

INTERNET-DRAFT                                            M. Nottingham
Document: draft-nottingham-http-roles-00.txt           1 September 1999
Expires: 1 March 2000



                     Server-Side Roles in the HTTP


Status of this Memo

   This document is an Internet-Draft and is in full conformance with
   all provisions of Section 10 of RFC2026.

   Internet-Drafts are working documents of the Internet Engineering
   Task Force (IETF), its areas, and its working groups. Note that
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   The list of current Internet-Drafts can be accessed at
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   Please send comments to Mark Nottingham <mnot@pobox.com>

1. Abstract

   Web servers are becoming more complex, and as a result are losing
   the full benefits of HTTP protocol compliance.

   This applicability statement defines classifications of Web server
   sub-components and clarifies their responsibilities in implementing
   HTTP/1.1 protocol features, with a discussion of the motivations for
   doing so.


2. Conventions used in this document

   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 RFC-2119 [1].


3. Introduction

   Although many Web servers available claim HTTP/1.1 [2] compliance,
   the majority do not issue compliant responses for every object
   served. This is especially noticeable for the performance-enhancing

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   provisions of the protocol, such as persistent connections, transfer
   encoding, and object validation [3].

   This behavior is most evident in objects which are created by non-
   traditional means [4]. Historically, the primary method of managing
   Web content was by storing it in a filesystem as discrete files. Web
   servers were designed to serve static files as efficiently as
   possible, while still having important (and readily available)
   metadata such as Last-Modified (for validation) and Content-length
   (for persistent connections) available.

   Dynamically generated content (such as CGI) was thought of as a
   small portion of the total traffic, and relatively inconsequential
   when optimizing performance.

   This is no longer necessarily the case. CGI and other specialized
   methods are being used to manage entire Web sites. The filesystem,
   with its easily generated metadata, is being replaced by a software
   layer that presents a much more opaque profile to the server.

   In common practice, this lack of clear responsibility between server
   components results in partial or no support for key protocol
   features, leaving compliance in the hands of the content publisher.

   Consequently, the performance benefits of HTTP/1.1 are unrealized
   for an increasing portion of Web traffic. Research has established
   that they are both able to be realized and feasible to implement
   [5].

   Additionally, future protocol extensions may not be useable for a
   large number of entities generated by a server, for the same
   reasons.


4. Goals

   The goals of this document are to:

   1. Define the roles and responsibilities of server-side components,
   with respect to implementing HTTP/1.1 protocol features.

   2. Promote interoperability of Web servers and increase the protocol
   compliance of objects served, without to regard to how they are
   generated.

   3. Prevent the burdening of content publishers with the
   responsibility for protocol compliance by shifting responsibility to
   the server, making the HTTP implementation transparent.






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5. Terminology

   HTTP/1.1 defines two primary roles implicitly:

   client
        A program that establishes connections for the purpose of
        sending requests.

   server
        An application program that accepts connections in order to
        service requests by sending back responses. Any given program
        may be capable of being both a client and a server; our use of
        these terms refers only to the role being performed by the
        program for a particular connection, rather than to the
        program's capabilities in general. Likewise, any server may act
        as an origin server, proxy, gateway, or tunnel, switching
        behavior based on the nature of each request.

   In this document, server means the component of the software that
   actually responds to the request. Also, we only consider generalized
   servers; that is, those that are capable of serving content for a
   number of purposes. Single-purpose (e.g., embedded) servers are out
   of scope.

   Additional roles, such as proxy, gateway and tunnel, are defined,
   but are considered out of scope for this proposal.

   The following terms are new:

   content
        The intended entity-body payload. It may or may not be actually
        transferred in a response.

   content generator
        A means by which requests are mapped to content, which is
        optionally transformed, generated or otherwise prepared, and
        then passed back to the server, which then sends it to the
        client. The generator may be, but is not necessarily, external
        to the server.

   publisher
        A person or persons who make content available on the server,
        possibly through a content generator.

   synthetic validator
        A validator generated by a server, without knowledge of the
        process used to produce the content it describes.






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6. Server Role and Responsibilities

   Implementation of HTTP protocol features should, in general, be the
   responsibility of the server. A server SHOULD NOT expect a content
   generator, content management system or publisher to handle them,
   except where it does not have sufficient information available to do
   so.

   Core features of interest are listed below; this is not meant to be
   exhaustive, but only an indication of those most critical and
   obvious.

