Multiprotocol Label Switching

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[note 1] 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 which 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 and/or egress routers to the VPN are often called PE (Provider Edge) routers. Devices that function only as transit routers are similarly called P (Provider) routers.[11] The job of a P router is significantly easier than that of a PE router, so they can be less complex and may be more dependable because of this.

Labels are distributed between LERs and LSRs using the Label Distribution Protocol (LDP).[12] 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.

When an unlabeled packet enters the ingress router and needs to be passed on to an MPLS tunnel, the router first determines the forwarding equivalence class (FEC) for the packet and then inserts one or more labels in the packet's newly created MPLS header. The packet is then passed on to the next hop router for this tunnel.

The MPLS Header is added between the network layer header and link layer header of the OSI model.[13]

When a labeled packet is received by an MPLS router, the topmost label is examined. Based on the contents of the label a swap, push (impose) or pop (dispose) operation is performed on the packet's label stack. Routers can have prebuilt lookup tables that tell them which kind of operation to do based on the topmost label of the incoming packet so they can process the packet very quickly.

• In a swap operation the label is swapped with a new label, and the packet is forwarded along the path associated with the new label.
• In a push operation a new label is pushed on top of the existing label, effectively "encapsulating" the packet in another layer of MPLS. This allows hierarchical routing of MPLS packets. Notably, this is used by MPLS VPNs.
• In a pop operation the label is removed from the packet, which may reveal an inner label below. This process is called "decapsulation". If the popped label was the last on the label stack, the packet "leaves" the MPLS tunnel. This can be done by the egress router, but see Penultimate Hop Popping (PHP) below.

During these operations, the contents of the packet below the MPLS Label stack are not examined. Indeed, transit routers typically need only to examine the topmost label on the stack. The forwarding of the packet is done based on the contents of the labels, which allows "protocol-independent packet forwarding" that does not need to look at a protocol-dependent routing table and avoids the expensive IP longest prefix match at each hop.

At the egress router, when the last label has been popped, only the payload remains. This can be an IP packet or any of a number of other kinds of payload packet. The egress router must, therefore, have routing information for the packet's payload since it must forward it without the help of label lookup tables. An MPLS transit router has no such requirement.

Usually (by default with only one label in the stack, accordingly to the MPLS specification), the last label is popped off at the penultimate hop (the hop before the egress router). This is called penultimate hop popping (PHP). This may be interesting in cases where the egress router has many packets leaving MPLS tunnels, and thus spends inordinate amounts of CPU time on this. By using PHP, transit routers connected directly to this egress router effectively offload it, by popping the last label themselves. In the label distribution protocols, this PHP label pop action is advertised as label value 3 « implicit-null» (which is never found in a label, since it means that the label is to be popped).

This optimisation is no longer that useful (like for initial rationales for MPLS – easier operations for the routers). Several MPLS services (including end-to-end QoS[14] management, and 6PE[15]) imply to keep a label even between the penultimate and the last MPLS router, with a label disposition always done on the last MPLS router: the «Ultimate Hop Popping» (UHP).[16][17] Some specific label values have been notably reserved[18][19] for this use:

• 0: «explicit-null» for IPv4
• 2: «explicit-null» for IPv6

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 a label edge router (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 called 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 set up an LSP in the other direction to compensate for this.

When protection is considered, LSPs could be categorized as primary (working), secondary (backup) and tertiary (LSP of last resort). As described above, LSPs are normally P2P (point to point). A new concept of LSPs, which are known as P2MP (point to multi-point), was introduced recently. These are mainly used for multicasting purposes.

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.[20][21] Furthermore, there exist extensions of the Border Gateway Protocol (BGP) that can be used to manage an MPLS path.[11][22][23]

An MPLS header does not identify the type of data carried inside the MPLS path. If one wants to carry two different types of traffic between the same two routers, with different treatment by the core routers for each type, one has to establish a separate MPLS path for each type of traffic.

Multicast was, for the most part, an after-thought in MPLS design. It was introduced by point-to-multipoint RSVP-TE.[24] It was driven by service provider requirements to transport broadband video over MPLS. Since the inception of RFC 4875 there has been a tremendous surge in interest and deployment of MPLS multicast and this has led to several new developments both in the IETF and in shipping products.

The hub&spoke multipoint LSP is also introduced by IETF, short as HSMP LSP. HSMP LSP is mainly used for multicast, time synchronization, and other purposes.

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.[27][28][29] 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.[27][29][30]

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 (excessive bandwidth overbooking ), while financially advantageous to the provider, can directly affect overall performance.

Telcos often sell 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 will reduce costs and improve manageability and performance of their wide area networks.[31]

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:Private Network-to-Network Interface 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 transports 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 to be established. Because 2 LSPs are required for connectivity, 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 SVC and PVC ATM connections are bidirectional. Check ITU-T I.150 3.1.3.1).

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 are able to 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.