XEP-0249: Direct MUC Invitations

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.

Table of Contents

1. Introduction

Multi-User Chat [1] defines a protocol for groupchat over XMPP. That specification includes a method for inviting a contact to a room, where the invitation is mediated by the room itself: the user sends the invitation to the room, which in turn sends it to the contact. Unfortunately, a mediated invitation might not be delivered to the contact, for example if the contact blocks communication with entities not in its roster as specified in Privacy Lists [2]. As privacy lists have become more common, MUC invitations have been increasingly blocked at the server side, resulting in an undesirable user experience. Therefore, this specification defines a method for sending an invitation directly from the user to the contact, which re-uses the original 'jabber:x:conference' namespace in use before XEP-0045 was written (with the addition of a 'reason' attribute for the sake of feature parity with XEP-0045).

2. How It Works

A user invites a contact to a room by sending a <message/> stanza containing only an <invitation/> element qualified by the 'jabber:x:conference' namespace.

Example 1. A direct invitation

<message
    from='crone1@shakespeare.lit/desktop'
    to='hecate@shakespeare.lit'>
  <x xmlns='jabber:x:conference'
     jid='darkcave@macbeth.shakespeare.lit'
     reason='Hey Hecate, this is the place for all good witches!'/>
</message>
  

The 'jid' attribute, which is REQUIRED, specifies the address of the groupchat room.

The 'reason' attribute, which is OPTIONAL, specifies a human-readable purpose for the invitation.

Upon receiving a direct invitation, the contact's client SHOULD present the invitation to the contact so that the contact can accept it or decline it. If the contact accepts the invitation, the contact's client shall join the room as described in XEP-0045. If the contact declines the invitation, it shall silently discard the invitation.

3. Determining Support

If an entity supports the protocol specified herein, it MUST advertise that fact by returning a feature of "jabber:x:conference" in response to Service Discovery [3] information requests (see Protocol Namespaces regarding issuance of one or more permanent namespaces).

Example 2. Service discovery information request

<iq from='crone1@shakespeare.lit/desktop'
    id='disco1'
    to='hecate@shakespeare.lit/broom'
    type='get'>
  <query xmlns='http://jabber.org/protocol/disco#info'/>
</iq>
  

Example 3. Service discovery information response

<iq from='hecate@shakespeare.lit/broom'
    id='disco1'
    to='crone1@shakespeare.lit/desktop'
    type='result'>
  <query xmlns='http://jabber.org/protocol/disco#info'>
    <feature var='jabber:x:conference'/>
  </query>
</iq>
  

In order for an application to determine whether an entity supports this protocol, where possible it SHOULD use the dynamic, presence-based profile of service discovery defined in Entity Capabilities [4]. However, if an application has not received entity capabilities information from an entity, it SHOULD use explicit service discovery instead.

4. Implementation Notes

Before inviting a contact to a members-only room, a user SHOULD check to see if the contact is already a member and, if not, add the contact to the member list (or ask a room administrator to do so).

If a client receives multiple invitations to the same room (e.g., a mediated invitation as defined in XEP-0045 and a direct invitation as defined here), the client SHOULD present only one of the invitations to a human user. If a client receives an invitation to a room in which the user is already an occupant, the client SHOULD silently discard the invitation.

5. Security Considerations

The following attacks are possible, in roughly the order of probability. See also Best Practices to Discourage Denial of Service Attacks [5] and RFC 3552 [6].

1. The sender of an invitation could overload the 'reason' attribute with malicious or offensive text. The recipient can mitigate this attack by blocking the sender using technologies such as Privacy Lists [7] and Simple Communications Blocking [8].

2. A passive attacker could flood the recipient with a large number of chatroom invitations. This attack, too, can be mitigated with Privacy Lists or Simple Communications Blocking.

3. A passive attacker could use a mimicked JID to fool the recipient into thinking that the sender is a known or trusted contact. This attack requires knowledge of the recipient's known or trusted contacts, and can be mitigated by following the recommendations in Best Practices to Prevent JID Mimicking [9].

4. In the absence of end-to-end encryption, a passive attacker could eavesdrop on the chatroom invitations that a user sends or receives.

5. In the absence of end-to-end encryption or signing, an active attacker could modify the invitation in transit so that the recipient is directed to a different room than intended by the sender.

6. IANA Considerations

This document requires no interaction with the Internet Assigned Numbers Authority (IANA) [10].

7. XMPP Registrar Considerations

8. XML Schema

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

<xs:schema
    xmlns:xs='http://www.w3.org/2001/XMLSchema'
    targetNamespace='jabber:x:conference'
    xmlns='jabber:x:conference'
    elementFormDefault='qualified'>

  <xs:element name='x'>
    <xs:complexType>
      <xs:simpleContent>
        <xs:extension base='empty'>
          <xs:attribute
              name='jid'
              type='xs:string'
              use='required'/>
          <xs:attribute
              name='reason'
              type='xs:string'
              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>
  

9. Acknowledgements

Thanks to Joe Hildebrand for his feedback.

Appendices

Appendix A: Document Information

Series: XEP
Number: 0249
Publisher: XMPP Standards Foundation
Status: Proposed
Type: Standards Track
Version: 0.3
Last Updated: 2009-06-22
Approving Body: XMPP Council
Dependencies: XMPP Core, XEP-0045
Supersedes: None
Superseded By: None
Short Name: x-conference
Source Control: HTML  RSS

Appendix B: Author Information

Appendix C: Legal Notices

Appendix D: Relation to XMPP

The Extensible Messaging and Presence Protocol (XMPP) is defined in the XMPP Core (RFC 3920) and XMPP IM (RFC 3921) 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.

Discussion on other xmpp.org discussion lists might also be appropriate; see <http://xmpp.org/about/discuss.shtml> for a complete list.

Errata can be sent to <editor@xmpp.org>.

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

Appendix H: Revision History

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