IRTF HIP Research Group J. Ahrenholz
Internet-Draft The Boeing Company
Expires: August 16, 2007 February 12, 2007
HIP DHT Interface
draft-ahrenholz-hiprg-dht-01
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Abstract
This document specifies a common interface for using HIP with a DHT
to provide a HIT-to-address lookup service and an unmanaged name-to-
HIT lookup service.
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3
2. The OpenDHT interface . . . . . . . . . . . . . . . . . . . . 4
3. HIP lookup services . . . . . . . . . . . . . . . . . . . . . 7
3.1. HIP address lookup . . . . . . . . . . . . . . . . . . . . 7
3.2. Secure HIP address lookup . . . . . . . . . . . . . . . . 10
3.3. HIP name to HIT lookup . . . . . . . . . . . . . . . . . . 12
3.4. When to use the HIP lookup services . . . . . . . . . . . 14
4. Issues with DHT support . . . . . . . . . . . . . . . . . . . 16
5. Security Considerations . . . . . . . . . . . . . . . . . . . 17
6. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 18
7. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 19
8. References . . . . . . . . . . . . . . . . . . . . . . . . . . 20
Appendix A. Change Log . . . . . . . . . . . . . . . . . . . . . 21
A.1. Changes from Version 00 to 01 . . . . . . . . . . . . . . 21
Author's Address . . . . . . . . . . . . . . . . . . . . . . . . . 22
Intellectual Property and Copyright Statements . . . . . . . . . . 23
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1. Introduction
The Host Identity Protocol [2] may benefit from a lookup service
based on Distributed Hash Tables (DHTs). The Host Identity namespace
is flat, consisting of public keys, in contrast to the hierarchical
Domain Name System. These keys are hashed to form Host Identity Tags
(HITs) which appear as large random numbers. The current DNS system
does not provide a suitable lookup mechanism for these flat, random
values, and has been heavily optimized for address lookup. DHTs
manage such data well by applying a hash function that distributes
data across a number of servers. DHTs also feature good support for
frequently updating stored values.
One freely available implementation of a DHT is the Bamboo DHT, which
is Java-based software that has been deployed on PlanetLab servers to
form a free service named OpenDHT. OpenDHT is available via the
Internet for any program to store and retrieve arbitrary data.
OpenDHT uses a well defined XML-RPC interface, featuring put, get,
and remove operations. This document discusses a common interface
for HIP to be used with OpenDHT, so that various HIP implementations
may leverage lookup services in an interoperable fashion.
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2. The OpenDHT interface
OpenDHT is a public deployment of Bamboo DHT servers running on about
150 PlanetLab nodes. While the Bamboo project provides the actual
software running on the servers, here we will refer only to OpenDHT,
which uses a certain defined interface for the XML-RPC calls. One
can run their own Bamboo nodes to set up a private ring of servers,
but here we are interested in providing a service for use with
multiple, different HIP implementations.
OpenDHT was chosen because it is a well-known, publicly available DHT
used within the research community. Its interface features a simple,
standards-based protocol that can be easily implemented by HIP
developers. This document does not aim to dictate that only the
services and servers described here should be used, but is rather
meant to act as a starting point to gain experience with these
services, choosing tools that are readily available.
OpenDHT stores values using (hash) keys. Keys are limited to 20
bytes in length, and values can be up to 1024 bytes. Values are
stored for a certain number of seconds, up to a maximum of 604,800
seconds (one week.) See the OpenDHT website:
Three RPC operations are supported: put, get, and rm (remove). Put
is called with key and value parameters, causing the value to be
stored using the key as its hash index. Get is called with the key
parameter, when you have a key and want to retrieve the value. Rm is
called with a hash of the value to be removed along with a secret
value, a hash of which was included in the put operation.
The definitions below are taken from
.
