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Versions: 00 01 02 03 04 05 06 rfc4953                                  
IETF TCPM WG                                                   J. Touch
Internet Draft                                                  USC/ISI
Expires: April 2006                                     October 8, 2005

                  Defending TCP Against Spoofing Attacks

Status of this Memo

   By submitting this Internet-Draft, each author represents that
   any applicable patent or other IPR claims of which he or she is
   aware have been or will be disclosed, and any of which he or she
   becomes aware will be disclosed, in accordance with Section 6 of
   BCP 79.

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   This Internet-Draft will expire on April 8, 2006.

Copyright Notice

   Copyright (C) The Internet Society (2005).  All Rights Reserved.


   Recent analysis of potential attacks on core Internet infrastructure
   indicates an increased vulnerability of TCP connections to spurious
   resets (RSTs), sent with forged IP source addresses (spoofing).  TCP
   has always been susceptible to such RST spoofing attacks, which were
   indirectly protected by checking that the RST sequence number was
   inside the current receive window, as well as via the obfuscation of

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   TCP endpoint and port numbers.  For pairs of well-known endpoints
   often over predictable port pairs, such as BGP or between web servers
   and well-known large-scale caches, increases in the path bandwidth-
   delay product of a connection have sufficiently increased the receive
   window space that off-path third parties can guess a viable RST
   sequence number. The susceptibility to attack increases as the square
   of the bandwidth, thus presents a significant vulnerability for
   recent high-speed networks. This document addresses this
   vulnerability, discussing proposed solutions at the transport level
   and their inherent challenges, as well as existing network level
   solutions and the feasibility of their deployment. This document
   focuses on vulnerabilities due to spoofed TCP segments, and includes
   a discussion of related ICMP spoofing attacks on TCP connections.

Table of Contents

   1. Introduction...................................................3
   2. Background.....................................................4
      2.1. Review of TCP Windows.....................................5
      2.2. Recent BGP Attacks Using TCP RSTs.........................5
      2.3. TCP RST Vulnerability.....................................6
      2.4. What Changed - the Ever Opening Advertised Receive Window.7
   3. Proposed Solutions and Mitigations.............................9
      3.1. Transport Layer Solutions................................10
         3.1.1. TCP MD5 Authentication..............................10
         3.1.2. TCP RST Window Attenuation..........................10
         3.1.3. TCP Timestamp Authentication........................11
         3.1.4. Other TCP Cookies...................................12
         3.1.5. Other TCP Considerations............................12
         3.1.6. Other Transport Protocol Solutions..................13
      3.2. Network Layer (IP) Solutions.............................13
         3.2.1. Address filtering...................................13
         3.2.2. IPsec...............................................14
   4. ICMP..........................................................15
   5. Issues........................................................16
      5.1. Transport Layer (e.g., TCP)..............................16
      5.2. Network Layer (IP).......................................17
      5.3. Application Layer........................................18
      5.4. Shim Transport/Application Layer.........................19
      5.5. Link Layer...............................................19
      5.6. Issues Discussion........................................19
   6. Security Considerations.......................................20
   7. IANA Considerations...........................................20
   8. Conclusions...................................................20
   9. Acknowledgments...............................................21
   10. References...................................................21

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      10.1. Normative References....................................21
      10.2. Informative References..................................21
   Author's Addresses...............................................24
   Intellectual Property Statement..................................24
   Disclaimer of Validity...........................................25
   Copyright Statement..............................................25

1. Introduction

   Analysis of the Internet infrastructure has been recently
   demonstrated new version of a vulnerability in BGP connections
   between core routers using an attack known for nearly six years
   [8][9][18][40].  These connections, typically using TCP, can be
   susceptible to off-path third-party reset (RST) segments with forged
   source addresses (spoofed), which terminate the TCP connection.  BGP
   routers react to a terminated TCP connection in various ways which
   can amplify the impact of an attack, ranging from restarting the
   connection to deciding that the other router is unreachable and thus
   flushing the BGP routes [32].  This sort of attack affects other
   protocols besides BGP, involving any long-lived connection between
   well-known endpoints.  The impact on Internet infrastructure can be
   substantial (esp. for the BGP case), and warrants immediate

   TCP, like many other protocols, can be susceptible to these off-path
   third-party spoofing attacks.  Such attacks rely on the increase of
   commodity platforms supporting public access to previously privileged
   resources, such as root-level access.  Given such access, it is
   trivial for anyone to generate a packet with any header desired.

   This, coupled with the lack of sufficient address filtering to drop
   such spoofed traffic, can increase the potential for off-path third-
   party spoofing attacks.  Proposed solutions include the deployment of
   existing Internet network and transport security as well as
   modifications to transport protocols that reduce its vulnerability to
   generated attacks.

   One way to defeat spoofing is to validate the segments of a
   connection, either at the transport level or the network level.  TCP
   with MD5 extensions provides this authentication at the transport
   level, and IPsec provides authentication at the network level. In
   both cases their deployment overhead may be prohibitive, e.g., it may
   not feasible for public services, such as web servers, to be
   configured with the appropriate certificate authorities of large
   numbers of peers (for IPsec using IKE), or shared secrets (for IPsec
   in shared-secret mode, or TCP/MD5), because many clients may need to

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   be configured rapidly without external assistance.  Services from
   public web servers connecting to large-scale caches to BGP with
   larger numbers of peers can experience this challenge.

   The remainder of this document outlines the recent attack scenario in
   detail and describes and compares a variety of solutions, including
   existing solutions based on TCP/MD5 and IPsec, as well as recently
   proposed solutions, including modifications to TCP's RST processing
   [10], modifications to TCP's timestamp processing [30], and
   modifications to IPsec and TCP/MD5 keying [38]. This document focuses
   on spoofing of TCP segments, although a discussion of related
   spoofing of ICMP packets based on spoofed TCP contents is also

   Note that the description of these attacks is not new; attacks using
   RSTs on BGP have been known since 1998, and were the reason for the
   development of TCP/MD5 [18]. The recent attack scenario was first
   documented by Convery at a NANOG meeting in 2003, but that analysis
   assumed the entire sequence space (2^32 packets) needed to be covered
   for an attack to succeed [9]. Watson's more detailed analysis
   discovered that a single packet anywhere in the current window could
   succeed at an attack [40]. This document adds the observation that
   susceptibility to attack goes as the square of bandwidth, due to the
   coupling between the linear increase in receive window size and
   linear increase in rate an attacker, as well as comparing the variety
   of more recent proposals, including modifications to TCP, use of
   IPsec, and use of TCP/MD5 to resist such attacks.

