forked from brchiu/asn1c
-
Notifications
You must be signed in to change notification settings - Fork 0
/
rfc3525.txt
11931 lines (8084 loc) · 429 KB
/
rfc3525.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 C. Groves
Request for Comments: 3525 M. Pantaleo
Obsoletes: 3015 LM Ericsson
Category: Standards Track T. Anderson
Consultant
T. Taylor
Nortel Networks
Editors
June 2003
Gateway Control Protocol Version 1
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 (2003). All Rights Reserved.
Abstract
This document defines the protocol used between elements of a
physically decomposed multimedia gateway, i.e., a Media Gateway and a
Media Gateway Controller. The protocol presented in this document
meets the requirements for a media gateway control protocol as
presented in RFC 2805.
This document replaces RFC 3015. It is the result of continued
cooperation between the IETF Megaco Working Group and ITU-T Study
Group 16. It incorporates the original text of RFC 3015, modified by
corrections and clarifications discussed on the Megaco
E-mail list and incorporated into the Study Group 16 Implementor's
Guide for Recommendation H.248. The present version of this document
underwent ITU-T Last Call as Recommendation H.248 Amendment 1.
Because of ITU-T renumbering, it was published by the ITU-T as
Recommendation H.248.1 (03/2002), Gateway Control Protocol Version 1.
Users of this specification are advised to consult the H.248 Sub-
series Implementors' Guide at http://www.itu.int/itudoc/itu-
t/com16/implgd for additional corrections and clarifications.
Groves, et al. Standards Track [Page 1]
RFC 3525 Gateway Control Protocol June 2003
Conventions used in this document
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in RFC 2119 [RFC2119].
Table of Contents
1 Scope.........................................................5
1.1 Changes From RFC 3015.....................................5
1.2 Differences From ITU-T Recommendation H.248.1 (03/2002)...5
2 References....................................................6
2.1 Normative references......................................6
2.2 Informative references....................................9
3 Definitions..................................................10
4 Abbreviations................................................11
5 Conventions..................................................12
6 Connection model.............................................13
6.1 Contexts.................................................16
6.2 Terminations.............................................17
6.2.1 Termination dynamics.................................21
6.2.2 TerminationIDs.......................................21
6.2.3 Packages.............................................22
6.2.4 Termination properties and descriptors...............23
6.2.5 Root Termination.....................................25
7 Commands.....................................................26
7.1 Descriptors..............................................27
7.1.1 Specifying parameters................................27
7.1.2 Modem descriptor.....................................28
7.1.3 Multiplex descriptor.................................28
7.1.4 Media descriptor.....................................29
7.1.5 TerminationState descriptor..........................29
7.1.6 Stream descriptor....................................30
7.1.7 LocalControl descriptor..............................31
7.1.8 Local and Remote descriptors.........................32
7.1.9 Events descriptor....................................35
7.1.10 EventBuffer descriptor..............................38
7.1.11 Signals descriptor..................................38
7.1.12 Audit descriptor....................................40
7.1.13 ServiceChange descriptor............................41
7.1.14 DigitMap descriptor.................................41
7.1.15 Statistics descriptor...............................46
7.1.16 Packages descriptor.................................47
7.1.17 ObservedEvents descriptor...........................47
7.1.18 Topology descriptor.................................47
7.1.19 Error Descriptor....................................50
7.2 Command Application Programming Interface................50
7.2.1 Add..................................................51
Groves, et al. Standards Track [Page 2]
RFC 3525 Gateway Control Protocol June 2003
7.2.2 Modify...............................................52
7.2.3 Subtract.............................................53
7.2.4 Move.................................................55
7.2.5 AuditValue...........................................56
7.2.6 AuditCapabilities....................................59
7.2.7 Notify...............................................60
7.2.8 ServiceChange........................................61
7.2.9 Manipulating and Auditing Context Attributes.........65
7.2.10 Generic Command Syntax..............................66
7.3 Command Error Codes......................................66
8 Transactions.................................................66
8.1 Common parameters........................................68
8.1.1 Transaction Identifiers..............................68
8.1.2 Context Identifiers..................................68
8.2 Transaction Application Programming Interface............69
8.2.1 TransactionRequest...................................69
8.2.2 TransactionReply.....................................69
8.2.3 TransactionPending...................................71
8.3 Messages.................................................72
9 Transport....................................................72
9.1 Ordering of Commands.....................................73
9.2 Protection against Restart Avalanche.....................74
10 Security Considerations.....................................75
10.1 Protection of Protocol Connections......................75
10.2 Interim AH scheme.......................................76
