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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 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).

	   short 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.

	   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 sockaddr *rt_genmask;
		   When	the rtalloc() family of	functions performs a cloning
		   operation as	requested by the RTF_CLONING or	RTF_PRCLONING
		   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 rt_metrics rt_rmx;
		   See 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 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.

     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
	   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	reso-
			    lution for this route.  (Used in conjunction with
	   RTF_LLINFO	    Indicates that this	route represents information
			    being 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    Like RTF_CLONING, only managed by an entire	proto-
			    col.  (E.g., IP uses this flag to manage a per-
			    host cache integrated with the routing table, for
			    those destinations which do	not have a link	layer
			    performing this function.)
	   RTF_WASCLONED    Indicates that this	route was generated as a re-
			    sult of cloning requested by the RTF_CLONING or
			    RTF_PRCLONING flag.	 When set, the rt_parent field
			    indicates the route	from which this	one was	gener-
	   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, defined by struct

	   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(2)) 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.  The RTF_PRCLONING	mechanism first	appeared in
     FreeBSD 2.0.

     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	8, 1996				   BSD


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