2 bytes gone in receiving window size

Question

Sn_MR(ALIGN) = 0 Sn_MR(protocol) = TCP receive buffer size is 8k. When I receive TCP data (PC to W5300) the window size updates with the ack. But with every TCP packet the window size decreases with packet size + 2 bytes. That would be correct if align was 1 (packet info is 2 bytes). image provided: PC is 2.121.10.50 W5300 is 2.121.17.1 Packet send is 1514 bytes on wire, 1460 in payload. window size is 880 after auto ack from W5300. After reading the data I sent a dummy byte and window size increases to 2342 (880+1460+2???) the same goes the other way as well. The problem is when the PC sends the last 880 bytes, but W5300 can only accept 878 bytes. image828×173 6.82 KB I read the datasize from RSR register and there are no 2 bytes as header when i read the FIFO. other info: internal phy mode 16 bit data and direct address mode

Eugeny · Answer

Datasheet says in 5.2.1.1: TCP data is composed of PACKET-INFO and DATA packet in case of Sn_MR(ALIGN)=‘0’. In case of Sn_MR(ALIGN) = ‘1’, TCP data has only DATA packet by removing PACKET-INFO. Therefore you have ALIGN bit set to 0, and have data size as first 2 bytes in the RX buffer before the data. And therefore you must ‘remove’ datasize+2 bytes from the RX buffer.