Wissensdatenbank: VoIP > Allgemeines
SIP Request Messages
Gepostet von Daniel Jörg, Letzte Änderung durch Daniel Jörg an 16 March 2015 03:28 PM
  • ACK – Used to reply to a SIP Status message in the range 200-699 while in a SIP INVITE dialog.
  • BYE – Used to end an already-established session, such as a SIP INVITE-based call.
  • CANCEL – Used to cancel a request (such as an INVITE or call request that is still in progress but unanswered)
  • INFO – Used to provide additional signaling, not necessarily related to a call in progress. SIP INFO Messages are typically used to provide notification of DTMF strokes (even though RFC 2833 type notifications are preferred), and is also one method used to establish MWI (message waiting indicator) functionality to SIP phones.
  • INVITE – Used to initiate a session dialog – typically to set up a phone call
  • NOTIFY – Used by the PBX Server to communicate statistics, typically to a SIP phone. Transferring of one leg of a call from one endpoint to another may be signaled using this SIP Request, within the context of a SIP REFER dialog. MWI information can also be delivered in this way. Normally a SIP Phone would have previously established a SIP dialog using a SIP SUBSCRIBE request, and the SIP NOTIFY messages would be delivered within the context of this dialog. Note however that some SIP Phones do not establish a dialog in this way, and expect to receive SIP NOTIFY messages “out-of-dialog”. Note also that some SIP Phones may attempt to establish a SIP dialog with SIP SUBSCRIBE using incorrect methodology (GrandStream GXP-2000 being a typical example), and working (if not entirely correct) functionality can be achieved with such devices if they can be configured to NOT attempt a SIP SUBSCRIBE.
  • OPTIONS – Used by a SIP client to query another SIP client or SIP proxy (such as the 3CX PBX Server) about its capabilities to discover information about the supported methods, content types, extensions, codecs, and so on, prior to, for example, establishing a call using the SIP INVITE method.
  • PRACK – Not used in 3CX Phone System
  • REFER – Used by a SIP client to subscribe for notification of changes in call flow, for instance. Typically seen when a phone requests to transfer a call to another party.
  • REGISTER – Used to register or unregister a SIP user-agent with a SIP registrar. An unREGISTERed phone CANNOT receive a SIP INVITE Request to receive a call.
  • SUBSCRIBE – Used by a SIP Phone to establish a SIP dialog for receiving statistics. A typical example is MWI information for the phone’s configured Extension Number.
  • UPDATE – Not used in 3CX Phone System


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