Appenix B. Automatic Recognition of PPP Frames
It is sometimes desirable to detect PPP frames, for example during a
login sequence. The following octet sequences all begin valid PPP
7e ff 03 c0 21
7e ff 7d 23 c0 21
7e 7d df 7d 23 c0 21
Note that the first two forms are not a valid username for Unix.
However, only the third form generates a correctly checksummed PPP
frame, whenever 03 and ff are taken as the control characters ETX and
DEL without regard to parity (they are correct for an even parity
link) and discarded.
Many implementations deal with this by putting the interface into
packet mode when one of the above username patterns are detected
during login, without examining the initial PPP checksum. The
initial incoming PPP frame is discarded, but a Configure-Request is