Section 6 Definitions

The following terms have special significance for SIP.
      Address-of-Record: An address-of-record (AOR) is a SIP or SIPS URI
         that points to a domain with a location service that can map
         the URI to another URI where the user might be available.
         Typically, the location service is populated through
         registrations.  An AOR is frequently thought of as the "public
         address" of the user.
      Back-to-Back User Agent: A back-to-back user agent (B2BUA) is a
         logical entity that receives a request and processes it as a
         user agent server (UAS).  In order to determine how the request
         should be answered, it acts as a user agent client (UAC) and
         generates requests.  Unlike a proxy server, it maintains dialog
         state and must participate in all requests sent on the dialogs
         it has established.  Since it is a concatenation of a UAC and
         UAS, no explicit definitions are needed for its behavior.
      Call: A call is an informal term that refers to some communication
         between peers, generally set up for the purposes of a
         multimedia conversation.
      Call Leg: Another name for a dialog [31]; no longer used in this
         specification.
      Call Stateful: A proxy is call stateful if it retains state for a
         dialog from the initiating INVITE to the terminating BYE
         request.  A call stateful proxy is always transaction stateful,
         but the converse is not necessarily true.
      Client: A client is any network element that sends SIP requests
         and receives SIP responses.  Clients may or may not interact
         directly with a human user.  User agent clients and proxies are
         clients.
      Conference: A multimedia session (see below) that contains
         multiple participants.
      Core: Core designates the functions specific to a particular type
         of SIP entity, i.e., specific to either a stateful or stateless
         proxy, a user agent or registrar.  All cores, except those for
         the stateless proxy, are transaction users.
      Dialog: A dialog is a peer-to-peer SIP relationship between two
         UAs that persists for some time.  A dialog is established by
         SIP messages, such as a 2xx response to an INVITE request.  A
         dialog is identified by a call identifier, local tag, and a
         remote tag.  A dialog was formerly known as a call leg in RFC
         2543.
      Downstream: A direction of message forwarding within a transaction
         that refers to the direction that requests flow from the user
         agent client to user agent server.
      Final Response: A response that terminates a SIP transaction, as
         opposed to a provisional response that does not.  All 2xx, 3xx,
         4xx, 5xx and 6xx responses are final.
      Header: A header is a component of a SIP message that conveys
         information about the message.  It is structured as a sequence
         of header fields.
      Header Field: A header field is a component of the SIP message
         header.  A header field can appear as one or more header field
         rows. Header field rows consist of a header field name and zero
         or more header field values. Multiple header field values on a
         given header field row are separated by commas. Some header
         fields can only have a single header field value, and as a
         result, always appear as a single header field row.
      Header Field Value: A header field value is a single value; a
         header field consists of zero or more header field values.
      Home Domain: The domain providing service to a SIP user.
         Typically, this is the domain present in the URI in the
         address-of-record of a registration.
      Informational Response: Same as a provisional response.
      Initiator, Calling Party, Caller: The party initiating a session
         (and dialog) with an INVITE request.  A caller retains this
         role from the time it sends the initial INVITE that established
         a dialog until the termination of that dialog.
      Invitation: An INVITE request.
      Invitee, Invited User, Called Party, Callee: The party that
         receives an INVITE request for the purpose of establishing a
         new session.  A callee retains this role from the time it
         receives the INVITE until the termination of the dialog
         established by that INVITE.
      Location Service: A location service is used by a SIP redirect or
         proxy server to obtain information about a callee's possible
         location(s).  It contains a list of bindings of address-of-
         record keys to zero or more contact addresses.  The bindings
         can be created and removed in many ways; this specification
         defines a REGISTER method that updates the bindings.
      Loop: A request that arrives at a proxy, is forwarded, and later
         arrives back at the same proxy.  When it arrives the second
         time, its Request-URI is identical to the first time, and other
         header fields that affect proxy operation are unchanged, so
         that the proxy would make the same processing decision on the
         request it made the first time.  Looped requests are errors,
         and the procedures for detecting them and handling them are
         described by the protocol.
      Loose Routing: A proxy is said to be loose routing if it follows
         the procedures defined in this specification for processing of
         the Route header field.  These procedures separate the
         destination of the request (present in the Request-URI) from
         the set of proxies that need to be visited along the way
         (present in the Route header field).  A proxy compliant to
         these mechanisms is also known as a loose router.
      Message: Data sent between SIP elements as part of the protocol.
         SIP messages are either requests or responses.
      Method: The method is the primary function that a request is meant
         to invoke on a server.  The method is carried in the request
         message itself.  Example methods are INVITE and BYE.
      Outbound Proxy: A proxy that receives requests from a client, even
         though it may not be the server resolved by the Request-URI.
         Typically, a UA is manually configured with an outbound proxy,
         or can learn about one through auto-configuration protocols.
      Parallel Search: In a parallel search, a proxy issues several
         requests to possible user locations upon receiving an incoming
         request.  Rather than issuing one request and then waiting for
         the final response before issuing the next request as in a
         sequential search, a parallel search issues requests without
         waiting for the result of previous requests.
      Provisional Response: A response used by the server to indicate
         progress, but that does not terminate a SIP transaction.  1xx
         responses are provisional, other responses are considered
         final.
      Proxy, Proxy Server: An intermediary entity that acts as both a
         server and a client for the purpose of making requests on
         behalf of other clients.  A proxy server primarily plays the
         role of routing, which means its job is to ensure that a
         request is sent to another entity "closer" to the targeted
         user.  Proxies are also useful for enforcing policy (for
         example, making sure a user is allowed to make a call).  A
         proxy interprets, and, if necessary, rewrites specific parts of
         a request message before forwarding it.
