draft-ietf-tcpm-tcp-antispoof-04.txt   draft-ietf-tcpm-tcp-antispoof-05.txt 
IETF TCPM WG J. Touch IETF TCPM WG J. Touch
Internet Draft USC/ISI Internet Draft USC/ISI
Expires: November 2006 May 15, 2006 Expires: April 2007 October 22, 2006
Defending TCP Against Spoofing Attacks Defending TCP Against Spoofing Attacks
draft-ietf-tcpm-tcp-antispoof-04.txt draft-ietf-tcpm-tcp-antispoof-05.txt
Status of this Memo Status of this Memo
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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), sent with forged IP source addresses (spoofing). TCP resets (RSTs), sent with forged IP source addresses (spoofing). TCP
has always been susceptible to such RST spoofing attacks, which were has always been susceptible to such RST spoofing attacks, which were
indirectly protected by checking that the RST sequence number was indirectly protected by checking that the RST sequence number was
inside the current receive window, as well as via the obfuscation of inside the current receive window, as well as via the obfuscation of
TCP endpoint and port numbers. For pairs of well-known endpoints TCP endpoint and port numbers. For pairs of well-known endpoints
often over predictable port pairs, such as BGP or between web servers often over predictable port pairs, such as BGP or between web servers
and well-known large-scale caches, increases in the path bandwidth- and well-known large-scale caches, increases in the path bandwidth-
delay product of a connection have sufficiently increased the receive delay product of a connection have sufficiently increased the receive
window space that off-path third parties can brute-force generate a window space that off-path third parties can brute-force generate a
viable RST sequence number. The susceptibility to attack increases viable RST sequence number. The susceptibility to attack increases
as the square of the bandwidth, thus presents a significant with the square of the bandwidth, thus presents a significant
vulnerability for recent high-speed networks. This document vulnerability for recent high-speed networks. This document
addresses this vulnerability, discussing proposed solutions at the addresses this vulnerability, discussing proposed solutions at the
transport level and their inherent challenges, as well as existing transport level and their inherent challenges, as well as existing
network level solutions and the feasibility of their deployment. network level solutions and the feasibility of their deployment.
This document focuses on vulnerabilities due to spoofed TCP segments, This document focuses on vulnerabilities due to spoofed TCP segments,
and includes a discussion of related ICMP spoofing attacks on TCP and includes a discussion of related ICMP spoofing attacks on TCP
connections. connections.
Table of Contents Table of Contents
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3.1. Transport Layer Solutions................................10 3.1. Transport Layer Solutions................................10
3.1.1. TCP MD5 Authentication..............................11 3.1.1. TCP MD5 Authentication..............................11
3.1.2. TCP RST Window Attenuation..........................11 3.1.2. TCP RST Window Attenuation..........................11
3.1.3. TCP Timestamp Authentication........................12 3.1.3. TCP Timestamp Authentication........................12
3.1.4. Other TCP Cookies...................................13 3.1.4. Other TCP Cookies...................................13
3.1.5. Other TCP Considerations............................13 3.1.5. Other TCP Considerations............................13
3.1.6. Other Transport Protocol Solutions..................14 3.1.6. Other Transport Protocol Solutions..................14
3.2. Network Layer (IP) Solutions.............................14 3.2. Network Layer (IP) Solutions.............................14
3.2.1. Address filtering...................................15 3.2.1. Address filtering...................................15
3.2.2. IPsec...............................................16 3.2.2. IPsec...............................................16
4. ICMP..........................................................16 4. ICMP..........................................................17
5. Issues........................................................17 5. Issues........................................................18
5.1. Transport Layer (e.g., TCP)..............................18 5.1. Transport Layer (e.g., TCP)..............................18
5.2. Network Layer (IP).......................................19 5.2. Network Layer (IP).......................................19
5.3. Application Layer........................................20 5.3. Application Layer........................................21
5.4. Link Layer...............................................21 5.4. Link Layer...............................................21
5.5. Issues Discussion........................................21 5.5. Issues Discussion........................................22
6. Security Considerations.......................................22 6. Security Considerations.......................................22
7. IANA Considerations...........................................22 7. IANA Considerations...........................................23
8. Conclusions...................................................22 8. Conclusions...................................................23
9. Acknowledgments...............................................23 9. Acknowledgments...............................................23
10. References...................................................23 10. References...................................................23
10.1. Normative References....................................23 10.1. Normative References....................................23
10.2. Informative References..................................23 10.2. Informative References..................................24
Author's Addresses...............................................27 Author's Addresses...............................................27
Intellectual Property Statement..................................27 Intellectual Property Statement..................................27
Disclaimer of Validity...........................................27 Disclaimer of Validity...........................................28
Copyright Statement..............................................28 Copyright Statement..............................................28
Acknowledgment...................................................28 Acknowledgment...................................................28
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 based on RST spoofing from off- between core routers using an attack based on RST spoofing from off-
path attackers [9][10][43]. This attack has been known for nearly path attackers [9][10][45]. This attack has been known for nearly
six years [19]. Such connections, typically using TCP, can be six years [20]. Such connections, typically using TCP, can be
susceptible to off-path third-party reset (RST) segments with forged susceptible to off-path third-party reset (RST) segments with forged
source addresses (spoofed), which terminate the TCP connection. BGP source addresses (spoofed), which terminate the TCP connection. BGP
routers react to a terminated TCP connection in various ways which routers react to a terminated TCP connection in various ways which
can amplify the impact of an attack, ranging from restarting the can amplify the impact of an attack, ranging from restarting the
connection to deciding that the other router is unreachable and thus connection to deciding that the other router is unreachable and thus
flushing the BGP routes [33]. This sort of attack affects other flushing the BGP routes [34]. This sort of attack affects other
protocols besides BGP, involving any long-lived connection between protocols besides BGP, involving any long-lived connection between
well-known endpoints. The impact on Internet infrastructure can be well-known endpoints. The impact on Internet infrastructure can be
substantial (esp. for the BGP case), and warrants immediate substantial (esp. for the BGP case), and warrants immediate
attention. 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 spoofing attacks. Such attacks rely on the increase of third-party spoofing attacks. Such attacks rely on the increase of
commodity platforms supporting public access to previously privileged commodity platforms supporting public access to previously privileged
resources, such as system-level (i.e., root) access. Given such resources, such as system-level (i.e., root) access. Given such
access, it is trivial for anyone to generate a packet with any header access, it is trivial for anyone to generate a packet with any header
desired. desired.
This, coupled with the lack of sufficient address filtering to drop This, coupled with the lack of sufficient address 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 [9][10][43]. Proposed solutions include the party spoofing attacks [9][10][45]. Proposed solutions include the
deployment of existing Internet network and transport security as deployment of existing Internet network and transport security as
well as modifications to transport protocols that reduce its well as modifications to transport protocols that reduce its
vulnerability to generated attacks [13][15][19][36][42]. vulnerability to generated attacks [13][15][20][38][44].
One way to defeat spoofing is to validate the segments of a One way to defeat spoofing is to validate the segments of a
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 level, and IPsec provides authentication at the network level
[18][19][22][25]. In both cases their deployment overhead may be [19][20][23][26]. In both cases their deployment overhead may be
prohibitive, e.g., it may not feasible for public services, such as prohibitive, e.g., it may not feasible for public services, such as
web servers, to be configured with the appropriate certificate web servers, to be configured with the appropriate certificate
authorities of large numbers of peers (for IPsec using IKE), or authorities of large numbers of peers (for IPsec using IKE), or
shared secrets (for IPsec in shared-secret mode, or TCP/MD5), because shared secrets (for IPsec in shared-secret mode, or TCP/MD5), because
many clients may need to be configured rapidly without external many clients may need to be configured rapidly without external
assistance. Services from public web servers connecting to large- assistance. Services from public web servers connecting to large-
scale caches to BGP with larger numbers of peers can fall into this scale caches to BGP with larger numbers of peers can fall into this
category. category.
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
[36], modifications to TCP's timestamp processing [31], and [38], modifications to TCP's timestamp processing [32], and
modifications to IPsec and TCP/MD5 keying [41]. This document modifications to IPsec and TCP/MD5 keying [43]. This document
focuses on spoofing of TCP segments, although a discussion of related focuses on spoofing of TCP segments, although a discussion of related
spoofing of ICMP packets based on spoofed TCP contents is also spoofing of ICMP packets based on spoofed TCP contents is also
discussed. discussed.
