Network Working Group J. Reschke
Internet-Draft greenbytes
Intended status: Standards Track S. Loreto
Expires: July 3, 2016 Ericsson
December 31, 2015
'Out-Of-Band' Content Coding for HTTP
draft-reschke-http-oob-encoding-02
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
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XML versions, latest edits, and issue tracking for this document are
available from and
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The changes in this draft are summarized in Appendix C.2.
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This Internet-Draft will expire on July 3, 2016.
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 4
2. Notational Conventions . . . . . . . . . . . . . . . . . . . . 4
3. 'Out-Of-Band' Content Coding . . . . . . . . . . . . . . . . . 4
3.1. Overview . . . . . . . . . . . . . . . . . . . . . . . . . 4
3.2. Definitions . . . . . . . . . . . . . . . . . . . . . . . 5
3.3. Problem Reporting . . . . . . . . . . . . . . . . . . . . 6
3.3.1. Server Not Reachable . . . . . . . . . . . . . . . . . 7
3.3.2. Resource Not Found . . . . . . . . . . . . . . . . . . 7
3.3.3. Payload Unusable . . . . . . . . . . . . . . . . . . . 7
3.4. Examples . . . . . . . . . . . . . . . . . . . . . . . . . 7
3.4.1. Basic Example . . . . . . . . . . . . . . . . . . . . 7
3.4.2. Example involving an encrypted resource . . . . . . . 9
3.4.3. Example For Problem Reporting . . . . . . . . . . . . 10
4. Feature Discovery . . . . . . . . . . . . . . . . . . . . . . 11
5. Security Considerations . . . . . . . . . . . . . . . . . . . 11
5.1. Content Modifications . . . . . . . . . . . . . . . . . . 11
5.2. Use in Requests . . . . . . . . . . . . . . . . . . . . . 11
6. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 11
7. References . . . . . . . . . . . . . . . . . . . . . . . . . . 11
7.1. Normative References . . . . . . . . . . . . . . . . . . . 11
7.2. Informative References . . . . . . . . . . . . . . . . . . 12
Appendix A. Alternatives, or: why not a new Status Code? . . . . 13
Appendix B. Open Issues . . . . . . . . . . . . . . . . . . . . . 13
B.1. Range Requests . . . . . . . . . . . . . . . . . . . . . . 13
B.2. Accessing the Secondary Resource Too Early . . . . . . . . 13
Appendix C. Change Log (to be removed by RFC Editor before
publication) . . . . . . . . . . . . . . . . . . . . 14
C.1. Changes since draft-reschke-http-oob-encoding-00 . . . . . 14
C.2. Changes since draft-reschke-http-oob-encoding-01 . . . . . 14
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Appendix D. Acknowledgements . . . . . . . . . . . . . . . . . . 14
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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 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, leveraging content
encryption, as described in [ENCRYPTENC].
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].
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 GET 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 wrapped HTTP message
5. Client unwraps that representation (obtaining a full HTTP
message)
6. Client combines above representation with additional
representation metadata obtained from the primary resource
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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) |---------------------------\
wrapped HTTP message |
(4) <---------------------------/
(5, 6)
Client and combines HTTP message 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 array of objects describing secondary resources, each
containing some of the members below:
'URI' A REQUIRED string containing the URI reference (Section 4.1 of
[RFC3986]) of the secondary resource.
'metadata' An OPTIONAL object containing additional members,
representing header field values to be recombined with the
metadata from the secondary resource and which can not appear as
header fields in the response message itself (header fields that
occur multiple times need to be combined into a single field value
as per Section 3.2.2 of [RFC7230]; header field names are lower-
cased).
The payload format uses a JSON array so that the origin server can
specify multiple secondary resources. When a client receives a
response containing multiple entries, it is free to choose which of
these to use.
The representation of the secondary resource needs to use a media
type capable of representing a full HTTP message. For now the only
supported type is "application/http" (Section 8.3.2 of [RFC7230]).
The client then obtains the original message by:
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1. Unwrapping the encapsulated HTTP message by removing any transfer
and content codings.
The latter might require additional metadata that could be
present in the "metadata" object, such as the "Crypto-Key" header
field described in Section 4 of [ENCRYPTENC].
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.
3. Replacing/setting any header fields with those present as members
in the "metadata" object. [[anchor3: Do we have a use case for
this?]]
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), 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 Section 3.3 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.
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.
3.3. Problem Reporting
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:
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3.3.1. Server Not Reachable
Used in case the server was not reachable.
Link relation:
http://purl.org/NET/linkrel/not-reachable
3.3.2. Resource Not Found
Used in case the server responded, but the object could not be
obtained.
Link relation:
http://purl.org/NET/linkrel/resource-not-found
3.3.3. Payload Unusable
Used in case the the payload could be obtained, but wasn't usable
(for instance, because integrity checks failed).
