Internet Engineering Task Force (IETF) F. Gont

Internet Engineering Task Force (IETF) F. Gont
Request for Comments: 5927 UTN/FRH
Category: Informational July 2010
ISSN: 2070-1721

ICMP Attacks against TCP

Abstract

This document discusses the use of the Internet Control Message
Protocol (ICMP) to perform a variety of attacks against the
Transmission Control Protocol (TCP). Additionally, this document
describes a number of widely implemented modifications to TCP's
handling of ICMP error messages that help to mitigate these issues.

Status of This Memo

This document is not an Internet Standards Track specification; it is
published for informational purposes.

This document is a product of the Internet Engineering Task Force
(IETF). It represents the consensus of the IETF community. It has
received public review and has been approved for publication by the
Internet Engineering Steering Group (IESG). Not all documents
approved by the IESG are a candidate for any level of Internet
Standard; see Section 2 of RFC 5741.

Information about the current status of this document, any errata,
and how to provide feedback on it may be obtained at
http://www.rfc-editor.org/info/rfc5927.

Gont Informational [Page 1]

RFC 5927 ICMP Attacks against TCP July 2010

Copyright Notice

Copyright (c) 2010 IETF Trust and the persons identified as the
document authors. All rights reserved.

This document is subject to BCP 78 and the IETF Trust's Legal
Provisions Relating to IETF Documents
(http://trustee.ietf.org/license-info) in effect on the date of
publication of this document. Please review these documents
carefully, as they describe your rights and restrictions with respect
to this document. Code Components extracted from this document must
include Simplified BSD License text as described in Section 4.e of
the Trust Legal Provisions and are provided without warranty as
described in the Simplified BSD License.

This document may contain material from IETF Documents or IETF
Contributions published or made publicly available before November
10, 2008. The person(s) controlling the copyright in some of this
material may not have granted the IETF Trust the right to allow
modifications of such material outside the IETF Standards Process.
Without obtaining an adequate license from the person(s) controlling
the copyright in such materials, this document may not be modified
outside the IETF Standards Process, and derivative works of it may
not be created outside the IETF Standards Process, except to format
it for publication as an RFC or to translate it into languages other
than English.

Gont Informational [Page 2]

RFC 5927 ICMP Attacks against TCP July 2010

Table of Contents

1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 4
2. Background . . . . . . . . . . . . . . . . . . . . . . . . . . 5
2.1. The Internet Control Message Protocol (ICMP) . . . . . . . 5
2.1.1. ICMP for IP version 4 (ICMPv4) . . . . . . . . . . . . 5
2.1.2. ICMP for IP version 6 (ICMPv6) . . . . . . . . . . . . 6
2.2. Handling of ICMP Error Messages . . . . . . . . . . . . . 6
2.3. Handling of ICMP Error Messages in the Context of IPsec . 7
3. Constraints in the Possible Solutions . . . . . . . . . . . . 8
4. General Counter-Measures against ICMP Attacks . . . . . . . . 10
4.1. TCP Sequence Number Checking . . . . . . . . . . . . . . . 10
4.2. Port Randomization . . . . . . . . . . . . . . . . . . . . 11
4.3. Filtering ICMP Error Messages Based on the ICMP Payload . 11
5. Blind Connection-Reset Attack . . . . . . . . . . . . . . . . 12
5.1. Description . . . . . . . . . . . . . . . . . . . . . . . 12
5.2. Attack-Specific Counter-Measures . . . . . . . . . . . . . 13
6. Blind Throughput-Reduction Attack . . . . . . . . . . . . . . 16
6.1. Description . . . . . . . . . . . . . . . . . . . . . . . 16
6.2. Attack-Specific Counter-Measures . . . . . . . . . . . . . 16
7. Blind Performance-Degrading Attack . . . . . . . . . . . . . . 16
7.1. Description . . . . . . . . . . . . . . . . . . . . . . . 16
7.2. Attack-Specific Counter-Measures . . . . . . . . . . . . . 18
7.3. The Counter-Measure for the PMTUD Attack in Action . . . . 22
7.3.1. Normal Operation for Bulk Transfers . . . . . . . . . 22
7.3.2. Operation during Path-MTU Changes . . . . . . . . . . 24
7.3.3. Idle Connection Being Attacked . . . . . . . . . . . . 25
7.3.4. Active Connection Being Attacked after Discovery
of the Path-MTU . . . . . . . . . . . . . . . . . . . 26
7.3.5. TCP Peer Attacked when Sending Small Packets Just
after the Three-Way Handshake . . . . . . . . . . . . 26
7.4. Pseudo-Code for the Counter-Measure for the Blind
Performance-Degrading Attack . . . . . . . . . . . . . . . 27
8. Security Considerations . . . . . . . . . . . . . . . . . . . 30
9. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 32
10. References . . . . . . . . . . . . . . . . . . . . . . . . . . 32
10.1. Normative References . . . . . . . . . . . . . . . . . . . 32
10.2. Informative References . . . . . . . . . . . . . . . . . . 33

Gont Informational [Page 3]

RFC 5927 ICMP Attacks against TCP July 2010

1. Introduction

ICMP [RFC0792] [RFC4443] is a fundamental part of the TCP/IP protocol
suite, and is used mainly for reporting network error conditions.
However, the current specifications do not recommend any kind of
validation checks on the received ICMP error messages, thus allowing
a variety of attacks against TCP [RFC0793] by means of ICMP, which
include blind connection-reset, blind throughput-reduction, and blind
performance-degrading attacks. All of these attacks can be performed
even when the attacker is off-path, without the need to sniff the
packets that correspond to the attacked TCP connection.

