[SIPForum-discussion] Header Replaces and Parameter Replaces

Thomas_Young at 3com.com Thomas_Young at 3com.com
Wed Jan 27 16:09:16 UTC 2010

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

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"

       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;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 ?, 

   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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