Distributed systems

There is quite a few definitions of "distributed" and "decentralized" in use, in this note I'm using the following ones:

Centralized
Clients interacting with a single server (either physical or controlled by the same entity).
Decentralized
Clients interacting with multiple servers (controlled by different entities), which often build a federated network.
Distributed
Clients interacting with other clients directly, acting as servers themselves.

Distributed systems are useful for various purposes, but the common/achievable niceties are:

These are mostly shared with federated systems, but take it further.

Usable systems

Actually usable (reliably working, specified, having users and decent software) systems so far are usually federated/decentralized; those can, in principle, be quite close to distributed systems (simply by setting their servers on user machines). So, generally it seems more useful to focus on those if the intention is to get things done: SMTP (email), NNTP (Usenet), XMPP (jabber), and HTTP (World Wide Web) are relatively well-supported, standardized, and usable for various kinds of communication.

Existing systems

There is quite a few of them; I am going to write mostly about those which work over Internet. There's also a related Wikipedia category.

Generic networks

Tor and I2P: both support "hidden services", on top of which many regular protocols can be used, but it is more about privacy (and a bit about routing) than about decentralisation: they provide NAT traversal, encryption, and static addresses, but that's it: the benefits for decentralisation (excluding privacy) are basically the same as in IPv6 with IPsec. Tor documentation is relatively nice, and there is plenty of I2P docs. Tor provides a nice C client, I2P uses Java.

Mesh networks

Some mesh networks, like Telehash, provide routing as well, though advantages for decentralisation seem to be similar to those of Tor and I2P; just better in that they extend it beyond Internet. Telehash documentation is also pretty nice and full of references.

Cjdns (or its name, at least) seems to be relatively well-known, but it relies on node.js. Netsukuku and B.A.T.M.A.N. are two more protocols the names of which are known.

Those would be nice to get someday, but they would require quite a lot of users to function, and various government restrictions seem to complicate their usage (this varies from jurisdiction to jurisdiction and from year to year, but seems to be pretty bad in Russia in 2018).

IM and other social services

See also: Distributed state and network topologies in chat systems.

File sharing and websites

Web crawling

YaCy and a few more (some of which are dead by now) distributed search engines exist. I have only tried YaCy, and it works, though haven't managed to find its technical documentation – so it's not clear how it works.

Other information

These networks include search for files, but by their names, not content-addressable (so they can't be easily verified, which brings additional challenges).

Related papers:

Cryptocurrencies

Plenty of those popped up recently. Bitcoin-like ones (usually with a proof of work and block chaining) look like quite a waste of resources (and perhaps a pyramid scheme) to me, though the idea itself is interesting. I was rather interested in "digital cash" payment systems before, but those didn't quite take off so far.

As of 2021, bitcoin-like cryptocurrencies seem to be eating other distributed projects: many of those are merged with their custom cryptocurrencie, or occasionally piggyback on existing ones, but either way they become more complicated and commercialized.

General P2P networking tools

GNUnet

Not sure how to classify it, but here are some links: gnunet.org, wiki://GNUnet, A Secure and Resilent Communication Infrastructure for Decentralized Networking Applications. Seems promising, but tricky to build, to figure how it all works, and to do anything with it now (a lack of documentation seems to be the primary issue, though probably there's more).

Taler and secushare (using PSYC) are getting built on top of it, but it's not clear how's it going, how abandoned or alive it is, etc. Their documentation also seems to be obsolete/outdated/abandoned/incomplete. Update (January 2018): apparently secushare prototype won't be released this year.

libp2p

libp2p apparently provides common primitives needed for peer-to-peer networking in the presence of NATs and other obstructions. At the time of writing there's no C API (so it's only usable from a few languages) and its website is quite broken. At the same time worldwide IPv6 adoption reaches more than 32%, so possibly NATs will disappear before workarounds will become usable.

Generic protocols

There are more or less generic network protocols that may be used, possibly together with Tor, to get working and secure peer-to-peer services.

SSH

SSH is quite nice and layered. Apparently its authentication is not designed for distributed systems (such as distributed IMs or file sharing), its connection layer looks rather bloated, and generally it's not particularly simple. Those are small bits of a large protocol, but they seem to make it not quite usable for peer-to-peer communication.

TLS

TLS may provide mutual authentication, and there is plenty of tools to work with it.

IPsec

Has uses similar to TLS, but is a generally better way to solve the same problems. IPv6 (or, rather, individual addresses) is needed for its wide deployment though. IPv6 gets adopted, but slowly. Once computers will become addressable individually, and transport layer encryption will be there by default, it'd render plenty of the contemporary higher-level network protocols obsolete.

Ad hoc messaging

Pretty much every distributed IM tries to reinvent everything, and virtually none are satisfactory, but at least some of the problems are already solved separately, and there are:

Combinations such as Tor + TLS + rlwrap may serve as ad hoc IMs rather easily. Well, if participants are willing and able to use those, which often isn't the case. In an attempt to try and facilitate things like that, I wrote TLSd and a P2P IM with a libpurple plugin as one of the examples.

Users

Distributed systems, particularly when used for social activities, require users – so that there would be somebody to send messages to in case of an IM. That's quite a problem, since even by sticking to federated networks it is easy to lose or decrease contact with most people one knows, even if those are tech savvy; apparently it's even easier when moving to distributed and less common ones.

In my experience so far, for personal communication it works like this:

Apart from not being backed by a huge company for marketing and not having a lot of users using them already, probably the primary issue for non-centralised systems is that there's usually little focus on finding new contacts and establishing new relationships.

Though XMPP MUCs and that microblogging fediverse thing (see OStatus above) may be suitable for it; not distributed, but at least closer to it. A nicer approach would be to use distributed search, but I'm not aware of generic/suitable systems of that kind: they tend to either be an easy target for spammers, or require existing social connections in a system (although some may be suitable still).

Search, FOAF, and the rest of RDF

Some kind of a distributed search/directory may connect small peer-to-peer islands into a usable network. While it is hard to decide on an algorithm, lists of known and/or somewhat trusted nodes are common for both structured and unstructured networks, as well as for use of social graphs: if those would be provided by peers, a client may decide by itself which algorithm to apply. This reduces the task to just including known nodes into local directory entries, which can be shipped over any other protocols (e.g., HTTP, possibly over Tor).

Knowledge representation, which is needed for a generic directory structure, is tricky, but there is RDF (resource description framework) already. There is FOAF (friend of a friend ontology), specifically for describing persons, their relationships (including linking the persons they know), and other social things. There's a FOAF search engine, too: it's centralised and searches mostly in large centralised places, but such an engine can relatively easily be set locally, searching for users and information they provide in a distributed, peer-to-peer network. See also: Semantic Web.