Roughtime
draft-ietf-ntp-roughtime-11

Network Time Protocols                                           W. Ladd
Internet-Draft                                       Akamai Technologies
Intended status: Informational                               M. Dansarie
Expires: 3 February 2025                                          Netnod
                                                           2 August 2024

                               Roughtime
                      draft-ietf-ntp-roughtime-11

Abstract

   This document specifies Roughtime - a protocol that aims to achieve
   rough time synchronization even for clients without any idea of what
   time it is.

About This Document

   This note is to be removed before publishing as an RFC.

   Status information for this document may be found at
   https://datatracker.ietf.org/doc/draft-ietf-ntp-roughtime/.

   Source for this draft and an issue tracker can be found at
   https://github.com/wbl/roughtime-draft.

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
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   This Internet-Draft will expire on 3 February 2025.

Copyright Notice

   Copyright (c) 2024 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

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Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   3
   2.  Conventions and Definitions . . . . . . . . . . . . . . . . .   3
   3.  Protocol Overview . . . . . . . . . . . . . . . . . . . . . .   3
   4.  The Guarantee . . . . . . . . . . . . . . . . . . . . . . . .   4
   5.  Message Format  . . . . . . . . . . . . . . . . . . . . . . .   4
     5.1.  Data types  . . . . . . . . . . . . . . . . . . . . . . .   5
       5.1.1.  int32 . . . . . . . . . . . . . . . . . . . . . . . .   5
       5.1.2.  uint32  . . . . . . . . . . . . . . . . . . . . . . .   5
       5.1.3.  uint64  . . . . . . . . . . . . . . . . . . . . . . .   5
       5.1.4.  Tag . . . . . . . . . . . . . . . . . . . . . . . . .   6
       5.1.5.  Timestamp . . . . . . . . . . . . . . . . . . . . . .   6
     5.2.  Header  . . . . . . . . . . . . . . . . . . . . . . . . .   6
   6.  Protocol Details  . . . . . . . . . . . . . . . . . . . . . .   7
     6.1.  Requests  . . . . . . . . . . . . . . . . . . . . . . . .   8
       6.1.1.  VER . . . . . . . . . . . . . . . . . . . . . . . . .   8
       6.1.2.  NONC  . . . . . . . . . . . . . . . . . . . . . . . .   8
       6.1.3.  SRV . . . . . . . . . . . . . . . . . . . . . . . . .   8
       6.1.4.  ZZZZ  . . . . . . . . . . . . . . . . . . . . . . . .   8
     6.2.  Responses . . . . . . . . . . . . . . . . . . . . . . . .   9
       6.2.1.  SIG . . . . . . . . . . . . . . . . . . . . . . . . .   9
       6.2.2.  VER . . . . . . . . . . . . . . . . . . . . . . . . .   9
       6.2.3.  NONC  . . . . . . . . . . . . . . . . . . . . . . . .   9
       6.2.4.  PATH  . . . . . . . . . . . . . . . . . . . . . . . .   9
       6.2.5.  SREP  . . . . . . . . . . . . . . . . . . . . . . . .  10
       6.2.6.  CERT  . . . . . . . . . . . . . . . . . . . . . . . .  10
       6.2.7.  INDX  . . . . . . . . . . . . . . . . . . . . . . . .  10
     6.3.  The Merkle Tree . . . . . . . . . . . . . . . . . . . . .  11
       6.3.1.  Root Value Validity Check Algorithm . . . . . . . . .  11
     6.4.  Validity of Response  . . . . . . . . . . . . . . . . . .  12
   7.  Integration into NTP  . . . . . . . . . . . . . . . . . . . .  12
   8.  Grease  . . . . . . . . . . . . . . . . . . . . . . . . . . .  12
   9.  Roughtime Clients . . . . . . . . . . . . . . . . . . . . . .  13
     9.1.  Necessary configuration . . . . . . . . . . . . . . . . .  13
     9.2.  Measurement sequence  . . . . . . . . . . . . . . . . . .  13
     9.3.  Malfeasence reporting . . . . . . . . . . . . . . . . . .  13
   10. Security Considerations . . . . . . . . . . . . . . . . . . .  13
   11. Privacy Considerations  . . . . . . . . . . . . . . . . . . .  14

