draft-ietf-tcpm-tcp-antispoof-00.txt   draft-ietf-tcpm-tcp-antispoof-01.txt 
IETF TCPM WG J. Touch IETF TCPM WG J. Touch
Internet Draft USC/ISI Internet Draft USC/ISI
Expires: August 2005 February 13, 2005 Expires: October 2005 April 26, 2005
Defending TCP Against Spoofing Attacks Defending TCP Against Spoofing Attacks
draft-ietf-tcpm-tcp-antispoof-00.txt draft-ietf-tcpm-tcp-antispoof-01.txt
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Abstract Abstract
Recent analysis of potential attacks on core Internet infrastructure Recent analysis of potential attacks on core Internet infrastructure
indicates an increased vulnerability of TCP connections to spurious indicates an increased vulnerability of TCP connections to spurious
resets (RSTs). TCP has always been susceptible to such RST spoof resets (RSTs), sent with forged IP source addresses (spoofing). TCP
attacks, which were indirectly protected by checking that the RST has always been susceptible to such RST spoofing attacks, which were
sequence number was inside the current receive window, as well as via indirectly protected by checking that the RST sequence number was
the obfuscation of TCP endpoint and port numbers. For pairs of well- inside the current receive window, as well as via the obfuscation of
known endpoints often over predictable port pairs, such as BGP or TCP endpoint and port numbers. For pairs of well-known endpoints
between web servers and well-known large-scale caches, increases in often over predictable port pairs, such as BGP or between web servers
the path bandwidth-delay product of a connection have sufficiently and well-known large-scale caches, increases in the path bandwidth-
increased the receive window space that off-path third parties can delay product of a connection have sufficiently increased the receive
guess a viable RST sequence number. This document addresses this window space that off-path third parties can guess a viable RST
sequence number. The susceptibility to attack increases as the square
of the bandwidth, thus presents a significant vulnerability for
recent high-speed networks. This document addresses this
vulnerability, discussing proposed solutions at the transport level vulnerability, discussing proposed solutions at the transport level
and their inherent challenges, as well as existing network level and their inherent challenges, as well as existing network level
solutions and the feasibility of their deployment. solutions and the feasibility of their deployment.
Conventions used in this document
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in RFC-2119 [1].
Table of Contents Table of Contents
1. Introduction...................................................3 1. Introduction...................................................3
2. Background.....................................................4 2. Background.....................................................4
2.1. Recent BGP Attacks Using TCP RSTs.........................4 2.1. Recent BGP Attacks Using TCP RSTs.........................4
2.2. TCP RST Vulnerability.....................................5 2.2. TCP RST Vulnerability.....................................5
2.3. What Changed -- the Ever Opening Window...................5 2.3. What Changed -- the Ever Opening Receiver Window..........6
3. Proposed solutions.............................................8 3. Proposed solutions.............................................8
3.1. Transport Layer Solutions.................................8 3.1. Transport Layer Solutions.................................8
3.1.1. TCP MD5 Authentication...............................8 3.1.1. TCP MD5 Authentication...............................9
3.1.2. TCP RST Window Attenuation...........................8 3.1.2. TCP RST Window Attenuation...........................9
3.1.3. TCP Timestamp Authentication.........................9 3.1.3. TCP Timestamp Authentication........................10
3.1.4. Other TCP Cookies...................................10 3.1.4. Other TCP Cookies...................................10
3.1.5. Other TCP Considerations............................10 3.1.5. Other TCP Considerations............................11
3.1.6. Other Transport Protocol Solutions..................11 3.1.6. Other Transport Protocol Solutions..................11
3.2. Network Layer (IP) Solutions.............................11 3.2. Network Layer (IP) Solutions.............................12
4. Issues........................................................12 3.2.1. Ingress filtering...................................12
4.1. Transport Layer (e.g., TCP)..............................12 3.2.2. IPsec...............................................13
4.2. Network Layer (IP).......................................13 4. Issues........................................................13
4.3. Application Layer........................................14 4.1. Transport Layer (e.g., TCP)..............................13
4.4. Shim Transport/Application Layer.........................14 4.2. Network Layer (IP).......................................14
4.5. Link Layer...............................................14 4.3. Application Layer........................................15
4.6. Conclusion...............................................15 4.4. Shim Transport/Application Layer.........................16
5. Security Considerations.......................................15 4.5. Link Layer...............................................16
6. Conclusions...................................................16 4.6. Issues Discussion........................................16
7. Acknowledgments...............................................16 5. Security Considerations.......................................17
8. References....................................................16 6. Conclusions...................................................17
8.1. Normative References.....................................16 7. Acknowledgments...............................................17
8. References....................................................18
8.1. Normative References.....................................18
8.2. Informative References...................................18 8.2. Informative References...................................18
Author's Addresses...............................................18 Author's Addresses...............................................21
Intellectual Property Statement..................................19 Intellectual Property Statement..................................21
Disclaimer of Validity...........................................19 Disclaimer of Validity...........................................21
Copyright Statement..............................................19 Copyright Statement..............................................22
Acknowledgment...................................................19 Acknowledgment...................................................22
1. Introduction 1. Introduction
Analysis of the Internet infrastructure has been recently Analysis of the Internet infrastructure has been recently
demonstrated new version of a vulnerability in BGP connections demonstrated new version of a vulnerability in BGP connections
between core routers using an attack known for nearly six years between core routers using an attack known for nearly six years
[3][4]. These connections, typically using TCP, can be susceptible [6][7][15][35]. These connections, typically using TCP, can be
to off-path (non man-in-the-middle) third-party reset (RST) segments, susceptible to off-path (non man-in-the-middle) third-party reset
which terminate the TCP connection. BGP routers react to a (RST) segments with forged source addresses (spoofed), which
terminated TCP connection in various ways, ranging from restarting terminate the TCP connection. BGP routers react to a terminated TCP
the connection to deciding that the other router is unreachable and connection in various ways which can amplify the impact of an attack,
thus flushing the BGP routes. This sort of attack affects other ranging from restarting the connection to deciding that the other
protocols besides BGP, involving any long-lived connection between router is unreachable and thus flushing the BGP routes [29]. This
well-known endpoints. The impact on Internet infrastructure can be sort of attack affects other protocols besides BGP, involving any
substantial (esp. for the BGP case), and warrants immediate long-lived connection between well-known endpoints. The impact on
attention. Internet infrastructure can be substantial (esp. for the BGP case),
and warrants immediate attention.
TCP, like many other protocols, can be susceptible to these off-path TCP, like many other protocols, can be susceptible to these off-path
third-party attacks. Such attacks rely on the increase of commodity third-party spoofing attacks. Such attacks rely on the increase of
platforms supporting public access to previously privileged commodity platforms supporting public access to previously privileged
resources, such as root-level access. Given such access, it is resources, such as root-level access. Given such access, it is
trivial for anyone to generate a packet with any header desired. trivial for anyone to generate a packet with any header desired.
This, coupled with the lack of sufficient ingress filtering to drop This, coupled with the lack of sufficient ingress filtering to drop
such spoofed traffic, can increase the potential for off-path third- such spoofed traffic, can increase the potential for off-path third-
party spoofing attacks. Proposed solutions include the deployment of party spoofing attacks. Proposed solutions include the deployment of
existing Internet network and transport security as well as existing Internet network and transport security as well as
modifications to transport protocols that reduce its vulnerability to modifications to transport protocols that reduce its vulnerability to
generated attacks. generated attacks.
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connection, either at the transport level or the network level. TCP connection, either at the transport level or the network level. TCP
with MD5 extensions provides this authentication at the transport with MD5 extensions provides this authentication at the transport
level, and IPsec provides authentication at the network level. In level, and IPsec provides authentication at the network level. In
both cases their deployment overhead may be prohibitive, e.g., it may both cases their deployment overhead may be prohibitive, e.g., it may
not feasible for public services, such as web servers, to be not feasible for public services, such as web servers, to be
configured with the appropriate certificate authorities of large configured with the appropriate certificate authorities of large
numbers of peers (for IPsec using IKE), or shared secrets (for IPsec 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 in shared-secret mode, or TCP/MD5), because many clients may need to
be configured rapidly without external assistance. Services from be configured rapidly without external assistance. Services from
public web servers connecting to large-scale caches to BGP with public web servers connecting to large-scale caches to BGP with
larger numbers of peers can experience this challege. larger numbers of peers can experience this challenge.
The remainder of this document outlines the recent attack scenario in The remainder of this document outlines the recent attack scenario in
detail and describes and compares a variety of solutions, including detail and describes and compares a variety of solutions, including
existing solutions based on TCP/MD5 and IPsec, as well as recently existing solutions based on TCP/MD5 and IPsec, as well as recently
proposed solutions, including modifications to TCP's RST processing proposed solutions, including modifications to TCP's RST processing
[24], modifications to TCP's timestamp processing [5], and [8], modifications to TCP's timestamp processing [27], and
modifications to IPsec and TCP/MD5 keying [6]. modifications to IPsec and TCP/MD5 keying [34].
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 [15]. The recent attack scenario was first
documented by Convery at a NANOG meeting in 2003, but that analysis
assumed the entire sequence space (2^32 packets) needed to be covered
for an attack to succeed [7]. Watson's more detailed analysis
discovered that a single packet anywhere in the current window could
succeed at an attack [35]. This document adds the observation that
susceptibility to attack goes as the square of bandwidth, due to the
coupling between the linear decrease in window size and linear
increase in rate an attacker, as well as comparing the variety of
more recent proposals, including modifications to TCP, use of IPsec,
and use of TCP/MD5 to resist such attacks.
2. Background 2. Background
The recent analysis of potential attacks on BGP has again raised the The recent analysis of potential attacks on BGP has again raised the
issue of TCP's vulnerability to off-path third-party spoofing attacks issue of TCP's vulnerability to off-path third-party spoofing attacks
[3]. A variety of such attacks have been known for several years, [6][7][35]. A variety of such attacks have been known for several
including sending RSTs, SYNs, and even ACKs in an attempt to affect years, including sending RSTs, SYNs, and even ACKs in an attempt to
an existing connection or to load down servers. Overall, such affect an existing connection or to load down servers. Overall, such
attacks are countered by the use of some form of authentication at 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 the network (e.g., IPsec), transport (e.g., SYN cookies, TCP/MD5), or
other layers. TCP already includes a weak form of such other layers. TCP already includes a weak form of such
authentication in its check of segment sequence numbers against the authentication in its check of segment sequence numbers against the
current receiver window. Increases in the bandwidth-delay product current receiver window. Increases in the bandwidth-delay product
for certain long connections have sufficiently weakened this type of for certain long connections have sufficiently weakened this type of
weak authentication in recent weeks, rendering it moot. weak authentication in recent weeks, rendering it moot.
2.1. Recent BGP Attacks Using TCP RSTs 2.1. Recent BGP Attacks Using TCP RSTs
BGP represents a particular vulnerability to spoofing attacks. Most BGP represents a particular vulnerability to spoofing attacks because
TCP connections are protected by multiple levels of obfuscation it uses TCP connectivity to infer routability, so losing a TCP
except at the endpoints of the connection: connection with a BGP peer can result in the flushing of routes to
that peer [29].
Until six years ago, such connections were assumed difficult to
attack because they were described by a few comparatively obscure
parameters [15]. 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 o Both endpoint addresses are usually not well-known; although server
addresses are advertised, clients are somewhat anonymous. addresses are advertised, clients are somewhat anonymous.
o Both port numbers are usually not well-known; the server's usually o Both port numbers are usually not well-known; the server's usually
is advertised (representing the service), but the client's is is advertised (representing the service), but the client's is
typically sufficiently unpredictable to an off-path third-party. typically sufficiently unpredictable to an off-path third-party.
o Valid sequence number space is not well-known. o Valid sequence number space is not well-known.
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only case). Both endpoints are well-known, notably as part of an AS only case). Both endpoints are well-known, notably as part of an AS
path. The destination port is typically fixed to indicate the BGP path. The destination port is typically fixed to indicate the BGP
service. The source port used by a BGP router is sometimes fixed and service. The source port used by a BGP router is sometimes fixed and
advertised to enable firewall configuration; even when not fixed, advertised to enable firewall configuration; even when not fixed,
there are only 65,384 valid source ports which may be exhaustively there are only 65,384 valid source ports which may be exhaustively
attacked. Connections are long-lived, and as noted before some BGP attacked. Connections are long-lived, and as noted before some BGP
implementations interpret successive TCP connection failures as implementations interpret successive TCP connection failures as
routing failures, discarding the corresponding routing information. routing failures, discarding the corresponding routing information.
As importantly and as will be shown below, the valid sequence number As importantly and as will be shown below, the valid sequence number
space once thought to provide some protection has been rendered space once thought to provide some protection has been rendered
useless by increasing congestion window sizes. useless by increasing advertised receive window sizes.
2.2. TCP RST Vulnerability 2.2. TCP RST Vulnerability
TCP has a known vulnerability to third-party spoofed segments. SYN TCP has a known vulnerability to third-party spoofed segments. SYN
flooding consumes server resources in half-open connections, flooding consumes server resources in half-open connections,
affecting the server's ability to open new connections. ACK spoofing affecting the server's ability to open new connections. ACK spoofing
can cause connections to transmit too much data too quickly, creating can cause connections to transmit too much data too quickly, creating
network congestion and segment loss, causing connections to slow to a network congestion and segment loss, causing connections to slow to a
crawl. In the most recent attacks on BGP, RSTs cause connections to crawl. In the most recent attacks on BGP, RSTs cause connections to
be dropped. As noted earlier, some BGP implementations interpret TCP be dropped. As noted earlier, some BGP implementations interpret TCP
connection termination, or a series of such failures, as a network connection termination, or a series of such failures, as a network
failure. This causes routers to drop the BGP routing information failure [29]. This causes routers to drop the BGP routing
already exchanged, in addition to inhibiting their ongoing exchanges. information already exchanged, in addition to inhibiting their
The result can affect routing paths throughout the Internet. 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, 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 even when used by the legitimate endpoints of a connection. TCP RSTs
cause the receiver to drop all connection state; because the source 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 is not required to maintain a TIME_WAIT state, such a RST can cause
premature reuse of address/port pairs, potentially allowing segments premature reuse of address/port pairs, potentially allowing segments
from a previous connection to contaminate the data of a new from a previous connection to contaminate the data of a new
connection, known as TIME_WAIT assassination [7]. In this case, connection, known as TIME_WAIT assassination [5]. In this case,
assassination occurs inadvertently as the result of duplicate assassination occurs inadvertently as the result of duplicate
segments from a legitimate source, and can be avoided by blocking RST segments from a legitimate source, and can be avoided by blocking RST
processing while in TIME_WAIT. However, assassination can be useful processing while in TIME_WAIT. However, assassination can be useful
to deliberately reduce the state held at servers; this requires that 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, 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 [8]. and that RSTs are not blocked in the TIME_WAIT state [9].
