Traffic engineering distributes bandwidth load across network links. Learn about the evolution of traffic engineering and its role in networks transitioning from Layer 2 to IP technology. Then dive into MPLS traffic engineering and all the benefits it provides for network engineers and designers as well as MPLS TE myths and half-truths.
In this guide:
Traditional service provider networks provided Layer 2 point-to-point virtual circuits with contractually predefined bandwidth. Regardless of the technology used to implement the service (X.25 Frame Relay or ATM) the traffic engineering (optimal distribution of load across all available network links) was inherent in the process.
In most cases the calculation of the optimum routing of virtual circuits was done off-line by a network management platform; advanced networks (offering Frame Relay or ATM switched virtual circuits) also offered real-time on-demand establishment of virtual circuits. However the process was always the same:
- The free network capacity was examined.
- The end-to-end hop-by-hop path throughout the network that satisfied the contractual requirements (and if needed met other criteria) was computed.
- A virtual circuit was established along the computed path.
Internet and most IP-based services including IP-based virtual private networks (VPNs) implemented with MPLS VPN IPsec or Layer 2 transport protocol (L2TP) follow a completely different service model:
- The traffic contract specifies ingress and egress bandwidth for each site not site-to-site traffic requirements.
- Every IP packet is routed through the network independently and every router in the path makes independent next-hop decisions.
- Once merged all packets toward the same destination take the same path (whereas multiple virtual circuits toward the same site could traverse different links).
Simplified to the extreme the two paradigms could be expressed as follows:
- Layer 2 switched networks assume that the bandwidth is expensive and try to optimize its usage resulting in complex circuit setup mechanisms and expensive switching methods.
- IP networks assume that the bandwidth is 'free' and focus on low-cost high-speed switching of a high volume of traffic.
The significant difference between the cost-per-switched-megabit of Layer 2 network (for example ATM) and routed (IP) network has forced nearly all service providers to build next-generation networks exclusively on IP. Even in modern fiber-optics networks however bandwidth is not totally free and there are always scenarios where you could use free resources of an underutilized link to ease the pressure on an overloaded path. Effectively you would need traffic engineering capabilities in routed IP networks but they are simply not available in the traditional hop-by-hop destination-only routing model that most IP networks use.
Various approaches (including creative designs as well as new technologies) have been tried to bring the traffic engineering capabilities to IP-based networks. We can group them roughly into these categories:
- The network core uses Layer 2 switched technology (ATM or Frame Relay) that has inherent traffic engineering capabilities. Virtual circuits are then established between edge routers as needed.
- IP routing tricks are used to modify the operation of IP routing protocols resulting in adjustments to the path the packets are taking through the network.
- Deployment of IP-based virtual circuit technologies including IP-over-IP tunnels and MPLS traffic engineering.
The Layer 2 network core design was used extensively when the service providers were introducing IP as an additional service into their WAN networks.