Multiprotocol Label Switching
Encyclopedia
Multiprotocol Label Switching (MPLS) is a mechanism in high-performance telecommunications network
s that directs data from one network node to the next based on short path labels rather than long network addresses, avoiding complex lookups in a routing table
. The labels identify virtual links (paths) between distant nodes rather than endpoints. MPLS can encapsulate packets of various network protocols. MPLS supports a range of access technologies, including T1
/E1
, ATM, Frame Relay and DSL
.
technology, such as Asynchronous Transfer Mode
(ATM), Frame Relay
, Synchronous Optical Networking
(SONET) or Ethernet
, and eliminate the need for multiple layer-2 networks to satisfy different types of traffic. MPLS belongs to the family of packet-switched networks
.
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 frame
s.
A number of different technologies were previously deployed with essentially identical goals, such as Frame Relay and ATM. MPLS technologies have evolved with the strengths and weaknesses of ATM in mind. Many network engineers agree that ATM should be replaced with a protocol that requires less overhead, while providing connection-oriented services for variable-length frames. MPLS is currently replacing some of these technologies in the marketplace. It is highly possible that MPLS will completely replace these technologies in the future, thus aligning these technologies with current and future technology needs.
In particular, 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 so fast (at 40 Gbit/s and beyond) that even full-length 1500 byte packets do not incur significant real-time queuing delays (the need to reduce such delays — e.g., to support voice traffic — was the motivation for the cell nature of ATM).
At the same time, MPLS attempts to preserve the traffic engineering
and out-of-band
control that made frame relay and ATM attractive for deploying large-scale networks.
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.
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". It 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 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).
.
Each label stack entry contains four fields:
These MPLS-labeled packets are switched after a label lookup/switch instead of a lookup into the IP table. As mentioned above, when MPLS was conceived, label lookup and label switching
were faster than a routing table
or RIB (Routing Information Base) lookup because they could take place directly within the switched fabric
and not the CPU.
Routers that perform routing based only on the label are called label switch router
s (LSRs). The entry and exit points of an MPLS network are called label edge router
s (LERs), which, respectively, push an MPLS label onto an incoming packetIn some applications, the packet presented to the LER already may have a label, so that the new LER pushes a second label onto the packet. and pop it off the outgoing packet. Alternatively, under penultimate hop popping
this function may instead be performed by the LSR directly connected to the LER.
Labels are distributed between LERs and LSRs using the Label Distribution Protocol
(LDP). 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 they can then use to forward packets. Label switch 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 PVCs in ATM or Frame Relay networks, except that they are not dependent on a particular layer-2 technology.
In the specific context of an MPLS-based virtual private network
(VPN), LERs that function as ingress
and/or egress router
s to the VPN are often called PE (Provider Edge) routers. Devices that function only as transit routers are similarly called P (Provider) routers. See RFC 4364. 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.
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) the packet should be in, 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.
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 can be 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 VPN
s.
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 is usually 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.
In some special cases, the last label can also be 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 lots of 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.
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.
, an extension of the Resource Reservation Protocol
(RSVP) for traffic engineering. Furthermore, there exist extensions of the BGP protocol that can be used to manage an MPLS path.
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.
routing protocols. MPLS LSP
s 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. LSR devices provide traffic engineering functions can be defined using
In a pure IP network, the shortest path to a destination is chosen even when it becomes more 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 OSPF or IS-IS and RSVP. Besides 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.
meets the requirements of real-time applications with recovery times comparable to those of SONET rings of less than 50 ms.
(telcos) to their customers, as clients were unlikely to be utilizing a data service 100 percent of the time. In more recent years, frame relay has acquired a bad reputation in some markets because of excessive bandwidth overbooking by these telcos.
Telcos often sell frame relay to businesses looking for a cheaper alternative to dedicated line
s; its use in different geographic areas depended greatly on governmental and telecommunication companies' policies.
AT&T
is currently (as of June 2007) the largest frame relay service provider in the United States, with local networks in 22 states, plus national and international networks. This number is expected to change between 2007 and 2009 when most of these frame relay contracts expire. Many customers are likely to migrate from frame relay to MPLS over IP or Ethernet within the next two years, which in many cases will reduce costs and improve manageability and performance of their wide area networks.
service for transporting data across computer networks. In both technologies, connections are signaled between endpoints, 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 byte) 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 tunnelling.
The biggest single 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.
