Description of problem:

UDP packet larger than MTU size can not be sent to its' own end point via the service. MTU here means "MTU size - OVNK header (100) - IP header (20)"

A pod ----(UDP)---> service IP ----------> A pod

Version-Release number of selected component (if applicable):

OCP 4.12.z 4.14.z 4.16.z 

How reproducible:

Always

Steps to Reproduce:

1. create a pod and a service with an open UDP port (19765). e.g.

$ cat service.yaml 
apiVersion: v1
kind: Service
metadata:
  name: udp-service
spec:
  selector:
    app: udp
  ports:
  - name: udp-port
    protocol: UDP
    port: 19765
    targetPort: 19765
  type: ClusterIP

$ cat udp-pod.yaml 
apiVersion: v1
kind: Pod
metadata:
  labels:
    app: udp
  name: udppod
spec:
  containers:
  - name: udp
    resources:
    securityContext:
      privileged: true
      capabilities:
        add: ["NET_RAW"]
    image: docker.io/dokken/centos-8 
    args:
      - bash
      - -c
      - sleep INF 

2. In the pod, create a binary file that is bigger than the MTU size. If the MTU is 1500:

# yes 1 | head -n 10000 | tr '\n' ' ' | dd of=datafile bs=1373 count=1 

2. Send the file in UDP protocol via the service IP of the same pod

 # nc -lu 19765 &
 # nc -u <service IP> 19765 < datafile

Actual results:

It fails to send the data

Expected results:

It can send the data

Additional info:

1. The same works when sending UDP packet smaller than MTU size.

 e.g.

# yes 1 | head -n 10000 | tr '\n' ' ' | dd of=datafile4 bs=1372 count=1

2. Using the pod IP instead of the service IP works regardless of the packet size

3. Communication between different pods works fine regardless of the packet size

 B pod/node_A –--- (UDP) ---->  service IP ----------> A pod/node_A

It is originally reported by a telco partner 

Affected Platforms: