Based on kernel version 3.13. Page generated on 2014-01-20 22:04 EST.
1 X.25 Device Driver Interface 1.1 2 3 Jonathan Naylor 26.12.96 4 5 This is a description of the messages to be passed between the X.25 Packet 6 Layer and the X.25 device driver. They are designed to allow for the easy 7 setting of the LAPB mode from within the Packet Layer. 8 9 The X.25 device driver will be coded normally as per the Linux device driver 10 standards. Most X.25 device drivers will be moderately similar to the 11 already existing Ethernet device drivers. However unlike those drivers, the 12 X.25 device driver has a state associated with it, and this information 13 needs to be passed to and from the Packet Layer for proper operation. 14 15 All messages are held in sk_buff's just like real data to be transmitted 16 over the LAPB link. The first byte of the skbuff indicates the meaning of 17 the rest of the skbuff, if any more information does exist. 18 19 20 Packet Layer to Device Driver 21 ----------------------------- 22 23 First Byte = 0x00 (X25_IFACE_DATA) 24 25 This indicates that the rest of the skbuff contains data to be transmitted 26 over the LAPB link. The LAPB link should already exist before any data is 27 passed down. 28 29 First Byte = 0x01 (X25_IFACE_CONNECT) 30 31 Establish the LAPB link. If the link is already established then the connect 32 confirmation message should be returned as soon as possible. 33 34 First Byte = 0x02 (X25_IFACE_DISCONNECT) 35 36 Terminate the LAPB link. If it is already disconnected then the disconnect 37 confirmation message should be returned as soon as possible. 38 39 First Byte = 0x03 (X25_IFACE_PARAMS) 40 41 LAPB parameters. To be defined. 42 43 44 Device Driver to Packet Layer 45 ----------------------------- 46 47 First Byte = 0x00 (X25_IFACE_DATA) 48 49 This indicates that the rest of the skbuff contains data that has been 50 received over the LAPB link. 51 52 First Byte = 0x01 (X25_IFACE_CONNECT) 53 54 LAPB link has been established. The same message is used for both a LAPB 55 link connect_confirmation and a connect_indication. 56 57 First Byte = 0x02 (X25_IFACE_DISCONNECT) 58 59 LAPB link has been terminated. This same message is used for both a LAPB 60 link disconnect_confirmation and a disconnect_indication. 61 62 First Byte = 0x03 (X25_IFACE_PARAMS) 63 64 LAPB parameters. To be defined. 65 66 67 68 Possible Problems 69 ================= 70 71 (Henner Eisen, 2000-10-28) 72 73 The X.25 packet layer protocol depends on a reliable datalink service. 74 The LAPB protocol provides such reliable service. But this reliability 75 is 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 84 The X.25 packet layer protocol will detect this and reset the virtual 85 call in question. But many upper layer protocols are not designed to 86 handle such N-Reset events gracefully. And frequent N-Reset events 87 will always degrade performance. 88 89 Thus, driver authors should make netif_rx() as reliable as possible: 90 91 SMP re-ordering will not occur if the driver's interrupt handler is 92 always executed on the same CPU. Thus, 93 94 - Driver authors should use irq affinity for the interrupt handler. 95 96 The probability of packet loss due to backlog congestion can be 97 reduced 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.