One document matched: draft-ietf-hip-mm-03.txt
Differences from draft-ietf-hip-mm-02.txt
Network Working Group T. Henderson (editor)
Internet-Draft The Boeing Company
Expires: August 28, 2006 February 24, 2006
End-Host Mobility and Multihoming with the Host Identity Protocol
draft-ietf-hip-mm-03
Status of this Memo
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This Internet-Draft will expire on August 28, 2006.
Copyright Notice
Copyright (C) The Internet Society (2006).
Abstract
This document defines mobility and multihoming extensions to the Host
Identity Protocol (HIP). Specifically, this document defines a
general "LOCATOR" parameter for HIP messages that allows for a HIP
host to notify peers about alternate addresses at which it may be
reached. This document also defines elements of procedure for
mobility of a HIP host-- the process by which a host dynamically
changes the primary locator that it uses to receive packets. While
the same LOCATOR parameter can also be used to support end-host
multihoming, detailed procedures are left for further study.
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Table of Contents
1. Introduction and Scope . . . . . . . . . . . . . . . . . . . . 4
2. Terminology and Conventions . . . . . . . . . . . . . . . . . 6
3. Protocol Model . . . . . . . . . . . . . . . . . . . . . . . . 7
3.1. Operating Environment . . . . . . . . . . . . . . . . . . 7
3.1.1. Locator . . . . . . . . . . . . . . . . . . . . . . . 9
3.1.2. Mobility overview . . . . . . . . . . . . . . . . . . 10
3.1.3. Multihoming overview . . . . . . . . . . . . . . . . . 10
3.2. Protocol Overview . . . . . . . . . . . . . . . . . . . . 10
3.2.1. Mobility with single SA pair (no rekeying) . . . . . . 11
3.2.2. Host multihoming . . . . . . . . . . . . . . . . . . . 13
3.2.3. Site multihoming . . . . . . . . . . . . . . . . . . . 15
3.2.4. Dual host multihoming . . . . . . . . . . . . . . . . 15
3.2.5. Combined mobility and multihoming . . . . . . . . . . 16
3.2.6. Using LOCATORs across addressing realms . . . . . . . 16
3.2.7. Network renumbering . . . . . . . . . . . . . . . . . 16
3.2.8. Initiating the protocol in R1 or I2 . . . . . . . . . 16
3.3. Other Considerations . . . . . . . . . . . . . . . . . . . 17
3.3.1. Address Verification . . . . . . . . . . . . . . . . . 17
3.3.2. Credit-Based Authorization . . . . . . . . . . . . . . 18
3.3.3. Preferred locator . . . . . . . . . . . . . . . . . . 19
3.3.4. Interaction with Security Associations . . . . . . . . 20
4. LOCATOR parameter format . . . . . . . . . . . . . . . . . . . 23
4.1. Traffic Type and Preferred Locator . . . . . . . . . . . . 24
4.2. Locator Type and Locator . . . . . . . . . . . . . . . . . 25
4.3. UPDATE packet with included LOCATOR . . . . . . . . . . . 25
5. Processing rules . . . . . . . . . . . . . . . . . . . . . . . 26
5.1. Locator data structure and status . . . . . . . . . . . . 26
5.2. Sending LOCATORs . . . . . . . . . . . . . . . . . . . . . 26
5.3. Handling received LOCATORs . . . . . . . . . . . . . . . . 28
5.4. Verifying address reachability . . . . . . . . . . . . . . 30
5.5. Credit-Based Authorization . . . . . . . . . . . . . . . . 31
5.5.1. Handling Payload Packets . . . . . . . . . . . . . . . 31
5.5.2. Credit Aging . . . . . . . . . . . . . . . . . . . . . 33
5.6. Changing the preferred locator . . . . . . . . . . . . . . 34
6. Security Considerations . . . . . . . . . . . . . . . . . . . 36
6.1. Impersonation attacks . . . . . . . . . . . . . . . . . . 36
6.2. Denial of Service attacks . . . . . . . . . . . . . . . . 37
6.2.1. Flooding Attacks . . . . . . . . . . . . . . . . . . . 37
6.2.2. Memory/Computational exhaustion DoS attacks . . . . . 38
6.3. Mixed deployment environment . . . . . . . . . . . . . . . 38
7. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 40
8. Authors . . . . . . . . . . . . . . . . . . . . . . . . . . . 41
9. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 42
10. References . . . . . . . . . . . . . . . . . . . . . . . . . . 43
10.1. Normative references . . . . . . . . . . . . . . . . . . . 43
10.2. Informative references . . . . . . . . . . . . . . . . . . 43
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Appendix A. Changes from previous versions . . . . . . . . . . . 44
A.1. From nikander-hip-mm-00 to nikander-hip-mm-01 . . . . . . 44
A.2. From nikander-hip-mm-01 to nikander-hip-mm-02 . . . . . . 44
A.3. From -02 to draft-ietf-hip-mm-00 . . . . . . . . . . . . . 44
A.4. From draft-ietf-hip-mm-00 to -01 . . . . . . . . . . . . . 45
A.5. From draft-ietf-hip-mm-01 to -02 . . . . . . . . . . . . . 45
A.6. From draft-ietf-hip-mm-02 to -03 . . . . . . . . . . . . . 45
Author's Address . . . . . . . . . . . . . . . . . . . . . . . . . 47
Intellectual Property and Copyright Statements . . . . . . . . . . 48
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1. Introduction and Scope
The Host Identity Protocol [1] (HIP) supports an architecture that
decouples the transport layer (TCP, UDP, etc.) from the
internetworking layer (IPv4 and IPv6) by using public/private key
pairs, instead of IP addresses, as host identities. When a host uses
HIP, the overlying protocol sublayers (e.g., transport layer sockets
and ESP Security Associations) are instead bound to representations
of these host identities, and the IP addresses are only used for
packet forwarding. However, each host must also know at least one IP
address at which its peers are reachable. Initially, these IP
addresses are the ones used during the HIP base exchange [2].
One consequence of such a decoupling is that new solutions to
network-layer mobility and host multihoming are possible. There are
potentially many variations of mobility and multihoming possible.
