Internet Engineering Task Force (IETF) A. Shpiner

Internet Engineering Task Force (IETF) A. Shpiner
Request for Comments: 8039 Mellanox
Category: Experimental R. Tse
ISSN: 2070-1721 Microsemi
C. Schelp
Oracle
T. Mizrahi
Marvell
December 2016

Multipath Time Synchronization

Abstract

Clock synchronization protocols are very widely used in IP-based
networks. The Network Time Protocol (NTP) has been commonly deployed
for many years, and the last few years have seen an increasingly
rapid deployment of the Precision Time Protocol (PTP). As time-
sensitive applications evolve, clock accuracy requirements are
becoming increasingly stringent, requiring the time synchronization
protocols to provide high accuracy. This memo describes a multipath
approach to PTP and NTP over IP networks, allowing the protocols to
run concurrently over multiple communication paths between the master
and slave clocks, without modifying these protocols. The multipath
approach can significantly contribute to clock accuracy, security,
and fault tolerance. The multipath approach that is presented in
this document enables backward compatibility with nodes that do not
support the multipath functionality.

Status of This Memo

This document is not an Internet Standards Track specification; it is
published for examination, experimental implementation, and
evaluation.

This document defines an Experimental Protocol for the Internet
community. This document is a product of the Internet Engineering
Task Force (IETF). It represents the consensus of the IETF
community. It has received public review and has been approved for
publication by the Internet Engineering Steering Group (IESG). Not
all documents approved by the IESG are a candidate for any level of
Internet Standard; see Section 2 of RFC 7841.

Information about the current status of this document, any errata,
and how to provide feedback on it may be obtained at
http://www.rfc-editor.org/info/rfc8039.

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Copyright Notice

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described in the Simplified BSD License.

Table of Contents

1. Introduction ....................................................3
2. Conventions Used in This Document ...............................4
2.1. Abbreviations ..............................................4
2.2. Terminology ................................................4
3. Multiple Paths in IP Networks ...................................5
3.1. Load Balancing .............................................5
3.2. Using Multiple Paths Concurrently ..........................5
3.3. Two-Way Paths ..............................................5
4. Solution Overview ...............................................6
4.1. Path Configuration and Identification ......................6
4.2. Combining ..................................................6
5. Multipath Time Synchronization over IP Networks .................7
5.1. Overview ...................................................7
5.2. Single-Ended Multipath Synchronization .....................8
5.2.1. Single-Ended MPPTP Synchronization Message
Exchange ............................................8
5.2.2. Single-Ended MPNTP Synchronization Message
Exchange ............................................9
5.3. Dual-Ended Multipath Synchronization ......................10
5.3.1. Dual-Ended MPPTP Synchronization Message Exchange ..10
5.3.2. Dual-Ended MPNTP Synchronization Message Exchange ..11
5.4. Using Traceroute for Path Discovery .......................12
5.5. Using Unicast Discovery for MPPTP .........................13
6. Combining Algorithm ............................................13
7. Security Considerations ........................................14
8. Scope of the Experiment ........................................14
9. References .....................................................15
9.1. Normative References ......................................15
9.2. Informative References ....................................15
Acknowledgments ...................................................17
Authors' Addresses ................................................17

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1. Introduction

The two most common time synchronization protocols in IP networks are
(1) the Network Time Protocol [NTP] and (2) the Precision Time
Protocol (PTP) as defined in the IEEE 1588 standard [IEEE1588].

The accuracy of the time synchronization protocols directly depends
on the stability and the symmetry of propagation delays in both
directions between the master and slave clocks. Depending on the
nature of the underlying network, time synchronization protocol
packets can be subject to variable network latency or path asymmetry
(e.g., [ASYMMETRY] [ASYMMETRY2]). As time-sensitive applications
evolve, accuracy requirements are becoming increasingly stringent.
Using a single network path in a clock synchronization protocol
closely ties the slave clock accuracy to the behavior of the specific
path, which may suffer from temporal congestion, faults, or malicious
attacks. Relying on multiple clock servers, as in NTP, solves these
problems but requires active maintenance of multiple accurate sources
in the network, which is not always possible. The usage of
Transparent Clocks (TCs) in PTP solves the congestion problem by
eliminating the queuing time from the delay calculations but does not
address security or fault-tolerance aspects.

____
______/ \_____
___/ \____
____/ \
____ / path 1 / ___
/ \ / ________________________ \ / \
/Master\____\___/ \____\________/Slave\
\Clock / / \________ _______________/ \ \Clock/
\____/ \ / \__ /
\____ path 2 __/
\_______ ______/
\_________/

Figure 1: Multipath Connection

Since master and slave clocks are often connected through more than
one path in the network, as shown in Figure 1, [SLAVEDIV] suggested
that a time synchronization protocol can be run over multiple paths,
providing several advantages. First, it can significantly increase
the clock accuracy as shown in [SLAVEDIV]. Second, this approach
provides additional security, allowing the mitigation of
man-in-the-middle attacks against the time synchronization protocol
[DELAY-ATT]. Third, using multiple paths concurrently provides an
inherent failure protection mechanism.

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This document introduces Multipath PTP (MPPTP) and Multipath NTP
(MPNTP). The functionality of the multipath approach is defined at
the network layer and does not require any changes in PTP or NTP.

MPPTP and MPNTP are defined over IP networks. As IP networks
typically combine ECMP routing, this property is leveraged for the
multiple paths used in MPPTP and MPNTP. The key property of the
multipath approach is that clocks in the network can use more than
one IP address. Each {master IP, slave IP} address pair defines a
path. Depending on the network topology and configuration, the IP
combination pairs can form multiple diverse paths used by the
multipath synchronization protocols. It has been shown [MULTI] that
using multiple IP addresses over the wide Internet indeed allows two
endpoints to attain multiple diverse paths.

This document introduces two variants of the multipath approach:
(1) a variant that requires both master and slave nodes to support
the multipath functionality, referred to as the dual-ended variant,
and (2) a backward-compatible variant that allows a multipath clock
to connect to a conventional single-path clock, referred to as the
single-ended variant.

2. Conventions Used in This Document

2.1. Abbreviations

BMC Best Master Clock [IEEE1588]

ECMP Equal-Cost Multipath

LAN Local Area Network

MPNTP Multipath Network Time Protocol

MPPTP Multipath Precision Time Protocol

NTP Network Time Protocol [NTP]

PTP Precision Time Protocol [IEEE1588]

2.2. Terminology

In the NTP terminology, a time synchronization protocol is run
between a client and a server, while PTP uses the terms 'master' and
'slave'. Throughout this document, the sections that refer to both
PTP and NTP generically use the terms 'master' and 'slave'.

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3. Multiple Paths in IP Networks

3.1. Load Balancing

Traffic sent across IP networks is often load-balanced across
multiple paths. The load-balancing decisions are typically based on
packet header fields: source and destination addresses, Layer 4
ports, the Flow Label field in IPv6, etc.

Three common load-balancing criteria are per-destination, per-flow,
and per-packet. The per-destination load balancers take a
load-balancing decision based on the destination IP address.
Per-flow load balancers use various fields in the packet header,
e.g., IP addresses and Layer 4 ports, for the load-balancing
decision. Per-packet load balancers use flow-blind techniques such
as round-robin without basing the choice on the packet content.

3.2. Using Multiple Paths Concurrently

To utilize the diverse paths that traverse per-destination
load balancers or per-flow load balancers, the packet transmitter can
vary the IP addresses in the packet header. The analysis in [PARIS2]
shows that a significant majority of the flows on the Internet
traverse per-destination or per-flow load balancing. It presents
statistics that 72% of the flows traverse per-destination
load balancing and 39% of the flows traverse per-flow load balancing,
while only a negligible part of the flows traverse per-packet
load balancing. These statistics show that the vast majority of the
traffic on the Internet is load-balanced based on packet header
fields.

The approaches in this document are based on varying the source and
destination IP addresses in the packet header. Possible extensions
have been considered that also vary the UDP ports. However, some of
the existing implementations of PTP and NTP use fixed UDP port values
in both the source and destination UDP port fields and thus do not
allow this approach.

