doc: add doc for policy troubleshooting

docs/policy_troubleshoot.md

@@ -0,0 +1,112 @@
+## Policies in CalicoVPP
+
+Calico enriches Kubernetes set of policies allowing to have ordering in policies, deny rules, policies applied to host interfaces, more flexible match rules. In CalicoVPP, we feed Felix messaged to our policy server (agent component), which then configures VPP to create those policies. 
+
+## Troubleshooting policies
+
+VPP cli allows to look at policies in details, here are the commands for that
+
+```bash
+    _______    _        _   _____  ___ 
+ __/ __/ _ \  (_)__    | | / / _ \/ _ \
+ _/ _// // / / / _ \   | |/ / ___/ ___/
+ /_/ /____(_)_/\___/   |___/_/  /_/    
+
+vpp# sh capo ?
+  show capo interfaces                     show capo interfaces
+  show capo ipsets                         show capo ipsets
+  show capo policies                       show capo policies [verbose]
+  show capo rules                          show capo rules
+```
+Basically, `sh capo interfaces` shows everything related to policies and where they are applied.
+
+### Example
+
+Let's create two pods:
+
+```bash
+apiVersion: v1
+kind: Pod
+metadata:
+  labels:
+    role: sender
+  name: ts1
+spec:
+  containers:
+    - name: pod
+      image: nicolaka/netshoot
+      command: ["tail", "-f", "/dev/null"]
+---
+apiVersion: v1
+kind: Pod
+metadata:
+  labels:
+    role: receiver
+  name: ts2
+spec:
+  containers:
+    - name: pod
+      image: nicolaka/netshoot
+      command: ["tail", "-f", "/dev/null"]
+```
+Here are our pods
+```bash
+NAME   READY   STATUS    RESTARTS   AGE     IP           NODE           NOMINATED NODE   READINESS GATES
+ts1    1/1     Running   0          3m41s   11.0.0.196   kind-worker3   <none>           <none>
+ts2    1/1     Running   0          3m41s   11.0.0.67    kind-worker2   <none>           <none>
+```
+If we check ts2 interface we only have the usual allow policies:
+```bash
+sh capo interfaces
+...
+[tun3 sw_if_index=11  addr=11.0.0.67 addr6=fd20::1cc0:b1ac:ad47:e7c2]
+  profiles:
+    [policy#10]
+      tx:[rule#15;allow][]
+      rx:[rule#16;allow][]
+    [policy#11]
+```
+Let's create this policy:
+```bash
+apiVersion: networking.k8s.io/v1
+kind: NetworkPolicy
+metadata:
+  name: test-network-policy
+spec:
+  podSelector:
+    matchLabels:
+      role: receiver
+  policyTypes:
+    - Ingress
+  ingress:
+    - from:
+        - podSelector:
+            matchLabels:
+              role: sender
+      ports:
+        - protocol: TCP
+          port: 5978
+```
+And recheck interfaces policies
+```bash
+sh capo interfaces
+...
+[tun3 sw_if_index=11  addr=11.0.0.67 addr6=fd20::1cc0:b1ac:ad47:e7c2]
+  tx:
+    [policy#2]
+      tx:[rule#0;allow][src==172.18.0.2/32,src==fc00:f853:ccd:e793::2/128,]
+    [policy#12]
+      tx:[rule#18;allow][proto==TCP,dst==5978,src==[ipset#1;prefix;11.0.0.196/32,fd20::58fd:b191:5c13:9cc3/128,],]
+  profiles:
+    [policy#10]
+      tx:[rule#15;allow][]
+      rx:[rule#16;allow][]
+    [policy#11]
+```
+We see that a rule (rule#18 in policy#12) allowing tcp connections from the sender pod on 5978 port is added.
+Note: policy#2 is added automatically, it is a failsafe policy allowing traffic from host to its own pods.
+We conduct a test using netcat, it shows that this port accepts connections, unlike other ports.
+
+Other resources can be leveraged to add policies and troubleshooting is the same.
+For reference: [hostendpoint](https://docs.tigera.io/calico/latest/reference/resources/hostendpoint), [globalNetworkPolicy](https://docs.tigera.io/calico/latest/reference/resources/globalnetworkpolicy).
+