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Network routing scheme based on labels identifying paths

Multiprotocol Label Switching (MPLS) is a routing technique in telecommunications networks that directs data from one node to the next based on labels rather than network addresses. Whereas network addresses identify endpoints, the labels identify established paths between endpoints. MPLS can encapsulate packets of various network protocols, hence the multiprotocol component of the name. MPLS supports a range of access technologies, including T1/E1, ATM, Frame Relay, and DSL.

In an MPLS network, labels are assigned to data packets. Packet forwarding decisions are made solely on the contents of this label, without the need to examine the packet itself. This allows one to create end to end circuits across any type of transport medium, using any protocol. The primary benefit is to eliminate dependence on a particular OSI model data link layer (layer 2) technology, and eliminate the need for multiple layer 2 networks to satisfy different types of traffic. Multiprotocol label switching belongs to the family of packet switched networks. MPLS operates at a layer that is generally considered to lie between traditional definitions of OSI Layer 2 (data link layer) and Layer 3 (network layer), and thus is often referred to as a layer 2.5 protocol. It was designed to provide a unified data carrying service for both circuit based clients and packet switching clients which provide a datagram service model. It can be used to carry many different kinds of traffic, including IP packets, as well as native Asynchronous Transfer Mode (ATM), Frame Relay, Synchronous Optical Networking (SONET) or Ethernet. A number of different technologies were previously deployed with essentially identical goals, such as Frame Relay and ATM. Frame Relay and ATM use labels to move frames or cells through a network. The header of the Frame Relay frame and the ATM cell refers to the virtual circuit that the frame or cell resides on. The similarity between Frame Relay, ATM, and MPLS is that at each hop throughout the network, the label value in the header is changed. This is different from the forwarding of IP packets. MPLS technologies have evolved with the strengths and weaknesses of ATM in mind. MPLS is designed to have lower overhead than ATM while providing connection oriented services for variable length frames, and has replaced much use of ATM in the market. MPLS dispenses with the cell switching and signaling protocol baggage of ATM. MPLS recognizes that small ATM cells are not needed in the core of modern networks, since modern optical networks are fast enough that even full length 1500 byte packets do not incur significant real time queuing delays. At the same time, MPLS attempts to preserve the traffic engineering (TE) and out of band control that made Frame Relay and ATM attractive for deploying large scale networks.

An MPLS router that performs routing based only on the label is called a label switch router (LSR) or transit router. This is a type of router located in the middle of an MPLS network. It is responsible for switching the labels used to route packets. When an LSR receives a packet, it uses the label included in the packet header as an index to determine the next hop on the label switched path (LSP) and a corresponding label for the packet from a lookup table. The old label is then removed from the header and replaced with the new label before the packet is routed forward.

A label edge router (LER, also known as edge LSR) is a router that operates at the edge of an MPLS network and acts as the entry and exit points for the network. LERs push an MPLS label onto an incoming packet and pop it off an outgoing packet. Alternatively, under penultimate hop popping this function may instead be performed by the LSR directly connected to the LER. When forwarding an IP datagram into the MPLS domain, a LER uses routing information to determine the appropriate label to be affixed, labels the packet accordingly, and then forwards the labeled packet into the MPLS domain. Likewise, upon receiving a labeled packet that is destined to exit the MPLS domain, the LER strips off the label and forwards the resulting IP packet using normal IP forwarding rules.

In the specific context of an MPLS based virtual private network (VPN), LERs that function as ingress or egress routers to the VPN are often called provider edge (PE) routers. Devices that function only as transit routers are similarly called provider (P) routers. The job of a P router is significantly easier than that of a PE router.

Labels may be distributed between LERs and LSRs using the Label Distribution Protocol (LDP) or Resource Reservation Protocol (RSVP). LSRs in an MPLS network regularly exchange label and reachability information with each other using standardized procedures in order to build a complete picture of the network so that they can then use that information to forward the packets.

Label switched paths (LSPs) are established by the network operator for a variety of purposes, such as to create network based IP virtual private networks or to route traffic along specified paths through the network. In many respects, LSPs are not different from permanent virtual circuits (PVCs) in ATM or Frame Relay networks, except that they are not dependent on a particular layer 2 technology.

A label switched path (LSP) is a path through an MPLS network set up by the NMS or by a signaling protocol such as LDP, RSVP TE, BGP (or the now deprecated CR LDP). The path is set up based on criteria in the FEC. The path begins at an LER, which makes a decision on which label to prefix to a packet based on the appropriate FEC. It then forwards the packet along to the next router in the path, which swaps the packet's outer label for another label, and forwards it to the next router. The last router in the path removes the label from the packet and forwards the packet based on the header of its next layer, for example IPv4. Due to the forwarding of packets through an LSP being opaque to higher network layers, an LSP is also sometimes referred to as an MPLS tunnel. The router which first prefixes the MPLS header to a packet is an ingress router. The last router in an LSP, which pops the label from the packet, is called an egress router. Routers in between, which need only swap labels, are called transit routers or label switch routers (LSRs). Note that LSPs are unidirectional; they enable a packet to be label switched through the MPLS network from one endpoint to another. Since bidirectional communication is typically desired, the aforementioned dynamic signaling protocols can automatically set up a separate LSP in the opposite direction. When link protection is considered, LSPs can be categorized as primary (working), secondary (backup) and tertiary (LSP of last resort).

There are two standardized protocols for managing MPLS paths: the Label Distribution Protocol (LDP) and RSVP TE, an extension of the Resource Reservation Protocol (RSVP) for traffic engineering. Furthermore, there exist extensions of the Border Gateway Protocol (BGP) that can be used to manage an MPLS path.

