linux/Documentation/networking/x25-iface.txt
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   1                        X.25 Device Driver Interface 1.1
   2
   3                           Jonathan Naylor 26.12.96
   4
   5This is a description of the messages to be passed between the X.25 Packet
   6Layer and the X.25 device driver. They are designed to allow for the easy
   7setting of the LAPB mode from within the Packet Layer.
   8
   9The X.25 device driver will be coded normally as per the Linux device driver
  10standards. Most X.25 device drivers will be moderately similar to the
  11already existing Ethernet device drivers. However unlike those drivers, the
  12X.25 device driver has a state associated with it, and this information
  13needs to be passed to and from the Packet Layer for proper operation.
  14
  15All messages are held in sk_buff's just like real data to be transmitted
  16over the LAPB link. The first byte of the skbuff indicates the meaning of
  17the rest of the skbuff, if any more information does exist.
  18
  19
  20Packet Layer to Device Driver
  21-----------------------------
  22
  23First Byte = 0x00 (X25_IFACE_DATA)
  24
  25This indicates that the rest of the skbuff contains data to be transmitted
  26over the LAPB link. The LAPB link should already exist before any data is
  27passed down.
  28
  29First Byte = 0x01 (X25_IFACE_CONNECT)
  30
  31Establish the LAPB link. If the link is already established then the connect
  32confirmation message should be returned as soon as possible.
  33
  34First Byte = 0x02 (X25_IFACE_DISCONNECT)
  35
  36Terminate the LAPB link. If it is already disconnected then the disconnect
  37confirmation message should be returned as soon as possible.
  38
  39First Byte = 0x03 (X25_IFACE_PARAMS)
  40
  41LAPB parameters. To be defined.
  42
  43
  44Device Driver to Packet Layer
  45-----------------------------
  46
  47First Byte = 0x00 (X25_IFACE_DATA)
  48
  49This indicates that the rest of the skbuff contains data that has been
  50received over the LAPB link.
  51
  52First Byte = 0x01 (X25_IFACE_CONNECT)
  53
  54LAPB link has been established. The same message is used for both a LAPB
  55link connect_confirmation and a connect_indication.
  56
  57First Byte = 0x02 (X25_IFACE_DISCONNECT)
  58
  59LAPB link has been terminated. This same message is used for both a LAPB
  60link disconnect_confirmation and a disconnect_indication.
  61
  62First Byte = 0x03 (X25_IFACE_PARAMS)
  63
  64LAPB parameters. To be defined.
  65
  66
  67
  68Possible Problems
  69=================
  70
  71(Henner Eisen, 2000-10-28)
  72
  73The X.25 packet layer protocol depends on a reliable datalink service.
  74The LAPB protocol provides such reliable service. But this reliability
  75is not preserved by the Linux network device driver interface:
  76
  77- With Linux 2.4.x (and above) SMP kernels, packet ordering is not
  78  preserved. Even if a device driver calls netif_rx(skb1) and later
  79  netif_rx(skb2), skb2 might be delivered to the network layer
  80  earlier that skb1.
  81- Data passed upstream by means of netif_rx() might be dropped by the
  82  kernel if the backlog queue is congested.
  83
  84The X.25 packet layer protocol will detect this and reset the virtual
  85call in question. But many upper layer protocols are not designed to
  86handle such N-Reset events gracefully. And frequent N-Reset events
  87will always degrade performance.
  88
  89Thus, driver authors should make netif_rx() as reliable as possible:
  90
  91SMP re-ordering will not occur if the driver's interrupt handler is
  92always executed on the same CPU. Thus,
  93
  94- Driver authors should use irq affinity for the interrupt handler.
  95
  96The probability of packet loss due to backlog congestion can be
  97reduced by the following measures or a combination thereof:
  98
  99(1) Drivers for kernel versions 2.4.x and above should always check the
 100    return value of netif_rx(). If it returns NET_RX_DROP, the
 101    driver's LAPB protocol must not confirm reception of the frame
 102    to the peer. 
 103    This will reliably suppress packet loss. The LAPB protocol will
 104    automatically cause the peer to re-transmit the dropped packet
 105    later.
 106    The lapb module interface was modified to support this. Its
 107    data_indication() method should now transparently pass the
 108    netif_rx() return value to the (lapb mopdule) caller.
 109(2) Drivers for kernel versions 2.2.x should always check the global
 110    variable netdev_dropping when a new frame is received. The driver
 111    should only call netif_rx() if netdev_dropping is zero. Otherwise
 112    the driver should not confirm delivery of the frame and drop it.
 113    Alternatively, the driver can queue the frame internally and call
 114    netif_rx() later when netif_dropping is 0 again. In that case, delivery
 115    confirmation should also be deferred such that the internal queue
 116    cannot grow to much.
 117    This will not reliably avoid packet loss, but the probability
 118    of packet loss in netif_rx() path will be significantly reduced.
 119(3) Additionally, driver authors might consider to support
 120    CONFIG_NET_HW_FLOWCONTROL. This allows the driver to be woken up
 121    when a previously congested backlog queue becomes empty again.
 122    The driver could uses this for flow-controlling the peer by means
 123    of the LAPB protocol's flow-control service.
 124
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