draft-ietf-tsvwg-ecn-tunnel-10.txt   rfc6040.txt 
Transport Area Working Group B. Briscoe Internet Engineering Task Force (IETF) B. Briscoe
Internet-Draft BT Request for Comments: 6040 BT
Updates: 3168, 4301, 4774 August 26, 2010 Updates: 3168, 4301, 4774 November 2010
(if approved) Category: Standards Track
Intended status: Standards Track ISSN: 2070-1721
Expires: February 27, 2011
Tunnelling of Explicit Congestion Notification Tunnelling of Explicit Congestion Notification
draft-ietf-tsvwg-ecn-tunnel-10
Abstract Abstract
This document redefines how the explicit congestion notification This document redefines how the explicit congestion notification
(ECN) field of the IP header should be constructed on entry to and (ECN) field of the IP header should be constructed on entry to and
exit from any IP in IP tunnel. On encapsulation it updates RFC3168 exit from any IP-in-IP tunnel. On encapsulation, it updates RFC 3168
to bring all IP in IP tunnels (v4 or v6) into line with RFC4301 IPsec to bring all IP-in-IP tunnels (v4 or v6) into line with RFC 4301
ECN processing. On decapsulation it updates both RFC3168 and RFC4301 IPsec ECN processing. On decapsulation, it updates both RFC 3168 and
to add new behaviours for previously unused combinations of inner and RFC 4301 to add new behaviours for previously unused combinations of
outer header. The new rules ensure the ECN field is correctly inner and outer headers. The new rules ensure the ECN field is
propagated across a tunnel whether it is used to signal one or two correctly propagated across a tunnel whether it is used to signal one
severity levels of congestion, whereas before only one severity level or two severity levels of congestion; whereas before, only one
was supported. Tunnel endpoints can be updated in any order without severity level was supported. Tunnel endpoints can be updated in any
affecting pre-existing uses of the ECN field, thus ensuring backward order without affecting pre-existing uses of the ECN field, thus
compatibility. Nonetheless, operators wanting to support two ensuring backward compatibility. Nonetheless, operators wanting to
severity levels (e.g. for pre-congestion notification--PCN) can support two severity levels (e.g., for pre-congestion notification --
require compliance with this new specification. A thorough analysis PCN) can require compliance with this new specification. A thorough
of the reasoning for these changes and the implications is included. analysis of the reasoning for these changes and the implications is
In the unlikely event that the new rules do not meet a specific need, included. In the unlikely event that the new rules do not meet a
RFC4774 gives guidance on designing alternate ECN semantics and this specific need, RFC 4774 gives guidance on designing alternate ECN
document extends that to include tunnelling issues. semantics, and this document extends that to include tunnelling
issues.
Status of This Memo Status of This Memo
This Internet-Draft is submitted in full conformance with the This is an Internet Standards Track document.
provisions of BCP 78 and BCP 79.
Internet-Drafts are working documents of the Internet Engineering
Task Force (IETF). Note that other groups may also distribute
working documents as Internet-Drafts. The list of current Internet-
Drafts is at http://datatracker.ietf.org/drafts/current/.
Internet-Drafts are draft documents valid for a maximum of six months This document is a product of the Internet Engineering Task Force
and may be updated, replaced, or obsoleted by other documents at any (IETF). It represents the consensus of the IETF community. It has
time. It is inappropriate to use Internet-Drafts as reference received public review and has been approved for publication by the
material or to cite them other than as "work in progress." Internet Engineering Steering Group (IESG). Further information on
Internet Standards is available in Section 2 of RFC 5741.
This Internet-Draft will expire on February 27, 2011. Information about the current status of this document, any errata,
and how to provide feedback on it may be obtained at
http://www.rfc-editor.org/info/rfc6040.
Copyright Notice Copyright Notice
Copyright (c) 2010 IETF Trust and the persons identified as the Copyright (c) 2010 IETF Trust and the persons identified as the
document authors. All rights reserved. document authors. All rights reserved.
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described in the Simplified BSD License. described in the Simplified BSD License.
Table of Contents Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 11 1. Introduction ....................................................4
1.1. Scope . . . . . . . . . . . . . . . . . . . . . . . . . . 13 1.1. Scope ......................................................5
2. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 13 2. Terminology .....................................................6
3. Summary of Pre-Existing RFCs . . . . . . . . . . . . . . . . . 14 3. Summary of Pre-Existing RFCs ....................................7
3.1. Encapsulation at Tunnel Ingress . . . . . . . . . . . . . 14 3.1. Encapsulation at Tunnel Ingress ............................7
3.2. Decapsulation at Tunnel Egress . . . . . . . . . . . . . . 15 3.2. Decapsulation at Tunnel Egress .............................8
4. New ECN Tunnelling Rules . . . . . . . . . . . . . . . . . . . 16 4. New ECN Tunnelling Rules ........................................9
4.1. Default Tunnel Ingress Behaviour . . . . . . . . . . . . . 17 4.1. Default Tunnel Ingress Behaviour ..........................10
4.2. Default Tunnel Egress Behaviour . . . . . . . . . . . . . 17 4.2. Default Tunnel Egress Behaviour ...........................10
4.3. Encapsulation Modes . . . . . . . . . . . . . . . . . . . 19 4.3. Encapsulation Modes .......................................12
4.4. Single Mode of Decapsulation . . . . . . . . . . . . . . . 21 4.4. Single Mode of Decapsulation ..............................14
5. Updates to Earlier RFCs . . . . . . . . . . . . . . . . . . . 22 5. Updates to Earlier RFCs ........................................15
5.1. Changes to RFC4301 ECN processing . . . . . . . . . . . . 22 5.1. Changes to RFC 4301 ECN Processing ........................15
5.2. Changes to RFC3168 ECN processing . . . . . . . . . . . . 22 5.2. Changes to RFC 3168 ECN Processing ........................16
5.3. Motivation for Changes . . . . . . . . . . . . . . . . . . 23 5.3. Motivation for Changes ....................................17
5.3.1. Motivation for Changing Encapsulation . . . . . . . . 24 5.3.1. Motivation for Changing Encapsulation ..............17
5.3.2. Motivation for Changing Decapsulation . . . . . . . . 25 5.3.2. Motivation for Changing Decapsulation ..............18
6. Backward Compatibility . . . . . . . . . . . . . . . . . . . . 27 6. Backward Compatibility .........................................21
6.1. Non-Issues Updating Decapsulation . . . . . . . . . . . . 27 6.1. Non-Issues Updating Decapsulation .........................21
6.2. Non-Update of RFC4301 IPsec Encapsulation . . . . . . . . 28 6.2. Non-Update of RFC 4301 IPsec Encapsulation ................21
6.3. Update to RFC3168 Encapsulation . . . . . . . . . . . . . 28 6.3. Update to RFC 3168 Encapsulation ..........................22
7. Design Principles for Alternate ECN Tunnelling Semantics . . . 29 7. Design Principles for Alternate ECN Tunnelling Semantics .......22
8. IANA Considerations (to be removed on publication): . . . . . 31 8. Security Considerations ........................................24
9. Security Considerations . . . . . . . . . . . . . . . . . . . 31 9. Conclusions ....................................................26
10. Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . 32 10. Acknowledgements ..............................................26
11. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 33 11. References ....................................................27
12. References . . . . . . . . . . . . . . . . . . . . . . . . . . 34 11.1. Normative References .....................................27
12.1. Normative References . . . . . . . . . . . . . . . . . . . 34 11.2. Informative References ...................................27
12.2. Informative References . . . . . . . . . . . . . . . . . . 34 Appendix A. Early ECN Tunnelling RFCs ............................29
Appendix A. Early ECN Tunnelling RFCs . . . . . . . . . . . . . . 35 Appendix B. Design Constraints ...................................29
Appendix B. Design Constraints . . . . . . . . . . . . . . . . . 36 B.1. Security Constraints ......................................29
B.1. Security Constraints . . . . . . . . . . . . . . . . . . . 36 B.2. Control Constraints .......................................31
B.2. Control Constraints . . . . . . . . . . . . . . . . . . . 38 B.3. Management Constraints ....................................32
B.3. Management Constraints . . . . . . . . . . . . . . . . . . 39 Appendix C. Contribution to Congestion across a Tunnel ...........33
Appendix C. Contribution to Congestion across a Tunnel . . . . . 40
Appendix D. Compromise on Decap with ECT(1) Inner and ECT(0) Appendix D. Compromise on Decap with ECT(1) Inner and ECT(0)
Outer . . . . . . . . . . . . . . . . . . . . . . . . 41 Outer ................................................34
Appendix E. Open Issues . . . . . . . . . . . . . . . . . . . . . 42 Appendix E. Open Issues ..........................................35
Request to the RFC Editor (to be removed on publication):
In the RFC index, RFC3168 should be identified as an update to
RFC2003. RFC4301 should be identified as an update to RFC3168.
Changes from previous drafts (to be removed by the RFC Editor)
Full text differences between IETF draft versions are available at
<http://tools.ietf.org/wg/tsvwg/draft-ietf-tsvwg-ecn-tunnel/>, and
between earlier individual draft versions at
<http://www.briscoe.net/pubs.html#ecn-tunnel>
From ietf-09 to ietf-10 (current):
* Editorial changes:
+ Clarified couple of sentences in Introduction and one in
section 6.3 to distinguish whether the terms 'RFC3168' &
'RFC4301' refer to implementations or documents.
+ Corrected garbled sentence in the introduction about
backward compatibility.
+ Made it clear that 'drop' in Fig 2, Fig 4 and the following
para is an action, not a codepoint.
+ In sections 5.1 & 5.2, specifically identified the updated
sections of RFC3168 & RFC4301.
+ Avoided describing compatibility mode as 'optional' at the
end of section 5.2 where it should have said 'not always
obligatory' instead, because in section 4 compatibility mode
is normatively defined as obligatory in some circumstances
(rather than always optional).
+ Added RFC5659 as informative reference on pseudowires and
clarified only some pseudowires might be relevant examples.
+ Deleted "The views expressed here are those of the author
only." in the acknowledgements.
+ Fixed a few nits.
From ietf-08 to ietf-09: Added change log entry for -07 to -08 that
was previously omitted.
* Changes to standards action text:
+ Added RFC4774 to 'Updates:' header (the draft always has
extended the advice in RFC4774 (BCP124) which said very
little about tunnels. The GENART reviewer merely pointed
out that the header did not highlight this fact.)
* Editorial changes:
+ Abstract: s/providing backward compatibility./thus ensuring
backward compatibility./
+ Moved PCN-related text motivating changes to decapsulation
from "Default Tunnel Egress Behaviour" (Section 4.2) to
"Motivation for Changing Decapsulation" (Section 5.3.2)
where it was merged with existing similar text.
+ In the non-normative Design Principles avoided using words
in lower case where they were in contexts that might make
them confusable with upper case RFC2119 normative language.
+ Added Stephen Hanna and Ben Campbell to acks and corrected
spelling of Agarwal.
+ Deleted endnote discussing corner case with IKEv2 manual
keying (identified as "to be removed before publication
following SecDir review").
+ Deleted Appendices D & E on why existing ingress & egress
tunnelling behavour impede PCN and the endnotes that
referred to them (identified as "to be removed before
publication").
+ Various minor corrections pointed out by reviewers.
From ietf-07 to ietf-08:
* Changes to standards actions:
+ Section 4: Changed non-RFC2119 phrase 'NOT RECOMMENDED' to
'SHOULD be avoided', wrt alternate ECN tunnelling schemes.
+ Section 4.2: Used upper-case in 'Alarms SHOULD be rate-
limited'.
+ Section 7: Made bullet #1 in the decapsulation guidelines
for alternate schemes more precise. Also changed any upper-
case keywords in this informative section to lower case.
* Editorial changes:
+ Changed copyright notice to allow for pre-5378 material.
+ Shifted supporting text intended for deletion on publication
into editorial comments.
+ Explained how to read the decapsulation matrices in their
captions.
+ Minor clarifications throughout.
From ietf-06 to ietf-07:
* Emphasised that this is the opposite of a fork in the RFC
series.
* Altered Section 5 to focus on updates to implementations of
earlier RFCs, rather than on updates to the text of the RFCs.
* Removed potential loop-holes in normative text that
implementers might have used to claim compliance without
implementing normal mode. Highlighted the deliberate
distinction between "MUST implement" and "SHOULD use" normal
mode.
* Added question for Security Directorate reviewers on whether to
mention a corner-case concerning manual keying of IPsec
tunnels.
* Minor clarifications, updated references and updated acks.
* Marked two appendices about PCN motivations for removal before
publication.
From ietf-05 to ietf-06:
* Minor textual clarifications and corrections.
From ietf-04 to ietf-05:
* Functional changes:
+ Section 4.2: ECT(1) outer with Not-ECT inner: reverted to
forwarding as Not-ECT (as in RFC3168 & RFC4301), rather than
dropping.
+ Altered rationale in bullet 3 of Section 5.3.2 to justify
this.
+ Distinguished alarms for dangerous and invalid combinations
and allowed combinations that are valid in some tunnel
configurations but dangerous in others to be alarmed at the
discretion of the implementer and/or operator.
+ Altered advice on designing alternate ECN tunnelling
semantics to reflect the above changes.
* Textual changes:
+ Changed "Future non-default schemes" to "Alternate ECN
Tunnelling Semantics" throughout.
+ Cut down Appendix D and Appendix E for brevity.
+ A number of clarifying edits & updated refs.
From ietf-03 to ietf-04:
* Functional changes: none
* Structural changes:
+ Added "Open Issues" appendix
* Textual changes:
+ Section title: "Changes from Earlier RFCs" -> "Updates to
Earlier RFCs"
+ Emphasised that change on decap to previously unused
combinations will propagate PCN encoding.
+ Acknowledged additional reviewers and updated references
From ietf-02 to ietf-03:
* Functional changes:
+ Corrected errors in recap of previous RFCs, which wrongly
stated the different decapsulation behaviours of RFC3168 &
RFC4301 with a Not-ECT inner header. This also required
corrections to the "Changes from Earlier RFCs" and the
Motivations for these changes.
+ Mandated that any future standards action SHOULD NOT use the
ECT(0) codepoint as an indication of congestion, without
giving strong reasons.
+ Added optional alarm when decapsulating ECT(1) outer,
ECT(0), but noted it would need to be disabled for
2-severity level congestion (e.g. PCN).
* Structural changes:
+ Removed Document Roadmap which merely repeated the Contents
(previously Section 1.2).
+ Moved "Changes from Earlier RFCs" (Section 5) before
Section 6 on Backward Compatibility and internally organised
both by RFC, rather than by ingress then egress.
+ Moved motivation for changing existing RFCs (Section 5.3) to
after the changes are specified.
+ Moved informative "Design Principles for Future Non-Default
Schemes" after all the normative sections.
+ Added Appendix A on early history of ECN tunnelling RFCs.
+ Removed specialist appendix on "Relative Placement of
Tunnelling and In-Path Load Regulation" (Appendix D in the
-02 draft)
+ Moved and updated specialist text on "Compromise on Decap
with ECT(1) Inner and ECT(0) Outer" from Security
Considerations to Appendix D
* Textual changes:
+ Simplified vocabulary for non-native-english speakers
+ Simplified Introduction and defined regularly used terms in
an expanded Terminology section.
+ More clearly distinguished statically configured tunnels
from dynamic tunnel endpoint discovery, before explaining
operating modes.
+ Simplified, cut-down and clarified throughout
+ Updated references.
From ietf-01 to ietf-02:
* Scope reduced from any encapsulation of an IP packet to solely
IP in IP tunnelled encapsulation. Consequently changed title
and removed whole section 'Design Guidelines for New
Encapsulations of Congestion Notification' (to be included in a
future companion informational document).
* Included a new normative decapsulation rule for ECT(0) inner
and ECT(1) outer that had previously only been outlined in the
non-normative appendix 'Comprehensive Decapsulation Rules'.
Consequently:
+ The Introduction has been completely re-written to motivate
this change to decapsulation along with the existing change
to encapsulation.
+ The tentative text in the appendix that first proposed this
change has been split between normative standards text in
Section 4 and Appendix D, which explains specifically why
this change would streamline PCN. New text on the logic of
the resulting decap rules added.
* If inner/outer is Not-ECT/ECT(0), changed decapsulation to
propagate Not-ECT rather than drop the packet; and added
reasoning.
* Considerably restructured:
+ "Design Constraints" analysis moved to an appendix
(Appendix B);
+ Added Section 3 to summarise relevant existing RFCs;
+ Structured Section 4 and Section 6 into subsections.
+ Added tables to sections on old and new rules, for precision
and comparison.
+ Moved Section 7 on Design Principles to the end of the
section specifying the new default normative tunnelling
behaviour. Rewritten and shifted text on identifiers and
in-path load regulators to Appendix B.1 [deleted in revision
-03].
From ietf-00 to ietf-01:
* Identified two additional alarm states in the decapsulation
rules (Figure 4) if ECT(X) in outer and inner contradict each
other.
* Altered Comprehensive Decapsulation Rules (Appendix D) so that
ECT(0) in the outer no longer overrides ECT(1) in the inner.
Used the term 'Comprehensive' instead of 'Ideal'. And
considerably updated the text in this appendix.
* Added Appendix D.1 (removed again in a later revision) to weigh
up the various ways the Comprehensive Decapsulation Rules might
be introduced. This replaces the previous contradictory
statements saying complex backwards compatibility interactions
would be introduced while also saying there would be no
backwards compatibility issues.
* Updated references.
From briscoe-01 to ietf-00:
* Re-wrote Appendix C giving much simpler technique to measure
contribution to congestion across a tunnel.
* Added discussion of backward compatibility of the ideal
decapsulation scheme in Appendix D
* Updated references. Minor corrections & clarifications
throughout.
From briscoe-00 to briscoe-01:
* Related everything conceptually to the uniform and pipe models
of RFC2983 on Diffserv Tunnels, and completely removed the
dependence of tunnelling behaviour on the presence of any in-
path load regulation by using the [1 - Before] [2 - Outer]
function placement concepts from RFC2983;
* Added specific cases where the existing standards limit new
proposals, particularly Appendix E;
* Added sub-structure to Introduction (Need for Rationalisation,
Roadmap), added new Introductory subsection on "Scope" and
improved clarity;
* Added Design Guidelines for New Encapsulations of Congestion
Notification;
* Considerably clarified the Backward Compatibility section
(Section 6);
* Considerably extended the Security Considerations section
(Section 9);
* Summarised the primary rationale much better in the
conclusions;
* Added numerous extra acknowledgements;
* Added Appendix E. "Why resetting CE on encapsulation harms
PCN", Appendix C. "Contribution to Congestion across a Tunnel"
and Appendix D. "Ideal Decapsulation Rules";
* Re-wrote Appendix B [deleted in a later revision], explaining
how tunnel encapsulation no longer depends on in-path load-
regulation (changed title from "In-path Load Regulation" to
"Non-Dependence of Tunnelling on In-path Load Regulation"), but
explained how an in-path load regulation function must be
carefully placed with respect to tunnel encapsulation (in a new
sub-section entitled "Dependence of In-Path Load Regulation on
Tunnelling").
1. Introduction 1. Introduction
Explicit congestion notification (ECN [RFC3168]) allows a forwarding Explicit congestion notification (ECN [RFC3168]) allows a forwarding
element (e.g. a router) to notify the onset of congestion without element (e.g., a router) to notify the onset of congestion without
having to drop packets. Instead it can explicitly mark a proportion having to drop packets. Instead, it can explicitly mark a proportion
of packets in the 2-bit ECN field in the IP header (Table 1 recaps of packets in the two-bit ECN field in the IP header (Table 1 recaps
the ECN codepoints). the ECN codepoints).
The outer header of an IP packet can encapsulate one or more IP The outer header of an IP packet can encapsulate one or more IP
headers for tunnelling. A forwarding element using ECN to signify headers for tunnelling. A forwarding element using ECN to signify
congestion will only mark the immediately visible outer IP header. congestion will only mark the immediately visible outer IP header.
When a tunnel decapsulator later removes this outer header, it When a tunnel decapsulator later removes this outer header, it
follows rules to propagate congestion markings by combining the ECN follows rules to propagate congestion markings by combining the ECN
fields of the inner and outer IP header into one outgoing IP header. fields of the inner and outer IP header into one outgoing IP header.
This document updates those rules for IPsec [RFC4301] and non-IPsec This document updates those rules for IPsec [RFC4301] and non-IPsec
[RFC3168] tunnels to add new behaviours for previously unused [RFC3168] tunnels to add new behaviours for previously unused
combinations of inner and outer header. It also updates the ingress combinations of inner and outer headers. It also updates the ingress
behaviour of RFC3168 tunnels to match that of RFC4301 tunnels. behaviour of RFC 3168 tunnels to match that of RFC 4301 tunnels.
Tunnel endpoints complying with the updated rules will be backward Tunnel endpoints complying with the updated rules will be backward
compatible when interworking with tunnel endpoints complying with compatible when interworking with tunnel endpoints complying with RFC
RFC4301, RFC3168 or any earlier specification. 4301, RFC 3168, or any earlier specification.
When ECN and its tunnelling was defined in RFC3168, only the minimum When ECN and its tunnelling was defined in RFC 3168, only the minimum
necessary changes to the ECN field were propagated through tunnel necessary changes to the ECN field were propagated through tunnel
endpoints--just enough for the basic ECN mechanism to work. This was endpoints -- just enough for the basic ECN mechanism to work. This
due to concerns that the ECN field might be toggled to communicate was due to concerns that the ECN field might be toggled to
between a secure site and someone on the public Internet--a covert communicate between a secure site and someone on the public Internet
channel. This was because a mutable field like ECN cannot be -- a covert channel. This was because a mutable field like ECN
protected by IPsec's integrity mechanisms--it has to be able to cannot be protected by IPsec's integrity mechanisms -- it has to be
change as it traverses the Internet. able to change as it traverses the Internet.
Nonetheless, the latest IPsec architecture [RFC4301] considered a Nonetheless, the latest IPsec architecture [RFC4301] considered a
bandwidth limit of 2 bits per packet on a covert channel to be a bandwidth limit of two bits per packet on a covert channel to be a
manageable risk. Therefore, for simplicity, an RFC4301 ingress manageable risk. Therefore, for simplicity, an RFC 4301 ingress
copied the whole ECN field to encapsulate a packet. RFC4301 copied the whole ECN field to encapsulate a packet. RFC 4301
dispensed with the two modes of RFC3168, one which partially copied dispensed with the two modes of RFC 3168, one which partially copied
the ECN field, and the other which blocked all propagation of ECN the ECN field, and the other which blocked all propagation of ECN
changes. changes.
Unfortunately, this entirely reasonable sequence of standards actions Unfortunately, this entirely reasonable sequence of standards actions
resulted in a perverse outcome; non-IPsec tunnels (RFC3168) blocked resulted in a perverse outcome; non-IPsec tunnels (RFC 3168) blocked
the 2-bit covert channel, while IPsec tunnels (RFC4301) did not--at the two-bit covert channel, while IPsec tunnels (RFC 4301) did not --
least not at the ingress. At the egress, both IPsec and non-IPsec at least not at the ingress. At the egress, both IPsec and non-IPsec
tunnels still partially restricted propagation of the full ECN field. tunnels still partially restricted propagation of the full ECN field.
The trigger for the changes in this document was the introduction of The trigger for the changes in this document was the introduction of
pre-congestion notification (PCN [RFC5670]) to the IETF standards pre-congestion notification (PCN [RFC5670]) to the IETF Standards
track. PCN needs the ECN field to be copied at a tunnel ingress and Track. PCN needs the ECN field to be copied at a tunnel ingress and
it needs four states of congestion signalling to be propagated at the it needs four states of congestion signalling to be propagated at the
egress, but pre-existing tunnels only propagate three in the ECN egress, but pre-existing tunnels only propagate three in the ECN
field. field.
This document draws on currently unused (CU) combinations of inner This document draws on currently unused (CU) combinations of inner
and outer headers to add tunnelling of four-state congestion and outer headers to add tunnelling of four-state congestion
signalling to RFC3168 and RFC4301. Operators of tunnels who signalling to RFC 3168 and RFC 4301. Operators of tunnels who
specifically want to support four states can require that all their specifically want to support four states can require that all their
tunnels comply with this specification. However, this is not a fork tunnels comply with this specification. However, this is not a fork
in the RFC series. It is an update that can be deployed first by in the RFC series. It is an update that can be deployed first by
those that need it, and subsequently by all tunnel endpoint those that need it, and subsequently by all tunnel endpoint
implementations (RFC4301, RFC3168, RFC2481, RFC2401, RFC2003), which implementations (RFC 4301, RFC 3168, RFC 2481, RFC 2401, RFC 2003),
can safely be updated to this new specification as part of general which can safely be updated to this new specification as part of
code maintenance. This will gradually add support for four general code maintenance. This will gradually add support for four
congestion states to the Internet. Existing three state schemes will congestion states to the Internet. Existing three state schemes will
continue to work as before. continue to work as before.
In fact, this document is the opposite of a fork. At the same time In fact, this document is the opposite of a fork. At the same time
as supporting a fourth state, the opportunity has been taken to draw as supporting a fourth state, the opportunity has been taken to draw
together divergent ECN tunnelling specifications into a single together divergent ECN tunnelling specifications into a single
consistent behaviour, harmonising differences such as perverse covert consistent behaviour, harmonising differences such as perverse covert
channel treatment. Then any tunnel can be deployed unilaterally, and channel treatment. Then, any tunnel can be deployed unilaterally,
it will support the full range of congestion control and management and it will support the full range of congestion control and
schemes without any modes or configuration. Further, any host or management schemes without any modes or configuration. Further, any
router can expect the ECN field to behave in the same way, whatever host or router can expect the ECN field to behave in the same way,
type of tunnel might intervene in the path. whatever type of tunnel might intervene in the path.
1.1. Scope 1.1. Scope
This document only concerns wire protocol processing of the ECN field This document only concerns wire protocol processing of the ECN field
at tunnel endpoints and makes no changes or recommendations at tunnel endpoints and makes no changes or recommendations
concerning algorithms for congestion marking or congestion response. concerning algorithms for congestion marking or congestion response.
This document specifies common ECN field processing at encapsulation This document specifies common ECN field processing at encapsulation
and decapsulation for any IP in IP tunnelling, whether IPsec or non- and decapsulation for any IP-in-IP tunnelling, whether IPsec or non-
IPsec tunnels. It applies irrespective of whether IPv4 or IPv6 is IPsec tunnels. It applies irrespective of whether IPv4 or IPv6 is
used for either of the inner and outer headers. It applies for used for either the inner or outer headers. It applies for packets
packets with any destination address type, whether unicast or with any destination address type, whether unicast or multicast. It
multicast. It applies as the default for all Diffserv per-hop applies as the default for all Diffserv per-hop behaviours (PHBs),
behaviours (PHBs), unless stated otherwise in the specification of a unless stated otherwise in the specification of a PHB (but Section 4
PHB (but Section 4 strongly deprecates such exceptions). It is strongly deprecates such exceptions). It is intended to be a good
intended to be a good trade off between somewhat conflicting trade off between somewhat conflicting security, control, and
security, control and management requirements. management requirements.
[RFC2983] is a comprehensive primer on differentiated services and [RFC2983] is a comprehensive primer on differentiated services and
tunnels. Given ECN raises similar issues to differentiated services tunnels. Given ECN raises similar issues to differentiated services
when interacting with tunnels, useful concepts introduced in RFC2983 when interacting with tunnels, useful concepts introduced in RFC 2983
are used throughout, with brief recaps of the explanations where are used throughout, with brief recaps of the explanations where
necessary. necessary.
2. Terminology 2. Terminology
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in RFC 2119 [RFC2119]. document are to be interpreted as described in RFC 2119 [RFC2119].
Table 1 recaps the names of the ECN codepoints [RFC3168]. Table 1 recaps the names of the ECN codepoints [RFC3168].
+------------------+----------------+---------------------------+ +------------------+----------------+---------------------------+
| Binary codepoint | Codepoint name | Meaning | | Binary codepoint | Codepoint name | Meaning |
+------------------+----------------+---------------------------+ +------------------+----------------+---------------------------+
| 00 | Not-ECT | Not ECN-capable transport | | 00 | Not-ECT | Not ECN-capable transport |
| 01 | ECT(1) | ECN-capable transport | | 01 | ECT(1) | ECN-capable transport |
| 10 | ECT(0) | ECN-capable transport | | 10 | ECT(0) | ECN-capable transport |
| 11 | CE | Congestion experienced | | 11 | CE | Congestion experienced |
+------------------+----------------+---------------------------+ +------------------+----------------+---------------------------+
Table 1: Recap of Codepoints of the ECN Field [RFC3168] in the IP Table 1: Recap of Codepoints of the ECN Field [RFC3168]
Header in the IP Header
Further terminology used within this document: Further terminology used within this document:
Encapsulator: The tunnel endpoint function that adds an outer IP Encapsulator: The tunnel endpoint function that adds an outer IP
header to tunnel a packet (also termed the 'ingress tunnel header to tunnel a packet (also termed the 'ingress tunnel
endpoint' or just the 'ingress' where the context is clear). endpoint' or just the 'ingress' where the context is clear).
Decapsulator: The tunnel endpoint function that removes an outer IP Decapsulator: The tunnel endpoint function that removes an outer IP
header from a tunnelled packet (also termed the 'egress tunnel header from a tunnelled packet (also termed the 'egress tunnel
endpoint' or just the 'egress' where the context is clear). endpoint' or just the 'egress' where the context is clear).
skipping to change at page 14, line 37 skipping to change at page 7, line 16
outer header to Not-ECT ("00"). outer header to Not-ECT ("00").
Resetting ECN: On encapsulation, setting the ECN field of the new Resetting ECN: On encapsulation, setting the ECN field of the new
outer header to be a copy of the ECN field in the incoming header outer header to be a copy of the ECN field in the incoming header
except the outer ECN field is set to the ECT(0) codepoint if the except the outer ECN field is set to the ECT(0) codepoint if the
incoming ECN field is CE. incoming ECN field is CE.
3. Summary of Pre-Existing RFCs 3. Summary of Pre-Existing RFCs
This section is informative not normative, as it recaps pre-existing This section is informative not normative, as it recaps pre-existing
RFCs. Earlier relevant RFCs that were either experimental or RFCs. Earlier relevant RFCs that were either Experimental or
incomplete with respect to ECN tunnelling (RFC2481, RFC2401 and incomplete with respect to ECN tunnelling (RFC 2481, RFC 2401, and
RFC2003) are briefly outlined in Appendix A. The question of whether RFC 2003) are briefly outlined in Appendix A. The question of
tunnel implementations used in the Internet comply with any of these whether tunnel implementations used in the Internet comply with any
RFCs is not discussed. of these RFCs is not discussed.
3.1. Encapsulation at Tunnel Ingress 3.1. Encapsulation at Tunnel Ingress
At the encapsulator, the controversy has been over whether to At the encapsulator, the controversy has been over whether to
propagate information about congestion experienced on the path so far propagate information about congestion experienced on the path so far
into the outer header of the tunnel. into the outer header of the tunnel.
Specifically, RFC3168 says that, if a tunnel fully supports ECN Specifically, RFC 3168 says that, if a tunnel fully supports ECN
(termed a 'full-functionality' ECN tunnel in [RFC3168]), the (termed a 'full-functionality' ECN tunnel in [RFC3168]), the
encapsulator must not copy a CE marking from the inner header into encapsulator must not copy a CE marking from the incoming header into
the outer header that it creates. Instead the encapsulator must set the outer header that it creates. Instead, the encapsulator must set
the outer header to ECT(0) if the ECN field is marked CE in the the outer header to ECT(0) if the ECN field is marked CE in the
arriving IP header. We term this 'resetting' a CE codepoint. arriving IP header. We term this 'resetting' a CE codepoint.
However, the new IPsec architecture in [RFC4301] reverses this rule, However, the new IPsec architecture in [RFC4301] reverses this rule,
stating that the encapsulator must simply copy the ECN field from the stating that the encapsulator must simply copy the ECN field from the
incoming header to the outer header. incoming header to the outer header.
RFC3168 also provided a Limited Functionality mode that turns off ECN RFC 3168 also provided a Limited Functionality mode that turns off
processing over the scope of the tunnel by setting the outer header ECN processing over the scope of the tunnel by setting the outer
to Not-ECT ("00"). Then such packets will be dropped to indicate header to Not-ECT ("00"). Then, such packets will be dropped to
congestion rather than marked with ECN. This is necessary for the indicate congestion, rather than marked with ECN. This is necessary
ingress to interwork with legacy decapsulators ([RFC2481], [RFC2401] for the ingress to interwork with legacy decapsulators ([RFC2481],
and [RFC2003]) that do not propagate ECN markings added to the outer [RFC2401], and [RFC2003]) that do not propagate ECN markings added to
header. Otherwise such legacy decapsulators would throw away the outer header. Otherwise, such legacy decapsulators would throw
congestion notifications before they reached the transport layer. away congestion notifications before they reached the transport
layer.
Neither Limited Functionality mode nor Full Functionality mode are Neither Limited Functionality mode nor Full Functionality mode are
used by an RFC4301 IPsec encapsulator, which simply copies the used by an RFC 4301 IPsec encapsulator, which simply copies the
incoming ECN field into the outer header. An earlier key-exchange incoming ECN field into the outer header. An earlier key-exchange
phase ensures an RFC4301 ingress will not have to interwork with a phase ensures an RFC 4301 ingress will not have to interwork with a
legacy egress that does not support ECN. legacy egress that does not support ECN.
These pre-existing behaviours are summarised in Figure 1. These pre-existing behaviours are summarised in Figure 1.
+-----------------+-----------------------------------------------+ +-----------------+-----------------------------------------------+
| Incoming Header | Outgoing Outer Header | | Incoming Header | Departing Outer Header |
| (also equal to +---------------+---------------+---------------+ | (also equal to +---------------+---------------+---------------+
| Outgoing Inner | RFC3168 ECN | RFC3168 ECN | RFC4301 IPsec | | departing Inner | RFC 3168 ECN | RFC 3168 ECN | RFC 4301 IPsec|
| Header) | Limited | Full | | | Header) | Limited | Full | |
| | Functionality | Functionality | | | | Functionality | Functionality | |
+-----------------+---------------+---------------+---------------+ +-----------------+---------------+---------------+---------------+
| Not-ECT | Not-ECT | Not-ECT | Not-ECT | | Not-ECT | Not-ECT | Not-ECT | Not-ECT |
| ECT(0) | Not-ECT | ECT(0) | ECT(0) | | ECT(0) | Not-ECT | ECT(0) | ECT(0) |
| ECT(1) | Not-ECT | ECT(1) | ECT(1) | | ECT(1) | Not-ECT | ECT(1) | ECT(1) |
| CE | Not-ECT | ECT(0) | CE | | CE | Not-ECT | ECT(0) | CE |
+-----------------+---------------+---------------+---------------+ +-----------------+---------------+---------------+---------------+
Figure 1: IP in IP Encapsulation: Recap of Pre-existing Behaviours Figure 1: IP-in-IP Encapsulation: Recap of Pre-Existing Behaviours
3.2. Decapsulation at Tunnel Egress 3.2. Decapsulation at Tunnel Egress
RFC3168 and RFC4301 specify the decapsulation behaviour summarised in RFC 3168 and RFC 4301 specify the decapsulation behaviour summarised
Figure 2. The ECN field in the outgoing header is set to the in Figure 2. The ECN field in the outgoing header is set to the
codepoint at the intersection of the appropriate incoming inner codepoint at the intersection of the appropriate arriving inner
header (row) and incoming outer header (column). header (row) and arriving outer header (column).
+---------+------------------------------------------------+ +---------+------------------------------------------------+
|Incoming | Incoming Outer Header | |Arriving | Arriving Outer Header |
| Inner +---------+------------+------------+------------+ | Inner +---------+------------+------------+------------+
| Header | Not-ECT | ECT(0) | ECT(1) | CE | | Header | Not-ECT | ECT(0) | ECT(1) | CE |
+---------+---------+------------+------------+------------+ +---------+---------+------------+------------+------------+
RFC3168->| Not-ECT | Not-ECT |Not-ECT |Not-ECT | <drop> | RFC 3168->| Not-ECT | Not-ECT |Not-ECT |Not-ECT | <drop> |
RFC4301->| Not-ECT | Not-ECT |Not-ECT |Not-ECT |Not-ECT | RFC 4301->| Not-ECT | Not-ECT |Not-ECT |Not-ECT |Not-ECT |
| ECT(0) | ECT(0) | ECT(0) | ECT(0) | CE | | ECT(0) | ECT(0) | ECT(0) | ECT(0) | CE |
| ECT(1) | ECT(1) | ECT(1) | ECT(1) | CE | | ECT(1) | ECT(1) | ECT(1) | ECT(1) | CE |
| CE | CE | CE | CE | CE | | CE | CE | CE | CE | CE |
+---------+---------+------------+------------+------------+ +---------+---------+------------+------------+------------+
In pre-existing RFCs, the ECN field in the outgoing header was set to In pre-existing RFCs, the ECN field in the outgoing header was set to
the codepoint at the intersection of the appropriate incoming inner the codepoint at the intersection of the appropriate arriving inner
header (row) and incoming outer header (column) , or the packet was header (row) and arriving outer header (column), or the packet was
dropped where indicated. dropped where indicated.
Figure 2: IP in IP Decapsulation; Recap of Pre-existing Behaviour Figure 2: IP in IP Decapsulation; Recap of Pre-Existing Behaviour
The behaviour in the table derives from the logic given in RFC3168 The behaviour in the table derives from the logic given in RFC 3168
and RFC4301, briefly recapped as follows: and RFC 4301, briefly recapped as follows:
o On decapsulation, if the inner ECN field is Not-ECT the outer is o On decapsulation, if the inner ECN field is Not-ECT the outer is
ignored. RFC3168 (but not RFC4301) also specified that the ignored. RFC 3168 (but not RFC 4301) also specified that the
decapsulator must drop a packet with a Not-ECT inner and CE in the decapsulator must drop a packet with a Not-ECT inner and CE in the
outer. outer.
o In all other cases, if the outer is CE, the outgoing ECN field is o In all other cases, if the outer is CE, the outgoing ECN field is
set to CE, but otherwise the outer is ignored and the inner is set to CE; otherwise, the outer is ignored and the inner is used
used for the outgoing ECN field. for the outgoing ECN field.
Section 9.2.2 of RFC3168 also made it an auditable event for an IPsec Section 9.2.2 of RFC 3168 also made it an auditable event for an
tunnel "if the ECN Field is changed inappropriately within an IPsec IPsec tunnel "if the ECN Field is changed inappropriately within an
tunnel...". Inappropriate changes were not specifically enumerated. IPsec tunnel...". Inappropriate changes were not specifically
RFC4301 did not mention inappropriate ECN changes. enumerated. RFC 4301 did not mention inappropriate ECN changes.
4. New ECN Tunnelling Rules 4. New ECN Tunnelling Rules
The standards actions below in Section 4.1 (ingress encapsulation) The standards actions below in Section 4.1 (ingress encapsulation)
and Section 4.2 (egress decapsulation) define new default ECN tunnel and Section 4.2 (egress decapsulation) define new default ECN tunnel
processing rules for any IP packet (v4 or v6) with any Diffserv processing rules for any IP packet (v4 or v6) with any Diffserv
codepoint. codepoint.
If these defaults do not meet a particular requirement, an alternate If these defaults do not meet a particular requirement, an alternate
ECN tunnelling scheme can be introduced as part of the definition of ECN tunnelling scheme can be introduced as part of the definition of
an alternate congestion marking scheme used by a specific Diffserv an alternate congestion marking scheme used by a specific Diffserv
PHB (see section 5 of [RFC3168] and [RFC4774]). When designing such PHB (see [RFC4774] and Section 5 of [RFC3168]). When designing such
alternate ECN tunnelling schemes, the principles in Section 7 should alternate ECN tunnelling schemes, the principles in Section 7 should
be followed. However, alternate ECN tunnelling schemes SHOULD be be followed. However, alternate ECN tunnelling schemes SHOULD be
avoided whenever possible as the deployment burden of handling avoided whenever possible as the deployment burden of handling
exceptional PHBs in implementations of all affected tunnels should exceptional PHBs in implementations of all affected tunnels should
not be underestimated. There is no requirement for a PHB definition not be underestimated. There is no requirement for a PHB definition
to state anything about ECN tunnelling behaviour if the default to state anything about ECN tunnelling behaviour if the default
behaviour in the present specification is sufficient. behaviour in the present specification is sufficient.
4.1. Default Tunnel Ingress Behaviour 4.1. Default Tunnel Ingress Behaviour
Two modes of encapsulation are defined here; a REQUIRED `normal mode' Two modes of encapsulation are defined here; a REQUIRED 'normal mode'
and a `compatibility mode', which is for backward compatibility with and a 'compatibility mode', which is for backward compatibility with
tunnel decapsulators that do not understand ECN. Note that these are tunnel decapsulators that do not understand ECN. Note that these are
modes of the ingress tunnel endpoint only, not the whole tunnel. modes of the ingress tunnel endpoint only, not the whole tunnel.
Section 4.3 explains why two modes are necessary and specifies the Section 4.3 explains why two modes are necessary and specifies the
circumstances in which it is sufficient to solely implement normal circumstances in which it is sufficient to solely implement normal
mode. mode.
Whatever the mode, an encapsulator forwards the inner header without Whatever the mode, an encapsulator forwards the inner header without
changing the ECN field. changing the ECN field.
In normal mode an encapsulator compliant with this specification MUST In normal mode, an encapsulator compliant with this specification
construct the outer encapsulating IP header by copying the 2-bit ECN MUST construct the outer encapsulating IP header by copying the
field of the incoming IP header. In compatibility mode it clears the two-bit ECN field of the incoming IP header. In compatibility mode,
ECN field in the outer header to the Not-ECT codepoint (the IPv4 it clears the ECN field in the outer header to the Not-ECT codepoint
header checksum also changes whenever the ECN field is changed). (the IPv4 header checksum also changes whenever the ECN field is
These rules are tabulated for convenience in Figure 3. changed). These rules are tabulated for convenience in Figure 3.
+-----------------+-------------------------------+ +-----------------+-------------------------------+
| Incoming Header | Outgoing Outer Header | | Incoming Header | Departing Outer Header |
| (also equal to +---------------+---------------+ | (also equal to +---------------+---------------+
| Outgoing Inner | Compatibility | Normal | | departing Inner | Compatibility | Normal |
| Header) | Mode | Mode | | Header) | Mode | Mode |
+-----------------+---------------+---------------+ +-----------------+---------------+---------------+
| Not-ECT | Not-ECT | Not-ECT | | Not-ECT | Not-ECT | Not-ECT |
| ECT(0) | Not-ECT | ECT(0) | | ECT(0) | Not-ECT | ECT(0) |
| ECT(1) | Not-ECT | ECT(1) | | ECT(1) | Not-ECT | ECT(1) |
| CE | Not-ECT | CE | | CE | Not-ECT | CE |
+-----------------+---------------+---------------+ +-----------------+---------------+---------------+
Figure 3: New IP in IP Encapsulation Behaviours Figure 3: New IP in IP Encapsulation Behaviours
4.2. Default Tunnel Egress Behaviour 4.2. Default Tunnel Egress Behaviour
To decapsulate the inner header at the tunnel egress, a compliant To decapsulate the inner header at the tunnel egress, a compliant
tunnel egress MUST set the outgoing ECN field to the codepoint at the tunnel egress MUST set the outgoing ECN field to the codepoint at the
intersection of the appropriate incoming inner header (row) and outer intersection of the appropriate arriving inner header (row) and outer
header (column) in Figure 4 (the IPv4 header checksum also changes header (column) in Figure 4 (the IPv4 header checksum also changes
whenever the ECN field is changed). There is no need for more than whenever the ECN field is changed). There is no need for more than
one mode of decapsulation, as these rules cater for all known one mode of decapsulation, as these rules cater for all known
requirements. requirements.
+---------+------------------------------------------------+ +---------+------------------------------------------------+
|Incoming | Incoming Outer Header | |Arriving | Arriving Outer Header |
| Inner +---------+------------+------------+------------+ | Inner +---------+------------+------------+------------+
| Header | Not-ECT | ECT(0) | ECT(1) | CE | | Header | Not-ECT | ECT(0) | ECT(1) | CE |
+---------+---------+------------+------------+------------+ +---------+---------+------------+------------+------------+
| Not-ECT | Not-ECT |Not-ECT(!!!)|Not-ECT(!!!)| <drop>(!!!)| | Not-ECT | Not-ECT |Not-ECT(!!!)|Not-ECT(!!!)| <drop>(!!!)|
| ECT(0) | ECT(0) | ECT(0) | ECT(1) | CE | | ECT(0) | ECT(0) | ECT(0) | ECT(1) | CE |
| ECT(1) | ECT(1) | ECT(1) (!) | ECT(1) | CE | | ECT(1) | ECT(1) | ECT(1) (!) | ECT(1) | CE |
| CE | CE | CE | CE(!!!)| CE | | CE | CE | CE | CE(!!!)| CE |
+---------+---------+------------+------------+------------+ +---------+---------+------------+------------+------------+
The ECN field in the outgoing header is set to the codepoint at the The ECN field in the outgoing header is set to the codepoint at the
intersection of the appropriate incoming inner header (row) and intersection of the appropriate arriving inner header (row) and
incoming outer header (column) , or the packet is dropped where arriving outer header (column), or the packet is dropped where
indicated. Currently unused combinations are indicated by '(!!!)' or indicated. Currently unused combinations are indicated by '(!!!)' or
'(!)' '(!)'
Figure 4: New IP in IP Decapsulation Behaviour Figure 4: New IP in IP Decapsulation Behaviour
This table for decapsulation behaviour is derived from the following This table for decapsulation behaviour is derived from the following
logic: logic:
o If the inner ECN field is Not-ECT the decapsulator MUST NOT o If the inner ECN field is Not-ECT, the decapsulator MUST NOT
propagate any other ECN codepoint onwards. This is because the propagate any other ECN codepoint onwards. This is because the
inner Not-ECT marking is set by transports that rely on dropped inner Not-ECT marking is set by transports that rely on dropped
packets as an indication of congestion and would not understand or packets as an indication of congestion and would not understand or
respond to any other ECN codepoint [RFC4774]. Specifically: respond to any other ECN codepoint [RFC4774]. Specifically:
* If the inner ECN field is Not-ECT and the outer ECN field is CE * If the inner ECN field is Not-ECT and the outer ECN field is
the decapsulator MUST drop the packet. CE, the decapsulator MUST drop the packet.
* If the inner ECN field is Not-ECT and the outer ECN field is * If the inner ECN field is Not-ECT and the outer ECN field is
Not-ECT, ECT(0) or ECT(1) the decapsulator MUST forward the Not-ECT, ECT(0), or ECT(1), the decapsulator MUST forward the
outgoing packet with the ECN field cleared to Not-ECT. outgoing packet with the ECN field cleared to Not-ECT.
o In all other cases where the inner supports ECN, the decapsulator o In all other cases where the inner supports ECN, the decapsulator
MUST set the outgoing ECN field to the more severe marking of the MUST set the outgoing ECN field to the more severe marking of the
outer and inner ECN fields, where the ranking of severity from outer and inner ECN fields, where the ranking of severity from
highest to lowest is CE, ECT(1), ECT(0), Not-ECT. This in no way highest to lowest is CE, ECT(1), ECT(0), Not-ECT. This in no way
precludes cases where ECT(1) and ECT(0) have the same severity; precludes cases where ECT(1) and ECT(0) have the same severity;
o Certain combinations of inner and outer ECN fields cannot result o Certain combinations of inner and outer ECN fields cannot result
from any transition in any current or previous ECN tunneling from any transition in any current or previous ECN tunneling
specification. These currently unused (CU) combinations are specification. These currently unused (CU) combinations are
indicated in Figure 4 by '(!!!)' or '(!)', where '(!!!)' means the indicated in Figure 4 by '(!!!)' or '(!)', where '(!!!)' means the
combination is CU and always potentially dangerous, while '(!)' combination is CU and always potentially dangerous, while '(!)'
means it is CU and possibly dangerous. In these cases, means it is CU and possibly dangerous. In these cases,
particularly the more dangerous ones, the decapsulator SHOULD log particularly the more dangerous ones, the decapsulator SHOULD log
the event and MAY also raise an alarm. the event and MAY also raise an alarm.
Just because the highlighted combinations are currently unused, Just because the highlighted combinations are currently unused,
does not mean that all the other combinations are always valid. does not mean that all the other combinations are always valid.
Some are only valid if they have arrived from a particular type of Some are only valid if they have arrived from a particular type of
legacy ingress, and dangerous otherwise. Therefore an legacy ingress, and dangerous otherwise. Therefore, an
implementation MAY allow an operator to configure logging and implementation MAY allow an operator to configure logging and
alarms for such additional header combinations known to be alarms for such additional header combinations known to be
dangerous or CU for the particular configuration of tunnel dangerous or CU for the particular configuration of tunnel
endpoints deployed at run-time. endpoints deployed at run-time.
Alarms SHOULD be rate-limited so that the anomalous combinations Alarms SHOULD be rate-limited so that the anomalous combinations
will not amplify into a flood of alarm messages. It MUST be will not amplify into a flood of alarm messages. It MUST be
possible to suppress alarms or logging, e.g. if it becomes possible to suppress alarms or logging, e.g., if it becomes
apparent that a combination that previously was not used has apparent that a combination that previously was not used has
started to be used for legitimate purposes such as a new standards started to be used for legitimate purposes such as a new standards
action. action.
The above logic allows for ECT(0) and ECT(1) to both represent the The above logic allows for ECT(0) and ECT(1) to both represent the
same severity of congestion marking (e.g. "not congestion marked"). same severity of congestion marking (e.g., "not congestion marked").
But it also allows future schemes to be defined where ECT(1) is a But it also allows future schemes to be defined where ECT(1) is a
more severe marking than ECT(0), in particular enabling the simplest more severe marking than ECT(0), in particular, enabling the simplest
possible encoding for PCN [I-D.ietf-pcn-3-in-1-encoding] (see possible encoding for PCN [PCN3in1] (see Section 5.3.2). Treating
Section 5.3.2). Treating ECT(1) as either the same as ECT(0) or as a ECT(1) as either the same as ECT(0) or as a higher severity level is
higher severity level is explained in the discussion of the ECN nonce explained in the discussion of the ECN nonce [RFC3540] in Section 8,
[RFC3540] in Section 9, which in turn refers to Appendix D. which in turn refers to Appendix D.
4.3. Encapsulation Modes 4.3. Encapsulation Modes
Section 4.1 introduces two encapsulation modes, normal mode and Section 4.1 introduces two encapsulation modes: normal mode, and
compatibility mode, defining their encapsulation behaviour (i.e. compatibility mode, defining their encapsulation behaviour (i.e.,
header copying or zeroing respectively). Note that these are modes header copying or zeroing, respectively). Note that these are modes
of the ingress tunnel endpoint only, not the tunnel as a whole. of the ingress tunnel endpoint only, not the tunnel as a whole.
