Does a TCP socket connection have a keep alive

Question

I have heard of HTTP keep-alive but for now I want to open a socket connection with a remote server  Now will this socket connection remain open forever or is there a timeout limit associated with it similar to HTTP keep-alive

User · Answer

You are looking for the SO KEEPALIVE socket option   The Java Socket API exposes  keep-alive  to applications via the setKeepAlive and getKeepAlive methods    EDIT  SO KEEPALIVE is implemented in the OS network protocol stacks without sending any  real  data   The keep-alive interval is operating system dependent  and may be tuneable via a kernel parameter   Since no data is sent  SO KEEPALIVE can only test the liveness of the network connection  not the liveness of the service that the socket is connected to   To test the latter  you need to implement something that involves sending messages to the server and getting a response

User · Answer

Now will this socket connection remain open forever or is there a timeout limit associated with it similar to HTTP keep-alive    The short answer is no it won t remain open forever  it will probably time out after a few hours  Therefore yes there is a timeout and it is enforced via TCP Keep-Alive   If you would like to configure the Keep-Alive timeout on your machine  see the  Changing TCP Timeouts  section below  Otherwise read through the rest of the answer to learn how TCP Keep-Alive works   Introduction  TCP connections consist of two sockets  one on each end of the connection  When one side wants to terminate the connection  it sends an RST packet which the other side acknowledges and both close their sockets   Until that happens  however  both sides will keep their socket open indefinitely  This leaves open the possibility that one side may close their socket  either intentionally or due to some error  without informing the other end via RST  In order to detect this scenario and close stale connections the TCP Keep Alive process is used   Keep-Alive Process  There are three configurable properties that determine how Keep-Alives work  On Linux they are1    tcp keepalive time   default 7200 seconds  tcp keepalive probes   default 9  tcp keepalive intvl   default 75 seconds    The process works like this    Client opens TCP connection If the connection is silent for tcp keepalive time seconds  send a single empty ACK packet 1 Did the server respond with a corresponding ACK of its own    No   Wait tcp keepalive intvl seconds  then send another ACK Repeat until the number of ACK probes that have been sent equals tcp keepalive probes  If no response has been received at this point  send a RST and terminate the connection   Yes  Return to step 2    This process is enabled by default on most operating systems  and thus dead TCP connections are regularly pruned once the other end has been unresponsive for 2 hours 11 minutes  7200 seconds   75   9 seconds    Gotchas  2 Hour Default  Since the process doesn t start until a connection has been idle for two hours by default  stale TCP connections can linger for a very long time before being pruned  This can be especially harmful for expensive connections such as database connections   Keep-Alive is Optional  According to RFC 1122 4 2 3 6  responding to and or relaying TCP Keep-Alive packets is optional      Implementors MAY include  keep-alives  in their TCP implementations    although this practice is not universally accepted  If keep-alives are   included  the application MUST be able to turn them on or off for each   TCP connection  and they MUST default to off                It is extremely   important to remember that ACK segments that contain no data are not   reliably transmitted by TCP    The reasoning being that Keep-Alive packets contain no data and are not strictly necessary and risk clogging up the tubes of the interwebs if overused   In practice however  my experience has been that this concern has dwindled over time as bandwidth has become cheaper  and thus Keep-Alive packets are not usually dropped  Amazon EC2 documentation for instance gives an indirect endorsement of Keep-Alive  so if you re hosting with AWS you are likely safe relying on Keep-Alive  but your mileage may vary   Changing TCP Timeouts  Per Socket  Unfortunately since TCP connections are managed on the OS level  Java does not support configuring timeouts on a per-socket level such as in java net Socket  I have found some attempts3 to use Java Native Interface  JNI  to create Java sockets that call native code to configure these options  but none appear to have widespread community adoption or support   Instead  you may be forced to apply your configuration to the operating system as a whole  Be aware that this configuration will affect all TCP connections running on the entire system   Linux  The currently configured TCP Keep-Alive settings can be found in    proc sys net ipv4 tcp keepalive time  proc sys net ipv4 tcp keepalive probes  proc sys net ipv4 tcp keepalive intvl   You can update any of these like so     Send first Keep-Alive packet when a TCP socket has been idle for 3 minutes   echo 180  gt   proc sys net ipv4 tcp keepalive time   Send three Keep-Alive probes      echo 3  gt   proc sys net ipv4 tcp keepalive probes       spaced 10 seconds apart    echo 10  gt   proc sys net ipv4 tcp keepalive intvl   Such changes will not persist through a restart  To make persistent changes  use sysctl   sysctl -w net ipv4 tcp keepalive time 180 net ipv4 tcp keepalive probes 3 net ipv4 tcp keepalive intvl 10   Mac OS X  The currently configured settings can be viewed with sysctl     sysctl net inet tcp   grep -E  keepidle keepintvl keepcnt  net inet tcp keepidle  7200000 net inet tcp keepintvl  75000 net inet tcp keepcnt  8   Of note  Mac OS X defines keepidle and keepintvl in units of milliseconds as opposed to Linux which uses seconds   The properties can be set with sysctl which will persist these settings across reboots   sysctl -w net inet tcp keepidle 180000 net inet tcp keepcnt 3 net inet tcp keepintvl 10000   Alternatively  you can add them to  etc sysctl conf  creating the file if it doesn t exist      cat  etc sysctl conf net inet tcp keepidle 180000 net inet tcp keepintvl 10000 net inet tcp keepcnt 3   Windows  I don t have a Windows machine to confirm  but you should find the respective TCP Keep-Alive settings in the registry at   HKEY LOCAL MACHINE System CurrentControlSet Services TCPIP Parameters  Footnotes  1  See man tcp for more information   2   This packet is often referred to as a  Keep-Alive  packet  but within the TCP specification it is just a regular ACK packet  Applications like Wireshark are able to label it as a  Keep-Alive  packet by meta-analysis of the sequence and acknowledgement numbers it contains in reference to the preceding communications on the socket   3  Some examples I found from a basic Google search are lucwilliams JavaLinuxNet and flonatel libdontdie

User · Answer

Here is some supplemental literature on keepalive which explains it in much finer detail   http   www tldp org HOWTO html single TCP-Keepalive-HOWTO  Since Java does not allow you to control the actual keepalive times  you can use the examples to change them if you re using a Linux kernel  or proc based OS

User · Answer

If you re behind a masquerading NAT  as most home users are these days   there is a limited pool of external ports  and these must be shared among the TCP connections  Therefore masquerading NATs tend to assume a connection has been terminated if no data has been sent for a certain time period   This and other such issues  anywhere in between the two endpoints  can mean the connection will no longer  work  if you try to send data after a reasonble idle period  However  you may not discover this until you try to send data   Using keepalives both reduces the chance of the connection being interrupted somewhere down the line  and also lets you find out about a broken connection sooner

User · Answer

For Windows according to Microsoft docs   KeepAliveTime  REG DWORD  milliseconds  by default is not set which means 7 200 000 000    2 hours  - analogue to tcp keepalive time KeepAliveInterval  REG DWORD  milliseconds  by default is not set which means 1 000   1 second  - analogue to tcp keepalive intvl Since Windows Vista there is no analogue to tcp keepalive probes  value is fixed to 10 and cannot be changed

User · Answer

In JAVA Socket     TCP connections are managed on the OS level  java net Socket does not provide any in-built function to set timeouts for keepalive packet on a per-socket level  But we can enable keepalive option for java socket but it takes 2 hours 11 minutes  7200 sec  by default to process after a stale tcp connections  This cause connection will be availabe for very long time before purge  So we found some solution to use Java Native Interface  JNI  that call native code c    to configure these options       Windows OS      In windows operating system keepalive time  amp  keepalive intvl can be configurable but tcp keepalive probes cannot be change By default  when a TCP socket is initialized sets the keep-alive timeout to 2 hours and the keep-alive interval to 1 second  The default system-wide value of the keep-alive timeout is controllable through the KeepAliveTime registry setting which takes a value in milliseconds   On Windows Vista and later  the number of keep-alive probes  data retransmissions  is set to 10 and cannot be changed   On Windows Server 2003  Windows XP  and Windows 2000  the default setting for number of keep-alive probes is 5  The number of keep-alive probes is controllable  For windows Winsock IOCTLs library is used to configure the tcp-keepalive parameters   int WSAIoctl  SocketFD     descriptor identifying a socket SIO KEEPALIVE VALS     dwIoControlCode  LPVOID  lpvInBuffer     pointer to tcp keepalive struct  DWORD  cbInBuffer     length of input buffer NULL     output buffer 0     size of output buffer  LPDWORD  lpcbBytesReturned     number of bytes returned NULL     OVERLAPPED structure NULL    completion routine     Linux OS  Linux has built-in support for keepalive which is need to be enabling TCP IP networking in order to use it  Programs must request keepalive control for their sockets using the setsockopt interface   int setsockopt int socket  int level  int optname  const void  optval  socklen t optlen   Each client socket will be created using java net Socket  File descriptor ID for each socket will retrieve using java reflection

User · Answer

TCP keepalive and HTTP keepalive are very different concepts  In TCP  the keepalive is the administrative packet sent to detect stale connection  In HTTP  keepalive means the persistent connection state   This is from TCP specification   Keep-alive packets MUST only be sent when no data or acknowledgement packets have been received for the connection within an interval  This interval MUST be configurable and MUST default to no less than two hours   As you can see  the default TCP keepalive interval is too long for most applications  You might have to add keepalive in your application protocol

User · Answer

TCP sockets remain open till they are closed   That said  it s very difficult to detect a broken connection  broken  as in a router died  etc  as opposed to closed  without actually sending data  so most applications do some sort of ping pong reaction every so often just to make sure the connection is still actually alive

[java] Does a TCP socket connection have a "keep alive"?

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