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Versions: 00                                                            
Network Working Group                                           J. Touch
Internet-Draft                                                   USC/ISI
Expires: January 8, 2005                                   July 10, 2004


                 Defending TCP Against Spoofing Attacks
                       draft-touch-tcp-antispoof-00

Status of this Memo

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   patent or other IPR claims of which I am aware have been disclosed,
   and any of which I become aware will be disclosed, in accordance with
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   This Internet-Draft will expire on January 8, 2005.

Copyright Notice

   Copyright (C) The Internet Society (2004).  All Rights Reserved.

Abstract

   Recent attacks on core Internet infrastructure indicate an increased
   vulnerability of TCP connections to spurious resets (RSTs).  TCP has
   always been susceptible to such RST spoof 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
   TCP endpoint and port numbers.  For pairs of well-known endpoints
   often over predictable port pairs, such as BGP, 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



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   viable RST sequence number.  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.

Table of Contents

   1.  Introduction . . . . . . . . . . . . . . . . . . . . . . . . .  3
   2.  Background . . . . . . . . . . . . . . . . . . . . . . . . . .  4
     2.1   Recent BGP Attacks Using TCP RSTs  . . . . . . . . . . . .  4
     2.2   TCP RST Vulnerability  . . . . . . . . . . . . . . . . . .  5
     2.3   What Changed -- the Ever Opening Window  . . . . . . . . .  5
   3.  Proposed solutions . . . . . . . . . . . . . . . . . . . . . .  9
     3.1   Transport Layer  . . . . . . . . . . . . . . . . . . . . .  9
       3.1.1   TCP MD5 Authentication . . . . . . . . . . . . . . . .  9
       3.1.2   TCP RST Window Attenuation . . . . . . . . . . . . . .  9
       3.1.3   TCP Timestamp Authentication . . . . . . . . . . . . . 10
       3.1.4   Other TCP Cookies  . . . . . . . . . . . . . . . . . . 11
       3.1.5   Other TCP Considerations . . . . . . . . . . . . . . . 11
       3.1.6   Other Protocols  . . . . . . . . . . . . . . . . . . . 12
     3.2   Network Layer (IP) . . . . . . . . . . . . . . . . . . . . 12
   4.  Issues . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13
     4.1   Transport Layer (e.g., TCP)  . . . . . . . . . . . . . . . 13
     4.2   Network Layer (IP) . . . . . . . . . . . . . . . . . . . . 14
     4.3   Application Layer  . . . . . . . . . . . . . . . . . . . . 15
     4.4   Shim Transport/Application Layer . . . . . . . . . . . . . 15
     4.5   Link Layer . . . . . . . . . . . . . . . . . . . . . . . . 15
     4.6   Need for Alternate Security Levels . . . . . . . . . . . . 15
   5.  The Need for High-Performance Anonymous Security . . . . . . . 17
     5.1   Anonymous Keying . . . . . . . . . . . . . . . . . . . . . 17
     5.2   Alternatives for Performance . . . . . . . . . . . . . . . 17
   6.  Security Considerations  . . . . . . . . . . . . . . . . . . . 20
   7.  Conclusions  . . . . . . . . . . . . . . . . . . . . . . . . . 21
   8.  Acknowledgments  . . . . . . . . . . . . . . . . . . . . . . . 22
   9.  References . . . . . . . . . . . . . . . . . . . . . . . . . . 23
   9.1   Normative References . . . . . . . . . . . . . . . . . . . . 23
   9.2   Informative References . . . . . . . . . . . . . . . . . . . 24
       Author's Address . . . . . . . . . . . . . . . . . . . . . . . 25
       Intellectual Property and Copyright Statements . . . . . . . . 26












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1.  Introduction

   The Internet infrastructure has recently seen a flurry of attacks on
   BGP connections between core routers using an attack known for nearly
   six years [1][2].  These connections, typically using TCP, can be
   susceptible to off-path (non man-in-the-middle) third-party reset
   (RST) segments, which terminate the TCP connection.  BGP routers
   react to a terminated TCP connection in various ways, ranging from
   restarting the connection to deciding that the other router is
   unreachable and thus flushing the BGP routes.  This sort of attack
   affects other protocols besides BGP, involving any long-lived
   connection between well-known endpoints.  The impact on Internet
   infrastructure has been substantial (esp.  for the BGP case), and
   warrants immediate attention.

