Internet Draft Bala Rajagopalan Internet Draft Tellium, Inc. draft-many-ip-optical-framework-02.txt James Luciani Expires on: 5/14/2001 Tollbridge Technologies Daniel Awduche UUNET (MCI Worldcom) Brad Cain, Bilel Jamoussi Nortel Networks Debanjan Saha Tellium, Inc. IP over Optical Networks: A Framework Status of this Memo This document is an Internet-Draft and is in full conformance with all provisions of Section 10 of RFC2026 except that the right to produce derivative works is not granted. Internet Drafts are working documents of the Internet Engineering Task Force (IETF), its areas, and its working groups. Note that other groups may also distribute working documents as Internet- Drafts. Internet-Drafts are draft documents valid for a maximum of six months and may be updated, replaced, or obsoleted by other documents at any time. It is inappropriate to use Internet- Drafts as reference material or to cite them other than as "work in progress." The list of current Internet-Drafts can be accessed at http://www.ietf.org/ietf/1id-abstracts.txt The list of Internet-Draft Shadow Directories can be accessed at http://www.ietf.org/shadow.html. 1. Abstract The Internet transport infrastructure is moving towards a model of high-speed routers interconnected by optical core networks. A consensus has emerged in the industry on utilizing IP-based protocols for the optical control plane. At the same time, there is ongoing activity in defining architectural models for IP transport over optical networks, specifically, the routing and signaling aspects. This draft defines a framework for IP over Optical networks, considering both the IP-based control plane for optical networks as well as IP transport over optical networks (together referred to as "IP over optical networks"). Expires on 5/24/2001 Page 1 of 36 draft-many-ip-optical-framework-02.txt Table of Contents ----------------- 1. Abstract ------------------------------------------------- 1 2. Conventions Used in this Document ------------------------- 3 3. Introduction ---------------------------------------------- 3 4. Terminology and Concepts ---------------------------------- 4 5. The Network Model ----------------------------------------- 7 5.1 Network Interconnection -------------------------------- 7 5.2 Control Structure -------------------------------------- 9 6. IP over Optical Service Models ---------------------------- 9 6.1 Domain Services Model ---------------------------------- 10 6.2 Unified Services Model --------------------------------- 12 6.3 Which Service Model? ----------------------------------- 13 6.4 What are the Possible Services? ------------------------ 13 7. IP Transport over Optical Networks ----------------------- 14 7.1 Interconnection Models --------------------------------- 14 7.2 Routing Approaches ------------------------------------- 15 7.3 Signaling Related ------------------------------------- 18 7.4 End-to-End Protection Models --------------------------- 19 8. IP-Based Optical Control Plane Issues -------------------- 20 8.1 Addressing -------------------------------------------- 21 8.2 Neighbor Discovery ------------------------------------- 22 8.3 Topology Discovery --------------------------------------23 8.4 Restoration Models ------------------------------------- 24 8.5 Route Computation ------------------------------------- 25 8.6 Signaling Issues -------------------------------------- 27 8.7 Optical Internetworking ------------------------------- 29 9. Other Issues --------------------------------------------- 30 9.1 WDM and TDM in the Same Network ------------------------ 30 9.2 Wavelength Conversion ---------------------------------- 30 9.3 Service Provider Peering Points ------------------------ 31 9.4 Rate of Lightpath Set-Up ------------------------------- 31 9.5 Distributed vs Centralized Provisioning --------------- 32 10. Evolution Path for IP over Optical Architecture --------- 33 11. Security Considerations --------------------------------- 33 12. Summary and Conclusions --------------------------------- 34 13. References ---------------------------------------------- 34 14. Acknowledgements ---------------------------------------- 35 Expires on 5/24/01 Page 2 of 36 draft-many-ip-optical-framework-02.txt 2. Conventions used in this document The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this document are to be interpreted as described in RFC 2119. 3. Introduction Optical network technologies are evolving rapidly in terms of functions and capabilities. The increasing importance of optical networks is evidenced by the copious amount of attention focused on IP over optical networks and related photonic and electronic interworking issues by all the major network service providers, telecommunications equipment vendors, and standards organizations. It has been realized that optical networks must be survivable, flexible, and controllable. There is, therefore, an ongoing trend to introduce intelligence in the control plane of optical networks to make them more versatile [1]. An essential attribute of intelligent optical networks is the capability to instantiate and route optical layer connections in real-time or near real-time, and to provide capabilities that enhance network survivability. Furthermore, there is a need for multi-vendor optical network interoperability, when an optical network may consist of interconnected vendor-specific optical sub-networks. The OTN must also be versatile because some service providers may offer generic optical layer services that may not be client- specific. It would therefore be necessary to have an optical network control layer that can handle such generic optical services. There is general consensus in the industry that the optical network control plane should utilize IP-based protocols for dynamic provisioning and restoration of lightpaths within and across optical sub-networks. This is based on the practical view that signaling and routing mechanisms developed for IP traffic engineering applications could be re-used in optical networks. Nevertheless, the issues and requirements that are specific to optical networking must be understood to suitably adopt the IP-based protocols. This is especially the case for restoration. Also, there are different views on the model for interaction between the optical network and client networks, such as IP networks. Reasonable architectural alternatives in this regard must be supported, with an understanding of their pros and cons. Thus, there are two fundamental issues related to IP over optical networks. The first is the adaptation and reuse of IP control plane protocols within the optical network control plane, irrespective of the types of digital clients that utilize the optical network. The second is the transport of IP traffic through an optical network together with the control and coordination issues that arise therefrom. Expires on 5/24/01 Page 3 of 36 draft-many-ip-optical-framework-02.txt This draft defines a framework for IP over optical networks covering the requirements and mechanisms for establishing an IP-centric optical control plane, and the architectural aspects of IP transport over optical networks. In this regard, it is recognized that the specific capabilities required for IP over optical networks would depend on the services expected at the IP-optical interface as well as the optical sub-network interfaces. Depending on the specific operational requirements, a progression of capabilities is possible, reflecting increasingly sophisticated interactions at these interfaces. This draft therefore advocates the definition of "capability sets" that define the evolution of functionality at the interfaces as more sophisticated operational requirements arise. This draft is organized as follows. In the next section, terminology covering certain concepts related to this framework are described. In Section 5, the network model pertinent to this framework is described. The service model and requirements for IP-optical, and multi-vendor optical internetworking are described in Section 6. This section presently considers certain general requirements. Specific operational requirements may be accommodated in this section as they arise. Section 7 considers the architectural models for IP-optical interworking, describing the pros and cons of each model. It should be noted that it is not the intent of this draft to promote any particular model over the others. However, particular aspects of the models that may make one approach more appropriate than another in certain circumstances are described. Section 8 describes IP-centric control plane mechanisms for optical networks, covering signaling and routing issues in support of provisioning and restoration. Section 9 describes certain specialized issues in relation to IP over optical networks. The approaches described in Section 7 and 8 range from the relatively simple to the sophisticated. Section 10 describes a possible evolution path for IP over optical networking capabilities in terms of increasingly sophisticated functionality supported. Section 11 considers security aspects. Finally, summary and conclusion are presented in Section 12. 4. Terminology and Concepts This section introduces some terminology for describing common concepts in IP over optical networks pertinent to this framework. WDM --- Wavelength Division Multiplexing (WDM) is a technology that allows multiple optical signals operating at different wavelengths to be multiplexed onto a single fiber so that they can be transported in parallel through the fiber. In general, each optical wavelength may carry digital client payloads at a different data rate (e.g., OC-3c, Expires on 5/24/01 Page 4 of 36 draft-many-ip-optical-framework-02.txt OC-12c, OC- 48c, OC-192c, etc.) and in a different format (SONET, Ethernet, ATM, etc.) Optical cross-connect (OXC) --------------------------- An OXC is a space-division switch that can switch an optical data stream on an input port to a output port. Such a switch may have optical-electrical conversion at the input port and electrical- optical conversion at the output port, or it can be all-optical. An OXC is assumed to have a control-plane processor that implements signaling and routing protocols necessary for realizing an optical network. Optical channel trail or Lightpath ---------------------------------- An optical channel trail is a point-to-point optical layer connection between two access points in an optical network. In this draft, the term "lightpath" is used interchangeably with optical channel trail. Optical mesh sub-network ------------------------ An optical sub-network, as considered in this framework, is a network of OXCs that supports end-to-end networking of optical channel trails providing functionality like routing, monitoring, grooming, and protection and restoration of optical channels. The interconnection of OXCs in this network can be based on a general topology (also called "mesh" topology) Underlying this network could be the following: (a) An optical multiplex section (OMS) layer network : The optical multiplex section layer provides the transport of the optical channels. The information contained in this layer is a data stream comprising a set of n optical