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netgraph(4)

NAME

     netgraph -- graph based kernel networking subsystem

DESCRIPTION

     The netgraph system provides a uniform and modular system for the imple-
     mentation of kernel objects which perform various networking functions.
     The objects, known as nodes, can be arranged into arbitrarily complicated
     graphs.  Nodes have hooks which are used to connect two nodes together,
     forming the edges in the graph.  Nodes communicate along the edges to
     process data, implement protocols, etc.

     The aim of netgraph is to supplement rather than replace the existing
     kernel networking infrastructure.	It provides:

     o	 A flexible way of combining protocol and link level drivers.
     o	 A modular way to implement new protocols.
     o	 A common framework for kernel entities to inter-communicate.
     o	 A reasonably fast, kernel-based implementation.

   Nodes and Types
     The most fundamental concept in netgraph is that of a node.  All nodes
     implement a number of predefined methods which allow them to interact
     with other nodes in a well defined manner.

     Each node has a type, which is a static property of the node determined
     at node creation time.  A node's type is described by a unique ASCII type
     name.  The type implies what the node does and how it may be connected to
     other nodes.

     In object-oriented language, types are classes, and nodes are instances
     of their respective class.  All node types are subclasses of the generic
     node type, and hence inherit certain common functionality and capabili-
     ties (e.g., the ability to have an ASCII name).

     Nodes may be assigned a globally unique ASCII name which can be used to
     refer to the node.  The name must not contain the characters `.' or `:',
     and is limited to NG_NODESIZ characters (including the terminating NUL
     character).

     Each node instance has a unique ID number which is expressed as a 32-bit
     hexadecimal value.  This value may be used to refer to a node when there
     is no ASCII name assigned to it.

   Hooks
     Nodes are connected to other nodes by connecting a pair of hooks, one
     from each node.  Data flows bidirectionally between nodes along connected
     pairs of hooks.  A node may have as many hooks as it needs, and may
     assign whatever meaning it wants to a hook.

     Hooks have these properties:

     o	 A hook has an ASCII name which is unique among all hooks on that node
	 (other hooks on other nodes may have the same name).  The name must
	 not contain the characters `.' or `:', and is limited to NG_HOOKSIZ
	 characters (including the terminating NUL character).

     o	 A hook is always connected to another hook.  That is, hooks are cre-

     o	 A hook may supply overriding receive data and receive message func-
	 tions which should be used for data and messages received through
	 that hook in preference to the general node-wide methods.

     A node may decide to assign special meaning to some hooks.  For example,
     connecting to the hook named debug might trigger the node to start send-
     ing debugging information to that hook.

   Data Flow
     Two types of information flow between nodes: data messages and control
     messages.	Data messages are passed in mbuf chains along the edges in the
     graph, one edge at a time.  The first mbuf in a chain must have the
     M_PKTHDR flag set.  Each node decides how to handle data coming in on its
     hooks.

     Along with data, nodes can also receive control messages.	There are
     generic and type-specific control messages.  Control messages have a com-
     mon header format, followed by a type-specific data, and are binary
     structures for efficiency.  However, node types may also support conver-
     sion of the type specific data between binary and ASCII formats, for
     debugging and human interface purposes (see the NGM_ASCII2BINARY and
     NGM_BINARY2ASCII generic control messages below).	Nodes are not required
     to support these conversions.

     There are three ways to address a control message.  If there is a
     sequence of edges connecting the two nodes, the message may be ``source
     routed'' by specifying the corresponding sequence of ASCII hook names as
     the destination address for the message (relative addressing).  If the
     destination is adjacent to the source, then the source node may simply
     specify (as a pointer in the code) the hook across which the message
     should be sent.  Otherwise, the recipient node global ASCII name (or
     equivalent ID based name) is used as the destination address for the mes-
     sage (absolute addressing).  The two types of ASCII addressing may be
     combined, by specifying an absolute start node and a sequence of hooks.
     Only the ASCII addressing modes are available to control programs outside
     the kernel, as use of direct pointers is limited of course to kernel mod-
     ules.

     Messages often represent commands that are followed by a reply message in
     the reverse direction.  To facilitate this, the recipient of a control
     message is supplied with a ``return address'' that is suitable for
     addressing a reply.

