400 lines
15 KiB
Plaintext
400 lines
15 KiB
Plaintext
--------------------------------------------------------------------------------
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+ ABSTRACT
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--------------------------------------------------------------------------------
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This file documents the CONFIG_PACKET_MMAP option available with the PACKET
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socket interface on 2.4 and 2.6 kernels. This type of sockets is used for
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capture network traffic with utilities like tcpdump or any other that uses
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the libpcap library.
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You can find the latest version of this document at
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http://pusa.uv.es/~ulisses/packet_mmap/
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Please send me your comments to
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Ulisses Alonso Camaró <uaca@i.hate.spam.alumni.uv.es>
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-------------------------------------------------------------------------------
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+ Why use PACKET_MMAP
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--------------------------------------------------------------------------------
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In Linux 2.4/2.6 if PACKET_MMAP is not enabled, the capture process is very
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inefficient. It uses very limited buffers and requires one system call
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to capture each packet, it requires two if you want to get packet's
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timestamp (like libpcap always does).
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In the other hand PACKET_MMAP is very efficient. PACKET_MMAP provides a size
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configurable circular buffer mapped in user space. This way reading packets just
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needs to wait for them, most of the time there is no need to issue a single
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system call. By using a shared buffer between the kernel and the user
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also has the benefit of minimizing packet copies.
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It's fine to use PACKET_MMAP to improve the performance of the capture process,
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but it isn't everything. At least, if you are capturing at high speeds (this
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is relative to the cpu speed), you should check if the device driver of your
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network interface card supports some sort of interrupt load mitigation or
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(even better) if it supports NAPI, also make sure it is enabled.
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--------------------------------------------------------------------------------
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+ How to use CONFIG_PACKET_MMAP
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--------------------------------------------------------------------------------
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From the user standpoint, you should use the higher level libpcap library, which
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is a de facto standard, portable across nearly all operating systems
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including Win32.
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Said that, at time of this writing, official libpcap 0.8.1 is out and doesn't include
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support for PACKET_MMAP, and also probably the libpcap included in your distribution.
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I'm aware of two implementations of PACKET_MMAP in libpcap:
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http://pusa.uv.es/~ulisses/packet_mmap/ (by Simon Patarin, based on libpcap 0.6.2)
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http://public.lanl.gov/cpw/ (by Phil Wood, based on lastest libpcap)
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The rest of this document is intended for people who want to understand
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the low level details or want to improve libpcap by including PACKET_MMAP
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support.
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--------------------------------------------------------------------------------
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+ How to use CONFIG_PACKET_MMAP directly
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--------------------------------------------------------------------------------
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From the system calls stand point, the use of PACKET_MMAP involves
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the following process:
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[setup] socket() -------> creation of the capture socket
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setsockopt() ---> allocation of the circular buffer (ring)
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mmap() ---------> mapping of the allocated buffer to the
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user process
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[capture] poll() ---------> to wait for incoming packets
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[shutdown] close() --------> destruction of the capture socket and
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deallocation of all associated
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resources.
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socket creation and destruction is straight forward, and is done
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the same way with or without PACKET_MMAP:
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int fd;
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fd= socket(PF_PACKET, mode, htons(ETH_P_ALL))
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where mode is SOCK_RAW for the raw interface were link level
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information can be captured or SOCK_DGRAM for the cooked
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interface where link level information capture is not
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supported and a link level pseudo-header is provided
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by the kernel.
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The destruction of the socket and all associated resources
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is done by a simple call to close(fd).
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Next I will describe PACKET_MMAP settings and it's constraints,
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also the mapping of the circular buffer in the user process and
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the use of this buffer.
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--------------------------------------------------------------------------------
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+ PACKET_MMAP settings
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--------------------------------------------------------------------------------
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To setup PACKET_MMAP from user level code is done with a call like
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setsockopt(fd, SOL_PACKET, PACKET_RX_RING, (void *) &req, sizeof(req))
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The most significant argument in the previous call is the req parameter,
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this parameter must to have the following structure:
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struct tpacket_req
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{
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unsigned int tp_block_size; /* Minimal size of contiguous block */
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unsigned int tp_block_nr; /* Number of blocks */
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unsigned int tp_frame_size; /* Size of frame */
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unsigned int tp_frame_nr; /* Total number of frames */
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};
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This structure is defined in /usr/include/linux/if_packet.h and establishes a
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circular buffer (ring) of unswappable memory mapped in the capture process.
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Being mapped in the capture process allows reading the captured frames and
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related meta-information like timestamps without requiring a system call.
