How do you design and implement SIP and SDP message format extensions and customizations?

1 SIP Message Format

A SIP message consists of a request line or a status line, followed by a set of headers and an optional message body. The request line or status line indicates the method, the URI, and the version of the protocol. The headers provide additional information about the sender, the receiver, the session, and the message. The message body can contain SDP or other data types. You can extend and customize the SIP message format by defining new methods, headers, parameters, or body types. To do so, you need to follow the syntax and semantics rules defined in RFC 3261 and other relevant specifications. You also need to register your extensions with the Internet Assigned Numbers Authority (IANA) to avoid conflicts and ensure interoperability.

2 SDP Message Format

An SDP message consists of a series of lines, each starting with a single character that identifies the type of information, followed by an equal sign and a value. The lines are grouped into session-level and media-level sections, separated by blank lines. The session-level section describes the global attributes and parameters of the session, such as the name, the originator, the timing, and the connection. The media-level section describes the specific attributes and parameters of each media stream, such as the type, the port, the format, and the codec. You can extend and customize the SDP message format by defining new types, attributes, or values. To do so, you need to follow the syntax and semantics rules defined in RFC 4566 and other relevant specifications. You also need to register your extensions with the IANA to avoid conflicts and ensure interoperability.

3 SIP-SDP Interaction

SIP and SDP interact with each other in several ways. SIP uses SDP as the default message body type to negotiate and establish the media sessions. SIP also uses SDP to modify or terminate the existing sessions. SIP can also use other message body types, such as XML or JSON, to convey additional information or functionality. SDP can also use SIP headers or parameters to refer to external resources or capabilities. For example, SDP can use the SIP URI scheme to identify the media sources or destinations. SIP and SDP can also use extensions or customizations to enhance their interaction. For example, SIP can use new methods or headers to support features such as session mobility, security, or quality of service. SDP can use new attributes or values to support features such as media encryption, multicast, or bandwidth management.

4 SIP Extension Frameworks

SIP has several extension frameworks that provide guidelines and mechanisms for designing and implementing SIP extensions and customizations, such as SIP Events, SIP Refer, SIP Service URIs, and SIP OPTIONS. The SIP Events framework defines how to use SIP to subscribe and notify about events or changes in the state of resources or sessions, as well as how to use SIP to publish or fetch event information. The SIP Refer framework defines how to use SIP to transfer or redirect a session to another destination or service. The SIP Service URIs framework defines how to use SIP URIs to identify and invoke services or applications that are not directly addressed by SIP. Lastly, the SIP OPTIONS framework defines how to use SIP to query and discover the capabilities and preferences of SIP entities or sessions.

5 SDP Extension Frameworks

SDP has several extension frameworks that provide guidelines and mechanisms for designing and implementing SDP extensions and customizations. These include SDP Offer/Answer, which defines how to use SDP to negotiate and establish media sessions in an interactive and incremental way, as well as modify or terminate them. Additionally, SDP Capability Negotiation defines how to use SDP to negotiate and select among multiple alternative media configurations. Furthermore, SDP Media Attributes outlines how to use SDP to specify and control various aspects of the media streams. Finally, the SDP Grouping Framework defines how to use SDP to group and associate multiple media streams or sessions for different purposes.

6 SIP-SDP Best Practices

When designing and implementing SIP and SDP message format extensions and customizations, you should adhere to some best practices to guarantee compatibility, interoperability, and scalability. For instance, it’s recommended to use existing extensions or customizations instead of creating new ones. It’s also important to follow the syntax and semantics rules defined in the RFCs and specifications, and register your extensions or customizations with the IANA. Additionally, you should document your extensions or customizations clearly and comprehensively, as well as provide examples and test cases. And don't forget to test your extensions or customizations with different implementations and scenarios, while handling errors and exceptions appropriately.

7 Here’s what else to consider

This is a space to share examples, stories, or insights that don’t fit into any of the previous sections. What else would you like to add?