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
|
<pre>Network Working Group T. Moncaster
Request for Comments: 5696 B. Briscoe
Category: Standards Track BT
M. Menth
University of Wuerzburg
November 2009
<span class="h1">Baseline Encoding and Transport of Pre-Congestion Information</span>
Abstract
The objective of the Pre-Congestion Notification (PCN) architecture
is to protect the quality of service (QoS) of inelastic flows within
a Diffserv domain. It achieves this by marking packets belonging to
PCN-flows when the rate of traffic exceeds certain configured
thresholds on links in the domain. These marks can then be evaluated
to determine how close the domain is to being congested. This
document specifies how such marks are encoded into the IP header by
redefining the Explicit Congestion Notification (ECN) codepoints
within such domains. The baseline encoding described here provides
only two PCN encoding states: Not-marked and PCN-marked. Future
extensions to this encoding may be needed in order to provide more
than one level of marking severity.
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) 2009 IETF Trust and the persons identified as the
document authors. All rights reserved.
This document is subject to <a href="https://www.rfc-editor.org/bcp/bcp78">BCP 78</a> and the IETF Trust's Legal
Provisions Relating to IETF Documents
(<a href="http://trustee.ietf.org/license-info">http://trustee.ietf.org/license-info</a>) in effect on the date of
publication of this document. Please review these documents
carefully, as they describe your rights and restrictions with respect
to this document. Code Components extracted from this document must
include Simplified BSD License text as described in Section 4.e of
the Trust Legal Provisions and are provided without warranty as
described in the BSD License.
<span class="grey">Moncaster, et al. Standards Track [Page 1]</span></pre>
<hr class='noprint'/><!--NewPage--><pre class='newpage'><span id="page-2" ></span>
<span class="grey"><a href="./rfc5696">RFC 5696</a> Baseline PCN Encoding November 2009</span>
This document may contain material from IETF Documents or IETF
Contributions published or made publicly available before November
10, 2008. The person(s) controlling the copyright in some of this
material may not have granted the IETF Trust the right to allow
modifications of such material outside the IETF Standards Process.
Without obtaining an adequate license from the person(s) controlling
the copyright in such materials, this document may not be modified
outside the IETF Standards Process, and derivative works of it may
not be created outside the IETF Standards Process, except to format
it for publication as an RFC or to translate it into languages other
than English.
Table of Contents
<a href="#section-1">1</a>. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . <a href="#page-3">3</a>
<a href="#section-2">2</a>. Requirements Notation . . . . . . . . . . . . . . . . . . . . <a href="#page-3">3</a>
<a href="#section-3">3</a>. Terminology and Abbreviations . . . . . . . . . . . . . . . . <a href="#page-3">3</a>
<a href="#section-3.1">3.1</a>. Terminology . . . . . . . . . . . . . . . . . . . . . . . <a href="#page-3">3</a>
<a href="#section-3.2">3.2</a>. List of Abbreviations . . . . . . . . . . . . . . . . . . <a href="#page-4">4</a>
<a href="#section-4">4</a>. Encoding Two PCN States in IP . . . . . . . . . . . . . . . . <a href="#page-4">4</a>
<a href="#section-4.1">4.1</a>. Marking Packets . . . . . . . . . . . . . . . . . . . . . <a href="#page-5">5</a>
<a href="#section-4.2">4.2</a>. Valid and Invalid Codepoint Transitions . . . . . . . . . <a href="#page-6">6</a>
<a href="#section-4.3">4.3</a>. Rationale for Encoding . . . . . . . . . . . . . . . . . . <a href="#page-7">7</a>
<a href="#section-4.4">4.4</a>. PCN-Compatible Diffserv Codepoints . . . . . . . . . . . . <a href="#page-7">7</a>
<a href="#section-4.4.1">4.4.1</a>. Co-Existence of PCN and Not-PCN Traffic . . . . . . . <a href="#page-8">8</a>
<a href="#section-5">5</a>. Rules for Experimental Encoding Schemes . . . . . . . . . . . <a href="#page-8">8</a>
<a href="#section-6">6</a>. Backward Compatibility . . . . . . . . . . . . . . . . . . . . <a href="#page-9">9</a>
<a href="#section-7">7</a>. Security Considerations . . . . . . . . . . . . . . . . . . . <a href="#page-9">9</a>
<a href="#section-8">8</a>. Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . <a href="#page-10">10</a>