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   12. Operational Considerations  . . . . . . . . . . . . . . . . .  14
   13. IANA Considerations . . . . . . . . . . . . . . . . . . . . .  14
     13.1.  Service Name and Transport Protocol Port Number
            Registry . . . . . . . . . . . . . . . . . . . . . . . .  14
     13.2.  Roughtime Version Registry . . . . . . . . . . . . . . .  15
     13.3.  Roughtime Tag Registry . . . . . . . . . . . . . . . . .  16
   14. References  . . . . . . . . . . . . . . . . . . . . . . . . .  17
     14.1.  Normative References . . . . . . . . . . . . . . . . . .  17
     14.2.  Informative References . . . . . . . . . . . . . . . . .  18
   Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . . .  19
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  19

1.  Introduction

   Time synchronization is essential to Internet security as many
   security protocols and other applications require synchronization
   [RFC738].  Unfortunately, widely deployed protocols such as the
   Network Time Protocol (NTP) [RFC5905] lack essential security
   features, and even newer protocols like Network Time Security (NTS)
   [RFC8915] lack mechanisms to ensure that the servers behave
   correctly.  Furthermore, clients may lack even a basic idea of the
   time, creating bootstrapping problems.  Roughtime is intended to
   permit devices to obtain a rough idea of the current time from fairly
   static configuration consisting of a key and a server.

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.  Protocol Overview

   Roughtime is a protocol for rough time synchronization that enables
   clients to provide cryptographic proof of server malfeasance.  It
   does so by having responses from servers include a signature over a
   value derived from a nonce in the client request.  This provides
   cryptographic proof that the timestamp was issued after the server
   received the client's request.  The derived value included in the
   server's response is the root of a Merkle tree which includes the
   hash of the client's nonce as the value of one of its leaf nodes.
   This enables the server to amortize the relatively costly signing
   operation over a number of client requests.

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   Single server mode: At its most basic level, Roughtime is a one round
   protocol in which a completely fresh client requests the current time
   and the server sends a signed response.  The response includes a
   timestamp and a radius used to indicate the server's certainty about
   the reported time.

   The server proves freshness of its response as follows.  The client's
   request contains a nonce which the server incorporates into its
   signed response.  The client can verify the server's signatures and -
   provided that the nonce has sufficient entropy - this proves that the
   signed response could only have been generated after the nonce.

4.  The Guarantee

   A Roughtime server guarantees that a response to a query sent at t_1,
   received at t_2, and with timestamp t_3 has been created between t_1
   and t_2.  If t_3 is not within that interval, a server inconsistency
   may be detected and used to impeach the server.  The propagation of
   such a guarantee and its use for time synchronization is discussed in
   Section 7.  No delay attacker may affect this: they may only expand
   the interval between t_1 and t_2, or of course stop the measurement
   in the first place.

5.  Message Format

   Roughtime messages are maps consisting of one or more (tag, value)
   pairs.  They start with a header, which contains the number of pairs,
   the tags, and value offsets.  The header is followed by a message
   values section which contains the values associated with the tags in
   the header.  Messages MUST be formatted according to Figure 1 as
   described in the following sections.

   Messages MAY be recursive, i.e. the value of a tag can itself be a
   Roughtime message.