   TCP, like many other protocols, has been susceptible to off-path
   third-party 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 ingress filtering to drop
   such spoofed traffic, has resulted in an increase in off-path
   third-party spoofing attacks.  As a result, a number of proposed
   solutions have been developed in collaboration among the Internet
   research and commercial router communities.  The foremost of these
   modifies TCP processing to defeat off-path third-party spoofs by
   further limiting viable sequence numbers in the RST segment.

   Such modifications are, at best, temporary patches to the ubiquitous
   vulnerability to spoofing attacks.  The obvious solution to spoofing
   is to validate the segments of a connection, either at the transport
   level or the network level.  TCP with MD5 extensions already provides
   this authentication at the transport level, and IPsec already intends
   to provide authentication of a network level, but in both cases their
   deployment overhead can be prohibitive.  E.g., it is not feasible for
   BGP routers 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 be configured rapidly without external
   assistance.  The same is true for other uses of long-lived TCP
   connections between well-known pairs.

   The remainder of this document outlines the attack 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 [22],
   modifications to TCP's timestamp processing [3], and modifications to
   IPsec and TCP/MD5 keying [4].



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2.  Background

   The recent attacks on BGP have raised the issue of TCP's
   vulnerability to off-path third-party spoofing attacks [1].  A number
   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
   (IPsec), transport (SYN cookies), or other layers.  TCP already
   includes a weak form of such authentication in its check of segment
   sequence numbers.  Increases in the bandwidth-delay product for
   certain long connections has made sufficiently weakened this
   authentication in recent weeks, rendering it moot.

2.1  Recent BGP Attacks Using TCP RSTs

   BGP represents a particular vulnerability to spoofing attacks.  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
      useful.

   BGP represents an exception to the above criteria (though not the
   only case).  Both endpoints are well-known, notably as 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 65,000 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 congestion window sizes.






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2.2  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.  This causes routers to drop the BGP routing information
   already exchanged, in addition to inhibiting their ongoing exchanges.
   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 [5].  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 [6].

   Firewalls and load balancers, so-called 'middleboxes', sometimes emit
   RSTs on behalf of transited connections to optimize server
   performance [7].  This is a 'man in the middle' RST attack, where the
   RSTs are sent for benign or beneficial intent.  There are numerous
   hazards with such use of RSTs, outlined in that RFC.

2.3  What Changed -- the Ever Opening 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 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



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   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
   connections.

   TCP uses two window mechanisms, a primary mechanism which uses a
   space of 32 bits, and a secondary mechanism which scales this window
   [8][9].  The valid receive window is a fraction, not to exceed
   approximately half, of this space, or ~2,000,000,000.  Under typical
   use, the majority of TCP connections open to a very small fraction of
   this space, e.g., 10,000-60,000 (approximately 5-100 segments).  On a
   low-loss path, the window should open to around the path
   bandwidth-delay product, including buffering delays (assume 1 packet/
   hop).  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 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.  Put differently, a blind (non
   man-in-the-middle) 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 result in
   bandwidth-delay products over 125 MB - approximately 1/10 of TCP's
   overall window size excluding scale, assuming the receiver allocates
   sufficient buffering (to be discussed later).  Even under networks
   that are ten times slower (1 Gbps), the active receiver window covers
   1/100th of the overall window size.  At these speeds, it takes only
   10-100 packets, or under 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):














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           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

   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 valid window (bw*delay) out of
   the overall window (2^32) is an insufficient obfuscation.

   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 most of the connection)

   3.  the receive socket buffers do not limiting the receive window

   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 will be addressed later.  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
       limitation

   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



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   BGP due to the vulnerability of TCP to RST spoofing attacks, due
   largely to recent increases in the fraction of the TCP window space
   in use for a single, long-lived connection.
















































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3.  Proposed solutions

   TCP currently authenticates received RSTs using the address and port
   pair numbers, and checks that the sequence number is inside the valid
   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 centering on increasing
   the authentication of received RSTs.

3.1  Transport Layer

   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
   third-party spoofs and man-in-the-middle attacks.  SCTP also has
   mechanisms to authenticate segments.

3.1.1  TCP MD5 Authentication

   An extension to TCP supporting MD5 authentication was developed
   around six years ago specifically to authenticate BGP connections
   [2].  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, which are 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.  It further is considered computationally
   expensive for either hosts or routers due to the overhead of MD5
   [10][11].

3.1.2  TCP RST Window Attenuation

   A recent proposal extends TCP to further constrain received RST to
   match the expected next sequence number [22].  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.  There are a number of
   concerns with this proposal, including the platitude "think twice
   before modifying TCP, then don't" [12], notably because this
   modification adds arcs to the TCP state diagram (in contrast to



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   adding MD5 signatures, which is orthogonal to the state machine
   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 modifies
   TCP so that under some circumstances a RST causes a reply, in
   violation of generally accepted practice, if not gentle
   recommendation.  The advantage to this proposal is that it can be
   deployed incrementally and has benefit to the endpoint on which it is
   deployed.

   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 [13].  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.

   The most obvious other 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 at
   best.  While there are many reasons to increase the TCP sequence
   number space, we believe authentication is not one of them.  Finally,
   such a modification is not obviously backward compatible, and would
   be thus difficult to deploy.

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 [3].  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 RST processing 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
   nonsensical data whose value is used to validate the packet.  In the
   case of MD5 checksums, the cookie is computed based on a shared
   secret.  Even a signature can be guessed, and presents a 1 in
   2^(cookie length) probability of attack anyway.  The primary
   difference is that MD5 signatures are effectively one-time cookies,
   not predictable based on man-in-the-middle snooping.  Window
   attenuation sequence numbers can be guessed by snooping the sequence
   number of current packets, and timestamps can may 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
   receive window opens to the maximum extent suggested by the
   bandwidth-delay product of the end-to-end path, and that the window
   opens 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 to for such a large window to occur.
   Expanding TCP's sequence number space is a direct way to further
   avoid such vulnerability, even for long connections over emerging
   bandwidths.

   Finally, it is often sufficient for the endpoint to limit the receive
   window in other ways, notably using 'socket options'.  If the receive
   socket buffer is limited, e.g., to the ubiquitous default of 65KB,
   the receive window cannot grow to vulnerable sizes even for very long
   connections over very high bandwidths.  The consequence is lower
   sustained throughput, where only one window's worth of data per round
   trip time (RTT) is exchanged.  Although this will keep the connection
   open longer, it also reduces the receive window; for long-lived
   connections 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 is small (i.e., unlikely to cover a substantial portion of
   the sequence space, even if long-lived), so is difficult to consider



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   where smaller receive buffers would not sufficiently address the
   immediate problem.

3.1.6  Other Protocols

   Segment authentication has been addressed at the transport layer in
   other protocols.  Both SCTP and DCCP* include cookies for connection
   establishment and uses them to authenticate a variety of other
   control messages [14][23].  The inclusion of such mechanism at the
   transport protocol, although emerging as standard practice,
   unnecessarily complicates the design and implementation of new
   protocols.  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.

   *[AUTH - DCCP may be removing cookies from the spec for the
   redundancies discussed above, because the use of cookies at the
   transport layer primarily supports dynamic multihoming (a design goal
   of SCTP, but not DCCP) rather than security.]

