Request Methods

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].