From the OSI Model's point of view: If we already have Ip fragmentation which happen due to the network limitation, why the transport layer provides the segmentation mechanism via TCP protocol? why this segmentation needed ?

  • This question actually provides the same question from me, but all answers there didn't provide the correct answer and drafted to answer the second question not the main question Jan 5 '20 at 19:48
  • It's very much the same question. You can place a comment there to explain why you think the answers are not complete.
    – Zac67
    Jan 5 '20 at 19:49

Segmentation and fragmentation, as the different names imply, serve different functions. Segmentation, which is a feature of transport protocols, serves to packetize a stream of data for reliable delivery. Fragmentation at the network level, ensures that the packet can be transmitted in a data link PDU.

The transport layer can't depend on the lower layers doing segmentation, because the lower layer has no concept of acknowledgement or retransmission. Moreover, not all lower layers fragment data -- IPv6 does not fragment packets.

Finally, don't forget that although TCP and IPv4 is nearly universal, they were just two of many protocols when the OSI model was developed.

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