Re: [Hls-interest] LL-HLS: status-code expected in the response to the PRELOAD request

[Pieter-Jan Speelmans <[e-mail]>]

Hi Will

I would indeed expect this behavior to be the case if the client is
configured to collapse part requests when the part hold off is configured
to three part duration.

P


On Wed, 26 Aug 2020, 17:44 Law, Will, <wilaw=[e-mail]>
wrote:

> @Roger – that’s good news. A clear directive within the spec will help
> achieve consistent player behavior as well as consistent CDN and origin
> response.
>
>
>
> One note on the applicability of RFC 8673 would be that it seems to apply
> for *any request for LL-HLS content that is open-ended,* not only for
> PRELOAD-HINT requests. The interesting thing about range-based addressing
> with LL-HLS is that the player on average only need make one request per
> media type per segment duration. This is true in nearly all start-up and
> switch conditions too, with the sole exception being when the PRELOAD-HINT
> represents the first part in a new segment. The following playlist snippets
> illustrate this point. Can you confirm if this is the expected client
> behavior?
>
>
>
> *Case #1:*
>
> #EXTINF:4.000,
>
> v1_1-7727.m4s
>
> #EXT-X-PROGRAM-DATE-TIME:2020-07-28T17:37:48.771Z
>
> #EXT-X-PART:DURATION=0.500,URI="v1_1-7728.m4s",BYTERANGE="64267@0
> ",INDEPENDENT=YES
>
> #EXT-X-PART:DURATION=0.500,URI="v1_1-7728.m4s",BYTERANGE="63033@64267"
>
> #EXT-X-PART:DURATION=0.500,URI="v1_1-7728.m4s",BYTERANGE="57810@127300"
>
> #EXT-X-PART:DURATION=0.500,URI="v1_1-7728.m4s",BYTERANGE="60558@185110"
>
> #EXT-X-PART:DURATION=0.500,URI="v1_1-7728.m4s",BYTERANGE="68575@245668"
>
> #EXT-X-PART:DURATION=0.500,URI="v1_1-7728.m4s",BYTERANGE="60880@314243"
>
> #EXT-X-PRELOAD-HINT:TYPE=PART,URI="v1_1-7728.m4s",BYTERANGE-START=375123
>
>
>
> A player seeing this playlist at startup would need to make a single
> request of the form
>
>
>
>    GET / v1_1-7728.m4s HTTP/2
>
>    Host: example.com
>
>    Range: bytes=0-9007199254740991
>
>
>
> The origin would respond by bursting the bytes it has (up to 375123) and
> then releasing the remainder as each part boundary becomes available. This
> would give it the independent part it needs to start, plus all the segments
> up to and including the HINTed part.
>
>
>
>   HTTP/2 206 Partial Content
>
>    Content-Range: bytes 0-9007199254740991/*
>
>
>
>
>
> *Case #2:*
>
>
>
> #EXTINF:4.000,
>
> v1_1-7727.m4s
>
> #EXT-X-PROGRAM-DATE-TIME:2020-07-28T17:37:48.771Z
>
> #EXT-X-PART:DURATION=0.500,URI="v1_1-7728.m4s",BYTERANGE="64267@0
> ",INDEPENDENT=YES
>
> #EXT-X-PART:DURATION=0.500,URI="v1_1-7728.m4s",BYTERANGE="63033@64267"
>
> #EXT-X-PART:DURATION=0.500,URI="v1_1-7728.m4s",BYTERANGE="57810@127300"
>
> #EXT-X-PART:DURATION=0.500,URI="v1_1-7728.m4s",BYTERANGE="60558@185110"
>
> #EXT-X-PART:DURATION=0.500,URI="v1_1-7728.m4s",BYTERANGE="68575@245668
> ",INDEPENDENT=YES
>
> #EXT-X-PART:DURATION=0.500,URI="v1_1-7728.m4s",BYTERANGE="60880@314243"
>
> #EXT-X-PRELOAD-HINT:TYPE=PART,URI="v1_1-7728.m4s",BYTERANGE-START=375123
>
>
>
> A player seeing this playlist at startup would make a single request of
> the form
>
>
>
>    GET / v1_1-7728.m4s HTTP/2
>
>    Host: example.com
>
>    Range: bytes=245668-9007199254740991
>
>
>
> Origin would respond by bursting the bytes from 245668 to 375123 and then
> releasing the remainder as each part boundary becomes available.
