Network Working Group J. Touch
Internet-Draft USC/ISI
Expires: January 8, 2005 July 10, 2004
Defending TCP Against Spoofing Attacks
draft-touch-tcp-antispoof-00
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Abstract
Recent attacks on core Internet infrastructure indicate an increased
vulnerability of TCP connections to spurious resets (RSTs). TCP has
always been susceptible to such RST spoof attacks, which were
indirectly protected by checking that the RST sequence number was
inside the current receive window, as well as via the obfuscation of
TCP endpoint and port numbers. For pairs of well-known endpoints
often over predictable port pairs, such as BGP, increases in the path
bandwidth-delay product of a connection have sufficiently increased
the receive window space that off-path third parties can guess a
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viable RST sequence number. This document addresses this
vulnerability, discussing proposed solutions at the transport level
and their inherent challenges, as well as existing network level
solutions and the feasibility of their deployment.
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3
2. Background . . . . . . . . . . . . . . . . . . . . . . . . . . 4
2.1 Recent BGP Attacks Using TCP RSTs . . . . . . . . . . . . 4
2.2 TCP RST Vulnerability . . . . . . . . . . . . . . . . . . 5
2.3 What Changed -- the Ever Opening Window . . . . . . . . . 5
3. Proposed solutions . . . . . . . . . . . . . . . . . . . . . . 9
3.1 Transport Layer . . . . . . . . . . . . . . . . . . . . . 9
3.1.1 TCP MD5 Authentication . . . . . . . . . . . . . . . . 9
3.1.2 TCP RST Window Attenuation . . . . . . . . . . . . . . 9
3.1.3 TCP Timestamp Authentication . . . . . . . . . . . . . 10
3.1.4 Other TCP Cookies . . . . . . . . . . . . . . . . . . 11
3.1.5 Other TCP Considerations . . . . . . . . . . . . . . . 11
3.1.6 Other Protocols . . . . . . . . . . . . . . . . . . . 12
3.2 Network Layer (IP) . . . . . . . . . . . . . . . . . . . . 12
4. Issues . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13
4.1 Transport Layer (e.g., TCP) . . . . . . . . . . . . . . . 13
4.2 Network Layer (IP) . . . . . . . . . . . . . . . . . . . . 14
4.3 Application Layer . . . . . . . . . . . . . . . . . . . . 15
4.4 Shim Transport/Application Layer . . . . . . . . . . . . . 15
4.5 Link Layer . . . . . . . . . . . . . . . . . . . . . . . . 15
4.6 Need for Alternate Security Levels . . . . . . . . . . . . 15
5. The Need for High-Performance Anonymous Security . . . . . . . 17
5.1 Anonymous Keying . . . . . . . . . . . . . . . . . . . . . 17
5.2 Alternatives for Performance . . . . . . . . . . . . . . . 17
6. Security Considerations . . . . . . . . . . . . . . . . . . . 20
7. Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . 21
8. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 22
9. References . . . . . . . . . . . . . . . . . . . . . . . . . . 23
9.1 Normative References . . . . . . . . . . . . . . . . . . . . 23
9.2 Informative References . . . . . . . . . . . . . . . . . . . 24
Author's Address . . . . . . . . . . . . . . . . . . . . . . . 25
Intellectual Property and Copyright Statements . . . . . . . . 26
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1. Introduction
The Internet infrastructure has recently seen a flurry of attacks on
BGP connections between core routers using an attack known for nearly
six years [1][2]. These connections, typically using TCP, can be
susceptible to off-path (non man-in-the-middle) third-party reset
(RST) segments, which terminate the TCP connection. BGP routers
react to a terminated TCP connection in various ways, ranging from
restarting the connection to deciding that the other router is
unreachable and thus flushing the BGP routes. This sort of attack
affects other protocols besides BGP, involving any long-lived
connection between well-known endpoints. The impact on Internet
infrastructure has been substantial (esp. for the BGP case), and
warrants immediate attention.
TCP, like many other protocols, has been susceptible to off-path
third-party attacks. Such attacks rely on the increase of commodity
platforms supporting public access to previously privileged
resources, such as root-level access. Given such access, it is
trivial for anyone to generate a packet with any header desired.
This, coupled with the lack of sufficient ingress filtering to drop
such spoofed traffic, has resulted in an increase in off-path
third-party spoofing attacks. As a result, a number of proposed
solutions have been developed in collaboration among the Internet
research and commercial router communities. The foremost of these
modifies TCP processing to defeat off-path third-party spoofs by
further limiting viable sequence numbers in the RST segment.
Such modifications are, at best, temporary patches to the ubiquitous
vulnerability to spoofing attacks. The obvious solution to spoofing
is to validate the segments of a connection, either at the transport
level or the network level. TCP with MD5 extensions already provides
this authentication at the transport level, and IPsec already intends
to provide authentication of a network level, but in both cases their
deployment overhead can be prohibitive. E.g., it is not feasible for
BGP routers to be configured with the appropriate certificate
authorities of large numbers of peers (for IPsec using IKE), or
shared secrets (for IPsec in shared-secret mode, or TCP/MD5), because
many clients may need to be configured rapidly without external
assistance. The same is true for other uses of long-lived TCP
connections between well-known pairs.
