6lo D. Migault
Internet-Draft Ericsson
Intended status: Standards Track T. Guggemos
Expires: January 9, 2017 LMU Munich
C. Bormann
Universitaet Bremen TZI
July 8, 2016
Diet-ESP: a flexible and compressed format for IPsec/ESP
draft-mglt-6lo-diet-esp-02.txt
Abstract
This document defines Diet-ESP that adapts IPsec/ESP for IoT. Diet-
ESP compresses fields of the Standard ESP. The compression is
defined by profiles based ROHC and ROHCoverIPsec as well as
parameters mentioned in the Diet-ESP Context agreed between the two
Diet-ESP peers for example using IKEv2.
Status of This Memo
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Table of Contents
1. Requirements notation . . . . . . . . . . . . . . . . . . . . 2
2. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 3
2.1. IoT context . . . . . . . . . . . . . . . . . . . . . . . 3
2.2. Document Overview . . . . . . . . . . . . . . . . . . . . 4
3. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 6
4. Diet-ESP Context . . . . . . . . . . . . . . . . . . . . . . 6
5. Diet-ESP Protocol Description . . . . . . . . . . . . . . . . 9
5.1. Robust Header Compression (ROHC) . . . . . . . . . . . . 10
5.2. Diet-ESP ROHC framework . . . . . . . . . . . . . . . . . 11
5.3. Diet-ESP header classification . . . . . . . . . . . . . 12
6. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 14
7. Security Considerations . . . . . . . . . . . . . . . . . . . 14
8. Privacy Considerations . . . . . . . . . . . . . . . . . . . 15
9. Acknowledgment . . . . . . . . . . . . . . . . . . . . . . . 16
10. References . . . . . . . . . . . . . . . . . . . . . . . . . 16
10.1. Normative References . . . . . . . . . . . . . . . . . . 16
10.2. Informational References . . . . . . . . . . . . . . . . 18
Appendix A. Example of light Diet-ESP implementation for sensor 18
Appendix B. Difference between Diet-ESP and ESP . . . . . . . . 20
B.1. Packet Alignment . . . . . . . . . . . . . . . . . . . . 21
B.2. SAD . . . . . . . . . . . . . . . . . . . . . . . . . . . 21
B.2.1. Inbound Security Association Lookup . . . . . . . . . 21
B.2.2. Outgoing Security Association Lookup . . . . . . . . 24
B.3. Sequence Number . . . . . . . . . . . . . . . . . . . . . 24
B.4. Outgoing Packet processing . . . . . . . . . . . . . . . 25
B.5. Inbound Packet processing . . . . . . . . . . . . . . . . 26
Appendix C. Interaction with other Compression Protocols . . . . 27
C.1. 6LoWPAN . . . . . . . . . . . . . . . . . . . . . . . . . 27
C.2. ROHC . . . . . . . . . . . . . . . . . . . . . . . . . . 27
C.3. ROHCoverIPsec and 6LoWPANoverIPsec . . . . . . . . . . . 28
Appendix D. Diet-ESP and Requirements . . . . . . . . . . . . . 29
Appendix E. Document Change Log . . . . . . . . . . . . . . . . 30
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 31
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].
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2. Introduction
2.1. IoT context
The IPsec/ESP [RFC4303] is represented in Figure 1. It was designed
to: 1) provide general purposes security protocol with high level of
security, 2) favor interoperability between implementations and 3)
scale on large infrastructures.
In order to match these goals, ESP format favor mandatory fields with
fixed sizes that are designed considering extreme or worst case
scenarios. This results in a kind of "unique" packet format common
to all considered scenarios using ESP. On the other hand ESP ends up
carrying "unnecessary" or "larger than required" fields. This cost
of additional bytes were considered as negligible versus
interoperability, making ESP very successful over the years.
With IoT, requirements become slightly different. For most devices,
like sensors, sending extra bytes directly impacts the battery and so
the life time of the sensor. As a result, IoT may look at reducing
the number of bytes sent on the wire.
This document describes Diet-ESP which compresses fields of the
Standard ESP. The compression is defined by profiles based ROHC and
ROHCoverIPsec as well as parameters mentioned in the Diet-ESP Context
agreed between the two Diet-ESP peers for example using IKEv2
[RFC7296]
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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) | ^
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ I
| Sequence Number (SN) | n
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ t--
| Payload Data* (variable) | e ^
~ ~ g c
| | r o
+ +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ i n
| | Padding (0-255 bytes) | t f
+-+-+-+-+-+-+-+-+ +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ y .
| | Pad Length | Next Header | v v
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ ---
| Integrity Check Value-ICV (variable) |
~ ~
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 1: ESP Packet Description
2.2. Document Overview
This document describes how to compress ESP fields sent on the wire.
Concerned fields are those of the ESP Header and the ESP Trailer.
Compression of the Payload Data, including the Initialization Vector
(IV) or the Integrity Check Value (ICV) are out of scope of the
document.
The compression mechanisms defined in this document are based on ROHC
[RFC3095], [RFC5225] and ROHCoverIPsec [RFC5856], [RFC5857],
[RFC5858].
ROHC defines mechanisms to compress/decompress fields of an IP
packet. These compressors are placed between the MAC layer and the
IP layer. In the case of ESP, ROHC can be used to compress/
decompress SPI, SN. However, ROHC cannot be used to compress
encrypted fields like Padding, Pad Length, Next Header -- and later
the Clear Text Data before encryption. In fact, at the MAC layer,
these fields are encrypted and their encrypted value is used generate
the ICV. As a result, compression an ESP packet at the MAC layer
requires to decrypt the packet to be able to compress fields like
Clear Text Data, Pad Length in order to be able to eventually remove
the Padding Field. Similarly, decompressing a compress ESP packet at
the MAC layer would require to decrypt the received packet,
decompress the packet the Clear Text Data as well as the other ESP
fields, before forwarding the ESP packet to the IP stack. Note that
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in some case decompression is not feasible. Consider for example an
ESP implementation that generates a random Padding. If this field is
removed by the compressor, it can hardly be recovered by the
decompressor. Using a different Padding field would result is ESP
rejecting the packet as the ICV check will not succeed. As a result
ROHC cannot be used alone.
On the other hand, ROHCoverIPsec makes compression possible before
the ESP payload is encrypted, and so the Clear Text Data can be
compressed, but not the ESP related fields like Padding, Pad Length
and Next Header.
ROHC and ROHCoverIPsec have been designed for bandwidth optimization,
but not necessarily for constraint devices. As a result, defining
ROHC and ROHCoverIPsec profiles is not sufficient to fulfill the
complete set of Diet-ESP requirements listed in
[I-D.mglt-6lo-diet-esp-requirements]. In fact Diet-ESP MUST result
in an light implementation that does not require implementation of
the full ROHC and ROHCoverIPsec frameworks.
In order to achieve ESP field compression, this document describes
the Diet-ESP Context. This context contains all necessary parameters
to compress an ESP packet. This Diet-ESP Context can be provided as
input to proceed to Diet-ESP compression / decompression. This
document uses the ROHC and ROHCoverIPsec framework to compress the
ESP packet. The advantage of using ROHC and ROHCoverIPsec is that
compression behavior follows a standardized compression framework.
On the other hand, ROHC and ROHCoverIPsec frameworks are used in a
stand alone mode, which means no ROHC communications between
compressor and decompressor are considered. This enables specific
and lighter implementations to perform Diet-ESP compression without
implementing the ROHC or ROHCoverIPsec frameworks. All Diet-ESP
implementations only have to agree on the Diet-ESP Context to become
inter-operable.
The remaining of the document is as follows. Section 4 described the
Diet-ESP Context. Section 5 describes how the parameters of the
Diet-ESP Context are used by the ROHC and ROHCoverIPsec framework to
compress the ESP packet. This requires definition of new profiles
and extensions. Finally, Section 7 and Section 8 provides a security
and privacy analysis on Diet-ESP over standard ESP. Informational
material have been added into the Appendix section. Appendix A
illustrates how an minimal Diet-ESP implementation may be used in IoT
devices. Appendix B lists the differences between Diet-ESP and
Standard ESP. Appendix C describes the interactions of Diet-ESP
interacts with other compression protocols such as 6lowPAN and ROHC
compression for other protocols than ESP. Finally, Appendix D
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describes how Diet-ESP matches the requirements for Diet-ESP
[I-D.mglt-6lo-diet-esp-requirements].
3. Terminology
This document uses the following terminology:
- IoT: Internet of Things
- LSB: Last Significant Bytes
- IP alignment: The necessary alignment for IPv4 (32 bits) resp.
IPv6 (64 bits)
- Clear Text Data: designates the original data that are carried by
ESP.
- Encrypted Payload: carries the encrypted Data Payload including
cryptographic material like the IV and the ESP Trailer.
4. Diet-ESP Context
The Diet-ESP context provides the necessary parameters for the
compressor and decompressor to perform the appropriated compression
and decompression of the ESP packet. Table 1 the different
parameters of the Diet-ESP Context.
+-----------------+-------------------------------------------------+
| Context Field | Overview |
| Name | |
+-----------------+-------------------------------------------------+
| ALIGN | Necessary Alignment for the specific device. |
| SPI_SIZE | Size in bytes of the SPI field in the sent |
| | packet. |
| SN_SIZE | Size in byte of the SN field in the sent |
| | packet. |
| NH | Presence of the Next Header field in the ESP |
| | Trailer. |
| PAD | Presence of the Pad Length field present in the |
| | ESP trailer. |
+-----------------+-------------------------------------------------+
Table 1: Diet-ESP Context.
ALIGN:
Alignment is the minimum alignment accepted by the hardware.
Constrains may come from various reasons. Hardware may have some
specific requirements, but also operating systems. For most
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servers CPU and OS have been designed with 32 bit or 64 bit
alignment. As a result, IP headers have been standardized with 32
bits (resp. 64 bits in IPv6) alignment for each IP extension
header. ESP is one of these extension headers with an Header (SPI
and SN) of 64 bits and the Trailer (NH, PL, PAD) of (2 + PL)
bytes. Since the trailer is part of the ESP extension header, it
must provide the necessary padding for a correct alignment of the
NH field to 32 (resp. 64) bits. The alignment may also be
relevant if Block-Ciphers like AES-CBC needs an aligned payload to
perform the encryption. To inter-oprate with the standard ESP, IP
alignment must be 32 bit for IPv4 and 64 bit for IPv6.
Diet-ESP reduces the ALIGN value from 32 bits for IPv4 or 64 bits
for IPv6 to 8, 16, 32 and 64 bit alignment.
Motivations to do so is to remove the Padding and other mandatory
fields of the ESP packet. Then, most IoT embeds small 8 or 16 bit
CPUs. Finally, even though ESP is an extension header, it is
often the last extension header of a header-only IP packet. The
ESP header is only read by the real receiver and is uninteresting
for other devices like routers, placed between the to peers. As a
result, there seems no real impact on the system if ESP extension
header is not aligned.
Note that the benefices of ALIGN also depends on the used
cryptographic mode. More specifically AES-CTR has a 8 bit block
whereas AES-CBC has a 128 bit block. As a result the use of AES-
CBC with small Clear Text Data results in large encrypted Data
with embedded padding. In other words, the alignment for one
packet is always MAX(CIPHER_BLOCK_SIZE, ALIGN).
SPI_SIZE:
ESP Security Policy Index is 4 byte long to identify the SAD-entry
for incoming traffic. To interoperate with the standard ESP, the
SPI_SIZE must be of 4 bytes.
Diet-ESP omits, leaves unchanged, or reduces the SPI sent on the
wire to the 0, 1, 2, 3 or 4 LSB.
Compression only impacts the data sent on the wire and therefore
SHOULD only deal with 4 byte decompressed SPIs in the SAD. This
allows systems to send and receive multiple SPI_SIZE with
different hosts. Decompressing the SPI at the receiver may
involve IP addresses (see Appendix B.2.1).
Compressing the SPI has significant security impacts as detailed
in Section 7. It should be guided by 1) the number of
simultaneous inbound SA the device is expected to handle and 2)
reliability of the IP addresses in order to identify the proper SA
for incoming packets. More specifically, a sensor with a single
connection to a Security Gateway, may bind incoming packets to the
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proper SA based only in its IP addresses. In that case, the SPI
may not be necessary. Other scenarios may consider using the SPI
to index the SAs or may consider having multiple ESP channels with
the same host from a single host. In that case one may choose a
reduced length for the SPI. Note also that the value 0 for the
SPI is not allowed to be sent on the wire as described in
[RFC4303].
SN_SIZE:
ESP Sequence Number is 32 bit and extended SN is 64 bit long and
used for anti-replay protection. To interoperate with standard
ESP the SN_SIZE must be of 4 bytes.
Diet-ESP omits, leaves unchanged or reduces SN sent on the wire to
0, 1, 2, 3 or 4 LSB.
Decompressing the SN at the receiver is guided by a linear
extrapolation of the expected received Sequence Number and the
LSB-SN sent on the wire. To avoid packet overhead, this
configuration is stored within the SA, whereas it remains valid
during its lifetime. Therefore an implementer should consider the
LSB window such that two consecutive received SN should not
present a difference of more than the LSB window.
In some cases, the received SN may increase by a high number e.g.
using the time as the SN or because of a high number of packet
loss. See Section 7 for the related security considerations.
Note that SN and SPI MUST be aligned to a multiple of the
Alignment value (ALIGN).
NH:
Next Header in ESP is used to identify the first header inside the
ESP payload. To interoperate with standard ESP, the Next Header
must be indicated and present.
Diet-ESP is able to remove the Next Header field from the ESP-
Trailer.
Removing the Next Header is possible only if the underlying
protocol can be derived from the Traffic Selector (TS) within the
Security Association (SA). More specifically, the Next Header
indicates whether the encrypted ESP payload is an IP packet, a UDP
packet, a TCP packet or no next header. The NH can only be
removed if this has been explicitly specified in the SA or if the
device has a single application.
