The overall protocol flow for negotiation of the Jingle ICE-UDP Transport Method is as follows (note: many of these events happen simultaneously, not in sequence).

Note: The examples in this document follow the scenario described in Section 17 of &icecore;, except that we substitute the Shakespearean characters "Romeo" and "Juliet" for the generic entities "L" and "R".

In order for the initiator in a Jingle exchange to start the negotiation, it sends a Jingle "session-initiate" stanza that includes at least one content type, as described in XEP-0166. If the initiator wishes to negotiate the ice-udp transport method for an application format, it MUST include a &TRANSPORT; child element qualified by the 'urn:xmpp:jingle:transports:ice-udp:1' namespace &VNOTE;. This element SHOULD in turn contain one &CANDIDATE; element for each of the initiator's higher-priority transport candidates as determined in accordance with the ICE methodology, but MAY instead be empty (with each candidate to be sent as the payload of a transport-info message).

The &TRANSPORT; element's 'pwd' and 'ufrag' attributes MUST be included whenever sending one or more candidates to the other party, e.g. in a session-initiate, session-accept, transport-info, content-add, or transport-replace message. The values for these attributes are separately generated for both the initiator and the responder, in accordance with &icecore; and as shown in the examples. The attributes of the <transport/> element are as follows.

The attributes of the <candidate/> element are as follows.

As described in XEP-0166, to acknowledge receipt of the session initiation request, the responder immediately returns an IQ-result.

Depending on the application type, 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. This message MUST contain a &TRANSPORT; element qualified by the 'urn:xmpp:jingle:transports:ice-udp:1' namespace, which SHOULD in turn contain one &CANDIDATE; element for each ICE-UDP candidate generated by or known to the responder, but MAY instead be empty (with each candidate to be sent as the payload of a transport-info message).

Note: See the Security Considerations section of this document regarding the exposure of IP addresses by the responder's client.

The initiator and responder negotiate connectivity over ICE by exchanging XML-formatted transport candidates for the channel. This negotiation proceeds immediately in order to maximize the possibility that connectivity can be established (and therefore media can be exchanged) as quickly as possible. In order to expedite session establishment, the initiator SHOULD include transport candidates in its session-initiate message but MAY also send additional transport candidates as soon as it learns of them, even before receiving acknowledgement of the session-initiate message (i.e., the initiator MUST consider the session to be live as soon as it sends the session-initiate message). Given in-order delivery as mandated by &xmppcore;, the responder will receive such transport-info messages after receiving the session-initiate message; if not, it is appropriate for the responder to return <unknown-session/> errors since according to its state machine the session does not exist.

The first step in negotiating connectivity is for each party to send transport candidates to the other party. The fact that both parties send candidates means that Jingle requires each party to be a full implementation of ICE, not a lite implementation as specified in &icecore;. These candidates SHOULD be gathered by following the procedure specified in Section 4.1.1 of &icecore; (typically by communicating with a standalone STUN server in order to discover the client's public IP address and port) and prioritized by following the procedure specified in Section 4.1.2 of &icecore;.

Each candidate shall be sent as a <candidate/> child of a &TRANSPORT; element qualified by the 'urn:xmpp:jingle:transports:ice-udp:1' namespace. The &TRANSPORT; element is sent via a Jingle message of type session-initiate, session-accept, or transport-info.

Either party MAY include multiple <candidate/> elements in one &TRANSPORT; element, especially in the session-initiate and session-accept messages sent at the beginning of the session negotiation. Including multiple candidates in the session-initiate and session-accept messages can help to ensure interoperability with entities that implement the SDP offer/answer model described in RFC 3264; in particular, an entity SHOULD include multiple candidates in its session-initiate or session-accept message if the other party advertises support for the "urn:ietf:rfc:3264" service discovery feature as described in the SDP Offer / Answer Support section of this document. However, including one candidate per subsequent transport-info message typically results in a faster negotiation because the candidates most likely to succeed are sent first (in the session-info and session-accept messages) and it is not necessary to gather all candidates before beginning to send any candidates; furthermore, because certain candidates can be more "expensive" in terms of bandwidth or processing power, either party might not want to advertise the existence of such candidates unless it is necessary to do so after other candidates have failed.