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The put operation takes the following arguments:
+----------------+--------------------------------------+
| field | type |
+----------------+--------------------------------------+
| application | string |
| | |
| client_library | string |
| | |
| key | byte array, 20 bytes max. |
| | |
| value | byte array, 1024 bytes max. |
| | |
| ttl_sec | four-byte integer, max. value 604800 |
| | |
| secret_hash | optional SHA-1 hash of secret value |
+----------------+--------------------------------------+
The server replies with an integer -- 0 for "success", 1 if it is
"over capacity", and 2 indicating "try again".
The get operation takes the following arguments:
+----------------+---------------------------------------------+
| field | type |
+----------------+---------------------------------------------+
| application | string |
| | |
| client_library | string |
| | |
| key | byte array, 20 bytes max. |
| | |
| maxvals | four-byte singed integer, max. value 2^31-1 |
| | |
| placemark | byte array, 100 bytes max. |
+----------------+---------------------------------------------+
The server replies with an array of values, and a placemark that can
be used for fetching additional values.
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The rm operation takes the following arguments:
+----------------+----------------------------------------------+
| field | type |
+----------------+----------------------------------------------+
| application | string |
| | |
| client_library | string |
| | |
| key | byte array, 20 bytes max. |
| | |
| value_hash | SHA-1 hash of value to remove |
| | |
| ttl_sec | four-byte integer, max. value 604800 |
| | |
| secret | secret value (SHA-1 of this was used in put) |
+----------------+----------------------------------------------+
The server replies with an integer -- 0 for "success", 1 if it is
"over capacity", and 2 indicating "try again".
This is the basic XML-RPC interface provided by OpenDHT. Each
"field" from the above tables are XML tags that enclose their
corresponding values. Below, specific uses for HIP are suggested,
along with values that can be used inside the fields shown above.
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3. HIP lookup services
Here we define an address lookup service for use with HIP, and
provide a second definition that includes Host Identity signatures to
protect against polluted DHT records. A HIT lookup service is also
defined for discovering a peer HIT given a name.
First is the basic address lookup service. Before a HIP association
can be initiated (a non-opportunistic initiation), a HIP host needs
the peer's HIT and the current address at which the peer is
reachable. Often the HIT will be pre-configured or available via DNS
lookup using a hostname lookup [3]. With HIP mobility [4], IP
addresses may be used as locators that are subject to change
frequently. The Host Identity and the HIT remain constant and can be
used to securely identify a host, so the HIT makes a good DHT key for
storing and retrieving addresses.
The address lookup service is then enhanced by including the peer's
Host Identity and a signature over the address and a timestamp. This
allows the DHT client or server to validate the address information
stored in the DHT.
Finally, the HIT lookup service can be used when legacy DNS servers
do not support HIP resource records, or when hosts do not have
administrative access to their DNS records. This unmanaged naming
service may help facilitate the HIP IRTF experiment.
These services reduce the amount of pre-configuration required at
each HIP host. The address of each peer no longer needs to be known
ahead of time, if peers also participate by publishing their
addresses. If peers choose to publish their HITs with a name, peer
HITs also no longer need pre-configuration. However, the DHT server
that will be contacted to service lookup requests is one additional
configuration item.
3.1. HIP address lookup
Functionally, this interface is: addr = get(HIT). Publish, lookup,
and remove operations are defined. Given a HIT, a lookup returns the
preferred address of the peer.
