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HIP                                                        H. Tschofenig
Internet-Draft                                    Nokia Siemens Networks
Intended status: Standards Track                                 D. Wing
Expires: December 23, 2007                                         Cisco
                                                           June 21, 2007


  Utilizing Interactive Connectivity Establishment (ICE) for the Host
                        Identity Protocol (HIP)
                    draft-tschofenig-hip-ice-00.txt

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   This Internet-Draft will expire on December 23, 2007.

Copyright Notice

   Copyright (C) The IETF Trust (2007).

Abstract

   This document describes how the Interactive Connectivity
   Establishment (ICE) methodology can be used for the Host Identity
   Protocol (HIP) to determine whether end-to-end communication is
   possible.  ICE makes use of the Session Traversal Utilities for NAT
   (STUN) protocol in addition to mechanisms for checking connectivity
   between peers.  After running the ICE the two HIP end points will be



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   able to communicate directly or through a relay via Network Address
   Translators (NATs), Network Address and Port Translators (NAPTs) and
   firewalls .


Table of Contents

   1.  Introduction . . . . . . . . . . . . . . . . . . . . . . . . .  3
     1.1.  Gathering Candidate Addresses  . . . . . . . . . . . . . .  5
     1.2.  Connectivity Checks  . . . . . . . . . . . . . . . . . . .  5
     1.3.  Sorting Candidates . . . . . . . . . . . . . . . . . . . .  5
     1.4.  Frozen Candidates  . . . . . . . . . . . . . . . . . . . .  6
     1.5.  Security for Checks  . . . . . . . . . . . . . . . . . . .  6
     1.6.  Concluding HIP-ICE . . . . . . . . . . . . . . . . . . . .  6
   2.  Terminology  . . . . . . . . . . . . . . . . . . . . . . . . .  6
   3.  Design Choices . . . . . . . . . . . . . . . . . . . . . . . .  6
   4.  Sending the Initial Offer  . . . . . . . . . . . . . . . . . .  8
   5.  Receiving the Initial Offer  . . . . . . . . . . . . . . . . .  8
   6.  Receiving the Initial Offer  . . . . . . . . . . . . . . . . .  8
   7.  Concluding ICE Processing  . . . . . . . . . . . . . . . . . .  9
   8.  Generating the Offer . . . . . . . . . . . . . . . . . . . . .  9
   9.  Keepalives . . . . . . . . . . . . . . . . . . . . . . . . . .  9
   10. Attribute Encoding . . . . . . . . . . . . . . . . . . . . . .  9
   11. Demultiplexing HIP and STUN  . . . . . . . . . . . . . . . . . 10
   12. Example  . . . . . . . . . . . . . . . . . . . . . . . . . . . 11
   13. Security Considerations  . . . . . . . . . . . . . . . . . . . 11
     13.1. Attacks on Connectivity Checks . . . . . . . . . . . . . . 11
     13.2. Attacks on Address Gathering . . . . . . . . . . . . . . . 14
     13.3. Attacks on the Offer/Answer Exchanges  . . . . . . . . . . 15
     13.4. Insider Attacks  . . . . . . . . . . . . . . . . . . . . . 15
       13.4.1.  HIP Amplification Attack  . . . . . . . . . . . . . . 15
       13.4.2.  STUN Amplification Attack . . . . . . . . . . . . . . 15
   14. IAB Considerations . . . . . . . . . . . . . . . . . . . . . . 16
     14.1. Problem Definition . . . . . . . . . . . . . . . . . . . . 16
     14.2. Exit Strategy  . . . . . . . . . . . . . . . . . . . . . . 17
     14.3. Brittleness Introduced by HIP-ICE  . . . . . . . . . . . . 17
     14.4. Requirements for a Long Term Solution  . . . . . . . . . . 18
     14.5. Issues with Existing NAPT Boxes  . . . . . . . . . . . . . 19
   15. Acknowledgments  . . . . . . . . . . . . . . . . . . . . . . . 19
   16. References . . . . . . . . . . . . . . . . . . . . . . . . . . 20
     16.1. Normative References . . . . . . . . . . . . . . . . . . . 20
     16.2. Informative References . . . . . . . . . . . . . . . . . . 20
   Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 21
   Intellectual Property and Copyright Statements . . . . . . . . . . 22







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1.  Introduction

   This document describes how the Interactive Connectivity
   Establishment (ICE [I-D.ietf-mmusic-ice]) methodology can be used for
   the Host Identity Protocol (HIP) to determine whether end-to-end
   communication is possible.  We call this usage HIP-ICE.  ICE makes
   use of the Session Traversal Utilities for NAT (STUN [RFC3489],
   STUNbis [I-D.ietf-behave-rfc3489bis]) protocol to learn candidate
   transport addresses and to verify connectivity between peers.  After
   running the ICE methodology, the two HIP end points will be able to
   communicate as directly possible -- directly with each other, through
   a HIP relay, via Network Address Translators (NATs), Network Address
   and Port Translators (NAPTs), and firewalls.

