Perhaps you're having difficulty figuring out how the loopback device receives packets because it never really anonymously receives anything -- it only receives whenever someone transmits something on device lo. On a transmit when the protocol stack determines the lo device is the actual device the packet needs to be transmitted out, the lo device's transmit routine (in loopback_init.c, dev->hard_start_xmit is set to loopback_xmit) will disassociate the packet from the transmit socket and place it on the incoming net receive queue. netif_rx takes care of the queueing. Then the net receive code finds anyone who wants the packet and then what you describe below takes place to queue the packet to the receive queue of the interested socket and wake up any process blocked on the socket (sk->data_ready) etc. For a regular net device (eth for instance), the hardware gets a packet, and an sk_buff is allocated out of the free pool. The code in loopback_xmit does the equivalent by "making the packet that needs to be transmitted meet the conditions that a freshly allocated skbuff is in" so it can be put on the net receive queue just like a real eth device does. For speed, when a single socket owns the packet, skb_orphan is called to call the sk_buff destructor function which actually decrements the transmit socket's sndbuf count -- this is what implements the send queue on a socket and keeps a single socket from taking up too many system buffers. The skbuff is then available to be placed on a new socket or in the general network receive queue. When there's more than one owner of the skb (skb->users > 1) then the existing skb information is left alone and skb_clone is called to create a new skb info portion that points to the same data and this is put on the general network receive queue. I'm not sure when more than one user of an sk_buff occurs in the system -- perhaps someone else on the list can come up with an example. Hope this helps. -Eric siva prasad wrote: > Hi all, > > In case of loopback , after searching > and finding the right 'sock' to send to, > sk->data_ready(sk,skb_len) is invoked . > The appropriate protocol's xxx_create > function initialises sk->data_ready() > to a function which takes care of the > rest of the story on the receiving side. > Where does the loopback driver come into > the scene..? Please explain... > > Thanking you in advance > > espe > > __________________________________________________ > Do You Yahoo!? > Make a great connection at Yahoo! Personals. > http://personals.yahoo.com > - > Kernelnewbies: Help each other learn about the Linux kernel. > Archive: http://mail.nl.linux.org/kernelnewbies/ > IRC Channel: irc.openprojects.net / #kernelnewbies > Web Page: http://www.kernelnewbies.org/ - Kernelnewbies: Help each other learn about the Linux kernel. Archive: http://mail.nl.linux.org/kernelnewbies/ IRC Channel: irc.openprojects.net / #kernelnewbies Web Page: http://www.kernelnewbies.org/
