Adding upstream version 6.6.15.upstream/6.6.15

Signed-off-by: Daniel Baumann <daniel.baumann@progress-linux.org>

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+.. SPDX-License-Identifier: GPL-2.0
+
+====================================
+Netfilter's flowtable infrastructure
+====================================
+
+This documentation describes the Netfilter flowtable infrastructure which allows
+you to define a fastpath through the flowtable datapath. This infrastructure
+also provides hardware offload support. The flowtable supports for the layer 3
+IPv4 and IPv6 and the layer 4 TCP and UDP protocols.
+
+Overview
+--------
+
+Once the first packet of the flow successfully goes through the IP forwarding
+path, from the second packet on, you might decide to offload the flow to the
+flowtable through your ruleset. The flowtable infrastructure provides a rule
+action that allows you to specify when to add a flow to the flowtable.
+
+A packet that finds a matching entry in the flowtable (ie. flowtable hit) is
+transmitted to the output netdevice via neigh_xmit(), hence, packets bypass the
+classic IP forwarding path (the visible effect is that you do not see these
+packets from any of the Netfilter hooks coming after ingress). In case that
+there is no matching entry in the flowtable (ie. flowtable miss), the packet
+follows the classic IP forwarding path.
+
+The flowtable uses a resizable hashtable. Lookups are based on the following
+n-tuple selectors: layer 2 protocol encapsulation (VLAN and PPPoE), layer 3
+source and destination, layer 4 source and destination ports and the input
+interface (useful in case there are several conntrack zones in place).
+
+The 'flow add' action allows you to populate the flowtable, the user selectively
+specifies what flows are placed into the flowtable. Hence, packets follow the
+classic IP forwarding path unless the user explicitly instruct flows to use this
+new alternative forwarding path via policy.
+
+The flowtable datapath is represented in Fig.1, which describes the classic IP
+forwarding path including the Netfilter hooks and the flowtable fastpath bypass.
+
+::
+
+					 userspace process
+					  ^              |
+					  |              |
+				     _____|____     ____\/___
+				    /          \   /         \
+				    |   input   |  |  output  |
+				    \__________/   \_________/
+					 ^               |
+					 |               |
+      _________      __________      ---------     _____\/_____
+     /         \    /          \     |Routing |   /            \
+  -->  ingress  ---> prerouting ---> |decision|   | postrouting |--> neigh_xmit
+     \_________/    \__________/     ----------   \____________/          ^
+       |      ^                          |               ^                |
+   flowtable  |                     ____\/___            |                |
+       |      |                    /         \           |                |
+    __\/___   |                    | forward |------------                |
+    |-----|   |                    \_________/                            |
+    |-----|   |                 'flow offload' rule                       |
+    |-----|   |                   adds entry to                           |
+    |_____|   |                     flowtable                             |
+       |      |                                                           |
+      / \     |                                                           |
+     /hit\_no_|                                                           |
+     \ ? /                                                                |
+      \ /                                                                 |
+       |__yes_________________fastpath bypass ____________________________|
+
+	       Fig.1 Netfilter hooks and flowtable interactions
+
+The flowtable entry also stores the NAT configuration, so all packets are
+mangled according to the NAT policy that is specified from the classic IP
+forwarding path. The TTL is decremented before calling neigh_xmit(). Fragmented
+traffic is passed up to follow the classic IP forwarding path given that the
+transport header is missing, in this case, flowtable lookups are not possible.
+TCP RST and FIN packets are also passed up to the classic IP forwarding path to
+release the flow gracefully. Packets that exceed the MTU are also passed up to
+the classic forwarding path to report packet-too-big ICMP errors to the sender.
+
+Example configuration
+---------------------
+
+Enabling the flowtable bypass is relatively easy, you only need to create a
+flowtable and add one rule to your forward chain::
+
+	table inet x {
+		flowtable f {
+			hook ingress priority 0; devices = { eth0, eth1 };
+		}
+		chain y {
+			type filter hook forward priority 0; policy accept;
+			ip protocol tcp flow add @f
+			counter packets 0 bytes 0
+		}
+	}
+
+This example adds the flowtable 'f' to the ingress hook of the eth0 and eth1
+netdevices. You can create as many flowtables as you want in case you need to
+perform resource partitioning. The flowtable priority defines the order in which
+hooks are run in the pipeline, this is convenient in case you already have a
+nftables ingress chain (make sure the flowtable priority is smaller than the
+nftables ingress chain hence the flowtable runs before in the pipeline).
