draft-ietf-ntp-refid-updates-02.txt   draft-ietf-ntp-refid-updates-03.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: June 7, 2018 Boston University Expires: December 8, 2018 Boston University
December 4, 2017 June 6, 2018
Network Time Protocol REFID Updates Network Time Protocol REFID Updates
draft-ietf-ntp-refid-updates-02 draft-ietf-ntp-refid-updates-03
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 at least three shortcomings to this approach,
and this proposal will address the three so noted. One is that and this proposal will address the three so noted. One is that
knowledge of the system peer is "abusable" information and should not knowledge of the system peer is "abusable" information and should not
skipping to change at page 1, line 43 skipping to change at page 1, line 43
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 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 June 7, 2018. This Internet-Draft will expire on December 8, 2018.
Copyright Notice Copyright Notice
Copyright (c) 2017 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.
This document is subject to BCP 78 and the IETF Trust's Legal This document is subject to BCP 78 and the IETF Trust's Legal
Provisions Relating to IETF Documents Provisions Relating to IETF Documents
(https://trustee.ietf.org/license-info) in effect on the date of (https://trustee.ietf.org/license-info) in effect on the date of
publication of this document. Please review these documents publication of this document. Please review these documents
carefully, as they describe your rights and restrictions with respect carefully, as they describe your rights and restrictions with respect
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
skipping to change at page 2, line 27 skipping to change at page 2, line 27
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 . . . . . . . . . . . . . . . . . . . . . . . 4
1.4. Leap-Smear REFID . . . . . . . . . . . . . . . . . . . . 4 1.4. Leap-Smear REFID . . . . . . . . . . . . . . . . . . . . 4
1.5. Requirements Language . . . . . . . . . . . . . . . . . . 5 1.5. Requirements Language . . . . . . . . . . . . . . . . . . 5
2. The NOT-YOU REFID . . . . . . . . . . . . . . . . . . . . . . 5 2. The NOT-YOU REFID . . . . . . . . . . . . . . . . . . . . . . 5
2.1. Proposal . . . . . . . . . . . . . . . . . . . . . . . . 5 2.1. Proposal . . . . . . . . . . . . . . . . . . . . . . . . 5
3. Augmenting the IPv6 REFID Hash . . . . . . . . . . . . . . . 6 3. Augmenting the IPv6 REFID Hash . . . . . . . . . . . . . . . 6
3.1. Background . . . . . . . . . . . . . . . . . . . . . . . 6 3.1. Background . . . . . . . . . . . . . . . . . . . . . . . 6
3.2. Potential Problems . . . . . . . . . . . . . . . . . . . 7 3.2. Potential Problems . . . . . . . . . . . . . . . . . . . 6
3.3. Questions . . . . . . . . . . . . . . . . . . . . . . . . 7 3.3. Questions . . . . . . . . . . . . . . . . . . . . . . . . 7
4. The REFID sent to clients during a Leap-Smear . . . . . . . . 7 4. The REFID sent to clients during a Leap-Smear . . . . . . . . 7
4.1. Background . . . . . . . . . . . . . . . . . . . . . . . 7 4.1. Background . . . . . . . . . . . . . . . . . . . . . . . 7
4.2. Leap Smear REFID . . . . . . . . . . . . . . . . . . . . 8 4.2. Leap Smear REFID . . . . . . . . . . . . . . . . . . . . 7
4.3. Questions . . . . . . . . . . . . . . . . . . . . . . . . 9 4.3. Questions . . . . . . . . . . . . . . . . . . . . . . . . 9
5. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 10 5. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 9
6. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 10 6. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 10
7. Security Considerations . . . . . . . . . . . . . . . . . . . 10 7. Security Considerations . . . . . . . . . . . . . . . . . . . 10
8. References . . . . . . . . . . . . . . . . . . . . . . . . . 10 8. References . . . . . . . . . . . . . . . . . . . . . . . . . 10
8.1. Normative References . . . . . . . . . . . . . . . . . . 10 8.1. Normative References . . . . . . . . . . . . . . . . . . 10
8.2. Informative References . . . . . . . . . . . . . . . . . 11 8.2. Informative References . . . . . . . . . . . . . . . . . 10
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 11 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
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publicly accessible IPv6 servers. Remote attackers can use this publicly accessible IPv6 servers. Remote attackers can use this
technique to attempt to identify the time sources used by a target, technique to attempt to identify the time sources used by a target,
and then send spoofed packets to the target or its time source in an and then send spoofed packets to the target or its time source in an
attempt to disrupt time service, as was done e.g., in [NDSS16] or attempt to disrupt time service, as was done e.g., in [NDSS16] or
[CVE-2015-8138]. [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" value, when it believes value for its REFID, called the NOT-YOU REFID 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. It can be The NOT-YOU REFID proposal is backwards-compatible and provides the
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. the other peer. This holds as long as responding NOT-YOU system can
accurately detect when it's getting a request from its system peer.
