Frame Decapsulation Frame is forwarded with destination MAC address of Server A. Server A compares interface MAC to frame destination MAC. RTB GO/0/1 FCS Data Ethernet II SFD Preamble D.MAC S.MAC Type(0x0800) Server A 172.16.10.1/24 08-07-06-05-04-AA Server B 172.16.10.2/24 08-07-06-05-04-8B Frames received at the ultimate destination will initially determine whether the frame has arrived at the intended location. The example shows two servers on a shared Ethernet network over which both receive a copy of the frame. The frame is ultimately discarded by server B since the destination MAC value and the interface MAC address of server B do not match. Server A however successfully receives the frame and learns that the MAC fields are the same, the integrity of the frame based on the FCS can also be understood to be correct. The frame will use the type field to identify 0x0800 as the next header, following which the frame header and trailer are discarded and the packet is received by IP. Packet Decapsulation Server A compares own IP to destination address of IP header. IP header is processed and discarded, data is directed to TCP. RTB Server A Data (If Applicable) IP 172.16.10.1/24 08-07-06-05-04-AA Ver. Header Length DS Field. Total Length Identification Flags Fragment Offset GO/0/1 Server B TTL Protocol: 0x06 Header Checksum 172.16.10.2/24 08-07-06-05-04-BB Source IP: 10.1.1.1 Destination IP: 172.16.10.1 IP Options Upon reaching the ultimate destination, the IP packet header must facilitate a number of processes. The first includes validating the integrity of the packet header through the checksum field, again applying a ones compliment value comparison based on a sum of the IP header fields. Where correct, the IP header will be used to determine whether the destination IP matches the IP address of the current end station, which in this case is true. If any fragmentation occurred during transmission between the source and the destination, the packet must be reassembled at this point. The identification field will collect the fragments belonging to a single data source together, the offset will determine the order and the flags field will specify when the reassembly should commence, since all fragments must be received firstly and a fragment with a flag of 0 will be recognized as the last fragment to be received. A timer will then proceed during which time the reassembly must be completed, should reassembly fail in this time period, all fragments will be discarded. The protocol field will be used to identify the next header for processing and the packet header will be discarded. It should be noted that the next header may not always be a transport layer header, a clear example of where this can be understood is in the case of ICMP, which is understood to also be a network layer protocol with a protocol field value of 0x01. Segment Decapsulation TCP header builds connection with the service at port 80. Parameters within the TCP header used to manage connection. Server A Data (If APL) TCP RTB 172.16.10.1/24 02-03-04-05-06-AA Source Port: 1027 Destination Port: 80 Sequence Number GO/0/1 Server B Acknowledgement Number Header Length Ress WCRC331 REGKHTNN NC EUAPRSF Window Checksum Urgent Pointer 172.16.10.2/24 02-03-04-05-06-BB Options Padding In the case where a packet header is discarded, the resulting segment or datagram is passed to the transport layer for application-to-application based processing. The header information is received in this case by TCP (0x06). In the example it can be understood that a TCP connection has already been established and the segment represents an acknowledgement for the transmission of HTTP traffic from the HTTP server to the acknowledging host. The host is represented by the port 1027 as a means to distinguish between multiple HTTP connections that may exist between the same source host and destination server. In receiving this acknowledgement, the HTTP server will continue to forward to the host within the boundaries of the window size of the host. 2. What happens when a frame is forwarded to a destination to which it is not intended? 3. How does the data in the frame ultimately reach the application it is intended for? 4. When multiple sessions of the same application are active (e.g. multiple web browsers), how does the return data reach the correct session?