draft-ietf-tsvwg-ecn-tunnel-07.txt   draft-ietf-tsvwg-ecn-tunnel-08.txt 
Transport Area Working Group B. Briscoe Transport Area Working Group B. Briscoe
Internet-Draft BT Internet-Draft BT
Updates: 3168, 4301 February 11, 2010 Updates: 3168, 4301 March 03, 2010
(if approved) (if approved)
Intended status: Standards Track Intended status: Standards Track
Expires: August 15, 2010 Expires: September 4, 2010
Tunnelling of Explicit Congestion Notification Tunnelling of Explicit Congestion Notification
draft-ietf-tsvwg-ecn-tunnel-07 draft-ietf-tsvwg-ecn-tunnel-08
Abstract Abstract
This document redefines how the explicit congestion notification This document redefines how the explicit congestion notification
(ECN) field of the IP header should be constructed on entry to and (ECN) field of the IP header should be constructed on entry to and
exit from any IP in IP tunnel. On encapsulation it updates RFC3168 exit from any IP in IP tunnel. On encapsulation it updates RFC3168
to bring all IP in IP tunnels (v4 or v6) into line with RFC4301 IPsec to bring all IP in IP tunnels (v4 or v6) into line with RFC4301 IPsec
ECN processing. On decapsulation it updates both RFC3168 and RFC4301 ECN processing. On decapsulation it updates both RFC3168 and RFC4301
to add new behaviours for previously unused combinations of inner and to add new behaviours for previously unused combinations of inner and
outer header. The new rules ensure the ECN field is correctly outer header. The new rules ensure the ECN field is correctly
propagated across a tunnel whether it is used to signal one or two propagated across a tunnel whether it is used to signal one or two
severity levels of congestion, whereas before only one severity level severity levels of congestion, whereas before only one severity level
was supported. Tunnel endpoints can be updated in any order without was supported. Tunnel endpoints can be updated in any order without
affecting pre-existing uses of the ECN field (backward compatible). affecting pre-existing uses of the ECN field, providing backward
Nonetheless, operators wanting to support two severity levels (e.g. compatibility. Nonetheless, operators wanting to support two
for pre-congestion notification--PCN) can require compliance with severity levels (e.g. for pre-congestion notification--PCN) can
this new specification. A thorough analysis of the reasoning for require compliance with this new specification. A thorough analysis
these changes and the implications is included. In the unlikely of the reasoning for these changes and the implications is included.
event that the new rules do not meet a specific need, RFC4774 gives In the unlikely event that the new rules do not meet a specific need,
guidance on designing alternate ECN semantics and this document RFC4774 gives guidance on designing alternate ECN semantics and this
extends that to include tunnelling issues. document extends that to include tunnelling issues.
Status of This Memo Status of This Memo
This Internet-Draft is submitted to IETF in full conformance with the This Internet-Draft is submitted to IETF in full conformance with the
provisions of BCP 78 and BCP 79. provisions of BCP 78 and BCP 79.
Internet-Drafts are working documents of the Internet Engineering Internet-Drafts are working documents of the Internet Engineering
Task Force (IETF), its areas, and its working groups. Note that Task Force (IETF), its areas, and its working groups. Note that
other groups may also distribute working documents as Internet- other groups may also distribute working documents as Internet-
Drafts. Drafts.
skipping to change at page 2, line 9 skipping to change at page 2, line 9
and may be updated, replaced, or obsoleted by other documents at any and may be updated, replaced, or obsoleted by other documents at any
time. It is inappropriate to use Internet-Drafts as reference time. It is inappropriate to use Internet-Drafts as reference
material or to cite them other than as "work in progress." material or to cite them other than as "work in progress."
The list of current Internet-Drafts can be accessed at The list of current Internet-Drafts can be accessed at
http://www.ietf.org/ietf/1id-abstracts.txt. http://www.ietf.org/ietf/1id-abstracts.txt.
The list of Internet-Draft Shadow Directories can be accessed at The list of Internet-Draft Shadow Directories can be accessed at
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This Internet-Draft will expire on August 15, 2010. This Internet-Draft will expire on September 4, 2010.
Copyright Notice Copyright Notice
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Table of Contents Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 9 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 9
1.1. Scope . . . . . . . . . . . . . . . . . . . . . . . . . . 11 1.1. Scope . . . . . . . . . . . . . . . . . . . . . . . . . . 11
2. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 11 2. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 11
3. Summary of Pre-Existing RFCs . . . . . . . . . . . . . . . . . 12 3. Summary of Pre-Existing RFCs . . . . . . . . . . . . . . . . . 12
3.1. Encapsulation at Tunnel Ingress . . . . . . . . . . . . . 12 3.1. Encapsulation at Tunnel Ingress . . . . . . . . . . . . . 12
3.2. Decapsulation at Tunnel Egress . . . . . . . . . . . . . . 13 3.2. Decapsulation at Tunnel Egress . . . . . . . . . . . . . . 13
4. New ECN Tunnelling Rules . . . . . . . . . . . . . . . . . . . 14 4. New ECN Tunnelling Rules . . . . . . . . . . . . . . . . . . . 14
4.1. Default Tunnel Ingress Behaviour . . . . . . . . . . . . . 15 4.1. Default Tunnel Ingress Behaviour . . . . . . . . . . . . . 15
4.2. Default Tunnel Egress Behaviour . . . . . . . . . . . . . 15 4.2. Default Tunnel Egress Behaviour . . . . . . . . . . . . . 15
4.3. Encapsulation Modes . . . . . . . . . . . . . . . . . . . 17 4.3. Encapsulation Modes . . . . . . . . . . . . . . . . . . . 17
4.4. Single Mode of Decapsulation . . . . . . . . . . . . . . . 19 4.4. Single Mode of Decapsulation . . . . . . . . . . . . . . . 19
5. Updates to Earlier RFCs . . . . . . . . . . . . . . . . . . . 20 5. Updates to Earlier RFCs . . . . . . . . . . . . . . . . . . . 20
5.1. Changes to RFC4301 ECN processing . . . . . . . . . . . . 20 5.1. Changes to RFC4301 ECN processing . . . . . . . . . . . . 20
5.2. Changes to RFC3168 ECN processing . . . . . . . . . . . . 21 5.2. Changes to RFC3168 ECN processing . . . . . . . . . . . . 20
5.3. Motivation for Changes . . . . . . . . . . . . . . . . . . 22 5.3. Motivation for Changes . . . . . . . . . . . . . . . . . . 22
5.3.1. Motivation for Changing Encapsulation . . . . . . . . 22 5.3.1. Motivation for Changing Encapsulation . . . . . . . . 22
5.3.2. Motivation for Changing Decapsulation . . . . . . . . 23 5.3.2. Motivation for Changing Decapsulation . . . . . . . . 23
6. Backward Compatibility . . . . . . . . . . . . . . . . . . . . 25 6. Backward Compatibility . . . . . . . . . . . . . . . . . . . . 25
6.1. Non-Issues Updating Decapsulation . . . . . . . . . . . . 25 6.1. Non-Issues Updating Decapsulation . . . . . . . . . . . . 25
6.2. Non-Update of RFC4301 IPsec Encapsulation . . . . . . . . 26 6.2. Non-Update of RFC4301 IPsec Encapsulation . . . . . . . . 26
6.3. Update to RFC3168 Encapsulation . . . . . . . . . . . . . 26 6.3. Update to RFC3168 Encapsulation . . . . . . . . . . . . . 26
7. Design Principles for Alternate ECN Tunnelling Semantics . . . 27 7. Design Principles for Alternate ECN Tunnelling Semantics . . . 27
8. Security Considerations . . . . . . . . . . . . . . . . . . . 29 8. Security Considerations . . . . . . . . . . . . . . . . . . . 29
9. Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . 30 9. Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . 30
10. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 31 10. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 31
11. References . . . . . . . . . . . . . . . . . . . . . . . . . . 31 11. References . . . . . . . . . . . . . . . . . . . . . . . . . . 31
11.1. Normative References . . . . . . . . . . . . . . . . . . . 31 11.1. Normative References . . . . . . . . . . . . . . . . . . . 31
11.2. Informative References . . . . . . . . . . . . . . . . . . 32 11.2. Informative References . . . . . . . . . . . . . . . . . . 32
Editorial Comments . . . . . . . . . . . . . . . . . . . . . . . .
