Encapsulation
The four data units (data, segment, packet, frame), the header each layer adds when sending and removes when receiving, and what each device reads and rewrites along the path of a web request.
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Encapsulation: what is it?
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When sending, each layer wraps what the layer above hands it by adding its own header.
An HTTP request gets a TCP header, then an IP header, then an Ethernet header: each layer knows only its own header and treats the rest as an opaque payload.
A router receives an Ethernet frame that contains an IP packet. What does it do with the frame?
- It strips the frame's header and trailer, then builds a new frame
- It forwards it unchanged out of the exit interface
- It also opens the TCP segment to read the destination port
- It only replaces the destination MAC and keeps the rest of the frame
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It strips the frame's header and trailer, then builds a new frame
A frame only exists on one link: the router decapsulates it, decides using the destination IP address, then builds a new frame for the next link, with its own source and destination MACs and a new FCS. The TCP segment is not opened (unless it also filters or does NAT).
The data unit of the application layer is …, that of the transport layer the …, that of the network layer the …, that of the data-link layer the ….
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The data unit of the application layer is data, that of the transport layer the segment, that of the network layer the packet, that of the data-link layer the frame.
Each name points to a layer: reading it in a capture tells you which header you are looking at. The data (or message) has not yet received any header from the network stack. Over UDP, the transport unit is called a datagram.
Decapsulation: what is it?
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When receiving, each layer removes its header and hands the rest to the layer above.
It is the reverse of sending: the network card reads the Ethernet header, IP reads its own, TCP reads its own, and the application only receives its request.
Sending a web request, from the application to the cable: which step comes after "The browser produces the HTTP request"?
- IP adds its header, with the IP addresses and the TTL
- Ethernet adds its header, with the MAC addresses, and its FCS
- The network card sends the bits onto the cable
- TCP adds its header, with the source and destination ports
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TCP adds its header, with the source and destination ports
You go down the layers one by one: the request becomes a segment, then a packet, then a frame. You cannot form the frame before the packet, nor the packet before the segment, and the network card only sends bits at the very end.