File: rfc3032.txt

package info (click to toggle)
doc-rfc 20181229-2
  • links: PTS, VCS
  • area: non-free
  • in suites: buster
  • size: 570,944 kB
  • sloc: xml: 285,646; sh: 107; python: 90; perl: 42; makefile: 14
file content (1291 lines) | stat: -rw-r--r-- 48,314 bytes parent folder | download | duplicates (5)
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
61
62
63
64
65
66
67
68
69
70
71
72
73
74
75
76
77
78
79
80
81
82
83
84
85
86
87
88
89
90
91
92
93
94
95
96
97
98
99
100
101
102
103
104
105
106
107
108
109
110
111
112
113
114
115
116
117
118
119
120
121
122
123
124
125
126
127
128
129
130
131
132
133
134
135
136
137
138
139
140
141
142
143
144
145
146
147
148
149
150
151
152
153
154
155
156
157
158
159
160
161
162
163
164
165
166
167
168
169
170
171
172
173
174
175
176
177
178
179
180
181
182
183
184
185
186
187
188
189
190
191
192
193
194
195
196
197
198
199
200
201
202
203
204
205
206
207
208
209
210
211
212
213
214
215
216
217
218
219
220
221
222
223
224
225
226
227
228
229
230
231
232
233
234
235
236
237
238
239
240
241
242
243
244
245
246
247
248
249
250
251
252
253
254
255
256
257
258
259
260
261
262
263
264
265
266
267
268
269
270
271
272
273
274
275
276
277
278
279
280
281
282
283
284
285
286
287
288
289
290
291
292
293
294
295
296
297
298
299
300
301
302
303
304
305
306
307
308
309
310
311
312
313
314
315
316
317
318
319
320
321
322
323
324
325
326
327
328
329
330
331
332
333
334
335
336
337
338
339
340
341
342
343
344
345
346
347
348
349
350
351
352
353
354
355
356
357
358
359
360
361
362
363
364
365
366
367
368
369
370
371
372
373
374
375
376
377
378
379
380
381
382
383
384
385
386
387
388
389
390
391
392
393
394
395
396
397
398
399
400
401
402
403
404
405
406
407
408
409
410
411
412
413
414
415
416
417
418
419
420
421
422
423
424
425
426
427
428
429
430
431
432
433
434
435
436
437
438
439
440
441
442
443
444
445
446
447
448
449
450
451
452
453
454
455
456
457
458
459
460
461
462
463
464
465
466
467
468
469
470
471
472
473
474
475
476
477
478
479
480
481
482
483
484
485
486
487
488
489
490
491
492
493
494
495
496
497
498
499
500
501
502
503
504
505
506
507
508
509
510
511
512
513
514
515
516
517
518
519
520
521
522
523
524
525
526
527
528
529
530
531
532
533
534
535
536
537
538
539
540
541
542
543
544
545
546
547
548
549
550
551
552
553
554
555
556
557
558
559
560
561
562
563
564
565
566
567
568
569
570
571
572
573
574
575
576
577
578
579
580
581
582
583
584
585
586
587
588
589
590
591
592
593
594
595
596
597
598
599
600
601
602
603
604
605
606
607
608
609
610
611
612
613
614
615
616
617
618
619
620
621
622
623
624
625
626
627
628
629
630
631
632
633
634
635
636
637
638
639
640
641
642
643
644
645
646
647
648
649
650
651
652
653
654
655
656
657
658
659
660
661
662
663
664
665
666
667
668
669
670
671
672
673
674
675
676
677
678
679
680
681
682
683
684
685
686
687
688
689
690
691
692
693
694
695
696
697
698
699
700
701
702
703
704
705
706
707
708
709
710
711
712
713
714
715
716
717
718
719
720
721
722
723
724
725
726
727
728
729
730
731
732
733
734
735
736
737
738
739
740
741
742
743
744
745
746
747
748
749
750
751
752
753
754
755
756
757
758
759
760
761
762
763
764
765
766
767
768
769
770
771
772
773
774
775
776
777
778
779
780
781
782
783
784
785
786
787
788
789
790
791
792
793
794
795
796
797
798
799
800
801
802
803
804
805
806
807
808
809
810
811
812
813
814
815
816
817
818
819
820
821
822
823
824
825
826
827
828
829
830
831
832
833
834
835
836
837
838
839
840
841
842
843
844
845
846
847
848
849
850
851
852
853
854
855
856
857
858
859
860
861
862
863
864
865
866
867
868
869
870
871
872
873
874
875
876
877
878
879
880
881
882
883
884
885
886
887
888
889
890
891
892
893
894
895
896
897
898
899
900
901
902
903
904
905
906
907
908
909
910
911
912
913
914
915
916
917
918
919
920
921
922
923
924
925
926
927
928
929
930
931
932
933
934
935
936
937
938
939
940
941
942
943
944
945
946
947
948
949
950
951
952
953
954
955
956
957
958
959
960
961
962
963
964
965
966
967
968
969
970
971
972
973
974
975
976
977
978
979
980
981
982
983
984
985
986
987
988
989
990
991
992
993
994
995
996
997
998
999
1000
1001
1002
1003
1004
1005
1006
1007
1008
1009
1010
1011
1012
1013
1014
1015
1016
1017
1018
1019
1020
1021
1022
1023
1024
1025
1026
1027
1028
1029
1030
1031
1032
1033
1034
1035
1036
1037
1038
1039
1040
1041
1042
1043
1044
1045
1046
1047
1048
1049
1050
1051
1052
1053
1054
1055
1056
1057
1058
1059
1060
1061
1062
1063
1064
1065
1066
1067
1068
1069
1070
1071
1072
1073
1074
1075
1076
1077
1078
1079
1080
1081
1082
1083
1084
1085
1086
1087
1088
1089
1090
1091
1092
1093
1094
1095
1096
1097
1098
1099
1100
1101
1102
1103
1104
1105
1106
1107
1108
1109
1110
1111
1112
1113
1114
1115
1116
1117
1118
1119
1120
1121
1122
1123
1124
1125
1126
1127
1128
1129
1130
1131
1132
1133
1134
1135
1136
1137
1138
1139
1140
1141
1142
1143
1144
1145
1146
1147
1148
1149
1150
1151
1152
1153
1154
1155
1156
1157
1158
1159
1160
1161
1162
1163
1164
1165
1166
1167
1168
1169
1170
1171
1172
1173
1174
1175
1176
1177
1178
1179
1180
1181
1182
1183
1184
1185
1186
1187
1188
1189
1190
1191
1192
1193
1194
1195
1196
1197
1198
1199
1200
1201
1202
1203
1204
1205
1206
1207
1208
1209
1210
1211
1212
1213
1214
1215
1216
1217
1218
1219
1220
1221
1222
1223
1224
1225
1226
1227
1228
1229
1230
1231
1232
1233
1234
1235
1236
1237
1238
1239
1240
1241
1242
1243
1244
1245
1246
1247
1248
1249
1250
1251
1252
1253
1254
1255
1256
1257
1258
1259
1260
1261
1262
1263
1264
1265
1266
1267
1268
1269
1270
1271
1272
1273
1274
1275
1276
1277
1278
1279
1280
1281
1282
1283
1284
1285
1286
1287
1288
1289
1290
1291