The remainder of this document outlines the attack in detail and
describes and compares a variety of solutions, including existing
solutions based on TCP/MD5 and IPsec, as well as recently proposed
solutions, including modifications to TCP's RST processing [22],
modifications to TCP's timestamp processing [3], and modifications to
IPsec and TCP/MD5 keying [4].
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2. Background
The recent attacks on BGP have raised the issue of TCP's
vulnerability to off-path third-party spoofing attacks [1]. A number
of such attacks have been known for several years, including sending
RSTs, SYNs, and even ACKs in an attempt to affect an existing
connection or to load down servers. Overall, such attacks are
countered by the use of some form of authentication at the network
(IPsec), transport (SYN cookies), or other layers. TCP already
includes a weak form of such authentication in its check of segment
sequence numbers. Increases in the bandwidth-delay product for
certain long connections has made sufficiently weakened this
authentication in recent weeks, rendering it moot.
2.1 Recent BGP Attacks Using TCP RSTs
BGP represents a particular vulnerability to spoofing attacks. Most
TCP connections are protected by multiple levels of obfuscation
except at the endpoints of the connection:
o Both endpoint addresses are usually not well-known; although
server addresses are advertised, clients are somewhat anonymous.
o Both port numbers are usually not well-known; the server's usually
is advertised (representing the service), but the client's is
typically sufficiently unpredictable to an off-path third-party.
o Valid sequence number space is not well-known.
o Connections are relatively short-lived and valid sequence space
changes, so any guess of the above information is unlikely to be
useful.
BGP represents an exception to the above criteria (though not the
only case). Both endpoints are well-known, notably as part of an AS
path. The destination port is typically fixed to indicate the BGP
service. The source port used by a BGP router is sometimes fixed and
advertised to enable firewall configuration; even when not fixed,
there are only 65,000 valid source ports which may be exhaustively
attacked. Connections are long-lived, and as noted before some BGP
implementations interpret successive TCP connection failures as
routing failures, discarding the corresponding routing information.
As importantly and as will be shown below, the valid sequence number
space once thought to provide some protection has been rendered
useless by increasing congestion window sizes.
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2.2 TCP RST Vulnerability
TCP has a known vulnerability to third-party spoofed segments. SYN
flooding consumes server resources in half-open connections,
affecting the server's ability to open new connections. ACK spoofing
can cause connections to transmit too much data too quickly, creating
network congestion and segment loss, causing connections to slow to a
crawl. In the most recent attacks on BGP, RSTs cause connections to
be dropped. As noted earlier, some BGP implementations interpret TCP
connection termination, or a series of such failures, as a network
failure. This causes routers to drop the BGP routing information
already exchanged, in addition to inhibiting their ongoing exchanges.
The result can affect routing paths throughout the Internet.
The dangerous effects of RSTs on TCP have been known for many years,
even when used by the legitimate endpoints of a connection. TCP RSTs
cause the receiver to drop all connection state; because the source
is not required to maintain a TIME_WAIT state, such a RST can cause
premature reuse of address/port pairs, potentially allowing segments
from a previous connection to contaminate the data of a new
connection, known as TIME_WAIT assassination [5]. In this case,
assassination occurs inadvertently as the result of duplicate
segments from a legitimate source, and can be avoided by blocking RST
processing while in TIME_WAIT. However, assassination can be useful
to deliberately reduce the state held at servers; this requires that
the source of the RSTs go into TIME_WAIT state to avoid such hazards,
and that RSTs are not blocked in the TIME_WAIT state [6].
Firewalls and load balancers, so-called 'middleboxes', sometimes emit
RSTs on behalf of transited connections to optimize server
performance [7]. This is a 'man in the middle' RST attack, where the
RSTs are sent for benign or beneficial intent. There are numerous
hazards with such use of RSTs, outlined in that RFC.
2.3 What Changed -- the Ever Opening Window
RSTs represent a hazard to TCP, especially when completely unchecked.
Fortunately, there are a number of obfuscation mechanisms that make
it difficult for off-path third parties to forge (spoof) valid RSTs,
as noted earlier. We have already shown it is easy to learn both
endpoint addresses and ports for some protocols, notably BGP. The
final obfuscation is the sequence number.
TCP segments include a sequence number which enables out-of-order
receiver processing, as well as duplicate detection. The sequence
number space is also used to manage congestion, and indicates the
index of the next byte to be transmitted or received. For RSTs, this
is relevant because legitimate RSTs use the next sequence number in
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the transmitter window, and the receiver checks that incoming RSTs
have a sequence number in the expected receive window. Such
processing is intended to eliminate duplicate segments (somewhat moot
for RSTs, though), and to drop RSTs which were part of previous
connections.
