One document matched: draft-moskowitz-hip-rfc4423-bis-01.xml
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<rfc docName="draft-moskowitz-hip-rfc4423-bis-01" category="std" obsoletes="4423" ipr="pre5378Trust200902">
<front>
<title>Host Identity Protocol Architecture</title>
<author initials="R." surname="Moskowitz"
fullname="Robert Moskowitz">
<organization abbrev="ICSAlabs">ICSAlabs, An Independent Division of Verizon Business Systems
</organization>
<address>
<postal>
<street>1000 Bent Creek Blvd, Suite 200</street>
<city>Mechanicsburg</city>
<region>PA</region>
<country>USA</country>
</postal>
<email>robert.moskowitz@icsalabs.com</email>
</address>
</author>
<author initials="P." surname="Nikander"
fullname="Pekka Nikander">
<organization>Ericsson Research Nomadic Lab</organization>
<address>
<postal>
<street />
<city>JORVAS</city>
<code>FIN-02420</code>
<country>FINLAND</country>
</postal>
<phone>+358 9 299 1</phone>
<email>pekka.nikander@nomadiclab.com</email>
</address>
</author>
<date month="December" year="2009" />
<area>Internet</area>
<keyword>Request for Comments</keyword>
<keyword>RFC</keyword>
<keyword>Internet Draft</keyword>
<keyword>I-D</keyword>
<abstract>
<t>This memo describes a new namespace, the Host Identity namespace,
and a new protocol layer, the Host Identity Protocol, between the
internetworking and transport layers. Herein are presented the
basics of the current namespaces, their strengths and
weaknesses, and how a new namespace will add completeness to
them. The roles of this new namespace in the protocols are
defined. </t>
<t>
This document obsoletes RFC 4423 and addresses the concerns raised by
the IESG, particularly that of crypto agility. It also incorporates
lessons learned from the implementations of RFC 5201.
</t>
</abstract>
</front>
<middle>
<section title="Introduction">
<t>The Internet has two important global namespaces: Internet
Protocol (IP) addresses and Domain Name Service (DNS) names.
These two namespaces have a set of features and abstractions
that have powered the Internet to what it is today. They also
have a number of weaknesses. Basically, since they are all we
have, we try and do too much with them. Semantic overloading
and functionality extensions have greatly complicated these
namespaces.</t>
<t>The proposed Host Identity namespace fills an important gap
between the IP and DNS namespaces. The Host Identity namespace
consists of Host Identifiers (HI). A Host Identifier is
cryptographic in its nature; it is the public key of an
asymmetric key-pair. Each host will have at least one Host
Identity, but it will typically have more than one. Each Host
Identity uniquely identifies a single host, i.e., no two hosts
have the same Host Identity. The Host Identity, and the
corresponding Host Identifier, can either be public
(e.g. published in the DNS), or unpublished. Client systems
will tend to have both public and unpublished Identities.</t>
<t>There is a subtle but important difference between Host
Identities and Host Identifiers. An Identity refers to the
abstract entity that is identified. An Identifier, on the other
hand, refers to the concrete bit pattern that is used in the
identification process.</t>
<t>Although the Host Identifiers could be used in many
authentication systems, such as <xref
target="RFC4306">IKEv2</xref>, the presented
architecture introduces a new protocol, called the Host Identity
Protocol (HIP), and a cryptographic exchange, called the HIP
base exchange; see also <xref target="esp"/>.
The HIP protocols under development provide for limited forms of
trust between systems, enhance mobility, multi-homing and
dynamic IP renumbering, aid in protocol translation / transition,
and reduce certain types of denial-of-service (DoS) attacks.
</t>
<t>When HIP is used, the actual payload traffic between two HIP
hosts is typically, but not necessarily, protected with IPsec.
The Host Identities are used to create the needed IPsec Security
Associations (SAs) and to authenticate the hosts. When IPsec is
used, the actual payload IP packets do not differ in any way
from standard IPsec protected IP packets.</t>
</section>
<section title="Terminology">
<?rfc compact="no"?>
<section title="Terms common to other documents">
<texttable>
<ttcol width="20%" align="left">Term</ttcol>
<ttcol align="left">Explanation</ttcol>
<c>Public key</c><c>The public key of an asymmetric
cryptographic key pair. Used as a publicly known identifier
for cryptographic identity authentication.</c>
<c>Private key</c><c>The private or secret key of an
asymmetric cryptographic key pair. Assumed to be known only
to the party identified by the corresponding public key.
Used by the identified party to authenticate its identity to
other parties.</c>
<c>Public key pair</c><c>An asymmetric cryptographic
key pair consisting of public and private keys. For
example, Rivest-Shamir-Adelman (RSA) and Digital Signature
Algorithm (DSA) key pairs are such key pairs.</c>
<c>End-point</c><c>A communicating entity. For
historical reasons, the term 'computing platform' is used in
this document as a (rough) synonym for end-point.</c>
</texttable>
</section>
<?rfc compact="yes"?>
<?rfc compact="no"?>
<section title="Terms specific to this and other HIP documents">
<t>It should be noted that many of the terms defined herein
are tautologous, self-referential or defined through circular
reference to other terms. This is due to the succinct nature
of the definitions. See the text elsewhere in this document
for more elaborate explanations.</t>
<texttable>
<ttcol width="20%" align="left">Term</ttcol>
<ttcol align="left">Explanation</ttcol>
<c>Computing platform</c><c>An entity capable of
communicating and computing, for example, a computer. See
the definition of 'End-point', above.</c>
<c>HIP base exchange</c><c>A cryptographic protocol;
see also <xref target="esp" />.</c>
<c>HIP packet</c><c>An IP packet that carries a 'Host
Identity Protocol' message.</c>
<c>Host Identity</c><c>An abstract concept assigned to
a 'computing platform'. See 'Host Identifier', below.</c>
<c>Host Identity namespace</c><c>A name space
formed by all possible Host Identifiers.</c>
<c>Host Identity Protocol</c><c>A protocol used to
carry and authenticate Host Identifiers and other
information. </c>
<c>Host Identity Tag</c><c>A 128-bit datum created by
taking a cryptographic hash over a Host Identifier.</c>
<c>Host Identity Hash</c><c>The cryptograhic hash used
in creating the Host Identity Tag from the Host Identity.</c>
<c>Host Identifier</c><c>A public key used as a name
for a Host Identity.</c>
<c>Local Scope Identifier</c><c>A 32-bit datum denoting
a Host Identity.</c>
<c>Public Host Identifier and Identity</c><c>A
published or publicly known Host Identfier used as a public
name for a Host Identity, and the corresponding
Identity.</c>
<c>Unpublished Host Identifier and Identity</c><c>A
Host Identifier that is not placed in any public directory,
and the corresponding Host Identity. Unpublished Host
Identities are typically short lived in nature, being often
replaced and possibly used just once.</c>
<c>Rendezvous Mechanism</c><c>A mechanism used to
locate mobile hosts based on their HIT.</c>
</texttable>
</section>
<?rfc compact="yes"?>
</section>
<section title="Background">
<t>The Internet is built from three principal components:
computing platforms (end-points), packet transport
(i.e., internetworking) infrastructure, and services
(applications). The Internet exists to service two principal
components: people and robotic services (silicon based people,
if you will). All these components need to be named in order to
interact in a scalable manner. Here we concentrate on naming
computing platforms and packet transport elements.</t>
<t>There are two principal namespaces in use in the Internet for
these components: IP numbers, and Domain Names.
Domain Names provide hierarchically assigned names for some
computing platforms and some services. Each hierarchy is
delegated from the level above; there is no anonymity in Domain
Names. Email, HTTP, and SIP addresses all reference Domain
Names.</t>
<t>IP numbers are a confounding of two namespaces, the names of
a host's networking interfaces and the names of the locations
('confounding' is a term used in statistics to discuss metrics
that are merged into one with a gain in indexing, but a loss in
informational value). The names of locations should be
understood as denoting routing direction vectors, i.e.,
information that is used to deliver packets to their
destinations.</t>
<t>IP numbers name networking interfaces, and typically only
when the interface is connected to the network. Originally, IP
numbers had long-term significance. Today, the vast number of
interfaces use ephemeral and/or non-unique IP numbers. That is,
every time an interface is connected to the network, it is
assigned an IP number.</t>
<t>In the current Internet, the transport layers are coupled to
the IP addresses. Neither can evolve separately from the other.
IPng deliberations were strongly shaped by the decision that a
corresponding TCPng would not be created.</t>
<t>There are three critical deficiencies with the current
namespaces. Firstly, dynamic readdressing cannot be directly
managed. Secondly, anonymity is not provided in a consistent,
trustable manner. Finally, authentication for systems and
datagrams is not provided. All of these deficiencies arise
because computing platforms are not well named with the current
namespaces. </t>
<section title="A desire for a namespace for computing platforms">
<t>An independent namespace for computing platforms could be
used in end-to-end operations independent of the evolution of
the internetworking layer and across the many internetworking
layers. This could support rapid readdressing of the
internetworking layer because of mobility, rehoming, or
renumbering.</t>
<t>If the namespace for computing platforms is based on
public-key cryptography, it can also provide authentication
services. If this namespace is locally created without
requiring registration, it can provide anonymity. </t>
<t>Such a namespace (for computing platforms) and the names in
it should have the following characteristics:
<list style="symbols">
<t>The namespace should be applied to the IP 'kernel'.
The IP kernel is the 'component' between applications and the
packet transport infrastructure.</t>
<t>The namespace should fully decouple the internetworking
layer from the higher layers. The names should replace
all occurrences of IP addresses within applications (like
in the Transport Control Block, TCB). This may require
changes to the current APIs. In the long run, it is
probable that some new APIs are needed.</t>
<t>The introduction of the namespace should not mandate
any administrative infrastructure. Deployment must come
from the bottom up, in a pairwise deployment.</t>
<t>The names should have a fixed length representation,
for easy inclusion in datagram headers and existing
programming interfaces (e.g the TCB).</t>
<t>Using the namespace should be affordable when used in
protocols. This is primarily a packet size issue. There
is also a computational concern in affordability.</t>
<t>Name collisions should be avoided as much as possible. The
mathematics of the birthday paradox can be used to estimate
the chance of a collision in a given population and hash space.
In general, for a random hash space of size n bits, we would
expect to obtain a collision after approximately 1.2*sqrt(2**n)
hashes were obtained. For 64 bits, this number is roughly
4 billion. A hash size of 64 bits may be too small to avoid
collisions in a large population; for example, there is a 1%
chance of collision in a population of 640M. For 100 bits
(or more), we would not expect a collision until approximately
2**50 (1 quadrillion) hashes were generated.</t>
<t>The names should have a localized abstraction so that
it can be used in existing protocols and APIs.</t>
<?rfc needLines="8"?>
<t>It must be possible to create names locally. This can
provide anonymity at the cost of making resolvability very
difficult.