<a href="#section-9">9</a>. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . <a href="#page-10">10</a>
<a href="#section-10">10</a>. References . . . . . . . . . . . . . . . . . . . . . . . . . . <a href="#page-10">10</a>
<a href="#section-10.1">10.1</a>. Normative References . . . . . . . . . . . . . . . . . . . <a href="#page-10">10</a>
<a href="#section-10.2">10.2</a>. Informative References . . . . . . . . . . . . . . . . . . <a href="#page-10">10</a>
<a href="#appendix-A">Appendix A</a>. PCN Deployment Considerations (Informative) . . . . . <a href="#page-11">11</a>
<a href="#appendix-A.1">A.1</a>. Choice of Suitable DSCPs . . . . . . . . . . . . . . . . . <a href="#page-11">11</a>
<a href="#appendix-A.2">A.2</a>. Rationale for Using ECT(0) for Not-Marked . . . . . . . . <a href="#page-12">12</a>
<a href="#appendix-B">Appendix B</a>. Co-Existence of PCN and ECN (Informative) . . . . . . <a href="#page-13">13</a>
<span class="grey">Moncaster, et al. Standards Track [Page 2]</span></pre>
<hr class='noprint'/><!--NewPage--><pre class='newpage'><span id="page-3" ></span>
<span class="grey"><a href="./rfc5696">RFC 5696</a> Baseline PCN Encoding November 2009</span>
<span class="h2"><a class="selflink" id="section-1" href="#section-1">1</a>. Introduction</span>
The objective of the Pre-Congestion Notification (PCN) architecture
[<a href="./rfc5559" title=""Pre-Congestion Notification (PCN) Architecture"">RFC5559</a>] is to protect the quality of service (QoS) of inelastic
flows within a Diffserv domain in a simple, scalable, and robust
fashion. The overall rate of PCN-traffic is metered on every link in
the PCN-domain, and PCN-packets are appropriately marked when certain
configured rates are exceeded. These configured rates are below the
rate of the link, thus providing notification before any congestion
occurs (hence "Pre-Congestion Notification"). The level of marking
allows the boundary nodes to make decisions about whether to admit or
block a new flow request, and (in abnormal circumstances) whether to
terminate some of the existing flows, thereby protecting the QoS of
previously admitted flows.
This document specifies how these PCN-marks are encoded into the IP
header by reusing the bits of the Explicit Congestion Notification
(ECN) field [<a href="./rfc3168" title=""The Addition of Explicit Congestion Notification (ECN) to IP"">RFC3168</a>]. It also describes how packets are identified
as belonging to a PCN-flow. Some deployment models require two PCN
encoding states, others require more. The baseline encoding
described here only provides for two PCN encoding states. However,
the encoding can be easily extended to provide more states. Rules
for such extensions are given in <a href="#section-5">Section 5</a>.
<span class="h2"><a class="selflink" id="section-2" href="#section-2">2</a>. Requirements Notation</span>
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 [<a href="./rfc2119" title=""Key words for use in RFCs to Indicate Requirement Levels"">RFC2119</a>].
<span class="h2"><a class="selflink" id="section-3" href="#section-3">3</a>. Terminology and Abbreviations</span>
<span class="h3"><a class="selflink" id="section-3.1" href="#section-3.1">3.1</a>. Terminology</span>
The terms PCN-capable, PCN-domain, PCN-node, PCN-interior-node, PCN-
ingress-node, PCN-egress-node, PCN-boundary-node, PCN-traffic, PCN-
packets and PCN-marking are used as defined in [<a href="./rfc5559" title=""Pre-Congestion Notification (PCN) Architecture"">RFC5559</a>]. The
following additional terms are defined in this document:
o PCN-compatible Diffserv codepoint - a Diffserv codepoint
indicating packets for which the ECN field is used to carry PCN-
markings rather than [<a href="./rfc3168" title=""The Addition of Explicit Congestion Notification (ECN) to IP"">RFC3168</a>] markings.
o PCN-marked codepoint - a codepoint that indicates packets that
have been marked at a PCN-interior-node using some PCN-marking
behaviour [<a href="./rfc5670" title=""Metering and Marking Behaviour of PCN- Nodes"">RFC5670</a>]. Abbreviated to PM.
<span class="grey">Moncaster, et al. Standards Track [Page 3]</span></pre>
<hr class='noprint'/><!--NewPage--><pre class='newpage'><span id="page-4" ></span>
<span class="grey"><a href="./rfc5696">RFC 5696</a> Baseline PCN Encoding November 2009</span>
o Not-marked codepoint - a codepoint that indicates packets that are
PCN-capable but that are not PCN-marked. Abbreviated to NM.
o not-PCN codepoint - a codepoint that indicates packets that are
not PCN-capable.
