At higher levels (OSI layers 3 and above), these frames are aggregated (a.k.a combined) into packets, which are logical collections of the phrases, if you will, that are part of a network conversation. The protocol suites and protocols we will examine in this chapter communicate by exchanging these packets of logical information in an agreed-upon fashion that both the sender and receiver understand.
Packets are made up of both system header data (describing things like the sender and intended receiver of the message) and user data (the actual network message contents), organized in a format that both the sender and receiver have agreed upon. Typically what happens as a message is sent on the network is that the application layer requests that a message like Hello, Leeanne, be sent to Leeannes PC, and this request travels down through successive protocol layers on its way to Leeannes PC. Along the way, some layers may add additional standardized information to the original Hello, Leeanne packet, specifying things like the address of Leeannes PC, the address of the senders PC, the current length of the entire packet, the priority, a checksum (a calculation designed to verify that a message was received without corruption), etc. Typically, a protocol layer that is adding information to a packet will add it to the beginning or the end, so as not to disturb the encapsulated message.
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