Selected Papers

Cut-through Optical Path Control
Technology for a Terabit-class Super-network
Kenichi Matsui†, Takeshi Yagi, Yuuichi Naruse,
and Junichi Murayama
Abstract
Based on traffic monitoring data, our cut-through optical path control technology allocates cutthrough optical paths between provider edge routers on routes with a high traffic demand. To optimize
the optical path arrangement, a network control server monitors traffic demand, calculates the optimal
path allocation, and continuously configures the optical paths. Since this server ensures highly efficient
network resource usage, high-performance packet forwarding is achieved economically.

1. Introduction
As described in the first paper [1] in this issue, weare designing a terabit-class super-network (TSN)
architecture that applies IPv6-based connectionlessforwarding to a generalized multiprotocol label
switching (GMPLS) [2] network. In the TSN architecture, traffic is concentrated at electrical provider(P) routers and this could decrease network capacity.
To ease this traffic concentration and keep the forwarding performance high, we apply cut-throughoptical paths. A cut-through optical path is established between provider edge (PE) routers that areexchanging heavy traffic. This path increases the
bandwidth capacity between PE routers because thecapacity of a cut-through optical path is larger thanthat of IPv6 routes passing through electrical P
routers. Moreover, the use of cut-through opticalpaths avoids traffic congestion at the electrical P
routers. As the traffic demand pattern changes, ineffectual, obsolete cut-through optical paths are deleted
and new ones are established, improving the efficiency of packet forwarding. Therefore, a TSN network
can achieve the same forwarding performance as a
GMPLS network with fewer optical paths.
To achieve the maximum efficiency, cut-through† NTT Information Sharing Platform Laboratories
Musashino-shi, 180-8585 Japan
E-mail: matsui.kenichi@lab.ntt.co.jp

32

optical paths should be recalculated and reassigned
dynamically according to changes in traffic demand.
Additionally, the reassignment should be optimized
according to traffic demand taking account of the network resources available. There are two issues in
implementing such cut-through optical path control
technology.
The first issue is resource management. At present,
multiprotocol label switching (MPLS) routers monitor traffic demand, exchange the monitored traffic
data, and calculate path allocations and set up thepaths accordingly [3]. If network resources are managed by each PE router in such a distributed manner,
it is difficult to achieve uniform resource allocation.
For example, there is a risk of deadlock in resource
allocation. Thus, this type of network resource management could not provide optimal resource sharing.
The second issue is calculation time. To achieve
highly efficient cut-through optical path, we shouldsolve the combinatorial problem concerning the mapping of IPv6 flows to an optical path [4]. Since thepath calculation time increases in proportion to thenumber of network constraints, it is difficult to finish
a calculation during a dynamic path reassignment.
To solve these problems and provide dynamic andoptimum cut-through optical path control, we propose two methods: a server-based resource management method and a method of grouping IPv6 flows
according to their efficiency. The first method enables
network resources to be managed under a unified polNTT Technical Review

Selected Papers

icy because a network control server (NCS) can manage resources centrally. The second method reduces
the path calculation time because only the effective
IPv6 flows are selected for calculation from among
the massive number of IP flows.
This paper is organized as follows. Section 2 briefly
reviews cut-through optical paths. Section 3
describes their control issues and section 4 proposes
methods that solve these issues. Section 5 describes
the steps in path assignment. Section 6 evaluates theeffectiveness of cut-through optical paths.
2. Cut-through optical paths
2.1 Optical path layouts in a GMPLS network
and TSNIn a GMPLS network, to ensure full reachability
between PE routers by optical paths alone, the paths
would need to be arranged in a mesh topology. Since
the number of optical paths is the same as the squareof the number of PE routers, this full-mesh arrangement is not scalable. In the TSN architecture, IP-inIPv6 overlay networking technology [5] arranges
optical paths in a hub-and-spoke topology with electrical P routers forming the hub nodes and PE routers

forming the leaf nodes. Since the number of opticalpaths is the same as the number of PE routers, thishub-and-spoke arrangement is scalable.
2.2 Cut-through optical path control
In a hub-and-spoke topology, an electrical P routercan become a forwarding bottleneck depending on
the traffic concentration. To ease this traffic concentration and keep the forwarding performance high,we apply cut-through optical paths (Fig. 1). Since a
cut-through optical path is established only between
PE routers that are exchanging heavy traffic, the total
number of optical paths is smaller than in a full-meshtopology. Therefore, when cut-through optical paths
are combined with the hub-and-spoke topology, thearrangement is still scalable. To achieve maximum
efficiency, cut-through optical paths should be reassigned dynamically according to changes in traffic
demand. The reassignment should be optimized
according to traffic demand taking into account the
network resources available. To operate cut-throughoptical paths, we require a control technology that
implements these functions.

