When does the Nginx error happen, recv() failed (110: Connection timed out) while proxying upgraded connection

Question

I am using Nginx 1.10.1 for my APP to balance connections on Websocket.

For example, the App A(multi-process) establish Websocket connections to B1, B2 through Nginx so that the connections could be balanced.

While using it, sometimes even though my App send messages through the Websocket, the end-point(B1 or B2) doesn't receive any message and after a while Nginx prints the error log message recv() failed (110: Connection timed out) while proxying upgraded connection.

The below is my config for Nginx.

Why does it happen and how to fix it ? Something like connection timeout over network would lead this situation ?

worker_processes 2;
error_log /app_log/sangmin/nginx/error.log info;
pid /app_log/sangmin/nginx/nginx.pid;
events {
    worker_connections 1024;
    use epoll;
}
http {
            map $http_upgrade $connection_upgrade {
                        default upgrade;
                        '' close;
            }
            client_body_buffer_size 5m;
            access_log /app_log/sangmin/nginx/access.log;

            upstream s2s_host{
                        hash $remote_port;
                        server 10.2.150.211:8080 max_fails=0 fail_timeout=10s;server 10.2.150.212:8080 max_fails=0 fail_timeout=10s;
            }
            server {
                        listen 10.2.14.195:10002;
                        location = /s2s {
                                    proxy_read_timeout 365h;
                                    proxy_send_timeout 365h;
                                    proxy_http_version 1.1;
                                    proxy_set_header Upgrade $http_upgrade;
                                    proxy_set_header Connection "Upgrade";

                                    proxy_pass http://s2s_host;
                        }

            }
}