MULTICAST(4)                 Device Drivers Manual                MULTICAST(4)


     multicast - multicast routing


     options MROUTING

     #include <sys/types.h>
     #include <sys/socket.h>
     #include <netinet/in.h>
     #include <netinet/ip_mroute.h>
     #include <netinet6/ip6_mroute.h>

     getsockopt(int s, IPPROTO_IP, MRT_INIT, void *optval, socklen_t *optlen);

     setsockopt(int s, IPPROTO_IP, MRT_INIT, const void *optval,
         socklen_t optlen);

     getsockopt(int s, IPPROTO_IPV6, MRT6_INIT, void *optval,
         socklen_t *optlen);

     setsockopt(int s, IPPROTO_IPV6, MRT6_INIT, const void *optval,
         socklen_t optlen);


     Multicast routing is used to efficiently propagate data packets to a set
     of multicast listeners in multipoint networks.  If unicast is used to
     replicate the data to all listeners, then some of the network links may
     carry multiple copies of the same data packets.  With multicast routing,
     the overhead is reduced to one copy (at most) per network link.

     All multicast-capable routers must run a common multicast routing
     protocol.  The Distance Vector Multicast Routing Protocol (DVMRP) was the
     first developed multicast routing protocol.  Later, other protocols such
     as Multicast Extensions to OSPF (MOSPF) and Core Based Trees (CBT) were
     developed as well.

     To start multicast routing, the user must enable multicast forwarding via
     the sysctl(8) variables net.inet.ip.mforwarding and/or
     net.inet.ip6.mforwarding, and set multicast to "YES" in rc.conf.local(8).
     The user must also run a multicast routing capable user-level process,
     such as mrouted(8).  From a developer's point of view, the programming
     guide described in the Programming Guide section should be used to
     control the multicast forwarding in the kernel.

   Programming Guide
     This section provides information about the basic multicast routing API.
     The so-called "advanced multicast API" is described in the Advanced
     Multicast API Programming Guide section.

     First, a multicast routing socket must be open.  That socket would be
     used to control the multicast forwarding in the kernel.  Note that most
     operations below require certain privilege (i.e., root privilege):

           /* IPv4 */
           int mrouter_s4;
           mrouter_s4 = socket(AF_INET, SOCK_RAW, IPPROTO_IGMP);

           int mrouter_s6;
           mrouter_s6 = socket(AF_INET6, SOCK_RAW, IPPROTO_ICMPV6);

     Note that if the router needs to open an IGMP or ICMPv6 socket (IPv4 or
     IPv6, respectively) for sending or receiving of IGMP or MLD multicast
     group membership messages, then the same mrouter_s4 or mrouter_s6 sockets
     should be used for sending and receiving respectively IGMP or MLD
     messages.  In the case of -derivedBSD kernels, it may be possible to open
     separate sockets for IGMP or MLD messages only.  However, some other
     kernels (e.g., Linux) require that the multicast routing socket must be
     used for sending and receiving of IGMP or MLD messages.  Therefore, for
     portability reasons, the multicast routing socket should be reused for
     IGMP and MLD messages as well.

     After the multicast routing socket is open, it can be used to enable or
     disable multicast forwarding in the kernel:

          /* IPv4 */
          int v = 1;        /* 1 to enable, or 0 to disable */
          setsockopt(mrouter_s4, IPPROTO_IP, MRT_INIT, (void *)&v, sizeof(v));

          /* IPv6 */
          int v = 1;        /* 1 to enable, or 0 to disable */
          setsockopt(mrouter_s6, IPPROTO_IPV6, MRT6_INIT, (void *)&v, sizeof(v));
          /* If necessary, filter all ICMPv6 messages */
          struct icmp6_filter filter;
          setsockopt(mrouter_s6, IPPROTO_ICMPV6, ICMP6_FILTER, (void *)&filter,

     For each network interface (e.g., physical or a virtual tunnel) that
     would be used for multicast forwarding, a corresponding multicast
     interface must be added to the kernel:

        /* IPv4 */
        struct vifctl vc;
        memset(&vc, 0, sizeof(vc));
        /* Assign all vifctl fields as appropriate */
        vc.vifc_vifi = vif_index;
        vc.vifc_flags = vif_flags;
        vc.vifc_threshold = min_ttl_threshold;
        vc.vifc_rate_limit = max_rate_limit;
        memcpy(&vc.vifc_lcl_addr, &vif_local_address, sizeof(vc.vifc_lcl_addr));
        if (vc.vifc_flags & VIFF_TUNNEL)
            memcpy(&vc.vifc_rmt_addr, &vif_remote_address,
        setsockopt(mrouter_s4, IPPROTO_IP, MRT_ADD_VIF, (void *)&vc,

     The vif_index must be unique per vif.  The vif_flags contains the VIFF_*
     flags as defined in <netinet/ip_mroute.h>.  The min_ttl_threshold
     contains the minimum TTL a multicast data packet must have to be
     forwarded on that vif.  Typically, it would be 1.  The max_rate_limit
     contains the maximum rate (in bits/s) of the multicast data packets
     forwarded on that vif.  A value of 0 means no limit.  The
     vif_local_address contains the local IP address of the corresponding
     local interface.  The vif_remote_address contains the remote IP address
     for DVMRP multicast tunnels.

           /* IPv6 */
           struct mif6ctl mc;
           memset(&mc, 0, sizeof(mc));
           /* Assign all mif6ctl fields as appropriate */
           mc.mif6c_mifi = mif_index;
           mc.mif6c_flags = mif_flags;
           mc.mif6c_pifi = pif_index;
           setsockopt(mrouter_s6, IPPROTO_IPV6, MRT6_ADD_MIF, (void *)&mc,

     The mif_index must be unique per vif.  The mif_flags contains the MIFF_*
     flags as defined in <netinet6/ip6_mroute.h>.  The pif_index is the
     physical interface index of the corresponding local interface.

     A multicast interface is deleted by:

           /* IPv4 */
           vifi_t vifi = vif_index;
           setsockopt(mrouter_s4, IPPROTO_IP, MRT_DEL_VIF, (void *)&vifi,

           /* IPv6 */
           mifi_t mifi = mif_index;
           setsockopt(mrouter_s6, IPPROTO_IPV6, MRT6_DEL_MIF, (void *)&mifi,

     After multicast forwarding is enabled, and the multicast virtual
     interfaces have been added, the kernel may deliver upcall messages (also
     called signals later in this text) on the multicast routing socket that
     was open earlier with MRT_INIT or MRT6_INIT.  The IPv4 upcalls have a
     struct igmpmsg header (see <netinet/ip_mroute.h>) with the im_mbz field
     set to zero.  Note that this header follows the structure of struct ip
     with the protocol field ip_p set to zero.  The IPv6 upcalls have a struct
     mrt6msg header (see <netinet6/ip6_mroute.h>) with the im6_mbz field set
     to zero.  Note that this header follows the structure of struct ip6_hdr
     with the next header field ip6_nxt set to zero.

     The upcall header contains the im_msgtype and im6_msgtype fields, with
     the type of the upcall IGMPMSG_* and MRT6MSG_* for IPv4 and IPv6,
     respectively.  The values of the rest of the upcall header fields and the
     body of the upcall message depend on the particular upcall type.

