Checksum computation is one of the operations that dominate packet processing time. Efficient checksum computation is difficult to implement since it is hardware dependent. Therefore, an operating system kernel usually implements several machine-dependent versions of the Internet checksum function in_cksum() to be used on different platforms. To simplify your task in projects 2 and 3, we provide you with a portable C version of the in_cksum() function (see $PDIR/include/in_cksum.h). This version is from the BSD TCP/IP implementation (though modified to use our pbuf structure, instead of the BSD mbuf). in_cksum() calculates the checksum of the packet specified by the first argument (a pointer to pbuf) with length specified by the second argument. You can use this function to compute all checksums in projects 2 and 3.