Fully Decentralized P2P?

I’m looking at creating a P2P system. During initial research, I’m reading from Peer-to-Peer – Harnessing the Power of Disruptive Technologies. That book states “a fully decentralized approach to instant messaging would not work on today's Internet.” Mostly blaming开发者_Python百科 firewalls and NATs. The copyright is 2001. Is this information old or still correct?

It's still largely correct. Most users still are behind firewalls or home routers that block incoming connections. Those can be opened easier today than in 2001 (using uPnP for example, requiring little user interaction and knowledge) but most commercial end-user-targeting applications - phone (Skype, VoIP), chat (the various Messengers), remote control - are centralized solutions to circumvent firewall problems.

I would say that it is just plain wrong, both now and then. Yes, you will have many nodes that will be firewalled, however, you will also have a significant number who are not. So, if end-to-end encryption is used to protect the traffic from snooping, then you can use non-firewalled clients to act as intermediaries between two firewalled clients that want to chat.

You will need to take care, however, to spread the load around, so that a few unfirewalled clients aren't given too much load.

Skype uses a similar idea. They even allow file transfers through intermediaries, though they limit the through-put so as not to over load the middle-men.

That being said, now in 2010, it is a lot easier to punch holes in firewalls than it was in 2001, as most routers will allow you to automate the opening of ports via UPNP, so you are likely to have a larger pool of unfirewalled clients to work with.

Firewalls and NATs still commonly disrupt direct peer-to-peer communication between home-based PCs (and also between home-based PCs and corporate desktops).

They can be configured to allow particular peer-to-peer protocols, but that remains a stumbling block for most unsavvy users.

I think the original statement is no longer correct. But the field of Decentralized Computing is still in its infancy, with little serious contenders.

Read this interesting post on ZeroTier (thanks to @joehand): The State of NAT Traversal:

NAT is Traversable

In reading the Internet chatter on this subject I've been shocked by how many people don't really understand this, hence the reason this post was written. Lots of people think NAT is a show-stopper for peer to peer communication, but it isn't. More than 90% of NATs can be traversed, with most being traversable in reliable and deterministic ways.

At the end of the day anywhere from 4% (our numbers) to 8% (an older number from Google) of all traffic over a peer to peer network must be relayed to provide reliable service. Providing relaying for that small a number is fairly inexpensive, making reliable and scalable P2P networking that always works quite achievable.

I personally know of Dat Project, a decentralized data sharing toolkit (based on their hypercore protocol for P2P streaming).

From their Dat - Distributed Dataset Synchronization And Versioning paper:

Peer Connections

After the discovery phase, Dat should have a list of potential data sources to try and contact. Dat uses either TCP, UTP, or HTTP. UTP is designed to not take up all available bandwidth on a network (e.g. so that other people sharing wifi can still use the Inter- net), and is still based on UDP so works with NAT traversal techniques like UDP hole punching.

HTTP is supported for compatibility with static file servers and web browser clients. Note that these are the protocols we support in the reference Dat implementation, but the Dat protocol itself is transport agnostic.

Furthermore you can use it with Bittorrent DHT. The paper also contains some references to other technologies that inspired Dat.

For implementation of peer discovery, see: discovery-channel

Then there is also IPFS, or 'The Interplanetary File System' which is currently best positioned to become a standard.

They have extensive documentation on their use of DHT and NAT traversal to achieve decentralized P2P.

The session messenger seem to have solved the issue with a truly decentralized p2p messenger by using a incentivized mixnet to relay and store messages. Its a fork of the Signal messenger with a mixnet added in. https://getsession.org -- whitepaper: https://getsession.org/wp-content/uploads/2020/02/Session-Whitepaper.pdf

It's very old and not correct. I believe there is a product out called Tribler (news article) which enables BitTorrent to function in a fully decentralized way.

If you want to go back a few years (even before that document) you could look at Windows. Windows networking used to function in a fully decentralized way. In some cases it still does.

UPNP is also decentralized in how it determines available devices on your local network.

In order to be decentralized you need to have a way to locate other peers. This can be done proactively by scanning the network (time consuming) or by having some means of the clients announcing that they are available.

The announcements can be simple UDP packets that get broadcast every so often to the subnet which other peers listen for. Another mechanism is broadcasting to IIRC channels (most common for command and control of botnets), etc. You might even use twitter or similar services. Use your imagination here.

Firewalls don't really play a part because they almost always leave open a few ports, such as 80 (http). Obviously you couldn't browse the network if that was closed. Now if the firewall is configured to only allow connections that originated from internal clients, then you'd have a little more work to do. But not much.

NATs are also not a concern for similiar issues.