Provided by: freebsd-manpages_12.2-1_all bug


     netmap — a framework for fast packet I/O


     device netmap


     netmap is a framework for extremely fast and efficient packet I/O for userspace and kernel
     clients, and for Virtual Machines.  It runs on FreeBSD Linux and some versions of Windows,
     and supports a variety of netmap ports, including

     physical NIC ports
           to access individual queues of network interfaces;

     host ports
           to inject packets into the host stack;

     VALE ports
           implementing a very fast and modular in-kernel software switch/dataplane;

     netmap pipes
           a shared memory packet transport channel;

     netmap monitors
           a mechanism similar to bpf(4) to capture traffic

     All these netmap ports are accessed interchangeably with the same API, and are at least one
     order of magnitude faster than standard OS mechanisms (sockets, bpf, tun/tap interfaces,
     native switches, pipes).  With suitably fast hardware (NICs, PCIe buses, CPUs), packet I/O
     using netmap on supported NICs reaches 14.88 million packets per second (Mpps) with much
     less than one core on 10 Gbit/s NICs; 35-40 Mpps on 40 Gbit/s NICs (limited by the
     hardware); about 20 Mpps per core for VALE ports; and over 100 Mpps for netmap pipes.  NICs
     without native netmap support can still use the API in emulated mode, which uses unmodified
     device drivers and is 3-5 times faster than bpf(4) or raw sockets.

     Userspace clients can dynamically switch NICs into netmap mode and send and receive raw
     packets through memory mapped buffers.  Similarly, VALE switch instances and ports, netmap
     pipes and netmap monitors can be created dynamically, providing high speed packet I/O
     between processes, virtual machines, NICs and the host stack.

     netmap supports both non-blocking I/O through ioctl(2), synchronization and blocking I/O
     through a file descriptor and standard OS mechanisms such as select(2), poll(2), kqueue(2)
     and epoll(7).  All types of netmap ports and the VALE switch are implemented by a single
     kernel module, which also emulates the netmap API over standard drivers.  For best
     performance, netmap requires native support in device drivers.  A list of such devices is at
     the end of this document.

     In the rest of this (long) manual page we document various aspects of the netmap and VALE
     architecture, features and usage.


     netmap supports raw packet I/O through a port, which can be connected to a physical
     interface (NIC), to the host stack, or to a VALE switch.  Ports use preallocated circular
     queues of buffers (rings) residing in an mmapped region.  There is one ring for each
     transmit/receive queue of a NIC or virtual port.  An additional ring pair connects to the
     host stack.

     After binding a file descriptor to a port, a netmap client can send or receive packets in
     batches through the rings, and possibly implement zero-copy forwarding between ports.

     All NICs operating in netmap mode use the same memory region, accessible to all processes
     who own /dev/netmap file descriptors bound to NICs.  Independent VALE and netmap pipe ports
     by default use separate memory regions, but can be independently configured to share memory.


     The following section describes the system calls to create and control netmap ports
     (including VALE and netmap pipe ports).  Simpler, higher level functions are described in
     the LIBRARIES section.

     Ports and rings are created and controlled through a file descriptor, created by opening a
     special device
           fd = open("/dev/netmap");
     and then bound to a specific port with an
           ioctl(fd, NIOCREGIF, (struct nmreq *)arg);

     netmap has multiple modes of operation controlled by the struct nmreq argument.  arg.nr_name
     specifies the netmap port name, as follows:

     OS network interface name (e.g., 'em0', 'eth1', ...)
           the data path of the NIC is disconnected from the host stack, and the file descriptor
           is bound to the NIC (one or all queues), or to the host stack;

           the file descriptor is bound to port PPP of VALE switch SSS.  Switch instances and
           ports are dynamically created if necessary.

           Both SSS and PPP have the form [0-9a-zA-Z_]+ , the string cannot exceed IFNAMSIZ
           characters, and PPP cannot be the name of any existing OS network interface.

