[OpenSIPStack] Upper Registration How To

[Joegen E. B. <jb...@so...>]

Upper Registration


   Upper Registration is the term used for the capability of OpenSBC to
   proxy Registrations towards an upstream Registrar.  In most cases,
   SIP Networks already have an existing Registrar prior to the
   deployment of a Border Controller such as OpenSBC.  For this reason,
   it would not be practical to transfer the existing SIP accounts to
   OpenSBC. 
   Upper Registration is the solution to this scenario.  While it is
   necessary that all SIP requests traverse OpenSBC for NAT continuity,
   Registrations MAY be allowed to be relayed towards an upstream
   Registrar while OpenSBC retains a copy of the AOR and masquerade on
   behalf of the the UA that send the registration request.




Understanding Local and Remote Domain Concept in OpenSBC


   To understand how Upper Registration works, it is important to
   distinguish when OpenSBC processes a request as a local domain
   request and when it is treated as a remote domain request.

   Local Domain Requests - requests with a request-URI not resolving to
   one of OpenSBC's listener interface and port either via DNS host
   lookup, DNS SRV lookup or in raw IP Address form.
   Remote Domain Requests - requests that, when Resolved, points to an
   address and port tuple other than the ones listed as a listener
   addresses of OpenSBC.

   If REGISTER requests hits OpenSBC and OpenSBC is in either Full Mode
   or B2B Upper Reg Mode the following steps would be followed:

   1.  OpenSBC checks if the request-URI is bound to a local or a
   remote domain.

   2.  If the request is bound to a local domain ( request-URI resolves
   to OpenSBC listener address ), OpenSBC will then check if the to-URI
   resolves as local domain either in IP address from or via DNS
   resolution.   If both criteria is true then OpenSBC will
   authenticate the REGISTER request against the accounts configured in
   the "Local Domain Accounts" section.  In this case, OpenSBC takes
   the role of the Registrar.

   3.  If OpenSBC sees that the request-URI or the to-URI resolves to
   an external address, then it checks if an upstream registrar route
   is available in "Upper Registration Routes".

   Upper Registration Route Example Entry:    
[sip:*@interop.opensipstack.org] sip:interop.opensipstack.org

   To explain further what the above entry means

   ** [sip:*@interop.opensipstack.org] ** -  This is a wild card match
   filter for the to-URI of a REGISTER hitting OpenSBC
   ** sip:interop.opensipstack.org ** - This is the address of the
   upstream registrar.

   CAVEAT:  Take note that if the request-URI or the to-URI resolves to
   OpenSBC either in IP address or as a result of a DNS resolution,
   upper registration routes will not be checked even if you have
   existing entries for to To-URI.  This is a common mistake that
   continously appears in the opensipstack-devel mailing list.

   4.  If a route to an upstream registrar exists, OpenSBC will change
   the Contact-URI of the REGISTER request to point to the address of
   OpenSBC before sending the request to the upstream registrar.  This
   is called contact hijacking in OpenSBC lingo.  The purpose of this
   is to always let calls towards the registering UA to pass through
   OpenSBC to assure proper NAT traversal.   As far as the upstream
   registrar is concerned, the UA that just registered to it is
   OpenSBC.   OpenSBC stores the original contact before it reqrites
   the contact address.  This would enable OpenSBC to properly route
   requests bound to the UA in the future.




A Real Life Example


   OpenSBC Address:            10.0.0.1:5060
   Upper Registrar Address:    10.0.0.2:5060
   Upper Registar FQDN:        sip:interop.opensipstack.org


   Register sent by the UA towards OpenSBC

   REGISTER sip:10.0.0.1:5060 SIP/2.0
   From: "Foo" <sip:fo...@in...>;tag=tag1234
   To: "Foo" <sip:fo...@in...>
   Contact: "Foo" <sip:foo@10.0.0.3:5060>
   Via: SIP/UDP sip:10.0.0.3:5060;branch=branch1234
   Content-Length: 0


   When this request hits opensbc, OpenSBC will resolve the to-URI to
   10.0.0.2:5060 and thus would treat it as remote domain request.    
Since there is a route entry in "Upper Registration Route" for an
   upstream registrar:

   [sip:*@interop.opensipstack.org] sip:interop.opensipstack.org

   OpenSBC will enable upper registration for this request and hijack
   the contact address.

   REGISTER sip:10.0.0.1:5060 SIP/2.0
   From: "Foo" <sip:fo...@in...>;tag=tag1234
   To: "Foo" <sip:fo...@in...>
   Contact: "Foo" <sip:foo@10.0.0.1:5060>
   Via: SIP/UDP sip:10.0.0.1:5060;branch=branch5678
   Via: SIP/UDP sip:10.0.0.3:5060;branch=branch1234
   Content-Length: 0