<span class="h3"><a class="selflink" id="section-3.2" href="#section-3.2">3.2</a>. List of Abbreviations</span>
The following abbreviations are used in this document:
o AF = Assured Forwarding [<a href="./rfc2597" title=""Assured Forwarding PHB Group"">RFC2597</a>]
o CE = Congestion Experienced [<a href="./rfc3168" title=""The Addition of Explicit Congestion Notification (ECN) to IP"">RFC3168</a>]
o CS = Class Selector [<a href="./rfc2474" title=""Definition of the Differentiated Services Field (DS Field) in the IPv4 and IPv6 Headers"">RFC2474</a>]
o DSCP = Diffserv codepoint
o ECN = Explicit Congestion Notification [<a href="./rfc3168" title=""The Addition of Explicit Congestion Notification (ECN) to IP"">RFC3168</a>]
o ECT = ECN Capable Transport [<a href="./rfc3168" title=""The Addition of Explicit Congestion Notification (ECN) to IP"">RFC3168</a>]
o EF = Expedited Forwarding [<a href="./rfc3246" title=""An Expedited Forwarding PHB (Per-Hop Behavior)"">RFC3246</a>]
o EXP = Experimental
o NM = Not-marked
o PCN = Pre-Congestion Notification
o PM = PCN-marked
<span class="h2"><a class="selflink" id="section-4" href="#section-4">4</a>. Encoding Two PCN States in IP</span>
The PCN encoding states are defined using a combination of the DSCP
and ECN fields within the IP header. The baseline PCN encoding
closely follows the semantics of ECN [<a href="./rfc3168" title=""The Addition of Explicit Congestion Notification (ECN) to IP"">RFC3168</a>]. It allows the
encoding of two PCN states: Not-marked and PCN-marked. It also
allows for traffic that is not PCN-capable to be marked as such (not-
PCN). Given the scarcity of codepoints within the IP header, the
baseline encoding leaves one codepoint free for experimental use.
The following table defines how to encode these states in IP:
<span class="grey">Moncaster, et al. Standards Track [Page 4]</span></pre>
<hr class='noprint'/><!--NewPage--><pre class='newpage'><span id="page-5" ></span>
<span class="grey"><a href="./rfc5696">RFC 5696</a> Baseline PCN Encoding November 2009</span>
+---------------+-------------+-------------+-------------+---------+
| ECN codepoint | Not-ECT | ECT(0) (10) | ECT(1) (01) | CE (11) |
| | (00) | | | |
+---------------+-------------+-------------+-------------+---------+
| DSCP n | not-PCN | NM | EXP | PM |
+---------------+-------------+-------------+-------------+---------+
Table 1: Encoding PCN in IP
In the table above, DSCP n is a PCN-compatible Diffserv codepoint
(see <a href="#section-4.4">Section 4.4</a>) and EXP means available for Experimental use. N.B.
we deliberately reserve this codepoint for experimental use only (and
not local use) to prevent future compatibility issues.
The following rules apply to all PCN-traffic:
o PCN-traffic MUST be marked with a PCN-compatible Diffserv
codepoint. To conserve DSCPs, Diffserv codepoints SHOULD be
chosen that are already defined for use with admission-controlled
traffic. <a href="#appendix-A.1">Appendix A.1</a> gives guidance to implementors on suitable
DSCPs. Guidelines for mixing traffic types within a PCN-domain
are given in [<a href="./rfc5670" title=""Metering and Marking Behaviour of PCN- Nodes"">RFC5670</a>].
o Any packet arriving at the PCN-ingress-node that shares a PCN-
compatible DSCP and is not a PCN-packet MUST be marked as not-PCN
within the PCN-domain.
o If a packet arrives at the PCN-ingress-node with its ECN field
already set to a value other than not-ECT, then appropriate action
MUST be taken to meet the requirements of [<a href="./rfc3168" title=""The Addition of Explicit Congestion Notification (ECN) to IP"">RFC3168</a>]. The simplest
appropriate action is to just drop such packets. However, this is
a drastic action that an operator may feel is undesirable.
<a href="#appendix-B">Appendix B</a> provides more information and summarises other
alternative actions that might be taken.
<span class="h3"><a class="selflink" id="section-4.1" href="#section-4.1">4.1</a>. Marking Packets</span>
[<a id="ref-RFC5670">RFC5670</a>] states that any encoding scheme document must specify the
required action to take if one of the marking algorithms indicates
that a packet needs to be marked. For the baseline encoding scheme,
the required action is simply as follows:
o If a marking algorithm indicates the need to mark a PCN-packet,
then that packet MUST have its PCN codepoint set to 11, PCN-
marked.
<span class="grey">Moncaster, et al. Standards Track [Page 5]</span></pre>
<hr class='noprint'/><!--NewPage--><pre class='newpage'><span id="page-6" ></span>
<span class="grey"><a href="./rfc5696">RFC 5696</a> Baseline PCN Encoding November 2009</span>
<span class="h3"><a class="selflink" id="section-4.2" href="#section-4.2">4.2</a>. Valid and Invalid Codepoint Transitions</span>
A PCN-ingress-node MUST set the Not-marked (10) codepoint on any
arriving packet that belongs to a PCN-flow. It MUST set the not-PCN
(00) codepoint on all other packets sharing a PCN-compatible Diffserv
codepoint.
The only valid codepoint transitions within a PCN-interior-node are
from NM to PM (which should occur if either meter indicates a need to
PCN-mark a packet [<a href="./rfc5670" title=""Metering and Marking Behaviour of PCN- Nodes"">RFC5670</a>]) and from EXP to PM. PCN-nodes that only
implement the baseline encoding MUST be able to PCN-mark packets that
arrive with the EXP codepoint. This should ease the design of
experimental schemes that want to allow partial deployment of
experimental nodes alongside nodes that only implement the baseline
encoding. The following table gives the full set of valid and
invalid codepoint transitions.
