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98-410: 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

196-484: A Label Information Base . 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

294-474: A network host that lists the routes to particular network destinations, and in some cases, metrics (distances) associated with those routes. The routing table contains information about the topology of the network immediately around it. The construction of routing tables is the primary goal of routing protocols . Static routes are entries that are fixed, rather than resulting from routing protocols and network topology discovery procedures. A routing table

392-477: A corresponding entity at the same layer in another host. Service definitions, like the OSI model, abstractly describe the functionality provided to a layer N by a layer N−1 , where N is one of the seven layers of protocols operating in the local host. At each level N , two entities at the communicating devices (layer N peers ) exchange protocol data units (PDUs) by means of a layer N protocol . Each PDU contains

490-526: 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

588-709: A format specified by the application layer during the encapsulation of outgoing messages while being passed down the protocol stack , and possibly reversed during the deencapsulation of incoming messages when being passed up the protocol stack. For this very reason, outgoing messages during encapsulation are converted into a format specified by the application layer, while the conversion for incoming messages during deencapsulation are reversed. The presentation layer handles protocol conversion, data encryption, data decryption, data compression, data decompression, incompatibility of data representation between operating systems, and graphic commands. The presentation layer transforms data into

686-461: A key aspect of certain security operations, such as unicast reverse path forwarding (uRPF). In this technique, which has several variants, the router also looks up, in the routing table, the source address of the packet. If there exists no route back to the source address, the packet is assumed to be malformed or involved in a network attack and is dropped. The need to record routes to large numbers of devices using limited storage space represents

784-409: 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

882-506: A lack of common protocols. For a period in the late 1980s and early 1990s, engineers, organizations and nations became polarized over the issue of which standard , the OSI model or the Internet protocol suite , would result in the best and most robust computer networks. However, while OSI developed its networking standards in the late 1980s, TCP/IP came into widespread use on multi-vendor networks for internetworking . The OSI model

980-584: A light pulse. For example, a 1 bit might be represented on a copper wire by the transition from a 0-volt to a 5-volt signal, whereas a 0 bit might be represented by the transition from a 5-volt to a 0-volt signal. As a result, common problems occurring at the physical layer are often related to the incorrect media termination, EMI or noise scrambling, and NICs and hubs that are misconfigured or do not work correctly. The data link layer provides node-to-node data transfer —a link between two directly connected nodes. It detects and possibly corrects errors that may occur in

1078-464: A major advance in the standardisation of network concepts. It promoted the idea of a consistent model of protocol layers, defining interoperability between network devices and software. The concept of a seven-layer model was provided by the work of Charles Bachman at Honeywell Information Systems . Various aspects of OSI design evolved from experiences with the NPL network, ARPANET, CYCLADES, EIN , and

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1176-582: A major challenge in routing table construction. In the Internet, the currently dominant address aggregation technology is a bitwise prefix matching scheme called Classless Inter-Domain Routing (CIDR). Supernetworks can also be used to help control routing table size. The routing table consists of at least three information fields: Depending on the application and implementation, it can also contain additional values that refine path selection: Shown below

1274-477: A network-layer protocol, if the encapsulation of the payload takes place only at the endpoint, GRE becomes closer to a transport protocol that uses IP headers but contains complete Layer 2 frames or Layer 3 packets to deliver to the endpoint. L2TP carries PPP frames inside transport segments. Although not developed under the OSI Reference Model and not strictly conforming to the OSI definition of

1372-518: 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

1470-456: 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

1568-509: 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:

1666-459: A payload, called the service data unit (SDU), along with protocol-related headers or footers. Data processing by two communicating OSI-compatible devices proceeds as follows: The OSI model was defined in ISO/IEC 7498 which consists of the following parts: ISO/IEC 7498-1 is also published as ITU-T Recommendation X.200. The recommendation X.200 describes seven layers, labelled 1 to 7. Layer 1

