-
Notifications
You must be signed in to change notification settings - Fork 0
/
draft-ietf-bier-te-arch-04.txt
2240 lines (1464 loc) · 85.4 KB
/
draft-ietf-bier-te-arch-04.txt
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
Network Working Group T. Eckert, Ed.
Internet-Draft Futurewei
Intended status: Standards Track G. Cauchie
Expires: April 25, 2020 Bouygues Telecom
M. Menth
University of Tuebingen
October 23, 2019
Traffic Engineering for Bit Index Explicit Replication (BIER-TE)
draft-ietf-bier-te-arch-04
Abstract
This memo introduces per-packet stateless strict and loose path
engineered replication and forwarding for Bit Index Explicit
Replication packets ([RFC8279]). This is called BIER-TE.
BIER-TE leverages the BIER architecture ([RFC8279]) and extends it
with a new semantic for bits in the bitstring. BIER-TE can leverage
BIER forwarding engines with little or no changes.
In BIER, the BitPositions (BP) of the packets bitstring indicate BIER
Forwarding Egress Routers (BFER), and hop-by-hop forwarding uses a
Routing Underlay such as an IGP.
In BIER-TE, BitPositions indicate adjacencies. The BIFT of each BFR
are only populated with BPs that are adjacent to the BFR in the BIER-
TE topology. The BIER-TE topology can consist of layer 2 or remote
(route) adjacencies. The BFR then replicates and forwards BIER
packets to those adjacencies. This results in the aforementioned
strict and loose path forwarding.
BIER-TE can co-exist with BIER forwarding in the same domain, for
example by using separate sub-domains. In the absence of routed
adjacencies, BIER-TE does not require a BIER routing underlay, and
can then be operated without requiring an IGP routing protocol.
BIER-TE operates without explicit in-network tree-building and
carries the multicast distribution tree in the packet header. It can
therefore be a good fit to support multicast path steering in Segment
Routing (SR) networks.
Status of This Memo
This Internet-Draft is submitted in full conformance with the
provisions of BCP 78 and BCP 79.
Eckert, et al. Expires April 25, 2020 [Page 1]
Internet-Draft BIER-TE ARCH October 2019
Internet-Drafts are working documents of the Internet Engineering
Task Force (IETF). Note that other groups may also distribute
working documents as Internet-Drafts. The list of current Internet-
Drafts is at https://datatracker.ietf.org/drafts/current/.
Internet-Drafts are draft documents valid for a maximum of six months
and may be updated, replaced, or obsoleted by other documents at any
time. It is inappropriate to use Internet-Drafts as reference
material or to cite them other than as "work in progress."
This Internet-Draft will expire on April 25, 2020.
Copyright Notice
Copyright (c) 2019 IETF Trust and the persons identified as the
document authors. All rights reserved.
This document is subject to BCP 78 and the IETF Trust's Legal
Provisions Relating to IETF Documents
(https://trustee.ietf.org/license-info) in effect on the date of
publication of this document. Please review these documents
carefully, as they describe your rights and restrictions with respect
to this document. Code Components extracted from this document must
include Simplified BSD License text as described in Section 4.e of
the Trust Legal Provisions and are provided without warranty as
described in the Simplified BSD License.
