Root/Documentation/networking/packet_mmap.txt

1--------------------------------------------------------------------------------
2+ ABSTRACT
3--------------------------------------------------------------------------------
4
5This file documents the mmap() facility 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 needs
8raw access to network interface.
9
10You can find the latest version of this document at:
11    http://wiki.ipxwarzone.com/index.php5?title=Linux_packet_mmap
12
13Howto can be found at:
14    http://wiki.gnu-log.net (packet_mmap)
15
16Please send your comments to
17    Ulisses Alonso CamarĂ³ <uaca@i.hate.spam.alumni.uv.es>
18    Johann Baudy <johann.baudy@gnu-log.net>
19
20-------------------------------------------------------------------------------
21+ Why use PACKET_MMAP
22--------------------------------------------------------------------------------
23
24In Linux 2.4/2.6 if PACKET_MMAP is not enabled, the capture process is very
25inefficient. It uses very limited buffers and requires one system call
26to capture each packet, it requires two if you want to get packet's
27timestamp (like libpcap always does).
28
29In the other hand PACKET_MMAP is very efficient. PACKET_MMAP provides a size
30configurable circular buffer mapped in user space that can be used to either
31send or receive packets. This way reading packets just needs to wait for them,
32most of the time there is no need to issue a single system call. Concerning
33transmission, multiple packets can be sent through one system call to get the
34highest bandwidth.
35By using a shared buffer between the kernel and the user also has the benefit
36of minimizing packet copies.
37
38It's fine to use PACKET_MMAP to improve the performance of the capture and
39transmission process, but it isn't everything. At least, if you are capturing
40at high speeds (this is relative to the cpu speed), you should check if the
41device driver of your network interface card supports some sort of interrupt
42load mitigation or (even better) if it supports NAPI, also make sure it is
43enabled. For transmission, check the MTU (Maximum Transmission Unit) used and
44supported by devices of your network.
45
46--------------------------------------------------------------------------------
47+ How to use mmap() to improve capture process
48--------------------------------------------------------------------------------
49
50From the user standpoint, you should use the higher level libpcap library, which
51is a de facto standard, portable across nearly all operating systems
52including Win32.
53
54Said that, at time of this writing, official libpcap 0.8.1 is out and doesn't include
55support for PACKET_MMAP, and also probably the libpcap included in your distribution.
56
57I'm aware of two implementations of PACKET_MMAP in libpcap:
58
59    http://wiki.ipxwarzone.com/ (by Simon Patarin, based on libpcap 0.6.2)
60    http://public.lanl.gov/cpw/ (by Phil Wood, based on lastest libpcap)
61
62The rest of this document is intended for people who want to understand
63the low level details or want to improve libpcap by including PACKET_MMAP
64support.
65
66--------------------------------------------------------------------------------
67+ How to use mmap() directly to improve capture process
68--------------------------------------------------------------------------------
69
70From the system calls stand point, the use of PACKET_MMAP involves
71the following process:
72
73
74[setup] socket() -------> creation of the capture socket
75            setsockopt() ---> allocation of the circular buffer (ring)
76                              option: PACKET_RX_RING
77            mmap() ---------> mapping of the allocated buffer to the
78                              user process
79
80[capture] poll() ---------> to wait for incoming packets
81
82[shutdown] close() --------> destruction of the capture socket and
83                              deallocation of all associated
84                              resources.
85
86
87socket creation and destruction is straight forward, and is done
88the same way with or without PACKET_MMAP:
89
90int fd;
91
92fd= socket(PF_PACKET, mode, htons(ETH_P_ALL))
93
94where mode is SOCK_RAW for the raw interface were link level
95information can be captured or SOCK_DGRAM for the cooked
96interface where link level information capture is not
97supported and a link level pseudo-header is provided
98by the kernel.
99
100The destruction of the socket and all associated resources
101is done by a simple call to close(fd).
102
103Next I will describe PACKET_MMAP settings and its constraints,
104also the mapping of the circular buffer in the user process and
105the use of this buffer.
106
107--------------------------------------------------------------------------------
108+ How to use mmap() directly to improve transmission process
109--------------------------------------------------------------------------------
110Transmission process is similar to capture as shown below.
