Internet Engineering Task Force (IETF) T. Henderson, Ed.

Internet Engineering Task Force (IETF) T. Henderson, Ed.
Request for Comments: 8047 University of Washington
Category: Standards Track C. Vogt
ISSN: 2070-1721 Independent
J. Arkko
Ericsson
February 2017

Host Multihoming with the Host Identity Protocol

Abstract

This document defines host multihoming extensions to the Host
Identity Protocol (HIP), by leveraging protocol components defined
for host mobility.

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 7841.

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/rfc8047.

Copyright Notice

Copyright (c) 2017 IETF Trust and the persons identified as the
document authors. All rights reserved.

This document is subject to BCP 78 and the IETF Trust's Legal
Provisions Relating to IETF Documents
(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 and Scope . . . . . . . . . . . . . . . . . . . 3
2. Terminology and Conventions . . . . . . . . . . . . . . . . . 4
3. Protocol Model . . . . . . . . . . . . . . . . . . . . . . . 4
4. Protocol Overview . . . . . . . . . . . . . . . . . . . . . . 4
4.1. Background . . . . . . . . . . . . . . . . . . . . . . . 5
4.2. Usage Scenarios . . . . . . . . . . . . . . . . . . . . . 6
4.2.1. Multiple Addresses . . . . . . . . . . . . . . . . . 6
4.2.2. Multiple Security Associations . . . . . . . . . . . 6
4.2.3. Host Multihoming for Fault Tolerance . . . . . . . . 7
4.2.4. Host Multihoming for Load Balancing . . . . . . . . . 9
4.2.5. Site Multihoming . . . . . . . . . . . . . . . . . . 10
4.2.6. Dual-Host Multihoming . . . . . . . . . . . . . . . . 10
4.2.7. Combined Mobility and Multihoming . . . . . . . . . . 11
4.2.8. Initiating the Protocol in R1, I2, or R2 . . . . . . 11
4.2.9. Using LOCATOR_SETs across Addressing Realms . . . . . 13
4.3. Interaction with Security Associations . . . . . . . . . 13
5. Processing Rules . . . . . . . . . . . . . . . . . . . . . . 14
5.1. Sending LOCATOR_SETs . . . . . . . . . . . . . . . . . . 14
5.2. Handling Received LOCATOR_SETs . . . . . . . . . . . . . 16
5.3. Verifying Address Reachability . . . . . . . . . . . . . 18
5.4. Changing the Preferred Locator . . . . . . . . . . . . . 18
6. Security Considerations . . . . . . . . . . . . . . . . . . . 19
7. References . . . . . . . . . . . . . . . . . . . . . . . . . 21
7.1. Normative References . . . . . . . . . . . . . . . . . . 21
7.2. Informative References . . . . . . . . . . . . . . . . . 21
Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . . . 22
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 22

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1. Introduction and Scope

The Host Identity Protocol (HIP) [RFC7401] supports an architecture
that decouples the transport layer (TCP, UDP, etc.) from the
internetworking layer (IPv4 and IPv6) by using public/private key
pairs, instead of IP addresses, as host identities. When a host uses
HIP, the overlying protocol sublayers (e.g., transport-layer sockets
and Encapsulating Security Payload (ESP) Security Associations (SAs))
are instead bound to representations of these host identities, and
the IP addresses are only used for packet forwarding. However, each
host must also know at least one IP address at which its peers are
reachable. Initially, these IP addresses are the ones used during
the HIP base exchange.

One consequence of such a decoupling is that new solutions to
network-layer mobility and host multihoming are possible. Basic host
mobility is defined in [RFC8046] and covers the case in which a host
has a single address and changes its network point of attachment
while desiring to preserve the HIP-enabled security association.
Host multihoming is somewhat of a dual case to host mobility, in
that, a host may simultaneously have more than one network point of
attachment. There are potentially many variations of host
multihoming possible. [RFC8046] specifies the format of the HIP
parameter (LOCATOR_SET parameter) used to convey IP addressing
information between peers, the procedures for sending and processing
this parameter to enable basic host mobility, and procedures for an
address verification mechanism. The scope of this document
encompasses messaging and elements of procedure for some basic host
multihoming scenarios of interest.

Another variation of multihoming that has been heavily studied is
site multihoming. Solutions for host multihoming in multihomed IPv6
networks have been specified by the IETF shim6 working group. The
Shim6 protocol [RFC5533] bears many architectural similarities to
HIP, but there are differences in the security model and in the
protocol.

While HIP can potentially be used with transports other than the ESP
transport format [RFC7402], this document largely assumes the use of
ESP and leaves other transport formats for further study.

Finally, making underlying IP multihoming transparent to the
transport layer has implications on the proper response of transport
congestion control, path MTU selection, and Quality of Service (QoS).
Transport-layer mobility triggers, and the proper transport response
to a HIP multihoming address change, are outside the scope of this
document.

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This specification relies on implementing Sections 4 ("LOCATOR_SET
Parameter Format") and 5 ("Processing Rules") of [RFC8046] as a
starting point for this implementation.

2. Terminology and Conventions

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].

The following terms used in this document are defined in [RFC8046]:
LOCATOR_SET, Locator, locator, Address, preferred locator, and
Credit-Based Authorization.

3. Protocol Model

The protocol model for HIP support of host multihoming extends the
model for host mobility described in Section 3 of [RFC8046]. This
section only highlights the differences.

