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RFC 4953 - Defending TCP Against Spoofing Attacks


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Network Working Group                                          J.  Touch
Request for Comments: 4953                                       USC/ISI
Category: Informational                                        July 2007

                 Defending TCP Against Spoofing Attacks

Status of This Memo

   This memo provides information for the Internet community.  It does
   not specify an Internet standard of any kind.  Distribution of this
   memo is unlimited.

Copyright Notice

   Copyright (C) The IETF Trust (2007).

Abstract

   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
   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 brute-force generate a
   viable RST sequence number.  The susceptibility to attack increases
   with the square of the bandwidth, and 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 ..........................6
      2.3. TCP RST Vulnerability ......................................6
      2.4. What Changed - the Ever-Opening Advertised Receive Window ..7
   3. Proposed Solutions and Mitigations .............................10
      3.1. Transport Layer Solutions .................................10
           3.1.1. TCP MD5 Authentication .............................11
           3.1.2. TCP RST Window Attenuation .........................11
           3.1.3. TCP Timestamp Authentication .......................12
           3.1.4. Other TCP Cookies ..................................13
           3.1.5. Other TCP Considerations ...........................13
           3.1.6. Other Transport Protocol Solutions .................14
      3.2. Network Layer (IP) Solutions ..............................14
           3.2.1. Address Filtering ..................................15
           3.2.2. IPsec ..............................................16
   4. ICMP ...........................................................17
   5. Issues .........................................................18
      5.1. Transport Layer (e.g., TCP) ...............................18
      5.2. Network Layer (IP) ........................................19
      5.3. Application Layer .........................................21
      5.4. Link Layer ................................................21
      5.5. Issues Discussion .........................................21
   6. Security Considerations ........................................22
   7. Conclusions ....................................................23
   8. Acknowledgments ................................................23
   9. Informative References .........................................24

1.  Introduction

   Analysis of the Internet infrastructure has recently demonstrated a
   new version of a vulnerability in BGP connections between core
   routers using an attack based on RST spoofing from off-path attackers
   [9][10][48].  The attack itself is not new, having been documented
   nearly six years earlier [20].  Such 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 [37].  This sort of
   attack affects other protocols besides BGP, involving any long-lived
   connection between well-known endpoints.  The impact on the Internet
   infrastructure can be substantial (especially for the BGP case), and
   warrants immediate attention.

   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 system-level (i.e., root) 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 [9][10][48].  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 [13][15][20][36][46].

   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
   [20][24][27].  In both cases, their deployment overhead may be
   prohibitive, e.g., it may not be feasible for public services, such
   as web servers, to be configured with the appropriate certificate
   authorities of large numbers of peers (for IPsec using the Internet
   Key Exchange Protocol (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.  Services located on
   public web servers connecting to large-scale caches or BGP with
   larger numbers of peers can fall into this category.

   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
   [36], modifications to TCP's timestamp processing [34], and
   modifications to IPsec and TCP/MD5 keying [45].  This document
   focuses on spoofing of TCP segments, although a discussion of related
   spoofing of ICMP packets based on spoofed TCP contents is also
   discussed.

   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 [20].  The recent attack scenario was first
   documented by Convery at a NANOG (North American Network Operators'
   Group) meeting in 2003, but that analysis assumed the entire sequence
   space (2^32 packets) needed to be covered for an attack to succeed
   [10].  Watson's more detailed analysis discovered that a single
   packet anywhere in the current window could succeed at an attack
   [48].  This document adds the observation that susceptibility to
   attack is directly proportional to the square of bandwidth, due to
   the coupling between the linear increase in receive window size and
   linear increase in rate of a potential attack, 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
   [9][10][48].  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.  These attacks
   often combine external knowledge (e.g., to indicate the IP addresses
   to attack, the destination port number, and sometimes the Initial
   Sequence Number (ISN)) with brute-force capabilities enabled by
   modern computers and network bandwidths (e.g., to scan all source
   ports or an entire window space).  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 to make reliance on it inadvisable.

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][35]:

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

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

   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 TCP connections in most modern implementations, 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 as can 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) [23][38].

   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 at which the attacker can send
   spoofed packets), not as the square of the bandwidth.  Note that
   either increase depends on the receive window itself, and is
   independent of the congestion state or amount of data transmitted.

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
   connection with a BGP peer can result in the flushing of routes to
   that peer [37].

