Transport Area Working Group B. Briscoe
Internet-Draft BT
Intended status: Standards Track March 24, 2009
Expires: September 25, 2009
Tunnelling of Explicit Congestion Notification
draft-ietf-tsvwg-ecn-tunnel-02
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Abstract
This document redefines how the explicit congestion notification
(ECN) field of the IP header should be constructed on entry to and
exit from any IP in IP tunnel. On encapsulation it brings all IP in
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IP tunnels (v4 or v6) into line with the way RFC4301 IPsec tunnels
now construct the ECN field. On decapsulation it redefines how the
ECN field in the forwarded IP header should be calculated for two
previously invalid combinations of incoming inner and outer headers,
in order that these combinations may be usefully employed in future
standards actions. It includes a thorough analysis of the reasoning
for these changes and the implications.
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 6
1.1. Scope . . . . . . . . . . . . . . . . . . . . . . . . . . 8
1.2. Document Roadmap . . . . . . . . . . . . . . . . . . . . . 9
2. Requirements Language . . . . . . . . . . . . . . . . . . . . 9
3. Summary of Pre-Existing RFCs . . . . . . . . . . . . . . . . . 10
3.1. Encapsulation at Tunnel Ingress . . . . . . . . . . . . . 10
3.2. Decapsulation at Tunnel Egress . . . . . . . . . . . . . . 12
4. New ECN Tunnelling Rules . . . . . . . . . . . . . . . . . . . 13
4.1. Default Tunnel Ingress Behaviour . . . . . . . . . . . . . 14
4.2. Default Tunnel Egress Behaviour . . . . . . . . . . . . . 14
4.3. Design Principles for Future Non-Default Schemes . . . . . 16
5. Backward Compatibility . . . . . . . . . . . . . . . . . . . . 17
5.1. Non-Issues Upgrading Any Tunnel Decapsulation . . . . . . 18
5.2. Non-Issues for RFC4301 IPsec Encapsulation . . . . . . . . 18
5.3. Upgrading Other IP in IP Tunnel Encapsulators . . . . . . 19
6. Changes from Earlier RFCs . . . . . . . . . . . . . . . . . . 20
7. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 21
8. Security Considerations . . . . . . . . . . . . . . . . . . . 21
9. Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . 23
10. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 24
11. Comments Solicited . . . . . . . . . . . . . . . . . . . . . . 25
12. References . . . . . . . . . . . . . . . . . . . . . . . . . . 25
12.1. Normative References . . . . . . . . . . . . . . . . . . . 25
12.2. Informative References . . . . . . . . . . . . . . . . . . 25
Appendix A. Design Constraints . . . . . . . . . . . . . . . . . 28
A.1. Security Constraints . . . . . . . . . . . . . . . . . . . 28
A.2. Control Constraints . . . . . . . . . . . . . . . . . . . 30
A.3. Management Constraints . . . . . . . . . . . . . . . . . . 31
Appendix B. Relative Placement of Tunnelling and In-Path Load
Regulation . . . . . . . . . . . . . . . . . . . . . 32
B.1. Identifiers and In-Path Load Regulators . . . . . . . . . 32
B.2. Non-Dependence of Tunnelling on In-path Load Regulation . 33
B.3. Dependence of In-Path Load Regulation on Tunnelling . . . 34
Appendix C. Contribution to Congestion across a Tunnel . . . . . 37
Appendix D. Why Not Propagating ECT(1) on Decapsulation
Impedes PCN . . . . . . . . . . . . . . . . . . . . . 38
D.1. Alternative Ways to Introduce the New Decapsulation
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Rules . . . . . . . . . . . . . . . . . . . . . . . . . . 39
Appendix E. Why Resetting CE on Encapsulation Impedes PCN . . . . 40
Author's Address . . . . . . . . . . . . . . . . . . . . . . . . . 40
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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.cs.ucl.ac.uk/staff/B.Briscoe/pubs.html#ecn-tunnel>
From ietf-01 to ietf-02 (current):
* 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 A);
+ Added Section 3 to summarise relevant existing RFCs;
+ Structured Section 4 and Section 5 into subsections.
+ Added tables to sections on old and new rules, for precision
and comparison.
+ Moved Section 4.3 on Design Principles to the end of the
section specifying the new default normative tunnelling
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behaviour. Rewritten and shifted text on identifiers and
in-path load regulators to Appendix B.1.
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 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 -00 to -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;
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* Added Design Guidelines for New Encapsulations of Congestion
Notification;
* Considerably clarified the Backward Compatibility section
(Section 5);
* Considerably extended the Security Considerations section
(Section 8);
* 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.2, 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
This document redefines how the explicit congestion notification
(ECN) field [RFC3168] in the IP header should be constructed for all
IP in IP tunnelling. Previously, tunnel endpoints blocked visibility
of transitions of the ECN field except the minimum necessary to allow
the basic ECN mechanism to work. Three main change are defined, one
on entry to and two on exit from any IP in IP tunnel. The newly
specified behaviours make all transitions to the ECN field visible
across tunnel end-points, so tunnels no longer restrict new uses of
the ECN field that were not envisaged when ECN was first designed.
The immediate motivation for opening up the ECN behaviour of tunnels
is because otherwise they impede the introduction of pre-congestion
notification (PCN [I-D.ietf-pcn-marking-behaviour]) in networks with
tunnels (Appendix E explains why). But these changes are not just
intended to ease the introduction of PCN; care has been taken to
ensure the resulting ECN tunnelling behaviour is simple and generic
for other potential future uses.
Given this is a change to behaviour at 'the neck of the hourglass',
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an extensive analysis of the trade-offs between control, management
and security constraints has been conducted in order to minimise
unexpected side-effects both now and in the future. Care has also
been taken to ensure the changes are fully backwards compatible with
all previous tunnelling behaviours.
The ECN protocol allows a forwarding element to notify the onset of
congestion of its resources without having to drop packets. Instead
it can explicitly mark a proportion of packets by setting the
congestion experienced (CE) codepoint in the 2-bit ECN field in the
IP header (see Table 1 for a recap of the ECN codepoints).
+------------------+----------------+---------------------------+
| Binary codepoint | Codepoint name | Meaning |
+------------------+----------------+---------------------------+
| 00 | Not-ECT | Not ECN-capable transport |
| 01 | ECT(1) | ECN-capable transport |
| 10 | ECT(0) | ECN-capable transport |
| 11 | CE | Congestion experienced |
+------------------+----------------+---------------------------+
Table 1: Recap of Codepoints of the ECN Field [RFC3168] in the IP
Header
The outer header of an IP packet can encapsulate one (or more)
additional IP headers tunnelled within it. A forwarding element that
is using ECN to signify congestion will only mark the outer IP header
that is immediately visible to it. When a tunnel decapsulator later
removes this outer header, it must follow rules to ensure the marking
is propagated into the IP header being forwarded onwards, otherwise
congestion notifications will disappear into a black hole leading to
potential congestion collapse.
The rules for constructing the ECN field to be forwarded after tunnel
decapsulation ensure this happens, but they are not wholly
straightforward, and neither are the rules for encapsulating one IP
header in another on entry to a tunnel. The factor that has
introduced most complication at both ends of a tunnel has been the
possibility that the ECN field might be used as a covert channel to
compromise the integrity of an IPsec tunnel.
A common use for IPsec is to create a secure tunnel between two
secure sites across the public Internet. A field like ECN that can
change as it traverses the Internet cannot be covered by IPsec's
integrity mechanisms. Therefore, the ECN field might be toggled
(with two bits per packet) to communicate between a secure site and
someone on the public Internet--a covert channel.
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Over the years covert channel restrictions have been added to the
design of ECN (with consequent backward compatibility complications).
However the latest IPsec architecture [RFC4301] takes the view that
simplicity is more important than closing off the covert channel
threat, which it deems manageable given its bandwidth is limited to
two bits per packet.
As a result, an unfortunate sequence of standards actions has left us
with nearly the worst of all possible combinations of outcomes,
despite the best endeavours of everyone concerned. The new IPsec
architecture [RFC4301] only updates the earlier specification of ECN
tunnelling behaviour [RFC3168] for the case of IPsec tunnels. For
the case of non-IPsec tunnels the earlier RFC3168 specification still
applies. At the time RFC3168 was standardised, covert channels
through the ECN field were restricted, whether or not IPsec was being
used. The perverse position now is that non-IPsec tunnels restrict
covert channels, while IPsec tunnels don't.
Actually, this statement needs some qualification. IPsec tunnels
only don't restrict the ECN covert channel at the ingress. At the
tunnel egress, the presumption that the ECN covert channel should be
restricted has not been removed from any tunnelling specifications,
whether IPsec or not.
Now that these historic 2-bit covert channel constraints are impeding
the introduction of PCN, this specification is designed to remove
them and at the same time streamline the whole ECN behaviour for the
future.
1.1. Scope
This document only concerns wire protocol processing at tunnel
endpoints and makes no changes or recommendations concerning
algorithms for congestion marking or congestion response.
