aboutsummaryrefslogtreecommitdiff
path: root/Documentation/networking/packet_mmap.txt
blob: 8e48e3b142275ff1500df754301d39c7f1639d71 (plain)
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
61
62
63
64
65
66
67
68
69
70
71
72
73
74
75
76
77
78
79
80
81
82
83
84
85
86
87
88
89
90
91
92
93
94
95
96
97
98
99
100
101
102
103
104
105
106
107
108
109
110
111
112
113
114
115
116
117
118
119
120
121
122
123
124
125
126
127
128
129
130
131
132
133
134
135
136
137
138
139
140
141
142
143
144
145
146
147
148
149
150
151
152
153
154
155
156
157
158
159
160
161
162
163
164
165
166
167
168
169
170
171
172
173
174
175
176
177
178
179
180
181
182
183
184
185
186
187
188
189
190
191
192
193
194
195
196
197
198
199
200
201
202
203
204
205
206
207
208
209
210
211
212
213
214
215
216
217
218
219
220
221
222
223
224
225
226
227
228
229
230
231
232
233
234
235
236
237
238
239
240
241
242
243
244
245
246
247
248
249
250
251
252
253
254
255
256
257
258
259
260
261
262
263
264
265
266
267
268
269
270
271
272
273
274
275
276
277
278
279
280
281
282
283
284
285
286
287
288
289
290
291
292
293
294
295
296
297
298
299
300
301
302
303
304
305
306
307
308
309
310
311
312
313
314
315
316
317
318
319
320
321
322
323
324
325
326
327
328
329
330
331
332
333
334
335
336
337
338
339
340
341
342
343
344
345
346
347
348
349
350
351
352
353
354
355
356
357
358
359
360
361
362
363
364
365
366
367
368
369
370
371
372
373
374
375
376
377
378
379
380
381
382
383
384
385
386
387
388
389
390
391
392
393
394
395
396
397
398
399
400
401
402
403
404
405
406
407
408
409
410
411
412
413
414
415
416
417
418
419
420
421
422
423
424
425
426
427
428
429
430
431
432
433
434
435
436
437
438
439
440
441
442
443
444
445
446
447
448
449
450
451
452
453
454
455
456
457
458
459
460
461
462
463
464
465
466
467
468
469
470
471
472
473
474
475
476
477
478
479
480
481
482
483
484
485
486
487
488
489
490
491
492
493
494
495
496
497
498
499
500
501
502
503
504
505
506
507
508
509
510
511
512
513
514
515
516
517
518
519
520
521
522
523
524
525
526
527
528
529
530
531
532
533
534
535
536
537
538
539
540
541
542
543
544
545
546
547
548
549
550
551
552
553
554
555
556
557
558
559
560
561
562
563
564
565
566
567
568
569
570
571
572
573
574
575
576
577
578
579
580
581
582
583
584
585
586
587
588
589
590
591
592
593
594
595
596
597
598
599
600
601
602
603
604
605
606
607
608
609
610
611
612
613
614
615
616
617
618
619
620
621
622
623
624
625
626
627
628
629
630
631
632
633
634
635
636
637
638
639
640
641
642
643
644
645
646
647
648
649
650
651
652
653
654
655
656
657
658
659
660
661
662
663
664
665
666
667
668
669
670
671
672
673
674
675
676
677
678
679
680
681
682
683
684
685
686
687
688
689
690
691
692
693
694
695
696
697
698
699
700
701
702
703
704
705
706
707
708
709
710
711
712
713
714
715
716
717
718
719
720
721
722
723
724
725
726
727
728
729
730
731
732
733
734
735
736
737
738
739
740
741
742
743
744
745
746
747
748
749
750
751
752
753
754
755
756
757
758
759
760
761
762
763
764
765
766
767
768
769
770
771
772
773
774
775
776
777
778
779
780
781
782
783
784
785
786
787
788
789
790
791
792
793
794
795
796
797
798
799
800
801
802
803
804
805
806
807
808
809
810
811
812
813
814
815
816
817
818
819
820
821
822
823
824
825
826
827
828
829
830
831
832
833
834
835
836
837
838
839
840
841
842
843
844
845
846
847
848
849
850
851
852
853
854
855
856
857
858
859
860
861
862
863
864
865
866
867
868
869
870
871
872
873
874
875
876
877
878
879
880
881
882
883
884
885
886
887
888
889
890
891
892
893
894
895
896
897
898
899
900
901
902
903
904
905
906
907
908
909
910
911
912
913
914
915
916
917
918
919
920
921
922
923
924
925
926
927
928
929
930
931
932
933
934
935
936
937
938
939
940
941
942
943
944
945
946
947
948
949
950
951
952
953
954
955
956
957
958
959
960
961
962
963
964
965
966
967
968
969
970
971
972
973
974
975
976
977
978
979
980
981
982
983
984
985
986
987
988
989
990
991
992
993
994
995
996
997
998
999
1000
1001
1002
1003
1004
1005
1006
1007
1008
1009
1010
1011
1012
1013
1014
1015
1016
1017
1018
1019
1020
1021
1022
--------------------------------------------------------------------------------
+ ABSTRACT
--------------------------------------------------------------------------------

