304 J. OPT. COMMUN. NETW./VOL. 9, NO. 4/APRIL 2017

Alparslan et al.

Dynamic Wavelength Allocation andAnalytical Model for Flow Assignment
in Optical Packet and Path Integrated
Networks
Onur Alparslan, Shin’ichi Arakawa, and Masayuki Murata

Abstract—A hybrid optical architecture combining path(circuit) and packet switching can be a good candidate
for future optical networks because it exploits the best ofboth worlds. In this paper, we present a control framework
called the Dynamic Optical Wavelength and Flow
Allocation Framework (DOWFAF), which can dynamicallychange the ratio of path and packet wavelengths and theflow size threshold in the hybrid path–packet integrated
networks in order to balance the utilization of path andpacket subnetworks and maximize the ratio of large flows
benefiting from the path switching. We propose an analytical model for calculating the flow size threshold and a
feedback control for estimating the wavelength allocation
ratio for varying traffic. DOWFAF can be implemented bysoftware-defined networking, which is getting a lot of
attention recently. We show that DOWFAF can greatly
increase the goodput of large Transmission Control
Protocol (TCP) flows for a wide range of traffic, while decreasing the cost and the power consumption.

goodput (speed) of Transmission Control Protocol (TCP)
flows because of the high packet drop rate in small buffers.
In comparison, path switching has many advantages overpacket switching, like low switch cost and power consumption, as its switching speed and frequency are lower.
Moreover, it does not need optical buffering in the corenodes, as there is no contention of packets, so the flows
can achieve higher goodput. Furthermore, it has an easierand more effective QoS support for flows with strict QoS
requirements. However, establishing a path for each flow
is not a good stand-alone solution for Internet traffic,
because a short flow may not fully utilize the capacity inthe dedicated channel due to TCP dynamics. Moreover,path switching needs prior reservation of channels, whichadds an additional delay to the flow completion time anddecreases the achievable utilization.

Index Terms—Blocking probability; Circuit switching;
Optical network; Packet switching; Path switching;Wavelength-division multiplexing.

Recently, some hybrid architectures combining path andpacket switching were proposed as a solution to these problems by exploiting the best of both worlds [1,2]. There are
two main approaches in the literature for realizing a hybrid
path and packet switching architecture. One of them is carrying both packet and path traffic on the same wavelength
at the same time like in ORION [3] and OpMiGua [4],where all wavelengths are principally used by paths.
The packet traffic is inserted into idle periods left fromthe path traffic on a wavelength. Another method is touse separate channels for path and packet switching anddistribute the traffic between them. For example, the network can carry short flows over a packet switching subnetwork, while carrying the large flows on an independent
path switching subnetwork as in CHEETAH [5]. The
CHEETAH project proposes two different methods to provide its service. In the first method, end hosts must be
equipped with second Ethernet network interface cards
that are connected directly to a synchronous optical network based CHEETAH network. End hosts directly initiate
end-to-end circuits. The authors state that this approach of
creating dedicated virtual circuits between end servers
located in enterprises and the closest core circuit/virtual
circuit network switch is only feasible for small numbersof users, so they proposed one more method, which is a
proxy-based internetworking architecture [6]. In thisarchitecture, they define a system called a circuit-aware
application gateway running modified Squid software to

I. INTRODUCTION

W

avelength-division multiplexing (WDM) is a promising solution to the high capacity, energy efficiency,
cost, and quality-of-service (QoS) requirements of fastgrowing Internet traffic. WDM provides different switching
granularities, e.g., packet, burst, and path (circuit) switching, each satisfying these requirements to some degree.
While optical packet switching benefits from high multiplexing gain and flexibility, it has disadvantages like highcost and high power consumption, as it needs fast switching fabric and fast header processing capability to achieve
high granularity. However, its cost and power consumption
is much higher than a path switch. Moreover, the current
optical buffering technology is not mature enough toprovide large buffering space, which may jeopardize theManuscript received June 27, 2016; revised January 20, 2017; accepted
January 31, 2017; published March 31, 2017 (Doc. ID 268929).
The authors are with the Graduate School of Information Science andTechnology, Osaka University, 1-5 Yamadaoka, Suita, Osaka 565-0871,
Japan (e-mail: a‑onur@ist.osaka‑u.ac.jp).
https://doi.org/10.1364/JOCN.9.000304

1943-0620/17/040304-15

Journal © 2017 Optical Society of America

Alparslan et al.

interconnect connectionless IP-routed segments at theedges (regional/enterprise subnetworks) with two subnetworks consisting of a circuit switching subnetwork andan IP-routed subnetwork. A long-haul TCP connection issplit into three concatenated connections. Gateways mustrun a complex Squid proxy server, cache data with disk buffering, and employ a special version of TCP to transport
data between gateways in the optical network for each flow.
However, considering that fiber speeds of over 1 Tbit∕s are
already achieved in commercial-grade hardware in a realworld environment, the processing speed may not beenough to employ as many functionalities at the gateways
as are necessary for CHEETAH. Another hybrid architecture that uses separate channels for path and packet
switching is OPCInet [7]. OPCInet uses waveband switching, where a waveband carries N wavelengths, so the number of ports required for packet switching fabric is
decreased. When an optical path is established and thenumber of reserved path switching wavelengths in the border waveband reaches N − 1, a set of N packet switching
wavelengths are converted to path switching. If all wavelengths in the path switching border waveband are unused
and the number of used wavelengths in the previous pathswitching waveband is less than N − 1, the unused pathswitching border waveband is converted to packet switching [8]. The authors implemented a messaging frameworkfor switch configuration based on OpenFlow [7,9]. However,
the authors do not propose a control framework to decide
which flows will be carried over the path subnetwork.
In this paper, we propose a control framework for an integrated hybrid path and packet switching network. We callit the Dynamic Optical Wavelength and Flow Allocation
Framework (DOWFAF), as it can dynamically change theratio of path and packet wavelengths. DOWFAF makes
use of an optical path switching fabric for large flows, while
carrying the short flows on an optical packet switching fabric. DOWFAF maximizes the ratio of large flows benefitingfrom dedicated path switching wavelengths by dynamicallychanging the ratio of path and packet wavelengths and theflow size threshold for using the path wavelengths according
to the network conditions. Its objectives are the following:1) increase the goodput of large flows by carrying them on
dedicated path switching wavelengths;
2) not discriminate the short flows on packet switching
wavelengths;3) decrease the power consumption by carrying the large
flows on a path switching fabric instead of a packet
switching fabric, which consumes much more power;4) decrease the manufacturing cost via decreasing thesize of the packet switching fabric by using only pathswitching fabric for some wavelengths.
DOWFAF differs from the other works in the literaturein the following aspects:• The same fiber is used for both packet and path switching by allocating separate wavelengths for each switching paradigm, so it is completely different from ORION
and OpMiGua, which use the same wavelength for bothVOL. 9, NO. 4/APRIL 2017/J. OPT. COMMUN. NETW. 305

