gVisor rack blog

A blog about RACK loss detection algorithm implemented in gVisor.
(https://datatracker.ietf.org/doc/rfc8985/)

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+# gVisor RACK
+
+gVisor has implemented the [RACK](https://datatracker.ietf.org/doc/html/rfc8985)
+(Recent ACKnowledgement) TCP loss-detection algorithm in our network stack,
+which improves throughput in the presence of packet loss and reordering.
+
+TCP is a connection-oriented protocol that detects and recovers from loss by
+retransmitting packets. [RACK](https://datatracker.ietf.org/doc/html/rfc8985) is
+one of the recent loss-detection methods implemented in Linux and BSD, which
+helps in identifying packet loss quickly and accurately in the presence of
+packet reordering and tail losses.
+
+## Background
+
+The TCP congestion window indicates the number of unacknowledged packets that
+can be sent at any time. When packet loss is identified, the congestion window
+is reduced depending on the type of loss. The sender will recover from the loss
+after all the packets sent before reducing the congestion window are
+acknowledged. If the loss is identified falsely by the connection, then the
+connection enters loss recovery unnecessarily, resulting in sending fewer
+packets.
+
+Packet loss is identified mainly in two ways:
+
+1.  Three duplicate acknowledgments, which will result in either
+    [Fast](https://datatracker.ietf.org/doc/html/rfc2001#section-4) or
+    [SACK](https://datatracker.ietf.org/doc/html/rfc6675) recovery. The
+    congestion window is reduced depending on the type of congestion control
+    algorithm. For example, in the
+    [Reno](https://en.wikipedia.org/wiki/TCP_congestion_control#TCP_Tahoe_and_Reno)
+    algorithm it is reduced to half.
+2.  RTO (Retransmission Timeout) which will result in Timeout recovery. The
+    congestion window is reduced to one
+    [MSS](https://en.wikipedia.org/wiki/Maximum_segment_size).
+
+Both of these cases result in reducing the congestion window, with RTO being
+more expensive. Most of the existing algorithms do not detect packet reordering,
+which get incorrectly identified as packet loss, resulting in an RTO.
+Furthermore, the loss of an ACK at the end of a sequence (known as "tail loss")
+will also trigger RTO and slow down future transmissions unnecessarily. RACK
+helps us to identify loss accurately in all these scenarios, and will avoid
+entering RTO.
+
+## Implementation of RACK
+
+Implementation of RACK requires support for:
+
+1.  Per-packet transmission timestamps: RACK detects loss depending on the
+    transmission times of the packet and the timestamp at which ACK was
+    received.
+2.  SACK and ability to detect DSACK: Selective Acknowledgement and Duplicate
+    SACK are used to adjust the timer window after which a packet can be marked
+    as lost.
+
+### Packet Reordering
+
+Packet reordering commonly occurs when different packets take different paths
+through a network. The diagram below shows the transmission of four packets
+which get reordered in transmission, and the resulting TCP behavior with and
+without RACK.
+
+![Figure 1](/assets/images/2021-08-31-rack-figure1.png "Packet reordering.")
+
+In the above example, the sender sees three duplicate acknowledgments. Without
+RACK, this is identified falsely as packet loss, and the congestion window will
+be reduced after entering Fast/SACK recovery.
+
+To detect packet reordering, RACK uses a reorder window, bounded between
+[[RTT](https://en.wikipedia.org/wiki/Round-trip_delay)/4, RTT]. The reorder
+timer is set to expire after _RTT+reorder\_window_. A packet is marked as lost
+when the packets following it were acknowledged using SACK and the reorder timer
+expires. The reorder window is increased when a DSACK is received (which
+indicates that there is a higher degree of reordering).
+
+### Tail Loss
+
+Tail loss occurs when the packets are lost at the end of data transmission. The
+diagram below shows an example of tail loss when the last three packets are
+lost, and how it is handled with and without RACK.
+
+![Figure 2](/assets/images/2021-08-31-rack-figure2.png "Tail loss figure 2.")
+
+For tail losses, RACK uses a Tail Loss Probe (TLP), which relies on a timer for
+the last packet sent. The TLP timer is set to _2 \* RTT,_ after which a probe is
+sent. The probe packet will allow the connection one more chance to detect a
+loss by triggering ACK feedback to avoid entering RTO. In the above example, the
+loss is recovered without entering the RTO.
+
+TLP will also help in cases where the ACK was lost but all the packets were
+received by the receiver. The below diagram shows that the ACK received for the
+probe packet avoided the RTO.
+
+![Figure 3](/assets/images/2021-08-31-rack-figure3.png "Tail loss figure 3.")
+
+If there was some loss, then the ACK for the probe packet will have the SACK
+blocks, which will be used to detect and retransmit the lost packets.
+
+In gVisor, we have support for
+[NewReno](https://datatracker.ietf.org/doc/html/rfc6582) and SACK loss recovery
+methods. We
+[added support for RACK](https://github.com/google/gvisor/issues/5243) recently,
+and it is the default when SACK is enabled. After enabling RACK, our internal
+benchmarks in the presence of reordering and tail losses and the data we took
+from internal users inside Google have shown ~50% reduction in the number of
+RTOs.
+
+While RACK has improved one aspect of TCP performance by reducing the timeouts
+in the presence of reordering and tail losses, in gVisor we plan to implement
+the undoing of congestion windows and
+[BBRv2](https://datatracker.ietf.org/doc/html/draft-cardwell-iccrg-bbr-congestion-control)
+(once there is an RFC available) to further improve TCP performance in less
+ideal network conditions.
+
+If you haven’t already, try gVisor. The instructions to get started are in our
+[Quick Start](https://gvisor.dev/docs/user_guide/quick_start/docker/). You can
+also get involved with the gVisor community via our
+[Gitter channel](https://gitter.im/gvisor/community),
+[email list](https://groups.google.com/forum/#!forum/gvisor-users),
+[issue tracker](https://gvisor.dev/issue/new), and
+[Github repository](https://github.com/google/gvisor).