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diff --git a/Documentation/networking/packet_mmap.txt b/Documentation/networking/packet_mmap.txt new file mode 100644 index 000000000000..8d4cf78258e4 --- /dev/null +++ b/Documentation/networking/packet_mmap.txt @@ -0,0 +1,399 @@ +-------------------------------------------------------------------------------- ++ 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, wich +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() ---------> maping 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 maping 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 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 +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 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. + +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> + +Suposse the following parameters, wich 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 byteas. 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) alignes 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 choosed to be a power of two or there will +be a waste of memory. + +-------------------------------------------------------------------------------- ++ Maping and use of the circular buffer (ring) +-------------------------------------------------------------------------------- + +The maping 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 wich + 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 + |
