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For a Microsoft Windows 2000 version of this article, see 169789.
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256986 Description of the Microsoft Windows Registry
If you use the TCP/IP protocol and you have a 100-megabit network adapter, you may notice slow network performance while you copy files. You may also experience this slow performance while you use Windows Explorer in Windows XP.
Note that although the slow performance is known to occur on several 100-megabit network adapters, this issue is not specific to Windows.
This issue is due to a high rate of early collisions on the network. The interframe gap (the amount of time that a workstation waits before attempting to transmit on the wire) is less than the IEEE 802.3 specification of 9.6 microseconds.
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To resolve this issue, contact the manufacturer of your network adapter forinformation about increasing the interframe gap.
Intel EtherExpress 100B network adapters have a registry parameter that controls the length of the interframe gap. To modify this parameter if you use an Intel EtherExpress 100B network adapter, follow these steps:
Start Registry Editor (Regedit.exe).
Locate the following key in the registry, where x is the number of your network adapter:
Click Edit, click Add Value, and then type the following information:
Value Name: Adaptive_ifs Value Type: REG_DWORD Data: 1 (enable adaptive algorithm)
The adaptive algorithm for detecting collisions and tuning the interframe gap is enabled by default. Setting the value data to 0 disables the adaptive algorithm. A value between 2 and 200 sets a predefined interframe gap. Begin by setting the value at 20, and then increase the value by 20s. Measure collisions and note performance, and set a final value that has a low collision rate and does not affect performance.
To work around this issue, you can create a TcpWindowSize registry parameter to have the sender wait to receive an ACK from the receiver before sending more data. Making this change reduces the potential for collision. Follow these steps:
Click Add, click Value, and then add the following information:
Value Name: TcpWindowSize Value Type: REG_DWORD Data: 2920 (decimal) - Number of bytes
Set the value to Decimal 2920 for Ethernet, or to twice the maximum TCP data size for other network topologies. This setting does not need to be modified for other networks, however, because networks that are based on topologies such as token ring or Fiber Distributed Data Interface (FDDI) do not have collisions as an Ethernet network does.
Note that modifying this parameter can significantly affect performance. In general, on a wide area network (WAN) or on a regular 10-megabit network, there is some latency between the sender and the receiver, depending on the network topology. Therefore, the potential for collisions because of TCP/IP acknowledgement (ACK) does not exist. Tuning TcpWindowSize in this case only reduces throughput.
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A "collision" occurs when two stations transmit simultaneously on the wire.An "early collision" is any collision that occurs before 512 bits of theframe are on the wire. Early collisions can occur regularly ina normally operating Ethernet network. There is no hardware malfunction ormisbehaving station.
The IEEE 802.3 specification states that a station must wait 9.6 microseconds (the interframe gap) before the station can attempt to transmit on the wire. Several adapter manufacturers design their adapters with a smaller interframe gap to achieve higher data transfer rates. This smaller interframe gap can lead to a high rate of collisions.
The potential for collisions is also influenced by the behavior of the upper layer protocol. The TCP/IP specification is sending an ACK (acknowledgement) for every other data frame that is received. That is, after a TCP host receives two data frames, the TCP host transmits an ACK to the sender. The potential for collisions is high if a client that has received two packets tries to send an ACK while the sender is trying to send more data to the client.
Analysis of Windows Explorer File Copy in Windows XP
When the same file copy is initiated by using a command prompt or by usingWindows Explorer in Windows XP, the pattern of data transfer isdifferent. In this case, the redirector issues a 60-kilobyte (KB) "bulk read" or "raw read." The data transfer pattern is shown below:
10 CLIENT SERVER SMB C read & X, FID = 0x1004, Read 0xf000 11 SERVER CLIENT SMB R read & X, Read 0xf000 12 SERVER CLIENT NBT SS: Session Message Cont., 1460 Bytes 13 SERVER CLIENT NBT SS: Session Message Cont., 1460 Bytes 14 SERVER CLIENT NBT SS: Session Message Cont., 1460 Bytes 15 SERVER CLIENT NBT SS: Session Message Cont., 1460 Bytes 16 CLIENT SERVER TCP .A...., len: 0, seq:404791-404791, ack 17 CLIENT SERVER TCP .A...., len: 0, seq:404791-404791, ack 18 SERVER CLIENT NBT SS: Session Message Cont., 1460 Bytes 19 SERVER CLIENT NBT SS: Session Message Cont., 1460 Bytes 20 SERVER CLIENT NBT SS: Session Message Cont., 1460 Bytes
At the TCP level:
10 CLIENT SERVER TCP len: 64, seq: 404727-404790, ack: 6992081 >Client sends 64 bytes of data (SMB command to read 4KB); 11 SERVER CLIENT TCP len: 1460, seq: 6992081-6993540, ack: 404791 >Server sends the piggyback ACK, and the SMB response with some data; 12 SERVER CLIENT TCP len: 1460, seq: 6993541-6995000, ack: 404791 13 SERVER CLIENT TCP len: 1460, seq: 6995001-6996460, ack: 404791 >Server sends rest of the data to the client. >**Collision likely at this point because the client tries to send an ACK. 14 SERVER CLIENT TCP len: 1460, seq: 6996461-6997920, ack: 404791 15 SERVER CLIENT TCP len: 1460, seq: 6997921-6999380, ack: 404791 >But server is able to get the wire and send more data. 16 CLIENT SERVER TCP len: 0, seq: 404791-404791, ack: 6996461 >Client is able to get the wire and send the ACK for data on frames 12 and 13. 17 CLIENT SERVER TCP len: 0, seq: 404791-404791, ack: 6999381 >Client is able to get the wire and send the ACK for data on frames 14 and 15. 18 SERVER CLIENT TCP len: 1460, seq: 6999381-7000840, ack: 404791 19 SERVER CLIENT TCP len: 1460, seq: 7000841-7002300, ack: 404791 >Server continues to send data. >**Collision likely at this point because the client tries to send an ACK. 20 SERVER CLIENT TCP len: 1460, seq: 7002301-7003760, ack: 404791 >But server is able to get the wire and send more data.
When you use the TCP/IP protocol, TCP acknowledgement (ACK) influences the collision. However, the problem is not due to TCP/IP or to the enhancement that makes Windows Explorer able to do 60-KB bulk reads. The problem can be illustrated by using FTP also. TCP/IP, the redirector, and Windows Explorer have absolutely no control over interframe gap. Interframe gap is at the physical layer, which is controlled by the chipset on the adapter. If your network experiences a high rate of collisions, contact the vendor of your network adapter.
For additional information about the TCP/IP protocol, see the white paper titled "TCP/IP Implementation Details," which is available at the following Microsoft anonymous FTP site: