DOCUMENT:Q200770 04-APR-2002 [winnt] TITLE :TCP/IP Connection Attempts Take Longer to Time Out PRODUCT :Microsoft Windows NT PROD/VER::4.0 OPER/SYS: KEYWORDS:kbnetwork kbWinNT400sp4fix ====================================================================== ------------------------------------------------------------------------------- The information in this article applies to: - Microsoft Windows NT Server version 4.0 - Microsoft Windows NT Workstation version 4.0 - Microsoft Windows NT Server version 4.0, Terminal Server Edition ------------------------------------------------------------------------------- SYMPTOMS ======== Applications that try to establish a session to a remote host using NetBIOS over TCP/IP may experience a time-out longer than expected when the remote host is unavailable. The "longer than expected" time-out occurs when the TCP connection attempt process, which starts with the original SYN segment, is repeated. For example, if the TcpMaxConnectRetransmissions registry parameter is set to 1, you would see the following incorrect behavior when the remote or target host is down: 1. The client computer sends the first SYN segment. 2. Three seconds later, the SYN segment is retransmitted 3. Six seconds later, a different initial SYN segment, with a different sequence number, is sent. 4. Three seconds later, the second initial SYN segment is retransmitted. 5. Six seconds later, TCP reports a time-out error to the application. The total time-out is approximately 18 seconds. CAUSE ===== This behavior occurs because NetBIOS over TCP/IP (NetBT) is requesting the connection a second time. RESOLUTION ========== A new NetBT.sys file included in Windows NT 4.0 Service Pack 4 fixes this problem. To resolve this problem, obtain the latest service pack for Windows NT 4.0 or Windows NT Server 4.0, Terminal Server Edition. For additional information, please see the following article in the Microsoft Knowledge Base: Q152734 How to Obtain the Latest Windows NT 4.0 Service Pack STATUS ====== Microsoft has confirmed this to be a problem in Windows NT 4.0 and Windows NT Server 4.0, Terminal Server Edition. This problem was first corrected in Windows NT 4.0 Service Pack 4.0 and Windows NT Server 4.0, Terminal Server Edition Service Pack 4. MORE INFORMATION ================ With the latest service pack, you will see the following behavior while trying to connect to another computer: TCP, acting as the transport for the application, sends a SYN segment to the destination host specifying the initial sequence number in an attempt to initialize the connection. If the destination host is unavailable, TCP will retransmit this original SYN packet the number of times specified in the TcpMaxConnectRetransmissions registry parameter (the default is three times). The retransmission time-out is doubled with each successive retransmission in a given connection attempt. The initial time-out value is three seconds. After retransmitting the number of times specified in the registry key mentioned above, the transport will notify the application of a time-out error. For example, if the TcpMaxConnectRetransmissions registry parameter is set to 1, you should see the following correct behavior when the destination host is down: 1. Client sends the first SYN segment 2. Three seconds later, the SYN packet is retransmitted 3. Six seconds later, TCP reports a timeout error to the application. This time, the total time-out is approximately nine seconds, which is the correct behavior with the default configuration settings. For additional information, please see the following article(s) in the Microsoft Knowledge Base: Q172983 Explanation of the Three-Way Handshake via TCP/IP Q120642 TCP/IP and NBT Configuration Parameters for Windows 2000 or Windows NT Additional query words: sp4 ====================================================================== Keywords : kbnetwork kbWinNT400sp4fix Technology : kbWinNTsearch kbWinNTWsearch kbWinNTW400 kbWinNTW400search kbWinNT400search kbWinNTSsearch kbWinNTS400search kbWinNTS400 kbNTTermServ400 kbNTTermServSearch Version : :4.0 Issue type : kbbug Solution Type : kbfix ============================================================================= THE INFORMATION PROVIDED IN THE MICROSOFT KNOWLEDGE BASE IS PROVIDED "AS IS" WITHOUT WARRANTY OF ANY KIND. MICROSOFT DISCLAIMS ALL WARRANTIES, EITHER EXPRESS OR IMPLIED, INCLUDING THE WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE. IN NO EVENT SHALL MICROSOFT CORPORATION OR ITS SUPPLIERS BE LIABLE FOR ANY DAMAGES WHATSOEVER INCLUDING DIRECT, INDIRECT, INCIDENTAL, CONSEQUENTIAL, LOSS OF BUSINESS PROFITS OR SPECIAL DAMAGES, EVEN IF MICROSOFT CORPORATION OR ITS SUPPLIERS HAVE BEEN ADVISED OF THE POSSIBILITY OF SUCH DAMAGES. SOME STATES DO NOT ALLOW THE EXCLUSION OR LIMITATION OF LIABILITY FOR CONSEQUENTIAL OR INCIDENTAL DAMAGES SO THE FOREGOING LIMITATION MAY NOT APPLY. Copyright Microsoft Corporation 2002.