Congestion and Pre Congestion T. Moncaster
Internet-Draft B. Briscoe
Intended status: Standards Track BT
Expires: March 8, 2010 M. Menth
University of Wuerzburg
September 4, 2009
Baseline Encoding and Transport of Pre-Congestion Information
draft-ietf-pcn-baseline-encoding-06
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Abstract
The objective of the pre-congestion notification (PCN) architecture
is to protect the 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.
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3
2. Requirements notation . . . . . . . . . . . . . . . . . . . . 6
3. Terminology and Abbreviations . . . . . . . . . . . . . . . . 6
3.1. List of Abbreviations . . . . . . . . . . . . . . . . . . 7
4. Encoding two PCN States in IP . . . . . . . . . . . . . . . . 7
4.1. Marking Packets . . . . . . . . . . . . . . . . . . . . . 8
4.2. Valid and Invalid Codepoint Transitions . . . . . . . . . 8
4.3. Rationale for Encoding . . . . . . . . . . . . . . . . . . 9
4.4. PCN-Compatible Diffserv Codepoints . . . . . . . . . . . . 10
4.4.1. Co-existence of PCN and not-PCN traffic . . . . . . . 10
5. Rules for Experimental Encoding Schemes . . . . . . . . . . . 11
6. Backwards Compatibility . . . . . . . . . . . . . . . . . . . 11
7. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 12
8. Security Considerations . . . . . . . . . . . . . . . . . . . 12
9. Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . 12
10. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 12
11. Comments Solicited . . . . . . . . . . . . . . . . . . . . . . 13
12. References . . . . . . . . . . . . . . . . . . . . . . . . . . 13
12.1. Normative References . . . . . . . . . . . . . . . . . . . 13
12.2. Informative References . . . . . . . . . . . . . . . . . . 13
Appendix A. PCN Deployment Considerations (Informational) . . . . 14
A.1. Choice of Suitable DSCPs . . . . . . . . . . . . . . . . . 14
A.2. Rationale for Using ECT(0) for Not-marked . . . . . . . . 15
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1. Introduction
The objective of the Pre-Congestion Notification (PCN) Architecture
[RFC5559] 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 the 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 re-using the bits of the Explicit Congestion Notification
(ECN) field [RFC3168]. 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 Section 5.
Changes from previous drafts (to be removed by the RFC Editor):
From -05 to -06:
Extensive changes to the text following IETF Last Call and Gen-ART
review comments.
Abstract updated following mailing list discussions after Gen-ART
review by Spencer Dawkins.
Added list of abbreviations
New [section 4.1] added to explain the required action when a node
indicates the need to mark a packet.
Clarified text and Table 2 in Section 4.2.
Improved explanation of rules for experimental encoding schemes in
Section 5. Removed any ambiguity about meaning of PCN-marked in
such a context. Added requirements for experimental schemes to
define which meter causes which mark.
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Clarified text in Section 6 relating to support for e2e ECN.
Added text in Section 8 relating to injection of PCN-marks into
the PCN-domain.
Changed text of Appendix A.1 to reflect comments from Spencer
Dawkins and Philip Eardey.
From -04 to -05:
Clarified throughout that the PCN WG is not requesting a specific
DSCP for PCN. Rather we are recommending a set of DSCPs that
might be suitable. Appendix A.1 has been re-written to reflect
this. References to maintaining a list of PCN-compatible DSCPs
have also been removed.
Last sentence of Section 6 altered.
Several spelling corrections.
References updated throughout.
From -03 to -04:
Major WGLC comments addressed:
* Added Section 4.4.1 to clarify why we need the not-PCN
codepoint.
* Stated that the PCN WG will maintain a list of PCN-compatible
DSCPs. This should help avoid inter-operability issues.
Also addressed a number of WGLC nits.
From -02 to -03:
Extensive changes to address comments made by Gorry Fairhurst
including:
* Abstract re-written.
* Clarified throughout that this re-uses the ECN bits in the IP
header.
* Re-arranged order of terminology section for clarity.
* Table 2 replaced with new table and text.
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* Security considerations re-written.
* Appendixes re-written to improve clarity.
* Numerous minor nits and language changes throughout.
Extensive other minor changes throughout.
From -01 to -02:
Removed Appendix A and replaced with reference to
[I-D.ietf-tsvwg-ecn-tunnel]
Moved Appendix B into main body of text.
Changed Appendix C to give deployment advice.
Minor changes throughout including checking consistency of
capitalisation of defined terms.
Clarified that LU was deliberately excluded from encoding.
From -00 to -01:
Added section on restrictions for extension encoding schemes.
Included table in Appendix showing encoding transitions at
different PCN nodes.
Checked for consistency of terminology.