   In a typical deployment, there are two endpoints, Agent L and Agent
   R, which want to communicate.  These two agents play the role of HIP
   initiators and HIP responders.  They are able to communicate
   indirectly via a combination of HIP and traversal via a HIP
   rendezvous server.

   At the beginning of the HIP-ICE process, the end points are ignorant
   of their own topologies.  They might or might not be behind a NAT (or
   multiple tiers of NATs) and might be behind firewalls that limit the
   ability to communicate in different ways between the end points.
   HIP-ICE allows these end points to discover enough information about
   their topologies to potentially find one or more paths by which they
   can communicate.

   Figure 1 shows a typical environment for HIP-ICE deployment.  The two
   end points are labelled L and R (for left and right).  Both L and R
   are behind their own respective NATs or firewalls though they may not
   be aware of it.  The type of NAT or firewall and their properties are
   also unknown.  L and R are capable of engaging in an end-to-end
   protocol exchange with the help of HIP rendezvous servers.

      If desired, TURN [I-D.ietf-behave-turn] could be used for relaying
      IPsec traffic rather than relying on dedicated HIP rendezvous
      servers.  This would allow HIP DNS [I-D.ietf-hip-dns] to be used.
      This aspect is for further study.

   In Figure 1 the STUN server is co-located with the HIP rendezvous
   servers for editorial reasons.  From a solution point of view this
   is, however, not necessary.








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                                    HIP
              +----------+       Signalling       +----------+
              |Rendezvous|<---------------------->|Rendezvous|
              |Server A  |                        |Server B  |
              +----------+                        +----------+
                  ^                                     ^
                  |                                     |
                  |                                     |
                  |                                     |
        HIP       |                                     |HIP
        Signalling|                                     |Signalling
                  |                                     |
                  |                                     |
              +---v----+   Optimal HIP/IPsec path   +---v----+
              | FW/NAT |<-------------------------->| FW/NAT |
              +---^----+                            +---^----+
                  |                                     |
                  |                                     |
                  v                                     v
              +--------+                            +--------+
              | Agent  |                            |  Agent |
              |   L    |                            |    R   |
              +--------+                            +--------+

                            Figure 1: Overview

   The basic idea behind HIP-ICE is as follows: each end point has a
   variety of candidate TRANSPORT ADDRESSES (combination of IP address,
   transport protocol (UDP), and port) it could use to communicate with
   the other end point.

      To avoid unnecessary UDP encapsulation of IPsec traffic, it is
      also possible to consider using IP addresses rather than focusing
      exclusively on TRANSPORT ADDRESSES.  For example, two HIP hosts
      behind the same NAT do not need to use UDP encapsulation.  As
      another example, a HIP host behind a HIP-friendly NAT or HIP-
      friendly firewall does not need UDP encapsulation.  The need and
      desire for this functionality will be analysed in a future version
      of this document.

   Potentially, any of L's candidate transport addresses can be used to
   communicate with any of R's candidate transport addresses.  In
   practice, however, many combinations do not work.  For instance, if L
   and R are both behind NATs, their directly attached interface
   addresses (e.g., 192.168.1.100) are unlikely to be able to
   communicate.  The purpose of HIP-ICE is to discover which pairs of
   addresses will work.  The way that HIP-ICE does this is to
   systematically try all possible pairs (in a carefully sorted order)



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   until it finds one or more that works.  Once found, the best pair is
   used for subsequent communication between the hosts.

1.1.  Gathering Candidate Addresses

   In order to execute ICE, an agent has to identify all of its address
   candidates.  A CANDIDATE is a transport address - a combination of IP
   address and port for a particular transport protocol.

   This document uses three types of candidates:

   1.  One viable candidate is a transport address obtained directly
       from a local interface.  Such a candidate is called a HOST
       CANDIDATE.
   2.  Translated addresses on the public side of a NAT (called SERVER
       REFLEXIVE CANDIDATES).  This address is obtained via STUN.
   3.  Addresses obtained via relaying traffic through the rendezvous
       server, called RELAYED CANDIDATES.

1.2.  Connectivity Checks

   Once L has gathered all of its candidates, it orders them in highest
   to lowest priority and sends them to R over the signalling channel.
   We refer to the signaling channel to the end-to-end HIP exchange.
   The extension to exchange candidates can be found in Section 10.

   When R receives the L's HIP I2 message, R performs the same candidate
   gathering process and responds with its own list of candidates.  At
   the end of this process, each agent has a complete list of both its
   candidates and its peer's candidates.  It pairs them up, resulting in
   CANDIDATE PAIRS.  To see which pairs work, each agent schedules a
   series of connectivity CHECKS.  Each check is a STUN transaction that
   the client will perform on a particular candidate pair by sending a
   STUN request from the local candidate to the remote candidate; a
   response indicates there is connectivity to the peer using that
   candidate address.

   It is important to note that the STUN requests are sent to and from
   the exact same IP addresses and ports that will be used for
   subsequent data traffic.