The scope of this document encompasses messaging and elements of
procedure for basic network-level mobility and simple multihoming,
leaving more complicated scenarios and other variations for further
study. Specifically,
This document defines a generalized LOCATOR parameter for use in
HIP messages. The LOCATOR parameter allows a HIP host to notify a
peer about alternate addresses at which it is reachable. The
LOCATORs may be merely IP addresses, or they may have additional
multiplexing and demultiplexing context to aid the packet handling
in the lower layers. For instance, an IP address may need to be
paired with an ESP SPI so that packets are sent on the correct SA
for a given address.
This document also specifies the messaging and elements of
procedure for end-host mobility of a HIP host-- the sequential
change in preferred IP address used to reach a host. In
particular, message flows to enable successful host mobility,
including address verification methods, are defined herein.
However, while the same LOCATOR parameter is intended to support
host multihoming (parallel support of a number of addresses), and
experimentation is encouraged, detailed elements of procedure for
host multihoming are left for further study.
While HIP can potentially be used with transports other than the ESP
transport format [5], this document largely assumes the use of ESP
and leaves other transport for further study.
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, simultaneous
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mobility of both hosts, and some modes of NAT traversal. In these
situations there is a need for some helper functionality in the
network, such as a HIP Rendezvous server [3]. Such functionality is
out of scope of this document. We also do not consider localized
mobility management extensions; this document is concerned with end-
to-end mobility. 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 multihoming address change, are outside the
scope of this document.
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2. Terminology and Conventions
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in RFC2119 [6].
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.
Credit Based Authorization. A host must verify a mobile or multi-
homed peer's reachability at a new locator. Credit-Based
Authorization authorizes the peer to receive a certain amount of
data at the new locator before the result of such verification is
known.
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3. Protocol Model
3.1. Operating Environment
The Host Identity Protocol (HIP) [2] is a key establishment and
parameter negotiation protocol. Its primary applications are for
authenticating host messages based on host identities, and
establishing security associations (SAs) for ESP transport format [5]
and possibly other protocols in the future.
+--------------------+ +--------------------+
| | | |
| +------------+ | | +------------+ |
| | Key | | HIP | | Key | |
| | Management | <-+-----------------------+-> | Management | |
| | Process | | | | Process | |
| +------------+ | | +------------+ |
| ^ | | ^ |
| | | | | |
| v | | v |
| +------------+ | | +------------+ |
| | IPsec | | ESP | | IPsec | |
| | Stack | <-+-----------------------+-> | Stack | |
| | | | | | | |
| +------------+ | | +------------+ |
| | | |
| | | |
| Initiator | | Responder |
+--------------------+ +--------------------+
Figure 1: HIP deployment model
The general deployment model for HIP is shown above, assuming
operation in an end-to-end fashion. This document specifies
extensions to the HIP protocol to enable end-host mobility and
multihoming. In summary, these extensions to the HIP protocol can
carry new addressing information to the peer and can enable direct
authentication of the message via a signature or keyed hash message
authentication code (HMAC) based on its host identity. This document
specifies the format of this new addressing (LOCATOR) parameter, the
procedures for sending and processing this parameter to enable basic
host mobility, and procedures for a concurrent address verification
mechanism.
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---------
| TCP | (sockets bound to HITs)
---------
|
---------
----> | ESP | {HIT_s, HIT_d} <-> SPI
| ---------
| |
---- ---------
| MH |-> | HIP | {HIT_s, HIT_d, SPI} <-> {IP_s, IP_d, SPI}
---- ---------
|
---------
| IP |
---------
Figure 2: Architecture for HIP mobility and multihoming
Figure 2 depicts a layered architectural view of a HIP-enabled stack
using ESP transport format. In HIP, upper-layer protocols (including
TCP and ESP in this figure) are bound to HITs and not IP addresses.
The HIP sublayer is responsible for maintaining the binding between
HITs and IP addresses. The SPI (or other context tag if ESP is not
used with HIP), and not necessarily the IP addresses, is used to
associate an incoming packet with the right HITs. The block labeled
"MH" is introduced below.
Consider first the case in which there is no mobility or multihoming,
as specified in the base protocol specification [2]. The HIP base
exchange establishes the HITs in use between the hosts, the SPIs to
use for ESP, and the IP addresses (used in the HIP signaling
packets). Note that there can only be one such binding in the
outbound direction for any given packet, and the only selectors for
the binding at the HIP layer are the fields exposed by ESP (the SPI
and HITs). For the inbound direction, the SPI is all that is
required to find the right host context. ESP rekeying events change
the mapping between the HIT pair and SPI, but do not change the IP
addresses.
Consider next a mobility event, in which a host is still single-homed
but moves to another IP address. Two things must occur in this case.
First, the peer must be notified of the address change using a HIP
UPDATE message. Second, each host must change its local bindings at
the HIP sublayer (new IP addresses). It may be that both the SPIs
and IP addresses are changed simultaneously in a single UPDATE; the
protocol described herein supports this. This document specifies the
messaging and elements of procedure for such a mobility event.
However, simultaneous movement of both hosts, notification of
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transport layer protocols of the path change, and procedures for
possibly traversing middleboxes are not covered by this document.
Finally, consider the case when a host is multihomed (has more than
one globally routable address) and wants to make these multiple
addresses available for use by the upper layer protocols, for fault
tolerance. Examples include the use of (possibly multiple) IPv4 and
IPv6 addresses on the same interface, or the use of multiple
interfaces attached to different service providers. Such host
multihoming generally necessitates that a separate ESP SA is
maintained for each interface in order to prevent packets that arrive
over different paths from falling outside of the ESP replay
protection window. Multihoming thus makes possible that the bindings
shown on the right side of Figure 2 are one to many (in the outbound
direction, one HIT pair to multiple SPIs, and possibly then to
multiple IP addresses). However, only one SPI and address can be
used for any given packet, so the job of the "MH" block depicted
above is to dynamically manipulate these bindings. Beyond locally
managing such multiple bindings, the peer-to-peer HIP signaling
protocol needs to be flexible enough to define the desired mappings
between HITs, SPIs, and addresses, and needs to ensure that UPDATE
messages are sent along the right network paths so that any HIP-aware
middleboxes can observe the SPIs. This document does not specify the
"MH" block, nor does it specify detailed elements of procedure for
how to handle various multihoming (perhaps combined with mobility)
scenarios. However, this document does describe a basic multihoming
case (one host adds one address to its initial address and notifies
the peer) and leave more complicated scenarios for experimentation
and future documents.
