draft-ietf-ntp-network-time-security-01.txt   draft-ietf-ntp-network-time-security-02.txt 
NTP Working Group D. Sibold NTP Working Group D. Sibold
Internet-Draft PTB Internet-Draft PTB
Intended status: Standards Track S. Roettger Intended status: Standards Track S. Roettger
Expires: April 21, 2014 TU-BS Expires: August 18, 2014
K. Teichel K. Teichel
PTB PTB
October 18, 2013 February 14, 2014
Network Time Security Network Time Security
draft-ietf-ntp-network-time-security-01.txt draft-ietf-ntp-network-time-security-02.txt
Abstract Abstract
This document describes the Network Time Security (NTS) protocol that This document describes the Network Time Security (NTS) protocol that
enables secure authentication of time servers using Network Time enables secure authentication of time servers using Network Time
Protocol (NTP) or Precision Time Protocol (PTP). Its design Protocol (NTP) or Precision Time Protocol (PTP). Its design
considers the special requirements of precise timekeeping, which are considers the special requirements of precise timekeeping, which are
described in Security Requirements of Time Protocols in Packet described in Security Requirements of Time Protocols in Packet
Switched Network [I-D.ietf-tictoc-security-requirements]. Switched Networks [I-D.ietf-tictoc-security-requirements].
Requirements Language Requirements Language
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in RFC 2119 [RFC2119]. document are to be interpreted as described in RFC 2119 [RFC2119].
Status of This Memo Status of This Memo
This Internet-Draft is submitted in full conformance with the This Internet-Draft is submitted in full conformance with the
skipping to change at page 1, line 44 skipping to change at page 1, line 44
Internet-Drafts are working documents of the Internet Engineering Internet-Drafts are working documents of the Internet Engineering
Task Force (IETF). Note that other groups may also distribute Task Force (IETF). Note that other groups may also distribute
working documents as Internet-Drafts. The list of current Internet- working documents as Internet-Drafts. The list of current Internet-
Drafts is at http://datatracker.ietf.org/drafts/current/. Drafts is at http://datatracker.ietf.org/drafts/current/.
Internet-Drafts are draft documents valid for a maximum of six months Internet-Drafts are draft documents valid for a maximum of six months
and may be updated, replaced, or obsoleted by other documents at any and may be updated, replaced, or obsoleted by other documents at any
time. It is inappropriate to use Internet-Drafts as reference time. It is inappropriate to use Internet-Drafts as reference
material or to cite them other than as "work in progress." material or to cite them other than as "work in progress."
This Internet-Draft will expire on April 21, 2014. This Internet-Draft will expire on August 18, 2014.
Copyright Notice Copyright Notice
Copyright (c) 2013 IETF Trust and the persons identified as the Copyright (c) 2014 IETF Trust and the persons identified as the
document authors. All rights reserved. document authors. All rights reserved.
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described in the Simplified BSD License. described in the Simplified BSD License.
Table of Contents Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 3 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 3
2. Security Threats . . . . . . . . . . . . . . . . . . . . . . 3 2. Security Threats . . . . . . . . . . . . . . . . . . . . . . 4
3. Objectives . . . . . . . . . . . . . . . . . . . . . . . . . 3 3. Objectives . . . . . . . . . . . . . . . . . . . . . . . . . 4
4. Terms and Abbreviations . . . . . . . . . . . . . . . . . . . 4 4. Terms and Abbreviations . . . . . . . . . . . . . . . . . . . 4
5. NTS Overview . . . . . . . . . . . . . . . . . . . . . . . . 4 5. NTS Overview . . . . . . . . . . . . . . . . . . . . . . . . 5
5.1. Symmetric and Client/Server Mode . . . . . . . . . . . . 4 5.1. Symmetric and Client/Server Mode . . . . . . . . . . . . 5
5.2. Broadcast Mode . . . . . . . . . . . . . . . . . . . . . 5 5.2. Broadcast Mode . . . . . . . . . . . . . . . . . . . . . 5
6. Protocol Sequence . . . . . . . . . . . . . . . . . . . . . . 5 6. Protocol Messages . . . . . . . . . . . . . . . . . . . . . . 6
6.1. Association Message . . . . . . . . . . . . . . . . . . . 5 6.1. Association Messages . . . . . . . . . . . . . . . . . . 6
6.2. Certificate Message . . . . . . . . . . . . . . . . . . . 5 6.1.1. Message type: "client_assoc" . . . . . . . . . . . . 6
6.3. Cookie Message . . . . . . . . . . . . . . . . . . . . . 6 6.1.2. Message type: "server_assoc" . . . . . . . . . . . . 6
6.4. Broadcast Parameter Message . . . . . . . . . . . . . . . 6 6.2. Certificate Messages . . . . . . . . . . . . . . . . . . 7
6.5. Time Request Message . . . . . . . . . . . . . . . . . . 7 6.2.1. Message type: "client_cert" . . . . . . . . . . . . . 7
6.6. Broadcast Message . . . . . . . . . . . . . . . . . . . . 7 6.2.2. Message type: "server_cert" . . . . . . . . . . . . . 7
6.7. Server Seed Refresh . . . . . . . . . . . . . . . . . . . 7 6.3. Cookie Messages . . . . . . . . . . . . . . . . . . . . . 8
7. Hash Algorithms and MAC Generation . . . . . . . . . . . . . 8 6.3.1. Message type: "client_cook" . . . . . . . . . . . . . 8
7.1. Hash Algorithms . . . . . . . . . . . . . . . . . . . . . 8 6.3.2. Message type: "server_cook" . . . . . . . . . . . . . 8
7.2. MAC Calculation . . . . . . . . . . . . . . . . . . . . . 8 6.4. Unicast Time Synchronisation Messages . . . . . . . . . . 8
8. Server Seed Considerations . . . . . . . . . . . . . . . . . 8 6.4.1. Message type: "time_request" . . . . . . . . . . . . 9
8.1. Server Seed Algorithm . . . . . . . . . . . . . . . . . . 9 6.4.2. Message type: "time_response" . . . . . . . . . . . . 9
8.2. Server Seed Live Time . . . . . . . . . . . . . . . . . . 9 6.5. Broadcast Parameter Messages . . . . . . . . . . . . . . 9
9. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 9 6.5.1. Message type: "client_bpar" . . . . . . . . . . . . . 10
10. Security Considerations . . . . . . . . . . . . . . . . . . . 9 6.5.2. Message type: "server_bpar" . . . . . . . . . . . . . 10
10.1. Initial Verification of the Server Certificates . . . . 9 6.6. Broadcast Message . . . . . . . . . . . . . . . . . . . . 10
10.2. Revocation of Server Certificates . . . . . . . . . . . 9 6.6.1. Message type: "server_broad" . . . . . . . . . . . . 11
10.3. Usage of NTP Pools . . . . . . . . . . . . . . . . . . . 10 7. Protocol Sequence . . . . . . . . . . . . . . . . . . . . . . 11
10.4. Denial-of-Service in Broadcast Mode . . . . . . . . . . 10 7.1. The client . . . . . . . . . . . . . . . . . . . . . . . 11
11. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 10 7.1.1. The client in unicast mode . . . . . . . . . . . . . 11
12. References . . . . . . . . . . . . . . . . . . . . . . . . . 10 7.1.2. The client in broadcast mode . . . . . . . . . . . . 12
12.1. Normative References . . . . . . . . . . . . . . . . . . 10 7.2. The server . . . . . . . . . . . . . . . . . . . . . . . 13
12.2. Informative References . . . . . . . . . . . . . . . . . 11 7.2.1. The server in unicast mode . . . . . . . . . . . . . 13
Appendix A. TICTOC Security Requirements . . . . . . . . . . . . 11 7.2.2. The server in broadcast mode . . . . . . . . . . . . 14
Appendix B. Broadcast Mode . . . . . . . . . . . . . . . . . . . 12 7.3. Server Seed Refresh . . . . . . . . . . . . . . . . . . . 14
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 12 8. Hash Algorithms and MAC Generation . . . . . . . . . . . . . 15
8.1. Hash Algorithms . . . . . . . . . . . . . . . . . . . . . 15
8.2. MAC Calculation . . . . . . . . . . . . . . . . . . . . . 15
9. Server Seed Considerations . . . . . . . . . . . . . . . . . 15
9.1. Server Seed Algorithm . . . . . . . . . . . . . . . . . . 16
9.2. Server Seed Live Time . . . . . . . . . . . . . . . . . . 16
10. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 16
11. Security Considerations . . . . . . . . . . . . . . . . . . . 16
11.1. Initial Verification of the Server Certificates . . . . 16
11.2. Revocation of Server Certificates . . . . . . . . . . . 16
11.3. Usage of NTP Pools . . . . . . . . . . . . . . . . . . . 17
11.4. Denial-of-Service in Broadcast Mode . . . . . . . . . . 17
12. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 17
13. References . . . . . . . . . . . . . . . . . . . . . . . . . 17
13.1. Normative References . . . . . . . . . . . . . . . . . . 17
13.2. Informative References . . . . . . . . . . . . . . . . . 18
Appendix A. Flow Diagrams of Client Behaviour . . . . . . . . . 18
Appendix B. Extension fields . . . . . . . . . . . . . . . . . . 20
Appendix C. TICTOC Security Requirements . . . . . . . . . . . . 21
Appendix D. Broadcast Mode . . . . . . . . . . . . . . . . . . . 22
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 22
1. Introduction 1. Introduction
Time synchronization protocols are more and more utilized to Time synchronization protocols are utilized more and more to
synchronize clocks in networked infrastructures. The reliable synchronize clocks in networked infrastructures. The reliable
performance of such infrastructures can be degraded seriously by performance of such infrastructures can be degraded seriously by
successful attacks against the time synchronization protocol. successful attacks against the time synchronization protocol.
Therefore, time synchronization protocols applied in critical Therefore, time synchronization protocols applied in critical
infrastructures have to provide security measures to defeat possible infrastructures have to provide security measures to defeat possible
adversaries. Consequently, the widespread Network Time Protocol adversaries. Consequently, the widespread Network Time Protocol
(NTP) [RFC5905] was supplemented by the autokey protocol [RFC5906] (NTP) [RFC5905] was supplemented by the autokey protocol [RFC5906]
which shall ensure authenticity of the NTP server and integrity of which shall ensure authenticity of the NTP server and integrity of
the protocol packets. Unfortunately,the autokey protocol exhibits the protocol packets. Unfortunately, the autokey protocol exhibits
various severe security vulnerabilities as revealed in a thorough various severe security vulnerabilities as revealed in a thorough
analysis of the protocol [Roettger]. For the Precision Time Protocol analysis of the protocol [Roettger]. For the Precision Time Protocol
(PTP) Annex K of the standard document IEEE 1588 [IEEE1588] defines (PTP), Annex K of the standard document IEEE 1588 [IEEE1588] defines
an informative security protocol that is still in experimental state. an informative security protocol that is still in experimental state.
Because of autokey's security vulnerabilities and the absence of a Because of autokey's security vulnerabilities and the absence of a
standardized security protocol for PTP these protocols cannot be standardized security protocol for PTP, these protocols cannot be
applied in environments in which compliance requirements demand applied in environments in which compliance requirements demand
authenticity and integrity protection. This document specifies a authenticity and integrity protection. This document specifies a
security protocol which ensures authenticity of the time server via a security protocol which ensures authenticity of the time server via a
Public Key Infrastructure and integrity of the time synchronization Public Key Infrastructure (PKI) and integrity of the time
protocol packets and which therefore enables the usage of NTP and PTP synchronization protocol packets and which therefore enables the
in such environments. usage of NTP and PTP in such environments.
The protocol is specified with the prerequisite in mind that precise The protocol is specified with the prerequisite in mind that precise
timekeeping can only be accomplished with stateless time timekeeping can only be accomplished with stateless time
synchronization communication, which excludes standard security synchronization communication, which excludes the utilization of
protocols like IPsec or TLS. This prerequisite corresponds with the standard security protocols like IPsec or TLS for time
synchronization messages. This prerequisite corresponds with the
requirement that a security mechanism for timekeeping must be requirement that a security mechanism for timekeeping must be
designed in such a way that it does not degrade the quality of the designed in such a way that it does not degrade the quality of the
time transfer [I-D.ietf-tictoc-security-requirements]. time transfer [I-D.ietf-tictoc-security-requirements].
Note: Note:
It is intended to formulate the protocol to be applicable to NTP The intent is to formulate the protocol to be applicable to NTP as
as well as PTP. In the current state the draft focuses on the well as PTP. In the current state the draft focuses on the
application to NTP. application to NTP.
2. Security Threats 2. Security Threats
A profound analysis of security threats and requirements for NTP and A profound analysis of security threats and requirements for NTP and
PTP can be found in the I-D [I-D.ietf-tictoc-security-requirements]. PTP can be found in the I-D [I-D.ietf-tictoc-security-requirements].
