Network Working Group J. Reschke
Internet-Draft greenbytes
Intended status: Standards Track S. Loreto
Expires: December 26, 2017 Ericsson
June 24, 2017
'Out-Of-Band' Content Coding for HTTP
draft-reschke-http-oob-encoding-12
Abstract
This document describes an Hypertext Transfer Protocol (HTTP) content
coding that can be used to describe the location of a secondary
resource that contains the payload.
Editorial Note (To be removed by RFC Editor before publication)
Distribution of this document is unlimited. Although this is not a
work item of the HTTPbis Working Group, comments should be sent to
the Hypertext Transfer Protocol (HTTP) mailing list at ietf-http-
wg@w3.org [1], which may be joined by sending a message with subject
"subscribe" to ietf-http-wg-request@w3.org [2].
Discussions of the HTTPbis Working Group are archived at
.
XML versions, latest edits, and issue tracking for this document are
available from and .
The changes in this draft are summarized in Appendix D.12.
Status of This Memo
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provisions of BCP 78 and BCP 79.
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This Internet-Draft will expire on December 26, 2017.
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 3
2. Notational Conventions . . . . . . . . . . . . . . . . . . . 3
3. 'Out-Of-Band' Content Coding . . . . . . . . . . . . . . . . 4
3.1. Overview . . . . . . . . . . . . . . . . . . . . . . . . 4
3.2. Definitions . . . . . . . . . . . . . . . . . . . . . . . 4
3.3. Processing Steps . . . . . . . . . . . . . . . . . . . . 5
3.4. Examples . . . . . . . . . . . . . . . . . . . . . . . . 7
3.4.1. Basic Example . . . . . . . . . . . . . . . . . . . . 7
3.4.2. Example for an attempt to use 'out-of-band' cross-
origin . . . . . . . . . . . . . . . . . . . . . . . 8
3.4.3. Example involving an encrypted resource . . . . . . . 9
3.4.4. Relation to Content Negotiation . . . . . . . . . . . 10
4. Content Codings and Range Requests . . . . . . . . . . . . . 11
5. Feature Discovery . . . . . . . . . . . . . . . . . . . . . . 12
6. Security Considerations . . . . . . . . . . . . . . . . . . . 12
6.1. Content Modifications . . . . . . . . . . . . . . . . . . 12
6.2. Content Stealing . . . . . . . . . . . . . . . . . . . . 13
6.3. Use in Requests . . . . . . . . . . . . . . . . . . . . . 13
7. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 13
7.1. Content Coding: out-of-band . . . . . . . . . . . . . . . 13
7.2. Internet Media Type: application/oob-stream . . . . . . . 14
8. References . . . . . . . . . . . . . . . . . . . . . . . . . 15
8.1. Normative References . . . . . . . . . . . . . . . . . . 15
8.2. Informative References . . . . . . . . . . . . . . . . . 16
Appendix A. Problem Reporting . . . . . . . . . . . . . . . . . 18
A.1. Server Not Reachable . . . . . . . . . . . . . . . . . . 18
A.2. Resource Not Found . . . . . . . . . . . . . . . . . . . 18
A.3. Payload Unusable . . . . . . . . . . . . . . . . . . . . 18
A.4. TLS Handshake Failure . . . . . . . . . . . . . . . . . . 18
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A.5. Example For Problem Reporting . . . . . . . . . . . . . . 19
Appendix B. Alternatives, or: why not a new Status Code? . . . . 19
Appendix C. Open Issues . . . . . . . . . . . . . . . . . . . . 20
C.1. Accessing the Secondary Resource Too Early . . . . . . . 20
C.2. Resource maps . . . . . . . . . . . . . . . . . . . . . . 20
C.3. Fragmenting . . . . . . . . . . . . . . . . . . . . . . . 20
C.4. Relation to Content Encryption . . . . . . . . . . . . . 21
C.5. Reporting . . . . . . . . . . . . . . . . . . . . . . . . 21
C.6. Controlling Transmission Of Various Request Header Fields 22
Appendix D. Change Log (to be removed by RFC Editor before
publication) . . . . . . . . . . . . . . . . . . . . 22
D.1. Changes since draft-reschke-http-oob-encoding-00 . . . . 22
D.2. Changes since draft-reschke-http-oob-encoding-01 . . . . 22
D.3. Changes since draft-reschke-http-oob-encoding-02 . . . . 22
D.4. Changes since draft-reschke-http-oob-encoding-03 . . . . 22
D.5. Changes since draft-reschke-http-oob-encoding-04 . . . . 23
D.6. Changes since draft-reschke-http-oob-encoding-05 . . . . 23
D.7. Changes since draft-reschke-http-oob-encoding-06 . . . . 23
D.8. Changes since draft-reschke-http-oob-encoding-07 . . . . 24
D.9. Changes since draft-reschke-http-oob-encoding-08 . . . . 24
D.10. Changes since draft-reschke-http-oob-encoding-09 . . . . 24
D.11. Changes since draft-reschke-http-oob-encoding-10 . . . . 24
D.12. Changes since draft-reschke-http-oob-encoding-11 . . . . 24
Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . . 24
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 24
1. Introduction
This document describes an Hypertext Transfer Protocol (HTTP) content
coding (Section 3.1.2.1 of [RFC7231]) that can be used to describe
the location of a secondary resource that contains the payload.
