Destination-initiating path restoration protocol for wavelength-routed WDM networks

J. Zheng and H.T. Mouftah

Abstract: Network survivability has been a crucial concern in wavelength-routed WDM networks. Due to the huge transmission capacity, a single network failure may cause a large amount of data loss, which would greatly degrade and even disrupt network services. In the paper, a destination- initiating path restoration protocol is proposed for surviving single-link failures in wavelength- routed WDM networks. Unlike existing path restoration protocols, the proposed protocol allows the destination node of a broken connection to initiate a connection restoration process. The objective is to reduce the connection restoration time so that a backup path can be provisioned rapidly for each broken connection that traverses a failed link. The major procedures of the protocol are described and its performance in terms of the connection restoration time is evaluated.

1 Introduction

Wavelength-routed WDM networks have been widely considered to be the potential network architecture for future core networks [I]. Due to the huge transmission capacity, however, a single network failure may cause a large amount of data loss, which would greatly degrade and even disrupt network services. For ths reason, network survivability has been a crucial concern in wavelength- routed WDM networks. To survive different types of network failures (e.g. a fibre cut or a node fault), a variety of optical-layer protection and restoration schemes have been proposed with the objective to provision backup paths rapidly and utilise network resources efficiently [2-51. All these schemes are based on two basic survivability paradigms: pre-configured protection and dynamic restora- tion [5]. In general, pre-configured protection is fast in service recovery but inefficient in resource utilisation, while dynamic restoration is efficient in resource utilisation but slow in service recovery. Accordingly, how to utilise network resources more efficiently with pre-configured protection and how to recover network services more rapidly with dynamic restoration have been a challenge for network designers. In this paper we study dynamic restoration and propose a destination-initiating path restoration protocol for surviving single-link failures in wavelength-routed WDM networks. Unlike existing path restoration protocols, the proposed protocol allows the destination node of a broken connection, rather than the source node, to initiate a connection restoration process. The objective is to reduce the connection restoration time so that a backup path can be provisioned rapidly for each broken connection that traverses a failed link.

0 IEE, 2002 IEE Proceedings online no. 20020306 DOL 10.1049jip-com: 20020306 Paper first received 13th March and in revised form 12th September 2001 The authors are with the Department of Electrical and Computer Engineering, Queen’s University, Kingston, Canada, ON K7L 3N6

2 Background

We consider the architecture of a wavelength-routed WDM network as shown in Fig. 1. It consists of network nodes interconnected by WDM links. Each network node consists of an optical switch that can perform wavelength switchmg optically, and an electronic controller that controls the optical switch. The optical switch can be either wavelength- conversion capable or incapable. The controller maintains network state information (e.g. network topology and wavelength usage) for wavelength routing, which can be either local or global. An access device may be connected to each node, which is used as the interface to a client network. Each WDM link consists of a pair of unidirectional fibre links that operate in WDM with a number of optical channels (or wavelengths) on each fibre link. The controllers communicate with each other over a dedicated optical channel on each fibre link. For clarity and conciseness, we assume no wavelength conversion hereafter; however, the work presented in this paper is applicable to both situations with and without wavelength conversion.

In a wavelength-routed WDM network, a lightpath must be established between a pair of source and destination nodes before data can be transferred. In the occurrence of a network failure, a backup path must be provisioned rapidly for each broken connection to recover the disrupted network services. Path restoration is one of the dynamic restoration schemes for surviving single-link failures. In path restoration, the source and destination nodes of each broken connection that traverses a failed link dynamically

Fig. 1 Architecture of a wavelength-routed WDM network

IEE Proc. Commun., Vol. 149, No. 1, February 2002 18

establish a backup path under distributed control on an end-to-end basis upon a link failure [5]. If no backup path can be established for a broken connection, the connection is blocked. To establish a backup path, existing path restoration protocols allow the source node of a broken connection to initiate a connection restoration process (i.e. source-initiating). Either the forward reservation protocol (FRP) or the backward reservation protocol (BRP) is employed for wavelength reservation.

With FRP [7], the source node (S-node) first performs a route-computing algorithm to decide a route and select a wavelength for the connection. Once a route is decided and a wavelength is selected, the source node sends a REQ packet to the destination node (D-node) along the decided route. At each intermediate node (I-node), the REQ packet tries to reserve the selected wavelength. If the REQ packet cannot reserve the wavelength, a NAK packet is sent back to the source node along the reverse route and the REQ packet is dropped. The NAK packet releases the wave- length reserved by the REQ packet and informs the source node of the reservation failure. If the REQ packet arrives at the destination node, the destination node will send an ACK packet back to the source node along the reverse route and the ACK packet will configure the optical switch at each intermediate node. When the ACK packet arrives at the source node, it implies that the connection has been established successfully and the source node can start to transfer data on it. Fig. 2 illustrates the forward reservation process. The shaded area represents the period during which a wavelength is reserved but not in use. Obviously, a lot of bandwidth on the reserved wavelength is wasted during the reservation period, which would greatly decrease the network resource utilisation.

A simple way to address this problem is to use BRP, as shown in Fig. 3. With BRP [7], the source node first sends a PROB packet to the destination node along the decided route. However, the PROB packet does not reserve any wavelength. Instead, it just collects the wavelength usage information on each link along the route. When the destination node receives the PROB packet, it selects a wavelength and then sends a RESV packet back to the source node along the reverse route. It is the RESV packet that reserves the selected wavelength and simultaneously configures the optical switch at each intermediate node. If the RESV packet cannot reserve the wavelength at an intermediate node, the node sends a FAIL packet to the destination node and a NACK packet to the source node. The FAIL packet disconfigures the optical switches and releases the wavelength already reserved by the RESV packet, while the NACK packet simply informs the source

S-node D-node S-node I-node p NAK

a b

Fig. 2 Forward reservation a Successful b Unsuccessful

D-node

S-node D-node

a

S-node I-node D-node

PROB

b

Fig. 3 Backward reservation a Successful b Unsuccessful

node of the reservation failure. Obviously, this can reduce the bandwidth waste significantly.

With either FRP or BRP, it takes a two-way delay to establish a backup path. A forward control packet (i.e. REQ or PROB) must first be sent to the destination node followed by a backward control packet (i.e. ACK or RESV) sent back to the source node. This may not be the most efficient way in path restoration. In path restoration, a connection restoration process does not necessarily have to be initiated by the source node. Since the destination node can learn related information on a broken connection, such as the source node and the route, it is feasible to allow the destination node to initiate a connection restoration process. This makes it possible to take a one-way delay to establish a backup path and can thus reduce the connection restoration time significantly.

3 Destination-initiating path restoration protocol

In this Section, we propose a destination-initiating path restoration protocol based on the preceding argument. To increase the possibility of restoration success, we introduce a retrying mechanism at the destination node and meanwhlle use a timer at the source node to control the maximum allowed restoration time. Since we only consider single-link failures, we assume that there is one link failure at a time. The route-computing algorithm used in the protocol is beyond the scope of this work, which can be those presented in [8], such as fixed routing, fixed-alternate routing, and adaptive routing. Refer to [8] for details. Accordingly, the major procedures involved in the restora- tion process for a broken connection that traverses a failed link can be described as follows, as shown in Figs. 4 and 5.

a Once the end nodes of the failed link detect the failure, both nodes send a link failure (L-Fail) packet to the source node and destination node of the broken connection, respectively. The L-Fail packet disconfigures the optical switch and releases the wavelength reserved for the connection at each intermediate node on its way.

When the destination node receives the L-Fail packet, it first performs a route-computing algorithm to decide a route from the source node to the destination node and select a wavelength for a backup path. Once a route is decided and a wavelength is selected, the node sends a request (D-REQ) packet to the source node along the reverse route, which carries both the route and wavelength information. At each intermediate node, the D-REQ packet

IEE Proc. Commun., Vol. 149, No. 1, February 2002 19

S-node ____)

S-ACK D-node h

b

Fig. 4 Restoration with a’estinatdon-initiating protocol - backup path _ _ _ _ working path u Successful b Unsuccessful

S-node D-node

data transfer

S-node I-node 0-node

timeout -1 S-REL

b

Fig. 5 Restoration u Successful b Unsuccessful

tries to reserve the selected wavelength, and simultaneously configures the opticaI switch.

0 When the source node receives the L-Fail packet, it first starts a timer that specifies the maximum allowed restora- tion time and then waits for a D-REQ packet from the destination node. 0 If the D-REQ packet cannot reserve the wavelength at an intermediate node, the node sends a negative acknowl- edgement (I-NAK) packet back to the destination node. The I-NAK packet disconfigures the optical switches and releases the wavelength already reserved by the D-REQ packet on its way back. Once the destination node receives the I-NAK packet, it retries. 0 If the D-REQ packet does arrive at the source node it implies that the backup path has been established success- fully. In this case, the source node sends an acknowl- edgement (S-ACK) packet to the destination node and then recovers the data transfer on the backup path. When the destination node receives the S-ACK packet, it starts to receive the data on the backup path and meanwhile terminates the restoration process. 0 If the timer at the source node timeouts without receiving a D-REQ packet, it implies that no backup path can be

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established during the specified restoration time. In this case, the source node sends a release (S-REL) packet to the destination node to terminate the restoration process. Since the control packets are delivered in the control channels, the S-REL packet can take any available route. If the source node receives a D-REQ packet after it sends out an S-REL packet, the D-REQ packet is ignored.

0 If the destination node receives an S-REL packet before it sends out a D-REQ packet, it simply terminates the restoration process. If the destination node receives an S-REL packet after it sends out a D-REQ packet, it first sends a release (D-REL) packet along the reverse route and then terminates the restoration process. The D-REL packet disconfigures all the optical switches and releases the wavelength already reserved by the D-REQ packet.

