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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|C|         Record Type         |          Body Length          |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|                                                               |
.                                                               .
.                           Record Body                         .
.                                                               .
|                                                               |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
        

Figure 1

[[Ed. Note: this ad-hoc binary format should be fine as long as we continue to keep things very simple. However, if we think there's any reasonable probability of wanting to include more complex data structures, we should consider using some semi-structured data format such as JSON, Protocol Buffers, or (ugh) ASN.1]]

The requirement that all NTS-KE messages be terminated by an End of Message record makes them self-delimiting.

The fields of an NTS-KE record are defined as follows:

• C (Critical Bit): Determines the disposition of unrecognized Record Types. Implementations which receive a record with an unrecognized Record Type MUST ignore the record if the Critical Bit is 0, and MUST treat it as an error if the Critical Bit is 1.
• Record Type: A 15-bit integer in network byte order (from most-to-least significant, its bits are record bits 7–1 and then 15–8). The semantics of record types 0–5 are specified in this memo; additional type numbers SHALL be tracked through the IANA Network Time Security Key Establishment Record Types registry.
• Body Length: the length of the Record Body field, in octets, as a 16-bit integer in network byte order. Record bodies may have any representable length and need not be aligned to a word boundary.
• Record Body: the syntax and semantics of this field shall be determined by the Record Type.

5.3.1. NTS-KE record types

The following NTS-KE Record Types are defined.

5.3.1.1. End of Message

The End of Message record has a Record Type number of 0 and an zero-length body. It MUST occur exactly once as the final record of every NTS-KE request and response. The Critical Bit MUST be set.

5.3.1.2. NTS Next Protocol Negotiation

The NTS Next Protocol Negotiation record has a record type of 1. It MUST occur exactly once in every NTS-KE request and response. Its body consists of a sequence of 16-octet strings. Each 16-octet string represents a Protocol Name from the IANA Network Time Security Next Protocols registry. The Critical Bit MUST be set.

The Protocol Names listed in the client's NTS Next Protocol Negotiation record denote those protocols which the client wishes to speak using the key material established through this NTS-KE session. The Protocol Names listed in the server's response MUST comprise a subset of those listed in the request, and denote those protocols which the server is willing and able to speak using the key material established through this NTS-KE session. The client MAY proceed with one or more of them. The request MUST list at least one protocol, but the response MAY be empty.

5.3.1.3. Error

The Error record has a Record Type number of 2. Its body is exactly two octets long, consisting of an unsigned 16-bit integer in network byte order, denoting an error code. The Critical Bit MUST be set.

Clients MUST NOT include Error records in their request. If clients receive a server response which includes an Error record, they MUST discard any negotiated key material and MUST NOT proceed to the Next Protocol.

The following error code are defined.

• Error code 0 means "Unrecognized Critical Record". The server MUST respond with this error code if the request included a record which the server did not understand and which had its Critical Bit set. The client SHOULD NOT retry its request without modification.
• Error code 1 means "Bad Request". The server MUST respond with this error if, upon the expiration of an implementation-defined timeout, it has not yet received a complete and syntactically well-formed request from the client. This error is likely to be the result of a dropped packet, so the client SHOULD start over with a new DTLS handshake and retry its request.

5.3.1.4. Warning

The Warning record has a Record Type number of 3. Its body is exactly two octets long, consisting of an unsigned 16-bit integer in network byte order, denoting a warning code. The Critical Bit MUST be set.

Clients MUST NOT include Warning records in their request. If clients receive a server response which includes an Warning record, they MAY discard any negotiated key material and abort without proceeding to the Next Protocol. Unrecognized warning codes MUST be treated as errors.

This memo defines no warning codes.

5.3.1.5. AEAD Algorithm Negotiation

The AEAD Algorithm Negotiation record has a Record Type number of 4. Its body consists of a sequence of unsigned 16-bit integers in network byte order, denoting Numeric Identifiers from the IANA AEAD registry [RFC5116]. The Critical Bit MAY be set.

If the NTS Next Protocol Negotiation record offers "ntp/4",this record MUST be included exactly once. Other protocols MAY require it as well.

When included in a request, this record denotes which AEAD algorithms the client is willing to use to secure the Next Protocol, in decreasing preference order. When included in a response, this record denotes which algorithm the server chooses to use, or is empty if the server supports none of the algorithms offered.. In requests, the list MUST include at least one algorithm. In responses, it MUST include at most one. Honoring the client's preference order is OPTIONAL: servers may select among any of the client's offered choices, even if they are able to support some other algorithm which the client prefers more.

Server implementations of NTS extensions for NTPv4 [nts-extensions-for-ntpv4] MUST support AEAD_AES_SIV_CMAC_256 [RFC5297] (Numeric Identifier 15). That is, if the client includes AEAD_AES_SIV_CMAC_256 in its AEAD Algorithm Negotiation record, and the server accepts the "ntp/4" protocol in its NTS Next Protocol Negotiation record, then the server's AEAD Algorithm Negotation record MUST NOT be empty.

5.3.1.6. New Cookie for NTPv4

The New Cookie for NTPv4 record has a Record Type number of 5. The contents of its body SHALL be implementation-defined and clients MUST NOT attempt to interpret them. See [[TODO]] for a RECOMMENDED construction.

Clients MUST NOT send records of this type. Servers MUST send at least one record of this type, and SHOULD send eight of them, if they accept "ntp/4" as a Next Protocol. The Critical Bit SHOULD NOT be set.

[[Ed. Note: the purpose of sending eight cookies is to allow the client to recover from dropped packets without reusing cookies or starting a new handshake. Discussion of cookie management should probably be broken out into its own section.]]

5.3.2. Key Extraction (generally)

Following a successful run of the NTS-KE protocol, key material SHALL be extracted according to RFC 5705 [RFC5705]. Inputs to the exporter function are to be constructed in a manner specific to the negotiated Next Protocol. However, all protocols which utilize NTS-KE MUST conform to the following two rules:

• The disambiguating label string MUST be "EXPORTER-network-time-security/1".
• The per-association context value MUST be provided, and MUST begin with the 16-octet Protocol Name which was negotiated as a Next Protocol.

5.4. NTS Extensions for NTPv4

5.4.1. Key Extraction (for NTPv4)

Following a successful run of the NTS-KE protocol wherein "ntp/4" is selected as a Next Protocol, two AEAD keys SHALL be extracted: a client-to-server (C2S) key and a server-to-client (S2C) key. These keys SHALL be computed according to RFC 5705 [RFC5705], using the following inputs.

• The disambiguating label string SHALL be "EXPORTER-network-time-security/1".
• The per-association context value SHALL consist of the following 19 octets:
  • The first 16 octets SHALL be (in hexadecimal):
    
      6E 74 70 2F 34 00 00 00 00 00 00 00 00 00 00 00
  • The next two octets SHALL be the Numeric Identifier of the negotiated AEAD Algorithm, in network byte order.
  • The final octet SHALL be 0x00 for the C2S key and 0x01 for the S2C key.

Implementations wishing to derive additional keys for private or experimental use MUST NOT do so by extending the above-specified syntax for per-association context values. Instead, they SHOULD use their own disambiguating label string. Note that RFC 5705 provides that disambiguating label strings beginning with "EXPERIMENTAL" MAY be used without IANA registration.

5.4.2. Packet structure overview

In general, an NTS-protected NTPv4 packet consists of:

• The usual 48-octet NTP header, which is authenticated but not encrypted.
• Some extensions which are authenticated but not encrypted.
• An NTS extension which contains AEAD output (i.e., an authentication tag and possible ciphertext). The corresponding plaintext, if non-empty, consists of some extensions which benefit from both encryption and authentication.
• Possibly, some additional extensions which are neither encrypted nor authenticated. These are discarded by the receiver. [[Ed. Note: right now there's no good reason for the sender to include anything here, but eventually there might be. We've seen Checksum Complement [RFC7821] and LAST-EF as two examples of semantically-void extensions that are included to satsify constraints imposed lower on the protocol stack, and while there's no reason to use either of these on NTS-protected packets, I think we could see similar examples in the future. So, rejecting packets with unauthenticated extensions could cause interoperability problems, while accepting and processing those extensions would of course be a security risk. Thus, I think "allow and discard" is the correct policy.]]

