I'm implementing a basic file transfer system between a client and server, for practice using UDP. I'm trying to implement some basic reliability.
Right now my server sends a chunk of data, and waits for an acknowledgement signal saying the chunk was received. Is there a way to send a more robust message without the use of signals?
Ideally I'd like to just be able to use sendto and recvfrom, but in the off chance that no packets are received, then recvfrom would just hang.
Is this possible, or do I need to use signals?
You can use select() to learn when there's input to be read from the socket on which you receive ACKs from the client. You can then specify a timeout, after which select() will return even if no input is available. This gives you a chance to keep the server "alive".
You can also use non-blocking I/O, which gives even more flexibility, but is a bit harder since you need to take care not to "busy loop" and consume too much CPU.
Set a socket read timeout with setsockopt() and SO_TIMEOUT.
Related
Should I use non-blocking or blocking TCP sockets when using an I/O multiplexing API like poll(2) or epoll(2)?
Some people suggest using non-blocking sockets here but the I/O multiplexing APIs inform you anyway if there is data to read so what is wrong with a blocking socket here?
If your TCP server is single-threaded and uses blocking I/O, then it's likely that any client that connects to it will be able to deny service to all of the other clients simply by sending only a partial-message, or alternatively by refusing to read any data from its TCP socket after the server sends data. In the former case, the server may block for a long time (perhaps forever) waiting for the entire message to be received from the client; during that time, the server will not be able to respond to other clients. In the latter case, the server will block for a long time (perhaps forever) waiting the client to read some TCP data so that the server-socket's send-buffer can be drained enough to fit some more outgoing data to that client.
One way to avoid that problem is to set all of the server's sockets to non-blocking I/O mode; that way the server knows it can never get "stuck" inside a recv() or a send() call, and thus can remain responsive to all clients regardless of whether any particular client is behaving nicely, or not. In the non-blocking design, the only place the server ever blocks is inside select() or poll() or similar, because those calls are designed to return whenever any client needs service, rather than blocking on only a single client. (the tradeoff is that with non-blocking I/O your server's buffering/queueing logic will need to be a bit more elaborate, since you can no longer assume that any particular fixed number of bytes will be sent or received during any given send or receive operation)
The other way to avoid the problem is to make a multi-threaded server; that has the advantage that each client gets its own thread, and therefore a badly-behaved client will block only its own thread and not the threads servicing other clients. The disadvantage is that now your server is multi-threaded, with all of the additional pitfalls that multithreading introduces.
(and, for completeness, the third approach is simply to ignore the possibility of badly-behaved/poorly-connected clients, and use a single-threaded/blocking model. That works fine for toy examples where clients are expected to be non-hostile, and where the network they are connecting over is reliable, but doesn't work so well in real life)
Non-blocking IO is used when you prefer an error response (EWOULDBLOCK / EAGAIN) over your thread waiting (blocking) until an IO operation becomes possible.
This leads to the question of how is the IO multiplexing achieved?
If you're using a thread-per-connection model (or a process-per-connection), using blocking IO might be more comfortable.
However, if the same thread is serving multiple IO objects, blocking IO would be hazardous and could bring the whole application to a halt.
It is better to use non-blocking IO when a single thread serves multiple IO objects.
Note that the issue might not be noticeable at first when polling (using select / poll or epoll/kqueue).
Since the IO operations are only performed by a code path that already "knows" that the IO operation will not block (it was polled and known to be an available operation).
This masks the issue that somewhere in the code an IO operation might be called directly without polling first, resulting in a blocking IO call that will grind the application to a halt.
I would like to use the recvmmsg call to read multiple UDP messages from ONE single socket at once. I'm reading data from a single multicast group.
When I read TCP data, I usually use poll/select with a non-blocking socket (and timeout) to be notified when that is ready to be read. I follow this approach as I am aware of the issue of spurious wakeup and potential troubles of having a blocking socket.
As my application must be very quick, if I follow the same approach with recvmmsg I will introduce an extra system call (poll/select) that might slow down the execution.
So my two questions are the following:
With UDP, can I safely read from BLOCKING sockets using recvmmsg without poll/select or do I have to apply the same principle I've used for TCP (non-blocking+poll)?
Suppose I have a huge amount of multicast traffic, would you go for non-blocking socket + recvmmsg only (no poll) and burn a lot of CPU?
I am using Linux: CentOS 7 and Oracle Linux.
You can always use blocking mode, with both TCP and UDP sockets.
If you want to impose a read timeout there is setsockopt() with the SO_RCVTIMEO option.
I follow this approach as I am aware of the issue of spurious wakeup
What spurious wakeup? Never seen it in 25 years of network programming.
and potential troubles of having a blocking socket.
Never heard of those either.
Using select() and non-blocking mode with a single socket is pointless unless your platform doesn't support SO_RCVTIMEO. It's an extra system call, for a start.
The option of using blocking or non-blocking depends on what is the final purpose of the application.
- Say it's just a sample chat application showing the usage of UDP combined with TCP then you can use either.
- But if you are planning to make this module a part of highly used application with lots of data flowing then probably creating multiple threads/processes to handle different tasks will come in handy. The parent thread will to wait for the message but for processing it will spawn a different child thread and hence make the parent available for the next message.
But in a nutshell I don't see any issue with your first option of using a blocking socket without poll/select for a UDP application considering it's just for homework purposes.
