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Versions: (draft-shpiner-multi-path-synchronization) 00 01 02 03 04 05 06 07 RFC 8039

Network Working Group                                         A. Shpiner
Internet Draft                                                  Mellanox
Intended status: Experimental                                     R. Tse
Expires: April 2017                                           PMC-Sierra
                                                               C. Schelp
                                                                  Oracle
                                                              T. Mizrahi
                                                                 Marvell
                                                        October 27, 2016

                      Multi-Path Time Synchronization
            draft-ietf-tictoc-multi-path-synchronization-07.txt


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 multi-path
   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 multi-path
   approach can significantly contribute to clock accuracy, security and
   fault tolerance. The multi-path approach that is presented in this
   document enables backward compatibility with nodes that do not
   support the multi-path functionality.

Status of this Memo

   This Internet-Draft is submitted to IETF in full conformance with the
   provisions of BCP 78 and BCP 79.

   Internet-Drafts are working documents of the Internet Engineering
   Task Force (IETF), its areas, and its working groups.  Note that
   other groups may also distribute working documents as Internet-
   Drafts.

   Internet-Drafts are draft documents valid for a maximum of six months
   and may be updated, replaced, or obsoleted by other documents at any
   time.  It is inappropriate to use Internet-Drafts as reference
   material or to cite them other than as "work in progress."

   The list of current Internet-Drafts can be accessed at
   http://www.ietf.org/ietf/1id-abstracts.txt.





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   The list of Internet-Draft Shadow Directories can be accessed at
   http://www.ietf.org/shadow.html.

   This Internet-Draft will expire on April 27, 2017.

Copyright Notice

   Copyright (c) 2016 IETF Trust and the persons identified as the
   document authors. All rights reserved.

   This document is subject to BCP 78 and the IETF Trust's Legal
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   include Simplified BSD License text as described in Section 4.e of
   the Trust Legal Provisions and are provided without warranty as
   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...............................................5
   3. Multiple Paths in IP Networks..................................5
      3.1. Load Balancing............................................5
      3.2. Using Multiple Paths Concurrently.........................5
      3.3. Two-Way Paths.............................................6
   4. Solution Overview..............................................6
      4.1. Path Configuration and Identification.....................6
      4.2. Combining.................................................7
   5. Multi-Path Time Synchronization over IP Networks...............7
      5.1. Overview..................................................7
      5.2. Single-Ended Multi-Path 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 Multi-Path 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. IANA Considerations...........................................14


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   10. Acknowledgments..............................................15
   11. References...................................................15
      11.1. Normative References....................................15
      11.2. Informative References..................................15


1. Introduction

   The two most common time synchronization protocols in IP networks are
   the Network Time Protocol [NTP], and the Precision Time Protocol
   (PTP), 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 on 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. [ASSYMETRY], [ASSYMETRY2]). 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 (TC) in PTP solves the congestion problem by
   eliminating the queueing time from the delay calculations, but does
   not address security or fault-tolerance aspects.
                                  ____
                           ______/    \_____
                       ___/                 \____
                  ____/                          \
      ____       /           path 1              /           ___
     /    \     /    ________________________    \          /   \
    /Master\____\___/                        \____\________/Slave\
    \Clock /    /   \________ _______________/     \       \Clock/
     \____/     \                                  /        \__ /
                 \____       path 2             __/
                      \_______           ______/
                              \_________/


                      Figure 1 Multi-Path Connection



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   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 to mitigate man-in-the-middle
   attacks against the time synchronization protocol [DELAY-ATT]. Third,
   using multiple paths concurrently provides an inherent failure
   protection mechanism.

   This document introduces Multi-Path PTP (MPPTP) and Multi-Path NTP
   (MPNTP). The functionality of the multi-path approach is defined at
   the network layer and does not require any changes in the PTP or in
   the NTP protocols.

   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
   multi-path 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 multi-
   path synchronization protocols. It has been shown [MULTI] that using
   multiple IP addresses over the wide Internet indeed allows two end-
   points to attain multiple diverse paths.

   This document introduces two variants of the multi-path approach; a
   variant that requires both master and slave nodes to support the
   multi-path functionality, referred to as the dual-ended variant, and
   a backward compatible variant that allows a multi-path 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 Multiple Path

   LAN      Local Area Network

   MPNTP    Multi-Path Network Time Protocol

   MPPTP    Multi-Path Precision Time Protocol



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   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.

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 draft 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.


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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.

   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 on 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 multi-path time synchronization protocols we present 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.





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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.

5. Multi-Path Time Synchronization over IP Networks

5.1. Overview

   This section presents two variants of MPPTP and MPNTP; single-ended
   multi-path time synchronization and dual-ended multi-path time
   synchronization. In the first variant, the multi-path 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 is can interoperate with
   existing implementations, which do not use multiple paths.  The dual-
   ended and single-ended approaches can co-exist 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.

