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RFC 5201

 
 
 

Host Identity Protocol

Part 3 of 3, p. 66 to 104
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6.  Packet Processing

   Each host is assumed to have a single HIP protocol implementation
   that manages the host's HIP associations and handles requests for new
   ones.  Each HIP association is governed by a conceptual state
   machine, with states defined above in Section 4.4.  The HIP
   implementation can simultaneously maintain HIP associations with more
   than one host.  Furthermore, the HIP implementation may have more
   than one active HIP association with another host; in this case, HIP
   associations are distinguished by their respective HITs.  It is not
   possible to have more than one HIP association between any given pair
   of HITs.  Consequently, the only way for two hosts to have more than
   one parallel association is to use different HITs, at least at one
   end.

   The processing of packets depends on the state of the HIP
   association(s) with respect to the authenticated or apparent
   originator of the packet.  A HIP implementation determines whether it
   has an active association with the originator of the packet based on
   the HITs.  In the case of user data carried in a specific transport
   format, the transport format document specifies how the incoming
   packets are matched with the active associations.

6.1.  Processing Outgoing Application Data

   In a HIP host, an application can send application-level data using
   an identifier specified via the underlying API.  The API can be a
   backwards-compatible API (see [HIP-APP]), using identifiers that look
   similar to IP addresses, or a completely new API, providing enhanced

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   services related to Host Identities.  Depending on the HIP
   implementation, the identifier provided to the application may be
   different; for example, it can be a HIT or an IP address.

   The exact format and method for transferring the data from the source
   HIP host to the destination HIP host is defined in the corresponding
   transport format document.  The actual data is transferred in the
   network using the appropriate source and destination IP addresses.

   In this document, conceptual processing rules are defined only for
   the base case where both hosts have only single usable IP addresses;
   the multi-address multi-homing case will be specified separately.

   The following conceptual algorithm describes the steps that are
   required for handling outgoing datagrams destined to a HIT.

   1.  If the datagram has a specified source address, it MUST be a HIT.
       If it is not, the implementation MAY replace the source address
       with a HIT.  Otherwise, it MUST drop the packet.

   2.  If the datagram has an unspecified source address, the
       implementation must choose a suitable source HIT for the
       datagram.

   3.  If there is no active HIP association with the given <source,
       destination> HIT pair, one must be created by running the base
       exchange.  While waiting for the base exchange to complete, the
       implementation SHOULD queue at least one packet per HIP
       association to be formed, and it MAY queue more than one.

   4.  Once there is an active HIP association for the given <source,
       destination> HIT pair, the outgoing datagram is passed to
       transport handling.  The possible transport formats are defined
       in separate documents, of which the ESP transport format for HIP
       is mandatory for all HIP implementations.

   5.  Before sending the packet, the HITs in the datagram are replaced
       with suitable IP addresses.  For IPv6, the rules defined in
       [RFC3484] SHOULD be followed.  Note that this HIT-to-IP-address
       conversion step MAY also be performed at some other point in the
       stack, e.g., before wrapping the packet into the output format.

6.2.  Processing Incoming Application Data

   The following conceptual algorithm describes the incoming datagram
   handling when HITs are used at the receiving host as application-
   level identifiers.  More detailed steps for processing packets are
   defined in corresponding transport format documents.

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   1.  The incoming datagram is mapped to an existing HIP association,
       typically using some information from the packet.  For example,
       such mapping may be based on the ESP Security Parameter Index
       (SPI).

   2.  The specific transport format is unwrapped, in a way depending on
       the transport format, yielding a packet that looks like a
       standard (unencrypted) IP packet.  If possible, this step SHOULD
       also verify that the packet was indeed (once) sent by the remote
       HIP host, as identified by the HIP association.

       Depending on the used transport mode, the verification method can
       vary.  While the HI (as well as HIT) is used as the higher-layer
       identifier, the verification method has to verify that the data
       packet was sent by a node identity and that the actual identity
       maps to this particular HIT.  When using ESP transport format
       [RFC5202], the verification is done using the SPI value in the
       data packet to find the corresponding SA with associated HIT and
       key, and decrypting the packet with that associated key.

   3.  The IP addresses in the datagram are replaced with the HITs
       associated with the HIP association.  Note that this IP-address-
       to-HIT conversion step MAY also be performed at some other point
       in the stack.

   4.  The datagram is delivered to the upper layer.  When
       demultiplexing the datagram, the right upper-layer socket is
       based on the HITs.

6.3.  Solving the Puzzle

   This subsection describes the puzzle-solving details.

   In R1, the values I and K are sent in network byte order.  Similarly,
   in I2, the values I and J are sent in network byte order.  The hash
   is created by concatenating, in network byte order, the following
   data, in the following order and using the RHASH algorithm:

      64-bit random value I, in network byte order, as appearing in R1
      and I2.

      128-bit Initiator's HIT, in network byte order, as appearing in
      the HIP Payload in R1 and I2.

      128-bit Responder's HIT, in network byte order, as appearing in
      the HIP Payload in R1 and I2.

      64-bit random value J, in network byte order, as appearing in I2.

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   In order to be a valid response puzzle, the K low-order bits of the
   resulting RHASH digest must be zero.

   Notes:

      i) The length of the data to be hashed is 48 bytes.

      ii) All the data in the hash input MUST be in network byte order.

      iii) The order of the Initiator's and Responder's HITs are
      different in the R1 and I2 packets; see Section 5.1.  Care must be
      taken to copy the values in the right order to the hash input.

   The following procedure describes the processing steps involved,
   assuming that the Responder chooses to precompute the R1 packets:

   Precomputation by the Responder:
      Sets up the puzzle difficulty K.
      Creates a signed R1 and caches it.

   Responder:
      Selects a suitable cached R1.
      Generates a random number I.
      Sends I and K in an R1.
      Saves I and K for a Delta time.

   Initiator:
      Generates repeated attempts to solve the puzzle until a matching J
      is found:
      Ltrunc( RHASH( I | HIT-I | HIT-R | J ), K ) == 0
      Sends I and J in an I2.


   Responder:
      Verifies that the received I is a saved one.
      Finds the right K based on I.
      Computes V := Ltrunc( RHASH( I | HIT-I | HIT-R | J ), K )
      Rejects if V != 0
      Accept if V == 0

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6.4.  HMAC and SIGNATURE Calculation and Verification

   The following subsections define the actions for processing HMAC,
   HIP_SIGNATURE and HIP_SIGNATURE_2 parameters.

6.4.1.  HMAC Calculation

   The following process applies both to the HMAC and HMAC_2 parameters.
   When processing HMAC_2, the difference is that the HMAC calculation
   includes a pseudo HOST_ID field containing the Responder's
   information as sent in the R1 packet earlier.

   Both the Initiator and the Responder should take some care when
   verifying or calculating the HMAC_2.  Specifically, the Responder
   should preserve other parameters than the HOST_ID when sending the
   R2.  Also, the Initiator has to preserve the HOST_ID exactly as it
   was received in the R1 packet.

   The scope of the calculation for HMAC and HMAC_2 is:

   HMAC: { HIP header | [ Parameters ] }

   where Parameters include all HIP parameters of the packet that is
   being calculated with Type values from 1 to (HMAC's Type value - 1)
   and exclude parameters with Type values greater or equal to HMAC's
   Type value.

   During HMAC calculation, the following applies:

   o  In the HIP header, the Checksum field is set to zero.

   o  In the HIP header, the Header Length field value is calculated to
      the beginning of the HMAC parameter.

   Parameter order is described in Section 5.2.1.

   HMAC_2: { HIP header | [ Parameters ] | HOST_ID }

   where Parameters include all HIP parameters for the packet that is
   being calculated with Type values from 1 to (HMAC_2's Type value - 1)
   and exclude parameters with Type values greater or equal to HMAC_2's
   Type value.

   During HMAC_2 calculation, the following applies:

   o  In the HIP header, the Checksum field is set to zero.

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   o  In the HIP header, the Header Length field value is calculated to
      the beginning of the HMAC_2 parameter and added to the length of
      the concatenated HOST_ID parameter length.

   o  HOST_ID parameter is exactly in the form it was received in the R1
      packet from the Responder.

   Parameter order is described in Section 5.2.1, except that the
   HOST_ID parameter in this calculation is added to the end.

   The HMAC parameter is defined in Section 5.2.9 and the HMAC_2
   parameter in Section 5.2.10.  The HMAC calculation and verification
   process (the process applies both to HMAC and HMAC_2 except where
   HMAC_2 is mentioned separately) is as follows:

   Packet sender:

   1.  Create the HIP packet, without the HMAC, HIP_SIGNATURE,
       HIP_SIGNATURE_2, or any other parameter with greater Type value
       than the HMAC parameter has.

