Separate Transports for IKE and ESP
draft-ietf-ipsecme-ikev2-reliable-transport-00
| Document | Type | Active Internet-Draft (ipsecme WG) | |
|---|---|---|---|
| Authors | Valery Smyslov , Tirumaleswar Reddy.K | ||
| Last updated | 2025-10-06 | ||
| Replaces | draft-smyslov-ipsecme-ikev2-reliable-transport | ||
| RFC stream | Internet Engineering Task Force (IETF) | ||
| Intended RFC status | (None) | ||
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| Additional resources | Mailing list discussion | ||
| Stream | WG state | WG Document | |
| Document shepherd | (None) | ||
| IESG | IESG state | I-D Exists | |
| Consensus boilerplate | Unknown | ||
| Telechat date | (None) | ||
| Responsible AD | (None) | ||
| Send notices to | (None) |
draft-ietf-ipsecme-ikev2-reliable-transport-00
Network Working Group V. Smyslov
Internet-Draft ELVIS-PLUS
Intended status: Standards Track T. Reddy
Expires: 9 April 2026 Nokia
6 October 2025
Separate Transports for IKE and ESP
draft-ietf-ipsecme-ikev2-reliable-transport-00
Abstract
The Internet Key Exchange protocol version 2 (IKEv2) can operate
either over unreliable (UDP) transport or over reliable (TCP)
transport. If TCP is used, then IPsec tunnels created by IKEv2 also
use TCP. This document specifies how to decouple IKEv2 and IPsec
transports so that IKEv2 can operate over TCP, while IPsec tunnels
use unreliable transport. This feature allows IKEv2 to effectively
exchange large blobs of data (e.g., when post-quantum algorithms are
employed) while avoiding performance problems that arise when IPsec
uses TCP.
Status of This Memo
This Internet-Draft is submitted in full conformance with the
provisions of BCP 78 and BCP 79.
Internet-Drafts are working documents of the Internet Engineering
Task Force (IETF). Note that other groups may also distribute
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Internet-Drafts are draft documents valid for a maximum of six months
and may be updated, replaced, or obsoleted by other documents at any
time. It is inappropriate to use Internet-Drafts as reference
material or to cite them other than as "work in progress."
This Internet-Draft will expire on 9 April 2026.
Copyright Notice
Copyright (c) 2025 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 publication of this document.
Please review these documents carefully, as they describe your rights
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and restrictions with respect to this document. Code Components
extracted from this document must include Revised BSD License text as
described in Section 4.e of the Trust Legal Provisions and are
provided without warranty as described in the Revised BSD License.
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2
2. Terminology and Notation . . . . . . . . . . . . . . . . . . 3
3. Protocol Details . . . . . . . . . . . . . . . . . . . . . . 3
3.1. Discovery of TCP Transport Support for IKEv2 . . . . . . 4
3.2. Using TCP Transport for IKEv2 from the Start . . . . . . 4
3.3. Notification Format . . . . . . . . . . . . . . . . . . . 5
3.4. ESP Behavior . . . . . . . . . . . . . . . . . . . . . . 6
4. Interaction with IKEv2 Extensions . . . . . . . . . . . . . . 6
5. Security Considerations . . . . . . . . . . . . . . . . . . . 6
6. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 6
7. References . . . . . . . . . . . . . . . . . . . . . . . . . 6
7.1. Normative References . . . . . . . . . . . . . . . . . . 6
7.2. Informative References . . . . . . . . . . . . . . . . . 7
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 8
1. Introduction
The Internet Key Exchange protocol version 2 (IKEv2) [RFC7296]
originally used unreliable transport (UDP) for its messages. Later
it was extended to use TCP [RFC9329] where UDP is blocked. UDP
remains the preferred transport for IKEv2, and TCP is only used if
UDP datagrams cannot get through.
