Light-Weight Implementation Guidance (lwig)                   D. Migault
Internet-Draft                                                  Ericsson
Intended status: Informational                               T. Guggemos
Expires: 10 October 25 November 2022                                     LMU Munich
                                                            8 April
                                                             24 May 2022

            Minimal IP Encapsulating Security Payload (ESP)
                     draft-ietf-lwig-minimal-esp-10
                     draft-ietf-lwig-minimal-esp-11

Abstract

   This document describes the minimal properties that an IP
   Encapsulating Security Payload (ESP) implementation needs to meet to
   remain interoperable with the standard RFC4303 ESP.  Such a minimal
   version of ESP is not intended to become a replacement of the RFC
   4303 ESP.  Instead, a minimal implementation is expected to be
   optimized for constrained environment environments while remaining interoperable
   with implementations of RFC 4303 ESP.  In addition, this document
   also provides some considerations to implement for implementing minimal ESP in a
   constrained environment which includes limiting the number of flash
   writes, handling frequent wakeup / sleep states, limiting wakeup
   time, or and reducing the use of random generation.

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

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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   Drafts is at https://datatracker.ietf.org/drafts/current/.

   Internet-Drafts are draft documents valid for a maximum of six months
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   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 10 October 25 November 2022.

Copyright Notice

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   document authors.  All rights reserved.

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   Please review these documents carefully, as they describe your rights
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   provided without warranty as described in the Revised BSD License.

Table of Contents

   1.  Requirements notation . . . . . . . . . . . . . . . . . . . .   2
   2.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   2
   3.  Security Parameter Index (SPI) (32 bit) . . . . . . . . . . .   4
     3.1.  Considerations over SPI generation  . . . . . . . . . . .   5   4
   4.  Sequence Number(SN) (32 bit)  . . . . . . . . . . . . . . . .   6
   5.  Padding . . . . . . . . . . . . . . . . . . . . . . . . . . .   8
   6.  Next Header (8 bit) . . . . . . . . . and Dummy Packets . . . . . . . . . . . .   9
   7.  ICV . . . . . . . . . . . . . . . . . . . . . . . . . . . . .  10
   8.  Cryptographic Suites  . . . . . . . . . . . . . . . . . . . .  10
   9.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .  12  11
   10. Security Considerations . . . . . . . . . . . . . . . . . . .  12  11
   11. Privacy Considerations  . . . . . . . . . . . . . . . . . . .  13  12
   12. Acknowledgment  . . . . . . . . . . . . . . . . . . . . . . .  13  12
   13. References  . . . . . . . . . . . . . . . . . . . . . . . . .  13
     13.1.  Normative References . . . . . . . . . . . . . . . . . .  13
     13.2.  Informative References . . . . . . . . . . . . . . . . .  14
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  15

1.  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 [RFC2119]. BCP
   14 [RFC2119] [RFC8174] when, and only when, they appear in all
   capitals, as shown here.

2.  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 (a form of partial
   sequence integrity) 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 (OS).  The  ESP and
   IPsec suite is
   usually implemented in a complete way with all of its features to fit the multiple
   purpose usage of these OSes.  However, completeness of the IPsec
   suite as well as multipurpose scope of these OSes is often performed OSes, at the expense of resources, or performance.  As a result,
   constrained resources and with no
   considerations for code size.  Constrained devices are likely to have
   their own implementation of ESP optimized and adapted to their specificities
   specific use, such as limiting the number of flash writes (for each
   packet or across wake time), handling frequent wakeup and sleep
   state, limiting wakeup time, or and reducing the use of random
   generation.  With the adoption of IPsec by IoT devices with minimal
   IKEv2 [RFC7815] and ESP Header Compression (EHC) with
   [I-D.mglt-ipsecme-diet-esp] or
   [I-D.mglt-ipsecme-ikev2-diet-esp-extension], it becomes crucial that these ESP implementation designed for constrained devices
   implementations MUST remain inter-
   operable interoperable with the standard ESP implementation to avoid a fragmented
   usage of ESP.
   implementations.  This document describes the minimal properties an
   ESP implementation needs to meet to remain interoperable with
   [RFC4303] ESP.  In addition, this document also provides a set of options advise to
   implement these properties under certain
   implementers for implementing ESP within constrained environments.
   This document does not update or modify RFC 4303, but provides a
   compact description of how to implement the minimal version of the
   protocol.  RFC 4303 remains the authoritative description. 4303.

