WARNING: This document has not yet been accepted for consideration or approved in any official manner by the XMPP Standards Foundation, and this document is not yet an XMPP Extension Protocol (XEP). If this document is accepted as a XEP by the XMPP Council, it will be published at <https://xmpp.org/extensions/> and announced on the <email@example.com> mailing list.
The IETFs RTCWEB working group has decided to use RFC 4566  as the API surface for WebRTC. For XMPP this opened up a number of questions about the future of Jingle, XMPPs native signalling protocol, since Jingle does not use SDP. Various proposals have been made, including mappingSDP to Jingle and vice versa over to a dumb transport of SDP (called SoX) inside <message/> stanzas. This document proposes a way to transport SDP over Jingle which respects the established session model of Jingle.
This SDP consists of a session level section and has two media description. It can easily be transformed to the following session-initiate by splitting it at "\r\nm=":
Note that the a=ice-ufrag, a=ice-pwd and a=fingerprint lines are removed from the SDP (either in session level or all of the mediadescriptions) and put into the transport element. The same rule applies to any a=candidate lines. 
The receiver can reconstruct the SDP by first extracting the session section and then appending the media descriptions. When reconstructing the mediasections, the transport information can simply be appended to the raw SDP.
This section is actually where things get interesting. It should talk about adding or removing content which is a concept that has been known in Jingle for ages and is now coming to SDP with the "partial offer/partial answer" concept from Unified Plan. This is the reason why the individual m-lines are transported in separate <content/> elements.
The initiator starts with the offer SDP as described in the first section, but with audio only. Video is added later. Thus it sends the following session-initiate:
After the session is established, the initiator wants to add video from his webcam. It creates a new (full) SDP offer which will look like the one in example 1, but with candidates already added. It then calculates the difference between the new offer and the initial offer  which may consist of more than one mediaparts.
When BUNDLE is used this will typically also contain a=candidate lines which are omitted for simplicity. This is transformed to a content-add. Note that the session-part must be resent if BUNDLE is used in the session since the BUNDLE group has changed:
The receiver extracts the new session part and the additional media description(s) and constructs the new SDP based on the previous one .
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In no event and under no legal theory, whether in tort (including negligence), contract, or otherwise, unless required by applicable law (such as deliberate and grossly negligent acts) or agreed to in writing, shall the XMPP Standards Foundation or any author of this Specification be liable for damages, including any direct, indirect, special, incidental, or consequential damages of any character arising from, out of, or in connection with the Specification or the implementation, deployment, or other use of the Specification (including but not limited to damages for loss of goodwill, work stoppage, computer failure or malfunction, or any and all other commercial damages or losses), even if the XMPP Standards Foundation or such author has been advised of the possibility of such damages.
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The Extensible Messaging and Presence Protocol (XMPP) is defined in the XMPP Core (RFC 6120) and XMPP IM (RFC 6121) specifications contributed by the XMPP Standards Foundation to the Internet Standards Process, which is managed by the Internet Engineering Task Force in accordance with RFC 2026. Any protocol defined in this document has been developed outside the Internet Standards Process and is to be understood as an extension to XMPP rather than as an evolution, development, or modification of XMPP itself.
The following requirements keywords as used in this document are to be interpreted as described in RFC 2119: "MUST", "SHALL", "REQUIRED"; "MUST NOT", "SHALL NOT"; "SHOULD", "RECOMMENDED"; "SHOULD NOT", "NOT RECOMMENDED"; "MAY", "OPTIONAL".
5. at some point, the browser may be able to do this, too
6. The Internet Assigned Numbers Authority (IANA) is the central coordinator for the assignment of unique parameter values for Internet protocols, such as port numbers and URI schemes. For further information, see <http://www.iana.org/>.