Clique

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Clique is the protocol (formerly called rMulticast) used by telepathy-salut to implement multi-user chat on Link Local XMPP.

The following is derived and extracted from the clique.xml distributed with telepathy-salut-0.2.2 in docs/.

Introduction

Clique is an extension of XEP-0174 to support multi-user chat. It consists of the following components:

  • a binary multicast message-passing protocol sometimes referred to as rMulticast, with retransmission/reliability, fragmentation/reassembly, causal ordering, and reliable membership semantics, in which messages are associated with a stream ID in the range 0 to 65535
  • XMPP-like XML messages sent with stream ID 0
  • optional auxiliary data (e.g. Tubes) sent with stream IDs 1-65535, with semantics signalled by XML messages in stream 0

The chat room behaves a lot like XMPP, and namespaces used in XMPP have their usual semantics. However, instead of a stream of XML as in XMPP, Clique's XMPP-like messages are individually well-formed XML documents.

XEP-0174 instance names are used for identification in a chat room.

The Clique XML namespace can also be used for certain elements sent over XEP-0174 unicast TCP streams, for instance to send invitations.

Use Cases

FIXME: there need to be some examples of the actual Clique protocol here

Advertising a public link-local chatroom via mDNS

In this example a chatroom called "Witchcraft" uses the multicast group 239.255.71.66, port 13251 (arbitrarily chosen).

We recommend that chatrooms use a random high port number in a random multicast group in the range 239.255.71.x. 

       ; A dummy A record for the chatroom's multicast group
       Witchcraft._clique._udp.local. IN A 239.255.71.66

       ; A DNS-SD service of type _clique._udp
       _clique._udp.local. IN PTR Witchcraft._clique._udp.local.
       Witchcraft._clique._udp.local. IN SRV 13251 ._clique._udp.local.

       ; For future expansion - implementations SHOULD NOT resolve this,
       ; unless they implement a future version of this protocol that
       ; defines some TXT keys for _clique._udp
       Witchcraft._clique._udp.local. IN TXT "txtvers=0"

Inviting a contact to a public or private room

In this example crone@desktop invites hecate@broom to the chatroom given above.

This would work just as well if the chatroom was not advertised in mDNS; this can be used to make "private" chatrooms. Note however that anyone with a network traffic sniffer can see the chatroom itself. 


     <message from='crone@desktop' to='hecate@broom'>
       <invite xmlns='http://telepathy.freedesktop.org/xmpp/clique'>
         <roomname>Witchcraft</roomname>
         <address>239.255.71.66</address>
         <port>13251</port>
         <reason>We need to plot Macbeth's doom, but the XMPP server is
           down</reason>
       </invite>
       <body>You got a Clique chatroom invitation</body>
     </message>
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