Transport Area Working Group B. Briscoe
Internet-Draft BT
Updates: 3168, 4301 June 25, 2010
(if approved)
Intended status: Standards Track
Expires: December 27, 2010
Tunnelling of Explicit Congestion Notification
draft-ietf-tsvwg-ecn-tunnel-09
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 updates RFC3168
to bring all IP in IP tunnels (v4 or v6) into line with RFC4301 IPsec
ECN processing. On decapsulation it updates both RFC3168 and RFC4301
to add new behaviours for previously unused combinations of inner and
outer header. The new rules ensure the ECN field is correctly
propagated across a tunnel whether it is used to signal one or two
severity levels of congestion, whereas before only one severity level
was supported. Tunnel endpoints can be updated in any order without
affecting pre-existing uses of the ECN field, thus ensuring backward
compatibility. Nonetheless, operators wanting to support two
severity levels (e.g. for pre-congestion notification--PCN) can
require compliance with this new specification. A thorough analysis
of the reasoning for these changes and the implications is included.
In the unlikely event that the new rules do not meet a specific need,
RFC4774 gives guidance on designing alternate ECN semantics and this
document extends that to include tunnelling issues.
Status of This Memo
This Internet-Draft is submitted in full conformance with the
provisions of BCP 78 and BCP 79.
Internet-Drafts are working documents of the Internet Engineering
Task Force (IETF). Note that other groups may also distribute
working documents as Internet-Drafts. The list of current Internet-
Drafts is at http://datatracker.ietf.org/drafts/current/.
Internet-Drafts are draft documents valid for a maximum of six months
and may be updated, replaced, or obsoleted by other documents at any
time. It is inappropriate to use Internet-Drafts as reference
material or to cite them other than as "work in progress."
This Internet-Draft will expire on December 27, 2010.
Briscoe Expires December 27, 2010 [Page 1]
Internet-Draft ECN Tunnelling June 2010
Copyright Notice
Copyright (c) 2010 IETF Trust and the persons identified as the
document authors. All rights reserved.
This document is subject to BCP 78 and the IETF Trust's Legal
Provisions Relating to IETF Documents
(http://trustee.ietf.org/license-info) in effect on the date of
publication of this document. Please review these documents
carefully, as they describe your rights and restrictions with respect
to this document. Code Components extracted from this document must
include Simplified BSD License text as described in Section 4.e of
the Trust Legal Provisions and are provided without warranty as
described in the Simplified BSD License.
This document may contain material from IETF Documents or IETF
Contributions published or made publicly available before November
10, 2008. The person(s) controlling the copyright in some of this
material may not have granted the IETF Trust the right to allow
modifications of such material outside the IETF Standards Process.
Without obtaining an adequate license from the person(s) controlling
the copyright in such materials, this document may not be modified
outside the IETF Standards Process, and derivative works of it may
not be created outside the IETF Standards Process, except to format
it for publication as an RFC or to translate it into languages other
than English.
Briscoe Expires December 27, 2010 [Page 2]
Internet-Draft ECN Tunnelling June 2010
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 10
1.1. Scope . . . . . . . . . . . . . . . . . . . . . . . . . . 12
2. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 12
3. Summary of Pre-Existing RFCs . . . . . . . . . . . . . . . . . 13
3.1. Encapsulation at Tunnel Ingress . . . . . . . . . . . . . 13
3.2. Decapsulation at Tunnel Egress . . . . . . . . . . . . . . 14
4. New ECN Tunnelling Rules . . . . . . . . . . . . . . . . . . . 15
4.1. Default Tunnel Ingress Behaviour . . . . . . . . . . . . . 16
4.2. Default Tunnel Egress Behaviour . . . . . . . . . . . . . 16
4.3. Encapsulation Modes . . . . . . . . . . . . . . . . . . . 18
4.4. Single Mode of Decapsulation . . . . . . . . . . . . . . . 20
5. Updates to Earlier RFCs . . . . . . . . . . . . . . . . . . . 21
5.1. Changes to RFC4301 ECN processing . . . . . . . . . . . . 21
5.2. Changes to RFC3168 ECN processing . . . . . . . . . . . . 21
5.3. Motivation for Changes . . . . . . . . . . . . . . . . . . 22
5.3.1. Motivation for Changing Encapsulation . . . . . . . . 23
5.3.2. Motivation for Changing Decapsulation . . . . . . . . 24
6. Backward Compatibility . . . . . . . . . . . . . . . . . . . . 26
6.1. Non-Issues Updating Decapsulation . . . . . . . . . . . . 26
6.2. Non-Update of RFC4301 IPsec Encapsulation . . . . . . . . 27
6.3. Update to RFC3168 Encapsulation . . . . . . . . . . . . . 27
7. Design Principles for Alternate ECN Tunnelling Semantics . . . 28
8. Security Considerations . . . . . . . . . . . . . . . . . . . 30
9. Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . 31
10. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 32
11. References . . . . . . . . . . . . . . . . . . . . . . . . . . 33
11.1. Normative References . . . . . . . . . . . . . . . . . . . 33
11.2. Informative References . . . . . . . . . . . . . . . . . . 33
Appendix A. Early ECN Tunnelling RFCs . . . . . . . . . . . . . . 34
Appendix B. Design Constraints . . . . . . . . . . . . . . . . . 35
B.1. Security Constraints . . . . . . . . . . . . . . . . . . . 35
B.2. Control Constraints . . . . . . . . . . . . . . . . . . . 37
B.3. Management Constraints . . . . . . . . . . . . . . . . . . 38
Appendix C. Contribution to Congestion across a Tunnel . . . . . 39
Appendix D. Compromise on Decap with ECT(1) Inner and ECT(0)
Outer . . . . . . . . . . . . . . . . . . . . . . . . 40
Appendix E. Open Issues . . . . . . . . . . . . . . . . . . . . . 41
Briscoe Expires December 27, 2010 [Page 3]
Internet-Draft ECN Tunnelling June 2010
Request to the RFC Editor (to be removed on publication):
In the RFC index, RFC3168 should be identified as an update to
RFC2003. RFC4301 should be identified as an update to RFC3168.
Changes from previous drafts (to be removed by the RFC Editor)
Full text differences between IETF draft versions are available at
, and
between earlier individual draft versions at
From ietf-08 to ietf-09 (current): Added change log entry for -07 to
-08 that was previously omitted.
* Editorial changes:
+ Abstract: s/providing backward compatibility./thus ensuring
backward compatibility./
+ Moved PCN-related text motivating changes to decapsulation
from "Default Tunnel Egress Behaviour" (Section 4.2) to
"Motivation for Changing Decapsulation" (Section 5.3.2)
where it was merged with existing similar text.
+ Added Stephen Hanna to acks and corrected spelling of
Agarwal.
+ Deleted endnote discussing corner case with IKEv2 manual
keying (identified as "to be removed before publication
following Sec").
+ Deleted Appendices D & E on why existing ingress & egress
tunnelling behavour impede PCN and the endnotes that
referred to them (identified as "to be removed before
publication").
From ietf-07 to ietf-08:
* Changes to standards actions:
+ Section 4: Changed non-RFC2119 phrase 'NOT RECOMMENDED' to
'SHOULD be avoided', wrt alternate ECN tunnelling schemes.
+ Section 4.2: Used upper-case in 'Alarms SHOULD be rate-
limited'.
Briscoe Expires December 27, 2010 [Page 4]
Internet-Draft ECN Tunnelling June 2010
+ Section 7: Made bullet #1 in the decapsulation guidelines
for alternate schemes more precise. Also changed any upper-
case keywords in this informative section to lower case.
* Editorial changes:
+ Changed copyright notice to allow for pre-5378 material.
+ Shifted supporting text intended for deletion on publication
into editorial comments.
+ Explained how to read the decapsulation matrices in their
captions.
+ Minor clarifications throughout.
From ietf-06 to ietf-07:
* Emphasised that this is the opposite of a fork in the RFC
series.
* Altered Section 5 to focus on updates to implementations of
earlier RFCs, rather than on updates to the text of the RFCs.
* Removed potential loop-holes in normative text that
implementers might have used to claim compliance without
implementing normal mode. Highlighted the deliberate
distinction between "MUST implement" and "SHOULD use" normal
mode.
* Added question for Security Directorate reviewers on whether to
mention a corner-case concerning manual keying of IPsec
tunnels.
* Minor clarifications, updated references and updated acks.
* Marked two appendices about PCN motivations for removal before
publication.