Link relation:
http://purl.org/NET/linkrel/payload-unusable
3.4. Examples
3.4.1. Basic Example
Client request of primary resource:
GET /test HTTP/1.1
Host: www.example.com
Accept-Encoding: gzip, out-of-band
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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: 76
Vary: Accept-Encoding
[{
"URI": "http://example.net/bae27c36-fa6a-11e4-ae5d-00059a3c7a00"
}]
(note that the Content-Type header field describes the media type of
the secondary's resource representation)
Client request for secondary resource:
GET /bae27c36-fa6a-11e4-ae5d-00059a3c7a00 HTTP/1.1
Host: example.net
Response:
HTTP/1.1 200 OK
Date: Thu, 14 May 2015 18:52:10 GMT
Content-Type: application/http
Cache-Control: private
Content-Length: 115
HTTP/1.1 200 OK
Date: Thu, 14 May 2015 17:00:00 GMT
Content-Length: 15
Content-Language: en
Hello, world.
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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
Content-Language: en
Hello, world.
In this example, Cache-Control, Content-Length, and Date have been
set/overwritten with data from the primary resource's representation.
3.4.2. Example involving an encrypted resource
Given the example HTTP message from Section 5.4 of [ENCRYPTENC], a
primary resource could use the "out-of-band" encoding to specify just
the location of the secondary resource plus the contents of the
"Crypto-Key" header field needed to decrypt the payload:
Response:
HTTP/1.1 200 OK
Date: Thu, 14 May 2015 18:52:00 GMT
Content-Encoding: out-of-band
Content-Type: text/plain
Content-Length: 194
Vary: Accept-Encoding
[{
"URI": "http://example.net/bae27c36-fa6a-11e4-ae5d-00059a3c7a00"
"metadata": {
"crypto-key": "keyid=\"a1\";
aesgcm128=\"csPJEXBYA5U-Tal9EdJi-w\""
}
}]
(note that the Content-Type header field describes the media type of
the secondary's resource representation)
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Response for secondary resource:
HTTP/1.1 200 OK
Date: Thu, 14 May 2015 18:52:10 GMT
Content-Type: application/http
Content-Length: ...
Cache-Control: private
HTTP/1.1 200 OK
Content-Length: 32
Content-Encoding: aesgcm128
Encryption: keyid="a1"; salt="vr0o6Uq3w_KDWeatc27mUg"
fuag8ThIRIazSHKUqJ5OduR75UgEUuM76J8UFwadEvg
(payload body shown in base64 here)
Final message after recombining header fields:
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
3.4.3. 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/NET/linkrel/resource-not-found"
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4. 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.
5. Security Considerations
5.1. 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]).
5.2. 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).
6. IANA Considerations
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
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,
.
[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,
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.
[RFC5988] Nottingham, M., "Web Linking", RFC 5988, DOI 10.17487/
RFC5988, October 2010,
.
[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,
.
7.2. Informative References
[CONTENTSIG] Thomson, M., "Content-Signature Header Field for HTTP",
draft-thomson-http-content-signature-00 (work in
progress), July 2015.
[ENCRYPTENC] Thomson, M., "Encrypted Content-Encoding for HTTP",
draft-ietf-httpbis-encryption-encoding-00 (work in
progress), December 2015.
[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,
.
[RFC6454] Barth, A., "The Web Origin Concept", RFC 6454,
DOI 10.17487/RFC6454, December 2011,
.
[RFC7232] Fielding, R., Ed. and J. Reschke, Ed., "Hypertext
Transfer Protocol (HTTP/1.1): Conditional Requests",
RFC 7232, DOI 10.17487/RFC7232, June 2014,
.
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URIs
[1]
[2]
Appendix A. 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 woudld 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.
Appendix B. Open Issues
B.1. Range Requests
We probably need to handle Range Requests. How would this work?
Passing down the Range request header field to the secondary
resource?
What about codes other than 200 and 206?
B.2. 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
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content to it).
In this particular case, it can be useful if the client was able to
"piggyback" the URI of 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/NET/linkrel/primary-resource"
(continuing the example from Section 3.4.1)
What's unclear is whether it's ok for the client to reveal the URI if
the primary resource, and under which conditions it's ok for the
secondary server to access it. All it needs is the potentially
encrypted payload, so maybe yet another URI on the origin server is
needed.
Appendix C. Change Log (to be removed by RFC Editor before publication)
C.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 ().
C.2. Changes since draft-reschke-http-oob-encoding-01
Updated ENCRYPTENC reference.
Appendix D. Acknowledgements
Thanks to Christer Holmberg, Daniel Lindstrom, Goran Eriksson, John
Mattsson, Kevin Smith, Mark Nottingham, Martin Thomson, and Roland
Zink for feedback on this document.
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Authors' Addresses
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
Hirsalantie 11
Jorvas 02420
Finland
EMail: salvatore.loreto@ericsson.com
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