   2.  In case of HMAC_2 calculation, add a HOST_ID (Responder)
       parameter to the end of the packet.

   3.  Calculate the Header Length field in the HIP header including the
       added HOST_ID parameter in case of HMAC_2.

   4.  Compute the HMAC using either HIP-gl or HIP-lg integrity key
       retrieved from KEYMAT as defined in Section 6.5.

   5.  In case of HMAC_2, remove the HOST_ID parameter from the packet.

   6.  Add the HMAC parameter to the packet and any parameter with
       greater Type value than the HMAC's (HMAC_2's) that may follow,
       including possible HIP_SIGNATURE or HIP_SIGNATURE_2 parameters

   7.  Recalculate the Length field in the HIP header.

   Packet receiver:

   1.  Verify the HIP header Length field.

   2.  Remove the HMAC or HMAC_2 parameter, as well as all other
       parameters that follow it with greater Type value including
       possible HIP_SIGNATURE or HIP_SIGNATURE_2 fields, saving the
       contents if they will be needed later.

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   3.  In case of HMAC_2, build and add a HOST_ID parameter (with
       Responder information) to the packet.  The HOST_ID parameter
       should be identical to the one previously received from the
       Responder.

   4.  Recalculate the HIP packet length in the HIP header and clear the
       Checksum field (set it to all zeros).  In case of HMAC_2, the
       length is calculated with the added HOST_ID parameter.

   5.  Compute the HMAC using either HIP-gl or HIP-lg integrity key as
       defined in Section 6.5 and verify it against the received HMAC.

   6.  Set Checksum and Header Length field in the HIP header to
       original values.

   7.  In case of HMAC_2, remove the HOST_ID parameter from the packet
       before further processing.

6.4.2.  Signature Calculation

   The following process applies both to the HIP_SIGNATURE and
   HIP_SIGNATURE_2 parameters.  When processing HIP_SIGNATURE_2, the
   only difference is that instead of HIP_SIGNATURE parameter, the
   HIP_SIGNATURE_2 parameter is used, and the Initiator's HIT and PUZZLE
   Opaque and Random #I fields are cleared (set to all zeros) before
   computing the signature.  The HIP_SIGNATURE parameter is defined in
   Section 5.2.11 and the HIP_SIGNATURE_2 parameter in Section 5.2.12.

   The scope of the calculation for HIP_SIGNATURE and HIP_SIGNATURE_2
   is:

   HIP_SIGNATURE: { HIP header | [ Parameters ] }

   where Parameters include all HIP parameters for the packet that is
   being calculated with Type values from 1 to (HIP_SIGNATURE's Type
   value - 1).

   During signature calculation, the following apply:

   o  In the HIP header, the Checksum field is set to zero.

   o  In the HIP header, the Header Length field value is calculated to
      the beginning of the HIP_SIGNATURE parameter.

   Parameter order is described in Section 5.2.1.

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   HIP_SIGNATURE_2: { HIP header | [ Parameters ] }

   where Parameters include all HIP parameters for the packet that is
   being calculated with Type values from 1 to (HIP_SIGNATURE_2's Type
   value - 1).

   During signature calculation, the following apply:

   o  In the HIP header, the Initiator's HIT field and Checksum fields
      are set to zero.

   o  In the HIP header, the Header Length field value is calculated to
      the beginning of the HIP_SIGNATURE_2 parameter.

   o  PUZZLE parameter's Opaque and Random #I fields are set to zero.

   Parameter order is described in Section 5.2.1.

   Signature calculation and verification process (the process applies
   both to HIP_SIGNATURE and HIP_SIGNATURE_2 except in the case where
   HIP_SIGNATURE_2 is separately mentioned):

   Packet sender:

   1.  Create the HIP packet without the HIP_SIGNATURE parameter or any
       parameters that follow the HIP_SIGNATURE parameter.

   2.  Calculate the Length field and zero the Checksum field in the HIP
       header.  In case of HIP_SIGNATURE_2, set Initiator's HIT field in
       the HIP header as well as PUZZLE parameter's Opaque and Random #I
       fields to zero.

   3.  Compute the signature using the private key corresponding to the
       Host Identifier (public key).

   4.  Add the HIP_SIGNATURE parameter to the packet.

   5.  Add any parameters that follow the HIP_SIGNATURE parameter.

   6.  Recalculate the Length field in the HIP header, and calculate the
       Checksum field.

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   Packet receiver:

   1.  Verify the HIP header Length field.

   2.  Save the contents of the HIP_SIGNATURE parameter and any
       parameters following the HIP_SIGNATURE parameter and remove them
       from the packet.

   3.  Recalculate the HIP packet Length in the HIP header and clear the
       Checksum field (set it to all zeros).  In case of
       HIP_SIGNATURE_2, set Initiator's HIT field in HIP header as well
       as PUZZLE parameter's Opaque and Random #I fields to zero.

   4.  Compute the signature and verify it against the received
       signature using the packet sender's Host Identifier (public key).

   5.  Restore the original packet by adding removed parameters (in step
       2) and resetting the values that were set to zero (in step 3).

   The verification can use either the HI received from a HIP packet,
   the HI from a DNS query, if the FQDN has been received in the HOST_ID
   packet, or one received by some other means.

6.5.  HIP KEYMAT Generation

   HIP keying material is derived from the Diffie-Hellman session key,
   Kij, produced during the HIP base exchange (Section 4.1.3).  The
   Initiator has Kij during the creation of the I2 packet, and the
   Responder has Kij once it receives the I2 packet.  This is why I2 can
   already contain encrypted information.

   The KEYMAT is derived by feeding Kij and the HITs into the following
   operation; the | operation denotes concatenation.

    KEYMAT = K1 | K2 | K3 | ...
          where

    K1   = RHASH( Kij | sort(HIT-I | HIT-R) | I | J | 0x01 )
    K2   = RHASH( Kij | K1 | 0x02 )
    K3   = RHASH( Kij | K2 | 0x03 )
    ...
    K255 = RHASH( Kij | K254 | 0xff )
    K256 = RHASH( Kij | K255 | 0x00 )
    etc.

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   Sort(HIT-I | HIT-R) is defined as the network byte order
   concatenation of the two HITs, with the smaller HIT preceding the
   larger HIT, resulting from the numeric comparison of the two HITs
   interpreted as positive (unsigned) 128-bit integers in network byte
   order.

   I and J values are from the puzzle and its solution that were
   exchanged in R1 and I2 messages when this HIP association was set up.
   Both hosts have to store I and J values for the HIP association for
   future use.

   The initial keys are drawn sequentially in the order that is
   determined by the numeric comparison of the two HITs, with comparison
   method described in the previous paragraph.  HOST_g denotes the host
   with the greater HIT value, and HOST_l the host with the lower HIT
   value.

   The drawing order for initial keys:

      HIP-gl encryption key for HOST_g's outgoing HIP packets

      HIP-gl integrity (HMAC) key for HOST_g's outgoing HIP packets

      HIP-lg encryption key (currently unused) for HOST_l's outgoing HIP
      packets

      HIP-lg integrity (HMAC) key for HOST_l's outgoing HIP packets

   The number of bits drawn for a given algorithm is the "natural" size
   of the keys.  For the mandatory algorithms, the following sizes
   apply:

   AES  128 bits

   SHA-1  160 bits

   NULL  0 bits

   If other key sizes are used, they must be treated as different
   encryption algorithms and defined separately.

6.6.  Initiation of a HIP Exchange

   An implementation may originate a HIP exchange to another host based
   on a local policy decision, usually triggered by an application
   datagram, in much the same way that an IPsec IKE key exchange can

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   dynamically create a Security Association.  Alternatively, a system
   may initiate a HIP exchange if it has rebooted or timed out, or
   otherwise lost its HIP state, as described in Section 4.5.4.

   The implementation prepares an I1 packet and sends it to the IP
   address that corresponds to the peer host.  The IP address of the
   peer host may be obtained via conventional mechanisms, such as DNS
   lookup.  The I1 contents are specified in Section 5.3.1.  The
   selection of which Host Identity to use, if a host has more than one
   to choose from, is typically a policy decision.

   The following steps define the conceptual processing rules for
   initiating a HIP exchange:

   1.  The Initiator gets the Responder's HIT and one or more addresses
       either from a DNS lookup of the Responder's FQDN, from some other
       repository, or from a local table.  If the Initiator does not
       know the Responder's HIT, it may attempt opportunistic mode by
       using NULL (all zeros) as the Responder's HIT.  See also "HIP
       Opportunistic Mode" (Section 4.1.6).