<list style="symbols">
<t>Sometimes the names may contain a delegation
component. This is the cost of resolvability.</t>
</list>
</t>
<t>The namespace should provide authentication services.</t>
<t>The names should be long lived, but replaceable at any
time. This impacts access control lists; short lifetimes
will tend to result in tedious list maintenance or require
a namespace infrastructure for central control of access
lists.</t>
</list>
</t>
<t>In this document, a new namespace approaching these ideas
is called the Host Identity namespace. Using Host Identities
requires its own protocol layer, the Host Identity Protocol,
between the internetworking and transport layers. The names
are based on public-key cryptography to supply authentication
services. Properly designed, it can deliver all of the above
stated requirements.</t>
</section>
</section>
<section title="Host Identity namespace">
<t>A name in the Host Identity namespace, a Host Identifier
(HI), represents a statistically globally unique name for naming
any system with an IP stack. This identity is normally
associated with, but not limited to, an IP stack. A system can
have multiple identities, some 'well known', some unpublished or
'anonymous'. A system may self-assert its own identity, or may
use a third-party authenticator like DNSSEC <xref
target="RFC2535" />, PGP, or X.509 to 'notarize' the identity
assertion. It is expected that the Host Identifiers will
initially be authenticated with DNSSEC and that all
implementations will support DNSSEC as a minimal baseline.</t>
<t>In theory, any name that can claim to be 'statistically
globally unique' may serve as a Host Identifier. However, in
the authors' opinion, a public key of a 'public key pair' makes
the best Host Identifier. As will be specified in the
Host Identity Protocol
specification, a public-key-based HI can authenticate the
HIP packets and protect them for man-in-the-middle attacks.
Since authenticated datagrams are mandatory to provide much of
HIP's denial-of-service protection, the Diffie-Hellman exchange
in HIP has to be authenticated. Thus, only public-key HI and
authenticated HIP messages are supported in practice. In this
document, the non-cryptographic forms of HI and HIP are
presented to complete the theory of HI, but they should not be
implemented as they could produce worse denial-of-service
attacks than the Internet has without Host Identity.</t>
<section title="Host Identifiers">
<t>Host Identity adds two main features to Internet protocols.
The first is a decoupling of the internetworking and transport
layers; see <xref target="sec-architecture" />. This
decoupling will allow for independent evolution of the two
layers. Additionally, it can provide end-to-end services over
multiple internetworking realms. The second feature is host
authentication. Because the Host Identifier is a public key,
this key can be used for authentication in security protocols
like IPsec.</t>
<t>The only completely defined structure of the Host Identity
is that of a public/private key pair. In this case, the Host
Identity is referred to by its public component, the public
key. Thus, the name representing a Host Identity in the Host
Identity namespace, i.e., the Host Identifier, is the public
key. In a way, the possession of the private key defines the
Identity itself. If the private key is possessed by more than
one node, the Identity can be considered to be a distributed
one.</t>
<t>Architecturally, any other Internet naming convention might
form a usable base for Host Identifiers. However,
non-cryptographic names should only be used in situations of
high trust - low risk. That is any place where host
authentication is not needed (no risk of host spoofing) and no
use of IPsec. However, at least for interconnected networks
spanning several operational domains, the set of environments
where the risk of host spoofing allowed by non-cryptographic
Host Identifiers is acceptable is the null set. Hence, the
current HIP documents do not specify how to use any other
types of Host Identifiers but public keys.</t>
<t>The actual Host Identities are never directly used in any
Internet protocols. The corresponding Host Identifiers
(public keys) may be stored in various DNS or LDAP directories
as identified elsewhere in this document, and they are passed
in the HIP base exchange. A Host Identity Tag (HIT) is used
in other protocols to represent the Host Identity. Another
representation of the Host Identities, the Local Scope
Identifier (LSI), can also be used in protocols and APIs.</t>
</section>
<section title="Storing Host Identifiers in DNS">
<t>The public Host Identifiers should be stored in DNS; the
unpublished Host Identifiers should not be stored anywhere
(besides the communicating hosts themselves). The (public) HI
along with the supported HIHs are stored in a new RR type. This RR type
is defined in <xref target="RFC5205">HIP DNS Extension</xref>.</t>
<t>Alternatively, or in addition to storing Host Identifiers
in the DNS, they may be stored in various kinds of Public Key
Infrastructure (PKI). Such a practice may allow them to be
used for purposes other than pure host identification.</t>
</section>
<section title="Host Identity Tag (HIT)">
<t>A Host Identity Tag is a 128-bit representation for a Host
Identity. It is created by taking a cryptographic hash over
the corresponding Host Identifier. There are two advantages
of using a hash over using the Host Identifier in protocols.
Firstly, its fixed length makes for easier protocol coding and
also better manages the packet size cost of this technology.
Secondly, it presents the identity in a consistent format to
the protocol independent of the cryptographic algorithms
used.</t>
<t>There can be multiple HITs per Host Identifier when multiple
hashes are supported. An Initator may have to initially guess
which HIT to use for the Responder, typically based on what it
perfers, until it learns the appropriate HIT through the HIP
exchange.</t>
<t>In the HIP packets, the HITs identify the sender and
recipient of a packet. Consequently, a HIT should be unique
in the whole IP universe as long as it is being used. In the
extremely rare case of a single HIT mapping to more than one
Host Identity, the Host Identifiers (public keys) will make
the final difference. If there is more than one public key
for a given node, the HIT acts as a hint for the correct
public key to use.</t>
</section>
<section title="Host Identity Hash (HIH)">
<t>The Host Identity Hash is the cryptographic hash used in
producing the HIT from the HI. It is also the hash used
through out the HIP protocol for consistancy and simplicity. It
is possible to for the two Hosts in the HIP exchange to use
different hashes.</t>
<t>Multiple HIHs within HIP is needed to address the moving
target of creation and eventual compromise of cryptographic
hashes. This significantly complicates HIP and offers an
attacker an additional downgrade attack that is mitigated
in the HIP protocol.</t>
</section>
<section title="Local Scope Identifier (LSI)">
<t>An LSI is a 32-bit localized representation for a Host
Identity. The purpose of an LSI is to facilitate using Host
Identities in existing protocols and APIs. LSI's advantage
over HIT is its size; its disadvantage is its local scope.
