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
|
/*
* Copyright (c) 2012 The WebRTC project authors. All Rights Reserved.
*
* Use of this source code is governed by a BSD-style license
* that can be found in the LICENSE file in the root of the source
* tree. An additional intellectual property rights grant can be found
* in the file PATENTS. All contributing project authors may
* be found in the AUTHORS file in the root of the source tree.
*/
#include "modules/rtp_rtcp/source/forward_error_correction.h"
#include <string.h>
#include <algorithm>
#include <utility>
#include "absl/algorithm/container.h"
#include "modules/include/module_common_types_public.h"
#include "modules/rtp_rtcp/include/rtp_rtcp_defines.h"
#include "modules/rtp_rtcp/source/byte_io.h"
#include "modules/rtp_rtcp/source/flexfec_03_header_reader_writer.h"
#include "modules/rtp_rtcp/source/forward_error_correction_internal.h"
#include "modules/rtp_rtcp/source/ulpfec_header_reader_writer.h"
#include "rtc_base/checks.h"
#include "rtc_base/logging.h"
#include "rtc_base/numerics/mod_ops.h"
namespace webrtc {
namespace {
// Transport header size in bytes. Assume UDP/IPv4 as a reasonable minimum.
constexpr size_t kTransportOverhead = 28;
constexpr uint16_t kOldSequenceThreshold = 0x3fff;
} // namespace
ForwardErrorCorrection::Packet::Packet() : data(0), ref_count_(0) {}
ForwardErrorCorrection::Packet::~Packet() = default;
int32_t ForwardErrorCorrection::Packet::AddRef() {
return ++ref_count_;
}
int32_t ForwardErrorCorrection::Packet::Release() {
int32_t ref_count;
ref_count = --ref_count_;
if (ref_count == 0)
delete this;
return ref_count;
}
// This comparator is used to compare std::unique_ptr's pointing to
// subclasses of SortablePackets. It needs to be parametric since
// the std::unique_ptr's are not covariant w.r.t. the types that
// they are pointing to.
template <typename S, typename T>
bool ForwardErrorCorrection::SortablePacket::LessThan::operator()(
const S& first,
const T& second) {
RTC_DCHECK_EQ(first->ssrc, second->ssrc);
return IsNewerSequenceNumber(second->seq_num, first->seq_num);
}
ForwardErrorCorrection::ReceivedPacket::ReceivedPacket() = default;
ForwardErrorCorrection::ReceivedPacket::~ReceivedPacket() = default;
ForwardErrorCorrection::RecoveredPacket::RecoveredPacket() = default;
ForwardErrorCorrection::RecoveredPacket::~RecoveredPacket() = default;
ForwardErrorCorrection::ProtectedPacket::ProtectedPacket() = default;
ForwardErrorCorrection::ProtectedPacket::~ProtectedPacket() = default;
ForwardErrorCorrection::ReceivedFecPacket::ReceivedFecPacket() = default;
ForwardErrorCorrection::ReceivedFecPacket::~ReceivedFecPacket() = default;
ForwardErrorCorrection::ForwardErrorCorrection(
std::unique_ptr<FecHeaderReader> fec_header_reader,
std::unique_ptr<FecHeaderWriter> fec_header_writer,
uint32_t ssrc,
uint32_t protected_media_ssrc)
: ssrc_(ssrc),
protected_media_ssrc_(protected_media_ssrc),
fec_header_reader_(std::move(fec_header_reader)),
fec_header_writer_(std::move(fec_header_writer)),
generated_fec_packets_(fec_header_writer_->MaxFecPackets()),
packet_mask_size_(0) {}
ForwardErrorCorrection::~ForwardErrorCorrection() = default;
std::unique_ptr<ForwardErrorCorrection> ForwardErrorCorrection::CreateUlpfec(
uint32_t ssrc) {
std::unique_ptr<FecHeaderReader> fec_header_reader(new UlpfecHeaderReader());
std::unique_ptr<FecHeaderWriter> fec_header_writer(new UlpfecHeaderWriter());
return std::unique_ptr<ForwardErrorCorrection>(new ForwardErrorCorrection(
std::move(fec_header_reader), std::move(fec_header_writer), ssrc, ssrc));
}
std::unique_ptr<ForwardErrorCorrection> ForwardErrorCorrection::CreateFlexfec(
uint32_t ssrc,
uint32_t protected_media_ssrc) {
std::unique_ptr<FecHeaderReader> fec_header_reader(
new Flexfec03HeaderReader());
std::unique_ptr<FecHeaderWriter> fec_header_writer(
new Flexfec03HeaderWriter());
return std::unique_ptr<ForwardErrorCorrection>(new ForwardErrorCorrection(
std::move(fec_header_reader), std::move(fec_header_writer), ssrc,
protected_media_ssrc));
}
int ForwardErrorCorrection::EncodeFec(const PacketList& media_packets,
uint8_t protection_factor,
int num_important_packets,
bool use_unequal_protection,
FecMaskType fec_mask_type,
std::list<Packet*>* fec_packets) {
const size_t num_media_packets = media_packets.size();
// Sanity check arguments.
