1-------------------------------------------------------------------------------- 2+ ABSTRACT 3-------------------------------------------------------------------------------- 4 5This file documents the CONFIG_PACKET_MMAP option available with the PACKET 6socket interface on 2.4 and 2.6 kernels. This type of sockets is used for 7capture network traffic with utilities like tcpdump or any other that uses 8the libpcap library. 9 10You can find the latest version of this document at 11 12 http://pusa.uv.es/~ulisses/packet_mmap/ 13 14Please send me your comments to 15 16 Ulisses Alonso Camar�� <uaca@i.hate.spam.alumni.uv.es> 17 18------------------------------------------------------------------------------- 19+ Why use PACKET_MMAP 20-------------------------------------------------------------------------------- 21 22In Linux 2.4/2.6 if PACKET_MMAP is not enabled, the capture process is very 23inefficient. It uses very limited buffers and requires one system call 24to capture each packet, it requires two if you want to get packet's 25timestamp (like libpcap always does). 26 27In the other hand PACKET_MMAP is very efficient. PACKET_MMAP provides a size 28configurable circular buffer mapped in user space. This way reading packets just 29needs to wait for them, most of the time there is no need to issue a single 30system call. By using a shared buffer between the kernel and the user 31also has the benefit of minimizing packet copies. 32 33It's fine to use PACKET_MMAP to improve the performance of the capture process, 34but it isn't everything. At least, if you are capturing at high speeds (this 35is relative to the cpu speed), you should check if the device driver of your 36network interface card supports some sort of interrupt load mitigation or 37(even better) if it supports NAPI, also make sure it is enabled. 38 39-------------------------------------------------------------------------------- 40+ How to use CONFIG_PACKET_MMAP 41-------------------------------------------------------------------------------- 42 43From the user standpoint, you should use the higher level libpcap library, which 44is a de facto standard, portable across nearly all operating systems 45including Win32. 46 47Said that, at time of this writing, official libpcap 0.8.1 is out and doesn't include 48support for PACKET_MMAP, and also probably the libpcap included in your distribution. 49 50I'm aware of two implementations of PACKET_MMAP in libpcap: 51 52 http://pusa.uv.es/~ulisses/packet_mmap/ (by Simon Patarin, based on libpcap 0.6.2) 53 http://public.lanl.gov/cpw/ (by Phil Wood, based on lastest libpcap) 54 55The rest of this document is intended for people who want to understand 56the low level details or want to improve libpcap by including PACKET_MMAP 57support. 58 59-------------------------------------------------------------------------------- 60+ How to use CONFIG_PACKET_MMAP directly 61-------------------------------------------------------------------------------- 62 63From the system calls stand point, the use of PACKET_MMAP involves 64the following process: 65 66 67[setup] socket() -------> creation of the capture socket 68 setsockopt() ---> allocation of the circular buffer (ring) 69 mmap() ---------> mapping of the allocated buffer to the 70 user process 71 72[capture] poll() ---------> to wait for incoming packets 73 74[shutdown] close() --------> destruction of the capture socket and 75 deallocation of all associated 76 resources. 77 78 79socket creation and destruction is straight forward, and is done 80the same way with or without PACKET_MMAP: 81 82int fd; 83 84fd= socket(PF_PACKET, mode, htons(ETH_P_ALL)) 85 86where mode is SOCK_RAW for the raw interface were link level 87information can be captured or SOCK_DGRAM for the cooked 88interface where link level information capture is not 89supported and a link level pseudo-header is provided 90by the kernel. 91 92The destruction of the socket and all associated resources 93is done by a simple call to close(fd). 94 95Next I will describe PACKET_MMAP settings and it's constraints, 96also the mapping of the circular buffer in the user process and 97the use of this buffer. 98 99-------------------------------------------------------------------------------- 100+ PACKET_MMAP settings 101-------------------------------------------------------------------------------- 102 103 104To setup PACKET_MMAP from user level code is done with a call like 105 106 setsockopt(fd, SOL_PACKET, PACKET_RX_RING, (void *) &req, sizeof(req)) 107 108The most significant argument in the previous call is the req parameter, 109this parameter must to have the following structure: 110 111 struct tpacket_req 112 { 113 unsigned int tp_block_size; /* Minimal size of contiguous block */ 114 unsigned int tp_block_nr; /* Number of blocks */ 115 unsigned int tp_frame_size; /* Size of frame */ 116 unsigned int tp_frame_nr; /* Total number of frames */ 117 }; 118 119This structure is defined in /usr/include/linux/if_packet.h and establishes a 120circular buffer (ring) of unswappable memory mapped in the capture process. 