libscion_checksum.c

#include <arpa/inet.h>
#include <assert.h>
#include <stdio.h>
#include <stdlib.h>
#include "libscion_checksum.h"

/*
 * Calculate RFC1071 checksum of supplied data chunks. The use of a gather
 * mechanism means there's 0 copies required to calculate the checksum.
 * in: a struct containing the number of chunks, for every chunk a length and a
 *     pointer to its start.
 */
uint16_t checksum(chk_input *in)
{
	int i;
	/* As the maximum packet size is 65535B, the 32-bit accumulator cannot overflow. */
	uint32_t sum = 0;

	// Iterate over the chunks
	for(i = 0; i < in->idx; i++) {
		int j = 0;
		int len = in->len[i];
		int len2 = len / 2;
		uint16_t *ptr = (uint16_t *)(in->ptr[i]);
		if(len == 0) {
			continue;
		}
		for(; j < len2; j++) {
			sum += ptr[j];
		}
		// Add left-over byte, if any
		if(len % 2 != 0) {
			sum += in->ptr[i][len - 1];
		}
	}
	// Fold 32-bit sum to 16 bits
	while(sum > 0xFFFF) {
		sum = (sum & 0xFFFF) + (sum >> 16);
	}

	// Return ones-complement.
	// XXX(kormat): this value is in network-byte order.
	return ~sum;
}

/*
 * Helper function to initialize a chk_input struct for checksum.
 * total: number of input chunks.
 */
chk_input *init_chk_input(chk_input *input, int total)
{
	input->idx = 0;
	if(total > SCION_MAX_CHECKSUM_CHUNKS) {
		return NULL;
	}
	input->total = total;
	return input;
}

/*
 * Helper function to populate a chk_input struct for checksum.
 * in: Pointer to the chk_input struct.
 * idx: Index of the current chunk.
 * ptr: Pointer to start of data chunk.
 * len: Length of data chunk.
 * return value: a pointer to the data following the current chunk.
 */
const uint8_t *chk_add_chunk(chk_input *in, const uint8_t *ptr, int len)
{
	assert(in->idx < in->total);
	in->len[in->idx] = len;
	in->ptr[in->idx] = ptr;
	in->idx++;
	return ptr + len;
}