-
Notifications
You must be signed in to change notification settings - Fork 0
/
rfc1662.txt
1440 lines (876 loc) · 46.9 KB
/
rfc1662.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 W. Simpson, Editor
Request for Comments: 1662 Daydreamer
STD: 51 July 1994
Obsoletes: 1549
Category: Standards Track
PPP in HDLC-like Framing
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.
Abstract
The Point-to-Point Protocol (PPP) [1] provides a standard method for
transporting multi-protocol datagrams over point-to-point links.
This document describes the use of HDLC-like framing for PPP
encapsulated packets.
Table of Contents
1. Introduction .......................................... 1
1.1 Specification of Requirements ................... 2
1.2 Terminology ..................................... 2
2. Physical Layer Requirements ........................... 3
3. The Data Link Layer ................................... 4
3.1 Frame Format .................................... 5
3.2 Modification of the Basic Frame ................. 7
4. Octet-stuffed framing ................................. 8
4.1 Flag Sequence ................................... 8
4.2 Transparency .................................... 8
4.3 Invalid Frames .................................. 9
4.4 Time Fill ....................................... 9
4.4.1 Octet-synchronous ............................... 9
4.4.2 Asynchronous .................................... 9
4.5 Transmission Considerations ..................... 10
4.5.1 Octet-synchronous ............................... 10
4.5.2 Asynchronous .................................... 10
Simpson [Page i]
RFC 1662 HDLC-like Framing July 1994
5. Bit-stuffed framing ................................... 11
5.1 Flag Sequence ................................... 11
5.2 Transparency .................................... 11
5.3 Invalid Frames .................................. 11
5.4 Time Fill ....................................... 11
5.5 Transmission Considerations ..................... 12
6. Asynchronous to Synchronous Conversion ................ 13
7. Additional LCP Configuration Options .................. 14
7.1 Async-Control-Character-Map (ACCM) .............. 14
APPENDICES ................................................... 17
A. Recommended LCP Options ............................... 17
B. Automatic Recognition of PPP Frames ................... 17
C. Fast Frame Check Sequence (FCS) Implementation ........ 18
C.1 FCS table generator ............................. 18
C.2 16-bit FCS Computation Method ................... 19
C.3 32-bit FCS Computation Method ................... 21
SECURITY CONSIDERATIONS ...................................... 24
REFERENCES ................................................... 24
ACKNOWLEDGEMENTS ............................................. 25
CHAIR'S ADDRESS .............................................. 25
EDITOR'S ADDRESS ............................................. 25
1. Introduction
This specification provides for framing over both bit-oriented and
octet-oriented synchronous links, and asynchronous links with 8 bits
of data and no parity. These links MUST be full-duplex, but MAY be
either dedicated or circuit-switched.
An escape mechanism is specified to allow control data such as
XON/XOFF to be transmitted transparently over the link, and to remove
spurious control data which may be injected into the link by
intervening hardware and software.
Some protocols expect error free transmission, and either provide
error detection only on a conditional basis, or do not provide it at
all. PPP uses the HDLC Frame Check Sequence for error detection.
This is commonly available in hardware implementations, and a
software implementation is provided.
Simpson [Page 1]
RFC 1662 HDLC-like Framing July 1994
1.1. Specification of Requirements
In this document, several words are used to signify the requirements
of the specification. These words are often capitalized.
MUST This word, or the adjective "required", means that the
definition is an absolute requirement of the specification.
MUST NOT This phrase means that the definition is an absolute
prohibition of the specification.
SHOULD This word, or the adjective "recommended", means that there
may exist valid reasons in particular circumstances to
ignore this item, but the full implications must be
understood and carefully weighed before choosing a
different course.
MAY This word, or the adjective "optional", means that this
item is one of an allowed set of alternatives. An
implementation which does not include this option MUST be
prepared to interoperate with another implementation which
does include the option.
1.2. Terminology
This document frequently uses the following terms:
datagram The unit of transmission in the network layer (such as IP).