3.1.1. Locator
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 of the stack. 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 [4]. Addresses may also be affiliated with transport ports in
certain tunneling scenarios. Or locators may merely be traditional
network addresses. In Section 4, a generalized HIP LOCATOR parameter
is defined that can contain one or more locators (addresses).
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3.1.2. Mobility overview
When a host moves to another address, it notifies its peer of the new
address by sending a HIP UPDATE packet containing a LOCATOR
parameter. This UPDATE packet is acknowledged by the peer, and is
protected by retransmission. The peer can authenticate the contents
of the UPDATE packet based on the signature and keyed hash of the
packet.
When using ESP Transport Format [5], the host may at the same time
decide to rekey its security association and possibly generate a new
Diffie-Hellman key; all of these actions are triggered by including
additional parameters in the UPDATE packet, as defined in the base
protocol specification [2] and ESP extension [5].
When using ESP (and possibly other transport modes in the future),
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 without necessarily
rekeying. However, 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 for the SA, thereby requiring
rekeying.
3.1.3. Multihoming overview
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 detailed policies and procedures such
as which 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 future documents.
3.2. Protocol Overview
In this section we briefly introduce a number of usage scenarios for
HIP mobility and multihoming. These scenarios assume that HIP is
being used with the ESP transform [5], 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
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specification [2]. 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 a mobile or
multihomed host informs a peer host about changes of IP addresses on
affected SPIs. The readdressing exchange is designed to be
piggybacked on existing HIP exchanges. The main packets on which the
LOCATOR parameters are expected to be carried are UPDATE packets.
However, some implementations may want to experiment with sending
LOCATOR parameters also on other packets, such as R1, I2, and NOTIFY.
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.
3.2.1. Mobility with single SA pair (no rekeying)
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, a
single pair of SAs (one inbound, one outbound), and no rekeying
occurs on the SAs. We also assume that the new IP addresses are
within the same address family (IPv4 or IPv6) as the first address.
This is the simplest scenario, depicted in Figure 3.
Mobile Host Peer Host
UPDATE(ESP_INFO, LOCATOR, SEQ)
----------------------------------->
UPDATE(ESP_INFO, SEQ, ACK, ECHO_REQUEST)
<-----------------------------------
UPDATE(ACK, ECHO_RESPONSE)
----------------------------------->
Figure 3: Readdress without rekeying, but with address check
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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 UPDATE
message also contains an ESP_INFO parameter with the "Old SPI"
and "New SPI" parameters both set to the value of the pre-
existing incoming SPI; this ESP_INFO does not trigger a rekeying
event but is instead included for possible parameter-inspecting
middleboxes on the path. The LOCATOR parameter contains the new
IP address (Locator Type of "1", defined below) and a locator
lifetime. The mobile host waits for this UPDATE to be
acknowledged, and retransmits if necessary, as specified in the
base specification [2].
2. The peer host receives the UPDATE, validates it, and updates any
local bindings between the HIP association and the mobile host's
destination address. The peer host MUST perform an address
verification by placing a nonce in the ECHO_REQUEST parameter of
hte UPDATE message sent back to the mobile host. It also
includes an ESP_INFO parameter with the "Old SPI" and "New SPI"
parameters both set to the value of the pre-existing incoming
SPI, and sends this UPDATE (with piggybacked acknowledgment) to
the mobile host at its new address. The peer MAY use the new
address immediately, but it MUST limit the amount of data it
sends to the address until address verification completes.
3. The mobile host completes the readdress by processing the UPDATE
ACK and echoing the nonce in an ECHO_RESPONSE. Once the peer
host receives this ECHO_RESPONSE, it considers the new address to
be verified and can put it into full use.
While the peer host is verifying the new address, the new address is
marked as UNVERIFIED in the interim, and the old address is
DEPRECATED. Once the peer host has received a correct reply to its
UPDATE challenge, it marks the new address as ACTIVE and removes the
old address.
3.2.1.1. Mobility with single SA pair (mobile-initiated rekey)
The mobile host may decide to rekey the SAs at the same time that it
is notifying the peer of the new address. In this case, the above
procedure described in Figure 3 is slightly modified. The UPDATE
message sent from the mobile host includes an ESP_INFO with the "Old
SPI" set to the previous SPI, the "New SPI" set to the desired new
SPI value for the incoming SA, and the Keymat Index desired.
Optionally, the host may include a DIFFIE_HELLMAN parameter for a new
Diffie-Hellman key. The peer completes the request for rekey as is
normally done for HIP rekeying, except that the new address is kept
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as UNVERIFIED until the UPDATE nonce challenge is received as
described above. Figure 4 illustrates this scenario.
Mobile Host Peer Host
UPDATE(ESP_INFO, LOCATOR, SEQ, [DIFFIE_HELLMAN])
----------------------------------->
UPDATE(ESP_INFO, SEQ, ACK, [DIFFIE_HELLMAN,] ECHO_REQUEST)
<-----------------------------------
UPDATE(ACK, ECHO_RESPONSE)
----------------------------------->
Figure 4: Readdress with mobile-initiated rekey
3.2.1.2. Mobility with single SA pair (peer-initiated rekey)
A second variation of this basic mobility scenario covers the case in
which the mobile host does not attempt to rekey the existing SAs, but
the peer host decides to do so. This typically results in a four
packet exchange, as shown in Figure 5. The initial UPDATE packet
from the mobile host is the same as in the scenario for which there
is no rekey (Figure 3). The peer may decide to rekey, however, in
which case the subsequent three packets follow the normal rekeying
procedure described in the ESP specification [5], with the addition
of the ECHO_REQUEST and ECHO_RESPONSE nonce for verification of the
new address.
