代理的级别

Anonymity levels - applies for http/https proxies only.

* Level 1: No anonymity; remote host knows your IP and knows you are using proxy.
* Level 4: Low anonymity; remote host does not know your IP, but it knows you are using proxy.
* Level 8: Medium anonymity; remote host knows you are using proxy, and thinks it knows your IP, but this is not yours (this is usually a multihomed proxy which shows its inbound interface as REMOTE_ADDR for a target host).
* Level 16: High anonymity; remote host does not know your IP and has no direct proof of proxy usage (proxy-connection family header strings). If such hosts do not send additional header strings it may be considered as high-anonymous. If a high-anonymous proxy supports keep-alive you can consider it to be extremely-anonymous. However, such a host is highly possible to be a honey-pot.

KA means "keep-alive".

参考资料:

http://www.proxynova.com/proxy-articles/proxy-anonymity-levels-explained/
http://www.freeproxy.ru/en/free_proxy/faq/proxy_anonymity.htm
http://forum.hidemyass.com/index.php/topic/1782-what-means-anonymity-level-highka/