3.1.5.12 SACK Mask Processing

When one or both of the optional SACK mask 32-bit fields is present, and one or more bits are set in the fields, the sender is indicating that it received a packet or packets out of order, presumably due to packet loss. The two 32-bit, little-endian fields MUST be considered as one 64-bit field, where dwSACKMask1 is the low 32 bits and dwSACKMask2 is the high 32 bits. If either 32-bit field is not available, the entire contents of the 64-bit field MUST be considered as all 0.

The receiver of a SACK mask SHOULD loop through each bit of the combined 64-bit value in the ascending order of significance. Each bit corresponds to a sequence ID after bNRcv. If the bit is set, it indicates that the corresponding packet was received out of order

The receiver of a SACK mask SHOULD shorten the retry timer for the first frame of the window to speed recovery from the packet loss. The recommended duration is 10 milliseconds. This value can be modified according to application and network requirements. The receiver MAY also choose to remove the selectively acknowledged packets from its list to retry.