TCP Socket no connection timeout
I open a TCP socket and connect it to another socket somewhere else on the network. I can then successfully send and receive data. I have a timer that sends something to the socket every second. I then rudely interrupt the connection by forcibly losing the connection (pulling out the Ethernet cable in this case). My socket is still reporting that it is successfully writing data out every second. This continues for approximately 1hour and 30 minutes, where a write error is eventually given. What specifies this time-out where a socket finally accepts the other end has disappeared? Is it the OS (Ubuntu 11.04), is it from the TCP/IP specification, or is it a socket configuration option?
3 Answers 3
Pulling the network cable will not break a TCP connection(1) though it will disrupt communications. You can plug the cable back in and once IP connectivity is established, all back-data will move. This is what makes TCP reliable, even on cellular networks.
When TCP sends data, it expects an ACK in reply. If none comes within some amount of time, it re-transmits the data and waits again. The time it waits between transmissions generally increases exponentially.
After some number of retransmissions or some amount of total time with no ACK, TCP will consider the connection «broken». How many times or how long depends on your OS and its configuration but it typically times-out on the order of many minutes.
tcp_retries2 (integer; default: 15; since Linux 2.2) The maximum number of times a TCP packet is retransmitted in established state before giving up. The default value is 15, which corresponds to a duration of approximately between 13 to 30 minutes, depending on the retransmission timeout. The RFC 1122 specified minimum limit of 100 seconds is typically deemed too short. This is likely the value you’ll want to adjust to change how long it takes to detect if your connection has vanished.
(1) There are exceptions to this. The operating system, upon noticing a cable being removed, could notify upper layers that all connections should be considered «broken».
Setting timeout on tcp
Is it correct that keep alive probes are not sent during retransmission? I also looked into Stevens to find a more formal definition of this, because I am wondering what «the last data packet sent» actually means when considering retransmissions.
Setting timeout on tcp
i was wondering if is there any variable that allows me to modifty the value of tcp’s timeout. I’m trying to send packages between two virtual machines. Thank you.
The setsockopt function allows controlling a number of configuration parameters, including timeouts.
Linux increasing or decreasing TCP sockets timeouts
There are a load of parameters that you can modify — see http://man7.org/linux/man-pages/man7/tcp.7.html.
As you question is vague it is difficult to tell you what ones you can modify
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How to find the connection time-out value in linux?
I am setting up a nginx webserver with php-fpm and (d)dos deflate to ban attacks.
Now currently there is no traffic to my server at all, as i’m testing things.
With this command i can see who is connected to my server, and how many connections they have open:
netstat -ntu | awk » | cut -d: -f1 | sort | uniq -c | sort -n
During testing I noticed that when I would load a test script which is basicly it would start 3 connections. I guess 1 for the HTML an 2 for the 2 images on that page. All is fine so far.
But I noticed it took well over a minute before those 3 connections where closed. I kept running the above netstat command to see if those 3 external connections would close.
My nginx.conf has a keep alive timeout of 4.
The connection was made via a default setup Chrome browser.
How come those connections stayed open so long, and is this normal? Also, is there a way I can close them sooner?
You can increase or decrease timeouts on TCP sockets using the file tcp_keepalive_time found on the directory /proc/sys/net/ipv4/ .
The default timeout value is 7200 (2 hours).
For example, to change into 1200 seconds issue the command as below:
#echo 1200 > /proc/sys/net/ipv4/tcp_keepalive_time Linux — How to find process using TCP port?, linux tcp debian. Share. Improve this question. Follow edited Apr 15, 2015 at 21:25. Jason Who is listening on a given TCP port on Mac OS X? Share. Improve this answer. Follow edited May 23, 2017 at 12:33. Is Romana still the only Time Lord to be one of the Doctor’s companions?
When is a TCP connection considered idle?
I have a requirement to enable TCP keepalive on any connections and now I am struggling with the results from our test case. I think this is because I do not really understand when the first keepalive probe is sent. I read the following in the documentation for tcp_keepalive_time on Linux:
the interval between the last data packet sent (simple ACKs are not considered data) and the first keepalive probe; after the connection is marked to need keepalive, this counter is not used any further
Some other sources state that this is the time a connection is idle, but they do not further define what this means. I also looked into Stevens to find a more formal definition of this, because I am wondering what «the last data packet sent» actually means when considering retransmissions.
In my test case, I have a connection where data is only sent from a server to a client at rather high rates. To test keepalive, we unplugged the cable on the client’s NIC. I can now see that the network stack tries to send the data and enters the retransmission state, but no keep alive probe is sent. Is it correct that keep alive probes are not sent during retransmission?
I have a connection where data is only sent from a server to a client at rather high rates.
Then you’ll never see keepalives. Keepalives are sent when there is «silence on the wire». RFC1122 has some explanation re keepalives.
