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156 Sparc2 (SunOS 4.1.1) or Sparc10 (SunOS 4.1.3) do 25 times send 16 MB Data ttcp TCP AAL3/4 end-of-message interrupt on incoming packets. The second generation Sbus adapter, SBA-200 [18][19], includes an embedded Intel i960 RISC control processor, and hardware support for both AAL 3/4 and AAL5 CRC calculation. It is an Sbus master device and uses DMA for data transfer on both the send and receive path. The segmentation and reassembly processing is performed by the control processor on the adapter using the host memory for storage of frames. The SBA- 200 adapters have the best performance when using AAL5, and they were configured to provide an AAL5 end-offrame interrupt on incoming packets. The ATM transmission capacity is not a bottleneck. Therefore, compared to AAL3/4 the reduced AAL5 segmentation and reassembly header overhead should not be a decisive factor on the measured throughput. Using AAL3/4, the on-adapter receive control processor turns out to be a bottleneck for large TCP window sizes. 2.2 Host architectures The memory-to-memory measurements presented in this paper are performed between two Sparc2 based Sun IPX machines, and between two Sparc10 based Axil 311/5.1 machines. The workstations were not tuned in any way, except for allowing only a single user during the measurements. All standard daemons and other network interfaces were running as normal. In the rest of this paper we name the two machine types Sparc2 and Sparc10. The MIPS rating (28.5 vs. 135.5) is about 4.5 times higher for the Sparc10 com- IP AAL ATM Acknowledgements Application Higher level protocols Network interface=driver+adapter SBA-100/AAL3/4 SBA-200/AAL5 ATM Figure 1 Measurement environment and set-up Sparc2 (SunOS 4.1.1) or Sparc10 (SunOS 4.1.3) Data do 25 times receive 16 MB Acknowledgements pared to the Sparc2. Apart from the CPU, the main difference between these machine architectures is the internal bus structure. While the CPU in the Sparc2 has direct access to the Sbus, the Sparc10 has separated the memory (Mbus) and I/O (Sbus) bus. Thus, access to the Sbus from the CPU must pass through an Mbus-to-Sbus interface (MSI) which certainly increases the latency when accessing the network adapter. 2.3 Measurement methods We used the ttcp application program to measure the TCP memory-to-memory throughput. The ttcp program uses the write and read system calls to send and receive data. We modified the ttcp program to set the socket options which influence the maximum TCP window size on a connection. Each measured throughput point is the average of 25 runs. Each run transfers 16 Mbyte memory-to-memory between the sender and the receiver. The CPU utilization is measured for different window sizes with the user data size set to 8192 bytes. The SunOS maintenance program vmstat, was run to register the average CPU load. For minimal interference with the throughput measurements, vmstat was run every 10 seconds in parallel with a transfer of 256 Mbyte between the sender and receiver. To analyze the segment flow on the ATM connections, we put small optimized probes in the network driver to log all packets on the ATM connections of dedicated runs. One log corresponds to one run which transfers 16 Mbytes. The probes parse the TCP/IP packets and register events in a large log table during the data transfer. Included in each event log is a time stamp, an event code and a length field. The time stamp is generated using the SunOS uniqtime() kernel function which accesses the internal µsec hardware clock. The length field is used to log, among other things, the announced window size, the TCP packet length, and sequence numbers as indicated by the event code. The contents of the log table is printed off-line using the kvm library functions available in SunOS 4.1.x. The probes were put in the network driver to only log traffic on the ATM network. To minimize the logging overhead the probes were placed on the machine with the least CPU utilization. Thus, for the SBA-100 adapters logging was done on the send side while logging was done on the receive side for the SBA-200 adapters. The throughput results with and without the logging mechanism indicate the logging overhead to be less than 5%. The log information is presented in primarily four kinds of graphs. The first three, covering the whole measurement period, present the size of transmitted segments, the number of outstanding bytes, and the receiver announced window size. Due to the granularity of the xaxis, fluctuations along the y-axis show up as black areas. The fourth kind of graph uses a finer time resolution and covers only a small interval of the connection life-time. Two values are displayed in the same figure; the announced window size and the number of outstanding unacknowledged bytes. The transmission of a segment is shown as a vertical increase in the number of outstanding bytes, while an acknowledgment is displayed as a drop. 3 Factors influencing performance There are several factors which influence the segment flow on a TCP connection. In addition to the protocol itself, the application interface and the operating system [10], the CPU and machine architecture, and the underlying network technology affect the segment flow. In this section we describe and quantify some of these factors. 3.1 Environment and implementation factors The interface to the TCP/IP protocol in BSD-based systems is through the socket layer [11]. Each socket has a send and a
