Network Working Group                                         M. Bagnulo
Internet-Draft                                                      UC3M
Intended status: Informational                         November 26,                          December 6, 2010
Expires: May 30, June 9, 2011

           Threat Analysis for Multi-addressed/Multi-path TCP


   Multi-path TCP (MPTCP for short) describes the extensions proposed
   for TCP so that each endpoint of a given TCP connection can use
   multiple IP addresses to exchange data (instead of a single IP
   address per endpoint as currently defined).  Such extensions enable
   the exchange of segments using different source-destination address
   pairs, resulting in the capability of using multiple paths in a
   significant number of scenarios.  In particular, some level of
   multihoming and mobility support can be achieved through these
   extensions.  However, the support for multiple IP addresses per
   endpoint may have implications on the security of the resulting MPTCP
   protocol.  This note includes a threat analysis for MPTCP.

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   This Internet-Draft will expire on May 30, June 9, 2011.

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Table of Contents

   1.  Introduction . . . . . . . . . . . . . . . . . . . . . . . . .  3
   2.  Scope  . . . . . . . . . . . . . . . . . . . . . . . . . . . .  3
   3.  Related work . . . . . . . . . . . . . . . . . . . . . . . . .  4
   4.  Basic MPTCP. . . . . . . . . . . . . . . . . . . . . . . . . .  6
   5.  Flooding attacks . . . . . . . . . . . . . . . . . . . . . . .  7
   6.  Hijacking attacks  . . . . . . . . . . . . . . . . . . . . . . 10
     6.1.  Hijacking attacks to the Basic MPTCP protocol  . . . . . . 10
     6.2.  Time-shifted hijacking attacks . . . . . . . . . . . . . . 12
     6.3.  NAT considerations . . . . . . . . . . . . . . . . . . . . 14
   7.  Reccommendation  Recommendation . . . . . . . . . . . . . . . . . . . . . . . . 15
   8.  Security Considerations  . . . . . . . . . . . . . . . . . . . 15
   9.  IANA Considerations  . . . . . . . . . . . . . . . . . . . . . 15
   10. Contributors . . . . . . . . . . . . . . . . . . . . . . . . . 15
   11. Acknowledgments  . . . . . . . . . . . . . . . . . . . . . . . 15
   12. Informative References . . . . . . . . . . . . . . . . . . . . 16
   Author's Address . . . . . . . . . . . . . . . . . . . . . . . . . 16

1.  Introduction

   Multi-path TCP (MPTCP for short) describes the extensions proposed
   for TCP [RFC0793] so that each endpoint of a given TCP connection can
   use multiple IP addresses to exchange data (instead of a single IP
   address per endpoint as currently defined).  Such extensions enable
   the exchange of segments using different source-destination address
   pairs, resulting in the capability of using multiple paths in a
   significant number of scenarios.  In particular, some level of
   multihoming and mobility support can be achieved through these
   extensions.  However, the support for multiple IP addresses per
   endpoint may have implications on the security of the resulting MPTCP
   protocol.  This note includes a threat analysis for MPTCP.  It should
   be noted that there are there may other ways to provide multiple
   paths for a TCP connection other than the usage of multiple
   addresses.  The threat analysis performed in this document is limited
   to the specific case of using multiple addresses per endpoint.

2.  Scope

   There are multiple ways to achieve Multi-path TCP.  Essentially what
   is needed is for different segments of the communication to be
   forwarded through different paths by enabling the sender to specify
   some form of path selector.  There are multiple options for such path
   selector, including the usage of different next hops, using tunnels
   to different egress points and so on.  In this note, we will focus on
   a particular approach, namely MPTCP approaches that rely on the usage
   of multiple IP address per endpoint and that use different source-
   destination address pairs as a mean to express different paths.  So,
   in the rest of this note, the MPTCP expression will refer to this
   Multi-addressed flavour of Multi-path TCP.