Always included among the authenticated or authenticated-and-encrypted extensions are a cookie extension and a unique-identifier extension. The purpose of the cookie extension is to enable the server to offload storage of session state onto the client. The purpose of the unique-identifier extension is to protect the client from replay attacks.

5.4.3. The Unique Identifier extension

The Unique Identifier extension has a Field Type of [[TBD]]. When the extension is included in a client packet (mode 3), its body SHALL consist of a string of octets generated uniformly at random. The string SHOULD be 32 octets long. When the extension is included in a server packet (mode 4), its body SHALL contain the same octet string as was provided in the client packet to which the server is responding. Its use in modes other than client/server is not defined.

The Unique Identifier extension provides the client with a cryptographically strong means of detecting replayed packets. It may also be used standalone, without NTS, in which case it provides the client with a means of detecting spoofed packets from off-path attackers. Historically, NTP's origin timestamp field has played both these roles, but for cryptographic purposes this is suboptimal because it is only 64 bits long and, depending on implementation details, most of those bits may be predictable. In contrast, the Unique Identifier extension enables a degree of unpredictability and collision-resistance more consistent with cryptographic best practice.

[[TODO: consider using separate extension types for request and response, thus allowing for use in symmetric mode. But proper handling in the presence of dropped packets needs to be documented and involves a lot of subtlety.]]

5.4.4. The NTS Cookie extension

The NTS Cookie extension has a Field Type of [[TBD]]. Its purpose is to carry information which enables the server to recompute keys and other session state without having to store any per-client state. The contents of its body SHALL be implementation-defined and clients MUST NOT attempt to interpret them. See [[TODO]] for a RECOMMENDED construction. The NTS Cookie extension MUST NOT be included in NTP packets whose mode is other than 3 (client) or 4 (server).

5.4.5. The NTS Cookie Placeholder extension

The NTS Cookie Placeholder extension has a Field Type of [[TBD]]. When this extension is included in a client packet (mode 3), it communicates to the server that the client wishes it to send additional cookies in its response. This extension MUST NOT be included in NTP packets whose mode is other than 3.

Whenever an NTS Cookie Placeholder extension is present, it MUST be accompanied by an NTS Cookie extension, and the body length of the NTS Cookie Placeholder extension MUST be the same as the body length of the NTS Cookie Extension. (This length requirement serves to ensure that the response will not be larger than the request, in order to improve timekeeping precision and prevent DDoS amplification). The contents of the NTS Cookie Placeholder extension's body are undefined and, aside from checking its length, MUST be ignored by the server.

5.4.6. The NTS Authenticator and Encrypted Extensions extension

The NTS Authenticator and Encrypted Extensions extension is the central cryptographic element of an NTS-protected NTP packet. Its Field Type is [[TBD]] and the format of its body SHALL be as follows:

• Nonce length: two octets in network byte order, giving the length of the Nonce field.
• Nonce: a nonce as required by the negotiated AEAD Algorithm.
• Ciphertext: the output of the negotiated AEAD Algorithm. The structure of this field is determined by the negotiated algorithm, but it typically contains an authentication tag in addition to the actual ciphertext.
• Padding: between 1 and 24 octets of padding, with every octet set to the number of padding octets included, e.g., "01", "02 02", or "03 03 03". The number of padding bytes should be chosen in order to comply with the RFC 7822 [RFC7822] requirement that (in the absence of a legacy MAC) extensions have a total length in octets (including the four octets for the type and length fields) which is at least 28 and divisible by 4. At least one octet of padding MUST be included, so that implementations can unambiguously delimit the end of the ciphertext from the start of the padding.

