Internet DRAFT - draft-sun-nmrg-hybrid-switching
draft-sun-nmrg-hybrid-switching
Network Working Group W. Sun
Internet-Draft J. Shao
Intended status: Informational W. Hu
Expires: August 12, 2024 SJTU
February 14, 2024
Resource Allocation Model for Hybrid Switching Networks
draft-sun-nmrg-hybrid-switching-09.txt
Abstract
The fast increase in traffic volumn within and outside Datacenters is
placing an unprecendented challenge on the underline network, in both
the capacity it can provide, and the way it delivers traffic. When a
large portion of network traffic is contributed by large flows,
providing high capacity and slow to change optical circuit switching
along side fine-granular packet services may potentially improve
network utility and reduce both CAPEX and OpEX. This gives rise to
the concept of hybrid switching - a paradigm that seeks to make the
best of packet and circuit switching.
However, the full potential of hybrid switching networks (HSNs) can
only be realized when such a network is optimally designed and
operated, in the sense that "an appropriate amount of resource is
used to handle an appropriate amount of traffic in both switching
planes." The resource allocation problem in HSNs is in fact complex
ineractions between three components: resource allocation between the
two switching planes, traffic partitioning between the two switching
planes, and the overall cost or performance constraints.
In this memo, we explore the challenges of planning and operating
hybrid switching networks, with a particular focus on the resource
allocation problem, and provide a high-level model that may guide
resource allocation in future hybrid switching networks.
Status of This Memo
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This Internet-Draft will expire on March 9, 2024.
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1. Introduction
In facing rapid increase of network traffic [Gantz12], as well as the
number of servers in cloud data centers [Cisco15], new architectures
and operation models of Data Center Networks (DCNs) gained wide
interests. One concept that attracted considerable and lasting
attention is the introduction of optical switching technologies into
DCNs, hoping that bypassing some of the traffic without performing
per-packet electronic processing will help reducing the Operational
Cost (OpEx), as well as the Capital Expenditure (CapEx) of DCNs.
This concept of combining electronic packet switching (EPS) and
optical switching (often optical circuit switching, OCS), is called
hybrid switching [Zukerman89]. In recent years, many hybrid
switching schemes have been proposed [Gauger06], and it is reasonable
to believe that when a DCN grows beyond a certain scale, the benefit
of introducing optical switching will emerge and become more evident
as the size of the DC continues to increase.
On the other hand, achieving the benefits of hybrid switching
requires careful design at the planning stage, and proper operation
during runtime. This poses challenges that goes far beyond the
topological or architectural aspects. For instance, at the planning
stage, one has to decide how much to invest in the two switching
planes, such that each could be fully utilized when the network
becomes operational. Under cases when dynamic resource allocation
between the two planes are possible, one has to decide how resource
is allocated between the two planes, and how traffic should be
directed to each of them, such that performance constraints can be
satisfied, and operational cost such as power consumption can be
minimized.
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This memo aims to explore the challenges of planning and operating
hybrid switching networks, and provide a high-level model that may
guide the resource allocation in future hybrid switching networks.
We will use hybrid switching DCN as an example to show one possible
application of this model.
2. 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.
3. Overview of Hybrid Switching Networks
Hybrid Switching Networks (HSNs) are networks that employ more than
one switching technology. The term started to attract attention when
Wavelength Dense Multiplexing (WDM) started to be deployed as a
underlying infrastructure of TCP/IP based packet networks [FENG17].
It continued to receive considerable attention, as the research on
future-looking optical switching schemes boomed, around and after the
begining of the 21th century.
The research on hybrid switching gained momentum again with the rapid
growth of cloud data centers. With a clearer context and real-life
prototyping efforts, a wider concensus regarding the benefits and
feasibility of HSN have been reached.
The challenges of planning and operating hybrid DCNs are rooted in
the fundamental differences between EPS and OCS [Farrington10],
[WANG10]. In principle, EPS is good at delivering traffic that is
bursty and difficult to predict. By aggregating the traffic from a
large number of communcating peers, high network utilization can be
achieved at modest cost. OCS, on the other hand, is suited for well
planed, or highly predictable traffic patterns. One good example is
the delivery of bulk flows, which can last up to a few minutes when
carried by a wavelength channel at full capacity.