RTC_DCHECK_GT(num_media_packets, 0);
RTC_DCHECK_GE(num_important_packets, 0);
RTC_DCHECK_LE(num_important_packets, num_media_packets);
RTC_DCHECK(fec_packets->empty());
const size_t max_media_packets = fec_header_writer_->MaxMediaPackets();
if (num_media_packets > max_media_packets) {
RTC_LOG(LS_WARNING) << "Can't protect " << num_media_packets
<< " media packets per frame. Max is "
<< max_media_packets << ".";
return -1;
}
// Error check the media packets.
for (const auto& media_packet : media_packets) {
RTC_DCHECK(media_packet);
if (media_packet->data.size() < kRtpHeaderSize) {
RTC_LOG(LS_WARNING) << "Media packet " << media_packet->data.size()
<< " bytes "
"is smaller than RTP header.";
return -1;
}
// Ensure the FEC packets will fit in a typical MTU.
if (media_packet->data.size() + MaxPacketOverhead() + kTransportOverhead >
IP_PACKET_SIZE) {
RTC_LOG(LS_WARNING) << "Media packet " << media_packet->data.size()
<< " bytes "
"with overhead is larger than "
<< IP_PACKET_SIZE << " bytes.";
}
}
// Prepare generated FEC packets.
int num_fec_packets = NumFecPackets(num_media_packets, protection_factor);
if (num_fec_packets == 0) {
return 0;
}
for (int i = 0; i < num_fec_packets; ++i) {
generated_fec_packets_[i].data.EnsureCapacity(IP_PACKET_SIZE);
memset(generated_fec_packets_[i].data.MutableData(), 0, IP_PACKET_SIZE);
// Use this as a marker for untouched packets.
generated_fec_packets_[i].data.SetSize(0);
fec_packets->push_back(&generated_fec_packets_[i]);
}
internal::PacketMaskTable mask_table(fec_mask_type, num_media_packets);
packet_mask_size_ = internal::PacketMaskSize(num_media_packets);
memset(packet_masks_, 0, num_fec_packets * packet_mask_size_);
internal::GeneratePacketMasks(num_media_packets, num_fec_packets,
num_important_packets, use_unequal_protection,
&mask_table, packet_masks_);
// Adapt packet masks to missing media packets.
int num_mask_bits = InsertZerosInPacketMasks(media_packets, num_fec_packets);
if (num_mask_bits < 0) {
RTC_LOG(LS_INFO) << "Due to sequence number gaps, cannot protect media "
"packets with a single block of FEC packets.";
fec_packets->clear();
return -1;
}
packet_mask_size_ = internal::PacketMaskSize(num_mask_bits);
// Write FEC packets to `generated_fec_packets_`.
GenerateFecPayloads(media_packets, num_fec_packets);
// TODO(brandtr): Generalize this when multistream protection support is
// added.
const uint32_t media_ssrc = ParseSsrc(media_packets.front()->data.data());
const uint16_t seq_num_base =
ParseSequenceNumber(media_packets.front()->data.data());
FinalizeFecHeaders(num_fec_packets, media_ssrc, seq_num_base);
return 0;
}
int ForwardErrorCorrection::NumFecPackets(int num_media_packets,
int protection_factor) {
// Result in Q0 with an unsigned round.
int num_fec_packets = (num_media_packets * protection_factor + (1 << 7)) >> 8;
// Generate at least one FEC packet if we need protection.
if (protection_factor > 0 && num_fec_packets == 0) {
num_fec_packets = 1;
}
RTC_DCHECK_LE(num_fec_packets, num_media_packets);
return num_fec_packets;
}
void ForwardErrorCorrection::GenerateFecPayloads(
const PacketList& media_packets,
size_t num_fec_packets) {
RTC_DCHECK(!media_packets.empty());
for (size_t i = 0; i < num_fec_packets; ++i) {
Packet* const fec_packet = &generated_fec_packets_[i];
size_t pkt_mask_idx = i * packet_mask_size_;
const size_t min_packet_mask_size = fec_header_writer_->MinPacketMaskSize(
&packet_masks_[pkt_mask_idx], packet_mask_size_);
const size_t fec_header_size =
fec_header_writer_->FecHeaderSize(min_packet_mask_size);
size_t media_pkt_idx = 0;
auto media_packets_it = media_packets.cbegin();
uint16_t prev_seq_num =
ParseSequenceNumber((*media_packets_it)->data.data());
while (media_packets_it != media_packets.end()) {
Packet* const media_packet = media_packets_it->get();
// Should `media_packet` be protected by `fec_packet`?
if (packet_masks_[pkt_mask_idx] & (1 << (7 - media_pkt_idx))) {
size_t media_payload_length =
media_packet->data.size() - kRtpHeaderSize;
size_t fec_packet_length = fec_header_size + media_payload_length;
if (fec_packet_length > fec_packet->data.size()) {
size_t old_size = fec_packet->data.size();
fec_packet->data.SetSize(fec_packet_length);
memset(fec_packet->data.MutableData() + old_size, 0,
fec_packet_length - old_size);
}
XorHeaders(*media_packet, fec_packet);
XorPayloads(*media_packet, media_payload_length, fec_header_size,
fec_packet);
}
media_packets_it++;
if (media_packets_it != media_packets.end()) {
uint16_t seq_num =
ParseSequenceNumber((*media_packets_it)->data.data());
media_pkt_idx += static_cast<uint16_t>(seq_num - prev_seq_num);
prev_seq_num = seq_num;
}
pkt_mask_idx += media_pkt_idx / 8;
media_pkt_idx %= 8;
}
RTC_DCHECK_GT(fec_packet->data.size(), 0)
<< "Packet mask is wrong or poorly designed.";
}
}
int ForwardErrorCorrection::InsertZerosInPacketMasks(
const PacketList& media_packets,
size_t num_fec_packets) {
size_t num_media_packets = media_packets.size();
if (num_media_packets <= 1) {
return num_media_packets;
}
uint16_t last_seq_num =
ParseSequenceNumber(media_packets.back()->data.data());
uint16_t first_seq_num =
ParseSequenceNumber(media_packets.front()->data.data());
size_t total_missing_seq_nums =
static_cast<uint16_t>(last_seq_num - first_seq_num) - num_media_packets +
1;
if (total_missing_seq_nums == 0) {
// All sequence numbers are covered by the packet mask.
// No zero insertion required.
return num_media_packets;
}
const size_t max_media_packets = fec_header_writer_->MaxMediaPackets();
if (total_missing_seq_nums + num_media_packets > max_media_packets) {
return -1;
}
// Allocate the new mask.
size_t tmp_packet_mask_size =
internal::PacketMaskSize(total_missing_seq_nums + num_media_packets);
memset(tmp_packet_masks_, 0, num_fec_packets * tmp_packet_mask_size);
auto media_packets_it = media_packets.cbegin();
uint16_t prev_seq_num = first_seq_num;
++media_packets_it;
// Insert the first column.
internal::CopyColumn(tmp_packet_masks_, tmp_packet_mask_size, packet_masks_,
packet_mask_size_, num_fec_packets, 0, 0);
size_t new_bit_index = 1;
size_t old_bit_index = 1;
// Insert zeros in the bit mask for every hole in the sequence.
while (media_packets_it != media_packets.end()) {
if (new_bit_index == max_media_packets) {
// We can only cover up to 48 packets.