The primary use case for this content coding is to enable origin
servers to securely delegate the delivery of content to a secondary
server that might be "closer" to the client (with respect to network
topology) and/or able to cache content ([SCD]), leveraging content
encryption ([RFC8188]).
2. 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 reuses terminology used in the base HTTP
specifications, namely Section 2 of [RFC7230] and Section 3 of
[RFC7231].
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3. 'Out-Of-Band' Content Coding
3.1. Overview
The 'Out-Of-Band' content coding is used to direct the recipient to
retrieve the actual message representation (Section 3 of [RFC7231])
from a secondary resource, such as a public cache:
1. Client performs a request
2. Received response specifies the 'out-of-band' content coding; the
payload of the response contains additional meta data, plus the
location of the secondary resource
3. Client performs GET request on secondary resource (usually again
via HTTP(s))
4. Secondary server provides payload
5. Client combines above representation with additional
representation metadata obtained from the primary resource
Client Secondary Server Origin Server
sends GET request with Accept-Encoding: out-of-band
(1) |---------------------------------------------------------\
status 200 and Content-Coding: out-of-band |
(2) <---------------------------------------------------------/
GET to secondary server
(3) |---------------------------\
payload |
(4) <---------------------------/
(5)
Client and combines payload received in (4)
with metadata received in (2).
3.2. Definitions
The name of the content coding is "out-of-band".
The payload format uses JavaScript Object Notation (JSON, [RFC7159]),
describing an object describing secondary resources; currently only
defining one member:
'sr' A REQUIRED array of JSON objects.
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Objects having a member named 'r' describe a secondary resource,
with the member's string value containing a URI reference
(Section 4.1 of [RFC3986]) of the secondary resource (URI
references that are relative references are resolved against the
URI of the primary resource).
An OPTIONAL member 'crypto-key' carries an array of strings, each
of which specifying keying material for use in encryption
encodings such as the 'aes128gcm' encoding defined in [RFC8188].
Values consist of the name of the content coding, a "=", and the
base64url encoded keying material (see Section 5 of [RFC4648]).
The payload format uses an array so that the origin server can
specify multiple secondary resources. The ordering within the array
reflects the origin server's preference (if any), with the most
preferred secondary resource location being first. Clients receiving
a response containing multiple entries are free to choose which of
these to use.
In some cases, the origin server might want to specify a "fallback
URI"; identifying a secondary resource served by the origin server
itself, but otherwise equivalent "regular" secondary resources. Any
secondary resource hosted by the origin server can be considered to
be a "fallback"; origin servers will usually list them last in the
"sr" array so that they only will be used by clients when there is no
other choice.
New specifications can define new OPTIONAL member fields, thus
clients MUST ignore unknown fields. Furthermore, new specifications
can define new object formats for the 'sr' array; however, they MUST
NOT use a member named 'r' unless the semantics are compatible with
those defined above.
Extension specifications will have to update this specification.
3.3. Processing Steps
Upon receipt of an 'out-of-band' encoded response, a client first
needs to obtain the secondary resource's presentation. This is done
using an HTTP GET request (independently of the original request
method).