2. Background

   The recent analysis of potential attacks on BGP has again raised the
   issue of TCP's vulnerability to off-path third-party spoofing attacks
   [8][9][40].  A variety of such attacks have been known for several
   years, including sending RSTs, SYNs, and even ACKs in an attempt to
   affect an existing connection or to load down servers.  Overall, such
   attacks are countered by the use of some form of authentication at
   the network (e.g., IPsec), transport (e.g., SYN cookies, TCP/MD5), or
   other layers.  TCP already includes a weak form of such
   authentication in its check of segment sequence numbers against the
   current receiver window.  Increases in the bandwidth-delay product
   for certain long connections have sufficiently weakened this type of
   weak authentication in recent weeks, rendering it moot.

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2.1. Review of TCP Windows

   Before proceeding, it is useful to review the terminology and
   components of TCP's windowing algorithm. TCP connections have three
   kinds of windows [1][31]:

   o  Send window (SND.WND): the latest advertised send window size.

   o  Receive window (RCV.WND): the latest advertised receive window

   o  Congestion window (CWND): the window determined by congestion
      feedback that limits how much of RCV.WND can be in-flight in a
      round trip time.

   For most modern TCP connections, SND.WND and RCV.WND are the size of
   the corresponding send and receive socket buffers, and are
   configurable using socket buffer resizing commands.

   CWND determines how much data can be in transit in a round trip time,
   SND.WND determines how much data the sender is willing to store on
   its side for possible retransmission due to loss, and RCV.WND
   determines the ability of the receiver to accommodate that loss and
   reorder received packets. CWND never grows beyond RCV.WND.

   High bandwidth-delay product networks need CWND to be sufficiently
   large to accommodate as much data would be in transit in a round trip
   time, otherwise their performance will suffer. As a result, it is
   recommended that users and various automatic programs increase
   RCV.WND to at least the size of bandwidth*delay (the bandwidth-delay
   product) [19][33].

   As the bandwidth-delay product of the network increases, however,
   such increases in the advertised receive window can cause increased
   susceptibility to spoofing attacks, as the remainder of this document
   shows. This assumes, however, that the receive window size (e.g., via
   increased receive socket buffer configuration) is increased with the
   increased bandwidth-delay product; if not, then connection
   performance will degrade, but susceptibility to spoofing attacks will
   increase only linearly (with the rate of the attacker to send spoofed
   packets), not as the square of the bandwidth.

2.2. Recent BGP Attacks Using TCP RSTs

   BGP represents a particular vulnerability to spoofing attacks because
   it uses TCP connectivity to infer routability, so losing a TCP

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   connection with a BGP peer can result in the flushing of routes to
   that peer [32].

   Until six years ago, such connections were assumed difficult to
   attack because they were described by a few comparatively obscure
   parameters [18]. Most TCP connections are protected by multiple
   levels of obfuscation except at the endpoints of the connection:

  o Both endpoint addresses are usually not well-known; although server
     addresses are advertised, clients are somewhat anonymous.

  o Both port numbers are usually not well-known; the server's usually
     is advertised (representing the service), but the client's is
     typically sufficiently unpredictable to an off-path third-party.

  o Valid sequence number space is not well-known.

  o Connections are relatively short-lived and valid sequence space
     changes, so any guess of the above information is unlikely to be

   BGP represents an exception to the above criteria (though not the
   only case).  Both endpoints can be well-known, using hints from part
   of an AS path.  The destination port is typically fixed to indicate
   the BGP service.  The source port used by a BGP router is sometimes
   fixed and advertised to enable firewall configuration; even when not
   fixed, there are only 65,384 valid source ports which may be
   exhaustively attacked.  Connections are long-lived, and as noted
   before some BGP implementations interpret successive TCP connection
   failures as routing failures, discarding the corresponding routing
   information. As importantly and as will be shown below, the valid
   sequence number space once thought to provide some protection has
   been rendered useless by increasing advertised receive window sizes.

2.3. TCP RST Vulnerability

   TCP has a known vulnerability to third-party spoofed segments.  SYN
   flooding consumes server resources in half-open connections,
   affecting the server's ability to open new connections.  ACK spoofing
   can cause connections to transmit too much data too quickly, creating
   network congestion and segment loss, causing connections to slow to a
   crawl.  In the most recent attacks on BGP, RSTs cause connections to
   be dropped.  As noted earlier, some BGP implementations interpret TCP
   connection termination, or a series of such failures, as a network
   failure [32].  This causes routers to drop the BGP routing
   information already exchanged, in addition to inhibiting their

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   ongoing exchanges, thus amplifying the impact of the attack.  The
   result can affect routing paths throughout the Internet.

   The dangerous effects of RSTs on TCP have been known for many years,
   even when used by the legitimate endpoints of a connection.  TCP RSTs
   cause the receiver to drop all connection state; because the source
   is not required to maintain a TIME_WAIT state, such a RST can cause
   premature reuse of address/port pairs, potentially allowing segments
   from a previous connection to contaminate the data of a new
   connection, known as TIME_WAIT assassination [7].  In this case,
   assassination occurs inadvertently as the result of duplicate
   segments from a legitimate source, and can be avoided by blocking RST
   processing while in TIME_WAIT.  However, assassination can be useful
   to deliberately reduce the state held at servers; this requires that
   the source of the RSTs go into TIME_WAIT state to avoid such hazards,
   and that RSTs are not blocked in the TIME_WAIT state [11].

