Network Working Group                                         R. Stewart
Internet-Draft                                                  M. Dalal
Expires: August December 17, 2006                                        Editor
                                                       February 13,
                                                           June 15, 2006

         Improving TCP's Robustness to Blind In-Window Attacks
                    draft-ietf-tcpm-tcpsecure-04.txt
                    draft-ietf-tcpm-tcpsecure-05.txt

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Copyright Notice

   Copyright (C) The Internet Society (2006).

Abstract

   A recent study indicates that some types of TCP connections have an
   increased vulnerability to spoofed packet injection attacks than
   previously believed [SITW].  TCP has historically been considered
   protected against spoofed packet injection attacks by relying on the
   fact that it is difficult to guess the 4-tuple (the source and
   destination IP addresses and the source and destination ports) in
   combination with the 32 bit sequence number(s).  A combination of
   increasing window sizes and applications using a longer term
   connections (e.g.  H-323 or Border Gateway Protocol [RFC1771]) have
   left modern TCP implementation more vulnerable to these types of
   spoofed packet injection attacks.

   Note: Both [SITW] and [DTASA] [I-D.touch-tcp-antispoof] provide charts which
   can give the reader an idea as to the time it takes to penetrate an
   unprotected system.

   Many of these long term TCP applications tend to have predictable IP
   addresses and ports which makes it far easier for the 4-tuple to be
   guessed.  Having guessed the 4-tuple correctly, an attacker can
   inject a RST, SYN or DATA segment into a TCP connection by carefly carefully
   crafting the sequence number of the spoofed segment to be in the
   current receive window.  This can cause the connection to either
   abort or possibly cause data corruption.  This document proposes requires
   small modifications to the way TCP handles inbound segments that can
   reduce the probability of such an attack.

Table of Contents

   1.  Introduction . . . . . . . . . . . . . . . . . . . . . . . . .  4
     1.1.  The RESET attack . . . . . . . . . . . . . . . . . . . . .  4
     1.2.  Attack probabilities . . . . . . . . . . . . . . . . . . .  5
   2.  Terminology  . . . . . . . . . . . . . . . . . . . . . . . . .  8
   3.  Blind reset attack using the RST bit . . . . . . . . . . . . .  9
     3.1.  Description of the attack  . . . . . . . . . . . . . . . .  9
     3.2.  Mitigation . . . . . . . . . . . . . . . . . . . . . . . .  9
   4.  Blind reset attack using the SYN bit . . . . . . . . . . . . . 11
     4.1.  Description of the attack  . . . . . . . . . . . . . . . . 11
     4.2.  Mitigation . . . . . . . . . . . . . . . . . . . . . . . . 11
   5.  Blind data injection attack  . . . . . . . . . . . . . . . . . 13
     5.1.  Description of the attack  . . . . . . . . . . . . . . . . 13
     5.2.  Mitigation . . . . . . . . . . . . . . . . . . . . . . . . 13
   6.  ACK throttling . . . . . . . . . . . . . . . . . . . . . . . . 15
   7.  Backward Compatibility and Other considerations  . . . . . . . 16
   8.  Middlebox considerations . . . . . . . . . . . . . . . . . . . 17
     8.1.  Middlebox that resend RST's  . . . . . . . . . . . . . . . 17
     8.2.  Middleboxes that advance sequence numbers  . . . . . . . . 17
   9.  Interoperability Testing . . . . . . . . . . . . . . . . . . . 19
   10.  Security Considerations  . . . . . . . . . . . . . . . . . . . 21
   11. 19
   10. IANA Considerations  . . . . . . . . . . . . . . . . . . . . . 22
   12. 20
   11. Contributors . . . . . . . . . . . . . . . . . . . . . . . . . 23
   13. 21
   12. Acknowledgments  . . . . . . . . . . . . . . . . . . . . . . . 24
   14. 22
   13. References . . . . . . . . . . . . . . . . . . . . . . . . . . 25
     14.1. 23
     13.1. Normative References . . . . . . . . . . . . . . . . . . . 25
     14.2. 23
     13.2. Informative References . . . . . . . . . . . . . . . . . . 25 23
   Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 26 24
   Intellectual Property and Copyright Statements . . . . . . . . . . 27 25

1.  Introduction

   TCP [RFC0793] is widely deployed and the most common reliable end to
   end transport protocol used for data communication in todays
   Internet.  Yet when it was defined over 20 years ago the Internet, as
   we know it, was a different place lacking many of the threats that
   are now common.  TCP spoofing attacks are one such attack that are
   seen on the Internet today.

   In a TCP spoofing attack, an off-path attacker crafts TCP packets by
   forging the IP source and destination addresses as well as the source
   and destination ports (commonly referred to as a 4-tuple value) so
   that a target TCP endpoint can associate such a packet with an
   existing TCP connection.  Note that in and of itself guessing this
   4-tuple value is not always easy for an attacker.  But there are some
   applications (e.g.  BGP [RFC1771]) that may have a tendency to use
   the same set(s) of ports on either endpoint making the odds of
   guessing correctly the 4-tuple value much easier.  When an attacker
   is successful in guessing the 4-tuple value, one of three types of
   injection attacks may be waged against a long-lived connection.

   RST - Where an attacker injects a reset segment hoping to cause the
      connection to be torn down.

   SYN - Where an attacker injects a 'SYN' hoping to cause the receiver
      to believe the peer has restarted and so tear down the connection
      state.

   DATA - Where an attacker tries to inject a "DATA" segment to corrupt
      the contents of the transmission.

1.1.  The RESET attack

   Focusing upon the RESET attack, let's examine this attack in more
   detail to get an overview as to how it works and how this document
   proposes addressing
   addresses the issue.  For this attack the goal is to cause one of the
   two endpoints of the connection to incorrectly tear down the
   connection state, effectively closing the connection.  To do this the
   attacker needs to have or guess several pieces of information
   (namely):

   1) The 4-tuple value containing the IP address and TCP port number of
      both ends of the connection.  For one side (usually the server)
      guessing the port number is a trivial exercise.  The client side
      may or may not be easy for an attacker to guess depending on a
      number of factors most notably the operating system and
      application involved.

   2) A sequence number that will be used in the RST.  This sequence
      number will be a starting point for a series of guesses to attempt
      to present a RST segment to a connection endpoint that would be
      acceptable to it.  Any random value may be used to guess the
      initial sequence number.

   3) The window size that the two endpoints are using.  This value does
      NOT have to be the exact window size since a smaller value used in
      lieu of the correct one will just cause the attacker to generate
      more segments before succeeding in his mischieve.  Most modern
      operating systems have a default window size which usually is
      applied to most connections.  Some applications however may change
      the window size to better suit the needs of the application.  So
      often times the attacker, with a fair degree of certainty (knowing
      the application that is under attack), can come up with a very
      close approximation as to the actual window size in use on the
      connection.

   After assembling the above set of information the attacker begins
   sending spoofed TCP segments with the RST bit set and a guessed TCP
   sequence number.  Each time a new RST segment is sent, the sequence
   number guess is incremented by the window size.  Without mitigation
   [SITW] has shown that such an attack is much easier to accomplish
   then previously assumed.  This is because RFC793 [RFC0793] specifies that
   any RST within the current window is acceptable.

   A slight modification to the TCP state machine can be made which
   makes such an attack much more difficult to accomplish.  If the
   receiver examines the incoming RST segment and validates that the
   sequence number exactly matches the sequence number that is next
   expected, then such an attack becomes much more difficult then
   outlined in [SITW] (i.e. the attacker would have to generate 1/2 the
   entire sequence space, on average).  This document will discuss the
   exact details of what needs to be changed within TCP's state machine
   to mitigate all three types of attacks (RST, SYN and DATA).

1.2.  Attack probabilities

   Every application has control of a number of factors that effect
   drastically the probability of a successful spoofing attack.  These
   factors include such things as:

   Window Size - Normally settable by the application but often times
      defaulting to 32,768 or 65,535 depending upon the operating system
      (Medina05 [Medina05]).

