# Packet\_in method implementation using P4Runtime
**URL:** https://forum.p4.org/t/packet-in-method-implementation-using-p4runtime/216
**Category:** Getting Started with P4
**Created:** [January 4, 2022, 5:35pm UTC](https://forum.p4.org/t/packet-in-method-implementation-using-p4runtime/216 "2022-01-04T17:35:39Z")
**Posts on this page:** 2
**Page:** 1
### Author:  [@ppavlidis](https://forum.p4.org/u/ppavlidis)
#### Post date: [January 4, 2022, 5:35pm UTC](https://forum.p4.org/t/packet-in-method-implementation-using-p4runtime/216/1 "2022-01-04T17:35:39Z")
I am trying to implement a packet\_in method in order to receive a packet from the switch to the controller, according to p4runtime.proto file.
My code does not work and i have no idea what is going wrong.
In the beginning, i thought that i had to send a StreamMessageRequest of packet out message to get a StreamMessageResponse of packet\_in message, but as i can see they work independent.
My method written in python into the switch.py file in p4runtime library looks like this:
```auto
def packet_in(self, **kwargs):
print("Before loop")
for response in self.stream_msg_resp:
print("I am into it")
if response.HasField("packet"):
print("Packet is set")
else:
print(response.WhichOneof("update"))
return(response)
```
When i call it from my\_controller.py program, it blocks after “Before loop” print message.
I suspect that switch, when it has to send a packet to the cpu\_port, it does not create a StreamMessageResponse of packet\_in message in order to send it through grpc StreamChannel that is created when a connection between the stub and the server is established.
Am i doing anything or thinking it wrong? Can anybody help me?
---
### Author:  [@andyfingerhut](https://forum.p4.org/u/andyfingerhut)
#### Post date: [January 4, 2022, 5:55pm UTC](https://forum.p4.org/t/packet-in-method-implementation-using-p4runtime/216/2 "2022-01-04T17:55:01Z")
I have a working PTF test written in Python, using the P4Runtime API, and a corresponding P4 program, such that there are times when the controller sends a PacketOut message to the switch (which it always received on a dedicated CPU port, not a regular switch port. In those tests the CPU port is numbered 510, but this is configurable on the simple\_switch\_grpc command line).
It also contains other test cases where the switch sends a message to the dedicated CPU port, and those become PacketIn messages to the controller.
> **[p4-guide/ptf-tests/packetinout at master · jafingerhut/p4-guide](https://github.com/jafingerhut/p4-guide/tree/master/ptf-tests/packetinout)**
>
> master/ptf-tests/packetinout
It uses some Python functions in the directory `testlib` that are common to some other PTF tests I have written, which are important to read and understand, too. You can focus on only the functions in `testlib` that are actually called by the PTF test and ignore the rest, since not all of them are needed by this PTF test.