Guest Writer: Roy Kruger

As companies and telecommunications operators begin to realise that the Internet Protocol (IP) as a standalone protocol does not offer the quality of service required for many services which can be offered on telephony networks, it becomes necessary to implement solutions which can offer efficiencies and the quality required by users of such networks. Examples are Voice over IP (VoIP) and IPTV which require defined latencies over specific networks to be able to function properly and to be able to deliver high quality services. MPLS can meet these requirements.

MPLS is one of the fastest growing protocols to be implemented in services based networks and as such BCX finds itself in the forefront of MPLS designed and implemented networking solutions throughout Africa.

Multiprotocol label switching enables the speeding up of packet forwarding in Internet protocol (IP) networks. As IP is a connectionless based protocol and provides no guarantee of Quality of Service, MPLS allows IP networks to become more like a connection-oriented network where the path between the source and the destination is calculated on a set of metrics prior to transmission of the packets. To speed up the forwarding scheme, an MPLS device uses labels rather than address matching to determine the next hop for a received packet. To provide traffic engineering, tables are used that represent the levels of quality of service (QoS) that the network can support.

The tables and the labels are used together to establish an end-to-end path called a label switched path (LSP). Traditional IP routing protocols (for example, open shortest path first (OSPF) and intermediate system to intermediate system (IS–IS) and extensions to existing signalling protocols (for example, resource reservation protocol (RSVP) and constraint-based routing–label distribution protocol (CR–LDP) comprise the suite of MPLS protocols.

MPLS extends the suite of IP protocols to expedite the forwarding scheme used by IP routers. Routers, to date, have used complex and time-consuming route lookups and address matching schemes to determine the next hop for a received packet, primarily by examining the destination address in the header of the packet. MPLS has greatly simplified this operation by basing the forwarding decision on a simple label. Another major feature of MPLS is its ability to place IP traffic on a defined path through the network. This capability was not previously possible with IP traffic. In this way, MPLS provides bandwidth guarantees and other differentiated service features for a specific user application (or flow).

Current IP–based MPLS networks are capable of providing advanced services such as bandwidth-based guaranteed service, priority-based bandwidth allocation, and pre-emption services. For each specific service a table of forwarding equivalence class (FEC) is created to represent a group of flows with the same traffic-engineering requirements. A specific label is then bound to an FEC. At the ingress of an MPLS network, incoming IP packets are examined and assigned a "label" by a label edge router (LER). The labelled packets are then forwarded along a label switched path (LSP), where each label-switched router (LSR) makes a switching decision based on the packet's label field. An LSR does not need to examine the IP headers of the packets to find an output port (next hop). An LSR simply strips off the existing label and applies a new label for the next hop. The label information base (LIB) provides an outgoing label (to be inserted into the packet) and an outgoing interface (based on an incoming label on an incoming interface).

Signalling to establish a traffic-engineered LSP is done using a label distribution protocol that runs on every MPLS node. There are a number of different label-distribution protocols. The two most popular RSVP–traffic engineering (RSVP–TE) and CR –LDP. RSVP–TE is an extended version of the original RSVP to piggyback and distribute labels on its messages and to provide traffic-engineering capability. CR–LDP was designed specifically for this purpose.

The emergence of optical transport systems has dramatically increased the raw capacity of optical networks and has enabled a slew of new, sophisticated applications. For example, network-based storage, bandwidth leasing, data mirroring, add/drop multiplexing (ADM), dense wavelength division multiplexing (DWDM), optical cross-connect (OXC), photonic cross-connect (PXC), and multiservice switching platforms are some of the devices that may make up an optical network and are expected to be the main carriers for the growth in data traffic.

The diversity and complexity in managing these devices have been the main driving factors in the evolution and enhancement of the MPLS suite of protocols to provide control for not only packet-based domains, but also time, wavelength, and space domains.

The MPLS framework includes extensions to existing IP link-state routing protocols. These protocols provide real-time coordination of the current network topology, including attributes of each link. MPLS extensions to OSPF and IS–IS allow nodes to not only exchange information about the network topology, but also resource information and even policy information—for example, IP addresses, available bandwidth, and load-balancing policies. Constraint-based routing algorithms use this information to compute the optimal paths for the LSPs through the network and allow complex traffic-engineering decisions to be made automatically when selecting routes through the network.

MPLS Evolution to GMPLS
Within the past two years, the International Engineering Task Force (IETF) has extended the MPLS suite of protocols to include devices that switch in time, wavelength, (e.g., DWDM) and space domains via GMPLS. This allows GMPLS–based networks to find and provision an optimal path based on user traffic requirements for a flow that potentially starts on an IP network, is then transported by SDH/SONET, and then is switched through a specific wavelength on a specific physical fibre.

The basic challenge for an all-encompassing control protocol is the establishment, maintenance, and management of traffic-engineered paths to allow the data plane to efficiently transport user data from the source to the destination. A user flow starting from its source is likely to travel several network spans–for example, an access or edge network that aggregates the flows from multiple users (for example., enterprise applications) to feed into a metro network that is SDH/SONET–based or ATM–based that itself aggregates multiple flows from various edge networks to feed into a long-haul network that uses lambdas to transport the aggregated flow of multiple metro networks. The reverse path is used to deliver data to its destination.

G-MPLS is a logical evolutionary advance from IP through MPLS. With support from the Internet Engineering Task Force (IETF) and the Optical Internetworking Forum (OIF), it is fast becoming an industry standard.

Kruger is Managing Director, BCX Networks Ltd., Nigeria, a subsidiary of Business Connexion (Pty) Ltd., South Africa. He can be reached at: roykruger_2@yahoo.com