Firewalls and load balancers, so-called 'middleboxes', sometimes emit Firewalls and load balancers, so-called 'middleboxes', sometimes emit
RSTs on behalf of transited connections to optimize server RSTs on behalf of transited connections to optimize server
performance [9]. This is effectively a 'man in the middle' RST performance [11]. This is effectively a 'man in the middle' RST
attack in which the RSTs are sent for benign or beneficial intent. attack in which the RSTs are sent for benign or beneficial intent.
There are numerous hazards with such use of RSTs, outlined in that There are numerous hazards with such use of RSTs, outlined in that
RFC. RFC.
2.3. What Changed -- the Ever Opening Window 2.3. What Changed -- the Ever Opening Receiver Window
RSTs represent a hazard to TCP, especially when completely unchecked. RSTs represent a hazard to TCP, especially when completely unchecked.
Fortunately, there are a number of obfuscation mechanisms that make Fortunately, there are a number of obfuscation mechanisms that make
it difficult for off-path third parties to forge (spoof) valid RSTs, 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 as noted earlier. We have already shown it is easy to learn both
endpoint addresses and ports for some protocols, notably BGP. The endpoint addresses and ports for some protocols, notably BGP. The
final obfuscation is the segment sequence number. final obfuscation is the segment sequence number.
TCP segments include a sequence number which enables out-of-order TCP segments include a sequence number which enables out-of-order
receiver processing, as well as duplicate detection. The sequence receiver processing as well as duplicate detection. The sequence
number space is also used to manage congestion, and indicates the number space is also used to manage congestion, and indicates the
index of the next byte to be transmitted or received. For RSTs, this index of the next byte to be transmitted or received. For RSTs, this
is relevant because legitimate RSTs use the next sequence number in is relevant because legitimate RSTs use the next sequence number in
the transmitter window, and the receiver checks that incoming RSTs the transmitter window, and the receiver checks that incoming RSTs
have a sequence number in the expected receive window. Such have a sequence number in the expected receive window. Such
processing is intended to eliminate duplicate segments (somewhat moot processing is intended to eliminate duplicate segments (somewhat moot
for RSTs, though), and to drop RSTs which were part of previous for RSTs, though), and to drop RSTs which were part of previous
connections. connections.
TCP uses two window mechanisms, a primary mechanism which uses a TCP uses two window mechanisms, a primary mechanism which uses a
space of 32 bits, and a secondary mechanism which scales this window space of 32 bits, and a secondary mechanism which scales this window
[10][11]. The valid receive window is a fraction, not to exceed [28][16]. The valid advertised receive window is a fraction, not to
approximately half, of this space, or ~2,000,000,000. Under typical exceed approximately half, of this space, or ~2,000,000,000. Under
use, the majority of TCP connections open to a very small fraction of typical use, the majority of TCP connections open to a very small
this space, e.g., 10,000-60,000(approximately 5-100 segments). On a fraction of this space, e.g., 10,000-60,000(approximately 5-100
low-loss path, the window should open to around the path bandwidth- segments). On a low-loss path, the advertised receive window should
delay product, including buffering delays (assume 1 packet/hop). open to around the path bandwidth-delay product, including buffering
Many paths in the Internet have end-to-end bandwidths of under 1 delays (assume 1 packet/hop). Many paths in the Internet have end-
Mbps, latencies under 100ms, and are under 15 hops, resulting in to-end bandwidths of under 1 Mbps, latencies under 100ms, and are
fairly small windows as above (under 35,000 bytes). Under these under 15 hops, resulting in fairly small windows as above (under
conditions, and further assuming that the initial sequence number is 35,000 bytes). Under these conditions, and further assuming that the
suitably (pseudo-randomly) chosen, a valid guessed sequence number initial sequence number is suitably (pseudo-randomly) chosen, a valid
would have odds of 1 in 57,000. Put differently, a blind (non man- guessed sequence number would have odds of 1 in 57,000 of falling
in-the-middle) attacker would need to send 57,000 RSTs with suitably within the advertised receive window. Put differently, a blind (non
spaced sequence number guesses to successfully reset a connection. man-in-the-middle) attacker would need to send 57,000 RSTs with
At 1 Mbps, 57,000 (40 byte) RSTs would take over 50 minutes to suitably spaced sequence number guesses to successfully reset a
transmit, and, as noted earlier, most current connections are fairly connection. At 1 Mbps, 57,000 (40 byte) RSTs would take over 50
brief by comparison. 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 Recent use of high bandwidth paths of 10 Gbps and higher result in
bandwidth-delay products over 125 MB - approximately 1/10 of TCP's bandwidth-delay products over 125 MB - approximately 1/10 of TCP's
overall window size excluding scale, assuming the receiver allocates overall maximum advertised receive window size excluding scale,
sufficient buffering (to be discussed later). Even under networks assuming the receiver allocates sufficient buffering (to be discussed
that are ten times slower (1 Gbps), the active receiver window covers later). Even under networks that are ten times slower (1 Gbps), the
1/100th of the overall window size. At these speeds, it takes only active advertised receiver window covers 1/100th of the overall
10-100 packets, or under 32 microseconds, to correctly guess a valid window size. At these speeds, it takes only 10-100 packets, or under
sequence number and kill a connection. A table of corresponding 32 microseconds, to correctly guess a valid sequence number and kill
exposure to various amounts of RSTs is shown below, for various line a connection. A table of corresponding exposure to various amounts
rates, assuming the more conventional 100ms latencies (though even of RSTs is shown below, for various line rates, assuming the more
100ms is large for BGP cases): conventional 100ms latencies (though even 100ms is large for BGP
cases):
BW BW*delay RSTs needed Time needed BW BW*delay RSTs needed Time needed
------------------------------------------------------------ ------------------------------------------------------------
10 Gbps 125 MB 35 1 us (microsecond) 10 Gbps 125 MB 35 1 us (microsecond)
1 Gbps 12.5 MB 344 110 us 1 Gbps 12.5 MB 344 110 us
100 Mbps 1.25 MB 3,436 10 ms (millisecond) 100 Mbps 1.25 MB 3,436 10 ms (millisecond)
10 Mbps 0.125 MB 34,360 1 second 10 Mbps 0.125 MB 34,360 1 second
1 Mbps 0.0125 MB 343,598 2 minutes 1 Mbps 0.0125 MB 343,598 2 minutes
100 Kbps 0.00125 MB 3,435,974 3 hours 100 Kbps 0.00125 MB 3,435,974 3 hours
Figure 1: Time needed to kill a connection Figure 1 Time needed to kill a connection
This table demonstrates that the effect of bandwidth on the This table demonstrates that the effect of bandwidth on the
vulnerability is squared; for every increase in bandwidth, there is a vulnerability is squared; for every increase in bandwidth, there is a
linear decrease in the number of sequence number guesses needed, as 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 well as a linear decrease in the time needed to send a set of
guesses. Notably, as inter-router link bandwidths approach 1 Mbps, guesses. Notably, as inter-router link bandwidths approach 1 Mbps,
an 'exhaustive' attack becomes practical. Checking that the RST an 'exhaustive' attack becomes practical. Checking that the RST
sequence number is somewhere in the valid window (bw*delay) out of sequence number is somewhere in the valid window (bw*delay) out of
the overall window (2^32) is an insufficient obfuscation. the overall advertised receive window (2^32) is an insufficient
obfuscation.
Note that this table makes a number of assumptions: Note that this table makes a number of assumptions:
1. the overall bandwidth-delay product is relatively fixed 1. the overall bandwidth-delay product is relatively fixed
2. traffic losses are negligible (insufficient to affect the 2. traffic losses are negligible (insufficient to affect the
congestion window over the duration of most of the connection)
3. congestion window over most of the connection) 3. the receive socket buffers do not limiting the receive window
4. the receive socket buffers do not limiting the receive window
5. the attack bandwidth is similar to the end-to-end path bandwidth 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 Of these assumptions, the last two are more notable. The issue of
receive socket buffers will be addressed later. The issue of the receive socket buffers will be addressed later. The issue of the
attack bandwidth is considered reasonable as follows: attack bandwidth is considered reasonable as follows:
1. RSTs are substantially easier to send than data; they can be 1. RSTs are substantially easier to send than data; they can be
precomputed and they are smaller than data packets (40 bytes). precomputed and they are smaller than data packets (40 bytes).
2. although susceptible connections use somewhat less ubiquitous 2. although susceptible connections use somewhat less ubiquitous
high-bandwidth paths, the attack may be distributed, at which high-bandwidth paths, the attack may be distributed, at which
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receive socket buffers will be addressed later. The issue of the receive socket buffers will be addressed later. The issue of the
attack bandwidth is considered reasonable as follows: attack bandwidth is considered reasonable as follows:
1. RSTs are substantially easier to send than data; they can be 1. RSTs are substantially easier to send than data; they can be
precomputed and they are smaller than data packets (40 bytes). precomputed and they are smaller than data packets (40 bytes).
2. although susceptible connections use somewhat less ubiquitous 2. although susceptible connections use somewhat less ubiquitous
high-bandwidth paths, the attack may be distributed, at which high-bandwidth paths, the attack may be distributed, at which
point only the ingress link of the attack is the primary point only the ingress link of the attack is the primary
limitation limitation
3. for the purposes of the above table, we assume that the ingress at 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 attack has the same bandwidth as the path, as an approximation
The previous sections discussed the nature of the recent attacks on The previous sections discussed the nature of the recent attacks on
BGP due to the vulnerability of TCP to RST spoofing attacks, due BGP due to the vulnerability of TCP to RST spoofing attacks, due
largely to recent increases in the fraction of the TCP window space largely to recent increases in the fraction of the TCP advertised
in use for a single, long-lived connection. receive window space in use for a single, long-lived connection.
3. Proposed solutions 3. Proposed solutions
TCP currently authenticates received RSTs using the address and port TCP currently authenticates received RSTs using the address and port
pair numbers, and checks that the sequence number is inside the valid pair numbers, and checks that the sequence number is inside the valid
receiver window. The previous section demonstrated how TCP has receiver window. The previous section demonstrated how TCP has
become more vulnerable to RST spoofing attacks due to the increases become more vulnerable to RST spoofing attacks due to the increases
in the receive window size. There are a number of current and in the receive window size. There are a number of current and
proposed solutions to this vulnerability, all attempting to increase proposed solutions to this vulnerability, all attempting to increase
the authentication of received RSTs. the authentication of received RSTs.
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authenticated before they affect connection management. TCP has a authenticated before they affect connection management. TCP has a
variety of current and proposed mechanisms to increase the variety of current and proposed mechanisms to increase the
authentication of segments, protecting against both off-path third- authentication of segments, protecting against both off-path third-
party spoofs and man-in-the-middle attacks. SCTP also has mechanisms party spoofs and man-in-the-middle attacks. SCTP also has mechanisms
to authenticate segments. to authenticate segments.
3.1.1. TCP MD5 Authentication 3.1.1. TCP MD5 Authentication
An extension to TCP supporting MD5 authentication was developed An extension to TCP supporting MD5 authentication was developed
around six years ago specifically to authenticate BGP connections around six years ago specifically to authenticate BGP connections
(although it can be used for any TCP connection) [4]. The extension (although it can be used for any TCP connection) [15]. The extension
relies on a pre-shared secret key to authenticate the entire TCP relies on a pre-shared secret key to authenticate the entire TCP
segment, including the data, TCP header, and TCP pseudo-header segment, including the data, TCP header, and TCP pseudo-header
(certain fields of the IP header). All segments are protected, (certain fields of the IP header). All segments are protected,
including RSTs, to be accepted only when their signature matches. including RSTs, to be accepted only when their signature matches.
This option, although widely deployed in Internet routers, is This option, although widely deployed in Internet routers, is
considered undeployable for widespread use because the need for pre- considered undeployable for widespread use because the need for pre-
shared keys. It further is considered computationally expensive for shared keys [2][24]. It further is considered computationally
either hosts or routers due to the overhead of MD5 [12][13]. expensive for either hosts or routers due to the overhead of MD5
[32][33].
3.1.2. TCP RST Window Attenuation 3.1.2. TCP RST Window Attenuation
A recent proposal extends TCP to further constrain received RST to A recent proposal extends TCP to further constrain received RST to
match the expected next sequence number [24]. This restores TCP's match the expected next sequence number [8]. This restores TCP's
resistance to spurious RSTs, effectively limiting the receive window resistance to spurious RSTs, effectively limiting the receive window
for RSTs to a single number. As a result, an attacker would need to 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. send 2^32 different packets to correctly guess the sequence number.
The extension further modifies the RST receiver to react to The extension further modifies the RST receiver to react to
incorrectly-numbered RSTs, by sending a zero-length ACK. If the RST incorrectly-numbered RSTs, by sending a zero-length ACK. If the RST
source is legitimate, upon receipt of an ACK the closed source would source is legitimate, upon receipt of an ACK the closed source would
presumably emit a RST with the sequence number matching the ACK, presumably emit a RST with the sequence number matching the ACK,
correctly resetting the intended recipient. This modification adds correctly resetting the intended recipient. This modification adds
arcs to the TCP state diagram, adding to its complexity and thus arcs to the TCP state diagram, adding to its complexity and thus
potentially affecting its correctness (in contrast to adding MD5 potentially affecting its correctness (in contrast to adding MD5
skipping to change at page 9, line 24 skipping to change at page 9, line 46
connections between the same pair of endpoints because RSTs flush the connections between the same pair of endpoints because RSTs flush the
TIME-WAIT (as mentioned earlier). Further, this modifies TCP so that TIME-WAIT (as mentioned earlier). Further, this modifies TCP so that
under some circumstances a RST causes a reply, in violation of under some circumstances a RST causes a reply, in violation of
generally accepted practice, if not gentle recommendation. The generally accepted practice, if not gentle recommendation. The
advantage to this proposal is that it can be deployed incrementally advantage to this proposal is that it can be deployed incrementally
and has benefit to the endpoint on which it is deployed. and has benefit to the endpoint on which it is deployed.
A variant of this proposal uses a different value to attenuate the A variant of this proposal uses a different value to attenuate the
window of viable RSTs. It requires RSTs to carry the initial window of viable RSTs. It requires RSTs to carry the initial
sequence number rather than the next expected sequence number, i.e., sequence number rather than the next expected sequence number, i.e.,
the value negotiated on connection establishment [15]. This proposal the value negotiated on connection establishment [31]. This proposal
has the advantage of using an explicitly negotiated value, but at the has the advantage of using an explicitly negotiated value, but at the
cost of changing the behavior of an unmodified endpoint to a cost of changing the behavior of an unmodified endpoint to a
currently valid RST. It would thus be more difficult, without currently valid RST. It would thus be more difficult, without
additional mechanism, to deploy incrementally. additional mechanism, to deploy incrementally.
The most obvious other variant of this proposal involves increasing The most obvious other variant of this proposal involves increasing
TCP's window space, rather than decreasing the valid range for RSTs, 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 i.e., increasing the sequence space from 32 bits to 64 bits. This
has the equivalent effect - the ratio of the valid sequence numbers has the equivalent effect - the ratio of the valid sequence numbers
for any segment to the overall sequence number space is significantly for any segment to the overall sequence number space is significantly
skipping to change at page 9, line 46 skipping to change at page 10, line 21
establish weak authentication using initial sequence numbers (ISNs), establish weak authentication using initial sequence numbers (ISNs),
is contingent on using suitably random values for the ISN. Such is contingent on using suitably random values for the ISN. Such
randomness adds additional complexity to TCP both in specification randomness adds additional complexity to TCP both in specification
and implementation, and provides only very weak authentication. Such and implementation, and provides only very weak authentication. Such
a modification is not obviously backward compatible, and would be a modification is not obviously backward compatible, and would be
thus difficult to deploy. thus difficult to deploy.
3.1.3. TCP Timestamp Authentication 3.1.3. TCP Timestamp Authentication
Another way to authenticate TCP segments is to utilize its timestamp Another way to authenticate TCP segments is to utilize its timestamp
option, using the value as a sort of authentication [5]. This option, using the value as a sort of authentication [27]. This
requires that the receiver TCP discard values whose timestamp is requires that the receiver TCP discard values whose timestamp is
outside the accepted window, which is derived from the timestamps of outside the accepted window, which is derived from the timestamps of
other packets from the same connection. This technique uses an other packets from the same connection. This technique uses an
existing TCP option, but also requires modified RST processing and existing TCP option, but also requires modified RST processing and
may be difficult to deploy incrementally without further may be difficult to deploy incrementally without further
modifications. Additionally, the timestamp value may be easier to modifications. Additionally, the timestamp value may be easier to
guess because it is derived from a predictable value. guess because it is derived from a predictable value.
3.1.4. Other TCP Cookies 3.1.4. Other TCP Cookies
All of the above techniques are variants of cookies, otherwise All of the above techniques are variants of cookies, otherwise
meaningless data whose value is used to validate the packet. In the meaningless data whose value is used to validate the packet. In the
case of MD5 checksums, the cookie is computed based on a shared 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 secret. Note that even a signature can be guessed, and presents a 1
in 2^(signature length) probability of attack. The primary in 2^(signature length) probability of attack. The primary
difference is that MD5 signatures are effectively one-time cookies, difference is that MD5 signatures are effectively one-time cookies,
not predictable based on man-in-the-middle snooping, because they are not predictable based on man-in-the-middle snooping, because they are
dependent on packet data and thus do not repeat. Window attenuation dependent on packet data and thus do not repeat. Window attenuation
sequence numbers can be guessed by snooping the sequence number of sequence numbers can be guessed by snooping the sequence number of
current packets, and timestamps can may be guessed even more current packets, and timestamps can be guessed even more remotely.
remotely. These variants of cookies are similar in spirit to TCP SYN These variants of cookies are similar in spirit to TCP SYN cookies,
cookies, again patching a vulnerability to off-path third-party again patching a vulnerability to off-path third-party spoofing
spoofing attacks based on a (fairly weak, excepting MD5) form of attacks based on a (fairly weak, excepting MD5) form of
authentication. Another form of cookie is the source port itself, authentication. Another form of cookie is the source port itself,
which can be randomized but provides only 16 bits of protection which can be randomized but provides only 16 bits of protection
(65,000 combinations), which may be exhaustively attacked. This can (65,000 combinations), which may be exhaustively attacked. This can
be combined with destination port randomization as well, but that be combined with destination port randomization as well, but that
would require a separate coordination mechanism (so both parties know would require a separate coordination mechanism (so both parties know
which ports to use), which is equivalent to (and as infeasible for which ports to use), which is equivalent to (and as infeasible for
large-scale deployments as) exchanging a shared secret. large-scale deployments as) exchanging a shared secret.
3.1.5. Other TCP Considerations 3.1.5. Other TCP Considerations
The analysis of the potential for RST spoofing above assumes that the The analysis of the potential for RST spoofing above assumes that the
receive window opens to the maximum extent suggested by the receive window opens to the maximum extent suggested by the
bandwidth-delay product of the end-to-end path, and that the window bandwidth-delay product of the end-to-end path, and that the window
opens to an appreciable fraction of the overall sequence number opens to an appreciable fraction of the overall sequence number
space. As noted earlier, for most common cases, connections are too 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. brief or over bandwidths too low for such a large window to occur.
Expanding TCP's sequence number space is a direct way to further Expanding TCP's sequence number space is a direct way to further
avoid such vulnerability, even for long connections over emerging avoid such vulnerability, even for long connections over emerging
bandwidths. bandwidths.
Finally, it is often sufficient for the endpoint to limit the receive Finally, it is often sufficient for the endpoint to limit the receive
window in other ways, notably using 'socket options'. If 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, socket buffer is limited, e.g., to the ubiquitous default of 65KB,
the receive window cannot grow to vulnerable sizes even for very long the receive window cannot grow to vulnerable sizes even for very long
connections over very high bandwidths. The consequence is lower connections over very high bandwidths. The consequence is lower
sustained throughput, where only one window's worth of data per round sustained throughput, where only one window's worth of data per round
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traffic is small (i.e., unlikely to cover a substantial portion of traffic is small (i.e., unlikely to cover a substantial portion of
the sequence space, even if long-lived), so is difficult to consider the sequence space, even if long-lived), so is difficult to consider
where smaller receive buffers would not sufficiently address the where smaller receive buffers would not sufficiently address the
immediate problem. immediate problem.
3.1.6. Other Transport Protocol Solutions 3.1.6. Other Transport Protocol Solutions
Segment authentication has been addressed at the transport layer in Segment authentication has been addressed at the transport layer in
other protocols. Both SCTP and DCCP* include cookies for connection other protocols. Both SCTP and DCCP* include cookies for connection
establishment and uses them to authenticate a variety of other establishment and uses them to authenticate a variety of other
control messages [16][25]. The inclusion of such mechanism at the control messages [30][23]. The inclusion of such mechanism at the
transport protocol, although emerging as standard practice, transport protocol, although emerging as standard practice,
unnecessarily complicates the design and implementation of new unnecessarily complicates the design and implementation of new
protocols. As new attacks are discovered (SYN floods, RSTs, etc.), protocols [25] As new attacks are discovered (SYN floods, RSTs,
each protocol must be modified individually to compensate. A network etc.), each protocol must be modified individually to compensate. A
solution may be more appropriate and efficient. network solution may be more appropriate and efficient.
*[AUTH - DCCP may be removing cookies from the spec for the *[AUTH - DCCP may be removing cookies from the spec for the
redundancies discussed above, because the use of cookies at the redundancies discussed above, because the use of cookies at the
transport layer primarily supports dynamic multihoming (a design goal transport layer primarily supports dynamic multihoming (a design goal
of SCTP, but not DCCP) rather than security.] of SCTP, but not DCCP) rather than security.]
3.2. Network Layer (IP) Solutions 3.2. Network Layer (IP) Solutions
There are two primary variants of network layer solutions to
spoofing: ingress filtering and IPsec. Ingress filtering is an
indirect system which relies on other parties to filter packets sent
upstream of an attack, but does not necessarily require participation
of the packet source. IPsec requires cooperation between the
endpoints wanting to avoid attack on their connection, which
currently involves pre-existing shared knowledge of either a shared
key or shared certificate authority.
3.2.1. Ingress filtering
Ingress filtering is often proposed as an alternative to protocol
mechanisms to defeat IP source address spoofing [1][10]. Ingress
filtering restricts traffic from downstream sources across transit
networks based on the IP source address. It cannot restrict traffic
from the core to edges, i.e., from upstream sources. As a result,
each ingress must perform the appropriate filtering for overall
protection to result; failure of any ingress to filter defeats the
protection of all network participants, ultimately.