channels, which have a defined aggregate bandwidth. (b) An optical transmission section (OTS) layer network : This provides functionality for transmission of the optical signal on optical media of different types. This framework does not address the interaction between the optical sub-network and the OMS, or between the OMS and OTS layer networks. Optical mesh network -------------------- An optical mesh network, as considered in draft, is a mesh-connected collection of optical sub-networks. Expires on 5/24/01 Page 5 of 36 draft-many-ip-optical-framework-02.txt Wavelength continuity property ------------------------------ A lightpath is said to satisfy the wavelength continuity property if it is transported over the same wavelength end-to-end. Wavelength continuity is required in optical networks with no wavelength conversion feature. Trust domain ------------ A trust domain is a network under a single technical administration in which most nodes in the network are considered to be secure or trusted in some fashion. An example of a trust domain is a campus or corporate network in which all routing protocol packets are considered to be authentic, without the need for additional security schemes to prevent unauthorized access to the network infrastructure. Generally, the "single" administrative control rule may be relaxed in practice if a set of administrative entities agree to trust one another to form an enlarged heterogeneous trust domain. However, to simplify the discussions in this draft, it will assumed, without loss of generality, that the term trust domain applies to a single administrative entity. Flow ---- For the purpose of this document, the term flow will be used to represent the smallest separable stream of data, as seen by an endpoint (source or destination node). It is to be noted that the term flow is heavily overloaded in the networking literature. Within the context of this document, it may be convenient to consider a wavelength as a flow under certain circumstances. However, if there is a method to partition the bandwidth of the wavelength, then each partition may be considered a flow, for example by quantizing time into some nicely manageable intervals, it may be feasible to consider each quanta of time within a given wavelength as a flow. Traffic Trunk ------------- A abstraction of traffic flow that follows the same path between two access points which allows some characteristics and attributes of the traffic to be parameterized. Expires on 5/24/01 Page 6 of 36 draft-many-ip-optical-framework-02.txt 5. The Network Model 5.1 Network Interconnection The network model considered in this draft consists of IP routers attached to an optical core network, and connected to their peers over dynamically established switched lightpaths. The optical core itself is assumed to be incapable of processing individual IP packets. The optical network is assumed to consist of multiple optical sub-networks interconnected by optical links in a general topology (referred to as an optical mesh network). This network may be multi- vendor. In the near term, it may be expected that each sub-network will consist of a single vendor switches. In the future, as standardization efforts mature, each optical sub-network may in fact contain optical switches from different vendors. In any case, each sub-network itself is assumed to be mesh-connected. In general, it can be expected that topologically adjacent OXCs in an optical mesh network will be connected via multiple, parallel (bi-directional) optical links. This network model is shown in Figure 1. Here, an optical sub-network may consist entirely of all-optical OXCs or OXCs with optical-electrical-optical (OEO) conversion. Interconnection between sub-networks is assumed to be through compatible physical interfaces, with suitable optical-electrical conversions where necessary. The routers that have direct physical connectivity with the optical network are referred to as "edge routers". As shown in the figure, other client networks (e.g., ATM) may connect to the optical network. The switching function in an OXC is controlled by appropriately configuring the cross-connect fabric. Conceptually, this may be viewed as setting up a cross-connect table whose entries are of the form , indicating that the data stream entering input port i will be switched to output port j. A lightpath from an ingress port in an OXC to an egress port in a remote OXC is established by setting up suitable cross-connects in the ingress, the egress and a set of intermediate OXCs such that a continuous physical path exists from the ingress to the egress port. Optical paths are assumed to be bi-directional, i.e., the return path from the egress port to the ingress port is routed along the same set of intermediate ports as the forward path. Multiple data streams output from an OXC may be multiplexed onto an optical link using WDM technology. The WDM functionality may exist outside of the OXC, and be transparent to the OXC. Or, this function may be built into the OXC. In the latter case, the cross-connect table (conceptually) consists of pairs of the form, <{input port i, Lambda(j)}, {output port k, Lambda(l)}>. This indicates that the data stream received on wavelength Lambda(j) over input port i is switched to output port k on Lambda(l). Automated establishment of Expires on 5/24/01 Page 7 of 36 draft-many-ip-optical-framework-02.txt lightpaths involves setting up the cross-connect table entries in the appropriate OXCs in a coordinated manner such that the desired physical path is realized. Optical Network +---------------------------------------+ | | +--------------+ | | | | | +------------+ +------------+ | | IP | | | | | | | | Network +--UNI--+ Optical +---NNI--+ Optical | | | | | Subnetwork | | Subnetwork | | +--------------+ | | | +-----+ | | | +------+-----+ | +------+-----+ | | | | | | | NNI NNI NNI | +--------------+ | | | | | | | | +------+-----+ | +------+-----+ | | IP +--UNI--| +--+ | | | | Network | | | Optical | | Optical | | | | | | Subnetwork +---NNI--+ Subnetwork | | +--------------+ | | | | | | | +------+-----+ +------+-----+ | | | | | +-------UNI-------------------UNI-------+ | | | | +------+-------+ +------------+ | | | | | Other Client | |Other Client| | Network | | Network | | (e.g., ATM) | | | +--------------+ +------------+ Figure 1: Optical Network Model Under this network model, a switched lightpath must be established between a pair of IP routers before they can communicate. This lightpath might traverse multiple optical sub-networks and be subject to different provisioning and restoration procedures in each sub-network. The IP-based control plane issue is that of designing standard signaling and routing protocols for coherent end-to-end provisioning and restoration of lightpaths across multiple optical sub-networks. Similarly, IP transport over such an optical network involves determining IP reachability and seamless establishment of paths from one IP endpoint to another over an optical core. Expires on 5/24/01 Page 8 of 36 draft-many-ip-optical-framework-02.txt 5.2 Control Structure There are two logical control interfaces identified in Figure 1. These are the client-optical network interface, and the optical sub- network interface. These interfaces are also referred to as the User-Network Interface (UNI) and the Network-Network Interface(NNI). The distinction between these interfaces arises out of the type and amount of control flow across them. The UNI represents a technology boundary between the client and optical networks. Thus, the control flow across the UNI is dependent of the set of services defined across it and the manner in which the services may be accessed. The service models are described in Section 7. Here, we merely note that since the optical network implements an IP-based control plane, it is possible in principle to harmonize the control flow across the UNI and the NNI and eliminate the distinction between them. On the other hand, it may be required to minimize control flow information, especially routing-related information, over UNI. In this case, UNI and NNI may look different in some respects. In this draft, these interfaces are treated as distinct. Each of these interfaces can also be categorized as public or private depending upon context and service models. If UNI (or NNI) is private, then routing information (ie, topology state information) can be exchanged across it. If UNI (or NNI) is public, then routing information is not exchanged across it, or such information may be exchanged across it with very explicit restrictions (including for example abstraction, filtration, etc). Thus, different relationships (e.g., peer or over-lay, Section 7) may occur across private and public logical interfaces. The physical control structure used to realize these logical interfaces may vary. For instance, for the UNI, some of the possibilities are: 1.Direct interface: An in-band or out-of-band IP control channel (IPCC) may be implemented between an edge router and each OXC that it connects to. This control channel is used for exchanging signaling and routing messages between the router and the OXC. With a direct interface, the edge router and the OXC it connects to are peers in the control plane. This is shown in Figure 2. The type of routing and signaling information exchanged across the direct interface would vary depending on the service definition. This issue is dealt with in the next section. Some choices for the routing protocol are OSPF/ISIS (with traffic engineering extensions) or BGP. Other directory-based routing information exchanges are also possible. Some of the signaling protocol choices are adaptations of RSVP-TE or CR-LDP. The details of how the IP control channel is realized is outside the scope of this draft. 2.Indirect interface: An out-of-band IP control channel may be implemented between the client and a device in the optical network to signal service requests and responses. For instance, a Expires on 5/24/01 Page 9 of 36 draft-many-ip-optical-framework-02.txt management system or a server in the optical network may receive service requests from clients. Similarly, out-of-band signaling may be used between management systems in client and optical networks to signal service requests. In these cases, there is no direct control interaction between clients and respective OXCs. One reason to have an indirect interface would be that the OXCs and/or clients do not support a direct signaling interface. 3. Provisioned interface: In this case, the optical network services are manually provisioned and there is no control interactions between the client and the optical network. +-----------------------------+ +-----------------------------+ | | | | | +---------+ +---------+ | | +---------+ +---------+ | | | | | | | | | | | | | | | Routing | |Signaling| | | | Routing | |Signaling| | | | Protocol| |Protocol | | | | Protocol| |Protocol | | | | | | | | | | | | | | | +-----+---+ +---+-----+ | | +-----+---+ +---+-----+ | | | | | | | | | | | | | | | | | | +--+-----------+---+ | | +--+-----------+---+ | | | | | | | | | | | IP Layer +......IPCC.......