From ietf-05 to ietf-06:
* Minor textual clarifications and corrections.
From ietf-04 to ietf-05:
* Functional changes:
Briscoe Expires December 27, 2010 [Page 5]
Internet-Draft ECN Tunnelling June 2010
+ Section 4.2: ECT(1) outer with Not-ECT inner: reverted to
forwarding as Not-ECT (as in RFC3168 & RFC4301), rather than
dropping.
+ Altered rationale in bullet 3 of Section 5.3.2 to justify
this.
+ Distinguished alarms for dangerous and invalid combinations
and allowed combinations that are valid in some tunnel
configurations but dangerous in others to be alarmed at the
discretion of the implementer and/or operator.
+ Altered advice on designing alternate ECN tunnelling
semantics to reflect the above changes.
* Textual changes:
+ Changed "Future non-default schemes" to "Alternate ECN
Tunnelling Semantics" throughout.
+ Cut down Appendix D and Appendix E for brevity.
+ A number of clarifying edits & updated refs.
From ietf-03 to ietf-04:
* Functional changes: none
* Structural changes:
+ Added "Open Issues" appendix
* Textual changes:
+ Section title: "Changes from Earlier RFCs" -> "Updates to
Earlier RFCs"
+ Emphasised that change on decap to previously unused
combinations will propagate PCN encoding.
+ Acknowledged additional reviewers and updated references
From ietf-02 to ietf-03:
* Functional changes:
+ Corrected errors in recap of previous RFCs, which wrongly
stated the different decapsulation behaviours of RFC3168 &
Briscoe Expires December 27, 2010 [Page 6]
Internet-Draft ECN Tunnelling June 2010
RFC4301 with a Not-ECT inner header. This also required
corrections to the "Changes from Earlier RFCs" and the
Motivations for these changes.
+ Mandated that any future standards action SHOULD NOT use the
ECT(0) codepoint as an indication of congestion, without
giving strong reasons.
+ Added optional alarm when decapsulating ECT(1) outer,
ECT(0), but noted it would need to be disabled for
2-severity level congestion (e.g. PCN).
* Structural changes:
+ Removed Document Roadmap which merely repeated the Contents
(previously Section 1.2).
+ Moved "Changes from Earlier RFCs" (Section 5) before
Section 6 on Backward Compatibility and internally organised
both by RFC, rather than by ingress then egress.
+ Moved motivation for changing existing RFCs (Section 5.3) to
after the changes are specified.
+ Moved informative "Design Principles for Future Non-Default
Schemes" after all the normative sections.
+ Added Appendix A on early history of ECN tunnelling RFCs.
+ Removed specialist appendix on "Relative Placement of
Tunnelling and In-Path Load Regulation" (Appendix D in the
-02 draft)
+ Moved and updated specialist text on "Compromise on Decap
with ECT(1) Inner and ECT(0) Outer" from Security
Considerations to Appendix D
* Textual changes:
+ Simplified vocabulary for non-native-english speakers
+ Simplified Introduction and defined regularly used terms in
an expanded Terminology section.
+ More clearly distinguished statically configured tunnels
from dynamic tunnel endpoint discovery, before explaining
operating modes.
Briscoe Expires December 27, 2010 [Page 7]
Internet-Draft ECN Tunnelling June 2010
+ Simplified, cut-down and clarified throughout
+ Updated references.
From ietf-01 to ietf-02:
* Scope reduced from any encapsulation of an IP packet to solely
IP in IP tunnelled encapsulation. Consequently changed title
and removed whole section 'Design Guidelines for New
Encapsulations of Congestion Notification' (to be included in a
future companion informational document).
* Included a new normative decapsulation rule for ECT(0) inner
and ECT(1) outer that had previously only been outlined in the
non-normative appendix 'Comprehensive Decapsulation Rules'.
Consequently:
+ The Introduction has been completely re-written to motivate
this change to decapsulation along with the existing change
to encapsulation.
+ The tentative text in the appendix that first proposed this
change has been split between normative standards text in
Section 4 and Appendix D, which explains specifically why
this change would streamline PCN. New text on the logic of
the resulting decap rules added.
* If inner/outer is Not-ECT/ECT(0), changed decapsulation to
propagate Not-ECT rather than drop the packet; and added
reasoning.
* Considerably restructured:
+ "Design Constraints" analysis moved to an appendix
(Appendix B);
+ Added Section 3 to summarise relevant existing RFCs;
+ Structured Section 4 and Section 6 into subsections.
+ Added tables to sections on old and new rules, for precision
and comparison.
+ Moved Section 7 on Design Principles to the end of the
section specifying the new default normative tunnelling
behaviour. Rewritten and shifted text on identifiers and
in-path load regulators to Appendix B.1 [deleted in revision
-03].
Briscoe Expires December 27, 2010 [Page 8]
Internet-Draft ECN Tunnelling June 2010
From ietf-00 to ietf-01:
* Identified two additional alarm states in the decapsulation
rules (Figure 4) if ECT(X) in outer and inner contradict each
other.
* Altered Comprehensive Decapsulation Rules (Appendix D) so that
ECT(0) in the outer no longer overrides ECT(1) in the inner.
Used the term 'Comprehensive' instead of 'Ideal'. And
considerably updated the text in this appendix.
* Added Appendix D.1 (removed again in a later revision) to weigh
up the various ways the Comprehensive Decapsulation Rules might
be introduced. This replaces the previous contradictory
statements saying complex backwards compatibility interactions
would be introduced while also saying there would be no
backwards compatibility issues.
* Updated references.
From briscoe-01 to ietf-00:
* Re-wrote Appendix C giving much simpler technique to measure
contribution to congestion across a tunnel.
* Added discussion of backward compatibility of the ideal
decapsulation scheme in Appendix D
* Updated references. Minor corrections & clarifications
throughout.
From briscoe-00 to briscoe-01:
* Related everything conceptually to the uniform and pipe models
of RFC2983 on Diffserv Tunnels, and completely removed the
dependence of tunnelling behaviour on the presence of any in-
path load regulation by using the [1 - Before] [2 - Outer]
function placement concepts from RFC2983;
* Added specific cases where the existing standards limit new
proposals, particularly Appendix E;
* Added sub-structure to Introduction (Need for Rationalisation,
Roadmap), added new Introductory subsection on "Scope" and
improved clarity;
* Added Design Guidelines for New Encapsulations of Congestion
Notification;
Briscoe Expires December 27, 2010 [Page 9]
Internet-Draft ECN Tunnelling June 2010
* Considerably clarified the Backward Compatibility section
(Section 6);
* 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 [deleted in a later revision], explaining
how tunnel encapsulation no longer depends on in-path load-
regulation (changed title from "In-path Load Regulation" to
"Non-Dependence of Tunnelling on In-path Load Regulation"), but
explained how an in-path load regulation function must be
carefully placed with respect to tunnel encapsulation (in a new
sub-section entitled "Dependence of In-Path Load Regulation on
Tunnelling").
1. Introduction
Explicit congestion notification (ECN [RFC3168]) allows a forwarding
element (e.g. a router) to notify the onset of congestion without
having to drop packets. Instead it can explicitly mark a proportion
of packets in the 2-bit ECN field in the IP header (Table 1 recaps
the ECN codepoints).
The outer header of an IP packet can encapsulate one or more IP
headers for tunnelling. A forwarding element using ECN to signify
congestion will only mark the immediately visible outer IP header.
When a tunnel decapsulator later removes this outer header, it
follows rules to propagate congestion markings by combining the ECN
fields of the inner and outer IP header into one outgoing IP header.
This document updates those rules for IPsec [RFC4301] and non-IPsec
[RFC3168] tunnels to add new behaviours for previously unused
combinations of inner and outer header. It also updates the tunnel
ingress behaviour of RFC3168 to match that of RFC4301. The updated
rules are backward compatible with RFC4301 and RFC3168 when
interworking with any other tunnel endpoint complying with any
earlier specification.
When ECN and its tunnelling was defined in RFC3168, only the minimum
Briscoe Expires December 27, 2010 [Page 10]
Internet-Draft ECN Tunnelling June 2010
necessary changes to the ECN field were propagated through tunnel
endpoints--just enough for the basic ECN mechanism to work. This was
due to concerns that the ECN field might be toggled to communicate
between a secure site and someone on the public Internet--a covert
channel. This was because a mutable field like ECN cannot be
protected by IPsec's integrity mechanisms--it has to be able to
change as it traverses the Internet.