Heavy traffic

Electrical
P routerPErouterPErouter

OXC

OXC

PE
routerPErouterOptical layer
IPv6 layer
Service provider network
Optical path

Cut-through optical path

PE router: provider edge router, P router: provider router, OXC: optical cross connect

Fig. 1. Cut-through optical path.

Vol. 2 No. 3 Mar. 200433

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3. Issues in cut-through optical path control
3.1 Problems in network resource management
For the present, some router vendors are implementing the dynamic label switched path (LSP) reassignment mechanism on MPLS routers. They are
doing this by the distributed approach, appending a
traffic engineering function to routers. In a service
provider (SP) network, the MPLS routers arearranged as PE routers and P routers. PE routers areinterconnected by LSPs in a full-mesh configuration.
Each PE router monitors traffic on the terminated

LSPs and exchanges the monitored traffic information by extended routing protocols. To ease the traffic
concentration on P routers, a PE router calculates andsets up an LSP arrangement according to traffic
demand.
Since each router calculates paths separately, it isdifficult to manage all of the network resources undera unified policy. Thus, quick and optimal resource
sharing is impossible. For example, if many routers
try to reserve the same link resource or the same PE
router interfaces, deadlock will result. Figure 2
shows PE router 1 and PE router 2 trying to reserve

LockedPE router 1

PE router 3

Link resource A
Waiting for unlocking

Waiting for unlocking
PE router 2

PE router 4
Link resource B
LockedFig. 2. Deadlock in a distributed resource management.

Forwarding to anelectrical P router
PE router

Forwarding toa PE router

IPv6 flow. Thickness of an IPv6 flow indicates its number of packets.
Default optical path
Cut-through optical path

Fig. 3. Mapping IPv6 flows to optical paths.

34

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link resources A and B simultaneously and ending up
deadlocked. This deadlock will cause a delay in path
setup and degrade the forwarding performance.
3.2 Problems in path calculation
The second problem is in path calculation. Since
optical paths are expensive to operate, the required
number of cut-through optical paths should be
reduced. To achieve highly efficient cut-through optical paths, as many as possible of the IPv6 flows
should be mapped to a cut-through optical path. To
optimize this problem, we should solve the combinatorial problem concerning the mapping of IPv6 flows
to an optical path (Fig. 3). The objective of the combinatorial problem of packing IPv6 packets is to maximize the number of IPv6 packets in an optical path.
The main constraint in the problem is the bandwidthof IPv6 flows [4].
This problem is similar to a knapsack problem*1 interms of finding the most valuable set of IPv6 flows

Step II.
Collecting traffic informationthat fit in an optical path of fixed capacity. The calculation time of the knapsack problem increases with
the number of constraints. To ensure the optical pathcalculation method can efficiently provide dynamic
cut-through optical path assignment, the calculation
time of the problem should be shortened.
4. Proposed methods for cut-through optical
path control
4.1 Server-based path control architecture
To solve the resource management problem, we use
a server-based optical path control architecture. As
shown in Fig. 4, the network control server (NCS)
calculates a path centrally and assigns an optical path.
*1 The knapsack problem is a combinatorial problem. Given items ofdifferent values and volumes, the problem is to select the mostvaluable set of items to fit in a container of fixed volume.

Step III.
Calculating optical path assignment

Step II.
Collecting traffic information

Management network
Network control server (NCS)Electrical
P routerStep IV.
Controlling paths
Step I.
Traffic monitoringPErouterPErouter
Step I.
Traffic monitoring

Step I.
Traffic monitoring

OXC

PE
router

OXC

PE
router

DWDM network
IPv6 network

Step I.
Traffic monitoring

Step I.
Traffic monitoring

Service provider network
Traffic information messages
Path control messages

Default optical path
Cut-through optical path

DWDM: dense wavelength division multiplexing

Fig. 4. Operations of cut-through optical path assignment.