In order to prevent any leakage of information, the GET request for
the secondary resource MUST only contain information provided by the
origin server or the secondary server itself, namely HTTP
authentication credentials ([RFC7235]) and cookies ([RFC6265]).
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Furthermore, the request MUST include an "Origin" header field
indicating the origin of the original resource ([RFC6454],
Section 7). The secondary server MUST verify that the specified
origin is authorized to retrieve the given payload (or otherwise
return an appropriate 4xx status code).
In addition to that, the secondary server's response MUST include a
"Content-Type" header field indicating an Internet media type of
"application/oob-stream". Clients MUST check for this media type and
abort out-of-band processing if no media type is specified, or if it
doesn't match this value.
After receipt of the secondary resource's payload, the client then
reconstructs the original message by:
1. Unwrapping the encapsulated HTTP message by removing any transfer
and content codings.
2. Replacing/setting any response header fields from the primary
response except for framing-related information such as Content-
Length, Transfer-Encoding and Content-Encoding.
If the client is unable to retrieve the secondary resource's
representation (host can't be reached, non 2xx response status code,
payload failing integrity check, etc.), it can choose an alternate
secondary resource (if specified), try the fallback URI (if given),
or simply retry the request to the origin server without including
'out-of-band' in the Accept-Encoding request header field. In the
latter case, it can be useful to inform the origin server about what
problems were encountered when trying to access the secondary
resource; see Appendix A for details.
Note that although this mechanism causes the inclusion of external
content, it will not affect the application-level security properties
of the reconstructed message, such as its web origin ([RFC6454]).
The cacheability of the response for the secondary resource does not
affect the cacheability of the reconstructed response message, which
is the same as for the origin server's response.
Use of the 'out-of-band' coding is similar to HTTP redirects
([RFC7231], Section 6.4) in that it can lead to cycles. Unless with
HTTP redirects, the client however is in full control: it does not
need to advertise support for the 'out-of-band' coding in requests
for secondary resources. Alternatively, it can protect itself just
like for HTTP redirects -- by limiting the number of indirections it
supports.
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Note that because the server's response depends on the request's
Accept-Encoding header field, the response usually will need to be
declared to vary on that. See Section 7.1.4 of [RFC7231] and
Section 2.3 of [RFC7232] for details.
3.4. Examples
3.4.1. Basic Example
Client request of primary resource at https://www.example.com/test:
GET /test HTTP/1.1
Host: www.example.com
Accept-Encoding: gzip, out-of-band
Response:
HTTP/1.1 200 OK
Date: Thu, 14 May 2015 18:52:00 GMT
Content-Type: text/plain
Cache-Control: max-age=10, public
Content-Encoding: out-of-band
Content-Length: 165
Vary: Accept-Encoding
{
"sr": [
{ "r" :
"http://example.net/bae27c36-fa6a-11e4-ae5d-00059a3c7a00"},
{ "r" :
"/c/bae27c36-fa6a-11e4-ae5d-00059a3c7a00"}
]
}
(note that the Content-Type header field describes the media type of
the secondary's resource representation, and the origin server
supplied a fallback URI)
Client request for secondary resource:
GET /bae27c36-fa6a-11e4-ae5d-00059a3c7a00 HTTP/1.1
Host: example.net
Origin: https://www.example.com
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Response:
HTTP/1.1 200 OK
Date: Thu, 14 May 2015 18:52:10 GMT
Cache-Control: private
Content-Type: application/oob-stream
Content-Length: 15
Hello, world.
Final message after recombining header fields:
HTTP/1.1 200 OK
Date: Thu, 14 May 2015 18:52:00 GMT
Content-Length: 15
Cache-Control: max-age=10, public
Content-Type: text/plain
Hello, world.
3.4.2. Example for an attempt to use 'out-of-band' cross-origin
Section 3.3 requires the client to include an "Origin" header field
in the request to a secondary server. The example below shows how
the server for the secondary resource would respond to a request
which contains an "Origin" header field identifying an unauthorized
origin.