TCP uses two window mechanisms, a primary mechanism which uses a
space of 32 bits, and a secondary mechanism which scales this window
[8][9]. The valid receive window is a fraction, not to exceed
approximately half, of this space, or ~2,000,000,000. Under typical
use, the majority of TCP connections open to a very small fraction of
this space, e.g., 10,000-60,000 (approximately 5-100 segments). On a
low-loss path, the window should open to around the path
bandwidth-delay product, including buffering delays (assume 1 packet/
hop). Many paths in the Internet have end-to-end bandwidths of under
1 Mbps, latencies under 100ms, and are under 15 hops, resulting in
fairly small windows as above (under 35,000 bytes). Under these
conditions, and further assuming that the initial sequence number is
suitably (pseudo-randomly) chosen, a valid guessed sequence number
would have odds of 1 in 57,000. Put differently, a blind (non
man-in-the-middle) attacker would need to send 57,000 RSTs with
suitably spaced sequence number guesses to successfully reset a
connection. At 1 Mbps, 57,000 (40 byte) RSTs would take over 50
minutes to transmit, and, as noted earlier, most current connections
are fairly brief by comparison.
Recent use of high bandwidth paths of 10 Gbps and result in
bandwidth-delay products over 125 MB - approximately 1/10 of TCP's
overall window size excluding scale, assuming the receiver allocates
sufficient buffering (to be discussed later). Even under networks
that are ten times slower (1 Gbps), the active receiver window covers
1/100th of the overall window size. At these speeds, it takes only
10-100 packets, or under 32 microseconds, to correctly guess a valid
sequence number and kill a connection. A table of corresponding
exposure to various amounts of RSTs is shown below, for various line
rates, assuming the more conventional 100ms latencies (though even
100ms is large for BGP cases):
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BW BW*delay RSTs needed Time needed
------------------------------------------------------------
10 Gbps 125 MB 35 1 us (microsecond)
1 Gbps 12.5 MB 344 110 us
100 Mbps 1.25 MB 3,436 10 ms (millisecond)
10 Mbps 0.125 MB 34,360 1 second
1 Mbps 0.0125 MB 343,598 2 minutes
100 Kbps 0.00125 MB 3,435,974 3 hours
Figure 1: Time needed to kill a connection
This table demonstrates that the effect of bandwidth on the
vulnerability is squared; for every increase in bandwidth, there is a
linear decrease in the number of sequence number guesses needed, as
well as a linear decrease in the time needed to send a set of
guesses. Notably, as inter-router link bandwidths approach 1 Mbps,
an 'exhaustive' attack becomes practical. Checking that the RST
sequence number is somewhere in the valid window (bw*delay) out of
the overall window (2^32) is an insufficient obfuscation.
Note that this table makes a number of assumptions:
1. the overall bandwidth-delay product is relatively fixed
2. traffic losses are negligible (insufficient to affect the
congestion window over most of the connection)
3. the receive socket buffers do not limiting the receive window
4. the attack bandwidth is similar to the end-to-end path bandwidth
Of these assumptions, the last two are more notable. The issue of
receive socket buffers will be addressed later. The issue of the
attack bandwidth is considered reasonable as follows:
1. RSTs are substantially easier to send than data; they can be
precomputed and they are smaller than data packets (40 bytes).
2. although susceptible connections use somewhat less ubiquitous
high-bandwidth paths, the attack may be distributed, at which
point only the ingress link of the attack is the primary
limitation
3. for the purposes of the above table, we assume that the ingress
at the attack has the same bandwidth as the path, as an
approximation
The previous sections discussed the nature of the recent attacks on
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BGP due to the vulnerability of TCP to RST spoofing attacks, due
largely to recent increases in the fraction of the TCP window space
in use for a single, long-lived connection.
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3. Proposed solutions
TCP currently authenticates received RSTs using the address and port
pair numbers, and checks that the sequence number is inside the valid
receiver window. The previous section demonstrated how TCP has
become more vulnerable to RST spoofing attacks due to the increases
in the receive window size. There are a number of current and
proposed solutions to this vulnerability, all centering on increasing
the authentication of received RSTs.
3.1 Transport Layer
The transport layer represents the last place that segments can be
authenticated before they affect connection management. TCP has a
variety of current and proposed mechanisms to increase the
authentication of segments, protecting against both off-path
third-party spoofs and man-in-the-middle attacks. SCTP also has
mechanisms to authenticate segments.
3.1.1 TCP MD5 Authentication
An extension to TCP supporting MD5 authentication was developed
around six years ago specifically to authenticate BGP connections
[2]. The extension relies on a pre-shared secret key to authenticate
the entire TCP segment, including the data, TCP header, and TCP
pseudo-header (certain fields of the IP header). All segments are
protected, including RSTs, which are accepted only when their
signature matches. This option, although widely deployed in Internet
routers, is considered undeployable for widespread use because the
need for pre-shared keys. It further is considered computationally
expensive for either hosts or routers due to the overhead of MD5
[10][11].