Network Working Group                                           E. Rosen
Request for Comments: 3032                                     D. Tappan
Category: Standards Track                                    G. Fedorkow
                                                     Cisco Systems, Inc.
                                                              Y. Rekhter
                                                        Juniper Networks
                                                            D. Farinacci
                                                                   T. Li
                                                  Procket Networks, Inc.
                                                                A. Conta
                                                  TranSwitch Corporation
                                                            January 2001


                       MPLS Label Stack Encoding

Status of this Memo

   This document specifies an Internet standards track protocol for the
   Internet community, and requests discussion and suggestions for
   improvements.  Please refer to the current edition of the "Internet
   Official Protocol Standards" (STD 1) for the standardization state
   and status of this protocol.  Distribution of this memo is unlimited.

Copyright Notice

   Copyright (C) The Internet Society (2001).  All Rights Reserved.

Abstract

   "Multi-Protocol Label Switching (MPLS)" [1] requires a set of
   procedures for augmenting network layer packets with "label stacks",
   thereby turning them into "labeled packets".  Routers which support
   MPLS are known as "Label Switching Routers", or "LSRs".  In order to
   transmit a labeled packet on a particular data link, an LSR must
   support an encoding technique which, given a label stack and a
   network layer packet, produces a labeled packet.  This document
   specifies the encoding to be used by an LSR in order to transmit
   labeled packets on Point-to-Point Protocol (PPP) data links, on LAN
   data links, and possibly on other data links as well.  On some data
   links, the label at the top of the stack may be encoded in a
   different manner, but the techniques described here MUST be used to
   encode the remainder of the label stack.  This document also
   specifies rules and procedures for processing the various fields of
   the label stack encoding.






Rosen, et al.               Standards Track                     [Page 1]

RFC 3032               MPLS Label Stack Encoding            January 2001


Table of Contents

    1      Introduction  ...........................................  2
    1.1    Specification of Requirements  ..........................  3
    2      The Label Stack  ........................................  3
    2.1    Encoding the Label Stack  ...............................  3
    2.2    Determining the Network Layer Protocol  .................  5
    2.3    Generating ICMP Messages for Labeled IP Packets  ........  6
    2.3.1  Tunneling through a Transit Routing Domain  .............  7
    2.3.2  Tunneling Private Addresses through a Public Backbone  ..  7
    2.4    Processing the Time to Live Field  ......................  8
    2.4.1  Definitions  ............................................  8
    2.4.2  Protocol-independent rules  .............................  8
    2.4.3  IP-dependent rules  .....................................  9
    2.4.4  Translating Between Different Encapsulations  ...........  9
    3      Fragmentation and Path MTU Discovery  ................... 10
    3.1    Terminology  ............................................ 11
    3.2    Maximum Initially Labeled IP Datagram Size  ............. 12
    3.3    When are Labeled IP Datagrams Too Big?  ................. 13
    3.4    Processing Labeled IPv4 Datagrams which are Too Big  .... 13
    3.5    Processing Labeled IPv6 Datagrams which are Too Big  .... 14
    3.6    Implications with respect to Path MTU Discovery  ........ 15
    4      Transporting Labeled Packets over PPP  .................. 16
    4.1    Introduction  ........................................... 16
    4.2    A PPP Network Control Protocol for MPLS  ................ 17
    4.3    Sending Labeled Packets  ................................ 18
    4.4    Label Switching Control Protocol Configuration Options  . 18
    5      Transporting Labeled Packets over LAN Media  ............ 18
    6      IANA Considerations  .................................... 19
    7      Security Considerations  ................................ 19
    8      Intellectual Property  .................................. 19
    9      Authors' Addresses  ..................................... 20
   10      References  ............................................. 22
   11      Full Copyright Statement  ............................... 23

1. Introduction

   "Multi-Protocol Label Switching (MPLS)" [1] requires a set of
   procedures for augmenting network layer packets with "label stacks",
   thereby turning them into "labeled packets".  Routers which support
   MPLS are known as "Label Switching Routers", or "LSRs".  In order to
   transmit a labeled packet on a particular data link, an LSR must
   support an encoding technique which, given a label stack and a
   network layer packet, produces a labeled packet.







Rosen, et al.               Standards Track                     [Page 2]

RFC 3032               MPLS Label Stack Encoding            January 2001


   This document specifies the encoding to be used by an LSR in order to
   transmit labeled packets on PPP data links and on LAN data links.
   The specified encoding may also be useful for other data links as
   well.

