Gb Switched Router

This paper gives a general overview of modern techniques used to improve router performance. In retrospect, it would have been much better to read this paper before reading the other paper. Too late now. This paper covers the operation and need for switched backplanes, as well as the iSLIP and ESLIP protocols.

The general idea behind routers is that there are various linecards which take incoming packets and forward them to a decision maker, and are also able to transmit outgoing packets. The key issue is the decision maker. In early routers, it was a single CPU. As this CPU became the bottleneck in routers, it was replaced with an individual decision maker for each linecard, with a bus connecting the various CPUs (so that packets can move from one linecard to another). Then, since the bus became the bottleneck, the need for switched backplanes arose.

A switched backplane is essentially a grid that allows any linecard to connect to any other linecard. The only caveat is that a linecard can only connect to a single other linecard at a time. Therefore, there needs to be a scheduler to open and close connections on the grid. The algorithm used to determine which connections to open/close is called iSLIP. It works by allowing the sources to request destinations, letting the destinations choose a source in round-robin fashion, and then letting the sources choose a destination in round-robin fashion. The sources have multiple requests because of virtual output queuing (VOQ). In VOQ, each source has multiple queues so that no packet is stuck behind other packets which cannot reach their destinations.

A few other improvements are possible, such as the use of ESLIP to accomodate for multicast messages, and the use of priority queues to more tightly control delay.

This paper introduces a classification system for blocking in the backplane that is quite useful. The first kind is called Head of Line (HOL) blocking. HOL blocking occurs when there is no VOQ, and a packet is unable to reach its destination even though that destination is idle because that packet is second in the queue; the first packet is destined for a destination that is currently busy. While VOQ solves this issue, it does not solve the issue of input blocking our output blocking (where a packet at a given input/output cannot be transmitted because that input/output is currently transmitting a different packet). These issues cannot be solved except in the case of multicast, when they are solved by ESLIP.