A «keep-alive» mechanism periodically probes the other end of a connection when the connection is otherwise idle , even when there is no data to be sent
Some other sources state that this is the time a connection is idle, but they do not further define what this means.
This is how long TCP will wait before poking the peer «hoy! still alive?».
$ cat /proc/sys/net/ipv4/tcp_keepalive_time 7200 In other words, you’ve been using a TCP connection and it has been great. However, for the past 2 hours there hasn’t been anything to send. Is it reasonable to assume the connection is still alive? Is it reasonable to assume all the middleboxes in the middle still have state about your connection? Opinions vary and keepalives aren’t part of RFC793.
The TCP specification does not include a keep-alive mechanism it could: (1) cause perfectly good connections to break during transient Internet failures; (2) consume unnecessary bandwidth («if no one is using the connection, who cares if it is still good?»)
To test keepalive, we unplugged the cable on the client’s NIC.
This isn’t testing keepalive. This is testing your TCPs retransmit strategy, i.e. how many times and how often TCP will try to get your message across. On a Linux box this (likely) ends up testing net.ipv4.tcp_retries2 :
How may times to retry before killing alive TCP connection. RFC 1122 says that the limit should be longer than 100 sec. It is too small number. Default value 15 corresponds to 13-30min depending on RTO.
But RFC5482 — TCP User Timeout Option provides more ways to influence it.
The TCP user timeout controls how long transmitted data may remain unacknowledged before a connection is forcefully closed.
Is it correct that keep alive probes are not sent during retransmission
It makes sense: TCP is already trying to elicit a response from the other peer, an empty keepalive would be superfluous.
Linux-specific (2.4+) options to influence keepalive
- TCP_KEEPCNT The maximum number of keepalive probes TCP should send before dropping the connection.
- TCP_KEEPIDLE The time (in seconds) the connection needs to remain idle before TCP starts sending keepalive probes, if the socket option SO_KEEPALIVE has been set on this socket
- TCP_KEEPINTVL The time (in seconds) between individual keepalive probes
Linux-specific (2.6.37+) option to influence TCP User Timeout
TCP_USER_TIMEOUT The maximum amount of time in milliseconds that transmitted data may remain unacknowledged before TCP will forcibly close connection.
So for example your application could use this option to determine how long the connection survives when there is no connectivity (similar to your NIC-unplugging example). E.g. if you have reason to believe the client will come back (perhaps they closed the laptop lid? spotty wireless access?) you can specify a timeout of 12 hours and when they do come back the connection will still function.
How to Use the timeout Command on Linux, We can run tcpdump for 10 seconds with its default options, and redirect its output to a file called capture.txt with the following command: timeout 10 sudo tcpdump > capture.txt ( tcpdump has its own options to save captured network traffic to a file. This is a quick hack because we’re discussing timeout, …
Linux TCP Connection timeout
I observe the TCP timeout from server to client. When TCP Three-way Handshake finished, and then the client does not anything for a long time. How many times will timeout TCP sessions? I consult the RFC 793 document, 3.8 Interfaces:
The timeout, if present, permits the caller to set up a timeout for all data submitted to TCP. If data is not successfully delivered to the destination within the timeout period, the TCP will abort the connection.
The present global default is five minutes
The following is the captured packet connection, More than 10 minutes have passed and there is no TCP disconnection. Am I misunderstanding somewhere? OS: Ubuntu 20 The following is my test code. Client Code:
#include #include #include int main(int argc, char *argv[]) < int socket_desc; struct sockaddr_in server; socket_desc = socket(AF_INET, SOCK_STREAM, 0); if(socket_desc == -1)< printf("Socket failed\n"); >server.sin_addr.s_addr = inet_addr("192.168.88.88"); server.sin_family = AF_INET; server.sin_port = htons(8888); if(connect(socket_desc, (struct sockaddr *)&server, sizeof(server)) <0)< printf("Connect failed\n"); >else < printf("Connected\n"); while(0); // When connected, do not anything. return 0; >#include #include #include int main(int argc, char *argv[]) < int socket_desc, new_socket, c; struct sockaddr_in server; socket_desc = socket(AF_INET, SOCK_STREAM, 0); if(socket_desc == -1)< printf("Socket failed\n"); >server.sin_addr.s_addr = INADDR_ANY; server.sin_family = AF_INET; server.sin_port = htons(8888); if(bind(socket_desc, (struct sockaddr *)&server, sizeof(server)) <0)< printf("bind failed\n"); >listen(socket_desc, 5); c = sizeof(struct sockaddr_in); new_socket = accept(socket_desc, (struct sockaddr *) &client, (socklen_t *) &c); if (new_socket < 0)< printf("Accept failed\n"); >else < printf("Accept\n"); while(1); // When accept , do not anything. >return 0; >