igher layer protocols, such as Transmission Control Protocol (TCP), which are able to detect packet congestion and loss and respond by throttling the rate at which data are sent across the network. Network Core reservation. The response messages back toward the head end then releases any reserved resources in upstream nodes. The head end then updates its routing database based on what it has learnt and tries to find another path. Not only do these mechanisms support failure scenarios but also they work in response to chang- ing traffic demands; for example, a new app going viral causing a temporary shift in traffic demands. In the figures, the links between core nodes have been shown as a single line. In practical appli- cations, a link is composed of many parallel optical interfaces that are logically bundled together into something called a link aggregation group (LAG). Even with the deployment of 100G inter- faces between the core nodes, LAGs are still required as the capacity demand on the largest links is rapidly approaching 1 terabit/s. The core routers on either side of the LAG distribute traffic across the member links using a hashing algorithm that ensures the same flow* always hashes to the same member link but provides good distribution of flows between member links. A control protocol ensures that the insertion/removal of member links is coordinated from both sides of the interface. Before bandwidth-aware routing was available, an entire LAG was brought down even when only one or a small number of member links within it failed. This was necessary because the link would not have sufficient capacity in its diminished state to carry offered traffic resulting in congestion and packet loss. With bandwidth-aware routing the remaining member links are kept in service and the remaining capacity is effectively used to carry traffic. TE has had an extremely positive impact on AT&T's network. It has dramatically improved overall customer experience, improving the utilization of the network while maintaining and 