   Server Port number - This value is normally a fixed value so that a
      client will know where to connect to the peer at.  Thus this value
      normally provides no additional protection.

   Client Port number - This value may be a random ephemeral value, if
      so, this makes a spoofing attack more difficult.  There are some
      clients, however, that for whatever reason either pick a fixed
      client port or have a very guessable one (due to the range of
      ephemeral ports available with their operating system or other
      application considerations) for such applications a spoofing
      attack becomes less difficult.

   For the purposes of the rest of this discussion we will assume that
   the attacker knows the 4-tuple values.  This assumption will help us
   focus on the effects of the window size verses the number of TCP
   packets an attacker must generate.  This assumption will rarely be
   true in the real Internet since at least the client port number will
   provide us with some amount of randomness (depending on operating
   system).

   To successfully inject a spoofed packet (RST, SYN or DATA), in the
   past, the entire sequence space (i.e. 2^32) was often considered
   available to make such an attack unlikely.  [SITW] demonstrated that
   this assumption was incorrect and that instead of [1/2 X 2^32]
   packets (assuming a random distribution) [1/2 X (2^32/window)]
   packets is required.

   Placing real numbers on this formula we see that for a window size of
   32,768, an average of 65,536 packets would need to be transmitted in
   order to "spoof" a TCP segment that would be acceptable to a TCP
   receiver.  A window size of 65,535 reduces this even further to
   32,768 packets.  With rises in bandwidth to both the home and office,
   it can only be expected that the values for default window sizes will
   continue to rise in order to better take advantage of the newly
   available bandwidth.

   As we can see from the above discussion this weakness lowers the bar
   quite considerably for likely attacks.  But there is one additional
   dependency which is the duration of the TCP connection.  A TCP
   connection that lasts only a few brief packets, as often is the case
   for web traffic, would not be subject to such an attack since the
   connection may not be established long enough for an attacker to
   generate enough traffic.  However there is a set of applications such
   as BGP [RFC1771] which is judged to be potentially most affected by
   this vulnerability.  BGP relies on a persistent TCP session between
   BGP peers.  Resetting the connection can result in medium term
   unavailability due to the need to rebuild routing tables and route
   flapping see [NISCC] for further details.

   It should be noted that there are existing alternative protection
   against the threats that this document addresses.  For further
   details regarding the attacks and the existing techniques, please
   refer to draft [DTASA] [I-D.touch-tcp-antispoof]

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 RFC2119 [RFC2119].  TCP
   terminology should be interpreted as described in RFC793 [RFC0793].

3.  Blind reset attack using the RST bit

3.1.  Description of the attack

   As described in the introduction, it is possible for an attacker to
   generate a "RST" segment that would be acceptable to a TCP receiver
   by guessing a "in-window" sequence numbers.  In particulary RFC 793, particularly
   [RFC0793], p37, states the following:

   "In all states except SYN-SENT, all reset (RST) segments are
   validated by checking their SEQ-fields [sequence numbers].  A reset
   is valid if its sequence number is in the window.  In the SYN-SENT
   state (a RST received in response to an initial SYN), the RST is
   acceptable if the ACK field acknowledges the SYN."

3.2.  Mitigation

   RFC793

   [RFC0793] currently requires handling of a segment with the RST bit
   when in a synchronized state to be processed as follows:

   1) If the RST bit is set and the sequence number is outside the
      current receive window (SEG.SEQ <= RCV.NXT || SEG.SEQ > RCV.NXT+
      RCV.WND) , silently drop the segment.

   2) If the RST bit is set and the sequence number is acceptable i.e.:
      (RCV.NXT <= SEG.SEQ < RCV.NXT+RCV.WND) then reset the connection.

   Instead, this document proposes requires that implementations MUST perform the
   following changes should be made
   to provide protection against such an attack. steps in place of those specified in [RFC0793](listed
   above).

   A) If the RST bit is set and the sequence number is outside the
      current receive window, silently drop the segment.

   B) If the RST bit is set and the sequence number exactly matches the
      next expected sequence number (RCV.NXT), then TCP MUST reset the
      connection.

