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], the SPI is a mandatory 32 bits field and is not allowed to be removed.

The SPI is used to index the Security Association. The SPI MUST be unique so that any incoming ESP packet can appropriately be bound to its association. Uniqueness of the SPI may be provided by random functions. However, the SPI does not need to be unpredictable. As a result, if random functions are too costly for some constraint devices, the SPI can be generated using predictable functions or even fixed values.

If a constraint device is designed to set a single ESP connection with a single remote device, it can use a fix value for the SPI. Since the constraint device uses a single connection, there is no risk of SPI collision by using a fix value. More specifically, the collision does not affect the remote device. In fact, when the SPI is proposed, it is used by the proposing entity to index inbound traffic. In the case two different constraint devices are using the same SPI, the remote device ends up with two outbound traffic identified by the same SPI. Should SPI collision for outbound traffic does not affect the remote device as the SPI will not be used by this device to index the traffic.

Similarly, if a constraint device establishes a single ESP connection with multiple remote devices, it may use the IPv4 or the interface ID of IPv6 addresses for example.

Values 0-255 SHOULD NOT be used. Values 1-255 are reserved and 0 is only allowed to be used internal and it MUST NOT be send on the wire.

[RFC4303] mentions :

-

"The SPI is an arbitrary 32-bit value that is used by a receiver to identify the SA to which an incoming packet is bound. The SPI field is mandatory. [...]"

-

"For a unicast SA, the SPI can be used by itself to specify an SA, or it may be used in conjunction with the IPsec protocol type (in this case ESP). Because the SPI value is generated by the receiver for a unicast SA, whether the value is sufficient to identify an SA by itself or whether it must be used in conjunction with the IPsec protocol value is a local matter. This mechanism for mapping inbound traffic to unicast SAs MUST be supported by all ESP implementations."

4. Sequence Number(SN) (32 bit)

According to [RFC4303], the sequence number 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 then the SN of packet A.

In IoT, constraint devices are expected to establish communication with specific devices, like a specific gateway, or nodes similar to them. As a result, the sender may know whereas the receiver implements anti-replay protection or not. Even though the sender may know the receiver does not implement anti replay protection, the sender MUST implement a always increasing function to generate the SN.

Usually, SN is generated by incrementing a counter for each packet sent. A constraint device may avoid maintaining this context. If the device has a clock, it may use the time indicated by the clock has a SN. This guarantees a strictly increasing function, and avoid storing any additional values or context related to the SN.

[RFC4303] mentions :

-

"This unsigned 32-bit field contains a counter value that increases by one for each packet sent, i.e., a per-SA packet sequence number. For a unicast SA or a single-sender multicast SA, the sender MUST increment this field for every transmitted packet. Sharing an SA among multiple senders is permitted, though generally not recommended. [...] The field is mandatory and MUST always be present even if the receiver does not elect to enable the anti-replay service for a specific SA."

5. Padding

[RFC4303] does not specify any way on how Padding bytes should be generated. These bytes may for example, be generated randomly or each byte may be numbered from \x01 to \xpad-length. A simplified implementation may consider a fix value, and consider all Padding bytes set to zero.

Note that Padding can also be defined by the encryption algorithm like AES in CBC mode [RFC3602]. In that case, Padding MUST be performed as described in [RFC3602]. However, [RFC3602] does not specify how Padding bytes are generated, and AES in CTR [RFC3686] or GCM[RFC4106] or CCM [RFC4309] mode do not consider Padding.

6. Next Header (8 bit)

According to [RFC4303], the Next Header is a mandatory 8 bits field in the packet. In some cases, devices are dedicated to a single application or a single transport protocol, in which case, the Next Header has a fix value.

[RFC4303] mentions :

-

"The Next Header is a mandatory, 8-bit field that identifies the type of data contained in the Payload Data field, e.g., an IPv4 or IPv6 packet, or a next layer header and data. [...] the protocol value 59 (which means "no next header") MUST be used to designate a "dummy" packet. A transmitter MUST be capable of generating dummy packets marked with this value in the next protocol field, and a receiver MUST be prepared to discard such packets, without indicating an error."

7. ICV

The ICV is an optional value with variable length. Unless the crypto-suite provides authentication without the use of the ICV field, the ICV field is often use to host the authentication part of the packet.

As detailed in Section 8 we recommend to use authentication, the ICV field is expected to be present that is to say with a size different from zero. This makes it a mandatory field which size is defined by the security recommendations only.