In practice, MPLS is mainly used to forward IP
datagrams and Ethernet traffic. Major applications of MPLS are telecommunications traffic engineering and MPLS VPN.
environment (using IPv4 routing protocols) and an IPv6 environment (using IPv6 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. 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.
The main competitors to MPLS are Provider Backbone Bridges (PBB), and MPLS-TP
. These also provide services such as service provider layer 2 and layer 3 VPNs. L2TPv3
has been suggested as a competitor, but has not reached any wider success. Some internet providers are offering different services to customers along with MPLS. These services mainly include National Private Lease Circuit (NPLC), ILL, IPLC etc.
As an example of NPLC, consider City A and City B. An organisation has an office in each city. The organisation requires connectivity between these two offices. The ISP
will have access to a PoP
in each city and therefore has a link between the PoPs. To connect the offices to the PoPs, a connection via the local loop will be commissioned for each office. In this way, an NPLC is delivered.
IEEE 1355
and Spacewire
are family of simplified physical-layer standards very similar in function at the hardware level to MPLS.
how does this get approved.
Telecommunications network
A telecommunications network is a collection of terminals, links and nodes which connect together to enable telecommunication between users of the terminals. Networks may use circuit switching or message switching. Each terminal in the network must have a unique address so messages or connections...
s that directs data from one network node to the next based on short path labels rather than long network addresses, avoiding complex lookups in a routing table
Routing table
In computer networking a routing table, or Routing Information Base , is a data table stored in a router or a networked computer that lists the routes to particular network destinations, and in some cases, metrics associated with those routes. The routing table contains information about the...
. The labels identify virtual links (paths) between distant nodes rather than endpoints. MPLS can encapsulate packets of various network protocols. MPLS supports a range of access technologies, including T1
Digital Signal 1
Digital signal 1 is a T-carrier signaling scheme devised by Bell Labs. DS1 is a widely used standard in telecommunications in North America and Japan to transmit voice and data between devices. E1 is used in place of T1 outside North America, Japan, and South Korea...
/E1
E-carrier
In digital telecommunications, where a single physical wire pair can be used to carry many simultaneous voice conversations by time-division multiplexing, worldwide standards have been created and deployed...
, ATM, Frame Relay and DSL
Digital Subscriber Line
Digital subscriber line is a family of technologies that provides digital data transmission over the wires of a local telephone network. DSL originally stood for digital subscriber loop. In telecommunications marketing, the term DSL is widely understood to mean Asymmetric Digital Subscriber Line ,...
.
Introduction
MPLS is a highly scalable, protocol agnostic, data-carrying mechanism. In an MPLS network, data packets are assigned labels. 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 data link layerData link layer
The data link layer is layer 2 of the seven-layer OSI model of computer networking. It corresponds to, or is part of the link layer of the TCP/IP reference model....
technology, such as Asynchronous Transfer Mode
Asynchronous Transfer Mode
Asynchronous Transfer Mode is a standard switching technique designed to unify telecommunication and computer networks. It uses asynchronous time-division multiplexing, and it encodes data into small, fixed-sized cells. This differs from approaches such as the Internet Protocol or Ethernet that...
(ATM), Frame Relay
Frame relay
Frame Relay is a standardized wide area network technology that specifies the physical and logical link layers of digital telecommunications channels using a packet switching methodology...
, Synchronous Optical Networking
Synchronous optical networking
Synchronous Optical Networking and Synchronous Digital Hierarchy are standardized multiplexing protocols that transfer multiple digital bit streams over optical fiber using lasers or highly coherent light from light-emitting diodes . At low transmission rates data can also be transferred via an...
(SONET) or Ethernet
Ethernet
Ethernet is a family of computer networking technologies for local area networks commercially introduced in 1980. Standardized in IEEE 802.3, Ethernet has largely replaced competing wired LAN technologies....
, and eliminate the need for multiple layer-2 networks to satisfy different types of traffic. MPLS belongs to the family of packet-switched networks
Packet switching
Packet switching is a digital networking communications method that groups all transmitted data – regardless of content, type, or structure – into suitably sized blocks, called packets. Packet switching features delivery of variable-bit-rate data streams over a shared network...
.
