Packet Number Vs. Segment Number?
I'm a high school student studying networks for AS Level Computer Science (although what I'm asking is a little outside the syllabus).
From what I've read, layer 4 splits and encapsulates the data with a segment header (in practice, I hear they use TCP, a type of protocol for this layer). The segment header contains many things, but namely a segment number so that a device like a router can reorganize the data into the correct order. However, I've been taught that packets also have packet numbers which functionally do the same thing – help the router organize the data.
So, why do segments have a segment number? If segments are already encapsulated with an IP header that contains the packer number in layer 3, then wouldn't the segment number just be unnecessary? I'm not sure what I'm missing.