Nonetheless, the latest IPsec architecture [RFC4301] considered a
bandwidth limit of 2 bits per packet on a covert channel made it a
manageable risk. Therefore, for simplicity, an RFC4301 ingress
copied the whole ECN field to encapsulate a packet. It dispensed
with the two modes of RFC3168, one which partially copied the ECN
field, and the other which blocked all propagation of ECN changes.
Unfortunately, this entirely reasonable sequence of standards actions
resulted in a perverse outcome; non-IPsec tunnels (RFC3168) blocked
the 2-bit covert channel, while IPsec tunnels (RFC4301) did not--at
least not at the ingress. At the egress, both IPsec and non-IPsec
tunnels still partially restricted propagation of the full ECN field.
The trigger for the changes in this document was the introduction of
pre-congestion notification (PCN [RFC5670]) to the IETF standards
track. PCN needs the ECN field to be copied at a tunnel ingress and
it needs four states of congestion signalling to be propagated at the
egress, but pre-existing tunnels only propagate three in the ECN
field.
This document draws on currently unused (CU) combinations of inner
and outer headers to add tunnelling of four-state congestion
signalling to RFC3168 and RFC4301. Operators of tunnels who
specifically want to support four states can require that all their
tunnels comply with this specification. However, this is not a fork
in the RFC series. It is an update that can be deployed first by
those that need it, and subsequently by all tunnel endpoint
implementations (RFC4301, RFC3168, RFC2481, RFC2401, RFC2003), which
can safely be updated to this new specification as part of general
code maintenance. This will gradually add support for four
congestion states to the Internet. Existing three state schemes will
continue to work as before.
In fact, this document is the opposite of a fork. At the same time
as supporting a fourth state, the opportunity has been taken to draw
together divergent ECN tunnelling specifications into a single
consistent behaviour, harmonising differences such as perverse covert
channel treatment. Then any tunnel can be deployed unilaterally, and
it will support the full range of congestion control and management
schemes without any modes or configuration. Further, any host or
Briscoe Expires December 27, 2010 [Page 11]
Internet-Draft ECN Tunnelling June 2010
router can expect the ECN field to behave in the same way, whatever
type of tunnel might intervene in the path.
1.1. Scope
This document only concerns wire protocol processing of the ECN field
at tunnel endpoints and makes no changes or recommendations
concerning algorithms for congestion marking or congestion response.
This document specifies common ECN field processing at encapsulation
and decapsulation for any IP in IP tunnelling, whether IPsec or non-
IPsec tunnels. It applies irrespective of whether IPv4 or IPv6 is
used for either of the inner and outer headers. It applies for
packets with any destination address type, whether unicast or
multicast. It applies as the default for all Diffserv per-hop
behaviours (PHBs), unless stated otherwise in the specification of a
PHB (but Section 4 strongly deprecates such exceptions). It is
intended to be a good trade off between somewhat conflicting
security, control and management requirements.
[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. Terminology
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].
Table 1 recaps the names of the ECN codepoints [RFC3168].
+------------------+----------------+---------------------------+
| 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
Further terminology used within this document:
Briscoe Expires December 27, 2010 [Page 12]
Internet-Draft ECN Tunnelling June 2010
Encapsulator: The tunnel endpoint function that adds an outer IP
header to tunnel a packet (also termed the 'ingress tunnel
endpoint' or just the 'ingress' where the context is clear).
Decapsulator: The tunnel endpoint function that removes an outer IP
header from a tunnelled packet (also termed the 'egress tunnel
endpoint' or just the 'egress' where the context is clear).
Incoming header: The header of an arriving packet before
encapsulation.
Outer header: The header added to encapsulate a tunnelled packet.
Inner header: The header encapsulated by the outer header.
Outgoing header: The header constructed by the decapsulator using
logic that combines the fields in the outer and inner headers.
Copying ECN: On encapsulation, setting the ECN field of the new
outer header to be a copy of the ECN field in the incoming header.
Zeroing ECN: On encapsulation, clearing the ECN field of the new
outer header to Not-ECT ("00").
Resetting ECN: On encapsulation, setting the ECN field of the new
outer header to be a copy of the ECN field in the incoming header
except the outer ECN field is set to the ECT(0) codepoint if the
incoming ECN field is CE.
3. Summary of Pre-Existing RFCs
This section is informative not normative, as it recaps pre-existing
RFCs. Earlier relevant RFCs that were either experimental or
incomplete with respect to ECN tunnelling (RFC2481, RFC2401 and
RFC2003) are briefly outlined in Appendix A. The question of whether
tunnel implementations used in the Internet comply with any of these
RFCs is not discussed.
3.1. Encapsulation at Tunnel Ingress
At the encapsulator, the controversy has been over whether to
propagate information about congestion experienced on the path so far
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
encapsulator must not copy a CE marking from the inner header into
the outer header that it creates. Instead the encapsulator must set
Briscoe Expires December 27, 2010 [Page 13]
Internet-Draft ECN Tunnelling June 2010
the outer header to ECT(0) if the ECN field is marked CE in the
arriving IP header. We term this 'resetting' a CE codepoint.
However, the new IPsec architecture in [RFC4301] reverses this rule,
stating that the encapsulator must simply copy the ECN field from the
incoming header to the outer header.
RFC3168 also provided a Limited Functionality mode that turns off ECN
processing over the scope of the tunnel by setting the outer header
to Not-ECT ("00"). Then such packets will be dropped to indicate
congestion rather than marked with ECN. This is necessary for the
ingress to interwork with legacy decapsulators ([RFC2481], [RFC2401]
and [RFC2003]) that do not propagate ECN markings added to the outer
header. Otherwise such legacy decapsulators would throw away
congestion notifications before they reached the transport layer.
Neither Limited Functionality mode nor Full Functionality mode are
used by an RFC4301 IPsec encapsulator, which simply copies the
incoming ECN field into the outer header. An earlier key-exchange
phase ensures an RFC4301 ingress will not have to interwork with a
legacy egress that does not support ECN.
These pre-existing behaviours are summarised in Figure 1.
+-----------------+-----------------------------------------------+
| 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 |
+-----------------+---------------+---------------+---------------+
Figure 1: IP in IP Encapsulation: Recap of Pre-existing Behaviours
3.2. Decapsulation at Tunnel Egress
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).
Briscoe Expires December 27, 2010 [Page 14]
Internet-Draft ECN Tunnelling June 2010
+---------+------------------------------------------------+
|Incoming | Incoming Outer Header |
| Inner +---------+------------+------------+------------+
| Header | Not-ECT | ECT(0) | ECT(1) | CE |
+---------+---------+------------+------------+------------+
RFC3168->| Not-ECT | Not-ECT |Not-ECT |Not-ECT | drop |
RFC4301->| Not-ECT | Not-ECT |Not-ECT |Not-ECT |Not-ECT |
| ECT(0) | ECT(0) | ECT(0) | ECT(0) | CE |
| ECT(1) | ECT(1) | ECT(1) | ECT(1) | CE |
| CE | CE | CE | CE | CE |
+---------+---------+------------+------------+------------+
In pre-existing RFCs, the ECN field in the outgoing header was set to
the codepoint at the intersection of the appropriate incoming inner
header (row) and incoming outer header (column).
Figure 2: IP in IP Decapsulation; Recap of Pre-existing Behaviour
The behaviour in the table derives from the logic given in RFC3168
and RFC4301, briefly recapped as follows:
o On decapsulation, if the inner ECN field is Not-ECT the outer is
ignored. RFC3168 (but not RFC4301) also specified that the
decapsulator must drop a packet with a Not-ECT inner and CE in the
outer.
o In all other cases, if the outer is CE, the outgoing ECN field is
set to CE, but otherwise the outer is ignored and the inner is
used for the outgoing ECN field.
Section 9.2.2 of RFC3168 also made it an auditable event for an IPsec
tunnel "if the ECN Field is changed inappropriately within an IPsec
tunnel...". Inappropriate changes were not specifically enumerated.
RFC4301 did not mention inappropriate ECN changes.
4. New ECN Tunnelling Rules
The standards actions below in Section 4.1 (ingress encapsulation)
and Section 4.2 (egress decapsulation) define new default ECN tunnel
processing rules for any IP packet (v4 or v6) with any Diffserv
codepoint.
If these defaults do not meet a particular requirement, an alternate
ECN tunnelling scheme 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
alternate ECN tunnelling schemes, the principles in Section 7 should
be followed. However, alternate ECN tunnelling schemes SHOULD be
Briscoe Expires December 27, 2010 [Page 15]
Internet-Draft ECN Tunnelling June 2010
avoided whenever possible as the deployment burden of handling
exceptional PHBs in implementations of all affected tunnels should
not be underestimated. There is no requirement for a PHB definition
to state anything about ECN tunnelling behaviour if the default
behaviour in the present specification is sufficient.
