XEP-0166: Jingle

Abstract:This specification defines an XMPP protocol extension for initiating and managing peer-to-peer media sessions between two XMPP entities in a way that is interoperable with existing Internet standards. The protocol provides a pluggable model that enables the core session management semantics (compatible with SIP) to be used for a wide variety of application types (e.g., voice chat, video chat, file transfer) and with a wide variety of transport methods (e.g., TCP, UDP, ICE, application-specific transports).
Authors:Scott Ludwig, Joe Beda, Peter Saint-Andre, Robert McQueen, Sean Egan, Joe Hildebrand
Copyright:© 1999 - 2012 XMPP Standards Foundation. SEE LEGAL NOTICES.
Status:Draft
Type:Standards Track
Version:1.1
Last Updated:2009-12-23

NOTICE: The protocol defined herein is a Draft Standard of the XMPP Standards Foundation. Implementations are encouraged and the protocol is appropriate for deployment in production systems, but some changes to the protocol are possible before it becomes a Final Standard.

Table of Contents


1. Introduction
2. How It Works
3. Requirements
4. Terminology
    4.1. Glossary
    4.2. Conventions
5. Concepts and Approach
    5.1. Overall Session Management
6. Session Flow
    6.1. Resource Determination
    6.2. Initiation
    6.3. Responder Response
       6.3.1. Acknowledgement
       6.3.2. Errors
    6.4. Negotiation
    6.5. Acceptance
    6.6. Modifying an Active Session
    6.7. Termination
    6.8. Informational Messages
7. Formal Definition
    7.1. Jingle Element
    7.2. Action Attribute
       7.2.1. content-accept
       7.2.2. content-add
       7.2.3. content-modify
       7.2.4. content-reject
       7.2.5. content-remove
       7.2.6. description-info
       7.2.7. security-info
       7.2.8. session-accept
       7.2.9. session-info
       7.2.10. session-initiate
       7.2.11. session-terminate
       7.2.12. transport-accept
       7.2.13. transport-info
       7.2.14. transport-reject
       7.2.15. transport-replace
       7.2.16. Tie Breaking Related to Jingle Actions
    7.3. Content Element
    7.4. Reason Element
8. Transport Types
    8.1. Datagram
    8.2. Streaming
9. Security Preconditions
10. Error Handling
11. Determining Support
12. Conformance by Using Protocols
    12.1. Application Formats
    12.2. Transport Methods
    12.3. Security Preconditions
13. Security Considerations
    13.1. Transport Security
    13.2. Denial of Service
    13.3. Communication Through Gateways
    13.4. Information Exposure
    13.5. Redirection
14. IANA Considerations
15. XMPP Registrar Considerations
    15.1. Protocol Namespaces
    15.2. Namespace Versioning
    15.3. Jingle Application Formats Registry
    15.4. Jingle Transport Methods Registry
16. XML Schemas
    16.1. Jingle
    16.2. Jingle Errors
17. History
18. Acknowledgements

Appendices
    A: Document Information
    B: Author Information
    C: Legal Notices
    D: Relation to XMPP
    E: Discussion Venue
    F: Requirements Conformance
    G: Notes
    H: Revision History

1. Introduction

The purpose of Jingle is to enable one-to-one, peer-to-peer media sessions between XMPP entities, where the negotiation occurs over the XMPP signalling channel and the media is exchanged over a data channel that is usually a dedicated non-XMPP transport. Jingle is designed in a modular way:

It is expected that most application types, transport methods, and security preconditions will be documented in specifications produced by the XMPP Standards Foundation (XSF) [14] or the Internet Engineering Task Force (IETF) [15]; however, developers can also define proprietary methods for custom functionality.

Although Jingle provides a general framework for session management, the original target application for Jingle was simple voice and video chat. We stress the word "simple". The purpose of Jingle was not to build a full-fledged telephony application that supports call waiting, call forwarding, call transfer, hold music, IVR systems, find-me-follow-me functionality, conference calls, and the like. These features are of interest to some user populations, but adding support for them to the core Jingle layer would introduce unnecessary complexity into a technology that is designed for simple but generalized session negotiation.