A datagram may be encapsulated in one or more packets
passed to the data link layer.
frame The unit of transmission at the data link layer. A frame
may include a header and/or a trailer, along with some
number of units of data.
packet The basic unit of encapsulation, which is passed across the
interface between the network layer and the data link
layer. A packet is usually mapped to a frame; the
exceptions are when data link layer fragmentation is being
performed, or when multiple packets are incorporated into a
single frame.
peer The other end of the point-to-point link.
silently discard
The implementation discards the packet without further
processing. The implementation SHOULD provide the
capability of logging the error, including the contents of
the silently discarded packet, and SHOULD record the event
in a statistics counter.
Simpson [Page 2]
RFC 1662 HDLC-like Framing July 1994
2. Physical Layer Requirements
PPP is capable of operating across most DTE/DCE interfaces (such as,
EIA RS-232-E, EIA RS-422, and CCITT V.35). The only absolute
requirement imposed by PPP is the provision of a full-duplex circuit,
either dedicated or circuit-switched, which can operate in either an
asynchronous (start/stop), bit-synchronous, or octet-synchronous
mode, transparent to PPP Data Link Layer frames.
Interface Format
PPP presents an octet interface to the physical layer. There is
no provision for sub-octets to be supplied or accepted.
Transmission Rate
PPP does not impose any restrictions regarding transmission rate,
other than that of the particular DTE/DCE interface.
Control Signals
PPP does not require the use of control signals, such as Request
To Send (RTS), Clear To Send (CTS), Data Carrier Detect (DCD), and
Data Terminal Ready (DTR).
When available, using such signals can allow greater functionality
and performance. In particular, such signals SHOULD be used to
signal the Up and Down events in the LCP Option Negotiation
Automaton [1]. When such signals are not available, the
implementation MUST signal the Up event to LCP upon
initialization, and SHOULD NOT signal the Down event.
Because signalling is not required, the physical layer MAY be
decoupled from the data link layer, hiding the transient details
of the physical transport. This has implications for mobility in
cellular radio networks, and other rapidly switching links.
When moving from cell to cell within the same zone, an
implementation MAY choose to treat the entire zone as a single
link, even though transmission is switched among several
frequencies. The link is considered to be with the central
control unit for the zone, rather than the individual cell
transceivers. However, the link SHOULD re-establish its
configuration whenever the link is switched to a different
administration.
Due to the bursty nature of data traffic, some implementations
have choosen to disconnect the physical layer during periods of
Simpson [Page 3]
RFC 1662 HDLC-like Framing July 1994
inactivity, and reconnect when traffic resumes, without informing
the data link layer. Robust implementations should avoid using
this trick over-zealously, since the price for decreased setup
latency is decreased security. Implementations SHOULD signal the
Down event whenever "significant time" has elapsed since the link
was disconnected. The value for "significant time" is a matter of
considerable debate, and is based on the tariffs, call setup
times, and security concerns of the installation.
3. The Data Link Layer
PPP uses the principles described in ISO 3309-1979 HDLC frame
structure, most recently the fourth edition 3309:1991 [2], which
specifies modifications to allow HDLC use in asynchronous
environments.
The PPP control procedures use the Control field encodings described
in ISO 4335-1979 HDLC elements of procedures, most recently the
fourth edition 4335:1991 [4].
This should not be construed to indicate that every feature of the
above recommendations are included in PPP. Each feature included
is explicitly described in the following sections.
To remain consistent with standard Internet practice, and avoid
confusion for people used to reading RFCs, all binary numbers in the
following descriptions are in Most Significant Bit to Least
Significant Bit order, reading from left to right, unless otherwise
indicated. Note that this is contrary to standard ISO and CCITT
practice which orders bits as transmitted (network bit order). Keep
this in mind when comparing this document with the international
standards documents.
Simpson [Page 4]
RFC 1662 HDLC-like Framing July 1994
3.1. Frame Format
A summary of the PPP HDLC-like frame structure is shown below. This
figure does not include bits inserted for synchronization (such as
start and stop bits for asynchronous links), nor any bits or octets
inserted for transparency. The fields are transmitted from left to
right.