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Address publish
+----------------+----------------------------------+---------------+
| field | value | data type |
+----------------+----------------------------------+---------------+
| application | "hip-addr" | string |
| | | |
| client_library | (implementation dependent) | string |
| | | |
| key | 128-bit HIT | base64 |
| | | encoded |
| | | |
| value | struct sockaddr | base64 |
| | | encoded |
| | | |
| ttl_sec | current address lifetime | numeric |
| | | string |
| | | |
| secret_hash | optional SHA-1 hash of secret | base64 |
| | value | encoded |
+----------------+----------------------------------+---------------+
Address lookup
+----------------+---------------------------------+----------------+
| field | value | data type |
+----------------+---------------------------------+----------------+
| application | "hip-addr" | string |
| | | |
| client_library | (implementation dependent) | string |
| | | |
| key | 128-bit HIT | base64 encoded |
| | | |
| maxvals | (implementation dependent) | numeric string |
| | | |
| placemark | (NULL, or used from server | base64 encoded |
| | reply) | |
+----------------+---------------------------------+----------------+
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Address remove (optional)
+----------------+------------------------------------+-------------+
| field | value | data type |
+----------------+------------------------------------+-------------+
| application | "hip-addr" | string |
| | | |
| client_library | (implementation dependent) | string |
| | | |
| key | 128-bit HIT | base64 |
| | | encoded |
| | | |
| value_hash | SHA-1 hash of address value to | base64 |
| | remove | encoded |
| | | |
| ttl_sec | old address lifetime | numeric |
| | | string |
| | | |
| secret | secret value (SHA-1 of this was | base64 |
| | used in put) | encoded |
+----------------+------------------------------------+-------------+
The application and client_library fields are used for logging in
OpenDHT. The client_library may vary between different
implementations, specifying the name of the XML-RPC library used or
the application that directly makes XML-RPC calls.
The key for both address publish and lookup is the 128-bits of the
HIT, base64 encoded [1]. The value used in the publish and lookup
response is the struct sockaddr that stores the address in network
byte order, base64 encoded. The struct sockaddr was chosen because
it is a standard interface that includes the address family. Other
fields in the struct such as flowlabel and port numbers should be
zeroed and ignored. The length of the struct may be longer than the
system's struct sockaddr, if for example the IPv6 struct sockaddr_in6
is used.
The ttl_sec field used with address publish includes the time-to-
live, the number of seconds for which the entry will be stored by the
DHT, which is set to the number of seconds remaining in the address
lifetime.
The secret_hash is an optional field used with address publish, used
if the value will later be removed with an rm operation. The
secret_hash contains the base64 encoded SHA-1 hash of some secret
value known only to the publishing host.
The max_vals and placemark fields used with address lookup are
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defined by the get XML-RPC interface. The get needs to know the
maximum number of values to retrieve. The placemark is a value found
in the server reply that causes the get to continue to retrieve
values starting at where it left off.
3.2. Secure HIP address lookup
The address lookup interface can be slightly modified to exploit the
secure properties of the HIT. Here, the Host Identity, a public key,
is sent in the put operation, and is used to sign the contents of the
record. Now the DHT server and/or the client can verify that the HIT
and address pair added to the DHT were added by the host possessing
the private key associated with the given HIT.
In order for the DHT server to support this HIT verification, the
standard OpenDHT server would need to be modified. Users running
their own server can run the Bamboo DHT software, on which OpenDHT is
based, with some modifications. For put operations that include a
signature, the signature needs to be verified using the given Host
Identity, and the HIT provided needs to match this Host Identity. If
either signature or HIT verification fails, the put is not recorded
into the DHT.
The signature is also calculated over a timestamp to prevent replay
attacks. When the put message is received, the timestamp from the
put is compared against the server's current time, and if the
difference is greater than a certain threshold, the message is
discarded. This threshold is a local policy decision; a recommended
value is 30 seconds.
The client can perform the same signature and HIT verification. If
the signature is invalid for the given Host Identity or the HIT from
the record does not match the Host Identity, the DHT record retrieved
must be ignored. Note that for client-only verification the DHT
server does not need to be modified, so this would work on the
existing OpenDHT PlanetLab deployment. The timestamp is not as
meaningful when received from the client get request, but the
timestamp may be compared to the client's current time, to see if it
exceeds the entry's time-to-live value.
This interface is: (addr,timestamp,HI,sig) = get(HIT). Publish,
lookup, and remove operations are defined. Given a HIT, a lookup
returns the preferred address of the peer concatenated with the
timestamp, Host Identity and a signature over the address and
timestamp.
