I was going through this Cisco Documentation - Resolve IP Fragmentation, MTU, MSS, and PMTUD Issues with GRE and IPSEC
I have a confusion with example 3. Screenshot below.
As I understand, with the total of the header size would be 44 (New IP Header (20) + GRE Header (4) + Original IP Header (20)). As the example 3 mentions, a router in between the path has an MTU of 1400 bytes. So that leaves 1400 - 44 = 1356 bytes for the TCP Payload.
However, the example shows "1352 bytes", that is 4 short of a minimum MTU along the path of 1400 bytes. The remaining data of 104 bytes has been carried over to the next fragment.
Why is the first packet not a complete 1400 bytes rather 1396 bytes (1352 + 44) and have 100 bytes of the TCP payload added on to the next fragment?