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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
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                   Number of pairs (uint32)                    |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                                                               |
   .                                                               .
   .                     N-1 offsets (uint32)                      .
   .                                                               .
   |                                                               |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                                                               |
   .                                                               .
   .                        N tags (uint32)                        .
   .                                                               .
   |                                                               |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                                                               |
   .                                                               .
   .                            Values                             .
   .                                                               .
   |                                                               |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

                        Figure 1: Roughtime Message

5.1.  Data types

5.1.1.  int32

   An int32 is a 32 bit signed integer.  It is serialized least
   significant byte first in sign-magnitude representation with the sign
   bit in the most significant bit.  The negative zero value
   (0x80000000) MUST NOT be used and any message with it is
   syntactically invalid and MUST be ignored.

5.1.2.  uint32

   A uint32 is a 32 bit unsigned integer.  It is serialized with the
   least significant byte first.

5.1.3.  uint64

   A uint64 is a 64 bit unsigned integer.  It is serialized with the
   least significant byte first.

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5.1.4.  Tag

   Tags are used to identify values in Roughtime messages.  A tag is a
   uint32 but can also be represented as a sequence of up to four ASCII
   characters [RFC20].  ASCII strings shorter than four characters can
   be unambiguously converted to tags by padding them with zero bytes.
   Tags MUST NOT contain any other bytes than capital letters (A-Z) or
   padding zero bytes.  For example, the ASCII string "NONC" would
   correspond to the tag 0x434e4f4e and "ZZZZ" would correspond to
   0x5a5a5a5a.  Note that when encoded into a message the ASCII values
   will be in the natural bytewise order.

5.1.5.  Timestamp

   A timestamp is a representation of UTC time as a uint64 count of
   seconds since 00:00:00 on 1 January 1970 (the Unix epoch), assuming
   every day has 86400 seconds.  This is a constant offset from the NTP
   timestamp in seconds.  Leap seconds do not have an unambiguous
   representation in a timestamp, and this has implications for the
   attainable accuracy and setting of the RADI tag.

5.2.  Header

   All Roughtime messages start with a header.  The first four bytes of
   the header is the uint32 number of tags N, and hence of (tag, value)
   pairs.

   The following 4*(N-1) bytes are offsets, each a uint32.  The last 4*N
   bytes in the header are tags.  Offsets refer to the positions of the
   values in the message values section.  All offsets MUST be multiples
   of four and placed in increasing order.  The first post-header byte
   is at offset 0.  The offset array is considered to have a not
   explicitly encoded value of 0 as its zeroth entry.

   The value associated with the ith tag begins at offset[i] and ends at
   offset[i+1]-1, with the exception of the last value which ends at the
   end of the message.  Values may have zero length.  All lengths and
   offsets are in bytes.

   Tags MUST be listed in the same order as the offsets of their values
   and MUST also be sorted in ascending order by numeric value.  A tag
   MUST NOT appear more than once in a header.

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6.  Protocol Details

   As described in Section 3, clients initiate time synchronization by
   sending requests containing a nonce to servers who send signed time
   responses in return.  Roughtime packets can be sent between clients
   and servers either as UDP datagrams or via TCP streams.  Servers
   SHOULD support the UDP transport mode and TCP mode.

   A Roughtime packet MUST be formatted according to Figure 2 and as
   described here.  The first field is a uint64 with the value
   0x4d49544847554f52 ("ROUGHTIM" in ASCII).  The second field is a
   uint32 and contains the length of the third field.  The third and
   last field contains a Roughtime message as specified in Section 5.

   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
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                  0x4d49544847554f52 (uint64)                  |
   |                        ("ROUGHTIM")                           |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                    Message length (uint32)                    |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                                                               |
   .                                                               .
   .                      Roughtime message                        .
   .                                                               .
   |                                                               |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

                         Figure 2: Roughtime packet

   Roughtime request and response packets MUST be transmitted in a
   single datagram when the UDP transport mode is used.  Setting the
   packet's don't fragment bit [RFC791] is OPTIONAL in IPv4 networks.

   Multiple requests and responses can be exchanged over an established
   TCP connection.  Clients MAY send multiple requests at once and
   servers MAY send responses out of order.  The connection SHOULD be
   closed by the client when it has no more requests to send and has
   received all expected responses.  Either side SHOULD close the
   connection in response to synchronization, format, implementation-
   defined timeouts, or other errors.