+ IP Layer | | | | | | | | | | | +------------------+ | | +------------------+ | | | | | | Edge Router | | OXC | +-----------------------------+ +-----------------------------+ Figure 2: Direct Interface Although different control structures are possible, further descriptions in this framework assume direct interfaces for IP- optical and optical sub-network control interactions. 6. IP over Optical Service Models and Requirements In this section, the service models and requirements at the IP- optical UNI and the optical sub-network NNI are considered. Two general models have emerged for the services at the IP-optical interface (which can also be applied at the optical sub-network interface). These models are as follows. 6.1 Domain Services Model Under this model, the optical network primarily offers high bandwidth connectivity in the form of lightpaths [2]. Standardized signaling across the UNI (Figure 1) is used to invoke the following services: Expires on 5/24/01 Page 10 of 36 draft-many-ip-optical-framework-02.txt 1. Lightpath creation: This service allows a lightpath with the specified attributes to be created between a pair of termination points in the optical network. Lightpath creation may be subject to network-defined policies (e.g., connectivity restrictions) and security procedures. 2. Lightpath deletion: This service allows an existing lightpath to be deleted. 3. Lightpath modification: This service allows certain parameters of the lightpath to be modified. 4. Lightpath status enquiry: This service allows the status of certain parameters of the lightpath (referenced by its ID) to be queried by the router that created the lightpath. Additionally, the following address resolution procedures may be made available over the UNI (more sophisticated routing information exchange over the UNI is also possible, as described later and covered in more detail in [3]): 1. Client Registration: This allows a client to register its address(es) and user group identifier(s) with the optical network. The registered address may be of different types, IP, ATM NSAP, E.164, etc. The optical network associates the client address and user group ID with an optical-network-administered address. 2. Client De-Registration: This allows a client to withdraw its address(es) and user group identifier(s) from the optical network. 3. Query: This allows a client to supply another clientÆs native address (e.g., ATM) and user group ID, and get back an optical- network-administered address that can be used in lightpath create messages. An end-system discovery procedure may be used over the UNI to verify local port connectivity between the optical and client devices, and allows each device to bootstrap the UNI control channel. Finally, a "service discovery" procedure may be employed as a precursor to obtaining UNI services. Service discovery allows a client to determine the static parameters of the interconnection with the optical network, including the UNI signaling protocols supported. The protocols for neighbor and service discovery are different from the UNI signaling protocol itself (for example, see LMP [4]). With regard to address resolution, the registration and de- registration procedures may be implemented using service discovery mechanisms. The query mechanism may be implemented as an additional UNI signaling procedure. Expires on 5/24/01 Page 11 of 36 draft-many-ip-optical-framework-02.txt Because a small set of well-defined services is offered across UNI, the signaling protocol requirements are minimal. Specifically, the signaling protocol is required to convey a few messages with certain attributes point-to-point between the router and the optical network. Such a protocol may be based on RSVP-TE or LDP, or even a messaging application over a TCP connection. The optical domain services model does not deal with the type and nature of routing protocols within the optical network. Furthermore, the integration of multiple, optical sub-networks across NNI will require the specification of a standard routing protocol, say, BGP. The optical domain services model would result in the establishment of a lightpath topology between routers at the edge of the optical network. The resulting overlay model for IP over optical networks is discussed in Section 7. 6.2 Unified Service Model Under this model, the IP and optical networks are treated together as a single integrated network that is managed and traffic engineered in a unified manner. In this regard, the OXCs are treated just like any other router as far as the control plane is considered. Thus, from a routing and signaling point of view, there is no distinction between UNI, NNI and any other router-to-router interface. It is assumed that this control plane is MPLS-based, as described in [1]. Under the unified service model, optical network services are obtained implicitly during end-to-end MPLS signaling. Specifically, an edge router can create a lightpath with specified attributes, or delete and modify lightpaths as it creates label-switched paths (LSPs). In this regard, the services obtained from the optical network are similar to the domain services model. These services, however, may be invoked in a more seamless manner as compared to the domain services model. For instance, in principle, a remote router could compute an end-to-end path across the optical network utilizing, say, OSPF with traffic engineering extensions [5]. It can then establish an LSP across the optical network. But the edge routers must still recognize that an LSP across the optical network is a lightpath, or a conduit for multiple LSPs. The concept of "forwarding adjacency" can be used to specify virtual links across optical networks in routing protocols such as OSPF [6]. In essence, once a lightpath is established across an optical network between two edge routers, it can be advertised as a forwarding adjacency (a virtual link) between these routers. Thus, from a data plane point of view, the lightpaths result in a overlay between edge routers. The decisions as to when to create such lightpaths, and the bandwidth management for these lightpaths is identical in both the domain services model and the unified service model. The routing and signaling models for unified services is described in Section 7. Expires on 5/24/01 Page 12 of 36 draft-many-ip-optical-framework-02.txt 6.3 Which Service Model? The pros and cons of the above service models can be debated at length, but the approach recommended in this framework is to define routing and signaling mechanisms in support of both. As pointed out above, signaling for service request can be unified to cover both models. The developments in GMPLS signaling [7] for the unified service model and its adoption for UNI signaling under the domain services model [8] essentially supports this view. The significant difference between the service models, however, is in routing protocols, as described in Section 7. 6.4 What are the Possible Services? Specialized services may be built atop the point-to-point connectivity service offered by the optical network. For example, 6.4.1 Virtual Private Networks (VPNs) Given that the data plane between IP routers over an optical network is an overlay, it is easy to imagine a virtual private network of lightpaths that interconnect routers (or any other set of clients). Indeed, in the case where the optical network provides connectivity for multiple sets of external client networks, there has to be a way to enforce routing policies that ensure routing separation between different sets of clients (i.e., VPN service). 7. IP transport over Optical Networks To examine the architectural alternatives for IP over optical networks, it is important to distinguish between the data and control planes over the UNI. As described in Section 6, the optical network provides a service to external entities in the form of fixed bandwidth transport pipes (optical paths). IP routers at the edge of the optical networks must necessarily establish such paths before communication at the IP layer can begin. Thus, the IP data plane over optical networks is realized over an overlay network of optical paths. On the other hand, IP routers and OXCs can have a peer relation on the control plane, especially for the implementation of a routing protocol that allows dynamic discovery of IP endpoints attached to the optical network. The IP over optical network architecture is defined essentially by the organization of the control plane. The assumption in this framework is that an MPLS- based control plane [1] is used. Depending on the service model(Section 6), however, the control planes in the IP and optical networks can be loosely or tightly coupled. This coupling determines o the details of the topology and routing information advertised by the optical network across UNI; o Level of control that IP routers can exercise in selecting specific paths for connections across the optical network; and Expires on 5/24/01 Page 13 of 36 draft-many-ip-optical-framework-02.txt o Policies regarding the dynamic provisioning of optical paths between routers. This includes access control, accounting and security issues. The following interconnection models are then possible: 7.1 Interconnection Models 7.1.1 The Peer Model Under the peer model, the IP/MPLS layers act as peers of the optical transport network, such that a single routing protocol instance runs over both the IP/MPLS and optical domains. Presumably a common IGP such as OSPF or IS-IS, with appropriate extensions, can be used to distribute topology information [3]. In the case of OSPF, opaque LSAs can be used to advertise topology state information. In the case of IS-IS, extended TLVs will have to be defined to propagate topology state information. One tacit assumption here is that a common addressing scheme will also be used for the optical and IP networks. A common address space can be trivially realized by using IP addresses in both IP and optical domains. Thus, the optical network elements become IP addressable entities as noted in [1]. 7.1.2 The Overlay Model Under the overlay model, the IP/MPLS routing, topology distribution, and signaling protocols are independent of the routing, topology distribution, and signaling protocols at the optical layer. This model is conceptually similar to the classical IP over ATM or MPOA models, but applied to a optical sub-network directly. In the overlay model, topology distribution, path computation and signaling protocols would have to be defined for the optical domain. In certain circumstances, it may also be feasible to statically configure the optical channels that provide connectivity in the overlay model through network management. Static configuration is, however, unlikely to scale in very large networks. 