Network implementation

[an7]

Hi, i am a french engineer who has some work to do on
TCP/IP implementation in the linux kernel.

I have three questions on it :

1) What are the advantages of using a ring buffer for
receiving and queuing packets ?

2) in tcp_minisocks.c in linux i get, in the function
which handles segment arrival in FIN_WAIT_2 state : 

if (th->syn && !before(TCP_SKB_CB(skb)->seq,
tw->rcv_nxt))
                         goto kill_with_rst;

Just to confirm : in the state FIN_WAIT_2 a SYN
segment is automatically killed by a RST, unless its
sequence number is superior to the waited sequence
number. But in this case , what happens ? I think the
RFC tells to send a ACK. Am i right ?

3) the function tcp_recv_skb() (tcp.c)

struct sk_buff *tcp_recv_skb(struct sock *sk, u32 seq,
u32 *off)
1383 {
1384         struct sk_buff *skb;
1385         u32 offset;
1386 
1387         skb_queue_walk(&sk->receive_queue, skb) {
1388                 offset = seq -
TCP_SKB_CB(skb)->seq;
1389                 if (skb->h.th->syn)
1390                         offset--;
1391                 if (offset < skb->len ||
skb->h.th->fin) {
1392                         *off = offset;
1393                         return skb;
1394                 }
1395         }
1396         return NULL;
1397 }

 
I just can't understand the role of offset (and thus,
i can't undersand why it is decremented with a syn
segment, and why we can return the skb structure when
we have an offset inferior to the skb len, or a FIN
segment)

Could you help please ?

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