T-Rex Traffic Generator - Stateless vs Stateful

I am in the beginning of learning the T-Rex Traffic Generator. Cisco developed this initially, but it is now an open source traffic generator. With all traffic generators, there is a learning curve associated with it. 

The first major question I had, was the modes that T-Rex can work in:

• Stateless (STL)
• Stateful (STF)
• Advanced Stateful (ASTF)

There are two T-Rex doc pages that discuss these distinctions, but they are not written from a comparative perspective. I will list those links here.

Trex Website: Trex Stateless 

Trex Website: Trex Stateful

While this has good information, however, it was a discussion on Reddit that I found most useful:

Reddit Discussion: STF vs. STL vs. ASTF

In the event that this Reddit thread becomes archived, I will (re) post that discussion here:

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Stateless STL - there is no IP stack so it can't communicate with another standard IP node. The framed packets are pre-built and just pumped out the NIC. Because there is no normal dynamic protocol stack STL mode is run between a TRex NIC pair where they just pass the framed packets between each other and track statistics.

Stateful [A]STF - there is an actual TCP stack running with some L7 support so the stream can communicate to a non t-rex node; or through a stateful firewall with NAT or load balancer etc.

More info and a quick comparison table is here - https://trex-tgn.cisco.com/trex/doc/trex_stateless.html#_stateful_vs_stateless

There is a fairly active community at https://groups.google.com/g/trex-tgn The developers are usually very responsive and will patch bugs usually within a day or two.

t-rex is an engineering tool that seems to be run by the developers and engineers so the documentation can be a little frustrating and the learning curve can be steep. It is however a flexible, powerful and extremely cost effective tool when compared to commercial equivalents.

Then making use of the API combined with your imagination you can also build things other than just stress testing hardware.

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