   2.  The Initiator sends an I1 to one of the Responder's addresses.
       The selection of which address to use is a local policy decision.

   3.  Upon sending an I1, the sender shall transition to state I1-SENT,
       start a timer whose timeout value should be larger than the
       worst-case anticipated RTT, and shall increment a timeout counter
       associated with the I1.

   4.  Upon timeout, the sender SHOULD retransmit the I1 and restart the
       timer, up to a maximum of I1_RETRIES_MAX tries.

6.6.1.  Sending Multiple I1s in Parallel

   For the sake of minimizing the session establishment latency, an
   implementation MAY send the same I1 to more than one of the
   Responder's addresses.  However, it MUST NOT send to more than three
   (3) addresses in parallel.  Furthermore, upon timeout, the
   implementation MUST refrain from sending the same I1 packet to
   multiple addresses.  That is, if it retries to initialize the
   connection after timeout, it MUST NOT send the I1 packet to more than
   one destination address.  These limitations are placed in order to
   avoid congestion of the network, and potential DoS attacks that might
   happen, e.g., because someone's claim to have hundreds or thousands
   of addresses could generate a huge number of I1 messages from the
   Initiator.

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   As the Responder is not guaranteed to distinguish the duplicate I1s
   it receives at several of its addresses (because it avoids storing
   states when it answers back an R1), the Initiator may receive several
   duplicate R1s.

   The Initiator SHOULD then select the initial preferred destination
   address using the source address of the selected received R1, and use
   the preferred address as a source address for the I2.  Processing
   rules for received R1s are discussed in Section 6.8.

6.6.2.  Processing Incoming ICMP Protocol Unreachable Messages

   A host may receive an ICMP 'Destination Protocol Unreachable' message
   as a response to sending a HIP I1 packet.  Such a packet may be an
   indication that the peer does not support HIP, or it may be an
   attempt to launch an attack by making the Initiator believe that the
   Responder does not support HIP.

   When a system receives an ICMP 'Destination Protocol Unreachable'
   message while it is waiting for an R1, it MUST NOT terminate the
   wait.  It MAY continue as if it had not received the ICMP message,
   and send a few more I1s.  Alternatively, it MAY take the ICMP message
   as a hint that the peer most probably does not support HIP, and
   return to state UNASSOCIATED earlier than otherwise.  However, at
   minimum, it MUST continue waiting for an R1 for a reasonable time
   before returning to UNASSOCIATED.

6.7.  Processing Incoming I1 Packets

   An implementation SHOULD reply to an I1 with an R1 packet, unless the
   implementation is unable or unwilling to set up a HIP association.
   If the implementation is unable to set up a HIP association, the host
   SHOULD send an ICMP Destination Protocol Unreachable,
   Administratively Prohibited, message to the I1 source address.  If
   the implementation is unwilling to set up a HIP association, the host
   MAY ignore the I1.  This latter case may occur during a DoS attack
   such as an I1 flood.

   The implementation MUST be able to handle a storm of received I1
   packets, discarding those with common content that arrive within a
   small time delta.

   A spoofed I1 can result in an R1 attack on a system.  An R1 sender
   MUST have a mechanism to rate-limit R1s to an address.

   It is RECOMMENDED that the HIP state machine does not transition upon
   sending an R1.

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   The following steps define the conceptual processing rules for
   responding to an I1 packet:

   1.  The Responder MUST check that the Responder's HIT in the received
       I1 is either one of its own HITs or NULL.

   2.  If the Responder is in ESTABLISHED state, the Responder MAY
       respond to this with an R1 packet, prepare to drop existing SAs,
       and stay at ESTABLISHED state.

   3.  If the Responder is in I1-SENT state, it must make a comparison
       between the sender's HIT and its own (i.e., the receiver's) HIT.
       If the sender's HIT is greater than its own HIT, it should drop
       the I1 and stay at I1-SENT.  If the sender's HIT is smaller than
       its own HIT, it should send R1 and stay at I1-SENT.  The HIT
       comparison goes similarly as in Section 6.5.

   4.  If the implementation chooses to respond to the I1 with an R1
       packet, it creates a new R1 or selects a precomputed R1 according
       to the format described in Section 5.3.2.

   5.  The R1 MUST contain the received Responder's HIT, unless the
       received HIT is NULL, in which case the Responder SHOULD select a
       HIT that is constructed with the MUST algorithm in Section 3,
       which is currently RSA.  Other than that, selecting the HIT is a
       local policy matter.

   6.  The Responder sends the R1 to the source IP address of the I1
       packet.

6.7.1.  R1 Management

   All compliant implementations MUST produce R1 packets.  An R1 packet
   MAY be precomputed.  An R1 packet MAY be reused for time Delta T,
   which is implementation dependent, and SHOULD be deprecated and not
   used once a valid response I2 packet has been received from an
   Initiator.  During an I1 message storm, an R1 packet may be re-used
   beyond this limit.  R1 information MUST NOT be discarded until Delta
   S after T.  Time S is the delay needed for the last I2 to arrive back
   to the Responder.

   An implementation MAY keep state about received I1s and match the
   received I2s against the state, as discussed in Section 4.1.1.

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6.7.2.  Handling Malformed Messages

   If an implementation receives a malformed I1 message, it SHOULD NOT
   respond with a NOTIFY message, as such practice could open up a
   potential denial-of-service danger.  Instead, it MAY respond with an
   ICMP packet, as defined in Section 5.4.

6.8.  Processing Incoming R1 Packets

   A system receiving an R1 MUST first check to see if it has sent an I1
   to the originator of the R1 (i.e., it is in state I1-SENT).  If so,
   it SHOULD process the R1 as described below, send an I2, and go to
   state I2-SENT, setting a timer to protect the I2.  If the system is
   in state I2-SENT, it MAY respond to an R1 if the R1 has a larger R1
   generation counter; if so, it should drop its state due to processing
   the previous R1 and start over from state I1-SENT.  If the system is
   in any other state with respect to that host, it SHOULD silently drop
   the R1.

   When sending multiple I1s, an Initiator SHOULD wait for a small
   amount of time after the first R1 reception to allow possibly
   multiple R1s to arrive, and it SHOULD respond to an R1 among the set
   with the largest R1 generation counter.

   The following steps define the conceptual processing rules for
   responding to an R1 packet:

   1.   A system receiving an R1 MUST first check to see if it has sent
        an I1 to the originator of the R1 (i.e., it has a HIP
        association that is in state I1-SENT and that is associated with
        the HITs in the R1).  Unless the I1 was sent in opportunistic
        mode (see Section 4.1.6), the IP addresses in the received R1
        packet SHOULD be ignored and, when looking up the right HIP
        association, the received R1 SHOULD be matched against the
        associations using only the HITs.  If a match exists, the system
        should process the R1 as described below.

   2.   Otherwise, if the system is in any other state than I1-SENT or
        I2-SENT with respect to the HITs included in the R1, it SHOULD
        silently drop the R1 and remain in the current state.

   3.   If the HIP association state is I1-SENT or I2-SENT, the received
        Initiator's HIT MUST correspond to the HIT used in the original,
        and the I1 and the Responder's HIT MUST correspond to the one
        used, unless the I1 contained a NULL HIT.

   4.   The system SHOULD validate the R1 signature before applying
        further packet processing, according to Section 5.2.12.

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   5.   If the HIP association state is I1-SENT, and multiple valid R1s
        are present, the system SHOULD select from among the R1s with
        the largest R1 generation counter.

   6.   If the HIP association state is I2-SENT, the system MAY reenter
        state I1-SENT and process the received R1 if it has a larger R1
        generation counter than the R1 responded to previously.

   7.   The R1 packet may have the A bit set -- in this case, the system
        MAY choose to refuse it by dropping the R1 and returning to
        state UNASSOCIATED.  The system SHOULD consider dropping the R1
        only if it used a NULL HIT in I1.  If the A bit is set, the
        Responder's HIT is anonymous and should not be stored.

   8.   The system SHOULD attempt to validate the HIT against the
        received Host Identity by using the received Host Identity to
        construct a HIT and verify that it matches the Sender's HIT.

   9.   The system MUST store the received R1 generation counter for
        future reference.

   10.  The system attempts to solve the puzzle in R1.  The system MUST
        terminate the search after exceeding the remaining lifetime of
        the puzzle.  If the puzzle is not successfully solved, the
        implementation may either resend I1 within the retry bounds or
        abandon the HIP exchange.

   11.  The system computes standard Diffie-Hellman keying material
        according to the public value and Group ID provided in the
        DIFFIE_HELLMAN parameter.  The Diffie-Hellman keying material
        Kij is used for key extraction as specified in Section 6.5.  If
        the received Diffie-Hellman Group ID is not supported, the
        implementation may either resend I1 within the retry bounds or
        abandon the HIP exchange.