   C) If the RST bit is set and the sequence number does not exactly
      match the next expected sequence value, yet is within the current
      receive window (RCV.NXT < SEG.SEQ < RCV.NXT+RCV.WND), TCP MUST
      send an acknowledgment (challenge ACK):

      <SEQ=SND.NXT><ACK=RCV.NXT><CTL=ACK>
      After sending the challenge ACK, TCP MUST drop the unacceptable
      segment and stop processing the incoming packet further.

      The previous text,quoted from RFC793 [RFC0793] pg 37 would thus become:

      In all states except SYN-SENT, all reset (RST) segments are
      validated by checking their SEQ-fields [sequence numbers].  A
      reset is valid if its sequence number exactly matches the next
      expected sequence number.  If the the RST arrives and its sequence
      number field does NOT match the next expected sequence number but
      is within the window, then the receiver should generate an ACK.
      In all other cases where the SEQ-field does not match and is
      outside the window, the receiver MUST silently discard the
      segment.

      In the SYN-SENT state (a RST received in response to an initial
      SYN), the RST is acceptable if the ACK field acknowledges the SYN.
      In all other cases the receiver MUST silently discard the segment.

      With the above slight change to the TCP state machine, it becomes
      much harder for an attacker to generate an acceptable reset
      segment.

      In cases where the remote peer did generate a RST but it fails to
      meet the above criteria (the RST sequence number was within the
      window but NOT the exact expected sequence number) when the
      challenge ACK is sent back, it will no longer have the
      transmission control block (TCB) related to this connection and
      hence as per RFC793 [RFC0793], the remote peer will send a second RST
      back.  The sequence number of the second RST is derived from the
      acknowledgment number of the incoming ACK.  This second RST if it
      reaches the sender will cause the connection to be aborted since
      the sequence number would now be an exact match.

   Note that the above mitigation may cause a non-amplification ACK
   exchange.  This concern is detailed in

4.  Blind reset attack using the SYN bit

4.1.  Description of the attack

   Analysis of the reset attack using the RST bit, highlights another
   possible avenue for a blind attacker using a similar set of sequence
   number guessing.  Instead of using the RST bit an attacker can use
   the SYN bit with the exact same symantics semantics to tear down a connection.

4.2.  Mitigation

   RFC793

   [RFC0793] currently requires handling of a segment with the SYN bit
   set in the synchronized state to be as follows:

   1) If the SYN bit is set and the sequence number is outside the
      expected window, send an ACK back to the sender.

   2) If the SYN bit is set and the sequence number is acceptable i.e.:
      (RCV.NXT <= SEG.SEQ <= RCV.NXT+RCV.WND) then send a RST segment to
      the sender.

   Instead, changing the handling of the SYN in the synchronized state
   to the following will mitigate this attack: MUST be
   performed as follows:

   A) If the SYN bit is set, irrespective of the sequence number, TCP
      MUST send an ACK (also referred to as challenge ACK) to the remote
      peer:

      <SEQ=SND.NXT><ACK=RCV.NXT><CTL=ACK>

      After sending the acknowledgment, TCP MUST drop the unacceptable
      segment and stop processing further.

   By sending an ACK, the remote end sender is challenged to confirm the
   loss of the previous connection and the request to start a new
   connection.  A legitimate peer, after restart, would not have a TCB
   in the synchronized state.  Thus when the ACK arrives the peer should
   send a RST segment back with the sequence number derived from the ACK
   field that caused the RST.

   This RST will confirm that the remote TCP endpoint has indeed closed
   the previous connection.  Upon receipt of a valid RST, the local TCP
   endpoint MUST terminate its connection.  The local TCP endpoint
   should then rely on SYN retransmission from the remote end to re-
   establish the connection.

   A spoofed SYN, on the other hand, will then have generated an
   additional ACK which the peer will discarded as a duplicate ACK and
   will not affect the established connection.

