Internet Engineering Task Force H. Stenn Internet-Draft Network Time Foundation Intended status: Standards Track March 14, 2016 Expires: September 15, 2016 Network Time Protocol IPv6 REFID Hash draft-stenn-ntp-ipv6-refid-hash-00 Abstract RFC 5905 [RFC5905], section 7.3, "Packet Header Variables", defines the value to be used as the REFID for network associations. For IPv4 associations the IPv4 address is used, and for IPv6 associations four octets of the MD5 hash of the IPv6 are used. Often, the REFID is simplistically and incorrectly used to identify upstream servers. While this works in an IPv4 network, it doesn't work for IPv6 associations and may have other problems in an environment with mixed use of IPv4 and IPv6. Specifically, the NTP Project has received a report where the generated IPv6 hash decoded to the IPv4 address of a different machine on the system peer's network. This proposal offers a way for a system to generate a REFID for a system peer that communicates over IPv6 that does not conflict with a valid IPv4-based REFID. Status of This Memo This Internet-Draft is submitted in full conformance with the provisions of BCP 78 and BCP 79. Internet-Drafts are working documents of the Internet Engineering Task Force (IETF). Note that other groups may also distribute working documents as Internet-Drafts. The list of current Internet- Drafts is at http://datatracker.ietf.org/drafts/current/. Internet-Drafts are draft documents valid for a maximum of six months and may be updated, replaced, or obsoleted by other documents at any time. It is inappropriate to use Internet-Drafts as reference material or to cite them other than as "work in progress." This Internet-Draft will expire on September 15, 2016. Copyright Notice Copyright (c) 2016 IETF Trust and the persons identified as the document authors. All rights reserved. Stenn Expires September 15, 2016 [Page 1] Internet-Draft Network Time Protocol IPv6 REFID Hash March 2016 This document is subject to BCP 78 and the IETF Trust's Legal Provisions Relating to IETF Documents (http://trustee.ietf.org/license-info) in effect on the date of publication of this document. Please review these documents carefully, as they describe your rights and restrictions with respect to this document. Code Components extracted from this document must include Simplified BSD License text as described in Section 4.e of the Trust Legal Provisions and are provided without warranty as described in the Simplified BSD License. Table of Contents 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2 1.1. Requirements Language . . . . . . . . . . . . . . . . . . 3 2. Augmenting the IPv6 REFID Hash . . . . . . . . . . . . . . . 3 3. Potential Problems . . . . . . . . . . . . . . . . . . . . . 3 4. Questions . . . . . . . . . . . . . . . . . . . . . . . . . . 4 5. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 4 6. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 4 7. Security Considerations . . . . . . . . . . . . . . . . . . . 4 8. Normative References . . . . . . . . . . . . . . . . . . . . 4 Author's Address . . . . . . . . . . . . . . . . . . . . . . . . 4 1. Introduction RFC 5905 [RFC5905], section 7.3, "Packet Header Variables", defines the value to be used as the REFID for network associations. It says: If using the IPv4 address family, the identifier is the four-octet IPv4 address. If using the IPv6 family, it is the first four octets of the MD5 hash of the IPv6 address. ... Often, the REFID is simplistically and incorrectly used to identify upstream servers. While this works in an IPv4 network, it doesn't work for IPv6 associations and may have other problems in an environment with mixed use of IPv4 and IPv6. Specifically, the NTP Project has received a report where the generated IPv6 hash decoded to the IPv4 address of a different machine on the system peer's network. This proposal offers a way for a system to generate a REFID for a system peer that communicates over IPv6 that does not conflict with a valid IPv4-based REFID. Stenn Expires September 15, 2016 [Page 2] Internet-Draft Network Time Protocol IPv6 REFID Hash March 2016 1.1. Requirements Language The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this document are to be interpreted as described in RFC 2119 [RFC2119]. 2. Augmenting the IPv6 REFID Hash When generating a REFID based on a network system peer, the NTPv4 specification says: If using the IPv4 address family, the identifier is the four-octet IPv4 address. If using the IPv6 family, it is the first four octets of the MD5 hash of the IPv6 address. ... This means that the IPv4 representation of the IPv6 hash would be: b1.b2.b3.b4 . The proposal is that the system MAY also use 255.b2.b3.b4 as its REFID. When using the REFID to check for a timing loop for an IPv6 association, if the code that checks the first four-octets of the hash fails to match then the code must check again, using 0xFF as the first octet of the hash. 3. Potential Problems There is a 1 in 16,777,216 chance that the REFID hashes of two IPv6 addresses will be identical, producing a false-positive loop detection. With a sufficient number of servers, the risk of this problem becomes a non-issue. The use of the "REFID Suggestion" extension field is also a way to mitigate this potential situation. Unrealistically, if only two instances of NTP are communicating via IPv6 and one side implements this new IPv4 REFID hash and the other side does not, the "other side" will not be able to detect this loop condition. In this case, the two machines will slowly increase their Stratum until they reach S16 and become unsynchronized. This situation is considered to be unrealistic because the only current way this could happen would be for there to only be these two instances of NTP available as time sources in a misconfigured "orphan mode" setup. There is no risk of this happening in an NTP network with 3 or more time sources, or in a properly-configured "time island" setup. Stenn Expires September 15, 2016 [Page 3] Internet-Draft Network Time Protocol IPv6 REFID Hash March 2016 4. Questions Should we ask IANA to allocate a pseudo Extension Field Type of 0xFFFF (for example) so the proposed "I-Do" exchange can report whether or not the "IPv6 REFID Hash" is supported? 5. Acknowledgements The author wishes to acknowledge Dan Mahoney (and perhaps others) for suggesting the idea of using an "impossible" first-octet value to indicate an IPv6 refid hash. The author wishes to acknowledge the contributions of Joey Saccadonuts. 6. IANA Considerations This memo makes no requests of IANA. 7. Security Considerations Additional information TBD 8. Normative References [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate Requirement Levels", BCP 14, RFC 2119, DOI 10.17487/RFC2119, March 1997, . [RFC5905] Mills, D., Martin, J., Ed., Burbank, J., and W. Kasch, "Network Time Protocol Version 4: Protocol and Algorithms Specification", RFC 5905, DOI 10.17487/RFC5905, June 2010, . Author's Address Harlan Stenn Network Time Foundation P.O. Box 918 Talent, OR 97540 US Email: stenn@nwtime.org Stenn Expires September 15, 2016 [Page 4]