Network Working Group P. Nikander
Internet-Draft J. Arkko
Expires: August 21, 2005 Ericsson Research Nomadic Lab
T. Henderson
The Boeing Company
February 20, 2005
End-Host Mobility and Multi-Homing with the Host Identity Protocol
draft-ietf-hip-mm-01
Status of this Memo
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Copyright Notice
Copyright (C) The Internet Society (2005).
Abstract
This document defines a "locator" parameter for the Host Identity
Protocol and specifies an end-host mobility mechanism.
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Table of Contents
1. Introduction and Scope . . . . . . . . . . . . . . . . . . . . 3
2. Conventions used in this document . . . . . . . . . . . . . . 5
3. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 6
4. LOCATOR parameter format . . . . . . . . . . . . . . . . . . . 7
4.1 Traffic Type and Preferred Locator . . . . . . . . . . . . 8
4.2 Locator Type and Locator . . . . . . . . . . . . . . . . . 8
4.3 UPDATE packet with included LOCATOR . . . . . . . . . . . 9
5. Overview of HIP basic mobility and multi-homing
functionality . . . . . . . . . . . . . . . . . . . . . . . . 10
5.1 Informing the peer about multiple or changed locator(s) . 10
5.2 Address verification . . . . . . . . . . . . . . . . . . . 13
5.3 Preferred locator . . . . . . . . . . . . . . . . . . . . 13
5.4 Locator data structure and status . . . . . . . . . . . . 14
6. Protocol overview . . . . . . . . . . . . . . . . . . . . . . 15
6.1 Mobility with single SA pair . . . . . . . . . . . . . . . 15
6.2 Host multihoming . . . . . . . . . . . . . . . . . . . . . 17
6.3 Site multi-homing . . . . . . . . . . . . . . . . . . . . 19
6.4 Dual host multi-homing . . . . . . . . . . . . . . . . . . 19
6.5 Combined mobility and multi-homing . . . . . . . . . . . . 20
6.6 Using LOCATORs across addressing realms . . . . . . . . . 20
6.7 Network renumbering . . . . . . . . . . . . . . . . . . . 20
6.8 Initiating the protocol in R1 or I2 . . . . . . . . . . . 20
7. Processing rules . . . . . . . . . . . . . . . . . . . . . . . 22
7.1 Sending LOCATORs . . . . . . . . . . . . . . . . . . . . . 22
7.2 Handling received LOCATORs . . . . . . . . . . . . . . . . 23
7.3 Verifying address reachability . . . . . . . . . . . . . . 24
7.4 Changing the preferred locator . . . . . . . . . . . . . . 24
8. Policy considerations . . . . . . . . . . . . . . . . . . . . 26
9. Security Considerations . . . . . . . . . . . . . . . . . . . 27
10. IANA Considerations . . . . . . . . . . . . . . . . . . . . 28
11. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . 29
12. References . . . . . . . . . . . . . . . . . . . . . . . . . 30
12.1 Normative references . . . . . . . . . . . . . . . . . . . . 30
12.2 Informative references . . . . . . . . . . . . . . . . . . . 30
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . 30
A. Changes from previous versions . . . . . . . . . . . . . . . . 32
A.1 From nikander-hip-mm-00 to nikander-hip-mm-01 . . . . . . 32
A.2 From nikander-hip-mm-01 to nikander-hip-mm-02 . . . . . . 32
A.3 From -02 to draft-ietf-hip-mm-00 . . . . . . . . . . . . . 32
A.4 From draft-ietf-hip-mm-00 to -01 . . . . . . . . . . . . . 33
Intellectual Property and Copyright Statements . . . . . . . . 34
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1. Introduction and Scope
The Host Identity Protocol [1] (HIP) defines a mechanism that
decouples the transport layer (TCP, UDP, etc) from the
internetworking layer (IPv4 and IPv6). When a host uses HIP, the
overlying protocol sublayers (e.g., transport layer sockets and ESP
Security Associations) are not bound to IP addresses but instead to
Host Identifiers. However, the hosts must also know at least one IP
address where their peers are reachable. Initially these IP
addresses are the ones used during the HIP base exchange.
This document defines a generalization of an address called a
"locator". A locator specifies a point-of-attachment to the network
but may also include additional end-to-end tunneling or per-host
demultiplexing context that affects how packets are handled below the
logical HIP sublayer. This generalization is useful because IP
addresses alone may not be sufficient to describe how packets should
be handled below HIP. For example, in a host multihoming context,
certain IP addresses may need to be associated with certain ESP SPIs,
to avoid violation of the ESP anti-replay window [2]. Addresses may
also be affiliated with transport ports in certain tunneling
scenarios. Or locators may merely be traditional network addresses.
Using the locator concept, this document specifies extensions to HIP
to allow a mobile host to directly inform a correspondent host, with
whom the host has an active HIP association, of a locator change.
The extensions consist of a new LOCATOR parameter for use in HIP
messages, packet processing procedures for using HIP messages to
securely notify the peer of a locator change, and additional
procedures such as an address check mechanism.
When using ESP, since the SAs are not bound to IP addresses, the host
is able to receive packets that are protected using a HIP created ESP
SA from any address. Thus, a host can change its IP address and
continue to send packets to its peers. However, unless the host is
sufficiently trusted by its peers, the peers are not able to reply
before they can reliably and securely update the set of addresses
that they associate with the sending host. Furthermore, mobility may
change the path characteristics in such a manner that reordering
occurs and packets fall outside the ESP anti-replay window.
A related operational configuration is host multihoming, in which a
host has multiple locators simultaneously rather than sequentially as
in the case of mobility. By using the locator parameter defined
herein, a host can inform its peers of additional (multiple) locators
at which it can be reached, and can declare a particular locator as a
"preferred" locator. Although this document defines a mechanism for
multihoming, it does not define associated policies such as which
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locators to choose when more than one pair is available, the
operation of simultaneous mobility and multihoming, and the
implications of multihoming on transport protocols and ESP
anti-replay windows. Additional definition of HIP-based multihoming
is expected to be part of a future document.
Due to the danger of flooding attacks (see [3]), the peers must
always check the reachability of the host at a new IP address, unless
a sufficient level of trust exists between the hosts. The
reachability check is implemented by the challenger sending some
piece of unguessable information to the new address, and waiting for
some acknowledgment from the responder that indicates reception of
the information at the new address. This may include exchange of a
nonce, or generation of a new SPI and observing data arriving on the
new SPI.
There are a number of situations where the simple end-to-end
readdressing functionality is not sufficient. These include the
initial reachability of a mobile host, location privacy, end-host and
site multi-homing with legacy hosts, and NAT traversal. In these
situations there is a need for some helper functionality in the
network. Such functionality is out of scope of this document.
Finally, making underlying IP mobility transparent to the transport
layer has implications on the proper response of transport congestion
control, path MTU selection, and QoS. Transport-layer mobility
triggers, and the proper transport response to a HIP mobility or
multi-homing address change, are outside the scope of this document.
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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 RFC2119 [4].
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3. Terminology
Locator. A name that controls how the packet is routed through the
network and demultiplexed by the end host. It may include a
concatenation of traditional network addresses such as an IPv6
address and end-to-end identifiers such as an ESP SPI. It may
also include transport port numbers or IPv6 Flow Labels as
demultiplexing context, or it may simply be a network address.
Address. A name that denotes a point-of-attachment to the network.
The two most common examples are an IPv4 address and an IPv6
address. The set of possible addresses is a subset of the set of
possible locators.
Preferred locator. A locator on which a host prefers to receive
data. With respect to a given peer, a host always has one active
preferred locator, unless there are no active locators. By
default, the locators used in the HIP base exchange are the
preferred locators.
New preferred locator. A new preferred locator sent by a host to its
peers. The reachability of the new preferred locator often needs
to be verified before it can be put into use. Consequently, there
may simultaneously be an active preferred locator, being used, and
a new preferred locator, the reachability of which is being
verified.
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4. LOCATOR parameter format
The LOCATOR parameter is a critical parameter as defined by [1]. The
LOCATOR parameter is also abbreviated as "LOC" in the figures herein.
It consists of the standard HIP parameter Type and Length fields,
plus one or more locator sub-parameters. Each Locator sub-parameter
contains a Traffic Type, Locator Type, Locator Length, Preferred
Locator bit, Locator Lifetime, and a Locator encoding.
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 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Traffic Type | Locator Type | Locator Length | Reserved |P|
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Locator Lifetime |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Locator |
| |
| |
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
. .
. .
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Traffic Type | Locator Type | Locator Length | Reserved |P|
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Locator Lifetime |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Locator |
| |
| |
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Type: 3
Length: Length in octets, excluding Type and Length fields, and
excluding padding.
Traffic Type: Defines whether the locator pertains to HIP signaling,
user data, or both.
Locator Type: Defines the semantics of the Locator field.
Locator Length: Defines the length of the Locator field, in units of
4-byte words (Locators up to a maximum of 4*255 bytes are
supported).
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Reserved: Zero when sent, ignored when received.
P: Preferred locator. Set to one if the locator is preferred for
that Traffic Type; otherwise set to zero.
Locator Lifetime: Locator lifetime, in seconds.
Locator: The locator whose semantics and encoding are indicated by
the Locator Type field. All Locator sub-fields are integral
multiples of four bytes in length.
The Locator Lifetime indicates how long the following locator is
expected to be valid. The lifetime is expressed in seconds. Each
locator MUST have a non-zero lifetime. The address is expected to
become deprecated when the specified number of seconds has passed
since the reception of the message. A deprecated address SHOULD NOT
be used as an destination address if an alternate (non-deprecated) is
available and has sufficient scope.
4.1 Traffic Type and Preferred Locator
The following Traffic Type values are defined:
0: Both signaling (HIP control packets) and user data.
1: Signaling packets only.
2: Data packets only.
The "P" bit, when set, has scope over the corresponding Traffic Type
that precedes it. That is, if a "P" bit is set for Traffic Type "2",
for example, that means that the locator is preferred for data
packets. If there is a conflict (for example, if P bit is set for
both "0" and "2"), the more specific Traffic Type rule applies. By
default, the IP addresses used in the base exchange are preferred
locators for both signaling and user data, unless a new preferred
locator supersedes them. If no locators are indicated as preferred
for a given Traffic Type, the implementation may use an arbitrary
locator from the set of active locators.
4.2 Locator Type and Locator
The following Locator Type values are defined, along with the
associated semantics of the Locator field:
0: An IPv6 address or an IPv4-in-IPv6 format IPv4 address [5] (128
bits long).
1: The concatenation of an ESP SPI (first 32 bits) followed by an
IPv6 address or an IPv4-in-IPv6 format IPv4 address (an additional
128 bits).
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4.3 UPDATE packet with included LOCATOR
A number of combinations of parameters in an UPDATE packet are
possible (e.g., see Section 6). Any UPDATE packet that includes a
LOCATOR parameter SHOULD include both an HMAC and a HIP_SIGNATURE
parameter.>
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5. Overview of HIP basic mobility and multi-homing functionality
HIP mobility and multi-homing is fundamentally based on the HIP
architecture [3], where the transport and internetworking layers are
decoupled from each other by an interposed host identity protocol
layer. In the HIP architecture, the transport layer sockets are
bound to the Host Identifiers (through HIT or LSI in the case of
legacy APIs), and the Host Identifiers are translated to the actual
IP address.
The HIP base protocol specification [1] is expected to be commonly
used with the ESP Transport Format [6] to establish a pair of
Security Associations (SA). The ESP SAs are then used to carry the
actual payload data between the two hosts, by wrapping TCP, UDP, and
other upper layer packets into transport mode ESP payloads. The IP
header uses the actual IP addresses in the network.
Although HIP may also be specified in the future to operate with an
alternative to ESP providing the per-packet HIP context, the
remainder of this document assumes that HIP is being used in
conjunction with ESP. Future documents may extend this document to
include other behaviors when ESP is not used.
The base specification does not contain any mechanisms for changing
the IP addresses that were used during the base HIP exchange. Hence,
in order to remain connected, any systems that implement only the
base specification and nothing else must retain the ability to
receive packets at their primary IP address; that is, those systems
cannot change the IP address on which they are using to receive
packets without causing loss of connectivity until a base exchange is
performed from the new address.
5.1 Informing the peer about multiple or changed locator(s)
This document specifies a new HIP protocol parameter, the LOCATOR
parameter (see Section 4), that allows the hosts to exchange
information about their locator(s), and any changes in their
locator(s). The logical structure created with LOCATOR parameters
has three levels: hosts, Security Associations (SAs) indexed by
Security Parameter Indices (SPIs), and addresses.
The relation between these entities for an association negotiated as
defined in the base specification [1] and ESP transform [6] is
illustrated in Figure 2.
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-<- SPI1a -- -- SPI2a ->-
host1 < > addr1a <---> addr2a < > host2
->- SPI2a -- -- SPI1a -<-
Figure 2: Relation between hosts, SPIs, and addresses (base
specification)
In Figure 2, host1 and host2 negotiate two unidirectional SAs, and
each host selects the SPI value for its inbound SA. The addresses
addr1a and addr2a are the source addresses that each host uses in the
base HIP exchange. These are the "preferred" (and only) addresses
conveyed to the peer for each SA; even though packets sent to any of
the hosts' interfaces can arrive on an inbound SPI, when a host sends
packets to the peer on an outbound SPI, it knows of a single
destination address associated with that outbound SPI (for host1, it
sends a packet on SPI2a to addr2a to reach host2), unless other
mechanisms exist to learn of new addresses.
In general, the bindings that exist in an implementation
corresponding to this draft can be depicted as shown in Figure 3. In
this figure, a host can have multiple inbound SPIs (and, not shown,
multiple outbound SPIs) between itself and another host.
Furthermore, each SPI may have multiple addresses associated with it.
These addresses bound to an SPI are not used as SA selectors.
Rather, the addresses are those addresses that are provided to the
peer host, as hints for which addresses to use to reach the host on
that SPI. The LOCATOR parameter allows for IP addresses and SPIs to
be combined to form generalized locators. The LOCATOR parameter is
used to change the set of addresses that a peer associates with a
particular SPI.
address11
/
SPI1 - address12
/
/ address21
host -- SPI2 <
\ address22
\
SPI3 - address31
\
address32
Figure 3: Relation between hosts, SPIs, and addresses (general case)
A host may establish any number of security associations (or SPIs)
with a peer. The main purpose of having multiple SPIs is to group
the addresses into collections that are likely to experience fate
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sharing. For example, if the host needs to change its addresses on
SPI2, it is likely that both address21 and address22 will
simultaneously become obsolete. In a typical case, such SPIs may
correspond with physical interfaces; see below. Note, however, that
especially in the case of site multi-homing, one of the addresses may
become unreachable while the other one still works. In the typical
case, however, this does not require the host to inform its peers
about the situation, since even the non-working address still
logically exists.
A basic property of HIP SAs is that the inbound IP address is not
used as a selector for the SA. Therefore, in Figure 3, it may seem
unnecessary for address31, for example, to be associated only with
SPI3-- in practice, a packet may arrive to SPI1 via destination
address address31 as well. 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, if
there is a concern that anti-replay windows might be violated
otherwise. 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 loose.
Moreover, even if 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, 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 uses to send its packets on a given SPI. Hosts
SHOULD consistently use the same source interface 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.
An address may appear on more than one SPI. This creates no
ambiguity since the receiver will ignore the IP addresses as SA
selectors anyway.
A single LOCATOR parameter contains data only about one SPI. To
simultaneously signal changes on several SPIs, it is necessary to
send several LOCATOR parameters. The packet structure supports this.
If the LOCATOR parameter is sent in an UPDATE packet, then the
receiver will respond with an UPDATE acknowledgment. If the LOCATOR
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parameter is sent in a NOTIFY, I2, or R2 packet, then the recipient
may consider the LOCATOR as informational, and act only when it needs
to activate a new address. The use of LOCATOR in a NOTIFY message
may not be compatible with middleboxes.
5.2 Address verification
When a HIP host receives a set of locators from another HIP host in a
LOCATOR, it does not necessarily know whether the other host is
actually reachable at the claimed addresses. In fact, a malicious
peer host may be intentionally giving bogus addresses in order to
cause a packet flood towards the given addresses [9]. Thus, before
the HIP host can actually use a new address, it must first check that
the peer is reachable at the new address.
A second benefit of performing an address check is to allow any
possible middleboxes in the network along the new path to obtain the
peer host's inbound SPI.
A simple technique to verify addresses is to send an UPDATE to the
host at the new address. The UPDATE packet SHOULD include a nonce,
unguessable by anyone not on the path to the new address, that forces
the host to reply in a manner that confirms reception of the nonce.
One direct way to perform this is to include an ECHO_REQUEST
parameter with some piece of unguessable information such as a random
number. If the host is sending a NES parameter, the ECHO_REQUEST MAY
contain the new SPI, for example. If the peer host is rekeying by
sending an UPDATE with NES to the new address, the arrival of data on
the new SPI can also be used to verify the address.
If middlebox traversal is possible along the path, and the peer host
is not rekeying, the peer host SHOULD include a SPI parameter as part
of its UPDATE, with the SPI corresponding to its active inbound SPI.
It is not specified how a host knows whether or not middleboxes might
lie on its path, so a conservative assumption may be to always
include the SPI parameter.
In certain networking scenarios, hosts may be trusted enough to
bypass performing address verification. In such a case, the host MAY
bypass the address verification step and put the addresses into
immediate service. Note that this may not be compatible with
middlebox traversal.
5.3 Preferred locator
When a host has multiple locators, the peer host must decide upon
which to use for outbound packets. It may be that a host would
prefer to receive data on a particular inbound interface. HIP allows
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a particular locator to be designated as a preferred locator, and
communicated to the peer (see Section 4).
In general, when multiple locators are used for a session, there is
the question of using multiple locators for failover only or for
load-balancing. Due to the implications of load-balancing on the
transport layer that still need to be worked out, this draft assumes
that multiple locators are used primarily for failover. An
implementation may use ICMP interactions, reachability checks, or
other means to detect the failure of a locator.
5.4 Locator data structure and status
In a typical implementation, each outgoing locator is represented as
a piece of state that contains the following data:
the actual bit pattern representing the locator,
lifetime (seconds),
status (UNVERIFIED, ACTIVE, DEPRECATED).
The status is used to track the reachability of the address embedded
within the LOCATOR parameter:
UNVERIFIED indicates that the reachability of the address has not
been verified yet,
ACTIVE indicates that the reachability of the address has been
verified and the address has not been deprecated,
DEPRECATED indicates that the locator lifetime has expired
The following state changes are allowed:
UNVERIFIED to ACTIVE The reachability procedure completes
successfully.
UNVERIFIED to DEPRECATED The locator lifetime expires while it is
UNVERIFIED.
ACTIVE to DEPRECATED The locator lifetime expires while it is ACTIVE.
ACTIVE to UNVERIFIED There has been no traffic on the address for
some time, and the local policy mandates that the address
reachability must be verified again before starting to use it
again.
DEPRECATED to UNVERIFIED The host receives a new lifetime for the
locator.
If a host is verifying reachability with another host, a DEPRECATED
address MUST NOT be changed to ACTIVE without first verifying its
reachability. If reachability is not being verified, then the
UNVERIFIED state is a transient state that transitions immediately to
ACTIVE.
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6. Protocol overview
In this section we briefly introduce a number of usage scenarios
where the HIP mobility and multi-homing facility is useful. These
scenarios assume that HIP is being used with the ESP Transform,
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 [1]. 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 SPI acts as an index to the right host-to-host context.
Each of the scenarios below assumes that the HIP base exchange has
completed, and the hosts each have a single outbound SA to the peer
host. Associated with this outbound SA is a single destination
address of the peer host-- the source address used by the peer during
the base exchange.
The readdressing protocol is an asymmetric protocol where one host,
called the mobile host, informs another host, called the peer host,
about changes of IP addresses on affected SPIs. The readdressing
exchange is designed to be piggybacked on a number of existing HIP
exchanges. The main packets on which the LOCATOR parameters are
expected to be carried on are UPDATE packets. However, some
implementations may want to experiment with sending LOCATOR
parameters also on other packets, such as R1, I2, and NOTIFY.
6.1 Mobility with single SA pair
A mobile host must sometimes change an IP address bound to an
interface. The change of an IP address might be needed due to a
change in the advertised IPv6 prefixes on the link, a reconnected PPP
link, a new DHCP lease, or an actual movement to another subnet. In
order to maintain its communication context, the host must inform its
peers about the new IP address. This first example considers the
case in which the mobile host has only one interface, IP address, and
a single pair of SAs (one inbound, one outbound).
1. The mobile host is disconnected from the peer host for a brief
period of time while it switches from one IP address to another.
Upon obtaining a new IP address, the mobile host sends a LOCATOR
parameter to the peer host in an UPDATE message. The LOCATOR
indicates the new IP address and the SPI associated with the new
IP address by using a Locator Type of "1", the locator lifetime,
and whether the new locator is a preferred locator. The mobile
host may optionally send a NES to create a new inbound SA, in
which case it transitions to state REKEYING. In this case, the
Locator contains the new SPI to use. Otherwise, the existing SPI
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is identified in the Locator parameter, and the host waits for
its UPDATE to be acknowledged.
2. Depending on whether the mobile host initiated a rekey, and on
whether the peer host itself wants to rekey or verify the mobile
host's new address, a number of responses are possible. Figure 4
illustrates an exchange for which neither side initiates a
rekeying, but for which the peer host performs an address check.
If the peer host chooses not to perform an address check, the
UPDATE that it sends will only acknowledge the mobile host's
update but will not solicit a response from the mobile host. If
the mobile host is rekeying, the peer will also rekey, as shown
in Figure 5. If the mobile host did not decide to rekey but the
peer desires to do so, then it initiates a rekey as illustrated
in Figure 6. The UPDATE messages sent from the peer back to the
mobile are sent to the newly advertised address.
3. If the peer host is verifying the new address, the address is
marked as UNVERIFIED in the interim. Once it has successfully
received a reply to its UPDATE challenge, or optionally, data on
the new SA, it marks the new address as ACTIVE and removes the
old address.
Mobile Host Peer Host
UPDATE(LOC, SEQ)
----------------------------------->
UPDATE(SPI, SEQ, ACK, ECHO_REQUEST)
<-----------------------------------
UPDATE(ACK, ECHO_RESPONSE)
----------------------------------->
Figure 4: Readdress without rekeying, but with address check
Mobile Host Peer Host
UPDATE(LOC, NES, SEQ, [DIFFIE_HELLMAN])
----------------------------------->
UPDATE(NES, SEQ, ACK, [DIFFIE_HELLMAN,] ECHO_REQUEST)
<-----------------------------------
UPDATE(ACK, ECHO_RESPONSE)
----------------------------------->
Figure 5: Readdress with mobile-initiated rekey
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Mobile Host Peer Host
UPDATE(LOC, SEQ)
----------------------------------->
UPDATE(NES, SEQ, ACK, [DIFFIE_HELLMAN], ECHO_REQUEST)
<-----------------------------------
UPDATE(NES, SEQ, ACK, [DIFFIE_HELLMAN,] ECHO_RESPONSE)
----------------------------------->
UPDATE(ACK)
<-----------------------------------
Figure 6: Readdress with peer-initiated rekey
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 parameter.
6.2 Host multihoming
A (mobile or stationary) host may sometimes have more than one
interface. The host may notify the peer host of the additional
interface(s) by using the LOCATOR parameter. To avoid problems with
the ESP anti-replay window, a host SHOULD use a different SA for each
interface used to receive packets from the peer host.
When more than one locator is provided to the peer host, the host
SHOULD indicate which locator is preferred. By default, the
addresses used in the base exchange are preferred until indicated
otherwise.
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 RECOMMENDED that inbound and outbound SAs be
created pairwise between hosts. When a NES arrives to rekey a
particular outbound SA, the corresponding inbound SA should be also
rekeyed at that time. Although asymmetric SA configurations might be
experimented with, their usage may constrain interoperability at this
time. However, it is recommended that implementations attempt to
support peers that prefer to use non-paired SAs. It is expected that
this section and behavior will be modified in future revisions of
this protocol, once the issue and its implications are better
understood.
To add both an additional interface and SA, the host sends a LOCATOR
with a NES. The host uses the same (new) SPI value in the LOCATOR
and both the "Old SPI" and "New SPI" values in the NES-- this
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indicates to the peer that the SPI is not replacing an existing SPI.
The multihomed host transitions to state REKEYING, waiting for a NES
from the peer and an ACK of its own UPDATE. As in the mobility case,
the peer host can perform an address check while it is rekeying.
Figure 7 illustrates the basic packet exchange.
Multi-homed Host Peer Host
UPDATE(LOC, NES, SEQ, [DIFFIE_HELLMAN])
----------------------------------->
UPDATE(NES, SEQ, ACK, [DIFFIE_HELLMAN,] ECHO_REQUEST)
<-----------------------------------
UPDATE(ACK, ECHO_RESPONSE)
----------------------------------->
Figure 7: Basic multihoming scenario
For the case in which multiple locators are advertised in a LOCATOR,
the peer does not need to send ACK for the UPDATE(LOCATOR) in every
subsequent message used for the address check procedure of the
multiple locators. Therefore, a sample packet exchange might look as
shown in Figure 8.
Multi-homed Host Peer Host
UPDATE(LOC(addr_1,addr_2), SEQ)
----------------------------------->
UPDATE(ACK)
<-----------------------------------
sent to addr_1:UPDATE(SPI, SEQ, ECHO_REQUEST)
<-----------------------------------
UPDATE(ACK, ECHO_RESPONSE)
----------------------------------->
sent to addr_2:UPDATE(SPI, SEQ, ECHO_REQUEST)
<-----------------------------------
UPDATE(ACK, ECHO_RESPONSE)
----------------------------------->
Figure 8: LOCATOR with multiple addresses
When processing inbound LOCATORs that establish new security
associations, a host uses the destination address of the UPDATE
containing LOCATOR as the local address to which the LOC plus NES is
targeted. Hosts may send LOCATOR with the same IP address to
different peer addresses-- this has the effect of creating multiple
inbound SAs implicitly affiliated with different source addresses.
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When rekeying in a multihoming situation in which there is an
asymmetric number of SAs between two hosts, a respondent to the NES/
UPDATE procedure may have some ambiguity as to which inbound SA it
should update in response to the peer's UPDATE. In such a case, the
host SHOULD choose an SA corresponding to the inbound interface on
which the UPDATE was received.
6.3 Site multi-homing
A host may have an interface that has multiple globally reachable 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. It is
desirable that the host can stay reachable with all or any subset of
the currently available globally routable addresses, independent on
how they are provided.
This case is handled the same as if there were different IP
addresses, described above in Section 6.2. Note that a single
interface may experience site multi-homing while the host itself may
have multiple interfaces.
Note that a host may be multi-homed and mobile simultaneously, and
that a multi-homed 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 presently specify additional site multihoming
extensions to HIP to further align it with the requirements of the
multi6 working group.
6.4 Dual host multi-homing
Consider the case in which both hosts would like to add an additional
address after the base exchange completes. In Figure 9, consider
that host1 wants to add address addr1b. It would send a LOCATOR to
host2 located at addr2a, and a new set of SPIs would be added between
hosts 1 and 2 (call them SPI1b and SPI2b). Next, consider host2
deciding to add addr2b to the relationship. host2 now has a choice
of which of host1's addresses to initiate LOCATOR to. It may choose
to initiate a LOCATOR to addr1a, addr1b, or both. If it chooses to
send to both, then a full mesh (four SA pairs) of SAs would exist
between the two hosts. This is the most general case; it may be
often the case that hosts primarily establish new SAs only with the
peer's preferred locator. The readdressing protocol is flexible
enough to accommodate this choice.
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-<- SPI1a -- -- SPI2a ->-
host1 < > addr1a <---> addr2a < > host2
->- SPI2a -- -- SPI1a -<-
addr1b <---> addr2b
Figure 9: Dual multihoming case in which each host uses LOCATOR to
add a second address
6.5 Combined mobility and multi-homing
It looks likely that in the future many mobile hosts will be
simultaneously mobile and multi-homed, 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 other(s) may
experience mobility (undergo IP address change).
The use of LOCATOR plus NES should be flexible enough to handle most
such scenarios, although more complicated scenarios have not been
studied so far.
6.6 Using LOCATORs across addressing realms
It is possible for HIP associations to migrate to a state in which
both parties are only using locators in different addressing realms.
For example, the two hosts may initiate the HIP association when both
are using IPv6 locators, then one host may loose its IPv6
connectivity and obtain an IPv4 address. 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.
If no mechanism exists, then the UPDATE message carrying the new
LOCATOR will likely not be acknowledged anyway, and the HIP state may
time out.
6.7 Network renumbering
It is expected that IPv6 networks will be renumbered much more often
than most IPv4 networks are. From an end-host point of view, network
renumbering is similar to mobility.
6.8 Initiating the protocol in R1 or I2
A Responder host MAY include one or more LOCATOR parameters in the R1
packet that it sends to the Initiator. These parameters MUST be
protected by the R1 signature. If the R1 packet contains LOCATOR
parameters, the Initiator SHOULD send the I2 packet to the new
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preferred locator. The I1 destination address and the new preferred
locator may be identical.
Initiator Responder
R1 with LOCATOR
<-----------------------------------
record additional addresses
change responder address
I2 with new SPI in SPI parameter
----------------------------------->
(process normally)
R2
<-----------------------------------
(process normally)
Figure 10: LOCATOR inclusion in R1
An Initiator MAY include one or more LOCATOR parameters in the I2
packet, independent on whether there was LOCATOR parameter(s) in the
R1 or not. These parameters MUST be protected by the I2 signature.
Even if the I2 packet contains LOCATOR parameters, the Responder MUST
still send the R2 packet to the source address of the I2. The new
preferred locator SHOULD be identical to the I2 source address.
Initiator Responder
I2 with LOCATOR
----------------------------------->
(process normally)
record additional addresses
R2 with new SPI in SPI parameter
<-----------------------------------
(process normally)
data on new SA
------------------------------------>
(process normally)
Figure 11: LOCATOR inclusion in I2
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7. Processing rules
7.1 Sending LOCATORs
The decision of when to send LOCATORs is basically a local policy
issue. However, it is RECOMMENDED that a host sends a LOCATOR
whenever it recognizes a change of its IP addresses, and assumes that
the change is going to last at least for a few seconds. Rapidly
sending conflicting LOCATORs SHOULD be avoided.
When a host decides to inform its peers about changes in its IP
addresses, it has to decide how to group the various addresses, and
whether to include any addresses on multiple SPIs. Since each SPI is
associated with a different Security Association, the grouping policy
may be based on ESP anti-replay protection considerations. In the
typical case, simply basing the grouping on actual kernel level
physical and logical interfaces is often the best policy. Virtual
interfaces, such as IPsec tunnel interfaces or Mobile IP home
addresses SHOULD NOT be announced.
Note that the purpose of announcing IP addresses in a LOCATOR is to
provide connectivity between the communicating hosts. In most cases,
tunnels (and therefore virtual interfaces) provide sub-optimal
connectivity. Furthermore, it should be possible to replace most
tunnels with HIP based "non-tunneling", therefore making most virtual
interfaces fairly unnecessary in the future. On the other hand,
there are clearly situations where tunnels are used for diagnostic
and/or testing purposes. In such and other similar cases announcing
the IP addresses of virtual interfaces may be appropriate.
Once the host has decided on the groups and assignment of addresses
to the SPIs, it creates a LOCATOR parameter for each group. If there
are multiple LOCATOR parameters, the parameters MUST be ordered so
that the new preferred locator is in the first LOCATOR parameter.
Only one locator (the first one, if at all) may be indicated as
preferred for each distinct Traffic Type in the LOCATOR parameter.
If addresses are being added to an existing SPI, the LOCATOR
parameter includes the full set of valid addresses for that SPI, each
using a Locator Type of "1" and each with the same value for SPI.
Any locators previously ACTIVE on that SPI that are not included in
the LOCATOR will be set to DEPRECATED by the receiver.
If a mobile host decides to change the SPI upon a readdress, it sends
a LOCATOR with the SPI field within the LOCATOR set to the new SPI,
and also a NES parameter with the Old SPI field set to the previous
SPI and the New SPI field set to the new SPI. If multiple LOCATOR
and NES parameters are included, the NES MUST be ordered such that
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they appear in the same order as the set of corresponding LOCATORs.
The decision as to whether to rekey and send a new Diffie-Hellman
parameter while performing readdressing is a local policy decision.
If new addresses and new SPIs are being created, the LOCATOR
parameter's SPI field contains the new SPI, and the NES parameter's
Old SPI field and New SPI fields are both set to the new SPI,
indicating that this is a new and not a replacement SPI.
If there are multiple LOCATOR parameters leading to a packet size
that exceeds the MTU, the host SHOULD send multiple packets, each
smaller than the MTU. In the case of R1 and I2, the additional
packets should be UPDATE packets that are sent after the base
exchange has been completed.
7.2 Handling received LOCATORs
A host SHOULD be prepared to receive LOCATOR parameters in any HIP
packets, excluding I1.
When a host receives a LOCATOR parameter, it first performs the
following operations:
1. For each locator listed in the LOCATOR 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.
2. For each address listed in the LOCATOR parameter, check if the
address is already bound to the SPI. 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. Mark all addresses on the SPI that were NOT
listed in the LOCATOR parameter as DEPRECATED. As a result, the
SPI now contains any addresses listed in the LOCATOR parameter
either as UNVERIFIED or ACTIVE, and any old addresses not listed
in the LOCATOR parameter as DEPRECATED.
3. If the LOCATOR is paired with a NES parameter, the NES parameter
is processed. If the LOCATOR is replacing the address on an
existing SPI, the SPI itself may be changed-- in this case, the
host proceeds according to HIP rekeying procedures. This case is
indicated by the NES parameter including an existing SPI in the
Old SPI field and a new SPI in the New SPI field, and the SPI
field in the LOCATOR matching the New SPI in the NES. If instead
the LOCATOR corresponds to a new SPI, the NES will include the
same SPI in both its Old SPI and New SPI fields.
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4. Mark all locators at the address group that were NOT listed in
the LOCATOR parameter as DEPRECATED.
Once the host has updated the SPI, if the LOCATOR parameter contains
a new preferred locator, the host SHOULD initiate a change of the
preferred locator. This usually requires that the host first
verifies reachability of the associated address, and only then
changes the preferred locator. See Section 7.4.
7.3 Verifying address reachability
A host MAY want to verify the reachability of any UNVERIFIED address
at any time. It typically does so by sending a nonce to the new
address. For example, if the host is changing its SPI and is sending
a NES to the peer, the new SPI value SHOULD be random and the value
MAY be copied into an ECHO_REQUEST sent in the rekeying UPDATE. If
the host is not rekeying, it MAY still use the ECHO_REQUEST parameter
in an UPDATE message sent to the new address. A host MAY also use
other message exchanges as confirmation of the address reachability.
Note that in the case of receiving a LOCATOR on an R1 and replying
with an I2, receiving the corresponding R2 is sufficient for marking
the Responder's primary address active.
In some cases, it may be sufficient to use the arrival of data on a
newly advertised SA as implicit address reachability verification,
instead of waiting for the confirmation via a HIP packet (e.g.,
Figure 12). In this case, a host advertising a new SPI as part of
its address reachability check SHOULD be prepared to receive traffic
on the new SA. Marking the address active as a part of receiving
data on the SA is an idempotent operation, and does not cause any
harm.
Mobile host Peer host
prepare incoming SA
new SPI in R2, or UPDATE
<-----------------------------------
switch to new outgoing SA
data on new SA
----------------------------------->
mark address ACTIVE
Figure 12: Address activation via use of new SA
7.4 Changing the preferred locator
A host MAY want to change the preferred outgoing locator for
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different reasons, e.g., because traffic information or ICMP error
messages indicate that the currently used preferred address may have
become unreachable. Another reason is receiving a LOCATOR parameter
that has the P-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. Once the verification has
succeeded, the preferred locator change is completed, unless a
new change has been initiated in the meantime.
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 arrive that deprecate the preferred locator,
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.
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8. Policy considerations
XXX: This section needs to be written.
The host may change the status of unused ACTIVE addresses into
UNVERIFIED after a locally configured period of inactivity.
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9. Security Considerations
Text contribution expected from Greg Perkins
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10. IANA Considerations
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11. Acknowledgments
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12. References
12.1 Normative references
[1] Moskowitz, R., "Host Identity Protocol", draft-ietf-hip-base-01
(work in progress), October 2004.
[2] Kent, S. and R. Atkinson, "IP Encapsulating Security Payload
(ESP)", RFC 2406, November 1998.
[3] Moskowitz, R., "Host Identity Protocol Architecture",
draft-ietf-hip-arch-02 (work in progress), January 2005.
[4] Bradner, S., "Key words for use in RFCs to Indicate Requirement
Levels", BCP 14, RFC 2119, March 1997.
[5] Hinden, R. and S. Deering, "IP Version 6 Addressing
Architecture", RFC 2373, July 1998.
[6] Jokela, P., "Host Identity Protocol", draft-ietf-hip-esp-00
(work in progress), February 2005.
12.2 Informative references
[7] Bellovin, S., "EIDs, IPsec, and HostNAT", IETF 41th, March 1998.
[8] Rescorla, E. and B. Korver, "Guidelines for Writing RFC Text on
Security Considerations", draft-iab-sec-cons-00 (work in
progress), August 2002.
[9] Nikander, P., "Mobile IP version 6 Route Optimization Security
Design Background", draft-nikander-mobileip-v6-ro-sec-02 (work
in progress), December 2003.
Authors' Addresses
Pekka Nikander
Ericsson Research Nomadic Lab
JORVAS FIN-02420
FINLAND
Phone: +358 9 299 1
EMail: pekka.nikander@nomadiclab.com
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Jari Arkko
Ericsson Research Nomadic Lab
JORVAS FIN-02420
FINLAND
Phone: +358 9 299 1
EMail: jari.arkko@nomadiclab.com
Tom Henderson
The Boeing Company
P.O. Box 3707
Seattle, WA
USA
EMail: thomas.r.henderson@boeing.com
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Appendix A. Changes from previous versions
A.1 From nikander-hip-mm-00 to nikander-hip-mm-01
The actual protocol has been largely revised, based on the new
symmetric New SPI (NES) design adopted in the base protocol draft
version -08. There are no more separate REA, AC or ACR packets, but
their functionality has been folded into the NES packet. At the same
time, it has become possible to send REA parameters in R1 and I2.
The Forwarding Agent functionality was removed, since it looks like
that it will be moved to the proposed HIP Research Group. Hence,
there will be two other documents related to that, a simple
Rendezvous server document (WG item) and a Forwarding Agent document
(RG item).
A.2 From nikander-hip-mm-01 to nikander-hip-mm-02
Alignment with base-00 draft (use of UPDATE and NOTIFY packets).
The "logical interface" concept was dropped, and the SA/SPI was
identified as the protocol component to which a HIP association binds
addresses to.
The RR was (again) made recommended, not mandatory, able to be
administratively overridden.
A.3 From -02 to draft-ietf-hip-mm-00
REA parameter type value is now "3" (was TBD before).
Recommend that in multihoming situations, that inbound/outbound SAs
are paired to avoid ambiguity when rekeying them.
Clarified that multihoming scenario for now was intended for failover
instead of load-balancing, due to transport layer issues.
Clarified that if HIP negotiates base exchange using link local
addresses, that a host SHOULD provide its peer with a globally
reachable address.
Clarified whether REAs sent for existing SPIs update the full set of
addresses associated with that SPI, or only perform an incremental
(additive) update. REAs for an existing SPI should list all current
addresses for that SPI, and any addresses previously in use on the
SPI but not in the new REA parameter should be DEPRECATED.
Clarified that address verification pertains to *outgoing* addresses.
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When discussing inclusion of REA in I2, the draft stated "The
Responder MUST make sure that the puzzle solution is valid BOTH for
the initial IP destination address used for I1 and for the new
preferred address." However, this statement conflicted with Appendix
D of the base specification, so it has been removed for now.
A.4 From draft-ietf-hip-mm-00 to -01
Introduction section reorganized. Some of the scope of the document
relating to multihoming was reduced.
Removed empty appendix "Implementation experiences"
Renamed REA parameter to LOCATOR and aligned to the discussion on
redefining this parameter that occurred on the RG mailing list.
Aligned with decoupling of ESP from base spec.
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