Core SIP Protocol

RFC3261
SIP: Session Initiation Protocol

RFC 3261 establishes the standard for SIP, a signaling protocol used for initiating, managing, and terminating real-time sessions that involve video, voice, messaging, and other communications applications and services between two or more endpoints on IP networks. It defines the structure and sequence of SIP messages, methods, responses, and the overall behavior of SIP agents. This RFC is fundamental to all SIP-based communications and provides the framework necessary for establishing and controlling multimedia sessions, ensuring interoperability across diverse systems.

 

SIP, originally defined in RFC 3261, is constantly evolving to adapt to the changing needs of Internet communications. As a result, numerous additions and updates were made to expand its functionality and solve new problems.  RFC3261 is updated by:

RFC 3265 Session Initiation Protocol (SIP)-Specific Event Notification

RFC 3265 specifies an extension to the Session Initiation Protocol (SIP) for subscribing to and receiving notifications of specific events. This mechanism allows SIP clients to request and receive real-time information about changes in system state or services, such as call progress or message summaries. It introduces the concept of event packages that define the specific types of events that can be subscribed to and the format of the notifications. This extension enhances the SIP's capability by enabling it to handle various events and notifications, making it more versatile and efficient in managing communications in dynamic environments.
 

RFC 5367: Subscriptions to Request-Contained Resource Lists in the Session Initiation Protocol (SIP)

RFC 5367 enhances RFC 3265 by introducing the ability for SIP clients to subscribe to a list of resources contained within a single request. This update simplifies the management of multiple subscriptions, reducing the overhead and complexity of handling individual subscriptions to each resource.

 

RFC 5727: Change Process for the Session Initiation Protocol (SIP) and the Real-time Applications and Infrastructure Area

RFC 5727, while primarily focusing on the administrative and procedural aspects of SIP and its related protocols, indirectly impacts RFC 3265 by establishing a structured change process. This ensures that updates and modifications are managed systematically, maintaining the integrity and coherence of SIP specifications, including event notification mechanisms.

RFC 7957: Management Incident Lightweight Exchange (MILE) Implementation Report

RFC 7957, although primarily focused on the Management Incident Lightweight Exchange (MILE), also updates RFC 5727 by providing insights and recommendations based on the implementation experiences of related protocols. It offers a practical perspective on the change process introduced in RFC 5727, highlighting the effectiveness, challenges, and areas for improvement in managing SIP and related protocol developments. This update contributes to refining the change processes and guidelines for SIP, ensuring they remain relevant and effective in light of evolving implementation experiences and technological advancements.

 

RFC 6446: Session Initiation Protocol (SIP) Event Notification Extension for Resource Lists

RFC 6446 builds on RFC 3265 by providing an extension that allows for efficient event notifications concerning a list of resources. This extension optimizes the process of monitoring multiple resources, enhancing the scalability and performance of SIP in large-scale environments.

 

RFC 6665: SIP-Specific Event Notification

RFC 6665 obsoletes RFC 3265, comprehensively revising the event notification mechanism. It consolidates previous updates and introduces new functionalities to improve the robustness, flexibility, and scalability of event notifications in SIP. This RFC refines the subscription and notification processes, ensuring better performance and compatibility across diverse SIP implementations.

RFC 7621: Session Initiation Protocol (SIP) “Join” Header Field

RFC 7621 enhances the functionality of the SIP by defining the “Join” header field. This header field allows a user agent to request that the call it is initiating be joined with an existing call. The "Join" header is used in scenarios such as call transfer and call merging, enabling more complex call control operations within the SIP framework. By allowing different sessions to be joined, RFC 7621 supports more flexible and dynamic communication scenarios, increasing the versatility of SIP in supporting diverse communication needs.

 

RFC 3853: S/MIME Advanced Encryption Standard (AES) Requirement for the Session Initiation Protocol (SIP)

RFC 3853 enhances the security features of SIP, as defined in RFC 3261, by specifying the use of the Advanced Encryption Standard (AES) with Secure/Multipurpose Internet Mail Extensions (S/MIME) for encrypting SIP messages. This RFC addresses the need for a robust encryption standard within SIP to ensure the confidentiality and integrity of SIP communications. By integrating AES, a widely recognized and strong encryption algorithm, with S/MIME in SIP, RFC 3853 significantly improves the security framework of SIP.