</t>
<t>Examples of how LSIs can be used include: as the address in
an FTP command and as the address in a socket call. Thus, LSIs
act as a bridge for Host Identities into IPv4-based protocols
and APIs. LSIs also make it possible for some IPv4 applications
to run over an IPv6 network.</t>
</section>
</section>
<section anchor="sec-architecture" title="New stack architecture">
<t>One way to characterize Host Identity is to compare the
proposed new architecture with the current one. As discussed
above, the IP addresses can be seen to be a confounding of
routing direction vectors and interface names. Using the
terminology from the <xref target="nsrg-report">IRTF
Name Space Research Group Report</xref> and, e.g., the
unpublished Internet-Draft <xref
target="chiappa-endpoints">Endpoints and Endpoint Names </xref>,
the IP addresses currently embody the dual role
of locators and end-point identifiers. That is, each IP address
names a topological location in the Internet, thereby acting as
a routing direction vector, or locator. At the same time, the IP
address names the physical network interface currently located
at the point-of-attachment, thereby acting as a end-point
name.</t>
<t>In the HIP architecture, the end-point names and locators are
separated from each other. IP addresses continue to act as
locators. The Host Identifiers take the role of end-point
identifiers. It is important to understand that the end-point
names based on Host Identities are slightly different from
interface names; a Host Identity can be simultaneously reachable
through several interfaces.</t>
<t>The difference between the bindings of the logical entities
are illustrated in <xref target="figure-bindings"/>.</t>
<figure anchor="figure-bindings">
<artwork src="draft-ietf-hip-arch-1.gif" type="gif">
Service ------ Socket Service ------ Socket
| |
| |
| |
| |
End-point | End-point --- Host Identity
\ | |
\ | |
\ | |
\ | |
Location --- IP address Location --- IP address
</artwork>
</figure>
<section title="Transport associations and end-points">
<t>Architecturally, HIP provides for a different binding of
transport-layer protocols. That is, the transport-layer
associations, i.e., TCP connections and UDP associations, are
no longer bound to IP addresses but to Host Identities.</t>
<t>It is possible that a single physical computer hosts
several logical end-points. With HIP, each of these
end-points would have a distinct Host Identity. Furthermore,
since the transport associations are bound to Host Identities,
HIP provides for process migration and clustered servers.
That is, if a Host Identity is moved from one physical
computer to another, it is also possible to simultaneously
move all the transport associations without breaking them.
Similarly, if it is possible to distribute the processing of a
single Host Identity over several physical computers, HIP
provides for cluster based services without any changes at the
client end-point.</t>
</section>
</section>
<?rfc needLines="8"?>
<section title="End-host mobility and multi-homing">
<t>HIP decouples the transport from the internetworking layer,
and binds the transport associations to the Host Identities
(through actually either the HIT or LSI). Consequently, HIP can
provide for a degree of internetworking mobility and
multi-homing at a low infrastructure cost. HIP mobility
includes IP address changes (via any method) to either party.
Thus, a system is considered mobile if its IP address can change
dynamically for any reason like PPP, DHCP, IPv6 prefix
reassignments, or a NAT device remapping its translation.
Likewise, a system is considered multi-homed if it has more than
one globally routable IP address at the same time. HIP links IP
addresses together, when multiple IP addresses correspond to the
same Host Identity, and if one address becomes unusable, or a
more preferred address becomes available, existing transport
associations can easily be moved to another address.</t>
<t>When a node moves while communication is already on-going,
address changes are rather straightforward. The peer of the
mobile node can just accept a HIP or an integrity protected
IPsec packet from any address and ignore the source address.
However, as discussed in <xref target="ssec-flooding" /> below,
a mobile node must send a HIP readdress packet to inform the
peer of the new address(es), and the peer must verify that the
mobile node is reachable through these addresses. This is
especially helpful for those situations where the peer node is
sending data periodically to the mobile node (that is
re-starting a connection after the initial connection).</t>
<section title="Rendezvous mechanism">
<t>Making a contact to a mobile node is slightly more
involved. In order to start the HIP exchange, the initiator
node has to know how to reach the mobile node. Although
infrequently moving HIP nodes could use Dynamic DNS <xref
target="RFC2136" /> to update their reachability information in
the DNS, an alternative to using DNS in this fashion is to use
a piece of new static infrastructure to facilitate rendezvous
between HIP nodes.</t>
<t>The mobile node keeps the rendezvous infrastructure
continuously updated with its current IP address(es). The
mobile nodes must trust the rendezvous mechanism to properly
maintain their HIT and IP address mappings.</t>
<t>The rendezvous mechanism is also needed if both of the
nodes happen to change their address at the same time, either
because they are mobile and happen to move at the same time,
because one of them is off-line for a while, or because of
some other reason. In such a case, the HIP readdress packets
will cross each other in the network and never reach the peer
node.</t>
<t>The HIP rendezvous mechanism is defined in
<xref target="RFC5204">HIP Rendezvous</xref>.</t>
</section>
<section anchor="ssec-flooding"
title="Protection against flooding attacks">
<t>Although the idea of informing about address changes by
simply sending packets with a new source address appears
appealing, it is not secure enough. That is, even if HIP does
not rely on the source address for anything (once the base
exchange has been completed), it appears to be necessary to
check a mobile node's reachability at the new address before
actually sending any larger amounts of traffic to the new
address.</t>
<t>Blindly accepting new addresses would potentially lead to
flooding Denial-of-Service attacks against third parties <xref
target="RFC4225" />. In a distributed
flooding attack an attacker opens high volume HIP connections
with a large number of hosts (using unpublished HIs), and then
claims to all of these hosts that it has moved to a target
node's IP address. If the peer hosts were to simply accept
the move, the result would be a packet flood to the target
node's address. To close this attack, HIP includes an address
check mechanism where the reachability of a node is separately
checked at each address before using the address for larger
amounts of traffic.</t>
<t>Whenever HIP is used between two hosts that fully trust
each other, the hosts may optionally decide to skip the
address tests. However, such performance optimization must be
restricted to peers that are known to be trustworthy and
capable of protecting themselves from malicious software.</t>
</section>
</section>
<section anchor="esp" title="HIP and IPsec">
<t>The preferred way of implementing HIP is to use IPsec to
carry the actual data traffic. As of today, the only completely
defined method is to use IPsec Encapsulated Security Payload
(ESP) to carry the data packets <xref target="RFC5202" />. In the
future, other ways of transporting payload data may be developed,
including ones that do not use cryptographic protection.</t>
<t>In practice, the HIP base exchange uses the cryptographic
Host Identifiers to set up a pair of ESP Security Associations
(SAs) to enable ESP in an end-to-end manner. This is
implemented in a way that can span addressing realms.</t>
<t>While it would be possible, at least in theory, to use some
existing cryptographic protocol, such as IKEv2 together with
Host Identifiers, to establish the needed SAs, HIP defines a new
protocol. There are a number of historical reasons for this,
and there are also a few architectural reasons. First, IKE (and
IKEv2) were not designed with middle boxes in mind. As adding a
new naming layer allows one to potentially add a new forwarding
layer (see <xref target="nat" />, below), it is very important
that the HIP protocols are friendly towards any middle
boxes.</t>
<t>Second, from a conceptual point of view, the IPsec Security
Parameter Index (SPI) in ESP provides a simple compression of
the HITs. This does require per-HIT-pair SAs (and SPIs), and a
decrease of policy granularity over other Key Management
Protocols, such as IKE and IKEv2. In particular, the current
thinking is limited to a situation where, conceptually, there is
only one pair of SAs between any given pair of HITs. In other
words, from an architectural point of view, HIP only supports
host-to-host (or endpoint-to-endpoint) Security Associations.
