Internet DRAFT - draft-ietf-http-negotiation-scenarios
draft-ietf-http-negotiation-scenarios
INTERNET-DRAFT Edward Hardie
Expires: December, 1997 NASA NIC
<draft-ietf-http-negotiate-scenario-00.txt>
Scenarios for the Delivery of Negotiated Content using HTTP
1. Status of this Memo
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2. Abstract
This document describes various problems which have arisen in attempts
to deliver negotiated content using HTTP and examines several
scenarios by which improvements in delivery might be accomplished.
This document does not discuss how HTTP might be used to negotiate the
use of other protocols to deliver content.
3. Problems with content negotiation in HTTP
Content providers on the World Wide Web would like to present their
material in the form most appropriate to each individual user. In
order to do this effectively, the content provider and content user
must be able negotiate among variants when more than one
representation of a resource is available.
The Hypertext Transport Protocol has since version 1.0 [1] used
certain request headers to indicate a list of the responses preferred
by the user agent making the request. The Accept request header is
used to indicate media types, Accept-Charset a list of preferred
character sets, Accept-Encoding supported content encodings, and
Accept-Language the set of natural languages preferred. In theory, a
server receiving these headers can use them to determine which of the
available variants of a particular item is most appropriate to the
requesting user agent.
Problems have emerged with this approach. First, the explosion of
content types in use on the World Wide Web has meant that Accept
headers would have to be extraordinarily long in order to adequately
state the user's Accept preferences. Increasing the length of the
Accept headers in order to negotiate consumes bandwidth for every
request, whether the resource has variants or not. Increasing their
complexity in order to account for all of the possible axes of
variation can also consume excessive amounts of server processing
power. Second, even if using a full list of Accept preferences did
not present resource problems, it could create privacy problems. A
full list of Accept preferences creates a user profile; in some
situations it could be combined with other information to track a
user's interaction with a server or set of servers even if the user
has taken steps to eliminate that possiblity.
Many content providers have attempted to circumvent the problems
associated with negotiation based on Accept headers. The most popular
method has been to examine the User-Agent header and to create content
tailored to the typical capabilities of that user agent. Because many
web user agents are both configurable and extensible, this method has
serious drawbacks. A user agent capable of using graphics but
configured to display only text may be presented with completely
unusable content. Conversely, a user agent which has been extended to
handle new media types may never be presented with them. The
usefulness of this method has been further degraded by some client
authors, who have chosen to use the User-Agent strings associated
with other clients in order to indicate compatibility; commonly, this
has been done even when the two clients are not, in fact, fully
compatible.
Content negotiation based on User-agent strings also creates
difficulties for caching proxies, as they have no way to determine
when the form of content delivered to two different user-agents would
be the same. The delivery of inappropriate content can be prevented
with the appropriate use of expiration times and cache control
directives, but only at the cost of serious losses in cache
efficiency.
4. Requirements for effective content negotiation
Content negotiation methods should improve the likelihood that the
best available resource variant is served to each user. If no
available resource variant is appropriate for a particular user, they
should furnish a method for the content provider to indicate that
lack.
Content negotiation methods should also accomplish their task without
substantially increasing the bandwidth required to deliver the
negotiated variant over that required to deliver similar invariant
content. Some increase must be expected, but the smallest possible
increase is preferred, and any overhead should be limited to those
resources for which variants are available. As a rule of thumb, any
method that increases bandwidth requirements by more than 50% is
unacceptable. For example, a method that delivered all available
variants to a client and expected the client to discard those that
were not used might be very successful in providing the best available
resource and in limiting overhead to those resources for which
variants existed; it would, nonetheless, be unacceptable for
deployment on the global Internet because of the wastage associated
with each delivery.
In addition, content negotiation methods should interact well with
caching proxies. They must prevent the delivery of inappropriate
content to proxy users. If possible, they should allow caches to
store sufficient information to participate in the negotiation
process.
5. Scenarios
5.1 Improvements to the current system
Content negotiation based on Accept headers or User-Agent strings is
undertaken by the server based on information present in all requests
from clients. The current problems with this method could be
ameliorated in a number of ways. Registration of feature bundles
could, for example, allow a client to indicate its capabilities
without the increase in packet size that individual enumeration of
capabilities requires. The use of registered feature bundles does,
however, require a fair amount of maintenance for both server and
clients, especially given their dynamic extendability.