This file documents the mmap() facility available with the PACKET
socket interface on 2.4/2.6/3.x kernels. This type of sockets is used for
i) capture network traffic with utilities like tcpdump, ii) transmit network
traffic, or any other that needs raw access to network interface.

You can find the latest version of this document at:
    http://wiki.ipxwarzone.com/index.php5?title=Linux_packet_mmap

Howto can be found at:
    http://wiki.gnu-log.net (packet_mmap)

Please send your comments to
    Ulisses Alonso Camaró <uaca@i.hate.spam.alumni.uv.es>
    Johann Baudy <johann.baudy@gnu-log.net>

-------------------------------------------------------------------------------
+ Why use PACKET_MMAP
--------------------------------------------------------------------------------

In Linux 2.4/2.6/3.x 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 that can be used to either
send or receive packets. This way reading packets just needs to wait for them,
most of the time there is no need to issue a single system call. Concerning
transmission, multiple packets can be sent through one system call to get the
highest bandwidth. 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 and
transmission 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. For transmission, check the MTU (Maximum Transmission Unit) used and
supported by devices of your network. CPU IRQ pinning of your network interface
card can also be an advantage.

--------------------------------------------------------------------------------
+ How to use mmap() to improve capture process
--------------------------------------------------------------------------------

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://wiki.ipxwarzone.com/		     (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 mmap() directly to improve capture process
--------------------------------------------------------------------------------

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)
                              option: PACKET_RX_RING
            mmap() ---------> mapping 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 = 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 its constraints,
also the mapping of the circular buffer in the user process and 
the use of this buffer.

--------------------------------------------------------------------------------
+ How to use mmap() directly to improve transmission process
--------------------------------------------------------------------------------
Transmission process is similar to capture as shown below.

[setup]          socket() -------> creation of the transmission socket
                 setsockopt() ---> allocation of the circular buffer (ring)
                                   option: PACKET_TX_RING
                 bind() ---------> bind transmission socket with a network interface
                 mmap() ---------> mapping of the allocated buffer to the
                                   user process

[transmission]   poll() ---------> wait for free packets (optional)
                 send() ---------> send all packets that are set as ready in
                                   the ring
                                   The flag MSG_DONTWAIT can be used to return
                                   before end of transfer.

[shutdown]  close() --------> destruction of the transmission socket and
                              deallocation of all associated resources.

Socket creation and destruction is also straight forward, and is done
the same way as in capturing described in the previous paragraph:

 int fd = socket(PF_PACKET, mode, 0);

The protocol can optionally be 0 in case we only want to transmit
via this socket, which avoids an expensive call to packet_rcv().
In this case, you also need to bind(2) the TX_RING with sll_protocol = 0
set. Otherwise, htons(ETH_P_ALL) or any other protocol, for example.

Binding the socket to your network interface is mandatory (with zero copy) to
know the header size of frames used in the circular buffer.

As capture, each frame contains two parts:

 --------------------
| struct tpacket_hdr | Header. It contains the status of
|                    | of this frame
|--------------------|
| data buffer        |
.                    .  Data that will be sent over the network interface.
.                    .
 --------------------

 bind() associates the socket to your network interface thanks to
 sll_ifindex parameter of struct sockaddr_ll.