Mobile Host Peer Host
UPDATE(ESP_INFO, LOCATOR, SEQ)
----------------------------------->
UPDATE(ESP_INFO, SEQ, ACK, [DIFFIE_HELLMAN], ECHO_REQUEST)
<-----------------------------------
UPDATE(ESP_INFO, SEQ, ACK, [DIFFIE_HELLMAN,] ECHO_RESPONSE)
----------------------------------->
UPDATE(ACK)
<-----------------------------------
Figure 5: Readdress with peer-initiated rekey
3.2.2. Host multihoming
A (mobile or stationary) host may sometimes have more than one
interface or global address. The host may notify the peer host of
the additional interface or address by using the LOCATOR parameter.
To avoid problems with the ESP anti-replay window, a host SHOULD use
a different SA for each interface or address used to receive packets
from the peer host.
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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 an ESP_INFO 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.
Consider the case between two single-homed hosts, in which one of the
host notifies the peer of an additional address. It is RECOMMENDED
that the host set up a new SA pair for use on this new address. To
do this, the multihomed host sends a LOCATOR with an ESP_INFO,
indicating the request for a new SA by setting the "Old SPI" value to
zero, and the "New SPI" value to the newly created incoming SPI. A
Locator Type of "1" is used to associate the new address with the new
SPI. The LOCATOR parameter also contains a second Type 1 locator:
that of the original address and SPI. To simplify parameter
processing and avoid explicit protocol extensions to remove locators,
each LOCATOR parameter must list all locators in use on a connection
(a complete listing of inbound locators and SPIs for the host). The
multihomed host transitions to state REKEYING, waiting for a ESP_INFO
(new outbound SA) from the peer and an ACK of its own UPDATE. As in
the mobility case, the peer host must perform an address verification
before putting the new address into active use. Figure 6 illustrates
the basic packet exchange.
Multi-homed Host Peer Host
UPDATE(ESP_INFO, LOCATOR, SEQ, [DIFFIE_HELLMAN])
----------------------------------->
UPDATE(ESP_INFO, SEQ, ACK, [DIFFIE_HELLMAN,] ECHO_REQUEST)
<-----------------------------------
UPDATE(ACK, ECHO_RESPONSE)
----------------------------------->
Figure 6: Basic multihoming scenario
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When processing inbound LOCATORs that establish new security
associations on an interface with multiple addresses, a host uses the
destination address of the UPDATE containing LOCATOR as the local
address to which the LOCATOR plus ESP_INFO is targeted. Hosts may
send UPDATEs with the same IP address in the LOCATOR to different
peer addresses-- this has the effect of creating multiple inbound SAs
implicitly affiliated with different peer source addresses.
3.2.3. Site multihoming
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 3.2.2. Note that a single
interface may experience site multihoming 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; further alignment with the IETF shim6 working
group may be considered in the future.
3.2.4. Dual host multihoming
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 an UPDATE with
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 to which of host1's addresses to initiate an UPDATE. It
may choose to initiate an UPDATE 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
often may be 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 <---> addr2a (second SA pair)
addr1a <---> addr2b (third SA pair)
addr1b <---> addr2b (fourth SA pair)
Figure 7: Dual multihoming case in which each host uses LOCATOR to
add a second address
3.2.5. Combined mobility and multihoming
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 ESP_INFO should be flexible enough to handle
most such scenarios, although more complicated scenarios have not
been studied so far.
3.2.6. 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 reach the destination anyway, and the HIP
state may time out.
3.2.7. 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.
3.2.8. 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
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parameters with a new preferred locator, the Initiator SHOULD
directly set the new preferred locator to status ACTIVE without
performing address verification first, and MUST send the I2 packet to
the new preferred locator. The I1 destination address and the new
preferred locator may be identical. All new non-preferred locators
must still undergo address verification.
Initiator Responder
R1 with LOCATOR
<-----------------------------------
record additional addresses
change responder address
I2 sent to newly indicated preferred address
----------------------------------->
(process normally)
R2
<-----------------------------------
(process normally, later verification of non-preferred locators)
Figure 8: LOCATOR inclusion in R1
An Initiator MAY include one or more LOCATOR parameters in the I2
packet, independent of whether there was a LOCATOR parameter 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. If
the I2 packet contains LOCATOR parameters, all new locators must
undergo address verification as usual.
Initiator Responder
I2 with LOCATOR
----------------------------------->
(process normally)
record additional addresses
R2 sent to source address of I2
<-----------------------------------
(process normally)
Figure 9: LOCATOR inclusion in I2
3.3. Other Considerations
3.3.1. Address Verification
When a HIP host receives a set of locators from another HIP host in a
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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 target addresses [8]. Likewise,
viral software may have compromised the peer host, programming it to
redirect packets to the target addresses. Thus, the HIP host must
first check that the peer is reachable at the new address.
An additional potential benefit of performing address verification is
to allow middleboxes in the network along the new path to obtain the
peer host's inbound SPI.
Address verification 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.
3.3.2. Credit-Based Authorization
Credit-Based Authorization allows a host to securely use a new
locator even though the peer's reachability at the address embedded
in this locator has not yet been verified. This is accomplished
based on the following three hypotheses:
1. A flooding attacker typically seeks to somehow multiply the
packets it generates itself for the purpose of its attack because
bandwidth is an ample resource for many attractive victims.
2. An attacker can always cause unamplified flooding by sending
packets to its victim directly.
3. Consequently, the additional effort required to set up a
redirection-based flooding attack would pay off for the attacker
only if amplification could be obtained this way.
On this basis, rather than eliminating malicious packet redirection
in the first place, Credit-Based Authorization prevents any
amplification that can be reached through it. This is accomplished
by limiting the data a host can send to an unverified address of a
peer by the data recently received from that peer. Redirection-based
flooding attacks thus become less attractive than, e.g., pure direct
flooding, where the attacker itself sends bogus packets to the
victim.
Figure 10 illustrates Credit-Based Authorization: Host B measures the
bytes recently received from peer A and, when A readdresses, sends
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packets to A's new, unverified address as long as the sum of their
sizes does not exceed the measured, received data volume. When
insufficient credit is left, B stops sending further packets to A
until A's address becomes ACTIVE. The address changes may be due to
mobility, due to multihoming, or due to any other reason.
+-------+ +-------+
| A | | B |
+-------+ +-------+
| |
address |------------------------->| credit += size(packet)
ACTIVE | |
|------------------------->| credit += size(packet)
|<-------------------------| don't change credit
| |
+ address change |
address |<-------------------------| credit -= size(packet)
UNVERIFIED |------------------------->| credit += size(packet)
|<-------------------------| credit -= size(packet)
| |
|<-------------------------| credit -= size(packet)
| X credit < size(packet)=> drop!
| |
+ address change |
address | |
ACTIVE |<-------------------------| don't change credit
| |
Figure 10: Readdressing Scenario
3.3.3. 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
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.
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3.3.4. Interaction with Security Associations
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 [2] and ESP transform [5] is
illustrated in Figure 11.
-<- SPI1a -- -- SPI2a ->-
host1 < > addr1a <---> addr2a < > host2
->- SPI2a -- -- SPI1a -<-
Figure 11: Relation between hosts, SPIs, and addresses (base
specification)
In Figure 11, 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 12.
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.
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address11
/
SPI1 - address12
/
/ address21
host -- SPI2 <
\ address22
\
SPI3 - address31
\
address32
Figure 12: 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 multihoming, 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 12, 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
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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 and address when
sending to a particular destination IP address and SPI. For this
reason, a host may find it useful to change its SPI or at least reset
its ESP anti-replay window when the peer host readdresses.
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. However, this document does not specify such
cases.
If the LOCATOR parameter is sent in an UPDATE packet, then the
receiver will respond with an UPDATE acknowledgment. If the LOCATOR
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.
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4. LOCATOR parameter format
The LOCATOR parameter is a critical parameter as defined by [2]. 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: 193
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.
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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).
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 an
address of Type "0" and a different address of Type "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.
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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 [7] (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).
4.3. UPDATE packet with included LOCATOR
A number of combinations of parameters in an UPDATE packet are
possible (e.g., see Section 3.2). Only one LOCATOR parameter is used
in any HIP packet, and this LOCATOR SHOULD list all of the locators
that the host wishes to make available for the HIP association. Any
UPDATE packet that includes a LOCATOR parameter SHOULD include both
an HMAC and a HIP_SIGNATURE parameter.
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5. Processing rules
5.1. Locator data structure and status
In a typical implementation, each outgoing locator is represented by
a piece of state that contains the following data:
o the actual bit pattern representing the locator,
o lifetime (seconds),
o 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.
A DEPRECATED address MUST NOT be changed to ACTIVE without first
verifying its reachability.
5.2. Sending LOCATORs
The decision of when to send LOCATORs is basically a local policy
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issue. However, it is RECOMMENDED that a host sends a LOCATOR
whenever it recognizes a change of its IP addresses in use on an
active HIP association, 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 with
SPIs. The grouping should consider also whether middlebox
interaction requires sending (the same) LOCATOR in separate UPDATEs
on different paths. Since each SPI is associated with a different
Security Association, the grouping policy may also be based on ESP
anti-replay protection considerations. In the typical case, simply
basing the grouping on actual kernel level physical and logical
interfaces may be the best policy. Grouping policy is outside of the
scope of this document.
Note that the purpose of announcing IP addresses in a LOCATOR is to
provide connectivity between the communicating hosts. In most cases,
tunnels or virtual interfaces such as IPsec tunnel interfaces or
Mobile IP home addresses 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. Therefore, virtual interfaces
SHOULD NOT be announced in general. 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 that serves as a complete
representation of the addresses and affiliated SPIs intended for
active use. We now describe a few cases introduced in Section 3.2.
We assume that the Traffic Type for each locator is set to "0" (other
values for Traffic Type may be specified in documents that separate
HIP control plane from data plane traffic). Other mobility and
multihoming cases are possible but are left for further
experimentation.
1. Host mobility with no multihoming and no rekeying. The mobile
host creates a single UPDATE containing a single ESP_INFO with a
single LOCATOR parameter. The ESP_INFO contains the current
value of the SPI in both the "Old SPI" and "New SPI" fields. The
LOCATOR contains a single Locator with a "Locator Type" of "1";
the SPI must match that of the ESP_INFO. The Preferred bit
SHOULD be set and the "Locator Lifetime" is set according to
local policy. The UPDATE also contains a SEQ parameter as usual
and is protected by retransmission. The UPDATE should be sent to
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the peer's preferred IP address with an IP source address
corresponding to the address in the LOCATOR parameter.
2. Host mobility with no multihoming but with rekeying. The mobile
host creates a single UPDATE containing a single ESP_INFO with a
single LOCATOR parameter (with a single address). The ESP_INFO
contains the current value of the SPI in the "Old SPI" and the
new value of the SPI in the "New SPI", and a "Keymat Index" as
selected by local policy. Optionally, the host may choose to
initiate a Diffie Hellman rekey by including a DIFFIE_HELLMAN
parameter. The LOCATOR contains a single Locator with "Locator
Type" of "1"; the SPI must match that of the "New SPI" in the
ESP_INFO. Otherwise, the steps are identical to the case when no
rekeying is initiated.
3. Host multihoming (addition of an address). We only describe the
simple case of adding an additional address to a single-homed,
non-mobile host. The host SHOULD set up a new SA pair between
this new address and the preferred address of the peer host. To
do this, the multihomed host creates a new inbound SA and creates
a new ESP_INFO parameter with an "Old SPI" parameter of "0", a
"New SPI" parameter corresponding to the new SPI, and a "Keymat
Index" as selected by local policy. The host adds to the UPDATE
message a LOCATOR with two Type "1" Locators: the original
address and SPI active on the association, and the new address
and new SPI being added (with the SPI matching the "New SPI"
contained in the ESP_INFO). The Preferred bit SHOULD be set
depending on the policy to tell the peer host which of the two
locators is preferred. The UPDATE also contains a SEQ parameter
and optionally a DIFFIE_HELLMAN parameter, and follows rekeying
procedures with respect to this new address. The UPDATE message
SHOULD be sent to the peer's preferred address with a source
address corresponding to the new locator.