     Each control message contains a 32 bit value called a typecookie indicat-
     ing the type of the message, i.e., how to interpret it.  Typically each
     type defines a unique typecookie for the messages that it understands.
     However, a node may choose to recognize and implement more than one type
     of messages.

     If a message is delivered to an address that implies that it arrived at
     that node through a particular hook (as opposed to having been directly
     addressed using its ID or global name) then that hook is identified to
     the receiving node.  This allows a message to be re-routed or passed on,
     should a node decide that this is required, in much the same way that
     data packets are passed around between nodes.  A set of standard messages
     for flow control and link management purposes are defined by the base
     system that are usually passed around in this manner.  Flow control mes-
     data delivery function.  This function in turn locates node B and calls
     B's ``receive data'' method.  There are exceptions to this.

     Each node has an input queue, and some operations can be considered to be
     writers in that they alter the state of the node.	Obviously, in an SMP
     world it would be bad if the state of a node were changed while another
     data packet were transiting the node.  For this purpose, the input queue
     implements a reader/writer semantic so that when there is a writer in the
     node, all other requests are queued, and while there are readers, a
     writer, and any following packets are queued.  In the case where there is
     no reason to queue the data, the input method is called directly, as men-
     tioned above.

     A node may declare that all requests should be considered as writers, or
     that requests coming in over a particular hook should be considered to be
     a writer, or even that packets leaving or entering across a particular
     hook should always be queued, rather than delivered directly (often use-
     ful for interrupt routines who want to get back to the hardware quickly).
     By default, all control message packets are considered to be writers
     unless specifically declared to be a reader in their definition.  (See
     NGM_READONLY in <ng_message.h>.)

     While this mode of operation results in good performance, it has a few
     implications for node developers:

     o	 Whenever a node delivers a data or control message, the node may need
	 to allow for the possibility of receiving a returning message before
	 the original delivery function call returns.

     o	 Netgraph provides internal synchronization between nodes.  Data
	 always enters a ``graph'' at an edge node.  An edge node is a node
	 that interfaces between netgraph and some other part of the system.
	 Examples of ``edge nodes'' include device drivers, the socket, ether,
	 tty, and ksocket node type.  In these edge nodes, the calling thread
	 directly executes code in the node, and from that code calls upon the
	 netgraph framework to deliver data across some edge in the graph.
	 From an execution point of view, the calling thread will execute the
	 netgraph framework methods, and if it can acquire a lock to do so,
	 the input methods of the next node.  This continues until either the
	 data is discarded or queued for some device or system entity, or the
	 thread is unable to acquire a lock on the next node.  In that case,
	 the data is queued for the node, and execution rewinds back to the
	 original calling entity.  The queued data will be picked up and pro-
	 cessed by either the current holder of the lock when they have com-
	 pleted their operations, or by a special netgraph thread that is
	 activated when there are such items queued.

     o	 It is possible for an infinite loop to occur if the graph contains
	 cycles.

     So far, these issues have not proven problematical in practice.

   Interaction with Other Parts of the Kernel
     A node may have a hidden interaction with other components of the kernel
     outside of the netgraph subsystem, such as device hardware, kernel proto-
     col stacks, etc.  In fact, one of the benefits of netgraph is the ability
     to join disparate kernel networking entities together in a consistent
     communication framework.
     hardware.

   Node Methods
     Nodes are notified of the following actions via function calls to the
     following node methods, and may accept or reject that action (by return-
     ing the appropriate error code):

     Creation of a new node
	 The constructor for the type is called.  If creation of a new node is
	 allowed, constructor method may allocate any special resources it
	 needs.  For nodes that correspond to hardware, this is typically done
	 during the device attach routine.  Often a global ASCII name corre-
	 sponding to the device name is assigned here as well.

     Creation of a new hook
	 The hook is created and tentatively linked to the node, and the node
	 is told about the name that will be used to describe this hook.  The
	 node sets up any special data structures it needs, or may reject the
	 connection, based on the name of the hook.

     Successful connection of two hooks
	 After both ends have accepted their hooks, and the links have been
	 made, the nodes get a chance to find out who their peer is across the
	 link, and can then decide to reject the connection.  Tear-down is
	 automatic.  This is also the time at which a node may decide whether
	 to set a particular hook (or its peer) into the queueing mode.

