&xep0166; can be used to initiate and negotiate a wide range of peer-to-peer sessions. One session type of interest is media such as voice or video. This document specifies an application format for negotiating Jingle media sessions, where the media is exchanged over the Realtime Transport Protocol (RTP; see &rfc3550;).

The Jingle application format defined herein is designed to meet the following requirements:

1. Enable negotiation of parameters necessary for media sessions using the Realtime Transport Protocol (RTP).
2. Map these parameters to Session Description Protocol (SDP; see &rfc4566;) to enable interoperability.
3. Define informational messages related to typical RTP uses such as audio chat and video chat (e.g., ringing, on hold, on mute).

In accordance with Section 10 of XEP-0166, this document specifies the following information related to the Jingle RTP application type:

1. The application format negotiation process is defined in the Negotiating a Jingle RTP Session section of this document.

2. The semantics of the &DESCRIPTION; element are defined in the Application Format section of this document.

3. A mapping of Jingle semantics to the Session Description Protocol is provided in the Mapping to Session Description Protocol section of this document.

4. A Jingle RTP session SHOULD use a datagram transport method such as &xep0177; or the "ice-udp" method specified in &xep0176;, but MAY use a streaming transport such as "ice-tcp" if a low-bandwidth codec is employed and the media negotiated is not unduly heavy (e.g., it might be possible to use a streaming transport for audio, but not for video).

5. Content is to be sent and received as follows:

   • For datagram transports, outbound content shall be encoded into RTP packets and each packet shall be sent individually over the transport. Each inbound packet received over the transport is an RTP packet.

   • For streaming transports, outbound content shall be encoded into RTP packets and each packet data shall be sent in succession over the transport. Incoming data received over the transport shall be processed as a stream of RTP packets, where each RTP packet boundary marks the location of the next packet.

A Jingle RTP session is described by a content type that contains one application format and one transport method. Each <content/> element defines a single RTP session. A Jingle negotiation MAY result in the establishment of multiple RTP sessions (e.g., one for audio and one for video). An application SHOULD consider all of the RTP sessions that are established via the same Jingle negotiation to be synchronized for purposes of streaming, playback, recording, etc.

The application format consists of one or more encodings contained within a wrapper <description/> element qualified by the 'urn:xmpp:jingle:apps:rtp:0' namespace &VNOTE;. In the language of RFC 4566 each encoding is a payload-type; therefore, each <payload-type/> element specifies an encoding that can be used for the RTP stream, as illustrated in the following example.

The &DESCRIPTION; element is intended to be a child of a Jingle &CONTENT; element as specified in XEP-0166.

The &DESCRIPTION; element MUST possess a 'media' attribute that specifies the media type, such as "audio" or "video", where the media type SHOULD be as registered at &ianamedia;.

The encodings SHOULD be provided in order of preference by placing the most-preferred payload type as the first &PAYLOADTYPE; child of the &DESCRIPTION; element and the least-preferred payload type as the last child.

The allowable attributes of the &PAYLOADTYPE; element are as follows:

In Jingle RTP, the encodings are used in the context of RTP. The most common encodings for the Audio/Video Profile (AVP) of RTP are listed in &rfc3551; (these "static" types are reserved from payload ID 0 through payload ID 95), although other encodings are allowed (these "dynamic" types use payload IDs 96 to 127) in accordance with the dynamic assignment rules described in Section 3 of RFC 3551. The payload IDs are represented in the 'id' attribute.

Each <payload-type/> element MAY contain one or more child elements that specify particular parameters related to the payload. For example, as described in &rtpspeex;, the "cng", "mode", and "vbr" parameters can be specified in relation to usage of the Speex See <http://www.speex.org/>. codec. Where such parameters are encoded via the "fmtp" SDP attribute, they shall be represented in Jingle via the following format:

The order of parameter elements MUST be ignored.

Parameter names MUST be treated as case-sensitive. However, parameter names are effectively guaranteed to be unique, since &IANA; maintains a registry of SDP parameters (see <http://www.iana.org/assignments/sdp-parameters>).

In general, the process for negotiating a Jingle RTP session is as follows:

When the initiator sends a session-initiate stanza to the responder, the &DESCRIPTION; element includes all of the payload types that the initiator can send and/or receive for Jingle RTP, each one encapsulated in a separate &PAYLOADTYPE; element (the rules specified in &rfc3264; SHOULD be followed regarding inclusion of payload types).

Upon receiving the session-initiate stanza, the responder determines whether it can proceed with the negotiation. The general Jingle error cases are specified in XEP-0166 and illustrated in the Scenarios section of this document.

If there is no error, the responder acknowledges the session initiation request.

After successful transport negotiation (not shown here), the responder accepts the session by sending a session-accept action to the initiator. The session-accept SHOULD include a subset of the payload types sent by the initiator, i.e., a list of the offered payload types that the responder can send and/or receive. The list that the responder sends SHOULD retain the ID numbers specified by the initiator. The order of the &PAYLOADTYPE; elements indicates the responder's preferences, with the most-preferred types first.

In the following example, we imagine that the responder supports Speex at clockrate of 8000 but not 16000, G729, and PCMA but not PMCU. Therefore the responder returns only two payload types (since PMCA was not offered).

And the initiator acknowledges session acceptance:

The initiator and responder would then exchange media using any of the codecs that meet the following criteria:

• If the value of the 'senders' attribute is "initiator" then the initiator MAY use any codec that it can send and the responder can receive.
• If the value of the 'senders' attribute is "responder" then the responder MAY use any codec that it can send and the initiator can receive.
• If the value of the 'senders' attribute is "both" then the parties MAY use any codec that both parties can send and receive.

