The IETF does not provide an authoring toolchain and does not recommend a specific authoring format, you will need to choose what works best for you. Writing Internet-Drafts is complex and time consuming and you can make that much easier by basing your choice on what markup languages and document editing tools you are familiar with, and how you intend to work.
If you know XML reasonably well, have access to a fully-featured XML editor, and will be collaborating with people in the same position then you should be Drafting in RFCXML. RFCXML is supported end-to-end in the authoring process.
If not then you should be Drafting in Markdown. Markdown is not supported end-to-end but the Markdown toolchains will generate RFCXML when required, normally hiding that complexity.
Drafting in any other format is not recommended for new authors.
The diagram below shows the different formats supported at each stage of the authoring process. Some of these stages only support RFCXML and plaintext, but if you are using Markdown then that should not be a problem as the toolchain will generate the RFCXML for you.
As explained, there are multiple formats for initial drafting but a careful choice is needed. New authors often report that they used a format and tool recommended by a fellow author and then later had to put in significant effort to convert it into something they could use more productively.
It is common for a group of authors to work on an I-D and so it is important to consider if all of your co-authors can work with your chosen format and have access to and familiarity with the same tools. Additionally, you need to consider if your I-Ds will be worked on using an integrated issue tracker and source control system such as GitHub. For more details see Collaborative editing.
Validation is an important step for a document to ensure that it is correctly formatted and for it to successfully pass the next steps of submission as an Internet-Draft or output rendering. For more details, see Document validation.
It is common for authors to render their documents into a easily read format such as PDF, HTML or plaintext. For more details see Rendering and converting.
Datatracker submission with document validation
When you are ready to share your I-D you need to submit it to the Datatracker at which point it will be automatically validated. Datatracker only accepts I-Ds in RFCXML or plaintext format. For more details see Submitting your Internet-Draft.
RFC Pre-publication review
If your document is chosen to become an RFC (each RFC Stream has a different process for this) then it will need to go through the AUTH48 pre-publication review process. At this stage you will need to work with RFCXML to address any issues raised by the editors in order for the RFC to be published.
The canonical format for published RFCs is RFCXML and the other published formats, PDF, HTML and plain text, are derived from this RFCXML. The published RFCXML looks quite different from Internet-Draft RFCXML as it has been passed through the 'prep' tool which makes it work better as a standalone document.
The following authoring formats are in use by the IETF community, though some by only a handful of people: