Hello everybody,
I'm reading and learning a lot about TCP/IP protocols, and i want to know how to calculate the IP header checksum manually. I've searched for it EVERYWHERE! and none of options work.
I've read a function that does it pretty good, but i can't "translate" it into a human form so i can do it manually.
here's the code:
Code:
unsigned short checksum(unsigned short *ptr, int length){
register int sum = 0;
u_short answer = 0;
register u_short *w = ptr;
register int nleft = len;
while(nleft > 1){
sum += *w++;
nleft -= 2;
}
sum = (sum >> 16) + (sum & 0xFFFF);
sum += (sum >> 16);
answer = ~sum;
return(answer);
}
People, i've tried, i understand and know how to calculate using ones complement, hex conversion, binary conversion, AND, OR, XOR, NOT, and everything, but i can't do this .
Does anyone know the answer to this?
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I am communicating with two devices using my computer over UDP protocol. The application is running fine. When I monitored the UDP traffic using Wireshark software, I found that there were too many Checksum errors.
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I have an input like this
1 2 3 4
2 3 4 5
4 5 6 7
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Discussion started by: verdepollo
3 Replies
LEARN ABOUT MINIX
lo
LO(4) BSD Kernel Interfaces Manual LO(4)NAME
lo -- software loopback network interface
SYNOPSIS
device loop
DESCRIPTION
The loop interface is a software loopback mechanism which may be used for performance analysis, software testing, and/or local communication.
As with other network interfaces, the loopback interface must have network addresses assigned for each address family with which it is to be
used. These addresses may be set or changed with the SIOCSIFADDR ioctl(2). The loopback interface should be the last interface configured,
as protocols may use the order of configuration as an indication of priority. The loopback should never be configured first unless no hard-
ware interfaces exist.
If the transmit checksum offload capability flag is enabled on a loopback interface, checksums will not be generated by IP, UDP, or TCP for
packets sent on the interface.
If the receive checksum offload capability flag is enabled on a loopback interface, checksums will not be validated by IP, UDP, or TCP for
packets received on the interface.
By default, both receive and transmit checksum flags will be enabled, in order to avoid the overhead of checksumming for local communication
where data corruption is unlikely. If transmit checksum generation is disabled, then validation should also be disabled in order to avoid
packets being dropped due to invalid checksums.
DIAGNOSTICS
lo%d: can't handle af%d. The interface was handed a message with addresses formatted in an unsuitable address family; the packet was
dropped.
SEE ALSO inet(4), intro(4)HISTORY
The lo device appeared in 4.2BSD. The current checksum generation and validation avoidance policy appeared in FreeBSD 8.0.
BSD March 15, 2009 BSD
08-24-2009
.