Network Working Group M. Nottingham
Internet-Draft November 24, 2016
Intended status: Standards Track
Expires: May 28, 2017
Site-Wide HTTP Headers
draft-nottingham-site-wide-headers-01
Abstract
This document specifies an alternative way for Web sites to send HTTP
response header fields that apply to an entire origin, to improve
efficiency.
Status of This Memo
This Internet-Draft is submitted in full conformance with the
provisions of BCP 78 and BCP 79.
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This Internet-Draft will expire on May 28, 2017.
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2
1.1. Selecting Site-Wide Headers . . . . . . . . . . . . . . . 3
1.2. Example . . . . . . . . . . . . . . . . . . . . . . . . . 4
1.3. Notational Conventions . . . . . . . . . . . . . . . . . 5
2. Server Operation . . . . . . . . . . . . . . . . . . . . . . 5
3. User Agent Operation . . . . . . . . . . . . . . . . . . . . 6
3.1. The "SH" HTTP Request Header Field . . . . . . . . . . . 6
3.2. The "HS" HTTP Response Header Field . . . . . . . . . . . 7
4. The "site-headers" well-known URI . . . . . . . . . . . . . . 7
4.1. The "text/site-headers" Media Type . . . . . . . . . . . 7
5. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 8
6. Security Considerations . . . . . . . . . . . . . . . . . . . 8
6.1. Injecting Headers . . . . . . . . . . . . . . . . . . . . 8
6.2. Differing Views of Headers . . . . . . . . . . . . . . . 8
7. References . . . . . . . . . . . . . . . . . . . . . . . . . 8
7.1. Normative References . . . . . . . . . . . . . . . . . . 8
7.2. Informative References . . . . . . . . . . . . . . . . . 9
Author's Address . . . . . . . . . . . . . . . . . . . . . . . . 10
1. Introduction
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;
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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.
1.1. Selecting Site-Wide Headers
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:
o Access-Control-Allow-Origin
o Alt-Svc
o Content-Security-Policy
o P3P
o Public-Key-Pins
o Public-Key-Pins-Report-Only
o Server
o Strict-Transport-Security
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.
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1.2. Example
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:
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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.
1.3. Notational Conventions
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].
This document uses the following ABNF rules from [RFC7230]: "OWS",
"CRLF", "header-field". From [RFC7232]: "entity-tag".
2. Server Operation
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.
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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).
3. User Agent Operation
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.
3.1. The "SH" HTTP Request Header Field
The "SH" HTTP request header field indicates that the user agent has
a fresh (as per [RFC7234]) copy of the well-known resource (see
Section 4) for the request's origin ([RFC6454]).
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).
For example:
SH: "abc123"
SH: *
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3.2. The "HS" HTTP Response Header Field
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.
For example:
HS: "abc123"
4. The "site-headers" well-known URI
The well-known URI [RFC5785] "site-headers" is a resource that, when
fetched, returns a representation in the "text/site-headers" format
(see Section 4.1).
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.
4.1. The "text/site-headers" Media Type
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.
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For example:
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).
5. IANA Considerations
TBD
6. Security Considerations
6.1. Injecting Headers
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.
6.2. Differing Views of Headers
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. References
7.1. Normative References
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, DOI 10.17487/
RFC2119, March 1997,
<http://www.rfc-editor.org/info/rfc2119>.
[RFC5785] Nottingham, M. and E. Hammer-Lahav, "Defining Well-Known
Uniform Resource Identifiers (URIs)", RFC 5785, DOI
10.17487/RFC5785, April 2010,
<http://www.rfc-editor.org/info/rfc5785>.
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[RFC6454] Barth, A., "The Web Origin Concept", RFC 6454, DOI
10.17487/RFC6454, December 2011,
<http://www.rfc-editor.org/info/rfc6454>.
[RFC7230] 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,
<http://www.rfc-editor.org/info/rfc7230>.
[RFC7232] Fielding, R., Ed. and J. Reschke, Ed., "Hypertext Transfer
Protocol (HTTP/1.1): Conditional Requests", RFC 7232, DOI
10.17487/RFC7232, June 2014,
<http://www.rfc-editor.org/info/rfc7232>.
[RFC7234] 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,
<http://www.rfc-editor.org/info/rfc7234>.
7.2. Informative References
[RFC6797] Hodges, J., Jackson, C., and A. Barth, "HTTP Strict
Transport Security (HSTS)", RFC 6797, DOI 10.17487/
RFC6797, November 2012,
<http://www.rfc-editor.org/info/rfc6797>.
[RFC7469] Evans, C., Palmer, C., and R. Sleevi, "Public Key Pinning
Extension for HTTP", RFC 7469, DOI 10.17487/RFC7469, April
2015, <http://www.rfc-editor.org/info/rfc7469>.
[RFC7540] Belshe, M., Peon, R., and M. Thomson, Ed., "Hypertext
Transfer Protocol Version 2 (HTTP/2)", RFC 7540, DOI
10.17487/RFC7540, May 2015,
<http://www.rfc-editor.org/info/rfc7540>.
[RFC7541] Peon, R. and H. Ruellan, "HPACK: Header Compression for
HTTP/2", RFC 7541, DOI 10.17487/RFC7541, May 2015,
<http://www.rfc-editor.org/info/rfc7541>.
[W3C.CR-CSP2-20150721]
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>.
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Author's Address
Mark Nottingham
Email: mnot@mnot.net
URI: https://www.mnot.net/
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