draft-ietf-ntp-refid-updates-03.txt   draft-ietf-ntp-refid-updates-04.txt 
Internet Engineering Task Force H. Stenn Internet Engineering Task Force H. Stenn
Internet-Draft Network Time Foundation Internet-Draft Network Time Foundation
Intended status: Standards Track S. Goldberg Intended status: Standards Track S. Goldberg
Expires: December 8, 2018 Boston University Expires: April 7, 2019 Boston University
June 6, 2018 October 4, 2018
Network Time Protocol REFID Updates Network Time Protocol REFID Updates
draft-ietf-ntp-refid-updates-03 draft-ietf-ntp-refid-updates-04
Abstract Abstract
RFC 5905 [RFC5905], section 7.3, "Packet Header Variables", defines RFC 5905 [RFC5905], section 7.3, "Packet Header Variables", defines
the value of the REFID, the system peer for the responding host. In the value of the REFID, the system peer for the responding host. In
the past, for IPv4 associations the IPv4 address is used, and for the past, for IPv4 associations the IPv4 address is used, and for
IPv6 associations the first four octets of the MD5 hash of the IPv6 IPv6 associations the first four octets of the MD5 hash of the IPv6
are used. There are at least three shortcomings to this approach, are used. There are two recognized shortcomings to this approach,
and this proposal will address the three so noted. One is that and this proposal addresses them. One is that knowledge of the
knowledge of the system peer is "abusable" information and should not system peer is "abusable" information and should not be generally
be generally available. The second is that the four octet hash of available. The second is that the four octet hash of the IPv6
the IPv6 address looks very much like an IPv4 address, and this is address looks very much like an IPv4 address, and this is confusing.
confusing. The third is that a growing number of low-stratum servers
want to offer leap-smeared time to their clients, and there is no
obvious way to know if a server is offering accurate time or leap-
smeared time.
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
provisions of BCP 78 and BCP 79. provisions of BCP 78 and BCP 79.
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
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Drafts is at https://datatracker.ietf.org/drafts/current/. Drafts is at https://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 December 8, 2018. This Internet-Draft will expire on April 7, 2019.
Copyright Notice Copyright Notice
Copyright (c) 2018 IETF Trust and the persons identified as the Copyright (c) 2018 IETF Trust and the persons identified as the
document authors. All rights reserved. document authors. All rights reserved.
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Provisions Relating to IETF Documents Provisions Relating to IETF Documents
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to this document. Code Components extracted from this document must to this document. Code Components extracted from this document must
include Simplified BSD License text as described in Section 4.e of include Simplified BSD License text as described in Section 4.e of
the Trust Legal Provisions and are provided without warranty as the Trust Legal Provisions and are provided without warranty as
described in the Simplified BSD License. described in the Simplified BSD License.
Table of Contents Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2
1.1. The REFID . . . . . . . . . . . . . . . . . . . . . . . . 2 1.1. The REFID . . . . . . . . . . . . . . . . . . . . . . . . 2
1.2. NOT-YOU REFID . . . . . . . . . . . . . . . . . . . . . . 3 1.2. NOT-YOU REFID . . . . . . . . . . . . . . . . . . . . . . 3
1.3. IPv6 REFID . . . . . . . . . . . . . . . . . . . . . . . 4 1.3. IPv6 REFID . . . . . . . . . . . . . . . . . . . . . . . 3
1.4. Leap-Smear REFID . . . . . . . . . . . . . . . . . . . . 4 1.4. Requirements Language . . . . . . . . . . . . . . . . . . 4
1.5. Requirements Language . . . . . . . . . . . . . . . . . . 5 2. The NOT-YOU REFID . . . . . . . . . . . . . . . . . . . . . . 4
2. The NOT-YOU REFID . . . . . . . . . . . . . . . . . . . . . . 5 2.1. Proposal . . . . . . . . . . . . . . . . . . . . . . . . 4
2.1. Proposal . . . . . . . . . . . . . . . . . . . . . . . . 5 3. Augmenting the IPv6 REFID Hash . . . . . . . . . . . . . . . 5
3. Augmenting the IPv6 REFID Hash . . . . . . . . . . . . . . . 6 3.1. Background . . . . . . . . . . . . . . . . . . . . . . . 5
3.1. Background . . . . . . . . . . . . . . . . . . . . . . . 6
3.2. Potential Problems . . . . . . . . . . . . . . . . . . . 6 3.2. Potential Problems . . . . . . . . . . . . . . . . . . . 6
3.3. Questions . . . . . . . . . . . . . . . . . . . . . . . . 7 3.3. Questions . . . . . . . . . . . . . . . . . . . . . . . . 6
