Getsockopt() Function Returns a Different Maximum UDP Message Size Than You Expect in Windows 2000 SP3

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SYMPTOMS

When you use the Getsockopt() function with the SO_MAX_MSG_SIZE option to obtain the maximum User Datagram Protocol (UDP) message size on a Windows 2000 Service Pack 3 (SP3)-based computer, a value of 65507 is returned. Before you installed Windows 2000 SP3, this value was 65527.

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CAUSE

This behavior occurs because the value that is returned by Getsockopt() with the SO_MAX_MSG_SIZE option is incorrect on pre-Windows 2000 SP3 installations.

The correct maximum UDP message size is 65507, as determined by the following formula:

0xffff - (sizeof(IP Header) + sizeof(UDP Header)) = 65535-(20+8) = 65507

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STATUS

Microsoft has confirmed that this is a problem in Windows 2000 Service Pack 2. This problem was first corrected in Microsoft Windows 2000 Service Pack 3.

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MORE INFORMATION

For additional information about how to obtain the latest Windows 2000 Service Pack, click the article number below to view the article in the Microsoft Knowledge Base:

260910 (http://support.microsoft.com/kb/260910/EN-US/ ) How to Obtain the Latest Windows 2000 Service Pack

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