Furthermore, Jingle is not intended to supplant or replace existing Internet technologies based on the Session Initiation Protocol (SIP; RFC 3261 [16]). Because dual-stack XMPP+SIP clients are difficult to build, Jingle was designed as a pure XMPP signalling protocol. However, Jingle is at the same time designed to interwork with SIP so that the millions of deployed XMPP clients can be added onto existing Voice over Internet Protocol (VoIP) networks, rather than limiting XMPP users to a separate and distinct network.

2. How It Works

This section provides a friendly introduction to Jingle.

In essence, Jingle enables two XMPP entities (e.g., romeo@montague.lit and juliet@capulet.lit) to set up, manage, and tear down a multimedia session. The negotiation takes place over XMPP, and the media transfer typically takes place outside of XMPP. A simplified session flow would be as follows: [17]

Romeo                         Juliet
  |                             |
  |   session-initiate          |
  |---------------------------->|
  |   ack                       |
  |<----------------------------|
  |   session-accept            |
  |<----------------------------|
  |   ack                       |
  |---------------------------->|
  |   MEDIA SESSION             |
  |<===========================>|
  |   session-terminate         |
  |<----------------------------|
  |   ack                       |
  |---------------------------->|
  |                             |

To illustrate the basic flow, we show a truncated example with a "stub" application format and transport method (skipping non-essential steps to enforce the most essential concepts and ignoring security preconditions for now).

Example 1. Initiator sends session-initiate (stub)

<iq from='romeo@montague.lit/orchard'
    id='zid615d9'
    to='juliet@capulet.lit/balcony'
    type='set'>
  <jingle xmlns='urn:xmpp:jingle:1'
          action='session-initiate'
          initiator='romeo@montague.lit/orchard'
          sid='a73sjjvkla37jfea'>
    <content creator='initiator' name='this-is-a-stub'>
      <description xmlns='urn:xmpp:jingle:apps:stub:0'/>
      <transport xmlns='urn:xmpp:jingle:transports:stub:0'/>
    </content>
  </jingle>
</iq>

In this example, the initiator (romeo@montague.lit/orchard) sends a session initiation offer to the responder (juliet@capulet.lit/balcony), where the session is defined as the exchange of "stub" media over a "stub" transport.

After the responding client acknowledges receipt of the session-initiate message (not shown here), it prompts the responding user (if any) to choose whether she wants to proceed with the session (however, it does not need to prompt the user if for example she has configured her client to automatically accept session requests from this particular initiator). If she wants to proceed she selects the appropriate interface element and her client sends a session-accept message to the initiator.

Example 2. Responder definitively accepts the session

<iq from='juliet@capulet.lit/balcony'
    id='rc61n59s'
    to='romeo@montague.lit/orchard'
    type='set'>
  <jingle xmlns='urn:xmpp:jingle:1'
          action='session-accept'
          responder='juliet@capulet.lit/balcony'
          sid='a73sjjvkla37jfea'>
    <content creator='initiator' name='this-is-a-stub'>
      <description xmlns='urn:xmpp:jingle:apps:stub:0'/>
      <transport xmlns='urn:xmpp:jingle:transports:stub:0'/>
    </content>
  </jingle>
</iq>

The initiating client acknowledges receipt of the session-accept message (not shown here) and the parties can exchange "stub" media data over the "stub" transport.

Eventually, one of the parties (here the responder) will terminate the session.

Example 3. Responder terminates the session

<iq from='juliet@capulet.lit/balcony'
    id='le71fa63'
    to='romeo@montague.lit/orchard'
    type='set'>
  <jingle xmlns='urn:xmpp:jingle:1'
          action='session-terminate'
          sid='a73sjjvkla37jfea'>
    <reason>
      <success/>
    </reason>
  </jingle>
</iq>

The initiating client acknowledges receipt of the session-terminate message (not shown here) and the session is ended.