 

RFC 4320: Actions Addressing Identified Issues with the Session Initiation Protocol's (SIP) Non-INVITE Transaction

RFC 4320 focuses on addressing specific issues identified in the SIP handling of non-INVITE transactions, as defined in RFC 3261. Non-INVITE transactions in SIP are used for various operations other than establishing a session, such as querying the status of a server or registering with a SIP service. RFC 4320 provides clarifications and proposes optimizations for the processing of non-INVITE transactions in SIP. This helps to improve the reliability and efficiency of these operations. It introduces improved mechanisms for error handling in non-INVITE transactions. 

 

RFC 4916: Connected Identity in the Session Initiation Protocol (SIP)

RFC 4916 enhances SIP by introducing the concept of connected identity. This concept allows parties in a SIP session to confirm the identity of the other party once the session has been established, providing a mechanism for verifying the identity of users during ongoing communications.
This feature reduces the risk of impersonation and fraud.

 

RFC 5393: Addressing an Amplification Vulnerability in Session Initiation Protocol (SIP) Forking Proxies

RFC 5393 addresses a specific vulnerability in the Session Initiation Protocol (SIP), particularly in the context of forking proxies. Forking proxies in SIP are used to send a single incoming call to multiple endpoints simultaneously. The vulnerability addressed in RFC 5393 involves the potential for these proxies to be used in amplification attacks, where an attacker sends small requests to a proxy, causing it to send large amounts of traffic to unintended recipients.
RFC 5393 identifies and provides solutions to mitigate the amplification vulnerability in SIP forking proxies. This reduces the potential for SIP-based systems to be exploited in denial-of-service (DoS) attacks.
By addressing this vulnerability, RFC 5393 enhances the overall security of the SIP ecosystem, protecting both service providers and users from potential attacks.

 

RFC 5621: Managing Client-Initiated Connections in the Session Initiation Protocol (SIP)

RFC 5621 addresses the management of client-initiated connections in SIP, enhancing the protocol's functionality by defining how SIP user agents should manage and utilize outbound connections. This management is crucial for improving the efficiency and reliability of SIP communications, especially in scenarios where clients initiate connections to servers.
It provides a framework for managing persistent connections established by clients, improving the efficiency of SIP communication by reducing the need for frequent setup and teardown of connections.
y facilitating the reuse of client-initiated connections, RFC 5621 optimizes network resource utilization and improves the performance of SIP-based systems.

 

RFC 8262: Connection Reuse in the Session Initiation Protocol (SIP)

RFC 8262 updates and extends RFC 5621 by further detailing the mechanisms for connection reuse in SIP. It clarifies how SIP entities can efficiently manage and reuse existing connections for multiple SIP transactions, thereby enhancing the protocol’s network efficiency and reducing latency in SIP message exchanges.
The update provided by RFC 8262 builds on the groundwork laid by RFC 5621, refining the process of connection management and reuse in SIP to ensure more robust and efficient communications, particularly in client-initiated scenarios.

 

RFC 5626: Managing Client-Initiated Connections in a Session Initiation Protocol (SIP) Network

RFC 5626 focuses on the management of client-initiated connections within a SIP network, addressing challenges associated with SIP signaling over unreliable transport mechanisms and in network environments that include Network Address Translators (NATs) and firewalls. This RFC aims to improve the reliability of SIP communications by enhancing the way SIP user agents handle client-initiated connections. 
It introduces mechanisms for SIP user agents to reuse existing connections for sending subsequent requests and responses. This reuse helps to maintain the reliability of the signaling path and reduce the number of new connections needed, improving the efficiency of network resource utilization.
RFC 5626 enhances the robustness and efficiency of SIP communications by providing a comprehensive framework for managing client-initiated connections, particularly in environments where NAT and firewall traversal issues are prevalent.

