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Based on kernel version 3.9. Page generated on 2013-05-02 23:12 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 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.
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