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
|
Network Working Group P. Christian
Request for Comments: 3147 Nortel Networks
Category: Informational July 2001
Generic Routing Encapsulation over CLNS Networks
Status of this Memo
This memo provides information for the Internet community. It does
not specify an Internet standard of any kind. Distribution of this
memo is unlimited.
Copyright Notice
Copyright (C) The Internet Society (2001). All Rights Reserved.
Abstract
This document proposes a method for transporting an arbitrary
protocol over a CLNS (Connectionless Network Service) network using
GRE (Generic Routing Encapsulation). This may then be used as a
method to tunnel IPv4 or IPv6 over CLNS.
1. Introduction
RFC 2784 Generic Routing Encapsulation (GRE) [1] provides a standard
method for transporting one arbitrary network layer protocol over
another arbitrary network layer protocol.
RFC 1702 Generic Routing Encapsulation over IPv4 networks [2]
provides a standard method for transporting an arbitrary network
layer protocol over IPv4 using GRE.
However no standard method exists for transporting other network
layer protocols over CLNS. This causes lack of interoperability
between different vendors' products as they provide solutions to
migrate from CLNS networks to IP networks. This is a problem
specifically in, but not limited to, the context of management
networks for SONET and SDH networks elements.
Large networks exist for the purpose of providing management
communications for SONET and SDH network elements. Standards
Bellcore GR-253-CORE [3] and ITU-T G.784 [4] mandate that these
networks are based on CLNS.
Christian Informational [Page 1]
^L
RFC 3147 Generic Routing Encapsulation over CLNS Networks July 2001
Many vendors have already started to offer SONET and SDH products
that are managed by IP instead of CLNS and a general migration from
CLNS towards IP is anticipated within the industry.
Part of any migration strategy from CLNS to IP should provide for the
co-existence of both CLNS managed and IP managed network elements in
the same network.
Such a migration strategy should foresee the need to manage existing
CLNS managed network elements that become isolated by a new IP
network. Such a scenario may be tackled by tunnelling CLNP PDUs over
IP using the existing GRE standard RFC 2784 [1] and informational RFC
1702 [2]. Networks have already been deployed that use this method.
Such a migration strategy should also foresee the need to manage new
IP managed network elements that are installed on the far side of
existing CLNS managed network. Such a scenario requires a method for
tunnelling IP over CLNS.
2. GRE over CLNS advantages
Using GRE to tunnel IP over CLNS offers some advantages.
In the absence of a standard for tunnelling IP over CLNS, GRE as
specified in RFC 2784 [1] is the most applicable standard that
exists.
The move from CLNS to IP comes at a time when IP is itself
migrating from IPv4 to IPv6. GRE defines a method to tunnel any
protocol that has an Ethernet Protocol Type. Therefore by
defining a method for CLNS to transport GRE, a method will then
exist for CLNS to transport any other protocol that has an
Ethernet Protocol Type defined in RFC 1700 [5]. Thus GRE over
CLNS can be used to tunnel both IPv4 and IPv6.
GRE is already commonly used to tunnel CLNP PDUs over IP and so
using GRE to tunnel IP over CLNS gives a common approach to
tunnelling and may simplify software within network elements that
initiate and terminate tunnels.
The only disadvantage of using GRE is the extra minimum of four bytes
that will be used between CLNP header and IP payload packet. Given
the large size of CLNP headers this will not make a significant
difference to the performance of any network that has IP over CLNP
PDUs present on it.
Christian Informational [Page 2]
^L
RFC 3147 Generic Routing Encapsulation over CLNS Networks July 2001
3. Transporting GRE packets over CLNS.
It is suggested that GRE should be transported over CLNS at the
lowest layer possible, which is as a transport layer protocol over
the network layer. This can be achieved by placing the entire GRE
packet inside a CLNP Data Type PDU (DT PDU) as data payload.
The GRE packet is a GRE packet as defined in RFC 2784 [1], in other
words GRE header plus payload packet.
Data payload is the part of a Data PDU that is described as "Data" in
the structure of a Data PDU in ISO/IEC 8473-1 [6].
Christian Informational [Page 3]
^L
RFC 3147 Generic Routing Encapsulation over CLNS Networks July 2001
For convenience the structure of a Data PDU is reproduced from
ISO/IEC 8473-1 [6] below:
Octet
----------------------------------------
| Network Layer Protocol Identifier | 1
----------------------------------------
| Length Indicator | 2
----------------------------------------
| Version/Protocol Id Extension | 3
----------------------------------------
| Lifetime | 4
----------------------------------------
| SP | MS | E/R | Type | 5
----------------------------------------
| Segment Length | 6,7
----------------------------------------
| Checksum | 8,9
----------------------------------------
| Destination Address Length Indicator | 10
----------------------------------------
| | 11
| Destination Address |
| | m-1
----------------------------------------
| Source Address Length Indicator | m
----------------------------------------
| | m+1
| Source Address |
| | n-1
----------------------------------------
| Data Unit Identifier | n,n+1
----------------------------------------
| Segment Offset | n+2,n+3
----------------------------------------
| Total Length | n+4,n+5
----------------------------------------
| | n+6
| Options |
| | p
----------------------------------------
| | p+1
| Data ( GRE packet ) |
| | z
----------------------------------------
Christian Informational [Page 4]
^L
RFC 3147 Generic Routing Encapsulation over CLNS Networks July 2001
4. NSAP selector (N-SEL) value.
Transport of GRE packets is a new type of Network Service (NS) user.
