Linux TCP Connection timeout

Question

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 <stdio.h>
#include <sys/socket.h>
#include <netinet/in.h>
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;
}

Server Code:

#include <stdio.h>
#include <sys/socket.h>
#include <netinet/in.h>
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;
}

Steffen Ullrich · Accepted Answer · 2021-01-17T09:08:33.350

8

There is no standardized idle timeout for TCP connections. Connections could in theory exist for days without any traffic. The idle timeout is instead specific for the application and is enforced there.

Often applications also employ TCP keep-alive to make sure that the connection state will be kept alive by intermediate devices like stateful firewalls and NAT routers and that a silent disconnect (like system switched off, connection broken) will be detected early. This way, TCP keep-alive helps to detect a broken connection even if the connection is idle.

... I consult the RFC 793 document, 3.8 Interfaces:

What you refer to has nothing to do with an idle timeout. It is the timeout when data are send but not ACKed by the peer.

edited Jan 17 '21 at 09:08

answered Jan 17 '21 at 08:47

Steffen Ullrich

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So, if the timeout of the TCP connection depends on TCP KeepAilve? For an application. How about the Idle timeout? – Rain Jan 17 '21 at 08:57
1

@Rain: There is no "THE timeout of the TCP connection". What you describe in your question is the idle timeout, i.e. who long the connection exist when no data are transferred and no ACK for transferred data are outstanding. As I wrote - this timeout __fully__ depends on the application. TCP keep alive is only employed to detect connection loss even when no data are transferred. It thus does not determine the idle timeout but makes sure that it can be distinguished between an idle and a broken connection. – Steffen Ullrich Jan 17 '21 at 09:06

Linux TCP Connection timeout

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