gmpls

Web ProForum Tutorials http://www.iec.org Copyright © The International Engineering Consortium 1/27 Generalized Multiprotocol Label Switching (GMPLS) Definition and Overview The premise of multiprotocol label switching (MPLS) is to speed up packet forwarding and provide for traffic engineering in Internet protocol (IP) networks. To accomplish this, the connectionless operation of IP networks becomes more like a connection-oriented network where the path between the source and the destination is precalculated based on user specifics. To speed up the forwarding scheme, an MPLS device uses labels rather than address matching to determine the next hop for a received packet. To provide traffic engineering, tables are used that represent the levels of quality of service (QoS) that the network can support. The tables and the labels are used together to establish an end-to-end path called a label switched path (LSP). Traditional IP routing protocols (e.g., open shortest path first [OSPF] and intermediate system to intermediate system [IS–IS]) and extensions to existing signaling protocols (e.g., resource reservation protocol [RSVP] and constraint-based routing–label distribution protocol [CR–LDP]) comprise the suite of MPLS protocols. Generalized MPLS (GMPLS) extends MPLS to provide the control plane (signaling and routing) for devices that switch in any of these domains: packet, time, wavelength, and fiber. This common control plane promises to simplify network operation and management by automating end-to-end provisioning of connections, managing network resources, and providing the level of QoS that is expected in the new, sophisticated applications. This tutorial focuses on the issues that GMPLS resolves in providing a common control plane to operate across dissimilar network types (e.g., packet, time division multiplexing [TDM], and optical). Initially, a brief overview of MPLS and its evolution to GMPLS is given. Next, a summary of GMPLS protocols and important extensions is presented. In-depth coverage of the issues is then provided. At the end, some of the current outstanding issues in GMPLS are explored. Web ProForum Tutorials http://www.iec.org Copyright © The International Engineering Consortium 2/27 Topics 1. Introduction 2. Evolution from MPLS 3. GMPLS Issues and their Resolutions 4. GMPLS Outstanding Issues Self-Test Correct Answers Glossary 1. Introduction The emergence of optical transport systems has dramatically increased the raw capacity of optical networks and has enabled a slew of new, sophisticated applications. For example, network-based storage, bandwidth leasing, data mirroring, add/drop multiplexing [ADM], dense wavelength division multiplexing [DWDM], optical cross-connect [OXC], photonic cross-connect [PXC], and multiservice switching platforms are some of the devices that may make up an optical network and are expected to be the main carriers for the growth in data traffic. The diversity and complexity in managing these devices have been the main driving factors in the evolution and enhancement of the MPLS suite of protocols to provide control for not only packet-based domains, but also time, wavelength, and space domains. GMPLS further extends the suite of IP-based protocols that manage and control the establishment and release of label switched paths (LSP) that traverse any combination of packet, TDM, and optical networks. An important economic impact of GMPLS is providing the ability to automate network resource management and the service provisioning of end-to-end traffic-engineered paths. Service provisioning has been a manual, lengthy, and costly process—e.g., synchronous optical network (SONET)–based ring networks. To manually provision an end-to-end high-speed connection, a carrier must determine which SONET rings the connection traverses and provision bandwidth on each ring manually. If any ring is at full capacity, the carrier must find an alternative ring path or upgrade the capacity of a ring and propagate the information to all sites manually. These are very time-consuming processes and can take months. The deployment of GMPLS–based nodes allows carriers to automate the provisioning and management of the network and promises to Web ProForum Tutorials http://www.iec.org Copyright © The International Engineering Consortium 3/27 lower the cost of operation by several orders of magnitude (days or even minutes instead of weeks or months). 