+-------------------------------------------------+
| Codepoint Out |
+--------------+-------------+-----------+-----------+-----------+
| Codepoint in | not-PCN(00) | NM(10) | EXP(01) | PM(11) |
+--------------+-------------+-----------+-----------+-----------+
| not-PCN(00) | Valid | Not valid | Not valid | Not valid |
+--------------+-------------+-----------+-----------+-----------+
| NM(10) | Not valid | Valid | Not valid | Valid |
+--------------+-------------+-----------+-----------+-----------+
| EXP(01)* | Not valid | Not valid | Valid | Valid |
+--------------+-------------+-----------+-----------+-----------+
| PM(11) | Not valid | Not valid | Not valid | Valid |
+--------------+-------------+-----------+-----------+-----------+
* This MAY cause an alarm to be raised at a management layer.
See paragraph above for an explanation of this transition.
Table 2: Valid and Invalid Codepoint Transitions for
PCN-Packets at PCN-Interior-Nodes
The codepoint transition constraints given here apply only to the
baseline encoding scheme. Constraints on codepoint transitions for
future experimental schemes are discussed in <a href="#section-5">Section 5</a>.
A PCN-egress-node SHOULD set the not-PCN (00) codepoint on all
packets it forwards out of the PCN-domain. The only exception to
this is if the PCN-egress-node is certain that revealing other
codepoints outside the PCN-domain won't contravene the guidance given
in [<a href="./rfc4774" title=""Specifying Alternate Semantics for the Explicit Congestion Notification (ECN) Field"">RFC4774</a>]. For instance, if the PCN-ingress-node has explicitly
informed the PCN-egress-node that this flow is ECN-capable, then it
might be safe to expose other codepoints.
<span class="grey">Moncaster, et al. Standards Track [Page 6]</span></pre>
<hr class='noprint'/><!--NewPage--><pre class='newpage'><span id="page-7" ></span>
<span class="grey"><a href="./rfc5696">RFC 5696</a> Baseline PCN Encoding November 2009</span>
<span class="h3"><a class="selflink" id="section-4.3" href="#section-4.3">4.3</a>. Rationale for Encoding</span>
The exact choice of encoding was dictated by the constraints imposed
by existing IETF RFCs, in particular [<a href="./rfc3168" title=""The Addition of Explicit Congestion Notification (ECN) to IP"">RFC3168</a>], [<a href="./rfc4301" title=""Security Architecture for the Internet Protocol"">RFC4301</a>], and
[<a href="./rfc4774" title=""Specifying Alternate Semantics for the Explicit Congestion Notification (ECN) Field"">RFC4774</a>]. One of the tightest constraints was the need for any PCN
encoding to survive being tunnelled through either an IP-in-IP tunnel
or an IPsec Tunnel. [<a href="#ref-ECN-TUN" title=""Tunnelling of Explicit Congestion Notification"">ECN-TUN</a>] explains this in more detail. The
main effect of this constraint is that any PCN-marking has to carry
the 11 codepoint in the ECN field since this is the only codepoint
that is guaranteed to be copied down into the forwarded header upon
decapsulation. An additional constraint is the need to minimise the
use of Diffserv codepoints because there is a limited supply of
Standards Track codepoints remaining. <a href="#section-4.4">Section 4.4</a> explains how we
have minimised this still further by reusing pre-existing Diffserv
codepoint(s) such that non-PCN-traffic can still be distinguished
from PCN-traffic.
There are a number of factors that were considered before choosing to
set 10 as the NM state instead of 01. These included similarity to
ECN, presence of tunnels within the domain, leakage into and out of
the PCN-domain, and incremental deployment (see <a href="#appendix-A.2">Appendix A.2</a>).
The encoding scheme above seems to meet all these constraints and
ends up looking very similar to ECN. This is perhaps not surprising
given the similarity in architectural intent between PCN and ECN.
<span class="h3"><a class="selflink" id="section-4.4" href="#section-4.4">4.4</a>. PCN-Compatible Diffserv Codepoints</span>
Equipment complying with the baseline PCN encoding MUST allow PCN to
be enabled for certain Diffserv codepoints. This document defines
the term "PCN-compatible Diffserv codepoint" for such a DSCP. To be
clear, any packets with such a DSCP will be PCN-enabled only if they
are within a PCN-domain and have their ECN field set to indicate a
codepoint other than not-PCN.
Enabling PCN-marking behaviour for a specific DSCP disables any other
marking behaviour (e.g., enabling PCN replaces the default ECN
marking behaviour introduced in [<a href="./rfc3168" title=""The Addition of Explicit Congestion Notification (ECN) to IP"">RFC3168</a>]) with the PCN-metering and
-marking behaviours described in [<a href="./rfc5670" title=""Metering and Marking Behaviour of PCN- Nodes"">RFC5670</a>]). This ensures compliance
with the Best Current Practice (BCP) guidance set out in [<a href="./rfc4774" title=""Specifying Alternate Semantics for the Explicit Congestion Notification (ECN) Field"">RFC4774</a>].
