Multi-Protocol Label Switching (MPLS) Conform…

In Layer 2 VPNs, the PE and CE routers need not be routing peers as required in Layer 3 VPNs. Instead, only a Layer 2 connection needs to exist between PE and CE, with the PE routers simply switching incoming traffic into tunnels configured to one or more other PE routers. A Layer 2 MPLS VPN determines reachability through the data plane by using address learning, in contrast with Layer 3 VPNs, which determine reachability through the control plane by exchanging BGP routes.

L2 VPNs use label stacking similar to Layer 3 VPNs. The outer tunnel label determines the hop-by-hop path through the network.

The inner Virtual Circuit (VC) label identifies the VLAN, VPN, or connection at the end point. In addition, there is an optional Control Word following the VC label that carries information about the enclosed Layer 2 packet.

Layer 2 MPLS VPNs have the distinct advantage of being able to transport any enterprise protocol ? whatever is being carried is transparent to the MPLS network. They can also run over nearly any transport medium, including ATM, Frame Relay, Packet over SONET, and Ethernet, enabling the integration of connectionless IP networks with connection-oriented networks. End customer expertise is minimal since no routing configuration is required.

On the downside, L2 VPNs may not scale as well as L3 VPNs. A full mesh of LSPs must be set up between all L2 VPN sites, a requirement that does not scale well with large numbers of sites. In addition, they cannot take advantage of automatic route discovery available with L3 VPNs, and so are better suited to situations with a smaller number of VPN sites and static routes.

QoS/CoS

One of the key shortcomings of IP-based networks, compared with Frame Relay and ATM networks, has been their inability to provide service guarantees for traffic. For example, real-time traffic such as voice or video needs high quality service (low latency, low jitter, etc.) to successfully traverse a network. Similarly, mission-critical data, such as e-commerce transactions, must have priority over normal web browser traffic.

MPLS抯 connection-oriented nature provides the framework necessary to give quality guarantees to IP traffic. While QoS and Class of Service (CoS) are not fundamental features of MPLS, they can be applied in MPLS networks where traffic engineering is used. This enables providers to establish Service Level Agreements (SLAs) with customers to guarantee service aspects such as network bandwidth, delay, and jitter. Value-added services can be delivered in addition to basic data transport, increasing revenue propositions and ultimately enabling the move to converged networks.

IntServ and DiffServ. Several mechanisms have developed over time to establish QoS/CoS within a network. In the IntServ (Integrated Services) model, RSVP was developed to signal QoS requirements across a network, allowing devices to negotiate and establish guaranteed traffic parameters such as bandwidth and latency end-to-end. It uses hard allocation of resources, guaranteeing services down to a per-flow basis. The DiffServ (Differentiated Services) model is less stringent, providing for CoS delivery by classifying traffic into relative priority levels for aggregate treatment, but without signaling or end-to-end guarantees of service. DiffServ redefines the Type of Service (TOS) field in the IP packet header to provide this classification.

While IntServ offers traffic bandwidth guarantees, it has proved to be not very scalable or practical to operate across large networks. The DiffServ architecture, on the other hand, is a scalable alternative, but does not provide guarantees. Recent work in the IETF has focused on combining DiffServ and MPLS traffic engineering elements to provide QoS guarantees in MPLS packet networks. The DiffServ information in IP packet headers is mapped into the label information of the MPLS packets. MPLS routers act upon the prioritization information in the packets to forward the data appropriately. Some of the mechanisms used include traffic shaping, queuing, and packet classification.

QoS is typically implemented at the edge of the MPLS cloud, where non-labeled traffic from the customer network enters the carrier network. At this entry point for example, delay-sensitive real-time traffic, such as IP voice or video conferencing traffic, can be prioritized for delivery over bulk data transmissions.

Traffic engineering

Another key shortcoming of IP, especially in public networks, is its inability to optimize network resource utilization. Using standard IP routing, all traffic between two points is sent over the shortest path even though multiple paths may exist. During periods of high traffic volume, this can result in traffic congestion on certain routes while alternative routes are underused. This is known as the hyper-aggregation problem (Figure 5).

Rather than adding bandwidth to handle traffic volume increases, MPLS traffic engineering uses existing bandwidth more efficiently by allowing packets to be routed along explicit routes and with specific bandwidth guarantees.

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