   12.  The system selects the HIP transform from the choices presented
        in the R1 packet and uses the selected values subsequently when
        generating and using encryption keys, and when sending the I2.
        If the proposed alternatives are not acceptable to the system,
        it may either resend I1 within the retry bounds or abandon the
        HIP exchange.

   13.  The system initializes the remaining variables in the associated
        state, including Update ID counters.

   14.  The system prepares and sends an I2, as described in
        Section 5.3.3.

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   15.  The system SHOULD start a timer whose timeout value should be
        larger than the worst-case anticipated RTT, and MUST increment a
        timeout counter associated with the I2.  The sender SHOULD
        retransmit the I2 upon a timeout and restart the timer, up to a
        maximum of I2_RETRIES_MAX tries.

   16.  If the system is in state I1-SENT, it shall transition to state
        I2-SENT.  If the system is in any other state, it remains in the
        current state.

6.8.1.  Handling Malformed Messages

   If an implementation receives a malformed R1 message, it MUST
   silently drop the packet.  Sending a NOTIFY or ICMP would not help,
   as the sender of the R1 typically doesn't have any state.  An
   implementation SHOULD wait for some more time for a possibly good R1,
   after which it MAY try again by sending a new I1 packet.

6.9.  Processing Incoming I2 Packets

   Upon receipt of an I2, the system MAY perform initial checks to
   determine whether the I2 corresponds to a recent R1 that has been
   sent out, if the Responder keeps such state.  For example, the sender
   could check whether the I2 is from an address or HIT that has
   recently received an R1 from it.  The R1 may have had Opaque data
   included that was echoed back in the I2.  If the I2 is considered to
   be suspect, it MAY be silently discarded by the system.

   Otherwise, the HIP implementation SHOULD process the I2.  This
   includes validation of the puzzle solution, generating the Diffie-
   Hellman key, decrypting the Initiator's Host Identity, verifying the
   signature, creating state, and finally sending an R2.

   The following steps define the conceptual processing rules for
   responding to an I2 packet:

   1.   The system MAY perform checks to verify that the I2 corresponds
        to a recently sent R1.  Such checks are implementation
        dependent.  See Appendix A for a description of an example
        implementation.

   2.   The system MUST check that the Responder's HIT corresponds to
        one of its own HITs.

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   3.   If the system's state machine is in the R2-SENT state, the
        system MAY check if the newly received I2 is similar to the one
        that triggered moving to R2-SENT.  If so, it MAY retransmit a
        previously sent R2, reset the R2-SENT timer, and the state
        machine stays in R2-SENT.

   4.   If the system's state machine is in the I2-SENT state, the
        system makes a comparison between its local and sender's HITs
        (similarly as in Section 6.5).  If the local HIT is smaller than
        the sender's HIT, it should drop the I2 packet, use the peer
        Diffie-Hellman key and nonce I from the R1 packet received
        earlier, and get the local Diffie-Hellman key and nonce J from
        the I2 packet sent to the peer earlier.  Otherwise, the system
        should process the received I2 packet and drop any previously
        derived Diffie-Hellman keying material Kij it might have formed
        upon sending the I2 previously.  The peer Diffie-Hellman key and
        the nonce J are taken from the just arrived I2 packet.  The
        local Diffie-Hellman key and the nonce I are the ones that were
        earlier sent in the R1 packet.

   5.   If the system's state machine is in the I1-SENT state, and the
        HITs in the I2 match those used in the previously sent I1, the
        system uses this received I2 as the basis for the HIP
        association it was trying to form, and stops retransmitting I1
        (provided that the I2 passes the below additional checks).

   6.   If the system's state machine is in any other state than R2-
        SENT, the system SHOULD check that the echoed R1 generation
        counter in I2 is within the acceptable range.  Implementations
        MUST accept puzzles from the current generation and MAY accept
        puzzles from earlier generations.  If the newly received I2 is
        outside the accepted range, the I2 is stale (perhaps replayed)
        and SHOULD be dropped.

   7.   The system MUST validate the solution to the puzzle by computing
        the hash described in Section 5.3.3 using the same RHASH
        algorithm.

   8.   The I2 MUST have a single value in the HIP_TRANSFORM parameter,
        which MUST match one of the values offered to the Initiator in
        the R1 packet.

   9.   The system must derive Diffie-Hellman keying material Kij based
        on the public value and Group ID in the DIFFIE_HELLMAN
        parameter.  This key is used to derive the HIP association keys,
        as described in Section 6.5.  If the Diffie-Hellman Group ID is
        unsupported, the I2 packet is silently dropped.

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   10.  The encrypted HOST_ID is decrypted by the Initiator encryption
        key defined in Section 6.5.  If the decrypted data is not a
        HOST_ID parameter, the I2 packet is silently dropped.

   11.  The implementation SHOULD also verify that the Initiator's HIT
        in the I2 corresponds to the Host Identity sent in the I2.
        (Note: some middleboxes may not able to make this verification.)

   12.  The system MUST verify the HMAC according to the procedures in
        Section 5.2.9.

   13.  The system MUST verify the HIP_SIGNATURE according to
        Section 5.2.11 and Section 5.3.3.

   14.  If the checks above are valid, then the system proceeds with
        further I2 processing; otherwise, it discards the I2 and its
        state machine remains in the same state.

   15.  The I2 packet may have the A bit set -- in this case, the system
        MAY choose to refuse it by dropping the I2 and the state machine
        returns to state UNASSOCIATED.  If the A bit is set, the
        Initiator's HIT is anonymous and should not be stored.

   16.  The system initializes the remaining variables in the associated
        state, including Update ID counters.

   17.  Upon successful processing of an I2 when the system's state
        machine is in state UNASSOCIATED, I1-SENT, I2-SENT, or R2-SENT,
        an R2 is sent and the system's state machine transitions to
        state R2-SENT.

   18.  Upon successful processing of an I2 when the system's state
        machine is in state ESTABLISHED, the old HIP association is
        dropped and a new one is installed, an R2 is sent, and the
        system's state machine transitions to R2-SENT.

   19.  Upon the system's state machine transitioning to R2-SENT, the
        system starts a timer.  The state machine transitions to
        ESTABLISHED if some data has been received on the incoming HIP
        association, or an UPDATE packet has been received (or some
        other packet that indicates that the peer system's state machine
        has moved to ESTABLISHED).  If the timer expires (allowing for
        maximal retransmissions of I2s), the state machine transitions
        to ESTABLISHED.

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6.9.1.  Handling Malformed Messages

   If an implementation receives a malformed I2 message, the behavior
   SHOULD depend on how many checks the message has already passed.  If
   the puzzle solution in the message has already been checked, the
   implementation SHOULD report the error by responding with a NOTIFY
   packet.  Otherwise, the implementation MAY respond with an ICMP
   message as defined in Section 5.4.

6.10.  Processing Incoming R2 Packets

   An R2 received in states UNASSOCIATED, I1-SENT, or ESTABLISHED
   results in the R2 being dropped and the state machine staying in the
   same state.  If an R2 is received in state I2-SENT, it SHOULD be
   processed.

   The following steps define the conceptual processing rules for an
   incoming R2 packet:

   1.  The system MUST verify that the HITs in use correspond to the
       HITs that were received in the R1.

   2.  The system MUST verify the HMAC_2 according to the procedures in
       Section 5.2.10.

   3.  The system MUST verify the HIP signature according to the
       procedures in Section 5.2.11.

   4.  If any of the checks above fail, there is a high probability of
       an ongoing man-in-the-middle or other security attack.  The
       system SHOULD act accordingly, based on its local policy.

   5.  If the system is in any other state than I2-SENT, the R2 is
       silently dropped.

   6.  Upon successful processing of the R2, the state machine moves to
       state ESTABLISHED.

6.11.  Sending UPDATE Packets

   A host sends an UPDATE packet when it wants to update some
   information related to a HIP association.  There are a number of
   likely situations, e.g., mobility management and rekeying of an
   existing ESP Security Association.  The following paragraphs define
   the conceptual rules for sending an UPDATE packet to the peer.
   Additional steps can be defined in other documents where the UPDATE
   packet is used.

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   The system first determines whether there are any outstanding UPDATE
   messages that may conflict with the new UPDATE message under
   consideration.  When multiple UPDATEs are outstanding (not yet
   acknowledged), the sender must assume that such UPDATEs may be
   processed in an arbitrary order.  Therefore, any new UPDATEs that
   depend on a previous outstanding UPDATE being successfully received
   and acknowledged MUST be postponed until reception of the necessary
   ACK(s) occurs.  One way to prevent any conflicts is to only allow one
   outstanding UPDATE at a time.  However, allowing multiple UPDATEs may
   improve the performance of mobility and multihoming protocols.