   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 RFC 4303 ESP, 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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ ----
|               Security Parameters Index (SPI)                 | ^Int.
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ |Cov-
|                      Sequence Number                          | |ered
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | ----
|                    Payload Data* (variable)                   | |   ^
~                                                               ~ |   |
|                                                               | |Conf.
+               +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ |Cov-
|               |     Padding (0-255 bytes)                     | |ered*
+-+-+-+-+-+-+-+-+               +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ |   |
|                               |  Pad Length   | Next Header   | v   v
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ ------
|         Integrity Check Value-ICV   (variable)                |
~                                                               ~
|                                                               |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

                   Figure 1: ESP Packet Description

3.  Security Parameter Index (SPI) (32 bit)

   According to the [RFC4303],

   [RFC4303] defines the SPI is as a mandatory 32 bits field and
   is not allowed to be removed. field.

   The SPI has a local significance to index the Security Association
   (SA).  From [RFC4301] section 4.1, nodes supporting only unicast
   communications can index their SA only using only the SPI.  On the other
   hand, nodes  Nodes
   supporting multicast communications must also require to use the IP
   addresses and thus SA lookup needs need to be performed using the longest
   match.

   For nodes supporting only unicast communications, this document
   recommends it is RECOMMENDED
   indexing the SA with using only the SPI only. SPI.  The index may be based on the
   full 32 bits of 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 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 it must not MUST NOT be sent on over the wire.

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

   However, some

   Some constrained nodes may be devices are less concerned by with the privacy
   properties associated to SPIs randomly generated. generated SPIs.  Examples of such
   nodes
   devices might include sensors looking to reduce their code complexity,
   in which case the
   complexity.  The use of a predictive function to generate the SPI
   might be preferred over the generation and handling of random values.
   An example implementation of such predictable function may consider could use the
   combination of a fixed value and the memory address of the 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 between SPI and SAD entries that is involved lookup function for the SPI to
   its SAD entry for every incoming packet.

3.1.  Considerations over SPI generation

   SPI

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

   Note that

   The SPI value is used only looked up for inbound traffic, as such the traffic.  The SPI
   negotiated with IKEv2 [RFC7296] or Minimal IKEv2 [RFC7815] by a peer, 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, that is
   performed by replacing an old SA by a new SA, both indexed with
   distinct SPIs.  As the SPI is only used for inbound traffic by the
   peer, this allows each peer to manage the set of SPIs used for its
   inbound traffic.  The necessary number of SPI reflects the number of
   inbound SAs as well as the ability to rekey these SAs.  Typically,
   rekeying a 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 SPI non-random SPIs is also
   limited to the incoming traffic.

   When alternate designs are considered, it is likely that the number
   of possible SPIs

   Alternatively, some constrained devices will not implement IKEv2 or
   Minimal IKEv2 and as such will not be limited.  This limit should both consider
   the able to manage a roll-over
   between two distinct SAs.  In addition, some of these constrained
   devices are also likely to have a limited number of inbound SAs - possibly per IP addresses - as well as likely to
   be indexed over 3 bytes only for example.  One possible way to enable
   a rekey mechanism with these devices is to use the ability SPI where for
   example the node to rekey. first 3 bytes designates the SA while the remaining byte
   indicates a rekey index.  SPI numbers can typically be used to implement a key update with the SPI indicating
   tracking the key inbound SAs when rekeying is being used.
   For example, taking place.  When
   rekeying a SPI, the new SPI might be encoded with could use the Security Association
   Database (SAD) entry on a subset of SPI bytes (for example 3 bytes),
   while the remaining byte indicates to indicate the rekey
   rekeying index.

   The use of a smaller number small limited set of SPIs SPI numbers across communications
   comes with privacy and security concerns.  Typically, some  Some specific values or
   subset of SPI values may could reveal the models or manufacturer of the
   node implementing ESP.  This may raise some privacy issues  It could also reveal some state such as an
   observer is likely to be able to determine the "not
   yet rekeyed" or "rekeyed 10 times".  If a constrained devices of
   the network.  In some cases, these nodes may host uses a
   very limited
   number of applications - typically a single application - in which
   case or even just one application, the SPI would provide some information related to the
   application itself could
   indicate what kind of traffic (eg the user.  In addition, the device or kind of application may
   be associated with some vulnerabilities, in which case specific SPI
   values may typically
   running) is transmitted.  This could be used further correlated by an attacker
   encrypted data size to discover vulnerabilities.

