Lasers!

Cool! You mean that I actually have frickin' switches with frickin' laser beams attached to their frickin' ports?

Dr. Evil is right, lasers are cool! The draft sFlow Optical Interface Structures specification exports metrics gathered from instrumentation built into Small Form-factor Pluggable (SFP) and Quad Small Form-factor Pluggable (QSFP) optics modules. This article provides some background on optical modules and discusses the value of including optical metrics in the sFlow telemetry stream exported from switches and hosts.

Pluggable optical modules are intelligent devices that do more than simply convert between optical and electrical signals. The functional diagram below shows the elements within a pluggable optical module.

The transmit and receive functions are shown in the upper half of the diagram. Incoming optical signals are received on a fiber and amplified as they are converted to electrical signals that can be handled by the switch. Transmit data drives the modulation of a laser diode which transmits the optical signal down a fiber.

The bottom half of the diagram shows the management components. Power, voltage and temperature sensors are monitored and the results are written into registers in an EEPROM that are accessible via a management interface.

The proposed sFlow extension standardizes the export of the module sensor data so that they can be exported along with switch port interface counters. A standard structure ensures multi-vendor interoperability and including the optical metrics as part of the sFlow export provides a scaleable method of monitoring all the optical modules in the network.

While the measurements from a single module are useful, the value increases when measurements from all modules can be combined at the sFlow collector. For example, matching pairs of modules allows problems with the transmitter, receiver and the cable connecting them to be isolated.

The sFlow counter push mechanism is an extremely efficient method of monitoring at scale and can collect metrics from all the optical modules in the data center. Combining data from all the modules makes it easier to find outliers.

Finally, incorporating optics monitoring as part of the comprehensive sFlow telemetry stream allows optical metrics to be correlated with switch port, traffic flow and application performance metrics. For example, an increase in application response time can be traced to the paths that the traffic takes across the network, packet discard rates on the ports, and signal strength from the optical sensors, to find a marginal fiber link.