draft-ietf-tcpm-syn-flood-05.txt   rfc4987.txt 
Network Working Group W. Eddy Network Working Group W. Eddy
Internet-Draft Verizon Federal Network Systems Request for Comments: 4987 Verizon
Intended status: Informational May 30, 2007 Category: Informational August 2007
Expires: December 1, 2007
TCP SYN Flooding Attacks and Common Mitigations TCP SYN Flooding Attacks and Common Mitigations
draft-ietf-tcpm-syn-flood-05
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Copyright Notice Copyright Notice
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Abstract Abstract
This document describes TCP SYN flooding attacks, which have been This document describes TCP SYN flooding attacks, which have been
well-known to the community for several years. Various well-known to the community for several years. Various
countermeasures against these attacks, and the trade-offs of each, countermeasures against these attacks, and the trade-offs of each,
are described. This document archives explanations of the attack and are described. This document archives explanations of the attack and
common defense techniques for the benefit of TCP implementers and common defense techniques for the benefit of TCP implementers and
administrators of TCP servers or networks, but does not make any administrators of TCP servers or networks, but does not make any
standards-level recommendations. standards-level recommendations.
Table of Contents Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 2
2. Attack Description . . . . . . . . . . . . . . . . . . . . . . 4 2. Attack Description . . . . . . . . . . . . . . . . . . . . . . 2
2.1. History . . . . . . . . . . . . . . . . . . . . . . . . . 4 2.1. History . . . . . . . . . . . . . . . . . . . . . . . . . 3
2.2. Theory of Operation . . . . . . . . . . . . . . . . . . . 4 2.2. Theory of Operation . . . . . . . . . . . . . . . . . . . 3
3. Common Defenses . . . . . . . . . . . . . . . . . . . . . . . 8 3. Common Defenses . . . . . . . . . . . . . . . . . . . . . . . 6
3.1. Filtering . . . . . . . . . . . . . . . . . . . . . . . . 8 3.1. Filtering . . . . . . . . . . . . . . . . . . . . . . . . 6
3.2. Increasing Backlog . . . . . . . . . . . . . . . . . . . . 8 3.2. Increasing Backlog . . . . . . . . . . . . . . . . . . . . 7
3.3. Reducing SYN-RECEIVED Timer . . . . . . . . . . . . . . . 8 3.3. Reducing SYN-RECEIVED Timer . . . . . . . . . . . . . . . 7
3.4. Recycling the Oldest Half-Open TCB . . . . . . . . . . . . 9 3.4. Recycling the Oldest Half-Open TCB . . . . . . . . . . . . 7
3.5. SYN Cache . . . . . . . . . . . . . . . . . . . . . . . . 9 3.5. SYN Cache . . . . . . . . . . . . . . . . . . . . . . . . 8
3.6. SYN Cookies . . . . . . . . . . . . . . . . . . . . . . . 10 3.6. SYN Cookies . . . . . . . . . . . . . . . . . . . . . . . 8
3.7. Hybrid Approaches . . . . . . . . . . . . . . . . . . . . 11 3.7. Hybrid Approaches . . . . . . . . . . . . . . . . . . . . 10
3.8. Firewalls and Proxies . . . . . . . . . . . . . . . . . . 12 3.8. Firewalls and Proxies . . . . . . . . . . . . . . . . . . 10
4. Analysis . . . . . . . . . . . . . . . . . . . . . . . . . . . 13 4. Analysis . . . . . . . . . . . . . . . . . . . . . . . . . . . 11
5. Security Considerations . . . . . . . . . . . . . . . . . . . 15 5. Security Considerations . . . . . . . . . . . . . . . . . . . 13
6. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 16 6. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 13
7. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 17 7. Informative References . . . . . . . . . . . . . . . . . . . . 13
8. Informative References . . . . . . . . . . . . . . . . . . . . 18 Appendix A. SYN Cookies Description . . . . . . . . . . . . . . . 16
Appendix A. SYN Cookies Description . . . . . . . . . . . . . . . 20
Author's Address . . . . . . . . . . . . . . . . . . . . . . . . . 23
Intellectual Property and Copyright Statements . . . . . . . . . . 24
1. Introduction 1. Introduction
The SYN flooding attack is a denial of service method affecting hosts The SYN flooding attack is a denial-of-service method affecting hosts
that run TCP server processes. The attack takes advantage of the that run TCP server processes. The attack takes advantage of the
state retention TCP performs for some time after receiving a SYN state retention TCP performs for some time after receiving a SYN
segment to a port that has been put into the LISTEN state. The basic segment to a port that has been put into the LISTEN state. The basic
idea is to exploit this behavior by causing a host to retain enough idea is to exploit this behavior by causing a host to retain enough
state for bogus half-connections that there are no resources left to state for bogus half-connections that there are no resources left to
establish new legitimate connections. establish new legitimate connections.
This SYN flooding attack has been well-known to the community for This SYN flooding attack has been well-known to the community for
many years, and has been observed in the wild by network operators many years, and has been observed in the wild by network operators
and end-hosts. A number of methods have been developed and deployed and end hosts. A number of methods have been developed and deployed
to make SYN flooding less effective. Despite the notoriety of the to make SYN flooding less effective. Despite the notoriety of the
attack, and the widely available countermeasures, the RFC series only attack, and the widely available countermeasures, the RFC series only
documented the vulnerability as an example motivation for ingress documented the vulnerability as an example motivation for ingress
filtering [RFC2827], and has not suggested any mitigation techniques filtering [RFC2827], and has not suggested any mitigation techniques
for TCP implementations. This document addresses both points, but for TCP implementations. This document addresses both points, but
does not define any standards. Formal specifications and does not define any standards. Formal specifications and
requirements of defense mechanisms are outside the scope of this requirements of defense mechanisms are outside the scope of this
document. Many defenses only impact an end-host's implementation document. Many defenses only impact an end host's implementation
without changing interoperability. These may not require without changing interoperability. These may not require
standardization, but their side-effects should at least be well standardization, but their side-effects should at least be well
understood. understood.