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Captured frames are grouped in blocks. Each block is a physically contiguous
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region of memory and holds tp_block_size/tp_frame_size frames. The total number
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of blocks is tp_block_nr. Note that tp_frame_nr is a redundant parameter because
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frames_per_block = tp_block_size/tp_frame_size
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indeed, packet_set_ring checks that the following condition is true
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frames_per_block * tp_block_nr == tp_frame_nr
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Lets see an example, with the following values:
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tp_block_size= 4096
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tp_frame_size= 2048
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tp_block_nr = 4
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tp_frame_nr = 8
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we will get the following buffer structure:
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block #1 block #2
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+---------+---------+ +---------+---------+
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| frame 1 | frame 2 | | frame 3 | frame 4 |
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+---------+---------+ +---------+---------+
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block #3 block #4
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+---------+---------+ +---------+---------+
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| frame 5 | frame 6 | | frame 7 | frame 8 |
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+---------+---------+ +---------+---------+
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A frame can be of any size with the only condition it can fit in a block. A block
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can only hold an integer number of frames, or in other words, a frame cannot
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be spawned accross two blocks, so there are some details you have to take into
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account when choosing the frame_size. See "Mapping and use of the circular
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buffer (ring)".
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--------------------------------------------------------------------------------
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+ PACKET_MMAP setting constraints
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--------------------------------------------------------------------------------
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In kernel versions prior to 2.4.26 (for the 2.4 branch) and 2.6.5 (2.6 branch),
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the PACKET_MMAP buffer could hold only 32768 frames in a 32 bit architecture or
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16384 in a 64 bit architecture. For information on these kernel versions
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see http://pusa.uv.es/~ulisses/packet_mmap/packet_mmap.pre-2.4.26_2.6.5.txt
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Block size limit
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------------------
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As stated earlier, each block is a contiguous physical region of memory. These
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memory regions are allocated with calls to the __get_free_pages() function. As
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the name indicates, this function allocates pages of memory, and the second
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argument is "order" or a power of two number of pages, that is
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(for PAGE_SIZE == 4096) order=0 ==> 4096 bytes, order=1 ==> 8192 bytes,
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order=2 ==> 16384 bytes, etc. The maximum size of a
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region allocated by __get_free_pages is determined by the MAX_ORDER macro. More
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precisely the limit can be calculated as:
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PAGE_SIZE << MAX_ORDER
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In a i386 architecture PAGE_SIZE is 4096 bytes
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In a 2.4/i386 kernel MAX_ORDER is 10
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In a 2.6/i386 kernel MAX_ORDER is 11
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So get_free_pages can allocate as much as 4MB or 8MB in a 2.4/2.6 kernel
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respectively, with an i386 architecture.
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User space programs can include /usr/include/sys/user.h and
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/usr/include/linux/mmzone.h to get PAGE_SIZE MAX_ORDER declarations.
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The pagesize can also be determined dynamically with the getpagesize (2)
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system call.
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Block number limit
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--------------------
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To understand the constraints of PACKET_MMAP, we have to see the structure
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used to hold the pointers to each block.
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Currently, this structure is a dynamically allocated vector with kmalloc
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called pg_vec, its size limits the number of blocks that can be allocated.
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+---+---+---+---+
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| x | x | x | x |
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+---+---+---+---+
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| | | |
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| | | v
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| | v block #4
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| v block #3
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v block #2
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block #1
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kmalloc allocates any number of bytes of physically contiguous memory from
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a pool of pre-determined sizes. This pool of memory is maintained by the slab
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allocator which is at the end the responsible for doing the allocation and
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hence which imposes the maximum memory that kmalloc can allocate.
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In a 2.4/2.6 kernel and the i386 architecture, the limit is 131072 bytes. The
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predetermined sizes that kmalloc uses can be checked in the "size-<bytes>"
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entries of /proc/slabinfo
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In a 32 bit architecture, pointers are 4 bytes long, so the total number of
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pointers to blocks is
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131072/4 = 32768 blocks
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PACKET_MMAP buffer size calculator
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------------------------------------
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Definitions:
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<size-max> : is the maximum size of allocable with kmalloc (see /proc/slabinfo)
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<pointer size>: depends on the architecture -- sizeof(void *)
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<page size> : depends on the architecture -- PAGE_SIZE or getpagesize (2)
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<max-order> : is the value defined with MAX_ORDER
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<frame size> : it's an upper bound of frame's capture size (more on this later)
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from these definitions we will derive
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<block number> = <size-max>/<pointer size>
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<block size> = <pagesize> << <max-order>
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so, the max buffer size is
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<block number> * <block size>
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and, the number of frames be
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<block number> * <block size> / <frame size>
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Suppose the following parameters, which apply for 2.6 kernel and an
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i386 architecture:
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<size-max> = 131072 bytes
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<pointer size> = 4 bytes
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<pagesize> = 4096 bytes
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<max-order> = 11
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and a value for <frame size> of 2048 bytes. These parameters will yield
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<block number> = 131072/4 = 32768 blocks
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<block size> = 4096 << 11 = 8 MiB.
