What's the difference between mbedtls release? Mbed TLS 2.xx.x and 2.x.xx

Question

In mbed.org website updates: https://tls.mbed.org/tech-updates/releases

We can see :

What's the difference btw this two release version? (e.g. why have this two branches release?)

How to decide which realse should be used?

Answer 1

The second version is a "long-term support" (LTS) version. This is made explicit in this announcement from 2018:

[...] we are pleased to announce that Mbed TLS 2.7 will now become our next LTS (Long Term Support) branch, and that we will continue to maintain it for the next 3 years, until at least February 2021.

This means that the library keeps two different branches: The main branch off which the most recent releases are made (as of now, 2.16.6) and a stable LTS branch based on the 2.7 version.

The development is done for the main branch, new features will be implemented there. Occasionally, the public API of the library will change, for example an old function might be removed or renamed, or the parameters of a function might change. When this happens, all software that uses the main version needs to be updated as well.

For the LTS branch, no new features are added. The public API remains as unchanged as much as possible. Only critical bug-fixes are back-ported from the main branch to the LTS branch.

For software projects that are interested in having the most recent features, the main branch would be preferred. However, for some software projects, it might be undesirable to frequently adapt the software to conform to changes made in the library, because making such changes requires resources (time, money) that might be better spend on other things. Such software might still be interested in receiving critical bug fixes made in the library (consider for example security vulnerabilities). For such cases, the LTS version is preferred: Updating the LTS version of the library will require a minimal amount of changes in the software, while still allowing to receive critical bug fixes.