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.
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 Lightweight Directory Access Protocol (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.
5.2. Storing Host Identifiers in DNS
The public Host Identifiers should be stored in DNS; the unpublished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 is stored in a new
Resource Record (RR) type, to be defined. This RR type is likely to
be quite similar to the IPSECKEY RR [6].
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.
5.3. Host Identity Tag (HIT)
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. First, its fixed length makes for
easier protocol coding and also better manages the packet size cost
of this technology. Second, it presents the identity in a consistent
format to the protocol independent of the cryptographic algorithms
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 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.
5.4. Local Scope Identifier (LSI)
A Local Scope Identifier (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.
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.
6. 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 [7] and, e.g., the unpublished
Internet Draft "Endpoints and Endpoint Names" [10] by Noel Chiappa,
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 an end-point name.
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.
The difference between the bindings of the logical entities is
illustrated in Figure 1.
Service ------ Socket Service ------ Socket
| |
| |
| |
| |
End-point | End-point --- Host Identity
\ | |
\ | |
\ | |
\ | |
Location --- IP address Location --- IP address
Figure 1
6.1. Transport Associations and End-points
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.
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.
7. 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 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, Dynamic Host
Configuration Protocol (DHCP), IPv6 prefix reassignments, or a
Network Address Translation (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.
When a node moves while communication is already ongoing, 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
Section 7.2 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 restarting a
connection after the initial connection).
7.1. Rendezvous Mechanism
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 [1] 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.
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.
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.
A separate document will specify the details of the HIP rendezvous
mechanism.
7.2. Protection against Flooding Attacks
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.
Blindly accepting new addresses would potentially lead to flooding
DoS attacks against third parties [8]. 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.
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.
8. 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 Encapsulating 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 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.
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 Section 9, below), it is
very important that the HIP protocols are friendly toward any middle
boxes.
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.
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 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 [12].
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.
9. 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) [4] or NAT Protocol Translation (NAT-PT) [3].
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.
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.
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.
9.1. HIP and TCP Checksums
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.
10. Multicast
Back in the Fall of 2003, there were little if any concrete thoughts
about how HIP might affect IP-layer or application-layer multicast.
11. 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 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.
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, describing the hosts allowed
to participate in HIP exchanges, and the preferred transforms and
local lifetimes.
12. Benefits of HIP
In the beginning, the network layer protocol (i.e., IP) had the
following four "classic" invariants:
o Non-mutable: The address sent is the address received.
o Non-mobile: The address does not change during the course of an
"association".
o Reversible: A return header can always be formed by reversing the
source and destination addresses.
o 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 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.
Realm Specific IP [5] 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. 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.
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-specific 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
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 do not care what network-layer address the partner
is using (non-omniscient).
12.1. HIP’s Answers to NSRG Questions
The IRTF Name Space Research Group has posed a number of evaluating
questions in its report [7]. In this section, we provide answers to
these questions.
1. How would a stack name improve the overall functionality of the
Internet?
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.
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?
The HIs live between the transport and internetworking layers.
5. How is it used on the end-points?
The Host Identifiers may be used directly or indirectly (in
the form of HITs or LSIs) by applications when they access
network services. In addition, 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?
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. Such a
new rendezvous mechanism may need new infrastructure to be
deployed.
7. If we add an additional layer, would it make the address list in
Stream Control Transmission Protocol (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 [11] 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 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.
13. 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.