| -------------------------------------------------------------------------------- |
| + 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() ---------> 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 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 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 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 |
| |
