Internet DRAFT - draft-ruellan-http-accept-push-policy
draft-ruellan-http-accept-push-policy
Httpbis H. Ruellan
Internet-Draft Y. Fablet
Intended status: Experimental R. Bellessort
Expires: October 15, 2016 F. Denoual
F. Maze
Canon CRF
April 13, 2016
Accept-Push-Policy Header Field
draft-ruellan-http-accept-push-policy-01
Abstract
The "Accept-Push-Policy" and "Push-Policy" header fields enable a
client and a server to negotiate the behaviour of the server
regarding the usage of push on a per-request basis.
Status of This Memo
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2
1.1. Terminology . . . . . . . . . . . . . . . . . . . . . . . 3
2. Push Control Use Cases . . . . . . . . . . . . . . . . . . . 3
2.1. Adapting Push Behaviour . . . . . . . . . . . . . . . . . 3
2.2. Load Balancer . . . . . . . . . . . . . . . . . . . . . . 4
2.3. MPEG-DASH Fast Start . . . . . . . . . . . . . . . . . . 4
2.4. Fast Page Load . . . . . . . . . . . . . . . . . . . . . 5
2.5. Use Cases Requirements . . . . . . . . . . . . . . . . . 5
3. Push Policy . . . . . . . . . . . . . . . . . . . . . . . . . 5
3.1. The Accept-Push-Policy Header Field . . . . . . . . . . . 6
3.2. Push-Policy Header Field . . . . . . . . . . . . . . . . 6
3.3. Push Policy Values . . . . . . . . . . . . . . . . . . . 7
3.3.1. None Push Policy . . . . . . . . . . . . . . . . . . 7
3.3.2. Head Push Policy . . . . . . . . . . . . . . . . . . 7
3.3.3. Default Push Policy . . . . . . . . . . . . . . . . . 8
3.3.4. Fast-Load Push Policy . . . . . . . . . . . . . . . . 8
4. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 9
5. Security Considerations . . . . . . . . . . . . . . . . . . . 9
6. References . . . . . . . . . . . . . . . . . . . . . . . . . 9
6.1. Normative References . . . . . . . . . . . . . . . . . . 9
6.2. Informative References . . . . . . . . . . . . . . . . . 9
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 10
1. Introduction
HTTP/2 [RFC7540], the new version of the HTTP protocol, not only
provides significant improvements compared to HTTP/1.1 (see [RFC7230]
and [RFC7231]), but also provides several new features. Among these
is Server Push, which enables a server to send responses to a client
without having received the corresponding requests.
The range of possibilities offered by Server Push is a new domain
wide open for experimentation. A first usage was foreseen early in
the addition of this feature into HTTP/2, which is to replace the
inlining of sub-resources inside a main resource, by pushing these
sub-resources in response to the request for the main resource. As
described in [HighPerformance], with HTTP/1.1, a web designer may
want to optimize the page load time by packing a whole web page into
a single HTTP response. This can be achieved by inlining the CSS,
JavaScript, and images inside the HTML document. By removing the
need for the client to send requests for these sub-resources, this
inlining technique can reduce the page load time by roughly a RTT.
With HTTP/2, the same results can be obtained by pushing the sub-
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resources instead of inlining them. Using push has the advantage of
keeping each sub-resource independent.
HTTP/2 provides a few ways of controlling Server Push from the client
side. First, the SETTINGS parameter "SETTINGS_ENABLE_PUSH" allows a
client to globally enable or disable push on a HTTP/2 connection. In
addition, HTTP/2 Flow Control can be used to limit the bandwidth used
by pushed resources.
These options provide only a coarse control of the usage of Server
Push from the client side. In some cases, a more fine-grained
control would be useful. This document describes several use cases
where controlling Server Push would be useful for the client. It
then proposes new header fields for realizing this control.
1.1. Terminology
In this document, the key words "MUST", "MUST NOT", "REQUIRED",
"SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY",
and "OPTIONAL" are to be interpreted as described in BCP 14, RFC 2119
[RFC2119] and indicate requirement levels for compliant
implementations.
This document uses the Augmented BNF defined in [RFC5234].
2. Push Control Use Cases
2.1. Adapting Push Behaviour
A browser may want to ask the server to adapt its behaviour for
pushing resources depending on the user's actions. For example,
after navigating through a site for some time, the browser may have
many sub-resources in its cache and may prefer that the server stop's
pushing sub-resources to prevent wasting bandwidth. This could be
further optimized with the browser asking the server to push only
response metadata (i.e., the responses pushed by the server
correspond to requests made with the HEAD method instead of requests
made with the GET method). By receiving in advance the list of sub-
resources corresponding to a specific request, the browser would be
able to fetch early on any sub-resource missing from its cache.
