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IETF RFC 9601
Last modified on Saturday, August 31st, 2024
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Internet Engineering Task Force (IETF) B. Briscoe
Request for Comments: 9601 Independent
Updates: 2661, 2784, 3931, 4380, 6040, 7450 August 2024
Category: Standards Track
ISSN: 2070-1721
Propagating Explicit Congestion Notification across IP Tunnel Headers
Separated by a Shim
 Abstract
RFC 6040 on "Tunnelling of Explicit Congestion Notification" made the
rules for propagation of Explicit Congestion Notification (ECN)
consistent for all forms of IP-in-IP tunnel. This specification
updates RFC 6040 to clarify that its scope includes tunnels where two
IP headers are separated by at least one shim header that is not
sufficient on its own for wide-area packet forwarding. It surveys
widely deployed IP tunnelling protocols that use such shim headers
and updates the specifications of those that do not mention ECN
propagation (including RFCs 2661, 3931, 2784, 4380 and 7450, which
specify L2TPv2, L2TPv3, Generic Routing Encapsulation (GRE), Teredo,
and Automatic Multicast Tunneling (AMT), respectively). This
specification also updates RFC 6040 with configuration requirements
needed to make any legacy tunnel ingress safe.
Status of This Memo
This is an Internet Standards Track document.
This document is a product of the Internet Engineering Task Force
(IETF). It represents the consensus of the IETF community. It has
received public review and has been approved for publication by the
Internet Engineering Steering Group (IESG). Further information on
Internet Standards is available in Section 2 of RFC 7841.
Information about the current status of this document, any errata,
and how to provide feedback on it may be obtained at
https://www.rfc-editor.org/info/RFC 9601.
Copyright Notice
Copyright (c) 2024 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
(https://trustee.ietf.org/license-info) in effect on the date of
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This document may contain material from IETF Documents or IETF
Contributions published or made publicly available before November
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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.
Table of Contents
1. Introduction
2. Terminology
3. Scope of RFC 6040
3.1. Feasibility of ECN Propagation between Tunnel Headers
3.2. Desirability of ECN Propagation between Tunnel Headers
4. Making a Non-ECN Tunnel Ingress Safe by Configuration
5. ECN Propagation and Fragmentation/Reassembly
6. IP-in-IP Tunnels with Tightly Coupled Shim Headers
6.1. Specific Updates to Protocols under IETF Change Control
6.1.1. L2TP (v2 and v3) ECN Extension
6.1.2. GRE
6.1.3. Teredo
6.1.4. AMT
7. IANA Considerations
8. Security Considerations
9. References
9.1. Normative References
9.2. Informative References
Acknowledgements
Author's Address
1. Introduction
[RFC 6040] on "Tunnelling of Explicit Congestion Notification" made
the rules for propagation of Explicit Congestion Notification (ECN)
[RFC 3168] consistent for all forms of IP-in-IP tunnel.
A common pattern for many tunnelling protocols is to encapsulate an
inner IP header (v4 or v6) with one or more shim headers then an
outer IP header (v4 or v6). Some of these shim headers are designed
as generic encapsulations, so they do not necessarily directly
encapsulate an inner IP header. Instead, they can encapsulate
headers such as link-layer (L2) protocols that, in turn, often
encapsulate IP. Thus, the abbreviation 'IP-shim-(L2)-IP' can be used
for tunnels that are in scope of this document.
To clear up confusion, this specification clarifies that the scope of
[RFC 6040] includes any IP-in-IP tunnel, including those with one or
more shim headers and other encapsulations between the IP headers.
Where necessary, it updates the specifications of the relevant
encapsulation protocols with the specific text necessary to comply
with [RFC 6040].
This specification also updates [RFC 6040] to state how operators
ought to configure a legacy tunnel ingress to avoid unsafe system
configurations.
2. Terminology
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and
"OPTIONAL" in this document are to be interpreted as described in
BCP 14 [RFC 2119] [RFC 8174] when, and only when, they appear in all
capitals, as shown here.
This specification uses the terminology defined in [RFC 6040].
3. Scope of RFC 6040
In Section 1.1 of [RFC 6040], its scope is defined as:
| ...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
| the inner or outer headers.
There are two problems with the above scoping statement:
Problem 1: It was intended to include cases where one or more shim
headers sit between the IP headers. Many tunnelling implementers
have interpreted the scope of [RFC 6040] as it was intended, but it is
ambiguous. Therefore, this specification updates [RFC 6040] by adding
the following scoping text after the sentences quoted above:
| It applies in cases where an outer IP header encapsulates an inner
| IP header either directly or indirectly by encapsulating other
| headers that in turn encapsulate (or might encapsulate) an inner
| IP header.
Problem 2: Like many IETF specifications, [RFC 6040] is written as a
specification that implementations can choose to claim compliance
with. This means it does not cover two important situations:
1. Cases where it is infeasible for an implementation to access an
inner IP header when adding or removing an outer IP header
2. Cases where implementations choose not to propagate ECN between
IP headers
However, the ECN field is a non-optional part of the IP header (v4
and v6), so any implementation that creates an outer IP header has to
give the ECN field some value. There is only one safe value a tunnel
ingress can use if it does not know whether the egress supports
propagation of the ECN field; it has to clear the ECN field in any
outer IP header to 0b00.
However, an RFC has no jurisdiction over implementations that choose
not to comply or cannot comply with the RFC, including all
implementations that predated it. Therefore, it would have been
unreasonable to add such a requirement to [RFC 6040]. Nonetheless, to
ensure safe propagation of the ECN field over tunnels, it is
reasonable to add requirements on operators to ensure they configure
their tunnels safely (where possible). Before resolving 'Problem 2'
by stating these configuration requirements (in Section 4), the
factors that determine whether propagating ECN is feasible or
desirable will be briefly introduced.