1764-451: A physical layer can be described in terms of the network topology . Physical layer specifications are included in the specifications for the ubiquitous Bluetooth , Ethernet , and USB standards. An example of a less well-known physical layer specification would be for the CAN standard. The physical layer also specifies how encoding occurs over a physical signal, such as electrical voltage or

1862-559: 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

1960-403: A remote database protocol to record reservations. Neither of these protocols have anything to do with reservations. That logic is in the application itself. The application layer has no means to determine the availability of resources in the network. Routing table In computer networking , a routing table , or routing information base ( RIB ), is a data table stored in a router or

2058-533: 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

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2156-430: 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 ,

2254-492: 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

2352-468: Is 1500 bytes, the minimum size of a TCP header is 20 bytes, and the minimum size of an IPv4 header is 20 bytes, so the maximum segment size is 1500−(20+20) bytes, or 1460 bytes. The process of dividing data into segments is called segmentation ; it is an optional function of the transport layer. Some connection-oriented transport protocols, such as TCP and the OSI connection-oriented transport protocol (COTP), perform segmentation and reassembly of segments on

2450-564: Is a data link layer protocol that can operate over several different physical layers, such as synchronous and asynchronous serial lines. The ITU-T G.hn standard, which provides high-speed local area networking over existing wires (power lines, phone lines and coaxial cables), includes a complete data link layer that provides both error correction and flow control by means of a selective-repeat sliding-window protocol . Security, specifically (authenticated) encryption, at this layer can be applied with MACsec . The network layer provides

2548-454: Is a database that keeps track of paths, like a map, and uses these to determine which way to forward traffic. A routing table is a data file in RAM that is used to store route information about directly connected and remote networks. Nodes can also share the contents of their routing table with other nodes. The primary function of a router is to forward a packet toward its destination network, which

2646-441: Is an example of what the table above could look like on a computer connected to the internet via a home router : Routing tables are generally not used directly for packet forwarding in modern router architectures; instead, they are used to generate the information for a simpler forwarding table . This forwarding table contains only the routes which are chosen by the routing algorithm as preferred routes for packet forwarding. It

2744-431: Is analogous to a distribution map in package delivery . Whenever a node needs to send data to another node on a network, it must first know where to send it. If the node cannot directly connect to the destination node, it has to send it via other nodes along a route to the destination node. Each node needs to keep track of which way to deliver various packages of data, and for this it uses a routing table. A routing table

2842-411: Is closest to TCP, although TCP contains functions, such as the graceful close, which OSI assigns to the session layer. Also, all OSI TP connection-mode protocol classes provide expedited data and preservation of record boundaries. Detailed characteristics of TP0–4 classes are shown in the following table: An easy way to visualize the transport layer is to compare it with a post office, which deals with

2940-468: 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

3038-403: Is known as peer-to-peer networking (also known as peer-to-peer communication). As a result, the OSI reference model has not only become an important piece among professionals and non-professionals alike, but also in all networking between one or many parties, due in large part to its commonly accepted user-friendly framework. The development of the OSI model started in the late 1970s to support

Multiprotocol Label Switching - Misplaced Pages Continue

3136-483: Is not usually a fatal problem. The OSI connection-oriented transport protocol defines five classes of connection-mode transport protocols, ranging from class 0 (which is also known as TP0 and provides the fewest features) to class 4 (TP4, designed for less reliable networks, similar to the Internet). Class 0 contains no error recovery and was designed for use on network layers that provide error-free connections. Class 4

3234-460: 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

3332-696: Is standardized by the IETF in RFC   3031 . 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

3430-486: Is still used as a reference for teaching and documentation; however, the OSI protocols originally conceived for the model did not gain popularity. Some engineers argue the OSI reference model is still relevant to cloud computing . Others say the original OSI model does not fit today's networking protocols and have suggested instead a simplified approach. Communication protocols enable an entity in one host to interact with

3528-426: Is the destination IP address of the packet. To do this, a router needs to search the routing information stored in its routing table. The routing table contains network/next hop associations. These associations tell a router that a particular destination can be optimally reached by sending the packet to a specific router that represents the next hop on the way to the final destination. The next hop association can also be

