&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 (e.g. &xep0177; or the "ice-udp" method specified in &xep0176;), but MAY use a streaming transport if the end-to-end link has minimal latency 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. Jingle RTP supports two components: one for RTP itself and one for the Real Time Control Protocol (RTCP). The component numbered "1" MUST be associated with RTP and the component numbered "2" MUST be associated with RTCP. Even if an implementation does not support RTCP, it MUST accept Jingle content types that include component "2" by mirroring the second component in its replies (however, it would simply ignore the RTCP-related data during the RTP session).

6. 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, framed in accordance with &rfc4571;, and 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.

RTP as defined in RFC 3550 is used in the context of various "profiles" that are defined by other specifications. Jingle RTP treats RTP profiles as follows:

1. By default the RTP profile in Jingle RTP MUST be considered "RTP/AVP" as defined in &rfc3551;.
2. If the session initiation request contains an <encryption/> element to specify use of SRTP as described under Negotiation of SRTP, then the RTP profile MUST instead be considered "RTP/SAVP" as defined in &rfc3711;.
3. Future versions of this specification might define how to use other RTP profiles, such as "RTP/AVPF" and "RTP/SAVPF" as defined in &rfc4585; and &rfc5124; respectively.

The application format consists of one or more encodings contained within a wrapper <description/> element qualified by the 'urn:xmpp:jingle:apps:rtp:1' 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 &DESCRIPTION; element MAY possess a 'ssrc' attribute that specifies the 32-bit synchronization source for this media stream, as defined in RFC 3550.

After inclusion of one or more &PAYLOADTYPE; child elements, the &DESCRIPTION; element MAY also contain a <bandwidth/> element that specifies the allowable or preferred bandwidth for use by this application type. The 'type' attribute of the <bandwidth/> element SHOULD be a value for the SDP "bwtype" parameter as listed in the &ianasdp;. For RTP sessions, often the <bandwidth/> element will specify the "session bandwidth" as described in Section 6.2 of RFC 3550, measured in kilobits per second as described in Section 5.2 of RFC 4566.

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 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 RFC 3551 (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 &rfc5574;, 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.

Note: 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 message 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 immediate error, the responder acknowledges the session initiation request.

Depending on user preferences or client configuration, a user agent controlled by a human user might need to wait for the user to affirm a desire to proceed with the session before continuing. When the user agent has received such affirmation (or if the user agent can automatically proceed for any reason, e.g. because no human intervention is expected or because a human user has configured the user agent to automatically accept sessions with a given entity), it returns a Jingle session-accept message. The session-accept message 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 type first.

In the following example, we imagine that the responder supports Speex at a 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).

If the responder supports none of the payload-types offered by the initiator, the responder SHOULD terminate the session and include a Jingle reason of <failed-application/>.

If the responder accepts the session, the initiator acknowledges the session-accept message:

The initiator and responder would then attempt to establish connectivity for the data channel, Once they do, they would 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. The media type is reflected in the Jingle 'media' attribute.

The Jingle <bandwidth/> element SHALL be mapped to an SDP b= line; in particular, the value of the 'type' attribute SHALL be mapped to the SDP <bwtype> parameter and the XML character data of the Jingle <bandwidth/> element SHALL be mapped to the SDP <bandwidth> parameter.

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

The SDP <media> parameter is "audio" or "video" or some other media type as specified by the Jingle 'media' attribute, the <port> parameter is the preferred port for such communications (which might be determined dynamically), the <transport> parameter corresponds to the RTP profile as described under Application Format, and the <fmt list> parameter 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:

&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:1' namespace.

If the initiator wishes to use SRTP, the session-initiate stanza shall include an <encryption/> element, which MUST contain at least one <crypto/> element and MAY include multiple instances of the <crypto/> element. The <encryption/> element MUST be a child of the <description/> element. If the initiator requires the session to be encrypted, the <encryption/> element MUST include a 'required' attribute whose logical value is TRUE and whose lexical value is "true" or "1" &BOOLEANNOTE;, where this attribute defaults to a logical value of FALSE (i.e., a lexical value of "false" or "0").