MPLS operates at an OSI model
OSI model
The Open Systems Interconnection model is a product of the Open Systems Interconnection effort at the International Organization for Standardization. It is a prescription of characterizing and standardizing the functions of a communications system in terms of abstraction layers. Similar...
layer that is generally considered to lie between traditional definitions of layer 2 (data link layer) and layer 3 (network layer
Network layer
The network layer is layer 3 of the seven-layer OSI model of computer networking.The network layer is responsible for packet forwarding including routing through intermediate routers, whereas the data link layer is responsible for media access control, flow control and error checking.The network...
), 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
Telecommunication circuit
A telecommunication circuit is any line, conductor, or other conduit by which information is transmitted.A dedicated circuit, private circuit, or leased line is a line that is dedicated to only one use...
-based clients and packet-switching clients which provide a datagram
Datagram
A datagram is a basic transfer unit associated with a packet-switched network in which the delivery, arrival time, and order are not guaranteed....
service model. It can be used to carry many different kinds of traffic, including IP packets, as well as native ATM, SONET, and Ethernet frame
Ethernet frame
A data packet on an Ethernet link is called an Ethernet frame. A frame begins with Preamble and Start Frame Delimiter. Following which, each Ethernet frame continues with an Ethernet header featuring destination and source MAC addresses. The middle section of the frame is payload data including any...
s.
A number of different technologies were previously deployed with essentially identical goals, such as Frame Relay and ATM. MPLS technologies have evolved with the strengths and weaknesses of ATM in mind. Many network engineers agree that ATM should be replaced with a protocol that requires less overhead, while providing connection-oriented services for variable-length frames. MPLS is currently replacing some of these technologies in the marketplace. It is highly possible that MPLS will completely replace these technologies in the future, thus aligning these technologies with current and future technology needs.
In particular, 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 so fast (at 40 Gbit/s and beyond) that even full-length 1500 byte packets do not incur significant real-time queuing delays (the need to reduce such delays — e.g., to support voice traffic — was the motivation for the cell nature of ATM).
At the same time, MPLS attempts to preserve the traffic engineering
Teletraffic engineering
Telecommunications traffic engineering, teletraffic engineering, or traffic engineering is the application of traffic engineering theory to telecommunications...
and out-of-band
Out-of-band
The term out-of-band has different uses in communications and telecommunication. In case of out-of-band control signaling, signaling bits are sent in special order in a dedicated signaling frame...
control that made frame relay and ATM attractive for deploying large-scale networks.
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.
History
In 1996 a group from Ipsilon NetworksIpsilon Networks
Ipsilon Networks was a computer networking company which specialised in IP switching during the 1990s.The first product called the IP Switch ATM 1600 was announced in March 1996 for US$46,000....
proposed a "flow management protocol".
Their "IP Switching" technology, which was defined only to work over ATM, did not achieve market dominance. Cisco Systems
Cisco Systems
Cisco Systems, Inc. is an American multinational corporation headquartered in San Jose, California, United States, that designs and sells consumer electronics, networking, voice, and communications technology and services. Cisco has more than 70,000 employees and annual revenue of US$...
introduced a related proposal, not restricted to ATM transmission, called "Tag Switching". It was a Cisco proprietary proposal, and was renamed "Label Switching". It was handed over to the Internet Engineering Task Force
Internet Engineering Task Force
The Internet Engineering Task Force develops and promotes Internet standards, cooperating closely with the W3C and ISO/IEC standards bodies and dealing in particular with standards of the TCP/IP and Internet protocol suite...
(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
Very-large-scale integration
Very-large-scale integration is the process of creating integrated circuits by combining thousands of transistors into a single chip. VLSI began in the 1970s when complex semiconductor and communication technologies were being developed. The microprocessor is a VLSI device.The first semiconductor...
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
Synchronous optical networking
Synchronous Optical Networking and Synchronous Digital Hierarchy are standardized multiplexing protocols that transfer multiple digital bit streams over optical fiber using lasers or highly coherent light from light-emitting diodes . At low transmission rates data can also be transferred via an...
(SONET/SDH).
MPLS operation
MPLS works by prefixing packets with an MPLS header, containing one or more labels. This is called a label stackStack (data structure)
In computer science, a stack is a last in, first out abstract data type and linear data structure. A stack can have any abstract data type as an element, but is characterized by only three fundamental operations: push, pop and stack top. The push operation adds a new item to the top of the stack,...
.
Each label stack entry contains four fields:
- A 20-bit label value.
- a 3-bit Traffic Class field for QoS (quality of serviceQuality of serviceThe quality of service refers to several related aspects of telephony and computer networks that allow the transport of traffic with special requirements...
