This document specifies an alternative way for Web sites to send HTTP response header fields that apply to an entire origin, to improve efficiency.
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HTTP response headers are being used for an increasing amount of metadata that applies to an entire Web site (i.e., the entire origin, as per [RFC6454]).
For example, Strict-Transport-Security [RFC6797] and Public-Key-Pins [RFC7469] both define headers that are explicitly scoped to an entire origin, and number of similar headers are under consideration.
Likewise, some HTTP header fields only sensibly have a single value per origin; for example, Server.
Furthermore, some headers are used uniformly across an origin. For example, a site might have a homogenous Content-Security-Policy [W3C.CR-CSP2-20150721] header.
HTTP/2’s HPACK [RFC7541] header compression mechanism was designed to reduce bandwidth usage for often-repeated headers, both in responses and requests. However, it limits the amount of compression contents usable for a connection (by default, 4K), and some sites are beginning to exceed this limit, thereby reducing the efficiency of HPACK itself.
For example, it is not uncommon for a CSP response header field to exceed 1K (and has been observed to be greater than 3K on popular sites). This forces site administrators to make an awkward choice; put the large header in the HPACK table, thereby crowding out other headers, or omit it, requiring its full content to be sent on every applicable response.
This document defines a way to specify one or more sets of HTTP response header fields in a well-known resource [RFC5785] that, when their use is negotiated, are appended to the header blocks of all HTTP responses on that site by the user agent. This allows common response headers to be omitted both from on-the-wire responses and the HPACK compression table, making both more efficient.
This approach is preferable to increasing the HTTP/2 SETTINGS_HEADER_TABLE_SIZE ([RFC7540], Section 6.5.2), because increasing that setting incurs a per-connection overhead on the server, whereas using the technique documented here does not.
Only certain header fields are suitable for being set for an entire origin. Therefore, a header field MUST be listed below, or its field name MUST start with the characters “site-“ (case insensitive) to be usable as a site-wide header.
The whitelisted field names are:
Note that inclusion in this list does not imply that a header field is always site-wide.
Future specifications SHOULD NOT update this whitelist; instead, they SHOULD use the “site-“ prefix.
If a user agent has a fresh copy of the well-known resource for an origin (see Section 4) (e.g., because it performed a GET, or HTTP/2 Server Push was used):
HTTP/1.1 200 OK Content-Type: text/site-headers Cache-Control: max-age=3600 ETag: "abc123" Content-Length: 284 Strict-Transport-Security: max-age=15768000 ; includeSubDomains Server: Apache/2.4.7 (Ubuntu) Public-Key-Pins: max-age=604800; pin-sha256="ZitlqPmA9wodcxkwOW/c7ehlNFk8qJ9FsocodG6GzdjNM="; pin-sha256="XRXP987nz4rd1/gS2fJSNVfyrZbqa00T7PeRXUPd15w="; report-uri="/lib/key-pin.cgi" Site-Foo: bar
and the user agent makes a subsequent request:
GET /images/foo.jpg HTTP/1.1 Host: www.example.com SH: "abc123"
That indicates that the user agent has processed the well-known resource (because the SH header field is present, and its value matches the current value of the ETag of the well-known resource). Therefore, the server can omit the nominated response header fields on the wire, replacing them with the HS response header field, whose value is the same as that of the SH field:
HTTP/1.1 200 OK Content-Type: image/jpeg Vary: SH, Accept-Encoding Cache-Control: max-age=3600 Transfer-Encoding: chunked HS: "abc123"
Upon receipt of that response, the user agent will consider it equivalent to:
HTTP/1.1 200 OK Content-Type: image/jpeg Vary: SH, Accept-Encoding Cache-Control: max-age=3600 Connection: close Strict-Transport-Security: max-age=15768000 ; includeSubDomains Server: Apache/2.4.7 (Ubuntu) Public-Key-Pins: max-age=604800; pin-sha256="ZitlqPmA9wodcxkwOW/c7ehlNFk8qJ9FsocodG6GzdjNM="; pin-sha256="XRXP987nz4rd1/gS2fJSNVfyrZbqa00T7PeRXUPd15w="; report-uri="/lib/key-pin.cgi" Site-Foo: bar
If a request omits the SH header field, or its field-value does not match the current ETag of the well-known resource, all of the header fields above will be sent by the server in the response, and HS will not be sent.
The key words “MUST”, “MUST NOT”, “REQUIRED”, “SHALL”, “SHALL NOT”, “SHOULD”, “SHOULD NOT”, “RECOMMENDED”, “MAY”, and “OPTIONAL” in this document are to be interpreted as described in [RFC2119].
When a server wishes to use site-wide HTTP headers, it places a file in the format specified in Section 4.1 at the well-known URI specified in Section 4. That file SHOULD NOT contain header fields not allowed by Section 1.1.
Then, when a request has a SH request header field (as per Section 3.1) whose value matches the current ETag of the well-known resource, the set of response header fields in the payload of the well-known resource are omitted from the corresponding response, and the HS response header field is sent with the same value as the SH request header field.
Servers MUST include SH in the field-value of the Vary response header field for all cacheable (as per [RFC7234]) responses of resources that behave in this manner, whether or not headers have been actually appended. This assures correct cache operation, and also advertises support for this specification.
Servers MAY use HTTP/2 Server Push ([RFC7540], Section 8.2) to proactively send the well-known resource to user agents (e.g., if they emit SH: *, indicating that they do not have a fresh copy of the well-known resource).
User agents that support this specification SHOULD always emit a SH header field in requests.
When a valid representation of the well-known resource is held (as defined in Section 4), its value will be its ETag. When one is not (e.g., because it has not been requested, the one held is syntactically invalid, or it is stale, as per [RFC7234]), its value is “*” (unquoted).
