NETMAP(4)						   BSD Kernel Interfaces Manual 						 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 both userspace and kernel clients.  It runs on FreeBSD and Linux, and
     includes VALE, a very fast and modular in-kernel software switch/dataplane, and netmap pipes, a shared memory packet transport channel.  All
     these are accessed interchangeably with the same API.

     netmap, VALE and netmap pipes are at least one order of magnitude faster than standard OS mechanisms (sockets, bpf, tun/tap interfaces,
     native switches, pipes), reaching 14.88 million packets per second (Mpps) with much less than one core on a 10 Gbit NIC, about 20 Mpps per
     core for VALE ports, and over 100 Mpps for netmap pipes.

     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, and netmap pipes can be created dynamically, providing high speed packet I/O between processes, virtual
     machines, NICs and the host stack.

     netmap suports both non-blocking I/O through ioctls(), synchronization and blocking I/O through a file descriptor and standard OS mechanisms
     such as select(2), poll(2), epoll(2), kqueue(2).  VALE and netmap pipes are implemented by a single kernel module, which also emulates the
     netmap API over standard drivers for devices without native netmap support.  For best performance, netmap requires explicit support in device
     drivers.

     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 section LIBRARIES.

     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 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;

     valeXXX:YYY (arbitrary XXX and YYY)
	   the file descriptor is bound to port YYY of a VALE switch called XXX, both dynamically created if necessary.  The string cannot exceed
	   IFNAMSIZ characters, and YYY 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 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.
     (NOTE: older versions of netmap used head/count format to indicate the content of a ring).

     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 avail-
     able.  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() wlll 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
	  it 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 detecting when packets have been send and a file descriptor can be closed.

     NS_FORWARD
	  When a ring is in 'transparent' mode (see TRANSPARENT MODE), packets marked with this flags 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 SRC 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 tx 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 inde-
	       pendent 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-spe-
	       cific functions (e.g.  ethtool ).

     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.

	   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 conveniency, 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, binds a file descriptor to a port.

	    ifname
		is a port name, in the form "netmap:XXX" for a NIC and "valeXXX:YYY" 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: em(4), igb(4), ixgbe(4), lem(4), re(4).

     On Linux e1000(4), e1000e(4), igb(4), ixgbe(4), mlx4(4), forcedeth(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 sig-
     nificantly higher than sockets, and approaching that of in-kernel solutions such as Linux's pktgen.

     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 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 and kqueue 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 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 has many options can be uses to set packet sizes, addresses, rates, and use multiple send/receive threads and cores.

     bridge 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 vale:x{3 -f rx # receiver on the master side
	   pkt-gen -i vale: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

     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

     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, 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.  Sev-
     eral 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 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.

BSD
								 February 13, 2014							       BSD