6.1 Persistent Connections

   Servers MUST support a persistent connection if the content
   generator supplies a Content-Length header. If it is not available,
   the server SHOULD attempt to buffer the response in order to
   generate one, although this MAY be circumvented if:
   * the server does not have resources (i.e., memory) to do so, or
   * the object is very large, and overall latency becomes
   unacceptable, or
   * the time required to generate the object adds unacceptable latency

   Because many clients begin rendering HTML as soon as it is
   available, servers MAY reduce size and time thresholds for text/html
   responses.

   There MUST be a method by which content generators can specify that
   content is not to be buffered; this MAY be performed by a pseudo-
   HTTP header that is consumed by the server.

   Servers MUST serve chunked encoding responses for all objects, if:
   * Content-Length is unavailable and impractical to generate
   and
   * the client advertises itself as HTTP/1.1 capable, or
   * the client includes 'chunked' in a TE header

6.2 Validators and Validation

   Servers MUST pass through any validator (e.g., Etag or Last-Modified
   date) sent by a Content Generator, without modification.

   Conditional request validation MUST be handled by regenerating the
   object, comparing the validators, and returning an appropriate
   response. Servers MAY use an alternate method of regenerating the
   validator without generating the entire object, if the validator
   produced is equivalent.

6.2.1 Synthetic Validators

   Servers SHOULD generate synthetic validators for objects that do not
   any associated, when possible. However, this MUST NOT be done when
   the generated object contains a Cache-Control: no-store header.

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   Synthetic validators MUST be strong, and MUST have a Cache-Control:
   no-cache and Expires time in the past associated, unless a freshness
   period is explicitly assigned.

   One possible method of synthetic validator generation would be to
   chunk-encode or buffer the response (as outlined above) and append a
   Trailer containing the MD5 hash of the body as a validator.

6.3 Partial Content

   Servers MUST respond to partial-content requests (i.e., those with
   Range headers) appropriately, by using the Content Generator to
   produce the response, and then sending back the requested range(s).

6.4 Transfer-Encoding

   Servers SHOULD reply to requests that allow transfer encoding of
   objects (i.e., TE header present) with appropriate encoding, in a
   fashion transparent to the content generator.

6.5 HEAD Requests

   HEAD requests SHOULD be responded to by using the content generator
   to produce a GET response, omitting the body before returning it to
   the client.

6.5 Publisher Access

   Servers SHOULD provide a suitable method of setting per-object and
   global metadata (such as Expires times and Cache-Control headers) to
   users. If WebDAV [6] is implemented, it MAY be the mechanism used.


7. Content Generator Role and Responsibilities

   Content generators SHOULD provide as much information as possible to
   the server, to facilitate efficient operation. This includes
   Content-Length, Etag and/or Last-Modified validators, and freshness
   information, as possible.

   Content generators SHOULD NOT specify unique URLs for multiple
   instances of identical content, if avoidable. When this does happen
   predictably, the generator SHOULD include a Cache-Control: no-store
   header.

   User-specific or transaction-based content SHOULD have an associated
   Cache-Control: no-store header, unless there is a possibility of
   reuse of the resource (by one or more clients), and side effects of
   the transaction are not important.

   Content generators SHOULD allow publishers to specify that buffering
   is not to be used for specific content.


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8.  Security Considerations

   There are no security implications specific to this document; see
   the HTTP/1.1 security section for general information.


9.  References

   1  Bradner, S., "Key words for use in RFCs to Indicate Requirement
      Levels", BCP 14, RFC 2119, March 1997
   2  Fielding, R. et al., "Hypertext Transfer Protocol - HTTP/1.1",
      RFC2616, June, 1999
   3  Krishnamurthy, B., Arlitt, M., "PRO-COW: Protocol Compliance on
      the Web", AT&T Labs - Research Technical Report # 990803-05-TM
   4  Nottingham, M., "Web Server Capabilities",
      http://www.mnot.net/papers/capabilities.html
   5  Wills, C., Mikhailov, M., "Examining the Cacheability of User-
      Requested Web Resources", 4th Web Caching Workshop, San Diego CA,
      March 1999
   6  Whitehead, E. et al., "HTTP Extensions for Distributed Authoring
      - WEBDAV", RFC2518, February 1999

10. 
    Acknowledgments

   The author would like to thank Mike Ciavarella for his constructive
   comments.


11. 
    Author's Addresses

   Mark Nottingham
   Email: mnot@pobox.com





















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