Internet Research Task Force (IRTF)                         J. Ahrenholz
Request for Comments: 6537                            The Boeing Company
Category: Experimental                                     February 2012
ISSN: 2070-1721

Host Identity Protocol Distributed Hash Table Interface

Abstract

   This document specifies a common interface for using the Host
   Identity Protocol (HIP) with a Distributed Hash Table (DHT) service
   to provide a name-to-Host-Identity-Tag lookup service and a Host-
   Identity-Tag-to-address lookup service.

Status of This Memo

   This document is not an Internet Standards Track specification; it is
   published for examination, experimental implementation, and
   evaluation.

   This document defines an Experimental Protocol for the Internet
   community.  This document is a product of the Internet Research Task
   Force (IRTF).  The IRTF publishes the results of Internet-related
   research and development activities.  These results might not be
   suitable for deployment.  This RFC represents the consensus of the
   HIP Research Group of the Internet Research Task Force (IRTF).
   Documents approved for publication by the IRSG are not a candidate
   for any level of Internet Standard; see Section 2 of RFC 5741.

   Information about the current status of this document, any errata,
   and how to provide feedback on it may be obtained at
   http://www.rfc-editor.org/info/rfc6537.

Copyright Notice

   Copyright (c) 2012 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

   This document is subject to BCP 78 and the IETF Trust's Legal
   Provisions Relating to IETF Documents
   (http://trustee.ietf.org/license-info) in effect on the date of
   publication of this document.  Please review these documents
   carefully, as they describe your rights and restrictions with respect
   to this document.

   1. Introduction ....................................................2
   2. The OpenDHT Interface ...........................................3
   3. HDRR - The HIP DHT Resource Record ..............................6
   4. HIP Lookup Services .............................................8
      4.1. HIP Name to HIT Lookup .....................................9
      4.2. HIP Address Lookup ........................................12
   5. Use Cases ......................................................15
   6. Issues with DHT Support ........................................16
   7. Security Considerations ........................................17
   8. IANA Considerations ............................................18
   9. Acknowledgments ................................................18
   10. References ....................................................19
      10.1. Normative References .....................................19
      10.2. Informative References ...................................19

1. Introduction

   The Host Identity Protocol (HIP) [RFC5201] 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 (DNS).  These keys are hashed and
   prefixed to form Host Identity Tags (HITs), which appear as large
   random numbers.  As the current DNS system has been heavily optimized
   for address lookup, it may be worthwhile to experiment with other
   services such as those defined here.  DHTs manage such data well by
   applying a hash function that distributes data across a number of
   servers.  DHTs are also designed to be updated more frequently than a
   DNS-based approach.  For an alternative method of using HITs to look
   up IP addresses using DNS, see [HIT2IP].

   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 was available via the
   Internet for any program to store and retrieve arbitrary data.
   OpenDHT used a well-defined Extensible Markup Language-Remote
   Procedure Calling (XML-RPC) interface, featuring put, get, and remove
   operations.  OpenLookup, while not implemented as a DHT, is another
   deployment of open source software compatible with this OpenDHT
   interface.  This document discusses a common way for HIP to use this
   OpenDHT interface, so that various HIP experimenters may employ
   lookup services in an interoperable fashion.

   This document is a product of the HIP research group (RG) of the
   IRTF.  The HIP research group reached consensus that this interface
   specification should be published as an Experimental RFC, based on

   document review by at least six RG members including the chairs, and
   based on implementation experience.  This document is not an IETF
   product and is not a standard.

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
   document are to be interpreted as described in [RFC2119].

2. The OpenDHT Interface

   OpenDHT [OPENDHT] was a public deployment of Bamboo DHT servers that
   ran on about 150 PlanetLab nodes, and was retired in July 2009.
   While the Bamboo project provided 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.  Another service compatible
   with this interface is OpenLookup.  One can run their own Bamboo
   nodes to set up a private ring of servers.

   OpenDHT was chosen because it was 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 and indexes those values by 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.)  For more information, see the
   OpenDHT website: <http://www.opendht.org/>.

   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 the OpenDHT users guide at
   <http://opendht.org/users-guide.html>.