packet and path switching at the same time. Basicallyboth ORION and OpMiGua use a path switching architecture, but they inject extra packets into the voidsin path switching.
• OPCInet proposes only a switch hardware architecture
and a messaging framework for a packet and circuit
integrated switch. The authors do not specify either
how to select the flows that will be carried over the pathwavelengths or how to optimize the ratio of path andpacket switching wavelengths. On the other hand, we
propose an analytical model for calculating the flow size
threshold for varying traffic that also optimizes the ratioof path and packet switching wavelengths. We do not propose a specific switch hardware architecture. DOWFAF
can be applied to any hybrid switch architectures in theliterature including OPCInet as long as they support
dynamically changing the ratio of path and packet
switching wavelengths.
• The optical network changes the ratio of path and packet
switching wavelengths dynamically, so the network canadapt itself to the current traffic in order to maximize theratio of large flows benefiting from path switching. On
the other hand, CHEETAH uses fixed subnetworks forpath switching and packet switching. Moreover,
CHEETAH does not specify how to choose the capacityof these two subnetworks.
• The large flows are carried over path switching like
CHEETAH, but we propose an analytical model for
estimating the flow size threshold for selecting the large
flows. CHEETAH’s analytical model for estimating theflow size threshold is based on the analysis of TCP
Reno’s additive increase/multiplicative decrease congestion control [10]. However, recent operating systems areusing much more aggressive TCP variants as the default,
where the TCP analytical model in CHEETAH is nolonger applicable. Unlike CHEETAH, our analyticalmodel does not employ any equations specific to a TCP
variant, so it is generic and also applicable in the future
regardless of changes in TCP congestion control.
• The core network in DOWFAF is transparent to the end
hosts (users). The only requirement is the application support, as the applications in the end hosts may request using
the path subnetwork when sending their flows. If DOWFAF
is available in the network, DOWFAF decides the subnetwork for the flow by using the flow size information.
This paper evaluates the performance of the proposed
control framework for DOWFAF by a simulation study
using CUBIC TCP [11], which has been the default TCP
variant in Linux kernels since version 2.6.19. As
CHEETAH does not specify the ratio of path and packet
switching wavelengths and is based on an analytical modelof TCP Reno congestion control, which is no longer the default in Linux kernels, direct comparison of DOWFAF andCHEETAH in terms of analysis or simulation is not possible. Therefore, a simulation study was carried out toevaluate the performance of DOWFAF and to show its
improvement over a pure packet switching network. The
simulation results revealed that DOWFAF is capable ofincreasing the goodput of large flows and dynamically306 J. OPT. COMMUN. NETW./VOL. 9, NO. 4/APRIL 2017

Alparslan et al.

optimizing the ratio of path/packet wavelengths in order tomaximize the number of large flows benefiting from thepath wavelengths without penalizing the short flows on
packet wavelengths, while decreasing the power consumption and manufacturing cost of the routers.
The paper is organized as follows. Section II presents thearchitecture of the path/packet integrated architecture andproposes a model dynamically changing the path/packet
wavelength ratio. Section III shows the simulation results
and discusses the performance of the architecture.
Section IV concludes the paper.

II. MODEL OF PATH/PACKET INTEGRATED NETWORKThis section describes the architecture in detail.

A. Network and Node Architecture
The DOWFAF network consists of a packet switching
subnetwork and a path switching subnetwork where separate wavelengths are allocated for each, as shown in
Fig. 1. DOWFAF dynamically changes the ratio of wavelengths assigned to the path and packet subnetworks according to the feedback from the edge routers. The pathswitching subnetwork is used for the DATA packets of only
selected large flows, while the remaining traffic is carriedover the packet switching subnetwork. A possible node architecture is shown in Fig. 2 that was used in the simulations of this paper. As an example, a node with three
wavelengths in red, blue, and green is plotted. It does
not make sense to reserve a path for short flows like flows
with a single packet size, so there should be at least one
packet switching wavelength that is only connected tothe packet switching fabric (OPS-SW), as shown by the
red wavelength in the figure. Moreover, User Datagram
Protocol (UDP) and TCP ACK packets require a packet
switching wavelength. The blue wavelength is connected
to both the OPS-SW and the path switching fabric (OCSSW), so it is possible to assign it to the packet switching
subnetwork or the path switching subnetwork. When it
is assigned to the packet switching subnetwork, the small
1 × 2 switch at the entrance forwards the packets on thiswavelength to the OPS-SW. Otherwise the packets are sent
to the OCS-SW. The packets in the packet switching wavelengths require header processing, so after their headers

Fig. 2. Simulated node architecture.

are sampled, they are delayed for a short time for header
processing before they enter the OPS-SW. The packets on
the path switching wavelengths do not need any header
processing, which requires a high amount of CPU power
and optical/electrical/optical (O/E/O) conversion of sampled
headers. The green wavelength is reserved for the pathswitching subnetwork, so it is not connected to the OPSSW, which decreases the size and cost of the required
OPS-SW fabric compared to a pure packet switching nodewith the same number of wavelengths.
Note that we do not propose a specific switch architecture for DOWFAF. DOWFAF can be applied to the hybrid
switch architectures in the literature as long as they support dynamically changing the ratio of path and packet
switching wavelengths. From a cost and power perspective,
OPCInet may be a promising path–packet integrated architecture, because OPCInet minimizes the number ofports of the packet switching fabric by using waveband
switching. However, waveband switching decreases the
wavelength distribution granularity. In the simulations
presented in this paper, we used the switch architecture
shown in Fig. 2, which does not employ waveband switching, in order to carry out a more sensitive analysis of the
wavelength allocation. While the cost of this node architecture is higher than OPCInet, as a larger optical switching
fabric is required, it can be implemented by using an
OSMOSIS [12] switch as the optical packet switching fabric. The OSMOSIS project demonstrated a 64 × 64 opticalpacket switch with a line rate of 40 Gbps per port and isscalable to 2048 ports by using multiple 64-port switch
modules in a multi-stage fat tree topology.

B. Analytical Model
Fig. 1. Network architecture. The packet plane uses IP routers
and the path plane uses optical cross connects (OXCs) for switching.

The primary objective of DOWFAF is improving the
goodput of large flows, by forwarding them to the pathAlparslan et al.

subnetwork. As the full capacity of a path wavelength isdedicated to a single flow, the flows using the path subnetwork can get higher goodput than in the packet subnetwork. CHEETAH [5] proposes choosing the subnetwork
for a new flow by estimating the goodput analytically inboth subnetworks and choosing the one with the shortest
transfer time. However, different variants of TCP using
distinctively different congestion control algorithms are
emerging, so it is difficult to propose a general analyticalframework to estimate the goodput of TCP flows.
CHEETAH’s analytical model is based on TCP Reno’s[10] congestion control. However, CUBIC TCP [11], which
has been used by default in Linux kernels since version
2.6.19, applies a completely different congestion control
in the congestion avoidance phase, so CHEETAH’s analysis
is no longer applicable. Instead of trying to estimate thetransfer time of a TCP flow, we propose an analytical modelthat considers the utilization of wavelengths in order to calculate a size threshold for classifying a flow as large or
short. In principle, we do not want to jeopardize the goodput of flows in the packet subnetwork when increasing the
goodput of large flows. Therefore, we calculate the flow size
threshold, which gives equal utilization at the problematic
links in both packet and path subnetworks of DOWFAF
and a pure packet switching network, while the utilizationof other links is even lower in the packet subnetwork of
DOWFAF than the path subnetwork and the pure packet
switching network. Keeping the utilization of packet
subnetworks equal to or lower than the path subnetwork
is important. For example, let’s say the traffic arrival rate
to a fiber with eight wavelengths is 800 Mbytes∕s. If
all wavelengths are packet switching wavelengths andthe traffic is distributed among wavelengths uniformly,
the traffic per packet switching wavelength becomes
100 Mbytes∕s. If we use an integrated architecture withfour packet and four path switching wavelengths and forward only 200 Mbytes∕s traffic (out of 800 Mbytes∕s total
traffic) to the path subnetwork, then the traffic per
packet switching wavelength becomes 150 Mbytes∕s,
which causes higher utilization in the packet subnetwork
compared to a pure packet switching network. As theutilization and the packet drop rate in the packet
wavelengths increase, the goodput of flows in the packet
wavelengths of the integrated architecture becomes worse
than in the pure packet switching network, which we wantto avoid.
In this paper, the flow size is assumed to have a bounded(truncated) Pareto distribution [13]. The bounded Pareto
distribution possesses the heavy-tailed property ofInternet traffic and has finite variance, so it is good formodeling the flow size distribution. The real flow size
distribution in an operating network can also be used in
the analysis by sampling the flow size distribution.
The bounded Pareto distribution has three parameters,
α, k, and p, where α denotes the shape, k denotes the minimum value, and p denotes the maximum value. As given in
Ref. [13], the mean of the bounded Pareto for α ≠ 1 is


kα
α
1
1
Ek;p 
:
(1)
·
·
−
1 − k∕pα α − 1 kα−1 pα−1VOL. 9, NO. 4/APRIL 2017/J. OPT. COMMUN. NETW. 307