4. Terms used in this document
o Electronic Packet Switching (EPS)
EPS in this memo refers to the off-the-shelf switching technology.
It provides "best-effort" packet delivery service. Since EPS
performs fine-granular per-packet processing, it is generally
regarded to be best suitable for traffic that is bursty and
difficult to predict. Existing researches show that when lightly
loaded, the performance of EPS can be rather reliable and
predictable. However, when the network is heavily loaded, the
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performance of EPS will deteriorate very quickly and result in
long queueing delay and high packet loss rate.
o Optical Circuit Switching (OCS)
OCS in this memo refers to connection oriented network services
based on optical switching technologies, such as MEMS or WSS based
switches, and the like. The connection oriented nature of OCS
requires the establishment of connections through signaling prior
to data transfer. The capacity of each connection, for instance,
a wavelength channel, often consumes a significant portion of the
overall network capacity. Request blocking is thus difficult to
eliminate in OCS, if not impossible.
o Hybrid Switching Networks (HSNs)
HSNs in this memo refers to networks that: i) employ both EPS and
OCS, and ii) accept data transfer request in both packet and
stream/flow form. Upon entering the network, requests in packets
form will be handled by the EPS plane, and requests in flow form
will be handled by OCS following the connection provisioning
procedures. This differs HSN from IP over WDM networks, where
both switching schemes exist, but services start and terminate
only on the IP layer, and standalone OCS service is not provided.
Note that the boundary between packet and flow requests may not
naturally exist. For instance, when flow level information is not
available from outside the network, it will be up to the network
to decide how traffic should be partitioned and then directed to
either EPS or OCS.
5. Performance Measures in Hybrid switching Networks
5.1. Performance Measures in Electronic Packet Switching
Without loss of generality, performance of packet switching networks
can be characterized by one or more of the following metrics:
o Packet loss rate - packet loss may happen when congestions occur.
Statistically, in a given network, packet loss rate can be seen as
a function of network load. Packet loss rarely happen when the
traffic load is low. But when the load increases to a certain
threshold in the network, or in part of it, packet loss rate may
increase quickly as load continues to increase.
Packet delay and jitter - like packet loss rate, packet delay is
mostly stable and jitter is small when the network is lightly
loaded. Delay and jitter will increase dramatically when network
load increases.
Flow completion time - flow completion time is a composite metric
that relies on both packet loss rate and packet delay.
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5.2. Performance Measures in Optical Circuit Switching
Performance of Optical Circuit Switching (OCS) is typically measured
by request blocking rate, defined as the number of admitted requests
over the total number of arrivals. In theory, blocking in OCS can
not be eliminated. The planning of OCS is thus often a tradeoff
between performance and cost, as in the case of conventional
telephone networks, in which trunk capacity can be dimensioned with
the Erlang-B formula.
6. BLOC - the Blocking LOss Curves
6.1. General Idea
To understand the resource allocation in HSNs, it is important to
understand the interactions between the three components in the
systems:
o Traffic partitioning
Traffic partitioning means the separation of incoming traffic into
two parts so that each part can be handled by the one of the two
switching planes. In today's networks, there might be many
traffic separation/differentiation mechanisms for the purpose of
enforcing differentiated policy based on traffic type. Traffic
partitioning in the context of HSN, however, aims to realize the
optimal separation of flows into the two planes, such that the
utility of the network can be maximized.
One traffic partitioning method is a flow length based method.
With a predefined threshold, flows are classified into short flows
and large flows, each served with the packet switching plane and
the circuit switching plane, respectively.