While the possible security implications of ICMP have been known in
the research community for a long time, there has never been an
official proposal on how to deal with these vulnerabilities. In
2005, a disclosure process was carried out by the UK's National
Infrastructure Security Co-ordination Centre (NISCC) (now CPNI,
Centre for the Protection of National Infrastructure), with the
collaboration of other computer emergency response teams. A large
number of implementations were found vulnerable to either all or a
subset of the attacks discussed in this document [NISCC][US-CERT].
The affected systems ranged from TCP/IP implementations meant for
desktop computers, to TCP/IP implementations meant for core Internet
routers.

It is clear that implementations should be more cautious when
processing ICMP error messages, to eliminate or mitigate the use of
ICMP to perform attacks against TCP [RFC4907].

This document aims to raise awareness of the use of ICMP to perform a
variety of attacks against TCP, and discusses several counter-
measures that eliminate or minimize the impact of these attacks.
Most of the these counter-measures can be implemented while still
remaining compliant with the current specifications, as they simply
describe reasons for not taking the advice provided in the
specifications in terms of "SHOULDs", but still comply with the
requirements stated as "MUSTs".

We note that the counter-measures discussed in this document are not
part of standard TCP behavior, and this document does not change that
state of affairs. The consensus of the TCPM WG (TCP Maintenance and
Minor Extensions Working Group) was to document this widespread
implementation of nonstandard TCP behavior but to not change the TCP
standard.

Section 2 provides background information on ICMP. Section 3
discusses the constraints in the general counter-measures that can be
implemented against the attacks described in this document.

Gont Informational [Page 4]

RFC 5927 ICMP Attacks against TCP July 2010

Section 4 describes several general validation checks that can be
implemented to mitigate any ICMP-based attack. Finally, Section 5,
Section 6, and Section 7, discuss a variety of ICMP attacks that can
be performed against TCP, and describe attack-specific counter-
measures that eliminate or greatly mitigate their impact.

The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in RFC 2119 [RFC2119].

2. Background

2.1. The Internet Control Message Protocol (ICMP)

The Internet Control Message Protocol (ICMP) is used in the Internet
architecture mainly to perform the fault-isolation function, that is,
the group of actions that hosts and routers take to determine that
there is some network failure [RFC0816].

When an intermediate router detects a network problem while trying to
forward an IP packet, it will usually send an ICMP error message to
the source system, to inform the source system of the network problem
taking place. In the same way, there are a number of scenarios in
which an end-system may generate an ICMP error message if it finds a
problem while processing a datagram. The received ICMP errors are
handed to the corresponding transport-protocol instance, which will
usually perform a fault recovery function.

It is important to note that ICMP error messages are transmitted
unreliably and may be discarded due to data corruption, network
congestion, or rate-limiting. Thus, while they provide useful
information, upper-layer protocols cannot depend on ICMP for correct
operation.

It should be noted that there are no timeliness requirements for ICMP
error messages. ICMP error messages could be delayed for various
reasons, and at least in theory could be received with an arbitrarily
long delay. For example, there are no existing requirements that a
router flush any queued ICMP error messages when it is rebooted.

2.1.1. ICMP for IP version 4 (ICMPv4)

[RFC0792] specifies the Internet Control Message Protocol (ICMP) to
be used with the Internet Protocol version 4 (IPv4) -- henceforth
"ICMPv4". It defines, among other things, a number of error messages
that can be used by end-systems and intermediate systems to report
errors to the sending system. The Host Requirements RFC [RFC1122]

Gont Informational [Page 5]

RFC 5927 ICMP Attacks against TCP July 2010

classifies ICMPv4 error messages into those that indicate "soft
errors", and those that indicate "hard errors", thus roughly defining
the semantics of them.

The ICMPv4 specification [RFC0792] also defines the ICMPv4 Source
Quench message (type 4, code 0), which is meant to provide a
mechanism for flow control and congestion control.

[RFC1191] defines a mechanism called "Path MTU Discovery" (PMTUD),
which makes use of ICMPv4 error messages of type 3 (Destination
Unreachable), code 4 (fragmentation needed and DF bit set) to allow
systems to determine the MTU of an arbitrary internet path.

Finally, [RFC4884] redefines selected ICMPv4 messages to include an
extension structure and a length attribute, such that those ICMPv4
messages can carry additional information by encoding that
information in the extension structure.

Appendix D of [RFC4301] provides information about which ICMPv4 error
messages are produced by hosts, intermediate routers, or both.

2.1.2. ICMP for IP version 6 (ICMPv6)

[RFC4443] specifies the Internet Control Message Protocol (ICMPv6) to
be used with the Internet Protocol version 6 (IPv6) [RFC2460].

[RFC4443] defines the "Packet Too Big" (type 2, code 0) error
message, which is analogous to the ICMPv4 "fragmentation needed and
DF bit set" (type 3, code 4) error message. [RFC1981] defines the
Path MTU Discovery mechanism for IP version 6, which makes use of
these messages to determine the MTU of an arbitrary internet path.

Finally, [RFC4884] redefines selected ICMPv6 messages to include an
extension structure and a length attribute, such that those ICMPv6
messages can carry additional information by encoding that
information in the extension structure.

Appendix D of [RFC4301] provides information about which ICMPv6 error
messages are produced by hosts, intermediate routers, or both.

2.2. Handling of ICMP Error Messages

The Host Requirements RFC [RFC1122] states in Section 4.2.3.9 that
TCP MUST act on an ICMP error message passed up from the IP layer,
directing it to the connection that triggered the error.

Gont Informational [Page 6]

RFC 5927 ICMP Attacks against TCP July 2010

In order to allow ICMP messages to be demultiplexed by the receiving
system, part of the original packet that triggered the message is
included in the payload of the ICMP error message. Thus, the
receiving system can use that information to match the ICMP error to
the transport protocol instance that triggered it.

Neither the Host Requirements RFC [RFC1122] nor the original TCP
specification [RFC0793] recommends any validation checks on the
received ICMP messages. Thus, as long as the ICMP payload contains
the information that identifies an existing communication instance,
it will be processed by the corresponding transport-protocol
instance, and the corresponding action will be performed.

Therefore, in the case of TCP, an attacker could send a crafted ICMP
error message to the attacked system, and, as long as he is able to
guess the four-tuple (i.e., Source IP Address, Source TCP port,
Destination IP Address, and Destination TCP port) that identifies the
communication instance to be attacked, he will be able to use ICMP to
perform a variety of attacks.

Generally, the four-tuple required to perform these attacks is not
known. However, as discussed in [Watson] and [RFC4953], there are a
number of scenarios (notably that of TCP connections established
between two BGP routers [RFC4271]) in which an attacker may be able
to know or guess the four-tuple that identifies a TCP connection. In
such a case, if we assume the attacker knows the two systems involved
in the TCP connection to be attacked, both the client-side and the
server-side IP addresses could be known or be within a reasonable
number of possibilities. Furthermore, as most Internet services use
the so-called "well-known" ports, only the client port number might
need to be guessed. In such a scenario, an attacker would need to
send, in principle, at most 65536 packets to perform any of the
attacks described in this document. These issues are exacerbated by
the fact that most systems choose the port numbers they use for
outgoing connections from a subset of the whole port number space,
thus reducing the amount of work needed to successfully perform these
attacks.

The need to be more cautious when processing received ICMP error
messages in order to mitigate or eliminate the impact of the attacks
described in this RFC has been documented by the Internet
Architecture Board (IAB) in [RFC4907].

2.3. Handling of ICMP Error Messages in the Context of IPsec

Section 5.2 of [RFC4301] describes the processing of inbound IP
traffic in the case of "unprotected-to-protected". In the case of
ICMP, when an unprotected ICMP error message is received, it is

Gont Informational [Page 7]

RFC 5927 ICMP Attacks against TCP July 2010

matched to the corresponding security association by means of the SPI
(Security Parameters Index) included in the payload of the ICMP error
message. Then, local policy is applied to determine whether to
accept or reject the message and, if accepted, what action to take as
a result. For example, if an ICMP Destination Unreachable message is
received, the implementation must decide whether to act on it, reject
it, or act on it with constraints. Section 8 ("Path MTU/DF
Processing") discusses the processing of unauthenticated ICMPv4
"fragmentation needed and DF bit set" (type 3, code 4) and ICMPv6
"Packet Too Big" (type 2, code 0) messages when an IPsec
implementation is configured to process (vs. ignore) such messages.

Section 6.1.1 of [RFC4301] notes that processing of unauthenticated
ICMP error messages may result in denial or degradation of service,
and therefore it would be desirable to ignore such messages.
However, it also notes that in many cases, ignoring these ICMP
messages can degrade service, e.g., because of a failure to process
PMTUD and redirection messages, and therefore there is also a
motivation for accepting and acting upon them. It finally states
that to accommodate both ends of this spectrum, a compliant IPsec
implementation MUST permit a local administrator to configure an
IPsec implementation to accept or reject unauthenticated ICMP
traffic, and that this control MUST be at the granularity of ICMP
type and MAY be at the granularity of ICMP type and code.
Additionally, an implementation SHOULD incorporate mechanisms and
parameters for dealing with such traffic.

Thus, the policy to apply for the processing of unprotected ICMP
error messages is left up to the implementation and administrator.

3. Constraints in the Possible Solutions

If a host wants to perform validation checks on the received ICMP
error messages before acting on them, it is limited by the piece of
the packet that triggered the error that the sender of the ICMP error
message chose to include in the ICMP payload. This constrains the
possible validation checks, as the number of bytes of the packet that
triggered the error message that is included in the ICMP payload is
limited.

For ICMPv4, [RFC0792] states that the IP header plus the first
64 bits of the packet that triggered the ICMPv4 message are to be
included in the payload of the ICMPv4 error message. Thus, it is
assumed that all data needed to identify a transport protocol
instance and process the ICMPv4 error message is contained in the
first 64 bits of the transport protocol header. Section 3.2.2 of
[RFC1122] states that "the Internet header and at least the first 8
data octets of the datagram that triggered the error" are to be

Gont Informational [Page 8]

RFC 5927 ICMP Attacks against TCP July 2010

included in the payload of ICMPv4 error messages, and that "more than
8 octets MAY be sent", thus allowing implementations to include more
data from the original packet than those required by the original
ICMPv4 specification. The "Requirements for IP Version 4 Routers"
RFC [RFC1812] states that ICMPv4 error messages "SHOULD contain as
much of the original datagram as possible without the length of the
ICMP datagram exceeding 576 bytes".

Thus, for ICMPv4 messages generated by hosts, we can only expect to
get the entire IP header of the original packet, plus the first
64 bits of its payload. For TCP, this means that the only fields
that will be included in the ICMPv4 payload are the source port
number, the destination port number, and the 32-bit TCP sequence
number. This clearly imposes a constraint on the possible validation
checks that can be performed, as there is not much information
available on which to perform them.

This means, for example, that even if TCP were signing its segments
by means of the TCP MD5 signature option [RFC2385], this mechanism
could not be used as a counter-measure against ICMP-based attacks,
because, as ICMP messages include only a piece of the TCP segment
that triggered the error, the MD5 [RFC1321] signature could not be
recalculated. In the same way, even if the attacked peer were
authenticating its packets at the IP layer [RFC4301], because only a
part of the original IP packet would be available, the signature used
for authentication could not be recalculated, and thus the
authentication header in the original packet could not be used as a
counter-measure for ICMP-based attacks against TCP.

[RFC4884] updated [RFC0792] and specified that ICMPv4 Destination
Unreachable (type 3), Time Exceeded (type 11), and Parameter Problem
(type 12) messages that have an ICMP Extension Structure appended
include at least 128 octets in the "original datagram" field. This
would improve the situation, but at the time of this writing,
[RFC4884] is not yet widely deployed for end-systems.

For IPv6, the payload of ICMPv6 error messages includes as many
octets from the IPv6 packet that triggered the ICMPv6 error message
as will fit without making the resulting ICMPv6 error message exceed
the minimum IPv6 MTU (1280 octets) [RFC4443]. Thus, more information
is available than in the IPv4 case.

Hosts could require ICMP error messages to be authenticated
[RFC4301], in order to act upon them. However, while this
requirement could make sense for those ICMP error messages sent by
hosts, it would not be feasible for those ICMP error messages
generated by routers, as this would imply either that the attacked
system should have a security association [RFC4301] with every

Gont Informational [Page 9]

RFC 5927 ICMP Attacks against TCP July 2010

existing intermediate system, or that it should be able to establish
one dynamically. Current levels of deployment of protocols for
dynamic establishment of security associations makes this unfeasible.
Additionally, this would require routers to use certificates with
paths compatible for all hosts on the network. Finally, there may be
some scenarios, such as embedded devices, in which the processing
power requirements of authentication might not allow IPsec
authentication to be implemented effectively.

4. General Counter-Measures against ICMP Attacks

The following subsections describe a number of mitigation techniques
that help to eliminate or mitigate the impact of the attacks
discussed in this document. Rather than being alternative counter-
measures, they can be implemented together to increase the protection
against these attacks.

4.1. TCP Sequence Number Checking

The current specifications do not impose any validity checks on the
TCP segment that is contained in the ICMP payload. For instance, no
checks are performed to verify that a received ICMP error message has
been triggered by a segment that was "in flight" to the destination.
Thus, even stale ICMP error messages will be acted upon.

Many TCP implementations have incorporated a validation check such
that they react only to those ICMP error messages that appear to
relate to segments currently "in flight" to the destination system.
These implementations check that the TCP sequence number contained in
the payload of the ICMP error message is within the range
SND.UNA =< SEG.SEQ < SND.NXT. This means that they require that the
sequence number be within the range of the data already sent but not
yet acknowledged. If an ICMP error message does not pass this check,
it is discarded.

Even if an attacker were able to guess the four-tuple that identifies
the TCP connection, this additional check would reduce the
possibility of considering a spoofed ICMP packet as valid to
Flight_Size/2^^32 (where Flight_Size is the number of data bytes
already sent to the remote peer, but not yet acknowledged [RFC5681]).
For connections in the SYN-SENT or SYN-RECEIVED states, this would
reduce the possibility of considering a spoofed ICMP packet as valid
to 1/2^^32. For a TCP endpoint with no data "in flight", this would
completely eliminate the possibility of success of these attacks.

This validation check has been implemented in Linux [Linux] for many
years, in OpenBSD [OpenBSD] since 2004, and in FreeBSD [FreeBSD] and
NetBSD [NetBSD] since 2005.

Gont Informational [Page 10]

RFC 5927 ICMP Attacks against TCP July 2010

It is important to note that while this check greatly increases the
number of packets required to perform any of the attacks discussed in
this document, this may not be enough in those scenarios in which
bandwidth is easily available and/or large TCP windows [RFC1323] are
in use. Additionally, this validation check does not help to prevent
on-path attacks, that is, attacks performed in scenarios in which the
attacker can sniff the packets that correspond to the target TCP
connection.

It should be noted that, as there are no timeliness requirements for
ICMP error messages, the TCP Sequence Number check described in this
section might cause legitimate ICMP error messages to be discarded.
Also, even if this check is enforced, TCP might end up responding to
stale ICMP error messages (e.g., if the Sequence Number for the
corresponding direction of the data transfer wraps around).

4.2. Port Randomization

As discussed in the previous sections, in order to perform any of the
attacks described in this document, an attacker would need to guess
(or know) the four-tuple that identifies the connection to be
attacked. Increasing the port number range used for outgoing TCP
connections, and randomizing the port number chosen for each outgoing
TCP connection, would make it harder for an attacker to perform any
of the attacks discussed in this document.

[PORT-RANDOM] recommends that transport protocols randomize the
ephemeral ports used by clients, and proposes a number of
randomization algorithms.

4.3. Filtering ICMP Error Messages Based on the ICMP Payload

The source address of ICMP error messages does not need to be spoofed
to perform the attacks described in this document, as the ICMP error
messages might legitimately come from an intermediate system.
Therefore, simple filtering based on the source address of ICMP error
messages does not serve as a counter-measure against these attacks.
However, a more advanced packet filtering can be implemented in
middlebox devices such as firewalls and NATs. Middleboxes
implementing such advanced filtering look at the payload of the ICMP
error messages, and perform ingress and egress packet filtering based
on the source address of the IP header contained in the payload of
the ICMP error message. As the source address contained in the
payload of the ICMP error message does need to be spoofed to perform
the attacks described in this document, this kind of advanced
filtering serves as a counter-measure against these attacks. As with
traditional egress filtering [IP-filtering], egress filtering based
on the ICMP payload can help to prevent users of the network being

Gont Informational [Page 11]

RFC 5927 ICMP Attacks against TCP July 2010

protected by the firewall from successfully performing ICMP attacks
against TCP connections established between external systems.
Additionally, ingress filtering based on the ICMP payload can prevent
TCP connections established between internal systems from being
attacked by external systems. [ICMP-Filtering] provides examples of
ICMP filtering based on the ICMP payload.

This filtering technique has been implemented in OpenBSD's Packet
Filter [OpenBSD-PF], which has in turn been ported to a number of
systems, including FreeBSD [FreeBSD].

5. Blind Connection-Reset Attack

5.1. Description

When TCP is handed an ICMP error message, it will perform its fault
recovery function, as follows:

o If the network problem being reported is a "hard error", TCP will
abort the corresponding connection.

o If the network problem being reported is a "soft error", TCP will
just record this information, and repeatedly retransmit its data
until they either get acknowledged, or the connection times out.

