This patch adds initial documentation for the Netfilter flowtable
infrastructure.
Reviewed-by: Florian Westphal <fw@xxxxxxxxx>
Signed-off-by: Pablo Neira Ayuso <pablo@xxxxxxxxxxxxx>
---
Documentation/networking/nf_flowtable.txt | 112 ++++++++++++++++++++++++++++++
1 file changed, 112 insertions(+)
create mode 100644 Documentation/networking/nf_flowtable.txt
diff --git a/Documentation/networking/nf_flowtable.txt b/Documentation/networking/nf_flowtable.txt
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+Netfilter's flowtable infrastructure
+====================================
+
+This documentation describes the software flowtable infrastructure available in
+Netfilter since Linux kernel 4.16.
+
+Overview
+--------
+
+Initial packets follow the classic forwarding path, once the flow enters the
+established state according to the conntrack semantics (ie. we have seen traffic
+in both directions), then you can decide to offload the flow to the flowtable
+from the forward chain via the 'flow offload' action available in nftables.
+
+Packets that find an entry in the flowtable (ie. flowtable hit) are sent to the
+output netdevice via neigh_xmit(), hence, they bypass the classic forwarding
+path (the visible effect is that you do not see these packets from any of the
+netfilter hooks coming after the ingress). In case of flowtable miss, the packet
+follows the classic forward path.
+
+The flowtable uses a resizable hashtable, lookups are based on the following
+7-tuple selectors: source, destination, layer 3 and layer 4 protocols, source
+and destination ports and the input interface (useful in case there are several
+conntrack zones in place).
+
+Flowtables are populated via the 'flow offload' nftables action, so the user can
+selectively specify what flows are placed into the flow table. Hence, packets
+follow the classic forwarding path unless the user explicitly instruct packets
+to use this new alternative forwarding path via nftables policy.
+
+This is represented in Fig.1, which describes the classic 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 matches the initial packets that went
+through the classic forwarding path. The TTL is decremented before calling
+neigh_xmit(). Fragmented traffic is passed up to follow the classic forwarding
+path given that the transport selectors are missing, therefore flowtable lookup
+is not possible.
+
+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 offload @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.
+
+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
+mailined [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
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2.11.0
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