Network Working GroupJ. Reschke
Intended status: Standards TrackS. Loreto
Expires: August 22, 2016Ericsson
February 19, 2016

'Out-Of-Band' Content Coding for HTTP


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, which may be joined by sending a message with subject "subscribe" to

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 C.3.

Status of This Memo

This Internet-Draft is submitted in full conformance with the provisions of BCP 78 and BCP 79.

Internet-Drafts are working documents of the Internet Engineering Task Force (IETF). Note that other groups may also distribute working documents as Internet-Drafts. The list of current Internet-Drafts is at

Internet-Drafts are draft documents valid for a maximum of six months and may be updated, replaced, or obsoleted by other documents at any time. It is inappropriate to use Internet-Drafts as reference material or to cite them other than as “work in progress”.

This Internet-Draft will expire on August 22, 2016.

Copyright Notice

Copyright (c) 2016 IETF Trust and the persons identified as the document authors. All rights reserved.

This document is subject to BCP 78 and the IETF Trust's Legal Provisions Relating to IETF Documents ( in effect on the date of publication of this document. Please review these documents carefully, as they describe your rights and restrictions with respect to this document. Code Components extracted from this document must include Simplified BSD License text as described in Section 4.e of the Trust Legal Provisions and are provided without warranty as described in the Simplified BSD License.

1. Introduction

This document describes an Hypertext Transfer Protocol (HTTP) content coding (Section 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 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) <---------------------------/

   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 plus OPTIONAL additional metadata:

A REQUIRED string array containing at least one URI reference (Section 4.1 of [RFC3986]) of a secondary resource.
An OPTIONAL string containing a URI reference of a fallback resource (see Appendix B.2). This URI reference, after resolution against the URI of the primary resource, MUST identify a resource on the same server as the primary resource.
An OPTIONAL object containing additional members, representing header field values 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 URIs, it is free to choose which of these to use.

New specifications can define new OPTIONAL header fields, thus clients MUST ignore unknown fields. Extension specifications will have to update this specification. [rfc.comment.1: or we define a registry]

The client then obtains 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.
  3. Replacing/setting any header fields with those present as members in the "metadata" object. [rfc.comment.2: 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), 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 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:

3.3.1. Server Not Reachable

Used in case the server was not reachable.

Link relation:

3.3.2. Resource Not Found

Used in case the server responded, but the object could not be obtained.

Link relation:

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:

3.4. Examples

3.4.1. Basic Example

Client request of primary resource:

GET /test HTTP/1.1
Accept-Encoding: gzip, out-of-band


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: 145
Vary: Accept-Encoding

  "URIs": [
  "fallback": "/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


HTTP/1.1 200 OK
Date: Thu, 14 May 2015 18:52:10 GMT
Cache-Control: private
Content-Length: 15

Hello, world.

(Note no Content-Type header field is present here because the secondary server truly does not know the media type of the payload)

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 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:


HTTP/1.1 200 OK
Date: Thu, 14 May 2015 18:52:00 GMT
Content-Encoding: aesgcm128, out-of-band
Content-Type: text/plain
Encryption: keyid="a1"; salt="vr0o6Uq3w_KDWeatc27mUg"
Crypto-Key: keyid="a1"; aesgcm128="csPJEXBYA5U-Tal9EdJi-w"
Content-Length: 87
Vary: Accept-Encoding

  "URIs": [

(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-Length: ...
Cache-Control: private


(payload body shown in base64 here)

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

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.


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
Accept-Encoding: gzip, out-of-band
Link: <>;

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] and [MICE]).

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:

Payload needs to be retrieved from a secondary resource
Section 3 of this document

7. References

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, <>.
Berners-Lee, T., Fielding, R., and L. Masinter, “Uniform Resource Identifier (URI): Generic Syntax”, STD 66, RFC 3986, DOI 10.17487/RFC3986, January 2005, <>.
Nottingham, M., “Web Linking”, RFC 5988, DOI 10.17487/RFC5988, October 2010, <>.
Bray, T., “The JavaScript Object Notation (JSON) Data Interchange Format”, RFC 7159, DOI 10.17487/RFC7159, March 2014, <>.
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, <>.
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

Thomson, M., “Content-Signature Header Field for HTTP”, Internet-Draft draft-thomson-http-content-signature-00 (work in progress), July 2015.
Thomson, M., “Encrypted Content-Encoding for HTTP”, Internet-Draft draft-ietf-httpbis-encryption-encoding-00 (work in progress), December 2015.
Thomson, M., “Merkle Integrity Content Encoding”, Internet-Draft draft-thomson-http-mice-00 (work in progress), January 2016.
Freed, N. and K. Moore, “Definition of the URL MIME External-Body Access-Type”, RFC 2017, DOI 10.17487/RFC2017, October 1996, <>.
Burger, E., “A Mechanism for Content Indirection in Session Initiation Protocol (SIP) Messages”, RFC 4483, DOI 10.17487/RFC4483, May 2006, <>.
Barth, A., “The Web Origin Concept”, RFC 6454, DOI 10.17487/RFC6454, December 2011, <>.
Fielding, R., Ed. and J. Reschke, Ed., “Hypertext Transfer Protocol (HTTP/1.1): Conditional Requests”, RFC 7232, DOI 10.17487/RFC7232, June 2014, <>.

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:

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 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
Link: <>;

(continuing the example from Section 3.4.1)

B.3. Resource maps

When out-of-band encoding 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:

  • as extension in the out-of-band encoding JSON payload, or
  • as separate resource identified by a "Link" response header field.

This specification does not define a format, nor a mechanism to transport the map, but it's a given that some specification using "out-of-band" encoding will do.

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.

C.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 B.2 accordingly).

Discuss extensibility a bit.


Thanks to Christer Holmberg, Daniel Lindstrom, Goran Eriksson, John Mattsson, Kevin Smith, Mark Nottingham, Martin Thomson, and Roland Zink for feedback on this document.

Authors' Addresses

Julian F. Reschke
greenbytes GmbH
Hafenweg 16
Muenster, NW 48155
Salvatore Loreto
Torshamnsgatan 21
Stochholm, 16483