If the party that receives a candidate in a Jingle message can successfully process a given candidate or set of candidates, it returns an IQ-result (if not, for example because the candidate data is improperly formatted, it returns an IQ-error). At this point, the receiving entity is only indicating receipt of the candidate or set of candidates, not telling the other party that the candidate will be used.

The initiator can keep sending candidates (without stopping to receive an acknowledgement of receipt from the responder for each candidate) until it has exhausted its supply of possible or desirable transport candidates. The responder can also keep sending potential candidates, which the initiator will acknowledge.

As the initiator and responder receive candidates, they probe the candidates for connectivity. In performing these connectivity checks, each party SHOULD follow the procedure specified in Section 7 of &icecore;. The following business rules apply:

1. Each party sends a STUN Binding Request (see &rfc5389;) from each local candidate it generated to each remote candidate it received.
2. In accordance with &icecore;, the STUN Binding Requests MUST include the PRIORITY attribute (computed according to Section 7.1.1.1. of &icecore;).
3. For the purposes of the Jingle ICE-UDP Transport Method, both parties are full ICE implementations and therefore the controlling role MUST be assumed by the initiator and the controlled role MUST be assumed by the responder.
4. The STUN Binding Requests generated by the initiator MAY include the USE-CANDIDATE attribute to indicate that the initiator wishes to cease checks for this component.
5. The STUN Binding Requests generated by the initiator MUST include the ICE-CONTROLLING attribute.
6. The STUN Binding Requests generated by the responder MUST include the ICE-CONTROLLED attribute.
7. The parties MUST use STUN short term credentials to authenticate requests and perform message integrity checks. As in &icecore;, the username in the STUN Binding Request is of the form "ufrag-of-sender:ufrag-of-peer" and the password is the value of the 'pwd' attribute provided by the peer. Thus when Romeo sends a STUN Binding Request to Juliet the credentials will be STUN username "8hhy:9uB6" and password "YH75Fviy6338Vbrhrlp8Yh" whereas when Juliet sends a STUN Binding Request to Romeo the credentials will be STUN username "9uB6:8hhy" and password "asd88fgpdd777uzjYhagZg".

When it receives a STUN Binding Request, each party MUST return a STUN Binding Response, which indicates either an error case or the success case. As described in Section 7.1.2.2 of &icecore;, a connectivity check succeeds if all of the following are true:

1. The STUN transaction generated a success response.
2. The source IP address and port of the response equals the destination IP address and port to which the Binding Request was sent.
3. The destination IP address and port of the response match the source IP address and port from which the Binding Request was sent.

For the candidates exchanged in the previous section, the connectivity checks would be as follows (this diagram mirrors the example in &icecore;).

Note: Here the initiator (controlling agent) is using "aggressive nomination" as described in Section 8.1.1.2 of &icecore; and therefore includes the USE-CANDIDATE attribute in the STUN Binding Requests it sends.

If, based on STUN connectivity checks, the parties determine that they will be able to exchange media between a given pair of local candidates and remote candidates (i.e., the pair is "nominated" and ICE processing is "completed"), they can then begin using that candidate pair to exchange media.

Once the parties have connectivity and therefore the initiator has completed ICE as explained in &icecore;, the initiator MAY communicate the in-use candidate pair in the signalling channel by sending a transport-info message that contains a <remote-candidate/> element (this maps to the SDP "remote-candidates" attribute as described in Section B.6 of &icecore;, i.e., remote candidates are "the actual candidates at R that were selected by the offerer", of which there will be only one at this stage of the ICE-UDP negotiation).

