title: Access to Scapy's inner protocol layers
body: <p>I'm writing my first program in Python.
  I'm using a third-party kit for interacting with network packets (Scapy).</p>
  
  <p>When I use this method:</p>
  
  <pre><code>p=sr1(IP(dst="google.com")/TCP())
  </code></pre>
  
  <p>the result is:</p>
  
  <pre><code>&lt;IP  version=4L ihl=5L tos=0x0 len=44 id=18972 flags= frag=0L ttl=44
  proto=tcp chksum=0x25bd src=173.194.39.134 dst=192.168.137.2
  options=[] |&lt;TCP  sport=www dport=ftp_data seq=1295300429 ack=1
  dataofs=6L reserved=0L flags=SA window=14300 chksum=0x3c7e urgptr=0
  options=[('MSS', 1430)] |&lt;Padding  load='\x00\x00' |&gt;&gt;&gt;
  </code></pre>
  
  <p>I can access to <code>chksum</code> of <code>IP</code> simply by <code>p.chksum</code>, but how can I access to <code>chksum</code> of <code>TCP</code>?</p>