10.3 Protection of Media Connections.........................77
11 MG-MGC Control Interface....................................78
11.1 Multiple Virtual MGs....................................78
11.2 Cold start..............................................79
11.3 Negotiation of protocol version.........................79
11.4 Failure of a MG.........................................80
11.5 Failure of an MGC.......................................81
12 Package definition..........................................82
12.1 Guidelines for defining packages........................82
12.1.1 Package.............................................83
12.1.2 Properties..........................................84
12.1.3 Events..............................................85
12.1.4 Signals.............................................85
12.1.5 Statistics..........................................86
12.1.6 Procedures..........................................86
12.2 Guidelines to defining Parameters to Events and Signals.86
12.3 Lists...................................................87
12.4 Identifiers.............................................87
12.5 Package registration....................................88
13 IANA Considerations.........................................88
13.1 Packages................................................88
13.2 Error codes.............................................89
13.3 ServiceChange reasons...................................89
Groves, et al. Standards Track [Page 3]
RFC 3525 Gateway Control Protocol June 2003
ANNEX A Binary encoding of the protocol.......................90
A.1 Coding of wildcards......................................90
A.2 ASN.1 syntax specification...............................92
A.3 Digit maps and path names...............................111
ANNEX B Text encoding of the protocol.........................113
B.1 Coding of wildcards.....................................113
B.2 ABNF specification......................................113
B.3 Hexadecimal octet coding................................127
B.4 Hexadecimal octet sequence..............................127
ANNEX C Tags for media stream properties......................128
C.1 General media attributes................................128
C.2 Mux properties..........................................130
C.3 General bearer properties...............................130
C.4 General ATM properties..................................130
C.5 Frame Relay.............................................134
C.6 IP......................................................134
C.7 ATM AAL2................................................134
C.8 ATM AAL1................................................136
C.9 Bearer capabilities.....................................137
C.10 AAL5 properties........................................147
C.11 SDP equivalents........................................148
C.12 H.245..................................................149
ANNEX D Transport over IP.....................................150
D.1 Transport over IP/UDP using Application Level Framing ..150
D.1.1 Providing At-Most-Once functionality................150
D.1.2 Transaction identifiers and three-way handshake.....151
D.1.3 Computing retransmission timers.....................152
D.1.4 Provisional responses...............................153
D.1.5 Repeating Requests, Responses and Acknowledgements..153
D.2 Using TCP...............................................155
D.2.1 Providing the At-Most-Once functionality............155
D.2.2 Transaction identifiers and three-way handshake.....155
D.2.3 Computing retransmission timers.....................156
D.2.4 Provisional responses...............................156
D.2.5 Ordering of commands................................156
ANNEX E Basic packages.......................................157
E.1 Generic.................................................157
E.2 Base Root Package.......................................159
E.3 Tone Generator Package..................................161
E.4 Tone Detection Package..................................163
E.5 Basic DTMF Generator Package............................166
E.6 DTMF detection Package..................................167
E.7 Call Progress Tones Generator Package...................169
E.8 Call Progress Tones Detection Package...................171
E.9 Analog Line Supervision Package.........................172
E.10 Basic Continuity Package...............................175
E.11 Network Package........................................178
E.12 RTP Package............................................180
Groves, et al. Standards Track [Page 4]
RFC 3525 Gateway Control Protocol June 2003
E.13 TDM Circuit Package....................................182
APPENDIX I EXAMPLE CALL FLOWS (INFORMATIVE)...................184
A.1 Residential Gateway to Residential Gateway Call.........184
A.1.1 Programming Residential GW Analog Line Terminations
for Idle Behavior...................................184
A.1.2 Collecting Originator Digits and Initiating
Termination.........................................186
APPENDIX II Changes From RFC 3015............................195
Intellectual Property Rights..................................210
Acknowledgments...............................................211
Authors' Addresses............................................212
Full Copyright Statement......................................213
1 Scope
The present document, which is identical to the published version of
ITU-T Recommendation H.248.1 (03/2002) except as noted below, defines
the protocols used between elements of a physically decomposed
multimedia gateway. There are no functional differences from a
system view between a decomposed gateway, with distributed sub-
components potentially on more than one physical device, and a
monolithic gateway such as described in ITU-T Recommendation H.246.
This document does not define how gateways, multipoint control units
or interactive voice response units (IVRs) work. Instead it creates
a general framework that is suitable for these applications.
Packet network interfaces may include IP, ATM or possibly others.
The interfaces will support a variety of Switched Circuit Network
(SCN) signalling systems, including tone signalling, ISDN, ISUP, QSIG
and GSM. National variants of these signalling systems will be
supported where applicable.
1.1 Changes From RFC 3015
The differences between this document and RFC 3015 are documented in
Appendix II.
1.2 Differences From ITU-T Recommendation H.248.1 (03/2002)
This document differs from the corresponding ITU-T publication in the
following respects:
- Added IETF front matter in place of the corresponding ITU-T
material.
- The ITU-T summary is too H.323-specific and has been omitted.
Groves, et al. Standards Track [Page 5]
RFC 3525 Gateway Control Protocol June 2003
- The IETF conventions have been stated as governing this document.
As discussed in section 5 below, this gives slightly greater
strength to "should" requirements.
- The Scope section (just above) has been edited slightly to suit
its IETF context.
- Added normative references to RFCs 2026 and 2119.
- Figures 4, 5, and 6 show the centre of the context for greater
clarity. Also added Figure 6a showing an important additional
example.
- Added a paragraph in section 7.1.18 which was approved in the
Implementor's Guide but lost inadvertently in the ITU-T approved
version.
- This document incorporates corrections to the informative examples
in Appendix I which also appear in H.248.1 version 2, but which
were not picked up in H.248.1 (03/2002).
- This document includes a new Appendix II listing all the changes
from RFC 3015.
- This document includes an Acknowledgements section listing the
authors of RFC 3015 but also many other people who contributed to
the development of the Megaco/H.248.x protocol.
- Moved the Intellectual Property declaration to its usual place in
an IETF document and added a reference to declarations on the IETF
web site.
2 References
The following ITU-T Recommendations and other references contain
provisions which, through reference in this text, constitute
provisions of this RFC. At the time of publication, the editions
indicated were valid. All Recommendations and other references are
subject to revision; all users of this RFC are therefore encouraged
to investigate the possibility of applying the most recent edition of
the Recommendations and other references listed below. A list of the
currently valid ITU-T Recommendations is regularly published.
2.1 Normative references
- ITU-T Recommendation H.225.0 (1999), Call signalling protocols and
media stream packetization for packet-based multimedia
communication systems.
Groves, et al. Standards Track [Page 6]
RFC 3525 Gateway Control Protocol June 2003
- ITU-T Recommendation H.235 (1998), Security and encryption for
H-Series (H.323 and other H.245-based) multimedia terminals.
- ITU-T Recommendation H.245 (1998), Control protocol for multimedia
communication.
- ITU-T Recommendation H.246 (1998), Interworking of H-series
multimedia terminals with H-series multimedia terminals and
voice/voiceband terminals on GSTN and ISDN.
