8. Current implementations
The only current implementation of this protocol is the IRC server, version 2.8. Earlier versions may implement some or all of the commands described by this document with NOTICE messages replacing many of the numeric replies. Unfortunately, due to backward compatibility requirements, the implementation of some parts of this document varies with what is laid out. On notable difference is:
The rest of this section deals with issues that are mostly of importance to those who wish to implement a server but some parts also apply directly to clients as well.
8.1 Network protocol: TCP - why it is best used here.
IRC has been implemented on top of TCP since TCP supplies a reliable network protocol which is well suited to this scale of conferencing. The use of multicast IP is an alternative, but it is not widely available or supported at the present time.
8.1.1 Support of Unix sockets
Given that Unix domain sockets allow listen/connect operations, the current implementation can be configured to listen and accept both client and server connections on a Unix domain socket. These are recognized as sockets where the hostname starts with a '/'.
When providing any information about the connections on a Unix domain socket, the server is required to supplant the actual hostname in place of the pathname unless the actual socket name is being asked for.
8.2 Command Parsing
To provide useful 'non-buffered' network IO for clients and servers, each connection is given its own private 'input buffer' in which the results of the most recent read and parsing are kept. A buffer size of 512 bytes is used so as to hold 1 full message, although, this will usually hold several commands. The private buffer is parsed after every read operation for valid messages. When dealing with multiple messages from one client in the buffer, care should be taken in case one happens to cause the client to be 'removed'.
8.3 Message delivery
It is common to find network links saturated or hosts to which you are sending data unable to send data. Although Unix typically handles this through the TCP window and internal buffers, the server often has large amounts of data to send (especially when a new server-server link forms) and the small buffers provided in the kernel are not enough for the outgoing queue. To alleviate this problem, a "send queue" is used as a FIFO queue for data to be sent. A typical "send queue" may grow to 200 Kbytes on a large IRC network with a slow network connection when a new server connects.
When polling its connections, a server will first read and parse all incoming data, queuing any data to be sent out. When all available input is processed, the queued data is sent. This reduces the number of write() system calls and helps TCP make bigger packets.
8.4 Connection 'Liveness'
To detect when a connection has died or become unresponsive, the server must ping each of its connections that it doesn't get a response from in a given amount of time.
If a connection doesn't respond in time, its connection is closed using the appropriate procedures. A connection is also dropped if its sendq grows beyond the maximum allowed, because it is better to close a slow connection than have a server process block.
8.5 Establishing a server to client connection
Upon connecting to an IRC server, a client is sent the MOTD (if present) as well as the current user/server count (as per the LUSER command). The server is also required to give an unambiguous message to the client which states its name and version as well as any other introductory messages which may be deemed appropriate.
After dealing with this, the server must then send out the new user's nickname and other information as supplied by itself (USER command) and as the server could discover (from DNS/authentication servers). The server must send this information out with NICK first followed by USER.
8.6 Establishing a server-server connection.
The process of establishing of a server-to-server connection is fraught with danger since there are many possible areas where problems can occur - the least of which are race conditions.
After a server has received a connection following by a PASS/SERVER pair which were recognised as being valid, the server should then reply with its own PASS/SERVER information for that connection as well as all of the other state information it knows about as described below.
When the initiating server receives a PASS/SERVER pair, it too then checks that the server responding is authenticated properly before accepting the connection to be that server.
8.6.1 Server exchange of state information when connecting
The order of state information being exchanged between servers is essential. The required order is as follows:
Information regarding servers is sent via extra SERVER messages, user information with NICK/USER/MODE/JOIN messages and channels with MODE messages.
NOTE: channel topics are *NOT* exchanged here because the TOPIC command overwrites any old topic information, so at best, the two sides of the connection would exchange topics.
By passing the state information about servers first, any collisions with servers that already exist occur before nickname collisions due to a second server introducing a particular nickname. Due to the IRC network only being able to exist as an acyclic graph, it may be possible that the network has already reconnected in another location, the place where the collision occurs indicating where the net needs to split.
8.7 Terminating server-client connections
When a client connection closes, a QUIT message is generated on behalf of the client by the server to which the client connected. No other message is to be generated or used.