The NOT-YOU REFID proposal does have a small risk, in that a system The NOT-YOU REFID proposal does have a small risk. Consider system A
that might return NOT-YOU does not have perfect information, and it that returns the NOT-YOU REFID and system B that has two network
is possible that the remote system peer is contacting "us" via a interfaces B1 and B2. Suppose that system A is using system B as his
different network interface. In this case, the remote system might time source, via network interface B1. Now suppose that system B
choose us as their system peer, and a degree-one timing loop will queries system A for time via network interface B2. In this case,
occur. In this case, however, the two systems will spiral into worse system A returns the NOT-YOU REFID value to system B, since system A
stratum positions with increasing root distances, and eventually the does not realize that network interface B1 and B2 belong to the same
loop will break. If any other systems are available as time servers, system. In this case, system B might choose system A as its time
one of them will become the new system peer. However, until this source, and a degree-one timing loop will occur. In this case,
happens the two spiraling systems will have degraded time quality. 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
become the new system peer. However, 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 fully backwards-compatible. It SHOULD be This proposal is not backwards-compatible. It SHOULD be implemented
implemented by both peers in an NTP association. In the scenario by both peers in an NTP association. In the scenario where A and B
where A and B are peering using IPv6, where A is the system peer and are peering using IPv6, where A is the system peer and does not
does not understand IPv6 REFID, and B is subordinate and is using understand IPv6 REFID, and B is subordinate and is using IPv6 REFID,
IPv6 REFID, A will not be able to determine that B is using A as its A will not be able to determine that B is using A as its system peer
system peer and a degree-one timing loop can form. 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. Leap-Smear REFID
RFC 5905 [RFC5905] and earlier versions of NTP are the overwhelming RFC 5905 [RFC5905] and earlier versions of NTP are the overwhelming
method of distributing time on networks. Leap Seconds will continue method of distributing time on networks. Leap Seconds will continue
to exist for a good number of years' time, and since the timescale to exist for a good number of years' time, and since the timescale
mandated by POSIX effectively ignores any instances where there are mandated by POSIX effectively ignores any instances where there are
not 86,400 seconds' time in a day something must be done to reliably not 86,400 seconds' time in a day something must be done to reliably
synchronize clocks during the application of leap second corrections. synchronize clocks during the application of leap second corrections.
One mechanism that has recently become visible to deal with the One way to deal with the insertion of a leap second is to apply the
insertion of a leap second is to apply the leap second using a leap second using a "smear", where the time reported by leap-second
"smear", where the time reported by leap-second aware servers is aware servers is gradually adjusted so there is no major disruption
gradually adjusted so there is no major disruption to time to time synchronization when processing a leap second.
synchronization when processing a leap second.
While the proper handling of leap seconds can be expected from up-to- 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 and time servers, there are large numbers of out-of-
date software installations and systems that are just not able to date software installations and systems that are just not able to
properly handle a leap second correction. properly handle a leap second correction.
This proposal offers a way for a system to generate a REFID that This proposal offers a way for a system to generate a REFID that
indicates that the time being supplied in the NTP packet already indicates that the time being supplied in the NTP packet already
contains an amount of leap smear correction, and what that amount is. contains an amount of leap smear correction, and what that amount is.
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address is 127.127.127.128 when the query is made from an address address is 127.127.127.128 when the query is made from an address
whose four-octet hash equals 127.127.127.127. whose four-octet hash equals 127.127.127.127.
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. However... to take time from the current system.
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 . The 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
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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 reach S16 and become unsynchronized. This situation is until they become unsynchronized. This situation is considered to be
considered to be unrealistic because, for this to happen, each system unrealistic because, for this to happen, each system would have to
would have to have only the other system available as a time source, have only the other system available as a time source, for example,
for example, in a misconfigured "orphan mode" setup. There is no in a misconfigured "orphan mode" setup. There is no risk of this
risk of this happening in an NTP network with 3 or more time sources, happening in an NTP network with 3 or more time sources, or in a
or in a properly-configured "time island" setup. 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 Should we ask IANA to allocate a pseudo Extension Field Type of
0xFFFF (for example) so the proposed "I-Do" exchange can report 0xFFFF (for example) so the proposed "I-Do" exchange can report
whether or not the "IPv6 REFID Hash" is supported? whether or not the "IPv6 REFID Hash" is supported?