   All requests and responses MUST contain the VER tag.  It contains a
   list of one or more uint32 version numbers.  The version of Roughtime
   specified by this memo has version number 1.

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   NOTE TO RFC EDITOR: remove this paragraph before publication.  For
   testing drafts of this memo, a version number of 0x80000000 plus the
   draft number is used.

6.1.  Requests

   A request MUST contain the tags VER and NONC.  It SHOULD include the
   tag SRV.  Other tags SHOULD be ignored by the server.  A future
   version of this protocol may mandate additional tags in the message
   and assign them semantic meaning.

   The size of the request message SHOULD be at least 1024 bytes when
   the UDP transport mode is used.  To attain this size the ZZZZ tag
   SHOULD be added to the message.  Responding to requests shorter than
   1024 bytes is OPTIONAL and servers MUST NOT send responses larger
   than the requests they are replying to.

6.1.1.  VER

   In a request, the VER tag contains a list of versions.  The VER tag
   MUST include at least one Roughtime version supported by the client.
   The client MUST ensure that the version numbers and tags included in
   the request are not incompatible with each other or the packet
   contents.

   The version numbers MUST NOT repeat.

6.1.2.  NONC

   The value of the NONC tag is a 32 byte nonce.  It SHOULD be generated
   in a manner indistinguishable from random.  BCP 106 contains specific
   guidelines regarding this [RFC4086].

6.1.3.  SRV

   The SRV tag is used by the client to indicate which long-term public
   key it expects to verify the response with.  The value of the SRV tag
   is H(0xff || public_key) where public_key is the server's long-term,
   32-byte Ed25519 public key and H is SHA512 truncated to the first 32
   bytes.

6.1.4.  ZZZZ

   The ZZZZ tag is used to expand the response to the minimum required
   length.  Its value MUST be a string of all zeros.

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6.2.  Responses

   The server begins the request handling process with a set of long-
   term keys.  It resolves which long-term key to use with the following
   procedure:

   1.  If the request contains a SRV tag, then the server looks up the
       long-term key indicated by the SRV value.  If no such key exists,
       then the server MUST ignore the request.

   2.  If the request contains no SRV tag, but the server has just one
       long-term key, it SHOULD select that key.  Otherwise, if the
       server has multiple long-term keys, then it MUST ignore the
       request.

   A response MUST contain the tags SIG, VER, NONC, PATH, SREP, CERT,
   and INDX.

6.2.1.  SIG

   In general, a SIG tag value is a 64 byte Ed25519 signature [RFC8032]
   over a concatenation of a signature context ASCII string and the
   entire value of a tag.  All context strings MUST include a
   terminating zero byte.

   The SIG tag in the root of a response MUST be a signature over the
   SREP value using the public key contained in CERT.  The context
   string MUST be "RoughTime v1 response signature".

6.2.2.  VER

   In a response, the VER tag MUST contain a single version number.  It
   SHOULD be one of the version numbers supplied by the client in its
   request.  The server MUST ensure that the version number corresponds
   with the rest of the packet contents.

6.2.3.  NONC

   The NONC tag MUST contain the nonce of the message being responded
   to.

6.2.4.  PATH

   The PATH tag value MUST be a multiple of 32 bytes long and represent
   a path of 32 byte hash values in the Merkle tree used to generate the
   ROOT value as described in a Section 6.3.  In the case where a
   response is prepared for a single request and the Merkle tree
   contains only the root node, the size of PATH MUST be zero.

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6.2.5.  SREP

   The SREP tag contains a time response.  Its value MUST be a Roughtime
   message with the tags ROOT, MIDP, and RADI.

   The ROOT tag MUST contain a 32 byte value of a Merkle tree root as
   described in Section 6.3.

