cho the Table 6.8 Extension Feature Tags Meaning events SIP Events [3] Join call control primitive [8] Path header field [9] rel100 Reliable provisional response (PRACK) support [10] replaces Replaces call control primitive [11] timer Session Timer feature [12] SIP Header Fields header field in the response to the request and may add another numerical time entry indicating the amount of delay. Unlike the Date header field, the time format is not specified. The most accurate time format should be used, includ- ing a decimal point. Examples are shown in Table 6.9. 6.1.15 To The To header field is a required header field in every SIP message used to indicate the recipient of the request. Any responses generated by a user agent will contain this header field with the addition of a tag. (Note that an RFC 2543 client will typically only generate a tag if more than one Via header field is present in the request.) Any response generated by a proxy must have a tag added to the To header field. A tag added to the header field in a 200 OK response is used throughout the call and incorporated into the dialog. The To header field is never used for routing-the Request-URI is used for this purpose. An optional display name can be present in the header field, in which case the SIP URI is enclosed in < >. If the URI contains any parameters or username parameters, the URI must be enclosed in < > even if no display name is present. The compact form of the header field is t. Examples are shown in Table 6.10. 6.1.16 User-Agent The User-Agent header field is used to convey information about the user agent originating the request. Based on the HTTP header field of the same name [2], it can contain manufacturer information, software version, or com- ments. The field may contain multiple tokens, with the ordering assumed to be from most general to most specific. This information can be used for logging or for generating a specific response for a specific user agent. For security reasons, this header field may be suppressed. For example, 