   Until six years ago, such connections were assumed difficult to
   attack because they were described by a few comparatively obscure
   parameters [20].  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 is
      usually 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 attempt to guess (e.g., by external knowledge or
      brute force) the above information is unlikely to be useful.

   BGP represents an exception to the above criteria (though not the
   only case).  Both endpoints can be well-known, or guessed 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 approximately 65,000 valid source
   ports, which thus 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.  In addition, the valid
   sequence number space once thought to provide some protection has
   been significantly weakened 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 [4][11].  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 [37].  This causes routers to
   drop the BGP routing information already exchanged, in addition to
   inhibiting their 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 [8].  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 [12].

   Firewalls and load balancers, so-called 'middleboxes', sometimes emit
   RSTs on behalf of transited connections to optimize server
   performance, as noted in RFC 3360 [14].  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
   unvalidated.  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 that were part of previous
   connections.

   TCP uses two window mechanisms, a primary mechanism for reordering
   and congestion control (which uses a space of 32 bits), and a
   secondary mechanism that scales this window [23][35].  The valid
   advertised receive window is a fraction, not to exceed approximately
   half, of this space, or ~2 billion (2 * 10^9, i.e., 2E9 or 2 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
   [38].

   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) [38].  Many paths in the
   Internet have end-to-end bandwidths of under 1 Mbps, latencies under
   100 ms, 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 within one round-trip time to successfully reset a
   connection.  At 1 Mbps, 57,000 (40 byte) RSTs would take only 20
   seconds to transmit, but this presumes that both IP addresses and
   both ports are known.  Absent knowledge of the source port, an off-
   path spoofer would need to try at least the entire range of 49152-
   65535, or 16,384 different ports, resulting in an attack that would
   take over 91 hours.  Because most TCP connections are comparatively
   short-lived, even this moderate variation in the source port is
   sufficient for such environments, although further port randomization
   may be recommended [29].

   Recent use of high bandwidth paths of 10 Gbps and higher results 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 Section 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 100-ms latencies (though even
   100 ms 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

   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
   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 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 Section 2.  Figure 1
   summarized the time to a successful attack based on large advertised
   receive windows, but many current commercial routers have limits of
   128 KB for large devices, 32 KB for medium, and as little as 4 KB for
   modest ones.  Figure 2 shows the time and bandwidths needed to
   accomplish an attack on BGP sessions in the time shown for 100-ms
   latencies; for even short-range network latencies (10 ms), these
   sessions can be still be attacked over short timescales (minutes to
   hours).

                   Receive
          BW     Buffer Size  RSTs needed     Time needed
      ------------------------------------------------------------
       10 Mbps     0.128 MB        33,555     1 second
        3 Mbps     0.032 MB       134,218    40 seconds
      300 Kbps     0.004 MB     1,073,742     1 hour

      Figure 2: Time needed to kill a connection with limited buffers

   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
   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
   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.  Other transport protocols, such
   as SCTP and DCCP, also have limited antispoofing mechanisms.

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) [20].  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][30].  It
   further is considered computationally expensive for either hosts or
   routers due to the overhead of MD5 [43][44].

   There are also concerns about the use of MD5 due to recent collision-
   based attacks [22].  Similar concerns exist for SHA-1, and the IETF
   is currently evaluating how these attacks impact the recommendation
   for using these hashes, both in TCP/MD5 and in the IPsec suite.  For
   the purposes of this discussion, the particular algorithm used in
   either protocol suite is not the focus, and there is ongoing work to
   allow TCP/MD5 to evolve to a more general TCP security option
   [6][47].

3.1.2.  TCP RST Window Attenuation

   A recent proposal extends TCP to further constrain received RST to
   match the expected next sequence number [36].  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 brute-force guess the sequence number
   (worst case, the average would be half that); this makes TCP's
   vulnerability to attack independent of the size of the receive window
   (RCV.WND).  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
   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

   deployed.  The other advantage to this proposal is that the window
   attenuation described here makes the vulnerability to spoofed RST
   packets independent of the size of the receive window.

   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 [42][49].  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 (RCV.WND) explicitly, 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 via its timestamp option,
   using the value as a sort of authentication [34].  This requires that
   the receiver TCP discard segments 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 can be derived predictably, either
   assuming it represents actual time at the host, or by probing the
   host using unrelated benign traffic.

