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