+----------+----------+----------+
| Flag | Address | Control |
| 01111110 | 11111111 | 00000011 |
+----------+----------+----------+
+----------+-------------+---------+
| Protocol | Information | Padding |
| 8/16 bits| * | * |
+----------+-------------+---------+
+----------+----------+-----------------
| FCS | Flag | Inter-frame Fill
|16/32 bits| 01111110 | or next Address
+----------+----------+-----------------
The Protocol, Information and Padding fields are described in the
Point-to-Point Protocol Encapsulation [1].
Flag Sequence
Each frame begins and ends with a Flag Sequence, which is the
binary sequence 01111110 (hexadecimal 0x7e). All implementations
continuously check for this flag, which is used for frame
synchronization.
Only one Flag Sequence is required between two frames. Two
consecutive Flag Sequences constitute an empty frame, which is
silently discarded, and not counted as a FCS error.
Address Field
The Address field is a single octet, which contains the binary
sequence 11111111 (hexadecimal 0xff), the All-Stations address.
Individual station addresses are not assigned. The All-Stations
address MUST always be recognized and received.
The use of other address lengths and values may be defined at a
later time, or by prior agreement. Frames with unrecognized
Addresses SHOULD be silently discarded.
Simpson [Page 5]
RFC 1662 HDLC-like Framing July 1994
Control Field
The Control field is a single octet, which contains the binary
sequence 00000011 (hexadecimal 0x03), the Unnumbered Information
(UI) command with the Poll/Final (P/F) bit set to zero.
The use of other Control field values may be defined at a later
time, or by prior agreement. Frames with unrecognized Control
field values SHOULD be silently discarded.
Frame Check Sequence (FCS) Field
The Frame Check Sequence field defaults to 16 bits (two octets).
The FCS is transmitted least significant octet first, which
contains the coefficient of the highest term.
A 32-bit (four octet) FCS is also defined. Its use may be
negotiated as described in "PPP LCP Extensions" [5].
The use of other FCS lengths may be defined at a later time, or by
prior agreement.
The FCS field is calculated over all bits of the Address, Control,
Protocol, Information and Padding fields, not including any start
and stop bits (asynchronous) nor any bits (synchronous) or octets
(asynchronous or synchronous) inserted for transparency. This
also does not include the Flag Sequences nor the FCS field itself.
When octets are received which are flagged in the Async-
Control-Character-Map, they are discarded before calculating
the FCS.
For more information on the specification of the FCS, see the
Appendices.
The end of the Information and Padding fields is found by locating
the closing Flag Sequence and removing the Frame Check Sequence
field.
Simpson [Page 6]
RFC 1662 HDLC-like Framing July 1994
3.2. Modification of the Basic Frame
The Link Control Protocol can negotiate modifications to the standard
HDLC-like frame structure. However, modified frames will always be
clearly distinguishable from standard frames.
Address-and-Control-Field-Compression
When using the standard HDLC-like framing, the Address and Control
fields contain the hexadecimal values 0xff and 0x03 respectively.
When other Address or Control field values are in use, Address-
and-Control-Field-Compression MUST NOT be negotiated.
On transmission, compressed Address and Control fields are simply
omitted.
On reception, the Address and Control fields are decompressed by
examining the first two octets. If they contain the values 0xff
and 0x03, they are assumed to be the Address and Control fields.
If not, it is assumed that the fields were compressed and were not
transmitted.
By definition, the first octet of a two octet Protocol field
will never be 0xff (since it is not even). The Protocol field
value 0x00ff is not allowed (reserved) to avoid ambiguity when
Protocol-Field-Compression is enabled and the first Information
field octet is 0x03.
Simpson [Page 7]
RFC 1662 HDLC-like Framing July 1994
4. Octet-stuffed framing
This chapter summarizes the use of HDLC-like framing with 8-bit
asynchronous and octet-synchronous links.
4.1. Flag Sequence
The Flag Sequence indicates the beginning or end of a frame. The
octet stream is examined on an octet-by-octet basis for the value
01111110 (hexadecimal 0x7e).
4.2. Transparency
An octet stuffing procedure is used. The Control Escape octet is
defined as binary 01111101 (hexadecimal 0x7d), most significant bit
first.