     On return, arg indicates the size of the shared memory region, and the number, size and
     location of all the netmap data structures, which can be accessed by mmapping the memory
           char *mem = mmap(0, arg.nr_memsize, fd);

     Non-blocking I/O is done with special ioctl(2) select(2) and poll(2) on the file descriptor
     permit blocking I/O.

     While a NIC is in netmap mode, the OS will still believe the interface is up and running.
     OS-generated packets for that NIC end up into a netmap ring, and another ring is used to
     send packets into the OS network stack.  A close(2) on the file descriptor removes the
     binding, and returns the NIC to normal mode (reconnecting the data path to the host stack),
     or destroys the virtual port.


     The data structures in the mmapped memory region are detailed in <sys/net/netmap.h>, which
     is the ultimate reference for the netmap API.  The main structures and fields are indicated

     struct netmap_if (one per interface)

          struct netmap_if {
              const uint32_t   ni_flags;      /* properties              */
              const uint32_t   ni_tx_rings;   /* NIC tx rings            */
              const uint32_t   ni_rx_rings;   /* NIC rx rings            */
              uint32_t         ni_bufs_head;  /* head of extra bufs list */

          Indicates the number of available rings (struct netmap_rings) and their position in the
          mmapped region.  The number of tx and rx rings (ni_tx_rings, ni_rx_rings) normally
          depends on the hardware.  NICs also have an extra tx/rx ring pair connected to the host
          stack.  NIOCREGIF can also request additional unbound buffers in the same memory space,
          to be used as temporary storage for packets.  The number of extra buffers is specified
          in the arg.nr_arg3 field.  On success, the kernel writes back to arg.nr_arg3 the number
          of extra buffers actually allocated (they may be less than the amount requested if the
          memory space ran out of buffers).  ni_bufs_head contains the index of the first of
          these extra buffers, which are connected in a list (the first uint32_t of each buffer
          being the index of the next buffer in the list).  A 0 indicates the end of the list.
          The application is free to modify this list and use the buffers (i.e., binding them to
          the slots of a netmap ring).  When closing the netmap file descriptor, the kernel frees
          the buffers contained in the list pointed by ni_bufs_head , irrespectively of the
          buffers originally provided by the kernel on NIOCREGIF.

     struct netmap_ring (one per ring)

          struct netmap_ring {
              const uint32_t num_slots;   /* slots in each ring            */
              const uint32_t nr_buf_size; /* size of each buffer           */
              uint32_t       head;        /* (u) first buf owned by user   */
              uint32_t       cur;         /* (u) wakeup position           */
              const uint32_t tail;        /* (k) first buf owned by kernel */
              uint32_t       flags;
              struct timeval ts;          /* (k) time of last rxsync()     */
              struct netmap_slot slot[0]; /* array of slots                */

          Implements transmit and receive rings, with read/write pointers, metadata and an array
          of slots describing the buffers.

     struct netmap_slot (one per buffer)

          struct netmap_slot {
              uint32_t buf_idx;           /* buffer index                 */
              uint16_t len;               /* packet length                */
              uint16_t flags;             /* buf changed, etc.            */
              uint64_t ptr;               /* address for indirect buffers */

          Describes a packet buffer, which normally is identified by an index and resides in the
          mmapped region.

     packet buffers
          Fixed size (normally 2 KB) packet buffers allocated by the kernel.

     The offset of the struct netmap_if in the mmapped region is indicated by the nr_offset field
     in the structure returned by NIOCREGIF.  From there, all other objects are reachable through
     relative references (offsets or indexes).  Macros and functions in <net/netmap_user.h> help
     converting them into actual pointers:

           struct netmap_if *nifp = NETMAP_IF(mem, arg.nr_offset);
           struct netmap_ring *txr = NETMAP_TXRING(nifp, ring_index);
           struct netmap_ring *rxr = NETMAP_RXRING(nifp, ring_index);

           char *buf = NETMAP_BUF(ring, buffer_index);


     Rings are circular queues of packets with three indexes/pointers (head, cur, tail); one slot
     is always kept empty.  The ring size (num_slots) should not be assumed to be a power of two.

     head is the first slot available to userspace;

     cur is the wakeup point: select/poll will unblock when tail passes cur;

     tail is the first slot reserved to the kernel.

