Packet Flow

• Logically,information is exchanged between peer protocol layers.
• Physically, information is transported by requesting a service from the next lower layer.
• Example: The transport layer requests from the network layer that a packet be transported to a host at a specified address.
• The details of routing and link layer transport are transparent to the originating host.
• The contents of the packet is of no concern to the network layer and lower layers.
• Only the transport layer will interpret the contents of the packet. Therefore, logically the two transport layers communicate.
• Practically, a layer will encapsulate a packet received from a higher layer by attaching additional information in a header.
• This header may contain information such as addresses that will be interpreted by its peer-layer.

  

Figure 2.41: Flow of packets