In host multihoming, a host has multiple locators simultaneously
rather than sequentially, as in the case of mobility. By using the
LOCATOR_SET parameter defined in [RFC8046], a host can inform its
peers of additional (multiple) locators at which it can be reached.
When multiple locators are available and announced to the peer, a
host can designate a particular locator as a "preferred" locator,
meaning that the host prefers that its peer send packets to the
designated address before trying an alternative address. Although
this document defines a basic mechanism for multihoming, it does not
define all possible policies and procedures, such as which locators
to choose when more than one is available, the operation of
simultaneous mobility and multihoming, source address selection
policies (beyond those specified in [RFC6724]), and the implications
of multihoming on transport protocols.

4. Protocol Overview

In this section, we briefly introduce a number of usage scenarios for
HIP multihoming. These scenarios assume that HIP is being used with
the ESP transport [RFC7402], although other scenarios may be defined
in the future. To understand these usage scenarios, the reader
should be at least minimally familiar with the HIP protocol
specification [RFC7401], the use of the ESP transport format
[RFC7402], and the HIP mobility specification [RFC8046]. However,
for the (relatively) uninitiated reader, it is most important to keep
in mind that in HIP, the actual payload traffic is protected with
ESP, and that the ESP Security Parameter Index (SPI) acts as an index
to the right host-to-host context.

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4.1. Background

The multihoming scenarios can be explained in contrast to the
non-multihoming case described in the base protocol specification
[RFC7401]. We review the pertinent details here. In the base
specification, when used with the ESP transport format, the HIP base
exchange will set up a single SA in each direction. The IP addresses
associated with the SAs are the same as those used to convey the HIP
packets. For data traffic, a security policy database (SPD) and
security association database (SAD) will likely exist, following the
IPsec architecture. One distinction between HIP and IPsec, however,
is that the host IDs, and not the IP addresses, are conceptually used
as selectors in the SPD. In the outbound direction, as a result of
SPD processing, when an outbound SA is selected, the correct IP
destination address for the peer must also be assigned. Therefore,
outbound SAs are conceptually associated with the peer IP address
that must be used as the destination IP address below the HIP layer.
In the inbound direction, the IP addresses may be used as selectors
in the SAD to look up the SA, but they are not strictly required; the
ESP SPI may be used alone. To summarize, in the non-multihoming
case, there is only one source IP address, one destination IP
address, one inbound SA, and one outbound SA.

The HIP readdressing protocol [RFC8046] is an asymmetric protocol in
which a mobile or multihomed host informs a peer host about changes
of IP addresses on affected SPIs. IP address and ESP SPI information
is carried in Locator fields in a HIP parameter called a LOCATOR_SET.
The HIP mobility specification [RFC8046] describes how the
LOCATOR_SET is carried in a HIP UPDATE packet.

To summarize the mobility elements of procedure, as background for
multihoming, the basic idea of host mobility is to communicate a
local IP address change to the peer when active HIP-maintained SAs
are in use. To do so, the IP address must be conveyed, any
association between the IP address and an inbound SA (via the SPI
index) may be conveyed, and protection against flooding attacks must
be ensured. The association of an IP address with an SPI is
performed by a Locator Type of "1", which is a concatenation of an
ESP SPI with an IP address.

An address verification method is specified in [RFC8046]. It is
expected that addresses learned in multihoming scenarios also are
subject to the same verification rules. At times, the scenarios
describe addresses as being in either an ACTIVE, VERIFIED, or
DEPRECATED state. From the perspective of a host, newly learned
addresses of the peer must be verified before put into active

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service, and addresses removed by the peer are put into a deprecated
state. Under limited conditions described in [RFC8046], an
UNVERIFIED address may be used.

With this background, we next describe an additional protocol to
facilitate scenarios in which one or both hosts have multiple IP
addresses available. Increasingly, this is the common case with
network-connected hosts on the Internet.

4.2. Usage Scenarios

4.2.1. Multiple Addresses

Hosts may have multiple IP addresses within different address
families (IPv4 and IPv6) and scopes available to support HIP
messaging and HIP-enabled SAs. The multiple addresses may be on a
single network interface or multiple network interfaces. It is
outside of the scope of this document to specify how a host decides
which of possibly multiple addresses may be used to support a HIP
association. Some IP addresses may be held back from usage due to
privacy, security, or cost considerations.

When multiple IP addresses are shared with a peer, the procedures
described in the HIP mobility specification [RFC8046] allow for a
host to set a preferred locator ("P") bit, requesting that one of the
multiple addresses be preferred for control- or data-plane traffic.
It is also permitted to leave the preferred bit unset for all
addresses, allowing the peer to make address selection decisions.

Hosts that use link-local addresses as source addresses in their HIP
handshakes may not be reachable by a mobile peer. Such hosts SHOULD
provide a globally routable address either in the initial handshake
or via the LOCATOR_SET parameter.

To support mobility, as described in the HIP mobility specification
[RFC8046], the LOCATOR_SET may be sent in a HIP UPDATE packet. To
support multihoming, the LOCATOR_SET may also be sent in R1, I2, or
R2 packets defined in the HIP protocol specification [RFC7401]. The
reason to consider sending LOCATOR_SET parameters in base exchange
packets is to convey all usable addresses for fault-tolerance or
load-balancing considerations.

4.2.2. Multiple Security Associations

When multiple addresses are available between peer hosts, a question
that arises is whether to use one or multiple SAs. The intent of
this specification is to support different use cases but to leave the
policy decision to the hosts.

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When one host has n addresses and the other host has m addresses, it
is possible to set up as many as (n * m) SAs in each direction. In
such a case, every combination of source and destination IP addresses
would have a unique SA, and the possibility of the reordering of
datagrams on each SA will be lessened (ESP SAs may have an anti-
replay window [RFC4303] sensitive to reordering). However, the
downside to creating a mesh of SAs is the signaling overhead required
(for exchanging UPDATE messages conveying ESP_INFO parameters) and
the state maintenance required in the SPD/SAD.

For load balancing, when multiple paths are to be used in parallel,
it may make sense to create different SAs for different paths. In
this use case, while a full mesh of 2 * (n * m) SAs may not be
required, it may be beneficial to create one SA pair per load-
balanced path to avoid anti-replay window issues.

For fault tolerance, it is more likely that a single SA and multiple
IP addresses associated with that SA can be used, and the alternative
addresses can be used only upon failure detection of the addresses in
use. Techniques for path failure detection are outside the scope of
this specification. An implementation may use ICMP interactions,
reachability checks, or other means to detect the failure of a
locator.

In summary, whether and how a host decides to leverage additional
addresses in a load-balancing or fault-tolerant manner is outside the
scope of the specification (although the academic literature on
multipath TCP schedulers may provide guidance on how to design such a
policy). However, in general, this document recommends that for
fault tolerance, it is likely sufficient to use a single SA pair for
all addresses, and for load balancing, to support a different SA pair
for all active paths being balanced across.

4.2.3. Host Multihoming for Fault Tolerance

A (mobile or stationary) host may have more than one interface or
global address. The host may choose to notify the peer host of the
additional interface or address by using the LOCATOR_SET parameter.
The LOCATOR_SET parameter may be included in an I2, R1, or R2 packet,
or it may be conveyed, after the base exchange completes in an UPDATE
packet.

When more than one locator is provided to the peer host, the host MAY
indicate which locator is preferred (the locator on which the host
prefers to receive traffic). By default, the address that a host
uses in the base exchange is its preferred locator (for the address

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family and address scope in use during the base exchange) until
indicated otherwise. It may be the case that the host does not
express any preferred locators.

In the multihoming case, the sender may also have multiple valid
locators from which to source traffic. In practice, a HIP
association in a multihoming configuration may have both a preferred
peer locator and a preferred local locator. The host should try to
use the peer's preferred locator unless policy or other circumstances
prevent such usage. A preferred local locator may be overridden if
source address selection rules on the destination address (peer's
preferred locator) suggest the use of a different source address.

Although the protocol may allow for configurations in which there is
an asymmetric number of SAs between the hosts (e.g., one host has two
interfaces and two inbound SAs, while the peer has one interface and
one inbound SA), it is suggested that inbound and outbound SAs be
created pairwise between hosts. When an ESP_INFO arrives to rekey a
particular outbound SA, the corresponding inbound SA should also be
rekeyed at that time. Section 4.3 discusses the interaction between
addresses and security associations in more detail.

Consider the case of two hosts, one single-homed and one multihomed.
The multihomed host may decide to inform the single-homed host about
its other address(es). It may choose to do so as follows.

If the multihomed host wishes to convey the additional address(es)
for fault tolerance, it should include all of its addresses in
Locator fields, indicating the Traffic Type, Locator Type, and
whether the locator is a preferred locator. If it wishes to bind any
particular address to an existing SPI, it may do so by using a
Locator Type of "1" as specified in the HIP mobility specification
[RFC8046]. It does not need to rekey the existing SA or request
additional SAs at this time.

Figure 1 illustrates this scenario. Note that the conventions for
message parameter notations in figures (use of parentheses and
brackets) is defined in Section 2.2 of [RFC7401].

Multihomed Host Peer Host

UPDATE(LOCATOR_SET, SEQ)
----------------------------------->
UPDATE(ACK)
<-----------------------------------

Figure 1: Basic Multihoming Scenario

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In this scenario, the peer host associates the multiple addresses
with the SA pair between it and the multihomed host. It may also
undergo address verification procedures to transition the addresses
to ACTIVE state. For inbound data traffic, it may choose to use the
addresses along with the SPI as selectors. For outbound data
traffic, it must choose among the available addresses of the
multihomed host, considering the state of address verification
[RFC8046] of each address, and also considering available information
about whether an address is in a working state.

4.2.4. Host Multihoming for Load Balancing

A multihomed host may decide to set up new SA pairs corresponding to
new addresses, for the purpose of load balancing. The decision to
load balance and the mechanism for splitting load across multiple SAs
is out of scope of this document. The scenario can be supported by
sending the LOCATOR_SET parameter with one or more ESP_INFO
parameters to initiate new ESP SAs. To do this, the multihomed host
sends a LOCATOR_SET with an ESP_INFO, indicating the request for a
new SA by setting the OLD SPI value to zero and the NEW SPI value to
the newly created incoming SPI. A Locator Type of "1" is used to
associate the new address with the new SPI. The LOCATOR_SET
parameter also contains a second Type "1" Locator, that of the
original address and SPI. To simplify parameter processing and avoid
explicit protocol extensions to remove locators, each LOCATOR_SET
parameter MUST list all locators in use on a connection (a complete
listing of inbound locators and SPIs for the host). The multihomed
host waits for a corresponding ESP_INFO (new outbound SA) from the
peer and an ACK of its own UPDATE. As in the mobility case, the peer
host must perform an address verification before actively using the
new address.

Figure 2 illustrates this scenario.

Multihomed Host Peer Host

UPDATE(ESP_INFO, LOCATOR_SET, SEQ, [DIFFIE_HELLMAN])
----------------------------------->
UPDATE(ESP_INFO, SEQ, ACK, [DIFFIE_HELLMAN,] ECHO_REQUEST)
<-----------------------------------
UPDATE(ACK, ECHO_RESPONSE)
----------------------------------->

Figure 2: Host Multihoming for Load Balancing

In multihoming scenarios, it is important that hosts receiving
UPDATEs associate them correctly with the destination address used in
the packet carrying the UPDATE. When processing inbound LOCATOR_SETs

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that establish new security associations on an interface with
multiple addresses, a host uses the destination address of the UPDATE
containing the LOCATOR_SET as the local address to which the
LOCATOR_SET plus ESP_INFO is targeted. This is because hosts may
send UPDATEs with the same (locator) IP address to different peer
addresses -- this has the effect of creating multiple inbound SAs
implicitly affiliated with different peer source addresses.

4.2.5. Site Multihoming

A host may have an interface that has multiple globally routable IP
addresses. Such a situation may be a result of the site having
multiple upper Internet Service Providers, or just because the site
provides all hosts with both IPv4 and IPv6 addresses. The host
should stay reachable at all or any subset of the currently available
global routable addresses, independent of how they are provided.

This case is handled the same as if there were different IP
addresses, described above in Sections 4.2.3 and 4.2.4. Note that a
single interface may have addresses corresponding to site multihoming
while the host itself may also have multiple network interfaces.

Note that a host may be multihomed and mobile simultaneously, and
that a multihomed host may want to protect the location of some of
its interfaces while revealing the real IP address of some others.

This document does not present additional site multihoming extensions
to HIP; such extensions are for further study.

4.2.6. Dual-Host Multihoming

Consider the case in which both hosts are multihomed and would like
to notify the peer of an additional address after the base exchange
completes. It may be the case that both hosts choose to simply
announce the second address in a LOCATOR_SET parameter using an
UPDATE message exchange. It may also be the case that one or both
hosts decide to ask for new SA pairs to be created using the newly
announced address. In the case that both hosts request this, the
result will be a full mesh of SAs as depicted in Figure 3. In such a
scenario, consider that host1, which used address addr1a in the base
exchange to set up SPI1a and SPI2a, wants to add address addr1b. It
would send an UPDATE with LOCATOR_SET (containing the address addr1b)
to host2, using destination address addr2a, and a new ESP_INFO, and a
new set of SPIs would be added between hosts 1 and 2 (call them SPI1b
and SPI2b; not shown in the figure). Next, consider host2 deciding
to add addr2b to the relationship. Host2 must select one of host1's
addresses towards which to initiate an UPDATE. It may choose to
initiate an UPDATE to addr1a, addr1b, or both. If it chooses to send

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to both, then a full mesh (four SA pairs) of SAs would exist between
the two hosts. This is the most general case; the protocol is
flexible enough to accommodate this choice.

-<- SPI1a -- -- SPI2a ->-
host1 < > addr1a <---> addr2a < > host2
->- SPI2a -- -- SPI1a -<-

addr1b <---> addr2a (second SA pair)
addr1a <---> addr2b (third SA pair)
addr1b <---> addr2b (fourth SA pair)

Figure 3: Dual-Multihoming Case in which Each Host Uses LOCATOR_SET
to Add a Second Address

4.2.7. Combined Mobility and Multihoming

Mobile hosts may be simultaneously mobile and multihomed, i.e., have
multiple mobile interfaces. Furthermore, if the interfaces use
different access technologies, it is fairly likely that one of the
interfaces may appear stable (retain its current IP address) while
some others may experience mobility (undergo IP address change).

The use of LOCATOR_SET plus ESP_INFO should be flexible enough to
handle most such scenarios, although more complicated scenarios have
not been studied so far.

4.2.8. Initiating the Protocol in R1, I2, or R2

A Responder host MAY include a LOCATOR_SET parameter in the R1 packet
that it sends to the Initiator. This parameter MUST be protected by
the R1 signature. If the R1 packet contains LOCATOR_SET parameters
with a new preferred locator, the Initiator SHOULD directly set the
new preferred locator to status ACTIVE without performing address
verification first, and it MUST send the I2 packet to the new
preferred locator. The I1 destination address and the new preferred
locator may be identical. All new non-preferred locators must still
undergo address verification once the base exchange completes. It is
also possible for the host to send the LOCATOR_SET without any
preferred bits set, in which case the exchange will continue as
normal and the newly learned addresses will be in an UNVERIFIED state
at the initiator.

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Initiator Responder

R1 with LOCATOR_SET
<-----------------------------------
record additional addresses
change Responder address
I2 sent to newly indicated preferred address
----------------------------------->
(process normally)
R2
<-----------------------------------
(process normally, later verification of non-preferred locators)

Figure 4: LOCATOR_SET Inclusion in R1

An Initiator MAY include one or more LOCATOR_SET parameters in the I2
packet, independent of whether or not there was a LOCATOR_SET
parameter in the R1. These parameters MUST be protected by the I2
signature. Even if the I2 packet contains LOCATOR_SET parameters,
the Responder MUST still send the R2 packet to the source address of
the I2. The new preferred locator, if set, SHOULD be identical to
the I2 source address. If the I2 packet contains LOCATOR_SET
parameters, all new locators must undergo address verification as
usual, and the ESP traffic that subsequently follows should use the
preferred locator.

Initiator Responder

I2 with LOCATOR_SET
----------------------------------->
(process normally)
record additional addresses
R2 sent to source address of I2
<-----------------------------------
(process normally)

Figure 5: LOCATOR_SET Inclusion in I2

The I1 and I2 may be arriving from different source addresses if the
LOCATOR_SET parameter is present in R1. In this case,
implementations simultaneously using multiple pre-created R1s,
indexed by Initiator IP addresses, may inadvertently fail the puzzle
solution of I2 packets due to a perceived puzzle mismatch. See, for
instance, the example in Appendix A of [RFC7401]. As a solution, the
Responder's puzzle indexing mechanism must be flexible enough to
accommodate the situation when R1 includes a LOCATOR_SET parameter.

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Finally, the R2 may be used to carry the LOCATOR_SET parameter. In
this case, the LOCATOR_SET is covered by the HIP_MAC_2 and
HIP_SIGNATURE. Including LOCATOR_SET in R2 as opposed to R1 may have
some advantages when a host prefers not to divulge additional
locators until after the I2 is successfully processed.

When the LOCATOR_SET parameter is sent in an UPDATE packet, the
receiver will respond with an UPDATE acknowledgment. When the
LOCATOR_SET parameter is sent in an R1, I2, or R2 packet, the base
exchange retransmission mechanism will confirm its successful
delivery.

4.2.9. Using LOCATOR_SETs across Addressing Realms

It is possible for HIP associations to use these mechanisms to
migrate their HIP associations and security associations from
addresses in the IPv4 addressing realm to IPv6, or vice versa. It
may be possible for a state to arise in which both hosts are only
using locators in different addressing realms, but in such a case,
some type of mechanism for interworking between the different realms
must be employed; such techniques are outside the scope of the
present text.

4.3. Interaction with Security Associations

A host may establish any number of security associations (or SPIs)
with a peer. The main purpose of having multiple SPIs with a peer is
to group the addresses into collections that are likely to experience
fate sharing, or to perform load balancing.

A basic property of HIP SAs is that the inbound IP address is not
used to look up the incoming SA. However, the use of different
source and destination addresses typically leads to different paths,
with different latencies in the network, and if packets were to
arrive via an arbitrary destination IP address (or path) for a given
SPI, the reordering due to different latencies may cause some packets
to fall outside of the ESP anti-replay window. For this reason, HIP
provides a mechanism to affiliate destination addresses with inbound
SPIs, when there is a concern that anti-replay windows might be
violated. In this sense, we can say that a given inbound SPI has an
"affinity" for certain inbound IP addresses, and this affinity is
communicated to the peer host. Each physical interface SHOULD have a
separate SA, unless the ESP anti-replay window is extended or
disabled.

Moreover, even when the destination addresses used for a particular
SPI are held constant, the use of different source interfaces may
also cause packets to fall outside of the ESP anti-replay window,

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since the path traversed is often affected by the source address or
interface used. A host has no way to influence the source interface
on which a peer sends its packets on a given SPI. A host SHOULD
consistently use the same source interface and address when sending
to a particular destination IP address and SPI. For this reason, a
host may find it useful to change its SPI or at least reset its ESP
anti-replay window when the peer host readdresses.

5. Processing Rules

Basic processing rules for the LOCATOR_SET parameter are specified in
[RFC8046]. This document focuses on multihoming-specific rules.

5.1. Sending LOCATOR_SETs

The decision of when to send a LOCATOR_SET, and which addresses to
include, is a local policy issue. [RFC8046] recommends that a host
"send a LOCATOR_SET whenever it recognizes a change of its IP
addresses in use on an active HIP association and [when it] assumes
that the change is going to last at least for a few seconds." It is
possible to delay the exposure of additional locators to the peer,
and to send data from previously unannounced locators, as might arise
in certain mobility or multihoming situations.

When a host decides to inform its peers about changes in its IP
addresses, it has to decide how to group the various addresses with
SPIs. If hosts are deployed in an operational environment in which
HIP-aware NATs and firewalls (that may perform parameter inspection)
exist, and different such devices may exist on different paths, hosts
may take that knowledge into consideration about how addresses are
grouped, and may send the same LOCATOR_SET in separate UPDATEs on the
different paths. However, more detailed guidelines about how to
operate in the presence of such HIP-aware NATs and firewalls are a
topic for further study. Since each SPI is associated with a
different security association, the grouping policy may also be based
on ESP anti-replay protection considerations. In the typical case,
simply basing the grouping on actual kernel-level physical and
logical interfaces may be the best policy. The grouping policy is
outside of the scope of this document.

Locators corresponding to tunnel interfaces (e.g., IPsec tunnel
interfaces or Mobile IP home addresses) or other virtual interfaces
MAY be announced in a LOCATOR_SET, but implementations SHOULD avoid
announcing such locators as preferred locators if more direct paths
may be obtained by instead preferring locators from non-tunneling
interfaces if such locators provide a more direct path to the HIP
peer.

