



Network Working Group                                         M. Bagnulo
Internet-Draft                                                      UC3M
Intended status: Informational                                 C. Paasch
Expires: September 28, 2015                                    UCLouvain
                                                                 F. Gont
                                                  SI6 Networks / UTN-FRH
                                                          O. Bonaventure
                                                               UCLouvain
                                                               C. Raiciu
                                                                     UPB
                                                          March 27, 2015


         Analysis of MPTCP residual threats and possible fixes
                      draft-ietf-mptcp-attacks-04

Abstract

   This documents performs an analysis of the residual threats for MPTCP
   and explores possible solutions to them.

Status of This Memo

   This Internet-Draft is submitted in full conformance with the
   provisions of BCP 78 and BCP 79.

   Internet-Drafts are working documents of the Internet Engineering
   Task Force (IETF).  Note that other groups may also distribute
   working documents as Internet-Drafts.  The list of current Internet-
   Drafts is at http://datatracker.ietf.org/drafts/current/.

   Internet-Drafts are draft documents valid for a maximum of six months
   and may be updated, replaced, or obsoleted by other documents at any
   time.  It is inappropriate to use Internet-Drafts as reference
   material or to cite them other than as "work in progress."

   This Internet-Draft will expire on September 28, 2015.

Copyright Notice

   Copyright (c) 2015 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

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



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   to this document.  Code Components extracted from this document must
   include Simplified BSD License text as described in Section 4.e of
   the Trust Legal Provisions and are provided without warranty as
   described in the Simplified BSD License.

Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   2
   2.  ADD_ADDR attack . . . . . . . . . . . . . . . . . . . . . . .   4
     2.1.  Possible security enhancements to prevent this attack . .  10
   3.  DoS attack on MP_JOIN . . . . . . . . . . . . . . . . . . . .  10
     3.1.  Possible security enhancements to prevent this attack . .  11
   4.  SYN flooding amplification  . . . . . . . . . . . . . . . . .  11
     4.1.  Possible security enhancements to prevent this attack . .  12
   5.  Eavesdropper in the initial handshake . . . . . . . . . . . .  12
     5.1.  Possible security enhancements to prevent this attack . .  13
   6.  SYN/JOIN attack . . . . . . . . . . . . . . . . . . . . . . .  13
     6.1.  Possible security enhancements to prevent this attack . .  14
   7.  Reccomendations . . . . . . . . . . . . . . . . . . . . . . .  14
     7.1.  Security enhancements for MPTCP . . . . . . . . . . . . .  15
   8.  Security considerations . . . . . . . . . . . . . . . . . . .  15
   9.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .  15
   10. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . .  15
   11. References  . . . . . . . . . . . . . . . . . . . . . . . . .  15
     11.1.  Normative References . . . . . . . . . . . . . . . . . .  16
     11.2.  Informative References . . . . . . . . . . . . . . . . .  16
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  17

1.  Introduction

   This document provides a complement to the threat analysis for
   Multipath TCP (MPTCP) [RFC6824] documented in RFC 6181 [RFC6181].
   RFC 6181 provided a threat analysis for the general solution space of
   extending TCP to operate with multiple IP addresses per connection.
   Its main goal was to leverage previous experience acquired during the
   design of other multi-address protocols, notably SHIM6 [RFC5533],
   SCTP [RFC4960] and MIPv6 [RFC6275] for designing MPTCP.  Thus, RFC
   6181 was produced before the actual MPTCP specification (RFC6824) was
   completed, and documented a set of reccommendations that were
   considered during the production of such specification.

   This document complements RFC 6181 with a vulnerability analysis of
   the specific mechanisms specified in RFC 6824.  The motivation for
   this analysis is to identify possible security issues with MPTCP as
   currently specified and propose security enhancements to address the
   identified security issues.





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   The goal of the security mechanisms defined in RFC 6824 were to make
   MPTCP no worse than currently available single-path TCP.  We believe
   that this goal is still valid, so we will perform our analysis on the
   same grounds.  This document describes all the threats identified
   that are specific to MPTCP (as defined in RFC6824) that are not
   possible with (single-path) TCP.  This means that threats that are
   common to TCP and MPTCP are not covered in this document.