To comply with this specification, a tunnel ingress MUST at least To comply with this specification, a tunnel ingress MUST at least
implement `normal mode'. Unless it will never be used with legacy implement normal mode. Unless it will never be used with legacy
tunnel egress nodes (RFC2003, RFC2401 or RFC2481 or the limited tunnel egress nodes (RFC 2003, RFC 2401, or RFC 2481 or the limited
functionality mode of RFC3168), an ingress MUST also implement functionality mode of RFC 3168), an ingress MUST also implement
`compatibility mode' for backward compatibility with tunnel egresses compatibility mode for backward compatibility with tunnel egresses
that do not propagate explicit congestion notifications [RFC4774]. that do not propagate explicit congestion notifications [RFC4774].
We can categorise the way that an ingress tunnel endpoint is paired We can categorise the way that an ingress tunnel endpoint is paired
with an egress as either static or dynamically discovered: with an egress as either static or dynamically discovered:
Static: Tunnel endpoints paired together by prior configuration. Static: Tunnel endpoints paired together by prior configuration.
Some implementations of encapsulator might always be statically Some implementations of encapsulator might always be statically
deployed, and constrained to never be paired with a legacy deployed, and constrained to never be paired with a legacy
decapsulator (RFC2003, RFC2401 or RFC2481 or the limited decapsulator (RFC 2003, RFC 2401 or RFC 2481 or the limited
functionality mode of RFC3168). In such a case, only normal mode functionality mode of RFC 3168). In such a case, only normal mode
needs to be implemented. needs to be implemented.
For instance, RFC4301-compatible IPsec tunnel endpoints invariably For instance, IPsec tunnel endpoints compatible with RFC 4301
use IKEv2 [RFC4306] for key exchange, which was introduced invariably use Internet Key Exchange Protocol version 2 (IKEv2)
alongside RFC4301. Therefore both endpoints of an RFC4301 tunnel [RFC5996] for key exchange, the original specification of which
can be sure that the other end is RFC4301-compatible, because the was introduced alongside RFC 4301. Therefore, both endpoints of
tunnel is only formed after IKEv2 key management has completed, at an RFC 4301 tunnel can be sure that the other end is compatible
which point both ends will be RFC4301-compliant by definition. with RFC 4301, because the tunnel is only formed after IKEv2 key
Therefore an IPsec tunnel ingress does not need compatibility management has completed, at which point both ends will be
mode, as it will never interact with legacy ECN tunnels. To compliant with RFC 4301 by definition. Therefore an IPsec tunnel
comply with the present specification, it only needs to implement ingress does not need compatibility mode, as it will never
the required normal mode, which is identical to the pre-existing interact with legacy ECN tunnels. To comply with the present
RFC4301 behaviour. specification, it only needs to implement the required normal
mode, which is identical to the pre-existing RFC 4301 behaviour.
Dynamic Discovery: Tunnel endpoints paired together by some form of Dynamic Discovery: Tunnel endpoints paired together by some form of
tunnel endpoint discovery, typically finding an egress on the path tunnel endpoint discovery, typically finding an egress on the path
taken by the first packet. taken by the first packet.
This specification does not require or recommend dynamic discovery This specification does not require or recommend dynamic discovery
and it does not define how dynamic negotiation might be done, but and it does not define how dynamic negotiation might be done, but
it recognises that proprietary tunnel endpoint discovery protocols it recognises that proprietary tunnel endpoint discovery protocols
exist. It therefore sets down some constraints on discovery exist. It therefore sets down some constraints on discovery
protocols to ensure safe interworking. protocols to ensure safe interworking.
If dynamic tunnel endpoint discovery might pair an ingress with a If dynamic tunnel endpoint discovery might pair an ingress with a
legacy egress (RFC2003, RFC2401 or RFC2481 or the limited legacy egress (RFC 2003, RFC 2401, or RFC 2481 or the limited
functionality mode of RFC3168), the ingress MUST implement both functionality mode of RFC 3168), the ingress MUST implement both
normal and compatibility mode. If the tunnel discovery process is normal and compatibility mode. If the tunnel discovery process is
arranged to only ever find a tunnel egress that propagates ECN arranged to only ever find a tunnel egress that propagates ECN
(RFC3168 full functionality mode, RFC4301 or this present (RFC 3168 full functionality mode, RFC 4301, or this present
specification), then a tunnel ingress can be compliant with the specification), then a tunnel ingress can be compliant with the
present specification without implementing compatibility mode. present specification without implementing compatibility mode.
While a compliant tunnel ingress is discovering an egress, it MUST While a compliant tunnel ingress is discovering an egress, it MUST
send packets in compatibility mode in case the egress it discovers send packets in compatibility mode in case the egress it discovers
is a legacy egress. If, through the discovery protocol, the is a legacy egress. If, through the discovery protocol, the
egress indicates that it is compliant with the present egress indicates that it is compliant with the present
specification, with RFC4301 or with RFC3168 full functionality specification, with RFC 4301 or with RFC 3168 full functionality
mode, the ingress can switch itself into normal mode. If the mode, the ingress can switch itself into normal mode. If the
egress denies compliance with any of these or returns an error egress denies compliance with any of these or returns an error
that implies it does not understand a request to work to any of that implies it does not understand a request to work to any of
these ECN specifications, the tunnel ingress MUST remain in these ECN specifications, the tunnel ingress MUST remain in
compatibility mode. compatibility mode.
If an ingress claims compliance with this specification it MUST NOT If an ingress claims compliance with this specification, it MUST NOT
permanently disable ECN processing across the tunnel (i.e. only using permanently disable ECN processing across the tunnel (i.e., only
compatibility mode). It is true that such a tunnel ingress is at using compatibility mode). It is true that such a tunnel ingress is
least safe with the ECN behaviour of any egress it may encounter, but at least safe with the ECN behaviour of any egress it may encounter,
it does not meet the central aim of this specification: introducing but it does not meet the central aim of this specification:
ECN support to tunnels. introducing ECN support to tunnels.
Instead, if the ingress knows that the egress does support Instead, if the ingress knows that the egress does support
propagation of ECN (full functionality mode of RFC3168 or RFC4301 or propagation of ECN (full functionality mode of RFC 3168 or RFC 4301
the present specification), it SHOULD use normal mode, in order to or the present specification), it SHOULD use normal mode, in order to
support ECN where possible. Note that this section started by saying support ECN where possible. Note that this section started by saying
an ingress "MUST implement "normal mode, while it has just said an an ingress "MUST implement" normal mode, while it has just said an
ingress "SHOULD use" normal mode. This distinction is deliberate, to ingress "SHOULD use" normal mode. This distinction is deliberate, to
allow the mode to be turned off in exceptional circumstances but to allow the mode to be turned off in exceptional circumstances but to
ensure all implementations make normal mode available. ensure all implementations make normal mode available.
Implementation note: If a compliant node is the ingress for multiple Implementation note: If a compliant node is the ingress for multiple
tunnels, a mode setting will need to be stored for each tunnel tunnels, a mode setting will need to be stored for each tunnel
ingress. However, if a node is the egress for multiple tunnels, ingress. However, if a node is the egress for multiple tunnels,
none of the tunnels will need to store a mode setting, because a none of the tunnels will need to store a mode setting, because a
compliant egress only needs one mode. compliant egress only needs one mode.
skipping to change at page 21, line 40 skipping to change at page 14, line 42
A compliant decapsulator only needs one mode of operation. However, A compliant decapsulator only needs one mode of operation. However,
if a compliant egress is implemented to be dynamically discoverable, if a compliant egress is implemented to be dynamically discoverable,
it may need to respond to discovery requests from various types of it may need to respond to discovery requests from various types of
legacy tunnel ingress. This specification does not define how legacy tunnel ingress. This specification does not define how
dynamic negotiation might be done by (proprietary) discovery dynamic negotiation might be done by (proprietary) discovery
protocols, but it sets down some constraints to ensure safe protocols, but it sets down some constraints to ensure safe
interworking. interworking.
Through the discovery protocol, a tunnel ingress compliant with the Through the discovery protocol, a tunnel ingress compliant with the
present specification might ask if the egress is compliant with the present specification might ask if the egress is compliant with the
present specification, with RFC4301 or with RFC3168 full present specification, with RFC 4301 or with RFC 3168 full
functionality mode. Or an RFC3168 tunnel ingress might try to functionality mode. Or an RFC 3168 tunnel ingress might try to
negotiate to use limited functionality or full functionality mode negotiate to use limited functionality or full functionality mode
[RFC3168]. In all these cases, a decapsulating tunnel egress [RFC3168]. In all these cases, a decapsulating tunnel egress
compliant with this specification MUST agree to any of these compliant with this specification MUST agree to any of these
requests, since it will behave identically in all these cases. requests, since it will behave identically in all these cases.
If no ECN-related mode is requested, a compliant tunnel egress MUST If no ECN-related mode is requested, a compliant tunnel egress MUST
continue without raising any error or warning, because its egress continue without raising any error or warning, because its egress
behaviour is compatible with all the legacy ingress behaviours that behaviour is compatible with all the legacy ingress behaviours that
do not negotiate capabilities. do not negotiate capabilities.
A compliant tunnel egress SHOULD raise a warning alarm about any A compliant tunnel egress SHOULD raise a warning alarm about any
requests to enter modes it does not recognise but, for 'forward requests to enter modes it does not recognise but, for 'forward
compatibility' with standards actions possibly defined after it was compatibility' with standards actions possibly defined after it was
implemented, it SHOULD continue operating. implemented, it SHOULD continue operating.
5. Updates to Earlier RFCs 5. Updates to Earlier RFCs
5.1. Changes to RFC4301 ECN processing 5.1. Changes to RFC 4301 ECN Processing
Ingress: An RFC4301 IPsec encapsulator is not changed at all by the Ingress: An RFC 4301 IPsec encapsulator is not changed at all by the
present specification. It uses the normal mode of the present present specification. It uses the normal mode of the present
specification, which defines packet encapsulation identically to specification, which defines packet encapsulation identically to
RFC4301. RFC 4301.
Egress: An RFC4301 egress will need to be updated to the new Egress: An RFC 4301 egress will need to be updated to the new
decapsulation behaviour in Figure 4, in order to comply with the decapsulation behaviour in Figure 4, in order to comply with the
present specification. However, the changes are backward present specification. However, the changes are backward
compatible; combinations of inner and outer that result from any compatible; combinations of inner and outer that result from any
protocol defined in the RFC series so far are unaffected. Only protocol defined in the RFC series so far are unaffected. Only
combinations that have never been used have been changed, combinations that have never been used have been changed,
effectively adding new behaviours to RFC4301 decapsulation without effectively adding new behaviours to RFC 4301 decapsulation
altering existing behaviours. The following specific updates to without altering existing behaviours. The following specific
section 5.1.2 of RFC4301 have been made: updates to Section 5.1.2 of RFC 4301 have been made:
* The outer, not the inner, is propagated when the outer is * The outer, not the inner, is propagated when the outer is
ECT(1) and the inner is ECT(0); ECT(1) and the inner is ECT(0);
* A packet with Not-ECT in the inner and an outer of CE is * A packet with Not-ECT in the inner and an outer of CE is
dropped rather than forwarded as Not-ECT; dropped rather than forwarded as Not-ECT;
* Certain combinations of inner and outer ECN field have been * Certain combinations of inner and outer ECN field have been
identified as currently unused. These can trigger logging identified as currently unused. These can trigger logging
and/or raise alarms. and/or raise alarms.
Modes: RFC4301 tunnel endpoints do not need modes and are not Modes: RFC 4301 tunnel endpoints do not need modes and are not
updated by the modes in the present specification. Effectively an updated by the modes in the present specification. Effectively,
RFC4301 IPsec ingress solely uses the REQUIRED normal mode of an RFC 4301 IPsec ingress solely uses the REQUIRED normal mode of
encapsulation, which is unchanged from RFC4301 encapsulation. It encapsulation, which is unchanged from RFC 4301 encapsulation. It
will never need the OPTIONAL compatibility mode as explained in will never need the OPTIONAL compatibility mode as explained in
Section 4.3. Section 4.3.
5.2. Changes to RFC3168 ECN processing 5.2. Changes to RFC 3168 ECN Processing
Ingress: On encapsulation, the new rule in Figure 3 that a normal Ingress: On encapsulation, the new rule in Figure 3 that a normal
mode tunnel ingress copies any ECN field into the outer header mode tunnel ingress copies any ECN field into the outer header
updates the full functionality behaviour of an RFC3168 ingress updates the full functionality behaviour of an RFC 3168 ingress
[RFC3168; section 9.1.1]. Nonetheless, the new compatibility mode (Section 9.1.1 of [RFC3168]). Nonetheless, the new compatibility
encapsulates packets identically to the limited functionality mode mode encapsulates packets identically to the limited functionality
of an RFC3168 ingress. mode of an RFC 3168 ingress.
Egress: An RFC3168 egress will need to be updated to the new Egress: An RFC 3168 egress will need to be updated to the new
decapsulation behaviour in Figure 4, in order to comply with the decapsulation behaviour in Figure 4, in order to comply with the
present specification. However, the changes are backward present specification. However, the changes are backward
compatible; combinations of inner and outer that result from any compatible; combinations of inner and outer that result from any
protocol defined in the RFC series so far are unaffected. Only protocol defined in the RFC series so far are unaffected. Only
combinations that have never been used have been changed, combinations that have never been used have been changed,
effectively adding new behaviours to RFC3168 decapsulation without effectively adding new behaviours to RFC 3168 decapsulation
altering existing behaviours. The following specific updates to without altering existing behaviours. The following specific
section 9.1.1 of RFC3168 have been made: updates to Section 9.1.1 of RFC 3168 have been made:
* The outer, not the inner, is propagated when the outer is * The outer, not the inner, is propagated when the outer is
ECT(1) and the inner is ECT(0); ECT(1) and the inner is ECT(0);
* Certain combinations of inner and outer ECN field have been * Certain combinations of inner and outer ECN field have been
identified as currently unused. These can trigger logging identified as currently unused. These can trigger logging
and/or raise alarms. and/or raise alarms.
Modes: An RFC3168 ingress will need to be updated if it is to comply Modes: An RFC 3168 ingress will need to be updated if it is to
with the present specification, whether or not it implemented the comply with the present specification, whether or not it
optional full functionality mode of section 9.1.1 of RFC3168. implemented the optional full functionality mode of Section 9.1.1
of RFC 3168.
Section 9.1 of RFC3168 defined a (required) limited functionality Section 9.1 of RFC 3168 defined a (required) limited functionality
mode and an (optional) full functionality mode for a tunnel. In mode and an (optional) full functionality mode for a tunnel. In
RFC3168, modes applied to both ends of the tunnel, while in the RFC 3168, modes applied to both ends of the tunnel, while in the
present specification, modes are only used at the ingress--a present specification, modes are only used at the ingress -- a
single egress behaviour covers all cases. single egress behaviour covers all cases.
The normal mode of encapsulation is an update to the encapsulation The normal mode of encapsulation is an update to the encapsulation
behaviour of the full functionality mode of an RFC3168 ingress. behaviour of the full functionality mode of an RFC 3168 ingress.
The compatibility mode of encapsulation is identical to the The compatibility mode of encapsulation is identical to the
encapsulation behaviour of the limited functionality mode of an encapsulation behaviour of the limited functionality mode of an
RFC3168 ingress, except it is not always obligatory. RFC 3168 ingress, except it is not always obligatory.
The constraints on how tunnel discovery protocols set modes in The constraints on how tunnel discovery protocols set modes in
Section 4.3 and Section 4.4 are an update to RFC3168, but they are Sections 4.3 and 4.4 are an update to RFC 3168, but they are
unlikely to require code changes as they document existing safe unlikely to require code changes as they document existing safe
practice. practice.
5.3. Motivation for Changes 5.3. Motivation for Changes
An overriding goal is to ensure the same ECN signals can mean the An overriding goal is to ensure the same ECN signals can mean the
same thing whatever tunnels happen to encapsulate an IP packet flow. same thing whatever tunnels happen to encapsulate an IP packet flow.
This removes gratuitous inconsistency, which otherwise constrains the This removes gratuitous inconsistency, which otherwise constrains the
available design space and makes it harder to design networks and new available design space and makes it harder to design networks and new
protocols that work predictably. protocols that work predictably.
5.3.1. Motivation for Changing Encapsulation 5.3.1. Motivation for Changing Encapsulation
The normal mode in Section 4 updates RFC3168 to make all IP in IP The normal mode in Section 4 updates RFC 3168 to make all IP-in-IP
encapsulation of the ECN field consistent--consistent with the way encapsulation of the ECN field consistent -- consistent with the way
both RFC4301 IPsec [RFC4301] and IP in MPLS or MPLS in MPLS both RFC 4301 IPsec [RFC4301] and IP-in-MPLS or MPLS-in-MPLS
encapsulation [RFC5129] construct the ECN field. encapsulation [RFC5129] construct the ECN field.
Compatibility mode has also been defined so that a non-RFC4301 Compatibility mode has also been defined so that an ingress compliant
ingress can still switch to using drop across a tunnel for backwards with a version of IPsec prior to RFC 4301 can still switch to using
compatibility with legacy decapsulators that do not propagate ECN drop across a tunnel for backward compatibility with legacy
correctly. decapsulators that do not propagate ECN.