   The following steps define the conceptual processing rules for
   sending UPDATE packets.

   1.  The first UPDATE packet is sent with Update ID of zero.
       Otherwise, the system increments its own Update ID value by one
       before continuing the below steps.

   2.  The system creates an UPDATE packet that contains a SEQ parameter
       with the current value of Update ID.  The UPDATE packet may also
       include an ACK of the peer's Update ID found in a received UPDATE
       SEQ parameter, if any.

   3.  The system sends the created UPDATE packet and starts an UPDATE
       timer.  The default value for the timer is 2 * RTT estimate.  If
       multiple UPDATEs are outstanding, multiple timers are in effect.

   4.  If the UPDATE timer expires, the UPDATE is resent.  The UPDATE
       can be resent UPDATE_RETRY_MAX times.  The UPDATE timer SHOULD be
       exponentially backed off for subsequent retransmissions.  If no
       acknowledgment is received from the peer after UPDATE_RETRY_MAX
       times, the HIP association is considered to be broken and the
       state machine should move from state ESTABLISHED to state CLOSING
       as depicted in Section 4.4.3.  The UPDATE timer is cancelled upon
       receiving an ACK from the peer that acknowledges receipt of the
       UPDATE.

6.12.  Receiving UPDATE Packets

   When a system receives an UPDATE packet, its processing depends on
   the state of the HIP association and the presence and values of the
   SEQ and ACK parameters.  Typically, an UPDATE message also carries
   optional parameters whose handling is defined in separate documents.

   For each association, the peer's next expected in-sequence Update ID
   ("peer Update ID") is stored.  Initially, this value is zero.  Update
   ID comparisons of "less than" and "greater than" are performed with
   respect to a circular sequence number space.

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   The sender may send multiple outstanding UPDATE messages.  These
   messages are processed in the order in which they are received at the
   receiver (i.e., no resequencing is performed).  When processing
   UPDATEs out-of-order, the receiver MUST keep track of which UPDATEs
   were previously processed, so that duplicates or retransmissions are
   ACKed and not reprocessed.  A receiver MAY choose to define a receive
   window of Update IDs that it is willing to process at any given time,
   and discard received UPDATEs falling outside of that window.

   The following steps define the conceptual processing rules for
   receiving UPDATE packets.

   1.  If there is no corresponding HIP association, the implementation
       MAY reply with an ICMP Parameter Problem, as specified in
       Section 5.4.4.

   2.  If the association is in the ESTABLISHED state and the SEQ (but
       not ACK) parameter is present, the UPDATE is processed and
       replied to as described in Section 6.12.1.

   3.  If the association is in the ESTABLISHED state and the ACK (but
       not SEQ) parameter is present, the UPDATE is processed as
       described in Section 6.12.2.

   4.  If the association is in the ESTABLISHED state and there is both
       an ACK and SEQ in the UPDATE, the ACK is first processed as
       described in Section 6.12.2, and then the rest of the UPDATE is
       processed as described in Section 6.12.1.

6.12.1.  Handling a SEQ Parameter in a Received UPDATE Message

   The following steps define the conceptual processing rules for
   handling a SEQ parameter in a received UPDATE packet.

   1.  If the Update ID in the received SEQ is not the next in the
       sequence of Update IDs and is greater than the receiver's window
       for new UPDATEs, the packet MUST be dropped.

   2.  If the Update ID in the received SEQ corresponds to an UPDATE
       that has recently been processed, the packet is treated as a
       retransmission.  The HMAC verification (next step) MUST NOT be
       skipped.  (A byte-by-byte comparison of the received and a stored
       packet would be OK, though.)  It is recommended that a host cache
       UPDATE packets sent with ACKs to avoid the cost of generating a
       new ACK packet to respond to a replayed UPDATE.  The system MUST
       acknowledge, again, such (apparent) UPDATE message
       retransmissions but SHOULD also consider rate-limiting such
       retransmission responses to guard against replay attacks.

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   3.  The system MUST verify the HMAC in the UPDATE packet.  If the
       verification fails, the packet MUST be dropped.

   4.  The system MAY verify the SIGNATURE in the UPDATE packet.  If the
       verification fails, the packet SHOULD be dropped and an error
       message logged.

   5.  If a new SEQ parameter is being processed, the parameters in the
       UPDATE are then processed.  The system MUST record the Update ID
       in the received SEQ parameter, for replay protection.

   6.  An UPDATE acknowledgment packet with ACK parameter is prepared
       and sent to the peer.  This ACK parameter may be included in a
       separate UPDATE or piggybacked in an UPDATE with SEQ parameter,
       as described in Section 5.3.5.  The ACK parameter MAY acknowledge
       more than one of the peer's Update IDs.

6.12.2.  Handling an ACK Parameter in a Received UPDATE Packet

   The following steps define the conceptual processing rules for
   handling an ACK parameter in a received UPDATE packet.

   1.  The sequence number reported in the ACK must match with an
       earlier sent UPDATE packet that has not already been
       acknowledged.  If no match is found or if the ACK does not
       acknowledge a new UPDATE, the packet MUST either be dropped if no
       SEQ parameter is present, or the processing steps in
       Section 6.12.1 are followed.

   2.  The system MUST verify the HMAC in the UPDATE packet.  If the
       verification fails, the packet MUST be dropped.

   3.  The system MAY verify the SIGNATURE in the UPDATE packet.  If the
       verification fails, the packet SHOULD be dropped and an error
       message logged.

   4.  The corresponding UPDATE timer is stopped (see Section 6.11) so
       that the now acknowledged UPDATE is no longer retransmitted.  If
       multiple UPDATEs are newly acknowledged, multiple timers are
       stopped.

6.13.  Processing NOTIFY Packets

   Processing NOTIFY packets is OPTIONAL.  If processed, any errors in a
   received NOTIFICATION parameter SHOULD be logged.  Received errors
   MUST be considered only as informational, and the receiver SHOULD NOT
   change its HIP state (Section 4.4.1) purely based on the received
   NOTIFY message.

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6.14.  Processing CLOSE Packets

   When the host receives a CLOSE message, it responds with a CLOSE_ACK
   message and moves to CLOSED state.  (The authenticity of the CLOSE
   message is verified using both HMAC and SIGNATURE).  This processing
   applies whether or not the HIP association state is CLOSING in order
   to handle CLOSE messages from both ends that cross in flight.

   The HIP association is not discarded before the host moves from the
   UNASSOCIATED state.

   Once the closing process has started, any need to send data packets
   will trigger creating and establishing of a new HIP association,
   starting with sending an I1.

   If there is no corresponding HIP association, the CLOSE packet is
   dropped.

6.15.  Processing CLOSE_ACK Packets

   When a host receives a CLOSE_ACK message, it verifies that it is in
   CLOSING or CLOSED state and that the CLOSE_ACK was in response to the
   CLOSE (using the included ECHO_RESPONSE_SIGNED in response to the
   sent ECHO_REQUEST_SIGNED).

   The CLOSE_ACK uses HMAC and SIGNATURE for verification.  The state is
   discarded when the state changes to UNASSOCIATED and, after that, the
   host MAY respond with an ICMP Parameter Problem to an incoming CLOSE
   message (see Section 5.4.4).

6.16.  Handling State Loss

   In the case of system crash and unanticipated state loss, the system
   SHOULD delete the corresponding HIP state, including the keying
   material.  That is, the state SHOULD NOT be stored on stable storage.
   If the implementation does drop the state (as RECOMMENDED), it MUST
   also drop the peer's R1 generation counter value, unless a local
   policy explicitly defines that the value of that particular host is
   stored.  An implementation MUST NOT store R1 generation counters by
   default, but storing R1 generation counter values, if done, MUST be
   configured by explicit HITs.

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7.  HIP Policies

   There are a number of variables that will influence the HIP exchanges
   that each host must support.  All HIP implementations MUST support
   more than one simultaneous HI, at least one of which SHOULD be
   reserved for anonymous usage.  Although anonymous HIs will be rarely
   used as Responders' HIs, they will be common for Initiators.  Support
   for more than two HIs is RECOMMENDED.

   Many Initiators would want to use a different HI for different
   Responders.  The implementations SHOULD provide for an ACL of
   Initiator's HIT to Responder's HIT.  This ACL SHOULD also include
   preferred transform and local lifetimes.

   The value of K used in the HIP R1 packet can also vary by policy.  K
   should never be greater than 20, but for trusted partners it could be
   as low as 0.