   Types of attackers: for all attacks considered in this document, we
   identify the type of attacker.  We can classify the attackers based
   on their location as follows:

   o  Off-path attacker.  This is an attacker that does not need to be
      located in any of the paths of the MPTCP session at any point in
      time during the lifetime of the MPTCP session.  This means that
      the Off-path attacker cannot eavesdrop any of the packets of the
      MPTCP session.

   o  Partial time On-path attacker.  This is an attacker that needs to
      be in at least one of the paths during part but not during the
      entire lifetime of the MPTCP session.  The attacker can be in the
      forward and/or backward directions, for the initial subflow and/or
      other subflows.  The specific needs of the attacker will be made
      explicit in the attack description.

   o  On-path attacker.  This attacker needs to be on at least one of
      the paths during the whole duration of the MPTCP session.  The
      attacker can be in the forward and/or backward directions, for the
      initial subflow and/or other subflows.  The specific needs of the
      attacker will be made explicit in the attack description.

   We can also classify the attackers based on their actions as follows:

   o  Eavesdropper.  The attacker is able to capture some of the packets
      of the MPTCP session to perform the attack, but it is not capable
      of changing, discarding or delaying any packet of the MPTCP
      session.  The attacker can be in the forward and/or backward
      directions, for the initial subflow and/or other subflows.  The
      specific needs of the attacker will be made explicit in the attack
      description.

   o  Active attacker.  The attacker is able to change, discard or delay
      some of the packets of the MPTCP session.  The attacker can be in
      the forward and/or backward directions, for the initial subflow
      and/or other subflows.  The specific needs of the attacker will be
      made explicit in the attack description.





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   In this document, we consider the following possible combinations of
   attackers:

   o  an On-path eavesdropper

   o  an On-path active attacker

   o  an Off-path active attacker

   o  a Partial-time On-path eavesdropper

   o  a Partial-time On-path active attacker

   In the rest of the document we describe different attacks that are
   possible against the MPTCP protocol specified in RFC6824 and we
   propose possible security enhancements to address them.

2.  ADD_ADDR attack

   Summary of the attack:

      Type of attack: MPTCP session hijack enabling Man-in-the-Middle.

      Type of attacker: Off-path, active attacker.

   Description:

   In this attack, the attacker uses the ADD_ADDR option defined in
   RFC6824 to hijack an ongoing MPTCP session and enables himself to
   perform a Man-in-the-Middle attack on the MPTCP session.

   Consider the following scenario.  Host A with address IPA has one
   MPTCP session with Host B with address IPB.  The MPTCP subflow
   between IPA and IPB is using port PA on host A and port PB on host B.
   The tokens for the MPTCP session are TA and TB for Host A and Host B
   respectively.  Host C is the attacker.  It owns address IPC.  The
   attack is executed as follows:

   1.  Host C sends a forged packet with source address IPA, destination
       address IPB, source port PA and destination port PB.  The packet
       has the ACK flag set.  The TCP sequence number for the segment is
       i and the ACK sequence number is j.  We will assume all these are
       valid, we discuss what the attacker needs to figure these ones
       later on.  The packet contains the ADD_ADDR option.  The ADD_ADDR
       option announces IPC as an alternative address for the
       connection.  It also contains an eight bit address identifier
       which does not bring any strong security benefit.




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   2.  Host B receives the ADD_ADDR message and it replies by sending a
       TCP SYN packet.  (Note: the MPTCP specification states that the
       host receiving the ADD_ADDR option may initiate a new subflow.
       If the host is configured so that it does not initiate a new
       subflow the attack will not succeed.  For example, on the current
       Linux implementation, the server does not create subflows.  Only
       the client does so.)  The source address for the packet is IPB,
       the destination address for the packet is IPC, the source port is
       PB' and the destination port is PA' (It is not required that
       PA=PA' nor that PB=PB').  The sequence number for this packet is
       the new initial sequence number for this subflow.  The ACK
       sequence number is not relevant as the ACK flag is not set.  The
       packet carries an MP_JOIN option and it carries the token TA.  It
       also carries a random nonce generated by Host B called RB.

