Multipath Traffic Engineering

Shortest path routing offers an easy-to-understand, easy-to-implement method of establishing loop-free connectivity in a network, but offers few other features. Equal-cost multipath (ECMP), a simple extension, uses multiple equal-cost paths between any two points in a network: at any node in a path (really, Directed Acyclic Graph), traffic can be (typically equally) load-balanced among the next hops. ECMP is easy to add on to shortest path routing, and offers a few more features, such as resiliency and load distribution, but the feature set is still quite limited. Traffic Engineering (TE), on the other hand, offers a very rich toolkit for managing traffic flows and the paths they take in a network. A TE network can have link attributes such as bandwidth, colors, risk groups and alternate metrics. A TE path can use these attributes to include or avoid certain links, increase path diversity, manage bandwidth reservations, improve service experience, and offer protection paths. However, TE typically doesn't offer multipathing as the tunnels used to implement TE usually take a single path. This memo proposes multipath traffic-engineering (MPTE), combining the best of ECMP and TE. The multipathing proposed here need not be strictly equal-cost, nor the load balancing equally weighted to each next hop. Moreover, the desired destination may be reachable via multiple egresses. The proposal includes a protocol for signaling MPTE paths using various types of tunnels, some of which are better suited to multipathing.