SIP Methods - NormB/sipnab GitHub Wiki
Every method in the IANA registry, the RFC section that defines it, and which dialog state machine sipnab runs it through.
Source. https://www.iana.org/assignments/sip-parameters/sip-parameters-6.csv — the IANA Methods registry, retrieved 2026-07-30. RFC 3261 defines six of these and later RFCs register the rest, the same arrangement as the response codes in SIP response codes.
SipMethod in src/sip/method.rs names all fourteen,
and covers_the_iana_methods_registry holds it to this list. A method outside
the registry parses as Custom and takes the generic dialog handler, which is
right for a private extension and wrong for a registered one nobody noticed.
Dialog state machines
sipnab dispatches on the method that opened the dialog, corrected for the four
methods that cannot open one. ACK, BYE, CANCEL and PRACK each
presuppose an INVITE (RFC 3261 section 13.2.2.4,
RFC 3261 section 15.1,
RFC 3261 section 9.1,
RFC 3262 section 4), so a capture
that opens on one is an INVITE dialog seen from its middle and runs through the
INVITE machine. Two more methods get their own machine and the rest share a
generic one.
Which machine handles a message is one question. Which transaction the
message belongs to is another. A response answers the transaction its CSeq
method names (RFC 3261 section 8.1.1.5).
So a 200 OK in an INVITE dialog means the callee picked up only when that
CSeq says INVITE. The same code answering a CANCEL means the cancellation
arrived, and nothing more.
Every dialog opens in Trying. From there each machine reaches only its own
states, and a state no machine reaches is a state no filter can select:
| Machine | Methods | States it reaches |
|---|---|---|
| INVITE | INVITE, ACK, BYE, CANCEL, PRACK |
Ringing, InCall, Transferring, Completed, Canceled, Redirected, Failed |
| REGISTER | REGISTER |
Registered, Expired, Redirected, Failed |
| SUBSCRIBE | SUBSCRIBE |
Pending, Active, Terminated, Redirected |
| generic | the other seven | Completed, Redirected, Failed |
Expired is a de-registration: a 2xx to a REGISTER whose Contact granted
zero seconds. Redirected is a 3xx, which every machine can draw.
| Method | Machine | Defined in | Description |
|---|---|---|---|
ACK |
INVITE | RFC 3261 section 17.1.1.3 | This section specifies the construction of ACK requests sent within the client transaction. A UAC core that generates an ACK for 2xx MUST instead follow the rules described in Section 13. |
BYE |
INVITE | RFC 3261 section 15.1 | A BYE request is constructed as would any other request within a dialog, as described in Section 12. Once the BYE is constructed, the UAC core creates a new non-INVITE client transaction, and passes it the BYE request. |
CANCEL |
INVITE | RFC 3261 section 9 | The previous section has discussed general UA behavior for generating requests and processing responses for requests of all methods. In this section, we discuss a general purpose method, called CANCEL. |
INFO |
generic | RFC 6086 section 4 | The INFO method provides a mechanism for transporting application level information that can further enhance a SIP application. Section 8 gives more details on the types of applications for which the use of INFO is appropriate. |
INVITE |
INVITE | RFC 3261 section 13 | When a user agent client desires to initiate a session (for example, audio, video, or a game), it formulates an INVITE request. The INVITE request asks a server to establish a session. |
MESSAGE |
generic | RFC 3428 section 3 | When one user wishes to send an instant message to another, the sender formulates and issues a SIP request using the new MESSAGE method defined by this document. |
NOTIFY |
generic | RFC 6665 section 3.2 | NOTIFY requests are sent to inform subscribers of changes in state to which the subscriber has a subscription. Subscriptions are created using the SUBSCRIBE method. |
OPTIONS |
generic | RFC 3261 section 11 | The SIP method OPTIONS allows a UA to query another UA or a proxy server as to its capabilities. This allows a client to discover information about the supported methods, content types, extensions, codecs, etc. |
PRACK |
INVITE | RFC 3262 section 6 | This specification defines a new SIP method, PRACK. The semantics of this method are described above. |
PUBLISH |
generic | RFC 3903 section 3 | This document defines a new SIP method, PUBLISH, for publishing event state. PUBLISH is similar to REGISTER in that it allows a user to create, modify, and remove state in another entity which manages this state on behalf of the user. |
REFER |
generic | RFC 3515 section 2 | REFER is a SIP method as defined by RFC 3261 [1]. The REFER method indicates that the recipient (identified by the Request-URI) should contact a third party using the contact information provided in the request. |
REGISTER |
REGISTER | RFC 3261 section 10 | 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. |
SUBSCRIBE |
SUBSCRIBE | RFC 6665 section 3.1 | The SUBSCRIBE method is used to request current state and state updates from a remote node. SUBSCRIBE requests are target refresh requests, as that term is defined in [RFC3261]. |
UPDATE |
generic | RFC 3311 section 3 | Operation of this extension is straightforward. The caller begins with an INVITE transaction, which proceeds normally. |
Registry references beyond the defining RFC, where a later document updates the
method: INVITE [RFC3261][RFC 6026].