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Secure address publish
+----------------+-------------------------------------+------------+
| field | value | data type |
+----------------+-------------------------------------+------------+
| application | "hip-secure-addr" | string |
| | | |
| client_library | (implementation dependent) | string |
| | | |
| key | 128-bit HIT | base64 |
| | | encoded |
| | | |
| value | struct sockaddr | timestamp | Host | base64 |
| | Identity | signature | encoded |
| | | |
| ttl_sec | current address lifetime | numeric |
| | | string |
| | | |
| secret_hash | optional SHA-1 hash of secret value | base64 |
| | | encoded |
+----------------+-------------------------------------+------------+
Address lookup
+----------------+---------------------------------+----------------+
| field | value | data type |
+----------------+---------------------------------+----------------+
| application | "hip-secure-addr" | string |
| | | |
| client_library | (implementation dependent) | string |
| | | |
| key | 128-bit HIT | base64 encoded |
| | | |
| maxvals | (implementation dependent) | numeric string |
| | | |
| placemark | (NULL, or used from server | base64 encoded |
| | reply) | |
+----------------+---------------------------------+----------------+
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Secure address remove (optional)
+----------------+-------------------------------------+------------+
| field | value | data type |
+----------------+-------------------------------------+------------+
| application | "hip-addr" | string |
| | | |
| client_library | (implementation dependent) | string |
| | | |
| key | 128-bit HIT | base64 |
| | | encoded |
| | | |
| value_hash | SHA-1 hash of | base64 |
| | (address|timestamp|HI|sig) value to | encoded |
| | remove | |
| | | |
| ttl_sec | old address lifetime | numeric |
| | | string |
| | | |
| secret | secret value (SHA-1 of this was | base64 |
| | used in put) | encoded |
+----------------+-------------------------------------+------------+
The three secure address lookup operations put, get, and rm are
identical to the address lookup operations, except that wherever the
struct sockaddr (the address) was included before, it is appended
with the concatenation of a timestamp, the Host Identity (in the
encoding used in the Host Identity field, from section 5.2.8 of [2])
and a signature calculated over the struct sockaddr value and the
timestamp. In the tables above, "|" signifies concatenation of
values. The signature is not computed over the Host Identity or the
HIT because any change to these values results in validation failure.
The timestamp is an unsigned 32-bit value representing the number of
seconds since midnight UTC starting Jan 1, 1970 (standard UNIX time
format). Before the client signs the put message, it sets the
timestamp to the current value of its internal clock. The timestamp
is meant to protect against replayed put messages as described above.
3.3. HIP name to HIT lookup
Functionally, this interface is: HIT = get(SHA-1(name)), where name
is any text string defined by a host. Given the SHA-1 hash of a
name, a lookup returns the HIT of the peer. The hash of a name is
used because OpenDHT keys are limited to 20 bytes, so this allows for
names longer than 20 bytes. Publish, lookup, and remove operations
are defined.
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HIT publish
+----------------+----------------------------------+---------------+
| field | value | data type |
+----------------+----------------------------------+---------------+
| application | "hip-name-hit" | string |
| | | |
| client_library | (implementation dependent) | string |
| | | |
| key | SHA-1 hash of name | base64 |
| | | encoded |
| | | |
| value | 128-bit HIT | base64 |
| | | encoded |
| | | |
| ttl_sec | name lifetime | numeric |
| | | string |
| | | |
| secret_hash | optional SHA-1 hash of secret | base64 |
| | value | encoded |
+----------------+----------------------------------+---------------+
HIT lookup
+----------------+---------------------------------+----------------+
| field | value | data type |
+----------------+---------------------------------+----------------+
| application | "hip-name-hit" | string |
| | | |
| client_library | (implementation dependent) | string |
| | | |
| key | SHA-1 hash of name | base64 encoded |
| | | |
| maxvals | (implementation dependent) | numeric string |
| | | |
| placemark | (NULL, or used from server | base64 encoded |
| | reply) | |
+----------------+---------------------------------+----------------+
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HIT remove (optional)
+----------------+------------------------------------+-------------+
| field | value | data type |
+----------------+------------------------------------+-------------+
| application | "hip-name-hit" | string |
| | | |
| client_library | (implementation dependent) | string |
| | | |
| key | SHA-1 hash of name | base64 |
| | | encoded |
| | | |
| value_hash | SHA-1 hash of address value to | base64 |
| | remove | encoded |
| | | |
| ttl_sec | name lifetime | numeric |
| | | string |
| | | |
| secret | secret value (SHA-1 of this was | base64 |
| | used in put) | encoded |
+----------------+------------------------------------+-------------+
The key for both HIT publish and lookup is the SHA-1 hash of the
name. The name does not necessarily need to be associated with a
valid DNS or host name. It does not need to be related to the Domain
Identifier found in HI TLV. OpenDHT limits the keys to 20 bytes in
length, so the SHA-1 hash is used to allow arbitrary name lengths.