 Initialization example:

 struct sockaddr_ll my_addr;
 struct ifreq s_ifr;
 ...

 strncpy (s_ifr.ifr_name, "eth0", sizeof(s_ifr.ifr_name));

 /* get interface index of eth0 */
 ioctl(this->socket, SIOCGIFINDEX, &s_ifr);

 /* fill sockaddr_ll struct to prepare binding */
 my_addr.sll_family = AF_PACKET;
 my_addr.sll_protocol = htons(ETH_P_ALL);
 my_addr.sll_ifindex =  s_ifr.ifr_ifindex;

 /* bind socket to eth0 */
 bind(this->socket, (struct sockaddr *)&my_addr, sizeof(struct sockaddr_ll));

 A complete tutorial is available at: http://wiki.gnu-log.net/

By default, the user should put data at :
 frame base + TPACKET_HDRLEN - sizeof(struct sockaddr_ll)

So, whatever you choose for the socket mode (SOCK_DGRAM or SOCK_RAW),
the beginning of the user data will be at :
 frame base + TPACKET_ALIGN(sizeof(struct tpacket_hdr))

If you wish to put user data at a custom offset from the beginning of
the frame (for payload alignment with SOCK_RAW mode for instance) you
can set tp_net (with SOCK_DGRAM) or tp_mac (with SOCK_RAW). In order
to make this work it must be enabled previously with setsockopt()
and the PACKET_TX_HAS_OFF option.

--------------------------------------------------------------------------------
+ PACKET_MMAP settings
--------------------------------------------------------------------------------

To setup PACKET_MMAP from user level code is done with a call like

 - Capture process
     setsockopt(fd, SOL_PACKET, PACKET_RX_RING, (void *) &req, sizeof(req))
 - Transmission process
     setsockopt(fd, SOL_PACKET, PACKET_TX_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.
Being mapped in the capture process allows reading the captured frames and 
related meta-information like timestamps without requiring a system call.

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 spawned across two blocks, so there are some details you have to take into 
account when choosing the frame_size. See "Mapping 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 physically contiguous memory from 
a pool of pre-determined sizes. This pool of memory is maintained 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 bytes. 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 necessary 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 beginning 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) aligns 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 chosen to be a power of two or there will
be a waste of memory.

--------------------------------------------------------------------------------
+ Mapping and use of the circular buffer (ring)
--------------------------------------------------------------------------------

The mapping 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 
contiguously 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:

+++ Capture process:
     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 
                        its 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.

++ Transmission process
Those defines are also used for transmission:

     #define TP_STATUS_AVAILABLE        0 // Frame is available
     #define TP_STATUS_SEND_REQUEST     1 // Frame will be sent on next send()
     #define TP_STATUS_SENDING          2 // Frame is currently in transmission
     #define TP_STATUS_WRONG_FORMAT     4 // Frame format is not correct

First, the kernel initializes all frames to TP_STATUS_AVAILABLE. To send a
packet, the user fills a data buffer of an available frame, sets tp_len to
current data buffer size and sets its status field to TP_STATUS_SEND_REQUEST.
This can be done on multiple frames. Once the user is ready to transmit, it
calls send(). Then all buffers with status equal to TP_STATUS_SEND_REQUEST are
forwarded to the network device. The kernel updates each status of sent
frames with TP_STATUS_SENDING until the end of transfer.
At the end of each transfer, buffer status returns to TP_STATUS_AVAILABLE.

    header->tp_len = in_i_size;
    header->tp_status = TP_STATUS_SEND_REQUEST;
    retval = send(this->socket, NULL, 0, 0);

The user can also use poll() to check if a buffer is available:
(status == TP_STATUS_SENDING)

    struct pollfd pfd;
    pfd.fd = fd;
    pfd.revents = 0;
    pfd.events = POLLOUT;
    retval = poll(&pfd, 1, timeout);

-------------------------------------------------------------------------------
+ What TPACKET versions are available and when to use them?
-------------------------------------------------------------------------------

 int val = tpacket_version;
 setsockopt(fd, SOL_PACKET, PACKET_VERSION, &val, sizeof(val));
 getsockopt(fd, SOL_PACKET, PACKET_VERSION, &val, sizeof(val));

where 'tpacket_version' can be TPACKET_V1 (default), TPACKET_V2, TPACKET_V3.

