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copied!<p>The following doesn't answer your question directly but you might find it interesting; it says that IP packets can be disassembled/reassembled, and therefore bigger than limit on the underling media (e.g. 1500-byte Ethernet): <a href="http://www.cisco.com/en/US/tech/tk827/tk369/technologies_white_paper09186a00800d6979.shtml" rel="noreferrer">Resolve IP Fragmentation, MTU, MSS, and PMTUD Issues with GRE and IPSEC</a></p> <hr> <p>More on this topic:</p> <ul> <li><a href="http://lists.apple.com/archives/darwin-development/2004/May/msg00069.html" rel="noreferrer">Re: UDP fragmentation</a> says you should use ICMP instead of UDP to discover MTU</li> <li><a href="http://msdn.microsoft.com/en-us/library/ms817967.aspx" rel="noreferrer">Path MTU Discovery</a> says that a TCP connection might include implicit MTU negotiation via ICMP</li> </ul> <p>I don't know about generating ICMP via an API on Windows: at one time such an API was proposed, and was controversial because people argued that would make it easy to write software that implements denial-of-service functionality by generating a flood of ICMP messages.</p> <p>No, it looks like it <em>is</em> implemented: see for example <a href="http://tangentsoft.net/wskfaq/examples/rawping.html" rel="noreferrer">Winsock Programmer's FAQ Examples: Ping: Raw Sockets Method</a>.</p> <p>So, to discover MTU, generate ping packets with the 'do not fragment' flag.</p> <p>Maybe there's an easier API than this, I don't know; but I hope I've given you to understand the underlying protocol[s].</p>

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