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[RFC8046] specifies that hosts MUST NOT announce broadcast or
multicast addresses in LOCATOR_SETs. Link-local addresses MAY be
announced to peers that are known to be neighbors on the same link,
such as when the IP destination address of a peer is also link local.
The announcement of link-local addresses in this case is a policy
decision; link-local addresses used as preferred locators will create
reachability problems when the host moves to another link. In any
case, link-local addresses MUST NOT be announced to a peer unless
that peer is known to be on the same link.

Once the host has decided on the groups and assignment of addresses
to the SPIs, it creates a LOCATOR_SET parameter that serves as a
complete representation of the addresses and associated SPIs intended
for active use. We now describe a few cases introduced in Section 4.
We assume that the Traffic Type for each locator is set to "0" (other
values for Traffic Type may be specified in documents that separate
the HIP control plane from data-plane traffic). Other mobility and
multihoming cases are possible but are left for further
experimentation.

1. Host multihoming (addition of an address). We only describe the
simple case of adding an additional address to a (previously)
single-homed, non-mobile host. The host MAY choose to simply
announce this address to the peer, for fault tolerance. To do
this, the multihomed host creates a LOCATOR_SET parameter
including the existing address and SPI as a Type "1" Locator, and
the new address as a Type "0" Locator. The host sends this in an
UPDATE message with the SEQ parameter, which is acknowledged by
the peer.

2. The host MAY set up a new SA pair between this new address and an
address of the peer host. To do this, the multihomed host
creates a new inbound SA and creates a new SPI. For the outgoing
UPDATE message, it inserts an ESP_INFO parameter with an OLD SPI
field of "0", a NEW SPI field corresponding to the new SPI, and a
KEYMAT Index as selected by local policy. The host adds to the
UPDATE message a LOCATOR_SET with two Type "1" Locators: the
original address and SPI active on the association, and the new
address and new SPI being added (with the SPI matching the NEW
SPI contained in the ESP_INFO). The preferred bit SHOULD be set
depending on the policy to tell the peer host which of the two
locators is preferred. The UPDATE also contains a SEQ parameter
and optionally a DIFFIE_HELLMAN parameter and follows rekeying
procedures with respect to this new address. The UPDATE message
SHOULD be sent to the peer's preferred address with a source
address corresponding to the new locator.

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The sending of multiple LOCATOR_SETs is unsupported. Note that the
inclusion of LOCATOR_SET in an R1 packet requires the use of Type "0"
Locators since no SAs are set up at that point.

5.2. Handling Received LOCATOR_SETs

A host SHOULD be prepared to receive a LOCATOR_SET parameter in the
following HIP packets: R1, I2, R2, and UPDATE.

This document describes sending both ESP_INFO and LOCATOR_SET
parameters in an UPDATE. The ESP_INFO parameter is included when
there is a need to rekey or key a new SPI and can otherwise be
included for the possible benefit of HIP-aware middleboxes. The
LOCATOR_SET parameter contains a complete map of the locators that
the host wishes to make or keep active for the HIP association.

In general, the processing of a LOCATOR_SET depends upon the packet
type in which it is included. Here, we describe only the case in
which ESP_INFO is present and a single LOCATOR_SET and ESP_INFO are
sent in an UPDATE message; other cases are for further study. The
steps below cover each of the cases described in Section 5.1.

The processing of ESP_INFO and LOCATOR_SET parameters is intended to
be modular and support future generalization to the inclusion of
multiple ESP_INFO and/or multiple LOCATOR_SET parameters. A host
SHOULD first process the ESP_INFO before the LOCATOR_SET, since the
ESP_INFO may contain a new SPI value mapped to an existing SPI, while
a Type "1" Locator will only contain a reference to the new SPI.

When a host receives a validated HIP UPDATE with a LOCATOR_SET and
ESP_INFO parameter, it processes the ESP_INFO as follows. The
ESP_INFO parameter indicates whether an SA is being rekeyed, created,
deprecated, or just identified for the benefit of middleboxes. The
host examines the OLD SPI and NEW SPI values in the ESP_INFO
parameter:

1. (no rekeying) If the OLD SPI is equal to the NEW SPI and both
correspond to an existing SPI, the ESP_INFO is gratuitous
(provided for middleboxes), and no rekeying is necessary.

2. (rekeying) If the OLD SPI indicates an existing SPI and the NEW
SPI is a different non-zero value, the existing SA is being
rekeyed and the host follows HIP ESP rekeying procedures by
creating a new outbound SA with an SPI corresponding to the NEW
SPI, with no addresses bound to this SPI. Note that locators in
the LOCATOR_SET parameter will reference this new SPI instead of
the old SPI.

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3. (new SA) If the OLD SPI value is zero and the NEW SPI is a new
non-zero value, then a new SA is being requested by the peer.
This case is also treated like a rekeying event; the receiving
host must create a new SA and respond with an UPDATE ACK.

4. (deprecating the SA) If the OLD SPI indicates an existing SPI and
the NEW SPI is zero, the SA is being deprecated and all locators
uniquely bound to the SPI are put into the DEPRECATED state.

If none of the above cases apply, a protocol error has occurred and
the processing of the UPDATE is stopped.

Next, the locators in the LOCATOR_SET parameter are processed. For
each locator listed in the LOCATOR_SET parameter, check that the
address therein is a legal unicast or anycast address. That is, the
address MUST NOT be a broadcast or multicast address. Note that some
implementations MAY accept addresses that indicate the local host,
since it may be allowed that the host runs HIP with itself.

