I have the thankless job of writing an IPv6 header parser.
I'm wondering if the version, traffic class and flow control labels could be parsed out using bitfields.
I wrote some test code. Executing on an x86 system I get unexpected results.
#include <stdint.h>
#include <stdio.h>
typedef struct __attribute__ ((__packed__)) {
uint32_t flow_label:20;
uint32_t traffic_class:8;
uint32_t ip_version:4;
} test_t;
int main(int argc, char **argv)
{
uint8_t data[] = { 0x60, 0x00, 0x00, 0x00 };
test_t *ipv6 = (void *)data;
printf("Size is %zu, version %u, traffic class %u, flow label %u\n", sizeof(test_t), ipv6->ip_version, ipv6->traffic_class, ipv6->flow_label);
}
I'd expect the first nibble to be available in ip_version, but it doesn't seem to be, instead I get:
Size is 4, version 0, traffic class 0, flow label 96
or with the field order inverted
Size is 4, version 0, traffic class 6, flow label 0
Can anyone explain why this happens?
With bitfields, it's implementation dependent how they are laid out. You're better off declaring a 32 bit field for the start of the packet and using bit shifting to extract the relevant fields.
uint8_t ipver = data[0] >> 4;
uint8_t tclass = ((data[0] & 0xf) << 4) | (data[1] >> 4);
uint32_t flowlbl = (((uint32_t)data[1] & 0xf) << 16) | ((uint32_t)data[2] << 8) | data[3];
Indeed, even the Linux netinet/ip6.h header doesn't use a bit field for the ipv6 header:
struct ip6_hdr
{
union
{
struct ip6_hdrctl
{
uint32_t ip6_un1_flow; /* 4 bits version, 8 bits TC,
20 bits flow-ID */
uint16_t ip6_un1_plen; /* payload length */
uint8_t ip6_un1_nxt; /* next header */
uint8_t ip6_un1_hlim; /* hop limit */
} ip6_un1;
uint8_t ip6_un2_vfc; /* 4 bits version, top 4 bits tclass */
} ip6_ctlun;
struct in6_addr ip6_src; /* source address */
struct in6_addr ip6_dst; /* destination address */
};