Documentation/networking/packet_mmap.txt - maze/linux - Git at Google

 --------------------------------------------------------------------------------
 + ABSTRACT
 --------------------------------------------------------------------------------

 This file documents the CONFIG_PACKET_MMAP option available with the PACKET
 socket interface on 2.4 and 2.6 kernels. This type of sockets is used for
 capture network traffic with utilities like tcpdump or any other that uses
 the libpcap library.

 You can find the latest version of this document at

     http://pusa.uv.es/~ulisses/packet_mmap/

 Please send me your comments to

     Ulisses Alonso Camaró <uaca@i.hate.spam.alumni.uv.es>

 -------------------------------------------------------------------------------
 + Why use PACKET_MMAP
 --------------------------------------------------------------------------------

 In Linux 2.4/2.6 if PACKET_MMAP is not enabled, the capture process is very
 inefficient. It uses very limited buffers and requires one system call
 to capture each packet, it requires two if you want to get packet's
 timestamp (like libpcap always does).

 In the other hand PACKET_MMAP is very efficient. PACKET_MMAP provides a size
 configurable circular buffer mapped in user space. This way reading packets just
 needs to wait for them, most of the time there is no need to issue a single
 system call. By using a shared buffer between the kernel and the user
 also has the benefit of minimizing packet copies.

 It's fine to use PACKET_MMAP to improve the performance of the capture process,
 but it isn't everything. At least, if you are capturing at high speeds (this
 is relative to the cpu speed), you should check if the device driver of your
 network interface card supports some sort of interrupt load mitigation or
 (even better) if it supports NAPI, also make sure it is enabled.

 --------------------------------------------------------------------------------
 + How to use CONFIG_PACKET_MMAP
 --------------------------------------------------------------------------------

 From the user standpoint, you should use the higher level libpcap library, which
 is a de facto standard, portable across nearly all operating systems
 including Win32.

 Said that, at time of this writing, official libpcap 0.8.1 is out and doesn't include
 support for PACKET_MMAP, and also probably the libpcap included in your distribution.

 I'm aware of two implementations of PACKET_MMAP in libpcap:

     http://pusa.uv.es/~ulisses/packet_mmap/  (by Simon Patarin, based on libpcap 0.6.2)
     http://public.lanl.gov/cpw/              (by Phil Wood, based on lastest libpcap)

 The rest of this document is intended for people who want to understand
 the low level details or want to improve libpcap by including PACKET_MMAP
 support.

 --------------------------------------------------------------------------------
 + How to use CONFIG_PACKET_MMAP directly
 --------------------------------------------------------------------------------

 From the system calls stand point, the use of PACKET_MMAP involves
 the following process:


 [setup]     socket() -------> creation of the capture socket
             setsockopt() ---> allocation of the circular buffer (ring)
             mmap() ---------> mapping of the allocated buffer to the
                               user process

 [capture]   poll() ---------> to wait for incoming packets

 [shutdown]  close() --------> destruction of the capture socket and
                               deallocation of all associated
                               resources.


 socket creation and destruction is straight forward, and is done
 the same way with or without PACKET_MMAP:

 int fd;

 fd= socket(PF_PACKET, mode, htons(ETH_P_ALL))

 where mode is SOCK_RAW for the raw interface were link level
 information can be captured or SOCK_DGRAM for the cooked
 interface where link level information capture is not
 supported and a link level pseudo-header is provided
 by the kernel.

 The destruction of the socket and all associated resources
 is done by a simple call to close(fd).

 Next I will describe PACKET_MMAP settings and it's constraints,
 also the mapping of the circular buffer in the user process and
 the use of this buffer.

 --------------------------------------------------------------------------------
 + PACKET_MMAP settings
 --------------------------------------------------------------------------------


 To setup PACKET_MMAP from user level code is done with a call like

      setsockopt(fd, SOL_PACKET, PACKET_RX_RING, (void *) &req, sizeof(req))

 The most significant argument in the previous call is the req parameter,
 this parameter must to have the following structure:

     struct tpacket_req
     {
         unsigned int    tp_block_size;  /* Minimal size of contiguous block */
         unsigned int    tp_block_nr;    /* Number of blocks */
         unsigned int    tp_frame_size;  /* Size of frame */
         unsigned int    tp_frame_nr;    /* Total number of frames */
     };

 This structure is defined in /usr/include/linux/if_packet.h and establishes a
 circular buffer (ring) of unswappable memory mapped in the capture process.
 Being mapped in the capture process allows reading the captured frames and
 related meta-information like timestamps without requiring a system call.

