One document matched: draft-ietf-pcn-baseline-encoding-06.xml
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<front>
<title abbrev="Baseline PCN Encoding">
Baseline Encoding and Transport of Pre-Congestion Information
</title>
<author initials="T." surname="Moncaster" fullname="Toby Moncaster">
<organization>BT</organization>
<address>
<postal>
<street>B54/70, Adastral Park</street>
<street>Martlesham Heath</street>
<city>Ipswich</city>
<code>IP5 3RE</code>
<country>UK</country>
</postal>
<phone>+44 1473 648734</phone>
<email>toby.moncaster@bt.com</email>
</address>
</author>
<author initials="B." surname="Briscoe" fullname="Bob Briscoe">
<organization>BT</organization>
<address>
<postal>
<street>B54/77, Adastral Park</street>
<street>Martlesham Heath</street>
<city>Ipswich</city>
<code>IP5 3RE</code>
<country>UK</country>
</postal>
<phone>+44 1473 645196</phone>
<email>bob.briscoe@bt.com</email>
</address>
</author>
<author initials="M." surname="Menth" fullname="Michael Menth">
<organization>University of Wuerzburg</organization>
<address>
<postal>
<street>room B206, Institute of Computer Science</street>
<street>Am Hubland</street>
<city> Wuerzburg</city>
<code>D-97074</code>
<country>Germany</country>
</postal>
<phone>+49 931 888 6644</phone>
<email>menth@informatik.uni-wuerzburg.de</email>
</address>
</author>
<date day="4" month="September" year="2009"></date>
<area>Transport</area>
<workgroup>Congestion and Pre Congestion</workgroup>
<keyword>Quality of Service</keyword>
<keyword>QoS</keyword>
<keyword>Differentiated Services</keyword>
<keyword>Admission Control</keyword>
<keyword>Codepoint</keyword>
<keyword>Protocol</keyword>
<abstract>
<t>
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.
</t>
</abstract>
<!-- ================================================================ -->
</front>
<middle>
<!-- ================================================================ -->
<section anchor="pcn_enc_intro" title="Introduction">
<t>
The objective of the Pre-Congestion Notification (PCN) Architecture <xref target="RFC5559"></xref> 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.</t>
<t>
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 <xref target="RFC3168"></xref>.
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
<xref target="pcn_enc_experiments"></xref>.
</t>
<t>
Changes from previous drafts (to be removed by the RFC Editor):
<list style="hanging">
<t hangText="From -05 to -06:"> </t>
<t>Extensive changes to the text following IETF Last Call and Gen-ART review comments.</t>
<t>Abstract updated following mailing list discussions after Gen-ART review by Spencer Dawkins.</t>
<t>Added list of abbreviations</t>
<t>New [section 4.1] added to explain the required action when a node indicates the need to mark a packet.</t>
<t>Clarified text and Table 2 in <xref target="pcn_enc_valid_invalid"></xref>. </t>
<t>Improved explanation of rules for experimental encoding schemes in <xref target="pcn_enc_experiments"></xref>. Removed
any ambiguity about meaning of PCN-marked in such a context. Added requirements for experimental schemes to define which meter causes which mark.</t>
<t>Clarified text in <xref target="pcn_enc_compat"></xref> relating to support for e2e ECN. </t>
<t>Added text in <xref target="pcn_enc_security"></xref> relating to injection of PCN-marks into the PCN-domain.</t>
<t>Changed text of <xref target="pcn_enc_app_DSCP_choice"></xref> to reflect comments from Spencer Dawkins and Philip Eardey. </t>
<t hangText="From -04 to -05:"> </t>
<t>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. <xref target="pcn_enc_app_DSCP_choice"></xref>
has been re-written to reflect this. References to maintaining a list of PCN-compatible DSCPs
have also been removed.</t>
<t>Last sentence of <xref target="pcn_enc_compat"></xref> altered.</t>
<t>Several spelling corrections.</t>
<t>References updated throughout.</t>