break;
}
uint16_t seq_num = ParseSequenceNumber((*media_packets_it)->data.data());
const int num_zeros_to_insert =
static_cast<uint16_t>(seq_num - prev_seq_num - 1);
if (num_zeros_to_insert > 0) {
internal::InsertZeroColumns(num_zeros_to_insert, tmp_packet_masks_,
tmp_packet_mask_size, num_fec_packets,
new_bit_index);
}
new_bit_index += num_zeros_to_insert;
internal::CopyColumn(tmp_packet_masks_, tmp_packet_mask_size, packet_masks_,
packet_mask_size_, num_fec_packets, new_bit_index,
old_bit_index);
++new_bit_index;
++old_bit_index;
prev_seq_num = seq_num;
++media_packets_it;
}
if (new_bit_index % 8 != 0) {
// We didn't fill the last byte. Shift bits to correct position.
for (uint16_t row = 0; row < num_fec_packets; ++row) {
int new_byte_index = row * tmp_packet_mask_size + new_bit_index / 8;
tmp_packet_masks_[new_byte_index] <<= (7 - (new_bit_index % 8));
}
}
// Replace the old mask with the new.
memcpy(packet_masks_, tmp_packet_masks_,
num_fec_packets * tmp_packet_mask_size);
return new_bit_index;
}
void ForwardErrorCorrection::FinalizeFecHeaders(size_t num_fec_packets,
uint32_t media_ssrc,
uint16_t seq_num_base) {
for (size_t i = 0; i < num_fec_packets; ++i) {
const FecHeaderWriter::ProtectedStream protected_streams[] = {
{.ssrc = media_ssrc,
.seq_num_base = seq_num_base,
.packet_mask = {&packet_masks_[i * packet_mask_size_],
packet_mask_size_}}};
fec_header_writer_->FinalizeFecHeader(protected_streams,
generated_fec_packets_[i]);
}
}
void ForwardErrorCorrection::ResetState(
RecoveredPacketList* recovered_packets) {
// Free the memory for any existing recovered packets, if the caller hasn't.
recovered_packets->clear();
received_fec_packets_.clear();
}
void ForwardErrorCorrection::InsertMediaPacket(
RecoveredPacketList* recovered_packets,
const ReceivedPacket& received_packet) {
RTC_DCHECK_EQ(received_packet.ssrc, protected_media_ssrc_);
// Search for duplicate packets.
for (const auto& recovered_packet : *recovered_packets) {
RTC_DCHECK_EQ(recovered_packet->ssrc, received_packet.ssrc);
if (recovered_packet->seq_num == received_packet.seq_num) {
// Duplicate packet, no need to add to list.
return;
}
}
std::unique_ptr<RecoveredPacket> recovered_packet(new RecoveredPacket());
// This "recovered packet" was not recovered using parity packets.
recovered_packet->was_recovered = false;
// This media packet has already been passed on.
recovered_packet->returned = true;
recovered_packet->ssrc = received_packet.ssrc;
recovered_packet->seq_num = received_packet.seq_num;
recovered_packet->pkt = received_packet.pkt;
// TODO(holmer): Consider replacing this with a binary search for the right
// position, and then just insert the new packet. Would get rid of the sort.
RecoveredPacket* recovered_packet_ptr = recovered_packet.get();
recovered_packets->push_back(std::move(recovered_packet));
recovered_packets->sort(SortablePacket::LessThan());
UpdateCoveringFecPackets(*recovered_packet_ptr);
}
void ForwardErrorCorrection::UpdateCoveringFecPackets(
const RecoveredPacket& packet) {
for (auto& fec_packet : received_fec_packets_) {
// Is this FEC packet protecting the media packet `packet`?
auto protected_it = absl::c_lower_bound(
fec_packet->protected_packets, &packet, SortablePacket::LessThan());
if (protected_it != fec_packet->protected_packets.end() &&
(*protected_it)->seq_num == packet.seq_num) {
// Found an FEC packet which is protecting `packet`.
