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Multi-protocol Label Switching explained

What is MPLS?

Multiprotocol Label Switching (MPLS) is a networking technology that directs data from one node to the next using labels rather than a network address. These labels identify paths between the desired end points, where the use of network addresses identify the end points themselves. Packet-forwarding decisions are made solely on the contents of this label, without the need to examine the packet itself.

MPLS does not directly align with the OSI seven-layer hierarchy and 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 who provide a datagram service model. MPLS has risen to some prominence due to its ability to encapsulate and forward a range of network protocols. This has made MPLS ideal for network and services convergence in both the service provider and enterprise markets and is often used in conjunction with IP, but can also carry ATM, Frame Relay, and TDM access technologies.

Enabling traffic engineering

With MPLS, the first time a packet enters the network, it is assigned a specific forwarding Class of Service (CoS). This is sometimes referred to as the forwarding equivalence class (FEC) and is identified by the appending of a label to the incoming packet and is often representative of the type of traffic being carried. For example, voice and video services may be given a label to represent their ‘real-time’ requirements, while other traffic could be labelled to represent its ‘mission-critical’ nature, and other internet or email traffic could be given a ‘best-effort’ label. This enables additional information to be attached to each packet, with the intention it receives the appropriate service quality across the network. For traffic to transit the network, MPLS routers establish a label-switch path (LSP), which is a predetermined path to forward the traffic across the MPLS network, based upon the criteria in the FEC. LSPs are unidirectional, which means that return traffic is sent over a different LSP.

There are two kinds of routers in an MPLS network, transit routers and edge routers. If the protocol is IP, then as an IP packet enters the MPLS network, it passes through an edge router first (it will also exit through an edge router). The MPLS edge router analyzes the information in the packet header to determine the class of service (CoS) required by the application data in the payload and then affixes a ‘label’ to the packet. Edge routers are where the most innovation occurs as vendors develop specialized network processors to perform complex calculations fast enough to match line speed — in other words, fast enough that the edge router is not adding latency and delay while it is trying to ensure QoS.

If you’ve ever been lucky enough to fly business class, you will have seen the airline personnel affix a bright orange label to your baggage. This (usually) ensures that your bag will be among the first group to arrive at the carousel. An MPLS label works in much the same way. The label indicates the CoS or QoS that the packet will receive throughout its journey. 

The label switch router (LSR) or transit router is a router located within an MPLS network, and that performs routing based on the label. It reads the incoming label and replaces it with a new label for the next hop in the LSP. 

Multiservice networks

Today we are seeing the replacement of many networking protocols by IP or IP/MPLS. This is because IP/MPLS is a multiservice network technology and can deliver the same level of performance, even for the most mission-critical applications. There are other factors, however, driving the adoption of IP/MPLS. 

The first is that application developers are writing IP-based applications. This means that any vertical business that wants to update its software to take advantage of the latest and greatest is going to, at some point, be forced to make the move to IP. 

The second factor is because of economies of scale due to the pervasive deployment of IP networking systems. There was a time, several decades ago, when choosing a specialized network technology for a specific application was likely only marginally more expensive. This was because there were many competing technologies and no technology had market dominance. But today, the world has settled on IP and IP/MPLS as the network technology. Even specialized mobile networks have moved to using IP and IP/MPLS since 4G/LTE. 

IP/MPLS and business services

Historically, IP/MPLS networks largely replaced circuit-based networks that businesses used for connecting branch offices to headquarters. These were originally run on T1 or E1 TDM (time-division multiplexing) dedicated L2 links. The advantage of TDM private line business services was that the link was dedicated to the branch office. It was not competing with other users for use of the line. Thus, business critical QoS could be assured. For similar reasons, TDM was used in many vertical applications such as power utility and air traffic control networks, where best-effort service and the associated delays were simply not acceptable.

The strength of TDM, however, was also its weakness. Because the line was dedicated to the branch or power utility control network, when it wasn’t in use, the network capacity was not being used. The great advantage of an IP packet-based network like the internet is that all users share the basic managed connectivity. It also means that the service can scale, taking more bandwidth when needed and less when not. 

With its QoS and reliability capabilities, IP/MPLS networks ushered in the age of virtual private networks or VPNs for both enterprises for delivering mission-critical services and within the service provider network themselves. As part of an enterprise’s digital transformation strategy, instead of using legacy T1/E1 lines, branch offices and specialized vertical applications can realize the cost advantages of a shared physical infrastructure. By using labels, the MPLS network can logically or virtually separate the VPN data from other data traversing the network. By adding line-rate encryption abilities to the edge router, it also becomes possible to make those packets completely secure as they pass across the network. 

The road ahead

As data networking continues to evolve, MPLS will have a continuing role in our data networks. Ultra-reliable MPLS is well suited for the delivery of mission-critical services and real-time applications, an area of increasing preeminence within enterprise digital transformation strategies. MPLS is often heavily embedded within service provider networks to deliver VPN-based services and other infrastructure capabilities, but moving forward, it can also be used in conjunction with new initiatives such as SD-WAN. Improvements in other networking technologies and protocols have made IP traffic more reliable, but there will always be a place for the ultra-high reliability of MPLS.

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