The trigger that motivated this update to RFC3168 encapsulation was a The trigger that motivated this update to RFC 3168 encapsulation was
standards track proposal for pre-congestion notification (PCN a Standards-Track proposal for pre-congestion notification (PCN
[RFC5670]). PCN excess rate marking only works correctly if the ECN [RFC5670]). PCN excess-traffic-marking only works correctly if the
field is copied on encapsulation (as in RFC4301 and RFC5129); it does ECN field is copied on encapsulation (as in RFC 4301 and RFC 5129);
not work if ECN is reset (as in RFC3168). This is because PCN excess it does not work if ECN is reset (as in RFC 3168). This is because
rate marking depends on the outer header revealing any congestion PCN excess-traffic-marking depends on the outer header revealing any
experienced so far on the whole path, not just since the last tunnel congestion experienced so far on the whole path, not just since the
ingress. last tunnel ingress.
PCN allows a network operator to add flow admission and termination PCN allows a network operator to add flow admission and termination
for inelastic traffic at the edges of a Diffserv domain, but without for inelastic traffic at the edges of a Diffserv domain, but without
any per-flow mechanisms in the interior and without the generous any per-flow mechanisms in the interior and without the generous
provisioning typical of Diffserv, aiming to significantly reduce provisioning typical of Diffserv, aiming to significantly reduce
costs. The PCN architecture [RFC5559] states that RFC3168 IP in IP costs. The PCN architecture [RFC5559] states that RFC 3168 IP-in-IP
tunnelling of the ECN field cannot be used for any tunnel ingress in tunnelling of the ECN field cannot be used for any tunnel ingress in
a PCN domain. Prior to the present specification, this left a stark a PCN domain. Prior to the present specification, this left a stark
choice between not being able to use PCN for inelastic traffic choice between not being able to use PCN for inelastic traffic
control or not being able to use the many tunnels already deployed control or not being able to use the many tunnels already deployed
for Mobile IP, VPNs and so forth. for Mobile IP, VPNs, and so forth.
The present specification provides a clean solution to this problem, The present specification provides a clean solution to this problem,
so that network operators who want to use both PCN and tunnels can so that network operators who want to use both PCN and tunnels can
specify that every tunnel ingress in a PCN region must comply with specify that every tunnel ingress in a PCN region must comply with
this latest specification. this latest specification.
Rather than allow tunnel specifications to fragment further into one Rather than allow tunnel specifications to fragment further into one
for PCN, one for IPsec and one for other tunnels, the opportunity has for PCN, one for IPsec, and one for other tunnels, the opportunity
been taken to consolidate the diverging specifications back into a has been taken to consolidate the diverging specifications back into
single tunnelling behaviour. Resetting ECN was originally motivated a single tunnelling behaviour. Resetting ECN was originally
by a covert channel concern that has been deliberately set aside in motivated by a covert channel concern that has been deliberately set
RFC4301 IPsec. Therefore the reset behaviour of RFC3168 is an aside in RFC 4301 IPsec. Therefore, the reset behaviour of RFC 3168
anomaly that we do not need to keep. Copying ECN on encapsulation is is an anomaly that we do not need to keep. Copying ECN on
anyway simpler than resetting. So, as more tunnel endpoints comply encapsulation is simpler than resetting. So, as more tunnel
with this single consistent specification, encapsulation will be endpoints comply with this single consistent specification,
simpler as well as more predictable. encapsulation will be simpler as well as more predictable.
Appendix B assesses whether copying rather than resetting CE on Appendix B assesses whether copying rather than resetting CE on
ingress will cause any unintended side-effects, from the three ingress will cause any unintended side effects, from the three
perspectives of security, control and management. In summary this perspectives of security, control, and management. In summary, this
analysis finds that: analysis finds that:
o From the control perspective either copying or resetting works for o From the control perspective, either copying or resetting works
existing arrangements, but copying has more potential for for existing arrangements, but copying has more potential for
simplifying control and resetting breaks at least one proposal simplifying control and resetting breaks at least one proposal
already on the standards track. that is already on the Standards Track.
o From the management and monitoring perspective copying is o From the management and monitoring perspective, copying is
preferable. preferable.
o From the traffic security perspective (enforcing congestion o From the traffic security perspective (enforcing congestion
control, mitigating denial of service etc) copying is preferable. control, mitigating denial of service, etc.), copying is
preferable.
o From the information security perspective resetting is preferable, o From the information security perspective, resetting is
but the IETF Security Area now considers copying acceptable given preferable, but the IETF Security Area now considers copying
the bandwidth of a 2-bit covert channel can be managed. acceptable given the bandwidth of a two-bit covert channel can be
managed.
Therefore there are two points against resetting CE on ingress while Therefore, there are two points against resetting CE on ingress while
copying CE causes no significant harm. copying CE causes no significant harm.
5.3.2. Motivation for Changing Decapsulation 5.3.2. Motivation for Changing Decapsulation
The specification for decapsulation in Section 4 fixes three problems The specification for decapsulation in Section 4 fixes three problems
with the pre-existing behaviours of both RFC3168 and RFC4301: with the pre-existing behaviours found in both RFC 3168 and RFC 4301:
1. The pre-existing rules prevented the introduction of alternate 1. The pre-existing rules prevented the introduction of alternate
ECN semantics to signal more than one severity level of ECN semantics to signal more than one severity level of
congestion [RFC4774], [RFC5559]. The four states of the 2-bit congestion [RFC4774], [RFC5559]. The four states of the two-bit
ECN field provide room for signalling two severity levels in ECN field provide room for signalling two severity levels in
addition to not-congested and not-ECN-capable states. But, the addition to not-congested and not-ECN-capable states. But, the
pre-existing rules assumed that two of the states (ECT(0) and pre-existing rules assumed that two of the states (ECT(0) and
ECT(1)) are always equivalent. This unnecessarily restricts the ECT(1)) are always equivalent. This unnecessarily restricts the
use of one of four codepoints (half a bit) in the IP (v4 & v6) use of one of four codepoints (half a bit) in the IP (v4 and v6)
header. The new rules are designed to work in either case; header. The new rules are designed to work in either case;
whether ECT(1) is more severe than or equivalent to ECT(0). whether ECT(1) is more severe than or equivalent to ECT(0).
As explained in Appendix B.1, the original reason for not As explained in Appendix B.1, the original reason for not
forwarding the outer ECT codepoints was to limit the covert forwarding the outer ECT codepoints was to limit the covert
channel across a decapsulator to 1 bit per packet. However, now channel across a decapsulator to 1 bit per packet. However, now
that the IETF Security Area has deemed that a 2-bit covert that the IETF Security Area has deemed that a two-bit covert
channel through an encapsulator is a manageable risk, the same channel through an encapsulator is a manageable risk, the same
should be true for a decapsulator. should be true for a decapsulator.
As well as being useful for general future-proofing, this problem As well as being useful for general future-proofing, this problem
is immediately pressing for standardisation of pre-congestion is immediately pressing for standardisation of pre-congestion
notification (PCN), which uses two severity levels of congestion. notification (PCN), which uses two severity levels of congestion.
If a congested queue used ECT(1) in the outer header to signal If a congested queue used ECT(1) in the outer header to signal
more severe congestion than ECT(0), the pre-existing more severe congestion than ECT(0), the pre-existing
decapsulation rules would have thrown away this congestion decapsulation rules would have thrown away this congestion
signal, preventing tunnelled traffic from ever knowing that it signal, preventing tunnelled traffic from ever knowing that it
should reduce its load. should reduce its load.
Before the present specification was written, the PCN working Before the present specification was written, the PCN working
group had to consider a number of wasteful or convoluted work- group had to consider a number of wasteful or convoluted work-
rounds to this problem. Without wishing to disparage the rounds to this problem. Without wishing to disparage the
ingenuity of these work-rounds, none were chosen for the ingenuity of these work-rounds, none were chosen for the
standards track because they were either somewhat wasteful, Standards Track because they were either somewhat wasteful,
imprecise or complicated. Instead a baseline PCN encoding was imprecise, or complicated. Instead, a baseline PCN encoding was
specified [RFC5696] that supported only one severity level of specified [RFC5696] that supported only one severity level of
congestion but allowed space for these work-rounds as congestion but allowed space for these work-rounds as
experimental extensions. experimental extensions.
But by far the simplest approach is that taken by the current By far the simplest approach is that taken by the current
specification: just to remove the covert channel blockages from specification: just to remove the covert channel blockages from
tunnelling behaviour--now deemed unnecessary anyway. Then tunnelling behaviour -- now deemed unnecessary anyway. Then,
network operators that want to support two congestion severity- network operators that want to support two congestion severity
levels for PCN can specify that every tunnel egress in a PCN levels for PCN can specify that every tunnel egress in a PCN
region must comply with this latest specification. Having taken region must comply with this latest specification. Having taken
this step, the simplest possible encoding for PCN with two this step, the simplest possible encoding for PCN with two
severity levels of congestion [I-D.ietf-pcn-3-in-1-encoding] can severity levels of congestion [PCN3in1] can be used.
be used.
Not only does this make two congestion severity-levels available Not only does this make two congestion severity levels available
for PCN, but also for other potential uses of the extra ECN for PCN, but also for other potential uses of the extra ECN
codepoint (e.g. [VCP]). codepoint (e.g., [VCP]).
2. Cases are documented where a middlebox (e.g. a firewall) drops 2. Cases are documented where a middlebox (e.g., a firewall) drops
packets with header values that were currently unused (CU) when packets with header values that were currently unused (CU) when
the box was deployed, often on the grounds that anything the box was deployed, often on the grounds that anything
unexpected might be an attack. This tends to bar future use of unexpected might be an attack. This tends to bar future use of
CU values. The new decapsulation rules specify optional logging CU values. The new decapsulation rules specify optional logging
and/or alarms for specific combinations of inner and outer header and/or alarms for specific combinations of inner and outer
that are currently unused. The aim is to give implementers a headers that are currently unused. The aim is to give
recourse other than drop if they are concerned about the security implementers a recourse other than drop if they are concerned
of CU values. It recognises legitimate security concerns about about the security of CU values. It recognises legitimate
CU values but still eases their future use. If the alarms are security concerns about CU values, but still eases their future
interpreted as an attack (e.g. by a management system) the use. If the alarms are interpreted as an attack (e.g., by a
offending packets can be dropped. But alarms can be turned off management system) the offending packets can be dropped.
if these combinations come into regular use (e.g. through a However, alarms can be turned off if these combinations come into
future standards action). regular use (e.g., through a future standards action).
3. While reviewing currently unused combinations of inner and outer, 3. While reviewing currently unused combinations of inner and outer
the opportunity was taken to define a single consistent behaviour headers, the opportunity was taken to define a single consistent
for the three cases with a Not-ECT inner header but a different behaviour for the three cases with a Not-ECT inner header but a
outer. RFC3168 and RFC4301 had diverged in this respect and even different outer. RFC 3168 and RFC 4301 had diverged in this
their common behaviours had never been justified. respect and even their common behaviours had never been
justified.
None of these combinations should result from Internet protocols None of these combinations should result from Internet protocols
in the RFC series, but future standards actions might put any or in the RFC series, but future standards actions might put any or
all of them to good use. Therefore it was decided that a all of them to good use. Therefore, it was decided that a
decapsulator must forward a Not-ECT inner unchanged when the decapsulator must forward a Not-ECT inner header unchanged when
arriving outer is ECT(0) or ECT(1). But for safety it must drop the arriving outer header is ECT(0) or ECT(1). For safety, it
a combination of Not-ECT inner and CE outer. Then, if some must drop a combination of Not-ECT inner and CE outer headers.
unfortunate misconfiguration resulted in a congested router Then, if some unfortunate misconfiguration resulted in a
marking CE on a packet that was originally Not-ECT, drop would be congested router marking CE on a packet that was originally
the only appropriate signal for the egress to propagate--the only Not-ECT, drop would be the only appropriate signal for the egress
signal a non-ECN-capable transport (Not-ECT) would understand. to propagate -- the only signal a non-ECN-capable transport
(Not-ECT) would understand.
It may seem contradictory that the same argument has not been It may seem contradictory that the same argument has not been
applied to the ECT(1) codepoint, given it is being proposed as an applied to the ECT(1) codepoint, given it is being proposed as an
intermediate level of congestion in a scheme progressing through intermediate level of congestion in a scheme progressing through
the IETF [I-D.ietf-pcn-3-in-1-encoding]. Instead, a decapsulator the IETF [PCN3in1]. Instead, a decapsulator must forward a
must forward a Not-ECT inner unchanged when its outer is ECT(1). Not-ECT inner unchanged when its outer is ECT(1). The rationale
The rationale for not dropping this CU combination is to ensure for not dropping this CU combination is to ensure it will be
it will be usable if needed in the future. If any usable if needed in the future. If any misconfiguration led to
misconfiguration led to ECT(1) congestion signals with a Not-ECT ECT(1) congestion signals with a Not-ECT inner, it would not be
inner, it would not be disastrous for the tunnel egress to disastrous for the tunnel egress to suppress them, because the
suppress them, because the congestion should then escalate to CE congestion should then escalate to CE marking, which the egress
marking, which the egress would drop, thus at least preventing would drop, thus at least preventing congestion collapse.
congestion collapse.
Problems 2 & 3 alone would not warrant a change to decapsulation, but Problems 2 and 3 alone would not warrant a change to decapsulation,
it was decided they are worth fixing and making consistent at the but it was decided they are worth fixing and making consistent at the
same time as decapsulation code is changed to fix problem 1 (two same time as decapsulation code is changed to fix problem 1 (two
congestion severity-levels). congestion severity levels).
6. Backward Compatibility 6. Backward Compatibility
A tunnel endpoint compliant with the present specification is A tunnel endpoint compliant with the present specification is
backward compatible when paired with any tunnel endpoint compliant backward compatible when paired with any tunnel endpoint compliant
with any previous tunnelling RFC, whether RFC4301, RFC3168 (see with any previous tunnelling RFC, whether RFC 4301, RFC 3168 (see
Section 3) or the earlier RFCs summarised in Appendix A (RFC2481, Section 3), or the earlier RFCs summarised in Appendix A (RFC 2481,
RFC2401 and RFC2003). Each case is enumerated below. RFC 2401, and RFC 2003). Each case is enumerated below.
6.1. Non-Issues Updating Decapsulation 6.1. Non-Issues Updating Decapsulation
At the egress, this specification only augments the per-packet At the egress, this specification only augments the per-packet
calculation of the ECN field (RFC3168 and RFC4301) for combinations calculation of the ECN field (RFC 3168 and RFC 4301) for combinations
of inner and outer headers that have so far not been used in any IETF of inner and outer headers that have so far not been used in any IETF
protocols. protocols.
Therefore, all other things being equal, if an RFC4301 IPsec egress Therefore, all other things being equal, if an RFC 4301 IPsec egress
is updated to comply with the new rules, it will still interwork with is updated to comply with the new rules, it will still interwork with
any RFC4301 compliant ingress and the packet outputs will be any ingress compliant with RFC 4301 and the packet outputs will be
identical to those it would have output before (fully backward identical to those it would have output before (fully backward
compatible). compatible).
And, all other things being equal, if an RFC3168 egress is updated to And, all other things being equal, if an RFC 3168 egress is updated
comply with the same new rules, it will still interwork with any to comply with the same new rules, it will still interwork with any
ingress complying with any previous specification (both modes of ingress complying with any previous specification (both modes of RFC
RFC3168, both modes of RFC2481, RFC2401 and RFC2003) and the packet 3168, both modes of RFC 2481, RFC 2401, and RFC 2003) and the packet
outputs will be identical to those it would have output before (fully outputs will be identical to those it would have output before (fully
backward compatible). backward compatible).
A compliant tunnel egress merely needs to implement the one behaviour A compliant tunnel egress merely needs to implement the one behaviour
in Section 4 with no additional mode or option configuration at the in Section 4 with no additional mode or option configuration at the
ingress or egress nor any additional negotiation with the ingress. ingress or egress nor any additional negotiation with the ingress.
The new decapsulation rules have been defined in such a way that The new decapsulation rules have been defined in such a way that
congestion control will still work safely if any of the earlier congestion control will still work safely if any of the earlier
versions of ECN processing are used unilaterally at the encapsulating versions of ECN processing are used unilaterally at the encapsulating
ingress of the tunnel (any of RFC2003, RFC2401, either mode of ingress of the tunnel (any of RFC 2003, RFC 2401, either mode of RFC
RFC2481, either mode of RFC3168, RFC4301 and this present 2481, either mode of RFC 3168, RFC 4301, and this present
specification). specification).
6.2. Non-Update of RFC4301 IPsec Encapsulation 6.2. Non-Update of RFC 4301 IPsec Encapsulation
An RFC4301 IPsec ingress can comply with this new specification An RFC 4301 IPsec ingress can comply with this new specification
without any update and it has no need for any new modes, options or without any update and it has no need for any new modes, options, or
configuration. So, all other things being equal, it will continue to configuration. So, all other things being equal, it will continue to
interwork identically with any egress it worked with before (fully interwork identically with any egress it worked with before (fully
backward compatible). backward compatible).
