1 files changed, 32 insertions, 0 deletions
diff --git a/Documentation/networking/tuntap.rst b/Documentation/networking/tuntap.rst
index 4d7087f727be..56c9dc7b96af 100644
--- a/Documentation/networking/tuntap.rst
+++ b/Documentation/networking/tuntap.rst
@@ -206,6 +206,38 @@ enable is true we enable it, otherwise we disable it::
       return ioctl(fd, TUNSETQUEUE, (void *)&ifr);
   }
 
+3.4 qdisc backpressure
+----------------------
+
+IFF_BACKPRESSURE can be set to enable qdisc backpressure. Without it, TX
+drops occur when the internal ring buffer is full, so any attached qdisc
+is effectively bypassed and applications only learn about congestion
+through those drops.
+
+With it, the kernel stops the queue instead, letting the qdisc hold and
+schedule packets, so its AQM, shaping and fairness actually apply. This
+helps protocols like TCP, which cut throughput in reaction to packet
+drops. With IFF_BACKPRESSURE, drops then only occur as a rare race.
+Backpressure requires a qdisc to be attached and has no effect with
+noqueue.
+
+The flag is a property of the TUN/TAP device rather than of the file
+descriptor it was set on, so it applies to all queues of the device,
+regardless of which process opened which queue.
+
+The flag can only be changed while the device has at most one queue. On a
+multiqueue device that already has a second queue attached or detached, a
+later TUNSETIFF succeeds but leaves the flag as it is. All the other
+TUNSETIFF flags behave the same way.
+
+The txqueuelen can be reduced alongside this flag to further shift
+buffering into the qdisc and reduce bufferbloat, at a possible
+performance cost.
+
+When running multiple network streams in parallel through a single
+TUN/TAP queue, the flag may reduce performance due to the extra overhead
+of the backpressure mechanism.
+
 Universal TUN/TAP device driver Frequently Asked Question
 =========================================================