 Captured frames are grouped in blocks. Each block is a physically contiguous
 region of memory and holds tp_block_size/tp_frame_size frames. The total number
 of blocks is tp_block_nr. Note that tp_frame_nr is a redundant parameter because

     frames_per_block = tp_block_size/tp_frame_size

 indeed, packet_set_ring checks that the following condition is true

     frames_per_block * tp_block_nr == tp_frame_nr


 Lets see an example, with the following values:

      tp_block_size= 4096
      tp_frame_size= 2048
      tp_block_nr  = 4
      tp_frame_nr  = 8

 we will get the following buffer structure:

         block #1                 block #2
 +---------+---------+    +---------+---------+
 | frame 1 | frame 2 |    | frame 3 | frame 4 |
 +---------+---------+    +---------+---------+

         block #3                 block #4
 +---------+---------+    +---------+---------+
 | frame 5 | frame 6 |    | frame 7 | frame 8 |
 +---------+---------+    +---------+---------+

 A frame can be of any size with the only condition it can fit in a block. A block
 can only hold an integer number of frames, or in other words, a frame cannot
 be spawned accross two blocks, so there are some details you have to take into
 account when choosing the frame_size. See "Mapping and use of the circular
 buffer (ring)".


 --------------------------------------------------------------------------------
 + PACKET_MMAP setting constraints
 --------------------------------------------------------------------------------

 In kernel versions prior to 2.4.26 (for the 2.4 branch) and 2.6.5 (2.6 branch),
 the PACKET_MMAP buffer could hold only 32768 frames in a 32 bit architecture or
 16384 in a 64 bit architecture. For information on these kernel versions
 see http://pusa.uv.es/~ulisses/packet_mmap/packet_mmap.pre-2.4.26_2.6.5.txt

  Block size limit
 ------------------

 As stated earlier, each block is a contiguous physical region of memory. These
 memory regions are allocated with calls to the __get_free_pages() function. As
 the name indicates, this function allocates pages of memory, and the second
 argument is "order" or a power of two number of pages, that is
 (for PAGE_SIZE == 4096) order=0 ==> 4096 bytes, order=1 ==> 8192 bytes,
 order=2 ==> 16384 bytes, etc. The maximum size of a
 region allocated by __get_free_pages is determined by the MAX_ORDER macro. More
 precisely the limit can be calculated as:

    PAGE_SIZE << MAX_ORDER

    In a i386 architecture PAGE_SIZE is 4096 bytes
    In a 2.4/i386 kernel MAX_ORDER is 10
    In a 2.6/i386 kernel MAX_ORDER is 11

 So get_free_pages can allocate as much as 4MB or 8MB in a 2.4/2.6 kernel
 respectively, with an i386 architecture.

 User space programs can include /usr/include/sys/user.h and
 /usr/include/linux/mmzone.h to get PAGE_SIZE MAX_ORDER declarations.

 The pagesize can also be determined dynamically with the getpagesize (2)
 system call.


  Block number limit
 --------------------

 To understand the constraints of PACKET_MMAP, we have to see the structure
 used to hold the pointers to each block.

 Currently, this structure is a dynamically allocated vector with kmalloc
 called pg_vec, its size limits the number of blocks that can be allocated.

     +---+---+---+---+
     | x | x | x | x |
     +---+---+---+---+
       |   |   |   |
       |   |   |   v
       |   |   v  block #4
       |   v  block #3
       v  block #2
      block #1


 kmalloc allocates any number of bytes of physically contiguous memory from
 a pool of pre-determined sizes. This pool of memory is maintained by the slab
 allocator which is at the end the responsible for doing the allocation and
 hence which imposes the maximum memory that kmalloc can allocate.

