Objects Relationship

socket object

userspace藉由system call API: sys_socket()產生，結構中有一個 sock object

struct socket {
    socket_state state;
    kmemcheck_bitfield_begin(type);
    short type;
    kmemcheck_bitfield_end(type);
    unsigned long flags;
    . . .
    struct file *file;
    struct sock *sk;
    const struct proto_ops *ops;
};
(include/linux/net.h)

sock object

L3的抽象代表層(for scoket object)，會擁有很多個sk_buff(也就是很多封包，用linked-list存)

struct sock {
    struct sk_buff_head sk_receive_queue;
    int sk_rcvbuf;
    unsigned long sk_flags;
    int sk_sndbuf;
    struct sk_buff_head sk_write_queue;
    . . .
    unsigned int sk_shutdown : 2,
    sk_no_check : 2,
    sk_protocol : 8,
    sk_type : 16;
    . . .
    void (*sk_data_ready)(struct sock *sk, int bytes);
    void (*sk_write_space)(struct sock *sk);
};

sk_type: socket的類型,像是:SOCK_STREAM， SOCK_RAW等等。(include/linux/net.h) sk_write_queue: 這就是retransmission queue

sk_buff_head object

這就是sk_buff的linked-list

sk_buff(skb)

當封包抵達device，net device driver就會allocate skb
有net_device，查詢硬體資訊，比方說MTU等等

當packet只是forward用的時候，sk=NULL(sock object)。

struct sk_buff {
 . . .
 struct sock *sk;
 struct net_device *dev;  // 下面有補充
 . . .
 __u8 pkt_type:3,   // L2決定:broadcast, muticast...
 . . .
 __be16 protocol;
 . . .
 sk_buff_data_t tail;
 sk_buff_data_t end;
 unsigned char *head,
 *data;
 sk_buff_data_t transport_header;
 sk_buff_data_t network_header;
 sk_buff_data_t mac_header;
 . . .
};
(include/linux/skbuff.h)

一個skb包含L2，L3，L4的 header & payload。
skb的操作方式:
- data: 正在處理的payload
- tail: end of payload
- len: tail - data

L2 -> L3

net_device

struct net_device {
    unsigned int irq; /* device IRQ number */
    . . .
    const struct net_device_ops *netdev_ops;
    . . .
    unsigned int mtu;
    . . .
    unsigned int promiscuity;
    . . .
    unsigned char *dev_addr;
    . . .
};
(include/linux/netdevice.h)

Objects relationship