The sending of multiple LOCATORs, locators with Locator Type "0", and
multiple ESP_INFO parameters is for further study.
5.3. Handling received LOCATORs
A host SHOULD be prepared to receive a LOCATOR parameter in any HIP
packet, excluding I1.
This document describes sending both ESP_INFO and LOCATOR parameters
in an UPDATE. The ESP_INFO parameter is included if there is a need
to rekey or key a new SPI, and is otherwise included for the possible
benefit of HIP-aware middleboxes. The LOCATOR parameter contains a
complete map of the locators that the host wishes to make or keep
active for the HIP association.
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In general, the processing of a LOCATOR depends upon the packet type
in which it is included and upon whether ESP_INFO parameter is
included. Here, we describe only the case in which ESP_INFO is
present and a single LOCATOR and ESP_INFO are sent in an UPDATE
message; other cases are for further study. The steps below cover
each of the cases described in Section 5.2.
When a host receives a LOCATOR parameter in a validated HIP packet,
it first performs the following operations:
1. The host checks if the New SPI listed in the ESP_INFO is a new
one. If it is a new one, it creates a new inbound SA with that
SPI that contains no addresses. If it is an existing one, it
prepares to change the address set on the existing SPI.
2. 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.
3. For each Type 1 address listed in the LOCATOR parameter, check if
the address is already bound to the SPI indicated. If the
address is already bound, its lifetime is updated. If the status
of the address is DEPRECATED, the status is changed to
UNVERIFIED. If the address is not already bound, the address is
added, and its status is set to UNVERIFIED. 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.
4. If the LOCATOR is paired with an ESP_INFO parameter, the ESP_INFO
parameter is processed as follows:
1. If the Old SPI indicates an existing SPI and the New SPI is a
different non-zero value, the existing SA is being rekeyed
and the host follows HIP ESP rekeying procedures. Note that
the Locators in the LOCATOR parameter will use this New SPI
instead of the Old SPI.
2. If the Old SPI value is zero and the New SPI is a new non-
zero value, then a new SA is being requested by the peer.
This case is also treated like a rekeying event; the
receiving host must create a new inbound SA and respond with
an UPDATE ACK.
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3. If the Old SPI indicates an existing SPI and the New SPI is
zero, the SPI is being deprecated and all locators uniquely
bound to the SPI are put into DEPRECATED state.
4. If the Old SPI equals the New SPI and both correspond to an
existing SPI, the ESP_INFO is gratuitous (provided for
middleboxes) and no rekeying is necessary.
5. Mark all locators on each SPI that were NOT listed in the LOCATOR
parameter as DEPRECATED.
As a result, each 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.
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 requires that the host first verifies
reachability of the associated address, and only then changes the
preferred locator. See Section 5.6.
5.4. Verifying address reachability
A host MUST verify the reachability of an UNVERIFIED address. The
status of a newly learned address MUST initially be set to UNVERIFIED
unless the new address is advertised in a R1 packet as a new
preferred locator. A host MAY also want to verify the reachability
of an ACTIVE address again after some time, in which case it would
set the status of the address to UNVERIFIED and reinitiate address
verification
A host typically starts the address-verification procedure by sending
a nonce to the new address. For example, if the host is changing its
SPI and is sending an ESP_INFO 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 proof of
reachability for the Responder's preferred address. Since further
address verification of such address can impede the HIP base
exchange, a host MUST NOT verify reachability of a new preferred
locator that was received on a R1.
In some cases, it may be sufficient to use the arrival of data on a
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newly advertised SA as implicit address reachability verification,
instead of waiting for the confirmation via a HIP packet (e.g.,
Figure 14). 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 14: Address activation via use of new SA
When address verification is in progress for a new preferred locator,
the host SHOULD select a different locator listed as ACTIVE, if one
such locator is available, to continue communications until address
verification completes. Alternatively, the host MAY use the new
preferred locator while in UNVERIFIED status to the extent Credit-
Based Authorization permits. Credit-Based Authorization is explained
in Section 5.5. Once address verification succeeds, the status of
the new preferred locator changes to ACTIVE.
5.5. Credit-Based Authorization
5.5.1. Handling Payload Packets
A host maintains a "credit counter" for each of its peers. Whenever
a packet arrives from a peer, the host SHOULD increase that peer's
credit counter by the size of the received packet. When the host has
a packet to be sent to the peer, if the peers preferred locator is
listed as UNVERIFIED and no alternative locator with status ACTIVE is
available, the host checks whether it can send the packet to the
UNVERIFIED locator: The packet SHOULD be sent if the value of the
credit counter is higher than the size of the outbound packet. If
the credit counter is too low, the packet MUST be discarded or
buffered until address verification succeeds. When a packet is sent
to a peer at an UNVERIFIED locator, the peer's credit counter MUST be
reduced by the size of the packet. The peer's credit counter is not
affected by packets that the host sends to an ACTIVE locator of that
peer.
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Figure 15 depicts the actions taken by the host when a packet is
received. Figure 16 shows the decision chain in the event a packet
is sent.
Inbound
packet
|
| +----------------+ +---------------+
| | Increase | | Deliver |
+-----> | credit counter |-------------> | packet to |
| by packet size | | application |
+----------------+ +---------------+
Figure 15: Receiving Packets with Credit-Based Authorization
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Outbound
packet
| _________________
| / \ +---------------+
| / Is the preferred \ No | Send packet |
+-----> | destination address |-------------> | to preferred |
\ UNVERIFIED? / | address |
\_________________/ +---------------+
|
| Yes
|
v
_________________
/ \ +---------------+
/ Does an ACTIVE \ Yes | Send packet |
| destination address |-------------> | to ACTIVE |
\ exist? / | address |
\_________________/ +---------------+
|
| No
|
v
_________________
/ \ +---------------+
/ Credit counter \ No | |
| >= |-------------> | Drop packet |
\ packet size? / | |
\_________________/ +---------------+
|
| Yes
|
v
+---------------+ +---------------+
| Reduce credit | | Send packet |
| counter by |----------------> | to preferred |
| packet size | | address |
+---------------+ +---------------+
Figure 16: Sending Packets with Credit-Based Authorization
5.5.2. Credit Aging
A host ensures that the credit counters it maintains for its peers
gradually decrease over time. Such "credit aging" prevents a
malicious peer from building up credit at a very slow speed and using
this, all at once, for a severe burst of redirected packets.