The PCN working group has chosen not to define a single DSCP for use
with PCN for several reasons. Firstly, the PCN mechanism is
applicable to a variety of different traffic classes. Secondly,
Standards Track DSCPs are in increasingly short supply. Thirdly, PCN
is not a scheduling behaviour -- rather, it should be seen as being
<span class="grey">Moncaster, et al. Standards Track [Page 7]</span></pre>
<hr class='noprint'/><!--NewPage--><pre class='newpage'><span id="page-8" ></span>
<span class="grey"><a href="./rfc5696">RFC 5696</a> Baseline PCN Encoding November 2009</span>
essentially a marking behaviour similar to ECN but intended for
inelastic traffic. More details are given in the informational
<a href="#appendix-A.1">Appendix A.1</a>.
<span class="h4"><a class="selflink" id="section-4.4.1" href="#section-4.4.1">4.4.1</a>. Co-Existence of PCN and Not-PCN Traffic</span>
The scarcity of pool 1 DSCPs, coupled with the fact that PCN is
envisaged as a marking behaviour that could be applied to a number of
different DSCPs, makes it essential that we provide a not-PCN state.
As stated above (and expanded in <a href="#appendix-A.1">Appendix A.1</a>), the aim is for PCN to
re-use existing DSCPs. Because PCN redefines the meaning of the ECN
field for such DSCPs, it is important to allow an operator to still
use the DSCP for non-PCN-traffic. This is achieved by providing a
not-PCN state within the encoding scheme. <a href="./rfc5559#section-3.5">Section 3.5 of [RFC5559]</a>
discusses how competing-non-PCN-traffic should be handled.
<span class="h2"><a class="selflink" id="section-5" href="#section-5">5</a>. Rules for Experimental Encoding Schemes</span>
Any experimental encoding scheme MUST follow these rules to ensure
backward compatibility with this baseline scheme:
o All PCN-interior-nodes within a PCN-domain MUST interpret the 00
codepoint in the ECN field as not-PCN and MUST NOT change it to
another value. Therefore, a PCN-ingress-node wishing to disable
PCN-marking for a packet with a PCN-compatible Diffserv codepoint
MUST set the ECN field to 00.
o The 11 codepoint in the ECN field MUST indicate that the packet
has been PCN-marked as the result of one or both of the meters
indicating a need to PCN-mark a packet [<a href="./rfc5670" title=""Metering and Marking Behaviour of PCN- Nodes"">RFC5670</a>]. The
experimental scheme MUST define which meter(s) trigger this
marking.
o The 01 Experimental codepoint in the ECN field MAY mean PCN-marked
or it MAY carry some other meaning. However, any experimental
scheme MUST define its meaning in the context of that experiment.
o If both the 01 and 11 codepoints are being used to indicate PCN-
marked, then the 11 codepoint MUST be taken to be the more severe
marking and the choice of which meter sets which mark MUST be
defined.
o Once set, the 11 codepoint in the ECN field MUST NOT be changed to
any other codepoint.
o Any experimental scheme MUST include details of all valid and
invalid codepoint transitions at any PCN-nodes.
<span class="grey">Moncaster, et al. Standards Track [Page 8]</span></pre>
<hr class='noprint'/><!--NewPage--><pre class='newpage'><span id="page-9" ></span>
<span class="grey"><a href="./rfc5696">RFC 5696</a> Baseline PCN Encoding November 2009</span>
<span class="h2"><a class="selflink" id="section-6" href="#section-6">6</a>. Backward Compatibility</span>
<a href="https://www.rfc-editor.org/bcp/bcp124">BCP 124</a> [<a href="./rfc4774" title=""Specifying Alternate Semantics for the Explicit Congestion Notification (ECN) Field"">RFC4774</a>] gives guidelines for specifying alternative
semantics for the ECN field. It sets out a number of factors to be
taken into consideration. It also suggests various techniques to
allow the co-existence of default ECN and alternative ECN semantics.
The baseline encoding specified in this document defines PCN-
compatible Diffserv codepoints as no longer supporting the default
ECN semantics. As such, this document is compatible with <a href="https://www.rfc-editor.org/bcp/bcp124">BCP 124</a>.
On its own, this baseline encoding cannot support both ECN marking
end-to-end (e2e) and PCN-marking within a PCN-domain. It is possible
to do this by carrying e2e ECN across a PCN-domain within the inner
header of an IP-in-IP tunnel, or by using a richer encoding such as
the proposed experimental scheme in [<a href="#ref-PCN-ENC" title=""A PCN encoding using 2 DSCPs to provide 3 or more states"">PCN-ENC</a>].
In any PCN deployment, traffic can only enter the PCN-domain through
PCN-ingress-nodes and leave through PCN-egress-nodes. PCN-ingress-
nodes ensure that any packets entering the PCN-domain have the ECN
field in their outermost IP header set to the appropriate PCN
codepoint. PCN-egress-nodes then guarantee that the ECN field of any
packet leaving the PCN-domain has the correct ECN semantics. This
prevents unintended leakage of ECN marks into or out of the PCN-
domain, and thus reduces backward-compatibility issues.