<t hangText="From -03 to -04:"> </t>
<t>Major WGLC comments addressed:
<list style="symbols">
<t>Added <xref target="pcn_enc_not-PCN"></xref> to clarify why we need the not-PCN codepoint.</t>
<t>Stated that the PCN WG will maintain a list of PCN-compatible DSCPs. This should help
avoid inter-operability issues.</t>
</list>
</t>
<t>Also addressed a number of WGLC nits.</t>
<t hangText="From -02 to -03:"> </t>
<t>Extensive changes to address comments made by Gorry Fairhurst including:
<list style="symbols">
<t>Abstract re-written.</t>
<t>Clarified throughout that this re-uses the ECN bits in the IP header.</t>
<t>Re-arranged order of terminology section for clarity.</t>
<t>Table 2 replaced with new table and text.</t>
<t>Security considerations re-written.</t>
<t>Appendixes re-written to improve clarity.</t>
<t>Numerous minor nits and language changes throughout.</t>
</list>
</t>
<t>Extensive other minor changes throughout.</t>
<t hangText="From -01 to -02:"> </t>
<t> Removed Appendix A and replaced with reference to <xref target="I-D.ietf-tsvwg-ecn-tunnel"></xref></t>
<t> Moved Appendix B into main body of text.</t>
<t> Changed Appendix C to give deployment advice.</t>
<t> Minor changes throughout including checking consistency of capitalisation of defined terms.</t>
<t> Clarified that LU was deliberately excluded from encoding.</t>
<t hangText="From -00 to -01:"> </t>
<t>Added section on restrictions for extension encoding schemes.</t>
<t>Included table in Appendix showing encoding transitions at different PCN nodes.</t>
<t>Checked for consistency of terminology.</t>
<t>Minor language changes for clarity.</t>
<t hangText="Changes from previous filename"> </t>
<t>Filename changed from draft-moncaster-pcn-baseline-encoding.</t>
<t>Terminology changed for clarity (PCN-compatible DSCP and PCN-enabled packet).</t>
<t>Minor changes throughout.</t>
<t> Modified meaning of ECT(1) state to EXP.</t>
<t>Moved text relevant to behaviour of nodes into appendix for
later transfer to new document on edge behaviours.
</t>
<t hangText="From draft-moncaster -01 to -02:"></t>
<t>Minor changes throughout including tightening up language to remain
consistent with the PCN Architecture terminology.</t>
<t hangText="From draft-moncaster -00 to -01:"> </t>
<t>Change of title from "Encoding and Transport of (Pre-)Congestion Information from within a
Diffserv Domain to the Egress"</t>
<t>Extensive changes to Introduction and abstract.</t>
<t>Added a section on the implications of re-using a DSCP.</t>
<t>Added appendix listing possible operator scenarios for using this baseline encoding.</t>
<t>Minor changes throughout.</t>
</list>
</t>
</section>
<!-- ================================================================ -->
<section anchor="pcn_enc_Reqs_notation" title="Requirements notation">
<t>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 <xref target="RFC2119"></xref>.
</t>
</section>
<!-- ================================================================ -->
<section anchor="pcn_enc_terminology" title="Terminology and Abbreviations">
<t> The following terms are defined in this document:
<list style="symbols">
<t>PCN-compatible Diffserv codepoint - a Diffserv codepoint for which the ECN field
is used to carry PCN markings rather than <xref target="RFC3168"></xref> markings.</t>
<t>PCN-marked - codepoint indicating packets that have been marked at a
PCN-interior-node using some PCN marking behaviour <xref target="I-D.ietf-pcn-marking-behaviour"></xref>. Abbreviated to PM.</t>
<t>Not-marked - codepoint indicating packets that are PCN-capable, but are not
PCN-marked. Abbreviated to NM.</t>
<t>PCN-enabled codepoints - collective term for all NM and PM codepoints.
By definition, packets carrying such codepoints are PCN-packets.</t>
<t>not-PCN - packets that are not PCN-enabled.</t>
</list>
In addition, the document uses the terminology defined in <xref target="RFC5559"></xref>.
