Pipeline Forwarding - Operating Principles

Operating Principles

As in other pipelining implementations, a common time reference (CTR) is needed to perform pipeline forwarding. In the context of global networks the CTR can be effectively realized by using UTC (coordinated universal time) that is globally available via GPS (global positioning system) or Galileo in the near future. For example the UTC second is divided into fixed duration time frames, which are grouped into time cycles so that in each UTC second there is a predefined integer number of time cycles. Alternatively, or complementary, the CTR can be obtained through the network by means of synchronization protocols such as IEEE 1588.

Packets are forwarded from node to node according to predefined schedules, as shown in the figure below, i.e., each node forwards packets of a certain flow during predefined time frames. The time cycles define the periodic re-occurrence of the various predefined schedules. The periodic scheduling within each node results in a periodic packet forwarding across the network, which is referred to as pipeline forwarding for the ordered, step-by-step fashion with which packets travel toward their destination.

Through a resource reservation procedure transmission capacity is booked for a flow on each link it traverses during the time frame (or time frames) predefined for its forwarding, thus setting up a synchronous virtual pipe (SVP). The capacity during each time frame can be is partially or totally reserved to one or more flows. Consequently, the time cycle provides the basis for a periodic repetition of the reservation that ensures enough transmission resources to be available on each link to forward the packets of each flow, which prevents delays due to resource contention and loss resulting to congestion.