Network Working Group P. Nikander
Internet-Draft J. Arkko
Expires: January 11, 2005 Ericsson Research Nomadic Lab
T. Henderson
The Boeing Company
July 13, 2004
End-Host Mobility and Multi-Homing with Host Identity Protocol
draft-nikander-hip-mm-02
Status of this Memo
This document is an Internet-Draft and is in full conformance with
all provisions of Section 10 of RFC2026.
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Copyright Notice
Copyright (C) The Internet Society (2004). All Rights Reserved.
Abstract
This document specifies basic end-host mobility and multi-homing
mechanisms for the Host Identity Protocol.
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3
2. Conventions used in this document . . . . . . . . . . . . . . 5
3. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 6
4. Overview of HIP basic mobility and multi-homing
functionality . . . . . . . . . . . . . . . . . . . . . . . . 7
4.1 Informing the peer about multiple or changed address(es) . . . 7
4.2 Address verification . . . . . . . . . . . . . . . . . . . . . 9
4.3 Preferred address . . . . . . . . . . . . . . . . . . . . . . 10
4.4 Address data structure and status . . . . . . . . . . . . . . 10
5. Protocol overview . . . . . . . . . . . . . . . . . . . . . . 12
5.1 Mobility with single SA pair . . . . . . . . . . . . . . . . . 12
5.2 Host multihoming . . . . . . . . . . . . . . . . . . . . . . . 14
5.3 Site multi-homing . . . . . . . . . . . . . . . . . . . . . . 15
5.4 Dual host multi-homing . . . . . . . . . . . . . . . . . . . . 15
5.5 Combined mobility and multi-homing . . . . . . . . . . . . . . 16
5.6 Network renumbering . . . . . . . . . . . . . . . . . . . . . 16
5.7 Initiating the protocol in R1 or I2 . . . . . . . . . . . . . 16
6. Parameter and packet formats . . . . . . . . . . . . . . . . . 18
6.1 REA parameter . . . . . . . . . . . . . . . . . . . . . . . . 18
6.2 UPDATE packet with included REA . . . . . . . . . . . . . . . 19
7. Processing rules . . . . . . . . . . . . . . . . . . . . . . . 20
7.1 Sending REAs . . . . . . . . . . . . . . . . . . . . . . . . . 20
7.2 Handling received REAs . . . . . . . . . . . . . . . . . . . . 21
7.3 Verifying address reachability . . . . . . . . . . . . . . . . 22
7.4 Changing the preferred address . . . . . . . . . . . . . . . . 22
8. Policy considerations . . . . . . . . . . . . . . . . . . . . 24
9. Security Considerations . . . . . . . . . . . . . . . . . . . 25
10. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 26
11. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 27
Normative references . . . . . . . . . . . . . . . . . . . . . 28
Informative references . . . . . . . . . . . . . . . . . . . . 29
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . 29
A. Changes from previous versions . . . . . . . . . . . . . . . . 30
A.1 From -00 to -01 . . . . . . . . . . . . . . . . . . . . . . . 30
A.2 From -01 to -02 . . . . . . . . . . . . . . . . . . . . . . . 30
B. Implementation experiences . . . . . . . . . . . . . . . . . . 31
Intellectual Property and Copyright Statements . . . . . . . . 32
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1. Introduction
This document specifies an extension to the Host Identity Protocol
[3] (HIP). The extension provides a means for keep their
communications on-going while having multiple IP addresses, either at
the same time or one after another. That is, the extension provides
basic end-to-end support for multi-homing, mobility, and simultaneous
multi-homing and mobility. Additionally, the extension allows
communications to continue even when multi-homing or mobility causes
a change of the IP version that is available in the network; that is,
if one of the communicating hosts has both IPv4 and IPv6
connectivity, either directly or through a proxy, the other host can
alternate between IPv4 and IPv6, without needing to tear down and
re-establish upper layer protocol connections or associations. In
other words, the way upper layer protocols need to react to
cross-IP-version handovers does not differ from the way they need to
react to intra-IP-version handovers.
This document does not specify any rendezvous or proxy services.
Those are subject to other specifications. Hence, this document
alone does not necessarily allow two mobile hosts to communicate,
unless they have other means for initial rendezvous and for solving
the simultaneous movement problem.