3.2  Network Layer (IP)

   TCP is susceptible to RSTs, but also to other spoofing and
   man-in-the-middle 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, it is clear that such attacks are fundamentally a
   network layer issue.  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, which most
   directly, explicitly, and completely addresses this security
   vulnerability.

   The larger problem with IPsec is that of CA 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
   signed X.509 certificates 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).









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4.  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.  These solutions are not a sufficient
   long-term strategy to dealing with such attacks, however.  Transport
   solutions require pervasive modification of every transport protocol
   and address the problem of packet origin identification at the wrong
   layer.  Current network solutions are computationally intensive and
   require pervasive registration of certificate authorities with every
   possible endpoint.  Neither application-layer nor link-layer
   solutions suffice to protect either the network or transport layers.
   This section explains these observations in detail.

4.1  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 pervasive
   modification of all transport protocols to include authentication.
   Not all transport layers support negotiated endpoint state (e.g.,
   UDP), and legacy protocols are notoriously hard to safely augment
   (e.g., TCP).  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 [15][22].

   Transport layer solutions are not only per-protocol, but often
   per-connection.  Each connection needs to negotiate and maintain
   authentication state separately.  Overhead is not amortized over
   multiple connections - this includes overheads in packet exchanges,
   design complexity, and implementation complexity.  Finally, because
   the authentication happens later in packet processing than is
   required, additional endpoint resources are consumed needlessly,
   e.g., in demultiplexing received packets, indexing connection
   identifiers, etc.




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4.2  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 quickly.  Network solutions protect all transport protocols,
   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
   [16][24].  IPsec specifies the overall architecture, including header
   authentication (AH) [17][25] and encapsulation (ESP) modes [18].  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 [19][26].

   IPsec already provides authentication of an IP header and its data
   contents sufficient to defeat both man-in-the-middle 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.

   IPsec does not suffice for many uses of BGP, however.  It is
   computationally intensive both in key management and individual
   packet authentication [10].  As importantly, IKE is not anonymous;
   keys can be exchanged between parties only if they trust each others'
   X.509 certificates.  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 themselves are the pre-shared keys
   which help IKE establish security association keys, which are then
   used in the authentication algorithms.

   IPsec, although widely available both in commercial routers and
   commodity end-systems, is not often utilized except between parties



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   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 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 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.

4.3  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.  It is the only way to protect the
   application data, ultimately.

   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 ever gets to check the data.  Authentication
   needs to winnow these packets and drop them before they interfere at
   these lower layers.

4.4  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, like application authentication, they do not protect
   the transport layer against spoofing attacks.

4.5  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 hop where it is examined.

4.6  Need for Alternate Security Levels

   The issues raised in this section suggest the need for alternate



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   security levels.  While it is already widely recognized that security
   needs to occur simultaneously at many protocol layers, the need for a
   variety of strengths at a single layer.  IPsec already supports a
   variety of algorithms (MD5, SHA, etc.  for authentication), but
   always assumes that:

   1.  the entire body of the packet is secured

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

   3.  both man-in-the-middle and off-path third-party spoofing attacks
       must be defeated

   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
   (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 spoofs, but not addressing
   man-in-the-middle at all.

























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5.  The Need for High-Performance Anonymous Security

   Internet servers need a form of anonymous security, to protect
   established connections from spoofing attacks, but without the
   heavyweight infrastructure typically assumed in conventional security
   architectures.  Such security would allow servers to interact with
   clients without a-priori shared keys or a key signing hierarchy.  It
   would be useful to also support high-performance variants, especially
   where only a nonce is sufficient to avoid off-path attacks.  Both
   these properties would address the concerns about deployability of
   existing IPsec and TCP/MD5 solutions.  There are a few different ways
   to establish anonymous security; the details of these approaches are
   discussed in detail elsewhere, and are briefly summarized here [4].