111
112[setup] socket() -------> creation of the transmission socket
113                 setsockopt() ---> allocation of the circular buffer (ring)
114                                   option: PACKET_TX_RING
115                 bind() ---------> bind transmission socket with a network interface
116                 mmap() ---------> mapping of the allocated buffer to the
117                                   user process
118
119[transmission] poll() ---------> wait for free packets (optional)
120                 send() ---------> send all packets that are set as ready in
121                                   the ring
122                                   The flag MSG_DONTWAIT can be used to return
123                                   before end of transfer.
124
125[shutdown] close() --------> destruction of the transmission socket and
126                              deallocation of all associated resources.
127
128Binding the socket to your network interface is mandatory (with zero copy) to
129know the header size of frames used in the circular buffer.
130
131As capture, each frame contains two parts:
132
133 --------------------
134| struct tpacket_hdr | Header. It contains the status of
135| | of this frame
136|--------------------|
137| data buffer |
138. . Data that will be sent over the network interface.
139. .
140 --------------------
141
142 bind() associates the socket to your network interface thanks to
143 sll_ifindex parameter of struct sockaddr_ll.
144
145 Initialization example:
146
147 struct sockaddr_ll my_addr;
148 struct ifreq s_ifr;
149 ...
150
151 strncpy (s_ifr.ifr_name, "eth0", sizeof(s_ifr.ifr_name));
152
153 /* get interface index of eth0 */
154 ioctl(this->socket, SIOCGIFINDEX, &s_ifr);
155
156 /* fill sockaddr_ll struct to prepare binding */
157 my_addr.sll_family = AF_PACKET;
158 my_addr.sll_protocol = ETH_P_ALL;
159 my_addr.sll_ifindex = s_ifr.ifr_ifindex;
160
161 /* bind socket to eth0 */
162 bind(this->socket, (struct sockaddr *)&my_addr, sizeof(struct sockaddr_ll));
163
164 A complete tutorial is available at: http://wiki.gnu-log.net/
165
166--------------------------------------------------------------------------------
167+ PACKET_MMAP settings
168--------------------------------------------------------------------------------
169
170
171To setup PACKET_MMAP from user level code is done with a call like
172
173 - Capture process
174     setsockopt(fd, SOL_PACKET, PACKET_RX_RING, (void *) &req, sizeof(req))
175 - Transmission process
176     setsockopt(fd, SOL_PACKET, PACKET_TX_RING, (void *) &req, sizeof(req))
177
178The most significant argument in the previous call is the req parameter,
179this parameter must to have the following structure:
180
181    struct tpacket_req
182    {
183        unsigned int tp_block_size; /* Minimal size of contiguous block */
184        unsigned int tp_block_nr; /* Number of blocks */
185        unsigned int tp_frame_size; /* Size of frame */
186        unsigned int tp_frame_nr; /* Total number of frames */
187    };
188
189This structure is defined in /usr/include/linux/if_packet.h and establishes a
190circular buffer (ring) of unswappable memory.
191Being mapped in the capture process allows reading the captured frames and
192related meta-information like timestamps without requiring a system call.
193
194Frames are grouped in blocks. Each block is a physically contiguous
195region of memory and holds tp_block_size/tp_frame_size frames. The total number
196of blocks is tp_block_nr. Note that tp_frame_nr is a redundant parameter because
197
198    frames_per_block = tp_block_size/tp_frame_size
199
200indeed, packet_set_ring checks that the following condition is true
201
202    frames_per_block * tp_block_nr == tp_frame_nr
203
204
205Lets see an example, with the following values:
206
207     tp_block_size= 4096
208     tp_frame_size= 2048
209     tp_block_nr = 4
210     tp_frame_nr = 8
211
212we will get the following buffer structure:
213
214        block #1 block #2
215+---------+---------+ +---------+---------+
216| frame 1 | frame 2 | | frame 3 | frame 4 |
217+---------+---------+ +---------+---------+
218
219        block #3 block #4
220+---------+---------+ +---------+---------+
221| frame 5 | frame 6 | | frame 7 | frame 8 |
222+---------+---------+ +---------+---------+
223
224A frame can be of any size with the only condition it can fit in a block. A block
225can only hold an integer number of frames, or in other words, a frame cannot
226be spawned accross two blocks, so there are some details you have to take into
227account when choosing the frame_size. See "Mapping and use of the circular
228buffer (ring)".
