INTERNET-DRAFT Link Security TOS Donald Eastlake, III
15 November 1992
Expires 14 May 1993
Physical Link Security Type of Service
Abstract
This draft proposes a type of service (TOS) to request maximum
physical link security. This would be an addition to the types of
service enumerated in RFC 1349: Type of Service in the Internet
Protocol Suite. This TOS would request 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 in
that it is based on physical link characteristics and will not
provide any particular guaranteed level of service.
This draft is intended to be submitted to the RFC editor as a
Proposed Standard. Distribution of this document is unlimited.
Please send any comments to the author, Donald Eastlake, III,
<dee@ranger.enet.dec.com>.
Status
This document is an Internet Draft. Internet Drafts are working
documents of the Internet Engineering Task Force (IETF), its Areas,
and its Working Groups. Note that other groups may also distribute
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This draft expires 14 May 1993
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INTERNET-DRAFT Link Security TOS Donald E. Eastlake, III
1. Nature of Requirement
This proposal 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 by 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 address, 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 entity inside 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 TOS facility.
Although the TOS field is provided in all current Internet packets
and routing based on TOS is provided in routing protocols such as
OSPF, there is no chance that all of the Internet will implement the
proposed additional TOS anytime 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 this capability.
1.2 Confidentiality
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
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INTERNET-DRAFT Link Security TOS Donald E. Eastlake, III
confidentiality.
There are cases where end-to-end encryption can not be used.
Examples include paths which incorporate links within nations which
severely restrict encryption, such as France, or which incorporate an
amateur radio link, where encryption is prohibited. In these cases,
link security is generally the only type of security available. The
proposed TOS will provide a way of requesting the best that the
network can do for the confidentiality of such unencrypted data.
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 differ widely in their susceptibility to surreptitious
observation of the traffic flowing over them. For example:
1) Land line media is usually harder to intercept than radio
broadcast media.
2) Between 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 intercepted, it would be much more difficult
to obtain any useful information.
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.
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INTERNET-DRAFT Link Security TOS Donald E. Eastlake, III
2. 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.
3. Note on Choice of TOS Value
The value 15 is at the maximum hamming distance from existing TOS
values. In addition, although the TOS field is no longer bit
encoded, this value is chosen so that it is binarily convenient to
specify any pair of the five defined TOS attributes should it be
decided to make such a pair a recognized TOS. The exclusive-or
(i.e., bitwise addition without carry) of any pair of the five TOS
values produces a new value not presently used for a defined TOS
which could be used to specify the combination of the two types of
service indicated by the values that were so combined.
4. Implementation
This TOS can be implemented in routing systems that offer TOS based
routing (as can be done with OSPF, see RFCs 1245 through 1248) 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
line distinctions.
An example of costs for a hypothetical router would be as follows:
Cost Type
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
It should 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
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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.
It should also be noted that using costs such as the sample given
above could result in using many more links than if the default class
of service were requested. For example, over 50 highly secure links
where two insecure links, such as a satellite hop and a radio link,
might otherwise have served.
Security Considerations
The entirety of this draft concerns an Internet Protocol Type of
Service to request maximum physical link security against
surreptitious interception.
Author's Address
Donald E. Eastlake, III
PO Box N, MIT Branch PO
Cambridge, MA 02139 USA
phone: +1 508 486 2358
email: dee@ranger.enet.dec.com
Expiration
This draft expires 14 May 1993.
Eastlake [Page 5]