              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.  The key is a byte array used to index the
   record for storage and retrieval from the DHT.  The value is a byte
   array of the data being stored in the DHT.  The application and
   client_library fields are metadata used only for logging purposes.
   The ttl_sec field specifies the number of seconds that the DHT should
   store the value.  The secret_hash field allows values to be later
   removed from the DHT.  The maxvals and placemark fields are for
   retrieving a maximum number of values and for iterating get results.

   The return code of 0 "success" indicates a successful put or remove
   operation.  The return code of 1 "over capacity" means that a client
   is using too much storage space on the server.  The return value of 2
   "try again" indicates that the client should retry the put operation
   because a temporary problem prevented the server from accepting the
   put.

   In the sections that follow, specific uses for these DHT operations
   and their XML fields are suggested for use with HIP.

3. HDRR - The HIP DHT Resource Record

   The two lookup services described in this document use a HIP DHT
   Resource Record (HDRR) defined in this section.  This record is a
   wrapper around data contained in TLVs, similar to a HIP control
   packet.  The data contained in each HDRR differs between the two
   services.

   The HDRR uses the same binary format as HIP packets (defined in
   [RFC5201].)  This packet encoding is used as a convenience, even
   though this data is actually a resource record stored and retrieved
   by the DHT servers, not a packet sent on the wire by a HIP protocol
   daemon.  Note that this HDRR format is different than the HIP RR used
   by the Domain Name System as defined in [RFC5205].  The reason it is
   different is that it is a different record from a functional point of
   view: in DNS, the query key is a Fully Qualified Domain Name (FQDN),
   and the return value is a HIT, while here, the query key is a HIT.

   HIP header values for the HDRR:

     HIP Header:
       Packet Type = 20 DHT Resource Record
       SRC HIT = Sender's HIT
       DST HIT = NULL

     HDRR used with HIT lookup:
     HIP ( [CERT] )

     HDRR used with address lookup:
     HIP ( LOCATOR, SEQ, HOST_ID, [CERT], HIP_SIGNATURE )

   The Initiator HIT (Sender's HIT, SRC HIT) MUST be set to the HIT that
   the host wishes to make available using the lookup service.  With the
   HIT lookup service, this is the main piece of information returned by
   a get operation.  For the address lookup service, this HIT MUST be
   the same one used to derive the HIT_KEY used as the DHT key.  The
   Responder HIT (Receiver's HIT, DST HIT) MUST be NULL (all zeroes)
   since the data is intended for any host.

   The only other TLV used with the HIT lookup service is an optional
   CERT parameter containing a certificate for validating the name that
   is used as the DHT key.  The CERT parameter is defined in [RFC6253].
   The DHT server SHOULD use the certificate to verify that the client
   is authorized to use the name used for the DHT key, using the hash of
   the name found in the certificate.  The Common Name (CN) field from
   the Distinguished Name (DN) of the X.509.v3 certificate MUST be used.
   Which certificates are considered trusted is a local policy issue.

   The remaining parameters described here are used with the address
   lookup service.

   The LOCATOR parameter contains the addresses that the host wishes to
   make available using the lookup service.  A host MAY publish its
   current preferred IPv4 and IPv6 locators, for example.

   The SEQ parameter contains an unsigned 32-bit sequence number, the
   Update ID.  This is typically initialized to zero and incremented by
   one for each new HDRR that is published by the host.  The host SHOULD
   retain the last Update ID value it used for each HIT across reboots,
   or perform a self lookup in the DHT.  The Update ID value may be
   retained in the DHT records and will determine the preferred address
   used by peers.

   The HOST_ID parameter contains the Host Identity that corresponds
   with the Sender's HIT.  (The encoding of this parameter is defined in
   Section 5.2.8 of [RFC5201].)

   The HOST_ID parameter and HIP_SIGNATURE parameter MUST be used with
   the HDRR so that HIP clients receiving the record can validate the
   sender and the included LOCATOR parameter.  The HIT_KEY used for the
   DHT key will also be verified against the Host Identity.

   The client that receives the HDRR from the DHT response MUST perform
   the signature and HIT_KEY verification.  If the signature is invalid
   for the given Host Identity or the HIT_KEY used to retrieve 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.

