Submit init version of Service Mesh probe protocol

docs/en/protocols/README.md

 # Protocols
 There are two types of protocols list here. 
 
-- **Probe Protocol**. Include the descriptions and defines about how agent send collected metric data and traces, also the formats of each entities.
+- **Probe Protocol**. Include the descriptions and definitions about how agent send collected metric data and traces, also the formats of each entities.
 
 - **Query Protocol**. The backend provide query capability to SkyWalking own UI and others. These queries are based on GraphQL.
 
-## Probe Protocol
+## Probe Protocols
 They also related to the probe group, for understand that, look [Concepts and Designs](../concepts-and-designs/README.md) document.
 These groups are **Language based native agent protocol**, **Service Mesh protocol** and **3rd-party instrument protocol**.  
 
 
-## Language based native agent protocol
+### Language based native agent protocol
 This protocol is combined from two parts:
 * [Cross Process Propagation Headers Protocol](Skywalking-Cross-Process-Propagation-Headers-Protocol-v1.md) is for in-wire propagation.
 which is usually used to carrier the necessary info to build trace.
 * [SkyWalking Trace Data Protocol](Trace-Data-Protocol.md) define the communication way and format between agent and backend.
 
 
-## Service Mesh probe protocol
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+### Service Mesh probe protocol
+The probe in sidecar or proxy could use this protocol to send data to backendEnd. This service provided by gRPC, requires 
+the following key info:
+
+1. Service Name or ID at both sides.
+1. Service Instance Name or ID at both sides.
+1. Endpoint. URI in HTTP, service method full signature in gRPC.
+1. Latency. In milliseconds.
+1. Response code in HTTP
+1. Status. Success or fail.
+1. Protocol. HTTP, gRPC
+1. DetectPoint. In Service Mesh sidecar, `client` or `server`. In normal L7 proxy, value is `proxy`.
+
+```proto
+syntax = "proto3";
+
+option java_multiple_files = true;
+option java_package = "org.apache.skywalking.apm.network.servicemesh";
+
+service ServiceMeshMetricService {
+    rpc collect(stream serviceMeshMetric) returns (Downstream) {
+    }
+}
+
+message serviceMeshMetric {
+    string sourceServiceName = 1;
+    int32 sourceServiceId = 2;
+    string sourceServiceInstance = 3;
+    int32 sourceServiceInstanceId = 4;
+    string destServiceName = 5;
+    int32 destServiceId = 6;
+    string destServiceInstance = 7;
+    int32 destServiceInstanceId = 8;
+    string endpoint = 9;
+    int32 latency = 10;
+    int32 responseCode = 11;
+    bool status = 12;
+    Protocol protocol = 13;
+    DetectPoint detectPoint = 14;
+}
+
+enum Protocol {
+    HTTP = 0;
+    gRPC = 1;
+}
+
+enum DetectPoint {
+    client = 0;
+    server = 1;
+    proxy = 2;
+}
+
+message Downstream {
+}
+
+```
+
+### 3rd-party instrument protocol
+3rd-party instrument protocols are not defined by SkyWalking. They are just protocols/formats, which SkyWalking is compatible and
+could receive from their existed libraries. SkyWalking starts with supporting Zipkin v1, v2 data formats.
+
+Backend is based on modulization principle, so very easy to extend a new receiver to support new protocol/format.
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