We now "fill in the blanks" for the <description/> and <transport/> elements with a more complex example: a voice chat session, where the application type is a Jingle RTP session (with several different codec possibilities) and the transport method is ICE-UDP.

Example 4. Initiator sends session-initiate

<iq from='romeo@montague.lit/orchard'
    id='ph37a419'
    to='juliet@capulet.lit/balcony'
    type='set'>
  <jingle xmlns='urn:xmpp:jingle:1'
          action='session-initiate'
          initiator='romeo@montague.lit/orchard'
          sid='a73sjjvkla37jfea'>
    <content creator='initiator' name='voice'>
      <description xmlns='urn:xmpp:jingle:apps:rtp:1' media='audio'>
        <payload-type id='96' name='speex' clockrate='16000'/>
        <payload-type id='97' name='speex' clockrate='8000'/>
        <payload-type id='18' name='G729'/>
        <payload-type id='0' name='PCMU' />
        <payload-type id='103' name='L16' clockrate='16000' channels='2'/>
        <payload-type id='98' name='x-ISAC' clockrate='8000'/>
      </description>
      <transport xmlns='urn:xmpp:jingle:transports:ice-udp:1'
                 pwd='asd88fgpdd777uzjYhagZg'
                 ufrag='8hhy'>
        <candidate component='1'
                   foundation='1'
                   generation='0'
                   id='el0747fg11'
                   ip='10.0.1.1'
                   network='1'
                   port='8998'
                   priority='2130706431'
                   protocol='udp'
                   type='host'/>
        <candidate component='1'
                   foundation='2'
                   generation='0'
                   id='y3s2b30v3r'
                   ip='192.0.2.3'
                   network='1'
                   port='45664'
                   priority='1694498815'
                   protocol='udp'
                   rel-addr='10.0.1.1'
                   rel-port='8998'
                   type='srflx'/>
      </transport>
    </content>
  </jingle>
</iq>

Upon receiving the session-initiate message, the responder determines whether it can proceed with the negotiation. If there is no error, the responder acknowledges the session initiation request.

Example 5. Responder acknowledges session-initiate

<iq from='juliet@capulet.lit/balcony'
    id='ph37a419'
    to='romeo@montague.lit/orchard'
    type='result'/>

When the responding user affirms that she would like to proceed with the session, the responding client sends a session-accept message to the initiator (including in this example the subset of offered codecs that the responding client supports and one or more transport candidates generated by the responder).

Example 6. Responder definitively accepts the session

<iq from='juliet@capulet.lit/balcony'
    id='yd71f495'
    to='romeo@montague.lit/orchard'
    type='set'>
  <jingle xmlns='urn:xmpp:jingle:1'
          action='session-accept'
          responder='juliet@capulet.lit/balcony'
          sid='a73sjjvkla37jfea'>
    <content creator='initiator' name='voice'>
      <description xmlns='urn:xmpp:jingle:apps:rtp:1' media='audio'>
        <payload-type id='97' name='speex' clockrate='8000'/>
        <payload-type id='18' name='G729'/>
      </description>
      <transport xmlns='urn:xmpp:jingle:transports:ice-udp:1'>
        <candidate component='1'
                   foundation='1'
                   generation='0'
                   id='or2ii2syr1'
                   ip='192.0.2.1'
                   network='0'
                   port='3478'
                   priority='2130706431'
                   protocol='udp'
                   type='host'/>
      </transport>
    </content>
  </jingle>
</iq>

And the initiating client acknowledges session acceptance:

Example 7. Initiator acknowledges session acceptance

<iq from='romeo@montague.lit/orchard'
    id='yd71f495'
    to='juliet@capulet.lit/balcony'
    type='result'/>

Once the parties finish the transport negotiation, they would then exchange media using any of the acceptable codecs.

Eventually, one of the parties (here the responder) will terminate the session.

