XMPP Working Group L. Stout, Ed.
Internet-Draft &yet
Intended status: Standards Track J. Moffitt
Expires: August 18, 2014 Mozilla
E. Cestari
cstar industries
February 14, 2014

An XMPP Sub-protocol for WebSocket


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.

Status of This Memo

This Internet-Draft is submitted in full conformance with the provisions of BCP 78 and BCP 79.

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This Internet-Draft will expire on August 18, 2014.

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Table of Contents

1. Introduction

Applications using 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] now exists to solve these kinds of problems. The WebSocket protocol is a bi-directional 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 the Extensible Messaging and Presence Protocol (XMPP).

2. Terminology

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 whose top-level element name is message. In this document, the word "message" will mean a WebSocket message, not an XMPP message stanza (see Section 3.2).

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].

3. XMPP Sub-Protocol

3.1. Handshake

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"
          version="1.0" />

3.2. Messages

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.

Unless noted in text, the word "message" will mean a WebSocket message composed of text data frames.

3.3. XMPP Stream Setup

The first message sent after the handshake is complete MUST be an <open /> element using the "urn:ietf:params:xml:ns:xmpp-framing" namespace, whose 'from', 'id', 'to' and 'version' attributes mirror those in 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.

3.4. Stream Errors

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.

3.5. Closing the Connection

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.

3.5.1. see-other-uri

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" />

3.6. Stanzas

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 messsages.

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" />

<error xmlns="http://etherx.jabber.org/streams">
  <host-unknown xmlns='urn:ietf:params:xml:ns:xmpp-streams'/>

<message xmlns="jabber:client" xml:lang="en">
  <body>Every WebSocket message is parsable by itself.</body>

3.7. Stream Restarts

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"
         version="1.0" />

3.8. Pings and Keepalives

XMPP servers send whitespace pings as keepalives between stanzas, 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.

3.9. Use of TLS

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.

3.10. Stream Management

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.

4. Discovering Connection Method

The XMPP extension Discovering Alternate XMPP Connection Methods [XEP-0156] provides mechanisms to discover the additional information needed to connect to an XMPP server outside of the procedure defined in in Section 3 of [RFC6120].

Servers MAY expose such discovery information, and clients MAY use such information to determine the WebSocket endpoint for a server.

Use of the HTTP lookup method in [XEP-0156] 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.

5. IANA Considerations

5.1. WebSocket Subprotocol Name

This specification requests IANA to register the WebSocket XMPP sub-protocol under the "WebSocket Subprotocol Name" Registry with the following data:

Subprotocol Identifier:
Subprotocol Common Name:
WebSocket Transport for the Extensible Messaging and Presence Protocol (XMPP)
Subprotocol Definition:

[[ NOTE TO RFC EDITOR: Please replace "XXXX" with the number assigned to this document upon publication as an RFC. ]]

5.2. URN Sub-Namespace

A URN sub-namespace for framing of Extensible Messaging and Presence Protocol (XMPP) streams is defined as follows.

This is the XML namespace name for framing of Extensible Messaging and Presence Protocol (XMPP) streams as defined by RFC XXXX.
Registrant Contact:
IESG <iesg@ietf.org>

[[ NOTE TO RFC EDITOR: Please replace "XXXX" with the number assigned to this document upon publication as an RFC. ]]

6. Security Considerations

Since application level TLS cannot be used (see Section 3.9), applications which need to protect the privacy of the XMPP traffic need to do so 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 HTTP discovery method in [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.

7. References

7.1. Normative References

[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.
[XEP-0156] Hildebrand, J. and P. Saint-Andre, "Discovering Alternative XMPP Connection Methods", XSF XEP 0156, June 2007.

7.2. Informative References

[RFC6121] Saint-Andre, P., "Extensible Messaging and Presence Protocol (XMPP): Instant Messaging and Presence", RFC 6121, March 2011.
[RFC6202] Loreto, S., Saint-Andre, P., Salsano, S. and G. Wilkins, "Known Issues and Best Practices for the Use of Long Polling and Streaming in Bidirectional HTTP", RFC 6202, April 2011.
[RFC6455] Fette, I. and A. Melnikov, "The WebSocket Protocol", RFC 6455, December 2011.
[XEP-0124] Paterson, I., Smith, D., Saint-Andre, P. and J. Moffitt, "Bidirectional-streams Over Synchronous HTTP (BOSH)", XSF XEP 0124, July 2010.
[XEP-0198] Karneges, J., Saint-Andre, P., Hildebrand, J., Forno, F., Cridland, D. and M. Wild, "Stream Management", XSF XEP 0198, June 2011.
[XEP-0199] Saint-Andre, P., "XMPP Ping", XSF XEP 0199, June 2009.
[XEP-0206] Paterson, I. and P. Saint-Andre, "XMPP Over BOSH", XSF XEP 0206, July 2010.
[XML-SCHEMA] Thompson, H., Maloney, M., Mendelsohn, N. and D. Beech, "XML Schema Part 1: Structures Second Edition", World Wide Web Consortium Recommendation REC-xmlschema-1-20041028, October 2004.

Appendix A. XML Schema

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:element name='open'>
        <xs:extension base='empty'>
          <xs:attribute name='from' type='xs:string' 
          <xs:attribute name='id' type='xs:string' 
          <xs:attribute name='see-other-uri' type='xs:anyURI' 
          <xs:attribute name='to' type='xs:string' 
          <xs:attribute name='version' type='xs:decimal' 
          <xs:attribute ref='xml:lang' 

  <xs:element name='close'>
        <xs:extension base='empty'>
          <xs:attribute name='from' type='xs:string' 
          <xs:attribute name='id' type='xs:string' 
          <xs:attribute name='see-other-uri' type='xs:anyURI' 
          <xs:attribute name='to' type='xs:string' 
          <xs:attribute name='version' type='xs:decimal' 
          <xs:attribute ref='xml:lang' 

  <xs:simpleType name='empty'>
    <xs:restriction base='xs:string'>
      <xs:enumeration value=''/>


Authors' Addresses

Lance Stout (editor) &yet EMail: lance@andyet.net
Jack Moffitt Mozilla EMail: jack@metajack.im
Eric Cestari cstar industries EMail: eric@cestari.info