On 7 Jul 2017, at 7:23 pm, Willy Tarreau <w@xxxxxx> wrote: > > Maybe the stronger wording should be oriented differently, such as > "Servers MUST not send 103 to HTTP/1.0 clients nor to any client > known not to support 1xx informational responses" ? This way it > leaves the possibility opened (ie rely on version and/or user-agent > or anything else once an exception is known). RFC7231 already says "Since HTTP/1.0 did not define any 1xx status codes, a server MUST NOT send a 1xx response to an HTTP/1.0 client." -- Mark Nottingham https://www.mnot.net/