     If the upcall message type is IGMPMSG_NOCACHE or MRT6MSG_NOCACHE, this is
     an indication that a multicast packet has reached the multicast router,
     but the router has no forwarding state for that packet.  Typically, the
     upcall would be a signal for the multicast routing user-level process to
     install the appropriate Multicast Forwarding Cache (MFC) entry in the

     An MFC entry is added by:

           /* IPv4 */
           struct mfcctl mc;
           memset(&mc, 0, sizeof(mc));
           memcpy(&mc.mfcc_origin, &source_addr, sizeof(mc.mfcc_origin));
           memcpy(&mc.mfcc_mcastgrp, &group_addr, sizeof(mc.mfcc_mcastgrp));
           mc.mfcc_parent = iif_index;
           for (i = 0; i < maxvifs; i++)
               mc.mfcc_ttls[i] = oifs_ttl[i];
           setsockopt(mrouter_s4, IPPROTO_IP, MRT_ADD_MFC,
                      (void *)&mc, sizeof(mc));

           /* IPv6 */
           struct mf6cctl mc;
           memset(&mc, 0, sizeof(mc));
           memcpy(&mc.mf6cc_origin, &source_addr, sizeof(mc.mf6cc_origin));
           memcpy(&mc.mf6cc_mcastgrp, &group_addr, sizeof(mf6cc_mcastgrp));
           mc.mf6cc_parent = iif_index;
           for (i = 0; i < maxvifs; i++)
               if (oifs_ttl[i] > 0)
                   IF_SET(i, &mc.mf6cc_ifset);
           setsockopt(mrouter_s4, IPPROTO_IPV6, MRT6_ADD_MFC,
                      (void *)&mc, sizeof(mc));

     The source_addr and group_addr fields are the source and group address of
     the multicast packet (as set in the upcall message).  The iif_index is
     the virtual interface index of the multicast interface the multicast
     packets for this specific source and group address should be received on.
     The oifs_ttl[] array contains the minimum TTL (per interface) a multicast
     packet should have to be forwarded on an outgoing interface.  If the TTL
     value is zero, the corresponding interface is not included in the set of
     outgoing interfaces.  Note that for IPv6 only the set of outgoing
     interfaces can be specified.

     An MFC entry is deleted by:

           /* IPv4 */
           struct mfcctl mc;
           memset(&mc, 0, sizeof(mc));
           memcpy(&mc.mfcc_origin, &source_addr, sizeof(mc.mfcc_origin));
           memcpy(&mc.mfcc_mcastgrp, &group_addr, sizeof(mc.mfcc_mcastgrp));
           setsockopt(mrouter_s4, IPPROTO_IP, MRT_DEL_MFC,
                      (void *)&mc, sizeof(mc));

           /* IPv6 */
           struct mf6cctl mc;
           memset(&mc, 0, sizeof(mc));
           memcpy(&mc.mf6cc_origin, &source_addr, sizeof(mc.mf6cc_origin));
           memcpy(&mc.mf6cc_mcastgrp, &group_addr, sizeof(mf6cc_mcastgrp));
           setsockopt(mrouter_s4, IPPROTO_IPV6, MRT6_DEL_MFC,
                      (void *)&mc, sizeof(mc));

     The following method can be used to get various statistics per installed
     MFC entry in the kernel (e.g., the number of forwarded packets per source
     and group address):

           /* IPv4 */
           struct sioc_sg_req sgreq;
           memset(&sgreq, 0, sizeof(sgreq));
           memcpy(&sgreq.src, &source_addr, sizeof(sgreq.src));
           memcpy(&sgreq.grp, &group_addr, sizeof(sgreq.grp));
           ioctl(mrouter_s4, SIOCGETSGCNT, &sgreq);

           /* IPv6 */
           struct sioc_sg_req6 sgreq;
           memset(&sgreq, 0, sizeof(sgreq));
           memcpy(&sgreq.src, &source_addr, sizeof(sgreq.src));
           memcpy(&sgreq.grp, &group_addr, sizeof(sgreq.grp));
           ioctl(mrouter_s6, SIOCGETSGCNT_IN6, &sgreq);

     The following method can be used to get various statistics per multicast
     virtual interface in the kernel (e.g., the number of forwarded packets
     per interface):

           /* IPv4 */
           struct sioc_vif_req vreq;
           memset(&vreq, 0, sizeof(vreq));
           vreq.vifi = vif_index;
           ioctl(mrouter_s4, SIOCGETVIFCNT, &vreq);

