C++ socket programming Max size of TCP/IP socket Buffer?
I am using C++ TCP/IP sockets. According to my requirements my client has to connect to a server and read the messages sent by it (that's something really new, isn't it) but... in my application I have to wait for some time (typically 1 - 2 hrs) before I actually start reading messages (through recv() or read()) and the server still keeps on sending messages.
I want to know whether there is a limit on the capacity of the buffer which keeps those messages in case they are not read and whose physical memory is used to buffer those messages? Sender's or receiver's?
Answer
TCP data is buffered at both sender and receiver. The size of the receiver's socket receive buffer determines how much data can be in flight without acknowledgement, and the size of the sender's send buffer determines how much data can be sent before the sender blocks or gets EAGAIN/EWOULDBLOCK, depending on blocking/non-blocking mode. You can set these socket buffers as large as you like up to 2^32-1 bytes, but if you set the client receive buffer higher than 2^16-1 you must do so before connecting the socket, so that TCP window scaling can be negotiated in the connect handshake, so that the upper 16 bits can come into play. [The server receive buffer isn't relevant here, but if you set it >= 64k you need to set it on the listening socket, from where it will be inherited by accepted sockets, again so the handshake can negotiate window scaling.]
However I agree entirely with Martin James that this is a silly requirement. It wastes a thread, a thread stack, a socket, a large socket send buffer, an FD, and all the other associated resources at the server for two hours, and possibly affects other threads and therefore other clients. It also falsely gives the server the impression that two hours' worth of data has been received, when it has really only been transmitted to the receive buffer, which may lead to unknown complications in recovery situations: for example, the server may be unable to reconstruct the data sent so far ahead. You would be better off not connecting until you are ready to start receiving the data, or else reading and spooling the data to yourself at the client for processing later.