     Destruction of a hook
	 The node is notified of a broken connection.  The node may consider
	 some hooks to be critical to operation and others to be expendable:
	 the disconnection of one hook may be an acceptable event while for
	 another it may effect a total shutdown for the node.

     Preshutdown of a node
	 This method is called before real shutdown, which is discussed below.
	 While in this method, the node is fully operational and can send a
	 ``goodbye'' message to its peers, or it can exclude itself from the
	 chain and reconnect its peers together, like the ng_tee(4) node type
	 does.

     Shutdown of a node
	 This method allows a node to clean up and to ensure that any actions
	 that need to be performed at this time are taken.  The method is
	 called by the generic (i.e., superclass) node destructor which will
	 get rid of the generic components of the node.  Some nodes (usually
	 associated with a piece of hardware) may be persistent in that a
	 shutdown breaks all edges and resets the node, but does not remove
	 it.  In this case, the shutdown method should not free its resources,
	 but rather, clean up and then call the NG_NODE_REVIVE() macro to sig-
	 nal the generic code that the shutdown is aborted.  In the case where
	 the shutdown is started by the node itself due to hardware removal or
	 unloading (via ng_rmnode_self()), it should set the NGF_REALLY_DIE
	 flag to signal to its own shutdown method that it is not to persist.

   Sending and Receiving Data
     Two other methods are also supported by all nodes:

     Receive data message

	 passed on to another node, by using the NG_FREE_ITEM() macro.	If the
	 item still holds references to mbufs at the time of freeing then they
	 will also be appropriately freed.  Therefore, if there is any chance
	 that the mbuf will be changed or freed separately from the item, it
	 is very important that it be retrieved using the NGI_GET_M() macro
	 that also removes the reference within the item.  (Or multiple frees
	 of the same object will occur.)

	 If it is only required to examine the contents of the mbufs, then it
	 is possible to use the NGI_M() macro to both read and rewrite mbuf
	 pointer inside the item.

	 If developer needs to pass any meta information along with the mbuf
	 chain, he should use mbuf_tags(9) framework.  Note that old netgraph
	 specific meta-data format is obsoleted now.

	 The receiving node may decide to defer the data by queueing it in the
	 netgraph NETISR system (see below).  It achieves this by setting the
	 HK_QUEUE flag in the flags word of the hook on which that data will
	 arrive.  The infrastructure will respect that bit and queue the data
	 for delivery at a later time, rather than deliver it directly.  A
	 node may decide to set the bit on the peer node, so that its own out-
	 put packets are queued.

	 The node may elect to nominate a different receive data function for
	 data received on a particular hook, to simplify coding.  It uses the
	 NG_HOOK_SET_RCVDATA(hook, fn) macro to do this.  The function
	 receives the same arguments in every way other than it will receive
	 all (and only) packets from that hook.

     Receive control message
	 This method is called when a control message is addressed to the
	 node.	As with the received data, an item is received, with a pointer
	 to the control message.  The message can be examined using the
	 NGI_MSG() macro, or completely extracted from the item using the
	 NGI_GET_MSG() which also removes the reference within the item.  If
	 the Item still holds a reference to the message when it is freed
	 (using the NG_FREE_ITEM() macro), then the message will also be freed
	 appropriately.  If the reference has been removed, the node must free
	 the message itself using the NG_FREE_MSG() macro.  A return address
	 is always supplied, giving the address of the node that originated
	 the message so a reply message can be sent anytime later.  The return
	 address is retrieved from the item using the NGI_RETADDR() macro and
	 is of type ng_ID_t.  All control messages and replies are allocated
	 with the malloc(9) type M_NETGRAPH_MSG, however it is more convenient
	 to use the NG_MKMESSAGE() and NG_MKRESPONSE() macros to allocate and
	 fill out a message.  Messages must be freed using the NG_FREE_MSG()
	 macro.

	 If the message was delivered via a specific hook, that hook will also
	 be made known, which allows the use of such things as flow-control
	 messages, and status change messages, where the node may want to for-
	 ward the message out another hook to that on which it arrived.