1.3.  Sorting Candidates

   Because the algorithm above searches all candidate pairs, if a
   working pair exists it will eventually find it no matter what order
   the candidates are tried in.  In order to produce faster (and better)
   results, the candidates are sorted in a specified order.  The
   resulting list of sorted candidate pairs is called the CHECK LIST.



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1.4.  Frozen Candidates

   The concept of frozen candidates is not applicable when ICE is
   applied to HIP.

1.5.  Security for Checks

   Because the ICE algorithm is used to discover which addresses can be
   used to send traffic between two end points, it is important to
   ensure that the process cannot be hijacked to send traffic to the
   wrong location.  Each STUN connectivity check is covered by a message
   authentication code (MAC) generated based on passwords exchanged via
   the HIP exchange.  This MAC provides message integrity and data
   origin authentication, thus stopping an attacker from forging or
   modifying connectivity check messages.

1.6.  Concluding HIP-ICE

   ICE checks are performed in a specific sequence, so that high
   priority candidate pairs are checked first, followed by lower
   priority ones.


2.  Terminology

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
   document are to be interpreted as described in RFC 2119 [RFC2119].

   This document heavily relies on the terminology introduced in
   [I-D.ietf-mmusic-ice].


3.  Design Choices

   The work in this document is guided by the following design choices,
   namely:

   o  The offer/answer exchange described in ICE [I-D.ietf-mmusic-ice]
      is mapped to the end-to-end HIP exchange.  An initial offer is
      carried inside the I2 message and an initial answer is carried
      inside the R2 message.  A later offer/answer exchange is conveyed
      in a NOTIFY packet (inside the NOTIFICATION parameter).  When
      Network Address Translators and firewalls are located along the
      path then direct end-to-end communication between the two end
      point is typically not possible and hence this protocol
      interaction is provided via HIP rendezvous nodes.




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   o  We assume that HIP initiators and HIP responders implement and use
      STUN.  For performing connectivity checks a couple of other
      alternatives are, however, possible:

         It would be possible to utilize REAP
         [I-D.ietf-shim6-failure-detection] but STUN provides the same
         support with a more likely chance for widespread deployment.
         REAP currently only provides IPv6 support.
         Custom HIP messages could be created as described in
         [I-D.ietf-hip-nat-traversal].
         HIP multi-homing and mobility support [I-D.ietf-hip-mm]
         provides a basic support for address checks.

   o  If one peer does not support STUN then the optimal results of HIP-
      ICE cannot be provided.  There is, however, the ability to make
      use of STUN LITE when a host is on the public address space and
      not behind a firewall.
   o  Obtaining Relay Addresses from STUN [I-D.ietf-behave-turn],
      formerly known as TURN, is intentionally not used.  For HIP, a HIP
      rendezvous server reverse tunneling functionality is used instead
      of TURN.
   o  This document makes use of the UDP-encapsulated of HIP packets, as
      specified in [I-D.ietf-hip-nat-traversal].
   o  This document focuses only on the data exchange between the two
      end points rather than on the communication between a HIP end
      point and a rendezvous server or on the ability to allow HIP
      signaling messages to traverse NATs and firewalls.
   o  Each STUN connectivity check is covered by a message
      authentication code (MAC) generated based on passwords exchanged
      via the HIP exchange.  This is similar to how ICE exchanges
      username and password fragments in the offer/answer exchange.

         Alternatively, it is also possible to generate keying material
         for the message authentication code used in STUN based on the
         key derivation procedure described in Section 6.5 of
         [I-D.ietf-hip-base].

   Note that the ICE description assumes usage within a VoIP environment
   where individual flows are controlled.  However, the protocol
   interaction described in this document operates at a lower layer
   where application specific message flows are not visible.  When a
   CANDIDATE PAIR, consisting of two TRANSPORT ADDRESSES, is created
   then it will typically refer to multiple flows then traffic between
   two end points experiences UDP encapsulation (due to the need to
   traverse a NAT or a stateful packet filtering firewall).

   The descriptions in the ICE specification related to SIP, ANAT, RTP,
   RTCP, third party call control, preconditions, forking, etc. are not



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   applicable to HIP and are not included in this document.

      From an editorial point of view it would be possible to copy-and-
      paste relevant parts of the ICE specification and to remove VoIP
      specific descriptions but for this version of the document we did
      not follow this approach.


   The main accomplishment of this document is the reuse of the well-
   established ICE specification that builds on STUN.  STUN enjoys
   widespread implementation support and maximum code re-use was one of
   the design criteria for this document.


4.  Sending the Initial Offer

   In order to send the initial offer in an offer/answer exchange, an
   agent must (1) gather candidates, (2) prioritize them, (3) choose
   default candidates, and then (4) formulate and send them to the other
   peer.

   Section 4 of ICE [I-D.ietf-mmusic-ice] is applicable to this document
   with the following two exceptions: First, TURN is not used in this
   document but instead the same functionality is accomplished via the
   HIP rendezvous mechanism.  Second, the description regarding encoding
   of candidates in SDP is not applicable and replaced by a HIP specific
   encoding described in Section 10.