This document intentionally focuses on SYN flooding attacks from an This document intentionally focuses on SYN flooding attacks from an
individual end-host or application's perspective, as a means to deny individual end host or application's perspective, as a means to deny
service to that specific entity. High packet-rate attacks that service to that specific entity. High packet-rate attacks that
target the network's packet processing capability and capacity have target the network's packet-processing capability and capacity have
been observed operationally. Since such attacks target the network, been observed operationally. Since such attacks target the network,
and not a TCP implementation, they are out of scope for this and not a TCP implementation, they are out of scope for this
document, whether or not they happen to use TCP SYN segments as part document, whether or not they happen to use TCP SYN segments as part
of the attack, as the nature of the packets used is irrelevant in of the attack, as the nature of the packets used is irrelevant in
comparison to the packet-rate in such attacks. comparison to the packet-rate in such attacks.
The majority of this document consists of three sections. Section 2 The majority of this document consists of three sections. Section 2
explains the SYN flooding attack in greater detail. Several common explains the SYN flooding attack in greater detail. Several common
mitigation techniques are described in Section 3. An analysis and mitigation techniques are described in Section 3. An analysis and
discussion of these techniques and their use is presented in discussion of these techniques and their use is presented in
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The SYN flooding attack was first publicized in 1996, with the The SYN flooding attack was first publicized in 1996, with the
release of a description and exploit tool in Phrack Magazine release of a description and exploit tool in Phrack Magazine
[P48-13]. Aside from some minor inaccuracies, this article is of [P48-13]. Aside from some minor inaccuracies, this article is of
high enough quality to be useful, and code from the article was high enough quality to be useful, and code from the article was
widely distributed and used. widely distributed and used.
By September of 1996, SYN flooding attacks had been observed in the By September of 1996, SYN flooding attacks had been observed in the
wild. Particularly, an attack against one ISP's mail servers caused wild. Particularly, an attack against one ISP's mail servers caused
well-publicized outages. CERT quickly released an advisory on the well-publicized outages. CERT quickly released an advisory on the
attack [CA-96.21]. SYN flooding was particularly serious in attack [CA-96.21]. SYN flooding was particularly serious in
comparison to other known denial of service attacks at the time. comparison to other known denial-of-service attacks at the time.
Rather than relying on the common brute-force tactic of simply Rather than relying on the common brute-force tactic of simply
exhausting the network's resources, SYN flooding targets end-host exhausting the network's resources, SYN flooding targets end-host
resources, which it requires fewer packets to deplete. resources, which require fewer packets to deplete.
The community quickly developed many widely-differing techniques for The community quickly developed many widely differing techniques for
preventing or limiting the impact of SYN flooding attacks. Many of preventing or limiting the impact of SYN flooding attacks. Many of
these have been deployed to varying degrees on the Internet, in both these have been deployed to varying degrees on the Internet, in both
end hosts and intervening routers. Some of these techniques have end hosts and intervening routers. Some of these techniques have
become important pieces of the TCP implementations in certain become important pieces of the TCP implementations in certain
operating systems, although some significantly diverge from the TCP operating systems, although some significantly diverge from the TCP
specification and none of these techniques have yet been standardized specification and none of these techniques have yet been standardized
or sanctioned by the IETF process. or sanctioned by the IETF process.
2.2. Theory of Operation 2.2. Theory of Operation
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any time. The current base TCP specification, RFC 793 [RFC0793], any time. The current base TCP specification, RFC 793 [RFC0793],
describes the standard processing of incoming SYN segments. RFC 793 describes the standard processing of incoming SYN segments. RFC 793
describes the concept of a Transmission Control Block (TCB) data describes the concept of a Transmission Control Block (TCB) data
structure to store all the state information for an individual structure to store all the state information for an individual
connection. In practice, operating systems may implement this connection. In practice, operating systems may implement this
concept rather differently, but the key is that each TCP connection concept rather differently, but the key is that each TCP connection
requires some memory space. requires some memory space.
Per RFC 793, when a SYN is received for a local TCP port where a Per RFC 793, when a SYN is received for a local TCP port where a
connection is in the LISTEN state, then the state transitions to SYN- connection is in the LISTEN state, then the state transitions to SYN-
RECEIVED and some of the TCB is initialized with information from the RECEIVED, and some of the TCB is initialized with information from
header fields of the received SYN segment. In practice, many the header fields of the received SYN segment. In practice, many
operating systems do not alter the TCB in LISTEN, but instead make a operating systems do not alter the TCB in LISTEN, but instead make a
copy of the TCB and perform the state transition and update on the copy of the TCB and perform the state transition and update on the
copy. This is done so that the local TCP port may be shared amongst copy. This is done so that the local TCP port may be shared amongst
several distinct connections. This TCB-copying behavior is not several distinct connections. This TCB-copying behavior is not
actually essential for this purpose, but influences the way in which actually essential for this purpose, but influences the way in which
applications that wish to handle multiple simultaneous connections applications that wish to handle multiple simultaneous connections
through a single TCP port are written. The crucial result of this through a single TCP port are written. The crucial result of this
behavior is that instead of updating already-allocated memory, new behavior is that, instead of updating already-allocated memory, new
(or unused) memory must be devoted to the copied TCB. (or unused) memory must be devoted to the copied TCB.
As an example, in the Linux 2.6.10 networking code, a "sock" As an example, in the Linux 2.6.10 networking code, a "sock"
structure is used to implement the TCB concept. By examination, this structure is used to implement the TCB concept. By examination, this
structure takes over 1300 bytes to store in memory. In other systems structure takes over 1300 bytes to store in memory. In other systems
that implement less complex TCP algorithms and options, the overhead that implement less-complex TCP algorithms and options, the overhead
may be less, although it typically exceeds 280 bytes [SKK+97]. may be less, although it typically exceeds 280 bytes [SKK+97].
To protect host memory from being exhausted by connection requests, To protect host memory from being exhausted by connection requests,
the number of TCB structures that can be resident at any time is the number of TCB structures that can be resident at any time is
usually limited by operating system kernels. Systems vary on whether usually limited by operating system kernels. Systems vary on whether
limits are globally applied or local to a particular port number. limits are globally applied or local to a particular port number.
