<title>Jingle SOCKS5 Bytestreams Transport Method</title>
<abstract>This specification defines a Jingle transport method that results in sending data via the SOCKS5 Bytestreams (S5B) protocol defined in XEP-0065. Essentially this transport method reuses XEP-0065 semantics for sending the data and defines native Jingle methods for starting and ending an S5B session.</abstract>
<remark><p>Per feedback from the XMPP Council, changed some implementation guidelines from normative to informative and modified the security considerations to remove user interface recommendations and the recommendation to use XTLS (since it is not longer being actively developed).</p></remark>
<remark><p>Added proxy-error action; added a block-size attribute in the transport-accept action when negotiating fallback to another transport, analogous to changes in XEP-0261; editorial review.</p></remark>
<remark><p>Major update to make Jingle S5B inherit more features from ICE and ICE-TCP. Added priorities and candidate identifiers. Renamed streamhost element to candidate element. Updated candidate selection to use priorities, and it is now required for both clients to send a candidate-used or candidate-error notification. Defined type attribute to differentiate between various kinds of candidates. More clearly described how S5B negotiation is completed, including an activated notification from responder to initiator when the candidate used is a proxy. Noted reuse of fast-mode methodology from S5B optimization specification. Because of incompatibilities with the previous version, changed the namespace to urn:xmpp:jingle:transports:s5b:1.</p></remark>
<remark><p>Minor changes to track modifications to XEP-0166; updated security considerations for consistency with other transport methods; added section on service discovery.</p></remark>
<remark><p>Specified that the responder can send <streamhost/> candidates, consistent with the earlier S5B optimization extension; this change required the introduction of streamhost-used and streamhost-error. Also added text to encourage the use of NAT-assisting protocols.</p></remark>
<p>&xep0166; defines a framework for negotiating and managing data sessions over XMPP. In order to provide a flexible framework, the base Jingle specification defines neither data transport methods nor application formats, leaving that up to separate specifications. The current document defines a transport method for establishing and managing data exchanges between XMPP entities using the existing SOCKS5 Bytestreams (S5B) protocol specified in &xep0065;. This "jingle-s5b" method results in a streaming transport method suitable for use in Jingle application types where packet loss cannot be tolerated (e.g., file transfer). Jingle-S5B reuses the protocol flow from <cite>XEP-0065</cite> for the communication with a SOCKS5 streamhost; the communication between two clients to negotiate the possible candidates differs from <cite>XEP-0065</cite> and shares similarities with &xep0176;</p>
<p>This flow is illustrated in the following examples (to simplify the presentation these use an "example" application instead of a real application type).</p>
<p>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:</p>
<ul>
<li>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).</li>
<li>Using the client's external IP address as discovered through an assisting NAT protocol or other means.</li>
<p>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 encourages the responder to also send offers, effectively equivalent to the "fast-mode" for SOCKS5 Bytestreams as previously described at <<linkurl='http://delta.affinix.com/specs/stream.html'>http://delta.affinix.com/specs/stream.html</link>>.</p>
<p>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 <cite>XEP-0065</cite> streamhosts and ICE candidates used in <cite>XEP-0176</cite>.</p>
<p>Just as with the &QUERY; element from <cite>XEP-0065</cite>, here the <transport/> element that contains the candidates MUST possess a 'sid' attribute that specifies the Stream ID for this bytestream. The <transport/> element MAY possess a 'mode' attribute whose value is "tcp" (the default) or "udp" (for the latter, see Section 8 of <cite>XEP-0065</cite>). The <transport/> element MAY also possess a 'dstaddr' attribute whose value is the initiator's calculated hash value for the SOCKS5 DST.ADDR field (see Section 7 of <cite>XEP-0065</cite>).</p>
<p>Note: As explained in Section 5.3.2 of <cite>XEP-0065</cite>, the DST.ADDR value in the SOCKS5 negotiation is the SHA1 hash of (SID + Requester JID + Target JID). In the context of Jingle SOCKS5 Bytestreams, the "Requester JID" is the XMPP address of the initiator and the "Target JID" is the XMPP address of the responder (in the examples used here, the values are "romeo@montague.lit/orchard" and "juliet@capulet.lit/balcony" respectively).</p>
<p>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.</p>
<p>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 an S5B 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. 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 S5B proxies.</p>
<p>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.</p>
<p>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. An implementation SHOULD use the following formula:</p>
<p>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.</p>
<p>After receiving its peer's candidates, a client start to connect to them in order of the priority. A detailed description of the protocol can be found in <cite>XEP-0065</cite>.</p>
<p>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 sent by its peer if it has not tried all candidates with a higher priority than the one successfully used by the peer.</p>
<p>If a client is unable to connect to <em>any</em> 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).</p>
<li>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 (but not necessarily) IBB; see the <linkurl='#fallback'>Fallback Methods</link> section of this document for details.</li>
<li>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.</li>
<li>If both parties send a candidate-used notification but the candidates have a different priority, then the candidate with the higher priority shall be considered the nominated candidate.</li>
<li>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 <cite>XEP-0166</cite>).</li>
<p>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 <cite>XEP-0065</cite>); it does so by sending a transport-info message containing an <activated/> element as follows.</p>
<p>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 failing party shall send a transport-info message containing an <proxy-error/> element.</p>
<p>The parties shall then consider the bytestream unsuccessful and SHOULD attempt to fall back to another transport as described in <linkurl='#fallback'>Fallback Methods</link>.</p>
<p>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.</p>
<examplecaption="Initiator terminates the session"><