   3.  Host C receives the SYN+MP_JOIN packet from Host B, and it alters
       it in the following way.  It changes the source address to IPC
       and the destination address to IPA.  It sends the modified packet
       to Host A, impersonating Host B.

   4.  Host A receives the SYN+MP_JOIN message and it replies with a
       SYN/ACK+MP_JOIN message.  The packet has source address IPA and
       destination address IPC, as well as all the other needed
       parameters.  In particular, Host A computes a valid HMAC and
       places it in the MP_JOIN option.

   5.  Host C receives the SYN/ACK+MP_JOIN message and it changes the
       source address to IPC and the destination address to IPB.  It
       sends the modified packet to IPB impersonating Host A.

   6.  Host B receives the SYN/ACK+MP_JOIN message.  Host B verifies the
       HMAC of the MP_JOIN option and confirms its validity.  It replies
       with an ACK+MP_JOIN packet.  The packet has source address IPB
       and destination address IPC, as well as all the other needed
       parameters.  The returned MP_JOIN option contains a valid HMAC
       computed by Host B.

   7.  Host C receives the ACK+MP_JOIN message from B and it alters it
       in the following way.  It changes the source address to IPC and
       the destination address to IPA.  It sends the modified packet to
       Host A impersonating Host B.

   8.  Host A receives the ACK+MP_JOIN message and creates the new
       subflow.

          At this point the attacker has managed to place itself as a
          MitM for one subflow for the existing MPTCP session.  It
          should be noted that there still exists the subflow between



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          address IPA and IPB that does not flow through the attacker,
          so the attacker has not completely intercepted all the packets
          in the communication (yet).  If the attacker wishes to
          completely intercept the MPTCP session it can do the following
          additional step.

   9.  Host C sends two TCP RST messages.  One TCP RST packet is sent to
       Host B, with source address IPA and destination address IPB and
       source and destination ports PA and PB, respectively.  The other
       TCP RST message is sent to Host A, with source address IPB and
       destination address IPA and source and destination ports PB and
       PA, respectively.  Both RST messages must contain a valid
       sequence number.  Note that figuring the sequence numbers to be
       used here for subflow A is the same difficulty as being able to
       send the initial ADD_ADDR option with valid Sequence number and
       ACK value.  If there are more subflows, then the attacker needs
       to find the Sequence Number and ACK for each subflow.

          At this point the attacker has managed to fully hijack the
          MPTCP session.

   Information required by the attacker to perform the described attack:

   In order to perform this attack the attacker needs to guess or know
   the following pieces of information: (The attacker need this
   information for one of the subflows belonging to the MPTCP session.)

   o  the four-tuple {Client-side IP Address, Client-side Port, Server-
      side Address, Servcer-side Port} that identifies the target TCP
      connection

   o  a valid sequence number for the subflow

   o  a valid ACK sequence number for the subflow

   o  a valid address identifier for IPC

   TCP connections are uniquely identified by the four-tuple {Source
   Address, Source Port, Destination Address, Destination Port}. Thus,
   in order to attack a TCP connection, an attacker needs to know or be
   able to guess each of the values in that four-tuple.  Assuming the
   two peers of the target TCP connection are known, the Source Address
   and the Destination Address can be assumed to be known.

      We note that in order to be able to successfully perform this
      attack, the attacker needs to be able to send packets with a
      forged source address.  This means that the attacker cannot be
      located in a network where techniques like ingress filtering



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      [RFC2827]  or source address validation [RFC7039] are deployed.
      However, ingress filtering is not as widely implemented as one
      would expect, and hence cannot be relied upon as a mitigation for
      this kind of attack.

   Assuming the attacker knows the application protocol for which the
   TCP connection is being employed, the server-side port can also be
   assumed to be known.  Finally, the client-side port will generally
   not be known, and will need to be guessed by the attacker.  The
   chances of an attacker guessing the client-side port will depend on
   the ephemeral port range employed by the client, and whether the
   client implements port randomization [RFC6056].

   Assuming TCP sequence number randomization is in place (see e.g.
   [RFC6528]), an attacker would have to blindly guess a valid TCP
   sequence number.  That is,

      RCV.NXT =< SEG.SEQ < RCV.NXT+RCV.WND or RCV.NXT =<
      SEG.SEQ+SEG.LEN-1 < RCV.NXT+RCV.WND

   As a result, the chances of an attacker to succeed will depend on the
   TCP receive window size at the target TCP peer.