- ITU-T Recommendation H.248.8 (2002), H.248 Error Codes and Service
Change Reasons.
- ITU-T Recommendation H.323 (1999), Packet-based multimedia
communication systems.
- ITU-T Recommendation I.363.1 (1996), B-ISDN ATM adaptation layer
(AAL) specification: Type 1 AAL.
- ITU-T Recommendation I.363.2 (1997), B-ISDN ATM adaptation layer
(AAL) specification: Type 2 AAL.
- ITU-T Recommendation I.363.5 (1996), B-ISDN ATM adaptation layer
(AAL) specification: Type 5 AAL.
- ITU-T Recommendation I.366.1 (1998), Segmentation and Reassembly
Service Specific Convergence Sublayer for the AAL type 2.
- ITU-T Recommendation I.366.2 (1999), AAL type 2 service specific
convergence sublayer for trunking.
- ITU-T Recommendation I.371 (2000), Traffic control and congestion
control in B-ISDN.
- ITU-T Recommendation Q.763 (1999), Signalling System No. 7 - ISDN
user part formats and codes.
- ITU-T Recommendation Q.765.5 (2001), Application transport
mechanism - Bearer independent call control (BICC).
- ITU-T Recommendation Q.931 (1998), ISDN user-network interface
layer 3 specification for basic call control.
- ITU-T Recommendation Q.2630.1 (1999), AAL type 2 signalling
protocol (Capability Set 1).
Groves, et al. Standards Track [Page 7]
RFC 3525 Gateway Control Protocol June 2003
- ITU-T Recommendation Q.2931 (1995), Digital Subscriber Signalling
System No. 2 (DSS2) - User-Network Interface (UNI) - Layer 3
specification for basic call/connection control.
- ITU-T Recommendation Q.2941.1 (1997), Digital Subscriber
Signalling System No. 2 - Generic identifier transport.
- ITU-T Recommendation Q.2961.1 (1995), Additional signalling
capabilities to support traffic parameters for the tagging option
and the sustainable call rate parameter set.
- ITU-T Recommendation Q.2961.2 (1997), Additional traffic
parameters: Support of ATM transfer capability in the broadband
bearer capability information element.
- ITU-T Recommendation Q.2965.1 (1999), Digital subscriber
signalling system No. 2 - Support of Quality of Service classes.
- ITU-T Recommendation Q.2965.2 (1999), Digital subscriber
signalling system No. 2 - Signalling of individual Quality of
Service parameters.
- ITU-T Recommendation V.76 (1996), Generic multiplexer using V.42
LAPM-based procedures.
- ITU-T Recommendation X.213 (1995), Information technology - Open
Systems Interconnection - Network service definition plus
Amendment 1 (1997), Addition of the Internet protocol address
format identifier.
- ITU-T Recommendation X.680 (1997), Information technology -
Abstract Syntax Notation One (ASN.1): Specification of basic
notation.
- ITU-T Recommendation X.690 (1997), Information Technology - ASN.1
Encoding Rules: Specification of Basic Encoding Rules (BER),
Canonical Encoding Rules (CER) and Distinguished Encoding Rules
(DER).
- ATM Forum (1996), ATM User-Network Interface (UNI) Signalling
Specification - Version 4.0.
[RFC 1006] Rose, M. and D. Cass, "ISO Transport Service on top of the
TCP, Version 3", STD 35, RFC 1006, May 1987.
[RFC 2026] Brander, S., "The Internet Standards Process -- Revision
3", BCP 9, RFC 2026, October 1996.
Groves, et al. Standards Track [Page 8]
RFC 3525 Gateway Control Protocol June 2003
[RFC 2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
[RFC 2234] Crocker, D., Ed. and P. Overell, "Augmented BNF for Syntax
Specifications: ABNF", RFC 2234, November 1997.
