DOCUMENT:Q229064 10-AUG-2001 [winnt] TITLE :Load Balanced Service May Not Work Properly PRODUCT :Microsoft Windows NT PROD/VER::1.0,2000,4.0 OPER/SYS: KEYWORDS:kbnetwork ====================================================================== ------------------------------------------------------------------------------- The information in this article applies to: - Microsoft Windows NT Server version 4.0 - Microsoft Windows NT Load Balancing Service 1.0 - Microsoft Windows 2000 Advanced Server ------------------------------------------------------------------------------- SYMPTOMS ======== A service being load balanced by the Windows NT Load Balancing service (WLBS) or the Windows 2000 Network Load Balancing (NLB) feature may not work at all or present unpredictable behavior. The client program may report connection errors. CAUSE ===== This problem occurs because datagrams or segments are being fragmented by the IP protocol (possibly by the Router forwarding the packets to the cluster's segment) when their size is greater than the Maximum Transfer Unit AND Affinity of None is being used in the cluster. The first packet that contains the TCP/UDP header is handled properly by the correct cluster host, however, the following fragmented packets do not include a TCP/UDP header and may be handled by different hosts randomly. RESOLUTION ========== To resolve this problem, configure a single port rule covering ports 0 - 65535 for both TCP and UDP AND use Affinity. This rule should be configured in the cluster whenever fragmentation is expected. MORE INFORMATION ================ When using no Affinity, WLBS or NLB checks the TCP OR UDP header of a packet arriving from a client (to obtain the client's port address), as well as the IP header. Fragmented IP packets contain just the IP header, preventing WLBS or NLB from finding out the port information needed to determine which WLBS host should accept the packet. Using Affinity causes WLBS or NLB to check just the IP header of a packet arriving from a client. REFERENCES ========== For more information, please see the following Microsoft Knowledge Base article: 103884 The OSI Model's Seven Layers Defined and Functions Explained Additional query words: ====================================================================== Keywords : kbnetwork Technology : kbWinNTsearch kbWinNT400search kbwin2000AdvServ kbwin2000AdvServSearch kbWinNTSsearch kbWinNTS400search kbWinNTS400 kbWinNTLBS100 kbWinNTLBSSearch kbwin2000Search kbWinAdvServSearch Version : :1.0,2000,4.0 Issue type : kbprb ============================================================================= 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 2001.