rwards The Max-Forwards header field is used to indicate the maximum number of hops that a SIP request may take. The value of the header field is decremented by each proxy that forwards the request. A proxy receiving the header field with a value of zero discards the message and sends a 483 Too Many Hops response back to the originator. Max-Forwards is a mandatory header field in requests generated by a RFC 3261 compliant UA. However, an RFC 2543 UA generally will not include the header field field. The suggested initial value is 70 hops. An example is: Max-Forwards: 10 6.2.19 Reason The Reason header field [18] can be used in BYE and CANCEL messages to indicate the reason why the session or call attempt is being terminated. It can carry a SIP response code or a Q.850 cause value (from an ISUP REL message, for example). For example, a forking proxy could include the following header field in CANCEL sent to a leg after one leg has answered the call: SIP: Understanding the Session Initiation Protocol Reason: SIP ; cause=200 text: "Call completed elsewhere" 6.2.20 Refer-To The Refer-To header field [19] is a required header field in a REFER request, which contains the URI or URL resource that is being referenced. It may contain any type of URI from a sip or sips to a tel URI to a http or mailto URI. For a sip or sips URI, the URI may contain a method or escaped header fields. For example, the following Refer-To header field: Refer-To: <sip:UserC@client.anywhere.com?Replaces sdjfdjfskdf@there.com%3Bto-tag%3D5f35a3%3Bfrom-tag%3D86753098 contains an escaped Replaces header field. The resulting INVITE message generated by this Refer-To header field would have a Request-URI of sip: UserC@client.anywhere.comand a Replaces: sdjfdjfskdf@there.com;to-tag=5f35a3;from-tag=8675309 header field. Note that the characters and = are replaced by their hex equiva- lents % 33 and %3D. In the next example, the header field contains a method: Refer-To: sip:UserC@client.anywhere.com?method=SUBSCRIBE> would cause a SUBSCRIBE request to be