4. The REFID sent to clients during a Leap-Smear . . . . . . . . 7 4. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 6
4.1. Background . . . . . . . . . . . . . . . . . . . . . . . 7 5. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 6
4.2. Leap Smear REFID . . . . . . . . . . . . . . . . . . . . 7 6. Security Considerations . . . . . . . . . . . . . . . . . . . 6
4.3. Questions . . . . . . . . . . . . . . . . . . . . . . . . 9 7. References . . . . . . . . . . . . . . . . . . . . . . . . . 7
5. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 9 7.1. Normative References . . . . . . . . . . . . . . . . . . 7
6. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 10 7.2. Informative References . . . . . . . . . . . . . . . . . 7
7. Security Considerations . . . . . . . . . . . . . . . . . . . 10 Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 7
8. References . . . . . . . . . . . . . . . . . . . . . . . . . 10
8.1. Normative References . . . . . . . . . . . . . . . . . . 10
8.2. Informative References . . . . . . . . . . . . . . . . . 10
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 11
1. Introduction 1. Introduction
1.1. The REFID 1.1. The REFID
The interpretation of a REFID is based on the stratum, as documented The interpretation of a REFID is based on the stratum, as documented
in RFC 5905 [RFC5905], section 7.3, "Packet Header Variables". The in RFC 5905 [RFC5905], section 7.3, "Packet Header Variables". The
core reason for the REFID in the NTP Protocol is to prevent a degree- core reason for the REFID in the NTP Protocol is to prevent a degree-
one timing loop, where server B decides to follow A as its time one timing loop, where server B decides to follow A as its time
source, and A then decides to follow B as its time source. source, and A then decides to follow B as its time source.
At Stratum 2+, which will be the case if two servers A and B are At Stratum 2+, which will be the case if two servers A and B are
exchanging timing information, then if server B follows A as its time exchanging timing information, then if server B follows A as its time
source, A's address will be B's REFID. When A uses IPv4, the default source, A's address will be B's REFID. When A uses IPv4, the default
REFID is A's IPv4 address. When A uses IPv6, the default REFID is a REFID is A's IPv4 address. When A uses IPv6, the default REFID is a
four-octet digest of A's IPv6 address. Now, if A queries B for its four-octet digest of A's IPv6 address. Now, if A queries B for its
time, then A will learn that B is using A as its time source by time, then A will learn that B is using A as its time source by
observing A's address in the REFID field of the response packet sent observing A's address in the REFID field of the response packet sent
by B. Thus, A will not select B as a potential time source, since by B. Thus, A will not select B as a potential time source, as this
this would cause a timing loop. would cause a timing loop.
1.2. NOT-YOU REFID 1.2. NOT-YOU REFID
This REFID mechanism, however, also allows a third-party C to learn The traditional REFID mechanism, however, also allows a third-party C
that A is the time source that is being used by B. When A is using to learn that A is the time source that is being used by B. When A
IPv4, C can learn this by querying B for its time, and observing that is using IPv4, C can learn this by querying B for its time, and
the REFID in B's response is the IPv4 address of A. Meanwhile, when observing that the REFID in B's response is the IPv4 address of A.
A is using IPv6, then C can again query B for its time, and then can Meanwhile, when A is using IPv6, then C can again query B for its
use an offline dictionary attack to attempt to determine the IPv6 time, and then can use an offline dictionary attack to attempt to
address that corresponds to the digest value in the response sent by determine the IPv6 address that corresponds to the digest value in
B. C could construct the necessary dictionary by compiling a list of the response sent by B. C could construct the necessary dictionary
publicly accessible IPv6 servers. Remote attackers can use this by compiling a list of publicly accessible IPv6 servers. Remote
technique to attempt to identify the time sources used by a target, attackers can use this technique to attempt to identify the time
and then send spoofed packets to the target or its time source in an sources used by a target, and then send spoofed packets to the target
attempt to disrupt time service, as was done e.g., in [NDSS16] or or its time source in an attempt to disrupt time service, as was done
[CVE-2015-8138]. e.g., in [NDSS16] or [CVE-2015-8138].