Note that the description of these attacks is not new; attacks using 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 RSTs on BGP have been known since 1998, and were the reason for the
development of TCP/MD5 [19]. The recent attack scenario was first development of TCP/MD5 [20]. The recent attack scenario was first
documented by Convery at a NANOG meeting in 2003, but that analysis documented by Convery at a NANOG meeting in 2003, but that analysis
assumed the entire sequence space (2^32 packets) needed to be covered assumed the entire sequence space (2^32 packets) needed to be covered
for an attack to succeed [10]. Watson's more detailed analysis for an attack to succeed [10]. Watson's more detailed analysis
discovered that a single packet anywhere in the current window could discovered that a single packet anywhere in the current window could
succeed at an attack [43]. This document adds the observation that succeed at an attack [45]. This document adds the observation that
susceptibility to attack goes as the square of bandwidth, due to the susceptibility to attack goes as the square of bandwidth, due to the
coupling between the linear increase in receive window size and coupling between the linear increase in receive window size and
linear increase in rate an attacker, as well as comparing the variety linear increase in rate a potential attack, as well as comparing the
of more recent proposals, including modifications to TCP, use of variety of more recent proposals, including modifications to TCP, use
IPsec, and use of TCP/MD5 to resist such attacks. 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
[9][10][43]. A variety of such attacks have been known for several [9][10][45]. A variety of such attacks have been known for several
years, including sending RSTs, SYNs, and even ACKs in an attempt to years, including sending RSTs, SYNs, and even ACKs in an attempt to
affect an existing connection or to load down servers. These attacks affect an existing connection or to load down servers. These attacks
often combine external knowledge (e.g., to indicate the IP addresses often combine external knowledge (e.g., to indicate the IP addresses
to attack, the destination port number, and sometimes the ISN) with to attack, the destination port number, and sometimes the ISN) with
brute-force capabilities enabled by modern computers and network brute-force capabilities enabled by modern computers and network
bandwidths (e.g., to scan all source ports or an entire window bandwidths (e.g., to scan all source ports or an entire window
space). Overall, such attacks are countered by the use of some form space). Overall, such attacks are countered by the use of some form
of authentication at the network (e.g., IPsec), transport (e.g., SYN of authentication at the network (e.g., IPsec), transport (e.g., SYN
cookies, TCP/MD5), or other layers. TCP already includes a weak form cookies, TCP/MD5), or other layers. TCP already includes a weak form
of such authentication in its check of segment sequence numbers of such authentication in its check of segment sequence numbers
against the current receiver window. Increases in the bandwidth- against the current receiver window. Increases in the bandwidth-
delay product for certain long connections have sufficiently weakened delay product for certain long connections have sufficiently weakened
this type of weak authentication to make reliance on it inadvisable. this type of weak authentication to make reliance on it inadvisable.
2.1. Review of TCP Windows 2.1. Review of TCP Windows
Before proceeding, it is useful to review the terminology and Before proceeding, it is useful to review the terminology and
components of TCP's windowing algorithm. TCP connections have three components of TCP's windowing algorithm. TCP connections have three
kinds of windows [1][32]: kinds of windows [1][33]:
o Send window (SND.WND): the latest send window size. o Send window (SND.WND): the latest send window size.
o Receive window (RCV.WND): the latest advertised receive window o Receive window (RCV.WND): the latest advertised receive window
size. size.
o Congestion window (CWND): the window determined by congestion o Congestion window (CWND): the window determined by congestion
feedback that limits how much of RCV.WND can be in-flight in a feedback that limits how much of RCV.WND can be in-flight in a
round trip time. round trip time.
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SND.WND determines how much data the sender is willing to store on SND.WND determines how much data the sender is willing to store on
its side for possible retransmission due to loss, and RCV.WND its side for possible retransmission due to loss, and RCV.WND
determines the ability of the receiver to accommodate that loss and determines the ability of the receiver to accommodate that loss and
reorder received packets. CWND never grows beyond RCV.WND. reorder received packets. CWND never grows beyond RCV.WND.
High bandwidth-delay product networks need CWND to be sufficiently High bandwidth-delay product networks need CWND to be sufficiently
large to accommodate as much data would be in transit in a round trip large to accommodate as much data would be in transit in a round trip
time, otherwise their performance will suffer. As a result, it is time, otherwise their performance will suffer. As a result, it is
recommended that users and various automatic programs increase recommended that users and various automatic programs increase
RCV.WND to at least the size of bandwidth*delay (the bandwidth-delay RCV.WND to at least the size of bandwidth*delay (the bandwidth-delay
product) [21][34]. product) [22][35].
As the bandwidth-delay product of the network increases, however, As the bandwidth-delay product of the network increases, however,
such increases in the advertised receive window can cause increased such increases in the advertised receive window can cause increased
susceptibility to spoofing attacks, as the remainder of this document susceptibility to spoofing attacks, as the remainder of this document
shows. This assumes, however, that the receive window size (e.g., shows. This assumes, however, that the receive window size (e.g.,
via increased receive socket buffer configuration) is increased with via increased receive socket buffer configuration) is increased with
the increased bandwidth-delay product; if not, then connection the increased bandwidth-delay product; if not, then connection
performance will degrade, but susceptibility to spoofing attacks will performance will degrade, but susceptibility to spoofing attacks will
increase only linearly (with the rate at which the attacker can send increase only linearly (with the rate at which the attacker can send
spoofed packets), not as the square of the bandwidth. Note that spoofed packets), not as the square of the bandwidth. Note that
either increase depends on the receive window itself, and is either increase depends on the receive window itself, and is
independent of the congestion state or amount of data transmitted. independent of the congestion state or amount of data transmitted.
2.2. Recent BGP Attacks Using TCP RSTs 2.2. Recent BGP Attacks Using TCP RSTs
BGP represents a particular vulnerability to spoofing attacks because BGP represents a particular vulnerability to spoofing attacks because
it uses TCP connectivity to infer routability, so losing a TCP it uses TCP connectivity to infer routability, so losing a TCP
connection with a BGP peer can result in the flushing of routes to connection with a BGP peer can result in the flushing of routes to
that peer [33]. that peer [34].
Until six years ago, such connections were assumed difficult to Until six years ago, such connections were assumed difficult to
attack because they were described by a few comparatively obscure attack because they were described by a few comparatively obscure
parameters [19]. Most TCP connections are protected by multiple parameters [20]. Most TCP connections are protected by multiple
levels of obfuscation except at the endpoints of the connection: 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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2.3. TCP RST Vulnerability 2.3. 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 [4][11]. ACK affecting the server's ability to open new connections [4][11]. ACK
spoofing can cause connections to transmit too much data too quickly, spoofing can cause connections to transmit too much data too quickly,
creating network congestion and segment loss, causing connections to creating network congestion and segment loss, causing connections to
slow to a crawl. In the most recent attacks on BGP, RSTs cause slow to a crawl. In the most recent attacks on BGP, RSTs cause
connections to be dropped. As noted earlier, some BGP connections to be dropped. As noted earlier, some BGP
implementations interpret TCP connection termination, or a series of implementations interpret TCP connection termination, or a series of
such failures, as a network failure [33]. This causes routers to such failures, as a network failure [34]. This causes routers to
drop the BGP routing information already exchanged, in addition to drop the BGP routing information already exchanged, in addition to
inhibiting their ongoing exchanges, thus amplifying the impact of the inhibiting their ongoing exchanges, thus amplifying the impact of the
attack. The result can affect routing paths throughout the Internet. 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
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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
[21][32]. The valid advertised receive window is a fraction, not to [22][33]. The valid advertised receive window is a fraction, not to
exceed approximately half, of this space, or ~2 billion (2 * 10^9, exceed approximately half, of this space, or ~2 billion (2 * 10^9,
i.e., 2E9 or 2 U.S. billion). Under typical configurations, the i.e., 2E9 or 2 U.S. billion). Under typical configurations, the
majority of TCP connections open to a very small fraction of this majority of TCP connections open to a very small fraction of this
space, e.g., 10,000-60,000(approximately 5-100 segments). This is space, e.g., 10,000-60,000(approximately 5-100 segments). This is
because the advertised receive window typically matches the receive because the advertised receive window typically matches the receive
socket buffer size. It is recommended that this buffer be tuned to socket buffer size. It is recommended that this buffer be tuned to
match the needs of the connection, either manually or by automatic match the needs of the connection, either manually or by automatic
external means [34]. external means [35].