3.3. Two-Way Paths

A key property of IP networks is that packets forwarded from A to B
do not necessarily traverse the same path as packets from B to A.
Thus, we define a two-way path for a master-slave connection as a
pair of one-way paths: the first from master to slave and the second
from slave to master.

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If possible, a traffic engineering approach can be used to verify
that time synchronization traffic is always forwarded through
bidirectional two-way paths, i.e., that each two-way path uses the
same route in the forward and reverse directions, thus allowing
propagation time symmetry. However, in the general case, two-way
paths do not necessarily use the same path for the forward and
reverse directions.

4. Solution Overview

The multipath time synchronization protocols we present here are
comprised of two building blocks: one is the path configuration and
identification, and the other is the algorithm used by the slave to
combine the information received from the various paths.

4.1. Path Configuration and Identification

The master and slave clocks must be able to determine the path of
transmitted protocol packets and to identify the path of incoming
protocol packets. A path is determined by a {master IP, slave IP}
address pair. The synchronization protocol message exchange is run
independently through each path.

Each IP address pair defines a two-way path and thus allows the
clocks to bind a transmitted packet to a specific path or to identify
the path of an incoming packet.

If possible, the routing tables across the network should be
configured with multiple traffic-engineered paths between the pair of
clocks. By carefully configuring the routers in such networks, it is
possible to create diverse paths for each of the IP address pairs
between two clocks in the network. However, in public and provider
networks, the load-balancing behavior is hidden from the end users.
In this case, the actual number of paths may be less than the number
of IP address pairs, since some of the address pairs may share common
paths.

4.2. Combining

Various methods can be used for combining the time information
received from the different paths. The output of the combining
algorithm is the accurate time offset. Combining methods are further
discussed in Section 6.

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5. Multipath Time Synchronization over IP Networks

5.1. Overview

This section presents two variants of MPPTP and MPNTP: single-ended
multipath time synchronization and dual-ended multipath time
synchronization. In the first variant, the multipath approach is
only implemented by the slave, and the master is not aware of its
usage. In the second variant, all clocks use multiple paths.

The dual-ended variant provides higher path diversity by using
multiple IP addresses at both ends, the master and slave, while the
single-ended variant only uses multiple addresses at the slave.
Consequently, the single-ended approach can interoperate with
existing implementations that do not use multiple paths. The
dual-ended and single-ended approaches can coexist in the same
network; each slave selects the connection(s) it wants to make with
the available masters. A dual-ended slave could switch to
single-ended mode if it does not see any dual-ended masters
available. A single-ended slave could connect to a single IP address
of a dual-ended master.

Multipath time synchronization, in both variants, requires clocks to
use multiple IP addresses. Using multiple IP addresses introduces a
trade-off. A large number of IP addresses allows a large number of
diverse paths, providing the advantages of slave diversity discussed
in Section 1. On the other hand, a large number of IP addresses is
more costly, requires the network topology to be more redundant, and
exacts extra management overhead.

If possible, the set of IP addresses for each clock should be chosen
in a way that enables the establishment of paths that are the most
different. If the load-balancing rules in the network are known, it
is possible to choose the IP addresses in a way that enforces path
diversity. However, even if the load-balancing scheme is not known,
a careful choice of the IP addresses can increase the probability of
path diversity. For example, choosing multiple addresses with
different prefixes is likely to produce higher path diversity, as BGP
routers are more likely to route these different prefixes through
different routes.

The use of Network Address Translation (NAT) may significantly reduce
the effectiveness of multipath synchronization in some cases. For
example, if a master uses multiple IP addresses that are translated
to a single IP address, the path diversity can be dramatically
reduced compared to a network that does not use NAT. Thus, path

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discovery should be used to identify the possible paths between the
master and slave. Path discovery is further discussed in
Section 5.4.

The concept of using multiple IP addresses or multiple interfaces is
well established and is being used today by various applications and
protocols, e.g., [MPTCP]. Using multiple interfaces introduces some
challenges and issues, which were presented and discussed in [MIF].

The descriptions in this section refer to the end-to-end scheme of
PTP but are similarly applicable to the peer-to-peer scheme. MPNTP,
as described in this document, refers to the NTP client-server mode,
although the concepts described here can be extended to include the
symmetric variant as well.

