Transport Area Working Group B. Briscoe
Internet-Draft BT
Intended status: Standards Track Oct 27, 2008
Expires: April 30, 2009
Layered Encapsulation of Congestion Notification
draft-ietf-tsvwg-ecn-tunnel-01
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Abstract
This document redefines how the explicit congestion notification
(ECN) field of the outer IP header of a tunnel should be constructed.
It brings all IP in IP tunnels (v4 or v6) into line with the way
IPsec tunnels now construct the ECN field. It includes a thorough
analysis of the reasoning for this change and the implications. It
also gives guidelines on the encapsulation of IP congestion
notification by any outer header, whether encapsulated in an IP
tunnel or in a lower layer header. Following these guidelines should
help interworking, if the IETF or other standards bodies specify any
new encapsulation of congestion notification.
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 4
1.1. The Need for Rationalisation . . . . . . . . . . . . . . . 5
1.2. Document Roadmap . . . . . . . . . . . . . . . . . . . . . 6
1.3. Scope . . . . . . . . . . . . . . . . . . . . . . . . . . 7
2. Requirements Language . . . . . . . . . . . . . . . . . . . . 8
3. Design Constraints . . . . . . . . . . . . . . . . . . . . . . 9
3.1. Security Constraints . . . . . . . . . . . . . . . . . . . 9
3.2. Control Constraints . . . . . . . . . . . . . . . . . . . 11
3.3. Management Constraints . . . . . . . . . . . . . . . . . . 12
4. Design Principles . . . . . . . . . . . . . . . . . . . . . . 13
4.1. Design Guidelines for New Encapsulations of Congestion
Notification . . . . . . . . . . . . . . . . . . . . . . . 14
5. Default ECN Tunnelling Rules . . . . . . . . . . . . . . . . . 16
6. Backward Compatibility . . . . . . . . . . . . . . . . . . . . 17
7. Changes from Earlier RFCs . . . . . . . . . . . . . . . . . . 19
8. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 20
9. Security Considerations . . . . . . . . . . . . . . . . . . . 20
10. Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . 22
11. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 23
12. Comments Solicited . . . . . . . . . . . . . . . . . . . . . . 23
13. References . . . . . . . . . . . . . . . . . . . . . . . . . . 23
13.1. Normative References . . . . . . . . . . . . . . . . . . . 23
13.2. Informative References . . . . . . . . . . . . . . . . . . 24
Editorial Comments . . . . . . . . . . . . . . . . . . . . . . . .
Appendix A. Why resetting CE on encapsulation harms PCN . . . . . 26
Appendix B. Contribution to Congestion across a Tunnel . . . . . 27
Appendix C. Comprehensive Decapsulation Rules . . . . . . . . . . 28
C.1. Ways to Introduce the Comprehensive Decapsulation Rules . 31
Appendix D. Non-Dependence of Tunnelling on In-path Load
Regulation . . . . . . . . . . . . . . . . . . . . . 32
D.1. Dependence of In-Path Load Regulation on Tunnelling . . . 33
Author's Address . . . . . . . . . . . . . . . . . . . . . . . . . 36
Intellectual Property and Copyright Statements . . . . . . . . . . 37
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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
, and
between earlier individual draft versions at
From ietf-00 to ietf-01 (current):
* Identified two additional alarm states in the decapsulation
rules (Figure 3) if ECT(X) in outer and inner contradict each
other.
* Altered Comprehensive Decapsulation Rules (Appendix C) 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 C.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 B giving much simpler technique to measure
contribution to congestion across a tunnel.
* Added discussion of backward compatibility of the ideal
decapsulation scheme in Appendix C
* 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 A;
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* 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 (Section 4.1);
* 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 A. "Why resetting CE on encapsulation harms
PCN", Appendix B. "Contribution to Congestion across a Tunnel"
and Appendix C. "Ideal Decapsulation Rules";
* Re-wrote Appendix D, 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] of the outer IP header of a tunnel should be
constructed. It brings all IP in IP tunnels (v4 or v6) into line
with the way IPsec tunnels [RFC4301] now construct the ECN field,
ensuring that the outer header reveals any congestion experienced so
far on the whole path, not just since the last tunnel ingress.