   This document also specifies rules and procedures for processing the
   various fields of the label stack encoding.  Since MPLS is
   independent of any particular network layer protocol, the majority of
   such procedures are also protocol-independent.  A few, however, do
   differ for different protocols.  In this document, we specify the
   protocol-independent procedures, and we specify the protocol-
   dependent procedures for IPv4 and IPv6.

   LSRs that are implemented on certain switching devices (such as ATM
   switches) may use different encoding techniques for encoding the top
   one or two entries of the label stack.  When the label stack has
   additional entries, however, the encoding technique described in this
   document MUST be used for the additional label stack entries.

1.1. Specification of Requirements

   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 [2].

2. The Label Stack

2.1. Encoding the Label Stack

   The label stack is represented as a sequence of "label stack
   entries".  Each label stack entry is represented by 4 octets.  This
   is shown in Figure 1.

 0                   1                   2                   3
 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ Label
|                Label                  | Exp |S|       TTL     | Stack
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ Entry

                    Label:  Label Value, 20 bits
                    Exp:    Experimental Use, 3 bits
                    S:      Bottom of Stack, 1 bit
                    TTL:    Time to Live, 8 bits

                              Figure 1






Rosen, et al.               Standards Track                     [Page 3]

RFC 3032               MPLS Label Stack Encoding            January 2001


   The label stack entries appear AFTER the data link layer headers, but
   BEFORE any network layer headers.  The top of the label stack appears
   earliest in the packet, and the bottom appears latest.  The network
   layer packet immediately follows the label stack entry which has the
   S bit set.

   Each label stack entry is broken down into the following fields:

      1. Bottom of Stack (S)

         This bit is set to one for the last entry in the label stack
         (i.e., for the bottom of the stack), and zero for all other
         label stack entries.

      2. Time to Live (TTL)

         This eight-bit field is used to encode a time-to-live value.
         The processing of this field is described in section 2.4.

      3. Experimental Use

         This three-bit field is reserved for experimental use.

      4. Label Value

         This 20-bit field carries the actual value of the Label.

         When a labeled packet is received, the label value at the top
         of the stack is looked up.  As a result of a successful lookup
         one learns:

         a) the next hop to which the packet is to be forwarded;

         b) the operation to be performed on the label stack before
            forwarding; this operation may be to replace the top label
            stack entry with another, or to pop an entry off the label
            stack, or to replace the top label stack entry and then to
            push one or more additional entries on the label stack.

         In addition to learning the next hop and the label stack
         operation, one may also learn the outgoing data link
         encapsulation, and possibly other information which is needed
         in order to properly forward the packet.








Rosen, et al.               Standards Track                     [Page 4]

RFC 3032               MPLS Label Stack Encoding            January 2001


         There are several reserved label values:

           i. A value of 0 represents the "IPv4 Explicit NULL Label".
              This label value is only legal at the bottom of the label
              stack.  It indicates that the label stack must be popped,
              and the forwarding of the packet must then be based on the
              IPv4 header.

          ii. A value of 1 represents the "Router Alert Label".  This
              label value is legal anywhere in the label stack except at
              the bottom.  When a received packet contains this label
              value at the top of the label stack, it is delivered to a
              local software module for processing.  The actual
              forwarding of the packet is determined by the label
              beneath it in the stack.  However, if the packet is
              forwarded further, the Router Alert Label should be pushed
              back onto the label stack before forwarding.  The use of
              this label is analogous to the use of the "Router Alert
              Option" in IP packets [5].  Since this label cannot occur
              at the bottom of the stack, it is not associated with a
              particular network layer protocol.

         iii. A value of 2 represents the "IPv6 Explicit NULL Label".
              This label value is only legal at the bottom of the label
              stack.  It indicates that the label stack must be popped,
              and the forwarding of the packet must then be based on the
              IPv6 header.

          iv. A value of 3 represents the "Implicit NULL Label".  This
              is a label that an LSR may assign and distribute, but
              which never actually appears in the encapsulation.  When
              an LSR would otherwise replace the label at the top of the
              stack with a new label, but the new label is "Implicit
              NULL", the LSR will pop the stack instead of doing the
              replacement.  Although this value may never appear in the
              encapsulation, it needs to be specified in the Label
              Distribution Protocol, so a value is reserved.

           v. Values 4-15 are reserved.

2.2. Determining the Network Layer Protocol

   When the last label is popped from a packet's label stack (resulting
   in the stack being emptied), further processing of the packet is
   based on the packet's network layer header.  The LSR which pops the
   last label off the stack must therefore be able to identify the
   packet's network layer protocol.  However, the label stack does not
   contain any field which explicitly identifies the network layer



Rosen, et al.               Standards Track                     [Page 5]

RFC 3032               MPLS Label Stack Encoding            January 2001


   protocol.  This means that the identity of the network layer protocol
   must be inferable from the value of the label which is popped from
   the bottom of the stack, possibly along with the contents of the
   network layer header itself.

   Therefore, when the first label is pushed onto a network layer
   packet, either the label must be one which is used ONLY for packets
   of a particular network layer, or the label must be one which is used
   ONLY for a specified set of network layer protocols, where packets of
   the specified network layers can be distinguished by inspection of
   the network layer header.  Furthermore, whenever that label is
   replaced by another label value during a packet's transit, the new
   value must also be one which meets the same criteria.  If these
   conditions are not met, the LSR which pops the last label off a
   packet will not be able to identify the packet's network layer
   protocol.

   Adherence to these conditions does not necessarily enable
   intermediate nodes to identify a packet's network layer protocol.
   Under ordinary conditions, this is not necessary, but there are error
   conditions under which it is desirable.  For instance, if an
   intermediate LSR determines that a labeled packet is undeliverable,
   it may be desirable for that LSR to generate error messages which are
   specific to the packet's network layer.  The only means the
   intermediate LSR has for identifying the network layer is inspection
   of the top label and the network layer header.  So if intermediate
   nodes are to be able to generate protocol-specific error messages for
   labeled packets, all labels in the stack must meet the criteria
   specified above for labels which appear at the bottom of the stack.

