Your task is to build a simple "echo" server.

• The server should automatically return to any client that connects exactly what it receives (it should echo all messages).
• You will also write a python script that, when run, will send a message to the server and receive the reply, printing it to stdout.
• Finally, you’ll do all of the above in such a way that it can be tested.

• Complete the code in echo_server.py to create a server that sends back whatever messages it receives from a client

• Complete the code in echo_client.py to create a client function that can send a message and receive a reply.

• Ensure that the tests in tests.py pass.

• Open one terminal while in this folder and execute this command:

  $ python echo_server.py

• Open a second terminal in this same folder and execute this command:

  $ python echo_client.py "This is a test message."

Once all tasks are completed, the server should print out a message indicating the message that it received from the client, and the client should print out a message indicating that it received the message back from the server.

• Open one terminal while in this folder and execute this command:

  $ python echo_server.py

• Open a second terminal in this same folder and execute this command:

  $ python tests.py

Look at demo_client.py and demo_server.py. These demonstrate basic client/server communication as shown in class. You can play the short video demo_client_server_behavior.mp4 to see an example how these two files can be called to work together.

To complete the assignment in echo_server.py and echo_client.py, you'll be using MOST of the same lines of code. The main difference is that the echo_server:

1. Has an outer loop that accepts a connection from a client, processes a message from that client, closes the client connection, and then repeats.
2. Has an inner loop that pulls bytes off the client connection 16 bytes at a time.
3. Also, you're putting all of this code lives inside of a function named server.

One more hint: how do you know when you're done pulling 16 byte chunks off of the client connection? You're done with recv returns fewer than 16 bytes.

Simple:

• Write a python function that lists the services provided by a given range of ports.

  • accept the lower and upper bounds as arguments
  • provide sensible defaults
  • Ensure that it only accepts valid port numbers (0-65535)

Challenging:

• The echo server as outlined will only process a connection from one client at a time. If a second client were to attempt a connection, it would have to wait until the first message was fully echoed before it could be dealt with.

• Python provides a module called select that allows waiting for I/O events in order to control flow. The select.select method can be used to allow our echo server to handle more than one incoming connection in "parallel".

• Read the documentation about the select module (https://docs.python.org/3/library/select.html) and attempt to write a second version of the echo server that can handle multiple client connections in "parallel". You do not need to invoke threading of any kind to do this.