3.1.5.1.5 SACK

If the bExtOpcode field indicates FRAME_EXOPCODE_SACK (0x06), the source address (for example, IPv4 address and port type when running on UDP) for the message SHOULD be checked. If the address does not correspond to one with a fully established connection, it MUST be ignored. If the connection used signing, the signature MUST be valid; otherwise, the packet MUST be ignored. The bNSeq, bNRcv, optional selective acknowledgment (SACK), and optional send mask fields are then processed by using the standard rules in sections 3.1.5.2.1 through 3.1.5.2.4.

A successfully validated SACK packet SHOULD count as a valid receive and thus restart the KeepAlive timer.