TPACKET_V1:
	- Default if not otherwise specified by setsockopt(2)
	- RX_RING, TX_RING available
	- VLAN metadata information available for packets
	  (TP_STATUS_VLAN_VALID)

TPACKET_V1 --> TPACKET_V2:
	- Made 64 bit clean due to unsigned long usage in TPACKET_V1
	  structures, thus this also works on 64 bit kernel with 32 bit
	  userspace and the like
	- Timestamp resolution in nanoseconds instead of microseconds
	- RX_RING, TX_RING available
	- How to switch to TPACKET_V2:
		1. Replace struct tpacket_hdr by struct tpacket2_hdr
		2. Query header len and save
		3. Set protocol version to 2, set up ring as usual
		4. For getting the sockaddr_ll,
		   use (void *)hdr + TPACKET_ALIGN(hdrlen) instead of
		   (void *)hdr + TPACKET_ALIGN(sizeof(struct tpacket_hdr))

TPACKET_V2 --> TPACKET_V3:
	- Flexible buffer implementation:
		1. Blocks can be configured with non-static frame-size
		2. Read/poll is at a block-level (as opposed to packet-level)
		3. Added poll timeout to avoid indefinite user-space wait
		   on idle links
		4. Added user-configurable knobs:
			4.1 block::timeout
			4.2 tpkt_hdr::sk_rxhash
	- RX Hash data available in user space
	- Currently only RX_RING available

-------------------------------------------------------------------------------
+ AF_PACKET fanout mode
-------------------------------------------------------------------------------

In the AF_PACKET fanout mode, packet reception can be load balanced among
processes. This also works in combination with mmap(2) on packet sockets.

Currently implemented fanout policies are:

  - PACKET_FANOUT_HASH: schedule to socket by skb's rxhash
  - PACKET_FANOUT_LB: schedule to socket by round-robin
  - PACKET_FANOUT_CPU: schedule to socket by CPU packet arrives on
  - PACKET_FANOUT_RND: schedule to socket by random selection
  - PACKET_FANOUT_ROLLOVER: if one socket is full, rollover to another

Minimal example code by David S. Miller (try things like "./test eth0 hash",
"./test eth0 lb", etc.):

#include <stddef.h>
#include <stdlib.h>
#include <stdio.h>
#include <string.h>

#include <sys/types.h>
#include <sys/wait.h>
#include <sys/socket.h>
#include <sys/ioctl.h>

#include <unistd.h>

#include <linux/if_ether.h>
#include <linux/if_packet.h>

#include <net/if.h>

static const char *device_name;
static int fanout_type;
static int fanout_id;

#ifndef PACKET_FANOUT
# define PACKET_FANOUT			18
# define PACKET_FANOUT_HASH		0
# define PACKET_FANOUT_LB		1
#endif

static int setup_socket(void)
{
	int err, fd = socket(AF_PACKET, SOCK_RAW, htons(ETH_P_IP));
	struct sockaddr_ll ll;
	struct ifreq ifr;
	int fanout_arg;

	if (fd < 0) {
		perror("socket");
		return EXIT_FAILURE;
	}

	memset(&ifr, 0, sizeof(ifr));
	strcpy(ifr.ifr_name, device_name);
	err = ioctl(fd, SIOCGIFINDEX, &ifr);
	if (err < 0) {
		perror("SIOCGIFINDEX");
		return EXIT_FAILURE;
	}

	memset(&ll, 0, sizeof(ll));
	ll.sll_family = AF_PACKET;
	ll.sll_ifindex = ifr.ifr_ifindex;
	err = bind(fd, (struct sockaddr *) &ll, sizeof(ll));
	if (err < 0) {
		perror("bind");
		return EXIT_FAILURE;
	}

	fanout_arg = (fanout_id | (fanout_type << 16));
	err = setsockopt(fd, SOL_PACKET, PACKET_FANOUT,
			 &fanout_arg, sizeof(fanout_arg));
	if (err) {
		perror("setsockopt");
		return EXIT_FAILURE;
	}

	return fd;
}

static void fanout_thread(void)
{
	int fd = setup_socket();
	int limit = 10000;

	if (fd < 0)
		exit(fd);

	while (limit-- > 0) {
		char buf[1600];
		int err;

		err = read(fd, buf, sizeof(buf));
		if (err < 0) {
			perror("read");
			exit(EXIT_FAILURE);
		}
		if ((limit % 10) == 0)
			fprintf(stdout, "(%d) \n", getpid());
	}