[RFC 2327] Handley, M. and V. Jacobson, "SDP: Session Description
Protocol", RFC 2327, April 1998.
[RFC 2402] Kent, S. and R. Atkinson, "IP Authentication Header", RFC
2402, November 1998.
[RFC 2406] Kent, S. and R. Atkinson, "IP Encapsulating Security
Payload (ESP)", RFC 2406, November 1998.
2.2 Informative references
- ITU-T Recommendation E.180/Q.35 (1998), Technical characteristics
of tones for the telephone service.
- CCITT Recommendation G.711 (1988), Pulse Code Modulation (PCM) of
voice frequencies.
- ITU-T Recommendation H.221 (1999), Frame structure for a 64 to
1920 kbit/s channel in audiovisual teleservices.
- ITU T Recommendation H.223 (1996), Multiplexing protocol for low
bit rate multimedia communication.
- ITU-T Recommendation H.226 (1998), Channel aggregation protocol
for multilink operation on circuit-switched networks
- ITU-T Recommendation Q.724 (1998), Signalling procedures.
- ITU-T Recommendation Q.764 (1999), Signalling system No. 7 - ISDN
user part signalling procedures.
- ITU-T Recommendation Q.1902.4 (2001), Bearer independent call
control protocol - Basic call procedures.
[RFC 768] Postel, J., "User Datagram Protocol", STD 6, RFC 768,
August 1980.
[RFC 791] Postel, J., "Internet Protocol", STD 5, RFC 791, September
1981.
[RFC 793] Postel, J., "Transmission Control Protocol", STD 7, RFC
793, September 1981.
Groves, et al. Standards Track [Page 9]
RFC 3525 Gateway Control Protocol June 2003
[RFC 1661] Simpson, W., Ed., "The Point-to-Point Protocol (PPP)", STD
51, RFC 1661, July 1994.
[RFC 1889] Schulzrinne, H., Casner, S., Frederick, R. and V.
Jacobson, "RTP: A Transport Protocol for Real-Time
Applications", RFC 1889, January 1996.
[RFC 1890] Schulzrinne, H. and G. Fokus, "RTP Profile for Audio and
Video Conferences with Minimal Control", RFC 1890,
January 1996.
[RFC 2401] Kent, S. and R. Atkinson, "Security Architecture for the
Internet Protocol", RFC 2401, November 1998.
[RFC 2460] Deering, S. and R. Hinden, "Internet Protocol, Version 6
(IPv6) Specification", RFC 2460, December 1998.
[RFC 2543] Handley, M., Schulzrinne, H., Schooler, E. and J.
Rosenberg, "SIP: Session Initiation Protocol", RFC 2543,
March 1999.
[RFC 2805] Greene, N., Ramalho, M. and B. Rosen, "Media Gateway
Control Protocol Architecture and Requirements", RFC 2805,
April 2000.
3 Definitions
This document defines the following terms:
Access gateway:
A type of gateway that provides a User-Network Interface (UNI) such
as ISDN.
Descriptor:
A syntactic element of the protocol that groups related properties.
For instance, the properties of a media flow on the MG can be set by
the MGC by including the appropriate descriptor in a command.
Media Gateway (MG):
The media gateway converts media provided in one type of network to
the format required in another type of network. For example, a MG
could terminate bearer channels from a switched circuit network
(e.g., DS0s) and media streams from a packet network (e.g., RTP
streams in an IP network). This gateway may be capable of processing
audio, video and T.120 alone or in any combination, and will be
capable of full duplex media translations. The MG may also play
audio/video messages and perform other IVR functions, or may perform
media conferencing.
Groves, et al. Standards Track [Page 10]
RFC 3525 Gateway Control Protocol June 2003
Media Gateway Controller (MGC):
Controls the parts of the call state that pertain to connection
control for media channels in a MG.
Multipoint Control Unit (MCU):
An entity that controls the setup and coordination of a multi-user
conference that typically includes processing of audio, video and
data.
