Internet Engineering Task Force M. Lichvar Internet-Draft Red Hat Updates: RFC7822 (if approved) April 25, 2019 Intended status: Standards Track Expires: October 27, 2019 NTPv4 Short Extension Fields draft-mlichvar-ntp-short-extension-fields-01 Abstract This document specifies a new packet format for the Network Time Protocol version 4 (NTPv4) which is compatible with RFC 7822, but allows NTPv4 packets to contain shorter extension fields. 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 Task Force (IETF). Note that other groups may also distribute working documents as Internet-Drafts. The list of current Internet- Drafts is at https://datatracker.ietf.org/drafts/current/. Internet-Drafts are draft documents valid for a maximum of six months and may be updated, replaced, or obsoleted by other documents at any time. It is inappropriate to use Internet-Drafts as reference material or to cite them other than as "work in progress." This Internet-Draft will expire on October 27, 2019. Copyright Notice Copyright (c) 2019 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 (https://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. Lichvar Expires October 27, 2019 [Page 1] Internet-Draft NTPv4 Short Extension Fields April 2019 1. Introduction RFC 7822 [RFC7822] specifies a minimum length of extension fields in NTPv4 packets in order to prevent ambiguities in their parsing. Without these rules, an extension field in a valid NTPv4 packet could be parsed as a Message Authentication Code (MAC), or a MAC could be parsed as an extension field. The minimum length of 28 octets forces extension fields that do not contain enough data to reach the minimum length to waste space. With multiple extension fields in a packet the wasted space accumulates. A different issue with extension fields in NTPv4 packets is that servers/clients cannot pad a response/request to a specific length, e.g. to make their length symmetric when they contain different extension fields, or the sums of their lengths are different, unless one of the extension fields included in the request/response supports padding. This document specifies a new NTPv4 format using three new extension fields: 1. An extension field which contains other extension fields with no requirements on minimum length 2. An extension field which does not contain any information and can always be used for padding 3. An extension field which contains MAC Together, these extension fields allow NTPv4 packets to contain short extension fields, minimize the wasted space, and allow the packets to be padded to any length that meets the requirements of RFC 7822. Older NTP implementations which follow RFC 7822 will parse a packet in the new format as a valid packet which contains a single unknown extension field, skipping all extension fields and/or MAC, and can respond as appropriate. An implementation which supports the new format will parse all extension fields and/or MAC contained in the packet. 1.1. Requirements Language 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 RFC 2119 [RFC2119]. Lichvar Expires October 27, 2019 [Page 2] Internet-Draft NTPv4 Short Extension Fields April 2019 2. New extension fields 2.1. Packing Field The Packing Field is an NTP extension field following RFC 7822 [RFC7822], which contains one or more other extension fields. The format of the extension field is shown below. 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 | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Subfield 1 Type | Subfield 1 Length | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ . . . Subfield 1 Data . . . +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ . . . . +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Subfield N Type | Subfield N Length | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ . . . Subfield N Data . . . +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ Figure 1: Format of Packing Field The extension field has the following fields: Field Type The type which identifies the Packing Field. TBD Length The length of the extension field, which is at least 28 octets. Subfield 1..N Type The types of the contained extension fields. Subfield 1..N Length The lengths of the contained extension field, which are divisible by 4 and can be smaller than 28. Subfield 1..N Data Lichvar Expires October 27, 2019 [Page 3] Internet-Draft NTPv4 Short Extension Fields April 2019 Data specific to the included extension fields. 2.2. Padding Field The Padding Field is an NTP extension field which does not contain any useful data. It does not follow the requirements from RFC 7822 [RFC7822] and it MUST be contained in the Packing Field. 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 | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ . . . Padding . . . +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ Figure 2: Format of Padding Field The extension field has the following fields: Field Type The type which identifies the Padding Field. TBD Length The length of the extension field. Padding Octets filling the space of the extension field with any value. 2.3. MAC Field The MAC Field is an NTP extension field which contains a MAC as specified in RFC 5905 [RFC5905]. It does not follow the requirements from RFC 7822 [RFC7822] and it MUST be contained in the Packing Field. Lichvar Expires October 27, 2019 [Page 4] Internet-Draft NTPv4 Short Extension Fields April 2019 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 | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Key Identifier | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ . . . Message Digest . . . +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ Figure 3: Format of MAC Field The extension field has the following fields: Field Type The type which identifies the MAC Field. TBD Length The length of the extension field. Key Identifier The ID of the key which is used for calculating the digest. Message Digest Digest calculated over all UDP data before the Key Identifier, including the length of the MAC Field and Packing field. 3. New NTPv4 format An NTPv4 packet in the new format consists of: 1. NTPv4 header per RFC 5905 [RFC5905](48 octets) 2. Field Type of the Packing Field (2 octets) 3. Length of all data following the NTP header (2 octets) 4. Extension fields with no restrictions on their minimum length, optionally including the Padding and/or MAC Fields (at least 24 octets) The packet MUST have exactly one Packing Field and it MUST contain all other extension fields. The packet MUST NOT have a MAC outside the Packing Field. If there is not enough data to reach the minimum Lichvar Expires October 27, 2019 [Page 5] Internet-Draft NTPv4 Short Extension Fields April 2019 length of 28 octets, the Packing Field MUST include at least one Padding Field to increase the length of the Packing Field. 4. Parsing of NTPv4 packets An implementation SHOULD check if the following applies to the UDP data before parsing it as an NTPv4 packet in the new format: 1. NTP version (in the first octet) is 4. 2. NTP mode (in the first octet) is 1, 2, 3, 4, or 5. 3. Length is at least 76 octets. 4. 49th and 50th octets contain the type of the Packing Field. 5. 51st and 52nd octets contain a value that is equal to the length of the UDP data minus 48. 5. IANA Considerations IANA is requested to allocate Extension Field Types for the Packing, Padding, and MAC Extension Fields. 6. 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, . Author's Address Lichvar Expires October 27, 2019 [Page 6] Internet-Draft NTPv4 Short Extension Fields April 2019 Miroslav Lichvar Red Hat Purkynova 115 Brno 612 00 Czech Republic Email: mlichvar@redhat.com Lichvar Expires October 27, 2019 [Page 7]