XMPP Instant Messaging
The roster and subscription model in RFC 6121 defines how federated chat contacts work. Essential for implementing XMPP clients, servers, or S2S (server-to-server) federation between XMPP deployments.
What It Defines
RFC 6121 defines the instant messaging and presence layer on top of XMPP Core: roster (contact list) management, subscription model (subscribe/subscribed/unsubscribe), presence semantics (available, away, dnd, xa), and message delivery guarantees. Combined with RFC 6120 this forms the complete XMPP IM specification.
Canonical (Normative)
Convenient (Practical)
The canonical publication point for finalized RFCs. If a protocol is standardized as an RFC, the RFC Editor text is the normative final reference. Published by the IETF, IRTF, IAB, and independent stream.
Related Specs
XMPP powers enterprise messaging (Cisco Jabber), IoT device management, and many open federated chat systems. WhatsApp internally uses a modified XMPP protocol for message delivery.
Matrix is the modern alternative to XMPP for federated messaging: bridges to Discord/Slack/WhatsApp, end-to-end encryption by default, and full decentralization. Used by the German government, French public sector, and Mozilla.