| ID Number: G00122178 |
An Introduction to MPLS
3 August 2004 Kathleen M. Adams Multiprotocol Label Switching is a labeling scheme that is used by network edge routers to create paths across the network with specific constraints, such as acceptable packet delay.
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| Table of Contents
|  | Technology Basics
|  | MPLS Network Components
|  | MPLS Routing/Switching
|  | Forwarding
|  | LSP
|  | MPLS Protocols
|  | Routing
|  | Signaling
|  | Technology Analysis
|  | Business Use
|  | VPNs
|  | Security
|  | MPLS VPN Types
|  | Traffic Engineering
|  | Fast Reroute
|  | Quality of Service/Class of Service
|  | DiffServ
|  | RSVP
|  | Benefits and Risks
|  | Benefits
|  | Risks
|  | Standards
|  | Technology Leaders
|  | Technology Alternatives
|  | Recommended Gartner Research
|  | Insight
| Analysis
| Technology Basics Multiprotocol Label Switching (MPLS) is an Internet Engineering Task Force (IETF) Committee-specified framework that provides for the efficient designation, routing, forwarding and switching of traffic flows through the network. MPLS is considered multiprotocol because its techniques are applicable to any network layer protocol, including IP, frame relay and asynchronous transfer mode (ATM). For example, in a frame relay network, frame relay switches can be upgraded to support MPLS features, and these MPLS-enabled frame relay switches can coexist with the platform's frame relay switches. Additionally, MPLS can operate in a mixed network, such as a frame/ATM backbone environment. MPLS is typically applied to pure IP networks or in networks where IP is integrated with an ATM core.
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MPLS Network Components The main components of an MPLS network are: Customer Edge (CE) router Provider Edge (PE) router, also known as the Label Edge Router (LER) Provider core router, also known as the Label Switch Router (LSR)
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MPLS Routing/Switching Traditional packet-routing schemes require each packet to be examined by each router on the network to determine where to send the packet until it arrives at its ultimate destination. This process is invariably inefficient, as each router must maintain forwarding tables and process each packet that comes its way. The packet headers contain more information than is needed to choose the next hop. With MPLS, a simpler and quicker technique called label switching is used instead — packets are routed at the PE and switched in the core. As a packet enters an MPLS network at the PE, the LER assigns a label to it. A label is a short fixed-length 20-bit marking, or tag, that is attached to each packet's IP header by the LER based on routing table information in an onboard database called the Label Information Base (LIB). The LIB contains a table that is used to assign a label to an IP packet (at ingress), or change/remove an established label (this is done in the core), and determine the outgoing interface to which the packet will be sent. The label, which is inserted between the local network header and the IP header, identifies the treatment that should be afforded that packet as it travels throughout the backbone. One of the main purposes of the label is to identify a packet's Forwarding Equivalence Class (FEC). An FEC is a distinct group of IP packets receiving similar treatment — they are forwarded in the same manner and over the same path. An FEC may represent packets requiring specific quality of service (QOS) treatment or characterize packets destined for the same egress node (although packets sent between the same endpoints could belong to different FECs and therefore would be labeled differently). Packets travel throughout the core over a specific route known as the Label Switched Path (LSP); thus, MPLS is considered a connection-oriented technology. An LSP is assigned for each FEC, then the packet is forwarded to successive core switches on the backbone, known as LSRs. The LSRs read only the label to identify the LSP and check it against the LIB to determine the best link over which to forward the packet and the label to use on this next hop — there is no further analysis of the packet's network layer header. Then label is replaced with a new one, and the packet is forwarded to the next LSR. Since LSRs examine only the label instead of the full IP address, packets are switched quickly throughout the backbone. When the packet arrives at the egress LER, the label is removed, the IP packet header is read and the packet is delivered to its final destination.
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Forwarding Forwarding in an MPLS environment consists of two steps: assigning a packet to an FEC and determining the next hop of each FEC. The full address of the IP packet is read only when entering (the ingress) or leaving (the egress) the MPLS "cloud." Ingress routers use the packet's destination address to determine which LSP to use. Forwarding of packets throughout the MPLS core is driven by labels. Packets may contain multiple labels, and this is called label stacking. In these cases, labels are organized in a last-in/first-out stack. Only the outermost label (the last label to be applied to the packet) is processed for forwarding. LSRs may add a label to a stack (known as label push) or remove a label from the stack (known as label pop). Label stacking is used to create tunnels through a network — when the LSRs insert an additional label at the front of a labeled packet, an encapsulated tunnel is formed that can be shared by multiple LSPs. MPLS supports unlimited stacking, enabling the aggregation of multiple flows of traffic.