<span class="h2"><a class="selflink" id="section-7" href="#section-7">7</a>. Security Considerations</span>
PCN-marking only carries a meaning within the confines of a PCN-
domain. This encoding document is intended to stand independently of
the architecture used to determine how specific packets are
authorised to be PCN-marked, which will be described in separate
documents on PCN-boundary-node behaviour.
This document assumes the PCN-domain to be entirely under the control
of a single operator, or a set of operators who trust each other.
However, future extensions to PCN might include inter-domain versions
where trust cannot be assumed between domains. If such schemes are
proposed, they must ensure that they can operate securely despite the
lack of trust. However, such considerations are beyond the scope of
this document.
One potential security concern is the injection of spurious PCN-marks
into the PCN-domain. However, these can only enter the domain if a
PCN-ingress-node is misconfigured. The precise impact of any such
misconfiguration will depend on which of the proposed PCN-boundary-
node behaviour schemes is used, but in general spurious marks will
lead to admitting fewer flows into the domain or potentially
terminating too many flows. In either case, good management should
<span class="grey">Moncaster, et al. Standards Track [Page 9]</span></pre>
<hr class='noprint'/><!--NewPage--><pre class='newpage'><span id="page-10" ></span>
<span class="grey"><a href="./rfc5696">RFC 5696</a> Baseline PCN Encoding November 2009</span>
be able to quickly spot the problem since the overall utilisation of
the domain will rapidly fall.
<span class="h2"><a class="selflink" id="section-8" href="#section-8">8</a>. Conclusions</span>
This document defines the baseline PCN encoding, utilising a
combination of a PCN-compatible DSCP and the ECN field in the IP
header. This baseline encoding allows the existence of two PCN
encoding states: Not-marked and PCN-marked. It also allows for the
co-existence of competing traffic within the same DSCP, so long as
that traffic does not require ECN support within the PCN-domain. The
encoding scheme is conformant with [<a href="./rfc4774" title=""Specifying Alternate Semantics for the Explicit Congestion Notification (ECN) Field"">RFC4774</a>]. The working group has
chosen not to define a single DSCP for use with PCN. The rationale
for this decision along with advice relating to the choice of
suitable DSCPs can be found in <a href="#appendix-A.1">Appendix A.1</a>.
<span class="h2"><a class="selflink" id="section-9" href="#section-9">9</a>. Acknowledgements</span>
This document builds extensively on work done in the PCN working
group by Kwok Ho Chan, Georgios Karagiannis, Philip Eardley, Anna
Charny, Joe Babiarz, and others. Thanks to Ruediger Geib and Gorry
Fairhurst for providing detailed comments on this document.
<span class="h2"><a class="selflink" id="section-10" href="#section-10">10</a>. References</span>
<span class="h3"><a class="selflink" id="section-10.1" href="#section-10.1">10.1</a>. Normative References</span>
[<a id="ref-RFC2119">RFC2119</a>] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", <a href="https://www.rfc-editor.org/bcp/bcp14">BCP 14</a>, <a href="./rfc2119">RFC 2119</a>, March 1997.
[<a id="ref-RFC3168">RFC3168</a>] Ramakrishnan, K., Floyd, S., and D. Black, "The Addition
of Explicit Congestion Notification (ECN) to IP",
<a href="./rfc3168">RFC 3168</a>, September 2001.
[<a id="ref-RFC4774">RFC4774</a>] Floyd, S., "Specifying Alternate Semantics for the
Explicit Congestion Notification (ECN) Field", <a href="https://www.rfc-editor.org/bcp/bcp124">BCP 124</a>,
<a href="./rfc4774">RFC 4774</a>, November 2006.
[<a id="ref-RFC5670">RFC5670</a>] Eardley, P., Ed., "Metering and Marking Behaviour of PCN-
Nodes", <a href="./rfc5670">RFC 5670</a>, November 2009.
<span class="grey">Moncaster, et al. Standards Track [Page 10]</span></pre>
<hr class='noprint'/><!--NewPage--><pre class='newpage'><span id="page-11" ></span>
<span class="grey"><a href="./rfc5696">RFC 5696</a> Baseline PCN Encoding November 2009</span>
<span class="h3"><a class="selflink" id="section-10.2" href="#section-10.2">10.2</a>. Informative References</span>
[<a id="ref-ECN-TUN">ECN-TUN</a>] Briscoe, B., "Tunnelling of Explicit Congestion
Notification", Work in Progress, July 2009.
[<a id="ref-PCN-ENC">PCN-ENC</a>] Moncaster, T., Briscoe, B., and M. Menth, "A PCN encoding
using 2 DSCPs to provide 3 or more states", Work
in Progress, April 2009.
[<a id="ref-RFC2474">RFC2474</a>] Nichols, K., Blake, S., Baker, F., and D. Black,
"Definition of the Differentiated Services Field (DS
Field) in the IPv4 and IPv6 Headers", <a href="./rfc2474">RFC 2474</a>,
December 1998.
