ADC? No, cancelled…

Back in the end of 2000’s when the 1.0 version of the ADC protocol was ready and the implementation had started to taking shape the protocol maintainers thought it’s a good idea to add information about the distinct new protocol of DC to Wikipedia. Besides linking to the technicals a brief description of what and why is ADC was added to the new Advanced Direct Connect Protocol page.

The page, professional and made according to the best Wikipedia standards, had been improved over time and stayed there for many years – until the end of last year when someone requested a complete removal, an uncontroversial deletion. This action was requested to be reverted (thanks to klondike) which means that per the Wikipedia rules the page itself cannot be requested to be removed again. However, a few weeks later, another admin made a lawnmover style deletion of the best part of the content of the page citing that the source of information, this very blog, where the most of the content are from the ADC protocol’s designers, maintainers and implementators, is unreliable.

Of course Wikipedia has its own rules and they have been controversial all the time. This time they’re clearly followed the rules unwisely and I guess it’s not worth to engage into an add/remove style fight with them anymore. Who knows, maybe cancel culture has reached Wikipedia as well or it’s just another attempt at making Wikipedia worse for technical purposes.

In any case we decided to preserve the removed document, originally added by Fredrik Ullner, here:

Advanced Direct Connect (ADC) is a peer-to-peer file sharing and chat protocol, using the same network topology, concepts and terminology as the Direct Connect (DC) protocol.

“ADC” unofficially an acronym for “Advanced Direct Connect”.[1]


  • 1 History
  • 2 Design and features
  • 3 Protocol
  • 4 See also
  • 5 References
  • 6 External links


ADC was created to allow an extensible protocol and to address some shortcomings of the Direct Connect protocol. It was initiated by Jacek Sieka, under the influence of Jan Vidar Krey’s DCTNG draft.[2] The first revision of ADC came in 2004 and the first official version in 2007-12-01.

Design and features

ADC is structured around clients that connect to a central hub, where the clients (users) can chat and download files from other clients (users). The hub provides routing between clients for chat, searches and requests for connections. The actual file transfers are between clients.

The protocol itself is split in two parts: a base protocol that every client and hub respectively must follow and extensions that are optional. The protocols allow signalling of protocol features (such as bloom filters), and messages can be constructed to only be routed to those who support that particular feature.

Each hub has their own rules and are commonly governed by hub operators.[3] Hubs may define different capabilities for hub operators. The hubs themselves do not regulate discussion and files, but the hub operators. The hub regulate minimum share and maximum amount of simultaneous hubs; things that are sent by the client, rather than the user.

Lists of hubs [4] exist where a hub’s name, description, address and rules are specified. With the hub list, users can choose hubs that are similar according to the user’s liking of discussion topics and files.

The peer-to-peer part of the protocol is based on a concept of “slots” [5] (similar to number of open positions for a job). These slots denote the number of people that are allowed to download from a user at any time. The slots are controlled by the user of respective client.

ADC require that all text must be sent in UTF-8, which means that users with different system encoding (say, Russian and Chinese) are able to chat with respective native characters.

The protocol natively supports IPv6.

There are two modes a user can be in: “active” or “passive”. Clients in active mode can download from anyone else on the network. Passive mode users can only download from active users. Passive clients will be sent search results through the hub, while active clients will receive the results directly. An active searcher will receive (at most) 10 results per user and a passive searcher will receive (at most) 5 results per user. NAT traversal exist as a protocol extension,[6] which allow passive users to connect to other passive users.

The base protocol does not require encryption, but extensions exist to provide encryption with TLS.[7]

Files in client connections are identified by their hash, most commonly the Tiger Tree Hash. The hash algorithm is negotiated with the hub and used throughout the client-hub session, as well as subsequent client-client connections.


The ADC protocol is a text-based protocol, where commands and their information are sent in clear text, except during password negotiation. The client-server (as well as client-client, where one acts as a “server”) aspect of the protocol stipulates that the client speak first when a connection has been made. For example, when a client connects to a hub’s socket, the client is the first to talk to the hub.

The protocol requires that all text must be sent as UTF-8 encoded Unicode, normalized in form C.

There are no port defaults, for hubs or clients.

Hub addresses are in the following form: adc://, where 411 is the port.

