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Applications and Real-Time HTTP content coding ouf-of-band 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. 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, which may be joined by sending a message with subject “subscribe” to ietf-http-wg-request@w3.org. 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 .

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. 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 .

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 . This document reuses terminology used in the base HTTP specifications, namely and .

The ‘Out-Of-Band’ content coding is used to direct the recipient to retrieve the actual message representation () 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 () |———————————————————\ status 200 and Content-Coding: out-of-band | () <———————————————————/ GET to secondary server () |—————————\ payload | () <—————————/ () Client and combines payload received in () with metadata received in ().

The name of the content coding is “out-of-band”. The payload format uses JavaScript Object Notation (JSON, ), describing an object describing secondary resources plus &OPTIONAL; additional metadata:

‘URIs’

A &REQUIRED; string array containing at least one URI reference () of a secondary resource.

‘fallback’

An &OPTIONAL; string containing a URI reference of a fallback resource (see ). This URI reference, after resolution against the URI of the primary resource, &MUST; identify a resource on the same server as the primary resource.

‘metadata’

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 ; 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. 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. 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 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 (). 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 and for details.

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 () 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:

Used in case the server was not reachable.

Link relation: http://purl.org/NET/linkrel/not-reachable

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

Link relation: http://purl.org/NET/linkrel/resource-not-found

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

Client request of primary resource: 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: Vary: Accept-Encoding { “URIs”: [ “http://example.net/bae27c36-fa6a-11e4-ae5d-00059a3c7a00” ], “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 Host: example.net

Response: HTTP/1.1 200 OK Date: Thu, 14 May 2015 18:52:10 GMT Cache-Control: private Content-Length: 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: Cache-Control: max-age=10, public Content-Type: text/plain Hello, world.

Given the example HTTP message from , 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: aesgcm128, out-of-band Content-Type: text/plain Encryption: keyid=”a1″; salt=”vr0o6Uq3w_KDWeatc27mUg” Crypto-Key: keyid=”a1″; aesgcm128=”csPJEXBYA5U-Tal9EdJi-w” Content-Length: Vary: Accept-Encoding { “URIs”: [ “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)

Response for secondary resource: HTTP/1.1 200 OK Date: Thu, 14 May 2015 18:52:10 GMT Content-Length: … Cache-Control: private fuag8ThIRIazSHKUqJ5OduR75UgEUuM76J8UFwadEvg (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: Content-Type: text/plain I am the walrus

Note: in this case, the ability to undo the “aescgm128” is needed to process the response. If “aescgm128” wasn’t listed as acceptable content encoding in the request, the origin server wouldn’t be able to use the “out-of-band” mechanism.

Client requests primary resource as in , 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: 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: <http://example.net/bae27c36-fa6a-11e4-ae5d-00059a3c7a00>; rel=”http://purl.org/NET/linkrel/resource-not-found”

New content codings can be deployed easily, as the client can use the “Accept-Encoding” header field () to signal which content codings are supported.

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 and ).

The Out-Of-Band content coding could be used to circumvent the same-origin policy () 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 out-of-band codings, would be handled as if it originated from the origin’s resource. This problem is not yet addressed by this specification. Possible defenses would be to rely on signatures and encryption, or to add an indication to the secondary resource’s response that would prevent further processing in responses from “bad” origins (not unlike the “Access-Control-Allow-Origin” header field defined in ).

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).

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:

of this document

Latest version available at .

A plausible alternative approach would be to implement this functionality one level up, using a new redirect status code (). However, this would have several drawbacks:

• 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.
• In redirect messages, representation metadata (), namely “Content-Type”, applies to the response message, not the redirected-to resource.
• 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 and refined for use in the Session Initiation Protocol (SIP) in . This approach though would share most of the drawbacks of the status code approach mentioned above.

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?

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: <http://example.com/c/bae27c36-fa6a-11e4-ae5d-00059a3c7a00>; rel=”http://purl.org/NET/linkrel/primary-resource” (continuing the example from )

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.

It might be a good idea to allow padding in the secondary resource’s payload, in order to even hide the precise content length. This could be accomplished by adding range information to the out-of-band metadata, allowing the client to throw away parts of the payload when reconstructing the response body.

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 ().

Updated ENCRYPTENC reference.

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 accordingly). Discuss extensibility a bit.

Mention “Content Stealing” thread. Mention padding.

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