include/linux/skbuff.h

struct sk_buff {
    char            cb[80] __aligned(8);
}

http://www.embeddedlinux.org.cn/linux_net/0596002556/understandlni-CHP-2-SECT-1.html

char cb[40]

This is a "control buffer," or storage for private information, maintained by each layer for internal use. It is statically allocated within the sk_buff

struct tcp_skb_cb {
    ... ... ...
    _ _u32        seq;        /* Starting sequence number */
    _ _u32        end_seq;    /* SEQ + FIN + SYN + datalen*/
    _ _u32        when;       /* used to compute rtt's    */
    _ _u8         flags;      /* TCP header flags.        */
    ... ... ...
};

And this is the macro used by the TCP code to access the structure. The macro consists simply of a pointer cast:

#define TCP_SKB_CB(_ _skb)    ((struct tcp_skb_cb *)&((_ _skb)->cb[0]))

Here is an example where the TCP subsystem fills in the structure upon receipt of a segment:

int tcp_v4_rcv(struct sk_buff *skb)
{
        ... ... ...
        th = skb->h.th;
        TCP_SKB_CB(skb)->seq = ntohl(th->seq);
        TCP_SKB_CB(skb)->end_seq = (TCP_SKB_CB(skb)->seq + th->syn + th->fin +
                                    skb->len - th->doff * 4);
        TCP_SKB_CB(skb)->ack_seq = ntohl(th->ack_seq);
        TCP_SKB_CB(skb)->when = 0;
        TCP_SKB_CB(skb)->flags = skb->nh.iph->tos;
        TCP_SKB_CB(skb)->sacked = 0;
        ... ... ...
}

To see how the parameters in the cb buffer are retrieved, take a look at the function tcp_transmit_skb in net/ipv4/tcp_output.c. That function is used by TCP to push a data segment down to the IP layer for transmission.

In Chapter 22, you will also see how IPv4 uses cb to store information about IP fragmentation.