	 The node may elect to nominate a different receive message function
	 for messages received on a particular hook, to simplify coding.  It
	 uses the NG_HOOK_SET_RCVMSG(hook, fn) macro to do this.  The function
	 receives the same arguments in every way other than it will receive
     The netgraph framework provides an unambiguous and simple to use method
     of specifically addressing any single node in the graph.  The naming of a
     node is independent of its type, in that another node, or external compo-
     nent need not know anything about the node's type in order to address it
     so as to send it a generic message type.  Node and hook names should be
     chosen so as to make addresses meaningful.

     Addresses are either absolute or relative.  An absolute address begins
     with a node name or ID, followed by a colon, followed by a sequence of
     hook names separated by periods.  This addresses the node reached by
     starting at the named node and following the specified sequence of hooks.
     A relative address includes only the sequence of hook names, implicitly
     starting hook traversal at the local node.

     There are a couple of special possibilities for the node name.  The name
     `.' (referred to as `.:') always refers to the local node.  Also, nodes
     that have no global name may be addressed by their ID numbers, by enclos-
     ing the hexadecimal representation of the ID number within the square
     brackets.	Here are some examples of valid netgraph addresses:

	   .:
	   [3f]:
	   foo:
	   .:hook1
	   foo:hook1.hook2
	   [d80]:hook1

     The following set of nodes might be created for a site with a single
     physical frame relay line having two active logical DLCI channels, with
     RFC 1490 frames on DLCI 16 and PPP frames over DLCI 20:

     [type SYNC ]		   [type FRAME] 		[type RFC1490]
     [ "Frame1" ](uplink)<-->(data)[<un-named>](dlci16)<-->(mux)[<un-named>  ]
     [	  A	]		   [	B     ](dlci20)<---+	[     C      ]
							   |
							   |	  [ type PPP ]
							   +>(mux)[<un-named>]
								  [    D     ]

     One could always send a control message to node C from anywhere by using
     the name ``Frame1:uplink.dlci16''.  In this case, node C would also be
     notified that the message reached it via its hook mux.  Similarly,
     ``Frame1:uplink.dlci20'' could reliably be used to reach node D, and node
     A could refer to node B as ``.:uplink'', or simply ``uplink''.  Con-
     versely, B can refer to A as ``data''.  The address ``mux.data'' could be
     used by both nodes C and D to address a message to node A.

     Note that this is only for control messages.  In each of these cases,
     where a relative addressing mode is used, the recipient is notified of
     the hook on which the message arrived, as well as the originating node.
     This allows the option of hop-by-hop distribution of messages and state
     information.  Data messages are only routed one hop at a time, by speci-
     fying the departing hook, with each node making the next routing deci-
     sion.  So when B receives a frame on hook data, it decodes the frame
     relay header to determine the DLCI, and then forwards the unwrapped frame
     to either C or D.

     In a similar way, flow control messages may be routed in the reverse

     [ type BRI ](B1)<--->(link1)[ type MPP  ]
     [	"ISDN1" ](B2)<--->(link2)[ (no name) ]
     [		](D) <-+
		       |
      +----------------+
      |
      +->(switch)[ type Q.921 ](term1)<---->(datalink)[ type Q.931 ]
		 [ (no name)  ] 		      [ (no name)  ]

   Netgraph Structures
     Structures are defined in <netgraph/netgraph.h> (for kernel structures
     only of interest to nodes) and <netgraph/ng_message.h> (for message defi-
     nitions also of interest to user programs).

     The two basic object types that are of interest to node authors are nodes
     and hooks.  These two objects have the following properties that are also
     of interest to the node writers.

     struct ng_node
	 Node authors should always use the following typedef to declare their
	 pointers, and should never actually declare the structure.

	 typedef struct ng_node *node_p;

	 The following properties are associated with a node, and can be
	 accessed in the following manner:

	 Validity
	     A driver or interrupt routine may want to check whether the node
	     is still valid.  It is assumed that the caller holds a reference
	     on the node so it will not have been freed, however it may have
	     been disabled or otherwise shut down.  Using the
	     NG_NODE_IS_VALID(node) macro will return this state.  Eventually
	     it should be almost impossible for code to run in an invalid node
	     but at this time that work has not been completed.

	 Node ID (ng_ID_t)
	     This property can be retrieved using the macro NG_NODE_ID(node).