 In a 2.4/2.6 kernel and the i386 architecture, the limit is 131072 bytes. The
 predetermined sizes that kmalloc uses can be checked in the "size-<bytes>"
 entries of /proc/slabinfo

 In a 32 bit architecture, pointers are 4 bytes long, so the total number of
 pointers to blocks is

      131072/4 = 32768 blocks


  PACKET_MMAP buffer size calculator
 ------------------------------------

 Definitions:

 <size-max>    : is the maximum size of allocable with kmalloc (see /proc/slabinfo)
 <pointer size>: depends on the architecture -- sizeof(void *)
 <page size>   : depends on the architecture -- PAGE_SIZE or getpagesize (2)
 <max-order>   : is the value defined with MAX_ORDER
 <frame size>  : it's an upper bound of frame's capture size (more on this later)

 from these definitions we will derive

 	<block number> = <size-max>/<pointer size>
 	<block size> = <pagesize> << <max-order>

 so, the max buffer size is

 	<block number> * <block size>

 and, the number of frames be

 	<block number> * <block size> / <frame size>

 Suppose the following parameters, which apply for 2.6 kernel and an
 i386 architecture:

 	<size-max> = 131072 bytes
 	<pointer size> = 4 bytes
 	<pagesize> = 4096 bytes
 	<max-order> = 11

 and a value for <frame size> of 2048 bytes. These parameters will yield

 	<block number> = 131072/4 = 32768 blocks
 	<block size> = 4096 << 11 = 8 MiB.

 and hence the buffer will have a 262144 MiB size. So it can hold
 262144 MiB / 2048 bytes = 134217728 frames


 Actually, this buffer size is not possible with an i386 architecture.
 Remember that the memory is allocated in kernel space, in the case of
 an i386 kernel's memory size is limited to 1GiB.

 All memory allocations are not freed until the socket is closed. The memory
 allocations are done with GFP_KERNEL priority, this basically means that
 the allocation can wait and swap other process' memory in order to allocate
 the nececessary memory, so normally limits can be reached.

  Other constraints
 -------------------

 If you check the source code you will see that what I draw here as a frame
 is not only the link level frame. At the begining of each frame there is a
 header called struct tpacket_hdr used in PACKET_MMAP to hold link level's frame
 meta information like timestamp. So what we draw here a frame it's really
 the following (from include/linux/if_packet.h):

 /*
    Frame structure:

    - Start. Frame must be aligned to TPACKET_ALIGNMENT=16
    - struct tpacket_hdr
    - pad to TPACKET_ALIGNMENT=16
    - struct sockaddr_ll
    - Gap, chosen so that packet data (Start+tp_net) aligns to
      TPACKET_ALIGNMENT=16
    - Start+tp_mac: [ Optional MAC header ]
    - Start+tp_net: Packet data, aligned to TPACKET_ALIGNMENT=16.
    - Pad to align to TPACKET_ALIGNMENT=16
  */


  The following are conditions that are checked in packet_set_ring

    tp_block_size must be a multiple of PAGE_SIZE (1)
    tp_frame_size must be greater than TPACKET_HDRLEN (obvious)
    tp_frame_size must be a multiple of TPACKET_ALIGNMENT
    tp_frame_nr   must be exactly frames_per_block*tp_block_nr

 Note that tp_block_size should be chosen to be a power of two or there will
 be a waste of memory.

 --------------------------------------------------------------------------------
 + Mapping and use of the circular buffer (ring)
 --------------------------------------------------------------------------------

 The mapping of the buffer in the user process is done with the conventional
 mmap function. Even the circular buffer is compound of several physically
 discontiguous blocks of memory, they are contiguous to the user space, hence
 just one call to mmap is needed:

     mmap(0, size, PROT_READ|PROT_WRITE, MAP_SHARED, fd, 0);

 If tp_frame_size is a divisor of tp_block_size frames will be
 contiguosly spaced by tp_frame_size bytes. If not, each
 tp_block_size/tp_frame_size frames there will be a gap between
 the frames. This is because a frame cannot be spawn across two
 blocks.