 

RFC 5630: The Use of the SIPS URI Scheme in the Session Initiation Protocol (SIP)

RFC 5630 addresses the use of the SIPS URI (Uniform Resource Identifier) scheme within SIP, which signifies secure SIP communications. The SIPS URI scheme is designed to ensure that all SIP messages transmitted between the client and server are encrypted from eavesdropping and tampering and transmitted securely over Transport Layer Security (TLS).
RFC 5630 provides clear guidelines on how and when to use the SIPS URI, detailing the expected behavior of SIP entities when handling SIPS URIs to maintain secure communication channels.

 

RFC 5922: Domain Certificates in the Session Initiation Protocol (SIP)

RFC 5922 defines the use of domain certificates in SIP, enhancing the security of SIP communications by establishing trusted certificate practices for domain validation. This RFC addresses the need for a standardized method of authenticating the identity of SIP domains, thereby preventing fraudulent activities and enhancing trust in SIP interactions.
RFC 5922 outlines the procedures for managing and validating certificates, including the use of Certificate Authorities (CAs) to issue and verify domain certificates for SIP communications.

 

RFC 5954: Essential Correction for IPv6 ABNF and URI Comparison in RFC 3261

RFC 5954 addresses specific corrections related to the use of IPv6 addressing and Uniform Resource Identifier (URI) comparison in SIP, as initially outlined in RFC 3261. The document focuses on updating the Augmented Backus-Naur Form (ABNF) syntax used in SIP for IPv6 addresses and clarifies the procedures for comparing URIs in SIP messages, ensuring accurate processing and routing in IPv6 environments.
RFC 5954 clarifies how SIP URIs should be compared, particularly when they contain IPv6 addresses. This clarification is crucial for the correct routing and handling of SIP messages in networks that use IPv6.
By providing these corrections and clarifications, RFC 5954 enhances the interoperability of SIP implementations in IPv6 networks, ensuring consistent behavior across different platforms and environments.

 

RFC 6026: Correct Transaction Handling for 2xx Responses to Session Initiation Protocol (SIP) INVITE Requests

RFC 6026 addresses an issue in the transaction handling for 2xx responses to SIP INVITE requests. The document focuses on improving the reliability of SIP by ensuring that 2xx responses to INVITE requests are correctly processed and acknowledged, which is crucial for establishing and maintaining SIP sessions.
By addressing the handling of 2xx responses, RFC 6026 significantly improves the reliability of call setup and maintenance in SIP, reducing the likelihood of call drops and other related issues.

 

RFC 6141: Re-INVITE and Target-Refresh Request Handling in the Session Initiation Protocol (SIP)

RFC 6141 addresses the procedures and mechanisms related to handling Re-INVITE and target-refresh requests in SIP. These types of requests are crucial for modifying existing sessions, such as changing the media stream or updating session parameters.
By defining clear procedures for these requests, RFC 6141 enhances the overall session management capabilities of SIP, allowing for more dynamic and flexible communication sessions.

 

RFC 6878: IANA Registry for the Session Initiation Protocol (SIP) "Priority" Header Field

RFC 6878 establishes an IANA (Internet Assigned Numbers Authority) registry for the "Priority" header field in SIP. This header field is used in SIP messages to indicate the priority level of the request, helping to manage the processing and handling of calls based on their urgency.
It creates a formal registry for the values that can be used in the "Priority" header field in SIP messages. This standardization ensures a consistent interpretation and handling of priority levels across different SIP implementations.
The establishment of a registry promotes interoperability between different SIP implementations, as it ensures that all parties use and understand the "Priority" header field values in a consistent manner.

 

RFC 7462: URNs for the Alert-Info Header Field of the Session Initiation Protocol (SIP)

RFC 7462 specifies the use of Uniform Resource Names (URNs) for the Alert-Info header field in SIP. This header field in SIP messages is used to provide additional information about the alerting or ringtone pattern that should be used for an incoming call. 
By using URNs in the Alert-Info header, SIP can more precisely control the user experience during call alerts, enabling the delivery of customized or context-specific ring tones or alert information.

