| | 28 | |
| | 29 | The above figure shows how a STA encapsulates and de-encapsulates different types of Ethernet frames. In general, the process is very similar to that of the AP's but is complicated by the addition of an extra hardware MAC address: the address of the client device connected to the WARP STA via Ethernet. The STA implementation must spoof all packets to "trick" the rest of the network into thinking that the Ethernet source address of the connected client is the wireless MAC address of the WARP STA. |
| | 30 | |
| | 31 | === Encapsulation === |
| | 32 | |
| | 33 | The transmitter address (addr) of all outgoing MPDUs must be the MAC address of the WARP STA -- otherwise, the AP will not accept these packets since the Ethernet address of the client device was not used during the authentication and association handshake. As such, the source address of all received Ethernet frames does not appear anywhere in the outgoing wireless MPDU transmission. Since this address will be needed to construct Ethernet transmissions, we save this address into memory. ''Note: This approach has a limitation that only one device should be plugged into the Ethernet port of the WARP STA. The implementation cannot handle seeing multiple source MAC addresses without implementing NAT.'' |
| | 34 | |
| | 35 | In addition to the standard address Ethernet to MPDU address translation, the STA must also modify the contents of a few specific types of packets: |
| | 36 | |
| | 37 | 1. ARP: ARP requests contain the Ethernet source address within the packet itself in a addition to its usual place at the front of the Ethernet header. The 802.11 STA implementation spoofs this address with the wireless MAC address of the WARP node. |
| | 38 | 1. DHCP: DHCP discovery and request packets can contain a flag that ensures that the DHCP handshake takes place via broadcast MAC addresses rather than unicast MAC addresses that the DHCP server learns. When in unicast mode, outgoing DHCP packets from the AP may be addressed to the client device connected to the 802.11 STA rather than the wireless address of the 802.11 STA -- this confuses the AP since the client address is not in the list of associated devices. By raising the broadcast flag in DHCP packets, the WARP STA can ensure that the packets pass through the wireless network unimpeded. |
| | 39 | |
| | 40 | === De-encapsulation === |