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 of an existing connection, and timestamps can be guessed even
   less directly, either by separate benign connections or by assuming
   they roughly correlate to local time.  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
   [39].

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) [38].

   It 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 64 KB, 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
   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 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, or a time-based signal, though either would be
   nonstandard).

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 [28][41].  The inclusion of such mechanism at the transport
   protocol, although emerging as standard practice, complicates the
   design and implementation of new protocols [32].  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.

   It should be noted that RST attacks, which rely on brute-force, are
   relatively easy for intrusion detection software to detect at the TCP
   layer.  Any connection that receives a large number of invalid --
   outside-window -- RSTs might have subsequent RSTs blocked, to defeat
   such attacks.  This would have the side-effect of blocking legitimate
   RSTs to that connection, which might then interfere with cleaning up
   the transport state between the endpoint peers.  This side-effect,
   coupled with the increased monitoring load, might render such
   solutions undesirable in the general case, but they might usefully be
   applied to special cases, e.g., for BGP for routers.

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 that 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 preexisting 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][13].  Address
   filtering restricts traffic from downstream sources across transit
   networks based on the IP source address.  A kind of filtering already
   occurs at the endpoints of a connection, because attack messages must
   match the socket pair to succeed; again, note that such attacks
   require knowing the entire socket pair, and are unlikely except in
   particular cases.  This section discusses filtering based on address
   only, typically done at the borders of an AS.

   It can also restrict core-to-edge paths to reject traffic that should
   have originated further toward the edge.  It cannot restrict traffic
   from edges lacking filtering through the core to a particular edge.
   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, and potentially those outside as well.  Address filtering
   at the border can protect those inside the border from some kinds of
   spoofing, i.e., connections among those inside a border, because only
   interior addresses should originate inside the border.  It cannot,
   however, protect connections including endpoints outside the border
   (i.e., those that traverse the AS boundary) 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, but may be applicable to connections among
   those inside the protected border in some scenarios.  Applying
   filtering can also be useful to reduce the network load of spoofed
   traffic [31].

   A more recent variant of address filtering checks the IP TTL (Time to
   Live) field, relying on the TTL set by the other end of the
   connection [15].  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,
   notably for "bump in the wire" (BITW) or BITW-equivalent scenarios
   [33] (see also Section 5.1 of [15] and [16]).  TTL filtering works
   only where all traffic from the other end of the tunnel is trusted,

   i.e., where it does not originate or transit spoofed traffic.  The
   use of TTL rather than link or network security also assumes an
   untampered point-to-point link, where no other traffic can be spoofed
   onto a link.

   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 that 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 in order to most
   directly and explicitly address 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 trusted third-party
   (e.g., Kerberos) key infrastructure to establish and exchange keying
   information (e.g., via IKE).  Each of these issues presents
   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 [45][46].  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).

4.  ICMP

   Just as spoofed TCP packets can terminate a connection, so too can
   spoofed ICMP packets.  ICMP can be used to launch a variety of
   attacks on TCP including connection resets, path-MTU attacks, and can
   also be used to attack the host with non-TCP 'ping of death' and
   'smurf attacks', etc. [40].  ICMP thus represents a substantial
   threat to TCP, but this is not the focus of this document, although a
   number of protections are discussed below because some are comparable
   to TCP anti-spoofing techniques.  Note also that ICMP attacks on TCP
   assume that the socket pair is known by the attacker, which is
   unlikely except for a subset of services between pairs of widely-
   known endpoints.

   TCP headers can be included inside certain ICMP messages [7].  There
   have been recent suggestions to validate the sequence number of TCP
   headers when they occur inside ICMP messages [18].  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.  One other proposed alternative is to change
   TCP's reaction to ICMPs after a connection is established; that may
   leave TCP susceptible during connection establishment and modifies
   TCP's reaction to certain valid network events [19].  This considers
   the context-sensitivity of ICMP messages, as does IPsec in some
   tunneled configurations, but the recommendations are ambiguous
   regarding such filtering [27].

   Ultimately, requiring TCP ICMP messages to be 'in window' may be
   insufficient protection, as this document shows for spoofed data.
   ICMP packets can be authenticated when originating at known, trusted
   endpoints, such as endpoints of connections or routers in known

   domains with preexisting IPsec associations.  Unfortunately, they
   also can originate at other places in the network.  In addition, some
   networks filter all ICMP packets because validation may not be
   possible, especially because they can be injected from anywhere in a
   network, and so cannot be easily and locally address filtered [27].
   As a result, they are not addressed separately in the issues or
   security considerations of this document further.

