Internet Engineering Task Force (IETF) P. Jokela

Internet Engineering Task Force (IETF) P. Jokela
Request for Comments: 7402 Ericsson Research NomadicLab
Obsoletes: 5202 R. Moskowitz
Category: Standards Track HTT Consulting
ISSN: 2070-1721 J. Melen
Ericsson Research NomadicLab
April 2015

Using the Encapsulating Security Payload (ESP) Transport Format
with the Host Identity Protocol (HIP)

Abstract

This memo specifies an Encapsulating Security Payload (ESP) based
mechanism for transmission of user data packets, to be used with the
Host Identity Protocol (HIP). This document obsoletes RFC 5202.

Status of This Memo

This is an Internet Standards Track document.

This document is a product of the Internet Engineering Task Force
(IETF). It represents the consensus of the IETF community. It has
received public review and has been approved for publication by the
Internet Engineering Steering Group (IESG). Further information on
Internet Standards is available in Section 2 of RFC 5741.

Information about the current status of this document, any errata,
and how to provide feedback on it may be obtained at
http://www.rfc-editor.org/info/rfc7402.

Copyright Notice

Copyright (c) 2015 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
(http://trustee.ietf.org/license-info) in effect on the date of
publication of this document. Please review these documents
carefully, as they describe your rights and restrictions with respect
to this document. Code Components extracted from this document must
include 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.

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Table of Contents

1. Introduction ....................................................3
2. Conventions Used in This Document ...............................4
3. Using ESP with HIP ..............................................4
3.1. ESP Packet Format ..........................................5
3.2. Conceptual ESP Packet Processing ...........................5
3.2.1. Semantics of the Security Parameter Index (SPI) .....6
3.3. Security Association Establishment and Maintenance .........6
3.3.1. ESP Security Associations ...........................6
3.3.2. Rekeying ............................................7
3.3.3. Security Association Management .....................8
3.3.4. Security Parameter Index (SPI) ......................8
3.3.5. Supported Ciphers ...................................8
3.3.6. Sequence Number .....................................9
3.3.7. Lifetimes and Timers ................................9
3.4. IPsec and HIP ESP Implementation Considerations ............9
3.4.1. Data Packet Processing Considerations ..............10
3.4.2. HIP Signaling Packet Considerations ................10
4. The Protocol ...................................................11
4.1. ESP in HIP ................................................11
4.1.1. IPsec ESP Transport Format Type ....................11
4.1.2. Setting Up an ESP Security Association .............11
4.1.3. Updating an Existing ESP SA ........................12
5. Parameter and Packet Formats ...................................13
5.1. New Parameters ............................................13
5.1.1. ESP_INFO ...........................................13
5.1.2. ESP_TRANSFORM ......................................15
5.1.3. NOTIFICATION Parameter .............................16
5.2. HIP ESP Security Association Setup ........................17
5.2.1. Setup during Base Exchange .........................17
5.3. HIP ESP Rekeying ..........................................18
5.3.1. Initializing Rekeying ..............................19
5.3.2. Responding to the Rekeying Initialization ..........19
5.4. ICMP Messages .............................................20
5.4.1. Unknown SPI ........................................20
6. Packet Processing ..............................................20
6.1. Processing Outgoing Application Data ......................20
6.2. Processing Incoming Application Data ......................21
6.3. HMAC and SIGNATURE Calculation and Verification ...........21
6.4. Processing Incoming ESP SA Initialization (R1) ............22
6.5. Processing Incoming Initialization Reply (I2) .............22
6.6. Processing Incoming ESP SA Setup Finalization (R2) ........23
6.7. Dropping HIP Associations .................................23
6.8. Initiating ESP SA Rekeying ................................23

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6.9. Processing Incoming UPDATE Packets ........................24
6.9.1. Processing UPDATE Packet: No Outstanding
Rekeying Request ...................................25
6.10. Finalizing Rekeying ......................................26
6.11. Processing NOTIFY Packets ................................26
7. Keying Material ................................................27
8. Security Considerations ........................................27
9. IANA Considerations ............................................28
10. References ....................................................29
10.1. Normative References .....................................29
10.2. Informative References ...................................30
Appendix A. A Note on Implementation Options ......................32
Appendix B. Bound End-to-End Tunnel Mode for ESP ..................32
B.1. Protocol Definition ........................................33
B.1.1. Changes to Security Association Data Structures .....33
B.1.2. Packet Format .......................................34
B.1.3. Cryptographic Processing ............................36
B.1.4. IP Header Processing ................................36
B.1.5. Handling of Outgoing Packets ........................37
B.1.6. Handling of Incoming Packets ........................38
B.1.7. Handling of IPv4 Options ............................39
Acknowledgments ...................................................40
Authors' Addresses ................................................40

1. Introduction

In the Host Identity Protocol Architecture [HIP-ARCH], hosts are
identified with public keys. The Host Identity Protocol (HIP)
[RFC7401] base exchange allows any two HIP-supporting hosts to
authenticate each other and to create a HIP association between
themselves. During the base exchange, the hosts generate a piece of
shared keying material using an authenticated Diffie-Hellman
exchange.

The HIP base exchange specification [RFC7401] does not describe any
transport formats or methods for user data to be used during the
actual communication; it only defines that it is mandatory to
implement the Encapsulating Security Payload (ESP) [RFC4303] based
transport format and method. This document specifies how ESP is used
with HIP to carry actual user data.

To be more specific, this document specifies a set of HIP protocol
extensions and their handling. Using these extensions, a pair of ESP
Security Associations (SAs) is created between the hosts during the
base exchange. The resulting ESP Security Associations use keys
drawn from the keying material (KEYMAT) generated during the base
exchange. After the HIP association and required ESP SAs have been

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established between the hosts, the user data communication is
protected using ESP. In addition, this document specifies methods to
update an existing ESP Security Association.

It should be noted that representations of Host Identity are not
carried explicitly in the headers of user data packets. Instead, the
ESP Security Parameter Index (SPI) is used to indicate the right host
context. The SPIs are selected during the HIP ESP setup exchange.
For user data packets, ESP SPIs (in possible combination with IP
addresses) are used indirectly to identify the host context, thereby
avoiding any additional explicit protocol headers.

HIP and ESP traffic have known issues with middlebox traversal (RFC
5207 [RFC5207]). Other specifications exist for operating HIP and
ESP over UDP. (RFC 5770 [RFC5770] is an experimental specification,
and others are being developed.) Middlebox traversal is out of scope
for this document.

This document obsoletes RFC 5202.

2. Conventions Used in This Document

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 RFC 2119 [RFC2119].

3. Using ESP with HIP

The HIP base exchange is used to set up a HIP association between two
hosts. The base exchange provides two-way host authentication and
key material generation, but it does not provide any means for
protecting data communication between the hosts. In this document,
we specify the use of ESP for protecting user data traffic after the
HIP base exchange. Note that this use of ESP is intended only for
host-to-host traffic; security gateways are not supported.

To support ESP use, the HIP base exchange messages require some minor
additions to the parameters transported. In the R1 packet, the
Responder adds the possible ESP transforms in an ESP_TRANSFORM
parameter before sending it to the Initiator. The Initiator gets the
proposed transforms, selects one of those proposed transforms, and
adds it to the I2 packet in an ESP_TRANSFORM parameter. In this I2
packet, the Initiator also sends the SPI value that it wants to be
used for ESP traffic flowing from the Responder to the Initiator.
This information is carried using the ESP_INFO parameter. When
finalizing the ESP SA setup, the Responder sends its SPI value to the
Initiator in the R2 packet, again using ESP_INFO.

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3.1. ESP Packet Format

The ESP specification [RFC4303] defines the ESP packet format for
IPsec. The HIP ESP packet looks exactly the same as the IPsec ESP
transport format packet. The semantics, however, are a bit different
and are described in more detail in the next subsection.

3.2. Conceptual ESP Packet Processing

ESP packet processing can be implemented in different ways in HIP.
It is possible to implement it in a way that a standards compliant,
unmodified IPsec implementation [RFC4303] can be used in conjunction
with some additional transport checksum processing above it, and if
IP addresses are used as indexes to the right host context.

When a standards compliant IPsec implementation that uses IP
addresses in the Security Policy Database (SPD) and Security
Association Database (SAD) is used, the packet processing may take
the following steps. For outgoing packets, assuming that the
upper-layer pseudo header has been built using IP addresses, the
implementation recalculates upper-layer checksums using Host Identity
Tags (HITs) and, after that, changes the packet source and
destination addresses back to corresponding IP addresses. The packet
is sent to the IPsec ESP for transport mode handling, and from there
the encrypted packet is sent to the network. When an ESP packet is
received, the packet is first put through the IPsec ESP transport
mode handling, and after decryption, the source and destination IP
addresses are replaced with HITs, and finally, upper-layer checksums
are verified before passing the packet to the upper layer.

An alternative way to implement packet processing is the BEET (Bound
End-to-End Tunnel) mode (see Appendix B). In BEET mode, the ESP
packet is formatted as a transport mode packet, but the semantics of
the connection are the same as for tunnel mode. The "outer"
addresses of the packet are the IP addresses, and the "inner"
addresses are the HITs. For outgoing traffic, after the packet has
been encrypted, the packet's IP header is changed to a new one that
contains IP addresses instead of HITs, and the packet is sent to the
network. When the ESP packet is received, the SPI value, together
with the integrity protection, allow the packet to be securely
associated with the right HIT pair. The packet header is replaced
with a new header containing HITs, and the packet is decrypted. BEET
mode is completely internal for a host and doesn't require that the
corresponding host implement it; instead, the corresponding host can
have ESP transport mode and do HIT IP conversions outside ESP.

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3.2.1. Semantics of the Security Parameter Index (SPI)

SPIs are used in ESP to find the right Security Association for
received packets. The ESP SPIs have added significance when used
with HIP; they are a compressed representation of a pair of HITs.
Thus, SPIs MAY be used by intermediary systems in providing services
like address mapping. Note that since the SPI has significance at
the receiver, only the < DST, SPI >, where DST is a destination IP
address, uniquely identifies the receiver HIT at any given point of
time. The same SPI value may be used by several hosts. A single
< DST, SPI > value may denote different hosts and contexts at
different points of time, depending on the host that is currently
reachable at the DST.

Each host selects for itself the SPI it wants to see in packets
received from its peer. This allows it to select different SPIs for
different peers. The SPI selection SHOULD be random; the rules of
Section 2.1 of the ESP specification [RFC4303] must be followed. A
different SPI SHOULD be used for each HIP exchange with a particular
host; this is to avoid a replay attack. Additionally, when a host
rekeys, the SPI MUST be changed. Furthermore, if a host changes over
to use a different IP address, it MAY change the SPI.

One method for SPI creation that meets the above criteria would be to
concatenate the HIT with a 32-bit random or sequential number, hash
this (using SHA1), and then use the high-order 32 bits as the SPI.

The selected SPI is communicated to the peer in the third (I2) and
fourth (R2) packets of the base HIP exchange. Changes in SPI are
signaled with ESP_INFO parameters.

3.3. Security Association Establishment and Maintenance

3.3.1. ESP Security Associations

In HIP, ESP Security Associations are set up between the HIP nodes
during the base exchange [RFC7401]. Existing ESP SAs can be updated
later using UPDATE messages. The reason for updating the ESP SA
later can be, for example, a need for rekeying the SA because of
sequence number rollover.

Upon setting up a HIP association, each association is linked to two
ESP SAs, one for incoming packets and one for outgoing packets. The
Initiator's incoming SA corresponds with the Responder's outgoing
one, and vice versa. The Initiator defines the SPI for its incoming
association, as defined in Section 3.2.1. This SA is herein called

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SA-RI, and the corresponding SPI is called SPI-RI. Respectively, the
Responder's incoming SA corresponds with the Initiator's outgoing SA
and is called SA-IR, with the SPI being called SPI-IR.

The Initiator creates SA-RI as a part of R1 processing, before
sending out the I2, as explained in Section 6.4. The keys are
derived from KEYMAT, as defined in Section 7. The Responder creates
SA-RI as a part of I2 processing; see Section 6.5.

The Responder creates SA-IR as a part of I2 processing, before
sending out R2; see Section 6.5. The Initiator creates SA-IR when
processing R2; see Section 6.6.

The initial session keys are drawn from the generated keying
material, KEYMAT, after the HIP keys have been drawn as specified in
[RFC7401].

When the HIP association is removed, the related ESP SAs MUST also be
removed.

3.3.2. Rekeying

After the initial HIP base exchange and SA establishment, both hosts
are in the ESTABLISHED state. There are no longer Initiator and
Responder roles, and the association is symmetric. In this
subsection, the party that initiates the rekey procedure is denoted
with I' and the peer with R'.

An existing HIP-created ESP SA may need updating during the lifetime
of the HIP association. This document specifies the rekeying of an
existing HIP-created ESP SA, using the UPDATE message. The ESP_INFO
parameter introduced above is used for this purpose.

I' initiates the ESP SA updating process when needed (see
Section 6.8). It creates an UPDATE packet with required information
and sends it to the peer node. The old SAs are still in use, local
policy permitting.

R', after receiving and processing the UPDATE (see Section 6.9),
generates new SAs: SA-I'R' and SA-R'I'. It does not take the new
outgoing SA into use, but still uses the old one, so there
temporarily exist two SA pairs towards the same peer host. The SPI
for the new outgoing SA, SPI-R'I', is specified in the received
ESP_INFO parameter in the UPDATE packet. For the new incoming SA, R'
generates the new SPI value, SPI-I'R', and includes it in the
response UPDATE packet.

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When I' receives a response UPDATE from R', it generates new SAs, as
described in Section 6.9: SA-I'R' and SA-R'I'. It starts using the
new outgoing SA immediately.

R' starts using the new outgoing SA when it receives traffic on the
new incoming SA or when it receives the UPDATE ACK confirming
completion of rekeying. After this, R' can remove the old SAs.
Similarly, when the I' receives traffic from the new incoming SA, it
can safely remove the old SAs.

3.3.3. Security Association Management

An SA pair is indexed by the 2 SPIs and 2 HITs (both local and remote
HITs since a system can have more than one HIT). An inactivity timer
is RECOMMENDED for all SAs. If the state dictates the deletion of an
SA, a timer is set to allow for any late arriving packets.

3.3.4. Security Parameter Index (SPI)

The SPIs in ESP provide a simple compression of the HIP data from all
packets after the HIP exchange. This does require a per HIT-pair
Security Association (and SPI), and a decrease of policy granularity
over other Key Management Protocols like Internet Key Exchange (IKE)
[RFC7296].

When a host updates the ESP SA, it provides a new inbound SPI to and
gets a new outbound SPI from its peer.

3.3.5. Supported Ciphers

All HIP implementations MUST support AES-128-CBC and AES-256-CBC
[RFC3602]. If the Initiator does not support any of the transforms
offered by the Responder, it should abandon the negotiation and
inform the peer with a NOTIFY message about a non-supported
transform.

In addition to AES-128-CBC, all implementations SHOULD implement the
ESP NULL encryption algorithm. When the ESP NULL encryption is used,
it MUST be used together with SHA-256 authentication as specified in
Section 5.1.2.

When an authentication-only suite is used (NULL, AES-CMAC-96, and
AES-GMAC are examples), the suite MUST NOT be accepted if offered by
the peer unless the local policy configuration regarding the peer
host is explicitly set to allow an authentication-only mode. This is
to prevent sessions from being downgraded to an authentication-only
mode when one side's policy requests privacy for the session.

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3.3.6. Sequence Number

The Sequence Number field is MANDATORY when ESP is used with HIP.
Anti-replay protection MUST be used in an ESP SA established with
HIP. When ESP is used with HIP, a 64-bit sequence number MUST be
used. This means that each host MUST rekey before its sequence
number reaches 2^64.

When using a 64-bit sequence number, the higher 32 bits are NOT
included in the ESP header, but are simply kept local to both peers.
See [RFC4301].

3.3.7. Lifetimes and Timers

HIP does not negotiate any lifetimes. All ESP lifetimes are local
policy. The only lifetimes a HIP implementation MUST support are
sequence number rollover (for replay protection), and SHOULD support
timing out inactive ESP SAs. An SA times out if no packets are
received using that SA. Implementations SHOULD support a
configurable SA timeout value. Implementations MAY support lifetimes
for the various ESP transforms. Each implementation SHOULD implement
per-HIT configuration of the inactivity timeout, allowing statically
configured HIP associations to stay alive for days, even when
inactive.

3.4. IPsec and HIP ESP Implementation Considerations

When HIP is run on a node where a standards compliant IPsec is used,
some issues have to be considered.

The HIP implementation must be able to co-exist with other IPsec
keying protocols. When the HIP implementation selects the SPI value,
it may lead to a collision if not implemented properly. To avoid the
possibility for a collision, the HIP implementation MUST ensure that
the SPI values used for HIP SAs are not used for IPsec or other SAs,
and vice versa.

Incoming packets using an SA that is not negotiated by HIP MUST NOT
be processed as described in Section 3.2, paragraph 2. The SPI will
identify the correct SA for packet decryption and MUST be used to
identify that the packet has an upper-layer checksum that is
calculated as specified in [RFC7401].

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3.4.1. Data Packet Processing Considerations

For outbound traffic, the SPD (or coordinated SPDs, if there are two
-- one for HIP and one for IPsec) MUST ensure that packets intended
for HIP processing are given a HIP-enabled SA and that packets
intended for IPsec processing are given an IPsec-enabled SA. The SP
then MUST be bound to the matching SA, and non-HIP packets will not
be processed by this SA. Data originating from a socket that is not
using HIP MUST NOT have the checksum recalculated (as described in
Section 3.2, paragraph 2), and data MUST NOT be passed to the SP or
SA created by HIP.

It is possible that in the case of overlapping policies, the outgoing
packet would be handled by both IPsec and HIP. In this case, it is
possible that the HIP association is end to end, while the IPsec SA
is for encryption between the HIP host and a security gateway. In
the case of a security gateway ESP association, the ESP always uses
tunnel mode.

In the case of IPsec tunnel mode, it is hard to see during the HIP SA
processing if the IPsec ESP SA has the same final destination. Thus,
traffic MUST be encrypted with both the HIP ESP SA and the IPsec SA
when the IPsec ESP SA is used in tunnel mode.

In the case of IPsec transport mode, the connection endpoints are the
same. However, for HIP data packets it is not possible to avoid HIP
SA processing, while mapping the HIP data packet's IP addresses to
the corresponding HITs requires SPI values from the ESP header. In
the case of a transport mode IPsec SA, the IPsec encryption MAY be
skipped to avoid double encryption, if the local policy allows.

3.4.2. HIP Signaling Packet Considerations

In general, HIP signaling packets should follow the same processing
as HIP data packets.

In the case of IPsec tunnel mode, the HIP signaling packets are
always encrypted using an IPsec ESP SA. Note that this hides the HIP
signaling packets from the eventual HIP middleboxes on the path
between the originating host and the security gateway.

In the case of IPsec transport mode, the HIP signaling packets MAY
skip the IPsec ESP SA encryption if the local policy allows. This
allows the eventual HIP middleboxes to handle the passing HIP
signaling packets.

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4. The Protocol

In this section, the protocol for setting up an ESP association to be
used with a HIP association is described.

4.1. ESP in HIP

4.1.1. IPsec ESP Transport Format Type

The HIP handshake signals the TRANSPORT_FORMAT_LIST parameter in the
R1 and I2 messages. This parameter contains a list of the supported
HIP transport formats of the sending host, in the order of
preference. The transport format type for IPsec ESP is the type
number of the ESP_TRANSFORM parameter, i.e., 4095.

4.1.2. Setting Up an ESP Security Association

Setting up an ESP Security Association between hosts using HIP is
performed by including parameters in the last three messages (R1, I2,
and R2 messages) of the four-message HIP base exchange.

Initiator Responder

I1
---------------------------------->

R1: ESP_TRANSFORM
<----------------------------------

I2: ESP_TRANSFORM, ESP_INFO
---------------------------------->

R2: ESP_INFO
<----------------------------------

The R1 message contains the ESP_TRANSFORM parameter, in which the
sending host defines the possible ESP transforms it is willing to use
for the ESP SA.

Including the ESP_TRANSFORM parameter in the R1 message adds clarity
to the TRANSPORT_FORMAT_LIST but may initiate negotiations for
possibly unselected transforms. However, resource-constrained
devices will most likely restrict support to a single transform for
the sake of minimizing ROM overhead, and the additional parameter
adds negligible overhead with unconstrained devices.

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The I2 message contains the response to an ESP_TRANSFORM received in
the R1 message. The sender must select one of the proposed ESP
transforms from the ESP_TRANSFORM parameter in the R1 message and
include the selected one in the ESP_TRANSFORM parameter in the I2
packet. In addition to the transform, the host includes the ESP_INFO
parameter containing the SPI value to be used by the peer host.

In the R2 message, the ESP SA setup is finalized. The packet
contains the SPI information required by the Initiator for the
ESP SA.

4.1.3. Updating an Existing ESP SA

The update process is accomplished using three messages. The HIP
UPDATE message is used to update the parameters of an existing ESP
SA. The UPDATE mechanism and message are defined in [RFC7401], and
the additional parameters for updating an existing ESP SA are
described here.

The following picture shows a typical exchange when an existing ESP
SA is updated. Messages include SEQ and ACK parameters required by
the UPDATE mechanism.

H1 H2
UPDATE: SEQ, ESP_INFO [, DIFFIE_HELLMAN]
----------------------------------------------------->

UPDATE: SEQ, ACK, ESP_INFO [, DIFFIE_HELLMAN]
<-----------------------------------------------------

UPDATE: ACK
----------------------------------------------------->

The host willing to update the ESP SA creates and sends an UPDATE
message. The message contains the ESP_INFO parameter containing the
old SPI value that was used, the new SPI value to be used, and the
index value for the keying material, giving the point from where the
next keys will be drawn. If new keying material must be generated,
the UPDATE message will also contain the DIFFIE_HELLMAN parameter
defined in [RFC7401].

The host receiving the UPDATE message requesting update of an
existing ESP SA MUST reply with an UPDATE message. In the reply
message, the host sends the ESP_INFO parameter containing the
corresponding values: old SPI, new SPI, and the keying material
index. If the incoming UPDATE contained a DIFFIE_HELLMAN parameter,
the reply packet MUST also contain a DIFFIE_HELLMAN parameter.

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5. Parameter and Packet Formats

In this section, new and modified HIP parameters are presented, as
well as modified HIP packets.

5.1. New Parameters

Two HIP parameters are defined for setting up ESP transport format
associations in HIP communication and for rekeying existing ones.
Also, the NOTIFICATION parameter, described in [RFC7401], has two
error values defined for this specification.

Parameter Type Length Data

ESP_INFO 65 12 Remote's old SPI,
new SPI, and other info
ESP_TRANSFORM 4095 variable ESP Encryption and
Authentication Transform(s)

5.1.1. ESP_INFO

During the establishment and update of an ESP SA, the SPI value of
both hosts must be transmitted between the hosts. In addition, hosts
need the index value to the KEYMAT when they are drawing keys from
the generated keying material. The ESP_INFO parameter is used to
transmit the SPI values and the KEYMAT index information between the
hosts.

During the initial ESP SA setup, the hosts send the SPI value that
they want the peer to use when sending ESP data to them. The value
is set in the NEW SPI field of the ESP_INFO parameter. In the
initial setup, an old value for the SPI does not exist; thus, the OLD
SPI field value is set to zero. The OLD SPI field value may also be
zero when additional SAs are set up between HIP hosts, e.g., in the
case of multihomed HIP hosts [RFC5206]. However, such use is beyond
the scope of this specification.

The KEYMAT index value points to the place in the KEYMAT from where
the keying material for the ESP SAs is drawn. The KEYMAT index value
is zero only when the ESP_INFO is sent during a rekeying process and
new keying material is generated.

During the life of an SA established by HIP, one of the hosts may
need to reset the Sequence Number to one and rekey. The reason for
rekeying might be an approaching sequence number wrap in ESP, or a
local policy on the use of a key. Rekeying ends the current SAs and
starts new ones on both peers.

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During the rekeying process, the ESP_INFO parameter is used to
transmit the changed SPI values and the keying material index.

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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Type | Length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Reserved | KEYMAT Index |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| OLD SPI |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| NEW SPI |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

Type 65
Length 12
KEYMAT Index index, in bytes, where to continue to draw ESP keys
from KEYMAT. If the packet includes a new
Diffie-Hellman key and the ESP_INFO is sent in an
UPDATE packet, the field MUST be zero. If the
ESP_INFO is included in base exchange messages, the
KEYMAT Index must have the index value of the point
from where the ESP SA keys are drawn. Note that
the length of this field limits the amount of
keying material that can be drawn from KEYMAT. If
that amount is exceeded, the packet MUST contain
a new Diffie-Hellman key.
OLD SPI old SPI for data sent to address(es) associated
with this SA. If this is an initial SA setup, the
OLD SPI value is zero.
NEW SPI new SPI for data sent to address(es) associated
with this SA.

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5.1.2. ESP_TRANSFORM

The ESP_TRANSFORM parameter is used during ESP SA establishment. The
first party sends a selection of transform families in the
ESP_TRANSFORM parameter, and the peer must select one of the proposed
values and include it in the response ESP_TRANSFORM parameter.

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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Type | Length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Reserved | Suite ID #1 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Suite ID #2 | Suite ID #3 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Suite ID #n | Padding |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

Type 4095
Length length in octets, excluding Type, Length, and
padding.
Reserved zero when sent, ignored when received.
Suite ID defines the ESP Suite to be used.

The following Suite IDs can be used:

Suite ID Value

RESERVED 0 [RFC7402]
AES-128-CBC with HMAC-SHA1 1 [RFC3602], [RFC2404]
DEPRECATED 2 [RFC7402]
DEPRECATED 3 [RFC7402]
DEPRECATED 4 [RFC7402]
DEPRECATED 5 [RFC7402]
DEPRECATED 6 [RFC7402]
NULL with HMAC-SHA-256 7 [RFC2410], [RFC4868]
AES-128-CBC with HMAC-SHA-256 8 [RFC3602], [RFC4868]
AES-256-CBC with HMAC-SHA-256 9 [RFC3602], [RFC4868]
AES-CCM-8 10 [RFC4309]
AES-CCM-16 11 [RFC4309]
AES-GCM with an 8-octet ICV 12 [RFC4106]
AES-GCM with a 16-octet ICV 13 [RFC4106]
AES-CMAC-96 14 [RFC4493], [RFC4494]
AES-GMAC 15 [RFC4543]

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The sender of an ESP transform parameter MUST make sure that there
are no more than six (6) Suite IDs in one ESP transform parameter.
Conversely, a recipient MUST be prepared to handle received transform
parameters that contain more than six Suite IDs. The limited number
of Suite IDs sets the maximum size of the ESP_TRANSFORM parameter.
As the default configuration, the ESP_TRANSFORM parameter MUST
contain at least one of the mandatory Suite IDs. There MAY be a
configuration option that allows the administrator to override this
default.

Mandatory implementations: AES-128-CBC with HMAC-SHA-256. NULL with
HMAC-SHA-256 SHOULD also be supported (see also Section 3.3.5).

Under some conditions, it is possible to use Traffic Flow
Confidentiality (TFC) [RFC4303] with ESP in BEET mode. However, the
definition of such an operation is left for future work and must be
done in a separate specification.

5.1.3. NOTIFICATION Parameter

The HIP base specification defines a set of NOTIFICATION error types.
The following error types are required for describing errors in ESP
Transform crypto suites during negotiation.

NOTIFICATION PARAMETER - ERROR TYPES Value
------------------------------------ -----

NO_ESP_PROPOSAL_CHOSEN 18

None of the proposed ESP Transform crypto suites was
acceptable.

INVALID_ESP_TRANSFORM_CHOSEN 19

The ESP Transform crypto suite does not correspond to
one offered by the Responder.

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5.2. HIP ESP Security Association Setup

The ESP Security Association is set up during the base exchange. The
following subsections define the ESP SA setup procedure using both
base exchange messages (R1, I2, R2) and UPDATE messages.

5.2.1. Setup during Base Exchange

5.2.1.1. Modifications in R1

The ESP_TRANSFORM contains the ESP modes supported by the sender,
in the order of preference. All implementations MUST support
AES-128-CBC [RFC3602] with HMAC-SHA-256 [RFC4868].

The following figure shows the resulting R1 packet layout.

The HIP parameters for the R1 packet:

IP ( HIP ( [ R1_COUNTER, ]
PUZZLE,
DIFFIE_HELLMAN,
HIP_CIPHER,
ESP_TRANSFORM,
HOST_ID,
[ ECHO_REQUEST, ]
HIP_SIGNATURE_2 )
[, ECHO_REQUEST ])

5.2.1.2. Modifications in I2

The ESP_INFO contains the sender's SPI for this association as well
as the KEYMAT index from where the ESP SA keys will be drawn. The
old SPI value is set to zero.

The ESP_TRANSFORM contains the ESP mode selected by the sender of R1.
All implementations MUST support AES-128-CBC [RFC3602] with
HMAC-SHA-256 [RFC4868].

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The following figure shows the resulting I2 packet layout.

The HIP parameters for the I2 packet:

IP ( HIP ( ESP_INFO,
[R1_COUNTER,]
SOLUTION,
DIFFIE_HELLMAN,
HIP_CIPHER,
ESP_TRANSFORM,
ENCRYPTED { HOST_ID },
[ ECHO_RESPONSE ,]
HMAC,
HIP_SIGNATURE
[, ECHO_RESPONSE] ) )

5.2.1.3. Modifications in R2

The R2 contains an ESP_INFO parameter, which has the SPI value of the
sender of the R2 for this association. The ESP_INFO also has the
KEYMAT index value specifying where the ESP SA keys are drawn.

The following figure shows the resulting R2 packet layout.

The HIP parameters for the R2 packet:

IP ( HIP ( ESP_INFO, HMAC_2, HIP_SIGNATURE ) )

5.3. HIP ESP Rekeying

In this section, the procedure for rekeying an existing ESP SA is
presented.

Conceptually, the process can be represented by the following message
sequence using the host names I' and R' defined in Section 3.3.2.
For simplicity, HMAC and HIP_SIGNATURE are not depicted, and
DIFFIE_HELLMAN keys are optional. The UPDATE with ACK_I need not be
piggybacked with the UPDATE with SEQ_R; it may be ACKed separately
(in which case the sequence would include four packets).

I' R'

UPDATE(ESP_INFO, SEQ_I, [DIFFIE_HELLMAN])
----------------------------------->
UPDATE(ESP_INFO, SEQ_R, ACK_I, [DIFFIE_HELLMAN])
<-----------------------------------
UPDATE(ACK_R)
----------------------------------->

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Below, the first two packets in this figure are explained.

5.3.1. Initializing Rekeying

When HIP is used with ESP, the UPDATE packet is used to initiate
rekeying. The UPDATE packet MUST carry an ESP_INFO and MAY carry a
DIFFIE_HELLMAN parameter.

Intermediate systems that use the SPI will have to inspect HIP
packets for those that carry rekeying information. The packet is
signed for the benefit of the intermediate systems. Since
intermediate systems may need the new SPI values, the contents cannot
be encrypted.

The following figure shows the contents of a rekeying initialization
UPDATE packet.

The HIP parameters for the UPDATE packet initiating rekeying:

IP ( HIP ( ESP_INFO,
SEQ,
[DIFFIE_HELLMAN, ]
HMAC,
HIP_SIGNATURE ) )

5.3.2. Responding to the Rekeying Initialization

The UPDATE ACK is used to acknowledge the received UPDATE rekeying
initialization. The acknowledgment UPDATE packet MUST carry an
ESP_INFO and MAY carry a DIFFIE_HELLMAN parameter.

Intermediate systems that use the SPI will have to inspect HIP
packets for packets carrying rekeying information. The packet is
signed for the benefit of the intermediate systems. Since
intermediate systems may need the new SPI values, the contents cannot
be encrypted.

The following figure shows the contents of a rekeying acknowledgment
UPDATE packet.

The HIP parameters for the UPDATE packet:

IP ( HIP ( ESP_INFO,
SEQ,
ACK,
[ DIFFIE_HELLMAN, ]
HMAC,
HIP_SIGNATURE ) )

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5.4. ICMP Messages

ICMP message handling is mainly described in the HIP base
specification [RFC7401]. In this section, we describe the actions
related to ESP security associations.

