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Stenn 3 Internet-Draft Network Time Foundation 4 Intended status: Standards Track March 14, 2016 5 Expires: September 15, 2016 7 Network Time Protocol IPv6 REFID Hash 8 draft-stenn-ntp-ipv6-refid-hash-00 10 Abstract 12 RFC 5905 [RFC5905], section 7.3, "Packet Header Variables", defines 13 the value to be used as the REFID for network associations. For IPv4 14 associations the IPv4 address is used, and for IPv6 associations four 15 octets of the MD5 hash of the IPv6 are used. Often, the REFID is 16 simplistically and incorrectly used to identify upstream servers. 17 While this works in an IPv4 network, it doesn't work for IPv6 18 associations and may have other problems in an environment with mixed 19 use of IPv4 and IPv6. Specifically, the NTP Project has received a 20 report where the generated IPv6 hash decoded to the IPv4 address of a 21 different machine on the system peer's network. 23 This proposal offers a way for a system to generate a REFID for a 24 system peer that communicates over IPv6 that does not conflict with a 25 valid IPv4-based REFID. 27 Status of This Memo 29 This Internet-Draft is submitted in full conformance with the 30 provisions of BCP 78 and BCP 79. 32 Internet-Drafts are working documents of the Internet Engineering 33 Task Force (IETF). Note that other groups may also distribute 34 working documents as Internet-Drafts. The list of current Internet- 35 Drafts is at http://datatracker.ietf.org/drafts/current/. 37 Internet-Drafts are draft documents valid for a maximum of six months 38 and may be updated, replaced, or obsoleted by other documents at any 39 time. It is inappropriate to use Internet-Drafts as reference 40 material or to cite them other than as "work in progress." 42 This Internet-Draft will expire on September 15, 2016. 44 Copyright Notice 46 Copyright (c) 2016 IETF Trust and the persons identified as the 47 document authors. All rights reserved. 49 This document is subject to BCP 78 and the IETF Trust's Legal 50 Provisions Relating to IETF Documents 51 (http://trustee.ietf.org/license-info) in effect on the date of 52 publication of this document. Please review these documents 53 carefully, as they describe your rights and restrictions with respect 54 to this document. Code Components extracted from this document must 55 include Simplified BSD License text as described in Section 4.e of 56 the Trust Legal Provisions and are provided without warranty as 57 described in the Simplified BSD License. 59 Table of Contents 61 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2 62 1.1. Requirements Language . . . . . . . . . . . . . . . . . . 3 63 2. Augmenting the IPv6 REFID Hash . . . . . . . . . . . . . . . 3 64 3. Potential Problems . . . . . . . . . . . . . . . . . . . . . 3 65 4. Questions . . . . . . . . . . . . . . . . . . . . . . . . . . 4 66 5. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 4 67 6. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 4 68 7. Security Considerations . . . . . . . . . . . . . . . . . . . 4 69 8. Normative References . . . . . . . . . . . . . . . . . . . . 4 70 Author's Address . . . . . . . . . . . . . . . . . . . . . . . . 4 72 1. Introduction 74 RFC 5905 [RFC5905], section 7.3, "Packet Header Variables", defines 75 the value to be used as the REFID for network associations. It says: 77 If using the IPv4 address family, the identifier is the four-octet 78 IPv4 address. If using the IPv6 family, it is the first four 79 octets of the MD5 hash of the IPv6 address. ... 81 Often, the REFID is simplistically and incorrectly used to identify 82 upstream servers. While this works in an IPv4 network, it doesn't 83 work for IPv6 associations and may have other problems in an 84 environment with mixed use of IPv4 and IPv6. Specifically, the NTP 85 Project has received a report where the generated IPv6 hash decoded 86 to the IPv4 address of a different machine on the system peer's 87 network. 89 This proposal offers a way for a system to generate a REFID for a 90 system peer that communicates over IPv6 that does not conflict with a 91 valid IPv4-based REFID. 93 1.1. Requirements Language 95 The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", 96 "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this 97 document are to be interpreted as described in RFC 2119 [RFC2119]. 99 2. Augmenting the IPv6 REFID Hash 101 When generating a REFID based on a network system peer, the NTPv4 102 specification says: 104 If using the IPv4 address family, the identifier is the four-octet 105 IPv4 address. If using the IPv6 family, it is the first four 106 octets of the MD5 hash of the IPv6 address. ... 108 This means that the IPv4 representation of the IPv6 hash would be: 109 b1.b2.b3.b4 . The proposal is that the system MAY also use 110 255.b2.b3.b4 as its REFID. 112 When using the REFID to check for a timing loop for an IPv6 113 association, if the code that checks the first four-octets of the 114 hash fails to match then the code must check again, using 0xFF as the 115 first octet of the hash. 117 3. Potential Problems 119 There is a 1 in 16,777,216 chance that the REFID hashes of two IPv6 120 addresses will be identical, producing a false-positive loop 121 detection. With a sufficient number of servers, the risk of this 122 problem becomes a non-issue. The use of the "REFID Suggestion" 123 extension field is also a way to mitigate this potential situation. 125 Unrealistically, if only two instances of NTP are communicating via 126 IPv6 and one side implements this new IPv4 REFID hash and the other 127 side does not, the "other side" will not be able to detect this loop 128 condition. In this case, the two machines will slowly increase their 129 Stratum until they reach S16 and become unsynchronized. This 130 situation is considered to be unrealistic because the only current 131 way this could happen would be for there to only be these two 132 instances of NTP available as time sources in a misconfigured "orphan 133 mode" setup. There is no risk of this happening in an NTP network 134 with 3 or more time sources, or in a properly-configured "time 135 island" setup. 137 4. Questions 139 Should we ask IANA to allocate a pseudo Extension Field Type of 140 0xFFFF (for example) so the proposed "I-Do" exchange can report 141 whether or not the "IPv6 REFID Hash" is supported? 143 5. Acknowledgements 145 The author wishes to acknowledge Dan Mahoney (and perhaps others) for 146 suggesting the idea of using an "impossible" first-octet value to 147 indicate an IPv6 refid hash. The author wishes to acknowledge the 148 contributions of Joey Saccadonuts. 150 6. IANA Considerations 152 This memo makes no requests of IANA. 154 7. Security Considerations 156 Additional information TBD 158 8. Normative References 160 [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate 161 Requirement Levels", BCP 14, RFC 2119, 162 DOI 10.17487/RFC2119, March 1997, 163 . 165 [RFC5905] Mills, D., Martin, J., Ed., Burbank, J., and W. Kasch, 166 "Network Time Protocol Version 4: Protocol and Algorithms 167 Specification", RFC 5905, DOI 10.17487/RFC5905, June 2010, 168 . 170 Author's Address 172 Harlan Stenn 173 Network Time Foundation 174 P.O. Box 918 175 Talent, OR 97540 176 US 178 Email: stenn@nwtime.org