There is also variation on whether the limits apply to fully- There is also variation on whether the limits apply to fully
established connections as well as those in SYN-RECEIVED. Commonly, established connections as well as those in SYN-RECEIVED. Commonly,
systems implement a parameter to the typical listen() system call systems implement a parameter to the typical listen() system call
that allows the application to suggest a value for this limit, called that allows the application to suggest a value for this limit, called
the backlog. When the backlog limit is reached, then either incoming the backlog. When the backlog limit is reached, then either incoming
SYN segments are ignored, or uncompleted connections in the backlog SYN segments are ignored, or uncompleted connections in the backlog
are replaced. The concept of using a backlog is not described in the are replaced. The concept of using a backlog is not described in the
standards documents, so the failure behavior when the backlog is standards documents, so the failure behavior when the backlog is
reached might differ between stacks (for instance, TCP RSTs might be reached might differ between stacks (for instance, TCP RSTs might be
generated). The exact failure behavior will determine whether generated). The exact failure behavior will determine whether
initiating hosts continue to retransmit SYN segments over time, or initiating hosts continue to retransmit SYN segments over time, or
quickly cease. These differences in implementation are acceptable quickly cease. These differences in implementation are acceptable
since they only affect the behavior of the local stack when its since they only affect the behavior of the local stack when its
resources are constrained, and do not cause interoperability resources are constrained, and do not cause interoperability
problems. problems.
The SYN flooding attack neither attempts to overload the network's The SYN flooding attack does not attempt to overload the network's
resources, nor the end host's memory, but merely to exhaust the resources or the end host's memory, but merely attempts to exhaust
backlog of half-open connections associated with a port number. The the backlog of half-open connections associated with a port number.
goal is to send a quick barrage of SYN segments from IP addresses The goal is to send a quick barrage of SYN segments from IP addresses
(often spoofed) that will not generate replies to the SYN-ACKs that (often spoofed) that will not generate replies to the SYN-ACKs that
are produced. By keeping the backlog full of bogus half-opened are produced. By keeping the backlog full of bogus half-opened
connections, legitimate requests will be rejected. Three important connections, legitimate requests will be rejected. Three important
attack parameters for success are the size of the barrage, the attack parameters for success are the size of the barrage, the
frequency with which barrages are generated, and the means of frequency with which barrages are generated, and the means of
selecting IP addresses to spoof. selecting IP addresses to spoof.
Barrage Size Barrage Size
To be effective, the size of the barrage must be made large enough To be effective, the size of the barrage must be made large enough
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several dozen, so the attack might be tailored to the particular several dozen, so the attack might be tailored to the particular
value determined by the victim host and application. On machines value determined by the victim host and application. On machines
intended to be servers, especially for a high volume of traffic, intended to be servers, especially for a high volume of traffic,
the backlogs are often administratively configured to higher the backlogs are often administratively configured to higher
values. values.
Barrage Frequency Barrage Frequency
To limit the lifetime of half-opened connection state, TCP To limit the lifetime of half-opened connection state, TCP
implementations commonly reclaim memory from half-opened implementations commonly reclaim memory from half-opened
connections if they do not become fully-opened after some time connections if they do not become fully opened after some time
period. For instance, a timer of 75 seconds [SKK+97] might be set period. For instance, a timer of 75 seconds [SKK+97] might be set
when the first SYN-ACK is sent, and on expiration cause SYN-ACK when the first SYN-ACK is sent, and on expiration cause SYN-ACK
retransmissions to cease and the TCB to be released. The TCP retransmissions to cease and the TCB to be released. The TCP
specifications do not include this behavior of giving up on specifications do not include this behavior of giving up on
connection establishment after an arbitrary time. Some purists connection establishment after an arbitrary time. Some purists
have expressed that the TCP implementation should continue have expressed that the TCP implementation should continue
retransmitting SYN and SYN-ACK segments without artificial bounds retransmitting SYN and SYN-ACK segments without artificial bounds
(but with exponential backoff to some conservative rate) until the (but with exponential backoff to some conservative rate) until the
application gives up. Despite this, common operating systems application gives up. Despite this, common operating systems
today do implement some artificial limit on half-open TCB today do implement some artificial limit on half-open TCB
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It is important to note that this attack is directed at particular It is important to note that this attack is directed at particular
listening applications on a host, and not the host itself or the listening applications on a host, and not the host itself or the
network. The attack also attempts to prevent only the establishment network. The attack also attempts to prevent only the establishment
of new incoming connections to the victim port, and does not impact of new incoming connections to the victim port, and does not impact
outgoing connection requests, nor previously established connections outgoing connection requests, nor previously established connections
to the victim port. to the victim port.
In practice, an attacker might choose not to use spoofed IP In practice, an attacker might choose not to use spoofed IP
addresses, but instead to use a multitude of hosts to initiate a SYN addresses, but instead to use a multitude of hosts to initiate a SYN
flooding attack. For instance, a collection of compromised hosts flooding attack. For instance, a collection of compromised hosts
under the attacker's control (i.e. a "botnet") could be used. In under the attacker's control (i.e., a "botnet") could be used. In
this case, each host utilized in the attack would have to suppress this case, each host utilized in the attack would have to suppress
its operating system's native response to the SYN-ACKs coming from its operating system's native response to the SYN-ACKs coming from
the target. It is also possible for the attack TCP segments to the target. It is also possible for the attack TCP segments to
arrive in a more continuous fashion than the "barrage" terminology arrive in a more continuous fashion than the "barrage" terminology
used here suggests; as long as the rate of new SYNs exceeds the rate used here suggests; as long as the rate of new SYNs exceeds the rate
at which TCBs are reaped, the attack will be successful. at which TCBs are reaped, the attack will be successful.
3. Common Defenses 3. Common Defenses
This section discusses a number of defense techniques which are known This section discusses a number of defense techniques that are known
to the community, many of which are available in off-the-shelf to the community, many of which are available in off-the-shelf
products. products.
3.1. Filtering 3.1. Filtering
Since in the absence of an army of controlled hosts, the ability to Since in the absence of an army of controlled hosts, the ability to
send packets with spoofed source IP addresses is required for this send packets with spoofed source IP addresses is required for this
attack to work, removing an attacker's ability to send spoofed IP attack to work, removing an attacker's ability to send spoofed IP
packets is an effective solution that requires no modifications to packets is an effective solution that requires no modifications to
TCP. The filtering techniques described in RFCs 2827, 3013, and 3704 TCP. The filtering techniques described in RFCs 2827, 3013, and 3704
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Once the entire backlog is exhausted, some implementations allow Once the entire backlog is exhausted, some implementations allow
incoming SYNs to overwrite the oldest half-open TCB entry. This incoming SYNs to overwrite the oldest half-open TCB entry. This
works under the assumption that legitimate connections can be fully works under the assumption that legitimate connections can be fully
established in less time than the backlog can be filled by incoming established in less time than the backlog can be filled by incoming
attack SYNs. This can fail when the attacking packet rate is high attack SYNs. This can fail when the attacking packet rate is high
and/or the backlog size is small, and is not a robust defense. and/or the backlog size is small, and is not a robust defense.