Partitioning can be performed according to a priori knowledge,
e.g., according to the information provided by the applications
that generate the traffic flows. It also can be performed in
network during runtime. The details on traffic classification and
partitioning may be found in [Cisco15] and are outside the scope
of this memo.
o Resource allocation
The resource here can be physical resources such as switch ports,
wavelengths or fibers. It also can be abstract resource such as
the overall budget.
o Performance/Cost Constraints
The cost constraint applies when the making of the hybrid
switching system is subject to limited budget. For any given
traffic demand, the cost and performance of carrying the traffic
through either EPS or OCS can be very different. A good design
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should, on the first hand, satisfy the performance constraint; on
the other, it should also leave space for future traffic demand
growth. The performance constraints specify the acceptable worst-
case performance of the system, for example, the maximum packet
loss rate, highest request-blocking rate, or longest packet delay
etc. Given traffic demand, the worst-case performance constraint
specifies the least amount of resource that should be allocated to
a switching plane.
As can easily be seen, the operation of HSNs involves close
interactions between the three components, and is a difficult
problem. The interactions can be summarized into the following
diagram.
+-----------------------+
| Cost/Performance |
| Constraint |
+-----------------------+
^
Constraint | Constraint
+---------------------+-------------------------+
| |
v v
+-----------------+ +-----------------+
| Traffic | Linkage | Resource |
| Partition |----------------------------| allocation |
| | | |
+-----------------+ +-----------------+
Interactions beween the three components in HSN
6.2. Modeling the curves
In a typical IP network with a given traffic load, the packet loss
rate decreases when the network capacity increases and vice versa.
Similarly, in circuit switching networks, the request blocking
probability will decrease when the bandwidth increases and vice
versa. In a hybrid switching system, the overall resource capacity
is constant. The resource allocation between EPS and OCS plane will
directly affect the network performance of both switching planes.
The network performance is also affected directly by how the traffic
is partitioned between EPS and OCS planes.
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^ packet loss rate
| o capacity resource 1
| * capacity resource 2 ____o _____* ____x
| x capacity resource 3 / / /
| _____o _____* _____x
| / / /
| ____o ____* ____x
| / / /
| ___o ___* ___x
| / / /
| __o __* __x
| / / /
| _o _* _x
| / / /
| _o _* _x
| / / /
| _o _* _x
| / / /
| o * x
+-------------------------------------------------------->
the traffic load for EPS
Fig. 1(a) the performance curves for EPS
^ request blocking rate
| o capacity resource 1
| * capacity resource 2 ____o _____* ____x
| x capacity resource 3 / / /
| _____o _____* _____x
| / / /
| ____o ____* ____x
| / / /
| ___o ___* ___x
| / / /
| __o __* __x
| / / /
| _o _* _x
| / / /
| _o _* _x
| / / /
| _o _* _x
| / / /
| o * x
+-------------------------------------------------------->
the traffic load for OCS
Fig. 1(b) the performance curves for OCS
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To clearly classify the influence from the traffic load partition and
network capacity allocation, we take Fig. 1(a) and 1(b) to show the
performance curves for EPS and OCS with varying traffic loads and
network capacities. In Fig. 1(a), we choose the packet loss rate as
the performance of EPS. When the traffic load for EPS increases, the
packet loss rate becomes worse under the constrained network
capacity. The extension of network capacity will bring a promoted
packet loss rate for EPS which is classified in Fig. 1(a) with the
capacity resource 1 <= capacity resource 2 <= capacity resource 3.
The performance curves for OCS is shown in Fig. 1(b) after choosing
the request blocking rate as the y-axis, and the relationship among
these curves is still resource 1 <= capacity resource 2 <= capacity
resource 3. Combining Fig. 1(a) and 1(b), the more network
capacities we allocate to EPS or OCS, the better service they will
provide under a heavier traffic load transmission.
6.3. The BLOC System
The BLOC framework comprises two types of curves, i.e., loss curves
(LCs) and blocking curves (BCs), in the same two-dimensional
coordinate system. An LC or a BC in the BLOC framework is a curve
that contains a series of points with the same packet loss rate or
request blocking probability. Using the percentage of traffic
delivered by EPS as the x-axis and the percentage of bandwidth
allocated to EPS plane as the y-axis, all of the curves in the BLOC
framework are monotonic. Another important component of the BLOC
framework is the feasible region. In this paper, "feasible" means
that as long as the traffic partitioning and resource allocation fall
within this area, the resulting packet loss rate will be smaller than
Pmax (the maximum packet loss rate) and the request blocking
probability will be lower than Bmax (the maximum request blocking
rate). Thus, the feasible region contains all the feasible
combinations of resource allocation and traffic partitioning that
satisfy the network performance requirements. Different resource
allocation strategies in hybrid switching networks can be achieved by
choosing a point from the feasible region.