5.4.1. Unknown SPI

If a HIP implementation receives an ESP packet that has an
unrecognized SPI number, it MAY respond (subject to rate limiting the
responses) with an ICMP packet with type "Parameter Problem", with
the pointer pointing to the beginning of the SPI field in the ESP
header.

6. Packet Processing

Packet processing is mainly defined in the HIP base specification
[RFC7401]. This section describes the changes and new requirements
for packet handling when the ESP transport format is used. Note that
all HIP packets (currently protocol 139) MUST bypass ESP processing.

6.1. Processing Outgoing Application Data

Outgoing application data handling is specified in the HIP base
specification [RFC7401]. When the ESP transport format is used, and
there is an active HIP session for the given < source, destination >
HIT pair, the outgoing datagram is protected using the ESP security
association. The following additional steps define the conceptual
processing rules for outgoing ESP protected datagrams.

1. Detect the proper ESP SA using the HITs in the packet header or
other information associated with the packet.

2. Process the packet normally, as if the SA was a transport
mode SA.

3. Ensure that the outgoing ESP protected packet has proper IP
header format, depending on the used IP address family, and
proper IP addresses in its IP header, e.g., by replacing HITs
left by the ESP processing. Note that this placement of proper
IP addresses MAY also be performed at some other point in the
stack, e.g., before ESP processing.

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6.2. Processing Incoming Application Data

Incoming HIP user data packets arrive as ESP protected packets. In
the usual case, the receiving host has a corresponding ESP security
association, identified by the SPI and destination IP address in the
packet. However, if the host has crashed or otherwise lost its HIP
state, it may not have such an SA.

The basic incoming data handling is specified in the HIP base
specification. Additional steps are required when ESP is used for
protecting the data traffic. The following steps define the
conceptual processing rules for incoming ESP protected datagrams
targeted to an ESP security association created with HIP.

1. Detect the proper ESP SA using the SPI. If the resulting SA is a
non-HIP ESP SA, process the packet according to standard IPsec
rules. If there are no SAs identified with the SPI, the host MAY
send an ICMP packet as defined in Section 5.4. How to handle
lost state is an implementation issue.

2. If the SPI matches with an active HIP-based ESP SA, the IP
addresses in the datagram are replaced with the HITs associated
with the SPI. Note that this IP-address-to-HIT conversion step
MAY also be performed at some other point in the stack, e.g.,
after ESP processing. Note also that if the incoming packet has
IPv4 addresses, the packet must be converted to IPv6 format
before replacing the addresses with HITs (such that the transport
checksum will pass if there are no errors).

3. The transformed packet is next processed normally by ESP, as if
the packet were a transport mode packet. The packet may be
dropped by ESP, as usual. In a typical implementation, the
result of successful ESP decryption and verification is a
datagram with the associated HITs as source and destination.

4. The datagram is delivered to the upper layer. Demultiplexing the
datagram to the right upper-layer socket is performed as usual,
except that the HITs are used in place of IP addresses during the
demultiplexing.

6.3. HMAC and SIGNATURE Calculation and Verification

The new HIP parameters described in this document, ESP_INFO and
ESP_TRANSFORM, must be protected using HMAC and signature
calculations. In a typical implementation, they are included in R1,
I2, R2, and UPDATE packet HMAC and SIGNATURE calculations as
described in [RFC7401].

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6.4. Processing Incoming ESP SA Initialization (R1)

The ESP SA setup is initialized in the R1 message. The receiving
host (Initiator) selects one of the ESP transforms from the presented
values. If no suitable value is found, the negotiation is
terminated. The selected values are subsequently used when
generating and using encryption keys, and when sending the reply
packet. If the proposed alternatives are not acceptable to the
system, it may abandon the ESP SA establishment negotiation, or it
may resend the I1 message within the retry bounds.

After selecting the ESP transform and performing other R1
processing, the system prepares and creates an incoming ESP security
association. It may also prepare a security association for outgoing
traffic, but since it does not have the correct SPI value yet, it
cannot activate it.

6.5. Processing Incoming Initialization Reply (I2)

The following steps are required to process the incoming ESP SA
initialization replies in I2. The steps below assume that the I2 has
been accepted for processing (e.g., has not been dropped due to HIT
comparisons as described in [RFC7401]).

o The ESP_TRANSFORM parameter is verified, and it MUST contain a
single value in the parameter; and it MUST match one of the values
offered in the initialization packet.

o The ESP_INFO NEW SPI field is parsed to obtain the SPI that will
be used for the Security Association outbound from the Responder
and inbound to the Initiator. For this initial ESP SA
establishment, the old SPI value MUST be zero. The KEYMAT Index
field MUST contain the index value to the KEYMAT from where the
ESP SA keys are drawn.

o The system prepares and creates both incoming and outgoing ESP
security associations.

o Upon successful processing of the initialization reply message,
the possible old Security Associations (as left over from an
earlier incarnation of the HIP association) are dropped and the
new ones are installed, and a finalizing packet, R2, is sent.
Possible ongoing rekeying attempts are dropped.

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6.6. Processing Incoming ESP SA Setup Finalization (R2)

Before the ESP SA can be finalized, the ESP_INFO NEW SPI field is
parsed to obtain the SPI that will be used for the ESP Security
Association inbound to the sender of the finalization message R2.
The system uses this SPI to create or activate the outgoing ESP
security association used for sending packets to the peer.

6.7. Dropping HIP Associations

When the system drops a HIP association, as described in the HIP base
specification, the associated ESP SAs MUST also be dropped.

6.8. Initiating ESP SA Rekeying

During ESP SA rekeying, the hosts draw new keys from the existing
keying material, or new keying material is generated from where the
new keys are drawn.

A system may initiate the SA rekeying procedure at any time. It MUST
initiate a rekey if its incoming ESP sequence counter is about to
overflow. The system MUST NOT replace its keying material until the
rekeying packet exchange successfully completes.

Optionally, a system may include a new Diffie-Hellman key for use in
new KEYMAT generation. New KEYMAT generation occurs prior to drawing
the new keys.

The rekeying procedure uses the UPDATE mechanism defined in
[RFC7401]. Because each peer must update its half of the security
association pair (including new SPI creation), the rekeying process
requires that each side both send and receive an UPDATE. A system
will then rekey the ESP SA when it has sent parameters to the peer
and has received both an ACK of the relevant UPDATE message and
corresponding peer's parameters. It may be that the ACK and the
required HIP parameters arrive in different UPDATE messages. This is
always true if a system does not initiate an ESP SA update but
responds to an update request from the peer, and may also occur if
two systems initiate update nearly simultaneously. In such a case,
if the system has an outstanding update request, it saves the one
parameter and waits for the other before completing rekeying.

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The following steps define the processing rules for initiating an ESP
SA update:

1. The system decides whether to continue to use the existing KEYMAT
or to generate a new KEYMAT. In the latter case, the system MUST
generate a new Diffie-Hellman public key.

2. The system creates an UPDATE packet, which contains the ESP_INFO
parameter. In addition, the host may include the optional
DIFFIE_HELLMAN parameter. If the UPDATE contains the
DIFFIE_HELLMAN parameter, the KEYMAT Index in the ESP_INFO
parameter MUST be zero, and the Diffie-Hellman Group ID must be
unchanged from that used in the initial handshake. If the UPDATE
does not contain DIFFIE_HELLMAN, the ESP_INFO KEYMAT Index MUST
be greater than or equal to the index of the next byte to be
drawn from the current KEYMAT.

3. The system sends the UPDATE packet. For reliability, the
underlying UPDATE retransmission mechanism MUST be used.

4. The system MUST NOT delete its existing SAs, but continue using
them if its policy still allows. The rekeying procedure SHOULD
be initiated early enough to make sure that the SA replay
counters do not overflow.

5. In case a protocol error occurs and the peer system acknowledges
the UPDATE but does not itself send an ESP_INFO, the system may
not finalize the outstanding ESP SA update request. To guard
against this, a system MAY re-initiate the ESP SA update
procedure after some time waiting for the peer to respond, or it
MAY decide to abort the ESP SA after waiting for an
implementation-dependent time. The system MUST NOT keep an
outstanding ESP SA update request for an indefinite time.

To simplify the state machine, a host MUST NOT generate new UPDATEs
while it has an outstanding ESP SA update request, unless it is
restarting the update process.

6.9. Processing Incoming UPDATE Packets

When a system receives an UPDATE packet, it must be processed if the
following conditions hold (in addition to the generic conditions
specified for UPDATE processing in Section 6.12 of [RFC7401]):

1. A corresponding HIP association must exist. This is usually
ensured by the underlying UPDATE mechanism.

2. The state of the HIP association is ESTABLISHED or R2-SENT.

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If the above conditions hold, the following steps define the
conceptual processing rules for handling the received UPDATE packet:

1. If the received UPDATE contains a DIFFIE_HELLMAN parameter, the
received KEYMAT Index MUST be zero and the Group ID must match
the Group ID in use on the association. If this test fails, the
packet SHOULD be dropped and the system SHOULD log an error
message.

2. If there is no outstanding rekeying request, the packet
processing continues as specified in Section 6.9.1.

3. If there is an outstanding rekeying request, the UPDATE MUST be
acknowledged, the received ESP_INFO (and possibly DIFFIE_HELLMAN)
parameters must be saved, and the packet processing continues as
specified in Section 6.10.

6.9.1. Processing UPDATE Packet: No Outstanding Rekeying Request

The following steps define the conceptual processing rules for
handling a received UPDATE packet with the ESP_INFO parameter:

1. The system consults its policy to see if it needs to generate a
new Diffie-Hellman key, and generates a new key (with same
Group ID) if needed. The system records any newly generated or
received Diffie-Hellman keys for use in KEYMAT generation upon
finalizing the ESP SA update.

2. If the system generated a new Diffie-Hellman key in the previous
step, or if it received a DIFFIE_HELLMAN parameter, it sets the
ESP_INFO KEYMAT Index to zero. Otherwise, the ESP_INFO KEYMAT
Index MUST be greater than or equal to the index of the next byte
to be drawn from the current KEYMAT. In this case, it is
RECOMMENDED that the host use the KEYMAT Index requested by the
peer in the received ESP_INFO.

3. The system creates an UPDATE packet, which contains an ESP_INFO
parameter and the optional DIFFIE_HELLMAN parameter. This UPDATE
would also typically acknowledge the peer's UPDATE with an ACK
parameter, although a separate UPDATE ACK may be sent.

4. The system sends the UPDATE packet and stores any received
ESP_INFO and DIFFIE_HELLMAN parameters. At this point, it only
needs to receive an acknowledgment for the newly sent UPDATE to
finish the ESP SA update. In the usual case, the acknowledgment
is handled by the underlying UPDATE mechanism.

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6.10. Finalizing Rekeying

A system finalizes rekeying when it has both received the
corresponding UPDATE acknowledgment packet from the peer and
successfully received the peer's UPDATE. The following steps
are taken:

1. If the received UPDATE messages contain a new Diffie-Hellman key,
the system has a new Diffie-Hellman key due to initiating an ESP
SA update, or both, the system generates a new KEYMAT. If there
is only one new Diffie-Hellman key, the old existing key is used
as the other key.

2. If the system generated a new KEYMAT in the previous step, it
sets the KEYMAT Index to zero, independent of whether the
received UPDATE included a Diffie-Hellman key or not. If the
system did not generate a new KEYMAT, it uses the greater KEYMAT
Index of the two (sent and received) ESP_INFO parameters.

3. The system draws keys for new incoming and outgoing ESP SAs,
starting from the KEYMAT Index, and prepares new incoming and
outgoing ESP SAs. The SPI for the outgoing SA is the new SPI
value received in an ESP_INFO parameter. The SPI for the
incoming SA was generated when the ESP_INFO was sent to the peer.
The order of the keys retrieved from the KEYMAT during the
rekeying process is similar to that described in Section 7. Note
that only IPsec ESP keys are retrieved during the rekeying
process, not the HIP keys.

4. The system starts to send to the new outgoing SA and prepares to
start receiving data on the new incoming SA. Once the system
receives data on the new incoming SA, it may safely delete the
old SAs.

6.11. Processing NOTIFY Packets

The processing of NOTIFY packets is described in the HIP base
specification.

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7. Keying Material

The keying material is generated as described in the HIP base
specification. During the base exchange, the initial keys are drawn
from the generated material. After the HIP association keys have
been drawn, the ESP keys are drawn in the following order:

SA-gl ESP encryption key for HOST_g's outgoing traffic

SA-gl ESP authentication key for HOST_g's outgoing traffic

SA-lg ESP encryption key for HOST_l's outgoing traffic

SA-lg ESP authentication key for HOST_l's outgoing traffic

HOST_g denotes the host with the greater HIT value, and HOST_l
denotes the host with the lower HIT value. When HIT values are
compared, they are interpreted as positive (unsigned) 128-bit
integers in network byte order.

The four HIP keys are only drawn from KEYMAT during a HIP I1->R2
exchange. Subsequent rekeys using UPDATE will only draw the four ESP
keys from KEYMAT. Section 6.9 describes the rules for reusing or
regenerating KEYMAT based on the rekeying.

The number of bits drawn for a given algorithm is the "natural" size
of the keys, as specified in Section 6.5 of [RFC7401].

8. Security Considerations

In this document, the usage of ESP [RFC4303] between HIP hosts to
protect data traffic is introduced. The security considerations for
ESP are discussed in the ESP specification.

There are different ways to establish an ESP Security Association
between two nodes. This can be done, e.g., using IKE [RFC7296].
This document specifies how the Host Identity Protocol is used to
establish ESP Security Associations.

The following issues are new or have changed from the standard ESP
usage:

o Initial keying material generation

o Updating the keying material

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The initial keying material is generated using the Host Identity
Protocol [RFC7401] using the Diffie-Hellman procedure. This document
extends the usage of the UPDATE packet, defined in the base
specification, to modify existing ESP SAs. The hosts may rekey,
i.e., force the generation of new keying material using the
Diffie-Hellman procedure. The initial setup of ESP SAs between the
hosts is done during the base exchange, and the message exchange is
protected using methods provided by the base exchange. Changes in
connection parameters basically mean that the old ESP SA is removed
and a new one is generated once the UPDATE message exchange has been
completed. The message exchange is protected using the HIP
association keys. Both HMAC and signing of packets are used.

9. IANA Considerations

The following changes to the "Host Identity Protocol (HIP)
Parameters" registries have been made. In all cases, the changes
updated the reference from [RFC5202] to this specification.

This document defines two Parameter Types and two NOTIFY Message
Types for the Host Identity Protocol [RFC7401].

The parameters and their type numbers are defined in Sections 5.1.1
and 5.1.2, and they have been added to the "Parameter Types"
namespace created by [RFC7401]. No new action regarding these values
is required by this specification, other than updating the reference
from [RFC5202] to this specification.

The new NOTIFICATION error types and their values are defined in
Section 5.1.3, and they have been added to the "Notify Message Types"
namespace created by [RFC7401]. No new action regarding these values
is required by this specification, other than updating the reference
from [RFC5202] to this specification.

Section 5.1.2 of this document defines values for "ESP Transform
Suite IDs", which are registered in a new IANA registry, with an
"IETF Review" registration procedure [RFC5226] for new values.

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10. References

10.1. Normative References

[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997,
<http://www.rfc-editor.org/info/rfc2119>.

[RFC2404] Madson, C. and R. Glenn, "The Use of HMAC-SHA-1-96 within
ESP and AH", RFC 2404, November 1998,
<http://www.rfc-editor.org/info/rfc2404>.

[RFC2410] Glenn, R. and S. Kent, "The NULL Encryption Algorithm and
Its Use With IPsec", RFC 2410, November 1998,
<http://www.rfc-editor.org/info/rfc2410>.

[RFC3602] Frankel, S., Glenn, R., and S. Kelly, "The AES-CBC Cipher
Algorithm and Its Use with IPsec", RFC 3602,
September 2003, <http://www.rfc-editor.org/info/rfc3602>.

[RFC4106] Viega, J. and D. McGrew, "The Use of Galois/Counter Mode
(GCM) in IPsec Encapsulating Security Payload (ESP)",
RFC 4106, June 2005, <http://www.rfc-editor.org/
info/rfc4106>.

[RFC4303] Kent, S., "IP Encapsulating Security Payload (ESP)",
RFC 4303, 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, December 2005, <http://www.rfc-editor.org/
info/rfc4309>.