7.1.3 The Augmented Model Under the augmented model, there are actually separate routing instances in the IP and optical domains, but information from one routing instance is passed through the other routing instance. For example, external IP addresses could be carried within the optical routing protocols to allow reachability information to be passed to IP clients. The routing approaches corresponding to these interconnection models are described below. Expires on 5/24/01 Page 14 of 36 draft-many-ip-optical-framework-02.txt 7.2 Routing Approaches 7.2.1 Integrated Routing This routing approach supports the peer model described above. Under this approach, the IP and optical networks are assumed to run the same instance of an IP routing protocol, e.g., OSPF with suitable "optical" extensions. These extensions must capture optical link parameters, and any constraints that are specific to optical networks. The topology and link state information maintained by all nodes (OXCs and routers) is identical. This permits a router to compute an end-to-end path to another router across the optical network. Suppose the path computation is triggered by the need to route a label switched path (LSP). Such an LSP can be established using MPLS signaling, e.g., RSVP-TE or CR-LDP. When the LSP is routed over the optical network, a lightpath must be established between two edge routers. This lightpath is in essence a tunnel across the optical network, and may have capacity much larger than that required to route the first LSP. Thus, it is essential that other routers in the network realize the availability of resources in the lightpath for other LSPs to be routed over it. The lightpath must therefore be advertised as a virtual link in the topology. The notion of "forwarding adjacency" (FA) described in [6] is essential in propagating lightpath information to routers. An FA is essentially a virtual link advertised into a link state routing protocol. Thus, an FA could be described by the same parameters that define resources in any regular link. While it is necessary to specify the mechanism for creating an FA, it is not necessary to specify how an FA is used by the routing scheme. Once an FA is advertised in a link state protocol, its usage for routing LSPs is defined by the route computation and traffic engineering algorithms implemented. It should be noted that at the IP-optical interface, the physical ports over which routers are connected to OXCs define the connectivity and resource availability. Suppose a router R1 is connected to OXC O1 over two ports, P1 and P2. Under integrated routing, the connectivity between R1 and O1 over the two ports would have been captured in the link state representation of the network. Now, suppose an FA at full port bandwidth is created from R1 to another router R2 over port P1. While this FA is advertised as a virtual link between R1 and R2, it is also necessary to remove the link R1-O1 (over P1) from the link state representation since that port is no longer available for creating a lightpath. Thus, as FAs are created, an overlaid set of virtual links is introduced into the link state representation, replacing the links previously advertised at the IP-Optical interface. Finally, the details of the optical network captured in the link state representation is replaced by a network of FAs. In this regard, there is a great deal of similarity between integrated routing and domain-specific routing (described next). Both ultimately deal with the creation of the overlaid lightpath topology to meet the traffic engineering objectives. Expires on 5/24/01 Page 15 of 36 draft-many-ip-optical-framework-02.txt 7.2.2 Domain-Specific Routing This routing approach supports the augmented interconnection model. Under this approach, routing within the optical and IP domains are separated, with a standard routing protocol running between domains. This is similar to the IP inter-domain routing model. Two choices for this are considered. 7.2.2.1 Domain-Specific Routing using BGP The inter-domain IP routing protocol, BGP [9], may be adapted for exchanging routing information between IP and optical domains. This would allow the routers to advertise IP address prefixes within their network to the optical network and to receive external IP address prefixes from the optical network. The optical network transports the reachability information from one IP network to others. For instance, edge routers and OXCs can run exterior BGP (EBGP). Within the optical network, interior BGP (IBGP) is used between border OXCs within the same sub-network, and EBGP is used between sub-networks (over NNI, Figure 1). Under this scheme, it is necessary to identify the egress points in the optical network corresponding to externally reachable IP addresses. This is due to the following. Suppose an edge router desires to establish an LSP to a destination reachable across the optical network. It could directly request a lightpath to that destination, without explicitly specifying the egress optical port for the lightpath as the optical network has knowledge of externally reachable IP addresses. However, if the same edge router has to establish another LSP to a different external destination, it must first determine whether there is a lightpath already available (with sufficient residual capacity) that leads to that destination. To identify this, it is necessary for edge routers to keep track of which egress ports in the optical network lead to which external destinations. Thus, a border OXC receiving external IP prefixes from an edge router through EBGP must include its own IP address as the egress point before propagating these prefixes to other border OXCs or edge routers. An edge router receiving this information need not propagate the egress address further, but it must keep the association between external IP addresses and egress OXC addresses. Specific BGP mechanisms for propagating egress OXC addresses are to be determined, considering prior examples described in [10]. When VPNs are implemented, the address prefixes advertised by the border OXCs must be accompanied by some VPN identification (for example, VPN IPv4 addresses, as defined in [10], may be used). 7.2.2.2 Domain Specific Routing using OSPF/ISIS The routing information exchanged across the IP-optical UNI could be summarized using a hierarchical routing protocol such as OSPF/ISIS. The following description is based on OSPF. Expires on 5/24/01 Page 16 of 36 draft-many-ip-optical-framework-02.txt OSPF supports a two-level hierarchical routing scheme through the use of OSPF areas. Routing within each area is flat, while detailed knowledge of an areaÆs topology is hidden from all other areas. Routers attached to two or more areas are called Area Border Routers (ABRs). ABRs propagate IP addressing information from one area to another using summary LSAs. Within an OSPF routing domain, all areas are attached directly to a special area called the OSPF backbone area. The exchange of information between areas can be controlled to implement domain specific routing in each area. For instance, the optical network can be a collection of one or more areas in which certain link parameters and information specific to optical networks is incorporated into a version of OSPF. The client network (e.g., IP) could be separate OSPF area(s), running OSPF with TE extensions. The summary LSAs exchanged between the optical and client areas can be designed such that optical domain specific information is hidden from client networks while providing adequate routing information for end-to-end routing of lightpaths. While the use of BGP or OSPF/ISIS allows edge routers to learn about reachability of destinations across the optical network, the determination of how many lightpaths to establish and to what egress points are traffic engineering decisions. 7.2.3 Overlay Routing This routing approach supports the overlay interconnection model. Under this approach, overlay mechanism that allows edge routers to register and query for external addresses is implemented. This is similar to address resolution for IP over ATM. Under this approach, the optical network could implement a registry that allows edge routers to register IP addresses and VPN identifiers. An edge router may be allowed to query for external addresses belonging to the same set of VPNs it belongs to. A successful query would return the address of the egress optical port through which the external destination can be reached. Because IP-optical interface connectivity is limited, the determination of how many lightpaths must be established and to what endpoints are traffic engineering decisions. Furthermore, after an initial set of such lightpaths are established, these may be used as adjacencies within VPNs for a VPN-wide routing scheme, for example, OSPF. With this approach, an edge router could first determine other edge routers of interest by querying the registry. After it obtains the appropriate addresses, an initial overlay lightpath topology may be formed. Routing adjacencies may then be established across the lightpaths and further routing information may be exchanged to establish VPN-wide routing. Routing approaches in optical networks are further described in [3]. Expires on 5/24/01 Page 17 of 36 draft-many-ip-optical-framework-02.txt 7.3 Signaling-Related 7.3.1 The Role of MPLS It is possible to model wavelengths, and potentially TDM channels within a wavelength as "labels". This concept was proposed in [1], and ôgeneralizedö MPLS (GMPLS) mechanisms for realizing this are described in [7]. MPLS signaling protocols with traffic engineering extensions, such as RSVP-TE and CR-LDP can be used for signaling lightpath requests. In the case of the domain services model, these protocols can be adapted for UNI signaling [11, 12]. In the case of the unified services model, lightpath establishment occurs to support end-to-end LSP establishment using these protocols (with suitable GMPLS enhancements [13, 14]). 7.3.2 Signaling Models With the domain-services model, the signaling control plane in the IP and optical network are completely separate as shown in Figure 3 below. This separation also implies the separation of IP and optical address spaces (even though the optical network would be using internal IP addressing). While RSVP-TE and LDP can be adapted for UNI signaling, the full functionality of these protocols will not be used. For example, UNI signaling does not require the specification of explicit routes [8]. On the other hand, based on the service attributes, new objects need to be signaled using these protocols as described in [11, 12]. MPLS Signaling UNI Signaling MPLS or other signaling | +-----------------------------+ | +-----------------------------+ | IP Network | | | Optical Network | | +---------+ +---------+ | | | +---------+ +---------+ | | | | | | | | | | | | | | | | Router +---+ Router +-----+------+ OXC +---+ OXC | | | | | | | | | | | | | | | | +-----+---+ +---+-----+ | | | +-----+---+ +---+-----+ | +-----------------------------+ | +-----------------------------+ | | Completely Separated Addressing and Control Planes Figure 3: Domain Services Signaling Model With the unified services model, the addressing is common in the IP and optical networks and the respective signaling control are related, as shown in Figure 4. It is understood that GMPLS signaling is implemented in the IP and optical networks, using suitably enhanced RSVP-TE and CR-LDP protocols. But the semantics of services within the optical network may be different from that in the IP network. As an example, the protection services offered in the optical network may be different from that end-to-end protection Expires on 5/24/01 Page 18 of 36 draft-many-ip-optical-framework-02.txt services offered by the IP network. Another example is with regard to bandwidth. While the IP network may offer a continuum of bandwidths, the optical network will offer only discrete bandwidths. Thus, the signaling attributes and services are defined independently for IP and optical networks. The routers at the edge of the optical network must therefore identify service boundaries and perform suitable translations in the signaling messages crossing the IP-optical boundary. This must occur even though the signaling control plane in both networks are GMPLS-based and there is tighter coupling of the control plane as compared to the domain services model. Service Boundary Service Boundary | | IP Layer GMPLS Signaling | Optical Layer GMPLS | IP Layer GMPLS | | +--------+ +--------+ | +-------+ +-------+ | +--------+ | | | | | | | | | | | | | IP LSR +--+ IP LSR +--+--+Optical+--+Optical+-+--+ IP LSR +--- | | | | | | LSR | | LSR | | | | +-----+--+ +---+----+ | +-----+-+ +---+---+ | +--------+ Common Address Space, Service Translation Figure 4: Unified Services Signaling Model Thus, as illustrated in Figure 4, the signaling in the case of unified services is actually multi-layered. The layering is based on the technology and functionality. As an example, the specific adaptations of GMPLS signaling for SONET layer (whose functionality is transport) are described in [15]. 