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Versions: 00 draft-ietf-httpbis-rand-access-live

Hypertext Transfer Protocol                                     C. Pratt
Internet-Draft                                                 CableLabs
Intended status: Informational                                  B. Stark
Expires: May 17, 2017                                               AT&T
                                                              D. Thakore
                                                               CableLabs
                                                       November 13, 2016


                  HTTP Random Access and Live Content
                draft-pratt-httpbis-rand-access-live-00

Abstract

   To accommodate byte range requests for content that has data appended
   over time, this document defines semantics that allow a HTTP client
   and server to perform byte-range GET and HEAD requests that start at
   an arbitrary byte offset within the representation and ends at an
   indeterminate offset.

Requirements Language

   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 [RFC2119].

Status of This Memo

   This Internet-Draft is submitted in full conformance with the
   provisions of BCP 78 and BCP 79.

   Internet-Drafts are working documents of the Internet Engineering
   Task Force (IETF).  Note that other groups may also distribute
   working documents as Internet-Drafts.  The list of current Internet-
   Drafts is at http://datatracker.ietf.org/drafts/current/.

   Internet-Drafts are draft documents valid for a maximum of six months
   and may be updated, replaced, or obsoleted by other documents at any
   time.  It is inappropriate to use Internet-Drafts as reference
   material or to cite them other than as "work in progress."

   This Internet-Draft will expire on May 17, 2017.

Copyright Notice

   Copyright (c) 2016 IETF Trust and the persons identified as the
   document authors.  All rights reserved.




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   This document is subject to BCP 78 and the IETF Trust's Legal
   Provisions Relating to IETF Documents
   (http://trustee.ietf.org/license-info) in effect on the date of
   publication of this document.  Please review these documents
   carefully, as they describe your rights and restrictions with respect
   to this document.  Code Components extracted from this document must
   include Simplified BSD License text as described in Section 4.e of
   the Trust Legal Provisions and are provided without warranty as
   described in the Simplified BSD License.

Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   2
   2.  Performing Range requests on Random-Access Aggregating
       ("live") Content  . . . . . . . . . . . . . . . . . . . . . .   3
     2.1.  Establishing the Randomly Accessible Byte Range . . . . .   3
     2.2.  Byte-Range Requests Beyond the Randomly Accessible Byte
           Range . . . . . . . . . . . . . . . . . . . . . . . . . .   4
     2.3.  Byte-Range Responses Beyond the Randomly Accessible Byte
           Range . . . . . . . . . . . . . . . . . . . . . . . . . .   5
   3.  Other Applications of Random-Access Aggregating Content . . .   6
     3.1.  Requests Starting at the Aggregation ("Live") Point . . .   6
     3.2.  Shift Buffer Representations  . . . . . . . . . . . . . .   6
   4.  Security Considerations . . . . . . . . . . . . . . . . . . .   7
   5.  Acknowledgements  . . . . . . . . . . . . . . . . . . . . . .   8
   6.  References  . . . . . . . . . . . . . . . . . . . . . . . . .   8
     6.1.  Normative References  . . . . . . . . . . . . . . . . . .   8
     6.2.  Informative References  . . . . . . . . . . . . . . . . .   8
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .   8

1.  Introduction

   Some Hypertext Transfer Protocol (HTTP) Clients use byte-range
   requests (Range requests using the "bytes" Range Unit) to transfer
   select portions of large representations.  And in some cases large
   representations require content to be continuously or periodically
   appended - such as representations consisting of live audio or video
   sources, blockchain databases, and log files.  Clients cannot access
   the appended/live content using a Range request with the bytes range
   unit using the currently defined byte-range semantics accepting
   performance or behavior sacrifices which are not acceptable for many
   applications.

   For instance, HTTP Clients have the ability to access appended
   content by simply transferring the entire accessible portion of the
   representation from the beginning and continuing to read the appended
   content as it's made available.  Obviously, this is highly
   inefficient for cases where the representation is large and only a



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   portion of the randomly accessible content is needed by the Client.
   And when bandwidth is limited, the client may never "catch up" with
   the appending content.

   Clients can also attempt to access appended content by sending
   periodic bytes Range requests using the last-known end byte position.
   Performing periodic bytes Range requests in this fashion (polling)
   introduces latency since the Client will necessarily be somewhat
   behind the aggregated content - mimicking the behavior (and latency)
   of segmented content representations such as HLS or MPEG-DASH.  And
   performing these Range requests at higher frequency incurs more
   processing overhead and HTTP traffic as the periodic requests will
   often return no content - since content is usually aggregated in
   groups of bytes (e.g. a video frame, audio sample, block, or log
   entry).