[RFC4493] Song, JH., Poovendran, R., Lee, J., and T. Iwata, "The
AES-CMAC Algorithm", RFC 4493, June 2006,
<http://www.rfc-editor.org/info/rfc4493>.

[RFC4494] Song, JH., Poovendran, R., and J. Lee, "The AES-CMAC-96
Algorithm and Its Use with IPsec", RFC 4494, June 2006,
<http://www.rfc-editor.org/info/rfc4494>.

[RFC4543] McGrew, D. and J. Viega, "The Use of Galois Message
Authentication Code (GMAC) in IPsec ESP and AH", RFC 4543,
May 2006, <http://www.rfc-editor.org/info/rfc4543>.

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[RFC4868] Kelly, S. and S. Frankel, "Using HMAC-SHA-256,
HMAC-SHA-384, and HMAC-SHA-512 with IPsec", RFC 4868,
May 2007, <http://www.rfc-editor.org/info/rfc4868>.

[RFC7401] Moskowitz, R., Ed., Heer, T., Jokela, P., and T.
Henderson, "Host Identity Protocol Version 2 (HIPv2)",
RFC 7401, April 2015, <http://www.rfc-editor.org/
info/rfc7401>.

10.2. Informative References

[HIP-ARCH] Moskowitz, R., Ed., and M. Komu, "Host Identity Protocol
Architecture", Work in Progress,
draft-ietf-hip-rfc4423-bis-09, October 2014.

[RFC0791] Postel, J., "Internet Protocol", STD 5, RFC 791,
September 1981, <http://www.rfc-editor.org/info/rfc791>.

[RFC4301] Kent, S. and K. Seo, "Security Architecture for the
Internet Protocol", RFC 4301, December 2005,
<http://www.rfc-editor.org/info/rfc4301>.

[RFC5202] Jokela, P., Moskowitz, R., and P. Nikander, "Using the
Encapsulating Security Payload (ESP) Transport Format with
the Host Identity Protocol (HIP)", RFC 5202, April 2008,
<http://www.rfc-editor.org/info/rfc5202>.

[RFC5206] Nikander, P., Henderson, T., Vogt, C., and J. Arkko,
"End-Host Mobility and Multihoming with the Host Identity
Protocol", RFC 5206, April 2008,
<http://www.rfc-editor.org/info/rfc5206>.

[RFC5207] Stiemerling, M., Quittek, J., and L. Eggert, "NAT and
Firewall Traversal Issues of Host Identity Protocol (HIP)
Communication", RFC 5207, April 2008,
<http://www.rfc-editor.org/info/rfc5207>.

[RFC5226] Narten, T. and H. Alvestrand, "Guidelines for Writing an
IANA Considerations Section in RFCs", BCP 26, RFC 5226,
May 2008, <http://www.rfc-editor.org/info/rfc5226>.

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RFC 7402 Using the ESP Transport Format with HIP April 2015

[RFC5770] Komu, M., Henderson, T., Tschofenig, H., Melen, J., and A.
Keranen, "Basic Host Identity Protocol (HIP) Extensions
for Traversal of Network Address Translators", RFC 5770,
April 2010, <http://www.rfc-editor.org/info/rfc5770>.

[RFC7296] Kaufman, C., Hoffman, P., Nir, Y., Eronen, P., and T.
Kivinen, "Internet Key Exchange Protocol Version 2
(IKEv2)", STD 79, RFC 7296, October 2014,
<http://www.rfc-editor.org/info/rfc7296>.

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Appendix A. A Note on Implementation Options

It is possible to implement this specification in multiple different
ways. As noted above, one possible way of implementing this is to
rewrite IP headers below IPsec. In such an implementation, IPsec is
used as if it was processing IPv6 transport mode packets, with the
IPv6 header containing HITs instead of IP addresses in the source and
destination address fields. In outgoing packets, after IPsec
processing, the HITs are replaced with actual IP addresses, based on
the HITs and the SPI. In incoming packets, before IPsec processing,
the IP addresses are replaced with HITs, based on the SPI in the
incoming packet. In such an implementation, all IPsec policies are
based on HITs and the upper layers only see packets with HITs in the
place of IP addresses. Consequently, support of HIP does not
conflict with other uses of IPsec as long as the SPI spaces are kept
separate. Appendix B describes another way to implement this
specification.

Appendix B. Bound End-to-End Tunnel Mode for ESP

This section introduces an alternative way of implementing the
necessary functions for HIP ESP transport. Compared to the option of
implementing the required address rewrites outside of IPsec, BEET has
one implementation-level benefit. In a BEET-mode-based
implementation, the address-rewriting information is kept in one
place, at the SAD. On the other hand, when address rewriting is
implemented separately, the implementation MUST make sure that the
information in the SAD and the information in the separate
address-rewriting database are kept in synchrony. As a result, the
BEET-mode-based way of implementing this specification is RECOMMENDED
over the separate implementation, as it binds the identities,
encryption, and locators tightly together. It should be noted that
implementing BEET mode doesn't require that corresponding hosts
implement it, as the behavior is only visible internally in a host.

BEET mode is a combination of IPsec tunnel and transport modes, and
it provides some of the features from both. HIP uses HITs as the
"inner" addresses and IP addresses as "outer" addresses, like IP
addresses are used in tunnel mode. Instead of tunneling packets
between hosts, a conversion between inner and outer addresses is made
at end hosts, and the inner address is never sent on the wire after
the initial HIP negotiation. BEET provides IPsec transport mode
syntax (no inner headers) with limited tunnel mode semantics (fixed
logical inner addresses -- the HITs -- and changeable outer IP
addresses).

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B.1. Protocol Definition

In this section, we define the exact protocol formats and operations.

B.1.1. Changes to Security Association Data Structures

A BEET mode Security Association contains the same data as a regular
tunnel mode Security Association, with the exception that the inner
selectors must be single addresses and cannot be subnets. The data
includes the following:

o A pair of inner IP addresses.

o A pair of outer IP addresses.

o Cryptographic keys and other data as defined in Section 4.4.2 of
RFC 4301 [RFC4301].

A conforming implementation MAY store the data in a way similar to a
regular tunnel mode Security Association.

Note that in a conforming implementation the inner and outer
addresses MAY belong to different address families. All
implementations that support both IPv4 and IPv6 SHOULD support both
IPv4-over-IPv6 and IPv6-over-IPv4 tunneling.

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B.1.2. Packet Format

The wire packet format is identical to the ESP transport mode wire
format as defined in Section 3.1.1 of [RFC4303]. However, the
resulting packet contains outer IP addresses instead of the inner IP
addresses received from the upper layer. The construction of the
outer headers is defined in Section 5.1.2 of RFC 4301 [RFC4301]. The
following diagram illustrates ESP BEET mode positioning for typical
IPv4 and IPv6 packets.

IPv4 INNER ADDRESSES
--------------------

BEFORE APPLYING ESP
------------------------------
| inner IP hdr | | |
| | TCP | Data |
------------------------------

AFTER APPLYING ESP, OUTER v4 ADDRESSES
----------------------------------------------------
| outer IP hdr | | | | ESP | ESP |
| (any options) | ESP | TCP | Data | Trailer | ICV |
----------------------------------------------------
|<---- encryption ---->|
|<-------- integrity ------->|

AFTER APPLYING ESP, OUTER v6 ADDRESSES
------------------------------------------------------
| outer | new ext | | | | ESP | ESP |
| IP hdr | hdrs | ESP | TCP | Data | Trailer| ICV |
------------------------------------------------------
|<--- encryption ---->|
|<------- integrity ------->|

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IPv4 INNER ADDRESSES with options
---------------------------------

BEFORE APPLYING ESP
------------------------------
| inner IP hdr | | |
| + options | TCP | Data |
------------------------------

AFTER APPLYING ESP, OUTER v4 ADDRESSES
----------------------------------------------------------
| outer IP hdr | | | | | ESP | ESP |
| (any options) | ESP | PH | TCP | Data | Trailer | ICV |
----------------------------------------------------------
|<------- encryption ------->|
|<----------- integrity ---------->|

AFTER APPLYING ESP, OUTER v6 ADDRESSES
------------------------------------------------------------
| outer | new ext | | | | | ESP | ESP |
| IP hdr | hdrs | ESP | PH | TCP | Data | Trailer| ICV |
------------------------------------------------------------
|<------ encryption ------->|
|<---------- integrity ---------->|

