Change Details

Since the dawn of the SACK feature, TSO was never enabled during SACK loss recovery episodes. With the use of LRO / GRO leading to ACK compression (partial ACKs and SACK blocks stepping by more than a singular segment), and also features like PRR which allow increasing the cwnd in the final phase of loss recovery (SSRB), this frequently results in multiple passes of the tcp_default_output() codepath. Initially restrict the maximum allowable size for a SACK TSO chunk to no more than 4 MSS in case this exposes rare cwnd issues.

Since the dawn of the SACK feature, TSO was never enabled during SACK loss recovery episodes. With the use of LRO / GRO leading to ACK compression (partial ACKs and SACK blocks stepping by more than a singular segment), and also features like PRR which allow increasing the cwnd in the final phase of loss recovery (SSRB), this frequently results in multiple passes of the tcp_default_output() codepath. It turns out that previously, the SACK transmission path would only really work as the restriction to 1 MSS per call to tcp_output() work, but allowing more than 1 MSS could result in invalid transmission selection. Refactoring of cwnd and moving the adjustment for SACKed data into tcp_output() - cwnd tracking the maximum extend from snd_una - allows both SACK loss recovery as well as SACK transmissions after RTO during slow start.

Since the dawn of the SACK feature, TSO was never enabled during SACK loss recovery episodes. With the use of LRO / GRO leading to ACK compression (partial ACKs and SACK blocks stepping by more than a singular segment), and also features like PRR which allow increasing the cwnd in the final phase of loss recovery (SSRB), this frequently results in multiple passes of the tcp_default_output() codepath. Initially restrictIt turns out that previously, the maximum allowable size for aSACK transmission path would SACK TSO chunk to no more than 4only really work as the restriction to 1 MSS in case thisper call to tcp_output() exposes rare cwnd issueswork, but allowing more than 1 MSS could result in invalid transmission selection. Refactoring of cwnd and moving the adjustment for SACKed data into tcp_output() - cwnd tracking the maximum extend from snd_una - allows both SACK loss recovery as well as SACK transmissions after RTO during slow start.