   To accommodate byte-range requests on large representations which
   have data appended over time efficiently and with low latency, this
   recommendation defines semantics whereby the HTTP Client performs
   byte-range requests using a combination of open-ended byte-range HEAD
   requests and GET requests using "Large Value" last-byte-pos values.

2.  Performing Range requests on Random-Access Aggregating ("live")
    Content

   There are two critical operations for accessing randomly accessing
   live/aggregating representations:

      Establishing the randomly accessible range of the representation,
      and

      Performing range requests that continue beyond the randomly
      accessible range.

2.1.  Establishing the Randomly Accessible Byte Range

   Establishing if a representation is continuously aggregating ("live")
   and determining the randomly accessible byte range can be performed
   using the existing definition for an open-ended byte-range request.
   Specifically, [RFC7233] defines:

      byte-range-spec = first-byte-pos "-" [ last-byte-pos ]

   which allows a Client to send a request with a first-byte-pos and
   leave last-byte-pos absent.  A Server that receives a satisfiable
   byte-range request (with first-byte-pos smaller than the current
   representation length) must respond with a 206 status code (Partial
   Content) with a Content-Range header indicating the satisfiable byte



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   range(s).  For example, a Client-issued HEAD request against a
   continuously aggregating representation hosted on a Server could
   contain a byte-range header of the form:

      Range: bytes=0-

   could return

      Content-Range: bytes 0-1234567/*

   from the Server indicating that (1) the complete representation
   length is unknown (via the "*" in place of the complete-length field)
   and (2) that only bytes 0-1234567 were accessable at the time the
   request was processed.  The Client can infer from this response that
   bytes 0-1234567 of the representation can be requested and returned
   in a timely fashion (the bytes are immediately available).

2.2.  Byte-Range Requests Beyond the Randomly Accessible Byte Range

   Once a Client has determined that a representation has an
   indeterminate length and established the byte range that can be
   accessed, it may want to perform a request that starts within the
   randomly accessible content range and ends at an indefinite "live"
   point - a point where the byte-range GET request is fulfilled on-
   demand as the content is aggregated.

   For example, for a large video asset, a client may wish to start a
   content transfer from the video "key" frame immediately before the
   point of aggregation and continue the content transfer indefinitely
   as content is aggregated - in order to support low-latency startup of
   a live video stream.

   Unlike a byte-range Range request, a byte-range Content-Range
   response header cannot be "open ended", per [RFC7233]:

      byte-content-range = bytes-unit SP ( byte-range-resp /
      unsatisfied-range )

      byte-range-resp = byte-range "/" ( complete-length / "*" )

      byte-range = first-byte-pos "-" last-byte-pos

      unsatisfied-range = "*/" complete-length

      complete-length = 1*DIGIT

   last-byte-pos is required in byte-range.  So in order to preserve
   interoperability with existing HTTP clients, servers, proxies, and



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   caches, this document proposes a mechanism for a Client to indicate
   support for handling an indeterminate-length byte-range response, and
   a mechanism for a Server to indicate if/when it's providing a
   indeterminate-length response.

   A Client can indicate support for indeterminate-length byte-ranges by
   providing a Very Large Value for the last-byte-pos in the byte-range
   request.  For example, a Client can perform a byte-range GET request
   of the form:

      Range: bytes=1230000-999999999999

   where the last-byte-pos in the Request is much larger than the last-
   byte-pos returned in response to an open-ended byte-range request.

2.3.  Byte-Range Responses Beyond the Randomly Accessible Byte Range

   A Server can indicate that it's supplying an continuously aggregating
   ("live") response by supplying the Client request's last-byte-pos in
   the Content-Range response header.

   For example:

      Range: bytes=1230000-999999999999

   could return

      Content-Range: bytes 1230000-999999999999/*

   from the Server to indicate that the response will start at byte
   1230000 and continues indefinitely to include all aggregated content,
   as it becomes available.

   A Server that doesn't support or supply a continuously aggregating
   ("live") response can supply a value other than the Client request's
   last-byte-pos in the Content-Range response header.