121Being mapped in the capture process allows reading the captured frames and 122related meta-information like timestamps without requiring a system call. 123 124Captured frames are grouped in blocks. Each block is a physically contiguous 125region of memory and holds tp_block_size/tp_frame_size frames. The total number 126of blocks is tp_block_nr. Note that tp_frame_nr is a redundant parameter because 127 128 frames_per_block = tp_block_size/tp_frame_size 129 130indeed, packet_set_ring checks that the following condition is true 131 132 frames_per_block * tp_block_nr == tp_frame_nr 133 134 135Lets see an example, with the following values: 136 137 tp_block_size= 4096 138 tp_frame_size= 2048 139 tp_block_nr = 4 140 tp_frame_nr = 8 141 142we will get the following buffer structure: 143 144 block #1 block #2 145+---------+---------+ +---------+---------+ 146| frame 1 | frame 2 | | frame 3 | frame 4 | 147+---------+---------+ +---------+---------+ 148 149 block #3 block #4 150+---------+---------+ +---------+---------+ 151| frame 5 | frame 6 | | frame 7 | frame 8 | 152+---------+---------+ +---------+---------+ 153 154A frame can be of any size with the only condition it can fit in a block. A block 155can only hold an integer number of frames, or in other words, a frame cannot 156be spawned accross two blocks, so there are some details you have to take into 157account when choosing the frame_size. See "Mapping and use of the circular 158buffer (ring)". 159 160 161-------------------------------------------------------------------------------- 162+ PACKET_MMAP setting constraints 163-------------------------------------------------------------------------------- 164 165In kernel versions prior to 2.4.26 (for the 2.4 branch) and 2.6.5 (2.6 branch), 166the PACKET_MMAP buffer could hold only 32768 frames in a 32 bit architecture or 16716384 in a 64 bit architecture. For information on these kernel versions 168see http://pusa.uv.es/~ulisses/packet_mmap/packet_mmap.pre-2.4.26_2.6.5.txt 169 170 Block size limit 171------------------ 172 173As stated earlier, each block is a contiguous physical region of memory. These 174memory regions are allocated with calls to the __get_free_pages() function. As 175the name indicates, this function allocates pages of memory, and the second 176argument is "order" or a power of two number of pages, that is 177(for PAGE_SIZE == 4096) order=0 ==> 4096 bytes, order=1 ==> 8192 bytes, 178order=2 ==> 16384 bytes, etc. The maximum size of a 179region allocated by __get_free_pages is determined by the MAX_ORDER macro. More 180precisely the limit can be calculated as: 181 182 PAGE_SIZE << MAX_ORDER 183 184 In a i386 architecture PAGE_SIZE is 4096 bytes 185 In a 2.4/i386 kernel MAX_ORDER is 10 186 In a 2.6/i386 kernel MAX_ORDER is 11 187 188So get_free_pages can allocate as much as 4MB or 8MB in a 2.4/2.6 kernel 189respectively, with an i386 architecture. 190 191User space programs can include /usr/include/sys/user.h and 192/usr/include/linux/mmzone.h to get PAGE_SIZE MAX_ORDER declarations. 193 194The pagesize can also be determined dynamically with the getpagesize (2) 195system call. 196 197 198 Block number limit 199-------------------- 200 201To understand the constraints of PACKET_MMAP, we have to see the structure 202used to hold the pointers to each block. 203 204Currently, this structure is a dynamically allocated vector with kmalloc 205called pg_vec, its size limits the number of blocks that can be allocated. 206 207 +---+---+---+---+ 208 | x | x | x | x | 209 +---+---+---+---+ 210 | | | | 211 | | | v 212 | | v block #4 213 | v block #3 214 v block #2 215 block #1 216 217 218kmalloc allocates any number of bytes of physically contiguous memory from 219a pool of pre-determined sizes. This pool of memory is maintained by the slab 220allocator which is at the end the responsible for doing the allocation and 221hence which imposes the maximum memory that kmalloc can allocate. 222 223In a 2.4/2.6 kernel and the i386 architecture, the limit is 131072 bytes. The 224predetermined sizes that kmalloc uses can be checked in the "size-<bytes>" 225entries of /proc/slabinfo 226 227In a 32 bit architecture, pointers are 4 bytes long, so the total number of 228pointers to blocks is 229 230 131072/4 = 32768 blocks 231 232 233 PACKET_MMAP buffer size calculator 234------------------------------------ 235 236Definitions: 237 238<size-max> : is the maximum size of allocable with kmalloc (see /proc/slabinfo) 239<pointer size>: depends on the architecture -- sizeof(void *) 240<page size> : depends on the architecture -- PAGE_SIZE or getpagesize (2) 241<max-order> : is the value defined with MAX_ORDER 242<frame size> : it's an upper bound of frame's capture size (more on this later) 243 244from these definitions we will derive 245 246 <block number> = <size-max>/<pointer size> 247 <block size> = <pagesize> << <max-order> 248 249so, the max buffer size is 250 251 <block number> * <block size> 252 253and, the number of frames be 254 255 <block number> * <block size> / <frame size> 256 257Suppose the following parameters, which apply for 2.6 kernel and an 258i386 architecture: 259 260 <size-max> = 131072 bytes 261 <pointer size> = 4 bytes 262 <pagesize> = 4096 bytes 263 <max-order> = 11 264 265and a value for <frame size> of 2048 bytes. These parameters will yield 266 267 <block number> = 131072/4 = 32768 blocks 268 <block size> = 4096 << 11 = 8 MiB. 269 270and hence the buffer will have a 262144 MiB size. So it can hold 271262144 MiB / 2048 bytes = 134217728 frames 272 273 274Actually, this buffer size is not possible with an i386 architecture. 