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Network Working Group D. Eastlake, III Request for Comments: 1455 Digital Equipment Corporation

                                                              May 1993

               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.

Abstract

 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.

Eastlake [Page 1]  RFC 1455 Link Security TOS May 1993

 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
 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 [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
 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
 confidentiality.

 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
 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.

Eastlake [Page 2]  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 [Page 3]  RFC 1455 Link Security TOS May 1993

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
 line distinctions.

 An example of "costs" at a hypothetical router could 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

 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

Eastlake [Page 4]  RFC 1455 Link Security TOS May 1993

 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.

References

 [1] Postel, J., "Internet Protocol - DARPA Internet Program Protocol
     Specification", STD 5, RFC 791, DARPA, September 1981.

 [2] Braden, R., Editor, "Requirements for Internet Hosts --
     Communication Layers", STD 3, RFC 1122, IETF, October 1989.

 [3] Braden, R., Editor, "Requirements for Internet Hosts --
     Application and Support", STD 3, RFC 1123, IETF, October 1989.

 [4] Almquist, P., "Type of Service in the Internet Protocol Suite",
     RFC 1349, Consultant, July 1992.

 [5] Moy, J., Editor, "OSPF Protocol Analysis", RFC 1245, Proteon,
     Inc., July 1991.

 [6] Moy, J., Editor, "Experience with the OSPF Protocol", RFC 1246,
     Proteon, Inc., July 1991.

 [7] Moy, J., "OSPF Version 2", RFC 1247, Proteon, Inc., July 1991.

Eastlake [Page 5]  RFC 1455 Link Security TOS May 1993

Security Considerations

 The entirety of this memo concerns an Internet Protocol Type of
 Service to request maximum physical link security against
 surreptitious interception.

Author's Address

 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)
 Email: dee@ranger.enet.dec.com

• Company affiliation given for identification only. This document

does not constitute a statement, official or otherwise, by Digital

 Equipment Corporation.

Eastlake [Page 6]