+
+The 'flow offload' action from the forward chain 'y' adds an entry to the
+flowtable for the TCP syn-ack packet coming in the reply direction. Once the
+flow is offloaded, you will observe that the counter rule in the example above
+does not get updated for the packets that are being forwarded through the
+forwarding bypass.
+
+You can identify offloaded flows through the [OFFLOAD] tag when listing your
+connection tracking table.
+
+::
+
+	# conntrack -L
+	tcp      6 src=10.141.10.2 dst=192.168.10.2 sport=52728 dport=5201 src=192.168.10.2 dst=192.168.10.1 sport=5201 dport=52728 [OFFLOAD] mark=0 use=2
+
+
+Layer 2 encapsulation
+---------------------
+
+Since Linux kernel 5.13, the flowtable infrastructure discovers the real
+netdevice behind VLAN and PPPoE netdevices. The flowtable software datapath
+parses the VLAN and PPPoE layer 2 headers to extract the ethertype and the
+VLAN ID / PPPoE session ID which are used for the flowtable lookups. The
+flowtable datapath also deals with layer 2 decapsulation.
+
+You do not need to add the PPPoE and the VLAN devices to your flowtable,
+instead the real device is sufficient for the flowtable to track your flows.
+
+Bridge and IP forwarding
+------------------------
+
+Since Linux kernel 5.13, you can add bridge ports to the flowtable. The
+flowtable infrastructure discovers the topology behind the bridge device. This
+allows the flowtable to define a fastpath bypass between the bridge ports
+(represented as eth1 and eth2 in the example figure below) and the gateway
+device (represented as eth0) in your switch/router.
+
+::
+
+                      fastpath bypass
+               .-------------------------.
+              /                           \
+              |           IP forwarding   |
+              |          /             \ \/
+              |       br0               eth0 ..... eth0
+              .       / \                          *host B*
+               -> eth1  eth2
+                   .           *switch/router*
+                   .
+                   .
+                 eth0
+               *host A*
+
+The flowtable infrastructure also supports for bridge VLAN filtering actions
+such as PVID and untagged. You can also stack a classic VLAN device on top of
+your bridge port.
+
+If you would like that your flowtable defines a fastpath between your bridge
+ports and your IP forwarding path, you have to add your bridge ports (as
+represented by the real netdevice) to your flowtable definition.
+
+Counters
+--------
+
+The flowtable can synchronize packet and byte counters with the existing
+connection tracking entry by specifying the counter statement in your flowtable
+definition, e.g.
+
+::
+
+	table inet x {
+		flowtable f {
+			hook ingress priority 0; devices = { eth0, eth1 };
+			counter
+		}
+	}
+
+Counter support is available since Linux kernel 5.7.
+
+Hardware offload
+----------------
+
+If your network device provides hardware offload support, you can turn it on by
+means of the 'offload' flag in your flowtable definition, e.g.
+
+::
+
+	table inet x {
+		flowtable f {
+			hook ingress priority 0; devices = { eth0, eth1 };
+			flags offload;
+		}
+	}
+
+There is a workqueue that adds the flows to the hardware. Note that a few
+packets might still run over the flowtable software path until the workqueue has
+a chance to offload the flow to the network device.
+
+You can identify hardware offloaded flows through the [HW_OFFLOAD] tag when
+listing your connection tracking table. Please, note that the [OFFLOAD] tag
+refers to the software offload mode, so there is a distinction between [OFFLOAD]
+which refers to the software flowtable fastpath and [HW_OFFLOAD] which refers
+to the hardware offload datapath being used by the flow.
+
+The flowtable hardware offload infrastructure also supports for the DSA
+(Distributed Switch Architecture).
+
+Limitations
+-----------
+
+The flowtable behaves like a cache. The flowtable entries might get stale if
+either the destination MAC address or the egress netdevice that is used for
+transmission changes.
+
+This might be a problem if:
+
+- You run the flowtable in software mode and you combine bridge and IP
+  forwarding in your setup.
+- Hardware offload is enabled.
+
+More reading
+------------
+
+This documentation is based on the LWN.net articles [1]_\ [2]_. Rafal Milecki
+also made a very complete and comprehensive summary called "A state of network
+acceleration" that describes how things were before this infrastructure was
+mainlined [3]_ and it also makes a rough summary of this work [4]_.
+
+.. [1] https://lwn.net/Articles/738214/
+.. [2] https://lwn.net/Articles/742164/
+.. [3] http://lists.infradead.org/pipermail/lede-dev/2018-January/010830.html
+.. [4] http://lists.infradead.org/pipermail/lede-dev/2018-January/010829.html