what exactly does proxy_http_version mean?

[admon.org]

Many thanks Francis!
Here are my configs, I'm using Nginx as a reverse proxy server to cache
static files from its upstream servers.

+++++++++++++++++++++++++++++++++++
    proxy_temp_path   /tmp/proxy_temp_dir;
    proxy_cache_path  /tmp/proxy_cache_dir  levels=1:2  
keys_zone=cache_one:200m inactive=1d max_size=30g;

    upstream backend_server {
        server   myserver01 weight=1 max_fails=2 fail_timeout=15s;
        server   myserver02 weight=1 max_fails=2 fail_timeout=15s;
    }

    server {
        listen       80;
        server_name  test.stg
        root   html;
        index  index.html index.htm;

        charset utf-8;

        location / {
            proxy_next_upstream http_502 http_504 error timeout
invalid_header;
            proxy_cache cache_one;
            proxy_cache_valid  200 304 12h;
            proxy_cache_key $host$uri$is_args$args;
            proxy_set_header Host  $host;
            proxy_set_header X-Forwarded-For  $remote_addr;
            proxy_set_header Connection "";

            proxy_pass http://backend_server;
            proxy_http_version   1.1;
            expires      1d;
        }

        #error_page  404              /404.html;
        # redirect server error pages to the static page /50x.html
        error_page   500 502 503 504  /50x.html;
        location = /50x.html {
             root   html;
        }
+++++++++++++++++++++++++++++++++++ END


I'm trying to implement a smart way to purge cache e.g. by touching the
file to change its etag. This needs the HTTP/1.1 support. But per my
tests, Nginx does not return expected value.

$ curl -I -H 'If-None-Match: "9cx7b1-x2e-44xa4x9axd0x0"'
http://test.stg/crossdomain.xml 

This command gives 200 OK return code, but if I use Varnish instead, it
gives the expected value 304 Not Modified.
So I'm wondering that it may be caused by the protocal between Nginx and
the upstream server.

Can you help have another check. Appriciated!

Posted at Nginx Forum: http://forum.nginx.org/read.php?2,228225,228245#msg-228245