XEP-0297: Stanza Forwarding

NOTICE: This document is currently within Last Call or under consideration by the XMPP Council for advancement to the next stage in the XSF standards process. The Last Call ends on 2012-12-14. Please send your feedback to the standards@xmpp.org discussion list.

1. Introduction
2. Requirements
3. Forwarding a stanza
    3.1. Overview
    3.2. Business rules
4. Determining Support
5. Security Considerations
    5.1. As-is
    5.2. Extensions
6. XML Schema
7. Acknowledgements

Appendices
    A: Document Information
    B: Author Information
    C: Legal Notices
    D: Relation to XMPP
    E: Discussion Venue
    F: Requirements Conformance
    G: Notes
    H: Revision History

1. Introduction

There are many situations is which an entity needs to forward a previously sent stanza to another entity, such as forwarding an interesting message to a friend, or a server forwarding stored messages from an archive. Here we specify a simple encapsulation method for such forwards. This format can be used in other specifications requiring the forwarding of stanzas, or used in isolation for a user to forward a message to another user (rather like email forwards).

2. Requirements

3. Forwarding a stanza

3.1 Overview

Let us suppose that a Romeo receives a message from Juliet:

Example 1. Receiving a message

      <message from='juliet@capulet.lit/orchard'
               id='0202197'
               to='romeo@montague.lit'
               type='chat'>
        <body>Yet I should kill thee with much cherishing.</body>
        <mood xmlns='http://jabber.org/protocol/mood'>
            <amorous/>
        </mood>
      </message>

To forward this to Mercutio, Romeo would send a new message with a <forwarded/> payload of namespace 'urn:xmpp:forward:0'.

Example 2. Forwarding a message

      <message to='mercutio@verona.lit' from='romeo@montague.lit/orchard' type='chat' id='28gs'>
        <body>A most courteous exposition!</body>
        <forwarded xmlns='urn:xmpp:forward:0'>
          <delay xmlns='urn:xmpp:delay' stamp='2010-07-10T23:08:25Z'/>
          <message from='juliet@capulet.lit/orchard'
                   id='0202197'
                   to='romeo@montague.lit'
                   type='chat'
                   xmlns='jabber:client'>
              <body>Yet I should kill thee with much cherishing.</body>
              <mood xmlns='http://jabber.org/protocol/mood'>
                  <amorous/>
              </mood>
          </message>
        </forwarded>
      </message>

3.2 Business rules

Forwarded stanzas SHOULD include all relevant child elements of the original stanza by default. However, an implementation MAY omit elements it deems irrelevant and safe to discard. An example would be omitting Chat State Notifications [1] elements from <message> stanzas which typically do not make sense outside the context of a conversation session. However it should be noted that removing such elements can invalidate any digital signature on a stanza. If preserving a signature is important in the context this extension is used then child elements SHOULD NOT be removed.
The forwarding entity SHOULD add a <delay/> child to the <forwarded/> element to indicate to the recipient the date/time that the forwarding entity received the original stanza. The format of this element is described in Delayed Delivery [2].
The namespace of the forwarded stanza MUST be preserved (this is typically 'jabber:client'). If no 'xmlns' is set for the stanza then as per XML namespacing rules it would inherit the 'urn:xmpp:forward:0' namespace, which is wrong.
When this extension is employed simply for a user to forward a given message to a contact, the outer <message/> SHOULD contain a body (even if empty) and a receiving client should pay particular attention to ensure it renders both the sender's text and the forwarded message unambiguously.
When a forwarded stanza forms part of an encapsulating protocol, the <forwarded/> element SHOULD be a child of a tag of that protocol, and SHOULD NOT be included as a direct child of the transmitted stanza.

4. Determining Support

Clients that implement this specification to display simple forwarded messages (i.e. those not part of another extension) SHOULD indicate their support via the 'urn:xmpp:forward:0' feature.