   Firewalls and load balancers, so-called 'middleboxes', sometimes emit
   RSTs on behalf of transited connections to optimize server
   performance [13].  This is effectively an on-path RST attack in which
   the RSTs are sent for benign or beneficial intent.  There are
   numerous hazards with such use of RSTs, outlined in that RFC.

2.4. What Changed - the Ever Opening Advertised Receive Window

   RSTs represent a hazard to TCP, especially when completely unchecked.
   Fortunately, there are a number of obfuscation mechanisms that make
   it difficult for off-path third parties to forge (spoof) valid RSTs,
   as noted earlier.  We have already shown it is easy to learn both
   endpoint addresses and ports for some protocols, notably BGP.  The
   final obfuscation is the segment sequence number.

   TCP segments include a sequence number which enables out-of-order
   receiver processing as well as duplicate detection.  The sequence
   number space is also used to manage congestion, and indicates the
   index of the next byte to be transmitted or received.  For RSTs, this
   is relevant because legitimate RSTs use the next sequence number in
   the transmitter window, and the receiver checks that incoming RSTs
   have a sequence number in the expected receive window.  Such
   processing is intended to eliminate duplicate segments (somewhat moot
   for RSTs, though), and to drop RSTs which were part of previous

   TCP uses two window mechanisms, a primary mechanism which uses a
   space of 32 bits, and a secondary mechanism which scales this window
   [19][31].  The valid advertised receive window is a fraction, not to
   exceed approximately half, of this space, or ~2 billion (2E9, i.e.,

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   U.S. billion).  Under typical configurations, the majority of TCP
   connections open to a very small fraction of this space, e.g.,
   10,000-60,000(approximately 5-100 segments).  This is because the
   advertised receive window typically matches the receive socket buffer
   size. It is recommended that this buffer be tuned to match the needs
   of the connection, either manually or by automatic external means

   On a low-loss path, the advertised receive window should be
   configured to match the path bandwidth-delay product, including
   buffering delays (assume 1 packet/hop) [33].  Many paths in the
   Internet have end-to-end bandwidths of under 1 Mbps, latencies under
   100ms, and are under 15 hops, resulting in fairly small advertised
   receive windows as above (under 35,000 bytes).  Under these
   conditions, and further assuming that the initial sequence number is
   suitably (pseudo-randomly) chosen, a valid guessed sequence number
   would have odds of 1 in 57,000 of falling within the advertised
   receive window.  Put differently, a blind (i.e., off-path) attacker
   would need to send 57,000 RSTs with suitably spaced sequence number
   guesses to successfully reset a connection.  At 1 Mbps, 57,000 (40
   byte) RSTs would take over 50 minutes to transmit, and, as noted
   earlier, most current connections are fairly brief by comparison.

   Recent use of high bandwidth paths of 10 Gbps and higher result in
   bandwidth-delay products over 125 MB - approximately 1/10 of TCP's
   overall maximum advertised receive window size (i.e., assuming the
   receive socket buffers are increased as much as possible) excluding
   scale, assuming the receiver allocates sufficient buffering (as
   discussed in Sec. 2. ).  Even under networks that are ten times
   slower (1 Gbps), the active advertised receive window covers 1/100th
   of the overall window size.  At these speeds, it takes only 10-100
   packets, or less than 32 microseconds, to correctly guess a valid
   sequence number and kill a connection.  A table of corresponding
   exposure to various amounts of RSTs is shown below, for various line
   rates, assuming the more conventional 100ms latencies (though even
   100ms is large for BGP cases):

          BW       BW*delay   RSTs needed     Time needed
       10 Gbps   125       MB          35     1 us (microsecond)
        1 Gbps    12.5     MB         344   110 us
      100 Mbps     1.25    MB       3,436    10 ms (millisecond)
       10 Mbps     0.125   MB      34,360     1 second
        1 Mbps     0.0125  MB     343,598     2 minutes
      100 Kbps     0.00125 MB   3,435,974     3 hours

                 Figure 1 Time needed to kill a connection

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   This table demonstrates that the effect of bandwidth on the
   vulnerability is squared; for every increase in bandwidth, there is a
   linear decrease in the number of sequence number guesses needed, as
   well as a linear decrease in the time needed to send a set of
   guesses.  Notably, as inter-router link bandwidths approach 1 Mbps,
   an 'exhaustive' attack becomes practical.  Checking that the RST
   sequence number is somewhere in the advertised receive window out of
   the overall maximum receive window (2^32) is an insufficient

   Note that this table makes a number of assumptions:

   1. the overall bandwidth-delay product is relatively fixed

   2. traffic losses are negligible (insufficient to affect the
      congestion window over the duration of most of the connection)

   3. the advertised receive window is a large fraction of the overall
      maximum receive window size, e.g., because the receive socket
      buffers are set to match a large bandwidth-delay product

   4. the attack bandwidth is similar to the end-to-end path bandwidth

   Of these assumptions, the last two are more notable.  The issue of
   receive socket buffers was discussed in Sec. 2.  The issue of the
   attack bandwidth is considered reasonable as follows:

   1. RSTs are substantially easier to send than data; they can be
      precomputed and they are smaller than data packets (40 bytes).

   2. although susceptible connections use somewhat less ubiquitous
      high-bandwidth paths, the attack may be distributed, at which
      point only the ingress link of the attack is the primary

   3. for the purposes of the above table, we assume that the ingress at
      the attack has the same bandwidth as the path, as an approximation

   The previous sections discussed the nature of the recent attacks on
   BGP due to the vulnerability of TCP to RST spoofing attacks, due
   largely to recent increases in the fraction of the TCP advertised
   receive window space in use for a single, long-lived connection.

3. Proposed Solutions and Mitigations

   TCP currently authenticates received RSTs using the address and port
   pair numbers, and checks that the sequence number is inside the valid

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   receiver window.  The previous section demonstrated how TCP has
   become more vulnerable to RST spoofing attacks due to the increases
   in the receive window size.  There are a number of current and
   proposed solutions to this vulnerability, all attempting to provide
   evidence that a received RST is legitimate.

