C-Docs: New structure

Change-Id: I73d107f94b28b138f3350a9e1eedb0555583a9ca
Signed-off-by: Tibor Frank <tifrank@cisco.com>

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----
-title: "TRex Traffic Generator"
-weight: 5
----
-
-# TRex Traffic Generator
-
-## Usage
-
-[TRex traffic generator](https://trex-tgn.cisco.com) is used for majority of
-CSIT performance tests. TRex is used in multiple types of performance tests,
-see [Data Plane Throughtput]({{< ref "data_plane_throughput/data_plane_throughput/#Data Plane Throughtput" >}})
-for more detail.
-
-## Traffic modes
-
-TRex is primarily used in two (mutually incompatible) modes.
-
-### Stateless mode
-
-Sometimes abbreviated as STL.
-A mode with high performance, which is unable to react to incoming traffic.
-We use this mode whenever it is possible.
-Typical test where this mode is not applicable is NAT44ED,
-as DUT does not assign deterministic outside address+port combinations,
-so we are unable to create traffic that does not lose packets
-in out2in direction.
-
-Measurement results are based on simple L2 counters
-(opackets, ipackets) for each traffic direction.
-
-### Stateful mode
-
-A mode capable of reacting to incoming traffic.
-Contrary to the stateless mode, only UDP and TCP is supported
-(carried over IPv4 or IPv6 packets).
-Performance is limited, as TRex needs to do more CPU processing.
-TRex suports two subtypes of stateful traffic,
-CSIT uses ASTF (Advanced STateFul mode).
-
-This mode is suitable for NAT44ED tests, as clients send packets from inside,
-and servers react to it, so they see the outside address and port to respond to.
-Also, they do not send traffic before NAT44ED has created the corresponding
-translation entry.
-
-When possible, L2 counters (opackets, ipackets) are used.
-Some tests need L7 counters, which track protocol state (e.g. TCP),
-but those values are less than reliable on high loads.
-
-## Traffic Continuity
-
-Generated traffic is either continuous, or limited (by number of transactions).
-Both modes support both continuities in principle.
-
-### Continuous traffic
-
-Traffic is started without any data size goal.
-Traffic is ended based on time duration, as hinted by search algorithm.
-This is useful when DUT behavior does not depend on the traffic duration.
-The default for stateless mode.
-
-### Limited traffic
-
-Traffic has defined data size goal (given as number of transactions),
-duration is computed based on this goal.
-Traffic is ended when the size goal is reached,
-or when the computed duration is reached.
-This is useful when DUT behavior depends on traffic size,
-e.g. target number of NAT translation entries, each to be hit exactly once
-per direction.
-This is used mainly for stateful mode.
-
-## Traffic synchronicity
-
-Traffic can be generated synchronously (test waits for duration)
-or asynchronously (test operates during traffic and stops traffic explicitly).
-
-### Synchronous traffic
-
-Trial measurement is driven by given (or precomputed) duration,
-no activity from test driver during the traffic.
-Used for most trials.
-
-### Asynchronous traffic
-
-Traffic is started, but then the test driver is free to perform
-other actions, before stopping the traffic explicitly.
-This is used mainly by reconf tests, but also by some trials
-used for runtime telemetry.
-
-## Trafic profiles
-
-TRex supports several ways to define the traffic.
-CSIT uses small Python modules based on Scapy as definitions.
-Details of traffic profiles depend on modes (STL or ASTF),
-but some are common for both modes.
-
-Search algorithms are intentionally unaware of the traffic mode used,
-so CSIT defines some terms to use instead of mode-specific TRex terms.
-
-### Transactions
-
-TRex traffic profile defines a small number of behaviors,
-in CSIT called transaction templates. Traffic profiles also instruct
-TRex how to create a large number of transactions based on the templates.
-
-Continuous traffic loops over the generated transactions.
-Limited traffic usually executes each transaction once
-(typically as constant number of loops over source addresses,
-each loop with different source ports).
-
-Currently, ASTF profiles define one transaction template each.
-Number of packets expected per one transaction varies based on profile details,
-as does the criterion for when a transaction is considered successful.
-
-Stateless transactions are just one packet (sent from one TG port,
-successful if received on the other TG port).
-Thus unidirectional stateless profiles define one transaction template,
-bidirectional stateless profiles define two transaction templates.
-
-### TPS multiplier
-
-TRex aims to open transaction specified by the profile at a steady rate.
-While TRex allows the transaction template to define its intended "cps" value,
-CSIT does not specify it, so the default value of 1 is applied,
-meaning TRex will open one transaction per second (and transaction template)
-by default. But CSIT invocation uses "multiplier" (mult) argument
-when starting the traffic, that multiplies the cps value,
-meaning it acts as TPS (transactions per second) input.
-
-With a slight abuse of nomenclature, bidirectional stateless tests
-set "packets per transaction" value to 2, just to keep the TPS semantics
-as a unidirectional input value.
-
-### Duration stretching
-
-TRex can be IO-bound, CPU-bound, or have any other reason
-why it is not able to generate the traffic at the requested TPS.
-Some conditions are detected, leading to TRex failure,
-for example when the bandwidth does not fit into the line capacity.
-But many reasons are not detected.
-
-Unfortunately, TRex frequently reacts by not honoring the duration
-in synchronous mode, taking longer to send the traffic,
-leading to lower then requested load offered to DUT.
-This usualy breaks assumptions used in search algorithms,
-so it has to be avoided.
-
-For stateless traffic, the behavior is quite deterministic,
-so the workaround is to apply a fictional TPS limit (max_rate)
-to search algorithms, usually depending only on the NIC used.
-
-For stateful traffic the behavior is not deterministic enough,
-for example the limit for TCP traffic depends on DUT packet loss.
-In CSIT we decided to use logic similar to asynchronous traffic.
-The traffic driver sleeps for a time, then stops the traffic explicitly.
-The library that parses counters into measurement results
-than usually treats unsent packets/transactions as lost/failed.
-
-We have added a IP4base tests for every NAT44ED test,
-so that users can compare results.
-If the results are very similar, it is probable TRex was the bottleneck.
-
-### Startup delay
-
-By investigating TRex behavior, it was found that TRex does not start
-the traffic in ASTF mode immediately. There is a delay of zero traffic,
-after which the traffic rate ramps up to the defined TPS value.
-
-It is possible to poll for counters during the traffic
-(fist nonzero means traffic has started),
-but that was found to influence the NDR results.
-
-Thus "sleep and stop" stategy is used, which needs a correction
-to the computed duration so traffic is stopped after the intended
-duration of real traffic. Luckily, it turns out this correction
-is not dependend on traffic profile nor CPU used by TRex,
-so a fixed constant (0.112 seconds) works well.
-Unfortunately, the constant may depend on TRex version,
-or execution environment (e.g. TRex in AWS).
-
-The result computations need a precise enough duration of the real traffic,
-luckily server side of TRex has precise enough counter for that.
-
-It is unknown whether stateless traffic profiles also exhibit a startup delay.
-Unfortunately, stateless mode does not have similarly precise duration counter,
-so some results (mostly MRR) are affected by less precise duration measurement
-in Python part of CSIT code.
-
-## Measuring Latency
-
-If measurement of latency is requested, two more packet streams are
-created (one for each direction) with TRex flow_stats parameter set to
-STLFlowLatencyStats. In that case, returned statistics will also include
-min/avg/max latency values and encoded HDRHistogram data.
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