	 Node name
	     Optional globally unique name, NUL terminated string.  If there
	     is a value in here, it is the name of the node.

		   if (NG_NODE_NAME(node)[0] != '\0') ...

		   if (strcmp(NG_NODE_NAME(node), "fred") == 0) ...

	 A node dependent opaque cookie
	     Anything of the pointer type can be placed here.  The macros
	     NG_NODE_SET_PRIVATE(node, value) and NG_NODE_PRIVATE(node) set
	     and retrieve this property, respectively.

	 Number of hooks
	     The NG_NODE_NUMHOOKS(node) macro is used to retrieve this value.

	 Hooks
	     The node may have a number of hooks.  A traversal method is pro-

	 Node authors should always use the following typedef to declare their
	 hook pointers.

	 typedef struct ng_hook *hook_p;

	 The following properties are associated with a hook, and can be
	 accessed in the following manner:

	 A hook dependent opaque cookie
	     Anything of the pointer type can be placed here.  The macros
	     NG_HOOK_SET_PRIVATE(hook, value) and NG_HOOK_PRIVATE(hook) set
	     and retrieve this property, respectively.

	 An associate node
	     The macro NG_HOOK_NODE(hook) finds the associated node.

	 A peer hook (hook_p)
	     The other hook in this connected pair.  The NG_HOOK_PEER(hook)
	     macro finds the peer.

	 References
	     The NG_HOOK_REF(hook) and NG_HOOK_UNREF(hook) macros increment
	     and decrement the hook reference count accordingly.  After decre-
	     ment you should always assume the hook has been freed unless you
	     have another reference still valid.

	 Override receive functions
	     The NG_HOOK_SET_RCVDATA(hook, fn) and NG_HOOK_SET_RCVMSG(hook,
	     fn) macros can be used to set override methods that will be used
	     in preference to the generic receive data and receive message
	     functions.  To unset these, use the macros to set them to NULL.
	     They will only be used for data and messages received on the hook
	     on which they are set.

	 The maintenance of the names, reference counts, and linked list of
	 hooks for each node is handled automatically by the netgraph subsys-
	 tem.  Typically a node's private info contains a back-pointer to the
	 node or hook structure, which counts as a new reference that must be
	 included in the reference count for the node.	When the node con-
	 structor is called, there is already a reference for this calculated
	 in, so that when the node is destroyed, it should remember to do a
	 NG_NODE_UNREF() on the node.

	 From a hook you can obtain the corresponding node, and from a node,
	 it is possible to traverse all the active hooks.

	 A current example of how to define a node can always be seen in
	 src/sys/netgraph/ng_sample.c and should be used as a starting point
	 for new node writers.

   Netgraph Message Structure
     Control messages have the following structure:

     #define NG_CMDSTRSIZ    32      /* Max command string (including nul) */

     struct ng_mesg {
       struct ng_msghdr {
	 u_char      version;	     /* Must equal NG_VERSION */
       char  data[0];		     /* Start of cmd/resp data */
     };

     #define NG_ABI_VERSION  5		     /* Netgraph kernel ABI version */
     #define NG_VERSION      4		     /* Netgraph message version */
     #define NGF_ORIG	     0x0000	     /* Command */
     #define NGF_RESP	     0x0001	     /* Response */

     Control messages have the fixed header shown above, followed by a vari-
     able length data section which depends on the type cookie and the com-
     mand.  Each field is explained below:

     version
	     Indicates the version of the netgraph message protocol itself.
	     The current version is NG_VERSION.

     arglen  This is the length of any extra arguments, which begin at data.

     flags   Indicates whether this is a command or a response control mes-
	     sage.

     token   The token is a means by which a sender can match a reply message
	     to the corresponding command message; the reply always has the
	     same token.

     typecookie
	     The corresponding node type's unique 32-bit value.  If a node
	     does not recognize the type cookie it must reject the message by
	     returning EINVAL.

	     Each type should have an include file that defines the commands,
	     argument format, and cookie for its own messages.	The typecookie
	     insures that the same header file was included by both sender and
	     receiver; when an incompatible change in the header file is made,
	     the typecookie must be changed.  The de-facto method for generat-
	     ing unique type cookies is to take the seconds from the Epoch at
	     the time the header file is written (i.e., the output of ``date
	     -u +%s'').