   The Sender's HIT in the HDRR MUST correspond with the key used for
   the lookup and Host Identity verification.  The Receiver's HIT MUST
   be NULL (all zeroes) in the HDRR header.

   When several HDRR records are returned by the server, the client
   SHOULD pick the most recent record as indicated by the Update ID in
   the SEQ TLV of the HDRR and perform verification on that record.  The
   order in which records are returned should not be considered.

   The DHT server MAY also verify the SIGNATURE and HOST_ID, with some
   modifications to the Bamboo DHT software and a new return code with
   the OpenDHT interface.  The signature in the put MUST be verified
   using the given Host Identity (public key), and the HIT_KEY provided
   as the lookup key MUST match this Host Identity according to the
   Overlay Routable Cryptographic Hash Identifiers (ORCHID) generation
   method defined by [RFC4843].  If either signature or HIT verification

   fails, the put MUST not be recorded into the DHT, and the server
   returns a failure code.  The failure code is an additional return
   code not defined by OpenDHT, with a value of 3.

   This server-side verification of records could introduce a source of
   a denial-of-service attack.  The server policy could require clients
   to have an active HIP association.  See Section 7 for further
   discussion.

4. HIP Lookup Services

   This document defines a HIT lookup and address lookup service for use
   with HIP.  The HIT lookup uses a text name to discover a peer's HIT.
   The address lookup uses a peer's HIT to discover its current
   addresses.

   The two lookups are defined below.  The abbreviated notation refers
   to the HIP parameter types; for example, HIP_SIG is the HIP signature
   parameter defined by [RFC5201].

         HDRR([CERT]) = get(SHA-1("name"))
         HDRR(LOCATOR, SEQ, HOST_ID, [CERT], HIP_SIG) = get(HIT_KEY)

   The HIT lookup service returns the Host Identity Tag of a peer given
   a name.  The name SHOULD be the FQDN, hostname, or some other alias.
   This HIT is found in the Sender's HIT field of the HDRR.  The HIT is
   the hash of the public-key-based Host Identity as described in
   [RFC5201].  There are no security properties of the name, unlike the
   HIT.  An optional certificate MAY be included in the record, for
   validating the name, providing some measure of security.  Which
   certificates are considered trusted is a local policy issue.  This
   service is intended for use when legacy DNS servers do not support
   HIP resource records, or when hosts do not have administrative access
   to publish their own DNS records.  Such an unmanaged naming service
   may help facilitate experimentation.

   The address lookup returns a locator and other validation data in the
   HDRR for a given HIT.  Before a HIP association can be initiated (not
   in opportunistic mode), a HIP host needs to know the peer's HIT and
   the current address at which the peer is reachable.  Often the HIT
   will be pre-configured, available via DNS lookup using a hostname
   lookup [RFC5205], or retrieved using the HIT lookup service defined
   in this document.  With HIP mobility [RFC5206], IP addresses may be
   used as locators and may often change.  The Host Identity and the HIT
   remain relatively constant and can be used to securely identify a
   host, so the HIT serves as a suitable DHT key for storing and
   retrieving addresses.

   The address lookup service includes the peer's Host Identity and a
   signature over the locators.  This allows the DHT client or server to
   validate the address information stored in the DHT.

   These two separate lookups are defined instead of one because the
   address record is expected to change more frequently, while the name-
   to-HIT binding should remain relatively constant.  For example, local
   policy may specify checking the name-to-HIT binding on a daily basis,
   while the address record is updated hourly for active peers.  Also,
   the client and server validation of the two records is different,
   with the HIT lookup using certificates verifying the name and the
   address lookup using a signature produced by the bearer of a
   particular Host Identity/HIT.

   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 require pre-configuration.  However, discovering
   an available DHT server for servicing these lookups will require some
   additional configuration.

4.1. HIP Name to HIT Lookup

   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 longer names.  Publish, lookup, and remove
   operations are defined below.

