Internet DRAFT - draft-stenn-ntp-ipv6-refid-hash
draft-stenn-ntp-ipv6-refid-hash
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.
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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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Internet-Draft Network Time Protocol IPv6 REFID Hash March 2016
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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.
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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].
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.
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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,
<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
Stenn Expires September 15, 2016 [Page 4]