ECN allows a congested resource to notify the onset of congestion
without having to drop packets, by explicitly marking a proportion of
packets with the congestion experienced (CE) codepoint. Because
congestion is exhaustion of a physical resource, if the transport
layer is to deal with congestion, congestion notification must
propagate upwards; from the physical layer to the transport layer.
The transport layer can directly detect loss of a packet (or frame)
by a lower layer. But if a lower layer marks rather than drops a
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forward-travelling data packet (or frame) in order to notify
incipient congestion, this marking has to be explicitly copied up the
layers at every header decapsulation. So, at each decapsulation of
an outer (lower layer) header a congestion marking has to be arranged
to propagate into the forwarded (upper layer) header. It must
continue upwards until it reaches the destination transport. Then
typically the destination feeds this congestion notification back to
the source transport. Given encapsulation by lower layer headers is
functionally similar to tunnelling, it is necessary to arrange
similar propagation of congestion notification up the layers. For
instance, ECN and its propagation up the layers has recently been
specified for MPLS [RFC5129].
As packets pass up the layers, current specifications of
decapsulation behaviours are largely all consistent and correct.
However, as packets pass down the layers, specifications of
encapsulation behaviours are not consistent. This document is
primarily aimed at rationalising encapsulation. (Nevertheless,
Appendix C explains why the consistency of decapsulation solutions
will not last for long and proposes a fix to decapsulation rules as
well. The IETF can then discuss whether to rationalise decapsulation
at the same time as encapsulation.)
1.1. The Need for Rationalisation
IPsec tunnel mode is a specific form of tunnelling that can hide the
inner headers. Because the ECN field has to be mutable, it cannot be
covered by IPsec encryption or authentication calculations.
Therefore concern has been raised in the past that the ECN field
could be used as a low bandwidth covert channel to communicate with
someone on the unprotected public Internet even if an end-host is
restricted to only communicate with the public Internet through an
IPsec gateway. However, the updated version of IPsec [RFC4301] chose
not to block this covert channel, deciding that the threat could be
managed given the channel bandwidth is so limited (ECN is a 2-bit
field).
An unfortunate sequence of standards actions leading up to this
latest change in IPsec has left us with nearly the worst of all
possible combinations of outcomes, despite the best endeavours of
everyone concerned. The controversy has been over whether to reveal
information about congestion experienced on the path upstream of the
tunnel ingress. Even though this has various uses if it is revealed
in the outer header of a tunnel, when ECN was standardised [RFC3168]
it was decided that all IP in IP tunnels should hide this upstream
congestion simply to avoid the extra complexity of two different
mechanisms for IPsec and non-IPsec tunnels. However, now that
[RFC4301] IPsec tunnels deliberately no longer hide this information,
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we are left in the perverse position where non-IPsec tunnels still
hide congestion information unnecessarily. This document is designed
to correct that anomaly.
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 has to set
the ECN field of the outer header to ECT(0) (i.e. codepoint 10). We
term this 'resetting' a CE codepoint. However, RFC4301 reverses
this, stating that the tunnel ingress must simply copy the ECN field
from the inner to the outer header. The main purpose of this
document 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.
Why does it matter if we have different ECN encapsulation behaviours
for IPsec and non-IPsec tunnels? The general argument is that
gratuitous inconsistency constrains the available design space and
makes it harder to design networks and new protocols that work
predictably.
Already complicated constraints have had to be added to a standards
track congestion marking proposal. The section of the pre-congestion
notification (PCN) architecture [I-D.ietf-pcn-architecture] on
tunnelling says PCN works correctly in the presence of RFC4301 IPsec
encapsulation (and RFC5129 MPLS encapsulation). However it doesn't
work with RFC3168 IP in IP encapsulation (Appendix A explains why).
