# A Design for an Internet Router with a Digital Optical Data Plane
## 1 Introduction
### Some History  
The Internet is the most recent stage of an evolution in communication architecture that began with telephony over 120 years ago. Early telephone network architectures used circuits, a dedicated physical path between endpoints. The need for more flexible channel sharing led to the development of virtual circuits, in which a circuit is emulated by dedicating fractions of resources (frequencies/wavelengths, time slots, or combinations) over multiple paths. Transmissions are multiplexed over these resource components to emulate a channel with a fixed capacity.  
Most communication, whether data exchanges, audio, or even compressed video, are more typically bursty and do not require fixed capacity. In this case, concurrent channels can share each other's spare capacity during idle periods. This is the basis of packet switching, which is a dynamic, adaptive, on-demand variant of time-division multiplexing. Unlike a circuit, a packet network reacts to each message that traverses it, but this was difficult to accomplish at high bandwidths in the early Internet (1970s-1990s).  
In the mid-1990s, the Internet's increasing need for the flexibility and more efficient channel sharing of packet switching led to the development of Tag Switching [3] (also known as Flow Switching [4] and more recently as Multiprotocol Label Switching, MPLS [5]). Groups of packets are relayed through a predetermined set of switches using configurations that are deployed in advance, making decisions only once for each group. These mechanisms allowed networks to leverage some of the efficiency of on-demand resource sharing while reducing the processing needed for each message, but also limited sharing to timescales of the packet bursts.  
The final step in the evolution towards increasingly shorter channel multiplexing timescales culminated in the development of electronic packet switching that was capable of relaying packets at full electronic transmission rates (so-called 'line-rate routers'), which occurred towards the turn of the last century. This native electronic packet switching performs on-demand resource sharing for each packet, and is the basis of electronic Internet routers. This evolution is shown in the top of Figure 1.  
Optics is currently recapitulating this evolution (bottom of Figure 1). It began with dedicated light paths and evolved to virtual light paths using multiplexing wavelengths (WDM), with wavelength conversion to avoid the need for global coordination of wavelength assignment. Optical WDM thus corresponds roughly to electronic virtual circuits. Optical Burst Switching [6] (another variant is known as Terabit Burst Switching [7]) is the optical corollary of Tag Switching, configuring paths for small groups of adjacent packets with the same destination, effectively providing very short timescale circuits, corresponding to electronic tag switching. The corollary of electronic line-rate packet switching is all-optical packet switching, and is the obvious conclusion of this evolution. An optical router assigns resources for each message, which results in much more efficient resource sharing and this sharing efficiency is why we focus on a packet switched approach.  
The alternative to optical packet switching would be to optimize the network to support less dynamic, adaptive, and efficient sharing. This is only possible if data exchange patterns are both known in advance and are stable over long periods. Such patterns currently occur in datacenters and other researchers are developing optical circuit systems for that environment [8]. We prefer to focus on the more general, most flexible solution of optical packet switching, which would support not only these known, long-scale traffic patterns but also the unknown, rapidly shifting patterns prominent elsewhere throughout the Internet.