Linux is everywhere, including optical networks!

These past two summers, I've had the honor of working in the open-access COSMOS wireless testbed, where researchers can remotely access radio and networking hardware in NYC and beyond! One of my assignments this year was to troubleshoot an unresponsive Calient S320 Optical Switch.

Diagnosing the Issue

As it would turn out, the embedded controller system ran an embedded Linux distro, with U-Boot to boot!

We found this out after plugging in a serial adapter from the manufacturer, and using PuTTY to interact with the serial console. From the console, we found out that the FPGA image used by the switch could not be found, and that the kernel image disappeared.

Recovery

We ran TFTP and NFS^[1] servers on a laptop connected to the same network as the optical switch, which provided system files we got from a manufacturer tarball.

TFTP

The Trivial File Transfer Protocol (TFTP), by virtue of its simplicity, is used in the bootloader stages for loading the FPGA image. It uses UDP port 69 to provide read/write access to files on the server.

On our TFTP server, we loaded the FPGA image so U-Boot could access it.

NFS

The Network File System (NFS) is used to share access to a server computer's files (read access, write access, along with other metadata) across multiple client computers. It is supported by the Linux kernel as one of its many mountable filesystems, so by running an NFS share off of an ext4 partition, we can run the optical switch's OS^[2] from the laptop's disk!

Execution

Using the serial connection, we could then configure the U-Boot environment variables to indicate that the path of the FPGA image to download over TFTP, and that the OS should be booted off of NFS.

Thanks to this setup, we were able to bring the optical switch back online, with the system working just like new!