The tag is present in Lintian version 2.109.3. That is the most recent version we know about.
The given source file is licensed under the newer RFC license.
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.