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Service provider network has a requirement to fast switching without any routing lookup in the core network and not to load the traffic in core network. If traffic congested in core network it makes a big delay whole network traffic switching. As a result MPLS technology has been introduced. MPLS is a packet forwarding technology used in service provider core network for fast switching of packets.
MPLS technology uses label technology to switch the packets rather than traditional destination IP based mechanism. Multi Protocol Label Switching (MPLS) is a packet forwarding technology used in the service provider core network. MPLS uses the labels to packet forwarding instead of traditional destination IP based mechanism to integrate the layer 2 information such as bandwidth, latency, utilization with layer 3 (IP) elements.
MPLS Labels usually correspond to IP destination networks. Labels also correspond to other parameters such as Quality of Service (QoS), source address or layer 2 circuits. Label switching is regardless of layer 3 protocol.
MPLS is called multiprotocol because it works with the Internet Protocol (IP), Asynchronous Transport Mode (ATM), and frame relay network protocols.
MPLS is a protocol neutral
MPLS is designed to integrate layer 2 information about network links (bandwidth, latency, utilization) into layer 3 (IP) elements. That allowed MPLS to work with ATM, Frame Relay and Ethernet at the core
Drawbacks of Traditional IP forwarding
Routing protocols are used to distribute Layer3 routing Information. Regardless of routing protocol, routers always forward packets based on the destination address only. Destination based routing does not provide any mechanism for load balancing across unequal paths. Routing lookups are performed on every hop. This is much over head to every hop and it makes delay on forwarding packets since, the routing table consists hundreds of thousands routes. Routing complexity depends on the size of routing table.
MPLS is adaptable
MPLS is able to support new application and services such as layer 2 layer 3 VPNs, Ethernet services and traffic engineering.
MPLS is cost effective solution
In case of VPN provision, Service providers use one centralized network to connect number of sites of a customer. Likewise several customers are served using a single MPLS network.
Traditional IP forwarding Technology
Traditional IP based technology designed based on routing protocols which used to distribute layer3 routing information, Destination based packet forwarding technology and routing lookup on every hop. When packets reach the nodes for routing, All the nodes (routers) in the network, will have a destination address based lookup in the routing table which is the data base provides for which destination packets has to be routed through which interface. Policy based routing is only exception for destination based routing. The following figure shows the traditional IP forwarding mechanism.
Figure2. Traditional IP forwarding Technology
As a packet of a connectionless network layer protocol travels from one router to the next, each router makes an independent forwarding decision for that packet. That is, each router analyzes the packet’s header, and each router runs a network layer routing algorithm. Each router independently chooses a next hop for the packet, based on its analysis of the packet’s header and the results of running the routing algorithm. Packet headers contain considerably more information than is needed simply to choose the next hop.
Choosing the next hop can therefore be thought of as the composition of two functions. The first function partitions the entire set of possible packets into a set of “Forwarding Equivalence Classes (FECs)”. The second maps each FEC to a next hop. Insofar as the forwarding decision is concerned, different packets which get mapped into the same FEC are indistinguishable. All packets which belong to a particular FEC and which travel from a particular node will follow the same path (or if certain kinds of multi-path routing are in use, they will all follow one of a set of paths associated with the FEC).
In conventional IP forwarding, a particular router will typically consider two packets to be in the same FEC if there is some address prefix X in that router’s routing tables such that X is the “longest match” for each packet’s destination address. As the packet traverses the network, each hop in turn re-examines the packet and assigns it to a FEC.
Drawbacks of Traditional IP forwarding
As shown in the diagram, router does a routing lookup for each packet in a large routing database. The destination based routing lookup is forward through longest prefix match of the destination IP address. Each router has to do the same job until the packet reaches the destination. It makes more latency on packet delivery, processing load for routers. Service provider core network is running with much loaded traffic. Normal routers canââ‚¬â„¢t perform packet forwarding based on traditional ip forwarding technology.
Virtual Private Network (VPN)
MPLS VPN application is one of the main targets of the project. MPLS provides a secure inter sites connectivity without any complexity for customers who have number of branches all over the country. VPN connectivity between the branches over MPLS is a big business for service providers and a main application over MPLS.
The Reason for MPLS VPN rapid growth amongst the customers is MPLS allows service providers to create new VPNs without having to install new hardware; it significantly reduces the cost of implementation, which in turn reduces the overall cost of VPNs.