Minor language changes for clarity.
Changes from previous filename
Filename changed from draft-moncaster-pcn-baseline-encoding.
Terminology changed for clarity (PCN-compatible DSCP and PCN-
enabled packet).
Minor changes throughout.
Modified meaning of ECT(1) state to EXP.
Moved text relevant to behaviour of nodes into appendix for later
transfer to new document on edge behaviours.
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From draft-moncaster -01 to -02:
Minor changes throughout including tightening up language to
remain consistent with the PCN Architecture terminology.
From draft-moncaster -00 to -01:
Change of title from "Encoding and Transport of (Pre-)Congestion
Information from within a Diffserv Domain to the Egress"
Extensive changes to Introduction and abstract.
Added a section on the implications of re-using a DSCP.
Added appendix listing possible operator scenarios for using this
baseline encoding.
Minor changes throughout.
2. Requirements notation
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 [RFC2119].
3. Terminology and Abbreviations
The following terms are defined in this document:
o PCN-compatible Diffserv codepoint - a Diffserv codepoint for which
the ECN field is used to carry PCN markings rather than [RFC3168]
markings.
o PCN-marked - codepoint indicating packets that have been marked at
a PCN-interior-node using some PCN marking behaviour
[I-D.ietf-pcn-marking-behaviour]. Abbreviated to PM.
o Not-marked - codepoint indicating packets that are PCN-capable,
but are not PCN-marked. Abbreviated to NM.
o PCN-enabled codepoints - collective term for all NM and PM
codepoints. By definition, packets carrying such codepoints are
PCN-packets.
o not-PCN - packets that are not PCN-enabled.
In addition, the document uses the terminology defined in [RFC5559].
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3.1. List of Abbreviations
The following abbreviations are used in this document:
o AF = Assured Forwarding [RFC2597]
o CE = Congestion Experienced [RFC3168]
o CS = Class Selector [RFC2474]
o DSCP = Diffserv codepoint
o ECN = Explicit Congestion Notification [RFC3168]
o ECT = ECN Capable Transport [RFC3168]
o EF = Expedited Forwarding [RFC3246]
o EXP = Experimental
o NM = Not-marked
o PCN = Pre-Congestion Notification
o PM = PCN-marked
4. Encoding two PCN States in IP
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 [RFC3168]. 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:
+---------------+-------------+-------------+-------------+---------+
| ECN codepoint | Not-ECT | ECT(0) (10) | ECT(1) (01) | CE (11) |
| | (00) | | | |
+---------------+-------------+-------------+-------------+---------+
| DSCP n | not-PCN | NM | EXP | PM |
+---------------+-------------+-------------+-------------+---------+
Where DSCP n is a PCN-compatible Diffserv codepoint (see Section 4.4)
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.
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Table 1: Encoding PCN in IP
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. Appendix A.1 gives guidance to implementors on suitable
DSCPs. Guidelines for mixing traffic-types within a PCN-domain
are given in [I-D.ietf-pcn-marking-behaviour].
o Any packet that is not-PCN but which shares the same Diffserv
codepoint as PCN-enabled traffic MUST have its ECN field set to
00.
4.1. Marking Packets
[I-D.ietf-pcn-marking-behaviour] 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.
4.2. Valid and Invalid Codepoint Transitions
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 [I-D.ietf-pcn-marking-behaviour]) 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.
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+-------------------------------------------------+
| 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 Section 5.
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 [RFC4774]. 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.
4.3. Rationale for Encoding
The exact choice of encoding was dictated by the constraints imposed
by existing IETF RFCs, in particular [RFC3168], [RFC4301] and
[RFC4774]. 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. [I-D.ietf-tsvwg-ecn-tunnel] 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
inner 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. Section 4.4
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.
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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
PCN-domain and incremental deployment (see Appendix A.2).
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.
4.4. PCN-Compatible Diffserv Codepoints
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 [RFC3168]) with the PCN metering and marking
behaviours described in [I-D.ietf-pcn-marking-behaviour]). This
ensures compliance with the BCP guidance set out in [RFC4774].
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
essentially a marking behaviour similar to ECN but intended for
inelastic traffic. More details are given in the informational
Appendix A.1.
4.4.1. Co-existence of PCN and not-PCN traffic
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 Appendix A.1) the aim is for PCN to
re-use existing DSCPs. Because PCN re-defines the meaning of the ECN
field for such DSCPs it is important to allow an operator to still
use the DSCP for traffic that isn't PCN-enabled. This is achieved by
providing a not-PCN state within the encoding scheme. S3.5 of
[RFC5559] discusses how competing-non-PCN-traffic should be handled.