   Note that this mitigation does leave one corner case un-handled which
   will prevent the reset of a connection when it should be reset (i.e.
   it is a non spoofed SYN wherein a peer really did restart).  This
   problem occurs when the restarting host chooses the exact same IP
   address and port number that it was using prior to its restart.  By
   chance the restarted host must also choose an initial sequence number
   of exactly (RCV.NXT - 1) of the remote TCP endpoint that is still in
   the established state.  Such a case would cause the receiver to
   generate a "challenge" ack as described above.  But since the ACK
   would be within the outgoing connections window the inbound ACK would
   be acceptable, and the sender of the SYN will do nothing with the
   response ACK.  This sequence will continue as the SYN sender
   continually times out and retransmits the SYN until such time as the
   connection attempt fails.

   This corner case is a result of the RFC793 [RFC0793] specification and is
   not introduced by the proposed mitigations. these new requirments.

   Note that the above mitigation may cause a non-amplification ACK
   exchange.  This concern is detailed in Section 10 9

5.  Blind data injection attack

5.1.  Description of the attack

   A third type of attack is also highlighted by both the RST and SYN
   attacks.  It is also possible to inject data into a TCP connection by
   simply guessing the a sequence number within the current receive
   window of the victim.  The ACK value of any data segment is
   considered valid as long as it does not acknowledge data ahead of the
   next segment to send.  In other words an ACK value is acceptable if
   it is (SND.UNA-(2^31-1)) <= SEG.ACK <= SND.NXT).  This means that an
   attacker has to guess two ACK values with every guessed sequence
   number so that the chances successfully injecting data into a
   connection are 1 in ((2^32 / RCV.WND) * 2).

   When an attacker successfully injects data into a connection the data
   will sit in the receiver's re-assembly queue until the peer sends
   enough data to bridge the gap between the RCV.NXT value and the
   injected data.  At that point one of two things will occur either:

   a) An packet war will ensue with the receiver indicating that it has
      received data up until RCV.NXT (which includes the attackers data)
      and the sender sending an ACK with an acknowledgment number less
      than RCV.NXT.

   b) The sender will send enough data to the peer which will move
      RCV.NXT even further along past the injected data.

   Depending upon the TCP implementation in question and the TCP traffic
   characteristics at that time, data corruption may result.  In case
   (a) the connection will eventually be reset by one of the sides
   unless the sender produces more data that will transform the ACK war
   into case (b).  The reset will usually occur via User Time Out (UTO)
   (see section 4.2.3.5 of [RFC1122]).

   Note that the protections illustrated in this section neither cause
   an ACK war nor prevent one from occurring if data is actually
   injected into a connection.  The ACK war is a product of the attack
   itself and cannot be prevented (other than by preventing the data
   from being injected).

5.2.  Mitigation

   An additional input check should MUST be added to any incoming segment.  The
   ACK value MUST be acceptable only if it is in the range of ((SND.UNA
   - MAX.SND.WND) <= SEG.ACK <= SND.NXT). SND.NXT).A new state variable
   MAX.SND.WND is defined as the largest window that the local receiver sender
   has ever
   advertised to received from its peer.  This window may be a scaled value
   i.e. the value may be larger than 65,535 bytes (RFC1323 [RFC1323]). ([RFC1323]).  This
   small check will greatly reduce the vulnerability to an attacker guessing a
   valid sequence number since not only must he/she guess the sequence
   number in window, but must also guess a proper ACK value within a
   scoped range.  This mitigation reduces but does not eliminate the
   ability to generate false segments.  It does however reduce the
   probability that invalid data will be injected.

   Note that the above mitigation may cause a non-amplification ACK
   exchange.  This concern is detailed in

6.  ACK throttling

   In order to alleviate multiple RSTs/SYNs from triggering multiple
   challenge ACKs, an ACK throttling mechanism SHOULD be implemented.
   Suggested values are to send implemented as
   follows:

   1) In any 5 second window, no more than 10 challenge ACKs in a 5
   second window. ACK's should be
      sent.

   2) The values for both the time and number of ACK's MUST be tunable
      by the system administrator to accommodate different perceived
      threats.

