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Network Working Group                                          S. Loreto
Internet-Draft                                                  Ericsson
Intended status: Informational                            P. Saint-Andre
Expires: April 22, 2010                                            Cisco
                                                              G. Wilkins
                                                                 Webtide
                                                              S. Salsano
                                             Univ. of Rome "Tor Vergata"
                                                            Oct 19, 2009


                Design Space for Bidirectional protocols
               draft-loreto-design-space-bidirectional-00

Status of this Memo

   This Internet-Draft is submitted to IETF in full conformance with the
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   This Internet-Draft will expire on April 22, 2010.

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   Copyright (c) 2009 IETF Trust and the persons identified as the
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Abstract

   As technologies that enable bidirectional communication over HTTP
   have become more pervasive, interest has grown in (1) defining
   improved mechanisms for support of existing technologies (short-term
   solutions such as new HTTP headers) and (2) laying the foundation for
   development of new bidirectional protocols (long-term solutions such
   as WebSocket, Bidirectional Web Transfer Protocol, and Reverse HTTP).
   In order to provide context for such work, this document provides a
   first tentative map of the design space for bidirectional HTTP, with
   special attention to deployed infrastructure (e.g. web clients,
   intermediaries, firewalls, NATs, web servers) and programming
   environments.


Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . . . 3
   2.  Concerns and criteria . . . . . . . . . . . . . . . . . . . . . 3
   3.  Server side . . . . . . . . . . . . . . . . . . . . . . . . . . 4
   4.  Clients . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5
   5.  Intermediaries  . . . . . . . . . . . . . . . . . . . . . . . . 6
   6.  Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . . 7
   7.  IANA Considerations . . . . . . . . . . . . . . . . . . . . . . 7
   8.  Security Considerations . . . . . . . . . . . . . . . . . . . . 7
   9.  Informative References  . . . . . . . . . . . . . . . . . . . . 7
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . . . 8
























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1.  Introduction

   As technologies that enable bidirectional communication over HTTP
   have become more pervasive, interest has grown in (1) defining
   improved mechanisms for support of existing technologies (short-term
   solutions such as new HTTP headers) and (2) laying the foundation for
   development of new bidirectional protocols (long-term solutions such
   as WebSocket, Bidirectional Web Transfer Protocol, and Reverse HTTP).
   In order to provide context for such work, this document provides a
   first tentative map of the design space for bidirectional HTTP, with
   special attention to deployed infrastructure (e.g. web clients,
   intermediaries, firewalls, NATs, web servers) and programming
   environments.

   In existing HTTP-based systems, the typical semantic is client-server
   Representational State Transfer (REST), where the resources served
   are closely associated with an entity known by a URI or URL.
   However, the introduction of bidirectionality can significantly
   change the normal HTTP patterns.

   As one example, the standard roles of client and server can be
   reversed and a server can request resources from a client.
   Unfortunately, due to a lack of client addressability URL's may not
   be applicable to client entities and new addressing paradigms may be
   required.

   Furthermore, bidirectionality introduces a message passing pattern
   into the traditional REST style.

   These additional semantics will influence the design of the
   bidirectional protocols, addressing, and APIs.  Without a strong
   association between an identified entity and a resource, new
   mechanisms for content distribution and caching messages might need
   to be considered.

   In the following sections we analyse the design space from the
   perspective of clients, servers, and intermediaries such as proxies,
   caching servers, and load balancers.


2.  Concerns and criteria

   The process of evaluating eventual improvements to the bidirectional
   solutions or development of new bidirection protocols should take
   into consideration the following concerns and criteria:






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   Complexity:  enables ease of implementation, understanding, and
      debugging

   Capability:  addresses all/most known bidirectional use-cases

   Extensibility:  has the capacity to handle new use-cases

   Latency:  defines minimal, average, and maximal latency for message
      delivery

   Bandwidth overhead:  minimizes overhead for idle and busy usage

   Scalability:  has the ability to handle large scale usage

   Footprint:  has the ability to handle small devices and/or massive
      replication ("cloud")

   AAA:  enables proper Authentication, Authorization and Accounting

   Security:  enables strong security for integral, confidential,
      endorsed, and cross domain deployments.

   Interoperability:  can work with and/or be integrated with existing
      bidirectional implementations

   Compatibility:  can work with existing web infrastructure for
      distributing, caching, manipulating, and displaying content.


3.  Server side

   The server side can be decomposed into three categories:

   Standard HTTP  In this category two scenarios are possible:

      In the first scenario the bidirectionality is part of the normal
      HTTP/Web server responsibilities.  HTTP is used for transport the
      server events.  Examples include Comet and (in some deployments)
      BOSH.

      In the second scenario two servers are involved: bidirectionality
      is not part of the normal HTTP/Web server responsibilities; the
      push service is offered by a separate server that might not even
      be on the same machi ne of the HTTP/Web server, neither written in
      the same language.  HTTP is used for transport the server events.






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      In the latter scenario, when a browser is used in the client side,
      a cross domain solution is needed to overcome the "same-source"
      web service restriction.  Examples include BOSH (in some
      deployments) and Lightstreamer.

   In-band non-HTTP:  Bidirectionality is part of the normal HTTP/Web
      server responsibilities.  However, server events are transported
      on an upgraded HTTP connection.  Examples include Bidirectional
      Web Transfer Protocol (BWTP) and WebSockets
      [I-D.hixie-thewebsocketprotocol].