           /* IPv6 */
           struct sioc_mif_req6 mreq;
           memset(&mreq, 0, sizeof(mreq));
           mreq.mifi = vif_index;
           ioctl(mrouter_s6, SIOCGETMIFCNT_IN6, &mreq);

   Advanced Multicast API Programming Guide
     Adding new features to the kernel makes it difficult to preserve backward
     compatibility (binary and API), and at the same time to allow user-level
     processes to take advantage of the new features (if the kernel supports

     One of the mechanisms that allows preserving the backward compatibility
     is a sort of negotiation between the user-level process and the kernel:

     1.   The user-level process tries to enable in the kernel the set of new
          features (and the corresponding API) it would like to use.

     2.   The kernel returns the (sub)set of features it knows about and is
          willing to be enabled.

     3.   The user-level process uses only that set of features the kernel has
          agreed on.

     To support backward compatibility, if the user-level process does not ask
     for any new features, the kernel defaults to the basic multicast API (see
     the Programming Guide section).  Currently, the advanced multicast API
     exists only for IPv4; in the future there will be IPv6 support as well.

     Below is a summary of the expandable API solution.  Note that all new
     options and structures are defined in <netinet/ip_mroute.h> and
     <netinet6/ip6_mroute.h>, unless stated otherwise.

     The user-level process uses new getsockopt()/setsockopt() options to
     perform the API features negotiation with the kernel.  This negotiation
     must be performed right after the multicast routing socket is open.  The
     set of desired/allowed features is stored in a bitset (currently, in
     uint32_t i.e., maximum of 32 new features).  The new
     getsockopt()/setsockopt() options are MRT_API_SUPPORT and MRT_API_CONFIG.
     An example:

        uint32_t v;
        getsockopt(sock, IPPROTO_IP, MRT_API_SUPPORT, (void *)&v, sizeof(v));

     This would set v to the pre-defined bits that the kernel API supports.
     The eight least significant bits in uint32_t are the same as the eight
     possible flags MRT_MFC_FLAGS_* that can be used in mfcc_flags as part of
     the new definition of struct mfcctl (see below about those flags), which
     leaves 24 flags for other new features.  The value returned by
     getsockopt(MRT_API_SUPPORT) is read-only; in other words,
     setsockopt(MRT_API_SUPPORT) would fail.

     To modify the API, and to set some specific feature in the kernel, then:

        uint32_t v = MRT_MFC_FLAGS_DISABLE_WRONGVIF;
        if (setsockopt(sock, IPPROTO_IP, MRT_API_CONFIG, (void *)&v, sizeof(v))
            != 0) {
            return (ERROR);
            return (OK);        /* Success */
            return (ERROR);

     In other words, when setsockopt(MRT_API_CONFIG) is called, the argument
     to it specifies the desired set of features to be enabled in the API and
     the kernel.  The return value in v is the actual (sub)set of features
     that were enabled in the kernel.  To obtain later the same set of
     features that were enabled, use:

           getsockopt(sock, IPPROTO_IP, MRT_API_CONFIG, (void *)&v, sizeof(v));

     The set of enabled features is global.  In other words,
     setsockopt(MRT_API_CONFIG) should be called right after

     Currently, the following set of new features is defined:

     #define MRT_MFC_FLAGS_DISABLE_WRONGVIF (1 << 0)/*disable WRONGVIF signals*/
     #define MRT_MFC_FLAGS_BORDER_VIF   (1 << 1)  /* border vif              */
     #define MRT_MFC_RP                 (1 << 8)  /* enable RP address       */
     #define MRT_MFC_BW_UPCALL          (1 << 9)  /* enable bw upcalls       */

     The advanced multicast API uses a newly defined struct mfcctl2 instead of
     the traditional struct mfcctl.  The original struct mfcctl is kept as is.
     The new struct mfcctl2 is:

      * The new argument structure for MRT_ADD_MFC and MRT_DEL_MFC overlays
      * and extends the old struct mfcctl.
     struct mfcctl2 {
             /* the mfcctl fields */
             struct in_addr  mfcc_origin;       /* ip origin of mcasts       */
             struct in_addr  mfcc_mcastgrp;     /* multicast group associated*/
             vifi_t          mfcc_parent;       /* incoming vif              */
             u_char          mfcc_ttls[MAXVIFS];/* forwarding ttls on vifs   */

             /* extension fields */
             uint8_t         mfcc_flags[MAXVIFS];/* the MRT_MFC_FLAGS_* flags*/
             struct in_addr  mfcc_rp;            /* the RP address           */

     The new fields are mfcc_flags[MAXVIFS] and mfcc_rp.  Note that for
     compatibility reasons they are added at the end.

     The mfcc_flags[MAXVIFS] field is used to set various flags per interface
     per (S,G) entry.  Currently, the defined flags are:

     #define MRT_MFC_FLAGS_DISABLE_WRONGVIF (1 << 0)/*disable WRONGVIF signals*/
     #define MRT_MFC_FLAGS_BORDER_VIF       (1 << 1) /* border vif          */

     The MRT_MFC_FLAGS_DISABLE_WRONGVIF flag is used to explicitly disable the
     IGMPMSG_WRONGVIF kernel signal at the (S,G) granularity if a multicast
     data packet arrives on the wrong interface.  However, it should not be
     delivered for interfaces that are not set in the outgoing interface, and
     that are not expecting to become an incoming interface.  Hence, if the
     MRT_MFC_FLAGS_DISABLE_WRONGVIF flag is set for some of the interfaces,
     then a data packet that arrives on that interface for that MFC entry will
     NOT trigger a WRONGVIF signal.  If that flag is not set, then a signal is
     triggered (the default action).

     Typically, a multicast routing user-level process would need to know the
     forwarding bandwidth for some data flow.

     The original solution for measuring the bandwidth of a dataflow was that
     a user-level process would periodically query the kernel about the number
     of forwarded packets/bytes per (S,G), and then based on those numbers it
     would estimate whether a source has been idle, or whether the source's
     transmission bandwidth is above a threshold.  That solution is far from
     being scalable, hence the need for a new mechanism for bandwidth

     Below is a description of the bandwidth monitoring mechanism.

     o   If the bandwidth of a data flow satisfies some pre-defined filter,
         the kernel delivers an upcall on the multicast routing socket to the
         multicast routing process that has installed that filter.

     o   The bandwidth-upcall filters are installed per (S,G).  There can be
         more than one filter per (S,G).

     o   Instead of supporting all possible comparison operations (i.e., < <=
         == != > >= ), there is support only for the <= and >= operations,
         because this makes the kernel-level implementation simpler, and
         because practically we need only those two.  Furthermore, the missing
         operations can be simulated by secondary user-level filtering of
         those <= and >= filters.  For example, to simulate !=, then we need
         to install filter "bw <= 0xffffffff", and after an upcall is
         received, we need to check whether "measured_bw != expected_bw".

     o   The bandwidth-upcall mechanism is enabled by
         setsockopt(MRT_API_CONFIG) for the MRT_MFC_BW_UPCALL flag.

     o   The bandwidth-upcall filters are added/deleted by the new
         setsockopt(MRT_ADD_BW_UPCALL) and setsockopt(MRT_DEL_BW_UPCALL)
         respectively (with the appropriate struct bw_upcall argument of

