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Internet Engineering Task Force (IETF) D. Migault Request for Comments: 9333 Ericsson Category: Informational T. Guggemos ISSN: 2070-1721 LMU Munich

                                                          January 2023

          Minimal IP Encapsulating Security Payload (ESP)

Abstract

 This document describes the minimal properties that an IP
 Encapsulating Security Payload (ESP) implementation needs to meet to
 remain interoperable with the standard ESP as defined in RFC 4303.
 Such a minimal version of ESP is not intended to become a replacement
 of ESP in RFC 4303.  Instead, a minimal implementation is expected to
 be optimized for constrained environments while remaining
 interoperable with implementations of ESP.  In addition, this
 document provides some considerations for implementing minimal ESP in
 a constrained environment, such as limiting the number of flash
 writes, handling frequent wakeup and sleep states, limiting wakeup
 time, and reducing the use of random generation.

 This document does not update or modify RFC 4303.  It provides a
 compact description of how to implement the minimal version of that
 protocol.  RFC 4303 remains the authoritative description.

Status of This Memo

 This document is not an Internet Standards Track specification; it is
 published for informational purposes.

 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).  Not all documents
 approved by the IESG are candidates for any level of Internet
 Standard; see 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/rfc9333.

Copyright Notice

 Copyright (c) 2023 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 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.  Requirements Notation
 3.  Security Parameters Index (SPI)
   3.1.  Considerations for SPI Generation
 4.  Sequence Number (SN)
 5.  Padding
 6.  Next Header and "Dummy" Packets
 7.  ICV
 8.  Cryptographic Suites
 9.  IANA Considerations
 10. Security Considerations
 11. Privacy Considerations
 12. References
   12.1.  Normative References
   12.2.  Informative References
 Acknowledgments
 Authors' Addresses

1. Introduction

 ESP [RFC4303] is part of the IPsec protocol suite [RFC4301].  IPsec
 is used to provide confidentiality, data origin authentication,
 connectionless integrity, an anti-replay service, and limited Traffic
 Flow Confidentiality (TFC) padding.

 Figure 1 describes an ESP packet.  Currently, ESP is implemented in
 the kernel of most major multipurpose Operating Systems (OSes).  ESP
 is usually implemented with all of its features to fit the
 multipurpose usage of these OSes, at the expense of resources and
 with no considerations for code size.  Constrained devices are likely
 to have their own implementation of ESP optimized and adapted to
 their specific use, such as limiting the number of flash writes (for
 each packet or across wake time), handling frequent wakeup and sleep
 states, limiting wakeup time, and reducing the use of random
 generation.  With the adoption of IPsec by Internet of Things (IoT)
 devices with minimal IKEv2 [RFC7815] and ESP Header Compression (EHC)
 [EHC-DIET-ESP] [EHC-IKEv2], these ESP implementations MUST remain
 interoperable with standard ESP implementations.  This document
 describes the minimal properties an ESP implementation needs to meet
 to remain interoperable with ESP [RFC4303].  In addition, this
 document provides advice to implementers for implementing ESP within
 constrained environments.  This document does not update or modify
 [RFC4303].

 For each field of the ESP packet represented in Figure 1, this
 document provides recommendations and guidance for minimal
 implementations.  The primary purpose of minimal ESP is to remain
 interoperable with other nodes implementing ESP [RFC4303], while
 limiting the standard complexity of the implementation.

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 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ —-

+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ |Cov-

+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | —-

~ ~ | |

+ +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ |Cov-

+-+-+-+-+-+-+-+-+ +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | |

+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ ——

~ ~

+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

                 Figure 1: ESP Packet Description

2. Requirements 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. Security Parameters Index (SPI)

 [RFC4303] defines the SPI as a mandatory 32-bit field.

 The SPI has local significance to index the Security Association
 (SA).  As described in Section 4.1 of [RFC4301], nodes supporting
 only unicast communications can index their SA using only the SPI.
 Nodes supporting multicast communications also require the use of IP
 addresses; thus, SA lookup needs to be performed using the longest
 match.

