5 Contact header field, 130-32 405 Method Not Allowed, 115 defined, 130 406 Not Acceptable, 115 examples, 131 407 Proxy Authentication feature tag, 131-32 Required, 115-16 in INVITE requests, 130 408 Request Timeout, 116 parameters, 131 410 Gone, 116 See also Header fields 411 Length Required, 116 Content-Disposition header field, 413 Request Entity Too Large, 117 Content-Encoding header field, 158 414 Request-URI Too Long, 117 Content-Language header field, 158 415 Unsupported Media Type, content-length calculation example, 21 Content-Length header field, 159 416 Unsupported URI Scheme, Content-Type header field, 159-60 CSeq header field, 132, 133 420 Bad Extension, 117 421 Extension Required, Date header field, 132-33 devices 117-18 422 Session Timer Interval initialization of, 33 Too Small, 118 URIs, 26 423 Interval Too Brief, 118 Domain Name Service (DNS), 1 428 Use Authentication defined, 10 Token, 118 lookups, 26 records, 10, 12 SIP: Understanding the Session Initiation Protocol Domain Name Service (DNS) (continued) Accept-Contact, 140-41 servers, 10, 11 Accept-Encoding, 141 dual tone multiple frequency (DTMF) digits, Accept-Language, 141-42 174,263 Alert-Info, 128-29 Dynamic Host Configuration Protocol Allow, 158 (DHCP), 9 Allow-Events, 129 Authentication-Info, 153-54 emergency calling design team, 263 Authorization, 142 encryption Call-ID, 129-30 end-to-end, 60 Call-Info, 142 hop-by-hop, 60 categories, 127 S/MIME, 59-60 compact forms, 128 Encryption header field, 133 Contact, 130-32 Error-Info header field, 154 Content-Disposition, 158 Event header field, 143 Content-Encoding, 158 Events extensions, 33 Content-Language, 158 Expires header field, 160 Content-Length, 159 firewalls, 61 Content-Type, 159-60 forking proxy operation, 51 CSeq, 132, 133 From header field, 133-34 Date, 132-33 future directions, 261-64 Encryption, 133 Error-Info, 154 gateways, 45-47 Event, 143 decomposition, 47 Expires, 160 defined, 45-46 From, 133-34 media (MG), 47 In-Reply-To, 143 network with, 46 inserted/modified by proxies, 12