i have found out that the cecksumcomputation over the payload is wrong

$//$ Compute checksum over payload data

uint$8\_$t byte $=0$;

for $($size$\_$t i $=0$; i $<$ sz; i$++)\{$

$//$ Extract each byte of the payload data and XOR it with the checksum

byte $=$ buffer$[$i$]$;

$//$ Convert byte to network byte order before XOR

$\}$

checksum $^=($byte $<<8)$;

checksum $^=($byte $>>8)$;

i have also tried this method

for $($size$\_$t i $=0$; i $<$ sz; i$++)\{$

$//$ XOR each byte of the payload data with the checksum

checksum $^=$ buffer$[$i$]$;

$\}$

both wrong, $($black box server gets different checksum when ther is data but, it gets equal checkum when there is no data$)$ so how do you compute the checksum over the data aka payload?

my entire checksum computation is within this function

int d$1\_$send$\_$data$($ D$1$Peer$*$ peer, char$*$ buffer, size$\_$t sz $)$

$\{$

$//$ Check if peer or buffer is NULL

if $($peer $==$ NULL $||$ buffer $==$ NULL$)\{$

return $-1$; $//$ Return negative value for error

$\}$

$//$ Calculate the total size of the packet $($header $+$ data$)$

size$\_$t packet$\_$size $=$ sizeof$($D$1$Header$)+$ sz;

$//$ Check if the packet size exceeds the maximum allowed size

if $($packet$\_$size $>1024)\{$

fprintf$($stderr$,$ "Error: Packet size exceeds maximum allowed size

$")$;

return $-1$; $//$ Return negative value for error

$\}$

$//$ Create a D$1$Header and populate its fields

D$1$Header header;

header.flags $=$ FLAG$\_$DATA;

header.size $=$ packet$\_$size;

$//$ Compute the checksum

uint$16\_$t checksum $=0$;

$//$ Compute checksum over flags and size fields

checksum $^=$ header.flags; $//$ checksum for flag field

checksum $^=$ header.size $>>16$; $//$ Compute checksum for the upper $16$ bits of size

checksum $^=$ header.size & $0$xFFFF; $//$ Compute checksum for the lower $16$ bits of size

$//$ Compute checksum over payload data

uint$8\_$t byte $=0$;

for $($size$\_$t i $=0$; i $<$ sz; i$++)\{$

$//$ Extract each byte of the payload data and XOR it with the checksum

byte $=$ buffer$[$i$]$;

$//$ Convert byte to network byte order before XOR

$\}$

checksum $^=($byte $<<8)$;

checksum $^=($byte $>>8)$;

$//$ Add padding byte if the payload size is odd

if $($sz $\%2!=0)\{$

printf$("$odd data"$)$;

checksum $^=0$x$00$;

$\}$

$//$ convert all fields to network byte order before sending the packet

header.flags $=$ htons$($header$.$flags$)$;

header.size $=$ htonl$($header$.$size$)$;

header.checksum $=$ htons$($checksum$)$; $//$ Convert checksum to network byte order

char packet$[1024]$;

memcpy$($packet$,$ &header, sizeof$($D$1$Header$))$;

memcpy$($packet $+$ sizeof$($D$1$Header$),$ buffer, sz$)$;

$//$ Send the packet header to the peer using sendto

printf$("$sending header

$")$;

ssize$\_$t bytes$\_$sent $=$ sendto$($peer$->$socket, packet, packet$\_$size, $0,$

$($struct sockaddr$*)$&peer$->$addr, sizeof$($struct sockaddr$\_$in$))$;

if $($bytes$\_$sent $==-1)\{$

perror$("$Error sending header"$)$;

return $-1$; $//$ Return negative value for error

$\}$

$//$ Receive and validate the ACK packet

D$1$Header ack$\_$packet;

printf$("$waiting for ack

$")$;

ssize$\_$t ack$\_$bytes$\_$received $=$ recv$($peer$->$socket, $($void$*)$&ack$\_$packet, sizeof$($D$1$Header$),0)$;

if $($ack$\_$bytes$\_$received $==-1)\{$

perror$("$Error receiving ACK packet"$)$;

return $-1$; $//$ Return negative value for error

$\}$

$//$ Validate the ACK packet

uint$16\_$t ack$\_$flags $=$ ntohs$($ack$\_$packet.flags$)$;

$//$ Check if received packet is an ACK and sequence number matches

if $(($ack$\_$flags & FLAG$\_$ACK$)$ && $(($ack$\_$flags & SEQNO$)==$ peer$->$next$\_$seqno$))\{$

$//$ Valid ACK received

peer$->$next$\_$seqno $=($peer$->$next$\_$seqno $+1)\%2$;

return bytes$\_$sent; $//$ Success: return number of bytes sent

$\}$

else $\{$

d$1\_$send$\_$data$($peer$,$ buffer, sz$)$;

$\}$

$\}$