PC Router Research Tool

What is a PC Router Research Tool?

The PC Router is a tool for investigating new routing algorithms. The fundamental idea is to use a dual-processor PC system to facilitate the development of router code. One processor runs unix and the other processor is dedicated to routing. This implementation is based on FreeBSD and uses Intel Pro 100/B network cards. The router processor runs a loop polling a list of devices to see if there is work to be done. There is supporting code in the driver and minor modifications to the boot-up code and the TCP/IP protocol code. The protocols themselves are not changed, only which code runs on which processor.

Design Info - Packet Flows

There are several packet flows to consider. The simplest flow to understand is routed packet: a packet arrives on an interface, its destination is looked up in the routing table, and it is sent out an interface. The minor variations on this are In the case of the locally generated outbound packet, the router processor sends it to the appropriate interface. In the case of the locally destined packet, the router processor queues it to the unix processor and sends an interprocessor interrupt to the unix processor. In the case of the unroutable packet, the router processor also queues it to the unix processor, sends an interprocessor interrupt, and lets the full-fledged routing code in the unix processor deal with the packet.

More Design Info - Buffer Management

At boot up time, each interface has several mbufs assigned to it. Each mbuf is tagged with the id of the interface to which it belongs. Initially an mbuf is set up to receive data on an interface. When it is full, it is queued to other subsystems for further processing and is, probably, enqueued on an interface for transmission. When the transmission is complete, the mbuf is returned to the interface that owned it originally.

What were the hard parts?

Conceptually this tool was straight forward to design. The challenges were

What's left to do?

Questions and comments to Dorothy Curtis dcurtis at csail.mit.edu