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5. Rules for Experimental Encoding Schemes
Any experimental encoding scheme MUST follow these rules to ensure
backward compatibility with this baseline scheme:
o All 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 an 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
[I-D.ietf-pcn-marking-behaviour]. 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.
6. Backwards Compatibility
BCP 124 [RFC4774] 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 BCP 124.
On its own, this baseline encoding cannot support both ECN marking
end to end 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 [I-D.ietf-pcn-3-state-encoding].
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7. IANA Considerations
This document makes no request to IANA.
8. Security Considerations
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 be
able to quickly spot the problem since the overall utilisation of the
domain will rapidly fall.
9. Conclusions
This document defines the baseline PCN encoding utilising a
combination of a PCN-enabled 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 [RFC4774]. 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 choice of suitable
DSCPs can be found in Appendix A.1.
10. Acknowledgements
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
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Fairhurst for providing detailed comments on this document.
11. Comments Solicited
(To be removed by the RFC-Editor.) Comments and questions are
encouraged and very welcome. They can be addressed to the IETF
congestion and pre-congestion working group mailing list
<pcn@ietf.org>, and/or to the authors.
12. References
12.1. Normative References
[I-D.ietf-pcn-marking-behaviour] Eardley, P., "Metering and marking
behaviour of PCN-nodes",
draft-ietf-pcn-marking-behaviour-05
(work in progress), August 2009.
[RFC2119] Bradner, S., "Key words for use in
RFCs to Indicate Requirement
Levels", BCP 14, RFC 2119,
March 1997.
[RFC3168] Ramakrishnan, K., Floyd, S., and D.
Black, "The Addition of Explicit
Congestion Notification (ECN) to
IP", RFC 3168, September 2001.
[RFC4774] Floyd, S., "Specifying Alternate
Semantics for the Explicit
Congestion Notification (ECN)
Field", BCP 124, RFC 4774,
November 2006.
12.2. Informative References
[I-D.ietf-pcn-3-state-encoding] Moncaster, T., Briscoe, B., and M.
Menth, "A PCN encoding using 2
DSCPs to provide 3 or more states",
draft-ietf-pcn-3-state-encoding-00
(work in progress), April 2009.
[I-D.ietf-tsvwg-ecn-tunnel] Briscoe, B., "Tunnelling of
Explicit Congestion Notification",
draft-ietf-tsvwg-ecn-tunnel-03
(work in progress), July 2009.
[RFC2474] Nichols, K., Blake, S., Baker, F.,
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and D. Black, "Definition of the
Differentiated Services Field (DS
Field) in the IPv4 and IPv6
Headers", RFC 2474, December 1998.
[RFC2597] Heinanen, J., Baker, F., Weiss, W.,
and J. Wroclawski, "Assured
Forwarding PHB Group", RFC 2597,
June 1999.
[RFC3246] 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)",
RFC 3246, March 2002.
[RFC3540] Spring, N., Wetherall, D., and D.
Ely, "Robust Explicit Congestion
Notification (ECN) Signaling with
Nonces", RFC 3540, June 2003.
[RFC4301] Kent, S. and K. Seo, "Security
Architecture for the Internet
Protocol", RFC 4301, December 2005.
[RFC4594] Babiarz, J., Chan, K., and F.
Baker, "Configuration Guidelines
for DiffServ Service Classes",
RFC 4594, August 2006.
[RFC5127] Chan, K., Babiarz, J., and F.
Baker, "Aggregation of DiffServ
Service Classes", RFC 5127,
February 2008.
[RFC5559] Eardley, P., "Pre-Congestion
Notification (PCN) Architecture",
RFC 5559, June 2009.
Appendix A. PCN Deployment Considerations (Informational)
A.1. Choice of Suitable DSCPs
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
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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 [RFC5127]. The PCN
Working Group suggests using admission control for the following
service classes (defined in [RFC4594]):
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 [RFC5127]). 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
[RFC5127].
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
certain service classes and whether to use PCN as their mechanism of
choice.
A.2. Rationale for Using ECT(0) for Not-marked
The choice of which ECT codepoint to use for the Not-marked state was
based on the following considerations:
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o [RFC3168] 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 [RFC3540], leakage of ECT(1) is less likely to
occur so might be considered safer.
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 [RFC3168].
Overall this seemed to suggest ECT(0) was most appropriate to use.
Authors' Addresses
Toby Moncaster
BT
B54/70, Adastral Park
Martlesham Heath
Ipswich IP5 3RE
UK
Phone: +44 1473 648734
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
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Michael Menth
University of Wuerzburg
room B206, Institute of Computer Science
Am Hubland
Wuerzburg D-97074
Germany
Phone: +49 931 888 6644
EMail: menth@informatik.uni-wuerzburg.de
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