 At the beginning of each frame there is an status field (see
 struct tpacket_hdr). If this field is 0 means that the frame is ready
 to be used for the kernel, If not, there is a frame the user can read
 and the following flags apply:

      from include/linux/if_packet.h

      #define TP_STATUS_COPY          2
      #define TP_STATUS_LOSING        4
      #define TP_STATUS_CSUMNOTREADY  8


 TP_STATUS_COPY        : This flag indicates that the frame (and associated
                         meta information) has been truncated because it's
                         larger than tp_frame_size. This packet can be
                         read entirely with recvfrom().

                         In order to make this work it must to be
                         enabled previously with setsockopt() and
                         the PACKET_COPY_THRESH option.

                         The number of frames than can be buffered to
                         be read with recvfrom is limited like a normal socket.
                         See the SO_RCVBUF option in the socket (7) man page.

 TP_STATUS_LOSING      : indicates there were packet drops from last time
                         statistics where checked with getsockopt() and
                         the PACKET_STATISTICS option.

 TP_STATUS_CSUMNOTREADY: currently it's used for outgoing IP packets which
                         it's checksum will be done in hardware. So while
                         reading the packet we should not try to check the
                         checksum.

 for convenience there are also the following defines:

      #define TP_STATUS_KERNEL        0
      #define TP_STATUS_USER          1

 The kernel initializes all frames to TP_STATUS_KERNEL, when the kernel
 receives a packet it puts in the buffer and updates the status with
 at least the TP_STATUS_USER flag. Then the user can read the packet,
 once the packet is read the user must zero the status field, so the kernel
 can use again that frame buffer.

 The user can use poll (any other variant should apply too) to check if new
 packets are in the ring:

     struct pollfd pfd;

     pfd.fd = fd;
     pfd.revents = 0;
     pfd.events = POLLIN|POLLRDNORM|POLLERR;

     if (status == TP_STATUS_KERNEL)
         retval = poll(&pfd, 1, timeout);

 It doesn't incur in a race condition to first check the status value and
 then poll for frames.

 --------------------------------------------------------------------------------
 + THANKS
 --------------------------------------------------------------------------------

    Jesse Brandeburg, for fixing my grammathical/spelling errors
	--------------------------------------------------------------------------------
	+ ABSTRACT
	--------------------------------------------------------------------------------

	This file documents the CONFIG_PACKET_MMAP option available with the PACKET
	socket interface on 2.4 and 2.6 kernels. This type of sockets is used for
	capture network traffic with utilities like tcpdump or any other that uses
	the libpcap library.

	You can find the latest version of this document at

	http://pusa.uv.es/~ulisses/packet_mmap/

	Please send me your comments to

	Ulisses Alonso Camaró <uaca@i.hate.spam.alumni.uv.es>

	-------------------------------------------------------------------------------
	+ Why use PACKET_MMAP
	--------------------------------------------------------------------------------

	In Linux 2.4/2.6 if PACKET_MMAP is not enabled, the capture process is very
	inefficient. It uses very limited buffers and requires one system call
	to capture each packet, it requires two if you want to get packet's
	timestamp (like libpcap always does).

	In the other hand PACKET_MMAP is very efficient. PACKET_MMAP provides a size
	configurable circular buffer mapped in user space. This way reading packets just
	needs to wait for them, most of the time there is no need to issue a single
	system call. By using a shared buffer between the kernel and the user
	also has the benefit of minimizing packet copies.

	It's fine to use PACKET_MMAP to improve the performance of the capture process,
	but it isn't everything. At least, if you are capturing at high speeds (this
	is relative to the cpu speed), you should check if the device driver of your
	network interface card supports some sort of interrupt load mitigation or
	(even better) if it supports NAPI, also make sure it is enabled.

	--------------------------------------------------------------------------------
	+ How to use CONFIG_PACKET_MMAP
	--------------------------------------------------------------------------------

	From the user standpoint, you should use the higher level libpcap library, which
	is a de facto standard, portable across nearly all operating systems
	including Win32.

	Said that, at time of this writing, official libpcap 0.8.1 is out and doesn't include
	support for PACKET_MMAP, and also probably the libpcap included in your distribution.

