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+.. SPDX-License-Identifier: GPL-2.0
+
+============================-
+X.25 Device Driver Interface
+============================-
+
+Version 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 module) 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.