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 3
1.1. Basic Examples . . . . . . . . . . . . . . . . . . . . . 4
1.2. BIER-TE Topology and adjacencies . . . . . . . . . . . . 7
1.3. Comparison with BIER . . . . . . . . . . . . . . . . . . 8
1.4. Requirements Language . . . . . . . . . . . . . . . . . . 8
2. Components . . . . . . . . . . . . . . . . . . . . . . . . . 8
2.1. The Multicast Flow Overlay . . . . . . . . . . . . . . . 9
2.2. The BIER-TE Controller Host . . . . . . . . . . . . . . . 9
2.2.1. Assignment of BitPositions to adjacencies of the
network topology . . . . . . . . . . . . . . . . . . 10
2.2.2. Changes in the network topology . . . . . . . . . . . 10
2.2.3. Set up per-multicast flow BIER-TE state . . . . . . . 10
2.2.4. Link/Node Failures and Recovery . . . . . . . . . . . 11
2.3. The BIER-TE Forwarding Layer . . . . . . . . . . . . . . 11
2.4. The Routing Underlay . . . . . . . . . . . . . . . . . . 11
3. BIER-TE Forwarding . . . . . . . . . . . . . . . . . . . . . 11
3.1. The Bit Index Forwarding Table (BIFT) . . . . . . . . . . 11
3.2. Adjacency Types . . . . . . . . . . . . . . . . . . . . . 13
3.2.1. Forward Connected . . . . . . . . . . . . . . . . . . 13
Eckert, et al. Expires April 25, 2020 [Page 2]
Internet-Draft BIER-TE ARCH October 2019
3.2.2. Forward Routed . . . . . . . . . . . . . . . . . . . 13
3.2.3. ECMP . . . . . . . . . . . . . . . . . . . . . . . . 13
3.2.4. Local Decap . . . . . . . . . . . . . . . . . . . . . 14
3.3. Encapsulation considerations . . . . . . . . . . . . . . 14
3.4. Basic BIER-TE Forwarding Example . . . . . . . . . . . . 14
3.5. Forwarding comparison with BIER . . . . . . . . . . . . . 17
3.6. Requirements . . . . . . . . . . . . . . . . . . . . . . 17
4. BIER-TE Controller Host BitPosition Assignments . . . . . . . 18
4.1. P2P Links . . . . . . . . . . . . . . . . . . . . . . . . 18
4.2. BFER . . . . . . . . . . . . . . . . . . . . . . . . . . 18
4.3. Leaf BFERs . . . . . . . . . . . . . . . . . . . . . . . 18
4.4. LANs . . . . . . . . . . . . . . . . . . . . . . . . . . 19
4.5. Hub and Spoke . . . . . . . . . . . . . . . . . . . . . . 19
4.6. Rings . . . . . . . . . . . . . . . . . . . . . . . . . . 19
4.7. Equal Cost MultiPath (ECMP) . . . . . . . . . . . . . . . 20
4.8. Routed adjacencies . . . . . . . . . . . . . . . . . . . 23
4.8.1. Reducing BitPositions . . . . . . . . . . . . . . . . 23
4.8.2. Supporting nodes without BIER-TE . . . . . . . . . . 23
5. Avoiding loops and duplicates . . . . . . . . . . . . . . . . 24
5.1. Loops . . . . . . . . . . . . . . . . . . . . . . . . . . 24
5.2. Duplicates . . . . . . . . . . . . . . . . . . . . . . . 24
6. BIER-TE Forwarding Pseudocode . . . . . . . . . . . . . . . . 24
7. Managing SI, subdomains and BFR-ids . . . . . . . . . . . . . 27
7.1. Why SI and sub-domains . . . . . . . . . . . . . . . . . 28
7.2. Bit assignment comparison BIER and BIER-TE . . . . . . . 29
7.3. Using BFR-id with BIER-TE . . . . . . . . . . . . . . . . 29
7.4. Assigning BFR-ids for BIER-TE . . . . . . . . . . . . . . 30
7.5. Example bit allocations . . . . . . . . . . . . . . . . . 31
7.5.1. With BIER . . . . . . . . . . . . . . . . . . . . . . 31
7.5.2. With BIER-TE . . . . . . . . . . . . . . . . . . . . 32
7.6. Summary . . . . . . . . . . . . . . . . . . . . . . . . . 33
8. BIER-TE and Segment Routing (SR) . . . . . . . . . . . . . . 33
9. Security Considerations . . . . . . . . . . . . . . . . . . . 34
10. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 35
11. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 35
12. Change log [RFC Editor: Please remove] . . . . . . . . . . . 35
13. References . . . . . . . . . . . . . . . . . . . . . . . . . 38
13.1. Normative References . . . . . . . . . . . . . . . . . . 38
13.2. Informative References . . . . . . . . . . . . . . . . . 39
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 39
1. Introduction
BIER-TE shares architecture, terminology and packet formats with BIER
as described in [RFC8279] and [RFC8296]. This document describes
BIER-TE in the expectation that the reader is familiar with these two
documents.
Eckert, et al. Expires April 25, 2020 [Page 3]
Internet-Draft BIER-TE ARCH October 2019
In BIER-TE, BitPositions (BP) indicate adjacencies. The BIFT of each
BFR is only populated with BP that are adjacent to the BFR in the
BIER-TE Topology. Other BPs are left without adjacency. The BFR
replicate and forwards BIER packets to adjacent BPs that are set in
the packet. BPs are normally also reset upon forwarding to avoid
duplicates and loops. This is detailed further below.
Note that related work [ICC], [I-D.ietf-roll-ccast] uses bloom
filters to represent leaves or edges of the intended delivery tree.
Bloom filters can support larger trees with fewer addressing bits,
but they introduce the heuristic risk of false positives and cannot
reset bits in the bitstring during forwarding to avoid loops. For
these reasons, BIER-TE does not use bloom filters, but explicit
bitstrings like BIER.
