Network Working Group R. Moskowitz
Internet-Draft ICSAlabs, a Division of TruSecure
Expires: March 1, 2004 Corporation
P. Nikander
Ericsson Research Nomadic Lab
Sep 2003
Host Identity Protocol Architecture
draft-moskowitz-hip-arch-05
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Copyright Notice
Copyright (C) The Internet Society (2003). All Rights Reserved.
Abstract
This memo describes the reasoning behind a proposed 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, strengths
and weaknesses, and how a new namespace will add completeness to
them. The roles of this new namespace in the protocols are defined.
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 3
2. Background . . . . . . . . . . . . . . . . . . . . . . . . . 4
2.1 A Desire for a Namespace for Computing Platforms . . . . . . 5
3. Host Identity Namespace . . . . . . . . . . . . . . . . . . 7
3.1 Host Identifiers . . . . . . . . . . . . . . . . . . . . . . 7
3.2 Storing Host Identifiers in DNS . . . . . . . . . . . . . . 8
3.3 Host Identity Tag (HIT) . . . . . . . . . . . . . . . . . . 8
3.4 Local Scope Identifier (LSI) . . . . . . . . . . . . . . . . 9
4. New Stack Architecture . . . . . . . . . . . . . . . . . . . 10
4.1 Transport associations and endpoints . . . . . . . . . . . . 10
5. End-Host Mobility and Multi-Homing . . . . . . . . . . . . . 12
5.1 Rendezvous server . . . . . . . . . . . . . . . . . . . . . 12
5.2 Protection against Flooding Attacks . . . . . . . . . . . . 13
6. HIP and IPsec . . . . . . . . . . . . . . . . . . . . . . . 14
7. HIP and NATs . . . . . . . . . . . . . . . . . . . . . . . . 15
7.1 HIP and TCP Checksum . . . . . . . . . . . . . . . . . . . . 15
8. HIP Policies . . . . . . . . . . . . . . . . . . . . . . . . 16
9. Benefits of HIP . . . . . . . . . . . . . . . . . . . . . . 17
9.1 HIP's Answers to NSRG questions . . . . . . . . . . . . . . 18
10. Security Considerations . . . . . . . . . . . . . . . . . . 20
10.1 HITs used in ACLs . . . . . . . . . . . . . . . . . . . . . 21
10.2 Non-security Considerations . . . . . . . . . . . . . . . . 22
11. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . 23
References (informative) . . . . . . . . . . . . . . . . . . 24
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . 24
Intellectual Property and Copyright Statements . . . . . . . 26
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1. Introduction
The Internet has created two 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.
The Host Identity namespace fills an important gap between the IP and
DNS namespaces. The Host Identity namespace consist of Host
Identifiers (HI). A Host Identifier is cryptographic in its nature;
it is the public key of an asymmetric key-pair. A Host Identity is
assigned to each host, or technically its networking kernel or stack.
Each host will have at least one Host Identity and a corresponding
Host Identifier, which can either be public (e.g. published in DNS),
or anonymous. Client systems will tend to have both public and
anonymous Identities.
Although the Host Identities could be used in many authentication
systems, the presented architecture introduces a new protocol, called
the Host Identity Protocol (HIP), and a cryptographic exchange,
called the HIP base exchange [4]. The new protocol provides for
limited forms of trust between systems. It enhances mobility,
multi-homing and dynamic IP renumbering [7], aids in protocol
translation / transition [4], and reduces certain types of
denial-of-service (DoS) attacks [4].
When HIP is used, the actual payload traffic between two HIP hosts is
typically protected with IPsec. The Host Identities are used to
create the needed IPsec Security Associations (SA) and to
authenticate the hosts. The actual payload IP packets do not differ
in any way from standard IPsec protected IP packets.
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2. Background
The Internet is built from three principle components: computing
platforms, packet transport (i.e. internetworking) infrastructure,
and services (applications). The Internet exists to service two
principal components: people and robotic processes (silicon based
people, if you will). All these components need to be named in order
to interact in a scalable manner.
There are two principal namespaces in use in the Internet for these
components: IP numbers, and Domain Names. Email, HTTP and SIP
addresses are really only extensions of Domain Names.
IP numbers are a confounding of two namespaces, the names of the
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.
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.
