6. The statement that "double translation is also a sort of tunneling in general sense" is IMHO confusing in view of its lack of IPv4 end-to-end transparency.
limited to the definition of "a source route that circumvents conventional routing mechanisms", IMHO, it has no business with the end-to-end transparency and therefore not confusing with explictly specifying "in general sense".
The statement that "It needs further investigation to ensure if 4rd-U is qualified to be called as a tunnel in the narrow sense" expresses a doubt without substance to justify it.
because 4rd-u draft calls itself a "tunnel". the commentary identifies it is surely a general-sense tunnel. the commentary tries to understand whether it is also a narrow-sense tunnel but did not yet concluded at the time of writing. now it looks we can conclude that 4rd-u is hard to be called as a tunnel in the narrow sense. see below, regarding the transparency.
4rd-U only claims that IPv4 packets traverse ISP domains transparently (unless they have IPv4 options in which case the domain signals it doesn't support them).
now we have clearified the checksum end-to-end transparency covering payload length and payload protocol type is lost for IPv4/ICMPv4 datagrams in 4rd-u. i suggest 4rd-u draft is revised, if anyway the work will continue, to reflect this understanding too, by explicitly claiming that 4rd-u keeps end-to-end transparency for most of IPv4 fields but checksum covering payload length and payload protocol type (instead of only mentioning options) when the payload is ICMPv4, less than the transparency provided by encapsulation.
commentary document will include this understanding in next revision, without judging how severe this concern is in practice.
if no further significant dissent, this subsubject is closed.