Appendix A. Early ECN Tunnelling RFCs . . . . . . . . . . . . . . 34 Appendix A. Early ECN Tunnelling RFCs . . . . . . . . . . . . . . 34
Appendix B. Design Constraints . . . . . . . . . . . . . . . . . 35 Appendix B. Design Constraints . . . . . . . . . . . . . . . . . 35
B.1. Security Constraints . . . . . . . . . . . . . . . . . . . 35 B.1. Security Constraints . . . . . . . . . . . . . . . . . . . 35
B.2. Control Constraints . . . . . . . . . . . . . . . . . . . 37 B.2. Control Constraints . . . . . . . . . . . . . . . . . . . 37
B.3. Management Constraints . . . . . . . . . . . . . . . . . . 38 B.3. Management Constraints . . . . . . . . . . . . . . . . . . 38
Appendix C. Contribution to Congestion across a Tunnel . . . . . 38 Appendix C. Contribution to Congestion across a Tunnel . . . . . 39
Appendix D. Why Losing ECT(1) on Decapsulation Impedes PCN Appendix D. Why Losing ECT(1) on Decapsulation Impedes PCN
(to be removed before publication) . . . . . . . . . 39 (to be removed before publication) . . . . . . . . . 40
Appendix E. Why Resetting ECN on Encapsulation Impedes PCN Appendix E. Why Resetting ECN on Encapsulation Impedes PCN
(to be removed before publication) . . . . . . . . . 41 (to be removed before publication) . . . . . . . . . 41
Appendix F. Compromise on Decap with ECT(1) Inner and ECT(0) Appendix F. Compromise on Decap with ECT(1) Inner and ECT(0)
Outer . . . . . . . . . . . . . . . . . . . . . . . . 41 Outer . . . . . . . . . . . . . . . . . . . . . . . . 42
Appendix G. Open Issues . . . . . . . . . . . . . . . . . . . . . 42 Appendix G. Open Issues . . . . . . . . . . . . . . . . . . . . . 43
Request to the RFC Editor (to be removed on publication): Request to the RFC Editor (to be removed on publication):
In the RFC index, RFC3168 should be identified as an update to In the RFC index, RFC3168 should be identified as an update to
RFC2003. RFC4301 should be identified as an update to RFC3168. RFC2003. RFC4301 should be identified as an update to RFC3168.
Changes from previous drafts (to be removed by the RFC Editor) Changes from previous drafts (to be removed by the RFC Editor)
Full text differences between IETF draft versions are available at Full text differences between IETF draft versions are available at
<http://tools.ietf.org/wg/tsvwg/draft-ietf-tsvwg-ecn-tunnel/>, and <http://tools.ietf.org/wg/tsvwg/draft-ietf-tsvwg-ecn-tunnel/>, and
skipping to change at page 9, line 32 skipping to change at page 9, line 32
regulation (changed title from "In-path Load Regulation" to regulation (changed title from "In-path Load Regulation" to
"Non-Dependence of Tunnelling on In-path Load Regulation"), but "Non-Dependence of Tunnelling on In-path Load Regulation"), but
explained how an in-path load regulation function must be explained how an in-path load regulation function must be
carefully placed with respect to tunnel encapsulation (in a new carefully placed with respect to tunnel encapsulation (in a new
sub-section entitled "Dependence of In-Path Load Regulation on sub-section entitled "Dependence of In-Path Load Regulation on
Tunnelling"). Tunnelling").
1. Introduction 1. Introduction
Explicit congestion notification (ECN [RFC3168]) allows a forwarding Explicit congestion notification (ECN [RFC3168]) allows a forwarding
element to notify the onset of congestion without having to drop element (e.g. a router) to notify the onset of congestion without
packets. Instead it can explicitly mark a proportion of packets in having to drop packets. Instead it can explicitly mark a proportion
the 2-bit ECN field in the IP header (Table 1 recaps the ECN of packets in the 2-bit ECN field in the IP header (Table 1 recaps
codepoints). the ECN codepoints).
The outer header of an IP packet can encapsulate one or more IP The outer header of an IP packet can encapsulate one or more IP
headers for tunnelling. A forwarding element using ECN to signify headers for tunnelling. A forwarding element using ECN to signify
congestion will only mark the immediately visible outer IP header. congestion will only mark the immediately visible outer IP header.
When a tunnel decapsulator later removes this outer header, it When a tunnel decapsulator later removes this outer header, it
follows rules to propagate congestion markings by combining the ECN follows rules to propagate congestion markings by combining the ECN
fields of the inner and outer IP header into one outgoing IP header. fields of the inner and outer IP header into one outgoing IP header.
This document updates those rules for IPsec [RFC4301] and non-IPsec This document updates those rules for IPsec [RFC4301] and non-IPsec
[RFC3168] tunnels to add new behaviours for previously unused [RFC3168] tunnels to add new behaviours for previously unused
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|Incoming | Incoming Outer Header | |Incoming | Incoming Outer Header |
| Inner +---------+------------+------------+------------+ | Inner +---------+------------+------------+------------+
| Header | Not-ECT | ECT(0) | ECT(1) | CE | | Header | Not-ECT | ECT(0) | ECT(1) | CE |
+---------+---------+------------+------------+------------+ +---------+---------+------------+------------+------------+
RFC3168->| Not-ECT | Not-ECT |Not-ECT |Not-ECT | drop | RFC3168->| Not-ECT | Not-ECT |Not-ECT |Not-ECT | drop |
RFC4301->| Not-ECT | Not-ECT |Not-ECT |Not-ECT |Not-ECT | RFC4301->| Not-ECT | Not-ECT |Not-ECT |Not-ECT |Not-ECT |
| ECT(0) | ECT(0) | ECT(0) | ECT(0) | CE | | ECT(0) | ECT(0) | ECT(0) | ECT(0) | CE |
| ECT(1) | ECT(1) | ECT(1) | ECT(1) | CE | | ECT(1) | ECT(1) | ECT(1) | ECT(1) | CE |
| CE | CE | CE | CE | CE | | CE | CE | CE | CE | CE |
+---------+---------+------------+------------+------------+ +---------+---------+------------+------------+------------+
| Outgoing Header |
+------------------------------------------------+ 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 Figure 2: IP in IP Decapsulation; Recap of Pre-existing Behaviour
The behaviour in the table derives from the logic given in RFC3168 The behaviour in the table derives from the logic given in RFC3168
and RFC4301, briefly recapped as follows: and RFC4301, briefly recapped as follows:
o On decapsulation, if the inner ECN field is Not-ECT the outer is o On decapsulation, if the inner ECN field is Not-ECT the outer is
ignored. RFC3168 (but not RFC4301) also specified that the ignored. RFC3168 (but not RFC4301) also specified that the
decapsulator must drop a packet with a Not-ECT inner and CE in the decapsulator must drop a packet with a Not-ECT inner and CE in the
outer. outer.
o In all other cases, if the outer is CE, the outgoing ECN field is o In all other cases, if the outer is CE, the outgoing ECN field is
set to CE, but otherwise the outer is ignored and the inner is set to CE, but otherwise the outer is ignored and the inner is
used for the outgoing ECN field. used for the outgoing ECN field.
RFC3168 also made it an auditable event for an IPsec tunnel "if the Section 9.2.2 of RFC3168 also made it an auditable event for an IPsec
ECN Field is changed inappropriately within an IPsec tunnel...". tunnel "if the ECN Field is changed inappropriately within an IPsec
Inappropriate changes were not specifically enumerated. RFC4301 did tunnel...". Inappropriate changes were not specifically enumerated.
not mention inappropriate ECN changes. RFC4301 did not mention inappropriate ECN changes.
4. New ECN Tunnelling Rules 4. New ECN Tunnelling Rules
The standards actions below in Section 4.1 (ingress encapsulation) The standards actions below in Section 4.1 (ingress encapsulation)
and Section 4.2 (egress decapsulation) define new default ECN tunnel and Section 4.2 (egress decapsulation) define new default ECN tunnel
processing rules for any IP packet (v4 or v6) with any Diffserv processing rules for any IP packet (v4 or v6) with any Diffserv
codepoint. codepoint.
If these defaults do not meet a particular requirement, an alternate If these defaults do not meet a particular requirement, an alternate
ECN tunnelling scheme can be introduced as part of the definition of ECN tunnelling scheme can be introduced as part of the definition of
an alternate congestion marking scheme used by a specific Diffserv an alternate congestion marking scheme used by a specific Diffserv
PHB (see S.5 of [RFC3168] and [RFC4774]). When designing such PHB (see S.5 of [RFC3168] and [RFC4774]). When designing such
alternate ECN tunnelling schemes, the principles in Section 7 should alternate ECN tunnelling schemes, the principles in Section 7 should
be followed. However, alternate ECN tunnelling schemes are NOT be followed. However, alternate ECN tunnelling schemes SHOULD be
RECOMMENDED as the deployment burden of handling exceptional PHBs in avoided whenever possible as the deployment burden of handling
implementations of all affected tunnels should not be underestimated. exceptional PHBs in implementations of all affected tunnels should
not be underestimated. There is no requirement for a PHB definition
There is no requirement for a PHB definition to state anything about to state anything about ECN tunnelling behaviour if the default
ECN tunnelling behaviour if the default behaviour in the present behaviour in the present specification is sufficient.
specification is sufficient.
4.1. Default Tunnel Ingress Behaviour 4.1. Default Tunnel Ingress Behaviour
Two modes of encapsulation are defined here; a REQUIRED `normal mode' Two modes of encapsulation are defined here; a REQUIRED `normal mode'
and a `compatibility mode', which is for backward compatibility with and a `compatibility mode', which is for backward compatibility with
tunnel decapsulators that do not understand ECN. Note that these are tunnel decapsulators that do not understand ECN. Note that these are
modes of the ingress tunnel endpoint only, not the whole tunnel. modes of the ingress tunnel endpoint only, not the whole tunnel.
Section 4.3 explains why two modes are necessary and specifies the Section 4.3 explains why two modes are necessary and specifies the
circumstances in which it is sufficient to solely implement normal circumstances in which it is sufficient to solely implement normal
mode. mode.
