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RTENTRY(9)		 BSD Kernel Developer's	Manual		    RTENTRY(9)

     rtentry --	structure of an	entry in the kernel routing table

     #include <sys/types.h>
     #include <sys/socket.h>
     #include <net/route.h>

     The kernel	provides a common mechanism by which all protocols can store
     and retrieve entries from a central table of routes.  Parts of this mech-
     anism are also used to interact with user-level processes by means	of a
     socket in the route(4) pseudo-protocol family.  The <net/route.h> header
     file defines the structures and manifest constants	used in	this facility.

     The basic structure of a route is defined by struct rtentry, which	in-
     cludes the	following fields:

	   struct radix_node rt_nodes[2];
		   Glue	used by	the radix-tree routines.  These	members	also
		   include in their substructure the key (i.e.,	destination
		   address) and	mask used when the route was created.  The
		   rt_key(rt) and rt_mask(rt) macros can be used to extract
		   this	information (in	the form of a struct sockaddr *) given
		   a struct rtentry *.

	   struct sockaddr *rt_gateway;
		   The "target"	of the route, which can	either represent a
		   destination in its own right	(some protocols	will put a
		   link-layer address here), or	some intermediate stop on the
		   way to that destination (if the RTF_GATEWAY flag is set).

	   u_long rt_flags;
		   See below.

	   struct ifnet	*rt_ifp;

	   struct ifaddr *rt_ifa;
		   These two fields represent the "answer", as it were,	to the
		   question posed by a route lookup; that is, they name	the
		   interface and interface address to be used in sending a
		   packet to the destination or	set of destinations which this
		   route represents.

	   struct rt_metrics_lite rt_rmx;
		   See below.

	   long	rt_refcnt;
		   Route entries are reference-counted;	this field indicates
		   the number of external (to the radix	tree) references.  If
		   the RTF_UP flag is not present, the rtfree()	function will
		   delete the route from the radix tree	when the last refer-
		   ence	drops.

	   struct sockaddr *rt_genmask;
		   When	the rtalloc() family of	functions performs a cloning
		   operation as	requested by the RTF_CLONING flag, this	field
		   is used as the mask for the new route which is inserted
		   into	the table.  If this field is a null pointer, then a
		   host	route is generated.

	   caddr_t rt_llinfo;
		   When	the RTF_LLINFO flag is set, this field contains	infor-
		   mation specific to the link layer represented by the	named
		   interface address.  (It is normally managed by the
		   rt_ifa-_ifa_rtrequest routine.)  Protocols such as arp(4)
		   use this field to reference per-destination state internal
		   to that protocol.

	   struct rtentry *rt_gwroute;
		   This	member is a reference to a route whose destination is
		   rt_gateway.	It is only used	for RTF_GATEWAY	routes.

	   struct rtentry *rt_parent;
		   A reference to the route from which this route was cloned,
		   or a	null pointer if	this route was not generated by
		   cloning.  See also the RTF_WASCLONED	flag.

	   struct mtx rt_mtx;
		   Mutex to lock this routing entry.

     The following flag	bits are defined:
	   RTF_UP	  The route is not deleted.
	   RTF_GATEWAY	  The route points to an intermediate destination and
			  not the ultimate recipient; the rt_gateway and
			  rt_gwroute fields name that destination.
	   RTF_HOST	  This is a host route.
	   RTF_REJECT	  The destination is presently unreachable.  This
			  should result	in an EHOSTUNREACH error from output
	   RTF_DYNAMIC	  This route was created dynamically by	rtredirect().
	   RTF_MODIFIED	  This route was modified by rtredirect().
	   RTF_DONE	  Used only in the route(4) protocol, indicating that
			  the request was executed.
	   RTF_CLONING	  When this route is returned as a result of a lookup,
			  automatically	create a new route using this one as a
			  template and rt_genmask (if present) as a mask.
	   RTF_XRESOLVE	  When this route is returned as a result of a lookup,
			  send a report	on the route(4)	interface requesting
			  that an external process perform resolution for this
			  route.  (Used	in conjunction with RTF_CLONING.)
	   RTF_LLINFO	  Indicates that this route represents information be-
			  ing managed by a link	layer's	adaptation layer
			  (e.g., ARP).
	   RTF_STATIC	  Indicates that this route was	manually added by
			  means	of the route(8)	command.
	   RTF_BLACKHOLE  Requests that	output sent via	this route be dis-
	   RTF_PROTO3	  Protocol-specific.
	   RTF_PRCLONING  This flag is obsolete	and simply ignored by facil-
	   RTF_WASCLONED  Indicates that this route was	generated as a result
			  of cloning requested by the RTF_CLONING flag.	 When
			  set, the rt_parent field indicates the route from
			  which	this one was generated.
	   RTF_PINNED	  (Reserved for	future use to indicate routes which
			  are not to be	modified by a routing protocol.)
	   RTF_LOCAL	  Indicates that the destination of this route is an
			  address configured as	belonging to this system.
	   RTF_BROADCAST  Indicates that the destination is a broadcast	ad-
	   RTF_MULTICAST  Indicates that the destination is a multicast	ad-

     Every route has associated	with it	a set of metrics, stored in struct
     rt_metrics_lite.  Metrics are supplied in struct rt_metrics passed	with
     routing control messages via route(4) API.	 Currently only	rmx_mtu,
     rmx_expire, and rmx_pksent	metrics	are used in struct rt_metrics_lite.
     All others	are ignored.

     The following metrics are defined by struct rt_metrics:

	   u_long rmx_locks;
		   Flag	bits indicating	which metrics the kernel is not	per-
		   mitted to dynamically modify.

	   u_long rmx_mtu;
		   MTU for this	path.

	   u_long rmx_hopcount;
		   Number of intermediate systems on the path to this destina-

	   u_long rmx_expire;
		   The time (a la time(3)) at which this route should expire,
		   or zero if it should	never expire.  It is the responsibil-
		   ity of individual protocol suites to	ensure that routes are
		   actually deleted once they expire.

	   u_long rmx_recvpipe;
		   Nominally, the bandwidth-delay product for the path from
		   the destination to this system.  In practice, this value is
		   used	to set the size	of the receive buffer (and thus	the
		   window in sliding-window protocols like TCP).

	   u_long rmx_sendpipe;
		   As before, but in the opposite direction.

	   u_long rmx_ssthresh;
		   The slow-start threshold used in TCP	congestion-avoidance.

	   u_long rmx_rtt;
		   The round-trip time to this destination, in units of
		   RMX_RTTUNIT per second.

	   u_long rmx_rttvar;
		   The average deviation of the	round-type time	to this	desti-
		   nation, in units of RMX_RTTUNIT per second.

	   u_long rmx_pksent;
		   A count of packets successfully sent	via this route.

	   u_long rmx_filler[4];
		   Empty space available for protocol-specific information.

     route(4), route(8), rtalloc(9)

     The rtentry structure first appeared in 4.2BSD.  The radix-tree represen-
     tation of the routing table and the rt_metrics structure first appeared
     in	4.3BSD-Reno.

     There are a number	of historical relics remaining in this interface.  The
     rt_gateway	and rmx_filler fields could be named better.

     There is some disagreement	over whether it	is legitimate for RTF_LLINFO
     to	be set by any process other than rt_ifa-_ifa_rtrequest.

     This manual page was written by Garrett Wollman.

BSD				October	7, 2004				   BSD


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