Figs. 6 ,7 and 8 further illustrate the procedures by using the finite state machine (FSM) at D-node, I-node and S-node, respectively. At the destination node, there are three possible states:

route unavailable

receive L-fail

receive S-REL

receive S-REL

Fig. 6 FSM at D-node

receive D-REQ

Fig. 7 FSM at I-node

time out send S-REL

receive L-fail start timer

receive D-REQ

Fig. 8 FSM at S-node

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monitoring state M(d): node monitors the arrival of an L-Fail packet; computing state C(d): node performs a route-computing algorithm to decide a route and select a wavelength for a backup path; waiting state W(d): node waits for the arrival of an S-ACK packet. It may also receive an I-NAK packet or an S-REL packet.

At an intermediate node, there are also three possible states:

monitoring state M(i): node monitors the arrival of a D- REQ packet; reserving state R(i): node tries to reserve a selected wavelength; waiting state W(i): node waits for the arrival of an S-ACK packet. It may also receive an I-NAK packet or a D-REL packet.

At the source node, there are two possible states:

monitoring state M(s): node monitors the arrivai of an L- Fail packet. It may also receive a D-REQ packet; waiting state W(s): node waits for the arrival of a D-REQ packet.

At each node, different actions are taken and the states transit to each other in response to some discrete events, such as the arrival of a control packet, the output of a routing decision, or the timeout of the timer.

4 Performance evaluation

To evaluate the performance, we compare the proposed destination-initiating protocol with an existing source- initiating protocol in terms of the connection restoration time. For the source-initiating protocol, we only consider BRP because both BRP and FRP take a two-way delay to establish a backup path and BRP generally performs better than FRP in terms of wavelength utilisation [7]. For comparison, we assume that both the source-initiating protocol and the destination-initiating protocol use the same route-computing algorithm. As a result, under the same network conditions, the average number of hops on a backup path established by either of the protocols should be identical. In view of the fact that both protocols use a backward control packet (i.e. RESV or D-REQ) to reserve a wavelength, the wavelength utilisation in both cases should also be identical. This implies that under the same network conditions, the average blocking probability experienced by a RESV packet is equal to that experienced by a D-REQ packet. For these reasons, we also assume that both the source-initiating protocol and the destination- initiating protocol can successfully establish a backup without experiencing a reservation failure and the number of hops on a backup path established by either of the protocols is identical to that on the working path. We define the connection restoration time for a broken connection as the time taken from the instant the link fails to the instant a backup path is established successfully. The other notations used are defined as follows.

T R : connection restoration time with the source-initiating protocol TL: connection restoration time with the destination- initiating protocol Tj time that a node takes to detect a link failure q,: time that a node takes to process a control packet

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T,: time that a node takes to compute a route T,: time that a node takes to reserve a wavelength Td: propagation delay on each link n,l: number of hops on the working path n3; number of hops from the source-end node of a failed link to the source node on the working path ndd: number of hops from the destination-end node of a failed link to the destination node on the working path; q,: number of hops on a backup path established by the source-initiating protocol n6 : number of hops on a backup path established by the destination-initiating protocol.

Accordingly, the connection restoration time with the source-initiating protocol can be estimated as

TR =T' + n,xG + (a , + l ) x T p +2nb

In contrast, the connection restoration time with the destination-initiating protocol can be estimated as

( 1 ) x(Td + T p ) + (nh + 1 ) x C

TA = Tj +nddxTd+(ndd+l lXTp+nbx(Td+TP) + (ab + l ) x T , (2)

( 3 )

Therefore the difference in the restoration time is

TR - TA = (2n,, + 1 ) x (Td + Tp)

where ndd = nsd - n,, - 1 (nss, ndd = 0, I , 2 , . . . , n,d - 1) and nb = n; = nsd. Obviously, the difference is always a positive value.

Table 1 gives some numerical results to show the difference in the connection restoration time. To get these results, we assumed that nrrt= 7, ?''=OS5 ms, T,=2 ms, Tp = 0.01 ms, T,I= 0.50 ms, and Tr = 0.05 ms. It is observed that no matter what value n,, takes, i.e. no matter on which link a failure occurs, the restoration time with the destination-initiating protocol is always smaller than that with the source-initiating protocol. The closer a failure is to the destination node, the smaller the restoration time.

Table 1: Comparison in restoration time (ms)

0 10.10 9.59 0.51

1 10.61 9.08 1.53

2 11.12 8.57 2.55

3 11.63 8.06 3.57

4 12.14 7.55 4.59

5 12.65 7.04 5.61

6 13.16 6.53 6.63

5 Conclusion

We have proposed a destination-initiating path restoration protocol for surviving single-link failures in wavelength- routed WDM networks. Unlike existing path restoration protocols, the proposed protocol allows the destination node of a broken connection to initiate a connection restoration process and meanwhile introduces a retrying mechanism to increase the possibility of restoration success. We showed through the performance analysis that the proposed protocol can significantly reduce the connection restoration time and can thus provision backup paths more

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rapidly than existing path restoration protocols. Although we assumed that there is no wavelength conversion at each node, the proposed protocol is also applicable to networks with wavelength conversion.

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References 7

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