3. Objectives 3. Objectives
The objectives of the NTS specifications are as follows: The objectives of the NTS specifications are as follows:
skipping to change at page 4, line 30 skipping to change at page 4, line 50
o Compatibility: o Compatibility:
* Unsecured NTP associations shall not be affected. * Unsecured NTP associations shall not be affected.
* An NTP server that does not support NTS shall not be affected * An NTP server that does not support NTS shall not be affected
by NTS authentication requests. by NTS authentication requests.
4. Terms and Abbreviations 4. Terms and Abbreviations
o TESLA: Time efficient stream loss-tolerant authentication o TESLA: Timed Efficient Stream Loss-Tolerant Authentication
5. NTS Overview 5. NTS Overview
5.1. Symmetric and Client/Server Mode 5.1. Symmetric and Client/Server Mode
Authenticity of the time server is verified once by a Public Key Authenticity of the time server is verified once by utilization of
Infrastructure. Authenticity and integrity of the NTP packets are X.509 certificates. Authenticity and integrity of the NTP packets
then ensured by a Message Authentication Code (MAC), which is are then ensured by a Message Authentication Code (MAC), which is
attached to the NTP packet. The calculation of the MAC includes the attached to the NTP packet. The calculation of the MAC includes the
whole NTP packet and the cookie which is shared between client and whole NTP packet and the cookie which is shared between client and
server. It is calculated according to: server. It is calculated according to:
cookie = MSB_128 (H(server seed || H(public key of client))), cookie = MSB_128 (HMAC(server seed, public key of client)),
where || indicates concatenation and in which H is a hash algorithm. with the server seed as key, and where the function MSB_128 cuts off
The function MSB_128 cuts off the 128 most significant bits of the the 128 most significant bits of the result of the HMAC function.
result of the hash function. The server seed is a 128 bit random The server seed is a 128 bit random value of the server, which has to
value of the server, which has to be kept secret. The cookie thus be kept secret. The cookie thus never changes as long as the server
never changes as long as the server seed stays the same. The server seed stays the same. The server seed has to be refreshed
seed has to be refreshed periodically in order to provide key periodically in order to provide key freshness as required in
freshness as required in [I-D.ietf-tictoc-security-requirements]. [I-D.ietf-tictoc-security-requirements]. The server does not keep a
The server does not keep a state of the client. Therefore it has to state of the client. Therefore it has to recalculate the cookie each
recalculate the cookie each time it receives a request from the time it receives a request from the client. To this end, the client
client. To this end, the client has to attach the hash value of its has to attach the hash value of its public key to each request (see
public key to each request (see Section 6.5). Section 6.4).
5.2. Broadcast Mode 5.2. Broadcast Mode
Just as in the case of the client server mode and symmetric mode, Just as in the case of the client server mode and symmetric mode,
authenticity and integrity of the NTP packets are ensured by a MAC, authenticity and integrity of the NTP packets are ensured by a MAC,
which is attached to the NTP packet by the sender. The verification which is attached to the NTP packet by the sender. The verification
of the authenticity is based on the TESLA protocol [RFC4082]. TESLA of the authenticity is based on the TESLA protocol, in particular on
is based on a one-way chain of keys, where each key is the output of its "Not Re-using Keys" scheme, see section 3.7.2 of [RFC4082].
a one-way function applied on the previous key in the chain. The TESLA is based on a one-way chain of keys, where each key is the
last element of the chain is shared securely with all clients. The output of a one-way function applied on the previous key in the
server splits time into intervals of uniform duration and assigns chain. The last element of the chain is shared securely with all
each key to an interval in reverse order, starting with the clients. The server splits time into intervals of uniform duration
penultimate. At each time interval, the server sends an NTP and assigns each key to an interval in reverse order, starting with
the penultimate. At each time interval, the server sends an NTP
broadcast packet appended by a MAC, calculated using the broadcast packet appended by a MAC, calculated using the
corresponding key, and the key of the previous interval. The client corresponding key, and the key of the previous disclosure interval.
verifies the MAC by buffering the packet until the disclosure of the The client verifies the MAC by buffering the packet until the
key in the next interval. In order to be able to verify the validity disclosure of the key in its associated disclosure interval. In
of the key, the client has to be loosely time synchronized to the order to be able to verify the validity of the key, the client has to
server. This has to be accomplished during the initial client server be loosely time synchronized to the server. This has to be
exchange between broadcast client and server. For a more detailed accomplished during the initial client server exchange between
description of the TESLA protocol see Appendix B. broadcast client and server. For a more detailed description of the
TESLA protocol see Appendix D.
6. Protocol Sequence 6. Protocol Messages
6.1. Association Message Note that this section currently describes realization of the message
format of NTS only for its utilization for NTP, in which the NTS
specific data are enclosed in extension fields on top of NTP packets.
A specification of NTS messages for PTP would have to be developed
accordingly.
The protocol sequence starts with the association message, in which The steps described in Section 6.1 - Section 6.4 belong to the
the client sends an NTP packet with an extension field of type unicast mode, while Section 6.5 and Section 6.6 explain the steps
association. It contains the hostname of the client and a status involved in the broadcast mode of NTS.
word which contains the algorithms used for the signatures and the
status of the connection. The response contains the hostname of the
server and the algorithms for the signatures. The server notifies
the cryptographic hash algorithms which it supports.
6.2. Certificate Message 6.1. Association Messages
In this step, the hash and signature algorithms that are used for the
rest of the protocol are negotiated.
6.1.1. Message type: "client_assoc"
The protocol sequence starts with the client sending an association
message, called client_assoc. This message contains
o the version number of NTS that the client wants to use (this
SHOULD be the highest version number that it supports),
o the hostname of the client,
o a selection of hash algorithms, and
o a selection of accepted algorithms for the signatures.
For NTP, this message is realized as a packet with an extension field
of type "association", which contains all this data.
6.1.2. Message type: "server_assoc"
This message is sent by the server upon receipt of client_assoc. It
contains
o the version number used for the rest of the protocol (which SHOULD
be determined as the minimum over the client's suggestion in the
client_assoc and the highest supported by the server),
o the hostname of the server, and
o the server's choice of algorithm for the signatures and
cryptographic hash algorithm, both of which MUST be chosen from
the client's proposals.
In the case of NTP, the data is enclosed in a packet's extension
field, also of type "association".
6.2. Certificate Messages
In this step, the client receives the certification chain up to a In this step, the client receives the certification chain up to a
trusted authority (TA). To this end, the client requests the trusted anchor. With the established certification chain the client
certificate for the subject name (hostname) of the NTP server. The is able to verify the server signatures and, hence, the authenticity
response contains the certificate with the issuer name. If the of the server messages with extension fields is ensured.
issuer name is different from the subject name, the client requests
the certificate for the issuer. This continues until it receives a
certificate in which the subject name and the issuer name are
identical, which indicates that it is issued by a TA. The client
then checks that the issuer is indeed on its list of issuers which
are accepted as TAs. The client has to check that each issuer in the
certificate chain is authorized to issue new certificates. To this
end, the certificates have to include the X.509v3 extension field
"CA:TRUE". With the established certification chain the client is
able to verify the server signatures and, hence, the authenticity of
the server messages with extension fields is ensured.
Discussion: Discussion:
Note that in this step the client validates the authenticity of Note that in this step the client validates the authenticity of
its NTP server only. It does not recursively validate the its immediate NTP server only. It does not recursively validate
authenticity of each NTP server on the time synchronization chain. the authenticity of each NTP server on the time synchronization
But each NTP server on the time synchronization chain validates chain. Recursive authentication (and authorization) as formulated
the NTP server to which it is synchronized. This conforms to the in [I-D.ietf-tictoc-security-requirements] depends on the chosen
recursive authentication requirement in the TICTOC security trust anchor.
requirements [I-D.ietf-tictoc-security-requirements].
6.3. Cookie Message 6.2.1. Message type: "client_cert"
The client requests a cookie from the server. It selects a hash This message is sent by the client, after it successfully verified
algorithm from the list of algorithms supported by the server. The the content of the received server_assoc message (see Section 7.1.1).
request includes its public key and the selected hash algorithm. The It contains
hash of the public key is used by the server to calculate the cookie
(see Section 5.1). The response of the server contains the cookie
and a signature of the cookie signed with the server's private key,
both encrypted with the client's public key.
6.4. Broadcast Parameter Message o the negotiated version number,
In the broadcast mode the client requests the following information o the client's hostname, and
from the server:
o the last key of the one-way key chain, o the signature algorithm negotiated during the association
messages.
o the disclosure schedule of the following keys. This contains: It is realized as an NTP packet with extension field of type
"certificate request" for the necessary data.
* time interval duration, time at which the next time interval 6.2.2. Message type: "server_cert"
will start and its associated index,
* key disclosure delay (number of time intervals for which a key This message is sent by the server, upon receipt of a client_cert
is valid). message, if the version number and choice of methods communicated in
that message are actually supported by the server. It contains
The server will sign all transmitted properties so that the client is o all the information necessary to authenticate the server to the
able to verify their authenticity. For this packet exchange a new client. This is a chain of certificates, which starts at the
extension field "broadcast parameters" is used. The client server and goes up to a trusted authority, where each certificate
synchronizes its time with the server in the client server mode and MUST be certified by the one directly following it.
saves an upper bound of its time offset with respect to the time of
the server. See Appendix B for more details.
6.5. Time Request Message This message is realized for NTP as a packet with extension field of
type "certificate" which holds the certification data.
The client request includes a new extension field "time request" 6.3. Cookie Messages
which contains the hash of its public key, a 128-bit nonce, and the
chosen hash algorithm . The server needs the hash of the public key During this step, the server transmits a secret cookie to the client
and the notified hash algorithm to recalculate the cookie for the securely. The cookie will be used for integrity protection during
client. The response is a NTP packet with a new extension field unicast time synchronization.
"time response" which contains the nonce and a MAC generated over the
time synchronization data, the cookie and the nonce. 6.3.1. Message type: "client_cook"
This message is sent by the client, upon successful authentication of
the server. In this message, the client requests a cookie from the
server. It contains
o the negotiated version number,
o the hash algorithm H negotiated between client and server during
the association messages,
o the client's public key.
For NTP, an extension field of type "cookie request" holds the listed
data.
6.3.2. Message type: "server_cook"
This message is sent by the server, upon receipt of a client_cook
message. The hash of the client's public key, as included in
client_cook, is used by the server to calculate the cookie (see
Section 5.1). This message contains
o a concatenated pair, encrypted with the client's public key, where
the pair consists of
* the cookie, and
* a signature of the cookie signed with the server's private key.
In the case of NTP, this is a packet with an extension field of type
"cookie transmit".
6.4. Unicast Time Synchronisation Messages
In this step, the usual time synchronization process is executed,
with the addition of integrity protection for all messages that the
server sends. This step can be repeated as often as the client
desires and as long as the integrity of the server's time responses
is verified successfully. Secure time synchronization by repetition
of this step is the goal of a unicast run.
6.4.1. Message type: "time_request"
This message is sent by the client when it requests time exchange.
To send this message, the client MUST have received server_cook and
successfully verified the cookie via the server's signature. It
contains
o the negotiated version number,
o the time synchronization data that the client wants to transmit,
o a 128-bit nonce,
o the negotiated hash algorithm H,
o the hash of the client's public key under H.
It is realized as an NTP packet with the time synchronization data
and an additional extension field of type "time request" for the rest
of the information.
6.4.2. Message type: "time_response"
This message is sent by the server, after it received a time_request
message. The server uses the hash of the client's public key and the
transmitted hash algorithm to recalculate the cookie for the client.
This message contains
o the server's time synchronization response data,
o the nonce transmitted in time_request,
o a MAC (generated with the cookie as key) for verification of the
above.
It is realized as an NTP packet with the necessary time
synchronization data and with a new extension field of type "time
response". This packet has an appended MAC that is generated over
the time synchronization data and the extension field, with the
cookie as the key.
6.5. Broadcast Parameter Messages
In this step, the client receives the necessary information to
execute the TESLA protocol in a secured broadcast association. The
client can only initiate a secure broadcast association after a
successful unicast run, see Section 7.1.2.
See Appendix D for more details on TESLA.
6.5.1. Message type: "client_bpar"
This message is sent by the client in order to establish a secured
time broadcast association with the server. It contains
o the version number negotiated during association in unicast mode,
o the client's hostname, and
o the signature algorithm negotiated during unicast.
For NTP, this message is realized as a packet with an extension field
of type "broadcast request".
6.5.2. Message type: "server_bpar"
This message is sent by the server upon receipt of a client_bpar
message during the broadcast loop of the server. It contains
o the one-way function used for building the one-way key chain,
o the last key of the one-way key chain, and
o the disclosure schedule of the keys. This contains:
* time interval duration,
* the disclosure delay (number of intervals between use and
disclosure of a key),
* the time at which the next time interval will start, and
* the next interval's associated index.
o The message also contains a signature signed by the server with
its private key, verifying all the data listed above.
It is realized for NTP as a packet with an extension field of type
"broadcast parameters", which contains all the given data.
