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-Description of the "concap" encapsulation protocol interface
-============================================================
-
-The "concap" interface is intended to be used by network device
-drivers that need to process an encapsulation protocol.
-It is assumed that the protocol interacts with a linux network device by
-- data transmission
-- connection control (establish, release)
-Thus, the mnemonic: "CONnection CONtrolling eNCAPsulation Protocol".
-
-This is currently only used inside the isdn subsystem. But it might
-also be useful to other kinds of network devices. Thus, if you want
-to suggest changes that improve usability or performance of the
-interface, please let me know. I'm willing to include them in future
-releases (even if I needed to adapt the current isdn code to the
-changed interface).
-
-
-Why is this useful?
-===================
-
-The encapsulation protocol used on top of WAN connections or permanent
-point-to-point links are frequently chosen upon bilateral agreement.
-Thus, a device driver for a certain type of hardware must support
-several different encapsulation protocols at once.
-
-The isdn device driver did already support several different
-encapsulation protocols. The encapsulation protocol is configured by a
-user space utility (isdnctrl). The isdn network interface code then
-uses several case statements which select appropriate actions
-depending on the currently configured encapsulation protocol.
-
-In contrast, LAN network interfaces always used a single encapsulation
-protocol which is unique to the hardware type of the interface. The LAN
-encapsulation is usually done by just sticking a header on the data. Thus,
-traditional linux network device drivers used to process the
-encapsulation protocol directly (usually by just providing a hard_header()
-method in the device structure) using some hardware type specific support
-functions. This is simple, direct and efficient. But it doesn't fit all
-the requirements for complex WAN encapsulations.
-
-
- The configurability of the encapsulation protocol to be used
- makes isdn network interfaces more flexible, but also much more
- complex than traditional lan network interfaces.
-
-
-Many Encapsulation protocols used on top of WAN connections will not just
-stick a header on the data. They also might need to set up or release
-the WAN connection. They also might want to send other data for their
-private purpose over the wire, e.g. ppp does a lot of link level
-negotiation before the first piece of user data can be transmitted.
-Such encapsulation protocols for WAN devices are typically more complex
-than encapsulation protocols for lan devices. Thus, network interface
-code for typical WAN devices also tends to be more complex.
-
-
-In order to support Linux' x25 PLP implementation on top of
-isdn network interfaces I could have introduced yet another branch to
-the various case statements inside drivers/isdn/isdn_net.c.
-This eventually made isdn_net.c even more complex. In addition, it made
-isdn_net.c harder to maintain. Thus, by identifying an abstract
-interface between the network interface code and the encapsulation
-protocol, complexity could be reduced and maintainability could be
-increased.
-
-
-Likewise, a similar encapsulation protocol will frequently be needed by
-several different interfaces of even different hardware type, e.g. the
-synchronous ppp implementation used by the isdn driver and the
-asynchronous ppp implementation used by the ppp driver have a lot of
-similar code in them. By cleanly separating the encapsulation protocol
-from the hardware specific interface stuff such code could be shared
-better in future.
-
-
-When operating over dial-up-connections (e.g. telephone lines via modem,
-non-permanent virtual circuits of wide area networks, ISDN) many
-encapsulation protocols will need to control the connection. Therefore,
-some basic connection control primitives are supported. The type and
-semantics of the connection (i.e the ISO layer where connection service
-is provided) is outside our scope and might be different depending on
-the encapsulation protocol used, e.g. for a ppp module using our service
-on top of a modem connection a connect_request will result in dialing
-a (somewhere else configured) remote phone number. For an X25-interface
-module (LAPB semantics, as defined in Documentation/networking/x25-iface.txt)
-a connect_request will ask for establishing a reliable lapb
-datalink connection.
-
-
-The encapsulation protocol currently provides the following
-service primitives to the network device.
-
-- create a new encapsulation protocol instance
-- delete encapsulation protocol instance and free all its resources
-- initialize (open) the encapsulation protocol instance for use.
-- deactivate (close) an encapsulation protocol instance.
-- process (xmit) data handed down by upper protocol layer
-- receive data from lower (hardware) layer
-- process connect indication from lower (hardware) layer
-- process disconnect indication from lower (hardware) layer
-
-
-The network interface driver accesses those primitives via callbacks
-provided by the encapsulation protocol instance within a
-struct concap_proto_ops.
-
-struct concap_proto_ops{
-
- /* create a new encapsulation protocol instance of same type */
- struct concap_proto * (*proto_new) (void);
-
- /* delete encapsulation protocol instance and free all its resources.