Residential gateway:
A gateway that interworks an analogue line to a packet network. A
residential gateway typically contains one or two analogue lines and
is located at the customer premises.
SCN FAS signalling gateway:
This function contains the SCN Signalling Interface that terminates
SS7, ISDN or other signalling links where the call control channel
and bearer channels are collocated in the same physical span.
SCN NFAS signalling gateway:
This function contains the SCN Signalling Interface that terminates
SS7 or other signalling links where the call control channels are
separated from bearer channels.
Stream:
Bidirectional media or control flow received/sent by a media gateway
as part of a call or conference.
Trunk:
A communication channel between two switching systems such as a DS0
on a T1 or E1 line.
Trunking gateway:
A gateway between SCN network and packet network that typically
terminates a large number of digital circuits.
4 Abbreviations
This RFC document uses the following abbreviations:
ALF Application Layer Framing
ATM Asynchronous Transfer Mode
CAS Channel Associated Signalling
DTMF Dual Tone Multi-Frequency
Groves, et al. Standards Track [Page 11]
RFC 3525 Gateway Control Protocol June 2003
FAS Facility Associated Signalling
GSM Global System for Mobile communications
GW GateWay
IANA Internet Assigned Numbers Authority (superseded by Internet
Corporation for Assigned Names and Numbers - ICANN)
IP Internet Protocol
ISUP ISDN User Part
IVR Interactive Voice Response
MG Media Gateway
MGC Media Gateway Controller
NFAS Non-Facility Associated Signalling
PRI Primary Rate Interface
PSTN Public Switched Telephone Network
QoS Quality of Service
RTP Real-time Transport Protocol
SCN Switched Circuit Network
SG Signalling Gateway
SS7 Signalling System No. 7
5 Conventions
In the H.248.1 Recommendation, "SHALL" refers to a mandatory
requirement, while "SHOULD" refers to a suggested but optional
feature or procedure. The term "MAY" refers to an optional course of
action without expressing a preference. Note that these definition
are overridden in the present document by the RFC 2119 conventions
stated at the beginning of this document. RFC 2119 has a more
precise definition of "should" than is provided by the ITU-T.
Groves, et al. Standards Track [Page 12]
RFC 3525 Gateway Control Protocol June 2003
6 Connection model
The connection model for the protocol describes the logical entities,
or objects, within the Media Gateway that can be controlled by the
Media Gateway Controller. The main abstractions used in the
connection model are Terminations and Contexts.
A Termination sources and/or sinks one or more streams. In a
multimedia conference, a Termination can be multimedia and sources or
sinks multiple media streams. The media stream parameters, as well
as modem, and bearer parameters are encapsulated within the
Termination.
A Context is an association between a collection of Terminations.
There is a special type of Context, the null Context, which contains
all Terminations that are not associated to any other Termination.
For instance, in a decomposed access gateway, all idle lines are
represented by Terminations in the null Context.
Following is a graphical depiction of these concepts. The diagram of
Figure 1 gives several examples and is not meant to be an
all-inclusive illustration. The asterisk box in each of the Contexts
represents the logical association of Terminations implied by the
Context.
