| XMPP Working Group | L. Stout, Ed. |
| Internet-Draft | &yet |
| Intended status: Standards Track | J. Moffitt |
| Expires: September 15, 2014 | Mozilla |
| E. Cestari | |
| cstar industries | |
| March 14, 2014 |
An XMPP Sub-protocol for WebSocket
draft-ietf-xmpp-websocket-02
This document defines a binding for the XMPP protocol over a WebSocket transport layer. A WebSocket binding for XMPP provides higher performance than the current HTTP binding for XMPP.
This Internet-Draft is submitted in full conformance with the provisions of BCP 78 and BCP 79.
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Applications using the Extensible Messaging and Presence Protocol (XMPP) (see [RFC6120] and [RFC6121]) on the Web currently make use of BOSH (see [XEP-0124] and [XEP-0206]), an XMPP binding to HTTP. BOSH is based on the HTTP long polling technique, and it suffers from high transport overhead compared to XMPP's native binding to TCP. In addition, there are a number of other known issues with long polling [RFC6202], which have an impact on BOSH-based systems.
It would be much better in most circumstances to avoid tunneling XMPP over HTTP long polled connections and instead use the XMPP protocol directly. However, the APIs and sandbox that browsers have provided do not allow this. The WebSocket protocol [RFC6455] exists to solve these kinds of problems. The WebSocket protocol is a bidirectional protocol that provides a simple message-based framing layer over raw sockets and allows for more robust and efficient communication in web applications.
The WebSocket protocol enables two-way communication between a client and a server, effectively emulating TCP at the application layer and therefore overcoming many of the problems with existing long-polling techniques for bidirectional HTTP. This document defines a WebSocket sub-protocol for XMPP.
The basic unit of framing in the WebSocket protocol is called a message. In XMPP, the basic unit is the stanza, which is a subset of the first-level children of each document in an XMPP stream (see Section 9 of [RFC6120]). XMPP also has a concept of messages, which are stanzas with a top-level element of <message/>. In this document, the word "message" will mean a WebSocket message, not an XMPP message stanza, unless otherwise noted.
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this document are to be interpreted as described in [RFC2119].
The XMPP sub-protocol is used to transport XMPP over a WebSocket connection. The client and server agree to this protocol during the WebSocket handshake (see Section 1.3 of [RFC6455]).
During the WebSocket handshake, the client MUST include the |Sec-WebSocket-Protocol| header in its handshake, and the value |xmpp| MUST be included in the list of protocols. The reply from the server MUST also contain |xmpp| in its own |Sec-WebSocket-Protocol| header in order for an XMPP sub-protocol connection to be established.
Once the handshake is complete, WebSocket messages sent or received will conform to the protocol defined in the rest of this document.
C: GET /xmpp-websocket HTTP/1.1
Host: example.com
Upgrade: websocket
Connection: Upgrade
Sec-WebSocket-Key: dGhlIHNhbXBsZSBub25jZQ==
Origin: http://example.com
...
Sec-WebSocket-Protocol: xmpp
Sec-WebSocket-Version: 13
S: HTTP/1.1 101 Switching Protocols
Upgrade: websocket
Connection: Upgrade
...
Sec-WebSocket-Accept: s3pPLMBiTxaQ9kYGzzhZRbK+xOo=
Sec-WebSocket-Protocol: xmpp
[WebSocket connection established]
C: <open xmlns="urn:ietf:params:xml:ns:xmpp-framing"
to="example.com"
version="1.0" />
Data frame messages in the XMPP sub-protocol MUST be of the text type and contain UTF-8 encoded data. The close control frame's contents are specified in Section 3.5. Control frames other than close are not restricted.
The first message sent after the handshake is complete MUST be an <open/> element qualified by the "urn:ietf:params:xml:ns:xmpp-framing" namespace. The 'from', 'id', 'to', and 'version' attributes of this element mirror those of the XMPP opening stream tag as defined for the 'http://etherx.jabber.org/streams' namespace in XMPP [RFC6120]. The '<' character of the open tag MUST be the first character of the text payload.
The server MUST respond with an <open /> element, or a <close /> element (see Section 3.5.1).
Clients MUST NOT attempt to multiplex XMPP streams for multiple JIDs over the same WebSocket.
Stream level errors in XMPP are terminal. Should such an error occur, the server MUST send the stream error as a complete element in a message to the client.
If the error occurs during the opening of a stream, the server MUST send the initial open element response, followed by the stream level error in a second WebSocket message frame. The server MUST then close the connection as specified in Section 3.5.
Either the server or the client may close the connection at any time. Before closing the connection, the closing party SHOULD close the XMPP stream, if it has been established, by sending a message with the <close/> element, qualified by the "urn:ietf:params:xml:ns:xmpp-framing" namespace. The stream is considered closed when a corresponding <close/> element is received from the other party.