On a low-loss path, the advertised receive window should be On a low-loss path, the advertised receive window should be
configured to match the path bandwidth-delay product, including configured to match the path bandwidth-delay product, including
buffering delays (assume 1 packet/hop) [34]. Many paths in the buffering delays (assume 1 packet/hop) [35]. Many paths in the
Internet have end-to-end bandwidths of under 1 Mbps, latencies under Internet have end-to-end bandwidths of under 1 Mbps, latencies under
100ms, and are under 15 hops, resulting in fairly small advertised 100ms, and are under 15 hops, resulting in fairly small advertised
receive windows as above (under 35,000 bytes). Under these receive windows as above (under 35,000 bytes). Under these
conditions, and further assuming that the initial sequence number is conditions, and further assuming that the initial sequence number is
suitably (pseudo-randomly) chosen, a valid guessed sequence number suitably (pseudo-randomly) chosen, a valid guessed sequence number
would have odds of 1 in 57,000 of falling within the advertised would have odds of 1 in 57,000 of falling within the advertised
receive window. Put differently, a blind (i.e., off-path) attacker receive window. Put differently, a blind (i.e., off-path) attacker
would need to send 57,000 RSTs with suitably spaced sequence number would need to send 57,000 RSTs with suitably spaced sequence number
guesses to successfully reset a connection. At 1 Mbps, 57,000 (40 guesses to successfully reset a connection. At 1 Mbps, 57,000 (40
byte) RSTs would take over 50 minutes to transmit, and, as noted byte) RSTs would take over 50 minutes to transmit, and, as noted
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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 was discussed in Sec. 2. Figure 1 summarized receive socket buffers was discussed in Sec. 2. Figure 1 summarized
the time to an successful attack based on large advertised receive the time to an successful attack based on large advertised receive
windows, but many current commercial routers have limits of 128KB for windows, but many current commercial routers have limits of 128KB for
large devices, 32KB for medium, and as little as 4KB for modest ones. large devices, 32KB for medium, and as little as 4KB for modest ones.
Figure 2 shows the time and bandwidths needed to accomplish an attack Figure 2 shows the time and bandwidths needed to accomplish an attack
BGP sessions in the time shown for 100ms latencies; for even short- BGP sessions in the time shown for 100ms latencies; for even short-
range network latencies (10ms), these sessions can be still be range network latencies (10ms), these sessions can be still be
attacked over short timescales (minutes to hours). attacked over short timescales (minutes to hours).
BW needed BW*delay RSTs needed Time needed BW BW*delay RSTs needed Time needed
------------------------------------------------------------ ------------------------------------------------------------
10 Mbps 0.128 MB 33,555 1 second 10 Mbps 0.128 MB 33,555 1 second
3 Mbps 0.032 MB 134,218 40 seconds 3 Mbps 0.032 MB 134,218 40 seconds
300 Kbps 0.004 MB 1,073,742 1 hour 300 Kbps 0.004 MB 1,073,742 1 hour
Figure 2 Time needed to kill a connection with limited buffers Figure 2 Time needed to kill a connection with limited buffers
The issue of the attack bandwidth is considered reasonable as The issue of the attack bandwidth is considered reasonable as
follows: follows:
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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 and on- authentication of segments, protecting against both off-path and on-
path third-party spoofing attacks. Other transport protocols, such path third-party spoofing attacks. Other transport protocols, such
as SCTP and DCCP, also have limited antispoofing mechanisms. as SCTP and DCCP, also have limited antispoofing mechanisms.
3.1.1. TCP MD5 Authentication 3.1.1. TCP MD5 Authentication
An extension to TCP supporting MD5 authentication was developed in An extension to TCP supporting MD5 authentication was developed in
1998 specifically to authenticate BGP connections (although it can be 1998 specifically to authenticate BGP connections (although it can be
used for any TCP connection) [19]. The extension relies on a pre- used for any TCP connection) [20]. The extension relies on a pre-
shared secret key to authenticate the entire TCP segment, including shared secret key to authenticate the entire TCP segment, including
the data, TCP header, and TCP pseudo-header (certain fields of the IP the data, TCP header, and TCP pseudo-header (certain fields of the IP
header). All segments are protected, including RSTs, to be accepted header). All segments are protected, including RSTs, to be accepted
only when their signature matches. This option, although widely only when their signature matches. This option, although widely
deployed in Internet routers, is considered undeployable for deployed in Internet routers, is considered undeployable for
widespread use because the need for pre-shared keys [3][27]. It widespread use because the need for pre-shared keys [3][28]. It
further is considered computationally expensive for either hosts or further is considered computationally expensive for either hosts or
routers due to the overhead of MD5 [39][40]. routers due to the overhead of MD5 [41][42].
There are also concerns about the use of MD5 due to recent collision- There are also concerns about the use of MD5 due to recent collision-
based attacks [20]. Similar concerns exist for SHA-1, and the IETF based attacks [21]. Similar concerns exist for SHA-1, and the IETF
is currently evaluating how these attacks impact the recommendation is currently evaluating how these attacks impact the recommendation
for using these hashes, both in TCP/MD5 and in the IPsec suite. For for using these hashes, both in TCP/MD5 and in the IPsec suite. For
the purposes of this discussion, the particular algorithm used in the purposes of this discussion, the particular algorithm used in
either protocol suite is not the focus, and there is ongoing work to either protocol suite is not the focus, and there is ongoing work to
allow TCP/MD5 to evolve to a more general TCP security option [6]. allow TCP/MD5 to evolve to a more general TCP security option [6].
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 [36]. This restores TCP's match the expected next sequence number [38]. 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 brute-force guess the sequence number send 2^32 different packets to brute-force guess the sequence number
(worst case, average would be half that); this makes TCP's (worst case, average would be half that); this makes TCP's
vulnerability to attack independent of the size of the receive window vulnerability to attack independent of the size of the receive window
(RCV.WND). The extension further modifies the RST receiver to react (RCV.WND). The extension further modifies the RST receiver to react
to incorrectly-numbered RSTs, by sending a zero-length ACK. If the to incorrectly-numbered RSTs, by sending a zero-length ACK. If the
RST source is legitimate, upon receipt of an ACK the closed source RST source is legitimate, upon receipt of an ACK the closed source
would presumably emit a RST with the sequence number matching the would presumably emit a RST with the sequence number matching the
ACK, correctly resetting the intended recipient. This modification ACK, correctly resetting the intended recipient. This modification
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recommendation - although this can be omitted, allowing timeouts to recommendation - although this can be omitted, allowing timeouts to
suffice. The advantage to this proposal is that it can be deployed suffice. The advantage to this proposal is that it can be deployed
incrementally and has benefit to the endpoint on which it is incrementally and has benefit to the endpoint on which it is
deployed. The other advantage to this proposal is that the window deployed. The other advantage to this proposal is that the window
attenuation described here makes the vulnerability to spoofed RST attenuation described here makes the vulnerability to spoofed RST
packets independent of the size of the receive window. packets independent of the size of the receive window.
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 [38][44]. This the value negotiated on connection establishment [40][46]. This
proposal has the advantage of using an explicitly negotiated value, proposal has the advantage of using an explicitly negotiated value,
but at the cost of changing the behavior of an unmodified endpoint to but at the cost of changing the behavior of an unmodified endpoint to
a currently valid RST. It would thus be more difficult, without a currently valid RST. It would thus be more difficult, without
additional mechanism, to deploy incrementally. additional mechanism, to deploy incrementally.