   Responders would need a similar ACL, representing which hosts they
   accept HIP exchanges, and the preferred transform and local
   lifetimes.  Wildcarding SHOULD be supported for this ACL also.

8.  Security Considerations

   HIP is designed to provide secure authentication of hosts.  HIP also
   attempts to limit the exposure of the host to various denial-of-
   service and man-in-the-middle (MitM) attacks.  In so doing, HIP
   itself is subject to its own DoS and MitM attacks that potentially
   could be more damaging to a host's ability to conduct business as
   usual.

   The 384-bit Diffie-Hellman Group is targeted to be used in hosts that
   either do not require or are not powerful enough for handling strong
   cryptography.  Although there is a risk that with suitable equipment
   the encryption can be broken in real time, the 384-bit group can
   provide some protection for end-hosts that are not able to handle any
   stronger cryptography.  When the security provided by the 384-bit
   group is not enough for applications on a host, the support for this
   group should be turned off in the configuration.

   Denial-of-service attacks often take advantage of the cost of start
   of state for a protocol on the Responder compared to the 'cheapness'
   on the Initiator.  HIP makes no attempt to increase the cost of the
   start of state on the Initiator, but makes an effort to reduce the
   cost to the Responder.  This is done by having the Responder start
   the 3-way exchange instead of the Initiator, making the HIP protocol
   4 packets long.  In doing this, packet 2 becomes a 'stock' packet
   that the Responder MAY use many times, until some Initiator has

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   provided a valid response to such an R1 packet.  During an I1 storm,
   the host may reuse the same D-H value also even if some Initiator has
   provided a valid response using that particular D-H value.  However,
   such behavior is discouraged and should be avoided.  Using the same
   Diffie-Hellman values and random puzzle #I value has some risks.
   This risk needs to be balanced against a potential storm of HIP I1
   packets.

   This shifting of the start of state cost to the Initiator in creating
   the I2 HIP packet, presents another DoS attack.  The attacker spoofs
   the I1 HIP packet and the Responder sends out the R1 HIP packet.
   This could conceivably tie up the 'Initiator' with evaluating the R1
   HIP packet, and creating the I2 HIP packet.  The defense against this
   attack is to simply ignore any R1 packet where a corresponding I1 was
   not sent.

   A second form of DoS attack arrives in the I2 HIP packet.  Once the
   attacking Initiator has solved the puzzle, it can send packets with
   spoofed IP source addresses with either an invalid encrypted HIP
   payload component or a bad HIP signature.  This would take resources
   in the Responder's part to reach the point to discover that the I2
   packet cannot be completely processed.  The defense against this
   attack is after N bad I2 packets, the Responder would discard any I2s
   that contain the given Initiator HIT.  This will shut down the
   attack.  The attacker would have to request another R1 and use that
   to launch a new attack.  The Responder could up the value of K while
   under attack.  On the downside, valid I2s might get dropped too.

   A third form of DoS attack is emulating the restart of state after a
   reboot of one of the partners.  A restarting host would send an I1 to
   a peer, which would respond with an R1 even if it were in the
   ESTABLISHED state.  If the I1 were spoofed, the resulting R1 would be
   received unexpectedly by the spoofed host and would be dropped, as in
   the first case above.

   A fourth form of DoS attack is emulating the end of state.  HIP
   relies on timers plus a CLOSE/CLOSE_ACK handshake to explicitly
   signal the end of a HIP association.  Because both CLOSE and
   CLOSE_ACK messages contain an HMAC, an outsider cannot close a
   connection.  The presence of an additional SIGNATURE allows
   middleboxes to inspect these messages and discard the associated
   state (for e.g., firewalling, SPI-based NATing, etc.).  However, the
   optional behavior of replying to CLOSE with an ICMP Parameter Problem
   packet (as described in Section 5.4.4) might allow an IP spoofer
   sending CLOSE messages to launch reflection attacks.

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   A fifth form of DoS attack is replaying R1s to cause the Initiator to
   solve stale puzzles and become out of synchronization with the
   Responder.  The R1 generation counter is a monotonically increasing
   counter designed to protect against this attack, as described in
   Section 4.1.4.

   Man-in-the-middle attacks are difficult to defend against, without
   third-party authentication.  A skillful MitM could easily handle all
   parts of HIP, but HIP indirectly provides the following protection
   from a MitM attack.  If the Responder's HI is retrieved from a signed
   DNS zone, a certificate, or through some other secure means, the
   Initiator can use this to validate the R1 HIP packet.

   Likewise, if the Initiator's HI is in a secure DNS zone, a trusted
   certificate, or otherwise securely available, the Responder can
   retrieve the HI (after having got the I2 HIP packet) and verify that
   the HI indeed can be trusted.  However, since an Initiator may choose
   to use an anonymous HI, it knowingly risks a MitM attack.  The
   Responder may choose not to accept a HIP exchange with an anonymous
   Initiator.

   The HIP Opportunistic Mode concept has been introduced in this
   document, but this document does not specify what the semantics of
   such a connection setup are for applications.  There are certain
   concerns with opportunistic mode, as discussed in Section 4.1.6.

   NOTIFY messages are used only for informational purposes and they are
   unacknowledged.  A HIP implementation cannot rely solely on the
   information received in a NOTIFY message because the packet may have
   been replayed.  It SHOULD NOT change any state information based
   purely on a received NOTIFY message.

   Since not all hosts will ever support HIP, ICMP 'Destination Protocol
   Unreachable' messages are to be expected and present a DoS attack.
   Against an Initiator, the attack would look like the Responder does
   not support HIP, but shortly after receiving the ICMP message, the
   Initiator would receive a valid R1 HIP packet.  Thus, to protect from
   this attack, an Initiator should not react to an ICMP message until a
   reasonable delta time to get the real Responder's R1 HIP packet.  A
   similar attack against the Responder is more involved.  Normally, if
   an I1 message received by a Responder was a bogus one sent by an
   attacker, the Responder may receive an ICMP message from the IP
   address the R1 message was sent to.  However, a sophisticated
   attacker can try to take advantage of such a behavior and try to
   break up the HIP exchange by sending such an ICMP message to the
   Responder before the Initiator has a chance to send a valid I2
   message.  Hence, the Responder SHOULD NOT act on such an ICMP
   message.  Especially, it SHOULD NOT remove any minimal state created

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   when it sent the R1 HIP packet (if it did create one), but wait for
   either a valid I2 HIP packet or the natural timeout (that is, if R1
   packets are tracked at all).  Likewise, the Initiator should ignore
   any ICMP message while waiting for an R2 HIP packet, and should
   delete any pending state only after a natural timeout.

9.  IANA Considerations

   IANA has reserved protocol number 139 for the Host Identity Protocol.

   This document defines a new 128-bit value under the CGA Message Type
   namespace [RFC3972], 0xF0EF F02F BFF4 3D0F E793 0C3C 6E61 74EA, to be
   used for HIT generation as specified in ORCHID [RFC4843].

   This document also creates a set of new namespaces.  These are
   described below.

   Packet Type

      The 7-bit Packet Type field in a HIP protocol packet describes the
      type of a HIP protocol message.  It is defined in Section 5.1.
      The current values are defined in Sections 5.3.1 through 5.3.8.

      New values are assigned through IETF Consensus [RFC2434].

   HIP Version

      The four-bit Version field in a HIP protocol packet describes the
      version of the HIP protocol.  It is defined in Section 5.1.  The
      only currently defined value is 1.  New values are assigned
      through IETF Consensus.

   Parameter Type

      The 16-bit Type field in a HIP parameter describes the type of the
      parameter.  It is defined in Section 5.2.1.  The current values
      are defined in Sections 5.2.3 through 5.2.20.

      With the exception of the assigned Type codes, the Type codes 0
      through 1023 and 61440 through 65535 are reserved for future base
      protocol extensions, and are assigned through IETF Consensus.

      The Type codes 32768 through 49141 are reserved for
      experimentation.  Types SHOULD be selected in a random fashion
      from this range, thereby reducing the probability of collisions.
      A method employing genuine randomness (such as flipping a coin)
      SHOULD be used.

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      All other Type codes are assigned through First Come First Served,
      with Specification Required [RFC2434].

   Group ID

      The eight-bit Group ID values appear in the DIFFIE_HELLMAN
      parameter and are defined in Section 5.2.6.  New values either
      from the reserved or unassigned space are assigned through IETF
      Consensus.

   Suite ID

      The 16-bit Suite ID values in a HIP_TRANSFORM parameter are
      defined in Section 5.2.7.  New values either from the reserved or
      unassigned space are assigned through IETF Consensus.

   DI-Type

      The four-bit DI-Type values in a HOST_ID parameter are defined in
      Section 5.2.8.  New values are assigned through IETF Consensus.

