Attempts to show data responsibility at the arch abstraction

While the architecture doc shouldn't focus too much on wire formatting,
it can clarify who's ultimately responsible for data quality, and use
examples in json or headers to illustrate that.

See #68

pages/architecture.md

@@ -33,6 +33,49 @@ Here's a diagram describing this flow:
 To see if an instrumentation library already exists for your platform, see the
 list of [existing instrumentations]({{ site.github.url}}/pages/existing_instrumentations).
 
+Data Ownership and Flow
+-----------------------
+
+As mentioned in the overview, identifiers are sent in-band and details are sent
+out-of-band to Zipkin. In both cases, trace instrumentation is responsible for
+creating valid traces and rendering them properly. For example, a tracer ensures
+parity between the data it sends in-band (downstream) and out-of-band (async to
+Zipkin).
+
+                                                              
+                            Propagated in band                
+                           ┌───────────────────┐              
+                           │ X─B3─TraceId      │              
+   TraceContext            │                   │              
+ ┌──────────────┐          │ X─B3─ParentSpanId │              
+ │ TraceId      │    ┌────▶│                   │              
+ │              │    │     │ X─B3─SpanId       │              
+ │ ParentSpanId │────┘     │                   │              
+ │              │          │ X─B3─Sampled      │              
+ │ SpanId       │──┐       └───────────────────┘              
+ │              │  │                                          
+ │ Sampled      │  │        Reported out of band              
+ └──────────────┘  │   ┌─────────────────────────────────────┐
+                   │   │{                                    │
+                   └──▶│  "traceId": "35cb9095c9e18d62",     │
+                       │  "parentId": "35cb9095c9e18d62",    │
+                       │  "id": "3132b7749db08a71",          │
+                       │  "name": "get",                     │
+                       │  "timestamp": 1484105262951000,     │
+                       │  "duration": 100000,                │
+                       │  "annotations": [                   │
+                       │    {                                │
+                       │      "timestamp": 1484105262951000, │
+                       │      "value": "cs",                 │
+                       │      "endpoint": {                  │
+                       │        "serviceName": "frontend",   │
+                       │        "ipv4": "127.0.0.1",         │
+                       │        "port": 8081                 │
+                       │      }                              │
+                       │    },                               │
+                       │--snip--                             │
+                       └─────────────────────────────────────┘
+
 
 Transport
 ---------