Question

I need to know what the largest UDP packet I can send to another computer is without fragmentation.

This size is commonly known as the MTU (Maximum Transmission Unit). Supposedly, between 2 computers, will be many routers and modems that may have different MTUs.

I read that the TCP implementation in windows automatically finds the maximum MTU in a path.

I was also experimenting, and I found out that the maximum MTU from my computer to a server was 57712 bytes+header. Anything above that was discarded. My computer is on a LAN, isn’t the MTU supposed to be around 1500 bytes?

Answer 1

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): Resolve IP Fragmentation, MTU, MSS, and PMTUD Issues with GRE and IPSEC

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More on this topic:

• Re: UDP fragmentation says you should use ICMP instead of UDP to discover MTU
• Path MTU Discovery says that a TCP connection might include implicit MTU negotiation via ICMP

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.

No, it looks like it is implemented: see for example Winsock Programmer’s FAQ Examples: Ping: Raw Sockets Method.

So, to discover MTU, generate ping packets with the ‘do not fragment’ flag.

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].