3626-556: Is the function of the payload that makes these belong to the network layer, not the protocol that carries them. The transport layer provides the functional and procedural means of transferring variable-length data sequences from a source host to a destination host from one application to another across a network, while maintaining the quality-of-service functions. Transport protocols may be connection-oriented or connectionless. This may require breaking large protocol data units or long data streams into smaller chunks called "segments", since

3724-563: Is the fundamental characteristic of the IP Internet layer and the OSI Network Layer . When a router interface is configured with an IP address and subnet mask, the interface becomes a host on that attached network. A directly connected network is a network that is directly attached to one of the router interfaces. The network address and subnet mask of the interface, along with the interface type and number, are entered into

3822-423: Is the layer of the OSI model that is closest to the end user, which means both the OSI application layer and the user interact directly with a software application that implements a component of communication between the client and server, such as File Explorer and Microsoft Word . Such application programs fall outside the scope of the OSI model unless they are directly integrated into the application layer through

3920-411: Is the lowest layer in this model. The physical layer is responsible for the transmission and reception of unstructured raw data between a device, such as a network interface controller , Ethernet hub , or network switch , and a physical transmission medium . It converts the digital bits into electrical, radio, or optical signals. Layer specifications define characteristics such as voltage levels,

4018-567: The International Network Working Group ( IFIP WG6.1). In this model, a networking system was divided into layers. Within each layer, one or more entities implement its functionality. Each entity interacted directly only with the layer immediately beneath it and provided facilities for use by the layer above it. The OSI standards documents are available from the ITU-T as the X.200 series of recommendations. Some of

Multiprotocol Label Switching - Misplaced Pages Continue

4116-747: 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 )

4214-399: 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

4312-492: The X.25 standard in the late 1970s. The Experimental Packet Switched System in the UK c.  1973 –1975 identified the need for defining higher level protocols. The UK National Computing Centre publication, Why Distributed Computing , which came from considerable research into future configurations for computer systems, resulted in the UK presenting the case for an international standards committee to cover this area at

4410-400: The teardown , between two or more computers, which is called a "session". Common functions of the session layer include user logon (establishment) and user logoff (termination) functions. Including this matter, authentication methods are also built into most client software, such as FTP Client and NFS Client for Microsoft Networks. Therefore, the session layer establishes, manages and terminates

4508-689: The ISO meeting in Sydney in March 1977. Beginning in 1977, the ISO initiated a program to develop general standards and methods of networking. A similar process evolved at the International Telegraph and Telephone Consultative Committee (CCITT, from French: Comité Consultatif International Téléphonique et Télégraphique ). Both bodies developed documents that defined similar networking models. The British Department of Trade and Industry acted as

4606-553: 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

4704-418: The OSI reference model, the communications between systems are split into seven different abstraction layers: Physical, Data Link, Network, Transport, Session, Presentation, and Application. The model partitions the flow of data in a communication system into seven abstraction layers to describe networked communication from the physical implementation of transmitting bits across a communications medium to

4802-440: 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). MPLS works by prefixing packets with an MPLS header, containing one or more labels. This is called a label stack . Each entry in the label stack contains four fields: These MPLS-labeled packets are switched based on

4900-499: The application layer, known as HTTP, FTP, SMB/CIFS, TFTP, and SMTP. When identifying communication partners, the application layer determines the identity and availability of communication partners for an application with data to transmit. The most important distinction in the application layer is the distinction between the application-entity and the application. For example, a reservation website might have two application-entities: one using HTTP to communicate with its users, and one for

4998-417: 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

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5096-477: 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

5194-558: The connections between the local and remote application. The session layer also provides for full-duplex , half-duplex , or simplex operation, and establishes procedures for checkpointing, suspending, restarting, and terminating a session between two related streams of data, such as an audio and a video stream in a web-conferencing application. Therefore, the session layer is commonly implemented explicitly in application environments that use remote procedure calls . The presentation layer establishes data formatting and data translation into