The Host Identity Protocol [3] (HIP) defines a mechanism that
decouples the transport layer (TCP, UDP, etc) from the
internetworking layer (IPv4 and IPv6), and introduces a new Host
Identity namespace. When a host uses HIP, the transport layer sockets
and IPsec Security Associations are not bound to IP addresses but to
Host Identifiers. This document specifies how the mapping from Host
Identifiers to IP addresses can be extended from a static one-to-one
mapping into a dynamic one-to-many mapping, thereby enabling end-host
mobility and multi-homing.
In practice, the HIP base exchange [3] creates a pair of IPsec
Security Associations (SA) between a pair of HIP enabled hosts.
These SAs are not bound to IP addresses, but to the Host Identifiers
(public keys) used to create them. 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.
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, 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
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characteristics in such a manner that reordering occurs and packets
fall outside the ESP anti-replay window.
This document specifies a mechanism that allows a HIP host to update
the set of addresses that its peers associate with it. The address
update is implemented with new HIP parameter types. Due to the danger
of flooding attacks (see [4]), the peers must always check the
reachability of the host at a new address, unless 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. This document does not address those needs.
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 [1].
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3. Terminology
Preferred address An address on which a host prefers to receive data.
With respect to a given peer, a host always has one active
preferred address. By default, the source address used in the HIP
base exchange is the preferred address.
New preferred address A new preferred address sent by a host to its
peers. The reachability of the new preferred address often needs
to be verified before it can be taken into use. Consequently,
there may simultaneously be an active preferred address, being
used, and a new preferred address, the reachability of which is
being verified.
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4. Overview of HIP basic mobility and multi-homing functionality
HIP mobility and multi-homing is fundamentally based on the HIP
architecture [4], 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.
In the HIP base protocol specification [3], it is defined how two
hosts exchange their Host Identifiers and establish a pair of ESP
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.
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.
4.1 Informing the peer about multiple or changed address(es)
This document specifies a new HIP protocol parameter, the REA
parameter (see Section 6.1), that allows the hosts to exchange
information about their IP address(es), and any changes in their
address(es). The logical structure created with REA parameters has
three levels: hosts, IPsec 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 [3] is illustrated in Figure 1.
-<- SPI1a -- -- SPI2a ->-
host1 < > addr1a <---> addr2a < > host2
->- SPI2a -- -- SPI1a -<-
Figure 1: Relation between hosts, SPIs, and addresses (base
specification)
In Figure 1, host1 and host2 negotiate two unidirectional IPsec SAs,
and each host selects the SPI value for its inbound SA. The
addresses addr1a and addr2a are the source addresses that each host
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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 2. 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 IPsec 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 REA 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 2: 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
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 2, it may seem
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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 IPsec ESP anti-replay window. For this reason, HIP
provides a mechanism to affiliate destination addresses with inbound
SPIs, if there is a concern that 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 reordering window is loose.
Moreover, even if the destination addresses used for a particular SPI
are held constant, the use of different source addresses may also
cause packets to fall outside of the ESP 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 address on which a
peer uses to send its packets on a given SPI. Hosts SHOULD
consistently use the same source 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 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 IPsec
selectors anyway.
A single REA parameter contains data only about one SPI. To
simultaneously signal changes on several SPIs, it is necessary to
send several REA parameters. The packet structure supports this.
If the REA parameter is sent in an UPDATE packet, then the receiver
will respond with an UPDATE acknowledgment. If the REA parameter is
sent in a NOTIFY, I2, or R2 packet, then the recipient may consider
the REA as informational, and act only when it needs to activate a
new address. The use of REA in a NOTIFY message may not be
compatible with middleboxes.
4.2 Address verification
When a HIP host receives a set of IP addresses from another HIP host
in a REA, 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 a bogus addresses in order to
cause a packet flood towards the given address [7]. Thus, before the
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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.
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.
4.3 Preferred address
When a host has multiple addresses and SPIs, the peer host must
decide upon which to use as a destination address. It may be that a
host would prefer to receive data on a particular inbound interface.
HIP allows a particular address to be designated as a preferred
address, and communicated to the peer.
