UDP socket - how to make a timer and how to handle packet loss

[Sagittarius]

I will first try to describe my problem in words.

I have a simple program, written in C++, that needs to send a single bytearray via a UDP socket to a microprocessor, which returns an answer, also via UDP.

Everything works just fine - exept when packet loss occurs (or nothing is recieved at the recieving socket).

If packet loss occurs, the program locks up, and it needs to be restarted!

Here comes the question:

How do I make the program wait for a given amount of time, to recieve something on the socket? If nothing has arrived, I will need to retransmit. If multiple retransmits wont help, an error should be returned.

What I dont know is:

- How do I 'check that something has arrived in the socket', now that the program 'locks up'?
- How do I, in an easy way, make the program wait, lets say 2 seconds, and then retransmit if nothing has arrived?

My problem is mainly the syntax, and also that I am very unexperienced in socket-programming, so what might be simple to everyone, is not that simple for me.

My code for the socket-is as follows:

WSADATA ws; //These two lines sets up a Windows Socket
WSAStartup(0x0101,&ws);
 
SOCKET udp_socket; //The socket that sends the bytearray RISP_Frame
struct sockaddr_in peer;
int peerlen;
 
  peer.sin_family = AF_INET;
  peer.sin_port = htons(PORT); //port
  peer.sin_addr.s_addr = inet_addr(IPADRESS); //IP adress
 
  udp_socket = socket(AF_INET, SOCK_DGRAM, 0);
 
  sendto(udp_socket, RISP_Frame, 10, 0, (struct sockaddr *)&peer, sizeof(peer)); //The bytearray is sent, no problems so far
 
 
   SOCKET udp_socket_rec; //Recieving socket
 
   char recvbuffer[10]; //Bytearray has a fixed lenth of 10
   int retval;
 
   peer.sin_family = AF_INET;
   peer.sin_port = htons(1228);
   peer.sin_addr.s_addr = htonl(INADDR_ANY);
 
   udp_socket_rec = socket(AF_INET, SOCK_DGRAM, 0);
 
   bind(udp_socket_rec,(struct sockaddr *)&peer,sizeof(peer));
 
   peerlen=sizeof(peer);
 
//When packet loss occurs, this is where the program ends
 
   retval = recvfrom(udp_socket_rec, recvbuffer, sizeof(recvbuffer), 0, (struct sockaddr *)&peer, &peerlen);
 
   closesocket(udp_socket_rec);
 

I hope that you are able to help me.

As said before - it works as it should - the only problem is to make it stabile.

I look forward to see if you have any solutiuon-proposals.

Best regards to all of you

/Sagi

[horace1]

where you call recvfrom() you have a loop calling either select() or poll() to determine if data has arrived - if so you then call recvfrom() to read the data. If nothing has arrived within your time limit up exit the loop and send a datagram to the micrprocessor again, etc etc.
see the recvfrom() man page, e.g.
http://www.die.net/doc/linux/man/man2/recvfrom.2.html

[Sagittarius]

where you call recvfrom() you have a loop calling either select() or poll() to determine if data has arrived - if so you then call recvfrom() to read the data. If nothing has arrived within your time limit up exit the loop and send a datagram to the micrprocessor again, etc etc.
see the recvfrom() man page, e.g.
http://www.die.net/doc/linux/man/man2/recvfrom.2.html

Thanks a lot buddy - it makes perfect sense :) The functionallity of select and poll was just what I was looking for.

I have chosen to use select().

However I am stuck with a new problem now - I cant figure out how I am supposed to call select()

I have found various examples on the web, and very good descriptions, but I just cant make it compile.

The arguments for Select are(as far as I know):

select(a, b, c, d, e)

a) The first is used for backward-compatibility, and can be set to any integer I want.

b) The second argument I am a bit confused about - according to my sources I have found, it is a File Descriptor which points to a socket that i wish to check for readability (which is what I want) - do I, in my case, just need to insert the socket in question (udp_socket_rec) here, or is that not allowed?

c & d) This is the same as argument b), only difference is that c) if for writeability-check and d) is for error check.

e) As far as I know, this is a timer, which in miliseconds state for how long the select-function os to listen on the socket. Since I am going to put my select()-function inside a loop, and therefore there is no need for a timer, what should i give as value to this argument.


Putting the function into a loop and how to retransmit the package is not a problem - what I really need to know is how to call select.

An example, how to check on the socket in my program above, just once, would be very appreciated :)

Again, thanks for your quick answer

Best regards and have a nice weekend everyone

/Sagi

P.S. my main source for my socket programming can be found here

[horace1]

have a look at this thread
http://www.ussg.iu.edu/hypermail/linux/kernel/0001.2/0756.html

[horace1]

have a look at the udp-client.cc on URL
http://sage.mc.yu.edu/kbeen/teaching/networking/resources/sockets.html
which uses select so

    // wait until answer comes back, for up to 1 second
    FD_ZERO(&readSet);
    FD_SET(socketDescriptor, &readSet);
    timeVal.tv_sec = 1;
    timeVal.tv_usec = 0;
 
    if (select(socketDescriptor+1, &readSet, NULL, NULL, &timeVal)) {
      // Read the modified line back from the server.
 

the UDP server and client work OK on my machine

[Sagittarius]

Thank you so much for your help :)

Everything works now.

It is really appreciated.

All the best regards

/Sagi

[horace1]

Thank you so much for your help :)

Everything works now.

It is really appreciated.

All the best regards

/Sagi

always pleased to help!