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3. SCHC Compound ACK

3. SCHC Compound ACK​

The SCHC Compound ACK is a failure SCHC ACK message that can contain several bitmaps, with each bitmap being identified by its corresponding window number. In [RFC8724], the failure SCHC ACK message only contains one bitmap corresponding to one window. The SCHC Compound ACK extends this format, allowing more windows to be acknowledged in a single ACK and reducing the total number of failure SCHC ACK messages, especially when fragment losses are present in intermediate windows.

The SCHC Compound ACK MAY be used in fragmentation modes that use windows and that allow reporting the bitmaps of multiple windows at the same time; otherwise, the SCHC Compound ACK MUST NOT be used.

The SCHC Compound ACK:

  • provides feedback only for windows with fragment losses,

  • has a variable size that depends on the number of windows with fragment losses being reported in the single SCHC Compound ACK,

  • includes the window number (i.e., W) of each bitmap,

  • might not cover all windows with fragment losses of a SCHC Packet, and

  • is distinguishable from the SCHC Receiver-Abort.

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