eceive buffer for outgoing and incoming data, respectively. The size of the buffers can be set by the socket options SO_SNDBUF and SO_RCVBUF. In SunOS 4.1.x the maximum size of these buffers is 52428 bytes. The user data size is the size of the message or user buffer in the write/read system calls. The segment size is the user data portion of a TCP/IP packet. On transmit, the user data size is the number of bytes which the write system call hands over to the socket layer. The socket layer in SunOS 4.1.x copies maximum 4096 bytes of the user data into the socket send buffer before TCP is called. If the user data size is larger than 4096 bytes, TCP is called more than once within the write system call. When there is no room in the socket send buffer, the application accessing the protocols through the socket interface does a sleep to await more space in the socket send buffer. On receive, the user data size is the byte size of the user buffer in which data is to be received. The read system call copies bytes from the socket receive buffer and returns either when the user buffer in the system call is filled, or when there are no more bytes in the socket receive buffer. However, before the system call returns, the socket layer calls TCP, which checks if a window update and acknowledgment should be returned. Figure 2 presents how the segment flow may depend on the send user data size. The segment size is the payload size of the TCP/IP packet. The figure is based on logs from Sparc10 machines with SBA-100/2.2.6 interfaces. For a window size of 8 kbytes, Figure 2 (a) shows a snap-shot of the segment flow on a connection with a user data size of 8192 bytes. Figure 2 (b) shows the same, but with a user data size of 8704 bytes. In Figure 2 (a), the write system call is called with a user data size of 8192 bytes. First, 4096 bytes are copied into the socket layer. Then, TCP is called and a 4096 byte segment is transmitted on the network. This is repeated with the next 4096 bytes of the 8192 byte user data size. In Figure 2 (b) there are 512 bytes left over after two segments of 4096 bytes have been transmitted. These bytes are transmitted as a separate 512 byte TCP segment. The last segment flow is clearly less efficient, because it does not fill the window in-between each acknowledgment, and it has a higher perbyte overhead. [byte] [byte] 16384 12288 8192 4096 16384 12288 0 10 15 20 25 Time [msec] 30 35 40 a) 8192 byte user data size 8192 4096 Window size Unacknowledged bytes Window size Unacknowledged bytes 0 10 15 20 25 Time [msec] 30 35 40 b) 8704 byte user data size Figure 2 Segment flow depends on user data size 3.2 TCP protocol factors TCP is a byte stream oriented protocol, and does not preserve boundaries between user data units. TCP uses a sliding window flow control mechanism. Hence, the window size is relative to the acknowledgment sequence number. In SunOS 4.1.x a window update is sent if the highest announced window sequence number edge will slide at least twice the maximum segment size, MSS, or if the highest advertised window sequence number edge will slide at least 35% of the maximum socket receive buffer [11], [13]. The available space in the socket receive buffer reflects the TCP window which is announced to the peer. The size of the socket receive buffer is the maximum window which can be announced. The TCP maximum segment size, MSS, depends on the maximum transmission unit, MTU, of the underlying network [14]. For our ATM network the MTU is 9188 bytes and TCP computes the MSS to 9148 bytes (the MTU size less the size of the TCP/IP header). The slow-start algorithm [15] is not an issue in our measurements since the sender and receiver reside on the same IP subnetwork. Nagle’s algorithm [16] was introduced as a solution to the “small-packet problem” which results in a high segment overhead if data is transmitted in many small segments. Sending a new TCP segment which is smaller than MSS bytes and smaller than half the announced window size is inhibited if any previously transmitted bytes are not acknowledged. The TCP delayed acknowledgment strategy piggybacks acknowledgments on either data segments or window updates. In addition, acknowledgments are generated periodically every 200 ms. An incoming acknowledgment releases space in the socket send buffer. The reception of an acknowledgment may therefore trigger transmissions of new segments based on the current number of bytes in the socket send buffer. The timer generated acknowledgment occurs asynchronously with other connection activities. Therefore, such an acknowledgment may not adhere to the window update rules above. As a conse- 157
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16 0 Local calls mean: 27 sec. Std.
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20 j k λ ij µji λ ik µ ki λ ir
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42 Solution of some problems in the
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56 Architectures for the modelling
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58 QoS user Service catagories user
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60 (N)-service-user (N)-service-pro
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62 ACSE Presentation layer Session
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Page 144 and 145: 142 Number of type 2 connections No
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Page 148 and 149: 146 Preliminary studies indicate th
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Page 152 and 153: 150 ATM cell header 2.1.1 Measuring
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Page 160 and 161: 158 Segment size [byte] 8192 6144 4
Page 162 and 163: 160 Throughput [Mbit/s] Throughput
Page 164 and 165: 162 Segment size [byte] Unacknowled
Page 166 and 167: 164 Throughput [Mbit/s] 30 20 4 kby
Page 168 and 169: 166 Throughput [Mbit/s] Throughput
Page 170 and 171: 168 TEL A TEL B Satellitedish Swiss
Page 172 and 173: 170 Cell Discard Ratio Cell Discard
Page 174 and 175: 172 Number of Type 2 Sources Number
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Page 178 and 179: 176 The type of the modulated proce
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