Internet Engineering Task Force (IETF) A. Antony

Internet Engineering Task Force (IETF) A. Antony
Request for Comments: 9611 secunet
Category: Standards Track T. Brunner
ISSN: 2070-1721 codelabs
S. Klassert
secunet
P. Wouters
Aiven
July 2024

Internet Key Exchange Protocol Version 2 (IKEv2) Support for
Per-Resource Child Security Associations (SAs)

Abstract

In order to increase the bandwidth of IPsec traffic between peers,
this document defines one Notify Message Status Types and one Notify
Message Error Types payload for the Internet Key Exchange Protocol
Version 2 (IKEv2) to support the negotiation of multiple Child
Security Associations (SAs) with the same Traffic Selectors used on
different resources, such as CPUs.

The SA_RESOURCE_INFO notification is used to convey information that
the negotiated Child SA and subsequent new Child SAs with the same
Traffic Selectors are a logical group of Child SAs where most or all
of the Child SAs are bound to a specific resource, such as a specific
CPU. The TS_MAX_QUEUE notify conveys that the peer is unwilling to
create more additional Child SAs for this particular negotiated
Traffic Selector combination.

Using multiple Child SAs with the same Traffic Selectors has the
benefit that each resource holding the Child SA has its own Sequence
Number Counter, ensuring that CPUs don't have to synchronize their
cryptographic state or disable their packet replay protection.

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/rfc9611.

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
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Trust Legal Provisions and are provided without warranty as described
in the Revised BSD License.

Table of Contents

1. Introduction
1.1. Requirements Language
1.2. Terminology
2. Performance Bottlenecks
3. Negotiation of Resource-Specific Child SAs
4. Implementation Considerations
5. Payload Format
5.1. SA_RESOURCE_INFO Notify Message Status Type Payload
5.2. TS_MAX_QUEUE Notify Message Error Type Payload
6. Operational Considerations
7. Security Considerations
8. IANA Considerations
9. References
9.1. Normative References
9.2. Informative References
Acknowledgements
Authors' Addresses

1. Introduction

Most IPsec implementations are currently limited to using one
hardware queue or a single CPU resource for a Child SA. Running
packet stream encryption in parallel can be done, but there is a
bottleneck of different parts of the hardware locking or waiting to
get their sequence number assigned for the packet being encrypted.
The result is that a machine with many such resources is limited to
using only one of these resources per Child SA. This severely limits
the throughput that can be attained. For example, at the time of
writing, an unencrypted link of 10 Gbps or more is commonly reduced
to 2-5 Gbps when IPsec is used to encrypt the link using AES-GCM. By
using the implementation specified in this document, aggregate
throughput increased from 5Gbps using 1 CPU to 40-60 Gbps using 25-30
CPUs.

While this could be (partially) mitigated by setting up multiple
narrowed Child SAs (for example, using Populate From Packet (PFP) as
specified in IPsec architecture [RFC4301]), this IPsec feature would
cause too many Child SAs (one per network flow) or too few Child SAs
(one network flow used on multiple CPUs). PFP is also not widely
implemented.

To make better use of multiple network queues and CPUs, it can be
beneficial to negotiate and install multiple Child SAs with identical
Traffic Selectors. IKEv2 [RFC7296] already allows installing
multiple Child SAs with identical Traffic Selectors, but it offers no
method to indicate that the additional Child SA is being requested
for performance increase reasons and is restricted to some resource
(queue or CPU).

When an IKEv2 peer is receiving more additional Child SAs for a
single set of Traffic Selectors than it is willing to create, it can
return an error notify of TS_MAX_QUEUE.

1.1. Requirements Language

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.

1.2. Terminology

This document uses the following terms defined in IKEv2 [RFC7296]:
Notification Data, Traffic Selector (TS), Traffic Selector initiator
(TSi), Traffic Selector responder (TSr), Child SA, Configuration
Payload (CP), IKE SA, CREATE_CHILD_SA, and NO_ADDITIONAL_SAS.

This document also uses the following terms defined in [RFC4301]:
Security Policy Database (SPD), SA.