Let the minimum and maximum flow sizes in the network be L and H, respectively. Then, the mean size ofall flows in the network is EL;H . Let t be the flow size
threshold for carrying the flows over the path switching
wavelengths. The flows shorter than t are sent to thepacket switching wavelengths. The bounded Pareto distribution still holds for the size distribution of flows sent toboth switching domains, so the mean size of flows sent
to the packet switching wavelengths is EL;t , while themean size of flows sent to the path switching wavelengths
is Et;H .
First, we find the utilization of a pure packet switching
network. TCP carries most of the traffic, so we estimate thetraffic of TCP flows, which consists of DATA, ACK and TCP
control packets (SYN and FIN).
1) DATA packets: Let λ be the arrival rate of TCP flows
carrying DATA traffic in the forward direction, SD be
the average size of TCP DATA packets, C be the capacity of each wavelength, and W total be the total numberof wavelengths on a fiber. Calculating C · W total gives
the capacity of a fiber. As λ is the arrival rate of theflows carrying DATA packets, which has a mean sizeof EL;H, calculating λ · EL;H gives the incoming traffic
rate of a fiber due to DATA packets. Dividing this by
the fiber capacity C · W total gives the utilization due to
DATA packets, which is denoted by U allD, as
λ · EL;H:(2)
C · W total
2) ACK and TCP control packets (SYN, FIN): SYN and
FIN control packets are regarded as types of ACK
packets in the analysis, as their only difference from
ACK is the setting of a control flag in the TCP segment
header. The calculation of utilization of a link due to
ACK packets is a bit difficult. Delayed ACK strategy
causes an ACK to be sent for every other full-size segment, so the number of generated ACK packets isaround half of the number of DATA packets in a large
file transfer. However, many short flows end up sending only one or two DATA packets, causing an ACK to
be generated for each DATA packet. Moreover, a delayed ACK strategy may change in future variantsof TCP. Let SA be the average size of ACK packets
and d be the ratio of the number of ACK and DATA
packets on an outgoing link estimated from trafficstats after each control period. As the number of
ACK packets is d times the number of DATA packets
and the size ratio of ACK and DATA packets isSA ∕SD , the ratio of the amount of traffic due to
ACK and DATA packets is d · SA ∕SD . Multiplying
this ratio with λ · EL;H , which is the incoming traffic
rate of an outgoing fiber due to DATA packets, gives
the incoming traffic rate of an outgoing fiber due to
ACK packets. Dividing this by the fiber capacity C ·W total gives the utilization due to ACK packets, which
is denoted by U allA, as
U allD 

U allA 

d · SA ∕SD  · λ · EL;H
:
C · W total(3)308 J. OPT. COMMUN. NETW./VOL. 9, NO. 4/APRIL 2017

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The utilization of a fiber (and also a wavelength) in apure packet switching network, which is denoted by
U all , can be calculated by summing the utilization
due to DATA and ACK packets, calculated by
Eqs. (2) and (3), as
U all  U allD  U allA ;λ · EL;H  d · SA ∕SD  · λ · EL;H
U all :C · W total

(4)

(5)

As given in Ref. [13], in a bounded Pareto flow size distribution with minimum flow size L and maximum flow
size H, the probability that the size of a flow is less than
the threshold t isPrX < t 

1 − Lα t−α:
1 − L∕Hα(6)

The types of traffic carried over the packet subnetworkby DOWFAF and their utilization are as follows:
1) DATA packets of flows shorter than t: These packets arecarried over the packet subnetwork. Let W packet be thenumber of wavelengths assigned to the packet subnetwork at a time. The network changes W packet according
to the traffic, so it is not fixed. The probability that aflow is shorter than t is PrX < t, so the DATA
traffic rate of flows shorter than t can be calculated
by multiplying this probability with the overall
arrival rate of DATA packets and their mean size:
PrX < t · λ · EL;t . The average utilization of packet
wavelengths due to the DATA packets of flows shorter
than t, which is denoted as U hybD , can be calculated by
dividing this traffic rate by the total capacity of thepacket wavelengths:
U hybD 

PrX < t · λ · EL;t
:
C · W packet(7)2) ACK packets: The ACK traffic rate on a link was foundpreviously in the pure packet switching architecture
as d · SA ∕SD  · λ · EL;H . As all ACK packets are carried in the packet subnetwork, the utilization due to
ACK packets denoted by U hybA can be found by dividing this traffic rate by the total capacity of the packet
wavelengths:
U hybA 

d · SA ∕SD  · λ · EL;H
:
C · W packet(8)3) DATA packets of flows that are larger than t but directly forwarded to the packet subnetwork due to lackof flow size information: Let req be the probability thata flow requests to be carried over the path subnetwork.
Then the probability that a flow is larger than t anddoes not request to be carried over the path networkis 1 − req · 1 − PrX < t. Multiplying this probability with the arrival rate of DATA flows on this fiber,

which is λ, and further multiplying with the mean
size of flows larger than t, which is Et;H , gives the
traffic rate of DATA packets of flows that are larger
than t but directly forwarded to the packet subnetwork
due to lack of flow size information. Its utilization,
denoted by U hybDi, can be found by dividing this traffic
rate by the total capacity of the packet wavelengths:
U hybDi 

1 − req · 1 − PrX < t · λ · Et;H
:C · W packet(9)4) DATA packets of flows that are larger than t and contain the size information but failed in reserving apath wavelength and thus are assigned to a packet
subnetwork: The architecture tries to keep the
maximum reservation blocking rate in the path subnetwork around a threshold parameter denoted as
T B , which will be explained in Section II.C. The
probability that a flow is larger than t and carries
flow size information but fails reservation isT B · req · 1 − PrX < t. Multiplying this probability
with the arrival rate of DATA flows on this fiber,
which is λ, and further multiplying with the mean sizeof flows larger than t, which is Et;H , gives the traffic
rate of DATA packets of flows that are larger than t
and carry flow size information, but that failed reservation. Its utilization, which is denoted by U hybDf , canbe found by dividing this traffic rate by the total
capacity of the packet wavelengths:
U hybDf 

T B · req · 1 − PrX < t · λ · Et;H
:C · W packet(10)5) Partial data sent to the packet subnetwork until theflows that are larger than t succeed in reserving a path:
If this traffic is kept low by choosing a low T B, retrial
count, and back-off time, it can be omitted in theanalysis.
6) Retransmissions due to packet losses in the packet
subnetwork: The packet drop rate in the packet
switching optical networks is expected to be low, so thisextra traffic can be omitted in the analysis.
7) Other packet types, e.g., control packets like ARP,
ICMP, and UDP traffic: The majority of Internet traffic
is TCP. Unless other types of packets have a significant
amount of traffic in the network, they can be carried in
the packet subnetwork together with TCP traffic andomitted in the analysis. Otherwise, they may be carried on dedicated and fixed packet wavelengths thatare not controlled by DOWFAF so that they do notaffect the control framework.
The total utilization per wavelength in the packet subnetwork, which is denoted by U hyb, can be estimated bysumming the utilization due to these traffic types:
U hyb  U hybD  U hybA  U hybDi  U hybDf :(11)

We want to calculate the flow size threshold, which gives
equal utilization on a link in the packet subnetwork ofAlparslan et al.

