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
|
Network Working Group B. Thompson
Request for Comments: 3336 T. Koren
Category: Standards Track Cisco Systems
B. Buffam
Seaway Networks
December 2002
PPP Over Asynchronous Transfer Mode Adaptation Layer 2 (AAL2)
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 (2002). All Rights Reserved.
Abstract
The Point-to-Point Protocol (PPP) provides a standard method for
transporting multi-protocol datagrams over point-to-point links.
This document describes the use of ATM Adaptation Layer 2 (AAL2) for
framing PPP encapsulated packets.
Applicability
This specification is intended for those implementations which desire
to use the facilities which are defined for PPP, such as the Link
Control Protocol, Network-layer Control Protocols, authentication,
and compression. These capabilities require a point-to-point
relationship between the peers, and are not designed for the multi-
point relationships which are available in ATM and other multi-access
environments.
Thompson, et. al. Standards Track [Page 1]
RFC 3336 PPP Over AAL2 December 2002
1. Introduction
PPP over AAL5 [2] describes the encapsulation format and operation of
PPP when used with the ATM AAL5 adaptation layer. While this
encapsulation format is well suited to PPP transport of IP, it is
bandwidth inefficient when used for transporting small payloads such
as voice. PPP over AAL5 is especially bandwidth inefficient when
used with RTP header compression [3].
PPP over AAL2 addresses the bandwidth efficiency issues of PPP over
AAL5 for small packet transport by making use of the AAL2 Common Part
Sublayer (CPS) [4] to allow multiple PPP payloads to be multiplexed
into a set of ATM cells.
2. Conventions
The keywords MUST, MUST NOT, REQUIRED, SHALL, SHALL NOT, SHOULD,
SHOULD NOT, RECOMMENDED, MAY, and OPTIONAL, when they appear in this
document, are to be interpreted as described in [6].
3. AAL2 Layer Service Interface
The PPP layer treats the underlying ATM AAL2 layer service as a bit-
synchronous point-to-point link. In this context, the PPP link
corresponds to an ATM AAL2 virtual connection. The virtual
connection MUST be full-duplex, point to point, and it MAY be either
dedicated (i.e., permanent, set up by provisioning) or switched (set
up on demand). In addition, the PPP/AAL2 service interface boundary
MUST meet the following requirements.
Interface Format - The PPP/AAL2 layer boundary presents an octet
service interface to the AAL2 layer. There is no provision for
sub-octets to be supplied or accepted.
Transmission Rate - The PPP layer does not impose any
restrictions regarding transmission rate on the underlying ATM
layer traffic descriptor parameters.
Control Signals - The AAL2 layer MUST provide control signals to
the PPP layer which indicate when the virtual connection link has
become connected or disconnected. These provide the "Up" and
"Down" events to the LCP state machine [1] within the PPP layer.
In the case of PPP over AAL2, the state of the link can be derived
from the type 3 fault management packets carried in-band within a
given AAL2 CID flow.
Thompson, et. al. Standards Track [Page 2]
RFC 3336 PPP Over AAL2 December 2002
4. PPP Operation with AAL2
PPP over AAL2 defines an encapsulation that uses the Service Specific
Segmentation and Reassembly Sublayer (SSSAR) [5] for AAL type 2. The
SSSAR sub-layer is used to segment PPP packets into frames that can
be transported using the AAL2 CPS. The SSSAR sub-layer uses
different AAL2 UUI code-points to indicate whether a segment is the
last segment of a packet or not.
The encapsulation of PPP over AAL2 provides a 16-bit CRC for PPP
payloads. There are 2 UUI code points assigned from SSSAR to
indicate intermediate fragments of a packet and the last fragment of
a packet. Code point 27 indicates intermediate frames of a
fragmented packet and code point 26 indicates the last frame of a
packet. The encapsulation format is more fully described in section
6.2.1.
An implementation of PPP over AAL2 MAY use one or more AAL2 Channel
Identifiers (CIDs) for transport of PPP packets associated with each
PPP session. Multiple CIDs could be used to implement a multiple
class real time transport service for PPP using the AAL2 layer for
link fragmentation and interleaving. A companion document [10]
describes class extensions for PPP over AAL2 using multiple AAL2
CIDs.
