The kernel provides a common mechanism by which all protocols can store
and retrieve entries from a central table of routes.
Parts of this
mechanism are also used to interact with user-level processes by means
of a socket in the
route(4)
pseudo-protocol family.
The
#include <net/route.h>
header file defines the structures and manifest constants used in this
facility.
The basic structure of a route is defined by
Vt struct rtentry ,
which includes the following fields:
Vt 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
Vt struct sockaddr * )
given a
Vt struct rtentry * .
Vt 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).
Vt u_long rt_flags
See below.
Vt struct ifnet *rt_ifp
Vt 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.
Vt struct rt_metrics_lite rt_rmx
See below.
Vt 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
reference drops.
Vt 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.
Vt caddr_t rt_llinfo
When the
RTF_LLINFO
flag is set, this field contains information 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.
Vt 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.
Vt 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.
Vt 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
Er EHOSTUNREACH
error from output routines.
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 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 discarded.
RTF_PROTO1
RTF_PROTO2
RTF_PROTO3
Protocol-specific.
RTF_PRCLONING
This flag is obsolete and simply ignored by facility.
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 address.
RTF_MULTICAST
Indicates that the destination is a multicast address.
Every route has associated with it a set of metrics, stored in
Vt struct rt_metrics_lite .
Metrics are supplied in
Vt struct rt_metrics
passed with routing control messages via
route(4)
API.
Currently only
Vt rmx_mtu , rmx_expire ,
and
Vt rmx_pksent
metrics are used in
Vt struct rt_metrics_lite .
All others are ignored.
The following metrics are defined by
Vt struct rt_metrics :
Vt u_long rmx_locks
Flag bits indicating which metrics the kernel is not permitted to
dynamically modify.
Vt u_long rmx_mtu
MTU for this path.
Vt u_long rmx_hopcount
Number of intermediate systems on the path to this destination.
Vt 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 responsibility of individual protocol suites to ensure that routes
are actually deleted once they expire.
Vt 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 )
Vt u_long rmx_sendpipe
As before, but in the opposite direction.
Vt u_long rmx_ssthresh
The slow-start threshold used in
TCP
congestion-avoidance.
Vt u_long rmx_rtt
The round-trip time to this destination, in units of
RMX_RTTUNIT
per second.
Vt u_long rmx_rttvar
The average deviation of the round-trip time to this destination, in
units of
RMX_RTTUNIT
per second.
Vt u_long rmx_pksent
A count of packets successfully sent via this route.
Vt u_long rmx_filler[4]
Empty space available for protocol-specific information.
The
Vt rtentry
structure first appeared in
BSD 4.2
The radix-tree representation of the routing table and the
Vt rt_metrics
structure first appeared in
BSD 4.3 reno
AUTHORS
This manual page was written by
An Garrett Wollman .
BUGS
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