   If a packet cannot be forwarded for some reason (e.g., it exceeds the
   data link MTU), and either its network layer protocol cannot be
   identified, or there are no specified protocol-dependent rules for
   handling the error condition, then the packet MUST be silently
   discarded.

2.3. Generating ICMP Messages for Labeled IP Packets

   Section 2.4 and section 3 discuss situations in which it is desirable
   to generate ICMP messages for labeled IP packets.  In order for a
   particular LSR to be able to generate an ICMP packet and have that
   packet sent to the source of the IP packet, two conditions must hold:

      1. it must be possible for that LSR to determine that a particular
         labeled packet is an IP packet;

      2. it must be possible for that LSR to route to the packet's IP
         source address.



Rosen, et al.               Standards Track                     [Page 6]

RFC 3032               MPLS Label Stack Encoding            January 2001


   Condition 1 is discussed in section 2.2.  The following two
   subsections discuss condition 2.  However, there will be some cases
   in which condition 2 does not hold at all, and in these cases it will
   not be possible to generate the ICMP message.

2.3.1. Tunneling through a Transit Routing Domain

   Suppose one is using MPLS to "tunnel" through a transit routing
   domain, where the external routes are not leaked into the domain's
   interior routers.  For example, the interior routers may be running
   OSPF, and may only know how to reach destinations within that OSPF
   domain.  The domain might contain several Autonomous System Border
   Routers (ASBRs), which talk BGP to each other.  However, in this
   example the routes from BGP are not distributed into OSPF, and the
   LSRs which are not ASBRs do not run BGP.

   In this example, only an ASBR will know how to route to the source of
   some arbitrary packet.  If an interior router needs to send an ICMP
   message to the source of an IP packet, it will not know how to route
   the ICMP message.

   One solution is to have one or more of the ASBRs inject "default"
   into the IGP.  (N.B.: this does NOT require that there be a "default"
   carried by BGP.)  This would then ensure that any unlabeled packet
   which must leave the domain (such as an ICMP packet) gets sent to a
   router which has full routing information.  The routers with full
   routing information will label the packets before sending them back
   through the transit domain, so the use of default routing within the
   transit domain does not cause any loops.

   This solution only works for packets which have globally unique
   addresses, and for networks in which all the ASBRs have complete
   routing information.  The next subsection describes a solution which
   works when these conditions do not hold.

2.3.2. Tunneling Private Addresses through a Public Backbone

   In some cases where MPLS is used to tunnel through a routing domain,
   it may not be possible to route to the source address of a fragmented
   packet at all.  This would be the case, for example, if the IP
   addresses carried in the packet were private (i.e., not globally
   unique) addresses, and MPLS were being used to tunnel those packets
   through a public backbone.  Default routing to an ASBR will not work
   in this environment.

   In this environment, in order to send an ICMP message to the source
   of a packet, one can copy the label stack from the original packet to
   the ICMP message, and then label switch the ICMP message.  This will



Rosen, et al.               Standards Track                     [Page 7]

RFC 3032               MPLS Label Stack Encoding            January 2001


   cause the message to proceed in the direction of the original
   packet's destination, rather than its source.  Unless the message is
   label switched all the way to the destination host, it will end up,
   unlabeled, in a router which does know how to route to the source of
   original packet, at which point the message will be sent in the
   proper direction.

   This technique can be very useful if the ICMP message is a "Time
   Exceeded" message or a "Destination Unreachable because fragmentation
   needed and DF set" message.

   When copying the label stack from the original packet to the ICMP
   message, the label values must be copied exactly, but the TTL values
   in the label stack should be set to the TTL value that is placed in
   the IP header of the ICMP message.  This TTL value should be long
   enough to allow the circuitous route that the ICMP message will need
   to follow.

   Note that if a packet's TTL expiration is due to the presence of a
   routing loop, then if this technique is used, the ICMP message may
   loop as well.  Since an ICMP message is  never sent as a result of
   receiving an ICMP message, and since many implementations throttle
   the rate at which ICMP messages can be generated, this is not
   expected to pose a problem.

2.4. Processing the Time to Live Field

2.4.1. Definitions

   The "incoming TTL" of a labeled packet is defined to be the value of
   the TTL field of the top label stack entry when the packet is
   received.

   The "outgoing TTL" of a labeled packet is defined to be the larger
   of:

      a) one less than the incoming TTL,
      b) zero.

2.4.2. Protocol-independent rules

   If the outgoing TTL of a labeled packet is 0, then the labeled packet
   MUST NOT be further forwarded; nor may the label stack be stripped
   off and the packet forwarded as an unlabeled packet.  The packet's
   lifetime in the network is considered to have expired.






Rosen, et al.               Standards Track                     [Page 8]

RFC 3032               MPLS Label Stack Encoding            January 2001


   Depending on the label value in the label stack entry, the packet MAY
   be simply discarded, or it may be passed to the appropriate
   "ordinary" network layer for error processing (e.g., for the
   generation of an ICMP error message, see section 2.3).

   When a labeled packet is forwarded, the TTL field of the label stack
   entry at the top of the label stack MUST be set to the outgoing TTL
   value.

   Note that the outgoing TTL value is a function solely of the incoming
   TTL value, and is independent of whether any labels are pushed or
   popped before forwarding.  There is no significance to the value of
   the TTL field in any label stack entry which is not at the top of the
   stack.

2.4.3. IP-dependent rules

   We define the "IP TTL" field to be the value of the IPv4 TTL field,
   or the value of the IPv6 Hop Limit field, whichever is applicable.