5. Comparison of PPP Over AAL2 with Existing Encapsulations
This document proposes the substitution of AAL2 transport for PPP in
scenarios where small packets are being transported over an ATM
network. This is most critical in applications such as voice
transport using RTP [9] where RTP Header compression [3] is used. In
applications such as voice transport, both bandwidth efficiency and
low delay are very important.
This section provides justification for the PPP over AAL2 service for
ATM transport by comparing it to existing PPP encapsulation formats
used for transport over ATM. Two encapsulation formats will be
examined here: PPP over AAL5 [2], and PPP with PPPMUX [8] over AAL5.
5.1 Comparison With PPP Over AAL5
This proposal uses ATM AAL2 [4] rather than AAL5 as the transport for
PPP. SSSAR along with the AAL2 CPS generates less ATM encapsulation
overhead per PPP payload. The payload encapsulation consists of a 2
byte CRC. The AAL2 CPS header consists of 3 bytes, and the AAL2
Start Field (STF) is 1 byte. This is a total encapsulation overhead
of 6 bytes. This compares to 8 bytes of overhead for the AAL5
trailer used for PPP over AAL5.
Thompson, et. al. Standards Track [Page 3]
RFC 3336 PPP Over AAL2 December 2002
The multiplexing function of the AAL2 CPS layer allows more bandwidth
efficient transport of PPP frames by multiplexing multiple PPP frames
into one or more ATM cells using the AAL2 CPS function. This removes
the pad overhead of AAL5 when used to transport short frames.
5.2 Comparison with PPPMUX over AAL5
PPP Multiplexing (PPPMUX) [8] is a new method for doing multiplexing
in the PPP layer. PPPMUX provides functionality similar to the CPS
based multiplexing function of AAL2. Using PPP multiplexing, a PPP
stack would look like PPP/PPPMUX/AAL5.
Both PPP/PPPMUX/AAL5 and PPP/AAL2 use multiplexing to reduce the
overhead of cell padding when frames are sent over an ATM virtual
circuit. However, the bandwidth utilization of PPP/AAL2 will
typically be better than the bandwidth used by PPP/PPPMUX/AAL5. This
is because multiplexed frames in PPP/PPPMUX/AAL5 must always be
encapsulated within an AAL5 frame before being sent. This
encapsulation causes an additional 8 bytes of AAL5 trailer to be
added to the PPPMUX encapsulation. In addition to the 8 bytes of
AAL5 trailer, PPPMUX will incur an average of 24 additional bytes of
AAL5 PAD. These 2 factors will end up reducing the effective
efficiency of PPPMUX when it is used over AAL5.
With PPP/AAL2, the AAL2 CPS layer treats individual PPP frames as a
series of CPS payloads that can be multiplexed. As long as PPP
frames arrive at the CPS layer before the CPS TIMER_CU expires, all
ATM cells coming from the CPS layer will be filled. Under these
conditions, PPP/AAL2 will have no PAD associated with it. When the
AAL2 CPS function causes no PAD to be generated, PPP/AAL2 will be
more bandwidth efficient than PPP/PPPMUX/AAL5.
In PPP/PPPMUX/AAL5, the AAL5 SAR and the PPP MUX/DEMUX are performed
in two different layers. Thus, the PPPMUX/AAL5 receiver must
reassemble a full AAL5 frame from the ATM layer before the PPPMUX
layer can extract the PPP payloads. To derive maximum PPP
Multiplexing efficiency, many PPP payloads may be multiplexed
together. This increases the size of the multiplexed frame to many
ATM cells. If one of these ATM cells is lost, the whole PPPMUX
packet will be discarded. Also, there may be a significant delay
incurred while the AAL5 layer waits for many ATM cell arrival times
until a full frame has been assembled before the full frame is passed
up to the PPP Multiplexing layer where the inverse PPP demux then
occurs. This same issue also applies to PPPMUX/AAL5 frames
progressing down the stack.
Thompson, et. al. Standards Track [Page 4]
RFC 3336 PPP Over AAL2 December 2002
With AAL2, both the segmentation and reassembly and multiplexing
functions are performed in the AAL2 CPS layer. Because of the
definition of the AAL2 CPS function, a multiplexed payload will be
extracted as soon as it is received. The CPS receiver does not wait
until the many payloads of an AAL2 multiplexed frame are received
before removing payloads from the multiplexed stream. The same
benefit also applies to AAL2 CPS sender implementations. Also, the
loss of an ATM cell causes the loss of the packets that are included
in that cell only.