Note that removing the Next Header impacts how encryption is
performed. For example, the use of AES-CBC [RFC3602] mode
requires the last block to be padded, reaching a 128 bit
alignment. In this case removing the Next Header increases the
padding by the Next Header length, which is 8 bits. In this case,
removing the Next Header provides few advantages, as it does not
reduce the ESP packet length. With AES-CBC, the only advantage of
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removing the Next Header would be for data with the last block of
15 bytes. In that case, ESP pads with 15 modulo 16 bytes, sets
the 1 byte pad length field to 15 and add the one byte Next Header
field. This leads to 15 + 15 + 1 + 1 = 32 bytes to be sent. On
the other hand, removing the Next Header would require only the
concatenation of the pad length byte with a 0 value, which leads
to 16 bytes to be sent.
Other modes like AES-CTR [RFC3686] do not have block alignment
requirements, so the only alignment constraint comes from the
device hardware alignment (ALIGN). Suppose A designates the
alignment constraint from OS, hardware, encryption, packet
format...). A is fixed and consider then any data of length k * A
+ A - 1 bytes with k an integer. Sending this data using ESP
takes advantage of removing the Next Header as it reduces the
number of bytes to be sent by A over the traditional ESP. As a
result, for 8 bit alignment hardware (A = 1) removing the Next
Header always prevent an unnecessary byte to be sent.
PAD:
With ESP, all packets have a Pad Length field. This field is
usually present because ESP requires IP alignment which is ensured
with padding. In order to interoperate with the standard ESP, the
Pad Length must be indicated and be present.
Diet-ESP considers removing the Padding and the Pad Length field.
If PAD is present, then it is computed according to ALIGN.
In fact, some devices might use an 8 bits alignment, in which case
padding is not necessary. Similarly, sensors may send application
data of fixed length matching the alignment. Note that alignment
may be required by the device (8-bit, 16-bit, or more generally
32-bit), but it may also be required by the encryption block size
(AES-CBC uses 128 bit blocks). With ESP these scenarios would
result in an unnecessary Pad Length field always set to zero.
Diet-ESP considers those case with no padding, and thus the Pad
Length field can be omitted.
Some additional parameters may be added to the Diet-ESP Context.
Such parameters are defined in other documents, like
[I-D.mglt-6lo-diet-esp-payload-compression] the compression of
protocol headers inside the encrypted ESP payload.
5. Diet-ESP Protocol Description
This section defines Diet-ESP on the top of the ROHC and
ROHCoverIPsec framework. Section 5.1 presents and explains the
choice of these frameworks. This section is informational and its
only goal is to position our work toward ROHC and ROHCoverIPsec.
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5.1. Robust Header Compression (ROHC)
ROHC enables the compression of different protocols of all layers.
It is designed as a framework, where protocol compression is defined
as profile. Each profile is defined for a specific layer, and ROHC
compression in [RFC3095] defines profiles for the following
protocols: uncompressed, UDP/IP, ESP/IP and RTP/UDP/IP. The
compression occurs between the IP and the MAC layer, and so remains
independent of an eventual IP alignment.
The general idea of ROHC is to classify the different protocol
fields. According to the classification, they can either completely
and always be omitted, omitted only after the fields has been sent
once and registered by the receiver or partly sent and be regenerated
by the receiver. For example, a static field value may be negotiated
out of band (for example IP version) and thus not be sent at all. In
some cases, the value is not negotiated out of band and is carried in
the first packet (for example SPI, UDP ports). As a result, the
first packet is usually not so highly compressed with ROHC. Finally,
some variable fields (for example Sequence Number) can be represented
partially by their Last Significant Bits (LSB) and regenerated by the
receiver.
The main issue encountered with ESP and ROHC is that ESP may contain
encrypted data which makes compression between the IP and MAC layer
complex to achieve. Therefore ROHC defines different compression of
the ESP protocol (see Figure 2), so compression of the Clear Text
Data can occur before the ESP encryption. Regular ROHC can compress
the ESP header. If the packet is not encrypted, the rate of
compression is extremely high as the whole packet including padding
can be compressed in the regular ROHC stack, too. For encrypted
payload ROHC defines ROHCoverIPsec ([RFC5856], [RFC5857], [RFC5858])
to compress the ESP payload before it is going to be encrypted. This
leads to a second ESP stack, where another ROHC compressor (resp.
decompressor) works (see Figure 2). Excluding the first packet which
initializes the ROHC context, this makes ESP compression highly
efficient.
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+-------------------------------+---
| Transport Layer | Layer 4
+---------------+---------------+---
| ROHCoverIPsec | Standard ESP | ^
+---------------+---------------+ | Layer 3
| IP Layer | v
+-------------------------------+---
| ROHC (de-)compressor | ^
+-------------------------------+ | Layer 2
| MAC Layer | v
+-------------------------------+---
Figure 2: The two different ROHC layers in the TCP/IP stack.
The first drawback for ROHC and ROHCoverIPsec is, that it leads to
two ROHC compression layers (ROHCoverIPsec before ESP encryption and
ROHC before the MAC layer) in addition to two ESP implementations
(Standard ESP and ROHCoverIPsec ESP). Both frameworks are quite
complex and require a lot of resources which does not fit IoT
requirements. Then ROHCoverIPsec also limits the compression of the
ESP protocol, according to the IP restrictions. Padding remains
necessary as IPsec is part of the IP stack which requires a 32 bits
(resp. 64 bits for IPv6) aligned packet. This makes compression
quite inefficient when small amount of data are sent.
Note that mechanism to compress encrypted fields may be possible with
ROHC only and without ROHCoverIPsec. Such mechanisms may be possible
for fields like the Next Header or the Padding and Pad Length when
the data sent is of fixed size. As the sizes of the fields are known
the compressor may simply remove these fields. However, even in this
case, it almost doubles the amount of computation on the receiver's
side. In fact, the ROHC compressor would almost decompress and re-
encrypt the compressed ESP payload before forwarding it to the IP
stack. In addition, since the receiver has to re-encrypt the
decompressed information before integrity of the packet can be
checked, one can easily construct a DoS attack. Flooding the
receiver with invalid packet causes the receiver to perform the
complex encryption and authentication algorithm for each packet.
5.2. Diet-ESP ROHC framework
This section defines how the compression of all ESP fields is
performed within the ROHC and ROHCoverIPsec frameworks. More
especially fields that are in the ESP Header (i.e. the SPI and the
SN) and the ICV are compressed by the ROHC framework. The other
fields, that is to say those of the ESP Trailer, are compressed by
the ROHCoverIPsec framework.
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Diet-ESP fits in the ROHC and the ROHCoverIPsec in a very specific
way.
1 - Diet-ESP does not need any ROHC signaling between the peers.
More specifically, ROHC Initialization and Refresh (IR), or
ROHC IR-DYN or ROHC Feed back packet are not considered with
Diet-ESP. The first reason is that fields are either STATIC or
PATTERN and their value or profile is defined through the Diet-
ESP Context agreement. This agreement is out of scope of ROHC,
it is expected to be agreed by other protocols like IKEv2 and
thus is considered as an out-of band agreement by the peers.
Then, the profiles are applied for each Security Association
that is unidirectional. In fact an IKEv2 negotiation results
in two unidirectional SA. As a result, each SA the packets are
sent in one direction only, which corresponds to the
Unidirectional mode -- U-mode of ROHC.
2 - Diet-ESP only exchanges compressed data. How the compression /
decompression occurs is defined by the Diet-ESP Context. Once
the Diet-ESP Context has been agreed, both peers are in a
Second Order (SO) State and exchange only compressed data.