If two hosts need more pairs of parallel SAs, they should use
separate HITs for that. However, future HIP extensions may
provide for more granularity and creation of several ESP SAs
between a pair of HITs.</t>
<t>Since HIP is designed for host usage, not for gateways or so
called Bump-in-the-Wire (BITW) implementations, only ESP
transport mode is supported. An ESP SA pair is indexed by the
SPIs and the two HITs (both HITs since a system can have more
than one HIT). The SAs need not to be bound to IP addresses;
all internal control of the SA is by the HITs. Thus, a host can
easily change its address using Mobile IP, DHCP, PPP, or IPv6
readdressing and still maintain the SAs. Since the transports
are bound to the SA (via an LSI or a HIT), any active transport
is also maintained. Thus, real-world conditions like loss of a
PPP connection and its re-establishment or a mobile handover
will not require a HIP negotiation or disruption of transport
services <xref target="Bel1998" />.</t>
<t>Since HIP does not negotiate any SA lifetimes, all lifetimes
are local policy. The only lifetimes a HIP implementation must
support are sequence number rollover (for replay protection),
and SA timeout. An SA times
out if no packets are received using that SA. Implementations
may support lifetimes for the various ESP transforms.</t>
</section>
<section anchor="MACsec" title="HIP and MAC Security">
<t>The IEEE 802 standards have been defining MAC layered security. Many
of these standards use EAP <xref target="RFC3748"></xref>
as a Key Management System (KMS) transport, but some like IEEE
802.15.4 <xref target="IEEE.802-15-4.2006"></xref> leave the
KMS and its transport as "Out of Scope".</t>
<t>HIP is well suited as a KMS in these environments.
<list style="symbols">
<t>HIP is independent of IP addressing and can be directly
transported over any network protocol.</t>
<t>Master Keys in 802 protocols are strictly pair-based with
group keys transported from the group controller using pair-wise
keys.</t>
<t>AdHoc 802 networks can be better served by a peer-to-peer
KMS than the EAP client/server model.</t>
<t>Some devices are very memory constrained and a common KMS
for both MAC and IP security represents a considerable code
savings.</t>
</list>
</t>
</section>
<section anchor="nat" title="HIP and NATs">
<t>Passing packets between different IP addressing realms
requires changing IP addresses in the packet header. This may
happen, for example, when a packet is passed between the public
Internet and a private address space, or between IPv4 and IPv6
networks. The address translation is usually implemented as
<xref target="RFC3022">Network Address Translation (NAT)</xref>
or <xref target="RFC2766"> NAT Protocol translation
(NAT-PT)</xref>.</t>
<t>In a network environment where identification is based on the
IP addresses, identifying the communicating nodes is difficult
when NAT is used. With HIP, the transport-layer end-points are
bound to the Host Identities. Thus, a connection between two
hosts can traverse many addressing realm boundaries. The IP
addresses are used only for routing purposes; they may be
changed freely during packet traversal.</t>
<t>For a HIP-based flow, a HIP-aware NAT or NAT-PT system tracks
the mapping of HITs, and the corresponding IPsec SPIs, to an IP
address. The NAT system has to learn mappings both from HITs
and from SPIs to IP addresses. Many HITs (and SPIs) can map to
a single IP address on a NAT, simplifying connections on address
poor NAT interfaces. The NAT can gain much of its knowledge
from the HIP packets themselves; however, some NAT configuration
may be necessary.</t>
<t>NAT systems cannot touch the datagrams within the IPsec
envelope, thus application-specific address translation must be
done in the end systems. HIP provides for 'Distributed NAT',
and uses the HIT or the LSI as a placeholder for embedded IP
addresses.</t>
<t>HIP and NAT interaction is defined in <xref
target="hip-nat-traversal"></xref>.</t>
<section title="HIP and TCP checksums">
<t>There is no way for a host to know if any of the IP
addresses in an IP header are the addresses used to calculate
the TCP checksum. That is, it is not feasible to calculate
the TCP checksum using the actual IP addresses in the pseudo
header; the addresses received in the incoming packet are not
necessarily the same as they were on the sending host.