   Scope of the analysis

   In this note we perform a threat analysis for MPTCP.  Introducing the
   support of multiple addresses per endpoint in a single TCP connection
   may result in additional vulnerabilities.  The scope of this note is
   to identify and characterize these new vulnerabilities.  So, the
   scope of the analysis is limited to the additional vulnerabilities
   resulting from the multi-address support compared to the current TCP
   protocol (where each endpoint only has one address available for use
   per connection).  In other words, a full analysis of the complete set
   of threats is explicitly out of the scope.  The goal of this analysis
   is to help the MPTCP protocol designers to create a MPTCP that is as
   secure as the current TCP.  It is a non goal of this analysis to help
   in the design of MPTCP that is more secure than regular TCP.

   In particular, we will focus on attackers that are not along the
   path, at least not during the whole duration of the connection.  In
   the current single path TCP, an on-path attacker can launch a
   significant number of attacks, including eavesdropping, connection
   hijacking Man-in-the-Middle attacks and so on.  However, it is not
   possible for the off-path attackers to launch such attacks.  There is
   a middle ground in case the attacker is located along the path for a
   short period of time to launch the attack and then moves away, but
   the attack effects still apply.  These are the so-called time-shifted
   attacks.  Since these are not possible in today's TCP, we will also
   consider them as part of the analysis.  So, summarizing, we will
   consider both attacks launched by off-path attackers and time-shifted
   attacks.  Attacks launched by on-path attackers are out of scope,
   since they also apply to current single-path TCP.
      It should be noted, however, that some current on-path attacks may
      become more difficult with multi-path TCP, since an attacker (on a
      single path) will not have visibility of the complete data stream.

3.  Related work

   There is significant amount of previous work in terms of analysis of
   protocols that support address agility.  In this section we present
   the most relevant ones and we relate them to the current MPTCP

   Most of the problems related to address agility have been deeply
   analyzed and understood in the context of Route Optimization support
   in Mobile IPv6 (MIPv6 RO) [RFC3775].  [RFC4225] includes the rational
   for the design of the security of MIPv6 RO.  All the attacks
   described in the aforementioned analysis apply here and are an
   excellent basis for our own analysis.  The main differences are:
   o  In MIPv6 RO, the address binding affects all the communications
      involving an address, while in the MPTCP case, a single connection
      is at stake.  In other words, if a binding between two address is
      created at the IP layer, this binding can and will affect all the
      connections that involve those addresses.  However, in MPTCP, if
      an additional address is added to an ongoing TCP connection, the
      additional address will/can only affect the connection at hand and
      not other connections even if the same address is being used for
      those other connections.  The result is that in MPTCP there is
      much less at stake and the resulting vulnerabilities are less.  On
      the other hand, it is very important to keep the assumption valid
      that the address bindings for a given connection do not affect
      other connections.  If reusing of binding or security information
      is to be considered, this assumption could be no longer valid and
      the full impact of the vulnerabilities must be assessed.
   o  In MIPv6 RO, there is the assumption that the original path
      through which the connection has been established is always
      available and in case it is not, the communication will be lost.

      In MPTCP, it is an explicit goal to provide communication
      resilience when one of the address pairs is no longer usable, so
      it is not possible to leverage on the original address pair to be
      always working.
   o  MIPv6 RO is of course designed for IPv6 and it is an explicit goal
      of MPTCP to support both IPv6 and IPv4.  Some MIPv6 RO security
      solutions rely on the usage of some characteristics of IPv6 (such
      as the usage of CGAs [RFC3972]), which will not be usable in the
      context of MPTCP.
   o  As opposed to MPTCP, MIPv6 RO does not have a connection state
      information, including sequence numbers, port numbers that could
      be leveraged to provide security in some form.