3.1.2 TCP RST Window Attenuation
A recent proposal extends TCP to further constrain received RST to
match the expected next sequence number [22]. This restores TCP's
resistance to spurious RSTs, effectively limiting the receive window
for RSTs to a single number. As a result, an attacker would need to
send 2^32 different packets to correctly guess the sequence number.
The extension further modifies the RST receiver to react to
incorrectly-numbered RSTs, by sending a zero-length ACK. If the RST
source is legitimate, upon receipt of an ACK the closed source would
presumably emit a RST with the sequence number matching the ACK,
correctly resetting the intended recipient. There are a number of
concerns with this proposal, including the platitude "think twice
before modifying TCP, then don't" [12], notably because this
modification adds arcs to the TCP state diagram (in contrast to
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adding MD5 signatures, which is orthogonal to the state machine
altogether). For example, there may be complications between RSTs of
different connections between the same pair of endpoints because RSTs
flush the TIME-WAIT (as mentioned earlier). Further, this modifies
TCP so that under some circumstances a RST causes a reply, in
violation of generally accepted practice, if not gentle
recommendation. The advantage to this proposal is that it can be
deployed incrementally and has benefit to the endpoint on which it is
deployed.
A variant of this proposal uses a different value to attenuate the
window of viable RSTs. It requires RSTs to carry the initial
sequence number rather than the next expected sequence number, i.e.,
the value negotiated on connection establishment [13]. This proposal
has the advantage of using an explicitly negotiated value, but at the
cost of changing the behavior of an unmodified endpoint to a
currently valid RST. It would thus be more difficult, without
additional mechanism, to deploy incrementally.
The most obvious other variant of this proposal involves increasing
TCP's window space, rather than decreasing the valid range for RSTs,
i.e., increasing the sequence space from 32 bits to 64 bits. This
has the equivalent effect - the ratio of the valid sequence numbers
for any segment to the overall sequence number space is significantly
reduced. The use of the larger space, as with current schemes to
establish weak authentication using initial sequence numbers (ISNs),
is contingent on using suitably random values for the ISN. Such
randomness adds additional complexity to TCP both in specification
and implementation, and provides only very weak authentication at
best. While there are many reasons to increase the TCP sequence
number space, we believe authentication is not one of them. Finally,
such a modification is not obviously backward compatible, and would
be thus difficult to deploy.
3.1.3 TCP Timestamp Authentication
Another way to authenticate TCP segments is to utilize its timestamp
option, using the value as a sort of authentication [3]. This
requires that the receiver TCP discard values whose timestamp is
outside the accepted window, which is derived from the timestamps of
other packets from the same connection. This technique uses an
existing TCP option, but also requires modified RST processing and
may be difficult to deploy incrementally without further
modifications. Additionally, the timestamp value may be easier to
guess because it is derived from a predictable value.
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3.1.4 Other TCP Cookies
All of the above techniques are variants of cookies, otherwise
nonsensical data whose value is used to validate the packet. In the
case of MD5 checksums, the cookie is computed based on a shared
secret. Even a signature can be guessed, and presents a 1 in
2^(cookie length) probability of attack anyway. The primary
difference is that MD5 signatures are effectively one-time cookies,
not predictable based on man-in-the-middle snooping. Window
attenuation sequence numbers can be guessed by snooping the sequence
number of current packets, and timestamps can may be guessed even
more remotely. These variants of cookies are similar in spirit to
TCP SYN cookies, again patching a vulnerability to off-path
third-party spoofing attacks based on a (fairly weak, excepting MD5)
form of authentication. Another form of cookie is the source port
itself, which can be randomized but provides only 16 bits of
protection (65,000 combinations), which may be exhaustively attacked.
This can be combined with destination port randomization as well, but
that would require a separate coordination mechanism (so both parties
know which ports to use), which is equivalent to (and as infeasible
for large-scale deployments as) exchanging a shared secret.
3.1.5 Other TCP Considerations
The analysis of the potential for RST spoofing above assumes that the
receive window opens to the maximum extent suggested by the
bandwidth-delay product of the end-to-end path, and that the window
opens to an appreciable fraction of the overall sequence number
space. As noted earlier, for most common cases, connections are too
brief or over bandwidths too low to for such a large window to occur.
Expanding TCP's sequence number space is a direct way to further
avoid such vulnerability, even for long connections over emerging
bandwidths.
Finally, it is often sufficient for the endpoint to limit the receive
window in other ways, notably using 'socket options'. If the receive
socket buffer is limited, e.g., to the ubiquitous default of 65KB,
the receive window cannot grow to vulnerable sizes even for very long
connections over very high bandwidths. The consequence is lower
sustained throughput, where only one window's worth of data per round
trip time (RTT) is exchanged. Although this will keep the connection
open longer, it also reduces the receive window; for long-lived
connections with continuous sourced data, this may continue to
present an attack opportunity, albeit a sparse and slow-moving
target. For the most recent case where BGP data is being exchanged
between Internet routers, the data is bursty and the aggregate
traffic is small (i.e., unlikely to cover a substantial portion of
the sequence space, even if long-lived), so is difficult to consider
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where smaller receive buffers would not sufficiently address the
immediate problem.