   While further leak information to an observer on the
   network.  In addition, use of randomly generated SPIs may reduce the leakage specific hardcoded SPI numbers could
   reveal a manufacturer or
   privacy of security related information device version.  If updated devices use
   different SPI numbers, an attacker could locate vulnerable devices by ESP itself, these pieces
   their use of information may also be leaked otherwise.  As a result, a specific SPI numbers.

   A privacy analysis should consider at least the type of information
   as well the traffic pattern before determining a non random SPI can be used.
   Typically, deciding whether non-random SPIs
   are safe to use.  Typically temperature sensors, wind sensors, used
   outdoors may not leak privacy sensitive information and most of its
   traffic is expected to be outbound traffic.  When used indoors, a
   sensor that reports every minute an encrypted status of the a door (closed or opened) may
   every minute, might not leak truly little privacy 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 these
   sensors are being used. If the risks associated to privacy and
   security are acceptable, a randomized SPI is not
   necessary.  In any case, for such constrained sensors, it remains
   better to have secure communications with non random non-randomized SPI as opposed
   to no security. can be used.

4.  Sequence Number(SN) (32 bit)

   According to [RFC4303], the

   The Sequence Number (SN) in [RFC4303] is a mandatory 32 bits 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: if packet B is sent after packet A, then SN of
   packet B is strictly greater higher than the SN of packet A.

   Some constrained devices may establish communication with specific
   devices, like a specific gateway, or nodes similar to them.  As a
   result, the sender may know
   devices where it is known whether or not the receiver peer implements anti-
   replay protection or not.  Even though the sender may know the
   receiver does not implement anti-replay protection, protection.  As per [RFC4303], the sender must MUST still implement an always
   a strictly increasing function to generate the SN.

   Usually, SN

   The RECOMMENDED way for multipurpose ESP implementation is generated by incrementing to
   increment a counter for each packet sent.  A  However, a constrained
   device may avoid maintaining this context and use another source that
   is known to always increase.  Typically, constrained nodes using devices use
   802.15.4 Time Slotted Channel Hopping (TSCH),
   whose (TSCH).  This communication is
   heavily dependent on time, time.  A contrained device can take advantage of their
   this clock mechanism to generate the SN.  A lot of IoT devices are in
   a sleep state most of the time and wake up and are only awake to perform a specific
   operation before going back to sleep.  They  These devices do have separate
   hardware that allows them to wake up after a certain timeout, timeout and
   most likely
   typically also timers that start running when the device was booted
   up, so they might have a concept of time with certain granularity.
   This requires to store any information in a stable storage - such as
   flash memory - that can be restored across sleeps.  Storing
   information associated with the SA such as SN requires some read and
   writing
   write operation on a stable storage after each packet is sent as
   opposed to a SPI number or cryptographic keys that are only written
   to stable storage at the creation of the SA.  Such  Write operations are likely to wear
   out the flash, and flash storage.  Write operations also slow down the system greatly,
   significantly, as writing to flash is not as fast as reading.
   Their much slower than reading from
   flash.  While these devices have internal clocks/timers clocks or timers that might
   not be very accurate, but they
   should be these are good enough to know guarantee that each
   time they wake the device wakes up from sleep that their time is greater than
   what it was last time they woke up. 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.  Of course, one
   should only consider use of a clock SN on flash.  In addition
   to generate SNs if the
   application will inherently time value, a RAM based counter can be used to ensure that no two if
   the device sends multiple packets with a given over an SA are sent with within one wake up
   period, that the same time value. serial numbers are still increasing and unique.
   Note however that standard receivers are generally configured with
   incrementing counters and, if not appropriately configured, the use
   of a significantly larger SN
   difference may than the previous packet can result in the
   that packet out falling outside of the receiver's windows and peer's receiver window which could
   cause that packet being to be discarded.