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 require distributed coordination (filtering) or can
   be computationally intensive and require pervasive registration of
   certificate authorities with every possible endpoint
   (authentication).  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 protocols 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][36].

   Transport layer solutions are not only per-protocol, but often per-
   connection.  This has both advantages and drawbacks.  One advantage
   to transport layer solutions is that they can protect the transport
   protocol when lower layers have failed, e.g., due to bugs in

   implementation.  TCP already includes a variety of packet validation
   mechanisms to protect in these cases, e.g., checking that RSTs are
   in-window.  More strict checks can increase the protections provided,
   e.g., to protect against misaddressed RSTs that end up in-window (via
   TCPsecure) or to protect against connection interruption due to RSTs,
   SYNs, or data injection from misaddressed packets (TCP/MD5) [36].

   Another advantage is that transport layer protections can be more
   specifically limited to a particular connection.  Because each
   connection negotiates its state separately, that state can be more
   specifically tied to that connection.  This is both an advantage and
   a drawback.  It can make it easier to tie security to an individual
   connection, although in practice a shared secret or certificate will
   generally be shared across multiple connections.

   As a drawback, each transport 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.  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, and
   continuing to buffer spoofed packets, etc., only to be dropped later
   at the transport layer.

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 IGMP, and some kinds of ICMP (Section 4), spoofing
   attacks.

   The IETF Proposed Standard protocol for network layer authentication
   is IPsec [27].  IPsec specifies the overall architecture, including
   header authentication (AH) [25] and encapsulation (ESP) modes [26].

   AH authenticates both the IP header and IP data, whereas ESP
   authenticates only the IP data (e.g., transport header and payload).
   AH is being phased out since ESP is more efficient and the Security
   Parameters Index (SPI) includes sufficient information to verify the
   IP header anyway [27].  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, e.g., IKE [24].

   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 [43].  This computational overhead can be
   prohibitive, and so often requires additional hardware, especially in
   commercial routers.  As importantly, IKE is not anonymous; keys can
   be exchanged between parties only if they trust each other's 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
   themselves are the pre-shared keys that help IKE establish security
   association keys, which are then used in the authentication
   algorithms.

   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 can be made available [45][46].  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 same communications path.

   IPsec, although widely available both in commercial routers and
   commodity end-systems, is not often used 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
   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.  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.

5.5.  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:

   1. The entire body of the packet is secured.

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

   3. Both on-path 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 spoof attacks, but not addressing on-
   path attacks at all.  Such potential solutions are discussed in the
   Better Than Nothing Security (BTNS) documents [5][45][46].  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 [17].

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.

   Note that Network Address Translators (NATs) and other middleboxes
   complicate the design and deployment of techniques to defeat spoofing
   attacks.  Devices such as these, that modify IP and/or TCP headers
   in-transit, generate traffic equivalent to a spoofing attack, and
   thus should be inhibited by antispoofing mechanisms.  Details of
   these middlebox-related problems are out of scope for this document,
   but issues thereof are addressed in RFCs and emerging documents that
   discuss the interactions between such devices and the Internet
   architecture, e.g., [21].  Fortunately, many of the most critical
   TCP-based connections -- in particular, those supporting routing
   protocols like BGP -- do not traverse such middleboxes, and are not
   affected by this limitation.

7.  Conclusions

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

8.  Acknowledgments

   This document was inspired by discussions in the TCPM WG
   <http://www.ietf.org/html.charters/tcpm-charter.html> about the
   recent spoofed RST attacks on BGP routers, including R. Stewart's
   document (whose author list has since evolved) [36][42].  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.  R. Atkinson suggested the UDP variant of TCP/MD5, P. Goyette
   suggested using the ISN to seed TCP/MD5, and L. 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
   F. Gont, P. Savola, and A. Hoenes.

   This document was prepared using 2-Word-v2.0.template.dot.

9.  Informative References

   [1]   Allman, M., Paxson, V., and W. Stevens, "TCP Congestion
         Control", RFC 2581, April 1999.

   [2]   Baker, F. and P. Savola, "Ingress Filtering for Multihomed
         Networks", BCP 84, RFC 3704, March 2004.

   [3]   Bellovin, S. and A. Zinin, "Standards Maturity Variance
         Regarding the TCP MD5 Signature Option (RFC 2385) and the BGP-4
         Specification", RFC 4278, January 2006.