As a minimum, sending implementations MUST escape the Flag Sequence
and Control Escape octets.
After FCS computation, the transmitter examines the entire frame
between the two Flag Sequences. Each Flag Sequence, Control Escape
octet, and any octet which is flagged in the sending Async-Control-
Character-Map (ACCM), is replaced by a two octet sequence consisting
of the Control Escape octet followed by the original octet
exclusive-or'd with hexadecimal 0x20.
This is bit 5 complemented, where the bit positions are numbered
76543210 (the 6th bit as used in ISO numbered 87654321 -- BEWARE
when comparing documents).
Receiving implementations MUST correctly process all Control Escape
sequences.
On reception, prior to FCS computation, each octet with value less
than hexadecimal 0x20 is checked. If it is flagged in the receiving
ACCM, it is simply removed (it may have been inserted by intervening
data communications equipment). Each Control Escape octet is also
removed, and the following octet is exclusive-or'd with hexadecimal
0x20, unless it is the Flag Sequence (which aborts a frame).
A few examples may make this more clear. Escaped data is transmitted
on the link as follows:
Simpson [Page 8]
RFC 1662 HDLC-like Framing July 1994
0x7e is encoded as 0x7d, 0x5e. (Flag Sequence)
0x7d is encoded as 0x7d, 0x5d. (Control Escape)
0x03 is encoded as 0x7d, 0x23. (ETX)
Some modems with software flow control may intercept outgoing DC1 and
DC3 ignoring the 8th (parity) bit. This data would be transmitted on
the link as follows:
0x11 is encoded as 0x7d, 0x31. (XON)
0x13 is encoded as 0x7d, 0x33. (XOFF)
0x91 is encoded as 0x7d, 0xb1. (XON with parity set)
0x93 is encoded as 0x7d, 0xb3. (XOFF with parity set)
4.3. Invalid Frames
Frames which are too short (less than 4 octets when using the 16-bit
FCS), or which end with a Control Escape octet followed immediately
by a closing Flag Sequence, or in which octet-framing is violated (by
transmitting a "0" stop bit where a "1" bit is expected), are
silently discarded, and not counted as a FCS error.
4.4. Time Fill
4.4.1. Octet-synchronous
There is no provision for inter-octet time fill.
The Flag Sequence MUST be transmitted during inter-frame time fill.
4.4.2. Asynchronous
Inter-octet time fill MUST be accomplished by transmitting continuous
"1" bits (mark-hold state).
Inter-frame time fill can be viewed as extended inter-octet time
fill. Doing so can save one octet for every frame, decreasing delay
and increasing bandwidth. This is possible since a Flag Sequence may
serve as both a frame end and a frame begin. After having received
any frame, an idle receiver will always be in a frame begin state.
Simpson [Page 9]
RFC 1662 HDLC-like Framing July 1994
Robust transmitters should avoid using this trick over-zealously,
since the price for decreased delay is decreased reliability. Noisy
links may cause the receiver to receive garbage characters and
interpret them as part of an incoming frame. If the transmitter does
not send a new opening Flag Sequence before sending the next frame,
then that frame will be appended to the noise characters causing an
invalid frame (with high reliability).
It is suggested that implementations will achieve the best results by
always sending an opening Flag Sequence if the new frame is not
back-to-back with the last. Transmitters SHOULD send an open Flag
Sequence whenever "appreciable time" has elapsed after the prior
closing Flag Sequence. The maximum value for "appreciable time" is
likely to be no greater than the typing rate of a slow typist, about
1 second.
4.5. Transmission Considerations
4.5.1. Octet-synchronous
The definition of various encodings and scrambling is the
responsibility of the DTE/DCE equipment in use, and is outside the
scope of this specification.
4.5.2. Asynchronous
All octets are transmitted least significant bit first, with one
start bit, eight bits of data, and one stop bit. There is no
provision for seven bit asynchronous links.
Simpson [Page 10]
RFC 1662 HDLC-like Framing July 1994
5. Bit-stuffed framing
This chapter summarizes the use of HDLC-like framing with bit-
synchronous links.
