2.4 KiB
Telnet
Channels
Telnet supports data multiplexing by way of 256 built-in sub-channels, each identified by a byte in the interval [\x00-\xFF]. By switching between channels, you can send completely separate streams of data through the same connection.
All channels start out closed by default. To open a channel, one host must request or offer a channel using IAC WILL <id> or IAC DO <id>. The remote host then responds with IAC DO <id> or IAC WILL <id>, respectively. Alternatively, the request may be denied using IAC DONT <id> or IAC WONT <id>, respectively.
In order to switch to a specific channel, the IAC SB <id> sequence must be used. All data sent afterwards will be routed through that specific channel. To switch back to the main channel, IAC SE must be used. Note that subchannels do not support any IAC sequences except IAC IAC (an escaped \xFF byte) and IAC SE (return to the main channel). In particular, you cannot switch directly from one subchannel to another: you must revert to the main channel first.
Due to the unbiased nature of Telnet, neither side of the connection is automatically recognized as the server or the client. However, a host may either request a channel (as a client) or offer a channel (as a server). The WILL/WONT commands are used in the role of server ("I will", "I wont"), while DO/DONT are used in the role of client ("You do", "You do not"). As such, a channel may be opened twice (even simultaneously).
As an example, lets assume a terminal is connected to a server using Telnet. The server offers MCCP (data compression), but wants to know what the terminal's window size is. The following communication might occur:
<server> IAC DO NAWS
<server> IAC WILL MCCP
<client> IAC WILL NAWS
<client> IAC SB NAWS \x50 \x00 \x50 \x00 IAC SE
<client> IAC DO MCCP
<server> IAC SB MCCP IAC SE
<server> (compressed data)
Notice that MCCP was negotiated such that the server offers the compression. Only the server-to-client flow of data is compressed; the client would not compress its data unless the channel was negotiated in the other direction as well.
In general, a specific subchannel is tied to a specific Telnet subprotocol. For example, the EXOPL subprotocol is assigned to channel 255, so that channel should be avoided for any other purpose. A full list of registered subprotocols can be found on the IANA website.