Allow implementations to use their own default for OTLP compression.

CHANGELOG.md

@@ -62,6 +62,8 @@ release.
 ### OpenTelemetry Protocol
 
 - Add environment variables for configuring the OTLP exporter protocol (`grpc`, `http/protobuf`, `http/json`) ([#1880](https://github.com/open-telemetry/opentelemetry-specification/pull/1880))
+- Allow implementations to use their own default for OTLP compression, with `none` denotating no compression
+  ([#1923](https://github.com/open-telemetry/opentelemetry-specification/pull/1923))
 
 ### SDK Configuration
 

specification/protocol/exporter.md

@@ -15,14 +15,18 @@ The following configuration options MUST be available to configure the OTLP expo
 | Insecure             | Whether to enable client transport security for the exporter's gRPC connection. This option only applies to OTLP/gRPC - OTLP/HTTP always uses the scheme provided for the `endpoint`. Implementations MAY choose to not implement the `insecure` option if it is not required or supported by the underlying gRPC client implementation. | `false` | `OTEL_EXPORTER_OTLP_INSECURE` `OTEL_EXPORTER_OTLP_SPAN_INSECURE` `OTEL_EXPORTER_OTLP_METRIC_INSECURE` |
 | Certificate File     | The trusted certificate to use when verifying a server's TLS credentials. Should only be used for a secure connection. | n/a               | `OTEL_EXPORTER_OTLP_CERTIFICATE` `OTEL_EXPORTER_OTLP_TRACES_CERTIFICATE` `OTEL_EXPORTER_OTLP_METRICS_CERTIFICATE` |
 | Headers              | Key-value pairs to be used as headers associated with gRPC or HTTP requests. See [Specifying headers](./exporter.md#specifying-headers-via-environment-variables) for more details.                   | n/a               | `OTEL_EXPORTER_OTLP_HEADERS` `OTEL_EXPORTER_OTLP_TRACES_HEADERS` `OTEL_EXPORTER_OTLP_METRICS_HEADERS` |
-| Compression          | Compression key for supported compression types. Supported compression: `gzip`| No value              | `OTEL_EXPORTER_OTLP_COMPRESSION` `OTEL_EXPORTER_OTLP_TRACES_COMPRESSION` `OTEL_EXPORTER_OTLP_METRICS_COMPRESSION` |
+| Compression          | Compression key for supported compression types. Supported compression: `gzip`| No value [1]          | `OTEL_EXPORTER_OTLP_COMPRESSION` `OTEL_EXPORTER_OTLP_TRACES_COMPRESSION` `OTEL_EXPORTER_OTLP_METRICS_COMPRESSION` |
 | Timeout              | Maximum time the OTLP exporter will wait for each batch export. | 10s               | `OTEL_EXPORTER_OTLP_TIMEOUT` `OTEL_EXPORTER_OTLP_TRACES_TIMEOUT` `OTEL_EXPORTER_OTLP_METRICS_TIMEOUT` |
 | Protocol             | The transport protocol. Options MAY include `grpc`, `http/protobuf`, and `http/json`. See [Specify Protocol](./exporter.md#specify-protocol) for more details. | n/a               | `OTEL_EXPORTER_OTLP_PROTOCOL` `OTEL_EXPORTER_OTLP_TRACES_PROTOCOL` `OTEL_EXPORTER_OTLP_METRICS_PROTOCOL` |
 
+**[1]**: If no compression value is explicitly specified, SIGs can default to the value they deem
+most useful among the supported options. This is especially important in the presence of technical constraints,
+e.g. directly sending telemetry data from mobile devices to backend servers.
+
 Supported values for `OTEL_EXPORTER_OTLP_*COMPRESSION` options:
 
-- If the value is missing, then compression is disabled.
-- `gzip` is the only specified compression method for now. Other options MAY be supported by language SDKs and should be documented for each particular language.
+- `none` if compression is disabled.
+- `gzip` is the only specified compression method for now.
 
 Example 1