Software-Assisted Congestion Signaling and Sub-Millisecond Telemetry in SONiC#

Arpitha Raghunandan (Google), Shane Gao (Google)

• Why: The deployable half of the Virgo story: how Google gets microsecond-scale telemetry and per-packet congestion signaling into SONiC — and, crucially, onto older ASICs that don't support it in hardware.
• What: A SONiC architecture proven at Google scale with two components: HST (High-Speed Switch Telemetry) — lightweight digests and high-resolution traces enabling microsecond-scale per-device profiling — and CSIG (Software-Assisted Congestion Signaling), which updates packet signals with real-time switch state to give end hosts high-frequency per-packet telemetry (from abstract only).
• How: Sub-millisecond runtime write-backs of per-port metrics via the HST framework, and reuse of unused match-action table resources in the ASIC pipeline to implement custom signal types the hardware doesn't natively support — enabling CSIG on existing networks with older silicon.
• Where applicable: Google-proven but designed for upstream SONiC; the "software-assisted on older ASICs" angle makes this broadly applicable to brownfield fabrics, not just new AI builds.