NTP Working Group                                               D. Mayer
Internet Draft                                   Network Time Foundation
Intended status: Standards Track                                H. Stenn
Updates: 5905                                    Network Time Foundation
Expires: September 2016                                   March 14, 2016

      The Network Time Protocol Version 4 (NTPv4) MAC Extension Field


   The Network Time Protocol Version 4 (NTPv4) defines in RFC5905 the
   optional usage of Message Authentication Code (MAC). The MAC is an
   optional component of the NTP packet at the end of the packet. There
   can only be one MAC segment in the packet but there is no way of
   knowing if the last data segment at the end of an NTP packet is a MAC
   or an extension field, which is also defined in RFC5905. This draft
   defines a MAC extension field which will allow the existing MAC
   segment to be moved into an extension field and have a known length
   and deprecates the existing MAC.

Status of this Memo

   This Internet-Draft is submitted to IETF in full conformance with the
   provisions of BCP 78 and BCP 79.

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   This Internet-Draft will expire on September 14, 2016.

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Copyright Notice

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

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   described in the Simplified BSD License.

Table of Contents

   1. Introduction...................................................2
   2. Conventions Used in this Document..............................3
      2.1. Terminology...............................................3
      2.2. Terms & Abbreviations.....................................3
   3. MAC Extension Field............................................3
   4. Security Considerations........................................5
   5. IANA Considerations............................................5
   6. Acknowledgments................................................6
   7. References.....................................................6
      7.1. Normative References......................................6
      7.2. Informative References....................................6

1. Introduction

   The NTP packet format consists of a set of fixed fields that may be
   followed by some optional fields. Two types of optional fields are
   currently defined, a Message Authentication Code (MAC), and extension
   fields, as defined in Section 7.5 of [RFC5905].

   If a MAC is used it resides at the end of the packet. This field has
   a length which depends on the digest algorithm being used. While
   extension fields have a known length specified in the extension field
   header, there is no simple way to unequivocally know if the final
   extra data segment in an NTP packet is a MAC or if it is an extension
   field.  There is also no currently implemented way to pad the length
   of a MAC to make it difficult to determine the digest algorithm being

   This document creates a MAC extension field to remove this ambiguity,
   clearly defining a MAC in an extension field with known size, and

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   allows us the possibility of deprecating the MAC as described in
   [RFC5905]. The content of the MAC extension field is almost identical
   to the existing MAC field but with a size specified in the extension
   field and the ability to have multiple MAC's within the extension
   field for different digest algorithms. We note that the only current
   potential use for multiple MAC algorithms would be for certain
   broadcast scenarios. By deprecating the original MAC field all parts
   of the NTP packet will have well-specified lengths.

2. Conventions Used in this Document

2.1. Terminology

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

2.2. Terms & Abbreviations

   NTPv4        Network Time Protocol Version 4 [RFC5905]

   MAC          Message Authentication Code

   Legacy MAC   MAC as defined in RFC5095

3. MAC Extension Field

   The MAC extension field is designed to allow one or more MAC digests
   to be present within the MAC extension field. The MAC extension field
   contains the unsigned number of MACs present followed by the unsigned
   size of each MAC.  The number of MACs listed in the MAC COUNT in this
   extension field MUST be greater than zero.  The MAC extension field
   SHOULD be the last extension field in the packet and a legacy MAC at
   the end of the packet is OPTIONAL. The extension field MAC supplants
   the use of a legacy MAC. All new extension fields that require a MAC
   SHOULD use this MAC extension field, if the recipient implements the
   MAC extension field. The MACs present in the extension field should
   perform the digest on all parts of the packet up to but not including
   the MAC extension field. A legacy MAC MAY be present at the end of
   the extension fields provided it covers all extension fields
   including the MAC extension field and is present only for reasons of
   interoperability with servers that do not understand the new MAC
   extension field but require a MAC for authentication of the packet.
   The layout of the data in a MAC extension field is as follows:

