Out of band signaling allows telnet to send control characters in special situations by preceding them with IAC and sending them to the remote process. This allows urgent data to override regular flow control and interrupt an application program stuck in an infinite loop that is not accepting input. The client sends an urgent TCP segment with the urgent bit set and urgent pointer pointing to a data mark character. The receiving process discards data before the data mark and handles remaining data normally, using the data mark to resynchronize from urgent to normal mode.

1. OUT OF BAND SIGNALING

2. OUT OF BAND SIGNALING
• To make control characters effective in special situation telnet
makes use of OUT OF BAND SIGNALING.
• In OUT OF BAND SIGNALING control characters are preceded
by IAC and are sent to the remote process.
• Imagine a situation in which the Application program running at
server site has gone into an infinite loop and does not accept any
input data.
• User wants to interrupt application program but application
program is not reading any data from the buffer.

3. OUT OF BAND SIGNALING
• TCP at the server site has found that buffer is full and it sends a
segment specifying that client window size should be zero.
• In other words TCP at the server site is announcing that no more
regular traffic is accepted.
• To remedy such a situation an urgent TCP segment must be sent
from client to server.
• The urgent segment overrides the regular flow control
mechanism.
• Although TCP is not accepting regular segments,it must accept
urgent segment.

4. OUT OF BAND SIGNALING
• When a telnet process(client or server) wants to send out of band sequence
of characters to other process(client or server),it embeds the sequence in the
data stream and inserts a special character called as DM(i.e. Data Mark).
• However,to inform the other party it creates a TCP segment in which urgent
bit set and Urgent Pointer pointing to DM character.
• when the receiving process receive the data ,it reads the data and discards
any data preceding the control characters(for example IP and IAC).
• when it reaches DM character the remaining data are handled normally.
• In other words DM character is used as synchronization character which
switches the receiving process from urgent mode to normal mode and
resynchronizes the two ends.

5. OUT OF BAND SIGNALING
DATA DM IP IAC DATA
SERVER
CLIENT
KEPT DISCARDED
In this way control characters(i.e. IP) is delivered out of band to the Operating System which uses the
Appropriate function to interrupt the running application program.