The AAL2 CPS function provides multiplexing in AAL2. This function
often needs to be implemented in hardware for performance reasons.
Because of this, a PPP/AAL2 implementation that takes advantage of an
AAL2 SAR implemented in hardware will have significant performance
benefits over a PPP/PPPMUX/AAL5 implementation where PPPMUX is
implemented in software. Also, the AAL2 specification has been
available significantly longer than the PPP Multiplexing
specification and because of this, optimized software and hardware
implementations of the AAL2 CPS function are further in development
than PPP Multiplexing implementations.
6. Detailed Protocol Operation Description
6.1 Background
6.1.1 AAL2 Multiplexing
ITU-T I.363.2 specifies ATM Adaptation Layer Type 2. This AAL type
provides for bandwidth efficient transmission of low-rate, short and
variable length packets in delay sensitive applications. More than
one AAL type 2 user information stream can be supported on a single
ATM connection. There is only one definition for the sub-layer
because it implements the interface to the ATM layer and is shared by
more than one SSCS layer.
6.1.2 AAL2 Service Specific Convergence Sub-layers
ITU-T I.366.1 and I.366.2 define Service Specific Convergence Sub-
layers (SSCS) that operate above the Common Part Sub-layer defined in
I.363.2. This layer specifies packet formats and procedures to
encode the different information streams in bandwidth efficient
transport. As the name implies, this sub-layer implements those
elements of service specific transport. While there is only one
definition of the Common Part Sublayer for AAL2, there can be
multiple SSCS functions defined to run over this CPS layer.
Different CIDs within an AAL2 virtual circuit may run different
SSCSs.
Thompson, et. al. Standards Track [Page 5]
RFC 3336 PPP Over AAL2 December 2002
6.1.3 AAL2 CPS Packet (CPS-PKT) Format
The CPS-PKT format over AAL2 as defined in I.363.2:
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| + + + + |
| CID + LI + UUI + HEC + CPS-INFO |
| + + + + |
| + + + + |
| (8) + (6) + (5) + (5) + (45/64 * 8) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Note: The size of the fields denote bit-width
The Channel ID (CID) identifies the sub-stream within the AAL2
connection. The Length indication (LI) indicates the length of the
CPS-INFO payload. The User-to-User Indication (UUI) carries
information between the SSCS/Application running above the CPS. The
SSSAR sub-layer as defined in I.366.1 uses the following code points:
UUI Code-point Packet Content
++++++++++++++ ++++++++++++++
0-26 Framed mode data, final packet.
27 Framed mode data, more to come.
This proposal uses two UUI code-points as follows:
UUI Code-point Packet Content
++++++++++++++ ++++++++++++++
27 non-final packet.
26 final packet.
Thompson, et. al. Standards Track [Page 6]
RFC 3336 PPP Over AAL2 December 2002
6.1.4 AAL2 CPS-PDU Format
The CPS-PDU format over AAL2 as defined in I.363.2:
+-+-+-+~+~+-+-+
|CPS| CPS-INFO|
|PKT| |
|HDR| |
+-+-+-+~+~+-+-+
| CPS-PKT |
| +-+-+-+~+~+-+-+
|CPS| CPS-INFO|
| |PKT| |
|HDR| |
| +-+-+-+~+~+-+-+
CPS-PKT
| | +-+-+-+~+~+-+-+
|CPS| CPS-INFO|
| | |PKT| |
|HDR| |
| | +-+-+-+~+~+-+-+
CPS-PKT
V V V V
+-+-+-+-+-+-+-+~+~+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Cell | | | |
Header | STF | CPS-PDU Payload | PAD |
| | | |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+~+~+
Note: The size of the fields denote bitwidth
The CPS-PDU format is used to carry one or more CPS-PKT's multiplexed
on a single CPS-PDU. The CPS header contains enough information to
identify the CPS packets within a CPS-PDU. In the event of cell loss,
the STF field is used to find the first CPS-PKT in the current cell.
Thompson, et. al. Standards Track [Page 7]
RFC 3336 PPP Over AAL2 December 2002
6.2 PPP Over AAL2 Encapsulation
PPP encapsulation over AAL2 uses the AAL2 CPS with no change.
Some PPP encapsulated protocols such as RTP header compression
require that the link layer provide packet error detection. Because
of this, PPP over AAL2 defines a 16-bit CRC that is used along with
the SSSAR sub-layer of I.366.1 to provide packet error detection.