3.1.6 Other Protocols
Segment authentication has been addressed at the transport layer in
other protocols. Both SCTP and DCCP* include cookies for connection
establishment and uses them to authenticate a variety of other
control messages [14][23]. The inclusion of such mechanism at the
transport protocol, although emerging as standard practice,
unnecessarily complicates the design and implementation of new
protocols. As new attacks are discovered (SYN floods, RSTs, etc.),
each protocol must be modified individually to compensate. A network
solution may be more appropriate and efficient.
*[AUTH - DCCP may be removing cookies from the spec for the
redundancies discussed above, because the use of cookies at the
transport layer primarily supports dynamic multihoming (a design goal
of SCTP, but not DCCP) rather than security.]
3.2 Network Layer (IP)
TCP is susceptible to RSTs, but also to other spoofing and
man-in-the-middle attacks, including SYN attacks. Other transport
protocols, such as UDP and RTP are equally susceptible. Although
emerging transport protocols attempt to defeat such attacks at the
transport layer, it is clear that such attacks are fundamentally a
network layer issue. The packet is coming from an endpoint who is
spoofing another endpoint, either upstream or somewhere else in the
Internet. IPsec was designed specifically to establish and enforce
authentication of a packet's source and contents, which most
directly, explicitly, and completely addresses this security
vulnerability.
The larger problem with IPsec is that of CA key distribution and use.
IPsec is often cumbersome, and has only recently been supported in
many end-system operating systems. More importantly, it relies on
signed X.509 certificates to establish and exchange keying
information (e.g., via IKE). These present challenges when using
IPsec to secure traffic to a well-known server, whose clients may not
support IPsec or may not have registered with a previously-known
certificate authority (CA).
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4. Issues
There are a number of existing and proposed solutions addressing the
vulnerability of transport protocols in general, and TCP in specific,
to off-path third-party spoofing attacks. As shown, these operate at
the transport or network layer. These solutions are not a sufficient
long-term strategy to dealing with such attacks, however. Transport
solutions require pervasive modification of every transport protocol
and address the problem of packet origin identification at the wrong
layer. Current network solutions are computationally intensive and
require pervasive registration of certificate authorities with every
possible endpoint. Neither application-layer nor link-layer
solutions suffice to protect either the network or transport layers.
This section explains these observations in detail.
4.1 Transport Layer (e.g., TCP)
Transport solutions rely on shared cookies to authenticate segments,
including data, transport header, and even pseudo-header (e.g., fixed
portions of the outer IP header in TCP). Because the Internet relies
on stateless network protocols, it makes sense to rely on state
establishment and maintenance available in some transport layers not
only for the connection but for authentication state. Three-way
handshakes and heartbeats can be used to negotiate authentication
state in conjunction with connection parameters, which can be stored
with connection state easily.
As noted earlier, transport layer solutions require pervasive
modification of all transport protocols to include authentication.
Not all transport layers support negotiated endpoint state (e.g.,
UDP), and legacy protocols are notoriously hard to safely augment
(e.g., TCP). Not all authentication solutions are created equal
either, and relying on a variety of transport solutions exposes
end-systems to increased potential for incorrectly specified or
implemented solutions. Transport authentication has often been
developed piece-wise, in response to specific attacks, e.g., SYN
cookies and RST window attenuation [15][22].
Transport layer solutions are not only per-protocol, but often
per-connection. Each connection needs to negotiate and maintain
authentication state separately. Overhead is not amortized over
multiple connections - this includes overheads in packet exchanges,
design complexity, and implementation complexity. Finally, because
the authentication happens later in packet processing than is
required, additional endpoint resources are consumed needlessly,
e.g., in demultiplexing received packets, indexing connection
identifiers, etc.
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4.2 Network Layer (IP)
A network layer solution avoids the hazards of multiple transport
variants, using a single shared endpoint authentication mechanism
early in receiver packet processing to discard unauthenticated
packets quickly. Network solutions protect all transport protocols,
including both legacy and emerging protocols, and reduces the
complexity of these protocols as well. A shared solution also
reduces protocol overhead, and decouples the management (and
refreshing) of authentication state from that of individual transport
connections. Finally, a network layer solution protects not only the
transport layer but the network layer as well, e.g., from ICMP, IGMP,
etc. spoofing attacks.
The ubiquitous protocol for network layer authentication is IPsec
[16][24]. IPsec specifies the overall architecture, including header
authentication (AH) [17][25] and encapsulation (ESP) modes [18]. AH
authenticates both the IP header and IP data, whereas ESP
authenticates only the IP data (e.g., transport header and payload).
AH is deprecated since ESP is more efficient and the SPI includes
sufficient information to verify the IP header anyway. These two
modes describe the security applied to individual packets within the
IPsec system; key exchange and management is performed either
out-of-band (via pre-shared keys) or by an automated key exchange
protocol IKE [19][26].