	     There is a predefined typecookie NGM_GENERIC_COOKIE for the
	     generic node type, and a corresponding set of generic messages
	     which all nodes understand.  The handling of these messages is
	     automatic.

     cmd     The identifier for the message command.  This is type specific,
	     and is defined in the same header file as the typecookie.

     cmdstr  Room for a short human readable version of command (for debugging
	     purposes only).

     Some modules may choose to implement messages from more than one of the
     header files and thus recognize more than one type cookie.

   Control Message ASCII Form
     Control messages are in binary format for efficiency.  However, for
     debugging and human interface purposes, and if the node type supports it,
     control messages may be converted to and from an equivalent ASCII form.
     The ASCII form is similar to the binary form, with two exceptions:
     C data type.  Netgraph includes parsing routines to support some pre-
     defined datatypes in ASCII with this simple syntax:

     o	 Integer types are represented by base 8, 10, or 16 numbers.

     o	 Strings are enclosed in double quotes and respect the normal C lan-
	 guage backslash escapes.

     o	 IP addresses have the obvious form.

     o	 Arrays are enclosed in square brackets, with the elements listed con-
	 secutively starting at index zero.  An element may have an optional
	 index and equals sign (`=') preceding it.  Whenever an element does
	 not have an explicit index, the index is implicitly the previous ele-
	 ment's index plus one.

     o	 Structures are enclosed in curly braces, and each field is specified
	 in the form fieldname=value.

     o	 Any array element or structure field whose value is equal to its
	 ``default value'' may be omitted.  For integer types, the default
	 value is usually zero; for string types, the empty string.

     o	 Array elements and structure fields may be specified in any order.

     Each node type may define its own arbitrary types by providing the neces-
     sary routines to parse and unparse.  ASCII forms defined for a specific
     node type are documented in the corresponding man page.

   Generic Control Messages
     There are a number of standard predefined messages that will work for any
     node, as they are supported directly by the framework itself.  These are
     defined in <netgraph/ng_message.h> along with the basic layout of mes-
     sages and other similar information.

     NGM_CONNECT
	     Connect to another node, using the supplied hook names on either
	     end.

     NGM_MKPEER
	     Construct a node of the given type and then connect to it using
	     the supplied hook names.

     NGM_SHUTDOWN
	     The target node should disconnect from all its neighbours and
	     shut down.  Persistent nodes such as those representing physical
	     hardware might not disappear from the node namespace, but only
	     reset themselves.	The node must disconnect all of its hooks.
	     This may result in neighbors shutting themselves down, and possi-
	     bly a cascading shutdown of the entire connected graph.

     NGM_NAME
	     Assign a name to a node.  Nodes can exist without having a name,
	     and this is the default for nodes created using the NGM_MKPEER
	     method.  Such nodes can only be addressed relatively or by their
	     ID number.

     NGM_RMHOOK
	     unique to that node.

     NGM_LISTHOOKS
	     This returns the information given by NGM_NODEINFO, but in addi-
	     tion includes an array of fields describing each link, and the
	     description for the node at the far end of that link.

     NGM_LISTNAMES
	     This returns an array of node descriptions (as for NGM_NODEINFO)
	     where each entry of the array describes a named node.  All named
	     nodes will be described.

     NGM_LISTNODES
	     This is the same as NGM_LISTNAMES except that all nodes are
	     listed regardless of whether they have a name or not.

     NGM_LISTTYPES
	     This returns a list of all currently installed netgraph types.

     NGM_TEXT_STATUS
	     The node may return a text formatted status message.  The status
	     information is determined entirely by the node type.  It is the
	     only ``generic'' message that requires any support within the
	     node itself and as such the node may elect to not support this
	     message.  The text response must be less than NG_TEXTRESPONSE
	     bytes in length (presently 1024).	This can be used to return
	     general status information in human readable form.

     NGM_BINARY2ASCII
	     This message converts a binary control message to its ASCII form.
	     The entire control message to be converted is contained within
	     the arguments field of the NGM_BINARY2ASCII message itself.  If
	     successful, the reply will contain the same control message in
	     ASCII form.  A node will typically only know how to translate
	     messages that it itself understands, so the target node of the
	     NGM_BINARY2ASCII is often the same node that would actually
	     receive that message.

