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Diffstat (limited to 'Documentation/networking/x25-iface.txt')
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1 files changed, 0 insertions, 123 deletions
diff --git a/Documentation/networking/x25-iface.txt b/Documentation/networking/x25-iface.txt deleted file mode 100644 index 7f213b556e85..000000000000 --- a/Documentation/networking/x25-iface.txt +++ /dev/null @@ -1,123 +0,0 @@ - X.25 Device Driver Interface 1.1 - - Jonathan Naylor 26.12.96 - -This is a description of the messages to be passed between the X.25 Packet -Layer and the X.25 device driver. They are designed to allow for the easy -setting of the LAPB mode from within the Packet Layer. - -The X.25 device driver will be coded normally as per the Linux device driver -standards. Most X.25 device drivers will be moderately similar to the -already existing Ethernet device drivers. However unlike those drivers, the -X.25 device driver has a state associated with it, and this information -needs to be passed to and from the Packet Layer for proper operation. - -All messages are held in sk_buff's just like real data to be transmitted -over the LAPB link. The first byte of the skbuff indicates the meaning of -the rest of the skbuff, if any more information does exist. - - -Packet Layer to Device Driver ------------------------------ - -First Byte = 0x00 (X25_IFACE_DATA) - -This indicates that the rest of the skbuff contains data to be transmitted -over the LAPB link. The LAPB link should already exist before any data is -passed down. - -First Byte = 0x01 (X25_IFACE_CONNECT) - -Establish the LAPB link. If the link is already established then the connect -confirmation message should be returned as soon as possible. - -First Byte = 0x02 (X25_IFACE_DISCONNECT) - -Terminate the LAPB link. If it is already disconnected then the disconnect -confirmation message should be returned as soon as possible. - -First Byte = 0x03 (X25_IFACE_PARAMS) - -LAPB parameters. To be defined. - - -Device Driver to Packet Layer ------------------------------ - -First Byte = 0x00 (X25_IFACE_DATA) - -This indicates that the rest of the skbuff contains data that has been -received over the LAPB link. - -First Byte = 0x01 (X25_IFACE_CONNECT) - -LAPB link has been established. The same message is used for both a LAPB -link connect_confirmation and a connect_indication. - -First Byte = 0x02 (X25_IFACE_DISCONNECT) - -LAPB link has been terminated. This same message is used for both a LAPB -link disconnect_confirmation and a disconnect_indication. - -First Byte = 0x03 (X25_IFACE_PARAMS) - -LAPB parameters. To be defined. - - - -Possible Problems -================= - -(Henner Eisen, 2000-10-28) - -The X.25 packet layer protocol depends on a reliable datalink service. -The LAPB protocol provides such reliable service. But this reliability -is not preserved by the Linux network device driver interface: - -- With Linux 2.4.x (and above) SMP kernels, packet ordering is not - preserved. Even if a device driver calls netif_rx(skb1) and later - netif_rx(skb2), skb2 might be delivered to the network layer - earlier that skb1. -- Data passed upstream by means of netif_rx() might be dropped by the - kernel if the backlog queue is congested. - -The X.25 packet layer protocol will detect this and reset the virtual -call in question. But many upper layer protocols are not designed to -handle such N-Reset events gracefully. And frequent N-Reset events -will always degrade performance. - -Thus, driver authors should make netif_rx() as reliable as possible: - -SMP re-ordering will not occur if the driver's interrupt handler is -always executed on the same CPU. Thus, - -- Driver authors should use irq affinity for the interrupt handler. - -The probability of packet loss due to backlog congestion can be -reduced by the following measures or a combination thereof: - -(1) Drivers for kernel versions 2.4.x and above should always check the - return value of netif_rx(). If it returns NET_RX_DROP, the - driver's LAPB protocol must not confirm reception of the frame - to the peer. - This will reliably suppress packet loss. The LAPB protocol will - automatically cause the peer to re-transmit the dropped packet - later. - The lapb module interface was modified to support this. Its - data_indication() method should now transparently pass the - netif_rx() return value to the (lapb module) caller. -(2) Drivers for kernel versions 2.2.x should always check the global - variable netdev_dropping when a new frame is received. The driver - should only call netif_rx() if netdev_dropping is zero. Otherwise - the driver should not confirm delivery of the frame and drop it. - Alternatively, the driver can queue the frame internally and call - netif_rx() later when netif_dropping is 0 again. In that case, delivery - confirmation should also be deferred such that the internal queue - cannot grow to much. - This will not reliably avoid packet loss, but the probability - of packet loss in netif_rx() path will be significantly reduced. -(3) Additionally, driver authors might consider to support - CONFIG_NET_HW_FLOWCONTROL. This allows the driver to be woken up - when a previously congested backlog queue becomes empty again. - The driver could uses this for flow-controlling the peer by means - of the LAPB protocol's flow-control service. |