It is RECOMMENDED that a client will offer as many <candidate/> elements as possible with itself as the host (i.e., non-proxy candidates). Such candidates might be found using the following methods:

• Opening the TCP port on all available interfaces the user wants to use (e.g., maybe not an expensive UMTS link), including the IPv4 and IPv6 addresses of that interface (if available).
• Using the client's external IP address as discovered through an assisting NAT protocol or other means.

If the client knows it is behind a NAT and the router announces UPnP IGD or NAT-PMP support, the client SHOULD map the open port to the external interface of the router and include the public IP address and port information in the <candidate/> offers. To increase the chance of success without using a proxy, this specification allows the responder to also send offers, effectively equivalent to the "fast-mode" for SOCKS5 Bytestreams as previously described at <http://delta.affinix.com/specs/stream.html>.

Once the initiator has a set of candidates, it sends a Jingle session-initiate request that contains one or more transport candidates which are a mixture of XEP-0065 streamhosts and ICE candidates used in XEP-0176.

The responder immediately acknowledges receipt.

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:s5b:1' namespace, which SHOULD in turn contain one <candidate/> element for each SOCKS5 Bytestreams candidate generated by or known to the responder, but MAY instead be empty if the responder does not wish to offer any candidates or wishes to send each candidate as the payload of a transport-info message.

If the responder sends candidates in the session-accept, the chances of a successful connection are increased. For example, the initiator might be behind a NAT or might have no access to a proxy, whereas the responder might have a public IP address, might know about a proxy, or might have NAT penetration support like NAT-PMP in a router. (Whereas this "fast-mode" was a special extension previously, it is the default in jingle-s5b.) However, the responder MUST NOT offer as a candidate any host/port combination that has already been offered by the initiator; this helps to prevent failure of negotiation with the proxy.

In the following example, Juliet's client opens one port. The provided <candidates/> are the (private) IPv4 address of the interface, a (public) IPv6 address, and the public IPv4 address created by mapping the private IP address/port using NAT-PMP.

The initiator acknowledges receipt and tries to connect to the offered candidates.

A client SHOULD check the offered candidates in order of their priority, starting with the highest value. How the priority is calculated depends on the actual available interfaces. It is RECOMMENDED to use the following formula:

The type preference is an integer value between 0 and 127. The following types and their suggested preference values are defined.

The local preference is used to rate different candidates of the same type, e.g. a DSL link might be preferred over a VPN connection. The value of the local preference SHOULD be between 0 and 65535. The proposed values are only guidelines. If a client wants to increase or decrease the value of a specific candidate it is free to do so. For instance, a client might have an expensive UMTS link as a last resort and might rate this link lower than all SOCKS5 relays.

After receiving its peer's candidates, a client start to connect to them in order of the priority. The protocol is described in XEP-0065 in detail. Once one client has successfully created a connection, it sends the <candidate-used/> element to the peer inside a Jingle transport-info message. If a client receives a candidate-used notification it SHOULD continue trying to connect to candidates of its peer if it has not tried all candidates with a higher priority than the one successfully used by the peer.

The peer immediately acknowledges receipt.

If a client is unable to connect to any candidate sent by its peer, or if it stopped trying to connect because its peer sent a candidate-used notification with a priority higher than its remaining candidate(s), it sends a candidate-error Jingle transport-info message (this is equivalent to the IQ-error with code='500' from the "fast-mode" extension).

The peer immediately acknowledges receipt.

The transport negotiation is completed in one of the following ways:

1. If both parties send a candidate-error notification then the SOCKS5 negotiation has failed and the parties need to fall back to some other transport method, typically IBB; see the Fallback to IBB section of this document for details.
2. If one of the parties sends a candidate-error notification and the other party sends a candidate-used notification, then the candidate-used shall be considered the nominated candidate.
3. If both parties send a candidate-used notification but the candidates have a different priority, then the candidate with the lower priority shall be considered the nominated candidate.
4. If both parties send a candidate-used notification with candidates having the same priority, then the candidate chosen by the initiator shall be considered the nominated candidate (this is consistent with the rules in XEP-0166).

The parties shall use the nominated candidate for the data transfer. If the nominated candidate is of the proxy type, then the peer has no way to know when it can send data. Therefore the party that offered the nominated candidate MUST send an activated notification to the peer once it has activated the bytestream (as described in XEP-0065); it does so by sending a transport-info message containing an <activated/> element as follows.

If the nominated candidate is of the proxy type and either party cannot connect to the proxy (for example because of a restrictive firewall), the party shall send a transport-info message containing an <proxy-error/> element.

The parties shall then consider the bytestream unsuccessful and fall back to IBB as described in Fallback to IBB.

Once the parties have chosen (and if necessary activated) a streamhost, they can exchange data as defined in XEP-0065.

Once the parties have finished using the bytestream (e.g., because a complete file has been sent), either party can send a Jingle session-terminate action.

The other party then acknowledges the session-terminate and the Jingle session is finished.

The exchange of candidates might result 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 the bytestream, such as those described in &xtls;.

The ®ISTRAR; shall add to its registry of Jingle transport methods a definition for the "jingle-s5b" transport method. The registry submission is as follows: