TCP keepalive:

TCP keepalive: 參考TCP Keepalive HOWTO

於RFC1122 TCP內建的option,由c socket或kernel設置參數:

• TCP_KEEPCNT - tcp_keepalive_probes (重試幾次決定斷線)
• TCP_KEEPINTVL - tcp_keepalive_intvl(ACK傳了多久沒回應要重試)
• TCP_KEEPIDLE - tcp_keepalive_time (閒置多久時間開始傳ACK)
  一般linux是預設socket不使用keepalive的,所以要在code開啟socket時就設定(類似example setsockopt),linux判定斷線之後kernel內部直接關閉連結,在上層的應用無法callback處理.

Heartbeat:

之前常用在即時通訊(IM)等服務, 在應用層利用TCP packet自訂的protocol.
利用TCP echo 封包來判定連線狀態,普遍傳遞時間很密集(可能幾分鐘就傳一次),自行判定斷線再呼叫關閉連線的後續程序.
在傳輸過程需要偷帶一些data或者是使用者斷線後需要做反應的情況下使用.

測試並觀察tcp keepalive 運作模式

以兩台device觀察比較容易製造斷線的情況,不然就需要防火牆設定

• 利用tcpdump/wireshark ,netcat
• nc -l之後以socket client 連入
• 開socket server運行後以nc連入
• ps -aux | grep tcp_ka observe child process pattern
• sudo lsof -i | grep [port] observerconnection state

the tcp_keepalive_client program

You run this program as:

    ./tcp_ka_client [-c CNT] [-d IDLE] [-i INTVL] HOST:PORT
    ./tcp_ka_client -c 3 -d 10 -i 5 192.168.2.95:8050

The program creates a TCP socket and sets the SO_KEEPALIVE socket option to 1. If any of the "-c", "-d", and "-i" options are specified, then the TCP_KEEPCNT, TCP_KEEPIDLE, and TCP_KEEPINTVL socket options are set on the socket to the respective option arguments. Then the program connects to HOST:PORT and enters a loop between calling poll(2) for the POLLIN event and reading data.

example pattern

    ./tcp_ka_client -c 3 -d 10 -i 5 192.168.2.95:8050

when 20 sec serverside D/C( no ack) , then send 3 probes with interval = 5sec . Judge D/C @ 35s, send RST then close connection.