Why would you send a UDP packet with no payload?

Question

It seems that a UDP packet can be sent without a payload.

The only thing I can think of that doesn't need a payload is for NAT hole punching. What else could this be used for?

This relates to my previous question Under Linux, can recv ever return 0 on UDP?

I suppose more to the point is that if it's been specified as part of some standard, then it's been thought to be useful somewhere right?

Answer 1

Anything! The UDP packet isn't empty -- it comes with the sender's identity. Therefore, such a packet could be used as a primitive kind of signal: maybe a hello, a goodbye, or a keep-alive.

With interfaces like sendmsg, an empty packet might be used in order to send auxiliary data, like a cmsg structure (which can be used for things like transferring file descriptors between two processes on Linux).

EDIT: One more use: NAT traversal algorithms such as STUN or UDP hole punching.