</t>
<section anchor="pcn_enc_abbreviations" title="List of Abbreviations">
<t> The following abbreviations are used in this document:
<list style="symbols">
<t>AF = Assured Forwarding <xref target="RFC2597"></xref></t>
<t>CE = Congestion Experienced <xref target="RFC3168"></xref></t>
<t>CS = Class Selector <xref target="RFC2474"></xref></t>
<t>DSCP = Diffserv codepoint</t>
<t>ECN = Explicit Congestion Notification <xref target="RFC3168"></xref> </t>
<t>ECT = ECN Capable Transport <xref target="RFC3168"></xref></t>
<t>EF = Expedited Forwarding <xref target="RFC3246"></xref></t>
<t>EXP = Experimental </t>
<t>NM = Not-marked </t>
<t>PCN = Pre-Congestion Notification</t>
<t>PM = PCN-marked </t>
</list>
</t>
</section>
</section>
<!-- ================================================================ -->
<section anchor="pcn_enc_IP" title="Encoding two PCN States in IP">
<t>
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:</t>
<texttable anchor="pcn_enc_Tab_Default_coding" title="Encoding PCN in IP">
<ttcol align="center">ECN codepoint</ttcol>
<ttcol align="center">Not-ECT (00)</ttcol>
<ttcol align="center">ECT(0) (10)</ttcol>
<ttcol align="center">ECT(1) (01)</ttcol>
<ttcol align="center">CE (11)</ttcol>
<c>DSCP n</c><c>not-PCN</c><c>NM</c><c>EXP</c><c>PM</c>
<postamble>Where DSCP n is a PCN-compatible Diffserv codepoint (see <xref target="pcn_enc_DSCPs"></xref>)
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.</postamble>
</texttable>
<t>
The following rules apply to all PCN traffic:
<list style="symbols">
<t>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. <xref target="pcn_enc_app_DSCP_choice"></xref> gives guidance to implementors on suitable
DSCPs. Guidelines for mixing traffic-types within
a PCN-domain are given in <xref target="I-D.ietf-pcn-marking-behaviour"></xref>.</t>
<t>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. </t>
</list>
</t>
<section anchor="pcn_enc_marking_action" title="Marking Packets">
<t><xref target="I-D.ietf-pcn-marking-behaviour"></xref> 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:
<list style="symbols">
<t> 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.</t>
</list></t>
</section>
<section anchor="pcn_enc_valid_invalid" title="Valid and Invalid Codepoint Transitions">
<t>
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.
</t>
<t>
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 <xref target="I-D.ietf-pcn-marking-behaviour"></xref>) 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.</t>
<artwork>
+-------------------------------------------------+
| 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
</artwork>
<t>The codepoint transition constraints given here apply only to the baseline encoding scheme.
Constraints on codepoint transitions for future experimental schemes are discussed in
<xref target="pcn_enc_experiments"></xref>.</t>
<t>
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 <xref target="RFC4774"></xref>. 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.
</t>
</section>
<!-- ================================================================ -->
<section anchor="pcn_enc_rationale" title="Rationale for Encoding">
<t>
The exact choice of encoding was dictated by the constraints imposed by existing
IETF RFCs, in particular <xref target="RFC3168"></xref>, <xref target="RFC4301"></xref> and
<xref target="RFC4774"></xref>. 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. <xref target="I-D.ietf-tsvwg-ecn-tunnel"></xref>
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.
<xref target="pcn_enc_DSCPs"></xref> 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.</t>
<t>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 <xref target="pcn_enc_app_rationale"></xref>).
</t><t>
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.
</t>
</section>
<!-- ================================================================ -->
<section anchor="pcn_enc_DSCPs" title="PCN-Compatible Diffserv Codepoints">
<t>
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.
</t> <t>
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
<xref target="RFC3168"></xref>) with the PCN metering and marking behaviours described in
<xref target="I-D.ietf-pcn-marking-behaviour"></xref>). This ensures compliance with the
BCP guidance set out in <xref target="RFC4774"></xref>.
</t>
<t> 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 <xref target="pcn_enc_app_DSCP_choice"></xref>.</t>
<section anchor="pcn_enc_not-PCN" title="Co-existence of PCN and not-PCN traffic">
<t>
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
<xref target="pcn_enc_app_DSCP_choice"></xref>) 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 <xref target="RFC5559"></xref>
discusses how competing-non-PCN-traffic should be handled.
</t>
</section>
</section>
<!-- ================================================================ -->
</section>
<section anchor="pcn_enc_experiments" title="Rules for Experimental Encoding Schemes">
<t>
Any experimental encoding scheme MUST follow these rules to ensure backward compatibility
with this baseline scheme:
<list style="symbols">
<t>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.
</t>
<t>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 <xref target="I-D.ietf-pcn-marking-behaviour"></xref>. The experimental
scheme MUST define which meter(s) trigger this marking.