	fprintf(stdout, "%d: Received 10000 packets\n", getpid());

	close(fd);
	exit(0);
}

int main(int argc, char **argp)
{
	int fd, err;
	int i;

	if (argc != 3) {
		fprintf(stderr, "Usage: %s INTERFACE {hash|lb}\n", argp[0]);
		return EXIT_FAILURE;
	}

	if (!strcmp(argp[2], "hash"))
		fanout_type = PACKET_FANOUT_HASH;
	else if (!strcmp(argp[2], "lb"))
		fanout_type = PACKET_FANOUT_LB;
	else {
		fprintf(stderr, "Unknown fanout type [%s]\n", argp[2]);
		exit(EXIT_FAILURE);
	}

	device_name = argp[1];
	fanout_id = getpid() & 0xffff;

	for (i = 0; i < 4; i++) {
		pid_t pid = fork();

		switch (pid) {
		case 0:
			fanout_thread();

		case -1:
			perror("fork");
			exit(EXIT_FAILURE);
		}
	}

	for (i = 0; i < 4; i++) {
		int status;

		wait(&status);
	}

	return 0;
}

-------------------------------------------------------------------------------
+ AF_PACKET TPACKET_V3 example
-------------------------------------------------------------------------------

AF_PACKET's TPACKET_V3 ring buffer can be configured to use non-static frame
sizes by doing it's own memory management. It is based on blocks where polling
works on a per block basis instead of per ring as in TPACKET_V2 and predecessor.

It is said that TPACKET_V3 brings the following benefits:
 *) ~15 - 20% reduction in CPU-usage
 *) ~20% increase in packet capture rate
 *) ~2x increase in packet density
 *) Port aggregation analysis
 *) Non static frame size to capture entire packet payload

So it seems to be a good candidate to be used with packet fanout.

Minimal example code by Daniel Borkmann based on Chetan Loke's lolpcap (compile
it with gcc -Wall -O2 blob.c, and try things like "./a.out eth0", etc.):

/* Written from scratch, but kernel-to-user space API usage
 * dissected from lolpcap:
 *  Copyright 2011, Chetan Loke <loke.chetan@gmail.com>
 *  License: GPL, version 2.0
 */

#include <stdio.h>
#include <stdlib.h>
#include <stdint.h>
#include <string.h>
#include <assert.h>
#include <net/if.h>
#include <arpa/inet.h>
#include <netdb.h>
#include <poll.h>
#include <unistd.h>
#include <signal.h>
#include <inttypes.h>
#include <sys/socket.h>
#include <sys/mman.h>
#include <linux/if_packet.h>
#include <linux/if_ether.h>
#include <linux/ip.h>

#ifndef likely
# define likely(x)		__builtin_expect(!!(x), 1)
#endif
#ifndef unlikely
# define unlikely(x)		__builtin_expect(!!(x), 0)
#endif

struct block_desc {
	uint32_t version;
	uint32_t offset_to_priv;
	struct tpacket_hdr_v1 h1;
};

struct ring {
	struct iovec *rd;
	uint8_t *map;
	struct tpacket_req3 req;
};

static unsigned long packets_total = 0, bytes_total = 0;
static sig_atomic_t sigint = 0;

static void sighandler(int num)
{
	sigint = 1;
}

static int setup_socket(struct ring *ring, char *netdev)
{
	int err, i, fd, v = TPACKET_V3;
	struct sockaddr_ll ll;
	unsigned int blocksiz = 1 << 22, framesiz = 1 << 11;
	unsigned int blocknum = 64;

	fd = socket(AF_PACKET, SOCK_RAW, htons(ETH_P_ALL));
	if (fd < 0) {
		perror("socket");
		exit(1);
	}

	err = setsockopt(fd, SOL_PACKET, PACKET_VERSION, &v, sizeof(v));
	if (err < 0) {
		perror("setsockopt");
		exit(1);
	}

	memset(&ring->req, 0, sizeof(ring->req));
	ring->req.tp_block_size = blocksiz;
	ring->req.tp_frame_size = framesiz;
	ring->req.tp_block_nr = blocknum;
	ring->req.tp_frame_nr = (blocksiz * blocknum) / framesiz;
	ring->req.tp_retire_blk_tov = 60;
	ring->req.tp_feature_req_word = TP_FT_REQ_FILL_RXHASH;