3.5. SYN Cache 3.5. SYN Cache
The SYN cache, best described by Lemon [Lem02], is based on The SYN cache, best described by Lemon [Lem02], is based on
minimizing the amount of state that a SYN allocates, i.e. not minimizing the amount of state that a SYN allocates, i.e., not
immediately allocating a full TCB. The full state allocation is immediately allocating a full TCB. The full state allocation is
delayed until the connection has been fully established. Hosts delayed until the connection has been fully established. Hosts
implementing a SYN cache have some secret bits that they select from implementing a SYN cache have some secret bits that they select from
the incoming SYN segments. The secret bits are hashed along with the the incoming SYN segments. The secret bits are hashed along with the
IP addresses and TCP ports of a segment, and the hash value IP addresses and TCP ports of a segment, and the hash value
determines the location in a global hash table where the incomplete determines the location in a global hash table where the incomplete
TCB is stored. There is a bucket limit for each hash value, and when TCB is stored. There is a bucket limit for each hash value, and when
this limit is reached, the oldest entry is dropped. this limit is reached, the oldest entry is dropped.
The SYN cache technique is effective because the secret bits prevent The SYN cache technique is effective because the secret bits prevent
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bits and 25 bits for the other data [Lem02]. One way to encode these bits and 25 bits for the other data [Lem02]. One way to encode these
bits has been to XOR the initial sequence number received with a bits has been to XOR the initial sequence number received with a
truncated cryptographic hash of the IP address and TCP port number truncated cryptographic hash of the IP address and TCP port number
pairs, and secret bits. In practice, this hash has been generated pairs, and secret bits. In practice, this hash has been generated
using MD5 [RFC1321]. Any similar one-way hash could be used instead using MD5 [RFC1321]. Any similar one-way hash could be used instead
without impacting interoperability since the hash value is checked by without impacting interoperability since the hash value is checked by
the same host who generates it. the same host who generates it.
The problem with SYN cookies is that commonly implemented schemes are The problem with SYN cookies is that commonly implemented schemes are
incompatible with some TCP options, if the cookie generation scheme incompatible with some TCP options, if the cookie generation scheme
does not consider them. For example, an encoding of the MSS does not consider them. For example, an encoding of the Maximum
advertised on the SYN has been accommodated by using 2 sequence Segment Size (MSS) advertised on the SYN has been accommodated by
number bits to represent 4 predefined common MSS values. Similar using 2 sequence number bits to represent 4 predefined common MSS
techniques would be required for some other TCP options, while values. Similar techniques would be required for some other TCP
negotiated use of other TCP options can be detected implicitly. A options, while negotiated use of other TCP options can be detected
timestamp on the ACK, as an example, indicates that Timestamp use was implicitly. A timestamp on the ACK, as an example, indicates that
successfully negotiated on the SYN and SYN-ACK, while the reception Timestamp use was successfully negotiated on the SYN and SYN-ACK,
of a SACK option at some point during the connection implies that while the reception of a Selective Acknowledgement (SACK) option at
SACK was negotiated. Note that SACK blocks should normally not be some point during the connection implies that SACK was negotiated.
sent by a host using TCP cookies unless they are first received. For Note that SACK blocks should normally not be sent by a host using TCP
the common unidirectional data flow in many TCP connections, this can cookies unless they are first received. For the common
be a problem, as it limits SACK usage. For this reason, SYN cookies unidirectional data flow in many TCP connections, this can be a
problem, as it limits SACK usage. For this reason, SYN cookies
typically are not used by default on systems that implement them, and typically are not used by default on systems that implement them, and
are only enabled either under high-stress conditions indicative of an are only enabled either under high-stress conditions indicative of an
attack, or via administrative action. attack, or via administrative action.
Recently, a new SYN cookie technique developed for release in FreeBSD Recently, a new SYN cookie technique developed for release in FreeBSD
7.0 leverages the bits of the Timestamp option in addition to the 7.0 leverages the bits of the Timestamp option in addition to the
sequence number bits for encoding state. Since the Timestamp value sequence number bits for encoding state. Since the Timestamp value
is echoed back in the Timestamp Echo field of the ACK packet, any is echoed back in the Timestamp Echo field of the ACK packet, any
state stored in the Timestamp option can be restored similarly to the state stored in the Timestamp option can be restored similarly to the
way that it is from the sequence number / acknowledgement in a basic way that it is from the sequence number / acknowledgement in a basic
SYN cookie. Using the Timestamp bits, it is possible to explicitly SYN cookie. Using the Timestamp bits, it is possible to explicitly
store state bits for things like send and receive window scales, store state bits for things like send and receive window scales,
SACK-allowed, and TCP-MD5-enabled, that there is no room for in a SACK-allowed, and TCP-MD5-enabled, for which there is no room in a
typical SYN cookie. This use of Timestamps to improve the typical SYN cookie. This use of Timestamps to improve the
compromises inherent in SYN cookies is unique to the FreeBSD compromises inherent in SYN cookies is unique to the FreeBSD
implementation, to our knowledge. A limitation is that the technique implementation, to our knowledge. A limitation is that the technique
can only be used if the SYN itself contains a Timestamp option, but can only be used if the SYN itself contains a Timestamp option, but
this option seems to be widely implemented today, and hosts that this option seems to be widely implemented today, and hosts that
support window scaling and SACK typically support timestamps as well. support window scaling and SACK typically support timestamps as well.
Similarly to SYN caches, SYN cookies do not handle application data Similarly to SYN caches, SYN cookies do not handle application data
piggybacked on the SYN segment. piggybacked on the SYN segment.
Another problem with SYN cookies is for applications where the first Another problem with SYN cookies is for applications where the first
application data is sent by the passive host. If this host is application data is sent by the passive host. If this host is
handling a large number of connections, then packet loss may be handling a large number of connections, then packet loss may be
likely. When a handshake-completing ACK from the initiator is lost, likely. When a handshake-completing ACK from the initiator is lost,
the passive side's application-layer never is notified of the the passive side's application layer never is notified of the
connection's existence and never sends data, even though the connection's existence and never sends data, even though the
initiator thinks that the connection has been successfully initiator thinks that the connection has been successfully
established. An example application where the first application- established. An example application where the first application-
layer data is sent by the passive side is SMTP, if implemented layer data is sent by the passive side is SMTP, if implemented
according to RFC 2821, where a "service ready" message is sent by the according to RFC 2821, where a "service ready" message is sent by the
passive side after the TCP handshake is completed. passive side after the TCP handshake is completed.
