There are basically two types of data transfers: With connectionless data transfer, data is sent at any time and without limits from the desired device to a target system without the path of the packages being set in advance. Each interconnected network node (usually a router) automatically knows how to forward the data stream. Connectionless data transfer offers a high degree of flexibility, but no guarantee that the necessary resources will be available.
In contrast, the path of the data packages in connection-oriented data transfer is decided from the beginning. The network nodes involved (generally switches) receive the corresponding information for the forwarding of the data from the preceding station, until the point when the packages have arrived at the target computer at the end of their path. In this way, the time-consuming routing process necessary for a connectionless transfer is considerably accelerated. It also allows for optimal control of available network resources and their distribution amongst individual participants. The so-called multiprotocol label switching (MPLS) allows this to be used for TCP/IP networks as well, though these are technically in the connectionless network category.
In the mid-1990s, large communication networks were characterized by a much higher proportion of voice communication (telephony) than data communication (internet). Telecommunication providers at this time were still operating separate networks for both transfer types, which was quite expensive, but also didn’t ensure a comprehensive quality of service. The high quality, connection-oriented communication networks stood in contrast to connectionless data networks, which lacked the necessary bandwidth. The introduction of the ATM protocol (Asynchronous Transfer Mode) largely solved this problem by allowing the voice and data to be transferred via a shared infrastructure. But multiprotocol label switching really provided a solution in the late 1990s for using available bandwidths efficiently.
To accomplish this, MPLS finally relieved the overloaded routing systems: Instead of defining the optimal route of a data package by the individual intermediate stations, as was done before, the new method offered the possibility to pre-define paths that define a package’s way from the input point (ingress router) to the starting point (egress router). The relay points (label switched router) recognize these paths by evaluating labels that contain the appropriate routing and service information, and are assigned to the respective data package. The evaluation takes place using the appropriate hardware (e.g. a switch) above the backup layer (layer 2), while the time-consuming routing on the switching layer (layer 3) is left out.
Thanks to the generalized MPLS extension, the technology originally developed only for IP networks is now also available for other network types, such as SONET/SDH (Synchronous Optical Networking / Synchronous Digital Hierarchy) or WSON (Wavelength Switched Optical Network).
The use of MPLS in IP networks requires a logical and physical infrastructure consisting of MPLS-capable routers. The labeling process operates primarily within an autonomous system (AS) – a congregation of different IP networks that are managed as a unit and connected via at least one common interior gateway protocol (IGP). Administrators of such systems are generally internet providers, universities, or international companies.
Before the individual paths can be built, the IGP being used needs to ensure that all routers of the autonomous system can reach one another. Then the corner points of the paths, which are also referred to as label switched paths (LSP), are defined. These previously mentioned ingress and egress routers are usually at the inputs and outputs of a system. Activation of the LSPs is then either manual, semi-automatic, or fully automatic:
Data packages sent in a configured MPLS network receive an additional MPLS header from the ingress router. This is inserted between the information of the second and third layers, and is also referred to as a push operation. During the transfer, the individual hops involved exchange the label with a customized version with its own connection information (i.e. latency, bandwidth, and destination hop) – this procedure is often called a swap operation. At the end of the path, the label is removed from the IP header as part of a pop operation.
MPLS extends the normal IP header by the so-called MPLS label stack entry, which is also known as the MPLS shim header. This entry is very short, with a length of 4 bytes (32 bits), which is why it can quickly be processed. The corresponding header line, which is inserted between the layer 2 and layer 3 headers, looks as follows:
| Bits 0-19 | Bits 20-22 | Bit 23 | Bits 24-31 |
|---|---|---|---|
| Label | TC | S | TTL |
The additional 32 bits of the MPLS label stack entry add four pieces of information to an IP package for the next network hop:
In the 1990s, MPLS helped providers with the rapid development and growth of their networks. But the initial speed advantage for data transfer has been pushed to the background with the new generation of high-performance routers with integrated network processors. As a procedure that can guarantee quality of service, though, it’s still used today by many service providers. This has to do with so-called traffic engineering – a process that deals with the analysis and optimization of data streams. In addition to the classification of individual data connections, an analysis of the bandwidths and capacities of individual network elements also takes place. Based on the results, the data load can then be optimally distributed to strengthen the entire network.
Another important area of application is virtual private networks (VPN) – self-contained, virtual communication networks that use public network infrastructures like the internet as a transport medium. In this way, devices can merge in a network without physically connecting with one another. There are two different types of such MPLS networks:
Operators of large WAN networks (Wide Area Network) profit from provider offers that are based on multiprotocol label switching: Correctly configured, the strategic label switched paths optimize data traffic and ensure to the greatest extent possible that all users can have the bandwidth that they need at any time – while their own effort remains limited. For campus networks, such as university or enterprise networks, the method is also a suitable solution, if the necessary budget is available.
Multiprotocol label switching competes as a technology for virtual networks with the IP protocol stack extension IPsec, among others. The security upgrade of the internet protocol is characterized, in particular, by its own encryption mechanisms and low costs. The implementation of infrastructure by means of IPsec isn’t the responsibility of the provider, though, but rather the user – as opposed to the MPLS method. This requires a higher level of effort, giving the MPLS method an advantage. This isn’t the only benefit of ‘label’ networks, as the following list shows:
The advantages of MPLS and the technology based on virtual private networks are especially interesting for companies and institutions that are spread over several sites and want to grant their customers access to their network. As a result, such virtual networks are often the first choice when building an IT infrastructure. This allows users to combine into one network without requiring a physical connection or public, routed IP address on the internet.
Basically, a multiprotocol label switching VPN is only available to users who have the appropriate data for setting up the connection. This fact alone doesn’t make the virtual networks immune to unauthorized access, though: The ‘private’ attribute isn’t used in such networks for secrecy and encryption, but to make the IP addresses only accessible internally. Without additional encryption, all information is transferred in plaintext. But a corresponding certification also doesn’t offer one hundred percent protection, even if the normal internet traffic via the transfer router (also called the ‘Provider Edge’ (PE)) runs between the MPLS network and customer LAN. Some possible risks when using the MPLS infrastructures are listed below:
In addition to encryption, each VPN should have additional security mechanisms to protect the provider edge router against external attacks. The primary recommendation for this is the establishment of a demilitarized zone between two firewalls, and the use of network monitoring systems. In addition, regular updates of software and hardware, as well as security measures against unauthorized physical access to the gateways, should be the standard.
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