     NGM_ASCII2BINARY
	     The opposite of NGM_BINARY2ASCII.	The entire control message to
	     be converted, in ASCII form, is contained in the arguments sec-
	     tion of the NGM_ASCII2BINARY and need only have the flags,
	     cmdstr, and arglen header fields filled in, plus the
	     NUL-terminated string version of the arguments in the arguments
	     field.  If successful, the reply contains the binary version of
	     the control message.

   Flow Control Messages
     In addition to the control messages that affect nodes with respect to the
     graph, there are also a number of flow control messages defined.  At
     present these are not handled automatically by the system, so nodes need
     to handle them if they are going to be used in a graph utilising flow
     control, and will be in the likely path of these messages.  The default
     action of a node that does not understand these messages should be to
     pass them onto the next node.  Hopefully some helper functions will
     assist in this eventually.  These messages are also defined in
     <netgraph/ng_message.h> and have a separate cookie NG_FLOW_COOKIE to help
     identify them.  They will not be covered in depth here.
     in your kernel configuration file.  You may also include selected node
     types in the kernel compilation, for example:

	   options NETGRAPH
	   options NETGRAPH_SOCKET
	   options NETGRAPH_ECHO

     Once the netgraph subsystem is loaded, individual node types may be
     loaded at any time as KLD modules via kldload(8).	Moreover, netgraph
     knows how to automatically do this; when a request to create a new node
     of unknown type type is made, netgraph will attempt to load the KLD mod-
     ule ng_<type>.ko.

     Types can also be installed at boot time, as certain device drivers may
     want to export each instance of the device as a netgraph node.

     In general, new types can be installed at any time from within the kernel
     by calling ng_newtype(), supplying a pointer to the type's struct ng_type
     structure.

     The NETGRAPH_INIT() macro automates this process by using a linker set.

EXISTING NODE TYPES

     Several node types currently exist.  Each is fully documented in its own
     man page:

     SOCKET  The socket type implements two new sockets in the new protocol
	     domain PF_NETGRAPH.  The new sockets protocols are NG_DATA and
	     NG_CONTROL, both of type SOCK_DGRAM.  Typically one of each is
	     associated with a socket node.  When both sockets have closed,
	     the node will shut down.  The NG_DATA socket is used for sending
	     and receiving data, while the NG_CONTROL socket is used for send-
	     ing and receiving control messages.  Data and control messages
	     are passed using the sendto(2) and recvfrom(2) system calls,
	     using a struct sockaddr_ng socket address.

     HOLE    Responds only to generic messages and is a ``black hole'' for
	     data.  Useful for testing.  Always accepts new hooks.

     ECHO    Responds only to generic messages and always echoes data back
	     through the hook from which it arrived.  Returns any non-generic
	     messages as their own response.  Useful for testing.  Always
	     accepts new hooks.

     TEE     This node is useful for ``snooping''.  It has 4 hooks: left,
	     right, left2right, and right2left.  Data entering from the right
	     is passed to the left and duplicated on right2left, and data
	     entering from the left is passed to the right and duplicated on
	     left2right.  Data entering from left2right is sent to the right
	     and data from right2left to left.

     RFC1490 MUX
	     Encapsulates/de-encapsulates frames encoded according to RFC
	     1490.  Has a hook for the encapsulated packets (downstream) and
	     one hook for each protocol (i.e., IP, PPP, etc.).

     FRAME RELAY MUX
	     Encapsulates/de-encapsulates Frame Relay frames.  Has a hook for
	     mbuf frames and sequential serial data, allowing a TTY to appear
	     as a netgraph node.  It has a programmable ``hotkey'' character.

     ASYNC   This node encapsulates and de-encapsulates asynchronous frames
	     according to RFC 1662.  This is used in conjunction with the TTY
	     node type for supporting PPP links over asynchronous serial
	     lines.

     ETHERNET
	     This node is attached to every Ethernet interface in the system.
	     It allows capturing raw Ethernet frames from the network, as well
	     as sending frames out of the interface.

     INTERFACE
	     This node is also a system networking interface.  It has hooks
	     representing each protocol family (IP, AppleTalk, IPX, etc.) and
	     appears in the output of ifconfig(8).  The interfaces are named
	     ``ng0'', ``ng1'', etc.