 For nodes supporting only unicast communications, indexing the SA
 using only the SPI is RECOMMENDED.  The index may be based on the
 full 32 bits of the SPI or a subset of these bits.  The node may
 require a combination of the SPI as well as other parameters (like
 the IP address) to index the SA.

 Values 0-255 MUST NOT be used.  As per Section 2.1 of [RFC4303],
 values 1-255 are reserved, and 0 is only allowed to be used
 internally and MUST NOT be sent over the wire.

 [RFC4303] does not require the 32-bit SPI to be randomly generated,
 although that is the RECOMMENDED way to generate SPIs as it provides
 some privacy and security benefits and avoids correlation between ESP
 communications.  To obtain a usable random 32-bit SPI, the node
 generates a random 32-bit value and checks it does not fall within
 the 0-255 range.  If the SPI has an acceptable value, it is used to
 index the inbound session.  Otherwise, the generated value is
 discarded, and the process repeats until a valid value is found.

 Some constrained devices are less concerned with the privacy
 properties associated with randomly generated SPIs.  Examples of such
 devices might include sensors looking to reduce their code
 complexity.  The use of a predictive function to generate the SPI
 might be preferred over the generation and handling of random values.
 An implementation of such predictable function could use the
 combination of a fixed value and the memory address of the Security
 Association Database (SAD) structure.  For every incoming packet, the
 node will be able to point to the SAD structure directly from the SPI
 value.  This avoids having a separate and additional binding and
 lookup function for the SPI to its SAD entry for every incoming
 packet.

3.1. Considerations for SPI Generation

 SPIs that are not randomly generated over 32 bits may have privacy
 and security concerns.  As a result, the use of alternative designs
 requires careful security and privacy reviews.  This section provides
 some considerations for the adoption of alternative designs.

 The SPI value is only looked up for inbound traffic.  The SPI
 negotiated with IKEv2 [RFC7296] or minimal IKEv2 [RFC7815] by a peer
 is the value used by the remote peer when it sends traffic.  The main
 advantage of using a rekeying mechanism is to enable a rekey, which
 is performed by replacing an old SA with a new SA, both indexed with
 distinct SPIs.  The SPI is only used for inbound traffic by the peer,
 which allows each peer to manage the set of SPIs used for its inbound
 traffic.  The necessary number of SPIs reflects the number of inbound
 SAs as well as the ability to rekey those SAs.  Typically, rekeying
 an SA is performed by creating a new SA (with a dedicated SPI) before
 the old SA is deleted.  This results in an additional SA and the need
 to support an additional SPI.  Similarly, the privacy concerns
 associated with the generation of non-random SPIs is also limited to
 the incoming traffic.

 Alternatively, some constrained devices will not implement IKEv2 or
 minimal IKEv2 and, as such, will not be able to manage a rollover
 between two distinct SAs.  In addition, some of these constrained
 devices are likely to have a limited number of SAs; for example, they
 are likely to be indexed over 3 bytes only.  One possible way to
 enable a rekeying mechanism with these devices is to use the SPI
 where, for example, the first 3 bytes designates the SA while the
 remaining byte indicates a rekey index.  SPI numbers can be used to
 implement tracking the inbound SAs when rekeying is taking place.
 When rekeying an SPI, the new SPI could use the SPI bytes to indicate
 the rekeying index.

 The use of a small, limited set of SPI numbers across communications
 comes with privacy and security concerns.  Some specific values or
 subsets of SPI values could reveal the model or manufacturer of the
 node implementing ESP or reveal a state such as "not yet rekeyed" or
 "rekeyed 10 times".  If a constrained host uses a very limited number
 of applications, eventually a single one, the SPI itself could
 indicate what kind of traffic is transmitted (e.g., the kind of
 application typically running).  This could also be correlated with
 encrypted data size to further leak information to an observer on the
 network.  In addition, use of specific hardcoded SPI numbers could
 reveal a manufacturer or device version.  If updated devices use
 different SPI numbers, an attacker could locate vulnerable devices by
 their use of specific SPI numbers.