   [4]   Bernstein, D., "SYN cookies", 1997,
         <http://cr.yp.to/syncookies.html>.

   [5]   Better Than Nothing Security [BTNS] WG web pages,
         <http://www.postel.org/anonsec>.

   [6]   Bonica, R., Weis, B., Viswanathan, S., Lange, A., and O.
         Wheeler, "Authentication for TCP-based Routing and Management
         Protocols", Work in Progress, February 2007.

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

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

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

   [10]  Convery, S., and M. Franz, "BGP Vulnerability Testing:
         Separating Fact from FUD", 2003,
         <http://www.nanog.org/mtg-0306/pdf/franz.pdf>.

   [11]  Eddy, W., "TCP SYN Flooding Attacks and Common Mitigations",
         Work in Progress, May 2007.

   [12]  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, Mar. 1999.

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

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

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

   [16]  Gill, V., Heasley, J., Meyer, D., Savola, P., Ed., and C.
         Pignataro, "The Generalized TTL Security Mechanism (GTSM)",
         Work in Progress, June 2007.

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

   [18]  Gont, F., "ICMP attacks against TCP", Work in Progress, May
         2007.

   [19]  Gont, F., "TCP's Reaction to Soft Errors", Work in Progress,
         June 2007.

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

   [21]  Holdrege, M. and P. Srisuresh, "Protocol Complications with the
         IP Network Address Translator", RFC 3027, January 2001.

   [22]  Housley, R., Post to IETF Discussion mailing list regarding his
         IETF 64 Security Area presentation,
         ID=7.0.0.10.2.20051124135914.00f50558@vigilsec.com, Nov. 24,
         2005, <http://www1.ietf.org/
         mail-archive/ietf/Current/maillist.html>.

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

   [24]  Kaufman, C., Ed., "Internet Key Exchange (IKEv2) Protocol", RFC
         4306, December 2005.

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

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

   [27]  Kent, S. and K. Seo, "Security Architecture for the Internet
         Protocol", RFC 4301, December 2005.

   [28]  Kohler, E., Handley, M., and S. Floyd, "Datagram Congestion
         Control Protocol (DCCP)", RFC 4340, March 2006.

   [29]  Larsen, M., and F. Gont, "Port Randomization", Work in
         Progress, February 2007.

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

   [31]  Moore, D., G. Voelker, and S. Savage, "Inferring Internet
         Denial-of-Service Activity", Proc. Usenix Security Symposium,
         Aug. 2001.

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

   [33]  Perkins, C., "IP Encapsulation within IP", RFC 2003, October
         1996.

   [34]  Poon, K., "Use of TCP timestamp option to defend against blind
         spoofing attack", Work in Progress, October 2004.

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

   [36]  Ramaiah, A., Stewart, R., and M. Dalal, "Improving TCP's
         Robustness to Blind In-Window Attacks", Work in Progress, July
         2007.

   [37]  Rekhter, Y., Ed., Li, T., Ed., and S. Hares, Ed., "A Border
         Gateway Protocol 4 (BGP-4)", RFC 4271, January 2006.

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

   [39]  Shepard, T., "Reassign Port Number option for TCP", Work in
         Progress, July 2004.

   [40]  Shirey, R., "Internet Security Glossary, Version 2", Work in
         Progress, November 2006.

   [41]  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.

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

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

   [44]  Touch, J., "Performance Analysis of MD5", Proc. Sigcomm 1995,
         pp. 77-86, Mar. 1999.

   [45]  Touch, J., "ANONsec: Anonymous Security to Defend Against
         Spoofing Attacks", Work in Progress, May 2004.

   [46]  Touch, J., Black, D., and Y. Wang, "Problem and Applicability
         Statement for Better Than Nothing Security (BTNS)", Work in
         Progress, February 2007.

   [47]  Touch, J. and A. Mankin, "The TCP Simple Authentication
         Option", Work in Progress, July 2007.

   [48]  Watson, P., "Slipping in the Window: TCP Reset attacks",
         Presentation at 2004 CanSecWest,
         <http://cansecwest.com/csw04archive.html>.

   [49]  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,
         <http://www1.ietf.org/mail-archive/web/tcpm/current/
         msg00213.html>.

Author's Addresses

   Joe Touch
   USC/ISI
   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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