3.1. Feasibility of ECN Propagation between Tunnel Headers
In many cases, one or more shim headers and an outer IP header are
always added to (or removed from) an inner IP packet as part of the
same procedure. We call these tightly coupled shim headers.
Processing a shim and outer header together is often necessary
because a shim is not sufficient for packet forwarding in its own
right; not unless complemented by an outer header. In these cases,
it will often be feasible for an implementation to propagate the ECN
field between the IP headers.
In some cases, a tunnel adds an outer IP header and a tightly coupled
shim header to an inner header that is not an IP header, but that, in
turn, encapsulates an IP header (or might encapsulate an IP header).
For instance, an inner Ethernet (or other link-layer) header might
encapsulate an inner IP header as its payload. We call this a
tightly coupled shim over an encapsulating header.
Digging to arbitrary depths to find an inner IP header within an
encapsulation is strictly a layering violation, so it cannot be a
required behaviour. Nonetheless, some tunnel endpoints already look
within a Layer 2 (L2) header for an IP header, for instance, to map
the Diffserv codepoint between an encapsulated IP header and an outer
IP header [RFC 2983]. In such cases at least, it should be feasible
to also (independently) propagate the ECN field between the same IP
headers. Thus, access to the ECN field within an encapsulating
header can be a useful and benign optimization. The guidelines in
Section 5 of [RFC 9599] give the conditions for this layering
violation to be benign.
3.2. Desirability of ECN Propagation between Tunnel Headers
Developers and network operators are encouraged to implement and
deploy tunnel endpoints compliant with [RFC 6040] (as updated by the
present specification) in order to provide the benefits of wider ECN
deployment [RFC 8087]. Nonetheless, propagation of ECN between IP
headers, whether separated by shim headers or not, has to be optional
to implement and to use, because:
* legacy implementations of tunnels without any ECN support already
exist;
* a network might be designed so that there is usually no bottleneck
within the tunnel; and
* if the tunnel endpoints would have to search within an L2 header
to find an encapsulated IP header, it might not be worth the
potential performance hit.
4. Making a Non-ECN Tunnel Ingress Safe by Configuration
Even when no specific attempt has been made to implement propagation
of the ECN field at a tunnel ingress, it ought to be possible for the
operator to render a tunnel ingress safe by configuration. The main
safety concern is to disable (clear to zero) the ECN capability in
the outer IP header at the ingress if the egress of the tunnel does
not implement ECN logic to propagate any ECN markings into the packet
forwarded beyond the tunnel. Otherwise, the non-ECN egress could
discard any ECN marking introduced within the tunnel, which would
break all the ECN-based control loops that regulate the traffic load
over the tunnel.
Therefore, this specification updates Section 4.3 of [RFC 6040] by
inserting the following text at the end of the section:
| Whether or not an ingress implementation claims compliance with
| [RFC 6040], [RFC 4301], or [RFC 3168], when the outer tunnel header
| is IP (v4 or v6), if possible, the ingress MUST be configured to
| zero the outer ECN field in all of the following cases:
|
| * if it is known that the tunnel egress does not support any of
| the RFCs that define propagation of the ECN field ([RFC 6040],
| [RFC 4301], or the full functionality mode of [RFC 3168]);
|
| * if the behaviour of the egress is not known or an egress with
| unknown behaviour might be dynamically paired with the ingress
| (one way for an operator of a tunnel ingress to determine the
| behaviour of an otherwise unknown egress is described in
| [decap-test]);
|
| * if an IP header might be encapsulated within a non-IP header
| that the tunnel ingress is encapsulating, but the ingress does
| not inspect within the encapsulation.
|
| For the avoidance of doubt, the above only concerns the outer IP
| header. The ingress MUST NOT alter the ECN field of the arriving
| IP header that will become the inner IP header.
|
| In order that the network operator can comply with the above
| safety rules, an implementation of a tunnel ingress:
|
| * MUST NOT treat the former Type of Service (ToS) octet (IPv4) or
| the former Traffic Class octet (IPv6) as a single 8-bit field.
| This is because the resulting linkage of ECN and Diffserv field
| propagation between inner and outer headers is not consistent
| with the definition of the 6-bit Diffserv field in [RFC 2474]
| and [RFC 3260].
|
| * SHOULD be able to be configured to zero the ECN field of the
| outer header.
|
| These last two rules apply even if an implementation of a tunnel
| ingress does not claim to support [RFC 6040], [RFC 4301], or the
| full functionality mode of [RFC 3168]
For instance, if a tunnel ingress with no ECN-specific logic had a
configuration capability to refer to the last 2 bits of the old ToS
Byte of the outer (e.g., with a 0x3 mask) and set them to zero, while
also being able to allow the DSCP to be re-mapped independently, that
would be sufficient to satisfy both implementation requirements
above.
There might be concern that the above "MUST NOT" makes compliant
implementations non-compliant at a stroke. However, by definition,
it solely applies to equipment that provides Diffserv configuration.
Any such Diffserv equipment that is configuring treatment of the
former ToS octet (IPv4) or the former Traffic Class octet (IPv6) as a
single 8-bit field must have always been non-compliant with the
definition of the 6-bit Diffserv field in [RFC 2474] and [RFC 3260].
If a tunnel ingress does not have any ECN logic, copying the ECN
field as a side effect of copying the DSCP is a seriously unsafe bug
that risks breaking the feedback loops that regulate load on a
tunnel, because it omits to check the ECN capability of the tunnel
egress.
Zeroing the outer ECN field of all packets in all circumstances would
be safe, but it would not be sufficient to claim compliance with
[RFC 6040] because it would not meet the aim of introducing ECN
support to tunnels (see Section 4.3 of [RFC 6040]).