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and hence the buffer will have a 262144 MiB size. So it can hold
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262144 MiB / 2048 bytes = 134217728 frames
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Actually, this buffer size is not possible with an i386 architecture.
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Remember that the memory is allocated in kernel space, in the case of
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an i386 kernel's memory size is limited to 1GiB.
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All memory allocations are not freed until the socket is closed. The memory
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allocations are done with GFP_KERNEL priority, this basically means that
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the allocation can wait and swap other process' memory in order to allocate
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the necessary memory, so normally limits can be reached.
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Other constraints
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-------------------
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If you check the source code you will see that what I draw here as a frame
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is not only the link level frame. At the beginning of each frame there is a
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header called struct tpacket_hdr used in PACKET_MMAP to hold link level's frame
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meta information like timestamp. So what we draw here a frame it's really
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the following (from include/linux/if_packet.h):
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/*
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Frame structure:
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- Start. Frame must be aligned to TPACKET_ALIGNMENT=16
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- struct tpacket_hdr
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- pad to TPACKET_ALIGNMENT=16
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- struct sockaddr_ll
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- Gap, chosen so that packet data (Start+tp_net) aligns to
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TPACKET_ALIGNMENT=16
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- Start+tp_mac: [ Optional MAC header ]
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- Start+tp_net: Packet data, aligned to TPACKET_ALIGNMENT=16.
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- Pad to align to TPACKET_ALIGNMENT=16
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*/
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The following are conditions that are checked in packet_set_ring
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tp_block_size must be a multiple of PAGE_SIZE (1)
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tp_frame_size must be greater than TPACKET_HDRLEN (obvious)
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tp_frame_size must be a multiple of TPACKET_ALIGNMENT
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tp_frame_nr must be exactly frames_per_block*tp_block_nr
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Note that tp_block_size should be chosen to be a power of two or there will
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be a waste of memory.
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--------------------------------------------------------------------------------
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+ Mapping and use of the circular buffer (ring)
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--------------------------------------------------------------------------------
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The mapping of the buffer in the user process is done with the conventional
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mmap function. Even the circular buffer is compound of several physically
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discontiguous blocks of memory, they are contiguous to the user space, hence
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just one call to mmap is needed:
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mmap(0, size, PROT_READ|PROT_WRITE, MAP_SHARED, fd, 0);
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If tp_frame_size is a divisor of tp_block_size frames will be
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contiguosly spaced by tp_frame_size bytes. If not, each
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tp_block_size/tp_frame_size frames there will be a gap between
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the frames. This is because a frame cannot be spawn across two
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blocks.
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At the beginning of each frame there is an status field (see
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struct tpacket_hdr). If this field is 0 means that the frame is ready
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to be used for the kernel, If not, there is a frame the user can read
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and the following flags apply:
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from include/linux/if_packet.h
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#define TP_STATUS_COPY 2
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#define TP_STATUS_LOSING 4
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#define TP_STATUS_CSUMNOTREADY 8
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TP_STATUS_COPY : This flag indicates that the frame (and associated
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meta information) has been truncated because it's
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larger than tp_frame_size. This packet can be
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read entirely with recvfrom().
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In order to make this work it must to be
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enabled previously with setsockopt() and
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the PACKET_COPY_THRESH option.
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The number of frames than can be buffered to
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be read with recvfrom is limited like a normal socket.
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See the SO_RCVBUF option in the socket (7) man page.
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TP_STATUS_LOSING : indicates there were packet drops from last time
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statistics where checked with getsockopt() and
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the PACKET_STATISTICS option.
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TP_STATUS_CSUMNOTREADY: currently it's used for outgoing IP packets which
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it's checksum will be done in hardware. So while
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reading the packet we should not try to check the
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checksum.
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for convenience there are also the following defines:
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#define TP_STATUS_KERNEL 0
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#define TP_STATUS_USER 1
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The kernel initializes all frames to TP_STATUS_KERNEL, when the kernel
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receives a packet it puts in the buffer and updates the status with
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at least the TP_STATUS_USER flag. Then the user can read the packet,
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once the packet is read the user must zero the status field, so the kernel
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can use again that frame buffer.
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The user can use poll (any other variant should apply too) to check if new
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packets are in the ring:
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struct pollfd pfd;
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pfd.fd = fd;
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pfd.revents = 0;
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pfd.events = POLLIN|POLLRDNORM|POLLERR;
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if (status == TP_STATUS_KERNEL)
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retval = poll(&pfd, 1, timeout);
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It doesn't incur in a race condition to first check the status value and
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then poll for frames.
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--------------------------------------------------------------------------------
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+ THANKS
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--------------------------------------------------------------------------------
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Jesse Brandeburg, for fixing my grammathical/spelling errors
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