1 Description of the "concap" encapsulation protocol interface
2 ============================================================
3
4 The "concap" interface is intended to be used by network device
5 drivers that need to process an encapsulation protocol.
6 It is assumed that the protocol interacts with a linux network device by
7 - data transmission
8 - connection control (establish, release)
9 Thus, the mnemonic: "CONnection CONtrolling eNCAPsulation Protocol".
10
11 This is currently only used inside the isdn subsystem. But it might
12 also be useful to other kinds of network devices. Thus, if you want
13 to suggest changes that improve usability or performance of the
14 interface, please let me know. I'm willing to include them in future
15 releases (even if I needed to adapt the current isdn code to the
16 changed interface).
17
18
19 Why is this useful?
20 ===================
21
22 The encapsulation protocol used on top of WAN connections or permanent
23 point-to-point links are frequently chosen upon bilateral agreement.
24 Thus, a device driver for a certain type of hardware must support
25 several different encapsulation protocols at once.
26
27 The isdn device driver did already support several different
28 encapsulation protocols. The encapsulation protocol is configured by a
29 user space utility (isdnctrl). The isdn network interface code then
30 uses several case statements which select appropriate actions
31 depending on the currently configured encapsulation protocol.
32
33 In contrast, LAN network interfaces always used a single encapsulation
34 protocol which is unique to the hardware type of the interface. The LAN
35 encapsulation is usually done by just sticking a header on the data. Thus,
36 traditional linux network device drivers used to process the
37 encapsulation protocol directly (usually by just providing a hard_header()
38 method in the device structure) using some hardware type specific support
39 functions. This is simple, direct and efficient. But it doesn't fit all
40 the requirements for complex WAN encapsulations.
41
42
43 The configurability of the encapsulation protocol to be used
44 makes isdn network interfaces more flexible, but also much more
45 complex than traditional lan network interfaces.
46
47
48 Many Encapsulation protocols used on top of WAN connections will not just
49 stick a header on the data. They also might need to set up or release
50 the WAN connection. They also might want to send other data for their
51 private purpose over the wire, e.g. ppp does a lot of link level
52 negotiation before the first piece of user data can be transmitted.
53 Such encapsulation protocols for WAN devices are typically more complex
54 than encapsulation protocols for lan devices. Thus, network interface
55 code for typical WAN devices also tends to be more complex.
56
57
58 In order to support Linux' x25 PLP implementation on top of
59 isdn network interfaces I could have introduced yet another branch to
60 the various case statements inside drivers/isdn/isdn_net.c.
61 This eventually made isdn_net.c even more complex. In addition, it made
62 isdn_net.c harder to maintain. Thus, by identifying an abstract
63 interface between the network interface code and the encapsulation
64 protocol, complexity could be reduced and maintainability could be
65 increased.
66
67
68 Likewise, a similar encapsulation protocol will frequently be needed by
69 several different interfaces of even different hardware type, e.g. the
70 synchronous ppp implementation used by the isdn driver and the
71 asynchronous ppp implementation used by the ppp driver have a lot of
72 similar code in them. By cleanly separating the encapsulation protocol
73 from the hardware specific interface stuff such code could be shared
74 better in future.
75
76
77 When operating over dial-up-connections (e.g. telephone lines via modem,
78 non-permanent virtual circuits of wide area networks, ISDN) many
79 encapsulation protocols will need to control the connection. Therefore,
80 some basic connection control primitives are supported. The type and
81 semantics of the connection (i.e the ISO layer where connection service
82 is provided) is outside our scope and might be different depending on
83 the encapsulation protocol used, e.g. for a ppp module using our service
84 on top of a modem connection a connect_request will result in dialing
85 a (somewhere else configured) remote phone number. For an X25-interface
86 module (LAPB semantics, as defined in Documentation/networking/x25-iface.txt)
87 a connect_request will ask for establishing a reliable lapb
88 datalink connection.
89
90
91 The encapsulation protocol currently provides the following
92 service primitives to the network device.
93
94 - create a new encapsulation protocol instance
95 - delete encapsulation protocol instance and free all its resources
96 - initialize (open) the encapsulation protocol instance for use.
97 - deactivate (close) an encapsulation protocol instance.
98 - process (xmit) data handed down by upper protocol layer
99 - receive data from lower (hardware) layer
100 - process connect indication from lower (hardware) layer
101 - process disconnect indication from lower (hardware) layer
102
103
104 The network interface driver accesses those primitives via callbacks
105 provided by the encapsulation protocol instance within a
106 struct concap_proto_ops.
107
108 struct concap_proto_ops{
109
110 /* create a new encapsulation protocol instance of same type */
111 struct concap_proto * (*proto_new) (void);
112
113 /* delete encapsulation protocol instance and free all its resources.
