Link: http://trac.tools.ietf.org/wg/httpbis/trac/ticket/53
Component: p1-messaging
Section 14.7 states:
"A proxy MUST NOT modify the Allow header field even if it does not understand all the methods specified, since the user agent might have other means of communicating with the origin server."
However, section 13.5.2 (Non-modifiable Headers) makes no mention of Allow. This seems like an error, but I'm not entirely sure what the fix should be -- remove 13.5.2 and push the (not-)modifiable information in the definition of the respective headers, or to maintain 13.5.2 in parallel with all of the header definitions, or to push all the information out of the header definitions into 13.5.2.
The easy fix for now would be to just make a mention of Allow in 13.5.2.
Additionally, Server should also be included.