To ensure we do not cause any unintended side-effects, Section 3
assesses whether copying or resetting CE would 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 deems is manageable).
1.2. Document Roadmap
Most of the document gives a thorough analysis of the knock-on
effects of the apparently minor change to tunnel encapsulation. The
reader may jump to Section 5 if only interested in standards actions
impacting implementation. The whole document is organised as
follows:
o 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 we cannot only discuss the ECN protocol
that RFC3168 gives as a default. Instead, Section 3 lays out the
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design constraints when tunnelling congestion notification without
assuming a particular congestion marking scheme.
o Then in Section 4 we resolve the tensions between these
constraints to give general design principles and guidelines on
how a tunnel should process congestion notification; principles
that could apply to any marking behaviour for any PHB, not just
the default in RFC3168. In particular, we examine the underlying
principles behind whether CE should be reset or copied into the
outer header at the ingress to a tunnel--or indeed at the ingress
of any layered encapsulation of headers with congestion
notification fields. We end this section with a bulleted list of
design guidelines for new encapsulations of congestion
notification.
o Section 5 then uses precise standards terminology to confirm the
rules for the default ECN tunnelling behaviour based on the above
design principles.
o Extending the new IPsec tunnel ingress behaviour to all IP in IP
tunnels requires consideration of backwards compatibility, which
is covered in Section 6 and changes from earlier RFCs are brought
together in Section 7.
o Finally, a number of security considerations are discussed and
conclusions are drawn.
1.3. 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 a common, default congestion encapsulation
for any IP in IP tunnelling, based on that now specified for IPsec.
It applies irrespective of whether IPv4 or IPv6 is used for either of
the inner and outer headers. It applies to all 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 should be followed as closely as
possible. There is no requirement for a PHB to state anything about
ECN tunnelling behaviour if the default behaviour is sufficient.
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Often lower layer resources (e.g. a point-to-point Ethernet link) are
arranged to be protected by higher layer buffers, so instead of
congestion occurring at the lower layer, it merely causes the queue
from the higher layer to overflow. Such non-blocking link and
physical layer technologies do not have to implement congestion
notification, which can be introduced solely in the active queue
management (AQM) from the IP layer. However, not all link layer
technologies are always protected from congestion by buffers at
higher layers (e.g. a subnetwork of Ethernet links and switches can
congest internally). In these cases, when adding congestion
notification to lower layers, we have to arrange for it to be
explicitly copied up the layers, just as when IP is tunnelled in IP.
As well as guiding alternate IP in IP tunnelling schemes, the design
guidelines of Section 4 are intended to be followed when IP packets
are encapsulated by any connectionless datagram/packet/frame where
the outer header is designed to support a congestion notification
capability. [RFC5129] already deals with handling ECN for IP in MPLS
and MPLS in MPLS, and S.9.3 of [RFC3168] lists IP encapsulated in
L2TP [RFC2661], GRE [RFC1701] or PPTP [RFC2637] as possible examples
where ECN may be added in future.
Of course, the IETF does not have standards authority over every link
or tunnel protocol, so this document merely aims to guide the
interface between IP ECN and lower layer congestion notification.
Then the IETF or the relevant standards body can be free to define
the specifics of each lower layer scheme, but a common interface
should ensure interworking across all technologies.
Note that just because there is forward congestion notification in a
lower layer protocol, if the lower layer has its own feedback and
load regulation, there is no need to propagate it up the layers. For
instance, FECN (forward ECN) has been present in Frame Relay and EFCI
(explicit forward congestion indication) in ATM [ITU-T.I.371] for a
long time. But so far they have been used for internal management
rather than being propagated to endpoint transports for them to
control end-to-end congestion.
[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.
2. Requirements Language
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
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"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in RFC 2119 [RFC2119].