>
>
>
>   HTTP/2 206 Partial Content
>
>    Content-Range: bytes 245668-9007199254740991/*
>
>
>
>
>
> *Case #3:*
>
>
>
> #EXTINF:4.000,
>
> v1_1-7727.m4s
>
> #EXT-X-PROGRAM-DATE-TIME:2020-07-28T17:37:48.771Z
>
> #EXT-X-PART:DURATION=0.500,URI="v1_1-7728.m4s",BYTERANGE="64267@0
> ",INDEPENDENT=YES
>
> #EXT-X-PART:DURATION=0.500,URI="v1_1-7728.m4s",BYTERANGE="63033@64267"
>
> #EXT-X-PART:DURATION=0.500,URI="v1_1-7728.m4s",BYTERANGE="57810@127300"
>
> #EXT-X-PART:DURATION=0.500,URI="v1_1-7728.m4s",BYTERANGE="60558@185110"
>
> #EXT-X-PART:DURATION=0.500,URI="v1_1-7728.m4s",BYTERANGE="68575@245668
> ",INDEPENDENT=YES
>
> #EXT-X-PART:DURATION=0.500,URI="v1_1-7728.m4s",BYTERANGE="60880@314243"
>
> #EXT-X-PART:DURATION=0.500,URI="v1_1-7728.m4s",BYTERANGE="62485@375123"
>
> #EXT-X-PART:DURATION=0.500,URI="v1_1-7728.m4s",BYTERANGE="61325@437608"
>
> #EXTINF:4.000,
>
> v1_1-7728.m4s
>
> #EXT-X-PRELOAD-HINT:TYPE=PART,URI="v1_1-7729.m4s",BYTERANGE-START=0
>
>
>
> This player would make a first request for
>
>
>
>    GET / v1_1-7728.m4s HTTP/2
>
>    Host: example.com
>
>    Range: bytes=245668-498933
>
>
>
> Note that this request is NOT using the RFC8673 convention for
> last-byte-pos, because the last-byte-pos is known. The origin would respond
> by adding in a content-length response header, signaling the total bytes in
> the content-range header and bursting all these bytes since all the content
> is fully available.
>
>
>
>   HTTP/2 206 Partial Content
>
>   Content-Length: 253265
>
>   Content-Range: bytes 245668-498933/498934
>
>
>
> The player would then make a second open ended range request for
>
>
>
>    GET / v1_1-7729.m4s HTTP/2
>
>    Host: example.com
>
>    Range: bytes=0-9007199254740991
>
>
>
> Origin would respond by releasing the content as each part becomes
> available.
>
>
>
>   HTTP/2 206 Partial Content
>
>    Content-Range: bytes 0-9007199254740991/*
>
>
>
> This is the steady state condition of the player from this point on. It
> keeps making a series of open-ended range requests against each new segment
> starting at an offset of 0. This continues until it needs to switch,
> encounters a discontinuity or playback pauses.  During this time it
> continues to update the media playlist at a period of the part duration, in
> order to maintain its knowledge of the internal structure of each segment
> and also to discover discontinuities.
>
>
>
> Cheers
>
> Will
>
>
>
>
>
> *From: *Roger Pantos <rpantos=[e-mail]>
> *Date: *Tuesday, August 25, 2020 at 12:38 PM
> *To: *"Law, Will" <[e-mail]>
> *Cc: *"[e-mail]" <[e-mail]>
> *Subject: *Re: [Hls-interest] LL-HLS: status-code expected in the
> response to the PRELOAD request
>
>
>
> Okay, I think that approach would work well enough. We’ll take a closer
> look at it once iOS 14 et al are in the can. Assuming it works out we can
> put a reference to that part of RFC 8673 into the EXT-X-PRELOAD-HINT
> section of the HLS spec.
