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