114 cprot may no loger be referenced after calling this */
115 void (*proto_del)(struct concap_proto *cprot);
116
117 /* initialize the protocol's data. To be called at interface startup
118 or when the device driver resets the interface. All services of the
119 encapsulation protocol may be used after this*/
120 int (*restart)(struct concap_proto *cprot,
121 struct net_device *ndev,
122 struct concap_device_ops *dops);
123
124 /* deactivate an encapsulation protocol instance. The encapsulation
125 protocol may not call any *dops methods after this. */
126 int (*close)(struct concap_proto *cprot);
127
128 /* process a frame handed down to us by upper layer */
129 int (*encap_and_xmit)(struct concap_proto *cprot, struct sk_buff *skb);
130
131 /* to be called for each data entity received from lower layer*/
132 int (*data_ind)(struct concap_proto *cprot, struct sk_buff *skb);
133
134 /* to be called when a connection was set up/down.
135 Protocols that don't process these primitives might fill in
136 dummy methods here */
137 int (*connect_ind)(struct concap_proto *cprot);
138 int (*disconn_ind)(struct concap_proto *cprot);
139 };
140
141
142 The data structures are defined in the header file include/linux/concap.h.
143
144
145 A Network interface using encapsulation protocols must also provide
146 some service primitives to the encapsulation protocol:
147
148 - request data being submitted by lower layer (device hardware)
149 - request a connection being set up by lower layer
150 - request a connection being released by lower layer
151
152 The encapsulation protocol accesses those primitives via callbacks
153 provided by the network interface within a struct concap_device_ops.
154
155 struct concap_device_ops{
156
157 /* to request data be submitted by device */
158 int (*data_req)(struct concap_proto *, struct sk_buff *);
159
160 /* Control methods must be set to NULL by devices which do not
161 support connection control. */
162 /* to request a connection be set up */
163 int (*connect_req)(struct concap_proto *);
164
165 /* to request a connection be released */
166 int (*disconn_req)(struct concap_proto *);
167 };
168
169 The network interface does not explicitly provide a receive service
170 because the encapsulation protocol directly calls netif_rx().
171
172
173
174
175 An encapsulation protocol itself is actually the
176 struct concap_proto{
177 struct net_device *net_dev; /* net device using our service */
178 struct concap_device_ops *dops; /* callbacks provided by device */
179 struct concap_proto_ops *pops; /* callbacks provided by us */
180 int flags;
181 void *proto_data; /* protocol specific private data, to
182 be accessed via *pops methods only*/
183 /*
184 :
185 whatever
186 :
187 */
188 };
189
190 Most of this is filled in when the device requests the protocol to
191 be reset (opend). The network interface must provide the net_dev and
192 dops pointers. Other concap_proto members should be considered private
193 data that are only accessed by the pops callback functions. Likewise,
194 a concap proto should access the network device's private data
195 only by means of the callbacks referred to by the dops pointer.
196
197
198 A possible extended device structure which uses the connection controlling
199 encapsulation services could look like this:
200
201 struct concap_device{
202 struct net_device net_dev;
203 struct my_priv /* device->local stuff */
204 /* the my_priv struct might contain a
205 struct concap_device_ops *dops;
206 to provide the device specific callbacks
207 */
208 struct concap_proto *cprot; /* callbacks provided by protocol */
209 };
210
211
212
213 Misc Thoughts
214 =============
215
216 The concept of the concap proto might help to reuse protocol code and
217 reduce the complexity of certain network interface implementations.
218 The trade off is that it introduces yet another procedure call layer
219 when processing the protocol. This has of course some impact on
220 performance. However, typically the concap interface will be used by
221 devices attached to slow lines (like telephone, isdn, leased synchronous
222 lines). For such slow lines, the overhead is probably negligible.
223 This might no longer hold for certain high speed WAN links (like
224 ATM).
225
226
227 If general linux network interfaces explicitly supported concap
228 protocols (e.g. by a member struct concap_proto* in struct net_device)
229 then the interface of the service function could be changed
230 by passing a pointer of type (struct net_device*) instead of
231 type (struct concap_proto*). Doing so would make many of the service
232 functions compatible to network device support functions.
233
234 e.g. instead of the concap protocol's service function
235
236 int (*encap_and_xmit)(struct concap_proto *cprot, struct sk_buff *skb);
237
238 we could have
239
240 int (*encap_and_xmit)(struct net_device *ndev, struct sk_buff *skb);
241
242 As this is compatible to the dev->hard_start_xmit() method, the device
243 driver could directly register the concap protocol's encap_and_xmit()
244 function as its hard_start_xmit() method. This would eliminate one
245 procedure call layer.
246
247
248 The device's data request function could also be defined as
249
250 int (*data_req)(struct net_device *ndev, struct sk_buff *skb);
251
252 This might even allow for some protocol stacking. And the network
253 interface might even register the same data_req() function directly
254 as its hard_start_xmit() method when a zero layer encapsulation
255 protocol is configured. Thus, eliminating the performance penalty
256 of the concap interface when a trivial concap protocol is used.
257 Nevertheless, the device remains able to support encapsulation
258 protocol configuration.
259
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