[<a id="ref-RFC2597">RFC2597</a>] Heinanen, J., Baker, F., Weiss, W., and J. Wroclawski,
"Assured Forwarding PHB Group", <a href="./rfc2597">RFC 2597</a>, June 1999.
[<a id="ref-RFC3246">RFC3246</a>] Davie, B., Charny, A., Bennet, J., Benson, K., Le Boudec,
J., Courtney, W., Davari, S., Firoiu, V., and D.
Stiliadis, "An Expedited Forwarding PHB (Per-Hop
Behavior)", <a href="./rfc3246">RFC 3246</a>, March 2002.
[<a id="ref-RFC3540">RFC3540</a>] Spring, N., Wetherall, D., and D. Ely, "Robust Explicit
Congestion Notification (ECN) Signaling with Nonces",
<a href="./rfc3540">RFC 3540</a>, June 2003.
[<a id="ref-RFC4301">RFC4301</a>] Kent, S. and K. Seo, "Security Architecture for the
Internet Protocol", <a href="./rfc4301">RFC 4301</a>, December 2005.
[<a id="ref-RFC4594">RFC4594</a>] Babiarz, J., Chan, K., and F. Baker, "Configuration
Guidelines for DiffServ Service Classes", <a href="./rfc4594">RFC 4594</a>,
August 2006.
[<a id="ref-RFC5127">RFC5127</a>] Chan, K., Babiarz, J., and F. Baker, "Aggregation of
DiffServ Service Classes", <a href="./rfc5127">RFC 5127</a>, February 2008.
[<a id="ref-RFC5559">RFC5559</a>] Eardley, P., "Pre-Congestion Notification (PCN)
Architecture", <a href="./rfc5559">RFC 5559</a>, June 2009.
<span class="grey">Moncaster, et al. Standards Track [Page 11]</span></pre>
<hr class='noprint'/><!--NewPage--><pre class='newpage'><span id="page-12" ></span>
<span class="grey"><a href="./rfc5696">RFC 5696</a> Baseline PCN Encoding November 2009</span>
<span class="h2"><a class="selflink" id="appendix-A" href="#appendix-A">Appendix A</a>. PCN Deployment Considerations (Informative)</span>
<span class="h3"><a class="selflink" id="appendix-A.1" href="#appendix-A.1">A.1</a>. Choice of Suitable DSCPs</span>
The PCN working group chose not to define a single DSCP for use with
PCN for several reasons. Firstly, the PCN mechanism is applicable to
a variety of different traffic classes. Secondly, Standards Track
DSCPs are in increasingly short supply. Thirdly, PCN is not a
scheduling behaviour -- rather, it should be seen as being a marking
behaviour similar to ECN but intended for inelastic traffic. The
choice of which DSCP is most suitable for a given PCN-domain is
dependent on the nature of the traffic entering that domain and the
link rates of all the links making up that domain. In PCN-domains
with sufficient aggregation, the appropriate DSCPs would currently be
those for the Real-Time Treatment Aggregate [<a href="./rfc5127" title=""Aggregation of DiffServ Service Classes"">RFC5127</a>]. The PCN
working group suggests using admission control for the following
service classes (defined in [<a href="./rfc4594" title=""Configuration Guidelines for DiffServ Service Classes"">RFC4594</a>]):
o Telephony (EF)
o Real-time interactive (CS4)
o Broadcast Video (CS3)
o Multimedia Conferencing (AF4)
CS5 is excluded from this list since PCN is not expected to be
applied to signalling traffic.
PCN-marking is intended to provide a scalable admission-control
mechanism for traffic with a high degree of statistical multiplexing.
PCN-marking would therefore be appropriate to apply to traffic in the
above classes, but only within a PCN-domain containing sufficiently
aggregated traffic. In such cases, the above service classes may
well all be subject to a single forwarding treatment (treatment
aggregate [<a href="./rfc5127" title=""Aggregation of DiffServ Service Classes"">RFC5127</a>]). However, this does not imply all such IP
traffic would necessarily be identified by one DSCP -- each service
class might keep a distinct DSCP within the highly aggregated region
[<a href="./rfc5127" title=""Aggregation of DiffServ Service Classes"">RFC5127</a>].
Additional service classes may be defined for which admission control
is appropriate, whether through some future standards action or
through local use by certain operators, e.g., the Multimedia
Streaming service class (AF3). This document does not preclude the
use of PCN in more cases than those listed above.
Note: The above discussion is informative not normative, as operators
are ultimately free to decide whether to use admission control for
<span class="grey">Moncaster, et al. Standards Track [Page 12]</span></pre>
<hr class='noprint'/><!--NewPage--><pre class='newpage'><span id="page-13" ></span>
<span class="grey"><a href="./rfc5696">RFC 5696</a> Baseline PCN Encoding November 2009</span>
certain service classes and whether to use PCN as their mechanism of
choice.