The REFID thus unnecessarily leaks information about a target's time The REFID thus unnecessarily leaks information about a target's time
server to remote attackers. The best way to mitigate this server to remote attackers. The best way to mitigate this
vulnerability is to decouple the IP address of the time source from vulnerability is to decouple the IP address of the time source from
the REFID. To do this, a system can use an otherwise-impossible the REFID. To do this, a system can use an otherwise-impossible
value for its REFID, called the NOT-YOU REFID value, when it believes value for its REFID, called the "not-you" value, when it believes
that a querying system is not its time source. that a querying system is not its time source.
The NOT-YOU REFID proposal is backwards-compatible and provides the The NOT-YOU REFID proposal is backwards-compatible. It can be
most basic diagnostic information to third parties. It can be
implemented by one peer in an NTP association without any changes to implemented by one peer in an NTP association without any changes to
the other peer. This holds as long as responding NOT-YOU system can the other peer.
accurately detect when it's getting a request from its system peer.
The NOT-YOU REFID proposal does have a small risk. Consider system A
that returns the NOT-YOU REFID and system B that has two network
interfaces B1 and B2. Suppose that system A is using system B as his
time source, via network interface B1. Now suppose that system B
queries system A for time via network interface B2. In this case,
system A returns the NOT-YOU REFID value to system B, since system A
does not realize that network interface B1 and B2 belong to the same
system. In this case, system B might choose system A as its time
source, and a degree-one timing loop will occur. In this case,
however, the two systems will spiral into worse stratum positions
with increasing root distances, and eventually the loop will break.
If any other systems are available as time servers, one of them will The NOT-YOU REFID proposal does have a small risk, in that a system
become the new system peer. However, until this happens the two that might return NOT-YOU does not have perfect information and it is
spiraling systems will have degraded time quality. possible that the remote system peer is contacting "us" via a
different network interface. In this case, the remote system might
choose us as their system peer, and a degree-one timing loop will
occur. In this case, however, the two systems will spiral into
worsening stratum positions with increasing root distances, and
eventually the loop will break. If any other systems are available
as time servers, one of them may become the new system peer.
However, unless or until this happens the two spiraling systems will
have degraded time quality.
1.3. IPv6 REFID 1.3. IPv6 REFID
In an environment where all time queries made to a server can be In an environment where all time queries made to a server can be
trusted, an operator might well choose to expose the real REFID. RFC trusted, an operator might well choose to expose the real REFID. RFC
5905 [RFC5905], section 7.3, "Packet Header Variables", explains how 5905 [RFC5905], section 7.3, "Packet Header Variables", explains how
a remote system peer is converted to a REFID. It says: a remote system peer is converted to a REFID. It says:
If using the IPv4 address family, the identifier is the four-octet If using the IPv4 address family, the identifier is the four-octet
IPv4 address. If using the IPv6 family, it is the first four IPv4 address. If using the IPv6 family, it is the first four
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However, the MD5 hash of an IPv6 address often looks like a valid However, the MD5 hash of an IPv6 address often looks like a valid
IPv4 address. When this happens, an operator cannot tell if the IPv4 address. When this happens, an operator cannot tell if the
REFID refers to an IPv6 address or and IPv4. Specifically, the NTP REFID refers to an IPv6 address or and IPv4. Specifically, the NTP
Project has received a report where the generated IPv6 hash decoded Project has received a report where the generated IPv6 hash decoded
to the IPv4 address of a different machine on the system peer's to the IPv4 address of a different machine on the system peer's
network. network.
This proposal offers a way for a system to generate a REFID for a This proposal offers a way for a system to generate a REFID for a
IPv6 system peer that does not conflict with an IPv4-based REFID. IPv6 system peer that does not conflict with an IPv4-based REFID.
This proposal is not backwards-compatible. It SHOULD be implemented This proposal is not fully backwards-compatible. It SHOULD be
by both peers in an NTP association. In the scenario where A and B implemented by both peers in an NTP association. In the scenario
are peering using IPv6, where A is the system peer and does not where A and B are peering using IPv6, where A is the system peer and
understand IPv6 REFID, and B is subordinate and is using IPv6 REFID, does not understand IPv6 REFID, and B is subordinate and is using
A will not be able to determine that B is using A as its system peer IPv6 REFID, A will not be able to determine that B is using A as its
and a degree-one timing loop can form. system peer and a degree-one timing loop can form.
