124 lines
4.8 KiB
Plaintext
124 lines
4.8 KiB
Plaintext
X.25 Device Driver Interface 1.1
|
|
|
|
Jonathan Naylor 26.12.96
|
|
|
|
This is a description of the messages to be passed between the X.25 Packet
|
|
Layer and the X.25 device driver. They are designed to allow for the easy
|
|
setting of the LAPB mode from within the Packet Layer.
|
|
|
|
The X.25 device driver will be coded normally as per the Linux device driver
|
|
standards. Most X.25 device drivers will be moderately similar to the
|
|
already existing Ethernet device drivers. However unlike those drivers, the
|
|
X.25 device driver has a state associated with it, and this information
|
|
needs to be passed to and from the Packet Layer for proper operation.
|
|
|
|
All messages are held in sk_buff's just like real data to be transmitted
|
|
over the LAPB link. The first byte of the skbuff indicates the meaning of
|
|
the rest of the skbuff, if any more information does exist.
|
|
|
|
|
|
Packet Layer to Device Driver
|
|
-----------------------------
|
|
|
|
First Byte = 0x00 (X25_IFACE_DATA)
|
|
|
|
This indicates that the rest of the skbuff contains data to be transmitted
|
|
over the LAPB link. The LAPB link should already exist before any data is
|
|
passed down.
|
|
|
|
First Byte = 0x01 (X25_IFACE_CONNECT)
|
|
|
|
Establish the LAPB link. If the link is already established then the connect
|
|
confirmation message should be returned as soon as possible.
|
|
|
|
First Byte = 0x02 (X25_IFACE_DISCONNECT)
|
|
|
|
Terminate the LAPB link. If it is already disconnected then the disconnect
|
|
confirmation message should be returned as soon as possible.
|
|
|
|
First Byte = 0x03 (X25_IFACE_PARAMS)
|
|
|
|
LAPB parameters. To be defined.
|
|
|
|
|
|
Device Driver to Packet Layer
|
|
-----------------------------
|
|
|
|
First Byte = 0x00 (X25_IFACE_DATA)
|
|
|
|
This indicates that the rest of the skbuff contains data that has been
|
|
received over the LAPB link.
|
|
|
|
First Byte = 0x01 (X25_IFACE_CONNECT)
|
|
|
|
LAPB link has been established. The same message is used for both a LAPB
|
|
link connect_confirmation and a connect_indication.
|
|
|
|
First Byte = 0x02 (X25_IFACE_DISCONNECT)
|
|
|
|
LAPB link has been terminated. This same message is used for both a LAPB
|
|
link disconnect_confirmation and a disconnect_indication.
|
|
|
|
First Byte = 0x03 (X25_IFACE_PARAMS)
|
|
|
|
LAPB parameters. To be defined.
|
|
|
|
|
|
|
|
Possible Problems
|
|
=================
|
|
|
|
(Henner Eisen, 2000-10-28)
|
|
|
|
The X.25 packet layer protocol depends on a reliable datalink service.
|
|
The LAPB protocol provides such reliable service. But this reliability
|
|
is not preserved by the Linux network device driver interface:
|
|
|
|
- With Linux 2.4.x (and above) SMP kernels, packet ordering is not
|
|
preserved. Even if a device driver calls netif_rx(skb1) and later
|
|
netif_rx(skb2), skb2 might be delivered to the network layer
|
|
earlier that skb1.
|
|
- Data passed upstream by means of netif_rx() might be dropped by the
|
|
kernel if the backlog queue is congested.
|
|
|
|
The X.25 packet layer protocol will detect this and reset the virtual
|
|
call in question. But many upper layer protocols are not designed to
|
|
handle such N-Reset events gracefully. And frequent N-Reset events
|
|
will always degrade performance.
|
|
|
|
Thus, driver authors should make netif_rx() as reliable as possible:
|
|
|
|
SMP re-ordering will not occur if the driver's interrupt handler is
|
|
always executed on the same CPU. Thus,
|
|
|
|
- Driver authors should use irq affinity for the interrupt handler.
|
|
|
|
The probability of packet loss due to backlog congestion can be
|
|
reduced by the following measures or a combination thereof:
|
|
|
|
(1) Drivers for kernel versions 2.4.x and above should always check the
|
|
return value of netif_rx(). If it returns NET_RX_DROP, the
|
|
driver's LAPB protocol must not confirm reception of the frame
|
|
to the peer.
|
|
This will reliably suppress packet loss. The LAPB protocol will
|
|
automatically cause the peer to re-transmit the dropped packet
|
|
later.
|
|
The lapb module interface was modified to support this. Its
|
|
data_indication() method should now transparently pass the
|
|
netif_rx() return value to the (lapb mopdule) caller.
|
|
(2) Drivers for kernel versions 2.2.x should always check the global
|
|
variable netdev_dropping when a new frame is received. The driver
|
|
should only call netif_rx() if netdev_dropping is zero. Otherwise
|
|
the driver should not confirm delivery of the frame and drop it.
|
|
Alternatively, the driver can queue the frame internally and call
|
|
netif_rx() later when netif_dropping is 0 again. In that case, delivery
|
|
confirmation should also be deferred such that the internal queue
|
|
cannot grow to much.
|
|
This will not reliably avoid packet loss, but the probability
|
|
of packet loss in netif_rx() path will be significantly reduced.
|
|
(3) Additionally, driver authors might consider to support
|
|
CONFIG_NET_HW_FLOWCONTROL. This allows the driver to be woken up
|
|
when a previously congested backlog queue becomes empty again.
|
|
The driver could uses this for flow-controlling the peer by means
|
|
of the LAPB protocol's flow-control service.
|