NTP Interleaved Modes
draft-ietf-ntp-interleaved-modes-05
Internet Engineering Task Force M. Lichvar
Internet-Draft Red Hat
Updates: 5905 (if approved) A. Malhotra
Intended status: Standards Track Boston University
Expires: October 10, 2021 Apr 8, 2021
NTP Interleaved Modes
draft-ietf-ntp-interleaved-modes-05
Abstract
This document extends the specification of Network Time Protocol
(NTP) version 4 in RFC 5905 with special modes called the NTP
interleaved modes, that enable NTP servers to provide their clients
and peers with more accurate transmit timestamps that are available
only after transmitting NTP packets. More specifically, this
document describes three modes: interleaved client/server,
interleaved symmetric, and interleaved broadcast.
Status of This Memo
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provisions of BCP 78 and BCP 79.
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This Internet-Draft will expire on October 10, 2021.
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2
1.1. Requirements Language . . . . . . . . . . . . . . . . . . 4
2. Interleaved Client/server mode . . . . . . . . . . . . . . . 4
3. Interleaved Symmetric mode . . . . . . . . . . . . . . . . . 8
4. Interleaved Broadcast mode . . . . . . . . . . . . . . . . . 10
5. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 10
6. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 11
7. Security Considerations . . . . . . . . . . . . . . . . . . . 11
8. References . . . . . . . . . . . . . . . . . . . . . . . . . 12
8.1. Normative References . . . . . . . . . . . . . . . . . . 12
8.2. Informative References . . . . . . . . . . . . . . . . . 12
8.3. URIs . . . . . . . . . . . . . . . . . . . . . . . . . . 12
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 12
1. Introduction
RFC 5905 [RFC5905] describes the operations of NTPv4 in a client/
server, symmetric, and broadcast mode. The transmit and receive
timestamps are two of the four timestamps included in every NTPv4
packet used for time synchronization.
For a highly accurate and stable synchronization, the transmit and
receive timestamp should be captured close to the beginning of the
actual transmission and the end of the reception respectively. An
asymmetry in the timestamping causes the offset measured by NTP to
have an error.
There are at least four options where a timestamp of an NTP packet
may be captured with a software NTP implementation running on an
operating system:
1. User space (software)
2. Network device driver or kernel (software)
3. Data link layer (hardware - MAC chip)
4. Physical layer (hardware - PHY chip)
Software timestamps captured in user space in the NTP implementation
itself are least accurate. They do not include system calls used for
sending and receiving packets, processing and queuing delays in the
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system, network device drivers, and hardware. Hardware timestamps
captured at the physical layer are most accurate.
A transmit timestamp captured in the driver or hardware is more
accurate than the user-space timestamp, but it is available to the
NTP implementation only after it sent the packet using a system call.
The timestamp cannot be included in the packet itself unless the
driver or hardware supports NTP and can modify the packet before or
during the actual transmission.
The protocol described in RFC 5905 does not specify any mechanism for
a server to provide its clients and peers with a more accurate
transmit timestamp that is known only after the transmission. A
packet that strictly follows RFC 5905, i.e. it contains a transmit
timestamp corresponding to the packet itself, is said to be in basic
mode.
Different mechanisms could be used to exchange timestamps known after
the transmission. The server could respond to each request with two
packets. The second packet would contain the transmit timestamp
corresponding to the first packet. However, such a protocol would
enable a traffic amplification attack, or it would use packets with
an asymmetric length, which would cause an asymmetry in the network
delay and an error in the measured offset.
This document describes an interleaved client/server, interleaved
symmetric, and interleaved broadcast mode. In these modes, the
server sends a single packet, which contains a transmit timestamp
corresponding to the previous packet that was sent to the client or
peer. This transmit timestamp can be captured at any of the four
places mentioned above. Both servers and clients/peers are required
to keep some state specific to the interleaved mode.
The protocol does not change the NTP packet header format, only the
semantics of some timestamp fields. An NTPv4 implementation that
supports the client/server and broadcast interleaved modes
interoperates with NTPv4 implementations without this capability. A
peer using the symmetric interleaved mode does not fully interoperate
with a peer which does not support it. The mode needs to be
configured specifically for each symmetric association.