      Recursion: A client recurses on a 3xx response when it generates a
         new request to one or more of the URIs in the Contact header
         field in the response.
      Redirect Server: A redirect server is a user agent server that
         generates 3xx responses to requests it receives, directing the
         client to contact an alternate set of URIs.
      Registrar: A registrar is a server that accepts REGISTER requests
         and places the information it receives in those requests into
         the location service for the domain it handles.
      Regular Transaction: A regular transaction is any transaction with
         a method other than INVITE, ACK, or CANCEL.
      Request: A SIP message sent from a client to a server, for the
         purpose of invoking a particular operation.
      Response: A SIP message sent from a server to a client, for
         indicating the status of a request sent from the client to the
         server.
      Ringback: Ringback is the signaling tone produced by the calling
         party's application indicating that a called party is being
         alerted (ringing).
      Route Set: A route set is a collection of ordered SIP or SIPS URI
         which represent a list of proxies that must be traversed when
         sending a particular request.  A route set can be learned,
         through headers like Record-Route, or it can be configured.
      Server: A server is a network element that receives requests in
         order to service them and sends back responses to those
         requests.  Examples of servers are proxies, user agent servers,
         redirect servers, and registrars.
      Sequential Search: In a sequential search, a proxy server attempts
         each contact address in sequence, proceeding to the next one
         only after the previous has generated a final response.  A 2xx
         or 6xx class final response always terminates a sequential
         search.
      Session: From the SDP specification: "A multimedia session is a
         set of multimedia senders and receivers and the data streams
         flowing from senders to receivers.  A multimedia conference is
         an example of a multimedia session." (RFC 2327 [1]) (A session
         as defined for SDP can comprise one or more RTP sessions.)  As
         defined, a callee can be invited several times, by different
         calls, to the same session.  If SDP is used, a session is
         defined by the concatenation of the SDP user name, session id,
         network type, address type, and address elements in the origin
         field.
      SIP Transaction: A SIP transaction occurs between a client and a
         server and comprises all messages from the first request sent
         from the client to the server up to a final (non-1xx) response
         sent from the server to the client.  If the request is INVITE
         and the final response is a non-2xx, the transaction also
         includes an ACK to the response.  The ACK for a 2xx response to
         an INVITE request is a separate transaction.
      Spiral: A spiral is a SIP request that is routed to a proxy,
         forwarded onwards, and arrives once again at that proxy, but
         this time differs in a way that will result in a different
         processing decision than the original request.  Typically, this
         means that the request's Request-URI differs from its previous
         arrival.  A spiral is not an error condition, unlike a loop.  A
         typical cause for this is call forwarding.  A user calls
         joe@example.com.  The example.com proxy forwards it to Joe's
         PC, which in turn, forwards it to bob@example.com.  This
         request is proxied back to the example.com proxy.  However,
         this is not a loop.  Since the request is targeted at a
         different user, it is considered a spiral, and is a valid
         condition.
      Stateful Proxy: A logical entity that maintains the client and
         server transaction state machines defined by this specification
         during the processing of a request, also known as a transaction
         stateful proxy.  The behavior of a stateful proxy is further
         defined in Section 16.  A (transaction) stateful proxy is not
         the same as a call stateful proxy.
      Stateless Proxy: A logical entity that does not maintain the
         client or server transaction state machines defined in this
         specification when it processes requests.  A stateless proxy
         forwards every request it receives downstream and every
         response it receives upstream.
      Strict Routing: A proxy is said to be strict routing if it follows
         the Route processing rules of RFC 2543 and many prior work in
         progress versions of this RFC.  That rule caused proxies to
         destroy the contents of the Request-URI when a Route header
         field was present.  Strict routing behavior is not used in this
         specification, in favor of a loose routing behavior.  Proxies
         that perform strict routing are also known as strict routers.
      Target Refresh Request: A target refresh request sent within a
         dialog is defined as a request that can modify the remote
         target of the dialog.
      Transaction User (TU): The layer of protocol processing that
         resides above the transaction layer.  Transaction users include
         the UAC core, UAS core, and proxy core.
      Upstream: A direction of message forwarding within a transaction
         that refers to the direction that responses flow from the user
         agent server back to the user agent client.
      URL-encoded: A character string encoded according to RFC 2396,
         Section 2.4 [5].
      User Agent Client (UAC): A user agent client is a logical entity
         that creates a new request, and then uses the client
         transaction state machinery to send it.  The role of UAC lasts
         only for the duration of that transaction.  In other words, if
         a piece of software initiates a request, it acts as a UAC for
         the duration of that transaction.  If it receives a request
         later, it assumes the role of a user agent server for the
         processing of that transaction.
      UAC Core: The set of processing functions required of a UAC that
         reside above the transaction and transport layers.
      User Agent Server (UAS): A user agent server is a logical entity
         that generates a response to a SIP request.  The response
         accepts, rejects, or redirects the request.  This role lasts
         only for the duration of that transaction.  In other words, if
         a piece of software responds to a request, it acts as a UAS for
         the duration of that transaction.  If it generates a request
         later, it assumes the role of a user agent client for the
         processing of that transaction.
      UAS Core: The set of processing functions required at a UAS that
         resides above the transaction and transport layers.
      User Agent (UA): A logical entity that can act as both a user
         agent client and user agent server.
   The role of UAC and UAS, as well as proxy and redirect servers, are
   defined on a transaction-by-transaction basis.  For example, the user
   agent initiating a call acts as a UAC when sending the initial INVITE
   request and as a UAS when receiving a BYE request from the callee.
   Similarly, the same software can act as a proxy server for one
   request and as a redirect server for the next request.
   Proxy, location, and registrar servers defined above are logical
   entities; implementations MAY combine them into a single application.