istio.io/content/en/about/faq/distributed-tracing/how-envoy-based-tracing-works.md

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How does Envoy-based tracing work?	11

For Envoy-based tracing integrations, Envoy (the sidecar proxy) sends tracing information directly to tracing backends on behalf of the applications being proxied.

Envoy:

• generates request IDs and trace headers (i.e., X-B3-TraceId) for requests as they flow through the proxy
• generates trace spans for each request based on request and response metadata (i.e., response time)
• sends the generated trace spans to the tracing backends
• forwards the trace headers to the proxied application

Istio supports the Envoy-based integrations of Lightstep and Zipkin, as well as all Zipkin API-compatible backends, including Jaeger.