[tests/ptfadapter] add README.md on how to use ptfadapter

Signed-off-by: Stepan Blyschak <stepanb@mellanox.com>

tests/ptfadapter/README.md

@@ -0,0 +1,47 @@
+# How to write traffic tests using PTF adapter
+
+## Overview
+
+```PtfTestAdapter``` provides an interface to send and receive traffic in the same way as ```ptf.base_tests.BaseTest``` object in PTF framework.
+It makes use of ```ptf_nn_agent.py``` script running on PTF host, connectes to it over TCP and intialize PTF data plane thread.
+
+**NOTE** a good network connection between sonic-mgmt node and PTF host is requiered for traffic tests to be stable.
+
+## Usage in pytest
+
+You can use ```ptfadapter``` fixture which runs ```ptf_nn_agent.py``` on PTF and yields ```PtfTestAdapter``` object.
+
+Example test case code using PTF adapter:
+
+```python
+import ptf.testutils as testutils
+import ptf.mask as mask
+
+def test_some_traffic(ptfadapter):
+    pkt = testutils.simple_tcp_packet(
+        eth_dst=host_facts['ansible_Ethernet0']['macaddress'],
+        eth_src=ptfadapter.dataplane.get_mac(0, 0),
+        ip_src='1.1.1.1',
+        ip_dst='192.168.0.1',
+        ip_ttl=64,
+        tcp_sport=1234,
+        tcp_dport=4321)
+
+    exp_pkt = pkt.copy()
+    exp_pkt = mask.Mask(exp_pkt)
+    exp_pkt.set_do_not_care_scapy(packet.Ether, 'dst')
+    exp_pkt.set_do_not_care_scapy(packet.Ether, 'src')
+    exp_pkt.set_do_not_care_scapy(packet.IP, 'ttl')
+    exp_pkt.set_do_not_care_scapy(packet.IP, 'chksum')
+
+    testutils.send(ptfadapter, 5, pkt)
+    testutils.verify_packet_any_port(ptfadapter, exp_pkt, ports=[28, 29, 30, 31])
+```
+
+If you have changed interface configuration on PTF host (like MAC address change) or you want to run PTF providing custom parameters you can use ```reinit``` method, e.g.:
+
+```python
+def test_some_traffic(ptfadapter):
+    ptfadapter.reinit({'qlen': 1000})
+    # rest of the test ...
+```