6.6. Broadcast Message 6.6. Broadcast Message
In broadcast mode the NTP packet includes a new extension field In this step, the server keeps sending broadcast time synchronization
"broadcast message" which contains the disclosed key of the previous messages to all participating clients.
disclosure interval (current time interval minus disclosure delay).
The NTP packet is appended by a MAC, calculated with the key for the
current time interval. When a client receives a broadcast message it
has to perform the following tests:
o Proof that the MAC is based on a key that is not yet disclosed. 6.6.1. Message type: "server_broad"
This is achieved via a disclosure schedule, so this is where loose
time synchronization is required. If verified the packet will be
buffered for later authentication otherwise it has to be
discarded. Note that the time information included in the packet
will not be used for synchronization until their authenticity
could be verified.
o The client checks whether it already knows the disclosed key. If This message is sent by the server over the course of its broadcast
so, the packet is discarded to avoid a buffer overrun. If not, schedule. It is part of any broadcast association. It contains
the client verifies that the disclosed key belongs to the one-way
key chain by applying the one-way function until equality with a
previous disclosed key is verified. If falsified the packet has
to be discarded.
o If the disclosed key is legitimate the client verifies the o time broadcast data,
authenticity of any packet that it received during the
corresponding time interval. If authenticity of a packet is
verified it is released from the buffer and the packet's time
information can be utilized. If the verification fails
authenticity is no longer given. In this case the client MUST
request authentic time from the server by means of a unicast time
request message.
See RFC 4082[RFC4082] for a detailed description of the packet o the index that belongs to the current interval (and therefore
verification process. identifies the current, yet undisclosed key)
6.7. Server Seed Refresh o the disclosed key of the previous disclosure interval (current
time interval minus disclosure delay).
o a MAC, calculated with the key for the current time interval,
verifying the time data
The message is realized as an NTP broadcast packet with the time
broadcast data and with an extension field of type "broadcast
message", which contains the rest of the listed data. The NTP packet
is then appended by a MAC verifying the time data, but not the
extension field.
7. Protocol Sequence
7.1. The client
7.1.1. The client in unicast mode
For a unicast run, the client performs the following steps:
1. It sends a client_assoc message to the server.
2. It waits for a reply in the form of a server_assoc message.
After receipt of the message it performs the following checks:
* The message MUST contain a conform version number.
* The client has to verify that the server has chosen the
signature and hash algorithms from its proposal sent in the
client_assoc message.
If one of the checks fails, the client MUST abort the run.
3. The client then sends a client_cert message to the server.
4. It awaits a reply in the form of a server_cert message and
performs an authenticity check. If this check fails, the client
MUST abort the run.
5. Next, it sends a client_cook message to the server.
6. It awaits a reply in the form of a server_cook message; upon
receipt it executes the following actions:
* It decrypts the message with its own private key.
* It checks that the decrypted message has the format of a 128
bit Cookie concatenated with its own signature value,
verifiable with the server's public key.
If the check fails, the client MAY abort the run.
7. The client sends a time_request message to the server.
8. It awaits a reply in the form of a time_response message. Upon
receipt, it checks:
* that the transmitted nonce belongs to the previous
time_request message and .
* that the appended MAC verifies the time data and the
transmitted nonce.
If the nonce is invalid, the client MUST ignore this
time_response message. If the MAC is invalid, the client MUST do
one of the following: abort the run or go back to step 5. If
both checks are successful, the client SHOULD continue time
synchronization by going back to step 7.
The client's behaviour in unicast mode is also expressed in Figure 1.
7.1.2. The client in broadcast mode
To establish a secure broadcast association with a broadcast server,
the client MUST initially authenticate the broadcast server and
securely synchronize its time to it up to an upper bound for its time
offset in unicast mode. After that, the client performs the
following steps:
1. It sends a client_bpar message to the server.
2. It waits for a reply in the form of a server_bpar message after
which it performs the following checks:
* The message must contain all the necessary information for the
TESLA protocol, as listed in Section 6.5.2.
* Verification of the message's signature.
If any information is missing or cannot be verified as signed by
the server, the client MUST abort the broadcast run.
3. The client awaits time synchronization data in the form of a
server_broadcast message. Upon receipt, it performs the
following checks:
1. Proof that the MAC is based on a key that is not yet
disclosed. This is achieved via a disclosure schedule, so
this is where loose time synchronization is required. If
verified the packet will be buffered for later
authentication. Otherwise, the client MUST discard it. Note
that the time information included in the packet will not be
used for synchronization until its authenticity could be
verified.
2. The client checks whether it already knows the disclosed key.
If so, the client SHOULD discard the packet to avoid a buffer
overrun. If not, the client verifies that the disclosed key
belongs to the one-way key chain by applying the one-way
function until equality with a previous disclosed key is
verified. If falsified, the client MUST discard the packet.
3. If the disclosed key is legitimate the client verifies the
authenticity of any packet that it received during the
corresponding time interval. If authenticity of a packet is
verified it is released from the buffer and the packet's time
information can be utilized. If the verification fails
authenticity is no longer given. In this case the client
MUST request authentic time from the server by means of a
unicast time request message.
See RFC 4082[RFC4082] for a detailed description of the packet
verification process.
The client's behaviour in broadcast mode can also be seen in Figure
2.
7.2. The server
The server's behaviour is not as easy to express in sequential terms
as the client's, not even for a single association with one client.
This is because the server does not keep state of any connection.
7.2.1. The server in unicast mode
A broadcast server MUST also support unicast mode, in order to
provide the initial time synchronization is a precondition for any
broadcast association. To support unicast mode, the server MUST be
ready to perform the following actions:
o Upon receipt of a client_assoc message, the server constructs and
sends a reply in the form of a server_assoc message as described
in Section 6.1.2.
o Upon receipt of a client_cert message, the server checks whether
it supports the given signature algorithm. If so, it constructs
and sends a server_cert message as described in Section 6.2.2.
o Upon receipt of a client_cook message, the server calculates the
cookie according to the formula given in Section 5.1. With this,
it constructs a server_cook message as described in Section 6.3.2.
o Upon receipt of a time_request message, the server re-calculates
the cookie, then computes the necessary time synchronization data
and constructs a time_response message as given in Section 6.4.2.
Also, it must adhere to the rule of server seed refreshing, as given
in [1]. More information on that can be found in Section 7.3.
7.2.2. The server in broadcast mode
To support NTS broadcast, the server MUST be ready to perform the
following actions:
o Upon receipt of a client_bpar message, the server constructs and
sends a server_bpar message as described in Section 6.5.2.
o The server follows the TESLA protocol in all other aspects, by
regularly sending server_broad messages as described in
Section 6.6.1, adhering to its own disclosure schedule.