4.1. Default Tunnel Ingress Behaviour
Two modes of encapsulation are defined here; a REQUIRED `normal mode'
and a `compatibility mode', which is for backward compatibility with
tunnel decapsulators that do not understand ECN. Note that these are
modes of the ingress tunnel endpoint only, not the whole tunnel.
Section 4.3 explains why two modes are necessary and specifies the
circumstances in which it is sufficient to solely implement normal
mode.
Whatever the mode, an encapsulator forwards the inner header without
changing the ECN field.
In normal mode an encapsulator compliant with this specification MUST
construct the outer encapsulating IP header by copying the 2-bit ECN
field of the incoming IP header. In compatibility mode it clears the
ECN field in the outer header to the Not-ECT codepoint (the IPv4
header checksum also changes whenever the ECN field is changed).
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
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 (the IPv4 header checksum also changes
whenever the ECN field is changed). There is no need for more than
one mode of decapsulation, as these rules cater for all known
Briscoe Expires December 27, 2010 [Page 16]
Internet-Draft ECN Tunnelling June 2010
requirements.
+---------+------------------------------------------------+
|Incoming | Incoming Outer Header |
| Inner +---------+------------+------------+------------+
| Header | Not-ECT | ECT(0) | ECT(1) | CE |
+---------+---------+------------+------------+------------+
| Not-ECT | Not-ECT |Not-ECT(!!!)|Not-ECT(!!!)| drop(!!!)|
| ECT(0) | ECT(0) | ECT(0) | ECT(1) | CE |
| ECT(1) | ECT(1) | ECT(1) (!) | ECT(1) | CE |
| CE | CE | CE | CE(!!!)| CE |
+---------+---------+------------+------------+------------+
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). Currently unused combinations are
indicated by '(!!!)' or '(!)'
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 onwards. This is because the
inner Not-ECT marking is set by transports that use drop as an
indication of congestion and would not understand or respond to
any other ECN codepoint [RFC4774]. Specifically:
* If the inner ECN field is Not-ECT and the outer ECN field is CE
the decapsulator MUST drop the packet.
* If the inner ECN field is Not-ECT and the outer ECN field is
Not-ECT, ECT(0) or ECT(1) the decapsulator MUST forward the
outgoing packet with the ECN field cleared to Not-ECT.
o In all other cases where the inner supports ECN, the decapsulator
MUST set the outgoing ECN field to the more severe marking of the
outer and inner ECN fields, where the ranking of severity from
highest to lowest is CE, ECT(1), ECT(0), Not-ECT. This in no way
precludes cases where ECT(1) and ECT(0) have the same severity;
o Certain combinations of inner and outer ECN fields cannot result
from any transition in any current or previous ECN tunneling
specification. These currently unused (CU) combinations are
indicated in Figure 4 by '(!!!)' or '(!)', where '(!!!)' means the
combination is CU and always potentially dangerous, while '(!)'
means it is CU and possibly dangerous. In these cases,
particularly the more dangerous ones, the decapsulator SHOULD log
Briscoe Expires December 27, 2010 [Page 17]
Internet-Draft ECN Tunnelling June 2010
the event and MAY also raise an alarm.
Just because the highlighted combinations are currently unused,
does not mean that all the other combinations are always valid.
Some are only valid if they have arrived from a particular type of
legacy ingress, and dangerous otherwise. Therefore an
implementation MAY allow an operator to configure logging and
alarms for such additional header combinations known to be
dangerous or CU for the particular configuration of tunnel
endpoints deployed at run-time.
Alarms SHOULD be rate-limited so that the anomalous combinations
will not amplify into a flood of alarm messages. It MUST be
possible to suppress alarms or logging, e.g. if it becomes
apparent that a combination that previously was not used has
started to be used for legitimate purposes such as a new standards
action.
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), in particular enabling the simplest
possible encoding for PCN [I-D.ietf-pcn-3-in-1-encoding] (see
Section 5.3.2). Treating ECT(1) as either the same as ECT(0) or as a
higher severity level is explained in the discussion of the ECN nonce
[RFC3540] in Section 8, which in turn refers to Appendix D.
4.3. Encapsulation Modes
Section 4.1 introduces two encapsulation modes, normal mode and
compatibility mode, defining their encapsulation behaviour (i.e.
header copying or zeroing respectively). Note that these are modes
of the ingress tunnel endpoint only, not the tunnel as a whole.
To comply with this specification, a tunnel ingress MUST at least
implement `normal mode'. Unless it will never be used with legacy
tunnel egress nodes (RFC2003, RFC2401 or RFC2481 or the limited
functionality mode of RFC3168), an ingress MUST also implement
`compatibility mode' for backward compatibility with tunnel egresses
that do not propagate explicit congestion notifications [RFC4774].
We can categorise the way that an ingress tunnel endpoint is paired
with an egress as either static or dynamically discovered:
Static: Tunnel endpoints paired together by prior configuration.
Some implementations of encapsulator might always be statically
deployed, and constrained to never be paired with a legacy
Briscoe Expires December 27, 2010 [Page 18]
Internet-Draft ECN Tunnelling June 2010
decapsulator (RFC2003, RFC2401 or RFC2481 or the limited
functionality mode of RFC3168). In such a case, only normal mode
needs to be implemented.
For instance, RFC4301-compatible IPsec tunnel endpoints invariably
use IKEv2 [RFC4306] for key exchange, which was introduced
alongside RFC4301. Therefore both endpoints of an RFC4301 tunnel
can be sure that the other end is RFC4301-compatible, because the
tunnel is only formed after IKEv2 key management has completed, at
which point both ends will be RFC4301-compliant by definition.
Therefore an IPsec tunnel ingress does not need compatibility
mode, as it will never interact with legacy ECN tunnels. To
comply with the present specification, it only needs to implement
the required normal mode, which is identical to the pre-existing
RFC4301 behaviour.
Dynamic Discovery: Tunnel endpoints paired together by some form of
tunnel endpoint discovery, typically finding an egress on the path
taken by the first packet.
This specification does not require or recommend dynamic discovery
and it does not define how dynamic negotiation might be done, but
it recognises that proprietary tunnel endpoint discovery protocols
exist. It therefore sets down some constraints on discovery
protocols to ensure safe interworking.
If dynamic tunnel endpoint discovery might pair an ingress with a
legacy egress (RFC2003, RFC2401 or RFC2481 or the limited
functionality mode of RFC3168), the ingress MUST implement both
normal and compatibility mode. If the tunnel discovery process is
arranged to only ever find a tunnel egress that propagates ECN
(RFC3168 full functionality mode, RFC4301 or this present
specification), then a tunnel ingress can be complaint with the
present specification without implementing compatibility mode.
While a compliant tunnel ingress is discovering an egress, it MUST
send packets in compatibility mode in case the egress it discovers
is a legacy egress. If, through the discovery protocol, the
egress indicates that it is compliant with the present
specification, with RFC4301 or with RFC3168 full functionality
mode, the ingress can switch itself into normal mode. If the
egress denies compliance with any of these or returns an error
that implies it does not understand a request to work to any of
these ECN specifications, the tunnel ingress MUST remain in
compatibility mode.
If an ingress claims compliance with this specification it MUST NOT
permanently disable ECN processing across the tunnel (i.e. only using
Briscoe Expires December 27, 2010 [Page 19]
Internet-Draft ECN Tunnelling June 2010
compatibility mode). It is true that such a tunnel ingress is at
least safe with the ECN behaviour of any egress it may encounter, but
it does not meet the central aim of this specification: introducing
ECN support to tunnels.
Instead, if the ingress knows that the egress does support
propagation of ECN (full functionality mode of RFC3168 or RFC4301 or
the present specification), it SHOULD use normal mode, in order to
support ECN where possible. Note that this section started by saying
an ingress "MUST implement "normal mode, while it has just said an
ingress "SHOULD use" normal mode. This distinction is deliberate, to
allow the mode to be turned off in exceptional circumstances but to
ensure all implementations make normal mode available.
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 only needs one mode.
4.4. Single Mode of Decapsulation
A compliant decapsulator only needs one mode of operation. However,
if a complaint egress is implemented to be dynamically discoverable,
it may need to respond to discovery requests from various types of
legacy tunnel ingress. This specification does not define how
dynamic negotiation might be done by (proprietary) discovery
protocols, but it sets down some constraints to ensure safe
interworking.