      We note that automatic TCP buffer tuning mechanisms have been
      become common for popular TCP implementations, and hence very
      large TCP window sizes of values up to 2 MB could end up being
      employed by such TCP implementations.

   According to [RFC0793], the Acknowledgement Number is considered
   valid as long as it does not acknowledge the receipt of data that has
   not yet been sent.  That is, the following expression must be true:

      SEG.ACK <= SND.NXT

   However, for implementations that support [RFC5961], the following
   (stricter) validation check is enforced:

      SND.UNA - SND.MAX.WND <= SEG.ACK <= SND.NXT

   Finally, in order for the address identifier to be valid, the only
   requirement is that it needs to be different than the ones already
   being used by Host A in that MPTCP session, so a random identifier is
   likely to work.

   Given that a large number of factors affect the chances of an
   attacker of successfully performing the aforementioned off-path
   attacks, we provide two general expressions for the expected number
   of packets the attacker needs to send to succeed in the attack: one



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   for MPTCP implementations that support [RFC5961], and another for
   MPTCP implementations that do not.

   Implementations that do not support RFC 5961

   Packets = (2^32/(RCV_WND)) * 2 * EPH_PORT_SIZE/2 * 1/MSS

   Where the new :

   Packets:
      Maximum number of packets required to successfully perform an off-
      path (blind) attack.

   RCV_WND:
      TCP receive window size (RCV.WND) at the target node.

   EPH_PORT_SIZE:
      Number of ports comprising the ephemeral port range at the
      "client" system.

   MSS:
      Maximum Segment Size, assuming the attacker will send full
      segments to maximize the chances to get a hit.

   Notes:
      The value "2^32" represents the size of the TCP sequence number
      space.
      The value "2" accounts for 2 different ACK numbers (separated by
      2^31) that should be employed to make sure the ACK number is
      valid.

   The following table contains some sample results for the number of
   required packets, based on different values of RCV_WND and
   EPH_PORT_SIZE for a MSS of 1500 bytes.

          +-------------+---------+---------+--------+---------+
          | Ports \ Win |  16 KB  |  128 KB | 256 KB | 2048 KB |
          +-------------+---------+---------+--------+---------+
          |     4000    |  699050 |  87381  | 43690  |   5461  |
          +-------------+---------+---------+--------+---------+
          |    10000    | 1747626 |  218453 | 109226 |  13653  |
          +-------------+---------+---------+--------+---------+
          |    50000    | 8738133 | 1092266 | 546133 |  68266  |
          +-------------+---------+---------+--------+---------+

           Table 1: Max. Number of Packets for Successful Attack

   Implementations that do support RFC 5961



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   Packets = (2^32/(RCV_WND)) * (2^32/(2 * SND_MAX_WND)) *
   EPH_PORT_SIZE/2 * 1/MSS

   Where:

   Packets:
      Maximum number of packets required to successfully perform an off-
      path (blind) attack.

   RCV_WND:
      TCP receive window size (RCV.WND) at the target MPTCP endpoint.

   SND_MAX_WND:
      Maximum TCP send window size ever employed by the target MPTCP
      end-point (SND.MAX.WND).

   EPH_PORT_SIZE:
      Number of ports comprising the ephemeral port range at the
      "client" system.

   Notes:
      The value "2^32" represents the size of the TCP sequence number
      space.
      The parameter "SND_MAX_WND" is specified in [RFC5961].
      The value "2*SND_MAX_WND" results from the expresion "SND.NXT -
      SND.UNA - MAX.SND.WND", assuming that, for connections that
      perform bulk data transfers, "SND.NXT - SND.UNA == MAX.SND.WND".
      If an attacker targets a TCP endpoint that is not actively
      transferring data, "2 * SND_MAX_WND" would become "SND_MAX_WND"
      (and hence a successful attack would typically require more
      packets).

   The following table contains some sample results for the number of
   required packets, based on different values of RCV_WND, SND_MAX_WND,
   and EPH_PORT_SIZE.  For these implementations, only a limited number
   of sample results are provided, just as an indication of how
   [RFC5961] increases the difficulty of performing these attacks.