     ONE2MANY
	     This node implements a simple round-robin multiplexer.  It can be
	     used for example to make several LAN ports act together to get a
	     higher speed link between two machines.

     Various PPP related nodes
	     There is a full multilink PPP implementation that runs in
	     netgraph.	The net/mpd port can use these modules to make a very
	     low latency high capacity PPP system.  It also supports PPTP VPNs
	     using the PPTP node.

     PPPOE   A server and client side implementation of PPPoE.	Used in con-
	     junction with either ppp(8) or the net/mpd port.

     BRIDGE  This node, together with the Ethernet nodes, allows a very flexi-
	     ble bridging system to be implemented.

     KSOCKET
	     This intriguing node looks like a socket to the system but
	     diverts all data to and from the netgraph system for further pro-
	     cessing.  This allows such things as UDP tunnels to be almost
	     trivially implemented from the command line.

     Refer to the section at the end of this man page for more nodes types.

NOTES

     Whether a named node exists can be checked by trying to send a control
     message to it (e.g., NGM_NODEINFO).  If it does not exist, ENOENT will be
     returned.

     All data messages are mbuf chains with the M_PKTHDR flag set.

     Nodes are responsible for freeing what they allocate.  There are three
     exceptions:

     1.   Mbufs sent across a data link are never to be freed by the sender.
	  In the case of error, they should be considered freed.

     2.   Messages sent using one of NG_SEND_MSG_*() family macros are freed

FILES

     <netgraph/netgraph.h>
	     Definitions for use solely within the kernel by netgraph nodes.

     <netgraph/ng_message.h>
	     Definitions needed by any file that needs to deal with netgraph
	     messages.

     <netgraph/ng_socket.h>
	     Definitions needed to use netgraph socket type nodes.

     <netgraph/ng_><type>.h
	     Definitions needed to use netgraph type nodes, including the type
	     cookie definition.

     /boot/kernel/netgraph.ko
	     The netgraph subsystem loadable KLD module.

     /boot/kernel/ng_<type>.ko
	     Loadable KLD module for node type type.

     src/sys/netgraph/ng_sample.c
	     Skeleton netgraph node.  Use this as a starting point for new
	     node types.

USER MODE SUPPORT

     There is a library for supporting user-mode programs that wish to inter-
     act with the netgraph system.  See netgraph(3) for details.

     Two user-mode support programs, ngctl(8) and nghook(8), are available to
     assist manual configuration and debugging.

     There are a few useful techniques for debugging new node types.  First,
     implementing new node types in user-mode first makes debugging easier.
     The tee node type is also useful for debugging, especially in conjunction
     with ngctl(8) and nghook(8).

     Also look in /usr/share/examples/netgraph for solutions to several common
     networking problems, solved using netgraph.

SEE ALSO

     socket(2), netgraph(3), ng_async(4), ng_atm(4), ng_atmllc(4),
     ng_atmpif(4), ng_bluetooth(4), ng_bpf(4), ng_bridge(4), ng_bt3c(4),
     ng_btsocket(4), ng_cisco(4), ng_device(4), ng_echo(4), ng_eiface(4),
     ng_etf(4), ng_ether(4), ng_fec(4), ng_frame_relay(4), ng_gif(4),
     ng_gif_demux(4), ng_h4(4), ng_hci(4), ng_hole(4), ng_hub(4), ng_iface(4),
     ng_ip_input(4), ng_ksocket(4), ng_l2cap(4), ng_l2tp(4), ng_lmi(4),
     ng_mppc(4), ng_netflow(4), ng_one2many(4), ng_ppp(4), ng_pppoe(4),
     ng_pptpgre(4), ng_rfc1490(4), ng_socket(4), ng_split(4), ng_sppp(4),
     ng_sscfu(4), ng_sscop(4), ng_tee(4), ng_tty(4), ng_ubt(4), ng_UI(4),
     ng_uni(4), ng_vjc(4), ng_vlan(4), ngctl(8), nghook(8)

HISTORY

     The netgraph system was designed and first implemented at Whistle Commu-
     nications, Inc. in a version of FreeBSD 2.2 customized for the Whistle
     InterJet.	It first made its debut in the main tree in FreeBSD 3.4.

AUTHORS

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