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^ 100%
|...% of resource allocated to the EPS plane (Y Axis)...
| _o _x
| (1) packet loss / /
| rate increase __o __x
| (2) packet loss / ^ /
| rate decrease _o . _x
| / . /
| __o .(2) __x
| / . /
| __o . __x
| / . /
| ___o . ___x
| / (1) ./
| ____o .............>____x
| / /
| _____o ____x
| / /
|_o____________________x
+-------------------------------------------------------->
(0,0) % of traffic offered to the EPS plane (X Axis) 100%
Fig. 2(a) the packet loss curves for EPS plane
^ 100%
| (100%,100%)
|...% of resource allocated to the EPS plane (Y Axis)... +
| o______________x______/
| / /
| _____o _____x
| / /
| ____o ____x
| / ^ /
| ___o . ___x
| / (2). /
| __o . __x
| / (1) ./
| __o ........> __x (1) request blocking
| / / rate decrease
| _o _x (2) request blocking
| / / rate increase
| _o _x
| / /
|o x
+-------------------------------------------------------->
% of traffic offered to the EPS plane (X Axis) 100%
Fig. 2(b) the request blocking curves for OCS plane
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Fig. 2(a) and 2(b) show an example of LCs and BCs when the overall
hybrid system capacity and the traffic volume are fixed. In Fig.
2(a), when the percentage of traffic to be transmitted by EPS
increases, the bandwidth allocated to EPS plane must also be
increased so the same packet loss rate can be achieved. Hence, each
LC is monotonically increasing. In addition, the LCs with smaller
loss rate values require a larger percentage of bandwidth for the
same amount of traffic. Therefore, the LCs moves to the top left
when the packet loss rate becomes smaller, as shown in Fig. 2(a).
All of the LCs pass through the origin (0, 0), so if no bandwidth is
allocated to PS plane, it cannot transmit any traffic. Similarly,
the BCs move downward to the right when the request blocking
probability becomes lower, and all of the BCs converge to the point
(100,100), where all of the bandwidth and traffic is assigned to PS
plane, as shown in Fig. 2(b).
^ 100%
|
|----- % of resource allocated to the EPS plane (Y Axis)
| +----------------------------------+
| | Request blocking curve with Bmax | /
| +------------------+---------------+ /
| | /
| +-----------------------------+ | / /
| | Packet loss curve with Pmax | | / /
| +------+----------------------+ | / /
| | v / /
| | /-------------------O---------X-/
| | / -- -- -- -- -- -- -- -- -- /
| | / -- -- -- -- -- -- -- -- -- /
| v / -- -- -- -- -- -- -- -- -- /
| /-O-.-----------------^-----------/
| / / |
| / / |
| / / +---------------------+
| / / | feasible region |
| / +---------------------+
| /
+-------------------------------------------------------->
% of traffic offered to the EPS plane (X Axis) 100%
Fig. 3 an examplery BLOC
We now consider a hybrid switching system with the maximal allowed
packet loss rate Pmax and the maximal allowed request blocking
probability Bmax [FENG16]. Fig. 3 shows a BLOC where the LCs and BCs
are placed in the same two-dimensional coordinate system. The
hatched area above the LC of Pmax and below the BC of Bmax contains
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all of the feasible combinations of traffic partitioning and resource
allocation. Choosing a point from the feasible region (i.e., a
combination of resource allocation and traffic partitioning) is
subject to various optimization objectives. For instance, from an
energy consumption perspective, we need to choose the point with the
minimal percentage of EPS resources from the feasible region (i.e.,
the lowest point in the feasible region), so that the overall energy
consumption would be minimized. In Section 5, we show that other
metrics can also be optimized with the BLOC, such as the packet delay
in EPS plane as a function of resource allocation and traffic
partitioning.