The value used in the publish and lookup response is the base64-
encoded 128-bit HIT.
The ttl_sec field specifies the number of seconds for which the entry
will be stored by the DHT, which is implementation dependent.
The secret_hash is an optional field used with HIT publish if the
value will later be removed with an rm operation. The secret_hash
contains the base64 encoded SHA-1 hash of some secret value known
only to the publishing host. The max_vals and placemark fields used
with the HIT lookup are defined by the get XML-RPC interface.
3.4. When to use the HIP lookup services
Below are some suggestions of when a HIP implementation may want to
use the address and HIT lookup services.
To learn of a peer's HIT, a host might first consult DNS using the
peer's hostname if the DNS server supports the HIP Resource Record
defined by [3]. Sometimes hosts do not have administrative authority
over their DNS entries and/or the DNS server is not able to support
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HIP resource records. Hosts may want to associate other non-DNS
names with their HITs. For these and other reasons, a host may use
the HIT publish service defined in Section 3.3. The peer HIT may be
learned by performing a DHT lookup of such a name.
Once a peer HIT is learned or configured, an address lookup could be
performed so that an address can be cached and immediately available
for when an association is requested. Implementations might load a
list of peer HITs on startup, resulting in several lookups that can
take some time to complete.
However, cached addresses may quickly become obsolete, depending on
how often the peer changes its preferred address. Performing an
address lookup before sending the I1 may be needed. At this time the
latency of a lookup may be intolerable, and a lookup could instead be
performed after the I1 retransmission timer fires -- when no R1 reply
has been received -- to detect any change in address.
A HIP host should publish its preferred address upon startup, so
other hosts may determine where it is reachable. The host needs to
periodically refresh this address entry because each entry carries a
TTL and will eventually expire. Also, when there is a change in
preferred address, usually associated with sending UPDATE packets
with included locator parameters, the host should update its address
with the DHT. The old address should be removed using the rm
operation, if a secret value was used in the put.
Addresses from the private address space should not be published to
the DHT. If the host is located behind a NAT, for example, the host
could publish the address of its RVS to the DHT if that is how it is
reachable. In this case however, a peer could instead simply use the
RVS field of the NATted host's HIP DNS record, which would eliminate
a separate DHT lookup.
A HIP host should also publish its HIT upon startup or whenever a new
HIT is configured, for use with the HIT lookup service, if desired.
The host should first check if the name already exists in the DHT by
performing a lookup, to avoid interfering with an existing name-to-
HIT mapping. The name-to-HIT binding needs to be refreshed
periodically before the TTL expires.
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4. Issues with DHT support
Each put operation appends the new value to any existing values. If
a host does not remove its old preferred address before adding
another, several entries may be present. Therefore when performing
an address lookup, the last address in the list should be used and
considered to be the current preferred address. If performing a
secure address lookup and several values are returned, the last valid
address (validated using the signature and the HIT) should be used
and considered to be the current preferred address. Before
performing each put a host should remove its old preferred address
using the rm operation.
All HITs carry the same prefix, so using them as a key to a DHT entry
may cause uneven distribution of the stored values. After some
experience is gained utilizing this draft, it may prove beneficial to
remove the HIT prefix and instead use the lower bits as the key.
In the case of the HIT lookup service, there is nothing preventing
different hosts from publishing the same name. A lookup performed on
this name will return multiple HITs that belong to different devices.