1.1. Basic Examples
BIER-TE forwarding is best introduced with simple examples.
Eckert, et al. Expires April 25, 2020 [Page 4]
Internet-Draft BIER-TE ARCH October 2019
BIER-TE Topology:
Diagram:
p5 p6
--- BFR3 ---
p3/ p13 \p7
BFR1 ---- BFR2 BFR5 ----- BFR6
p1 p2 p4\ p14 /p10 p11 p12
--- BFR4 ---
p8 p9
(simplified) BIER-TE Bit Index Forwarding Tables (BIFT):
BFR1: p1 -> local_decap
p2 -> forward_connected to BFR2
BFR2: p1 -> forward_connected to BFR1
p5 -> forward_connected to BFR3
p8 -> forward_connected to BFR4
BFR3: p3 -> forward_connected to BFR2
p7 -> forward_connected to BFR5
p13 -> local_decap
BFR4: p4 -> forward_connected to BFR2
p10 -> forward_connected to BFR5
p14 -> local_decap
BFR5: p6 -> forward_connected to BFR3
p9 -> forward_connected to BFR4
p12 -> forward_connected to BFR6
BFR6: p11 -> forward_connected to BFR5
p12 -> local_decap
Figure 1: BIER-TE basic example
Consider the simple network in the above BIER-TE overview example
picture with 6 BFRs. p1...p14 are the BitPositions (BP) used. All
BFRs can act as ingress BFR (BFIR), BFR1, BFR3, BFR4 and BFR6 can
also be egress BFR (BFER). Forward_connected is the name for
adjacencies that are representing subnet adjacencies of the network.
Local_decap is the name of the adjacency to decapsulate BIER-TE
packets and pass their payload to higher layer processing.
Eckert, et al. Expires April 25, 2020 [Page 5]
Internet-Draft BIER-TE ARCH October 2019
Assume a packet from BFR1 should be sent via BFR4 to BFR6. This
requires a bitstring (p2,p8,p10,p12). When this packet is examined
by BIER-TE on BFR1, the only BitPosition from the bitstring that is
also set in the BIFT is p2. This will cause BFR1 to send the only
copy of the packet to BFR2. Similarly, BFR2 will forward to BFR4
because of p8, BFR4 to BFR5 because of p10 and BFR5 to BFR6 because
of p12. p12 also makes BFR6 receive and decapsulate the packet.
To send in addition to BFR6 via BFR4 also a copy to BFR3, the
bitstring needs to be (p2,p5,p8,p10,p12,p13). When this packet is
examined by BFR2, p5 causes one copy to be sent to BFR3 and p8 one
copy to BFR4. When BFR3 receives the packet, p13 will cause it to
receive and decapsulate the packet.
If instead the bitstring was (p2,p6,p8,p10,p12,p13), the packet would
be copied by BFR5 towards BFR3 because p6 instead of BFR2 to BFR5
because of p6 in the prior case. This is showing the ability of the
shown BIER-TE Topology to make the traffic pass across any possible
path and be replicated where desired.
BIER-TE has various options to minimize BP assignments, many of which
are based on assumptions about the required multicast traffic paths
and bandwidth consumption in the network.
The following picture shows a modified example, in which Rtr2 and
Rtr5 are assumed not to support BIER-TE, so traffic has to be unicast
encapsulated across them. Unicast tunneling of BIER-TE packets can
leverage any feasible mechanism such as MPLS or IP, these
encapsulations are out of scope of this document. To emphasize non-
native forwarding of BIER-TE packets, these adjacencies are called
"forward_routed", but otherwise there is no difference in their
processing over the aforementioned "forward_connected" adjacencies.
In addition, bits are saved in the following example by assuming that
BFR1 only needs to be BFIR but not BFER or transit BFR.
Eckert, et al. Expires April 25, 2020 [Page 6]
Internet-Draft BIER-TE ARCH October 2019
BIER-TE Topology:
Diagram:
p1 p3 p7
....> BFR3 <.... p5
........ ........>
BFR1 (Rtr2) (Rtr5) BFR6
........ ........>
....> BFR4 <.... p6
p2 p4 p8
(simplified) BIER-TE Bit Index Forwarding Tables (BIFT):
BFR1: p1 -> forward_routed to BFR3
p2 -> forward_routed to BFR4
BFR3: p3 -> local_decap
p5 -> forward_routed to BFR6
BFR4: p4 -> local_decap
p6 -> forward_routed to BFR6
BFR6: p5 -> local_decap
p6 -> local_decap
p7 -> forward_routed to BFR3
p8 -> forward_routed to BFR4
Figure 2: BIER-TE basic overlay example
To send a BIER-TE packet from BFR1 via BFR3 to BFR6, the bitstring is
(p1,p5). From BFR1 via BFR4 to BFR6 it is (p2,p6). A packet from
BFR1 to BFR3,BFR4 and BFR6 can use (p1,p2,p3,p4,p5) or
(p1,p2,p3,p4,p6), or via BFR6 (p2,p3,p4,p6,p7) or (p1.p3,p4,p5,p8).
