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+Load Balancer plugin
+====================
+
+Version
+-------
+
+The load balancer plugin is currently in *beta* version. Both CLIs and
+APIs are subject to *heavy* changes, which also means feedback is really
+welcome regarding features, apis, etc…
+
+Overview
+--------
+
+This plugin provides load balancing for VPP in a way that is largely
+inspired from Google’s MagLev:
+http://research.google.com/pubs/pub44824.html
+
+The load balancer is configured with a set of Virtual IPs (VIP, which
+can be prefixes), and for each VIP, with a set of Application Server
+addresses (ASs).
+
+There are four encap types to steer traffic to different ASs: 1).
+IPv4+GRE ad IPv6+GRE encap types: Traffic received for a given VIP (or
+VIP prefix) is tunneled using GRE towards the different ASs in a way
+that (tries to) ensure that a given session will always be tunneled to
+the same AS.
+
+2). IPv4+L3DSR encap types: L3DSR is used to overcome Layer 2
+limitations of Direct Server Return Load Balancing. It maps VIP to DSCP
+bits, and reuse TOS bits to transfer DSCP bits to server, and then
+server will get VIP from DSCP-to-VIP mapping.
+
+Both VIPs or ASs can be IPv4 or IPv6, but for a given VIP, all ASs must
+be using the same encap. type (i.e. IPv4+GRE or IPv6+GRE or IPv4+L3DSR).
+Meaning that for a given VIP, all AS addresses must be of the same
+family.
+
+3). IPv4/IPv6 + NAT4/NAT6 encap types: This type provides kube-proxy
+data plane on user space, which is used to replace linux kernel’s
+kube-proxy based on iptables.
+
+Currently, load balancer plugin supports three service types: a) Cluster
+IP plus Port: support any protocols, including TCP, UDP. b) Node IP plus
+Node Port: currently only support UDP. c) External Load Balancer.
+
+For Cluster IP plus Port case: kube-proxy is configured with a set of
+Virtual IPs (VIP, which can be prefixes), and for each VIP, with a set
+of AS addresses (ASs).
+
+For a specific session received for a given VIP (or VIP prefix), first
+packet selects a AS according to internal load balancing algorithm, then
+does DNAT operation and sent to chosen AS. At the same time, will create
+a session entry to store AS chosen result. Following packets for that
+session will look up session table first, which ensures that a given
+session will always be routed to the same AS.
+
+For returned packet from AS, it will do SNAT operation and sent out.
+
+Please refer to below for details:
+https://schd.ws/hosted_files/ossna2017/1e/VPP_K8S_GTPU_OSSNA.pdf
+
+Performance
+-----------
+
+The load balancer has been tested up to 1 millions flows and still
+forwards more than 3Mpps per core in such circumstances. Although 3Mpps
+seems already good, it is likely that performance will be improved in
+next versions.
+
+Configuration
+-------------
+
+Global LB parameters
+~~~~~~~~~~~~~~~~~~~~
+
+The load balancer needs to be configured with some parameters:
+
+::
+
+   lb conf [ip4-src-address <addr>] [ip6-src-address <addr>]
+           [buckets <n>] [timeout <s>]
+
+ip4-src-address: the source address used to send encap. packets using
+IPv4 for GRE4 mode. or Node IP4 address for NAT4 mode.
+
+ip6-src-address: the source address used to send encap. packets using
+IPv6 for GRE6 mode. or Node IP6 address for NAT6 mode.
+
+buckets: the *per-thread* established-connections-table number of
+buckets.
+
+timeout: the number of seconds a connection will remain in the
+established-connections-table while no packet for this flow is received.
+
+Configure the VIPs
+~~~~~~~~~~~~~~~~~~
+
+::
+
+   lb vip <prefix> [encap (gre6|gre4|l3dsr|nat4|nat6)] \
+     [dscp <n>] [port <n> target_port <n> node_port <n>] [new_len <n>] [del]
+
+new_len is the size of the new-connection-table. It should be 1 or 2
+orders of magnitude bigger than the number of ASs for the VIP in order
+to ensure a good load balancing. Encap l3dsr and dscp is used to map VIP
+to dscp bit and rewrite DSCP bit in packets. So the selected server
+could get VIP from DSCP bit in this packet and perform DSR. Encap
+nat4/nat6 and port/target_port/node_port is used to do kube-proxy data
+plane.
