Host Identity Protocol                                           T. Heer
Internet-Draft                           Distributed Systems Group, RWTH
Intended status: Experimental                          Aachen University
Expires: May 14, 2008                                  November 11, 2007

              End-Host Authentication for HIP Middleboxes

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Copyright Notice

   Copyright (C) The IETF Trust (2007).


   The Host Identity Protocol is a signaling protocol for secure
   communication, mobility, and multihoming by introducing a
   cryptographic namespace.  This document specifies an extension for
   HIP that enables middleboxes to unambiguously verify the identities
   of hosts that communicate across them.  This extension enables

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   middleboxes to verify the liveness and freshness of a HIP association
   and, thus, enables reliable and secure access control in middleboxes.

Requirements Language

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   document are to be interpreted as described in RFC 2119.


   [x]        indicates that x is optional.

   {x}        indicates that x is under signature.

   Initiator  is the host which initiates a HIP association
              (cf. HIP base protocol).

   Responder  is the host which responds to the INITIATOR
              (cf. HIP base protocol).

   -->        signifies "Initiator to Responder" communication.

   <--        signifies "Responder to Initiator" communication.

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Table of Contents

   1.  Introduction . . . . . . . . . . . . . . . . . . . . . . . . .  4
     1.1.  Authentication and Replay Attacks  . . . . . . . . . . . .  5
   2.  Protocol Overview  . . . . . . . . . . . . . . . . . . . . . .  5
     2.1.  Signed Middlebox Nonces  . . . . . . . . . . . . . . . . .  6
     2.2.  Identity Verification by Middleboxes . . . . . . . . . . .  8
     2.3.  Failure Signaling  . . . . . . . . . . . . . . . . . . . . 11
     2.4.  Fragmentation  . . . . . . . . . . . . . . . . . . . . . . 11
   3.  HIP Parameters . . . . . . . . . . . . . . . . . . . . . . . . 11
     3.1.  ECHO_REQUEST_M . . . . . . . . . . . . . . . . . . . . . . 12
     3.2.  ECHO_RESONSE_M . . . . . . . . . . . . . . . . . . . . . . 12
     3.3.  PUZZLE_M . . . . . . . . . . . . . . . . . . . . . . . . . 13
     3.4.  SOLUTION_M . . . . . . . . . . . . . . . . . . . . . . . . 14
   4.  Security Considerations  . . . . . . . . . . . . . . . . . . . 15
   5.  IANA Considerations  . . . . . . . . . . . . . . . . . . . . . 15
   6.  Acknowledgments  . . . . . . . . . . . . . . . . . . . . . . . 15
   7.  Normative References . . . . . . . . . . . . . . . . . . . . . 15
   Author's Address . . . . . . . . . . . . . . . . . . . . . . . . . 16
   Intellectual Property and Copyright Statements . . . . . . . . . . 17

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1.  Introduction

   The Host Identity Protocol (HIP) introduces a new cryptographic
   namespace, based on public keys, in order to secure Internet
   communication.  This namespace allows hosts to authenticate their
   peers.  HIP was designed to be middlebox-friendly and allows
   middleboxes to inspect HIP control traffic.  Such middleboxes are
   e.g. firewalls and Network Address Translators (NATs).

   In this context, one can distinguish HIP-aware middleboxes, which
   were designed to process HIP packets, and other middleboxes, which
   are not aware of the Host Identity Protocol.  This document addresses
   only on HIP-aware middleboxes while the behavior of HIP in
   combination with non-HIP-aware middleboxes is specified elsewhere
   [I-D.ietf-hip-nat-traversal].  Moreover, the scope of this document
   is restricted to middleboxes that use HIP in order to enforce access
   regulation and, thus, need to authenticate the communicating peers
   that send traffic over the middlebox.  The class of middleboxes, this
   document focuses on, does not require explicit registration via a
   handshake with the middlebox.  HIP behavior for interacting and
   registering to such middleboxes is specified in
   [I-D.ietf-hip-registration].  Thus, we focus on middleboxes that
   build their state-base from packets it forwards.

