The tag is present in Lintian version 2.104.319. That is the most recent version we know about.
The given source file is licensed under the non-free RFC license (BCP78).
The majority of IETF documents, such as RFCs, are not licensed under DFSG-free terms, and should thus not be included in Debian main.
If this file is multi-licensed, please override the tag.
Please refer to https://wiki.debian.org/NonFreeIETFDocuments for details.