5.  Receiving the Initial Offer

   When an agent receives an initial offer, it will check if the offerer
   supports sufficient ICE functionality to proceed (i.e., if both
   offerer and answerer are lite implementations, ICE cannot proceed),
   determine its own role, gather candidates, prioritize them, choose
   default candidates, encode and send an answer, and for full
   implementations, form the check lists and begin connectivity checks.

   Again, the description regarding encoding of candidates in SDP is not
   applicable to this document and is replaced by a HIP specific
   encoding described in Section 10.  Note that only the encoding is
   different but not the semantic.  As such, the description in Section
   5 of [I-D.ietf-mmusic-ice] is applicable to this document.


6.  Receiving the Initial Offer

   Section 6 of ICE [I-D.ietf-mmusic-ice] describes the procedures that



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   an agent follows when it receives the answer from the peer.  It
   verifies that its peer supports ICE, determines its role, and for
   full implementations, forms the check list and begins performing
   periodic checks.


7.  Concluding ICE Processing

   The description in Section of ICE [I-D.ietf-mmusic-ice] illustrates
   processing rules that apply only to full implementations.  Concluding
   ICE involves nominating pairs by the controlling agent and updating
   of state machinery


8.  Generating the Offer

   Either agent may generate a subsequent offer at any time.  The rules
   in Section 8 of ICE [I-D.ietf-mmusic-ice] will cause the controlling
   agent to send an updated offer at the conclusion of ICE processing
   when ICE has selected different candidate pairs from the default
   pairs.  Section 9 of ICE [I-D.ietf-mmusic-ice] defines rules for
   construction of subsequent offers and answers.

   Note that the term "media stream" in Section 9 of ICE
   [I-D.ietf-mmusic-ice] translates to an individual UDP-encapsulated
   data flow exchanged between the two HIP peers.


9.  Keepalives

   Section 10 of ICE [I-D.ietf-mmusic-ice] describes a keepalive
   mechanism.  The RTP description, such as RTP No-Op and RTP comfort
   noise, is not applicable to this document.  Other useful keepalive
   techniques are described in [I-D.marjou-behave-app-rtp-keepalive] and
   may be useful for HIP; a recommendation will be made in a subsequent
   version of this document.


10.  Attribute Encoding

   This specification reuses seven SDP attributes, the "candidate",
   "remote-candidates", "ice-lite", "ice-mismatch", "ice- ufrag", "ice-
   pwd" and "ice-options" attributes, and places them in a single HIP
   TLVs.

   For this version of the document we decided to utilize the same gamma
   as used in Section 15 of ICE [I-D.ietf-mmusic-ice].




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   All HIP TLV parameters have a length (including Type and Length
   fields) which is a multiple of 8 bytes.  When needed, padding MUST be
   added to the end of the parameter so that the total length becomes a
   multiple of 8 bytes.

   Section 15.1 to Section 15.5 of ICE [I-D.ietf-mmusic-ice] describe
   the content of the attributes.


       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            |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      ~ ICE attribute                                                 ~
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

      Type           TBD
      Length         variable
      ICE attribute  Attributes specified in ICE

                       Figure 2: Attribute Encoding

   This TLV MUST be protected by the ENCRYPTED TLV.


11.  Demultiplexing HIP and STUN

   When HIP and STUN are run over the same port it is necessary to
   demultiplex them.

   A STUN packet always has the fixed value 0x2112A442 in its Magic
   Cookie field (bits 32-64 from the beginning of the UDP payload):

       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
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |0 0|     STUN Message Type     |         Message Length        |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |0 0 1 0 0 0 0 1 0 0 0 1 0 0 1 0 1 0 1 0 0 1 0 0 0 1 0 0 0 0 1 0|
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
                                    . . .

                           Figure 3: STUN Header







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   In this same offset from the UDP header, the HIP header has its
   Checksum field and its Controls field.  The Controls field currently
   only defines its last bit (bit 64); the rest of the bits in the
   Control field are for extensions and must be 0.

       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
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      | Next Header   | Header Length |0| Packet Type |  VER. | RES.|1|
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |          Checksum             |           Controls            |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
                                    . . .

                           Figure 4: HIP Header

   Bit 16 of STUN's magic cookie is 1.  To ensure HIP and STUN can
   always be demultiplexed, HIP should require the first bit in the same
   position always be 0.  That bit is first bit of the HIP Controls
   field, which is currently undefined and currently must be zero
   [I-D.ietf-hip-base].


12.  Example

   [Editor's Note: We will provide a message flow in a future version of
   this document.]


13.  Security Considerations

   There are several types of attacks possible in an HIP-ICE system.
   This section considers these attacks and their countermeasures.

13.1.  Attacks on Connectivity Checks

   An attacker might attempt to disrupt the STUN connectivity checks.
   Ultimately, all of these attacks fool an agent into thinking
   something incorrect about the results of the connectivity checks.
   The possible false conclusions an attacker can try and cause are:

   False Invalid:

      An attacker can fool a pair of agents into thinking a candidate
      pair is invalid, when it isn't.  This can be used to cause an
      agent to prefer a different candidate (such as one injected by the
      attacker), or to disrupt a call by forcing all candidates to fail.