Continuing with the example from Section 3.4.1, and a secondary
server that is configured to allow only access for requests initiated
by "https://www.example.org":
Client request for secondary resource:
GET /bae27c36-fa6a-11e4-ae5d-00059a3c7a00 HTTP/1.1
Host: example.net
Origin: https://www.example.com
Response:
HTTP/1.1 403 Forbidden
Date: Thu, 14 May 2015 18:52:10 GMT
Note that a request missing the "Origin" header field would be
treated the same way.
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[[CREF1: Any reason why to *mandate* a specific 4xx code?]]
3.4.3. Example involving an encrypted resource
Given the example HTTP message from Section 3.1 of [RFC8188], a
primary resource could use the 'out-of-band' coding to specify just
the location of the secondary resource plus the keying material
needed to decrypt the payload:
Response:
HTTP/1.1 200 OK
Date: Thu, 14 May 2015 18:52:00 GMT
Content-Encoding: aes128gcm, out-of-band
Content-Type: text/plain
Content-Length: 171
Vary: Accept-Encoding
{
"sr": [
{ "r" :
"http://example.net/bae27c36-fa6a-11e4-ae5d-00059a3c7a00",
"crypto-key" :
[ "aes128gcm=yqdlZ-tYemfogSmv7Ws5PQ" ] }
]
}
(note that the Content-Type header field describes the media type of
the secondary's resource representation)
Response for secondary resource:
HTTP/1.1 200 OK
Date: Thu, 14 May 2015 18:52:10 GMT
Content-Type: application/oob-stream
Content-Length: 54
I1BsxtFttlv3u_Oo94xnmwAAEAAA-NAVub2qFgBEuQKRapoZu-IxkIva3MEB1PD-
ly8Thjg
(payload body shown in base64 for presentation purposes)
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Final message undoing all content codings:
HTTP/1.1 200 OK
Date: Thu, 14 May 2015 18:52:00 GMT
Content-Length: 15
Content-Type: text/plain
I am the walrus
Note: in this case, the ability to undo the 'aes128gcm' is needed
to process the response. If 'aes128gcm' wasn't listed as
acceptable content coding in the request, the origin server
wouldn't be able to use the 'out-of-band' mechanism.
3.4.4. Relation to Content Negotiation
Use of the 'out-of-band' encoding is a case of "proactive content
negotiation", as defined in Section 3.4 of [RFC7231].
This however does not rule out combining it with other content
codings. As an example, the possible iteractions with the 'gzip'
content coding ([RFC7230], Section 4.2.3) are described below:
Case 1: Primary resource does not support 'gzip' encoding
In this case, the response for the primary resource will never
include 'gzip' in the Content-Encoding header field. The secondary
resource however might support it, in which case the client could
negotiate compression by including "Accept-Encoding: gzip" in the
request to the secondary resource.
Case 2: Primary resource does support 'gzip' encoding
Here, the origin server would actually use two different secondary
resources, one of them being gzip-compressed. For instance -- going
back to the first example in Section 3.4.1 -- it might reply with:
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HTTP/1.1 200 OK
Date: Thu, 14 May 2015 18:52:00 GMT
Content-Type: text/plain
Cache-Control: max-age=10, public
Content-Encoding: gzip, out-of-band
Content-Length: 165
Vary: Accept-Encoding
{
"sr": [
{ "r" :
"http://example.net/bae27c36-fa6a-11e4-ae5d-00059a3c7a01"},
{ "r" :
"/c/bae27c36-fa6a-11e4-ae5d-00059a3c7a01"}
]
}
which would mean that the payload for the secondary resource already
is gzip-compressed.
Note: The origin server could also apply gzip compression to the
out-of-band payload, in which case the Content-Encoding field
value would become: "gzip, out-of-band, gzip".
4. Content Codings and Range Requests
The combination of content codings ([RFC7231], Section 3.1.2 with
range requests ([RFC7233]) can lead to surprising results, as
applying the range request happens after applying content codings.
Thus, for a request for the bytes starting at position 100000 of a
video:
GET /test.mp4 HTTP/1.1
Host: www.example.com
Range: bytes=100000-
Accept-Encoding: identity
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...a successful response would use status code 206 (Partial Content)
and have a payload containing the octets starting at position 100000.