   For inbound traffic, this document recommends it is RECOMMENDED that receivers
   implements anti-replay protection, and the implement anti-
   replay protection.  The size of the window depends should depend on the ability
   property of the network to deliver packets out of order.  As a
   result, in  In an
   environment where out of order packets is are not possible possible, the window
   size can be set to one.  However, while recommended, there
   are no requirements  An ESP implementation may choose to not
   implement an anti-replay protection mechanism
   implemented by IPsec.  Similarly to the SN the protection.  An implementation of anti anti-
   replay protection may require the device to write the received SN for
   every packet, which may in some cases come with packet to stable storage.  This will have the same drawbacks issues as
   those exposed for
   discussed earlier with the SN.  As a result, some  Some constrained device
   implementations may drop a
   non required anti replay protection especially when the necessary
   resource involved overcomes the benefit of the mechanism.  These
   resources need also choose to balance that absence of not implement the optional anti-replay mechanism,
   may lead to unnecessary integrity check operations that might be
   significantly more expensive as well.
   protection.  A typical example might consider an IoT device such as a
   temperature sensor that is sending a temperature measurement every 60
   seconds, and that receives an acknowledgment from the receiver.  In
   such cases, the ability to spoof and replay an acknowledgement is of
   limited interest and might not justify the implementation of an anti anti-
   replay mechanism.  Receiving peers may also use ESP anti-replay
   mechanism adapted to a specific application.  Typically, when the
   sending peer is using SN based on time, anti-replay may be
   implemented by discarding any packets that present a SN whose value
   is too much in the past.  Note
   that such  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.  When a packet  It could accept any SN as long as it is
   received at a regular time interval, some variant of time based
   mechanisms may not even use the value of higher than the SN, but instead
   previously received SN.  Another mechanism could be used where only
   consider
   the receiving received time of on the device is used to consider a packet as
   valid, without looking at the packet. SN at all.

   The SN can be encoded over represented as a 32 bits bit number, or as a 64 bits - bit number,
   known as Extended Sequence Number (ESN).  As per [RFC4303], the support
   of ESN is not
   mandatory.  The determination of the mandatory and its use of is negotiated via IKEv2
   [RFC7296].  A ESN is based on 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
   largest possible value a SN can take over a session.  When from rolling over.  This assumes the SN is incremented by 1
   for each packet, the number of packets sent over the
   lifetime of a session may be considered.  However, when packet.  When the SN is incremented differently - such as
   when time is used - the maximum
   value SN rekeying needs to be considered instead.  Note that happen based on how the limit of
   messages being sent SN is primarily determined by the security
   associated with the key rather than
   incremented to prevent the SN. 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 challenging predicatable and as such large margins should be considered
   cautiously
   used espcially as nodes could be online for much more time than
   expected.  Even for constrained devices, this document recommends implementing it is RECOMMENDED to
   implement some rekey mechanisms (see Section 10).

5.  Padding

   The purpose of padding

   Padding is required to respect keep the 32 bit alignment of ESP or ESP.  It is also
   required for some encryption transforms that need a specific block
   size expected by an encryption transform - of input, such as AES-CBC for
   example. ENCR_AES_CBC.  ESP must have at least one specifies padding byte in the
   Pad Length that
   indicates the padding length.  ESP padding bytes are generated byte, followed by a
   succession of unsigned bytes starting with 1, 2, 3 with the last byte
   set up to Pad Length, where Pad Length designates the length 255 bytes of the
   padding bytes. padding.

   Checking the padding structure is not mandatory, so the constrained
   device
   devices may omit such checks, however, in order to interoperate with
   existing these checks on received ESP implementations, it must build the packets.  For outgoing
   ESP packets, padding bytes must be applied as
   recommended required by ESP.

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

   ESP [RFC4303] also additionally provides Traffic Flow Confidentiality
   (TFC) as a way to perform padding to hide traffic characteristics, which differs
   from respecting a 32 bit alignment. characteristics.
   TFC is not mandatory and must be is negotiated with the SA management protocol.
   protocol, such as IKEv2.  TFC has not been widely
   adopted implemented but it is
   not widely deployed for standard ESP traffic.  One possible reason is that it
   requires to shape the traffic according to one traffic pattern that
   needs to be maintained.  This  It is likely NOT RECOMMENDED to require extra processing
   as well as providing
   implement TFC for a "well recognized" traffic shape which could
   end up being counterproductive.  As such, this document does not
   recommend that minimal ESP implementation supports TFC. ESP.

   As a result, consequence, communication protection that was relying relies on TFC will would
   be more sensitive to traffic shaping. patterns without TFC.  This could expose the can leak
   application information as well as the devices manifacturor or model of the
   device used to a passive monitoring attacker.  Such information could can
   be used used, for example, by the an attacker in case a vulnerability is
   disclosed on known
   for the specific device. device or application.  In addition, some
   application use - such as health applications - may also reveal could leak important
   privacy oriented information.