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   False Valid:

      An attacker can fool a pair of agents into thinking a candidate
      pair is valid, when it isn't.  This can cause an agent to proceed
      with a session, but then not be able to receive any data traffic.

   False Peer-Reflexive Candidate:

      An attacker can cause an agent to discover a new peer reflexive
      candidate, when it shouldn't have.  This can be used to redirect
      data traffic to a DoS target or to the attacker, for eavesdropping
      or other purposes.

   False Valid on False Candidate:

      An attacker has already convinced an agent that there is a
      candidate with an address that doesn't actually route to that
      agent (for example, by injecting a false peer reflexive candidate
      or false server reflexive candidate).  It must then launch an
      attack that forces the agents to believe that this candidate is
      valid.

   Of the various techniques for creating faked STUN messages described
   in [I-D.ietf-behave-rfc3489bis], many are not applicable for the
   connectivity checks.  Compromises of STUN servers are not much of a
   concern, since the STUN servers are embedded in endpoints and
   distributed throughout the network.  Thus, compromising the peer's
   embedded STUN server is equivalent to compromising the end point, and
   if that happens, far more problematic attacks are possible than those
   against ICE.  When a rendezvous server that also operators a STUN
   server is compromised then various attacks are possible since the
   rendezvous server maintains also mappings between identifiers and
   locators.

   Injection of fake responses and relaying modified requests all can be
   handled in ICE with the countermeasures discussed below.

   To force the false invalid result, the attacker has to wait for the
   connectivity check from one of the agents to be sent.  When it is,
   the attacker needs to inject a fake response with an unrecoverable
   error response, such as a 600.  However, since the candidate is, in
   fact, valid, the original request may reach the peer agent, and
   result in a success response.  The attacker needs to force this
   packet or its response to be dropped, through a DoS attack, layer 2
   network disruption, or other technique.  If it doesn't do this, the
   success response will also reach the originator, alerting it to a
   possible attack.  Fortunately, this attack is mitigated completely
   through the STUN message integrity mechanism.  The attacker needs to



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   inject a fake response, and in order for this response to be
   processed, the attacker needs the password.  If the candidates are
   exchange in HIP messages and therefore secured, the attacker will not
   have the password.

   Forcing the fake valid result works in a similar way.  The agent
   needs to wait for the Binding Request from each agent, and inject a
   fake success response.  The attacker won't need to worry about
   disrupting the actual response since, if the candidate is not valid,
   it presumably wouldn't be received anyway.  However, like the fake
   invalid attack, this attack is mitigated completely through the STUN
   message integrity and offer/answer security techniques.

   Forcing the false peer reflexive candidate result can be done either
   with fake requests or responses, or with replays.  We consider the
   fake requests and responses case first.  It requires the attacker to
   send a Binding Request to one agent with a source IP address and port
   for the false candidate.  In addition, the attacker must wait for a
   Binding Request from the other agent, and generate a fake response
   with a XOR-MAPPED-ADDRESS attribute containing the false candidate.
   Like the other attacks described here, this attack is mitigated by
   the STUN message integrity mechanisms and secure offer/answer
   exchanges.

   Forcing the false peer reflexive candidate result with packet replays
   is different.  The attacker waits until one of the agents sends a
   check.  It intercepts this request, and replays it towards the other
   agent with a faked source IP address.  It must also prevent the
   original request from reaching the remote agent, either by launching
   a DoS attack to cause the packet to be dropped, or forcing it to be
   dropped using layer 2 mechanisms.  The replayed packet is received at
   the other agent, and accepted, since the integrity check passes (the
   integrity check cannot and does not cover the source IP address and
   port).  It is then responded to.  This response will contain a XOR-
   MAPPED-ADDRESS with the false candidate, and will be sent to that
   false candidate.  The attacker must then receive it and relay it
   towards the originator.

   The other agent will then initiate a connectivity check towards that
   false candidate.  This validation needs to succeed.  This requires
   the attacker to force a false valid on a false candidate.  Injecting
   of fake requests or responses to achieve this goal is prevented using
   the integrity mechanisms of STUN and the offer/answer exchange.
   Thus, this attack can only be launched through replays.  To do that,
   the attacker must intercept the check towards this false candidate,
   and replay it towards the other agent.  Then, it must intercept the
   response and replay that back as well.




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   This attack is very hard to launch unless the attacker is identified
   by the fake candidate.  This is because it requires the attacker to
   intercept and replay packets sent by two different hosts.  If both
   agents are on different networks (for example, across the public
   Internet), this attack can be hard to coordinate, since it needs to
   occur against two different endpoints on different parts of the
   network at the same time.

   If the attacker them self is identified by the fake candidate the
   attack is easier to coordinate.  However, since HIP utilizes IPsec
   ESP to protect the data traffic end-to-end, the attacker will not be
   able to inspect any application data, they will only be able to
   discard them.  However, this attack requires the agent to disrupt
   packets in order to block the connectivity check from reaching the
   target.  In that case, if the goal is to disrupt the end-to-end
   communication, its much easier to just disrupt it with the same
   mechanism, rather than attack ICE.