The negotiation in the protocol is implicit. The origin timestamp
enables servers and peers to detect requests conforming to the
interleaved mode. A response can be valid only in one mode. If a
client or peer that does not support interleaved mode received a
response conforming to the interleaved mode, it would be rejected as
bogus. An explicit negotiation would require a new extension field,
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which would not work well with implementations that do not respond to
requests with unknown extension fields.
Requests and responses cannot always be formed in interleaved mode.
Servers, clients, and peers are required to support both interleaved
and basic modes.
This document assumes familiarity with RFC 5905.
1.1. Requirements Language
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and
"OPTIONAL" in this document are to be interpreted as described in BCP
14 [RFC2119] [RFC8174] when, and only when, they appear in all
capitals, as shown here.
2. Interleaved Client/server mode
The interleaved client/server mode is similar to the basic client/
server mode. The only difference between the two modes is in the
meaning of the transmit and origin timestamp fields.
The origin timestamp is a cookie, which is used to detect a packet
which is not a response to the last packet sent in the other
direction. Such packets are called bogus packets in RFC 5905.
A client request in the basic mode has an origin timestamp equal to
the transmit timestamp from the previous server response, or is zero.
A server response in the basic mode has an origin timestamp equal to
the transmit timestamp from the client's request. The transmit
timestamps correspond to the packets in which they are included.
A client request in the interleaved mode has an origin timestamp
equal to the receive timestamp from the previous server response. A
server response in the interleaved mode has an origin timestamp equal
to the receive timestamp from the client's request. The transmit
timestamps correspond to the previous packets that were sent to the
server or client.
A server which supports the interleaved mode needs to save pairs of
local receive and transmit timestamps. The server SHOULD discard old
timestamps to limit the amount of memory needed to support clients
using the interleaved mode. The server MAY separate the timestamps
by IP addresses, but it SHOULD NOT separate them by port numbers,
i.e. clients are allowed to change their source port between
requests.
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The server MAY restrict the interleaved mode to specific IP addresses
and/or authenticated clients.
Both servers and clients that support the interleaved mode MUST NOT
send a packet that has a transmit timestamp equal to the receive
timestamp in order to reliably detect whether received packets
conform to the interleaved mode.
The transmit and receive timestamps in server responses need to be
unique to prevent two different clients from sending requests with
the same origin timestamp and the server responding in the
interleaved mode with an incorrect transmit timestamp. If the
timestamps are not guaranteed to be monotonically increasing, the
server SHOULD check that the transmit and receive timestamp is not
already saved as a receive timestamp of a previous request (from the
same IP address if the server separates timestamps by addresses), and
generate a new timestamp if necessary.
When the server receives a request from a client, it SHOULD respond
in the interleaved mode if the following conditions are met:
1. The request does not have a receive timestamp equal to the
transmit timestamp.
2. The origin timestamp from the request matches the local receive
timestamp of a previous request that the server has saved (for
the IP address if it separates timestamps by addresses).
A response in the interleaved mode MUST contain the transmit
timestamp of the response which contained the receive timestamp
matching the origin timestamp from the request. The server SHOULD
drop the timestamps after sending the response. The receive
timestamp MUST NOT be used again to detect a request conforming to
the interleaved mode.
If the conditions are not met (i.e. the request is not detected to
conform to the interleaved mode), the server MUST NOT respond in the
interleaved mode. The server MAY always respond in the basic mode.
In any case, the server SHOULD save the new receive and transmit
timestamps.
The first request from a client is always in the basic mode and so is
the server response. It has a zero origin timestamp and zero receive
timestamp. Only when the client receives a valid response from the
server, it will be able to send a request in the interleaved mode.
The protocol recovers from packet loss. When a client request or
server response is lost, the client will use the same origin
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timestamp in the next request. The server can respond in the
interleaved mode if it still has the timestamps corresponding to the
origin timestamp. If the server already responded to the timestamp
in the interleaved mode, or it had to drop the timestamps for other
reasons, it will respond in the basic mode and save new timestamps,
which will enable an interleaved response to the subsequent request.
The client SHOULD limit the number of requests in the interleaved
mode between server responses to prevent processing of very old
timestamps in case a large number of consecutive requests is lost.
