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Message M is generated by layer 5 and passed to layer 4 for transmission
Layer 4 adds header H4 in front of message and sends to Layer 3. H4 might contain sequences/sizes of messages, times etc.
Layer 4 has no size restrictions on messages but layer 3 almost certainly has. Therefore M is broken into two (or more) messages M1, M2, … of size equal to a packet (max size that is). H3 is prepended.
Layer 3 decides which of multiple outgoing lines to use and passes message to layer 2.
Layer 2 adds header and trailer to packets and sends resulting unit to the physical layer for transmission.
The receiving machine strips off headers and trailers and reconstitutes the message.
Network software/Protocol Hierarch.