This document specifies common, default ECN field processing at
encapsulation and decapsulation for any IP in IP tunnelling. It
applies irrespective of whether IPv4 or IPv6 is used for either of
the inner and outer headers. It applies to all Diffserv per-hop
behaviours (PHBs), unless stated otherwise in the specification of a
PHB. It is intended to be a good trade off between somewhat
conflicting security, control and management requirements.
Nonetheless, if necessary, an alternate congestion encapsulation
behaviour can be introduced as part of the definition of an alternate
congestion marking scheme used by a specific Diffserv PHB (see S.5 of
[RFC3168] and [RFC4774]). When designing such new encapsulation
schemes, the principles in Section 4.3 should be followed as closely
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as possible. There is no requirement for a PHB to state anything
about ECN tunnelling behaviour if the new default behaviour is
sufficient.
[RFC2983] is a comprehensive primer on differentiated services and
tunnels. Given ECN raises similar issues to differentiated services
when interacting with tunnels, useful concepts introduced in RFC2983
are used throughout, with brief recaps of the explanations where
necessary.
1.2. Document Roadmap
The body of the document focuses solely on standards actions
impacting implementation. Appendices record the analysis that
motivates and justifies these actions. The whole document is
organised as follows:
o Section 3 recaps relevant existing RFCs and explains exactly why
changes are needed, referring to Appendix D and Appendix E in
order to explain in detail why current tunnelling behaviours
impede PCN deployment, at egress and ingress respectively.
o Section 4 uses precise standards terminology to specify the new
ECN tunnelling behaviours. It refers to Appendix A for analysis
of the trade-offs between security, control and management design
constraints that led to these particular standards actions.
o Extending the new IPsec tunnel ingress behaviour to all IP in IP
tunnels requires consideration of backwards compatibility, which
is covered in Section 5 and detailed changes from earlier RFCs are
brought together in Section 6.
o Finally, a number of security considerations are discussed and
conclusions are drawn.
o Additional specialist issues are deferred to appendices in
addition to those already referred to above, in particular
Appendix B discusses specialist tunnelling issues that could arise
when ECN is fed back to a load regulation function on a middlebox,
rather than at the source of the path.
2. Requirements Language
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in RFC 2119 [RFC2119].
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3. Summary of Pre-Existing RFCs
This section is informative not normative. It merely recaps pre-
existing RFCs to help motivate changing these behaviours. Earlier
relevant RFCs that were either experimental or incomplete with
respect to ECN tunnelling (RFC2481, RFC2401 and RFC2003) are not
discussed, although the backwards compatibility considerations in
Section 5 take them into account. The question of whether tunnel
implementations used in the Internet comply with any of these RFCs is
also not discussed.
3.1. Encapsulation at Tunnel Ingress
The controversy at tunnel ingress has been over whether to propagate
information about congestion experienced on the path upstream of the
tunnel ingress into the outer header of the tunnel.
Specifically, RFC3168 says that, if a tunnel fully supports ECN
(termed a 'full-functionality' ECN tunnel in [RFC3168]), the tunnel
ingress must not copy a CE marking from the inner header into the
outer header that it creates. Instead the tunnel ingress must set
the outer header to ECT(0) (i.e. codepoint 10) if the ECN field is
marked CE (codepoint 11) in the arriving IP header. We term this
'resetting' a CE codepoint.
However, the new IPsec architecture in [RFC4301] reverses this rule,
stating that the tunnel ingress must simply copy the ECN field from
the arriving to the outer header. The main purpose of the present
specification is to carry the new behaviour of IPsec over to all IP
in IP tunnels, so all tunnel ingress nodes consistently copy the ECN
field.
RFC3168 also provided a Limited Functionality mode that turns off ECN
processing over the scope of the tunnel. This is necessary if the
ingress does not know whether the tunnel egress supports propagation
of ECN markings. Neither Limited Functionality mode nor Full
Functionality mode are used in RFC4301 IPsec.
These pre-existing behaviours are summarised in Figure 1.
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+-----------------+-----------------------------------------------+
| Incoming Header | Outgoing Outer Header |
| (also equal to +---------------+---------------+---------------+
| Outgoing Inner | RFC3168 ECN | RFC3168 ECN | RFC4301 IPsec |
| Header) | Limited | Full | |
| | Functionality | Functionality | |
+-----------------+---------------+---------------+---------------+
| Not-ECT | Not-ECT | Not-ECT | Not-ECT |
| ECT(0) | Not-ECT | ECT(0) | ECT(0) |
| ECT(1) | Not-ECT | ECT(1) | ECT(1) |
| CE | Not-ECT | ECT(0) | CE e|
+-----------------+---------------+---------------+---------------+
Figure 1: IP in IP Encapsulation: Recap of Pre-existing Behaviours
For encapsulation, the specification in Section 4 below brings all IP
in IP tunnels (v4 or v6) into line with the way IPsec tunnels
[RFC4301] now construct the ECN field, except where a legacy tunnel
egress might not understand ECN at all. This removes the now
redundant full functionality mode in the middle column of Figure 1.
Wherever possible it ensures that the outer header reveals any
congestion experienced so far on the whole path, not just since the
last tunnel ingress.
Why does it matter if we have different ECN encapsulation behaviours
for IPsec and non-IPsec tunnels? A general answer is that gratuitous
inconsistency constrains the available design space and makes it
harder to design networks and new protocols that work predictably.
But there is also a specific need not to reset the CE codepoint. The
standards track proposal for excess rate pre-congestion notification
(PCN [I-D.ietf-pcn-marking-behaviour]) only works correctly in the
presence of RFC4301 IPsec encapsulation or [RFC5129] MPLS
encapsulation, but not with RFC3168 IP in IP encapsulation
(Appendix E explains why). The PCN architecture
[I-D.ietf-pcn-architecture] states that the regular RFC3168 rules for
IP in IP tunnelling of the ECN field should not be used for PCN. But
if non-IPsec tunnels are already present within a network to which
PCN is being added, that is not particularly helpful advice.
The present specification provides a clean solution to this problem,
so that network operators who want to use PCN and tunnels can specify
that all tunnel endpoints in a PCN region need to be upgraded to
comply with this specification. Also, whether using PCN or not, as
more tunnel endpoints comply with this specification, it should make
ECN behaviour simpler, faster and more predictable.
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To ensure copying rather than resetting CE on ingress will not cause
unintended side-effects, Appendix A assesses whether either harm any
security, control or management functions. It finds that resetting
CE makes life difficult in a number of directions, while copying CE
harms nothing (other than opening a low bit-rate covert channel
vulnerability which the IETF Security Area now deems is manageable).
3.2. Decapsulation at Tunnel Egress
Both RFC3168 and RFC4301 specify the decapsulation behaviour
summarised in Figure 2. The ECN field in the outgoing header is set
to the codepoint at the intersection of the appropriate incoming
inner header (row) and incoming outer header (column).
+------------------+----------------------------------------------+
| Incoming Inner | Incoming Outer Header |
| Header +---------+------------+------------+----------+
| | Not-ECT | ECT(0) | ECT(1) | CE |
+------------------+---------+------------+------------+----------+
| Not-ECT | Not-ECT | drop(!!!)| drop(!!!)| drop(!!!)|
| ECT(0) | ECT(0) | ECT(0) | ECT(0) | CE |
| ECT(1) | ECT(1) | ECT(1) | ECT(1) | CE |
| CE | CE | CE | CE | CE |
+------------------+---------+------------+------------+----------+
| Outgoing Header |
+----------------------------------------------+
Figure 2: IP in IP Decapsulation; Recap of Pre-existing Behaviour
The behaviour in the table derives from the logic given in RFC3168,
briefly recapped as follows:
o On decapsulation, if the inner ECN field is Not-ECT but the outer
ECN field is anything except Not-ECT the decapsulator must drop
the packet. Drop is mandated because known legal protocol
transitions should not be able to lead to these cases (indicated
in the table by '(!!!)'), therefore the decapsulator may also
raise an alarm;
o In all other cases, the outgoing ECN field is set to the more
severe marking of the outer and inner ECN fields, where the
ranking of severity from highest to lowest is CE, ECT, Not-ECT;
o ECT(0) and ECT(1) are considered of equal severity (indicated by
just 'ECT' in the rank order above). Where the inner and outer
ECN fields are both ECT but they differ, the packet is forwarded
with the codepoint of the inner ECN field, which prevents ECT
codepoints being used for a covert channel.
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The specification for decapsulation in Section 4 fixes two problems
with this pre-existing behaviour:
o Firstly, forwarding the codepoint of the inner header in the cases
where both inner and outer are different values of ECT effectively
implies that any distinction between ECT(0) and ECT(1) cannot be
introduced in the future wherever a tunnel might be deployed.
Therefore, the currently specified tunnel decapsulation behaviour
unnecessarily wastes one of four codepoints (effectively wasting
half a bit) in the IP (v4 & v6) header. As explained in
Appendix A.1, the original reason for not using the outer ECT
codepoints for onward forwarding was to limit the covert channel
across a decapsulator to 1 bit per packet. However, now that the
IETF Security Area has deemed that a 2-bit covert channel through
an encapsulator is a manageable risk, the same should be true for
a decapsulator.
As well as being a general future-proofing issue, this problem is
immediately pressing for standardisation of pre-congestion
notification (PCN). PCN solutions generally require three
encoding states in addition to Not-ECT: one for 'not marked' and
two increasingly severe levels of marking. Although the ECN field
gives sufficient codepoints for these three states, they cannot
all be used for PCN because a change between ECT(0) and ECT(1) in
any tunnelled packet would be lost when the outer header was
decapsulated, dangerously discarding congestion signalling. A
number of wasteful or convoluted work-rounds to this problem are
being considered for standardisation by the PCN working group (see
Appendix D), but by far the simplest approach is just to remove
the covert channel blockages from tunnelling behaviour, that are
now deemed unnecessary anyway. Not only will this streamline PCN
standardisation, but it could also streamline other future uses of
these codepoints.
o Secondly, mandating drop is not always a good idea just because a
combination of headers seems invalid. There are many cases where
it has become nearly impossible to deploy new standards because
legacy middleboxes drop packets carrying header values they don't
expect. Where possible, the new decapsulation behaviour specified
in Section 4 below is more liberal in its response to unexpected
combinations of headers.
4. New ECN Tunnelling Rules
The ECN tunnel processing rules below in Section 4.1 (ingress
encapsulation) and Section 4.2 (egress decapsulation) are the default
for a packet with any DSCP. If required, different ECN encapsulation
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rules MAY be defined as part of the definition of an appropriate
Diffserv PHB using the guidelines that follow in Section 4.3.
However, the deployment burden of handling exceptional PHBs in
implementations of all affected tunnels and lower layer link
protocols should not be underestimated.
4.1. Default Tunnel Ingress Behaviour
A tunnel ingress compliant with this specification MUST implement a
`normal mode'. It might also need to implement a `compatibility
mode' for backward compatibility with legacy tunnel egresses that do
not understand ECN (see Section 5 for when compatibility mode is
required). Note that these are modes of the ingress tunnel endpoint
only, not the tunnel as a whole.
Whatever the mode, the tunnel ingress forwards the inner header
without changing the ECN field. In normal mode a tunnel ingress
compliant with this specification MUST construct the outer
encapsulating IP header by copying the 2-bit ECN field of the
arriving IP header. In compatibility mode it clears the ECN field in
the outer header to the Not-ECT codepoint. These rules are tabulated
for convenience in Figure 3.
+-----------------+-------------------------------+
| Incoming Header | Outgoing Outer Header |
| (also equal to +---------------+---------------+
| Outgoing Inner | Compatibility | Normal |
| Header) | Mode | Mode |
+-----------------+---------------+---------------+
| Not-ECT | Not-ECT | Not-ECT |
| ECT(0) | Not-ECT | ECT(0) |
| ECT(1) | Not-ECT | ECT(1) |
| CE | Not-ECT | CE |
+-----------------+---------------+---------------+
Figure 3: New IP in IP Encapsulation Behaviours
Compatibility mode is the same per packet behaviour as the ingress
end of RFC3168's limited functionality mode. Normal mode is the same
per packet behaviour as the ingress end of RFC4301 IPsec.
4.2. Default Tunnel Egress Behaviour
To decapsulate the inner header at the tunnel egress, a compliant
tunnel egress MUST set the outgoing ECN field to the codepoint at the
intersection of the appropriate incoming inner header (row) and outer
header (column) in Figure 4.
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+------------------+----------------------------------------------+
| Incoming Inner | Incoming Outer Header |
| Header +---------+------------+------------+----------+
| | Not-ECT | ECT(0) | ECT(1) | CE |
+------------------+---------+------------+------------+----------+
| Not-ECT | Not-ECT |Not-ECT(!!!)| drop(!!!)| drop(!!!)|
| ECT(0) | ECT(0) | ECT(0) | ECT(1) | CE |
| ECT(1) | ECT(1) | ECT(1)(!!!)| ECT(1) | CE |
| CE | CE | CE | CE(!!!)| CE |
+------------------+---------+------------+------------+----------+
| Outgoing Header |
+----------------------------------------------+
Figure 4: New IP in IP Decapsulation Behaviour
This table for decapsulation behaviour is derived from the following
logic:
o If the inner ECN field is Not-ECT the decapsulator MUST NOT
propagate any other ECN codepoint in the outer header onwards.
This is because the inner Not-ECT marking is set by transports
that would not understand the ECN protocol. Instead:
* If the inner ECN field is Not-ECT and the outer ECN field is
ECT(1) or CE the decapsulator MUST drop the packet.
Reasoning: these combinations of codepoints either imply some
illegal protocol transition has occurred within the tunnel, or
that some locally defined mechanism is being used within the
tunnel that might be signalling congestion. In either case,
the only appropriate signal to the transport is a packet drop.
It would have been nice to allow packets with ECT(1) in the
outer to be forwarded, but drop has had to be mandated in case
future multi-level ECN schemes are defined. Then ECT(1) and CE
can be used in the future to signify two levels of congestion
severity.
* If the inner ECN field is Not-ECT and the outer ECN field is
ECT(0) or Not-ECT the decapsulator MUST forward the packet with
the ECN field cleared to Not-ECT.
Reasoning: Although no known legal protocol transition would
lead to ECT(0) in the outer and Not-ECT in the inner, no known
or proposed protocol uses ECT(0) as a congestion signal either.
Therefore in this case the packet can be forwarded rather than
dropped, which will allow future standards actions to use this
combination.
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o In all other cases, the outgoing ECN field is set to the more
severe marking of the outer and inner ECN fields, where the
ranking of severity from highest to lowest is CE, ECT(1), ECT(0),
Not-ECT;
o There are cases where no currently legal transition in any current
or previous ECN tunneling specification would result in certain
combinations of inner and outer ECN fields. These cases are
indicated in Figure 4 by '(!!!)'). In these cases, the
decapsulator SHOULD log the event and MAY also raise an alarm, but
not so often that the illegal combinations would amplify into a
flood of alarm messages.
The above logic allows for ECT(0) and ECT(1) to both represent the
same severity of congestion marking (e.g. "not congestion marked").
But it also allows future schemes to be defined where ECT(1) is a
more severe marking than ECT(0). This approach is discussed in
Appendix D and in the discussion of the ECN nonce [RFC3540] in
Section 8.
4.3. Design Principles for Future Non-Default Schemes
This section is informative not normative.
S.5 of RFC3168 permits the Diffserv codepoint (DSCP)[RFC2474] to
'switch in' different behaviours for marking the ECN field, just as
it switches in different per-hop behaviours (PHBs) for scheduling.
Therefore here we give guidance for designing possibly different
marking schemes.
In one word the guidance is "Don't". If a scheme requires tunnels to
implement special processing of the ECN field for certain DSCPs, it
is highly unlikely that every implementer of every tunnel will want
to add the required exception and that operators will want to deploy
the required configuration options. Therefore it is highly likely
that some tunnels within a network will not implement this special
case. Therefore, designers should avoid non-default tunnelling
schemes if at all possible.
That said, if a non-default scheme for processing the ECN field is
really required, the following guidelines may prove useful in its
design:
o For any new scheme, a tunnel ingress should not set the ECN field
of the outer header if it cannot guarantee that any corresponding
tunnel egress will understand how to handle such an ECN field.
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o On encapsulation in any new scheme, an outer header capable of
carrying congestion markings should reflect accumulated congestion
since the last interface designed to regulate load (see
Appendix A.2 for the definition of a Load Regulator, which is
usually but not always the data source). This implies that new
schemes for tunnelling congestion notification should copy
congestion notification into the outer header of each new
encapsulating header that supports it.
Reasoning: The constraints from the three perspectives of
security, control and management in Appendix A are somewhat in
tension as to whether a tunnel ingress should copy congestion
markings into the outer header it creates or reset them. From the
control perspective either copying or resetting works for existing
arrangements, but copying has more potential for simplifying
control. From the management perspective copying is preferable.
From the security perspective resetting is preferable but copying
is now considered acceptable given the bandwidth of a 2-bit covert
channel can be managed. Therefore, on balance, copying is simpler
and more useful than resetting and does minimal harm.
o For any new scheme, a tunnel egress should not forward any ECN
codepoint if the arriving inner header implies the transport will
not understand how to process it.
o On decapsulation in any new scheme, if a combination of inner and
outer headers is encountered that should not have been possible,
this event should be logged and an alarm raised. But the packet
should still be forwarded with a safe codepoint setting if at all
possible. This increases the chances of 'forward compatibility'
with possible future protocol extensions.
o On decapsulation in any new scheme, the ECN field that the tunnel
egress forwards should reflect the more severe congestion marking
of the arriving inner and outer headers.
5. Backward Compatibility
Note: in RFC3168, a whole tunnel was considered in one of two modes:
limited functionality or full functionality. The new modes defined
in this specification are only modes of the tunnel ingress. The new
tunnel egress behaviour has only one mode and doesn't need to know
what mode the ingress is in.
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5.1. Non-Issues Upgrading Any Tunnel Decapsulation
This specification only changes the egress per-packet calculation of
the ECN field for combinations of inner and outer headers that have
so far not been used in any IETF protocols. Therefore, a tunnel
egress complying with any previous specification (RFC4301, both modes
of RFC3168, both modes of RFC2481, RFC2401 and RFC2003) can be
upgraded to comply with this new decapsulation specification without
any backwards compatibility issues.
The proposed tunnel egress behaviour also requires no additional mode
or option configuration at the ingress or egress nor any additional
negotiation with the ingress. A compliant tunnel egress merely needs
to implement the one behaviour in Section 4. The reduction to one
mode at the egress has no backwards compatibility issues, because
previously the egress produced the same output whichever mode the
tunnel was in.
These new decapsulation rules have been defined in such a way that
congestion control will still work safely if any of the earlier
versions of ECN processing are used unilaterally at the encapsulating
ingress of the tunnel (any of RFC2003, RFC2401, either mode of
RFC2481, either mode of RFC3168, RFC4301 and this present
specification). If a tunnel ingress tries to negotiate to use
limited functionality mode or full functionality mode [RFC3168], a
decapsulating tunnel egress compliant with this specification MUST
agree to either request, as its behaviour will be the same in both
cases.
For 'forward compatibility', a compliant tunnel egress SHOULD raise a
warning about any requests to enter modes it doesn't recognise, but
it can continue operating. If no ECN-related mode is requested, a
compliant tunnel egress can continue without raising any error or
warning as its egress behaviour is compatible with all the legacy
ingress behaviours that don't negotiate capabilities.
5.2. Non-Issues for RFC4301 IPsec Encapsulation
The new normal mode of ingress behaviour defined above (Section 4.1)
brings all IP in IP tunnels into line with [RFC4301]. If one end of
an IPsec tunnel is compliant with [RFC4301], the other end is
guaranteed to also be RFC4301-compliant (there could be corner cases
where manual keying is used, but they will be set aside here).
Therefore the new normal ingress behaviour introduces no backward
compatibility isses with IKEv2 [RFC4306] IPsec [RFC4301] tunnels, and
no need for any new modes, options or configuration.
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5.3. Upgrading Other IP in IP Tunnel Encapsulators
At the tunnel ingress, this specification effectively extends the
scope of RFC4301's ingress behaviour to any IP in IP tunnel. If any
other IP in IP tunnel ingress (i.e. not RFC4301 IPsec) is upgraded to
be compliant with this specification, it has to cater for the
possibility that it is talking to a legacy tunnel egress that may not
know how to process the ECN field. If ECN capable outer headers were
sent towards a legacy (e.g. [RFC2003]) egress, it would most likely
simply disregard the outer headers, dangerously discarding
information about congestion experienced within the tunnel. ECN-
capable traffic sources would not see any congestion feedback and
instead continually ratchet up their share of the bandwidth without
realising that cross-flows from other ECN sources were continually
having to ratchet down.
This specification introduces no new backward compatibility issues
when a compliant ingress talks with a legacy egress, but it has to
provide similar sfaeguards to those already defined in RFC3168.
Therefore, to comply with this specification, a tunnel ingress that
does not always know the ECN capability of its tunnel egress MUST
implement a 'normal' mode and a 'compatibility' mode, and for safety
it MUST initiate each negotiated tunnel in compatibility mode.
However, a tunnel ingress can be compliant even if it only implements
the 'normal mode' of encapsulation behaviour, but only as long as it
is designed or configured so that all possible tunnel egress nodes it
will ever talk to will have at least full ECN functionality
(complying with either RFC3168 full functionality mode, RFC4301 or
this present specification).
Before switching to normal mode, a compliant tunnel ingress that does
not know the egress ECN capability MUST negotiate with the tunnel
egress. If the egress says it is compliant with this specification
or with RFC3168 full functionality mode, the ingress puts itself into
normal mode. If the egress denies compliance with all of these or
doesn't understand the question, the tunnel ingress MUST remain in
compatibility mode.
The encapsulation rules for normal mode and compatibility mode are
defined in Section 4 (i.e. header copying or zeroing respectively).
An ingress cannot claim compliance with this specification simply by
disabling ECN processing across the tunnel (only implementing
compatibility mode). Although such a tunnel ingress is at least safe
with the ECN behaviour of any egress it may encounter (any of
RFC2003, RFC2401, either mode of RFC2481 and RFC3168's limited
functionality mode), it doesn't meet the aim of introducing ECN.
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Therefore, a compliant tunnel ingress MUST at least implement `normal
mode' and, if it might be used with arbitrary tunnel egress nodes, it
MUST also implement `compatibility mode'.
Implementation note: if a compliant node is the ingress for multiple
tunnels, a mode setting will need to be stored for each tunnel
ingress. However, if a node is the egress for multiple tunnels, none
of the tunnels will need to store a mode setting, because a compliant
egress can only be in one mode.
6. Changes from Earlier RFCs
On encapsulation, the rule that a normal mode tunnel ingress MUST
copy any ECN field into the outer header is a change to the ingress
behaviour of RFC3168, but it is the same as the rules for IPsec
tunnels in RFC4301.
On decapsulation, the rules for calculating the outgoing ECN field at
a tunnel egress are similar to the full functionality mode of ECN in
RFC3168 and to RFC4301, with the following exceptions:
o The outer, not the inner, is propagated when the outer is ECT(1)
and the inner is ECT(0);
o A packet with Not-ECT in the inner may be forwarded as Not-ECT
rather than dropped, if the outer is ECT(0);
o The following extra illegal combinations have been identified,
which may require logging and/or an alarm: outer ECT(1) with inner
CE; outer ECT(0) with inner ECT(1)
The rules for how a tunnel establishes whether the egress has full
functionality ECN capabilities are an update to RFC3168. For all the
typical cases, RFC4301 is not updated by the ECN capability check in
this specification, because a typical RFC4301 tunnel ingress will
have already established that it is talking to an RFC4301 tunnel
egress (e.g. if it uses IKEv2). However, there may be some corner
cases (e.g. manual keying) where an RFC4301 tunnel ingress talks with
an egress with limited functionality ECN handling. Strictly, for
such corner cases, the requirement to use compatibility mode in this
specification updates RFC4301, but this is unlikely to be necessary
to implement for this corner case in practice.
The optional ECN Tunnel field in the IPsec security association
database (SAD) and the optional ECN Tunnel Security Association
Attribute defined in RFC3168 are no longer needed. The security
association (SA) has no policy on ECN usage, because all RFC4301
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tunnels now support ECN without any policy choice.
RFC3168 defines a (required) limited functionality mode and an
(optional) full functionality mode for a tunnel, but RFC4301 doesn't
need modes. In this specification only the ingress might need two
modes: a normal mode (required) and a compatibility mode (required in
some scenarios, optional in others). The egress needs only one mode
which correctly handles any ingress ECN behaviour.
Additional changes to the RFC Index (to be removed by the RFC Editor):
In the RFC index, RFC3168 should be identified as an update to
RFC2003. RFC4301 should be identified as an update to RFC3168.
This specification updates RFC3168 and RFC4301.
7. IANA Considerations
This memo includes no request to IANA.
8. Security Considerations
Appendix A.1 discusses the security constraints imposed on ECN tunnel
processing. The new rules for ECN tunnel processing (Section 4)
trade-off between security (covert channels) and congestion
monitoring & control. In fact, ensuring congestion markings are not
lost is itself another aspect of security, because if we allowed
congestion notification to be lost, any attempt to enforce a response
to congestion would be much harder.
If alternate congestion notification semantics are defined for a
certain PHB (e.g. the pre-congestion notification architecture
[I-D.ietf-pcn-architecture]), the scope of the alternate semantics
might typically be bounded by the limits of a Diffserv region or
regions, as envisaged in [RFC4774]. The inner headers in tunnels
crossing the boundary of such a Diffserv region but ending within the
region can potentially leak the external congestion notification
semantics into the region, or leak the internal semantics out of the
region. [RFC2983] discusses the need for Diffserv traffic
conditioning to be applied at these tunnel endpoints as if they are
at the edge of the Diffserv region. Similar concerns apply to any
processing or propagation of the ECN field at the edges of a Diffserv
region with alternate ECN semantics. Such edge processing must also
be applied at the endpoints of tunnels with one end inside and the
other outside the domain. [I-D.ietf-pcn-architecture] gives specific
advice on this for the PCN case, but other definitions of alternate
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semantics will need to discuss the specific security implications in
each case.
With the decapsulation rules as they stood in RFC3168 and RFC4301, a
small part of the protection of the ECN nonce [RFC3540] was
compromised. The new decapsulation rules do not solve this problem.
The minor problem is as follows: The ECN nonce was defined to enable
the data source to detect if a CE marking had been applied then
subsequently removed. The source could detect this by weaving a
pseudo-random sequence of ECT(0) and ECT(1) values into a stream of
packets, which is termed an ECN nonce. By the decapsulation rules in
RFC3168 and RFC4301, if the inner and outer headers carry
contradictory ECT values only the inner header is preserved for
onward forwarding. So if a CE marking added to the outer ECN field
in a tunnel has been illegally (or accidentally) suppressed by a
subsequent node in the tunnel, the decapsulator will revert the ECN
field to its value before tampering, hiding all evidence of the crime
from the onward feedback loop. We chose not to close this minor
loophole for all the following reasons:
1. This loophole is only applicable in the corner case where the
attacker controls a network node downstream of a congested node
in the same tunnel;
2. In tunnelling scenarios, the ECN nonce is already vulnerable to
suppression by nodes downstream of a congested node in the same
tunnel, if they can copy the ECT value in the inner header to the
outer header (any node in the tunnel can do this if the inner
header is not encrypted, and an IPsec tunnel egress can do it
whether or not the tunnel is encrypted);
3. Although the new decapsulation behaviour removes evidence of
congestion suppression from the onward feedback loop, the
decapsulator itself can at least detect that congestion within
the tunnel has been suppressed;
4. The ECN nonce [RFC3540] currently has experimental status and
there has been no evidence that anyone has implemented it beyond
the author's prototype.
We could have fixed this loophole by specifying that the outer header
should always be propagated onwards if inner and outer are both ECT.
Although this would close the minor loophole in the nonce, it would
raise a minor safety issue if multilevel ECN or PCN were used. A
less severe marking in the inner header would override a more severe
one in the outer. Both are corner cases so it is difficult to decide
which is more important:
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1. The loophole in the nonce is only for a minor case of one tunnel
node attacking another in the same tunnel;
2. The severity inversion for multilevel congestion notification
would not result from any legal codepoint transition.
We decided safety against misconfiguration was slightly more
important than securing against an attack that has little, if any,
clear motivation.
If a legacy security policy configures a legacy tunnel ingress to
negotiate to turn off ECN processing, a compliant tunnel egress will
agree to a request to turn off ECN processing but it will actually
still copy CE markings from the outer to the forwarded header.
Although the tunnel ingress 'I' in Figure 5 (Appendix A.1) will set
all ECN fields in outer headers to Not-ECT, 'M' could still toggle CE
on and off to communicate covertly with 'B', because we have
specified that 'E' only has one mode regardless of what mode it says
it has negotiated. We could have specified that 'E' should have a
limited functionality mode and check for such behaviour. But we
decided not to add the extra complexity of two modes on a compliant
tunnel egress merely to cater for a legacy security concern that is
now considered manageable.
9. Conclusions
This document updates the ingress tunnelling encapsulation of RFC3168
ECN for all IP in IP tunnels to bring it into line with the new
behaviour in the IPsec architecture of RFC4301. It copies rather
than resets a congestion experienced (CE) marking when creating outer
headers.
It also specifies new rules that update both RFC3168 and RFC4301 for
calculating the outgoing ECN field on tunnel decapsulation. The new
rules update egress behaviour for two specific combinations of inner
and outer header that have no current legal usage, but will now be
possible to use in future standards actions, rather than being wasted
by current tunnelling behaviour.
The new rules propagate changes to the ECN field across tunnel end-
points that were previously blocked due to a perceived covert channel
vulnerability. The new IPsec architecture deems the two-bit covert
channel that the ECN field opens up is a manageable threat, so these
new rules bring all IP in IP tunnelling into line with this new more
permissive attitude. The result is a single specification for all
future tunnelling of ECN, whether IPsec or not. Then equipment can
be specified against a single ECN behaviour and ECN markings can have
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a well-defined meaning wherever they are measured in a network. This
new certainty will enable new uses of the ECN field that would
otherwise be confounded by ambiguity.
The immediate motivation for making these changes is to allow the
introduction of multi-level pre-congestion notification (PCN). But
great care has been taken to ensure the resulting ECN tunnelling
behaviour is simple and generic for other potential future uses.
The change to encapsulation has been analysed from the three
perspectives of security, control and management. They are somewhat
in tension as to whether a tunnel ingress should copy congestion
markings into the outer header it creates or reset them. From the
control perspective either copying or resetting works for existing
arrangements, but copying has more potential for simplifying control
and resetting breaks at least one proposal already on the standards
track. From the management and monitoring perspective copying is
preferable. From the network security perspective (theft of service
etc) copying is preferable. From the information security
perspective resetting is preferable, but the IETF Security Area now
considers copying acceptable given the bandwidth of a 2-bit covert
channel can be managed. Therefore there are no points against
copying and a number against resetting CE on ingress.
The only downside of the changes to decapsulation is that the same
2-bit covert channel is opened up as at the ingress, but this is now
deemed to be a manageable threat. The changes at decapsulation have
been found to be free of any backwards compatibility issues.
10. Acknowledgements
Thanks to Anil Agawaal for pointing out a case where it's safe for a
tunnel decapsulator to forward a combination of headers it doesn't
understand. Thanks to David Black for explaining a better way to
think about function placement and to Louise Burness for a better way
to think about multilayer transports and networks, having read
[Patterns_Arch]. Also thanks to Arnaud Jacquet for the idea for
Appendix C. Thanks to Michael Menth, Bruce Davie, Toby Moncaster,
Gorry Fairhurst, Sally Floyd, Alfred Hoenes and Gabriele Corliano for
their thoughts and careful review comments.
Bob Briscoe is partly funded by Trilogy, a research project (ICT-
216372) supported by the European Community under its Seventh
Framework Programme. The views expressed here are those of the
author only.
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11. Comments Solicited
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
[RFC2003] Perkins, C., "IP Encapsulation within IP", RFC 2003,
October 1996.
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
[RFC2474] Nichols, K., Blake, S., Baker, F., and D. Black,
"Definition of the Differentiated Services Field (DS
Field) in the IPv4 and IPv6 Headers", RFC 2474,
December 1998.
[RFC3168] Ramakrishnan, K., Floyd, S., and D. Black, "The Addition
of Explicit Congestion Notification (ECN) to IP",
RFC 3168, September 2001.
[RFC4301] Kent, S. and K. Seo, "Security Architecture for the
Internet Protocol", RFC 4301, December 2005.
12.2. Informative References
[I-D.briscoe-pcn-3-in-1-encoding]
Briscoe, B., "PCN 3-State Encoding Extension in a single
DSCP", draft-briscoe-pcn-3-in-1-encoding-00 (work in
progress), October 2008.
[I-D.charny-pcn-single-marking]
Charny, A., Zhang, X., Faucheur, F., and V. Liatsos, "Pre-
Congestion Notification Using Single Marking for Admission
and Termination", draft-charny-pcn-single-marking-03
(work in progress), November 2007.
[I-D.ietf-pcn-architecture]
Eardley, P., "Pre-Congestion Notification (PCN)
Architecture", draft-ietf-pcn-architecture-10 (work in
progress), March 2009.
[I-D.ietf-pcn-baseline-encoding]
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Moncaster, T., Briscoe, B., and M. Menth, "Baseline
Encoding and Transport of Pre-Congestion Information",
draft-ietf-pcn-baseline-encoding-02 (work in progress),
February 2009.
[I-D.ietf-pcn-marking-behaviour]
Eardley, P., "Marking behaviour of PCN-nodes",
draft-ietf-pcn-marking-behaviour-02 (work in progress),
March 2009.
[I-D.ietf-pwe3-congestion-frmwk]
Bryant, S., Davie, B., Martini, L., and E. Rosen,
"Pseudowire Congestion Control Framework",
draft-ietf-pwe3-congestion-frmwk-01 (work in progress),
May 2008.
[I-D.menth-pcn-psdm-encoding]
Menth, M., Babiarz, J., Moncaster, T., and B. Briscoe,
"PCN Encoding for Packet-Specific Dual Marking (PSDM)",
draft-menth-pcn-psdm-encoding-00 (work in progress),
July 2008.
[I-D.moncaster-pcn-3-state-encoding]
Moncaster, T., Briscoe, B., and M. Menth, "A three state
extended PCN encoding scheme",
draft-moncaster-pcn-3-state-encoding-01 (work in
progress), March 2009.
[I-D.satoh-pcn-st-marking]
Satoh, D., Maeda, Y., Phanachet, O., and H. Ueno, "Single
PCN Threshold Marking by using PCN baseline encoding for
both admission and termination controls",
draft-satoh-pcn-st-marking-01 (work in progress),
March 2009.
[IEEE802.1au]
IEEE, "IEEE Standard for Local and Metropolitan Area
Networks--Virtual Bridged Local Area Networks - Amendment
10: Congestion Notification", 2008,
<http://www.ieee802.org/1/pages/802.1au.html>.
(Work in Progress; Access Controlled link within page)
[ITU-T.I.371]
ITU-T, "Traffic Control and Congestion Control in B-ISDN",
ITU-T Rec. I.371 (03/04), March 2004.
[PCNcharter]
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IETF, "Congestion and Pre-Congestion Notification (pcn)",
IETF w-g charter , Feb 2007,
<http://www.ietf.org/html.charters/pcn-charter.html>.
[Patterns_Arch]
Day, J., "Patterns in Network Architecture: A Return to
Fundamentals", Pub: Prentice Hall ISBN-13: 9780132252423,
Jan 2008.
[RFC1254] Mankin, A. and K. Ramakrishnan, "Gateway Congestion
Control Survey", RFC 1254, August 1991.
[RFC2205] Braden, B., Zhang, L., Berson, S., Herzog, S., and S.
Jamin, "Resource ReSerVation Protocol (RSVP) -- Version 1
Functional Specification", RFC 2205, September 1997.
[RFC2983] Black, D., "Differentiated Services and Tunnels",
RFC 2983, October 2000.
[RFC3426] Floyd, S., "General Architectural and Policy
Considerations", RFC 3426, November 2002.
[RFC3540] Spring, N., Wetherall, D., and D. Ely, "Robust Explicit
Congestion Notification (ECN) Signaling with Nonces",
RFC 3540, June 2003.
[RFC4306] Kaufman, C., "Internet Key Exchange (IKEv2) Protocol",
RFC 4306, December 2005.
[RFC4423] Moskowitz, R. and P. Nikander, "Host Identity Protocol
(HIP) Architecture", RFC 4423, May 2006.
[RFC4774] Floyd, S., "Specifying Alternate Semantics for the
Explicit Congestion Notification (ECN) Field", BCP 124,
RFC 4774, November 2006.
[RFC5129] Davie, B., Briscoe, B., and J. Tay, "Explicit Congestion
Marking in MPLS", RFC 5129, January 2008.
[Shayman] "Using ECN to Signal Congestion Within an MPLS Domain",
2000, <http://www.ee.umd.edu/~shayman/papers.d/
draft-shayman-mpls-ecn-00.txt>.
(Expired)
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Appendix A. Design Constraints
Tunnel processing of a congestion notification field has to meet
congestion control and management needs without creating new
information security vulnerabilities (if information security is
required). This appendix documents the analysis of the tradeoffs
between these factors that led to the new encapsulation rules in
Section 4.1.
A.1. Security Constraints
Information security can be assured by using various end to end
security solutions (including IPsec in transport mode [RFC4301]), but
a commonly used scenario involves the need to communicate between two
physically protected domains across the public Internet. In this
case there are certain management advantages to using IPsec in tunnel
mode solely across the publicly accessible part of the path. The
path followed by a packet then crosses security 'domains'; the ones
protected by physical or other means before and after the tunnel and
the one protected by an IPsec tunnel across the otherwise unprotected
domain. We will use the scenario in Figure 5 where endpoints 'A' and
'B' communicate through a tunnel. The tunnel ingress 'I' and egress
'E' are within physically protected edge domains, while the tunnel
spans an unprotected internetwork where there may be 'men in the
middle', M.
physically unprotected physically
<-protected domain-><--domain--><-protected domain->
+------------------+ +------------------+
| | M | |
| A-------->I=========>==========>E-------->B |
| | | |
+------------------+ +------------------+
<----IPsec secured---->
tunnel
Figure 5: IPsec Tunnel Scenario
IPsec encryption is typically used to prevent 'M' seeing messages
from 'A' to 'B'. IPsec authentication is used to prevent 'M'
masquerading as the sender of messages from 'A' to 'B' or altering
their contents. But 'I' can also use IPsec tunnel mode to allow 'A'
to communicate with 'B', but impose encryption to prevent 'A' leaking
information to 'M'. Or 'E' can insist that 'I' uses tunnel mode
authentication to prevent 'M' communicating information to 'B'.
Mutable IP header fields such as the ECN field (as well as the TTL/
Hop Limit and DS fields) cannot be included in the cryptographic
calculations of IPsec. Therefore, if 'I' copies these mutable fields
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into the outer header that is exposed across the tunnel it will have
allowed a covert channel from 'A' to M that bypasses its encryption
of the inner header. And if 'E' copies these fields from the outer
header to the inner, even if it validates 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
from a congested resource towards downstream nodes. Typically a
downstream transport might feed the information back somehow to the
point upstream of the congestion that can regulate the load on the
congested resource, but other actions are possible (see [RFC3168]
S.6). In terms of the above unicast scenario, ECN is typically
intended to create an information channel from 'M' to 'B' (for 'B' to
feed back to 'A'). Therefore the goals of IPsec and ECN are mutually
incompatible.
With respect to the DS or ECN fields, S.5.1.2 of RFC4301 says,
"controls are provided to manage the bandwidth of this [covert]
channel". Using the ECN processing rules of RFC4301, the channel
bandwidth is two bits per datagram from 'A' to 'M' and one bit per
datagram from 'M' to 'A' (because 'E' limits the combinations of the
2-bit ECN field that it will copy). In both cases the covert channel
bandwidth is further reduced by noise from any real congestion
marking. RFC4301 therefore implies that these covert channels are
sufficiently limited to be considered a manageable threat. However,
with respect to the larger (6b) DS field, the same section of RFC4301
says not copying is the default, but a configuration option can allow
copying "to allow a local administrator to decide whether the covert
channel provided by copying these bits outweighs the benefits of
copying". Of course, an administrator considering copying of the DS
field has to take into account that it could be concatenated with the
ECN field giving an 8b per datagram covert channel.
Thus, for tunnelling the 6b Diffserv field two conceptual models have
had to be defined so that administrators can trade off security
against the needs of traffic conditioning [RFC2983]:
The uniform model: where the DIffserv field is preserved end-to-end
by copying into the outer header on encapsulation and copying from
the outer header on decapsulation.
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
from any interaction with nodes along the tunnel.
However, for ECN, the new IPsec security architecture in RFC4301 only
standardised one tunnelling model equivalent to the uniform model.
It deemed that simplicity was more important than allowing
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administrators the option of a tiny increment in security, especially
given not copying congestion indications could seriously harm
everyone's network service.
A.2. Control Constraints
Congestion control requires that any congestion notification marked
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.
To be precise, rather than calling this function the data source, we
will call it the Load Regulator. This will allow us to deal with
exceptional cases where load is not regulated by the data source, but
usually the two terms will be synonymous. Note the term "a function
_capable of_ controlling the load" deliberately includes a source
application that doesn't actually control the load but ought to (e.g.
an application without congestion control that uses UDP).
A--->R--->I=========>M=========>E-------->B
Figure 6: Simple Tunnel Scenario
We now consider a similar tunnelling scenario to the IPsec one just
described, but without the different security domains so we can just
focus on ensuring the control loop and management monitoring can work
(Figure 6). If we want resources in the tunnel 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 any CE marking
from the outer header to the inner header for onward transmission to
'B', otherwise congestion notification from resources like 'M' cannot
be fed back to the Load Regulator ('A'). But it doesn't seem
necessary for 'I' to copy CE markings from the inner to the outer
header. For instance, if resource 'R' is congested, it can send
congestion information to 'B' using the congestion field in the inner
header without 'I' copying the congestion field into the outer header
and 'E' copying it back to the inner header. 'E' can still write any
additional congestion marking introduced across the tunnel into the
congestion field of the inner header.
It might be useful for the tunnel egress to be able to tell whether
congestion occurred across a tunnel or upstream of it. If outer
header congestion marking was reset by the tunnel ingress ('I'), at
the end of a tunnel ('E') the outer headers would indicate congestion
experienced across the tunnel ('I' to 'E'), while the inner header
would indicate congestion upstream of 'I'. But similar information
can be gleaned even if the tunnel ingress copies the inner to the
outer headers. At the end of the tunnel ('E'), any packet with an
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_extra_ mark in the outer header relative to the inner header
indicates congestion across the tunnel ('I' to 'E'), while the inner
header would still indicate congestion upstream of ('I'). Appendix C
gives a simple and precise method for a tunnel egress to infer the
congestion level introduced across a tunnel.
All this shows that 'E' can preserve the control loop irrespective of
whether 'I' copies congestion notification into the outer header or
resets it.
That is the situation for existing control arrangements but, because
copying reveals more information, it would open up possibilities for
better control system designs. For instance, Appendix E describes
how resetting CE marking at a tunnel ingress confuses a proposed
congestion marking scheme on the standards track. It ends up
removing excessive amounts of traffic unnecessarily. Whereas copying
CE markings at ingress leads to the correct control behaviour.
A.3. Management Constraints
As well as control, there are also management constraints.
Specifically, a management system may monitor congestion markings in
passing packets, perhaps at the border between networks as part of a
service level agreement. For instance, monitors at the borders of
autonomous systems may need to measure how much congestion has
accumulated since the original source, perhaps to determine between
them how much of the congestion is contributed by each domain.
Therefore, when monitoring the middle of a path, it should be
possible to establish how far back in the path congestion markings
have accumulated from. In this document we term this the baseline of
congestion marking (or the Congestion Baseline), i.e. the source of
the layer that last reset (or created) the congestion notification
field. Given some tunnels cross domain borders (e.g. consider M in
Figure 6 is monitoring a border), it would therefore be desirable for
'I' to copy congestion accumulated so far into the outer headers
exposed across the tunnel.
Appendix B.2 discusses various scenarios where the Load Regulator
lies in-path, not at the source host as we would typically expect.
It concludes that a Congestion Baseline is determined by where the
Load Regulator function is, which should be identified in the
transport layer, not by addresses in network layer headers. This
applies whether the Load Regulator is at the source host or within
the path. The appendix also discusses where a Load Regulator
function should be located relative to a local tunnel encapsulation
function.