<iq xmlns='jabber:client'
    type='get'
    from='romeo@montague.lit/orchard'
    to='juliet@capulet.lit/balcony'
    id='info1'>
  <query xmlns='http://jabber.org/protocol/disco#info'/>
</iq>

<iq xmlns='jabber:client'
    type='result'
    to='romeo@montague.net/orchard'
    from='juliet@capulet.lit/balcony'
    id='info1'>
  <query xmlns='http://jabber.org/protocol/disco#info'>
...
    <feature var='urn:xmpp:forward:0'/>
...
  </query>
</iq>

Senders SHOULD NOT forward messages using this protocol to recipients that have not indicated support for it. However they may still format the forward as plain text inside the <body> of a standard message stanza for compatibility with clients lacking support. Such a plain text version SHOULD NOT be included in a stanza using this extension (e.g. as a 'fallback'), as receiving entities are expected to display the <body> of a message as well as any forwarded stanza therein.

5. Security Considerations

Forwarding stanzas can reveal information about the original sender, including possible presence leaks as well as the stanza payloads themselves. Any extensions using this format must therefore consider the implications of this.

Forwarding can either be used as-is, or in the context of another specification, with different security considerations as described below.

5.1 As-is

Receipt of a forwarded stanza from a third-party does not guarantee that the original stanza was actually received, or that the content has not been modified, by the forwarder. Integrity of the original stanza can only be determined through cryptographic signatures for example, which are beyond the scope of this specification.

Considering the above an end-user client should take special care in its rendering of forwarded stanzas, such as forwarded messages, to ensure that the user cannot mistake it for a message received directly from the original sender.

An entity SHOULD NOT trust that forwards are genuine when receiving them unprovoked (i.e. outside the scope of another specification).

Forwarded stanzas MUST NOT be processed as if they were non-forwarded stanzas. Furthermore a client SHOULD ignore non-<message> stanzas that are not embedded within another extension.

5.2 Extensions

While security considerations are ultimately dependent upon the specifications using the format defined herein, forwarding introduces scope for stanza forgery such that authors of derivative specifications will need to address security considerations themselves. These need to cover which entities a client should accept forwards from, and which entities those are permitted to forward stanzas for. For example, a specification may choose to only trust forwards if they are received from the user's client, another client on the bare JID, or the user's server.

6. XML Schema

<?xml version='1.0' encoding='UTF-8'?>

<xs:schema
    xmlns:xs='http://www.w3.org/2001/XMLSchema'
    targetNamespace='urn:xmpp:forward:0'
    xmlns='urn:xmpp:forward:0'
    elementFormDefault='qualified'>

  <xs:import namespace='jabber:client'/>
  <xs:import namespace='jabber:server'/>

  <xs:element name='forwarded'>
    <xs:complexType>
      <xs:sequence xmlns:delay='urn:xmpp:delay'
                   xmlns:client='jabber:client'
                   xmlns:server='jabber:server'>
        <xs:element ref='delay:delay' minOccurs='0' maxOccurs='1' />
        <xs:choice minOccurs='0' maxOccurs='1'>
          <xs:choice>
            <xs:element ref='client:message'/>
            <xs:element ref='client:presence'/>
            <xs:element ref='client:iq'/>
          </xs:choice>
          <xs:choice>
            <xs:element ref='server:message'/>
            <xs:element ref='server:presence'/>
            <xs:element ref='server:iq'/>
          </xs:choice>
        </xs:choice>
      </xs:sequence>
    </xs:complexType>
  </xs:element>
</xs:schema>

7. Acknowledgements

Thanks to Matt Miller, Florian Zeitz, Jefry Lagrange, Kim Alvefur and Kurt Zeilenga for their feedback.