Although SYN cookie implementations exist and are deployed, the use Although SYN cookie implementations exist and are deployed, the use
of SYN cookies is often disabled in default configurations, so it is of SYN cookies is often disabled in default configurations, so it is
unclear how much operational experience actually exists with them, or unclear how much operational experience actually exists with them or
if using them opens up new vulnerabilities. Anecdotes of incidents if using them opens up new vulnerabilities. Anecdotes of incidents
where SYN cookies have been used on typical web servers seem to where SYN cookies have been used on typical web servers seem to
indicate that the added processing burden of computing MD5 sums for indicate that the added processing burden of computing MD5 sums for
every SYN packet received is not significant in comparison to the every SYN packet received is not significant in comparison to the
loss of application availability when undefended. For some loss of application availability when undefended. For some
computationally-constrained mobile or embedded devices, this computationally constrained mobile or embedded devices, this
situation might be different. situation might be different.
3.7. Hybrid Approaches 3.7. Hybrid Approaches
The SYN cache and SYN cookie techniques can be combined. For The SYN cache and SYN cookie techniques can be combined. For
example, in the event that the cache becomes full, then SYN cookies example, in the event that the cache becomes full, then SYN cookies
can be sent instead of purging cache entries upon the arrival of new can be sent instead of purging cache entries upon the arrival of new
SYNs. Such hybrid approaches may provide a strong combination of the SYNs. Such hybrid approaches may provide a strong combination of the
positive aspects of each approach. Lemon has demonstrated the positive aspects of each approach. Lemon has demonstrated the
utility of this hybrid [Lem02]. utility of this hybrid [Lem02].
3.8. Firewalls and Proxies 3.8. Firewalls and Proxies
Firewall-based tactics may also be used to defend end-hosts from SYN Firewall-based tactics may also be used to defend end hosts from SYN
flooding attacks. The basic concept is to offload the connection flooding attacks. The basic concept is to offload the connection
establishment procedures onto a firewall that screens connection establishment procedures onto a firewall that screens connection
attempts until they are completed and then proxies them back to attempts until they are completed and then proxies them back to
protected end hosts. This moves the problem away from end-hosts to protected end hosts. This moves the problem away from end hosts to
become the firewall's or proxy's problem, and may introduce other become the firewall's or proxy's problem, and may introduce other
problems related to altering TCP's expected end-to-end semantics. A problems related to altering TCP's expected end-to-end semantics. A
common tactic used in these firewall and proxy products is to common tactic used in these firewall and proxy products is to
implement one of the end-host based techniques discussed above, and implement one of the end host based techniques discussed above, and
screen incoming SYNs from the protected network until the connection screen incoming SYNs from the protected network until the connection
is fully established. This is accomplished by spoofing the source is fully established. This is accomplished by spoofing the source
addresses of several packets to the initiator and listener at various addresses of several packets to the initiator and listener at various
stages of the handshake [Eddy06]. stages of the handshake [Eddy06].
4. Analysis 4. Analysis
Several of the defenses discussed in the previous section rely on Several of the defenses discussed in the previous section rely on
changes to behavior inside the network; via router filtering, changes to behavior inside the network; via router filtering,
firewalls, and proxies. These may be highly effective, and often firewalls, and proxies. These may be highly effective, and often
require no modification or configuration of end host software. Given require no modification or configuration of end-host software. Given
the mobile nature and dynamic connectivity of many end hosts, it is the mobile nature and dynamic connectivity of many end hosts, it is
optimistic for TCP implementers to assume the presence of such optimistic for TCP implementers to assume the presence of such
protective devices. TCP implementers should provide some means of protective devices. TCP implementers should provide some means of
defense to SYN flooding attacks in end host implementations. defense to SYN flooding attacks in end-host implementations.
Among end host modifications, the SYN cache and SYN cookie approaches Among end-host modifications, the SYN cache and SYN cookie approaches
seem to be the only viable techniques discovered to date. Increasing seem to be the only viable techniques discovered to date. Increasing
the backlog and reducing the SYN-RECEIVED timer are measurably the backlog and reducing the SYN-RECEIVED timer are measurably
problematic. The SYN cache implies a higher memory footprint than problematic. The SYN cache implies a higher memory footprint than
SYN cookies, however, SYN cookies may not be fully compatible with SYN cookies; however, SYN cookies may not be fully compatible with
some TCP options, and may hamper development of future TCP extensions some TCP options, and may hamper development of future TCP extensions
that require state. For these reasons, SYN cookies should not be that require state. For these reasons, SYN cookies should not be
enabled by default on systems that provide them. SYN caches do not enabled by default on systems that provide them. SYN caches do not
have the same negative implications and may be enabled as a default have the same negative implications and may be enabled as a default
mode of processing. mode of processing.
In October of 1996, Dave Borman implemented a SYN cache at BSDi for In October of 1996, Dave Borman implemented a SYN cache at BSDi for
BSD/OS, which was given to the community with no restrictions. This BSD/OS, which was given to the community with no restrictions. This
code seems to be the basis for the SYN cache implementations adopted code seems to be the basis for the SYN cache implementations adopted
later in other BSD variants. The cache was used when the backlog later in other BSD variants. The cache was used when the backlog
skipping to change at page 14, line 20 skipping to change at page 12, line 31
kernel for several years previous to 2.6.5. kernel for several years previous to 2.6.5.
When a SYN cache and/or SYN cookies are implemented with IPv6, the When a SYN cache and/or SYN cookies are implemented with IPv6, the
IPv6 flow label value used on the SYN-ACK should be consistent with IPv6 flow label value used on the SYN-ACK should be consistent with
the flow label used for the rest of the packets within that flow. the flow label used for the rest of the packets within that flow.
There have been implementation bugs that caused random flow labels to There have been implementation bugs that caused random flow labels to
be used in SYN-ACKs generated by SYN cache and SYN cookie code be used in SYN-ACKs generated by SYN cache and SYN cookie code
[MM05]. [MM05].