(*protected_it)->pkt = packet.pkt;
}
}
}
void ForwardErrorCorrection::InsertFecPacket(
const RecoveredPacketList& recovered_packets,
const ReceivedPacket& received_packet) {
RTC_DCHECK_EQ(received_packet.ssrc, ssrc_);
// Check for duplicate.
for (const auto& existing_fec_packet : received_fec_packets_) {
RTC_DCHECK_EQ(existing_fec_packet->ssrc, received_packet.ssrc);
if (existing_fec_packet->seq_num == received_packet.seq_num) {
// Drop duplicate FEC packet data.
return;
}
}
std::unique_ptr<ReceivedFecPacket> fec_packet(new ReceivedFecPacket());
fec_packet->pkt = received_packet.pkt;
fec_packet->ssrc = received_packet.ssrc;
fec_packet->seq_num = received_packet.seq_num;
// Parse ULPFEC/FlexFEC header specific info.
bool ret = fec_header_reader_->ReadFecHeader(fec_packet.get());
if (!ret) {
return;
}
RTC_CHECK_EQ(fec_packet->protected_streams.size(), 1);
if (fec_packet->protected_streams[0].ssrc != protected_media_ssrc_) {
RTC_LOG(LS_INFO)
<< "Received FEC packet is protecting an unknown media SSRC; dropping.";
return;
}
if (fec_packet->protected_streams[0].packet_mask_offset +
fec_packet->protected_streams[0].packet_mask_size >
fec_packet->pkt->data.size()) {
RTC_LOG(LS_INFO) << "Received corrupted FEC packet; dropping.";
return;
}
// Parse packet mask from header and represent as protected packets.
for (uint16_t byte_idx = 0;
byte_idx < fec_packet->protected_streams[0].packet_mask_size;
++byte_idx) {
uint8_t packet_mask =
fec_packet->pkt
->data[fec_packet->protected_streams[0].packet_mask_offset +
byte_idx];
for (uint16_t bit_idx = 0; bit_idx < 8; ++bit_idx) {
if (packet_mask & (1 << (7 - bit_idx))) {
std::unique_ptr<ProtectedPacket> protected_packet(
new ProtectedPacket());
// This wraps naturally with the sequence number.
protected_packet->ssrc = protected_media_ssrc_;
protected_packet->seq_num = static_cast<uint16_t>(
fec_packet->protected_streams[0].seq_num_base + (byte_idx << 3) +
bit_idx);
protected_packet->pkt = nullptr;
fec_packet->protected_packets.push_back(std::move(protected_packet));
}
}
}
if (fec_packet->protected_packets.empty()) {
// All-zero packet mask; we can discard this FEC packet.
RTC_LOG(LS_WARNING) << "Received FEC packet has an all-zero packet mask.";
} else {
AssignRecoveredPackets(recovered_packets, fec_packet.get());
// TODO(holmer): Consider replacing this with a binary search for the right
// position, and then just insert the new packet. Would get rid of the sort.
received_fec_packets_.push_back(std::move(fec_packet));
received_fec_packets_.sort(SortablePacket::LessThan());
const size_t max_fec_packets = fec_header_reader_->MaxFecPackets();
if (received_fec_packets_.size() > max_fec_packets) {
received_fec_packets_.pop_front();
}
RTC_DCHECK_LE(received_fec_packets_.size(), max_fec_packets);
}
}
void ForwardErrorCorrection::AssignRecoveredPackets(
const RecoveredPacketList& recovered_packets,
ReceivedFecPacket* fec_packet) {
ProtectedPacketList* protected_packets = &fec_packet->protected_packets;
std::vector<RecoveredPacket*> recovered_protected_packets;
// Find intersection between the (sorted) containers `protected_packets`
// and `recovered_packets`, i.e. all protected packets that have already
// been recovered. Update the corresponding protected packets to point to
// the recovered packets.
auto it_p = protected_packets->cbegin();
auto it_r = recovered_packets.cbegin();
SortablePacket::LessThan less_than;
while (it_p != protected_packets->end() && it_r != recovered_packets.end()) {
if (less_than(*it_p, *it_r)) {
++it_p;
} else if (less_than(*it_r, *it_p)) {
++it_r;
} else { // *it_p == *it_r.
// This protected packet has already been recovered.
(*it_p)->pkt = (*it_r)->pkt;
++it_p;
++it_r;
}
}
}
void ForwardErrorCorrection::InsertPacket(
const ReceivedPacket& received_packet,
RecoveredPacketList* recovered_packets) {
// Discard old FEC packets such that the sequence numbers in
// `received_fec_packets_` span at most 1/2 of the sequence number space.