3 - Diet-ESP only compresses ESP packets, it may include inner
packet compression, but Diet-ESP does not make any assumption
on the IP compression. This is made in order to make Diet-ESP
interoperable with multiple IP compression protocols.
4 - Diet-ESP compresses partially STATIC fields as they are used as
indexes by the receiver, and may not completely be removed.
5.3. Diet-ESP header classification
The ROHC header field classifications are defined in Appendix A.1 of
[RFC3095] and Appendix A of [RFC5225].
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+---------+------------+---------------+-----------+----------------+
| Field | ROHC class | Framework | Encoding | Diet-ESP |
| | | | Method | Context |
| | | | | Parameters |
+---------+------------+---------------+-----------+----------------+
| SPI | STATIC-DEF | ROHC | LSB | SPI_SIZE |
| SN | PATTERN | ROHC | LSB | SN_SIZE |
| Padding | PATTERN | ROHCoverIPsec | Removed | PAD, ALIGN |
| Pad | PATTERN | ROHCoverIPsec | Removed | PAD, ALIGN |
| Length | | | | |
| Next | STATIC-DEF | ROHCoverIPsec | Removed | NH |
| Header | | | | |
+---------+------------+---------------+-----------+----------------+
Table 2: Diet-ESP ROHC profile.
SPI:
The SPI indexes the SA, is negotiated by the two peers (e.g. via
IKEv2 or manually) and remains the same during the session.
Therefore, as defined in Appendix A.6 of [RFC5225] this field is
classified as STATIC-DEF. The compressed SPI consists in the
SPI_SIZE LSB of the negotiated 32 bit SPI, and SPI_SIZE is
provided by the Diet-ESP Context.
SN:
The SN is used for anti-replay protection and is modified in every
packet. In default cases, the ESP Sequence Number will be
incremented by one for each packet sent. Therefore, as defined in
Appendix A.6 of [RFC5225] this field is classified as PATTERN.
The compressed SN consists in the SN_SIZE LSB of the 32 bit or 64
bit SN, and SN_SIZE is provided by the Diet-ESP Context.
Padding:
Padding is used for alignment purposes and is computed on a per-
packet basis. Therefore it is classified as PATTERN. The
compressed Padding is defined by PAD and ALIGN provided by the the
Diet-ESP Context. If PAD is set the Padding and Pad Length fields
are removed. If PAD is unset, Padding is computed according to
the ALIGN and the padding length is indicated in the PAD Length
field.
Pad Length:
Pad Length indicates the length of the Padding field and is
computed on a per-packet basis. Therefore it is classified as
PATTERN. See Padding for the compressed Pad Length.
Next Header:
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The Next Header indicates the next layer in the inner ESP Payload.
To be compressed the Next Header MUST remain the same during the
session. This means that it MUST have been negotiated (e.g. by
IKEv2) and can be derived from the Traffic Selectors. If this
condition is met and the Next Header compression is requested by
the peers with NH set in the Diet-ESP Context, then the Next
Header field MUST be removed.
6. IANA Considerations
There are no IANA consideration for this document.
7. Security Considerations
This section lists security considerations related to the Diet-ESP
protocol.
Security Parameter Index (SPI):
The Security Parameter Index (SPI) is used by the receiver to
index the Security Association that contains appropriated
cryptographic material. If the SPI is not found, the packet is
rejected as no further checks can be performed. In Diet-ESP, the
value of the SPI is not reduced, but compressed why the SPI value
may not be fully provided between the compressor and the
decompressor. On the other hand, its uncompressed value is
provided to the ESP-procession and no weakness is introduced to
ESP itself. On an implementation perspective, it is strongly
recommended that decompression is deterministic. Compression and
decompression adds some additional treatment to the ESP packet,
which might be used by an attacker. In order to minimize the load
associated to decompression, decompression is expected to be
deterministic. The incoming compressed SPI with the associated IP
addresses should output a single and unique uncompressed SPI
value. If n uncompressed SPI values have to be considered, then
the receiver could end in n signature checks which may be used by
an attacker for a DoS attack. On a privacy perspective, until
Diet-ESP is not deployed outside the scope of IoT and small
devices, the use of a compressed SPI may provide an indication
that one of the endpoint is a sensor. Such information may be
used, for example, to evaluate the number of appliances deployed,
or - in addition with other information, such as the time
interval, the geographic location - be used to derive the type of
data transmitted.
Sequence Numer (SN):
The Sequence Number (SN) is used as an anti-replay attack
mechanism. Compression and decompression of the SN is already
part of the standard ESP namely the Extended Sequence Number
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(ESN). The SN in a standard ESP packet is 32 bit long, whether
Diet-ESP enables to reduce it to 0 bytes and the main limitation
to the compression a deterministic decompression. SN compression
consists in indicating the least significant bits of the
uncompressed SN on the wire. The size of the compressed SN must
consider the maximum reordering index such that the probability
that a later sent packet arrives before an earlier one. In
addition the size of SN should also consider maximum consecutive
packets lost during transmission. In the case of ESP, this number
is set to 2^32 which is, in most real world case, largely over-
provisioned. When the compression of the SN is not appropriately
provisioned, the most significant bit value may be desynchronized
between the sending and receiving parties. Although IKEv2
provides some re-synchronization mechanisms, in case of IoT the
de-synchronization will most likely result in a renegotiation and
thus DoS possibilities. Note that IoT communication may also use
some external parameters, i.e. other than the compressed SN, to
define whether a packet be considered or not and eventually derive
the SN. One such scenario may be the use of time windows.
Suppose a device is expected to send some information every hour
or every week. In this case, for example, the SN may be
compressed to zero bytes. Instead the SN may be derived by
incrementing the SN every hour after the last received valid
packet. Considering the time the packet is received make it
possible to consider the time derivation of the sensor clock.
Note also that the anti-replay mechanism needs to define the size
of the anti-replay window. [RFC4303] provides guidance to set the
window size and are similar to those used to define the size of
the compressed SN
ESP Trailer:
Padding, Pad Length and Next Header are fields stored inside the
encrypted payload. They are part of the ESP payload so that ESP
can be seen as an IP option. IP extension headers must have 32
bit Byte-Alignment in IPv4 [43] and 64 bit Byte-Alignment in IPv6
[6]. The main motivation for the alignment is the improvement of
packet processing on 32 or 64 bit processors. As a result,
compression of these static fields does not impact the security of
Diet-ESP compared to the one provided by ESP.
8. Privacy Considerations
Security Parameter Index (SPI):
Until Diet-ESP is not deployed outside the scope of IoT and small
devices, the use of a compressed SPI may provide an indication
that one of the endpoint is a sensor. Such information may be
used, for example, to evaluate the number of appliances deployed,
or - in addition with other information, such as the time
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interval, the geographic location - be used to derive the type of
data transmitted.
Sequence Number (SN): If incremented for each ESP packet, the SN may
leak some information like the amount of transmitted data or the
age of the sensor. The age of the sensor may be correlated with
the software used and the potential bugs. On the other hand, re-
keying will re-initialize the SN, but the cost of a re-keying may
not be negligible and thus, frequent re-keying can be considered.
