NETMAP(4)

NAME
netmap - a framework for fast packet I/O VALE - a fast VirtuAl Local Ethernet using the netmap API

netmap pipes - a shared memory packet transport channel

SYNOPSIS
device netmap

DESCRIPTION
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), epoll(2), and kqueue(2). 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.

ARCHITECTURE
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.

ENTERING AND EXITING NETMAP MODE
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;

valeSSS:PPP
    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. epoll(2) and kqueue(2) are not supported on netmap file descriptors.

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.

DATA STRUCTURES
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 below:

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. ni_bufs_head contains the index of the first of these free rings, 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.

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, BUFFERS AND DATA I/O
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.

  TRANSMIT RINGS
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.

  RECEIVE RINGS
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....]

SLOTS AND PACKET BUFFERS
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:

NS_BUF_CHANGED
    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.

NS_REPORT
    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.

NS_FORWARD
    When a ring is in 'transparent' mode (see TRANSPARENT MODE), packets marked with this flag 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.

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

NS_INDIRECT
    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.

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

SCATTER GATHER I/O
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 set.

IOCTLS
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).

NIOCGINFO
    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.

    nr_memsize
        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) ).

NIOCREGIF
    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 name.

    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(4) 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.

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

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

SELECT, POLL, EPOLL, KQUEUE.
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(2) 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 and kqueue, NETMAP_NO_TX_POLL and NETMAP_DO_RX_POLL only have an effect when some event is posted for the file descriptor.

LIBRARIES
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
before
    #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(3pcap), binds a file descriptor to a port.

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

    req 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.

    arg 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

    flags
        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

SUPPORTED DEVICES
netmap natively supports the following devices:

On FreeBSD: cxgbe(4), em(4), igb(4), ixgbe(4), ixl(4), lem(4), re(4).

On Linux e1000(4), e1000e(4), i40e(4), igb(4), ixgbe(4), r8169(4).

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.

SYSCTL VARIABLES AND MODULE PARAMETERS
Some aspect of the operation of netmap are controlled through sysctl variables on FreeBSD (dev.netmap.*) and module parameters on Linux (/sys/module/netmap_lin/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_ringsize: 1024
    Ring size used for emulated netmap mode

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

dev.netmap.mmap_unreg: 0

dev.netmap.fwd: 0
    Forces NS_FORWARD mode

dev.netmap.flags: 0

dev.netmap.txsync_retry: 2

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

dev.netmap.mitigate: 1
    Propagates interrupt mitigation to user processes

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

SYSTEM CALLS
netmap uses select(2), poll(2), epoll(2) 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 switches.

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.

EXAMPLES

  TEST PROGRAMS
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 distributions.

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 ix0 -i ix1
or even connect the NIC to the host stack using netmap
    bridge -i ix0 -i ix0

  USING THE NATIVE API
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;
        fds.events = 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);
    	}
        }
    }

  HELPER FUNCTIONS
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);
        fds.events = POLLIN;
        for (;;) {
    	poll(&fds, 1, -1);
            while ( (buf = nm_nextpkt(d, &h)) )
    	    consume_pkt(buf, h->len);
        }
        nm_close(d);
    }

  ZERO-COPY FORWARDING
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);
        ...

  ACCESSING THE HOST STACK
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.

  VALE SWITCH
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:
    vale-ctl -h vale2:em0
Other netmap clients attached to the same switch can now communicate with the network card or the host.

SEE ALSO
pkt-gen(8), bridge(8)

http://info.iet.unipi.it/~luigi/netmap/

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

AUTHORS
-nosplit 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).

CAVEATS
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 performance.

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.

  SPECIAL NIC FEATURES
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.