Example 8. Responder terminates the session

<iq from='juliet@capulet.lit/balcony'
    id='vua614d9'
    to='romeo@montague.lit/orchard'
    type='set'>
  <jingle xmlns='urn:xmpp:jingle:1'
          action='session-terminate'
          sid='a73sjjvkla37jfea'>
    <reason>
      <success/>
      <text>Sorry, gotta go!</text>
    </reason>
  </jingle>
</iq>

The other party then acknowledges termination of the session:

Example 9. Initiator acknowledges termination

<iq from='romeo@montague.lit/orchard'
    id='vua614d9'
    to='juliet@capulet.lit/balcony'
    type='result'/>

3. Requirements

The protocol defined herein is designed to meet the following requirements:

  1. Make it possible to manage a wide variety of peer-to-peer sessions (including but not limited to voice and video) within XMPP.
  2. When a peer-to-peer connection cannot be negotiated, make it possible to fall back to relayed communications.
  3. Clearly separate the signalling channel (XMPP) from the data channel.
  4. Clearly separate the application format (e.g., RTP audio) from the transport method (e.g., UDP).
  5. Make it possible to add, modify, and remove both application types and transport methods in an existing session.
  6. Make it relatively easy to implement support for the protocol in standard Jabber/XMPP clients.
  7. Where communication with non-XMPP entities is needed, push as much complexity as possible onto server-side gateways between the XMPP network and the non-XMPP network.

This document defines the signalling protocol only. Additional documents specify the following:

4. Terminology

4.1 Glossary

Application Format
The data format of the content type being established, which formally declares one purpose of the session (e.g., "audio" or "video"). This is the 'what' of the session (i.e., the bits to be transferred), such as the acceptable codecs when establishing a voice conversation. In Jingle XML syntax the application format is the namespace of the <description/> element.
Component
A numbered stream of data that needs to be transmitted between the endpoints for a given content type in the context of a given session. It is up to the transport to negotiate the details of each component. Depending on the content type, multiple components might be needed (e.g., one to transmit an RTP stream and another to transmit RTCP timing information).
Content Type
A pair formed by the combination of one application format and one transport method.
Session
One or more content types negotiated between two entities. It is delimited in time by a session-initiate action and a session-terminate action. During the lifetime of a session, content types can be added or removed. A session consists of at least one content type at a time.
Transport Method
The method for establishing data stream(s) between entities. Possible transports might include ICE-UDP, ICE-TCP, Raw UDP, In-Band Bytestreams, SOCKS5 Bytestreams, etc. This is the 'how' of the session. In Jingle XML syntax this is the namespace of the <transport/> element. The transport method defines how to transfer bits from one host to another. Each transport method MUST specify whether it is "datagram" or "streaming" as described in the Transport Types section of this document.

4.2 Conventions

In diagrams, the following conventions are used:

5. Concepts and Approach

Jingle consists of three parts, each with its own syntax and semantics:

  1. Overall session management
  2. Application types (the "what")
  3. Transport methods (the "how")

This document defines the semantics and syntax for overall session management. It also provides pluggable "slots" for application formats and transport methods, which are specified in separate documents.

At the most basic level, the process for initial negotiation of a Jingle session is as follows:

  1. One user (the "initiator") sends to another user (the "responder") a session-initiate message containing at least one content definition, each of which defines one application type, one transport method, and optionally one security precondition.
  2. If the responder wishes to proceed, it sends a session-accept message to the initiator, optionally including one or more transport candidates (depending on the transport method specified in the session-initiate message).
  3. The parties attempt to establish connectivity over the offered transport method as defined in the relevant specification, which might involve the exchange of transport-info messages for additional transport candidates; if connectivity cannot be established then the parties might attempt to fall back to another transport method using the transport-replace and transport-accept messages.
  4. Optionally, the parties attempt to establish security for the transport method before using it to exchange application data.
  5. Optionally, either party can add or remove content definitions, or change the direction of the media flow, using the content-add, content-remove, and content-modify messages.
  6. Optionally, either party can send session-info messages (e.g., to inform the other party that its device is ringing).
  7. As soon as the initiator and responder determine that data can flow over the negotiated transport (potentially only after a security precondition has been met), they start sending application data over the transport.

