[test] save packets as pcap file in thread-cert tests (#2965)

tests/scripts/thread-cert/pcap.py

@@ -0,0 +1,69 @@
+#
+#  Copyright (c) 2018, The OpenThread Authors.
+#  All rights reserved.
+#
+#  Licensed under the Apache License, Version 2.0 (the "License");
+#  you may not use this file except in compliance with the License.
+#  You may obtain a copy of the License at
+#
+#  http://www.apache.org/licenses/LICENSE-2.0
+#
+#  Unless required by applicable law or agreed to in writing, software
+#  distributed under the License is distributed on an "AS IS" BASIS,
+#  WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+#  See the License for the specific language governing permissions and
+#  limitations under the License.
+#
+
+""" Module to provide codec utilities for .pcap formatters. """
+
+import struct
+import time
+
+DLT_IEEE802_15_4 = 195
+PCAP_MAGIC_NUMBER = 0xa1b2c3d4
+PCAP_VERSION_MAJOR = 2
+PCAP_VERSION_MINOR = 4
+
+
+class PcapCodec(object):
+    """ Utility class for .pcap formatters. """
+
+    def __init__(self, filename):
+        self._pcap_file = open('%s.pcap' % filename, 'wb')
+        self._pcap_file.write(self.encode_header())
+        self._epoch = time.time()
+
+    def encode_header(self):
+        """ Returns a pcap file header. """
+        return struct.pack("<LHHLLLL",
+                           PCAP_MAGIC_NUMBER,
+                           PCAP_VERSION_MAJOR,
+                           PCAP_VERSION_MINOR,
+                           0, 0, 256,
+                           DLT_IEEE802_15_4)
+
+    def encode_frame(self, frame, sec, usec):
+        """ Returns a pcap encapsulation of the given frame. """
+        frame = frame[1:]
+        length = len(frame)
+        pcap_frame = struct.pack("<LLLL", sec, usec, length, length)
+        pcap_frame += frame
+        return pcap_frame
+
+    def _get_timestamp(self):
+        """ Returns the internal timestamp. """
+        timestamp = time.time() - self._epoch
+        timestamp_sec = int(timestamp)
+        timestamp_usec = int((timestamp - timestamp_sec) * 1000000)
+        return timestamp_sec, timestamp_usec
+
+    def append(self, frame, timestamp=None):
+        """ Appends a frame. """
+        if timestamp is None:
+            timestamp = self._get_timestamp()
+        pkt = self.encode_frame(frame, *timestamp)
+        self._pcap_file.write(pkt)
+
+    def __del__(self):
+        self._pcap_file.close()