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LSP LSPs are virtual IP circuits similar to permanent virtual circuits (PVCs) in a frame relay or ATM network and can be provisioned using a couple of methods, including the Label Distribution Protocol (LDP). Fundamental to label switching is the idea that LSRs must agree on what labels they should use to forward traffic. When LDP is implemented, each LSR informs upstream nodes which label they should apply to packets that will be heading to a downstream destination. LSPs are unidirectional, that is, return traffic must use a separate LSP. LSPs have several purposes, such as to create IP tunnels for network-based IP virtual private networks (VPNs), to guarantee a certain level of performance and to route traffic along specified paths through the network to avoid congested points.
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MPLS Protocols MPLS defines the forwarding mechanism; however, it uses other protocols to establish the LSPs. Two protocols are needed to perform this task: a routing protocol and a signaling protocol.
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Routing The routing protocol distributes network topology information through the network so that the LSP can be calculated. Following are protocols that can be implemented: Open Shortest Path First (OSPF): calculates routes based on the number of routers, transmission speed, delays and route cost Intermediate System to Intermediate System (IS-IS): routers exchange information based on a single metric to determine network topology
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Signaling The signaling protocol informs the switches along the route which labels and links to use for each LSP. One of two signaling protocols is used: Resource Reservation Protocol-Traffic Engineering (RSVP-TE) is used when traffic engineering is required. LDP is implemented when traffic engineering is not required and when less management is needed.
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Technology Analysis Business Use The following applications are enabled by MPLS:
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VPNs MPLS VPNs are implemented over the infrastructure that runs MPLS in the service provider core. MPLS VPN functionality is enabled at the edge of an MPLS network — at the PE router (LER). Various types of VPNs can be formed using MPLS, including intranet VPNs (with the option of connecting to the Internet), extranets and access VPNs for remote dial access.
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Security With MPLS-based VPNs, enterprise/group traffic traverses over an IP network separately from other group traffic; therefore, it is impossible to send packets between one business and another. As with frame relay, MPLS does not encrypt traffic and therefore the security of traffic will depend on the underlying network of the provider backbone. In network configuration where the backbone is purely MPLS (a dedicated IP network for MPLS VPNs only), security is considered equal to that of frame relay. However, in configurations where the backbone carries a mix of MPLS tunnels and native Internet traffic (a shared Internet/MPLS backbone), security can be a concern, since outsiders can attempt to gain access to data via the Internet. There are several measures that providers can implement to decrease this threat; however, none will make the security on a shared backbone equal to that of a private IP backbone. Companies can increase security on an Internet shared backbone by encrypting traffic via an IPsec overlay.
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MPLS VPN Types Layer 2 MPLS VPN This type of VPN emulates Layer 2 services, such as frame relay, ATM or Ethernet (that is, it appears to be a Layer 2 connection to the end user), but is transported over an IP/MPLS backbone. Layer 2 services use the switching functionality of the MPLS network to deliver end-to-end Layer 2 services. Customers specify routing of VPNs in the same manner in which they specify Layer 2 network PVCs. There are three major components of Layer 2 VPNs: Layer 2 Transport over MPLS: Layer 2 circuit carried transparently over an MPLS-enabled backbone (also known as any transport [or protocol] over ATM [AToM]) Virtual Private Wire Services: the ability to add signaling to AToM Virtual Private LAN Services: MPLS is used to create virtual LAN services based on Ethernet Layer 3 MPLS VPN This type of VPN, also known as a Border Gateway Protocol (BGP) VPN, relies on IP routing to build label switched paths within an IP network. Layer 3 IP VPNs require only one entry point into the service provider "cloud" and can create fully meshed as well as hub-and-spoke networks within the core. A variety of Layer 2 interfaces into a Layer 3 VPN can be used, that is, one business site can access the Layer 3 VPN via an ATM interface while a second site uses a frame relay interface, even though both sites are part of a single VPN. This type of configuration involves protocol conversion at the ingress PE, mapping the originating site protocol (for example, ATM) to IP and then mapping IP to the terminating site protocol (for example, frame relay) at the egress PE. It is important to note that MPLS VPNs support a variety of access types in addition to frame relay and ATM, including direct leased line, DSL, Ethernet, cable and wireless. Outside the United States, frame relay and ATM are almost never used to connect to an MPLS VPN.
