feat(docs): Add Methodology

Signed-off-by: pmikus <peter.mikus@protonmail.ch>
Change-Id: I5b2e4c14cc258d821b630d2e54b23a8468820764

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+---
+bookToc: false
+title: "Packet Flow Ordering"
+weight: 9
+---
+
+# Packet Flow Ordering
+
+TRex Traffic Generator (TG) supports two main ways how to cover
+address space (on allowed ranges) in scale tests.
+
+In most cases only one field value (e.g. IPv4 destination address) is
+altered, in some cases two fields (e.g. IPv4 destination address and UDP
+destination port) are altered.
+
+## Incremental Ordering
+
+This case is simpler to implement and offers greater control.
+
+When changing two fields, they can be incremented synchronously, or one
+after another. In the latter case we can specify which one is
+incremented each iteration and which is incremented by "carrying over"
+only when the other "wraps around". This way also visits all
+combinations once before the "carry" field also wraps around.
+
+It is possible to use increments other than 1.
+
+## Randomized Ordering
+
+This case chooses each field value at random (from the allowed range).
+In case of two fields, they are treated independently.
+TRex allows to set random seed to get deterministic numbers.
+We use a different seed for each field and traffic direction.
+The seed has to be a non-zero number, we use 1, 2, 3, and so on.
+
+The seeded random mode in TRex requires a "limit" value,
+which acts as a cycle length limit (after this many iterations,
+the seed resets to its initial value).
+We use the maximal allowed limit value (computed as 2^24 - 1).
+
+Randomized profiles do not avoid duplicated values,
+and do not guarantee each possible value is visited,
+so it is not very useful for stateful tests.