SIP Extensions and Methods

RFC 2976: The SIP INFO Method

RFC 2976 describes the INFO method in SIP, which is used for sending mid-session information that does not modify the session state. This can include signals such as DTMF digits, feature keys, and other information payloads that are needed during a call but are not related to establishing, modifying, or terminating the call session. The INFO method provides a mechanism for transferring this information within an existing SIP dialogue, thus offering a means to convey session-related control information.
RFC 2976 plays a role in enriching the capabilities of SIP by detailing how supplementary control information can be delivered without affecting the session's established signaling flow.

RFC 6086: Session Initiation Protocol (SIP) INFO Method and Package Framework

RFC 6086 obsoletes RFC 2976, which initially defined the SIP INFO method. This method is used for sending mid-session information that does not modify the session state. RFC 6086 provides a comprehensive update to the original definition by not only detailing the INFO method itself but also introducing a package framework to standardize how different types of information are sent using this method. This enhancement addresses the limitations and ambiguities in RFC 2976, ensuring a more robust and flexible implementation of the INFO method in SIP communications.

RFC 3311: The Session Initiation Protocol (SIP) UPDATE Method

RFC 3311 introduces the UPDATE method to SIP, which allows a client to update parameters of a session before the final response is received. This method is particularly useful for adjusting session characteristics (such as media streams) mid-dialogue without needing to reinitiate the session entirely. The UPDATE method can be used for actions like changing the codec or modifying the bandwidth settings for a media stream, providing flexibility in managing ongoing SIP sessions. It is designed to be used after a session has been established but before it has been confirmed, filling a gap in the SIP protocol that previously required more cumbersome methods to achieve similar outcomes.
RFC 3311's specification of the UPDATE method enhances SIP's session management capabilities, making it more efficient and flexible in handling changes to session parameters without the need for session re-establishment.

RFC 3515: The Session Initiation Protocol (SIP) Refer Method

RFC 3515 defines the REFER method for the Session Initiation Protocol (SIP), which allows a participant to request that the recipient refer to a third resource. This mechanism is instrumental in enabling functionalities such as call transfer by notifying the referring party about the outcome of the reference. It introduces the Refer-To request header and the refer event package, essential for implementing the REFER method within SIP-based applications. The REFER method facilitates a variety of applications by allowing a more integrated and flexible handling of SIP requests, particularly in scenarios requiring actions on behalf of another party.
RFC 3515's specification of the REFER method and associated headers significantly enhances the SIP's functionality, enabling sophisticated interactions such as call transfers, by allowing users to refer calls to third-party resources seamlessly.

RFC 7647: Clarifications for the Use of REFER with RFC 6665

RFC 7647 directly updates RFC 3515 by detailing the implementation aspects of the REFER method within the context of the SIP-Specific Event Notification framework outlined in RFC 6665. It revises the original specifications in RFC 3515, specifically addressing how REFER requests create implicit subscriptions to SIP events. The clarifications include the introduction of the "norefersub" parameter to allow the suppression of these implicit subscriptions, the necessity of using Globally Routable User Agent URIs (GRUU) for the notifiers, rules against the reuse of dialog identifiers, the obsolescence of the 202 response code, and enhanced security measures for handling implicit subscriptions.

RFC 8217: Clarifications for When to Use the name-addr Production in SIP Messages

RFC 8217 indirectly updates RFC 3515 by providing clear guidelines on when to use the name-addr production in SIP message header fields. Although RFC 3515 primarily deals with the REFER method, RFC 8217's corrections and clarifications impact the broader SIP messaging format, which includes messages defined in RFC 3515. It establishes precise rules for using the name-addr format in SIP headers, especially when the URI contains certain characters, thereby influencing the formatting and parsing of SIP messages related to the REFER method and beyond.

RFC 3891: The Session Initiation Protocol (SIP) "Replaces" Header

RFC 3891 specifies the "Replaces" header field for use in Session Initiation Protocol (SIP) communications. This header is crucial for call control features, particularly call transfer and call replacement scenarios within SIP-based communication systems. The "Replaces" header allows a new incoming SIP INVITE request to specify that it intends to replace an existing call. This mechanism is essential for implementing attended and unattended call transfers, where an ongoing call is seamlessly transitioned to a new participant. By referencing the unique identifiers of the call to be replaced, the "Replaces" header facilitates the smooth handover of calls between parties, ensuring continuity of communication and efficient management of SIP sessions.

RFC 3892: The Session Initiation Protocol (SIP) Referred-By Mechanism

RFC 3892 introduces the "Referred-By" mechanism for SIP, enhancing the REFER method to allow a referrer to send information about a REFER request to the target through an intermediary. This includes the referrer's identity and the referred URI. Optional S/MIME protection guards against malicious intermediaries, though targets may reject unprotected requests.

RFC 3903: Session Initiation Protocol (SIP) Extension for Event State Publication

RFC 3903 introduces a method that allows the publication of event state in the Session Initiation Protocol (SIP) environment. This extension to SIP enables users to publish their state (such as whether they are busy or available) to a server, which then can be disseminated to other users or services subscribed to this information. This functionality is crucial for services like presence, where the availability of a user needs to be communicated across the network. The RFC specifies how SIP PUBLISH requests are used for this purpose, detailing the request format and the handling of such requests by SIP servers.