* 歴史

** SSTP

[REFS[
- [1] [CITE[伺か - SSTPプロトコル]] ([TIME[2007-09-14 12:30:05 +09:00]] 版) <http://usada.sakura.vg/contents/sstp.html##statuscode>
]REFS]

[2]
>
420 Refuse	
ゴーストが SSTP の受信を拒否

** PEP (HTTP)

[REFS[
- [3] [CITE[PEP - an Extension Mechanism for HTTP]] ([TIME[1998-04-07 21:31:46 +09:00]] 版) <http://www.w3.org/TR/WD-http-pep#_Toc404743960>
]REFS]

> 420 Policy Not Fulfilled

** SIP

[REFS[
- [4] [CITE@en[RFC 3261 - SIP: Session Initiation Protocol]] ([TIME[2012-01-09 20:48:30 +09:00]] 版) <http://tools.ietf.org/html/rfc3261#section-21.4.15>
]REFS]

>420 Bad Extension


[5] [CITE[Error Codes & Responses | Twitter Developers]]
( ([TIME[2012-02-19 13:24:15 +09:00]] 版))
<https://dev.twitter.com/docs/error-codes-responses>

[6] [CITE[Rate Limiting | Twitter Developers]]
( ([TIME[2012-08-17 02:45:08 +09:00]] 版))
<https://dev.twitter.com/docs/rate-limiting>

[7] [CITE@en[RFC 2660 - The Secure HyperText Transfer Protocol]]
( ([TIME[2013-07-20 22:21:48 +09:00]] 版))
<http://tools.ietf.org/html/rfc2660#section-5.2.2>

[8] [CITE@en[List of HTTP status codes - Wikipedia, the free encyclopedia]]
( ([TIME[2013-10-01 11:44:11 +09:00]] 版))
<http://en.wikipedia.org/wiki/List_of_HTTP_status_codes#420>