In the current Internet, the transport layers are coupled to the IP
addresses. Neither can evolve separately from the other. IPng
deliberations were framed by concerns of requiring a TCPng effort as
well.
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 addresses provide naming for both humans and autonomous
applications. Email addresses are extensions of Domain Names, only
in so far as a named service is responsible for managing a person's
mail. There is some anonymity in Email addresses.
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 because computing platforms are not well named with the current
namespaces.
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2.1 A Desire for a Namespace for Computing Platforms
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
either from mobility or renumbering.
If the namespace for computing platforms is cryptographically based,
it can also provide authentication services. If this namespace is
locally created without requiring registration, it can provide
anonymity.
Such a namespace (for computing platforms) and the names in it should
have the following characteristics:
The namespace should be applied to the IP 'kernel'. The IP kernel
is the 'component' between services and the packet transport
infrastructure.
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 TCB). This may require
changes to the current APIs. In the long run, it is probable that
some new APIs are needed.
The introduction of the namespace should not mandate any
administrative infrastructure. Deployment must come from the
bottom up, in a pairwise deployment.
The names should have a fixed length representation, for easy
inclusion in datagrams and programming interfaces (e.g the TCB).
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.
The names must be statistically globally unique. 64 bits is
inadequate (1% chance of collision in a population of 640M); thus
approximately 100 or more bits should be used.
The names should have a localized abstraction so that it can be
used in existing protocols and APIs.
It must be possible to create names locally. This can provide
anonymity at the cost of making resolvability very difficult.
Sometimes the names may contain a delegation component. This is
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the cost of resolvability.
The namespace should provide authentication services. This is a
preferred function.
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.
In this document, such a new namespace 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.
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3. Host Identity Namespace
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, but not
limited to, an IP stack. A system can have multiple identities, some
'well known', some anonymous. A system may self assert its identity,
or may use a third-party authenticator like DNSSEC, 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.
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.
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
Identifiers. As documented in the Host Identity Protocol
specification [4], 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.
3.1 Host Identifiers
Host Identity adds two main features to Internet protocols. The first
is a decoupling of the internetworking and transport layers; see
Section 4. 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 to authenticate security protocols like IPsec.
The only completely defined structure of the Host Identity is that of
a public 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
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possessed by more than one node, the Identity can be considered to be
a distributed one.
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. The current HIP documents do not
specify how to use any other types of Host Identifiers but public
keys.
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
Identities. Another representation of the Host Identities, the Local
Scope Identifier (LSI), can also be used in protocols and APIs.
3.2 Storing Host Identifiers in DNS
The Host Identifiers should be stored in DNS. The exception to this
is anonymous identities. The HI is stored in a new RR type, to be
defined. This RR type is likely to be quite similar to the IPSECKEY
RR [5].
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.
3.3 Host Identity Tag (HIT)
A Host Identity Tag is an 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 whatever
underlying technology is used.
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. In the extremely rare case that a single HIT happens to
map to more than one Host Identities, 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
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public key to use.
3.4 Local Scope Identifier (LSI)
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. The generation of LSIs is defined in
the Host Identity Protocol specification [4].
Examples of how LSIs can be used include: as the address in a FTP
command and as the address in a socket call. Thus, LSIs act as a
bridge for Host Identities into old protocols and APIs.
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4. New Stack Architecture
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 IRTF
Name Space Research Group Report [6] and, e.g., the unpublished
Internet-Draft Endpoints and Endpoint Names [9] by Noel Chiappa, the
IP addresses currently embody the dual role of locators and endpoint
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 endpoint name.
In the HIP architecture, the endpoint names and locators are
separated from each other. IP addresses continue to act as locators.
The Host Identifiers take the role of endpoint identifiers. It is
important to understand that the endpoint names based on Host
Identities are slightly different from interface names; a Host
Identity can be simultaneously reachable through several interfaces.
The difference between the bindings of the logical entities are
illustrated in Figure 1.
Process ------ Socket Process ------ Socket
| |
| |
| |
| |
Endpoint | Endpoint --- Host Identity
\ | |
\ | |
\ | |
\ | |
Location --- IP address Location --- IP address
Figure 1
4.1 Transport associations and endpoints
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 more bound to IP
addresses but to Host Identities.