3.1. Transport Layer Solutions

   The transport layer represents the last place that segments can be
   authenticated before they affect connection management.  TCP has a
   variety of current and proposed mechanisms to increase the
   authentication of segments, protecting against both off-path and on-
   path third-party spoofing attacks.  SCTP also has mechanisms to
   authenticate segments.

3.1.1. TCP MD5 Authentication

   An extension to TCP supporting MD5 authentication was developed in
   1998 specifically to authenticate BGP connections (although it can be
   used for any TCP connection) [18].  The extension relies on a pre-
   shared secret key to authenticate the entire TCP segment, including
   the data, TCP header, and TCP pseudo-header (certain fields of the IP
   header).  All segments are protected, including RSTs, to be accepted
   only when their signature matches.  This option, although widely
   deployed in Internet routers, is considered undeployable for
   widespread use because the need for pre-shared keys [3][27].  It
   further is considered computationally expensive for either hosts or
   routers due to the overhead of MD5 [36][37].

3.1.2. TCP RST Window Attenuation

   A recent proposal extends TCP to further constrain received RST to
   match the expected next sequence number [10].  This restores TCP's
   resistance to spurious RSTs, effectively limiting the receive window
   for RSTs to a single number.  As a result, an attacker would need to
   send 2^32 different packets to correctly guess the sequence number.
   The extension further modifies the RST receiver to react to
   incorrectly-numbered RSTs, by sending a zero-length ACK.  If the RST
   source is legitimate, upon receipt of an ACK the closed source would
   presumably emit a RST with the sequence number matching the ACK,
   correctly resetting the intended recipient.  This modification
   changes TCP's control processing, adding to its complexity and thus
   potentially affecting its correctness (in contrast to adding MD5
   signatures, which is orthogonal to TCP control processing
   altogether).  For example, there may be complications between RSTs of
   different connections between the same pair of endpoints because RSTs
   flush the TIME-WAIT (as mentioned earlier).  Further, this proposal

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   modifies TCP so that under some circumstances a RST causes a reply
   (an ACK), in violation of generally accepted practice, if not gentle
   recommendation - although this can be omitted, allowing timeouts to
   suffice.  The advantage to this proposal is that it can be deployed
   incrementally and has benefit to the endpoint on which it is

   A variant of this proposal uses a different value to attenuate the
   window of viable RSTs.  It requires RSTs to carry the initial
   sequence number rather than the next expected sequence number, i.e.,
   the value negotiated on connection establishment [35][41].  This
   proposal has the advantage of using an explicitly negotiated value,
   but at the cost of changing the behavior of an unmodified endpoint to
   a currently valid RST.  It would thus be more difficult, without
   additional mechanism, to deploy incrementally.

   Another variant of this proposal involves increasing TCP's window
   space, rather than decreasing the valid range for RSTs, i.e.,
   increasing the sequence space from 32 bits to 64 bits.  This has the
   equivalent effect - the ratio of the valid sequence numbers for any
   segment to the overall sequence number space is significantly
   reduced.  The use of the larger space, as with current schemes to
   establish weak authentication using initial sequence numbers (ISNs),
   is contingent on using suitably random values for the ISN.  Such
   randomness adds additional complexity to TCP both in specification
   and implementation, and provides only very weak authentication.  Such
   a modification is not obviously backward compatible, and would be
   thus difficult to deploy.

   A converse variant of increasing TCP's window space is to decrease
   the receive window, which would further reduce the effectiveness of
   spoofed RSTs with random sequence numbers. This alternative may
   reduce the throughput of the connection, if the advertised receive
   window is smaller than the bandwidth-delay product of the connection.

3.1.3. TCP Timestamp Authentication

   Another way to authenticate TCP segments is to utilize its timestamp
   option, using the value as a sort of authentication [30].  This
   requires that the receiver TCP discard values whose timestamp is
   outside the accepted window, which is derived from the timestamps of
   other packets from the same connection.  This technique uses an
   existing TCP option, but also requires modified TCP control
   processing (with the same caveats) and may be difficult to deploy
   incrementally without further modifications.  Additionally, the
   timestamp value may be easier to guess because it is derived from a
   predictable value.

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3.1.4. Other TCP Cookies

   All of the above techniques are variants of cookies, otherwise
   meaningless data whose value is used to validate the packet.  In the
   case of MD5 checksums, the cookie is computed based on a shared
   secret.  Note that even a signature can be guessed, and presents a 1
   in 2^(signature length) probability of attack.  The primary
   difference is that MD5 signatures are effectively one-time cookies,
   not predictable based on on-path snooping, because they are dependent
   on packet data and thus do not repeat.  Window attenuation sequence
   numbers can be guessed by snooping the sequence number of current
   packets, and timestamps can be guessed even more remotely.  These
   variants of cookies are similar in spirit to TCP SYN cookies, again
   patching a vulnerability to off-path third-party spoofing attacks
   based on a (fairly weak, excepting MD5) form of authentication.
   Another form of cookie is the source port itself, which can be
   randomized but provides only 16 bits of protection (65,000
   combinations), which may be exhaustively attacked. This can be
   combined with destination port randomization as well, but that would
   require a separate coordination mechanism (so both parties know which
   ports to use), which is equivalent to (and as infeasible for large-
   scale deployments as) exchanging a shared secret.

3.1.5. Other TCP Considerations

   The analysis of the potential for RST spoofing above assumes that the
   advertised receive window is opened to the maximum extent suggested
   by the bandwidth-delay product of the end-to-end path, and that the
   window is opened to an appreciable fraction of the overall sequence
   number space.  As noted earlier, for most common cases, connections
   are too brief or over bandwidths too low for such a large window to
   be useful. Expanding TCP's sequence number space is a direct way to
   further avoid such vulnerability, even for long connections over
   emerging bandwidths. If either manual tuning or automatic tuning of
   the advertised receive window (via receive buffer tuning) is not
   provided, this is not an issue (although connection performance will
   suffer) [33].

   It is may be sufficient for the endpoint to limit the advertised
   receive window by deliberately leaving it small.  If the receive
   socket buffer is limited, e.g., to the ubiquitous default of 64KB,
   the advertised receive window will not be as vulnerable even for very
   long connections over very high bandwidths. The vulnerability will
   grow linearly with the increased network speed, but not as the
   square.  The consequence is lower sustained throughput, where only
   one window's worth of data per round trip time (RTT) is exchanged.
   This will keep the connection open longer; for long-lived connections

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   with continuous sourced data, this may continue to present an attack
   opportunity, albeit a sparse and slow-moving target. For the most
   recent case where BGP data is being exchanged between Internet
   routers, the data is bursty and the aggregate traffic may be small
   (i.e., unlikely to cover a substantial portion of the sequence space,
   even if long-lived), so is smaller advertised receive windows (via
   small receiver buffers) may, in some cases, sufficiently address the
   immediate problem. This assumes that the routing tables can be
   exchanged quickly enough with bandwidth reduced due to the smaller
   buffers, or perhaps that the advertised receive window is opened only
   during a large burst exchange (e.g., via some other signal between
   the two routers).

3.1.6. Other Transport Protocol Solutions

   Segment authentication has been addressed at the transport layer in
   other protocols.  Both SCTP and DCCP include cookies for connection
   establishment and use them to authenticate a variety of other control
   messages [26][34].  The inclusion of such mechanism at the transport
   protocol, although emerging as standard practice, unnecessarily
   complicates the design and implementation of new protocols [28]  As
   new attacks are discovered (SYN floods, RSTs, etc.), each protocol
   must be modified individually to compensate.  A network solution may
   be more appropriate and efficient.

3.2. Network Layer (IP) Solutions

   There are two primary variants of network layer solutions to
   spoofing: address filtering and IPsec. Address filtering is an
   indirect system which relies on other parties to filter packets sent
   upstream of an attack, but does not necessarily require participation
   of the packet source. IPsec requires cooperation between the
   endpoints wanting to avoid attack on their connection, which
   currently involves pre-existing shared knowledge of either a shared
   key or shared certificate authority.

3.2.1. Address filtering

   Address filtering is often proposed as an alternative to protocol
   mechanisms to defeat IP source address spoofing [2][12]. Address
   filtering restricts traffic from downstream sources across transit
   networks based on the IP source address. It cannot restrict traffic

   from the core to edges, i.e., from upstream sources. As a result,
   each border router must perform the appropriate filtering for overall
   protection to result; failure of any border router to filter defeats
   the protection of all participants inside the border, ultimately.
   Address filtering at the border can protect those inside the border

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   from some kinds of spoofing, because only interior addresses should
   originate inside the border. It cannot, however, protect connections
   originating outside the border except to restrict where the traffic
   enters from, e.g., if it expected from one AS and not another.

   As a result, address filtering is not a local solution that can be
   deployed to protect communicating pairs, but rather relies on a
   distributed infrastructure of trusted gateways filtering forged
   traffic where it enters the network. It is not feasible for local,
   incremental deployment, and relies too heavily on distributed
   cooperation. Although useful to reduce the load of spoofed traffic,
   it is insufficient to protect particular connections from attack.

   A more recent variant of address filtering checks the IP TTL field,
   relying on the TTL set by the other end of the connection [14]. This
   technique has been used to provide filtering for BGP. It assumes the
   connection source TTL is set to 255; packets at the receiver are
   checked for TTL=255, and others are dropped. This restricts traffic
   to one hop upstream of the receiver (i.e., a BGP router), but those
   hops could include other user programs at those nodes (e.g., the BGP
   router's peer) or any traffic those nodes accept via tunnels -
   because tunnels need not decrement TTLs [29] (see Sec. 5.1 of [14]).
   This method of filtering works best where traffic originates one hop
   away, so that the address filtering is based on the trust of only
   directly-connected (tunneled or otherwise) nodes. Like conventional
   address filtering, this reduces spoofing traffic in general, but is
   not considered a reliable security mechanism because it relies on
   distributed filtering (e.g., the fact that upstream nodes do not
   terminate tunnels arbitrarily).

3.2.2. IPsec

   TCP is susceptible to RSTs, but also to other off-path and on-path
   spoofing attacks, including SYN attacks.  Other transport protocols,
   such as UDP and RTP are equally susceptible.  Although emerging
   transport protocols attempt to defeat such attacks at the transport
   layer, such attacks take advantage of network layer identity
   spoofing.  The packet is coming from an endpoint who is spoofing
   another endpoint, either upstream or somewhere else in the Internet.
   IPsec was designed specifically to establish and enforce
   authentication of a packet's source and contents, to most directly
   and explicitly addresses this security vulnerability.

   The larger problem with IPsec is that of key distribution and use.
   IPsec is often cumbersome, and has only recently been supported in
   many end-system operating systems.  More importantly, it relies on
   preshared keys, signed X.509 certificates, or a third-party (e.g.,

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   Kerberos) public key infrastructure to establish and exchange keying
   information (e.g., via IKE).  These present challenges when using
   IPsec to secure traffic to a well-known server, whose clients may not
   support IPsec or may not have registered with a previously-known
   certificate authority (CA).

   These keying challenges are being addressed in the IETF in ways that
   will enable servers secure associations with other parties without
   advance coordination [38][39]. This can be especially useful for
   publicly-available servers, or for protecting connections to servers
   that - for whatever reason - have not, or will not deploy
   conventional IPsec certificates (i.e., core Internet BGP routers).


   Just as spoofed TCP packets can kill a connection, so too can spoofed
   ICMP packets. TCP headers can occur inside certain ICMP messages [6].
   There have been recent suggestions to validate the sequence number of
   TCP headers when they occur inside ICMP messages [16]. This sequence
   checking is similar to checks that would occur for conventional data
   packets in TCP, but is being proposed in the spirit of the RST window
   attenuation described in Section 3.1.2.

   Some such checks may be reasonable, especially where they parallel
   the validations already performed by TCP processing, notably where
   they emulate the semantics of such processing. For example, the TCP
   checksum should be validated (if the entire TCP segment is contained
   in the ICMP message) before any fields of the TCP header are
   examined, to avoid reacting to corrupted packets. Similarly, if the
   TCP MD5 option is present, its signature should probably be validated
   before considering the contents of the message.