<span class="h3"><a class="selflink" id="appendix-A.2" href="#appendix-A.2">A.2</a>. Rationale for Using ECT(0) for Not-Marked</span>
The choice of which ECT codepoint to use for the Not-marked state was
based on the following considerations:
o [<a href="./rfc3168" title=""The Addition of Explicit Congestion Notification (ECN) to IP"">RFC3168</a>] full-functionality tunnel within the PCN-domain: Either
ECT is safe.
o Leakage of traffic into PCN-domain: Because of the lack of take-up
of the ECN nonce [<a href="./rfc3540" title=""Robust Explicit Congestion Notification (ECN) Signaling with Nonces"">RFC3540</a>], leakage of ECT(1) is less likely to
occur and so might be considered safer.
<span class="grey">Moncaster, et al. Standards Track [Page 13]</span></pre>
<hr class='noprint'/><!--NewPage--><pre class='newpage'><span id="page-14" ></span>
<span class="grey"><a href="./rfc5696">RFC 5696</a> Baseline PCN Encoding November 2009</span>
o Leakage of traffic out of PCN-domain: Either ECT is equally unsafe
(since this would incorrectly indicate the traffic was ECN-capable
outside the controlled PCN-domain).
o Incremental deployment: Either codepoint is suitable, providing
that the codepoints are used consistently.
o Conceptual consistency with other schemes: ECT(0) is conceptually
consistent with [<a href="./rfc3168" title=""The Addition of Explicit Congestion Notification (ECN) to IP"">RFC3168</a>].
Overall, this seemed to suggest that ECT(0) was most appropriate to
use.
<span class="h2"><a class="selflink" id="appendix-B" href="#appendix-B">Appendix B</a>. Co-Existence of PCN and ECN (Informative)</span>
This baseline encoding scheme redefines the ECN codepoints within the
PCN-domain. As packets with a PCN-compatible DSCP leave the PCN-
domain, their ECN field is reset to not-ECT (00). This is a problem
for the operator if packets with a PCN-compatible DSCP arrive at the
PCN-domain with any ECN codepoint other than not-ECN. If the ECN-
codepoint is ECT(0) (10) or ECT(1) (01), resetting the ECN field to
00 effectively turns off end-to-end ECN. This is undesirable as it
removes the benefits of ECN, but [<a href="./rfc3168" title=""The Addition of Explicit Congestion Notification (ECN) to IP"">RFC3168</a>] states that it is no worse
than dropping the packet. However, if a packet was marked with CE
(11), resetting the ECN field to 00 at the PCN egress node violates
the rule that CE-marks must never be lost except as a result of
packet drop [<a href="./rfc3168" title=""The Addition of Explicit Congestion Notification (ECN) to IP"">RFC3168</a>].
A number of options exist to overcome this issue. The most
appropriate option will depend on the circumstances and has to be a
decision for the operator. The definition of the action is beyond
the scope of this document, but we briefly explain the four broad
categories of solution below: tunnelling the packets, using an
extended encoding scheme, signalling to the end systems to stop using
ECN, or re-marking packets to a different DSCP.
o Tunnelling the packets across the PCN-domain (for instance, in an
IP-in-IP tunnel from the PCN-ingress-node to the PCN-egress-node)
preserves the original ECN marking on the inner header.
o An extended encoding scheme can be designed that preserves the
original ECN codepoints. For instance, if the PCN-egress-node can
determine from the PCN codepoint what the original ECN codepoint
was, then it can reset the packet to that codepoint. [<a href="#ref-PCN-ENC" title=""A PCN encoding using 2 DSCPs to provide 3 or more states"">PCN-ENC</a>]
partially achieves this but is unable to recover ECN markings if
the packet is PCN-marked in the PCN-domain.
<span class="grey">Moncaster, et al. Standards Track [Page 14]</span></pre>
<hr class='noprint'/><!--NewPage--><pre class='newpage'><span id="page-15" ></span>
<span class="grey"><a href="./rfc5696">RFC 5696</a> Baseline PCN Encoding November 2009</span>
o Explicit signalling to the end systems can indicate to the source
that ECN cannot be used on this path (because it does not support
ECN and PCN at the same time). Dropping the packet can be thought
of as a form of silent signal to the source, as it will see any
ECT-marked packets it sends being dropped.
o Packets that are not part of a PCN-flow but which share a PCN-
compatible DSCP can be re-marked to a different local-use DSCP at
the PCN-ingress-node with the original DSCP restored at the PCN-
egress. This preserves the ECN codepoint on these packets but
relies on there being spare local-use DSCPs within the PCN-domain.
Authors' Addresses
Toby Moncaster
BT
B54/70, Adastral Park
Martlesham Heath
Ipswich IP5 3RE
UK
Phone: +44 7918 901170
EMail: toby.moncaster@bt.com
Bob Briscoe
BT
B54/77, Adastral Park
Martlesham Heath
Ipswich IP5 3RE
UK
Phone: +44 1473 645196
EMail: bob.briscoe@bt.com
Michael Menth
University of Wuerzburg
Institute of Computer Science
Am Hubland
Wuerzburg D-97074
Germany
Phone: +49 931 318 6644
EMail: menth@informatik.uni-wuerzburg.de
Moncaster, et al. Standards Track [Page 15]
</pre>
|
