ovn-sb.xml: Update and improve documentation.

Some of the abbreviations at the head of this document, like LN and PN,
turn out to not be very useful, so expand them for clarity.

Some of the statements in this document are more about planning the design
than the current design.  Remove these for clarity.

Port_Binding rows used to all be about physical locations, except for
patch ports, but there are more kinds of rows now.  Elaborate for clarity.

Expand on the purpose of the Datapath_Binding table.

Signed-off-by: Ben Pfaff <[email protected]>
Acked-by: Russell Bryant <[email protected]>

ovn/ovn-sb.xml

@@ -21,60 +21,61 @@
     different properties, as described in the sections below.
   </p>
 
-  <h3>Physical Network (PN) data</h3>
+  <h3>Physical network</h3>
 
   <p>
-    PN tables contain information about the chassis nodes in the system.  This
-    contains all the information necessary to wire the overlay, such as IP
-    addresses, supported tunnel types, and security keys.
+    Physical network tables contain information about the chassis nodes in the
+    system.  This contains all the information necessary to wire the overlay,
+    such as IP addresses, supported tunnel types, and security keys.
   </p>
 
   <p>
-    The amount of PN data is small (O(n) in the number of chassis) and it
-    changes infrequently, so it can be replicated to every chassis.
+    The amount of physical network data is small (O(n) in the number of
+    chassis) and it changes infrequently, so it can be replicated to every
+    chassis.
   </p>
 
   <p>
-    The <ref table="Chassis"/> table comprises the PN tables.
+    The <ref table="Chassis"/> and <ref table="Encap"/> tables are the physical
+    network tables.
   </p>
 
-  <h3>Logical Network (LN) data</h3>
+  <h3>Logical Network</h3>
 
   <p>
-    LN tables contain the topology of logical switches and routers, ACLs,
-    firewall rules, and everything needed to describe how packets traverse a
-    logical network, represented as logical datapath flows (see Logical
-    Datapath Flows, below).
+    Logical network tables contain the topology of logical switches and
+    routers, ACLs, firewall rules, and everything needed to describe how
+    packets traverse a logical network, represented as logical datapath flows
+    (see Logical Datapath Flows, below).
   </p>
 
   <p>
-    LN data may be large (O(n) in the number of logical ports, ACL rules,
-    etc.).  Thus, to improve scaling, each chassis should receive only data
-    related to logical networks in which that chassis participates.  Past
-    experience shows that in the presence of large logical networks, even
-    finer-grained partitioning of data, e.g. designing logical flows so that
-    only the chassis hosting a logical port needs related flows, pays off
-    scale-wise.  (This is not necessary initially but it is worth bearing in
-    mind in the design.)
+    Logical network data may be large (O(n) in the number of logical ports, ACL
+    rules, etc.).  Thus, to improve scaling, each chassis should receive only
+    data related to logical networks in which that chassis participates.
   </p>
 
   <p>
-    The LN is a slave of the cloud management system running northbound of OVN.
-    That CMS determines the entire OVN logical configuration and therefore the
-    LN's content at any given time is a deterministic function of the CMS's
-    configuration, although that happens indirectly via the
-    <ref db="OVN_Northbound"/> database and <code>ovn-northd</code>.
+    The logical network data is ultimately controlled by the cloud management
+    system (CMS) running northbound of OVN.  That CMS determines the entire OVN
+    logical configuration and therefore the logical network data at any given
+    time is a deterministic function of the CMS's configuration, although that
+    happens indirectly via the <ref db="OVN_Northbound"/> database and
+    <code>ovn-northd</code>.
   </p>
 
   <p>
-    LN data is likely to change more quickly than PN data.  This is especially
-    true in a container environment where VMs are created and destroyed (and
-    therefore added to and deleted from logical switches) quickly.
+    Logical network data is likely to change more quickly than physical network
+    data.  This is especially true in a container environment where containers
+    are created and destroyed (and therefore added to and deleted from logical
+    switches) quickly.
   </p>
 
   <p>
-    <ref table="Logical_Flow"/> and <ref table="Multicast_Group"/> contain LN
-    data.
+    The <ref table="Logical_Flow"/>, <ref table="Multicast_Group"/>, <ref
+    table="Address_Group"/>, <ref table="DHCP_Options"/>, <ref
+    table="DHCPv6_Options"/>, and <ref table="DNS"/> tables contain logical
+    network data.
   </p>
 
   <h3>Logical-physical bindings</h3>
@@ -178,7 +179,7 @@
   <table name="Chassis" title="Physical Network Hypervisor and Gateway Information">
     <p>
       Each row in this table represents a hypervisor or gateway (a chassis) in
-      the physical network (PN).  Each chassis, via
+      the physical network.  Each chassis, via
       <code>ovn-controller</code>/<code>ovn-controller-vtep</code>, adds
       and updates its own row, and keeps a copy of the remaining rows to
       determine how to reach other hypervisors.
@@ -1655,11 +1656,18 @@ tcp.flags = RST;
 
   <table name="Datapath_Binding" title="Physical-Logical Datapath Bindings">
     <p>
-      Each row in this table identifies physical bindings of a logical
-      datapath.  A logical datapath implements a logical pipeline among the
-      ports in the <ref table="Port_Binding"/> table associated with it.  In
-      practice, the pipeline in a given logical datapath implements either a
-      logical switch or a logical router.
+      Each row in this table represents a logical datapath, which implements a
+      logical pipeline among the ports in the <ref table="Port_Binding"/> table
+      associated with it.  In practice, the pipeline in a given logical
+      datapath implements either a logical switch or a logical router.
+    </p>
+
+    <p>
+      The main purpose of a row in this table is provide a physical binding for
+      a logical datapath.  A logical datapath does not have a physical
+      location, so its physical binding information is limited: just <ref
+      column="tunnel_key"/>.  The rest of the data in this table does not
+      affect packet forwarding.
     </p>
 
     <column name="tunnel_key">
@@ -1725,9 +1733,12 @@ tcp.flags = RST;
 
   <table name="Port_Binding" title="Physical-Logical Port Bindings">
     <p>
-      Most rows in this table identify the physical location of a logical port.
-      (The exceptions are logical patch ports, which do not have any physical
-      location.)
+      Each row in this table binds a logical port to a realization.  For most
+      logical ports, this means binding to some physical location, for example
+      by binding a logical port to a VIF that belongs to a VM running on a
+      particular hypervisor.  Other logical ports, such as logical patch ports,
+      can be realized without a specific physical location, but their bindings
+      are still expressed through rows in this table.
     </p>
 
     <p>