6.3. Update to RFC3168 Encapsulation 6.3. Update to RFC 3168 Encapsulation
The encapsulation behaviour of the new normal mode copies the ECN The encapsulation behaviour of the new normal mode copies the ECN
field whereas an RFC3168 ingress in full functionality mode reset it. field, whereas an RFC 3168 ingress in full functionality mode reset
However, all other things being equal, if an RFC3168 ingress is it. However, all other things being equal, if an RFC 3168 ingress is
updated to the present specification, the outgoing packets from any updated to the present specification, the outgoing packets from any
tunnel egress will still be unchanged. This is because all variants tunnel egress will still be unchanged. This is because all variants
of tunnelling at either end (RFC4301, both modes of RFC3168, both of tunnelling at either end (RFC 4301, both modes of RFC 3168, both
modes of RFC2481, RFC2401, RFC2003 and the present specification) modes of RFC 2481, RFC 2401, RFC 2003, and the present specification)
have always propagated an incoming CE marking through the inner have always propagated an incoming CE marking through the inner
header and onward into the outgoing header, whether the outer header header and onward into the outgoing header; whether the outer header
is reset or copied. Therefore, If the tunnel is considered as a is reset or copied. Therefore, if the tunnel is considered a black
black box, the packets output from any egress will be identical with box, the packets output from any egress will be identical with or
or without an update to the ingress. Nonetheless, if packets are without an update to the ingress. Nonetheless, if packets are
observed within the black box (between the tunnel endpoints), CE observed within the black box (between the tunnel endpoints), CE
markings copied by the updated ingress will be visible within the markings copied by the updated ingress will be visible within the
black box, whereas they would not have been before. Therefore, the black box, whereas they would not have been before. Therefore, the
update to encapsulation can be termed 'black-box backwards update to encapsulation can be termed 'black-box backward compatible'
compatible' (i.e. identical unless you look inside the tunnel). (i.e., identical unless you look inside the tunnel).
This specification introduces no new backward compatibility issues This specification introduces no new backward compatibility issues
when a compliant ingress talks with a legacy egress, but it has to when a compliant ingress talks with a legacy egress, but it has to
provide similar safeguards to those already defined in RFC3168. provide similar safeguards to those already defined in RFC 3168. RFC
RFC3168 laid down rules to ensure that an RFC3168 ingress turns off 3168 laid down rules to ensure that an RFC 3168 ingress turns off ECN
ECN (limited functionality mode) if it is paired with a legacy egress (limited functionality mode) if it is paired with a legacy egress
(RFC 2481, RFC2401 or RFC2003), which would not propagate ECN (RFC 2481, RFC 2401, or RFC 2003), which would not propagate ECN
correctly. The present specification carries forward those rules correctly. The present specification carries forward those rules
(Section 4.3). It uses compatibility mode whenever RFC3168 would (Section 4.3). It uses compatibility mode whenever RFC 3168 would
have used limited functionality mode, and their per-packet behaviours have used limited functionality mode, and their per-packet behaviours
are identical. Therefore, all other things being equal, an ingress are identical. Therefore, all other things being equal, an ingress
using the new rules will interwork with any legacy tunnel egress in using the new rules will interwork with any legacy tunnel egress in
exactly the same way as an RFC3168 ingress (still black-box backward exactly the same way as an RFC 3168 ingress (still black-box backward
compatible). compatible).
7. Design Principles for Alternate ECN Tunnelling Semantics 7. Design Principles for Alternate ECN Tunnelling Semantics
This section is informative not normative. This section is informative, not normative.
Section 5 of RFC3168 permits the Diffserv codepoint (DSCP)[RFC2474] Section 5 of RFC 3168 permits the Diffserv codepoint (DSCP)[RFC2474]
to 'switch in' alternative behaviours for marking the ECN field, just to 'switch in' alternative behaviours for marking the ECN field, just
as it switches in different per-hop behaviours (PHBs) for scheduling. as it switches in different per-hop behaviours (PHBs) for scheduling.
[RFC4774] gives best current practice for designing such alternative [RFC4774] gives best current practice for designing such alternative
ECN semantics and very briefly mentions in section 5.4 that ECN semantics and very briefly mentions in Section 5.4 that
tunnelling needs to be considered. The guidance below complements tunnelling needs to be considered. The guidance below complements
and extends RFC4774, giving additional guidance on designing any and extends RFC 4774, giving additional guidance on designing any
alternate ECN semantics that would also require alternate tunnelling alternate ECN semantics that would also require alternate tunnelling
semantics. semantics.
The overriding guidance is: "Avoid designing alternate ECN tunnelling The overriding guidance is: "Avoid designing alternate ECN tunnelling
semantics, if at all possible." If a scheme requires tunnels to semantics, if at all possible". If a scheme requires tunnels to
implement special processing of the ECN field for certain DSCPs, it implement special processing of the ECN field for certain DSCPs, it
will be hard to guarantee that every implementer of every tunnel will will be hard to guarantee that every implementer of every tunnel will
have added the required exception or that operators will have have added the required exception or that operators will have
ubiquitously deployed the required updates. It is unlikely a single ubiquitously deployed the required updates. It is unlikely a single
authority is even aware of all the tunnels in a network, which may authority is even aware of all the tunnels in a network, which may
include tunnels set up by applications between endpoints, or include tunnels set up by applications between endpoints, or
dynamically created in the network. Therefore it is highly likely dynamically created in the network. Therefore, it is highly likely
that some tunnels within a network or on hosts connected to it will that some tunnels within a network or on hosts connected to it will
not implement the required special case. not implement the required special case.
That said, if a non-default scheme for tunnelling the ECN field is That said, if a non-default scheme for tunnelling the ECN field is
really required, the following guidelines might prove useful in its really required, the following guidelines might prove useful in its
design: design:
On encapsulation in any alternate scheme: On encapsulation in any alternate scheme:
1. The ECN field of the outer header ought to be cleared to Not- 1. The ECN field of the outer header ought to be cleared to Not-
ECT ("00") unless it is guaranteed that the corresponding ECT ("00") unless it is guaranteed that the corresponding
tunnel egress will correctly propagate congestion markings tunnel egress will correctly propagate congestion markings
introduced across the tunnel in the outer header. introduced across the tunnel in the outer header.
2. If it has established that ECN will be correctly propagated, 2. If it has established that ECN will be correctly propagated,
an encapsulator ought to also copy incoming congestion an encapsulator also ought to copy incoming congestion
notification into the outer header. The general principle notification into the outer header. The general principle
here is that the outer header should reflect congestion here is that the outer header should reflect congestion
accumulated along the whole upstream path, not just since the accumulated along the whole upstream path, not just since the
tunnel ingress (Appendix B.3 on management and monitoring tunnel ingress (Appendix B.3 on management and monitoring
explains). explains).
In some circumstances (e.g. PCN [RFC5559] and perhaps some In some circumstances (e.g., PCN [RFC5559] and perhaps some
pseudowires [RFC5659]), the whole path is divided into pseudowires [RFC5659]), the whole path is divided into
segments, each with its own congestion notification and segments, each with its own congestion notification and
feedback loop. In these cases, the function that regulates feedback loop. In these cases, the function that regulates
load at the start of each segment will need to reset load at the start of each segment will need to reset
congestion notification for its segment. Often the point congestion notification for its segment. Often, the point
where congestion notification is reset will also be located at where congestion notification is reset will also be located at
the start of a tunnel. However, the resetting function can be the start of a tunnel. However, the resetting function can be
thought of as being applied to packets after the encapsulation thought of as being applied to packets after the encapsulation
function--two logically separate functions even though they function -- two logically separate functions even though they
might run on the same physical box. Then the code module might run on the same physical box. Then, the code module
doing encapsulation can keep to the copying rule and the load doing encapsulation can keep to the copying rule and the load
regulator module can reset congestion, without any code in regulator module can reset congestion, without any code in
either module being conditional on whether the other is there. either module being conditional on whether the other is there.
On decapsulation in any new scheme: On decapsulation in any alternate scheme:
1. If the arriving inner header is Not-ECT it implies the 1. If the arriving inner header is Not-ECT, the transport will
transport will not understand other ECN codepoints. If the not understand other ECN codepoints. If the outer header
outer header carries an explicit congestion marking, the carries an explicit congestion marking, the alternate scheme
alternate scheme would be expected to drop the packet--the would be expected to drop the packet -- the only indication of
only indication of congestion the transport will understand. congestion the transport will understand. If the alternate
If the alternate scheme recommends forwarding rather than scheme recommends forwarding rather than dropping such a
dropping such a packet, it will need to clearly justify this packet, it will need to clearly justify this decision. If the
decision. If the inner is Not-ECT and the outer carries any inner is Not-ECT and the outer carries any other ECN codepoint
other ECN codepoint that does not indicate congestion, the that does not indicate congestion, the alternate scheme can
alternate scheme can forward the packet, but probably only as forward the packet, but probably only as Not-ECT.
Not-ECT.
2. If the arriving inner header is other than Not-ECT, the ECN 2. If the arriving inner header is one other than Not-ECT, the
field that the alternate decapsulation scheme forwards ought ECN field that the alternate decapsulation scheme forwards
to reflect the more severe congestion marking of the arriving ought to reflect the more severe congestion marking of the
inner and outer headers. arriving inner and outer headers.
3. Any alternate scheme will need to define a behaviour for all 3. Any alternate scheme will need to define a behaviour for all
combinations of inner and outer headers, even those that would combinations of inner and outer headers, even those that would
not be expected to result from standards known at the time and not be expected to result from standards known at the time and
even those that would not be expected from the tunnel ingress even those that would not be expected from the tunnel ingress
paired with the egress at run-time. Consideration should be paired with the egress at run-time. Consideration should be
given to logging such unexpected combinations and raising an given to logging such unexpected combinations and raising an
alarm, particularly if there is a danger that the invalid alarm, particularly if there is a danger that the invalid
combination implies congestion signals are not being combination implies congestion signals are not being
propagated correctly. The presence of currently unused propagated correctly. The presence of currently unused
combinations may represent an attack, but the new scheme combinations may represent an attack, but the new scheme
should try to define a way to forward such packets, at least should try to define a way to forward such packets, at least
if a safe outgoing codepoint can be defined. if a safe outgoing codepoint can be defined.
Raising an alarm allows a management system to decide whether Raising an alarm allows a management system to decide whether
the anomaly is indeed an attack, in which case it can decide the anomaly is indeed an attack, in which case it can decide
to drop such packets. This is a preferable approach to hard- to drop such packets. This is a preferable approach to hard-
coded discard of packets that seem anomalous today, but may be coded discard of packets that seem anomalous today, but may be
needed tomorrow in future standards actions. needed tomorrow in future standards actions.
8. IANA Considerations (to be removed on publication): 8. Security Considerations
This memo includes no request to IANA.
9. Security Considerations
Appendix B.1 discusses the security constraints imposed on ECN tunnel Appendix B.1 discusses the security constraints imposed on ECN tunnel
processing. The new rules for ECN tunnel processing (Section 4) processing. The new rules for ECN tunnel processing (Section 4)
trade-off between information security (covert channels) and traffic trade-off between information security (covert channels) and traffic
security (congestion monitoring & control). Ensuring congestion security (congestion monitoring and control). Ensuring congestion
markings are not lost is itself an aspect of security, because if we markings are not lost is itself an aspect of security, because if we
allowed congestion notification to be lost, any attempt to enforce a allowed congestion notification to be lost, any attempt to enforce a
response to congestion would be much harder. response to congestion would be much harder.
Security issues in unlikely but possible scenarios: Security issues in unlikely, but possible, scenarios:
Tunnels intersecting Diffserv regions with alternate ECN semantics: Tunnels intersecting Diffserv regions with alternate ECN semantics:
If alternate congestion notification semantics are defined for a If alternate congestion notification semantics are defined for a
certain Diffserv PHB, the scope of the alternate semantics might certain Diffserv PHB, the scope of the alternate semantics might
typically be bounded by the limits of a Diffserv region or typically be bounded by the limits of a Diffserv region or
regions, as envisaged in [RFC4774] (e.g. the pre-congestion regions, as envisaged in [RFC4774] (e.g., the pre-congestion
notification architecture [RFC5559]). The inner headers in notification architecture [RFC5559]). The inner headers in
tunnels crossing the boundary of such a Diffserv region but ending tunnels crossing the boundary of such a Diffserv region but ending
within the region can potentially leak the external congestion within the region can potentially leak the external congestion
notification semantics into the region, or leak the internal notification semantics into the region, or leak the internal
semantics out of the region. [RFC2983] discusses the need for semantics out of the region. [RFC2983] discusses the need for
Diffserv traffic conditioning to be applied at these tunnel Diffserv traffic conditioning to be applied at these tunnel
endpoints as if they are at the edge of the Diffserv region. endpoints as if they are at the edge of the Diffserv region.
Similar concerns apply to any processing or propagation of the ECN Similar concerns apply to any processing or propagation of the ECN
field at the endpoints of tunnels with one end inside and the field at the endpoints of tunnels with one end inside and the
other outside the domain. [RFC5559] gives specific advice on this other outside the domain. [RFC5559] gives specific advice on this
for the PCN case, but other definitions of alternate semantics for the PCN case, but other definitions of alternate semantics
will need to discuss the specific security implications in each will need to discuss the specific security implications in each
case. case.
ECN nonce tunnel coverage: The new decapsulation rules improve the ECN nonce tunnel coverage: The new decapsulation rules improve the
coverage of the ECN nonce [RFC3540] relative to the previous rules coverage of the ECN nonce [RFC3540] relative to the previous rules
in RFC3168 and RFC4301. However, nonce coverage is still not in RFC 3168 and RFC 4301. However, nonce coverage is still not
perfect, as this would have led to a safety problem in another perfect, as this would have led to a safety problem in another
case. Both are corner-cases, so discussion of the compromise case. Both are corner-cases, so discussion of the compromise
between them is deferred to Appendix D. between them is deferred to Appendix D.
Covert channel not turned off: A legacy (RFC3168) tunnel ingress Covert channel not turned off: A legacy (RFC 3168) tunnel ingress
could ask an RFC3168 egress to turn off ECN processing as well as could ask an RFC 3168 egress to turn off ECN processing as well as
itself turning off ECN. An egress compliant with the present itself turning off ECN. An egress compliant with the present
specification will agree to such a request from a legacy ingress, specification will agree to such a request from a legacy ingress,
but it relies on the ingress always sending Not-ECT in the outer. but it relies on the ingress always sending Not-ECT in the outer
If the egress receives other ECN codepoints in the outer it will header. If the egress receives other ECN codepoints in the outer
process them as normal, so it will actually still copy congestion it will process them as normal, so it will actually still copy
markings from the outer to the outgoing header. Referring for congestion markings from the outer to the outgoing header.
example to Figure 5 (Appendix B.1), although the tunnel ingress Referring, for example, to Figure 5 (Appendix B.1), although the
'I' will set all ECN fields in outer headers to Not-ECT, 'M' could tunnel ingress 'I' will set all ECN fields in outer headers to
still toggle CE or ECT(1) on and off to communicate covertly with Not-ECT, 'M' could still toggle CE or ECT(1) on and off to
'B', because we have specified that 'E' only has one mode communicate covertly with 'B', because we have specified that 'E'
regardless of what mode it says it has negotiated. We could have only has one mode regardless of what mode it says it has
specified that 'E' should have a limited functionality mode and negotiated. We could have specified that 'E' should have a
check for such behaviour. But we decided not to add the extra limited functionality mode and check for such behaviour. However,
complexity of two modes on a compliant tunnel egress merely to we decided not to add the extra complexity of two modes on a
cater for an historic security concern that is now considered compliant tunnel egress merely to cater for an historic security
manageable. concern that is now considered manageable.
10. Conclusions 9. Conclusions
This document allows tunnels to propagate an extra level of This document allows tunnels to propagate an extra level of
congestion severity. It uses previously unused combinations of inner congestion severity. It uses previously unused combinations of inner
and outer header to augment the rules for calculating the ECN field and outer headers to augment the rules for calculating the ECN field
when decapsulating IP packets at the egress of IPsec (RFC4301) and when decapsulating IP packets at the egress of IPsec (RFC 4301) and
non-IPsec (RFC3168) tunnels. non-IPsec (RFC 3168) tunnels.
This document also updates the ingress tunnelling encapsulation of This document also updates the ingress tunnelling encapsulation of
RFC3168 ECN to bring all IP in IP tunnels into line with the new RFC 3168 ECN to bring all IP-in-IP tunnels into line with the new
behaviour in the IPsec architecture of RFC4301, which copies rather behaviour in the IPsec architecture of RFC 4301, which copies rather
than resets the ECN field when creating outer headers. than resets the ECN field when creating outer headers.
