Light-Weight Implementation Guidance (lwig) D. Migault
Internet-Draft Ericsson
Intended status: Informational T. Guggemos
Expires: November 16, 2017 LMU Munich
May 15, 2017

Minimal ESP


This document describes a minimal implementation of the IP Encapsulation Security Payload (ESP) described in RFC 4303. Its purpose is to enable implementation of ESP with a minimal set of options that makes the minimal implementation compatible with ESP as described in RFC 4303. A minimal version of ESP is not intended to become a replacement of the RFC 4303 ESP, but instead to enable a limited implementation to interoperate with implementations of RFC 4303 ESP.

This document describes what is required from RFC 4303 ESP as well as various ways to optimize compliance with RFC 4303 ESP.

This document does not update or modify RFC 4303, but provides a compact description of how to implement the minimal version of the protocol. If this document and RFC 4303 conflicts then RFC 4303 is 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 working documents as Internet-Drafts. The list of current Internet-Drafts is at

Internet-Drafts are draft documents valid for a maximum of six months and may be updated, replaced, or obsoleted by other documents at any time. It is inappropriate to use Internet-Drafts as reference material or to cite them other than as "work in progress."

This Internet-Draft will expire on November 16, 2017.

Copyright Notice

Copyright (c) 2017 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 ( 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 Simplified BSD License text as described in Section 4.e of the Trust Legal Provisions and are provided without warranty as described in the Simplified BSD License.

1. Requirements notation

The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this document are to be interpreted as described in [RFC2119].

2. Introduction

ESP [RFC4303] is part of the IPsec suite protocol [RFC4301] . It 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.

Figure 1 describes an ESP Packet. Currently ESP is implemented in the kernel of major multi purpose Operating Systems (OS). The ESP and IPsec stack implemented is usually complete to fit multiple purpose usage of these OS. Completeness of the IPsec stack as well as multi purpose of these OS is often performed at the expense of resources, or a lack of performance, and so devices especially constraint devices like sensors have developed their own specific and task specific OS. This document provides a minimal ESP implementation guideline so these devices can implement ESP and benefit from IPsec.

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

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 the SPI. On the other hand, nodes supporting multicast communications must also use the IP addresses and thus SA lookup needs to be performed using the longest match.

For nodes supporting only unicast communications, it is RECOMMENDED to index SA with the SPI only. Some other local constraints on the node may require a combination of the SPI as well as other parameters to index the SA.

It is RECOMMENDED to randomly generate the SPI indexing each inbound session. A random generation provides a stateless way to generate the SPIs, while keeping the probability of collision between SPIs relatively low. In case of collision, the SPI is simply re-generated.

However, for some constraint nodes, generating a random SPI may consume to much resource, in which case SPI can be generated using predictable functions or even a fix value. In fact, the SPI does not need to the SPI does not need to be random.

When a constraint node uses fix value for SPIs, it imposes some limitations on the number of inbound SA. This limitation can be alleviate by how the SA look up is performed. When fix SPI are used, it is RECOMMENDED the constraint node has as many SPI values as ESP session per host IP address, and that lookup includes the IP addresses.

Note that SPI value is used only for inbound traffic, as such the SPI negotiated with IKEv2 [RFC7296] or [RFC7815] by a peer, is the value used by the remote peer when its sends traffic.

The use of fix SPI should not be considered as a way to avoid strong random generators. Such generator will be required in order to provide strong cryptographic protection. Instead, the use of a fix SPI should only considered as a way to overcome the resource limitations of the node, when this is feasible.

The use of a limited number of fix SPI also come with security or privacy drawbacks. Typically, a passive attacker may derive information such as the number of constraint devices connecting the remote peer, and in conjunction with data rate, the attacker may eventually determine the application the constraint device is associated to. In addition, if the fix value SPI is fixed by a manufacturer or by some software application, the SPI may leak in an obvious way the type of sensor, the application involved or the model of the constraint device. As a result, the use of a unpredictable SPI is preferred to provide better privacy.

As far as security is concerned, revealing the type of application or model of the constraint device could be used to identify the vulnerabilities the constraint device is subject to. This is especially sensitive for constraint device where patches or software updates will be challenging to operate. As a result, these devices may remain vulnerable for relatively long period. In addition, predictable SPI enable an attacker to forge packets with a valid SPI. Such packet will not be rejected due to an SPI mismatch, but instead after the signature check which requires more resource and thus make DoS more efficient, especially for devices powered by batteries.

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 :

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. When the use of a clock is considered, one should take care that packets associated to a given SA are not sent with the same time value.

[RFC4303] mentions :

5. Padding

The purpose of padding is to respect the 32 bit alignment of ESP. ESP MUST have at least one padding byte Pad Length that indicates the padding length. ESP padding bytes are generated by a succession of unsigned bytes starting with 1, 2, 3 with the last byte set to Pad Length, where Pad Length designates the length of the padding bytes.