Through the discovery protocol, a tunnel ingress compliant with the
present specification might ask if the egress is compliant with the
present specification, with RFC4301 or with RFC3168 full
functionality mode. Or an RFC3168 tunnel ingress might try to
negotiate to use limited functionality or full functionality mode
[RFC3168]. In all these cases, a decapsulating tunnel egress
compliant with this specification MUST agree to any of these
requests, since it will behave identically in all these cases.
If no ECN-related mode is requested, a compliant tunnel egress MUST
continue without raising any error or warning, because its egress
behaviour is compatible with all the legacy ingress behaviours that
do not negotiate capabilities.
A compliant tunnel egress SHOULD raise a warning alarm about any
requests to enter modes it does not recognise but, for 'forward
compatibility' with standards actions possibly defined after it was
implemented, it SHOULD continue operating.
Briscoe Expires December 27, 2010 [Page 20]
Internet-Draft ECN Tunnelling June 2010
5. Updates to Earlier RFCs
5.1. Changes to RFC4301 ECN processing
Ingress: An RFC4301 IPsec encapsulator is not changed at all by the
present specification. It uses the normal mode of the present
specification, which defines packet encapsulation identically to
RFC4301.
Egress: An RFC4301 egress will need to be updated to the new
decapsulation behaviour in Figure 4, in order to comply with the
present specification. However, the changes are backward
compatible; combinations of inner and outer that result from any
protocol defined in the RFC series so far are unaffected. Only
combinations that have never been used have been changed,
effectively adding new behaviours to RFC4301 decapsulation without
altering existing behaviours. The following specific updates have
been made:
* The outer, not the inner, is propagated when the outer is
ECT(1) and the inner is ECT(0);
* A packet with Not-ECT in the inner and an outer of CE is
dropped rather than forwarded as Not-ECT;
* Certain combinations of inner and outer ECN field have been
identified as currently unused. These can trigger logging
and/or raise alarms.
Modes: RFC4301 tunnel endpoints do not need modes and are not
updated by the modes in the present specification. Effectively an
RFC4301 IPsec ingress solely uses the REQUIRED normal mode of
encapsulation, which is unchanged from RFC4301 encapsulation. It
will never need the OPTIONAL compatibility mode as explained in
Section 4.3.
5.2. Changes to RFC3168 ECN processing
Ingress: On encapsulation, the new rule in Figure 3 that a normal
mode tunnel ingress copies any ECN field into the outer header
updates the full functionality behaviour of an RFC3168 ingress.
Nonetheless, the new compatibility mode encapsulates packets
identically to the limited functionality mode of an RFC3168
ingress.
Briscoe Expires December 27, 2010 [Page 21]
Internet-Draft ECN Tunnelling June 2010
Egress: An RFC3168 egress will need to be updated to the new
decapsulation behaviour in Figure 4, in order to comply with the
present specification. However, the changes are backward
compatible; combinations of inner and outer that result from any
protocol defined in the RFC series so far are unaffected. Only
combinations that have never been used have been changed,
effectively adding new behaviours to RFC3168 decapsulation without
altering existing behaviours. The following specific updates have
been made:
* The outer, not the inner, is propagated when the outer is
ECT(1) and the inner is ECT(0);
* Certain combinations of inner and outer ECN field have been
identified as currently unused. These can trigger logging
and/or raise alarms.
Modes: An RFC3168 ingress will need to be updated if it is to comply
with the present specification, whether or not it implemented the
optional full functionality mode of RFC3168.
RFC3168 defined a (required) limited functionality mode and an
(optional) full functionality mode for a tunnel. In RFC3168,
modes applied to both ends of the tunnel, while in the present
specification, modes are only used at the ingress--a single egress
behaviour covers all cases.
The normal mode of encapsulation is an update to the encapsulation
behaviour of the full functionality mode of an RFC3168 ingress.
The compatibility mode of encapsulation is identical to the
encapsulation behaviour of the limited functionality mode of an
RFC3168 ingress, except it is optional.
The constraints on how tunnel discovery protocols set modes in
Section 4.3 and Section 4.4 are an update to RFC3168, but they are
unlikely to require code changes as they document safe practice.
5.3. Motivation for Changes
An overriding goal is to ensure the same ECN signals can mean the
same thing whatever tunnels happen to encapsulate an IP packet flow.
This removes gratuitous inconsistency, which otherwise constrains the
available design space and makes it harder to design networks and new
protocols that work predictably.
Briscoe Expires December 27, 2010 [Page 22]
Internet-Draft ECN Tunnelling June 2010
5.3.1. Motivation for Changing Encapsulation
The normal mode in Section 4 updates RFC3168 to make all IP in IP
encapsulation of the ECN field consistent--consistent with the way
both RFC4301 IPsec [RFC4301] and IP in MPLS or MPLS in MPLS
encapsulation [RFC5129] construct the ECN field.
Compatibility mode has also been defined so that a non-RFC4301
ingress can still switch to using drop across a tunnel for backwards
compatibility with legacy decapsulators that do not propagate ECN
correctly.
The trigger that motivated this update to RFC3168 encapsulation was a
standards track proposal for pre-congestion notification (PCN
[RFC5670]). PCN excess rate marking only works correctly if the ECN
field is copied on encapsulation (as in RFC4301 and RFC5129); it does
not work if ECN is reset (as in RFC3168). This is because PCN excess
rate marking depends on the outer header revealing any congestion
experienced so far on the whole path, not just since the last tunnel
ingress.
PCN allows a network operator to add flow admission and termination
for inelastic traffic at the edges of a Diffserv domain, but without
any per-flow mechanisms in the interior and without the generous
provisioning typical of Diffserv, aiming to significantly reduce
costs. The PCN architecture [RFC5559] states that RFC3168 IP in IP
tunnelling of the ECN field cannot be used for any tunnel ingress in
a PCN domain. Prior to the present specification, this left a stark
choice between not being able to use PCN for inelastic traffic
control or not being able to use the many tunnels already deployed
for Mobile IP, VPNs and so forth.
The present specification provides a clean solution to this problem,
so that network operators who want to use both PCN and tunnels can
specify that every tunnel ingress in a PCN region must comply with
this latest specification.
Rather than allow tunnel specifications to fragment further into one
for PCN, one for IPsec and one for other tunnels, the opportunity has
been taken to consolidate the diverging specifications back into a
single tunnelling behaviour. Resetting ECN was originally motivated
by a covert channel concern that has been deliberately set aside in
RFC4301 IPsec. Therefore the reset behaviour of RFC3168 is an
anomaly that we do not need to keep. Copying ECN on encapsulation is
anyway simpler than resetting. So, as more tunnel endpoints comply
with this single consistent specification, encapsulation will be
simpler as well as more predictable.
Briscoe Expires December 27, 2010 [Page 23]
Internet-Draft ECN Tunnelling June 2010
Appendix B assesses whether copying rather than resetting CE on
ingress will cause any unintended side-effects, from the three
perspectives of security, control and management. In summary this
analysis finds that:
o 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.
o From the management and monitoring perspective copying is
preferable.
o From the traffic security perspective (enforcing congestion
control, mitigating denial of service etc) copying is preferable.
o 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 two points against resetting CE on ingress while
copying CE causes no significant harm.
5.3.2. Motivation for Changing Decapsulation
The specification for decapsulation in Section 4 fixes three problems
with the pre-existing behaviours of both RFC3168 and RFC4301:
1. The pre-existing rules prevented the introduction of alternate
ECN semantics to signal more than one severity level of
congestion [RFC4774], [RFC5559]. The four states of the 2-bit
ECN field provide room for signalling two severity levels in
addition to not-congested and not-ECN-capable states. But, the
pre-existing rules assumed that two of the states (ECT(0) and
ECT(1)) are always equivalent. This unnecessarily restricts the
use of one of four codepoints (half a bit) in the IP (v4 & v6)
header. The new rules are designed to work in either case;
whether ECT(1) is more severe than or equivalent to ECT(0).
As explained in Appendix B.1, the original reason for not
forwarding the outer ECT codepoints 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 useful for general future-proofing, this problem
is immediately pressing for standardisation of pre-congestion
Briscoe Expires December 27, 2010 [Page 24]
Internet-Draft ECN Tunnelling June 2010
notification (PCN), which uses two severity levels of congestion.
