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Network Working Group T. Moncaster
Request for Comments: 5696 B. Briscoe
Category: Standards Track BT
M. Menth
University of Wuerzburg
November 2009
Baseline Encoding and Transport of Pre-Congestion Information
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 BCP 78 and the IETF Trust's Legal
Provisions Relating to IETF Documents
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publication of this document. Please review these documents
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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.
Moncaster, et al. Standards Track [Page 1]
RFC 5696 Baseline PCN Encoding November 2009
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
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3
2. Requirements Notation . . . . . . . . . . . . . . . . . . . . 3
3. Terminology and Abbreviations . . . . . . . . . . . . . . . . 3
3.1. Terminology . . . . . . . . . . . . . . . . . . . . . . . 3
3.2. List of Abbreviations . . . . . . . . . . . . . . . . . . 4
4. Encoding Two PCN States in IP . . . . . . . . . . . . . . . . 4
4.1. Marking Packets . . . . . . . . . . . . . . . . . . . . . 5
4.2. Valid and Invalid Codepoint Transitions . . . . . . . . . 6
4.3. Rationale for Encoding . . . . . . . . . . . . . . . . . . 7
4.4. PCN-Compatible Diffserv Codepoints . . . . . . . . . . . . 7
4.4.1. Co-Existence of PCN and Not-PCN Traffic . . . . . . . 8
5. Rules for Experimental Encoding Schemes . . . . . . . . . . . 8
6. Backward Compatibility . . . . . . . . . . . . . . . . . . . . 9
7. Security Considerations . . . . . . . . . . . . . . . . . . . 9
8. Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . 10
9. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 10
10. References . . . . . . . . . . . . . . . . . . . . . . . . . . 10
10.1. Normative References . . . . . . . . . . . . . . . . . . . 10
10.2. Informative References . . . . . . . . . . . . . . . . . . 10
Appendix A. PCN Deployment Considerations (Informative) . . . . . 11
A.1. Choice of Suitable DSCPs . . . . . . . . . . . . . . . . . 11
A.2. Rationale for Using ECT(0) for Not-Marked . . . . . . . . 12
Appendix B. Co-Existence of PCN and ECN (Informative) . . . . . . 13
Moncaster, et al. Standards Track [Page 2]
RFC 5696 Baseline PCN Encoding November 2009
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 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 [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.
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
3.1. Terminology
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 [RFC5559]. 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 [RFC3168] markings.
o PCN-marked codepoint - a codepoint that indicates packets that
have been marked at a PCN-interior-node using some PCN-marking
behaviour [RFC5670]. Abbreviated to PM.
Moncaster, et al. Standards Track [Page 3]
RFC 5696 Baseline PCN Encoding November 2009
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.
3.2. 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:
Moncaster, et al. Standards Track [Page 4]
RFC 5696 Baseline PCN Encoding November 2009
+---------------+-------------+-------------+-------------+---------+
| 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 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.
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 [RFC5670].
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 [RFC3168]. The simplest
appropriate action is to just drop such packets. However, this is
a drastic action that an operator may feel is undesirable.
Appendix B provides more information and summarises other
alternative actions that might be taken.
4.1. Marking Packets
[RFC5670] 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.
Moncaster, et al. Standards Track [Page 5]
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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 [RFC5670]) 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 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.
Moncaster, et al. Standards Track [Page 6]
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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. [ECN-TUN] 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. 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.
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 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 [RFC5670]). This ensures compliance
with the Best Current Practice (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
Moncaster, et al. Standards Track [Page 7]
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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 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. Section 3.5 of [RFC5559]
discusses how competing-non-PCN-traffic should be handled.
5. Rules for Experimental Encoding Schemes
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 [RFC5670]. 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.
Moncaster, et al. Standards Track [Page 8]
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6. Backward 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 (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 [PCN-ENC].
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.
7. 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
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RFC 5696 Baseline PCN Encoding November 2009
be able to quickly spot the problem since the overall utilisation of
the domain will rapidly fall.
8. Conclusions
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 [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 the choice of
suitable DSCPs can be found in Appendix A.1.
9. 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
Fairhurst for providing detailed comments on this document.
10. References
10.1. Normative References
[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.
[RFC5670] Eardley, P., Ed., "Metering and Marking Behaviour of PCN-
Nodes", RFC 5670, November 2009.
Moncaster, et al. Standards Track [Page 10]
RFC 5696 Baseline PCN Encoding November 2009
10.2. Informative References
[ECN-TUN] Briscoe, B., "Tunnelling of Explicit Congestion
Notification", Work in Progress, July 2009.
[PCN-ENC] Moncaster, T., Briscoe, B., and M. Menth, "A PCN encoding
using 2 DSCPs to provide 3 or more states", Work
in Progress, April 2009.
[RFC2474] Nichols, K., Blake, S., Baker, F., 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.
Moncaster, et al. Standards Track [Page 11]
RFC 5696 Baseline PCN Encoding November 2009
Appendix A. PCN Deployment Considerations (Informative)
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
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
Moncaster, et al. Standards Track [Page 12]
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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:
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 and so might be considered safer.
Moncaster, et al. Standards Track [Page 13]
RFC 5696 Baseline PCN Encoding November 2009
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 that ECT(0) was most appropriate to
use.
Appendix B. Co-Existence of PCN and ECN (Informative)
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 [RFC3168] 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 [RFC3168].
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. [PCN-ENC]
partially achieves this but is unable to recover ECN markings if
the packet is PCN-marked in the PCN-domain.
Moncaster, et al. Standards Track [Page 14]
RFC 5696 Baseline PCN Encoding November 2009
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]