     From an application point of view, a developer needs to know about the

      * Structure for installing or delivering an upcall if the
      * measured bandwidth is above or below a threshold.
      * User programs (e.g. daemons) may have a need to know when the
      * bandwidth used by some data flow is above or below some threshold.
      * This interface allows the userland to specify the threshold (in
      * bytes and/or packets) and the measurement interval. Flows are
      * all packet with the same source and destination IP address.
      * At the moment the code is only used for multicast destinations
      * but there is nothing that prevents its use for unicast.
      * The measurement interval cannot be shorter than some Tmin (3s).
      * The threshold is set in packets and/or bytes per_interval.
      * Measurement works as follows:
      * For >= measurements:
      * The first packet marks the start of a measurement interval.
      * During an interval we count packets and bytes, and when we
      * pass the threshold we deliver an upcall and we are done.
      * The first packet after the end of the interval resets the
      * count and restarts the measurement.
      * For <= measurement:
      * We start a timer to fire at the end of the interval, and
      * then for each incoming packet we count packets and bytes.
      * When the timer fires, we compare the value with the threshold,
      * schedule an upcall if we are below, and restart the measurement
      * (reschedule timer and zero counters).

     struct bw_data {
             struct timeval  b_time;
             uint64_t        b_packets;
             uint64_t        b_bytes;

     struct bw_upcall {
             struct in_addr  bu_src;         /* source address            */
             struct in_addr  bu_dst;         /* destination address       */
             uint32_t        bu_flags;       /* misc flags (see below)    */
     #define BW_UPCALL_UNIT_PACKETS (1 << 0) /* threshold (in packets)    */
     #define BW_UPCALL_UNIT_BYTES   (1 << 1) /* threshold (in bytes)      */
     #define BW_UPCALL_GEQ          (1 << 2) /* upcall if bw >= threshold */
     #define BW_UPCALL_LEQ          (1 << 3) /* upcall if bw <= threshold */
     #define BW_UPCALL_DELETE_ALL   (1 << 4) /* delete all upcalls for s,d*/
             struct bw_data  bu_threshold;   /* the bw threshold          */
             struct bw_data  bu_measured;    /* the measured bw           */

     /* max. number of upcalls to deliver together */
     #define BW_UPCALLS_MAX                          128
     /* min. threshold time interval for bandwidth measurement */

     The bw_upcall structure is used as an argument to
     setsockopt(MRT_ADD_BW_UPCALL) and setsockopt(MRT_DEL_BW_UPCALL).  Each
     setsockopt(MRT_ADD_BW_UPCALL) installs a filter in the kernel for the
     source and destination address in the bw_upcall argument, and that filter
     will trigger an upcall according to the following pseudo-algorithm:

      if (bw_upcall_oper IS ">=") {
         if (((bw_upcall_unit & PACKETS == PACKETS) &&
              (measured_packets >= threshold_packets)) ||
             ((bw_upcall_unit & BYTES == BYTES) &&
              (measured_bytes >= threshold_bytes)))
            SEND_UPCALL("measured bandwidth is >= threshold");
       if (bw_upcall_oper IS "<=" && measured_interval >= threshold_interval) {
         if (((bw_upcall_unit & PACKETS == PACKETS) &&
              (measured_packets <= threshold_packets)) ||
             ((bw_upcall_unit & BYTES == BYTES) &&
              (measured_bytes <= threshold_bytes)))
            SEND_UPCALL("measured bandwidth is <= threshold");

     In the same bw_upcall, the unit can be specified in both BYTES and
     PACKETS.  However, the GEQ and LEQ flags are mutually exclusive.

     Basically, an upcall is delivered if the measured bandwidth is >= or <=
     the threshold bandwidth (within the specified measurement interval).  For
     practical reasons, the smallest value for the measurement interval is 3
     seconds.  If smaller values are allowed, then the bandwidth estimation
     may be less accurate, or the potentially very high frequency of the
     generated upcalls may introduce too much overhead.  For the >= operation,
     the answer may be known before the end of threshold_interval, therefore
     the upcall may be delivered earlier.  For the <= operation however, we
     must wait until the threshold interval has expired to know the answer.


