HTTP Random Access and Live Content
draft-ietf-httpbis-rand-access-live-02

HTTP Working Group                                              C. Pratt
Internet-Draft                                                 CableLabs
Intended status: Experimental                                   B. Stark
Expires: May 18, 2018                                               AT&T
                                                              D. Thakore
                                                               CableLabs
                                                       November 14, 2017

                  HTTP Random Access and Live Content
                 draft-ietf-httpbis-rand-access-live-02

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 May 18, 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
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   (https://trustee.ietf.org/license-info) in effect on the date of
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   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 . . . . . . . . . . . . . . . . . . . . . . . . . .   5
   3.  Other Applications of Random-Access Aggregating Content . . .   7
     3.1.  Requests Starting at the Aggregation ("Live") Point . . .   7
     3.2.  Shift Buffer Representations  . . . . . . . . . . . . . .   7
   4.  Security Considerations . . . . . . . . . . . . . . . . . . .   8
   5.  References  . . . . . . . . . . . . . . . . . . . . . . . . .   9
     5.1.  Normative References  . . . . . . . . . . . . . . . . . .   9
     5.2.  Informative References  . . . . . . . . . . . . . . . . .   9
   Appendix A.  Acknowledgements . . . . . . . . . . . . . . . . . .   9
   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 on an indeterminate-length resource by transferring the
   entire 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 the
   most recently appended content is needed by the client.

   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 while performing these
   Range requests at higher frequency can reduce this latency, it also
   incurs more processing overhead and HTTP exchanges as many of the
   requests will return no content - since content is usually aggregated
   in groups of bytes (e.g. a video frame, audio sample, block, or log
   entry).

   This document describes a usage model for range requests which
   enables efficient retrieval of representations that are appended to
   over time by using large values and associated semantics for
   communicating range end positions.  This model allows representations
   to be progressively delivered by servers as new content is added.  It
   also ensures compatibility with servers and intermediaries that don't
   support this technique.

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

   This document recommends a two-step process for accessing resources
   that have indeterminate length representations.  Two steps are
   necessary because of limitations with the Range request header and
   the Content-Range response header fields.  A server cannot know from
   a range request that a client wishes to receive a response that does
   not have a definite end.  More critically, the header fields do not
   allow the server to signal that a resource has indeterminate length
   without also providing a fixed portion of the resource.  A client
   first learns that the resource has a representation of indeterminate
   length by requesting a range of the resource.  The server responds
   with the range that is available, but indicates that the length of

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   the representation is unknown using the existing Content-Range
   syntax.  See Section 2.1 for details and examples.  Once the client
   knows the resource has indeterminate length, it can request a range
   with a very large end position from the resource.  The client chooses
   an explicit end value larger than can be transferred in the
   foreseeable term.  A server which supports range requests of
   indeterminate length signals its understanding of the client's
   indeterminate range request by indicating that the range it is
   providing has a range end that exactly matches the client's requested
   range end rather than a range that is bounded by what is currently
   available.  See Section 2.2 for details.

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) may respond with a 206 status code (Partial
   Content) with a Content-Range header indicating the currently
   satisfiable byte range.  For example:

         HEAD /resource HTTP/1.1
         Range: bytes=0-

   returns a response of the form:

       HTTP/1.1 206 Partial Content
       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).

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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.

   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:

       GET /resource HTTP/1.1
       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.

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   In response, 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:

       GET /resource HTTP/1.1
       Range: bytes=1230000-999999999999

   returns

       HTTP/1.1 206 Partial Content
       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 will supply the currently satisfiable byte range,
   as it would with an open-ended byte request.

   For example:

       GET /resource HTTP/1.1
       Range: bytes=1230000-999999999999

   will return

       HTTP/1.1 206 Partial Content
       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 the end
   of the transfer, per section 4.1 of [RFC7230].

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3.  Other Applications of Random-Access Aggregating Content

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

   A client that wishes to only receive newly-aggregated portions of a
   resource (i.e., start at the "live" point), can use a HEAD request to
   learn what range the server has currently available and initiate an
   indeterminate-length transfer.  For example:

       HEAD /resource HTTP/1.1
       Range: bytes=0-

   With the Content-Range response header indicating the (or ranges)
   available.  For example:

       206 Partial Content
       Content-Range: bytes 0-1234567/*

   The client can then issue a request for a range starting at the end
   value (using a very large value for the end of a range) and receive
   only new content.

       GET /resource HTTP/1.1
       Range: bytes=1234567-999999999999

   with a server returning a Content-Range response indicating that an
   indeterminate-length response body will be provided

       206 Partial Content
       Content-Range: bytes 1234567-999999999999/*

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 Range request containing:

       HEAD /resource HTTP/1.1
       Range: bytes=0-

   returns

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       206 Partial Content
       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

       GET /resource HTTP/1.1
       Range: bytes=1020000-999999999999

   with the server returning

       206 Partial Content
       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.  Also, some implementations
   (e.g.  JavaScript-based clients and servers) are not able to
   represent all values beyond 2^^53.  So similarly, if there's no
   expectation that a representation will ever exceed 2^^53 bytes,
   values smaller than this limit should be used for the last-byte-pos
   in byte-range requests.

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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.

Authors' Addresses

   Craig Pratt
   CableLabs
   858 Coal Creek Circle
   Louisville, CO  80027

   Email: pratt@acm.org

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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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