The <crypto/> element is defined as empty (i.e., not containing any child elements); 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 of that data to SDP is as follows.

When the responder receives a session-initiate message containing an <encryption/> element, the responder MUST do one of the following:

1. Attempt to proceed with an encrypted session by including the acceptable credentials (i.e., the relevant <crypto/> element) in its session-accept message.
2. Attempt to proceed with an unencrypted session by not including any <crypto/> element in its session-accept message (it is up to the initiator to reject this attempt if desired).
3. Reject the initiator's offer by sending a session-terminate message with a Jingle reason of <security-error/> (typically with an RTP-specific condition of <invalid-crypto/>).

Which of these the responder does is a matter of personal security policies or client configuration.

If the responder requires encryption but the initiator did not include an <encryption/> element in its offer, the responder MUST reject the offer by sending a session-terminate message with a Jingle reason of <security-error/> and an RTP-specific condition of <crypto-required/>.

If the initiator requires encryption but the responder does not include an <encryption/> element in its session acceptance, the initiator MUST terminate the session with a Jingle reason of <security-error/> and an RTP-specific condition of <crypto-required/>.

Informational messages can be sent by either party within the context of Jingle to communicate the status of a Jingle RTP session, device, or principal. The informational message MUST be an IQ-set containing a &JINGLE; element of type "session-info", where the informational message is a payload element qualified by the 'urn:xmpp:jingle:apps:rtp:info:1' namespace. The following payload elements are defined. A <trying/> element (equivalent to the SIP 100 Trying response code) is not necessary, since each session-level message is acknowledged via XMPP IQ semantics.

Note: Because an informational message is sent in an IQ-set, the receiving party MUST return either an IQ-result or an IQ-error (normally an IQ-result simply to acknowledge receipt).

Before or during an RTP session, either party can share suggested application parameters with the other party by sending a Jingle stanza with an action of "description-info". The stanza shall contain only a &DESCRIPTION; element, which specifies suggested parameters for a given application type (e.g., a change to the height and width for display of a video stream). An example follows.

The description-info message SHOULD include only the modified codecs, not the complete set of codecs (if those codecs have not changed). Their order is NOT meaningful. Furthermore, the data provided is purely advisory; the session SHOULD NOT fail if the receiving party cannot adjust its parameters accordingly.

To advertise its support for Jingle RTP Sessions and specific media types for RTP, when replying to &xep0030; information requests an entity MUST return the following features:

• URNs for any version of this protocol that the entity supports -- e.g., "urn:xmpp:jingle:apps:rtp:1" for this version and "urn:xmpp:jingle:apps:rtp:0" for the previous version &VNOTE;
• URNs for all of the media types that the entity supports -- e.g., "urn:xmpp:jingle:apps:rtp:audio" for RTP audio and "urn:xmpp:jingle:apps:rtp:video" for RTP video Support for the "audio" or "video" media type does not necessarily mean that the application supports all sub-types associated with those media types.

An example follows.

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 RTP data transport. It is RECOMMENDED to use SRTP as defined in the Negotiation of SRTP section of this document. The SRTP keying material SHOULD (1) be tied to a separate, secure connection such as provided by DTLS (&rfc4347;) where the keys are established as described in &dtlssrtp; and/or (2) protected by sending the Jingle signalling over a secure channel that protects the confidentiality and integrity of the SRTP-related signalling data.

While it is also possible to use native RTP methods, such as &rfc6189; as described in &xep0262;, this specification does not actively encourage or discourage the use of such methods.

This document requires no interaction with &IANA;.

Thanks to Milton Chen, Paul Chitescu, Olivier Crête, Tim Julien, Steffen Larsen, Jeff Muller, Mike Ruprecht, Sjoerd Simons, Will Thompson, Justin Uberti, Unnikrishnan Vikrama Panicker, and Paul Witty for their feedback.