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DOWFAF and a pure packet switching network architecture. Therefore, we equalize the utilization of the packet
subnetwork of DOWFAF and a pure packet switching
network architecture by
U all  U hyb :(12)Equalizing the utilization of the packet subnetwork of
DOWFAF and the pure packet switching network also
implies equalizing them to the utilization of the path subnetwork. After expanding Eq. (12) and applying reduction,
we getW packet · EL;H · 1  d · SA ∕SD  W total · PrX < t · EL;t  d · SA ∕SD  · EL;H 1  req · T B − 1 · 1 − PrX < t · Et;H ;(13)which gives the flow size threshold t at a selected packet
wavelength count W packet . This equation does not dependon C and λ, so it is independent of the wavelength capacity
and the flow arrival rate to a link. Moreover, the equation
does not require the traffic matrix (traffic among source–
destination pairs) in the network. The parameters in theequation can be estimated and used according to traffic
statistics by sampling the packets passing through thelink. In the case of difficulty in estimating the ACK
traffic, the ACK packets may be carried on dedicatedand fixed packet wavelengths so that they do not affectthe analysis. In that case, d in Eq. (13) is set to zero.
By solving Eq. (13), t is calculated for each link, and thelowest one is applied to the network. The value of t decreases with decreasing req and increasing T B and d.
The TCP DATA packets are the main sources of traffic
on the links, and the controller tries to keep the ratio of
DATA traffic on the packet and path subnetworks the sameat all links by applying the same t throughout the network.
However, the ratio of ACK packets is higher on some links,
which increases their d and causes them to have a lower tthan other links. Moreover, the packet subnetwork of somelinks may be carrying more large flows that failed reservation due to high utilization, which causes them to have alower effective t than other links. By choosing the lowest
t among the t values calculated for each link, the network
equalizes the utilization of the corresponding pure packet
switching network and path and packet subnetworks of thelink that has the highest ratio of extra traffic. As a result,the flows on the packet subnetwork of this link are not discriminated when compared with a pure packet switching
network. On the other hand, when the lowest t is applied,
the links with a higher calculated t will have a lower utilization in their packet subnetworks compared to a pure
packet switch, so the flows on these links can get evenhigher goodput than in pure packet switching.

C. Dynamic Wavelength Allocation
When the t value calculated by the analysis decreases,
this allows more flows to be carried on the path subnetworkand to benefit from the advantages of using a dedicatedpath. However, this decreases the average flow size inthe path subnetwork. The packet switching wavelengths
can achieve a high level of utilization independent of theflow size distribution, due to the high granularity multiplexing of packets of many flows on the same wavelength.
However, the path switching suffers from wavelength reservation signaling delays and the slow convergence speedof TCP. The short flows decrease the utilization efficiency
in the path subnetwork, because most of the wavelength
capacity is wasted until a TCP flow increases its congestion
window size high enough to achieve high utilization on afast wavelength. Moreover, a wavelength becomes idle
during the signaling and switch configuration when establishing a path reservation, so the utilization efficiency
decreases as the reservation frequency increases due toshorter flows. This low efficiency may cause a high reservation blocking rate in the path subnetwork, even when theutilization is low. As a result, some of the flows larger than t
may fail path reservation and end up using the packet subnetwork and further increase the utilization and the packet
drop rate in the packet subnetwork, which decreases the
goodput of flows. Therefore, it is necessary to find the optimum number of path switching wavelengths that can
carry the maximum number of large flows while not passing the path blocking threshold and not increasing the utilization of the packet subnetwork more than the utilizationof a pure packet switching network.
As stated before, it is difficult to estimate the goodput of
TCP flows, as new TCP variants have different congestion
control algorithms. Moreover, the goodput depends on
many factors, like the packet drops that the flow experiences outside the optical domain and the processing speed ofend hosts. Therefore, we apply a heuristic algorithm withfeedback control in order to detect the maximum number ofwavelengths that can be allocated to the path subnetwork.
DOWFAF changes the ratio of wavelengths for path andpacket subnetworks according to the recent reservation
blocking statistics in the path subnetwork, so it can adapt
itself to varying traffic without requiring the traffic matrix
information. In the optical domain, all nodes use the same
wavelength ratio and t, so a centralized control is easier toimplement. In the network, one of the routers acts as acenter node, which decides on both parameters. All nodes
in the optical network send the path reservation successrate information to the center node by control packets, periodically. The architecture does not need the traffic matrix
or the total traffic arrival rate information. Other parameters like the flow size distribution, the req parameter, andthe ratio of forward DATA and reverse ACK packets onlinks are estimated from the traffic statistics on a link
and sent to the center node only when there is a drasticchange. The center node decides the wavelength ratio
and the flow size threshold to be used network-wide in
the next control period and informs the other nodes bycontrol packets.
If the center node decides to convert some packet switching wavelengths to path switching, the nodes move theflows assigned to these wavelengths to other packet switching wavelengths. If some path switching wavelengths are310 J. OPT. COMMUN. NETW./VOL. 9, NO. 4/APRIL 2017

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TCP packet from an end host or an edge router arrives atthe optical domain, first the hash of certain fields in thepacket header is computed and compared with the hash table in the node. If the computed hash is available in thehash table, this means that the packet is a part of a currently active and known flow. If there is a path established
for this flow according to the hash table info, the packet issent directly to the next node over the path switching wavelength reserved for this flow. When there is packet contention in a packet switching wavelength, the contending
packet is delayed by a fiber delay line (FDL) buffer until
the wavelength is available, or dropped if the delay by
the FDL is not enough to solve the contention.

Fig. 3. Flow chart of the dynamic wavelength allocation
algorithm.

converted to packet switching, the nodes try to reserve newpaths on the remaining switching wavelengths for theflows on the to-be-converted wavelengths. If the reservation succeeds, the remaining packets of a flow are sent
to the new path over another wavelength. Otherwise,
the flow is switched to one of the packet switching wavelengths. The router may retry path reservation for theseflows. When there are no packets left in transit on the
to-be-converted wavelengths, the routers inform the central node and start using these wavelengths according to
the switching paradigm assigned by the center node after
updating their switch configurations.
Unless the traffic matrix is varying too fast or the network has too many wavelengths, a heuristic trial-and-error
algorithm can be enough to optimize the ratio of wavelengths assigned to the path and packet subnetworks
dynamically by using the path blocking rate statistics collected periodically. Figure 3 shows the flow chart of the algorithm used for controlling the wavelength allocation.
Initially, the ratio of packet wavelengths is maximized tobe sure that the network can carry the current traffic.
After a control period, all nodes start sending their stats,like path reservation success rate, to the center node bysending control packets. If the maximum blocking ratein the network is lower than a threshold T B , the controller
node increases the number of wavelengths assigned to thepath subnetwork by converting one of the wavelengths in
the packet subnetwork to the path subnetwork. Also, t isdecreased according to the analytical model. If the highest
blocking rate in the network is greater than T B , one of thepath switching wavelengths is converted to a packet
switching wavelength and t is increased according to theanalytical model.