The Ciphertext field SHALL be formed by providing the following inputs to the negotiated AEAD Algorithm:

• K: For packets sent from the client to the server, the C2S key SHALL be used. For packets sent from the server to the client, the S2C key SHALL be used.
• A: The associated data SHALL consist of the portion of the NTP packet beginning from the start of the NTP header and ending at the end of the last extension which precedes the NTS Authenticator and Encrypted Extensions extension.
• P: The plaintext SHALL consist of all (if any) extensions to be encrypted.
• N: The nonce SHALL be formed however required by the negotiated AEAD Algorithm.

The NTS Authenticator and Encrypted Extensions extension MUST NOT be included in NTP packets whose mode is other than 3 (client) or 4 (server).

5.4.7. Protocol details

A client sending an NTS-protected request SHALL include the following extensions:

• Exactly one Unique Identifier extension, which MUST be authenticated and MUST NOT be encrypted [[Ed. Note: so that if the server can't decrypt the request, it can still echo back the Unique Identifier in the NTS NAK it sends]]. MUST NOT duplicate those of any previous request.
• Exactly one NTS Cookie extension, which MUST be authenticated and MUST NOT be encrypted. The cookie MUST be one which the server previously provided the client; it may have been provided during the NTS-KE handshake or in response to a previous NTS-protected NTP request. To protect client's privacy, the same cookie SHOULD NOT be included in multiple requests. If the client does not have any cookies that it has not already sent, it SHOULD re-run the NTS-KE protocol before continuing.
• Exactly one NTS Authenticator and Encrypted Extensions extension, generated using an AEAD Algorithm and C2S key established through NTS-KE.

The client MAY include one or more NTS Cookie Placeholder extensions, which MUST be authenticated and MAY be encrypted. The number of NTS Cookie Placeholder extensions that the client includes SHOULD be such that if the client includes N placeholders and the server sends back N+1 cookies, the number of unused cookies stored by the client will come to eight. When both the client and server adhere to all cookie-management guidance provided in this memo, the number of placeholder extensions will equal the number of dropped packets since the last successful volley.

The client MAY include additional (non-NTS-related) extensions, which MAY appear prior to the NTS Authenticator and Encrypted Extensions extension (therefore authenticated but not encrypted), within it (therefore encrypted and authenticated), or after it (therefore neither encrypted nor authenticated). In general, however, the server MUST discard any unauthenticated extensions and process the packet as though they were not present. Servers MAY implement exceptions to this requirement for particular extensions if their specification explicitly provides for such.

Upon receiving an NTS-protected request, the server SHALL (through some implementation-defined mechanism) use the cookie to recover the AEAD Algorithm, C2S key, and S2C key associated with the request, and then use the C2S key to authenticate the packet and decrypt the ciphertext. If the cookie is valid and authentication and decryption succeed, then the server SHALL include the following extensions in its response:

• Exactly one Unique Identifier extension, which MUST be authenticated, MUST NOT be encrypted, and whose contents SHALL echo those provided by the client.
• Exactly one NTS Authenticator and Encrypted Extensions extension, generated using the AEAD algorithm and S2C key recovered from the cookie provided by the client.
• One or more NTS Cookie extensions, which MUST be authenticated and encrypted. The number of NTS Cookie extensions included SHOULD be equal to, and MUST NOT exceed, one plus the number of valid NTS Cookie Placeholder extensions included in the request.

The server MAY include additional (non-NTS-related) extensions, which MAY appear prior to the NTS Authenticator and Encrypted Extensions extension (therefore authenticated but not encrypted), within it (therefore encrypted and authenticated), or after it (therefore neither encrypted nor authenticated). In general, however, the client MUST discard any unauthenticated extensions and process the packet as though they were not present. Clients MAY implement exceptions to this requirement for particular extensions if their specification explicitly provides for such.

If the server is unable to validate the cookie or authenticate the request, it SHOULD respond with a Kiss-o'-Death packet (see RFC 5905, Section 7.4) [RFC5905]) with kiss code "NTSN" (meaning "NTS NAK"). Such a response MUST include exactly one Unique Identifier extension whose contents SHALL echo those provided by the client. It MUST NOT include any NTS Cookie or NTS Authenticator and Encrypted Extensions extension. [[Ed. Note: RFC 5905 already provides the kiss code "CRYP" meaning "Cryptographic authentication or identification failed" but I think this is meant to be Autokey-specific.]]