1.2. BIER-TE Topology and adjacencies
The key new component in BIER-TE to control where replication can or
should happens and how to minimize the required BP for segments is -
as shown in these two examples - the BIER-TE topology.
The BIER-TE Topology effectively consists of the BIFT of all the BFR
and can also be expressed in a diagram as a graph where the edges are
the adjacencies between the BFR. Adjacencies are naturally
unidirectional. BP can be reused across multiple adjacencies as long
as this does not lead to undesired duplicates or loops as explained
further down in the text.
Eckert, et al. Expires April 25, 2020 [Page 7]
Internet-Draft BIER-TE ARCH October 2019
If the BIER-TE topology represents the underlying (layer 2) topology
of the network, this is called "native" BIER-TE as shown in the first
example. This can be freely mixed with "overlay" BIER-TE, in
"forward_routed" adjacencies are used.
1.3. Comparison with BIER
The key differences over BIER are:
o BIER-TE replaces in-network autonomous path calculation by
explicit paths calculated off-path by the BIER-TE controller host.
o In BIER-TE every BitPosition of the BitString of a BIER-TE packet
indicates one or more adjacencies - instead of a BFER as in BIER.
o BIER-TE in each BFR has no routing table but only a BIER-TE
Forwarding Table (BIFT) indexed by SI:BitPosition and populated
with only those adjacencies to which the BFR should replicate
packets to.
BIER-TE headers use the same format as BIER headers.
BIER-TE forwarding does not require/use the BFIR-ID. The BFIR-ID can
still be useful though for coordinated BFIR/BFER functions, such as
the context for upstream assigned labels for MPLS payloads in MVPN
over BIER-TE.
If the BIER-TE domain is also running BIER, then the BFIR-ID in BIER-
TE packets can be set to the same BFIR-ID as used with BIER packets.
If the BIER-TE domain is not running full BIER or does not want to
reduce the need to allocate bits in BIER bitstrings for BFIR-ID
values, then the allocation of BFIR-ID values in BIER-TE packets can
be done through other mechanisms outside the scope of this document,
as long as this is appropriately agreed upon between all BFIR/BFER.
1.4. Requirements Language
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in RFC 2119 [RFC2119].
2. Components
End to end BIER-TE operations consists of four mayor components: The
"Multicast Flow Overlay", the "BIER-TE control plane" consisting of
the "BIER-TE Controller Host" and its signaling channels to the BFR,
the "Routing Underlay" and the "BIER-TE forwarding layer". The Bier-
Eckert, et al. Expires April 25, 2020 [Page 8]
Internet-Draft BIER-TE ARCH October 2019
TE Controller Host is the new architectural component in BIER-TE
compared to BIER.
Picture 2: Components of BIER-TE
<------BGP/PIM----->
|<-IGMP/PIM-> multicast flow <-PIM/IGMP->|
overlay
[BIER-TE Controller Host] <=> [BIER-TE Topology]
BIER-TE control plane
^ ^ ^
/ | \ BIER-TE control protocol
| | | e.g. Netconf/Restconf/Yang
v v v
Src -> Rtr1 -> BFIR-----BFR-----BFER -> Rtr2 -> Rcvr
|<----------------->|
BIER-TE forwarding layer
|<- BIER-TE domain->|
|<--------------------->|
Routing underlay
Figure 3: BIER-TE architecture
2.1. The Multicast Flow Overlay
The Multicast Flow Overlay operates as in BIER. See [RFC8279].
Instead of interacting with the BIER forwarding layer (as in BIER),
it interacts with the BIER-TE Controller Host.
2.2. The BIER-TE Controller Host
The BIER-TE controller host is representing the control plane of
BIER-TE. It communicates two sets of information with BFRs:
During initial provisioning or modifications of the network topology,
the controller discovers the network topology and creates the BIER-TE
topology from it: determine which adjacencies are required/desired
and assign BitPositions to them. Then it signals the resulting of
BitPositions and their adjacencies to each BFR to set up their BIER-
TE BIFTs.
