Internet DRAFT - draft-jiang-scsn-gap-analysis
draft-jiang-scsn-gap-analysis
Network Working Group Y. Jiang
X. Liu
Internet-Draft Huawei
L. Geng
Intended status: Informational China Mobile
D. P. Venmani
Orange Labs
Expires: September 2016 March 21, 2016
Gap Analysis of Scalable Synchronization Networks
draft-jiang-scsn-gap-analysis-00.txt
Abstract
This draft provides a gap analysis for the Scalable Synchronization
Networks (SCSN). The document provides an overview of the existing
standardization work on synchronization solutions, and outlines some
of the important features with regard to scalability that are still
missing in current synchronization networks.
Status of this Memo
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Table of Contents
1. Introduction .............................................. 2
1.1. Conventions used in this document ...................... 3
1.2. Terminology ............................................ 3
2. Related Standardization Work on Synchronization Network ... 4
2.1. A Survey of ITU-T work ................................. 4
2.2. A Survey of IEEE work .................................. 5
2.3. A Survey of IETF work .................................. 6
3. Discussions ............................................... 6
4. Security Considerations ................................... 7
5. IANA Considerations ....................................... 7
6. References ................................................ 7
6.1. Informative References ................................. 7
7. Acknowledgments ........................................... 9
1. Introduction
Traditionally, telecommunication systems rely heavily on accurate
frequency and/or time synchronization for their proper working. This
is especially true for the case of cellular networks (3G, 4G/LTE,
etc.), where base stations need accurate and stable frequency clocks
in order to obtain their carrier radio frequencies, arbitrate the
frequency-shared and time-shared access of terminals, coordinate the
handover of terminals between adjacent cells, and etc.
Over the years, time-division multiplexing (TDM) transmission
technologies such as SDH are used to provide frequency distribution,
typically by provisioning a tree-based hierarchy of clocks over the
transport network beforehand. Due to the advantages of higher
flexibility, lower operation costs, economies of scale and better
integration with higher layer IP-based services, telecommunication
operators are migrating their networks from TDM technology to packet-
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switching technology, and evolving to "all-IP" architecture. As a
result, Synchronous Ethernet (SyncE) is proposed to integrate
synchronization distribution capabilities into packet switching
systems. Similar to SDH nodes, a SyncE node can acquire the reference
clock from the signal received from a specific input port, use it to
correct the local clock, and regenerate frequency in the signals
transmitted over the output ports.
Following this, IEEE 1588-2008/PTPv2 has been specified which
provides time synchronization capabilities.
This draft analyzes the existing works on synchronization solutions
and provides a non-exhaustive list of them. This includes the
synchronization solutions developed for telecom networks and other
industries as well including but not limited to power generation and
transmission industries, finance and trading, scientific computing,
road traffic control, and etc., It outlines some of the missing
features with regard to scalability that are important for
synchronization networks. The aim of this document is to provide
guidance for some further synchronization work in the IETF.
1.1. Conventions used in this document
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in [RFC2119].
1.2. Terminology
AVB audio/video bridging
BMCA Best Master Clock Algorithm
MIB Management Information Base
NTP Network Time Protocol
OAM Operation Administration and Maintenance
PTP Precision Time Protocol
PTPv2 Precision Time Protocol Version 2
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SDH Synchronous Digital Hierarchy
SMIv2 Structure of Management Information Version 2
SNTP Simple Network Time Protocol
SSM Synchronization Status Message
TDM Time Division Multiplex
UDP User Datagram Protocol
2. Related Standardization Work on Synchronization Network
2.1. A Survey of ITU-T work
ITU-T has approved a series of Recommendations to transport and
distribute synchronization over telecom networks. It describes
different aspects of synchronization in a TDM network. [G.803]
specifies the SDH-based synchronization network architecture. Based
on the architecture, [G.781] specifies synchronization principles and
defines synchronization layer functions which obey the combination
rules given in [G.783] to specify synchronization functionality of
network elements. [G.823], [G.824] and [G.825] specify the maximum
network limits of jitter and wander and the minimum equipment
tolerance to jitter and wander respectively for networks based on the
2048 Kbit/s hierarchy, 1544 Kbit/s hierarchy and SDH.
Moving forward, ITU-T approved SyncE, a physical layer method, which
uses synchronous physical layer for the transport and distribution of
frequency over a packet network. [G.8261], [G.8262] and [G.8264]
describe the physical layer frequency distribution in packet-based
networks. [G.8261] focuses on the distribution of synchronization
network clock signals (PNT domain) and of service clock signals (CES
domain) over a packet network. It also defines network limits of
jitter and wander for the synchronous Ethernet interface. Its latest
amendment 1 adds the network jitter limits for several kinds of
multilane interfaces consisting of 10G lanes and of 25G lanes.
