Traffic Engineering

The task of mapping traffic flows onto an existing physical topology is called traffic engineering. Traffic engineering provides the capability to move traffic flow away from the shortest path selected by the interior gateway protocol (IGP) and onto a potentially less congested physical path across a network.

Traffic engineering provides the capabilities to do the following:

• Route primary paths around known bottlenecks or points of congestion in the network.

• Provide precise control over how traffic is rerouted when the primary path is faced with single or multiple failures.

• Provide more efficient use of available aggregate bandwidth and long-haul fiber by ensuring that subsets of the network do not become overutilized while other subsets of the network along potential alternate paths are underutilized .

• Maximize operational efficiency.

• Enhance the traffic-oriented performance characteristics of the network by minimizing packet loss, minimizing prolonged periods of congestion, and maximizing throughput.

• Enhance statistically bound performance characteristics of the network (such as loss ratio, delay variation, and transfer delay) required to support a multiservices Internet.

Traffic Engineering Components

In the JUNOS software, traffic engineering is implemented with Multiprotocol Label Switching (MPLS) and the Resource Reservation Protocol (RSVP). Traffic engineering consists of four functional components: the packet forwarding, information distribution, path selection, and signaling.

The packet forwarding component of the JUNOS traffic engineering architecture is MPLS, which is responsible for directing a flow of IP packets along a predetermined path across a network. This path is called a label-switched path (LSP). LSPs are simplex; that is, the traffic flows in one direction from the head-end (ingress) router to a tail-end (egress) router. Duplex traffic requires two LSPs: one LSP to carry traffic in each direction. An LSP is created by the concatenation of one or more label-switched hops, allowing a packet to be forwarded from one router to another across the MPLS domain.

When an ingress router receives an IP packet, it adds an MPLS header to the packet and forwards it to the next router in the LSP. The labeled packet is forwarded along the LSP by each router until it reaches the egress of the LSP, at which point the MPLS header is removed and the packet is forwarded based on Layer 3 information such as the IP destination address. The key purpose in this scheme is that the physical path of the LSP not be limited to what the IGP would choose as the shortest path to reach the destination IP address.

The packet forwarding process at each router is based on the concept of label swapping. This concept is similar to what occurs at each ATM switch in a PVC. Each MPLS packet carries a 4-byte encapsulation header that contains a 20-bit, fixed-length label field. When a packet containing a label arrives at a router, the router examines the label and uses it as an index into its MPLS forwarding table. Each entry in the forwarding table contains an interface-inbound label pair mapped to a set of forwarding information that is applied to all packets arriving on the specific interface with the same inbound label.

Traffic engineering consists of detailed knowledge about the network topology as well as dynamic information about network loading. The information distribution component is implemented by defining relatively simple extensions to the IGPs so that link attributes are included as part of each router's link-state advertisement. IS-IS extensions include the definition of new type-length values (TLVs), whereas OSPF extensions are implemented with opaque link-state advertisements (LSAs). The standard flooding algorithm used by the link-state IGPs ensures that link attributes are distributed to all routers in the routing domain. Some of the traffic engineering extensions to be added to the IGP LSA include maximum link bandwidth, maximum reserved link bandwidth, current bandwidth reservation, and link coloring.

Each router maintains network link attributes and topology information in a specialized traffic engineering database (TED). The TED is used exclusively for calculating explicit paths for the placement of LSPs across the physical topology. A separate database is maintained so that the subsequent traffic engineering computation is independent of the IGP and the IGP's link-state database. Meanwhile, the IGP continues its operation without modification, performing the traditional shortest-path calculation based on information contained in the router's link-state database.

After network link attributes and topology information are flooded by the IGP and placed in the TED, each ingress router uses the TED to calculate the paths for its own set of LSPs across the routing domain. The path for each LSP can be represented by either a strict or loose explicit route. An explicit route is a preconfigured sequence of routers that should be part of the physical path of the LSP. If the ingress router specifies all the routers in the LSP, the LSP is said to be identified by a strict explicit route. If the ingress router specifies only some of the routers in the LSP, the LSP is described as a loose explicit route. Support for strict and loose explicit routes allows the path selection process to be given broad latitude whenever possible, but to be constrained when necessary.

The ingress router determines the physical path for each LSP by applying a Constrained Shortest-Path First (CSPF) algorithm to the information in the TED. CSPF is a shortest-path-first algorithm that has been modified to take into account specific restrictions when calculating the shortest path across the network. Input into the CSPF algorithm includes the following:

• Topology link-state information learned from the IGP and maintained in the TED.

• Attributes associated with the state of network resources (such as total link bandwidth, reserved link bandwidth, available link bandwidth, and link color ) that are carried by IGP extensions and stored in the TED.

• Administrative attributes required to support traffic traversing the proposed LSP (such as bandwidth requirements, maximum hop count, and administrative policy requirements) that are obtained from user configuration.

As CSPF considers each candidate node and link for a new LSP, it either accepts or rejects a specific path component based on resource availability or whether selecting the component violates user policy constraints. The output of the CSPF calculation is an explicit route consisting of a sequence of router addresses that provides the shortest path through the network that meets the constraints. This explicit route is then passed to the signaling component, which establishes forwarding state in the routers along the LSP.

An LSP is not known to be workable until it is actually established by the signaling component. The signaling component, which is responsible for establishing LSP state and distributing labels, relies on a number of extensions to the RSVP:

• Explicit Route Object ”Allows an RSVP path message to traverse an explicit sequence of routers that is independent of conventional shortest-path IP routing. The explicit route can be either strict or loose.

• Label Request Object ”Permits the RSVP path message to request that intermediate routers provide a label binding for the LSP that it is establishing.

• Label Object ”Allows RSVP to support the distribution of labels without having to change its existing mechanisms. Because the RSVP resv message follows the reverse path of the RSVP path message, the label object supports the distribution of labels from downstream nodes to upstream nodes.