Network Working Group L. Nageleisen
Internet-Draft April 8, 2014
Expires: October 10, 2014
Chunked Progress extension to HTTP
draft-lnageleisen-http-chunked-progress-00
Abstract
This document describes Chunked Progress, an extension to
Transfer-Encoding: Chunked as defined in RFC2616 [RFC2616]. Chunked
Progress introduces a backwards-compatible, RFC2616 compliant method
to notify the client of transfer advancement in situations where the
server has knowledge of progress but cannot know the resource size
ahead of time.
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1. Overview
User agents have been displaying progress as feedback to the user for
some time already. As the nature of applications using HTTP as
evolved to an increaingly dynamic nature, having the User Agent
estimate progress based of bytes received is getting less useful, as
data has to be generated a priori as a whole before being sent over
the wire together with Content-Size, or Content-Size has to be
omitted altogether, even when the server could actually estimate
progress on the fly. In fact, Content-Size is used as a proxy for
actual progress towards completion.
While informing the client of the requested resource content size is
useful, it is useless when the exact resource size in bytes is not
known ahead of time. Nonetheless, the server may have knowledge of
the progress expressable as a unitless number towards completion.
It is important to distinguish the two notions of progress and
completion. Completion is handled by having received Content-Size
bytes, or with chunked content encoding without a Content-Size
header, when a zero-length chunk is received. Content-size is a
measure of completion, from which progress can be derived in
increasingly limited cases.
Even then, it can be an awkward proxy: imagine a scenario where a
known number of files of various sizes will have their SHA1 value
computed and sent over the wire. The content size is known beforehand
by a simple operation, yet progress is best measured by the number of
bytes read, not the number of bytes sent.
Thankfully, Transfer-Encoding: chunked amounts to a form of
multiplexing, where metadata can be sent interleaved with data on the
same channel. By providing complementary information establishing
progress towards completion, this extension aims to reduce latency
and resource usage while increasing feedback in a backwards
compatible way. Real world typical scenarios unable to generate
progress even though it can be known on the server side include:
- gzipping files on the fly
- generating data from result rows of a database request
- generating data by walking a tree of files
2. Progress Chunk Extension
2.1. Request Header
The client SHOULD send the Chunk-Extensions: progress header in its
request if it supports the feature. If the client sent the
Chunk-Extensions: progress header, the server SHOULD include the
progress chunk extension in the response chunks, compliant with
either basic or extended mode of operation. Otherwise, the server
MAY include the progress chunk extension, but MUST comply with the
basic mode of operation.
2.2 Basic mode of operation
Since all HTTP/1.1 applications MUST be able to receive and decode
the "chunked" transfer-coding, and MUST ignore chunk-extension
extensions they do not understand, the server MAY reliably include
the extension in chunks regardless of actual client-side support.
The chunk extension name MUST be the string "progress", while the
chunk extension value MUST be a short "floating point" number
comprised between 0 and 1, representing absolute progress towards
completion. The special, negative value "-1" SHOULD be used when
server progress tracking has been lost or compromised, and means
progress status is "undefined".
chunk-ext-name = "progress"
chunk-ext-val = "-1"
| ( "0" [ "." 0*3DIGIT ] )
| ( "1" [ "." 0*3("0") ] )
The progress chunk extension MAY be omitted. If no progress chunk
extension is present, the client MUST assume the current progress
chunk extension value to be equal to the previous progress chunk
extension value. If no previous chunk extension has been encountered
yet, the chunk extension value MUST be assumed to be "undefined".
The client MUST NOT assume the progress value to be monotonically
increasing, as the server MAY send any value it deems significant,
including "undefined". Nonetheless, the client MAY implement logic
presenting this information as monotonically increasing.
The client MUST NOT assume the value 1 to mean completion, due to
possible rounding errors and insufficient precision.
If the client did not send the appropriate Request Header, the server
MUST NOT send zero-length chunks unless all data has been sent, this
to ensure backwards compatibility. To comply with Transfer-Encoding:
chunked, the client, having not sent the request header, MUST accept
a zero-length chunk as an end of data, whether or not this chunk has
progress extension.
2.3 Enhanced mode of operation
If the client sent the appropriate Request Header, the server MAY
send zero-length chunks with progress information. In such a case,
the client MUST NOT assume that all data has been sent as is the case
with naked transfer encoding chunked, and MUST wait for a zero-length
chunk without a progress extension to assume completion. Indeed in
most cases, the server MAY skip sending the last progress chunk and
end the data stream with this last zero-length bare chunk instead of
sending two consecutive zero-length chunks. Nonetheless, the server
MAY, out of courtesy, send both, one notifying progress having
reached 1 and one marking completion of chunked data transfer.
3. Notes
The mechanism by which this extension sends additional metadata via
chunked extensions, even when there is no actual data to be sent,
effectively implements a much more general multiplexing extension,
which other chunked extensions may use. This area needs particular
scrutiny, as it alters the Transfer-Encoding: chunked mode of
operation. Mabye this should best be extracted in a newly defined
Transfer-Encoding, entirely distinct from chunked.
Suggestions of improvements are welcome to increase compatibility
with proxies, especially in the enhanced mode of operation.
It is regrettable that much state handling has to be done in order to
implement those modes of operations, but it is necessary to extend
current functionality all the while keeping backwards compatibility.
Clients and servers may restrict themselves to implement only basic
mode of operation, greatly simplifying the required work but limiting
potential functionality in the most dynamic cases.
Author's Addresses
Loic Nageleisen
Email: loic.nageleisen@gmail.com