IPsec already provides authentication of an IP header and its data
contents sufficient to defeat both man-in-the-middle and off-path
third-party spoofing attacks. IKE can configure authentication
between two endpoints on a per-endpoint, per-protocol, or
per-connection basis, as desired. IKE also can perform automatic
periodic re-keying, further defeating crypto-analysis based on
snooping (clandestine data collection). The use of IPsec is already
commonly strongly recommended for protected infrastructure.
IPsec does not suffice for many uses of BGP, however. It is
computationally intensive both in key management and individual
packet authentication [10]. As importantly, IKE is not anonymous;
keys can be exchanged between parties only if they trust each others'
X.509 certificates. These certificates provide identification (the
other party knows who you are) only where the certificates themselves
are signed by certificate authorities (CAs) that both parties already
trust. To a large extent, the CAs themselves are the pre-shared keys
which help IKE establish security association keys, which are then
used in the authentication algorithms.
IPsec, although widely available both in commercial routers and
commodity end-systems, is not often utilized except between parties
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that already have a preexisting relationship (employee/employer,
between two ISPs, etc.) Servers to anonymous clients (e.g., customer/
business) or more open services (e.g., BGP, where routers may large
numbers of peers) are unmanageable, due to the breadth and flux of
CAs. New endpoints cannot establish IPsec associations with such
servers unless their certificate is signed by a CA already trusted by
the server. Different servers - even within the same overall system
(e.g., BGP) - often cannot or will not trust overlapping subsets of
CAs in general.
4.3 Application Layer
There are a number of application layer authentication mechanisms,
often implicit within end-to-end encryption. Application-layer
security (e.g., TLS, SSH, or MD5 checksums within a BGP stream)
provides the ultimate protection of application data from all
intermediaries, including network routers as well as exposure at
other layers in the end-systems. It is the only way to protect the
application data, ultimately.
Application authentication cannot protect either the network or
transport protocols from spoofing attacks, however. Spoofed packets
interfere with network processing or reset transport connections
before the application ever gets to check the data. Authentication
needs to winnow these packets and drop them before they interfere at
these lower layers.
4.4 Shim Transport/Application Layer
Security can also be provided over the transport layer but below the
application layer, in a kind of 'shim' protocol, such as SSL or TLS.
These protocols provide data protection for a variety of applications
over a single, legacy transport protocol, such as SSL/TCP for HTTPS.
Unfortunately, like application authentication, they do not protect
the transport layer against spoofing attacks.
4.5 Link Layer
Link layer security operates separately on each hop of an Internet.
Such security can be critical in protecting link resources, such as
bandwidth and link management protocols. Protection at this layer
cannot suffice for network or transport layers, because it cannot
authenticate the endpoint source of a packet. Link authentication
ensures only the source of the current hop where it is examined.
4.6 Need for Alternate Security Levels
The issues raised in this section suggest the need for alternate
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security levels. While it is already widely recognized that security
needs to occur simultaneously at many protocol layers, the need for a
variety of strengths at a single layer. IPsec already supports a
variety of algorithms (MD5, SHA, etc. for authentication), but
always assumes that:
1. the entire body of the packet is secured
2. security associations are established only where identity is
authenticated by a know certificate authority or other pre-shared
key
3. both man-in-the-middle and off-path third-party spoofing attacks
must be defeated
These assumptions are prohibitive, especially in many cases of
spoofing attacks. For spoofing, the primary issue is whether packets
are coming from the same party the server can reach. Only the IP
header is fundamentally in question, so securing the entire packet
(1) is computational overkill. It is sufficient to authenticate the
other party as "a party you have exchanged packets with", rather than
establishing their trusted identity ("Bill" vs. "Bob") as in (2).
Finally, many cookie systems use clear-text (unencrypted), fixed
cookie values, providing reasonable (1 in 2^{cookie-size}) protection
against off-path third-party spoofs, but not addressing
man-in-the-middle at all.
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5. The Need for High-Performance Anonymous Security
Internet servers need a form of anonymous security, to protect
established connections from spoofing attacks, but without the
heavyweight infrastructure typically assumed in conventional security
architectures. Such security would allow servers to interact with
clients without a-priori shared keys or a key signing hierarchy. It
would be useful to also support high-performance variants, especially
where only a nonce is sufficient to avoid off-path attacks. Both
these properties would address the concerns about deployability of
existing IPsec and TCP/MD5 solutions. There are a few different ways
to establish anonymous security; the details of these approaches are
discussed in detail elsewhere, and are briefly summarized here [4].
5.1 Anonymous Keying
It would be useful to allow IPsec where CAs are not pre-shared, or
TCP/MD5 and/or IPsec shared-key mode without a-prior shared keys.
The former is a kind of anonymous IKE, where a key is negotiated and
exchanged but without requiring pre-agreed CAs. This is already
supported by IKE in shared-secret mode, where the shared secret is
open, i.e., a public string such as "password". IKE allows such a
public shared secret to be used to negotiate a pairwise private
secret via the Diffie-Hellman exchange.