In addition to the re-key operation, the SN may be generated in
order to reduce the accuracy of the information leaked. In fact,
the SN does not have to be incremented by one for each packet it
just has to be an increasing function. Using a function such as a
clock may prevent characterizing the age or the use of the sensor.
Note that the use of such function may also impact the compression
efficiency and result in larger compressed SN.
9. Acknowledgment
The current work on Diet-ESP results from exchange and cooperation
between Orange, Ludwig-Maximilians-Universitaet Munich, Universite
Pierre et Marie Curie. We thank Daniel Palomares and Carsten Bormann
for their useful remarks, comments and guidances on the design. We
thank Sylvain Killian for implementing an open source Diet-ESP on
Contiki and testing it on the FIT IoT-LAB [fit-iot-lab] funded by the
French Ministry of Higher Education and Research. We thank the IoT-
Lab Team and the INRIA for maintaining the FIT IoT-LAB platform and
for providing feed backs in an efficient way. We thank Ana Minaburo
for her ROHC review.
10. References
10.1. 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,
<http://www.rfc-editor.org/info/rfc2119>.
[RFC3095] Bormann, C., Burmeister, C., Degermark, M., Fukushima, H.,
Hannu, H., Jonsson, L-E., Hakenberg, R., Koren, T., Le,
K., Liu, Z., Martensson, A., Miyazaki, A., Svanbro, K.,
Wiebke, T., Yoshimura, T., and H. Zheng, "RObust Header
Compression (ROHC): Framework and four profiles: RTP, UDP,
ESP, and uncompressed", RFC 3095, DOI 10.17487/RFC3095,
July 2001, <http://www.rfc-editor.org/info/rfc3095>.
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[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,
<http://www.rfc-editor.org/info/rfc3602>.
[RFC3686] Housley, R., "Using Advanced Encryption Standard (AES)
Counter Mode With IPsec Encapsulating Security Payload
(ESP)", RFC 3686, DOI 10.17487/RFC3686, January 2004,
<http://www.rfc-editor.org/info/rfc3686>.
[RFC4104] Pana, M., Ed., Reyes, A., Barba, A., Moron, D., and M.
Brunner, "Policy Core Extension Lightweight Directory
Access Protocol Schema (PCELS)", RFC 4104,
DOI 10.17487/RFC4104, June 2005,
<http://www.rfc-editor.org/info/rfc4104>.
[RFC4301] Kent, S. and K. Seo, "Security Architecture for the
Internet Protocol", RFC 4301, DOI 10.17487/RFC4301,
December 2005, <http://www.rfc-editor.org/info/rfc4301>.
[RFC4303] Kent, S., "IP Encapsulating Security Payload (ESP)",
RFC 4303, DOI 10.17487/RFC4303, December 2005,
<http://www.rfc-editor.org/info/rfc4303>.
[RFC4309] Housley, R., "Using Advanced Encryption Standard (AES) CCM
Mode with IPsec Encapsulating Security Payload (ESP)",
RFC 4309, DOI 10.17487/RFC4309, December 2005,
<http://www.rfc-editor.org/info/rfc4309>.
[RFC4555] Eronen, P., "IKEv2 Mobility and Multihoming Protocol
(MOBIKE)", RFC 4555, DOI 10.17487/RFC4555, June 2006,
<http://www.rfc-editor.org/info/rfc4555>.
[RFC5225] Pelletier, G. and K. Sandlund, "RObust Header Compression
Version 2 (ROHCv2): Profiles for RTP, UDP, IP, ESP and
UDP-Lite", RFC 5225, DOI 10.17487/RFC5225, April 2008,
<http://www.rfc-editor.org/info/rfc5225>.
[RFC5857] Ertekin, E., Christou, C., Jasani, R., Kivinen, T., and C.
Bormann, "IKEv2 Extensions to Support Robust Header
Compression over IPsec", RFC 5857, DOI 10.17487/RFC5857,
May 2010, <http://www.rfc-editor.org/info/rfc5857>.
[RFC5858] Ertekin, E., Christou, C., and C. Bormann, "IPsec
Extensions to Support Robust Header Compression over
IPsec", RFC 5858, DOI 10.17487/RFC5858, May 2010,
<http://www.rfc-editor.org/info/rfc5858>.
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[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, <http://www.rfc-editor.org/info/rfc7296>.
[RFC7321] McGrew, D. and P. Hoffman, "Cryptographic Algorithm
Implementation Requirements and Usage Guidance for
Encapsulating Security Payload (ESP) and Authentication
Header (AH)", RFC 7321, DOI 10.17487/RFC7321, August 2014,
<http://www.rfc-editor.org/info/rfc7321>.
10.2. Informational References
[fit-iot-lab]
"Future Internet of Things (FIT) IoT-LAB",
<https://www.iot-lab.info>.
[I-D.mglt-6lo-diet-esp-payload-compression]
Migault, D. and T. Guggemos, "Diet-IPsec: ESP Payload
Compression of IPv6 / UDP / TCP / UDP-Lite", January 2015.
[I-D.mglt-6lo-diet-esp-requirements]
Migault, D. and T. Guggemos, "Requirements for Diet-ESP
the IPsec/ESP protocol for IoT", draft-mglt-6lo-diet-esp-
requirements-01 (work in progress), February 2015.
[I-D.raza-6lowpan-ipsec]
Raza, S., Duquennoy, S., and G. Selander, "Compression of
IPsec AH and ESP Headers for Constrained Environments",
draft-raza-6lowpan-ipsec-01 (work in progress), September
2013.
[RFC5856] Ertekin, E., Jasani, R., Christou, C., and C. Bormann,
"Integration of Robust Header Compression over IPsec
Security Associations", RFC 5856, DOI 10.17487/RFC5856,
May 2010, <http://www.rfc-editor.org/info/rfc5856>.
Appendix A. Example of light Diet-ESP implementation for sensor
Diet-ESP has been designed to enable light implementation. This
section illustrates the case of a sensor sending a specific amount of
data periodically. This section is not normative and has only an
illustrative purpose. In this scenario the sensor measures a
temperature every minute and sends its value to a gateway, which is
assumed to collect the data. The data is sent in an UDP packet and
there is no other connection between the two peers. The
communication between the sensor and the gateway should be secured by
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a Diet-ESP connection in transport mode. Therefore the following
context is chosen:
ALIGN: 8 bit
Sensors are not expected to be 32 or 64 bit CPU, and micro-
controllers are expected to support 8 bit alignment.
SPI_SIZE: 0
As it is a single connection, the SA can be identified by using
the IP addresses. As a result the SPI is not needed.
SN_SIZE: 0
Because only one packet every minute is sent, the packets will
arrive at the receiver in an ordered way. The receiver can
rebuild the SN which should be present in the packet, assuming the
SN is incremented by one for each packet. Note that setting SN to
0 does not mean there is no anti replay protection. In fact, the
SN is needed for the computation of the Diet-ESP ICV.
NH: Remove Next Header
Since the protocol is always UDP, the Next header can be omitted.
PAD: Remove Padding
With 8 bit alignment Padding has always a Pad Length of 0.
Setting PAD to "Remove Padding" removes the Pad Length field.
Diet-ESP_ICV_SIZE: 4 bytes
The ICV is chosen to be 32 bits in order to find a fair trade-off
between security and energy costs.