An example of packets in a client/server exchange using the
interleaved mode is shown in Figure 1. The packets in the basic and
interleaved mode are indicated with B and I respectively. The
timestamps t1~, t3~ and t11~ point to the same transmissions as t1,
t3 and t11, but they may be less accurate. The first exchange is in
the basic mode followed by a second exchange in the interleaved mode.
For the third exchange, the client request is in the interleaved
mode, but the server response is in the basic mode, because the
server did not have the pair of timestamps t6 and t7 (e.g. they were
dropped to save timestamps for other clients using the interleaved
mode).
Server t2 t3 t6 t7 t10 t11
-----+----+----------------+----+----------------+----+-----
/ \ / \ / \
Client / \ / \ / \
--+----------+----------+----------+----------+----------+--
t1 t4 t5 t8 t9 t12
Mode: B B I I I B
+----+ +----+ +----+ +----+ +----+ +----+
Org | 0 | | t1~| | t2 | | t4 | | t6 | | t5 |
Rx | 0 | | t2 | | t4 | | t6 | | t8 | |t10 |
Tx | t1~| | t3~| | t1 | | t3 | | t5 | |t11~|
+----+ +----+ +----+ +----+ +----+ +----+
Figure 1: Packet timestamps in interleaved client/server mode
When the client receives a response from the server, it performs the
tests described in RFC 5905. Two of the tests are modified for the
interleaved mode:
1. The check for duplicate packets SHOULD compare both receive and
transmit timestamps in order to not drop a valid response in the
interleaved mode if it follows a response in the basic mode and
they contain the same transmit timestamp.
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2. The check for bogus packets SHOULD compare the origin timestamp
with both transmit and receive timestamps from the request. If
the origin timestamp is equal to the transmit timestamp, the
response is in the basic mode. If the origin timestamp is equal
to the receive timestamp, the response is in the interleaved
mode.
The client SHOULD NOT update its NTP state when an invalid response
is received to not lose the timestamps which will be needed to
complete a measurement when the subsequent response in the
interleaved mode is received.
If the packet passed the tests and conforms to the interleaved mode,
the client can compute the offset and delay using the formulas from
RFC 5905 and one of two different sets of timestamps. The first set
is RECOMMENDED for clients that filter measurements based on the
delay. The corresponding timestamps from Figure 1 are written in
parentheses.
T1 - local transmit timestamp of the previous request (t1)
T2 - remote receive timestamp from the previous response (t2)
T3 - remote transmit timestamp from the latest response (t3)
T4 - local receive timestamp of the previous response (t4)
The second set gives a more accurate measurement of the current
offset, but the delay is much more sensitive to a frequency error
between the server and client due to a much longer interval between
T1 and T4.
T1 - local transmit timestamp of the latest request (t5)
T2 - remote receive timestamp from the latest response (t6)
T3 - remote transmit timestamp from the latest response (t3)
T4 - local receive timestamp of the previous response (t4)
Clients MAY filter measurements based on the mode. The maximum
number of dropped measurements in the basic mode SHOULD be limited in
case the server does not support or is not able to respond in the
interleaved mode. Clients that filter measurements based on the
delay will implicitly prefer measurements in the interleaved mode
over the basic mode, because they have a shorter delay due to a more
accurate transmit timestamp (T3).
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The server MAY limit saving of the receive and transmit timestamps to
requests which have an origin timestamp specific to the interleaved
mode in order to not waste resources on clients using the basic mode.
Such an optimization will delay the first interleaved response of the
server to a client by one exchange.
A check for a non-zero origin timestamp works with clients that
implement NTP data minimization [I-D.ietf-ntp-data-minimization]. To
detect requests in the basic mode from clients that do not implement
the data minimization, the server can encode in low-order bits of the
receive and transmit timestamps below precision of the clock a bit
indicating whether the timestamp is a receive timestamp. If the
server receives a request with a non-zero origin timestamp which does
not indicate it is a receive timestamp of the server, the request is
in the basic mode and it is not necessary to save the new receive and
transmit timestamp.
3. Interleaved Symmetric mode
The interleaved symmetric mode uses the same principles as the
interleaved client/server mode. A packet in the interleaved
symmetric mode has a transmit timestamp which corresponds to the
previous packet sent to the peer and an origin timestamp equal to the
receive timestamp from the last packet received from the peer.
