Load Balanced Service May Not Work Properly

• Microsoft Windows NT Server version 4.0
• Windows NT Load Balancing Service 1.0

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
Version : winnt:1.0,4.0
Platform : winnt
Issue type : kbprb