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- 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.