) priority (experimental) and ECN (Explicit Congestion Notification). - 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 liveTime to liveTime to live is a mechanism that limits the lifespan of data in a computer or network. TTL may be implemented as a counter or timestamp attached to or embedded in the data. Once the prescribed event count or timespan has elapsed, data is discarded. In computer networking, TTL prevents a data...
) field.
These MPLS-labeled packets are switched after a label lookup/switch instead of a lookup into the IP table. As mentioned above, when MPLS was conceived, label lookup and label switching
Label switching
Label switching is a technique of network relaying to overcome the problems perceived by traditional IP-table switching...
were faster than a routing table
Routing table
In computer networking a routing table, or Routing Information Base , is a data table stored in a router or a networked computer that lists the routes to particular network destinations, and in some cases, metrics associated with those routes. The routing table contains information about the...
or RIB (Routing Information Base) lookup because they could take place directly within the switched fabric
Switched fabric
Switched fabric, switching fabric, or just fabric, is a network topology where network nodes connect with each other via one or more network switches . The term is popular in telecommunication, Fibre Channel storage area networks and other high-speed networks, including InfiniBand...
and not the CPU.
Routers that perform routing based only on the label are called label switch router
Label Switch Router
A label switch router , is a type of a router located in the middle of a Multiprotocol Label Switching network...
s (LSRs). The entry and exit points of an MPLS network are called label edge router
Label Edge Router
A label edge router is a router that operates at the edge of an Multiprotocol Label Switching network....
s (LERs), which, respectively, push an MPLS label onto an incoming packetIn some applications, the packet presented to the LER already may have a label, so that the new LER pushes a second label onto the packet. and pop it off the outgoing packet. Alternatively, under penultimate hop popping
Penultimate Hop Popping
Penultimate Hop Popping is a function performed by certain routers in an MPLS enabled network. It refers to the process whereby the outermost label of an MPLS tagged packet is removed by a Label Switch Router before the packet is passed to an adjacent Label Edge Router .The process is important...
this function may instead be performed by the LSR directly connected to the LER.
Labels are distributed between LERs and LSRs using the Label Distribution Protocol
Label Distribution Protocol
Label Distribution Protocol is a protocol in which two Label Edge Routers exchange label mapping information. The two LERs are called LDP peers and the exchange of information is bi-directional....
(LDP). 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 they can then use to forward packets. Label switch 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 PVCs in ATM or Frame Relay networks, except that they are not dependent on a particular layer-2 technology.
In the specific context of an MPLS-based virtual private network
Virtual private network
A virtual private network is a network that uses primarily public telecommunication infrastructure, such as the Internet, to provide remote offices or traveling users access to a central organizational network....
(VPN), LERs that function as ingress
Ingress router
An "ingress router" is a Label Switch Router that is a starting point for a given Label Switched Path . An ingress router may be an egress router or an intermediate router for any other LSP. Hence the role of ingress and egress routers is LSP specific...
and/or egress router
Egress router
An egress router is a Label Switch Router that is an end point for a given Label Switched Path . An egress router may be an ingress router or an intermediate router for any other LSP. Hence the role of egress and ingress routers is LSP specific...
s to the VPN are often called PE (Provider Edge) routers. Devices that function only as transit routers are similarly called P (Provider) routers. See RFC 4364. The job of a P router is significantly easier than that of a PE router
PE router
A Provider Edge router is a router between one network service provider's area and areas administered by other network providers...
, so they can be less complex and may be more dependable because of this.
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
Forwarding equivalence class
A Forwarding Equivalence Class is a term used in Multiprotocol Label Switching to describe a set of packets with similar and / or identical characteristics which may be forwarded the same way; that is, they may be bound to the same MPLS label....
(FEC) the packet should be in, 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.
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 can be 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
Hierarchical routing
Hierarchical routing is method of routing in networks that is based on hierarchical addressing.-Background:Most Transmission Control Protocol/Internet Protocol routing is based on a two-level hierarchical routing in which an IP address is divided into a network portion and a host portion...
of MPLS packets. Notably, this is used by MPLS VPN
MPLS VPN
MPLS VPN is a family of methods for harnessing the power of Multiprotocol Label Switching to create virtual private networks . MPLS is well suited to the task as it provides traffic isolation and differentiation without substantial overhead....
s.
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 is usually 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
Longest prefix match
Longest prefix match refers to an algorithm used by routers in Internet Protocol networking to select an entry from a routing table....
