number is present in the username portion of the URI. The assumed default is that it is not, indicated by the value ip. If a telephone number is present, it is indicated by the value phone. This parameter must not be used to guess at the characteristics or capabilities of the user agent. For example, the presence of a user=phone parameter must not be interpreted that the user agent is a SIP telephone (which may have limited display/processing capabilities). In a teleph- ony environment, IP telephones and IP/PSTN gateways may in fact use the reverse assumption, interpreting any digits in a username as digits regardless if user=phone is present. The method parameter is used to indicate the method to be used. The default is INVITE. This parameter has no meaning in To or From header fields or in a Request-URI but can be used in Contact headers for registra- tion, for example, or in a Refer-To header field. The ttl parameter is the time-to-live, which must only be used if the maddr parameter contains a multicast address and the transport parameter con- tains udp. The default value is 1. This value scopes the multicast session broad- cast, as described in Section 1.8. The maddr usually contains the multicast address to which the request should be directed, overriding the address in the host portion of the URI. It can also contain, however, a unicast address of an alternative server for requests. The method, maddr, ttl, and header parameters must not be included in To or From headers, but may be included in Contact headers or in Request-URIs. In addition to these parameters, a SIP URI may contain other user-defined parameters. Following the "?" parameter, names can be specified to be included in the request. This is similar to the operation of the mailto URL, which allows SIP: Understanding the Session Initiation Protocol Subject and Priority to be set for the request. Additional headers can be specified, separated by a "&". The header name body indicates that the con- tents of a message body for an INVITE request is be