         HDRR([CERT]) = get(SHA-1("name"))
         put(SHA-1("name"), HDRR([CERT]), [SHA-1(secret)])
         rm(SHA-1("name"), SHA-1(HDRR), secret)

                           HIT remove (optional)

   +----------------+----------------------------------------+---------+
   | field          | value                                  | data    |
   |                |                                        | type    |
   +----------------+----------------------------------------+---------+
   | application    | "hip-name-hit"                         | string  |
   |                |                                        |         |
   | client_library | (implementation dependent)             | string  |
   |                |                                        |         |
   | key            | SHA-1 hash of a name                   | base64  |
   |                |                                        | encoded |
   |                |                                        |         |
   | value_hash     | SHA-1 hash of HDRR (value used during  | base64  |
   |                | publish) to remove                     | encoded |
   |                |                                        |         |
   | ttl_sec        | lifetime for the remove should be      | numeric |
   |                | greater than or equal to the amount of | string  |
   |                | time remaining for the record          |         |
   |                |                                        |         |
   | secret         | secret value (SHA-1 of this was used   | base64  |
   |                | 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 the 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 MUST be the base64-
   encoded HDRR containing the HIT, and MAY include an optional
   certificate.  The HIT MUST be stored in the Sender's HIT field in the
   HDRR header and is a 128-bit value that can be identified as a HIT
   both by its length and by the ORCHID prefix [RFC4843] that it starts
   with.

   If a certificate is included in this HIT record, the name used for
   the DHT key MUST be listed in the certificate.  The CERT parameter is
   defined in [RFC6253].  The Common Name (CN) field from the
   Distinguished Name (DN) of the X.509.v3 certificate MUST be used.
   The server can hash this name to verify it matches the DHT key.

   The ttl_sec field specifies the number of seconds requested by the
   client that the entry should be stored by the DHT server, which is
   implementation or policy dependent.

   The secret_hash is an optional field used with HIT publish if the
   value will later be removed with an rm operation.  It is RECOMMENDED
   that clients support these rm operations for the values they publish.
   The secret_hash contains the base64-encoded SHA-1 hash of some secret
   value known only to the publishing host.  A different secret value
   SHOULD be used for each put because rm requests are visible on the
   network.  The max_vals and placemark fields used with the HIT lookup
   are defined by the get XML-RPC interface.

4.2. HIP Address Lookup

   Given a HIT, a lookup returns the IP address of the peer.  The
   address is contained in a LOCATOR TLV inside the HDRR, along with
   other validation data.  This interface has publish, lookup, and
   remove operations.  A summary of these three operations is listed
   below.  The abbreviated notation refers to the HIP parameter types;
   for example, HIP_SIG is the HIP signature parameter defined by
   [RFC5201].  The details of these DHT operations is then described in
   greater detail.

         HDRR(LOCATOR, SEQ, HOST_ID, [CERT], HIP_SIG) = get(HIT_KEY)
         put(HIT_KEY, HDRR(LOCATOR, SEQ, HOST_ID, [CERT], HIP_SIG),
             [SHA-1(secret)])
         rm(HIT_KEY, SHA-1(HDRR), secret)

   The HDRR is defined in Section 3.  It contains one or more locators
   that the peer wants to publish, a sequence number, the peer's Host
   Identity, an optional certificate, and a signature over the contents.

   The HIT_KEY is comprised of the last 100 bits of the HIT appended
   with 60 zero bits.  This is the portion of the HIT used as a DHT key.
   The last 100 bits are used to avoid uneven distribution of the stored
   values across the DHT servers.  The HIT's ORCHID Prefix (defined by
   [RFC4843]) is comprised of the first 28 bits, and this prefix is
   dropped because it is the same for all HITs, which would cause this
   uneven distribution.  Zero padding is appended to this 100-bit value
   to fill the length required by the DHT, 160 bits total.

                         Address remove (optional)

   +----------------+-------------------------------------+------------+
   | field          | value                               | data type  |
   +----------------+-------------------------------------+------------+
   | application    | "hip-addr"                          | string     |
   |                |                                     |            |
   | client_library | (implementation dependent)          | string     |
   |                |                                     |            |
   | key            | HIT_KEY                             | base64     |
   |                |                                     | encoded    |
   |                |                                     |            |
   | value_hash     | SHA-1 hash of HDRR (value used      | base64     |
   |                | during publish) to 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 used with the address lookup and with publishing the address
   is the HIT_KEY as defined above, 160 bits base64 encoded [RFC2045].
   The value used in the publish and lookup response is the base64-
   encoded HDRR containing one or more LOCATORs.