I'm writing a server in C ++ for both Windows and Unix systems.
A key feature of this server is that it must be able to receive and send network packets at any time.
Specifically, the server must be able to send data to the client not only in response to their messages, but also be able to send packets to them asynch in push.
I'm having difficulty in implementing a solution that uses the select() function in the scenario described above.
The solution I have currently implemented does not convince me at all and I think it can be implemented with better patterns/solutions.
I currently have a dedicated thread (selector) that performs the select by listening on events in the reading for the server socket (to accept new connections) and for the sockets connected to the server.
This is the main select() loop:
if((sel_res_ = select(nfds_+1, &read_FDs_, NULL, &excep_FDs_, &sel_timeout)) > 0){
if(FD_ISSET(serv_socket, &read_FDs_)){
//we have to handle a newly connection.
...
if(sel_res_ > 1){
//in addition to the newly connection, there is also some other message incoming on client sockets.
...
}
}else{
//we have to handle incoming messages on client sockets
...
}
}
This solution works well for receiving the data and to respond to client requests in synchronous form.
However, the server must also be able to send asynchronous data, and send when necessary, packets in push.
To do this I currently use separate threads that perform directly the send() on the client sockets.
This solution does not convince me, and I would like to centralize the packets receiving and sending on the selector thread.
The main difficulty is that the select() by its nature is blocking and I have no control until a client does not send any packet or the timeout is triggered.
The solution to set a timeout very low does not convince me; I see it as an easy solution that is actually doing active wait, and not only, however, the worst case I would pay the price of the timeout before sending the push packet.
I thought a more 'elegant' solution; I think, will work well, but only for a Unix/Linux platform.
I thought to use an anonymous pipe and insert into the select() read_FDs_ the anonymous pipe read descriptor.
In this way, when a thread wants to send a data in push, it writes something on this pipe, interrupting the select() and returning control to the selector that can then predispose to send the data to the client, without significant loss of time.
I think that this solution, unfortunately, cannot be implemented on Windows because the select() function on that system works only with fds that are actually sockets.
So the question is: Is there some well known solution that can be used to address this kind of scenario (both Linux and Windows)?
You can create a self connected UDP socket, this works equally well on Windows and Linux.
Basically, you create a UDP socket, bind() it to INADDR_LOOPBACK and port 0, and connect() it to itself (with the address taken from getsockname()).
At this point, you can send yourself a single byte message (or something more specific) to wake yourself up.
I want to send regularly UDP messages through sockets.
Is it better to use one socket, or to recreate a socket for each message and close them just after?
Since UDP is stateless, does it even change anything?
If you're expecting to receive anything over the socket, it had better exist for the entire time over which you're expecting to receive.
If you're expecting to send multiple messages without receiving any replies you already have a design mistake, but there doesn't seem to be a good reason why you would keep opening and closing UDP sockets for that purpose. Surely it makes more sense to keep the socket open as long as necessary?
Since UDP is a packet-oriented protocol, there is no "connection" to open or to close. If you are going to send a lot of messages to a particular address you can call connect on a UDP socket, and then use send and recv to communicate instead of sendto and recvfrom. This way you don't have to specify the address where you want the data sent on every function call, but that doesn't mean that a connection is established.
So, whether it's better to create one socket and reuse it, or create a new one every time, depends more on your programming style: If you create lots of sockets and store them for long periods of time without using them, you may eventually run out of sockets. Then again, asking the operating system to create new sockets frequently may become a performance bottleneck if you do it in a tight loop.
I'm breaking my mind trying to understand how make a client/server write by myself accept multiple socket connections.
The connection is a datagram (UDP), for now was implemented based on getaddrinfo(3) man page works nice, but each client needs to wait the process of early connections be processed.
I've heard about select, but in its man page says:
select() can be used to solve many problems in a
portable and efficient way that naive programmers try to solve in a
more complicated manner using threads, forking, IPCs, signals, memory
sharing, and so on.
and more:
The Linux-specific epoll(7) API provides an interface that is
more
efficient than select(2) and poll(2) when monitoring large numbers of
file descriptors.
So, it is? epoll is simply better than select? Or it depends? If it depends, on what?
epoll man pages has a partial sample, so I'm trying to understand it.
At now, (on server) I think, I need a thread to listen in a thread and write in another. But how to control the completion of a partial message? If two clients send a partial message interleaved, how to identify? By the sockaddr? if it's the only need, I can manage it without a pool, so why use a epoll?
Can anyone try to explain to me, how to make, or where to learn about a multi-connection client-server UDP app?
I think there is a misunderstanding here about UDP. UDP is not a connection oriented protocol which means there is no permanent connection like TCP. UDP just bind to an address/port and waits for packets from everyone. At the server there is only one socket listening per address/port number. When a packet is received you may find out who is the sender by the packet's source IP, you can reply to the sender thru this IP.
As I see it, there is no need for poll() o select() you bind to an address/port and asynchronously receive packets. That is, when a packet is received you get a signal/message alerting your asynchronous function. This function should be reentrant, it means that in the middle of a reception another signal could be received and care must be taken when accessing/modifying global stuff (variables/objects). When dealing with an incoming packet it should be processed as soon as possible or, in case the process takes too long, you better keep the packet in a packet spool and process them in another [less priority] thread.
For UDP packet size read this question.
For UDP fragmentation read this
For UDP packet header read this