RFC 3514          The Security Flag in the IPv4 Header      1 April 2003


2. Syntax

   The high-order bit of the IP fragment offset field is the only unused
   bit in the IP header.  Accordingly, the selection of the bit position
   is not left to IANA.

   The bit field is laid out as follows:


   Currently-assigned values are defined as follows:

   0x0  If the bit is set to 0, the packet has no evil intent.  Hosts,
        network elements, etc., SHOULD assume that the packet is
        harmless, and SHOULD NOT take any defensive measures.  (We note
        that this part of the spec is already implemented by many common
        desktop operating systems.)

   0x1  If the bit is set to 1, the packet has evil intent.  Secure
        systems SHOULD try to defend themselves against such packets.
        Insecure systems MAY chose to crash, be penetrated, etc.