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Re: [EME] Support for multiple initData values in a single session?

From: Mark Watson <watsonm@netflix.com>
Date: Tue, 19 Jun 2012 15:27:25 +0000
To: David Dorwin <ddorwin@google.com>
CC: "<public-html-media@w3.org>" <public-html-media@w3.org>
Message-ID: <41CB9B90-CD90-433C-AC2B-6243DD21E553@netflix.com>
Hi David,

If we define a session by the rule that different sessions represent CDM state that needs a different lifecycle (create/update/delete), then there could at least be a one-to-many relationship between initData and sessions. i.e. the same initData block could be used to create multiple sessions.

I don't equate this as being a "single license exchange" - as per the other discussion on heartbeat and key rotation, the CDM can ask for a message exchange at any time and whether these message exchanges result in new 'licenses' inside the session state depends on how you define 'license'. Since we're not defining 'license' ourselves it must be up to the CDM.

There are two approaches for the use case where a single session can be used to decrypt multiple files (e.g. audio/video).
1) we require that all the necessary initData is included in both files, so the session can be initialized with one initData and when the other file is encountered the CDM will discover that there is a session already created with all the necessary state (for example the initData contains the key ids and the CDM recognizes it already has those keys).
2) we would need to provide multiple initData blocks to a single session

In (2), it's not certain that providing additional initData to a session will result in a new message exchange.

Also, when new initData is encountered, who should decide whether a new session is required or whether this initData can be absorbed into an existing session ? Probably the CDM, right ? If we use the object-oriented design, do we just overload the method which creates a new session (from some initData) so that it can either create a new one or return an existing one ?


On Jun 11, 2012, at 10:34 PM, David Dorwin wrote:

In the current draft, each successful call to generateKeyRequest() generates a new session. Since initData is passed to generateKeyRequest(), each initData would be in a separate session. I'd like to get feedback on whether separating each initData into a separate session or object would be a problem.

Possible issues with only allowing one initData value per session include:
 * Each session is a separate license exchange.
 * Applications that care about session IDs will need to deal with multiple IDs.
 * In the potential sessions as objects design (https://www.w3.org/Bugs/Public/show_bug.cgi?id=16613), each session would be a separate object.

The following are some possible scenarios where multiple initData values may occur:
 * Audio and video are in separate files and have different initData.
 * Different tracks or bitrates have different initData.
 * A container supports multiple initData values.

To allow multiple initData values per session, we would either need to separate session creation from providing initData or allow an array of initData to be provided. Both solutions would work in both the current and sessions as objects designs. However, note that since needkey events may occur at different times, the application might not know when it has all initData values that it needs to send. In addition, initData values that are encountered later (i.e. after a stream switch or later in the container) could not be added to the session. Addressing that would probably require changing the 1:1 license/session ratio.
Received on Tuesday, 19 June 2012 15:27:56 UTC

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