This is an unmanaged free-for-all service, so this issue will not be
solved here; it is recommended that a host simply pick another name.
Selecting an appropriate DHT server to use is not covered here. If a
particular server becomes unavailable, the connect will timeout and
some server selection algorithm should be performed, such as trying
the next server in a configured list. OpenDHT does provide a DNS-
based anycast service, when you perform a lookup of
"opendht.nyuld.net", it will return the two nearest OpenDHT servers.
Because the put and get calls rely on outside servers located across
the Internet, operations may have a latency involved that should be
considered when using these services with HIP.
The maximum size of 1024 bytes for the value field will limit the
maximum size of the Host Identity that may be used with the secure
address lookup service.
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5. Security Considerations
There are two possible attacks on this information exchange between
host and DHT server: attacks on the validity of the information
provided by the DHT to the host (such as a spoofed DHT response) and
attacks on the DHT records themselves (such as polluted records for a
given key).
For the address lookup based on HIT (Section 3.1), the validity of
the DHT response can be checked by an initiating host after the R1
message is received. The Host Identity provided in the R1 can be
hashed to obtain a HIT that can be checked against the original HIT.
However, the OpenDHT service does not currently prevent an attacker
from polluting the DHT records for a known HIT, thereby causing a
denial-of-service attack. It is possible to implement the use of HIP
signatures to validate put() requests for HITs as defined in
Section 3.2. When server validation of put() requests is not
possible, such as in the OpenDHT service deployed today, the client
should validate the record as described in Section 3.2.
A timestamp is included in Section 3.2 to protect against replayed
put messages. Without checking the timestamp, an attacker could re-
send the signed put message, possibly causing the entry to exist
beyond its valid lifetime or even after it has been removed.
Relying solely on client validation may be harmful. If an attacker
replays the signed put message and changes some aspect each time, and
if the server is not performing signature and HIT validation, there
could be a multitude of invalid entries stored in the DHT. When a
client retrieves these records it would need to perform signature and
HIT verification on each one, which could cause unacceptable amounts
of delay or computation.
For the HIT lookup based on name (Section 3.3), there are no
guarantees on the validity of the HIT. Users concerned with the
validity of HITs found in the DHT should simply exchange HITs out-of-
band with peers. Including a signature will not help here because
the HIT that identifies the Host Identity for signing is not known
ahead of time.
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6. IANA Considerations
This document has no actions for IANA.
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7. Acknowledgments
Thanks to Tom Henderson, Samu Varjonen, Andrei Gurtov, Miika Komu,
and Kristian Slavov for providing comments.
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8. References
[1] Freed, N. and N. Borenstein, "Multipurpose Internet Mail
Extensions (MIME) Part One: Format of Internet Message Bodies",
RFC 2045, November 1996.
[2] Moskowitz, R., "Host Identity Protocol", draft-ietf-hip-base-06
(work in progress), June 2006.
[3] Nikander, P. and J. Laganier, "Host Identity Protocol (HIP)
Domain Name System (DNS) Extensions", draft-ietf-hip-dns-08
(work in progress), October 2006.
[4] Nikander, P., "End-Host Mobility and Multihoming with the Host
Identity Protocol", draft-ietf-hip-mm-04 (work in progress),
June 2006.
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Appendix A. Change Log
A.1. Changes from Version 00 to 01
Removed the HIT lookup service -- using the LSI as a key to return a
HIT as the value -- and added a HIT lookup service using names.
Added support for OpenDHT remove. Changed all occurrences of "Open
DHT" to "OpenDHT".
Added the Host Identity and a signature as a secure address lookup
service, with text about running a modified OpenDHT server that can
verify signed put messages based on Host Identity signatures.
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Internet-Draft HIP DHT Interface February 2007
Author's Address
Jeff Ahrenholz
The Boeing Company
P.O. Box 3707
Seattle, WA
USA
Email: jeffrey.m.ahrenholz@boeing.com
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Internet-Draft HIP DHT Interface February 2007
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