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diff --git a/docs/content/methodology/trex_traffic_generator.md b/docs/content/methodology/trex_traffic_generator.md new file mode 100644 index 0000000000..1e394d9e50 --- /dev/null +++ b/docs/content/methodology/trex_traffic_generator.md @@ -0,0 +1,196 @@ +--- +bookToc: false +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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