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Internet Engineering Task Force (IETF)
Request for Comments: 7822
Updates: 5905
Category: Standards Track
ISSN: 2070-1721
T. Mizrahi
Marvell
D. Mayer
Network Time Foundation
March 2016

Network Time Protocol Version 4 (NTPv4) Extension Fields

Abstract

The Network Time Protocol version 4 (NTPv4) defines the optional usage of extension fields. An extension field, as defined in RFC 5905, is an optional field that resides at the end of the NTP header and that can be used to add optional capabilities or additional information that is not conveyed in the standard NTP header. This document updates RFC 5905 by clarifying some points regarding NTP extension fields and their usage with Message Authentication Codes (MACs).

Status of This Memo

This is an Internet Standards Track document.

This document is a product of the Internet Engineering Task Force (IETF). It represents the consensus of the IETF community. It has received public review and has been approved for publication by the Internet Engineering Steering Group (IESG). Further information on Internet Standards is available in Section 2 of RFC 5741.

Information about the current status of this document, any errata, and how to provide feedback on it may be obtained at http://www.rfc-editor.org/info/rfc7822.

Copyright Notice

Copyright © 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 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
   2. Conventions Used in This Document ...............................3
      2.1. Terminology ................................................3
      2.2. Terms and Abbreviations ....................................3
   3. NTP Extension Fields - RFC 5905 Update ..........................3
   4. Security Considerations .........................................6
   5. References ......................................................7
      5.1. Normative References .......................................7
      5.2. Informative References .....................................7
   Acknowledgments ....................................................8
   Authors' Addresses .................................................8

1. Introduction

The NTP header format consists of a set of fixed fields that may be followed by some optional fields. Two types of optional fields are defined: Message Authentication Codes (MACs), and extension fields as defined in Section 7.5 of [NTPv4].

If a MAC is used, it resides at the end of the packet. This field can be either 24 octets long, 20 octets long, or a 4-octet crypto-NAK.

NTP extension fields were defined in [NTPv4] as a generic mechanism that allows the addition of future extensions and features without modifying the NTP header format (Section 16 of [NTPv4]).

The only currently defined extension fields are those fields used by the Autokey protocol [Autokey] and the Checksum Complement [RFC7821]. The Autokey extension field is always followed by a MAC, and Section 10 of [Autokey] specifies the parsing rules that allow a host to distinguish between an extension field and a MAC. However, a MAC is not mandatory after an extension field; an NTPv4 packet can include one or more extension fields without including a MAC. This behavior is specified in Section 7.5 of [NTPv4] and in [Err3627], and is further clarified in this document.

This document updates [NTPv4] (RFC 5905) by clarifying some points regarding the usage of extension fields. These updates include changes to address errors found after the publication of [NTPv4] with respect to extension fields. Specifically, this document updates Section 7.5 of [NTPv4], clarifying the relationship between extension fields and MACs, and defining the behavior of a host that receives an unknown extension field.

2. Conventions Used in This Document

2.1. Terminology

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 [KEYWORDS].

2.2. Terms and Abbreviations

   MAC          Message Authentication Code
   
   NTPv4        Network Time Protocol version 4 [NTPv4]

3. NTP Extension Fields - RFC 5905 Update

This document updates Section 7.5 of [NTPv4] as follows:

OLD:

7.5. NTP Extension Field Format

In NTPv4, one or more extension fields can be inserted after the header and before the MAC, which is always present when an extension field is present. Other than defining the field format, this document makes no use of the field contents. An extension field contains a request or response message in the format shown in Figure 14. 

       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           |            Length             |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      .                                                               .
      .                            Value                              .
      .                                                               .
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |                       Padding (as needed)                     |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

Figure 14: Extension Field Format

All extension fields are zero-padded to a word (four octets) boundary. The Field Type field is specific to the defined function and is not elaborated here. While the minimum field length containing required fields is four words (16 octets), a maximum field length remains to be established.

The Length field is a 16-bit unsigned integer that indicates the length of the entire extension field in octets, including the Padding field.

NEW:

7.5. NTP Extension Field Format

In NTPv4, one or more extension fields can be inserted after the header and before the MAC, if a MAC is present.

Other than defining the field format, this document makes no use of the field contents. An extension field contains a request or response message in the format shown in Figure 14. 

       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           |            Length             |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      .                                                               .
      .                            Value                              .
      .                                                               .
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |                       Padding (as needed)                     |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

Figure 14: Extension Field Format

All extension fields are zero-padded to a word (four octets) boundary.