      +-------------+-------------+-----------+-----------+---------+
      | Ports \ Win |    16 KB    |   128 KB  |   256 KB  | 2048 KB |
      +-------------+-------------+-----------+-----------+---------+
      |     4000    | 45812984490 | 715827882 | 178956970 | 2796202 |
      +-------------+-------------+-----------+-----------+---------+

           Table 2: Max. Number of Packets for Successful Attack

   Note:




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      In the aforementioned table, all values are computed with RCV_WND
      equal to SND_MAX_WND.

2.1.  Possible security enhancements to prevent this attack

   1.  To include the token of the connection in the ADD_ADDR option.
       This would make it harder for the attacker to launch the attack,
       since he needs to either eavesdrop the token (so this can no
       longer be a blind attack) or to guess it, but a random 32 bit
       number is not so easy to guess.  However, this would imply that
       any eavesdropper that is able to see the token, would be able to
       launch this attack.  This solution then increases the
       vulnerability window against eavesdroppers from the initial 3-way
       handshake for the MPTCP session to any exchange of the ADD_ADDR
       messages.

   2.  To include the HMAC of the address contained in the ADD_ADDR
       option.  The key used for the HMAC is the concatenation of the
       key of the receiver and the key of the sender (in the same way
       they are used for generating the HMAC of the MP_JOIN message).
       This makes it much more secure, since it requires the attacker to
       have both keys (either by eavesdropping it in the first exchange
       or by guessing it).  Because this solution relies on the key used
       in the MPTCP session, the protection of this solution would
       increase if new key generation methods are defined for MPTCP
       (e.g. using SSL keys as has been proposed).

   3.  To include the destination address of the SYN packet in the HMAC
       of the MP_JOIN message.  As the attacker requires to change the
       destination address to perform the described attack, protecting
       it would prevent the attack.  It wouldn't allow hosts behind NATs
       to be reached by an address in the ADD_ADDR option, even with
       static NAT bindings (like a web server at home).

   Out of the options described above, option 2 is reccommended as it
   achieves a higher security level while preserving the required
   functionality (i.e.  NAT compatibility).

3.  DoS attack on MP_JOIN

   Summary of the attack:

      Type of attack: MPTCP Denial-of-Service attack, preventing the
      hosts from creating new subflows.

      Type of attacker: Off-path, active attacker

   Description:



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   As currently specified, the initial SYN+MP_JOIN message of the 3-way
   handshake for additional subflows creates state in the host receiving
   the message.  This is because the SYN+MP_JOIN contains the 32-bit
   token that allows the receiver to identify the MPTCP-session and the
   32-bit random nonce, used in the HMAC calculation.  As this
   information is not resent in the third ACK of the 3-way handshake, a
   host must create state upon reception of a SYN+MP_JOIN.

   Assume that there exists an MPTCP-session between host A and host B,
   with token Ta and Tb.  An attacker, sending a SYN+MP_JOIN to host B,
   with the valid token Tb, will trigger the creation of state on host
   B.  The number of these half-open connections a host can store per
   MPTCP-session is limited by a certain number, and it is
   implementation-dependent.  The attacker can simply exhaust this limit
   by sending multiple SYN+MP_JOINs with different 5-tuples.  The
   (possibly forged) source address of the attack packets will typically
   correspond to an address that is not in use, or else the SYN/ACK sent
   by Host B would elicit a RST from the impersonated node, thus
   removing the corresponding state at Host B.  Further discussion of
   traditional SYN-flood attacks and common mitigations can be found in
   [RFC4987]

   This effectively prevents the host A from sending any more
   SYN+MP_JOINs to host B, as the number of acceptable half-open
   connections per MPTCP-session on host B has been exhausted.

   The attacker needs to know the token Tb in order to perform the
   described attack.  This can be achieved if it is a Partial-time On-
   path eavesdropper , observing the 3-way handshake of the
   establishment of an additional subflow between host A and host B.  If
   the attacker is never on-path, it has to guess the 32-bit token.

