L3/L4 checksum verification pseudo code

gistfile1.lua

-- Return three values:
--   valid - is the header checksum valid (always true for IPv6)
--   offset - where the inner checksum starts, or nil if no more to checksum
--   initial - initial value for inner checksum ("pseudo header checksum")
function check_header (p)
   if is_ipv4(p) then
      -- check header checksum;
      -- find start of tcp/udp header;
      -- calculate pseudo header checksum
   elseif is_ipv6(p) then
      -- (no header checksum to check)
      -- find start of tcp/udp header;
      -- calculate pseudo header checksum
   end
end

-- Return true if the header and data checksums of p are known to be correct.
function check_packet (p)
   local valid, offset, pseudoheader_csum = check_header(p)
   if not valid then return false end -- header was invalid
   if not offset then return true end -- nothing more to check

   return ipsum(p.data + offset, p.length - offset, pseudoheader_csum) == 0
   -- ^^^ if the checksum is okay then ipsum() will return 0.
   -- That is because the packet includes its own checksum, in the
   -- checksum field, and if we calculate the correct checksum then
   -- these will "cancel out".
end