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Network Time Protocol Working Group                            D. Franke
Internet-Draft                                                    Akamai
Intended status: Standards Track                           June 22, 2016
Expires: December 24, 2016

   Clarifying Processing Expectations for Packets with keyid 0 in the
                    Network Time Protocol Version 4


   This memo clarifies that when a Network Time Protocol Version 4
   packet has a keyid field of zero, the MAC is present solely to
   satisfy certain syntactic constraints, and is to be ignored.

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
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   Internet-Drafts are draft documents valid for a maximum of six months
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   material or to cite them other than as "work in progress."

   This Internet-Draft will expire on December 24, 2016.

Copyright Notice

   Copyright (c) 2016 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

   This document is subject to BCP 78 and the IETF Trust's Legal
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1.  Introduction

   A Network Time Protocol Version 4 (NTPv4) packet consists of 48
   octets of required fields, followed by zero or more extension fields,
   possibly followed by a keyid field and a MAC.  RFC 5905 [RFC5905]
   (section 7.5) specifies that the MAC "is always present when an
   extension field is present".  RFC 7822 [RFC7822] relaxes this
   requirement by permitting the keyid and MAC fields to be omitted,
   provided that the last extension field has a length of at least 28
   octets.  This minimum length requirement is necessary to prevent
   syntactic ambiguity.

   Neither RFC 5905 nor RFC 7822 provides any clear guidance on what to
   do when it is necessary to construct a packet which contains at least
   one extension field but none with a length of 28 octets or more, and
   no key has been agreed which could be used to compute a valid MAC.
   This memo resolves this situation by codifying the convention,
   already observed by the RFC 5905 reference implementation and other
   existing implementations, that a keyid field of zero is a dummy value
   indicating that the MAC field is to be ignored.

2.  Requirements Terminology

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   document are to be interpreted as described in RFC 2119 [RFC2119].

3.  Processing Expectations

   In an NTPv4 packet, a keyid field with a value of zero denotes that
   the keyid field and the MAC field which follows it have been inserted
   solely to satisfy a syntactic requirement for the presence of a MAC
   field.  Implementations which receive such a packet MUST process it
   in the same manner that they would if the keyid and MAC fields were
   omitted (supposing this were syntactically possible).  In particular,
   implementations MUST NOT attempt to verify the MAC, and MUST NOT
   respond to the sender with a crypto-NAK.

4.  Security Considerations

   The security considerations of time protocols in general are
   discussed in RFC 7384 [RFC7384], and the security considerations of
   NTP are discussed in RFC 5905 [RFC5905].

   Legacy MAC fields containing dummy values do not contribute any
   information regarding the authenticity or inauthenticity of an NTP
   packet.  NTP packets with dummy MAC fields MAY prove their
   authenticity by other mechanisms, e.g.

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   [draft-mayer-ntp-mac-extension-field].  See the previously-cited RFC
   7384 and RFC 5905 for discussion of the security considerations
   surrounding accepting unauthenticated time packets.

   Whenever two cooperating principals have conflicting processing
   expectations for a similar message, "confused deputy" vulnerabilities
   may arise [confused-deputy].  Without speculating as to any specifics
   as to how this class of vulnerability could arise from this instance
   of confusion, by making the processing expectations clear we preclude
   the possibility of it doing so.

5.  IANA Considerations


6.  References

6.1.  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,

   [RFC7822]  Mizrahi, T. and D. Mayer, "Network Time Protocol Version 4
              (NTPv4) Extension Fields", RFC 7822, DOI 10.17487/RFC7822,
              March 2016, <http://www.rfc-editor.org/info/rfc7822>.

6.2.  Informative References

   [RFC7384]  Mizrahi, T., "Security Requirements of Time Protocols in
              Packet Switched Networks", RFC 7384, DOI 10.17487/RFC7384,
              October 2014, <http://www.rfc-editor.org/info/rfc7384>.

              Hardy, N., "The Confused Deputy: (or why capabilities
              might have been invented)", ACM SIGOPS Operating Systems
              Review Volume 22 Issue 4, pp. 36-38, October 1988.

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              Mayer, D. and H. Stenn, "The Network Time Protocol Version
              4 (NTPv4) MAC Extension Field", March 2016,

              Work in progress.

Author's Address

   Daniel Fox Franke
   Akamai Technologies, Inc.
   150 Broadway
   Cambridge, MA  02142
   United States

   Email: dafranke@akamai.com
   URI:   https://www.dfranke.us

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