// This is important for keeping `received_fec_packets_` sorted, and may
// also reduce the possibility of incorrect decoding due to sequence number
// wrap-around.
if (!received_fec_packets_.empty() &&
received_packet.ssrc == received_fec_packets_.front()->ssrc) {
// It only makes sense to detect wrap-around when `received_packet`
// and `front_received_fec_packet` belong to the same sequence number
// space, i.e., the same SSRC. This happens when `received_packet`
// is a FEC packet, or if `received_packet` is a media packet and
// RED+ULPFEC is used.
auto it = received_fec_packets_.begin();
while (it != received_fec_packets_.end()) {
uint16_t seq_num_diff = MinDiff(received_packet.seq_num, (*it)->seq_num);
if (seq_num_diff > kOldSequenceThreshold) {
it = received_fec_packets_.erase(it);
} else {
// No need to keep iterating, since `received_fec_packets_` is sorted.
break;
}
}
}
if (received_packet.is_fec) {
InsertFecPacket(*recovered_packets, received_packet);
} else {
InsertMediaPacket(recovered_packets, received_packet);
}
DiscardOldRecoveredPackets(recovered_packets);
}
bool ForwardErrorCorrection::StartPacketRecovery(
const ReceivedFecPacket& fec_packet,
RecoveredPacket* recovered_packet) {
// Ensure pkt is initialized.
recovered_packet->pkt = new Packet();
// Sanity check packet length.
if (fec_packet.pkt->data.size() <
fec_packet.fec_header_size + fec_packet.protection_length) {
RTC_LOG(LS_WARNING)
<< "The FEC packet is truncated: it does not contain enough room "
"for its own header.";
return false;
}
if (fec_packet.protection_length >
std::min(size_t{IP_PACKET_SIZE - kRtpHeaderSize},
IP_PACKET_SIZE - fec_packet.fec_header_size)) {
RTC_LOG(LS_WARNING) << "Incorrect protection length, dropping FEC packet.";
return false;
}
// Initialize recovered packet data.
recovered_packet->pkt->data.EnsureCapacity(IP_PACKET_SIZE);
recovered_packet->pkt->data.SetSize(fec_packet.protection_length +
kRtpHeaderSize);
recovered_packet->returned = false;
recovered_packet->was_recovered = true;
// Copy bytes corresponding to minimum RTP header size.
// Note that the sequence number and SSRC fields will be overwritten
// at the end of packet recovery.
memcpy(recovered_packet->pkt->data.MutableData(),
fec_packet.pkt->data.cdata(), kRtpHeaderSize);
// Copy remaining FEC payload.
if (fec_packet.protection_length > 0) {
memcpy(recovered_packet->pkt->data.MutableData() + kRtpHeaderSize,
fec_packet.pkt->data.cdata() + fec_packet.fec_header_size,
fec_packet.protection_length);
}
return true;
}
bool ForwardErrorCorrection::FinishPacketRecovery(
const ReceivedFecPacket& fec_packet,
RecoveredPacket* recovered_packet) {
uint8_t* data = recovered_packet->pkt->data.MutableData();
// Set the RTP version to 2.
data[0] |= 0x80; // Set the 1st bit.
data[0] &= 0xbf; // Clear the 2nd bit.
// Recover the packet length, from temporary location.
const size_t new_size =
ByteReader<uint16_t>::ReadBigEndian(&data[2]) + kRtpHeaderSize;
if (new_size > size_t{IP_PACKET_SIZE - kRtpHeaderSize}) {
RTC_LOG(LS_WARNING) << "The recovered packet had a length larger than a "
"typical IP packet, and is thus dropped.";
return false;
}
size_t old_size = recovered_packet->pkt->data.size();
recovered_packet->pkt->data.SetSize(new_size);
data = recovered_packet->pkt->data.MutableData();
if (new_size > old_size) {
memset(data + old_size, 0, new_size - old_size);
}
// Set the SN field.
ByteWriter<uint16_t>::WriteBigEndian(&data[2], recovered_packet->seq_num);
// Set the SSRC field.