   The MIDP tag value MUST be the timestamp of the moment of processing.

   The RADI tag value MUST be a uint32 representing the server's
   estimate of the accuracy of MIDP in seconds.  Servers MUST ensure
   that the true time is within (MIDP-RADI, MIDP+RADI) at the time they
   transmit the response message.

   The value of the RADI tag MUST be at least 3 seconds.  Otherwise leap
   seconds will impact the observed correctness of Roughtime servers.

6.2.6.  CERT

   The CERT tag contains a public-key certificate signed with the
   server's long-term key.  Its value is a Roughtime message with the
   tags DELE and SIG, where SIG is a signature over the DELE value.  The
   context string used to generate SIG MUST be "RoughTime v1 delegation
   signature--".

   The DELE tag contains a delegated public-key certificate used by the
   server to sign the SREP tag.  Its value is a Roughtime message with
   the tags MINT, MAXT, and PUBK.  The purpose of the DELE tag is to
   enable separation of a long-term public key from keys on devices
   exposed to the public Internet.

   The MINT tag is the minimum timestamp for which the key in PUBK is
   trusted to sign responses.  MIDP MUST be more than or equal to MINT
   for a response to be considered valid.

   The MAXT tag is the maximum timestamp for which the key in PUBK is
   trusted to sign responses.  MIDP MUST be less than or equal to MAXT
   for a response to be considered valid.

   The PUBK tag contains a temporary 32 byte Ed25519 public key which is
   used to sign the SREP tag.

6.2.7.  INDX

   The INDX tag value is a uint32 determining the position of NONC in
   the Merkle tree used to generate the ROOT value as described in
   Section 6.3.

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6.3.  The Merkle Tree

   A Merkle tree is a binary tree where the value of each non-leaf node
   is a hash value derived from its two children.  The root of the tree
   is thus dependent on all leaf nodes.

   In Roughtime, each leaf node in the Merkle tree represents the nonce
   in one request.  Leaf nodes are indexed left to right, beginning with
   zero.

   The values of all nodes are calculated from the leaf nodes and up
   towards the root node using the first 32 bytes of the output of the
   SHA-512 hash algorithm [RFC6234].  For leaf nodes, the byte 0x00 is
   prepended to the nonce before applying the hash function.  For all
   other nodes, the byte 0x01 is concatenated with first the left and
   then the right child node value before applying the hash function.

   The value of the Merkle tree's root node is included in the ROOT tag
   of the response.

   The index of a request's nonce node is included in the INDX tag of
   the response.

   The values of all sibling nodes in the path between a request's nonce
   node and the root node are stored in the PATH tag so that the client
   can reconstruct and validate the value in the ROOT tag using its
   nonce.  These values are each 32 bytes and are stored one after the
   other with no additional padding or structure.  The order in which
   they are stored is described in the next section.

6.3.1.  Root Value Validity Check Algorithm

   This section describes how to compute the root hash of the Merkle
   tree from the values in the tags PATH, INDX, and NONC.  The bits of
   INDX are ordered from least to most significant.  H(x) denotes the
   first 32 bytes of the SHA-512 hash digest of x and || denotes
   concatenation.

   The algorithm maintains a current hash value.  At initialization,
   hash is set to H(0x00 || nonce).  When no more entries remain in
   PATH, the current hash is the hash of the Merkle tree.  All remaining
   bits of INDX MUST be zero at that time.  Otherwise, let node be the
   next 32 bytes in PATH.  If the current bit in INDX is 0 then hash =
   H(0x01 || node || hash), else hash = H(0x01 || hash || node).

   PATH is thus the siblings from the leaf to the root.

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6.4.  Validity of Response

   A client MUST check the following properties when it receives a
   response.  We assume the long-term server public key is known to the
   client through other means.

   The signature in CERT was made with the long-term key of the server.

   The DELE timestamps and the MIDP value are consistent.