If a congested queue used ECT(1) in the outer header to signal
more severe congestion than ECT(0), the pre-existing
decapsulation rules would have thrown away this congestion
signal, preventing tunnelled traffic from ever knowing that it
should reduce its load.
Before the present specification was written, the PCN working
group had to consider a number of wasteful or convoluted work-
rounds to this problem. Without wishing to disparage the
ingenuity of these work-rounds, none were chosen for the
standards track because they were either somewhat wasteful,
imprecise or complicated. Instead a baseline PCN encoding was
specified [RFC5696] that supported only one severity level of
congestion but allowed space for these work-rounds as
experimental extensions.
But by far the simplest approach is that taken by the current
specification: just to remove the covert channel blockages from
tunnelling behaviour--now deemed unnecessary anyway. Then
network operators that want to support two congestion severity-
levels for PCN can specify that every tunnel egress in a PCN
region must comply with this latest specification. Having taken
this step, the simplest possible encoding for PCN with two
severity levels of congestion [I-D.ietf-pcn-3-in-1-encoding] can
be used.
Not only does this make two congestion severity-levels available
for PCN, but also for other potential uses of the extra ECN
codepoint (e.g. [VCP]).
2. Cases are documented where a middlebox (e.g. a firewall) drops
packets with header values that were currently unused (CU) when
the box was deployed, often on the grounds that anything
unexpected might be an attack. This tends to bar future use of
CU values. The new decapsulation rules specify optional logging
and/or alarms for specific combinations of inner and outer header
that are currently unused. The aim is to give implementers a
recourse other than drop if they are concerned about the security
of CU values. It recognises legitimate security concerns about
CU values but still eases their future use. If the alarms are
interpreted as an attack (e.g. by a management system) the
offending packets can be dropped. But alarms can be turned off
if these combinations come into regular use (e.g. through a
future standards action).
3. While reviewing currently unused combinations of inner and outer,
the opportunity was taken to define a single consistent behaviour
Briscoe Expires December 27, 2010 [Page 25]
Internet-Draft ECN Tunnelling June 2010
for the three cases with a Not-ECT inner header but a different
outer. RFC3168 and RFC4301 had diverged in this respect and even
their common behaviours had never been justified.
None of these combinations should result from Internet protocols
in the RFC series, but future standards actions might put any or
all of them to good use. Therefore it was decided that a
decapsulator must forward a Not-ECT inner unchanged when the
arriving outer is ECT(0) or ECT(1). But for safety it must drop
a combination of Not-ECT inner and CE outer. Then, if some
unfortunate misconfiguration resulted in a congested router
marking CE on a packet that was originally Not-ECT, drop would be
the only appropriate signal for the egress to propagate--the only
signal a non-ECN-capable transport (Not-ECT) would understand.
It may seem contradictory that the same argument has not been
applied to the ECT(1) codepoint, given it is being proposed as an
intermediate level of congestion in a scheme progressing through
the IETF [I-D.ietf-pcn-3-in-1-encoding]. Instead, a decapsulator
must forward a Not-ECT inner unchanged when its outer is ECT(1).
The rationale for not dropping this CU combination is to ensure
it will be usable if needed in the future. If any
misconfiguration led to ECT(1) congestion signals with a Not-ECT
inner, it would not be disastrous for the tunnel egress to
suppress them, because the congestion should then escalate to CE
marking, which the egress would drop, thus at least preventing
congestion collapse.
Problems 2 & 3 alone would not warrant a change to decapsulation, but
it was decided they are worth fixing and making consistent at the
same time as decapsulation code is changed to fix problem 1 (two
congestion severity-levels).
6. Backward Compatibility
A tunnel endpoint compliant with the present specification is
backward compatible when paired with any tunnel endpoint compliant
with any previous tunnelling RFC, whether RFC4301, RFC3168 (see
Section 3) or the earlier RFCs summarised in Appendix A (RFC2481,
RFC2401 and RFC2003). Each case is enumerated below.
6.1. Non-Issues Updating Decapsulation
At the egress, this specification only augments the per-packet
calculation of the ECN field (RFC3168 and RFC4301) for combinations
of inner and outer headers that have so far not been used in any IETF
protocols.
Briscoe Expires December 27, 2010 [Page 26]
Internet-Draft ECN Tunnelling June 2010
Therefore, all other things being equal, if an RFC4301 IPsec egress
is updated to comply with the new rules, it will still interwork with
any RFC4301 compliant ingress and the packet outputs will be
identical to those it would have output before (fully backward
compatible).
And, all other things being equal, if an RFC3168 egress is updated to
comply with the same new rules, it will still interwork with any
ingress complying with any previous specification (both modes of
RFC3168, both modes of RFC2481, RFC2401 and RFC2003) and the packet
outputs will be identical to those it would have output before (fully
backward compatible).
A compliant tunnel egress merely needs to implement the one behaviour
in Section 4 with no additional mode or option configuration at the
ingress or egress nor any additional negotiation with the ingress.
The 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).
6.2. Non-Update of RFC4301 IPsec Encapsulation
An RFC4301 IPsec ingress can comply with this new specification
without any update and it has no need for any new modes, options or
configuration. So, all other things being equal, it will continue to
interwork identically with any egress it worked with before (fully
backward compatible).
6.3. Update to RFC3168 Encapsulation
The encapsulation behaviour of the new normal mode copies the ECN
field whereas RFC3168 full functionality mode reset it. However, all
other things being equal, if RFC3168 ingress is updated to the
present specification, the outgoing packets from any tunnel egress
will still be unchanged. This is because all variants of tunnelling
at either end (RFC4301, both modes of RFC3168, both modes of RFC2481,
RFC2401, RFC2003 and the present specification) have always
propagated an incoming CE marking through the inner header and onward
into the outgoing header, whether the outer header is reset or
copied. Therefore, If the tunnel is considered as a black box, the
packets output from any egress will be identical with or without an
update to the ingress. Nonetheless, if packets are observed within
the black box (between the tunnel endpoints), CE markings copied by
the updated ingress will be visible within the black box, whereas
they would not have been before. Therefore, the update to
Briscoe Expires December 27, 2010 [Page 27]
Internet-Draft ECN Tunnelling June 2010
encapsulation can be termed 'black-box backwards compatible' (i.e.
identical unless you look inside the tunnel).
This specification introduces no new backward compatibility issues
when a compliant ingress talks with a legacy egress, but it has to
provide similar safeguards to those already defined in RFC3168.
RFC3168 laid down rules to ensure that an RFC3168 ingress turns off
ECN (limited functionality mode) if it is paired with a legacy egress
(RFC 2481, RFC2401 or RFC2003), which would not propagate ECN
correctly. The present specification carries forward those rules
(Section 4.3). It uses compatibility mode whenever RFC3168 would
have used limited functionality mode, and their per-packet behaviours
are identical. Therefore, all other things being equal, an ingress
using the new rules will interwork with any legacy tunnel egress in
exactly the same way as an RFC3168 ingress (still black-box backward
compatible).
7. Design Principles for Alternate ECN Tunnelling Semantics
This section is informative not normative.
S.5 of RFC3168 permits the Diffserv codepoint (DSCP)[RFC2474] to
'switch in' alternative behaviours for marking the ECN field, just as
it switches in different per-hop behaviours (PHBs) for scheduling.
[RFC4774] gives best current practice for designing such alternative
ECN semantics and very briefly mentions in section 5.4 that
tunnelling should be considered. The guidance below extends RFC4774,
giving additional guidance on designing any alternate ECN semantics
that would also require alternate tunnelling semantics.
The overriding guidance is: "Avoid designing alternate ECN tunnelling
semantics, if at all possible." If a scheme requires tunnels to
implement special processing of the ECN field for certain DSCPs, it
will be hard to guarantee that every implementer of every tunnel will
have added the required exception or that operators will have
ubiquitously deployed the required updates. It is unlikely a single
authority is even aware of all the tunnels in a network, which may
include tunnels set up by applications between endpoints, or
dynamically created in the network. Therefore it is highly likely
that some tunnels within a network or on hosts connected to it will
not implement the required special case.
That said, if a non-default scheme for tunnelling the ECN field is
really required, the following guidelines may prove useful in its
design:
Briscoe Expires December 27, 2010 [Page 28]
Internet-Draft ECN Tunnelling June 2010
On encapsulation in any alternate scheme:
1. The ECN field of the outer header should be cleared to Not-ECT
("00") unless it is guaranteed that the corresponding tunnel
egress will correctly propagate congestion markings introduced
across the tunnel in the outer header.