6.4. An example
A hybrid switching Datacenter network is shown in Fig. 4 [FENG17].
Among all s+p uplink interfaces on each ToR switch, s of them connect
the switch to the EPS plane and the rest, p, connect the ToR switch
to the OCS network. As the cost of supporting an OCS connection can
be very different from that of an EPS port, different combinations of
s and p will result in significant difference in building cost.
Different combinations of s and p will also lead to different
performance and running cost, such as power consumption.
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+-------------------------+ +-----------------------+
| | | |
| EPS Network | | OCS Network |
| +-+ +-+ +-+ +-+ +-+ +-+ | |+-+ +-+ +-+ +-+ +-+ +-+|
+-+-+-+-+-+-+-+-+-+-+-+-+-+ ++-+-+-+-+-+-+-+-+-+-+-++
^ ^ ^ ^ ^ ^ ^ ^ ^ ^ ^ ^
| | | | \ \ / | / / \ \
| | | | \ \ / / / / \ \
| | | | \ \ / / / / | |
| | | \ \ \-------*-----*-*---*---\ | |
| | | \ \---------*-----*-*---*\ \ \ \
.+---+. \ *---------------/ / / / \ \ \ \
s -->( | | ) \ / \ /-----------------/ / / \ \ | \
`+---+' \ / *-------\ / / \ \ | |
| | *---*-----\ \ / / \ | | |
| | .X--./ \ \ / / | | | |
p -----+---+-->(/ /) \ \ / / | | | |
v v v`---' v v v v v v v v
+-+-+-+-+-+-+-+-+-+ +-+-+-+-+-+-+-+-+-+ +-+-+-+-+-+-+-+-+-+
| +-+ +-+ +-+ +-+ | | +-+ +-+ +-+ +-+ | | +-+ +-+ +-+ +-+ |
| ToR Switch | | ToR Switch | | ToR Switch |
+-----------------+ +-----------------+ +-----------------+
+-----------------+ +-----------------+ +-----------------+
+-----------------+ +-----------------+ +-----------------+
| ... | | ... | | ... |
+-----------------+ +-----------------+ +-----------------+
+-----------------+ +-----------------+ +-----------------+
+-----------------+ +-----------------+ +-----------------+
Fig. 4 switch ports allocation in hybrid DCN
The costs of network interconnecting devices in the EPS and OCS
networks are determined by allocation of uplink interfaces. Thus,
for each ToR, the cost constraint can be presented as Cp(s) + Cc(p)<=
C, in which Cp(s) stands for the cost of EPS with s uplinks, and
Cc(p) stands for the OCS cost with p uplinks.
The total volume of flows to be transmitted on a ToR switch is V.
The traffic is carried by either EPS or OCS:Vp + Vc = V.
The performance requirements specify the acceptable worst-case
performance of the system, such as the longest flow completion time
and the highest request blocking probability. A proper resource
allocation and traffic partitioning should satisfy the performance
requirements in both EPS and OCS networks: Tp(s,Vp) <= Tmax, Bc(p,Vc)
<= Bmax, where Tmax and Bmax are the flow completion time and request
blocking probability requirements in EPS and OCS, respectively.