   Some constrained nodes

   Constrained devices that have limited battery lifetime may also prefer avoiding to
   avoid sending extra padding bytes.  However, the same nodes
   may also be very specific to an application and device.  As a result,
   they are also likely to be the main target for traffic shaping.  In most cases, the payload
   carried by these nodes devices is quite small, and the standard padding
   mechanism may also can be used as an alternative to
   TFC, with a sufficient tradeoff between the required energy to send
   additional payload and the exposure to traffic shaping attacks.  In
   addition, the TFC.  Alternatively, any
   information leaked by leak based on the traffic shaping may size - or presence - of the packet can
   also be addressed by at the application level.  For example, it level, before the packet is preferred
   encrypted with ESP.  If application packets vary between 1 to
   have 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 sending some changed state, such as "temperature
   changed" or "door opened", an application could send this information
   at regular time interval, rather than when a specific event is happening.  Typically, a
   happening, even if the sensor
   monitoring the temperature, or a door is expected to send regularly
   the information - i.e. the temperature of the room or whether the
   door is closed or open) instead of only sending the information when
   the temperature has raised or when the door is being opened. state did not change.

6.  Next Header (8 bit)

   According to [RFC4303], and Dummy Packets

   ESP [RFC4303] defines the Next Header is as a mandatory 8 bits field in
   the packet.  The Next header header, only visible after decryption,
   specifies the data contained in the
   payload as well as dummy packet, i.e. packets with the Next Header
   with a value 59 meaning "no next header". payload.  In addition, the Next
   Header may also carry an indication on how to process the packet
   [I-D.nikander-esp-beet-mode].  The Next Header can point to a dummy
   packet, i.e. packets with the Next Header value set to 59 meaning "no
   next header".  The data following to "no next header" is unstructured
   dummy data.

   The ability to generate and to 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 MUST be able to process and discard dummy packets
   without indicating an error.  Note that such
   recommendation only applies for nodes receiving packets, and that
   nodes designed to only send data might not implement this capability.

   As the generation of dummy packets is subject to local management and
   based on a per-SA basis, a minimal ESP implementation may not
   generate such dummy packet.  Specifically, in

   In constrained environment environments, sending dummy packets may have too much
   impact on the device lifetime, and in which case dummy packets should not
   be avoided. generated and sent.  On the other hand, constrained
   nodes 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 dedicated used
   to specific applications, in which case, hide some traffic pattern characteristics (see Section 5), dummy packet
   may expose the application or also reveal some patterns that can be used to identify the type of node.
   application.  For
   these nodes, not sending example, an application may send dummy data to hide
   some traffic pattern.  Suppose such such application sends a 1 byte
   data when a change occurs.  This results in sending a packet
   notifying a change has occurred.  Dummy packet may have some privacy
   implication that needs be used to prevent
   such information to be measured.  However, for leaked by sending a 1 byte packet every second
   when the same reasons
   exposed in Section 5 traffic shaping at information is not changed.  After an upgrade the data
   becomes two bytes.  At that point, the IPsec layer may also
   introduce some traffic pattern, dummy packets do not hide
   anything and on constrained devices having 1 byte regularly versus 2 bytes make even the
   application is probably
   identification of the most appropriated layer application, version easier to limit identify.  This
   generaly makes the risk use of leaking information by traffic shaping. dummy packets more appropriated on high
   speed links.

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

   Specific processing indications have not been standardized yet
   [I-D.nikander-esp-beet-mode] and is expected to result from an
   agreement between the peers.  As a result, it should not SHOULD NOT be part of a
   minimal implementation of ESP.

7.  ICV

   The ICV depends on the cryptographic suite used.  Currently,
   [RFC8221] only recommends cryptographic suites with an ICV which
   makes the ICV a mandatory field.  As detailed in
   [RFC8221] authentication or authenticated encryption are recommended 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 cryptographic suites implemented are an important component of
   ESP.  The recommended algorithms to use are expected to evolve over time
   and implementers should SHOULD follow the recommendations provided by
   [RFC8221] and updates.

   This section lists some of the criteria that may be considered to
   select the 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 transform.  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 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 (AEAD) is RECOMMENDED [RFC8221].

   2.  Resilience to nonce re-use: Some transforms -including AES-GCM -
       are very sensitive 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 a key. these encryption
       algorithms.  When the key is likely to be re-used across reboots,
       algorithms that are nonce misuse resistant such as, for example,
       AES-SIV [RFC5297], AES-GCM-SIV [RFC8452] or Deoxys-II [DeoxysII]
       are RECOMMENDED.  Note however that currently none of them has these are
       yet been defined for use with ESP.

