[SIPForum-discussion] Header Replaces and Parameter Replaces

[Thomas_Young at 3com.com]

Hi All,

    RFC 3515 defines a Refer method and Refer-to header, and gives us an 
example of Refer-to header with Replaces parameter below:
      Refer-To: <sip:dave at denver.example.org
?Replaces=12345%40192.168.118.3%3Bto-tag%3D12345%3Bfrom-tag%3D5FFE-3994>

Unfortunately, I can't see any formal description about this Replaces 
parameter this spec.
 

 
    RFC3891 defines the Replaces header with following syntax/format:
       Replaces = "Replaces" HCOLON callid *(SEMI replaces-param)
       replaces-param = to-tag / from-tag / early-flag / generic-param
       to-tag = "to-tag" EQUAL token
       from-tag = "from-tag" EQUAL token
       early-flag = "early-only"

    Examples:
       Replaces: 98732 at sip.example.com ;from-tag=r33th4x0r ;to-tag=ff87ff
       Replaces: 12adf2f34456gs5;to-tag=12345;from-tag=54321;early-only
       Replaces: 87134 at 171.161.34.23;to-tag=24796;from-tag=0

Please notice: this Replaces header has little bit different format from 
tha above Replace parameter which has some unfixed separators like ?, 
%3B,%D.
 

   Also RFC3891specifies that Replaces header can be only used in Invite 
method as below:
The Replaces header field indicates that a single dialog identified by the 
header field is to be shut down and logically replaced by the incoming 
INVITE in which it is contained. It is a request header  only, and defined 
only for INVITE requests......

      Header field    where   proxy   ACK  BYE  CAN  INV  OPT  REG  MSG
      ------------    -----   -----   ---  ---  ---  ---  ---  ---  ---
      Replaces          R              -    -    -    o    -    -    -


                                      SUB  NOT  REF  INF  UPD  PRA  PUB
                                      ---  ---  ---  ---  ---  ---  ---
      Replaces          R              -    -    -    -    -    -    -




 Here I'm just wondering why RFC3515 just gives an example of Replaces 
parameter in the Refer-to header without any formal description about its 
syntax or format(how to parse the unfixed separators), even I know 
RFC5359(SIP Service Examples) and RFC5589(SIP Call Control-Transfer)tell 
us how to use it.


Thanks and best regards

 Thomas Young
 

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