As another example, when a user opens many pages on the same site,
the browser may want to receive pushed sub-resources only for the
foreground tab and not for any background tab. This results in a
better optimization of the page load time for the tab that is visible
to the user.
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2.2. Load Balancer
A second use case is a load balancer serving both HTTP/1.1 and HTTP/2
clients, and using HTTP/2 to connect to the back-end servers, as
described in [LoadBalancer].
The load balancer uses the same HTTP/2 connection towards a back-end
server to forward the requests received from several clients. When
the client is a HTTP/1.1 client, the load balancer doesn't want the
back-end server to push any resource in response to the client's
request. On the contrary, when the client is a HTTP/2 client, the
load balancer would like the back-end server to push sub-resources
associated to the client's request.
The load balancer would like to be able to enable or disable push on
a per-request basis. This would enable it to optimize the server
behaviour depending on the client's capacity.
2.3. MPEG-DASH Fast Start
Controlling the server behaviour regarding push may also be useful
for specific applications. As an example, MPEG-DASH [DASH] is a
technology for streaming media content over HTTP. The media content
is split into small file-based segments that can be retrieved through
HTTP requests. Potentially, the media content is made available with
different quality levels. A media presentation description (MPD)
describes the organization of the media.
To render a media, an MPEG-DASH client needs to first download the
MPD, process it, and then request the necessary media segments. When
requesting a MPD to play the associated media content, it would be
useful for a DASH client to be able to ask the server to push some
initial content (for example, the initialization segments, and
possibly the first content segments).
However, there are also cases when it is not useful for the DASH
client to receive in advance this initial content. For example, in a
video program guide, the DASH client may want to download several
MPDs corresponding to different media content, but doesn't want to
receive the initial content for all of these. Therefore, it is
useful for the DASH client to be able to specify in a request for a
MPD whether it wants the server to push some initial content.
In addition, when the DASH client asks the server to push some
initial content, it could be useful for it to have some feedback from
the server. This feedback would indicate whether the server is
intending to push this initial content. The client could adapt its
behaviour depending on this indication. For example, the client
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could start rendering the media sooner if it knows that the server is
pushing the initial content.
2.4. Fast Page Load
The previous use case can be expanded to the more generic use case of
downloading quickly a web page. As described in
[Breaking1000msBarrier], it is important for the user perception to
keep the perceived latency of loading a web page under 1000 ms. This
can be difficult when using a mobile connection with a high latency.
Part of the solution proposed in [Breaking1000msBarrier] for HTTP/1.1
is to inline all the sub-resources necessary for achieving a first
rendering of the web page. With HTTP/2, the inlining of these sub-
resources can be replaced by having the server push them.
Therefore, a client detecting that it is using a high-latency network
could improve the user perceived latency by asking the server to push
all the sub-resources necessary for a first display of a web page.
2.5. Use Cases Requirements
The analysis of these use cases enables to build a list of
requirements for defining a fine-grained control over the usage of
push by a server.
o The client can ask the server not to push any resource in response
to a request.
o The client can ask the server to only push response metadata.
o The client can ask the server to use an application-defined
behaviour regarding push.
o The server can indicate to the client its behaviour regarding push
when processing a request.
3. Push Policy
A _push policy_ defines the behaviour of a HTTP server regarding push
when processing a request. Different push policies can be used when
processing different requests.
This section defines new HTTP header fields enabling a client and a
server to negotiate the push policy used by the server to process a
given request.
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The new "Accept-Push-Policy" header field enables a client to express
its expectations regarding the server's push policy for processing a
request.
The "Push-Policy" header field enables a server to indicate which
push policy it selected for processing a request.
3.1. The Accept-Push-Policy Header Field
A client can express the desired push policy for a request by sending
an "Accept-Push-Policy" header field in the request.
Accept-Push-Policy = token ; a push policy name
The header field value contains the push policy that the client
expects the server to use when processing the request.
Possibly, the "Accept-Push-Policy" header field could be extended to
support carrying multiple policies, as a comma-separated list of
tokens. The server could choose its preferred policy among those
proposed by the client.
3.2. Push-Policy Header Field
A server can indicate to a client the push policy it used when
processing a request by sending a "Push-Policy" header field in the
corresponding response.
Push-Policy = token ; a push policy name
The server MUST follow the indicated push policy when processing the
client request associated to the response.