TCP/MD5 would need to be augmented to support anonymous automatic
keying, since it currently assumes only manual keying. One solution
would be to use the TCP connection's Initial Sequence Number (ISN) as
the key, since it is visible only during connection establishment.
Another solution is to add a Diffie-Hellman exchange to the
connection establishment phase, inside the TCP/MD5 option.
Note that the ISN or key established by Diffie-Hellman both represent
shared, per-connection information that cannot be obtained after
connection establishment or keying, respectively. As such, either
can be used as the key itself if only off-path attacks are of
concern.
5.2 Alternatives for Performance
An independent impediment to deploying IPsec or TCP/MD5 is
performance; in both cases, there is a substantial impact on the
throughput of individual connections, as well as the computational
load of the endpoints. There are variants of IPsec and TCP/MD5 which
can address these performance issues, also discussed in further
detail elsewhere [4]. They involve protecting a subset of the packet
(focusing on the headers), and/or relaxing the authentication
algorithms.
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Conventional authentication involves ESP or ESP-like processing. The
entire IP payload is authenticated using existing algorithms, e.g.,
MD5, SHA, etc. This provides full protection of both all headers and
data against all third-party spoofing, as well as tampering.
Unfortunately, this alternative is as computationally expensive as
other forms of authentication, e.g., TCP MD5, since similar
algorithms are used over the bulk of the packet [10][11].
Header-only authentication uses truncated ESP processing over only
the fixed portions of the IP (in IPsec) or IP/TCP (in TCP/MD5)
header, possibly with some short prefix of the payload, for padding
and to avoid small-block hash problems. This mode protects headers
from all third-party spoofing and tampering. It does not protect the
payload (outside that optionally used to pad the hash) from
tampering. Since the IP header is usually sufficiently smaller than
a typical hash block (e.g., 64 bytes for MD5), part or all of the TCP
header would typically be protected for either IPsec or TCP/MD5.
This then completely protects TCP against RST attacks. Header-only
mode does not protect from man-in-the-middle replay of signed headers
with alternate payloads. For packets substantially larger than the
hash block size (again, 64 bytes for MD5), header-only mode can
represent a significant computational savings over full-packet
authentication.
Both header-only and nonce mode authentication require modifications
to IPsec or TCP/MD5. These modes would be negotiated during IKE for
IPsec or at connection establishment for TCP/MD5.
Nonce authentication ignores the context of individual packets
completely when computing the authentication signature. The
signature may be fixed or vary, but is independent of the packet
header and contents. The signature is effectively a cookie; it may
even be the case that the SPI itself (for IKE) or ISN (for TCP/MD5)
are sufficient for this purpose, though we do not recommend either
(SPI values are not chosen pseudo-randomly, and ISNs are not always
chosen pseudorandomly). The cookie may the keys established during
IKE or the augmented auto-key version of TCP/MD5. The benefit of
nonce mode is performance; only header modification is required,
avoiding all computational overhead. Nonce mode protects only from
off-path third-party spoofing.
Conventional IPsec and TCP/MD5 and header-only mode both protect TCP
from all third-party spoofing attacks. Nonce-only mode TCP protects
only against off-path third-party attacks, but has much lower
computational cost.
In summary, there are two components needed to enable more widespread
use of existing IPsec or TCP/MD5 spoofing protection. Automatic
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anonymous keying is already supported by IPsec using IKE with a
public shared secret; similar keying can be added to TCP/MD5 by
adding a Diffie-Hellman exchange therein, or even by just using the
TCP ISN. High-performance variants of both schemes are possible.
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6. Security Considerations
This entire document focuses on increasing the security of transport
protocols and their resistance to spoofing attacks. Security is
addressed throughout.
This document describes a number of techniques for defeating spoofing
attacks. Those relying on clear-text cookies, either explicit or
implicit (e.g., window sequence attenuation) do not protect from
man-in-the-middle spoofing attacks, since valid values can be learned
from prior traffic. Those relying on true authentication algorithms
are stronger, protecting even from man-in-the-middle, because the
authentication hash in a single packet approaches the behavior of
"one time" cookies.
Security at various levels of the protocol stack are addressed.
Spoofing attacks are fundamentally identity masquerading, so we
believe the most appropriate solutions defeat these at the network
layer, where end-to-end identity lies. Some transport protocols
subsume endpoint identity information from the network layer (e.g.,
TCP pseudo-headers), while others establish per-connection identity
based on exchanged nonces (e.g., SCTP). It is reasonable, if not
recommended, to address security at all layers of the protocol stack.
We believe that the current attacks are most directly addressed at
the network layer.
Some new solutions discussed weaken overall security compared to
existing solutions. However, a stronger solution which is not
deployed, due to its complexity or performance, is equivalent to no
security at all. Providing a more easily deployed anonymous variant
of IKE or TCP/MD5 together with authentication that can be adjusted
to trade strength for performance may constitute an overall benefit
toward the increased deployment of security solutions.