Another variant of this proposal involves increasing TCP's window Another variant of this proposal involves increasing TCP's window
space, rather than decreasing the valid range for RSTs, i.e., space, rather than decreasing the valid range for RSTs, i.e.,
increasing the sequence space from 32 bits to 64 bits. This has the increasing the sequence space from 32 bits to 64 bits. This has the
equivalent effect - the ratio of the valid sequence numbers for any equivalent effect - the ratio of the valid sequence numbers for any
segment to the overall sequence number space is significantly segment to the overall sequence number space is significantly
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A converse variant of increasing TCP's window space is to decrease A converse variant of increasing TCP's window space is to decrease
the receive window (RCV.WND) explicitly, which would further reduce the receive window (RCV.WND) explicitly, which would further reduce
the effectiveness of spoofed RSTs with random sequence numbers. This the effectiveness of spoofed RSTs with random sequence numbers. This
alternative may reduce the throughput of the connection, if the alternative may reduce the throughput of the connection, if the
advertised receive window is smaller than the bandwidth-delay product advertised receive window is smaller than the bandwidth-delay product
of the connection. of the connection.
3.1.3. TCP Timestamp Authentication 3.1.3. TCP Timestamp Authentication
Another way to authenticate TCP segments is via its timestamp option, Another way to authenticate TCP segments is via its timestamp option,
using the value as a sort of authentication [31]. This requires that using the value as a sort of authentication [32]. This requires that
the receiver TCP discard segments whose timestamp is outside the the receiver TCP discard segments whose timestamp is outside the
accepted window, which is derived from the timestamps of other accepted window, which is derived from the timestamps of other
packets from the same connection. This technique uses an existing packets from the same connection. This technique uses an existing
TCP option, but also requires modified TCP control processing (with TCP option, but also requires modified TCP control processing (with
the same caveats) and may be difficult to deploy incrementally the same caveats) and may be difficult to deploy incrementally
without further modifications. Additionally, the timestamp value may without further modifications. Additionally, the timestamp value may
be easier to guess because it can be derived predictably, either be easier to guess because it can be derived predictably, either
assuming it represents actual time at the host, or by probing the assuming it represents actual time at the host, or by probing the
host using unrelated benign traffic. host using unrelated benign traffic.
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reasonably correlation to local time. These variants of cookies are reasonably correlation to local time. These variants of cookies are
similar in spirit to TCP SYN cookies, again patching a vulnerability similar in spirit to TCP SYN cookies, again patching a vulnerability
to off-path third-party spoofing attacks based on a (fairly weak, to off-path third-party spoofing attacks based on a (fairly weak,
excepting MD5) form of authentication. Another form of cookie is the excepting MD5) form of authentication. Another form of cookie is the
source port itself, which can be randomized but provides only 16 bits source port itself, which can be randomized but provides only 16 bits
of protection (65,000 combinations), which may be exhaustively of protection (65,000 combinations), which may be exhaustively
attacked. This can be combined with destination port randomization attacked. This can be combined with destination port randomization
as well, but that would require a separate coordination mechanism (so as well, but that would require a separate coordination mechanism (so
both parties know which ports to use), which is equivalent to (and as both parties know which ports to use), which is equivalent to (and as
infeasible for large-scale deployments as) exchanging a shared secret infeasible for large-scale deployments as) exchanging a shared secret
[35]. [36].
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
advertised receive window is opened to the maximum extent suggested advertised receive window is opened to the maximum extent suggested
by the bandwidth-delay product of the end-to-end path, and that the by the bandwidth-delay product of the end-to-end path, and that the
window is opened to an appreciable fraction of the overall sequence window is opened to an appreciable fraction of the overall sequence
number space. As noted earlier, for most common cases, connections number space. As noted earlier, for most common cases, connections
are too brief or over bandwidths too low for such a large window to are too brief or over bandwidths too low for such a large window to
be useful. Expanding TCP's sequence number space is a direct way to be useful. Expanding TCP's sequence number space is a direct way to
further avoid such vulnerability, even for long connections over further avoid such vulnerability, even for long connections over
emerging bandwidths. If either manual tuning or automatic tuning of emerging bandwidths. If either manual tuning or automatic tuning of
the advertised receive window (via receive buffer tuning) is not the advertised receive window (via receive buffer tuning) is not
provided, this is not an issue (although connection performance will provided, this is not an issue (although connection performance will
suffer) [34]. suffer) [35].
It is may be sufficient for the endpoint to limit the advertised It is may be sufficient for the endpoint to limit the advertised
receive window by deliberately leaving it small. If the receive receive window by deliberately leaving it small. If the receive
socket buffer is limited, e.g., to the ubiquitous default of 64KB, socket buffer is limited, e.g., to the ubiquitous default of 64KB,
the advertised receive window will not be as vulnerable even for very the advertised receive window will not be as vulnerable even for very
long connections over very high bandwidths. The vulnerability will long connections over very high bandwidths. The vulnerability will
grow linearly with the increased network speed, but not as the grow linearly with the increased network speed, but not as the
square. The consequence is lower sustained throughput, where only square. The consequence is lower sustained throughput, where only
one window's worth of data per round trip time (RTT) is exchanged. one window's worth of data per round trip time (RTT) is exchanged.
This will keep the connection open longer; for long-lived connections This will keep the connection open longer; for long-lived connections
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exchanged quickly enough with bandwidth reduced due to the smaller exchanged quickly enough with bandwidth reduced due to the smaller
buffers, or perhaps that the advertised receive window is opened only buffers, or perhaps that the advertised receive window is opened only
during a large burst exchange (e.g., via some other signal between during a large burst exchange (e.g., via some other signal between
the two routers). the two routers).
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 use them to authenticate a variety of other control establishment and use them to authenticate a variety of other control
messages [26][37]. The inclusion of such mechanism at the transport messages [27][39]. The inclusion of such mechanism at the transport
protocol, although emerging as standard practice, complicates the protocol, although emerging as standard practice, complicates the
design and implementation of new protocols [29]. As new attacks are design and implementation of new protocols [30]. As new attacks are
discovered (SYN floods, RSTs, etc.), each protocol must be modified discovered (SYN floods, RSTs, etc.), each protocol must be modified
individually to compensate. A network solution may be more individually to compensate. A network solution may be more
appropriate and efficient. appropriate and efficient.
It should be noted that RST attacks which rely on brute-force are It should be noted that RST attacks which rely on brute-force are
relatively easy for intrusion detection software to detect at the TCP relatively easy for intrusion detection software to detect at the TCP
layer. Any connection that receives a large number of invalid - layer. Any connection that receives a large number of invalid -
outside-window - RSTs might have subsequent RSTs blocked, to defeat outside-window - RSTs might have subsequent RSTs blocked, to defeat
such attacks. This would have the side-effect of blocking legitimate such attacks. This would have the side-effect of blocking legitimate
RSTs to that connection, which might then interfere with cleaning up RSTs to that connection, which might then interfere with cleaning up
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of the packet source. IPsec requires cooperation between the of the packet source. IPsec requires cooperation between the
endpoints wanting to avoid attack on their connection, which endpoints wanting to avoid attack on their connection, which
currently involves pre-existing shared knowledge of either a shared currently involves pre-existing shared knowledge of either a shared
key or shared certificate authority. key or shared certificate authority.
3.2.1. Address filtering 3.2.1. Address filtering
Address filtering is often proposed as an alternative to protocol Address filtering is often proposed as an alternative to protocol
mechanisms to defeat IP source address spoofing [2][13]. Address mechanisms to defeat IP source address spoofing [2][13]. Address
filtering restricts traffic from downstream sources across transit filtering restricts traffic from downstream sources across transit
networks based on the IP source address. It can also restrict core- networks based on the IP source address. A kind of filtering already
to-edge paths to reject traffic that should have originated further occurs at the endpoints of a connection, because attack messages must
toward the edge. It cannot restrict traffic from edges lacking match the socket pair to succeed; again, note that such attacks
filtering through the core to a particular edge, i.e., from upstream require knowing the entire socket pair, and are unlikely except in
sources. As a result, each border router must perform the particular cases. This section discusses filtering based on address
appropriate filtering for overall protection to result; failure of only, typically done at the borders of an AS.
any border router to filter defeats the protection of all
participants inside the border, ultimately. Address filtering at the It can also restrict core-to-edge paths to reject traffic that should
border can protect those inside the border from some kinds of have originated further toward the edge. It cannot restrict traffic
spoofing, because only interior addresses should originate inside the from edges lacking filtering through the core to a particular edge.
border. It cannot, however, protect connections originating outside As a result, each border router must perform the appropriate
the border except to restrict where the traffic enters from, e.g., if filtering for overall protection to result; failure of any border
it expected from one AS and not another. router to filter defeats the protection of all participants inside
the border, and potentially those outside as well. Address filtering
at the border can protect those inside the border from some kinds of
spoofing, i.e., connections among those inside a border, because only
interior addresses should originate inside the border. It cannot,
however, protect connections including connections outside the border
except to restrict where the traffic enters from, e.g., if it
expected from one AS and not another.