2. If it has established that ECN will be correctly propagated,
an encapsulator should also copy incoming congestion
notification into the outer header. The general principle
here is that the outer header should reflect congestion
accumulated along the whole upstream path, not just since the
tunnel ingress (Appendix B.3 on management and monitoring
explains).
In some circumstances (e.g. pseudowires, PCN), the whole path
is divided into segments, each with its own congestion
notification and feedback loop. In these cases, the function
that regulates load at the start of each segment will need to
reset congestion notification for its segment. Often the
point where congestion notification is reset will also be
located at the start of a tunnel. However, the resetting
function should be thought of as being applied to packets
after the encapsulation function--two logically separate
functions even though they might run on the same physical box.
Then the code module doing encapsulation can keep to the
copying rule and the load regulator module can reset
congestion, without any code in either module being
conditional on whether the other is there.
On decapsulation in any new scheme:
1. If the arriving inner header is Not-ECT it implies the
transport will not understand other ECN codepoints. If the
outer header carries an explicit congestion marking, the
alternate scheme would be expected to drop the packet--the
only indication of congestion the transport will understand.
If the alternate scheme recommends forwarding rather than
dropping such a packet, it must clearly justify this decision.
If the inner is Not-ECT and the outer carries any other ECN
codepoint that does not indicate congestion, the alternate
scheme can forward the packet, but probably only as Not-ECT.
2. If the arriving inner header is other than Not-ECT, the ECN
field that the alternate decapsulation scheme forwards should
reflect the more severe congestion marking of the arriving
inner and outer headers.
Briscoe Expires December 27, 2010 [Page 29]
Internet-Draft ECN Tunnelling June 2010
3. Any alternate scheme must define a behaviour for all
combinations of inner and outer headers, even those that would
not be expected to result from standards known at the time and
even those that would not be expected from the tunnel ingress
paired with the egress at run-time. Consideration should be
given to logging such unexpected combinations and raising an
alarm, particularly if there is a danger that the invalid
combination implies congestion signals are not being
propagated correctly. The presence of currently unused
combinations may represent an attack, but the new scheme
should try to define a way to forward such packets, at least
if a safe outgoing codepoint can be defined.
Raising an alarm allows a management system to decide whether
the anomaly is indeed an attack, in which case it can decide
to drop such packets. This is a preferable approach to hard-
coded discard of packets that seem anomalous today, but may be
needed tomorrow in future standards actions.
8. Security Considerations
Appendix B.1 discusses the security constraints imposed on ECN tunnel
processing. The new rules for ECN tunnel processing (Section 4)
trade-off between information security (covert channels) and traffic
security (congestion monitoring & control). Ensuring congestion
markings are not lost is itself an aspect of security, because if we
allowed congestion notification to be lost, any attempt to enforce a
response to congestion would be much harder.
Specialist security issues:
Tunnels intersecting Diffserv regions with alternate ECN semantics:
If alternate congestion notification semantics are defined for a
certain Diffserv PHB, the scope of the alternate semantics might
typically be bounded by the limits of a Diffserv region or
regions, as envisaged in [RFC4774] (e.g. the pre-congestion
notification architecture [RFC5559]). 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 endpoints of tunnels with one end inside and the
other outside the domain. [RFC5559] gives specific advice on this
for the PCN case, but other definitions of alternate semantics
will need to discuss the specific security implications in each
Briscoe Expires December 27, 2010 [Page 30]
Internet-Draft ECN Tunnelling June 2010
case.
ECN nonce tunnel coverage: The new decapsulation rules improve the
coverage of the ECN nonce [RFC3540] relative to the previous rules
in RFC3168 and RFC4301. However, nonce coverage is still not
perfect, as this would have led to a safety problem in another
case. Both are corner-cases, so discussion of the compromise
between them is deferred to Appendix D.
Covert channel not turned off: A legacy (RFC3168) tunnel ingress
could ask an RFC3168 egress to turn off ECN processing as well as
itself turning off ECN. An egress compliant with the present
specification will agree to such a request from a legacy ingress,
but it relies on the ingress always sending Not-ECT in the outer.
If the egress receives other ECN codepoints in the outer it will
process them as normal, so it will actually still copy congestion
markings from the outer to the outgoing header. Referring for
example to Figure 5 (Appendix B.1), although the tunnel ingress
'I' will set all ECN fields in outer headers to Not-ECT, 'M' could
still toggle CE or ECT(1) 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 an historic security concern that is now considered
manageable.
9. Conclusions
This document allows tunnels to propagate an extra level of
congestion severity. It uses previously unused combinations of inner
and outer header to augment the rules for calculating the ECN field
when decapsulating IP packets at the egress of IPsec (RFC4301) and
non-IPsec (RFC3168) tunnels.
This document also updates the ingress tunnelling encapsulation of
RFC3168 ECN to bring all IP in IP tunnels into line with the new
behaviour in the IPsec architecture of RFC4301, which copies rather
than resets the ECN field when creating outer headers.
The need for both these updated behaviours was triggered by the
introduction of pre-congestion notification (PCN) onto the IETF
standards track. Operators wanting to support PCN or other alternate
ECN schemes that use an extra severity level can require that their
tunnels comply with the present specification. This is not a fork in
the RFC series, it is an update that can be deployed first by those
that need it, and subsequently by all tunnel endpoint implementations
Briscoe Expires December 27, 2010 [Page 31]
Internet-Draft ECN Tunnelling June 2010
during general code maintenance. It is backward compatible with all
previous tunnelling behaviours, so existing single severity level
schemes will continue to work as before, but support for two severity
levels will gradually be added to the Internet.
The new rules propagate changes to the ECN field across tunnel end-
points that previously blocked them to restrict the bandwidth of a
potential covert channel. Limiting the channel's bandwidth to 2 bits
per packet is now considered sufficient.
At the same time as removing these legacy constraints, the
opportunity has been taken to draw together diverging tunnel
specifications into a single consistent behaviour. Then any tunnel
can be deployed unilaterally, and it will support the full range of
congestion control and management schemes without any modes or
configuration. Further, any host or router can expect the ECN field
to behave in the same way, whatever type of tunnel might intervene in
the path. This new certainty could enable new uses of the ECN field
that would otherwise be confounded by ambiguity.
10. Acknowledgements
Thanks to David Black for his insightful reviews and patient
explanations of better ways to think about function placement and
alarms. Thanks to David and to Anil Agarwal for pointing out cases
where it is safe to forward CU combinations of headers. Also thanks
to Arnaud Jacquet for the idea for Appendix C. Thanks to Gorry
Fairhurst, Teco Boot, Michael Menth, Bruce Davie, Toby Moncaster,
Sally Floyd, Alfred Hoenes, Gabriele Corliano, Ingemar Johansson and
Phil Eardley for their thoughts and careful review comments, and to
Stephen Hanna for conducting the Security Directorate review.
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.
Comments Solicited (to be removed by the RFC Editor):
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.
11. References
Briscoe Expires December 27, 2010 [Page 32]
Internet-Draft ECN Tunnelling June 2010
11.1. 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.
[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.
11.2. Informative References
[I-D.ietf-pcn-3-in-1-encoding] Briscoe, B. and T. Moncaster, "PCN
3-State Encoding Extension in a
single DSCP",
draft-ietf-pcn-3-in-1-encoding-02
(work in progress), March 2010.
[RFC2401] Kent, S. and R. Atkinson, "Security
Architecture for the Internet
Protocol", RFC 2401, November 1998.
[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.
[RFC2481] Ramakrishnan, K. and S. Floyd, "A
Proposal to add Explicit Congestion
Notification (ECN) to IP", RFC 2481,
January 1999.
[RFC2983] Black, D., "Differentiated Services
and Tunnels", RFC 2983, October 2000.
[RFC3540] Spring, N., Wetherall, D., and D.
Ely, "Robust Explicit Congestion
Notification (ECN) Signaling with
Nonces", RFC 3540, June 2003.
Briscoe Expires December 27, 2010 [Page 33]
Internet-Draft ECN Tunnelling June 2010
[RFC4306] Kaufman, C., "Internet Key Exchange
(IKEv2) Protocol", RFC 4306,
December 2005.
[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.
[RFC5559] Eardley, P., "Pre-Congestion
Notification (PCN) Architecture",
RFC 5559, June 2009.
[RFC5670] Eardley, P., "Metering and Marking
Behaviour of PCN-Nodes", RFC 5670,
November 2009.
[RFC5696] Moncaster, T., Briscoe, B., and M.