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Credit aging may be implemented by multiplying credit counters with a
factor, CreditAgingFactor, less than one in fixed time intervals of
CreditAgingInterval length. Choosing appropriate values for
CreditAgingFactor and CreditAgingInterval is important to ensure that
a host can send packets to an address in state UNVERIFIED even when
the peer sends at a lower rate than the host itself. When
CreditAgingFactor or CreditAgingInterval are too small, the peer's
credit counter might be too low to continue sending packets until
address verification concludes.
The parameter values proposed in this document are as follows:
CreditAgingFactor 7/8
CreditAgingInterval 5 seconds
These parameter values work well when the host transfers a file to
the peer via a TCP connection and the end-to-end round-trip time does
not exceed 500 milliseconds. Alternative credit-aging algorithms may
use other parameter values or different parameters, which may even be
dynamically established.
5.6. Changing the preferred locator
A host MAY want to change the preferred outgoing locator for
different reasons, e.g., because traffic information or ICMP error
messages indicate that the currently used preferred address may have
become unreachable. Another reason may be due to receiving a LOCATOR
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. The host SHOULD use a
different locator listed as ACTIVE until address verification
completes if one such locator is available. Alternatively, the
host MAY use the new preferred locator, even though in UNVERIFIED
status, to the extent Credit-Based Authorization permits. Once
address verification succeeds, the status of the new preferred
locator changes to ACTIVE and its use is no longer governed by
Credit-Based Authorization.
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
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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. If the selected address is UNVERIFIED, this includes
address verification as described above.
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6. Security Considerations
The HIP mobility mechanism provides a secure means of updating a
host's IP address via HIP UPDATE packets. Upon receipt, a HIP host
cryptographically verifies the sender of an UPDATE, so forging or
replaying a HIP UPDATE packet is very difficult (see [2]).
Therefore, security issues reside in other attack domains. The two
we consider are malicious redirection of legitimate connections as
well as redirection-based flooding attacks using this protocol. This
can be broken down into the following:
Impersonation attacks
- direct conversation with the misled victim
- man-in-the-middle attack
DoS attacks
- flooding attacks (== bandwidth-exhaustion attacks)
* tool 1: direct flooding
* tool 2: flooding by zombies
* tool 2: redirection-based flooding
- memory-exhaustion attacks
- computational exhaustion attacks
We consider these in more detail in the following sections.
In Section 6.1 and Section 6.2, we assume that all users are using
HIP. In Section 6.3 we consider the security ramifications when we
have both HIP and non-HIP users.
6.1. Impersonation attacks
An attacker wishing to impersonate will try to mislead its victim
into directly communicating with them, or carry out a man in the
middle attack between the victim and the victim's desired
communication peer. Without mobility support, both attack types are
possible only if the attacker resides on the routing path between its
victim and the victim's desired communication peer, or if the
attacker tricks its victim into initiating the connection over an
incorrect routing path (e.g., by acting as a router or using spoofed
DNS entries).
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The HIP extensions defined in this specification change the situation
in that they introduce an ability to redirect a connection (like
IPv6), both before and after establishment. If no precautionary
measures are taken, an attacker could misuse this feature to
impersonate a victim's peer from any arbitrary location. The
authentication and authorization mechanisms of the HIP base exchange
[2] and the signatures in the UPDATE message prevent this attack.
Furthermore, ownership of a HIP association is securely linked to a
HIP HI/HIT. If an attacker somehow uses a bug in the implementation
or weakness in some protocol to redirect a HIP connection, the
original owner can always reclaim their connection (they can always
prove ownership of the private key associated with their public HI).
MitM attacks are always possible if the attacker is present during
the initial HIP base exchange and if the hosts do not authenticate
each other's identities, but once the base exchange has taken place
even a MitM cannot steal an opportunistic HIP connection because it
is very difficult for an attacker to create an UPDATE packet (or any
HIP packet) that will be accepted as a legitimate update. UPDATE
packets use HMAC and are signed. Even when an attacker can snoop
packets to obtain the SPI and HIT/HI, they still cannot forge an
UPDATE packet without knowledge of the secret keys.
6.2. Denial of Service attacks
6.2.1. Flooding Attacks
The purpose of a denial-of-service attack is to exhaust some resource
of the victim such that the victim ceases to operate correctly. A
denial-of-service attack can aim at the victim's network attachment
(flooding attack), its memory, or its processing capacity. In a
flooding attack the attacker causes an excessive number of bogus or
unwanted packets to be sent to the victim, which fills their
available bandwidth. Note that the victim does not necessarily need
to be a node; it can also be an entire network. The attack basically
functions the same way in either case.
An effective DoS strategy is distributed denial of service (DDoS).
Here, the attacker conventionally distributes some viral software to
as many nodes as possible. Under the control of the attacker, the
infected nodes, or "zombies", jointly send packets to the victim.
With such an 'army', an attacker can take down even very high
bandwidth networks/victims.
With the ability to redirect connections, an attacker could realize a
DDoS attack without having to distribute viral code. Here, the
attacker initiates a large download from a server, and subsequently
redirects this download to its victim. The attacker can repeat this
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with multiple servers. This threat is mitigated through reachability
checks and credit-based authorization. Both strategies do not
eliminate flooding attacks per se, but they preclude: (i) their use
from a location off the path towards the flooded victim; and (ii) any
amplification in the number and size of the redirected packets. As a
result, the combination of a reachability check and credit-based
authorization makes a HIP redirection-based flooding attack as
effective and applicable as a normal, direct flooding attack in which
the attacker itself sends the flooding traffic to the victim.
