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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