Even after application data is being exchanged, the parties can adjust the session definition by sending additional Jingle messages, such as content-modify, content-remove, content-add, description-info, security-info, session-info, and transport-replace.

5.1 Overall Session Management

The state machine for overall session management (i.e., the state per Session ID) is as follows:

         o
         |
         | session-initiate
         |
         | +---------->--------------+
         |/                          |
PENDING  o-----------------------+   |
         |   | content-accept,   |   |
         |   | content-add,      |   |
         |   | content-modify,   |   |
         |   | content-reject,   |   |
         |   | content-remove,   |   |
         |   | description-info, |   |
        \|/  | session-info,     |   |
         |   | transport-accept, |   |
         |   | transport-info,   |   |
         |   | transport-reject, |   |
         |   | transport-replace |   |
         |   +-------------------+   |
         |                           |
         | session-accept           \|/
         |                           |
 ACTIVE  o-----------------------+   |
         |   | content-accept,   |   |
         |   | content-add,      |   |
         |   | content-modify,   |   |
         |   | content-reject,   |   |
         |   | content-remove,   |   |
         |   | description-info, |   |
        \|/  | session-info,     |   |
         |   | transport-accept, |   |
         |   | transport-info,   |   |
         |   | transport-reject, |   |
         |   | transport-replace |   |
         |   +-------------------+   |
         |                           |
         +------------>--------------+
                                     |
                                     | session-terminate
                                     |
                                     o ENDED

As shown, there are three overall session states:

  1. PENDING
  2. ACTIVE
  3. ENDED

Note: While it is allowed to send all actions while in the PENDING state, typically the responder will send a session-accept message as quickly as possible in order to expedite the transport negotiation; see the Security Considerations section of this document regarding information exposure when the responder sends transport candidates to the initiator.

The actions related to management of the overall Jingle session are as follows (detailed definitions are provided in the Action Attribute section of this document).

content-accept
Accept a content-add action received from another party.
content-add
Add one or more new content definitions to the session.
content-modify
Change the directionality of media sending.
content-reject
Reject a content-add action received from another party.
content-remove
Remove one or more content definitions from the session.
description-info
Exchange information about parameters for an application type.
session-accept
Definitively accept a session negotiation.
session-info
Send session-level information, such as a ping or a ringing message.
session-initiate
Request negotiation of a new Jingle session.
session-terminate
End an existing session.
transport-accept
Accept a transport-replace action received from another party.
transport-info
Exchange transport candidates.
transport-reject
Reject a transport-replace action received from another party.
transport-replace
Redefine a transport method or replace it with a different method.

6. Session Flow

This section defines the high-level flow of a Jingle session. More detailed descriptions are provided in the specifications for Jingle application formats and transport methods.

6.1 Resource Determination

In order to initiate a Jingle session, the initiator needs to determine which of the responder's XMPP resources is best for the desired application format. Methods for doing so are out of scope for this specification. However, see the Determining Support section of this document for relevant information.

6.2 Initiation

Once the initiator has discovered which of the responder's XMPP resources is ideal for the desired application format, it sends a session initiation request to the responder. This request is an IQ-set containing a <jingle/> element qualified by the 'urn:xmpp:jingle:1' namespace (see Namespace Versioning regarding the possibility of incrementing the version number), where the value of the 'action' attribute is "session-initiate" and where the <jingle/> element contains one or more <content/> elements. Each <content/> element defines a content type to be transferred during the session, and each <content/> element in turn contains one <description/> child element that specifies a desired application format and one <transport/> child element that specifies a potential transport method, as well as (optionally) one <security/> element that specifies a security precondition that needs to be met before the parties can exchange application data over the negotiated transport.

