amples are shown in Table 6.14. 6.2.6 Call-Info The Call-Info header field is included in a request by a UAC or proxy to provide a URI with information relating to the session setup. It may be present in an INVITE, OPTIONS or REGISTER request. The header field parame- ter purpose indicates the purpose of the URI and may have the values icon, info, card, or other IANA registered tokens. An example follows: Call-Info: <http://www.code.com/my_picture.jpg>;purpose=icon Table 6.14 Example of Authorization Header Field Header Field Meaning Authorization: Digest username= 'Cust1", This HTTP digest authorization header realm="company.com" field contains the credentials of Cust1; nonce="9c8e88df84f1cec4341ae6e5a359" the nonce was supplied by the SIP opaque=" , uri="sip:user2@company.co server located at the uri specified. esponse="e56131d19580cd833064787ecc" The response contains the encrypted username and password. No opaque string is present. SIP Header Fields 6.2.7 Event The Event header field is used in a SUBSCRIBE (see Section 4.1.8) or NOTIFY (see Section 4.1.9) methods to indicate which event package is being used by the method. In a SUBSCRIBE, it lists the event package to which the client would like to subscribe. In a NOTIFY, it lists the event package that the notification contains state information about. Currently defined event packages are listed in Table 4.8. The compact form is O. An example follows: Event: dialog O : refer 6.2.8 Hide The Hide header field was defined in RFC 2543 but has been deprecated from RFC 3261. It was intended to be used by user agents or proxies to request that the next hop proxy encrypts the Via header fields to hide message routing path information. Encrypted Via headers were identified with the hidden Via parameter. However, the security provided and the mechanism requiring next hop trust made the value of this header field minimal. 6.2.9 In-Reply-To The In-Reply-To header field is used to indicate the Call - ID that this request references or is returning. For example, a missed 