Defending TCP Against Spoofing Attacks
RFC 4953
| Document | Type | RFC - Informational (July 2007; No errata) | |
|---|---|---|---|
| Author | Joseph Touch | ||
| Last updated | 2015-10-14 | ||
| Stream | Internet Engineering Task Force (IETF) | ||
| Formats | plain text html pdf htmlized bibtex | ||
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| Stream | WG state | (None) | |
| Document shepherd | No shepherd assigned | ||
| IESG | IESG state | RFC 4953 (Informational) | |
| Action Holders |
(None)
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| Consensus Boilerplate | Unknown | ||
| Telechat date | |||
| Responsible AD | Lars Eggert | ||
| Send notices to | (None) | ||
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.
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RFC 4953 Defending TCP Against Spoofing Attacks July 2007
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
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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.
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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.
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2.1. Review of TCP Windows
Before proceeding, it is useful to review the terminology and
components of TCP's windowing algorithm. TCP connections have three
kinds of windows [1][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.
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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
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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.
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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):
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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 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).
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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.
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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
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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.
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3.1.4. Other TCP Cookies
All of the above techniques are variants of cookies, otherwise
meaningless data whose value is used to validate the packet. In the
case of MD5 checksums, the cookie is computed based on a shared
secret. Note that even a signature can be guessed, and presents a 1
in 2^(signature length) probability of attack. The primary
difference is that MD5 signatures are effectively one-time cookies,
not predictable based on on-path snooping, because they are dependent
on packet data and thus do not repeat. Window attenuation sequence
numbers can be guessed by snooping the sequence number of current
packets 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.
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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.
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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,
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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).
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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
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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
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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].
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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
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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
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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
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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.
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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.
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[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.
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[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.
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[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
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Phone: +1 (310) 448-9151
Fax: +1 (310) 448-9300
EMail: touch@isi.edu
URI: http://www.isi.edu/touch
Touch Informational [Page 27]
RFC 4953 Defending TCP Against Spoofing Attacks July 2007
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