5. ECN Propagation and Fragmentation/Reassembly
The following requirements update [RFC 6040], which omitted handling
of the ECN field during fragmentation or reassembly. These changes
might alter how many ECN-marked packets are propagated by a tunnel
that fragments packets, but this would not raise any backward
compatibility issues.
If a tunnel ingress fragments a packet, it MUST set the outer ECN
field of all the fragments to the same value as it would have set if
it had not fragmented the packet.
Section 5.3 of [RFC 3168] specifies ECN requirements for reassembly of
sets of 'outer fragments' into packets (in 'outer fragmentation', the
fragmentation is visible in the outer header so that the tunnel
egress can reassemble the fragments [INTAREA-TUNNELS]).
Additionally, the following requirements apply at a tunnel egress:
* During reassembly of outer fragments, the packet MUST be discarded
if the ECN fields of the outer headers being reassembled into a
single packet consist of a mixture of Not ECN-Capable Transport
(Not-ECT) and other ECN codepoints.
* If there is mix of ECT(0) and ECT(1) outer fragments, then the
reassembled packet MUST be set to ECT(1).
Reasoning: [RFC 3168] originally defined ECT(0) and ECT(1) as
equivalent, but [RFC 3168] has been updated by [RFC 8311] to make
ECT(1) available for congestion marking differences. The rule is
independent of the current experimental use of ECT(1) for Low
Latency, Low Loss, and Scalable throughput (L4S) [RFC 9331]. The
rule is compatible with Pre-Congestion Notification (PCN)
[RFC 6660], which uses 2 levels of congestion severity, with the
ranking of severity from highest to lowest being Congestion
Experienced (CE), ECT(1), ECT(0). The decapsulation rules in
[RFC 6040] take a similar approach.
6. IP-in-IP Tunnels with Tightly Coupled Shim Headers
Below is a list of specifications of encapsulations with tightly
coupled shim header(s) in rough chronological order. This list is
confined to Standards Track or widely deployed protocols. So, for
the avoidance of doubt, the updated scope of [RFC 6040] is defined in
Section 3 and is not limited to this list.
* Point-to-Point Tunneling Protocol (PPTP) [RFC 2637]
* Layer Two Tunneling Protocol (L2TP), specifically L2TPv2 [RFC 2661]
and L2TPv3 [RFC 3931], which not only includes all the L2-specific
specializations of L2TP, but also derivatives such as the Keyed
IPv6 Tunnel [RFC 8159]
* Generic Routing Encapsulation (GRE) [RFC 2784] and Network
Virtualization using GRE (NVGRE) [RFC 7637]
* GPRS Tunnelling Protocol (GTP), specifically GTPv1 [GTPv1], GTP v1
User Plane [GTPv1-U], and GTP v2 Control Plane [GTPv2-C]
* Teredo [RFC 4380]
* Control And Provisioning of Wireless Access Points (CAPWAP)
[RFC 5415]
* Locator/Identifier Separation Protocol (LISP) [RFC 9300]
* Automatic Multicast Tunneling (AMT) [RFC 7450]
* Virtual eXtensible Local Area Network (VXLAN) [RFC 7348] and
Generic Protocol Extensions for VXLAN (VXLAN-GPE) [NVO3-VXLAN-GPE]
* The Network Service Header (NSH) [RFC 8300] for Service Function
Chaining (SFC)
* Geneve [RFC 8926]
* Direct tunnelling of an IP packet within a UDP/IP datagram (see
Section 3.1.11 of [RFC 8085])
* TCP Encapsulation of Internet Key Exchange Protocol (IKE) and
IPsec Packets (see Section 9.5 of [RFC 9329])
Some of the listed protocols enable encapsulation of a variety of
network layer protocols as inner and/or outer. This specification
applies to the cases where there is an inner and outer IP header as
described in Section 3. Otherwise, [RFC 9599] gives guidance on how
to design propagation of ECN into other protocols that might
encapsulate IP.
Where protocols in the above list need to be updated to specify ECN
propagation and are under IETF change control, update text is given
in the following subsections. For those not under IETF control, it
is RECOMMENDED that implementations of encapsulation and
decapsulation comply with [RFC 6040]. It is also RECOMMENDED that
their specifications are updated to add a requirement to comply with
[RFC 6040] (as updated by the present document).
PPTP is not under the change control of the IETF, but it has been
documented in an Informational RFC [RFC 2637]. However, there is no
need for the present specification to update PPTP because L2TP has
been developed as a standardized replacement.
NVGRE is not under the change control of the IETF, but it has been
documented in an Informational RFC [RFC 7637]. NVGRE is a specific
use case of GRE (it re-purposes the key field from the initial
specification of GRE [RFC 1701] as a Virtual Subnet ID). Therefore,
the text that updates GRE in Section 6.1.2 below is also intended to
update NVGRE.
Although the definition of the various GTP shim headers is under the
control of the Third Generation Partnership Project (3GPP), it is
hard to determine whether the 3GPP or the IETF controls
standardization of the _process_ of adding both a GTP and an IP
header to an inner IP header. Nonetheless, the present specification
is provided so that the 3GPP can refer to it from any of its own
specifications of GTP and IP header processing.
The specification of CAPWAP already specifies [RFC 3168] ECN
propagation and ECN capability negotiation. Without modification,
the CAPWAP specification already interworks with the backward-
compatible updates to [RFC 3168] in [RFC 6040].
LISP made the ECN propagation procedures in [RFC 3168] mandatory from
the start. [RFC 3168] has since been updated by [RFC 6040], but the
changes are backwards compatible, so there is still no need for LISP
tunnel endpoints to negotiate their ECN capabilities.
VXLAN is not under the change control of the IETF, but it has been
documented in an Informational RFC. It is RECOMMENDED that VXLAN
implementations comply with [RFC 6040] when the VXLAN header is
inserted between (or removed from between) IP headers. The authors
of any future update of the VXLAN spec are also encouraged to add a
requirement to comply with [RFC 6040] as updated by the present
specification. In contrast, VXLAN-GPE is being documented under IETF
change control and it does require compliance with [RFC 6040].
