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166 Throughput [Mbit/s] Throughput [Mbit/s] 70 60 50 40 30 20 10 0 0 4096 8192 12288 16384 20480 24576 28672 32768 MSS segments to be transmitted back-toback. There is also a throughput increase from a 40k to a 52428 byte window size. This increase is also caused by the size of the socket buffers. The segment flow is still primarily two MSSs in-between acknowledgments for these window sizes. Both window sizes have bytes for two MSS segments ready in the socket buffer when the acknowledgment arrives. 24576 byte window 32768 byte window 40960 byte window 52428 byte window User data size [byte] (a) TCP throughput dependent on window and user data size 60 50 40 30 20 10 0 0 16384 32768 49152 65536 Window size [byte] (b)Throughput dependent on window size 4096 byte window 8192 byte window 16384 byte window CPU utilization [%] 100 75 50 25 Transmit Receive 0 0 16384 32768 49152 65536 Window size [byte] (c) CPU utilization Figure 15 TCP throughput, Sparc10 SBA-200 With a 52428 byte window size, the sender has buffer space for more than 5*MSS bytes. It sometimes manages to transmit 3 MSSs before the acknowledgment arrives. In short, the window is better utilized on connections with a 52428 byte window size which results in a higher throughput. For large window sizes and large user data sizes there is no degradation in throughput as with the SBA-100 adapters. With the SBA-200 adapter, the receiver is less loaded than the sender. Thus, the read system calls return before the entire user receive buffer is filled. Thereby, the socket layer more often calls TCP to check if a window update should be returned. 7.1 Throughput peak patterns For small window sizes there are throughput peaks which are even more characteristic than peaks observed with the SBA-100 interfaces on the Sparc10s. For example, with an 8 kbyte window the variation can be as high as 10 Mbit/s for different user data sizes. This is caused by a mismatch between user data size and window size which directly affects the segment flow. Figure 15 shows that the throughput pattern to some extent repeats for every 4096 bytes. With the SBA-200 interface, the high byte-dependent overhead of the segment processing is reduced. This implies a relative increase in the fixed overhead per segment, and the number of segments will have a larger impact on throughput performance. Window and user data sizes that result in the least number of segments are expected to give the highest performance. This is reflected in the throughput peaks for user data sizes of an integer multiple of 4096 bytes. With the SBA-100 adapters, the byte dependent processing was much higher, and the throughput was therefore less dependent on the number of transmitted segments. For large window sizes there are no throughput peak patterns. For these window sizes, the number of outstanding unacknowledged bytes seldom reaches the window size. The segment flow is primarily maximum sized segments of 9148 bytes with an acknowledgment returned for every other segment. 8 Summary and conclusions In this paper we have presented TCP/IP throughput measurements over a local area ATM network from FORE Systems. The purpose of these measurements was to show how and why end-system hardware and software parameters influence achievable throughput. Both the host architecture and the host network interface (driver + adapter) as well as software configurable parameters such as window and user data size affect the measured throughput.
We used small optimized probes in the network device driver to log information about the segment flow on the TCP connection. The probes parsed the TCP/IP header of outgoing and incoming segments to log the window size, the segment lengths, and sequence numbers. Our reference architecture is two Sparc2 based machines, Sun IPXs, each equipped with a programmed I/O based ATM network interface, SBA-100 with device driver version 2.0 or 2.2.6 using ATM adaptation layer 3/4. We measured memory-to-memory throughput as a function of user data and window size. From these measurements we conclude: - The maximum throughput is approximately 21 Mbit/s when using the 2.0 network driver version. A software upgrade of the network driver to version 2.2.6 gave a maximum throughput of approximately 26 Mbit/s. - A window size above 32 kbytes contributes little to an increase in performance. - Increasing the window size may not result in a throughput gain using an arbitrarily chosen user data size. - The large ATM MTU results in TCP computing a large MSS. Because of the large MSS, for small window sizes the TCP behavior is stop-and-go within the window size. Then, we performed the same measurements on a Sparc10 based machine, Axil 311/5.1. The MIPS rating of the Sparc10 is about 4.5 times higher than the Sparc2 MIPS rating. However, due to the machine architecture, the latency between the host CPU and the Sbus network adapter is higher on the Sparc10. We presented measurements of the send and receive path of the network driver which support this. For small segment sizes the total send and receive times are also