On Wed, Dec 27, 2000 at 01:54:13AM +1000, Richard Sharpe wrote:
> There is a LAPB driver for Ethernet, lapbether, available for Linux, which
> allows Linux to Linux LAPB over ethernet, over which you can shove X.25 ...
>
> I am trying to dissect it; it is carried in frames of type 0x6000, an
> unused type allocated to DEC.
>
> It seems that there is a two byte length on the front of each frame ...
>
> Anyone know anything about it?
>From "drivers/net/lapbether.c":
/*
* Receive a LAPB frame via an ethernet interface.
*/
static int lapbeth_rcv(struct sk_buff *skb, struct device *dev, struct packet_type *ptype)
{
int len, err;
struct lapbethdev *lapbeth;
skb->sk = NULL; /* Initially we don't know who it's for */
dev = lapbeth_get_x25_dev(dev);
if (dev == NULL || dev->start == 0) {
kfree_skb(skb);
return 0;
}
lapbeth = (struct lapbethdev *)dev->priv;
lapbeth->stats.rx_packets++;
len = skb->data[0] + skb->data[1] * 256;
skb_pull(skb, 2); /* Remove the length bytes */
skb_trim(skb, len); /* Set the length of the data */
if ((err = lapb_data_received(lapbeth, skb)) != LAPB_OK) {
kfree_skb(skb);
printk(KERN_DEBUG "lapbether: lapb_data_received err - %d\n", err);
}
return 0;
}
If I'm correctly interpreting the lines
len = skb->data[0] + skb->data[1] * 256;
skb_pull(skb, 2); /* Remove the length bytes */
skb_trim(skb, len); /* Set the length of the data */
if ((err = lapb_data_received(lapbeth, skb)) != LAPB_OK) {
and correctly interpreting "lapb_data_received()", "lapb_data_input()",
and "lapb_decode()", the frame appears to be a two-byte length field (in
little-endian byte order), followed by a one-byte address field
(LAPB_ADDR_{A,B,C,D}), from "include/net/lapb.h", followed by the
control field of the LAPD header.
I think this means you get the length field, use it to construct the
next tvbuff, and hand the frame to "dissect_lapb()".