	I'm aware of two implementations of PACKET_MMAP in libpcap:

	http://pusa.uv.es/~ulisses/packet_mmap/ (by Simon Patarin, based on libpcap 0.6.2)
	http://public.lanl.gov/cpw/ (by Phil Wood, based on lastest libpcap)

	The rest of this document is intended for people who want to understand
	the low level details or want to improve libpcap by including PACKET_MMAP
	support.

	--------------------------------------------------------------------------------
	+ How to use CONFIG_PACKET_MMAP directly
	--------------------------------------------------------------------------------

	From the system calls stand point, the use of PACKET_MMAP involves
	the following process:


	[setup] socket() -------> creation of the capture socket
	setsockopt() ---> allocation of the circular buffer (ring)
	mmap() ---------> mapping of the allocated buffer to the
	user process

	[capture] poll() ---------> to wait for incoming packets

	[shutdown] close() --------> destruction of the capture socket and
	deallocation of all associated
	resources.


	socket creation and destruction is straight forward, and is done
	the same way with or without PACKET_MMAP:

	int fd;

	fd= socket(PF_PACKET, mode, htons(ETH_P_ALL))

	where mode is SOCK_RAW for the raw interface were link level
	information can be captured or SOCK_DGRAM for the cooked
	interface where link level information capture is not
	supported and a link level pseudo-header is provided
	by the kernel.

	The destruction of the socket and all associated resources
	is done by a simple call to close(fd).

	Next I will describe PACKET_MMAP settings and it's constraints,
	also the mapping of the circular buffer in the user process and
	the use of this buffer.

	--------------------------------------------------------------------------------
	+ PACKET_MMAP settings
	--------------------------------------------------------------------------------


	To setup PACKET_MMAP from user level code is done with a call like

	setsockopt(fd, SOL_PACKET, PACKET_RX_RING, (void *) &req, sizeof(req))

	The most significant argument in the previous call is the req parameter,
	this parameter must to have the following structure:

	struct tpacket_req
	{
	unsigned int tp_block_size; /* Minimal size of contiguous block */
	unsigned int tp_block_nr; /* Number of blocks */
	unsigned int tp_frame_size; /* Size of frame */
	unsigned int tp_frame_nr; /* Total number of frames */
	};

	This structure is defined in /usr/include/linux/if_packet.h and establishes a
	circular buffer (ring) of unswappable memory mapped in the capture process.
	Being mapped in the capture process allows reading the captured frames and
	related meta-information like timestamps without requiring a system call.

	Captured frames are grouped in blocks. Each block is a physically contiguous
	region of memory and holds tp_block_size/tp_frame_size frames. The total number
	of blocks is tp_block_nr. Note that tp_frame_nr is a redundant parameter because

	frames_per_block = tp_block_size/tp_frame_size

	indeed, packet_set_ring checks that the following condition is true

	frames_per_block * tp_block_nr == tp_frame_nr


	Lets see an example, with the following values:

	tp_block_size= 4096
	tp_frame_size= 2048
	tp_block_nr = 4
	tp_frame_nr = 8

	we will get the following buffer structure:

	block #1 block #2
	+---------+---------+ +---------+---------+
	\| frame 1 \| frame 2 \| \| frame 3 \| frame 4 \|
	+---------+---------+ +---------+---------+

	block #3 block #4
	+---------+---------+ +---------+---------+
	\| frame 5 \| frame 6 \| \| frame 7 \| frame 8 \|
	+---------+---------+ +---------+---------+

	A frame can be of any size with the only condition it can fit in a block. A block
	can only hold an integer number of frames, or in other words, a frame cannot
	be spawned accross two blocks, so there are some details you have to take into
	account when choosing the frame_size. See "Mapping and use of the circular
	buffer (ring)".


