[2] [DFN[[CODE(HTTP)[426 Upgrade Required]]]]

* 仕様書

[REFS[
- [1] [CITE@en[RFC 2817 - Upgrading to TLS Within HTTP/1.1]] ([TIME[2012-01-09 20:05:09 +09:00]] 版) <http://tools.ietf.org/html/rfc2817#section-4.2>
]REFS]

[3] [CITE@en[draft-ietf-tip-usehttp-01 - Using the Transaction Internet Protocol (TIP) with HTTP]]
( ([TIME[2012-01-17 01:38:02 +09:00]] 版))
<http://tools.ietf.org/html/draft-ietf-tip-usehttp-01#page-4>