Physical Link Security Type of Service
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This is an older version of an Internet-Draft that was ultimately published as RFC 1455.
|Donald E. Eastlake 3rd
|2013-03-02 (Latest revision 1992-11-10)
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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, <email@example.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 working documents as Internet Drafts. Internet Drafts are draft documents valid for a maximum of six months. Internet Drafts may be updated, replaced, or obsoleted by other documents at any time. It is not appropriate to use Internet Drafts as reference material or to cite them other than as a ``working draft'' or ``work in progress.'' Please check the 1id- abstracts.txt listing contained in the internet-drafts Shadow Directories on nic.ddn.mil, nnsc.nsf.net, nic.nordu.net, ftp.nisc.sri.com, or munnari.oz.au to learn the current status of any Internet Draft. This draft expires 14 May 1993 Eastlake [Page 1] 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 Eastlake [Page 2] 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. Eastlake [Page 3] 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 Eastlake [Page 4] INTERNET-DRAFT Link Security TOS Donald E. Eastlake, III 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: firstname.lastname@example.org Expiration This draft expires 14 May 1993. Eastlake [Page 5]