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October 1999

MPLS And Quality Of Service In Next-Generation Networks


There is quiet transformation occurring within the telecommunications network infrastructure. At the moment, equipment manufacturers are focusing on it, but the transformation is not really visible to users yet. Soon, however, it will be apparent to all of the end users, businesses, and organizations that use the network.

This transformation is occurring as the network moves towards a packet- and cell-based infrastructure. Whether the core backbone of the next-generation network will be IP based or ATM based is still undecided, but there is one common thread that has gained a great deal of momentum over the past year or so. Multi-Protocol Label Switching (MPLS) will play an important role in the routing, switching, and forwarding of packets through the next-generation network.

Packets and cells introduce uncertainty into what, until recently, had been a fairly deterministic network. Delay, jitter, and potential information loss now become serious issues that must be addressed to ensure that the appropriate quality of service is available for a wide range of network users. This situation will become increasingly troublesome as the number of users and volume of traffic continues to increase by a few hundred percent per year.

Understanding Traffic Patterns
Recent studies on usage and traffic patterns within the Internet have observed that these patterns exhibit characteristics of fractals. This means that traffic will look the same whether traffic is observed at a macroscopic level or a microscopic level (the scale of the axis simply changes). Variations in traffic patterns arise because of varying user patterns and varying traffic itself (since traffic can be in the form of short e-mail messages, continuous video streams, long file transfers as part of Web browsing, or voice). Understanding these patterns and building an infrastructure that can provide the high bandwidth, low delay, low jitter, and scalability in the face of these patterns will be critical to ensuring the success of existing and new services that will be offered over the next-generation network infrastructure.

Building Capacity
Nevertheless, the evolution of network models to support high throughput, low delay, low jitter, and scalability is a work in progress. The simplest way to ensure a high Quality of Service (QoS) is to engineer the network so that it has sufficient capacity in the form of processors, buffers, and high-speed links to process, store, and move packets through the network. If the capacity is sufficiently large, then, by implication, information will move quickly and with minimal delay and jitter through the network.

The downside of this network model may be that it is inefficient and potentially expensive to build. Some recently deployed networks are initially relying on their excess capacity to provide fairly high levels of QoS, but as their traffic increases, they will have to depend on additional mechanisms to maintain the QoS. These mechanisms fall into the realm of “traffic engineering.”

Traffic Engineering
The key concept in traffic engineering is that the traffic and information flowing between applications can be differentiated (for example, voice, video, e-mail, and Web browsing) and moved through the network with different levels of service. Traffic engineering uses either reservation-based or reservationless mechanisms.

  • Reservation-based mechanisms: These mechanisms assume that there is a certain amount of capacity available for each type of service. Capacity is reserved on an “as needed” basis from processors, buffers, and links.
  • Reservationless mechanisms: These mechanisms do not reserve capacity; instead, they assign different priorities to traffic and information flows. As capacity is used up, higher priority traffic and flows are maintained, and the service provided to the lower priority flows is degraded.

In both cases, if the network cannot provide the required level of service, then it may use mechanisms to prevent additional traffic from entering the network so that the already existing traffic and flows are not impacted.

Supporting QoS At The Routing Level
The “nuts and bolts” of supporting the different QoS mechanisms occurs at the level of the packet routing, switching, and forwarding mechanisms. There are two fundamentally different ways to route packets between a source and a destination: “hop-by-hop” routing and “source” routing. Each approach has advantages and disadvantages.

  • Hop-by-hop routing: The routers perform significant processing as packets are received, examined, and forwarded on to the next “hop” towards the destination. Unfortunately, as the traffic increases, these routers can become bottlenecks, and it becomes a challenge to scale to large networks.
  • Source routing: This alternative to hop-by-hop routing presents other challenges. It introduces additional complexity and setup delay because it requires that a connection be established between the source and destination.

There are several possible solutions that address the bandwidth, latency, and scalability issues mentioned earlier. These solutions include: layer 3 switching, layer 2 switching, cut-through layer 2 switching, and cut-through layer 3 switching. The differences among these approaches result from 1) whether the routing and forwarding occur at the data link level or the packet level and 2) whether a flow is identified locally or end-to-end.

The various approaches address the challenges from different angles, and until recently there has been no clearly discernable winner. But lately, momentum has been building rapidly among network operators and equipment manufacturers to use MPLS as the mechanism of choice to manage traffic flows in IP-, ATM-, and frame relay-based networks. MPLS combines various aspects of the different approaches to successfully provide efficient and scalable routing of packets and cells through what will be the next-generation network infrastructure.

What is MPLS? It’s an Internet Engineering Task Force (IETF) specified protocol that provides for the efficient designation, routing, forwarding, and switching of traffic flows through the network. MPLS can manage traffic flows of various granularities, such as flows between different hardware or even flows between different applications.

MPLS is independent of the layer 2 and layer 3 protocols. It provides a means to map IP addresses to simple, fixed-length labels used by different packet-forwarding and packet-switching technologies. MPLS can interface into existing routing protocols such as RSVP and OSPF, and it can support the IP, ATM, and frame relay layer 2 protocols.

The layer 3 protocol forwards the first few packets of a flow. As the flow is identified and classified (based on various QoS requirements), a series of layer 2 high-speed switching paths are set up between the routers along the path between the source and destination of the flow. The layer 2 switching paths are established by assigning labels to each link connecting the routers.

Associating these labels within each router and binding these labels to each other across the entire path of the flow is performed by a simple signaling protocol. The label assignment can be topology driven (for example, between source and destination devices), flow driven (say, via RSVP), or control driven (policy based, perhaps). High-speed switching is possible because the fixed-length labels (also known as tags) are inserted at the very beginning of the packet or cell and can be used by hardware to quickly switch packets between links.

MPLS allows what is known as “ships in the night operation.” That is, MPLS can be introduced into a network without impacting the existing operation of other routing, switching, and forwarding protocols within the network. This will allow for the gradual deployment of MPLS without having to replace the network infrastructure all at once.

Activity is underway to monitor and control MPLS networks by integrating some of the MPLS signaling with SS7 signaling so that network operators can manage the flows associated with their voice and data traffic more effectively, use their core and access resources more efficiently, and provide integrated management of different network infrastructures.

Future applications and services must be aligned with developments in the underlying network infrastructure to take full advantage of its capabilities. In the future, MPLS and other associated protocols will define, to some extent, the network’s capabilities. Understanding these capabilities will ensure that, as services are developed and deployed, they will behave as expected and not present any unwelcome surprises to users and service providers. In the future, applications running on the network will be able to identify the types of flows that are generated and to take maximum advantage of the network’s ability to manage its resources efficiently and cost-effectively.

Jeff Lawrence is president and CEO of Trillium Digital Systems, a leading provider of communications software solutions for computer and communications equipment manufacturers. Trillium develops, licenses, and supports standards-based communications software solutions for SS7, ATM, ISDN, frame relay, V5, IP, and X.25/X.75 technologies. For more information, visit the company’s Web site at

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