   In the Shim6 [RFC5533] design, similar issues related to address
   agility were considered and a threat analysis was also performed
   [RFC4218].  The analysis performed for Shim6 also largely applies to
   the MPTCP context, the main difference being:
   o  Similarly to the MPTCP case, the Shim6 protocol is a layer 3
      protocol so all the communications involving the target address
      are at stake, as opposed to the MPTCP case, where the impact can
      be limited to a single TCP connection.
   o  Similarly to MIPv6 RO, Shim6 only uses IPv6 addresses as
      identifiers and leverages on some of their properties to provide
      the security, such as relying on CGAs or HBAs [RFC5535], which is
      not possible in the MPTCP case where IPv4 addresses must be
   o  Similarly to MIPv6 RO, Shim6 does not have a connection state
      information, including sequence numbers, port that could be
      leveraged to provide security in some form.

   SCTP [RFC2960]is [RFC4960]is a transport protocol that supports multiple
   addresses per endpoint and as such, the security implications are
   very close to the ones of MPTCP.  A security analysis, identifying a
   set of attacks and proposed solutions was performed in [RFC5062].
   The results of this analysis apply directly to the case of MPTCP.
   However, the analysis was performed after the base SCTP protocol was
   designed and the goal of the document was essentially to improve the
   security of SCTP.  As such, the document is very specific to the
   actual SCTP specification and relies on the SCTP messages and
   behaviour to characterize the issues.  While some them can be
   translated to the MPTCP case, some may be caused by specific
   behaviour of SCTP as defined.

   So, the conclusion is that while we do have a significant amount of
   previous work that is closely related and we can and will use it as a
   basis for this analysis, there are a set of characteristics that are
   specific to MPTCP that grant the need for a specific analysis for
   MPTCP.  The goal of this analysis is to help MPTCP protocol designers
   to include a set of security mechanisms that prevent the introduction
   of new vulnerabilities to the Internet due to the adoption of MPTCP.

4.  Basic MPTCP.

   As we stated earlier, the goal of this document is to serve as input
   for MPTCP protocol designers to properly take into account the
   security issues.  As such, the analysis cannot be performed for a
   specific MPTCP specification, but must be a general analysis that
   applies to the widest possible set of MPTCP designs.  In order to do
   that, we will characterize what are the fundamental features that any
   MPTCP protocol must provide and attempt to perform the security
   implications only assuming those.  In some cases, we will have a
   design choice that will significantly influence the security aspects
   of the resulting protocol.  In that case we will consider both
   options and try to characterize both designs.

   We assume that any MPTCP will behave in the case of a single address
   per endpoint as TCP.  This means that a MPTCP connection will be
   established by using the TCP 3-way handshake and will use a single
   address pair.

   The addresses used for the establishment of the connection do have a
   special role in the sense that this is the address used as identifier
   by the upper layers.  In particular, the address used as destination
   address in the SYN packet is the address that the application is
   using to identify the peer and has been obtained either through the
   DNS (with or without DNSSEC validation) or passed by a referral or
   manually introduced by the user.  As such, the initiator does have a
   certain amount of trust in the fact that it is establishing a
   communication with that particular address.  If due to MPTCP, packets
   end up being delivered to an alternative address, the trust that the
   initiator has placed on that address would be deceived.  In any case,
   the adoption of MPTCP necessitates a slight evolution of the
   traditional TCP trust model, in that the initiator is additionally
   trusting the peer to provide additional addresses which it will trust
   to the same degree as the original pair.  An application or
   implementation that cannot trust the peer in this way should not make
   use of multiple paths.

   During the 3-way handshake, the sequence number will be synchronized
   for both ends, as in regular TCP.  We assume that a MPTCP connection
   will use a single sequence number for the data, even if the data is
   exchanged through different paths, as MPTCP provides an in-order
   delivery service of bytes

   Once the connection is established, the MPTCP extensions can be used
   to add addresses for each of the endpoints.  In order to do that each
   end will need to send a control message containing the additional
   address(es).  In order to associate the additional address to an
   ongoing connection, the connection needs to be identified.  We assume
   that the connection can be identified by the 4-tuple of source
   address, source port, destination address, destination port used for
   the establishment of the connection.  So, at least, the control
   message that will convey the additional address information can also
   contain the 4-tuple in order to inform about what connection the
   address belong to (if no other connection identifier is defined).
   There are two different ways to convey address information:
   o  Explicit mode: the control message contain a list of addresses.
   o  Implicit mode: the address added is the one included in the source
      address field of the IP header

