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Linus Torvalds1da177e2005-04-16 15:20:36 -07001--------------------------------------------------------------------------------
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 CamarĂ³ <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, wich
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() ---------> maping 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 maping 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 spawn accross two blocks so there are some datails you have to take into
157account when choosing the frame_size. See "Maping 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 phisically contiguous memory from
219a pool of pre-determined sizes. This pool of memory is mantained by the slab
220allocator wich is at the end the responsible for doing the allocation and
221hence wich 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
257Suposse the following parameters, wich 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 byteas. 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 nececessary 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 begining 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) alignes 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 choosed to be a power of two or there will
315be a waste of memory.
316
317--------------------------------------------------------------------------------
318+ Maping and use of the circular buffer (ring)
319--------------------------------------------------------------------------------
320
321The maping 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 wich
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