The "Push-Policy" header field can be used as an acknowledgement from
the server after receiving a request containing the "Accept-Push-
Policy" header field.
If the "Accept-Push-Policy" header field can contain a list of push
policy names, the "Push-Policy" header field can be used to express
which push policy was selected by the server.
The server can also choose a push policy not corresponding to the
client's expectation as expressed in the "Accept-Push-Policy" header,
and specify the selected push policy in the "Push-Policy" header
field.
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3.3. Push Policy Values
This section defines some generic push policies. Other push policies
can be standardized for either a generic usage, or for an
application-specific usage. In addition, private push policies can
be used by a web application.
TBD: select the form of private push policies (URN, "X-" values...).
3.3.1. None Push Policy
The "None" push policy value indicates that no resource is pushed
when processing a request.
None-Push-Policy = "none" ; 'None' push policy token
For example, a browser sending a request for a background tab could
ask the server not to push any resources in response to this request
by sending an "Accept-Push-Policy" header with the "None" value.
This would result in the following HTTP/2 header block:
:method = GET
:scheme = https
:path = /index.html
host = example.org
accept = text/html
accept-push-policy = none
3.3.2. Head Push Policy
The "Head" push policy value indicates that only response metadata
are pushed (the server is pushing responses corresponding to requests
made with the HEAD method).
Head-Push-Policy = "head" ; 'Head' push policy token
For example, a browser may already have many resources from a web
site in its cache. It could ask the server to push only response
metadata. This would allow the browser to know early on the
resources useful for rendering a web page (i.e., before receiving and
parsing the HTML document), without taking the risk of wasting
bandwidth with resources already in its cache. In this example, the
browser's request would contain the following HTTP/2 header block:
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:method = GET
:scheme = https
:path = /index.html
host = example.org
accept = text/html
accept-push-policy = head
3.3.3. Default Push Policy
The "Default" push policy value indicates that the server is using
its default behaviour for pushing resources when processing a
request.
Default-Push-Policy = "default" ; 'Default' push policy token
For example, a server not fulfilling a client's expectation regarding
the push policy could indicate this with the "Default" push policy.
It would send the following HTTP/2 header block in its response:
:status 200
push-policy = default
3.3.4. Fast-Load Push Policy
The "Fast-Load" push policy value indicates that the sub-resources
necessary for a first rendering of a main resource are pushed
alongside the response containing this main resource.
Fast-Load-Push-Policy = "fast-load" ; 'Fast-Load' push policy token
A server using the "Fast-Load" push policy while processing a request
can push sub-resources not necessary for a first rendering, but
SHOULD prioritize sub-resources necessary for this first rendering.
For example, a client detecting that it is using a high-latency
network can try to improve the user perceived latency by asking the
server to push the sub-resources necessary for a first rendering of a
main page by including an "Accept-Push-Policy" header with the "Fast-
Load" value. This would result in the following HTTP/2 header block:
:method = GET
:scheme = https
:path = /index.html
host = example.org
accept = text/html
accept-push-policy = fast-load
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4. IANA Considerations
TBD
5. Security Considerations
TBD
6. References
6.1. 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>.
[RFC5234] Crocker, D., Ed. and P. Overell, "Augmented BNF for Syntax
Specifications: ABNF", STD 68, RFC 5234, DOI 10.17487/
RFC5234, January 2008,
<http://www.rfc-editor.org/info/rfc5234>.
[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>.
6.2. Informative References
[Breaking1000msBarrier]
Grigorik, I., "Breaking the 1000 ms mobile barrier",
November 2013.
[DASH] "Dynamic adaptive streaming over HTTP (DASH)", ISO/IEC:
23009-1:2014 , 2014.
[HighPerformance]
Grigorik, I., "High Performance Browser Networking",
September 2013.
[LoadBalancer]
Douglas, S., "PUSH_PROMISE and load balancers", September
2014, <https://lists.w3.org/Archives/Public/ietf-http-
wg/2014JulSep/2374.html>.
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[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>.
[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,
<http://www.rfc-editor.org/info/rfc7231>.
Authors' Addresses
Herve Ruellan
Canon CRF
Email: herve.ruellan@crf.canon.fr
Youenn Fablet
Canon CRF
Email: youenn.fablet@crf.canon.fr
Romain Bellessort
Canon CRF
Email: romain.bellessort@crf.canon.fr
Franck Denoual
Canon CRF
Email: franck.denoual@crf.canon.fr
Frederic Maze
Canon CRF
Email: frederic.maze@crf.canon.fr
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