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EXPERIMENTAL

Internet Engineering Task Force (IETF)                          C. Pratt
Request for Comments: 8673
Category: Experimental                                        D. Thakore
ISSN: 2070-1721                                                CableLabs
                                                                B. Stark
                                                                    AT&T
                                                           November 2019


                  HTTP Random Access and Live Content

Abstract

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

Status of This Memo

   This document is not an Internet Standards Track specification; it is
   published for examination, experimental implementation, and
   evaluation.

   This document defines an Experimental Protocol for the Internet
   community.  This document is a product of the Internet Engineering
   Task Force (IETF).  It represents the consensus of the IETF
   community.  It has received public review and has been approved for
   publication by the Internet Engineering Steering Group (IESG).  Not
   all documents approved by the IESG are candidates for any level of
   Internet Standard; see Section 2 of RFC 7841.

   Information about the current status of this document, any errata,
   and how to provide feedback on it may be obtained at
   https://www.rfc-editor.org/info/rfc8673.

Copyright Notice

   Copyright (c) 2019 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
   (https://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
     1.1.  Notational Conventions
   2.  Performing Range Requests on Random-Access Aggregating (Live)
           Content
     2.1.  Establishing the Randomly Accessible Byte Range
     2.2.  Byte-Range Requests beyond the Randomly Accessible Byte
           Range
   3.  Other Applications of Random-Access Aggregating Content
     3.1.  Requests Starting at the Aggregation/Live Point
     3.2.  Shift-Buffer Representations
   4.  Recommendations for Byte-Range Request last-byte-pos Values
   5.  IANA Considerations
   6.  Security Considerations
   7.  References
     7.1.  Normative References
     7.2.  Informative References
   Acknowledgements
   Authors' Addresses

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 [RFC7233].  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 that
   are not acceptable for many applications.

   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 access appended content by sending
   periodic, open-ended byte-range requests using the last known end
   byte position as the range start.  Performing low-frequency periodic
   byte-range requests in this fashion (polling) introduces latency
   since the client will necessarily be somewhat behind in transferring
   the aggregated content, effectively resulting in the same kind of
   latency issues with the segmented content transfer mechanisms in
   "HTTP Live Streaming" (HLS) [RFC8216] and "Dynamic Adaptive Streaming
   over HTTP" [MPEG-DASH].  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 that 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.  Notational Conventions

   This document cites Augmented Backus-Naur Form (ABNF) productions
   from [RFC7233], using the notation defined in [RFC5234].

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 fields 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 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 that 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

   Determining if a representation is continuously aggregating ("live")
   and determining the randomly accessible byte range can both be
   performed using the existing definition for an open-ended byte-range
   request.  Specifically, Section 2.1 of [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 field indicating the currently
   satisfiable byte range.  For example:

   HEAD /resource HTTP/1.1
   Host: example.com
   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) only bytes 0-1234567 were accessible 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
   transfer can be performed immediately.

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 request header field, a byte-content-range
   response header field cannot be "open-ended", per Section 4.2 of
   [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 an 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
   Host: example.com
   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, and much larger than the expected
   maximum size of the representation.  See Section 6 for range value
   considerations.

   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 field.

   For example:

   GET /resource HTTP/1.1
   Host: example.com
   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 continue 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
   Host: example.com
   Range: bytes=1230000-999999999999

   returns

   HTTP/1.1 206 Partial Content
   Content-Range: bytes 1230000-1234567/*

   from the server to indicate that the response will start at byte
   1230000, end at byte 1234567, and 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 field.  A 0-length chunk indicates the end of
   the transfer, per Section 4.1 of [RFC7230].

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
   Host: example.com
   Range: bytes=0-

   with the Content-Range response header field indicating the range (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.

   For example:

   GET /resource HTTP/1.1
   Host: example.com
   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 removal 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 becomes 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
   Host: example.com
   Range: bytes=0-

   returns

   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
   Host: example.com
   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.

   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
   Host: example.com
   Range: bytes=0-999999999999

   returns

   HTTP/1.1 206 Partial Content
   Content-Range: bytes 1020000-1254567/*

   from the server to indicate that the response will start at byte
   1020000, end at byte 1254567, and 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.