As a result, address filtering is not a local solution that can be As a result, address filtering is not a local solution that can be
deployed to protect communicating pairs, but rather relies on a deployed to protect communicating pairs, but rather relies on a
distributed infrastructure of trusted gateways filtering forged distributed infrastructure of trusted gateways filtering forged
traffic where it enters the network. It is not feasible for local, traffic where it enters the network. It is not feasible for local,
incremental deployment, and relies too heavily on distributed incremental deployment, and relies heavily on distributed
cooperation. Although useful to reduce the load of spoofed traffic, cooperation. Although useful to reduce the load of spoofed traffic,
it is insufficient to protect particular connections from attack it is insufficient to protect particular connections from attack
[28]. [29].
A more recent variant of address filtering checks the IP TTL field, A more recent variant of address filtering checks the IP TTL field,
relying on the TTL set by the other end of the connection [15]. This relying on the TTL set by the other end of the connection [15]. This
technique has been used to provide filtering for BGP. It assumes the technique has been used to provide filtering for BGP. It assumes the
connection source TTL is set to 255; packets at the receiver are connection source TTL is set to 255; packets at the receiver are
checked for TTL=255, and others are dropped. This restricts traffic checked for TTL=255, and others are dropped. This restricts traffic
to one hop upstream of the receiver (i.e., a BGP router), but those to one hop upstream of the receiver (i.e., a BGP router), but those
hops could include other user programs at those nodes (e.g., the BGP hops could include other user programs at those nodes (e.g., the BGP
router's peer) or any traffic those nodes accept via tunnels - router's peer) or any traffic those nodes accept via tunnels -
because tunnels need not decrement TTLs [30] (see Sec. 5.1 of [15]). because tunnels need not decrement TTLs, notaby for "bump in the
This method works only where all traffic from the other end of the wire" (BITW) or BITW-equivalent scenarios [31] (see also Sec. 5.1 of
tunnel is trusted, i.e., where it does not originate or transit [15]). TTL filtering works only where all traffic from the other end
spoofed traffic. The use of TTL rather than link or network security of the tunnel is trusted, i.e., where it does not originate or
also assumes an untampered point-to-point link, where no other transit spoofed traffic. The use of TTL rather than link or network
traffic can be spoofed onto a link. security also assumes an untampered point-to-point link, where no
other traffic can be spoofed onto a link.
This method of filtering works best where traffic originates one hop This method of filtering works best where traffic originates one hop
away, so that the address filtering is based on the trust of only away, so that the address filtering is based on the trust of only
directly-connected (tunneled or otherwise) nodes. Like conventional directly-connected (tunneled or otherwise) nodes. Like conventional
address filtering, this reduces spoofing traffic in general, but is address filtering, this reduces spoofing traffic in general, but is
not considered a reliable security mechanism because it relies on not considered a reliable security mechanism because it relies on
distributed filtering (e.g., the fact that upstream nodes do not distributed filtering (e.g., the fact that upstream nodes do not
terminate tunnels arbitrarily). terminate tunnels arbitrarily).
3.2.2. IPsec 3.2.2. IPsec
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many end-system operating systems. More importantly, it relies on many end-system operating systems. More importantly, it relies on
preshared keys, signed X.509 certificates, or a third-party (e.g., preshared keys, signed X.509 certificates, or a third-party (e.g.,
Kerberos) public key infrastructure to establish and exchange keying Kerberos) public key infrastructure to establish and exchange keying
information (e.g., via IKE). These present challenges when using information (e.g., via IKE). These present challenges when using
IPsec to secure traffic to a well-known server, whose clients may not IPsec to secure traffic to a well-known server, whose clients may not
support IPsec or may not have registered with a previously-known support IPsec or may not have registered with a previously-known
certificate authority (CA). certificate authority (CA).
These keying challenges are being addressed in the IETF in ways that These keying challenges are being addressed in the IETF in ways that
will enable servers secure associations with other parties without will enable servers secure associations with other parties without
advance coordination [41][42]. This can be especially useful for advance coordination [43][44]. This can be especially useful for
publicly-available servers, or for protecting connections to servers publicly-available servers, or for protecting connections to servers
that - for whatever reason - have not, or will not deploy that - for whatever reason - have not, or will not deploy
conventional IPsec certificates (i.e., core Internet BGP routers). conventional IPsec certificates (i.e., core Internet BGP routers).
4. ICMP 4. ICMP
Just as spoofed TCP packets can terminate a connection, so too can Just as spoofed TCP packets can terminate a connection, so too can
spoofed ICMP packets. TCP headers can be included inside certain spoofed ICMP packets. ICMP can be used to launch a variety of
ICMP messages [7]. There have been recent suggestions to validate attacks on TCP including connection resets, path-MTU attacks, and can
the sequence number of TCP headers when they occur inside ICMP also be used to attack the host with non-TCP 'ping of death' and
messages [17]. This sequence checking is similar to checks that 'smurf attacks', etc. [37]. ICMP thus represents a substantial
would occur for conventional data packets in TCP, but is being threat to TCP, but this is not the focus of this document, although a
proposed in the spirit of the RST window attenuation described in number of protections are discussed below because some are comparable
Section 3.1.2. to TCP anti-spoofing techniques. Note also that ICMP attacks on TCP
assume that the socket pair is known by the attacker, which is
unlikely except for a subset of services between pairs of widely-
known endpoints.
TCP headers can be included inside certain ICMP messages [7]. There
have been recent suggestions to validate the sequence number of TCP
headers when they occur inside ICMP messages [17]. This sequence
checking is similar to checks that would occur for conventional data
packets in TCP, but is being proposed in the spirit of the RST window
attenuation described in Section 3.1.2.
Some such checks may be reasonable, especially where they parallel Some such checks may be reasonable, especially where they parallel
the validations already performed by TCP processing, notably where the validations already performed by TCP processing, notably where
they emulate the semantics of such processing. For example, the TCP they emulate the semantics of such processing. For example, the TCP
checksum should be validated (if the entire TCP segment is contained checksum should be validated (if the entire TCP segment is contained
in the ICMP message) before any fields of the TCP header are in the ICMP message) before any fields of the TCP header are
examined, to avoid reacting to corrupted packets. Similarly, if the examined, to avoid reacting to corrupted packets. Similarly, if the
TCP MD5 option is present, its signature should probably be validated TCP MD5 option is present, its signature should probably be validated
before considering the contents of the message. before considering the contents of the message. Such validation can
ensure that the packet was not corrupted prior to the ICMP generation
Such validation can ensure that the packet was not corrupted prior to (checksum), that the packet was one sent by the source (IPsec or
the ICMP generation (checksum), that the packet was one sent by the TCP/MD5 authenticated), or that the packet was not in the network for
source (IPsec or TCP/MD5 authenticated), or that the packet was not an excess of 2*MSL (valid sequence number).
in the network for an excess of 2*MSL (valid sequence number).
ICMP presents a particular challenge because some messages can reset ICMP presents a particular challenge because some messages can reset
a connection more easily - with less validation - than even some a connection more easily - with less validation - than even some
spoofed TCP segments. However, fixing such messages to be 'in spoofed TCP segments. One other proposed alternative is to change
window' is insufficient protection, as this document shows for TCP's reaction to ICMPs after a connection is established; that may
spoofed data. ICMP packets can be authenticated when originating at leave TCP susceptible during connection establishment and modifies
known, trusted endpoints, such as endpoints of connections or routers TCP's reaction to certain valid network events [18]. This considers
in known domains with pre-existing IPsec associations. Unfortunately, the context-sensitivity of ICMP messages, as does IPsec in some
they also can originate at other places in the network. As a result, tunneled configurations, but the recommendations are ambiguous
many networks filter all ICMP packets because validation may not be regarding such filtering [26].
Ultimately, requiring TCP ICMP messages to be 'in window' may be
insufficient protection, as this document shows for spoofed data.