The Host Requirements RFC [RFC1122] states (in Section 4.2.3.9) that
a host SHOULD abort the corresponding connection when receiving an
ICMPv4 error message that indicates a "hard error", and states that
ICMPv4 error messages of type 3 (Destination Unreachable), codes 2
(protocol unreachable), 3 (port unreachable), and 4 (fragmentation
needed and DF bit set) should be considered as indicating "hard
errors". In the case of ICMPv4 port unreachables, the specifications
are ambiguous, as Section 4.2.3.9 of [RFC1122] states that TCP SHOULD
abort the corresponding connection in response to them, but
Section 3.2.2.1 of the same RFC ([RFC1122]) states that TCP MUST
abort the connection in response to them.

While [RFC4443] did not exist when [RFC1122] was published, one could
extrapolate the concept of "hard errors" to ICMPv6 error messages of
type 1 (Destination Unreachable), codes 1 (communication with
destination administratively prohibited), and 4 (port unreachable).

Thus, an attacker could use ICMP to perform a blind connection-reset
attack by sending any ICMP error message that indicates a "hard
error" to either of the two TCP endpoints of the connection. Because
of TCP's fault recovery policy, the connection would be immediately
aborted.

Gont Informational [Page 12]

RFC 5927 ICMP Attacks against TCP July 2010

Some stacks are known to extrapolate ICMP "hard errors" across TCP
connections, increasing the impact of this attack, as a single ICMP
packet could bring down all the TCP connections between the
corresponding peers.

It is important to note that even if TCP itself were protected
against the blind connection-reset attack described in [Watson] and
[TCPM-TCPSECURE] by means of authentication at the network layer
[RFC4301], by means of the TCP MD5 signature option [RFC2385], by
means of the TCP-AO [RFC5925], or by means of the mechanism specified
in [TCPM-TCPSECURE], the blind connection-reset attack described in
this document would still succeed.

5.2. Attack-Specific Counter-Measures

An analysis of the circumstances in which ICMP messages that indicate
"hard errors" may be received can shed some light on opportunities to
mitigate the impact of ICMP-based blind connection-reset attacks.

ICMPv4 type 3 (Destination Unreachable), code 2 (protocol
unreachable)

This ICMP error message indicates that the host sending the ICMP
error message received a packet meant for a transport protocol it
does not support. For connection-oriented protocols such as TCP,
one could expect to receive such an error as the result of a
connection-establishment attempt. However, it would be strange to
get such an error during the life of a connection, as this would
indicate that support for that transport protocol has been removed
from the system sending the error message during the life of the
corresponding connection.

ICMPv4 type 3 (Destination Unreachable), code 3 (port unreachable)

This error message indicates that the system sending the ICMP
error message received a packet meant for a socket (IP address,
port number) on which there is no process listening. Those
transport protocols that have their own mechanisms for signaling
this condition should not be receiving these error messages, as
the protocol would signal the port unreachable condition by means
of its own mechanisms. Assuming that once a connection is
established it is not usual for the transport protocol to change
(or be reloaded), it should be unusual to get these error
messages.

ICMPv4 type 3 (Destination Unreachable), code 4 (fragmentation needed
and DF bit set)

Gont Informational [Page 13]

RFC 5927 ICMP Attacks against TCP July 2010

This error message indicates that an intermediate node needed to
fragment a datagram, but the DF (Don't Fragment) bit in the IP
header was set. It is considered a "soft error" when TCP
implements PMTUD, and a "hard error" if TCP does not implement
PMTUD. Those TCP/IP stacks that do not implement PMTUD (or have
disabled it) but support IP fragmentation/reassembly should not be
sending their IP packets with the DF bit set, and thus should not
be receiving these ICMP error messages. Some TCP/IP stacks that
do not implement PMTUD and that do not support IP fragmentation/
reassembly are known to send their packets with the DF bit set,
and thus could legitimately receive these ICMP error messages.

ICMPv6 type 1 (Destination Unreachable), code 1 (communication with
destination administratively prohibited)

This error message indicates that the destination is unreachable
because of an administrative policy. For connection-oriented
protocols such as TCP, one could expect to receive such an error
as the result of a connection-establishment attempt. Receiving
such an error for a connection in any of the synchronized states
would mean that the administrative policy changed during the life
of the connection. However, in the same way this error condition
(which was not present when the connection was established)
appeared, it could get solved in the near term.

ICMPv6 type 1 (Destination Unreachable), code 4 (port unreachable)

This error message is analogous to the ICMPv4 type 3 (Destination
Unreachable), code 3 (port unreachable) error message discussed
above. Therefore, the same considerations apply.

The Host Requirements RFC [RFC1122] states in Section 4.2.3.9 that
TCP SHOULD abort the corresponding connection in response to ICMPv4
messages of type 3 (Destination Unreachable), codes 2 (protocol
unreachable), 3 (port unreachable), and 4 (fragmentation needed and
DF bit set). However, Section 3.2.2.1 states that TCP MUST accept an
ICMPv4 port unreachable (type 3, code 3) for the same purpose as a
RST. Therefore, for ICMPv4 messages of type 3, codes 2 and 4, there
is room to go against the advice provided in the existing
specifications, while in the case of ICMPv4 messages of type 3,
code 3, there is ambiguity in the specifications that may or may not
provide some room to go against that advice.

Based on this analysis, most popular TCP implementations treat all
ICMP "hard errors" received for connections in any of the
synchronized states (ESTABLISHED, FIN-WAIT-1, FIN-WAIT-2, CLOSE-WAIT,
CLOSING, LAST-ACK, or TIME-WAIT) as "soft errors". That is, they do
not abort the corresponding connection upon receipt of them.

