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20.20 From

   The From header field indicates the initiator of the request.  This
   may be different from the initiator of the dialog.  Requests sent by
   the callee to the caller use the callee's address in the From header
   field.

   The optional "display-name" is meant to be rendered by a human user
   interface.  A system SHOULD use the display name "Anonymous" if the
   identity of the client is to remain hidden.  Even if the "display-
   name" is empty, the "name-addr" form MUST be used if the "addr-spec"
   contains a comma, question mark, or semicolon.  Syntax issues are
   discussed in Section 7.3.1.

   Two From header fields are equivalent if their URIs match, and their
   parameters match. Extension parameters in one header field, not
   present in the other are ignored for the purposes of comparison. This
   means that the display name and presence or absence of angle brackets
   do not affect matching.

   See Section 20.10 for the rules for parsing a display name, URI and
   URI parameters, and header field parameters.

   The compact form of the From header field is f.

   Examples:

      From: "A. G. Bell" <sip:
 This e-mail address is being protected from spambots. You need JavaScript enabled to view it
 > ;tag=a48s
      From: sip:+
 This e-mail address is being protected from spambots. You need JavaScript enabled to view it
 ;tag=887s
      f: Anonymous <sip:
 This e-mail address is being protected from spambots. You need JavaScript enabled to view it
 >;tag=hyh8

20.21 In-Reply-To

   The In-Reply-To header field enumerates the Call-IDs that this call
   references or returns.  These Call-IDs may have been cached by the
   client then included in this header field in a return call.

      This allows automatic call distribution systems to route return
      calls to the originator of the first call.  This also allows
      callees to filter calls, so that only return calls for calls they
      originated will be accepted.  This field is not a substitute for
      request authentication.


   Example:

      In-Reply-To: 
 This e-mail address is being protected from spambots. You need JavaScript enabled to view it
 , 
 This e-mail address is being protected from spambots. You need JavaScript enabled to view it
 

20.22 Max-Forwards

   The Max-Forwards header field must be used with any SIP method to
   limit the number of proxies or gateways that can forward the request
   to the next downstream server.  This can also be useful when the
   client is attempting to trace a request chain that appears to be
   failing or looping in mid-chain.

   The Max-Forwards value is an integer in the range 0-255 indicating
   the remaining number of times this request message is allowed to be
   forwarded.  This count is decremented by each server that forwards
   the request.  The recommended initial value is 70.

   This header field should be inserted by elements that can not
   otherwise guarantee loop detection.  For example, a B2BUA should
   insert a Max-Forwards header field.

   Example:

      Max-Forwards: 6

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