ChorusOS man pages section 7P: Protocols

route(7P)

NAME | SYNOPSIS | DESCRIPTION | ATTRIBUTES | SEE ALSO

NAME

route- microkernel packet forwarding database

SYNOPSIS

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

family

DESCRIPTION

The ChorusOS operating system provides a number of packet routing facilities. The microkernel maintains a routing information database, which is used in selecting the appropriate network interface when transmitting packets.

A user process (or possibly multiple cooperating processes) maintains this database by sending messages over a special kind of socket. This replaces the fixed size ioctl(2POSIX) used in earlier releases. Routing table changes may only be carried out by the super user.

The operating system may spontaneously emit routing messages in response to external events, such as receipt of a re-direct or failure to locate a suitable route for a request. The message types are described in greater detail below.

Routing database entries come in two flavors: for a specific host, or for all hosts on a generic subnetwork (specified by a bit mask and value under the mask). The effect of teh wildcard or default route may be achieved by using a mask of all zeros, and there may be hierarchical routes.

When the system is booted and addresses are assigned to the network interfaces, each protocol family installs a routing table entry for each interface when it is ready for traffic. Normally, the protocol specifies the route through each interface as a "direct" connection to the destination host or network. If the route is direct, the transport layer of a protocol family usually requests that the packet be sent to the same host as specified in the packet. Otherwise, the interface is requested to address the packet to the gateway listed in the routing entry (in other words, the packet is forwarded).

When routing a packet, the microkernel will attempt to find the most specific route to the destination. (If there are two different mask and value-under-the-mask pairs that match, the more specific is the one with more bits in the mask. A route to a host is assumed to be supplied with a mask of as many "1"'s as there are bits in the destination). If no entry is found, the destination is declared to be unreachable, and a routing-miss message is generated if there are any listeners on the routing control socket (described below).

A wildcard routing entry is specified with a zero destination address value, and a mask of all zeroes. Wildcard routes will be used when the system fails to find other routes matching the destination. The combination of wildcard routes and routing redirects can provide an economical mechanism for routing traffic.

One opens the channel for passing routing control messages by using the socket() call shown in the synopsis above.

The family parameter can be AF_UNSPEC, which will provide routing information for all address families. It can be restricted to a specific address family by specifying which one is desired. There can be more than one routing socket open per system.

Messages are formed by a header followed by a number of sockadders (now variable length, particularly in the ISO case), these are interpreted by position, and delimited by the new length entry in the sockaddr. An example of a message with four addresses might be an ISO redirect: Destination, Netmask, Gateway, and Author of the redirect. The interpretation of which addresses are present is provided by a bit mask within the header, and the sequence is least significant to most significant bit within the vector.

Any messages sent to the microkernel are returned, and copies are sent to all interested listeners. The microkernel provides the process ID for the sender, and the sender may use an additional sequence field to distinguish between outstanding messages. However, message replies may be lost when microkernel buffers are exhausted.

The microkernel may reject certain messages, and will indicate this by filling in the rtm_errno field. The routing code returns one of the following error conditions:

EEXIST: if requested to duplicate an existing entry,
ESRCH: if requested to delete a non-existent entry,
ENOBUFS: if insufficient resources were available to install a new route.

In the current implementation, all routing process run locally, and the values for rtm_errno are available through the normal errno mechanism, even if the routing reply message is lost.

A process may avoid the expense of reading replies to its own messages by issuing a setsockopt(2POSIX) call indicating that the SO_USELOOPBACK option at the SOL_SOCKET level is to be turned off. A process may ignore all messages from the routing socket by doing a shutdown(2POSIX) system call for further input.

If a route is in use when it is deleted, the routing entry will be marked down and removed from the routing table, but the resources associated with it will not be reclaimed until all references to it are released. User processes can obtain information about the routing entry to a specific destination by using a RTM_GET message, or by calling sysctl(3POSIX).

Messages include:

#define	RTM_ADD         0x1    /* Add Route */ 
#define	RTM_DELETE      0x2    /* Delete Route */ 
#define	RTM_CHANGE      0x3    /* Change Metrics, Flags, or Gateway */
#define	RTM_GET         0x4    /* Report Information */ 
#define	RTM_LOSING     0x5    /* Microkernel Suspects Partitioning */ 
#define	RTM_REDIRECT    0x6    /* Told to use different route */ 
#define	RTM_MISS        0x7    /* Lookup failed on this address */ 
#define	RTM_LOCK       0x8    /* Fix specified metrics */ 
#define	RTM_RESOLVE     0xb    /* Request to resolve dst to LL addr */
#define	RTM_NEWADDR        0xc    /* Address being added to iface */ 
#define	RTM_DELADDR        0xd    /* Address being removed from iface */ 
#define	RTM_IFINFO        0xe    /* iface going up/down etc. */ 
#define	RTM_NEWMADDR        0xf    /* mcast group membership being added to if */ 
#define	RTM_DELMADDR        0xg    /* mcast group membership being deleted */

A message header consists of one of the follwing:

struct rt_msghdr {
       u_short  rtm_msglen;     /* to skip over non-understood messages*/
       u_char   rtm_version;    /* future binary compatibility */
       u_char   rtm_type;       /* message type */
       u_short  rmt_index;      /* index for associated ifp */
       int      rtm_flags;      /* flags, incl kern & message, e.g. DONE */
       int      rtm_addrs;      /* bit mask identifying sockaddrs in msg */
       pid_t    rmt_pid;        /* identify sender */
       int      rtm_seq;        /* for sender to identify action */
       int      rtm_errno;      /* why failed */
       int      rtm_use;        /* from rtentry */
       u_long   rtm_inits;      /* which metrics we are initializing */
       struct   rt_metrics rtm_rmx; /* metrics themselves */ 
};

struct if_msghdr {
        u_short   ifm_msglen;      /* to skip over non-understood messages */
        u_char    ifm_version;        /* future binary compatability */
        u_char    ifm_type;   /* message type */
        int       ifm_addrs;     /* like rtm_addrs */
        int       ifm_flags;   /* value of if_flags */
        u_short   ifm_index;   /* index for assocated */
        u_struct  ifm_data ifm_data;   /* statistics and other data about it */
};

struct ifa_msghdr {
       u_short ifam_msglen;        /* to skip over non-understood messages */
         u_char  ifam_version;       /* future binary compatability */
         u_char  ifam_type;          /* message type */
         int     ifam_addrs;         /* like rtm_addrs */
         int     ifam_flags;         /* value of ifa_flags */
         u_short ifam_index;         /* index for associated ifp */
         int     ifam_metric;        /* value of ifa_metric */
     };

struct ifma_msghdr {
         u_short ifmam_msglen;       /* to skip over non-understood messages */
         u_char  ifmam_version;      /* future binary compatability */
         u_char  ifmam_type;         /* message type */
         int     ifmam_addrs;        /* like rtm_addrs */
         int     ifmam_flags;        /* value of ifa_flags */
         u_short ifmam_index;        /* index for associated ifp */
     };

The RTM_IFINFO message uses a if_msghdr header, the RTM_NEWADDR and RTM_DELADDR messages use a ifa_msghdr header, the RTM_NEWMADDR and RTM_DELMADDR messages use a ifma_msghdr, and all other messages use the rt_msghdr header.

Specifiers for metric values in rmx_locks and rtm_inits are:

#define RTV_MTU         0x1     /* init or lock _mtu */
#define RTV_HOPCOUNT    0x2     /* init or lock _hopcount */
#define RTV_EXPIRE      0x4     /* init or lock _hopcount */
#define RTV_RPIPE       0x8     /* init or lock _recvpipe */
#define RTV_SPIPE       0x10    /* init or lock _sendpipe */
#define RTV_SSTHRESH    0x20    /* init or lock _ssthresh */
#define RTV_RTT         0x40    /* init or lock _rtt */
#define RTV_RTTVAR      0x80    /* init or lock _rttvar */

Specifiers for which addresses are present in the messages are:

#define RTA_DST         0x1     /* destination sockaddr present */
#define RTA_GATEWAY     0x2     /* gateway sockaddr present */
#define RTA_NETMASK     0x4     /* netmask sockaddr present */
#define RTA_GENMASK     0x8     /* cloning mask sockaddr present */
#define RTA_IFP         0x10    /* interface name sockaddr present */
#define RTA_IFA         0x20    /* interface addr sockaddr present */
#define RTA_AUTHOR      0x40    /* sockaddr for author of redirect */
#define RTA_BRD         0x80    /* for NEWADDR, broadcast or p-p dest addr */

ATTRIBUTES

See attributes(5) for descriptions of the following attributes:

ATTRIBUTE TYPE	ATTRIBUTE VALUE
Interface Stability	Evolving