HTTP/1.1 206 Partial Content
Date: Thu, 08 September 2015 16:49:00 GMT
Content-Type: video/mp4
Content-Length: 134567
Content-Range: bytes 100000-234566/234567
(binary data)
However, if the request would have allowed the use of 'out-of-band'
coding:
GET /test.mp4 HTTP/1.1
Host: www.example.com
Range: bytes=100000-
Accept-Encoding: out-of-band
...a server might return an empty payload (if the out-of-band coded
response body would be shorter than 100000 bytes, as would be usually
the case).
Thus, in order to avoid unnecessary network traffic, servers SHOULD
NOT apply range request processing to responses using ouf-of-band
content coding (or, in other words: ignore "Range" request header
fields in this case).
5. Feature Discovery
New content codings can be deployed easily, as the client can use the
"Accept-Encoding" header field (Section 5.3.4 of [RFC7231]) to signal
which content codings are supported.
6. Security Considerations
6.1. Content Modifications
This specification does not define means to verify that the payload
obtained from the secondary resource really is what the origin server
expects it to be. Content signatures can address this concern (see
[CONTENTSIG] and [MICE]).
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6.2. Content Stealing
The 'out-of-band' content coding could be used to circumvent the
same-origin policy ([RFC6454], Section 3) of user agents: an
attacking site which knows the URI of a secondary resource would use
the 'out-of-band' coding to trick the user agent to read the contents
of the secondary resource, which then, due to the security properties
of this coding, would be handled as if it originated from the
origin's resource.
This scenario is addressed by the client requirement to include the
"Origin" request header field and the server requirement to verify
that the request was initiated by an authorized origin. In addition,
the restriction of the secondary server response's media type to
"application/oob-stream" protects existing content on "regular"
servers not implementing this specification.
Note: similarities with the "Cross-Origin Resource Sharing"
protocol ([CORS]) are intentional.
Requiring the secondary resource's payload to be encrypted
([RFC8188]) is an additional mitigation.
6.3. Use in Requests
In general, content codings can be used in both requests and
responses. This particular content coding has been designed for
responses. When supported in requests, it creates a new attack
vector where the receiving server can be tricked into including
content that the client might not have access to otherwise (such as
HTTP resources behind a firewall).
7. IANA Considerations
7.1. Content Coding: out-of-band
The IANA "HTTP Content Coding Registry", located at
, needs to be
updated with the registration below:
Name: out-of-band
Description: Payload needs to be retrieved from a secondary resource
Reference: Section 3 of this document
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7.2. Internet Media Type: application/oob-stream
IANA maintains the registry of Internet media types [BCP13] at
.
This document serves as the specification for the Internet media type
"application/oob-stream". The following is to be registered with
IANA.
The "application/oob-stream" media type represents a sequence of
octets sent as part of the "out-of-band" content coding protocol
exchange. The sender does not have any further information about the
type of the enclosed data. This type is different from "application/
octet-stream" as it is known not to be in use for pre-existing
content.
Type name: application
Subtype name: oob-stream
Required parameters: N/A
Optional parameters: N/A
Encoding considerations: always "binary"
Security considerations: see Section 6
Interoperability considerations: N/A
Published specification: This specification (see Section 7.2).
Applications that use this media type: HTTP servers for secondary
resources as defined by this specification.
Fragment identifier considerations: N/A
Additional information:
Additional information:
Magic number(s): N/A
Deprecated alias names for this type: N/A
File extension(s): N/A
Macintosh file type code(s): N/A
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Person and email address to contact for further information: See Aut
hors' Addresses section.
Intended usage: COMMON
Restrictions on usage: N/A
Author: See Authors' Addresses section.
Change controller: IESG
8. References
8.1. 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,
.
[RFC3986] Berners-Lee, T., Fielding, R., and L. Masinter, "Uniform
Resource Identifier (URI): Generic Syntax", STD 66,
RFC 3986, DOI 10.17487/RFC3986, January 2005,
.
[RFC5988] Nottingham, M., "Web Linking", RFC 5988,
DOI 10.17487/RFC5988, October 2010,
.
[RFC6265] Barth, A., "HTTP State Management Mechanism", RFC 6265,
DOI 10.17487/RFC6265, April 2011,
.