229
230
231--------------------------------------------------------------------------------
232+ PACKET_MMAP setting constraints
233--------------------------------------------------------------------------------
234
235In kernel versions prior to 2.4.26 (for the 2.4 branch) and 2.6.5 (2.6 branch),
236the PACKET_MMAP buffer could hold only 32768 frames in a 32 bit architecture or
23716384 in a 64 bit architecture. For information on these kernel versions
238see http://pusa.uv.es/~ulisses/packet_mmap/packet_mmap.pre-2.4.26_2.6.5.txt
239
240 Block size limit
241------------------
242
243As stated earlier, each block is a contiguous physical region of memory. These
244memory regions are allocated with calls to the __get_free_pages() function. As
245the name indicates, this function allocates pages of memory, and the second
246argument is "order" or a power of two number of pages, that is
247(for PAGE_SIZE == 4096) order=0 ==> 4096 bytes, order=1 ==> 8192 bytes,
248order=2 ==> 16384 bytes, etc. The maximum size of a
249region allocated by __get_free_pages is determined by the MAX_ORDER macro. More
250precisely the limit can be calculated as:
251
252   PAGE_SIZE << MAX_ORDER
253
254   In a i386 architecture PAGE_SIZE is 4096 bytes
255   In a 2.4/i386 kernel MAX_ORDER is 10
256   In a 2.6/i386 kernel MAX_ORDER is 11
257
258So get_free_pages can allocate as much as 4MB or 8MB in a 2.4/2.6 kernel
259respectively, with an i386 architecture.
260
261User space programs can include /usr/include/sys/user.h and
262/usr/include/linux/mmzone.h to get PAGE_SIZE MAX_ORDER declarations.
263
264The pagesize can also be determined dynamically with the getpagesize (2)
265system call.
266
267
268 Block number limit
269--------------------
270
271To understand the constraints of PACKET_MMAP, we have to see the structure
272used to hold the pointers to each block.
273
274Currently, this structure is a dynamically allocated vector with kmalloc
275called pg_vec, its size limits the number of blocks that can be allocated.
276
277    +---+---+---+---+
278    | x | x | x | x |
279    +---+---+---+---+
280      | | | |
281      | | | v
282      | | v block #4
283      | v block #3
284      v block #2
285     block #1
286
287
288kmalloc allocates any number of bytes of physically contiguous memory from
289a pool of pre-determined sizes. This pool of memory is maintained by the slab
290allocator which is at the end the responsible for doing the allocation and
291hence which imposes the maximum memory that kmalloc can allocate.
292
293In a 2.4/2.6 kernel and the i386 architecture, the limit is 131072 bytes. The
294predetermined sizes that kmalloc uses can be checked in the "size-<bytes>"
295entries of /proc/slabinfo
296
297In a 32 bit architecture, pointers are 4 bytes long, so the total number of
298pointers to blocks is
299
300     131072/4 = 32768 blocks
301
302
303 PACKET_MMAP buffer size calculator
304------------------------------------
305
306Definitions:
307
308<size-max> : is the maximum size of allocable with kmalloc (see /proc/slabinfo)
309<pointer size>: depends on the architecture -- sizeof(void *)
310<page size> : depends on the architecture -- PAGE_SIZE or getpagesize (2)
311<max-order> : is the value defined with MAX_ORDER
312<frame size> : it's an upper bound of frame's capture size (more on this later)
313
314from these definitions we will derive
315
316    <block number> = <size-max>/<pointer size>
317    <block size> = <pagesize> << <max-order>
318
319so, the max buffer size is
320
321    <block number> * <block size>
322
323and, the number of frames be
324
325    <block number> * <block size> / <frame size>
326
327Suppose the following parameters, which apply for 2.6 kernel and an
328i386 architecture:
329
330    <size-max> = 131072 bytes
331    <pointer size> = 4 bytes
332    <pagesize> = 4096 bytes
333    <max-order> = 11
334
335and a value for <frame size> of 2048 bytes. These parameters will yield
336
337    <block number> = 131072/4 = 32768 blocks
338    <block size> = 4096 << 11 = 8 MiB.
