Overview ofoptical control plane design

For IP networks, distributed management schemes, such as Multi-Protocol Label Switching (MPLS), are used to provide the control plane necessary to ensure automated provisioning, maintain connections, and manage the network resources (including providing Quality of Service (QoS) and Traffic Engineer-ing (TE)). In recent years, industry organizations like the OIF and the IETF have been continuously working on extending the MPLS-framework to support not only devices that perform packet switching, but also those that perform switching in time, wavelength, and space (Generalized-MPLS (GMPLS)) [4,40].

Thus, GMPLS can now be applied as the control plane for wavelength-routed optical networks. GMPLS includes extensions of signaling and routing protocols

44 G. Ellinas, A. Hadjiantonis, A. Khalil, N. Antoniades, and M. A. Ali

developed for MPLS traffic engineering [35], and also supports the new feature of link management.

3.2.1 Link Management Protocol

The Link Management Protocol (LMP) in GMPLS is responsible primarily for neighbor discovery (automated determination of the connectivity in the network) that is subsequently used for up-to-date topology and resource discovery in the network (used for opaque, as well as transparent network architectures) [39].

Other tasks performed by LMP include management of the control channel, link bundling, and link fault isolation. Information that is collected by the neighbor discovery protocol includes the physical properties of a fiber link interconnecting two nodes (length, available bandwidth, etc.), node, port, and link identifica-tion parameters, etc. Informaidentifica-tion exchange concerning these parameters can be achieved via an out-of-band control channel, via in-band signaling, or via a com-pletely separate out-of-band data communications network (DCN). When the network topology is ascertained, this information is either kept at a central loca-tion (central network manager) or distributed to the network nodes (distributed network control) to be subsequently used by the routing protocol to determine the routing paths for the various network connections (see Section 3.2.2).

The Link Management Protocol uses the periodic exchange of “Hello” and

“Configuration” messages between neighboring nodes so that each node obtains the required information about its neighbors. These messages are also utilized to monitor the health of the communication channel used for the LMP sessions. To minimize the information exchange between neighboring nodes that are linked with a large number of fiber-optic links, a “link bundling” technique is used that bundles together a number of these links that have the same characteristics for routing purposes [36]. This group of links is then called a “TE link” (with a corresponding TE link ID). An additional mechanism, namely, link verification, is then utilized to separate the component links used for different connections.

As LMP has information on the physical adjacencies between neighboring nodes, it can also be used to isolate a fault in the case of a network failure (fiber link cut, laser failure, etc.). In the event of a fault that has propagated downstream, a simple “backtracking mechanism” is used in the upstream direction to determine the location of the fault.

3.2.2 GMPLS routing protocol

To implement the GMPLS routing protocol in optical networks, extensions to the routing approaches used for MPLS, such as the Open Shortest Path First protocol with Traffic Engineering extensions (OSPF-TE), are utilized [14, 29, 31,37,41, 42,47]. OSPF is a distributed, link-state shortest path routing algo-rithm (based on Dijkstra’s algoalgo-rithm [18] which computes a shortest path tree from a node to all other nodes) that uses a table in each node for routing

The optical control plane 45

purposes. These tables contain the complete network topology, as well as other link parameters (such as link costs). This information is created by exchang-ing information on the state of links between the nodes via Link State Adver-tisements (LSAs) [31, 41, 42]. In these types of networks, information on the links is required in order to successfully route an optical connection. For exam-ple, optical transmission impairments and wavelength continuity (for transparent connections) [16, 17, 46], bit-rates and modulation formats (for opaque connec-tions), bandwidth availability, etc., must be taken into account when routing an optical connection. The topology information at each node is modified when a change occurs in the network (i.e., a new node or link is added to the network), or is updated periodically (e.g., every 30 minutes), in order to ensure that the topology information is correct and up-to-date.

OSPF also supports hierarchical routing in the case where the network is divided in multiple areas with a hierarchical structure. If routing takes place across multiple areas, no information, complete information, or summary infor-mation can flow between different areas. For example, in the case of summary information distribution, summarized TE LSAs can be distributed to the entire Autonomous System (AS) [14, 35, 47].

3.2.2.1 Extensions of OSPF-TE

While MPLS routing is implemented on a hop-by-hop basis, in optical networks routing is source-based (explicit from the source to the destination node). Fur-thermore, there are specific requirements needed to route optical connections (impairments, wavelength continuity, link types, etc.) and the number of links between different nodes may be quite large. Thus, several extensions to the OSPF-TE utilized in MPLS are required for the case of implementing a GMPLS routing protocol in optical networks. The most important extensions to OSPF-TE are as follows:

• Dissemination of link state information: Additional information is adver-tised utilizing opaque LSAs that is either optical network specific or is needed for protection purposes [37]. Such information includes a link’s

– Protection Type – Encoding Type – Bandwidth Parameter – Cost Metric

– Interface Switching Capability Descriptor – Shared Risk Link Group (SRLG).