ICMP packets can be authenticated when originating at known, trusted
endpoints, such as endpoints of connections or routers in known
domains with pre-existing IPsec associations. Unfortunately, they
also can originate at other places in the network. In addition, some
networks filter all ICMP packets because validation may not be
possible, especially because they can be injected from anywhere in a possible, especially because they can be injected from anywhere in a
network, and so cannot be selectively address filtered. As a result, network, and so cannot be easily and locally address filtered [26].
they are not addressed separately in the issues or security As a result, they are not addressed separately in the issues or
considerations of this document further. security considerations of this document further.
5. Issues 5. Issues
There are a number of existing and proposed solutions addressing the There are a number of existing and proposed solutions addressing the
vulnerability of transport protocols in general (and TCP in specific) vulnerability of transport protocols in general (and TCP in specific)
to off-path third-party spoofing attacks. As shown, these operate at to off-path third-party spoofing attacks. As shown, these operate at
the transport or network layer. Transport solutions require separate the transport or network layer. Transport solutions require separate
modification of each transport protocol, addressing network identity modification of each transport protocol, addressing network identity
spoofing separately in the context of each transport association. spoofing separately in the context of each transport association.
Network solutions require distributed coordination (filtering) or can Network solutions require distributed coordination (filtering) or can
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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 protocols support negotiated endpoint state (e.g., Not all transport protocols 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 [4][36]. window attenuation [4][38].
Transport layer solutions are not only per-protocol, but often per- Transport layer solutions are not only per-protocol, but often per-
connection. This has both advantages and drawbacks. One advantage connection. This has both advantages and drawbacks. One advantage
to transport layer solutions is that they can protect the transport to transport layer solutions is that they can protect the transport
protocol when lower layers have failed, e.g., due to bugs in protocol when lower layers have failed, e.g., due to bugs in
implementation. TCP already includes a variety of packet validation implementation. TCP already includes a variety of packet validation
mechanisms to protect in these cases, e.g., checking that RSTs are mechanisms to protect in these cases, e.g., checking that RSTs are
in-window. More strict checks can increase the protections provided, in-window. More strict checks can increase the protections provided,
e.g., to protect against misaddressed RSTs that end up in-window (via e.g., to protect against misaddressed RSTs that end up in-window (via
TCPsecure) or to protect against connection interruption due to RSTs, TCPsecure) or to protect against connection interruption due to RSTs,
SYNs, or data injection from misaddressed packets (TCP/MD5) [36]. SYNs, or data injection from misaddressed packets (TCP/MD5) [38].
Another advantage is that transport layer protections can be more Another advantage is that transport layer protections can be more
specifically limited to a particular connection. Because each specifically limited to a particular connection. Because each
connection negotiates its state separately, that state can be more connection negotiates its state separately, that state can be more
specifically tied to that connection. This is both an advantage and specifically tied to that connection. This is both an advantage and
a drawback. It can make it easier to tie security to an individual a drawback. It can make it easier to tie security to an individual
connection, although in practice a shared secret or certificate will connection, although in practice a shared secret or certificate will
generally be shared across multiple connections. generally be shared across multiple connections.
As a drawback, each transport connection needs to negotiate and As a drawback, each transport connection needs to negotiate and
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packets it emits. Such a network level solution protects all packets it emits. Such a network level solution protects all
transport protocols as a result, including both legacy and emerging transport protocols as a result, including both legacy and emerging
protocols, and reduces the complexity of these protocols as well. A protocols, and reduces the complexity of these protocols as well. A
shared solution also reduces protocol overhead, and decouples the shared solution also reduces protocol overhead, and decouples the
management (and refreshing) of authentication state from that of management (and refreshing) of authentication state from that of
individual transport connections. Finally, a network layer solution individual transport connections. Finally, a network layer solution
protects not only the transport layer but the network layer as well, protects not only the transport layer but the network layer as well,
e.g., from IGMP, and some kinds of ICMP (Sec. 4), spoofing attacks. e.g., from IGMP, and some kinds of ICMP (Sec. 4), spoofing attacks.
The IETF Proposed Standard protocol for network layer authentication The IETF Proposed Standard protocol for network layer authentication
is IPsec [25]. IPsec specifies the overall architecture, including is IPsec [26]. IPsec specifies the overall architecture, including
header authentication (AH) [23] and encapsulation (ESP) modes [24]. header authentication (AH) [24] and encapsulation (ESP) modes [25].
AH authenticates both the IP header and IP data, whereas ESP AH 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 Security AH is deprecated since ESP is more efficient and the Security
Parameters Index (SPI) includes sufficient information to verify the Parameters Index (SPI) includes sufficient information to verify the
IP header anyway. These two modes describe the security applied to IP header anyway. These two modes describe the security applied to
individual packets within the IPsec system; key exchange and individual packets within the IPsec system; key exchange and
management is performed either out-of-band (via pre-shared keys) or management is performed either out-of-band (via pre-shared keys) or
by an automated key exchange protocol IKE [18][22]. by an automated key exchange protocol IKE [19][23].
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 on-path and off-path third-party contents sufficient to defeat both on-path and off-path third-party
spoofing attacks. IKE can configure authentication between two spoofing attacks. IKE can configure authentication between two
endpoints on a per-endpoint, per-protocol, or per-connection basis, endpoints on a per-endpoint, per-protocol, or per-connection basis,
as desired. IKE also can perform automatic periodic re-keying, as desired. IKE also can perform automatic periodic re-keying,
further defeating crypto-analysis based on snooping (clandestine data further defeating crypto-analysis based on snooping (clandestine data
collection). The use of IPsec is already commonly strongly collection). The use of IPsec is already commonly strongly
recommended for protected infrastructure. recommended for protected infrastructure.
Existing IPsec is not appropriate for many deployments. It is Existing IPsec is not appropriate for many deployments. It is
computationally intensive both in key management and individual computationally intensive both in key management and individual
packet authentication [39]. This computational overhead can be packet authentication [41]. This computational overhead can be
prohibitive, and so often requires additional hardware, especially in prohibitive, and so often requires additional hardware, especially in
commercial routers. As importantly, IKE is not anonymous; keys can commercial routers. As importantly, IKE is not anonymous; keys can
be exchanged between parties only if they trust each others' X.509 be exchanged between parties only if they trust each others' X.509
certificates, trust some other third-party to help with key certificates, trust some other third-party to help with key
generation (e.g., Kerberos), or pre-share a key. These certificates generation (e.g., Kerberos), or pre-share a key. These certificates
provide identification (the other party knows who you are) only where provide identification (the other party knows who you are) only where
the certificates themselves are signed by certificate authorities the certificates themselves are signed by certificate authorities
(CAs) that both parties already trust. To a large extent, the CAs (CAs) that both parties already trust. To a large extent, the CAs
themselves are the pre-shared keys which help IKE establish security themselves are the pre-shared keys which help IKE establish security
association keys, which are then used in the authentication association keys, which are then used in the authentication
algorithms. algorithms.
Alternative mechanisms are under development to address this Alternative mechanisms are under development to address this
limitation, to allow publicly-accessible servers to secure limitation, to allow publicly-accessible servers to secure
connections to clients not known in advance, or to allow unilateral connections to clients not known in advance, or to allow unilateral
relaxation of identity validation so that the remaining protections relaxation of identity validation so that the remaining protections
of IPsec to be available [41][42]. In particular, these mechanisms of IPsec to be available [43][44]. In particular, these mechanisms
can prevent a client (but without knowing who that client is) from can prevent a client (but without knowing who that client is) from
being affected by spoofing from other clients, even when the being affected by spoofing from other clients, even when the
attackers are on the same communications path. attackers are on the same communications path.
IPsec, although widely available both in commercial routers and IPsec, although widely available both in commercial routers and
commodity end-systems, is not often used except between parties that commodity end-systems, is not often used except between parties that
already have a preexisting relationship (employee/employer, between already have a preexisting relationship (employee/employer, between
two ISPs, etc.). Servers to anonymous clients (e.g., customer/ two ISPs, etc.). Servers to anonymous clients (e.g., customer/
business) or more open services (e.g., BGP, where routers may have business) or more open services (e.g., BGP, where routers may have
large numbers of peers) are unmanageable, due to the breadth and flux large numbers of peers) are unmanageable, due to the breadth and flux
skipping to change at page 22, line 11 skipping to change at page 22, line 36
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 spoof attacks, but not addressing on- against off-path third-party spoof attacks, but not addressing on-
path attacks at all. Such potential solutions are discussed in the path attacks at all. Such potential solutions are discussed in the
BTNS documents [5][41][42]. Note also that NULL Encryption in IPsec BTNS documents [5][43][44]. Note also that NULL Encryption in IPsec
applies a variant of this cookie, where the SPI is the cookie, and no applies a variant of this cookie, where the SPI is the cookie, and no
further encryption is applied [16]. further encryption is applied [16].