   It should be noted that These numbers are empirical in nature and
   have been obtained from the RST throttling mechanism implemented in
   some OS's.
   These value MUST be tunable by a system administrator to accommodate
   different preceived threats.

   An alternative mechanism may also be used that does not involve an
   additional timer.  In such an implementation a sender would only send
   X acks between any window advancment.  Note  Also note that such a limitation
   will not require a no timer but must be implemented with care is needed to avoid a
   deadlock in implement the face of ack loss. above
   mechanism, instead a timestamp and a counter can be used.

   An implementation SHOULD include a ACK throttling mechanism to be
   conservative.  Currently there is no known bad behavior that can be
   attributed to the lack of ACK throttling, but as a general principle,
   if ever invoked, something incorrect is occuring occurring and such a mechanisn
   mechanism will act as a failsafe that protects both the sender and
   the network.

   An administrator who is more concerned about protecting his bandwidth
   and CPU utilization may set smaller ACK thottling throttling values whereas an
   administrator who is more interested in faster cleanup of stale
   connections (i.e. concerned about excess TCP state) may decide to set
   a higher value thus allowing more RST's to be processed in any given
   time period.

   The time limit SHOULD be tunable to help timeout brute force attacks
   faster than a potential legitimate flood of RSTs.

7.  Backward Compatibility and Other considerations

   1) All of the proposed new required mitigation techniques in this document are
      totally compatible with existing (RFC793 [RFC0793]) ([RFC0793]) compliant TCP
      implementations as this document introduces no new assumptions or
      conditions.

   2) There is a corner scenario in the above proposed mitigations which will
      require more than one round trip time to successfully abort the
      connection as per the figure below.  This scenario is similar to
      the one in which the original RST was lost in the network.

          TCP A                                                 TCP B
   1.a. ESTAB        <-- <SEQ=300><ACK=101><CTL=ACK><DATA> <--  ESTAB
     b. (delayed)    ... <SEQ=400><ACK=101><CTL=ACK><DATA> <--  ESTAB
     c. (in flight)  ... <SEQ=500><ACK=101><CTL=RST>       <--  CLOSED
   2.   ESTAB        --> <SEQ=101><ACK=400><CTL=ACK>       -->  CLOSED
       (ACK for 1.a)
                     ... <SEQ=400><ACK=0><CTL=RST>         <--  CLOSED
   3.   CHALLENGE    --> <SEQ=101><ACK=400><CTL=ACK>       -->  CLOSED
        (for 1.c)
                     ... <SEQ=400><ACK=0><CTL=RST>         <--  RESPONSE
   4.a. ESTAB        <-- <SEQ=400><ACK=101><CTL=ACK><DATA> 1.b reaches A
     b. ESTAB        --> <SEQ=101><ACK=500><CTL=ACK>
     c. (in flight)  ... <SEQ=500><ACK=0><CTL=RST>         <--  CLOSED
   5.   RESPONSE arrives at A, but dropped since its outside of window.
   6.   ESTAB        <-- <SEQ=500><ACK=0><CTL=RST>         4.c reaches A
   7.   CLOSED                                                   CLOSED

   3) For the mitigation to be maximally effective against the
      vulnerabilities discussed in this document, both ends of the TCP
      connection need to have the fix.  Although, having the mitagations mitigations
      at one end might prevent that end from being exposed to the
      attack, the connection is still vulnerable at the other end.

8.  Middlebox considerations

8.1.  Middlebox that resend RST's

   Consider a middlebox M-B tracking connection between two TCP endhosts
   E-A and E-C.  If E-C sends a RST with a sequence number that is
   within the window but not an exact match to reset the connection and
   M-B does not have the fix proposed required here, it may clear the connection
   and forward the RST to E-A saving an incorrect sequence number.  If
   E-A does not have the fix it will clear the connection and everything
   will be fine.  However if E-A does have the proposed required fix above, it
   will send a challenge ACK to E-C.  M-B, being a middlebox, may
   intercept this ACK and resend the RST on behalf of E-C with the old
   sequence number.  This RST, will again, not acceptable and may
   trigger a challenge ACK.

