[Hls-interest] LL-HLS Amendment Proposal: Optional part response headers for CDN efficiency.

[Andrew Crowe <[e-mail]>]

Hello,

CMAF content packaged and delivered using LL-DASH and range-based LL-HLS
are easily managed as duplicate content by CDNs as they specify only
segment files. In fact, once the segment is complete, it can then be served
out of CDN cache for players that are not Low Latency capable - effectively
reducing latency for them as well. Part-based LL-HLS introduces
individually named part files that then collapse to the separately named
segment file upon final part completion. This then means that on first
request for the whole collapsed segment file the CDN will have to go back
to origin to request bytes that it likely already has in the individually
named part files. CDNs can improve cache efficiency, origin hit rate, and
whole segment delivery times with a little bit of additional information
from origin.


On request for a named part file an origin may provide a set of response
headers:

*X-HLS-Part-Sequence*
A multi value header that represents the current part sequence (index=1)
and the total number of parts for the segment. The values will be separated
by a forward slash ("/"). For example a 2 second segment with 8 parts per
segment will respond to the 2nd part request
(vid720_segment_1521.part2.m4v) like
X-HLS-Part-Sequence: 2/8


*X-HLS-Part-Offset*
A single value header that represents the byte offset of the part in the
segment. The first part of a segment will always be 0 while, for example
the second .25s part of a 2mpbs stream (vid720_segment_1521.part2.m4v) may
have a value like 623751


*X-HLS-Part-Root-Segment*
A single value header that provides the name of the root segment of the
current part. This lets the CDN/proxy know which root file to concatenate
the parts into. vid720_segment_1521.part2.m4v would have a value of
vid720_segment_1521.m4v


With the information from these three headers the CDN can recognize the
individually named part files as ranges of a larger file, store them
effectively and deliver a better experience to viewers across all formats.

Regards,
-Andrew
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