This header field, along with Authorization, breaks the general rules about multiple header field names. Although not a comma-separated list, this header field name may be present multiple times, and MUST NOT be combined into a single header line.
An UAC sends a request to a proxy server containing a Proxy-Authorization header field, so that the proxy can authenticate the UAC before processing the request. A proxy can challenge for credentials by rejecting a request with a 407 (Proxy Authentication Required) status code upon which a UAC may provide credentials for the requested resource in the Proxy-Authorization header.
A Proxy-Authorization header field value applies only to the proxy whose realm is identified in the "realm" parameter. When multiple proxies are used in a chain, a Proxy-Authorization header field value MUST NOT be consumed by any proxy whose realm does not match the "realm" parameter specified in that value. Note that if an authentication scheme that does not support realms is used in the Proxy-Authorization header field, a proxy server MUST attempt to parse all Proxy-Authorization header field values to determine whether one of them has what the proxy server considers to be valid credentials.
Proxy-Authorization: Digest username="Alice", realm="atlanta.com", nonce="c60f3082ee1212b402a21831ae", response="245f23415f11432b3434341c022"