6. Security Considerations 6. 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
skipping to change at page 23, line 12 skipping to change at page 23, line 35
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.
9. Acknowledgments 9. Acknowledgments
This document was inspired by discussions in the TCPM WG This document was inspired by discussions in the TCPM WG
<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 (which is now edited by M. Dalal) [36][38]. The analysis of draft (which is now edited by M. Dalal) [38][40]. The analysis of
the attack issues, alternate solutions, and the anonymous security the attack issues, alternate solutions, and the anonymous security
proposed solutions were the result of discussions on that list as proposed solutions were the result of discussions on that list as
well as with USC/ISI's T. Faber, A. Falk, G. Finn, and Y. Wang. R. well as with USC/ISI's T. Faber, A. Falk, G. Finn, and Y. Wang. R.
Atkinson suggested the UDP variant of TCP/MD5, P. Goyette suggested Atkinson suggested the UDP variant of TCP/MD5, P. Goyette suggested
using the ISN to seed TCP/MD5, and L. Wood suggested using the ISN to using the ISN to seed TCP/MD5, and L. Wood suggested using the ISN to
validate RSTs. Other improvements are due to the input of various validate RSTs. Other improvements are due to the input of various
members of the IETF's TCPM WG, notably detailed feedback from P. members of the IETF's TCPM WG, notably detailed feedback from F. Gont
Savola. and P. Savola.
10. References 10. References
10.1. Normative References 10.1. Normative References
None. None.
10.2. Informative References 10.2. Informative References
[1] Allman, M., V. Paxson, W. Stephens, "TCP Congestion Control," [1] Allman, M., V. Paxson, W. Stephens, "TCP Congestion Control,"
RFC 2581, Apr. 1999. RFC 2581, Apr. 1999.
[2] Baker, F. and P. Savola, "Ingress Filtering for Multihomed [2] Baker, F. and P. Savola, "Ingress Filtering for Multihomed
Networks," RFC 3704 / BCP 84, Mar. 2004. Networks," RFC 3704 / BCP 84, Mar. 2004.
[3] Bellovin, S. and A. Zinin, "Standards Maturity Variance [3] Bellovin, S. and A. Zinin, "Standards Maturity Variance
Regarding the TCP MD5 Signature Option (RFC 2385) and the BGP-4 Regarding the TCP MD5 Signature Option (RFC 2385) and the BGP-4
Specification," (work in progress), Specification," RFC 4278 (Informational), Jan. 2006.
draft-iesg-tcpmd5app-01.txt, Sept. 2004.
[4] Bernstein, D., "SYN cookies - http://cr.yp.to/syncookies.html", [4] Bernstein, D., "SYN cookies - http://cr.yp.to/syncookies.html",
1997. 1997.
[5] Better Than Nothing Security [BTNS] WG web pages, [5] Better Than Nothing Security [BTNS] WG web pages,
http://www.postel.org/anonsec http://www.postel.org/anonsec
[6] Bonica, R., et al., "Authentication for TCP-based Routing and [6] Bonica, R., et al., "Authentication for TCP-based Routing and
Management Protocols," draft-bonica-tcp-auth-04, (work in Management Protocols," draft-bonica-tcp-auth-05, (work in
progress), Feb. 2006. progress), Jul. 2006.
[7] Braden, R., "Requirements for Internet Hosts -- Communication [7] Braden, R., "Requirements for Internet Hosts -- Communication
Layers," RFC 1122, Oct. 1989. Layers," RFC 1122, Oct. 1989.
[8] Braden, R., "TIME-WAIT Assassination Hazards in TCP", RFC 1337, [8] Braden, R., "TIME-WAIT Assassination Hazards in TCP", RFC 1337,
May 1992. May 1992.
[9] CERT alert: "Technical Cyber Security Alert TA04-111A: [9] CERT alert: "Technical Cyber Security Alert TA04-111A:
Vulnerabilities in TCP -- Vulnerabilities in TCP --
http://www.us-cert.gov/cas/techalerts/TA04-111A.html", April 20 http://www.us-cert.gov/cas/techalerts/TA04-111A.html", April 20
2004. 2004.
[10] Convery, S. and M. Franz, "BGP Vulnerability Testing: [10] Convery, S. and M. Franz, "BGP Vulnerability Testing:
Separating Fact from FUD", 2003, Separating Fact from FUD", 2003,
http://www.nanog.org/mtg-0306/pdf/franz.pdf http://www.nanog.org/mtg-0306/pdf/franz.pdf
[11] Eddy, W., "TCP SYN Flooding Attacks and Common Mitigations," [11] Eddy, W., "TCP SYN Flooding Attacks and Common Mitigations,"
draft-eddy-syn-flood-02.txt (work in progress), April 2006. draft-ietf-tcpm-syn-flood-00.txt (work in progress), Jul. 2006.
[12] Faber, T., J. Touch, and W. Yue, "The TIME-WAIT state in TCP [12] Faber, T., J. Touch, and W. Yue, "The TIME-WAIT state in TCP
and Its Effect on Busy Servers", Proc. Infocom 1999 pp. 1573- and Its Effect on Busy Servers", Proc. Infocom 1999 pp. 1573-
1583, Mar. 1999. 1583, Mar. 1999.
[13] Ferguson, P. and D. Senie, "Network Ingress Ingress Filtering: [13] Ferguson, P. and D. Senie, "Network Ingress Ingress Filtering:
Defeating Denial of Service Attacks which employ IP Address Defeating Denial of Service Attacks which employ IP Address
Spoofing," RFC 2827 / BCP 38, May 2000. Spoofing," RFC 2827 / BCP 38, May 2000.
[14] Floyd, S., "Inappropriate TCP Resets Considered Harmful", BCP [14] Floyd, S., "Inappropriate TCP Resets Considered Harmful", BCP
60, RFC 3360, Aug. 2002. 60, RFC 3360, Aug. 2002.
[15] Gill, V., J. Heasley, and D. Meyer, "The Generalized TTL [15] Gill, V., J. Heasley, and D. Meyer, "The Generalized TTL
Security Mechanism (GTSM)," RFC 3682 (Experimental), Feb. 2004. Security Mechanism (GTSM)," RFC 3682 (Experimental), Feb. 2004.
[16] Glenn, R. and S. Kent, "The NULL Encryption Algorithm and Its [16] Glenn, R. and S. Kent, "The NULL Encryption Algorithm and Its
Use With IPsec", RFC 2410 (Standards Track), Nov. 1998. Use With IPsec", RFC 2410 (Standards Track), Nov. 1998.
[17] Gont, F., "ICMP attacks against TCP," draft-gont-tcpm-icmp- [17] Gont, F., "ICMP attacks against TCP," draft-gont-tcpm-icmp-
attacks-05.txt, (work in progress), Oct. 2005. attacks-05.txt, (expired work in progress), Oct. 2005.
[18] Harkins, D. and D. Carrel, "The Internet Key Exchange (IKE)", [18] Gont, F., "TCP's Reaction to Soft Errors," draft-ietf-tcpm-tcp-
soft-errors-02.txt, (work in progress), Oct. 2006.
[19] Harkins, D. and D. Carrel, "The Internet Key Exchange (IKE)",
RFC 2409 (Standards Track), Nov. 1998. RFC 2409 (Standards Track), Nov. 1998.
[19] Heffernan, A., "Protection of BGP Sessions via the TCP MD5 [20] Heffernan, A., "Protection of BGP Sessions via the TCP MD5
Signature Option", RFC 2385 (Standards Track), Aug. 1998. Signature Option", RFC 2385 (Standards Track), Aug. 1998.