Different Network Service users are identified by using different
NSAP selector bytes also known as N-SEL bytes.
This is a similar concept to the use of the IP Protocol Type used in
IP packets.
Whilst it is not strictly necessary for all vendors to use the same
N-SEL values, they must use the same N-SEL value for it to be
possible for one vendor's CLNS device or network element to initiate
a GRE tunnel which is then terminated on a different vendor's CLNS
device.
Although N-SEL values (other than zero) are not defined in CLNS/CLNP
standards, some are defined when CLNS is used in SONET networks by
Bellcore GR-253-CORE [3] whilst others are in common use.
As the IP protocol number for GRE is 47, as defined in RFC 1702 [2],
and as 47 is not commonly used as an N-SEL value, it is suggested
that 47 (decimal) should be used as an N-SEL value to indicate to the
CLNS stack that the Data portion of the Data Type PDU contains a GRE
packet.
The N-SEL byte should be set to 47 (decimal) in both the source
address and the destination address of the CLNP PDU.
The N-SEL value of 47 should indicate only that the payload is GRE,
and the device or network element that transmits the PDU should use
the GRE header to indicate what protocol (for example IPv4 or IPv6)
is encapsulated within the GRE packet in conformance with RFC 2784
[1]. Similarly the device or network element that receives the PDU
should then inspect the GRE header to ascertain what protocol is
contained within the GRE packet in conformance with RFC 2784 [1].
5. Segmentation Permitted (SP) value.
It is recommended that the SP flag in all CLNP PDUs containing GRE
packets should be set.
If the SP flag is not set, and a CLNP PDU is too large for a
particular link, then a CLNS device or network element will drop the
PDU. The originator of the packet that is inside the GRE packet will
not have visibility of the packet loss or the reason for the packet
loss, and a black hole may form.
Christian Informational [Page 5]
^L
RFC 3147 Generic Routing Encapsulation over CLNS Networks July 2001
6. Interaction with Path MTU Discovery (PMTU), RFC 1191 [7].
A tunnel entry point for a GRE tunnel should treat IP packets that
are bigger than the MTU size of the GRE tunnel as per RFC 1191 [7].
If the oversize IP packet that is about to enter the GRE tunnel does
not have its Don't Fragment (DF) bit set then it should be fragmented
before entering the tunnel.
If the oversize IP packet that is about to enter the GRE tunnel has
its DF bit set then the packet should be discarded, and an ICMP
unreachable error message (in particular the "fragmentation needed
and DF set" code) should be sent back to the originator of the packet
as described in RFC 1191 [7].
7. Security Considerations
CLNS and GRE do not provide any security when employed in the way
recommended in this document.
If security is required, then it must be provided by other methods
and applied to the payload protocol before it is transported by GRE
over CLNS.
8. References
[1] Farinacci, D., Li, T., Hanks, S., Meyer, D. and P. Traina,
"Generic Routing Encapsulation (GRE)", RFC 2784, March 2000.
[2] Hanks, S., Li, T., Farinacci, D. and P. Traina, "Generic Routing
Encapsulation over IPv4", RFC 1702, October 1994.
[3] Bellcore Publication GR-253-Core "Synchronous Optical Network
(SONET) Transport Systems: Common Generic Criteria", January 1999
[4] ITU-T Recommendation G.784 "Synchronous Digital Hierarchy (SDH)
management", June 1999
[5] Reynolds, J. and J. Postel, "Assigned Numbers", STD 2, RFC 1700,
October 1994.
[6] "Information technology - Protocol for providing the
connectionless-mode network service", ISO/IEC 8473-1, 1994
[7] Mogul, J. and S. Deering, "Path MTU Discovery", RFC 1191,
November 1990.
Christian Informational [Page 6]
^L
RFC 3147 Generic Routing Encapsulation over CLNS Networks July 2001
9. Acknowledgements
Chris Murton, Paul Fee, Mike Tate for their contribution in writing
this document.
10. Author's Address
Philip Christian
Nortel Networks Harlow Laboratories
London Road, Harlow,
Essex, CM17 9NA UK
EMail: christi@nortelnetworks.com
Christian Informational [Page 7]
^L
RFC 3147 Generic Routing Encapsulation over CLNS Networks July 2001
11. Full Copyright Statement
Copyright (C) The Internet Society (2001). All Rights Reserved.
This document and translations of it may be copied and furnished to
others, and derivative works that comment on or otherwise explain it
or assist in its implementation may be prepared, copied, published
and distributed, in whole or in part, without restriction of any
kind, provided that the above copyright notice and this paragraph are
included on all such copies and derivative works. However, this
document itself may not be modified in any way, such as by removing
the copyright notice or references to the Internet Society or other
Internet organizations, except as needed for the purpose of
developing Internet standards in which case the procedures for
copyrights defined in the Internet Standards process must be
followed, or as required to translate it into languages other than
English.
The limited permissions granted above are perpetual and will not be
revoked by the Internet Society or its successors or assigns.
This document and the information contained herein is provided on an
"AS IS" basis and THE INTERNET SOCIETY AND THE INTERNET ENGINEERING
TASK FORCE DISCLAIMS ALL WARRANTIES, EXPRESS OR IMPLIED, INCLUDING
BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF THE INFORMATION
HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED WARRANTIES OF
MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE.
Acknowledgement
Funding for the RFC Editor function is currently provided by the
Internet Society.
Christian Informational [Page 8]
^L
|
