TCP over Wireless Networks:

Transcription

1 TCP over Wireless Networks: Issues Solutions Gürkan Gür NETLAB Seminar Series

2 TCP overview Outline TCP problems due to wireless link characteristics (focus: satellite) Performance enhancing proxies (PEPs) TCP variants & enhancements Open issues and potential solutions Conclusion 2

3 TCP Overview TCP: Connection oriented, reliable, end-to-end Self clocking mechanism How to start transmission? Slow Start : Exponential growth of cwnd Congestion Avoidance: Linear growth of cwnd Threshold between SS & CA: ssthresh 3

4 TCP and UDP Segment Format TCP Segment Format Source port (16) Destination port (16) Sequence number (32) Acknowledgment number (32) Header size (4) Reserved (6) 6 Control bits (6) Window size (16) Checksum (16) Urgent Pointer (16) Options Padding Data (variable length) UDP Segment Format Source port (16) Destination port (16) Length (16) Checksum (16) Data (variable length) 4

5 Slow-start 5

6 TCP Reno modifications Fast Retransmit Do not wait for retransmission timeout (RTO) Duplicate ACKs (3 or more) Fast Recovery Ssthresh = cwnd/2 Cwnd increased 1 MSS with each DUPACK ACK for new data : cwnd = ssthresh 6

7 Fast Recovery & Fast Retransmit 7

8 TCP Problems Long delay: RTT > 500ms (GEO) More time in slow start. Bandwidth is not utilized efficiently. Increasing the initial window (May cause burst of data) Multiple TCP connections Bandwidth x delay product: Unacknowledged packets to keep pipeline full = cwnd Cwnd limitation in TCP (64KB results in 1Mbps) Ka-band channel bandwidth=20mbps Window scale option to increase max. window size 8

9 Asymmetry: TCP Problems (cont.) Reason: power requirements, economical factors, system structure Throughput depends both on the forward and the reverse path. Congestion on return path cause throughput degradation (self clocking). Transmission errors: TCP is designed for low BER. Default assumption : loss is due to congestion RTT Fairness : A short RTT connection will unfairly capture a larger portion of the network bandwidth than a large RTT connection particularly in the presence of congestion and subsequent loss 9

10 Bandwidth-Delay Product 10

11 PEPs Different mechanisms: ACK spacing: to decrease sending rate Local ACKs: to speed up the slow start in networks with large BDP. Local retransmissions: to speed up the data recovery process. ACK filtering: prevent congestion caused by ACKs in highly asymmetric networks. Tunneling: encapsulation of messages to carry them over a particular path. Compression: can be applied to link layer and TCP or IP headers. Handling link outages: Sending the last ACK before the disconnection with cwnd = 0. 11

12 PEP Implementations Split TCP: divide the connection at the border of the terrestrial and satellite network and apply a well suited intermediate protocol for the network segment 12

13 PEPs (contd) TCP spoofing: local ACK mechanism is employed. Security: PEPs interfere with IPsec. If the PEP is trusted non transparent implementation of PEPs can operate with IPsec. 13

14 Satellite TCP Performance Example [11] Multiple TCP Connections over Satellite Link Without Enhancement With Protocol Gateway Enhancement Protocol gateway (splitting) improve throughput for carriers with TCP/IP traffic on satellite links up to a delay of 700 ms TCP/IP throughput is not affected as long as link BER is better than 1 x 10-7 Study the impact of BERs < 10-7 Review recommendation ITU-R S in view of satellite IP 14

15 SACK Selective acknowledgments option: Solution to multiple packet loss problem Informs the sender about the successfully received segments SACK is allowed with a SYN at conneciton establishment. Containes blocks representing received bytes of data SACKed segments are flagged in the queue If timeout occurs retransmission begins with segment on the left edge of the window. 15

16 TCP Vegas Proactive congestion detection Throughput changes determines the cwnd to obtain the optimum sending rate without a loss. Fine grained RTT measurments. RTT for DUPACKs are compared with RTO. Vegas does not wait for 3 DUPAC for retransmission. 16

17 TCP Vegas (contd) Window size is decreased only if a retransmission occurs after the last window decrease. An expected throughput is calculated depending on the minimum RTT measured. Window size is adjusted according to the difference of the expected and the actual throughput. Rerouting problem 17

18 SCPS-TP Space Communication Protocol Standard Transport Protocol (SCPS-TP): Uses the tools in TCP Vegas with modifications ECN is used Threshold between SS and CA is determined by throughput estimation. To identify link corruption an average number of packets that failed in CRC is used. 18

19 NewReno Solves the multiple packet drop problem in Reno. Last sequence number send when the third DUPAC received is holded in recover. If the ACK does not acknowledge all data upto recover, first unacknowledged segment is retransmitted, cwnd is deflated by the size of acknowledged segments and increased by one. 19

20 TCP Westwood Bandwidth estimation is calculated from the arrival rate af ACKs. ACK gives information about the size of the successfully received data Depending on the bandwidth estimation optimum values for cwnd and ssthresh is calculated DUPACKs also count for bandwidth with an average segment size. 20

21 TCP Jersey Available Bandwidth Estimation (ABE) is employed on sender side. ACKs are employed as in Westwood. Optimum window size is calculated with ABE. CW topology is used to determine congestion. (Similar to ECN). Ssthresh is set to ownd If congestion is notified cwnd is set to ssthresh 21