Furthermore, it is not possible to recompute the upper-layer
checksums in the NAT/NAT-PT system, since the traffic is IPsec
protected. Consequently, the TCP and UDP checksums are
calculated using the HITs in the place of the IP addresses in
the pseudo header. Furthermore, only the IPv6 pseudo header
format is used. This provides for IPv4 / IPv6 protocol
translation.</t>
</section>
</section>
<section title="Multicast">
<t>There was little if any concrete
thoughts about how HIP might affect IP-layer or
application-layer multicast.</t>
</section>
<section title="HIP policies">
<t>There are a number of variables that will influence the HIP
exchanges that each host must support. All HIP implementations
should support at least 2 HIs, one to publish in DNS and an
unpublished one for anonymous usage. Although unpublished HIs
will be rarely used as responder HIs, they are likely be common
for initiators. Support for multiple HIs is recommended.</t>
<t>Many initiators would want to use a different HI for
different responders. The implementations should provide for a
policy of initiator HIT to responder HIT. This policy should
also include preferred transforms and local lifetimes. </t>
<t>Responders would need a similar policy, describing the hosts
allowed to participate in HIP exchanges, and the preferred
transforms and local lifetimes.</t>
</section>
<section title="Benefits of HIP">
<t>In the beginning, the network layer protocol (i.e., IP) had
the following four "classic" invariants:
<list style="symbols">
<t>Non-mutable: The address sent is the address
received.</t>
<t>Non-mobile: The address doesn't change during the course
of an "association".</t>
<t>Reversible: A return header can always be formed by
reversing the source and destination addresses.</t>
<t>Omniscient: Each host knows what address a partner host
can use to send packets to it.</t>
</list>
</t>
<t>Actually, the fourth can be inferred from 1 and 3, but it is
worth mentioning for reasons that will be obvious soon if not
already.</t>
<t>In the current "post-classic" world, we are intentionally
trying to get rid of the second invariant (both for mobility and
for multi-homing), and we have been forced to give up the first
and the fourth. <xref target="RFC3102">Realm Specific IP</xref>
is an attempt to reinstate the fourth invariant without the
first invariant. IPv6 is an attempt to reinstate the first
invariant.</t>
<t>Few systems on the Internet have DNS names that are
meaningful. That is, if they have a Fully Qualified Domain Name
(FQDN), that name typically belongs to a NAT device or a dial-up
server, and does not really identify the system itself but its
current connectivity. FQDNs (and their extensions as email
names) are application-layer names; more frequently naming
services than a particular system. This is why many systems on
the Internet are not registered in the DNS; they do not have
services of interest to other Internet hosts.</t>
<t>DNS names are references to IP addresses. This only
demonstrates the interrelationship of the networking and
application layers. DNS, as the Internet's only deployed,
distributed database is also the repository of other namespaces,
due in part to DNSSEC and application specific key records.
Although each namespace can be stretched (IP with v6, DNS with
KEY records), neither can adequately provide for host
authentication or act as a separation between internetworking
and transport layers.</t>
<t>The Host Identity (HI) namespace fills an important gap
between the IP and DNS namespaces. An interesting thing about
the HI is that it actually allows one to give up all but the 3rd
network-layer invariant. That is to say, as long as the source
and destination addresses in the network-layer protocol are
reversible, then things work ok because HIP takes care of host
identification, and reversibility allows one to get a packet
back to one's partner host. You do not care if the
network-layer address changes in transit (mutable) and you don't
care what network-layer address the partner is using
(non-omniscient).</t>
<!--
<t>Since all systems can have a Host Identity, every system can
have an entry in the DNS. The mobility features in HIP make it
attractive to trusted 3rd parties to offer rendezvous
servers.</t>
-->
<section title="HIP's answers to NSRG questions">
<t>The IRTF Name Space Research Group has posed a number of
evaluating questions in <xref
target="nsrg-report">their report</xref>. In this
section, we provide answers to these questions.
<list style="numbers">
<t>How would a stack name improve the overall
functionality of the Internet?
<list style="empty">
<t>HIP decouples the internetworking layer from the
transport layer, allowing each to evolve separately.
The decoupling makes end-host mobility and
multi-homing easier, also across IPv4 and IPv6
networks. HIs make network renumbering easier, and
they also make process migration and clustered servers
easier to implement. Furthermore, being cryptographic
in nature, they provide the basis for solving the
security problems related to end-host mobility and
multi-homing.</t>
</list>
</t>
<t>What does a stack name look like?
<list style="empty">
<t>A HI is a cryptographic public key. However,
instead of using the keys directly, most protocols use
a fixed size hash of the public key.</t>
</list>
</t>
<t>What is its lifetime?
<list style="empty">
<t>HIP provides both stable and temporary Host
Identifiers. Stable HIs are typically long lived,
with a lifetime of years or more. The lifetime of
temporary HIs depends on how long the upper-layer
connections and applications need them, and can range
from a few seconds to years.</t>
</list>
</t>
<t>Where does it live in the stack?
<list style="empty">
<t>The HIs live between the transport and
internetworking layers.</t>
</list>
</t>
<t>How is it used on the end points
<list style="empty">
<t>The Host Identifiers may be used directly or
indirectly (in the form of HITs or LSIs) by
applications when they access network services.
Additionally, the Host Identifiers, as public keys,
are used in the built in key agreement protocol,
called the HIP base exchange, to authenticate the
hosts to each other.</t>
</list>
</t>
<t>What administrative infrastructure is needed to support
it?
<list style="empty">
<t>In some environments, it is possible to use HIP
opportunistically, without any infrastructure.
However, to gain full benefit from HIP, the HIs must
be stored in the DNS or a PKI, and a new rendezvous
mechanism is needed <xref target="RFC5205" />.</t>
</list>
</t>
<t>If we add an additional layer would it make the address
list in SCTP unnecessary?
<list style="empty">
<t>Yes</t>
</list>
</t>
<t>What additional security benefits would a new naming
scheme offer?