3.1.  Possible security enhancements to prevent this attack

   The third packet of the 3-way handshake could be extended to contain
   also the 32-bit token and the random nonce that has been sent in the
   SYN+MP_JOIN.  Further, host B will have to generate its own random
   nonce in a reproducible fashion (e.g., a Hash of the 5-tuple +
   initial sequence-number + local secret).  This will allow host B to
   reply to a SYN+MP_JOIN without having to create state.  Upon the
   reception of the third ACK, host B can then verify the correctness of
   the HMAC and create the state.

4.  SYN flooding amplification

   Summary of the attack:





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      Type of attack: The attacker can use the SYN+MP_JOIN messages to
      amplify the SYN flooding attack.

      Type of attacker: Off-path, active attacker

   Description:

   SYN flooding attacks [RFC4987] use SYN messages to exhaust the
   server's resources and prevent new TCP connections.  A common
   mitigation is the use of SYN cookies [RFC4987] that allow the
   stateless processing of the initial SYN message.

   With MPTCP, the initial SYN can be processed in a stateless fashion
   using the aforementioned SYN cookies.  However, as we described in
   the previous section, as currently specified, the SYN+MP_JOIN
   messages are not processed in a stateless manner.  This opens a new
   attack vector.  The attacker can now open a MPTCP session by sending
   a regular SYN and creating the associated state but then send as many
   SYN+MP_JOIN messages as supported by the server with different source
   address source port combinations, consuming server's resources
   without having to create state in the attacker.  This is an
   amplification attack, where the cost on the attacker side is only the
   cost of the state associated with the initial SYN while the cost on
   the server side is the state for the initial SYN plus all the state
   associated to all the following SYN+MP_JOIN.

4.1.  Possible security enhancements to prevent this attack

   1.  The solution described for the previous DoS attack on MP_JOIN
       would also prevent this attack.

   2.  Limiting the number of half open subflows to a low number (e.g. 3
       subflows) would also limit the impact of this attack.

5.  Eavesdropper in the initial handshake

   Summary of the attack

      Type of attack: An eavesdropper present in the initial handshake
      where the keys are exchanged can hijack the MPTCP session at any
      time in the future.

      Type of attacker: a Partial-time On-path eavesdropper

   Description:

   In this case, the attacker is present along the path when the initial
   3-way handshake takes place, and therefore is able to learn the keys



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   used in the MPTCP session.  This allows the attacker to move away
   from the MPTCP session path and still be able to hijack the MPTCP
   session in the future.  This vulnerability was readily identified at
   the moment of the design of the MPTCP security solution and the
   threat was considered acceptable.

5.1.  Possible security enhancements to prevent this attack

   There are many techniques that can be used to prevent this attack and
   each of them represents different tradeoffs.  At this point, we limit
   ourselves to enumerate them and provide useful pointers.

   1.  Use of hash-chains.  The use of hash chains for MPTCP has been
       explored in [hash-chains]

   2.  Use of SSL keys for MPTCP security as described in
       [I-D.paasch-mptcp-ssl]

   3.  Use of Cryptographically-Generated Addresses (CGAs) for MPTCP
       security.  CGAs [RFC3972] have been used in the past to secure
       multi addressed protocols like SHIM6 [RFC5533].

   4.  Use of TCPCrypt [I-D.bittau-tcp-crypt]

   5.  Use DNSSEC.  DNSSEC has been proposed to secure the Mobile IP
       protocol [dnssec]

6.  SYN/JOIN attack

   Summary of the attack

      Type of attack: An attacker that can intercept the SYN/JOIN
      message can alter the source address being added.

      Type of attacker: a Partial-time On-path eavesdropper

   Description:

   The attacker is present along the path when the SYN/JOIN exchange
   takes place, and this allows the attacker to add any new address it
   wants to by simply substituting the source address of the SYN/JOIN
   packet for one it chooses.  This vulnerability was readily identified
   at the moment of the design of the MPTCP security solution and the
   threat was considered acceptable.