   This may cause a RST/ACK war to occur.  However we believe that if
   such a case exists in the Internet the middle box design itself is
   flawed.  Consider a similar scenario where the RST from M-B to E-A
   gets lost, E-A will continue to hold the connection and E-A might
   send an ACK an arbitrary time later after the connection state was
   destroyed at M-B.  For this case, M-B will have to cache the RST for
   an arbitrary amount of time till until it is confirmed that the
   connection has been cleared at E-A.  Further, this is not compliant
   to RFC793 [RFC0793] which dictates that the sequence number of a RST has to
   be derived from the acknowledgment number of the incoming ACK
   segment.

8.2.  Middleboxes that advance sequence numbers

   Some middleboxes may compute RST sequence numbers at the higher end
   of the acceptable window.  The scenario is the same as the earlier
   case, but in this case instead of sending the cached RST, the
   middlebox (M-B) sends a RST that computes its sequence number as a
   sum of the ack field in the ACK and the window advertised by the ACK
   that was sent by E-A to challenge the RST as depicted below.  The
   difference in the sequence numbers between step 1 and 2 below is due
   to data lost in the network.

      TCP A                                                   Middlebox

   1. ESTABLISHED  <-- <SEQ=500><ACK=100><CTL=RST>          <--  CLOSED

   2. ESTABLISHED  --> <SEQ=100><ACK=300><WND=500><CTL=ACK> -->  CLOSED

   3. ESTABLISHED  <-- <SEQ=800><ACK=100><CTL=RST>          <--  CLOSED

   4. ESTABLISHED  --> <SEQ=100><ACK=300><WND=500><CTL=ACK> -->  CLOSED

   5. ESTABLISHED  <-- <SEQ=800><ACK=100><CTL=RST>          <--  CLOSED

   Although the authors are not aware of an implementation that does the
   above, it could be mitigated by implementing the ACK throttling
   mechanism described earlier.

9.  Interoperability Testing

   Interoperability testing was performed among the operating systems
   from Juniper Networks, WindRiver Systems, QNX Software and Cisco
   Systems.  The following topology was used:

   +---------+           +---------+
   |TCP    A |----+------|Victim B |
   +---------+    |      +---------+
                  |
   +---------+    |
   |Attacker |----+
   +---------+

   In the above topology B is  Security Considerations

   These changes to the unit under test. TCP A state machine do NOT protect an
   implementation from on-path attacks.  Situations where an on-path
   attack is a remote
   peer and the attacker workstation is used to generate malicious
   spoofed packets.

   First, an unmodifed stack had the following tests performed upon it.
   A TCP connection was brought up between TCP endpoint A and B. The
   4-tuple of the connection was manually populated in the brute force
   attack script running on the attacker workstation.  The script
   crafted TCP packets by using the 4-tuple and threat should be mitigated by generating sequence
   numbers that incremented from 0 to the max permissible sequence
   number (2^32 -1) in varying increments of window size use of 8K to 64K
   (the window size agreed by A IPSEC-AH
   [RFC4302] and B was larger than 8K, but was
   ignored to IPSEC-ESP [RFC4303].  It should also be emphasised that
   while mitigations within this document make the test conservative).  The time it took to cause
   the connection harder for off-path
   attackers to reset/corrupt was then recorded. inject segments, it does NOT make it impossible.  The test was repeated by then generating sequence numbers that were
   window (the one agreed between endpoints A and B) size apart.  It was
   observed that increasing the window size caused the connection
   only way to be
   reset faster than with a smaller window.  Further, it was also
   observed that the initial sequence number (ISN) selection also played
   a role in how fast fully protect a TCP connection was aborted, with ISN selection in
   the lower half of the sequence space aborting sooner than an ISN in
   the upper half.  Results were also found to be influenced by any
   active data transfer from both on the connection.

   Active data transfer sometimes caused the connection to be reset
   faster and some other times to slow the attack.  The window size
   selection at the attacker workstation and how it compares to off path
   attacks is the
   actual window size between A use of both IPSEC-AH [RFC4302] and B was also found to IPSEC-ESP
   [RFC4303].

   Another consideration that should be noted is a factor.
   The tests were repeated with a stack that did have the fix and as
   expected it became difficult as compared to the previous results to
   cause the connection to abort.

   [Editors Note: Add time test data gathered at inter-op here!!]

10.  Security Considerations

   A reflector attack is
   possible with the proposed required RST/SYN mitigation techniques.  Here  In this
   attack an off-path attacker can cause a victim to send an ACK segment
   for each spoofed RST/SYN segment that lies within the current receive
   window of the victim.  This,  It should be noted, however, that this does
   not cause any sort of amplification since the attacker must generate a
   segment for each one that the victim will generate.

11.

10.  IANA Considerations

   This document contains no IANA considerations.

12.

11.  Contributors

   Mitesh Dalal and Amol Khare of Cisco Systems came up with the
   solution for the RST/SYN attacks.  Anantha Ramaiah and Randall
   Stewart of Cisco Systems discovered the data injection vulnerability
   and together with Patrick Mahan and Peter Lei of Cisco Systems found
   solutions for the same.  Paul Goyette, Mark Baushke, Frank
   Kastenholz, Art Stine and David Wang of Juniper Networks provided the
   insight that apart from RSTs, SYNs could also result in formidable
   attacks.  Shrirang Bage of Cisco Systems, Qing Li and Preety Puri of
   Wind River Systems and Xiaodan Tang of QNX Software along with the
   folks above helped in ratifying and testing the interoperability of
   the suggested solutions.

13.

   Ack throttling was introduced to this document by combining the
   suggestions from the tcpm working group.

12.  Acknowledgments

   Special thanks to Mark Allman, Ted Faber, Steve Bellovin, Vern
   Paxson, Allison Mankin, Sharad Ahlawat, Damir Rajnovic, John Wong Wong,
   Joe Touch, and other members of the tcpm WG members for suggestions
   and comments.  Some of the text
   in this document has been derived from

14.

13.  References

14.1.

13.1.  Normative References

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

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

14.2.

   [RFC4302]  Kent, S., "IP Authentication Header", RFC 4302,
              December 2005.

   [RFC4303]  Kent, S., "IP Encapsulating Security Payload (ESP)",
              RFC 4303, December 2005.

13.2.  Informative References

   [DTASA]

   [I-D.touch-tcp-antispoof]
              Touch, J., "Defending TCP Against Spoofing Attacks",
              draft-touch-tcp-antispoof-00 (work in progress),
              July 2004.

   [Medina05]
              Medina, A., Allman, M., and S. Floyd, "Measuring the
              Evolution of Transport Protocols in the Internet. ACM
              Computer Communication Review, 35(2), April 2005.
              http://www.icir.org/mallman/papers/tcp-evo-ccr05.ps
              (figure 6)".

   [NISCC]    NISCC, "NISCC Vulnerability Advisory 236929 -
              Vulnerability Issues in TCP".

   [RFC1122]  Braden, R., "Requirements for Internet Hosts -
              Communication Layers", STD 3, RFC 1122, October 1989.

   [RFC1323]  Jacobson, V., Braden, B., and D. Borman, "TCP Extensions
              for High Performance", RFC 1323, May 1992.

   [RFC1771]  Rekhter, Y. and T. Li, "A Border Gateway Protocol 4
              (BGP-4)", RFC 1771, March 1995.

   [RFC3562]  Leech, M., "Key Management Considerations for the TCP MD5
              Signature Option", RFC 3562, July 2003.

   [SITW]     Watson, P., "Slipping in the Window: TCP Reset attacks". attacks,
              Presentation at 2004 CanSecWest
              http://www.cansecwest.com/archives.html".

Authors' Addresses

   Randall R. Stewart
   Editor
   4875 Forest Drive
   Suite 200
   Columbia, SC  29206
   USA

   Phone:
   Email: rrs@cisco.com

   Mitesh Dalal
   Editor
   170 Tasman Drive
   San Jose, CA  95134
   USA

   Phone: +1-408-853-5257
   Email: mdalal@cisco.com

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