2. Evolution from MPLS MPLS Background and Operation MPLS extended the suite of IP protocols to expedite the forwarding scheme used by IP routers. Routers, to date, have used complex and time-consuming route lookups and address matching schemes to determine the next hop for a received packet, primarily by examining the destination address in the header of the packet. MPLS has greatly simplified this operation by basing the forwarding decision on a simple label. Another major feature of MPLS is its ability to place IP traffic on a defined path through the network. This capability was not previously possible with IP traffic. In this way, MPLS provides bandwidth guarantees and other differentiated service features for a specific user application (or flow). Current IP–based MPLS networks are capable of providing advanced services such as bandwidth-based guaranteed service, priority-based bandwidth allocation, and preemption services. For each specific service a table of forwarding equivalence class (FEC) is created to represent a group of flows with the same traffic-engineering requirements. A specific label is then bound to an FEC. At the ingress of an MPLS network, incoming IP packets are examined and assigned a "label" by a label edge router (LER). The labeled packets are then forwarded along an LSP, where each label- switched router (LSR) makes a switching decision based on the packet's label field. An LSR does not need to examine the IP headers of the packets to find an output port (next hop). An LSR simply strips off the existing label and applies a new label for the next hop. The label information base (LIB) provides an outgoing label (to be inserted into the packet) and an outgoing interface (based on an incoming label on an incoming interface). Signaling to establish a traffic-engineered LSP is done using a label distribution protocol that runs on every MPLS node. There are a number of different label- distribution protocols. The two most popular RSVP–traffic engineering (RSVP– TE) and CR–LDP. RSVP–TE is an extended version of the original RSVP to piggyback and distribute labels on its messages and to provide traffic- engineering capability. CR–LDP was designed specifically for this purpose. Figure 1 shows the flow of label distribution that is carried out by the downstream LER (in this case LER2) while the LSP flow is the reverse. Web ProForum Tutorials http://www.iec.org Copyright © The International Engineering Consortium 4/27 Figure 1. Figure 1: An MPLS-Based Network The MPLS framework includes extensions to existing IP link-state routing protocols. These protocols provide real-time coordination of the current network topology, including attributes of each link. MPLS extensions to OSPF and IS–IS allow nodes to not only exchange information about the network topology, but also resource information and even policy information—for example, IP addresses, available bandwidth, and load-balancing policies. Constraint-based routing algorithms use this information to compute the optimal paths for the LSPs through the network and allow complex traffic-engineering decisions to be made automatically when selecting routes through the network. MPLS Evolution to GMPLS Within the past year, the International Engineering Task Force (IETF) has extended the MPLS suite of protocols to include devices that switch in time, wavelength, (e.g., DWDM) and space domains (e.g., OXC) via GMPLS. This allows GMPLS–based networks to find and provision an optimal path based on user traffic requirements for a flow that potentially starts on an IP network, is then transported by SONET, and then is switched through a specific wavelength on a specific physical fiber. Table 1 gives a summary of the GMPLS framework. Table 1. GMPLS Framework Switching Domain Traffic Type Forwarding Scheme Example of Device Nomenclature Packet, cell IP, asynchronous transfer mode (ATM) Label as shim header, virtual channel connection (VCC) IP router, ATM switch Packet switch capable (PSC) Web ProForum Tutorials http://www.iec.org Copyright © The International Engineering Consortium 5/27 Time TDM/SONET Time slot in repeating cycle Digital cross- connect system (DCS), ADM TDM capable Wavelength Transparent Lambda DWDM Lambda switch capable (LSC) Physical space Transparent Fiber, line OXC Fiber switch capable (FSC) The basic challenge for an all-encompassing control protocol is the establishment, maintenance, and management of traffic-engineered paths to allow the data plane to efficiently transport user data from the source to the destination. A user flow starting from its source is likely to travel several network spans–for example, an access or edge network that aggregates the flows from multiple users (e.g., enterprise applications) to feed into a metro network that is SONET– based or ATM–based that itself aggregates multiple flows from various edge networks to feed into a long-haul network that uses lambdas to transport the aggregated flow of multiple metro networks. The reverse path is used to deliver data to its destination. These networks and the typical devices used are shown in Figure 2. Figure 2. Dissimilar Networks That Carry End-User Traffic (Click on image for full-size version.) Summary of the GMPLS Protocol Suite The evolution of MPLS into GMPLS has extended the signaling (RSVP–TE, CR– LDP) and routing protocols (OSPF–TE, IS–IS–TE). The extensions accommodate the characteristics of TDM/SONET and optical networks. Web ProForum Tutorials http://www.iec.org Copyright © The International Engineering Consortium 6/27 A new protocol, link-management protocol (LMP), has been introduced to manage and maintain the health of the control and data planes between two neighboring nodes. LMP is an IP-based protocol that includes extensions to RSVP–TE and CR–LDP. Table 2 summarizes these protocols and the extensions for GMPLS. Table 1. GMPLS Protocols Protocols Description Routing OSPF–TE, IS–IS–TE Routing protocols for the auto-discovery of network topology, advertise resource availability (e.g., bandwidth or protection type). The major enhancements are as follows: Advertising of link-protection type (1+1, 1:1, unprotected, extra traffic) Implementing derived links (forwarding adjacency) for improved scalability Accepting and advertising links with no IP address—link ID Incoming and outgoing interface ID Route discovery for back-up that is different from the primary path (shared-risk link group) Signaling RSVP–TE, CR–LDP Signaling protocols for the establishment of traffic- engineered LSPs. The major enhancements are as follows: Label exchange to include non-packet networks (generalized labels) Establishment of bidirectional LSPs Signaling for the establishment of a back-up path (protection Information) Expediting label assignment via suggested label Waveband switching support—set of contiguous wavelengths switched together Link Management LMP Control-Channel Management: Established by negotiating link parameters (e.g., frequency in sending keep-alive messages) and ensuring the health of a link (hello protocol) Link-Connectivity Verification: Ensures the physical connectivity of the link between the neighboring nodes using a PING–like test message Web ProForum Tutorials http://www.iec.org Copyright © The International Engineering Consortium 7/27 Link-Property Correlation: Identification of the link properties of the adjacent nodes (e.g., protection mechanism) Fault Isolation: Isolates a single or multiple faults in the optical domain The details of each protocol and their enhancements are found in the references at the end of this tutorial. The protocol stack is shown in Figure 3. Figure 3. The GMPLS Protocol Stack Note that the IS–IS–TE routing protocol stack is similar to OSPF–TE with the exception that, instead of IP, connectionless network protocol (CLNP) is used to carry IS–IS–TE's information. 3. GMPLS Issues and Their Resolutions For a control plane to be used for all of these dissimilar networks types, the following issues must be considered: 1. Data forwarding is now not limited to that of merely packet forwarding. The general solution must be able to retain the simplicity of forwarding using a label for a variety of devices that switch in time or wavelength, or space (physical ports). 2. Not every type of network is capable of looking into the contents of the received data and of extracting a label. For instance, packet networks are able to parse the headers of the packets, check the label, and carry out decisions for the output interface (forwarding path) that they have to use. This is not the case for TDM or optical networks. The equipments in these Web ProForum Tutorials http://www.iec.org Copyright © The International Engineering Consortium 8/27 types of networks are not designed to have the ability to examine the content of the data that is fed into them. 3. Unlike packet networks, in TDM, LSC, and FSC interfaces, bandwidth allocation for an LSP can be performed only in discrete units. For example, a packet-based network may have flows of 1 Mbps to 10 or 100 Mbps. However, an optical network will use links that have fixed bandwidths: optical carrier (OC)–3, OC–12, OC–48, etc. When a 10 Mbps LSP is initiated by a PSC device and must be carried by optical connections with fixed bandwidths—e.g., an OC–12 line—it would not make sense to allocate an entire 622M line for a 10M flow. 4. Scalability is an important issue in designing large networks to accommodate changes in the network quickly and gracefully. The resources that must be managed in a TDM or optical network are expected to be much larger in scope than in a packet-based network. For optical networks, it is expected that hundreds to thousands of wavelengths (lambdas) will be transporting user data on hundreds of fibers. 