[RFC7159] Bray, T., "The JavaScript Object Notation (JSON) Data
Interchange Format", RFC 7159, DOI 10.17487/RFC7159, March
2014, .
[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,
.
[RFC7231] Fielding, R., Ed. and J. Reschke, Ed., "Hypertext Transfer
Protocol (HTTP/1.1): Semantics and Content", RFC 7231,
DOI 10.17487/RFC7231, June 2014,
.
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[RFC7235] Fielding, R., Ed. and J. Reschke, Ed., "Hypertext Transfer
Protocol (HTTP/1.1): Authentication", RFC 7235,
DOI 10.17487/RFC7235, June 2014,
.
8.2. Informative References
[BCP13] Freed, N., Klensin, J., and T. Hansen, "Media Type
Specifications and Registration Procedures", BCP 13,
RFC 6838, January 2013,
.
[CONTENTSIG]
Thomson, M., "Content-Signature Header Field for HTTP",
draft-thomson-http-content-signature-00 (work in
progress), July 2015.
[CORS] van Kesteren, A., "Cross-Origin Resource Sharing", W3C
Recommendation REC-cors-20140116, January 2014,
.
Latest version available at .
[MICE] Thomson, M., "Merkle Integrity Content Encoding", draft-
thomson-http-mice-02 (work in progress), October 2016.
[RFC2017] Freed, N. and K. Moore, "Definition of the URL MIME
External-Body Access-Type", RFC 2017,
DOI 10.17487/RFC2017, October 1996,
.
[RFC4483] Burger, E., "A Mechanism for Content Indirection in
Session Initiation Protocol (SIP) Messages", RFC 4483,
DOI 10.17487/RFC4483, May 2006,
.
[RFC4648] Josefsson, S., "The Base16, Base32, and Base64 Data
Encodings", RFC 4648, DOI 10.17487/RFC4648, October 2006,
.
[RFC5246] Dierks, T. and E. Rescorla, "The Transport Layer Security
(TLS) Protocol Version 1.2", RFC 5246,
DOI 10.17487/RFC5246, August 2008,
.
[RFC6454] Barth, A., "The Web Origin Concept", RFC 6454,
DOI 10.17487/RFC6454, December 2011,
.
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[RFC7232] Fielding, R., Ed. and J. Reschke, Ed., "Hypertext Transfer
Protocol (HTTP/1.1): Conditional Requests", RFC 7232,
DOI 10.17487/RFC7232, June 2014,
.
[RFC7233] Fielding, R., Ed., Lafon, Y., Ed., and J. Reschke, Ed.,
"Hypertext Transfer Protocol (HTTP/1.1): Range Requests",
RFC 7233, DOI 10.17487/RFC7233, June 2014,
.
[RFC8188] Thomson, M., "Encrypted Content-Encoding for HTTP",
RFC 8188, DOI 10.17487/RFC8188, June 2017,
.
[RMAP] Eriksson, G., Holmberg, C., Sarker, Z., and J. Reschke,
"Resource Maps", draft-eriksson-http-resource-map-00 (work
in progress), October 2016.
[SCD] Thomson, M., Eriksson, G., and C. Holmberg, "An
Architecture for Secure Content Delegation using HTTP",
draft-thomson-http-scd-02 (work in progress), October
2016.
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Appendix A. Problem Reporting
[[erwip: This is a rough proposal for an error reporting mechanism.
Is it good enough? Is it needed at all? Note that Alt-Svc doesn't
have anything like this.]]
When the client fails to obtain the secondary resource, it can be
useful to inform the origin server about the condition. This can be
accomplished by adding a "Link" header field ([RFC5988]) to a
subsequent request to the origin server, detailing the URI of the
secondary resource and the failure reason.
The following link extension relations are defined:
A.1. Server Not Reachable
Used in case the server was not reachable.
Link relation:
http://purl.org/linkrel/not-reachable
A.2. Resource Not Found
Used in case the server responded, but the object could not be
obtained.
Link relation:
http://purl.org/linkrel/resource-not-found
A.3. Payload Unusable
Used in case the payload could be obtained, but wasn't usable (for
instance, because integrity checks failed).
Link relation:
http://purl.org/linkrel/payload-unusable
A.4. TLS Handshake Failure
Used in case of a TLS handshare failure ([RFC5246]).
