Because Istio Telemetry V2 lacks a central component (Mixer) with access to K8s metadata, the proxies themselves require the metadata necessary to provide rich metrics. Additionally, features provided by Mixer had to be added to the Envoy proxies to replace the Mixer-based telemetry. Istio Telemetry V2 uses two custom Envoy plugins to achieve just that.
In-proxy service-level metrics in Telemetry V2 are provided by two custom plugins, metadata-exchange and stats.
By default, in Istio 1.5, Telemetry V2 is enabled as compiled in Istio proxy filters, mainly for performance reasons. The same filters are also compiled to WebAssembly (WASM) modules and shipped with Istio proxy. Performance will be continuously improved in forthcoming releases.
For HTTP-based traffic this is accomplished via custom HTTP headers (envoy.wasm.metadata_exchange.upstream, envoy.wasm.metadata_exchange.downstream) in the request/response that contains the metadata attributes of the other side.
For generic TCP traffic the metadata exchange uses ALPN-based tunneling and a prefix based protocol. A new protocol istio-peer-exchange is defined, which is advertised and prioritized by the client and the server sidecars in the mesh. ALPN negotiation resolves the protocol to istio-peer-exchange for connections between Istio enabled proxies, but not between an Istio enabled proxy and any client.
https://blog.christianposta.com/understanding-istio-telemetry-v2/