PH Pseudo Header for IPv4 options

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IPv6 INNER ADDRESSES
--------------------

BEFORE APPLYING ESP
------------------------------------------
| | ext hdrs | | |
| inner IP hdr | if present | TCP | Data |
------------------------------------------

AFTER APPLYING ESP, OUTER v6 ADDRESSES
--------------------------------------------------------------
| outer | new ext | | dest | | | ESP | ESP |
| IP hdr | hdrs | ESP | opts.| TCP | Data | Trailer | ICV |
--------------------------------------------------------------
|<---- encryption ---->|
|<------- integrity ------>|

AFTER APPLYING ESP, OUTER v4 ADDRESSES
----------------------------------------------------
| outer | | dest | | | ESP | ESP |
| IP hdr | ESP | opts.| TCP | Data | Trailer | ICV |
----------------------------------------------------
|<------- encryption -------->|
|<----------- integrity ----------->|

B.1.3. Cryptographic Processing

The outgoing packets MUST be protected exactly as in ESP transport
mode [RFC4303]. That is, the upper-layer protocol packet is wrapped
into an ESP header, encrypted, and authenticated exactly as if
regular transport mode was used. The resulting ESP packet is subject
to IP header processing as defined in Appendices B.1.4 and B.1.5.
The incoming ESP protected messages are verified and decrypted
exactly as if regular transport mode was used. The resulting
cleartext packet is subject to IP header processing as defined in
Appendices B.1.4 and B.1.6.

B.1.4. IP Header Processing

The biggest difference between BEET mode and the other two modes is
in IP header processing. In the regular transport mode, the IP
header is kept intact. In the regular tunnel mode, an outer IP
header is created on output and discarded on input. In BEET mode,
the IP header is replaced with another one on both input and output.

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On the BEET mode output side, the IP header processing MUST first
ensure that the IP addresses in the original IP header contain the
inner addresses as specified in the SA. This MAY be ensured by
proper policy processing, and it is possible that no checks are
needed at the time of SA processing. Once the IP header has been
verified to contain the right IP inner addresses, it is discarded. A
new IP header is created, using the fields of the discarded inner
header (except the IP addresses) to populate the fields of the new
outer header. The IP addresses in the new header MUST be the outer
tunnel addresses.

On the input side, the received IP header is simply discarded. Since
the packet has been decrypted and verified, no further checks are
necessary. A new IP header corresponding to a BEET mode inner header
is created, using the fields of the discarded outer header (except
the IP addresses) to populate the fields of the new inner header.
The IP addresses in the new header MUST be the inner addresses.

As the outer header fields are used as a hint for creating the inner
header, it must be noted that the inner header differs as compared to
a tunnel mode inner header. In BEET mode, the inner header will have
the Time to Live (TTL), Don't Fragment (DF) bit, and other option
values from the outer header. The TTL, DF bit, and other option
values of the inner header MUST be processed by the stack.

B.1.5. Handling of Outgoing Packets

The outgoing BEET mode packets are processed as follows:

1. The system MUST verify that the IP header contains the inner
source and destination addresses, exactly as defined in the SA.
This verification MAY be explicit, or it MAY be implicit, for
example, as a result of prior policy processing. Note that in
some implementations there may be no real IP header at this time
but the source and destination addresses may be carried out of
band. If the source address is still unassigned, it SHOULD be
ensured that the designated inner source address would be
selected at a later stage.

2. The IP payload (the contents of the packet beyond the IP header)
is wrapped into an ESP header as defined in Section 3.3 of
[RFC4303].

3. A new IP header is constructed, replacing the original one. The
new IP header MUST contain the outer source and destination
addresses, as defined in the SA. Note that in some
implementations there may be no real IP header at this time but
the source and destination addresses may be carried out of band.

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In the case where the source address must be left unassigned, it
SHOULD be ensured that the right source address is selected at a
later stage. Other than the addresses, it is RECOMMENDED that
the new IP header copies the fields from the original IP header.

4. If there are any IPv4 options in the original packet, it is
RECOMMENDED that they are discarded. If the inner header
contains one or more options that need to be transported between
the tunnel endpoints, the sender MUST encapsulate the options as
defined in Appendix B.1.7.

Instead of literally discarding the IP header and constructing a new
one, a conforming implementation MAY simply replace the addresses in
an existing header. However, if the RECOMMENDED feature of allowing
the inner and outer addresses from different address families is
used, this simple strategy does not work.

B.1.6. Handling of Incoming Packets

The incoming BEET mode packets are processed as follows:

1. The system MUST verify and decrypt the incoming packet
successfully, as defined in Section 3.4 of [RFC4303]. If the
verification or decryption fails, the packet MUST be discarded.

2. The original IP header is simply discarded, without any checks.
Since the ESP verification succeeded, the packet can be safely
assumed to have arrived from the right sender.

3. A new IP header is constructed, replacing the original one. The
new IP header MUST contain the inner source and destination
addresses, as defined in the SA. If the sender has set the ESP
Next Header field to 94 and included the pseudo header as
described in Appendix B.1.7, the receiver MUST include the
options after the constructed IP header. Note that in some
implementations the real IP header may have already been
discarded and the source and destination addresses are carried
out of band. In such a case, the out-of-band addresses MUST be
the inner addresses. Other than the addresses, it is RECOMMENDED
that the new IP header copies the fields from the original IP
header.

Instead of literally discarding the IP header and constructing a new
one, a conforming implementation MAY simply replace the addresses in
an existing header. However, if the RECOMMENDED feature of allowing
the inner and outer addresses from different address families is
used, this simple strategy does not work.

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B.1.7. Handling of IPv4 Options

In BEET mode, if IPv4 options are transported inside the tunnel, the
sender MUST include a pseudo header after the ESP header. The
pseudo header indicates that IPv4 options from the original packet
are to be applied to the packet on the input side.

The sender MUST set the Next Header field in the ESP header to 94.
The resulting pseudo header, including the IPv4 options, MUST be
padded to an 8-octet boundary. The padding length is expressed in
octets; valid padding lengths are 0 or 4 octets, as the original IPv4
options are already padded to a 4-octet boundary. The padding MUST
be filled with No Operation (NOP) options as defined in Section 3.1
("Internet Header Format") of [RFC0791] ("Internet Protocol"). The
padding is added in front of the original options to ensure that the
receiver is able to reconstruct the original IPv4 datagram. The
Header Length field contains the length of the IPv4 options, and
padding in 8-octet units.

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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Next Header | Header Len | Pad Len | Reserved |
+---------------+---------------+-------------------------------+
| Padding (if needed) |
+---------------------------------------------------------------+
| IPv4 options ... |
| |
+---------------------------------------------------------------+

Next Header identifies the data following this header.
Length in octets 8-bit unsigned integer. Length of the
pseudo header in 8-octet units, not
including the first 8 octets.

The receiver MUST remove this pseudo header and padding as a part of
BEET processing, in order to reconstruct the original IPv4 datagram.
The IPv4 options included in the pseudo header MUST be added after
the reconstructed IPv4 (inner) header on the receiving side.

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Acknowledgments

This document was separated from the base Host Identity Protocol
specification in the beginning of 2005. Since then, a number of
people have contributed to the text by providing comments and
modification proposals. The list of people includes Tom Henderson,
Jeff Ahrenholz, Jan Melen, Jukka Ylitalo, and Miika Komu.
Especially, the authors want to thank Pekka Nikander for his
invaluable contributions to the document since the first draft
version. The authors also want to thank Charlie Kaufman for
reviewing the document with his eye on the usage of crypto
algorithms.

Due to the history of this document, most of the ideas are inherited
from the base Host Identity Protocol specification. Thus, the list
of people in the Acknowledgments section of that specification is
also valid for this document. Many people have given valuable
feedback, and our apologies to anyone whose name is missing.

Authors' Addresses

Petri Jokela
Ericsson Research NomadicLab
JORVAS FIN-02420
Finland

Phone: +358 9 299 1
EMail: [email protected]

Robert Moskowitz
HTT Consulting
Oak Park, MI
United States

EMail: [email protected]

Jan Melen
Ericsson Research NomadicLab
JORVAS FIN-02420
Finland

Phone: +358 9 299 1
EMail: [email protected]

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