Upon receiving an NTS-protected response, the client MUST verify that the Unique Identifier matches that of an outstanding request, and that the packet is authentic under the S2C key associated with that request. If either of these checks fails, the packet MUST be discarded without further processing.

Upon receiving an NTS NAK, the client MUST verify that the Unique Identifier matches that of an outstanding request. If this check fails, the packet MUST be discarded without further processing. If this check passes, the client SHOULD discard all cookies and AEAD keys associated with the server which sent the NAK and initiate a fresh NTS-KE handshake.

5.5. Recommended format for NTS cookies

This section provides a RECOMMENDED way for servers to construct NTS cookies. Clients MUST NOT examine the cookie under the assumption that it is constructed according to this section.

The role of cookies in NTS is closely analagous to that of session cookies in TLS. Accordingly, the thematic resemblance of this section to RFC 5077 [RFC5077] is deliberate, and the reader should likewise take heed of its security considerations.

Servers should select an AEAD algorithm which they will use to encrypt and authenticate cookies. The chosen algorithm should be one such as AEAD_AES_SIV_CMAC_256 [RFC5297] which resists accidential nonce reuse, and it need not be the same as the one that was negotiated with the client. Servers should randomly generate and store a master AEAD key `K`. Servers should additionally choose a non-secret, unique value `I` as key-identifier for `K`.

Servers should periodically (e.g., once daily) generate a new pair (I,K) and immediately switch to using these values for all newly-generated cookies. Immediately following each such key rotation, servers should securely erase any keys generated two or more rotation periods prior. Servers should continue to accept any cookie generated using keys that they have not yet erased, even if those keys are no longer current. Erasing old keys provides for forward secrecy, limiting the scope of what old information can be stolen if a master key is somehow compromised. Holding on to a limited number of old keys allows clients to seamlessly transition from one generation to the next without having to perform a new NTS-KE handshake.

[[TODO: discuss key management considerations for load-balanced servers]]

To form a cookie, servers should first form a plaintext `P` consisting of the following fields:

• The AEAD algorithm negotiated during NTS-KE
• The S2C key
• The C2S key

Servers should the generate a nonce `N` uniformly at random, and form AEAD output `C` by encrypting `P` under key `K` with nonce `N` and no associated data.

The cookie should consist of the tuple `(I,N,C)`.

[[TODO: explicitly specify how to verify and decrypt a cookie, not just how to form one]]

6. IANA Considerations

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

• Service Name: nts
• Transport Protocol: udp
• Assignee: IESG <iesg@ietf.org>
• Contact: IETF Chair <chair@ietf.org>
• Description: Network Time Security
• Reference: [[this memo]]
• Port Number: selected by IANA from the user port range

IANA is requested to allocate the following two entries in the Application-Layer Protocol Negotation (ALPN) Protocol IDs registry:

• Protocol: Network Time Security Key Establishment, version 1
• Identification Sequence:
    0x6E 0x74 0x73 0x6B 0x65 0x2F 0x31 ("ntske/1")
• Reference: [[this memo]]

• Protocol: Network Time Protocol, version 4
• Identification Sequence:
    0x6E 0x74 0x70 0x2F 0x34 ("ntp/4")
• Reference: [[this memo]]

IANA is requested to allocate the following entry in the TLS Exporter Label Registry:

IANA is requested to allocate the following entries in the registry of NTP Kiss-o'-Death codes:

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

IANA is requested to create a new registry entitled "Network Time Security Key Establishment Record Types". Entries SHALL have the following fields:

• Type Number (REQUIRED): An integer in the range 0–32767 inclusive
• Description (REQUIRED): short text description of the purpose of the field
• Set Critical Bit (REQUIRED): One of "MUST", "SHOULD", "MAY", "SHOULD NOT", or "MUST NOT"
• Reference (REQUIRED): A reference to a document specifying the semantics of the record.