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7. Conclusions
This document describes the details of the recent BGP spoofing
attacks involving spurious RSTs used to shutdown TCP connections. It
summarizes and discusses a variety of current and proposed solutions
at various protocol layers.
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8. Acknowledgments
This document was inspired by discussions on the
<http://www.ietf.org/html.charters/tcpm-charter.html> about the
recent spoofed RST attacks on BGP routers, including R. Stewart's
draft [13][22]. The analysis of the attack issues, alternate
solutions, and the anonymous security proposed solutions were the
result of discussions on that list as well as with USC/ISI's T.
Faber, A. Falk, G. Finn, and Y. Wang. Ran Atkinson suggested the
UDP variant of TCP/MD5, and Paul Goyette suggested applying
headerANON-style processing to TCP/MD5 and using the ISN to seed TCP/
MD5. Other improvements are due to the input of various members of
the IETF's TCPM WG.
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9. References
9.1 Normative References
[1] "Technical Cyber Security Alert TA04-111A: Vulnerabilities in
TCP -- http://www.us-cert.gov/cas/techalerts/TA04-111A.html",
, April 20 2004.
[2] Heffernan, A., "Protection of BGP Sessions via the TCP MD5
Signature Option", RFC 2385, August 1998.
[3] Poon, K., "Use of TCP timestamp option to defend against blind
spoofing attack", (work in progress) , June 2004.
[4] Touch, J., "ANONsec: Anonymous Security to Defend Against
Spoofing Attacks", (work in progress) , July 2004.
[5] Braden, B., "TIME-WAIT Assassination Hazards in TCP", RFC 1337,
May 1992.
[6] Faber, T., Touch, J. and W. Yue, "The TIME-WAIT state in TCP
and Its Effect on Busy Servers", Proc. Infocom 1999 pp.
1573-1583, March 1999.
[7] Floyd, S., "Inappropriate TCP Resets Considered Harmful", BCP
60, RFC 3360, August 2002.
[8] Postel, J., "Transmission Control Protocol", STD 7, RFC 793,
September 1981.
[9] Jacobson, V., Braden, B. and D. Borman, "TCP Extensions for
High Performance", RFC 1323, May 1992.
[10] Touch, J., "Report on MD5 Performance", RFC 1810, June 1995.
[11] Touch, J., "Performance Analysis of MD5", Proc. Sigcomm 1995
77-86., March 1999.
[12] O'Malley, S. and L. Peterson, "TCP Extensions Considered
Harmful", RFC 1263, October 1991.
[13] "IETF TCPM Working Group and mailing list -
http://www.ietf.org/html.charters/tcpm-charter.html", .
[14] Stewart, R., Xie, Q., Morneault, K., Sharp, C., Schwarzbauer,
H., Taylor, T., Rytina, I., Kalla, M., Zhang, L. and V. Paxson,
"Stream Control Transmission Protocol", RFC 2960, October 2000.
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[15] Bernstein, D., "SYN cookies --
http://cr.yp.to/syncookies.html", , 1997.
[16] Kent, S. and R. Atkinson, "Security Architecture for the
Internet Protocol", RFC 2401, November 1998.
[17] Kent, S. and R. Atkinson, "IP Authentication Header", RFC 2402,
November 1998.
[18] Kent, S. and R. Atkinson, "IP Encapsulating Security Payload
(ESP)", RFC 2406, November 1998.
[19] Harkins, D. and D. Carrel, "The Internet Key Exchange (IKE)",
RFC 2409, November 1998.
[20] Glenn, R. and S. Kent, "The NULL Encryption Algorithm and Its
Use With IPsec", RFC 2410, November 1998.
[21] Maughan, D., Schneider, M. and M. Schertler, "Internet Security
Association and Key Management Protocol (ISAKMP)", RFC 2408,
November 1998.
9.2 Informative References
[22] Stewart, R., "Transmission Control Protocol security
considerations", draft-ietf-tcpm-tcpsecure-01 (work in
progress), June 2004.
[23] Kohler, E., "Datagram Congestion Control Protocol (DCCP)",
draft-ietf-dccp-spec-06 (work in progress), February 2004.
[24] Kent, S. and K. Seo, "Security Architecture for the Internet
Protocol", draft-ietf-ipsec-rfc2401bis-02 (work in progress),
April 2004.
[25] Kent, S., "IP Authentication Header",
draft-ietf-ipsec-rfc2402bis-07 (work in progress), March 2004.
[26] Kaufman, C., "Internet Key Exchange (IKEv2) Protocol",
draft-ietf-ipsec-ikev2-14 (work in progress), June 2004.
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Author's Address
Joe Touch
USC/Information Sciences Institute
4676 Admiralty Way
Marina del Rey, CA 90292-6695
U.S.A.
Phone: +1 (310) 448-9151
Fax: +1 (310) 448-9300
EMail: touch@isi.edu
URI: http://www.isi.edu/touch
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