 A privacy analysis should consider at least the type of information
 as well as the traffic pattern before deciding whether non-random
 SPIs are safe to use.  Typically, temperature and wind sensors that
 are used outdoors do not leak privacy-sensitive information, and most
 of their traffic is expected to be outbound traffic.  When used
 indoors, a sensor that reports an encrypted status of a door (closed
 or opened) every minute might not leak sensitive information outside
 the local network.  In these examples, the privacy aspect of the
 information itself might be limited.  Being able to determine the
 version of the sensor to potentially take control of it may also have
 some limited security consequences.  Of course, this depends on the
 context in which these sensors are being used.  If the risks
 associated to privacy and security are acceptable, a non-randomized
 SPI can be used.

4. Sequence Number (SN)

 The Sequence Number (SN) in [RFC4303] is a mandatory 32-bit field in
 the packet.

 The SN is set by the sender so the receiver can implement anti-replay
 protection.  The SN is derived from any strictly increasing function
 that guarantees the following: if packet B is sent after packet A,
 then the SN of packet B is higher than the SN of packet A.

 Some constrained devices may establish communication with specific
 devices where it is known whether or not the peer implements anti-
 replay protection.  As per [RFC4303], the sender MUST still implement
 a strictly increasing function to generate the SN.

 It is RECOMMENDED that multipurpose ESP implementations increment a
 counter for each packet sent.  However, a constrained device may
 avoid maintaining this context and use another source that is known
 to always increase.  Typically, constrained devices use 802.15.4 Time
 Slotted Channel Hopping (TSCH).  This communication is heavily
 dependent on time.  A constrained device can take advantage of this
 clock mechanism to generate the SN.  A lot of IoT devices are in a
 sleep state most of the time and wake up only to perform a specific
 operation before going back to sleep.  These devices have separate
 hardware that allows them to wake up after a certain timeout and
 typically also have timers that start running when the device is
 booted up, so they might have a concept of time with certain
 granularity.  This requires devices to store any information in
 stable storage that can be restored across sleeps (e.g., flash
 memory).  Storing information associated with the SA (such as the SN)
 requires some read and write operations on stable storage after each
 packet is sent as opposed to an SPI number or cryptographic keys that
 are only written to stable storage at the creation of the SA.  Write
 operations wear out the flash storage.  Write operations also slow
 down the system significantly, as writing to flash is much slower
 than reading from flash.  While these devices have internal clocks or
 timers that might not be very accurate, they are good enough to
 guarantee that each time the device wakes up from sleep, the time is
 greater than what it was before the device went to sleep.  Using time
 for the SN would guarantee a strictly increasing function and avoid
 storing any additional values or context related to the SN on flash.
 In addition to the time value, a RAM-based counter can be used to
 ensure that the serial numbers are still increasing and unique if the
 device sends multiple packets over an SA within one wakeup period.

 For inbound traffic, it is RECOMMENDED that receivers implement anti-
 replay protection.  The size of the window should depend on the
 network characteristic to deliver packets out of order.  In an
 environment where out-of-order packets are not possible, the window
 size can be set to one.  An ESP implementation may choose to not
 implement anti-replay protection.  An implementation of anti-replay
 protection may require the device to write the received SN for every
 packet to stable storage.  This will have the same issues as
 discussed earlier with the SN.  Some constrained device
 implementations may choose to not implement the optional anti-replay
 protection.  A typical example is an IoT device such as a temperature
 sensor that sends a temperature measurement every 60 seconds and
 receives an acknowledgment from the receiver.  In a case like this,
 the ability to spoof and replay an acknowledgement is of limited
 interest and might not justify the implementation of an anti-replay
 mechanism.  Receiving peers may also use an ESP anti-replay mechanism
 adapted to a specific application.  Typically, when the sending peer
 is using an SN based on time, anti-replay may be implemented by
 discarding any packets that present an SN whose value is too much in
 the past.  Such mechanisms may consider clock drifting in various
 ways in addition to acceptable delay induced by the network to avoid
 the anti-replay windows rejecting legitimate packets.  Receiving
 peers could accept any SN as long as it is higher than the previously
 received SN.  Another mechanism could be used where only the received
 time on the device is used to consider a packet to be valid, without
 looking at the SN at all.

