Prior Group ID Gap (Extension Header Type 0x3C) is a variable length integer
containing the number of Groups prior to the current Group that do not and will
never exist. This is equivalent to receiving an End of Group status with
Object ID 0 for each skipped Group. For example, if the Original Publisher is
publishing an Object in Group 7 and knows it will never publish any Objects in
Group 8 or Group 9, it can include Prior Group ID Gap = 2 in any number of
Objects in Group 10, as it sees fit. A Track is considered malformed (see
Section 2.5) if any of the following conditions are detected:
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An Object contains more than one instance of Prior Group ID Gap
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A Group contains more than one Object with different values for Prior Group ID Gap
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An Object has a Prior Group ID Gap larger than the Group ID
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An endpoint receives an Object with a Prior Group ID Gap covering an Object it previously received
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An endpoint receives an Object with a Group ID within a previously communicated gap
This extension is optional, as publishers might not know the prior gap gize, or there may not be a gap. If Prior Group ID Gap is not present, the receiver cannot infer any information about the existence of prior groups (see Section 2.3.1).
This extension can be added by the Original Publisher, but MUST NOT be added by relays. This extension MUST NOT be modified or removed.