Menth, "Baseline Encoding and
Transport of Pre-Congestion
Information", RFC 5696,
November 2009.
[VCP] Xia, Y., Subramanian, L., Stoica, I.,
and S. Kalyanaraman, "One more bit is
enough", Proc. SIGCOMM'05, ACM
CCR 35(4)37--48, 2005, .
Appendix A. Early ECN Tunnelling RFCs
IP in IP tunnelling was originally defined in [RFC2003]. On
encapsulation, the incoming header was copied to the outer and on
decapsulation the outer was simply discarded. Initially, IPsec
tunnelling [RFC2401] followed the same behaviour.
When ECN was introduced experimentally in [RFC2481], legacy (RFC2003
or RFC2401) tunnels would have discarded any congestion markings
added to the outer header, so RFC2481 introduced rules for
calculating the outgoing header from a combination of the inner and
outer on decapsulation. RC2481 also introduced a second mode for
IPsec tunnels, which turned off ECN processing (Not-ECT) in the outer
header on encapsulation because an RFC2401 decapsulator would discard
the outer on decapsulation. For RFC2401 IPsec this had the side-
Briscoe Expires December 27, 2010 [Page 34]
Internet-Draft ECN Tunnelling June 2010
effect of completely blocking the covert channel.
In RFC2481 the ECN field was defined as two separate bits. But when
ECN moved from the experimental to the standards track [RFC3168], the
ECN field was redefined as four codepoints. This required a
different calculation of the ECN field from that used in RFC2481 on
decapsulation. RFC3168 also had two modes; a 'full functionality
mode' that restricted the covert channel as much as possible but
still allowed ECN to be used with IPsec, and another that completely
turned off ECN processing across the tunnel. This 'limited
functionality mode' both offered a way for operators to completely
block the covert channel and allowed an RFC3168 ingress to interwork
with a legacy tunnel egress (RFC2481, RFC2401 or RFC2003).
The present specification includes a similar compatibility mode to
interwork safely with tunnels compliant with any of these three
earlier RFCs. However, unlike RFC3168, it is only a mode of the
ingress, as decapsulation behaviour is the same in either case.
Appendix B. 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.
B.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. The scenario in Figure 5 will be used 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.
Briscoe Expires December 27, 2010 [Page 35]
Internet-Draft ECN Tunnelling June 2010
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. 'I' can 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
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 effectively
intends to create an information channel (for congestion signalling)
from 'M' to 'B' (for 'B' to feed back to 'A'). Therefore the goals
of IPsec and ECN are mutually incompatible, requiring some
compromise.
With respect to using the DS or ECN fields as covert channels,
S.5.1.2 of RFC4301 says, "controls are provided to manage the
bandwidth of this 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 implies that these covert
Briscoe Expires December 27, 2010 [Page 36]
Internet-Draft ECN Tunnelling June 2010
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.
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
administrators the option of a tiny increment in security, especially
given not copying congestion indications could seriously harm
everyone's network service.
B.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, it
will be called the Load Regulator. This allows for 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).
Briscoe Expires December 27, 2010 [Page 37]
Internet-Draft ECN Tunnelling June 2010
A--->R--->I=========>M=========>E-------->B
Figure 6: Simple Tunnel Scenario
A similar tunnelling scenario to the IPsec one just described will
now be considered, but without the different security domains,
because the focus now shifts to whether the control loop and
management monitoring work (Figure 6). If resources in the tunnel
are to be able to explicitly notify congestion and the feedback path
is from 'B' to 'A', it will certainly be necessary for 'E' to copy
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 does
not 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.
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, resetting CE marking on
encapsulation breaks the standards track PCN congestion marking
scheme [RFC5670]. It ends up removing excessive amounts of traffic
unnecessarily. Whereas copying CE markings at ingress leads to the
correct control behaviour.
B.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 so far along the path, perhaps to determine between them
how much of the congestion is contributed by each domain.
In this document the baseline of congestion marking (or the
Congestion Baseline) is defined as the source of the layer that
created (or most recently reset) the congestion notification field.
When monitoring congestion it would be desirable if the Congestion
Briscoe Expires December 27, 2010 [Page 38]
Internet-Draft ECN Tunnelling June 2010
Baseline did not depend on whether packets were tunnelled or not.
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, so that it
is exposed across the tunnel.
For management purposes it might be useful for the tunnel egress to
be able to monitor whether congestion occurred across a tunnel or
upstream of it. Superficially it appears that copying congestion
markings at the ingress would make this difficult, whereas it was
straightforward when an RFC3168 ingress reset them. However,
Appendix C gives a simple and precise method for a tunnel egress to
infer the congestion level introduced across a tunnel. It works
irrespective of whether the ingress copies or resets congestion
markings.
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. Say 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 measure. It
is simply the proportion of packets not marked in the inner header
(70) that have a CE marking in the outer header (12). This technique
works whether the ingress copies or resets CE markings, so it can be
used by an egress that is not sure which RFC the ingress complies
with.
Figure 7 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 introduced across the tunnel.
Briscoe Expires December 27, 2010 [Page 39]
Internet-Draft ECN Tunnelling June 2010
^ outer header marking
|
100% +-----+---------+ The large square
| | | represents 100 packets
| 30 | |
| | | p_t = 12/(100-30)
p_t + +---------+ = 12/70
| | 12 | = 17%
0 +-----+---------+--->
0 30% 100% inner header marking
Figure 7: Tunnel Marking of Packets Already Marked at Ingress
Appendix D. Compromise on Decap with ECT(1) Inner and ECT(0) Outer
A packet with an ECT(1) inner and an ECT(0) outer should never arise
from any known IETF protocol. Without giving a reason, RFC3168 and
RFC4301 both say the outer should be ignored when decapsulating such
a packet. This appendix explains why it was decided not to change
this advice.
In summary, ECT(0) always means 'not congested' and ECT(1) may imply
the same [RFC3168] or it may imply a higher severity congestion
signal [RFC4774], [I-D.ietf-pcn-3-in-1-encoding], depending on the
transport in use. Whether they mean the same or not, at the ingress
the outer should have started the same as the inner and only a broken
or compromised router could have changed the outer to ECT(0).
The decapsulator can detect this anomaly. But the question is,
should it correct the anomaly by ignoring the outer, or should it
reveal the anomaly to the end-to-end transport by forwarding the
outer?
On balance, it was decided that the decapsulator should correct the
anomaly, but log the event and optionally raise an alarm. This is
the safe action if ECT(1) is being used as a more severe marking than
ECT(0), because it passes the more severe signal to the transport.
However, it is not a good idea to hide anomalies, which is why an
optional alarm is suggested. It should be noted that this anomaly
may be the result of two changes to the outer: a broken or
compromised router within the tunnel might be erasing congestion
markings introduced earlier in the same tunnel by a congested router.
In this case, the anomaly would be losing congestion signals, which
needs immediate attention.
The original reason for defining ECT(0) and ECT(1) as equivalent was
so that the data source could use the ECN nonce [RFC3540] to detect
if congestion signals were being erased. However, in this case, the
Briscoe Expires December 27, 2010 [Page 40]
Internet-Draft ECN Tunnelling June 2010
decapsulator does not need a nonce to detect any anomalies introduced
within the tunnel, because it has the inner as a record of the header
at the ingress. Therefore, it was decided that the best compromise
would be to give precedence to solving the safety issue over
revealing the anomaly, because the anomaly could at least be detected
and dealt with internally.
Superficially, the opposite case where the inner and outer carry
different ECT values, but with an ECT(1) outer and ECT(0) inner,
seems to require a similar compromise. However, because that case is
reversed, no compromise is necessary; it is best to forward the outer
whether the transport expects the ECT(1) to mean a higher severity
than ECT(0) or the same severity. Forwarding the outer either
preserves a higher value (if it is higher) or it reveals an anomaly
to the transport (if the two ECT codepoints mean the same severity).
Appendix E. Open Issues
The new decapsulation behaviour defined in Section 4.2 adds support
for propagation of 2 severity levels of congestion. However
transports have no way to discover whether there are any legacy
tunnels on their path that will not propagate 2 severity levels. It
would have been nice to add a feature for transports to check path
support, but this remains an open issue that will have to be
addressed in any future standards action to define an end-to-end
scheme that requires 2-severity levels of congestion. PCN avoids
this problem because it is only for a controlled region, so all
legacy tunnels can be upgraded by the same operator that deploys PCN.
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://bobbriscoe.net/
Briscoe Expires December 27, 2010 [Page 41]