If both peers implement the IPv6 REFID this situation cannot happen. If both peers implement the IPv6 REFID this situation cannot happen.
[If at least one of the peers implements the proposed I-DO protocol [If at least one of the peers implements the proposed I-DO protocol
this situation cannot happen.] this situation cannot happen.]
1.4. Leap-Smear REFID 1.4. Requirements Language
RFC 5905 [RFC5905] and earlier versions of NTP are the overwhelming
method of distributing time on networks. Leap Seconds will continue
to exist for a good number of years' time, and since the timescale
mandated by POSIX effectively ignores any instances where there are
not 86,400 seconds' time in a day something must be done to reliably
synchronize clocks during the application of leap second corrections.
One way to deal with the insertion of a leap second is to apply the
leap second using a "smear", where the time reported by leap-second
aware servers is gradually adjusted so there is no major disruption
to time synchronization when processing a leap second.
While the proper handling of leap seconds can be expected from up-to-
date software and time servers, there are large numbers of out-of-
date software installations and systems that are just not able to
properly handle a leap second correction.
This proposal offers a way for a system to generate a REFID that
indicates that the time being supplied in the NTP packet already
contains an amount of leap smear correction, and what that amount is.
This proposal is backwards-compatible in all but poorly-designed NTP
networks. The entire point of providing NTP servers that offer leap-
smeared time in response to CLIENT requests is to provide smooth time
to clients that are unable to properly handle leap seconds. If an
operator is skilled enough to provide leap-smeared time to a subset
of clients that cannot properly handle leap seconds, they can be
expected to know enough to avoid using leap-smeared time between time
servers that are expected to be able to properly handle leap seconds.
Leap smears are expected to be implemented on a limited number of
time servers where there is a base of client systems that cannot
handle a leap second correction. Furthermore, even in a poorly-
designed NTP network the "window of risk" lasts only as long as it
takes for the leap second to be smeared.
1.5. 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].
2. The NOT-YOU REFID 2. The NOT-YOU REFID
2.1. Proposal 2.1. Proposal
When enabled, this proposal allows the one-degree loop detection to When enabled, this proposal allows the one-degree loop detection to
work and useful diagnostic information to be provided to trusted work and useful diagnostic information to be provided to trusted
partners while keeping potentially abusable information from being partners while keeping potentially abusable information from being
disclosed to ostensibly uninterested parties. It does this by disclosed to ostensibly uninterested parties. It does this by
returning the normal REFID to queries that come from trusted returning the normal REFID to queries that come from trusted
addresses or from an address that the current system believes is its addresses or from an address that the current system believes is its
time source (aka its "system peer"), and otherwise returning a time source (aka its "system peer"), and otherwise returning one of
special IP address that is interpreted to mean "not you". The "not two special IP addresses that is interpreted to mean "not you". The
you" IP address is 127.127.127.127 when the query is made from an "not you" IP addresses are 127.127.127.127 and 127.127.127.128. If
IPv4 address, or when the query is made from an IPv6 address whose an IPv6 query is received from an address whose four-octet hash
four-octet hash does not equal 127.127.127.127. The "not you" IP equals one of these two addresses and we believe the querying host is
address is 127.127.127.128 when the query is made from an address not our system peer, the other NOT-YOU address is returned as the
whose four-octet hash equals 127.127.127.127. REFID.
This mechanism is correct and transparent when the system responding This mechanism is correct and transparent when the system responding
with a NOT-YOU can accurately detect when it's getting a timing query with a NOT-YOU can accurately detect when it's getting a timing query
from its system peer. A querying system that uses IPv4 continues to from its system peer. A querying system that uses IPv4 continues to
check that its IPv4 address does not appear in the REFID before check that its IPv4 address does not appear in the REFID before
deciding whether to take time from the current system. A querying deciding whether to take time from the current system. A querying
system that uses IPv6 continues to check that the four-octet hash of system that uses IPv6 continues to check that the four-octet hash of
its IPv6 address does not appear in the REFID before deciding whether its IPv6 address does not appear in the REFID before deciding whether
to take time from the current system. to take time from the current system. However...