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+---------+------------------------------------------------+ +---------+------------------------------------------------+
|Incoming | Incoming Outer Header | |Incoming | Incoming Outer Header |
| Inner +---------+------------+------------+------------+ | Inner +---------+------------+------------+------------+
| Header | Not-ECT | ECT(0) | ECT(1) | CE | | Header | Not-ECT | ECT(0) | ECT(1) | CE |
+---------+---------+------------+------------+------------+ +---------+---------+------------+------------+------------+
| Not-ECT | Not-ECT |Not-ECT(!!!)|Not-ECT(!!!)| drop(!!!)| | Not-ECT | Not-ECT |Not-ECT(!!!)|Not-ECT(!!!)| drop(!!!)|
| ECT(0) | ECT(0) | ECT(0) | ECT(1) | CE | | ECT(0) | ECT(0) | ECT(0) | ECT(1) | CE |
| ECT(1) | ECT(1) | ECT(1) (!) | ECT(1) | CE | | ECT(1) | ECT(1) | ECT(1) (!) | ECT(1) | CE |
| CE | CE | CE | CE(!!!)| CE | | CE | CE | CE | CE(!!!)| CE |
+---------+---------+------------+------------+------------+ +---------+---------+------------+------------+------------+
| Outgoing Header |
+------------------------------------------------+ The ECN field in the outgoing header is set to the codepoint at the
Currently unused combinations are indicated by '(!!!)' or '(!)' 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 Figure 4: New IP in IP Decapsulation Behaviour
This table for decapsulation behaviour is derived from the following This table for decapsulation behaviour is derived from the following
logic: logic:
o If the inner ECN field is Not-ECT the decapsulator MUST NOT o If the inner ECN field is Not-ECT the decapsulator MUST NOT
propagate any other ECN codepoint onwards. This is because the propagate any other ECN codepoint onwards. This is because the
inner Not-ECT marking is set by transports that use drop as an inner Not-ECT marking is set by transports that use drop as an
indication of congestion and would not understand or respond to indication of congestion and would not understand or respond to
skipping to change at page 17, line 12 skipping to change at page 17, line 15
Just because the highlighted combinations are currently unused, Just because the highlighted combinations are currently unused,
does not mean that all the other combinations are always valid. does not mean that all the other combinations are always valid.
Some are only valid if they have arrived from a particular type of Some are only valid if they have arrived from a particular type of
legacy ingress, and dangerous otherwise. Therefore an legacy ingress, and dangerous otherwise. Therefore an
implementation MAY allow an operator to configure logging and implementation MAY allow an operator to configure logging and
alarms for such additional header combinations known to be alarms for such additional header combinations known to be
dangerous or CU for the particular configuration of tunnel dangerous or CU for the particular configuration of tunnel
endpoints deployed at run-time. endpoints deployed at run-time.
Alarms should be rate-limited so that the anomalous combinations Alarms SHOULD be rate-limited so that the anomalous combinations
will not amplify into a flood of alarm messages. It MUST be will not amplify into a flood of alarm messages. It MUST be
possible to suppress alarms or logging, e.g. if it becomes possible to suppress alarms or logging, e.g. if it becomes
apparent that a combination that previously was not used has apparent that a combination that previously was not used has
started to be used for legitimate purposes such as a new standards started to be used for legitimate purposes such as a new standards
action. action.
The above logic allows for ECT(0) and ECT(1) to both represent the The above logic allows for ECT(0) and ECT(1) to both represent the
same severity of congestion marking (e.g. "not congestion marked"). same severity of congestion marking (e.g. "not congestion marked").
But it also allows future schemes to be defined where ECT(1) is a But it also allows future schemes to be defined where ECT(1) is a
more severe marking than ECT(0), in particular enabling the simplest more severe marking than ECT(0), in particular enabling the simplest
possible encoding for PCN [I-D.ietf-pcn-3-in-1-encoding]. This possible encoding for PCN [I-D.ietf-pcn-3-in-1-encoding]. Before the
approach is discussed in Appendix D and in the discussion of the ECN present specification was written, the PCN working-group had proposed
nonce [RFC3540] in Section 8, which in turn refers to Appendix F. a number of work-rounds to the problem of a tunnel egress not
propagating two severity levels of congestion. 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 [Note_PCN_egress]. 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 F.
4.3. Encapsulation Modes 4.3. Encapsulation Modes
Section 4.1 introduces two encapsulation modes, normal mode and Section 4.1 introduces two encapsulation modes, normal mode and
compatibility mode, defining their encapsulation behaviour (i.e. compatibility mode, defining their encapsulation behaviour (i.e.
header copying or zeroing respectively). Note that these are modes header copying or zeroing respectively). Note that these are modes
of the ingress tunnel endpoint only, not the tunnel as a whole. of the ingress tunnel endpoint only, not the tunnel as a whole.
To comply with this specification, a tunnel ingress MUST at least To comply with this specification, a tunnel ingress MUST at least
implement `normal mode'. Unless it will never be used with legacy implement `normal mode'. Unless it will never be used with legacy
tunnel egress nodes (RFC2003, RFC2401 or RFC2481 or the limited tunnel egress nodes (RFC2003, RFC2401 or RFC2481 or the limited
functionality mode of RFC3168), an ingress MUST also implement functionality mode of RFC3168), an ingress MUST also implement
`compatibility mode' for backward compatibility with tunnel egresses `compatibility mode' for backward compatibility with tunnel egresses
that do not propagate explicit congestion notifications [RFC4774]. that do not propagate explicit congestion notifications [RFC4774].
We can categorise the way that an ingress tunnel endpoint is paired We can categorise the way that an ingress tunnel endpoint is paired
with an egress as either: with an egress as either static or dynamically discovered:
static: those paired together by prior configuration or;
dynamically discovered: those paired together by some form of tunnel Static: Tunnel endpoints paired together by prior configuration.
endpoint discovery, typically finding an egress on the path taken
by the first packet.
Static: Some implementations of encapsulator might always be Some implementations of encapsulator might always be statically
statically deployed, and constrained to never be paired with a deployed, and constrained to never be paired with a legacy
legacy decapsulator (RFC2003, RFC2401 or RFC2481 or the limited decapsulator (RFC2003, RFC2401 or RFC2481 or the limited
functionality mode of RFC3168). In such a case, only normal mode functionality mode of RFC3168). In such a case, only normal mode
needs to be implemented. needs to be implemented.
For instance, RFC4301-compatible IPsec tunnel endpoints invariably For instance, RFC4301-compatible IPsec tunnel endpoints invariably
use IKEv2 [RFC4306] for key exchange, which was introduced use IKEv2 [RFC4306] for key exchange, which was introduced
alongside RFC4301. Therefore both endpoints of an RFC4301 tunnel alongside RFC4301. Therefore both endpoints of an RFC4301 tunnel
can be sure that the other end is RFC4301-compatible, because the can be sure that the other end is RFC4301-compatible, because the
tunnel is only formed after IKEv2 key management has completed, at tunnel is only formed after IKEv2 key management has completed, at
which point both ends will be RFC4301-compliant by definition. which point both ends will be RFC4301-compliant by definition.
Therefore an IPsec tunnel ingress does not need compatibility Therefore an IPsec tunnel ingress does not need compatibility
mode, as it will never interact with legacy ECN tunnels. To mode, as it will never interact with legacy ECN tunnels. To
comply with the present specification, it only needs to implement comply with the present specification, it only needs to implement
the required normal mode, which is identical to the pre-existing the required normal mode, which is identical to the pre-existing
RFC4301 behaviour. RFC4301 behaviour.
Dynamic Discovery: This specification does not require or recommend Dynamic Discovery: Tunnel endpoints paired together by some form of
dynamic discovery and it does not define how dynamic negotiation tunnel endpoint discovery, typically finding an egress on the path
might be done, but it recognises that proprietary tunnel endpoint taken by the first packet.
discovery protocols exist. It therefore sets down some
constraints on discovery protocols to ensure safe interworking. 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 If dynamic tunnel endpoint discovery might pair an ingress with a
legacy egress (RFC2003, RFC2401 or RFC2481 or the limited legacy egress (RFC2003, RFC2401 or RFC2481 or the limited
functionality mode of RFC3168), the ingress MUST implement both functionality mode of RFC3168), the ingress MUST implement both
normal and compatibility mode. If the tunnel discovery process is normal and compatibility mode. If the tunnel discovery process is
arranged to only ever find a tunnel egress that propagates ECN arranged to only ever find a tunnel egress that propagates ECN
(RFC3168 full functionality mode, RFC4301 or this present (RFC3168 full functionality mode, RFC4301 or this present
specification), then a tunnel ingress can be complaint with the specification), then a tunnel ingress can be complaint with the
present specification without implementing compatibility mode. present specification without implementing compatibility mode.
skipping to change at page 19, line 42 skipping to change at page 19, line 50
Through the discovery protocol, a tunnel ingress compliant with the Through the discovery protocol, a tunnel ingress compliant with the
present specification might ask if the egress is compliant with the present specification might ask if the egress is compliant with the
present specification, with RFC4301 or with RFC3168 full present specification, with RFC4301 or with RFC3168 full
functionality mode. Or an RFC3168 tunnel ingress might try to functionality mode. Or an RFC3168 tunnel ingress might try to
negotiate to use limited functionality or full functionality mode negotiate to use limited functionality or full functionality mode
[RFC3168]. In all these cases, a decapsulating tunnel egress [RFC3168]. In all these cases, a decapsulating tunnel egress
compliant with this specification MUST agree to any of these compliant with this specification MUST agree to any of these
requests, since it will behave identically in all these cases. requests, since it will behave identically in all these cases.
If no ECN-related mode is requested, a compliant tunnel egress MUST If no ECN-related mode is requested, a compliant tunnel egress MUST
continue without raising any error or warning as its egress behaviour continue without raising any error or warning, because its egress
is compatible with all the legacy ingress behaviours that do not behaviour is compatible with all the legacy ingress behaviours that
negotiate capabilities. do not negotiate capabilities.
A compliant tunnel egress SHOULD raise a warning alarm about any A compliant tunnel egress SHOULD raise a warning alarm about any
requests to enter modes it does not recognise but, for 'forward requests to enter modes it does not recognise but, for 'forward
compatibility' with standards actions possibly defined after it was compatibility' with standards actions possibly defined after it was
implemented, it SHOULD continue operating. implemented, it SHOULD continue operating.
5. Updates to Earlier RFCs 5. Updates to Earlier RFCs
5.1. Changes to RFC4301 ECN processing 5.1. Changes to RFC4301 ECN processing
skipping to change at page 20, line 38 skipping to change at page 20, line 44
dropped rather than forwarded as Not-ECT; dropped rather than forwarded as Not-ECT;
* Certain combinations of inner and outer ECN field have been * Certain combinations of inner and outer ECN field have been
identified as currently unused. These can trigger logging identified as currently unused. These can trigger logging
and/or raise alarms. and/or raise alarms.
Modes: RFC4301 tunnel endpoints do not need modes and are not Modes: RFC4301 tunnel endpoints do not need modes and are not
updated by the modes in the present specification. Effectively an updated by the modes in the present specification. Effectively an
RFC4301 IPsec ingress solely uses the REQUIRED normal mode of RFC4301 IPsec ingress solely uses the REQUIRED normal mode of
encapsulation, which is unchanged from RFC4301 encapsulation. It encapsulation, which is unchanged from RFC4301 encapsulation. It
will never need the OPTIONAL compatibility mode as explained in will never [Note_Manual_Keying] need the OPTIONAL compatibility
Section 4.3 (except in one corner-case described below). mode as explained in Section 4.3.
{ToDo: Question to Security Directorate: Although this corner-case
theoretically exists, it would be preferable to delete any mention
of it for simplicity & clarity. Agree?}
One corner case can exist where an RFC4301 ingress does not use
IKEv2, but uses manual keying instead. Then an RFC4301 ingress
could conceivably be configured to tunnel to an egress with
limited functionality ECN handling. Strictly, for this corner-
case, the requirement to use compatibility mode in this
specification updates RFC4301. However, this is such a remote
possibility that RFC4301 IPsec implementations are NOT REQUIRED to
implement compatibility mode.
5.2. Changes to RFC3168 ECN processing 5.2. Changes to RFC3168 ECN processing
Ingress: On encapsulation, the new rule in Figure 3 that a normal Ingress: On encapsulation, the new rule in Figure 3 that a normal
mode tunnel ingress copies any ECN field into the outer header mode tunnel ingress copies any ECN field into the outer header
updates the full functionality behaviour of an RFC3168 ingress. updates the full functionality behaviour of an RFC3168 ingress.
Nonetheless, the new compatibility mode encapsulates packets Nonetheless, the new compatibility mode encapsulates packets
identically to the limited functionality mode of an RFC3168 identically to the limited functionality mode of an RFC3168
ingress. ingress.
Egress: An RFC3168 egress will need to be updated to the new Egress: An RFC3168 egress will need to be updated to the new
decapsulation behaviour in Figure 4, in order to comply with the decapsulation behaviour in Figure 4, in order to comply with the
present specification. However, the changes are backward present specification. However, the changes are backward
skipping to change at page 22, line 32 skipping to change at page 22, line 32
compatibility with legacy decapsulators that do not propagate ECN compatibility with legacy decapsulators that do not propagate ECN
correctly. correctly.
The trigger that motivated this update to RFC3168 encapsulation was a The trigger that motivated this update to RFC3168 encapsulation was a
standards track proposal for pre-congestion notification (PCN standards track proposal for pre-congestion notification (PCN
[RFC5670]). PCN excess rate marking only works correctly if the ECN [RFC5670]). PCN excess rate marking only works correctly if the ECN
field is copied on encapsulation (as in RFC4301 and RFC5129); it does 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 not work if ECN is reset (as in RFC3168). This is because PCN excess
rate marking depends on the outer header revealing any congestion rate marking depends on the outer header revealing any congestion
experienced so far on the whole path, not just since the last tunnel experienced so far on the whole path, not just since the last tunnel
ingress (see Appendix E for a full explanation). ingress [Note_PCN_ingress].