   Note that responses to GET requests performed on shift-buffer
   representations using Range headers can be cached by intermediaries,
   since the Content-Range response header indicates which portion of
   the representation is being returned in the response body.  However,
   GET requests without a Range header cannot be cached since the first
   byte of the response body can vary from request to request.  To
   ensure GET requests without Range headers on shift-buffer
   representations are not cached, servers hosting a shift-buffer
   representation should either not return a 200-level response (e.g.,
   send a 300-level redirect response with a URI that represents the
   current start of the shift buffer) or indicate the response is non-
   cacheable.  See [RFC7234] for details on HTTP cache control.

4.  Recommendations for Byte-Range Request last-byte-pos Values

   While it would be ideal to define a single large last-byte-pos value
   for byte-range requests, there's no single value that would work for
   all applications and platforms.  For example, JavaScript numbers
   cannot represent all integer values above 2^^53, so a JavaScript
   application may want to use 2^^53-1 for last-byte-pos.  This value,
   however, would not be sufficient for all applications, such as long-
   duration high-bitrate streams.  So 2^^53-1 (9007199254740991) is
   recommended as a last-byte-pos unless an application has a good
   justification to use a smaller or larger value.  For example, if it
   is always known that the resource won't exceed a value smaller than
   the recommended last-byte-pos for an application, a smaller value can
   be used.  If it's likely that an application will utilize resources
   larger than the recommended last-byte-pos (such as a continuously
   aggregating high-bitrate media stream), a larger value should be
   used.

   Note that, in accordance with the semantics defined above, servers
   that support random-access live content will need to return the last-
   byte-pos provided in the byte-range request in some cases -- even if
   the last-byte-pos cannot be represented as a numerical value
   internally by the server.  As is the case with any continuously
   aggregating/live resource, the server should terminate the content
   transfer when the end of the resource is reached -- whether the end
   is due to termination of the content source or the content length
   exceeds the server's maximum representation length.

5.  IANA Considerations

   This document has no IANA actions.

6.  Security Considerations

   As described above, servers need to be prepared to receive last-byte-
   pos values in range requests that are numerically larger than the
   server implementation supports and return these values in Content-
   Range response header fields.  Servers should check the last-byte-pos
   value before converting and storing them into numeric form to ensure
   the value doesn't cause an overflow or index incorrect data.  The
   simplest way to satisfy the live-range semantics defined in this
   document without potential overflow issues is to store the last-byte-
   pos as a string value and return it in the byte-range Content-Range
   response header's last-byte-pos field.

7.  References

7.1.  Normative References

   [RFC5234]  Crocker, D., Ed. and P. Overell, "Augmented BNF for Syntax
              Specifications: ABNF", STD 68, RFC 5234,
              DOI 10.17487/RFC5234, January 2008,
              <https://www.rfc-editor.org/info/rfc5234>.

   [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>.

   [RFC7234]  Fielding, R., Ed., Nottingham, M., Ed., and J. Reschke,
              Ed., "Hypertext Transfer Protocol (HTTP/1.1): Caching",
              RFC 7234, DOI 10.17487/RFC7234, June 2014,
              <https://www.rfc-editor.org/info/rfc7234>.

7.2.  Informative References

   [MPEG-DASH]
              ISO, "Information technology -- Dynamic adaptive streaming
              over HTTP (DASH) -- Part 1: Media presentation description
              and segment formats", ISO/IEC 23009-1, August 2019,
              <https://www.iso.org/standard/75485.html>.

   [RFC8216]  Pantos, R., Ed. and W. May, "HTTP Live Streaming",
              RFC 8216, DOI 10.17487/RFC8216, August 2017,
              <https://www.rfc-editor.org/info/rfc8216>.

Acknowledgements

   The authors would like to thank Mark Nottingham, Patrick McManus,
   Julian Reschke, Remy Lebeau, Rodger Combs, Thorsten Lohmar, Martin
   Thompson, Adrien de Croy, K. Morgan, Roy T. Fielding, and Jeremy
   Poulter.

Authors' Addresses

   Craig Pratt
   Portland, OR 97229
   United States of America

   Email: pratt@acm.org


   Darshak Thakore
   CableLabs
   858 Coal Creek Circle
   Louisville, CO 80027
   United States of America

   Email: d.thakore@cablelabs.com


   Barbara Stark
   AT&T
   Atlanta, GA
   United States of America

   Email: barbara.stark@att.com


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