D. Flow Switching
In order to prevent the packet reordering, which
decreases the TCP goodput, the architecture employs
flow-based switching, so all packets of a flow use the same
wavelength regardless of the switching type. When a new

If the hash is not available in the hash table and thepacket type is SYN or SYN-ACK of TCP, then this meansthat the router received the first packet of a new TCP flow.
The router forwards the large flows to the path switching
subnetwork, so the router should know or detect whether
this is a large flow or not. One possible solution to detect
the large flows is to use the flow stats. The router may initially forward a flow to the packet subnetwork and record
its stats. The router can classify a flow as a large flow andswitch it to the path subnetwork if the total size of thepackets of the flow carried in the network passes a threshold in a time window. However, in this method the flow ends
up using the packet subnetwork until it transfers enoughpackets to pass the threshold, which increases the utilization of the packet subnetwork. Moreover, large flows may
experience packet losses in the packet switching subnetwork, which may limit the growth of the TCP congestion
window size and cause the flows to spend too much timein the packet subnetwork before they transfer enough data
to pass the threshold. Therefore, we consider informing therouters about the size of the flows that are candidates forthe path switching subnetwork. One way to inform therouters about the size of a flow can be that the application
at the end host may write the flow size information to the
TCP header option field or the TCP payload of the initial
SYN or SYN-ACK packet. Moreover, some extra information like the link speed of the end host may be added toeliminate the end hosts that do not have a connection fastenough to achieve high utilization in a path wavelength.
Section 3.4 of IETF RFC 793 allows data to be carried in
the SYN packets [14]. For example, TCP Fast Open protocol, which was added to the Linux kernel between versions
3.6 and 3.7 and was turned on by default in 3.13 and supported by Google Chrome and Chromium browsers, proposes carrying initial data in the SYN and SYN-ACK
packets. Another possible way to detect the flow size inHypertext Transfer Protocol (HTTP) traffic is to check
the Content-Length field in the HTTP header, but this is
more processing intensive.
Another possible way to inform the routers about thesize of a flow is to use software-defined networking(SDN) like OpenFlow [9], which is getting a lot of attention
recently. If SDN is available, the agents at the end hosts
can directly inform the SDN controller about the size ofthe flow and the preferred switching plane (path or packet
switching) instead of writing the flow size information to
the header or payload of SYN or SYN-ACK packets of theAlparslan et al.

VOL. 9, NO. 4/APRIL 2017/J. OPT. COMMUN. NETW. 311

flow. The SDN controller can choose the switching plane andassign the flow based on this information using DOWFAF.
Reference [7] reported that a messaging framework for resource requests and a centralized configuration based on
OpenFlow have already been implemented on OPCInet.
When a new flow arrives at the network, the edge router
tries to reserve a path for the flow inside the optical network
if the flow size information is available and the size is higher
than the current t. The initial TCP packet is buffered in therouter until the reservation attempt concludes. If the reservation succeeds, the router adds the flow to the hash tablewith the path information and the initial TCP packet is sent
over the established path. The ACK packets are carried overthe packet subnetwork. If the reservation fails, the flow issent to the packet subnetwork. The router may back offsome time and retry the path reservation for the flow. If
the retrial succeeds in reserving a path, the flow switches
to the established path. Otherwise the flow uses the same
wavelength on the packet subnetwork until it finishes.
The end host does not need to be directly connected to theoptical network. A flow may also traverse other network domains and use the path subnetwork of the optical networkon its route as long as it achieves high goodput. The end hostis not informed about the switching paradigm used for its
packets in the optical network, so the end host uses the same
TCP connection during the data transfer regardless ofwhether the path or packet subnetwork is used. If the application does not want the flow to be a candidate for the pathsubnetwork, it does not write flow size information to theinitial packet. The architecture directly assigns these flows
to the packet subnetwork. It is possible that a path wavelength is reserved for a flow upon the request of the application, but the flow cannot achieve high utilization in thepath wavelength due to a problem like processing limitations of the end host, behavior of the application at theend host, or a high packet drop rate on a link between
the end host and the optical network. The edge node, where
the packets of this flow enters the optical network, samples
the incoming traffic rate and reassigns such flows to thepacket subnetwork by tearing the path connection.
For path reservation, we used destination-initiated reservation (DIR), which is one of the most popular reservation
algorithms in the literature [15]. Resource Reservation
Protocol with extensions for traffic engineering (RSVPTE) [16] signaling protocol in generalized multiprotocollabel switching [17] networks uses DIR for wavelength reservation [18].

with 14 nodes and 21 bidirectional links, shown in Fig. 4, by
using a packet-based simulator that we implemented.
There were 80 wavelengths per link. Each wavelength
had a speed of 1 Gbps, so the total capacity of a link
was 80 Gbps. Although WDM allows much higher wavelength speeds, we used 1 Gbps due to simulation time restrictions. Link delay was 10 ms. Shortest-path routing wasapplied. The maximum and the average hop count were 3
and 2.1, so the maximum and average round trip time(RTT) were around 60 and 42 ms. The minimum flow size
was 1000 bytes, and the maximum flow size was 50 Gbytes.
The bounded Pareto shape parameter was 1.01. We applied
the traffic demand matrix in [19] with shortest-path routing. The flows between each node pair arrived according toa Poisson process. The core links used FDL buffering with
30 delay lines and the FDL granularity was 500 bytes, so
the maximum buffer size was 15 Kbytes per wavelength. If
a packet contention was not solvable with the available
buffering space, the contending packet was dropped. The
maximum packet size was 1500 bytes. At this FDL configuration, the maximum achievable wavelength utilization
was around 60% due to the voids occurring in FDLs. T B
was selected as 0.05. The total simulation time was
220 min. The maximum number of path reservation trials
was three times. The back-off time between reservation retrials was 300 ms. We used the CUBIC TCP code from
Ref. [11] written for ns-2 [20] and updated its code touse in our simulator.
The maximum flow size threshold was calculated by using the analytical model in Section II.B. Figure 5 shows thethreshold for req  1 and req  0.6 values calculated bysolving Eq. (13) using the Mathematica software package[21], which solved the equation almost instantly. The x axis
is the number of path wavelengths and the y axis is t, whichis the maximum flow size threshold, on a log scale. As seenin the figure, t decreases as req decreases.
In order to evaluate the transient wavelength allocation
ratio in DOWFAF, the total traffic applied to the network
was varied, as shown in Fig. 6. Initially, the total flow
arrival rate was set to 400,000 flows/s. At 100 min, thetotal traffic was increased to 1,000,000 flows/s to simulate
an instant traffic surge and network congestion. When
1,000,000 flows/s traffic was applied to a pure packet wavelength switching network, the expected utilization level ofthe most congested link in the network was around 30%.
Most service providers operate their backbone networks
at lower utilization [22]. Finally, the traffic decreased to aIII. EVALUATIONThis section discusses the performance of DOWFAF byevaluating the transient wavelength allocation ratio, thegoodput of TCP flows, and the cost and power consumptionof the architecture.

8
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6

We evaluated the performance of DOWFAF on the
National Science Foundation Network (NSFNET) topology

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A. Simulation Settings
2

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Fig. 4. NSFNET topology.

312 J. OPT. COMMUN. NETW./VOL. 9, NO. 4/APRIL 2017

Alparslan et al.
Path Wavelength Distribution

11

1x10

10

50

DOWFAF-1
DOWFAF-0.6

req=1
req=0.6

Path Wavelength Count

Maximum Threshold (Bytes)

Flow Size Threshold
1x10

1x109
8

1x10

7

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100,000
10,000
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0

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Number of Path Wavelengths

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Time (min)(a)
Fig. 5. Maximum flow size threshold.
Packet Wavelength Distribution

Two traffic scenarios were simulated. In the first scenario, all large flows carried the flow size information(req  1). In the second scenario, only 60% of the flows carried the flow size information, so 40% of the flows weredirectly forwarded to the packet subnetwork (req  0.6).

B. Simulation Results

80

Packet Wavelength Count

mid-level of 600,000 flows/s at 160 min. In a single simulation, more than 8 × 109 TCP flows were carried in the
network.

70

60

50

40
DOWFAF-1
DOWFAF-0.6

300

All wavelengths were shared by the path and packet subnetworks, like the blue wavelength in Fig. 2, so the maximum required number of wavelengths for the OCS-SW and
OPS-SW can be evaluated by looking at the transient wavelength allocation ratio. Figure 7 shows the number of wavelengths allocated to the path and packet subnetworks by
DOWFAF for req  1, denoted by DOWFAF-1, and forreq  0.6, denoted by DOWFAF-0.6. Initially, all wavelengths were allocated to the packet subnetwork, so the
network operated like a pure packet switching architecture. At the end of each control period, the controller node
received the path blocking rate information from all nodes.

50

100

150

200

Time (min)(b)
Fig. 7. Number of wavelengths allocated to the path and packet
subnetworks by the integrated architecture versus time.

