tcp: fix DSACK-based undo on non-duplicate ACK

Fixes DSACK-based undo when sender is in Open State and
an ACK advances snd_una.

Example scenario:
- Sender goes into recovery and makes some spurious rtx.
- It comes out of recovery and enters into open state.
- It sends some more packets, let's say 4.
- The receiver sends an ACK for the first two, but this ACK is lost.
- The sender receives ack for first two, and DSACK for previous
  spurious rtx.

Signed-off-by: Priyaranjan Jha <>
Signed-off-by: Yuchung Cheng <>
Signed-off-by: Neal Cardwell <>
Acked-by: Yousuk Seung <>
Signed-off-by: David S. Miller <>
1 file changed