[G.8262] defines the synchronous Ethernet Equipment Clock as well as
its requirements for clocks, e.g., bandwidth, frequency accuracy,
holdover and noise generation. [G.8264] defines the SSM protocol and
formats for SyncE as well as the Ethernet Synchronization Messaging
Channel (ESMC). Its latest amendment 1 adds text to describe the ESMC
operation with link aggregation.
On the other hand, [G.8263] and [G.8265] describe the packet-based
mechanisms based on IEEE 1588 to transport frequency over a packet
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network in the absence of physical layer timing. [G.8265] describes
the master-slave architecture and requirements for packet-based
frequency distribution in telecom networks. According to the
architecture, [G.8263] outlines minimum requirements, e.g., frequency
accuracy, noise generation, packet delay variation noise tolerance
and holdover, for the packet slave clocks. [G.8265.1] further defines
a PTP telecom profile for frequency distribution using only unicast
mode leaving the use of mixed unicast/multicast operation for further
study.
PTPv2 was developed based on IEEE 1588-2008 for the transport of
phase and time. [G.8271] presents the need for time and phase
synchronization in a carrier environment and specifies the time-phase
synchronization methods and interfaces as well as its related
performance. The architecture and requirements for packet-based time
and phase distribution using PTP is described in [G.8275]. According
to the architecture, [G.8275.1] defines the PTP profile for telecom
networks for time and phase distribution.
2.2. A Survey of IEEE work
[IEEE 1588] defines the PTP protocol to synchronize Wide Area
Networks. It includes synchronization methodology, datasets and state
machine maintained by each clock, to synchronize clocks of
distributed nodes in a system using packet-based networks and the
Best Master Clock Algorithm (BMCA). It allows all nodes to
synchronize system-wide in the sub-microsecond range. Its second
version [IEEE 1588-2008] enhances the usability and precision for
large networks by defining shorter synchronization frames, mappings
to UDP/IP and other protocols, options for redundancy and fault
tolerance, and by specifying message extensions using TLV, asymmetry
corrections and optional unicast messaging in addition to multicast.
It provides flexible configuration by means of configuration sets
known as "profiles" used by specific devices to guarantee the proper
behavior and performance. Its latest revision [IEEE 1588-20XX]
currently under development defines optional data sets required by
PTP options, complements the description about granting port
operations and about using an alternate timescale, and introduces an
optional mechanism for external configuration for a node's port state.
It permits synchronization accuracies better than 1 ns. Definitions
of a common MIB enabling the use of 1588 in a heterogeneous
environment and a link state protocol used to establish redundant
synchronization path are currently under study.
[IEEE 802.1AS] specifies synchronization based on [IEEE 1588-2008] in
Audio/Video Bridging (AVB) networks. It defines a PTP profile with
the time-aware bridge acting as boundary clock and time-aware end
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station acting as ordinary clock. The BMCA specified in this standard
is an alternate BMCA which is similar but not identical to that
specified in [IEEE 1588-2008]. This standard also defines a complete
SMIv2 Management Information Base (MIB) set for all features it
specifies. Its revision [IEEE 802.1ASbt/D0.7] enhances support for
redundant grandmasters and/or paths using multiple gPTP domains, link
aggregation and new media types with additional parameter sets for
non-Audio/Video applications. Its latest revision [IEEE 802.1AS-Rev]
enhances the way needed for a port to determine whether its neighbor
is asCapable in a particular domain by adding the per-port global
variable tmFtmSupport. Details about redundancy-related definitions,
functions and algorithms are currently under study.
2.3. A Survey of IETF work
[RFC5905] defines the Network Time Protocol version 4 (NTPv4), which
is widely used to synchronize system clocks among a set of
distributed time servers and clients with potential accuracies in the
low microseconds range. NTPv4 obsoletes both [RFC1305] (NTPv3) and
[RFC4330] (SNTP)) and describes the core architecture, protocol,
state machines, data structures and algorithms.
[draft-ietf-tictoc-ptp-mib-06] defines some managed objects used for
managing PTP devices of IEEE 1588 including the ordinary clock,
transparent clock, boundary clocks. But this MIB is read-only and not
intended to provide the ability to configure PTP clocks.
3. Discussions
With the worldwide deployment of 4G/LTE networks, we have already
seen tens of thousands of network nodes deployed in a single metro
network. In the future, 5G mobile networks will have a greater number
of cells and it is expected that the backhaul network will grow even
larger. Therefore, the computation of distribution path and
configuration for such a large synchronization network will pose a
great challenge.