(In accordance with Jingle core, the responder will also acknowledge the transport-info message.)

In the unlikely event that one of the parties determines that it cannot establish connectivity even after sending and checking lower-priority candidates, it SHOULD terminate the session as described in XEP-0166.

Even after media has begun to flow, either party MAY continue to send additional candidates to the other party (e.g., because the user agent has become aware of a new media proxy or network interface card). Such candidates are shared by sending a transport-info message.

The receiving party MUST acknowledge receipt of the candidate.

The parties would check the newly-offered candidate for connectivity, as described previously. If the parties determine that media can flow over the candidate, they MAY then use the new candidate in subsequent communications.

At any time, either party MAY restart the process of ICE negotiation by sending a candidate with a 'generation' value that is greater than the previous generation of candidates; when it does so, it MUST generate new values for the 'pwd' and 'ufrag' attributes, consistent with the definition of an ICE restart in Section 9.1.1.1 of &icecore;. As explained in &icecore;, typically the ICE negotiation would be restarted to change the media target (e.g., an IP address change for one of the parties) and certain third-party-call-control scenarios.

The recipient then acknowledges receipt.

The parties would then exchange new candidates to renegotiate connectivity and would check the new candidates for connectivity, as described previously. If the parties determine that media can flow over one of the new candidates, they can then use the successful candidate in subsequent communications. However, while ICE is being renegotiated the parties can continue to send media with the existing candidate-in-use.

To advertise its support for the Jingle ICE-UDP Transport Method, when replying to &xep0030; information requests an entity MUST return URNs for any version of this protocol that the entity supports -- e.g., "urn:xmpp:jingle:transports:ice-udp:1" for this version and "urn:xmpp:jingle:transports:ice-udp:0" for the previous version &VNOTE;.

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.

If an entity supports the SDP offer / answer model described in RFC 3264 and therefore prefers to receive multiple candidates in a single transport-info message, it MUST advertise support for the "urn:ietf:rfc:3264" service discovery feature. Typically this feature will be advertised only by gateways between Jingle and SIP.

By definition, the exchange of transport candidates results in exposure of the sender's IP addresses, which comprise a form of personally identifying information. A Jingle client MUST enable a user to control which entities will be allowed to receive such information. If a human user explicitly accepts a session request, then the client SHOULD consider that action to imply approval of IP address sharing. However, waiting for a human user to explicitly accept the session request can result in delays during session setup, since it is more efficient to immediately begin sharing transport candidates. Therefore, it is RECOMMENDED for the client to immediately send transport candidates to a contact (without waiting for explicit user approval of the session request) in the following cases:

1. The user has permanently and formally authorized the contact to view the user's presence information via a presence subscription as reflected in an XMPP roster item (see &xmppim;).
2. The user has temporarily and dynamically shared presence with the contact via "directed presence" as described in RFC 3921.
3. The user has explicitly added the contact to a "whitelist" of entities who are allowed to access the user's personally-identifying information.

A Jingle implementation SHOULD support security preconditions that are enforced before application media is allowed to flow over a UDP association, such as those described in &xtls;.

Application types that use the Jingle ICE-UDP transport method MAY also define their own application-specific encryption methods, such as the Secure Real-time Transport Protocol (SRTP) for RTP exchanges as described in &xep0167;.

This specification defines the following XML namespace:

• urn:xmpp:jingle:transports:ice-udp:1

Upon advancement of this specification from a status of Experimental to a status of Draft, the ®ISTRAR; shall add the foregoing namespaces to the registry located at &NAMESPACES;, as described in Section 4 of &xep0053;.

If an entity supports the SDP offer / answer model described in RFC 3264 and therefore prefers to receive one transport-info message with multiple candidates, it MUST advertise support for the "urn:ietf:rfc:3264" feature.

The registry submission is as follows.

The XMPP Registrar shall include "ice-udp" in its registry of Jingle transport methods. The registry submission is as follows: