Advanced Direct Connect


Advanced Direct Connect is a peer-to-peer file sharing and chat protocol, using the same network topology, concepts and terminology as the Direct Connect protocol.
"ADC" unofficially an acronym for "Advanced Direct Connect".

History

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 draft. 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 can chat and download files from other clients. 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, 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. 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 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" . 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 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 10 results per user and a passive searcher will receive 5 results per user. NAT traversal exist as a protocol extension, 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.
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.

Protocol

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 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://example.com:411, 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 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". 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 ' '. The character '\' is used as an escape sequence. Allowed escape sequences are "\n", "\s" and "\\".
The protocol allows for extensions such as compression with bzip2 or encryption with TLS. While the protocol does not mandate that these extensions be implemented, hubs may require them.