>
>
>
> We can also update our clients to conform to that behavior, although it
> might take a while to land that in an iOS/macOS release.
>
>
>
>
>
> Roger.
>
>
>
> On Aug 19, 2020, at 5:11 PM, Law, Will <wilaw=[e-mail]>
> wrote:
>
>
>
> @Roger – re “*Note that when using H2 (or H3) that there is a fourth
> option, which is to return 206 and not supply any Content-Length header at
> all.”*
>
> This is a good point however it still leaves open the question of what
> content-range header should be returned. Per the H2 spec (snippet below),
> the requirements of HTTP/1.1 Range Requests are carried forward under
> HTTP/2. These requirements RFC7233 state that the “the server generating
> the 206 response MUST generate a Content-Range header field,” and also that
> the byte-range must contain a last-byte-pos value and that a value of * can
> be used for an unknown total length of the object but it cannot be used for
> last-byte-pos. Since the last-byte-pos is not known in this case, we have
> the same problem with H2 that we have with H1.
>
>
>
> The solution proposed by https://tools.ietf.org/html/rfc8673
> <https://urldefense.proofpoint.com/v2/url?u=https-3A__tools.ietf.org_html_rfc8673&d=DwMFaQ&c=96ZbZZcaMF4w0F4jpN6LZg&r=KkevKJerDHRF9WRs8nW8Ew&m=QUdz0dxv2aY_mBTjkxa9DgyIAgM1TzH8j_w8i_spX2w&s=oMR4zfPXrho88f5S0AzpDC5z3D0bwKPKsOm2kymBHBk&e=>
>  would seem to work for H2 as well as it would for H1. Per this solution,
> the client should never make an open ended range request if it is expecting
> an aggregated response from a fixed offset. It should instead send a
> request with a very large number (9007199254740991 has been proposed in
> this thread) as the last-byte-pos in the range request. This would signal
> the server (or origin) to begin a response that starts at the requested
> offset and aggregates over time until the object is completely transferred.
>
>
>
>
> https://tools.ietf.org/html/rfc7540#section-8
> <https://urldefense.proofpoint.com/v2/url?u=https-3A__tools.ietf.org_html_rfc7540-23section-2D8&d=DwMFaQ&c=96ZbZZcaMF4w0F4jpN6LZg&r=KkevKJerDHRF9WRs8nW8Ew&m=QUdz0dxv2aY_mBTjkxa9DgyIAgM1TzH8j_w8i_spX2w&s=BzKvOAFjcWwd1oNZnRb_NH1b50kZYcJ245JTW83T53g&e=>
>
> 8. HTTP Message Exchanges
>
> HTTP/2 is intended to be as compatible as possible with current uses
> of HTTP. This means that, from the application perspective, the
> features of the protocol are largely unchanged. To achieve this, all
> request and response semantics are preserved, although the syntax of
> conveying those semantics has changed.
>
> Thus, the specification and requirements of HTTP/1.1 Semantics and
> Content [RFC7231], Conditional Requests [RFC7232], Range Requests
> [RFC7233], Caching [RFC7234], and Authentication [RFC7235] are
> applicable to HTTP/2.