Example 10. Initiator sends session-initiate

<iq from='romeo@montague.lit/orchard'
    id='xs51r0k4'
    to='juliet@capulet.lit/balcony'
    type='set'>
  <jingle xmlns='urn:xmpp:jingle:1'
          action='session-initiate'
          initiator='romeo@montague.lit/orchard'
          sid='a73sjjvkla37jfea'>
    <content creator='initiator' name='voice'>
      <description xmlns='urn:xmpp:jingle:apps:rtp:1' media='audio'>
        <payload-type id='96' name='speex' clockrate='16000'/>
        <payload-type id='97' name='speex' clockrate='8000'/>
        <payload-type id='18' name='G729'/>
        <payload-type id='0' name='PCMU' />
        <payload-type id='103' name='L16' clockrate='16000' channels='2'/>
        <payload-type id='98' name='x-ISAC' clockrate='8000'/>
      </description>
      <transport xmlns='urn:xmpp:jingle:transports:ice-udp:1'
                 pwd='asd88fgpdd777uzjYhagZg'
                 ufrag='8hhy'>
        <candidate component='1'
                   foundation='1'
                   generation='0'
                   id='el0747fg11'
                   ip='10.0.1.1'
                   network='1'
                   port='8998'
                   priority='2130706431'
                   protocol='udp'
                   type='host'/>
        <candidate component='1'
                   foundation='2'
                   generation='0'
                   id='y3s2b30v3r'
                   ip='192.0.2.3'
                   network='1'
                   port='45664'
                   priority='1694498815'
                   protocol='udp'
                   rel-addr='10.0.1.1'
                   rel-port='8998'
                   type='srflx'/>
      </transport>
    </content>
  </jingle>
</iq>

Application types ought not to be mixed beyond necessity within a single session. Therefore the session initiation request (along with subsequent additions) will include only content-types that can be grouped together into a coherent session within a given Jingle application. For example, two parties might start an audio call but then add a video aspect to that call. If one of the parties decides to send a file to the other party as a result of discussion over the audio/video session or a text chat conversation, conceptually that is probably a separate session (unless file exchange or screen sharing or some other application type is an integral part of a broader collaboration experience and needs to be calibrated with the audio/video session).

Note: The syntax and semantics of the <description/>, <transport/>, and <security/> elements are out of scope for this document, since they are defined in related specifications. The syntax and semantics of the <jingle/> and <content/> elements are specified in this document under Formal Definition.

6.3 Responder Response

6.3.1 Acknowledgement

Unless one of the following errors occurs, the responder MUST acknowledge receipt of the initiation request.

Example 11. Responder acknowledges session-initiate

<iq from='juliet@capulet.lit/balcony'
    id='xs51r0k4'
    to='romeo@montague.lit/orchard'
    type='result'/>

However, after acknowledging the session initiation request, the responder might subsequently determine that it cannot proceed with negotiation of the session (e.g., because it does not support any of the offered application formats or transport methods, because a human user is busy or unable to accept the session, because a human user wishes to formally decline the session, etc.). In these cases, the responder SHOULD immediately acknowledge the session initiation request but then terminate the session with an appropriate reason as described in the Termination section of this document.

6.3.2 Errors

There are several reasons why the responder might immediately return an error instead of acknowledging receipt of the initiation request:

If the initiator is unknown to the responder (e.g., via presence subscription as defined in RFC 3921 [22]) and the responder has a policy of not communicating via Jingle with unknown entities, it MUST return a <service-unavailable/> error.

Example 12. Initiator is unknown to responder

<iq from='juliet@capulet.lit/balcony'
    id='xs51r0k4'
    to='romeo@montague.lit/orchard'
    type='error'>
  <error type='cancel'>
    <service-unavailable xmlns='urn:ietf:params:xml:ns:xmpp-stanzas'/>
  </error>
</iq>

If the responder does not support Jingle, it MUST return a <service-unavailable/> error.