Vol. 2 No. 3 Mar. 2004

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IPv6 flow information table

Electrical P
router No.2

PriorityCount

IP-flow#1

1

10

IP-flow#2

1

9

IP-flow#3

1

8

IP-flow#4

1

IP-flow#5

1

6

IP-flow#6

2

10

IP-flow#7

2

9

IP-flow#82

8

IP-flow#93

10

IP-flow#10

3

9

IP-flow#11

510······

·····

····

IP addr pair

Priority······

7

IPv6 flow group table
IP flow

PriorityCount

IP-flow#1

1

67

IP-flow#2

1

28

IP-flow#3

1

27

1

26

1

25

IP-flow#6

2

29

IP-flow#7

2

20

Count

IP-flow#8

2

10

IP-flow#9

3

20

·····

····

IP-flow#10

310······

·····

····

IP-flow#11

IP addr pair

PriorityCount······

·····

····

······

·····

····

IP-flow#4
IP-flow#5······
Electrical P
router No.3

Group

I

II

4

10

·····

····

Calculate the problem group by group

Electrical P
router No.1

IP addr pair

Dividing IPv6 flows into small groups.

•••

Fig. 5. Grouping IPv6 flows.

Since the server approach makes it possible to manage all of the network resource information and traffic information, it can consider the priority of allpaths and allocate the network resources effectively(Details of Fig. 4 are described in section 5.).
4.2 Grouping IPv6 flows according to theirefficiency
To shorten the calculation time of the combinatorial problem of packing IP flows, we propose a methodthat reduces the number of IP-flows to be calculated.
In this method, we focus on the difference in influence of IP-flows. IP-flows with a high traffic volume
and high priority will improve the efficiency of cutthrough optical paths more than ones with a low traffic volume and priority. Therefore, the efficiency of
cut-through optical paths that are calculated using
only a subset of IP-flows that have high traffic volume and high priority may differ little from the efficiency of those calculated using the whole set of IPflows.
We divide the IPv6 flows into small groups as
36

shown in Fig. 5. First, we make an IPv6 flow list ofeach electrical P router and sort it according to priority and number of IPv6 packets. Second, we merge
these tables and sort the flows and divide them into
small groups. Each group consists of IPv6 flows having the same priority and number of packets. Third,
we sort the groups according to total priority andnumber of IPv6 packets. Then, we start to calculate
the combinatorial problem of packing IP flows fromthe top of the group list. When we finish the calculation for one group, we go on to the next group in thelist. If there are few optical path resources, the calculation is terminated. Since each group contains asmall number of IPv6 flows, the calculation time ofthis step should be shorter.
5. Cut-through optical path assignment
With the methods mentioned in section 3, cutthrough optical path will be reassigned by the following steps: monitoring IPv6 traffic demands, collecting the traffic information, calculating optical path
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assignment, and setting up optical paths (Fig. 4). In
this section, we briefly explain what the NCS, PE
routers, and electrical P routers do during each step.
5.1 Traffic monitoring
Traffic monitoring in TSN is economicallyachieved by use of IP-in-IPv6 connectionless forwarding technology. To suppress the amount of information collected, each PE router counts only the
number of IPv6 packets sent over cut-through opticalpaths; the total number of IPv6 packets from each PE
router sent to electrical P routers is counted at theelectrical P routers themselves. This monitoring
method is discussed in more detail in section 3.1 of[5].
5.2 Collecting traffic information
The NCS periodically accesses electrical P routers
and PE routers and collects the monitored information from them. Then it combines these two sets of
information to provide an overall IPv6 traffic picture.
Additionally, to detect a significant upsurge in thenumber of IPv6 packets in IP flows between pollingintervals, electrical P routers and PE routers send a
trap message to the NCS if the number exceeds athreshold value. When the NCS receives the trap, it
polls the routers immediately to update the present
distribution of IP flows.
5.3 Calculating optical path assignment
Based on the collected traffic information, the NCS
periodically optimizes the cut-through optical patharrangement that maximizes the efficiency of opticalpaths. To do this, first, the NCS calculates the combinatorial problem of packing IP flows with the IPv6
flow grouping method and allocates network resources
optimally. Second, the NCS compares the total number of packets transmitted over the currently assigned
cut-through optical paths with that in the newly calculated ones. It subtracts the former from the latter. Then,
if the difference exceeds a threshold value, the NCS
decides to reassign the cut-through optical paths.
5.4 Setting up cut-through optical paths
When the NCS decides to reassign the cut-though
optical path, it removes one cut-through optical pathand sets up another. To set up a cut-through opticalpath, it orders the ingress PE router to set up an optical path to the egress PE router. The ingress PE routersets up a new one by generating a path setup signaling message. The signaling message will be forwarded by the optical P routers on the optical path route,Vol. 2 No. 3 Mar. 2004