The encapsulation format is described below.
6.2.1 PPP Payload Encapsulation Over AAL2 with 16-bit CRC (CRC-16).
The payload encapsulation of PPP appends a two byte CRC to each PPP
frame before using the SSSAR layer to send the PPP packet as a series
of AAL2 frames.
The format of a PPP over AAL2 packet is shown in the diagram below.
Note that the diagram below shows the payload encapsulation when the
packet is not segmented (UUI=26). When the PPP packet is segmented,
the PPP Protocol ID, Information field, and CRC-16 fields will be
split across multiple SSSAR frames. In this case, the UUI field will
be set to 27 for all frames except the last frame. In the last frame,
the UUI field will be set to 26.
Payload Encapsulation
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| + + + + + + |
| CID + LI + UUI + HEC + Protocol + + |
| + + + + ID + Information + CRC-16 |
| + + + + + + |
| (8) + (6) + (5) + (5) + (8/16) + + (16) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Note: The size of the fields denote bit-width
The algorithms used for computing and verifying the CRC-16 field are
identical to the algorithms specified for the Frame Check Sequence
(FCS) field in Q.921 [13]. The algorithms from Q.921 are included in
this section for ease of access.
The CRC-16 field is filled with the value of a CRC calculation which
is performed over the contents of the PPP packet, including the PPP
Protocol ID and the information field. The CRC field shall contain
the ones complement of the sum (modulo 2) of:
1) the remainder of x^k (x^15 + x^14 + ... + x + 1) divided (modulo
2) by the generator polynomial, where k is the number of bits of
the information over which the CRC is calculated; and
Thompson, et. al. Standards Track [Page 8]
RFC 3336 PPP Over AAL2 December 2002
2) the remainder of the division (modulo 2) by the generator
polynomial of the product of x^16 by the information over which
the CRC is calculated.
The CRC-16 generator polynomial is:
G(x) = x^16 + x^12 + x^5 + 1
The result of the CRC calculation is placed with the least
significant bit right justified in the CRC field.
As a typical implementation at the transmitter, the initial content
of the register of the device computing the remainder of the division
is preset to all "1"s and is then modified by division by the
generator polynomial (as described above) on the information over
which the CRC is to be calculated; the ones complement of the
resulting remainder is put into the CRC field.
As a typical implementation at the receiver, the initial content of
the register of the device computing the remainder of the division is
preset to all "1"s. The final remainder, after multiplication by
x^16 and then division (modulo 2) by the generator polynomial of the
serial incoming PPP packet (including the Protocol ID, the
information and the CRC fields), will be 0001110100001111 (x^15
through x^0, respectively) in the absence of transmission errors.
6.3 Use of AAL2 CPS-PKT CIDs
An implementation of PPP over AAL2 MAY use a single AAL2 Channel
Identifier (CID) or multiple CIDs for transport of all PPP packets.
In order for the endpoints of a PPP session to work with AAL2, they
MUST both agree on the number, SSCS mapping, and values of AAL2 CIDs
that will be used for a PPP session. The values of AAL2 CIDs to be
used for a PPP session MAY be obtained from either static
provisioning in the case of a dedicated AAL2 connection (PVC) or from
Q.2630.2 [7] signaling in the case of an AAL2 switched virtual
circuit (SPVC or SVC).
Using this proposal it is possible to support the use of conventional
AAL2 in CIDs that are not used to support PPP over AAL2. This
proposal allows the co-existence of multiple types of SSCS function
within the same AAL2 VCC.
Thompson, et. al. Standards Track [Page 9]
RFC 3336 PPP Over AAL2 December 2002
6.4 PPP over AAL2 Operation
PPP operation with AAL2 will perform basic PPP encapsulation with the
PPP protocol ID. A 16-bit CRC is calculated as described above and
appended to the payload. The SSSAR sub-layer of AAL2 is used for
transport.
Applications implementing PPP over AAL2 MUST meet all the
requirements of PPP [1].
7. Example implementation of PPP/AAL2
This section describes an example implementation of how PPP can be
encapsulated over AAL2. The example shows two application stacks
generating IP packets that are sent to the same interface running
PPP/AAL2. One Application stack is generating RTP packets and
another application is generating IP Datagrams. The PPP/AAL2
interface shown in this example is running an RFC 2508 compliant
version of RTP header compression.
