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
+capture network traffic with utilities like tcpdump or any other that needs
+raw access to network interface.
+You can find the latest version of this document at:
http://pusa.uv.es/~ulisses/packet_mmap/
-Please send me your comments to
+Howto can be found at:
+ http://wiki.gnu-log.net (packet_mmap)
- Ulisses Alonso Camaró <uaca@i.hate.spam.alumni.uv.es>
+Please send your comments to
+ Ulisses Alonso Camaró <uaca@i.hate.spam.alumni.uv.es>
+ Johann Baudy <johann.baudy@gnu-log.net>
-------------------------------------------------------------------------------
+ Why use PACKET_MMAP
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.
+configurable circular buffer mapped in user space that can be used to either
+send or receive packets. This way reading packets just needs to wait for them,
+most of the time there is no need to issue a single system call. Concerning
+transmission, multiple packets can be sent through one system call to get the
+highest bandwidth.
+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 and
+transmission 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. For transmission, check the MTU (Maximum Transmission Unit) used and
+supported by devices of your network.
--------------------------------------------------------------------------------
-+ How to use CONFIG_PACKET_MMAP
++ How to use CONFIG_PACKET_MMAP to improve capture process
--------------------------------------------------------------------------------
-From the user standpoint, you should use the higher level libpcap library, wich
+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.
support.
--------------------------------------------------------------------------------
-+ How to use CONFIG_PACKET_MMAP directly
++ How to use CONFIG_PACKET_MMAP directly to improve capture process
--------------------------------------------------------------------------------
From the system calls stand point, the use of PACKET_MMAP involves
[setup] socket() -------> creation of the capture socket
setsockopt() ---> allocation of the circular buffer (ring)
- mmap() ---------> maping of the allocated buffer to the
+ option: PACKET_RX_RING
+ mmap() ---------> mapping of the allocated buffer to the
user process
[capture] poll() ---------> to wait for incoming packets
is done by a simple call to close(fd).
Next I will describe PACKET_MMAP settings and it's constraints,
-also the maping of the circular buffer in the user process and
+also the mapping of the circular buffer in the user process and
the use of this buffer.
--------------------------------------------------------------------------------
++ How to use CONFIG_PACKET_MMAP directly to improve transmission process
+--------------------------------------------------------------------------------
+Transmission process is similar to capture as shown below.
+
+[setup] socket() -------> creation of the transmission socket
+ setsockopt() ---> allocation of the circular buffer (ring)
+ option: PACKET_TX_RING
+ bind() ---------> bind transmission socket with a network interface
+ mmap() ---------> mapping of the allocated buffer to the
+ user process
+
+[transmission] poll() ---------> wait for free packets (optional)
+ send() ---------> send all packets that are set as ready in
+ the ring
+ The flag MSG_DONTWAIT can be used to return
+ before end of transfer.
+
+[shutdown] close() --------> destruction of the transmission socket and
+ deallocation of all associated resources.
+
+Binding the socket to your network interface is mandatory (with zero copy) to
+know the header size of frames used in the circular buffer.
+
+As capture, each frame contains two parts:
+
+ --------------------
+| struct tpacket_hdr | Header. It contains the status of
+| | of this frame
+|--------------------|
+| data buffer |
+. . Data that will be sent over the network interface.
+. .
+ --------------------
+
+ bind() associates the socket to your network interface thanks to
+ sll_ifindex parameter of struct sockaddr_ll.
+
+ Initialization example:
+
+ struct sockaddr_ll my_addr;
+ struct ifreq s_ifr;
+ ...
+
+ strncpy (s_ifr.ifr_name, "eth0", sizeof(s_ifr.ifr_name));
+
+ /* get interface index of eth0 */
+ ioctl(this->socket, SIOCGIFINDEX, &s_ifr);
+
+ /* fill sockaddr_ll struct to prepare binding */
+ my_addr.sll_family = AF_PACKET;
+ my_addr.sll_protocol = ETH_P_ALL;
+ my_addr.sll_ifindex = s_ifr.ifr_ifindex;
+
+ /* bind socket to eth0 */
+ bind(this->socket, (struct sockaddr *)&my_addr, sizeof(struct sockaddr_ll));
+
+ A complete tutorial is available at: http://wiki.gnu-log.net/
+
+--------------------------------------------------------------------------------
+ PACKET_MMAP settings
--------------------------------------------------------------------------------
To setup PACKET_MMAP from user level code is done with a call like
+ - Capture process
setsockopt(fd, SOL_PACKET, PACKET_RX_RING, (void *) &req, sizeof(req))
+ - Transmission process
+ setsockopt(fd, SOL_PACKET, PACKET_TX_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:
};
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.