The Field Type, Value, and Padding fields are specific to the defined function and are not elaborated here; the Field Type value is defined in an IANA registry, and its Length, Value, and Padding values are defined by the document referred to by the registry. If a host receives an extension field with an unknown Field Type, the host SHOULD ignore the extension field and MAY drop the packet altogether if policy requires it.

While the minimum field length containing required fields is four words (16 octets), the maximum field length cannot be longer than 65532 octets, due to the maximum size of the Length field.

The Length field is a 16-bit unsigned integer that indicates the length of the entire extension field in octets, including the Padding field. 

   7.5.1.  Extension Fields and MACs

7.5.1.1. Extension Fields in the Presence of a MAC

An extension field can be used in an NTP packet that includes a MAC -- for example, as defined in [Autokey]. A specification that defines a new extension field MUST specify whether the extension field requires a MAC or not. If the extension field requires a MAC, the extension field specification MUST define the algorithm to be used to create the MAC and the length of the MAC thus created. An extension field MAY allow for the use of more than one algorithm, in which case the information about which algorithm was used MUST be included in the extension field itself.

7.5.1.2. Multiple Extension Fields with a MAC

If there are multiple extension fields that require a MAC, they MUST all require the use of the same algorithm and MAC length. Extension fields that do not require a MAC can be included with extension fields that do require a MAC.

An NTP packet MUST NOT be sent with two or more extension fields that require a MAC with different algorithms.

If an NTP packet is received with two or more extension fields that this receiver recognizes and those fields require a MAC with different algorithms, the packet MUST be discarded.

7.5.1.3. MAC in the Absence of an Extension Field

A MAC MUST NOT be longer than 24 octets if there is no extension field present, unless a longer MAC is agreed upon by both client and server. The client and server can negotiate this behavior using a previous exchange of packets with an extension field that defines the size and algorithm of the MAC transmitted in NTP packets.

7.5.1.4. Extension Fields in the Absence of a MAC

If a MAC is not present, one or more extension fields can be inserted after the header, according to the following rules:

  • If the packet includes a single extension field, the length of the extension field MUST be at least 7 words, i.e., at least 28 octets.
  • If the packet includes more than one extension field, the length of the last extension field MUST be at least 28 octets. The length of the other extension fields in this case MUST be at least 16 octets each.

4. Security Considerations

The security considerations of time protocols in general are discussed in [SecTime], and the security considerations of NTP are discussed in [NTPv4].

Distributed Denial-of-Service (DDoS) attacks on NTP servers involve flooding a server with a high rate of NTP packets. Malicious usage of extension fields cannot amplify such DDoS attacks; such malicious attempts are mitigated by NTP servers, since the servers ignore unknown extension fields (as discussed in Section 3) and only respond, if needed, with known extension fields. Extension fields from incoming packets are neither propagated by NTP servers nor included in any response. NTP servers create their own extension fields if needed for a response. A large number of extension fields should be flagged by an NTP server as a potential attack. Large extension field sizes should also be flagged, unless they are expected to be large.

Middleboxes such as firewalls MUST NOT filter NTP packets based on their extension fields. Such middleboxes should not examine extension fields in the packets, since NTP packets may contain new extension fields that the middleboxes have not been updated to recognize.

5. References

5.1. Normative References

   [KEYWORDS]  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>.
   
   [NTPv4]     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>.

5.2. Informative References

   [Autokey]   Haberman, B., Ed., and D. Mills, "Network Time Protocol
               Version 4: Autokey Specification", RFC 5906,
               DOI 10.17487/RFC5906, June 2010,
               <http://www.rfc-editor.org/info/rfc5906>.
   
   [Err3627]   RFC Errata, Erratum ID 3627, RFC 5905.
   
   [RFC7821]   Mizrahi, T., "UDP Checksum Complement in the Network Time
               Protocol (NTP)", RFC 7821, DOI 10.17487/RFC7821,
               March 2016, <http://www.rfc-editor.org/info/rfc7821>.
   
   [SecTime]   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>.

Acknowledgments

The authors gratefully acknowledge Dave Mills for his insightful comments. The authors also thank Tim Chown, Sean Turner, Miroslav Lichvar, Suresh Krishnan, and Jari Arkko for their thorough review and helpful comments.

Authors' Addresses

Tal Mizrahi
Marvell
6 Hamada St.
Yokneam, 20692
Israel

Email: 

          talmi@marvell.com
   
   Danny Mayer
   Network Time Foundation
   PO Box 918
   Talent, OR  97540
   United States

Email: 

          mayer@ntp.org