/
draft-ietf-bier-te-arch-00.txt
1680 lines (1111 loc) · 64.6 KB
/
draft-ietf-bier-te-arch-00.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 Huawei
Intended status: Standards Track G. Cauchie
Expires: May 3, 2018 Bouygues Telecom
W. Braun
M. Menth
University of Tuebingen
October 30, 2017
Traffic Engineering for Bit Index Explicit Replication BIER-TE
draft-ietf-bier-te-arch-00
Abstract
This document proposes an architecture for BIER-TE: Traffic
Engineering for Bit Index Explicit Replication (BIER).
BIER-TE shares part of its architecture with BIER as described in
[I-D.ietf-bier-architecture]. It also proposes to share the packet
format with BIER.
BIER-TE forwards and replicates packets like BIER based on a
BitString in the packet header but it does not require an IGP. It
does support traffic engineering by explicit hop-by-hop forwarding
and loose hop forwarding of packets. It does support Fast ReRoute
(FRR) for link and node protection and incremental deployment.
Because BIER-TE like BIER operates without explicit in-network tree-
building but also supports traffic engineering, it is more similar to
SR than RSVP-TE.
Status of This Memo
This Internet-Draft is submitted in full conformance with the
provisions of BCP 78 and BCP 79.
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 May 3, 2018.
Eckert, et al. Expires May 3, 2018 [Page 1]
Internet-Draft BIER-TE ARCH October 2017
Copyright Notice
Copyright (c) 2017 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. Overview . . . . . . . . . . . . . . . . . . . . . . . . 3
1.2. Requirements Language . . . . . . . . . . . . . . . . . . 4
2. Layering . . . . . . . . . . . . . . . . . . . . . . . . . . 4
2.1. The Multicast Flow Overlay . . . . . . . . . . . . . . . 5
2.2. The BIER-TE Controller Host . . . . . . . . . . . . . . . 5
2.2.1. Assignment of BitPositions to adjacencies of the
network topology . . . . . . . . . . . . . . . . . . 6
2.2.2. Changes in the network topology . . . . . . . . . . . 6
2.2.3. Set up per-multicast flow BIER-TE state . . . . . . . 6
2.2.4. Link/Node Failures and Recovery . . . . . . . . . . . 6
2.3. The BIER-TE Forwarding Layer . . . . . . . . . . . . . . 7
2.4. The Routing Underlay . . . . . . . . . . . . . . . . . . 7
3. BIER-TE Forwarding . . . . . . . . . . . . . . . . . . . . . 7
3.1. The Bit Index Forwarding Table (BIFT) . . . . . . . . . . 7
3.2. Adjacency Types . . . . . . . . . . . . . . . . . . . . . 8
3.2.1. Forward Connected . . . . . . . . . . . . . . . . . . 8
3.2.2. Forward Routed . . . . . . . . . . . . . . . . . . . 9
3.2.3. ECMP . . . . . . . . . . . . . . . . . . . . . . . . 9
3.2.4. Local Decap . . . . . . . . . . . . . . . . . . . . . 9
3.3. Encapsulation considerations . . . . . . . . . . . . . . 10
3.4. Basic BIER-TE Forwarding Example . . . . . . . . . . . . 10
3.5. Forwarding comparison with BIER . . . . . . . . . . . . . 12
4. BIER-TE Controller Host BitPosition Assignments . . . . . . . 13
4.1. P2P Links . . . . . . . . . . . . . . . . . . . . . . . . 13
4.2. BFER . . . . . . . . . . . . . . . . . . . . . . . . . . 14
4.3. Leaf BFERs . . . . . . . . . . . . . . . . . . . . . . . 14
4.4. LANs . . . . . . . . . . . . . . . . . . . . . . . . . . 14
4.5. Hub and Spoke . . . . . . . . . . . . . . . . . . . . . . 15
4.6. Rings . . . . . . . . . . . . . . . . . . . . . . . . . . 15
4.7. Equal Cost MultiPath (ECMP) . . . . . . . . . . . . . . . 16
Eckert, et al. Expires May 3, 2018 [Page 2]
Internet-Draft BIER-TE ARCH October 2017
4.8. Routed adjacencies . . . . . . . . . . . . . . . . . . . 18
4.8.1. Reducing BitPositions . . . . . . . . . . . . . . . . 18
4.8.2. Supporting nodes without BIER-TE . . . . . . . . . . 18
5. Avoiding loops and duplicates . . . . . . . . . . . . . . . . 18
5.1. Loops . . . . . . . . . . . . . . . . . . . . . . . . . . 18
5.2. Duplicates . . . . . . . . . . . . . . . . . . . . . . . 19
6. BIER-TE Forwarding Pseudocode . . . . . . . . . . . . . . . . 19
7. Managing SI, subdomains and BFR-ids . . . . . . . . . . . . . 20
7.1. Why SI and sub-domains . . . . . . . . . . . . . . . . . 21
7.2. Bit assignment comparison BIER and BIER-TE . . . . . . . 22
7.3. Using BFR-id with BIER-TE . . . . . . . . . . . . . . . . 22
7.4. Assigning BFR-ids for BIER-TE . . . . . . . . . . . . . . 23
7.5. Example bit allocations . . . . . . . . . . . . . . . . . 24
7.5.1. With BIER . . . . . . . . . . . . . . . . . . . . . . 24
7.5.2. With BIER-TE . . . . . . . . . . . . . . . . . . . . 25
7.6. Summary . . . . . . . . . . . . . . . . . . . . . . . . . 26
8. BIER-TE and Segment Routing . . . . . . . . . . . . . . . . . 26
9. Security Considerations . . . . . . . . . . . . . . . . . . . 27
10. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 27
11. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 27
12. Change log [RFC Editor: Please remove] . . . . . . . . . . . 27
13. References . . . . . . . . . . . . . . . . . . . . . . . . . 29
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 29
1. Introduction
1.1. Overview
This document specifies the architecture for BIER-TE: traffic
engineering for Bit Index Explicit Replication BIER.
BIER-TE shares architecture and packet formats with BIER as described
in [I-D.ietf-bier-architecture].
BIER-TE forwards and replicates packets like BIER based on a
BitString in the packet header but it does not require an IGP. It
does support traffic engineering by explicit hop-by-hop forwarding
and loose hop forwarding of packets. It does support incremental
deployment and a Fast ReRoute (FRR) extension for link and node
protection is given in [I-D.eckert-bier-te-frr]. Because BIER-TE
like BIER operates without explicit in-network tree-building but also
supports traffic engineering, it is more similar to Segment Routing
(SR) than RSVP-TE.
The key differences over BIER are:
o BIER-TE replaces in-network autonomous path calculation by
explicit paths calculated offpath by the BIER-TE controller host.
Eckert, et al. Expires May 3, 2018 [Page 3]
Internet-Draft BIER-TE ARCH October 2017
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.
1.2. 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. Layering
End to end BIER-TE operations consists of four components: The
"Multicast Flow Overlay", the "BIER-TE Controller Host", the "Routing
Underlay" and the "BIER-TE forwarding layer".
Eckert, et al. Expires May 3, 2018 [Page 4]
Internet-Draft BIER-TE ARCH October 2017
Picture 2: Layers of BIER-TE
<------BGP/PIM----->
|<-IGMP/PIM-> multicast flow <-PIM/IGMP->|
overlay
[Bier-TE Controller Host]
^ ^ ^
/ | \ BIER-TE control protocol
| | | eg.: Netconf/Restconf/Yang
v v v
Src -> Rtr1 -> BFIR-----BFR-----BFER -> Rtr2 -> Rcvr
|--------------------->|
BIER-TE forwarding layer
|<- BIER-TE domain-->|
|<--------------------->|
Routing underlay
2.1. The Multicast Flow Overlay
The Multicast Flow Overlay operates as in BIER. See
[I-D.ietf-bier-architecture]. Instead of interacting with the BIER
layer, 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 bring-up or modifications of the network topology, the
controller discovers the network topology, assigns BitPositions to
adjacencies and signals the resulting mapping of BitPositions to
adjacencies to each BFR connecting to the adjacency.
During day-to-day operations of the network, the controller signals
to BFIRs what multicast flows are mapped to what BitStrings.
Communications between the BIER-TE controller host to BFRs is ideally
via standardized protocols and data-models such as Netconf/Retconf/
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).
Eckert, et al. Expires May 3, 2018 [Page 5]
Internet-Draft BIER-TE ARCH October 2017
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 tracks 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
(eg.: CSFP, Steiner Tree,...). It then pushes the calculated
BitString into the BFIR.
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
Eckert, et al. Expires May 3, 2018 [Page 6]
Internet-Draft BIER-TE ARCH October 2017
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 BitPositions in the BitString of
the packet to which it can create a copy. 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, eg.: 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.
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
Eckert, et al. Expires May 3, 2018 [Page 7]
Internet-Draft BIER-TE ARCH October 2017
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
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 do not have to have the same adjacencies. This is up
to the controller. BPs for p2p links are one case (see below).
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
Eckert, et al. Expires May 3, 2018 [Page 8]
Internet-Draft BIER-TE ARCH October 2017
copies of the packet made towards other adjacencies. The 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 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.
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.
Eckert, et al. Expires May 3, 2018 [Page 9]
Internet-Draft BIER-TE ARCH October 2017
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 [I-D.ietf-bier-mpls-encapsulation] 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 eg: MPLS label stacks or appropriate header extensions
(eg: for IPv6).
3.4. Basic BIER-TE Forwarding Example
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.
Eckert, et al. Expires May 3, 2018 [Page 10]
Internet-Draft BIER-TE ARCH October 2017
Picture 1: Forwarding Example
[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
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 BFR9 on the LAN connecting to BFER2.
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)
...and so on.
Traffic needs to flow from BFIR2 towards Rcv1, Rcv2. The controller
determines it wants it to pass across the following paths:
Eckert, et al. Expires May 3, 2018 [Page 11]
Internet-Draft BIER-TE ARCH October 2017
-> BFER1 ---------------> Rcv1
BFIR2 -> BFR3
-> BFR4 -> BFR5 -> BFER2 -> Rcv2
These paths equal to the following BitString: p2, p5, p7, p8, p10,
p11, p12.
This BitString is set up in BFIR2. Multicast packets arriving at
BFIR2 from Src are assigned this BitString.
BFIR2 forwards based on that BitString. It has p2 and p13 populated.
Only p13 is in BitString which has an adjacency towards BFR3. BFIR2
resets p2 in 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
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. Like BIER, the core of BIER-TE forwarding are BIFTs with
bitstring size number of entries: One for each bit of the bitstring
in the processed packet (consider that 256 is the most common size).
When a packet is received, the BIFT to process needs to be selected.
This is based on SI and subdomain like in BIER. How SI and subdomain
are indicated is subject to the BIER-TE encapsulation, but not BIER-T
itself. It is expected that the mechanisms for encapsulation will be
very similar if not the same to BIER, but this is subject to followup
work.
There are some key difference between the BIFT in BIER and BIER-TE:
In BIER-TE, each entry in the BIFT can have a list of 0 or more
adjacencies. A separate copy of the packet is made for each
adjacency. In BIER, each BIFT entry has at most one adjacency (BFR-
Eckert, et al. Expires May 3, 2018 [Page 12]
Internet-Draft BIER-TE ARCH October 2017
NBR). In BIER, different bits can not be processed independently
directly: Only one packet copy is to be sent for all bits in the
packet with the same adjacency, which is why the forwarding procedure
specifies how to sequentially identify those bits and avoid
duplication. In BIER-TE there are no mutual dependencies between bit
adjacencies, so all bits of a BIER-TE bitstring could be procssed
independently in parallel.
In BIER the BIFT has adjacencies for all BFR-ids assigned to BFER and
reachable in the IGP. In BIER-TE the BIFT only has adjacencies for
bits that are adjacent hops - intermediate or BFER. In forwarding,
this can be treated via the same lookup logic except that in BIER-TE
there is no step modifyin the original packet and the packet copy
bitstring with the FBM. Instead, all the bits locally processed are
reset in the original packet before looking up bits in the BIFT
(~MyBitsOfInterest). Only for an adjacency with the "DNR" (Do Not
Reset) bit set would the bit in the bitstring not be set again as
part of processing of the adjacency.
In summary, implementations of BIER forwarding that are to be
extended to also support BIER-TE forwarding primarily need to
consider how they can ensure that individual bit lookups can result
in a sequence of more than one copy to be made (as opposed to one in
BIER), and they need to see that they can accordingly reset bits in
the bitstring differently for BIER (per-packet) vs. BIER-TE (per-
paket-copy).
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.
Eckert, et al. Expires May 3, 2018 [Page 13]
Internet-Draft BIER-TE ARCH October 2017
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).
4.4. LANs
In a LAN, the adjacency to each neighboring BFR on the LAN is given a
unique BitPosition. The adjacency of this BitPosition is a
forward_connected adjacency towards the BFR and this BitPosition is
populated into the BIFT of all the other BFRs on that LAN.
BFR1
|p1
LAN1-+-+---+-----+
p3| p4| p2|
BFR3 BFR4 BFR7
If Bandwidth on the LAN is not an issue and most BIER-TE traffic
should be copied to all neighbors on a LAN, then BitPositions can be
saved by assigning just a single BitPosition to the LAN and
populating the BitPosition of the BIFTs of each BFRs on the LAN with
a list of forward_connected adjacencies to all other neighbors on the
LAN.
This optimization does not work in the face of BFRs redundantly
connected to more than one LANs with this optimization because these
BFRs would receive duplicates and forward those duplicates into the
opposite LANs. Adjacencies of such BFRs into their LANs still need a
separate BitPosition.
Eckert, et al. Expires May 3, 2018 [Page 14]
Internet-Draft BIER-TE ARCH October 2017
4.5. Hub and Spoke
In a setup with a hub and multiple spokes connected via separate p2p
links to the hub, all p2p links can share the same BitPosition. The
BitPosition on the hubs BIFT is set up with a list of
forward_connected adjacencies, one for each Spoke.
This option is similar to the BitPosition optimization in LANs:
Redundantly connected spokes need their own BitPositions.
4.6. Rings
In L3 rings, instead of assigning a single BitPosition for every p2p
link in the ring, it is possible to save BitPositions by setting the
"Do Not Reset" (DNR) flag on forward_connected adjacencies.
For the rings shown in the following picture, a single BitPosition
will suffice to forward traffic entering the ring at BFRa or BFRb all
the way up to BFR1:
On BFRa, BFRb, BFR30,... BFR3, the BitPosition is populated with a
forward_connected adjacency pointing to the clockwise neighbor on the
ring and with DNR set. On BFR2, the adjacency also points to the
clockwise neighbor BFR1, but without DNR set.
Handling DNR this way ensures that copies forwarded from any BFR in
the ring to a BFR outside the ring will not have the ring BitPosition
set, therefore minimizing the chance to create loops.
v v
| |
L1 | L2 | L3
/-------- BFRa ---- BFRb --------------------\
| |
\- BFR1 - BFR2 - BFR3 - ... - BFR29 - BFR30 -/
| | L4 | |
p33| p15|
BFRd BFRc
Note that this example only permits for packets to enter the ring at
BFRa and BFRb, and that packets will always travel clockwise. If
packets should be allowed to enter the ring at any ring BFR, then one
would have to use two ring BitPositions. One for clockwise, one for
counterclockwise.
Both would be set up to stop rotating on the same link, eg: L1. When
the ingress ring BFR creates the clockwise copy, it will reset the
counterclockwise BitPosition because the DNR bit only applies to the
Eckert, et al. Expires May 3, 2018 [Page 15]
Internet-Draft BIER-TE ARCH October 2017
bit for which the replication is done. Likewise for the clockwise
BitPosition for the counterclockwise copy. In result, the ring
ingress BFR will send a copy in both directions, serving BFRs on
either side of the ring up to L1.
4.7. Equal Cost MultiPath (ECMP)
The ECMP adjacency allows to use just one BP per link bundle between
two BFRs instead of one BP for each p2p member link of that link
bundle. In the following picture, one BP is used across L1,L2,L3 and
BFR1/BFR2 have for the BP
--L1-----
BFR1 --L2----- BFR2
--L3-----
BIFT entry in BFR1:
------------------------------------------------------------------
| Index | Adjacencies |
==================================================================
| 0:6 | ECMP({L1-to-BFR2,L2-to-BFR2,L3-to-BFR2}, seed) |
------------------------------------------------------------------
BIFT entry in BFR2:
------------------------------------------------------------------
| Index | Adjacencies |
==================================================================
| 0:6 | ECMP({L1-to-BFR1,L2-to-BFR1,L3-to-BFR1}, seed) |
------------------------------------------------------------------
In the following example, all traffic from BFR1 towards BFR10 is
intended to be ECMP load split equally across the topology. This
example is not mean as a likely setup, but to illustrate that ECMP
can be used to share BPs not only across link bundles, and it
explains the use of the seed parameter.
Eckert, et al. Expires May 3, 2018 [Page 16]
Internet-Draft BIER-TE ARCH October 2017
BFR1
/ \
/L11 \L12
BFR2 BFR3
/ \ / \
/L21 \L22 /L31 \L32
BFR4 BFR5 BFR6 BFR7
\ / \ /
\ / \ /
BFR8 BFR9
\ /
\ /
BFR10
BIFT entry in BFR1:
------------------------------------------------------------------
| 0:6 | ECMP({L11-to-BFR2,L12-to-BFR3}, seed) |
------------------------------------------------------------------
BIFT entry in BFR2:
------------------------------------------------------------------
| 0:6 | ECMP({L21-to-BFR4,L22-to-BFR5}, seed) |
------------------------------------------------------------------
BIFT entry in BFR3:
------------------------------------------------------------------
| 0:6 | ECMP({L31-to-BFR6,L32-to-BFR7}, seed) |
------------------------------------------------------------------
With the setup of ECMP in above topology, traffic would not be
equally load-split. Instead, links L22 and L31 would see no traffic
at all: BFR2 will only see traffic from BFR1 for which the ECMP hash
in BFR1 selected the first adjacency in a list of 2 adjacencies: link
L11-to-BFR2. When forwarding in BFR2 performs again an ECMP with two
adjacencies on that subset of traffic, then it will again select the
first of its two adjacencies to it: L21-to-BFR4. And therefore L22
and BFR5 sees no traffic.
To resolve this issue, the ECMP adjacency on BFR1 simply needs to be
set up with a different seed than the ECMP adjacencies on BFR2/BFR3
This issue is called polarization. It depends on the ECMP hash. It
is possible to build ECMP that does not have polarization, for
example by taking entropy from the actual adjacency members into
account, but that can make it harder to achieve evenly balanced load-
splitting on all BFR without making the ECMP hash algorithm
potentially too complex for fast forwarding in the BFRs.
Eckert, et al. Expires May 3, 2018 [Page 17]
Internet-Draft BIER-TE ARCH October 2017
4.8. Routed adjacencies
4.8.1. Reducing BitPositions
Routed adjacencies can reduce the number of BitPositions required
when the traffic engineering requirement is not hop-by-hop explicit
path selection, but loose-hop selection.
............... ...............
BFR1--... Redundant ...--L1-- BFR2... Redundant ...---
\--... Network ...--L2--/ ... Network ...---
BFR4--... Segment 1 ...--L3-- BFR3... Segment 2 ...---
............... ...............
Assume the requirement in above network is to explicitly engineer
paths such that specific traffic flows are passed from segment 1 to
segment 2 via link L1 (or via L2 or via L3).
To achieve this, BFR1 and BFR4 are set up with a forward_routed
adjacency BitPosition towards an address of BFR2 on link L1 (or link
L2 BFR3 via L3).
For paths to be engineered through a specific node BFR2 (or BFR3),
BFR1 and BFR4 are set up up with a forward_routed adjacency
BitPosition towards a loopback address of BFR2 (or BFR3).
4.8.2. Supporting nodes without BIER-TE
Routed adjacencies also enable incremental deployment of BIER-TE.
Only the nodes through which BIER-TE traffic needs to be steered -
with or without replication - need to support BIER-TE. Where they
are not directly connected to each other, forward_routed adjacencies
are used to pass over non BIER-TE enabled nodes.
5. Avoiding loops and duplicates
5.1. Loops
Whenever BIER-TE creates a copy of a packet, the BitString of that
copy will have all BitPositions cleared that are associated with
adjacencies in the BFR. This inhibits looping of packets. The only
exception are adjacencies with DNR set.
With DNR set, looping can happen. Consider in the ring picture that