Updated Zipkin spec for remoteEndpoint mapping.

specification/trace/sdk_exporters/zipkin.md

@@ -41,7 +41,6 @@ OpenTelemetry fields:
 Zipkin fields:
 
 - Local_endpoint
-- Remote_endpoint
 - debug
 - Shared
 
@@ -88,6 +87,36 @@ OpenTelemetry Span's `InstrumentationLibrary` MUST be reported as `tags` to Zipk
 | `InstrumentationLibrary.name`|`otel.instrumentation_library.name`|
 | `InstrumentationLibrary.version`|`otel.instrumentation_library.version`|
 
+### Remote endpoint
+
+#### OTLP -> Zipkin
+
+If Zipkin `SpanKind` resolves to either `SpanKind.CLIENT` or `SpanKind.PRODUCER`
+then the service SHOULD specify remote endpoint otherwise Zipkin won't treat the
+Span as a dependency. `peer.service` is the preferred attribute but is not
+always available. The following table lists the possible attributes for
+`remoteEndpoint` by preferred ranking:
+
+|Rank|Attribute Name|Reason|
+|---|---|---|
+|1|peer.service|[OpenTelemetry adopted attribute for remote service.](../semantic_conventions/span-general.md#general-remote-service-attributes)|
+|2|net.peer.name|[OpenTelemetry adopted attribute for remote hostname, or similar.](../semantic_conventions/span-general.md#general-network-connection-attributes)|
+|3|net.peer.ip & net.peer.port|[OpenTelemetry adopted attribute for remote address of the peer.](../semantic_conventions/span-general.md#general-network-connection-attributes)|
+|4|peer.hostname|Remote hostname defined in OpenTracing specification.|
+|5|peer.address|Remote address defined in OpenTracing specification.|
+|6|http.host|Commonly used HTTP host header attribute for Http Spans.|
+|7|db.name|Commonly used database name attribute for DB Spans.|
+
+* Ranking should control the selection order. For example, `net.peer.name` (Rank
+  2) should be selected before `http.host` (Rank 6).
+* `net.peer.ip` can be used by itself as `remoteEndpoint` but should be combined
+  with `net.peer.port` if it is also present.
+
+#### Zipkin -> OTLP
+
+When mapping from Zipkin to OTLP set `peer.service` tag from `remoteEndpoint`
+unless there is a `peer.service` tag defined explicitly.
+
 ### Attribute
 
 OpenTelemetry Span and Resource `Attribute`(s) MUST be reported as `tags` to
@@ -153,4 +182,4 @@ Frameworks made before then use a more complex v1 [Thrift](https://github.com/op
 
 Consider using [V1SpanConverter.java](https://github.com/openzipkin/zipkin/blob/master/zipkin/src/main/java/zipkin2/v1/V1SpanConverter.java) as a reference implementation for converting v1 model to OpenTelemetry.
 
-The span timestamp and duration were late additions to the V1 format. As in the code link above, it is possible to heuristically derive them from annotations.
+The span timestamp and duration were late additions to the V1 format. As in the code link above, it is possible to heuristically derive them from annotations.