   The INDX and PATH values prove NONC was included in the Merkle tree
   with value ROOT using the algorithm in Section 6.3.1.

   The signature of SREP in SIG validates with the public key in DELE.

   A response that passes these checks is said to be valid.  Validity of
   a response does not prove the time is correct, but merely that the
   server signed it, and thus promises that it began to compute the
   signature at a time in the interval (MIDP-RADI, MIDP+RADI).

7.  Integration into NTP

   We assume that there is a bound PHI on the frequency error in the
   clock on the machine.  Given a measurement taken at a local time t,
   we know the true time is in (t-delta-sigma, t-delta+sigma).  After d
   seconds have elapsed we know the true time is within (t-delta-sigma-
   d_PHI, t-delta+sigma+d_PHI).  A simple and effective way to mix with
   NTP or PTP discipline of the clock is to trim the observed intervals
   in NTP to fit entirely within this window or reject measurements that
   fall to far outside.  This assumes time has not been stepped.  If the
   NTP process decides to step the time, it MUST use Roughtime to ensure
   the new truetime estimate that will be stepped to is consistent with
   the true time.  Should this window become too large, another
   Roughtime measurement is called for.  The definition of "too large"
   is implementation defined.  Implementations MAY use other, more
   sophisticated means of adjusting the clock respecting Roughtime
   information.  Other applications such as X.509 verification may wish
   to apply different rules.

8.  Grease

   Servers SHOULD send back a fraction of responses that are
   syntactically invalid or contain invalid signatures as well as
   incorrect times.  Clients MUST properly reject such responses.
   Servers MUST NOT send back responses with incorrect times and valid
   signatures.  Either signature MAY be invalid for this application.

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9.  Roughtime Clients

9.1.  Necessary configuration

   To carry out a Roughtime measurement, a client must be equipped with
   a list of servers, a minimum of three of which are operational, not
   run by the same parties.  It must also have a means of reporting to
   the provider of such a list, such as an OS vendor or software vendor,
   a failure report as described below.

9.2.  Measurement sequence

   The client randomly permutes three servers from the list, and
   sequentially queries them.  The first probe uses a NONC that is
   randomly generated.  The second query uses H(resp || rand) where rand
   is a random 32 byte value and resp is the entire response to the
   first probe.  The third query uses H(resp || rand) for a different 32
   byte value.  If the times reported are consistent with the causal
   ordering, and the delay is within a system provided parameter, the
   measurement succeeds.  If they are not consistent, there has been
   malfeasance and the client SHOULD store a report for evaluation,
   alert the operator, and make another measurement.

9.3.  Malfeasence reporting

   A malfeasance report is a JSON [RFC8259] object with keys "nonces",
   containing an array of the rand values as base64-encoded [RFC4648]
   strings, and "responses", containing an array of the responses as
   base64-encoded strings.

   Malfeasence reports MAY be transported by any means to the relevant
   vendor or server operator for discussion.  A malfeasance report is
   cryptographic proof that the responses arrived in that order, and can
   be used to demonstrate that at least one server sent the wrong time.
   The venues for sharing such reports and what to do about them are
   outside the scope of this document.

10.  Security Considerations

   Since the only supported signature scheme, Ed25519, is not quantum
   resistant, the Roughtime version described in this memo will not
   survive the advent of quantum computers.

   Maintaining a list of trusted servers and adjudicating violations of
   the rules by servers is not discussed in this document and is
   essential for security.  Roughtime clients MUST regularly update
   their view of which servers are trustworthy in order to benefit from
   the detection of misbehavior.

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   Validating timestamps made on different dates requires knowledge of
   leap seconds in order to calculate time intervals correctly.

   Servers carry out a significant amount of computation in response to
   clients, and thus may experience vulnerability to denial of service
   attacks.

   This protocol does not provide any confidentiality.  Given the nature
   of timestamps, such impact is minor.

