Add sw6 header document. (#1767)

docs/en/protocols/README.md

@@ -22,7 +22,9 @@ service ID.
 
 ### 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.
+* [**SW6** Cross Process Propagation Headers Protocol](Skywalking-Cross-Process-Propagation-Headers-Protocol-v2.md) is the new protocol for 
+in-wire context propagation, started in 6.0.0-beta release. It will replace the old **SW3** protocol in the future, now both of them are supported.
+* [**SW3** Cross Process Propagation Headers Protocol](Skywalking-Cross-Process-Propagation-Headers-Protocol-v1.md) is for in-wire propagation.
 By following this protocol, the trace segments in different processes could be linked.
 * [SkyWalking Trace Data Protocol](Trace-Data-Protocol.md) define the communication way and format between agent and backend.
 

docs/en/protocols/Skywalking-Cross-Process-Propagation-Headers-Protocol-v2.md

@@ -0,0 +1,43 @@
+# SkyWalking Cross Process Propagation Headers Protocol
+* Version 2.0
+
+## Abstract
+SkyWalking Cross Process Propagation Headers Protocol v2 is also named as sw6 protocol. This protocol keeps the same purposes 
+of [v1(a.k.a. sw3)](Skywalking-Cross-Process-Propagation-Headers-Protocol-v1.md), which is keep context propagation works.
+
+## Differences from v1 
+The major differences of v2 and v1, comes from SkyWalking's evolution, including
+1. Mesh and languages are not same always, some info in headers should be optional.
+1. BASE64 encoding required.
+1. Sampling flag is including.
+
+## Header Item
+* Header Name: `sw6`
+* Header Value: Split by `-`, the parts are following. The length of header value should be less than 2k(default).
+
+Value format example, `XXXXX-XXXXX-XXXX-XXXX`
+
+## Values
+Values include the following segments, all String type values are in BASE64 encoding.
+
+- Required(s)
+1. Sample. 0 or 1. 0 means context exists, but could(most likely will) ignore. 1 means this trace need to be sampled and send to backend. 
+1. Trace Id. **String(BASE64 encoded)**. Three Longs split by `.` to represent the unique id of this trace.
+1. Parent trace segment Id. **String(BASE64 encoded)**. Three Longs split by `.` to represent the unique id of parent segment in parent service.
+1. Parent span Id. Integer. Begin with 0. This span id points to the parent span in parent trace segment. 
+1. Parent service instance Id. Integer. The instance ID of parent service.
+1. Entrance service instance Id. Integer. The instance ID of the entrance service. 
+1. Target address of this request. **String(BASE64 encoded)**. The network address(not must be IP + port) used at client side to access this target
+service. _This value can use exchange/compress collector service to get the id(integer) to represent the string. If you use the string, it must start with `#`, others use integer directly._
+
+- Optional(s)
+
+Optional values could not exist if the agent/SDK haven't those info or the length of header is over the threshold(2k default).  
+1. Entry endpoint of the trace. **String(BASE64 encoded)**. 
+_This value can use exchange/compress collector service to get the id(integer) to represent the string. If you use the string, it must start with `#`, others use integer directly._
+1. Parent endpoint of the parent service. **String(BASE64 encoded)**. 
+_This value can use exchange/compress collector service to get the id(integer) to represent the string. If you use the string, it must start with `#`, others use integer directly._
+
+## Sample values
+1. Short version, `1-TRACEID-SEGMENTID-3-5-2-IPPORT`
+1. Complete version, `1-TRACEID-SEGMENTID-3-5-2-IPPORT-PARENTURI-ENTRYURI`