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 module) 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