   These two modes have different security properties for some type of
   attacks.  The explicit mode seems to be the more vulnerable to abuse.
   In particular, the implicit mode may benefit from forms of ingress
   filtering security, which would reduce the possibility of an attacker
   to add any arbitrary address to an ongoing connection.  However, it
   should be noted that ingress filtering deployment is far from
   universal, and as such it is unwise to rely on it as a basis for the
   protection of MPTCP.

   In addition, further consideration about the interaction between
   ingress filtering and implicit mode signaling is needed in the case
   that we need to remove an address that is no longer available from
   the MPTCP connection.  In particular a host attached to a network
   that performs ingress filtering and using implicit signaling would
   not be able to remove an address that is no longer available (either
   because of a failure or due to a mobility event) from an ongoing
   MPTCP connection.

   In addition, we will assume that MPTCP will use all the address pairs
   that it has available for sending packets and that it will distribute
   the load based on congestion among the different paths.

5.  Flooding attacks

   The first type of attacks that are introduced by address agility are
   the so called flooding (or bombing) attacks.  The setup for this
   attack is depicted in the following figure:

               +--------+        (step 1)           +------+
               |Attacker| ------------------------- |Source|
               |    A   |IPA                     IPS|  S   |
               +--------+                          /+------+
                                        (step 2) /
                                               v IPT
                                           |  T   |

   The scenario consists of an attacker A who has an IP address IPA.  A
   server that can generate a significant amount of traffic (such as a
   streaming server), called source S and that has IP address IPS.  In
   addition, we have the target of the flooding attack, target T which
   has an IP address IPT.

   In the first step of this attack (depicted as step 1 in the figure),
   the attacker A establishes a MPTCP connection with the source of the
   traffic server S and starts downloading a significant amount of
   traffic.  The initial connection only involves one IP address per
   endpoint, namely IPA and IPS.  Once that the download is on course,
   the second step of the attack (depicted as step 2 in the figure) is
   that the attacker A adds IPT as one of the available addresses for
   the communication.  How the additional address is added depends on
   the MPTCP address management mode.  In explicit address management,
   the attacker A only needs to send a signaling packet conveying
   address IPT.  In implicit mode, the attacker A would need to send a
   packet with IPT as the source address.  Depending on whether ingress
   filtering is deployed and the location of the attacker, it may be
   possible or not for the attacker to send such packet.  At this stage,
   the MPTCP connection still has a single address for the Source S i.e.
   IPS but has two addresses for the Attacker A, namely IPA and IPT.
   The attacker now attempts to get the Source S to send the traffic of
   the ongoing download to the Target T IP address i.e.  IPT.  The
   attacker can do that by pretending that the path between IPA and IPT
   is congested but that the path between IPS and IPT is not.  In order
   to do that, it needs to send ACKs for the data that flows through the
   path between IPS and IPT and do not send ACKs for the data that is
   sent to IPA.  The actual details of this will depend on how the data
   sent through the different paths is ACKed.  One possibility is that
   ACKs for the data sent using a given a given address pair should come
   in packets containing the same address pair.  If so, the attacker
   would need to send ACKs using packets containing IPT as the source
   address to keep the attack flowing.  This may be possible or not
   depending on the deployment of ingress filtering and the location of
   the attacker.  The attacker would also need to guess the sequence
   number of the data being sent to the Target.  Once the attacker
   manages to perform these actions the attack is on place and the
   download will hit the target.  It should be noted that in this type
   of attacks, the Source S still thinks it is sending packets to the
   Attacker A while in reality it is sending the packet to Target T.