   Such validation can ensure that the packet was not corrupted prior to
   the ICMP generation (checksum), that the packet was one sent by the
   source (IPsec or TCP/MD5 authenticated), or that the packet was not
   in the network for an excess of 2*MSL (valid sequence number).

   ICMP presents a particular challenge because some messages can reset
   a connection more easily - with less validation - than even some
   spoofed TCP segments. However, fixing such messages to be 'in window'
   is insufficient protection, as this document shows for spoofed data.
   In addition, many networks filter all ICMP packets because validation
   may not be possible, especially because they can be injected from any
   point on a path, and so cannot be selectively address filtered. As a
   result, they are not addressed separately in the issues or security
   considerations of this document further.

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5. Issues

   There are a number of existing and proposed solutions addressing the
   vulnerability of transport protocols in general, and TCP in specific,
   to off-path third-party spoofing attacks.  As shown, these operate at
   the transport or network layer.  Transport solutions require separate
   modification of each transport protocol, addressing network identity
   spoofing separately in the context of each transport association.
   Network solutions are computationally intensive and require pervasive
   registration of certificate authorities with every possible endpoint.
   This section explains these observations further.

5.1. Transport Layer (e.g., TCP)

   Transport solutions rely on shared cookies to authenticate segments,
   including data, transport header, and even pseudo-header (e.g., fixed
   portions of the outer IP header in TCP).  Because the Internet relies
   on stateless network protocols, it makes sense to rely on state
   establishment and maintenance available in some transport layers not
   only for the connection but for authentication state.  Three-way
   handshakes and heartbeats can be used to negotiate authentication
   state in conjunction with connection parameters, which can be stored
   with connection state easily.

   As noted earlier, transport layer solutions require separate
   modification of all transport protocols to include authentication.
   Not all transport layers support negotiated endpoint state (e.g.,
   UDP), and legacy protocols have been notoriously difficult to safely
   augment.  Not all authentication solutions are created equal either,
   and relying on a variety of transport solutions exposes end-systems
   to increased potential for incorrectly specified or implemented
   solutions.  Transport authentication has often been developed piece-
   wise, in response to specific attacks, e.g., SYN cookies and RST
   window attenuation [4][10].

   Transport layer solutions are not only per-protocol, but often per-
   connection.  Each connection needs to negotiate and maintain
   authentication state separately.  Some overhead is not amortized over
   multiple connections, e.g., overheads in packet exchanges, whereas
   other overheads are not amortized over different transport protocols,
   e.g., design and implementation complexity - both as would be the
   case in a network layer solution.  Finally, because the
   authentication happens later in packet processing than is required,
   additional endpoint resources may be needlessly consumed, e.g., in
   demultiplexing received packets, indexing connection identifiers,
   etc., only to be dropped later at the transport layer.

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5.2. Network Layer (IP)

   A network layer solution avoids the hazards of multiple transport
   variants, using a single shared endpoint authentication mechanism
   early in receiver packet processing to discard unauthenticated
   packets at the network layer instead.  This defeats spoofing entirely
   because spoofing involves masquerading as another endpoint, and
   network layer security validates the endpoint as the source of the
   packets it emits. Such a network level solution protects all
   transport protocols as a result, including both legacy and emerging
   protocols, and reduces the complexity of these protocols as well.  A
   shared solution also reduces protocol overhead, and decouples the
   management (and refreshing) of authentication state from that of
   individual transport connections.  Finally, a network layer solution
   protects not only the transport layer but the network layer as well,
   e.g., from ICMP, IGMP, etc., spoofing attacks.

   The ubiquitous protocol for network layer authentication is IPsec
   [22][25].  IPsec specifies the overall architecture, including header
   authentication (AH) [21][23] and encapsulation (ESP) modes [24].  AH
   authenticates both the IP header and IP data, whereas ESP
   authenticates only the IP data (e.g., transport header and payload).
   AH is deprecated since ESP is more efficient and the SPI includes
   sufficient information to verify the IP header anyway.  These two
   modes describe the security applied to individual packets within the
   IPsec system; key exchange and management is performed either out-of-
   band (via pre-shared keys) or by an automated key exchange protocol
   IKE [17][20].

   IPsec already provides authentication of an IP header and its data
   contents sufficient to defeat both on-path and off-path third-party
   spoofing attacks.  IKE can configure authentication between two
   endpoints on a per-endpoint, per-protocol, or per-connection basis,
   as desired.  IKE also can perform automatic periodic re-keying,
   further defeating crypto-analysis based on snooping (clandestine data
   collection).  The use of IPsec is already commonly strongly
   recommended for protected infrastructure.

   Existing IPsec is not appropriate for many deployments.  It is
   computationally intensive both in key management and individual
   packet authentication [36].  As importantly, IKE is not anonymous;
   keys can be exchanged between parties only if they trust each others'
   X.509 certificates, trust some other third-party to help with key
   generation (e.g., Kerberos), or pre-share a key.  These certificates
   provide identification (the other party knows who you are) only where
   the certificates themselves are signed by certificate authorities
   (CAs) that both parties already trust.  To a large extent, the CAs

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   themselves are the pre-shared keys which help IKE establish security
   association keys, which are then used in the authentication

   Alternative mechanisms are under development to address this
   limitation, to allow publicly-accessible servers to secure
   connections to clients not known in advance, or to allow unilateral
   relaxation of identity validation so that the remaining protections
   of IPsec to be available [38][39]. In particular, these mechanisms
   can prevent a client (but without knowing who that client is) from
   being affected by spoofing from other clients, even when the
   attackers are on the communications path.

   IPsec, although widely available both in commercial routers and
   commodity end-systems, is not often utilized except between parties
   that already have a preexisting relationship (employee/employer,
   between two ISPs, etc.) Servers to anonymous clients (e.g., customer/
   business) or more open services (e.g., BGP, where routers may have
   large numbers of peers) are unmanageable, due to the breadth and flux
   of CAs.  New endpoints cannot establish IPsec associations with such
   servers unless their own certificate is signed by a CA already
   trusted by the server.  Different servers - even within the same
   overall system (e.g., BGP) - often cannot or will not trust
   overlapping subsets of CAs in general.

5.3. Application Layer

   There are a number of application layer authentication mechanisms,
   often implicit within end-to-end encryption.  Application-layer
   security (e.g., TLS, SSH, or MD5 checksums within a BGP stream)
   provides the ultimate protection of application data from all
   intermediaries, including network routers as well as exposure at
   other layers in the end-systems.  This is the only way to ultimately
   protect the application data.

   Application authentication cannot protect either the network or
   transport protocols from spoofing attacks, however.  Spoofed packets
   interfere with network processing or reset transport connections
   before the application checks the data.  Authentication needs to
   winnow these packets and drop them before they interfere at these
   lower layers.

   An alternate application layer solution would involve resilience to
   reset connections. If the application can recover from such
   connection interruptions, then such attacks have less impact.
   Unfortunately, attackers still affect the application, e.g., in the
   cost of restarting connections, delays until connections are

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   restarted, or increased connection establishment messages on the
   network. Some applications - notably BGP - even interpret TCP
   connection reliability as an indicator of route path stability, which
   is why attacks on BGP have such substantial consequences.

5.4. Shim Transport/Application Layer

   Security can also be provided over the transport layer but below the
   application layer, in a kind of 'shim' protocol, such as SSL or TLS.
   These protocols provide data protection for a variety of applications
   over a single, legacy transport protocol, such as SSL/TCP for HTTPS.
   Unfortunately, as with application authentication, they do not
   protect the transport layer against spoofing attacks.

5.5. Link Layer

   Link layer security operates separately on each hop of an Internet.
   Such security can be critical in protecting link resources, such as
   bandwidth and link management protocols.  Protection at this layer
   cannot suffice for network or transport layers, because it cannot
   authenticate the endpoint source of a packet.  Link authentication
   ensures only the source of the current link hop where it is examined.

5.6. Issues Discussion

   The issues raised in this section suggest that there are challenges
   with all solutions to transport protection from spoofing attacks.
   This raises the potential need for alternate security levels.  While
   it is already widely recognized that security needs to occur
   simultaneously at many protocol layers, there also may be utility in
   supporting a variety of strengths at a single layer.  For example,
   IPsec already supports a variety of algorithms (MD5, SHA1, etc. for
   authentication), but always assumes that:

   4. the entire body of the packet is secured

   5. security associations are established only where identity is
      authenticated by a know certificate authority or other pre-shared

   6. both on-path and off-path third-party spoofing attacks   must be

   These assumptions are prohibitive, especially in many cases of
   spoofing attacks.  For spoofing, the primary issue is whether packets
   are coming from the same party the server can reach.  Only the IP
   header is fundamentally in question, so securing the entire packet

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   (1) is computational overkill.  It is sufficient to authenticate the
   other party as "a party you have exchanged packets with", rather than
   establishing their trusted identity ("Bill" vs.  "Bob") as in (2).
   Finally, many cookie systems use clear-text (unencrypted), fixed
   cookie values, providing reasonable (1 in 2^{cookie-size}) protection
   against off-path third-party spoof attacks, but not addressing on-
   path attacks at all. Such potential solutions are discussed in the
   ANONsec document, in the BTNS (Better Than Nothing Security) BOF
   [5][38][39]. Note also that NULL Encryption in IPsec applies a
   variant of this cookie, where the SPI is the cookie, and no further
   encryption is applied [15].

6. Security Considerations

   This entire document focuses on increasing the security of transport
   protocols and their resistance to spoofing attacks.  Security is
   addressed throughout.

   This document describes a number of techniques for defeating spoofing
   attacks.  Those relying on clear-text cookies, either explicit or
   implicit (e.g., window sequence attenuation) do not protect from on-
   path spoofing attacks, since valid values can be learned from prior
   traffic.  Those relying on true authentication algorithms are
   stronger, protecting even from on-path attacks, because the
   authentication hash in a single packet approaches the behavior of
   "one time" cookies.

   The security of various levels of the protocol stack is addressed.
   Spoofing attacks are fundamentally identity masquerading, so we
   believe the most appropriate solutions defeat these at the network
   layer, where end-to-end identity lies.  Some transport protocols
   subsume endpoint identity information from the network layer (e.g.,
   TCP pseudo-headers), whereas others establish per-connection identity
   based on exchanged nonces (e.g., SCTP).  It is reasonable, if not
   recommended, to address security at all layers of the protocol stack.

7. IANA Considerations

   There are no IANA considerations in this document.

   This section should be removed by the RFC-Editor upon publication as
   an RFC.

8. Conclusions

   This document describes the details of the recent BGP spoofing
   attacks involving spurious RSTs which could be used to shutdown TCP

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   connections.  It summarizes and discusses a variety of current and
   proposed solutions at various protocol layers.

9. Acknowledgments

   This document was inspired by discussions on the
   <http://www.ietf.org/html.charters/tcpm-charter.html> about the
   recent spoofed RST attacks on BGP routers, including R. Stewart's
   draft (which is now edited by M. Dalal) [10][35].  The analysis of
   the attack issues, alternate solutions, and the anonymous security
   proposed solutions were the result of discussions on that list as
   well as with USC/ISI's T. Faber, A. Falk, G. Finn, and Y. Wang.  Ran
   Atkinson suggested the UDP variant of TCP/MD5, Paul Goyette suggested
   using the ISN to seed TCP/MD5, and Lloyd Wood suggested using the ISN
   to validate RSTs.  Other improvements are due to the input of various
   members of the IETF's TCPM WG, notably detailed feedback from Pekka

10. References

10.1. Normative References


10.2. Informative References

   [1]   Allman, M., V. Paxson, W. Stephens, "TCP Congestion Control,"
         RFC 2581, Apr. 1999.

   [2]   Baker, F. and P. Savola, "Ingress Filtering for Multihomed
         Networks," RFC 2827 / BCP 84, March 2004.