     Slot indexes must only move forward; for convenience, the function
           nm_ring_next(ring, index)
     returns the next index modulo the ring size.

     head and cur are only modified by the user program; tail is only modified by the kernel.
     The kernel only reads/writes the struct netmap_ring slots and buffers during the execution
     of a netmap-related system call.  The only exception are slots (and buffers) in the range
     tail ... head-1, that are explicitly assigned to the kernel.

     On transmit rings, after a netmap system call, slots in the range head ... tail-1 are
     available for transmission.  User code should fill the slots sequentially and advance head
     and cur past slots ready to transmit.  cur may be moved further ahead if the user code needs
     more slots before further transmissions (see SCATTER GATHER I/O).

     At the next NIOCTXSYNC/select()/poll(), slots up to head-1 are pushed to the port, and tail
     may advance if further slots have become available.  Below is an example of the evolution of
     a TX ring:

         after the syscall, slots between cur and tail are (a)vailable
                   head=cur   tail
                    |          |
                    v          v
          TX  [.....aaaaaaaaaaa.............]

         user creates new packets to (T)ransmit
                     head=cur tail
                         |     |
                         v     v
          TX  [.....TTTTTaaaaaa.............]

         NIOCTXSYNC/poll()/select() sends packets and reports new slots
                     head=cur      tail
                         |          |
                         v          v
          TX  [..........aaaaaaaaaaa........]

     select() and poll() will block if there is no space in the ring, i.e.,
           ring->cur == ring->tail
     and return when new slots have become available.

     High speed applications may want to amortize the cost of system calls by preparing as many
     packets as possible before issuing them.

     A transmit ring with pending transmissions has
           ring->head != ring->tail + 1 (modulo the ring size).
     The function int nm_tx_pending(ring) implements this test.

     On receive rings, after a netmap system call, the slots in the range head... tail-1 contain
     received packets.  User code should process them and advance head and cur past slots it
     wants to return to the kernel.  cur may be moved further ahead if the user code wants to
     wait for more packets without returning all the previous slots to the kernel.

     At the next NIOCRXSYNC/select()/poll(), slots up to head-1 are returned to the kernel for
     further receives, and tail may advance to report new incoming packets.

     Below is an example of the evolution of an RX ring:

         after the syscall, there are some (h)eld and some (R)eceived slots
                head  cur     tail
                 |     |       |
                 v     v       v
          RX  [..hhhhhhRRRRRRRR..........]

         user advances head and cur, releasing some slots and holding others
                    head cur  tail
                      |  |     |
                      v  v     v
          RX  [..*****hhhRRRRRR...........]

         NICRXSYNC/poll()/select() recovers slots and reports new packets
                    head cur        tail
                      |  |           |
                      v  v           v
          RX  [.......hhhRRRRRRRRRRRR....]


     Normally, packets should be stored in the netmap-allocated buffers assigned to slots when
     ports are bound to a file descriptor.  One packet is fully contained in a single buffer.

     The following flags affect slot and buffer processing:

          must be used when the buf_idx in the slot is changed.  This can be used to implement
          zero-copy forwarding, see ZERO-COPY FORWARDING.

          reports when this buffer has been transmitted.  Normally, netmap notifies transmit
          completions in batches, hence signals can be delayed indefinitely.  This flag helps
          detect when packets have been sent and a file descriptor can be closed.