339
340and hence the buffer will have a 262144 MiB size. So it can hold
341262144 MiB / 2048 bytes = 134217728 frames
342
343
344Actually, this buffer size is not possible with an i386 architecture.
345Remember that the memory is allocated in kernel space, in the case of
346an i386 kernel's memory size is limited to 1GiB.
347
348All memory allocations are not freed until the socket is closed. The memory
349allocations are done with GFP_KERNEL priority, this basically means that
350the allocation can wait and swap other process' memory in order to allocate
351the necessary memory, so normally limits can be reached.
352
353 Other constraints
354-------------------
355
356If you check the source code you will see that what I draw here as a frame
357is not only the link level frame. At the beginning of each frame there is a
358header called struct tpacket_hdr used in PACKET_MMAP to hold link level's frame
359meta information like timestamp. So what we draw here a frame it's really
360the following (from include/linux/if_packet.h):
361
362/*
363   Frame structure:
364
365   - Start. Frame must be aligned to TPACKET_ALIGNMENT=16
366   - struct tpacket_hdr
367   - pad to TPACKET_ALIGNMENT=16
368   - struct sockaddr_ll
369   - Gap, chosen so that packet data (Start+tp_net) aligns to
370     TPACKET_ALIGNMENT=16
371   - Start+tp_mac: [ Optional MAC header ]
372   - Start+tp_net: Packet data, aligned to TPACKET_ALIGNMENT=16.
373   - Pad to align to TPACKET_ALIGNMENT=16
374 */
375           
376 
377 The following are conditions that are checked in packet_set_ring
378
379   tp_block_size must be a multiple of PAGE_SIZE (1)
380   tp_frame_size must be greater than TPACKET_HDRLEN (obvious)
381   tp_frame_size must be a multiple of TPACKET_ALIGNMENT
382   tp_frame_nr must be exactly frames_per_block*tp_block_nr
383
384Note that tp_block_size should be chosen to be a power of two or there will
385be a waste of memory.
386
387--------------------------------------------------------------------------------
388+ Mapping and use of the circular buffer (ring)
389--------------------------------------------------------------------------------
390
391The mapping of the buffer in the user process is done with the conventional
392mmap function. Even the circular buffer is compound of several physically
393discontiguous blocks of memory, they are contiguous to the user space, hence
394just one call to mmap is needed:
395
396    mmap(0, size, PROT_READ|PROT_WRITE, MAP_SHARED, fd, 0);
397
398If tp_frame_size is a divisor of tp_block_size frames will be
399contiguously spaced by tp_frame_size bytes. If not, each
400tp_block_size/tp_frame_size frames there will be a gap between
401the frames. This is because a frame cannot be spawn across two
402blocks.
403
404At the beginning of each frame there is an status field (see
405struct tpacket_hdr). If this field is 0 means that the frame is ready
406to be used for the kernel, If not, there is a frame the user can read
407and the following flags apply:
408
409+++ Capture process:
410     from include/linux/if_packet.h
411
412     #define TP_STATUS_COPY 2
413     #define TP_STATUS_LOSING 4
414     #define TP_STATUS_CSUMNOTREADY 8
415
416
417TP_STATUS_COPY : This flag indicates that the frame (and associated
418                        meta information) has been truncated because it's
419                        larger than tp_frame_size. This packet can be
420                        read entirely with recvfrom().
421                        
422                        In order to make this work it must to be
423                        enabled previously with setsockopt() and
424                        the PACKET_COPY_THRESH option.
425
426                        The number of frames than can be buffered to
427                        be read with recvfrom is limited like a normal socket.
428                        See the SO_RCVBUF option in the socket (7) man page.
429
430TP_STATUS_LOSING : indicates there were packet drops from last time
431                        statistics where checked with getsockopt() and
432                        the PACKET_STATISTICS option.