+
+Examples:
+
+::
+
+   lb vip 2002::/16 encap gre6 new_len 1024
+   lb vip 2003::/16 encap gre4 new_len 2048
+   lb vip 80.0.0.0/8 encap gre6 new_len 16
+   lb vip 90.0.0.0/8 encap gre4 new_len 1024
+   lb vip 100.0.0.0/8 encap l3dsr dscp 2 new_len 32
+   lb vip 90.1.2.1/32 encap nat4 port 3306 target_port 3307 node_port 30964 new_len 1024
+   lb vip 2004::/16 encap nat6 port 6306 target_port 6307 node_port 30966 new_len 1024
+
+Configure the ASs (for each VIP)
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+
+::
+
+   lb as <vip-prefix> [<address> [<address> [...]]] [del]
+
+You can add (or delete) as many ASs at a time (for a single VIP). Note
+that the AS address family must correspond to the VIP encap. IP family.
+
+Examples:
+
+::
+
+   lb as 2002::/16 2001::2 2001::3 2001::4
+   lb as 2003::/16 10.0.0.1 10.0.0.2
+   lb as 80.0.0.0/8 2001::2
+   lb as 90.0.0.0/8 10.0.0.1
+
+Configure SNAT
+~~~~~~~~~~~~~~
+
+::
+
+   lb set interface nat4 in <intfc> [del]
+
+Set SNAT feature in a specific interface. (applicable in NAT4 mode only)
+
+::
+
+   lb set interface nat6 in <intfc> [del]
+
+Set SNAT feature in a specific interface. (applicable in NAT6 mode only)
+
+Monitoring
+----------
+
+The plugin provides quite a bunch of counters and information. These are
+still subject to quite significant changes.
+
+::
+
+   show lb
+   show lb vip
+   show lb vip verbose
+
+   show node counters
+
+Design notes
+------------
+
+Multi-Threading
+~~~~~~~~~~~~~~~
+
+MagLev is a distributed system which pseudo-randomly generates a
+new-connections-table based on AS names such that each server configured
+with the same set of ASs ends up with the same table. Connection
+stickiness is then ensured with an established-connections-table. Using
+ECMP, it is assumed (but not relied on) that servers will mostly receive
+traffic for different flows.
+
+This implementation pushes the parallelism a little bit further by using
+one established-connections table per thread. This is equivalent to
+assuming that RSS will make a job similar to ECMP, and is pretty useful
+as threads don’t need to get a lock in order to write in the table.
+
+Hash Table
+~~~~~~~~~~
+
+A load balancer requires an efficient read and write hash table. The
+hash table used by ip6-forward is very read-efficient, but not so much
+for writing. In addition, it is not a big deal if writing into the hash
+table fails (again, MagLev uses a flow table but does not heavily
+relies on it).
+
+The plugin therefore uses a very specific (and stupid) hash table. -
+Fixed (and power of 2) number of buckets (configured at runtime) - Fixed
+(and power of 2) elements per buckets (configured at compilation time)
+
+Reference counting
+~~~~~~~~~~~~~~~~~~
+
+When an AS is removed, there is two possible ways to react. - Keep using
+the AS for established connections - Change AS for established
+connections (likely to cause error for TCP)
+
+In the first case, although an AS is removed from the configuration, its
+associated state needs to stay around as long as it is used by at least
+one thread.
+
+In order to avoid locks, a specific reference counter is used. The
+design is quite similar to clib counters but: - It is possible to
+decrease the value - Summing will not zero the per-thread counters -
+Only the thread can reallocate its own counters vector (to avoid
+concurrency issues)
+
+This reference counter is lock free, but reading a count of 0 does not
+mean the value can be freed unless it is ensured by *other* means that
+no other thread is concurrently referencing the object. In the case of
+this plugin, it is assumed that no concurrent event will take place
+after a few seconds.