PIM: examples: document the IPv4 pim examples (dm, sm, iptv)

The IPv4 PIM examples had no explanatory documentation (just a stale ANSA
header and an unhelpful description). Add a README to each example folder
describing the scenario, topology, what it demonstrates, and how to run it:
dm (PIM-DM flood-and-prune), sm (PIM-SM with a static RP, four
join/send-ordering scenarios), and iptv (a 4x4 PIM mesh over OSPF run under
both PIM-SM and PIM-DM). Also add a short network doc-comment to each .ned,
and give the dm config a meaningful description string. No changes to
simulation behavior -- fingerprints unchanged.

Author: avarga

examples/pim/dm/README

@@ -0,0 +1,19 @@
+PIM-DM (Dense Mode) Example
+===========================
+
+This example demonstrates the flood-and-prune behavior of PIM-DM
+(Protocol Independent Multicast - Dense Mode, RFC 3973).
+
+Three multicast routers R1, R2 and R3 run PIM-DM, connecting two sources
+(Source1, Source2) and a receiver host (Host2). Source1 sends to group
+226.1.1.1, which Host2 joins; Source2 sends to group 226.2.2.2, which
+nobody joins.
+
+Dense mode initially floods each stream to every router (broadcast), then
+prunes the branches that have no downstream listeners: 226.2.2.2 is pruned
+everywhere (it has no receiver), while 226.1.1.1 is delivered along the
+branch toward Host2 and pruned on the other branches.
+
+To run the example:
+
+    inet -u Cmdenv -c General

examples/pim/dm/omnetpp.ini

@@ -1,6 +1,6 @@
 [General]
 sim-time-limit = 300s
-description = "PIM Final Test"
+description = "PIM-DM flood-and-prune (dense mode)"
 network = pimDMFinal
 
 **.hasPim = true

examples/pim/dm/pimDMFinal.ned

@@ -17,6 +17,11 @@ import inet.node.inet.MulticastRouter;
 import inet.node.inet.StandardHost;
 
 
+//
+// Demonstrates PIM-DM (Dense Mode) flood-and-prune across three multicast
+// routers (R1/R2/R3): Source1 feeds the group Host2 joins, Source2 feeds a group
+// with no listeners (pruned everywhere). See omnetpp.ini for the scenario.
+//
 network pimDMFinal
 {
     submodules:

examples/pim/iptv/Network.ned

@@ -14,7 +14,9 @@ import inet.node.inet.StandardHost;
 
 
 //
-// A generated network with grid topology.
+// A 4x4 mesh of PIM multicast routers (R00..R33) with a server and a client,
+// used to distribute an IPTV-style multicast stream. OSPF provides the unicast
+// underlay for PIM's RPF. The omnetpp.ini runs it under both PIM-SM and PIM-DM.
 //
 network Network
 {

examples/pim/iptv/README

@@ -0,0 +1,30 @@
+PIM IPTV Example
+================
+
+This example models IPTV-style multicast distribution over a 4x4 mesh of
+PIM routers (R00..R33). OSPF runs as the unicast underlay, so that PIM's
+RPF (Reverse Path Forwarding) checks use dynamically learned routes rather
+than static ones.
+
+A 'server' streams to multicast group 239.0.0.11 and a 'client' joins it;
+the multicast tree is built across the mesh toward the client.
+
+The [General] section is abstract; the same scenario runs under two PIM
+modes:
+
+
+1. PIM_SM
+---------
+
+Sparse mode, with the Rendezvous Point located at R10 (192.168.13.1).
+
+
+2. PIM_DM
+---------
+
+Dense mode (flood-and-prune).
+
+
+To run, e.g.:
+
+    inet -u Cmdenv -c PIM_SM

examples/pim/sm/PIM-SM.ned

@@ -17,6 +17,12 @@ import inet.node.inet.MulticastRouter;
 import inet.node.inet.StandardHost;
 
 
+//
+// PIM-SM (Sparse Mode) topology with a static Rendezvous Point: receiver-side
+// designated routers (DR_R1/DR_R2), the RP, and source-side designated routers
+// (DR_S1/DR_S2) connecting two sources and two receivers. The omnetpp.ini
+// scenarios exercise the RP shared tree, Register, and SPT switchover.
+//
 network PIM_SM_Network
 {
     submodules:

examples/pim/sm/README

@@ -0,0 +1,45 @@
+PIM-SM (Sparse Mode) Examples
+=============================
+
+These examples demonstrate PIM-SM (Protocol Independent Multicast - Sparse
+Mode, RFC 7761) with a statically configured Rendezvous Point (RP).
+
+Topology: receiver-side designated routers DR_R1/DR_R2, the RP (10.2.2.2),
+and source-side designated routers DR_S1/DR_S2, connecting two sources and
+two receivers. Sources register their traffic to the RP, which forwards it
+down the shared (*,G) tree to the receivers; last-hop routers then switch
+over to the (S,G) shortest-path tree.
+
+The [General] section is abstract; run one of the scenarios below. They
+differ in the order and timing of joins versus sends.
+
+
+1. Scenario1
+------------
+
+Receivers join the group before the source starts sending.
+
+
+2. Scenario2
+------------
+
+The source starts sending before the receivers join, and the receivers
+come and go during the simulation.
+
+
+3. Scenario3
+------------
+
+Long send and keepalive intervals exercise the PIM-SM timers.
+
+
+4. Scenario4
+------------
+
+Two sources send to two different groups (239.0.0.11 and 239.0.0.22) at the
+same time, with two receivers each joining one of the groups.
+
+
+To run a scenario, e.g.:
+
+    inet -u Cmdenv -c Scenario1