higher on the Sparc10. From these measurements we conclude: - Maximum throughput is approximately 34 Mbit/s. - Increasing the window size results in higher performance. - Access to the network adapter is a larger bottleneck than on the Sparc2. - For small windows and user data sizes the measured throughput is actually lower than on Sparc2. The largest throughput gain is achieved by upgrading the network adapter to the SBA-200/2.2.6. The DMA-based SBA- 200 adapter relieves the host from the time-consuming access to the adapter. Thus, the host resources can be assigned to higher-level protocol and application processing. From these measurements we conclude: - Maximum throughput is approximately 62 Mbit/s. - Increasing the window size clearly results in higher performance. - For small windows this configuration is more vulnerable to an inefficient segment flow, because byte dependent overhead is relatively much lower compared to the fixed segmentdependent overhead. - Variation in throughput within one window size is the highest for small window sizes. Since primarily maximum sized segments are transmitted with large windows, the higher the window size, the lower the probability of an inefficient segment flow. References 1 Jacobsen, V, Braden, B, Borman, D. TCP extensions for high-performance. I: RFC 1323, May 1992. 2 Cabrera, L P et al. User-process communication performance in networks of computers. IEEE transactions on software engineering, 14, 38–53, 1988. 3 Clark, D et al. An analysis of TCP processing overhead. IEEE communications magazine, 27, 23–29, 1989. 4 Nicholson, A et al. High speed networking at Cray Research. ACM computer communication review, 21 (1), 99–110, 1991. 5 Caceres, R et al. Characteristics of Wide-Area TCP/IP Conversations. I: Proceeding of SIGCOMM ‘91, Zürich, Switzerland, 101–112, 3–6 September 1991. 6 Mogul, J C. Observing TCP dynamics in real networks. I: Proceedings of ACM SIGCOMM’92, 305–317, Baltimore, USA, 17–20 August 1992. 7 Papadopoulos, C, Parulkar, G M. Experimental evaluation of SUNOS IPC and TCP/IP protocol implementation. IEEE/ACM transaction on networking, 1, 199–216, 1993. 8 Postel, J. Transmission control protocol, protocol specification. I: RFC 793, September 1981. 9 Lyle, J D. SBus : information, applications and experience. New York, Springer, 1992, ISBN 0-387-97862- 3. 10 Moldeklev, K, Gunningberg, P. Deadlock situations in TCP over ATM. I: Protocols for high-speed networks ’94, Vancouver, Canada, 219–235, August 1994. 11 Leffler, S J et al. 4.3 BSD Unix operating system. Reading, Mass., Addison-Wesley, 1989, ISBN 0-201- 06196-1. 12 Advanced Micro Devices. TAXIchip TM Integrated Circuits, transparent asynchronous transmitter/receiver interface. Am7968/Am7969-175, data sheet and technical manual, 1992. 13 Requirements for Internet hosts – communication layers. I: RFC 1122, R. Braden (ed.). October 1989. 14 Postel, J. The TCP maximum segment size and related topics. I: RFC 879, November 1983. 15 Jacobsen, V. Congestion avoidance and control. I: Proceedings of ACM SIGCOMM’88, 314–329, Palo-Alto, USA, 16–19 August 1988. 16 Nagle, J. Congestion control in TCP/IP internetworks. I: RFC 896, January 1984. 17 FORE Systems. SBA-100 SBus ATM computer interface – user’s manual. 1992. 18 Cooper, E et al. Host interface design for ATM LANs. I: Proceedings of 16th conference on local computer networks, 247–258, October 1991. 19 FORE Systems. 200-Series ATM adapter – design and architecture. January 1994. 167
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Contents Feature Guest editorial, A
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20 j k λ ij µji λ ik µ ki λ ir
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30 Frame 6 Basic queuing model From
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32 Traffic sources (N) ⇒ III - -
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34 Since Gw (t) is the survivor fun
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38 Figure 38 Mixed international da
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40 3 Gaaren, S. LAN interconnection
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42 Solution of some problems in the
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44 Table 3 a. The “Loss” (in
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48 Telephone Systems” (published
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50 The life and work of Conny Palm
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52 mind that the seventies were bef
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56 Architectures for the modelling
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62 ACSE Presentation layer Session
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82 throughput 1 Introduction Commun
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Table 4 Combined signalling and pac
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user access device 106 sound/speech
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Page 162 and 163: 160 Throughput [Mbit/s] Throughput
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