Those key value pairs in chunked encoding ("chunk extensions") are spec'ed to only be hop-by-hop, which makes them more or less completely unsuitable for actually using by end applications. Any proxy or reverse proxy are allowed to strip them. Indeed it can be argued that a conformant proxy is required to strip them, due to MUST ignore unknown extensions value requirement. (I suspect most do not strip them, and there is an argument to be made that blindly passing them through if not changing encoding could be considered ignoring them, but I'm not certain that is actually a conforming interpretation).
Plus surely there are many crusty middleboxes that will break if anybody tried to use that feature. Remember all the hoops websockets had to jump through to have much of a chance working for most people because of those? Many break badly if anything they were not programmed to handle tries to pass through.
> Those key value pairs in chunked encoding ("chunk extensions") are spec'ed to only be hop-by-hop, which makes them more or less completely unsuitable for actually using by end applications.
Oof, I hadn't mentally connected those dots, but you're completely right. (As Transfer-Encoding is hop-by-hop, not end-to-end…)
Plus surely there are many crusty middleboxes that will break if anybody tried to use that feature. Remember all the hoops websockets had to jump through to have much of a chance working for most people because of those? Many break badly if anything they were not programmed to handle tries to pass through.