5.1  Anonymous Keying

   It would be useful to allow IPsec where CAs are not pre-shared, or
   TCP/MD5 and/or IPsec shared-key mode without a-prior shared keys.
   The former is a kind of anonymous IKE, where a key is negotiated and
   exchanged but without requiring pre-agreed CAs.  This is already
   supported by IKE in shared-secret mode, where the shared secret is
   open, i.e., a public string such as "password".  IKE allows such a
   public shared secret to be used to negotiate a pairwise private
   secret via the Diffie-Hellman exchange.

   TCP/MD5 would need to be augmented to support anonymous automatic
   keying, since it currently assumes only manual keying.  One solution
   would be to use the TCP connection's Initial Sequence Number (ISN) as
   the key, since it is visible only during connection establishment.
   Another solution is to add a Diffie-Hellman exchange to the
   connection establishment phase, inside the TCP/MD5 option.

   Note that the ISN or key established by Diffie-Hellman both represent
   shared, per-connection information that cannot be obtained after
   connection establishment or keying, respectively.  As such, either
   can be used as the key itself if only off-path attacks are of
   concern.

5.2  Alternatives for Performance

   An independent impediment to deploying IPsec or TCP/MD5 is
   performance; in both cases, there is a substantial impact on the
   throughput of individual connections, as well as the computational
   load of the endpoints.  There are variants of IPsec and TCP/MD5 which
   can address these performance issues, also discussed in further
   detail elsewhere [4].  They involve protecting a subset of the packet
   (focusing on the headers), and/or relaxing the authentication
   algorithms.



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   Conventional authentication involves ESP or ESP-like processing.  The
   entire IP payload is authenticated using existing algorithms, e.g.,
   MD5, SHA, etc.  This provides full protection of both all headers and
   data against all third-party spoofing, as well as tampering.
   Unfortunately, this alternative is as computationally expensive as
   other forms of authentication, e.g., TCP MD5, since similar
   algorithms are used over the bulk of the packet [10][11].

   Header-only authentication uses truncated ESP processing over only
   the fixed portions of the IP (in IPsec) or IP/TCP (in TCP/MD5)
   header, possibly with some short prefix of the payload, for padding
   and to avoid small-block hash problems.  This mode protects headers
   from all third-party spoofing and tampering.  It does not protect the
   payload (outside that optionally used to pad the hash) from
   tampering.  Since the IP header is usually sufficiently smaller than
   a typical hash block (e.g., 64 bytes for MD5), part or all of the TCP
   header would typically be protected for either IPsec or TCP/MD5.
   This then completely protects TCP against RST attacks.  Header-only
   mode does not protect from man-in-the-middle replay of signed headers
   with alternate payloads.  For packets substantially larger than the
   hash block size (again, 64 bytes for MD5), header-only mode can
   represent a significant computational savings over full-packet
   authentication.

   Both header-only and nonce mode authentication require modifications
   to IPsec or TCP/MD5.  These modes would be negotiated during IKE for
   IPsec or at connection establishment for TCP/MD5.

   Nonce authentication ignores the context of individual packets
   completely when computing the authentication signature.  The
   signature may be fixed or vary, but is independent of the packet
   header and contents.  The signature is effectively a cookie; it may
   even be the case that the SPI itself (for IKE) or ISN (for TCP/MD5)
   are sufficient for this purpose, though we do not recommend either
   (SPI values are not chosen pseudo-randomly, and ISNs are not always
   chosen pseudorandomly).  The cookie may the keys established during
   IKE or the augmented auto-key version of TCP/MD5.  The benefit of
   nonce mode is performance; only header modification is required,
   avoiding all computational overhead.  Nonce mode protects only from
   off-path third-party spoofing.

   Conventional IPsec and TCP/MD5 and header-only mode both protect TCP
   from all third-party spoofing attacks.  Nonce-only mode TCP protects
   only against off-path third-party attacks, but has much lower
   computational cost.