Multipath synchronization by nature requires protocol messages to be
sent as unicast. Specifically in PTP, the following messages must be
sent as unicast in MPPTP: Sync, Delay_Req, Delay_Resp, PDelay_Req,
PDelay_Resp, Follow_Up, and PDelay_Resp_Follow_Up. Note that
[IEEE1588] allows these messages to be sent either as multicast or as
unicast.

5.2. Single-Ended Multipath Synchronization

In the single-ended approach, only the slave is aware of the fact
that multiple paths are used, while the master is agnostic to the
usage of multiple paths. This approach allows a hybrid network,
where some of the clocks are multipath clocks and others are
conventional one-path clocks. A single-ended multipath clock
presents itself to the network as N independent clocks, using N IP
addresses, as well as N clockIdentity [IEEE1588] values (in PTP).
Thus, the usage of multiple slave identities by a slave clock is
transparent from the master's point of view, such that it treats each
of the identities as a separate slave clock.

5.2.1. Single-Ended MPPTP Synchronization Message Exchange

The single-ended MPPTP message exchange procedure is as follows.

o Each single-ended MPPTP clock has a fixed set of N IP addresses
and N corresponding clockIdentities. Each clock arbitrarily
defines one of its IP addresses and clockIdentity values as the
clock primary identity.

o A single-ended MPPTP port sends Announce messages only from its
primary identity, according to the BMC algorithm.

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o The BMC algorithm at each clock determines the master, based on
the received Announce messages.

o A single-ended MPPTP port that is in the 'slave' state uses
unicast negotiation to request the master to transmit unicast
messages to each of the N slave clockIdentity values. The slave
port periodically sends N Signaling messages to the master, using
each of its N identities. The Signaling message includes the
REQUEST_UNICAST_TRANSMISSION TLV [IEEE1588].

o The master periodically sends unicast Sync messages from its
primary identity, identified by the sourcePortIdentity [IEEE1588]
and IP address, to each of the slave identities.

o The slave, upon receiving a Sync message, identifies its path
according to the destination IP address. The slave sends a
Delay_Req unicast message to the primary identity of the master.
The Delay_Req is sent using the slave identity corresponding to
the path through which the Sync was received. Note that the rate
of Delay_Req messages may be lower than the Sync message rate, and
thus a Sync message is not necessarily followed by a Delay_Req.

o The master, in response to a Delay_Req message from the slave,
responds with a Delay_Resp message using the IP address and
sourcePortIdentity from the Delay_Req message.

o Upon receiving the Delay_Resp message, the slave identifies the
path using the destination IP address and the
requestingPortIdentity [IEEE1588]. The slave can then compute the
corresponding path delay and the offset from the master.

o The slave combines the information from all negotiated paths.

5.2.2. Single-Ended MPNTP Synchronization Message Exchange

The single-ended MPNTP message exchange procedure is as follows.

o A single-ended MPNTP client has N separate identities, i.e., N IP
addresses. The assumption is that the server information,
including its IP address, is known to the NTP clients. This is a
fair assumption, as typically the address(es) of the NTP server(s)
is provided to the NTP client by configuration.

o A single-ended MPNTP client initiates NTP with an NTP server
N times, using each of its N identities.

o NTP is maintained between the server and each of the N client
identities.

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o The client sends NTP messages to the master using each of its
N identities.

o The server responds to the client's NTP messages using the IP
address from the received NTP packet.

o The client, upon receiving an NTP packet, uses the IP destination
address to identify the path through which it came, and it uses
the time information accordingly.

o The client combines the information from all paths.

5.3. Dual-Ended Multipath Synchronization

In dual-ended multipath synchronization, each clock has N IP
addresses. Time synchronization messages are exchanged between some
of the combinations of {master IP, slave IP} addresses, allowing
multiple paths between the master and slave. Note that the actual
number of paths between the master and slave may be less than the
number of chosen {master IP, slave IP} address pairs.

Once the multiple two-way connections are established, a separate
synchronization protocol exchange instance is run through each
of them.