   An example for such a middlebox is a firewall that only allows
   traffic from certain hosts to traverse.  We assume that access
   regulation is performed based on Host Identities (HIs).  Such an
   authenticating middlebox needs to observe the HIP Base EXchange (BEX)
   or a HIP mobility update [I-D.ietf-hip-mm]" and check the Host
   Identifiers (HIs) in the packets.

   Along the lines of [I-D.tschofenig-hiprg-hip-natfw-traversal], an
   authentication solution for middleboxes must have some vital
   properties.  For one, the middlebox must be able to unambiguously
   identify one or both of the communicating peers.  For another, the
   solution must not allow for new attacks against the middlebox.  This
   document specifies a HIP extension that allows middleboxes to
   participate in the HIP handshake and the HIP update process in order
   to enable these devices to reliably verify the identities of the
   communicating peers.  To this end, this HIP extension defines how
   middleboxes can interact with end-hosts in order to verify the
   identity of the end-hosts.

   Verifying public-key (PK) signatures is costly in terms of CPU
   cycles.  Thus, in addition to authentication capabilities, it is also
   necessary to provide middleboxes with a way of defending against
   resource-exhaustion attacks that target PK signature verification.
   This document defines how middleboxes can utilize the HIP puzzle

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   mechanism defined in [I-D.ietf-hip-base] to slow down resource-
   exhaustion attacks.

1.1.  Authentication and Replay Attacks

   Middleboxes need to be able to verify the HIs in the HIP base
   exchange messages to perform access control based on Host Identities.
   However, passive verification of identifiers in the messages is not
   sufficient to verify the identity of an end-host.  Moreover, it is
   necessary to also ensure the freshness and authenticity of the
   communication to prevent replay attacks.  The basic HIP protocol as
   specified in [I-D.ietf-hip-base] does not provide adequate protection
   against these attacks.  To illustrate the need for additional
   security features, we briefly outline a possible replay attack
   targeted at middleboxes:

   Assume that a middlebox M checks HIP HIs in order to restrict traffic
   passing through the box.  Further assume that the legitimate owner of
   HIT X establishes a HIP association with the legitimate owner of HIT
   Y at some point in time and an attacker A overhears the base exchange
   and records it.  Note that it is not required that the middlebox M is
   on the communication path between the peers at that time.

   At some later point in time, A collaborates with another attacker B.
   They replay the very same BEX with the middlebox M on the
   communication path.  The middlebox has no way to distinguish X and Y
   A and B as it can only overhear the BEX passively and does not
   participate in the authentication process.  If A and B have agreed on
   a shared secret beforehand, they can make fake ESP traffic traverse
   the middlebox by using the SPIs that A and B negotiated in the
   original BEX.  This is problematic in cases for which the middlebox
   needs to know who is communicating across it.  Examples for such
   cases are access restriction, logging of activities, and accounting
   for traffic volume or connection duration.

   So far, this attack is not addressed by the HIP specifications.
   Therefore, this document specifies a HIP extension that allows
   middleboxes to defend against it.

2.  Protocol Overview

   The following section gives an overview of the interaction between
   hosts and authenticating middleboxes.

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2.1.  Signed Middlebox Nonces

   The aforementioned attack scenario clearly shows the necessity for
   unambiguous end-host identity verification by middleboxes.  Relying
   on nonces generated by the end-hosts is not possible because
   middleboxes can not verify the freshness of these nonces.
   Introducing time-stamps restricts the attack to a certain time frame
   but requires global time synchronization.

   The following sections specify how HIP hosts can prove their identity
   by performing a challenge-response protocol between the middlebox and
   the end-hosts.  As the challenge, the middlebox add data (e.g.
   nonces) to HIP control packets which end-hosts must echo with applied
   PK signatures.