Example 13. Responder does not support Jingle

<iq from='juliet@capulet.lit/balcony'
    id='xs51r0k4'
    to='romeo@montague.lit/orchard'
    type='error'>
  <error type='cancel'>
    <service-unavailable xmlns='urn:ietf:params:xml:ns:xmpp-stanzas'/>
  </error>
</iq>

If the responder wishes to redirect the request to another address, it MUST return a <redirect/> error.

Example 14. Responder redirection

<iq from='juliet@capulet.lit/balcony'
    id='xs51r0k4'
    to='romeo@montague.lit/orchard'
    type='error'>
  <error type='modify'>
    <redirect xmlns='urn:ietf:params:xml:ns:xmpp-stanzas'>
      xmpp:voicemail@capulet.lit
    </redirect>
  </error>
</iq>

If the responder does not have sufficient resources to participate in a session, it MUST return a <resource-constraint/> error.

Example 15. Responder has insufficent resources

<iq from='juliet@capulet.lit/balcony'
    id='xs51r0k4'
    to='romeo@montague.lit/orchard'
    type='error'>
  <error type='wait'>
    <resource-constraint xmlns='urn:ietf:params:xml:ns:xmpp-stanzas'/>
  </error>
</iq>

If the initiation request was malformed, the responder MUST return a <bad-request/> error.

Example 16. Initiation request malformed

<iq from='juliet@capulet.lit/balcony'
    id='xs51r0k4'
    to='romeo@montague.lit/orchard'
    type='error'>
  <error type='cancel'>
    <bad-request xmlns='urn:ietf:params:xml:ns:xmpp-stanzas'/>
  </error>
</iq>

6.4 Negotiation

Although in general it is preferable for the responder to send a session-accept message as soon as possible, some forms of negotiation might be necessary before the parties can agree on an acceptable set of application formats and transport methods. There are many potential parameter combinations, as defined in the relevant specifications for various application formats and transport methods.

The allowable negotiations (e.g., content-level and transport-level negotiations) include:

These forms of negotiation can also occur after the session has been accepted.

6.5 Acceptance

As soon as possible after receiving the session-initiate message, the responder informs the initiator that she wishes to proceed with the session by sending a session-accept message.

Example 17. Responder accepts the session

<iq from='juliet@capulet.lit/balcony'
    id='jd82f517'
    to='romeo@montague.lit/orchard'
    type='set'>
  <jingle xmlns='urn:xmpp:jingle:1'
          action='session-accept'
          responder='juliet@capulet.lit/balcony'
          sid='a73sjjvkla37jfea'>
    <content creator='initiator' name='voice'>
      <description xmlns='urn:xmpp:jingle:apps:rtp:1' media='audio'>
        <payload-type id='97' name='speex' clockrate='8000'/>
        <payload-type id='18' name='G729'/>
      </description>
      <transport xmlns='urn:xmpp:jingle:transports:ice-udp:1'>
        <candidate component='1'
                   foundation='1'
                   generation='0'
                   id='or2ii2syr1'
                   ip='192.0.2.1'
                   network='0'
                   port='3478'
                   priority='2130706431'
                   protocol='udp'
                   type='host'/>
      </transport>
    </content>
  </jingle>
</iq>

Note: After receiving and acknowledging the "session-initiate" action received from the initiator, the responding client SHOULD present an interface element that enables a human user to explicitly agree to proceeding with the session (e.g., an "Accept Incoming Call?" pop-up window including "Yes" and "No" buttons). However, the responding client SHOULD NOT return a "session-accept" action to the initiator until the responder has explicitly agreed to proceed with the session (unless the initiator is on a list of entities whose sessions are automatically accepted).

The initiator then acknowledges the responder's definitive acceptance.

Example 18. Initiator acknowledges session acceptance

<iq from='romeo@montague.lit/orchard'
    id='jd82f517'
    to='juliet@capulet.lit/balcony'
    type='result'/>

The session is now in the ACTIVE state. However, this does not necessarily mean that the parties can exchange application data yet, be

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