At first, the path subnetwork carried the assigned large
flows with a low blocking rate because t was high.
Figure 8 shows the value of t on a log scale versus time.
As t was high, the congestion control algorithm of theassigned TCP flows had enough time and data to increase
the congestion window size to fully utilize the path

Traffic

Flow Size Threshold
11

1x106

DOWFAF-1
DOWFAF-0.6

10

1x10

Threshold (Bytes)

Flow Arrival Rate (flows/s)

1x10
Traffic

800,000

600,000

400,000

200,000

1x10

1x108
1x107
1x10

0

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6

100,000
0

50

100

150

200

Time (min)Fig. 6. Flow arrival rate at the network versus time.

0

50

100

150

Time (min)Fig. 8. Flow size threshold versus time.

200

Alparslan et al.

The congestion in the network was relieved by decreasing the traffic to 600,000 flows/s at 160 min. The utilizationand the reservation blocking rate of the path subnetwork
decreased, so the controller node again converted thepacket switching wavelengths to path switching while
decreasing t.
Figure 7 shows that when the traffic arrival rate was increased suddenly at the end of the first epoch, the biggest
change in the wavelength distribution occurred with
DOWFAF-1, where the system adapted to the new trafficand stabilized by increasing the number of packet wavelengths from 39 to 52 in only 7.5 min. In the worst-case scenario, if the number of packet switching wavelengths
increases from zero to the maximum possible number ofpacket switching wavelengths, the minimum time required
to change the wavelength distribution would be
0.5 × 80  40 min , when the control period is 0.5 min,the maximum number of packet wavelengths is 80, andthe number of converted wavelengths at each control
period is one. The convergence speed of the trial-and-error
algorithm can be increased by decreasing the control period
time and/or increasing the number of wavelengths converted after each period. If the traffic is varying too fastor the network has too many wavelengths, more advanced
wavelength allocation algorithms may be employed.
Designing such advanced wavelength allocation algorithms is left to future work.

Link Utilization
0.5

0.4

Utilization

When the incoming traffic was increased to
1,000,000 flows/s after 100 min, the system started to sendmore flows to the path subnetwork, which caused the path
blocking rate to pass T B . The controller responded by decreasing the number of path switching wavelengths by converting them to packet switching wavelengths. As thenumber of path wavelengths decreased, the correspondingt and the average flow size in the path subnetwork increased. As larger flows utilized the path wavelengths more
efficiently, the path subnetwork started to operate at higher
utilization while satisfying the blocking rate threshold.

Figure 9 shows the average utilization rate of the wavelengths on the fiber from node 4 to 3, which is the most congested fiber in the network. As there are many simulation
results, the results are split into two subfigures [Figs. 9(a)and 9(b)]. As a reference, the utilization estimated from theapplied traffic matrix, which is denoted as applied, is plotted in both subfigures. It is calculated by dividing the sumof the sizes of the incoming flows in a 500 s time window by
the fiber capacity. It assumes that incoming flows werecarried instantaneously. Therefore, applied shows the
optimum utilization that can be achieved. Figure 9(a)compares DOWFAF with two extreme cases of wavelength
distribution. The first one is pure-packet, which shows theaverage utilization when the traffic pattern was simulatedwith a pure packet switching network. The second one is79-1 path, which shows the average utilization when 79
wavelengths were dedicated to path switching and a single
wavelength was dedicated to packet switching. As control
packets require a packet switching wavelength, it is notpossible to simulate a pure path switching network, but
a close result is obtained by simulating with 79 path wavelengths. As the blocking rate was high due to the low efficiency of the path wavelengths and a single packet
wavelength was not able to carry all blocked flows, the

0.3

0.2

applied
DOWFAF-1
DOWFAF-0.6
pure-packet
79-1 path

0.1

0
0

50

100

150

200

Time (min)(a)
Link Utilization
0.5

0.4

Utilization

wavelengths. Moreover, the time wasted during the configuration and reservation in the path subnetwork wasmuch lower than the flow duration, so the utilization efficiency was high. While the maximum blocking rate was below T B, the architecture kept converting the wavelengths
from packet switching to path switching. As the numberof path wavelengths increased, t calculated by the analytical model decreased and allowed more flows to benefit fromthe dedicated path wavelengths. However, the efficiency ofthe path subnetwork decreased as t decreased. The capacity wasted during the TCP slow start and the path reservation phase became significant. Therefore, the path
reservation blocking rate started to oscillate around thepath blocking rate threshold T B and the system stabilized.
Figure 5 shows that t per path wavelength decreases withdecreasing req. Lower t causes the wasted capacity to behigher when req is lower at the same number of path wavelengths. As a result, the path blocking rate reached T B
when there were fewer path wavelengths. Therefore, thenumber of path wavelengths was lower when req  0.6,
as seen in Fig. 7(a).

VOL. 9, NO. 4/APRIL 2017/J. OPT. COMMUN. NETW. 313

0.3

0.2

0.1

applied40-40-wot
40-40-wt-1
40-40-wt-0.6

0
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50

100

Time (min)(b)
Fig. 9. Link utilization.

150

200

314 J. OPT. COMMUN. NETW./VOL. 9, NO. 4/APRIL 2017

Figure 9(b) shows the average utilization rate of the
wavelengths when the wavelength distribution is fixed
to an equal separation of 40 path and 40 packet wavelengths. 40-40-wot shows the case when there is no flow
size threshold, so all incoming flows first try to reserve
one of the 40 path wavelengths and use a packet wavelength in case the reservation fails. 40-40-wt-1 and40-40-wt-0.6 show the cases when the flow size thresholdcalculated by DOWFAF for a 40–40 wavelength distribution is applied when 100% and 60% of the flows carry
the flow size information, respectively. The only difference
between 40-40-wt and DOWFAF simulations is that the
number of path and packet wavelengths is fixed to 40 in
40-40-wt, so it shows the case when the wavelength distribution and the threshold are not updated according to thetraffic. Again, applied is plotted as a reference. Figure 9(b)In order to show that DOWFAF can equalize the utilization of path and packet subnetworks by distributing theflows effectively, the transient utilization of path andpacket wavelengths on the fiber from node 4 to 3 are plottedin Figs. 10 and 11 for req  1 and req  0.6, respectively. As
this was the most congested fiber, DOWFAF was expected
to equalize the utilization of this fiber in the path andpacket subnetworks of DOWFAF and a pure packet switching network by solving Eq. (12) and applying the calculated
t network-wide. The average utilization of the packet andpath wavelengths of each simulation are denoted as packet
and path, respectively. The path utilization is calculated by
the total amount of data carried on the path wavelengths ina time window. Figure 10 shows the first scenario when alllarge flows carried the flow size information (req  1).
As there are many simulation results, again the
results are divided into two subfigures [Figs. 10(a) and10(b)]. Figure 10(a) reveals that DOWFAF-1 path and
DOWFAF-1 packet by DOWFAF and applied had similar
utilization. Both DOWFAF-1 path and DOWFAF-1 packet
Wavelength Utilization (req=1)
0.5
applied
DOWFAF-1 packet
DOWFAF-1 pathpure-packet
79-1 path

0.4

Utilization

When pure-packet and applied are compared, we see thatpure-packet had a bit lower utilization at the beginning, butconverged to applied at the end of the first epoch. The reason for slow convergence was the slow increase in the TCP
congestion window of the large flows due to the packet
drops in the packet switching network. Therefore, the utilization in the pure-packet architecture converged to applied only after many large flows accumulated on thelink over a wide time span. The problem became more visible after the traffic increased in the second epoch. As thepacket drop rate increased further, the congestion windowof the TCP flows increased more slowly, so it could not converge to the utilization level of applied in the second epoch.
In the third epoch, there were already many large flowsthat had accumulated from the previous epochs, so purepacket converged faster in this epoch. When we checkthe result of DOWFAF, we see that the simulation with
DOWFAF-1 converged to applied in a short time. As
DOWFAF carried most of the large flows on dedicatedwavelengths, the large flows increased their congestion
windows faster and finished sending their data in a shorttime, so the utilization closely followed the pattern of applied. Forty percent of the flows were directly forwarded tothe packet subnetwork due to lack of flow size information
in DOWFAF-0.6. Even though the packet drops degraded
the performance of the large flows directly forwarded tothe packet subnetwork, the majority of the large flows still
benefited from the dedicated wavelengths, so the utilization in the second epoch was higher than pure-packet.
Finally, 79-1 path, which shows the average utilization
when 79 wavelengths were dedicated to path switching
and a single wavelength was dedicated to packet switching,gave a very low utilization with an average of 0.0018, so it
is difficult to see in the graph. While DOWFAF controlled
the ratio of wavelengths to keep the maximum blocking
rate in the network around 0.05, the maximum blocking
rate was around 0.995 when the path wavelength count
was fixed to 79. Most of the flows that succeeded reservation could not utilize the capacity of path wavelengths dueto their small size and caused extremely low utilization.

shows that none of the simulations could achieve the samelevel of utilization as applied and DOWFAF when the incoming traffic rate was 1,000,000 in the second epoch.