To enable synchronization in both directions of a symmetric
association, both peers need to support the interleaved mode. For
this reason, it SHOULD be disabled by default and enabled with an
option in the configuration of the active side of the association.
In order to prevent the peer from matching the transmit timestamp
with an incorrect packet when the peers' transmissions do not
alternate (e.g. they use different polling intervals) and a previous
packet was lost, the use of the interleaved mode in symmetric
associations requires additional restrictions.
Peers which have an association need to count valid packets received
between their transmissions to determine in which mode a packet
should be formed. A valid packet in this context is a packet which
passed all NTP tests for duplicate, replayed, bogus, and
unauthenticated packets. Other received packets may update the NTP
state to allow the (re)initialization of the association, but they do
not change the selection of the mode.
A peer A SHOULD send a peer B a packet in the interleaved mode only
when the following conditions are met:
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1. The peer A has an active association with the peer B which was
specified with the option enabling the interleaved mode, OR the
peer A received at least one valid packet in the interleaved mode
from the peer B.
2. The peer A did not send a packet to the peer B since it received
the last valid packet from the peer B.
3. The previous packet that the peer A sent to the peer B was the
only response to a packet received from the peer B.
An example of packets exchanged in a symmetric association is shown
in Figure 2. The minimum polling interval of the peer A is twice as
long as the maximum polling interval of the peer B. The first
packets sent by the peers are in the basic mode. The second and
third packet sent by the peer A is in the interleaved mode. The
second packet sent by the peer B is in the interleaved mode, but the
following packets sent by the peer are in the basic mode, because
multiple responses are sent per request.
Peer A t2 t3 t6 t8 t9 t12 t14 t15
-----+--+--------+-----------+--+--------+-----------+--+-----
/ \ / / \ / / \
Peer B / \ / / \ / / \
--+--------+--+-----------+--------+--+-----------+--------+--
t1 t4 t5 t7 t10 t11 t13 t16
Mode: B B I B I B B I
+----+ +----+ +----+ +----+ +----+ +----+ +----+ +----+
Org | 0 | | t1~| | t2 | | t3~| | t4 | | t3 | | t3 | |t10 |
Rx | 0 | | t2 | | t4 | | t4 | | t8 | |t10 | |t10 | |t14 |
Tx | t1~| | t3~| | t1 | | t7~| | t3 | |t11~| |t13~| | t9 |
+----+ +----+ +----+ +----+ +----+ +----+ +----+ +----+
Figure 2: Packet timestamps in interleaved symmetric mode
If the peer A has no association with the peer B and it responds with
symmetric passive packets, it does not need to count the packets in
order to meet the restrictions, because each request has at most one
response. The peer SHOULD process the requests in the same way as a
server which supports the interleaved client/server mode. It MUST
NOT respond in the interleaved mode if the request was not in the
interleaved mode.
The peers SHOULD compute the offset and delay using one of the two
sets of timestamps specified in the client/server section. They MAY
switch between them to minimize the interval between T1 and T4 in
order to reduce the error in the measured delay.
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4. Interleaved Broadcast mode
A packet in the interleaved broadcast mode contains two transmit
timestamps. One corresponds to the packet itself and is saved in the
transmit timestamp field. The other corresponds to the previous
packet and is saved in the origin timestamp field. The packet is
compatible with the basic mode, which uses a zero origin timestamp.
An example of packets sent in the broadcast mode is shown in
Figure 3.
Server t1 t3 t5 t7
------+------------+------------+------------+---------
\ \ \ \
Client \ \ \ \
---------+------------+------------+------------+------
t2 t4 t6 t8
Mode: B I I I
+----+ +----+ +----+ +----+
Org | 0 | | t1 | | t3 | | t5 |
Rx | 0 | | 0 | | 0 | | 0 |
Tx | t1~| | t3~| | t5~| | t7~|
+----+ +----+ +----+ +----+
Figure 3: Packet timestamps in interleaved broadcast mode
A client which does not support the interleaved mode ignores the
origin timestamp and processes all packets as if they were in the
basic mode.