275Remember that the memory is allocated in kernel space, in the case of 276an i386 kernel's memory size is limited to 1GiB. 277 278All memory allocations are not freed until the socket is closed. The memory 279allocations are done with GFP_KERNEL priority, this basically means that 280the allocation can wait and swap other process' memory in order to allocate 281the necessary memory, so normally limits can be reached. 282 283 Other constraints 284------------------- 285 286If you check the source code you will see that what I draw here as a frame 287is not only the link level frame. At the beginning of each frame there is a 288header called struct tpacket_hdr used in PACKET_MMAP to hold link level's frame 289meta information like timestamp. So what we draw here a frame it's really 290the following (from include/linux/if_packet.h): 291 292/* 293 Frame structure: 294 295 - Start. Frame must be aligned to TPACKET_ALIGNMENT=16 296 - struct tpacket_hdr 297 - pad to TPACKET_ALIGNMENT=16 298 - struct sockaddr_ll 299 - Gap, chosen so that packet data (Start+tp_net) aligns to 300 TPACKET_ALIGNMENT=16 301 - Start+tp_mac: [ Optional MAC header ] 302 - Start+tp_net: Packet data, aligned to TPACKET_ALIGNMENT=16. 303 - Pad to align to TPACKET_ALIGNMENT=16 304 */ 305 306 307 The following are conditions that are checked in packet_set_ring 308 309 tp_block_size must be a multiple of PAGE_SIZE (1) 310 tp_frame_size must be greater than TPACKET_HDRLEN (obvious) 311 tp_frame_size must be a multiple of TPACKET_ALIGNMENT 312 tp_frame_nr must be exactly frames_per_block*tp_block_nr 313 314Note that tp_block_size should be chosen to be a power of two or there will 315be a waste of memory. 316 317-------------------------------------------------------------------------------- 318+ Mapping and use of the circular buffer (ring) 319-------------------------------------------------------------------------------- 320 321The mapping of the buffer in the user process is done with the conventional 322mmap function. Even the circular buffer is compound of several physically 323discontiguous blocks of memory, they are contiguous to the user space, hence 324just one call to mmap is needed: 325 326 mmap(0, size, PROT_READ|PROT_WRITE, MAP_SHARED, fd, 0); 327 328If tp_frame_size is a divisor of tp_block_size frames will be 329contiguosly spaced by tp_frame_size bytes. If not, each 330tp_block_size/tp_frame_size frames there will be a gap between 331the frames. This is because a frame cannot be spawn across two 332blocks. 333 334At the beginning of each frame there is an status field (see 335struct tpacket_hdr). If this field is 0 means that the frame is ready 336to be used for the kernel, If not, there is a frame the user can read 337and the following flags apply: 338 339 from include/linux/if_packet.h 340 341 #define TP_STATUS_COPY 2 342 #define TP_STATUS_LOSING 4 343 #define TP_STATUS_CSUMNOTREADY 8 344 345 346TP_STATUS_COPY : This flag indicates that the frame (and associated 347 meta information) has been truncated because it's 348 larger than tp_frame_size. This packet can be 349 read entirely with recvfrom(). 350 351 In order to make this work it must to be 352 enabled previously with setsockopt() and 353 the PACKET_COPY_THRESH option. 354 355 The number of frames than can be buffered to 356 be read with recvfrom is limited like a normal socket. 357 See the SO_RCVBUF option in the socket (7) man page. 358 359TP_STATUS_LOSING : indicates there were packet drops from last time 360 statistics where checked with getsockopt() and 361 the PACKET_STATISTICS option. 362 363TP_STATUS_CSUMNOTREADY: currently it's used for outgoing IP packets which 364 it's checksum will be done in hardware. So while 365 reading the packet we should not try to check the 366 checksum. 367 368for convenience there are also the following defines: 369 370 #define TP_STATUS_KERNEL 0 371 #define TP_STATUS_USER 1 372 373The kernel initializes all frames to TP_STATUS_KERNEL, when the kernel 374receives a packet it puts in the buffer and updates the status with 375at least the TP_STATUS_USER flag. Then the user can read the packet, 376once the packet is read the user must zero the status field, so the kernel 377can use again that frame buffer. 378 379The user can use poll (any other variant should apply too) to check if new 380packets are in the ring: 381 382 struct pollfd pfd; 383 384 pfd.fd = fd; 385 pfd.revents = 0; 386 pfd.events = POLLIN|POLLRDNORM|POLLERR; 387 388 if (status == TP_STATUS_KERNEL) 389 retval = poll(&pfd, 1, timeout); 390 391It doesn't incur in a race condition to first check the status value and 392then poll for frames. 393 394-------------------------------------------------------------------------------- 395+ THANKS 396-------------------------------------------------------------------------------- 397 398 Jesse Brandeburg, for fixing my grammathical/spelling errors 399