   3.  Interoperability: Interoperability considers the encryption
       algorithm transforms shared with the other nodes.  Note that it
       is not because an constrained devices usually only implement one
       or very few different encryption algorithm transform is widely
       deployed that it is secured.  As a result, security should not be
       weakened for interoperability. transforms.  [RFC8221] and successors consider
       takes the life cycle of encryption algorithm transforms sufficiently
       long to provide interoperability.  Constrained devices may have
       limited interoperability requirements which makes possible to
       reduces the number of encryption algorithm transforms to
       implement. and
       device manufactoring into consideration in its recommendations
       for mandatory-to-implement ("MTI") algorithms.

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

   5.  Power Consumption and Bandwidth Consumption: Similarly to the
       encryption algorithm transform complexity, reducing Reducing the payload
       sent,
       sent may significantly reduce the energy consumption of the
       device.  As a result, encryption  Encryption algorithm transforms with low overhead may be considered. are
       strongly preferred.  To reduce the overall payload size one may,
       for example:

       1.  Use of counter-based ciphers without fixed block length (e.g.
           AES-CTR, or ChaCha20-Poly1305).

       2.  Use of ciphers with capability of using implicit IVs
           [RFC8750].

       3.  Use of ciphers recommended for IoT [RFC8221].

       4.  Avoid Padding by sending payload data which are aligned to
           the cipher block length - 2 for the ESP trailer.

9.  IANA Considerations

   There are no IANA consideration for this document.

10.  Security Considerations

   Security considerations Considerations are those of [RFC4303].  In addition, this
   document provided security recommendations and guidance over 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 MUST ensure that the mechanisms put in
   place remain valid across reboot as well.

   This document recommends

   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 prevent a session key being use beyond its
   lifetime.  When such mechanisms cannot be implemented and implemented, such as when
   the the session key is, for example, is provisioned, the nodes must device MUST ensure that keys
   are not used beyond their lifetime and that the appropriate use
   of the key remains used
   in compliance will all security requirements across reboots - e.g.
   conditions on counters and nonces remains valid.

   When a node device generates its own key or when random value such as
   nonces are generated, the random generation must MUST follow [RFC4086].
   In addition, [SP-800-90A-Rev-1] provides appropriated 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 result 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 some minimalistic way to implement ESP.  In some cases,
   this may result in potentially exposing revealing privacy sensitive pieces of
   information.  This document describes these privacy implications so
   the designer implementer can take the appropriate decisions given the
   specificities of a given environment and deployment.

   The main risks 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 some very
   specific context of non randomly generated SPI might in some cases
   ease the determination of the device of or the application.  Similarly,
   padding provides limited capabilities to obfuscate the traffic
   compared to those provided by TFC.  Such consequence on privacy as
   detailed in Section 5.

12.  Acknowledgment

   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 provided also provided multiple clarifications
   and examples of deployment ESP 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.

13.  References

13.1.  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>.

13.2.  Informative References

   [DeoxysII] Jeremy, J. J., Ivica, I. N., Thomas, T. P., and Y. S.
              Yannick, "Deoxys v1.41", October 2016,
              <https://competitions.cr.yp.to/round3/deoxysv141.pdf>.

   [I-D.mglt-ipsecme-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-07, 11 March
              2019, draft-mglt-ipsecme-diet-esp-08, 13 May
              2022, <https://www.ietf.org/archive/id/draft-mglt-ipsecme-
              diet-esp-07.txt>.
              diet-esp-08.txt>.

   [I-D.mglt-ipsecme-ikev2-diet-esp-extension]
              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-01, 26
              June 2018, draft-mglt-ipsecme-ikev2-diet-esp-extension-02, 13
              May 2022, <https://www.ietf.org/archive/id/draft-mglt-
              ipsecme-ikev2-diet-esp-extension-01.txt>.
              ipsecme-ikev2-diet-esp-extension-02.txt>.

   [I-D.nikander-esp-beet-mode]
              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://www.ietf.org/archive/id/draft-nikander-esp-beet-
              mode-09.txt>.

   [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]
              Elain, E. B. and J. K. Kelsey, "Recommendation for Random
              Number Generation Using Deterministic Random Bit
              Generators", <https://csrc.nist.gov/publications/detail/
              sp/800-90a/rev-1/final>.

Authors' Addresses

   Daniel Migault
   Ericsson
   8400 boulevard Decarie
   Montreal, QC H4P 2N2
   Canada
   Email: daniel.migault@ericsson.com

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