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+A65: mTLS Credentials in xDS Bootstrap File
+----
+* Author(s): @markdroth
+* Approver: @ejona86
+* Status: {Draft, In Review, Ready for Implementation, Implemented}
+* Implemented in: <language, ...>
+* Last updated: 2023-05-24
+* Discussion at: https://groups.google.com/g/grpc-io/c/h_LQnTimyt4
+
+## Abstract
+
+This proposal adds support for configuring the use of mTLS for
+communicating with the xDS server via the xDS bootstrap file.
+
+## Background
+
+[gRFC A27][A27] defines the xDS bootstrap file format for setting
+the channel creds used to communicate with the xDS server, with the
+initial options of `google_default` and `insecure`.  It suggested that a
+general-purpose mechanism could be added to configure arbitrary channel
+creds types, which has subsequently been done, albeit not intended as
+a public API.
+
+[gRFC A29][A29] describes the certificate provider framework used for
+configuring mTLS for data plane communication in xDS.  It also defines
+the file-watcher certificate provider.
+
+This proposal defines a new channel creds type to be used in the
+bootstrap file for using mTLS with the xDS server, leveraging the
+functionality of the file-watcher certificate provider.
+
+### Related Proposals: 
+* [gRFC A27: xDS-Based Global Load Balancing][A27]
+* [gRFC A29: xDS-Based Security for gRPC Clients and Servers][A29]
+
+## Proposal
+
+We will define a new credential type in the bootstrap file called `tls`.
+Its configuration will be essentially the same as that of the file-watcher
+certificate provider described in [gRFC A29][A29].  Specifically, the
+config will look like this:
+
+```json
+{
+  // Path to CA certificate file.
+  // If unset, system-wide root certs are used.
+  "ca_certificate_file": <string>,
+
+  // Paths to identity certificate file and private key file.
+  // If either of these fields are set, both must be set.
+  // If set, mTLS will be used; if unset, normal TLS will be used.
+  "certificate_file": <string>,
+  "private_key_file": <string>,
+
+  // How often to re-read the certificate files.
+  // Value is the JSON format described for a google.protobuf.Duration
+  // message in https://protobuf.dev/programming-guides/proto3/#json.
+  // If unset, defaults to "600s".
+  "refresh_interval": <string>
+}
+```
+
+The only difference between the file-watcher certificate provider config
+and this one is that in the file-watcher certificate provider, at least
+one of the "certificate_file" or "ca_certificate_file" fields must be
+specified, whereas in this configuration, it is acceptable to specify
+neither one.
+
+Implementations should be able to internally configure the use of the
+file-watcher certificate provider for the certificate-reloading
+functionality.
+
+### Temporary environment variable protection
+
+This feature is not enabled via remote I/O, and we don't have a good way
+to interop test it, so we will not use env var protection for this feature.
+Unit tests in individual languages should be sufficient to verify the
+functionality.
+
+## Rationale
+
+We considered phrasing the credential config in a way that exposes the
+certificate provider framework directly, since that would have allowed it
+to automatically support any new cert provider implementations we may
+add in the future.  However, the plumbing for that turned out to be a
+bit challenging, so we decided to only directly expose the file-watcher
+certificate provider mechanism.  If we add other certificate providers
+in the future, we can consider adding fields to expose them in this
+configuration.
+
+## Implementation
+
+Implemented in C-core in https://github.com/grpc/grpc/pull/33234.
+
+## Open issues (if applicable)
+
+N/A
+
+[A27]: A27-xds-global-load-balancing.md
+[A29]: A29-xds-tls-security.md