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.
In some special cases, the last label can also be 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 lots of 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.
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.
Installing and removing paths
There are two standardized protocols for managing MPLS paths: the Label Distribution Protocol (LDP) and RSVP-TERSVP-TE
Resource Reservation Protocol - Traffic Engineering is an extension of the resource reservation protocol for traffic engineering. It supports the reservation of resources across an IP network. Applications running on IP end systems can use RSVP to indicate to other nodes the nature of the packet...
, an extension of the Resource Reservation Protocol
Resource Reservation Protocol
The Resource Reservation Protocol is a Transport Layer protocol designed to reserve resources across a network for an integrated services Internet. RSVP operates over an IPv4 or IPv6 Internet Layer and provides receiver-initiated setup of resource reservations for multicast or unicast data flows...
(RSVP) for traffic engineering. Furthermore, there exist extensions of the BGP protocol that can be used to manage an MPLS path.
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
Multicast was for the most part an after-thought 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. Since the inception of RFC 4875 there has been 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.MPLS and IP
MPLS cannot be compared to IP as a separate entity because it works in conjunction with IP and IP's IGPInterior gateway protocol
An interior gateway protocol is a routing protocol that is used to exchange routing information within an autonomous system ....
routing protocols. MPLS LSP
Label Switched Path
In MPLS networking, a Label Switched Path is a path through an MPLS network, set up by a signaling protocol such as LDP, RSVP-TE, BGP or CR-LDP. The path is set up based on criteria in the forwarding equivalence class ....
s 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
IPv4
Internet Protocol version 4 is the fourth revision in the development of the Internet Protocol and the first version of the protocol to be widely deployed. Together with IPv6, it is at the core of standards-based internetworking methods of the Internet...
, IPv6
IPv6
Internet Protocol version 6 is a version of the Internet Protocol . It is designed to succeed the Internet Protocol version 4...
, ATM, Frame Relay, etc.). MPLS-capable devices are referred to as LSRs. LSR devices provide traffic engineering functions can be defined using
- explicit hop-by-hop configuration,
- dynamically routed by the Constrained Shortest Path First (CSPF) algorithm, or
- configured as a loose route that avoids a particular IP or that is partly explicit and partly dynamic.
In a pure IP network, the shortest path to a destination is chosen even when it becomes more 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 OSPF or IS-IS and RSVP. Besides 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.
MPLS local protection (Fast Reroute)
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 protectionMPLS local protection
MPLS Fast Reroute is a local restoration network resiliency mechanism. It is actually a feature of RSVP Traffic Engineering . In MPLS local protection each LSP passing through a facility is protected by a backup path which originates at the node immediately upstream to that facility...
meets the requirements of real-time applications with recovery times comparable to those of SONET rings of less than 50 ms.
With Frame Relay
Frame relay aimed to make more efficient use of existing physical resources, which allow for the underprovisioning of data services by telecommunications companiesTelephone company
A telephone company is a service provider of telecommunications services such as telephony and data communications access. Many were at one time nationalized or state-regulated monopolies...
(telcos) to their customers, as clients were unlikely to be utilizing a data service 100 percent of the time. In more recent years, frame relay has acquired a bad reputation in some markets because of excessive bandwidth overbooking by these telcos.
Telcos often sell frame relay to businesses looking for a cheaper alternative to dedicated line
Dedicated line
In computer networks and telecommunications, a dedicated line is a communications cable or other facility dedicated to a specific application, in contrast with a shared resource such as the telephone network or the Internet....
s; its use in different geographic areas depended greatly on governmental and telecommunication companies' policies.
AT&T
AT&T
AT&T Inc. is an American multinational telecommunications corporation headquartered in Whitacre Tower, Dallas, Texas, United States. It is the largest provider of mobile telephony and fixed telephony in the United States, and is also a provider of broadband and subscription television services...
is currently (as of June 2007) the largest frame relay service provider in the United States, with local networks in 22 states, plus national and international networks. This number is expected to change between 2007 and 2009 when most of these frame relay contracts expire. Many customers are likely to migrate from frame relay to MPLS over IP or Ethernet within the next two years, which in many cases will reduce costs and improve manageability and performance of their wide area networks.