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It is possible that a single physical computer hosts several logical
endpoints. With HIP, each of these endpoints 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 endpoint.
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5. End-Host Mobility and Multi-Homing
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 very 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 allows these IP
addresses to be linked with each other, and if one address becomes
unusable (e.g. due to a network failure), existing transport
associations can be easily moved to another address.
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 totally ignore the source address. However, as discussed
in Section 5.2 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).
5.1 Rendezvous server
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 Dynamic DNS could be used for
this function for infrequently moving nodes, an alternative to using
DNS in this fashion is to use a piece of new static infrastructure
called a HIP rendezvous server. Instead of registering its current
dynamic address to the DNS server, the mobile node registers the
address(es) of its rendezvous server(s). The mobile node keeps the
rendezvous server(s) continuously updated with its current IP
address(es). A rendezvous server simply forwards the initial HIP
packet from an initiator to the mobile node at its current location.
All further packets flow between the initiator and the mobile node.
There is typically very little activity on a rendezvous server,
address updates and initial HIP packet forwarding. Thus, one server
can support a large number of potential mobile nodes. The mobile
nodes must trust the rendezvous server to properly maintain their HIT
and IP address mappings.
The rendezvous server is also needed if both of the nodes are mobile
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and happen to move at the same time. In that case, the HIP readdress
packets will cross each other in the network and never reach the peer
node. To solve this situation, the nodes should remember the
rendezvous server address, and re-send the HIP readdress packet to
the rendezvous server if no reply is received.
The mobile node keeps its address current on the rendezvous server by
setting up a HIP association with the rendezvous server and sending
HIP readdress packets to it. A rendezvous server will permit two
mobile systems to use HIP without any extraneous infrastructure (in
addition to the rendezvous server itself), including DNS if they have
a method other than a DNS query to get each other's HI and HIT.
5.2 Protection against Flooding Attacks
While 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.
Blindly accepting new addresses would potentially lead to flooding
Denial-of-Service attacks against third parties [8]. In a
distributed flooding attack an attacker opens (anonymous) high volume
HIP connections with a large number of hosts, 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.
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.
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6. HIP and IPsec
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. In the future, other ways of transporting payload data may
be developed, including ones that do not use cryptographic
protection.
In practise, 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.
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. Future HIP extensions may provide for more granularity and
creation of several ESP SAs between a pair of HITs
Since HIP is designed for host usage, not for gateways, 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.
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.
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7. HIP and NATs
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 Network Address
Translation (NAT) [2] or NAT Protocol translation (NAT-PT) [1].
In a network environment where the identification is based on the IP
addresses, identifying the communicating nodes is difficult when NAT
is used. With HIP, the transport layer endpoints 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; the IP addresses may be changed freely during
packet traversal.
For a HIP based flow, a NAT or NAT-PT system tracks the mapping of
HITs and the corresponding IPsec SPIs to an IP address. Many HITs
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.
The 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 place holder for embedded IP addresses.
7.1 HIP and TCP Checksum
There is no way for a host to know if any of the IP addresses in the
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.
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8. HIP Policies
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 one for anonymous usage.
Although anonymous HIs will be rarely used as responder HIs, they are
likely be common for initiators. Support for multiple HIs is
recommended.
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.
Responders would need a similar policy, representing which hosts they
accept HIP exchanges, and the preferred transforms and local
lifetimes.
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9. Benefits of HIP
In the beginning, the network layer protocol (i.e. IP) had the
following four "classic" invariants:
Non-mutable: The address sent is the address received.
Non-mobile: The address doesn't change during the course of an
"association".
Reversible: A return header can always be formed by reversing the
source and destination addresses.
Omniscient: Each host knows what address a partner host can use to
send packets to it.
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.
In the current "post-classic" world, we are trying intentionally 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. Realm Specific IP [3] is an attempt to reinstate the fourth
invariant without the first invariant. IPv6 is an attempt to
reinstate the first invariant.
Few systems on the Internet have DNS names that are meaningful to
them. That is, if they have a Fully Qualified Domain Name (FQDN),
that typically belongs to a NAT device or a dial-up server, and does
not really identify the system itself but its current connectivity.
FQDN names (and their extensions as email names) are Application
Layer names; more frequently naming processes than a particular
system. This is why many systems on the internet are not registered
in DNS; they do not have processes of interest to other Internet
hosts.
