IP header checksum does not check for the correct order of 16 bit values within the header. To calculate the TCP segment headers Checksum field, the TCP pseudo header is first constructed and placed, logically, before the TCP segment. Taking the ones' complement (flipping every bit) yields 0000, which indicates that no error is detected. When verifying a checksum, the same procedure is used as above, except that the original header checksum is not omitted.Ĥ500 + 0073 + 0000 + 4000 + 4011 + b861 + c0a8 + 0001 + c0a8 + 00c7 = 2fffd To obtain the checksum we take the ones' complement of this result: b861 (as shown underlined in the original IP packet header). The first digit is the carry count and is added to the sum:Ģ + 479c = 479e (if another carry is generated by this addition, another 1 must be added to the sum) To calculate the checksum, we can first calculate the sum of each 16 bit value within the header, skipping only the checksum field itself. If another carry is generated by the correction, another 1 is added to the sum. A carry check and correction can be performed with each addition or as a post-process after all additions. The header is shown in bold and the checksum is underlined.įor ones' complement addition, each time a carry occurs, we must add a 1 to the sum. Take the following truncated excerpt of an IPv4 packet. (with errata), to cover the case in routers which need to recompute the header checksum during packet forwarding when only a single field has changed.Įxamples Calculating the IPv4 header checksum Optimisations are presented in RFC 1624 "Computation of the Internet Checksum via Incremental Update" The procedure is explained in detail in RFC 1071 "Computing the Internet Checksum". The router must adjust the checksum if it changes the IP header (such as when decrementing the TTL). Packets with checksum mismatch are discarded. If there is no corruption, the result of summing the entire IP header, including checksum, should be zero.
For purposes of computing the checksum, the value of the checksum field is zero. The checksum field is the 16-bit ones' complement of the ones' complement sum of all 16-bit words in the header. The checksum calculation is defined in RFC 791: 2.1 Calculating the IPv4 header checksum.
0 kommentar(er)