5.1. Flag Sequence
The Flag Sequence indicates the beginning or end of a frame, and is
used for frame synchronization. The bit stream is examined on a
bit-by-bit basis for the binary sequence 01111110 (hexadecimal 0x7e).
The "shared zero mode" Flag Sequence "011111101111110" SHOULD NOT be
used. When not avoidable, such an implementation MUST ensure that
the first Flag Sequence detected (the end of the frame) is promptly
communicated to the link layer. Use of the shared zero mode hinders
interoperability with bit-synchronous to asynchronous and bit-
synchronous to octet-synchronous converters.
5.2. Transparency
After FCS computation, the transmitter examines the entire frame
between the two Flag Sequences. A "0" bit is inserted after all
sequences of five contiguous "1" bits (including the last 5 bits of
the FCS) to ensure that a Flag Sequence is not simulated.
On reception, prior to FCS computation, any "0" bit that directly
follows five contiguous "1" bits is discarded.
5.3. Invalid Frames
Frames which are too short (less than 4 octets when using the 16-bit
FCS), or which end with a sequence of more than six "1" bits, are
silently discarded, and not counted as a FCS error.
5.4. Time Fill
There is no provision for inter-octet time fill.
The Flag Sequence SHOULD be transmitted during inter-frame time fill.
However, certain types of circuit-switched links require the use of
Simpson [Page 11]
RFC 1662 HDLC-like Framing July 1994
mark idle (continuous ones), particularly those that calculate
accounting based on periods of bit activity. When mark idle is used
on a bit-synchronous link, the implementation MUST ensure at least 15
consecutive "1" bits between Flags during the idle period, and that
the Flag Sequence is always generated at the beginning of a frame
after an idle period.
This differs from practice in ISO 3309, which allows 7 to 14 bit
mark idle.
5.5. Transmission Considerations
All octets are transmitted least significant bit first.
The definition of various encodings and scrambling is the
responsibility of the DTE/DCE equipment in use, and is outside the
scope of this specification.
While PPP will operate without regard to the underlying
representation of the bit stream, lack of standards for transmission
will hinder interoperability as surely as lack of data link
standards. At speeds of 56 Kbps through 2.0 Mbps, NRZ is currently
most widely available, and on that basis is recommended as a default.
When configuration of the encoding is allowed, NRZI is recommended as
an alternative, because of its relative immunity to signal inversion
configuration errors, and instances when it MAY allow connection
without an expensive DSU/CSU. Unfortunately, NRZI encoding
exacerbates the missing x1 factor of the 16-bit FCS, so that one
error in 2**15 goes undetected (instead of one in 2**16), and triple
errors are not detected. Therefore, when NRZI is in use, it is
recommended that the 32-bit FCS be negotiated, which includes the x1
factor.
At higher speeds of up to 45 Mbps, some implementors have chosen the
ANSI High Speed Synchronous Interface [HSSI]. While this experience
is currently limited, implementors are encouraged to cooperate in
choosing transmission encoding.
Simpson [Page 12]
RFC 1662 HDLC-like Framing July 1994
6. Asynchronous to Synchronous Conversion
There may be some use of asynchronous-to-synchronous converters (some
built into modems and cellular interfaces), resulting in an
asynchronous PPP implementation on one end of a link and a
synchronous implementation on the other. It is the responsibility of
the converter to do all stuffing conversions during operation.
To enable this functionality, synchronous PPP implementations MUST
always respond to the Async-Control-Character-Map Configuration
Option with the LCP Configure-Ack. However, acceptance of the
Configuration Option does not imply that the synchronous
implementation will do any ACCM mapping. Instead, all such octet
mapping will be performed by the asynchronous-to-synchronous
converter.
Simpson [Page 13]
RFC 1662 HDLC-like Framing July 1994
7. Additional LCP Configuration Options
The Configuration Option format and basic options are already defined
for LCP [1].
Up-to-date values of the LCP Option Type field are specified in the
most recent "Assigned Numbers" RFC [10]. This document concerns the
following values:
2 Async-Control-Character-Map
7.1. Async-Control-Character-Map (ACCM)
Description
This Configuration Option provides a method to negotiate the use
of control character transparency on asynchronous links.
