Create new webdriver session manually

I am trying to play a little bit with WebDriver Protocol using Python and requests module.

I started chromedriver binary:

$ chromedriver_2.31 
Starting ChromeDriver 2.31.488763 (092de99f48a300323ecf8c2a4e2e7cab51de5ba8) on port 9515
Only local connections are allowed.

So far so good.

But the problem is that I get session not created exception when trying to create a session:

import requests

r = requests.post("http://127.0.0.1:9515/session", {})

print("Status: " + str(r.status_code))
print("Body: " + str(r.content))

Execution output:

Status: 200
Body: b'{"sessionId":"286421fcd381ee0471418ebce7f3e125","status":33,"value":{"message":"session not created exception: Missing or invalid capabilities\\n  (Driver info: chromedriver=2.31.488763 (092de99f48a300323ecf8c2a4e2e7cab51de5ba8),platform=Linux 4.4.0-91-generic x86_64)"}}'

I searched through the WebDriver Protocol docs, but I couldn't find information about which capabilities are mandatory or something like that.

So, I tried with some random capabilities:

import requests

data = {
        "browserName": "chrome",
        "version": "",
        "platform": "LINUX",
        "javascriptEnabled": "true",
        "acceptInsecureCerts": "false",
        "cssSelectorsEnabled": "true"
        }

r = requests.post("http://127.0.0.1:9515/session", data)

print("Status: " + str(r.status_code))
print("Body: " + str(r.content))

But again fail:

Status: 400
Body: b'missing command parameters'

Do you have any ideas what is the problem and how to fix it?

UPDATE

Also tried:

import requests


data = """
        {
            desiredCapabilities: {  
            "browserName": "chrome",
            "version": "",
            "platform": "ANY"
            }
        }
    """

headers = {'Content-type': 'application/json'}

r = requests.post("http://127.0.0.1:9515/session", json=data, headers=headers)

print("Status: " + str(r.status_code))
print("Body: " + str(r.content))

Again error:

Status: 400
Body: b'missing command parameters'