This analysis leads to the following two points. First, when a
reachability packet is received, this nonce packet MUST be ignored if
the HIT is not one that is currently active. Second, if the attacker
is a MitM and can capture this nonce packet then it can respond to
it, in which case it is possible for an attacker to redirect the
connection. Note, this attack will always be possible when a
reachability packet is not sent.
6.2.2. Memory/Computational exhaustion DoS attacks
We now consider whether or not the proposed extensions to HIP add any
new DoS attacks (consideration of DoS attacks using the base HIP
exchange and updates is discussed in [2]). A simple attack is to
send many UPDATE packets containing many IP addresses that are not
flagged as preferred. The attacker continues to send such packets
until the number of IP addresses associated with the attacker's HI
crashes the system. Therefore, there SHOULD be a limit to the number
of IP addresses that can be associated with any HI. Other forms of
memory/computationally exhausting attacks via the HIP UPDATE packet
are handled in the base HIP draft [2].
6.3. Mixed deployment environment
We now assume an environment with both HIP and non-HIP aware hosts.
Four cases exist.
1. A HIP user redirects their connection onto a non-HIP user. The
non-HIP user will drop the reachability packet so this is not a
threat unless the HIP user is a MitM and can respond to the
reachability packet.
2. A non-HIP user attempts to redirect their connection onto a HIP
user. This falls into IPv4 and IPv6 security concerns, which are
outside the scope of this document.
3. A non-HIP user attempts to steal a HIP user's session (assume
that Secure Neighbor Discovery is not active for the following).
The non-HIP user contacts the service that a HIP user has a
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connection with and then attempts to use a IPv6 change of address
request to steal the HIP user's connection. What will happen in
this case is implementation dependent but such a request should
be ignored/dropped. Even if the attack is successful, the HIP
user can reclaim its connection via HIP.
4. A HIP user attempts to steal a non-HIP user's session. This
could be problematic since HIP sits 'on top of' layer 3. A HIP
user could spoof the non-HIP user's IP address during the base
exchange or set the non-HIP user's IP address as their preferred
address via an UPDATE. Other possibilities exist but a simple
solution is to add a check which does not allow any HIP session
to be moved to or created upon an already existing IP address.
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7. IANA Considerations
This document defines a LOCATOR parameter for the Host Identity
Protocol [2]. This parameter is defined in Section 4 with a Type of
193.
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8. Authors
Pekka Nikander originated this Internet Draft. Tom Henderson, Jari
Arkko, Greg Perkins, and Christian Vogt have each contributed
sections to this draft.
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9. Acknowledgments
The authors thank Mika Kousa, Jeff Ahrenholz, and Jan Melen for many
improvements to the draft.
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10. References
10.1. Normative references
[1] Moskowitz, R. and P. Nikander, "Host Identity Protocol
Architecture", draft-ietf-hip-arch-03 (work in progress),
August 2005.
[2] Moskowitz, R., "Host Identity Protocol", draft-ietf-hip-base-04
(work in progress), October 2005.
[3] Laganier, J. and L. Eggert, "Host Identity Protocol (HIP)
Rendezvous Extension", draft-ietf-hip-rvs-04 (work in progress),
October 2005.
[4] Kent, S., "IP Encapsulating Security Payload (ESP)", RFC 4303,
December 2005.
[5] Jokela, P., "Using ESP transport format with HIP",
draft-ietf-hip-esp-01 (work in progress), October 2005.
[6] Bradner, S., "Key words for use in RFCs to Indicate Requirement
Levels", BCP 14, RFC 2119, March 1997.
[7] Hinden, R. and S. Deering, "IP Version 6 Addressing
Architecture", RFC 2373, July 1998.
10.2. Informative references
[8] Nikander, P., Arkko, J., Aura, T., Montenegro, G., and E.
Nordmark, "Mobile IP Version 6 Route Optimization Security
Design Background", RFC 4225, December 2005.
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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.
A.5. From draft-ietf-hip-mm-01 to -02
Aligned with draft-ietf-hip-base-03 and draft-ietf-hip-esp-00
Address verification is a MUST (C. Vogt, list post on 06/12/05)
If UPDATE exceeds MTU because of too many locators, do not split into
multiple UPDATEs, but instead rely on IP fragmentation (C. Vogt, list
post on 06/12/05)
New value for LOCATOR parameter type (193), per 05/31/05 discussion
on the WG list
Various additions related to Credit-Based Authorization due to C.
Vogt
Security section contributed by Greg Perkins, with subsequent editing
from C. Vogt and P. Nikander
Reorganization according to RFC 4101 guidance on writing protocol
models
Open issue: LOCATOR parameter semantics (implicit/explicit removal)
A.6. From draft-ietf-hip-mm-02 to -03
Aligned with draft-ietf-hip-base-05 and draft-ietf-hip-esp-02
Further clarification that the scope of this draft is primarily
limited to the case in which ESP is used
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New layered architectural overview in Section 3
Limited the scope of multihoming description to just a single host
adding a single new address; other cases left for further study
Require that ESP_INFO be included on all UPDATE packets relating to
mobility and multihoming (for middleboxes)
New convention for use of "Old SPI" and "New SPI" values to signal
new SPIs (Old SPI == 0, New SPI != 0) and gratuitous ESP_INFOs with
no rekeying (Old SPI == New SPI != 0).
Only specify the use of Locator Type of 1 when using ESP, for
simplicity of receiver processing.
Removed multiple addresses in LOCATOR example of section 3.2.2,
because it is not clear that the example is correct (requires further
study)
Corrected mention of sending ECHO_REQUEST nonce in R2 (should be sent
in separate UPDATE because R2 is not an acknowledged packet)
Removed first four paragraphs of Section 5, which were redundant with
previous introductory material.
Rewrote Sections 5.2 and 5.3 on sending and receiving LOCATOR, to
more explicitly cover the scenario scope of this document.
Removed unwritten "Policy Considerations" section
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Author's Address
Tom Henderson
The Boeing Company
P.O. Box 3707
Seattle, WA
USA
Email: thomas.r.henderson@boeing.com
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