As a result, ingress filtering is not a local solution that can be
deployed to protect communicating pairs, but rather relies on a
distributed infrastructure of trusted gateways filtering forged
traffic where it enters the network. It is not feasible for local,
incremental deployment, and relies too heavily on distributed
cooperation. Although useful to reduce the load of spoofed traffic,
it is insufficient to protect particular connections from attack.
A more recent variant of ingress filtering checks the IP TTL field,
relying on the TTL set by the other end of the connection [12]. 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, but those hops could include
other user programs at those nodes or any traffic those nodes accept
via tunnels - because tunnels need not decrement TTLs [26]. This
method of filtering works best where traffic originates one hop away,
so that the ingress filtering is based on the trust of only directly-
connected (tunneled or otherwise) nodes. Like conventional ingress
filtering, this reduces spoofing traffic in general, but is not
considered a reliable security mechanism because it relies on
distributed filtering (that upstream nodes do not terminate tunnels
arbitrarily, e.g.).
3.2.2. IPsec
TCP is susceptible to RSTs, but also to other spoofing and man-in- TCP is susceptible to RSTs, but also to other spoofing and man-in-
the-middle attacks, including SYN attacks. Other transport the-middle attacks, including SYN attacks. Other transport
protocols, such as UDP and RTP are equally susceptible. Although protocols, such as UDP and RTP are equally susceptible. Although
emerging transport protocols attempt to defeat such attacks at the emerging transport protocols attempt to defeat such attacks at the
transport layer, such attacks take advantage of network layer transport layer, such attacks take advantage of network layer
identity spoofing. The packet is coming from an endpoint who is identity spoofing. The packet is coming from an endpoint who is
spoofing another endpoint, either upstream or somewhere else in the spoofing another endpoint, either upstream or somewhere else in the
Internet. IPsec was designed specifically to establish and enforce Internet. IPsec was designed specifically to establish and enforce
authentication of a packet's source and contents, to most directly authentication of a packet's source and contents, to most directly
and explicitly addresses this security vulnerability. and explicitly addresses this security vulnerability.
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As noted earlier, transport layer solutions require separate As noted earlier, transport layer solutions require separate
modification of all transport protocols to include authentication. modification of all transport protocols to include authentication.
Not all transport layers support negotiated endpoint state (e.g., Not all transport layers support negotiated endpoint state (e.g.,
UDP), and legacy protocols have been notoriously difficult to safely UDP), and legacy protocols have been notoriously difficult to safely
augment. Not all authentication solutions are created equal either, augment. Not all authentication solutions are created equal either,
and relying on a variety of transport solutions exposes end-systems and relying on a variety of transport solutions exposes end-systems
to increased potential for incorrectly specified or implemented to increased potential for incorrectly specified or implemented
solutions. Transport authentication has often been developed piece- solutions. Transport authentication has often been developed piece-
wise, in response to specific attacks, e.g., SYN cookies and RST wise, in response to specific attacks, e.g., SYN cookies and RST
window attenuation [17][24]. window attenuation [3][8].
Transport layer solutions are not only per-protocol, but often per- Transport layer solutions are not only per-protocol, but often per-
connection. Each connection needs to negotiate and maintain connection. Each connection needs to negotiate and maintain
authentication state separately. Overhead is not amortized over authentication state separately. Overhead is not amortized over
multiple connections - this includes overheads in packet exchanges, multiple connections - this includes overheads in packet exchanges,
design complexity, and implementation complexity. Finally, because design complexity, and implementation complexity. Finally, because
the authentication happens later in packet processing than is the authentication happens later in packet processing than is
required, additional endpoint resources may be needlessly consumed, required, additional endpoint resources may be needlessly consumed,
e.g., in demultiplexing received packets, indexing connection e.g., in demultiplexing received packets, indexing connection
identifiers, etc., only to be dropped later at the transport layer. identifiers, etc., only to be dropped later at the transport layer.
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packets quickly. Network solutions protect all transport protocols, packets quickly. Network solutions protect all transport protocols,
including both legacy and emerging protocols, and reduce the including both legacy and emerging protocols, and reduce the
complexity of these protocols as well. A shared solution also complexity of these protocols as well. A shared solution also
reduces protocol overhead, and decouples the management (and reduces protocol overhead, and decouples the management (and
refreshing) of authentication state from that of individual transport refreshing) of authentication state from that of individual transport
connections. Finally, a network layer solution protects not only the connections. Finally, a network layer solution protects not only the
transport layer but the network layer as well, e.g., from ICMP, IGMP, transport layer but the network layer as well, e.g., from ICMP, IGMP,
etc., spoofing attacks. etc., spoofing attacks.
The ubiquitous protocol for network layer authentication is IPsec The ubiquitous protocol for network layer authentication is IPsec
[18][26]. IPsec specifies the overall architecture, including header [19][22]. IPsec specifies the overall architecture, including header
authentication (AH) [19][27] and encapsulation (ESP) modes [20]. AH authentication (AH) [20][18] and encapsulation (ESP) modes [21]. AH
authenticates both the IP header and IP data, whereas ESP authenticates both the IP header and IP data, whereas ESP
authenticates only the IP data (e.g., transport header and payload). authenticates only the IP data (e.g., transport header and payload).
AH is deprecated since ESP is more efficient and the SPI includes AH is deprecated since ESP is more efficient and the SPI includes
sufficient information to verify the IP header anyway. These two sufficient information to verify the IP header anyway. These two
modes describe the security applied to individual packets within the modes describe the security applied to individual packets within the
IPsec system; key exchange and management is performed either out-of- IPsec system; key exchange and management is performed either out-of-
band (via pre-shared keys) or by an automated key exchange protocol band (via pre-shared keys) or by an automated key exchange protocol
IKE [21][28]. IKE [14][17].
IPsec already provides authentication of an IP header and its data IPsec already provides authentication of an IP header and its data
contents sufficient to defeat both man-in-the-middle and off-path contents sufficient to defeat both man-in-the-middle and off-path
third-party spoofing attacks. IKE can configure authentication third-party spoofing attacks. IKE can configure authentication
between two endpoints on a per-endpoint, per-protocol, or per- between two endpoints on a per-endpoint, per-protocol, or per-
connection basis, as desired. IKE also can perform automatic connection basis, as desired. IKE also can perform automatic
periodic re-keying, further defeating crypto-analysis based on periodic re-keying, further defeating crypto-analysis based on
snooping (clandestine data collection). The use of IPsec is already snooping (clandestine data collection). The use of IPsec is already
commonly strongly recommended for protected infrastructure. commonly strongly recommended for protected infrastructure.
IPsec is not appropriate for many deployments. It is computationally IPsec is not appropriate for many deployments. It is computationally
intensive both in key management and individual packet authentication intensive both in key management and individual packet authentication
[12]. As importantly, IKE is not anonymous; keys can be exchanged [32]. As importantly, IKE is not anonymous; keys can be exchanged
between parties only if they trust each others' X.509 certificates or between parties only if they trust each others' X.509 certificates or
pre-share a key. These certificates provide identification (the pre-share a key. These certificates provide identification (the
other party knows who you are) only where the certificates themselves other party knows who you are) only where the certificates themselves
are signed by certificate authorities (CAs) that both parties already are signed by certificate authorities (CAs) that both parties already
trust. To a large extent, the CAs themselves are the pre-shared keys trust. To a large extent, the CAs themselves are the pre-shared keys
which help IKE establish security association keys, which are then which help IKE establish security association keys, which are then
used in the authentication algorithms. used in the authentication algorithms.
IPsec, although widely available both in commercial routers and IPsec, although widely available both in commercial routers and
commodity end-systems, is not often utilized except between parties commodity end-systems, is not often utilized except between parties
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4.5. Link Layer 4.5. Link Layer
Link layer security operates separately on each hop of an Internet. Link layer security operates separately on each hop of an Internet.
