AAMP DASH Architecture Overview

DASH DRM Flow (analogous to MSE/EME)

1. PrivateStreamAbstractionMPD::ProcessContentProtection

• Checks if PSSH is available in manifest and, initiates DRM session creation, if PSSH is available.
• This will be the starting point for DRM session creation, if PSSH is available in manifest.
• Also Queue Protection Event using the extracted PSSH, for the cases where PSSH is missing in stream.
• This protection event is published to gstreamer layer from AMPGstPlayer_SendPendingEvents
  • 1.A) If yes extracts the PSSH and metadata for available DRM systems
  • 1.B) If multiple DRMs are present selects the preferred one
  • 1.C) If lastProcessedKeyID != newKeyID

Spawns thread to create DRM session for that keyID

2. AampDRMSessionManager::createDrmSession

• CreateDrmSession, creates and/or returns the decrypt context by calling DRM API's through OpenCDMSession
• We have two slots for DRM sessions, if the new request matches none of the already processed keyIDs, clears the oldest one.
  • 2.A) Returns drm context to decrypt if a session is present with requested keyID
  • 2.B) Else, creates new drm session and stores the keyID to keyID cache.

   AampDrmSessionFactory::GetDrmSession(systemId);

• Initializes and returns appropriate DrmSession ie, PlayReady or WideWine
• drm session state will be KEY_INIT on success

   drmSession->generateAampDRMSession(initDataPtr, dataLength);

• Binds the initdata with the initialized drm session
• drm session state will be KEY_INIT on success

    drmSession->aampGenerateKeyRequest(destinationURL);

•  Generates license request from the drm session
• drm session state will be KEY_PENDING on success

    SecClient_AcquireLicense(....);     OR  AampDRMSessionManager's getLicense()

• Gets the license key by posting a proper request to license server, created
•  using KeyRequest, destinationURL, accessToken etc.

    drmSession->aampDRMProcessKey(key);

• Key returned from AcquireLicense is fed to drm.
• drm session state will be KEY_READY on success

3. Handling GST_EVENT_PROTECTION in gst_aampcdmidecryptor_sink_event

• In normal cases, where PSSH is present in both manifest and stream AAMP will initiate to create drm session from step 1 itself and drm session will be almost ready, when qtdemux publishes the GST_EVENT_PROTECTION 
• In cases where PSSH is not in stream this will be the starting point of DRM session creation
  • 3.A) Parse the initdata in PROTECTION_EVENT and request drm session.

4. qtdemux and playready/widevine plugins.

• AAMP playready/widevine plugins are initialized with GST_RANK_PRIMARY
• This rank is changed to GST_RANK_PRIMARY + 111, on first AAMP tune.
• This is to make sure AAMP plugins are not interfering with WPE plugins

    4.A) qtdemux detects stream encryption

    4.B) qtdemux sends need context message for preferred-drm

    4.C) bus_sync_handler in AAMPGstPlayer responds with preferred-drm

    4.D) qtdemux sends protection event for preferred-drm

    4.E) playready/widevine plugins receives the protection event, only the preferred plugin process it and other one discards it.

    4.F) gst_aampcdmidecryptor_transform_ip is invoked, where buffer is decrypted

Below is the diagram showing MSE-EME’s stack overview, we follow almost same flow in AAMP.

https://www.w3.org/TR/encrypted-media/stack_overview.svg

https://w3c.github.io/encrypted-media/format-registry/stream/mp4.html

https://w3c.github.io/encrypted-media/format-registry/initdata/cenc.html