A client which supports the interleaved mode SHOULD check if the
origin timestamp is not zero to detect packets in the interleaved
mode. The client SHOULD also compare the origin timestamp with the
transmit timestamp from the previous packet to detect lost packets.
If the difference is larger than a specified maximum (e.g. 1 second),
the packet SHOULD NOT be used for synchronization.
The client SHOULD compute the offset using the origin timestamp from
the received packet and the local receive timestamp of the previous
packet. If the client needs to measure the network delay, it SHOULD
use the interleaved client/server mode.
5. Acknowledgements
The interleaved modes described in this document are based on the
implementation written by David Mills in the NTP project [1]. The
specification of the broadcast mode is based purely on this
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implementation. The specification of the symmetric mode has some
modifications. The client/server mode is specified as a new mode
compatible with the symmetric mode, similarly to the basic symmetric
and client/server modes.
The authors would like to thank Theresa Enghardt, Daniel Franke, Erik
Kline, Tal Mizrahi, Steven Sommars, Harlan Stenn, and Kristof Teichel
for their useful comments.
6. IANA Considerations
This memo includes no request to IANA.
7. Security Considerations
The security considerations of time protocols in general are
discussed in RFC 7384 [RFC7384], and specifically the security
considerations of NTP are discussed in RFC 5905.
Security issues that apply to the basic modes apply also to the
interleaved modes. They are described in The Security of NTP's
Datagram Protocol [SECNTP].
Clients and peers SHOULD NOT leak the receive timestamp in packets
sent to other peers or clients (e.g. as a reference timestamp) to
prevent off-path attackers from easily getting the origin timestamp
needed to make a valid response in the interleaved mode.
Clients using the interleaved mode SHOULD randomize all bits of both
receive and transmit timestamps, as recommended for the transmit
timestamp in the NTP client data minimization
[I-D.ietf-ntp-data-minimization], to make it more difficult for off-
path attackers to guess the origin timestamp. It is not possible to
zero the origin timestamp to prevent passive observers from easily
tracking clients moving between different networks.
Attackers can force the server to drop its timestamps in order to
prevent clients from getting an interleaved response. They can send
a large number of requests, send requests with a spoofed source
address, or replay an authenticated request if the interleaved mode
is enabled only for authenticated clients. Clients SHOULD NOT rely
on servers to be able to respond in the interleaved mode.
Protecting symmetric associations in the interleaved mode against
replay attacks is even more difficult than in the basic mode. The
NTP state needs to be protected not only between the reception and
transmission in order to send the peer a packet with a valid origin
timestamp, but all the time to not lose the timestamps which will be
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needed to complete a measurement when the following packet in the
interleaved mode is received.
8. References
8.1. Normative References
[I-D.ietf-ntp-data-minimization]
Franke, D. and A. Malhotra, "NTP Client Data
Minimization", draft-ietf-ntp-data-minimization-04 (work
in progress), March 2019.
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119,
DOI 10.17487/RFC2119, March 1997,
<https://www.rfc-editor.org/info/rfc2119>.
[RFC5905] Mills, D., Martin, J., Ed., Burbank, J., and W. Kasch,
"Network Time Protocol Version 4: Protocol and Algorithms
Specification", RFC 5905, DOI 10.17487/RFC5905, June 2010,
<https://www.rfc-editor.org/info/rfc5905>.
[RFC8174] Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174,
May 2017, <https://www.rfc-editor.org/info/rfc8174>.
8.2. Informative References
[RFC7384] Mizrahi, T., "Security Requirements of Time Protocols in
Packet Switched Networks", RFC 7384, DOI 10.17487/RFC7384,
October 2014, <https://www.rfc-editor.org/info/rfc7384>.
[SECNTP] Malhotra, A., Gundy, M., Varia, M., Kennedy, H., Gardner,
J., and S. Goldberg, "The Security of NTP's Datagram
Protocol", 2016, <http://eprint.iacr.org/2016/1006>.
8.3. URIs
[1] http://www.ntp.org
Authors' Addresses
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Miroslav Lichvar
Red Hat
Purkynova 115
Brno 612 00
Czech Republic
Email: mlichvar@redhat.com
Aanchal Malhotra
Boston University
111 Cummington St
Boston 02215
USA
Email: aanchal4@bu.edu
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