To initiate closing the WebSocket connection, the closing party MUST send a normal WebSocket close message with an empty body. The connection is considered closed when a matching close message is received (see Section 1.4 of [RFC6455]).
An example of ending an XMPP over WebSocket session by first closing the XMPP stream layer and then the WebSocket connection layer:
Client (XMPP WSS) Server | | | | | | <close xmlns="urn:ietf:params:xml:ns:xmpp-framing /> | | | |------------------------------------------------------------>| | | | <close xmlns="urn:ietf:params:xml:ns:xmpp-framing" /> | | | |<------------------------------------------------------------| | | | | | | | (XMPP Stream Closed) | | | +-------------------------------------------------------------+ | | | | WS CLOSE FRAME | |------------------------------------------------------------------>| | WS CLOSE FRAME | |<------------------------------------------------------------------| | | | (Connection Closed) | +-------------------------------------------------------------------+
If a client closes the WebSocket connection without closing the XMPP stream after having enabled stream management (see Section 3.10), the server SHOULD keep the XMPP session alive for a period of time based on server policy, as specified in [XEP-0198]. If the client has not negotiated the use of [XEP-0198], there is no distinction between a stream that was closed as described above and a simple disconnection; the stream is then considered implicitly closed and the XMPP session ended.
If the server (or a connection mananger intermediary) wishes to instruct the client to move to a different WebSocket endpoint (e.g. for load balancing purposes), the server MAY send a <close/> element and set the "see-other-uri" attribute to the URI of the new WebSocket endpoint.
Clients MUST NOT accept suggested endpoints with a lower security context (e.g. moving from a "wss://" endpoint to a "ws://" endpoint).
An example of the server closing a stream and instructing the client to connect at a different WebSocket endpoint:
S: <close xmlns="urn:ietf:params:xml:ns:xmpp-framing"
see-other-uri="wss://otherendpoint.example/xmpp-bind" />
Every XMPP stanza or other XML element sent directly over the XMPP stream (e.g. <features xmlns="http://etherx.jabber.org/streams"/>) MUST be sent in its own message. As such, every WebSocket text message that is received MUST be a complete and parsable XML fragment, with all relevant xmlns and xml:lang declarations specified.
As it is already mandated that the content of each message is UTF-8 encoded, XML text declarations SHOULD NOT be included in messages.
Examples of WebSocket messages that contain independently parsable XML fragments (note that for stream features and errors, there is no parent context element providing the "stream" namespace prefix as in [RFC6120], and thus the stream namespace MUST be declared):
<features xmlns="http://etherx.jabber.org/streams"> <bind xmlns="urn:ietf:params:xml:ns:xmpp-bind"/> </features> <error xmlns="http://etherx.jabber.org/streams"> <host-unknown xmlns='urn:ietf:params:xml:ns:xmpp-streams'/> </error> <message xmlns="jabber:client" xml:lang="en"> <body>Every WebSocket message is parsable by itself.</body> </message>
After successful SASL authentication, an XMPP stream needs to be restarted. In these cases, as soon as the message is sent (or received) containing the success indication, both the server and client streams are implicitly closed, and new streams need to be opened. The client MUST open a new stream as in Section 3.3 and MUST NOT send a closing <close/> element.
S: <success xmlns="urn:ietf:params:xml:ns:xmpp-sasl" />
[Streams implicitly closed]
C: <open xmlns="urn:ietf:params:xml:ns:xmpp-framing"
to="example.com"
version="1.0" />
XMPP servers often send "whitespace keepalives" (see Section 4.6.1 of [RFC6120]) between stanzas to maintain an XML stream, and XMPP clients can do the same as these extra whitespace characters are not significant in the protocol. Servers and clients SHOULD use WebSocket ping control frames instead for this purpose.
In some cases, the WebSocket connection might be served by an intermediary connection manager and not the XMPP server. In these situations, the use of WebSocket ping messages are insufficient to test that the XMPP stream is still alive. Both the XMPP Ping extension [XEP-0199] and the XMPP Stream Management extension [XEP-0198] provide mechanisms to ping the XMPP server, and either extension (or both) MAY be used to determine the state of the connection.
TLS cannot be used at the XMPP sub-protocol layer because the sub-protocol does not allow for raw binary data to be sent. Instead, enabling TLS SHOULD be done at the WebSocket layer using secure WebSocket connections via the |wss| URI scheme. (See Section 10.6 of [RFC6455].)
Because TLS is to be provided outside of the XMPP sub-protocol layer, a server MUST NOT advertise TLS as a stream feature (see Section 4.6 of [RFC6120]), and a client MUST ignore any advertised TLS stream feature, when using the XMPP sub-protocol.
In order to alleviate the problems of temporary disconnections, the XMPP Stream Management extension [XEP-0198] MAY be used to confirm when stanzas have been received by the server.
In particular, the use of session resumption in [XEP-0198] MAY be used to allow for recreating the same stream session state after a temporary network unavailability or after navigating to a new URL in a browser.
