nfirms the session timer by including the Session- Expires header field in the response to the request. A UAS may also shorten the interval by reducing the interval. An example is: Session-Expires: 3600 6.2.31 Subscription-State The Subscription-State header field [3] is a required header field in a NOTIFY request. It indicates the current state of the subscription. Values defined include active, pending, or terminated. Additional parame- ters include expires, reason, and retry-after. Values defined for the reason parameter include deactivated, giveup, probation, noresource, rejected, and timeout. An example is: Subscription-State: terminated ; reason=rejected 6.3 Response Header Fields These header fields are present only in responses. 6.3.1 Authentication-Info The Authentication-Info header field can be inserted in responses when performing mutual authentication using HTTP Digest. In normal HTTP Digest as described in Section 3.8, the server challenges the client to provide a shared secret, which the client then provides in a repeat of the request containing an Authorization or WWW-Authenticate header field. To do mutual authentication, the server would then provide an Authentication-Info header field containing either a next nonce or a SIP: Understanding the Session Initiation Protocol response in an repath parameter. The response auth digest is calculated by the server on the SIP response using the same algorithm as the successful request authentication and using the same shared secret (client's username and pass- word). In this way, the server proves that it also knows the client's secret, provid- ing mutual authentication. The credentials are carried in the rspauth parameter in the header field. An example is: Authentication-Info: rspauth="9105jr98li459jgfp" 6.3.2 Error-Info The Error-Info header field is used in failure response to convey more infor- mation about an error. A UAC receiving the header field in a failure response may fetch and render the URI to the user. The header field can be used to give t