**2. Traffic engineering's role in next-generation networks**

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.

aggregated and sent through the same path. Therefore a link may be congested despite the presence of under-utilized link in the network. And delay sensitive traffic like voice-over-IP calls may travel over a path with high propagation delay because this is the shortest path

As illustrated in the above figure 1 the shortest path from router 1 to 5 is the path (1-3-5). All traffic passing through router 1 with destination router 5 (or another router with router 5 in the shortest path) will travel through this shortest path if the shortest path algorithm is used for forwarding in this network. Although there is an alternative path (1-2-4-5) available that

Traffic engineering is the process of controlling how traffic flows through a network to optimize resource utilization and network performance [4]. Traffic engineering is basically concerned with two problems that occur from routing protocols that only use the shortest

The shortest paths from different sources overlap at some links, causing congestion on those links. The traffic from a source to a destination exceeds the capacity of the shortest path,

MPLS can be used as a traffic engineering tool to direct traffic in a network in a more efficient way then original IP shortest path routing. MPLS can be used to control which paths traffic travels through the network and therefore a more efficient use of the network resources can be achieved. Paths in the network can be reserved for traffic that is sensitive, and links and router

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

could be used to distribute traffic more evenly in the network.

while a longer path between these two routers is under-utilized.

that is more secure and not known to fail can be used for this kind of traffic.

**2. Traffic engineering's role in next-generation networks** 

across all available network links) was inherent in the process.

path as constraint when they construct a routing table.

while a low latency path is available.

Fig. 1. Traffic Engineering

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:


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:


Simplified to the extreme, the two paradigms could be expressed as follows:


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:


Traffic Engineering 251

1. **Information Distribution:** Traffic engineering requires detailed knowledge about the network topology as well as dynamic information about network loading. This can be implemented by using simple extensions to IGP so that link attributes (such as maximum link bandwidth, current bandwidth usage, current bandwidth reservation) are included as part of routers link-state advertisements. The standard flooding algorithm used by link-state IGP ensures that link attributes are distributed to all routers in ISPs routing domain. Each LSR maintains network link attributes and topology information in a specialized TE database (TED), which is used exclusively for

2. **Path Selection:** On the basis of the network topology and link attributes in the TED and some administrative attributes obtained from user configuration, each ingress LSR calculates the explicit paths for its LSPs, which may be strict or loose. A strict explicit route is one in which the ingress LSR specifies all the LSRs in the LSP, while only some LSRs are specified in a loose explicit path. LSP calculations may also be done offline for

3. **Signaling and Path-Setup:** The path calculated by the path selection component is not known to be workable, until LSP is actually established by the signaling component, because it is calculated on the basis of information present in TED, which may not be up-to-date. The signaling component is responsible for establishing LSP state and label

4. **Packet-Forwarding:** Once the path is set-up, packet forwarding process begins at the

MPLS is strategically significant for Traffic Engineering because it can potentially provide most of the functionality available from the overlay model, in an integrated manner, and at a lower cost than the currently competing alternatives. Equally importantly, MPLS offers the

The concept of MPLS traffic trunks is used, according to Li and Rekhter [7], a traffic trunk is an aggregation of traffic flows of the same class which are placed inside a Label Switched Path. Essentially, a traffic trunk is an abstract representation of traffic to which specific characteristics can be associated. It is useful to view traffic trunks as objects that can be routed; that is, the path through which a traffic trunk traverses can be changed. In this respect, traffic trunks are similar to virtual circuits in ATM and Frame Relay networks. It is important, however, to emphasize that there is a fundamental distinction between a traffic trunk and the path, and indeed the LSP, through which it traverses. An LSP is a specification of the label switched path through which the traffic traverses. In practice, the

The attractiveness of MPLS for Traffic Engineering can be attributed to the following

Label Switch Router (LSR) and is based on the concept of label switching.

calculating explicit paths for placement of LSPs on physical topology.

3. Signaling and path set-up 4. Packet forwarding

Now, discussing each of the components in detail:

optimal utilization of network resources.

**5. MPLS and traffic engineering** 

factors:

binding and distribution in the path set-up process.

possibility to automate aspects of the Traffic Engineering function.

terms LSP and traffic trunk are often used synonymously.

The Layer 2 network core design was used extensively when the service providers were introducing IP as an additional service into their WAN networks. Many large service providers have already dropped this approach because it does not result in the cost reduction or increase in switching speed that pure IP-based networks bring
