NTP Server Identifier Extension

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Table of Contents

1. Introduction

Operators of NTP services may choose to have system architectures which result in multiple servers responding for the same IP address. This is particularly notable in the case of deployments which use load balancing of UDP traffic, or the use of anycast IP addresses. In such situations, the server operator may want to provide an indication which actual server responded to a request. This information can be useful in identifying infrastructure, providing ongoing monitoring and assist in triaging faults or issues with services.¶

2. Conventions and Definitions

The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and "OPTIONAL" in this document are to be interpreted as described in BCP 14 [RFC2119] [RFC8174] when, and only when, they appear in all capitals, as shown here.¶

3. Current Deployments

Many operators use various methods to include diagnostic information about the network segment, datacentre location, region, or other details pertaining to their infrastructure in NTP responses with the NTP reference identifier most commonly being used.¶

4. The Server Identifier Extension Field

The Server Identifier field contains the following structure, based on the general structure of an NTP extension field defined in [RFC5905]:¶

 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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|       Type = TBD              |             Length            |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
.                                                               .
.                      Server Identifier                        .
.                                                               .
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

Field Type:

The type which identifies the Server Identifier extension field. To be determined by IANA. (Draft implementations: 0xf101)¶

Length:

Length of the Server Identifier field. The length is in octets expressed as an unsigned 16-bit integer and it includes the header itself. Implementations SHALL keep the length of this extension field at less than 256 bytes.¶

Server Identifier:

A Unicode string containing information about the server. In requests, this field shall be filled with zeroes. The length of this field SHALL be chosen such that the length requirements on extension fields from the NTP version in use are satisfied. If doing this requires padding of the string, the sender shall use zeroes to pad this field to the required length.¶

5. Security Considerations

Operators should note that NTP packets are not confidential and that revealing this information to clients may expose sensitive details about their network, services, or their configuration. Deployments not wishing to expose this data to clients should be configured to filter or ignore this extension on non-monitoring interfaces.¶

There is a risk of amplification with this extension field if servers blindly respond with the full server identifier on any request. To avoid this risk, it is important servers stick closely to the guidance provided in section Section 4.1.¶

6. IANA Considerations

IANA is requested to allocate the following entries in the NTP Extension Field Types registry [RFC5905]:¶

7. 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, <https://www.rfc-editor.org/rfc/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, <https://www.rfc-editor.org/rfc/rfc5905>.

[RFC8174]

Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC 2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174, May 2017, <https://www.rfc-editor.org/rfc/rfc8174>.

Acknowledgments

The authors would like to acknowledge Marco Davids, Miroslav Lichvar, Giovane Moura, and Ruben Nijveld for providing thoughtful discussion and inspiration for this document.¶

Authors' Addresses