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Appendix B. Relative Placement of Tunnelling and In-Path Load
Regulation
B.1. Identifiers and In-Path Load Regulators
The Load Regulator is the node to which congestion feedback should be
returned by the next downstream node with a transport layer feedback
function (typically but not always the data receiver). The Load
Regulator is often, but not always the data source. It is not always
(or even typically) the same thing as the node identified by the
source address of the outermost exposed header. In general the
addressing of the outermost encapsulation header says nothing about
the identifiers of either the upstream or the downstream transport
layer functions. As long as the transport functions know each
other's addresses, they don't have to be identified in the network
layer or in any link layer. It was only a convenience that a TCP
receiver assumed that the address of the source transport is the same
as the network layer source address of an IP packet it receives.
More generally, the return transport address for feedback could be
identified solely in the transport layer protocol. For instance, a
signalling protocol like RSVP [RFC2205] breaks up a path into
transport layer hops and informs each hop of the address of its
transport layer neighbour without any need to identify these hops in
the network layer. RSVP can be arranged so that these transport
layer hops are bigger than the underlying network layer hops. The
host identity protocol (HIP) architecture [RFC4423] also supports the
same principled separation (for mobility amongst other things), where
the transport layer sender identifies its transport address for
feedback to be sent to, using an identifier provided by a shim below
the transport layer.
Keeping to this layering principle deliberately doesn't require a
network layer packet header to reveal the origin address from where
congestion notification accumulates (its Congestion Baseline). It is
not necessary for the network and lower layers to know the address of
the Load Regulator. Only the destination transport needs to know
that. With forward congestion notification, the network and link
layers only notify congestion forwards; they aren't involved in
feeding it backwards. If they are (e.g. backward congestion
notification (BCN) in Ethernet [IEEE802.1au] or EFCI in ATM
[ITU-T.I.371]), that should be considered as a transport function
added to the lower layer, which must sort out its own addressing.
Indeed, this is one reason why ICMP source quench is now deprecated
[RFC1254]; when congestion occurs within a tunnel it is complex
(particularly in the case of IPsec tunnels) to return the ICMP
messages beyond the tunnel ingress back to the Load Regulator.
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Similarly, if a management system is monitoring congestion and needs
to know the Congestion Baseline, the management system has to find
this out from the transport; in general it cannot tell solely by
looking at the network or link layer headers.
B.2. Non-Dependence of Tunnelling on In-path Load Regulation
We have said that at any point in a network, the Congestion Baseline
(where congestion notification starts from zero) should be the
previous upstream Load Regulator. We have also said that the ingress
of an IP in IP tunnel must copy congestion indications to the
encapsulating outer headers it creates. If the Load Regulator is in-
path rather than at the source, and also a tunnel ingress, these two
requirements seem to be contradictory. A tunnel ingress must not
reset incoming congestion, but a Load Regulator must be the
Congestion Baseline, implying it needs to reset incoming congestion.
In fact, the two requirements are not contradictory, because a Load
Regulator and a tunnel ingress are not the names of machines, but the
names of functions within a machine that typically occur in sequence
on a stream of packets, not at the same point. Figure 7 is borrowed
from [RFC2983] (which was making a similar point about the location
of Diffserv traffic conditioning relative to the encapsulation
function of a tunnel). An in-path Load Regulator can act on packets
either at [1 - Before] encapsulation or at [2 - Outer] after
encapsulation. Load Regulation does not ever need to be integrated
with the [Encapsulate] function (but it can be for efficiency).
Therefore we can still mandate that the [Encapsulate] function always
copies CE into the outer header.
>>-----[1 - Before]--------[Encapsulate]----[3 - Inner]---------->>
\
\
+--------[2 - Outer]------->>
Figure 7: Placement of In-Path Load Regulator Relative to Tunnel
Ingress
Then separately, if there is a Load Regulator at location [2 -
Outer], it might reset CE to ECT(0), say. Then the Congestion
Baseline for the lower layer (outer) will be [2 - Outer], while the
Congestion Baseline of the inner layer will be unchanged. But how
encapsulation works has nothing to do with whether a Load Regulator
is present or where it is.
If on the other hand a Load Regulator resets CE at [1 - Before], the
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Congestion Baseline of both the inner and outer headers will be [1 -
Before]. But again, encapsulation is independent of load regulation.
B.3. Dependence of In-Path Load Regulation on Tunnelling
Although encapsulation doesn't need to depend on in-path load
regulation, the reverse is not true. The placement of an in-path
Load Regulator must be carefully considered relative to
encapsulation. Some examples are given in the following for
guidance.
In the traditional Internet architecture one tends to think of the
source host as the Load Regulator for a path. It is generally not
desirable or practical for a node part way along the path to regulate
the load. However, various reasonable proposals for in-path load
regulation have been made from time to time (e.g. fair queuing,
traffic engineering, flow admission control). The IETF has recently
chartered a working group to standardise admission control across a
part of a path using pre-congestion notification (PCN) [PCNcharter].
This is of particular relevance here because it involves congestion
notification with an in-path Load Regulator, it can involve
tunnelling and it certainly involves encapsulation more generally.
We will use the more complex scenario in Figure 8 to tease out all
the issues that arise when combining congestion notification and
tunnelling with various possible in-path load regulation schemes. In
this case 'I1' and 'E2' break up the path into three separate
congestion control loops. The feedback for these loops is shown
going right to left across the top of the figure. The 'V's are arrow
heads representing the direction of feedback, not letters. But there
are also two tunnels within the middle control loop: 'I1' to 'E1' and
'I2' to 'E2'. The two tunnels might be VPNs, perhaps over two MPLS
core networks. M is a congestion monitoring point, perhaps between
two border routers where the same tunnel continues unbroken across
the border.
______ _______________________________________ _____
/ \ / \ / \
V \ V M \ V \
A--->R--->I1===========>E1----->I2=========>==========>E2------->B
Figure 8: Complex Tunnel Scenario
The question is, should the congestion markings in the outer exposed
headers of a tunnel represent congestion only since the tunnel
ingress or over the whole upstream path from the source of the inner
header (whatever that may mean)? Or put another way, should 'I1' and
'I2' copy or reset CE markings?
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Based on the design principles in Section 4.3, the answer is that the
Congestion Baseline should be the nearest upstream interface designed
to regulate traffic load--the Load Regulator. In Figure 8 'A', 'I1'
or 'E2' are all Load Regulators. We have shown the feedback loops
returning to each of these nodes so that they can regulate the load
causing the congestion notification. So the Congestion Baseline
exposed to M should be 'I1' (the Load Regulator), not 'I2'.
Therefore I1 should reset any arriving CE markings. In this case,
'I1' knows the tunnel to 'E1' is unrelated to its load regulation
function. So the load regulation function within 'I1' should be
placed at [1 - Before] tunnel encapsulation within 'I1' (using the
terminology of Figure 7). Then the Congestion Baseline all across
the networks from 'I1' to 'E2' in both inner and outer headers will
be 'I1'.
The following further examples illustrate how this answer might be
applied:
o We argued in Appendix E that resetting CE on encapsulation could
harm PCN excess rate marking, which marks excess traffic for
removal in subsequent round trips. This marking relies on not
marking packets if another node upstream has already marked them
for removal. If there were a tunnel ingress between the two which
reset CE markings, it would confuse the downstream node into
marking far too much traffic for removal. So why do we say that
'I1' should reset CE, while a tunnel ingress shouldn't? The
answer is that it is the Load Regulator function at 'I1' that is
resetting CE, not the tunnel encapsulator. The Load Regulator
needs to set itself as the Congestion Baseline, so the feedback it
gets will only be about congestion on links it can relieve itself
(by regulating the load into them). When it resets CE markings,
it knows that something else upstream will have dealt with the
congestion notifications it removes, given it is part of an end-
to-end admission control signalling loop. It therefore knows that
previous hops will be covered by other Load Regulators.
Meanwhile, the tunnel ingresses at both 'I1' and 'I2' should
follow the new rule for any tunnel ingress and copy congestion
marking into the outer tunnel header. The ingress at 'I1' will
happen to copy headers that have already been reset just
beforehand. But it doesn't need to know that.
o [Shayman] suggested feedback of ECN accumulated across an MPLS
domain could cause the ingress to trigger re-routing to mitigate
congestion. This case is more like the simple scenario of
Figure 6, with a feedback loop across the MPLS domain ('E' back to
'I'). I is a Load Regulator because re-routing around congestion
is a load regulation function. But in this case 'I' should only
reset itself as the Congestion Baseline in outer headers, as it is
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not handling congestion outside its domain, so it must preserve
the end-to-end congestion feedback loop for something else to
handle (probably the data source). Therefore the Load Regulator
within 'I' should be placed at [2 - Outer] to reset CE markings
just after the tunnel ingress has copied them from arriving
headers. Again, the tunnel encapsulation function at 'I' simply
copies incoming headers, unaware that the load regulator will
subsequently reset its outer headers.
o The PWE3 working group of the IETF is considering the problem of
how and whether an aggregate edge-to-edge pseudo-wire emulation
should respond to congestion [I-D.ietf-pwe3-congestion-frmwk].
Although the study is still at the requirements stage, some
(controversial) solution proposals include in-path load regulation
at the ingress to the tunnel that could lead to tunnel
arrangements with similar complexity to that of Figure 8.