PCN allows a network operator to add flow admission and termination PCN allows a network operator to add flow admission and termination
for inelastic traffic at the edges of a Diffserv domain, but without for inelastic traffic at the edges of a Diffserv domain, but without
any per-flow mechanisms in the interior and without the generous any per-flow mechanisms in the interior and without the generous
provisioning typical of Diffserv, aiming to significantly reduce provisioning typical of Diffserv, aiming to significantly reduce
costs. The PCN architecture [RFC5559] states that RFC3168 IP in IP costs. The PCN architecture [RFC5559] states that RFC3168 IP in IP
tunnelling of the ECN field cannot be used for any tunnel ingress in tunnelling of the ECN field cannot be used for any tunnel ingress in
a PCN domain. Prior to the present specification, this left a stark a PCN domain. Prior to the present specification, this left a stark
choice between not being able to use PCN for inelastic traffic choice between not being able to use PCN for inelastic traffic
control or not being able to use the many tunnels already deployed control or not being able to use the many tunnels already deployed
skipping to change at page 24, line 20 skipping to change at page 24, line 20
As well as being useful for general future-proofing, this problem As well as being useful for general future-proofing, this problem
is immediately pressing for standardisation of pre-congestion is immediately pressing for standardisation of pre-congestion
notification (PCN), which uses two severity levels of congestion. notification (PCN), which uses two severity levels of congestion.
If a congested queue used ECT(1) in the outer header to signal If a congested queue used ECT(1) in the outer header to signal
more severe congestion than ECT(0), the pre-existing more severe congestion than ECT(0), the pre-existing
decapsulation rules would have thrown away this congestion decapsulation rules would have thrown away this congestion
signal, preventing tunnelled traffic from ever knowing that it signal, preventing tunnelled traffic from ever knowing that it
should reduce its load. should reduce its load.
The PCN working group has had to consider a number of wasteful or The PCN working group has had to consider a number of wasteful or
convoluted work-rounds to this problem (see Appendix D). But by convoluted work-rounds to this problem [Note_PCN_egress]. But by
far the simplest approach is just to remove the covert channel far the simplest approach is just to remove the covert channel
blockages from tunnelling behaviour--now deemed unnecessary blockages from tunnelling behaviour--now deemed unnecessary
anyway. Then network operators that want to support two anyway. Then network operators that want to support two
congestion severity-levels for PCN can specify that every tunnel congestion severity-levels for PCN can specify that every tunnel
egress in a PCN region must comply with this latest egress in a PCN region must comply with this latest
specification. specification.
Not only does this make two congestion severity-levels available Not only does this make two congestion severity-levels available
for PCN standardisation, but also for other potential uses of the for PCN standardisation, but also for other potential uses of the
extra ECN codepoint (e.g. [VCP]). extra ECN codepoint (e.g. [VCP]).
skipping to change at page 28, line 33 skipping to change at page 28, line 33
Then the code module doing encapsulation can keep to the Then the code module doing encapsulation can keep to the
copying rule and the load regulator module can reset copying rule and the load regulator module can reset
congestion, without any code in either module being congestion, without any code in either module being
conditional on whether the other is there. conditional on whether the other is there.
On decapsulation in any new scheme: On decapsulation in any new scheme:
1. If the arriving inner header is Not-ECT it implies the 1. If the arriving inner header is Not-ECT it implies the
transport will not understand other ECN codepoints. If the transport will not understand other ECN codepoints. If the
outer header carries an explicit congestion marking, the outer header carries an explicit congestion marking, the
alternate scheme will probably need to drop the packet--the alternate scheme would be expected to drop the packet--the
only indication of congestion the transport will understand. only indication of congestion the transport will understand.
If the outer carries any other ECN codepoint that does not If the alternate scheme recommends forwarding rather than
indicate congestion, the alternate scheme can forward the dropping such a packet, it must clearly justify this decision.
packet, but probably only as Not-ECT. 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 2. If the arriving inner header is other than Not-ECT, the ECN
field that the alternate decapsulation scheme forwards should field that the alternate decapsulation scheme forwards should
reflect the more severe congestion marking of the arriving reflect the more severe congestion marking of the arriving
inner and outer headers. inner and outer headers.
3. Any alternate scheme MUST define a behaviour for all 3. Any alternate scheme must define a behaviour for all
combinations of inner and outer headers, even those that would combinations of inner and outer headers, even those that would
not be expected to result from standards known at the time and not be expected to result from standards known at the time and
even those that would not be expected from the tunnel ingress even those that would not be expected from the tunnel ingress
paired with the egress at run-time. Consideration should be paired with the egress at run-time. Consideration should be
given to logging such unexpected combinations and raising an given to logging such unexpected combinations and raising an
alarm, particularly if there is a danger that the invalid alarm, particularly if there is a danger that the invalid
combination implies congestion signals are not being combination implies congestion signals are not being
propagated correctly. The presence of currently unused propagated correctly. The presence of currently unused
combinations may represent an attack, but the new scheme combinations may represent an attack, but the new scheme
should try to define a way to forward such packets, at least should try to define a way to forward such packets, at least
skipping to change at page 34, line 15 skipping to change at page 34, line 15
Information", RFC 5696, Information", RFC 5696,
November 2009. November 2009.
[VCP] Xia, Y., Subramanian, L., Stoica, [VCP] Xia, Y., Subramanian, L., Stoica,
I., and S. Kalyanaraman, "One more I., and S. Kalyanaraman, "One more
bit is enough", Proc. SIGCOMM'05, bit is enough", Proc. SIGCOMM'05,
ACM CCR 35(4)37--48, 2005, <http:// ACM CCR 35(4)37--48, 2005, <http://
doi.acm.org/10.1145/ doi.acm.org/10.1145/
1080091.1080098>. 1080091.1080098>.
Editorial Comments
[Note_Manual_Keying] Bob Briscoe: Note (To be removed by the RFC
Editor): One corner case can exist where an
RFC4301 ingress does not use IKEv2, but uses
manual keying instead. Then an RFC4301 ingress
could conceivably be configured to tunnel to an
egress with limited functionality ECN handling.
