date session parameters. However, neither party in a pending session (INVITE sent but no final response received) may re-INVITE-instead, the UPDATE method is used. Possible uses of UPDATE include muting or placing on hold pending media streams, performing QoS or other end-to-end attribute negotiation prior to session establishment The following is an example UPDATE message (Figure 4.12): UPDATE sips:beale@bufords.bedford.va SIP/2.0 Via SIP/2.0/TLS client.crypto.org: 5060 ;branch=z9hG4bK342 Max-Forwards 70 To: T. Beale <sips:beale@bufords.bedford.va.us>;tag=71 From: Blaise Vigenere ksips:bvigenere@crypto.org>;tag=19438 Call-ID: 170189761183162948 CSeq: 94 UPDATE Contact: <sips:client.crypto.org> Content-Type: application/sdp Content-Length: (SDP Message body not shown Table 4.14 lists the mandatory header fields in an UPDATE request. Proxy INVITE INVITE 183 (INVITE) 183 (INVITE) PRACK PRACK 200 OK (PRACK) 200 OK (PRACK) UPDATE UPDATE 200 OK (UPDATE) 200 OK (UPDATE) 200 OK (INVITE) 200 OK (INVITE) Media session Figure 4.12 Example of UPDATE method. SIP: Understanding the Session Initiation Protocol Table 4.14 Mandatory Header Fields for an UPDATE Call-ID Max-Forwards Contact 4.2 URI and URL Schemes Used by SIP SIP supports a number of URI and URL schemes including sip, sips, tel, pres, and im for SIP, secure SIP, telephone, presence, and instant message URIs as described in the following sections. In addition, other URI schemes can be present in SIP header fields as listed in Table 4.15. 4.2.1 SIP and SIPS URIs The addressing scheme of SIP URLs and URIs has been previously mentioned. SIP URIs are used in a number of places including the To, From, and Contact headers, as well as the Request-URI, which indicates the destination. SIP URIs are similar to the mailto URL [20] and can be used in hyperlinks Table 4.15 Common URI Schemes Used in SIP Messages Scheme Meaning SIP URI Most common in To, From, Contact, Request-URIs, and so forth [1] Secure SIP URI Same as SIP, provides end-to-end encryption using TLS [1] Telepho