          When a ring is in 'transparent' mode, packets marked with this flag by the user
          application are forwarded to the other endpoint at the next system call, thus restoring
          (in a selective way) the connection between a NIC and the host stack.

          tells the forwarding code that the source MAC address for this packet must not be used
          in the learning bridge code.

          indicates that the packet's payload is in a user-supplied buffer whose user virtual
          address is in the 'ptr' field of the slot.  The size can reach 65535 bytes.

          This is only supported on the transmit ring of VALE ports, and it helps reducing data
          copies in the interconnection of virtual machines.

          indicates that the packet continues with subsequent buffers; the last buffer in a
          packet must have the flag clear.


     Packets can span multiple slots if the NS_MOREFRAG flag is set in all but the last slot.
     The maximum length of a chain is 64 buffers.  This is normally used with VALE ports when
     connecting virtual machines, as they generate large TSO segments that are not split unless
     they reach a physical device.

     NOTE: The length field always refers to the individual fragment; there is no place with the
     total length of a packet.

     On receive rings the macro NS_RFRAGS(slot) indicates the remaining number of slots for this
     packet, including the current one.  Slots with a value greater than 1 also have NS_MOREFRAG


     netmap uses two ioctls (NIOCTXSYNC, NIOCRXSYNC) for non-blocking I/O.  They take no
     argument.  Two more ioctls (NIOCGINFO, NIOCREGIF) are used to query and configure ports,
     with the following argument:

     struct nmreq {
         char      nr_name[IFNAMSIZ]; /* (i) port name                  */
         uint32_t  nr_version;        /* (i) API version                */
         uint32_t  nr_offset;         /* (o) nifp offset in mmap region */
         uint32_t  nr_memsize;        /* (o) size of the mmap region    */
         uint32_t  nr_tx_slots;       /* (i/o) slots in tx rings        */
         uint32_t  nr_rx_slots;       /* (i/o) slots in rx rings        */
         uint16_t  nr_tx_rings;       /* (i/o) number of tx rings       */
         uint16_t  nr_rx_rings;       /* (i/o) number of rx rings       */
         uint16_t  nr_ringid;         /* (i/o) ring(s) we care about    */
         uint16_t  nr_cmd;            /* (i) special command            */
         uint16_t  nr_arg1;           /* (i/o) extra arguments          */
         uint16_t  nr_arg2;           /* (i/o) extra arguments          */
         uint32_t  nr_arg3;           /* (i/o) extra arguments          */
         uint32_t  nr_flags           /* (i/o) open mode                */

     A file descriptor obtained through /dev/netmap also supports the ioctl supported by network
     devices, see netintro(4).

           returns EINVAL if the named port does not support netmap.  Otherwise, it returns 0 and
           (advisory) information about the port.  Note that all the information below can change
           before the interface is actually put in netmap mode.

               indicates the size of the netmap memory region.  NICs in netmap mode all share the
               same memory region, whereas VALE ports have independent regions for each port.

           nr_tx_slots, nr_rx_slots
               indicate the size of transmit and receive rings.

           nr_tx_rings, nr_rx_rings
               indicate the number of transmit and receive rings.  Both ring number and sizes may
               be configured at runtime using interface-specific functions (e.g., ethtool(8) ).

           binds the port named in nr_name to the file descriptor.  For a physical device this
           also switches it into netmap mode, disconnecting it from the host stack.  Multiple
           file descriptors can be bound to the same port, with proper synchronization left to
           the user.

           The recommended way to bind a file descriptor to a port is to use function nm_open(..)
           (see LIBRARIES) which parses names to access specific port types and enable features.
           In the following we document the main features.

           NIOCREGIF can also bind a file descriptor to one endpoint of a netmap pipe, consisting
           of two netmap ports with a crossover connection.  A netmap pipe share the same memory
           space of the parent port, and is meant to enable configuration where a master process
           acts as a dispatcher towards slave processes.

           To enable this function, the nr_arg1 field of the structure can be used as a hint to
           the kernel to indicate how many pipes we expect to use, and reserve extra space in the
           memory region.