Checking the padding structure is not mandatory, so the constraint device may not proceed to such checks, however, in order to interoperate with existing ESP implementations, it MUST build the padding bytes as recommended by ESP.

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

[RFC4303] mentions :

ESP [RFC4303] also provides Traffic Flow Confidentiality (TFC) as a way to perform padding to hide traffic characteristics, which differs from respecting a 32 bit alignment. TFC is not mandatory and MUST be negotiated with the SA management protocol. As a result, TFC is not expected to be supported by a minimal ESP implementation. On the other hand, disabling TFC should be carefully measured and understood as it exposes the node to traffic shaping. This could expose the application as well as the devices used to a passive monitoring attacker. Such information could be used by the attacker in case a vulnerability is disclosed on the specific device. In addition, some application use - such as health applications - may also reveal important privacy oriented informations.

Some constraint nodes that have limited battery life time may also prefer avoiding 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 is quite small, and the standard padding mechanism may also be used as an alternative to TFC, with a sufficient trade off between the require energy to send additional payload and the exposure to traffic shaping attacks.

6. Next Header (8 bit)

According to [RFC4303], the Next Header is a mandatory 8 bits field in the packet. Next header is intended to specify the data contained in the payload as well as dummy packet. In addition, the Next Header may also carry an indication on how to process the packet [I-D.nikander-esp-beet-mode].

The ability to generate and receive dummy packet is required by [RFC4303]. For interoperability, it is RECOMMENDED a minimal ESP implementation discards dummy packets. Note that such recommendation only applies for nodes receiving packets, and that nodes designed to only send data may 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. More especially, in constraint environment sending dummy packets may have too much impact on the device life time, and so may be avoided. On the other hand, constraint nodes may be dedicated to specific applications, in which case, traffic pattern may expose the application or the type of node. For these nodes, not sending dummy packet may have some privacy implication that needs to be measured.

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.

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 is not expected to be part of a minimal implementation of ESP.

[RFC4303] mentions :

7. ICV

The ICV depends on the crypto-suite used. Currently recommended [I-D.ietf-ipsecme-rfc7321bis] only recommend crypto-suites with an ICV which makes the ICV a mandatory field.

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 :

8. Cryptographic Suites

The cryptographic suites implemented are an important component of ESP. The recommended suites to use are expect to evolve over time and implementer SHOULD follow the recommendations provided by [I-D.ietf-ipsecme-rfc7321bis] and updates. Recommendations are provided for standard nodes as well as constraint nodes.

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

  1. 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 (see [I-D.ietf-ipsecme-rfc7321bis] for outdated ciphers). ESP can be used to authenticate only or to encrypt the communication. In the later case, authenticated encryption must always be considered [I-D.ietf-ipsecme-rfc7321bis].
  2. Interoperability: Interoperability considers the cipher suites shared with the other 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. [I-D.ietf-ipsecme-rfc7321bis] and successors consider the life cycle of cipher suites sufficiently long to provide interoperability. Constraint devices may have limited interoperability requirements which makes possible to reduces the number of cipher suites to implement.
  3. 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 determines 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 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.
  4. 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, one MAY consider:
    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 [I-D.mglt-ipsecme-implicit-iv].
    3. Use of ciphers recommended for IoT [I-D.ietf-ipsecme-rfc7321bis].
    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 are those of [RFC4303]. In addition, this document provided security recommendations an guidances over the implementation choices for each fields.

11. Acknowledgment

The authors would like to thank Scott Fluhrer, Tero Kivinen, Valery Smyslov, Yoav Nir, Michael Richardson for their valuable comments.

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.
[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.
[RFC7815] Kivinen, T., "Minimal Internet Key Exchange Version 2 (IKEv2) Initiator Implementation", RFC 7815, DOI 10.17487/RFC7815, March 2016.

12.2. Informative References

[I-D.ietf-ipsecme-rfc7321bis] Migault, D., Mattsson, J., Wouters, P., Nir, Y. and T. Kivinen, "Cryptographic Algorithm Implementation Requirements and Usage Guidance for Encapsulating Security Payload (ESP) and Authentication Header (AH)", Internet-Draft draft-ietf-ipsecme-rfc7321bis-05, February 2017.
[I-D.mglt-ipsecme-implicit-iv] Migault, D., Guggemos, T. and Y. Nir, "Implicit IV for Counter-based Ciphers in IPsec", Internet-Draft draft-mglt-ipsecme-implicit-iv-02, November 2016.
[I-D.nikander-esp-beet-mode] NikanderMelen, PJ., "A Bound End-to-End Tunnel (BEET) mode for ESP", Internet-Draft draft-nikander-esp-beet-mode-09, February 2017.

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

Daniel Migault Ericsson 8400 boulevard Decarie Montreal, QC H4P 2N2, Canada EMail:
Tobias Guggemos LMU Munich MNM-Team Oettingenstr. 67 80538 Munich, Bavaria Germany EMail:

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