docs: Improve & link cnat doc

Type: docs

Change-Id: Ib985ed6a644ae3f4c330bf6a27dc69c49a489a2f
Signed-off-by: Nathan Skrzypczak <nathan.skrzypczak@gmail.com>

3 files changed, 81 insertions, 13 deletions

diff --git a/docs/gettingstarted/developers/cnat.rst b/docs/gettingstarted/developers/cnat.rst
new file mode 120000
index 00000000000..c261d501228
--- /dev/null
+++ b/docs/gettingstarted/developers/cnat.rst
@@ -0,0 +1 @@
+../../../src/plugins/cnat/cnat.rst
\ No newline at end of file
diff --git a/docs/gettingstarted/developers/index.rst b/docs/gettingstarted/developers/index.rst
index 93f4e33ee7f..378b9fb83c0 100644
--- a/docs/gettingstarted/developers/index.rst
+++ b/docs/gettingstarted/developers/index.rst
@@ -41,3 +41,4 @@ The Developers section covers the following areas:
    punt
    quic_plugin
    cross_compile_macos.rst
+   cnat
diff --git a/src/plugins/cnat/cnat.rst b/src/plugins/cnat/cnat.rst
index f1b3deff55e..3cb08bc8c6c 100644
--- a/src/plugins/cnat/cnat.rst
+++ b/src/plugins/cnat/cnat.rst
@@ -17,27 +17,45 @@ translations of the packets destined to them
 Terminology & Usage
 ___________________
 
-Setting up the NAT will consist in the creation of a translation
-that has several backends. A translation is 3-tuple containing :
+Setting up the NAT will consist in the creation of a ``translation``
+that has several backends. A ``translation`` is 3-tuple containing :
 a fully qualified IP address a port and a protocol. All packets
 destined to it (ip, port) will then choose one of the backends,
 and follow its rewrite rules.
 
-A backend consists of four rewrites components (source & destination
+A ``backend`` consists of four rewrites components (source & destination
 address, source & destination port) that shall be applied to packets
 on the way in, and reverted on the way back.
 
 Backends are equally load-balanced with a flow hash. The choice
-of a backend for a flow will trigger the creation of a NAT session,
+of a ``backend`` for a flow will trigger the creation of a NAT ``session``,
 that will store the packet rewrite to do and the one to undo
 until the flow is reset or a timeout is reached
 
+A ``session`` is a fully resolved 9-tuple of ``src_ip, src_port, dest_ip, dest_port, proto``
+to match incoming packets, and their new attributes ``new_src_ip, new_src_port, new_dest_ip, new_dest_port``. It allows for ``backend`` stickyness and a fast-path for established connections.
+
+These ``sessions`` expire after 30s for regular ``sessions`` and 1h for estabished
+TCP connections. These can be changed in vpp's configuration file
+
+.. code-block:: console
+
+  cnat {
+      session-max-age 60
+      tcp-max-age 3600
+  }
+
+Traffic is matched by inserting FIB entries, that are represented
+by a ``client``. These maintain a refcount of the number of ``sessions``
+and/or ``translations`` depending on them and be cleaned up when
+all have gone.
+
 Translating Addresses
 ---------------------
 
-In this example, all packets destined to 30.0.0.2:80 will be
-rewritten so that their destination IP is 20.0.0.1 and destination
-port 8080. Here 30.0.0.2 has to be a virtual IP, it cannot be
+In this example, all packets destined to ``30.0.0.2:80`` will be
+rewritten so that their destination IP is ``20.0.0.1`` and destination
+port ``8080``. Here ``30.0.0.2`` has to be a virtual IP, it cannot be
 assigned to an interface
 
 .. code-block:: console
@@ -45,9 +63,9 @@ assigned to an interface
   cnat translation add proto TCP vip 30.0.0.2 80 to ->20.0.0.1 8080
 
 
-If 30.0.0.2 is the address of an interface, we can use the following
+If ``30.0.0.2`` is the address of an interface, we can use the following
 to do the same translation, and additionnaly change the source.
-address with 1.2.3.4
+address with ``1.2.3.4``
 
 .. code-block:: console
 
@@ -67,15 +85,63 @@ SourceNATing outgoing traffic
 A independant part of the plugin allows changing the source address
 of outgoing traffic on a per-interface basis.
 
+In the following example, all traffic comming from ``tap0`` and NOT
+going to ``20.0.0.0/24`` will be source NAT-ed with ``30.0.0.1``.
+On the way back the translation will be undone.
+
+NB: ``30.0.0.1`` should be and address known to the FIB (e.g. the
+address assigned to an interface)
+
 .. code-block:: console
 
-  cnat snat with 30::1
-  cnat snat exclude 20::/100
-  ex_ctl _calico_master cnat snat exclude 10::/100
-  ex_ctl _calico_master set interface feature tap0 ip6-cnat-snat arc ip6-unicast
+  cnat snat with 30.0.0.1
+  cnat snat exclude 20.0.0.0/24
+  set interface feature tap0 ip4-cnat-snat arc ip4-unicast
+
+Other parameters
+----------------
+
+In vpp's startup file, you can also configure the bihash sizes for
+
+* the translation bihash ``(proto, port) -> translation``
+* the session bihash ``src_ip, src_port, dest_ip, dest_port, proto -> new_src_ip, new_src_port, new_dest_ip, new_dest_port``
+* the snat bihash for searching ``snat exclude`` prefixes
 
+.. code-block:: console
 
+  cnat {
+      translation-db-memory 64K
+      translation-db-buckets 1024
+      session-db-memory 1M
+      session-db-buckets 1024
+      snat-db-memory 64M
+      snat-db-buckets 1024
+  }
 
 Extending the NAT
 _________________
 
+This plugin is built to be extensible. For now two NAT types are defined, ``cnat_node_vip.c`` and ``cnat_node_snat.c``. They both inherit from ``cnat_node.h`` which provides :
+
+* Session lookup : ``rv`` will be set to ``0`` if a session was found
+* Translation primitives ``cnat_translation_ip4`` based on sessions
+* A session creation primitive ``cnat_session_create``
+
+Creating a session will also create a reverse session (for matching return traffic),
+and call a NAT node back that will perform the translation.
+
+Known limitations
+_________________
+
+This plugin is still under developpment, it lacks the following features :
+* Load balancing doesn't support parametric probabilities
+* VRFs aren't supported. All rules apply to fib table 0 only
+* Programmatic session handling (deletion, lifetime updates) aren't supported
+* ICMP is not yet supported
+* Traffic matching is only done based on ``(proto, dst_addr, dst_port)`` source matching isn't supported
+* Statistics & session tracking are still rudimentary.
+
+
+
+
+