trouble compiling and using netinet/tcp.h with -std=c99

Question

#include<stdint.h>
#include<netinet/ip.h>
#include<netinet/tcp.h>


int main()
{
        struct tcphdr *tcpHdr = ((struct tcphdr*)NULL) ;
        struct iphdr *ipHdr = ((struct iphdr*)NULL);

        uint8_t offset = (unsigned int)(tcpHdr->doff)*4;
        uint16_t port =  tcpHdr->dest;

        unsigned int hdr_len = (ipHdr->ihl)*4;

        return 0;
}

after compilation gcc -std=c99 test1.c:

test1.c: In function ‘main’:
test1.c:13:40: error: dereferencing pointer to incomplete type
test1.c:14:25: error: dereferencing pointer to incomplete type

problem:

1). It is not giving any error for struct iphdr

2). If I don't include -std=c99, it compiles and works fine.