   Once that the traffic from the Source S start hitting the Target T,
   the target will react.  In particular, since the packets are likely
   to belong to a non existent TCP connection, the Target T will issue
   RST packets.  It is relevant then to understand how MPTCP reacts to
   incoming RST packets.  It seems that the at least the MPTCP that
   receives a RST packet should terminate the packet exchange
   corresponding to the particular address pair (maybe not the complete
   MPTCP connection, but at least it should not send more packets with
   the address pair involved in the RST packet).  However, if the
   attacker, before redirecting the traffic has managed to increase the
   window size considerably, the flight size could be enough to impose a
   significant amount of traffic to the Target node.  There is a subtle
   operation that the attacker needs to achieve in order to launch a
   significant attack.  On the one hand it needs to grow the window
   enough so that the flight size is big enough to cause enough effect
   and on the other hand the attacker needs to be able to simulate
   congestion on the IPA-IPS path so that traffic is actually redirected
   to the alternative path without significantly reducing the window.
   This will heavily depend on how the coupling of the windows between
   the different paths works, in particular how the windows are
   increased.  Some designs of the congestion control window coupling
   could render this attack ineffective.  In particular, if the MPTCP
   protocol requires performing slow start per subflow, then the
   flooding will be limited by the slow-start initial window size.

   Previous protocols, such as MIPv6 RO and SCTP, that have to deal with
   this type of attacks have done so by adding a reachability check
   before actually sending data to a new address.  In other words, the
   solution used in other protocols, would include the Source S to
   explicitly asking the host sitting in the new address (in this case
   the Target T sitting in IPT) whether it is willing to accept packets
   from the MPTCP connection identified by the 4-tuple IPA, port A, IPS,
   port S. Since this is not part of the established connection that
   Target T has, T would not accept the request and Source S would not
   use IPT to send packets for this MPTCP connection.  Usually, the
   request also includes a nonce that cannot be guessed by the attacker
   A so that it cannot fake the reply to the request easily.  In
   particular, In the case of SCTP, it sends a message with a 64-bit
   nonce (in a HEARTBEAT).

   One possible approach to do this reachability test would be to
   perform a 3-way handshake for each new address pair that is going to
   be used in a MPTCP connection.  While there are other reasons for
   doing this (such as NAT traversal), such approach would also act as a
   reachability test and would prevent the flooding attacks described in
   this section.

   Another type of flooding attack that could potentially be performed
   with MPTCP is one where the attacker initiates a a communication with
   a peer and includes a long list of alternative addresses in explicit
   mode.  If the peer decides to establish subflows with all the
   available addresses, the attacker have managed to achieve an
   amplified attack, since by sending a single packet containing all the
   alternative addresses it triggers the peer to generate packets to all
   the destinations.

6.  Hijacking attacks

6.1.  Hijacking attacks to the Basic MPTCP protocol

   The hijacking attacks essentially use the MPTCP address agility to
   allow an attacker to hijack a connection.  This means that the victim
   of a connection thinks that it is talking to a peer, while it is
   actually exchanging packets with the attacker.  In some sense it is
   the dual of the flooding attacks (where the victim thinks it is
   exchanging packets with the attacker but in reality is sending the
   packets to the target).

   The scenario for a hijacking attack is described in the next figure.

                +------+                           +------+
                | Node | ------------------------- | Node |
                |   1  |IP1                     IP2|  2   |
                +------+                          /+------+
                                              v IPA
                                         |    A   |