13.2.  Attacks on Address Gathering

   ICE endpoints make use of STUN for gathering candidates from a STUN
   server in the network.  This is corresponds to the Binding Discovery
   usage of STUN described in [I-D.ietf-behave-rfc3489bis].  As a
   consequence, the attacks against STUN itself that are described in
   that specification can still be used against the binding discovery
   usage when utilized with ICE.

   However, the additional mechanisms provided by ICE actually
   counteract such attacks, making binding discovery with STUN more
   secure when combined with ICE.

   Consider an attacker which is able to provide an agent with a faked
   mapped address in a STUN Binding Request that is used for address
   gathering.  This is the primary attack primitive described in
   [I-D.ietf-behave-rfc3489bis].  This address will be used as a server
   reflexive candidate in the ICE exchange.  For this candidate to
   actually be used for media, the attacker must also attack the
   connectivity checks, and in particular, force a false valid on a
   false candidate.  This attack is very hard to launch if the false
   address identifies a fourth party (neither the offerer, answerer, or
   attacker), since it requires attacking the checks generated by each
   agent in the session.

   If the attacker elects not to attack the connectivity checks, the
   worst it can do is prevent the server reflexive candidate from being
   used.  However, if the peer agent has at least one candidate that is
   reachable by the agent under attack, the STUN connectivity checks
   themselves will provide a peer reflexive candidate that can be used



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   for the exchange of media.  Peer reflexive candidates are generally
   preferred over server reflexive candidates.  As such, an attack
   solely on the STUN address gathering will normally have no impact on
   a session at all.

13.3.  Attacks on the Offer/Answer Exchanges

   An attacker that can modify or disrupt the offer/answer exchanges
   themselves can readily launch a variety of attacks with HIP-ICE.
   They could direct data traffic to a target of a DoS attack, they
   could insert themselves into the data exchange, and so on.  The
   security considerations of HIP [I-D.ietf-hip-base] apply.

13.4.  Insider Attacks

   In addition to attacks where the attacker is a third party trying to
   insert fake offers, answers or STUN messages, there are several
   attacks possible with ICE when the attacker is an authenticated and
   valid participant in the HIP-ICE exchange.

13.4.1.  HIP Amplification Attack

   In this attack, the attacker initiates communication to other agents,
   and maliciously includes the IP address and port of a DoS target as
   the destination for data traffic signaled in the HIP exchange.

   This could causes substantial amplification; a single offer/answer
   exchange can create a continuing flood of data packets, possibly at
   high rates (consider video sources).  This attack is not specific to
   ICE, but ICE can help provide remediation.

   Specifically, if ICE is used, the agent receiving the malicious SDP
   will first perform connectivity checks to the target of media before
   sending media there.  If this target is a third party host, the
   checks will not succeed, and media is never sent.

   Unfortunately, ICE doesn't help if its not used, in which case an
   attacker could simply send the offer without the ICE parameters.
   However, in environments where the set of clients are known, and
   limited to ones that support ICE, the server can reject any offers or
   answers that don't indicate ICE support.

13.4.2.  STUN Amplification Attack

   The STUN amplification attack is similar to the HIP amplification
   attack.  However, instead of data packets being directed to the
   target, STUN connectivity checks are directed to the target.  The
   attacker sends an offer with a large number of candidates, say 50.



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   The answerer receives the offer, and starts its checks, which are
   directed at the target, and consequently, never generate a response.
   The answerer will start a new connectivity check every 20ms, and each
   check is a STUN transaction consisting of 7 transmissions of a
   message 65 bytes in length (plus 28 bytes for the IP/UDP header) that
   runs for 7.9 seconds, for a total of 58 bytes/second per transaction
   on average.  In the worst case, there can be 395 transactions in
   progress at once (7.9 seconds divided by 20ms), for a total of 182
   kbps, just for STUN requests.

   It is impossible to eliminate the amplification, but the volume can
   be reduced through a variety of heuristics.  Agents SHOULD limit the
   total number of connectivity checks they perform to 100.
   Additionally, agents MAY limit the number of candidates they'll
   accept in an offer or answer.


14.  IAB Considerations

   The IAB has studied the problem of "Unilateral Self Address Fixing",
   which is the general process by which a agent attempts to determine
   its address in another realm on the other side of a NAT through a
   collaborative protocol reflection mechanism [RFC3424].  HIP-ICE is an
   example of a protocol that performs this type of function.
   Interestingly, the process for HIP-ICE is not unilateral, but
   bilateral, and the difference has a significant impact on the issues
   raised by IAB.  HIP-ICE can be considered a B-SAF (Bilateral Self-
   Address Fixing) protocol, rather than an UNSAF protocol.  Regardless,
   the IAB has mandated that any protocols developed for this purpose
   document a specific set of considerations.  This section meets those
   requirements.

