**7. MPLS traffic engineering essentials**

Multi-Protocol Label Switching (MPLS) is the end result of the efforts to integrate Layer 3 switching, better known as routing, with Layer 2 WAN backbones, primarily ATM. Even though the IP+ATM paradigm is mostly gone today because of the drastic shift to IP-only networks in the last few years, MPLS retains a number of useful features from Layer 2 technologies. One of the most notable is the ability to send packets across the network through a virtual circuit called Label Switched Path, or LSP, in MPLS terminology.

While the Layer 2 virtual circuits are almost always bidirectional (although the traffic contracts in each direction can be different), the LSPs are always unidirectional. If you need bidirectional connectivity between a pair of routers, you have to establish two LSPs.

1. Explicit label switched paths which are not constrained by the destination based forwarding paradigm can be easily created through manual administrative action or

4. A set of attributes can be associated with traffic trunks which modulate their behavioral

5. A set of attributes can be associated with resources which constrain the placement of

6. MPLS allows for both traffic aggregation and disaggregating whereas classical

In [3] the MPLS domain is described as "a contiguous set of nodes which operate using MPLS routing and forwarding". This domain is typically managed and controlled by one administration. The MPLS domain concept is therefore similar to the notion of an AS (autonomous system), as the term is used in conventional IP routing i.e. a set of related

The MPLS domain can be divided into MPLS core and MPLS edge. The core consists of nodes neighboring only to MPLS capable nodes, while the edge consists of nodes neighboring both MPLS capable and incapable nodes. The nodes in the MPLS domain are often called LSRs (Label Switch Routers). The nodes in the core are called transit LSRs and the nodes in the MPLS edge are called LERs (Label Edge Routers). If a LER is the first node in the path for a packet traveling through the MPLS domain this node is called the ingress LER, if it is the last node in a path it's called the egress LER. Note that these terms are applied according to the direction of a flow in the network, one node can therefore be both ingress and egress LER depending on which flow is considered. The terms upstream and downstream routers are also often used to indicate in which order the routers are traversed. If a LSR is upstream from another LSR, traffic is passed through that LSR before the other

Multi-Protocol Label Switching (MPLS) is the end result of the efforts to integrate Layer 3 switching, better known as routing, with Layer 2 WAN backbones, primarily ATM. Even though the IP+ATM paradigm is mostly gone today because of the drastic shift to IP-only networks in the last few years, MPLS retains a number of useful features from Layer 2 technologies. One of the most notable is the ability to send packets across the network

While the Layer 2 virtual circuits are almost always bidirectional (although the traffic contracts in each direction can be different), the LSPs are always unidirectional. If you need

7. It is relatively easy to integrate a "constraint-based routing" framework with MPLS. 8. A good implementation of MPLS can offer significantly lower overhead than competing

through automated action by the underlying protocols.

3. Traffic trunks can be instantiated and mapped onto LSPs.

destination only based IP forwarding permits only aggregation.

routers that are usually under one administrative and management control.

(downstream). A schematic view of the MPLS domain is illustrated in figure 2.

through a virtual circuit called Label Switched Path, or LSP, in MPLS terminology.

bidirectional connectivity between a pair of routers, you have to establish two LSPs.

2. LSPs can potentially be efficiently maintained.

LSPs and traffic trunks across them.

alternatives for Traffic Engineering.

**7. MPLS traffic engineering essentials** 

characteristics.

**6. The MPLS domain** 

Fig. 2. The MPLS domain

The LSPs in MPLS networks are usually established based on the contents of IP routing tables in core routers. However, there is nothing that would prevent LSPs being established and used through other means, provided that:


The other routers along the LSP do not inspect the packets traversing the LSP and are thus oblivious to their content; they just need to understand the signaling protocol that is used to establish the LSP.

With the necessary infrastructure in place, it was only a matter of time before someone would get the idea to use LSPs to implement MPLS-based traffic engineering. The MPLS traffic engineering technology has evolved and matured significantly since then, but the concepts have not changed much since its introduction:


**NOTE:** The first MPLS TE implementations supported only static hop-by-hop definitions. These can still be used in situations where you need a very tight hop-by-hop control over the path the MPLS TE LSP will take or in networks using a routing protocol that does not have MPLS TE extensions.

Traffic Engineering 255

features, attributes and characteristics (e.g. link and nodal attributes). A network model can facilitate analysis or simulation, and thus can be useful to predict the network performance. Network modelling can be classified as structural or behavioural module. Structural modules focus on the organization of the network and its components. Behavioral modules focus on the dynamics of the networks and its traffic workload. Because of the complexity of realistic quantitative analysis of network behavior, certain aspects of network performance

**Optimization:** Network performance optimization can be called corrective when a solution to a problem is made, or perfective, where an improvement to the network performance is made, even when there is no problem. Many actions could be taken such as adding additional links, increasing link capacity or adding additional hardware. Planning for future improvement in the network (e.g. network design, network capacity or network

Traditionally, there have been three parameters that describe the quality of a connection: bandwidth, delay, and packet loss. A connection with high bandwidth, low delay, and low packet loss is considered to be better than one with low bandwidth, high delay, and high packet loss. The following parameters can be considered when selecting the best traffic route: **Congestion:** Congestion decreases the available bandwidth and increases delay and packet

**Distance:** Two routes may have different paths. Some networks interconnect only at relatively few locations, so they may have to transport traffic over long distances to get it to its destination. Others have better interconnection, so the traffic does not have to take a detour. There may be reasons not to prefer the more direct route, such as lower bandwidth

**Hops:** The number of hops (e.g. routers) that shows up on the path to the destination increases the delay. Each hop potentially adds additional delay, because packets have to wait in a queue before they are transmitted, and the extra equipment in a path means that a

Congestion in a packet switching network is a state in which the performance of the network degrades because of the saturation of network resources. Congestion could result in degradation of service quality to users. To avoid congestion, certain mechanisms have to

Congestion control policies can be categorized differently based on the objective of the policy, the time period of applying the policy, and the action taken to avoid congestion. In

failure somewhere along the way is more likely. So, paths with fewer hops are better.

studies can only be conducted effectively using simulation.

architecture) is considered as a part of network optimization.

**9. Criteria for selecting the best traffic route** 

loss. It is important to avoid routes over congested paths.

or congestion, but generally a shorter geographic path is better.

be provided; such mechanisms are usually called congestion control.

**10. Congestion control** 

**10.1 Categories of congestion control** 

the following we will explain some of these policies.


The tight integration of MPLS traffic engineering with the IP routing protocols provides an important advantage over the traditional Layer 2 WAN networks. In the Layer 2 backbones, the operator had to establish all the virtual circuits across the backbone (using a network management platform or by configuring switched virtual circuits on edge devices), whereas the MPLS TE can automatically augment and enhance the mesh of LSPs already established based on network topology discovered by IP routing protocols. You can thus use MPLS traffic engineering as a short-term measure to relieve the temporary network congestion or as a network core optimization tool without involving the edge routers.

In recent years, MPLS traffic engineering technology (and its implementation) has grown well beyond features offered by traditional WAN networks. For example:

