checksum() example added in doc

doc/scapy/functions.rst

+***********************
+Calling Scapy functions
+***********************
+
+This section provides some examples that show how to benefit from Scapy
+functions in your own code.
+
+UDP checksum
+============
+
+The following example explains howto use the checksum() function to compute and
+UDP checksum manually. The following steps must be performed:
+
+1. compute the UDP pseudo header as described in RFC768
+2. build an UDP packet with Scapy with p[UDP].chksum=0
+3. call checksum() with the pseudo header and the UDP packet
+
+::
+
+  from scapy.all import *
+
+  def pseudo_header(ip_src, ip_dst, ip_proto, length):
+      """
+      Return a pseudo header according to RFC768
+      """
+      # Prepare the binary representation of the pseudo header
+      return struct.pack("!4s4sHH", inet_aton(ip_src), inet_aton(ip_dst), ip_proto, length)
+
+  # Get the UDP checksum computed by Scapy
+  packet = IP(dst="10.11.12.13", src="10.11.12.14")/UDP()/DNS()
+  packet_raw = str(packet)
+  checksum_scapy = IP(packet_raw)[UDP].chksum
+
+  # Set the UDP checksum to 0 and compute the checksum 'manually'
+  packet = IP(dst="10.11.12.13", src="10.11.12.14")/UDP(chksum=0)/DNS()
+  packet_raw = str(packet)
+  udp_raw = packet_raw[20:]
+  phdr = pseudo_header(packet.src, packet.dst, socket.IPPROTO_UDP, len(udp_raw))
+
+  assert(checksum_scapy == checksum(phdr + udp_raw))

doc/scapy/index.rst

@@ -21,6 +21,7 @@ This document is under a `Creative Commons Attribution - Non-Commercial
    advanced_usage
    extending
    build_dissect
+   functions
 
    troubleshooting
    development