>
>
>
> -Will
>
>
>
> *From: *Roger Pantos <rpantos=[e-mail]>
> *Date: *Wednesday, August 19, 2020 at 1:17 PM
> *To: *"[e-mail]" <[e-mail]>
> *Subject: *Re: [Hls-interest] LL-HLS: status-code expected in the
> response to the PRELOAD request
>
>
>
>
>
>
>
>
> On Aug 17, 2020, at 4:30 PM, Law, Will <wilaw=[e-mail]>
> wrote:
>
>
>
> Hello
>
>
>
> I am requesting a clarification on the expected server response status
> code when a client makes an open-ended range request against a media
> segment when playing back LL-HLS.  Consider the following media playlist
> snippet:
>
>
>
> #EXTINF:4.000,
>
> v1_1-7727.m4s
>
> #EXT-X-PROGRAM-DATE-TIME:2020-07-28T17:37:48.771Z
>
> #EXT-X-PART:DURATION=0.500,URI="v1_1-7728.m4s",BYTERANGE="64267@0
> ",INDEPENDENT=YES
>
> #EXT-X-PART:DURATION=0.500,URI="v1_1-7728.m4s",BYTERANGE="63033@64267"
>
> #EXT-X-PART:DURATION=0.500,URI="v1_1-7728.m4s",BYTERANGE="57810@127300"
>
> #EXT-X-PART:DURATION=0.500,URI="v1_1-7728.m4s",BYTERANGE="60558@185110"
>
> #EXT-X-PART:DURATION=0.500,URI="v1_1-7728.m4s",BYTERANGE="68575@245668"
>
> #EXT-X-PART:DURATION=0.500,URI="v1_1-7728.m4s",BYTERANGE="60880@314243"
>
> #EXT-X-PRELOAD-HINT:TYPE=PART,URI="v1_1-7728.m4s",BYTERANGE-START=375123
>
>
>
> To start-up, the client would issue 6 GET range-requests , starting at the
> independent part The server would respond with a 206 response for each and
> a content-range response header indicating the range being returned.
>
>
>
> The next request would be for the PRELOAD-HINT part. This would be a GET
> request with a “range: 375123-“. The client is indicating it wants to
> receive this object starting at offset 375123 and continuing to the end of
> the segment.
>
>
>
> How should the origin (or proxy server) respond to this PRELOAD request?
> Three possible options
>
>    1. It holds back any response until the end of segment, returning at
>    that time a 206 response with a content-range of 375123 – T/T+1 (where
>    represents total size of segment). This would ruin the low latency behavior
>    for the client.
>    2. It starts an immediate response, signaling 206 with content-range:
>    375123 - */*. This is actually forbidden by the RFC’s, which indicate that
>    the last-byte-pos cannot hold a value of “*”.
>    3. It starts an immediate response, signaling a 200 response and if
>    serving H1 to a proxy server which then serves H2 to the client, a
>    "Transfer-Encoding: chunked" response header.
>
>
>
>
>
> Note that when using H2 (or H3) that there is a fourth option, which is to
> return 206 and not supply any Content-Length header at all. This (I
> believe) became the expected behavior when HTTP/2 removed support for
> chunked transfer encoding:
> https://httpwg.org/specs/rfc7540.html#HttpSequence
> <https://urldefense.proofpoint.com/v2/url?u=https-3A__httpwg.org_specs_rfc7540.html-23HttpSequence&d=DwMFaQ&c=96ZbZZcaMF4w0F4jpN6LZg&r=KkevKJerDHRF9WRs8nW8Ew&m=R-IP9-Hj--z3sXAZZX9rokMs5qnnChNcbFO4PLft4_M&s=QsYGd9vU3GGNCIKQjBOnvGSnXVh73OH0Kyi1KcPWpDw&e=>
>
>
>
>
>
> Roger Pantos
>
> Apple Inc.
>
>
>
>
> Option [3] looks to be the most correct however it raises the question of
>  whether a 200 response (chunked-transfer or not) implies to the client
> that is has received the complete object i.e starting at offset 0 instead
> of offset 375123. As a CDN, we need to build a behavior that is robustly
> supported by all HTTP clients and not just a particular class of
> application. The proxy-server cannot tell if is serving a LL-HLS client or
> some other client, therefore we need a consistent behavior when it is asked
> for an open-ended range request against an object of unknown size.
>
>
>
> So the questions are:
>
>    1. Is a 200 status-code expected in the response to the PRELOAD
>    request?
>    2. If so, are there other clients or applications on the internet that
>    would break if we did so for *all open-ended range requests against
>    objects of unknown size* (outside of the application space of LL-HLS)?
>
>
>
> Cheers
>
> Will
>
>
>
>
>
>
>
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