To date, most users use router to access Internet. Generally, no problem occurs to ping packet, but Outlook cannot be used normally or some web pages cannot be accessed. This is because there are big packets in both applications (the packet from devault negotiation by TCP may be very big).
Outlook fails to receive mails, or web pages cannot be accessed.
Use tcp mss command to limite the size of negotiation packet (1024); that is, packet fragments are less than the value. Configuring mss to 1024 means in TCP negotiation, the application program is notified not to transmit big packets to avoid down-layer fragment. Without fragments, NAT could process the packets normally.
In the fragment packets of IP, only the first fragment has the five-element group information of IP (source and destination IP addresses, source and destination port numbers, protocol number). In the succeeding fragments, there is no port number information etc. , failing NAT to process the packets.