5.3.1. Dual-Ended MPPTP Synchronization Message Exchange

The dual-ended MPPTP message exchange procedure is as follows.

o Every clock has N IP addresses but uses a single clockIdentity.

o The BMC algorithm at each clock determines the master. The master
is identified by its clockIdentity, allowing other clocks to know
the multiple IP addresses it uses.

o When a clock sends an Announce message, it sends it from each of
its IP addresses with its clockIdentity.

o A dual-ended MPPTP port that is in the 'slave' state uses unicast
negotiation to request the master to transmit unicast messages to
some or all of its N_s IP addresses. This negotiation is done
individually between a slave IP address and the corresponding
master IP address with which the slave desires a connection. The
slave port periodically sends Signaling messages to the master,
using some or all of its N_s IP addresses as the source, to the
corresponding master's N_m IP addresses. The Signaling message
includes the REQUEST_UNICAST_TRANSMISSION TLV [IEEE1588].

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('N_s' and 'N_m' indicate the number of IP addresses of the slave
and master, respectively.)

o The master periodically sends unicast Sync messages from each of
its IP addresses to the corresponding slave IP addresses for which
a unicast connection was negotiated.

o The slave, upon receiving a Sync message, identifies its path
according to the {source IP, destination IP} addresses. The slave
sends a Delay_Req unicast message, swapping the source and
destination IP addresses from the Sync message. Note that the
rate of Delay_Req messages may be lower than the Sync message
rate, and thus a Sync message is not necessarily followed by a
Delay_Req.

o The master, in response to a Delay_Req message from the slave,
responds with a Delay_Resp message using the sourcePortIdentity
from the Delay_Req message and swapping the IP addresses from the
Delay_Req.

o Upon receiving the Delay_Resp message, the slave identifies the
path using the {source IP, destination IP} address pair. The
slave can then compute the corresponding path delay and the offset
from the master.

o The slave combines the information from all negotiated paths.

5.3.2. Dual-Ended MPNTP Synchronization Message Exchange

The MPNTP message exchange procedure is as follows.

o Each NTP clock has a set of N IP addresses. The assumption is
that the server information, including its multiple IP addresses,
is known to the NTP clients.

o The MPNTP client chooses N_svr server IP addresses and N_c client
IP addresses and initiates the N_svr*N_c instances of the
protocol, one for each {server IP, client IP} address pair,
allowing the client to combine the information from the N_s*N_c
paths.

('N_svr' and 'N_c' indicate the number of IP addresses of the
server and client, respectively, with which a client chooses to
connect.)

o The client sends NTP messages to the master using each of the
source-destination address combinations.

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o The server responds to the client's NTP messages using the IP
address combination from the received NTP packet.

o Using the {source IP, destination IP} address pair in the received
packets, the client identifies the path and performs its
computations for each of the paths accordingly.

o The client combines the information from all paths.

5.4. Using Traceroute for Path Discovery

The approach described thus far uses multiple IP addresses in a
single clock to create multiple paths. However, although each
two-way path is defined by a different {master IP, slave IP} address
pair, some of the IP address pairs may traverse exactly the same
network path, making them redundant.

Traceroute-based path discovery can be used for filtering only the IP
addresses that obtain diverse paths. 'Paris traceroute' [PARIS] and
'TraceFlow' [TRACEFLOW] are examples of tools that discover the paths
between two points in the network. It should be noted that this
filtering approach is effective only if the Traceroute implementation
uses the same IP addresses and UDP ports as the synchronization
protocol packets. Since some Traceroute implementations vary the UDP
ports, they may not be effective in identifying and filtering
redundant paths in synchronization protocols.

Traceroute-based filtering can be implemented by both master and
slave nodes, or it can be restricted to run only on slave nodes to
reduce the overhead on the master. For networks that guarantee that
the path of the timing packets in the forward and reverse directions
are the same, path discovery should only be performed at the slave.

Since network routes change over time, path discovery and redundant
path filtering should be performed periodically. Two {master IP,
slave IP} address pairs that produce two diverse paths may be
rerouted to use the same paths. Thus, the set of addresses that are
used by each clock should be reassessed regularly.

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5.5. Using Unicast Discovery for MPPTP

As presented above, MPPTP uses Announce messages and the BMC
algorithm to discover the master. The unicast discovery option of
PTP can be used as an alternative.