The need for both these updated behaviours was triggered by the The need for both these updated behaviours was triggered by the
introduction of pre-congestion notification (PCN) onto the IETF introduction of pre-congestion notification (PCN) onto the IETF
standards track. Operators wanting to support PCN or other alternate Standards Track. Operators wanting to support PCN or other alternate
ECN schemes that use an extra severity level can require that their ECN schemes that use an extra severity level can require that their
tunnels comply with the present specification. This is not a fork in tunnels comply with the present specification. This is not a fork in
the RFC series, it is an update that can be deployed first by those the RFC series, it is an update that can be deployed first by those
that need it, and subsequently by all tunnel endpoint implementations that need it, and subsequently by all tunnel endpoint implementations
during general code maintenance. It is backward compatible with all during general code maintenance. It is backward compatible with all
previous tunnelling behaviours, so existing single severity level previous tunnelling behaviours, so existing single severity level
schemes will continue to work as before, but support for two severity schemes will continue to work as before, but support for two severity
levels will gradually be added to the Internet. levels will gradually be added to the Internet.
The new rules propagate changes to the ECN field across tunnel end- The new rules propagate changes to the ECN field across tunnel
points that previously blocked them to restrict the bandwidth of a endpoints that previously blocked them to restrict the bandwidth of a
potential covert channel. Limiting the channel's bandwidth to 2 bits potential covert channel. Limiting the channel's bandwidth to two
per packet is now considered sufficient. bits per packet is now considered sufficient.
At the same time as removing these legacy constraints, the At the same time as removing these legacy constraints, the
opportunity has been taken to draw together diverging tunnel opportunity has been taken to draw together diverging tunnel
specifications into a single consistent behaviour. Then any tunnel specifications into a single consistent behaviour. Then, any tunnel
can be deployed unilaterally, and it will support the full range of can be deployed unilaterally, and it will support the full range of
congestion control and management schemes without any modes or congestion control and management schemes without any modes or
configuration. Further, any host or router can expect the ECN field configuration. Further, any host or router can expect the ECN field
to behave in the same way, whatever type of tunnel might intervene in to behave in the same way, whatever type of tunnel might intervene in
the path. This new certainty could enable new uses of the ECN field the path. This new certainty could enable new uses of the ECN field
that would otherwise be confounded by ambiguity. that would otherwise be confounded by ambiguity.
11. Acknowledgements 10. Acknowledgements
Thanks to David Black for his insightful reviews and patient Thanks to David Black for his insightful reviews and patient
explanations of better ways to think about function placement and explanations of better ways to think about function placement and
alarms. Thanks to David and to Anil Agarwal for pointing out cases alarms. Thanks to David and to Anil Agarwal for pointing out cases
where it is safe to forward CU combinations of headers. Also thanks where it is safe to forward CU combinations of headers. Also, thanks
to Arnaud Jacquet for the idea for Appendix C. Thanks to Gorry to Arnaud Jacquet for the idea for Appendix C. Thanks to Gorry
Fairhurst, Teco Boot, Michael Menth, Bruce Davie, Toby Moncaster, Fairhurst, Teco Boot, Michael Menth, Bruce Davie, Toby Moncaster,
Sally Floyd, Alfred Hoenes, Gabriele Corliano, Ingemar Johansson, Sally Floyd, Alfred Hoenes, Gabriele Corliano, Ingemar Johansson,
Philip Eardley and David Harrington for their thoughts and careful Philip Eardley, and David Harrington for their thoughts and careful
review comments, and to Stephen Hanna, Ben Campbell and members of review comments, and to Stephen Hanna, Ben Campbell, and members of
the IESG for respectively conducting the Security Directorate, the IESG for respectively conducting the Security Directorate,
General Area and IESG reviews. General Area, and IESG reviews.
Bob Briscoe is partly funded by Trilogy, a research project (ICT- Bob Briscoe is partly funded by Trilogy, a research project (ICT-
216372) supported by the European Community under its Seventh 216372) supported by the European Community under its Seventh
Framework Programme. Framework Programme.
Comments Solicited (to be removed by the RFC Editor): 11. References
Comments and questions are encouraged and very welcome. They can be
addressed to the IETF Transport Area working group mailing list
<tsvwg@ietf.org>, and/or to the authors.
12. References
12.1. Normative References 11.1. Normative References
[RFC2003] Perkins, C., "IP Encapsulation within [RFC2003] Perkins, C., "IP Encapsulation within IP", RFC 2003,
IP", RFC 2003, October 1996. October 1996.
[RFC2119] Bradner, S., "Key words for use in [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
RFCs to Indicate Requirement Levels", Requirement Levels", BCP 14, RFC 2119, March 1997.
BCP 14, RFC 2119, March 1997.
[RFC3168] Ramakrishnan, K., Floyd, S., and D. [RFC3168] Ramakrishnan, K., Floyd, S., and D. Black, "The Addition
Black, "The Addition of Explicit of Explicit Congestion Notification (ECN) to IP",
Congestion Notification (ECN) to IP", RFC 3168, September 2001.
RFC 3168, September 2001.
[RFC4301] Kent, S. and K. Seo, "Security [RFC4301] Kent, S. and K. Seo, "Security Architecture for the
Architecture for the Internet Internet Protocol", RFC 4301, December 2005.
Protocol", RFC 4301, December 2005.
12.2. Informative References 11.2. Informative References
[I-D.ietf-pcn-3-in-1-encoding] Briscoe, B., Moncaster, T., and M. [PCN3in1] Briscoe, B., Moncaster, T., and M. Menth, "Encoding 3 PCN-
Menth, "Encoding 3 PCN-States in the States in the IP header using a single DSCP", Work
IP header using a single DSCP", in Progress, July 2010.
draft-ietf-pcn-3-in-1-encoding-03
(work in progress), July 2010.
[RFC2401] Kent, S. and R. Atkinson, "Security [RFC2401] Kent, S. and R. Atkinson, "Security Architecture for the
Architecture for the Internet Internet Protocol", RFC 2401, November 1998.
Protocol", RFC 2401, November 1998.
[RFC2474] Nichols, K., Blake, S., Baker, F., [RFC2474] Nichols, K., Blake, S., Baker, F., and D. Black,
and D. Black, "Definition of the "Definition of the Differentiated Services Field (DS
Differentiated Services Field (DS Field) in the IPv4 and IPv6 Headers", RFC 2474,
Field) in the IPv4 and IPv6 Headers", December 1998.
RFC 2474, December 1998.
[RFC2481] Ramakrishnan, K. and S. Floyd, "A [RFC2481] Ramakrishnan, K. and S. Floyd, "A Proposal to add Explicit
Proposal to add Explicit Congestion Congestion Notification (ECN) to IP", RFC 2481,
Notification (ECN) to IP", RFC 2481, January 1999.
January 1999.
[RFC2983] Black, D., "Differentiated Services [RFC2983] Black, D., "Differentiated Services and Tunnels",
and Tunnels", RFC 2983, October 2000. RFC 2983, October 2000.
[RFC3540] Spring, N., Wetherall, D., and D. [RFC3540] Spring, N., Wetherall, D., and D. Ely, "Robust Explicit
Ely, "Robust Explicit Congestion Congestion Notification (ECN) Signaling with Nonces",
Notification (ECN) Signaling with RFC 3540, June 2003.
Nonces", RFC 3540, June 2003.
[RFC4306] Kaufman, C., "Internet Key Exchange [RFC4774] Floyd, S., "Specifying Alternate Semantics for the
(IKEv2) Protocol", RFC 4306, Explicit Congestion Notification (ECN) Field", BCP 124,
December 2005. RFC 4774, November 2006.
[RFC4774] Floyd, S., "Specifying Alternate [RFC5129] Davie, B., Briscoe, B., and J. Tay, "Explicit Congestion
Semantics for the Explicit Congestion Marking in MPLS", RFC 5129, January 2008.
Notification (ECN) Field", BCP 124,
RFC 4774, November 2006.
[RFC5129] Davie, B., Briscoe, B., and J. Tay, [RFC5559] Eardley, P., "Pre-Congestion Notification (PCN)
"Explicit Congestion Marking in Architecture", RFC 5559, June 2009.
MPLS", RFC 5129, January 2008.
[RFC5559] Eardley, P., "Pre-Congestion [RFC5659] Bocci, M. and S. Bryant, "An Architecture for Multi-
Notification (PCN) Architecture", Segment Pseudowire Emulation Edge-to-Edge", RFC 5659,
RFC 5559, June 2009. October 2009.
[RFC5659] Bocci, M. and S. Bryant, "An [RFC5670] Eardley, P., "Metering and Marking Behaviour of PCN-
Architecture for Multi-Segment Nodes", RFC 5670, November 2009.
Pseudowire Emulation Edge-to-Edge",
RFC 5659, October 2009.
[RFC5670] Eardley, P., "Metering and Marking [RFC5696] Moncaster, T., Briscoe, B., and M. Menth, "Baseline
Behaviour of PCN-Nodes", RFC 5670, Encoding and Transport of Pre-Congestion Information",
November 2009. RFC 5696, November 2009.
[RFC5696] Moncaster, T., Briscoe, B., and M. [RFC5996] Kaufman, C., Hoffman, P., Nir, Y., and P. Eronen,
Menth, "Baseline Encoding and "Internet Key Exchange Protocol Version 2 (IKEv2)",
Transport of Pre-Congestion RFC 5996, September 2010.
Information", RFC 5696,
November 2009.
[VCP] Xia, Y., Subramanian, L., Stoica, I., [VCP] Xia, Y., Subramanian, L., Stoica, I., and S. Kalyanaraman,
and S. Kalyanaraman, "One more bit is "One more bit is enough", Proc. SIGCOMM'05, ACM
enough", Proc. SIGCOMM'05, ACM CCR 35(4)37--48, 2005,
CCR 35(4)37--48, 2005, <http:// <http://doi.acm.org/10.1145/1080091.1080098>.
doi.acm.org/10.1145/1080091.1080098>.
Appendix A. Early ECN Tunnelling RFCs Appendix A. Early ECN Tunnelling RFCs
IP in IP tunnelling was originally defined in [RFC2003]. On IP-in-IP tunnelling was originally defined in [RFC2003]. On
encapsulation, the incoming header was copied to the outer and on encapsulation, the incoming header was copied to the outer and on
decapsulation the outer was simply discarded. Initially, IPsec decapsulation, the outer was simply discarded. Initially, IPsec
tunnelling [RFC2401] followed the same behaviour. tunnelling [RFC2401] followed the same behaviour.
When ECN was introduced experimentally in [RFC2481], legacy (RFC2003 When ECN was introduced experimentally in [RFC2481], legacy (RFC 2003
or RFC2401) tunnels would have discarded any congestion markings or RFC 2401) tunnels would have discarded any congestion markings
added to the outer header, so RFC2481 introduced rules for added to the outer header, so RFC 2481 introduced rules for
calculating the outgoing header from a combination of the inner and calculating the outgoing header from a combination of the inner and
outer on decapsulation. RC2481 also introduced a second mode for outer on decapsulation. RFC 2481 also introduced a second mode for
IPsec tunnels, which turned off ECN processing (Not-ECT) in the outer IPsec tunnels, which turned off ECN processing (Not-ECT) in the outer
header on encapsulation because an RFC2401 decapsulator would discard header on encapsulation because an RFC 2401 decapsulator would
the outer on decapsulation. For RFC2401 IPsec this had the side- discard the outer on decapsulation. For RFC 2401 IPsec, this had the
effect of completely blocking the covert channel. side effect of completely blocking the covert channel.
In RFC2481 the ECN field was defined as two separate bits. But when In RFC 2481, the ECN field was defined as two separate bits. But
ECN moved from the experimental to the standards track [RFC3168], the when ECN moved from Experimental to Standards Track [RFC3168], the
ECN field was redefined as four codepoints. This required a ECN field was redefined as four codepoints. This required a
different calculation of the ECN field from that used in RFC2481 on different calculation of the ECN field from that used in RFC 2481 on
decapsulation. RFC3168 also had two modes; a 'full functionality decapsulation. RFC 3168 also had two modes; a 'full functionality
mode' that restricted the covert channel as much as possible but mode' that restricted the covert channel as much as possible but
still allowed ECN to be used with IPsec, and another that completely still allowed ECN to be used with IPsec, and another that completely
turned off ECN processing across the tunnel. This 'limited turned off ECN processing across the tunnel. This 'limited
functionality mode' both offered a way for operators to completely functionality mode' both offered a way for operators to completely
block the covert channel and allowed an RFC3168 ingress to interwork block the covert channel and allowed an RFC 3168 ingress to interwork
with a legacy tunnel egress (RFC2481, RFC2401 or RFC2003). with a legacy tunnel egress (RFC 2481, RFC 2401, or RFC 2003).
The present specification includes a similar compatibility mode to The present specification includes a similar compatibility mode to
interwork safely with tunnels compliant with any of these three interwork safely with tunnels compliant with any of these three
earlier RFCs. However, unlike RFC3168, it is only a mode of the earlier RFCs. However, unlike RFC 3168, it is only a mode of the
ingress, as decapsulation behaviour is the same in either case. ingress, as decapsulation behaviour is the same in either case.
Appendix B. Design Constraints Appendix B. Design Constraints
Tunnel processing of a congestion notification field has to meet Tunnel processing of a congestion notification field has to meet
congestion control and management needs without creating new congestion control and management needs without creating new
information security vulnerabilities (if information security is information security vulnerabilities (if information security is
required). This appendix documents the analysis of the tradeoffs required). This appendix documents the analysis of the trade-offs
between these factors that led to the new encapsulation rules in between these factors that led to the new encapsulation rules in
Section 4.1. Section 4.1.
B.1. Security Constraints B.1. Security Constraints
Information security can be assured by using various end to end Information security can be assured by using various end-to-end
security solutions (including IPsec in transport mode [RFC4301]), but security solutions (including IPsec in transport mode [RFC4301]), but
a commonly used scenario involves the need to communicate between two a commonly used scenario involves the need to communicate between two
physically protected domains across the public Internet. In this physically protected domains across the public Internet. In this
case there are certain management advantages to using IPsec in tunnel case, there are certain management advantages to using IPsec in
mode solely across the publicly accessible part of the path. The tunnel mode solely across the publicly accessible part of the path.
path followed by a packet then crosses security 'domains'; the ones The path followed by a packet then crosses security 'domains'; the
protected by physical or other means before and after the tunnel and ones protected by physical or other means before and after the tunnel
the one protected by an IPsec tunnel across the otherwise unprotected and the one protected by an IPsec tunnel across the otherwise
domain. The scenario in Figure 5 will be used where endpoints 'A' unprotected domain. The scenario in Figure 5 will be used where
and 'B' communicate through a tunnel. The tunnel ingress 'I' and endpoints 'A' and 'B' communicate through a tunnel. The tunnel
egress 'E' are within physically protected edge domains, while the ingress 'I' and egress 'E' are within physically protected edge
tunnel spans an unprotected internetwork where there may be 'men in domains, while the tunnel spans an unprotected internetwork where
the middle', M. there may be 'men in the middle', M.
physically unprotected physically physically unprotected physically
<-protected domain-><--domain--><-protected domain-> <-protected domain-><--domain--><-protected domain->
+------------------+ +------------------+ +------------------+ +------------------+
| | M | | | | M | |
| A-------->I=========>==========>E-------->B | | A-------->I=========>==========>E-------->B |
| | | | | | | |
+------------------+ +------------------+ +------------------+ +------------------+
<----IPsec secured----> <----IPsec secured---->
tunnel tunnel
skipping to change at page 37, line 29 skipping to change at page 30, line 36
Figure 5: IPsec Tunnel Scenario Figure 5: IPsec Tunnel Scenario
IPsec encryption is typically used to prevent 'M' seeing messages IPsec encryption is typically used to prevent 'M' seeing messages
from 'A' to 'B'. IPsec authentication is used to prevent 'M' from 'A' to 'B'. IPsec authentication is used to prevent 'M'
masquerading as the sender of messages from 'A' to 'B' or altering masquerading as the sender of messages from 'A' to 'B' or altering
their contents. 'I' can use IPsec tunnel mode to allow 'A' to their contents. 'I' can use IPsec tunnel mode to allow 'A' to
communicate with 'B', but impose encryption to prevent 'A' leaking communicate with 'B', but impose encryption to prevent 'A' leaking
information to 'M'. Or 'E' can insist that 'I' uses tunnel mode information to 'M'. Or 'E' can insist that 'I' uses tunnel mode
authentication to prevent 'M' communicating information to 'B'. authentication to prevent 'M' communicating information to 'B'.
Mutable IP header fields such as the ECN field (as well as the TTL/ Mutable IP header fields such as the ECN field (as well as the Time
Hop Limit and DS fields) cannot be included in the cryptographic to Live (TTL) / Hop Limit and DS fields) cannot be included in the
calculations of IPsec. Therefore, if 'I' copies these mutable fields cryptographic calculations of IPsec. Therefore, if 'I' copies these
into the outer header that is exposed across the tunnel it will have mutable fields into the outer header that is exposed across the
allowed a covert channel from 'A' to M that bypasses its encryption tunnel it will have allowed a covert channel from 'A' to 'M' that
of the inner header. And if 'E' copies these fields from the outer bypasses its encryption of the inner header. And if 'E' copies these
header to the inner, even if it validates authentication from 'I', it fields from the outer header to the outgoing, even if it validates
will have allowed a covert channel from 'M' to 'B'. authentication from 'I', it will have allowed a covert channel from
'M' to 'B'.