   In this case, we have a MPTCP connection established between Node 1
   and Node 2.  The connection is using only one address per endpoint,
   namely IP1 and IP2.  The attacker then launches the hijacking attack
   by adding IPA as an additional address for Node 1.  In this case,
   there is not much difference between explicit or implicit address
   management, since in both cases the Attacker A could easily send a
   control packet adding the address IPA, either as control data or as
   the source address of the control packet.  In order to be able to
   hijack the connection, the attacker needs to know the 4-tuple that
   identifies the connection, including the pair of addresses and the
   pair of ports.  It seems reasonable to assume that knowing the source
   and destination IP addresses and the port of the server side is
   fairly easy for the attacker.  Learning the port of the client (i.e.
   of the initiator of the connection) may prove to be more challenging.
   The attacker would need to guess what the port is or to learn it by
   intercepting the packets.  Assuming that the attacker can gather the
   4-tuple and issue the message adding IPA to the addresses available
   for the MPTCP connection, then the attacker A has been able to
   participate in the communication.  In particular:
   o  Segments flowing from the Node 2:Depending how the usage of
      addresses is defined, Node 2 will start using IPA to send data to.
      In general, since the main goal is to achieve multi-path
      capabilities, we can assume that unless there are already many IP
      address pairs in use in the MPTCP connection, Node 2 will start
      sending data to IPA.  This means that part of the data of the
      communication will reach the Attacker but probably not all of it.
      This per se, already has negative effects, since Node 1 will not
      receive all the data from Node 2.  Moreover, from the application
      perspective, this would result in DoS attack, since the byte flow
      will stop waiting for the missing data.  However, it is not enough
      to achieve full hijacking of the connection, since part of data
      will be still delivered to IP1, so it would reach Node 1 and not
      the Attacker.  In order for the attacker to receive all the data
      of the MPTCP connection, the Attacker must somehow remove IP1 of
      the set of available addresses for the connection. in the case of
      implicit address management, this operation is likely to imply
      sending a termination packet with IP1 as source address, which may
      or not be possible for the attacker depending on whether ingress
      filtering is in place and the location of the attacker.  If
      explicit address management is used, then the attacker will send a
      remove address control packet containing IP1.  The result is that
      once IP1 is removed, all the data sent by Node 2 will reach the
      Attacker and the incoming traffic has been hijacked.
   o  Segments flowing to the Node 2: As soon as IPA is accepted by Node
      2 as part of the address set for the MPTCP connection, the
      Attacker can send packets using IPA and those packets will be
      considered by Node 2 as part of MPTCP connection.  This means that
      the attacker will be able to inject data into the MPTCP
      connection, so from this perspective, the attacker has hijacked
      part of the outgoing traffic.  However, Node 1 would still be able
      to send traffic that will be received by Node 2 as part of the
      MPTCP connection.  This means that there will be two source of
      data i.e.  Node 1 and the attacker, potentially preventing the
      full hijacking of the outgoing traffic by the attacker.  In order
      to achieve a full hijacking, the attacker would need to remove IP1
      from the set of available addresses.  This can be done using the
      same techniques described in the previous paragraph.

   A related attack that can be achieved using similar techniques would
   be a Man-in-the-Middle (MitM) attack.  The scenario for the attack is
   depicted in the figure below.

                        +------+                 +------+
                        | Node | --------------- | Node |
                        |   1  |IP1           IP2|  2   |
                        +------+ \              /+------+
                                  \            /
                                   \          /
                                    \        /
                                    v IPA  v
                                   |    A   |

   In this case, there is an established connection between Node 1 and
   Node 2.  The Attacker A will use the MPTCP address agility
   capabilities to place itself as a MitM.  In order to do so, it will
   add IP address IPA as an additional address for the MPTCP connection
   on both Node 1 and Node 2.  This is essentially the same technique
   described earlier in this section, only that it is used against both
   nodes involved in the communication.  The main difference is that in
   this case, the attacker can simply sniff the content of the
   communication that is forwarded through it and in turn forward the
   data to the peer of the communication.  The result is that the
   attacker can place himself in the middle of the communication and
   sniff part of the traffic unnoticed.  Similar considerations about
   how the attacker can manage to get to see all the traffic by removing
   the genuine address of the peer apply.