Other reason is small and Medium Enterprise (SME) customers donââ‚¬â„¢t want to put the IT infrastructure with new hardware such as Firewalls to interconnect the sites (branches). Since, it is a big investment for them and more complex to manage the inter connectivity between branches.
Next of all, MPLS provides a centralised control over the connectivity of branches. Customers only need to provide only one connection from their office router to the service provider rather than setting up and managing individual points between each office. This central control effectively removes the need for additional trained manpower. Additional benefits can be realised through this central management as a business is given greater control of Internet usage as well. The following figure shows how a service provider connects several customers using MPLS as a centralised point without complexity
Another reason is VPN over MPLS is more secure than connecting entire sites using firewalls because there is no separate private network for a customer in such a situation. Each siteââ‚¬â„¢s firewall will establish connectivity over the internet using IPSEC. It makes much security threat over the data.
Other MPLS Applications
MPLS quality of service helps to classify the traffic such as voice, data, signaling, best effort and other traffics and guarantee the allocated bandwidth
One of the most obvious advantages of MPLS is that it provides customers with a number of tools for traffic engineering. An MPLS network can offer the same sort of quality of service guarantees that data transport services like Frame Relay or ATM can, without requiring the use of any dedicated lines.
Protocol Independent Multicast (PIM) is the control protocol used to create FEC tables; extensions of version 2 of the PIM protocol are used to exchange FEC-label binding.
These can be used to evolve legacy networks and services, such as Frame Relay, ATM, PPP, High-Level Data Link Control (HDLC), and Ethernet. Traffic is accepted into the network via a variety of access technologies, labeled at the edge, and transported over a common MPLS core. At the network egress, the label is removed and delivered in a manner similar to the ingress implementation.
Generalized MPLS (GMPLS)
The goal of GMPLS is to integrate control of the routing layer with that of the optical transmission layer, thus facilitating the implementation of traffic engineering across the network. Optical cross-connect platforms do not examine traffic passing through themââ‚¬”in contrast to routers, for example. GMPLS deployment links capacity provisioning in the optical layer for an automated execution of resource reservation (for example, bandwidth brokering and provisioning).
MPLS architecture consists of two planes such as Forwarding plane and Control plane. The above diagram shows the conceptual diagram of MPLS architecture. The MPLS control plane is a collection of protocols that collectively establish network level functionality in MPLS networks. The protocols are implemented as software in routers. They will communicate with each other and transfer signaling information. Protocols specify the message formats, syntax, semantics, and transaction sequence for the message exchange. The main functionality performed by the control plane is to establish the Label Switched Path for packet forwarding.
The data plane is used for the transport of packets (or label swapping algorithm). This separation permits applications to be developed and deployed in a scalable and flexible manner.
The above diagram shows the MPLS functionality. The edge routers of the MPLS cloud is known as Label Edge Routers (LERs) or Provide Edge routers (PE Routers). These edge routers are designed to inspect IP packets entering the network and add MPLS headers, as well as removing the headers from packets leaving the MPLS network. In central of the MPLS cloud there are four backbone routers placed. These routers are known as Label Switch Routers (LSRs) or P routers look for an MPLS label on each packet that passes through them, looking up and following the instructions contained in those labels, routing them based on a list of instructions.
MPLS allows administrators to define routes known as Label Switched Paths (LSPs) from one LER to another, through a series of LSRS, across the MPLS network. These LSPs are pre-assigned and pre-engineered paths that packets with a certain label should follow.
Forwarding Equivalency Class (FEC) is used in MPLS to describe the identical characteristics packets which may forward in the same way. Characteristics determine the FEC of the packets but typically it is at least destination IP address.
FEC consists of a group of IP destinations for which a fixed-length identifier is assigned which is called label. The path corresponding to each FEC between the ingress (PE router which accept the packets to MPLS cloud) and egress (PE router which send off the packets out to MPLS cloud) LSRs is called Label Switched Paths (LSP). An FEC, therefore, determines how packets are mapped to an LSP.
A label is assigned to the FEC imposition operation either by tagging an existing field or as a complement in the packet header. The label is pivotal to the establishment of the LSP through all the routers. Each LSR analyzes the incoming packet label. Then after consulting a label table that permits it to recognize the LSP, the LSR switches the packet to the next LSR after changing the value of the label. The label is removed at the egress LSR or a disposition operation is performed.
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