Network, Sockets API programming

Call sequence for server/client

Socket address structure “polymorphism”

// More info at : man 7 ip/ipv6

struct sockaddr {
  sa_family_t     sin6_family; 
  char sa_data[14];
}

struct sockaddr_in {
  sa_family_t    sin_family; // AF_INET
  in_port_t      sin_port;

  struct in_addr sin_addr;
  struct in_addr {
    uint32_t    s_addr;
  };
};

struct sockaddr_in6 {
  sa_family_t     sin6_family;   // AF_INET6
  in_port_t       sin6_port;
  uint32_t        sin6_flowinfo; // RFC6294 admits this do not have a really consistent semantic
  uint32_t        sin6_scope_id; // holds nic id for link local addresses

  struct in6_addr sin6_addr;
  struct in6_addr {
    unsigned char   s6_addr[16];
  };
};

The case for flow labels

The main idea is to be able to apply some kind of policy to the packets identified by the tuple : (src_addr, dst_addr, flow_lbl) This enforces layer isolation since policy only have to look at layer 3. Applying policy based on the 5-tuple (src_addr, src_port, dst_addr, dst_port, protocol) implies looking at transport layer.

Interoperability IPv4/IPv6

IPv6 server IPv4 client have some compatibility

IPv4 server IPv6 client have NO compatibility

The strange case of link local addresses

You can check the addresses assigned to a NIC (and their scope) using the ip command.