3.2. SCHC Compound ACK Behavior

3.2. SCHC Compound ACK Behavior

The SCHC ACK-on-Error behavior is described in Section 8.4.3 of [RFC8724]. The present document slightly modifies this behavior. In the baseline SCHC specification, a SCHC ACK reports only one bitmap for the reception of exactly one window of tiles. The present SCHC Compound ACK specification extends the SCHC ACK message format so that it can contain several bitmaps, with each bitmap being identified by its corresponding window number.

As presented in [RFC8724], the SCHC ACK format can be considered a special SCHC Compound ACK case in which it reports only the tiles of one window. Therefore, the SCHC Compound ACK is backwards compatible with the SCHC ACK format presented in [RFC8724]. The receiver can assume that the sender does not support the SCHC Compound ACK if, although the SCHC Compound ACK sent by the receiver reports losses in more than one window, the sender does not resend any tiles from windows other than the first window reported in the SCHC Compound ACK. In that case, the receiver can send SCHC Compound ACKs with only one window of tiles.

Also, some flexibility is introduced with respect to [RFC8724] in that the receiver has the capability to respond (or not) to the All-0 with a SCHC Compound ACK, depending on certain parameters, like network conditions, sender buffer/cache size, and supported application delay. Note that even though the protocol allows for such flexibility, the actual decision criteria is not specified in this document. The application MUST set expiration timer values according to when the feedback is expected to be received, e.g., after the All-0 or after the All-1.

Section 3.2.1 (and its subsections) replaces the complete Section 8.4.3 (and its subsections) of [RFC8724].