Encapsulating the outgoing Diet-ESP packet is proceeded as follows:
1) SAD lookup for outgoing traffic
2) Compress ESP payload incl. Transport Header (UDP)
3) Encrypt IP payload
4) Build ESP header
5) Calculate Diet-ESP ICV
6) Compress ESP header
7) Add ${Diet-ESP_ICV_SIZE} LSB of ICV to the packet.
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Diet-ESP | Standard ESP
+--------------------+ | +--------------------+
| orig | | | | | orig | | |
|IP hdr | UDP | Data | | |IP hdr | UDP | Data |
+--------------------+ | +--------------------+
| | |
V | V
+-----------+ | +-----------+
| Diet-ESP | | | ESP |
+-----------+ | +-----------+
| | |
V | V
+----------------------+|+---------------------------------------+
| orig | | Diet-ESP||| orig | ESP | | | ESP | ESP|
|IP hdr |Data| ICV |||IP hdr | Hdr | UDP |Data| Trailer | ICV|
+----------------------+|+---------------------------------------+
Figure 3: Minimal Example - Input and Output of the Diet-ESP function
vs. Standard ESP.
Incoming Diet-ESP packet is processed as follows:
1) SAD lookup for incoming traffic traffic
2) Decompress ESP-header incl. Transport Header (UDP)
3) Calculate packet Diet-ESP ICV
4) Check integrity with ${Diet-ESP_ICV_SIZE} LSB of Diet-ESP ICV
5) Check anti-replay
6) Decrypt IP payload (excluding ICV)
7) Decompress ESP payload
Appendix B. Difference between Diet-ESP and ESP
This section details how to use Diet-ESP to send and receive
messages. The use of Diet-ESP is based on the IPsec architecture
[RFC4301] and ESP [RFC4303]. We suppose the reader to be familiar
with these documents and we list here possible adaptations that may
be involved by Diet-ESP.
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B.1. Packet Alignment
Each ESP packet has a fixed alignment to 32 bits (resp. 64 bits in
IPv6). For Diet-ESP each device has an internal parameter that
defines the minimal acceptable alignment. ALIGN SHOULD be a the
maximum of the peer's minimal alignment.
Diet-ESP Context with SPI_SIZE + SN_SIZE that is not a multiple of
ALIGN MUST be rejected.
B.2. SAD
B.2.1. Inbound Security Association Lookup
For devices that are configured with a single SPI_SIZE value can
process inbound packet as defined in [RFC4301]. As such, no
modifications is required by Diet-ESP.
Detecting Inbound Security Association: Identifying the SA for
incoming packets is a one of the main reasons the SPI is send in each
packet on the wire. For regular ESP (and AH) packets, the Security
Association is detected as follows:
1. Search the SAD for a match on {SPI, destination address, source
address}. If an SAD entry matches, then process the inbound ESP
packet with that matching SAD entry. Otherwise, proceed to step
2.
2. Search the SAD for a match on {SPI, destination address}. If the
SAD entry matches, then process the inbound ESP packet with that
matching SAD entry. Otherwise, proceed to step 3.
3. Search the SAD for a match on only {SPI} if the receiver has
chosen to maintain a single SPI space for AH and ESP, or on {SPI,
protocol} otherwise. If an SAD entry matches, then process the
inbound ESP packet with that matching SAD entry. Otherwise,
discard the packet and log an audible event.
For device that are dealing with different SPI_SIZE SPI, the way
inbound packets are handled differs from the [RFC4301]. In fact,
when a inbound packet is received, the peer does not know the
SPI_SIZE. As a result, it does not know the SPI that applies to the
incoming packet. The different values could be the 0 (resp. 1, 2, 3
and 4) first bytes of the IP payload.
Since the size of the SPI is not known for incoming packets, the
detection of inbound SAs has to be redefined in a Diet-ESP
environment. In order to ensure a detection of a SA the above
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described regular detection have to be done for each supported SPI
size (in most cases 5 times). In most common cases this will return
a unique Security Association.
If there is more than one SA matching the lookup, the authentication
MUST be performed for all found SAs to detect the SA with the correct
key. In case there is no match, the packet MUST be dropped. Of
course this can lead into DoS vulnerability as an attacker recognizes
an overlap of one or more IP-SPI combinations. Therefore it is
highly recommended to avoid different values of the SPI_SIZE for one
tuple of Source and Destination IP address. Furthermore this
recommendation becomes mandatory if NULL authentication is supported.
This is easy to implement as long as the sensors are not mobile and
do not change their IP address.
The following optimizations MAY be considered for sensor that are not
likely to perform mobility or multihoming features provided by MOBIKE
[RFC4555] or any change of IP address during the lifetime of the SA.
Optimization 1 - SPI_SIZE is mentioned inside the SPI:
The SPI_SIZE is defined as part of the SPI sent in each packet.
Therefore the receiver has to choose the most significant 2 bits
of the SPI in the following way in order to recognize the right
size for incoming Diet-ESP packets:
00: SPI_SIZE of 1 byte is used.
01: SPI_SIZE of 2 byte is used.
10: SPI_SIZE of 3 byte is used.
11: SPI_SIZE of 4 byte is used.
If the the value 0 is chosen for the SPI_SIZE this option is not
feasible.
Optimization 2 - IP address based lookup:
IP address based search is one optimization one may choose to
avoid several SAD lookups. It is based on the IP address and the
stored SPI_SIZE, which MUST be the same value for each SA of one
IP address. Otherwise it can't neither be ensured that an SA is
found nor that the correct one is found. Note that in case of
mobile IP the SPI_SIZE MUST be updated for all SAs related to the
new IP address which may cause renegotiation. Figure 4 shows this
lookup described below.
1. Search most significant SA as follows:
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1.1 Search the first SA for a match on {destination address,
source address}. If an SA entry matches, then process to
step 2. Otherwise, proceed to step 1.2.
1.2 Search the first SA for a match on {source address}. If an
SA entry matches, then process to step 2. Otherwise, drop
the packet.
2. Identify the size of the compressed SPI for the found SA,
stored in the Diet-ESP context. Note that all SAs to one IP
address MUST have the same value for the SPI_SIZE. Then go to
step 3.
3. If the SPI_SIZE is NOT zero, read the SPI_SIZE SPI from the
packet and perform a regular SAD lookup as described in
[RFC4301]. If the SPI_SIZE is zero, the SA from step 1 is
unique and can be used.
Note that some implementations may collect all SPI matching the IP
addresses in step 2 to avoid an additional lookup over the whole
SAD. This is implementation dependent.
If the sensor is likely to change its IP address, the outcome may
be a given IP address associated to different SPI_SIZE. This case
may occur if one IP address has been used by a device not anymore
online, but the SA has not been removed. The IP has then been
provided to another device. In this case the Diet-ESP Context
SHOULD NOT be accepted by the Security Gateway when the new Diet-
ESP Context is provided to the Security Gateway. At least the
Security Gateway can check the previous peer is reachable and then
delete the SA before accepting the new SA.
Another case may be that a sensor got two interfaces with
different IP addresses, negotiates a different SPI_SIZE on each
interface and then use MOBIKE to move the IPsec channels from one
interface to the other. In this case, the Security Gateway SHOULD
NOT accept the update, or force a renegotiation of the SPI_SIZE
for all SAs, basically by re-keying the SAs.