During day-to-day operations of the network, the controller signals
to BFIRs what multicast flows are mapped to what BitStrings.
Eckert, et al. Expires April 25, 2020 [Page 9]
Internet-Draft BIER-TE ARCH October 2019
Communications between the BIER-TE controller host to BFRs is ideally
via standardized protocols and data-models such as Netconf/Restconf/
Yang. This is currently outside the scope of this document. Vendor-
specific CLI on the BFRs is also a possible stopgap option (as in
many other SDN solutions lacking definition of standardized data
model).
For simplicity, the procedures of the BIER-TE controller host are
described in this document as if it is a single, centralized
automated entity, such as an SDN controller. It could equally be an
operator setting up CLI on the BFRs. Distribution of the functions
of the BIER-TE controller host is currently outside the scope of this
document.
2.2.1. Assignment of BitPositions to adjacencies of the network
topology
The BIER-TE controller host tracks the BFR topology of the BIER-TE
domain. It determines what adjacencies require BitPositions so that
BIER-TE explicit paths can be built through them as desired by
operator policy.
The controller then pushes the BitPositions/adjacencies to the BIFT
of the BFRs, populating only those SI:BitPositions to the BIFT of
each BFR to which that BFR should be able to send packets to -
adjacencies connecting to this BFR.
2.2.2. Changes in the network topology
If the network topology changes (not failure based) so that
adjacencies that are assigned to BitPositions are no longer needed,
the controller can re-use those BitPositions for new adjacencies.
First, these BitPositions need to be removed from any BFIR flow state
and BFR BIFT state, then they can be repopulated, first into BIFT and
then into the BFIR.
2.2.3. Set up per-multicast flow BIER-TE state
The BIER-TE controller host interacts with the multicast flow overlay
to determine what multicast flow needs to be sent by a BFIR to which
set of BFER. It calculates the desired distribution tree across the
BIER-TE domain based on algorithms outside the scope of this document
(e.g. CSFP, Steiner Tree, ...). It then pushes the calculated
BitString into the BFIR.
See [I-D.ietf-bier-multicast-http-response] for a solution describing
this interaction.
Eckert, et al. Expires April 25, 2020 [Page 10]
Internet-Draft BIER-TE ARCH October 2019
2.2.4. Link/Node Failures and Recovery
When link or nodes fail or recover in the topology, BIER-TE can
quickly respond with the optional FRR procedures described in [I-
D.eckert-bier-te-frr]. It can also more slowly react by
recalculating the BitStrings of affected multicast flows. This
reaction is slower than the FRR procedure because the controller
needs to receive link/node up/down indications, recalculate the
desired BitStrings and push them down into the BFIRs. With FRR, this
is all performed locally on a BFR receiving the adjacency up/down
notification.
2.3. The BIER-TE Forwarding Layer
When the BIER-TE Forwarding Layer receives a packet, it simply looks
up the BitPositions that are set in the BitString of the packet in
the Bit Index Forwarding Table (BIFT) that was populated by the BIER-
TE controller host. For every BP that is set in the BitString, and
that has one or more adjacencies in the BIFT, a copy is made
according to the type of adjacencies for that BP in the BIFT. Before
sending any copy, the BFR resets all BP in the BitString of the
packet for which the BFR has one or more adjacencies in the BIFT,
except when the adjacency indicates "DoNotReset" (DNR, see
Section 3.2.1). This is done to inhibit that packets can loop.
2.4. The Routing Underlay
BIER-TE is sending BIER packets to directly connected BIER-TE
neighbors as L2 (unicasted) BIER packets without requiring a routing
underlay. BIER-TE forwarding uses the Routing underlay for
forward_routed adjacencies which copy BIER-TE packets to not-
directly-connected BFRs (see below for adjacency definitions).
If the BFR intends to support FRR for BIER-TE, then the BIER-TE
forwarding plane needs to receive fast adjacency up/down
notifications: Link up/down or neighbor up/down, e.g. from BFD.
Providing these notifications is considered to be part of the routing
underlay in this document.
3. BIER-TE Forwarding
3.1. The Bit Index Forwarding Table (BIFT)
The Bit Index Forwarding Table (BIFT) exists in every BFR. For every
subdomain in use, it is a table indexed by SI:BitPosition and is
populated by the BIER-TE control plane. Each index can be empty or
contain a list of one or more adjacencies.
Eckert, et al. Expires April 25, 2020 [Page 11]
Internet-Draft BIER-TE ARCH October 2019
BIER-TE can support multiple subdomains like BIER. Each one with a
separate BIFT
In the BIER architecture, indices into the BIFT are explained to be
both BFR-id and SI:BitString (BitPosition). This is because there is
a 1:1 relationship between BFR-id and SI:BitString - every bit in
every SI is/can be assigned to a BFIR/BFER. In BIER-TE there are
more bits used in each BitString than there are BFIR/BFER assigned to
the bitstring. This is because of the bits required to express the
(traffic engineered) path through the topology. The BIER-TE
forwarding definitions do therefore not use the term BFR-id at all.
Instead, BFR-ids are only used as required by routing underlay, flow
overlay of BIER headers. Please refer to Section 7 for explanations
how to deal with SI, subdomains and BFR-id in BIER-TE.
------------------------------------------------------------------
| Index: | Adjacencies: |
| SI:BitPosition | <empty> or one or more per entry |
==================================================================
| 0:1 | forward_connected(interface,neighbor,DNR) |
------------------------------------------------------------------
| 0:2 | forward_connected(interface,neighbor,DNR) |
| | forward_connected(interface,neighbor,DNR) |
------------------------------------------------------------------
| 0:3 | local_decap({VRF}) |
------------------------------------------------------------------
| 0:4 | forward_routed({VRF,}l3-neighbor) |
------------------------------------------------------------------
| 0:5 | <empty> |
------------------------------------------------------------------
| 0:6 | ECMP({adjacency1,...adjacencyN}, seed) |
------------------------------------------------------------------
...
| BitStringLength | ... |
------------------------------------------------------------------
Bit Index Forwarding Table
Figure 4: BIFT adjacencies
The BIFT is programmed into the data plane of BFRs by the BIER-TE
controller host and used to forward packets, according to the rules
specified in the BIER-TE Forwarding Procedures.
Adjacencies for the same BP when populated in more than one BFR by
the controller does not have to have the same adjacencies. This is
up to the controller. BPs for p2p links are one case (see below).
Eckert, et al. Expires April 25, 2020 [Page 12]
Internet-Draft BIER-TE ARCH October 2019
3.2. Adjacency Types
3.2.1. Forward Connected
A "forward_connected" adjacency is towards a directly connected BFR
neighbor using an interface address of that BFR on the connecting
interface. A forward_connected adjacency does not route packets but
only L2 forwards them to the neighbor.
Packets sent to an adjacency with "DoNotReset" (DNR) set in the BIFT
will not have the BitPosition for that adjacency reset when the BFR
creates a copy for it. The BitPosition will still be reset for
copies of the packet made towards other adjacencies. This can be
used for example in ring topologies as explained below.
3.2.2. Forward Routed
A "forward_routed" adjacency is an adjacency towards a BFR that is
not a forward_connected adjacency: towards a loopback address of a
BFR or towards an interface address that is non-directly connected.
Forward_routed packets are forwarded via the Routing Underlay.
If the Routing Underlay has multiple paths for a forward_routed
adjacency, it will perform ECMP independent of BIER-TE for packets
forwarded across a forward_routed adjacency.
If the Routing Underlay has FRR, it will perform FRR independent of
BIER-TE for packets forwarded across a forward_routed adjacency.
3.2.3. ECMP
The ECMP mechanisms in BIER are tied to the BIER BIFT and are
therefore not directly useable with BIER-TE. The following
procedures describe ECMP for BIER-TE that we consider to be
lightweight but also well manageable. It leverages the existing
entropy parameter in the BIER header to keep packets of the flows on
the same path and it introduces a "seed" parameter to allow
engineering traffic to be polarized or randomized across multiple
hops.
An "Equal Cost Multipath" (ECMP) adjacency has a list of two or more
adjacencies included in it. It copies the BIER-TE to one of those
adjacencies based on the ECMP hash calculation. The BIER-TE ECMP
hash algorithm must select the same adjacency from that list for all
packets with the same "entropy" value in the BIER-TE header if the
same number of adjacencies and same seed are given as parameters.
Further use of the seed parameter is explained below.
Eckert, et al. Expires April 25, 2020 [Page 13]
Internet-Draft BIER-TE ARCH October 2019
3.2.4. Local Decap
A "local_decap" adjacency passes a copy of the payload of the BIER-TE
packet to the packets NextProto within the BFR (IPv4/IPv6,
Ethernet,...). A local_decap adjacency turns the BFR into a BFER for
matching packets. Local_decap adjacencies require the BFER to
support routing or switching for NextProto to determine how to
further process the packet.
3.3. Encapsulation considerations
Specifications for BIER-TE encapsulation are outside the scope of
this document. This section gives explanations and guidelines.
Because a BFR needs to interpret the BitString of a BIER-TE packet
differently from a BIER packet, it is necessary to distinguish BIER
from BIER-TE packets. This is subject to definitions in BIER
encapsulation specifications.
MPLS encapsulation [RFC8296] for example assigns one label by which
BFRs recognizes BIER packets for every (SI,subdomain) combination.
If it is desirable that every subdomain can forward only BIER or
BIER-TE packets, then the label allocation could stay the same, and
only the forwarding model (BIER/BIER-TE) would have to be defined per
subdomain. If it is desirable to support both BIER and BIER-TE
forwarding in the same subdomain, then additional labels would need
to be assigned for BIER-TE forwarding.
"forward_routed" requires an encapsulation permitting to unicast
BIER-TE packets to a specific interface address on a target BFR.
With MPLS encapsulation, this can simply be done via a label stack
with that addresses label as the top label - followed by the label
assigned to (SI,subdomain) - and if necessary (see above) BIER-TE.
With non-MPLS encapsulation, some form of IP tunneling (IP in IP,
LISP, GRE) would be required.
The encapsulation used for "forward_routed" adjacencies can equally
support existing advanced adjacency information such as "loose source
routes" via e.g. MPLS label stacks or appropriate header extensions
(e.g. for IPv6).
3.4. Basic BIER-TE Forwarding Example
[RFC Editor: remove this section.]
THIS SECTION TO BE REMOVED IN RFC BECAUSE IT WAS SUPERCEEDED BY
SECTION 1.1 EXAMPLE - UNLESS REVIEWERS CHIME IN AND EXPRESS DESIRE TO
KEEP THIS ADDITIONAL EXAMPLE SECTION.
Eckert, et al. Expires April 25, 2020 [Page 14]
Internet-Draft BIER-TE ARCH October 2019
Step by step example of basic BIER-TE forwarding. This does not use
ECMP or forward_routed adjacencies nor does it try to minimize the
number of required BitPositions for the topology.
[Bier-Te Controller Host]
/ | \
v v v
| p13 p1 |
+- BFIR2 --+ |
| | p2 p6 | LAN2
| +-- BFR3 --+ |
| | | p7 p11 |
Src -+ +-- BFER1 --+
| | p3 p8 | |
| +-- BFR4 --+ +-- Rcv1
| | | |
| |
| p14 p4 |
+- BFIR1 --+ |
| +-- BFR5 --+ p10 p12 |
LAN1 | p5 p9 +-- BFER2 --+
| +-- Rcv2
|
LAN3
IP |..... BIER-TE network......| IP
Figure 5: BIER-TE Forwarding Example
pXX indicate the BitPositions number assigned by the BIER-TE
controller host to adjacencies in the BIER-TE topology. For example,
p9 is the adjacency towards BFR5 on the LAN connecting to BFER2.
Eckert, et al. Expires April 25, 2020 [Page 15]
Internet-Draft BIER-TE ARCH October 2019
BIFT BFIR2:
p13: local_decap()
p2: forward_connected(BFR3)
BIFT BFR3:
p1: forward_connected(BFIR2)
p7: forward_connected(BFER1)
p8: forward_connected(BFR4)
BIFT BFER1:
p11: local_decap()
p6: forward_connected(BFR3)
p8: forward_connected(BFR4)
Figure 6: BIER-TE Forwarding Example Adjacencies
...and so on.
For example, we assume that some multicast traffic seen on LAN1 needs
to be sent via BIER-TE by BFIR2 towards Rcv1 and Rcv2. The
controller determines it wants it to pass this traffic across the
following paths:
-> BFER1 ---------------> Rcv1
BFIR2 -> BFR3
-> BFR4 -> BFR5 -> BFER2 -> Rcv2
Figure 7: BIER-TE Forwarding Example Paths
These paths equal to the following BitString: p2, p5, p7, p8, p10,
p11, p12.
This BitString is assigned by BFIR2 to the example multicast traffic
received from LAN1.
Then BFIR2 forwards this multicast traffic with BIER-TE based on that
BitString. The BIFT of BFIR2 has only p2 and p13 populated. Only p2
is in the BitString and this is an adjacency towards BFR3. BFIR2
therefore resets p2 in the BitString and sends a copy towards BFR2.
BFR3 sees a BitString of p5,p7,p8,p10,p11,p12. It is only interested
in p1,p7,p8. It creates a copy of the packet to BFER1 (due to p7)
and one to BFR4 (due to p8). It resets p7, p8 before sending.
BFER1 sees a BitString of p5,p10,p11,p12. It is only interested in
p6,p7,p8,p11 and therefore considers only p11. p11 is a "local_decap"
adjacency installed by the BIER-TE controller host because BFER1
should pass packets to IP multicast. The local_decap adjacency
Eckert, et al. Expires April 25, 2020 [Page 16]
Internet-Draft BIER-TE ARCH October 2019
instructs BFER1 to create a copy, decapsulate it from the BIER header
and pass it on to the NextProtocol, in this example IP multicast. IP
multicast will then forward the packet out to LAN2 because it did
receive PIM or IGMP joins on LAN2 for the traffic.
Further processing of the packet in BFR4, BFR5 and BFER2 accordingly.
3.5. Forwarding comparison with BIER
Forwarding of BIER-TE is designed to allow common forwarding hardware
with BIER. In fact, one of the main goals of this document is to
encourage the building of forwarding hardware that cannot only
support BIER, but also BIER-TE - to allow experimentation with BIER-
TE and support building of BIER-TE control plane code.
The pseudocode in Section 6 shows how existing BIER/BIFT forwarding
can be amended to support basic BIER-TE forwarding, by using BIER
BIFT's F-BM. Only the masking of bits due to avoid duplicates must
be skipped when forwarding is for BIER-TE.
Whether to use BIER or BIER-TE forwarding can simply be a configured
choice per subdomain and accordingly be set up by a BIER-TE
controller host. The BIER packet encapsulation [RFC8296] too can be
reused without changes except that the currently defined BIER-TE ECMP
adjacency does not leverage the entropy field so that field would be
unused when BIER-TE forwarding is used.
3.6. Requirements
Basic BIER-TE forwarding MUST support to configure Subdomains to use
basic BIER-TE forwarding rules (instead of BIER). With basic BIER-TE
forwarding, every bit MUST support to have zero or one adjacency. It
MUST support the adjacency types forward_connected without DNR flag,
forward_routed and local_decap. All other BIER-TE forwarding
features are optional. These basic BIER-TE requirements make BIER-TE
forwarding exactly the same as BIER forwarding with the exception of
skipping the aforementioned F-BM masking on egress.
BIER-TE forwarding SHOULD support the DNR flag, as this is highly
useful to save bits in rings (see Section 4.6).
BIER-TE forwarding MAY support more than one adjacency on a bit and
ECMP adjacencies. The importance of ECMP adjacencies is unclear when
traffic engineering is used because it may be more desirable to
explicitly steer traffic across non-ECMP paths to make per-path
traffic calculation easier for controllers. Having more than one
adjacency for a bit allows further savings of bits in hub&spoke
scenarios, but unlike rings it is less "natural" to flood traffic
Eckert, et al. Expires April 25, 2020 [Page 17]
Internet-Draft BIER-TE ARCH October 2019
across multiple links unconditional. Both ECMP and multiple
adjacencies are forwarding plane features that should be possible to
support later when needed as they do not impact the basic BIER-TE
replication loop. This is true because there is no inter-copy
dependency through resetting of F-BM as in BIER.
4. BIER-TE Controller Host BitPosition Assignments
This section describes how the BIER-TE controller host can use the
different BIER-TE adjacency types to define the BitPositions of a
BIER-TE domain.
Because the size of the BitString is limiting the size of the BIER-TE
domain, many of the options described exist to support larger
topologies with fewer BitPositions (4.1, 4.3, 4.4, 4.5, 4.6, 4.7,
4.8).
4.1. P2P Links
Each P2p link in the BIER-TE domain is assigned one unique
BitPosition with a forward_connected adjacency pointing to the
neighbor on the p2p link.
4.2. BFER
Every BFER is given a unique BitPosition with a local_decap
adjacency.
4.3. Leaf BFERs
Leaf BFERs are BFERs where incoming BIER-TE packets never need to be
forwarded to another BFR but are only sent to the BFER to exit the
BIER-TE domain. For example, in networks where PEs are spokes
connected to P routers, those PEs are Leaf BFIRs unless there is a
U-turn between two PEs.
All leaf-BFER in a BIER-TE domain can share a single BitPosition.
This is possible because the BitPosition for the adjacency to reach
the BFER can be used to distinguish whether or not packets should
reach the BFER.
This optimization will not work if an upstream interface of the BFER
is using a BitPosition optimized as described in the following two
sections (LAN, Hub and Spoke).