When using unicast discovery, the MPPTP slave ports maintain a list
of the IP addresses of the master. The slave port uses unicast
negotiation to request unicast service from the master as follows:

o In single-ended MPPTP, the slave uses unicast negotiation from
each of its identities to the master's (only) identity.

o In dual-ended MPPTP, the slave uses unicast negotiation from its
IP addresses, each to a corresponding master IP address, to
request unicast synchronization messages.

Afterwards, the message exchange continues as described in
Sections 5.2.1 and 5.3.1.

The unicast discovery option can be used in networks that do not
support multicast or in networks in which the master clocks are known
in advance. In particular, unicast discovery avoids multicasting
Announce messages.

6. Combining Algorithm

Previous sections discussed the methods of creating the multiple
paths and obtaining the time information required by the slave
algorithm. Once the time information is received through each of the
paths, the slave should use a combining algorithm, which consolidates
the information from the different paths into a single clock.
Various methods have been suggested for combining information from
different paths or from different clocks, e.g., [NTP] [SLAVEDIV]
[HIGH-AVAI] [KALMAN]. The choice of the combining algorithm is local
to the slave and does not affect interoperability. Hence, this
document does not define a specific method to be used by the slave.
The combining algorithm should be chosen carefully based on the
system properties, as different combining algorithms provide
different advantages. For example, some combining algorithms (e.g.,
[NTP] [DELAY-ATT]) are intended to be robust in the face of security
attacks, while other combining algorithms (e.g., [KALMAN]) are more
resilient to random delay variation.

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7. Security Considerations

The security aspects of time synchronization protocols are discussed
in detail in [RFC7384]. The methods described in this document
propose to run a time synchronization protocol through redundant
paths and thus allow the detection and mitigation of
man-in-the-middle attacks, as described in [DELAY-ATT].
Specifically, multipath synchronization can mitigate the following
threats (as per [RFC7384]):

o Packet manipulation (Section 3.2.1 of [RFC7384]).

o Packet interception and removal (Section 3.2.5 of [RFC7384]).

o Packet delay manipulation (Section 3.2.6 of [RFC7384]).

It should be noted that when using multiple paths, these paths may
partially overlap, and thus an attack that takes place in a common
segment of these paths is not mitigated by the redundancy. Moreover,
an on-path attacker may in some cases have access to more than one
router or may be able to migrate from one router to another.
Therefore, when using multiple paths, it is important for the paths
to be as diverse and as independent as possible, making the
redundancy scheme more tolerant to on-path attacks.

It should be noted that the multipath approach requires the master
(or NTP server) to dedicate more resources to each slave (client)
than the conventional single-path approach. Hence, well-known
Distributed Denial-of-Service (DDoS) attacks may potentially be
amplified when the multipath approach is enabled.

8. Scope of the Experiment

This memo is published as an Experimental RFC. The purpose of the
experimental period is to allow the community to analyze and to
verify the methods defined in this document. An experimental
evaluation of some of these methods has been published in [MULTI].
It is expected that the experimental period will allow the methods to
be further investigated and verified by the community. The duration
of the experiment is expected to be no less than two years from the
publication of this document.

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9. References

9.1. Normative References

[IEEE1588] IEEE Instrumentation and Measurement Society, "IEEE
Standard for a Precision Clock Synchronization Protocol
for Networked Measurement and Control Systems", IEEE
Std 1588-2008, DOI 10.1109/IEEESTD.2008.4579760.

[NTP] 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,
<http://www.rfc-editor.org/info/rfc5905>.

9.2. Informative References

[ASYMMETRY]
He, Y., Faloutsos, M., Krishnamurthy, S., and B. Huffaker,
"On routing asymmetry in the Internet", IEEE GLOBECOM,
DOI 10.1109/GLOCOM.2005.1577769, December 2005.

[ASYMMETRY2]
Pathak, A., Pucha, H., Zhang, Y., Hu, C., and Z. Mao, "A
measurement study of internet delay asymmetry",
International Conference on Passive and Active Network
Measurement 2008, DOI 10.1007/978-3-540-79232-1_19,
April 2008.