0.3

0.2

0.1

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50

100

150

200

Time (min)(a)
Wavelength Utilization (req=1)
0.5
applied40-40-wot packet
40-40-wot path40-40-wt-1 packet
40-40-wt-1 path

0.4

Utilization

flows that failed reservation were deleted from the networkin 79-1 path. Only the control packets of DOWFAF and the
ACK, SYN, and FIN packets of TCP flows were carried overthe single packet wavelength.

Alparslan et al.

0.3

0.2

0.1

0
0

50

100

150

200

Time (min)(b)
Fig. 10. Comparison of wavelength utilization when req  1.

Alparslan et al.

Figure 10(b) shows that when the distribution of wavelengths was fixed to an equal distribution of 40 path and 40
packet switching wavelengths, the network could not balance the utilization of path and packet subnetworks. When
there was no flow size threshold set, the path subnetwork
experienced very low utilization (40-40-wot path) due tolow efficiency, so most of the flows ended up being carriedin the packet subnetwork, which caused very high utilization in the packet switching wavelengths (40-40-wot
packet) compared to applied. When the flow size threshold
was set according to the DOWFAF analysis at req  1 and40 path wavelengths, which was also the average numberof path wavelengths that DOWFAF converged to in
the first epoch, the path (40-40-wt-1 path) and packet(40-40-wt-1 packet) subnetworks achieved almost equal
utilization in the first epoch as expected. However, as
the flow arrival rate increased in the second epoch, by applying the same flow size threshold, the path subnetwork
caused a high level of path reservation blocking. Blocked
large flows ended up using the packet subnetwork, so
the utilization of the packet subnetwork was much higher
than applied in the second and third epochs.
Figure 11 shows the result of the scenario when 60% of
the flows carried the flow size information (req  0.6).
When req  1, there were few large flows in the packet
subnetwork of DOWFAF, so packet was close to path andapplied. When req is lower, it takes some time for the utilization of packet switching wavelengths to converge to astable level due to the high number of large flows in thepacket subnetwork, which increases their TCP congestion
window size more slowly than in the path switching wavelengths due to packet drops. As expected, DOWFAF-0.6
packet in Fig. 11(a) was a bit lower than DOWFAF-1 packet
in Fig. 10(a) due to slow convergence of large flows, while
DOWFAF-0.6 path and applied were close to each other.
Figure 11(b) shows that when the number of path andpacket wavelengths were fixed to 40 and the flow size
threshold was set to the value calculated by DOWFAF
for 40 path wavelengths and req  0.6 (40-40-wt-0.6), thepath subnetwork got very low utilization with a high blocking rate, which caused high utilization in the packet subnetwork throughout the simulation. The reason was that at
this traffic arrival rate, the number of packet wavelengths

Wavelength Utilization (req=0.6)
0.5
applied
DOWFAF-0.6 packet
DOWFAF-0.6 pathpure-packet
79-1 pathUtilization

0.4

0.3

0.2

0.1

0
0

50

100

150

200

Time (min)(a)
Wavelength Utilization (req=0.6)
0.5
applied40-40-wot packet
40-40-wot path40-40-wt-0.6 packet
40-40-wt-0.6 path

0.4

Utilization

closely followed applied, which is the optimum utilization,and both had a fast convergence to applied when the traffic
rate changed at 100 and 160 min, showing that a good loadbalance was achieved by DOWFAF. The result denoted by79-1 path, which was the average utilization of path wavelengths when 79 wavelengths were dedicated to pathswitching and a single wavelength was dedicated to packet
switching, shows that the utilization of path wavelengths
was very low due to the high blocking rate because of thelow efficiency of the short flows. On the other hand,
DOWFAF kept the maximum blocking rate around T B
by controlling the ratio of path and packet wavelengths
in order to maximize the ratio of path wavelengths andthe ratio of large flows switched to the path subnetwork
while balancing the utilization of path and packet subnetworks and preventing high blocking.

VOL. 9, NO. 4/APRIL 2017/J. OPT. COMMUN. NETW. 315

0.3

0.2

0.1

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0

50

100

150

200

Time (min)(b)
Fig. 11. Comparison of wavelength utilization when req  0.6.

and t should be much higher, as shown in Fig. 7, in order tobalance the utilization of path and packet subnetworks.
Figure 12 compares the average goodput of all TCP flows
larger than 106 bytes. The figures are divided into threeepochs between 50 and 100 min, 120 and 160 min, and180 and 220 min. The first 50 min and also the 20 min after
the traffic rate changed were ignored, because the wavelength allocation was still in transition. The x axis is thelog-scale histogram of the flow size in bytes, while the y axis
is the average transfer time in terms of seconds in log scale.
In the first epoch, as the traffic was low, t was low in all
DOWFAF simulations, as shown in Fig. 8. As a result, awide range of flows were carried in the path subnetworkand their average transfer time was shorter than in thepure packet switching architecture when t > 2 × 106 , asshown in Fig. 12(a). Even though around 40% of the large
flows were carried in the packet subnetwork when
DOWFAF-0.6, the average transfer time of the large flows
in DOWFAF was up to 40 times lower than pure-packet.
In the second epoch, the high traffic arrival rate increasedt, so fewer flows were carried in the path subnetwork of
DOWFAF, as seen in Fig. 12(b). However, the flows in
the path subnetwork continued to transfer much fasterthan in pure-packet. Moreover, the flows in the packet

316 J. OPT. COMMUN. NETW./VOL. 9, NO. 4/APRIL 2017

Alparslan et al.

decreased t adaptively and more flows benefited fromthe path subnetwork.