Such security can be critical in protecting link resources, such as Such security can be critical in protecting link resources, such as
bandwidth and link management protocols. Protection at this layer bandwidth and link management protocols. Protection at this layer
cannot suffice for network or transport layers, because it cannot cannot suffice for network or transport layers, because it cannot
authenticate the endpoint source of a packet. Link authentication authenticate the endpoint source of a packet. Link authentication
ensures only the source of the current link hop where it is examined. ensures only the source of the current link hop where it is examined.
4.6. Conclusion 4.6. Issues Discussion
The issues raised in this section suggest that there are challenges The issues raised in this section suggest that there are challenges
with all solutions to transport protection from spoofing attacks. with all solutions to transport protection from spoofing attacks.
This raises the potential need for alternate security levels. While This raises the potential need for alternate security levels. While
it is already widely recognized that security needs to occur it is already widely recognized that security needs to occur
simultaneously at many protocol layers, the also may be utility in simultaneously at many protocol layers, the also may be utility in
supporting a variety of strengths at a single layer. For example, supporting a variety of strengths at a single layer. For example,
IPsec already supports a variety of algorithms (MD5, SHA, etc. for IPsec already supports a variety of algorithms (MD5, SHA, etc. for
authentication), but always assumes that: authentication), but always assumes that:
1. the entire body of the packet is secured 4. the entire body of the packet is secured
2. security associations are established only where identity is 5. security associations are established only where identity is
authenticated by a know certificate authority or other pre-shared authenticated by a know certificate authority or other pre-shared
key key
3. both man-in-the-middle and off-path third-party spoofing attacks 6. both man-in-the-middle and off-path third-party spoofing attacks
must be defeated must be defeated
These assumptions are prohibitive, especially in many cases of These assumptions are prohibitive, especially in many cases of
spoofing attacks. For spoofing, the primary issue is whether packets spoofing attacks. For spoofing, the primary issue is whether packets
are coming from the same party the server can reach. Only the IP are coming from the same party the server can reach. Only the IP
header is fundamentally in question, so securing the entire packet header is fundamentally in question, so securing the entire packet
(1) is computational overkill. It is sufficient to authenticate the (1) is computational overkill. It is sufficient to authenticate the
other party as "a party you have exchanged packets with", rather than other party as "a party you have exchanged packets with", rather than
establishing their trusted identity ("Bill" vs. "Bob") as in (2). establishing their trusted identity ("Bill" vs. "Bob") as in (2).
Finally, many cookie systems use clear-text (unencrypted), fixed Finally, many cookie systems use clear-text (unencrypted), fixed
cookie values, providing reasonable (1 in 2^{cookie-size}) protection cookie values, providing reasonable (1 in 2^{cookie-size}) protection
against off-path third-party spoofs, but not addressing man-in-the- against off-path third-party spoofs, but not addressing man-in-the-
middle at all. Such potential solutions are discussed in the ANONsec middle at all. Such potential solutions are discussed in the ANONsec
document, in the BTNS (Better Than Nothing Security) BOF [2][6]. document, in the BTNS (Better Than Nothing Security) BOF [4][34].
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 [13].
5. Security Considerations 5. Security Considerations
This entire document focuses on increasing the security of transport This entire document focuses on increasing the security of transport
protocols and their resistance to spoofing attacks. Security is protocols and their resistance to spoofing attacks. Security is
addressed throughout. addressed throughout.
This document describes a number of techniques for defeating spoofing This document describes a number of techniques for defeating spoofing
attacks. Those relying on clear-text cookies, either explicit or attacks. Those relying on clear-text cookies, either explicit or
implicit (e.g., window sequence attenuation) do not protect from man- implicit (e.g., window sequence attenuation) do not protect from man-
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This document describes the details of the recent BGP spoofing This document describes the details of the recent BGP spoofing
attacks involving spurious RSTs which could be used to shutdown TCP attacks involving spurious RSTs which could be used to shutdown TCP
connections. It summarizes and discusses a variety of current and connections. It summarizes and discusses a variety of current and
proposed solutions at various protocol layers. proposed solutions at various protocol layers.
7. Acknowledgments 7. Acknowledgments
This document was inspired by discussions on the This document was inspired by discussions on the
<http://www.ietf.org/html.charters/tcpm-charter.html> about the <http://www.ietf.org/html.charters/tcpm-charter.html> about the
recent spoofed RST attacks on BGP routers, including R. Stewart's recent spoofed RST attacks on BGP routers, including R. Stewart's
draft [15][24]. The analysis of the attack issues, alternate draft (which is now edited by M. Dalal) [31][8]. The analysis of the
solutions, and the anonymous security proposed solutions were the attack issues, alternate solutions, and the anonymous security
result of discussions on that list as well as with USC/ISI's T. proposed solutions were the result of discussions on that list as
Faber, A. Falk, G. Finn, and Y. Wang. Ran Atkinson suggested the well as with USC/ISI's T. Faber, A. Falk, G. Finn, and Y. Wang. Ran
UDP variant of TCP/MD5, and Paul Goyette suggested using the ISN to Atkinson suggested the UDP variant of TCP/MD5, and Paul Goyette
seed TCP/MD5. Other improvements are due to the input of various suggested using the ISN to seed TCP/MD5. Other improvements are due
members of the IETF's TCPM WG. to the input of various members of the IETF's TCPM WG.
8. References 8. References
(NB: to be reordered and repartitioned)
8.1. Normative References 8.1. Normative References
[1] Bradner, S., "Key words for use in RFCs to Indicate Requirement As this is not a standards document, this section has no meaning.
Levels", BCP 14, RFC 2119, March 1997.
[2] Better Than Nothing Security [BTNS] BOF, IETF-61, Wash. DC., 8.2. Informative References
http://www.ietf.org/ietf/04nov/btns.txt.
[3] "Technical Cyber Security Alert TA04-111A: Vulnerabilities in [1] Baker, F. and P. Savola, "Ingress Filtering for Multihomed
TCP -- http://www.us-cert.gov/cas/techalerts/TA04-111A.html", Networks," RFC 2827 / BCP 84, March 2004.
April 20 2004.
[4] Heffernan, A., "Protection of BGP Sessions via the TCP MD5 [2] Bellovin, S. and A. Zinin, "Standards Maturity Variance
Signature Option", RFC 2385, August 1998. Regarding the TCP MD5 Signature Option (RFC 2385) and the BGP-4
Specification," (work in progress),
draft-iesg-tcpmd5app-01.txt, Sept. 2004.
[5] Poon, K., "Use of TCP timestamp option to defend against blind [3] Bernstein, D., "SYN cookies - http://cr.yp.to/syncookies.html",
spoofing attack", (work in progress), June 2004. 1997.
[6] Touch, J., "ANONsec: Anonymous Security to Defend Against [4] Better Than Nothing Security [BTNS] BOF, IETF-61, Wash. DC.,
Spoofing Attacks", (work in progress), July 2004. http://www.ietf.org/ietf/04nov/btns.txt
[7] Braden, B., "TIME-WAIT Assassination Hazards in TCP", RFC 1337, [5] Braden, B., "TIME-WAIT Assassination Hazards in TCP", RFC 1337,
May 1992. May 1992.
[8] Faber, T., Touch, J. and W. Yue, "The TIME-WAIT state in TCP [6] CERT alert: "Technical Cyber Security Alert TA04-111A:
Vulnerabilities in TCP --
http://www.us-cert.gov/cas/techalerts/TA04-111A.html", April 20
2004.