Groves, et al. Standards Track [Page 13]
RFC 3525 Gateway Control Protocol June 2003
+------------------------------------------------------+
|Media Gateway |
| +-------------------------------------------------+ |
| |Context +-------------+ | |
| | | Termination | | |
| | |-------------| | |
| | +-------------+ +->| SCN Bearer |<---+->
| | | Termination | +-----+ | | Channel | | |
| | |-------------| | |---+ +-------------+ | |
<-+--->| RTP Stream |---| * | | |
| | | | | |---+ +-------------+ | |
| | +-------------+ +-----+ | | Termination | | |
| | | |-------------| | |
| | +->| SCN Bearer |<---+->
| | | Channel | | |
| | +-------------+ | |
| +-------------------------------------------------+ |
| |
| |
| +------------------------------+ |
| (NULL Context) |Context | |
| +-------------+ | +-------------+ | |
| | Termination | | +-----+ | Termination | | |
| |-------------| | | | |-------------| | |
| | SCN Bearer | | | * |------| SCN Bearer |<---+->
| | Channel | | | | | Channel | | |
| +-------------+ | +-----+ +-------------+ | |
| +------------------------------+ |
| |
| |
| +-------------------------------------------------+ |
| |Context | |
| | +-------------+ +-------------+ | |
| | | Termination | +-----+ | Termination | | |
| | |-------------| | | |-------------| | |
<-+--->| SCN Bearer |---| * |------| SCN Bearer |<---+->
| | | Channel | | | | Channel | | |
| | +-------------+ +-----+ +-------------+ | |
| +-------------------------------------------------+ |
| ___________________________________________________ |
+------------------------------------------------------+
Figure 1: Examples of Megaco/H.248 Connection Model
Groves, et al. Standards Track [Page 14]
RFC 3525 Gateway Control Protocol June 2003
The example in Figure 2 shows an example of one way to accomplish a
call-waiting scenario in a decomposed access gateway, illustrating
the relocation of a Termination between Contexts. Terminations T1
and T2 belong to Context C1 in a two-way audio call. A second audio
call is waiting for T1 from Termination T3. T3 is alone in Context
C2. T1 accepts the call from T3, placing T2 on hold. This action
results in T1 moving into Context C2, as shown in Figure 3.
+------------------------------------------------------+
|Media Gateway |
| +-------------------------------------------------+ |
| |Context C1 | |
| | +-------------+ +-------------+ | |
| | | Term. T2 | +-----+ | Term. T1 | | |
| | |-------------| | | |-------------| | |
<-+--->| RTP Stream |---| * |------| SCN Bearer |<---+->
| | | | | | | Channel | | |
| | +-------------+ +-----+ +-------------+ | |
| +-------------------------------------------------+ |
| |
| +-------------------------------------------------+ |
| |Context C2 | |
| | +-------------+ | |
| | +-----+ | Term. T3 | | |
| | | | |-------------| | |
| | | * |------| SCN Bearer |<---+->
| | | | | Channel | | |
| | +-----+ +-------------+ | |
| +-------------------------------------------------+ |
+------------------------------------------------------+
Figure 2: Example Call Waiting Scenario / Alerting Applied to T1
Groves, et al. Standards Track [Page 15]
RFC 3525 Gateway Control Protocol June 2003
+------------------------------------------------------+
|Media Gateway |
| +-------------------------------------------------+ |
| |Context C1 | |
| | +-------------+ | |
| | | Term. T2 | +-----+ | |
| | |-------------| | | | |
<-+--->| RTP Stream |---| * | | |
| | | | | | | |
| | +-------------+ +-----+ | |
| +-------------------------------------------------+ |
| |
| +-------------------------------------------------+ |
| |Context C2 | |
| | +-------------+ +-------------+ | |
| | | Term. T1 | +-----+ | Term. T3 | | |
| | |-------------| | | |-------------| | |
<-+--->| SCN Bearer |---| * |------| SCN Bearer |<---+->
| | | Channel | | | | Channel | | |
| | +-------------+ +-----+ +-------------+ | |
| +-------------------------------------------------+ |
+------------------------------------------------------+
Figure 3. Example Call Waiting Scenario / Answer by T1
6.1 Contexts
A Context is an association between a number of Terminations. The
Context describes the topology (who hears/sees whom) and the media
mixing and/or switching parameters if more than two Terminations are
involved in the association.
There is a special Context called the null Context. It contains
Terminations that are not associated to any other Termination.
Terminations in the null Context can have their parameters examined
or modified, and may have events detected on them.
In general, an Add command is used to add Terminations to Contexts.
