istio

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apiVersion: release-notes/v2
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kind: promotion
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area: traffic-management
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issue:
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  - 47949
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releaseNotes:
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  - |
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    **Enabled** [Delta xDS](https://www.envoyproxy.io/docs/envoy/latest/api-docs/xds_protocol#incremental-xds) by default. See upgrade notes for more information.
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upgradeNotes:
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  - title: Delta xDS on by default
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    content: |
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      In previous versions, Istio used the "State of the world" xDS protocol to configure Envoy.
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      In this release, the ["Delta"](https://www.envoyproxy.io/docs/envoy/latest/api-docs/xds_protocol#incremental-xds) xDS protocol is enabled by default.
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      This should be an internal implementation detail, but because this controls the core configuration protocol in Istio,
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      an upgrade notice is present in an abundance of caution.
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      The expected impacts of this change is improved performance of configuration distribution.
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      This may result in reduced CPU and memory utilization in Istiod and proxies, as well as less network traffic between the two.
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      Note that while this release changes the *protocol* to be incremental, Istio does not yet send perfect minimal incremental updates.
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      However, there are already optimizations in place for a variety of critical code paths, and this change enables us to continue optimizations.
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      If you experience unexpected impacts of this change, please set the `ISTIO_DELTA_XDS=false` environment variable in proxies
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      and file a GitHub issue.
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