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6. Response
The response header fields allow the server to pass additional information about the response which cannot be placed in the Status-Line. These header fields are not intended to give information about an Entity-Body returned in the response, but about the server itself.
Response-Header = Public
| Retry-After
| Server
| WWW-Authenticate
| extension-header
Although additional response header fields may be implemented via the extension mechanism, applications which do not recognize those fields should treat them as Entity-Header fields.
6.3.1 Public
The Public header field lists the set of non-standard methods supported by the server. The purpose of this field is strictly to inform the recipient of the capabilities of the server regarding unusual methods. The methods listed may or may not be applicable to the Request-URI; the Allow header field (Section 7.1.1) should be used to indicate methods allowed for a particular URI. This does not prevent a client from trying other methods. The field value should not include the methods predefined for HTTP/1.0 in Section 5.2.
Public = "Public" ":" 1#method
Example of use:
Public: OPTIONS, MGET, MHEAD
This header field applies only to the current connection. If the response passes through a proxy, the proxy must either remove the Public header field or replace it with one applicable to its own capabilities.
6.3.2 Retry-After
The Retry-After header field can be used with "503 Service Unavailable" to indicate how long the service is expected to be unavailable to the requesting client. The value of this field can be either an full HTTP-date or an integer number of seconds (in decimal) after the time of the response.
Retry-After = "Retry-After" ":" ( HTTP-date | delta-seconds )
Two examples of its use are
Retry-After: Wed, 14 Dec 1994 18:22:54 GMT
Retry-After: 120
In the latter example, the delay is 2 minutes.
The Server header field contains information about the software being used by the origin server program handling the request. The field is analogous to the User-Agent field and has the following format:
Server = "Server" ":" 1*( product )
Example:
Server: CERN/3.0 libwww/2.17
If the response is being forwarded through a proxy, the proxy application must not add its data to the product list. Instead, it should include a Forwarded field, as described in Section 4.3.2.
The WWW-Authenticate header field must be included as part of a "401 Unauthorized" response. The field value consists of a challenge that indicates the authentication scheme and parameters applicable to the Request-URI.
WWW-Authenticate = "WWW-Authenticate" ":" challenge
The HTTP access authentication process is described in Section 10.
T. Berners-Lee, R. T. Fielding, H. Frystyk Nielsen - 12 MAR 95
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