Notices

No, you don't.

So explain me how to do offline message without server?
Or maybe define offline messaging how you understand.

How about, your sending device queues the message, and if immediate delivery is not possible retries it after a timeout.

Hence, no central server needed

And that doesnt work?
Offline messaging for me is
1. i´m sending and go offline
2. counterpart is offline so he not will receive.
3. when he go online, message arrive

So message has been stored somewhere since sending was fine (central server)

Central server is considered poison, we want nothing like that ever near our precious messages

Remeber, we are talking about peer-to-peer, distributed network.
Of course there are ways to make offline messaging work like you described without a central server. This requires that the message is queued on some other peers.

Consider this scenario, then;
1.) I want to send a message to recipient B who is offline.
2.) My message goes to several peers (X, Y, Z in the network).
3.) I might log off myself from the network or not
4.) Recipient B comes online, and when any of peers X, Y or Z notice that, they forward the message.

again, no central server needed

ok. This form i agree

So the delivery is based on the willingness of X, Y or Z? How long do they store the message? How large message are they willing to store? 160 chars? A photo? A video?

I'd love to see a fully p2p decentralized system, but offline messages are a requirement for me. Running my own XMPP-server makes this possible.

https://ring.cx/en/about/technical#OpenDHT

Ring seems to provide offline messages. Not sure about the specifics. I hope it will only work for text though to avoid clogging the network.
To be successful and have widespread adoption it needs to be snappy.

Well, I don't know about that document, but it definitely doesn't work the way a normal person would expect yet (the, quite reasonable, expectation is: everything is delivered, always): https://tuleap.ring.cx/plugins/tracker/?aid=765

Yeah, that's the main issue IMHO - it's not easy to do in a way where it can't be easily misused to DoS the network.

Limiting what you can send can send to pears that are offline could definitely help - eq. only text messages for example, and even that in a limited amount.

That's still IMHO the main use-case - you don't usually send a wall of text to someone who is offline - rather something like "checkout this youtube video, LOL" or "the meeting tomorrow at 5 was moved by an hour", etc.

A hybrid solution might also work - you could "send" anything to an offline pear, but the message(s) will actually be queued on your side, while a short text-only summary would be stored in the network.

If the peer comes online before you go offline, the message(s) will go directly from you to the peer and the summary stored in the network would be canceled/ignored on delivery.

If you go offline before the peer comes online, the peer will only get the summary - kinda like how you might get SMS messages that someone called you while you were not available. If the size limit for the "summary" is big enough (eq. a few hundred chars,
compressed during transit) this should be fairly transparent to the users.

Now thinking about it, peer- side storage actually already happens in Bitcoin and other crypto currency networks - the peers store & validate the transactions + some metadata. It's not far from that to peers storing messages in transit - and you avoid the problem of an ever growing blockchain crypto currencies have by throwing messages away if they are not delivered after a while (7 days ?).

Well, one could still run his own ring agent somewhere (own server, a VPS, RPi behind the fridge, etc.) - but fully de-centralized would certainly be better.