Appendices

Appendix A: Document Information

Series: XEP
Number: 0297
Publisher: XMPP Standards Foundation
Status: Proposed
Type: Standards Track
Version: 0.5
Last Updated: 2013-06-05
Approving Body: XMPP Council
Dependencies: XMPP Core
Supersedes: None
Superseded By: None
Short Name: forwarding
Schema: <>
Source Control: HTML
This document in other formats: XML PDF

Appendix B: Author Information

Appendix C: Legal Notices

Copyright

Permissions

Permission is hereby granted, free of charge, to any person obtaining a copy of this specification (the "Specification"), to make use of the Specification without restriction, including without limitation the rights to implement the Specification in a software program, deploy the Specification in a network service, and copy, modify, merge, publish, translate, distribute, sublicense, or sell copies of the Specification, and to permit persons to whom the Specification is furnished to do so, subject to the condition that the foregoing copyright notice and this permission notice shall be included in all copies or substantial portions of the Specification. Unless separate permission is granted, modified works that are redistributed shall not contain misleading information regarding the authors, title, number, or publisher of the Specification, and shall not claim endorsement of the modified works by the authors, any organization or project to which the authors belong, or the XMPP Standards Foundation.

Disclaimer of Warranty

## NOTE WELL: This Specification is provided on an "AS IS" BASIS, WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, express or implied, including, without limitation, any warranties or conditions of TITLE, NON-INFRINGEMENT, MERCHANTABILITY, or FITNESS FOR A PARTICULAR PURPOSE. ##

Limitation of Liability

In no event and under no legal theory, whether in tort (including negligence), contract, or otherwise, unless required by applicable law (such as deliberate and grossly negligent acts) or agreed to in writing, shall the XMPP Standards Foundation or any author of this Specification be liable for damages, including any direct, indirect, special, incidental, or consequential damages of any character arising from, out of, or in connection with the Specification or the implementation, deployment, or other use of the Specification (including but not limited to damages for loss of goodwill, work stoppage, computer failure or malfunction, or any and all other commercial damages or losses), even if the XMPP Standards Foundation or such author has been advised of the possibility of such damages.

IPR Conformance

This XMPP Extension Protocol has been contributed in full conformance with the XSF's Intellectual Property Rights Policy (a copy of which can be found at <http://xmpp.org/about-xmpp/xsf/xsf-ipr-policy/> or obtained by writing to XMPP Standards Foundation, 1899 Wynkoop Street, Suite 600, Denver, CO 80202 USA).

Appendix D: Relation to XMPP

The Extensible Messaging and Presence Protocol (XMPP) is defined in the XMPP Core (RFC 6120) and XMPP IM (RFC 6121) specifications contributed by the XMPP Standards Foundation to the Internet Standards Process, which is managed by the Internet Engineering Task Force in accordance with RFC 2026. Any protocol defined in this document has been developed outside the Internet Standards Process and is to be understood as an extension to XMPP rather than as an evolution, development, or modification of XMPP itself.

Appendix E: Discussion Venue

The primary venue for discussion of XMPP Extension Protocols is the <standards@xmpp.org> discussion list.

Appendix F: Requirements Conformance

The following requirements keywords as used in this document are to be interpreted as described in RFC 2119: "MUST", "SHALL", "REQUIRED"; "MUST NOT", "SHALL NOT"; "SHOULD", "RECOMMENDED"; "SHOULD NOT", "NOT RECOMMENDED"; "MAY", "OPTIONAL".

Appendix G: Notes

1. XEP-0085: Chat State Notifications <http://xmpp.org/extensions/xep-0085.html>.

2. XEP-0203: Delayed Delivery <http://xmpp.org/extensions/xep-0203.html>.

Appendix H: Revision History

Version 0.5 (2013-06-05)

Incorporated Last Call feedback, including; clarify that client handling of non-message stanzas, add a disco feature, remove references to deferred XEPs, tighten up the schema to allow only stanzas to be forwarded.

(mw)

Version 0.4 (2012-07-05)

Added recommendation that forwarded messages as part of another specification should be nested under an element of that protocol's namespace.

Adapted text to indicate that stanzas other than messages may be forwarded. Updated title to reflect this.

(mw)

Version 0.3 (2011-07-11)

Made security considerations more explicit.

(ks)

Version 0.2 (2011-07-06)

Corrected XML namespace to reflect official publication.

(psa)

Version 0.1 (2011-04-11)

Initial published version.