6.2.  Time-shifted hijacking attacks

   A simple way to prevent off-path attackers to launch hijacking
   attacks is to provide security of the control messages that add and
   remove addresses by the usage of a cookie.  In this type of
   approaches, the peers involved in the MPTCP connection agree on a
   cookie, that is exchanged in plain text during the establishment of
   the connection and that needs to be presented in every control packet
   that adds or removes an address for any of the peers.  The result is
   that the attacker needs to know the cookie in order to launch any of
   the hijacking attacks described earlier.  This implies that off path
   attackers can no longer perform the hijacking attacks and that only
   on-path attackers can do so, so one may consider that a cookie based
   approach to secure MPTCP connection results in similar security than
   current TCP.  While it is close, it is not entirely true.

   The main difference between the security of a MPTCP protocol secured
   through cookies and the current TCP protocol are the time shifted
   attacks.  As we described earlier, a time shifted attack is one where
   the attacker is along the path during a period of time, and then
   moves away but the effects of the attack still remains, after the
   attacker is long gone.  In the case of a MPTCP protocol secured
   through the usage of cookies, the attacker needs to be along the path
   until the cookie is exchanged.  After the attacker has learnt the
   cookie, it can move away from the path and can still launch the
   hijacking attacks described in the previous section.

   There are several type of approaches that provide some protection
   against hijacking attacks and that are vulnerable to some forms of
   time-shifted attacks.  We will next present some general taxonomy of
   solutions and we describe the residual threats:
   o  Cookie-based solution: As we described earlier, one possible
      approach is to use a cookie, that is sent in clear text in every
      MPTCP control message that adds a new address to the existing
      connection.  The residual threat in this type of solution is that
      any attacker that can sniff any of these control messages will
      learn the cookie and will be able to add new addresses at any
      given point in the lifetime of the connection.  Moreover, the
      endpoints will not detect the attack since the original cookie is
      being used by the attacker.  Summarizing, the vulnerability window
      of this type of attacks includes all the flow establishment
      exchanges and it is undetectable by the endpoints.
   o  Shared secret exchanged in plain text: An alternative option that
      is somehow more secure than the cookie based approach is to
      exchange a key in clear text during the establishment of the first
      subflow and then validate the following subflows by using an keyed
      HMAC signature using the shared key.  This solution would be
      vulnerable to attackers sniffing the message exchange for the
      establishment of the first subflow, but after that, the shared key
      is not transmitted any more, so the attacker cannot learn it
      through sniffing any other message.  Unfortunately, in order to be
      compatible with NATs (see analysis below) even though this
      approach includes a keyed HMAC signature, this signature cannot
      cover the IP address that is being added.  This basically means
      that this type of approaches are also vulnerable to integrity
      attacks of the exchanged messages.  This means that even though
      the attacker cannot learn the shared key by sniffing the
      subsequent subflow establishment, the attacker can modify the
      subflow establishment message and change the address that is being
      added.  So, the vulnerability window for confidentially to the
      shared key is limited to the establishment of the first subflow,
      but the vulnerability window for integrity attacks still includes
      all the subflow establishment exchanges.  These attacks are still
      undetectable by the endpoints.  It should be noted that the SCTP
      security falls in this category.
   o  Strong crypto anchor exchange.  Another approach that could be
      used would be to exchange some strong crypto anchor while the
      establishment of the first subflow, such as a public key or a hash
      chain anchor.  In this case, subsequent subflows could be
      protected by using the crypto material associated to that anchor.
      An attacker in this case would need to change the crypto material
      exchanged in the connection establishment phase.  As a result the
      vulnerability window for forging the crypto anchor is limited to
      the initial connection establishment exchange.  Similarly to the
      previous case, due to NAT traversal considerations, the
      vulnerability window for integrity attacks include all the subflow
      establishment exchanges.  As opposed to the previous one, because
      the attacker needs to change the crypto anchor, this approach are
      detectable by the endpoints, if they communicate directly.