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+-----------+
| START |
+-----------+
|
V
+-----------------+
| find 1st SA to |
| IP address |------------------+
+-----------------+ |
| |
____V____ V
yes / \ +-----+
+------/ SPI_SIZE \ /----->|'---'|
| \ = 0 ? / / | SAD |
| \_________/ / + +
| |no / '---'
| V /
| +-----------------+ /
| | find 1st SA to |--/
| | SPI +IP |
| | address |
| +-----------------+
| |
| V
| +-----------------+
| | Diet-ESP packet |
+-->| procession |
+-----------------+
Figure 4: SAD lookup for incoming packets.
B.2.2. Outgoing Security Association Lookup
Outgoing lookups for the SPI are performed in the same way as it is
done in ESP. The Traffic Selector for the packet is searched and the
right SA is read from the SA. The SPI used in the packet MUST be
reduced to the value stored in SPI_SIZE.
B.3. Sequence Number
Sequence number in ESP [RFC4303] can be of 4 bytes or 8 bytes for
extended ESP. Diet-ESP introduces different sizes. One way to deal
with this is to add a MAX_SN value that stores the maximum value the
SN can have. Any new value of the SN will be check against this
MAX_SN.
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B.4. Outgoing Packet processing
NH, TH, IH, P indicate fields or payloads that are removed from the
Diet-ESP packet. How the Diet-ESP packet is generated depends on the
length Payload Data LPD, BLCK the block size of the encryption
algorithm and the device alignment ALIGN. We note M = MAX(BLCK,
ALIGN).
- 1: Compress the headers inside the ESP payload.
- 2: if PAD and NH are set to present: Diet-ESP considers both
fields Pad Length and Next Header. The Diet-ESP Payload is the
encryption of the following clear text:
Payload Data | Padding of Pad Length bytes | Pad Length field |
Next Header field.
The Pad Length value is (LPD + 2) mod [M].
- 3: if PAD is set to present and NH is set to removed: Diet-ESP
considers the Pad Length field but removes the Next Header
field. The ESP Payload is the encryption of the following
clear text: Payload Data | Padding of Pad Length bytes | Pad
Length field | Next Header field. The Pad Length value is (LPD
+ 1) mod [M].
- 4: if PAD is set to removed and NH is set to present: Diet-ESP
considers the Next Header but do not consider the Pad Length
field or the Padding Field. This is valid as long as (LPD + 1)
mod [M] = 0. If M = 1 as it is the case for AES-CTR this
equation is always true. On the other hand the use of specific
block size requires the application to send specific length of
application data.
- 5: if PAD and NH are set to removed: Diet-ESP does consider
neither the Next Header field nor the Pad Length field nor the
Padding Field. This is valid as long as LPD mod [M] = 0. If M
= 1 as it is the case for AES-CTR this equation is always true.
On the other hand the use of specific block size requires the
application to send specific length of application data.
- 6: Encrypt the Diet-ESP payload.
- 7: Add ESP header.
- 8: Generate and add Diet-ESP ICV.
- 9: Compress ESP header.
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B.5. Inbound Packet processing
Decryption is for performed the other way around.
After SAD lookup, authenticating and decrypting the Diet-ESP payload
the original packet is rebuild as follows:
- 1: Decompress ESP header.
- 2: Generate Diet-ESP ICV and check ICV send in the packet.
- 3: Check anti-replay
- 4: Remove compressed header.
- 5: Encrypt the Diet-ESP payload.
- 6: if PAD and NH are set to removed: Diet-ESP does consider
neither the Next Header field nor the Pad Length field nor the
Padding Field. The Next Header field of the IP packet is set
to the protocol defined for incoming traffic within the
Traffic Selector of the SA. Because there is no Padding it is
disregarded.
- 7: if PAD is set to removed and NH is set to present: Diet-ESP
considers the Next Header but do not consider the Pad Length
field or the Padding Field. The Next Header field of the IP
packet is set to the value within the Diet-ESP trailer.
- 8: if PAD is set to present and NH is set to removed: Diet-ESP
considers the Pad Length field but removes the Next Header
field. The Next Header field of the IP packet is set to the
protocol defined for incoming traffic within the Traffic
Selector of the SA. The Pad Length field is read and the
Padding is removed from the Data Payload which results the
original Data Payload.
- 9: if PAD and NH are set to present: Diet-ESP considers both
fields Pad Length and Next Header. The Next Header field of
the IP packet is set to the value within the Diet-ESP trailer.
The Pad Length field is read and the Padding is removed from
the Data Payload which results the original Data Payload.
- 10: Decompress the headers inside the ESP payload.
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Appendix C. Interaction with other Compression Protocols
Diet-ESP exclusively defines compression for the ESP protocol as well
as the ESP payload. It does not consider compression of the IP
protocol. ROHC or 6LoWPAN may be used by a sensor to compress the IP
(resp. IPv6) header. Since compression usually occurs between the
MAC and IP layers, there are no expected complications with this
family of compression protocols.
C.1. 6LoWPAN
Diet-ESP smoothly interacts with 6LoWPAN. Every 6LoWPAN compression
header (NHC_EH) has an NH bit. This one is set to 1 if the following
header is compressed with 6LoWPAN. Similarly, the NH bit is set to 0
if the following header is not compressed with 6LowPAN. Thus,
interactions between 6LowPAN and Diet-ESP considers two case: 1) NH
set to 0: 6LowPAN indicates the Diet-ESP payload is not compressed
and 2) NH set to 1: 6LowPAN indicates the Diet-ESP payload is
compressed.
Suppose 6LowPAN indicates the Next Header ESP is not compressed by
6LowPAN. If the peers have agreed to use Diet-ESP, then the ESP
layer on each peers receives the expected Diet-ESP packet. Diet-ESP
is fully compatible with 6LowPAN ESP compression disabled.
Suppose 6LowPAN indicates the Next Header ESP is compressed by
6LowPAN. ESP compression with 6LowPAN [I-D.raza-6lowpan-ipsec]
considers the compression of the ESP Header, that is to say the
compression of the SPI and SN fields. As a result 6LowPAN
compression expects a 4 byte SPI and a 4 byte SN from the ESP layer.
Similarly 6LowPAN decompression provides a 4 byte SPI and a 4 byte SN
to the ESP layer. If the peers have agreed to use Diet-ESP and one
of them uses 6LowPAN ESP compression, then the Diet-ESP MUST use SPI
SIZE and the SN SIZE MUST be set to 4 bytes.
C.2. ROHC
ROHC and ROHCoverIPsec have been used to describe Diet-ESP. This
means the ROHC and ROHCoverIPsec concepts and terminology have been
used to describe Diet-ESP. In that sens Diet-ESP is compatible with
the ROHC and ROHCoverIPsec framework. The remaining of the section
describes how Diet-ESP interacts with ROHC and ROHCoverIPsec profiles
and payloads.
ROHC compress packets between the MAC and the IP layer. Compression
can only be performed over non encrypted packets. As a results, this
section considers the case of an ESP encrypted packet and an ESP non
encrypted packet.
