-------------------------------------------------------------------------------- + ABSTRACT -------------------------------------------------------------------------------- This file documents the mmap() facility 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 needs raw access to network interface. You can find the latest version of this document at: http://wiki.ipxwarzone.com/index.php5?title=Linux_packet_mmap Howto can be found at: http://wiki.gnu-log.net (packet_mmap) Please send your comments to Ulisses Alonso Camaró Johann Baudy ------------------------------------------------------------------------------- + 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 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 mmap() to improve capture process -------------------------------------------------------------------------------- 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://wiki.ipxwarzone.com/ (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 mmap() directly to improve capture process -------------------------------------------------------------------------------- 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) option: PACKET_RX_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 its constraints, also the mapping of the circular buffer in the user process and the use of this buffer. -------------------------------------------------------------------------------- + How to use 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 = htons(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: 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. Being mapped in the capture process allows reading the captured frames and related meta-information like timestamps without requiring a system call. 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 across 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-" 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: : is the maximum size of allocable with kmalloc (see /proc/slabinfo) : depends on the architecture -- sizeof(void *) : depends on the architecture -- PAGE_SIZE or getpagesize (2) : is the value defined with MAX_ORDER : it's an upper bound of frame's capture size (more on this later) from these definitions we will derive = / = << so, the max buffer size is * and, the number of frames be * / Suppose the following parameters, which apply for 2.6 kernel and an i386 architecture: = 131072 bytes = 4 bytes = 4096 bytes = 11 and a value for of 2048 bytes. These parameters will yield = 131072/4 = 32768 blocks = 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 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 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): /* 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 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. 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: +++ Capture process: 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 its 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. ++ 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); ------------------------------------------------------------------------------- + PACKET_TIMESTAMP ------------------------------------------------------------------------------- The PACKET_TIMESTAMP setting determines the source of the timestamp in the packet meta information. If your NIC is capable of timestamping packets in hardware, you can request those hardware timestamps to used. Note: you may need to enable the generation of hardware timestamps with SIOCSHWTSTAMP. PACKET_TIMESTAMP accepts the same integer bit field as SO_TIMESTAMPING. However, only the SOF_TIMESTAMPING_SYS_HARDWARE and SOF_TIMESTAMPING_RAW_HARDWARE values are recognized by PACKET_TIMESTAMP. SOF_TIMESTAMPING_SYS_HARDWARE takes precedence over SOF_TIMESTAMPING_RAW_HARDWARE if both bits are set. int req = 0; req |= SOF_TIMESTAMPING_SYS_HARDWARE; setsockopt(fd, SOL_PACKET, PACKET_TIMESTAMP, (void *) &req, sizeof(req)) If PACKET_TIMESTAMP is not set, a software timestamp generated inside the networking stack is used (the behavior before this setting was added). See include/linux/net_tstamp.h and Documentation/networking/timestamping for more information on hardware timestamps. -------------------------------------------------------------------------------- + THANKS -------------------------------------------------------------------------------- Jesse Brandeburg, for fixing my grammathical/spelling errors