   The compromise of a PUBK's private key, even past MAXT, is a problem
   as the private key can be used to sign invalid times that are in the
   range MINT to MAXT, and thus violate the good behavior guarantee of
   the server.

   Servers MUST NOT send response packets larger than the request
   packets sent by clients, in order to prevent amplification attacks.

11.  Privacy Considerations

   This protocol is designed to obscure all client identifiers.  Servers
   necessarily have persistent long-term identities essential to
   enforcing correct behavior.  Generating nonces in a nonrandom manner
   can cause leaks of private data or enable tracking of clients as they
   move between networks.

12.  Operational Considerations

   It is expected that clients identify a server by its long-term public
   key.  In multi-tenancy environments, where multiple servers may be
   listening on the same IP or port space, the protocol is designed so
   that the client indicates which server it expects to respond.  This
   is done with the SRV tag.

13.  IANA Considerations

13.1.  Service Name and Transport Protocol Port Number Registry

   IANA is requested to allocate the following entry in the Service Name
   and Transport Protocol Port Number Registry:

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     Service Name: Roughtime

     Transport Protocol: tcp,udp

     Assignee: IESG <iesg@ietf.org>

     Contact: IETF Chair <chair@ietf.org>

     Description: Roughtime time synchronization

     Reference: [[this memo]]

     Port Number: [[TBD1]], selected by IANA from the User Port range

13.2.  Roughtime Version Registry

   IANA is requested to create a new registry entitled "Roughtime
   Version Registry".  Entries shall have the following fields:

   Version ID (REQUIRED): a 32-bit unsigned integer

   Version name (REQUIRED): A short text string naming the version being
   identified.

   Reference (REQUIRED): A reference to a relevant specification
   document.

   The policy for allocation of new entries SHOULD be: IETF Review.

   The initial contents of this registry shall be as follows:

     +=======================+======================+===============+
     | Version ID            | Version name         | Reference     |
     +=======================+======================+===============+
     | 0x0                   | Reserved             | [[this memo]] |
     +-----------------------+----------------------+---------------+
     | 0x1                   | Roughtime version 1  | [[this memo]] |
     +-----------------------+----------------------+---------------+
     | 0x2-0x7fffffff        | Unassigned           |               |
     +-----------------------+----------------------+---------------+
     | 0x80000000-0xffffffff | Reserved for Private | [[this memo]] |
     +-----------------------+----------------------+---------------+
     |                       | or Experimental use  |               |
     +-----------------------+----------------------+---------------+

                                 Table 1

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13.3.  Roughtime Tag Registry

   IANA is requested to create a new registry entitled "Roughtime Tag
   Registry".  Entries SHALL have the following fields:

   Tag (REQUIRED): A 32-bit unsigned integer in hexadecimal format.

   ASCII Representation (REQUIRED): The ASCII representation of the tag
   in accordance with Section 5.1.4 of this memo, if applicable.

   Reference (REQUIRED): A reference to a relevant specification
   document.

   The policy for allocation of new entries in this registry SHOULD be:
   Specification Required.

   The initial contents of this registry SHALL be as follows:

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           +============+======================+===============+
           |        Tag | ASCII Representation | Reference     |
           +============+======================+===============+
           | 0x00474953 | SIG                  | [[this memo]] |
           +------------+----------------------+---------------+
           | 0x00565253 | SRV                  | [[this memo]] |
           +------------+----------------------+---------------+
           | 0x00524556 | VER                  | [[this memo]] |
           +------------+----------------------+---------------+
           | 0x434e4f4e | NONC                 | [[this memo]] |
           +------------+----------------------+---------------+
           | 0x454c4544 | DELE                 | [[this memo]] |
           +------------+----------------------+---------------+
           | 0x48544150 | PATH                 | [[this memo]] |
           +------------+----------------------+---------------+
           | 0x49444152 | RADI                 | [[this memo]] |
           +------------+----------------------+---------------+
           | 0x4b425550 | PUBK                 | [[this memo]] |
           +------------+----------------------+---------------+
           | 0x5044494d | MIDP                 | [[this memo]] |
           +------------+----------------------+---------------+
           | 0x50455253 | SREP                 | [[this memo]] |
           +------------+----------------------+---------------+
           | 0x544e494d | MINT                 | [[this memo]] |
           +------------+----------------------+---------------+
           | 0x544f4f52 | ROOT                 | [[this memo]] |
           +------------+----------------------+---------------+
           | 0x54524543 | CERT                 | [[this memo]] |
           +------------+----------------------+---------------+
           | 0x5458414d | MAXT                 | [[this memo]] |
           +------------+----------------------+---------------+
           | 0x58444e49 | INDX                 | [[this memo]] |
           +------------+----------------------+---------------+
           | 0x5a5a5a5a | ZZZZ                 | [[this memo]] |
           +------------+----------------------+---------------+