Each end of the asynchronous link maintains two Async-Control-
Character-Maps. The receiving ACCM is 32 bits, but the sending
ACCM may be up to 256 bits. This results in four distinct ACCMs,
two in each direction of the link.
For asynchronous links, the default receiving ACCM is 0xffffffff.
The default sending ACCM is 0xffffffff, plus the Control Escape
and Flag Sequence characters themselves, plus whatever other
outgoing characters are flagged (by prior configuration) as likely
to be intercepted.
For other types of links, the default value is 0, since there is
no need for mapping.
The default inclusion of all octets less than hexadecimal 0x20
allows all ASCII control characters [6] excluding DEL (Delete)
to be transparently communicated through all known data
communications equipment.
The transmitter MAY also send octets with values in the range 0x40
through 0xff (except 0x5e) in Control Escape format. Since these
octet values are not negotiable, this does not solve the problem
of receivers which cannot handle all non-control characters.
Also, since the technique does not affect the 8th bit, this does
not solve problems for communications links that can send only 7-
bit characters.
Simpson [Page 14]
RFC 1662 HDLC-like Framing July 1994
Note that this specification differs in detail from later
amendments, such as 3309:1991/Amendment 2 [3]. However, such
"extended transparency" is applied only by "prior agreement".
Use of the transparency methods in this specification
constitute a prior agreement with respect to PPP.
For compatibility with 3309:1991/Amendment 2, the transmitter
MAY escape DEL and ACCM equivalents with the 8th (most
significant) bit set. No change is required in the receiving
algorithm.
Following ACCM negotiation, the transmitter SHOULD cease
escaping DEL.
However, it is rarely necessary to map all control characters, and
often it is unnecessary to map any control characters. The
Configuration Option is used to inform the peer which control
characters MUST remain mapped when the peer sends them.
The peer MAY still send any other octets in mapped format, if it
is necessary because of constraints known to the peer. The peer
SHOULD Configure-Nak with the logical union of the sets of mapped
octets, so that when such octets are spuriously introduced they
can be ignored on receipt.
A summary of the Async-Control-Character-Map Configuration Option
format is shown below. The fields are transmitted from left to
right.
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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Type | Length | ACCM
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
ACCM (cont) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Type
2
Length
6
Simpson [Page 15]
RFC 1662 HDLC-like Framing July 1994
ACCM
The ACCM field is four octets, and indicates the set of control
characters to be mapped. The map is sent most significant octet
first.
Each numbered bit corresponds to the octet of the same value. If
the bit is cleared to zero, then that octet need not be mapped.
If the bit is set to one, then that octet MUST remain mapped. For
example, if bit 19 is set to zero, then the ASCII control
character 19 (DC3, Control-S) MAY be sent in the clear.
Note: The least significant bit of the least significant octet
(the final octet transmitted) is numbered bit 0, and would map
to the ASCII control character NUL.
Simpson [Page 16]
RFC 1662 HDLC-like Framing July 1994
A. Recommended LCP Options
The following Configurations Options are recommended:
High Speed links
Magic Number
Link Quality Monitoring
No Address and Control Field Compression
No Protocol Field Compression
Low Speed or Asynchronous links
Async Control Character Map
Magic Number
Address and Control Field Compression
Protocol Field Compression
B. Automatic Recognition of PPP Frames
It is sometimes desirable to detect PPP frames, for example during a
login sequence. The following octet sequences all begin valid PPP
LCP frames:
7e ff 03 c0 21
7e ff 7d 23 c0 21
7e 7d df 7d 23 c0 21
Note that the first two forms are not a valid username for Unix.
However, only the third form generates a correctly checksummed PPP
frame, whenever 03 and ff are taken as the control characters ETX and
DEL without regard to parity (they are correct for an even parity
link) and discarded.
Many implementations deal with this by putting the interface into
packet mode when one of the above username patterns are detected
during login, without examining the initial PPP checksum. The
initial incoming PPP frame is discarded, but a Configure-Request is
sent immediately.
Simpson [Page 17]
RFC 1662 HDLC-like Framing July 1994
C. Fast Frame Check Sequence (FCS) Implementation