8.8 Terminating server-server connections
If a server-server connection is closed, either via a remotely generated SQUIT or 'natural' causes, the rest of the connected IRC network must have its information updated with by the server which detected the closure. The server then sends a list of SQUITs (one for each server behind that connection) and a list of QUITs (again, one for each client behind that connection).
8.9 Tracking nickname changes
All IRC servers are required to keep a history of recent nickname changes. This is required to allow the server to have a chance of keeping in touch of things when nick-change race conditions occur with commands which manipulate them. Commands which must trace nick changes are:
No other commands are to have nick changes checked for.
In the above cases, the server is required to first check for the existence of the nickname, then check its history to see who that nick currently belongs to (if anyone!). This reduces the chances of race conditions but they can still occur with the server ending up affecting the wrong client. When performing a change trace for an above command it is recommended that a time range be given and entries which are too old ignored.
For a reasonable history, a server should be able to keep previous nickname for every client it knows about if they all decided to change. This size is limited by other factors (such as memory, etc).
8.10 Flood control of clients
With a large network of interconnected IRC servers, it is quite easy for any single client attached to the network to supply a continuous stream of messages that result in not only flooding the network, but also degrading the level of service provided to others. Rather than require every 'victim' to be provide their own protection, flood protection was written into the server and is applied to all clients except services. The current algorithm is as follows:
which in essence means that the client may send 1 message every 2 seconds without being adversely affected.
8.11 Non-blocking lookups
In a real-time environment, it is essential that a server process do as little waiting as possible so that all the clients are serviced fairly. Obviously this requires non-blocking IO on all network read/write operations. For normal server connections, this was not difficult, but there are other support operations that may cause the server to block (such as disk reads). Where possible, such activity should be performed with a short timeout.
8.11.1 Hostname (DNS) lookups
Using the standard resolver libraries from Berkeley and others has meant large delays in some cases where replies have timed out. To avoid this, a separate set of DNS routines were written which were setup for non-blocking IO operations and then polled from within the main server IO loop.
8.11.2 Username (Ident) lookups
Although there are numerous ident libraries for use and inclusion into other programs, these caused problems since they operated in a synchronous manner and resulted in frequent delays. Again the solution was to write a set of routines which would cooperate with the rest of the server and work using non-blocking IO.
8.12 Configuration File
To provide a flexible way of setting up and running the server, it is recommended that a configuration file be used which contains instructions to the server on the following:
In specifying hostnames, both domain names and use of the 'dot' notation (127.0.0.1) should both be accepted. It must be possible to specify the password to be used/accepted for all outgoing and incoming connections (although the only outgoing connections are those to other servers).
The above list is the minimum requirement for any server which wishes to make a connection with another server. Other items which may be of use are:
8.12.1 Allowing clients to connect
A server should use some sort of 'access control list' (either in the configuration file or elsewhere) that is read at startup and used to decide what hosts clients may use to connect to it.
Both 'deny' and 'allow' should be implemented to provide the required flexibility for host access control.
The granting of operator privileges to a disruptive person can have dire consequences for the well-being of the IRC net in general due to the powers given to them. Thus, the acquisition of such powers should not be very easy. The current setup requires two 'passwords' to be used although one of them is usually easy guessed. Storage of oper passwords in configuration files is preferable to hard coding them in and should be stored in a crypted format (ie using crypt(3) from Unix) to prevent easy theft.
8.12.3 Allowing servers to connect
The interconnection of server is not a trivial matter: a bad connection can have a large impact on the usefulness of IRC. Thus, each server should have a list of servers to which it may connect and which servers may connect to it. Under no circumstances should a server allow an arbitrary host to connect as a server. In addition to which servers may and may not connect, the configuration file should also store the password and other characteristics of that link.
To provide accurate and valid replies to the ADMIN command (see section 4.3.7), the server should find the relevant details in the configuration.
8.13 Channel membership
The current server allows any registered local user to join upto 10 different channels. There is no limit imposed on non-local users so that the server remains (reasonably) consistant with all others on a channel membership basis