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6.1.  Possible security enhancements to prevent this attack

   It should be noted that this vulnerability is fundamental due to the
   NAT support requirement.  In other words, MPTCP must work through
   NATs in order to be deployable in the current Internet.  NAT behavior
   is unfortunately indistinguishable from this attack.  It is
   impossible to secure the source address, since doing so would prevent
   MPTCP to work though NATs.  This basically implies that the solution
   cannot rely on securing the address.  A more promising approach would
   be then to look into securing the payload exchanged, limiting the
   impact that the attack would have in the communication (e.g.
   TCPCrypt [I-D.bittau-tcp-crypt] or similar).

7.  Reccomendations

   Current MPTCP specification [RFC6824] is experimental.  There is an
   ongoing effort to move it to Standards track.  We believe that the
   work on MPTCP security should follow two threads:

   o  The work on improving MPTCP security so that is enough to become a
      Standard Track document.

   o  The work on analyzing possible additional security enhancements to
      provide a more secure version of MPTCP.

   We will expand on these two next.

   MPTCP security for a Standard Track specification.

   We believe that in order for MPTCP to progress to Standard Track, the
   ADD_ADDR attack must be addressed.  We believe that the solution that
   should be adopted in order to deal with this attack is to include an
   HMAC to the ADD_ADDR message (with the address being added used as
   input to the HMAC, as well as the key).  This would make the ADD_ADDR
   message as secure as the JOIN message.  In addition, this implies
   that if we implement a more secure way to create the key used in the
   MPTCP connection, then the security of both the MP_JOIN and the
   ADD_ADDR messages is automatically improved (since both use the same
   key in the HMAC).

   We believe that this is enough for MPTCP to progress as a Standard
   track document, because the security level is similar to single path
   TCP, as results from our previous analysis.  Moreover, the security
   level achieved with these changes is exactly the same as other
   Standard Track documents.  In particular, this would be the same
   security level as SCTP with dynamic addresses as defined in
   [RFC5061].  The Security Consideration section of RFC5061 (which is a
   Standard Track document) reads:



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      The addition and or deletion of an IP address to an existing
      association does provide an additional mechanism by which existing
      associations can be hijacked.  Therefore, this document requires
      the use of the authentication mechanism defined in [RFC4895] to
      limit the ability of an attacker to hijack an association.
      Hijacking an association by using the addition and deletion of an
      IP address is only possible for an attacker who is able to
      intercept the initial two packets of the association setup when
      the SCTP-AUTH extension is used without pre-shared keys.  If such
      a threat is considered a possibility, then the [RFC4895] extension
      must be used with a preconfigured shared endpoint pair key to
      mitigate this threat.

   This is the same security level that would be achieved by MPTCP plus
   the ADD_ADDR security measure reccommended.

7.1.  Security enhancements for MPTCP

   We also believe that is worthwhile exploring alternatives to secure
   MPTCP.  As we identified earlier, the problem is securing JOIN
   messages is fundamentally incompatible with NAT support, so it is
   likely that a solution to this problem involves the protection of the
   data itself.  Exploring the integration of MPTCP and approaches like
   TCPCrypt [I-D.bittau-tcp-crypt] or integration with SSL seem
   promising venues.

8.  Security considerations

   This whole document is about security considerations for MPTCP.

9.  IANA Considerations

   There are no IANA considerations in this memo.

10.  Acknowledgments

   We would like to thank Mark Handley for his comments on the attacks
   and countermeasures discussed in this document.  thanks to Alissa
   Copper, Phil Eardley, Yoshifumi Nishida, Barry Leiba, Stephen
   Farrell, Stefan Winter for the comments and review.  Marcelo Bagnulo,
   Christophe Paasch, Oliver Bonaventure and Costin Raiciu are partially
   funded by the EU Trilogy 2 project.

11.  References







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11.1.  Normative References

   [RFC0793]  Postel, J., "Transmission Control Protocol", STD 7, RFC
              793, September 1981.

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

   [RFC5961]  Ramaiah, A., Stewart, R., and M. Dalal, "Improving TCP's
              Robustness to Blind In-Window Attacks", RFC 5961, August
              2010.

   [RFC6056]  Larsen, M. and F. Gont, "Recommendations for Transport-
              Protocol Port Randomization", BCP 156, RFC 6056, January
              2011.

   [RFC6528]  Gont, F. and S. Bellovin, "Defending against Sequence
              Number Attacks", RFC 6528, February 2012.