[RFC4303] mentions :

-

"The Integrity Check Value is a variable-length field computed over the ESP header, Payload, and ESP trailer fields. Implicit ESP trailer fields (integrity padding and high-order ESN bits, if applicable) are included in the ICV computation. The ICV field is optional. It is present only if the integrity service is selected and is provided by either a separate integrity algorithm or a combined mode algorithm that uses an ICV. The length of the field is specified by the integrity algorithm selected and associated with the SA. The integrity algorithm specification MUST specify the length of the ICV and the comparison rules and processing steps for validation."

8. Cryptographic Suites

Light implementations of ESP will probably implement a reduce number of cipher suites. When choosing the cipher suites it is recommended to balance the number of cipher suites as well as the cipher itself with other criteria. This section attempts to provide some generic guidances for choosing the appropriated cipher suites.

This section lists some of the criteria that may be consider. The list is not expected to be exhaustive and may also evolve overtime. As a result, the list is provided as indicative:

- Security :

Security is the criteria that should be considered first when a selection of cipher suites is performed. The security of cipher suites is expected to evolve over time, and it is of primary importance to follow up-to-date security guidances and recommendations. The chosen cipher suites MUST NOT be known vulnerable or weak. ESP can be used to authenticate only a communication or the encrypt the communication. In the later case, encryption should be always considered in conjunction with authentication. [RFC4303] allows combined encryption and authentication ciphers, which enables the use of modes like GCM [RFC4106] or CCM. Note that the use of AES-CTR for encryption requires the authentication with a non zero length ICV.

- Interoperability :

Interoperability considers the cipher suites shared by the greatest number of nodes. Note that it is not because a cipher suite is widely deployed that is secured. As a result, security should not be weaken for interoperability. Life cycle of cipher suites is expected to be long enough so interoperability can still be provided with secure cipher suites.

- Power Consumption and Cipher Suite Complexity :

Complexity of the cipher suite or the energy associated to it are especially considered when devices have limited resources or are using some batteries, in which case the battery main determine the life of the device. The choice of a cryptographic function may 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, it may prefer AUTH_AES-XCBC over a SHA2 based function for its authentication. In addition, some devices may also embed radio modules with hardware acceleration for AES-CCM, in which case, this mode may be preferred.

- Power Consumption and Bandwidth Consumption :

Similarly to the cipher suite complexity, reducing the payload sent, may significantly reduce the energy consumption of the device. As a result, cipher suites with low overhead may be considered. To reduce the overall payload size one may for example, consider the length of the ICV associated to the cipher suite, the use of implicit IV [I-D.mglt-6lo-aes-implicit-iv], the block size used by the cipher suite. Note that the size of the payload must not be performed at the expense of acceptable security. As a result, reducing the size of the ICV MUST follow the security recommendations. Regarding the block size, AES-CBC consumes a lot of bandwidth compared to other proposed modes. AES in CBC mode has a 128 bit alignment which for small packets of a few bytes length generates a large overhead in term of extra padding bytes.

9. IANA Considerations

There are no IANA consideration for this document.

10. Security Considerations

Security considerations are those of [RFC4303].

11. Acknowledgment

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.
[RFC3602]	Frankel, S., Glenn, R. and S. Kelly, "The AES-CBC Cipher Algorithm and Its Use with IPsec", RFC 3602, DOI 10.17487/RFC3602, September 2003.
[RFC3686]	Housley, R., "Using Advanced Encryption Standard (AES) Counter Mode With IPsec Encapsulating Security Payload (ESP)", RFC 3686, DOI 10.17487/RFC3686, January 2004.
[RFC4106]	Viega, J. and D. McGrew, "The Use of Galois/Counter Mode (GCM) in IPsec Encapsulating Security Payload (ESP)", RFC 4106, DOI 10.17487/RFC4106, June 2005.
[RFC4301]	Kent, S. and K. Seo, "Security Architecture for the Internet Protocol", RFC 4301, DOI 10.17487/RFC4301, December 2005.
[RFC4303]	Kent, S., "IP Encapsulating Security Payload (ESP)", RFC 4303, DOI 10.17487/RFC4303, December 2005.
[RFC4309]	Housley, R., "Using Advanced Encryption Standard (AES) CCM Mode with IPsec Encapsulating Security Payload (ESP)", RFC 4309, DOI 10.17487/RFC4309, December 2005.

12.2. Informative References

[I-D.mglt-6lo-aes-implicit-iv]

Migault, D. and T. Guggemos, "Implicit IV for AES-CBC, AES-CTR, AES-CCM and AES-GCM", Internet-Draft draft-mglt-6lo-aes-implicit-iv-01, February 2015.

Appendix A. Document Change Log

[RFC Editor: This section is to be removed before publication]

-00: First version published.

-01: Clarified description

-02: Clarified description

Authors' Addresses