6.3.  NAT considerations

   In order to be widely adopted MPTCP must work through NATs.  NATs are
   an interesting device from a security perspective.  In terms of MPTCP
   they essentially behave as a Man-in-the-Middle attacker.  As we have
   described earlier, MPTCP security goal is to prevent from any
   attacker to insert their addresses as valid addresses for a given
   MPTCP connection.  But that is exactly what a NAT does, they modify
   the addresses.  So, if MPTCP is to work through NATs, MPTCP must
   accept address rewritten by NATs as valid addresses for a given
   session.  The most direct corollary is that the MPTCP messages that
   add addresses in the implicit mode (i.e. the SYN of new subflows)
   cannot be protected against integrity attacks, since they must allow
   for NATs to change their addresses.  This basically rules out any
   solution that would rely on providing integrity protection to prevent
   an attacker from changing the address used in a subflow establishment
   exchange.  This implies that alternative creative mechanisms are
   needed to protect from integrity attacks to the MPTCP signaling that
   adds new addresses to a connection.  It is far from obvious how one
   such creative approach could look like at this point.

7.  Reccommendation  Recommendation

   The presented analysis shows that there is a tradeoff between the
   complexity of the security solution and the residual threats.  In
   order to define a proper security solution, we need to assess the
   tradeoff and make a recommendation.  After evaluating the different
   aspects in the MPTCP WG, our conclusion are: are as follows:

   MPTCP should implement some form of reachabilty reachability check using a random
   nonce (e.g.  TCP 3-way handshake) before adding a new address to an
   ongoing communication in order to prevent flooding attacks.

   The default security mechanisms for MPTCP should be to exchange a key
   in clear text in the establishment of the first subflow and then
   secure following address additions by using a keyed HMAC using the
   exchanged key.

   MPTCP security mechanism should support using a pre-shared key to be
   used in the keyed HMAC, providing a higher level of protection than
   the previous one.

   A mechanism to prevent replay attacks using these messages should be
   provided e.g. a sequence number protected by the HMAC

   The MPTCP protocol should be extensible and it should able to
   accommodate multiple security solutions, in order to enable the usage
   of more secure mechanisms if needed.

8.  Security Considerations

   This note contains a security analysis for MPTCP, so no further
   security considerations need to be described in this section

9.  IANA Considerations

   This document does not require any action from IANA.

10.  Contributors

   Alan Ford - Roke Manor Research Ltd.


11.  Acknowledgments

   Rolf Winter, Randall Stewart, Andrew McDonald, Michael Tuexen,
   Michael Scharf, Tim Shepard Shepard, Yoshifumi Nishida reviewed an earlier
   version of this document and provided comments to improve it.

   Mark Handley pointed out the problem with NATs and integrity
   protection of MPTCP signaling.

   Marcelo Bagnulo is partly funded by Trilogy, a research project
   supported by the European Commission under its Seventh Framework


12.  Informative References

   [RFC4225]  Nikander, P., Arkko, J., Aura, T., Montenegro, G., and E.
              Nordmark, "Mobile IP Version 6 Route Optimization Security
              Design Background", RFC 4225, December 2005.

   [RFC4218]  Nordmark, E. and T. Li, "Threats Relating to IPv6
              Multihoming Solutions", RFC 4218, October 2005.

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

   [RFC5062]  Stewart, R., Tuexen, M., and G. Camarillo, "Security
              Attacks Found Against the Stream Control Transmission
              Protocol (SCTP) and Current Countermeasures", RFC 5062,
              September 2007.

   [RFC5535]  Bagnulo, M., "Hash-Based Addresses (HBA)", RFC 5535,
              June 2009.

   [RFC3775]  Johnson, D., Perkins, C., and J. Arkko, "Mobility Support
              in IPv6", RFC 3775, June 2004.

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


   [RFC4960]  Stewart, R., Xie, Q., Morneault, K., Sharp, C.,
              Schwarzbauer, H., Taylor, T., Rytina, I., Kalla, M.,
              Zhang, L., and V. Paxson, "Stream Control Transmission Protocol",
              RFC 2960, October 2000. 4960, September 2007.

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

Author's Address

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

   Phone: 34 91 6248814