</t>
<t> 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.</t>
<t>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.</t>
<t>Once set, the 11 codepoint in the ECN field MUST NOT be changed to any other codepoint.
</t>
<t>Any experimental scheme MUST include details of all valid and invalid codepoint
transitions at any PCN nodes.</t>
</list>
</t>
</section>
<section anchor="pcn_enc_compat" title="Backwards Compatibility">
<t> BCP 124 <xref target="RFC4774"></xref> 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.</t>
<t>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 <xref target="I-D.ietf-pcn-3-state-encoding"></xref>.</t>
</section>
<!-- ================================================================ -->
<!-- ================================================================ -->
<section title="IANA Considerations">
<t>This document makes no request to IANA.
</t>
</section>
<!-- ================================================================ -->
<section anchor="pcn_enc_security" title="Security Considerations">
<t>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.
</t>
<t>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.
</t>
<t>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.</t>
</section>
<!-- ================================================================ -->
<section anchor="pcn_enc_Conclusions" title="Conclusions">
<t>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 <xref target="RFC4774"></xref>. 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 <xref target="pcn_enc_app_DSCP_choice"></xref>.
</t>
</section>
<!-- ================================================================ -->
<section anchor="pcn_enc_Acknowledgements" title="Acknowledgements">
<t>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.
</t>
</section>
<!-- ================================================================ -->
<section anchor="pcn_enc_Comments_Solicited" title="Comments Solicited">
<t>(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.
</t>
</section>
</middle>
<back>
<!-- ================================================================ -->
<references title="Normative References">
<?rfc include="reference.RFC.2119" ?>
<?rfc include="reference.RFC.4774" ?>
<?rfc include="reference.RFC.3168" ?>
<?rfc include="reference.I-D.ietf-pcn-marking-behaviour" ?>
</references>
<references title="Informative References">
<?rfc include="reference.RFC.5559" ?>
<?rfc include="reference.RFC.3246" ?>
<?rfc include="reference.RFC.2597" ?>
<?rfc include="reference.RFC.2474" ?>
<?rfc include="reference.RFC.3540" ?>
<?rfc include="reference.RFC.4301" ?>
<?rfc include="reference.RFC.4594" ?>
<?rfc include="reference.RFC.5127" ?>
<?rfc include="reference.I-D.ietf-tsvwg-ecn-tunnel" ?>
<?rfc include="reference.I-D.ietf-pcn-3-state-encoding" ?>
</references>
<section anchor="pcn_enc_app_deployment" title="PCN Deployment Considerations (Informational)">
<section anchor="pcn_enc_app_DSCP_choice" title="Choice of Suitable DSCPs">
<t>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 <xref target="RFC5127"></xref>. The PCN Working Group suggests
using admission control for the following service classes (defined in <xref target="RFC4594"></xref>):
<list style="symbols">
<t>Telephony (EF)</t>
<t>Real-time interactive (CS4)</t>
<t>Broadcast Video (CS3)</t>
<t>Multimedia Conferencing (AF4)</t>
</list>
CS5 is excluded from this list since PCN is not expected to be applied to signalling traffic.
</t>
<t>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 <xref target="RFC5127"></xref>). 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 <xref target="RFC5127"></xref>.</t>
<t>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.</t>
<t> 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.</t>
</section>
<section anchor="pcn_enc_app_rationale" title="Rationale for Using ECT(0) for Not-marked">
<t> The choice of which ECT codepoint to use for the Not-marked state was based on the following considerations:
<list style="symbols">
<t><xref target="RFC3168"></xref> full functionality tunnel within the PCN-domain: Either ECT is safe.</t>
<t> Leakage of traffic into PCN-domain: because of the lack of take-up of the ECN nonce <xref target="RFC3540"></xref>,
leakage of ECT(1) is less likely to occur so might be considered safer.</t>
<t> 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).</t>
<t> Incremental deployment: Either codepoint is suitable providing that the codepoints are used consistently.</t>
<t> Conceptual consistency with other schemes: ECT(0) is conceptually consistent with <xref target="RFC3168"></xref>.</t>
</list>
Overall this seemed to suggest ECT(0) was most appropriate to use.
</t>
</section>
</section>
</back>
</rfc>
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