   The challenge-response mechanism is similar to the ECHO_REQUEST/
   ECHO_RESPONSE mechanism used by HIP end-hosts to authenticate their
   peers.  Middleboxes may add ECHO_REQUEST_M parameters to HIP control
   packets and verify ECHO_RESPONSE_M parameters.  By echoing the data
   in the ECHO_REQUEST_M parameter as ECHO_RESPONSE_M parameter in the
   signed part of its response, an end-host proves that it is in
   possession of the private key that corresponds to the HI it uses.


   Middleboxes MAY add ECHO_REQUEST_M parameters to the the R1, I2, and
   to any UPDATE packet.  This parameter contains an opaque data block
   of variable size which is used by the middlebox to carry arbitrary
   data.  Each of the afore-mentioned HIP packets may contain multiple
   ECHO_REQUEST_M parameters.  As all middleboxes on the path may need
   to add ECHO_REQUEST_M parameters, the length of the data field of
   each parameter SHOULD not exceed a maximum of 32 bytes.  The total
   length of the packets SHOULD not exceed 1280 bytes to avoid IPv6
   fragmentation (cf. Section Section 2.4).

   The ECHO_REQUEST_M parameter is added to the unprotected part of a
   HIP message.  Thus it does not corrupt any HMAC or public-key
   signatures.  However, it is necessary to recompute the IP- and HIP
   header checksums.  The UDP headers of UDP encapsulated HIP packets
   MUST also be recomputed if UDP encapsulation, as defined in
   [I-D.ietf-hip-nat-traversal], is applied.

   An end-host that receives a HIP control packet containing one or
   multiple ECHO_REQUEST_M parameters must copy the contents of each
   parameter without modification to an ECHO_RESPONSE_M parameter.  This
   parameter MUST be sent within the signed part of its reply.  Note
   that middleboxes MAY also rewrite the ECHO_REQUEST_UNSIGNED parameter
   as specified in [I-D.ietf-hip-base] when the receiver of the

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   parameter is not required to sign the contents of the ECHO_REQUEST_M.

   Middleboxes can delay state creation by utilizing the ECHO_RESPONSE_M
   and ECHO_REQUEST_M parameter.  Encrypted or otherwise protected
   information about previous authentication steps can be hidden in the
   opaque blob.


   When a middlebox injects an opaque blob of data via an ECHO_REQUEST_M
   parameter, it expects to receive the same data without modification
   as part of an ECHO_RESPONSE_M parameter in a subsequent packet.  The
   opaque data MUST be copied as it is from the corresponding
   ECHO_REQUEST_M parameter.  In case of multiple ECHO_REQUEST_M
   parameters, their order MUST be preserved by the corresponding
   ECHO_RESPONSE_M parameters.

   The ECHO_REQUEST_M and ECHO_RESPONSE_M parameters MAY be used for any
   purpose, in particular when a middlebox needs to carry state or
   recognizable information in a HIP packet and receive it in a
   subsequent response packet.  The ECHO_RESPONSE_M MUST be covered by

   The ECHO_RESPONSE_M parameter is non critical.  Depending on its
   local policy, a middlebox can react differently on a missing
   ECHO_RESPONSE_M parameter.  Possible actions range from degraded or
   restricted service such as bandwidth limitation up to refusing
   connections and reporting access violations.

2.1.3.  Middlebox Puzzles

   As public-key (PK) operations are costly in terms of CPU cycles, it
   is necessary to provide some way for the middlebox to defend against
   resource-exhaustion attacks.  The HIP base protocol
   [I-D.ietf-hip-base] specifies a puzzle mechanism to protect the
   Responder from I2 floods that require numerous public-key operations.
   However, middleboxes can not utilize this mechanism as there is no
   defense against a collaborative replay attack, which involves a
   malicious Initiator and a malicious Responder.  This section
   specifies how middleboxes can utilize the puzzle mechanism to add
   their own puzzles to R1, I2, and any UPDATE packets.  This allows
   middleboxes to shelter against Service (DoS) attacks on PK

   To defend against attacks, a middlebox adds a puzzle in a PUZZLE_M
   parameter to I2, R2 and UPDATE packets.  Depending on the packet to
   which the puzzle was added, either the Initiator or the Responder of
   a BEX or the receiver of an UPDATE packet must solve it.

