<p>How does Istio support IP based routing between pods in the same Service (or ReplicaSet to be more specific)?</p>

<p>We would like to deploy a Tomcat application with replica > 1 within an Istio mesh. The app runs Infinispan, which is using JGroups to sort out communication and clustering. JGroups need to identify its cluster members and for that purpose there is the KUBE_PING (Kubernetes discovery protocol for JGroups). It will consult K8S API with a lookup comparable to <em>kubectl get pods</em>. The cluster members can be both pods in other services and pods within the same Service/Deployment.</p>

<p>Despite our issue being driven by rather specific needs the topic is generic. How do we enable pods to communicate with each other within a replica set?</p>

<p>Example: as a showcase we deploy the demo application <a href="https://github.com/jgroups-extras/jgroups-kubernetes" rel="noreferrer">https://github.com/jgroups-extras/jgroups-kubernetes</a>. The relevant stuff is:</p>

<pre><code>apiVersion: v1
items:
- apiVersion: extensions/v1beta1
  kind: Deployment
  metadata:
    name: ispn-perf-test
    namespace: my-non-istio-namespace
  spec:
    replicas: 3
&lt; -- edited for brevity -- &gt;
</code></pre>

<p>Running <strong>without Istio</strong>, the three pods will find each other and form the cluster. Deploying the same <strong>with Istio</strong> in <em>my-istio-namespace</em> and adding a basic Service definition:</p>

<pre><code>kind: Service
apiVersion: v1
metadata:
  name: ispn-perf-test-service
  namespace: my-istio-namespace
spec:
  selector:
    run : ispn-perf-test
  ports:
  - protocol: TCP
    port: 7800
    targetPort: 7800
    name: "one"
  - protocol: TCP
    port: 7900
    targetPort: 7900
    name: "two"
  - protocol: TCP
    port: 9000
    targetPort: 9000
    name: "three"
</code></pre>

<p>Note that output below is wide - you might need to scroll right to get the IPs</p>

<pre><code>kubectl get pods -n my-istio-namespace -o wide
NAME                                                READY     STATUS    RESTARTS   AGE       IP            NODE
ispn-perf-test-558666c5c6-g9jb5                     2/2       Running   0          1d        10.44.4.63    gke-main-pool-4cpu-15gb-98b104f4-v9bl
ispn-perf-test-558666c5c6-lbvqf                     2/2       Running   0          1d        10.44.4.64    gke-main-pool-4cpu-15gb-98b104f4-v9bl
ispn-perf-test-558666c5c6-lhrpb                     2/2       Running   0          1d        10.44.3.22    gke-main-pool-4cpu-15gb-98b104f4-x8ln

kubectl get service ispn-perf-test-service -n my-istio-namespace
NAME                     TYPE        CLUSTER-IP    EXTERNAL-IP   PORT(S)                      AGE
ispn-perf-test-service   ClusterIP   10.41.13.74   &lt;none&gt;        7800/TCP,7900/TCP,9000/TCP   1d
</code></pre>

<p>Guided by <a href="https://istio.io/help/ops/traffic-management/proxy-cmd/#deep-dive-into-envoy-configuration" rel="noreferrer">https://istio.io/help/ops/traffic-management/proxy-cmd/#deep-dive-into-envoy-configuration</a>, let's peek into the resulting Envoy conf of one of the pods:</p>

<pre><code>istioctl proxy-config listeners ispn-perf-test-558666c5c6-g9jb5 -n my-istio-namespace
ADDRESS          PORT      TYPE
10.44.4.63       7900      TCP
10.44.4.63       7800      TCP
10.44.4.63       9000      TCP
10.41.13.74      7900      TCP
10.41.13.74      9000      TCP
10.41.13.74      7800      TCP
&lt; -- edited for brevity -- &gt;
</code></pre>

<p>The Istio doc describes the listeners above as</p>

<blockquote>
  <p>Receives outbound non-HTTP traffic for relevant IP:PORT pair from
  listener <code>0.0.0.0_15001</code></p>
</blockquote>

<p>and this all makes sense. The pod <em>ispn-perf-test-558666c5c6-g9jb5</em> can reach itself on 10.44.4.63 and the service via 10.41.13.74. But... what if the pod sends packets to 10.44.4.64 or 10.44.3.22? Those IPs are not present among the listeners so afaik the two "sibling" pods are non-reachable for <em>ispn-perf-test-558666c5c6-g9jb5</em>.</p>

<p>Can Istio support this today - then how?</p>

<p>You are right that HTTP routing only supports local access or remote access by service name or service VIP.</p>

<p>That said, for your particular example, above, where the service ports are named "one", "two", "three", the routing will be plain TCP as described <a href="https://istio.io/docs/setup/kubernetes/spec-requirements/" rel="nofollow noreferrer">here</a>. Therefore, your example should work. The pod ispn-perf-test-558666c5c6-g9jb5 can reach itself on 10.44.4.63 and the other pods at 10.44.4.64 and 10.44.3.22.</p>

<p>If you rename the ports to "http-one", "http-two", and "http-three" then HTTP routing will kick in and the RDS config will restrict the remote calls to ones using recognized service domains.</p>

<p>To see the difference in the RDF config look at the output from the following command when the port is named "one", and when it is changed to "http-one".</p>

<pre><code>istioctl proxy-config routes ispn-perf-test-558666c5c6-g9jb5 -n my-istio-namespace  --name 7800 -o json
</code></pre>

<p>With the port named "one" it will return no routes, so TCP routing will apply, but in the "http-one" case, the routes will be restricted.</p>

<p>I don't know if there is a way to add additional remote pod IP addresses to the RDS domains in the HTTP case. I would suggest opening an Istio issue, to see if it's possible.</p>
