1228 Generating the Request

   A valid SIP request formulated by a UAC MUST, at a minimum, contain

   the following header fields: To, From, CSeq, Call-ID, Max-Forwards,

   and Via; all of these header fields are mandatory in all SIP

   requests.  These six header fields are the fundamental building

   blocks of a SIP message, as they jointly provide for most of the

   critical message routing services including the addressing of

   messages, the routing of responses, limiting message propagation,

   ordering of messages, and the unique identification of transactions.

   These header fields are in addition to the mandatory request line,

   which contains the method, Request-URI, and SIP version.

The initial Request-URI of the message SHOULD be set to the value of

   the URI in the To field.

   In some special circumstances, the presence of a pre-existing route

   set can affect the Request-URI of the message.  A pre-existing route

   set is an ordered set of URIs that identify a chain of servers, to

   which a UAC will send outgoing requests that are outside of a dialog.

   Commonly, they are configured on the UA by a user or service provider

   manually, or through some other non-SIP mechanism.  When a provider

   wishes to configure a UA with an outbound proxy, it is RECOMMENDED

   that this be done by providing it with a pre-existing route set with

   a single URI, that of the outbound proxy.