Strictly, for this corner-case, the requirement
to use compatibility mode in this specification
updates RFC4301. However, this is such a remote
possibility that RFC4301 IPsec implementations
are not required to implement compatibility
mode. It is planned to remove this note after
the review process has completed to avoid
unnecessarily complicating the document with a
largely theoretical corner case.
[Note_PCN_egress] Bob Briscoe: During the review process Appendix
D is provided to expand on this point, but it
will be deleted before publication.
[Note_PCN_ingress] Bob Briscoe: During the review process Appendix
E is provided to expand on this point, but it
will be deleted before publication.
Appendix A. Early ECN Tunnelling RFCs Appendix A. Early ECN Tunnelling RFCs
IP in IP tunnelling was originally defined in [RFC2003]. On IP in IP tunnelling was originally defined in [RFC2003]. On
encapsulation, the incoming header was copied to the outer and on encapsulation, the incoming header was copied to the outer and on
decapsulation the outer was simply discarded. Initially, IPsec decapsulation the outer was simply discarded. Initially, IPsec
tunnelling [RFC2401] followed the same behaviour. tunnelling [RFC2401] followed the same behaviour.
When ECN was introduced experimentally in [RFC2481], legacy (RFC2003 When ECN was introduced experimentally in [RFC2481], legacy (RFC2003
or RFC2401) tunnels would have discarded any congestion markings or RFC2401) tunnels would have discarded any congestion markings
added to the outer header, so RFC2481 introduced rules for added to the outer header, so RFC2481 introduced rules for
skipping to change at page 35, line 25 skipping to change at page 35, line 47
Information security can be assured by using various end to end Information security can be assured by using various end to end
security solutions (including IPsec in transport mode [RFC4301]), but security solutions (including IPsec in transport mode [RFC4301]), but
a commonly used scenario involves the need to communicate between two a commonly used scenario involves the need to communicate between two
physically protected domains across the public Internet. In this physically protected domains across the public Internet. In this
case there are certain management advantages to using IPsec in tunnel case there are certain management advantages to using IPsec in tunnel
mode solely across the publicly accessible part of the path. The mode solely across the publicly accessible part of the path. The
path followed by a packet then crosses security 'domains'; the ones path followed by a packet then crosses security 'domains'; the ones
protected by physical or other means before and after the tunnel and protected by physical or other means before and after the tunnel and
the one protected by an IPsec tunnel across the otherwise unprotected the one protected by an IPsec tunnel across the otherwise unprotected
domain. We will use the scenario in Figure 5 where endpoints 'A' and domain. The scenario in Figure 5 will be used where endpoints 'A'
'B' communicate through a tunnel. The tunnel ingress 'I' and egress and 'B' communicate through a tunnel. The tunnel ingress 'I' and
'E' are within physically protected edge domains, while the tunnel egress 'E' are within physically protected edge domains, while the
spans an unprotected internetwork where there may be 'men in the tunnel spans an unprotected internetwork where there may be 'men in
middle', M. the middle', M.
physically unprotected physically physically unprotected physically
<-protected domain-><--domain--><-protected domain-> <-protected domain-><--domain--><-protected domain->
+------------------+ +------------------+ +------------------+ +------------------+
| | M | | | | M | |
| A-------->I=========>==========>E-------->B | | A-------->I=========>==========>E-------->B |
| | | | | | | |
+------------------+ +------------------+ +------------------+ +------------------+
<----IPsec secured----> <----IPsec secured---->
tunnel tunnel
skipping to change at page 36, line 22 skipping to change at page 36, line 45
from a congested resource towards downstream nodes. Typically a from a congested resource towards downstream nodes. Typically a
downstream transport might feed the information back somehow to the downstream transport might feed the information back somehow to the
point upstream of the congestion that can regulate the load on the point upstream of the congestion that can regulate the load on the
congested resource, but other actions are possible (see [RFC3168] congested resource, but other actions are possible (see [RFC3168]
S.6). In terms of the above unicast scenario, ECN effectively S.6). In terms of the above unicast scenario, ECN effectively
intends to create an information channel (for congestion signalling) intends to create an information channel (for congestion signalling)
from 'M' to 'B' (for 'B' to feed back to 'A'). Therefore the goals from 'M' to 'B' (for 'B' to feed back to 'A'). Therefore the goals
of IPsec and ECN are mutually incompatible, requiring some of IPsec and ECN are mutually incompatible, requiring some
compromise. compromise.
With respect to the DS or ECN fields, S.5.1.2 of RFC4301 says, With respect to using the DS or ECN fields as covert channels,
"controls are provided to manage the bandwidth of this [covert] S.5.1.2 of RFC4301 says, "controls are provided to manage the
channel". Using the ECN processing rules of RFC4301, the channel bandwidth of this channel". Using the ECN processing rules of
bandwidth is two bits per datagram from 'A' to 'M' and one bit per RFC4301, the channel bandwidth is two bits per datagram from 'A' to
datagram from 'M' to 'A' (because 'E' limits the combinations of the 'M' and one bit per datagram from 'M' to 'A' (because 'E' limits the
2-bit ECN field that it will copy). In both cases the covert channel combinations of the 2-bit ECN field that it will copy). In both
bandwidth is further reduced by noise from any real congestion cases the covert channel bandwidth is further reduced by noise from
marking. RFC4301 implies that these covert channels are sufficiently any real congestion marking. RFC4301 implies that these covert
limited to be considered a manageable threat. However, with respect channels are sufficiently limited to be considered a manageable
to the larger (6b) DS field, the same section of RFC4301 says not threat. However, with respect to the larger (6b) DS field, the same
copying is the default, but a configuration option can allow copying section of RFC4301 says not copying is the default, but a
"to allow a local administrator to decide whether the covert channel configuration option can allow copying "to allow a local
provided by copying these bits outweighs the benefits of copying". administrator to decide whether the covert channel provided by
Of course, an administrator considering copying of the DS field has copying these bits outweighs the benefits of copying". Of course, an
to take into account that it could be concatenated with the ECN field administrator considering copying of the DS field has to take into
giving an 8b per datagram covert channel. 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 For tunnelling the 6b Diffserv field two conceptual models have had
to be defined so that administrators can trade off security against to be defined so that administrators can trade off security against
the needs of traffic conditioning [RFC2983]: the needs of traffic conditioning [RFC2983]:
The uniform model: where the Diffserv field is preserved end-to-end The uniform model: where the Diffserv field is preserved end-to-end
by copying into the outer header on encapsulation and copying from by copying into the outer header on encapsulation and copying from
the outer header on decapsulation. the outer header on decapsulation.