   When an IP packet is first labeled, the TTL field of the label stack
   entry MUST BE set to the value of the IP TTL field.  (If the IP TTL
   field needs to be decremented, as part of the IP processing, it is
   assumed that this has already been done.)

   When a label is popped, and the resulting label stack is empty, then
   the value of the IP TTL field SHOULD BE replaced with the outgoing
   TTL value, as defined above.  In IPv4 this also requires modification
   of the IP header checksum.

   It is recognized that there may be situations where a network
   administration prefers to decrement the IPv4 TTL by one as it
   traverses an MPLS domain, instead of decrementing the IPv4 TTL by the
   number of LSP hops within the domain.

2.4.4. Translating Between Different Encapsulations

   Sometimes an LSR may receive a labeled packet over, e.g., a label
   switching controlled ATM (LC-ATM) interface [9], and may need to send
   it out over a PPP or LAN link.  Then the incoming packet will not be
   received using the encapsulation specified in this document, but the
   outgoing packet will be sent using the encapsulation specified in
   this document.

   In this case, the value of the "incoming TTL" is determined by the
   procedures used for carrying labeled packets on, e.g., LC-ATM
   interfaces.  TTL processing then proceeds as described above.




Rosen, et al.               Standards Track                     [Page 9]

RFC 3032               MPLS Label Stack Encoding            January 2001


   Sometimes an LSR may receive a labeled packet over a PPP or a LAN
   link, and may need to send it out, say, an LC-ATM interface.  Then
   the incoming packet will be received using the encapsulation
   specified in this document, but the outgoing packet will not be sent
   using the encapsulation specified in this document.  In this case,
   the procedure for carrying the value of the "outgoing TTL" is
   determined by the procedures used for carrying labeled packets on,
   e.g., LC-ATM interfaces.

3. Fragmentation and Path MTU Discovery

   Just as it is possible to receive an unlabeled IP datagram which is
   too large to be transmitted on its output link, it is possible to
   receive a labeled packet which is too large to be transmitted on its
   output link.

   It is also possible that a received packet (labeled or unlabeled)
   which was originally small enough to be transmitted on that link
   becomes too large by virtue of having one or more additional labels
   pushed onto its label stack.  In label switching, a packet may grow
   in size if additional labels get pushed on.  Thus if one receives a
   labeled packet with a 1500-byte frame payload, and pushes on an
   additional label, one needs to forward it as frame with a 1504-byte
   payload.

   This section specifies the rules for processing labeled packets which
   are "too large".  In particular, it provides rules which ensure that
   hosts implementing Path MTU Discovery [4], and hosts using IPv6
   [7,8], will be able to generate IP datagrams that do not need
   fragmentation, even if those datagrams get labeled as they traverse
   the network.

   In general, IPv4 hosts which do not implement Path MTU Discovery [4]
   send IP datagrams which contain no more than 576 bytes.  Since the
   MTUs in use on most data links today are 1500 bytes or more, the
   probability that such datagrams will need to get fragmented, even if
   they get labeled, is very small.

   Some hosts that do not implement Path MTU Discovery [4] will generate
   IP datagrams containing 1500 bytes, as long as the IP Source and
   Destination addresses are on the same subnet.  These datagrams will
   not pass through routers, and hence will not get fragmented.

   Unfortunately, some hosts will generate IP datagrams containing 1500
   bytes, as long the IP Source and Destination addresses have the same
   classful network number.  This is the one case in which there is any
   risk of fragmentation when such datagrams get labeled.  (Even so,




Rosen, et al.               Standards Track                    [Page 10]

RFC 3032               MPLS Label Stack Encoding            January 2001


   fragmentation is not likely unless the packet must traverse an
   ethernet of some sort between the time it first gets labeled and the
   time it gets unlabeled.)

   This document specifies procedures which allow one to configure the
   network so that large datagrams from hosts which do not implement
   Path MTU Discovery get fragmented just once, when they are first
   labeled.  These procedures make it possible (assuming suitable
   configuration) to avoid any need to fragment packets which have
   already been labeled.

3.1. Terminology

   With respect to a particular data link, we can use the following
   terms:

      -  Frame Payload:

         The contents of a data link frame, excluding any data link
         layer headers or trailers (e.g., MAC headers, LLC headers,
         802.1Q headers, PPP header, frame check sequences, etc.).

         When a frame is carrying an unlabeled IP datagram, the Frame
         Payload is just the IP datagram itself.  When a frame is
         carrying a labeled IP datagram, the Frame Payload consists of
         the label stack entries and the IP datagram.

      -  Conventional Maximum Frame Payload Size:

         The maximum Frame Payload size allowed by data link standards.
         For example, the Conventional Maximum Frame Payload Size for
         ethernet is 1500 bytes.

      -  True Maximum Frame Payload Size:

         The maximum size frame payload which can be sent and received
         properly by the interface hardware attached to the data link.

         On ethernet and 802.3 networks, it is believed that the True
         Maximum Frame Payload Size is 4-8 bytes larger than the
         Conventional Maximum Frame Payload Size (as long as neither an
         802.1Q header nor an 802.1p header is present, and as long as
         neither can be added by a switch or bridge while a packet is in
         transit to its next hop).  For example, it is believed that
         most ethernet equipment could correctly send and receive
         packets carrying a payload of 1504 or perhaps even 1508 bytes,
         at least, as long as the ethernet header does not have an
         802.1Q or 802.1p field.



Rosen, et al.               Standards Track                    [Page 11]

RFC 3032               MPLS Label Stack Encoding            January 2001


         On PPP links, the True Maximum Frame Payload Size may be
         virtually unbounded.

      -  Effective Maximum Frame Payload Size for Labeled Packets:

         This is either the Conventional Maximum Frame Payload Size or
         the True Maximum Frame Payload Size, depending on the
         capabilities of the equipment on the data link and the size of
         the data link header being used.