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   A puzzle increases the delay and computational cost for establishing
   or updating a HIP association, a middlebox SHOULD only add puzzles to
   packets if it is under attack conditions.  Moreover, middleboxes
   SHOULD distinguish attack directions.  If the majority of the CPU
   load is caused by verifying HIP control messages that arrive from a
   certain interface, middleboxes MAY add puzzles with higher difficulty
   to HIP control packets that leave the interface.

   Middleboxes MAY decide to use only the PUZZLE_M parameter instead of
   using PUZZLE_M in combination with ECHO_REQUEST_M because the
   PUZZLE_M parameter also contains an opaque data field that guarantees
   the freshness of the signature.  However, the opaque data field in
   the PUZZLE_M and the corresponding SOLUTION_M parameter is restricted
   to 6 bytes which may not be sufficient for all purposes.

2.2.  Identity Verification by Middleboxes

   This section describes how middleboxes can interact with the BEX and
   the HIP update process in order to verify the identity of the HIP

2.2.1.  Identity Verification During BEX

   Middleboxes MAY add ECHO_REQUEST_M and PUZZLE_M parameters to R1 and
   I2 packets in order to verify the identities of the participating
   parties.  Middleboxes can choose to either authenticate the
   Initiator, the Responder, or both.  Middleboxes MUST NOT add
   ECHO_REQUEST_M or PUZZLE_M parameters to I1 messages because this
   would expose the Responder to DoS attacks.  Thus, middleboxes MUST
   let unauthenticated minimal I1 packets traverse.  Minimal means that
   the packet MUST NOT contain more than the minimal set of parameters
   specified by HIP standards or internet drafts.  In particular, the I1
   packet MUST NOT contain any attached payload.  Figure 1 illustrates
   the authentication process during the BEX.

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   Figure 1: Middlebox authentication of a HIP base exchange.

  Main path:

  Initiator                Middlebox                         Responder
   I1                  |                 |  I1
  -------------------> |                 | ---------------------------->
                       |                 |
   R1, + EQ1, [PM1]    | Add EQ1, PM1    |  R1
  <------------------- |                 | <----------------------------
                       |                 |
   I2, {ER1, SM1}      | Verify SM1, EQ1 |  I2, {ER1, SM1} + EQ2, [PM2]
  -------------------> | Add EQ2, PM2    | --------------------------->
                       |                 |
                       |                 |
   R2, {ER2, SM2}      | Verify SM2, ER2 |  R2, {ER2, SM2}
  -------------------> |                 | ---------------------------->

  EQ: Middlebox Echo reQuest
  ER: Middlebox Echo Response
  PM: Puzzle of the Middlebox
  SM: Solution of Middlebox puzzle

2.2.2.  Identity Verification During Mobility Updates

   Multihomed hosts may use multiple communication paths during an HIP
   mobility update.  Depending on whether the middlebox is located on
   the communication path between the preferred locators or not, the
   middlebox forwards different packets and, thus, needs to interact
   differently with the updates.  Figure 1 illustrates an update with
   Middlebox 1 on the path between the Initiator's and the RECEIVER's
   preferred locators and with Middlebox 2 on an alternative path.

   Middlebox 1 receives the first UPDATE packet, which contains e.g. the
   set of new locators.  As the middlebox has no adequate way of
   identifying replay attacks of U1 (first UPDATE message) and, moreover
   cannot defend against U1 flooding attacks, the middlebox may decide
   not to verify the signature in the U1 packet.  In the case it is
   necessary to verify the identity of the Responder and the freshness
   of the UPDATE packets, the middlebox MAY add an ECHO_REQUEST_M (EQ1)
   to the U1.