Average Transfer Time

Transfer Time (s)10,000

1000

100

10

1
DOWFAF-1
DOWFAF-0.6
pure-packet

0.1
1x106

1x107

1x108

1x109

1x10101x1011Size (Bytes)(a)Average Transfer Time

Transfer Time (s)10,000

1000

100

10

Figure 13 shows the goodput histogram of the flows sized
between 108 and 109 bytes in the second epoch between 120
and 160 min. The x axis is the flow goodput split into bins
on a log scale and the y axis is the number of flows in eachbin. Figure 13(a) shows that when req  1, most of thelarge flows in DOWFAF achieved high goodput, as they
were carried in the path subnetwork. Only a negligible
number of large flows, which were carried in the packet
subnetwork due to failed reservation in the path subnetwork, got lower goodput, but their goodput was in the rangeof the goodput of flows in pure-packet. When the wavelength distribution was fixed to 40–40 and the calculated
flow size threshold was applied (40-40-wt-1), the numberof the flows carried in the path subnetwork was lower thanin DOWFAF due to the high blocking rate because of thehigh traffic in the second epoch and low flow size threshold,even though the number of employed path switching wavelengths was higher than in DOWFAF. Therefore, manylarge flows ended up being carried in the packet subnetwork, and their goodput was lower than in pure-packet
due to the higher utilization and packet drop rate in thepacket subnetwork than in pure-packet. Figure 13(b) shows
Goodput Histogram

1

6000

DOWFAF-1
DOWFAF-0.6
pure-packet

0.1
1x106

1x107

1x108

1x109

1x1010Flow Count

Size (Bytes)(b)
Average Transfer Time
10,000Transfer Time (s)5000

1x1011

1000

DOWFAF-1 path
DOWFAF-1 packet
pure-packet
40-40-wt-1 path40-40-wt-1 packet

4000
3000
2000
1000
0
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100

1x10

7

8

1x10

9

1x10

Goodput (bps)10(a)
Goodput Histogram

1

6000

DOWFAF-1
DOWFAF-0.6
pure-packet

0.1
6
1x10

7

1x10

8

1x10

9

1x10

10

1x10

5000

11

1x10

DOWFAF-0.6 path
DOWFAF-0.6 packet
pure-packet
40-40-wt-0.6 path40-40-wt-0.6 packet(c)Fig. 12. Comparison of average transfer time of TCP flows inthree epochs.

subnetwork achieved almost the same goodput as in purepacket, which shows that DOWFAF succeeded in equalizing the utilization of the packet subnetwork, the pathsubnetwork of DOWFAF, and the pure packet switchingarchitecture. Therefore, many large flows achieved up to
40 times higher goodput without discriminating the shortflows, which achieved similar goodput as in the pure packet
switching architecture. Figure 12(a) shows that when theflow arrival rate decreased in the third epoch, DOWFAF

Flow Count

Size (Bytes)
4000
3000
2000
1000
0
6
1x10

1x10

7

8

1x10

9

1x10

Goodput (bps)(b)
Fig. 13. Comparison of the average goodput distributions in thepure packet switching architecture and DOWFAF.

Alparslan et al.

VOL. 9, NO. 4/APRIL 2017/J. OPT. COMMUN. NETW. 317

that when req  0.6, the number of flows with high goodput
in DOWFAF-0.6 path was lower than in DOWFAF-1 path,because 40% of the large flows were directly sent to thepacket subnetwork. As DOWFAF equalized the utilizationof the packet subnetwork of DOWFAF and the pure packet
switching architecture, the flows carried in the packet subnetwork of DOWFAF achieved similar goodput to the flows
in pure-packet. On the other hand, the goodput of the fixedwavelength distribution (40-40-wt-0.6) was much lower
than that of DOWFAF, because the applied wavelength distribution was very different from the optimum distribution
found by DOWFAF-0.6 path.
Finally, Fig. 14 shows the average of the packet loss rateof the flows in the packet subnetwork. Only the flows sized
between 106 and 3 × 106 bytes were included. Larger flows
were not included because they may span multiple epochswith different packet drop rates, and short flows were notincluded because they may finish before entering the congestion avoidance phase. The flows in the path subnetwork
did not experience any packet drops inside the network, sothey were not included either. The figure reveals that there
were no sudden peaks in the packet loss rate when the
wavelength allocation was still in transition when a newepoch started. As the packet subnetwork was underutilized, most of the packet drops were mainly due to packet
contentions in the small-sized FDL buffers. Factors likesynchronization of packet drops, traffic burstiness, andinteractions among TCP flows have a big impact on the rateof packet drops in FDL buffers, so the shape of the packet
drop rate in Fig. 14 is different from the flow arrival rate in
Fig. 6. The packet drop rates of DOWFAF and pure-packet
were very similar. Faster wavelength allocation with moreaggressive algorithms may cause sudden increases in thepacket loss rate, but their evaluation is left to future work.
Some types of traffic were omitted in the analysis basedon the assumption that their traffic ratio is low enough tobe omitted. Their traffic ratios were checked in the simulation. They are the following:
1) Partial data sent to the packet subnetwork until theflows that are larger than t succeed in reserving apath: In the simulations the maximum number ofPacket Drop Rate
0.03

Drop Rate

0.025
0.02
0.015
0.01
0.005

DOWFAF-1
DOWFAF-0.6
pure-packet

0
0

50

100

150

200

reservation trials was three and the back-off timewas 300 ms. Therefore, the maximum duration a large
flow may send partial data to the packet network until
it succeeds in reserving a path is the back-off timespent for two failed reservation trials, which is
600 ms, plus the time spent during the last reservation
control packet exchange, which is one RTT. As the
maximum RTT is 60 ms, the maximum duration forsending partial data is 660 ms. The lowest numberof packet wavelengths was in the first epoch, so theextra traffic had the highest impact in the first epochof the simulation. Figure 12(a) shows that the flows
carried around 106 bytes of data on average in thepacket subnetwork in the first 700 ms. The average
size of flows in the path subnetwork in the first epoch
was around 2 × 107 . Even if all the large flows carriedin the path subnetwork had failed reservation twice
before succeeding the reservation, they would havesent only around 5% of their initial data to the packet
subnetwork. Therefore, this traffic was low enough tobe omitted in the analysis.
2) Retransmissions due to packet losses in the packet
subnetwork: The maximum average packet drop rate
experienced by the flows in the packet subnetwork was
1.9%, so the ratio of retransmitted packets was low
enough to be omitted in the analysis.

C. Cost and Energy Consumption
DOWFAF not only increases the goodput of large flows
but is also capable of decreasing the building cost and theenergy consumption of the switch. Figure 7(b) revealed
that DOWFAF required fewer packet switching wavelengths than a pure packet architecture by carrying someof the traffic over a path switching subnetwork. The cost of
OCS-SW fabric is much lower than OPS-SW fabric, which
requires much faster switching. Moreover, it is difficult tomanufacture OPS-SW fabric with many ports, so even asmall decrease in the number of ports of OPS-SW cangreatly decrease the manufacturing cost of the switch.
Furthermore, as OCS-SW has a much lower switching frequency than OPS-SW, carrying some of the traffic on thepath subnetwork decreases the power consumption of
the switch. The minimum flow transfer time in the pathswitching subnetwork was around 1 s. Compared to an
OPS-SW with a 109 bps wavelength speed, which should
be capable of switching packets with a duration on the order of 10−7 s, the OCS-SW can operate at a much lower
granularity. The low granularity allows the use of a switching technology like in microelectromechanical systems,
which is slower but consumes much less power and is muchcheaper to manufacture [23]. As a result, DOWFAF can decrease the overall manufacturing cost and the energy consumption of a switch.

IV. CONCLUSIONSTime (min)Fig. 14. Average packet drop rate.

In this paper, we proposed a control framework called
DOWFAF for integrated optical architectures combining318 J. OPT. COMMUN. NETW./VOL. 9, NO. 4/APRIL 2017

path and packet switching for future optical networks. We
proposed an analytical model for calculating the flow size
threshold and a feedback control for estimating the wavelength allocation ratio for varying traffic without requiring
the traffic matrix information in the network. The simulation results on a mesh network revealed that DOWFAF
adaptively changed the ratio of path and packet wavelengths in order to balance the utilization of path andpacket subnetworks and to maximize the ratio of large
flows benefiting from the path switching, which greatly increased the goodput of the large TCP flows. Moreover, thearchitecture decreased the power consumption of theswitch by carrying part of the traffic over a low power pathswitching fabric and decreased the manufacturing cost by
decreasing the size of the packet switching fabric.
In future work, we will work on real-world implementation issues. We will explore the effect of flow size distributions on the performance of the architecture. If the traffic isvarying too fast or the network has too many wavelengths,
more advanced wavelength allocation algorithms may be
employed than the trial-and-error method. Designing such
advanced wavelength allocation algorithms is left to future work.

Alparslan et al.
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ACKNOWLEDGMENT
This work was supported by the National Institute ofInformation and Communications Technology (NICT).

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