UDP server, how can do that each thread will receive the right client's packet and not others?

Question

Well, i have to do a udp server in C on linux.

This is my issue, for example: I have that each thread will manage a client. But recvfrom will accept packets from any source, so, how can do that each thread will receive the right client's packet and not others?

I hope I explained my issue well.

Thank you!

(I mean that my server will make a new thread for each client that want to be served. So with TCP i can use accept to have a "dedicated" socket for each thread-client connection, but with UDP i can't do the same.)

Answer 1

You can't have multiple threads reading from the same socket at the same time.

What you should do instead is either:

1. have a dedicated thread that receives all inbound packets, looks at the source IP/Port, and routes the data to the appropriate processing thread as needed.

2. give each processing thread its own socket that is bind() 'ed to the same local IP/Port and connect() 'ed to the particular source IP/Port it is interested in, then each thread can call recvfrom() independently and it will only return packets that match the source that the thread is expecting.

Answer 2

UDP is a message based connection-less protocol. Here, there is no definite connection / setup done between the client and server prior sending the data. It does not maintain any states for communication at either client or server. UDP uses datagram socket(SOCK_DGRAM) . This ensures each packet sent or received in any order but with preserved message boundaries.

TCP is a stream based connection oriented protocol. Here , a definite logical connection is setup between the client and server prior to the exchange of data. TCP states are maintained for communication. TCP uses stream socket(SOCK_STREAM). This provides a connection-oriented, sequenced flow of data with mechanisms for creating/releasing connection and for detecting errors.

In header of TCP and UDP , only port number will be present. It is the responsibility of the IP header to add the necessary destination IP address for routing. However the source ip address shall be obtained from the source host.

In both TCP server and UDP server, once the socket is created using the particular socket type, it is bound to the port using bind system call. This is done so that the client can address to this port of the server. The procedure of binding is not necessary in either the TCP client or UDP client, because it does not matter in whatever port the client communicates. Till this point, it is same for both TCP server and UDP server.

In the case of TCP server, the listen system call will be invoked to listen for connections on the particular socket. The listen indicates the limit on the number of connections that can be queued up and that the server process is ready to accept any new incoming connections on the socket. The server calls the accept system call which is a blocking call that waits for incoming connections. Once a new connection comes, a new socket descriptor is created and the connection gets established between the server and client. However, as server, it has to keep listening for new connections. This is where the multiprocessing begins to get into picture. Since connections can be triggered by any client at any time by using connect system call, we need to have the accept call(blocking call)for the new connections. Further, to have these new connections processed in parallel, one option is to fork and create a child process which will in-turn take care of further processing of the new connections while the parent process waits for new connections on accept call. This is how individual client connections are identified and processed/handled in the server. In simple terms, in concurrent server architecture, there will be one listening socket and multiple connected socket end points at the server at any time.

Note : The connect system call from client triggers the TCP SYN from client for starting the 3 way handshake procedure. At this point of time, the client socket state is SYN_SENT and once the SYN reaches the server, the server socket state is SYN_RCVD. Then the server responds with SYN_ACK to client for which client will respond with ACK and this establishes the connection between client and server.

In the case of UDP server, there is no need for this initial connection establishment as it is a message based connection-less protocol using datagram socket(SOCK_DGRAM). So, UDP client will not need the connect call for connection establishment which turn strips off the need for listen and accept calls in the UDP server. So, the UDP client server architecture shall be a iterative architecture where every client request/response shall be sequentially processed as iterative server architecture are applicable for services like UDP(light-weight) that consume less processing time. Also note that since the time consumed for processing is less, iterative servers will be state less.

In the case of requirement of multiple threads listening on same socket, you can also go for select or epoll system call based software architecture, where every thread will be waiting on select call but in this scenario, it may not be of great help. An alternate design that can meet your requirement can be a combination of separate thread for receive,parse & dispatch that will direct multiple processing threads.

Answer 3

That's why you have a port number.

If your server is listening on say, port 8080, no other process will be allowed to bind a socket to that port.

All clients that want to connect to this server will need send their data to port 8080 on the server's IP.

Answer 4

The UDP datagram has a source IP and port - you know where it came from, so you can route it appropriately.

You may well need a dedicated thread to send to the port, maintain state and later match up replies with requests so that the originating thread can be signaled that it's own reply is available. This could be done by providing a callback in the request struct that the rx thread signals when the matching reply arrives. The callback could signal a synchro object that the originating thread is waiting on.