           On return, it gives the same info as NIOCGINFO, with nr_ringid and nr_flags indicating
           the identity of the rings controlled through the file descriptor.

           nr_flags nr_ringid selects which rings are controlled through this file descriptor.
           Possible values of nr_flags are indicated below, together with the naming schemes that
           application libraries (such as the nm_open indicated below) can use to indicate the
           specific set of rings.  In the example below, "netmap:foo" is any valid netmap port

           NR_REG_ALL_NIC netmap:foo
                  (default) all hardware ring pairs

           NR_REG_SW netmap:foo^
                  the ``host rings'', connecting to the host stack.

           NR_REG_NIC_SW netmap:foo+
                  all hardware rings and the host rings

           NR_REG_ONE_NIC netmap:foo-i
                  only the i-th hardware ring pair, where the number is in nr_ringid;

           NR_REG_PIPE_MASTER netmap:foo{i
                  the master side of the netmap pipe whose identifier (i) is in nr_ringid;

           NR_REG_PIPE_SLAVE netmap:foo}i
                  the slave side of the netmap pipe whose identifier (i) is in nr_ringid.

                  The identifier of a pipe must be thought as part of the pipe name, and does not
                  need to be sequential.  On return the pipe will only have a single ring pair
                  with index 0, irrespective of the value of i.

           By default, a poll(2) or select(2) call pushes out any pending packets on the transmit
           ring, even if no write events are specified.  The feature can be disabled by or-ing
           NETMAP_NO_TX_POLL to the value written to nr_ringid.  When this feature is used,
           packets are transmitted only on ioctl(NIOCTXSYNC) or select() / poll() are called with
           a write event (POLLOUT/wfdset) or a full ring.

           When registering a virtual interface that is dynamically created to a VALE switch, we
           can specify the desired number of rings (1 by default, and currently up to 16) on it
           using nr_tx_rings and nr_rx_rings fields.

           tells the hardware of new packets to transmit, and updates the number of slots
           available for transmission.

           tells the hardware of consumed packets, and asks for newly available packets.


     select(2) and poll(2) on a netmap file descriptor process rings as indicated in TRANSMIT
     RINGS and RECEIVE RINGS, respectively when write (POLLOUT) and read (POLLIN) events are
     requested.  Both block if no slots are available in the ring (ring->cur == ring->tail).
     Depending on the platform, epoll(7) and kqueue(2) are supported too.

     Packets in transmit rings are normally pushed out (and buffers reclaimed) even without
     requesting write events.  Passing the NETMAP_NO_TX_POLL flag to NIOCREGIF disables this
     feature.  By default, receive rings are processed only if read events are requested.
     Passing the NETMAP_DO_RX_POLL flag to NIOCREGIF updates receive rings even without read
     events. Note that on epoll(7) and kqueue(2), NETMAP_NO_TX_POLL and NETMAP_DO_RX_POLL only
     have an effect when some event is posted for the file descriptor.


     The netmap API is supposed to be used directly, both because of its simplicity and for
     efficient integration with applications.

     For convenience, the <net/netmap_user.h> header provides a few macros and functions to ease
     creating a file descriptor and doing I/O with a netmap port.  These are loosely modeled
     after the pcap(3) API, to ease porting of libpcap-based applications to netmap.  To use
     these extra functions, programs should
           #define NETMAP_WITH_LIBS
           #include <net/netmap_user.h>

     The following functions are available:

     struct nm_desc * nm_open(const char *ifname, const struct nmreq *req, uint64_t flags, const
            struct nm_desc *arg)
            similar to pcap_open_live(3), binds a file descriptor to a port.

                is a port name, in the form "netmap:PPP" for a NIC and "valeSSS:PPP" for a VALE

                provides the initial values for the argument to the NIOCREGIF ioctl.  The
                nm_flags and nm_ringid values are overwritten by parsing ifname and flags, and
                other fields can be overridden through the other two arguments.