3. Design Constraints
Tunnel processing of a congestion notification field has to meet
congestion control needs without creating new information security
vulnerabilities (if information security is required).
3.1. 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 1 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 1: 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
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calculations of IPsec. Therefore, if 'I' copies these mutable fields
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.
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It deemed that simplicity was more important than allowing
administrators the option of a tiny increment in security, especially
given not copying congestion indications could seriously harm
everyone's network service.
3.2. 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 2: 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 2). 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
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outer headers. At the end of the tunnel ('E'), any packet with an
_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 B
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 A 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.
3.3. 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 2 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 D 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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4. Design Principles
The constraints from the three perspectives of security, control and
management in Section 3 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 an outer encapsulating header capable of carrying
congestion markings SHOULD reflect accumulated congestion since the
last interface designed to regulate load (the Load Regulator). This
implies congestion notification SHOULD be copied into the outer
header of each new encapsulating header that supports it.
We have said that a tunnel ingress SHOULD (as opposed to MUST) copy
incoming congestion notification into an outer encapsulating header
that supports it. In the case of 2-bit ECN, the IETF security area
has deemed the benefit always outweighs the risk. Therefore for
2-bit ECN we can and we will say 'MUST' (Section 5). But in this
section where we are setting down general design principles, we leave
it as a 'SHOULD'. This allows for future multi-bit congestion
notification fields where the risk from the covert channel created by
copying congestion notification might outweigh the congestion control
benefit of copying.
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 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
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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.
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.
4.1. Design Guidelines for New Encapsulations of Congestion
Notification
The following guidelines are for specifications of new schemes for
encapsulating congestion notification (e.g. for specialised Diffserv
PHBs in IP, or for lower layer technologies):
1. Congestion notification in outer headers SHOULD be relative to a
Congestion Baseline at the node expected to regulate the load on
the link in question (the Load Regulator). This implies incoming
congestion notifications from the higher layer SHOULD be copied
into encapsulating headers. This guideline is particularly
important where outer headers might cross trust boundaries, but
less important otherwise.
2. Congestion notification MUST NOT simply be copied from outer
headers to the forwarded header on decapsulation. The forwarded
congestion notification field SHOULD be calculated from the inner
and outer headers, taking account of the following, in the order
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given:
1. If the inner header does not support congestion notification,
or indicates that the transport does not support congestion
notification, any explicit congestion notifications in the
outer header will not be understood if propagated further, so
if the only way to indicate congestion to onward nodes is to
drop the packet, it MUST be dropped.
2. If the outer header does not support explicit congestion
notification, but the inner header does, the inner header
SHOULD be forwarded unchanged.
3. Congestion indications may be ranked by strength. For
instance no congestion would be the weakest indication, with
possibly increasing levels of congestion given increasingly
stronger indications.
4. Where the inner and outer headers carry indications of
congestion of different strengths, the stronger indication
SHOULD be forwarded in preference to the weaker. Obviously,
if the strengths in both inner and outer are the same, the
same strength should be forwarded.
5. If the outer header carries a weaker indication of congestion
than the inner, it MAY be appropriate to raise a warning, as
this would be in illegal combination if Guideline Paragraph 1
had been followed.
3. Where framing boundaries are different between the two layers,
congestion indications SHOULD be propagated on the basis that a
congestion indication in a packet or frame applies to all the
octets in the frame/packet. On average, a tunnel endpoint SHOULD
approximately preserve the number of marked octets arriving and
leaving. An algorithm for spreading congestion indications over
multiple smaller `fragments' SHOULD propagate congestion
indications as soon as they arrive, and SHOULD NOT hold them back
for later frames.
4. Assumptions on incremental deployment MUST be stated.
Regarding incremental deployment, the Per-Domain ECT Checking
of[RFC5129] is a good example to follow. In this example, header
space in the lower layer protocol (MPLS) was extremely limited, so no
ECN-capable transport codepoint was added to the MPLS header.