It is also the server's responsibility to watch for the expiration
date of the one-way key chain and generate a new key chain
accordingly.
7.3. Server Seed Refresh
According to the requirements in According to the requirements in
[I-D.ietf-tictoc-security-requirements] the server has to refresh its [I-D.ietf-tictoc-security-requirements] the server has to refresh its
server seed periodically. As a consequence the cookie used in the server seed periodically. As a consequence the cookie used in the
time request messages becomes invalid. In this case the client time request messages becomes invalid. In this case the client
cannot verify the attached MAC and has to respond accordingly by re- cannot verify the attached MAC and has to respond accordingly by re-
initiating the protocol with a cookie request (Section 6.3). This is initiating the protocol with a cookie request (Section 6.3). This is
true for the unicast and broadcast mode, respectively. true for the unicast and broadcast mode, respectively.
Additionally, in broadcast mode the client has to restart the Additionally, in broadcast mode the client has to restart the
broadcast sequence with a time request message if the one-way key broadcast sequence with a time request message if the one-way key
chain expires. chain expires.
During certificate message exchange the client reads the expiration During certificate message exchange the client reads the expiration
date of the period of validity of the server certificate. The client date of the period of validity of the server certificate. The client
MAY restart the protocol sequence with the association message before MAY restart the protocol sequence with the association message before
the server certificate expires. the server certificate expires.
7. Hash Algorithms and MAC Generation 8. Hash Algorithms and MAC Generation
7.1. Hash Algorithms 8.1. Hash Algorithms
Hash algorithms are used at different points: calculation of the Hash algorithms are used at different points: calculation of the
cookie and the MAC, and hashing of the public key. The client cookie and the MAC, and hashing of the public key. Client and server
selects the hash algorithm from the list of hash algorithms which are negotiate a hash algorithm H during the association message exchange
supported by the server. This list is notified during the (Section 6.1) at the beginning of a unicast run. The selected
association message exchange (Section 6.1). The selected algorithm algorithm H is used for all hashing processes in that run.
is used for all hashing processes in the protocol.
In the broadcast mode hash algorithm are used as pseudo random In broadcast mode, hash algorithms are used as pseudo random
functions to construct the one-way key chain. functions to construct the one-way key chain. Here, the utilized
hash algorithm is communicated by the server and non-negotiable.
The list of the hash algorithms supported by the server has to The list of the hash algorithms supported by the server has to
fulfill the following requirements: fulfill the following requirements:
o it MUST NOT include MD5 or weaker algorithms, o it MUST NOT include MD5 or weaker algorithms,
o it MUST include SHA-256 or stronger algorithms. o it MUST include SHA-256 or stronger algorithms.
7.2. MAC Calculation 8.2. MAC Calculation
For the calculation of the MAC client and server are using a Keyed- For the calculation of the MAC client and server are using a Keyed-
Hash Message Authentication Code (HMAC) approach [RFC2104]. The HMAC Hash Message Authentication Code (HMAC) approach [RFC2104]. The HMAC
is generated with the hash algorithm specified by the client (see is generated with the hash algorithm specified by the client (see
Section 7.1). Section 8.1).
8. Server Seed Considerations 9. Server Seed Considerations
The server has to calculate a random seed which has to be kept secret The server has to calculate a random seed which has to be kept secret
and which has to be changed periodically. The server has to generate and which MUST be changed periodically. The server MUST generate a
a seed for each supported hash algorithm. seed for each supported hash algorithm.
8.1. Server Seed Algorithm 9.1. Server Seed Algorithm
8.2. Server Seed Live Time 9.2. Server Seed Live Time
9. IANA Considerations 10. IANA Considerations
This document makes no request of IANA. This document makes no request of IANA.
Note to RFC Editor: this section may be removed on publication as an Note to RFC Editor: this section may be removed on publication as an
RFC. RFC.
10. Security Considerations 11. Security Considerations
10.1. Initial Verification of the Server Certificates 11.1. Initial Verification of the Server Certificates
The client has to verify the validity of the certificates during the The client has to verify the validity of the certificates during the
certification message exchange (Section 6.2). Since it generally has certification message exchange (Section 6.2). Since it generally has
no reliable time during this initial communication phase, it is no reliable time during this initial communication phase, it is
impossible to verify the period of validity of the certificates. impossible to verify the period of validity of the certificates.
Therefore, the client MUST use one of the following approaches: Therefore, the client MUST use one of the following approaches:
o The validity of the certificates is preconditioned. Usually this o The validity of the certificates is preconditioned. Usually this
will be the case in corporate networks. will be the case in corporate networks.
skipping to change at page 9, line 43 skipping to change at page 16, line 43
end, the client uses the Online Certificate Status Protocol (OCSP) end, the client uses the Online Certificate Status Protocol (OCSP)
defined in [RFC6277]. defined in [RFC6277].
o The client requests a different service to get an initial time o The client requests a different service to get an initial time
stamp in order to be able to verify the certificates' periods of stamp in order to be able to verify the certificates' periods of
validity. To this end, it can, e.g., use a secure shell validity. To this end, it can, e.g., use a secure shell
connection to a reliable host. Another alternative is to request connection to a reliable host. Another alternative is to request
a time stamp from a Time Stamping Authority (TSA) by means of the a time stamp from a Time Stamping Authority (TSA) by means of the
Time-Stamp Protocol (TSP) defined in [RFC3161]. Time-Stamp Protocol (TSP) defined in [RFC3161].
10.2. Revocation of Server Certificates 11.2. Revocation of Server Certificates
According to Section Section 6.7 it is the client's responsibility to According to Section 7.3, it is the client's responsibility to
initiate a new association with the server after the server's initiate a new association with the server after the server's
certificate expires. To this end the client reads the expiration certificate expires. To this end the client reads the expiration
date of the certificate during the certificate message exchange date of the certificate during the certificate message exchange
(Section 6.2). Besides, certificates may also be revoked prior to (Section 6.2). Besides, certificates may also be revoked prior to
the normal expiration date. To increase security the client MAY the normal expiration date. To increase security the client MAY
verify the state of the server's certificate via OCSP periodically. verify the state of the server's certificate via OCSP periodically.
10.3. Usage of NTP Pools 11.3. Usage of NTP Pools
The certification based authentication scheme described in Section 6 The certification based authentication scheme described in Section 6
is not applicable to the concept of NTP pools. Therefore, NTS is not is not applicable to the concept of NTP pools. Therefore, NTS is not
able to provide secure usage of NTP pools. able to provide secure usage of NTP pools.