2. Performance Bottlenecks

There are several pragmatic reasons why most implementations must
restrict a Child Security Association (SA) to a single specific
hardware resource. A primary limitation arises from the challenges
associated with sharing cryptographic states, counters, and sequence
numbers among multiple CPUs. When these CPUs attempt to
simultaneously utilize shared states, it becomes impractical to do so
without incurring a significant performance penalty. It is necessary
to negotiate and establish multiple Child SAs with identical Traffic
Selector initiator (TSi) and Traffic Selector responder (TSr) on a
per-resource basis.

3. Negotiation of Resource-Specific Child SAs

An initial IKEv2 exchange is used to set up an IKE SA and the initial
Child SA. If multiple Child SAs with the same Traffic Selectors that
are bound to a single resource are desired, the initiator will add
the SA_RESOURCE_INFO notify payload to the Exchange negotiating the
Child SA (e.g., IKE_AUTH or CREATE_CHILD_SA). If this initial Child
SA will be tied to a specific resource, it MAY indicate this by
including an identifier in the Notification Data. A responder that
is willing to have multiple Child SAs for the same Traffic Selectors
will respond by also adding the SA_RESOURCE_INFO notify payload in
which it MAY add a non-zero Notification Data.

Additional resource-specific Child SAs are negotiated as regular
Child SAs using the CREATE_CHILD_SA exchange and are similarly
identified by an accompanying SA_RESOURCE_INFO notification.

Upon installation, each resource-specific Child SA is associated with
an additional local selector, such as the CPU. These resource-
specific Child SAs MUST be negotiated with identical Child SA
properties that were negotiated for the initial Child SA. This
includes cryptographic algorithms, Traffic Selectors, Mode (e.g.,
transport mode), compression usage, etc. However, each Child SA does
have its own keying material that is individually derived according
to the regular IKEv2 process. The SA_RESOURCE_INFO notify payload
MAY be empty or MAY contain some identifying data. This identifying
data SHOULD be a unique identifier within all the Child SAs with the
same TS payloads, and the peer MUST only use it for debugging
purposes.

Additional Child SAs can be started on demand or can be started all
at once. Peers may also delete specific per-resource Child SAs if
they deem the associated resource to be idle.

During the CREATE_CHILD_SA rekey for the Child SA, the
SA_RESOURCE_INFO notification MAY be included, but regardless of
whether or not it is included, the rekeyed Child SA should be bound
to the same resource(s) as the Child SA that is being rekeyed.

4. Implementation Considerations

There are various considerations that an implementation can use to
determine the best procedure to install multiple Child SAs.

A simple procedure could be to install one additional Child SA on
each CPU. An implementation can ensure that one Child SA can be used
by all CPUs, so that while negotiating a new per-CPU Child SA, which
typically takes 1 RTT delay, the CPU with no CPU-specific Child SA
can still encrypt its packets using the Child SA that is available
for all CPUs. Alternatively, if an implementation finds it needs to
encrypt a packet but the current CPU does not have the resources to
encrypt this packet, it can relay that packet to a specific CPU that
does have the capability to encrypt the packet, although this will
come with a performance penalty.

Performing per-CPU Child SA negotiations can result in both peers
initiating additional Child SAs simultaneously. This is especially
likely if per-CPU Child SAs are triggered by individual SADB_ACQUIRE
messages [RFC2367]. Responders should install the additional Child
SA on a CPU with the least amount of additional Child SAs for this
TSi/TSr pair.

When the number of queue or CPU resources are different between the
peers, the peer with the least amount of resources may decide to not
install a second outbound Child SA for the same resource, as it will
never use it to send traffic. However, it must install all inbound
Child SAs because it has committed to receiving traffic on these
negotiated Child SAs.

If per-CPU packet trigger (e.g., SADB_ACQUIRE) messages are
implemented (see Section 6), the Traffic Selector (TSi) entry
containing the information of the trigger packet should be included
in the TS set similarly to regular Child SAs as specified in IKEv2
[RFC7296], Section 2.9. Based on the trigger TSi entry, an
implementation can select the most optimal target CPU to install the
additional Child SA on. For example, if the trigger packet was for a
TCP destination to port 25 (SMTP), it might be able to install the
Child SA on the CPU that is also running the mail server process.
Trigger packet Traffic Selectors are documented in IKEv2 [RFC7296],
Section 2.9.

As per IKEv2, rekeying a Child SA SHOULD use the same (or wider)
Traffic Selectors to ensure that the new Child SA covers everything
that the rekeyed Child SA covers. This includes Traffic Selectors
negotiated via Configuration Payloads such as INTERNAL_IP4_ADDRESS,
which may use the original wide TS set or use the narrowed TS set.

5. Payload Format

The Notify Payload format is defined in IKEv2 [RFC7296],
Section 3.10, and is copied here for convenience.

All multi-octet fields representing integers are laid out in big
endian order (also known as "most significant byte first", or
"network byte order").

5.1. SA_RESOURCE_INFO Notify Message Status Type Payload

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
+-------------------------------+-------------------------------+
| Next Payload |C| RESERVED | Payload Length |
+---------------+---------------+-------------------------------+
| Protocol ID | SPI Size | Notify Message Type |
+---------------+---------------+-------------------------------+
| |
~ Resource Identifier (optional) ~
| |
+-------------------------------+-------------------------------+

(C)ritical flag - MUST be 0.

Protocol ID (1 octet) - MUST be 0. MUST be ignored if not 0.

SPI Size (1 octet) - MUST be 0. MUST be ignored if not 0.

Notify Status Message Type value (2 octets) - set to 16444.

Resource Identifier (optional) - This opaque data may be set to
convey the local identity of the resource.

5.2. TS_MAX_QUEUE Notify Message Error Type Payload

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
+---------------+---------------+-------------------------------+
| Next Payload |C| RESERVED | Payload Length |
+---------------+---------------+-------------------------------+
| Protocol ID | SPI Size | Notify Message Type |
+---------------+---------------+-------------------------------+

(C)ritical flag - MUST be 0.

Protocol ID (1 octet) - MUST be 0. MUST be ignored if not 0.

SPI Size (1 octet) - MUST be 0. MUST be ignored if not 0.

Notify Message Error Type (2 octets) - set to 48.

There is no data associated with this Notify type.

6. Operational Considerations

Implementations supporting per-CPU SAs SHOULD extend their local SPD
selector, and the mechanism of on-demand negotiation that is
triggered by traffic to include a CPU (or queue) identifier in their
packet trigger (e.g., SADB_ACQUIRE) message from the SPD to the IKE
daemon. An implementation that does not support receiving per-CPU
packet trigger messages MAY initiate all its Child SAs immediately
upon receiving the (only) packet trigger message it will receive from
the IPsec stack. Such an implementation also needs to be careful
when receiving a Delete Notify request for a per-CPU Child SA, as it
has no method to detect when it should bring up such a per-CPU Child
SA again later. Also, bringing the deleted per-CPU Child SA up again
immediately after receiving the Delete Notify might cause an infinite
loop between the peers. Another issue with not bringing up all its
per-CPU Child SAs is that if the peer acts similarly, the two peers
might end up with only the first Child SA without ever activating any
per-CPU Child SAs. It is therefore RECOMMENDED to implement per-CPU
packet trigger messages.

Peers SHOULD be flexible with the maximum number of Child SAs they
allow for a given TSi/TSr combination in order to account for corner
cases. For example, during Child SA rekeying, there might be a large
number of additional Child SAs created before the old Child SAs are
torn down. Similarly, when using on-demand Child SAs, both ends
could trigger multiple Child SA requests as the initial packet
causing the Child SA negotiation might have been transported to the
peer via the first Child SA, where its reply packet might also
trigger an on-demand Child SA negotiation to start. As additional
Child SAs consume little additional resources, allowing at the very
least double the number of available CPUs is RECOMMENDED. An
implementation MAY allow unlimited additional Child SAs and only
limit this number based on its generic resource protection strategies
that are used to require COOKIES or refuse new IKE or Child SA
negotiations. Although having a very large number (e.g., hundreds or
thousands) of SAs may slow down per-packet SAD lookup.