When an ETag is sent and the response contains the HS response header field (see Section 3.2), user agents MUST confirm that the value of the “HS” response header is character-for-character identical (after removing leading and trailing whitespace) to that of the SH request header field it sent. If it is not, the response MUST be considered invalid and MUST NOT be used; the user agent MAY retry the request without the SH request header field if its method was safe, MAY attempt to re-fetch the well-known location beforehand, and MAY take alternative recovery strategies.
If the values match, the user agent MUST append the contents of the well-known resource that are currently held to be appended to the response headers received, but MUST NOT include any headers not allowed by Section 1.1.
SH = "*" / entity-tag
Its value is the entity-tag [RFC7232] of the freshest valid well-known location response held by the user agent. If none is held, it should be * (without quotes).
SH: "abc123" SH: *
The HS HTTP response header field indicates that the server has chosen to omit the headers in the well-known resource’s response that shares its ETag with the field value.
HS = entity-tag
Its value is the entity-tag [RFC7232] of the well-known response whose headers are being used, and MUST match that received in the SH header field of the request.
Its media type SHOULD be generated as text/site-headers, although user agents SHOULD NOT reject responses with other types (particularly, application/octet-stream and text/plain).
Its representation MUST contain an ETag response header [RFC7232].
User agents SHOULD NOT consider it valid if it fails to parse, but MAY attempt to recover from errors in a manner similar to how headers are normally handled.
User agents SHOULD consider it to be valid for its freshness lifetime (as per [RFC7234]). If it does not have an explicit freshness lifetime, they SHOULD consider it to have a heuristic freshness lifetime of 120 seconds.
The text/site-headers media type is used to indicate that a file contains a set of HTTP header fields, as defined in [RFC7230], Section 3.
site-headers = OWS *( header-field CRLF ) OWS
As in HTTP itself, implementations need to be forgiving about line endings; specifically, bare CR MUST be considered to be a line ending.
Strict-Transport-Security: max-age=15768000 ; includeSubDomains Server: Apache/2.4.7 (Ubuntu) Public-Key-Pins: max-age=604800; pin-sha256="ZitlqPmA9wodcxkwOW/c7ehlNFk8qJ9FsocodG6GzdjNM="; pin-sha256="XRXP987nz4rd1/gS2fJSNVfyrZbqa00T7PeRXUPd15w="; report-uri="/lib/key-pin.cgi" Content-Security-Policy: default-src 'self'; img-src 'self' *.staticflickr.com; frame-ancestors 'none'; report-uri https://mnot.report-uri.io/r/default/csp/enforce
Note that the Public-Key-Pins and Content-Security-Policy header fields are line-folded; as in HTTP, this form of header is deprecated in this format, and SHOULD NOT be used (except in documentation, as we see here).
Site-wide headers allow a single resource to inject HTTP response headers for an entire origin. Accordingly, the ability to write to that resource needs to be carefully controlled by the origin server.
Because headers sent via this mechanism will not be seen by user agents and intermediaries that do not implement this specification, they will potentially have a different view of the response headers.
7.1. Normative References
- Bradner, S., “Key words for use in RFCs to Indicate Requirement Levels”, BCP 14, RFC 2119, DOI 10.17487/RFC2119, March 1997, <https://www.rfc-editor.org/info/rfc2119>.
- Nottingham, M. and E. Hammer-Lahav, “Defining Well-Known Uniform Resource Identifiers (URIs)”, RFC 5785, DOI 10.17487/RFC5785, April 2010, <https://www.rfc-editor.org/info/rfc5785>.
- Barth, A., “The Web Origin Concept”, RFC 6454, DOI 10.17487/RFC6454, December 2011, <https://www.rfc-editor.org/info/rfc6454>.
- Fielding, R., Ed. and J. Reschke, Ed., “Hypertext Transfer Protocol (HTTP/1.1): Message Syntax and Routing”, RFC 7230, DOI 10.17487/RFC7230, June 2014, <https://www.rfc-editor.org/info/rfc7230>.
- Fielding, R., Ed. and J. Reschke, Ed., “Hypertext Transfer Protocol (HTTP/1.1): Conditional Requests”, RFC 7232, DOI 10.17487/RFC7232, June 2014, <https://www.rfc-editor.org/info/rfc7232>.
- Fielding, R., Ed., Nottingham, M., Ed., and J. Reschke, Ed., “Hypertext Transfer Protocol (HTTP/1.1): Caching”, RFC 7234, DOI 10.17487/RFC7234, June 2014, <https://www.rfc-editor.org/info/rfc7234>.
7.2. Informative References
- Hodges, J., Jackson, C., and A. Barth, “HTTP Strict Transport Security (HSTS)”, RFC 6797, DOI 10.17487/RFC6797, November 2012, <https://www.rfc-editor.org/info/rfc6797>.
- Evans, C., Palmer, C., and R. Sleevi, “Public Key Pinning Extension for HTTP”, RFC 7469, DOI 10.17487/RFC7469, April 2015, <https://www.rfc-editor.org/info/rfc7469>.
- Belshe, M., Peon, R., and M. Thomson, Ed., “Hypertext Transfer Protocol Version 2 (HTTP/2)”, RFC 7540, DOI 10.17487/RFC7540, May 2015, <https://www.rfc-editor.org/info/rfc7540>.
- Peon, R. and H. Ruellan, “HPACK: Header Compression for HTTP/2”, RFC 7541, DOI 10.17487/RFC7541, May 2015, <https://www.rfc-editor.org/info/rfc7541>.
- West, M., Barth, A., and D. Veditz, “Content Security Policy Level 2”, World Wide Web Consortium CR CR-CSP2-20150721, July 2015, <http://www.w3.org/TR/2015/CR-CSP2-20150721>.