Registrations in SIP

SIP offers a discovery capability.  If a user wants to initiate a
session with another user, SIP must discover the current host(s) at
which the destination user is reachable.  This discovery process is
frequently accomplished by SIP network elements such as proxy servers
and redirect servers which are responsible for receiving a request,
determining where to send it based on knowledge of the location of
the user, and then sending it there.  To do this, SIP network
elements consult an abstract service known as a location service,
which provides address bindings for a particular domain.  These
address bindings map an incoming SIP or SIPS URI, sip:
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 ,
for example, to one or more URIs that are somehow "closer" to the
desired user, sip:
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 , for example.
Ultimately, a proxy will consult a location service that maps a
received URI to the user agent(s) at which the desired recipient is
currently residing.

10.1 Overview

   Registration creates bindings in a location service for a particular
   domain that associates an address-of-record URI with one or more
   contact addresses.  Thus, when a proxy for that domain receives a
   request whose Request-URI matches the address-of-record, the proxy
   will forward the request to the contact addresses registered to that
   address-of-record.  Generally, it only makes sense to register an
   address-of-record at a domain's location service when requests for
   that address-of-record would be routed to that domain.  In most
   cases, this means that the domain of the registration will need to
   match the domain in the URI of the address-of-record.

   There are many ways by which the contents of the location service can
   be established.  One way is administratively.  In the above example,
   Bob is known to be a member of the engineering department through
   access to a corporate database.  However, SIP provides a mechanism
   for a UA to create a binding explicitly.  This mechanism is known as
   registration.

   Registration entails sending a REGISTER request to a special type of
   UAS known as a registrar.  A registrar acts as the front end to the
   location service for a domain, reading and writing mappings based on
   the contents of REGISTER requests.  This location service is then
   typically consulted by a proxy server that is responsible for routing
   requests for that domain.

   An illustration of the overall registration process is given in
   Figure 2.  Note that the registrar and proxy server are logical roles
   that can be played by a single device in a network; for purposes of
   clarity the two are separated in this illustration.  Also note that
   UAs may send requests through a proxy server in order to reach a
   registrar if the two are separate elements.

   SIP does not mandate a particular mechanism for implementing the
   location service.  The only requirement is that a registrar for some
   domain MUST be able to read and write data to the location service,
   and a proxy or a redirect server for that domain MUST be capable of
   reading that same data.  A registrar MAY be co-located with a
   particular SIP proxy server for the same domain.

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