   The ttl_sec field used with address publish indicates the time-to-
   live (TTL).  This is the number of seconds for which the entry will
   be stored by the DHT.  The TTL SHOULD be set to the number of seconds
   remaining in the address lifetime.

   The secret_hash is an optional field that MAY be used with address
   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 that MUST be known only to the publishing host.  Clients SHOULD
   include the secret_hash and remove outdated values to reduce the
   amount of data the peer needs to handle.  A different secret value
   SHOULD be used for each put because rm requests are visible on the
   network.

   The max_vals and placemark fields used with address lookup are
   defined by the get XML-RPC interface.  The get operation 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 where it left off.

5. Use Cases

   Below are some suggestions of when a HIP implementation MAY want to
   use the HIT and address 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 [RFC5205].  Sometimes hosts do not have administrative
   authority over their DNS entries and/or the DNS server is not able to
   support 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 4.1.  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 MAY be
   performed so that the LOCATORs 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 LOCATORs 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 LOCATORs upon startup, so
   other hosts may determine where it is reachable.  The host SHOULD
   periodically refresh its HDRR entry because each entry carries a TTL
   and will eventually expire.  Also, when there is a change in the
   preferred address, usually associated with sending UPDATE packets
   with included locator parameters, the host SHOULD update its HDRR
   with the DHT.  The old HDRR 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 Rendezvous Server (RVS, from
   [RFC5204]) 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 NATed
   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.

   When publishing data to the DHT server, care should be taken to check
   the response from the server.  The server may respond with an "over
   capacity" code, indicating that its resources are too burdened to
   honor the given size and TTL.  The host SHOULD then select another
   server for publishing or reduce the TTL and retry the put operation.

6. Issues with DHT Support

   The DHT put operation does not replace existing values.  If a host
   does not remove its old HDRR before adding another, several entries
   may be present.  A client performing a lookup SHOULD determine the
   most recent address based on the Update ID from the SEQ TLV of the
   HDRR.  The order of values returned in the server's response may not
   be guaranteed.  Before performing each put, a host SHOULD remove its
   old HDRR data using the rm operation.

   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.
   The server may enforce a policy that requires clients to include a
   certificate when publishing a HIT, and only store HITs with a name
   that has been authorized by some trusted certificate.  Otherwise,
   this is an unmanaged free-for-all service, and 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 formerly provided a
   DNS-based anycast service; when one performed a lookup of
   "opendht.nyuld.net", it returned the two nearest OpenDHT servers.

   The latency involved with the DHT put and get operations should be
   considered when using these services with HIP.  The calls rely on
   servers that may be located across the Internet and may trigger
   communications between servers that add delay.  The DHT operations
   themselves may be slow to produce a response.

   The maximum size of 1024 bytes for the value field will limit the
   maximum size of the Host Identity and certificates that may be used
   within the HDRR.

7. Security Considerations

   There are two classes of attacks on this information exchange between
   the 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).  Without the server performing some measure of
   verification, not much can be done to prevent these attacks.

   For the HIT lookup based on a name (Section 4.1), 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.  A certificate MAY be included with the HIT, which
   guarantees that the name used for the lookup has been authorized by
   some third-party authority.  Which certificates are considered
   trusted is a local policy issue.

   For the address lookup based on HIT (Section 4.2), the validity of
   the DHT response MUST be checked with the HOST_ID and SIGNATURE
   parameters in the HDRR.  A HIP initiating host SHOULD also validate
   the DHT response after the R1 message is received during a HIP
   exchange.  The Host Identity provided in the R1 can be hashed to
   obtain a HIT that MUST be checked against the original HIT.  However,
   a legacy OpenDHT service without server modifications does not
   prevent an attacker from polluting the DHT records for a known HIT,
   thereby causing a denial-of-service attack, since server validation
   is not performed.

   Relying solely on client validation may be harmful.  An attacker can
   replay the put packets containing the signed HDRR, possibly causing
   stale or invalid information to exist in the DHT.  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.

   To protect against this type of attack, the DHT server SHOULD perform
   signature and HIT verification of each put operation as described in
   Section 3.  Another option would be the server running HIP itself and
   requiring client authentication with a HIP association before
   accepting HDRR puts.  Further validation would be only accepting HIT
   and address records from the association bound to the same HIT.