                points to a struct nm_desc containing arguments (e.g., from a previously open
                file descriptor) that should override the defaults.  The fields are used as
                described below

                can be set to a combination of the following flags: NETMAP_NO_TX_POLL,
                NETMAP_DO_RX_POLL (copied into nr_ringid); NM_OPEN_NO_MMAP (if arg points to the
                same memory region, avoids the mmap and uses the values from it); NM_OPEN_IFNAME
                (ignores ifname and uses the values in arg); NM_OPEN_ARG1, NM_OPEN_ARG2,
                NM_OPEN_ARG3 (uses the fields from arg); NM_OPEN_RING_CFG (uses the ring number
                and sizes from arg).

     int nm_close(struct nm_desc *d)
            closes the file descriptor, unmaps memory, frees resources.

     int nm_inject(struct nm_desc *d, const void *buf, size_t size)
            similar to pcap_inject(), pushes a packet to a ring, returns the size of the packet
            is successful, or 0 on error;

     int nm_dispatch(struct nm_desc *d, int cnt, nm_cb_t cb, u_char *arg)
            similar to pcap_dispatch(), applies a callback to incoming packets

     u_char * nm_nextpkt(struct nm_desc *d, struct nm_pkthdr *hdr)
            similar to pcap_next(), fetches the next packet


     netmap natively supports the following devices:

     On FreeBSD: cxgbe(4), em(4), iflib(4) (providing igb, em and lem), ixgbe(4), ixl(4), re(4),

     On Linux e1000, e1000e, i40e, igb, ixgbe, ixgbevf, r8169, virtio_net, vmxnet3.

     NICs without native support can still be used in netmap mode through emulation.  Performance
     is inferior to native netmap mode but still significantly higher than various raw socket
     types (bpf, PF_PACKET, etc.).  Note that for slow devices (such as 1 Gbit/s and slower NICs,
     or several 10 Gbit/s NICs whose hardware is unable to sustain line rate), emulated and
     native mode will likely have similar or same throughput.

     When emulation is in use, packet sniffer programs such as tcpdump could see received packets
     before they are diverted by netmap.  This behaviour is not intentional, being just an
     artifact of the implementation of emulation.  Note that in case the netmap application
     subsequently moves packets received from the emulated adapter onto the host RX ring, the
     sniffer will intercept those packets again, since the packets are injected to the host stack
     as they were received by the network interface.

     Emulation is also available for devices with native netmap support, which can be used for
     testing or performance comparison.  The sysctl variable dev.netmap.admode globally controls
     how netmap mode is implemented.


     Some aspects of the operation of netmap and VALE are controlled through sysctl variables on
     FreeBSD (dev.netmap.*) and module parameters on Linux (/sys/module/netmap/parameters/*):

     dev.netmap.admode: 0
             Controls the use of native or emulated adapter mode.

             0 uses the best available option;

             1 forces native mode and fails if not available;

             2 forces emulated hence never fails.

     dev.netmap.generic_rings: 1
             Number of rings used for emulated netmap mode

     dev.netmap.generic_ringsize: 1024
             Ring size used for emulated netmap mode

     dev.netmap.generic_mit: 100000
             Controls interrupt moderation for emulated mode

     dev.netmap.fwd: 0
             Forces NS_FORWARD mode

     dev.netmap.txsync_retry: 2
             Number of txsync loops in the VALE flush function

     dev.netmap.no_pendintr: 1
             Forces recovery of transmit buffers on system calls

     dev.netmap.no_timestamp: 0
             Disables the update of the timestamp in the netmap ring

     dev.netmap.verbose: 0
             Verbose kernel messages

     dev.netmap.buf_num: 163840

     dev.netmap.buf_size: 2048

     dev.netmap.ring_num: 200

     dev.netmap.ring_size: 36864

     dev.netmap.if_num: 100

     dev.netmap.if_size: 1024
             Sizes and number of objects (netmap_if, netmap_ring, buffers) for the global memory
             region.  The only parameter worth modifying is dev.netmap.buf_num as it impacts the
             total amount of memory used by netmap.

