Introduction In computer networking and telecommunications, Multi Protocol Label Switching (MPLS) is a data-carrying mechanism that belongs to the family of packet-switched networks. When it comes to getting network traffic from point A to point B, no single way suits every application. Voice and video applications require minimum delay variation, while mission-critical applications require hard guarantees-of-service and rerouting. So far, only circuit-switched networks have provided the differentiated services and guarantees required by many of these applications.
But a new technology called Multiprotocol Label Switching (MPLS) is changing all that. With MPLS, you can support all the above applications on an IP network without having to run large subsets of the network with completely different transport mechanisms, routing protocols, and addressing plans. Although the standard is a work in progress, many vendors and service providers are announcing MPLS products and services. As such, now seems like a good time to learn how the technology works, how it can be deployed, and what issues still need to be addressed.
MPLS operates at an OSI Model layer that is generally considered to lie between traditional definitions of 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 ATM, SONET, and Ethernet frames.
MPLS was originally proposed by a group of engineers from Ipsilon Networks, but their “IP Switching” technology, which was defined only to work over ATM, did not achieve market dominance. Cisco Systems, Inc. introduced a related proposal, not restricted to ATM transmission, called “Tag Switching” when it was a Cisco proprietary proposal, and was renamed “Label Switching” when it was handed over to the IETF for open standardization. The IETF work involved proposals from other vendors, and development of a consensus protocol that combined features from several vendors’ work.
One original motivation was to allow the creation of simple high-speed switches, since for a significant length of time it was impossible to forward IP packets entirely in hardware. However, advances in VLSI have made such devices possible. Therefore the advantages of MPLS primarily revolve around the ability to support multiple service models and perform traffic management. MPLS also offers a robust recovery framework that goes beyond the simple protection rings of synchronous optical networking (SONET/SDH).
While the traffic management benefits of migrating to MPLS are quite valuable (better reliability, increased performance), there is a significant loss of visibility and access into the MPLS cloud for IT departments. How MPSL Works MPLS works by prefixing packets with an MPLS header, containing one or more ‘labels’. This is called a label stack. Each label stack entry contains four fields: •a 20-bit label value. •a 3-bit field for QoS (Quality of Service) priority (experimental). •a 1-bit bottom of stack flag.
If this is set, it signifies that the current label is the last in the stack. •an 8-bit TTL (time to live) field. A label is a short, four-byte, fixed-length, locally-significant identifier which is used to identify a Forwarding Equivalence Class (FEC). The label which is put on a particular packet represents the FEC to which that packet is assigned. •Label—Label Value (Unstructured), 20 bits •Exp—Experimental Use, 3 bits; currently used as a Class of Service (CoS) field. •S—Bottom of Stack, 1 bit •TTL—Time to Live, 8 bits
The label is imposed between the data link layer (Layer 2) header and network layer (Layer 3) header. The top of the label stack appears first in the packet, and the bottom appears last. The network layer packet immediately follows the last label in the label stack. Traditional IP forwarding techniques analyze the destination IP address contained in the network layer header for every packet at each hop in the network. This process is called hop-by-hop destination-based routing. The route that packets take is based solely on the destination unicast address.
Layer 3 routing protocols do not traditionally have any interaction with Layer 2 network characteristics, making the implementation of Quality of Service (QOS) and loading features difficult. Multiprotocol Label Switching (MPLS) is a vendor-independent protocol (based on Cisco’s tag-switching protocol) that applies labels to packets providing QOS and advance route selection functions. MPSL Implementation There are several terms used in MPLS implementations. Label Header applied to a packet by an edge label switch router (edge LSR) and used by label switch routers (LSR) to forward packets.
Label forwarding information base (LFIB) Table that indicates where and how to forward frames. Created by label switch-capable devices, the LFIB contains a list of entries consisting of ingress and one or more egress subentries (outgoing label, outgoing interface, outgoing link-level components). The LFIB is constructed based on information the LSRs gain from interaction with the routing protocols. Label Switch Router (LSR) Device (switch or router) that forwards based upon label values. Neighboring LSRs establish a TCP connection to transfer label bindings.
Edge Label Switch Router (edge LSR) Device on the edge of a label network that applies or ultimately removes labels from the packet. Label-switched Path (LSP) The end-to-end path defined by labels on a packet. Label virtual circuit (LVC) An LSP through an ATM system. Label Switch Controller (LSC) An LSR that communicates with an ATM switch to provide label information. Label Distribution Protocol (LDP) IETF standard label binding protocol used to distribute label information to LSRs. MPLS has two major components.
The control component (also known as the control plane) is responsible for bindings between the labels and the network layer routes. It creates and maintains the LFIB. The forwarding component uses the LFIB to perform forwarding based on the labels contained in the packets. Labels are distributed using LDP. MPLS can be implemented over any media-type and can be used with point-to-point, multipoint and ATM links. MPLS can be used with different network layer protocols (hence the term multiprotocol) by using a control component specific to the desired protocol.
Labels can be applied to packets in different ways depending on the network type. In ATM, labels are applied to the layer 2 header of the packets (cells). IPv6 supports label switching via the Flow Label field. For most other protocols, the label is applied as a small, shim label header between the Layer 2 and Layer 3 headers. Conclusion and Advantages of MPLS Up to now, only circuit-switched networks have provided the quality of service, performance guarantees, and reliability that many of today’s mission-critical applications require.
But Multiprotocol Label Switching (MPLS) is changing all that by providing comparable levels of service over IP networks. Following are seven objections to MPLS implementation and the rebuttals: 1. New control protocols often require changes to routing hardware. MPLS minimizes this by separating routing and switching functions so that routing can be changed without significantly affecting switching operations. This should make the introduction of new routing protocols much easier and make current hardware designs simpler, faster, and less error prone. . Voice and video work best with circuit-switching transport because it minimizes delay variations. MPLS will make the quality of voice and video much easier to control by establishing the proper per-hop behavior for delay-sensitive traffic. In addition, the reservation and dynamic placement options of Constraint-based Label Distribution Protocol (CR-LDP) will help keep this potentially disruptive traffic under control—unlike current UDP-based video, which can wreak havoc on other traffic if not properly controlled. 3.
Traffic engineering requires circuit-switching so that accurate bandwidth reservations can be made and taken into account when laying down new traffic. Because MPLS permits bandwidth reservation and dynamic path computation, it allows flexibility when placing traffic flows. Unlike pure IP, which requires that all traffic to a destination follow the same path, MPLS will allow flow control over a wide range of available paths to a destination. 4. Traffic engineering requires the ability to automatically and manually compute routes that meet arbitrary constraints, including available bandwidth.
MPLS will permit bandwidth and a set of 32 other constraints to be considered when computing routes. Links that don’t meet these constraints will not be considered. 5. Traffic shaping requires a circuit to be established so that it can be identified and then shaped. MPLS traffic carries a label so that flow-based traffic shaping can be done in hardware as easily as it is currently done with ATM. 6. Mission-critical applications require hard guarantees of service and rerouting, which only a circuit-switched network can provide.
MPLS provides rankings to the individual flows so that during failure, or times when bandwidth is not available, more important flows can take precedence—either by being rerouted or by preempting less-important flows. 7. VPNs require controllable, efficient tunnels. MPLS transport does not look at the headers of the packets it is transporting; therefore, the addressing used in those packets may be private. ? References www. technopedia. com. pk www. networkcomputing. com www. miercom. com www. cisco. com www. bitpipe. com www. wikipedia. org