10.4. Denial-of-Service in Broadcast Mode 11.4. Denial-of-Service in Broadcast Mode
TESLA authentication buffers packets for delayed authentication. TESLA authentication buffers packets for delayed authentication.
This makes the protocol vulnerable to flooding attacks, causing the This makes the protocol vulnerable to flooding attacks, causing the
client to buffer excessive numbers of packets. To add stronger DoS client to buffer excessive numbers of packets. To add stronger DoS
protection to the protocol client and server SHALL use the "Not Re- protection to the protocol client and server SHALL use the "Not Re-
using Keys" scheme of TESLA as pointed out in section 3.7.2 of RFC using Keys" scheme of TESLA as pointed out in section 3.7.2 of RFC
4082 [RFC4082]. In this scheme the server never uses a key for the 4082 [RFC4082]. In this scheme the server never uses a key for the
MAC generation more than once. Therefore the client can discard any MAC generation more than once. Therefore the client can discard any
packet that contains a disclosed key it knows already, thus packet that contains a disclosed key it knows already, thus
preventing memory flooding attacks. preventing memory flooding attacks.
Note, an alternative approach to enhance TESLA's resistance against Note, an alternative approach to enhance TESLA's resistance against
DoS attacks involves the addition of a group MAC to each packet. DoS attacks involves the addition of a group MAC to each packet.
This requires the exchange of an additional shared key common to the This requires the exchange of an additional shared key common to the
whole group. This adds additional complexity to the protocol and whole group. This adds additional complexity to the protocol and
hence is currently not considered in this document. hence is currently not considered in this document.
11. Acknowledgements 12. Acknowledgements
12. References The authors would like to thank David Mills and Kurt Roeckx for
discussions and comments on the design of NTS. Also, thanks to
Harlan Stenn for his technical review and specific text contributions
to this document.
12.1. Normative References 13. References
13.1. Normative References
[IEEE1588] [IEEE1588]
IEEE Instrumentation and Measurement Society. TC-9 Sensor IEEE Instrumentation and Measurement Society. TC-9 Sensor
Technology, "IEEE standard for a precision clock Technology, "IEEE standard for a precision clock
synchronization protocol for networked measurement and synchronization protocol for networked measurement and
control systems", 2008. control systems", 2008.
[RFC2104] Krawczyk, H., Bellare, M., and R. Canetti, "HMAC: Keyed- [RFC2104] Krawczyk, H., Bellare, M., and R. Canetti, "HMAC: Keyed-
Hashing for Message Authentication", RFC 2104, February Hashing for Message Authentication", RFC 2104, February
1997. 1997.
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997. Requirement Levels", BCP 14, RFC 2119, March 1997.
[RFC3161] Adams, C., Cain, P., Pinkas, D., and R. Zuccherato, [RFC3161] Adams, C., Cain, P., Pinkas, D., and R. Zuccherato,
"Internet X.509 Public Key Infrastructure Time-Stamp "Internet X.509 Public Key Infrastructure Time-Stamp
Protocol (TSP)", RFC 3161, August 2001. Protocol (TSP)", RFC 3161, August 2001.
[RFC4082] Perrig, A., Song, D., Canetti, R., Tygar, J.D., and B. [RFC4082] Perrig, A., Song, D., Canetti, R., Tygar, J., and B.
Briscoe, "Timed Efficient Stream Loss-Tolerant Briscoe, "Timed Efficient Stream Loss-Tolerant
Authentication (TESLA): Multicast Source Authentication Authentication (TESLA): Multicast Source Authentication
Transform Introduction", RFC 4082, June 2005. Transform Introduction", RFC 4082, June 2005.
[RFC5905] Mills, D., Martin, J., Burbank, J., and W. Kasch, "Network [RFC5905] Mills, D., Martin, J., Burbank, J., and W. Kasch, "Network
Time Protocol Version 4: Protocol and Algorithms Time Protocol Version 4: Protocol and Algorithms
Specification", RFC 5905, June 2010. Specification", RFC 5905, June 2010.
[RFC5906] Haberman, B. and D. Mills, "Network Time Protocol Version [RFC5906] Haberman, B. and D. Mills, "Network Time Protocol Version
4: Autokey Specification", RFC 5906, June 2010. 4: Autokey Specification", RFC 5906, June 2010.
[RFC6277] Santesson, S. and P. Hallam-Baker, "Online Certificate [RFC6277] Santesson, S. and P. Hallam-Baker, "Online Certificate
Status Protocol Algorithm Agility", RFC 6277, June 2011. Status Protocol Algorithm Agility", RFC 6277, June 2011.
12.2. Informative References 13.2. Informative References
[I-D.ietf-tictoc-security-requirements] [I-D.ietf-tictoc-security-requirements]
Mizrahi, T., "Security Requirements of Time Protocols in Mizrahi, T., "Security Requirements of Time Protocols in
Packet Switched Networks", draft-ietf-tictoc-security- Packet Switched Networks", draft-ietf-tictoc-security-
requirements-05 (work in progress), April 2013. requirements-05 (work in progress), April 2013.
[Roettger] [Roettger]
Roettger, S., "Analysis of the NTP Autokey Procedures", Roettger, S., "Analysis of the NTP Autokey Procedures",
February 2012. February 2012.
Appendix A. TICTOC Security Requirements Appendix A. Flow Diagrams of Client Behaviour
+---------------------+
|Association Messages |
+----------+----------+
|
v
+---------------------+
|Certificate Messages |
+----------+----------+
|
+------------------------------>o
| |
| v
| +---------------+
| |Cookie Messages|
| +-------+-------+
| |
| o<------------------------------+
| | |
| 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-----------+
Figure 1: The client's behaviour in NTS unicast mode.
+-----------------------------+
|Broadcast Parameter Messages |
+--------------+--------------+
|
o<--------------------------+
| |
v |
+-----------------------------+ |
|Broadcast Time Sync. Message | |
+--------------+--------------+ |
| |
+-------------------------------------->o |
| | |
| v |
| +-------------------+ |
| |Key and Auth. Check| |
| +---------+---------+ |
| | |
| /----------------*----------------\ |
| v v |
| .---------. .---------. |
| ( Verified ) ( Falsified ) |
| '----+----' '----+----' |
| | | |
| v v |
| +-------------+ +-------+ |
| |Store Message| |Discard| |
| +------+------+ +---+---+ |
| | | |
| v +---------o
| +---------------+ |
| |Check Previous | |
| +-------+-------+ |
| | |
| /--------*--------\ |
| v v |
| .---------. .---------. |
| ( Verified ) ( Falsified ) |
| '----+----' '----+----' |
| | | |
| v v |
| +-------------+ +-----------------+ |
| |Sync. Process| |Discard Previous | |
| +------+------+ +--------+--------+ |
| | | |
+-----------+ +-----------------------------------+
Figure 2: The client's behaviour in NTS broadcast mode.