Link relation:
http://purl.org/linkrel/tls-handshake-failure
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A.5. Example For Problem Reporting
Client requests primary resource as in Section 3.4.1, but the attempt
to access the secondary resource fails.
Response:
HTTP/1.1 404 Not Found
Date: Thu, 08 September 2015 16:49:00 GMT
Content-Type: text/plain
Content-Length: 20
Resource Not Found
Client retries with the origin server and includes Link header field
reporting the problem:
GET /test HTTP/1.1
Host: www.example.com
Accept-Encoding: gzip, out-of-band
Link: ;
rel="http://purl.org/linkrel/resource-not-found"
Appendix B. Alternatives, or: why not a new Status Code?
A plausible alternative approach would be to implement this
functionality one level up, using a new redirect status code
(Section 6.4 of [RFC7231]). However, this would have several
drawbacks:
o Servers will need to know whether a client understands the new
status code; thus some additional signal to opt into this protocol
would always be needed.
o In redirect messages, representation metadata (Section 3.1 of
[RFC7231]), namely "Content-Type", applies to the response
message, not the redirected-to resource.
o The origin-preserving nature of using a content coding would be
lost.
Another alternative would be to implement the indirection on the
level of the media type using something similar to the type "message/
external-body", defined in [RFC2017] and refined for use in the
Session Initiation Protocol (SIP) in [RFC4483]. This approach though
would share most of the drawbacks of the status code approach
mentioned above.
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Appendix C. Open Issues
C.1. Accessing the Secondary Resource Too Early
One use-case for this protocol is to enable a system of "blind
caches", which would serve the secondary resources. These caches
might only be populated on demand, thus it could happen that whatever
mechanism is used to populate the cache hasn't finished when the
client hits it (maybe due to race conditions, or because the cache is
behind a middlebox which doesn't allow the origin server to push
content to it).
In this particular case, it can be useful if the client was able to
"piggyback" the URI of the fallback for the primary resource, giving
the secondary server a means by which it could obtain the payload
itself. This information could be provided in yet another Link
header field:
GET /bae27c36-fa6a-11e4-ae5d-00059a3c7a00 HTTP/1.1
Host: example.net
Link: ;
rel="http://purl.org/linkrel/fallback-resource"
(continuing the example from Section 3.4.1)
C.2. Resource maps
When 'out-of-band' coding is used as part of a caching solution, the
additional round trips to the origin server can be a significant
performance problem; in particular, when many small resources need to
be loaded (such as scripts, images, or video fragments). In cases
like these, it could be useful for the origin server to provide a
"resource map", allowing to skip the round trips to the origin server
for these mapped resources. Plausible ways to transmit the resource
map could be:
o as extension in the 'out-of-band' coding JSON payload, or
o as separate resource identified by a "Link" response header field.
See [RMAP] for further information.
C.3. Fragmenting
It might be interesting to divide the original resource's payload
into fragments, each of which being mapped to a distinct secondary
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resource. This would allow to not store the full payload of a
resource in a single cache, thus
o distribute load,
o caching different parts of the resource with different
characteristics (such as only distribute the first minutes of a
long video), or
o fetching specific parts of a resource (similar to byte range
requests), or
o hiding information from the secondary server.
Another benefit might be that it would allow the origin server to
only serve the first part of a resource itself (reducing time to play
of a media resource), while delegating the remainder to a cache
(however, this might require further adjustments of the 'out-of-band'
payload format).
C.4. Relation to Content Encryption
Right now this specification is orthogonal to [RFC8188]/[MICE]; that
is, it could be used for public content such as software downloads.
However, the lack of mandatory encryption affects the security
considerations (which currently try to rule attack vectors caused by
ambient authority ([RFC6265], Section 8.2). We need to decide
whether we need this level of independence.
C.5. Reporting
This specification already defines hooks through which a client can
report failures when accessing secondary resources (see Appendix A).
However, it would be useful if there were also ways to report on
statistics such as:
o Success (Cache Hit) rates, and
o Bandwidth to secondary servers.
This could be implemented using a new service endpoint and a (JSON?)
payload format.
Similarly, a reporting facility for use by the secondary servers
could be useful.
