How can i access and modify my UDP header?

Question

I am just starting out with network programming and I am having trouble keeping up with some of the stuff. Namely because there is a lot of auto-magic stuff going on under the hood that is hidden away from me, and I cant get my head around it all. In my current app, I take user input like so:

char buff[1024];
cout << "Enter a message:";
fgets(buff, 1024, stdin);

And i can easily send that off by essentially doing this:

sendto(m_socket, buff, size, flags, (SOCKADDR*)&addr, sizeof(addr));

The other variables (size,flags,etc) are obviously initialized and configured somewhere between the input and sendto call.

This all works fine, and the other end receives the data fine. the server does

recvfrom(m_socket, (char*)data, max, flags, (SOCKADDR*)&fromAddr, &fromLength) 

and I can then just print out 'data' into the console, and it'd be my message.

So where exactly is my UDP packets header? Did i have to specify it? when i pass in some data, is the 'data' part of a standard UDP packet filled with the data i specify, and the header automatically populated for me (source IP, destination IP)? If so how do i even access it?

A lot of websites I have looked at talk about headers and what not. It seems to be this very specific thing. But I dont understand how this illusive header works.. I'd like to define my own header with things like segment number, total segments, etc. but i dont know how to go about this at all. My googleing isnt sending me in the right direction either..!

Answer 1

The UDP headers are not available to the application when you use the standard interfaces from the operating system (socket() with SOCK_DGRAM, sendto(), recvfrom() and such). They are automatically handled by the operating system's network stack. They are automatically added when you send the message and automatically stripped when you receive.

Depending on the operating system, there are some means you can write and manage the UDP headers directly over the IP routing layer, but that is certainly unusual, and would probably require administrative privileges.

If you want to define your own headers for your own purposes, you must do so inside the body of the message, i.e. parse and interpret what you send and receive sendto() and recvfrom(), thus creating what is called an application protocol. Networking is a layered architecture, where applications sits upon UDP or TCP, that sits upon IP that (usually) sits upon Ethernet or Wi-Fi. Each one has its own headers that is stripped when the data is handled to the above layer, and you only gets what you send on the application layer (maybe you can miss some packets or get them out of order, because UDP doesn't give you those guarantees, as TCP does).

Answer 2

UDP does not contain such things as headers, segment numbers and total segments. In fact, a UDP datagram contains nothing except the buffer that you sendto.

You can add extra information by including them with your message. Instead of simply putting your message in your buffer, make a larger buffer into which you put whatever "header" information you would like and then put your message afterwards.

From UDP's perspective, all it sees is data. It is up to the program on the receiving end to parse this data and realize that a certain part it is metadata and the other part of it is regular data.

You will have to define your message format such that it is easy for the receiving program to extract the "header" portion of the buffer. There are two common schemes: fixed headers and variable length headers.

If you are going for a fixed header, you assume that the first X bytes of every UDP message are metadata. You pick the X and it never changes per message. The recipient then knows to read X bytes as the header and all the rest as a message.

If you are going for a variable length header, you need to be able to tell the receiving end how long the header is for each particular message. A common scheme is that the first 4 bytes of the header contain an integer which says how long the header part of the message is. Then the receiving end reads that many bytes and interprets that as the header. Then it reads the rest of the data and interprets that as the message.

Lastly, the source IP and the destination IP are not properties of your UDP messages but rather properties of your socket. You can retrieve this information by inspecting the socket itself.