Table of Contents:

Definition and Overview

1. Introduction

2. Traditional Routing and Packet Switching

3. MPLS and Its Components

4. MPLS Operation

5. MPLS Protocol Stack Architecture

6. MPLS Applications

7. Standard Groups

Self-Test

Glossary

PDF of this tutorial

What Is MPLS?

MPLS is an Internet Engineering Task Force (IETF)–specified framework that provides for the efficient designation, routing, forwarding, and switching of traffic flows through the network.

MPLS performs the following functions:

• specifies mechanisms to manage traffic flows of various granularities, such as flows between different hardware, machines, or even flows between different applications
• remains independent of the Layer-2 and Layer-3 protocols
• provides a means to map IP addresses to simple, fixed-length labels used by different packet-forwarding and packet-switching technologies
• interfaces to existing routing protocols such as resource reservation protocol (RSVP) and open shortest path first (OSPF)
• supports the IP, ATM, and frame-relay Layer-2 protocols

In MPLS, data transmission occurs on label-switched paths (LSPs). LSPs are a sequence of labels at each and every node along the path from the source to the destination. LSPs are established either prior to data transmission (control-driven) or upon detection of a certain flow of data (data-driven). The labels, which are underlying protocol-specific identifiers, are distributed using label distribution protocol (LDP) or RSVP or piggybacked on routing protocols like border gateway protocol (BGP) and OSPF. Each data packet encapsulates and carries the labels during their journey from source to destination. High-speed switching of data is possible because the fixed-length labels are inserted at the very beginning of the packet or cell and can be used by hardware to switch packets quickly between links.

LSRs and LERs

The devices that participate in the MPLS protocol mechanisms can be classified into label edge routers (LERs) and label switching routers (LSRs).

An LSR is a high-speed router device in the core of an MPLS network that participates in the establishment of LSPs using the appropriate label signaling protocol and high-speed switching of the data traffic based on the established paths.

An LER is a device that operates at the edge of the access network and MPLS network. LERs support multiple ports connected to dissimilar networks (such as frame relay, ATM, and Ethernet) and forwards this traffic on to the MPLS network after establishing LSPs, using the label signaling protocol at the ingress and distributing the traffic back to the access networks at the egress. The LER plays a very important role in the assignment and removal of labels, as traffic enters or exits an MPLS network.

FEC

The forward equivalence class (FEC) is a representation of a group of packets that share the same requirements for their transport. All packets in such a group are provided the same treatment en route to the destination. As opposed to conventional IP forwarding, in MPLS, the assignment of a particular packet to a particular FEC is done just once, as the packet enters the network. FECs are based on service requirements for a given set of packets or simply for an address prefix. Each LSR builds a table to specify how a packet must be forwarded. This table, called a label information base (LIB), is comprised of FEC–to-label bindings.

Labels and Label Bindings

A label, in its simplest form, identifies the path a packet should traverse. A label is carried or encapsulated in a Layer-2 header along with the packet. The receiving router examines the packet for its label content to determine the next hop. Once a packet has been labeled, the rest of the journey of the packet through the backbone is based on label switching. The label values are of local significance only, meaning that they pertain only to hops between LSRs.

Once a packet has been classified as a new or existing FEC, a label is assigned to the packet. The label values are derived from the underlying data link layer. For data link layers (such as frame relay or ATM), Layer-2 identifiers, such as data link connection identifiers (DLCIs) in the case of frame-relay networks or virtual path identifiers (VPIs)/virtual channel identifiers (VCIs) in case of ATM networks, can be used directly as labels. The packets are then forwarded based on their label value.

Labels are bound to an FEC as a result of some event or policy that indicates a need for such binding. These events can be either data-driven bindings or control-driven bindings. The latter is preferable because of its advanced scaling properties that can be used in MPLS.

Label assignment decisions may be based on forwarding criteria such as the following:

• destination unicast routing
• traffic engineering
• multicast
• virtual private network (VPN)
• QoS

The generic label format is illustrated in Figure 1. The label can be embedded in the header of the data link layer (the ATM VCI/VPI shown in Figure 2 and the frame-relay DLCI shown in Figure 3) or in the shim (between the Layer-2 data-link header and Layer-3 network layer header, as shown in Figure 4).

Figure 1. MPLS Generic Label Format

Figure 2. ATM as the Data Link Layer

Figure 3. Frame Relay as the Data Link Layer

Figure 4. Point-to-Point (PPP)/Ethernet as the Data Link Layer

Label Creation

There are several methods used in label creation:

• topology-based method—uses normal processing of routing protocols (such as OSPF and BGP)
• request-based method—uses processing of request-based control traffic (such as RSVP)
• traffic-based method—uses the reception of a packet to trigger the assignment and distribution of a label

The topology- and request-based methods are examples of control-driven label bindings, while the traffic-based method is an example of data-driven bindings.

Label Distribution

MPLS architecture does not mandate a single method of signaling for label distribution. Existing routing protocols, such as the border gateway protocol (BGP), have been enhanced to piggyback the label information within the contents of the protocol. The RSVP has also been extended to support piggybacked exchange of labels. The Internet Engineering Task Force (IETF) has also defined a new protocol known as the label distribution protocol (LDP) for explicit signaling and management of the label space. Extensions to the base LDP protocol have also been defined to support explicit routing based on QoS and CoS requirements. These extensions are captured in the constraint-based routing (CR)–LDP protocol definition.