Gont Informational [Page 14]

RFC 5927 ICMP Attacks against TCP July 2010

Additionally, they do not extrapolate ICMP errors across TCP
connections. This policy is based on the premise that TCP should be
as robust as possible. Aborting the connection would be to ignore
the valuable feature of the Internet -- that for many internal
failures, it reconstructs its function without any disruption of the
endpoints [RFC0816].

It should be noted that treating ICMP "hard errors" as "soft errors"
for connections in any of the synchronized states may prevent TCP
from responding quickly to a legitimate ICMP error message.

It is interesting to note that, as ICMP error messages are
transmitted unreliably, transport protocols should not depend on them
for correct functioning. In the event one of these messages were
legitimate, the corresponding connection would eventually time out.
Also, applications may still be notified asynchronously about the
error condition, and thus may still abort their connections on their
own if they consider it appropriate.

In scenarios such as that in which an intermediate system sets the DF
bit in the segments transmitted by a TCP that does not implement
PMTUD, or the TCP at one of the endpoints of the connection is
dynamically disabled, TCP would only abort the connection after a
USER TIMEOUT [RFC0793], losing responsiveness. However, these
scenarios are very unlikely in production environments, and it is
probably preferable to potentially lose responsiveness for the sake
of robustness. It should also be noted that applications may still
be notified asynchronously about the error condition, and thus may
still abort their connections on their own if they consider it
appropriate.

In scenarios of multipath routing or route changes, failures in some
(but not all) of the paths may elicit ICMP error messages that would
likely not cause a connection abort if any of the counter-measures
described in this section were implemented. However, aborting the
connection would be to ignore the valuable feature of the Internet --
that for many internal failures, it reconstructs its function without
any disruption of the endpoints [RFC0816]. That is, communication
should survive if there is still a working path to the destination
system [DClark]. Additionally, applications may still be notified
asynchronously about the error condition, and thus may still abort
their connections on their own if they consider it appropriate.

This counter-measure has been implemented in BSD-derived TCP/IP
implementations (e.g., [FreeBSD], [NetBSD], and [OpenBSD]) for more
than ten years [Wright][McKusick]. The Linux kernel has also
implemented this policy for more than ten years [Linux].

Gont Informational [Page 15]

RFC 5927 ICMP Attacks against TCP July 2010

6. Blind Throughput-Reduction Attack

6.1. Description

The Host Requirements RFC [RFC1122] states in Section 4.2.3.9 that
hosts MUST react to ICMPv4 Source Quench messages by slowing
transmission on the connection. Thus, an attacker could send ICMPv4
Source Quench (type 4, code 0) messages to a TCP endpoint to make it
reduce the rate at which it sends data to the other endpoint of the
connection. [RFC1122] further adds that the RECOMMENDED procedure is
to put the corresponding connection in the slow-start phase of TCP's
congestion control algorithm [RFC5681]. In the case of those
implementations that use an initial congestion window of one segment,
a sustained attack would reduce the throughput of the attacked
connection to about SMSS (Sender Maximum Segment Size) [RFC5681]
bytes per RTT (round-trip time). The throughput achieved during an
attack might be a little higher if a larger initial congestion window
is in use [RFC3390].

6.2. Attack-Specific Counter-Measures

As discussed in the "Requirements for IP Version 4 Routers" RFC
[RFC1812], research seems to suggest that ICMPv4 Source Quench
messages are an ineffective (and unfair) antidote for congestion.
[RFC1812] further states that routers SHOULD NOT send ICMPv4 Source
Quench messages in response to congestion. Furthermore, TCP
implements its own congestion control mechanisms ([RFC5681]
[RFC3168]) that do not depend on ICMPv4 Source Quench messages.

Based on this reasoning, a large number of implementations completely
ignore ICMPv4 Source Quench messages meant for TCP connections. This
behavior has been implemented in, at least, Linux [Linux] since 2004,
and in FreeBSD [FreeBSD], NetBSD [NetBSD], and OpenBSD [OpenBSD]
since 2005. However, it must be noted that this behavior violates
the requirement in [RFC1122] to react to ICMPv4 Source Quench
messages by slowing transmission on the connection.

7. Blind Performance-Degrading Attack

7.1. Description

When one IP system has a large amount of data to send to another
system, the data will be transmitted as a series of IP datagrams. It
is usually preferable that these datagrams be of the largest size
that does not require fragmentation anywhere along the path from the
source to the destination. This datagram size is referred to as the
Path MTU (PMTU) and is equal to the minimum of the MTUs of each hop
in the path. A technique called "Path MTU Discovery" (PMTUD) lets IP

Gont Informational [Page 16]

RFC 5927 ICMP Attacks against TCP July 2010

systems determine the Path MTU of an arbitrary internet path.
[RFC1191] and [RFC1981] specify the PMTUD mechanism for IPv4 and
IPv6, respectively.

The PMTUD mechanism for IPv4 uses the Don't Fragment (DF) bit in the
IP header to dynamically discover the Path MTU. The basic idea
behind the PMTUD mechanism is that a source system assumes that the
MTU of the path is that of the first hop, and sends all its datagrams
with the DF bit set. If any of the datagrams is too large to be
forwarded without fragmentation by some intermediate router, the
router will discard the corresponding datagram and will return an
ICMPv4 "Destination Unreachable, fragmentation needed and DF set"
(type 3, code 4) error message to the sending system. This message
will report the MTU of the constricting hop, so that the sending
system can reduce the assumed Path-MTU accordingly.

For IPv6, intermediate systems do not fragment packets. Thus,
there's an "implicit" DF bit set in every packet sent on a network.
If any of the datagrams is too large to be forwarded without
fragmentation by some intermediate router, the router will discard
the corresponding datagram, and will return an ICMPv6 "Packet Too
Big" (type 2, code 0) error message to the sending system. This
message will report the MTU of the constricting hop, so that the
sending system can reduce the assumed Path-MTU accordingly.