+circular buffer (ring) of unswappable memory.
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
+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
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 spawn accross two blocks so there are some datails you have to take into
-account when choosing the frame_size. See "Maping and use of the circular
+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)".
block #1
-kmalloc allocates any number of bytes of phisically contiguous memory from
-a pool of pre-determined sizes. This pool of memory is mantained by the slab
-allocator wich is at the end the responsible for doing the allocation and
-hence wich imposes the maximum memory that kmalloc can allocate.
+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>"
<block number> * <block size> / <frame size>
-Suposse the following parameters, wich apply for 2.6 kernel and an
+Suppose the following parameters, which apply for 2.6 kernel and an
i386 architecture:
<size-max> = 131072 bytes
<pagesize> = 4096 bytes
<max-order> = 11
-and a value for <frame size> of 2048 byteas. These parameters will yield
+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.
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.
+the necessary 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
+is not only the link level frame. At the beginning 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):
- struct tpacket_hdr
- pad to TPACKET_ALIGNMENT=16
- struct sockaddr_ll
- - Gap, chosen so that packet data (Start+tp_net) alignes to
+ - 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.
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 choosed to be a power of two or there will
+Note that tp_block_size should be chosen to be a power of two or there will
be a waste of memory.
--------------------------------------------------------------------------------
-+ Maping and use of the circular buffer (ring)
++ Mapping and use of the circular buffer (ring)
--------------------------------------------------------------------------------
-The maping of the buffer in the user process is done with the conventional
+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
+contiguously 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.
to be used for the kernel, If not, there is a frame the user can read
and the following flags apply:
++++ Capture process:
from include/linux/if_packet.h
#define TP_STATUS_COPY 2
statistics where checked with getsockopt() and
the PACKET_STATISTICS option.
-TP_STATUS_CSUMNOTREADY: currently it's used for outgoing IP packets wich
+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.
It doesn't incur in a race condition to first check the status value and
then poll for frames.
+
+++ Transmission process
+Those defines are also used for transmission:
+
+ #define TP_STATUS_AVAILABLE 0 // Frame is available
+ #define TP_STATUS_SEND_REQUEST 1 // Frame will be sent on next send()
+ #define TP_STATUS_SENDING 2 // Frame is currently in transmission
+ #define TP_STATUS_WRONG_FORMAT 4 // Frame format is not correct
+
+First, the kernel initializes all frames to TP_STATUS_AVAILABLE. To send a
+packet, the user fills a data buffer of an available frame, sets tp_len to
+current data buffer size and sets its status field to TP_STATUS_SEND_REQUEST.
+This can be done on multiple frames. Once the user is ready to transmit, it
+calls send(). Then all buffers with status equal to TP_STATUS_SEND_REQUEST are
+forwarded to the network device. The kernel updates each status of sent
+frames with TP_STATUS_SENDING until the end of transfer.
+At the end of each transfer, buffer status returns to TP_STATUS_AVAILABLE.
+
+ header->tp_len = in_i_size;
+ header->tp_status = TP_STATUS_SEND_REQUEST;
+ retval = send(this->socket, NULL, 0, 0);
+
+The user can also use poll() to check if a buffer is available:
+(status == TP_STATUS_SENDING)
+
+ struct pollfd pfd;
+ pfd.fd = fd;
+ pfd.revents = 0;
+ pfd.events = POLLOUT;
+ retval = poll(&pfd, 1, timeout);
+
--------------------------------------------------------------------------------
+ THANKS
--------------------------------------------------------------------------------
git://ftp.safe.ca / safe / jmp / linux-2.6 / blobdiff