For each Type "1" address listed in the LOCATOR_SET parameter, the
host checks whether the address is already bound to the SPI
indicated. If the address is already bound, its lifetime is updated.
If the status of the address is DEPRECATED, the status is changed to
UNVERIFIED. If the address is not already bound, the address is
added, and its status is set to UNVERIFIED. If there exist remaining
addresses corresponding to the SPI that were NOT listed in the
LOCATOR_SET parameter, the host sets the status of such addresses to
DEPRECATED.

For each Type "0" address listed in the LOCATOR_SET parameter, if the
status of the address is DEPRECATED, or the address was not
previously known, the status is changed to UNVERIFIED. The host MAY
choose to associate this address with one or more SAs. The
association with different SAs is a local policy decision, unless the
peer has indicated that the address is preferred, in which case the
address should be put into use on an SA that is prioritized in the
security policy database.

As a result, at the end of processing, the addresses listed in the
LOCATOR_SET parameter have a state of either UNVERIFIED or ACTIVE,
and any old addresses on the old SA not listed in the LOCATOR_SET
parameter have a state of DEPRECATED.

Once the host has processed the locators, if the LOCATOR_SET
parameter contains a new preferred locator, the host SHOULD initiate
a change of the preferred locator. This requires that the host first
verifies reachability of the associated address and only then changes
the preferred locator; see Section 5.4.

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If a host receives a locator with an unsupported Locator Type, and
when such a locator is also declared to be the preferred locator for
the peer, the host SHOULD send a NOTIFY error with a Notify Message
Type of LOCATOR_TYPE_UNSUPPORTED, with the Notification Data field
containing the locator(s) that the receiver failed to process.
Otherwise, a host MAY send a NOTIFY error if a (non-preferred)
locator with an unsupported Locator Type is received in a LOCATOR_SET
parameter.

5.3. Verifying Address Reachability

Address verification is defined in [RFC8046].

When address verification is in progress for a new preferred locator,
the host SHOULD select a different locator listed as ACTIVE, if one
such locator is available, to continue communications until address
verification completes. Alternatively, the host MAY use the new
preferred locator while in UNVERIFIED status to the extent Credit-
Based Authorization permits. Credit-Based Authorization is explained
in [RFC8046]. Once address verification succeeds, the status of the
new preferred locator changes to ACTIVE.

5.4. Changing the Preferred Locator

A host MAY want to change the preferred outgoing locator for
different reasons, e.g., because traffic information or ICMP error
messages indicate that the currently used preferred address may have
become unreachable. Another reason may be due to receiving a
LOCATOR_SET parameter that has the preferred bit set.

To change the preferred locator, the host initiates the following
procedure:

1. If the new preferred locator has ACTIVE status, the preferred
locator is changed and the procedure succeeds.

2. If the new preferred locator has UNVERIFIED status, the host
starts to verify its reachability. The host SHOULD use a
different locator listed as ACTIVE until address verification
completes if one such locator is available. Alternatively, the
host MAY use the new preferred locator, even though in UNVERIFIED
status, to the extent Credit-Based Authorization permits. Once
address verification succeeds, the status of the new preferred
locator changes to ACTIVE, and its use is no longer governed by
Credit-Based Authorization.

Henderson, et al. Standards Track [Page 18]

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3. If the peer host has not indicated a preference for any address,
then the host picks one of the peer's ACTIVE addresses randomly
or according to policy. This case may arise if, for example,
ICMP error messages that deprecate the preferred locator arrive,
but the peer has not yet indicated a new preferred locator.

4. If the new preferred locator has DEPRECATED status and there is
at least one non-deprecated address, the host selects one of the
non-deprecated addresses as a new preferred locator and
continues. If the selected address is UNVERIFIED, the address
verification procedure described above will apply.

6. Security Considerations

This document extends the scope of host mobility solutions defined in
[RFC8046] to also include host multihoming, and as a result, many of
the same security considerations for mobility also pertain to
multihoming. In particular, [RFC8046] describes how HIP host
mobility is resistant to different types of impersonation attacks and
denial-of-service (DoS) attacks.

The security considerations for this document are similar to those of
[RFC8046] because the strong authentication capabilities for mobility
also carry over to end-host multihoming. [RFC4218] provides a threat
analysis for IPv6 multihoming, and the remainder of this section
first describes how HIP host multihoming addresses those previously
described threats, and then it discusses some additional security
considerations.

The high-level threats discussed in [RFC4218] involve redirection
attacks for the purposes of packet recording, data manipulation, and
availability. There are a few types of attackers to consider:
on-path attackers, off-path attackers, and malicious hosts.

[RFC4218] also makes the comment that in identifier/locator split
solutions such as HIP, application security mechanisms should be tied
to the identifier, not the locator, and attacks on the identifier
mechanism and on the mechanism binding locators to the identifier are
of concern. This document does not consider the former issue
(application-layer security bindings) to be within scope. The latter
issue (locator bindings to identifier) is directly addressed by the
cryptographic protections of the HIP protocol, in that locators
associated to an identifier are listed in HIP packets that are signed
using the identifier key.