If the MGC does not specify an existing Context to which the
Termination is to be added, the MG creates a new Context. A
Termination may be removed from a Context with a Subtract command,
and a Termination may be moved from one Context to another with a
Move command. A Termination SHALL exist in only one Context at a
time.
Groves, et al. Standards Track [Page 16]
RFC 3525 Gateway Control Protocol June 2003
The maximum number of Terminations in a Context is a MG property.
Media gateways that offer only point-to-point connectivity might
allow at most two Terminations per Context. Media gateways that
support multipoint conferences might allow three or more Terminations
per Context.
6.1.1 Context attributes and descriptors
The attributes of Contexts are:
- ContextID.
- The topology (who hears/sees whom).
The topology of a Context describes the flow of media between the
Terminations within a Context. In contrast, the mode of a
Termination (send/receive/...) describes the flow of the media at
the ingress/egress of the media gateway.
- The priority is used for a Context in order to provide the MG with
information about a certain precedence handling for a Context.
The MGC can also use the priority to control autonomously the
traffic precedence in the MG in a smooth way in certain
situations (e.g., restart), when a lot of Contexts must be handled
simultaneously. Priority 0 is the lowest priority and a priority
of 15 is the highest priority.
- An indicator for an emergency call is also provided to allow a
preference handling in the MG.
6.1.2 Creating, deleting and modifying Contexts
The protocol can be used to (implicitly) create Contexts and modify
the parameter values of existing Contexts. The protocol has commands
to add Terminations to Contexts, subtract them from Contexts, and to
move Terminations between Contexts. Contexts are deleted implicitly
when the last remaining Termination is subtracted or moved out.
6.2 Terminations
A Termination is a logical entity on a MG that sources and/or sinks
media and/or control streams. A Termination is described by a number
of characterizing Properties, which are grouped in a set of
Descriptors that are included in commands. Terminations have unique
identities (TerminationIDs), assigned by the MG at the time of their
creation.
Groves, et al. Standards Track [Page 17]
RFC 3525 Gateway Control Protocol June 2003
Terminations representing physical entities have a semi-permanent
existence. For example, a Termination representing a TDM channel
might exist for as long as it is provisioned in the gateway.
Terminations representing ephemeral information flows, such as RTP
flows, would usually exist only for the duration of their use.
Ephemeral Terminations are created by means of an Add command. They
are destroyed by means of a Subtract command. In contrast, when a
physical Termination is Added to or Subtracted from a Context, it is
taken from or to the null Context, respectively.
Terminations may have signals applied to them (see 7.1.11).
Terminations may be programmed to detect Events, the occurrence of
which can trigger notification messages to the MGC, or action by the
MG. Statistics may be accumulated on a Termination. Statistics are
reported to the MGC upon request (by means of the AuditValue command,
see 7.2.5) and when the Termination is taken out of the call it is
in.
Multimedia gateways may process multiplexed media streams. For
example, Recommendation H.221 describes a frame structure for
multiple media streams multiplexed on a number of digital 64 kbit/s
channels. Such a case is handled in the connection model in the
following way. For every bearer channel that carries part of the
multiplexed streams, there is a physical or ephemeral "bearer
Termination". The bearer Terminations that source/sink the digital
channels are connected to a separate Termination called the
"multiplexing Termination". The multiplexing termination is an
ephemeral termination representing a frame-oriented session. The
MultiplexDescriptor for this Termination describes the multiplex used
(e.g., H.221 for an H.320 session) and indicates the order in which
the contained digital channels are assembled into a frame.
Multiplexing terminations may be cascades (e.g., H.226 multiplex of
digital channels feeding into a H.223 multiplex supporting an H.324
session).
The individual media streams carried in the session are described by
StreamDescriptors on the multiplexing Termination. These media
streams can be associated with streams sourced/sunk by Terminations
in the Context other than the bearer Terminations supporting the
multiplexing Termination. Each bearer Termination supports only a