Beginning with Windows 2000, Microsoft's Windows operating systems Beginning with Windows 2000, Microsoft's Windows operating systems
have had a "TCP SYN attack protection" feature which can be toggled have had a "TCP SYN attack protection" feature, which can be toggled
on or off in the registry. This defaulted to off, until Windows 2003 on or off in the registry. This defaulted to off, until Windows 2003
SP1, in which it is on by default. With this feature enabled, when SP1, in which it is on by default. With this feature enabled, when
the number of half-open connections and half-open connections with the number of half-open connections and half-open connections with
retransmitted SYN-ACKs exceeds configurable thresholds, then the retransmitted SYN-ACKs exceeds configurable thresholds, then the
number of times which SYN-ACKs are retransmitted before giving up is number of times that SYN-ACKs are retransmitted before giving up is
reduced, and the "Route Cache Entry" creation is delayed, which reduced, and the "Route Cache Entry" creation is delayed, which
prevents some features (e.g. window scaling) from being used prevents some features (e.g., window scaling) from being used
[win2k3-wp]. [win2k3-wp].
Several vendors of commercial firewall products sell devices that can Several vendors of commercial firewall products sell devices that can
mitigate SYN flooding's effects on end hosts by proxying connections. mitigate SYN flooding's effects on end hosts by proxying connections.
Discovery and exploitation of the SYN flooding vulnerability in TCP's Discovery and exploitation of the SYN flooding vulnerability in TCP's
design provided a valuable lesson for protocol designers. The Stream design provided a valuable lesson for protocol designers. The Stream
Control Transmission Protocol [RFC2960], which was designed more Control Transmission Protocol [RFC2960], which was designed more
recently, incorporated a 4-way handshake with a stateless cookie- recently, incorporated a 4-way handshake with a stateless cookie-
based component for the listening end. In this way, the passive- based component for the listening end. In this way, the passive-
opening side has better evidence that the initiator really exists at opening side has better evidence that the initiator really exists at
the given address before it allocates any state. The Host Identity the given address before it allocates any state. The Host Identity
Protocol base exchange [MNJH07] is similarly designed as a 4-way Protocol base exchange [MNJH07] is similarly designed as a 4-way
handshake, but also involves a puzzle sent to the initiator which handshake, but also involves a puzzle sent to the initiator that must
must be solved before any state is reserved by the responder. The be solved before any state is reserved by the responder. The general
general concept of designing statelessness into protocol setup to concept of designing statelessness into protocol setup to avoid
avoid denial of service attacks has been discussed by Aura and denial-of-service attacks has been discussed by Aura and Nikander
Nikander [AN97]. [AN97].
5. Security Considerations 5. Security Considerations
The SYN flooding attack on TCP has been described in numerous other The SYN flooding attack on TCP has been described in numerous other
publications, and the details and code needed to perform the attack publications, and the details and code needed to perform the attack
have been easily available for years. Describing the attack in this have been easily available for years. Describing the attack in this
document does not pose any danger of further publicizing this document does not pose any danger of further publicizing this
weakness in unmodified TCP stacks. Several widely-deployed operating weakness in unmodified TCP stacks. Several widely deployed operating
systems implement the mitigation techniques that this document systems implement the mitigation techniques that this document
discusses for defeating SYN flooding attacks. In at least some discusses for defeating SYN flooding attacks. In at least some
cases, these operating systems do not enable these countermeasures by cases, these operating systems do not enable these countermeasures by
default, however, the mechanisms for defeating SYN flooding are well default; however, the mechanisms for defeating SYN flooding are well
deployed, and easily enabled by end-users. The publication of this deployed, and easily enabled by end-users. The publication of this
document should not influence the number of SYN flooding attacks document should not influence the number of SYN flooding attacks
observed, and might increase the robustness of the Internet to such observed, and might increase the robustness of the Internet to such
attacks by encouraging use of the commonly available mitigations. attacks by encouraging use of the commonly available mitigations.
6. IANA Considerations 6. Acknowledgements
This document does not update or create any IANA registries.
7. Acknowledgements
A conversation with Ted Faber was the impetus for writing this A conversation with Ted Faber was the impetus for writing this
document. Comments and suggestions from Joe Touch, Dave Borman, document. Comments and suggestions from Joe Touch, Dave Borman,
Fernando Gont, Jean-Baptiste Marchand, Christian Huitema, Caitlin Fernando Gont, Jean-Baptiste Marchand, Christian Huitema, Caitlin
Bestler, Pekka Savola, Andre Oppermann, Alfred Hoenes, Mark Allman, Bestler, Pekka Savola, Andre Oppermann, Alfred Hoenes, Mark Allman,
Lars Eggert, Pasi Eronen, Warren Kumari, David Malone, Ron Bonica, Lars Eggert, Pasi Eronen, Warren Kumari, David Malone, Ron Bonica,
and Lisa Dusseault were useful in strengthening this document. The and Lisa Dusseault were useful in strengthening this document. The
original work on TCP SYN cookies presented in Appendix A is due to original work on TCP SYN cookies presented in Appendix A is due to
D.J. Bernstein. D.J. Bernstein.
Work on this document was performed at NASA's Glenn Research Center. Work on this document was performed at NASA's Glenn Research Center.
Funding was partially provided by a combination of NASA's Advanced Funding was partially provided by a combination of NASA's Advanced
Communications, Navigation, and Surveillance Architectures and System Communications, Navigation, and Surveillance Architectures and System
Technologies (ACAST) project, the Sensis Corporation, NASA's Space Technologies (ACAST) project, the Sensis Corporation, NASA's Space
Communications Architecture Working Group, and NASA's Earth Science Communications Architecture Working Group, and NASA's Earth Science
Technology Office. Technology Office.
8. Informative References 7. Informative References
[AN97] Aura, T. and P. Nikander, "Stateless Connections", [AN97] Aura, T. and P. Nikander, "Stateless Connections",
Proceedings of the First International Conference on Proceedings of the First International Conference on
Information and Communication Security , 1997. Information and Communication Security , 1997.
[All07] Allman, M., "personal communication", February 2007. [All07] Allman, M., "personal communication", February 2007.
[B96] Bennahum, D., "PANIX ATTACK", MEME 2.12 [B96] Bennahum, D., "PANIX ATTACK", MEME 2.12, October 1996,
(http://memex.org/meme2-12.html), October 1996. <http://memex.org/meme2-12.html>.
[B97] Borman, D., "Re: SYN/RST cookies (was Re: a quick [B97] Borman, D., "Re: SYN/RST cookies (was Re: a quick
clarification...)", IETF tcp-impl mailing list, June 1997. clarification...)", IETF tcp-impl mailing list,
June 1997.