7.4 End-to-End Protection Models Suppose an LSP is established from an ingress IP router to an egress router across an ingress IP network, a transit optical network and an egress IP network. If this LSP is to be afforded protection in the IP layer, how is the service coordinated between the IP and optical layers? Under this scenario, there are two approaches to end-to-end protection: 7.4.1 Segment-Wise Protection The protection services in the IP layer could utilize optical layer protection services for the LSP segment that traverses the optical network. Thus, the end-to-end LSP would be treated as a concatenation of three LSP segments from the protection point of view: a segment in the ingress IP network, a segment in the optical Expires on 5/24/01 Page 19 of 36 draft-many-ip-optical-framework-02.txt network and a segment in the egress IP network. The protection services at the IP layer for an end-to-end LSP must be mapped onto suitable protection services offered by the optical network. Suppose that 1+1 protection is offered to LSPs at the IP layer, i.e., each protected LSP has a pre-established hot stand-by. In case of a failure of the primary LSP, traffic can be immediately switched to the stand-by. This type of protection can be realized end-to-end as follows. With reference to Figure 5, let an LSP originate at (ingress) router interface A and terminate at (egress) router interface F. Under the first protection option, a primary path for the LSP must be established first. Let this path be as shown in the figure, traversing router interface B in the ingress network, optical ports C (ingress) and D (egress), and router interface E in the egress network. Next, 1+1 protection is realized separately in each network by establishing a protection path between points A and B, C and D and E and F. Furthermore, the segments B-C and D-E must themselves be 1+1 protected, using drop-side protection. For the segment between C and D, the optical network must offer a 1+1 service similar to that offered in the IP networks. +----------------+ +-----------------+ +---------------+ | | | | | | A Ingress IP Net B----C Optical Network D----E Egress IP Net F | | | | | | +----------------+ +-----------------+ +---------------+ Figure 5: End-to-End Protection Example 7.4.2 Single-Layer Protection The protection services in the IP layer do not rely on any protection services offered in the optical network. Thus, with reference to Figure 5, two SRLG-disjoint LSPs are established between A and F. The corresponding segments in the optical network are treated as independent lightpaths in the optical network. These lightpaths may be unprotected in the optical network. 7.4.3 Differences A distinction between these two choices is as follows. Under the first choice, the optical network is actively involved in end-to-end protection, whereas under the second choice, any protection service offered in the optical network is not utilized. Also, under the first choice, the protection in the optical network may apply collectively to a number of IP LSPs. That is, with reference to Figure 5, many LSPs may be aggregated into a single lightpath between C and D. The optical network protection may then be applied to all of them at once leading to some scalability. Under the second choice, each IP LSP must be separately protected. Finally, the first choice allows different restoration signaling to be implemented in the IP and optical network. These restoration protocols are "patched Expires on 5/24/01 Page 20 of 36 draft-many-ip-optical-framework-02.txt up" at the service boundaries to realize end-to-end protection. A further advantage of this is that restoration is entirely contained within the network where the failure occurs, thereby improving the restoration latency. For instance, if there is a failure in the optical network, optical network protocols restore the segment within. Under the second choice, restoration signaling is always end-to-end between IP routers, essentially by-passing the optical network. A result of this is that restoration latency could be higher. In addition, restoration protocol in the IP layer must run transparently over the optical network in the overlay mode. 8. IP-based Optical Control Plane Issues Provisioning and restoring lightpaths end-to-end between IP networks requires protocol and signaling support within optical sub-networks and across the interface NNI. In this regard, a distinction is made between control procedures within an optical sub-network (Figure 1) and those between sub-networks. The general guideline followed in this framework is to separate these two cases, and allow the possibility that different control procedures are followed inside different sub-networks, while a common set of procedures are followed across sub-networks (over interface NNI). Clearly, it is possible to follow the same control procedures inside a sub-network as defined for control across sub-networks. But this is left as a choice as per this framework, rather than a mandate. In the following, signaling and routing within and across sub-networks are considered. 8.1 Addressing For interoperability across optical sub-networks using an IP-centric control plane, the fundamental issue is that of addressing. What entities should be identifiable from a signaling and routing point of view? How should they be addressed? This section presents some guidelines on this. Identifiable entities in optical networks includes OXCs, optical links, optical channels and sub-channels, Shared Risk Link Groups (SRLGs), etc. An issue here is how granular the identification should be as far as the establishment of optical trails are concerned. The scheme for identification must accommodate the specification of the termination points in the optical network with adequate granularity when establishing optical trails. For instance, an OXC could have many ports, each of which may in turn terminate many optical channels, each of which contain many subchannels etc. It is perhaps not reasonable to assume that every sub-channel or channel termination, or even OXC ports could be assigned a unique IP address. Also, the routing of an optical trail within the network does not depend on the precise termination point information, but rather only on the terminating OXC. Thus, finer granularity identification of termination points is of relevance only to the terminating OXC and not to intermediate OXCs (of course, resource Expires on 5/24/01 Page 21 of 36 draft-many-ip-optical-framework-02.txt allocation at each intermediate point would depend on the granularity of resources requested). This suggests an identification scheme whereby OXCs are identified by a unique IP address and a "selector" identifies further fine-grain information of relevance at an OXC. This, of course, does not preclude the identification of these termination points directly with IP addresses(with a null selector). The selector can be formatted to have adequate number of bits and a structure that expresses port, channel, sub-channel, etc, identification. Within the optical network, the establishment of trail segments between adjacent OXCs require the identification of specific port, channel, sub-channel, etc. With an MPLS-based control plane, a label serves this function. The structure of the "optical label" must be such that it can encode the required information [7]. Another entity that must be identified is the SRLG [16]. An SRLG is an identifier assigned to a group of optical links that share a physical resource. For instance, all optical channels routed over the same fiber could belong to the same SRLG. Similarly, all fibers routed over a conduit could belong to the same SRLG. The notable characteristic of SRLGs is that a given link could belong to more than one SRLG, and two links belonging to a given SRLG may individually belong to two other SRLGs. This is illustrated in Figure 6. Here, the links 1,2,3 and 4 may belong to SRLG 1, links 1,2 and 3 could belong to SRLG 2 and link 4 could belong to SRLG 3. Similarly, links 5 and 6 could belong to SRLG 1, and links 7 and 8 could belong to SRLG 4. (In this example, the same SRLG, i.e., 1, contains links from two different adjacencies). While the classification of physical resources into SRLGs is a manual operation, the assignment of unique identifiers to these SRLGs within an optical network is essential to ensure correct SRLG-disjoint path computation for protection. SRLGs could be identified with a flat identifier (e.g., 32 bit integer). Finally, optical links between adjacent OXCs may be bundled for advertisementinto a link state protocol [16]. A bundled interface may be numbered or unnumbered. In either case, the component links within the bundle must be identifiable. In concert with SRLG identification, this information is necessary for correct path computation [16]. 8.2 Neighbor Discovery Routing within the optical network relies on knowledge of network topology and resource availability. This information may be gathered and used by a centralized system, or by a distributed link state routing protocol. In either case, the first step towards network- wide link state determination is the discovery of the status of local links to all neighbors by each OXC. Specifically, each OXC must determine the up/down status of each optical link, the Expires on 5/24/01 Page 22 of 36 draft-many-ip-optical-framework-02.txt bandwidth and other parameters of the link, and the identity of the remote end of the link (e.g., remote port number). The last piece of information is used to specify an appropriate label when signaling for lightpath provisioning. The determination of these parameters could be based on a combination of manual configuration and an automated protocol running between adjacent OXCs. The characteristics of such a protocol would depend on the type of OXCs that are adjacent (e.g., transparent or opaque). Generically, the protocol may be refered to as the "Neighbor Discovery Protocol (NDP)" although other functions such as link management and fault isolation may be performed as part of the protocol (e.g., LMP [4]). NDP would typically require in-band communication on the bearer channels to determine local connectivity and link status. In the case of opaque OXCs with SONET termination, one instance of NDP would run on each OXC port, communicating with the corresponding NDP instance at the neighboring OXC. The protocol would utilize the SONET overhead bytes to transmit the (configured) local attributes periodically to the neighbor. Thus, two neighboring switches can automatically determine the identities of each other and the local connectivity,and also keep track of the up/down status of local links. Neighbor discovery with transparent OXCs is described in [4]. +--------------+ +------------+ +------------+ | +-1:OC48---+ +-5:OC192-+ | | +-2:OC48---+ +-6:OC192-+ | | OXC1 +-3:OC48---+ OXC2 +-7:OC48--+ OXC3 | | +-4:OC192--+ +-8:OC48--+ | | | | | +------+ | +--------------+ +----+-+-----+ | +----+------+-----+ | | | | | | | | | | +--------------+ | | | | | | | +----+-+-----+ | | +------+-----+ | +----------+ +--+ | | | | OXC4 +----------+ +----+ | | | +----------+ OXC5 +--------+ OXC6 | | | | +--------+ | +--------------+ | | | | +------+-----+ +------+-----+ Figure 6: Mesh Optical Network with SRLGs 8.3 Topology Discovery Topology discovery is the procedure by which the topology and resource state of all the links in a sub-network are determined. Topology discovery may be done using a link state routing protocol (e.g., OSPF, ISIS), or it can be through a management protocol (in Expires on 5/24/01 Page 23 of 36 draft-many-ip-optical-framework-02.txt the case of centralized path computation). The focus in this framework is on fully distributed route computation using an IP link state protocol. In general, most of the link state routing functionality is maintained when applied to optical networks. However, the representation of optical links, as well as some link parameters, are changed in this setting. Specifically, o The link state information may consist of link bundles [16]. Each link bundle is represented as an abstract link in the network topology. Different bundling representations are possible. For instance, the parameters of the abstract link may include the number, bandwidth and the type of optical links contained in the underlying link bundle [16]. Also, the SRLGs corresponding to each optical link in the bundle may be included as a parameter. o The link state information should capture restoration-related parameters for optical links. Specifically, with shared protection (Section 8.5), the link state updates must have information that allows the computation of shared protection paths. o A single routing adjacency could be maintained between neighbors which may have multiple optical links (or even multiple link bundles) between them. This reduces the protocol messaging overhead. o Since link availability information changes dynamically, a flexible policy for triggering link state updates based on availability thresholds may be implemented. For instance, changes in availability of links of a given bandwidth (e.g., OC-48) may trigger updates only after the availability figure changes by a certain percentage. These concepts are relatively well-understood. On the other hand, the resource representation models and the topology discovery process for hierarchical routing (e.g., OSPF with multiple areas) are areas that need further work. 8.4 Restoration Models Automatic restoration of lightpaths is a service offered by optical networks. There could be local and end-to-end mechanisms for restoration of lightpaths within a sub-network. Local mechanisms are used to select an alternate link between two adjacent OXCs when a failure affects the primary link over which the (protected) lightpath is being routed. Local restoration does not affect the end-to-end route of the lightpath. When local restoration is not possible (e.g., no alternate link is available between the adjacent OXCs in question), end-to-end restoration may be performed. With this, the affected lightpath may be rerouted over an alternate path that completely avoids the OXCs or the link segment where the Expires on 5/24/01 Page 24 of 36 draft-many-ip-optical-framework-02.txt failure occurred. For end-to-end restoration, alternate paths are typically pre-computed. Such back-up paths may have to be physically diverse from the corresponding primary paths. End-to-end restoration may be based on two types of protection schemes; "1 + 1" protection or shared protection. Under 1 + 1 protection, a back-up path is established for the protected primary path along a physically diverse route. Both paths are active and the failure along the primary path results in an immediate switch-over to the back-up path. Under shared protection, back-up paths corresponding to physically diverse primary paths may share the same network resources. When a failure affects a primary path, it is assumed that the same failure will not affect the other primary paths whose back-ups share resources. 8.5 Route Computation The computation of a primary route for a lightpath within an optical sub-network is essentially a constraint-based routing problem. The constraint is typically the bandwidth required for the lightpath, perhaps along with administrative and policy constraints. The objective of path computation could be to minimize the total capacity required for routing lightpaths [17]. Route computation with constraints may be accomplished using a number of algorithms [18]. When 1+1 protection is used, a back-up path that does not traverse on any link which is part of the same SRLG as links in the primary path must be computed. Thus, it is essential that the SRLGs in the primary path be known during alternate path computation, along with the availability of resources in links that belong to other SRLGs. This requirement has certain implications on optical link bundling. Specifically, a bundled LSA must include adequate information such that a remote OXC can determine the resource availability under each SRLG that the bundled link refers to, and the relationship between links belonging to different SRLGs in the bundle. For example, considering Figure 3, if links 1,2,3 and 4 are bundled together in an LSA, the bundled LSA must indicate that there are three SRLGs which are part of the bundle (i.e., 1, 2 and 3), and that links in SRLGs 2 and 3 are also part of SRLG 1. It is somewhat complex to encode the SRLG relationships in a link bundle LSA. This information, however, is naturally captured if the link bundle is encoded as a set of "link groups", each specifying the links that belong to exactly the same set of SRLGs. Within the link group, it is possible to specify the number of links of a particular type, for example, OC-48. With reference to Figure 3, for example, a bundle LSA can be advertised for the entire set of links between OXC1 and OXC2, with the following information: Expires on 5/24/01 Page 25 of 36 draft-many-ip-optical-framework-02.txt Link Group ID SRLGs Link Type Number Other Info ------------- ----- --------- ------ ---------- 1 1,2 OC-48 3 --- 2 1,3 OC-192 1 --- Assuming that the above information is available for each bundle at every node, there are several approaches possible for path computation. For instance, 1. The primary path can be computed first, and the (exclusive or shared) back-up is computed next based on the SRLGs chosen for the primary path. In this regard, o The primary path itself can be computed by taking into account specific link groups in a bundle. That is, the primary path computation procedure can output a series of link groups the path is routed over. Since a link group is uniquely identified with a set of SRLGs, the alternate path can be computed right away based on this knowledge. In this case, if the primary path set up does not succeed for lack of resources in a chosen link group, the primary and backup paths muse be recomputed. o It might be desirable to compute primary paths using bundle- level information (i.e., resource availability in all link groups in a bundle) rather than specific link group level information. In this case, the primary path computation procedure would output a series of bundles the path traverses. Each OXC in the path would have the freedom to choose the particular link group to route that segment of the primary path. This procedure would increase the chances of successfully setting up the primary path when link state information is not up to date everywhere. But the specific link group chosen, and hence the SRLGs in the primary path, must be captured during primary path set-up, for example, using the RSVP-TE Route Record Object [19]. This SRLG information is then used for computing the back-up path. The back-up path may also be established specifying only which SRLGs to AVOID in a given segment, rather than which link groups to use. This would maximize the chances of establishing the back-up. 2. The primary path and the back-up path are comptuted together in one step, for example, using Suurbaale's algorithm [20]. In this case, the paths must be computed using specific link group information. To summarize, it is essential to capture sufficient information in link bundle LSAs to accommodate different path computation procedures and to maximize the chances of successful path establishment. Depending on the path computation procedure used, the type of support needed during path establishment (e.g., the recording of link group or SRLG information during path establishment) may differ. Expires on 5/24/01 Page 26 of 36 draft-many-ip-optical-framework-02.txt When shared protection is used, the route computation algorithm must take into account the possibility of sharing links among multiple back-up paths. Under shared protection, the back-up paths corresponding to SRLG-disjoint primary paths can be assigned the same links. The assumption here is that since the primary paths are not routed over links that have the same SRLG, a given failure will affect only one of them. Furthermore, it is assumed that multiple failure events affecting links belonging to more than one SRLG will not occur concurrently. Unlike the case of 1+1 protection, the back-up paths are not established apriori. Rather, a failure event triggers the establishment of a single back-up path corresponding to the affected primary path. The distributed implementation of route computation for shared back- up paths require knowledge about the routing of all primary and back-up paths at every node. This raises scalability concerns. For this reason, it may be practical to consider the centralization of the route computation algorithm in a route server that has complete knowledge of the link state and path routes. Heuristics for fully distributed route computation without complete knowledge of path routes are to be determined. Path computation for restoration is further described in [17, 21]. 