A summary of the various schemes for label exchange is as follows:

• LDP—maps unicast IP destinations into labels
• RSVP, CR–LDP—used for traffic engineering and resource reservation
• protocol-independent multicast (PIM)—used for multicast states label mapping
• BGP—external labels (VPN)

Label-Switched Paths (LSPs)

• A collection of MPLS—enabled devices represents an MPLS domain. Within an MPLS domain, a path is set up for a given packet to travel based on an FEC. The LSP is set up prior to data transmission. MPLS provides the following two options to set up an LSP.

• hop-by-hop routing—Each LSR independently selects the next hop for a given FEC. This methodology is similar to that currently used in IP networks. The LSR uses any available routing protocols, such as OSPF, ATM private network-to-network interface (PNNI), etc.

• explicit routing—Explicit routing is similar to source routing. The ingress LSR (i.e., the LSR where the data flow to the network first starts) specifies the list of nodes through which the ER–LSP traverses. The path specified could be nonoptimal, as well. Along the path, the resources may be reserved to ensure QoS to the data traffic. This eases traffic engineering throughout the network, and differentiated services can be provided using flows based on policies or network management methods.

The LSP setup for an FEC is unidirectional in nature. The return traffic must take another LSP.

Label Spaces

The labels used by an LSR for FEC–label bindings are categorized as follows:

• per platform—The label values are unique across the whole LSR. The labels are allocated from a common pool. No two labels distributed on different interfaces have the same value.

• per interface—The label ranges are associated with interfaces. Multiple label pools are defined for interfaces, and the labels provided on those interfaces are allocated from the separate pools. The label values provided on different interfaces could be the same.

Label Merging

The incoming streams of traffic from different interfaces can be merged together and switched using a common label if they are traversing the network toward the same final destination. This is known as stream merging or aggregation of flows.

If the underlying transport network is an ATM network, LSRs could employ virtual path (VP) or virtual channel (VC) merging. In this scenario, cell interleaving problems, which arise when multiple streams of traffic are merged in the ATM network, need to be avoided.

Label Retention

MPLS defines the treatment for label bindings received from LSRs that are not the next hop for a given FEC. Two modes are defined.

• conservative—In this mode, the bindings between a label and an FEC received from LSRs that are not the next hop for a given FEC are discarded. This mode requires an LSR to maintain fewer labels. This is the recommended mode for ATM–LSRs.

• liberal—In this mode, the bindings between a label and an FEC received from LSRs that are not the next hop for a given FEC are retained. This mode allows for quicker adaptation to topology changes and allows for the switching of traffic to other LSPs in case of changes.

Label Control

MPLS defines modes for distribution of labels to neighboring LSRs.

• independent—In this mode, an LSR recognizes a particular FEC and makes the decision to bind a label to the FEC independently to distribute the binding to its peers. The new FECs are recognized whenever new routes become visible to the router.

• ordered—In this mode, an LSR binds a label to a particular FEC if and only if it is the egress router or it has received a label binding for the FEC from its next hop LSR. This mode is recommended for ATM–LSRs.

Signaling Mechanisms

• label request—Using this mechanism, an LSR requests a label from its downstream neighbor so that it can bind to a specific FEC. This mechanism can be employed down the chain of LSRs up until the egress LER (i.e., the point at which the packet exits the MPLS domain).

• label mapping—In response to a label request, a downstream LSR will send a label to the upstream initiator using the label mapping mechanism.

The above concepts for label request and label mapping are explained in Figure 5.

Figure 5. Signaling Mechanisms

Label Distribution Protocol (LDP)

The LDP is a new protocol for the distribution of label binding information to LSRs in an MPLS network. It is used to map FECs to labels, which, in turn, create LSPs. LDP sessions are established between LDP peers in the MPLS network (not necessarily adjacent). The peers exchange the following types of LDP messages:

• discovery messages—announce and maintain the presence of an LSR in a network
• session messages—establish, maintain, and terminate sessions between LDP peers
• advertisement messages—create, change, and delete label mappings for FECs
• notification messages—provide advisory information and signal error information

Label Stack

The label stack mechanism allows for hierarchical operation in the MPLS domain. It basically allows MPLS to be used simultaneously for routing at the fine-grain level (e.g., between individual routers within an Internet service provider [ISP] and at a higher domain-by-domain level). Each level in a label stack pertains to some hierarchical level. This facilitates a tunneling mode of operation in MPLS.

Traffic Engineering

Traffic engineering is a process that enhances overall network utilization by attempting to create a uniform or differentiated distribution of traffic throughout the network. An important result of this process is the avoidance of congestion on any one path. It is important to note that traffic engineering does not necessarily select the shortest path between two devices. It is possible that, for two packet data flows, the packets may traverse completely different paths even though their originating node and the final destination node are the same. This way, the less-exposed or less-used network segments can be used and differentiated services can be provided.

In MPLS, traffic engineering is inherently provided using explicitly routed paths. The LSPs are created independently, specifying different paths that are based on user-defined policies. However, this may require extensive operator intervention. RSVP and CR–LDP are two possible approaches to supply dynamic traffic engineering and QoS in MPLS.

CR

Constraint-based routing (CR) takes into account parameters, such as link characteristics (bandwidth, delay, etc.), hop count, and QoS. The LSPs that are established could be CR–LSPs, where the constraints could be explicit hops or QoS requirements. Explicit hops dictate which path is to be taken. QoS requirements dictate which links and queuing or scheduling mechanisms are to be employed for the flow.

When using CR, it is entirely possible that a longer (in terms of cost) but less loaded path is selected. However, while CR increases network utilization, it adds more complexity to routing calculations, as the path selected must satisfy the QoS requirements of the LSP. CR can be used in conjunction with MPLS to set up LSPs. The IETF has defined a CR–LDP component to facilitate constraint-based routes.