ECN at the IP layer is designed to carry information about congestion ECN at the IP layer is designed to carry information about congestion
from a congested resource towards downstream nodes. Typically a from a congested resource towards downstream nodes. Typically, a
downstream transport might feed the information back somehow to the downstream transport might feed the information back somehow to the
point upstream of the congestion that can regulate the load on the point upstream of the congestion that can regulate the load on the
congested resource, but other actions are possible [RFC3168; section congested resource, but other actions are possible [RFC3168], Section
6]. In terms of the above unicast scenario, ECN effectively intends 6. In terms of the above unicast scenario, ECN effectively intends
to create an information channel (for congestion signalling) from 'M' to create an information channel (for congestion signalling) from 'M'
to 'B' (for 'B' to feed back to 'A'). Therefore the goals of IPsec to 'B' (for 'B' to feed back to 'A'). Therefore, the goals of IPsec
and ECN are mutually incompatible, requiring some compromise. and ECN are mutually incompatible, requiring some compromise.
With respect to using the DS or ECN fields as covert channels, With respect to using the DS or ECN fields as covert channels,
section 5.1.2 of RFC4301 says, "controls are provided to manage the Section 5.1.2 of RFC 4301 says, "controls are provided to manage the
bandwidth of this channel". Using the ECN processing rules of bandwidth of this channel". Using the ECN processing rules of RFC
RFC4301, the channel bandwidth is two bits per datagram from 'A' to 4301, the channel bandwidth is two bits per datagram from 'A' to 'M'
'M' and one bit per datagram from 'M' to 'A' (because 'E' limits the and one bit per datagram from 'M' to 'B' (because 'E' limits the
combinations of the 2-bit ECN field that it will copy). In both combinations of the 2-bit ECN field that it will copy). In both
cases the covert channel bandwidth is further reduced by noise from cases, the covert channel bandwidth is further reduced by noise from
any real congestion marking. RFC4301 implies that these covert any real congestion marking. RFC 4301 implies that these covert
channels are sufficiently limited to be considered a manageable channels are sufficiently limited to be considered a manageable
threat. However, with respect to the larger (6b) DS field, the same threat. However, with respect to the larger (six-bit) DS field, the
section of RFC4301 says not copying is the default, but a same section of RFC 4301 says not copying is the default, but a
configuration option can allow copying "to allow a local configuration option can allow copying "to allow a local
administrator to decide whether the covert channel provided by administrator to decide whether the covert channel provided by
copying these bits outweighs the benefits of copying". Of course, an copying these bits outweighs the benefits of copying". Of course, an
administrator considering copying of the DS field has to take into administrator who plans to copy the DS field has to take into account
account that it could be concatenated with the ECN field giving an 8b that it could be concatenated with the ECN field, creating a covert
per datagram covert channel. channel with eight bits per datagram.
For tunnelling the 6b Diffserv field two conceptual models have had For tunnelling the six-bit Diffserv field, two conceptual models have
to be defined so that administrators can trade off security against had to be defined so that administrators can trade off security
the needs of traffic conditioning [RFC2983]: against the needs of traffic conditioning [RFC2983]:
The uniform model: where the Diffserv field is preserved end-to-end The uniform model: where the Diffserv field is preserved end-to-end
by copying into the outer header on encapsulation and copying from by copying into the outer header on encapsulation and copying from
the outer header on decapsulation. the outer header on decapsulation.
The pipe model: where the outer header is independent of that in the The pipe model: where the outer header is independent of that in the
inner header so it hides the Diffserv field of the inner header inner header so it hides the Diffserv field of the inner header
from any interaction with nodes along the tunnel. from any interaction with nodes along the tunnel.
However, for ECN, the new IPsec security architecture in RFC4301 only However, for ECN, the new IPsec security architecture in RFC 4301
standardised one tunnelling model equivalent to the uniform model. only standardised one tunnelling model equivalent to the uniform
It deemed that simplicity was more important than allowing model. It deemed that simplicity was more important than allowing
administrators the option of a tiny increment in security, especially administrators the option of a tiny increment in security, especially
given not copying congestion indications could seriously harm given not copying congestion indications could seriously harm
everyone's network service. everyone's network service.
B.2. Control Constraints B.2. Control Constraints
Congestion control requires that any congestion notification marked Congestion control requires that any congestion notification marked
into packets by a resource will be able to traverse a feedback loop into packets by a resource will be able to traverse a feedback loop
back to a function capable of controlling the load on that resource. back to a function capable of controlling the load on that resource.
To be precise, rather than calling this function the data source, it To be precise, rather than calling this function the data source, it
will be called the Load Regulator. This allows for exceptional cases will be called the 'Load Regulator'. This allows for exceptional
where load is not regulated by the data source, but usually the two cases where load is not regulated by the data source, but usually the
terms will be synonymous. Note the term "a function _capable of_ two terms will be synonymous. Note the term "a function _capable of_
controlling the load" deliberately includes a source application that controlling the load" deliberately includes a source application that
doesn't actually control the load but ought to (e.g. an application doesn't actually control the load but ought to (e.g., an application
without congestion control that uses UDP). without congestion control that uses UDP).
A--->R--->I=========>M=========>E-------->B A--->R--->I=========>M=========>E-------->B
Figure 6: Simple Tunnel Scenario Figure 6: Simple Tunnel Scenario
A similar tunnelling scenario to the IPsec one just described will A similar tunnelling scenario to the IPsec one just described will
now be considered, but without the different security domains, now be considered, but without the different security domains,
because the focus now shifts to whether the control loop and because the focus now shifts to whether the control loop and
management monitoring work (Figure 6). If resources in the tunnel management monitoring work (Figure 6). If resources in the tunnel
are to be able to explicitly notify congestion and the feedback path are to be able to explicitly notify congestion and the feedback path
is from 'B' to 'A', it will certainly be necessary for 'E' to copy is from 'B' to 'A', it will certainly be necessary for 'E' to copy
any CE marking from the outer header to the inner header for onward any CE marking from the outer header to the outgoing header for
transmission to 'B', otherwise congestion notification from resources onward transmission to 'B'; otherwise, congestion notification from
like 'M' cannot be fed back to the Load Regulator ('A'). But it does resources like 'M' cannot be fed back to the Load Regulator ('A').
not seem necessary for 'I' to copy CE markings from the inner to the But it does not seem necessary for 'I' to copy CE markings from the
outer header. For instance, if resource 'R' is congested, it can incoming to the outer header. For instance, if resource 'R' is
send congestion information to 'B' using the congestion field in the congested, it can send congestion information to 'B' using the
inner header without 'I' copying the congestion field into the outer congestion field in the inner header without 'I' copying the
header and 'E' copying it back to the inner header. 'E' can still congestion field into the outer header and 'E' copying it back to the
write any additional congestion marking introduced across the tunnel outgoing header. 'E' can still write any additional congestion
into the congestion field of the inner header. marking introduced across the tunnel into the congestion field of the
outgoing header.
All this shows that 'E' can preserve the control loop irrespective of All this shows that 'E' can preserve the control loop irrespective of
whether 'I' copies congestion notification into the outer header or whether 'I' copies congestion notification into the outer header or
resets it. resets it.
That is the situation for existing control arrangements but, because That is the situation for existing control arrangements but, because
copying reveals more information, it would open up possibilities for copying reveals more information, it would open up possibilities for
better control system designs. For instance, resetting CE marking on better control system designs. For instance, resetting CE marking on
encapsulation breaks the standards track PCN congestion marking encapsulation breaks the Standards-Track PCN congestion marking
scheme [RFC5670]. It ends up removing excessive amounts of traffic scheme [RFC5670]. It ends up removing excessive amounts of traffic
unnecessarily. Whereas copying CE markings at ingress leads to the unnecessarily (Section 5.3.1). Whereas copying CE markings at
correct control behaviour. ingress leads to the correct control behaviour.
B.3. Management Constraints B.3. Management Constraints
As well as control, there are also management constraints. As well as control, there are also management constraints.
Specifically, a management system may monitor congestion markings in Specifically, a management system may monitor congestion markings in
passing packets, perhaps at the border between networks as part of a passing packets, perhaps at the border between networks as part of a
service level agreement. For instance, monitors at the borders of service level agreement. For instance, monitors at the borders of
autonomous systems may need to measure how much congestion has autonomous systems may need to measure how much congestion has
accumulated so far along the path, perhaps to determine between them accumulated so far along the path, perhaps to determine between them
how much of the congestion is contributed by each domain. how much of the congestion is contributed by each domain.
In this document the baseline of congestion marking (or the In this document, the baseline of congestion marking (or the
Congestion Baseline) is defined as the source of the layer that Congestion Baseline) is defined as the source of the layer that
created (or most recently reset) the congestion notification field. created (or most recently reset) the congestion notification field.
When monitoring congestion it would be desirable if the Congestion When monitoring congestion, it would be desirable if the Congestion
Baseline did not depend on whether packets were tunnelled or not. Baseline did not depend on whether or not packets were tunnelled.
Given some tunnels cross domain borders (e.g. consider M in Figure 6 Given some tunnels cross domain borders (e.g., consider 'M' in
is monitoring a border), it would therefore be desirable for 'I' to Figure 6 is monitoring a border), it would therefore be desirable for
copy congestion accumulated so far into the outer headers, so that it 'I' to copy congestion accumulated so far into the outer headers, so
is exposed across the tunnel. that it is exposed across the tunnel.
For management purposes it might be useful for the tunnel egress to For management purposes, it might be useful for the tunnel egress to
be able to monitor whether congestion occurred across a tunnel or be able to monitor whether congestion occurred across a tunnel or
upstream of it. Superficially it appears that copying congestion upstream of it. Superficially, it appears that copying congestion
markings at the ingress would make this difficult, whereas it was markings at the ingress would make this difficult, whereas it was
straightforward when an RFC3168 ingress reset them. However, straightforward when an RFC 3168 ingress reset them. However,
Appendix C gives a simple and precise method for a tunnel egress to Appendix C gives a simple and precise method for a tunnel egress to
infer the congestion level introduced across a tunnel. It works infer the congestion level introduced across a tunnel. It works
irrespective of whether the ingress copies or resets congestion irrespective of whether the ingress copies or resets congestion
markings. markings.
Appendix C. Contribution to Congestion across a Tunnel Appendix C. Contribution to Congestion across a Tunnel
This specification mandates that a tunnel ingress determines the ECN This specification mandates that a tunnel ingress determines the ECN
field of each new outer tunnel header by copying the arriving header. field of each new outer tunnel header by copying the arriving header.
Concern has been expressed that this will make it difficult for the Concern has been expressed that this will make it difficult for the
tunnel egress to monitor congestion introduced only along a tunnel, tunnel egress to monitor congestion introduced only along a tunnel,
which is easy if the outer ECN field is reset at a tunnel ingress which is easy if the outer ECN field is reset at a tunnel ingress
(RFC3168 full functionality mode). However, in fact copying CE marks (RFC 3168 full functionality mode). However, in fact copying CE
at ingress will still make it easy for the egress to measure marks at ingress will still make it easy for the egress to measure
congestion introduced across a tunnel, as illustrated below. congestion introduced across a tunnel, as illustrated below.
Consider 100 packets measured at the egress. Say it measures that 30 Consider 100 packets measured at the egress. Say it measures that 30
are CE marked in the inner and outer headers and 12 have additional are CE marked in the inner and outer headers and 12 have additional
CE marks in the outer but not the inner. This means packets arriving CE marks in the outer but not the inner. This means packets arriving
at the ingress had already experienced 30% congestion. However, it at the ingress had already experienced 30% congestion. However, it
does not mean there was 12% congestion across the tunnel. The does not mean there was 12% congestion across the tunnel. The
correct calculation of congestion across the tunnel is p_t = 12/ correct calculation of congestion across the tunnel is p_t = 12/
(100-30) = 12/70 = 17%. This is easy for the egress to measure. It (100-30) = 12/70 = 17%. This is easy for the egress to measure. It
is simply the proportion of packets not marked in the inner header is simply the proportion of packets not marked in the inner header
(70) that have a CE marking in the outer header (12). This technique (70) that have a CE marking in the outer header (12). This technique
works whether the ingress copies or resets CE markings, so it can be works whether the ingress copies or resets CE markings, so it can be
used by an egress that is not sure which RFC the ingress complies used by an egress that is not sure with which RFC the ingress
with. complies.
Figure 7 illustrates this in a combinatorial probability diagram. Figure 7 illustrates this in a combinatorial probability diagram.
The square represents 100 packets. The 30% division along the bottom The square represents 100 packets. The 30% division along the bottom
represents marking before the ingress, and the p_t division up the represents marking before the ingress, and the p_t division up the
side represents marking introduced across the tunnel. side represents marking introduced across the tunnel.
^ outer header marking ^ outer header marking
| |
100% +-----+---------+ The large square 100% +-----+---------+ The large square
| | | represents 100 packets | | | represents 100 packets
skipping to change at page 41, line 21 skipping to change at page 34, line 26
p_t + +---------+ = 12/70 p_t + +---------+ = 12/70
| | 12 | = 17% | | 12 | = 17%
0 +-----+---------+---> 0 +-----+---------+--->
0 30% 100% inner header marking 0 30% 100% inner header marking
Figure 7: Tunnel Marking of Packets Already Marked at Ingress Figure 7: Tunnel Marking of Packets Already Marked at Ingress
Appendix D. Compromise on Decap with ECT(1) Inner and ECT(0) Outer Appendix D. Compromise on Decap with ECT(1) Inner and ECT(0) Outer
A packet with an ECT(1) inner and an ECT(0) outer should never arise A packet with an ECT(1) inner and an ECT(0) outer should never arise
from any known IETF protocol. Without giving a reason, RFC3168 and from any known IETF protocol. Without giving a reason, RFC 3168 and
RFC4301 both say the outer should be ignored when decapsulating such RFC 4301 both say the outer should be ignored when decapsulating such
a packet. This appendix explains why it was decided not to change a packet. This appendix explains why it was decided not to change
this advice. this advice.
In summary, ECT(0) always means 'not congested' and ECT(1) may imply In summary, ECT(0) always means 'not congested' and ECT(1) may imply
the same [RFC3168] or it may imply a higher severity congestion the same [RFC3168] or it may imply a higher severity congestion
signal [RFC4774], [I-D.ietf-pcn-3-in-1-encoding], depending on the signal [RFC4774], [PCN3in1], depending on the transport in use.
transport in use. Whether they mean the same or not, at the ingress Whether or not they mean the same, at the ingress the outer should
the outer should have started the same as the inner and only a broken have started the same as the inner, and only a broken or compromised
or compromised router could have changed the outer to ECT(0). router could have changed the outer to ECT(0).
The decapsulator can detect this anomaly. But the question is, The decapsulator can detect this anomaly. But the question is,
should it correct the anomaly by ignoring the outer, or should it should it correct the anomaly by ignoring the outer, or should it
reveal the anomaly to the end-to-end transport by forwarding the reveal the anomaly to the end-to-end transport by forwarding the
outer? outer?
On balance, it was decided that the decapsulator should correct the On balance, it was decided that the decapsulator should correct the
anomaly, but log the event and optionally raise an alarm. This is anomaly, but log the event and optionally raise an alarm. This is
the safe action if ECT(1) is being used as a more severe marking than the safe action if ECT(1) is being used as a more severe marking than
ECT(0), because it passes the more severe signal to the transport. ECT(0), because it passes the more severe signal to the transport.
skipping to change at page 42, line 23 skipping to change at page 35, line 30
seems to require a similar compromise. However, because that case is seems to require a similar compromise. However, because that case is
reversed, no compromise is necessary; it is best to forward the outer reversed, no compromise is necessary; it is best to forward the outer
whether the transport expects the ECT(1) to mean a higher severity whether the transport expects the ECT(1) to mean a higher severity
than ECT(0) or the same severity. Forwarding the outer either than ECT(0) or the same severity. Forwarding the outer either
preserves a higher value (if it is higher) or it reveals an anomaly preserves a higher value (if it is higher) or it reveals an anomaly
to the transport (if the two ECT codepoints mean the same severity). to the transport (if the two ECT codepoints mean the same severity).
Appendix E. Open Issues Appendix E. Open Issues
The new decapsulation behaviour defined in Section 4.2 adds support The new decapsulation behaviour defined in Section 4.2 adds support
for propagation of 2 severity levels of congestion. However for propagation of two severity levels of congestion. However,
transports have no way to discover whether there are any legacy transports have no way to discover whether there are any legacy
tunnels on their path that will not propagate 2 severity levels. It tunnels on their path that will not propagate two severity levels.
would have been nice to add a feature for transports to check path It would have been nice to add a feature for transports to check path
support, but this remains an open issue that will have to be support, but this remains an open issue that will have to be
addressed in any future standards action to define an end-to-end addressed in any future standards action to define an end-to-end
scheme that requires 2-severity levels of congestion. PCN avoids scheme that requires two severity levels of congestion. PCN avoids
this problem because it is only for a controlled region, so all this problem because it is only for a controlled region, so all
legacy tunnels can be upgraded by the same operator that deploys PCN. legacy tunnels can be upgraded by the same operator that deploys PCN.
Author's Address Author's Address
Bob Briscoe Bob Briscoe
BT BT
B54/77, Adastral Park B54/77, Adastral Park
Martlesham Heath Martlesham Heath
Ipswich IP5 3RE Ipswich IP5 3RE
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