Use of the NOT-YOU REFID proposal will hide the current system's
system peer from querying systems that the current system believes
are not the current system's system peer. Should the current system
return the "not you" REFID to a query from its system peer, for
example in the case where the system peer sends its query from an
unexpected IP address, a one-degree timing loop can occur. Put
another way, the responding system has imperfect knowledge about
whether or not the sender is its system peer and there are cases
where it will offer a NOT-YOU response to its system peer, which can
then produce a degree-one timing loop.
Note that this mechanism fully supports degree-one loop detection in
the case where the responding NOT-YOU system can accurately detect
when it's getting a request from its system peer, and otherwise
provides the most basic diagnostic information to third parties.
3. Augmenting the IPv6 REFID Hash 3. Augmenting the IPv6 REFID Hash
3.1. Background 3.1. Background
In a trusted network, the S2+ REFID is generated based on the network In a trusted network, the S2+ REFID is generated based on the network
system peer. RFC 5905 [RFC5905] says: system peer. RFC 5905 [RFC5905] says:
If using the IPv4 address family, the identifier is the four-octet If using the IPv4 address family, the identifier is the four-octet
IPv4 address. If using the IPv6 family, it is the first four IPv4 address. If using the IPv6 family, it is the first four
octets of the MD5 hash of the IPv6 address. octets of the MD5 hash of the IPv6 address. ...
This means that the IPv4 representation of the IPv6 hash would be: This means that the IPv4 representation of the IPv6 hash would be:
b1.b2.b3.b4 . The proposal is that the system MAY also use b1.b2.b3.b4 . This proposal is that the system MAY also use
255.b2.b3.b4 as its REFID. This reduces the risk of ambiguity, since 255.b2.b3.b4 as its REFID. This reduces the risk of ambiguity, since
addresses beginning with 255 are "reserved", and thus will not addresses beginning with 255 are "reserved", and thus will not
collide with valid IPv4 on the network. collide with valid IPv4 on the network.
When using the REFID to check for a timing loop for an IPv6 When using the REFID to check for a timing loop for an IPv6
association, if the code that checks the first four-octets of the association, if the code that checks the first four-octets of the
hash fails to match then the code must check again, using 0xFF as the hash fails to match then the code must check again, using 0xFF as the
first octet of the hash. first octet of the hash.
3.2. Potential Problems 3.2. Potential Problems
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There is a 1 in 16,777,216 chance that the REFID hashes of two IPv6 There is a 1 in 16,777,216 chance that the REFID hashes of two IPv6
addresses will be identical, producing a false-positive loop addresses will be identical, producing a false-positive loop
detection. With a sufficient number of servers, the risk of this detection. With a sufficient number of servers, the risk of this
problem becomes a non-issue. [The use of the NOT-YOU REFID and/or problem becomes a non-issue. [The use of the NOT-YOU REFID and/or
the proposed "REFID Suggestion" or "I-DO" extension fields are ways the proposed "REFID Suggestion" or "I-DO" extension fields are ways
to mitigate this potential situation.] to mitigate this potential situation.]
Unrealistically, if only two instances of NTP are communicating via Unrealistically, if only two instances of NTP are communicating via
IPv6 and system A implements this new IPv6 REFID hash and system B IPv6 and system A implements this new IPv6 REFID hash and system B
does not, system B will not be able to detect this loop condition. does not, system B will not be able to detect this loop condition.
In this case, the two machines will slowly increase their stratum In this case, the two machines will slowly increase their Stratum
until they become unsynchronized. This situation is considered to be until they reach S16 and become unsynchronized. This situation is
unrealistic because, for this to happen, each system would have to considered to be unrealistic because, for this to happen, each system
have only the other system available as a time source, for example, would have to have only the other system available as a time source,
in a misconfigured "orphan mode" setup. There is no risk of this for example, in a misconfigured "orphan mode" setup. There is no
happening in an NTP network with 3 or more time sources, or in a risk of this happening in an NTP network with 3 or more time sources,
properly-configured "time island" setup. or in a properly-configured "time island" setup.
3.3. Questions 3.3. Questions
Should we reference the REFID Suggestion and I-DO proposals here? Should we reference the REFID Suggestion and I-DO proposals here?
