How to find the largest UDP packet I can send without fragmenting?

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

Your own MTU is available in the registry, but the MTU in practice is going to the smallest MTU in the path between your machine and the destination. Its both variable and can only be determined empirically. There are a number of RFCs showing how to determine it.

LAN's can internally have very large MTU values, since the network hardware is typically homogeneous or at least centrally administrated.

Answer 2

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].

Answer 3

In addition to all the previous answers, quoting the classic:

IPv4 and IPv6 define minimum reassembly buffer size, the minimum datagram size that we are guaranteed any implementation must support. For IPv4, this is 576 bytes. IPv6 raises this to 1,500 bytes. ...

This pretty much means that you want to limit your datagram size to under 576 if you work over public internet and you control only one side of the exchange - that's what most of the standard UDP-based protocols do.

Also note that PMTU is a dynamic property of the path. This is one of the things TCP deals with for you. Unless you are ready to re-implement lots of sequencing, timing, and retransmission logic, use TCP for any critical networking. Benchmark, test, profile, i.e. prove that TCP is your bottleneck, only then consider UDP.