5292-447: 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

5390-545: 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 such as ASIC , TCAM and CAM -based switching able to forward plain IPv4 as fast as MPLS labeled packets. Now, therefore,

5488-551: The dispatch and classification of mail and parcels sent. A post office inspects only the outer envelope of mail to determine its delivery. Higher layers may have the equivalent of double envelopes, such as cryptographic presentation services that can be read by the addressee only. Roughly speaking, tunnelling protocols operate at the transport layer, such as carrying non-IP protocols such as IBM 's SNA or Novell 's IPX over an IP network, or end-to-end encryption with IPsec . While Generic Routing Encapsulation (GRE) might seem to be

5586-576: The emergence of the diverse computer networking methods that were competing for application in the large national networking efforts in the world (see OSI protocols and Protocol Wars ). In the 1980s, the model became a working product of the Open Systems Interconnection group at the International Organization for Standardization (ISO). While attempting to provide a comprehensive description of networking,

5684-576: The fact; the reverse of the traditional approach to developing standards. Although not a standard itself, it was a framework in which future standards could be defined. In May 1983, the CCITT and ISO documents were merged to form The Basic Reference Model for Open Systems Interconnection , usually referred to as the Open Systems Interconnection Reference Model , OSI Reference Model , or simply OSI model . It

5782-670: The form that the application layer accepts, to be sent across a network. Since the presentation layer converts data and graphics into a display format for the application layer, the presentation layer is sometimes called the syntax layer. For this reason, the presentation layer negotiates the transfer of syntax structure through the Basic Encoding Rules of Abstract Syntax Notation One (ASN.1), with capabilities such as converting an EBCDIC -coded text file to an ASCII -coded file, or serialization of objects and other data structures from and to XML . The application layer

5880-407: The functional and procedural means of transferring packets from one node to another connected in "different networks". A network is a medium to which many nodes can be connected, on which every node has an address and which permits nodes connected to it to transfer messages to other nodes connected to it by merely providing the content of a message and the address of the destination node and letting

5978-413: The functions of communication, as is the case with applications such as web browsers and email programs . Other examples of software are Microsoft Network Software for File and Printer Sharing and Unix/Linux Network File System Client for access to shared file resources. Application-layer functions typically include file sharing, message handling, and database access, through the most common protocols at

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6076-530: 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

6174-426: The highest-level representation of data of a distributed application . Each intermediate layer serves a class of functionality to the layer above it and is served by the layer below it. Classes of functionality are implemented in software development using established communication protocols . Each layer in the OSI model has well-defined functions, and the methods of each layer communicate and interact with those of

6272-507: The idea of using labels to represent destination prefixes that was central to tag switching. One original motivation was to allow the creation of simple high-speed switches since for a significant length of time it was considered impractical to forward IP packets entirely in hardware. Advances in VLSI and in forwarding algorithms have made hardware forwarding of IP packets possible and common. The current advantages of MPLS primarily revolve around

6370-529: The label instead of a lookup in the IP routing table . When MPLS was conceived, label switching was faster than a routing table lookup because switching could take place directly within the switched fabric and avoided CPU and software involvement. The presence of such a label has to be indicated to the switch. In the case of Ethernet frames this is done through the use of EtherType values 0x8847 and 0x8848, for unicast and multicast connections respectively. An MPLS router that performs routing based only on

6468-405: 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

6566-401: The last MPLS router, ultimate hop popping (UHP). Some specific label values have been notably reserved for this use. In this scenario the remaining label stack entry conveys information to the last hop (such as its Traffic Class field for QoS information), while also instructing the last hop to pop the label stack using one of the following reserved label values: An MPLS header does not identify

6664-407: The last label themselves. In the label distribution protocols, this PHP label pop action is advertised as label value 3 (implicit null) and is never found in a label, since it means that the label is to be popped. Several MPLS services including end-to-end QoS management, and 6PE , require keeping a label even between the penultimate and the last MPLS router, with a label disposition always done on