   The following figure illustrates the authentication for middleboxes
   on the path between the preferred locators (main path) and other

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   paths between two HIP peers (alternative path).

   Figure 1: Middlebox authentication of a HIP mobility update over
   different paths.

  Main path:

  Initiator                     Middlebox 1                  Responder
   U1                            |      |  U1 + EQ1, [PM1]
  -----------------------------> |      | ---------------------------->
                                 |      |
   U2, {ER1, [SM1]} + EQ2, [PM2] |      |  U2, {ER1, [SM1]}
  <----------------------------- |OK    | <----------------------------
                                 |      |
   U3, {ER2, SM2}                |      |  U3, {ER2, SM2}
  -----------------------------> |    OK| ---------------------------->

  Alternative path:

  Initiator                     Middlebox 2                  Responder
   U2, {ER1, [SM1]} + P3, [PM3]  |      |  U2, {ER1, [SM1]}
  <----------------------------- | wrong| <----------------------------
                                 |      |
   U3', {ER3, SM3}               |      |  U3', {ER3, SM3} + EQ4, PM4
  -----------------------------> |OK    | ----------------------------->
                                 |      |
   U4, {ER4, [SM4]}              |      |  U4, {ER1, [SM1]}
  <----------------------------- |    OK| <----------------------------
  EQ: Middlebox Echo reQuest
  ER: Middlebox Echo Response
  PM: Puzzle of the Middlebox
  SM: Solution of Middlebox puzzle

   Middlebox 1 can verify the identity of the Responder by checking its
   PK signature and the presence of the ECHO_RESPONSE_M in the U2
   packet.  If necessary, the middlebox MAY add an ECHO_REQUEST_M for
   the Initiator of the update.  The middlebox can verify the
   Initiator's identity by verifying its signature and the
   ECHO_RESPONSE_M in the U3 packet.

   A middlebox that is not located on the path between preferred
   locators of the HIP end-hosts does not receive the U1 message.

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   Therefore, it will not recognize any ER1 or SM1 in the second UPDATE
   packet.  Thus, if a middlebox encounters non-matching or missing
   ECHO_RESPONSE_M parameters, the middlebox SHOULD ignore these.

   When receiving an UPDATE message with an ECHO_REQUEST_M, a HIP host
   SHOULD send an UPDATE message containing the corresponding
   ECHO_RESPONSE_M covered by a HIP_SIGNATURE parameter.  Otherwise the
   middlebox may refuse to make the communication path available to the
   HIP host.

2.2.3.  UPDATE Verification

   As middleboxes need to be able to rapidly verify and forward HIP
   packets, these devices need to be supplied with all information
   necessary to do so.  If, due to host mobility, a new communication
   path is used, middleboxes need to be able to learn the Host
   Identifiers (HIs) from the UPDATE packets.  Therefore, HIP hosts MUST
   include the HOST_ID parameter in all UPDATE packets that use
   combinations of locators that have not been used before.  Thus,
   UPDATE packets that contain ECHO_REQUEST or ECHO_RESPONSE parameters
   MUST contain the HOST_ID parameter.  Moreover, all packets that
   contain an ECHO_RESPONSE_M parameter MUST contain the HOST_ID

2.3.  Failure Signaling

   Middleboxes SHOULD inform the sender of a BEX or update message if it
   does not satisfy the requirements of the middlebox.  Reasons for non-
   satisfactory packets are missing HOST_ID, ECHO_RESPONSE_M, and
   SOLUTION_M parameters.  Options for expressing such shortcomings are
   ICMP or HIP_NOTIFY packets.  Defining this signaling mechanism is
   future work.

2.4.  Fragmentation

   Analogously to the specification in [I-D.ietf-hip-base], HIP aware
   middleboxes SHOULD support IP-level fragmentation and reassembly for
   IPv6 and MUST support IP-level fragmentation and reassembly for IPv4.
   However, when adding ECHO_REQUEST_M and PUZZLE_M parameters, a
   middlebox SHOULD keep the total packet size below 1280 bytes to avoid
   packet fragmentation in IPv6.

3.  HIP Parameters

   This HIP extension specifies four new HIP parameters that allow
   middleboxes to authenticate HIP end-hosts and to protect against DoS

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   The ECHO_REQUEST_M parameter MAY be added to R1, I2, and UPDATE
   packets by HIP-aware middleboxes.  The structure of the
   ECHO_REQUEST_M parameter is depicted below:

   0                   1                   2                   3
   0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
   |             Type              |             Length            |
   |                 Opaque data (variable length)                 |

   Type         65332
   Length       Variable
   Opaque data  Opaque data, supposed to be meaningful only to the
                middlebox that adds ECHO_REQUEST_M and receives a
                corresponding ECHO_RESPONSE_M.


   The ECHO_RESPONSE_M is the reply to the ECHO_REQUEST_M parameter.
   The receiver of an ECHO_RESPONSE_M parameter SHOULD reply with n
   ECHO_RESPONSE_M. If not, the middlebox that added the parameter MAY
   decide to degrade or deny its service.  The contents of the
   ECHO_REQUEST_M parameter must be copied to the ECHO_RESPONSE_M
   parameter without any modification.  The ECHO_RESPONSE_M parameter is
   non-critical and covered by the SIGNATURE.  The structure of the
   ECHO_RESPONSE_M parameter is depicted below:

   0                   1                   2                   3
   0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
   |             Type              |             Length            |
   |                 Opaque data (variable length)                 |

   Type         962
   Length       Variable
   Opaque data  Opaque data, supposed to be meaningful only to the
                middlebox that adds adds ECHO_REQUEST_M and receives a

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                corresponding ECHO_RESPONSE_M.

3.3.  PUZZLE_M

   A middlebox MAY add a PUZZLE_M parameter to R1, I2, and UPDATE
   packets.  A HIP packet may contain multiple PUZZLE_M parameters as
   multiple middleboxes may be located on a communication path.  These
   puzzles serve as defense against DoS attacks.  Hosts that receive a
   PUZZLE_M parameter SHOULD reply with a SOLUTION_M parameter in the
   subsequent I2, R2, or UPDATE packet.  With the exception of an
   extended opaque field, the format and meaning of the puzzle are
   defined in [I-D.ietf-hip-base].  The reader is advised to refer to
   that document for a detailed specification of the puzzle mechanism.
   The extended opaque data field helps middleboxes to recognize their
   puzzles and solutions, respectively, if a packet contains more than
   one puzzle.

   A middlebox MUST preserve the order of PUZZLE_M parameters in a
   packet and attach its own PUZZLE_M parameter after all other PUZZLE_M
   parameters.  Preserving the order of PUZZLE_M parameters may help
   middleboxes to recognize the puzzles and solutions relevant to a

    0                   1                   2                   3
    0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
   |             Type              |             Length            |
   | K, 1 byte     |    Lifetime   |        Opaque, 6 bytes        /
   /                                                               |
   | Random # I, 8 bytes                                           |
   |                                                               |

   Type           65334
   Length         16
   K              K is the number of verified bits
   Lifetime       Puzzle lifetime 2^(value-32) seconds
   Opaque         Data set by the middlebox, indexing the middlebox
   Random #I      Random number

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   The SOLUTION_M parameter contains the solution for the corresponding
   PUZZLE_M parameter.  End-hosts that receive a PUZZLE_M parameter
   SHOULD solve the puzzle according to the specification in
   [I-D.ietf-hip-base] and send the resulting solution in the SOLUTION_M
   parameter.  Exclusion of a solution MAY result in degraded or denied
   service by the middlebox that added the PUZZLE_M parameter.  The
   format and meaning of the fields in the SOLUTION_M parameter resemble
   the specifications of the SOLUTION parameter in [I-D.ietf-hip-base].
   The reader is advised to refer to that document for further details.
   The extended opaque data field helps middleboxes to recognize their
   puzzles and the resulting solutions, respectively, when a packet
   contains multiple puzzles.