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For encrypted ESP packet, ROHC profiles that enable ESP compression
(e.g. profile 0x0003 and 0x1003) compresses only the ESP Header and
the IP header. To enable ROHC compression a Diet-ESP packet MUST
present an similar header as the ESP Header, that is a 4 byte SPI and
a 4 byte SN. This is accomplished by setting SPI_SIZE = 4 and
SN_SIZE = 4 in the Diet-ESP Context. Reversely, if the Diet-ESP
packet presents a 4 byte SPI and a 4 byte SN, ROHC can proceed to the
compression. Note that Diet-ESP does not consider the IP header,
then ESP and Diet-ESP are encrypted, thus ROHC can hardly make the
difference between Diet-ESP and ESP packets. For encrypted packets,
the only difference at the MAC layer might be the alignment.
For non encrypted ESP packet, ROHC MAY proceed to the compression of
different fields of ESP and other layers, as the payload appears in
clear. ROHC compressor are unlikely to deal with ESP fields
compressed by Diet-ESP. As a result, it is recommended not to
combine Diet-ESP and ROHC ESP compression with non encrypted ESP
packets.
C.3. ROHCoverIPsec and 6LoWPANoverIPsec
ROHC or 6LoWPAN are not able to compress the ESP payload, as long as
it is encrypted. Diet-ESP describes how to compress the ESP-Trailer,
which is part of the encrypted payload can be compressed.
6LowPANoverIPsec (section 2 of [I-D.raza-6lowpan-ipsec]) and
ROHCoverIPsec define the compression of the ESP payload, more
specifically the upper layer headers (e.g. IP header or Transport
layer header). These protocols need a second, modified ESP stack in
order to make the payload compression possible. Then the packets
with compressed payload are forwarded to this second ESP stack which
can compress or decompress the payload.
Diet-ESP and its extensions also needs a modified ESP stack in order
to perform the compression of ESP payload possible. In addition,
fields that are subject to compression are most likely to be the same
with Diet6ESP and 6LowPANoverIPsec and/or ROHCoverIPsec. Therefore,
if a device implements Diet-ESP and 6LowPANoverIPsec and/or
ROHCoverIPsec the developer needs to define an order the various
frameworks perform the compression. Currently this order has not
been defined, and Diet-ESP is unlikely to be compatible with
6LowPANoverIPsec and/or ROHCoverIPsec. Integration of Diet-ESP and
6LowPANoverIPsec and/or ROHCoverIPsec has not been considered in the
current document as Diet-ESP has been designed to avoid
implementations of 6LowPANoverIPsec and/or ROHCoverIPsec frameworks
to be implemented into the devices. Diet-ESP has been designed to be
more lightweight than 6LowPANoverIPsec and/or ROHCoverIPsec by
avoiding negotiations between compressors and decompressors.
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Appendix D. Diet-ESP and Requirements
[I-D.mglt-6lo-diet-esp-requirements] lists the requirements for Diet-
ESP. This section position Diet-ESP described in this document
toward these requirements.
R1: Diet-ESP is completly based on IPsec/ESP and as such benefits
from the standard IPsec security.
R2: Diet-ESP does not introduces vulnerabilities to IPsec/ESP. The
only difference is that compression results in sending less
bytes on the wire. In return the bytes sent over the wire are
decompress to feed the IPsec stack with the appropriated bytes.
Compression MAY require a non standard IPsec/ESP
implementation, as some fields may have been removed. This
fields are packet descriptors (like padding, length...), and
are not related to the security of the standard IPsec/ESP.
R3: Diet-ESP is fully derived from IPsec/ESP ROHC and ROHCoverIPsec
and as such relies on the security of these protocols.
R4: Diet-ESP is able to handle alignments of 8, 16, 32 and 64 bits.
R5: is not in the scope of Diet-ESP. Announcement of the Byte-
Alignment should be performed by IKEv2.
R6: Diet-ESP does not modify how encryption occurs. It only
changes the encrypted payload, which is one of the parameters
for the encryption function. Therefore Diet-ESP is able to
work with any encryption defined in [RFC7321] which also
includes AES-CCM [RFC4309].
Combined Mode algorithm (e.g. AES-CCM, AES-GCM) have an
additional parameter, called Addition Authentication Data
(AAD). This AAD requires the uncompressed ESP header that is
to say the full SPI and SN. These parameters are not removed
by Diet-ESP. There are well known by the two peer. The ESP
Header MUST be uncompressed before proceeding to encryption/
decryption.
R7: Diet-ESP can remove all static and compress fields from the
protocol.
R8: The inner payload compression mechanisms are not defined in
this document. This aspect is the purpose of [draft-mglt-
inner-compression]
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R9: Diet-ESP compressed packet fields are always a number of bytes
-- that is Diet-ESP do not result in compressed fields that are
not expressed in a natural number of bytes.
R10: Diet-ESP allows the developer define the maximum compression
within the Diet-ESP context. The way the agreement is done, is
out of scope of this document and is described in [draft-mglt-
diet-esp-ikev2].
R11: Each field in the packet can be compressed separately, which
provides high flexibility.
R12: Since Diet-ESP does not propose compression method flexibility.
The proposed methods are generic enough and there is not
advantage for this flexibility and so it does not seems
appropriated for Diet-ESP.
R13: Each Diet-ESP client can have his own set of supported
contexts. The negotiation is out of scope of this document and
described in [draft-mglt-diet-esp-ikev2].
R14: Diet-ESP adds small complexity to Standard ESP, like described
in Appendix B. In- and Outbound packet procession is straight-
forward, like shown in Appendix B.5 and Appendix B.5.
Appendix A provides a implementation guideline for a minimal
use case. This one can be ported to any other use case.
R15: Diet-ESP is easy to configure and provides a default-context if
a developer does not want to dive into the details of Diet-ESP.
R16: Diet-ESP can interact with 6LoWPAN and ROHC IP compression, but
SHOULD be able to interact with all future compression applying
after the IP layer as well.
R17: Compatibility with Standard ESP 1: Diet-ESP can be implemented
instead of, nearby or like an add-on to an existing Standard
ESP implementation.
R18: Compatibility with Standard ESP 2: Diet-ESP is able to work
without compression and works with 32 and 64 bits alignment,
which makes it compatible with Standard ESP.
Appendix E. Document Change Log
[draft-mglt-6lo-diet-esp-00.txt]:
Changing affiliation
[draft-mglt-6lo-diet-esp-00.txt]:
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Updating references
[draft-mglt-ipsecme-diet-esp-01.txt]:
Diet ESP described in the ROHC framework
ESP is not modified.
[draft-mglt-ipsecme-diet-esp-00.txt]:
NAT consideration added.
Comparison actualized to new Version of 6LoWPAN ESP.
[draft-mglt-dice-diet-esp-00.txt]: First version published.
Authors' Addresses
Daniel Migault
Ericsson
8400 boulevard Decarie
Montreal, QC H4P 2N2
Canada
Email: daniel.migault@ericsson.com
Tobias Guggemos
LMU Munich
Oettingenstr. 67
80538 Muenchen, Bavaria
Germany
Email: guggemos@mnm-team.org
Carsten Bormann
Universitaet Bremen TZI
Postfach 330440
Bremen D-28359
Germany
Phone: +49-421-218-63921
Email: cabo@tzi.org
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