	--------------------------------------------------------------------------------
	+ PACKET_MMAP setting constraints
	--------------------------------------------------------------------------------

	In kernel versions prior to 2.4.26 (for the 2.4 branch) and 2.6.5 (2.6 branch),
	the PACKET_MMAP buffer could hold only 32768 frames in a 32 bit architecture or
	16384 in a 64 bit architecture. For information on these kernel versions
	see http://pusa.uv.es/~ulisses/packet_mmap/packet_mmap.pre-2.4.26_2.6.5.txt

	Block size limit
	------------------

	As stated earlier, each block is a contiguous physical region of memory. These
	memory regions are allocated with calls to the __get_free_pages() function. As
	the name indicates, this function allocates pages of memory, and the second
	argument is "order" or a power of two number of pages, that is
	(for PAGE_SIZE == 4096) order=0 ==> 4096 bytes, order=1 ==> 8192 bytes,
	order=2 ==> 16384 bytes, etc. The maximum size of a
	region allocated by __get_free_pages is determined by the MAX_ORDER macro. More
	precisely the limit can be calculated as:

	PAGE_SIZE << MAX_ORDER

	In a i386 architecture PAGE_SIZE is 4096 bytes
	In a 2.4/i386 kernel MAX_ORDER is 10
	In a 2.6/i386 kernel MAX_ORDER is 11

	So get_free_pages can allocate as much as 4MB or 8MB in a 2.4/2.6 kernel
	respectively, with an i386 architecture.

	User space programs can include /usr/include/sys/user.h and
	/usr/include/linux/mmzone.h to get PAGE_SIZE MAX_ORDER declarations.

	The pagesize can also be determined dynamically with the getpagesize (2)
	system call.


	Block number limit
	--------------------

	To understand the constraints of PACKET_MMAP, we have to see the structure
	used to hold the pointers to each block.

	Currently, this structure is a dynamically allocated vector with kmalloc
	called pg_vec, its size limits the number of blocks that can be allocated.

	+---+---+---+---+
	\| x \| x \| x \| x \|
	+---+---+---+---+
	\| \| \| \|
	\| \| \| v
	\| \| v block #4
	\| v block #3
	v block #2
	block #1


	kmalloc allocates any number of bytes of physically contiguous memory from
	a pool of pre-determined sizes. This pool of memory is maintained by the slab
	allocator which is at the end the responsible for doing the allocation and
	hence which imposes the maximum memory that kmalloc can allocate.

	In a 2.4/2.6 kernel and the i386 architecture, the limit is 131072 bytes. The
	predetermined sizes that kmalloc uses can be checked in the "size-<bytes>"
	entries of /proc/slabinfo

	In a 32 bit architecture, pointers are 4 bytes long, so the total number of
	pointers to blocks is

	131072/4 = 32768 blocks


	PACKET_MMAP buffer size calculator
	------------------------------------

	Definitions:

	<size-max> : is the maximum size of allocable with kmalloc (see /proc/slabinfo)
	<pointer size>: depends on the architecture -- sizeof(void *)
	<page size> : depends on the architecture -- PAGE_SIZE or getpagesize (2)
	<max-order> : is the value defined with MAX_ORDER
	<frame size> : it's an upper bound of frame's capture size (more on this later)

	from these definitions we will derive

	<block number> = <size-max>/<pointer size>
	<block size> = <pagesize> << <max-order>

	so, the max buffer size is

	<block number> * <block size>

	and, the number of frames be

	<block number> * <block size> / <frame size>

	Suppose the following parameters, which apply for 2.6 kernel and an
	i386 architecture:

	<size-max> = 131072 bytes
	<pointer size> = 4 bytes
	<pagesize> = 4096 bytes
	<max-order> = 11

	and a value for <frame size> of 2048 bytes. These parameters will yield

	<block number> = 131072/4 = 32768 blocks
	<block size> = 4096 << 11 = 8 MiB.

	and hence the buffer will have a 262144 MiB size. So it can hold
	262144 MiB / 2048 bytes = 134217728 frames


	Actually, this buffer size is not possible with an i386 architecture.
	Remember that the memory is allocated in kernel space, in the case of
	an i386 kernel's memory size is limited to 1GiB.

	All memory allocations are not freed until the socket is closed. The memory
	allocations are done with GFP_KERNEL priority, this basically means that
	the allocation can wait and swap other process' memory in order to allocate
	the nececessary memory, so normally limits can be reached.