   The relative order of SOLUTION_M parameters in a HIP control packet
   MUST match the order of the PUZZLE_M parameters in the previously
   received packet.  Preserving the order of PUZZLE_M for the
   corresponding SOLUTION_M parameters may help middleboxes to recognize
   the puzzles and solutions relevant to them.

    0                   1                   2                   3
    0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
   |             Type              |             Length            |
   | K, 1 byte     |    Reserved   |        Opaque, 6 bytes        /
   /                                                               |
   | Random # I, 8 bytes                                           |
   |                                                               |
   | Puzzle solution #J, 8 bytes                                   |
   |                                                               |

   Type             322
   Length           20
   K                K is the number of verified bits
   Reserved         Zero when sent, ignored when received
   Opaque           Copied unmodified from the received PUZZLE
   Random #I        Random number
   Puzzle solution  Random number

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4.  Security Considerations

   This HIP extension specifies how HIP-aware middleboxes interact with
   the handshake and mobility-signaling of the Host Identity Protocol.
   Its scope is restricted to the authentication of end-hosts and does
   not include the issue of authenticating ESP traffic on the middlebox.

   Providing middleboxes with a way of adding puzzles to the HIP control
   packets may cause both HIP peers, including the Responder, to spend
   CPU time on solving these puzzles.  Thus, it is advised that HIP
   implementations for servers employ mechanisms to prevent middlebox
   puzzles from being used as DoS attacks.  Under high CPU load, servers
   can e.g. prioritize packets that do not contain difficult middlebox

   If multiple middleboxes add ECHO_REQUEST_M parameters to a HIP
   control packet, the remaining space in the packet might not be
   sufficient for further parameters to be added.  Moreover, as the
   ECHO_REQUEST_M must be echoed within an ECHO_RESPONSE_M, the space in
   the subsequent packet may not be sufficient to add all ECHO_RESONSE_M
   parameters.  Thus, middleboxes SHOULD keep the size of the nonces

5.  IANA Considerations

   This document specifies four new HIP parameter types.  The
   preliminary parameter type numbers are 322, 962, 65332, and 65334.

6.  Acknowledgments

   Thanks to Shaohui Li, Miika Komu, and Janne Lindqvist for the
   fruitful discussions on this topic.  Many thanks to Stefan Goetz and
   Rene Hummen commenting and helping to improve the quality of this

7.  Normative References

              Moskowitz, R., Nikander, P., Jokela, P., and T. Henderson,
              "Host Identity Protocol", draft-ietf-hip-base-10 (work in
              progress), October 2007.

              Henderson, T., "End-Host Mobility and Multihoming with the
              Host Identity Protocol", draft-ietf-hip-mm-05 (work in

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              progress), March 2007.

              Schmitt, V., "HIP Extensions for the Traversal of Network
              Address Translators", draft-ietf-hip-nat-traversal-02
              (work in progress), July 2007.

              Laganier, J., "Host Identity Protocol (HIP) Registration
              Extension", draft-ietf-hip-registration-02 (work in
              progress), June 2006.

              Tschofenig, H. and M. Shanmugam, "Traversing HIP-aware
              NATs and Firewalls: Problem Statement and  Requirements",
              draft-tschofenig-hiprg-hip-natfw-traversal-06 (work in
              progress), July 2007.

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

Author's Address

   Tobias Heer
   Distributed Systems Group, RWTH Aachen University
   Ahornstrasse 55
   Aachen  52062

   Phone: +49 241 80 214 36

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