	err = setsockopt(fd, SOL_PACKET, PACKET_RX_RING, &ring->req,
			 sizeof(ring->req));
	if (err < 0) {
		perror("setsockopt");
		exit(1);
	}

	ring->map = mmap(NULL, ring->req.tp_block_size * ring->req.tp_block_nr,
			 PROT_READ | PROT_WRITE, MAP_SHARED | MAP_LOCKED, fd, 0);
	if (ring->map == MAP_FAILED) {
		perror("mmap");
		exit(1);
	}

	ring->rd = malloc(ring->req.tp_block_nr * sizeof(*ring->rd));
	assert(ring->rd);
	for (i = 0; i < ring->req.tp_block_nr; ++i) {
		ring->rd[i].iov_base = ring->map + (i * ring->req.tp_block_size);
		ring->rd[i].iov_len = ring->req.tp_block_size;
	}

	memset(&ll, 0, sizeof(ll));
	ll.sll_family = PF_PACKET;
	ll.sll_protocol = htons(ETH_P_ALL);
	ll.sll_ifindex = if_nametoindex(netdev);
	ll.sll_hatype = 0;
	ll.sll_pkttype = 0;
	ll.sll_halen = 0;

	err = bind(fd, (struct sockaddr *) &ll, sizeof(ll));
	if (err < 0) {
		perror("bind");
		exit(1);
	}

	return fd;
}

static void display(struct tpacket3_hdr *ppd)
{
	struct ethhdr *eth = (struct ethhdr *) ((uint8_t *) ppd + ppd->tp_mac);
	struct iphdr *ip = (struct iphdr *) ((uint8_t *) eth + ETH_HLEN);

	if (eth->h_proto == htons(ETH_P_IP)) {
		struct sockaddr_in ss, sd;
		char sbuff[NI_MAXHOST], dbuff[NI_MAXHOST];

		memset(&ss, 0, sizeof(ss));
		ss.sin_family = PF_INET;
		ss.sin_addr.s_addr = ip->saddr;
		getnameinfo((struct sockaddr *) &ss, sizeof(ss),
			    sbuff, sizeof(sbuff), NULL, 0, NI_NUMERICHOST);

		memset(&sd, 0, sizeof(sd));
		sd.sin_family = PF_INET;
		sd.sin_addr.s_addr = ip->daddr;
		getnameinfo((struct sockaddr *) &sd, sizeof(sd),
			    dbuff, sizeof(dbuff), NULL, 0, NI_NUMERICHOST);

		printf("%s -> %s, ", sbuff, dbuff);
	}

	printf("rxhash: 0x%x\n", ppd->hv1.tp_rxhash);
}

static void walk_block(struct block_desc *pbd, const int block_num)
{
	int num_pkts = pbd->h1.num_pkts, i;
	unsigned long bytes = 0;
	struct tpacket3_hdr *ppd;

	ppd = (struct tpacket3_hdr *) ((uint8_t *) pbd +
				       pbd->h1.offset_to_first_pkt);
	for (i = 0; i < num_pkts; ++i) {
		bytes += ppd->tp_snaplen;
		display(ppd);

		ppd = (struct tpacket3_hdr *) ((uint8_t *) ppd +
					       ppd->tp_next_offset);
	}

	packets_total += num_pkts;
	bytes_total += bytes;
}

static void flush_block(struct block_desc *pbd)
{
	pbd->h1.block_status = TP_STATUS_KERNEL;
}

static void teardown_socket(struct ring *ring, int fd)
{
	munmap(ring->map, ring->req.tp_block_size * ring->req.tp_block_nr);
	free(ring->rd);
	close(fd);
}

int main(int argc, char **argp)
{
	int fd, err;
	socklen_t len;
	struct ring ring;
	struct pollfd pfd;
	unsigned int block_num = 0, blocks = 64;
	struct block_desc *pbd;
	struct tpacket_stats_v3 stats;

	if (argc != 2) {
		fprintf(stderr, "Usage: %s INTERFACE\n", argp[0]);
		return EXIT_FAILURE;
	}

	signal(SIGINT, sighandler);

	memset(&ring, 0, sizeof(ring));
	fd = setup_socket(&ring, argp[argc - 1]);
	assert(fd > 0);

	memset(&pfd, 0, sizeof(pfd));
	pfd.fd = fd;
	pfd.events = POLLIN | POLLERR;
	pfd.revents = 0;

