacity is filled and the sender cannot send any more packets until an ACK arrives to acknowledge this flight. The lull occurs because the sender is doing nothing but waiting for an ACK over a slow ACK channel. For each flight, the spacing in time of transmitted packets corresponds to the spacing in time of the returning ACKs. The window is often half filled when VJHC is disabled but often fully filled when VJHC is enabled. Provided that the default buffer size of 65536 bytes is configured for both configurations, this is happening because enabling the VJHC compresses the size of packet header and makes more packets to arrive and be buffered at the receiver. This makes the FTP application unable to read the data on time. For this reason, the progress of data transfer is limited by the sender waiting for an ACK to open the window. The packets are sent as soon as an ACK is received and the window opens. In comparison, for VJHC disabled, the window often stays open and the progress is CORRESPONDENCE

merely limited by waiting for the returning ACKs. However, for both connections, throughput is mainly limited by the slow data rate of 115200 bit/s and the slower ACK rate of 2400 bit/s. The throughput comparison for asymmetric channel shown in Fig. 14 is very similar to the comparison for symmetric channel in Fig. 4. The average throughput for VJHC enabled is much higher than that for VJHC disabled. The throughput comparison can be explained using the outstanding data graphs (ODGs) shown in Fig. 15. The outstanding data represents the number of unacknowledged data bytes and is often used to estimate the congestion window size. The congestion window size specifies the number of data bytes that the sender can transmit and generally represents transmission throughput. An increase in outstanding data at a given time generally reflects an increase of throughput. In Fig. 15, the outstanding data with VJHC enabled rises sharply in a short time and then tends to remain steady. The outstanding data with VJHC disabled is much lower and has two drops around 60 s and 150 s. The two drops of outstanding data correspond to two retransmission scenarios in TSG and decreases in TPG. Fig. 16 gives the TSG comparison pair for VJHC enabled and disabled over an asymmetric channel with BER = 10¡5 . Two TSGs are closely bound together with many Rs and Ss overlapped representing frequent retransmissions, as seen from the enlarged segment in Fig. 17. The TSGs in Fig. 16 show about the same slope. This means that the two transmissions perform similarly and have nearly equal performance. 691

The t-test result indicates that their performance differences are not statistically significant. The performance equivalence is also reflected from the throughput comparisons in Fig. 18. A closer look at the TSG plot shows that packets are transmitted in a similar pattern as the symmetric channel in Fig. 5 with the only difference being the longer ACK time due to slower ACK channel rate. Based on the above analysis for the GEO-satellite asymmetric channel, the following conclusions can be derived: TCP/IP over an asymmetric channel shows similar traits as over a symmetric channel and VJHC benefits the transmission when BER is not very high (< 10¡5 ), but brings no advantages when BER is as high as 10¡5 .

REFERENCES [1]

[2]

[3]

[4]

[5]

[6]

III. CONCLUSIONS In both symmetric and asymmetric channel environments, VJHC benefits the transmission and brings a higher throughput when the BER is not very high (around 10¡6 or less). However, an increase in BER affects the performance difference between two header compression configurations. As BER increases, VJHC does not bring significant advantages for either channel rate. At a very high BER (around 10¡5 ), VJHC hurts the performance of TCP/IP for symmetric channel and brings no advantage for the asymmetric channel. This result is attributed to the VJHC ineffectiveness of a large number of unnecessary packets retransmissions and difficulty of synchronization recovery due to its go-back-n behavior. This suggests that it is better to enable TCP/IP’s VJHC in a lossy channel (with BER around 10¡6 or less) but to disable it in a very lossy channel (with BER around 10¡5 ), especially in symmetric channel environment.

[7]

[8]

[9]

[10]

[11]

[12]

ACKNOWLEDGMENTS The authors would like to express appreciation to the Lamar University Department of Electrical Engineering and the Office of Graduate Studies and Research, as well as K. Scott, P. Feighery, and R. Durst of MITRE Inc., for their support and assistance in establishing the test-bed and conducting the experiments. RUHAI WANG Department of Electrical Engineering Lamar University Beaumont, TX 77710-1029 E-mail: ([email protected]) STEPHEN HORAN Klipsch School of Electrical and Computer Engineering New Mexico State University Las Cruces, NM 88003-8001 692

[13]

[14]

[15]

[16]

[17]

Jacobson, V. Compressing TCP/IP headers. IETF Request for Comments RFC 1144, Feb. 1990. Lin, S., and Costello, D. Error Control Coding: Fundamentals and Applications. Englewood Cliffs, NJ: Prentice-Hall, 1983, 459—460. Pink, S., and Mutka, M. Dependency removal for transport protocol header compression over noisy channels. In Proceedings of ICC’97. Degermark, M., Nordgren, B., and Pink, S. IP header compression. IETF Request for Comments RFC 2507, Feb. 1999. Casner, S., and Jacobson, V. Compressing IP/UDP/RTP headers for low-speed serial links. IETF Request for Comments RFC 2508, Feb. 1999. Space Communications Protocol Specification (SCPS)–Network Protocol (SCPS-NP). Recommendation for Space Data System Standards, CCSDS 713.0-B-1. Blue Book. Issue 1. Washington, D.C.: CCSDS, May 1999. Burmeister, C., Degermark, M., Fukushima, H., et al. RObust Header Compression (ROHC): Framework and four profiles: RTP, UDP, ESP, and uncompressed. Internet Draft hdraft-ietf-rohc-rtp-09.txti, Feb. 2001. Liao, H., et al. TCP/IP-Aware RObust Header Compression (TAROC). Internet Draft hdraft-ietf-rohc-TCP/IP-taroc-01.txti, Internet Engineering Task Force, Mar. 2001. Ishac, J. A. Survey of header compression techniques. National Aeronautics and Space Administration, Glenn Research Center, Sept. 2001. ISCPS Team, Computer Science Corporation. Inter-spacecraft communication protocol study (ISCPS)–Final report, vol. 1. Prepared for Goddard Space, Mar. 1999. Durst, R., Miller, G., and Travis, E. TCP/IP extensions for space communications. In Proceedings of the 2nd ACM Conference on Mobile Computing and Network, Nov. 1996. Horan, S., and Wang, R. Design of a space channel simulator using virtual instrumentation software. IEEE Transactions on Instrumentation and Measurement, 51, 5 (Oct. 2002), 912—916. Horan, S., and Wang, R. Design of a channel error simulator using virtual instrument techniques for the initial testing of TCP/IP and SCPS protocols. NMSU-ECE-99-002, vol. 1, 1999. Wang, R., Bonasu, S., and Bagasrawala, S. The impact of TCP/IP header compression on performance over satellite links. In Proceedings of IEEE Wireless Communication and Network Conference (WCNC), Atlanta, GA, Mar. 2004. National Aeronautics and Space Administration. Space Network User’s Guide, Rev. 7. Goddard Space Flight Center, Greenbelt, MD, 2001. Dowdy, S., and Wearden, S. Statistics for Research (2nd ed.). New York: Wiley, 1991, 303—305. SAS Institute Inc. SAS User’s Guide: Statistics, Version 8 Edition. SAS Institute Inc., SAS Circle, Cary, NC 2001.