	Other constraints
	-------------------

	If you check the source code you will see that what I draw here as a frame
	is not only the link level frame. At the begining of each frame there is a
	header called struct tpacket_hdr used in PACKET_MMAP to hold link level's frame
	meta information like timestamp. So what we draw here a frame it's really
	the following (from include/linux/if_packet.h):

	/*
	Frame structure:

	- Start. Frame must be aligned to TPACKET_ALIGNMENT=16
	- struct tpacket_hdr
	- pad to TPACKET_ALIGNMENT=16
	- struct sockaddr_ll
	- Gap, chosen so that packet data (Start+tp_net) aligns to
	TPACKET_ALIGNMENT=16
	- Start+tp_mac: [ Optional MAC header ]
	- Start+tp_net: Packet data, aligned to TPACKET_ALIGNMENT=16.
	- Pad to align to TPACKET_ALIGNMENT=16
	*/


	The following are conditions that are checked in packet_set_ring

	tp_block_size must be a multiple of PAGE_SIZE (1)
	tp_frame_size must be greater than TPACKET_HDRLEN (obvious)
	tp_frame_size must be a multiple of TPACKET_ALIGNMENT
	tp_frame_nr must be exactly frames_per_block*tp_block_nr

	Note that tp_block_size should be chosen to be a power of two or there will
	be a waste of memory.

	--------------------------------------------------------------------------------
	+ Mapping and use of the circular buffer (ring)
	--------------------------------------------------------------------------------

	The mapping of the buffer in the user process is done with the conventional
	mmap function. Even the circular buffer is compound of several physically
	discontiguous blocks of memory, they are contiguous to the user space, hence
	just one call to mmap is needed:

	mmap(0, size, PROT_READ\|PROT_WRITE, MAP_SHARED, fd, 0);

	If tp_frame_size is a divisor of tp_block_size frames will be
	contiguosly spaced by tp_frame_size bytes. If not, each
	tp_block_size/tp_frame_size frames there will be a gap between
	the frames. This is because a frame cannot be spawn across two
	blocks.

	At the beginning of each frame there is an status field (see
	struct tpacket_hdr). If this field is 0 means that the frame is ready
	to be used for the kernel, If not, there is a frame the user can read
	and the following flags apply:

	from include/linux/if_packet.h

	#define TP_STATUS_COPY 2
	#define TP_STATUS_LOSING 4
	#define TP_STATUS_CSUMNOTREADY 8


	TP_STATUS_COPY : This flag indicates that the frame (and associated
	meta information) has been truncated because it's
	larger than tp_frame_size. This packet can be
	read entirely with recvfrom().

	In order to make this work it must to be
	enabled previously with setsockopt() and
	the PACKET_COPY_THRESH option.

	The number of frames than can be buffered to
	be read with recvfrom is limited like a normal socket.
	See the SO_RCVBUF option in the socket (7) man page.

	TP_STATUS_LOSING : indicates there were packet drops from last time
	statistics where checked with getsockopt() and
	the PACKET_STATISTICS option.

	TP_STATUS_CSUMNOTREADY: currently it's used for outgoing IP packets which
	it's checksum will be done in hardware. So while
	reading the packet we should not try to check the
	checksum.

	for convenience there are also the following defines:

	#define TP_STATUS_KERNEL 0
	#define TP_STATUS_USER 1

	The kernel initializes all frames to TP_STATUS_KERNEL, when the kernel
	receives a packet it puts in the buffer and updates the status with
	at least the TP_STATUS_USER flag. Then the user can read the packet,
	once the packet is read the user must zero the status field, so the kernel
	can use again that frame buffer.

	The user can use poll (any other variant should apply too) to check if new
	packets are in the ring:

	struct pollfd pfd;

	pfd.fd = fd;
	pfd.revents = 0;
	pfd.events = POLLIN\|POLLRDNORM\|POLLERR;

	if (status == TP_STATUS_KERNEL)
	retval = poll(&pfd, 1, timeout);

	It doesn't incur in a race condition to first check the status value and
	then poll for frames.

	--------------------------------------------------------------------------------
	+ THANKS
	--------------------------------------------------------------------------------

	Jesse Brandeburg, for fixing my grammathical/spelling errors