|Title||Physical Link Security Type of Service
|Author||D. Eastlake 3rd
Network Working Group D. Eastlake, III
Request for Comments: 1455 Digital Equipment Corporation
Physical Link Security Type of Service
Status of this Memo
This memo defines an Experimental Protocol for the Internet
community. Discussion and suggestions for improvement are requested.
Please refer to the current edition of the "IAB Official Protocol
Standards" for the standardization state and status of this protocol.
Distribution of this memo is unlimited.
This RFC documents an experimental protocol providing a Type of
Service (TOS) to request maximum physical link security. This is an
addition to the types of service enumerated in RFC 1349: Type of
Service in the Internet Protocol Suite. The new TOS requests the
network to provide what protection it can against surreptitious
observation by outside agents of traffic so labeled. The purpose is
protection against traffic analysis and as an additional possible
level of data confidentiality. This TOS is consistent with all other
defined types of service for IP version 4 in that it is based on link
level characteristics and will not provide any particular guaranteed
level of service.
1. Nature of Requirement
This Internet Protocol addition addresses two potential security
requirements: resistance to traffic analysis and confidentiality.
These are described in the two subsections below followed by a
discussion of why links have different levels of physical security so
that it is meaningful to request that more secure links be used.
1.1 Traffic Analysis
At this time all Internet Protocol (IP) packets must have most of
their header information, including the "from" and "to" addresses, in
the clear. This is required for routers to properly handle the
traffic even if a higher level protocol fully encrypts all bytes in
the packet after the IP header. This renders even end-to-end
encrypted IP packets subject to traffic analysis if the data stream
can be observed. While traffic statistics are normally less
sensitive than the data content of packets, in some cases activities
of hosts or users are deducible from traffic information.
It is essential that routers have access to header information, so it
is hard to protect traffic statistics from an adversary with inside
access to the network. However, use of more secure physical links
will make traffic observation by entities outside of the network more
difficult thus improving protection from traffic analysis.
No doubt users would like to be able to request a guaranteed level of
link security, just as they would like to be able to request a
guaranteed bandwidth or delay through the network. However, such
guarantees require a resource reservation and/or policy routing
scheme and are beyond the scope of the current IP Type of Service
Although the TOS field is provided in all current Internet packets
and routing based on TOS is provided in routing protocols such as
OSPF [See 5,6,7], there is no realistic chance that all of the
Internet will implement this additional TOS any time in the
foreseeable future. Nevertheless, users concerned about traffic
analysis need to be able to request that the physical security of the
links over which their packets will be pass be maximized in
preference to other link characteristics. The proposed TOS provides
Use of physical links with greater physical security provides a layer
of protection for the confidentiality of the data in the packets as
well as traffic analysis protection. If the content of the packets
are otherwise protected by end-to-end encryption, using secure links
makes it harder for an external adversary to obtain the encrypted
data to attack. If the content of the packets is unencrypted plain
text, secure links may provide the only protection of data
There are cases where end-to-end encryption can not be used.
Examples include paths which incorporate links within nations which
restrict encryption, such as France or Australia, and paths which
incorporate an amateur radio link, where encryption is prohibited.
In these cases, link security is generally the only type of
confidentiality available. The proposed TOS will provide a way of
requesting the best that the network can do for the security of such
This TOS is required for improved confidentiality, especially in
cases where encryption can not be used, despite the fact that it does
not provide the guarantees that many users would like. See
discussion at the end of the Traffic Analysis section above.
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
1) Land line media is usually harder to intercept than radio
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
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.)
3. Protocol Implementation
This TOS can be implemented in routing systems that offer TOS based
routing (as can be done with OSPF, see RFCs 1245 through 1247) by
assigning costs to links. Establishing the "cost" for different
links for this TOS is a local policy function.
In principle services are incomparable when criterion such as those
given in the Nature of Requirement section above conflict. For
example, a choice between an encrypted broadcast system and an
unencrypted fiber optic land line. In practice, link encryption
would probably dominate all other forms of protection and physical
security as mentioned in criterion 5 above would dominate other land
An example of "costs" at a hypothetical router could be as follows:
1 Strong encryption with secure key distribution
2 Physically secure point-to-point line
6 Typical point-to-point line
8 Typical local multi-point media
12 Metropolitan area multi-point media
24 Local radio broadcast
32 Satellite link
Link costs should be chosen so as to be in the same ratio as the
probability of interception. Thus the above example costs imply a
local policy assumption that interception is 32 times more likely on
a satellite link and associated router than on a strongly encrypted
line and its associated router. It is not necessary to estimate the
absolute probability of interception on any particular link. It is
sufficient to estimate the ratio between interception probabilities
on different links.
It should be noted that using costs such as the example given above
could result in using many more links than if the default type of
service were requested. For example, the use of over 50 highly
secure links could be better than using two insecure links, such as
an unencrypted satellite hop and radio link. However, if the costs
have been properly set in proportion to the probability of
interception, this larger number of links will be more secure than
the shorter default routing. This consideration should make it clear
why it is necessary to estimate router security as well as link
security. An excessive cost ratio based solely on the security of a
communications line could cause packets to go through many routers
which were less secure than the lines in question. This necessity to
take router characteristics into account is also present for all
other defined TOS values.
It should also be noted that routing algorithms typically compute the
sum of the costs of the links. For this particular type of service,
the product of the link probabilities of secure transmission would be
more appropriate. However, the same problem is present for the high
reliability TOS and the use of a sum is an adequate approximation for
most uses as noted in RFC 1349.
 Postel, J., "Internet Protocol - DARPA Internet Program Protocol
Specification", STD 5, RFC 791, DARPA, September 1981.
 Braden, R., Editor, "Requirements for Internet Hosts --
Communication Layers", STD 3, RFC 1122, IETF, October 1989.
 Braden, R., Editor, "Requirements for Internet Hosts --
Application and Support", STD 3, RFC 1123, IETF, October 1989.
 Almquist, P., "Type of Service in the Internet Protocol Suite",
RFC 1349, Consultant, July 1992.
 Moy, J., Editor, "OSPF Protocol Analysis", RFC 1245, Proteon,
Inc., July 1991.
 Moy, J., Editor, "Experience with the OSPF Protocol", RFC 1246,
Proteon, Inc., July 1991.
 Moy, J., "OSPF Version 2", RFC 1247, Proteon, Inc., July 1991.
The entirety of this memo concerns an Internet Protocol Type of
Service to request maximum physical link security against
Donald E. Eastlake, III
Digital Equipment Corporation*
30 Porter Road, MS: LJO2/I4
Littleton, MA 01460
Phone: +1 508 486 2358 (w), +1 617 244 2679 (h)
*Company affiliation given for identification only. This document
does not constitute a statement, official or otherwise, by Digital