4.4 Address data structure and status
In a typical implementation, each remote address is represented as a
piece of state that contains the following data:
the actual bit pattern representing the IPv4 or IPv6 address,
lifetime (seconds),
status (UNVERIFIED, ACTIVE, DEPRECATED).
The status is used to track the reachability of the address:
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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 address lifetime has expired
The following state changes are allowed:
UNVERIFIED to ACTIVE The reachability procedure completes
successfully.
UNVERIFIED to DEPRECATED The address lifetime expires while it is
UNVERIFIED.
ACTIVE to DEPRECATED The address 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
address.
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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5. Protocol overview
In this section we briefly introduce a number of usage scenarios
where the HIP mobility and multi-homing facility is useful. To
understand these usage scenarios, the reader should be at least
minimally familiar with the HIP protocol specification [3]. 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 one of its SPIs. The readdressing
exchange is designed to be piggybacked on a number of existing HIP
exchanges. The main packets on which the REA parameters are expected
to be carried on are UPDATE packets. However, some implementations
may want to experiment with sending REA parameters also on other
packets, such as R1, I2, and NOTIFY.
5.1 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 REA
parameter to the peer host in an UPDATE message. The REA
indicates the following: the new IP address, the SPI associated
with the new IP address, the address lifetime, and whether the
new address is a preferred address. 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 REA contains
the new SPI to use. Otherwise, the existing SPI is identified in
the REA parameter, and the host waits for its UPDATE to be
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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 3
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 4. 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 5. 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(REA, SEQ)
----------------------------------->
UPDATE(SPI, SEQ, ACK, ECHO_REQUEST)
<-----------------------------------
UPDATE(ACK, ECHO_RESPONSE)
----------------------------------->
Figure 3: Readdress without rekeying, but with address check
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Mobile Host Peer Host
UPDATE(REA, NES, SEQ, [DIFFIE_HELLMAN])
----------------------------------->
UPDATE(NES, SEQ, ACK, [DIFFIE_HELLMAN,] ECHO_REQUEST)
<-----------------------------------
UPDATE(ACK, ECHO_RESPONSE)
----------------------------------->
Figure 4: Readdress with mobile-initiated rekey
Mobile Host Peer Host
UPDATE(REA, SEQ)
----------------------------------->
UPDATE(NES, SEQ, ACK, [DIFFIE_HELLMAN], ECHO_REQUEST)
<-----------------------------------
UPDATE(NES, SEQ, ACK, [DIFFIE_HELLMAN,] ECHO_RESPONSE)
----------------------------------->
UPDATE(ACK)
<-----------------------------------
Figure 5: Readdress with peer-initiated rekey
5.2 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 REA parameter. To avoid problems with the
ESP reordering window, a host SHOULD use a different SA for each
interface used to receive packets from the peer host.
When more than one address is provided to the peer host, the host
SHOULD indicate which address is preferred. By default, the addresses
used in the base exchange are preferred until indicated otherwise.
To add an additional interface and SA, the host sends a REA with a
NES. The host uses the same (new) SPI value in the REA and both the
"Old SPI" and "New SPI" values in the NES-- this 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 6
illustrates the basic packet exchange.
When processing inbound REAs that establish new security
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associations, a host uses the destination address of the UPDATE
containing REA as the local address to which the REA plus NES is
targeted. Hosts may send REA with the same IP address to different
peer addresses-- this has the effect of creating multiple inbound SAs
implicitly affiliated with different source addresses.
Multi-homed Host Peer Host
UPDATE(REA, NES, SEQ, [DIFFIE_HELLMAN])
----------------------------------->
UPDATE(NES, SEQ, ACK, [DIFFIE_HELLMAN,] ECHO_REQUEST)
<-----------------------------------
UPDATE(ACK, ECHO_RESPONSE)
----------------------------------->
Figure 6: Basic multihoming scenario
5.3 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 5.2. 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.
5.4 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 7, consider that
host1 wants to add address addr1b. It would send a REA 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
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to add addr2b to the relationship. host2 now has a choice of which of
host1's addresses to initiate REA to. It may choose to initiate a
REA 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 address.
The readdressing protocol is flexible enough to accommodate this
choice.
-<- SPI1a -- -- SPI2a ->-
host1 < > addr1a <---> addr2a < > host2
->- SPI2a -- -- SPI1a -<-
addr1b <---> addr2b
Figure 7: Dual multihoming case in which each host uses REA to add a
second address
5.5 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 REA plus NES should be flexible enough to handle most such
scenarios, although more complicated scenarios have not been studied
so far.
5.6 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.
5.7 Initiating the protocol in R1 or I2
A Responder host MAY include one or more REA 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 REA
parameters, the Initiator SHOULD send the I2 packet to the new
preferred address. 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. The I1 destination address
and the new preferred address may be identical.
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Initiator Responder
R1 with REA
<-----------------------------------
record additional addresses
change responder address
I2 with new SPI in SPI parameter
----------------------------------->
(process normally)
R2
<-----------------------------------
(process normally)
Figure 8: REA inclusion in R1
An Initiator MAY include one or more REA parameters in the I2 packet,
independent on whether there was REA parameter(s) in the R1 or not.
These parameters MUST be protected by the I2 signature. Even if the
I2 packet contains REA parameters, the Responder MUST still send the
R2 packet to the source address of the I2. The new preferred address
SHOULD be identical to the I2 source address.
Initiator Responder
I2 with REA
----------------------------------->
(process normally)
record additional addresses
R2 with new SPI in SPI parameter
<-----------------------------------
(process normally)
data on new SA
------------------------------------>
(process normally)
Figure 9: REA inclusion in I2
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6. Parameter and packet formats
6.1 REA parameter
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Type | Length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| SPI |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Address Lifetime |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|P| Reserved |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Address |
| |
| |
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
. .
. .
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Address Lifetime |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Reserved |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Address |
| |
| |
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Type: TBD (to be determined)
Length: Length in octets, excluding Type and Length fields.
SPI: Security Parameter Index (SPI) corresponding to Addresses
P: Preferred address. Set to one if the first address in this REA is
the new preferred address; otherwise set to zero.
Reserved: Zero when sent, ignored when received.
Address Lifetime: Address lifetime, in seconds.
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Address: An IPv6 address or an IPv4-in-IPv6 format IPv4 address [2].
The SPI field identifies the SPI that this parameter applies to. It
is implicitly qualified by the Host Identity of the sending host.
The sending host is free to introduce new SPIs at will. That is, if
a received REA has a new SPI, it means that all the old addresses,
assigned to the other SPIs, are also supposed to still work, while
the new addresses in the newly received REA are supposed to be
associated with a new SPI. On the other hand, if a received REA has
an SPI that the receiver already knows about, it would replace (all)
the address(es) currently associated with the SPI with the new
one(s).
The Address Lifetime indicates how long the following address is
expected to be valid. The lifetime is expressed in seconds. Each
address 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. Since IP addresses are ignored
upon reception, deprecation status does not have any affect on the
receiver.
The Address field contains an IPv6 address, or an IPv4 address in the
IPv4-in-IPv6 format [2]. The latter format denotes a plain IPv4
address that can be used to reach the Mobile Host.
6.2 UPDATE packet with included REA
A number of combinations of parameters in an UPDATE packet are
possible (e.g., see Section 5). Any UPDATE packet that includes a REA
parameter SHOULD include both an HMAC and a HIP_SIGNATURE parameter.>
If there are multiple REA parameters to be sent in a single UPDATE,
each of them must be matched with a NES parameter:
IP ( HIP ( REA1, REA2, NES1, NES2, [ DH, ] ... ) )
If there are multiple REA parameters to be sent and not all are
paired with a NES, then multiple UPDATEs must be used (some with NES,
some without) to avoid ambiguity in the pairing of REA with NES.
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7. Processing rules
7.1 Sending REAs
The decision of when to send REAs is basically a local policy issue.
However, it is RECOMMENDED that a host sends a REA 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 REAs SHOULD be avoided.
When a host decided 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 IPsec 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 REA 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 REA parameter for each group. If there are
multiple REA parameters, the parameters MUST be ordered so that the
new preferred address is in the first REA parameter. Only one address
(the first one, if at all) may be indicated as preferred in the REA
parameter.
If addresses are being added to an existing SPI, the REA parameter
indicates the existing SPI and one or more addresses to add to the
SPI. It is not necessary to repeat addresses already known by the
peer host, unless the address lifetime is to be extended.
If a mobile host decides to change the SPI upon a readdress, it sends
a REA with the SPI field within the REA set to the new address, 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 REA and NES
parameters are included, the NES MUST be ordered such that they
appear in the same order as the set of corresponding REAs. The
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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 REA parameter's
SPI field contains the new SPI, and the NES parameter's the 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 REA 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 REAs
A host SHOULD be prepared to receive REA parameters in any HIP
packets, excluding I1.
When a host receives a REA parameter, it first performs the following
operations:
1. The host checks if the SPI listed is a new one. If it is a new
one, it creates a new SPI that contains no addresses. If it is
an existing one, it prepares to add addresses to the existing
SPI.
2. For each address listed in the REA parameter, check that the
address 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.
3. For each address listed in the REA 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.
4. If the REA is paired with a NES parameter, the NES parameter is
processed. If the REA 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 REA matching the New SPI in the NES. If instead the
REA corresponds to a new SPI, the NES will include the same SPI
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in both its Old SPI and New SPI fields.
Once the host has updated the SPI, if the REA parameter contains a
new preferred address, the host SHOULD initiate a change of the
preferred address. This usually requires that the host first
verifies reachability of the address, and only then changes the
preferred address. 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 REA 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 address
A host MAY want to change the preferred outgoing address for
different reasons, e.g., because traffic information or ICMP error
messages indicate that the currently used preferred address may have
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become unreachable. Another reason is receiving a REA parameter that
has the P-bit set.
To change the preferred address, the host initiates the following
procedure:
1. If the new preferred address has ACTIVE status, the preferred
address is changed and the procedure succeeds.
2. If the new preferred address has UNVERIFIED status, the host
starts to verify its reachability. Once the verification has
succeeded, the preferred address 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 erro messages arrive that deprecate the preferred address,
but the peer has not yet indicated a new preferred address.
4. If the new preferred address 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 address 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
XXX: This section requires lots of more work.
(Initial text by Jari Arkko): If not controlled in some manner,
messaging related to address changes would create the following types
of vulnerabilities:
Revealing the contents of the (cleartext) communications
Hijacking communications and man-in-the-middle attacks
Denial of service for the involved nodes, by disabling their
ability to receive the desired communications
Denial of service for third parties, by redirecting a large amount
of traffic to them
Revealing the location of the nodes to other parties
In HIP, communications are bound to the public keys of the end-points
and not to IP addresses. The REA message is signed with the sender's
public key, and hence it becomes impossible to hijack the
communications of another host through the use of the REA message.
Similarly, since only the host itself can sign messages to move its
traffic flows to a new IP address, denial of service attacks through
REA can not cause the traffic flows to be sent to an IP address that
the host did not wish to use. Finally, in HIP all communications are
encrypted with ESP, so a hijack attempt would also be unable to
reveal the contents of the communications.
Malicious nodes that use HIP can, however, try to cause a denial of
service attack by establishing a high-volume traffic flow, such as a
video stream, and then redirecting it to a victim. However, the
return routability check provides some assurance that the given
address is willing to accept the new traffic. Only attackers who are
on the path between the peer and the new address could respond to the
test.
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10. IANA Considerations
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11. Acknowledgments
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Normative references
[1] Bradner, S., "Key words for use in RFCs to Indicate Requirement
Levels", BCP 14, RFC 2119, March 1997.
[2] Hinden, R. and S. Deering, "IP Version 6 Addressing
Architecture", RFC 2373, July 1998.
[3] Moskowitz, R., Nikander, P. and P. Jokela, "Host Identity
Protocol", draft-moskowitz-hip-09 (work in progress), February
2004.
[4] Moskowitz, R., "Host Identity Protocol Architecture",
draft-moskowitz-hip-arch-05 (work in progress), October 2003.
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Informative references
[5] Bellovin, S., "EIDs, IPsec, and HostNAT", IETF 41th, March 1998.
[6] Rescorla, E. and B. Korver, "Guidelines for Writing RFC Text on
Security Considerations", draft-iab-sec-cons-00 (work in
progress), August 2002.
[7] 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
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 -00 to -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 -01 to -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.
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Appendix B. Implementation experiences
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