           struct bw_upcall bw_upcall;
           /* Assign all bw_upcall fields as appropriate */
           memset(&bw_upcall, 0, sizeof(bw_upcall));
           memcpy(&bw_upcall.bu_src, &source, sizeof(bw_upcall.bu_src));
           memcpy(&bw_upcall.bu_dst, &group, sizeof(bw_upcall.bu_dst));
           bw_upcall.bu_threshold.b_data = threshold_interval;
           bw_upcall.bu_threshold.b_packets = threshold_packets;
           bw_upcall.bu_threshold.b_bytes = threshold_bytes;
           if (is_threshold_in_packets)
               bw_upcall.bu_flags |= BW_UPCALL_UNIT_PACKETS;
           if (is_threshold_in_bytes)
               bw_upcall.bu_flags |= BW_UPCALL_UNIT_BYTES;
           do {
               if (is_geq_upcall) {
                   bw_upcall.bu_flags |= BW_UPCALL_GEQ;
               if (is_leq_upcall) {
                   bw_upcall.bu_flags |= BW_UPCALL_LEQ;
               return (ERROR);
           } while (0);
           setsockopt(mrouter_s4, IPPROTO_IP, MRT_ADD_BW_UPCALL,
                     (void *)&bw_upcall, sizeof(bw_upcall));

     To delete a single filter, use MRT_DEL_BW_UPCALL, and the fields of
     bw_upcall must be set to exactly same as when MRT_ADD_BW_UPCALL was

     To delete all bandwidth filters for a given (S,G), then only the bu_src
     and bu_dst fields in struct bw_upcall need to be set, and then just set
     only the BW_UPCALL_DELETE_ALL flag inside field bw_upcall.bu_flags.

     The bandwidth upcalls are received by aggregating them in the new upcall

           #define IGMPMSG_BW_UPCALL  4  /* BW monitoring upcall */

     This message is an array of struct bw_upcall elements (up to
     BW_UPCALLS_MAX = 128).  The upcalls are delivered when there are 128
     pending upcalls, or when 1 second has expired since the previous upcall
     (whichever comes first).  In an struct upcall element, the bu_measured
     field is filled in to indicate the particular measured values.  However,
     because of the way the particular intervals are measured, the user should
     be careful how bu_measured.b_time is used.  For example, if the filter is
     installed to trigger an upcall if the number of packets is >= 1, then
     bu_measured may have a value of zero in the upcalls after the first one,
     because the measured interval for >= filters is "clocked" by the
     forwarded packets.  Hence, this upcall mechanism should not be used for
     measuring the exact value of the bandwidth of the forwarded data.  To
     measure the exact bandwidth, the user would need to get the forwarded
     packets statistics with the ioctl(SIOCGETSGCNT) mechanism (see the
     Programming Guide section) .

     Note that the upcalls for a filter are delivered until the specific
     filter is deleted, but no more frequently than once per
     bu_threshold.b_time.  For example, if the filter is specified to deliver
     a signal if bw >= 1 packet, the first packet will trigger a signal, but
     the next upcall will be triggered no earlier than bu_threshold.b_time
     after the previous upcall.


     getsockopt(2), recvfrom(2), recvmsg(2), setsockopt(2), socket(2),
     icmp6(4), inet(4), inet6(4), intro(4), ip(4), ip6(4), mrouted(8),


     The original multicast code was written by David Waitzman (BBN Labs), and
     later modified by the following individuals: Steve Deering (Stanford),
     Mark J. Steiglitz (Stanford), Van Jacobson (LBL), Ajit Thyagarajan
     (PARC), Bill Fenner (PARC).

     The IPv6 multicast support was implemented by the KAME project
     (, and was based on the IPv4 multicast code.  The
     advanced multicast API and the multicast bandwidth monitoring were
     implemented by Pavlin Radoslavov (ICSI) in collaboration with Chris Brown

     This manual page was written by Pavlin Radoslavov (ICSI).

OpenBSD 6.4                      March 7, 2018                     OpenBSD 6.4

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