     dev.netmap.buf_curr_num: 0

     dev.netmap.buf_curr_size: 0

     dev.netmap.ring_curr_num: 0

     dev.netmap.ring_curr_size: 0

     dev.netmap.if_curr_num: 0

     dev.netmap.if_curr_size: 0
             Actual values in use.

     dev.netmap.priv_buf_num: 4098

     dev.netmap.priv_buf_size: 2048

     dev.netmap.priv_ring_num: 4

     dev.netmap.priv_ring_size: 20480

     dev.netmap.priv_if_num: 2

     dev.netmap.priv_if_size: 1024
             Sizes and number of objects (netmap_if, netmap_ring, buffers) for private memory
             regions.  A separate memory region is used for each VALE port and each pair of
             netmap pipes.

     dev.netmap.bridge_batch: 1024
             Batch size used when moving packets across a VALE switch.  Values above 64 generally
             guarantee good performance.

     dev.netmap.ptnet_vnet_hdr: 1
             Allow ptnet devices to use virtio-net headers


     netmap uses select(2), poll(2), epoll(7) and kqueue(2) to wake up processes when significant
     events occur, and mmap(2) to map memory.  ioctl(2) is used to configure ports and VALE

     Applications may need to create threads and bind them to specific cores to improve
     performance, using standard OS primitives, see pthread(3).  In particular,
     pthread_setaffinity_np(3) may be of use.


     netmap comes with a few programs that can be used for testing or simple applications.  See
     the examples/ directory in netmap distributions, or tools/tools/netmap/ directory in FreeBSD

     pkt-gen(8) is a general purpose traffic source/sink.

     As an example
           pkt-gen -i ix0 -f tx -l 60
     can generate an infinite stream of minimum size packets, and
           pkt-gen -i ix0 -f rx
     is a traffic sink.  Both print traffic statistics, to help monitor how the system performs.

     pkt-gen(8) has many options can be uses to set packet sizes, addresses, rates, and use
     multiple send/receive threads and cores.

     bridge(4) is another test program which interconnects two netmap ports.  It can be used for
     transparent forwarding between interfaces, as in
           bridge -i netmap:ix0 -i netmap:ix1
     or even connect the NIC to the host stack using netmap
           bridge -i netmap:ix0

     The following code implements a traffic generator

     #include <net/netmap_user.h>
     void sender(void)
         struct netmap_if *nifp;
         struct netmap_ring *ring;
         struct nmreq nmr;
         struct pollfd fds;

         fd = open("/dev/netmap", O_RDWR);
         bzero(&nmr, sizeof(nmr));
         strcpy(nmr.nr_name, "ix0");
         nmr.nm_version = NETMAP_API;
         ioctl(fd, NIOCREGIF, &nmr);
         p = mmap(0, nmr.nr_memsize, fd);
         nifp = NETMAP_IF(p, nmr.nr_offset);
         ring = NETMAP_TXRING(nifp, 0);
         fds.fd = fd; = POLLOUT;
         for (;;) {
             poll(&fds, 1, -1);
             while (!nm_ring_empty(ring)) {
                 i = ring->cur;
                 buf = NETMAP_BUF(ring, ring->slot[i].buf_index);
                 ... prepare packet in buf ...
                 ring->slot[i].len = ... packet length ...
                 ring->head = ring->cur = nm_ring_next(ring, i);

     A simple receiver can be implemented using the helper functions
     #define NETMAP_WITH_LIBS
     #include <net/netmap_user.h>
     void receiver(void)
         struct nm_desc *d;
         struct pollfd fds;
         u_char *buf;
         struct nm_pkthdr h;
         d = nm_open("netmap:ix0", NULL, 0, 0);
         fds.fd = NETMAP_FD(d); = POLLIN;
         for (;;) {
             poll(&fds, 1, -1);
             while ( (buf = nm_nextpkt(d, &h)) )
                 consume_pkt(buf, h->len);

