TLS1.3 Supported K6 Version

I am using following K6 Build Version and Trying to send TLS1.3 Version(tlsVersion: {
min: http.TLS_1_3,
max: http.TLS_1_3,
}) with TLS1.3 Ciphers mentioned in this link:


but I see in TCPDumps, the Ciphers that are sent is with TLS Version 1.2 as opposed to TLS Version 1.3.

k6 Build Version:

k6 v0.31.1 (2021-03-17T13:23:23+0000/e9d8349, go1.15.8, linux/amd64)

Please let me know if k6 supports TLS1.3 and if Yes what should be the k6 build version that I have to choose.

Hi @srikanthr1,

K6 does support TLS1.3 since at least 0.25.1 (from my testing, no idea why 0.24.0 didn’t :person_shrugging: ).

I have also verified this using wireshark

hitting the official k6 site.

I guess the confusion comes because 1.3 is negotied differently from previous versions and one of the things that is different is that 1.2 is put in a lot of the fields as you can see in

But this is still 1.3 as that is what is listed in “supported_versions”. I am don’t understand the exact reason this needed to be done like that but here is another explanation.

Additionally, as you can see the chosen cipher is TLS_AES_128_GCM_SHA256 which is only supported with 1.3.

I would guess that you just read the wrong field :).

Hope this helps

p.s. I would still recommend that you update your k6 version as the one you have is now more than 1 year old :wink:

Yes @mstoykov you exactly read my mind, Below is the capture screenshot:

Where I see in two places TLSv1.3(Highlighted in Green) which confirms me that request are sending via TLS1.3 but I also see in some places TLS1.0(Marked in Yellow) and TLS1.2(Marked in Red) which based on your snip from RFC is expected. Am I correct here? If so please do close the case

If wireshark is saying v1.3 on top I trust it :). All the others are basically some fields that neede to be interpreted.

The supported_versions having 1.3 does mean that k6 (as this is the client hello) specifically requests 1.3. Everything else AFAIK has the values it has as TLS1.3 need to have TLS1.2< compatible handshake. Which needs to have 1.1 compatible … and so on.

I don’t think I have ever known the exact differences and the reasons for them, but I do remember that 1.3 was specifically very different. But ultimately it still needs to be compatible with the previous ones at least in the handshake phase.