<list style="empty">
<t>HIP reduces dependency on IP addresses, making the
so called address ownership <xref target="Nik2001" />
problems easier to solve. In practice, HIP provides
security for end-host mobility and multi-homing.
Furthermore, since HIP Host Identifiers are public
keys, standard public key certificate infrastructures
can be applied on the top of HIP.</t>
</list>
</t>
<t>What would the resolution mechanisms be, or what
characteristics of a resolution mechanisms would be
required?
<list style="empty">
<t>For most purposes, an approach where DNS names are
resolved simultaneously to HIs and IP addresses is
sufficient. However, if it becomes necessary to
resolve HIs into IP addresses or back to DNS names, a
flat resolution infrastructure is needed. Such an
infrastructure could be based on the ideas of
Distributed Hash Tables, but would require significant
new development and deployment.</t>
</list>
</t>
</list>
</t>
</section>
</section>
<section title="Security considerations">
<t>HIP takes advantage of the new Host Identity paradigm to
provide secure authentication of hosts and to provide a fast key
exchange for IPsec. HIP also attempts to limit the exposure of
the host to various denial-of-service (DoS) and
man-in-the-middle (MitM) attacks. In so doing, HIP itself is
subject to its own DoS and MitM attacks that potentially could
be more damaging to a host's ability to conduct business as
usual.</t>
<t>Resource exhausting denial-of-service attacks take advantage
of the cost of setting up a state for a protocol on the
responder compared to the 'cheapness' on the initiator. HIP
allows a responder to increase the cost of the start of state on
the initiator and makes an effort to reduce the cost to the
responder. This is done by having the responder start the
authenticated Diffie-Hellman exchange instead of the initiator,
making the HIP base exchange 4 packets long. There are more
details on this process in the Host Identity Protocol
under development. </t>
<t>HIP optionally supports opportunistic negotiation. That is,
if a host receives a start of transport without a HIP
negotiation, it can attempt to force a HIP exchange before
accepting the connection. This has the potential for DoS
attacks against both hosts. If the method to force the start of
HIP is expensive on either host, the attacker need only spoof a
TCP SYN. This would put both systems into the expensive
operations. HIP avoids this attack by having the responder send
a simple HIP packet that it can pre-build. Since this packet is
fixed and easily replayed, the initiator only reacts to it if it
has just started a connection to the responder.</t>
<t>Man-in-the-middle attacks are difficult to defend against,
without third-party authentication. A skillful MitM could
easily handle all parts of the HIP base exchange, but HIP
indirectly provides the following protection from a MitM attack.
If the responder's HI is retrieved from a signed DNS zone or
secured by some other means, the initiator can use this to
authenticate the signed HIP packets. Likewise, if the
initiator's HI is in a secure DNS zone, the responder can
retrieve it and validate the signed HIP packets. However, since
an initiator may choose to use an unpublished HI, it knowingly
risks a MitM attack. The responder may choose not to accept a
HIP exchange with an initiator using an unknown HI.</t>
<t>The need to support multiple hashes for generating the HIT
from the HI affords the MitM a potentially powerful downgrade
attack due to the a-priori need of the HIT in the HIP base
exchange. The base exchange has been augmented to deal with
such an attack by restarting on detecting the attack. At
worst this would only lead to a situation in which the
base exchange would never finish (or would be aborted after
some retries). As a drawback, this leads to an 6-way base
exchange which may seem bad at first. However, since this
only happens in an attack scenario and since the attack can
be handled (so it is not interesting to mount anymore), we
assume the additional messages are not a problem at all. Since
the MitM cannot be successful with a downgrade attack, these
sorts of attacks will only occur as 'nuisance' attacks. So,
the base exchange would still be usually just four packets
even though implementations must be prepared to protect
themselves against the downgrade attack.</t>
<t>In HIP, the Security Association for IPsec is indexed by the
SPI; the source address is always ignored, and the destination
address may be ignored as well. Therefore, HIP-enabled IPsec
Encapsulated Security Payload (ESP) is IP address independent.
This might seem to make it easier for an attacker, but ESP with
replay protection is already as well protected as possible, and
the removal of the IP address as a check should not increase the
exposure of IPsec ESP to DoS attacks.</t>
<t>Since not all hosts will ever support HIP, ICMPv4
'Destination Unreachable, Protocol Unreachable' and ICMPv6
'Parameter Problem, Unrecognized Next Header' messages are to be
expected and present a DoS attack. Against an initiator, the
attack would look like the responder does not support HIP, but
shortly after receiving the ICMP message, the initiator would
receive a valid HIP packet. Thus, to protect against this
attack, an initiator should not react to an ICMP message until a
reasonable time has passed, allowing it to get the real
responder's HIP packet. A similar attack against the responder
is more involved.</t>
<t>Another MitM attack is simulating a responder's
administrative rejection of a HIP initiation. This is a simple
ICMP 'Destination Unreachable, Administratively Prohibited'
message. A HIP packet is not used because it would either have
to have unique content, and thus difficult to generate,
resulting in yet another DoS attack, or just as spoofable as the
ICMP message. Like in the previous case, the defense against
this attack is for the initiator to wait a reasonable time
period to get a valid HIP packet. If one does not come, then
the initiator has to assume that the ICMP message is valid.
Since this is the only point in the HIP base exchange where this
ICMP message is appropriate, it can be ignored at any other
point in the exchange.</t>
<section title="HITs used in ACLs">
<t>It is expected that HITs will be used in ACLs. Future
firewalls can use HITs to control egress and ingress to
networks, with an assurance level difficult to achieve today.
As discussed above in <xref target="esp" />, once a HIP
session has been established, the SPI value in an IPsec packet
may be used as an index, indicating the HITs. In practice,
firewalls can inspect HIP packets to learn of the bindings
between HITs, SPI values, and IP addresses. They can even
explicitly control IPsec usage, dynamically opening IPsec ESP
only for specific SPI values and IP addresses. The signatures
in HIP packets allow a capable firewall to ensure that
the HIP exchange is indeed happening between two known hosts.
