\(identifier\) "RFC \(identifier\)") [ 9110 ](https://datatracker.ietf.org/doc/html/rfc9110)  
[ P3P ](/wiki/P3P "P3P") |  This field is supposed to set [ P3P ](/wiki/P3P "P3P") policy, in the form of ` P3P:CP="your_compact_policy" ` . However, P3P did not take off,  [53]  most browsers have never fully implemented it, a lot of websites set this field with fake policy text, that was enough to fool browsers the existence of P3P policy and grant permissions for [ third party cookies ](/wiki/Third_party_cookie "Third party cookie") .  |  ` P3P: CP="This is not a P3P policy! See https://en.wikipedia.org/wiki/Special:CentralAutoLogin/P3P for more info." ` |  Permanent  |   
Pragma  |  Implementation-specific fields that may have various effects anywhere along the request-response chain.  |  ` Pragma: no-cache ` |  Permanent  |  [ RFC ](/wiki/RFC_\(identifier\) "RFC \(identifier\)") [ 9111 ](https://datatracker.ietf.org/doc/html/rfc9111)  
Preference-Applied  |  Indicates which Prefer tokens were honored by the server and applied to the processing of the request.  |  ` Preference-Applied: return=representation ` |  Permanent  |  RFC 7240   
Proxy-Authenticate  |  Request authentication to access the proxy.  |  ` Proxy-Authenticate: Basic ` |  Permanent  |  [ RFC ](/wiki/RFC_\(identifier\) "RFC \(identifier\)") [ 9110 ](https://datatracker.ietf.org/doc/html/rfc9110)  
Public-Key-Pins  [54]  |  [ HTTP Public Key Pinning ](/wiki/HTTP_Public_Key_Pinning "HTTP Public Key Pinning") , announces hash of website's authentic [ TLS ](/wiki/Transport_Layer_Security "Transport Layer Security") certificate  |  ` Public-Key-Pins: max-age=2592000; pin-sha256="E9CZ9INDbd+2eRQozYqqbQ2yXLVKB9+xcprMF+44U1g="; ` |  Permanent  |  [ RFC ](/wiki/RFC_\(identifier\) "RFC \(identifier\)") [ 7469 ](https://datatracker.ietf.org/doc/html/rfc7469)  
Retry-After  |  If an entity is temporarily unavailable, this instructs the client to try again later. Value could be a specified period of time (in seconds) or a HTTP-date.  [55]  | 

  * Example 1: ` Retry-After: 120 `
  * Example 2: ` Retry-After: Fri, 07 Nov 2014 23:59:59 GMT `

|  Permanent  |  [ RFC ](/wiki/RFC_\(identifier\) "RFC \(identifier\)") [ 9110 ](https://datatracker.ietf.org/doc/html/rfc9110)  
Server  |  A name for the server  |  ` Server: Apache/2.4.1 (Unix) ` |  Permanent  |  [ RFC ](/wiki/RFC_\(identifier\) "RFC \(identifier\)") [ 9110 ](https://datatracker.ietf.org/doc/html/rfc9110)  
Set-Cookie  |  An [ HTTP cookie ](/wiki/HTTP_cookie "HTTP cookie") |  ` Set-Cookie: UserID=JohnDoe; Max-Age=3600; Version=1 ` |  Permanent: standard  |  [ RFC ](/wiki/RFC_\(identifier\) "RFC \(identifier\)") [ 6265 ](https://datatracker.ietf.org/doc/html/rfc6265)  
[ Strict-Transport-Security ](/wiki/HTTP_Strict_Transport_Security "HTTP Strict Transport Security") |  A HSTS Policy informing the HTTP client how long to cache the HTTPS only policy and whether this applies to subdomains.  |  ` Strict-Transport-Security: max-age=16070400; includeSubDomains ` |  Permanent: standard  |   
Trailer  |  The Trailer general field value indicates that the given set of header fields is present in the trailer of a message encoded with [ chunked transfer coding ](/wiki/Chunked_transfer_coding "Chunked transfer coding") .  |  ` Trailer: Max-Forwards ` |  Permanent  |  [ RFC ](/wiki/RFC_\(identifier\) "RFC \(identifier\)") [ 9110 ](https://datatracker.ietf.org/doc/html/rfc9110)  
Transfer-Encoding  |  The form of encoding used to safely transfer the entity to the user. [ Currently defined methods ](https://www.iana.org/assignments/http-parameters) are: [ chunked ](/wiki/Chunked_transfer_encoding "Chunked transfer encoding") , compress, deflate, gzip, identity.  Must not be used with HTTP/2.  [14]  |  ` Transfer-Encoding: chunked ` |  Permanent  |  [ RFC ](/wiki/RFC_\(identifier\) "RFC \(identifier\)") [ 9110 ](https://datatracker.ietf.org/doc/html/rfc9110)  
Tk  |  Tracking Status header, value suggested to be sent in response to a