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       0                   1                   2                   3
       0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
      |          Field Type           |      Field Length             |
      |          MAC Count            |        MAC 1 Length           |
      |          MAC 2 Length         |        MAC 3 Length           |
      .                         MAC 1 Key ID                          .
      .                                       +-+-+-+-+-+-+-+-+-+-+-+-.
      .         MAC 1 Key Data                | Random Data Padding   .
      .                         MAC 2 Key ID                          .
      .                                     +-+-+-+-+-+-+-+-+-+-+-+-+-.
      .        MAC 2 Key Data               | Random Data Padding     .
      .                         MAC 3 Key ID                          .
      .                                           +-+-+-+-+-+-+-+-+-+-.
      .          MAC 3 Key Data                   |Random Data Padding.
      |                       Padding (as needed)                     |
                   Figure 1: MAC Extension Field Format

   A Field Type of 0 and a Length of 0 means this extension field is a
   CRYPTO-NAK, as defined by RFC5905. Otherwise, a Field Type value of
   TBD identifies this extension field as a MAC Extension field. The MAC
   Count is an unsigned 16-bit field, as is each MAC length field. If
   there are an even number of MACs specified there is an unused 16-bit
   field which SHOULD be 0x0000 at the end of the set of MAC length
   values so that the subsequent MAC data is longword (4-octet) aligned.
   Each MAC SHALL be padded so that any subsequent MAC starts on a 4-
   octet boundary.

   A MAC SHOULD not be present if there is a crypto-NAK present in the

   Each MAC within the extension field consists of a 32-bit key
   identifier which SHOULD be unique to the set of key identifiers in
   this MAC extension field followed by ((MAC Length) - 4) octets of
   data, optionally followed by random octets to pad the key data to the
   length specified earlier in the extension field. That key identifier

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   is a shared secret which defines the algorithm to be used and a
   cookie or secret to be used in generating the digest. The MAC digest
   is produced by hashing the data from the beginning of the NTP packet
   up to but not including the start of the MAC extension field.  The
   calculation of the digest SHOULD be a hash of this data concatenated
   with the 32-bit keyid (in network-order), and the key. When sending
   or receiving a key identifier each side needs to agree on the key
   identifier, algorithm and cookie to be used to produce the digest
   along with the digest lengths. Note that the sender may send more
   bytes than are required by the digest algorithm. This would be done
   to make it more difficult for a casual observer to identify the
   algorithm being used based on the length of the data.  The digest
   data begins immediately after the key ID, and any padding octets
   SHOULD be random.

   MAC values should be processed until either one of the MACs is
   validated, in which case the entire packet up to the beginning of the
   MAC extension field is considered to be validated, or no more MAC
   values are left to be validated, in which case the NTP packet is
   considered to have failed MAC validation.

4. Security Considerations

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

   Digests MD5, DES and SHA-1 are considered compromised and should not
   be used [COMP].

   If possible each MAC length should be at least 68 octets long to
   allow for 4 octets of key ID and at least 64 octets of digest and
   random padding. This means that for SHA-256 digests there are 4
   octets of key ID, 32 bytes digest and 32 random octets of padding.
   Using larger minimum MAC lengths makes it difficult for an attacker
   to know which digest algorithms are used.

5. IANA Considerations

   IANA is requested to allocate the NTP extension Field Type value of
   0x0000 for CRYPTO-NAK.

   IANA is requested to allocate an NTP extension Field Type value for
   the MAC extension.  We recommend 0x3003.

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6. Acknowledgments

   The authors gratefully acknowledge Dave Mills for his insightful

   This document was prepared using 2-Word-v2.0.template.dot.

7. References

7.1. Normative References

   [KEYWORDS]    Bradner, S., "Key words for use in RFCs to Indicate
                 Requirement Levels", BCP 14, RFC 2119, March 1997.

   [RFC5905]     Mills, D., Martin, J., Burbank, J., Kasch, W.,
                 "Network Time Protocol Version 4: Protocol and
                 Algorithms Specification", RFC 5905, June 2010.

7.2. Informative References

   [RFC5906]     Haberman, B., Mills, D., "Network Time Protocol
                 Version 4: Autokey Specification", RFC 5906, June

   [COMP]       TBF

Authors' Addresses

   Danny Mayer
   Network Time Foundation
   PO Box 918
   Talent OR 97540

   Email: mayer@ntp.org

   Harlan Stenn
   Network Time Foundation
   PO Box 918
   Talent OR 97540

   Email: stenn@nwtime.org

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