1.. SPDX-License-Identifier: GPL-2.0 2 3============================- 4X.25 Device Driver Interface 5============================- 6 7Version 1.1 8 9 Jonathan Naylor 26.12.96 10 11This is a description of the messages to be passed between the X.25 Packet 12Layer and the X.25 device driver. They are designed to allow for the easy 13setting of the LAPB mode from within the Packet Layer. 14 15The X.25 device driver will be coded normally as per the Linux device driver 16standards. Most X.25 device drivers will be moderately similar to the 17already existing Ethernet device drivers. However unlike those drivers, the 18X.25 device driver has a state associated with it, and this information 19needs to be passed to and from the Packet Layer for proper operation. 20 21All messages are held in sk_buff's just like real data to be transmitted 22over the LAPB link. The first byte of the skbuff indicates the meaning of 23the rest of the skbuff, if any more information does exist. 24 25 26Packet Layer to Device Driver 27----------------------------- 28 29First Byte = 0x00 (X25_IFACE_DATA) 30 31This indicates that the rest of the skbuff contains data to be transmitted 32over the LAPB link. The LAPB link should already exist before any data is 33passed down. 34 35First Byte = 0x01 (X25_IFACE_CONNECT) 36 37Establish the LAPB link. If the link is already established then the connect 38confirmation message should be returned as soon as possible. 39 40First Byte = 0x02 (X25_IFACE_DISCONNECT) 41 42Terminate the LAPB link. If it is already disconnected then the disconnect 43confirmation message should be returned as soon as possible. 44 45First Byte = 0x03 (X25_IFACE_PARAMS) 46 47LAPB parameters. To be defined. 48 49 50Device Driver to Packet Layer 51----------------------------- 52 53First Byte = 0x00 (X25_IFACE_DATA) 54 55This indicates that the rest of the skbuff contains data that has been 56received over the LAPB link. 57 58First Byte = 0x01 (X25_IFACE_CONNECT) 59 60LAPB link has been established. The same message is used for both a LAPB 61link connect_confirmation and a connect_indication. 62 63First Byte = 0x02 (X25_IFACE_DISCONNECT) 64 65LAPB link has been terminated. This same message is used for both a LAPB 66link disconnect_confirmation and a disconnect_indication. 67 68First Byte = 0x03 (X25_IFACE_PARAMS) 69 70LAPB parameters. To be defined. 71 72 73 74Possible Problems 75================= 76 77(Henner Eisen, 2000-10-28) 78 79The X.25 packet layer protocol depends on a reliable datalink service. 80The LAPB protocol provides such reliable service. But this reliability 81is not preserved by the Linux network device driver interface: 82 83- With Linux 2.4.x (and above) SMP kernels, packet ordering is not 84 preserved. Even if a device driver calls netif_rx(skb1) and later 85 netif_rx(skb2), skb2 might be delivered to the network layer 86 earlier that skb1. 87- Data passed upstream by means of netif_rx() might be dropped by the 88 kernel if the backlog queue is congested. 89 90The X.25 packet layer protocol will detect this and reset the virtual 91call in question. But many upper layer protocols are not designed to 92handle such N-Reset events gracefully. And frequent N-Reset events 93will always degrade performance. 94 95Thus, driver authors should make netif_rx() as reliable as possible: 96 97SMP re-ordering will not occur if the driver's interrupt handler is 98always executed on the same CPU. Thus, 99 100- Driver authors should use irq affinity for the interrupt handler. 101 102The probability of packet loss due to backlog congestion can be 103reduced by the following measures or a combination thereof: 104 105(1) Drivers for kernel versions 2.4.x and above should always check the 106 return value of netif_rx(). If it returns NET_RX_DROP, the 107 driver's LAPB protocol must not confirm reception of the frame 108 to the peer. 109 This will reliably suppress packet loss. The LAPB protocol will 110 automatically cause the peer to re-transmit the dropped packet 111 later. 112 The lapb module interface was modified to support this. Its 113 data_indication() method should now transparently pass the 114 netif_rx() return value to the (lapb module) caller. 115(2) Drivers for kernel versions 2.2.x should always check the global 116 variable netdev_dropping when a new frame is received. The driver 117 should only call netif_rx() if netdev_dropping is zero. Otherwise 118 the driver should not confirm delivery of the frame and drop it. 119 Alternatively, the driver can queue the frame internally and call 120 netif_rx() later when netif_dropping is 0 again. In that case, delivery 121 confirmation should also be deferred such that the internal queue 122 cannot grow to much. 123 This will not reliably avoid packet loss, but the probability 124 of packet loss in netif_rx() path will be significantly reduced. 125(3) Additionally, driver authors might consider to support 126 CONFIG_NET_HW_FLOWCONTROL. This allows the driver to be woken up 127 when a previously congested backlog queue becomes empty again. 128 The driver could uses this for flow-controlling the peer by means 129 of the LAPB protocol's flow-control service. 130