8.6 Signaling Issues Signaling within an optical sub-network for lightpath provisioning is a relatively simple operation. After a route is determined for a lightpath, each OXC in the path must establish appropriate cross- connects in a coordinated fashion. This coordination is akin to selecting incoming and outgoing labels in a label-switched environment. Thus, protocols like RSVP-TE [14] and CR-LDP [13] can be used for this. A few new concerns, however, must be addressed. 8.6.1 Bi-Directional Lightpath Establishment Lightpaths are typically bi-directional. That is, the output port selected at an OXC for the forward direction is also the input port for the reverse direction of the path. Since signaling for optical paths may be autonomously initiated by different nodes, it is possible that two path set-up attempts are in progress at the same time. Specifically, while setting up an optical path, an OXC A may select output port i which is connected to input port j of the "next" OXC B. Concurrently, OXC B may select output port j for setting up a different optical path, where the "next" OXC is A. This results in a "collision". Similarly, when WDM functionality is built into OXCs, a collision occurs when adjacent OXCs choose directly connected output ports and the same wavelength for two different optical paths. There are two ways to deal with such collisions. First, collisions may be detected and the involved paths may be torn down and re-established. Or, collisions may be avoided altogether. Expires on 5/24/01 Page 27 of 36 draft-many-ip-optical-framework-02.txt 8.6.2 Failure Recovery The impact of transient partial failures must be minimized in an optical network. Specifically, optical paths that are not directly affected by a failure must not be torn down due to the failure. For example, the control processor in an OXC may fail, affecting signaling and other internodal control communication. Similarly, the control channel between OXCs may be affected temporarily by a failure. These failure may not affect already established optical paths passing through the OXC fabric. The detection of such failures by adjacent nodes, for example, through a keepalive mechanism between signaling peers, must not result in these optical paths being torn down. It is likely that when the above failures occur, a backup processor or a backup control channel will be activated. The signaling protocol must be designed such that it is resilient to transient failures. During failure recovery, it is desirable to recover local state at the concerned OXC with least disruption to existing optical paths. 8.6.3 Restoration Signaling for restoration has two distict phases. There is a reservation phase in which capacity for the protection path is established. Then, there is an activation phase in which the back-up path is actually put in service. The former phase typically is not subject to strict time constraints, while the latter is. Signaling to establish a "1+1" back-up path is relatively straight- forward. This signaling is very similar to signaling used for establishing the primary path. Signaling to establish a shared back- up path is a little bit different. Here, each OXC must understand which back-up paths can share resources. The signaling message must itself indicate shared reservation. The sharing rule is as described in Section 8.4: back-up paths corresponding to physically diverse primary paths may share the same network resources. It is therefore necessary for the signaling message to carry adequate information that allows an OXC to verify that back-up paths that share a certain resources are allowed to do so. Under both 1+1 and shared protection, the activation phase has two parts: propagation of failure information to the source OXC from the point of failure, and activation of the back-up path. The signaling for these two phases must be very fast in order to realize response times in the order of tens of milliseconds. When optical links are SONET-based, in-band signals may be used, resulting in quick response. With out-of-band control, it is necessary to consider fast signaling over the control channel using very short IP packets and prioritized processing. While it is possible to use RSVP or CR- LDP for activating protection paths, these protocols do not provide any means to give priority to restoration signaling as opposed to signaling for provisioning. For instance, it is possible for a Expires on 5/24/01 Page 28 of 36 draft-many-ip-optical-framework-02.txt restoration-related RSVP message to be queued behind a number of provisioning messages thereby delaying restoration. It is therefore necessary to develop a definition of QoS for restoration signaling and incorporate mechanisms in existing signaling protocols to achieve this. Or, a new signaling protocol may be developed exclusively for activating protection paths during restoration. 8.7 Optical Internetworking Ideally, a set of interconnected optical sub-networks must be functionally similar to a single optical sub-network. Thus, it must be possible to dynamically provision and restore lightpaths across optical sub-networks. Therefore: o A standard scheme for uniquely identifying lightpath end-points in different sub-networks is required. o A protocol is required for determining reachability of end-points across sub-networks. o A standard signaling protocol is required for provisioning lightpaths across sub-networks. o A standard procedure is required for the restoration of lightpaths across sub-networks. o It should be possible to apply proprietary provisioning and restoration procedures for the segment of a lightpath passing through a given sub-net. The IP-centric control architecture for optical sub-networks can be extended to satisfy the functional requirements of optical internetworking. Routing and signaling interaction between optical sub-networks can be standardized across the interface NNI (Figure 1). For the joint control and management of the network, an integration of the sub-network management systems is required. The functionality provided across NNI is as follows. 8.7.1 Neighbor Discovery NDP, as described in Section 8.2, can be used for this. Indeed, a single protocol should be standardized for neighbor discovery within and across sub-networks. 8.7.2 Addressing and Routing Model The addressing mechanisms described in Section 8.1 can be used to identify OXCs, ports, channels and sub-channels in each sub-network. It is essential that the OXC IP addresses are unique network-wide. Provisioning an end-to-end lightpath across multiple sub-networks involves the establishment of path segments in each sub-network Expires on 5/24/01 Page 29 of 36 draft-many-ip-optical-framework-02.txt sequentially. Thus, a path segment is established from the source OXC to a border OXC in the source sub-network. From this border OXC, signaling across NNI is used to establish a path segment to a border OXC in the next sub-network. Provisioning then continues in the next sub-network and so on until the destination OXC is reached. A version of BGP may be used to determine the routes to destinations across sub-networks. Using exterior BGP, adjacent border OXCs in different sub-networks can exchange reachability of OXCs and other external IP endpoints (border routers). Using interior BGP, the same information is propagated from one border OXC to others in the same sub-network. Thus, every border OXC eventually learns of all IP addresses reachable across different neighboring sub-networks. These addresses may be propagated to other OXCs within the sub-network thereby allowing them to select appropriate border OXCs as exit points for external destinations. To support VPNs, the external reachability information should include VPN identifiers. 8.7.3 Restoration It is likely that proprietary restoration schemes may be implemented within optical sub-networks. It is therefore necessary to consider a two-level restoration mechanism. Path failures within an optical sub-network should be handled using procedures specific to the sub-network. If this fails, end-to-end restoration across sub- networks should be invoked. The border OXC that is the ingress to a sub-network can act as the source for restoration procedures within a sub-network. The signaling for invoking end-to-end restoration across NNI is similar to the signaling described in Section 8.6.3. The computation of the back-up path for end-to-end restoration may be based on various criteria. It is assumed that the back-up path is computed by the source OXC, and signaled using standard methods. 9. Other Issues 9.1 WDM and TDM in the Same Network A practical assumption would be that if SONET (or some other TDM mechanism that is capable partitioning the bandwidth of a wavelength) is used, then TDM is leveraged as an additional method to differentiate between "flows." In such cases, wavelengths and time intervals (sub-channels) within a wavelength become analogous to labels (as noted in [1]) which can be used to make switching decisions. This would be somewhat akin to using VPI (e.g., wavelength) and VCI (e.g., TDM sub-channel) in ATM networks. More generally, this will be akin to label stacking and to LSP nesting within the context of Multi-Protocol Lambda Switching [1]. GMPLS signaling [7] supports this type of multiplexing. 9.2 Wavelength Conversion Some form of wavelength conversion may exist at some switching elements. This however may not be case in some pure optical Expires on 5/24/01 Page 30 of 36 draft-many-ip-optical-framework-02.txt switching elements. A switching element is essentially anything more sophisticated than a simple repeater, that is capable of switching and converting a wavelength Lambda(k) from an input port to a wavelength Lambda(l) on an output port. In this display, it is not necessarily the case that Lambda(k) = Lambda(l), nor is it necessarily the case that the data carried on Lambda(k) is switched through the device without being examined or modified. It is not necessary to have a wavelength converter at every switching element. A number of studies have attempted to address the issue of the value of wavelength conversion in an optical network. Such studies typically use the blocking probability (the probability that a lightpath cannot be established because the requisite wavelengths are not available) as a metric to adjudicate the effectiveness of wavelength conversion. The IP over optical architecture must take into account hybrid networks with some OXCs capable of wavelength conversion and others incapable of this. The GMPLS "label set" mechanism [7] supports the selection of the same label (i.e., wavelength) across an optical sub-network. 9.3 Service Provider Peering Points There are proposed inter-network interconnect models which allow certain types of peering relationships to occur at the optical layer. This is consistent with the need to support optical layer services independent of higher layers payloads. In the context of IP over optical networks, peering relationships between different trust domains will eventually have to occur at the IP layer, on IP routing elements, even though non-IP paths may exist between the peering routers. 