These are not contrived scenarios--they could be a lot worse. For
instance, a host may create a tunnel for IPsec which is placed inside
a tunnel for Mobile IP over a remote part of its path. And around
this all we may have MPLS labels being pushed and popped as packets
pass across different core networks. Similarly, it is possible that
subnets could be built from link technology (e.g. future Ethernet
switches) so that link headers being added and removed could involve
congestion notification in future Ethernet link headers with all the
same issues as with IP in IP tunnels.
One reason we introduced the concept of a Load Regulator was to allow
for in-path load regulation. In the traditional Internet
architecture one tends to think of a host and a Load Regulator as
synonymous, but when considering tunnelling, even the definition of a
host is too fuzzy, whereas a Load Regulator is a clearly defined
function. Similarly, the concept of innermost header is too fuzzy to
be able to (wrongly) say that the source address of the innermost
header should be the Congestion Baseline. Which is the innermost
header when multiple encapsulations may be in use? Where do we stop?
If we say the original source in the above IPsec-Mobile IP case is
the host, how do we know it isn't tunnelling an encrypted packet
stream on behalf of another host in a p2p network?
We have become used to thinking that only hosts regulate load. The
end to end design principle advises that this is a good idea
[RFC3426], but it also advises that it is solely a guiding principle
intended to make the designer think very carefully before breaking
it. We do have proposals where load regulation functions sit within
a network path for good, if sometimes controversial, reasons, e.g.
PCN edge admission control gateways [I-D.ietf-pcn-architecture] or
traffic engineering functions at domain borders to re-route around
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congestion [Shayman]. Whether or not we want in-path load
regulation, we have to work round the fact that it will not go away.
Appendix C. Contribution to Congestion across a Tunnel
This specification mandates that a tunnel ingress determines the ECN
field of each new outer tunnel header by copying the arriving header.
Concern has been expressed that this will make it difficult for the
tunnel egress to monitor congestion introduced only along a tunnel,
which is easy if the outer ECN field is reset at a tunnel ingress
(RFC3168 full functionality mode). However, in fact copying CE marks
at ingress will still make it easy for the egress to measure
congestion introduced across a tunnel, as illustrated below.
Consider 100 packets measured at the egress. It measures that 30 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 at
the ingress had already experienced 30% congestion. However, it does
not mean there was 12% congestion across the tunnel. The correct
calculation of congestion across the tunnel is p_t = 12/(100-30) =
12/70 = 17%. This is easy for the egress to to measure. It is the
packets with additional CE marking in the outer header (12) as a
proportion of packets not marked in the inner header (70).
Figure 9 illustrates this in a combinatorial probability diagram.
The square represents 100 packets. The 30% division along the bottom
represents marking before the ingress, and the p_t division up the
side represents marking along the tunnel.
+-----+---------+100%
| | |
| 30 | |
| | | The large square
| +---------+p_t represents 100 packets
| | 12 |
+-----+---------+0
0 30% 100%
inner header marking
Figure 9: Tunnel Marking of Packets Already Marked at Ingress
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Appendix D. Why Not Propagating ECT(1) on Decapsulation Impedes PCN
Multi-level congestion notification is currently on the IETF's
standards track agenda in the Congestion and Pre-Congestion
Notification (PCN) working group. The PCN working group eventually
requires three congestion states (not marked and two increasingly
severe levels of congestion marking) [I-D.ietf-pcn-architecture].
The aim is for the less severe level of marking to stop admitting new
traffic and the more severe level to terminate sufficient existing
flows to bring a network back to its operating point after a serious
failure.
Although the ECN field gives sufficient codepoints for these three
states, current ECN tunnelling RFCs prevent the PCN working group
from using three ECN states in case any tunnel decapsulations occur
within a PCN region (see Appendix A of
[I-D.ietf-pcn-baseline-encoding]). If a node in a tunnel sets the
ECN field to ECT(0) or ECT(1), this change will be discarded by a
tunnel egress compliant with RFC4301 or RFC3168. This can be seen in
Figure 2 (Section 3.2), where ECT values in the outer header are
ignored unless the inner header is the same. Effectively one ECT
codepoint is wasted; the ECT(0) and ECT(1) codepoints have to be
treated as just one codepoint when they could otherwise have been
used for their intended purpose of congestion notification.
As a consequence, the PCN w-g has initially confined itself to two
encoding states as a baseline encoding
[I-D.ietf-pcn-baseline-encoding]. And it has had to propose an
experimental extension using extra Diffserv codepoint(s) to encode
the extra states [I-D.moncaster-pcn-3-state-encoding], using up the
rapidly exhausting DSCP space while leaving ECN codepoints unused.
Another PCN encoding has been proposed that would survive tunnelling
without an extra DSCP [I-D.menth-pcn-psdm-encoding], but it requires
the PCN edge gateways to somehow share state so the egress can
determine which marking a packet started with at the ingress. Also a
PCN ingress node can game the system by initiating packets with
inappropriate markings. Yet another work-round to the ECN tunnelling
problem proposes a more involved marking algorithm in the forwarding
plane to encode the three congestion notification states using only
two ECN codepoints [I-D.satoh-pcn-st-marking]. Still another
proposal compromises the precision of the admission control
mechanism, but manages to work with just two encoding states and a
single marking algorithm [I-D.charny-pcn-single-marking].
Rather than require the IETF to bless any of these work-rounds, this
specification fixes the root cause of the problem so that operators
deploying PCN can simply ask that tunnel end-points within a PCN
region should comply with this new ECN tunnelling specification.
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Then PCN can use the trivially simple experimental 3-state ECN
encoding defined in [I-D.briscoe-pcn-3-in-1-encoding].
D.1. Alternative Ways to Introduce the New Decapsulation Rules
There are a number of ways for the new decapsulation rules to be
introduced:
o They could be specified in the present standards track proposal
(preferred) or in an experimental extension;
o They could be specified as a new default for all Diffserv PHBs
(preferred) or as an option to be configured only for Diffserv
PHBs requiring them (e.g. PCN).
The argument for making this change now, rather than in a separate
experimental extension, is to avoid the burden of an extra standard
to be compliant with and to be backwards compatible with--so we don't
add to the already complex history of ECN tunnelling RFCs. The
argument for a separate experimental extension is that we may never
need this change (if PCN is never successfully deployed and if no-one
ever needs three ECN or PCN encoding states rather than two).
However, the change does no harm to existing mechanisms and stops
tunnels wasting of quarter of a bit (a 2-bit codepoint).
The argument for making this new decapsulation behaviour the default
for all PHBs is that it doesn't change any expected behaviour that
existing mechanisms rely on already. Also, by ending the present
waste of a codepoint, in the future a use of that codepoint could be
proposed for all PHBs, even if PCN isn't successfully deployed.
In practice, if these new decapsulation rules are specified
straightaway as the normative default for all PHBs, a network
operator deploying 3-state PCN would be able to request that tunnels
comply with the latest specification. Implementers of non-PCN
tunnels would not need to comply but, if they did, their code would
be future proofed and no harm would be done to legacy operations.
Therefore, rather than branching their code base, it would be easiest
for implementers to make all their new tunnel code comply with this
specfication, whether or not it was for PCN. But they could leave
old code untouched, unless it was for PCN.
The alternatives are worse. Implementers would otherwise have to
provide configurable decapsulation options and operators would have
to configure all IPsec and IP in IP tunnel endpoints for the
exceptional behaviour of certain PHBs. The rules for tunnel
endpoints to handle both the Diffserv field and the ECN field should
'just work' when handling packets with any Diffserv codepoint.
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Appendix E. Why Resetting CE on Encapsulation Impedes PCN
Regarding encapsulation, the section of the PCN architecture
[I-D.ietf-pcn-architecture] on tunnelling says that header copying
(RFC4301) allows PCN to work correctly. Whereas resetting CE
markings confuses PCN marking.
The specific issue here concerns PCN excess rate marking
[I-D.ietf-pcn-marking-behaviour], i.e. the bulk marking of traffic
that exceeds a configured threshold rate. One of the goals of excess
rate marking is to enable the speedy removal of excess admission
controlled traffic following re-routes caused by link failures or
other disasters. This maintains a share of the capacity for traffic
in lower priority classes. After failures, traffic re-routed onto
remaining links can often stress multiple links along a path.
Therefore, traffic can arrive at a link under stress with some
proportion already marked for removal by a previous link. By design,
marked traffic will be removed by the overall system in subsequent
round trips. So when the excess rate marking algorithm decides how
much traffic to mark for removal, it doesn't include traffic already
marked for removal by another node upstream (the `Excess traffic
meter function' of [I-D.ietf-pcn-marking-behaviour]).
However, if an RFC3168 tunnel ingress intervenes, it resets the ECN
field in all the outer headers, hiding all the evidence of problems
upstream. Thus, although excess rate marking works fine with RFC4301
IPsec tunnels, with RFC3168 tunnels it typically removes large
volumes of traffic that it didn't need to remove at all.
Author's Address
Bob Briscoe
BT
B54/77, Adastral Park
Martlesham Heath
Ipswich IP5 3RE
UK
Phone: +44 1473 645196
Email: bob.briscoe@bt.com
URI: http://www.cs.ucl.ac.uk/staff/B.Briscoe/
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