From 7801148a816a2ab1c2f9437c8992c86722361147 Mon Sep 17 00:00:00 2001 From: Denys Vlasenko Date: Thu, 25 Jul 2013 14:00:37 +0200 Subject: Add notes about TCP programming quirks. Signed-off-by: Denys Vlasenko --- docs/tcp.txt | 65 ++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++ 1 file changed, 65 insertions(+) create mode 100644 docs/tcp.txt (limited to 'docs/tcp.txt') diff --git a/docs/tcp.txt b/docs/tcp.txt new file mode 100644 index 000000000..7951e1c8b --- /dev/null +++ b/docs/tcp.txt @@ -0,0 +1,65 @@ + Some less-widely known details of TCP connections. + + Properly closing the connection. + +After this code sequence: + + sock = socket(AF_INET, SOCK_STREAM, 0); + connect(sock, &remote, sizeof(remote)); + write(sock, buffer, 1000000); + +a large block of data is only buffered by kernel, it can't be sent all at once. +What will happen if we close the socket? + +"A host MAY implement a 'half-duplex' TCP close sequence, so that + an application that has called close() cannot continue to read + data from the connection. If such a host issues a close() call + while received data is still pending in TCP, or if new data is + received after close() is called, its TCP SHOULD send a RST + to show that data was lost." + +IOW: if we just close(sock) now, kernel can reset the TCP connection, +discarding some not-yet sent data. + +What can be done about it? + +Solution #1: block until sending is done: + + /* When enabled, a close(2) or shutdown(2) will not return until + * all queued messages for the socket have been successfully sent + * or the linger timeout has been reached. + */ + struct linger { + int l_onoff; /* linger active */ + int l_linger; /* how many seconds to linger for */ + } linger; + linger.l_onoff = 1; + linger.l_linger = SOME_NUM; + setsockopt(sock, SOL_SOCKET, SO_LINGER, &linger, sizeof(linger)); + close(sock); + +Solution #2: tell kernel that you are done sending. +This makes kernel send FIN, not RST: + + shutdown(sock, SHUT_WR); + close(sock); + + + Defeating Nagle. + +Method #1: manually control whether partial sends are allowed: + +This prevents partially filled packets being sent: + + int state = 1; + setsockopt(fd, IPPROTO_TCP, TCP_CORK, &state, sizeof(state)); + +and this forces last, partially filled packet (if any) to be sent: + + int state = 0; + setsockopt(fd, IPPROTO_TCP, TCP_CORK, &state, sizeof(state)); + +Method #2: make any write to immediately send data, even if it's partial: + + int state = 1; + setsockopt(fd, IPPROTO_TCP, TCP_NODELAY, &state, sizeof(state)); -- cgit v1.2.3