433
434TP_STATUS_CSUMNOTREADY: currently it's used for outgoing IP packets which
435                        its checksum will be done in hardware. So while
436                        reading the packet we should not try to check the
437                        checksum.
438
439for convenience there are also the following defines:
440
441     #define TP_STATUS_KERNEL 0
442     #define TP_STATUS_USER 1
443
444The kernel initializes all frames to TP_STATUS_KERNEL, when the kernel
445receives a packet it puts in the buffer and updates the status with
446at least the TP_STATUS_USER flag. Then the user can read the packet,
447once the packet is read the user must zero the status field, so the kernel
448can use again that frame buffer.
449
450The user can use poll (any other variant should apply too) to check if new
451packets are in the ring:
452
453    struct pollfd pfd;
454
455    pfd.fd = fd;
456    pfd.revents = 0;
457    pfd.events = POLLIN|POLLRDNORM|POLLERR;
458
459    if (status == TP_STATUS_KERNEL)
460        retval = poll(&pfd, 1, timeout);
461
462It doesn't incur in a race condition to first check the status value and
463then poll for frames.
464
465
466++ Transmission process
467Those defines are also used for transmission:
468
469     #define TP_STATUS_AVAILABLE 0 // Frame is available
470     #define TP_STATUS_SEND_REQUEST 1 // Frame will be sent on next send()
471     #define TP_STATUS_SENDING 2 // Frame is currently in transmission
472     #define TP_STATUS_WRONG_FORMAT 4 // Frame format is not correct
473
474First, the kernel initializes all frames to TP_STATUS_AVAILABLE. To send a
475packet, the user fills a data buffer of an available frame, sets tp_len to
476current data buffer size and sets its status field to TP_STATUS_SEND_REQUEST.
477This can be done on multiple frames. Once the user is ready to transmit, it
478calls send(). Then all buffers with status equal to TP_STATUS_SEND_REQUEST are
479forwarded to the network device. The kernel updates each status of sent
480frames with TP_STATUS_SENDING until the end of transfer.
481At the end of each transfer, buffer status returns to TP_STATUS_AVAILABLE.
482
483    header->tp_len = in_i_size;
484    header->tp_status = TP_STATUS_SEND_REQUEST;
485    retval = send(this->socket, NULL, 0, 0);
486
487The user can also use poll() to check if a buffer is available:
488(status == TP_STATUS_SENDING)
489
490    struct pollfd pfd;
491    pfd.fd = fd;
492    pfd.revents = 0;
493    pfd.events = POLLOUT;
494    retval = poll(&pfd, 1, timeout);
495
496-------------------------------------------------------------------------------
497+ PACKET_TIMESTAMP
498-------------------------------------------------------------------------------
499
500The PACKET_TIMESTAMP setting determines the source of the timestamp in
501the packet meta information. If your NIC is capable of timestamping
502packets in hardware, you can request those hardware timestamps to used.
503Note: you may need to enable the generation of hardware timestamps with
504SIOCSHWTSTAMP.
505
506PACKET_TIMESTAMP accepts the same integer bit field as
507SO_TIMESTAMPING. However, only the SOF_TIMESTAMPING_SYS_HARDWARE
508and SOF_TIMESTAMPING_RAW_HARDWARE values are recognized by
509PACKET_TIMESTAMP. SOF_TIMESTAMPING_SYS_HARDWARE takes precedence over
510SOF_TIMESTAMPING_RAW_HARDWARE if both bits are set.
511
512    int req = 0;
513    req |= SOF_TIMESTAMPING_SYS_HARDWARE;
514    setsockopt(fd, SOL_PACKET, PACKET_TIMESTAMP, (void *) &req, sizeof(req))
515
516If PACKET_TIMESTAMP is not set, a software timestamp generated inside
517the networking stack is used (the behavior before this setting was added).
518
519See include/linux/net_tstamp.h and Documentation/networking/timestamping
520for more information on hardware timestamps.
521
522--------------------------------------------------------------------------------
523+ THANKS
524--------------------------------------------------------------------------------
525   
526   Jesse Brandeburg, for fixing my grammathical/spelling errors
527
528

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