XO: XMPP Overlay Service for Distributed Chat

Lass, Macker, Millar, Regli, Taylor

xmpp chat distributed serverless group communication

@article{lass:milcom-2010,
  title={{XO}: {XMPP} Overlay Service for Distributed Chat},
  author={Lass, R.N. and Macker, J. and Millar, D. and
          Regli, W.C. and Taylor, I.},
  inproceedings={Military Communications ({MILCOM})},
  year={2010}
}

Multi-user chat not designed for MANET

• MUC management server
• TCP
• Inefficient use of shared links

XO adapts XMPP MUC to serverless multicast

• Provides standard XMPP TCP interfaces to wrap legacy clients and servers

Unmodified XMPP client connects to XOP (XO Proxy) agent

• XOP mediates XMPP protocol to XO multicast and discovery
• XOG (XO Gateway) bridges XOP MUC multicast to conventional XMPP MUC, i.e., standard servers

Related XMPP Extensions

• XEP-0174 uses zero-conf networking (mDNS) to discover services
  • But only link-local discovery and point-to-point messaging over TCP
• XEP-0100 specifies gatewaying to non-XMPP chat
• XEP-0045 specifies XMPP Mult-User Chat (MUC)
  • Rooms are associated with servers, all messages for a room go to its server

Uses SMF to ensure multihop multicast forwarding

Uses GUMP (Generic Unicast-to-Multicast) library to manage communications

GUMP in turn here uses NORM for reliable multicast delivery

Topics are hashed to multicast addresses within a given range

• This prevents all messages being on the same address
  • Not clear why this matters until there's actual underlying multicast structure

Several options for gatewaying:

• One TCP connection per client to the server, each proxying the client
  • Inefficient, brittle
• Custom server plugin
  • May not be able to modify server, less flexible
• Gateway acts as XMPP server
  • What is implemented in XOG
    • Reopens connections to server if they drop

Of note:

• Reference point group mobility model (RPGM) from Colorado School of Mines mobility generation tool