It’s a bit perplexing that mnot’s draft cites XMPP as decentralised, given MUCs are very much centralised to a single provider which entirely controls that conversation, and if that provider goes down the conversation is dead. But I guess that’s because XMPP is submitted to the IETF, and Matrix isn’t yet.
XMPP is still a decentralized protocol by design. That you can't send messages to a conference hosted on a server that is offline doesn't make it 'centralized'.
There are probably five broad levels of decentralisation here:
1. Open network, but each user lives on a single server, each conversation is dependent on a single server: (XMPP MUCs)
2. Open network, but each user lives on a single server (with some ability to manually migrate between servers), conversations are replicated across all participating servers: (Matrix, ActivityPub, SMTP, NNTP)
3. Open network, users are replicated across multiple servers, conversations are replicated across all participating servers: (Matrix + MSC1228 or MSC2787 or similar)
4. Open network, users live on a single P2P node, conversations are replicated across all participating nodes: (Briar, today's P2P Matrix)
5. Open network, users are replicated across multiple P2P nodes, conversations are replicated across all participating nodes: (P2P Matrix + MSC2787 etc).
So yes, XMPP is decentralised by some definition, but it's kinda useful to map out the whole space.
Interesting to see Wire, and Matrix making an effort in this. Unlike Signal which still requires your phone number and is completely centralized to their servers whist promoting their 85% pre-mined cryptocurrency that they can dump at any time.
On the Matrix side we’re working on fully decentralising it (as per https://matrix.uhoreg.ca/mls/ordering.html). There’s also a cool similar project from Matthew Weidner: https://dl.acm.org/doi/10.1145/3460120.3484542
It’s a bit perplexing that mnot’s draft cites XMPP as decentralised, given MUCs are very much centralised to a single provider which entirely controls that conversation, and if that provider goes down the conversation is dead. But I guess that’s because XMPP is submitted to the IETF, and Matrix isn’t yet.