Section 3 of [RFC6120] defines a procedure for connecting to an XMPP server, including ways to discover the TCP/IP address and port of the server. When using the WebSocket binding as specified in this document (instead of the TCP binding as specified in [RFC6120]), a client needs an alternative way to discover information about the server's connection methods, since web browsers and other WebSocket-capable software applications typically cannot obtain such information from the Domain Name System.
The alternative lookup process uses Web Host Metadata [RFC6415] and Web Linking [RFC5988], where the link relation type is "urn:xmpp:alt-connections:websocket" as described in Discovering Alternate XMPP Connection Methods [XEP-0156]. An example follows.
<XRD xmlns='http://docs.oasis-open.org/ns/xri/xrd-1.0'>
<Link rel="urn:xmpp:alt-connections:websocket"
href="wss://webcm.example.net:443/ws" />
</XRD>
Servers MAY expose discovery information using host-meta documents, and clients MAY use such information to determine the WebSocket endpoint for a server.
Use of web-host metadata MAY be used to establish trust between the XMPP server domain and the WebSocket endpoint, particularly in multi-tenant situations where the same WebSocket endpoint is serving multiple XMPP domains.
This specification requests IANA to register the WebSocket XMPP sub-protocol under the "WebSocket Subprotocol Name" Registry with the following data:
A URN sub-namespace for framing of Extensible Messaging and Presence Protocol (XMPP) streams is defined as follows.
Since application level TLS cannot be used (see Section 3.9), applications need to protect the privacy of XMPP traffic at the WebSocket or other appropriate layer.
Browser based applications are not able to inspect and verify at the application layer the certificate used for the WebSocket connection to ensure that it corresponds to the domain specified as the "to" address of the XMPP stream. For hosts whose domain matches the origin for the WebSocket connection, that check is already performed by the browser. However, in situations where the domain of the XMPP server might not match the origin for the WebSocket endpoint (especially multi-tenant hosting situations), the web host metadata method (see [RFC6415] and [XEP-0156]) MAY be used to delegate trust from the XMPP server domain to the WebSocket origin.
When presented with a new WebSocket endpoint via the "see-other-uri" attribute of a <close/> element, clients MUST NOT accept the suggestion if the security context of the new endpoint is lower than the current one in order to prevent downgrade attacks from a "wss://" endpoint to "ws://".
The Security Considerations for both WebSocket (see Section 10 of [RFC6455] and XMPP (see Section 13 of [RFC6120]) apply to the WebSocket XMPP sub-protocol.
| [RFC2119] | Bradner, S., "Key words for use in RFCs to Indicate Requirement Levels", BCP 14, RFC 2119, March 1997. |
| [RFC6120] | Saint-Andre, P., "Extensible Messaging and Presence Protocol (XMPP): Core", RFC 6120, March 2011. |
| [RFC6455] | Fette, I. and A. Melnikov, "The WebSocket Protocol", RFC 6455, December 2011. |
The following schema formally defines the 'urn:ietf:params:xml:ns:xmpp-framing' namespace used in this document, in conformance with W3C XML Schema [XML-SCHEMA]. Because validation of XML streams and stanzas is optional, this schema is not normative and is provided for descriptive purposes only.
<?xml version='1.0' encoding='UTF-8'?>
<xs:schema
xmlns:xs='http://www.w3.org/2001/XMLSchema'
targetNamespace='urn:ietf:params:xml:ns:xmpp-framing'
xmlns='urn:ietf:params:xml:ns:xmpp-framing'
elementFormDefault='unqualified'>
<xs:element name='open'>
<xs:complexType>
<xs:simpleContent>
<xs:extension base='empty'>
<xs:attribute name='from' type='xs:string'
use='optional'/>
<xs:attribute name='id' type='xs:string'
use='optional'/>
<xs:attribute name='to' type='xs:string'
use='optional'/>
<xs:attribute name='version' type='xs:decimal'
use='optional'/>
<xs:attribute ref='xml:lang'
use='optional'/>
</xs:extension>
</xs:simpleContent>
</xs:complexType>
</xs:element>
<xs:element name='close'>
<xs:complexType>
<xs:simpleContent>
<xs:extension base='empty'>
<xs:attribute name='from' type='xs:string'
use='optional'/>
<xs:attribute name='id' type='xs:string'
use='optional'/>
<xs:attribute name='see-other-uri' type='xs:anyURI'
use='optional'/>
<xs:attribute name='to' type='xs:string'
use='optional'/>
<xs:attribute name='version' type='xs:decimal'
use='optional'/>
<xs:attribute ref='xml:lang'
use='optional'/>
</xs:extension>
</xs:simpleContent>
</xs:complexType>
</xs:element>
<xs:simpleType name='empty'>
<xs:restriction base='xs:string'>
<xs:enumeration value=''/>
</xs:restriction>
</xs:simpleType>
</xs:schema>