Re: Hello,Arnaldo.Can you do me a favor?

[Arnaldo Carvalho de Melo <acme@xxxxxxxxxx>]

Em Wed, Jan 07, 2009 at 12:27:22PM +0800, shizibo escreveu:

> Hello,  I am just studying the codes of DCCP, I encounter some
> questions now. I send this mail to <dccp@xxxxxxxxxxxxxxx>, but the
> yahoo mail box inform me that it can not be delivered.So sorry to
> bother you.Will you help me.

No problem, I'm replying to you and to the list, so that the answers can
help others in the future.
 
> in dccp_connect():
> skb = alloc_skb(sk->sk_prot->max_header, sk->sk_allocation);
> where is the sk->sk_prot->max_header set before it's used here?

Use grep:

[acme@doppio linux-2.6]$ grep "max_header.\+=" net/dccp/*.c
net/dccp/ipv4.c:	.max_header		= MAX_DCCP_HEADER,
net/dccp/ipv6.c:	.max_header	   = MAX_DCCP_HEADER,
[acme@doppio linux-2.6]$

I.e. it depends on the layer 3 used. Now look at net/dccp/ipv4.c to
see exactly how it is initialized:

static struct proto dccp_v4_prot = {
<SNIP>
        .max_header             = MAX_DCCP_HEADER,
<SNIP>
};

And this, in turn, is referenced here:

static struct inet_protosw dccp_v4_protosw = {
        .type           = SOCK_DCCP,
        .protocol       = IPPROTO_DCCP,
        .prot           = &dccp_v4_prot,
        .ops            = &inet_dccp_ops,
        .capability     = -1,
        .no_check       = 0,
        .flags          = INET_PROTOSW_ICSK,
};

If you now search where dccp_v4_protosw is used:

static int __init dccp_v4_init(void)
<SNIP>
	inet_register_protosw(&dccp_v4_protosw);
<SNIP>
}

inet_register_protosw will let net/ipv4/af_inet.c that when a PF_INET
socket of type SOCK_DCCP is asked for we should use dccp_v4_prot, etc.

Please take a look at a paper a wrote that explains these details:

http://ols.fedoraproject.org/OLS/Reprints-2005/melo-Reprint.pdf

Look at chapter 3.
 
> and, there is macro:
> #define DCCP_SKB_CB(__skb) ((struct dccp_skb_cb *)&((__skb)->cb[0]))
 
> It is said the skb->cb is used to hold some private control message
> per layer. For dccp, where is it initialized? What does the other
> bytes, cb[1], cb[2]..., content?

This is a scratch pad, always casted to some struct that is layer
specific, for instance, for DCCP, it is 'struct dccp_skb_cb', that is
defined in net/dccp/dccp.h:

/**
 * dccp_skb_cb  -  DCCP per-packet control information
 * @dccpd_type: one of %dccp_pkt_type (or unknown)
 * @dccpd_ccval: CCVal field (5.1), see e.g. RFC 4342, 8.1
 * @dccpd_reset_code: one of %dccp_reset_codes
 * @dccpd_reset_data: Data1..3 fields (depend on @dccpd_reset_code)
 * @dccpd_opt_len: total length of all options (5.8) in the packet
 * @dccpd_seq: sequence number
 * @dccpd_ack_seq: acknowledgment number subheader field value
 * This is used for transmission as well as for reception.
 */
struct dccp_skb_cb {
        union {
                struct inet_skb_parm    h4;
#if defined(CONFIG_IPV6) || defined (CONFIG_IPV6_MODULE)
                struct inet6_skb_parm   h6;
#endif
        } header;
        __u8  dccpd_type:4;
        __u8  dccpd_ccval:4;
        __u8  dccpd_reset_code,
              dccpd_reset_data[3];
        __u16 dccpd_opt_len;
        __u64 dccpd_seq;
        __u64 dccpd_ack_seq;
};

So we don't access, say, skb->cb[5], but instead we cast it to a
variable (or use it directly as DCCP_SKB_CB(skb)->dccpd_type), like in
dccp_v4_rcv in net/dccp/ipv4.c:

static int dccp_v4_rcv(struct sk_buff *skb)
{
        const struct dccp_hdr *dh;
<SNIP>
        dh = dccp_hdr(skb);

        DCCP_SKB_CB(skb)->dccpd_seq  = dccp_hdr_seq(dh);
        DCCP_SKB_CB(skb)->dccpd_type = dh->dccph_type;
<SNIP>

So, in this case, it is initialized when we receive a packet.

Later, in the same function (dccp_v4_rcv), if we don't find a socket to
queue this packet into, we use the skb->cb area in another fashion, this
time setting dccpd_reset_code to the sensible value and calling a
routine (dccp_v4_ctl_send_reset) that will then get this value and do
something it it:

no_dccp_socket:
        if (!xfrm4_policy_check(NULL, XFRM_POLICY_IN, skb))
                goto discard_it;
        /*
         * Step 2:
         *      If no socket ...
         *              Generate Reset(No Connection) unless P.type ==
         *              Reset
         *              Drop packet and return
         */
        if (dh->dccph_type != DCCP_PKT_RESET) {
                DCCP_SKB_CB(skb)->dccpd_reset_code =
                                        DCCP_RESET_CODE_NO_CONNECTION;
                dccp_v4_ctl_send_reset(sk, skb);
        }
 
- Arnaldo
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