6762-408: The layers immediately above and below as appropriate. The Internet protocol suite as defined in RFC   1122 and RFC   1123 is a model of networking developed contemporarily to the OSI model, and was funded primarily by the U.S. Department of Defense. It was the foundation for the development of the Internet . It assumed the presence of generic physical links and focused primarily on

6860-426: The main benefit of MPLS is to implement limited traffic engineering and layer 3 or layer 2 service provider type VPNs over IPv4 networks. OSI model The Open Systems Interconnection ( OSI ) model is a reference model from the International Organization for Standardization (ISO) that "provides a common basis for the coordination of standards development for the purpose of systems interconnection." In

6958-796: The model failed to garner reliance during the design of the Internet , which is reflected in the less prescriptive Internet Protocol Suite , principally sponsored under the auspices of the Internet Engineering Task Force (IETF). In the early- and mid-1970s, networking was largely either government-sponsored ( NPL network in the UK, ARPANET in the US, CYCLADES in France) or vendor-developed with proprietary standards, such as IBM 's Systems Network Architecture and Digital Equipment Corporation 's DECnet . Public data networks were only just beginning to emerge, and these began to use

7056-405: 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

7154-502: The network find the way to deliver the message to the destination node, possibly routing it through intermediate nodes. If the message is too large to be transmitted from one node to another on the data link layer between those nodes, the network may implement message delivery by splitting the message into several fragments at one node, sending the fragments independently, and reassembling the fragments at another node. It may, but does not need to, report delivery errors. Message delivery at

7252-449: The network layer imposes a maximum packet size called the maximum transmission unit (MTU), which depends on the maximum packet size imposed by all data link layers on the network path between the two hosts. The amount of data in a data segment must be small enough to allow for a network-layer header and a transport-layer header. For example, for data being transferred across Ethernet , the MTU

7350-426: The network layer is not necessarily guaranteed to be reliable; a network layer protocol may provide reliable message delivery, but it does not need to do so. A number of layer-management protocols, a function defined in the management annex , ISO 7498/4, belong to the network layer. These include routing protocols, multicast group management, network-layer information and error, and network-layer address assignment. It

7448-414: The outgoing or exit interface to the final destination. With hop-by-hop routing, each routing table lists, for all reachable destinations, the address of the next device along the path to that destination: the next hop . Assuming that the routing tables are consistent, the simple algorithm of relaying packets to their destination's next hop thus suffices to deliver data anywhere in a network. Hop-by-hop

7546-537: 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, the last label is popped off at the penultimate hop (the hop before the egress router). This is called penultimate hop popping (PHP). This is useful in cases where the egress router has many packets leaving MPLS tunnels and thus spends significant CPU resources on these transitions. By using PHP, transit routers connected directly to this egress router effectively offload it, by popping

7644-435: The physical layer. It defines the protocol to establish and terminate a connection between two physically connected devices. It also defines the protocol for flow control between them. IEEE 802 divides the data link layer into two sublayers: The MAC and LLC layers of IEEE 802 networks such as 802.3 Ethernet , 802.11 Wi-Fi , and 802.15.4 Zigbee operate at the data link layer. The Point-to-Point Protocol (PPP)

7742-482: The protocol specifications were also available as part of the ITU-T X series. The equivalent ISO/IEC standards for the OSI model were available from ISO. Not all are free of charge. OSI was an industry effort, attempting to get industry participants to agree on common network standards to provide multi-vendor interoperability. It was common for large networks to support multiple network protocol suites, with many devices unable to interoperate with other devices because of

7840-436: The receiving side; connectionless transport protocols, such as UDP and the OSI connectionless transport protocol (CLTP), usually do not. The transport layer also controls the reliability of a given link between a source and destination host through flow control, error control, and acknowledgments of sequence and existence. Some protocols are state- and connection-oriented . This means that the transport layer can keep track of