Appendix B. Extension fields
In Section 6, some new extension fields for NTP packets are
introduced. They are listed here again, for reference.
+------------------------+---------------+
| name | used in |
+------------------------+---------------+
| "association" | client_assoc |
| | server_assoc |
| | |
| "certificate request" | client_cert |
| | |
| "certificate" | server_cert |
| | |
| "cookie request" | client_cook |
| | |
| "cookie transmit" | server_cook |
| | |
| "time request" | time_request |
| | |
| "time response" | time_response |
| | |
| "broadcast request" | client_bpar |
| | |
| "broadcast parameters" | server_bpar |
| | |
| "broadcast message" | server_broad |
+------------------------+---------------+
Appendix C. TICTOC Security Requirements
The following table compares the NTS specifications against the The following table compares the NTS specifications against the
TICTOC security requirements [I-D.ietf-tictoc-security-requirements]. TICTOC security requirements [I-D.ietf-tictoc-security-requirements].
+---------+--------------------------------+---------------+--------+ +---------+--------------------------------+---------------+--------+
| Section | Requirement from I-D tictoc | Requirement | NTS | | Section | Requirement from I-D tictoc | Requirement | NTS |
| | security-requirements-05 | level | | | | security-requirements-05 | level | |
+---------+--------------------------------+---------------+--------+ +---------+--------------------------------+---------------+--------+
| 5.1.1 | Authentication of Servers | MUST | OK | | 5.1.1 | Authentication of Servers | MUST | OK |
| 5.1.1 | Authorization of Servers | MUST | - | +---------+--------------------------------+---------------+--------+
| 5.1.2 | Recursive Authentication of | MUST | NO | | 5.1.1 | Authorization of Servers | MUST | OK |
+---------+--------------------------------+---------------+--------+
| 5.1.2 | Recursive Authentication of | MUST | OK |
| | Servers (Stratum 1) | | | | | Servers (Stratum 1) | | |
| 5.1.2 | Recursive Authorization of | MUST | - | +---------+--------------------------------+---------------+--------+
| 5.1.2 | Recursive Authorization of | MUST | OK |
| | Servers (Stratum 1) | | | | | Servers (Stratum 1) | | |
+---------+--------------------------------+---------------+--------+
| 5.1.3 | Authentication and | MAY | - | | 5.1.3 | Authentication and | MAY | - |
| | Authorization of Slaves | | | | | Authorization of Slaves | | |
+---------+--------------------------------+---------------+--------+
| 5.2 | Integrity protection. | MUST | OK | | 5.2 | Integrity protection. | MUST | OK |
+---------+--------------------------------+---------------+--------+
| 5.3 | Protection against DoS attacks | SHOULD | OK | | 5.3 | Protection against DoS attacks | SHOULD | OK |
+---------+--------------------------------+---------------+--------+
| 5.4 | Replay protection | MUST | OK | | 5.4 | Replay protection | MUST | OK |
+---------+--------------------------------+---------------+--------+
| 5.5.1 | Key freshness. | MUST | OK | | 5.5.1 | Key freshness. | MUST | OK |
+---------+--------------------------------+---------------+--------+
| 5.5.2 | Security association. | SHOULD | OK | | 5.5.2 | Security association. | SHOULD | OK |
+---------+--------------------------------+---------------+--------+
| 5.5.3 | Unicast and multicast | SHOULD | OK | | 5.5.3 | Unicast and multicast | SHOULD | OK |
| | associations. | | | | | associations. | | |
+---------+--------------------------------+---------------+--------+
| 5.6 | Performance: no degradation in | MUST | OK | | 5.6 | Performance: no degradation in | MUST | OK |
| | quality of time transfer. | | | | | quality of time transfer. | | |
+---------+--------------------------------+---------------+--------+
| | Performance: lightweight | SHOULD | OK | | | Performance: lightweight | SHOULD | OK |
| | computation | | | | | computation | | |
+---------+--------------------------------+---------------+--------+
| | Performance: storage, | SHOULD | OK | | | Performance: storage, | SHOULD | OK |
| | bandwidth | | | | | bandwidth | | |
+---------+--------------------------------+---------------+--------+
| 5.7 | Confidentiality protection | MAY | NO | | 5.7 | Confidentiality protection | MAY | NO |
+---------+--------------------------------+---------------+--------+
| 5.8 | Protection against Packet | SHOULD | NA*) | | 5.8 | Protection against Packet | SHOULD | NA*) |
| | Delay and Interception Attacks | | | | | Delay and Interception Attacks | | |
+---------+--------------------------------+---------------+--------+
| 5.9.1 | Secure mode | MUST | - | | 5.9.1 | Secure mode | MUST | - |
+---------+--------------------------------+---------------+--------+
| 5.9.2 | Hybrid mode | MAY | - | | 5.9.2 | Hybrid mode | MAY | - |
+---------+--------------------------------+---------------+--------+ +---------+--------------------------------+---------------+--------+
*) Ensured by NTP via multi-source configuration. *) Ensured by NTP via multi-source configuration.
Comparsion of NTS sepecification against TICTOC security Comparsion of NTS sepecification against TICTOC security
requirements. requirements.
Appendix B. Broadcast Mode Appendix D. Broadcast Mode
Authors' Addresses Authors' Addresses
Dieter Sibold Dieter Sibold
Physikalisch-Technische Bundesanstalt Physikalisch-Technische Bundesanstalt
Bundesallee 100 Bundesallee 100
Braunschweig D-38116 Braunschweig D-38116
Germany Germany
Phone: +49-(0)531-592-8420 Phone: +49-(0)531-592-8420
Fax: +49-531-592-698420 Fax: +49-531-592-698420
Email: dieter.sibold@ptb.de Email: dieter.sibold@ptb.de
Stephen Roettger Stephen Roettger
Technische Universitaet Braunschweig
Email: stephen.roettger@googlemail.com Email: stephen.roettger@googlemail.com
Kristof Teichel Kristof Teichel
Physikalisch-Technische Bundesanstalt Physikalisch-Technische Bundesanstalt
Bundesallee 100 Bundesallee 100
Braunschweig D-38116 Braunschweig D-38116
Germany Germany
Email: kristof.teichel@ptb.de Email: kristof.teichel@ptb.de
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