[CA-96.21] [CA-96.21] CERT, "CERT Advisory CA-1996-21 TCP SYN Flooding and IP
CERT, "CERT Advisory CA-1996-21 TCP SYN Flooding and IP
Spoofing Attacks", September 1996. Spoofing Attacks", September 1996.
[CB94] Cheswick, W. and S. Bellovin, "Firewalls and Internet [CB94] Cheswick, W. and S. Bellovin, "Firewalls and Internet
Security", ISBN: 0201633574, January 1994. Security", ISBN: 0201633574, January 1994.
[Eddy06] Eddy, W., "Defenses Against TCP SYN Flooding Attacks", [Eddy06] Eddy, W., "Defenses Against TCP SYN Flooding Attacks",
Cisco Internet Protocol Journal Volume 8, Number 4, Cisco Internet Protocol Journal Volume 8, Number 4,
December 2006. December 2006.
[GN00] Griffin, M. and J. Nelson, "T/TCP: TCP for Transactions", [GN00] Griffin, M. and J. Nelson, "T/TCP: TCP for
Linux Journal, February 2000. Transactions", Linux Journal, February 2000.
[Lem02] Lemon, J., "Resisting SYN Flood DoS Attacks with a SYN [Lem02] Lemon, J., "Resisting SYN Flood DoS Attacks with a SYN
Cache", BSDCON 2002, February 2002. Cache", BSDCON 2002, February 2002.
[MM05] McGann, O. and D. Malone, "Flow Label Filtering [MM05] McGann, O. and D. Malone, "Flow Label Filtering
Feasibility", European Conference on Computer Network Feasibility", European Conference on Computer Network
Defense 2005, December 2005. Defense 2005, December 2005.
[MNJH07] Moskowitz, R., Nikander, P., Jokela, P., and T. Henderson, [MNJH07] Moskowitz, R., Nikander, P., Jokela, P., and T.
"Host Identity Protocol", (draft-ietf-hip-base-07), Henderson, "Host Identity Protocol", Work in Progress,
February 2007. June 2007.
[P48-13] daemon9, route, and infinity, "Project Neptune", Phrack [P48-13] daemon9, route, and infinity, "Project Neptune", Phrack
Magazine, Volume 7, Issue 48, File 13 of 18, July 1996. Magazine, Volume 7, Issue 48, File 13 of 18, July 1996.
[RFC0793] Postel, J., "Transmission Control Protocol", STD 7, [RFC0793] Postel, J., "Transmission Control Protocol", STD 7,
RFC 793, September 1981. RFC 793, September 1981.
[RFC1144] Jacobson, V., "Compressing TCP/IP headers for low-speed [RFC1144] Jacobson, V., "Compressing TCP/IP headers for low-speed
serial links", RFC 1144, February 1990. serial links", RFC 1144, February 1990.
[RFC1321] Rivest, R., "The MD5 Message-Digest Algorithm", RFC 1321, [RFC1321] Rivest, R., "The MD5 Message-Digest Algorithm",
April 1992. RFC 1321, April 1992.
[RFC1644] Braden, B., "T/TCP -- TCP Extensions for Transactions [RFC1644] Braden, B., "T/TCP -- TCP Extensions for Transactions
Functional Specification", RFC 1644, July 1994. Functional Specification", RFC 1644, July 1994.
[RFC2827] Ferguson, P. and D. Senie, "Network Ingress Filtering: [RFC2827] Ferguson, P. and D. Senie, "Network Ingress Filtering:
Defeating Denial of Service Attacks which employ IP Source Defeating Denial of Service Attacks which employ IP
Address Spoofing", BCP 38, RFC 2827, May 2000. Source Address Spoofing", BCP 38, RFC 2827, May 2000.
[RFC2960] Stewart, R., Xie, Q., Morneault, K., Sharp, C., [RFC2960] Stewart, R., Xie, Q., Morneault, K., Sharp, C.,
Schwarzbauer, H., Taylor, T., Rytina, I., Kalla, M., Schwarzbauer, H., Taylor, T., Rytina, I., Kalla, M.,
Zhang, L., and V. Paxson, "Stream Control Transmission Zhang, L., and V. Paxson, "Stream Control Transmission
Protocol", RFC 2960, October 2000. Protocol", RFC 2960, October 2000.
[RFC3013] Killalea, T., "Recommended Internet Service Provider [RFC3013] Killalea, T., "Recommended Internet Service Provider
Security Services and Procedures", BCP 46, RFC 3013, Security Services and Procedures", BCP 46, RFC 3013,
November 2000. November 2000.
[RFC3704] Baker, F. and P. Savola, "Ingress Filtering for Multihomed [RFC3704] Baker, F. and P. Savola, "Ingress Filtering for
Networks", BCP 84, RFC 3704, March 2004. Multihomed Networks", BCP 84, RFC 3704, March 2004.
[RFC4413] West, M. and S. McCann, "TCP/IP Field Behavior", RFC 4413, [RFC4413] West, M. and S. McCann, "TCP/IP Field Behavior",
March 2006. RFC 4413, March 2006.
[SD01] Seddigh, N. and M. Devetsikiotis, "Studies of TCP's [SD01] Seddigh, N. and M. Devetsikiotis, "Studies of TCP's
Retransmission Timeout Mechanism", Proceedings of the 2001 Retransmission Timeout Mechanism", Proceedings of the
IEEE International Conference on Communications (ICC 2001 IEEE International Conference on Communications
2001), volume 6, pages 1834-1840, June 2001. (ICC 2001), volume 6, pages 1834-1840, June 2001.
[SKK+97] Schuba, C., Krsul, I., Kuhn, M., Spafford, E., Sundaram, [SKK+97] Schuba, C., Krsul, I., Kuhn, M., Spafford, E., Sundaram,
A., and D. Zamboni, "Analysis of a Denial of Service A., and D. Zamboni, "Analysis of a Denial of Service
Attack on TCP", Proceedings of the 1997 IEEE Symposium on Attack on TCP", Proceedings of the 1997 IEEE Symposium
Security and Privacy 1997. on Security and Privacy 1997.
[Ste95] Stevens, W. and G. Wright, "TCP/IP Illustrated, Volume 2: [Ste95] Stevens, W. and G. Wright, "TCP/IP Illustrated, Volume
The Implementation", January 1995. 2: The Implementation", January 1995.
