I'm new in network programming and I'm trying to understand how functions like send and recv work under the hood in a TCP connection.I know that in a connection between a client and a server for example ,when the client manage to send a message to the server ,the message is split in different packages and at it's arrival,the server part checks to see if the sum of the packages is the same as it was before sending,and if is ok it sends a message back to the client as an approval.If a problem appears the client resends the message.
What I don't understand is that if you send a message from the client and you sleep the server for 10 seconds,you can still do what you want in the client,like the send function is executing in another thread ,or if you use multiple times send function in these 10 seconds,the message arrives as a combination of the messages used in that time.
If anyone can explain the situation ,I'll be very grateful !
This is implemented by the TCP/IP networking stack of your operating system.
The TCP/IP stack ...
provides a send buffer. When your program sends, the OS first fills internal buffers. You app can send immediately until the buffers are full. Then your send will block.
takes data from the internal buffer and sends it out onto the network in single packets.
receives data over the network and fills internal receive buffers with that data.
gives your program the data from the internal buffers when you call receive.
takes care of the TCP/IP protocol stuff like establishing connections, acknowledging received data, resending data if no receive acknowledge was received.
In the case you wrote the client is filling the sender OS's send buffer and the receiver OS's receive buffer. Your client can send non-blocking until both buffers are full. Then it will block until the server calls recv again.
Related
i'm having some sync trouble with threads and sockets. I need one thread to recive incoming connections on socket (and remember client data to respond) and other thread to setup frames and send current frame to listed clients. So i was wondering if its possible to (kinda) put my data frames into server socket, so that everyone could just read current frame from socket without server knowing.
Server will just spam its socket with some data and client will get data without server actions. Is this possible? how?
I'm currently doing it pretty messed up way which i dont like:
server is listening on one thread for incoming transmissions and upon reciving such, add client data to list.
on other thread server is sending data to all clients from list.
EDIT:
I want to send data to some kind of buffer from which clients are allowed to read. (client doesnt have to read all messages server sends, just the one buffer contains at the moment of clients request), i dont want server to even notice that clients are reading from buffer if possible.
Right now threads are syncronised using uniqe_lock
What you're describing is probably MultiCast. Specifically, IP MultiCast (I think).
Searching finds a number of useful resources. This one looks concise, and includes coded examples (although I'm not sure how current it is).
If you're only transmitting to a LAN then broadcast will work too.
Consider the following scenario:
I have a server and a client, and they communicate in a custom defined application protocol.
A typical conversation is like this: (Assuming a connection between the two)
Client sends message to server, and waits for acknowledgment (don't
confuse with TCP) that it's message has been processed
Server parses the message and as soon as it reached the end it sends
an acknowledgment back to the client that it has processed it's
message
Client gets the acknowledgment and can now freely send another
message and wait again for an acknowledgment etc etc.
I have come to a problem implementing this. I am looping recv() parsing the message but as soon as recv has nothing more to return it blocks, and my code can't proceed to argue that I've received the whole message so that it sends an acknowledgment.
I've managed to somehow come around this by sending a message size in the header of the message and counting each time whether I've read as many bytes as the message size, but this is not ideal; what if the client machine bugged and it sent an incorrect size?
Also, I've read in another question that this whole acknowledgment process I'm doing is not necessary when using tcp, and that tcp already does that for you; but isn't the handshake only done once per connection? What if I don't close the connection between the two? Then only one handshake at the creation of the connection would have happened.
Do I have to create a new connection every time I want to be sending something? And isn't the handshake done before the data is sent, and only when making the connection? So this might not be what I'm looking for in my case.
I try to make a server "messages" via websocket under boost.
Currently, I can often send large messages or series of messages from the server.
when I hit "send", it sends tons of data.
The difficulty is that when the server receives a command in a websocket message like "Stop", "Pause" ... this command runs until the end of the previous message. I try to stop the execution of the previous command.
I tried to read the buffer between sending data. but it does not work. I try to check if there is one receiving orders with async_read_some.
