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sarah.grant.ietf@gmail.com

david@venhoek.nl

Internet Network Time Protocols This document defines an extension field that gives operators of NTP services the ability to provide additional information about their servers to clients which request it. About This Document Status information for this document may be found at . Discussion of this document takes place on the Network Time Protocols Working Group mailing list (), which is archived at . Subscribe at . Source for this draft and an issue tracker can be found at .

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 assisting in triaging faults or issues with services.

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 when, and only when, they appear in all capitals, as shown here.

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.

The Server Identifier Extension Field The Server Identifier Extension Field contains the following structure, based on the general structure of an NTP extension field defined in :

Extension Field Structure

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 Extension 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 64 octets.

Server Identifier:

A UTF-8 encoded Unicode string containing information about the server. In requests, this field shall be filled with zeroes. The length of this field should be chosen such that the length requirements on extension fields from the NTP version in use are satisfied. If the string is shorter than the field, the sender SHALL pad the remainder of the field with zeroes. The identifier ends at the first zero octet or at the end of the field, whichever comes first.

Use of the Server Identifier Extension Field To request the server identifier, a client includes a Server Identifier Extension Field in its request, long enough to hold the identifier it expects. Clients wishing to reliably receive the identifier should use the maximum permitted length. On receiving a Server Identifier Extension Field, a server MAY choose to send its server identifier in a Server Identifier Extension Field with matching length to the one in the request. If the server's identifier doesn't fit within that length, the server should not include the extension field in its response.

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. As the extension field in a response is never bigger than the one in the corresponding request (), this extension cannot increase the DDoS amplification risk of NTP. The identifier sent in responses doesn't depend on the requested length, so varying the request length discloses no additional information. Operators concerned about enumeration of servers should consider limiting the extension to trusted clients.

IANA Considerations IANA is requested to allocate the following entries in the NTP Extension Field Types registry :

Field Type	Meaning	Reference
TBD	Server Identifier	this memo

Normative References The Network Time Protocol (NTP) is widely used to synchronize computer clocks in the Internet. This document describes NTP version 4 (NTPv4), which is backwards compatible with NTP version 3 (NTPv3), described in RFC 1305, as well as previous versions of the protocol. NTPv4 includes a modified protocol header to accommodate the Internet Protocol version 6 address family. NTPv4 includes fundamental improvements in the mitigation and discipline algorithms that extend the potential accuracy to the tens of microseconds with modern workstations and fast LANs. It includes a dynamic server discovery scheme, so that in many cases, specific server configuration is not required. It corrects certain errors in the NTPv3 design and implementation and includes an optional extension mechanism. In many standards track documents several words are used to signify the requirements in the specification. These words are often capitalized. This document defines these words as they should be interpreted in IETF documents. This document specifies an Internet Best Current Practices for the Internet Community, and requests discussion and suggestions for improvements. RFC 2119 specifies common key words that may be used in protocol specifications. This document aims to reduce the ambiguity by clarifying that only UPPERCASE usage of the key words have the defined special meanings.

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