14.1.  Problem Definition

   From RFC 3424 [RFC3424] any UNSAF proposal must provide:

   Precise definition of a specific, limited-scope problem that is to be
   solved with the UNSAF proposal.  A short term fix should not be
   generalized to solve other problems; this is why "short term fixes
   usually aren't".

   The specific problems being solved by HIP-ICE are:

   Provide a means for two peers to determine the set of transport
   addresses which can be used for communication.

   Provide a means for resolving many of the limitations of other UNSAF
   mechanisms by wrapping them in an additional layer of processing (the



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   HIP-ICE methodology).

   Provide a means for a agent to determine an address that is reachable
   by another peer with which it wishes to communicate.

14.2.  Exit Strategy

   From RFC 3424, any UNSAF proposal must provide:

   Description of an exit strategy/transition plan.  The better short
   term fixes are the ones that will naturally see less and less use as
   the appropriate technology is deployed.

   HIP-ICE itself doesn't easily get phased out.  However, it is useful
   even in a globally connected Internet, to serve as a means for
   detecting whether communication paths are disrupted.  HIP-ICE also
   helps prevent certain security attacks which have nothing to do with
   NAT.  However, what HIP-ICE does is help phase out other UNSAF
   mechanisms.  HIP-ICE effectively selects amongst those mechanisms,
   prioritizing ones that are better, and deprioritizing ones that are
   worse.  Local IPv6 addresses can be preferred.  As NATs begin to
   dissipate as IPv6 is introduced, server reflexive and relayed
   candidates (both forms of UNSAF mechanisms) simply never get used,
   because higher priority connectivity exists to the native host
   candidates.  Therefore, the servers get used less and less, and can
   eventually be remove when their usage goes to zero.

   Indeed, HIP-ICE can assist in the transition from IPv4 to IPv6.  It
   can be used to determine whether to use IPv6 or IPv4 when two dual-
   stack hosts communicate.  It can also allow a network with both 6to4
   and native v6 connectivity to determine which address to use when
   communicating with a peer.

14.3.  Brittleness Introduced by HIP-ICE

   From RFC3424, any UNSAF proposal must provide:

   Discussion of specific issues that may render systems more "brittle".
   For example, approaches that involve using data at multiple network
   layers create more dependencies, increase debugging challenges, and
   make it harder to transition.

   HIP-ICE uses ICE that is utilizes [I-D.ietf-behave-rfc3489bis]
   instead of traditional STUN, RFC 3489 [RFC3489]).  RFC 3489 has
   several points of brittleness.  One of them is the discovery process
   which requires a agent to try and classify the type of NAT it is
   behind.  This process is error-prone.  With HIP-ICE, that discovery
   process is simply not used.  Rather than unilaterally assessing the



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   validity of the address, its validity is dynamically determined by
   measuring connectivity to a peer.  The process of determining
   connectivity is very robust.

   Another point of brittleness in any other unilateral mechanism is its
   absolute reliance on an additional server on the public Internet,
   namely the rendezvous server.  ICE makes use of a server for
   allocating unilateral addresses, but allows agents to directly
   connect if possible.  Therefore, in some cases, the failure of a STUN
   server would still allow data packets to be exchanged in an end-to-
   end fashion when ICE is used.

   Another point of brittleness in traditional STUN is that it assumes
   that the STUN server is on the public Internet.  Interestingly, with
   HIP-ICE, that is not necessary.  There can be a multitude of STUN
   servers in a variety of address realms.  ICE will discover the one
   that has provided a usable address.

   The most troubling point of brittleness in traditional STUN is that
   it does not work in all network topologies.  In cases where there is
   a shared NAT between each agent and the STUN server, traditional STUN
   may not work.  With ICE, that restriction is removed.

   Traditional STUN also introduces some security considerations.
   Fortunately, those security considerations are also mitigated by ICE.

   Consequently, ICE serves to repair the brittleness introduced in
   other UNSAF mechanisms, and does not introduce any additional
   brittleness into the system.

   With HIP-ICE rendezvous servers are used and they are assumed to be
   located on the public Internet to allow HIP to work.  Without the
   usage of TURN servers it is, however, not possible to allow HIP usage
   without rendezvous server and solely with HIP DNS [I-D.ietf-hip-dns]
   to work in presence of NATs and firewalls.

14.4.  Requirements for a Long Term Solution

   From RFC 3424, any UNSAF proposal must provide:

   Identify requirements for longer term, sound technical solutions --
   contribute to the process of finding the right longer term solution.

   HIP-ICE provides a long term solution by utilizing ICE concepts that
   have received a lot of peer review in the VoIP community and to apply
   them to HIP.  The only other possible long term solutions are (a) to
   get rid of middleboxes, such as NATs and firewalls or to (b) interact
   with them.  Regarding (b) extensions for STUN to allow the protocol



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   to be deployed on NATs and firewalls is currently being investigated
   in [I-D.wing-behave-nat-control-stun-usage].

14.5.  Issues with Existing NAPT Boxes

   From RFC 3424, any UNSAF proposal must provide:

   Discussion of the impact of the noted practical issues with existing,
   deployed NA[P]Ts and experience reports.