Originally IKEv2 peers exchanged only a small amount of data, so that
simple retransmission mechanism on top of UDP with no congestion
control sufficed. The situation has changed when post-quantum
cryptographic (PQC) algorithms began to be incorporated into IKEv2
[RFC9370]. Most of post-quantum algorithms require IKE peers to
exchange much more data, than classical algorithms, up to tens (or
even hundreds) Kbytes. A few proposals exist that allow to overcome
the 64 Kbytes limitation on the size of an IKE payload
([I-D.nir-ipsecme-big-payload],
[I-D.smyslov-ipsecme-ikev2-extended-pld],
[I-D.tjhai-ikev2-beyond-64k-limit]).
When IKE messages grow to tens or even hundreds of kilobytes, using
UDP as a transport becomes challenging. The use of IKE fragmentation
[RFC7383] helps mitigate IP fragmentation issues and ensures that
each IKE message fragment fits into a UDP datagram, even if the
original message does not. However, all IKE fragments are always
sent (and retransmitted) simultaneously, meaning that as the number
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of fragments increases and congestion control remains absent, the
simple retransmission mechanism of IKEv2 will perform poorly
potentially causing even more problems for the network.
Using reliable transport (e.g., TCP) for IKEv2 could be a solution to
the problem. However, the current use of TCP as defined in [RFC9329]
implies that ESP SAs are also encapsulated in TCP, which has negative
impact on IPsec performance (see Section 9 of [RFC9329]).
In some cases, a pure PQC Key Exchange may be required for specific
deployments, particularly those governed by regulatory or compliance
mandates that necessitate exclusive use of post-quantum cryptography.
Examples include high-security environments or sectors governed by
stringent cryptographic standards. In this case larger amount of
data need to be sent in the IKE_SA_INIT exchnage, that makes using
UDP problematic. For PQ KEM algorithms, if TCP is used for IKEv2 and
peers do not require traditional algorithms, then PQ KEM can be used
directly within the IKE_SA_INIT message when TCP transport is enabled
for IKEv2. This approach allows IKEv2 to avoid UDP fragmentation
concerns while enabling a purely post-quantum key exchange for
deployments requiring exclusive PQC use.
The proposed mechanism enables the use of all parameter sets of a
post-quantum key exchange algorithm in IKE_SA_INIT as a quantum-
resistant-only key exchange. Unlike approaches that limit the
parameter sets based on message size constraints, this specification
supports the use of larger parameter sets when IKEv2 is transported
over TCP. This allows deployments requiring a pure post-quantum key
exchange to establish keys during the IKE_SA_INIT exchange without
concerns about exceeding typical network MTUs.
This specification allows to decouple IKE and IPsec transports,
making it possible to use a reliable transport for IKEv2 while
continuing to use an unreliable transport for IPsec.
The idea to decouple IKE and IPsec transports was originally
presented in [I-D.tjhai-ikev2-beyond-64k-limit].
2. Terminology and Notation
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 [RFC2119] [RFC8174] when, and only when, they appear in all
capitals, as shown here.
3. Protocol Details
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3.1. Discovery of TCP Transport Support for IKEv2
If the initiator supports this extension, is configured to use it,
and does not know whether the responder supports IKEv2 over TCP, the
initiator starts the IKE_SA_INIT exchange over UDP port 4500, as per
[RFC7296]. In this case, the initiator includes the
SEPARATE_TRANSPORTS notification (<TBA by IANA>) in the IKE_SA_INIT
request. This allows the initiator to discover whether the responder
supports the use of separate transports for IKE (over TCP) and ESP
(over UDP). Using UDP port 4500 ensures that IPsec traffic can
traverse NATs and intermediate devices that allow UDP encapsulation.
If the responder supports this extension and receives the
SEPARATE_TRANSPORTS notification in the IKE_SA_INIT request, it
responds with the same notification in the IKE_SA_INIT response.
Upon receiving the SEPARATE_TRANSPORTS notification in the response,
the initiator MUST switch to TCP port 4500 for subsequent exchanges
(IKE_INTERMEDIATE or IKE_AUTH). The responder MUST be prepared to
receive these exchanges over TCP.
Initiator (UDP:4500) Responder (UDP:4500)
-------------------------------------------------------------------
IKE_SA_INIT request:
HDR , SAi1, KEi1, Ni,
[N(NAT_DETECTION_SOURCE_IP),
N(NAT_DETECTION_DESTINATION_IP),]
N(SEPARATE_TRANSPORTS) --->
IKE_SA_INIT response:
HDR, SAr1, KEr1, Nr,
[N(NAT_DETECTION_SOURCE_IP),
N(NAT_DETECTION_DESTINATION_IP),]
<--- N(SEPARATE_TRANSPORTS)
=> Initiator switches to TCP:4500 for IKE_INTERMEDIATE /
IKE_AUTH / subsequent IKEv2 exchanges
=> ESP over UDP or IP if possible, else over TCP
3.2. Using TCP Transport for IKEv2 from the Start
Alternatively, the initiator may start IKE_SA_INIT over TCP port 4500
directly, as specified in [RFC9329], for example, when large key
exchange payloads (with large public keys) are expected. In this
case, the initiator includes the SEPARATE_TRANSPORTS notification in
the IKE_SA_INIT request to indicate its preference to use separate
transports; IKEv2 over TCP and ESP over UDP, provided that UDP is not
blocked in the network path.
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If the responder supports this extension, it includes the
SEPARATE_TRANSPORTS notification in the IKE_SA_INIT response. In
this case, Child SAs are created as specified in [RFC7296], with ESP
sent over UDP (or directly over IP) if possible. If both UDP and IP
are blocked, ESP is sent over TCP as described in [RFC9329].
If the responder does not support the SEPARATE_TRANSPORTS
notification (i.e., the notification is not echoed in the response),
the initiator MUST treat this as an indication that the responder
does not support separate transports. In this case, both IKEv2 and
ESP MUST use TCP transport for all subsequent exchanges, as per
[RFC9329].
In both scenarios, once the IKEv2 exchange switches to TCP transport,
either after IKE_SA_INIT or if TCP was used from the beginning, all
subsequent IKEv2 exchanges MUST continue to use TCP.
Initiator (TCP) Responder (TCP:4500)
-------------------------------------------------------------------
IKE_SA_INIT request:
HDR , SAi1, KEi1, Ni,
[N(NAT_DETECTION_SOURCE_IP),
N(NAT_DETECTION_DESTINATION_IP),]
N(SEPARATE_TRANSPORTS) --->
IKE_SA_INIT response:
HDR, SAr1, KEr1, Nr,
[N(NAT_DETECTION_SOURCE_IP),
N(NAT_DETECTION_DESTINATION_IP),]
<--- N(SEPARATE_TRANSPORTS)
=> All subsequent IKEv2 messages continue over TCP
=> ESP over UDP or IP if possible, else over TCP
3.3. Notification Format
The SEPARATE_TRANSPORTS notification has Protocol ID set to 0 and SPI
Size set to 0. This specification does not define any notification
data, the notification is sent with no data. Future specifications
may define data for this notification. Peers conforming to this
specification MUST ignore any data if present.
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3.4. ESP Behavior
Child SAs are created as specified in [RFC7296]. ESP packets either
use direct transport over IP or are UDP encapsulated if NAT is
detected. If UDP transport for ESP becomes unavailable (e.g.,
blocked by a firewall), peers MAY switch ESP to use TCP transport as
specified in [RFC9329]. Peers are responsible for maintaining NAT
mappings by sending NAT keepalive packets (see Section 2.23 of
[RFC7296]) when ESP over UDP is used. If ESP is transported over a
different protocol than IKE, intermediate devices might apply
different filtering rules. To detect possible connectivity issues
with ESP traffic, the encrypted ESP ping mechanism defined in
[I-D.ietf-ipsecme-encrypted-esp-ping] MAY be used.
4. Interaction with IKEv2 Extensions
MOBIKE [RFC4555] allows an IKE SA, along with its Child SAs, to
migrate from one IP address to another. Section 7.1 of [RFC9329]
specifies that when using TCP as the IKE transport, a peer should
attempt to switch back to UDP in the event of an IP address change.
This specification updates that requirement: when separate transports
are used for IKE and ESP, peers MUST NOT attempt to switch the IKE SA
transport from TCP to UDP. However, an ESP SA MAY switch from UDP to
TCP if UDP is blocked at the new IP address.
5. Security Considerations
Section 10 of [RFC9329] discusses security implications of using TCP
as IKE transport.
6. IANA Considerations
This document defines a new Notify Message Type in the "IKEv2 Notify
Message Status Types" registry:
<TBA> SEPARATE_TRANSPORTS
7. References
7.1. Normative References
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119,
DOI 10.17487/RFC2119, March 1997,
<https://www.rfc-editor.org/info/rfc2119>.
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[RFC8174] Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174,
May 2017, <https://www.rfc-editor.org/info/rfc8174>.
[RFC7296] 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/RFC7296, October
2014, <https://www.rfc-editor.org/info/rfc7296>.
[RFC9329] Pauly, T. and V. Smyslov, "TCP Encapsulation of Internet
Key Exchange Protocol (IKE) and IPsec Packets", RFC 9329,
DOI 10.17487/RFC9329, November 2022,
<https://www.rfc-editor.org/info/rfc9329>.
7.2. Informative References
[RFC4555] Eronen, P., "IKEv2 Mobility and Multihoming Protocol
(MOBIKE)", RFC 4555, DOI 10.17487/RFC4555, June 2006,
<https://www.rfc-editor.org/info/rfc4555>.
[RFC7383] Smyslov, V., "Internet Key Exchange Protocol Version 2
(IKEv2) Message Fragmentation", RFC 7383,
DOI 10.17487/RFC7383, November 2014,
<https://www.rfc-editor.org/info/rfc7383>.
[RFC9370] Tjhai, CJ., Tomlinson, M., Bartlett, G., Fluhrer, S., Van
Geest, D., Garcia-Morchon, O., and V. Smyslov, "Multiple
Key Exchanges in the Internet Key Exchange Protocol
Version 2 (IKEv2)", RFC 9370, DOI 10.17487/RFC9370, May
2023, <https://www.rfc-editor.org/info/rfc9370>.
[I-D.nir-ipsecme-big-payload]
Nir, Y., "A Larger Internet Key Exchange version 2 (IKEv2)
Payload", Work in Progress, Internet-Draft, draft-nir-
ipsecme-big-payload-06, 14 September 2025,
<https://datatracker.ietf.org/doc/html/draft-nir-ipsecme-
big-payload-06>.
[I-D.tjhai-ikev2-beyond-64k-limit]
Tjhai, C., Heider, T., and V. Smyslov, "Beyond 64KB Limit
of IKEv2 Payloads", Work in Progress, Internet-Draft,
draft-tjhai-ikev2-beyond-64k-limit-03, 28 July 2022,
<https://datatracker.ietf.org/doc/html/draft-tjhai-ikev2-
beyond-64k-limit-03>.
[I-D.smyslov-ipsecme-ikev2-extended-pld]
Smyslov, V., "Extended IKEv2 Payload Format", Work in
Progress, Internet-Draft, draft-smyslov-ipsecme-ikev2-
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extended-pld-01, 6 March 2023,
<https://datatracker.ietf.org/doc/html/draft-smyslov-
ipsecme-ikev2-extended-pld-01>.
[I-D.ietf-ipsecme-encrypted-esp-ping]
Antony, A. and S. Klassert, "Encrypted ESP Echo Protocol",
Work in Progress, Internet-Draft, draft-ietf-ipsecme-
encrypted-esp-ping-00, 3 April 2025,
<https://datatracker.ietf.org/doc/html/draft-ietf-ipsecme-
encrypted-esp-ping-00>.
Authors' Addresses
Valery Smyslov
ELVIS-PLUS
Russian Federation
Email: svan@elvis.ru
Tirumaleswar Reddy
Nokia
India
Email: kondtir@gmail.com
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