   [RFC5061]  Stewart, R., Xie, Q., Tuexen, M., Maruyama, S., and M.
              Kozuka, "Stream Control Transmission Protocol (SCTP)
              Dynamic Address Reconfiguration", RFC 5061, September
              2007.

   [RFC4895]  Tuexen, M., Stewart, R., Lei, P., and E. Rescorla,
              "Authenticated Chunks for the Stream Control Transmission
              Protocol (SCTP)", RFC 4895, August 2007.

   [RFC6824]  Ford, A., Raiciu, C., Handley, M., and O. Bonaventure,
              "TCP Extensions for Multipath Operation with Multiple
              Addresses", RFC 6824, January 2013.

11.2.  Informative References

   [RFC6181]  Bagnulo, M., "Threat Analysis for TCP Extensions for
              Multipath Operation with Multiple Addresses", RFC 6181,
              March 2011.

   [RFC5533]  Nordmark, E. and M. Bagnulo, "Shim6: Level 3 Multihoming
              Shim Protocol for IPv6", RFC 5533, June 2009.

   [RFC4960]  Stewart, R., "Stream Control Transmission Protocol", RFC
              4960, September 2007.

   [RFC6275]  Perkins, C., Johnson, D., and J. Arkko, "Mobility Support
              in IPv6", RFC 6275, July 2011.





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   [RFC2827]  Ferguson, P. and D. Senie, "Network Ingress Filtering:
              Defeating Denial of Service Attacks which employ IP Source
              Address Spoofing", BCP 38, RFC 2827, May 2000.

   [RFC7039]  Wu, J., Bi, J., Bagnulo, M., Baker, F., and C. Vogt,
              "Source Address Validation Improvement (SAVI) Framework",
              RFC 7039, October 2013.

   [RFC4987]  Eddy, W., "TCP SYN Flooding Attacks and Common
              Mitigations", RFC 4987, August 2007.

   [I-D.paasch-mptcp-ssl]
              Paasch, C. and O. Bonaventure, "Securing the MultiPath TCP
              handshake with external keys", draft-paasch-mptcp-ssl-00
              (work in progress), October 2012.

   [I-D.bittau-tcp-crypt]
              Bittau, A., Boneh, D., Hamburg, M., Handley, M., Mazieres,
              D., and Q. Slack, "Cryptographic protection of TCP Streams
              (tcpcrypt)", draft-bittau-tcp-crypt-04 (work in progress),
              February 2014.

   [hash-chains]
              Diez, J., Bagnulo, M., Valera, F., and I. Vidal, "Security
              for multipath TCP: a constructive approach", International
              Journal of Internet Protocol Technology 6, 2011.

   [dnssec]   Kukec, A., Bagnulo, M., Ayaz, S., Bauer, C., and W. Eddy,
              "OAM-DNSSEC: Route Optimization for Aeronautical Mobility
              using DNSSEC", 4th ACM International Workshop on Mobility
              in the Evolving Internet Architecture MobiArch 2009, 2009.

Authors' Addresses

   Marcelo Bagnulo
   Universidad Carlos III de Madrid
   Av. Universidad 30
   Leganes, Madrid  28911
   SPAIN

   Phone: 34 91 6249500
   Email: marcelo@it.uc3m.es
   URI:   http://www.it.uc3m.es








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   Christoph Paasch
   UCLouvain
   Place Sainte Barbe, 2
   Louvain-la-Neuve,   1348
   Belgium

   Email: christoph.paasch@uclouvain.be


   Fernando Gont
   SI6 Networks / UTN-FRH
   Evaristo Carriego 2644
   Haedo, Provincia de Buenos Aires  1706
   Argentina

   Phone: +54 11 4650 8472
   Email: fgont@si6networks.com
   URI:   http://www.si6networks.com


   Olivier Bonaventure
   UCLouvain
   Place Sainte Barbe, 2
   Louvain-la-Neuve,   1348
   Belgium

   Email: olivier.bonaventure@uclouvain.be


   Costin Raiciu
   Universitatea Politehnica Bucuresti
   Splaiul Independentei 313a
   Bucuresti
   Romania

   Email: costin.raiciu@cs.pub.ro















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