   Performing server-side verification adds to the processing burden of
   the DHT server and may be a source for a denial-of-service attack.
   Requiring a HIP association before accepting HDRR puts may help here.
   The HIT verification is less computationally intensive by design,
   using a hash algorithm.  Certificate validation (for name lookups)
   and signature verification (for HDRRs) may cause unacceptable amounts
   of computation.  A server may rate limit the number of puts that it
   allows.

   The SHA-1 message digest algorithm is used in two ways in this
   document, and the security of using this algorithm should be
   considered within the context of [RFC6194].  The first use is with
   the OpenDHT put and remove operations, described in Section 2, and
   the second is to reduce the size of the name string for the HIT
   lookup service in Section 4.1.

   The first use is intended to protect the secret values used to store
   records in the DHT as described by the OpenDHT interface.  An
   attacker would be able to remove a record, after capturing the
   plaintext put, if a secret value could be found that produces the
   same secret hash.  The purpose of this document is to maintain
   interoperable compatibility with that interface, which prescribes the
   use of SHA-1.  Future revisions of that interface should consider
   hash algorithm agility.  The OpenDHT FAQ states that future support
   for other hash algorithms is planned.

   The second use of the SHA-1 algorithm is to reduce the arbitrarily
   sized name strings to fit the fixed OpenDHT key size.  No security
   properties of the SHA-1 algorithm are used in this context.

8. IANA Considerations

   This document defines a new HIP Packet Type, the "HIP Distributed
   Hash Table Resource Record (HDRR)".  This packet type is defined in
   Section 3 with a value of 20.

9. Acknowledgments

   Thanks to Tom Henderson, Samu Varjonen, Andrei Gurtov, Miika Komu,
   Kristian Slavov, Ken Rimey, Ari Keranen, and Martin Stiemerling for
   providing comments.  Samu most notably contributed the resolver
   packet and its suggested parameters, which became the HDRR here.

10. References

10.1. Normative References

   [OPENDHT]  Rhea, S., Godfrey, B., Karp, B., Kubiatowicz, J.,
              Ratnasamy, S., Shenker, S., Stocia, I., and H. Yu,
              "OpenDHT: A Public DHT Service and Its Uses", Proceedings
              of ACM SIGCOMM 2005, August 2005.

   [RFC2045]  Freed, N. and N. Borenstein, "Multipurpose Internet Mail
              Extensions (MIME) Part One: Format of Internet Message
              Bodies", RFC 2045, November 1996.

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119, March 1997.

   [RFC4843]  Nikander, P., Laganier, J., and F. Dupont, "An IPv6 Prefix
              for Overlay Routable Cryptographic Hash Identifiers
              (ORCHID)", RFC 4843, April 2007.

   [RFC5201]  Moskowitz, R., Nikander, P., Jokela, P., and T. Henderson,
              "Host Identity Protocol", RFC 5201, April 2008.

   [RFC5205]  Nikander, P. and J. Laganier, "Host Identity Protocol
              (HIP) Domain Name System (DNS) Extensions", RFC 5205,
              April 2008.

   [RFC6194]  Polk, T., Chen, L., Turner, S., and P. Hoffman, "Security
              Considerations for the SHA-0 and SHA-1 Message-Digest
              Algorithms", RFC 6194, March 2011.

   [RFC6253]  Heer, T. and S. Varjonen, "Host Identity Protocol
              Certificates", RFC 6253, May 2011.

10.2. Informative References

   [HIT2IP]   Ponomarev, O. and A. Gurtov, "Embedding Host Identity Tags
              Data in DNS", Work in Progress, July 2009.

   [RFC5204]  Laganier, J. and L. Eggert, "Host Identity Protocol (HIP)
              Rendezvous Extension", RFC 5204, April 2008.

   [RFC5206]  Nikander, P., Henderson, T., Vogt, C., and J. Arkko, "End-
              Host Mobility and Multihoming with the Host Identity
              Protocol", RFC 5206, April 2008.

Author's Address

   Jeff Ahrenholz
   The Boeing Company
   P.O. Box 3707
   Seattle, WA
   USA

   EMail: jeffrey.m.ahrenholz@boeing.com