The SDP media type for Jingle RTP is "audio" (see Section 8.2.1 of RFC 4566) for audio media, "video" (see Section 8.2.1 of RFC 4566) for video media, etc.

If the payload type is static (payload-type IDs 0 through 95 inclusive), it MUST be mapped to a media field defined in RFC 4566. The generic format for the media field is as follows:

In the context of Jingle audio sessions, the <media> is "audio" or "video" or some other media type as specified by the 'media' attribute, the <port> is the preferred port for such communications (which might be determined dynamically), and the <fmt list> is the payload-type ID.

For example, consider the following static payload-type:

That Jingle-formatted information would be mapped to SDP as follows:

If the payload type is dynamic (payload-type IDs 96 through 127 inclusive), it SHOULD be mapped to an SDP media field plus an SDP attribute field named "rtpmap".

For example, consider a payload of 16-bit linear-encoded stereo audio sampled at 16KHz associated with dynamic payload-type 96:

That Jingle-formatted information would be mapped to SDP as follows:

As noted, if additional parameters are to be specified, they shall be represented as attributes of the <parameter/> child of the &PAYLOADTYPE; element, as in the following example.

That Jingle-formatted information would be mapped to SDP as follows:

The formatting is similar for video parameters, as shown in the following example.

That Jingle-formatted information would be mapped to SDP as follows:

The term "early media" refers to media that is exchanged before a responder has definitively accepted a session request generated by an initiator. Early media is typically used to send ringing tones and announcements, using either audio streams or Dual Tone Multi-Frequency (DTMF) events.

In Jingle, the exchange of early media is established through use of the "content-add" action. In order to match the usage specified in &rfc3959; and &rfc3960;, when adding a content definition for early media the value of the &CONTENT; element's 'disposition' attribute MUST be "early-session" for mapping to a SIP Content-Disposition header value of "early-session" (if necessary). This enables endpoints or intermediate gateways to apply the application server model described in RFC 3960.

An entity that generates a content-add for early media SHOULD specify the same codecs for both session media and early media (however, it is possible that the entity that generates the early media does not generate the session media, for example in the case of an intermediate gateway or application server; in this case the entity MUST use one of the codecs advertised by the initiator).

Upon receiving a content-add action specifying the use of early media, the initiator's client SHOULD acknowledge the content-add, complete any required transpor negotiation, and then send a content-accept (or content-reject) to the sender. When the responder subsequently sends a session-accept action, the acceptance MUST NOT be construed to include the content definition whose disposition is "early-session".

In handling early media and deciding whether to generate local ringing or to play early media received from the responder or an intermediate gateway, the initiator's client SHOULD proceed as follows:

1. If no ringing notification is received via a session-info event containing a <ringing/> condition, do not generate local ringing.
2. If a ringing notification is received and no early media is received, generate local ringing.
3. If a ringing notification is received but early media is received, play the early media and do not generate local media.
4. Once the responder has accepted the session and the session (as opposed to early session) media has begun to flow, stop local ringing or stop playing early media.

For examples of early media, see the Jingle Audio via RTP with Early Media section of this document.

&rfc3711; defines the Secure Real-time Transport Protocol, and &rfc4568; defines the SDP "crypto" attribute for signalling and negotiating the use of SRTP in the context of offer-answer protocols such as SIP. To enable the use of SRTP and gatewaying to non-XMPP technologies that make use of the "crypto" SDP attribute, we define a corresponding <crypto/> element qualified by the 'urn:xmpp:jingle:apps:rtp:0' namespace.

If the initiator wishes to use SRTP, the session-initiate MUST include at least one <crypto/> element and MAY include multiple instances of the element. The <crypto/> element MUST be a child of the <description/> element.

The XML attributes of the <crypto/> element are as follows:

• crypto-suite -- this maps to the SDP "crypto-suite" parameter and has the same semantics (i.e., it is an identifier that describes the encryption and authentication algorithms).
• key-params -- this maps to the SDP "key-params" parameter and has the same semantics (i.e., it provides one or more sets of keying material for the crypto-suite in question).
• session-params -- this maps to the SDP "session-params" parameter and has the same semantics (i.e., it provides transport-specific parameters for SRTP negotiation).
• tag -- this maps to the SDP "tag" parameter and has the same semantics (i.e., it is a decimal number used as an identifier for a particular crypto element).

An example follows.

The mapping to SDP is as follows.

When the responder receives a session-initiate action containing one or more instances of the <crypto/> element, it MUST either accept one of the <crypto/> elements or reject the offer by sending a session-terminate action with a Jingle reason of <general-error/> and an RTP-specific condition of <invalid-crypto/>; see the Scenarios section of this document for examples.

If an entity supports Jingle RTP session, it MUST advertise that fact by returning a feature of "urn:xmpp:jingle:apps:rtp:0" &VNOTE; in response to &xep0030; information requests.

In order for an application to determine whether an entity supports this protocol, where possible it SHOULD use the dynamic, presence-based profile of service discovery defined in &xep0115;. However, if an application has not received entity capabilities information from an entity, it SHOULD use explicit service discovery instead.

The following sections show a number of Jingle RTP scenarios, roughly in order of increasing complexity.

In order to secure the data stream, implementations SHOULD use encryption methods appropriate to the transport method and media being exchanged. Such encryption methods are out of scope for this specification.

This document requires no interaction with &IANA;.

Thanks to Milton Chen, Olivier Crête, Tim Julien, Steffen Larsen, Jeff Muller, Mike Ruprecht, and Paul Witty for their feedback.