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Message Forwarding This document defines a protocol to forward a message from one entity to another. &LEGALNOTICE; 0297 Proposed 2012-03-09 Standards Track Standards XMPP Core forwarding Matthew Wild me@matthewwild.co.uk me@matthewwild.co.uk &ksmith; 0.3 2011-07-11 ks

Made security considerations more explicit.

0.2 2011-07-06 psa

Corrected XML namespace to reflect official publication.

0.1 2011-04-11 psa

Initial published version.

0.0.1 2011-03-22 mw/ks

First draft.

There are many situations is which an entity needs to forward a previously sent message 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 forwarded messages. This format can be used in other specifications requiring the forwarding of messages, or used in isolation for a user to forward a message to another user (rather like email forwards).

Several properties are desirable when forwarding messages:

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

Yet I should kill thee with much cherishing. ]]>

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

A most courteous exposition! Yet I should kill thee with much cherishing. ]]>
  • Forwarded messages SHOULD include all relevant child elements of the original message by default. However, an implementation MAY omit elements it deems irrelevant and safe to discard. An example would be omitting &xep0085; elements 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 message. The format of this element is described in &xep0203;.
  • 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.

Forwarding messages can reveal information about the original sender, including possible presence leaks as well as the message 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:

Receipt of a forwarded message from a third-party does not guarantee that the original message was actually received, or that the content has not been modified, by the forwarder. Integrity of the original message can only be determined through digital signing mechanisms such as described in &xep0285; and &xep0290;.

Considering the above an end-user client should take special care in its rendering of a forwarded message 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).

While security considerations are ultimately dependent upon the specifications using the format defined herein, forwarding introduces scope for message 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 messages 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.

The protocol documented by this schema is defined in XEP-xxxx: http://www.xmpp.org/extensions/inbox/forwarding.html ]]>

Thanks to Matt Miller and Florian Zeitz for feedback.