[DELAY-ATT]
Mizrahi, T., "A Game Theoretic Analysis of Delay Attacks
against Time Synchronization Protocols", IEEE
International Symposium on Precision Clock Synchronization
for Measurement, Control and Communication (ISPCS),
DOI 10.1109/ISPCS.2012.6336612, September 2012.

[HIGH-AVAI]
Ferrari, P., Flammini, A., Rinaldi, S., and G. Prytz,
"High availability IEEE 1588 nodes over IEEE 802.1 aq
Shortest Path Bridging networks", IEEE International
Symposium on Precision Clock Synchronization for
Measurement, Control and Communication (ISPCS),
DOI 10.1109/ISPCS.2013.6644760, September 2013.

[KALMAN] Giorgi, G. and C. Narduzzi, "Kalman filtering for
multi-path network synchronization", IEEE International
Symposium on Precision Clock Synchronization for
Measurement, Control and Communication (ISPCS),
DOI 10.1109/ISPCS.2014.6948693, September 2014.

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RFC 8039 Multipath Time Synchronization December 2016

[MIF] Blanchet, M. and P. Seite, "Multiple Interfaces and
Provisioning Domains Problem Statement", RFC 6418,
DOI 10.17487/RFC6418, November 2011,
<http://www.rfc-editor.org/info/rfc6418>.

[MPTCP] Ford, A., Raiciu, C., Handley, M., and O. Bonaventure,
"TCP Extensions for Multipath Operation with Multiple
Addresses", RFC 6824, DOI 10.17487/RFC6824, January 2013,
<http://www.rfc-editor.org/info/rfc6824>.

[MULTI] Shpiner, A., Revah, Y., and T. Mizrahi, "Multi-path Time
Protocols", IEEE International Symposium on Precision
Clock Synchronization for Measurement, Control and
Communication (ISPCS), DOI 10.1109/ISPCS.2013.6644754,
September 2013.

[PARIS] Augustin, B., Friedman, T., and R. Teixeira, "Measuring
Load-balanced Paths in the Internet", 7th ACM SIGCOMM
conference on Internet measurement (IMC '07),
DOI 10.1145/1298306.1298329, October 2007.

[PARIS2] Augustin, B., Friedman, T., and R. Teixeira, "Measuring
Multipath Routing in the Internet", IEEE/ACM Transactions
on Networking, 19(3), pp. 830-840,
DOI 10.1109/TNET.2010.2096232, June 2011.

[RFC7384] Mizrahi, T., "Security Requirements of Time Protocols in
Packet Switched Networks", RFC 7384, DOI 10.17487/RFC7384,
October 2014, <http://www.rfc-editor.org/info/rfc7384>.

[SLAVEDIV] Mizrahi, T., "Slave Diversity: Using Multiple Paths to
Improve the Accuracy of Clock Synchronization Protocols",
IEEE International Symposium on Precision Clock
Synchronization for Measurement, Control and Communication
(ISPCS), DOI 10.1109/ISPCS.2012.6336621, September 2012.

[TRACEFLOW]
Narasimhan, J., Venkataswami, B., Groves, R., and P.
Hoose, "Traceflow", Work in Progress,
draft-janapath-intarea-traceflow-00, January 2012.

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RFC 8039 Multipath Time Synchronization December 2016

Acknowledgments

The authors would like to thank Yoram Revah for his contribution to
this work. The authors also gratefully acknowledge the useful
comments provided by Peter Meyer, Doug Arnold, Joe Abley, Zhen Cao,
Watson Ladd, and Mirja Kuehlewind, as well as other comments received
from the TICTOC working group participants.

Authors' Addresses

Alex Shpiner
Mellanox Technologies, Ltd.
Hakidma 26
Ofer Industrial Park
Yokneam, 2069200
Israel

Email: [email protected]

Richard Tse
Microsemi
8555 Baxter Place
Burnaby, BC V5A 4V7
Canada

Email: [email protected]

Craig Schelp
Oracle

Email: [email protected]

Tal Mizrahi
Marvell
6 Hamada St.
Yokneam, 2066721
Israel

Email: [email protected]

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