9.4 Rate of Lightpath Set-Up Dynamic establishment of optical channel trails and lightpaths is quite desirable in IP over optical networks, especially when such instantiations are driven by a stable traffic engineering control system, or in response to authenticated and authorized requests from client. However, there are many proposals suggesting the use of dynamic, data-driven shortcut-lightpath setups in IP over optical networks. The arguments put forth in such proposals are quite reminiscent of similar discussions regarding ATM deployment in the core of IP networks. Deployment of highly dynamic data driven shortcuts within core networks has not been widely adopted by carriers and ISPs for a number of reasons: possible CPU overhead in core network elements, complexity of proposed solutions, stability concerns, and lack of true economic drivers for this type of service. This draft assumes that this paradigm will not change and that highly dynamic, data- driven shortcut lightpath setups are for future investigation. Instead, the optical channel trails and lightpaths that are expected to be widely used at the initial phases in the evolution of IP over optical networks will include the following: Expires on 5/24/01 Page 31 of 36 draft-many-ip-optical-framework-02.txt o Dynamic connections for control plane traffic and default path routed data traffic, o Establishment and re-arrangement of arbitrary virtual topologies over rings and other physical layer topologies. o Use of stable traffic engineering control systems to engineer lightpath connections to enhance network performance, either for explicit demand based QoS reasons or for load balancing). Other issues surrounding dynamic connection setup within the core center around resource usage at the edge of the optical domain. One potential issue pertains to the number of flows that can be processed by an ingress or egress network element either because of aggregate bandwidth limitations or because of a limitation on the number of flows (e.g., lightpaths) that can be processed concurrently. Another possible short term reason for dynamic shortcut lightpath setup would be to quickly pre-provisioned paths based on some criteria (TOD, CEO wants a high BW reliable connection, etc.). In this scenario, a set of paths is pre-provisioned, but not actually instantiated until the customer initiates an authenticated and authorized setup requests, which is consistent with existing agreements between the provider and the customer. In a sense, the provider may have already agreed to supply this service, but will only instantiate it by setting up a lightpath when the customer submits an explicit request. 9.5 Distributed vs. Centralized Provisioning This draft has mainly dealt with a distributed model for lightpath provisioning, in which all nodes maintain a synchronized topology database, and advertise topology state information to maintain and refresh the database. A constraint-based routing entity in each node then uses the information in the topology database and other relevant details to compute appropriate paths through the optical domain. Once a path is computed, a signaling protocol (e.g., [14]) is used to instantiate the lightpath. Another provisioning model is to have a centralized server which has complete knowledge of the physical topology, the available wavelengths, and where applicable, relevant time domain information. A corresponding client will reside on each network element that can source or sink a lightpath. The source client would query the server in order to set up a lightpath from the source to the destination. The server would then check to see if such a lightpath can be established based on prevailing conditions. Furthermore, depending on the specifics of the model, the server may either setup the lightpath on behalf of the client or provide the necessary Expires on 5/24/01 Page 32 of 36 draft-many-ip-optical-framework-02.txt information to the client or to some other entity to allow the lightpath to be instantiated. Centralization aids in implementing complex capacity optimization schemes, and may be the near-term provisioning solution in optical networks with interconnected multi-vendor optical sub-networks. In the long term, however, the distributed solution with centralization of some control procedures (e.g., traffic engineering) is likely to be the approach followed. 10. Evolution Path for IP over Optical Architecture The architectural models described in Section 7 imply a certain degree of implementation complexity. Specifically, the overlay model was described as the least complex for near term deployment and the peer model the most complex. Nevertheless, each model has certain advantages and this raises the question as to the evolution path for IP over optical network architectures. The evolution approach recommended in this framework is the definition of capability sets that start with simpler functionality in the beginning and include more complex functionality later. In this regard, it is realistic to expect that initial IP over optical deployments will be based on the domain services model (with overlay interconnection), with no routing exchange between the IP and optical domains. Under this model, direct signaling between IP routers and optical networks is likely to be triggered by offline traffic engineering decisions. The next step in the evolution of IP- optical interaction is the introduction of reachability information exchange between the two domains. This would potentially allow lightpaths to be established as part of end-to-end LSP set-up. The final phase is the support for the full peer model with more sophisticated routing interaction between IP and optical domains. Using a common signaling framework (based on GMPLS) from the beginning facilitates this type of evolution. For the domain services model, implementation agreement based on GMPLS UNI signaling is being developed in the Optical Interworking Forum (OIF) [8, 11, 12]. This agreement is aimed at near term deployment and this could be the precursor to a future peer model architecture. In this evolution, the signaling capability and semantics at the IP- optical boundary would become more sophisticated, but the basic structure of signaling would remain. This would allow incremental developments as the interconnection model becomes more sophisticated, rather than complete re-development of signaling capabilities. 11. Security Considerations TBD. Expires on 5/24/01 Page 33 of 36 draft-many-ip-optical-framework-02.txt 12. Summary and Conclusions The objective of this draft was to define a framework for IP over optical networks, considering the service models, routing and signaling issues. There are a diversity of choices, as described in this draft, for IP-optical interconnection, service models and protocol mechanisms. The approach advocated in this draft was to allow different service models and proprietary enhancements in optical networks, and define complementary signaling and routing mechanisms that would support these. An evolution scenario, based on a common GMPLS-based signaling framework with increasing interworking functionality was suggested. Under this scenario, the IP-optical interaction is first based on the domain services model with overlay interconnection that eventually evolves to support full peer interaction. 13. References 1. D. Awduche, Y. Rekhter, J. Drake, R. Coltun, "Multi-Protocol Lambda Switching: Combining MPLS Traffic Engineering Control With Optical Crossconnects," draft-awduche-mpls-te-optical- 02.txt, Work in Progress, July, 2000. 2. K. Arvind, et. al, "Optical Domain Services Interconnect (ODSI) Signaling Control Specification, Version 1.4.5" www.odsi- coalition.com, March, 2000. 3. D. Pendarakis, B. Rajagopalan and D. Saha, "Routing Information Exchange in Optical Networks," draft-prs-optical-routing-01.ps, Internet Draft, Work in Progress, November, 2000. 4. J. P. Lang, et. al., "Link Management Protocol," draft-ietf-mpls- lmp-01.txt, Internet Draft, Work in progress, November, 2000. 5. K. Kompella et al, "OSPF Extensions in Support of Generalized MPLS," draft-kompella-ospf-gmpls-extensions-00.txt, Work in Progress, November, 2000. 6. K. Kompella and Y. Rekhter, "LSP Hierarchy with MPLS TE," draft- ietf-mpls-lsp-hierarchy-01.txt, Work in Progress, November, 2000. 7. P. Ashwood-Smith et. al, "Generalized MPLS - Signaling Functional Description", draft-ietf-mpls-generalized-signaling-00.txt, Internet Draft, Work in Progress, November, 2000. 8. O. Abul-Magd, et. al., "Signaling Requirements at the Optical UNI," draft-bala-mpls-optical-uni-signaling-01.txt, Internet Draft, Work in Progress, November, 2000. 9. Y. Rekhter and T. Li, "A Border Gateway Protocol 4 (BGP4)",RFC 1771, March, 1995. Expires on 5/24/01 Page 34 of 36 draft-many-ip-optical-framework-02.txt 10. E. Rosen and Y. Rekhter, "BGP/MPLS VPNs", RFC 2547, March, 1999. 11. E. Gray, et. al., "RSVP Extensions in Support of OIF Optical UNI Signaling," draft-gray-mpls-rsvp-oif-uni-ext-00.txt, Internet Draft, Work in Progress, October, 2000. 12. O. Abul-Magd, et. al., "LDP Extensions for Optical UNI Signaling," draft-ietf-mpls-ldp-optical-uni-00.txt, Internet Draft, Work in Progress, October, 2000. 13. P. Ashwood-Smith, et. al., "Generalized MPLS - CR-LDP Signaling Functional Description," draft-ietf-mpls-generalized-cr-ldp- 00.txt, Internet Draft, Work in Progress, November, 2000. 14. P. Ashwood-Smith, et. al., "Generalized MPLS - RSVP-TE Signaling Functional Description," draft-ietf-mpls-generalized- rsvp-te-00.txt, Internet Draft, Work in Progress, November, 2000. 15. B. Mack-Crane, et. al., "Enhancements to GMPLS Signaling for Optical Technologies," draft-mack-crane-gmpls-signaling- enchancements-00.txt, Internet Draft, Work in Progress, November, 2000. 16. B. Rajagopalan and D. Saha, "Link Bundling in Optical Networks," draft-rs-optical-bundling-01.txt, Internet Draft, Work in Progress, October, 2000. 17. B. Doshi, S. Dravida, P. Harshavardhana, et. al, "Optical Network Design and Restoration," Bell Labs Technical Journal, Jan-March, 1999. 18. E. Crawley, R. Nair, B. Rajagopalan and H. Sandick, "A Framework for QoS-based Routing in the Internet," RFC 2386, August, 1998. 19. D. Awduche, L. Berger, Der-Hwa Gan, T. Li, G. Swallow, V. Srinivasan, "RSVP-TE: Extensions to RSVP for LSP Tunnels,"draft- ietf-mpls-rsvp-lsp-tunnel-07.txt, Internet Draft, Work in progress, October, 2000. 20. J. Suurballe, "Disjoint Paths in a Network," Networks, vol. 4, 1974. 21. S. Ramamurthy, Z. Bogdanowicz, S. Samieian, et al., "Capacity Performance of Dynamic Provisioning in Optical Networks", to appear in J. of Lightwave Technology. 14. Acknowledgments We would like to thank Zouheir Mansourati and Ian Duncan of Nortel Networks for their comments on this draft. Expires on 5/24/01 Page 35 of 36 draft-many-ip-optical-framework-02.txt 15. Author's Addresses Bala Rajagopalan James V. Luciani Debanjan Saha TollBridge Technologies Tellium, Inc. P,O. Box 1010 2 Crescent Place Concord, MA 01742 P.O. Box 901 Email: james_luciani@mindspring.com Oceanport, NJ 07757-0901 Email: {braja, dsaha}@tellium.com Daniel O. Awduche Brad Cain, Bilel Jamoussi UUNET (MCI Worldcom) Nortel Networks Loudoun County Parkway 600 Tech Park Ashburn, VA 20247 Billerica, MA 01821 Phone: 703-886-5277 Phone: 978-288-4734 Email: awduche@uu.net Email: bcain@nortelnetworks.com jamoussi@nortelnetworks.com ******** This draft expires on May, 24, 2001 *********** Expires on 5/24/01 Page 36 of 36