The policy for allocation of new entries in this registry SHALL vary by the Type Number, as follows:

• 0–1023: Standards Action
• 1024–16383: Specification Required
• 16384–32767: Private and Experimental Use

Applications for new entries SHALL specify the contents of the Description, Set Critical Bit and Reference fields and which of the above ranges the Type Number should be allocated from. Applicants MAY request a specific Type Number, and such requests MAY be granted at the registrar's discretion.

The initial contents of this registry SHALL be as follows:

IANA is requested to create a new registry entitled "Network Time Security Next Protocols". Entries SHALL have the following fields:

• Protocol Name (REQUIRED): a sequence of 16 octets. Shorter sequences SHALL implicitly be right-padded with null octets (0x00).
• Human-Readable Name (OPTIONAL): if the sequence of octets making up the protocol name intentionally represent a valid UTF-8 [RFC3629] string, this field SHALL consist of that string.
• Reference (RECOMMENDED): a reference to a relevant specification document. If no relevant document exists, a point-of-contact for questions regarding the entry SHOULD be listed here in lieu.

Applications for new entries in this registry SHALL specify all desired fields, and SHALL be granted on a First Come, First Serve basis. Protocol Names beginning with 0x78 0x2D ("x-") SHALL be reserved for Private or Experimental Use, and SHALL NOT be registered. The reserved entry "ptp/2" may be updated or released by a future Standards Action.

The initial contents of this registry SHALL be as follows:

IANA is requested to create two new registries entitled "Network Time Security Error Codes" and "Network Time Security Warning Codes". Entries in each SHALL have the following fields:

• Number (REQUIRED): a 16-bit unsigned integer
• Description (REQUIRED): a short text description of the condition.
• Reference (REQUIRED): a reference to a relevant specification document.

The policy for allocation of new entries in these registries SHALL vary by their Number, as follows:

• 0–1023: Standards Action
• 1024–32767: Specification Required
• 32768–65535: Private and Experimental Use

The initial contents of the Network Time Security Error Codes Registry SHALL be as follows:

The Network Time Security Warning Codes Registry SHALL initially be empty.

7. Security Considerations

All security considerations described in [I-D.ietf-ntp-network-time-security] have to be taken into account. The application of NTS to NTP requires the following additional considerations.

7.1. Random Number Generation

At various points of the protocol, the generation of random numbers is required. The employed methods of generation need to be cryptographically secure. See [RFC4086] for guidelines concerning this topic.

7.2. Usage of NTP Pools

The certification-based authentication scheme described in [I-D.ietf-ntp-network-time-security] is not applicable to the concept of NTP pools. Therefore, NTS is unable to provide secure usage of NTP pools.

7.3. Initial Verification of the Server Certificates

The client may wish to verify the validity of certificates during the initial association phase. Since it generally has no reliable time during this initial communication phase, it is impossible to verify the period of validity of the certificates.

7.4. Treatment of Initial Messages

NTP packets which contains extension fields with key exchange messages do not provide integrity and authenticity protection of the included time stamps. Therefore these NTP packets MUST NOT be used for clock synchronization. Otherwise an initial attack on the client's clock [attacking-ntp] can potentially circumvent the employed security measures of later messages [delorean].

7.5. DTLS-Related Issues

... TBD

7.6. Delay Attack

In a packet delay attack, an adversary with the ability to act as a MITM delays time synchronization packets between client and server asymmetrically [RFC7384]. This prevents the client from accurately measuring the network delay, and hence its time offset to the server [Mizrahi]. The delay attack does not modify the content of the exchanged synchronization packets. Therefore, cryptographic means do not provide a feasible way to mitigate this attack. However, the maximum error that an adversary can introduced is bounded by half of the round trip delay. Also, several non-cryptographic precautions can be taken in order to detect this attack.