Multicast was, for the most part, an afterthought in MPLS design. It was introduced by point to multipoint RSVP TE. It was driven by service provider requirements to transport broadband video over MPLS. The hub and spoke multipoint LSP (HSMP LSP) was also introduced by IETF. HSMP LSP is mainly used for multicast, time synchronization, and other purposes.

MPLS works in conjunction with the Internet Protocol (IP) and its routing protocols, usually Interior Gateway Protocols (IGPs). MPLS LSPs provide dynamic, transparent virtual networks with support for traffic engineering, the ability to transport layer 3 (IP) VPNs with overlapping address spaces, and support for layer 2 pseudowires using Pseudowire Emulation Edge to Edge (PWE3) that are capable of transporting a variety of transport payloads (IPv4, IPv6, ATM, Frame Relay, etc.). MPLS capable devices are referred to as LSRs. The paths an LSR knows can be defined using explicit hop by hop configuration, or are dynamically routed by the Constrained Shortest Path First (CSPF) algorithm, or are configured as a loose route that avoids a particular IP address or that is partly explicit and partly dynamic. In a pure IP network, the shortest path to a destination is chosen even when the path becomes congested. Meanwhile, in an IP network with MPLS Traffic Engineering CSPF routing, constraints such as the RSVP bandwidth of the traversed links can also be considered, such that the shortest path with available bandwidth will be chosen. MPLS Traffic Engineering relies upon the use of TE extensions to Open Shortest Path First (OSPF) or Intermediate System to Intermediate System (IS IS) and RSVP. In addition to the constraint of RSVP bandwidth, users can also define their own constraints by specifying link attributes and special requirements for tunnels to route (or not to route) over links with certain attributes. For end users the use of MPLS is not visible directly, but can be assumed when doing a traceroute: only nodes that do full IP routing are shown as hops in the path, thus not the MPLS nodes used in between, therefore when you see that a packet hops between two very distant nodes and hardly any other 'hop' is seen in that provider's network (or AS) it is very likely that network uses MPLS.

In the event of a network element failure when recovery mechanisms are employed at the IP layer, restoration may take several seconds which may be unacceptable for real time applications such as VoIP. In contrast, MPLS local protection meets the requirements of real time applications with recovery times comparable to those of shortest path bridging networks or SONET rings of less than 50 ms.

MPLS can make use of existing ATM network or Frame Relay infrastructure, as its labeled flows can be mapped to ATM or Frame Relay virtual circuit identifiers, and vice versa.

Frame Relay aimed to make more efficient use of existing physical resources, which allow for the underprovisioning of data services by telecommunications companies (telcos) to their customers, as clients were unlikely to be utilizing a data service 100 percent of the time. Consequently, oversubscription of capacity by the telcos, while financially advantageous to the provider, can directly affect overall performance. Telcos often sold Frame Relay to businesses looking for a cheaper alternative to dedicated lines; its use in different geographic areas depended greatly on governmental and telecommunication companies' policies. Many customers migrated from Frame Relay to MPLS over IP or Ethernet, which in many cases reduced costs and improved manageability and performance of their wide area networks.

While the underlying protocols and technologies are different, both MPLS and ATM provide a connection oriented service for transporting data across computer networks. In both technologies, connections are signaled between endpoints, the connection state is maintained at each node in the path, and encapsulation techniques are used to carry data across the connection. Excluding differences in the signaling protocols (RSVP/LDP for MPLS and PNNI for ATM) there still remain significant differences in the behavior of the technologies. The most significant difference is in the transport and encapsulation methods. MPLS is able to work with variable length packets while ATM uses fixed length (53 bytes) cells. Packets must be segmented, transported and re assembled over an ATM network using an adaptation layer, which adds significant complexity and overhead to the data stream. MPLS, on the other hand, simply adds a label to the head of each packet and transmits it on the network. Differences exist, as well, in the nature of the connections. An MPLS connection (LSP) is unidirectional, allowing data to flow in only one direction between two endpoints. Establishing two way communications between endpoints requires a pair of LSPs be established. Because two LSPs are used, data flowing in the forward direction may use a different path from data flowing in the reverse direction. ATM point to point connections (virtual circuits), on the other hand, are bidirectional, allowing data to flow in both directions over the same path. Both ATM and MPLS support tunneling of connections inside connections. MPLS uses label stacking to accomplish this while ATM uses virtual paths. MPLS can stack multiple labels to form tunnels within tunnels. The ATM virtual path indicator (VPI) and virtual circuit indicator (VCI) are both carried together in the cell header, limiting ATM to a single level of tunneling. The biggest advantage that MPLS has over ATM is that it was designed from the start to be complementary to IP. Modern routers can support both MPLS and IP natively across a common interface allowing network operators great flexibility in network design and operation. ATM's incompatibilities with IP require complex adaptation, making it comparatively less suitable for today's predominantly IP networks.

MPLS is standardized by the IETF in It is deployed to connect as few as two facilities to very large deployments. In practice, MPLS is mainly used to forward IP protocol data units (PDUs) and Virtual Private LAN Service (VPLS) Ethernet traffic. Major applications of MPLS are telecommunications traffic engineering, and MPLS VPN.

MPLS was originally proposed to allow high performance traffic forwarding and traffic engineering in IP networks. However, it evolved in Generalized MPLS (GMPLS) to also allow the creation of LSPs in non native IP networks, such as SONET/SDH networks and wavelength switched optical networks.

MPLS can exist in both an IPv4 and an IPv6 environment, using appropriate routing protocols. The major goal of MPLS development was the increase of routing speed. This goal is no longer relevant because of the usage of newer switching methods (able to forward plain IPv4 as fast as MPLS labelled packets), such as ASIC, TCAM and CAM based switching. Now, therefore, the main application of MPLS is to implement limited traffic engineering and layer 3 / layer 2 “service provider type” VPNs over IPv4 networks.