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C.6. Controlling Transmission Of Various Request Header Fields
Clients by default might include request header fields such as "User-
Agent" (or some of the newly defined "Client Hints") into their
requests to the secondary server. If the secondary server does not
perform any content negotiation, none of these header fields is
actually useful, so suppressing them by default might be a good idea
to reduce fingerprinting. In this case, we could allow the origin
server to opt into sending some of them though.
Appendix D. Change Log (to be removed by RFC Editor before publication)
D.1. Changes since draft-reschke-http-oob-encoding-00
Mention media type approach.
Explain that clients can always fall back not to use oob when the
secondary resource isn't available.
Add Vary response header field to examples and mention that it'll
usually be needed ().
Experimentally add problem reporting using piggy-backed Link header
fields ().
D.2. Changes since draft-reschke-http-oob-encoding-01
Updated ENCRYPTENC reference.
D.3. Changes since draft-reschke-http-oob-encoding-02
Add MICE reference.
Remove the ability of the secondary resource to contain anything but
the payload ().
Changed JSON payload to be an object containing an array of URIs plus
additional members. Specify "fallback" as one of these additional
members, and update Appendix C.1 accordingly).
Discuss extensibility a bit.
D.4. Changes since draft-reschke-http-oob-encoding-03
Mention "Content Stealing" thread.
Mention padding.
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D.5. Changes since draft-reschke-http-oob-encoding-04
Reduce information leakage by disallowing ambient authority
information being sent to the secondary resource. Require "Origin"
to be included in request to secondary resource, and require
secondary server to check it.
Mention "Origin" + server check on secondary resource as defense to
content stealing.
Update ENCRYPTENC reference, add SCD reference.
Mention fragmentation feature.
Discuss relation with range requests.
D.6. Changes since draft-reschke-http-oob-encoding-05
Remove redundant Cache-Control: private from one example response
(the response payload is encrypted anyway).
Mention looping.
Remove 'metadata' payload element.
Align with changes in ENCRYPTENC spec.
Fix incorrect statement about what kind of cookies/credentials can be
used in the request to the secondary resource.
Rename "URIs" to "sr" ("secondary resources") and treat the fallback
URI like a regular secondary resource.
Mention reporting protocol ideas.
D.7. Changes since draft-reschke-http-oob-encoding-06
Changed the link relation name to the fallback resource from
"primary" to "fallback". Added link relation for reporting TLS
handshake failures.
Added an example about the interaction with 'gzip' coding.
Update ENCRYPTENC, MICE, and SCD references.
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D.8. Changes since draft-reschke-http-oob-encoding-07
Restrict the valid media types for the response of the secondary
server to "application/oob-stream".
Changed JSON format to allow annotation (optional flags) and entirely
new types of entries.
D.9. Changes since draft-reschke-http-oob-encoding-08
Moved error reporting into appendix (because it's optional and we're
not sure about the utility of it). See
.
Updated references for ENCRYPTENC, MICE, and SCD.
Mention that we could suppress certain request header fields in the
request to the secondary server.
D.10. Changes since draft-reschke-http-oob-encoding-09
Updated reference for ENCRYPTENC. Added RMAP reference. Use all-
lowercase PURLs and remove "/net" in them.
D.11. Changes since draft-reschke-http-oob-encoding-10
Updated reference for ENCRYPTENC: instead of using Crypto-Key
response header field move the key material into the OOB payload.
D.12. Changes since draft-reschke-http-oob-encoding-11
Removed note about registration of purl URIs for link relations.
Updated reference for ENCRYPTENC (now RFC 8188).
Acknowledgements
Thanks to Christer Holmberg, Daniel Lindstrom, Erik Nygren, Goran
Eriksson, John Mattsson, Kevin Smith, Magnus Westerlund, Mark
Nottingham, Martin Thomson, and Roland Zink for feedback on this
document.
Authors' Addresses
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Julian F. Reschke
greenbytes GmbH
Hafenweg 16
Muenster, NW 48155
Germany
EMail: julian.reschke@greenbytes.de
URI: http://greenbytes.de/tech/webdav/
Salvatore Loreto
Ericsson
Torshamnsgatan 21
Stochholm 16483
Sweden
EMail: salvatore.loreto@ericsson.com
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