As discussed in both [RFC1191] and [RFC1981], the Path-MTU Discovery
mechanism can be used to attack TCP. An attacker could send a
crafted ICMPv4 "Destination Unreachable, fragmentation needed and DF
set" packet (or their ICMPv6 counterpart) to the sending system,
advertising a small Next-Hop MTU. As a result, the attacked system
would reduce the size of the packets it sends for the corresponding
connection accordingly.

The effect of this attack is two-fold. On one hand, it will increase
the headers/data ratio, thus increasing the overhead needed to send
data to the remote TCP endpoint. On the other hand, if the attacked
system wanted to keep the same throughput it was achieving before
being attacked, it would have to increase the packet rate. On
virtually all systems, this will lead to an increased processing
overhead, thus degrading the overall system performance.

A particular scenario that may take place is one in which an attacker
reports a Next-Hop MTU smaller than or equal to the amount of bytes
needed for headers (IP header, plus TCP header). For example, if the
attacker reports a Next-Hop MTU of 68 bytes, and the amount of bytes
used for headers (IP header, plus TCP header) is larger than
68 bytes, the assumed Path-MTU will not even allow the attacked
system to send a single byte of application data without

Gont Informational [Page 17]

RFC 5927 ICMP Attacks against TCP July 2010

fragmentation. This particular scenario might lead to unpredictable
results. Another possible scenario is one in which a TCP connection
is being secured by means of IPsec. If the Next-Hop MTU reported by
the attacker is smaller than the amount of bytes needed for headers
(IP and IPsec, in this case), the assumed Path-MTU will not even
allow the attacked system to send a single byte of the TCP header
without fragmentation. This is another scenario that may lead to
unpredictable results.

For IPv4, the reported Next-Hop MTU could be as small as 68 octets,
as [RFC0791] requires every internet module to be able to forward a
datagram of 68 octets without further fragmentation. For IPv6, while
the required minimum IPv6 MTU is 1280, the reported Next-Hop MTU can
be smaller than 1280 octets [RFC2460]. If the reported Next-Hop MTU
is smaller than the minimum IPv6 MTU, the receiving host is not
required to reduce the Path-MTU to a value smaller than 1280, but is
required to include a fragmentation header in the outgoing packets to
that destination from that moment on.

7.2. Attack-Specific Counter-Measures

The IETF has standardized a Path-MTU Discovery mechanism called
"Packetization Layer Path MTU Discovery" (PLPMTUD) that does not
depend on ICMP error messages. Implementation of the aforementioned
mechanism in replacement of the traditional PMTUD (specified in
[RFC1191] and [RFC1981]) eliminates this vulnerability. However, it
can also lead to an increase in PMTUD convergence time.

This section describes a modification to the PMTUD mechanism
specified in [RFC1191] and [RFC1981] that has been incorporated in
OpenBSD and NetBSD (since 2005) to improve TCP's resistance to the
blind performance-degrading attack described in Section 7.1. The
described counter-measure basically disregards ICMP messages when a
connection makes progress, without violating any of the requirements
stated in [RFC1191] and [RFC1981].

Henceforth, we will refer to both ICMPv4 "fragmentation needed and DF
bit set" and ICMPv6 "Packet Too Big" messages as "ICMP Packet Too
Big" messages.

In addition to the general validation check described in Section 4.1,
these implementations include a modification to TCP's reaction to
ICMP "Packet Too Big" error messages that disregards them when a
connection makes progress, and honors them only after the
corresponding data have been retransmitted a specified number of
times. This means that upon receipt of an ICMP "Packet Too Big"

Gont Informational [Page 18]

RFC 5927 ICMP Attacks against TCP July 2010

error message, TCP just records this information, and honors it only
when the corresponding data have already been retransmitted a
specified number of times.

While this basic policy would greatly mitigate the impact of the
attack against the PMTUD mechanism, it would also mean that it might
take TCP more time to discover the Path-MTU for a TCP connection.
This would be particularly annoying for connections that have just
been established, as it might take TCP several transmission attempts
(and the corresponding timeouts) before it discovers the PMTU for the
corresponding connection. Thus, this policy would increase the time
it takes for data to begin to be received at the destination host.

In order to protect TCP from the attack against the PMTUD mechanism,
while still allowing TCP to quickly determine the initial Path-MTU
for a connection, the aforementioned implementations have divided the
traditional PMTUD mechanism into two stages: Initial Path-MTU
Discovery and Path-MTU Update.

The Initial Path-MTU Discovery stage is when TCP tries to send
segments that are larger than the ones that have so far been sent and
acknowledged for this connection. That is, in the Initial Path-MTU
Discovery stage, TCP has no record of these large segments getting to
the destination host, and thus these implementations believe the
network when it reports that these packets are too large to reach the
destination host without being fragmented.

The Path-MTU Update stage is when TCP tries to send segments that are
equal to or smaller than the ones that have already been sent and
acknowledged for this connection. During the Path-MTU Update stage,
TCP already has knowledge of the estimated Path-MTU for the given
connection. Thus, in this case, these implementations are more
cautious with the errors being reported by the network.

In order to allow TCP to distinguish segments between those
performing Initial Path-MTU Discovery and those performing Path-MTU
Update, two new variables are introduced to TCP: maxsizesent and
maxsizeacked.

The maxsizesent variable holds the size (in octets) of the largest
packet that has so far been sent for this connection. It is
initialized to 68 (the minimum IPv4 MTU) when the underlying Internet
Protocol is IPv4, and is initialized to 1280 (the minimum IPv6 MTU)
when the underlying Internet Protocol is IPv6. Whenever a packet
larger than maxsizesent octets is sent, maxsizesent is set to that
value.