   [3]   Bellovin, S. and A. Zinin, "Standards Maturity Variance
         Regarding the TCP MD5 Signature Option (RFC 2385) and the BGP-4
         Specification," (work in progress),
         draft-iesg-tcpmd5app-01.txt, Sept. 2004.

   [4]   Bernstein, D., "SYN cookies - http://cr.yp.to/syncookies.html",

   [5]   Better Than Nothing Security [BTNS] BOF, IETF-61, Wash. DC.,

   [6]   Braden, R., "Requirements for Internet Hosts -- Communication
         Layers," RFC 1122, Oct. 1989.

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   [7]   Braden, R., "TIME-WAIT Assassination Hazards in TCP", RFC 1337,
         May 1992.

   [8]   CERT alert: "Technical Cyber Security Alert TA04-111A:
         Vulnerabilities in   TCP --
         http://www.us-cert.gov/cas/techalerts/TA04-111A.html", April 20

   [9]   Convery, S. and M. Franz, "BGP Vulnerability Testing:
         Separating Fact from FUD", 2003,

   [10]  Dalal, M., (ed.), "Transmission Control Protocol security
         considerations", draft-ietf-tcpm-tcpsecure-02 (work in
         progress), Nov. 2004.

   [11]  Faber, T., J. Touch, and W. Yue, "The TIME-WAIT state in TCP
         and Its Effect on Busy Servers", Proc. Infocom 1999 pp. 1573-
         1583, March 1999.

   [12]  Ferguson, P. and D. Senie, Network Ingress Ingress Filtering:
         Defeating Denial of Service Attacks which employ IP Address
         Spoofing," RFC 2267 / BCP 38, May 2000.

   [13]  Floyd, S., "Inappropriate TCP Resets Considered Harmful", BCP
         60, RFC 3360, August 2002.

   [14]  Gill, V., J. Heasley, and D. Meyer, "The Generalized TTL
         Security Mechanism (GTSM)," RFC 3682 (Experimental), Feb. 2004.

   [15]  Glenn, R. and S. Kent, "The NULL Encryption Algorithm and Its
         Use With IPsec", RFC 2410 (Standards Track), November 1998.

   [16]  Gont, F., "ICMP attacks against TCP," (work in progress), Sept.

   [17]  Harkins, D. and D. Carrel, "The Internet Key Exchange (IKE)",
         RFC 2409 (Standards Track), November 1998.

   [18]  Heffernan, A., "Protection of BGP Sessions via the TCP MD5
         Signature Option", RFC 2385 (Standards Track), August 1998.

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

   [20]  Kaufman, C., "Internet Key Exchange (IKEv2) Protocol",
         draft-ietf-ipsec-ikev2-14 (work in progress), June 2004.

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   [21]  Kent, S., "IP Authentication Header",
         draft-ietf-ipsec-rfc2402bis-07 (work in progress), March 2004.

   [22]  Kent, S. and R. Atkinson, "Security Architecture for the
         Internet Protocol", RFC 2401 (Standards Track), November 1998.

   [23]  Kent, S. and R. Atkinson, "IP Authentication Header", RFC 2402
         (Standards Track),   November 1998.

   [24]  Kent, S. and R. Atkinson, "IP Encapsulating Security Payload
         (ESP)", RFC 2406 (Standards Track), November 1998.

   [25]  Kent, S. and K. Seo, "Security Architecture for the Internet
         Protocol", draft-ietf-ipsec-rfc2401bis-06 (work in progress),
         April 2005.

   [26]  Kohler, E., M. Handley, and S. Floyd, "Datagram Congestion
         Control Protocol (DCCP)",   draft-ietf-dccp-spec-11 (work in
         progress), March 2005.

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

   [28]  O'Malley, S. and L. Peterson, "TCP Extensions Considered
         Harmful", RFC 1263, October 1991.

   [29]  Perkins, C., "IP Encapsulation within IP," RFC 2003 (Standards
         Track), Oct. 1996.

   [30]  Poon, K., "Use of TCP timestamp option to defend against blind
         spoofing attack," draft-poon-tcp-tstamp-mod-01 (work in
         progress), Oct. 2004.

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

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

   [33]  Semke, J., J. Mahdavi, M. Mathis, "Automatic TCP Buffer
         Tuning", ACM SIGCOMM '98/ Computer Communication Review, volume
         28, number 4, October 1998

   [34]  Stewart, R., Q. Xie, K. Morneault, C. Sharp, H. Schwarzbauer,
         T. Taylor, I. Rytina, M. Kalla, L. Zhang, and V. Paxson,
         "Stream Control Transmission Protocol," RFC 2960 (Standards
         Track), October 2000.

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   [35]  TCPM: IETF TCPM Working Group and mailing list-

   [36]  Touch, J., "Report on MD5 Performance," RFC 1810
         (Informational), June 1995.

   [37]  Touch, J., "Performance Analysis of MD5," Proc. Sigcomm 1995
         77-86., March 1999.

   [38]  Touch, J., "ANONsec: Anonymous Security to Defend Against
         Spoofing Attacks," draft-touch-anonsec-00 (work in progress),
         May 2004.

   [39]  Touch, J., D. Black, and Y. Wang, "Problem and Applicability
         Statement for Better Than Nothing Security (BTNS)," draft-ietf-
         btns-prob-and-applic-01 (work in progress), Sept. 2005.

   [40]  Watson, P., "Slipping in the Window: TCP Reset attacks,"
         Presentation at 2004 CanSecWest.

   [41]  Wood, L., Post to TCPM mailing list regarding use of ISN in
         RSTs, ID=Pine.GSO.4.50.0404232249570.5889-
         100000@argos.ee.surrey.ac.uk, Apr. 23, 2004.

Author's Addresses

   Joe Touch
   4676 Admiralty Way
   Marina del Rey, CA 90292-6695

   Phone: +1 (310) 448-9151
   Fax:   +1 (310) 448-9300
   Email: touch@isi.edu
   URI:   http://www.isi.edu/touch

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