I based on the example of
http://www.codeproject.com/Articles/443660/Building-a-basic-HTML5-client-server-application
and HTTP server boost
http://www.boost.org/doc/libs/1_53_0/doc/html/boost_asio/examples.html
Do you have any idea? I reworked my code several times but I can not execute the new real-time control as it appears at the end ..
thank you
If the data has already been sent to the network adapter, there is very little you can do to alter the order of packets. Network adapter will send the packets as and when it gets round to it, in the order you've queued them.
If you want to be able to send "higher priority" messages, then don't send off all the data in one go, but hold it in a queue waiting for the device to accept more data, and if a high priority message comes in, send that before you send any of the other packets off.
Don't make the packets TOO small, but I guess if you make packets that are a few kilobytes or so at a time, it will work quite swiftly and still allow good control over the flow.
Of course, this also will require that the receiver has the understanding of "there may be different 'flows' or 'streams' of information, and if you send a 'pause' command, it means that the previously sent stream is not going to receive anything until 'resume' is sent" obviously adjust this as needed for the behaviour you need, but you do need some way to not just say "put 'STOP' as data into the rest of the flow", but interpret it as a command.
If you send large message in the network as a single packet by the time server receives all the data the server receives stop message you may not have control over it until you complete receiving data.
It's better you implement priority message queue. Send the message as small chunks from client and assemble server instead of single large packet. Give message packets like stop(cancel) high priority. While receiving the messages at server end if any high priority message exists like stop(cancel) you don't need to accept remaining messages you can close the websocket connection at server.
Read the thread Chunking WebSocket Transmission for more info.
As you are using Boost, have you looked at WebSocket++ (Boost/ASIO based)?
I have 2 UDP sockets (SOCKET), one for sending and one for receiving on a Windows machine. They both work well, but the trouble is that a program that is receiving messages from my send socket replies to the same port which sent the message.
I know that if I don't bind the send socket, using sendto will pick an ephemeral port to send on.
I'd like to know if it is possible to any of the following, and if so, what is the recommended way to do it:
Bind both the send and receive sockets to a chosen port so that when the external program sends a message back it can be received.
Update the port to which the receive socket is bound in such a way as to receive on the port from which I last sent a message (not sure if this would create a race condition).
Some other correct method.
So far I have tried:
Not binding the send socket (it sends from some open port to the destination port). I can successfully receive messages on that port for as long as it doesn't change, but eventually it does change.
Binding both the send and receive sockets to a desired port. This creates the desired behaviour when I watch the packets using a sniffer, but the receive socket never receives the messages even though I see them being transmitted to the correct port and IP.
Packets are being received from more than one outside entity, and not guaranteed to be in any particular order.
Thank you in advance!
Looks like you are trying to use threads to separate sending and receiving data. I would question this approach, since UDP is so easy to handle in one thread. Nevertheless, you can just use the same socket from both threads if you want (see related question: Are parallel calls to send/recv on the same socket valid?). Just bind(2) it and, optionally, connect(2) it.
I am working on TCP client server application using c++.third party lib are now allowed in this project.
Here exchange between client server takes using well define protocol format.once the client receives the packet it will send it for parsing.I have protocol manager which will take care of the parsing activity.
I have following doubt
When the data arrives at client from the network,
the OS buffers it until application calls recv() function.
So two message msg1 and msg2 arrives at the buffer a call to recv will return msg1+msg2.
Now this may result in failure of the parsing activity.
My queries
1. whether above mentioned assumption is correct or not ?
2. If above mentioned assuption is correct then how can resolve this issue.
TCP emulates a stream, so in TCP there is no notion of messages. If you want messages, your application has to have some protocol to seperate them.
UDP does have a messages, so there it is possible to retrieve separate messages.
You can use a LV protocol (Length-Value) for your message.
Encode the message length in the first (1-4) bytes, then put the message.
Something like this for example : 14"Hello world\0"
In your server, when a client is sending something you'll have to recv() the first (1-4) bytes then recv() the length of the message.