Interior nodes in a domain were allowed to set explicit congestion
indications without checking whether the frame was destined for a
transport that would understand them. This was made safe by
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emphasising repeatedly that all the decapsulating edges of a whole
domain had to be upgraded at once, so there would always be a check
that the higher layer transport was ECN-capable on decapsulation. If
the decapsulator discovered that the higher layer showed the
transport would not understand ECN, it dropped the packet on behalf
of the earlier congestion node (see Guideline Paragraph 2.1).
Note that such a deployment strategy that assumes a savvy operator
was only appropriate because MPLS is targeted solely at professional
operators. This strategy would not be appropriate for other link
technologies (e.g. Ethernet) targeted at deployment by the general
public.
5. Default ECN Tunnelling Rules
The following ECN tunnel processing rules are the default for a
packet with any DSCP. If required, different ECN encapsulation rules
MAY be defined as part of the definition of an appropriate Diffserv
PHB using the guidelines in Section 4. However, the burden of
handling exceptional PHBs in implementations of all affected tunnels
and lower layer link protocols should not be underestimated.
A tunnel ingress compliant with this specification MUST copy the
2-bit ECN field of the arriving IP header into the outer
encapsulating IP header, for all types of IP in IP tunnel. This
encapsulation behaviour MUST only be used if the tunnel ingress is in
`normal state'. A `compatibility state' with a different
encapsulation behaviour is also specified in Section 6 for backward
compatibility with legacy tunnel egresses that do not understand ECN.
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 3.
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+----------------------------------------------+
| Incoming Outer Header |
+------------------+---------+------------+------------+----------+
| Incoming Inner | Not-ECT | ECT(0) | ECT(1) | CE |
| Header | | | | |
+------------------+---------+------------+------------+----------+
| 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 3: IP in IP Decapsulation
The exclamation marks '(!!!)' in Figure 3 indicate that this
combination of inner and outer headers should not be possible if only
legal transitions have taken place. So, the decapsulator should drop
or mark the ECN field as the table in Figure 3 specifies, but it MAY
also raise an appropriate alarm. It MUST NOT raise an alarm so often
that the illegal combinations would amplify into a flood of alarm
messages.
6. Backward Compatibility
Note: in RFC3168, a tunnel was in one of two modes: limited
functionality or full functionality. Rather than working with modes
of the tunnel as a whole, this specification uses the term `state' to
refer separately to the state of each tunnel end point, which is how
implementations have to work.
If one end of an IPsec tunnel is compliant with [RFC4301], the other
end can be guaranteed to also be [RFC4301] compliant (there could be
corner cases where manual keying is used, but they will be ignored
here). So there is no backward compatibility problem with IKEv2
RFC4301 IPsec tunnels. But once we extend our scope to any IP in IP
tunnel, we have to cater for the possibility that a legacy tunnel
egress may not know how to process an ECN field, so 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.
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To be compliant with this specification a tunnel ingress that does
not always know the ECN capability of its tunnel egress MUST
implement a 'normal' state and a 'compatibility' state, and it MUST
initiate each negotiated tunnel in the compatibility state.
However, a tunnel ingress can be compliant even if it only implements
the 'normal state' 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 full ECN functionality (RFC3168 full
functionality mode, RFC4301 and this present specification). The
`normal state' is that defined in Section 5 (i.e. header copying).
Note that a [RFC4301] tunnel ingress that has used IKEv2 key
management [RFC4306] can guarantee that its tunnel egress is also
RFC4301-compliant and therefore need not further negotiate ECN
capabilities.
Before switching to normal state, a compliant tunnel ingress that
does not know the egress ECN capability MUST negotiate with the
tunnel egress. If the egress says it is in full functionality state
(or mode), the ingress puts itself into normal state. In normal
state the ingress follows the encapsulation rule in Section 5 (i.e.
header copying). If the egress says it is not in full-functionality
state/mode or doesn't understand the question, the tunnel ingress
MUST remain in compatibility state.
