RFC 1455 (rfc1455) - Page 3 of 6

Physical Link Security Type of Service

RFC 1455                   Link Security TOS                    May 1993


1.3 Link Physical Security Characteristics

   Physical links, which are composed of lines and routers, differ
   widely in their susceptibility to surreptitious observation of the
   information flowing over them.  For examples of line security see the
   following list:

      1) Land line media is usually harder to intercept than radio
         broadcast media.

      2) Between different radio broadcast media, spread spectrum or
         other low probability of intercept systems, are harder to
         intercept than normal broadcast systems.  At the other extreme,
         systems with a large footprint on the earth, such as some
         satellite down links, may be particularly accessible.

      3) Between land lines, point to point systems are generally harder
         to intercept than multi-point systems such as Ethernet or FDDI.

      4) Fiber optic land lines are generally harder to intercept than
         metallic paths because fiber is harder to tap.

      5) A secure land line, such as one in pressurized conduit with
         pressure alarms or one installed so as to be observable by
         guards, is harder to intercept than an unsecured land line.

      6) An encrypted link would be preferable to an unencrypted link
         because, even if it was accessed, it would be much more
         difficult to obtain any useful information.

   Routers also have different levels of security against interception
   depending on the physical security of the router site and the like.

   The above comparisons show that there are significant real
   differences between the security of the physical links in use in the
   Internet.  Choosing links where it is hard for an outside observer to
   observe the traffic improves confidentiality and protection against
   traffic analysis.

2. Protocol Specification

   The value 15 decimal (F hex) in the four-bit Type of Service IP
   header field requests routing the packet to minimize the chance of
   surreptitious observation of its contents by agents external to the
   network.  (This value is chosen to be at the maximum hamming distance
   from the existing other TOS values.)





Eastlake