• Link bundling: As previously discussed, link bundling is a technique used to combine several parallel links having the same properties for purposes of routing, into a single logical group, called a TE link [36]. When link bundling is used, explicit routing takes into account only the TE links and not the individual links in the bundle. The specific link used to route the connection is only decided locally during the signaling process.

46 G. Ellinas, A. Hadjiantonis, A. Khalil, N. Antoniades, and M. A. Ali

• Nested label switched paths (LSPs): Ahierarchy in the LSPs is created by introducing “optical LSPs” that are groups of LSPs that have the same source and destination nodes [6, 37, 38]. Anumber of LSPs (with different bandwidth values) can now use this optical LSP provided that the bandwidth of the optical LSP can support all of them.

3.2.3 GMPLS signaling protocol

Two types of signaling are in place in optical networks: signaling between the client and the transport network (that takes place at the User–Network Inter-face (UNI) [48]), and signaling between intermediate network nodes (that takes place at the Network–Network Interface (NNI)). Signaling at the UNI is used so that the clients can request connections across the transport network, speci-fying such parameters as the bandwidth of the connection, the Class of Service (CoS) requirements, and the protection type for the connection. After a route is determined, signaling is required to establish, maintain and teardown a connec-tion. Furthermore, in the event of a fault, signaling is also employed to restore connections. An enhanced Resource Reservation Protocol with Traffic Engineer-ing extensions (RSVP-TE) is a possible signalEngineer-ing protocol that can be used for optical networks [8].

3.2.3.1 Extensions of RSVP-TE for GMPLS

RSVP is a signaling protocol that utilizes the path computed by the routing protocol (e.g., OSPF) to reserve the necessary network resources for the estab-lishment of a session (supports both point-to-point and multicast traffic in IP networks with QoS requirements) [12]. In RSVP, a Path message, containing information on the traffic characteristics of the session, is sent downstream from the source to the destination and is being processed by each intermediate node.

The destination node then sends back a Resv message along the path, which allocates resources on the downstream link. When the reservation is complete, data can flow from source-to-destination adhering to the QoS requirements spec-ified. RSVP also utilizes other message types that are used for error notifica-tion (PathErrandResvErr), for connection establishment confirmation purposes (ResvConf), and for deleting reservations (PathTearandResvTear).

RSVP with traffic engineering extensions (RSVP-TE) supports the establish-ment and manageestablish-ment of LSP tunnels (including specifying an explicit route and specifying traffic characteristics and attributes of the LSP). It also supports fault detection capabilities by introducing a “Hello” protocol between adjacent label switched routers (LSRs) [5]. When RSVP is extended for GMPLS sup-port, it is adapted for circuit-switched rather than packet-switched connections and it accommodates the independence between the control and data planes.

To this end, new label formats are defined in RSVP-TE in order to support a variety of switching and multiplexing types (such as wavelength switching, waveband switching, fiber switching, etc.) [7, 9]. Thus, several new objects are

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defined in GMPLS RSVP-TE, such as (a) the Generalized Label Request object that includes LSP encoding type, switching type, generalized protocol ID (the type of payload), source and destination endpoints, and connection bandwidth, (b) the Upstream Label object that is used for bi-directional connections (not supported in MPLS RSVP-TE), and (c) the Interface Identification object that identifies the data link on which labels are being assigned. This is essential for optical networks where control and data planes are separate.

3.2.3.2 Signaling for lightpath establishment

As in MPLS networks,Path and Resvare again used to establish lightpaths in optical networks utilizing GMPLS RSVP-TE [30]. ThePath message now car-ries extra information concerning the connection to be provisioned (LSP tunnel information, explicit route information, etc.) and is again forwarded downstream from the source to the destination node, and is being processed at intermediate nodes. When the message reaches the destination node, aResv message is cre-ated that is forwarded upstream. At each intermediate node processing theResv message the cross-connects are now set in order to establish a bi-directional con-nection. Note that, in contrast to signaling in IP networks, in optical networks the DCN does not necessarily use the data transfer links.

3.2.3.3 Signaling for protection/restoration

In networks where protection is offered, GMPLS signaling can be used to pro-vision secondary protection paths during the connection propro-visioning phase. In the case of reactive protection (in which case alternative paths are precomputed anticipating a failure), when a failure occurs it is detected by the source (with the help of a Notify message that contains the failed connection information) and GMPLS RSVP-TE is used to activate the precomputed protection path in the same way that a regular connection is provisioned in an optical network [7].