Should we ask IANA to allocate a pseudo Extension Field Type of 4. Acknowledgements
0xFFFF (for example) so the proposed "I-Do" exchange can report
whether or not the "IPv6 REFID Hash" is supported?
4. The REFID sent to clients during a Leap-Smear
4.1. Background
This proposal offers a way for a system to generate a REFID that
indicates that the time being supplied in the NTP packet already
contains an amount of leap smear correction, and what that amount is.
4.2. Leap Smear REFID
RFC 5905 [RFC5905] defines the data type of NTP time values in
Section 6, "Data Types":
All NTP time values are represented in twos-complement format,
with bits numbered in big-endian (as described in Appendix A of
[RFC0791]) fashion from zero starting at the left, or high-order,
position. ...
The 32 bit signed integer seconds portion and the 32 bit unsigned
fractional seconds portion, or 32:32 format is:
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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Seconds |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Fraction |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
NTP Timestamp Format (32:32)
This format provides coverage for 136 years' time to a precision of
232 picoseconds. If a leap-second addition is being completely
smeared just before before the stroke of the next POSIX second then
the smear correction will be (0,1). [That's mathematical domain
range notation - how to cite it?] If this was the only way to apply
a leap smear correction then we could simply use an unsigned value to
represent the correction. But while the first popular leap smear
implementation applied the correction over an appropriate number of
hours' time before the actual leap second, so the system time was
again correct at the stroke of 00:00, that meant that the difference
between system time and UTC spent half of the duration of the smear
application at [.5,1) "off" of correct time. The second popular
implementation of the leap smear applied the first half-second
correction before the stroke of 00:00 for a correction range of
(0,.5] and the last half-second correction starting at the stroke of
00:00 for a [-.5,0) correction range. This also means we need a
signed value to represent the amount of correction.
The REFID of a system that is supplying smeared time to client
requests while leap-smear correction is active would be 254.b1.b2.b3,
where the three octets (b1, b2, and b3) are a 2:22 formatted value,
yielding 2 signed bits of integer time and 22 bits of unsigned
fractional subseconds, with a precision to 238 nanoseconds, or about
a quarter of a microsecond. Signed time is needed to implement the
mathematical range described in the previous paragraph.
[How should we cite the 2:22 notation? This is the same general
format that we use for NTP timestamps.]
[Sharon says: I suggest adding a concrete example of the scheme, so
that the above paragraph is easier to understand.]
The client is not expected to do anything with this information.
Indeed, the whole point of offering smeared time is that there is
reason to believe the clients are unable to properly handle a leap
second correction. In this case, clients cannot be expected to do
anything with data embedded in the REFID, either. However,
monitoring systems that use tools that show a host's system peer,
like the 'ntpq' and 'sntp' programs in the reference implementation,
[HMS: how to cite this?] can use this information to make sure that
clients are following a leap-smearing server and can see fairly
accurately what the smear is for each client.
Note that if an NTP server decides to offer smeared time corrections
to clients, it SHOULD only offer this time in response to CLIENT time
requests. An NTP server that is offering smeared time SHOULD NOT
send smeared time in any peer exchanges. Also, system that sync
their time via CLIENT requests SHOULD NOT be distributing time
(smeared or otherwise) to other systems.
[Sharon asks: Consider a client that doesn't know he is getting
smeared time (b\c he is outdated etc). How is a this client supposed
to know that he should not be distributing smeared time? Note that
its perfectly normal for a stratum 2 server that gets his time via
CLIENT requests from a stratum 1 server to then offer time to stratum
3 systems.]
We also note that during the application of a leap smear, the REFID
from a system offering smeared time cannot provide detection of a
timing loop. This is not expected to be a problem because time
server systems are not expected to make CLIENT connections with each
other, so they should not be receiving smeared time. [Sharon asks: I
don't understand this point, see my question above.] Moreso, if a
time server is configured to make CLIENT connections to a server that
offers smeared time, with the mechanism described here it can detect
when it is getting smeared time, and either ignore time from that
source, or "undo" the leap smear correction and use the corrected
time for that sample.
This proposal is not an attempt to justify servers offering leap
smeared time. It is only an attempt to make it easy and visible to
identify when a server is offering or client is receiving smeared
time, and provide the client a means to know the amount of smear
correction as of the latest successful poll.