ByteWriter<uint32_t>::WriteBigEndian(&data[8], recovered_packet->ssrc);
return true;
}
void ForwardErrorCorrection::XorHeaders(const Packet& src, Packet* dst) {
uint8_t* dst_data = dst->data.MutableData();
const uint8_t* src_data = src.data.cdata();
// XOR the first 2 bytes of the header: V, P, X, CC, M, PT fields.
dst_data[0] ^= src_data[0];
dst_data[1] ^= src_data[1];
// XOR the length recovery field.
uint8_t src_payload_length_network_order[2];
ByteWriter<uint16_t>::WriteBigEndian(src_payload_length_network_order,
src.data.size() - kRtpHeaderSize);
dst_data[2] ^= src_payload_length_network_order[0];
dst_data[3] ^= src_payload_length_network_order[1];
// XOR the 5th to 8th bytes of the header: the timestamp field.
dst_data[4] ^= src_data[4];
dst_data[5] ^= src_data[5];
dst_data[6] ^= src_data[6];
dst_data[7] ^= src_data[7];
// Skip the 9th to 12th bytes of the header.
}
void ForwardErrorCorrection::XorPayloads(const Packet& src,
size_t payload_length,
size_t dst_offset,
Packet* dst) {
// XOR the payload.
RTC_DCHECK_LE(kRtpHeaderSize + payload_length, src.data.size());
RTC_DCHECK_LE(dst_offset + payload_length, dst->data.capacity());
if (dst_offset + payload_length > dst->data.size()) {
size_t old_size = dst->data.size();
size_t new_size = dst_offset + payload_length;
dst->data.SetSize(new_size);
memset(dst->data.MutableData() + old_size, 0, new_size - old_size);
}
uint8_t* dst_data = dst->data.MutableData();
const uint8_t* src_data = src.data.cdata();
for (size_t i = 0; i < payload_length; ++i) {
dst_data[dst_offset + i] ^= src_data[kRtpHeaderSize + i];
}
}
bool ForwardErrorCorrection::RecoverPacket(const ReceivedFecPacket& fec_packet,
RecoveredPacket* recovered_packet) {
if (!StartPacketRecovery(fec_packet, recovered_packet)) {
return false;
}
for (const auto& protected_packet : fec_packet.protected_packets) {
if (protected_packet->pkt == nullptr) {
// This is the packet we're recovering.
recovered_packet->seq_num = protected_packet->seq_num;
recovered_packet->ssrc = protected_packet->ssrc;
} else {
XorHeaders(*protected_packet->pkt, recovered_packet->pkt.get());
XorPayloads(*protected_packet->pkt,
protected_packet->pkt->data.size() - kRtpHeaderSize,
kRtpHeaderSize, recovered_packet->pkt.get());
}
}
if (!FinishPacketRecovery(fec_packet, recovered_packet)) {
return false;
}
return true;
}
size_t ForwardErrorCorrection::AttemptRecovery(
RecoveredPacketList* recovered_packets) {
size_t num_recovered_packets = 0;
auto fec_packet_it = received_fec_packets_.begin();
while (fec_packet_it != received_fec_packets_.end()) {
// Search for each FEC packet's protected media packets.
int packets_missing = NumCoveredPacketsMissing(**fec_packet_it);
// We can only recover one packet with an FEC packet.
if (packets_missing == 1) {
// Recovery possible.
std::unique_ptr<RecoveredPacket> recovered_packet(new RecoveredPacket());
recovered_packet->pkt = nullptr;
if (!RecoverPacket(**fec_packet_it, recovered_packet.get())) {
// Can't recover using this packet, drop it.
fec_packet_it = received_fec_packets_.erase(fec_packet_it);
continue;
}
++num_recovered_packets;
auto* recovered_packet_ptr = recovered_packet.get();
// Add recovered packet to the list of recovered packets and update any
// FEC packets covering this packet with a pointer to the data.
// TODO(holmer): Consider replacing this with a binary search for the
// right position, and then just insert the new packet. Would get rid of
// the sort.
recovered_packets->push_back(std::move(recovered_packet));
recovered_packets->sort(SortablePacket::LessThan());
UpdateCoveringFecPackets(*recovered_packet_ptr);
DiscardOldRecoveredPackets(recovered_packets);
fec_packet_it = received_fec_packets_.erase(fec_packet_it);
// A packet has been recovered. We need to check the FEC list again, as
// this may allow additional packets to be recovered.
