RFC 1455 (rfc1455) - Page 3 of 6
Physical Link Security Type of Service
Alternative Format: Original Text Document
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