It’s the English version of http1.1 503 service temporarily unavailable, for the people who came from English Speaking countries.
Not long ago, I met HTTP/1.1 503 Service Temporarily Unavailable while I was using “fopen” to open a long-distance page, then I was thinking whether my sites run out of some limits, but the online operator told me that’s not the server’s reason. So I checked the error log on another server of mine, which the long distance page I opened was on, then I found the reason. That’s due to too much connection requests from my server to the page, so it limited the requests from my server.
It was using mod_limitipconn.c, a apache mod, to limit the ip connections, here’s the introduction:
Apache module mod_limitipconn.c, which allows web server administrators to limit the number of simultaneous downloads permitted from a single IP address.
The introduction here: mod_limitipconn.c.