RFC1662 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
   LCP frames:

      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
   sent immediately.