[20] Housley, R., Post to IETF Discussion mailing list regarding his [21] Housley, R., Post to IETF Discussion mailing list regarding his
IETF 64 Security Area presentation, IETF 64 Security Area presentation,
ID=7.0.0.10.2.20051124135914.00f50558@vigilsec.com, Nov. 24, ID=7.0.0.10.2.20051124135914.00f50558@vigilsec.com, Nov. 24,
2005, http://www1.ietf.org/mail- 2005, http://www1.ietf.org/mail-
archive/ietf/Current/maillist.html archive/ietf/Current/maillist.html
[21] Jacobson, V., B. Braden, and D. Borman, "TCP Extensions for [22] Jacobson, V., B. Braden, and D. Borman, "TCP Extensions for
High Performance", RFC 1323, May 1992. High Performance", RFC 1323, May 1992.
[22] Kaufman, C., "Internet Key Exchange (IKEv2) Protocol", RFC 4306 [23] Kaufman, C., "Internet Key Exchange (IKEv2) Protocol", RFC 4306
(Standards Track), Dec. 2005. (Standards Track), Dec. 2005.
[23] Kent, S., "IP Authentication Header", RFC 4302 (Standards [24] Kent, S., "IP Authentication Header", RFC 4302 (Standards
Track), Dec. 2005. Track), Dec. 2005.
[24] Kent, S, "IP Encapsulating Security Payload (ESP)", RFC 4303 [25] Kent, S, "IP Encapsulating Security Payload (ESP)", RFC 4303
(Standards Track), Dec. 2005. (Standards Track), Dec. 2005.
[25] Kent, S. and K. Seo, "Security Architecture for the Internet [26] Kent, S. and K. Seo, "Security Architecture for the Internet
Protocol", RFC 4301, Dec. 2005. Protocol", RFC 4301, Dec. 2005.
[26] Kohler, E., M. Handley, and S. Floyd, "Datagram Congestion [27] Kohler, E., M. Handley, and S. Floyd, "Datagram Congestion
Control Protocol (DCCP)", draft-ietf-dccp-spec-13 (work in Control Protocol (DCCP)", RFC 4340 (Proposed Standard), Dec.
progress), Dec. 2005. 2005.
[27] Leech, M., "Key Management Considerations for the TCP MD5 [28] Leech, M., "Key Management Considerations for the TCP MD5
Signature Option," RFC 3562 (Informational), July 2003. Signature Option," RFC 3562 (Informational), July 2003.
[28] Moore, D., G. Voelker, and S. Savage, "Inferring Internet [29] Moore, D., G. Voelker, and S. Savage, "Inferring Internet
Denial-of-Service Activity," Proc. Usenix Security Symposium, Denial-of-Service Activity," Proc. Usenix Security Symposium,
Aug. 2001. Aug. 2001.
[29] O'Malley, S. and L. Peterson, "TCP Extensions Considered [30] O'Malley, S. and L. Peterson, "TCP Extensions Considered
Harmful", RFC 1263, October 1991. Harmful", RFC 1263, October 1991.
[30] Perkins, C., "IP Encapsulation within IP," RFC 2003 (Standards [31] Perkins, C., "IP Encapsulation within IP," RFC 2003 (Standards
Track), Oct. 1996. Track), Oct. 1996.
[31] Poon, K., "Use of TCP timestamp option to defend against blind [32] Poon, K., "Use of TCP timestamp option to defend against blind
spoofing attack," draft-poon-tcp-tstamp-mod-01 (expired work in spoofing attack," draft-poon-tcp-tstamp-mod-01 (expired work in
progress), Oct. 2004. progress), Oct. 2004.
[32] Postel, J., "Transmission Control Protocol," RFC 793 / STD 7, [33] Postel, J., "Transmission Control Protocol," RFC 793 / STD 7,
Sep. 1981. Sep. 1981.
[33] Rekhter, Y. and T. Li, (eds.), "A Border Gateway Protocol 4 [34] Rekhter, Y. and T. Li, (eds.), "A Border Gateway Protocol 4
(BGP-4)," RFC 1771 (Standards Track), Mar. 1995. (BGP-4)," RFC 1771 (Standards Track), Mar. 1995.
[34] Semke, J., J. Mahdavi, M. Mathis, "Automatic TCP Buffer [35] Semke, J., J. Mahdavi, M. Mathis, "Automatic TCP Buffer
Tuning", ACM SIGCOMM '98/ Computer Communication Review, volume Tuning", ACM SIGCOMM '98/ Computer Communication Review, volume
28, number 4, Oct. 1998. 28, number 4, Oct. 1998.
[35] Shepard, T., "Reassign Port Number option for TCP," draft- [36] Shepard, T., "Reassign Port Number option for TCP," draft-
sheard-tcp-reassign-port-number-00.txt (work in progress), Jul. sheard-tcp-reassign-port-number-00.txt (expired work in
2004. progress), Jul. 2004.
[36] Stewart, R. and M. Dalal, "Improving TCP's Robustness to Blind [37] Shirey, R., "Internet Security Glossary, Version 2," draft-
In-Window Attacks", draft-ietf-tcpm-tcpsecure-04 (work in shirey-secgloss-v2-07.txt (work in progress), Sep. 2006.
progress), Feb. 2005.
[37] Stewart, R., Q. Xie, K. Morneault, C. Sharp, H. Schwarzbauer, [38] Stewart, R. and M. Dalal, "Improving TCP's Robustness to Blind
In-Window Attacks", draft-ietf-tcpm-tcpsecure-05 (work in
progress), Jun. 2006.
[39] Stewart, R., Q. Xie, K. Morneault, C. Sharp, H. Schwarzbauer,
T. Taylor, I. Rytina, M. Kalla, L. Zhang, and V. Paxson, T. Taylor, I. Rytina, M. Kalla, L. Zhang, and V. Paxson,
"Stream Control Transmission Protocol," RFC 2960 (Standards "Stream Control Transmission Protocol," RFC 2960 (Standards
Track), Oct. 2000. Track), Oct. 2000.
[38] TCPM: IETF TCPM Working Group and mailing list, [40] TCPM: IETF TCPM Working Group and mailing list,
http://www.ietf.org/html.charters/tcpm-charter.html. http://www.ietf.org/html.charters/tcpm-charter.html.
[39] Touch, J., "Report on MD5 Performance," RFC 1810 [41] Touch, J., "Report on MD5 Performance," RFC 1810
(Informational), Jun. 1995. (Informational), Jun. 1995.
[40] Touch, J., "Performance Analysis of MD5," Proc. Sigcomm 1995 [42] Touch, J., "Performance Analysis of MD5," Proc. Sigcomm 1995
pp. 77-86, Mar. 1999. pp. 77-86, Mar. 1999.
[41] Touch, J., "ANONsec: Anonymous Security to Defend Against [43] Touch, J., "ANONsec: Anonymous Security to Defend Against
Spoofing Attacks," draft-touch-anonsec-00 (expired work in Spoofing Attacks," draft-touch-anonsec-00 (expired work in
progress), May 2004. progress), May 2004.
[42] Touch, J., D. Black, and Y. Wang, "Problem and Applicability [44] Touch, J., D. Black, and Y. Wang, "Problem and Applicability
Statement for Better Than Nothing Security (BTNS)," Statement for Better Than Nothing Security (BTNS),"
draft-ietf-btns-prob-and-applic-02 (work in progress), Feb. draft-ietf-btns-prob-and-applic-04 (work in progress), Sep.
2006. 2006.
[43] Watson, P., "Slipping in the Window: TCP Reset attacks," [45] Watson, P., "Slipping in the Window: TCP Reset attacks,"
Presentation at 2004 CanSecWest. Presentation at 2004 CanSecWest.
http://www.cansecwest.com/archives.html http://www.cansecwest.com/archives.html
[44] Wood, L., Post to TCPM mailing list regarding use of ISN in [46] Wood, L., Post to TCPM mailing list regarding use of ISN in
RSTs, ID=Pine.GSO.4.50.0404232249570.5889- RSTs, ID=Pine.GSO.4.50.0404232249570.5889-
100000@argos.ee.surrey.ac.uk, Apr. 23, 2004. 100000@argos.ee.surrey.ac.uk, Apr. 23, 2004.
http://www1.ietf.org/mail- http://www1.ietf.org/mail-
archive/web/tcpm/current/msg00213.html archive/web/tcpm/current/msg00213.html
Author's Addresses Author's Addresses
Joe Touch Joe Touch
USC/ISI USC/ISI
4676 Admiralty Way 4676 Admiralty Way
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