and each optical P router sets up the optical path. In
this way the cut-through optical path is set up. The
details of setting up cut-through optical paths, such as
the cooperative reassignment of IPv6 routes and optical paths, are described in [4].
5.5 Feedback
The above four steps in the operating process arerepeated continually. This iterative operation keeps
high efficiency of cut-through optical paths in spite ofchanges in traffic demands.
6. Evaluation of the effect of cut-through opticalpaths
As described in section 2, the cut-through optical
path control technology can achieve the most efficient optical paths by matching the forwarding capacity of optical paths to traffic demand changes. In thissection, for the same traffic demand situation, we
compare the number of optical paths required for aconventional full-mesh optical path arrangement andthe cut-through optical path arrangement. Throughthis comparison, we evaluate the efficiency of cutthrough optical paths.
6.1 Calculation of the number of optical paths• Notation
Nop total number of optical paths
Npe total number of PE routers
Tij number of IPv6 packets between the nth PE
router (PEi) and PEj
Titotal number of IPv6 packets from PEi
Ndop total number of default optical paths
Ncopi number of cut-through optical paths from PEi
Ncop total number of cut-through optical paths• Nop in a full-mesh optical path arrangement
When optical paths are arranged between PE
routers in a full-mesh topology, the number of optical paths (Nop) is given by
Nop = Npe (Npe – 1).

(1)• Nop in a hub-and-spoke optical path arrangementwith cut-through optical paths
The number of optical paths (Nop) in a hub-andspoke topology is given by the sum of the numberof default optical paths (Ndop) and the total numberof cut-through optical paths (Ncop).
• Calculation of Ndop
A default optical path is assigned from each PE
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router to an electrical P router. Thus, the total number of default optical paths between PE routers andelectrical P routers is the same as the number of PE
routers (Npe). Therefore, Ndop is given by
Ndop = Npe.

(2)

• Calculation of Ncop
A cut-through optical path is assigned according
to the traffic distribution between PE routers. In thispaper, we hypothesize that the traffic distribution
between PE routers follows Zipf’s law. The total
number of IPv6 packets from PEi (Tij) is given by
Eq. (3), where C is a constant that normalizes thevalue of the original Zipf’s law.
Tij =

CTi
1
, where C = N pe
j
1
∑i
i =1

(3)

A cut-through optical path is assigned to a pair ofPE routers that are exchanging a high volume oftraffic. We assume that a proportion α (0 ≤ α ≤ 1) of
the IP flows from PEi to other PEs is accommodated by cut-through optical paths. Equation (4)expresses this situation.
NCOPi∑ Tij = αTi

(4)

j =1

Substituting Eq. (3) into Eq. (4) yields
NCOPi

N pe

1

1

∑ j = α∑ i
j =1

(5)

i =1

Equation (5) is a type of a harmonic progression*2. Since we hypothesize that Npe is large, we
can calculate approximate values of the total number of cut-through optical paths from PEi (Ncopi).
Ncopi is given by Eq. (6) where γ is Euler’s number*3.
NCOPi = e{α (γ + ln N pe ) −γ }(6)optical paths is given by the sum of Eqs. (2) and (6):Nop = Ndop + Ncop
N pe

= N pe + ∑ NCOPi = Npe e{α (γ + ln Npe ) −γ }i =1

6.2 Evaluation
We compared the number of optical paths (Nop) ofa full-mesh optical path arrangement (Eq. (1)) andthat of a hub-and-spoke optical path arrangementwith cut-through optical paths (Eq. (7)). As an example, consider the case where α =0.3. This means 70%of the traffic in an SP network is forwarded via electrical P routers and 30% via cut-through opticalpaths. Figure 6 is a graph showing Nop in the situation. For 10,000 optical paths, the hub-and-spoke
path arrangement can accommodate 1441 PE routerswhile the full-mesh path arrangement can accommodate only 100 PE routers. Thus, cut-through optical
path technology increases the number of PE routersthat can be accommodated by 1441%. This result
indicates that cut-through optical paths improve theforwarding capacity of optical paths.
7. Conclusion
Cut-through optical path control technology provides highly effective dynamic cut-through optical
path assignment. The network control server monitors all of the traffic information using a small
amount of traffic monitored data, assigns network
resources optimally, and calculates the cut-throughoptical path allocation in a short time. Applying thistechnology to a terabit-class super-network willachieve high-performance packet forwarding andeffective network resource utilization.
8. Acknowledgment
This research was supported by a grant from the
Telecommunications Advancement Organization ofJapan (TAO).
*2 Harmonic progression (Hn) is a progression of terms whose reciprocals from an arithmetic progression.
Hn is defined as Hn = 1 +• Nop in a hub-and-spoke optical path arrangementwith cut-through optical paths
The number of optical paths (Nop) in a hub-andspoke optical path arrangement with cut-through38+1 . (7)

