HTTP Random Access and Live Content
draft-ietf-httpbis-rand-access-live-01

Versions: 00 01                                                         
HTTP Working Group                                              C. Pratt
Internet-Draft                                                 CableLabs
Intended status: Experimental                                   B. Stark
Expires: March 8, 2018                                              AT&T
                                                              D. Thakore
                                                               CableLabs
                                                       September 4, 2017


                  HTTP Random Access and Live Content
                 draft-ietf-httpbis-rand-access-live-01

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.

Editorial Note (To be removed by RFC Editor before publication)

   Discussion of this draft takes place on the HTTPBIS working group
   mailing list (ietf-http-wg@w3.org), which is archived at
   <https://lists.w3.org/Archives/Public/ietf-http-wg/>.

   Working Group information can be found at <http://httpwg.github.io/>;
   source code and issues list for this draft can be found at
   <https://github.com/httpwg/http-extensions/labels/rand-access-live>.

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
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   This Internet-Draft will expire on March 8, 2018.





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Copyright Notice

   Copyright (c) 2017 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

   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
     1.1.  Requirements Language . . . . . . . . . . . . . . . . . .   3
   2.  Performing Range requests on Random-Access Aggregating
       ("live") Content  . . . . . . . . . . . . . . . . . . . . . .   3
     2.1.  Establishing the Randomly Accessible Byte Range . . . . .   4
     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  . . . . . . . . . . . . . .   7
   4.  Security Considerations . . . . . . . . . . . . . . . . . . .   8
   5.  References  . . . . . . . . . . . . . . . . . . . . . . . . .   8
     5.1.  Normative References  . . . . . . . . . . . . . . . . . .   8
     5.2.  Informative References  . . . . . . . . . . . . . . . . .   8
   Appendix A.  Acknowledgements . . . . . . . . . . . . . . . . . .   8
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .   9

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 without
   accepting performance or behavior sacrifices which are not acceptable
   for many applications.



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   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
   portion of the randomly-accessible content is needed by the Client.
   And when bandwidth is limited, the Client may never be able to
   transfer all of the currently accessible potion and "catch up" with
   the appending content.

   Alternatively, Clients can also access appended content by sending
   periodic open-ended bytes Range requests using the last-known end
   byte position as the range start.  Performing low-frequency 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
   exchanges as the 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.

1.1.  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].

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

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

   o  Establishing the randomly-accessible range of the representation,
      and

   o  Performing range requests that continue beyond the randomly-
      accessible range.






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2.1.  Establishing the Randomly Accessible Byte Range

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

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

   which allows a Client to send a HEAD 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 currently
   satisfiable byte range.  For example, a Client-issued HEAD request
   performed against a continuously aggregating representation hosted on
   a Server could contain a byte-range header of the form:

      Range: bytes=0-

   could return the header

      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 by the Server.  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 with a start position
   within the randomly-accessible content range and an end position 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.




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   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

   Specifically, last-byte-pos is required in byte-range.  So in order
   to preserve interoperability with existing HTTP clients, servers,
   proxies, and 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 handling indeterminate-length byte-
   range responses 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 HEAD
   request, as described above.

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

   A Server may indicate that it is supplying a 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.




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   A Server that doesn't support or supply a continuously aggregating
   ("live") response should supply the currently satisfiable byte range,
   as it would with an open-ended byte request.

   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 typical HTTP Server -
   one that doesn't support byte-range requests on aggregating 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 does return 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,
   per section 4.1 of [RFC7233].

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



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   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 (the "0"
   end) of the randomly-accessible region that become inaccessible over
   time.  Examples of this kind of representation would be an audio-
   video time-shift buffer or a rolling log file.

   For example a HEAD Range 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 Range 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.



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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.

5.  References

5.1.  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, <https://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,
              <https://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,
              <https://www.rfc-editor.org/info/rfc7233>.

5.2.  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, <https://www.rfc-editor.org/info/rfc4234>.

Appendix A.  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.






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Authors' Addresses

   Craig Pratt
   CableLabs
   858 Coal Creek Circle
   Louisville, CO  80027

   Email: pratt@acm.org


   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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