# A Design for an Internet Router with a Digital Optical Data Plane
## 1 Introduction
### Our Assumptions  
We focus on the Internet router as the device that shares capacity between interconnected optical links. As a result, we assume optical links that do not include dimensions that introduce their own, distinct sharing mechanism--i.e., they are individual, directional channels with no further partitioning by wavelength, phase, polarization, etc. As with electronic Internet channels, we assume that these dimensions may be used to encode information, e.g., symbols that represent multiple bits, but that they are not otherwise used to partition packets within a link.  
Similarly, we assume point-to-point (i.e., unicast) links. Shared links require a media sharing arbitration mechanism, often known as a Medium Access Control (MAC) protocol; this arbitration is more efficient when coordinated explicitly by the multiplexing mechanism inside a packet switch, which is why Ethernet evolved from shared, MAC-controlled links to dedicated unicast links between packet switches.  
We also avoid assumptions on the size of packets, other than that they conform to Internet requirements. IPv4 packets (also called 'datagrams') are a minimum of 20 bytes, but typically exhibit a bimodal distribution of 40 and around 1500 bytes [9]. IPv6 packets (these too are called 'datagrams') are a minimum of 40 bytes and typically exhibit a bimodal distribution at 60 and around 1500 bytes [10]. Packets on a link are assumed to have bytes in the order specified in the IPv4 or IPv6 formats--header bytes arrive in the order that they are defined in the IP standards. We also do not assume adjacent (or nearly-adjacent) packet sequences with the same destination, i.e., we do not assume typical flow or burst sequences of packets. We want a solution that works in the absence of such flow properties, without advance configuration.  
This paper focuses on whether implementing an Internet router as an all-optical device is feasible by focusing on its four defining capabilities--hop count decrement, header lookup (for packet demultiplexing by Internet routing rules), checksum update, and packet traffic merging (multiplexing  
Figure 1: Evolution of electronic channel sharing (**top**) and optical channel sharing (**bottom**).  
and output port contention). This approach is intended to support rates far beyond those possible with electronics, but is not expected to reduce either power or cost. Although we do not assume the need for new materials, our approach requires integration of active and passive optical devices on a single substrate, as noted in Section 8.  
Finally, we assume an optical encoding compatible with optical processing because conversion between transmission and processing encodings is as complex and costly when both are optical as when converting from optical to electrical, and we want to avoid that cost and complexity. Our preliminary implementations utilize binary encodings (e.g., on-off keying, return-to-zero (RZ) encoding, or binary phase-shift keying (BPSK)); in Section 7, we address extending these designs to higher-density, multiple bit symbol encodings (e.g., M-PSK,, where \"M\" refers to an integer, as in \"8-PSK\" or \"16-PSK\") for more efficient long-distance transmission.