   Out-of-band non-HTTP:  Bidirectionality is offered by a dedicated
      server using non HTTP protocol for transport server events.
      Examples include WebSockets [I-D.hixie-thewebsocketprotocol].

   The Standard HTTP based servers can work with existing HTTP standards
   or enhanced HTTP.  Enhanced HTTP would be a standardized set of
   headers and behaviours to assist with bidirectionality and cross
   domain concerns.

   At present, a protocol that interacts heavily with JavaScript on the
   client side implies some constraints also on the server side, such
   that they have to use XML or JSON encapsulation.


4.  Clients

   Clients can be involved in bidirectional transport in the following
   capacities:

   In browser open standards with HTTP transport:  For applications
      written in a web browser scripting language (e.g.  JavaScript)
      within a browser client.  Examples include Comet and BOSH.

   In browser open standards with enhanced HTTP transport:  For
      applications written in a web browser scripting language (e.g.
      JavaScript) within a browser client.  Examples include Comet with
      cross domain extensions.

      Browsers using either standard HTTP transport or enhanced HTTP
      transport typically use XMLHttpRequest (XHR) API
      [W3C.WD-XMLHttpRequest2-20090820] allowing scripts to perform HTTP
      client functionality.  The XHR object can be used to perform
      bidirectional HTTP using both "long polling" and "HTTP streaming"
      mechanisms.






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      XHR also supports the cross domain extension
      [W3C.WD-cors-20090317], which overcomes the same-origin
      restrictions that prevent a Web application running from one
      origin from obtaining data retrieved from another origin.
      However, XHR presents some limitations on the headers that can be
      set by the user.

      An alternative to XMLHttpRequest is using the Server-Sent Events
      API [W3C.WD-eventsource-20090423].  This "EventSource: interface
      enables servers to push data to Web pages over HTTP or using
      dedicated server-push protocols.

   In browser open standards with non HTTP transport:  For applications
      written in JavaScript within a browser client.  Examples include
      BWTP and WebSockets [I-D.hixie-thewebsocketprotocol].

   In browser with plugin:  This is not of particular interest to IETF,
      but should be noted as part of the design space.  Examples include
      BlazeDS.

   Non-browser HTTP:  A bidirectional client may be written outside of a
      browser and use bidirectional web transports.  Typically this is
      done when a rich or minimal client requires a mature transport for
      application protocol with request / response semantics and/or the
      ability to bypass firewalls to access public servers over HTTP
      transports.  Examples include the Comet Java client and the Second
      Life Viewer.

   Non browser enhanced HTTP:  [Definition and examples to follow.]

   Non browser non-HTTP:  [Definition and examples to follow.]


5.  Intermediaries

   Intermediaries (e.g. proxies, gateways, caching servers, load
   balancers, etc) can be involved in bidirectional transport in several
   capacities:

   Legal HTTP relay:  Transports such as long polling use intermediaries
      to carry legal HTTP requests and response.  Any capabilities that
      may interfere with bidirectional flow (e.g. caching) are
      controlled with standard headers or cookies.  The intermediary may
      be a participant in the transport (e.g., changing framing or
      encapsulation).






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   Defacto HTTP relay:  Some streaming transports use the common
      behavior of HTTP intermediaries to forward content packet-by-
      packet.  This relies on intermediaries to not cache or buffer
      content.

   Enhanced HTTP relay:  Uses yet-to-be-defined HTTP headers to assist
      HTTP based bidirectional transports.  The intermediary may be a
      participant in the transport (e.g., changing framing or
      encapsulation).

   Upgraded HTTP relay:  Uses HTTP upgrade to relay a non-HTTP protocol.

   Tunneled relay:  Uses (or abuses?) the CONNECT mechanism to simulate
      an SSL connection to be used as a tunnel for a non-HTTP transport.


6.  Acknowledgments


7.  IANA Considerations

   This document does not require any actions by the IANA.


8.  Security Considerations

   To follow.


9.  Informative References

   [I-D.hixie-thewebsocketprotocol]
              Hickson, I., "The Web Socket protocol",
              draft-hixie-thewebsocketprotocol-48 (work in progress),
              October 2009.

   [W3C.WD-XMLHttpRequest2-20090820]
              Kesteren, A., "XMLHttpRequest Level 2", World Wide Web
              Consortium WD WD-XMLHttpRequest2-20090820, August 2009,
              <http://www.w3.org/TR/2009/WD-XMLHttpRequest2-20090820>.

   [W3C.WD-cors-20090317]
              Kesteren, A., "Cross-Origin Resource Sharing", World Wide
              Web Consortium WD WD-cors-20090317, March 2009,
              <http://www.w3.org/TR/2009/WD-cors-20090317>.

   [W3C.WD-eventsource-20090423]
              Hickson, I., "Server-Sent Events", World Wide Web



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              Consortium WD WD-eventsource-20090423, April 2009,
              <http://www.w3.org/TR/2009/WD-eventsource-20090423>.


Authors' Addresses

   Salvatore Loreto
   Ericsson
   Hirsalantie 11
   Jorvas  02420
   Finland

   Email: salvatore.loreto@ericsson.com


   Peter Saint-Andre
   Cisco

   Email: psaintan@cisco.com


   Greg Wilkins
   Webtide

   Email: gregw@webtide.com


   Stefano Salsano
   Univ. of Rome "Tor Vergata"
   Via del Politecnico, 1
   Rome  00133
   Italy

   Email: stefano.salsano@uniroma2.it

















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