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Traffic Engineering Through the use of LSPs, MPLS provides a connection-oriented environment that enables traffic engineering of packet networks. Traffic engineering refers to the process of selecting the paths chosen by data traffic to balance the traffic load on the various links, routers and switches in the network. This technique enables providers to manage the traffic flow and provide the best possible performance by choosing a specific route and not necessarily the most inexpensive route. MPLS was first used by network service providers (NSPs) as a traffic-engineering tool on the backbone. Most Internet service providers use traffic engineering today as a method for controlling link backbone use (steering traffic onto specific links), improving performance due to reduced router processing and providing faster recovery. Traffic engineering is typically done without implementing QOS.
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Fast Reroute Based on MPLS traffic engineering, MPLS fast reroute pre-calculates alternate LSPs so that in the event of a failure, traffic can be switched over to a backup LSP very quickly, approaching Synchronous Digital Hierarchy/Synchronous Optical Network recovery times.
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Quality of Service/Class of Service There are quite a few different approaches to applying QOS on MPLS. Some operators use separate FECs end-to-end across the network; however, there are a substantial number of providers that run the backbone with one QOS and only use prioritization on the access line (ingress and egress). NSPs are applying traffic-engineering techniques to assign QOS metrics for business traffic via class of service (COS) distinctions. COS identifies specific service types (voice, video, data or even different types of applications) as requiring particular QOS treatment and marks each of these packets with a COS mark to ensure they are routed/treated in the network — with special emphasis on meeting QOS goals, which typically are tied to service-level agreements (SLAs) offered to customers. The LSP performs in a similar fashion as frame relay or ATM PVCs by forming a designated path through the network. The LDP, used commonly with MPLS, assigns an FEC to mark specific data for a particular LSP. As packets move along an LSP to the LSRs, these nodes apply QOS to the LSP and its associated packets. Priority LSPs receive preferential treatment and are expedited throughout the network, avoiding congested areas. MPLS must be combined with technologies that provide traffic flows with class-specific treatment. QOS techniques, such as Differentiated Services (DiffServ) and RSVP, determine how to use network resources under specific conditions (traffic shaping) and how much bandwidth to allot to certain types of applications (bandwidth prioritization).
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DiffServ MPLS is typically used in conjunction with DiffServ to deliver COSs. DiffServ is a prioritization-based Layer 3 protocol for specifying and controlling network traffic by class-enabling certain class types to receive precedence over others. Prioritization techniques classify network traffic into a set of classes, and network nodes provide preferential treatment to those classes deemed as a priority. DiffServ aims to divide traffic into classes and treat the classes differently, especially when there is a shortage of resources, such as bandwidth, as it prioritizes important traffic at the expense of lower-priority traffic. The protocol by itself does not guarantee QOS, but the priority scheme translates into higher throughput for high-priority classes by organizing traffic into aggregate traffic flows to receive better or worse treatment relative to each other.
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RSVP RSVP can be mixed with other protocols, such as DiffServ and MPLS. RSVP reserves resources, such as bandwidth and buffer space, so that applications can secure their needs. Resources for traffic are reserved specifically to an application's request before the data is transmitted. The protocol relies on end devices to request their priority so that resources can be reserved for flows to facilitate the requested QOS. Each router along the path to the end device attempts to honor the RSVP request by maintaining connection state information. If this cannot be honored, service is denied and packets are dropped. While RSVP is implemented today on some switches and routers, it is considered more of a business protocol, since its complexity can impair performance on backbone networks. RSVP requires all routers to support it, and this can prove challenging on the Internet.
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Benefits and Risks Benefits Among the benefits of MPLS are its: Efficiency: MPLS decreases the forwarding overhead on the label switched router. Label switching results in less route processing, which ultimately translates into lower latency and greater efficiency. Scalability: MPLS services offer more bandwidth (over 100 Mbps) than frame relay. Ubiquity: Full mesh connectivity can be achieved with only one access link to the MPLS cloud. MPLS-based VPNs can be set up as hub-and-spoke networks as well. IP MPLS services offer any-to-any connection in company networks, which is particularly useful for accessing backup data centers and distributed servers. Traffic engineering-enabling technology: Due to its connection-oriented nature and traffic-engineering mechanisms, MPLS is a QOS-enabling technology. Using DiffServ, MPLS networks can apply COS distinctions to specific traffic types. Flexibility: Adding, moving or changing VPN configurations is relatively simple with an MPLS-based IP VPN. Access-neutral: Frame relay, ATM, DSL, dial, cable and wireless can all be used for access into the VPN. Privacy: MPLS ensures privacy over a public shared network infrastructure. Security: In a network configuration where the backbone is purely MPLS (a dedicated IP network for MPLS VPNs only), security is considered equal to that of frame relay. Ability to accommodate Layer 2 technologies: Ethernet, frame relay and ATM traffic can be carried over an IP/MPLS core. SLAs: Service providers are offering COS-specific SLAs with their MPLS-based VPN offerings.
