So I expect that a packet is build as follows:

In the second half of the 80 I did some serial port programming.

Serial transmission is a point to point connection with out those source and destination addresses.Ĭomplicated factor is that you RX (received) example contains also the given checksum, but not add the end of the packet. The calculation of the checksum of a UDP packet contains also the source and destination address from and to where the buffer contents is send to. You start to calculate the checksum over the serial buffer contents. I understand from your story that you are going to send a buffer serially. I think reporting a reference to UDP is just too short-sighted. The third parameter is the number of bytes for the CRC calculation.The second variable I suspect is buffer string.Without this parameter no CRC calculation can be done. The CRC calculation starts with a secret offset value “uint16_t init”.This therefore requires the algorithm to be modified slightly to take into consideration the carry that may result at bit 15. However, most processors only provide a 32-bit add instruction, and do not provide an instruction that independently adds two 16-bit quantities contained in one 32-bit word. If the computed checksum is 0, it is transmitted as all ones (the equivalent in ones complement arithmetic).

If the datagram is odd-numbered in length, a zero octet is virtually added at the end, so that 16-bit maths can be used throughout. Protection of the data is optional for applications using UDP RFC768 for IPv4, but is required for IPv6. The IP (IPv4) header checksum protects only the IPv4 header, while the TCP, DCCP, ICMP, IGMP, and UDP checksums provide end-to-end error detection for both the transport header (including network and transport layer information) and the transport payload data. It may be, and is, also (on occassions) caused by bugs in host and router hardware and software.Įven if every link implemented strong error detection in the form of frame CRCs, it is still essential that end-to-end checksums at and above the IP level are used at the receiving end host. Packet corruption is not only caused by errors introduced by the physical layer.

The cost of the ability to detect more types of error is the increased cost of computing the checksum.