SABR in the extractor

SABR is YouTube's session-based delivery protocol. The protocol itself, the rationale, UMP, BotGuard, attestation, is the SABR Guide. This section is the extractor-side map: the ~47 classes in services/youtube/sabr, what each one actually does, and how a playing session is driven end to end. It assumes you've read the guide.

Where it sits

When YouTube returns SABR formats, the stream extractor surfaces them with DeliveryMethod.SABR (see Streams and delivery). Unlike progressive or DASH, there is no per-format URL to hand the player. The bytes live behind one endpoint, serverAbrStreamingUrl, that only speaks the SABR request/response dance: you POST a binary VideoPlaybackAbrRequest, you get back a UMP body, you decode it, you advance, you repeat.

The division of labour:

• The extractor owns the protocol. It probes the player response, opens the session, builds each binary request, POSTs it, decodes the UMP, tracks per-track buffered state, reassembles segments, and bounds every failure mode.
• The client owns the rhythm and the screen. Its pump decides when to ask for more, caches the segments, feeds the decoder, drives seeks, and supplies the one thing the extractor can't produce: a PO token (minted in a WebView).

The extractor never mints a token and never decodes a pixel.

Read in this order

1. Starting a session — the probe: player response, serverAbrStreamingUrl, the client profiles, formats.
2. The request — the VideoPlaybackAbrRequest proto, field by field, and the wire format.
3. UMP and decoding — the UMP framing, the full part-type table, the decoded response.
4. Media, segments, and the index — how media bytes become playable segments, and how time maps to a segment number.
5. The buffered-range model and seeking — the contiguous-vs-max model that prevents starvation, and how seeking works.
6. The session driver — the pump loop, the cache, eviction, and every bounded retry.
7. Control parts reference — every server instruction and its effect.

Class map

Area	Classes
Session lifecycle	YoutubeSabrProbe, YoutubeSabrProbeResult, YoutubeSabrInfo, YoutubeSabrSession, YoutubeSabrStreamState, YoutubeSabrClientProfile
Formats	YoutubeSabrFormat, SabrSelectableFormats, SabrFormatSelectionConfig, SabrFormatInitializationMetadata
Request	YoutubeSabrRequestBuilder, SabrSegmentRequest, SabrRequestIdentifier, SabrColdStartPoToken, SabrProto, SabrRequestDumper
UMP + decode	UmpReader, UmpPart, SabrResponseDecoder, SabrDecodedResponse
Media + segments	SabrMediaHeader, SabrMediaSegment, SabrMediaSegmentCollector, SabrSegmentIndex, SabrMp4SegmentIndexParser, SabrWebmSegmentIndexParser, SabrOnesieData, SabrOnesieHeader, SabrOnesieInnertubeResponse
Control parts	SabrNextRequestPolicy, SabrStreamProtectionStatus, SabrRedirect, SabrSeek, SabrReloadPlayerResponse, SabrPlaybackStartPolicy, SabrContextSendingPolicy, SabrContextUpdate, SabrContextValue, SabrRequestCancellationPolicy, SabrPrewarmConnection, SabrLiveMetadata, SabrError, SabrSnackbarMessage
Buffered state	SabrBufferedRange, SabrPlaybackCookie
Tokens	SabrPoTokenProvider
Errors	SabrProtocolException

The boundary

The extractor understands and drives the protocol; it does not mint tokens and does not render media. For the protocol itself, the request and response shapes, the session model, BotGuard, and attestation, read the SABR Guide.