1 ... 1
+ + , where n is an integer.
2
n

1
 1

*3 Euler’s number ( γ ) is defined as γ = lim 1 + + ... + − log n .
n →∞ 
2n
Hn can be approximated as Hn = γ + logn if n is large.

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108
Hub-and-spoke assignment with cut-through optical paths
Full-mesh assignment

106

105

10,000 optical paths

Number of optical paths (Nop)

107

104

103

102
100 PE routers
101 1
10

1441 PE routers

2

3

10

10

104

Number of PE routers (Npe)Fig. 6. Comparison of the number of optical paths.

References
[1]

[2]

[3]

J. Murayama, K. Matsui, K. Matsuda, and M. Makino, “A Conceptual Design for a Terabit-class Super-networking Architecture,” NTT
Technical Review, Vol. 2, No. 3, pp. 12-20, 2004.
A. Benerjee, J. Drake, J. Lang, B. Turner, D. Awduche, L. Berger, K.
Kompella, and Y. Rekhter, “Generalized multiprotocol label switching: An overview of signaling enhancement and recovery techniques,” IEEE Commun. Mag., pp. 144-151, Jul. 2001.
X. Xipeng, A. Hannan, B. Bailey, and L. M.Ni, “Traffic engineering

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[4]

[5]

with MPLS in the Internet,” Network, IEEE, Vol.14, issue 2, pp. 2833, Mar./Apr. 2000.
K. Matsui, T. Sakurai, M. Kaneda, and H. Ishii, “A Multi-Layered
Traffic Engineering Architecture for the Electrical/Optical Hybrid
Network,” Proc. of 2003 IEEE Pacific Rim Conference on Communications, Computers and Signal Processing (PACRIM’03), Victoria,
B.C., Canada, Vol. II, pp. 293-296, Aug. 2003.
Y. Naruse, T. Yagi, K. Matsui, and J. Murayama, “IP-in-IPv6 Overlay Networking Technology for a Terabit-class Super-network,” NTT
Technical Review, Vol. 2, No. 3, pp. 21-31, 2004.

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40

Kenichi Matsui

Yuuichi Naruse

Secure Communication Project, NTT Information Sharing Platform Laboratories.
He received the B.E. degree in informationengineering and the M.S. degree in information
sciences from Tohoku University, Sendai, Miyagi in 1995 and 1997, respectively. He joined
NTT in 1997. His work focuses on IP networking
and his research interests include traffic engineering for optical IP networks and MPLS, ondemand QoS management, and managed IP multicast platforms. He is a member of the Instituteof Electronics, Information and Communication
Engineers of Japan (IEICE), the Information Processing Society of Japan, and the IEEE Computer Society.

Research Engineer, Secure Communication
Project, NTT Information Sharing Platform Laboratories.
He received the B.S. and M.S. degrees inphysics from Tohoku University, Sendai, Miyagi
in 1990 and 1992, respectively. He joined NTT
in 1992. He has been engaged in R&D of ATMLAN systems, Fibre-Channel network systems,
and IP-VPN service platforms.

Takeshi Yagi

Junichi Murayama

Secure Communication Project, NTT Information Sharing Platform Laboratories.
He received the B.E. degree in electrical andelectronic engineering and the M.S. degree inscience and technology from Chiba University,
Chiba in 2000 and 2002, respectively. He joined
NTT in 2002. His studies focus on IP-VPN architecture and his current research interests include
IP routing and forwarding technology, trafficmonitoring technology, and layer cooperation
technology. He is a member of the Institute ofElectrical Engineers of Japan (IEEJ) and IEICE.

Senior Research Engineer, Secure Communication Project, NTT Information Sharing Platform Laboratories.
He received the B.E. and M. E. degrees in electronics and communication engineering fromWaseda University, Tokyo in 1989 and 1991,
respectively. Since joining NTT in 1991, he has
been engaged in R&D of ATM networks, largescale IP networks, and IP-VPN service platforms. He is a member of IEICE.

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