                                  Table 2

14.  References

14.1.  Normative References

   [RFC20]    Cerf, V., "ASCII format for network interchange", STD 80,
              RFC 20, DOI 10.17487/RFC0020, October 1969,
              <https://www.rfc-editor.org/rfc/rfc20>.

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   [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>.

   [RFC4086]  Eastlake 3rd, D., Schiller, J., and S. Crocker,
              "Randomness Requirements for Security", BCP 106, RFC 4086,
              DOI 10.17487/RFC4086, June 2005,
              <https://www.rfc-editor.org/rfc/rfc4086>.

   [RFC4648]  Josefsson, S., "The Base16, Base32, and Base64 Data
              Encodings", RFC 4648, DOI 10.17487/RFC4648, October 2006,
              <https://www.rfc-editor.org/rfc/rfc4648>.

   [RFC6234]  Eastlake 3rd, D. and T. Hansen, "US Secure Hash Algorithms
              (SHA and SHA-based HMAC and HKDF)", RFC 6234,
              DOI 10.17487/RFC6234, May 2011,
              <https://www.rfc-editor.org/rfc/rfc6234>.

   [RFC791]   Postel, J., "Internet Protocol", STD 5, RFC 791,
              DOI 10.17487/RFC0791, September 1981,
              <https://www.rfc-editor.org/rfc/rfc791>.

   [RFC8032]  Josefsson, S. and I. Liusvaara, "Edwards-Curve Digital
              Signature Algorithm (EdDSA)", RFC 8032,
              DOI 10.17487/RFC8032, January 2017,
              <https://www.rfc-editor.org/rfc/rfc8032>.

   [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>.

   [RFC8259]  Bray, T., Ed., "The JavaScript Object Notation (JSON) Data
              Interchange Format", STD 90, RFC 8259,
              DOI 10.17487/RFC8259, December 2017,
              <https://www.rfc-editor.org/rfc/rfc8259>.

14.2.  Informative References

   [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>.

   [RFC738]   Harrenstien, K., "Time server", RFC 738,
              DOI 10.17487/RFC0738, October 1977,
              <https://www.rfc-editor.org/rfc/rfc738>.

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   [RFC8915]  Franke, D., Sibold, D., Teichel, K., Dansarie, M., and R.
              Sundblad, "Network Time Security for the Network Time
              Protocol", RFC 8915, DOI 10.17487/RFC8915, September 2020,
              <https://www.rfc-editor.org/rfc/rfc8915>.

Acknowledgments

   Aanchal Malhotra and Adam Langley authored early drafts of this memo.
   Thomas Peterson corrected multiple nits.  Peter Löthberg, Tal
   Mizrahi, Ragnar Sundblad, Kristof Teichel, and the other members of
   the NTP working group contributed comments and suggestions.

Authors' Addresses

   Watson Ladd
   Akamai Technologies
   Email: watsonbladd@gmail.com

   Marcus Dansarie
   Netnod
   Email: marcus@dansarie.se
   URI:   https://orcid.org/0000-0001-9246-0263

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