Thompson, et. al. Standards Track [Page 10]
RFC 3336 PPP Over AAL2 December 2002
Here are the paths an Application packet can take in this
implementation:
+---+---+---+---+--+ +
| Application A | |
+---+---+---+---+--+ |
| RTP | |
+---+---+---+---+--+ +---+---+---+---+---+ Application
| UDP | | Application B | |
+---+---+---+---+--+ +---+---+---+---+---+ |
| IP | | IP | |
+---+---+---+---+--+ +---+---+---+---+---+ +
| |
+---------------+------------+
|
|
+---+---+---+---+---+--+ +
| Compression Filter | |
+---+---+---+---+---+--+ |
| |
| |
+---------+-----------+ |
| | |
RTP | | Non-RTP |
Packets V | Packets |
+---+---+---+---+---+---+ | |
| CRTP | | |
+---+---+---+---+---+---+---+---+---+---+---+---+ Transport
| PPP | |
+---+---+---+---+---+---+---+---+---+---+---+---+ |
| |
+---+---+---+---+---+---+---+ +--+---+---+---+---+--+--+-+ |
| Segmentation (SSSAR) | |
+---+---+---+---+---+---+---+ +--+---+---+---+---+--+--+-+ |
+---+---+---+---+---+---+---+---+---+---+---+---+---+----+ |
| AAL2 CPS | |
+---+---+---+---+---+---+---+---+---+---+---+---+---+----+ |
| ATM Layer | |
+---+---+---+---+---+---+---+---+---+---+---+---+---+----+ +
In the picture above, application A is an RTP application generating
RTP packets. Application B is an IP application generating IP
datagrams. Application A gathers the RTP data and formats an RTP
packet. Lower level layers of application A add UDP and IP headers
to form a complete IP packet. Application B is generating datagrams
to the IP layer. These datagrams may not have UDP or RTP headers.
Thompson, et. al. Standards Track [Page 11]
RFC 3336 PPP Over AAL2 December 2002
In the above picture, a protocol stack is configured to apply
CRTP/PPP/AAL2 compression on an interface to a destination host. All
packets that are sent to this interface will be tested to see if they
can be compressed using RTP header compression. As packets appear at
the interface, they will be tested by a compression filter to
determine if they are candidates for header compression. If the
compression filter determines that the packet is a candidate for
compression, the packet will be sent to the CRTP compressor. If the
packet is not a candidate for compression, it will be sent directly
to the PPP layer for encapsulation as an IP packet encapsulated in
PPP.
The destination UDP port number and packet length are examples of
criteria that may be used by the compression filter to select the
interface.
In this example, packets from application A will be passed to the
CRTP compressor which then hands the compressed packet to the PPP
layer for encapsulation as one of the compressed header types of
CRTP. The PPP layer will add the appropriate CRTP payload type for
the compressed packet.
Packets from application B will be sent directly to the PPP layer for
encapsulation as an IP/PPP packet. The PPP layer will add the PPP
payload type for an IP packet encapsulated in PPP.
PPP packets are then segmented using I.366.1 segmentation with SSSAR.
The resulting AAL2 frame mode PDU is passed down as a CPS SDU to the
CPS Layer for multiplexing accompanied by the CPS-UUI and the CPS-
CID. The CPS Layer multiplexes the CPS-PKT onto a CPS-PDU. CPS-PDUs
are passed to the ATM layer as ATM SDUs to be carried end-to-end
across the ATM network.
At the receiving end, the ATM SDU's arrive and are passed up to the
AAL2 CPS. As the AAL2 CPS PDU is accumulated, complete CPS-PKT's are
reassembled by the SSSAR SSCS. Reassembled packets are checked for
errors using the CRC algorithm.
At this point, the PPP layer on the receiving side uses the PPP
payload type to deliver the packet to either the CRTP decompressor or
the IP layer depending on the value of the PPP payload type.
Thompson, et. al. Standards Track [Page 12]
RFC 3336 PPP Over AAL2 December 2002
8. LCP Configuration Options
By default, PPP over AAL2 will use the 16 bit CRC encapsulation for
all packets.
The default Maximum-Receive-Unit (MRU) is 1500 bytes.
9. Security Considerations
This memo defines mechanisms for PPP encapsulation over ATM. There
is an element of trust in any encapsulation protocol: a receiver
should be able to trust that the sender has correctly identified the
protocol being encapsulated and that the sender has not been spoofed
or compromised. A receiver should also be able to trust that the
transport network between sender and receiver has not been
compromised.
