On 4/16/07, R. wen <rwen2012@xxxxxxxxx> wrote:
HI, guys :
The implement of the 3 way handshaking confuse me:
This is what the tcp_v4_do_rcv() mainly do:
if (sk->sk_state == TCP_LISTEN) {
struct sock *nsk = tcp_v4_hnd_req(sk, skb);
...
if (nsk != sk) {
if (tcp_child_process(sk, nsk, skb)) {
...
}
return 0;
}
}
...
if (tcp_rcv_state_process(sk, skb, skb-> h.th, skb->len)) {
...
}
It seems that, if the server is in the LISTEN state, it execs the line 2,
which is tcp_v4_hnd_req(sk, skb); , That is the function really confuse me:
static struct sock *tcp_v4_hnd_req(struct sock *sk, struct sk_buff *skb)
{
struct tcphdr *th = skb-> h.th;
struct iphdr *iph = skb->nh.iph;
struct sock *nsk;
struct request_sock **prev;
The comment talk for itself
/* Find possible connection requests. */
struct request_sock *req = inet_csk_search_req(sk, &prev, th->source,
iph->saddr, iph->daddr);
If found some req with same th->source, iph->saddr and iph->daddr,
check for if isn't already a established session.
if (req)
return tcp_check_req(sk, skb, req, prev);
nsk = inet_lookup_established(&tcp_hashinfo,
skb->nh.iph->saddr,
th->source, skb-> nh.iph->daddr,
th->dest, inet_iif(skb));
// if is an already established, session, check if isn't a socket in
timewait state.
if (nsk) {
// if not, lock the child socket and return
if (nsk->sk_state != TCP_TIME_WAIT) {
bh_lock_sock(nsk);
return nsk;
}
inet_twsk_put(inet_twsk(nsk));
return NULL;
}
#ifdef CONFIG_SYN_COOKIES
if (!th->rst && !th->syn && th->ack)
sk = cookie_v4_check(sk, skb, &(IPCB(skb)->opt));
#endif
// else return sk
return sk;
}
Because the server is running just now and at the LISTEN state, so there
is nothing in the listen queue's request_sock array. So the return value of
the Line 8 -- inet_csk_search_req() should be NULL, and then the " return
tcp_check_req(sk, skb, req, prev);" at Line 11 should NOT be executed.
IS that Right? Could you please explain this function for me? I am very
grateful.
R.wen
If you have more doubts please feel free to contact me, I worked
almost one year and a half with the tcp subsystem.
--
Best Regards
Alan Menegotto
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