5. Configuring the switching fabric in electronic or optical switches may be a time-consuming process. For instance, in a DCS that is capable of switching tens of thousands of digital signal (DS)–1 physical ports, identifying the connection between the input/output ports could be time consuming as fewer ports become available to accommodate incoming user traffic. Latency in setting up an LSP within these types of networks could have a cumulative delaying effect in setting up an end-to-end flow. 6. SONET networks have the inherent ability to perform a fast switchover from a failed path to a working one (50 milliseconds). GMPLS' control plane must be able to accommodate this and other levels of protection granularity. It also needs to provide restoration of failed paths via static (pre-allocated) or dynamic reroute, depending on the required class of service (CoS). These issues are summarized in Table 3 and discussed in subsequent sections in more detail. Table 3. Summary of Issues in a Common Control-Plane Approach Issue GMPLS Solution(s) Protocol(s) Notes Switching Generalized label Signaling: RSVP–TE, LSP to start and end on Web ProForum Tutorials http://www.iec.org Copyright © The International Engineering Consortium 9/27 diversity CR–LDP the same type of device Forwarding diversity Logical or physical separation of control and data All Signaling and routing to travel out of band Configuration Suggested label Bidirectional LSPs Signaling Expedite LSP set-up Scalability Forwarding adjacency Link bundling Hierarchical LSPs Routing and signaling: OSPF–TE, IS–IS–TE Lower link database size Bandwidth scalability Reliability Protection and restoration (M:N, 1+1) Shared-risk link group for path diversity LMP Routing: OSPF–TE, IS–IS–TE Simulate SONET bidirectional line- switched ring (BLSR), unidirectional path- switched ring (UPSR) User disjoint route for back-up Efficient use of network resources Hierarchical LSP Unnumbered links Signaling/routing Save on excess use of scarce IP addresses Switching Diversity Generalized Label and Its Distribution To be able to support devices that switch in different domains, GMPLS introduces new additions to the format of the labels. The new label format is referred to as a "generalized label" that contains information to allow the receiving device to program its switch and forward data regardless of its construction (packet, TDM, lambda, etc.). A generalized label can represent a single wavelength, a single fiber, or a single time-slot. Traditional MPLS labels— e.g., ATM, VCC, or IP shim—are also included. The information that is embedded in a generalized label includes the following: 1. LSP encoding type that indicates what type of label is being carried (e.g., packet, lambda, SONET, etc.) Web ProForum Tutorials http://www.iec.org Copyright © The International Engineering Consortium 10/27 2. Switching type that indicates whether the node is capable of switching packets, time-slot, wavelength, or fiber 3. A general payload identifier to indicate what payload is being carried by the LSP (e.g., virtual tributary [VT], DS-3, ATM, Ethernet, etc.) Details of a GMPLS label can be found in reference [2]. Similar to MPLS, label distribution starts from the upstream LSR requesting a label from the downstream LSR. GMPLS takes this further by allowing the upstream LSR to suggest a label for a LSP that can be overridden by the downstream LSR. (Suggested labels are covered in a later section.) LSP Creation in GMPLS-Based Networks Establishing an LSP in a GMPLS network is similar to that of MPLS networks. Figure 4 shows a packet network (PSC) connected via an OC–12 pipe to DCS1 in the upper TDM network. Both of the TDM networks shown use a SONET UPSR OC–48 ring architecture. The two TDM networks are connected via two OXCs capable of delivering multiple OC–192 lambdas. The goal is to establish an LSP (LSPpc) between LSR1 and LSR4. Figure 4. Establishing an LSP through Heterogeneous Networks with GMPLS To establish the LSPpc between LSR1 and LSR4, other LSPs in the other networks must be established to tunnel the LSPs in the lower hierarchy. For example, per Figure 4, LSP1T1 will carry LSP1, LSP2, and LSP3 if the sum of the traffic- engineering requirements of the packet LSPs can be accommodated by it. This is done by sending a PATH/Label Request message downstream to the destination that will carry the lower hierarchy LSP. For example, DSCi sends this message to OXC1, destined for DSCe. When received by OXC1, it will then create Web ProForum Tutorials http://www.iec.org Copyright © The International Engineering Consortium 11/27 an LSP between it and OXC2. Only when this LSP (LSPl) is established will an LSP between DSCi to DSCe be established (LSPtdi). The PATH/Label Request message contains a Generalized Label Request with the type of LSP (i.e., the layer concerned), and its payload type (e.g., DS–3, VT, etc.). Specific parameters—such as type of signal, local protection, bidirectional LSP, and suggested labels—are