[7] Convery, S. and M. Franz, "BGP Vulnerability Testing:
Separating Fact from FUD", 2003,
http://www.nanog.org/mtg-0306/pdf/franz.pdf
[8] Dalal, M., (ed.), "Transmission Control Protocol security
considerations", draft-ietf-tcpm-tcpsecure-02 (work in
progress), Nov. 2004.
[9] Faber, T., J. Touch, and W. Yue, "The TIME-WAIT state in TCP
and Its Effect on Busy Servers", Proc. Infocom 1999 pp. 1573- and Its Effect on Busy Servers", Proc. Infocom 1999 pp. 1573-
1583, March 1999. 1583, March 1999.
[9] Floyd, S., "Inappropriate TCP Resets Considered Harmful", BCP [10] Ferguson, P. and D. Senie, Network Ingress Ingress Filtering:
Defeating Denial of Service Attacks which employ IP Address
Spoofing," RFC 2267 / BCP 38, May 2000.
[11] Floyd, S., "Inappropriate TCP Resets Considered Harmful", BCP
60, RFC 3360, August 2002. 60, RFC 3360, August 2002.
[10] Postel, J., "Transmission Control Protocol", STD 7, RFC 793, [12] Gill, V., J. Heasley, and D. Meyer, "The Generalized TTL
September 1981. Security Mechanism (GTSM)," RFC 3682 (Experimental), Feb. 2004.
[11] Jacobson, V., Braden, B. and D. Borman, "TCP Extensions for [13] Glenn, R. and S. Kent, "The NULL Encryption Algorithm and Its
Use With IPsec", RFC 2410 (Standards Track), November 1998.
[14] Harkins, D. and D. Carrel, "The Internet Key Exchange (IKE)",
RFC 2409 (Standards Track), November 1998.
[15] Heffernan, A., "Protection of BGP Sessions via the TCP MD5
Signature Option", RFC 2385 (Standards Track), August 1998.
[16] Jacobson, V., B. Braden, and D. Borman, "TCP Extensions for
High Performance", RFC 1323, May 1992. High Performance", RFC 1323, May 1992.
[12] Touch, J., "Report on MD5 Performance", RFC 1810, June 1995. [17] Kaufman, C., "Internet Key Exchange (IKEv2) Protocol",
draft-ietf-ipsec-ikev2-14 (work in progress), June 2004.
[13] Touch, J., "Performance Analysis of MD5", Proc. Sigcomm 1995 [18] Kent, S., "IP Authentication Header",
77-86., March 1999. draft-ietf-ipsec-rfc2402bis-07 (work in progress), March 2004.
[14] O'Malley, S. and L. Peterson, "TCP Extensions Considered [19] Kent, S. and R. Atkinson, "Security Architecture for the
Harmful", RFC 1263, October 1991. Internet Protocol", RFC 2401 (Standards Track), November 1998.
[15] "IETF TCPM Working Group and mailing list - [20] Kent, S. and R. Atkinson, "IP Authentication Header", RFC 2402
http://www.ietf.org/html.charters/tcpm-charter.html". (Standards Track), November 1998.
[16] Stewart, R., Xie, Q., Morneault, K., Sharp, C., Schwarzbauer, [21] Kent, S. and R. Atkinson, "IP Encapsulating Security Payload
H., Taylor, T., Rytina, I., Kalla, M., Zhang, L. and V. Paxson, (ESP)", RFC 2406 (Standards Track), November 1998.
"Stream Control Transmission Protocol", RFC 2960, October 2000.
[17] Bernstein, D., "SYN cookies - http://cr.yp.to/syncookies.html", [22] Kent, S. and K. Seo, "Security Architecture for the Internet
1997. Protocol", draft-ietf-ipsec-rfc2401bis-06 (work in progress),
April 2005.
[18] Kent, S. and R. Atkinson, "Security Architecture for the [23] Kohler, E., M. Handley, and S. Floyd, "Datagram Congestion
Internet Protocol", RFC 2401, November 1998. Control Protocol (DCCP)", draft-ietf-dccp-spec-11 (work in
progress), March 2005.
[19] Kent, S. and R. Atkinson, "IP Authentication Header", RFC 2402, [24] Leech, M., "Key Management Considerations for the TCP MD5
November 1998. Signature Option," RFC 3562 (Informational), July 2003.
[20] Kent, S. and R. Atkinson, "IP Encapsulating Security Payload [25] O'Malley, S. and L. Peterson, "TCP Extensions Considered
(ESP)", RFC 2406, November 1998. Harmful", RFC 1263, October 1991.
[21] Harkins, D. and D. Carrel, "The Internet Key Exchange (IKE)", [26] Perkins, C., "IP Encapsulation within IP," RFC 2003 (Standards
RFC 2409, November 1998. Track), Oct. 1996.
[22] Glenn, R. and S. Kent, "The NULL Encryption Algorithm and Its [27] Poon, K., "Use of TCP timestamp option to defend against blind
Use With IPsec", RFC 2410, November 1998. spoofing attack," draft-poon-tcp-tstamp-mod-01 (work in
progress), Oct. 2004.
[23] Maughan, D., Schneider, M. and M. Schertler, "Internet Security [28] Postel, J., "Transmission Control Protocol," RFC 793 / STD 7,
Association and Key Management Protocol (ISAKMP)", RFC 2408, September 1981.
November 1998.
8.2. Informative References [29] Rekhter, Y. and T. Li, (eds.), "A Border Gateway Protocol 4
(BGP-4)," RFC 1771 (Standards Track), March 1995.
[24] Stewart, R., "Transmission Control Protocol security [30] Stewart, R., Q. Xie, K. Morneault, C. Sharp, H. Schwarzbauer,
considerations", draft-ietf-tcpm-tcpsecure-01 (work in T. Taylor, I. Rytina, M. Kalla, L. Zhang, and V. Paxson,
progress), June 2004. "Stream Control Transmission Protocol," RFC 2960 (Standards
Track), October 2000.
[25] Kohler, E., "Datagram Congestion Control Protocol (DCCP)", [31] TCPM: IETF TCPM Working Group and mailing list-
draft-ietf-dccp-spec-06 (work in progress), February 2004. http://www.ietf.org/html.charters/tcpm-charter.html.
[26] Kent, S. and K. Seo, "Security Architecture for the Internet [32] Touch, J., "Report on MD5 Performance," RFC 1810
Protocol", draft-ietf-ipsec-rfc2401bis-02 (work in progress), (Informational), June 1995.
April 2004.
[27] Kent, S., "IP Authentication Header", draft-ietf-ipsec- [33] Touch, J., "Performance Analysis of MD5," Proc. Sigcomm 1995
rfc2402bis-07 (work in progress), March 2004. 77-86., March 1999.
[28] Kaufman, C., "Internet Key Exchange (IKEv2) Protocol", draft- [34] Touch, J., "ANONsec: Anonymous Security to Defend Against
ietf-ipsec-ikev2-14 (work in progress), June 2004. Spoofing Attacks," draft-touch-anonsec-00 (work in progress),
May 2004.
[35] Watson, P., "Slipping in the Window: TCP Reset attacks,"
Presentation at 2004 CanSecWest.
http://www.cansecwest.com/archives.html
Author's Addresses Author's Addresses
Joe Touch Joe Touch
USC/ISI USC/ISI
4676 Admiralty Way 4676 Admiralty Way
Marina del Rey, CA 90292-6695 Marina del Rey, CA 90292-6695
U.S.A. U.S.A.
Phone: +1 (310) 448-9151 Phone: +1 (310) 448-9151
skipping to change at page 19, line 27 skipping to change at page 21, line 40
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such proprietary rights by implementers or users of this such proprietary rights by implementers or users of this
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