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Risks Limited provider and vendor interoperability at the features and services levels: Interoperability with other MPLS networks at the networking level is possible only via proprietary MPLS network interconnect platforms that map QOS between networks (for example, Nexagent and Vanco). Security: In configurations where the backbone carries a mix of MPLS tunnels and native Internet traffic (a shared Internet/MPLS backbone), security can be a concern, since outsiders can attempt to gain access to data via the Internet.
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Standards MPLS traces its roots to several preceding technology efforts at combining IP and ATM technologies and to improving the throughput and delay performance of IP. These early technologies (and the vendors which supported them) included Tag Switching (Cisco), IP Switching (Ipsilon), Aggregate Route-based IP Switching (or ARIS) (IBM), Cell Switched Router (Toshiba) and IP Navigator (Cascade Communications). In early 1997, the IETF proposed to define a standard version of IP switching and called it MPLS. The MPLS Working Group issued its first set of Proposed Standards in 2001. Many standards are currently in the "Internet Draft" phase, although several have been elevated into the Request for Comment (RFC)-STD phase. Several standards have been proposed for allowing service providers to use MPLS to provision VPN services. These include: IETF 2547BGP/MPLS VPNs: This standard defines a VPN solution that uses MPLS to forward customer traffic using per-customer labels. The BGP is used to distribute VPN information. RFC-2702—Requirements for Traffic Engineering Over MPLS: This RFC describes the requirements of a service provider wanting to use RSVP to traffic engineer paths over the core network. Martini Draft: This set of Internet drafts, co-authored by Luca Martini of Level 3 Communications, defines how MPLS can be used to support Layer 2 transport services. Martini drafts define Layer 2 encapsulation methods, as well as Layer 2 transport signaling methods. Pseudowire Emulation Edge-to-Edge: The Pseudo Wire Emulation Working Group is currently developing standards for Layer 2 encapsulation (supporting the Martini draft). These approaches are known as AToM/APoM.
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Technology Leaders NSPs are implementing MPLS on their networks for several reasons, including: The ability to reduce operating costs by integrating multiple networks (frame relay, ATM, IP) over a single converged IP core To offer more revenue-generating services, such as VPNs To provision customer IP VPNs more efficiently To extend SLAs with meaningful performance-related criteria The following U.S. NSPs are in the process or have completed implementation of MPLS on their core networks: AT&T, BellSouth, Equant, Global Crossing, Infonet, MCI, Masergy, Qwest, Savvis, SBC, Sprint and Verizon. Most of the incumbent operators and many alternate providers in Western Europe have been offering MPLS services for several years, including BT, France Telecom, Deutsche Telekom, Swisscom, Belgacom and KPN, among others. The major equipment manufacturers of MPLS equipment include Alcatel, Avici Systems, Cisco Systems, Juniper Networks, Lucent Technologies, Marconi, Nortel Networks, Riverstone Networks and Siemens.
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Technology Alternatives A Layer 3 MPLS-based IP VPN is considered a replacement solution to a frame relay or ATM enterprise network. Therefore, technology alternatives to MPLS include primarily frame relay, ATM and private line. However, frame relay does not scale as easily as MPLS and becomes cost-prohibitive as meshing among sites increases. Additionally, frame relay does not achieve the high levels of bandwidth that are available with MPLS.
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Recommended Gartner Research IP VPN Comparison Columns, G00121274 An Introduction to IP VPNs, DPRO-119844 Quality of Service over IP Networks, DPRO-101188 MPLS Networks: Drivers Beat Inhibitors in 2003, M-17-1953
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Insight MPLS is gaining universal protocol status as evidenced by its rapid and growing deployment on leading NSP backbones. As carrier networks increase in size, they also need to become more streamlined. MPLS provides these vendors with the capability to collapse their Layer 1, 2 and 3 networks into one platform, while also providing other important benefits. Its ability to enhance network scalability and performance while providing advanced services and functionality, such as IP VPNs and traffic engineering, make MPLS a highly valued technology.
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