   Notify Message Type

      The 16-bit Notify Message Type values in a NOTIFICATION parameter
      are defined in Section 5.2.16.

      Notify Message Type values 1-10 are used for informing about
      errors in packet structures, values 11-20 for informing about
      problems in parameters containing cryptographic related material,
      values 21-30 for informing about problems in authentication or
      packet integrity verification.  Parameter numbers above 30 can be
      used for informing about other types of errors or events.  Values
      51-8191 are error types reserved to be allocated by IANA.  Values
      8192-16383 are error types for experimentation.  Values 16385-
      40959 are status types to be allocated by IANA, and values 40960-
      65535 are status types for experimentation.  New values in ranges
      51-8191 and 16385-40959 are assigned through First Come First
      Served, with Specification Required.

10.  Acknowledgments

   The drive to create HIP came to being after attending the MALLOC
   meeting at the 43rd IETF meeting.  Baiju Patel and Hilarie Orman
   really gave the original author, Bob Moskowitz, the assist to get HIP
   beyond 5 paragraphs of ideas.  It has matured considerably since the
   early versions thanks to extensive input from IETFers.  Most
   importantly, its design goals are articulated and are different from
   other efforts in this direction.  Particular mention goes to the

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   members of the NameSpace Research Group of the IRTF.  Noel Chiappa
   provided valuable input at early stages of discussions about
   identifier handling and Keith Moore the impetus to provide
   resolvability.  Steve Deering provided encouragement to keep working,
   as a solid proposal can act as a proof of ideas for a research group.

   Many others contributed; extensive security tips were provided by
   Steve Bellovin.  Rob Austein kept the DNS parts on track.  Paul
   Kocher taught Bob Moskowitz how to make the puzzle exchange expensive
   for the Initiator to respond, but easy for the Responder to validate.
   Bill Sommerfeld supplied the Birthday concept, which later evolved
   into the R1 generation counter, to simplify reboot management.  Erik
   Nordmark supplied the CLOSE-mechanism for closing connections.
   Rodney Thayer and Hugh Daniels provided extensive feedback.  In the
   early times of this document, John Gilmore kept Bob Moskowitz
   challenged to provide something of value.

   During the later stages of this document, when the editing baton was
   transferred to Pekka Nikander, the input from the early implementors
   was invaluable.  Without having actual implementations, this document
   would not be on the level it is now.

   In the usual IETF fashion, a large number of people have contributed
   to the actual text or ideas.  The list of these people include Jeff
   Ahrenholz, Francis Dupont, Derek Fawcus, George Gross, Andrew
   McGregor, Julien Laganier, Miika Komu, Mika Kousa, Jan Melen, Henrik
   Petander, Michael Richardson, Tim Shepard, Jorma Wall, and Jukka
   Ylitalo.  Our apologies to anyone whose name is missing.

   Once the HIP Working Group was founded in early 2004, a number of
   changes were introduced through the working group process.  Most
   notably, the original document was split in two, one containing the
   base exchange and the other one defining how to use ESP.  Some
   modifications to the protocol proposed by Aura, et al., [AUR03] were
   added at a later stage.

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11.  References

11.1.  Normative References

   [FIPS95]       NIST, "FIPS PUB 180-1: Secure Hash Standard",
                  April 1995.

   [RFC0768]      Postel, J., "User Datagram Protocol", STD 6, RFC 768,
                  August 1980.

   [RFC1035]      Mockapetris, P., "Domain names - implementation and
                  specification", STD 13, RFC 1035, November 1987.

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

   [RFC2404]      Madson, C. and R. Glenn, "The Use of HMAC-SHA-1-96
                  within ESP and AH", RFC 2404, November 1998.

   [RFC2451]      Pereira, R. and R. Adams, "The ESP CBC-Mode Cipher
                  Algorithms", RFC 2451, November 1998.

   [RFC2460]      Deering, S. and R. Hinden, "Internet Protocol, Version
                  6 (IPv6) Specification", RFC 2460, December 1998.

   [RFC2463]      Conta, A. and S. Deering, "Internet Control Message
                  Protocol (ICMPv6) for the Internet Protocol Version 6
                  (IPv6) Specification", RFC 2463, December 1998.

   [RFC2536]      Eastlake, D., "DSA KEYs and SIGs in the Domain Name
                  System (DNS)", RFC 2536, March 1999.

   [RFC2898]      Kaliski, B., "PKCS #5: Password-Based Cryptography
                  Specification Version 2.0", RFC 2898, September 2000.

   [RFC3110]      Eastlake, D., "RSA/SHA-1 SIGs and RSA KEYs in the
                  Domain Name System (DNS)", RFC 3110, May 2001.

   [RFC3484]      Draves, R., "Default Address Selection for Internet
                  Protocol version 6 (IPv6)", RFC 3484, February 2003.

   [RFC3526]      Kivinen, T. and M. Kojo, "More Modular Exponential
                  (MODP) Diffie-Hellman groups for Internet Key Exchange
                  (IKE)", RFC 3526, May 2003.

   [RFC3602]      Frankel, S., Glenn, R., and S. Kelly, "The AES-CBC
                  Cipher Algorithm and Its Use with IPsec", RFC 3602,
                  September 2003.

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   [RFC3972]      Aura, T., "Cryptographically Generated Addresses
                  (CGA)", RFC 3972, March 2005.

   [RFC4034]      Arends, R., Austein, R., Larson, M., Massey, D., and
                  S. Rose, "Resource Records for the DNS Security
                  Extensions", RFC 4034, March 2005.

   [RFC4282]      Aboba, B., Beadles, M., Arkko, J., and P. Eronen, "The
                  Network Access Identifier", RFC 4282, December 2005.

   [RFC4307]      Schiller, J., "Cryptographic Algorithms for Use in the
                  Internet Key Exchange Version 2 (IKEv2)", RFC 4307,
                  December 2005.

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

   [RFC5202]      Jokela, P., Moskowitz, R., and P. Nikander, "Using the
                  Encapsulating Security Payload (ESP) Transport Format
                  with the Host Identity Protocol (HIP)", RFC 5202,
                  April 2008.

11.2.  Informative References

   [AUR03]        Aura, T., Nagarajan, A., and A. Gurtov, "Analysis of
                  the HIP Base Exchange Protocol", in Proceedings
                  of 10th Australasian Conference on Information
                  Security and  Privacy, July 2003.

   [CRO03]        Crosby, SA. and DS. Wallach, "Denial of Service via
                  Algorithmic Complexity Attacks", in Proceedings
                  of Usenix Security Symposium 2003,  Washington, DC.,
                  August 2003.

   [DIF76]        Diffie, W. and M. Hellman, "New Directions in
                  Cryptography", IEEE Transactions on Information
                  Theory vol. IT-22, number 6, pages 644-654, Nov 1976.

   [FIPS01]       NIST, "FIPS PUB 197: Advanced Encryption Standard",
                  Nov 2001.

   [HIP-APP]      Henderson, T., Nikander, P., and M. Komu, "Using the
                  Host Identity Protocol with Legacy Applications", Work
                  in Progress, November 2007.

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   [IPsec-APIs]   Richardson, M., Williams, N., Komu, M., and S.
                  Tarkoma, "IPsec Application Programming Interfaces",
                  Work in Progress, February 2008.

   [KAU03]        Kaufman, C., Perlman, R., and B. Sommerfeld, "DoS
                  protection for UDP-based protocols", ACM Conference on
                  Computer and Communications Security , Oct 2003.

   [KRA03]        Krawczyk, H., "SIGMA: The 'SIGn-and-MAc' Approach to
                  Authenticated Diffie-Hellman and Its Use in the IKE-
                  Protocols", in Proceedings of CRYPTO 2003, pages 400-
                  425, August 2003.

   [RFC0792]      Postel, J., "Internet Control Message Protocol",
                  STD 5, RFC 792, September 1981.

   [RFC2412]      Orman, H., "The OAKLEY Key Determination Protocol",
                  RFC 2412, November 1998.

   [RFC2434]      Narten, T. and H. Alvestrand, "Guidelines for Writing
                  an IANA Considerations Section in RFCs", BCP 26,
                  RFC 2434, October 1998.

   [RFC4306]      Kaufman, C., "Internet Key Exchange (IKEv2) Protocol",
                  RFC 4306, December 2005.

   [RFC4423]      Moskowitz, R. and P. Nikander, "Host Identity Protocol
                  (HIP) Architecture", RFC 4423, May 2006.

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

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

   [RFC5206]      Henderson, T., Ed., "End-Host Mobility and Multihoming
                  with the Host Identity Protocol", RFC 5206,
                  April 2008.