7938-435: 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

8036-466: 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. From an OSI model perspective, the MPLS Header is added between the network layer header and link layer header. When a labeled packet is received by an MPLS router,

8134-450: The routing table as a directly connected network. A remote network is a network that can only be reached by sending the packet to another router. Routing table entries to remote networks may be either dynamic or static. Dynamic routes are routes to remote networks that were learned automatically by the router through a dynamic routing protocol. Static routes are routes that a network administrator manually configured. Routing tables are also

8232-514: 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. In 1996 a group from Ipsilon Networks proposed a flow management protocol . Their IP Switching technology, which was defined only to work over ATM, did not achieve market dominance. Cisco Systems introduced a related proposal, not restricted to ATM transmission, called Tag Switching with its Tag Distribution Protocol (TDP). It

8330-736: The secretariat, and universities in the United Kingdom developed prototypes of the standards. The OSI model was first defined in raw form in Washington, D.C. , in February 1978 by French software engineer Hubert Zimmermann , and the refined but still draft standard was published by the ISO in 1980. The drafters of the reference model had to contend with many competing priorities and interests. The rate of technological change made it necessary to define standards that new systems could converge to rather than standardizing procedures after

8428-585: The segments and retransmit those that fail delivery through the acknowledgment hand-shake system. The transport layer will also provide the acknowledgement of the successful data transmission and sends the next data if no errors occurred. Reliability, however, is not a strict requirement within the transport layer. Protocols like UDP, for example, are used in applications that are willing to accept some packet loss, reordering, errors or duplication. Streaming media , real-time multiplayer games and voice over IP (VoIP) are examples of applications in which loss of packets

8526-557: The software layers of communication, with a similar but much less rigorous structure than the OSI model. In comparison, several networking models have sought to create an intellectual framework for clarifying networking concepts and activities, but none have been as successful as the OSI reference model in becoming the standard model for discussing and teaching networking in the field of information technology . The model allows transparent communication through equivalent exchange of protocol data units (PDUs) between two parties, through what

8624-448: 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

8722-433: The timing of voltage changes, physical data rates, maximum transmission distances, modulation scheme, channel access method and physical connectors. This includes the layout of pins , voltages , line impedance , cable specifications, signal timing and frequency for wireless devices. Bit rate control is done at the physical layer and may define transmission mode as simplex , half duplex , and full duplex . The components of

8820-537: The topmost label is examined. Based on the contents of the label a swap , push or pop 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. During these operations, the contents of the packet below the MPLS Label stack are not examined. Indeed, transit routers typically need only to examine

8918-485: 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 type of packet. The egress router must, therefore, have routing information for

9016-651: The transport layer, the Transmission Control Protocol (TCP) and the User Datagram Protocol (UDP) of the Internet Protocol Suite are commonly categorized as layer 4 protocols within OSI. Transport Layer Security (TLS) does not strictly fit inside the model either. It contains characteristics of the transport and presentation layers. The session layer creates the setup, controls the connections, and ends

9114-469: The type of data carried inside the MPLS path. To carry two different types of traffic between the same two routers, with different treatment by the core routers for each type, a separate MPLS path for each type of traffic is required. 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

9212-498: 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

9310-733: 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

9408-541: Was a Cisco proprietary proposal, and was renamed Label Switching . It was handed over to the Internet Engineering Task Force (IETF) for open standardization. The IETF formed the MPLS Working Group in 1997. Work involved proposals from other vendors, and development of a consensus protocol that combined features from several vendors' work. Some time later it was recognized that the work on threaded indices by Girish Chandranmenon and George Varghese had invented

9506-599: 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

9604-668: Was published in 1984 by both the ISO, as standard ISO 7498, and the renamed CCITT (now called the Telecommunications Standardization Sector of the International Telecommunication Union or ITU-T ) as standard X.200. OSI had two major components: an abstract model of networking, called the Basic Reference Model or seven-layer model, and a set of specific protocols . The OSI reference model was

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