Until now, little work has been done on the scalability of
synchronization. SyncE work is focused on the general synchronization
requirements and architecture (TDM and packet network), it is assumed
that a loop-free distribution path will be computed for each end node
externally and configured beforehand. Though Synchronization Status
Message (SSM) message provides some indication on the quality of the
synchronization signal, it cannot locate a fault in synchronization
path. Thus, it mainly resorts to onerous, error-prone and time-
consuming manual operations at present.
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In IEEE 1588, each node can automatically compute and select
synchronization source based on knowledge of the network topology in
a domain, but it is difficult to compute and configure a large
network in such a distributed manner. Furthermore, it is also assumed
that the network operator has assigned specific ports for
synchronization. IEEE 1588 does not provide fault diagnosis
capability, and resilience is realized by automatic re-computation of
a new distribution path based on the new converged network.
Therefore, it is crucial to provide a synchronization path
computation, configuration and restoration tools for a large
synchronization network, and provide necessary OAM tools for its
diagnosis. These tools must be generic, vendor-independent and
protocol-neutral.
4. Security Considerations
This document analyzes the standardization work on synchronization in
different SDOs. As no solution is proposed in this document, no
security concerns are raised.
5. IANA Considerations
There are no IANA actions required by this document.
6. References
6.1. Informative References
[RFC1305] Mills, D., "Network Time Protocol (Version 3) Specification,
Implementation and Analysis", RFC 1305, March 1992
[RFC4330] Mills, D., "Simple Network Time Protocol (SNTP) Version 4
for IPv4, IPv6 and OSI", RFC 4330, January 2006
[RFC5905] Mills, D., "Network Time Protocol Version 4: Protocol and
Algorithms Specification", RFC 5905, June 2010
[G.803] ITU-T, Architecture of transport networks based on the
synchronous digital hierarchy (SDH), March, 2000.
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[G.823] ITU-T, The control of jitter and wander within digital
networks which are based on the 2048 Kbit/s hierarchy,
March, 2000.
[G.824] ITU-T, The control of jitter and wander within digital
networks which are based on the 1544 Kbit/s hierarchy,
March, 2000.
[G.825] ITU-T, The control of jitter and wander within digital
networks which are based on the synchronous digital
hierarchy (SDH), March, 2000.
[G.781] ITU-T, Synchronization layer functions, September, 2008.
[G.783] ITU-T, Characteristics of synchronous digital hierarchy (SDH)
equipment functional blocks, March, 2006.
[G.8261] ITU-T, Timing and synchronization aspects in packet networks,
August, 2013.
[G.8262] ITU-T, Timing characteristics of synchronous Ethernet
equipment slave clock (EEC), January, 2015.
[G.8263] ITU-T, Timing characteristics of packet based equipment
clocks, February, 2012.
[G.8264] ITU-T, Distribution of timing information through packet
networks, May, 2014.
[G.8265] ITU-T, Architecture and requirements for packet-based
frequency delivery, October, 2010.
[G.8265.1] ITU-T, Precision time protocol telecom profile for
frequency synchronization, July, 2014.
[G.8271] ITU-T, Time and phase synchronization aspects of packet
networks, February, 2012.
[G.8275] ITU-T, Architecture and requirements for packet-based time
and phase distribution, November, 2013.
[G.8275.1] ITU-T, Precision time protocol telecom profile for
phase/time synchronization with full timing support from
the network, July, 2014.
[IEEE-1588] IEEE, Precision Clock Synchronization Protocol for
Networked Measurement and Control Systems, July, 2008.
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[IEEE 802.1AS] IEEE, Timing and Synchronization for Time-Sensitive
Applications in Bridged Local Area Networks, March, 2011.
[IEEE 802.1ASbt/D0.7] IEEE, Timing and Synchronization for Time-
sensitive Applications, November, 2014.
[IEEE 802.1AS-Rev/D2.0] IEEE, Timing and Synchronization for Time-
Sensitive Applications, October, 2015.
[draft-ietf-tictoc-ptp-mib-06] IETF, Precision Time Protocol Version
2 (PTPv2) Management Information Base, work in progress.
7. Acknowledgments
TBD
Authors' Addresses
Yuanlong Jiang
Huawei Technologies Co., Ltd.
Bantian, Longgang district
Shenzhen 518129, China
Email: jiangyuanlong@huawei.com
Xian Liu
Huawei Technologies Co., Ltd.
Bantian, Longgang district
Shenzhen 518129, China
Email: lene.liuxian@huawei.com
Liang Geng
China Mobile
Xuanwumenxi Ave, Xuanwu District
Beijing 100053, China
Email: gengliang@chinamobile.com
Daniel Philip Venmani
Orange Labs
2, avenue Pierre Marzin,
Lannion 22307, France
Email: danielphilip.venmani@orange.com
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