 

RFC 7463: Shared Appearances of a Session Initiation Protocol (SIP) Address of Record (AOR)

RFC 7463 addresses the concept of shared appearances for a SIP Address of Record (AOR). This functionality allows multiple endpoints to share the same SIP AOR, meaning that they can all make and receive calls as if they were a single identity. This is particularly useful in environments like offices or call centers, where multiple devices need to operate under the same telephone number or address.
RFC 7463 outlines mechanisms for keeping the shared devices in sync with the call state, ensuring that all devices reflecting a shared AOR have consistent information about ongoing calls.
By defining how shared appearances should be handled in SIP, RFC 7463 enhances the protocol’s capabilities in supporting complex telephony scenarios, allowing for more versatile and collaborative communication environments.

 

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

RFC 8217 provides clarifications on the use of the name-addr production in Session Initiation Protocol (SIP) messages. The name-addr format in SIP is used to specify a display name along with a SIP URI, enclosed in angle brackets. This format is crucial in SIP messaging for differentiating between the display name and the SIP address.
RFC 8217 addresses common misunderstandings and ambiguities in the use of 'name-addr' and 'addr-spec' (which is just the URI without a display name or brackets) in SIP messages, ensuring that implementations handle these formats correctly.
RFC 8217 plays a critical role in standardizing how SIP addresses are formatted and interpreted in messages, enhancing the overall functionality and reliability of SIP communications by ensuring clear and unambiguous address specifications.

 

RFC 8591: SIP-Based Messaging with S/MIME

RFC 8591 specifies the use of Secure/Multipurpose Internet Mail Extensions (S/MIME) in SIP based messaging, providing a method for securing SIP messages through encryption and digital signatures. S/MIME is a standard for public key encryption and signing of MIME data, which is used here to enhance the security of SIP-based communications.
It details how S/MIME can be used to encrypt SIP messages and ensure their integrity through digital signatures, offering protection against eavesdropping, tampering, and impersonation.
By leveraging S/MIME, RFC 8591 enhances the privacy and authentication capabilities of SIP messaging, allowing users to verify the sender’s identity and ensure that messages remain confidential.

 

RFC 8760: The Session Initiation Protocol (SIP) Digest Access Authentication Scheme

RFC 8760 updates the Digest Access Authentication scheme in SIP, enhancing the security of authentication processes. This RFC builds upon previous specifications for digest authentication in SIP, addressing known vulnerabilities and limitations to provide a more secure and robust authentication mechanism.
RFC 8760 advocates for the use of more secure hash functions than MD5, which was commonly used in earlier SIP implementations. This change is crucial for improving the resilience of the authentication process against hash collision and other cryptographic attacks.

 

RFC 8898: Third-Party Token-Based Authentication and Authorization for Session Initiation Protocol (SIP)

RFC 8898 introduces a method for third-party token-based authentication and authorization in SIP, offering a mechanism that allows SIP entities to be authenticated and authorized by means other than direct user credentials. This approach leverages tokens issued by a trusted third party, enhancing the flexibility and security of authentication and authorization processes in SIP communications.
It specifies how SIP can use tokens, such as those conforming to the OAuth 2.0 framework, for authenticating users. This allows SIP entities to verify the user's identity based on tokens provided by an authentication server, rather than using traditional credentials like passwords.
RFC 8898 advances the SIP protocol by integrating modern, secure, and flexible authentication and authorization mechanisms, thereby improving the security and operational efficiency of SIP-based communication systems.

 

RFC 8996: Deprecating TLS 1.0 and TLS 1.1

RFC 8996 marks the formal deprecation of Transport Layer Security (TLS) versions 1.0 and 1.1. This deprecation affects all protocols that rely on TLS, including SIP, to ensure enhanced security in network communications. The document advocates for the adoption of more modern and secure versions of TLS, specifically TLS 1.2 and higher, to address the vulnerabilities and limitations identified in the earlier TLS versions.
The deprecation of TLS 1.0 and 1.1 is driven by the need to mitigate security risks associated with these older protocols, such as weak cryptographic algorithms and vulnerability to attacks like BEAST and POODLE.
By deprecating older, less secure versions of TLS, RFC 8996 contributes to a more secure ecosystem for SIP.