4.3. Questions
Should we ask IANA to allocate a pseudo Extension Field Type of
0xFFFE (for example) so the proposed "I-Do" exchange can report
whether or not this server will offer leap smeared time in response
to CLIENT time requests, identifying the amount of correction using
the above REFID?
5. Acknowledgements
For the "not-you" REFID, we acknowledge useful discussions with For the "not-you" REFID, we acknowledge useful discussions with
Aanchal Malhotra and Matthew Van Gundy. Aanchal Malhotra and Matthew Van Gundy.
For the IPv6 REFID, we acknowledge Dan Mahoney (and perhaps others) For the IPv6 REFID, we acknowledge Dan Mahoney (and perhaps others)
for suggesting the idea of using an "impossible" first-octet value to for suggesting the idea of using an "impossible" first-octet value to
indicate an IPv6 refid hash. indicate an IPv6 refid hash.
For the Leap Smear REFID, we acknowledge useful discussions with 5. IANA Considerations
Juergen Perlinger.
6. IANA Considerations
This memo makes no requests of IANA. This memo requests IANA to allocate a pseudo Extension Field Type of
0xFFFF so the proposed "I-Do" exchange can report whether or not the
"IPv6 REFID Hash" is supported.
7. Security Considerations 6. Security Considerations
Many systems running NTP are configured to return responses to timing Many systems running NTP are configured to return responses to timing
queries by default. These responses contain a REFID field, which queries by default. These responses contain a REFID field, which
generally reveals the address of the system's time source if that generally reveals the address of the system's time source if that
source is an IPv4 address. This behavior can be exploited by remote source is an IPv4 address. This behavior can be exploited by remote
attackers who wish to first learn the address of a target's time attackers who wish to first learn the address of a target's time
source, and then attack the target and/or its time source. As such, source, and then attack the target and/or its time source. As such,
the NOT-YOU REFID proposal is designed to harden NTP against these the "not-you" REFID proposal is designed to harden NTP against these
attacks by limiting the amount of information leaked in the REFID attacks by limiting the amount of information leaked in the REFID
field. field.
Systems running NTP should reveal the identity of their system in Systems running NTP should reveal the identity of their system in
peer in their REFID only when they are on a trusted network. The peer in their REFID only when they are on a trusted network. The
IPv6 REFID proposal provides one way to do this, when the system peer IPv6 REFID proposal provides one way to do this, when the system peer
uses addresses in the IPv6 family. uses addresses in the IPv6 family.
8. References 7. References
8.1. Normative References 7.1. Normative References
[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, Requirement Levels", BCP 14, RFC 2119,
DOI 10.17487/RFC2119, March 1997, DOI 10.17487/RFC2119, March 1997,
<https://www.rfc-editor.org/info/rfc2119>. <https://www.rfc-editor.org/info/rfc2119>.
[RFC5905] Mills, D., Martin, J., Ed., Burbank, J., and W. Kasch, [RFC5905] Mills, D., Martin, J., Ed., Burbank, J., and W. Kasch,
"Network Time Protocol Version 4: Protocol and Algorithms "Network Time Protocol Version 4: Protocol and Algorithms
Specification", RFC 5905, DOI 10.17487/RFC5905, June 2010, Specification", RFC 5905, DOI 10.17487/RFC5905, June 2010,
<https://www.rfc-editor.org/info/rfc5905>. <https://www.rfc-editor.org/info/rfc5905>.
8.2. Informative References 7.2. Informative References
[CVE-2015-8138] [CVE-2015-8138]
Van Gundy, M. and J. Gardner, "Network Time Protocol Van Gundy, M. and J. Gardner, "Network Time Protocol
Origin Timestamp Check Impersonation Vulnerability (CVE- Origin Timestamp Check Impersonation Vulnerability (CVE-
2015-8138)", in TALOS VULNERABILITY REPORT (TALOS- 2015-8138)", in TALOS VULNERABILITY REPORT (TALOS-
2016-0077), 2016. 2016-0077), 2016.
[NDSS16] Malhotra, A., Cohen, I., Brakke, E., and S. Goldberg, [NDSS16] Malhotra, A., Cohen, I., Brakke, E., and S. Goldberg,
"Attacking the Network Time Protocol", in ISOC Network and "Attacking the Network Time Protocol", in ISOC Network and
Distributed System Security Symposium 2016 (NDSS'16), Distributed System Security Symposium 2016 (NDSS'16),
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