In order to be able to use our software tools on an existing network where packet traffics of any type can be found, the system uses a specific type which is FACEh; this number is configurable. By using a unique type, the software is not required to check the entire content of a packet in order to accept it. Only the type field is checked and packets using a type different from FACEh are discarded. In this way, computers using different network applications may co-exist on the same physical wire. However, to avoid conflict with future systems, the type FACEh would have to be offically registered to keep its authenticity. The Ethernet Frame format used by our system is illustrated in figure 4.
|Preamble||Destination Physical Address||Source Physical Address||Frame Type||Frame Data||FCS|
|64 bits||48 bits||48 bits||16 bits||368-12K bits||32 bits|
The Preamble also holds a Start Frame Delimiter (SFD). It is used for synchronisation of all receivers. The preamble must be equal to 10101010b and the SFD equal to 10101011b. This field is handled by the Network Interface Card (NIC) interface. The Destination Physical Address is the address of the receiver found on its NIC interface and the Source Physical Address is the address of the originator. They are unique. A destination address equal to FFFFFFh indicates a broadcast frame which is addressed to all NIC interfaces configured to receive these type of frames. The Frame Type value is always set to FACEh. The Frame Data field holds the data to transmit or receive. According to the Ethernet specifications, this field must hold at least 46 bytes of data; if not, padding is required. The maximum data length is 1500 bytes. We designed a data structure to fill this field where the selected software tool and its associated options can be found. The Frame Check Sequence (FCS) is a cyclic redundancy checksum (CRC) used for frame error detection and correction. This field is handled by the NIC interface.