1. Usage of multiple time servers: this enables the client to detect the attack, provided that the adversary is unable to delay the synchronization packets between the majority of servers. This approach is commonly used in NTP to exclude incorrect time servers [RFC5905].
2. Multiple communication paths: The client and server utilize different paths for packet exchange. The client can detect the attack, provided that the adversary is unable to manipulate the majority of the available paths [Shpiner]. Note that this approach is not yet available, neither for NTP nor for PTP.
3. Usage of an encrypted connection: the client exchanges all packets with the time server over an encrypted connection (e.g. IPsec). This measure does not mitigate the delay attack, but it makes it more difficult for the adversary to identify the time synchronization packets.
4. Introduction of a threshold value for the delay time of the synchronization packets. The client can discard a time server if the packet delay time of this time server is larger than the threshold value.

8. Privacy Considerations

8.1. Confidentiality

The actual time synchronization data in NTP packets does not involve any information that needs to be kept secret. There also does not seem to be any necessity to disguise the nature of an NTP association. This is why content confidentiality is a non-objective for this document.

8.2. Unlinkability

Unlinkability prevents a device from being tracked when it changes network addresses (e.g. because said device moved between different networks). In other words, unlinkability thwarts an attacker that seeks to link a new network address used by a device with a network address that it was formerly using, because of recognizable data that the device persistently sends as part of an NTS-secured NTP association. This is the justification for continually supplying the client with fresh cookies, so that a cookie never represents recognizable data in the sense outlined above.

NTS's unlinkability objective is merely to not leak any additional data that could be used to link a device's network address. NTS does not rectify legacy linkability issues that are already present in NTP. Thus, a client that requires unlinkability MUST also minimize information transmitted in a client query (mode 3) packet as described in the draft [I-D.dfranke-ntp-data-minimization].

The unlinkability objective only holds for time synchronization traffic, as opposed to key exchange traffic. This implies that it cannot be guaranteed for devices that function not only as time clients, but also as time servers (because the latter can be externally triggered to send authentication data).

It should also be noted that it could be possible to link devices that operate as time servers from their time synchronization traffic, using information exposed in (mode 4) server response packets (e.g. reference ID, reference time, stratum, poll).  Also, devices that respond to NTP control queries could be linked using the information revealed by control queries.

9. Acknowledgements

The authors would like to thank Richard Barnes, Steven Bellovin, Sharon Goldberg, Russ Housley, Martin Langer, Miroslav Lichvar, Aanchal Malhotra, Dave Mills, Danny Mayer, Karen O'Donoghue, Eric K. Rescorla, Stephen Roettger, Kurt Roeckx, Kyle Rose, Rich Salz, Brian Sniffen, Susan Sons, Douglas Stebila, Harlan Stenn, Martin Thomson, and Richard Welty for contributions to this document. on the design of NTS.

10. References

10.1. Normative References

10.2. Informative References

Appendix A. Flow Diagrams of Client Behaviour

                                             .------------.
+------------------------------>o<----------( No More Keys )<---+
|                               |            '------------'     |
|                               v                               |
|                        +-------------+                        |
|                        |Key Exchange |                        |
|                        +------+------+                        |
|                               |           .--------------.    |
|                               o<---------( Keys Remaining )<--+
|                               |           '--------------'    |
|                               v                               |
|                     +-------------------+                     |
|                     |Time Sync. Messages|                     |
|                     +---------+---------+                     |
|                               |                               |
|                               v                               |
|                            +-----+                            |
|                            |Check|                            |
|                            +--+--+                            |
|                               |                               |
|            /------------------+------------------\            |
|           v                   v                   v           |
|     .-----------.      .-------------.        .-------.       |
|    ( MAC Failure )    ( Nonce Failure )      ( Success )      |
|     '-----+-----'      '------+------'        '---+---'       |
|           |                   |                   |           |
|           v                   v                   v           |
|    +-------------+     +-------------+     +--------------+   |
|    |Discard Data |     |Discard Data |     |Sync. Process |   |
|    +-------------+     +------+------+     +------+-------+   |
|           |                   |                   |           |
|           |                   |                   v           |
+-----------+                   +------------------>o-----------+