   A number of NAT boxes are now being deployed into the market which
   try and provide "generic" ALG functionality.  These generic ALGs hunt
   for IP addresses, either in text or binary form within a packet, and
   rewrite them if they match a binding.  This interferes with
   traditional STUN.  However, the update to STUN
   [I-D.ietf-behave-rfc3489bis] uses an encoding which hides these
   binary addresses from generic ALGs.

   Existing NAPT boxes have non-deterministic and typically short
   expiration times for UDP-based bindings.  This requires
   implementations to send periodic keepalives to maintain those
   bindings.  ICE uses a default of 15s, which is a very conservative
   estimate.  Eventually, over time, as NAT boxes become compliant to
   behave [RFC4787], this minimum keepalive will become deterministic
   and well-known, and the ICE timers can be adjusted.  Having a way to
   discover and control the minimum keepalive interval would be far
   better still.


15.  Acknowledgments

   The authors would like to thank Jonathan Rosenberg for his work on
   the ICE specification.  This document copy-and-pastes text from the
   ICE specification.

   We would also like to thank the authors of
   [I-D.ietf-hip-nat-traversal] for the discussions on the IETF HIP
   mailing list.  Note, however, that the approach described in this
   document is considerably different than the approach outlined in
   [I-D.ietf-hip-nat-traversal].

   Finally, we would like to thank Thomas Schreck for his help on
   discussions various aspects in this document.


16.  References





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16.1.  Normative References

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

   [I-D.ietf-mmusic-ice]
              Rosenberg, J., "Interactive Connectivity Establishment
              (ICE): A Protocol for Network Address  Translator (NAT)
              Traversal for Offer/Answer Protocols",
              draft-ietf-mmusic-ice-16 (work in progress), June 2007.

16.2.  Informative References

   [I-D.ietf-behave-turn]
              Rosenberg, J., "Obtaining Relay Addresses from Simple
              Traversal Underneath NAT (STUN)",
              draft-ietf-behave-turn-03 (work in progress), March 2007.

   [I-D.ietf-shim6-failure-detection]
              Arkko, J. and I. Beijnum, "Failure Detection and Locator
              Pair Exploration Protocol for IPv6  Multihoming",
              draft-ietf-shim6-failure-detection-07 (work in progress),
              December 2006.

   [I-D.ietf-hip-nat-traversal]
              Schmitt, V., "HIP Extensions for the Traversal of Network
              Address Translators", draft-ietf-hip-nat-traversal-01
              (work in progress), March 2007.

   [I-D.ietf-hip-mm]
              Henderson, T., "End-Host Mobility and Multihoming with the
              Host Identity Protocol", draft-ietf-hip-mm-05 (work in
              progress), March 2007.

   [I-D.ietf-hip-dns]
              Nikander, P. and J. Laganier, "Host Identity Protocol
              (HIP) Domain Name System (DNS) Extensions",
              draft-ietf-hip-dns-09 (work in progress), April 2007.

   [I-D.ietf-hip-base]
              Moskowitz, R., "Host Identity Protocol",
              draft-ietf-hip-base-08 (work in progress), June 2007.

   [RFC3489]  Rosenberg, J., Weinberger, J., Huitema, C., and R. Mahy,
              "STUN - Simple Traversal of User Datagram Protocol (UDP)
              Through Network Address Translators (NATs)", RFC 3489,
              March 2003.




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   [I-D.ietf-behave-rfc3489bis]
              Rosenberg, J., "Session Traversal Utilities for (NAT)
              (STUN)", draft-ietf-behave-rfc3489bis-06 (work in
              progress), March 2007.

   [RFC3424]  Daigle, L. and IAB, "IAB Considerations for UNilateral
              Self-Address Fixing (UNSAF) Across Network Address
              Translation", RFC 3424, November 2002.

   [I-D.wing-behave-nat-control-stun-usage]
              Wing, D. and J. Rosenberg, "Discovering, Querying, and
              Controlling Firewalls and NATs using STUN",
              draft-wing-behave-nat-control-stun-usage-02 (work in
              progress), June 2007.

   [I-D.marjou-behave-app-rtp-keepalive]
              Marjou, X., "Application Mechanism for maintaining alive
              the Network Address Translator  (NAT) mappings associated
              to RTP flows.", draft-marjou-behave-app-rtp-keepalive-01
              (work in progress), February 2007.

   [RFC4787]  Audet, F. and C. Jennings, "Network Address Translation
              (NAT) Behavioral Requirements for Unicast UDP", BCP 127,
              RFC 4787, January 2007.


Authors' Addresses

   Hannes Tschofenig
   Nokia Siemens Networks
   Otto-Hahn-Ring 6
   Munich, Bavaria  81739
   Germany

   Email: Hannes.Tschofenig@nsn.com
   URI:   http://www.tschofenig.com


   Dan Wing
   Cisco
   170 West Tasman Drive
   San Jose, CA  95134
   USA

   Email: dwing@cisco.com






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Full Copyright Statement

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