[cr.yp.to] [cr.yp.to] Bernstein, D., "SYN cookies", visited in December 2005,
Bernstein, D., "URL: http://cr.yp.to/syncookies.html", <http://cr.yp.to/syncookies.html>.
visited in December 2005.
[win2k3-wp] [win2k3-wp] Microsoft Corporation, "Microsoft Windows Server 2003
Microsoft Corporation, "Microsoft Windows Server 2003
TCP/IP Implementation Details", White Paper, July 2005. TCP/IP Implementation Details", White Paper, July 2005.
Appendix A. SYN Cookies Description Appendix A. SYN Cookies Description
This information is taken from Bernstein's web page on SYN cookies This information is taken from Bernstein's web page on SYN cookies
[cr.yp.to]. This is a rewriting of the technical information on that [cr.yp.to]. This is a rewriting of the technical information on that
web page and not a full replacement. There are other slightly web page and not a full replacement. There are other slightly
different ways of implementing the SYN cookie concept than the exact different ways of implementing the SYN cookie concept than the exact
means described here, although the basic idea of encoding data into means described here, although the basic idea of encoding data into
the SYN-ACK sequence number is constant. the SYN-ACK sequence number is constant.
skipping to change at page 21, line 20 skipping to change at page 17, line 20
establishing the secret values is also not discussed). The remainder establishing the secret values is also not discussed). The remainder
of this section is excerpted from Bernstein's email [cr.yp.to]: of this section is excerpted from Bernstein's email [cr.yp.to]:
Here's what an implementation would involve: Here's what an implementation would involve:
Maintain two (constant) secret keys, sec1 and sec2. Maintain two (constant) secret keys, sec1 and sec2.
Maintain a (constant) sorted table of 8 common MSS values, Maintain a (constant) sorted table of 8 common MSS values,
msstab[8]. msstab[8].
Keep track of a ``last overflow time.'' Keep track of a "last overflow time".
Maintain a counter that increases slowly over time and never Maintain a counter that increases slowly over time and never
repeats, such as ``number of seconds since 1970, shifted right repeats, such as "number of seconds since 1970, shifted right 6
6 bits.'' bits".
When a SYN comes in from (saddr,sport) to (daddr,dport) with When a SYN comes in from (saddr,sport) to (daddr,dport) with
ISN x, find the largest i for which msstab[i] <= the incoming ISN x, find the largest i for which msstab[i] <= the incoming
MSS. Compute MSS. Compute
z = MD5(sec1,saddr,sport,daddr,dport,sec1) z = MD5(sec1,saddr,sport,daddr,dport,sec1)
+ x + x
+ (counter << 24) + (counter << 24)
+ (MD5(sec2,counter,saddr,sport,daddr,dport,sec2) % (1 << + (MD5(sec2,counter,saddr,sport,daddr,dport,sec2) % (1 <<
24)) 24))
and then and then
y = (i << 29) + (z % (1 << 29)) y = (i << 29) + (z % (1 << 29))
Create a TCB as usual, with y as our ISN. Send back a SYNACK. Create a TCB as usual, with y as our ISN. Send back a SYNACK.
Exception: _If_ we're out of memory for TCBs, set the ``last Exception: _If_ we're out of memory for TCBs, set the "last
overflow time'' to the current time. Send the SYNACK anyway, overflow time" to the current time. Send the SYNACK anyway,
with all fancy options turned off. with all fancy options turned off.
When an ACK comes back, follow this procedure to find a TCB: When an ACK comes back, follow this procedure to find a TCB:
(1) Look for a (saddr,sport,daddr,dport) TCB. If it's (1) Look for a (saddr,sport,daddr,dport) TCB. If it's there,
there, done. done.
(2) If the ``last overflow time'' is earlier than a few (2) If the "last overflow time" is earlier than a few minutes
minutes ago, give up. ago, give up.
(3) Figure out whether our alleged ISN makes sense. This (3) Figure out whether our alleged ISN makes sense. This
means recomputing y as above, for each of the counters that means recomputing y as above, for each of the counters
could have been used in the last few minutes (say, the last that could have been used in the last few minutes (say,
four counters), and seeing whether any of the y's match the the last four counters), and seeing whether any of the y's
ISN in the bottom 29 bits. If none of them do, give up. match the ISN in the bottom 29 bits. If none of them do,
give up.
(4) Create a new TCB. The top three bits of our ISN give a (4) Create a new TCB. The top three bits of our ISN give a
usable MSS. Turn off all fancy options. usable MSS. Turn off all fancy options.
Author's Address Author's Address
Wesley M. Eddy Wesley M. Eddy
Verizon Federal Network Systems Verizon Federal Network Systems
NASA Glenn Research Center NASA Glenn Research Center
21000 Brookpark Rd, MS 54-5 21000 Brookpark Rd, MS 54-5
Cleveland, OH 44135 Cleveland, OH 44135
Phone: 216-433-6682 Phone: 216-433-6682
Email: weddy@grc.nasa.gov EMail: weddy@grc.nasa.gov
Full Copyright Statement Full Copyright Statement
Copyright (C) The IETF Trust (2007). Copyright (C) The IETF Trust (2007).
This document is subject to the rights, licenses and restrictions This document is subject to the rights, licenses and restrictions
contained in BCP 78, and except as set forth therein, the authors contained in BCP 78, and except as set forth therein, the authors
retain all their rights. retain all their rights.
This document and the information contained herein are provided on an This document and the information contained herein are provided on an
skipping to change at page 24, line 45 skipping to change at page 19, line 45
such proprietary rights by implementers or users of this such proprietary rights by implementers or users of this
specification can be obtained from the IETF on-line IPR repository at specification can be obtained from the IETF on-line IPR repository at
http://www.ietf.org/ipr. http://www.ietf.org/ipr.
The IETF invites any interested party to bring to its attention any The IETF invites any interested party to bring to its attention any
copyrights, patents or patent applications, or other proprietary copyrights, patents or patent applications, or other proprietary
rights that may cover technology that may be required to implement rights that may cover technology that may be required to implement
this standard. Please address the information to the IETF at this standard. Please address the information to the IETF at
ietf-ipr@ietf.org. ietf-ipr@ietf.org.
Acknowledgment Acknowledgement
Funding for the RFC Editor function is provided by the IETF Funding for the RFC Editor function is currently provided by the
Administrative Support Activity (IASA). Internet Society.
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