22 TCP Peach Sudden Start: Low priority dummy segments probes the avaliability of the bandwidth. Send every (RTT/rwnd) In case of a congestion TCP Peach behaves like TCP Reno. Rapid Recovery: Cwnd is halved 2*cwdn dummy segments are allowed. If a retransmitted segment get lost sudden start begins 22

23 Countermeasures 4 different strategies [20] The congestion detection approach, a collection of methods that measure the current network conditions to determine whether network congestion has actually occurred and choose a proper traffic control strategy based on the measured information. In other words, it aims to perform proper traffic control by differentiating the congestive issues from the noncongestive ones. The state suspension approach represents a group of techniques that detects the current state of the network so as to decide when the communication activity of a TCP connection is suspended and when it can be resumed in order to avoid noncongestive losses. The state of the connection may or may not be readjusted based on the network conditions after the suspension. The response postponement approach The response postponement approach is a class of solutions in which a TCP client delays triggering a traffic control response in order to alleviate the problems in wireless networks. The hybrid approach is a collection of methods that can be classified by more than one approach described above. Specifically, a TCP client may make use of a combination of mechanisms to collectively improve the TCP performance in wireless networks. Any solution that can be classified in the hybrid approach is not considered as a member of any other three approaches. 23

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25 Points to attack! Burst Losses Temporary Disconnections Multiple segment losses in the same congestion window. Cross-layer approach with low complexity Smart discriminator for wireless problems 25

26 Conclusions Standard TCP can not utilize the available bandwidth in wireless/satellite networks. Different solutions are suitable for different topologies Security needs and fairness should be taken into consideration. 26

27 References [1] HENDERSON, T. R., and KATZ, R. H., Transport Protocols for Internet-compatible Satellite Networks, IEEE Journal on Selected Areas in Communications, vol. 17, no. 2: [2] XYLOMENOS, G., POLYZOS, G. C., MAHONEN, P., and SAARANEN, M., TCP Performance Issue over Wireless Links, IEEE Communications Magazine, vol. 39, no. 4: [3] TSAOUSSIDIS, V., and MATTA, I., Open Issues on TCP for Mobile Computing, The Journal of Wireless Communications and Mobile Computing, John Wiley & Sons, Issue 1, Vol. 2. [4] TIAN, Y., XU, K., and ANSARI, N., TCP in Wireless Environments: Problems and Solutions, IEEE (Radio) Communications Magazine, Vol. 43, No. 3, pp. S27 - S32. [5] EHSAN, N., LIU, M., and RAGLAND, R., Evaluation of Performance Enhancing Proxies in Internet over Satellite, Wiley International Journal of Communications, vol. 16, no. 5. [6] GRIECO, L. A., and MASCOLO, S., Performance evaluation and comparison of Westwood+, New Reno, and Vegas TCP congestion control, ACM Comp. Comm. Rev., vol. 34, no. 2, pp [7] FLOYD, S. and HENDERSON, T., RFC2582: The NewReno Modification to TCP's Fast Recovery Algorithm. [8] MATHIS, M., MAHDAVI, J., FLOYD, S., and ROMANOW, A., RFC2018: TCP Selective Acknowledgment Options. [9] FLOYD, S., MAHDAVI, J., MATHIS, M., and PODOLSKY, M., RFC2883 : An Extension to the Selective Acknowledgement (SACK) Option for TCP. [10] HASSAN, M. and JAIN, R., High Performance TCP/IP Networking, Prentice Hall. [11] KOTA, S. L., PAHLAVAN, K., and LEPPÄNEN, P. A., Broadband Satellite Communications for Internet Access, Springer. [12] LUGLIO, M., SANADIDI, M. Y., STEPANEK, J., and GERLA, M., On-board Satellite Split TCP Proxy, IEEE Journal on Selected Areas in Communications, special issue on Broadband IP Networks via Satellites, Vol. 22, n. 2, pp [13] FANG, J. and AKAN, Ö. B., TCP-Peach++: Enhancement of TCP-Peach+ for Satellite IP Networks with Asymmetrical Bandwidth and Persistent Fades, LNCS, Springer-Verlag, vol. 3733, pp [14] MARCHESE, M., ROSSI, M., and MORABITO, G., PETRA: Performance Enhancing Transport Architecture for Satellite Communications, IEEE Journal on Selected Areas in Communications, Vol. 22, No. 2, pp: [15] XU, K., TIAN, Y., and ANSARI, N., TCP-Jersey for Wireless IP Communications, IEEE Journal on Selected Areas in Communications, Vol. 22, No. 4, pp [16] MARCONDES, C.A.C., PERSSON, A., CHEN, L., SANADIDI, M. Y., and GERLA, M., TCP Probe: A TCP with built-in Path Capacity Estimation, in Eighth IEEE Global Internet Symposium, Miami, USA. [17] A.TALEB, N. KATO, N.NEMETO, "REFWA ", IEEE Wireless Communications, vol. 12, no. 5, October 2005 [18] BHANDARKAR, S., SADRY, N., REDDY, A. L. N., and VAIDYA, N., TCP-DCR: A Novel Protocol for Tolerating Wireless Channel Errors, IEEE Transactions on Mobile Computing, vol. 4, no. 5, pp [19] MIORANDI, D. and GIAMBENE, G., Performance Evaluation of Scalable TCP and HighSpeed TCP over Geostationary Satellite Links, in Proc. of IEEE VTC05 Spring. [20] LEUNG, K. and LI, V. O. K., Transmission Control Protocol (TCP) in Wireless Networks: Issues, Approaches and Challenges, IEEE Communications Surveys, vol. 8, no. 4, 4 th Quarter

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