Address #337

Author: SergeyKanzhelev

spec/20-http_header_format.md

@@ -102,12 +102,19 @@ trace-flags      = 2HEXDIGLC   ; 8 bit flags. Currently, only one bit is used. S
 
 This is the ID of the whole trace forest and is used to uniquely identify a [distributed trace](https://w3c.github.io/trace-context/#dfn-distributed-traces) through a system. It is represented as a 16-byte array, for example, `4bf92f3577b34da6a3ce929d0e0e4736`. All bytes as zero (`00000000000000000000000000000000`) is considered an invalid value.
 
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 A vendor SHOULD generate globally unique values for `trace-id`. Many unique identification generation algorithms create IDs where one part of the value is constant (often time- or host-based), and the other part is a randomly generated value. Because tracing systems may make sampling decisions based on the value of `trace-id`, for increased interoperability vendors MUST keep the random part of `trace-id` ID on the left side.
 
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-When a system operates with a `trace-id` that is shorter than 16 bytes, it SHOULD fill-in the extra bytes with random values rather than zeroes. Let's say the system works with an 8-byte `trace-id` like `3ce929d0e0e4736`. Instead of setting `trace-id` value to `0000000000000003ce929d0e0e4736` it SHOULD generate a value like `4bf92f3577b34da6a3ce929d0e0e4736` where `4bf92f3577b34da6a` is a random value or a function of time and host value.
-
+When a system operates with a `trace-id` that is shorter than 16 bytes, on new
+trace-id generation it SHOULD fill-in the extra bytes with random values rather
+than zeroes. Let's say the system works with an 8-byte `trace-id` like
+`23ce929d0e0e4736`. Instead of setting `trace-id` value to
+`23ce929d0e0e47360000000000000000` (note, that random part is kept on the left
+side as mentioned one paragraph above) it SHOULD generate a value like
+`4bf92f3577b34da623ce929d0e0e4736` where `4bf92f3577b34da6` is a random value or
+a function of time and host value. Note, that on receiving a `trace-id` which is
+longer than what system operates with, even though `trace-id` may be recorded
+with the shorter id, the entire trace-id needs to be propagated to the
+downstream components.
 
 **Note**: Even though a system may operate with a shorter `trace-id` for [distributed trace](https://w3c.github.io/trace-context/#dfn-distributed-traces) reporting, the full `trace-id` MUST be propagated to conform to the specification.