 The SN can be represented as a 32-bit number or as a 64-bit number,
 known as an "Extended Sequence Number (ESN)".  As per [RFC4303],
 support of ESN is not mandatory, and its use is negotiated via IKEv2
 [RFC7296].  An ESN is used for high-speed links to ensure there can
 be more than 2^32 packets before the SA needs to be rekeyed to
 prevent the SN from rolling over.  This assumes the SN is incremented
 by 1 for each packet.  When the SN is incremented differently -- such
 as when time is used -- rekeying needs to happen based on how the SN
 is incremented to prevent the SN from rolling over.  The security of
 all data protected under a given key decreases slightly with each
 message, and a node must ensure the limit is not reached, even though
 the SN would permit it.  Estimation of the maximum number of packets
 to be sent by a node is not always predictable, and large margins
 should be used, especially as nodes could be online for much more
 time than expected.  Even for constrained devices, it is RECOMMENDED
 to implement some rekeying mechanisms (see Section 10).

5. Padding

 Padding is required to keep the 32-bit alignment of ESP.  It is also
 required for some encryption transforms that need a specific block
 size of input, such as ENCR_AES_CBC.  ESP specifies padding in the
 Pad Length byte, followed by up to 255 bytes of padding.

 Checking the padding structure is not mandatory, so constrained
 devices may omit these checks on received ESP packets.  For outgoing
 ESP packets, padding must be applied as required by ESP.

 In some situations, the padding bytes may take a fixed value.  This
 would typically be the case when the Payload Data is of fixed size.

 ESP [RFC4303] additionally provides Traffic Flow Confidentiality
 (TFC) as a way to perform padding to hide traffic characteristics.
 TFC is not mandatory and is negotiated with the SA management
 protocol, such as IKEv2.  TFC has been widely implemented, but it is
 not widely deployed for ESP traffic.  It is NOT RECOMMENDED to
 implement TFC for minimal ESP.

 As a consequence, communication protection that relies on TFC would
 be more sensitive to traffic patterns without TFC.  This can leak
 application information as well as the manufacturer or model of the
 device used to a passive monitoring attacker.  Such information can
 be used, for example, by an attacker if a vulnerability is known for
 the specific device or application.  In addition, some applications
 (such as health applications) could leak important privacy-oriented
 information.

 Constrained devices that have a limited battery lifetime may prefer
 to avoid sending extra padding bytes.  In most cases, the payload
 carried by these devices is quite small, and the standard padding
 mechanism can be used as an alternative to TFC.  Alternatively, any
 information leak based on the size -- or presence -- of the packet
 can also be addressed at the application level before the packet is
 encrypted with ESP.  If application packets vary between 1 to 30
 bytes, the application could always send 32-byte responses to ensure
 all traffic sent is of identical length.  To prevent leaking
 information that a sensor changed state, such as "temperature
 changed" or "door opened", an application could send this information
 at regular time intervals, rather than when a specific event is
 happening, even if the sensor state did not change.

6. Next Header and "Dummy" Packets

 ESP [RFC4303] defines the Next Header as a mandatory 8-bit field in
 the packet.  The Next Header, only visible after decryption,
 specifies the data contained in the payload.  In addition, the Next
 Header may carry an indication on how to process the packet
 [BEET-ESP].  The Next Header can point to a "dummy" packet, i.e., a
 packet with the Next Header value set to 59, meaning "no next
 header".  The data following "no next header" is unstructured "dummy"
 data.  (Note that this document uses the term "dummy" for consistency
 with [RFC4303].)

 The ability to generate, receive, and ignore "dummy" packets is
 required by [RFC4303].  An implementation can omit ever generating
 and sending "dummy" packets.  For interoperability, a minimal ESP
 implementation MUST be able to process and discard "dummy" packets
 without indicating an error.

 In constrained environments, sending "dummy" packets may have too
 much impact on the device lifetime, in which case, "dummy" packets
 should not be generated and sent.  On the other hand, constrained
 devices running specific applications that would leak too much
 information by not generating and sending "dummy" packets may
 implement this functionality or even implement something similar at
 the application layer.  Note also that similarly to padding and TFC
 that can be used to hide some traffic characteristics (see
 Section 5), "dummy" packets may also reveal some patterns that can be
 used to identify the application.  For example, an application may
 send "dummy" data to hide a traffic pattern.  Suppose such an
 application sends a 1-byte data when a change occurs.  This results
 in sending a packet notifying a change has occurred.  "Dummy" packets
 may be used to prevent such information from being leaked by sending
 a 1-byte packet every second when the information is not changed.
 After an upgrade, the data becomes 2 bytes.  At that point, the
 "dummy" packets do not hide anything, and having 1 byte regularly
 versus 2 bytes makes even the identification of the application
 version easier to identify.  This generally makes the use of "dummy"
 packets more appropriate on high-speed links.

 In some cases, devices are dedicated to a single application or a
 single transport protocol.  In this case, the Next Header has a fixed
 value.

 Specific processing indications have not been standardized yet
 [BEET-ESP] and are expected to result from an agreement between the
 peers.  As a result, they SHOULD NOT be part of a minimal
 implementation of ESP.

7. ICV

 The ICV depends on the cryptographic suite used.  As detailed in
 [RFC8221], authentication or authenticated encryption is RECOMMENDED,
 and as such, the ICV field must be present with a size different from
 zero.  Its length is defined by the security recommendations only.

8. Cryptographic Suites

 The recommended algorithms to use are expected to evolve over time,
 and implementers SHOULD follow the recommendations provided by
 [RFC8221] and updates.

 This section lists some of the criteria that may be considered to
 select an appropriate cryptographic suite.  The list is not expected
 to be exhaustive and may also evolve over time.

 1.  Security: Security is the criteria that should be considered
     first for the selection of encryption algorithm transforms.  The
     security of encryption algorithm transforms is expected to evolve
     over time, and it is of primary importance to follow up-to-date
     security guidance and recommendations.  The chosen encryption
     algorithm MUST NOT be vulnerable or weak (see [RFC8221] for
     outdated ciphers).  ESP can be used to authenticate only
     (ENCR_NULL) or to encrypt the communication.  In the latter case,
     Authenticated Encryption with Associated Data (AEAD) is
     RECOMMENDED [RFC8221].

 2.  Resilience to Nonce Reuse: Some transforms, including AES-GCM,
     are vulnerable to nonce collision with a given key.  While the
     generation of the nonce may prevent such collision during a
     session, the mechanisms are unlikely to provide such protection
     across sleep states or reboot.  This causes an issue for devices
     that are configured using static keys (called "manual keying"),
     and manual keying should not be used with these encryption
     algorithms.  When the key is likely to be reused across reboots,
     algorithms that are resistant to nonce misuse (for example, AES-
     SIV [RFC5297], AES-GCM-SIV [RFC8452], and Deoxys-II [DeoxysII])
     are RECOMMENDED.  Note, however, that none of these are currently
     defined for use with ESP.

 3.  Interoperability: Constrained devices usually only implement one
     or very few different encryption algorithm transforms.  [RFC8221]
     takes the life cycle of encryption algorithm transforms and
     device manufacturing into consideration in its recommendations
     for mandatory-to-implement (MTI) algorithms.

 4.  Power Consumption and Cipher Suite Complexity: Complexity of the
     encryption algorithm transform and the energy cost associated
     with it are especially important considerations for devices that
     have limited resources or are battery powered.  The battery life
     might determine the lifetime of the entire device.  When choosing
     a cryptographic function, reusing specific libraries or taking
     advantage of hardware acceleration provided by the device should
     be considered.  For example, if the device benefits from AES
     hardware modules and uses ENCR_AES_CTR, it may prefer AUTH_AES-
     XCBC for its authentication.  In addition, some devices may embed
     radio modules with hardware acceleration for AES-CCM, in which
     case, this transform may be preferred.

