Mailbox switch: a scalable two-stage switch architecture for conflict resolution of ordered packets

Traditionally, conflict resolution in an input-buffered switch is solved by finding a matching between inputs and outputs per time slot. To do this, a switch not only needs to gather the information of the virtual output queues at the inputs, hut also uses the gathered information to compute a matching. As such, both the communication overhead and the computation overhead make it difficult to scale. Recent works on the two-stage switch architecture in (6|, [7], [12], (8| showed that conflict resolution can be easily solved over time and space without communication and computation overhead. However, the main problem of such a two-stage switch architecture is that packets might be out of sequence. The main objective of this paper is to propose a scalable solution, called the mailbox switch, that solves the out-of-sequence problem in the two-stage switch architecture. The key idea of the mailbox switch is to use a set of symmetric connection patterns to create a feedback path for packet departure times. With the information of packet departure times, the mailbox switch can schedule packets so that they depart in the order of their arrivals. Despite the simplicity of the mailbox switch, we show via both the theoretical models and simulations that the throughput of the mailbox switch can be as high as 75%. With limited resequencing delay, a modified version of the mailbox switch achieves 95% throughput. We also propose a recursive way to construct the switch fabrics for the set of symmetric connection patterns. If the number of inputs, N, is a power of 2, we show that the switch fabric for the mailbox switch can be built with N/2 log₂ N 2times2 switches