A tunnel ingress in compatibility state MUST set all outer headers to
Not-ECT. This is the same per packet behaviour as the ingress end of
RFC3168's limited functionality mode.
A tunnel ingress that only implements compatibility state 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). But an ingress cannot claim compliance with
this specification simply by disabling ECN processing across the
tunnel. A compliant tunnel ingress MUST at least implement `normal
state' and, if it might be used with arbitrary tunnel egress nodes,
it MUST also implement `compatibility state'.
A compliant tunnel egress on the other hand merely needs to implement
the one behaviour in Section 5, which we term 'full-functionality'
state, as it is the same as the egress end of the full-functionality
mode of [RFC3168]. It is also the same as the [RFC4301] egress
behaviour.
The decapsulation rules for the egress of the tunnel in Section 5
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
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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 states or modes it doesn't
recognise, but it can continue operating. If no ECN-related state or
mode is requested, a compliant tunnel egress need not raise an error
or warning as its egress behaviour is compatible with all the legacy
ingress behaviours that don't negotiate capabilities.
Implementation note: if a compliant node is the ingress for multiple
tunnels, a state 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 state setting, because a
compliant egress can only be in one state.
7. Changes from Earlier RFCs
The rule that a normal state 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.
The rules for calculating the outgoing ECN field on decapsulation at
a tunnel egress are in line with the full functionality mode of ECN
in RFC3168 and with RFC4301, except that neither identified the
following illegal combinations: outer ECT(1) with inner ECT(0) or
with 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.
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
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association (SA) has no policy on ECN usage, because all RFC4301
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
states: a normal state (required) and a compatibility state (required
in some scenarios, optional in others). The egress needs only full-
functionality state which handles ECN the same as either mode of
RFC3168 or RFC4301.
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 and RFC4301 should be identified as an update to RFC3168.
This specification updates RFC3168. It also suggests a minor
optional warning and a corner-case change to RFC4301, but these don't
really count as an update.
8. IANA Considerations
This memo includes no request to IANA.
9. Security Considerations
Section 3.1 discusses the security constraints imposed on ECN tunnel
processing. The Design Principles of 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
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region with alternate ECN semantics. Such edge processing must also
be applied at the endpoints of tunnels with ends both inside and
outside the domain. [I-D.ietf-pcn-architecture] gives specific
advice on this for the PCN case, but other definitions of alternate
semantics will need to discuss the specific security implications in
their case.
With the rules as they stand in RFC3168 and RFC4301, a small part of
the protection of the ECN nonce [RFC3540] is compromised. One reason
two ECT codepoints were defined was 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 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. To close this
minor loophole, we could have specified that an outer header value of
ECT should overwrite a contradictory ECT value in the inner header.
But currently we choose to keep the 'broken' behaviour defined in
RFC3168 & RFC4301 for all the following reasons:
1. We wanted to avoid any changes to IPsec tunnelling behaviour;
2. Allowing ECT values in the outer header to override the inner
header would have increased the bandwidth of the covert channel
through the egress gateway from 1 to 1.5 bit per datagram,
potentially threatening to upset the consensus established in the
security area that says that the bandwidth of this covert channel
can now be safely managed;
3. This loophole is only applicable in the corner case where the
attacker is a network node downstream of a congested node in the
same tunnel;
4. 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);
5. Although the 'broken' decapsulation behaviour removes evidence of
congestion suppression from the onward feedback loop, the
decapsulator itself can at least detect that congestion within
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the tunnel has been suppressed;
6. The ECN nonce [RFC3540] currently has experimental status and
there has been no evidence that anyone has implemented it beyond
the author's prototype.
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 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.
10. 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.
At a tunnel egress, header decapsulation for the default ECN marking
behaviour is broadly unchanged except that one exceptional case has
been catered for. At the ingress, for all forms of IP in IP tunnel,
encapsulation has been brought into line with the new IPsec rules in
RFC4301 which copy rather than reset CE markings when creating outer
headers.