The Network Service Header (NSH) [RFC 8300] has been defined as a
shim-based encapsulation to identify the Service Function Path (SFP)
in the Service Function Chaining (SFC) architecture [RFC 7665]. A
proposal has been made for the processing of ECN when handling
transport encapsulation [SFC-NSH-ECN].
The specification of Geneve already refers to [RFC 6040] for ECN
encapsulation.
Section 3.1.11 of [RFC 8085] already explains that a tunnel that
encapsulates an IP header within a UDP/IP datagram needs to follow
[RFC 6040] when propagating the ECN field between inner and outer IP
headers. Section 3 of the present specification updates [RFC 6040] to
clarify that its scope includes cases with a shim header between the
IP headers. So it indirectly updates the scope of [RFC 8085] to
include cases with a shim header as well as a UDP header between the
IP headers.
The requirements in Section 4 update [RFC 6040], and hence also
indirectly update the UDP usage guidelines in [RFC 8085] to add the
important but previously unstated requirement that, if the UDP tunnel
egress does not, or might not, support ECN propagation, a UDP tunnel
ingress has to clear the outer IP ECN field to 0b00, e.g., by
configuration.
Section 9.5 of [RFC 9329] already recommends the compatibility mode of
[RFC 6040] in this case because there is not a one-to-one mapping
between inner and outer packets when TCP encapsulates IKE or IPsec.
6.1. Specific Updates to Protocols under IETF Change Control
6.1.1. L2TP (v2 and v3) ECN Extension
The L2TP terminology used here is defined in [RFC 2661] and [RFC 3931].
L2TPv3 [RFC 3931] is used as a shim header between any packet-switched
network (PSN) header (e.g., IPv4, IPv6, and MPLS) and many types of
L2 headers. The L2TPv3 shim header encapsulates an L2-specific sub-
layer, then an L2 header that is likely to contain an inner IP header
(v4 or v6). Then this whole stack of headers can be encapsulated
within an optional outer UDP header and an outer PSN header that is
typically IP (v4 or v6).
L2TPv2 is used as a shim header between any PSN header and a PPP
header, which is in turn likely to encapsulate an IP header.
Even though these shims are rather fat (particularly in the case of
L2TPv3), they still fit the definition of a tightly coupled shim
header over an encapsulating header (Section 3.1) because all the
headers encapsulating the L2 header are added (or removed) together.
L2TPv2 and L2TPv3 are therefore within the scope of [RFC 6040], as
updated by Section 3.
Implementation of the ECN extension to L2TPv2 and L2TPv3 defined in
Section 6.1.1.2 is RECOMMENDED in order to provide the benefits of
ECN [RFC 8087] whenever a node within an L2TP tunnel becomes the
bottleneck for an end-to-end traffic flow.
6.1.1.1. Safe Configuration of a "Non-ECN" Ingress LCCE
The following text is appended to both Section 5.3 of [RFC 2661] and
Section 4.5 of [RFC 3931] as an update to the base L2TPv2 and L2TPv3
specifications:
| The operator of an LCCE that does not support the ECN extension in
| Section 6.1.1.2 of RFC 9601 MUST follow the configuration
| requirements in Section 4 of RFC 9601 to ensure it clears the
| outer IP ECN field to 0b00 when the outer PSN header is IP (v4 or
| v6).
In particular, for an L2TP Control Connection Endpoint (LCCE)
implementation that does not support the ECN extension, this means
that configuration of how it propagates the ECN field between inner
and outer IP headers MUST be independent of any configuration of the
Diffserv extension of L2TP [RFC 3308].
6.1.1.2. ECN Extension for L2TP (v2 or v3)
When the outer PSN header and the payload inside the L2 header are
both IP (v4 or v6), an LCCE will propagate the ECN field at ingress
and egress by following the rules in Section 4 of [RFC 6040].
Before encapsulating any data packets, [RFC 6040] requires an ingress
LCCE to check that the egress LCCE supports ECN propagation as
defined in [RFC 6040] or one of its compatible predecessors ([RFC 4301]
or the full functionality mode of [RFC 3168]). If the egress supports
ECN propagation, the ingress LCCE can use the normal mode of
encapsulation (copying the ECN field from inner to outer).
Otherwise, the ingress LCCE has to use compatibility mode [RFC 6040]
(clearing the outer IP ECN field to 0b00).
An LCCE can determine the remote LCCE's support for ECN either
statically (by configuration) or by dynamic discovery during setup of
each control connection between the LCCEs using the ECN Capability
Attribute-Value Pair (AVP) defined in Section 6.1.1.2.1.
Where the outer PSN header is some protocol other than IP that
supports ECN, the appropriate ECN propagation specification will need
to be followed, e.g., [RFC 5129] for MPLS. Where no specification
exists for ECN propagation by a particular PSN, [RFC 9599] gives
general guidance on how to design ECN propagation into a protocol
that encapsulates IP.
6.1.1.2.1. ECN Capability AVP for Negotiation between LCCEs
The ECN Capability AVP defined here has Attribute Type 103. The AVP
has the following format:
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|M|H|0|0|0|0| Length | Vendor ID |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| 103 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 1: ECN Capability AVP for L2TP (v2 or v3)
This AVP MAY be present in the Start-Control-Connection-Request
(SCCRQ) and Start-Control-Connection-Reply (SCCRP) message types.
This AVP MAY be hidden (the H-bit is set to 0 or 1) and is optional
(the M-bit is not set). The length (before hiding) of this AVP is 6
octets. The Vendor ID is the IETF Vendor ID of 0.
When an LCCE sends an ECN Capability AVP, it indicates that it
supports ECN propagation. When no ECN Capability AVP is present, it
indicates that the sender does not support ECN propagation.
If an LCCE initiating a control connection supports ECN propagation,
it will send an SCCRQ containing an ECN Capability AVP. If the
tunnel terminator supports ECN, it will return an SCCRP that also
includes an ECN Capability AVP. Then, for any sessions created by
that control connection, both ends of the tunnel can use the normal
mode of [RFC 6040]; i.e., they can copy the IP ECN field from inner to
outer when encapsulating data packets.
On the other hand, if the tunnel terminator does not support ECN, it
will ignore the ECN Capability AVP and send an SCCRP to the tunnel
initiator without an ECN Capability AVP. The tunnel initiator
interprets the absence of the ECN Capability flag in the SCCRP as an
indication that the tunnel terminator is incapable of supporting ECN.
When encapsulating data packets for any sessions created by that
control connection, the tunnel initiator will then use the
compatibility mode of [RFC 6040] to clear the ECN field of the outer
IP header to 0b00.
If the tunnel terminator does not support this ECN extension, the
network operator is still expected to configure it to comply with the
safety provisions set out in Section 6.1.1.1 when it acts as an
ingress LCCE.
If ECN support by the ingress and egress LCCEs is configured
statically, as allowed in Section 6.1.1.2, they both ignore the
presence or absence of any ECN capability AVP.
6.1.2. GRE
The GRE terminology used here is defined in [RFC 2784]. GRE is often
used as a tightly coupled shim header between IP headers. Sometimes,
the GRE shim header encapsulates an L2 header, which might in turn
encapsulate an IP header. Therefore, GRE is within the scope of
[RFC 6040] as updated by Section 3.
Implementation of support for [RFC 6040] as updated by the present
specification is RECOMMENDED for GRE tunnel endpoints in order to
provide the benefits of ECN [RFC 8087] whenever a node within a GRE
tunnel becomes the bottleneck for an end-to-end IP traffic flow
tunnelled over GRE using IP as the delivery protocol (outer header).
GRE itself does not support dynamic setup and configuration of
tunnels. However, control plane protocols, such as Next Hop
Resolution Protocol (NHRP) [RFC 2332], Mobile IPv4 (MIP4) [RFC 5944],
Mobile IPv6 (MIP6) [RFC 6275], Proxy Mobile IP (PMIP) [RFC 5845], and
IKEv2 [RFC 7296], are sometimes used to set up GRE tunnels
dynamically.
When these control protocols set up IP-in-IP or IPsec tunnels, it is
likely that the resulting tunnels will propagate the ECN field as
defined in [RFC 6040] or one of its compatible predecessors ([RFC 4301]
or the full functionality mode of [RFC 3168]). However, if they use a
GRE encapsulation, this presumption is less sound.
Therefore, if the outer delivery protocol is IP (v4 or v6), the
operator is obliged to follow the safe configuration requirements in
Section 4. Section 6.1.2.1 updates the base GRE specification with
this requirement to emphasize its importance.
Where the delivery protocol is some protocol other than IP that
supports ECN, the appropriate ECN propagation specification will need
to be followed, e.g., [RFC 5129] for MPLS. Where no specification
exists for ECN propagation by a particular PSN, [RFC 9599] gives more
general guidance on how to propagate ECN to and from protocols that
encapsulate IP.
6.1.2.1. Safe Configuration of a "Non-ECN" GRE Ingress
The following text is appended to Section 3 of [RFC 2784] as an update
to the base GRE specification:
| The operator of a GRE tunnel ingress MUST follow the configuration
| requirements in Section 4 of RFC 9601 when the outer delivery
| protocol is IP (v4 or v6).
6.1.3. Teredo
Teredo [RFC 4380] provides a way to tunnel IPv6 over an IPv4 network
with a UDP-based shim header between the two.
For Teredo tunnel endpoints to provide the benefits of ECN, the
Teredo specification would have to be updated to include negotiation
of the ECN capability between Teredo tunnel endpoints. Otherwise, it
would be unsafe for a Teredo tunnel ingress to copy the ECN field to
the IPv6 outer.
Those implementations known to the authors at the time of writing do
not support propagation of ECN, but they do safely zero the ECN field
in the outer IPv6 header. However, the specification does not
mention anything about this.
To make existing Teredo deployments safe, it would be possible to add
ECN capability negotiation to those that are subject to remote OS
update. However, for those implementations not subject to remote OS
update, it will not be feasible to require them to be configured
correctly because Teredo tunnel endpoints are generally deployed on
hosts.
Therefore, until ECN support is added to the specification of Teredo,
the only feasible further safety precaution available here is to
update the specification of Teredo implementations with the following
text as a new section:
| 5.1.3. Safe "Non-ECN" Teredo Encapsulation
|
| A Teredo tunnel ingress implementation that does not support ECN
| propagation as defined in [RFC 6040] or one of its compatible
| predecessors ([RFC 4301] or the full functionality mode of
| [RFC 3168]) MUST zero the ECN field in the outer IPv6 header.
6.1.4. AMT
AMT [RFC 7450] is a tightly coupled shim header that encapsulates an
IP packet and is encapsulated within a UDP/IP datagram. Therefore,
AMT is within the scope of [RFC 6040] as updated by Section 3.
Implementation of support for [RFC 6040] as updated by the present
specification is RECOMMENDED for AMT tunnel endpoints in order to
provide the benefits of ECN [RFC 8087] whenever a node within an AMT
tunnel becomes the bottleneck for an IP traffic flow tunnelled over
AMT.
To comply with [RFC 6040], an AMT relay and gateway will follow the
rules for propagation of the ECN field at ingress and egress,
respectively, as described in Section 4 of [RFC 6040].
Before encapsulating any data packets, [RFC 6040] requires an ingress
AMT relay to check that the egress AMT gateway supports ECN
propagation as defined in [RFC 6040] or one of its compatible
predecessors ([RFC 4301] or the full functionality mode of [RFC 3168]).
If the egress gateway supports ECN, the ingress relay can use the
normal mode of encapsulation (copying the IP ECN field from inner to
outer). Otherwise, the ingress relay has to use compatibility mode,
which means it has to clear the outer ECN field to zero [RFC 6040].
An AMT tunnel is created dynamically (not manually), so the relay
will need to determine the remote gateway's support for ECN using the
ECN capability declaration defined in Section 6.1.4.2.
6.1.4.1. Safe Configuration of a "Non-ECN" Ingress AMT Relay
The following text is appended to Section 4.2.2 of [RFC 7450] as an
update to the AMT specification:
| The operator of an AMT relay that does not support [RFC 6040] or
| one of its compatible predecessors ([RFC 4301] or the full
| functionality mode of [RFC 3168]) MUST follow the configuration
| requirements in Section 4 of RFC 9601 to ensure it clears the
| outer IP ECN field to zero.
6.1.4.2. ECN Capability Declaration of an AMT Gateway
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| V=0 |Type=3 | Reserved |E|P| Reserved |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Request Nonce |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 2: Updated AMT Request Message Format
Bit 14 of the AMT Request Message counting from 0 (or bit 7 of the
Reserved field counting from 1) is defined here as the AMT Gateway
ECN Capability flag (E) as shown in Figure 2. The definitions of all
other fields in the AMT Request Message are unchanged from [RFC 7450].
When the E flag is set to 1, it indicates that the sender of the
message supports [RFC 6040] ECN propagation. When it is cleared to
zero, it indicates the sender of the message does not support
[RFC 6040] ECN propagation. An AMT gateway "that supports [RFC 6040]
ECN propagation" means one that propagates the ECN field to the
forwarded data packet based on the combination of arriving inner and
outer ECN fields as defined in Section 4 of [RFC 6040].
The other bits of the Reserved field remain reserved. They will
continue to be cleared to zero when sent and ignored when either
received or forwarded as specified in Section 5.1.3.3 of [RFC 7450].
An AMT gateway that does not support [RFC 6040] MUST NOT set the E
flag of its Request Message to 1.
An AMT gateway that supports [RFC 6040] ECN propagation MUST set the E
flag of its Relay Discovery Message to 1.
The action of the corresponding AMT relay that receives a Request
message with the E flag set to 1 depends on whether the relay itself
supports [RFC 6040] ECN propagation:
* If the relay supports [RFC 6040] ECN propagation, it will store the
ECN capability of the gateway along with its address. Then,
whenever it tunnels datagrams towards this gateway, it MUST use
the normal mode of [RFC 6040] to propagate the ECN field when
encapsulating datagrams (i.e., it copies the IP ECN field from
inner to outer header).
* If the discovered AMT relay does not support [RFC 6040] ECN
propagation, it will ignore the E flag in the Reserved field as
per Section 5.1.3.3 of [RFC 7450].
If the AMT relay does not support [RFC 6040] ECN propagation, the
network operator is still expected to configure it to comply with
the safety provisions set out in Section 6.1.4.1.
7. IANA Considerations
IANA has assigned the following AVP in the L2TP "Control Message
Attribute Value Pairs" registry:
+================+================+===========+
| Attribute Type | Description | Reference |
+================+================+===========+
| 103 | ECN Capability | RFC 9601 |
+----------------+----------------+-----------+
Table 1
8. Security Considerations
The Security Considerations in [RFC 6040] and [RFC 9599] apply equally
to the scope defined for the present specification.
9. References
9.1. Normative References
[RFC 2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119,
DOI 10.17487/RFC 2119, March 1997,
<https://www.rfc-editor.org/info/RFC 2119>.
[RFC 2474] 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,
DOI 10.17487/RFC 2474, December 1998,
<https://www.rfc-editor.org/info/RFC 2474>.
[RFC 2661] Townsley, W., Valencia, A., Rubens, A., Pall, G., Zorn,
G., and B. Palter, "Layer Two Tunneling Protocol "L2TP"",
RFC 2661, DOI 10.17487/RFC 2661, August 1999,
<https://www.rfc-editor.org/info/RFC 2661>.
[RFC 2784] Farinacci, D., Li, T., Hanks, S., Meyer, D., and P.
Traina, "Generic Routing Encapsulation (GRE)", RFC 2784,
DOI 10.17487/RFC 2784, March 2000,
<https://www.rfc-editor.org/info/RFC 2784>.
[RFC 3168] Ramakrishnan, K., Floyd, S., and D. Black, "The Addition
of Explicit Congestion Notification (ECN) to IP",
RFC 3168, DOI 10.17487/RFC 3168, September 2001,
<https://www.rfc-editor.org/info/RFC 3168>.
[RFC 3931] Lau, J., Ed., Townsley, M., Ed., and I. Goyret, Ed.,
"Layer Two Tunneling Protocol - Version 3 (L2TPv3)",
RFC 3931, DOI 10.17487/RFC 3931, March 2005,
<https://www.rfc-editor.org/info/RFC 3931>.
[RFC 4301] Kent, S. and K. Seo, "Security Architecture for the
Internet Protocol", RFC 4301, DOI 10.17487/RFC 4301,
December 2005, <https://www.rfc-editor.org/info/RFC 4301>.
[RFC 4380] Huitema, C., "Teredo: Tunneling IPv6 over UDP through
Network Address Translations (NATs)", RFC 4380,
DOI 10.17487/RFC 4380, February 2006,
<https://www.rfc-editor.org/info/RFC 4380>.
[RFC 5129] Davie, B., Briscoe, B., and J. Tay, "Explicit Congestion
Marking in MPLS", RFC 5129, DOI 10.17487/RFC 5129, January
2008, <https://www.rfc-editor.org/info/RFC 5129>.
[RFC 6040] Briscoe, B., "Tunnelling of Explicit Congestion
Notification", RFC 6040, DOI 10.17487/RFC 6040, November
2010, <https://www.rfc-editor.org/info/RFC 6040>.
[RFC 6660] Briscoe, B., Moncaster, T., and M. Menth, "Encoding Three
Pre-Congestion Notification (PCN) States in the IP Header
Using a Single Diffserv Codepoint (DSCP)", RFC 6660,
DOI 10.17487/RFC 6660, July 2012,
<https://www.rfc-editor.org/info/RFC 6660>.
[RFC 7450] Bumgardner, G., "Automatic Multicast Tunneling", RFC 7450,
DOI 10.17487/RFC 7450, February 2015,
<https://www.rfc-editor.org/info/RFC 7450>.
[RFC 9599] Briscoe, B. and J. Kaippallimalil, "Guidelines for Adding
Congestion Notification to Protocols that Encapsulate IP",
RFC 9599, DOI 10.17487/RFC 9599, August 2024,
<https://www.rfc-editor.org/info/RFC 9599>.
9.2. Informative References
[decap-test]
Briscoe, B., "A Test for IP-ECN Propagation by a Remote
Tunnel Endpoint", Technical Report, TR-BB-2023-003,
DOI 10.48550/arXiv.2311.16825, November 2023,
<https://arxiv.org/abs/2311.16825>.
[GTPv1] 3GPP, "General Packet Radio Service (GPRS); GPRS
Tunnelling Protocol (GTP) across the Gn and Gp interface",
Technical Specification 29.060.
[GTPv1-U] 3GPP, "General Packet Radio System (GPRS) Tunnelling
Protocol User Plane (GTPv1-U)", Technical
Specification 29.281.
[GTPv2-C] 3GPP, "3GPP Evolved Packet System (EPS); Evolved General
Packet Radio Service (GPRS) Tunnelling Protocol for
Control plane (GTPv2-C); Stage 3", Technical
Specification 29.274.
[INTAREA-TUNNELS]
Touch, J. D. and M. Townsley, "IP Tunnels in the Internet
Architecture", Work in Progress, Internet-Draft, draft-
ietf-intarea-tunnels-13, 26 March 2023,
<https://datatracker.ietf.org/doc/html/draft-ietf-intarea-
tunnels-13>.
[NVO3-VXLAN-GPE]
Maino, F., Kreeger, L., and U. Elzur, "Generic Protocol
Extension for VXLAN (VXLAN-GPE)", Work in Progress,
Internet-Draft, draft-ietf-nvo3-vxlan-gpe-13, 4 November
2023, <https://datatracker.ietf.org/doc/html/draft-ietf-
nvo3-vxlan-gpe-13>.
[RFC 1701] Hanks, S., Li, T., Farinacci, D., and P. Traina, "Generic
Routing Encapsulation (GRE)", RFC 1701,
DOI 10.17487/RFC 1701, October 1994,
<https://www.rfc-editor.org/info/RFC 1701>.
[RFC 2332] Luciani, J., Katz, D., Piscitello, D., Cole, B., and N.
Doraswamy, "NBMA Next Hop Resolution Protocol (NHRP)",
RFC 2332, DOI 10.17487/RFC 2332, April 1998,
<https://www.rfc-editor.org/info/RFC 2332>.
[RFC 2637] Hamzeh, K., Pall, G., Verthein, W., Taarud, J., Little,
W., and G. Zorn, "Point-to-Point Tunneling Protocol
(PPTP)", RFC 2637, DOI 10.17487/RFC 2637, July 1999,
<https://www.rfc-editor.org/info/RFC 2637>.
[RFC 2983] Black, D., "Differentiated Services and Tunnels",
RFC 2983, DOI 10.17487/RFC 2983, October 2000,
<https://www.rfc-editor.org/info/RFC 2983>.
[RFC 3260] Grossman, D., "New Terminology and Clarifications for
Diffserv", RFC 3260, DOI 10.17487/RFC 3260, April 2002,
<https://www.rfc-editor.org/info/RFC 3260>.
[RFC 3308] Calhoun, P., Luo, W., McPherson, D., and K. Peirce, "Layer
Two Tunneling Protocol (L2TP) Differentiated Services
Extension", RFC 3308, DOI 10.17487/RFC 3308, November 2002,
<https://www.rfc-editor.org/info/RFC 3308>.
[RFC 5415] Calhoun, P., Ed., Montemurro, M., Ed., and D. Stanley,
Ed., "Control And Provisioning of Wireless Access Points
(CAPWAP) Protocol Specification", RFC 5415,
DOI 10.17487/RFC 5415, March 2009,
<https://www.rfc-editor.org/info/RFC 5415>.
[RFC 5845] Muhanna, A., Khalil, M., Gundavelli, S., and K. Leung,
"Generic Routing Encapsulation (GRE) Key Option for Proxy
Mobile IPv6", RFC 5845, DOI 10.17487/RFC 5845, June 2010,
<https://www.rfc-editor.org/info/RFC 5845>.
[RFC 5944] Perkins, C., Ed., "IP Mobility Support for IPv4, Revised",
RFC 5944, DOI 10.17487/RFC 5944, November 2010,
<https://www.rfc-editor.org/info/RFC 5944>.
[RFC 6275] Perkins, C., Ed., Johnson, D., and J. Arkko, "Mobility
Support in IPv6", RFC 6275, DOI 10.17487/RFC 6275, July
2011, <https://www.rfc-editor.org/info/RFC 6275>.
[RFC 7059] Steffann, S., van Beijnum, I., and R. van Rein, "A
Comparison of IPv6-over-IPv4 Tunnel Mechanisms", RFC 7059,
DOI 10.17487/RFC 7059, November 2013,
<https://www.rfc-editor.org/info/RFC 7059>.
[RFC 7296] Kaufman, C., Hoffman, P., Nir, Y., Eronen, P., and T.
Kivinen, "Internet Key Exchange Protocol Version 2
(IKEv2)", STD 79, RFC 7296, DOI 10.17487/RFC 7296, October
2014, <https://www.rfc-editor.org/info/RFC 7296>.
[RFC 7348] Mahalingam, M., Dutt, D., Duda, K., Agarwal, P., Kreeger,
L., Sridhar, T., Bursell, M., and C. Wright, "Virtual
eXtensible Local Area Network (VXLAN): A Framework for
Overlaying Virtualized Layer 2 Networks over Layer 3
Networks", RFC 7348, DOI 10.17487/RFC 7348, August 2014,
<https://www.rfc-editor.org/info/RFC 7348>.
[RFC 7637] Garg, P., Ed. and Y. Wang, Ed., "NVGRE: Network
Virtualization Using Generic Routing Encapsulation",
RFC 7637, DOI 10.17487/RFC 7637, September 2015,
<https://www.rfc-editor.org/info/RFC 7637>.
[RFC 7665] Halpern, J., Ed. and C. Pignataro, Ed., "Service Function
Chaining (SFC) Architecture", RFC 7665,
DOI 10.17487/RFC 7665, October 2015,
<https://www.rfc-editor.org/info/RFC 7665>.
[RFC 8085] Eggert, L., Fairhurst, G., and G. Shepherd, "UDP Usage
Guidelines", BCP 145, RFC 8085, DOI 10.17487/RFC 8085,
March 2017, <https://www.rfc-editor.org/info/RFC 8085>.
[RFC 8087] Fairhurst, G. and M. Welzl, "The Benefits of Using
Explicit Congestion Notification (ECN)", RFC 8087,
DOI 10.17487/RFC 8087, March 2017,
<https://www.rfc-editor.org/info/RFC 8087>.
[RFC 8159] Konstantynowicz, M., Ed., Heron, G., Ed., Schatzmayr, R.,
and W. Henderickx, "Keyed IPv6 Tunnel", RFC 8159,
DOI 10.17487/RFC 8159, May 2017,
<https://www.rfc-editor.org/info/RFC 8159>.
[RFC 8174] Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC 8174,
May 2017, <https://www.rfc-editor.org/info/RFC 8174>.
[RFC 8300] Quinn, P., Ed., Elzur, U., Ed., and C. Pignataro, Ed.,
"Network Service Header (NSH)", RFC 8300,
DOI 10.17487/RFC 8300, January 2018,
<https://www.rfc-editor.org/info/RFC 8300>.
[RFC 8311] Black, D., "Relaxing Restrictions on Explicit Congestion
Notification (ECN) Experimentation", RFC 8311,
DOI 10.17487/RFC 8311, January 2018,
<https://www.rfc-editor.org/info/RFC 8311>.
[RFC 8926] Gross, J., Ed., Ganga, I., Ed., and T. Sridhar, Ed.,
"Geneve: Generic Network Virtualization Encapsulation",
RFC 8926, DOI 10.17487/RFC 8926, November 2020,
<https://www.rfc-editor.org/info/RFC 8926>.
[RFC 9300] Farinacci, D., Fuller, V., Meyer, D., Lewis, D., and A.
Cabellos, Ed., "The Locator/ID Separation Protocol
(LISP)", RFC 9300, DOI 10.17487/RFC 9300, October 2022,
<https://www.rfc-editor.org/info/RFC 9300>.
[RFC 9329] Pauly, T. and V. Smyslov, "TCP Encapsulation of Internet
Key Exchange Protocol (IKE) and IPsec Packets", RFC 9329,
DOI 10.17487/RFC 9329, November 2022,
<https://www.rfc-editor.org/info/RFC 9329>.
[RFC 9331] De Schepper, K. and B. Briscoe, Ed., "The Explicit
Congestion Notification (ECN) Protocol for Low Latency,
Low Loss, and Scalable Throughput (L4S)", RFC 9331,
DOI 10.17487/RFC 9331, January 2023,
<https://www.rfc-editor.org/info/RFC 9331>.
[SFC-NSH-ECN]
Eastlake 3rd, D., Briscoe, B., Zhuang, S., Malis, A., and
X. Wei, "Explicit Congestion Notification (ECN) and
Congestion Feedback Using the Network Service Header (NSH)
and IPFIX", Work in Progress, Internet-Draft, draft-ietf-
sfc-nsh-ecn-support-13, 15 April 2024,
<https://datatracker.ietf.org/doc/html/draft-ietf-sfc-nsh-
ecn-support-13>.
Acknowledgements
Thanks to Ing-jyh (Inton) Tsang for initial discussions on the need
for ECN propagation in L2TP and its applicability. Thanks also to
Carlos Pignataro, Tom Herbert, Ignacio Goyret, Alia Atlas, Praveen
Balasubramanian, Joe Touch, Mohamed Boucadair, David Black, Jake
Holland, Sri Gundavelli, Gorry Fairhurst, and Martin Duke for helpful
advice and comments. [RFC 7059] helped to identify a number of
tunnelling protocols to include within the scope of this document.
Bob Briscoe was part-funded by the Research Council of Norway through
the TimeIn project for early drafts, and he was funded by Apple Inc.
for later draft versions (from -17). The views expressed here are
solely those of the authors.
Author's Address
Bob Briscoe
Independent
United Kingdom
Email: ietf@bobbriscoe.net
URI: https://bobbriscoe.net/
RFC TOTAL SIZE: 50799 bytes
PUBLICATION DATE: Saturday, August 31st, 2024
LEGAL RIGHTS: The IETF Trust (see BCP 78)
|