This may increase firewall security.</t>
<!-- <t>[add here wildcarding]</t> -->
<t>There has been considerable bad experience with distributed
ACLs that contain public key related material, for example,
with SSH. If the owner of a key needs to revoke it for any
reason, the task of finding all locations where the key is
held in an ACL may be impossible. If the reason for the
revocation is due to private key theft, this could be a
serious issue.</t>
<t>A host can keep track of all of its partners that might use
its HIT in an ACL by logging all remote HITs. It should only
be necessary to log responder hosts. With this information,
the host can notify the various hosts about the change to the
HIT. There has been no attempt to develop a secure method to
issue the HIT revocation notice.</t>
<t>HIP-aware NATs, however, are transparent to the HIP aware
systems by design. Thus, the host may find it difficult to
notify any NAT that is using a HIT in an ACL. Since most
systems will know of the NATs for their network, there should
be a process by which they can notify these NATs of the change
of the HIT. This is mandatory for systems that function as
responders behind a NAT. In a similar vein, if a host is
notified of a change in a HIT of an initiator, it should
notify its NAT of the change. In this manner, NATs will get
updated with the HIT change.</t>
</section>
<section title="Non-security considerations">
<t>The definition of the Host Identifier states that the HI
need not be a public key. It implies that the HI could be any
value; for example a FQDN. This document does not describe
how to support such a non-cryptographic HI. A
non-cryptographic HI would still offer the services of the HIT
or LSI for NAT traversal. It would be possible to carry HITs
in HIP packets that had neither privacy nor authentication.
Since such a mode would offer so little additional
functionality for so much addition to the IP kernel, it has
not been defined. Given how little public key cryptography
HIP requires, HIP should only be implemented using public key
Host Identities.</t>
<t>If it is desirable to use HIP in a low security situation
where public key computations are considered expensive, HIP
can be used with very short Diffie-Hellman and Host Identity
keys. Such use makes the participating hosts vulnerable to
MitM and connection hijacking attacks. However, it does not
cause flooding dangers, since the address check mechanism
relies on the routing system and not on cryptographic
strength.</t>
</section>
</section>
<section title="IANA considerations">
<t> This document has no actions for IANA.</t>
</section>
<section title="Acknowledgments">
<t>For the people historically involved in the early stages of
HIP, see the Acknowledgements section in the
Host Identity Protocol specification.</t>
<t>During the later stages of this document, when the editing
baton was transfered to Pekka Nikander, the comments from the
early implementors and others, including Jari Arkko, Tom
Henderson, Petri Jokela, Miika Komu, Mika Kousa, Andrew
McGregor, Jan Melen, Tim Shepard, Jukka Ylitalo, and Jorma Wall,
were invaluable. Finally, Lars Eggert, Spencer Dawkins and Dave
Crocker provided valuable input during the final stages of
publication, most of which was incorporated but some of which
the authors decided to ignore in order to get this document
published in the first place.</t>
<t>The authors want to express their special thanks to
Tom Henderson, who took the burden of editing the document
in response to IESG comments at the time when both of the
authors were busy doing other things. Without his perseverance
original document might have never made it as RFC4423.</t>
<t>This latest effort to update and move HIP forward within the IETF
process owes its impetuous to the three HIP development teams:
Boeing, HIIT (Helsinki Institute for Information Technology),
and NomadicLab of Ericsson. Without their collective efforts
HIP would have withered as on the IETF vine as a nice concept.</t>
</section>
</middle>
<back>
<references title="Normative References">
&RFC5202;
&RFC5204;
&RFC5205;
</references>
<references title="Informative references">
&RFC2136;
&RFC2535;
&RFC2766;
&RFC3022;
&RFC3102;
&RFC3748;
&RFC4025;
&RFC4225;
&RFC4306;
&hip-nat-traversal;
&nsrg-report;
&IEEE.802-15-4.2006;
<!-- Removed per Russ Housley IESG comment
&I-D.ietf-hip-mm;
-->
<reference anchor="chiappa-endpoints">
<front>
<title>Endpoints and Endpoint Names: A Proposed Enhancement
to the Internet Architecture</title>
<author initials="J. N." surname="Chiappa">
<organization />
</author>
<date year="1999" />
</front>
<seriesInfo name="URL"
value="http://www.chiappa.net/~jnc/tech/endpoints.txt" />
<format type="txt"
target="http://www.chiappa.net/~jnc/tech/endpoints.txt" />
</reference>
<reference anchor="Nik2001">
<front>
<title>Denial-of-Service, Address Ownership, and Early
Authentication in the IPv6 World</title>
<author initials="P." surname="Nikander">
<organization />
</author>
<date year="2002" />
</front>
<seriesInfo name="in Proceesings of"
value="Security Protocols, 9th International Workshop" />
<seriesInfo name=""
value="Cambridge, UK, April 25-27 2001" />
<seriesInfo name="LNCS" value="2467" />
<seriesInfo name="pp." value="12-26" />
<seriesInfo name="" value="Springer" />
<format type="pdf"
target="http://www.tml.hut.fi/~pnr/publications/cam2001.pdf"
/>
</reference>
<reference anchor="Bel1998">
<front>
<title>EIDs, IPsec, and HostNAT</title>
<author initials="S." surname="Bellovin">
<organization />
</author>
<date year="1998" month="March" />
</front>
<seriesInfo name="in Proceedings of"
value="41th IETF, Los Angeles, CA" />
<seriesInfo name="URL"
value="http://www1.cs.columbia.edu/~smb/talks/hostnat.pdf" />
<format type="pdf"
target="http://www1.cs.columbia.edu/~smb/talks/hostnat.pdf"
/>
</reference>
</references>
</back>
</rfc>
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