Implementations might support dynamically moving a per-CPU Child SA
from one CPU to another CPU. If this method is supported,
implementations must be careful to move both the inbound and outbound
SAs. If the IPsec endpoint is a gateway, it can move the inbound SA
and outbound SA independently of each other. It is likely that for a
gateway, IPsec traffic would be asymmetric. If the IPsec endpoint is
the same host responsible for generating the traffic, the inbound and
outbound SAs SHOULD remain as a pair on the same CPU. If a host
previously skipped installing an outbound SA because it would be an
unused duplicate outbound SA, it will have to create and add the
previously skipped outbound SA to the SAD with the new CPU ID. The
inbound SA may not have a CPU ID in the SAD. Adding the outbound SA
to the SAD requires access to the key material, whereas updating the
CPU selector on an existing outbound SAs might not require access to
key material. To support this, the IKE software might have to hold
on to the key material longer than it normally would, as it might
actively attempt to destroy key material from memory that the IKE
daemon no longer needs access to.

An implementation that does not accept any further resource-specific
Child SAs MUST NOT return the NO_ADDITIONAL_SAS error because it
could be misinterpreted by the peer to mean that no other Child SA
with a different TSi and/or TSr is allowed either. Instead, it MUST
return TS_MAX_QUEUE.

7. Security Considerations

Similar to how an implementation should limit the number of half-open
SAs to limit the impact of a denial-of-service attack, it is
RECOMMENDED that an implementation limits the maximum number of
additional Child SAs allowed per unique TSi/TSr.

Using multiple resource-specific child SAs makes sense for high-
volume IPsec connections on IPsec gateway machines where the
administrator has a trust relationship with the peer's administrator
and abuse is unlikely and easily escalated to resolve.

This trust relationship is usually not present for the deployments of
remote access VPNs, and allowing per-CPU Child SAs is NOT RECOMMENDED
in these scenarios. Therefore, it is also NOT RECOMMENDED to allow
per-CPU Child SAs by default.

The SA_RESOURCE_INFO notify contains an optional data payload that
can be used by the peer to identify the Child SA belonging to a
specific resource. Notification data SHOULD NOT be an identifier
that can be used to gain information about the hardware. For
example, using the CPU number itself as the identifier might give an
attacker knowledge of which packets are handled by which CPU ID, and
it might optimize a brute-force attack against the system.

8. IANA Considerations

IANA has registered one new value in the "IKEv2 Notify Message Status
Types" registry.

+=======+============================+===========+
| Value | Notify Message Status Type | Reference |
+=======+============================+===========+
| 16444 | SA_RESOURCE_INFO | RFC 9611 |
+-------+----------------------------+-----------+

Table 1

IANA has registered one new value in the "IKEv2 Notify Message Error
Types" registry.

+=======+===========================+===========+
| Value | Notify Message Error Type | Reference |
+=======+===========================+===========+
| 48 | TS_MAX_QUEUE | RFC 9611 |
+-------+---------------------------+-----------+

Table 2

9. References

9.1. 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>.

[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>.

[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>.

9.2. Informative References

[RFC2367] McDonald, D., Metz, C., and B. Phan, "PF_KEY Key
Management API, Version 2", RFC 2367,
DOI 10.17487/RFC2367, July 1998,
<https://www.rfc-editor.org/info/rfc2367>.

[RFC4301] Kent, S. and K. Seo, "Security Architecture for the
Internet Protocol", RFC 4301, DOI 10.17487/RFC4301,
December 2005, <https://www.rfc-editor.org/info/rfc4301>.

Acknowledgements

The following people provided reviews and valuable feedback: Roman
Danyliw, Warren Kumari, Tero Kivinen, Murray Kucherawy, John Scudder,
Valery Smyslov, Gunter van de Velde, and Éric Vyncke.

Authors' Addresses

Antony Antony
secunet Security Networks AG
Email: [email protected]

Tobias Brunner
codelabs GmbH
Email: [email protected]

Steffen Klassert
secunet Security Networks AG
Email: [email protected]

Paul Wouters
Aiven
Email: [email protected]