Difference between IP fragmentation and TCP segmentation


What is the difference between IP fragmentation and TCP segmentation ? Could you summarise of the reassembly process ? Kind Regards.

Hello Burak

TCP segmentation occurs at layer 4 of the OSI model. TCP will take the data received from the upper layers and separate it into segments. The header of each segment will include information such as source and destination ports, sequence numbers, acknowledgement numbers, window size, checksum and others that are necessary for the reassembly on the other end.

So, when Layer 4 receives the data from the upper layers, it separates it into pieces each with its own header. The size of these pieces depends on several factors. In general, the larger the segments, the more efficient the transmission (the less overhead from headers). If however, the segment is too big, it won’t fit within an IP packet at Layer 3, and something called IP fragmentation will occur (which can also reduce efficiency). TCP will usually determine the maximum segment size (MSS) based on the maximum transmission unit (MTU) of Layer 3 (IP layer).

More about MSS and MTU can be found at this lesson.

As for IP fragmentation, the logic is similar. If a TCP segment is encapsulated into an IP packet, this IP packet in turn must be encapsulated into a frame. The MTU is the maximum size of a frame on the medium. If an IP packet does not fit in the frame, it must be broken or fragmented into two to be sent separately. There are mechanisms that can be implemented that do not allow fragmentation on specific packets either allowing them to be transmitted if they are small enough or having them dropped if they are too large.

I hope this along with the lessons that I’ve linked to has been helpful!


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