Applications need to send data. Sometimes the data is small—in some cases, a single byte. In other cases, for instance, with a file transfer, the data might be millions of bytes.
Each different type of data link protocol typically has a limit on the maximum transmission unit (MTU) that can be sent. MTU refers to the size of the "data," according to the data link layer—in other words, the size of the Layer 3 packet that sits inside the data field of a frame. For many data link protocols, Ethernet included, the MTU is 1500 bytes.
TCP handles the fact that an application might give it millions of bytes to send by segmenting the data into smaller pieces, called segments. Because an IP packet can often be no more than 1500 bytes, and because IP and TCP headers are 20 bytes each, TCP typically segments large data into 1460 byte (or smaller) segments.
The TCP receiver performs re-ssembly when it receives the segments. To reassemble the data, TCP must recover lost segments, as was previously covered. However, the TCP receiver must also reorder segments that arrive out of sequence. Because IP routing can choose to balance traffic across multiple links, the actual segments may be delivered out of order. So, the TCP
receiver also must perform ordered data transfer by reassembling the data into the original order. The process is not hard to imagine: If segments arrive with the sequence numbers 1000, 3000, and 2000, each with 1000 bytes of data, the receiver can reorder them and no retransmissions are required.
You should also be aware of some terminology related to TCP segmentation. The TCP header, along with the data field, together are called a TCP segment. This term is similar to a data link frame and an IP packet, in that the terms refer to the headers and trailers for the respective layers, plus the encapsulated data. The term L4PDU also can be used instead of the term TCP segment because TCP is a Layer 4 protocol.
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