def _group_transaction_messages(self, messages):
''' Group together the SIP transactions. '''
transactions = {}
for message in sorted(messages, key=itemgetter('utc')):
if message['protocol'] == 'SIP':
sipmessage = SipMessage()
sipmessage.load(message)
key = sipmessage.get_transaction_id()
if key in transactions.keys():
transactions[key].append(sipmessage)
else:
transactions[key] = [sipmessage]
return transactions