   For example:

      Range: bytes=1230000-999999999999

   could return

      Content-Range: bytes 1230000-1234567/*

   from the Server to indicate that the response will start at byte
   1230000 and end at byte 1234567 and will not include any aggregated
   content.  This is the response expected from a typically-configured



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   HTTP Server - one that doesn't support byte-range requests on
   aggregated content.

   A Client that doesn't receive a response indicating it is
   continuously aggregating must use other means to access aggregated
   content (e.g. periodic byte-range polling).

   A Server that returns a continuously aggregating ("live") response
   should return data using chunked transfer coding and not provide a
   Content-Length header.  A 0-length chunk indicates that aggregation
   of the transferring resource is permanently discontinued.

3.  Other Applications of Random-Access Aggregating Content

3.1.  Requests Starting at the Aggregation ("Live") Point

   If a Client would like to start the content transfer at the
   Aggregation ("live") point without including any randomly accessible
   portion of the representation, then it should supply the last-byte-
   pos from the most-recently received byte-range-spec and a Very Large
   Value for the last-byte-pos in the byte-range request.

   For example a HEAD request containing:

      Range: bytes=0-

   could return

      Content-Range: bytes 0-1234567/*

   and a GET request containing

      Range: bytes=1234567-999999999999

   could return

      Content-Range: bytes 1234567-999999999999/*

   with the response body starting with continuously aggregating
   ("live") data and continuing indefinitely.

3.2.  Shift Buffer Representations

   Some representations lend themselves to front-end content deletion in
   addition to aggregation.  While still supporting random access,
   representations of this type have a portion at the beginning ("0"
   end) of the randomly accessible region become inaccessible over time.




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   Examples of this kind of representation would be a audio-video time-
   shift buffer or a rolling log file.

   For example a HEAD request containing:

      Range: bytes=0-

   could return

      Content-Range: bytes 1000000-1234567/*

   indicating that the first 1000000 bytes were not accessible at the
   time the HEAD request was processed.  Subsequent HEAD requests could
   return:

      Content-Range: bytes 1000000-1234567/*

      Content-Range: bytes 1010000-1244567/*

      Content-Range: bytes 1020000-1254567/*

   Note though that the difference between the first-byte-pos and last-
   byte-pos need not be constant.

   The Client could then follow-up with a GET request containing

      Range: bytes=1020000-999999999999

   with the Server returning

      Content-Range: bytes 1020000-999999999999/*

   with the response body returning bytes 1020000-1254567 immediately
   and aggregated ("live") data being returned as the content is
   aggregated.

4.  Security Considerations

   One potential issue with this recommendation is related to the use of
   very-large last-byte-pos values.  Some Client and Server
   implementations may not be prepared to deal with byte position values
   of 2^^63 and beyond.  So in applications where there's no expectation
   that the representation will ever exceed 2^^63, a value smaller than
   this value should be used as the Very Large last-byte-pos in a byte-
   seek request or content-range response.






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5.  Acknowledgements

   Mark Nottingham, Patrick McManus, Julian Reschke, Remy Lebeau, Rodger
   Combs, Thorsten Lohmar, Martin Thompson, Adrien de Croy, K.  Morgan,
   Roy T.  Fielding, Jeremy Poulter

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>.

   [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>.

   [RFC7233]  Fielding, R., Ed., Lafon, Y., Ed., and J. Reschke, Ed.,
              "Hypertext Transfer Protocol (HTTP/1.1): Range Requests",
              RFC 7233, DOI 10.17487/RFC7233, June 2014,
              <http://www.rfc-editor.org/info/rfc7233>.

6.2.  Informative References

   [RANGE-UNIT-REGISTRY]
              IANA, "Hypertext Transfer Protocol (HTTP) Parameters",
              2016, <http://www.iana.org/assignments/http-parameters/
              http-parameters.xhtml#range-units>.

   [RFC4234]  Crocker, D., Ed. and P. Overell, "Augmented BNF for Syntax
              Specifications: ABNF", RFC 4234, DOI 10.17487/RFC4234,
              October 2005, <http://www.rfc-editor.org/info/rfc4234>.

Authors' Addresses

   Craig Pratt
   CableLabs
   Portland, OR  97229
   US

   Email: craig@ecaspia.com







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   Barbara Stark
   AT&T
   Atlanta, GA
   US

   Email: barbara.stark@att.com


   Darshak Thakore
   CableLabs
   858 Coal Creek Circle
   Louisville, CO  80027

   Email: d.thakore@cablelabs.com





































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