   In summary, there are two components needed to enable more widespread
   use of existing IPsec or TCP/MD5 spoofing protection.  Automatic



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   anonymous keying is already supported by IPsec using IKE with a
   public shared secret; similar keying can be added to TCP/MD5 by
   adding a Diffie-Hellman exchange therein, or even by just using the
   TCP ISN.  High-performance variants of both schemes are possible.















































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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
   man-in-the-middle spoofing attacks, since valid values can be learned
   from prior traffic.  Those relying on true authentication algorithms
   are stronger, protecting even from man-in-the-middle, because the
   authentication hash in a single packet approaches the behavior of
   "one time" cookies.

   Security at various levels of the protocol stack are 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), while 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.
   We believe that the current attacks are most directly addressed at
   the network layer.

   Some new solutions discussed weaken overall security compared to
   existing solutions.  However, a stronger solution which is not
   deployed, due to its complexity or performance, is equivalent to no
   security at all.  Providing a more easily deployed anonymous variant
   of IKE or TCP/MD5 together with authentication that can be adjusted
   to trade strength for performance may constitute an overall benefit
   toward the increased deployment of security solutions.


















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7.  Conclusions

   This document describes the details of the recent BGP spoofing
   attacks involving spurious RSTs used to shutdown TCP connections.  It
   summarizes and discusses a variety of current and proposed solutions
   at various protocol layers.













































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8.  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 [13][22].  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, and Paul Goyette suggested applying
   headerANON-style processing to TCP/MD5 and using the ISN to seed TCP/
   MD5.  Other improvements are due to the input of various members of
   the IETF's TCPM WG.






































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9.  References

9.1  Normative References

   [1]   "Technical Cyber Security Alert TA04-111A: Vulnerabilities in
         TCP -- http://www.us-cert.gov/cas/techalerts/TA04-111A.html",
         , April 20 2004.

   [2]   Heffernan, A., "Protection of BGP Sessions via the TCP MD5
         Signature Option", RFC 2385, August 1998.

   [3]   Poon, K., "Use of TCP timestamp option to defend against blind
         spoofing attack", (work in progress) , June 2004.

   [4]   Touch, J., "ANONsec: Anonymous Security to Defend Against
         Spoofing Attacks", (work in progress) , July 2004.

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

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

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

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

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

   [10]  Touch, J., "Report on MD5 Performance", RFC 1810, June 1995.

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

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

   [13]  "IETF TCPM Working Group and mailing list -
         http://www.ietf.org/html.charters/tcpm-charter.html",  .

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




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   [15]  Bernstein, D., "SYN cookies --
         http://cr.yp.to/syncookies.html",  , 1997.

   [16]  Kent, S. and R. Atkinson, "Security Architecture for the
         Internet Protocol", RFC 2401, November 1998.

   [17]  Kent, S. and R. Atkinson, "IP Authentication Header", RFC 2402,
         November 1998.

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

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

   [20]  Glenn, R. and S. Kent, "The NULL Encryption Algorithm and Its
         Use With IPsec", RFC 2410, November 1998.

   [21]  Maughan, D., Schneider, M. and M. Schertler, "Internet Security
         Association and Key Management Protocol (ISAKMP)", RFC 2408,
         November 1998.

9.2  Informative References

   [22]  Stewart, R., "Transmission Control Protocol security
         considerations", draft-ietf-tcpm-tcpsecure-01 (work in
         progress), June 2004.

   [23]  Kohler, E., "Datagram Congestion Control Protocol (DCCP)",
         draft-ietf-dccp-spec-06 (work in progress), February 2004.

   [24]  Kent, S. and K. Seo, "Security Architecture for the Internet
         Protocol", draft-ietf-ipsec-rfc2401bis-02 (work in progress),
         April 2004.

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

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











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Author's Address

   Joe Touch
   USC/Information Sciences Institute
   4676 Admiralty Way
   Marina del Rey, CA  90292-6695
   U.S.A.

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







































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