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^ 100%
| % of resource to EPS
| +------------+ +-----------+ *
| |Tmax=0.02ms | | Bmax=0.01 | *
| +------------+ +-----------+ **----
| | | */ ////
| | | */ ////
| | | */ /////
| v ***********************/ //////
| ***O******** v / / +++++
| ** /-----------------O----------/ / +
| ** / ////////////////////////// / / +
| * / ////////////////////////// / / +
| * /--------O-------------------/ / +
| * / ////////^////////////////////// +
| ** / /////////|///////////////////// +
| * / // +++++++|+++++++++++++O++++++++
| * /+++++ | ^
| * /+ | |
| * ++ +-----------+ +-----------+
| * + |Bmax=0.001 | | Tmax=1ms |
| ** ++ +-----------+ +-----------+
| ***+++
| *** 100%
+-------------------------------------------------------->
% of traffic offered to the EPS
Fig. 5 BLOC for hybrid DCN
Fig. 5 shows the BLOC with different network performance
requirements. When Tmax equals 1 ms and Bmax equals 0.01, there is a
feasible region between the curves with Tmax and Bmax. When the
performance requirements are higher (i.e., smaller Tmax and Bmax),
the feasibleregion will be smaller or may even disappear. For
example, when Tmax and Bmax decrease respectively to 0.02 ms and
0.001, the feasible region cannot be found. That means it is not
possible to find a resource allocation that can satisfy Tmax and Bmax
simultaneously. As the hybrid switching system is an interaction
between the three components, when the network performance
requirements cannot be satisfied, the system should have a greater
budget or carry less traffic to obtain a feasible resource
allocation.
7. Security Considerations
This document does not impose any new challenges to the current
Internet.
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8. IANA Considerations
This document makes no requests for IANA action.
9. Acknowledgements
We are grateful to the valuable discussions and inputs from the
community. We thank the support from NSFC.
10. Informative References
[Cisco15] Cisco, Cisco., "Cisco global cloud index: Forecast and
methodology, 2015-2020. white paper", http://www.cisco.com
/en/US/solutions/collateral/ns341/ns525/ns537/ns705/
ns1175/Cloud_Index_White_Paper.html#wp9000816 1-29, 2015.
[Farrington10]
Farrington, Nathan., Porter, George., Radhakrishnan,
Sivasankar., Bazzaz,, Hamid., Subramanya, Vikram.,
Fainman, Yeshaiahu., Papen, George., and Amin. Vahdat,
"Helios: a hybrid electrical/optical switch architecture
for modular data centers", SIGCOMM'10 339-350,
DOI 10.1145/1851182.1851223, August 2010.
[FENG16] Feng, Z., Sun, W., and W. Hu, "BLOC: A Generic Resource
Allocation Framework for Hybrid Packet/Circuit-Switched
Networks", J. Opt. Commun. Netw. 8, 689-700,
DOI 10.1364/JOCN.8.000689, August 2016.
[FENG17] Feng, Z., Sun, W., Zhu, J., Shao, J., and W. Hu, "Resource
Allocation in Electrical/Optical Hybrid Switching Data
Center Networks", J. Opt. Commun. Netw. 9, 648-657,
DOI 10.1364/JOCN.8.000689, August 2017.
[Gantz12] Gantz, John. and David. Reinsel, "The digital universe in
2020: Big data, bigger digital shadows, and biggest growth
in the far east", International Data Corporation 1414_v2,
December 2012.
[Gauger06]
Gauger, C., Kuhn, P., Breusegem, E., Pickavet, M., and P.
Demeester, "Hybrid optical network architectures: Bringing
packets and circuits together", IEEE Commun. Mag. 44(8),
36-42, DOI 10.1109/MCOM.2006.1678107, August 2006.
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[WANG10] Wang, Guohui., Andersen, David., Kaminsky, Michael.,
Papagiannaki, Konstantina., Eugene Ng, T., Kozuch,
Michael., and Michael. Ryan, "c-Through: part-time optics
in data centers", SIGCOMM'10 327-338,
DOI 10.1145/1851182.1851223, August 2010.
[Zukerman89]
Zukerman, M., "Bandwidth allocation for bursty isochronous
traffic in a hybrid switching system", IEEE Transactions
on Communications 37(12), 1367-1371, DOI 10.1109/26.44208,
December 1989.
Authors' Addresses
Weiqiang Sun
Shanghai Jiao Tong University
800 Dongchuan Road
Shanghai 200240
China
Phone: +86 21 3420 5359
EMail: sun.weiqiang@gmail.com
Junyi Shao
Shanghai Jiao Tong University
800 Dongchuan Road
Shanghai 200240
China
EMail: shaojunyi@sjtu.edu.cn
Weisheng Hu
Shanghai Jiao Tong University
800 Dongchuan Road
Shanghai 200240
China
EMail: wshu@sjtu.edu.cn
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