The request

Part of SABR in the extractor. YoutubeSabrRequestHelper encodes the immutable YoutubeSabrRequest as the binary VideoPlaybackAbrRequest sent to YouTube. The wire labels below are the fields emitted by the current encoder.

Request lifecycle

There are two public request factories:

• YoutubeSabrRequest.preparation(playerTimeMs, preferredFormats) asks for initialization data and format timelines without declaring active tracks;
• YoutubeSabrRequest.playback(playerTimeMs, playbackRate, tracks) declares the active audio/video tracks and their buffered timeline positions.

Every request must contain at least one track. It cannot contain two audio tracks, two video tracks, or the same itag as both audio and video. A track may carry a YoutubeSabrFormatTimeline and the last buffered sequence number.

The HTTP URL is updated by YoutubeSabrRequestHelper with alr=yes, the session CPN, and rn=<requestNumber>. rn starts at zero and is replaced on every request. The body is sent as application/x-protobuf; the response must be application/vnd.yt-ump.

Top-level fields

#	Wire	Carries	When emitted
1	message	clientAbrState	Every request
2	message	selected formatId	Playback state is included
3	message	bufferedRange	A track has a timeline and buffered sequence
4	varint	top-level playerTimeMs	Playback state is included
5	bytes	decoded videoPlaybackUstreamerConfig	Every request
16	message	preferred audio formatId	When an audio format is present
17	message	preferred video formatId	When a video format is present
19	message	streamerContext	Every request

Playback state is included for a follow-up request, a non-zero player position, or a request that carries buffered ranges. A preparation request at time zero is therefore the minimal cold start; it still carries the ustreamer config and any preferred formats.

formatId is the shared nested message used for selected and preferred formats: field 1 is the itag, field 2 is lastModified when positive, and field 3 is xtags when non-empty.

clientAbrState

The current encoder writes the following fields:

#	Meaning
18 / 19	video width/height, only when playback state is included
21	video resolution, at least 360 when a video format exists
23	bandwidth estimate on follow-ups, or an estimate from active bitrates
28	playerTimeMs
34	visibility (1)
35	playback rate, defaulting to 1.0
40	enabled track mode (1 audio-only, 2 video-only, 0 both and omitted)
46	DRC enabled when the selected audio format is DRC
69	selected audio track id, when present

The nested client info in streamerContext identifies MWEB (client id 2), the client version, and the en-US/US localization used by the helper.

Buffered ranges

For a track with a parsed timeline and bufferedThrough > 0, the helper writes a range containing:

Field	Value
formatId	itag, last-modified value and xtags
startTimeMs	0
durationMs	end time of the last buffered sequence
startSegmentIndex / endSegmentIndex	1 / the bounded buffered sequence
time-range timescale	1000

YoutubeSabrFormatTimeline is built from initialization bytes with the MP4 or WebM segment-index parser. It maps sequence numbers to start/end times and maps a requested time back to the first segment whose end is after that time.

streamerContext

The context contains client info, and optionally:

• the current PO token (field 2);
• the playback cookie from NEXT_REQUEST_POLICY (field 3);
• active opaque SABR context values (field 5);
• context types not currently sent (field 6).

Token and cookie payloads are never printed by the diagnostic summaries.

Wire format

SabrProto is the small protobuf reader/writer used by the request and response paths. It supports varints, fixed32, fixed64 and length-delimited fields. A nested message is a length-delimited byte array; field tags use (fieldNumber << 3) | wireType. Invalid field numbers, unsupported wire types, truncated input and oversized lengths raise SabrProtocolException.

Next: UMP and decoding.