This change to encapsulation has been motivated by analysis 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.
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The change ensures ECN processing in all IP in IP tunnels reflects
this slightly more permissive attitude to revealing congestion
information in the new IPsec architecture. Once all tunnelling of
ECN works the same, ECN markings will have a defined meaning when
measured at any point in a network. This new certainty will enable
new uses of the ECN field that would otherwise be confounded by
ambiguity.
Also, this document defines more generic principles to guide the
design of alternate forms of tunnel processing of congestion
notification, if required for specific Diffserv PHBs or for other
lower layer encapsulating protocols that might support congestion
notification in the future.
11. Acknowledgements
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 B. Thanks to 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.
12. Comments Solicited
Comments and questions are encouraged and very welcome. They can be
addressed to the IETF Transport Area working group mailing list
, and/or to the authors.
13. References
13.1. 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.
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[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.
13.2. 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.ietf-pcn-architecture]
Eardley, P., "Pre-Congestion Notification (PCN)
Architecture", draft-ietf-pcn-architecture-08 (work in
progress), October 2008.
[I-D.ietf-pcn-baseline-encoding]
Moncaster, T., Briscoe, B., and M. Menth, "Baseline
Encoding and Transport of Pre-Congestion Information",
draft-ietf-pcn-baseline-encoding-01 (work in progress),
October 2008.
[I-D.ietf-pcn-marking-behaviour]
Eardley, P., "Marking behaviour of PCN-nodes",
draft-ietf-pcn-marking-behaviour-01 (work in progress),
October 2008.
[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]
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Moncaster, T., Briscoe, B., and M. Menth, "A three state
extended PCN encoding scheme",
draft-moncaster-pcn-3-state-encoding-00 (work in
progress), June 2008.
[IEEE802.1au]
IEEE, "IEEE Standard for Local and Metropolitan Area
Networks--Virtual Bridged Local Area Networks - Amendment
10: Congestion Notification", 2008,
.
(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]
IETF, "Congestion and Pre-Congestion Notification (pcn)",
IETF w-g charter , Feb 2007,
.
[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.
[RFC1701] Hanks, S., Li, T., Farinacci, D., and P. Traina, "Generic
Routing Encapsulation (GRE)", RFC 1701, October 1994.
[RFC2205] Braden, B., Zhang, L., Berson, S., Herzog, S., and S.
Jamin, "Resource ReSerVation Protocol (RSVP) -- Version 1
Functional Specification", RFC 2205, September 1997.
[RFC2637] Hamzeh, K., Pall, G., Verthein, W., Taarud, J., Little,
W., and G. Zorn, "Point-to-Point Tunneling Protocol",
RFC 2637, July 1999.
[RFC2661] Townsley, W., Valencia, A., Rubens, A., Pall, G., Zorn,
G., and B. Palter, "Layer Two Tunneling Protocol "L2TP"",
RFC 2661, August 1999.
[RFC2983] Black, D., "Differentiated Services and Tunnels",
RFC 2983, October 2000.
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[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, .
(Expired)
Editorial Comments
[Note_Nonce_Compr] Note that even the tentatively proposed
Comprehensive Decapsulation Rules in Appendix C
do not fix the minor compromise to the protection
of the ECN nonce that RFC3168 and RFC4301 both
suffer from (described under Security
Considerations above). An attacker with control
over a tunnel interior node can revert a packet
previously marked CE within the same tunnel to
its original marking. It can do this by changing
CE markings to ECT(0) because the decapsulator
rules give precedence to the inner header if the
outer is ECT(0). To fix this, we could have
specified that the outgoing header should be
ECT(0) when the incoming outer is ECT(0) but the
inner is ECT(1). 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
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decide which is more important: i) the loophole
in the nonce is only for a minor case of one
tunnel node attacking another in the same tunnel;
and ii) the severity inversion would not result
from any legal codepoint transition. If the
Comprehensive Decapsulation Rules of Appendix C
are taken up, we currently believe i) safety
against misconfiguration is slightly more
important than ii) securing against an attack
that has little, if any, clear motivation.