     Since physical interfaces share the same memory region, it is possible to do packet
     forwarding between ports swapping buffers.  The buffer from the transmit ring is used to
     replenish the receive ring:
         uint32_t tmp;
         struct netmap_slot *src, *dst;
         src = &src_ring->slot[rxr->cur];
         dst = &dst_ring->slot[txr->cur];
         tmp = dst->buf_idx;
         dst->buf_idx = src->buf_idx;
         dst->len = src->len;
         dst->flags = NS_BUF_CHANGED;
         src->buf_idx = tmp;
         src->flags = NS_BUF_CHANGED;
         rxr->head = rxr->cur = nm_ring_next(rxr, rxr->cur);
         txr->head = txr->cur = nm_ring_next(txr, txr->cur);

     The host stack is for all practical purposes just a regular ring pair, which you can access
     with the netmap API (e.g., with
           nm_open("netmap:eth0^", ...);
     All packets that the host would send to an interface in netmap mode end up into the RX ring,
     whereas all packets queued to the TX ring are send up to the host stack.

     A simple way to test the performance of a VALE switch is to attach a sender and a receiver
     to it, e.g., running the following in two different terminals:
           pkt-gen -i vale1:a -f rx # receiver
           pkt-gen -i vale1:b -f tx # sender
     The same example can be used to test netmap pipes, by simply changing port names, e.g.,
           pkt-gen -i vale2:x{3 -f rx # receiver on the master side
           pkt-gen -i vale2:x}3 -f tx # sender on the slave side

     The following command attaches an interface and the host stack to a switch:
           valectl -h vale2:em0
     Other netmap clients attached to the same switch can now communicate with the network card
     or the host.


     vale(4), valectl(8), bridge(8), lb(8), nmreplay(8), pkt-gen(8)

     Luigi Rizzo, Revisiting network I/O APIs: the netmap framework, Communications of the ACM,
     55 (3), pp.45-51, March 2012

     Luigi Rizzo, netmap: a novel framework for fast packet I/O, Usenix ATC'12, June 2012, Boston

     Luigi Rizzo, Giuseppe Lettieri, VALE, a switched ethernet for virtual machines, ACM
     CoNEXT'12, December 2012, Nice

     Luigi Rizzo, Giuseppe Lettieri, Vincenzo Maffione, Speeding up packet I/O in virtual
     machines, ACM/IEEE ANCS'13, October 2013, San Jose


     The netmap framework has been originally designed and implemented at the Universita` di Pisa
     in 2011 by Luigi Rizzo, and further extended with help from Matteo Landi, Gaetano Catalli,
     Giuseppe Lettieri, and Vincenzo Maffione.

     netmap and VALE have been funded by the European Commission within FP7 Projects CHANGE
     (257422) and OPENLAB (287581).


     No matter how fast the CPU and OS are, achieving line rate on 10G and faster interfaces
     requires hardware with sufficient performance.  Several NICs are unable to sustain line rate
     with small packet sizes.  Insufficient PCIe or memory bandwidth can also cause reduced

     Another frequent reason for low performance is the use of flow control on the link: a slow
     receiver can limit the transmit speed.  Be sure to disable flow control when running high
     speed experiments.

     netmap is orthogonal to some NIC features such as multiqueue, schedulers, packet filters.

     Multiple transmit and receive rings are supported natively and can be configured with
     ordinary OS tools, such as ethtool(8) or device-specific sysctl variables.  The same goes
     for Receive Packet Steering (RPS) and filtering of incoming traffic.

     netmap does not use features such as checksum offloading, TCP segmentation offloading,
     encryption, VLAN encapsulation/decapsulation, etc.  When using netmap to exchange packets
     with the host stack, make sure to disable these features.