During hub-client protocol information exchange, the client offers a set of hashes it supports. The hub will select one of these hashes, and that hash will be used throughout the hub-client session. If the hub deems that the client doesn’t support an (arbitrary) appropriate hash set, an error is raised.

The global identification scheme is based on the hash set producing two end-hashes, where one of them depends on the output of the other. During hub-client information exchange, the client will send these end-hashes, encoded with base32, which the hub will confirm to match. One of these base32 encoded hashes will be further sent to other clients in the network. The global identification scheme is this last string. The client may change its end-hashes on a hub-to-hub basis.

Each user, during a hub session, is assigned a hash that only lasts that particular session. This hash will be used for all client references in that hub. There is no dependency on nicks.

Each client information notification is incrementally sent.

An incoming request for a client-client connection is linked to an actual connection, with the use of a token.

Searches use a token, as well, to identify each result of a search.

There is no out-of-the-box ability for a client to kick or redirect another client from a hub. The hub, however, can kick and redirect arbitrarily. The hub can also require that all other clients in the hub must terminate their transfers with the kicked/redirected client. If a client is redirected to another hub, the redirecting client must use a referrer, similar to the HTTP referrer. The kicked/redirected client is not required to receive a notification message.

The peer-to-peer part of the protocol is based on a concept of “slots” (similar to number of open positions for a job). These slots denote the number of people that are allowed to download from a user at any time. These slots are controlled by the client. Automatic slot allocation is supported by the protocol.

The token in the client-client connection decides who should be allowed to download first.

Downloads are transported using TCP. Searches can be transported using TCP or UDP.

An active client has a listening port for TCP and another for UDP, though the ports don’t depend on each other.

Protocol delimiters are ‘\n’ and ‘ ‘ (space). The character ‘\’ is used as an escape sequence. Allowed escape sequences are “\n” (new line), “\s” (space) and “\\” (backslash).

The protocol allows for extensions such as compression with bzip2 or encryption with TLS.[8] While the protocol does not mandate that these extensions be implemented, hubs may require them.

See also


  1. Fredrik Ullner (March 2007). “ADC: The run down”. DC++: Just These Guys, Ya Know? blog. Retrieved 2010-12-13.

2. Jan Vidar Krey (August 2006). “ADC: Protocol simplicity”.

3. Jan Vidar Krey. Archived from the original on 2013-01-30. Retrieved 2006-09-23.

4. Fredrik Ullner (March 2006). “Power + Person = Operator”. DC++: Just These Guys, Ya Know? blog. Retrieved 2010-12-13.

5. Fredrik Ullner (January 2007). “The parts of a hub list”. DC++: Just These Guys, Ya Know? blog. Retrieved 2010-12-13.

6. Fredrik Ullner (March 2006). “Slots, slots, slots…”. DC++: Just These Guys, Ya Know? blog. Retrieved 2010-12-13.

7. Fredrik Ullner (December 2010). “ADC Extensions – NATT – NAT traversal”. ADC Project. Retrieved 2010-12-13.

8. Fredrik Ullner (December 2010). “ADC Extensions – ADCS – Symmetrical Encryption in ADC”. ADC Project. Retrieved 2010-12-13.

9. En_Dator (March 2009). “TLS and Encryption”. ADCPortal. Archived from the original on 2011-07-07. Retrieved 2009-03-01.

External links

Click here to see how the original Wikipedia page looked like before the content removal.

We’ve once had a very good overview of ADC at Wikipedia, a brief explanation of what it is all about so interested people can go further, contact, etc…. Now we have almost nothing. To compensate that I’ll also try to preserve this document another way by adding it to the ADC project site later.

May people with the powers to destruct valid and current information sleep better after each time they’re acting so.

About emtee
I started to use DC using DC++ in 2003 when its version number was around 0.260. Since then I've been amazed by the DC network: a professional but still easy-to-use way of P2P file sharing. I was invited to the DC++ team in 2006 where - in the beginning - I had been doing user support and some testing only. A few years later I started to add small contributions to the DC++ code as well so for many years I'd been doing mostly bug fixes, testing, feature proposals and improvements. At the same time I worked on improving the documentation for both DC++ and ADCH++ as well. These days I'm trying to maintain the whole code and the infrastructure behind to keep these software secure and usable for a prolonged time. My ultimate goal is to help making the DC network as more user friendly as possible.

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