A PPP session that runs over an ATM Virtual Circuit must follow the
PPP link operation state machine described in RFC 1661 [1]. This
state machine includes the ability to enforce the use of an
authentication phase using the PAP/CHAP authentication protocols
before any network layer packets are exchanged. Using PPP level
authentication, a PPP receiver can authenticate a PPP sender.
System security may also be compromised by the attacks of the ATM
transport network itself. The ATM Forum has published a security
framework [11] and a security specification [12] that define
procedures to guard against common threats to an ATM transport
network.
PPP level authentication does not guard against man in the middle
attacks. These attacks could occur if an attacker was able to
compromise the security infrastructure of an ATM switching network.
Applications that require protection against threats to an ATM
switching network are encouraged to use authentication headers, or
encrypted payloads, and/or the ATM-layer security services described
in [12].
When PPP over AAL2 is used on a set of CIDs in a virtual connection,
there may be other non PPP encapsulated AAL2 CIDs running on the same
virtual connection. Because of this, an end point cannot assume that
the PPP session authentication and related security mechanisms also
secure the non PPP encapsulated CIDs on that same virtual connection.
Thompson, et. al. Standards Track [Page 13]
RFC 3336 PPP Over AAL2 December 2002
10. Acknowledgements
The authors would like to thank Rajesh Kumar, Mike Mclaughlin, Pietro
Schicker, James Carlson and John O'Neil for their contributions to
this proposal.
11. References
[1] Simpson, W., Editor, "The Point-to-Point Protocol (PPP)", STD
51, RFC 1661, July 1994.
[2] Gross, G., Kaycee, M., Li, A., Malis, A. and J. Stephens, "PPP
over AAL5", STD 51, RFC 2364, July 1998.
[3] Casner, S. and V. Jacobson, "Compressing IP/UDP/RTP Headers for
Low-Speed Serial Links", RFC 2508, February 1999.
[4] International Telecommunications Union, "BISDN ATM Adaptation
layer specification: Type 2 AAL(AAL2)", ITU-T Recommendation
I.363.2, September 1997.
[5] International Telecommunications Union, "Segmentation and
Reassembly Service Specific Convergence Sublayer for the AAL
type 2", ITU-T Recommendation I.366.1, June 1998.
[6] Bradner, S., "Key words for use in RFCs to Indicate Requirement
Levels", BCP 14, RFC 2119, March 1997.
[7] ITU-T, "ITU-T RECOMMENDATION Q.2630.2", December 2000.
[8] Pazhyannur, R, Ali, I. and C. Fox, "PPP Multiplexing", RFC 3153,
August 2001.
[9] Schulzrinne, H., Casner, S., Frederick, R. and V. Jacobson,
"RTP: A Transport Protocol for Real-Time Applications", RFC
1889, January 1996.
[10] Thompson, B., Koren, T. and B. Buffam, "Class Extensions for PPP
over Asynchronous Transfer Mode Adaptation Layer 2", RFC 3337,
December 2002.
[11] The ATM Forum, "ATM Security Framework Version 1.0", af-sec-
0096.000, February 1998.
[12] The ATM Forum, "ATM Security Specification v1.1", af-sec-
0100.002, March 2001.
Thompson, et. al. Standards Track [Page 14]
RFC 3336 PPP Over AAL2 December 2002
[13] International Telecommunications Union, ISDN User-Network
Interface-Data Link Layer Specification, ITU-T Recommendation
Q.921, March 1993.
12. Authors' Addresses
Bruce Thompson
Cisco Systems, Inc.
170 West Tasman Drive
San Jose, CA 95134
USA
Phone: +1 408 527-0446
EMail: brucet@cisco.com
Tmima Koren
Cisco Systems, Inc.
170 West Tasman Drive
San Jose, CA 95134
USA
Phone: +1 408 527-6169
EMail: tmima@cisco.com
Bruce Buffam
Seaway Networks
One Chrysalis Way,
Suite 300,
Ottawa, Canada
K2G-6P9
Phone: +1 613 723-9161
EMail: bruce@seawaynetworks.com
Thompson, et. al. Standards Track [Page 15]
RFC 3336 PPP Over AAL2 December 2002
13. Full Copyright Statement
Copyright (C) The Internet Society (2002). 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.
Thompson, et. al. Standards Track [Page 16]
|
