What are the final TTL and the destination IP enclosed in the set of IP datagrams when they arrive at the webserver www.? Does this webserver send a UDP segment or an ICMP message back to the host? What are the type and code enclosed in such UDP segment or ICMP message? What’s the source IP in the IP datagram carrying this UDP/ICMP packet? Whose IP is used as the destination IP for this UDP/ICMP packet? (hint: websever’s, my computer’s, router 147.153.69.30’s, or …?)

particular layer N, a PDU is a complete message that implements the protocol at that layer. However, when this “layer N PDU” is passed down to layer N-1, it becomes the data that the layer N-1 protocol is supposed to service. Thus, the layer N protocol data unit (PDU) is called the layer N-1 service data unit (SDU). The job of layer N-1 is to transport this SDU, which it does in turn by placing the layer N SDU into its own PDU format, preceding the SDU with its own headers and appending footers as necessary. This process is called data encapsulation, because the entire contents of the higher-layer message are encapsulated as the data payload of the message at the lower layer.

What does layer N-1 do with its PDU? It of course passes it down to the next lower layer, where it is treated as a layer N-2 SDU. Layer N-2 creates a layer N-2 PDU containing the layer N-1 SDU and layer N-2’s headers and footers. And the so the process continues, all the way down to the physical layer. In the theoretical model, what you end up with is a message at layer 1 that consists of application-layer data that is encapsulated with headers and/or footers from each of layers 7 through

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