// Restart for first FEC packet.
fec_packet_it = received_fec_packets_.begin();
} else if (packets_missing == 0 ||
IsOldFecPacket(**fec_packet_it, recovered_packets)) {
// Either all protected packets arrived or have been recovered, or the FEC
// packet is old. We can discard this FEC packet.
fec_packet_it = received_fec_packets_.erase(fec_packet_it);
} else {
fec_packet_it++;
}
}
return num_recovered_packets;
}
int ForwardErrorCorrection::NumCoveredPacketsMissing(
const ReceivedFecPacket& fec_packet) {
int packets_missing = 0;
for (const auto& protected_packet : fec_packet.protected_packets) {
if (protected_packet->pkt == nullptr) {
++packets_missing;
if (packets_missing > 1) {
break; // We can't recover more than one packet.
}
}
}
return packets_missing;
}
void ForwardErrorCorrection::DiscardOldRecoveredPackets(
RecoveredPacketList* recovered_packets) {
const size_t max_media_packets = fec_header_reader_->MaxMediaPackets();
while (recovered_packets->size() > max_media_packets) {
recovered_packets->pop_front();
}
RTC_DCHECK_LE(recovered_packets->size(), max_media_packets);
}
bool ForwardErrorCorrection::IsOldFecPacket(
const ReceivedFecPacket& fec_packet,
const RecoveredPacketList* recovered_packets) {
if (recovered_packets->empty()) {
return false;
}
const uint16_t back_recovered_seq_num = recovered_packets->back()->seq_num;
const uint16_t last_protected_seq_num =
fec_packet.protected_packets.back()->seq_num;
// FEC packet is old if its last protected sequence number is much
// older than the latest protected sequence number received.
return (MinDiff(back_recovered_seq_num, last_protected_seq_num) >
kOldSequenceThreshold);
}
uint16_t ForwardErrorCorrection::ParseSequenceNumber(const uint8_t* packet) {
return (packet[2] << 8) + packet[3];
}
uint32_t ForwardErrorCorrection::ParseSsrc(const uint8_t* packet) {
return (packet[8] << 24) + (packet[9] << 16) + (packet[10] << 8) + packet[11];
}
ForwardErrorCorrection::DecodeFecResult ForwardErrorCorrection::DecodeFec(
const ReceivedPacket& received_packet,
RecoveredPacketList* recovered_packets) {
RTC_DCHECK(recovered_packets);
const size_t max_media_packets = fec_header_reader_->MaxMediaPackets();
if (recovered_packets->size() == max_media_packets) {
const RecoveredPacket* back_recovered_packet =
recovered_packets->back().get();
if (received_packet.ssrc == back_recovered_packet->ssrc) {
const unsigned int seq_num_diff =
MinDiff(received_packet.seq_num, back_recovered_packet->seq_num);
if (seq_num_diff > max_media_packets) {
// A big gap in sequence numbers. The old recovered packets
// are now useless, so it's safe to do a reset.
RTC_LOG(LS_INFO) << "Big gap in media/ULPFEC sequence numbers. No need "
"to keep the old packets in the FEC buffers, thus "
"resetting them.";
ResetState(recovered_packets);
}
}
}
InsertPacket(received_packet, recovered_packets);
DecodeFecResult decode_result;
decode_result.num_recovered_packets = AttemptRecovery(recovered_packets);
return decode_result;
}
size_t ForwardErrorCorrection::MaxPacketOverhead() const {
return fec_header_writer_->MaxPacketOverhead();
}
FecHeaderReader::FecHeaderReader(size_t max_media_packets,
size_t max_fec_packets)
: max_media_packets_(max_media_packets),
max_fec_packets_(max_fec_packets) {}
FecHeaderReader::~FecHeaderReader() = default;
size_t FecHeaderReader::MaxMediaPackets() const {
return max_media_packets_;
}
size_t FecHeaderReader::MaxFecPackets() const {
return max_fec_packets_;
}
FecHeaderWriter::FecHeaderWriter(size_t max_media_packets,
size_t max_fec_packets,
size_t max_packet_overhead)
: max_media_packets_(max_media_packets),
max_fec_packets_(max_fec_packets),
max_packet_overhead_(max_packet_overhead) {}
FecHeaderWriter::~FecHeaderWriter() = default;
size_t FecHeaderWriter::MaxMediaPackets() const {
return max_media_packets_;
}
size_t FecHeaderWriter::MaxFecPackets() const {
return max_fec_packets_;
}
size_t FecHeaderWriter::MaxPacketOverhead() const {
return max_packet_overhead_;
}
} // namespace webrtc
|