   [SHIM6-PROTO]  Nordmark, E. and M. Bagnulo, "Shim6: Level 3
                  Multihoming Shim Protocol for IPv6", Work in Progress,
                  February 2008.

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Appendix A.  Using Responder Puzzles

   As mentioned in Section 4.1.1, the Responder may delay state creation
   and still reject most spoofed I2s by using a number of pre-calculated
   R1s and a local selection function.  This appendix defines one
   possible implementation in detail.  The purpose of this appendix is
   to give the implementors an idea on how to implement the mechanism.
   If the implementation is based on this appendix, it MAY contain some
   local modification that makes an attacker's task harder.

   The Responder creates a secret value S, that it regenerates
   periodically.  The Responder needs to remember the two latest values
   of S.  Each time the S is regenerated, the R1 generation counter
   value is incremented by one.

   The Responder generates a pre-signed R1 packet.  The signature for
   pre-generated R1s must be recalculated when the Diffie-Hellman key is
   recomputed or when the R1_COUNTER value changes due to S value
   regeneration.

   When the Initiator sends the I1 packet for initializing a connection,
   the Responder gets the HIT and IP address from the packet, and
   generates an I value for the puzzle.  The I value is set to the pre-
   signed R1 packet.

        I value calculation:
        I = Ltrunc( RHASH ( S | HIT-I | HIT-R | IP-I | IP-R ), 64)

   The RHASH algorithm is the same that is used to generate the
   Responder's HIT value.

   From an incoming I2 packet, the Responder gets the required
   information to validate the puzzle: HITs, IP addresses, and the
   information of the used S value from the R1_COUNTER.  Using these
   values, the Responder can regenerate the I, and verify it against the
   I received in the I2 packet.  If the I values match, it can verify
   the solution using I, J, and difficulty K.  If the I values do not
   match, the I2 is dropped.

        puzzle_check:
        V := Ltrunc( RHASH( I2.I | I2.hit_i | I2.hit_r | I2.J ), K )
        if V != 0, drop the packet

   If the puzzle solution is correct, the I and J values are stored for
   later use.  They are used as input material when keying material is
   generated.

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   Keeping state about failed puzzle solutions depends on the
   implementation.  Although it is possible for the Responder not to
   keep any state information, it still may do so to protect itself
   against certain attacks (see Section 4.1.1).

Appendix B.  Generating a Public Key Encoding from an HI

   The following pseudo-code illustrates the process to generate a
   public key encoding from an HI for both RSA and DSA.

   The symbol := denotes assignment; the symbol += denotes appending.
   The pseudo-function encode_in_network_byte_order takes two
   parameters, an integer (bignum) and a length in bytes, and returns
   the integer encoded into a byte string of the given length.

   switch ( HI.algorithm )
   {

   case RSA:
    buffer := encode_in_network_byte_order ( HI.RSA.e_len,
              ( HI.RSA.e_len > 255 ) ? 3 : 1 )
    buffer += encode_in_network_byte_order ( HI.RSA.e, HI.RSA.e_len )
    buffer += encode_in_network_byte_order ( HI.RSA.n, HI.RSA.n_len )
    break;

   case DSA:
    buffer := encode_in_network_byte_order ( HI.DSA.T , 1 )
    buffer += encode_in_network_byte_order ( HI.DSA.Q , 20 )
    buffer += encode_in_network_byte_order ( HI.DSA.P , 64 +
                                             8 * HI.DSA.T )
    buffer += encode_in_network_byte_order ( HI.DSA.G , 64 +
                                             8 * HI.DSA.T )
    buffer += encode_in_network_byte_order ( HI.DSA.Y , 64 +
                                             8 * HI.DSA.T )
    break;

   }

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Appendix C.  Example Checksums for HIP Packets

   The HIP checksum for HIP packets is specified in Section 5.1.1.
   Checksums for TCP and UDP packets running over HIP-enabled security
   associations are specified in Section 3.5.  The examples below use IP
   addresses of 192.168.0.1 and 192.168.0.2 (and their respective IPv4-
   compatible IPv6 formats), and HITs with the prefix of 2001:10
   followed by zeros, followed by a decimal 1 or 2, respectively.

   The following example is defined only for testing a checksum
   calculation.  The address format for the IPv4-compatible IPv6 address
   is not a valid one, but using these IPv6 addresses when testing an
   IPv6 implementation gives the same checksum output as an IPv4
   implementation with the corresponding IPv4 addresses.

C.1.  IPv6 HIP Example (I1)

      Source Address:                 ::192.168.0.1
      Destination Address:            ::192.168.0.2
      Upper-Layer Packet Length:      40              0x28
      Next Header:                    139             0x8b
      Payload Protocol:               59              0x3b
      Header Length:                  4               0x4
      Packet Type:                    1               0x1
      Version:                        1               0x1
      Reserved:                       1               0x1
      Control:                        0               0x0
      Checksum:                       446             0x1be
      Sender's HIT  :                 2001:10::1
      Receiver's HIT:                 2001:10::2

C.2.  IPv4 HIP Packet (I1)

   The IPv4 checksum value for the same example I1 packet is the same as
   the IPv6 checksum (since the checksums due to the IPv4 and IPv6
   pseudo-header components are the same).

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C.3.  TCP Segment

   Regardless of whether IPv6 or IPv4 is used, the TCP and UDP sockets
   use the IPv6 pseudo-header format [RFC2460], with the HITs used in
   place of the IPv6 addresses.

      Sender's HIT:                   2001:10::1
      Receiver's HIT:                 2001:10::2
      Upper-Layer Packet Length:      20              0x14
      Next Header:                    6               0x06
      Source port:                    65500           0xffdc
      Destination port:               22              0x0016
      Sequence number:                1               0x00000001
      Acknowledgment number:          0               0x00000000
      Header length:                  20              0x14
      Flags:                          SYN             0x02
      Window size:                    65535           0xffff
      Checksum:                       28618           0x6fca
      Urgent pointer:                 0               0x0000


        0x0000:  6000 0000 0014 0640 2001 0010 0000 0000
        0x0010:  0000 0000 0000 0001 2001 0010 0000 0000
        0x0020:  0000 0000 0000 0002 ffdc 0016 0000 0001
        0x0030:  0000 0000 5002 ffff 6fca 0000

Appendix D.  384-Bit Group

   This 384-bit group is defined only to be used with HIP.  NOTE: The
   security level of this group is very low!  The encryption may be
   broken in a very short time, even real-time.  It should be used only
   when the host is not powerful enough (e.g., some PDAs) and when
   security requirements are low (e.g., during normal web surfing).

   This prime is: 2^384 - 2^320 - 1 + 2^64 * { [ 2^254 pi] + 5857 }

   Its hexadecimal value is:

       FFFFFFFF FFFFFFFF C90FDAA2 2168C234 C4C6628B 80DC1CD1
       29024E08 8A67CC74 020BBEA6 3B13B202 FFFFFFFF FFFFFFFF

   The generator is: 2.

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Appendix E.  OAKLEY Well-Known Group 1

   See also [RFC2412] for definition of OAKLEY well-known group 1.

   OAKLEY Well-Known Group 1: A 768-bit prime

   The prime is 2^768 - 2^704 - 1 + 2^64 * { [2^638 pi] + 149686 }.

   The hexadecimal value is:

       FFFFFFFF FFFFFFFF C90FDAA2 2168C234 C4C6628B 80DC1CD1
       29024E08 8A67CC74 020BBEA6 3B139B22 514A0879 8E3404DD
       EF9519B3 CD3A431B 302B0A6D F25F1437 4FE1356D 6D51C245
       E485B576 625E7EC6 F44C42E9 A63A3620 FFFFFFFF FFFFFFFF

   This has been rigorously verified as a prime.

   The generator is: 22 (decimal)

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Authors' Addresses

   Robert Moskowitz
   ICSAlabs, An Independent Division of Verizon Business Systems
   1000 Bent Creek Blvd, Suite 200
   Mechanicsburg, PA
   USA

   EMail: rgm@icsalabs.com


   Pekka Nikander
   Ericsson Research NomadicLab
   JORVAS  FIN-02420
   FINLAND

   Phone: +358 9 299 1
   EMail: pekka.nikander@nomadiclab.com


   Petri Jokela (editor)
   Ericsson Research NomadicLab
   JORVAS  FIN-02420
   FINLAND

   Phone: +358 9 299 1
   EMail: petri.jokela@nomadiclab.com


   Thomas R. Henderson
   The Boeing Company
   P.O. Box 3707
   Seattle, WA
   USA

   EMail: thomas.r.henderson@boeing.com

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Full Copyright Statement

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