DNS names are indirect 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.
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
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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 don't 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).
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.
9.1 HIP's Answers to NSRG questions
The IRTF Name Space Research Group has posed a number of evaluating
questions in their report [6]. In this section, we provide answers
to these questions.
1. How would a stack name improve the overall functionality of the
Internet?
At the fundamental level, HI decouples the internetworking
layer from the transport layer, allowing each to evolve
separately. At the same time, the decoupling makes end-host
mobility and multi-homing easier. It also allows mobility and
multi-homing across the IPv4 and IPv6 networks. HIs make
network renumbering easier. At the conceptual level, 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.
2. What does a stack name look like?
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.
3. What is its lifetime?
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.
4. Where does it live in the stack?
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The HIs live between the transport and internetworking layers.
5. How is it used on the end points
The Host Identifiers, in the form of HITs or LSIs, are used by
legacy applications as if they were IP addresses.
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.
6. What administrative infrastructure is needed to support it?
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
infrastructure of rendezvous servers is needed.
7. If we add an additional layer would it make the address list in
SCTP unnecessary?
Yes
8. What additional security benefits would a new naming scheme
offer?
HIP reduces dependency on IP addresses, making the so called
address ownership 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.
9. What would the resolution mechanisms be, or what characteristics
of a resolution mechanisms would be required?
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, hash based 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.
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10. Security Considerations
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.
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
specification [4].
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.
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 anonymous HI, it
knowingly risks a MitM attack. The responder may choose not to
accept a HIP exchange with an anonymous initiator.
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
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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.
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.
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.
10.1 HITs used in ACLs
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 Section 6,
once a HIP session has been established, the SPI value in an IPsec
packet may be used as an index, indicating the HITs. In practise,
the firewalls can inspect the 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 the HIP packets allow
a capable firewall to make sure that the HIP exchange is indeed
happening between two known hosts. This may increase firewall
security.
There has been considerable bad experience with distributed ACLs that
contain public key related material, for example, with SSH. If the
owner of the 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.
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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 (like in CMP and CMC) to issue the HIT
revocation notice.
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.
10.2 Non-security Considerations
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
an 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
carry the 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.
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.
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11. Acknowledgments
For the people historically involved in the early stages of HIP, see
the Acknowledgements section in the Host Identity Protocol
specification [4].
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.
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References (informative)
[1] Tsirtsis, G. and P. Srisuresh, "Network Address Translation -
Protocol Translation (NAT-PT)", RFC 2766, February 2000.
[2] Srisuresh, P. and K. Egevang, "Traditional IP Network Address
Translator (Traditional NAT)", RFC 3022, January 2001.
[3] Borella, M., Lo, J., Grabelsky, D. and G. Montenegro, "Realm
Specific IP: Framework", RFC 3102, October 2001.
[4] Moskowitz, R., Nikander, P. and P. Jokela, "Host Identity
Protocol", draft-moskowitz-hip-07 (work in progress), June 2003.
[5] Richardson, M., "A method for storing IPsec keying material in
DNS", draft-ietf-ipseckey-rr-07 (work in progress), September
2003.
[6] Lear, E. and R. Droms, "What's In A Name:Thoughts from the
NSRG", draft-irtf-nsrg-report-10 (work in progress), September
2003.
[7] Nikander, P., "End-Host Mobility and Multi-Homing with Host
Identity Protocol", draft-nikander-hip-mm-00 (work in progress),
June 2003.
[8] Nikander, P., "Mobile IP version 6 Route Optimization Security
Design Background", draft-nikander-mobileip-v6-ro-sec-01 (work
in progress), July 2003.
[9] Chiappa, J., "Endpoints and Endpoint Names: A Proposed
Enhancement to the Internet Architecture", URL http://
users.exis.net/~jnc/tech/endpoints.txt, 1999.
Authors' Addresses
Robert Moskowitz
ICSAlabs, a Division of TruSecure Corporation
1000 Bent Creek Blvd, Suite 200
Mechanicsburg, PA
USA
EMail: rgm@icsalabs.com
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Pekka Nikander
Ericsson Research Nomadic Lab
JORVAS FIN-02420
FINLAND
Phone: +358 9 299 1
EMail: pekka.nikander@nomadiclab.com
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