Data Packets

• Data and messages travel across networks and the Internet in a structure called a packet. The data may have to be broken up into segments to get to its destination this is because there may be more data than can fit in a packet of the length dictated by the networks MTU. The data segment has to be less than the MTU size, because the packet also contains a number of headers. Headers contain address information for the packet and also describe the data within the packet body. The number and size of headers on a packet depend on the application requirements of the sending program. However, there will be at least two headers: a transport header and an IP header. The transport protocol assembles the packet. The IP header is dictated by the Internet Protocol, which governs the address format for computers connected to the Internet, called the IP address. The IP header contains the source and destination addresses for the packet.
  
  

Fragmentation

• The sender of a packet cannot dictate which route that packet will take. Even the router on the sender's network does not know which path the packet will follow. It only chooses the first hop of the packet's journey. The next router that receives the packet then decides where it will next send the packet and so on until the packet reaches its destination. One of the routers in the chain, may work for a network with an MTU smaller than the size of the arriving packet. It will break up the packet into smaller packets, below its MTU so that the data can continue on its journey. This process is called fragmentation. The packet remains fragmented until it reaches its destination.
  
  

IP Headers

• A flag in the IP header dictates whether receivers of the packet are allowed to fragment the packet. If the fragmentation flag forbids fragmentation, the router will not pass on the packet. It drops the packet and returns an error message back to the originator of the packet.
  
  

Change MTU Settings

• The MTU settings are stored in a computer's registry, so Windows users need to use the Regedit Utility to change it. The registry entry is called REG_DWORD. Its value depends on the type of connection the computer uses to the Internet. Windows has a default MTU for different Internet connection types and will ignore the MTU in the registry if it larger than the default. The user's preference is only implemented if it is smaller than the standard MTU used by Windows for each connection type.