If you've ever found yourself browsing through RFCs published by the IETF, you know it can be a bit of a chore if you're just researching some tag name or error code. You shouldn't need to read through the entire spec for a protocol just to do some troubleshooting. Right? Well, here comes IANA to the rescue. IANA has conveniently compiled a central repository for the many codes and numbers contained in Internet protocols. You can find it at their protocol registry.

Personally I was just looking for a simple breakdown of all the tags available for DMARC. Of course, there's also a DMARC tag registry table embedded in the DMARC RFC (RFC 7489). But I was having trouble wading through all 73 pages of the spec. So, in this case, it wasn't really necessary. However, for something like the SMTP service extensions, it can be much more useful as the service extensions are spread across many different RFCs.