      -  Initially Labeled IP Datagram:

         Suppose that an unlabeled IP datagram is received at a
         particular LSR, and that the the LSR pushes on a label before
         forwarding the datagram.  Such a datagram will be called an
         Initially Labeled IP Datagram at that LSR.

      -  Previously Labeled IP Datagram:

         An IP datagram which had already been labeled before it was
         received by a particular LSR.

3.2. Maximum Initially Labeled IP Datagram Size

   Every LSR which is capable of

      a) receiving an unlabeled IP datagram,
      b) adding a label stack to the datagram, and
      c) forwarding the resulting labeled packet,

   SHOULD support a configuration parameter known as the "Maximum
   Initially Labeled IP Datagram Size", which can be set to a non-
   negative value.

   If this configuration parameter is set to zero, it has no effect.

   If it is set to a positive value, it is used in the following way.
   If:

      a) an unlabeled IP datagram is received, and
      b) that datagram does not have the DF bit set in its IP header,
         and
      c) that datagram needs to be labeled before being forwarded, and
      d) the size of the datagram (before labeling) exceeds the value of
         the parameter,
   then
      a) the datagram must be broken into fragments, each of whose size
         is no greater than the value of the parameter, and



Rosen, et al.               Standards Track                    [Page 12]

RFC 3032               MPLS Label Stack Encoding            January 2001


      b) each fragment must be labeled and then forwarded.

   For example, if this configuration parameter is set to a value of
   1488, then any unlabeled IP datagram containing more than 1488 bytes
   will be fragmented before being labeled.  Each fragment will be
   capable of being carried on a 1500-byte data link, without further
   fragmentation, even if as many as three labels are pushed onto its
   label stack.

   In other words, setting this parameter to a non-zero value allows one
   to eliminate all fragmentation of Previously Labeled IP Datagrams,
   but it may cause some unnecessary fragmentation of Initially Labeled
   IP Datagrams.

   Note that the setting of this parameter does not affect the
   processing of IP datagrams that have the DF bit set; hence the result
   of Path MTU discovery is unaffected by the setting of this parameter.

3.3. When are Labeled IP Datagrams Too Big?

   A labeled IP datagram whose size exceeds the Conventional Maximum
   Frame Payload Size of the data link over which it is to be forwarded
   MAY be considered to be "too big".

   A labeled IP datagram whose size exceeds the True Maximum Frame
   Payload Size of the data link over which it is to be forwarded MUST
   be considered to be "too big".

   A labeled IP datagram which is not "too big" MUST be transmitted
   without fragmentation.

3.4. Processing Labeled IPv4 Datagrams which are Too Big

   If a labeled IPv4 datagram is "too big", and the DF bit is not set in
   its IP header, then the LSR MAY silently discard the datagram.

   Note that discarding such datagrams is a sensible procedure only if
   the "Maximum Initially Labeled IP Datagram Size" is set to a non-zero
   value in every LSR in the network which is capable of adding a label
   stack to an unlabeled IP datagram.

   If the LSR chooses not to discard a labeled IPv4 datagram which is
   too big, or if the DF bit is set in that datagram, then it MUST
   execute the following algorithm:

      1. Strip off the label stack entries to obtain the IP datagram.





Rosen, et al.               Standards Track                    [Page 13]

RFC 3032               MPLS Label Stack Encoding            January 2001


      2. Let N be the number of bytes in the label stack (i.e, 4 times
         the number of label stack entries).

      3. If the IP datagram does NOT have the "Don't Fragment" bit set
         in its IP header:

         a. convert it into fragments, each of which MUST be at least N
            bytes less than the Effective Maximum Frame Payload Size.

         b. Prepend each fragment with the same label header that would
            have been on the original datagram had fragmentation not
            been necessary.

         c. Forward the fragments

      4. If the IP datagram has the "Don't Fragment" bit set in its IP
         header:

         a. the datagram MUST NOT be forwarded

         b. Create an ICMP Destination Unreachable Message:

             i. set its Code field [3] to "Fragmentation Required and DF
                Set",

            ii. set its Next-Hop MTU field [4] to the difference between
                the Effective Maximum Frame Payload Size and the value
                of N

         c. If possible, transmit the ICMP Destination Unreachable
            Message to the source of the of the discarded datagram.

3.5. Processing Labeled IPv6 Datagrams which are Too Big

   To process a labeled IPv6 datagram which is too big, an LSR MUST
   execute the following algorithm:

      1. Strip off the label stack entries to obtain the IP datagram.

      2. Let N be the number of bytes in the label stack (i.e., 4 times
         the number of label stack entries).

      3. If the IP datagram contains more than 1280 bytes (not counting
         the label stack entries), or if it does not contain a fragment
         header, then:






Rosen, et al.               Standards Track                    [Page 14]

RFC 3032               MPLS Label Stack Encoding            January 2001


         a. Create an ICMP Packet Too Big Message, and set its Next-Hop
            MTU field to the difference between the Effective Maximum
            Frame Payload Size and the value of N

         b. If possible, transmit the ICMP Packet Too Big Message to the
            source of the datagram.

         c. discard the labeled IPv6 datagram.

      4. If the IP datagram is not larger than 1280 octets, and it
         contains a fragment header, then

         a. Convert it into fragments, each of which MUST be at least N
            bytes less than the Effective Maximum Frame Payload Size.

         b. Prepend each fragment with the same label header that would
            have been on the original datagram had fragmentation not
            been necessary.

         c. Forward the fragments.

         Reassembly of the fragments will be done at the destination
         host.

3.6. Implications with respect to Path MTU Discovery

   The procedures described above for handling datagrams which have the
   DF bit set, but which are "too large", have an impact on the Path MTU
   Discovery procedures of RFC 1191 [4].  Hosts which implement these
   procedures will discover an MTU which is small enough to allow n
   labels to be pushed on the datagrams, without need for fragmentation,
   where n is the number of labels that actually get pushed on along the
   path currently in use.

   In other words, datagrams from hosts that use Path MTU Discovery will
   never need to be fragmented due to the need to put on a label header,
   or to add new labels to an existing label header.  (Also, datagrams
   from hosts that use Path MTU Discovery generally have the DF bit set,
   and so will never get fragmented anyway.)

   Note that Path MTU Discovery will only work properly if, at the point
   where a labeled IP Datagram's fragmentation needs to occur, it is
   possible to cause an ICMP Destination Unreachable message to be
   routed to the packet's source address.  See section 2.3.







Rosen, et al.               Standards Track                    [Page 15]

RFC 3032               MPLS Label Stack Encoding            January 2001


   If it is not possible to forward an ICMP message from within an MPLS
   "tunnel" to a packet's source address, but the network configuration
   makes it possible for the LSR at the transmitting end of the tunnel
   to receive packets that must go through the tunnel, but are too large
   to pass through the tunnel unfragmented, then:

      -  The LSR at the transmitting end of the tunnel MUST be able to
         determine the MTU of the tunnel as a whole.  It MAY do this by
         sending packets through the tunnel to the tunnel's receiving
         endpoint, and performing Path MTU Discovery with those packets.

      -  Any time the transmitting endpoint of the tunnel needs to send
         a packet into the tunnel, and that packet has the DF bit set,
         and it exceeds the tunnel MTU, the transmitting endpoint of the
         tunnel MUST send the ICMP Destination Unreachable message to
         the source, with code "Fragmentation Required and DF Set", and
         the Next-Hop MTU Field set as described above.

4. Transporting Labeled Packets over PPP

   The Point-to-Point Protocol (PPP) [6] provides a standard method for
   transporting multi-protocol datagrams over point-to-point links.  PPP
   defines an extensible Link Control Protocol, and proposes a family of
   Network Control Protocols for establishing and configuring different
   network-layer protocols.

   This section defines the Network Control Protocol for establishing
   and configuring label Switching over PPP.

4.1. Introduction

   PPP has three main components:

      1. A method for encapsulating multi-protocol datagrams.

      2. A Link Control Protocol (LCP) for establishing, configuring,
         and testing the data-link connection.

      3. A family of Network Control Protocols for establishing and
         configuring different network-layer protocols.

   In order to establish communications over a point-to-point link, each
   end of the PPP link must first send LCP packets to configure and test
   the data link.  After the link has been established and optional
   facilities have been negotiated as needed by the LCP, PPP must send
   "MPLS Control Protocol" packets to enable the transmission of labeled
   packets.  Once the "MPLS Control Protocol" has reached the Opened
   state, labeled packets can be sent over the link.



Rosen, et al.               Standards Track                    [Page 16]

RFC 3032               MPLS Label Stack Encoding            January 2001


   The link will remain configured for communications until explicit LCP
   or MPLS Control Protocol packets close the link down, or until some
   external event occurs (an inactivity timer expires or network
   administrator intervention).

4.2. A PPP Network Control Protocol for MPLS

   The MPLS Control Protocol (MPLSCP) is responsible for enabling and
   disabling the use of label switching on a PPP link.  It uses the same
   packet exchange mechanism as the Link Control Protocol (LCP).  MPLSCP
   packets may not be exchanged until PPP has reached the Network-Layer
   Protocol phase.  MPLSCP packets received before this phase is reached
   should be silently discarded.

   The MPLS Control Protocol is exactly the same as the Link Control
   Protocol [6] with the following exceptions:

      1. Frame Modifications

         The packet may utilize any modifications to the basic frame
         format which have been negotiated during the Link Establishment
         phase.

      2. Data Link Layer Protocol Field

         Exactly one MPLSCP packet is encapsulated in the PPP
         Information field, where the PPP Protocol field indicates type
         hex 8281 (MPLS).

      3. Code field

         Only Codes 1 through 7 (Configure-Request, Configure-Ack,
         Configure-Nak, Configure-Reject, Terminate-Request, Terminate-
         Ack and Code-Reject) are used.  Other Codes should be treated
         as unrecognized and should result in Code-Rejects.

      4. Timeouts

         MPLSCP packets may not be exchanged until PPP has reached the
         Network-Layer Protocol phase.  An implementation should be
         prepared to wait for Authentication and Link Quality
         Determination to finish before timing out waiting for a
         Configure-Ack or other response.  It is suggested that an
         implementation give up only after user intervention or a
         configurable amount of time.






Rosen, et al.               Standards Track                    [Page 17]

RFC 3032               MPLS Label Stack Encoding            January 2001


      5. Configuration Option Types

         None.

4.3. Sending Labeled Packets

   Before any labeled packets may be communicated, PPP must reach the
   Network-Layer Protocol phase, and the MPLS Control Protocol must
   reach the Opened state.

   Exactly one labeled packet is encapsulated in the PPP Information
   field, where the PPP Protocol field indicates either type hex 0281
   (MPLS Unicast) or type hex 0283 (MPLS Multicast).  The maximum length
   of a labeled packet transmitted over a PPP link is the same as the
   maximum length of the Information field of a PPP encapsulated packet.

   The format of the Information field itself is as defined in section
   2.

   Note that two codepoints are defined for labeled packets; one for
   multicast and one for unicast.  Once the MPLSCP has reached the
   Opened state, both label switched multicasts and label switched
   unicasts can be sent over the PPP link.

4.4. Label Switching Control Protocol Configuration Options

   There are no configuration options.

5. Transporting Labeled Packets over LAN Media

   Exactly one labeled packet is carried in each frame.

   The label stack entries immediately precede the network layer header,
   and follow any data link layer headers, including, e.g., any 802.1Q
   headers that may exist.

   The ethertype value 8847 hex is used to indicate that a frame is
   carrying an MPLS unicast packet.

   The ethertype value 8848 hex is used to indicate that a frame is
   carrying an MPLS multicast packet.

   These ethertype values can be used with either the ethernet
   encapsulation or the 802.3 LLC/SNAP encapsulation to carry labeled
   packets.  The procedure for choosing which of these two
   encapsulations to use is beyond the scope of this document.





Rosen, et al.               Standards Track                    [Page 18]

RFC 3032               MPLS Label Stack Encoding            January 2001


6. IANA Considerations

   Label values 0-15 inclusive have special meaning, as specified in
   this document, or as further assigned by IANA.

   In this document, label values 0-3 are specified in section 2.1.

   Label values 4-15 may be assigned by IANA, based on IETF Consensus.

7. Security Considerations

   The MPLS encapsulation that is specified herein does not raise any
   security issues that are not already present in either the MPLS
   architecture [1] or in the architecture of the network layer protocol
   contained within the encapsulation.

   There are two security considerations inherited from the MPLS
   architecture which may be pointed out here:

      -  Some routers may implement security procedures which depend on
         the network layer header being in a fixed place relative to the
         data link layer header.  These procedures will not work when
         the MPLS encapsulation is used, because that encapsulation is
         of a variable size.

      -  An MPLS label has its meaning by virtue of an agreement between
         the LSR that puts the label in the label stack (the "label
         writer"), and the LSR that interprets that label (the "label
         reader").  However, the label stack does not provide any means
         of determining who the label writer was for any particular
         label.  If labeled packets are accepted from untrusted sources,
         the result may be that packets are routed in an illegitimate
         manner.

8. Intellectual Property

   The IETF has been notified of intellectual property rights claimed in
   regard to some or all of the specification contained in this
   document.  For more information consult the online list of claimed
   rights.











Rosen, et al.               Standards Track                    [Page 19]

RFC 3032               MPLS Label Stack Encoding            January 2001


9. Authors' Addresses

   Eric C. Rosen
   Cisco Systems, Inc.
   250 Apollo Drive
   Chelmsford, MA, 01824

   EMail: erosen@cisco.com


   Dan Tappan
   Cisco Systems, Inc.
   250 Apollo Drive
   Chelmsford, MA, 01824

   EMail: tappan@cisco.com


   Yakov Rekhter
   Juniper Networks
   1194 N. Mathilda Avenue
   Sunnyvale, CA 94089

   EMail: yakov@juniper.net


   Guy Fedorkow
   Cisco Systems, Inc.
   250 Apollo Drive
   Chelmsford, MA, 01824

   EMail: fedorkow@cisco.com


   Dino Farinacci
   Procket Networks, Inc.
   3910 Freedom Circle, Ste. 102A
   Santa Clara, CA 95054

   EMail: dino@procket.com











Rosen, et al.               Standards Track                    [Page 20]

RFC 3032               MPLS Label Stack Encoding            January 2001


   Tony Li
   Procket Networks, Inc.
   3910 Freedom Circle, Ste. 102A
   Santa Clara, CA 95054

   EMail: tli@procket.com


   Alex Conta
   TranSwitch Corporation
   3 Enterprise Drive
   Shelton, CT, 06484

   EMail: aconta@txc.com





































Rosen, et al.               Standards Track                    [Page 21]

RFC 3032               MPLS Label Stack Encoding            January 2001


10. References

   [1] Rosen, E., Viswanathan, A., and R. Callon, "Multiprotocol Label
       Switching Architecture", RFC 3031, January 2001.

   [2] Bradner, S., "Key words for use in RFCs to Indicate Requirement
       Levels", BCP 14, RFC 2119, March 1997.

   [3] Postel, J., "Internet Control Message Protocol", STD 5, RFC 792,
       September 1981.

   [4] Mogul, J. and S. Deering, "Path MTU Discovery", RFC 1191,
       November 1990.

   [5] Katz, D., "IP Router Alert Option", RFC 2113, February 1997.

   [6] Simpson, W., Editor, "The Point-to-Point Protocol (PPP)", STD 51,
       RFC 1661, July 1994.

   [7] Conta, A. and S. Deering, "Internet Control Message Protocol
       (ICMPv6) for the Internet Protocol Version 6 (IPv6)
       Specification", RFC 1885, December 1995.

   [8] McCann, J., Deering, S. and J. Mogul, "Path MTU Discovery for IP
       version 6", RFC 1981, August 1996.

   [9] Davie, B., Lawrence, J., McCloghrie, K., Rekhter, Y., Rosen, E.
       and G. Swallow, "MPLS Using LDP and ATM VC Switching", RFC 3035,
       January 2001.






















Rosen, et al.               Standards Track                    [Page 22]

RFC 3032               MPLS Label Stack Encoding            January 2001


11. Full Copyright Statement

   Copyright (C) The Internet Society (2001).  All Rights Reserved.

   This document and translations of it may be copied and furnished to
   others, and derivative works that comment on or otherwise explain it
   or assist in its implementation may be prepared, copied, published
   and distributed, in whole or in part, without restriction of any
   kind, provided that the above copyright notice and this paragraph are
   included on all such copies and derivative works.  However, this
   document itself may not be modified in any way, such as by removing
   the copyright notice or references to the Internet Society or other
   Internet organizations, except as needed for the purpose of
   developing Internet standards in which case the procedures for
   copyrights defined in the Internet Standards process must be
   followed, or as required to translate it into languages other than
   English.

   The limited permissions granted above are perpetual and will not be
   revoked by the Internet Society or its successors or assigns.

   This document and the information contained herein is provided on an
   "AS IS" basis and THE INTERNET SOCIETY AND THE INTERNET ENGINEERING
   TASK FORCE DISCLAIMS ALL WARRANTIES, EXPRESS OR IMPLIED, INCLUDING
   BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF THE INFORMATION
   HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED WARRANTIES OF
   MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE.

Acknowledgement

   Funding for the RFC Editor function is currently provided by the
   Internet Society.



















Rosen, et al.               Standards Track                    [Page 23]