4. The REFID sent to clients during a Leap-Smear 4. The REFID sent to clients during a Leap-Smear
4.1. Background 4.1. Background
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 mechanism for dealing with the application that has recently
become visible 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 not able to properly
handle a leap second correction.
This proposal offers a way for a system to generate a REFID that This proposal offers a way for a system to generate a REFID that
indicates that the time being supplied in the NTP packet already indicates that the time being supplied in the NTP packet already
contains an amount of leap smear correction, and what that amount is. contains an amount of leap smear correction, and what that amount is.
4.2. Leap Smear REFID 4.2. Leap Smear REFID
RFC 5905 [RFC5905] defines the data type of NTP time values in RFC 5905 [RFC5905] defines the data type of NTP time values in
Section 6, "Data Types": Section 6, "Data Types":
All NTP time values are represented in twos-complement format, All NTP time values are represented in twos-complement format,
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requests while leap-smear correction is active would be 254.b1.b2.b3, 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, 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 yielding 2 signed bits of integer time and 22 bits of unsigned
fractional subseconds, with a precision to 238 nanoseconds, or about fractional subseconds, with a precision to 238 nanoseconds, or about
a quarter of a microsecond. Signed time is needed to implement the a quarter of a microsecond. Signed time is needed to implement the
mathematical range described in the previous paragraph. mathematical range described in the previous paragraph.
[How should we cite the 2:22 notation? This is the same general [How should we cite the 2:22 notation? This is the same general
format that we use for NTP timestamps.] 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. The client is not expected to do anything with this information.
Indeed, the whole point of offering smeared time is that there is Indeed, the whole point of offering smeared time is that there is
reason to believe the clients are unable to properly handle a leap reason to believe the clients are unable to properly handle a leap
second correction. In this case, clients cannot be expected to do second correction. In this case, clients cannot be expected to do
anything with data embedded in the REFID, either. However, anything with data embedded in the REFID, either. However,
monitoring systems that use tools that show a host's system peer, monitoring systems that use tools that show a host's system peer,
like the 'ntpq' and 'sntp' programs in the reference implementation, like the 'ntpq' and 'sntp' programs in the reference implementation,
[HMS: how to cite this?] can use this information to make sure that [HMS: how to cite this?] can use this information to make sure that
clients are following a leap-smearing server and can see fairly clients are following a leap-smearing server and can see fairly
accurately what the smear is for each client. accurately what the smear is for each client.
Note that if an NTP server decides to offer smeared time corrections 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 to clients, it SHOULD only offer this time in response to CLIENT time
requests. An NTP server that is offering smeared time SHOULD NOT requests. An NTP server that is offering smeared time SHOULD NOT
send smeared time in any peer exchanges. Also, system that sync send smeared time in any peer exchanges. Also, system that sync
their time via CLIENT requests SHOULD NOT be distributing time their time via CLIENT requests SHOULD NOT be distributing time
(smeared or otherwise) to other systems. (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 We also note that during the application of a leap smear, the REFID
from a system offering smeared time cannot provide detection of a from a system offering smeared time cannot provide detection of a
timing loop. This is not expected to be a problem because time timing loop. This is not expected to be a problem because time
server systems are not expected to make CLIENT connections with each server systems are not expected to make CLIENT connections with each
other, so they should not be receiving smeared time. Moreso, if a 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 time server is configured to make CLIENT connections to a server that
offers smeared time, with the mechanism described here it can detect offers smeared time, with the mechanism described here it can detect
when it is getting smeared time, and either ignore time from that when it is getting smeared time, and either ignore time from that
source, or "undo" the leap smear correction and use the corrected source, or "undo" the leap smear correction and use the corrected
time for that sample. time for that sample.
This proposal is not an attempt to justify servers offering leap 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 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 identify when a server is offering or client is receiving smeared
time, and provide the client a means to know the amount of smear time, and provide the client a means to know the amount of smear
skipping to change at page 10, line 31 skipping to change at page 10, line 17
This memo makes no requests of IANA. This memo makes no requests of IANA.
7. Security Considerations 7. 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 8. References
 End of changes. 22 change blocks. 
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