   Multi-path time synchronization, in both variants, requires clocks to
   use multiple IP addresses. Using multiple IP addresses introduces a
   tradeoff. 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.


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   The use of Network Address Translation (NAT) may significantly reduce
   the effectiveness of multi-path 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
   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
   a well-established concept that 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.

   Multi-path 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 Multi-Path 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 multi-path clocks, and others are conventional
   one-path clocks. A single-ended multi-path clock presents itself to
   the network as N independent clocks, using N IP addresses, as well as
   N clock identity 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.






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   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.

   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 clock identities. 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.

   o  The master periodically sends unicast Sync messages from its
      primary identity, identified by the sourcePortIdentity 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 the Sync was received through. 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. 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.





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   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)
      are provided to the NTP client by configuration.

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

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

   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 it came through, and uses the time
      information accordingly.

   o  The client combines the information from all paths.

5.3. Dual-Ended Multi-Path Synchronization

   In dual-ended multi-path 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, 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.



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   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 that the slave desires a connection with.  The
      slave port periodically sends Signaling messages to the master,
      using some or all of its N_s IP addresses as source, to the
      corresponding master's N_m IP addresses. The Signaling message
      includes the REQUEST_UNICAST_TRANSMISSION_TLV.

   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, 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, 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.






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   o  The MPNTP client chooses N_svr of the N server IP addresses and
      N_c of the N client IP addresses and initiates the N_svr*N_c
      instances of the protocol, one for each {server IP, client IP}
      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, which a client chooses to connect with)

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

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

   o  Using the {source, 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, slave} 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.

   The 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 direction
   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,slave}
   pairs that produce two diverse paths may be rerouted to use the same


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   paths. Thus, the set of addresses that are used by each clock should
   be reassessed regularly.

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 to detect and mitigate man-in-the-middle
   attacks, as described in [DELAY-ATT]. Specifically, multi-path
   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 multi-path 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 porentially be
   amplified when the multi-path 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.

9. IANA Considerations

   There are no IANA actions required by this document.

   RFC Editor: please delete this section before publication.


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10. Acknowledgments

   The authors 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.

   This document was prepared using 2-Word-v2.0.template.dot.

11. References

11.1. 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.

   [NTP]         D. Mills, J. Martin, J. Burbank, W. Kasch, "Network
                 Time Protocol Version 4: Protocol and Algorithms
                 Specification", IETF, RFC 5905, 2010.

11.2. Informative References

   [ASSYMETRY]   Yihua He and Michalis Faloutsos and Srikanth
                 Krishnamurthy and Bradley Huffaker, "On routing
                 asymmetry in the internet", IEEE Globecom, 2005.

   [ASSYMETRY2]  Abhinav Pathak, Himabindu Pucha, Ying Zhang, Y.
                 Charlie Hu, and Z. Morley Mao, "A measurement study of
                 internet delay asymmetry", PAM'08, 2008.

   [DELAY-ATT]   T. Mizrahi, "A Game Theoretic Analysis of Delay
                 Attacks against Time Synchronization Protocols",
                 ISPCS, 2012.

   [HIGH-AVAI]   P. Ferrari, A. Flammini, S. Rinaldi, G. Prytz, "High
                 availability IEEE 1588 nodes over IEEE 802.1 aq
                 Shortest Path Bridging networks" ISPCS, 2013.

   [KALMAN]      G. Giorgi, C. Narduzzi, "Kalman filtering for multi-
                 path network synchronization" ISPCS, 2014.

   [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>.


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   [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]       A. Shpiner, Y. Revah, T. Mizrahi, "Multi-path Time
                 Protocols" ISPCS, 2013.

   [PARIS]       Brice Augustin, Timur Friedman and Renata Teixeira,
                 "Measuring Load-balanced Paths in the Internet", IMC,
                 2007.

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

   [RFC7384]     T. Mizrahi, "Security Requirements of Time Protocols
                 in Packet Switched Networks", IETF, RFC 7384, 2014.

   [SLAVEDIV]    T. Mizrahi, "Slave Diversity: Using Multiple Paths to
                 Improve the Accuracy of Clock Synchronization
                 Protocols", ISPCS, 2012.

   [TRACEFLOW]   J. Narasimhan, B. V. Venkataswami, R. Groves and P.
                 Hoose, "Traceflow", IETF, draft-janapath-intarea-
                 traceflow, work in progress, 2012.



Authors' Addresses

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

   Email: alexshp@mellanox.com










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   Richard Tse
   PMC-Sierra
   8555 Baxter Place
   Burnaby, BC
   Canada
   V5A 4V7

   Email: Richard.Tse@pmcs.com



   Craig Schelp
   Oracle

   Email: craig.schelp@gmail.com



   Tal Mizrahi
   Marvell
   6 Hamada St.
   Yokneam, 20692, Israel

   Email: talmi@marvell.com

























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