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

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   provisions of BCP 78 and BCP 79.

   Internet-Drafts are working documents of the Internet Engineering
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   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.




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   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
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   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

.  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.








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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].

.  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.

.  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.








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.  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?

.  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.

.  IANA Considerations

   This memo makes no requests of IANA.

.  Security Considerations

   Additional information TBD

.  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,
              <http://www.rfc-editor.org/info/rfc2119>.

   [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,
              <http://www.rfc-editor.org/info/rfc5905>.

Author's Address

   Harlan Stenn
   Network Time Foundation
   P.O. Box 918
   Talent, OR  97540
   US

   Email: stenn@nwtime.org









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