Content providers could also improve the cachability of the variants
served by issuing redirects to unique URLs specific to each variant;
proxy users with equivalent feature bundles or user agents would then
be able to retrieve the cached variant once they received a redirect.
Some content providers already use this method to good effect.
5.2 Client-based negotiation
Improvements in the current method tend, however, to increase demands
on the processing power of the server, which raises serious problems
of scalability. Shifting all or part of the processing involved in
the negotiation to the client may improve scalability and shifts the
locus of negotiation to the party most aware of both the current and
potential possibilities for presentation to the user.
Client-based content negotiation can take two basic forms: elective
negotiation and "active content" negotiation.
5.2.1. Elective negotiation
Elective negotiation relies on the idea that the first response to a
request for a resource which has variants should be a statement naming
the choices and/or the axes along which they vary. The simplest form
of elective negotiation occurs when a content provider creates a
meta-resource with links to several variants and explanations of the
features needed to present each variant. This basic form presumes a
least common denominator for presentation of the links and an active
user selecting among the presentations. "Transport Content
Negotiation in HTTP" [2] provides for a more complex version of
elective negotiation. It describes a standard method for delineating
the axes along which a resource varies and a set of methods by which
caches can participate in the negotiation process.
Elective negotiation limits the process of selecting among variants to
the axes along which they vary. This eliminates the need for clients'
enumerating their capabilities and user preferences. It also provides
both a bandwidth savings and an increase in privacy. Many times,
however, this process requires a user to actively select among the
resources provided, which reduces perceived efficiency and increases
perceived latency.
5.2.2 "Active content" negotiation
"Active content" negotiation takes place when the request for a
resource which has variants returns content for execution in the
client context rather than for presentation to the user. This active
content determines the capabilities of the user agent and the
preferences of the user and then retrieves the best available variant
or provides an error status if no variants are appropriate. This
method has several advantages: the negotiation takes place without the
need for user intervention; the processing for selection of variants
is moved completely off the server; and the active content itself
should be cachable.
There are, however, several limitations to active content negotiation.
The most severe problem is that there is not yet a cross-platform
standard for active content. Without such a standard, servers may
have to engage in Accept header or User-Agent negotiation prior to
using active content in order to identify the correct form of active
content to send in response to a particular request. Some user agents
will also be unable to use active content, either because they have
limited processing power or because of security considerations (see
Section 7). For these user agents, a completely different method must
be provided. As with server-side negotiation, each variant will
require a unique URL to ensure cache correctness.
6. Summary
Content negotiation using HTTP currently presents problems in
accuracy, scalability, and privacy. Amelioration of the problems in
the current design is possible, but other content negotiation methods
may provide significant improvements. The type of content negotiation
most appropriate to a specific context will depend on the type of
variable content being served and the capabilities of the parties to
the negotiation. Before deployment on the global Internet, however,
any client-based content negotiation method MUST meet the following
tests:
1) It MUST provide at least as good a content negotiation facility as
the current Accept header and User-Agent methods, and it SHOULD
increase the likelihood that the best available variant is served to
each client.
2) It MUST NOT significantly increase the bandwidth required to
deliver the content.
3) It MUST NOT allow inappropriate content to be delivered to the
users of caching proxies. It SHOULD reduce overall bandwidth
requirements by providing cachable content and allowing proxies to
participate in the negotiation process.
Experimentation with various content negotiation methods may be
necessary to determine their usefulness for different types of
resources and contexts. Interoperability will be best maintained,
however, if a single method of each type is made standard as soon as
experience permits.
7. Security Considerations
"Active content" negotiation presumes that clients will execute code
provided by servers and that those clients will allow that code to
examine user preferences and user agent capabilities in order to
select among variants. Any such system would have to be tightly
bounded to prevent the active content from executing arbitrary code on
the client. The examination of user preferences and user agent
capabilities also presents a possible privacy problem, especially as
the selection of a particular variant provides an opportunity for the
active content to communicate its discoveries.
8. References
[1] Tim Berners-Lee, Roy Fielding, Henrik Frystk. Hypertext
Transfer Protocol -- HTTp/1.0. RFC RFC1945. May 1996.
[2] Koen Holtman and Andrew Mutz. Transport Content Negotiation in
HTTP. Internet-Draft draft-ietf-http-negotiation-02.txt, HTTP Working
Group, May 26, 1997.
9. Author's Address
Edward Hardie
NASA Network Information Center
MS 204-14
Moffett Field, CA 94035-1000 USA
+1 415 604 0134
hardie@nasa.gov
