Hi Patrick,
Thanks for your comments so far. More questions below.
Patrick McHardy wrote:
James Chapman wrote:
Patrick McHardy wrote:
The interaction with UDP sockets looks pretty horrible IMO. On the
send side I don't see why you can't simply build the UDP header
yourself instead of doing these set_fs + sendmsgs hacks.
Wouldn't that effectively duplicate the code in udp_sendmsg()? If I
don't use a socket, I'd also have to build an IP header and feed the
frame into the IP stack for outbound routing. It doesn't feel like the
right thing to do.
Thats what other tunnel drivers do. Sending UDP is pretty simple, I'd
expect that it comes down to less code than now.
I'm still not getting this. :( What other tunnel drivers do this?
I've looked at other socket code examples. I see sendmsg code like pppoe
which builds a skb and does a dev_queue_xmit() or code like cifs that
uses kernel_sendmsg() which does the set_fs hack internally.
ESP does build its own UDP header. But ESP is working at a different
level (IP packet type) rather than L2TP which is really just UDP data.
Most of the stuff done in udp_sendmsg() is relevant for L2TP, i.e.
socket param checks, UDP checksum, flow classification, route lookup, IP
output, MIB counter updates etc. If I do all of this in
pppol2tp_sendmsg() and insert the IP and UDP header, should I also
insert the netdevice's MAC address and then dev_queue_xmit()? I'd just
like to be clear in my own mind what you are recommending before I code
it up. :)
One reason for using the L2TP tunnel's UDP socket directly was to have
the data traffic carried by all sessions in that tunnel use the tunnel's
socket buffer, thereby ensuring that one tunnel's data can't starve
another tunnel.
If you use encapsulation sockets and process packets immediately
that should still not be possible.
My understanding is that encapsulation is used where a header is
inserted _before_ UDP/TCP headers, not after. In the L2TP case, the
kernel has some data that it wants to send over UDP. The UDP socket
doesn't need to be special.
- pppol2tp_fget: why do you want to open sockets for other processes?
I hope this can go together with the sendmsg hacks
There are two userspace daemons: l2tpd and pppd. L2ptd opens a UDP
tunnel socket and sets up one or more L2TP sessions in that tunnel. When
a new session is established, l2tpd spawns a pppd process (per session).
The pppd process creates a PPPoL2TP socket (this driver). The PPPoL2TP
socket is associated with the tunnel UDP socket that was created by
l2tpd. So on creating a new PPPoX socket, the connect() handling needs
to find the UDP socket of the tunnel. Since connect() runs in the
context of pppd, it needs a way of doing a sock lookup to find the UDP
socket that was created by l2tpd.
How about just passing the file descriptor from l2tpd to pppd?
I do already. :) The problem is that the UDP (tunnel) fd is created and
owned by l2tpd and the PPPoX socket is created and owned by pppd. The
PPPoX connect() is done by pppd on its PPPoX socket. The connect
parameters include the fd of the UDP socket which is to be used to carry
the PPPoX data. Hence pppol2tp_connect() needs to locate the UDP socket
that was created by l2tpd, while in the context of pppd.
Thanks again for your comments!
--
James Chapman
Katalix Systems Ltd
http://www.katalix.com
Catalysts for your Embedded Linux software development
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