	while (likely(!sigint)) {
		pbd = (struct block_desc *) ring.rd[block_num].iov_base;

		if ((pbd->h1.block_status & TP_STATUS_USER) == 0) {
			poll(&pfd, 1, -1);
			continue;
		}

		walk_block(pbd, block_num);
		flush_block(pbd);
		block_num = (block_num + 1) % blocks;
	}

	len = sizeof(stats);
	err = getsockopt(fd, SOL_PACKET, PACKET_STATISTICS, &stats, &len);
	if (err < 0) {
		perror("getsockopt");
		exit(1);
	}

	fflush(stdout);
	printf("\nReceived %u packets, %lu bytes, %u dropped, freeze_q_cnt: %u\n",
	       stats.tp_packets, bytes_total, stats.tp_drops,
	       stats.tp_freeze_q_cnt);

	teardown_socket(&ring, fd);
	return 0;
}

-------------------------------------------------------------------------------
+ PACKET_TIMESTAMP
-------------------------------------------------------------------------------

The PACKET_TIMESTAMP setting determines the source of the timestamp in
the packet meta information for mmap(2)ed RX_RING and TX_RINGs.  If your
NIC is capable of timestamping packets in hardware, you can request those
hardware timestamps to be used. Note: you may need to enable the generation
of hardware timestamps with SIOCSHWTSTAMP (see related information from
Documentation/networking/timestamping.txt).

PACKET_TIMESTAMP accepts the same integer bit field as
SO_TIMESTAMPING.  However, only the SOF_TIMESTAMPING_SYS_HARDWARE
and SOF_TIMESTAMPING_RAW_HARDWARE values are recognized by
PACKET_TIMESTAMP.  SOF_TIMESTAMPING_SYS_HARDWARE takes precedence over
SOF_TIMESTAMPING_RAW_HARDWARE if both bits are set.

    int req = 0;
    req |= SOF_TIMESTAMPING_SYS_HARDWARE;
    setsockopt(fd, SOL_PACKET, PACKET_TIMESTAMP, (void *) &req, sizeof(req))

For the mmap(2)ed ring buffers, such timestamps are stored in the
tpacket{,2,3}_hdr structure's tp_sec and tp_{n,u}sec members. To determine
what kind of timestamp has been reported, the tp_status field is binary |'ed
with the following possible bits ...

    TP_STATUS_TS_SYS_HARDWARE
    TP_STATUS_TS_RAW_HARDWARE
    TP_STATUS_TS_SOFTWARE

... that are equivalent to its SOF_TIMESTAMPING_* counterparts. For the
RX_RING, if none of those 3 are set (i.e. PACKET_TIMESTAMP is not set),
then this means that a software fallback was invoked *within* PF_PACKET's
processing code (less precise).

Getting timestamps for the TX_RING works as follows: i) fill the ring frames,
ii) call sendto() e.g. in blocking mode, iii) wait for status of relevant
frames to be updated resp. the frame handed over to the application, iv) walk
through the frames to pick up the individual hw/sw timestamps.

Only (!) if transmit timestamping is enabled, then these bits are combined
with binary | with TP_STATUS_AVAILABLE, so you must check for that in your
application (e.g. !(tp_status & (TP_STATUS_SEND_REQUEST | TP_STATUS_SENDING))
in a first step to see if the frame belongs to the application, and then
one can extract the type of timestamp in a second step from tp_status)!

If you don't care about them, thus having it disabled, checking for
TP_STATUS_AVAILABLE resp. TP_STATUS_WRONG_FORMAT is sufficient. If in the
TX_RING part only TP_STATUS_AVAILABLE is set, then the tp_sec and tp_{n,u}sec
members do not contain a valid value. For TX_RINGs, by default no timestamp
is generated!

See include/linux/net_tstamp.h and Documentation/networking/timestamping
for more information on hardware timestamps.

-------------------------------------------------------------------------------
+ Miscellaneous bits
-------------------------------------------------------------------------------

- Packet sockets work well together with Linux socket filters, thus you also
  might want to have a look at Documentation/networking/filter.txt

--------------------------------------------------------------------------------
+ THANKS
--------------------------------------------------------------------------------
   
   Jesse Brandeburg, for fixing my grammathical/spelling errors