Fragment payload size should be a multiple of eight octets

**Expected behavior:** Per RFC 791 3.1, the fragment offset for packet fragments is measured in units of 8 octets (64 bits). Therefore payloads must be fragmented in multiples of 8 (see also the discussion in RFC 791 3.2). **Actual behavior:** Fragments are not aligned to 8-octets, but are as large as the maximum transmission unit allows for the packet. This can cause problems when trying to determine where a received packet's payload actually begins and ends.