With ATM
While the underlying protocols and technologies are different, both MPLS and ATM provide a connection-orientedConnection-oriented
Connection-oriented communication is a data communication mode in telecommunications whereby the devices at the end points use a protocol to establish an end-to-end logical or physical connection before any data may be sent. In case of digital transmission, in-order delivery of a bit stream or...
service for transporting data across computer networks. In both technologies, connections are signaled between endpoints, 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 byte) 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
Two-way communication
Two-way communication - uses communication to negotiate with the public, resolve conflict, and promote mutual understanding and respect between the organization and its public. Two-Way Communication in Public Relations...
, 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 tunnelling.
The biggest single 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.
MPLS deployment
MPLS is currently in use in IP-only networks and is standardized by the IETF in RFC 3031. It is deployed to connect as few as two facilities to very large deployments. For example, in the retail sector, it is not uncommon to see deployments of 2000 to 5000 locations to communicate transaction data to a headquarters data center.In practice, MPLS is mainly used to forward IP
Internet Protocol
The Internet Protocol is the principal communications protocol used for relaying datagrams across an internetwork using the Internet Protocol Suite...
datagrams and Ethernet traffic. Major applications of MPLS are telecommunications traffic engineering and MPLS VPN.
Competitors to MPLS
MPLS can exist in both an IPv4IPv4
Internet Protocol version 4 is the fourth revision in the development of the Internet Protocol and the first version of the protocol to be widely deployed. Together with IPv6, it is at the core of standards-based internetworking methods of the Internet...
environment (using IPv4 routing protocols) and an IPv6 environment (using IPv6 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
Application-specific integrated circuit
An application-specific integrated circuit is an integrated circuit customized for a particular use, rather than intended for general-purpose use. For example, a chip designed solely to run a cell phone is an ASIC...
, TCAM and CAM
Content-addressable memory
Content-addressable memory is a special type of computer memory used in certain very high speed searching applications. It is also known as associative memory, associative storage, or associative array, although the last term is more often used for a programming data structure...
-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.
The main competitors to MPLS are Provider Backbone Bridges (PBB), and MPLS-TP
MPLS-TP
MPLS-TP or MPLS Transport Profile is a profile of MPLS whose definition has been commenced by the IETF. It will be designed for use as a network layer technology in transport networks. Its design will be a continuation of the work started by the transport network experts of the ITU-T, specifically...
. These also provide services such as service provider layer 2 and layer 3 VPNs. L2TPv3
L2TPv3
Layer 2 Tunneling Protocol Version 3 is an IETF standard related to L2TP that can be used as an alternative protocol to Multiprotocol Label Switching for encapsulation of multiprotocol Layer 2 communications traffic over IP networks...
has been suggested as a competitor, but has not reached any wider success. Some internet providers are offering different services to customers along with MPLS. These services mainly include National Private Lease Circuit (NPLC), ILL, IPLC etc.
As an example of NPLC, consider City A and City B. An organisation has an office in each city. The organisation requires connectivity between these two offices. The ISP
Internet service provider
An Internet service provider is a company that provides access to the Internet. Access ISPs directly connect customers to the Internet using copper wires, wireless or fiber-optic connections. Hosting ISPs lease server space for smaller businesses and host other people servers...
will have access to a PoP
Point of presence
A point of presence is an artificial demarcation point or interface point between communications entities. It may include a meet-me-room.In the US, this term became important during the court-ordered breakup of the Bell Telephone system...
in each city and therefore has a link between the PoPs. To connect the offices to the PoPs, a connection via the local loop will be commissioned for each office. In this way, an NPLC is delivered.
IEEE 1355
IEEE 1355
IEEE Standard 1355-1995, IEC 14575, or ISO 14575 is a data communications standard for Heterogeneous Interconnect . It is a low-cost, low latency, scalable serial interconnection system, originally intended for communication between large numbers of inexpensive computers. It lacks many of the...
and Spacewire
SpaceWire
SpaceWire is a spacecraft communication network based in part on the IEEE 1355 standard of communications. It is coordinated by the European Space Agency in collaboration with international space agencies including NASA, JAXA and RKA...
are family of simplified physical-layer standards very similar in function at the hardware level to MPLS.
Books
- "Deploying IP and MPLS QoS for Multiservice Networks: Theory and Practice" by John Evans, Clarence Filsfils (Morgan Kaufmann, 2007, ISBN 0-12-370549-5)
- Rick Gallaher's MPLS Training Guide (ISBN 1932266003)
how does this get approved.
External links
- MPLS Working Group, IETF.
- MPLS IP Specifications, Broadband Forum.