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Versions: 00 01 02                                                      
INTERNET-DRAFT                                            Edward Hardie
Expires:  May, 1998                                            NASA NIC

        Scenarios for the Delivery of Negotiated Content

1.  Status of this Memo

This document is an Internet-Draft.  Internet-Drafts are working
documents of the Internet Engineering Task Force (IETF), its areas, and
its working groups.  Note that other groups may also distribute working
documents as Internet-Drafts.

Internet-Drafts are draft documents valid for a maximum of six months
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or to cite them other than as ``work in progress.''

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Distribution of this memo is unlimited.  Please send comments to the

2.  Abstract

A number of Internet application protocols have a need to provide
content negotiation for the resources with which they interact.  While
MIME media types [1] provide a standard method for handling one
major axis of variation, resources also vary in ways which cannot be
expressed using currently available MIME headers.  This paper asserts
that a cross-protocol negotiation framework is needed, both to
leverage work already done for specific protocols and to allow for
content negotiation when resources will pass through several
protocols on their way from provider to recipient.  To justify the
need, this paper puts forward several content negotiation scenarios
and discusses how they affect the requirements for such a framework.

3.  Introduction

When a content provider makes a particular resource available in a
number of different representations, content negotiation may be used
to determine which of the available variants should be provided to a
particular recipient.  This negotiation entails the exchange of
information between the resource provider and the recipient about
their respective capabilities and preferences.  Among the key pieces
of information needed in this exchange are:

1) How the available representations of the resource vary.

2) The preferences of the provider among the available representations.

3) The capabilities and preferences of the recipient.

3.1  Representing variation

The MIME Content-type header can be used to differentiate
representations which differ by media type, and it is already used by
many protocols for that purpose.  Many representations differ,
however, by features which are not reflected in media types; two
representations using the image/gif type may, for example, differ in
color depth.  There are several possible methods for referring to
these features:

a) using a feature tag registry to record specific feature tags which can
be used to describe variants.

b) using URIs to point to descriptions of specific features.

c) using standardized reference documents as pointers to descriptions of
the features.

In each case, the association of a particular feature with a
registered tag, URI, or standardized reference must take place prior
to any content negotiation using that feature.  The selection of which
method to use to represent variation will depend primarily on how the
need for extensibility contrasts with the need for a stable reference.
A single feature tag registry provides a highly stable core set of
features, but is likely to be relatively slow in deployment of new
features.  A URI-based system is likely to be highly adaptable to new
features, but susceptible to duplication and loss of information.  The
use of pointers to standardized reference documents allows more than one
organization to participate in the vetting of new features, but this
method may engender confusion about which organization should review a

3.2  Provider preferences

Provider preferences are commonly expressed as opinions on the quality
of various representations of the resource.  The determination of
quality may be made by reference to a particular standard; the
determination of the lossiness of a JPEG image may, for example, be
made with reference to the JPEG standards. Quality may also be
assigned on a subjective basis, however; an author might, for example,
decide that the loss of figures in an ASCII representation of resource
implied a loss in quality, but she would have to invent a figure to
describe that loss.

The numerical expression of quality indicators can be ascending or
descending.  In HTTP 1.1 [2], lower qvalues represent increasing
degradation of quality; in NTP [3], in contrast, quality indicators
like peer.dispersion use increasing values to represent increasing
degradation.  A standard method for indicating provider preferences
for content-negotiating protocols is clearly needed for any system
based on numerical expressions of quality to work.  Any system based
on non-numerical expressions of quality will similarly need
standardization of the terms by which quality is expressed.

3.3  Recipient capabilities and preferences

The expression of recipient capabilities and preferences presents all
of the problems of feature description and provider preferences, as
well as some unique issues.  If a standardized method of expressing
features is available (cf 3.1, above), recipient capabilities may be
expressed with reference to Content-type and one or more features.
Similarly, recipient preferences can be expressed using a numerical
system like those described above in 3.2, with many of the same
problems of how to apply subjective criteria being present.

When to send indications of recipient preferences and capabilities and
how to indicate dependencies among them present problems unique to the
recipient side of the equation.  A content provider knows when
multiple variants are present and on what basis the representations
vary.  The recipient, in contrast, does not know a particular resource
has variants unless the content provider chooses to present that
information.  Depending on the type of exchange, a recipient may be
able to choose when or if to present its capabilities and preferences.
If it can and chooses to do so prior to every exchange, it will be
wasting network and processor resources whenever the resource has no
variants.  If it never presents its capabilities it may get a less
than optimal variant.  If it waits for an indication that variants
exist, a lag in the exchange is introduced which may be quite
significant for store-and-forward protocols.

Expressing the dependencies among various capabilities also presents
problems.  A printer may, for example, be able to print either in
color or at a high density; if the relationship between the two
features is not fixed, however, expressing it appropriately may be
very difficult.

4.0 Scenarios

Content negotiation may be provider-based, recipient-based, or based
on active content.

4.1 Provider-based negotiation

Provider-based content negotiation occurs when the provider of a
resource selects among the available variants without (or prior to)
notifying the potential recipient that alternatives exist.  In HTTP,
for example, content providers may configure their servers to deliver
certain variants based on the user-agent of the requester or the
Accept headers available in the request.

This method is obviously limited to protocols in which recipients
initiate requests for resources and provide information about their
capabilities in those requests.  Beyond that limitation are several
problems of scalability.  As features multiply, the overhead of
providing information on capabilities for each potential feature
grows, and since that overhead is incurred for every exchange it can
rapidly lead to network resource problems.  The use of feature
"bundles" would ameliorate this somewhat, but it would increase the
problems associated with maintenance for both provider and recipients.

Provider-based selection may also present privacy problems, as the
full set of preferences and capabilities must be released to the
provider for the negotiation to operate at its best.  This creates an
effective user profile, which could be combined with other information
to reveal more about potential recipients than they intend or expect.

Lastly, it is common for a single provider to serve multiple
recipients.  As the number of potential recipients grows, the
processing power required to handle variant selection also grows,
and this can strain providers.

4.2 Recipient-based negotiation

Recipient-based selection occurs whenever the provider notifies
the recipient that alternatives exist and allows the recipient to
select among them.  The Multipart-alternative MIME type is currently
the chief method by which providers notify potential recipients that
alternatives exist.  This message type allows the provider to send
either all of the available variants or, with external message bodies,
pointers to each of them.  The alternatives are ranked by the provider
according to the provider's preferences, and the recipient selects the
first of the alternatives that it can handle.

An Alternates header [4] has been proposed to both extend and
simplify the Multipart alternative method for delivering information
on available variants.  Using an Alternates header, a provider can
notify a potential recipient of available variants without the
overhead of MIME multipart.  The Alternates header also proposes a
method by which the provider can indicate on what basis the variants
differ, so that recipients can select based on the variation rather
than the rank assigned by the provider.

This form of negotiation limits the process of selection among
variants to the axes along which they vary.  This eliminates the need
for recipients to enumerate their capabilities and preferences, which
increases the recipients' privacy and reduces the possibility of
bandwidth wastage or provider processor strain.  This shift to
recipient-based selection increases, however, the processing demanded
of the recipient.  Depending on how the alternatives are presented, it
also either wastes bandwidth in the delivery of all available variants
or requires an additional step in the secondary retrieval of the one
selected.  If no resources are available or if the available methods
for selecting among them are inadequate, this method may also require
direct user intervention.

4.3  Active content negotiation

Active content negotiation takes place when the resource provided
contains content for execution in the recipient's environment; this
active content then determines the capabilities and preferences of the
recipient and retrieves or produces the best available variant.

This method has several advantages: it reduces the possible need for
active user intervention; it has the potential to use more complex
algorithms for selection than are available with Multipart-alternative
or Alternates-based systems; and the active content can be cached for
later use.

There are, however, several limitations to the usefulness of active
content negotiation.  The most severe problem is that there is no
cross-platform or cross-protocol standard for active content.  Without
such a standard, providers must engage in other forms of content
negotiation before providing the correct form of active content.  The
use of active content also raises the bar for recipients again, and
some recipients will be unable to use it, either because of limited
processing power or because of security considerations (cf 6, below).

5. Requirements and Desiderata

The limitations of each of the current methods of content negotiation
signal the need for a comprehensive framework for negotiation.  For
that framework, this author believes that there are three principal
requirements and three additional desiderata:

1) A cross-protocol negotiation framework must include a standardized
method for reflecting content features.

2) A cross-protocol negotiation framework must include a standardized
method for associating variants with features.

3) A cross-protocol negotiation framework must provide a method for
indicating provider and recipient preferences for specific features

4) A cross-protocol negotiation framework should have the minimum
possible impact on network resource consumption, especially in terms
of bandwidth and number of round-trips required.

5) A cross-protocol negotiation framework should protect the privacy
of users' profiles and providers' inventories of variants.

6) A cross-protocol negotiation framework should allow intelligent
gateways, proxies, or caches to participate in the negotiation.

6. Security Consideration

Active content negotiation presumes that recipients will execute code
provided and will allow that code to examine user preferences and
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.

Detailed listings of user preferences and capabilities present
possible privacy problems, whether that listing occurs in
provider-based negotiation or via active content.  Some content
providers may also find that exhaustive listings of available
representations may compromise their privacy.

7. Acknowledgments

Larry Masinter, Koen Holtman, Graham Klyne, Scott Lawrence, Dan Wing,
Andy Mutz, and Neil Joffe provided valuable comments on previous
versions of this draft.  Discussions within the HTTP working group and
the IETF-FAX mailing list also provided insights into the scope of
negotiation needed in different contexts.

8.  References

[1] Freed, N., and N. Borenstein, "Multipurpose Internet Mail
Extensions (MIME) Part One: Format of Internet Message Bodies", RFC

[2] Fielding, R., et al, "Hypertext Transfer Protocol -- HTTP/1.1",
RFC 2068

[3] Mills, D. "Network Time Protocol (Version 3) Specification,
Implementation", RFC 1305

[4] Holtman, K., et al, "The Alternates Header Field", Internet-Draft
draft-ietf-http-alternates-01.txt, HTTP working group, November 1997.

9.  Author's Address

Edward Hardie
NASA Network Information Center
MS 204-14
Moffett Field, CA 94035-1000
+1 650 604 0134