I wanted to capture, for statistical purposes, the inter-arrival time of packets sent from a remote machine.
In scapy, all packets have a parameter "time", which contains the unix time of the system when the packet is received.

If I send the two following packets:

hybrid:~ diogomonica$ scapy 
Welcome to Scapy (2.0.1) 
>>> send(IP(dst="XXX.XXX.XXX.XXX")/UDP(dport=8000)/"TAG1") 
Sent 1 packets. 
>>> send(IP(dst="XXX.XXX.XXX.XXX")/UDP(dport=8000)/"TAG2") 
Sent 1 packets. 

The following script is able to extract their inter-arrival time (this assumes you already have a .pcap where you sniffed the arrival of both packets):

from scapy.all import * 
capture = rdpcap(sys.argv[1]) 
for packet in capture: 
   if packet.haslayer(UDP) and packet.haslayer(IP) and packet.haslayer(Raw): 
      if packet.getlayer(Raw).load == sys.argv[2]: 
         start = packet.time 
      if packet.getlayer(Raw).load == sys.argv[3]: 
         end = packet.time 
         print (end - start)*1000 

Executing the script with the pcap file, and both payload strings, as arguments, we get the time (in milliseconds) between the arrival of both packets:

hybrid:~ diogomonica$ python interarrival.py XXX.pcap TAG1 TAG2