- cprot may no longer be referenced after calling this */
- void (*proto_del)(struct concap_proto *cprot);
-
- /* initialize the protocol's data. To be called at interface startup
- or when the device driver resets the interface. All services of the
- encapsulation protocol may be used after this*/
- int (*restart)(struct concap_proto *cprot,
- struct net_device *ndev,
- struct concap_device_ops *dops);
-
- /* deactivate an encapsulation protocol instance. The encapsulation
- protocol may not call any *dops methods after this. */
- int (*close)(struct concap_proto *cprot);
-
- /* process a frame handed down to us by upper layer */
- int (*encap_and_xmit)(struct concap_proto *cprot, struct sk_buff *skb);
-
- /* to be called for each data entity received from lower layer*/
- int (*data_ind)(struct concap_proto *cprot, struct sk_buff *skb);
-
- /* to be called when a connection was set up/down.
- Protocols that don't process these primitives might fill in
- dummy methods here */
- int (*connect_ind)(struct concap_proto *cprot);
- int (*disconn_ind)(struct concap_proto *cprot);
-};
-
-
-The data structures are defined in the header file include/linux/concap.h.
-
-
-A Network interface using encapsulation protocols must also provide
-some service primitives to the encapsulation protocol:
-
-- request data being submitted by lower layer (device hardware)
-- request a connection being set up by lower layer
-- request a connection being released by lower layer
-
-The encapsulation protocol accesses those primitives via callbacks
-provided by the network interface within a struct concap_device_ops.
-
-struct concap_device_ops{
-
- /* to request data be submitted by device */
- int (*data_req)(struct concap_proto *, struct sk_buff *);
-
- /* Control methods must be set to NULL by devices which do not
- support connection control. */
- /* to request a connection be set up */
- int (*connect_req)(struct concap_proto *);
-
- /* to request a connection be released */
- int (*disconn_req)(struct concap_proto *);
-};
-
-The network interface does not explicitly provide a receive service
-because the encapsulation protocol directly calls netif_rx().
-
-
-
-
-An encapsulation protocol itself is actually the
-struct concap_proto{
- struct net_device *net_dev; /* net device using our service */
- struct concap_device_ops *dops; /* callbacks provided by device */
- struct concap_proto_ops *pops; /* callbacks provided by us */
- int flags;
- void *proto_data; /* protocol specific private data, to
- be accessed via *pops methods only*/
- /*
- :
- whatever
- :
- */
-};
-
-Most of this is filled in when the device requests the protocol to
-be reset (opend). The network interface must provide the net_dev and
-dops pointers. Other concap_proto members should be considered private
-data that are only accessed by the pops callback functions. Likewise,
-a concap proto should access the network device's private data
-only by means of the callbacks referred to by the dops pointer.
-
-
-A possible extended device structure which uses the connection controlling
-encapsulation services could look like this:
-
-struct concap_device{
- struct net_device net_dev;
- struct my_priv /* device->local stuff */
- /* the my_priv struct might contain a
- struct concap_device_ops *dops;
- to provide the device specific callbacks
- */
- struct concap_proto *cprot; /* callbacks provided by protocol */
-};
-
-
-
-Misc Thoughts
-=============
-
-The concept of the concap proto might help to reuse protocol code and
-reduce the complexity of certain network interface implementations.
-The trade off is that it introduces yet another procedure call layer
-when processing the protocol. This has of course some impact on
-performance. However, typically the concap interface will be used by
-devices attached to slow lines (like telephone, isdn, leased synchronous
-lines). For such slow lines, the overhead is probably negligible.
-This might no longer hold for certain high speed WAN links (like
-ATM).
-
-
-If general linux network interfaces explicitly supported concap
-protocols (e.g. by a member struct concap_proto* in struct net_device)
-then the interface of the service function could be changed
-by passing a pointer of type (struct net_device*) instead of
-type (struct concap_proto*). Doing so would make many of the service
-functions compatible to network device support functions.
-
-e.g. instead of the concap protocol's service function
-
- int (*encap_and_xmit)(struct concap_proto *cprot, struct sk_buff *skb);
-
-we could have
-
- int (*encap_and_xmit)(struct net_device *ndev, struct sk_buff *skb);
-
-As this is compatible to the dev->hard_start_xmit() method, the device
-driver could directly register the concap protocol's encap_and_xmit()
-function as its hard_start_xmit() method. This would eliminate one
-procedure call layer.
-
-
-The device's data request function could also be defined as
-
- int (*data_req)(struct net_device *ndev, struct sk_buff *skb);
-
-This might even allow for some protocol stacking. And the network
-interface might even register the same data_req() function directly
-as its hard_start_xmit() method when a zero layer encapsulation
-protocol is configured. Thus, eliminating the performance penalty
-of the concap interface when a trivial concap protocol is used.
-Nevertheless, the device remains able to support encapsulation
-protocol configuration.
-