Appendix A. Why resetting CE on encapsulation harms 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
competing admission controlled traffic and 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.
Appendix B. 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
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tunnel egress to monitor congestion introduced 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 4 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 4: Tunnel Marking of Packets Already Marked at Ingress
Appendix C. Comprehensive Decapsulation Rules
This appendix is not currently normative. Compliance with this
appendix is NOT REQUIRED for compliance with the present
specification.
Given this specification requests a standards action to update the
RFC3168 encapsulation behaviour, this appendix explores a further
change to decapsulation that we ought to specify at the same time.
If instead this further change is added later, it will add another
optional mode to the already complicated change history of ECN
tunnelling.
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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 them 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 3, where the ECT values in the
outer header are ignored unless the inner header is the same.
Effectively 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.
Although this issue is currently most pressing for the PCN working
group, it is more general. The currently standardised tunnel
decapsulation behaviour unnecessarily wastes a quarter of two bits
(i.e. half a bit) in the IP (v4 & v6) header. As explained in
Section 3.1, the original reason for not copying down 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.
Figure 5 proposes a more comprehensive layered decapsulation
behaviour that would properly support a simpler experimental 3-state
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ECN encodings such as
[I-D.briscoe-pcn-3-in-1-encoding].[Note_Nonce_Compr] Note that the
proposal tabulated in Figure 5 is only to support discussion. It is
not currently proposed for standards action. The only difference
from Figure 3 (which _is_ proposed for standards action) is the
change to the cell highlighted as *ECT(1)*.
+----------------------------------------------+
| Incoming Outer Header |
+------------------+---------+------------+------------+----------+
| Incoming Inner | Not-ECT | ECT(0) | ECT(1) | CE |
| Header | | | | |
+------------------+---------+------------+------------+----------+
| Not-ECT | Not-ECT | drop(!!!)| 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 5: Comprehensive IP in IP Decapsulation (currently
informative, not normative)
The table is derived from the following logic:
o On decapsulation, if the inner ECN field is Not-ECT but the outer
ECN field is anything but Not-ECT the decapsulator must drop the
packet. This is because the Not-ECT marking on the inner header
is set by transports that do not know how to respond to an
explicit congestion marking;
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 legal transition in any current or
previous ECN tunneling specification would result in certain
combinations of inner and outer ECN fields. In these cases
(indicated in the table by '(!!!)'), the decapsulator may also
raise an alarm, but not so often that the illegal combinations
would amplify into a flood of alarm messages.
If this more comprehensive decapsulation proposal were taken up, it
would be backwards compatible with all previous encapsulations of ECN
at the ingress (RFC4301, both modes of RFC3168, both modes of RFC2481
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and RFC2003). The outgoing header is different for one combination
of inner & outer headers, but that combination was previously illegal
anyway, so no known mechanisms in the Internet rely on the previous
behaviour. The proposed tunnel egress requires no additional option
configuration at the ingress or egress nor any additional negotiation
with the ingress.
C.1. Ways to Introduce the Comprehensive Decapsulation Rules
There would be a number of ways for this more comprehensive
decapsulation proposal to be introduced:
o It could be specified in the present standards track proposal
(preferred) or in an experimental extension;
o it could be specified as a new default for all Diffserv PHBs
(preferred) or as an option to be configured only for Diffserv
PHBs requiring it.
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 this comprehensive decapsulation was 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
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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.
Appendix D. 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 functions within a node that
typically occur in sequence on a stream of packets, not at the same
point. Figure 6 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 6: 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.
D.1. 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 7 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 7: 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, the answer is that the
Congestion Baseline should be the nearest upstream interface designed
to regulate traffic load--the Load Regulator. In Figure 7 '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 6). 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 A 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 2, 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 7.
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.
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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