 5.  Power Consumption and Bandwidth Consumption: Reducing the payload
     sent may significantly reduce the energy consumption of the
     device.  Encryption algorithm transforms with low overhead are
     strongly preferred.  To reduce the overall payload size, one may,
     for example:

• Use counter-based ciphers without fixed block length (e.g.,

AES-CTR or ChaCha20-Poly1305).

• Use ciphers capable of using implicit Initialization Vectors

(IVs) [RFC8750].

• Use ciphers recommended for IoT [RFC8221].

• Avoid padding by sending payload data that are aligned to the

cipher block length – 2 bytes for the ESP trailer.

9. IANA Considerations

 This document has no IANA actions.

10. Security Considerations

 The security considerations in [RFC4303] apply to this document as
 well.  In addition, this document provides security recommendations
 and guidance for the implementation choices for each ESP field.

 The security of a communication provided by ESP is closely related to
 the security associated with the management of that key.  This
 usually includes mechanisms to prevent a nonce from repeating, for
 example.  When a node is provisioned with a session key that is used
 across reboot, the implementer MUST ensure that the mechanisms put in
 place remain valid across reboot as well.

 It is RECOMMENDED to use ESP in conjunction with key management
 protocols such as, for example, IKEv2 [RFC7296] or minimal IKEv2
 [RFC7815].  Such mechanisms are responsible for negotiating fresh
 session keys as well as preventing a session key being used beyond
 its lifetime.  When such mechanisms cannot be implemented, such as
 when the session key is provisioned, the device MUST ensure that keys
 are not used beyond their lifetime and that the key remains used in
 compliance with all security requirements across reboots (e.g.,
 conditions on counters and nonces remain valid).

 When a device generates its own key or when random values such as
 nonces are generated, the random generation MUST follow [RFC4086].
 In addition, [SP-800-90A-Rev-1] provides guidance on how to build
 random generators based on deterministic random functions.

11. Privacy Considerations

 Preventing the leakage of privacy-sensitive information is a hard
 problem to solve and usually results in balancing the information
 potentially being leaked to the cost associated with the counter
 measures.  This problem is not inherent to the minimal ESP described
 in this document and also concerns the use of ESP in general.

 This document targets minimal implementations of ESP and, as such,
 describes a minimalistic way to implement ESP.  In some cases, this
 may result in potentially revealing privacy-sensitive pieces of
 information.  This document describes these privacy implications so
 the implementer can make the appropriate decisions given the
 specificities of a given environment and deployment.

 The main risk associated with privacy is the ability to identify an
 application or a device by analyzing the traffic, which is designated
 as "traffic shaping".  As discussed in Section 3, the use in a very
 specific context of non-randomly generated SPIs might ease the
 determination of the device or the application in some cases.
 Similarly, padding provides limited capabilities to obfuscate the
 traffic compared to those provided by TFC.  Such consequences on
 privacy are detailed in Section 5.

12. References

12.1. 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>.

 [RFC4086]  Eastlake 3rd, D., Schiller, J., and S. Crocker,
            "Randomness Requirements for Security", BCP 106, RFC 4086,
            DOI 10.17487/RFC4086, June 2005,
            <https://www.rfc-editor.org/info/rfc4086>.

 [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>.

 [RFC4303]  Kent, S., "IP Encapsulating Security Payload (ESP)",
            RFC 4303, DOI 10.17487/RFC4303, December 2005,
            <https://www.rfc-editor.org/info/rfc4303>.

 [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>.

 [RFC7815]  Kivinen, T., "Minimal Internet Key Exchange Version 2
            (IKEv2) Initiator Implementation", RFC 7815,
            DOI 10.17487/RFC7815, March 2016,
            <https://www.rfc-editor.org/info/rfc7815>.

 [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>.

 [RFC8221]  Wouters, P., Migault, D., Mattsson, J., Nir, Y., and T.
            Kivinen, "Cryptographic Algorithm Implementation
            Requirements and Usage Guidance for Encapsulating Security
            Payload (ESP) and Authentication Header (AH)", RFC 8221,
            DOI 10.17487/RFC8221, October 2017,
            <https://www.rfc-editor.org/info/rfc8221>.