The pipe model: where the outer header is independent of that in the The pipe model: where the outer header is independent of that in the
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It deemed that simplicity was more important than allowing It deemed that simplicity was more important than allowing
administrators the option of a tiny increment in security, especially administrators the option of a tiny increment in security, especially
given not copying congestion indications could seriously harm given not copying congestion indications could seriously harm
everyone's network service. everyone's network service.
B.2. Control Constraints B.2. Control Constraints
Congestion control requires that any congestion notification marked Congestion control requires that any congestion notification marked
into packets by a resource will be able to traverse a feedback loop into packets by a resource will be able to traverse a feedback loop
back to a function capable of controlling the load on that resource. back to a function capable of controlling the load on that resource.
To be precise, rather than calling this function the data source, we To be precise, rather than calling this function the data source, it
will call it the Load Regulator. This will allow us to deal with will be called the Load Regulator. This allows for exceptional cases
exceptional cases where load is not regulated by the data source, but where load is not regulated by the data source, but usually the two
usually the two terms will be synonymous. Note the term "a function terms will be synonymous. Note the term "a function _capable of_
_capable of_ controlling the load" deliberately includes a source controlling the load" deliberately includes a source application that
application that doesn't actually control the load but ought to (e.g. doesn't actually control the load but ought to (e.g. an application
an application without congestion control that uses UDP). without congestion control that uses UDP).
A--->R--->I=========>M=========>E-------->B A--->R--->I=========>M=========>E-------->B
Figure 6: Simple Tunnel Scenario Figure 6: Simple Tunnel Scenario
We now consider a similar tunnelling scenario to the IPsec one just A similar tunnelling scenario to the IPsec one just described will
described, but without the different security domains so we can just now be considered, but without the different security domains,
focus on ensuring the control loop and management monitoring can work because the focus now shifts to whether the control loop and
(Figure 6). If we want resources in the tunnel to be able to management monitoring work (Figure 6). If resources in the tunnel
explicitly notify congestion and the feedback path is from 'B' to are to be able to explicitly notify congestion and the feedback path
'A', it will certainly be necessary for 'E' to copy any CE marking is from 'B' to 'A', it will certainly be necessary for 'E' to copy
from the outer header to the inner header for onward transmission to any CE marking from the outer header to the inner header for onward
'B', otherwise congestion notification from resources like 'M' cannot transmission to 'B', otherwise congestion notification from resources
be fed back to the Load Regulator ('A'). But it does not seem like 'M' cannot be fed back to the Load Regulator ('A'). But it does
necessary for 'I' to copy CE markings from the inner to the outer not seem necessary for 'I' to copy CE markings from the inner to the
header. For instance, if resource 'R' is congested, it can send outer header. For instance, if resource 'R' is congested, it can
congestion information to 'B' using the congestion field in the inner send congestion information to 'B' using the congestion field in the
header without 'I' copying the congestion field into the outer header inner header without 'I' copying the congestion field into the outer
and 'E' copying it back to the inner header. 'E' can still write any header and 'E' copying it back to the inner header. 'E' can still
additional congestion marking introduced across the tunnel into the write any additional congestion marking introduced across the tunnel
congestion field of the inner header. into the congestion field of the inner header.
All this shows that 'E' can preserve the control loop irrespective of All this shows that 'E' can preserve the control loop irrespective of
whether 'I' copies congestion notification into the outer header or whether 'I' copies congestion notification into the outer header or
resets it. resets it.
That is the situation for existing control arrangements but, because That is the situation for existing control arrangements but, because
copying reveals more information, it would open up possibilities for copying reveals more information, it would open up possibilities for
better control system designs. For instance, Appendix E describes better control system designs. For instance, resetting CE marking on
how resetting CE marking on encapsulation breaks a proposed encapsulation breaks the standards track PCN congestion marking
congestion marking scheme on the standards track. It ends up scheme [RFC5670]. It ends up removing excessive amounts of traffic
removing excessive amounts of traffic unnecessarily. Whereas copying unnecessarily. Whereas copying CE markings at ingress leads to the
CE markings at ingress leads to the correct control behaviour. correct control behaviour.
B.3. Management Constraints B.3. Management Constraints
As well as control, there are also management constraints. As well as control, there are also management constraints.
Specifically, a management system may monitor congestion markings in Specifically, a management system may monitor congestion markings in
passing packets, perhaps at the border between networks as part of a passing packets, perhaps at the border between networks as part of a
service level agreement. For instance, monitors at the borders of service level agreement. For instance, monitors at the borders of
autonomous systems may need to measure how much congestion has autonomous systems may need to measure how much congestion has
accumulated so far along the path, perhaps to determine between them accumulated so far along the path, perhaps to determine between them
how much of the congestion is contributed by each domain. how much of the congestion is contributed by each domain.
In this document we define the baseline of congestion marking (or the In this document the baseline of congestion marking (or the
Congestion Baseline) as the source of the layer that created (or most Congestion Baseline) is defined as the source of the layer that
recently reset) the congestion notification field. When monitoring created (or most recently reset) the congestion notification field.
congestion it would be desirable if the Congestion Baseline did not When monitoring congestion it would be desirable if the Congestion
depend on whether packets were tunnelled or not. Given some tunnels Baseline did not depend on whether packets were tunnelled or not.
cross domain borders (e.g. consider M in Figure 6 is monitoring a Given some tunnels cross domain borders (e.g. consider M in Figure 6
border), it would therefore be desirable for 'I' to copy congestion is monitoring a border), it would therefore be desirable for 'I' to
accumulated so far into the outer headers, so that it is exposed copy congestion accumulated so far into the outer headers, so that it
across the tunnel. is exposed across the tunnel.
For management purposes it might be useful for the tunnel egress to For management purposes it might be useful for the tunnel egress to
be able to monitor whether congestion occurred across a tunnel or be able to monitor whether congestion occurred across a tunnel or
upstream of it. Superficially it appears that copying congestion upstream of it. Superficially it appears that copying congestion
markings at the ingress would make this difficult, whereas it was markings at the ingress would make this difficult, whereas it was
straightforward when an RFC3168 ingress reset them. However, straightforward when an RFC3168 ingress reset them. However,
Appendix C gives a simple and precise method for a tunnel egress to Appendix C gives a simple and precise method for a tunnel egress to
infer the congestion level introduced across a tunnel. It works infer the congestion level introduced across a tunnel. It works
irrespective of whether the ingress copies or resets congestion irrespective of whether the ingress copies or resets congestion
markings. markings.
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