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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).

	   int rt_flags;
		   See below.  If the RTF_UP flag is not present, the rtfree()
		   function will delete	the route from the radix tree when the
		   last	reference drops.

	   int rt_refcnt;
		   Route entries are reference-counted;	this field indicates
		   the number of external (to the radix	tree) references.

	   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.

	   u_long rt_mtu;
		   See description of rmx_mtu below.

	   u_long rt_weight;
		   See description of rmx_weight below.

	   u_long rt_expire;
		   See description of rmx_expire below.

	   counter64_t rt_pksent;
		   See description of rmx_pksent below.

	   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 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_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
	   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_PINNED	  Indicates that this route is immutable to a routing
	   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-

     Several 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	supplied.  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-trip 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.

     This manual page was written by Garrett Wollman.

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

BSD				 March 5, 2014				   BSD


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