title: Scapy: fields to recompute when forging packets
body: <p>I'm forging with Scapy the TTL value in the IP header in some data packet that I captured. Besides the checksum in both IP and transport layer headers, is there anything else I should recompute?</p>
  
  <p>Right now I'm doing:</p>
  
  <pre><code>for p in myPackets:
      p[IP].ttl = targetTTL
      del(p[IP].chksum)
      del(p[IP].payload.chksum) 
  
  for i in range(len(myPackets)):
      myPackets[i] = myPackets[i].__class__(str(myPackets[i])) 
  </code></pre>
  
  <p>I'm asking this because Scapy's <code>sr</code> function, which matches packets with their responses (ICMP in my case) has been returning data with a few RTTs > 1 second, which is absurd since I'm using ttl=1.</p>