Section 3.1 of [RFC4218] lists several classes of security
configurations in use in the Internet. HIP maps to the fourth
(strong identifier) and fifth ("leap-of-faith") categories, the

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latter being associated with the optional opportunistic mode of HIP
operation. The remainder of Section 3 describes existing security
problems in the Internet and comments that the goal of a multihoming
solution is not to solve them specifically but rather not to make any
of them worse. HIP multihoming should not increase the severity of
the identified risks. One concern for both HIP mobility and
multihoming is the susceptibility of the mechanisms to misuse
flooding-based redirections due to a malicious host. The mechanisms
described in [RFC8046] for address verification are important in this
regard.

Regarding the new types of threats introduced by multihoming
(Section 4 of [RFC4218]), HIP multihoming should not introduce new
concerns. Classic and premeditated redirection are prevented by the
strong authentication in HIP messages. Third-party DoS attacks are
prevented by the address verification mechanism. Replay attacks can
be avoided via use of replay protection in ESP SAs. In addition,
accepting packets from unknown locators is protected by either the
strong authentication in the HIP control packets or by the ESP-based
encryption in use for data packets.

The HIP mechanisms are designed to limit the ability to introduce DoS
on the mechanisms themselves (Section 7 of [RFC4218]). Care is taken
in the HIP base exchange to avoid creating state or performing much
work before hosts can authenticate one another. A malicious host
involved in HIP multihoming with another host might attempt to misuse
the mechanisms for multihoming by, for instance, increasing the state
required or inducing a resource limitation attack by sending too many
candidate locators to the peer host. Therefore, implementations
supporting the multihoming extensions should consider avoiding
accepting large numbers of peer locators and rate limiting any UPDATE
messages being exchanged.

The exposure of a host's IP addresses through HIP mobility and
multihoming extensions may raise the following privacy concern. The
administrator of a host may be trying to hide its location in some
context through the use of a VPN or other virtual interfaces.
Similar privacy issues also arise in other frameworks such as WebRTC
and are not specific to HIP. Implementations SHOULD provide a
mechanism to allow the host administrator to block the exposure of
selected addresses or address ranges.

Finally, some implementations of VPN tunneling have experienced
instances of 'leakage' of flows that were intended to have been
protected by a security tunnel but are instead sent in the clear,
perhaps because some of the addresses used fall outside of the range
of addresses configured for the tunnel in the security policy or
association database. Implementors are advised to take steps to

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ensure that the usage of multiple addresses between hosts does not
cause accidental leakage of some data session traffic outside of the
ESP-protected envelope.

7. References

7.1. 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>.

[RFC6724] Thaler, D., Ed., Draves, R., Matsumoto, A., and T. Chown,
"Default Address Selection for Internet Protocol Version 6
(IPv6)", RFC 6724, DOI 10.17487/RFC6724, September 2012,
<http://www.rfc-editor.org/info/rfc6724>.

[RFC7401] Moskowitz, R., Ed., Heer, T., Jokela, P., and T.
Henderson, "Host Identity Protocol Version 2 (HIPv2)",
RFC 7401, DOI 10.17487/RFC7401, April 2015,
<http://www.rfc-editor.org/info/rfc7401>.

[RFC7402] Jokela, P., Moskowitz, R., and J. Melen, "Using the
Encapsulating Security Payload (ESP) Transport Format with
the Host Identity Protocol (HIP)", RFC 7402,
DOI 10.17487/RFC7402, April 2015,
<http://www.rfc-editor.org/info/rfc7402>.

[RFC8046] Henderson, T., Ed., Vogt, C., and J. Arkko, "Host Mobility
with the Host Identity Protocol", RFC 8046,
DOI 10.17487/RFC8046, February 2017,
<http://www.rfc-editor.org/info/rfc8046>.

7.2. Informative References

[RFC4218] Nordmark, E. and T. Li, "Threats Relating to IPv6
Multihoming Solutions", RFC 4218, DOI 10.17487/RFC4218,
October 2005, <http://www.rfc-editor.org/info/rfc4218>.

[RFC4303] Kent, S., "IP Encapsulating Security Payload (ESP)",
RFC 4303, DOI 10.17487/RFC4303, December 2005,
<http://www.rfc-editor.org/info/rfc4303>.

[RFC5533] Nordmark, E. and M. Bagnulo, "Shim6: Level 3 Multihoming
Shim Protocol for IPv6", RFC 5533, DOI 10.17487/RFC5533,
June 2009, <http://www.rfc-editor.org/info/rfc5533>.

Henderson, et al. Standards Track [Page 21]

RFC 8047 HIP Multihoming February 2017

Acknowledgments

This document contains content that was originally included in RFC
5206. Pekka Nikander and Jari Arkko originated RFC 5206, and
Christian Vogt and Thomas Henderson (editor) later joined as
coauthors. Also in RFC 5206, Greg Perkins contributed the initial
draft of the security section, and Petri Jokela was a coauthor of the
initial individual submission.

The authors thank Miika Komu, Mika Kousa, Jeff Ahrenholz, and Jan
Melen for many improvements to the document. Concepts from a paper
on host multihoming across address families, by Samu Varjonen, Miika
Komu, and Andrei Gurtov, contributed to this revised specification.

Authors' Addresses

Thomas R. Henderson (editor)
University of Washington
Campus Box 352500
Seattle, WA
United States of America

Email: [email protected]

Christian Vogt
Independent
3473 North First Street
San Jose, CA 95134
United States of America

Email: [email protected]

Jari Arkko
Ericsson
Jorvas, FIN-02420
Finland

Phone: +358 40 5079256
Email: [email protected]

Henderson, et al. Standards Track [Page 22]