 [RFC8750]  Migault, D., Guggemos, T., and Y. Nir, "Implicit
            Initialization Vector (IV) for Counter-Based Ciphers in
            Encapsulating Security Payload (ESP)", RFC 8750,
            DOI 10.17487/RFC8750, March 2020,
            <https://www.rfc-editor.org/info/rfc8750>.

12.2. Informative References

 [BEET-ESP] Nikander, P. and J. Melen, "A Bound End-to-End Tunnel
            (BEET) mode for ESP", Work in Progress, Internet-Draft,
            draft-nikander-esp-beet-mode-09, 5 August 2008,
            <https://datatracker.ietf.org/doc/html/draft-nikander-esp-
            beet-mode-09>.

 [DeoxysII] Jean, J., Nikolić, I., Peyrin, T., and Y. Seurin, "Deoxys
            v1.41", October 2016,
            <https://competitions.cr.yp.to/round3/deoxysv141.pdf>.

 [EHC-DIET-ESP]
            Migault, D., Guggemos, T., Bormann, C., and D. Schinazi,
            "ESP Header Compression and Diet-ESP", Work in Progress,
            Internet-Draft, draft-mglt-ipsecme-diet-esp-08, 13 May
            2022, <https://datatracker.ietf.org/doc/html/draft-mglt-
            ipsecme-diet-esp-08>.

 [EHC-IKEv2]
            Migault, D., Guggemos, T., and D. Schinazi, "Internet Key
            Exchange version 2 (IKEv2) extension for the ESP Header
            Compression (EHC) Strategy", Work in Progress, Internet-
            Draft, draft-mglt-ipsecme-ikev2-diet-esp-extension-02, 13
            May 2022, <https://datatracker.ietf.org/doc/html/draft-
            mglt-ipsecme-ikev2-diet-esp-extension-02>.

 [RFC5297]  Harkins, D., "Synthetic Initialization Vector (SIV)
            Authenticated Encryption Using the Advanced Encryption
            Standard (AES)", RFC 5297, DOI 10.17487/RFC5297, October
            2008, <https://www.rfc-editor.org/info/rfc5297>.

 [RFC8452]  Gueron, S., Langley, A., and Y. Lindell, "AES-GCM-SIV:
            Nonce Misuse-Resistant Authenticated Encryption",
            RFC 8452, DOI 10.17487/RFC8452, April 2019,
            <https://www.rfc-editor.org/info/rfc8452>.

 [SP-800-90A-Rev-1]
            Barker, E. and J. Kelsey, "Recommendation for Random
            Number Generation Using Deterministic Random Bit
            Generators", NIST SP 800-90A Rev 1,
            DOI 10.6028/NIST.SP.800-90Ar1, June 2015,
            <https://csrc.nist.gov/publications/detail/sp/800-90a/rev-
            1/final>.

Acknowledgments

 The authors would like to thank Daniel Palomares, Scott Fluhrer, Tero
 Kivinen, Valery Smyslov, Yoav Nir, Michael Richardson, Thomas Peyrin,
 Eric Thormarker, Nancy Cam-Winget, and Bob Briscoe for their valuable
 comments.  In particular, Scott Fluhrer suggested including the rekey
 index in the SPI.  Tero Kivinen also provided multiple clarifications
 and examples of ESP deployment within constrained devices with their
 associated optimizations.  Thomas Peyrin, Eric Thormarker, and Scott
 Fluhrer suggested and clarified the use of transform resilient to
 nonce misuse.  The authors would also like to thank Mohit Sethi for
 his support as the LWIG Working Group Chair.

Authors' Addresses

 Daniel Migault
 Ericsson
 8275 Rte Transcanadienne
 Saint-Laurent QC H4S 0B6
 Canada
 Email: daniel.migault@ericsson.com

 Tobias Guggemos
 LMU Munich
 MNM-Team
 Oettingenstr. 67
 80538 Munich
 Germany
 Email: guggemos@mnm-team.org