Gont Informational [Page 19]

RFC 5927 ICMP Attacks against TCP July 2010

On the other hand, maxsizeacked holds the size (in octets) of the
largest packet (data, plus headers) that has so far been acknowledged
for this connection. It is initialized to 68 (the minimum IPv4 MTU)
when the underlying Internet Protocol is IPv4, and is initialized to
1280 (the minimum IPv6 MTU) when the underlying Internet Protocol is
IPv6. Whenever an acknowledgement for a packet larger than
maxsizeacked octets is received, maxsizeacked is set to the size of
that acknowledged packet. Note that because of TCP's cumulative
acknowledgement, a single ACK may acknowledge the receipt of more
than one packet. When that happens, the algorithm may "incorrectly"
assume it is in the "Path-MTU Update" stage, rather than the "Initial
Path-MTU Discovery" stage (as described below).

Upon receipt of an ICMP "Packet Too Big" error message, the Next-Hop
MTU claimed by the ICMP message (henceforth "claimedmtu") is compared
with maxsizesent. If claimedmtu is larger than maxsizesent, then the
ICMP error message is silently discarded. The rationale for this is
that the ICMP error message cannot be legitimate if it claims to have
been triggered by a packet larger than the largest packet we have so
far sent for this connection.

If this check is passed, claimedmtu is compared with maxsizeacked.
If claimedmtu is equal to or larger than maxsizeacked, TCP is
supposed to be at the Initial Path-MTU Discovery stage, and thus the
ICMP "Packet Too Big" error message is honored immediately. That is,
the assumed Path-MTU is updated according to the Next-Hop MTU claimed
in the ICMP error message. Also, maxsizesent is reset to the minimum
MTU of the Internet Protocol in use (68 for IPv4, and 1280 for IPv6).

On the other hand, if claimedmtu is smaller than maxsizeacked, TCP is
supposed to be in the Path-MTU Update stage. At this stage, these
implementations are more cautious with the errors being reported by
the network, and therefore just record the received error message,
and delay the update of the assumed Path-MTU.

To perform this delay, one new variable and one new parameter are
introduced to TCP: nsegrto and MAXSEGRTO. The nsegrto variable holds
the number of times a specified segment has timed out. It is
initialized to zero, and is incremented by one every time the
corresponding segment times out. MAXSEGRTO specifies the number of
times a given segment must time out before an ICMP "Packet Too Big"
error message can be honored, and can be set, in principle, to any
value greater than or equal to 0.

Gont Informational [Page 20]

RFC 5927 ICMP Attacks against TCP July 2010

Thus, if nsegrto is greater than or equal to MAXSEGRTO, and there's a
pending ICMP "Packet Too Big" error message, the corresponding error
message is processed. At that point, maxsizeacked is set to
claimedmtu, and maxsizesent is set to 68 (for IPv4) or 1280 (for
IPv6).

If, while there is a pending ICMP "Packet Too Big" error message, the
TCP SEQ claimed by the pending message is acknowledged (i.e., an ACK
that acknowledges that sequence number is received), then the
"pending error" condition is cleared.

The rationale behind performing this delayed processing of ICMP
"Packet Too Big" messages is that if there is progress on the
connection, the ICMP "Packet Too Big" errors must be a false claim.
By checking for progress on the connection, rather than just for
staleness of the received ICMP messages, TCP is protected from attack
even if the offending ICMP messages are "in window", and as a
corollary, is made more robust to spurious ICMP messages triggered
by, for example, corrupted TCP segments.

MAXSEGRTO can be set, in principle, to any value greater than or
equal to 0. Setting MAXSEGRTO to 0 would make TCP perform the
traditional PMTUD mechanism defined in [RFC1191] and [RFC1981]. A
MAXSEGRTO of 1 provides enough protection for most cases. In any
case, implementations are free to choose higher values for this
constant. MAXSEGRTO could be a function of the Next-Hop MTU claimed
in the received ICMP "Packet Too Big" message. That is, higher
values for MAXSEGRTO could be imposed when the received ICMP "Packet
Too Big" message claims a Next-Hop MTU that is smaller than some
specified value. Both OpenBSD and NetBSD set MAXSEGRTO to 1.

In the event a higher level of protection is desired at the expense
of a higher delay in the discovery of the Path-MTU, an implementation
could consider TCP to always be in the Path-MTU Update stage, thus
always delaying the update of the assumed Path-MTU.

Section 7.3 shows this counter-measure in action. Section 7.4 shows
this counter-measure in pseudo-code.

It is important to note that the mechanism described in this section
is an improvement to the current Path-MTU discovery mechanism, to
mitigate its security implications. The current PMTUD mechanism, as
specified by [RFC1191] and [RFC1981], still suffers from some
functionality problems [RFC2923] that this document does not aim to
address. A mechanism that addresses those issues is described in
[RFC4821].

Gont Informational [Page 21]

RFC 5927 ICMP Attacks against TCP July 2010

7.3. The Counter-Measure for the PMTUD Attack in Action

This section illustrates the operation of the counter-measure for the
ICMP attack against the PMTUD mechanism that has been implemented in
OpenBSD and NetBSD. It shows both how the fix protects TCP from
being attacked and how the counter-measure works in normal scenarios.
As discussed in Section 7.2, this section assumes the PMTUD-specific
counter-measure is implemented in addition to the TCP sequence number
checking described in Section 4.1.

Figure 1 illustrates a hypothetical scenario in which two hosts are
connected by means of three intermediate routers. It also shows the
MTU of each hypothetical hop. All the following subsections assume
the network setup of this figure.

Also, for simplicity's sake, all subsections assume an IP header of
20 octets and a TCP header of 20 octets. Thus, for example, when the
PMTU is assumed to be 1500 octets, TCP will send segments that
contain, at most, 1460 octets of data.

For simplicity's sake, all the following subsections assume the TCP
implementation at Host 1 (H1) has chosen a MAXSEGRTO of 1.

+----+ +----+ +----+ +----+ +----+
| H1 |--------| R1 |--------| R2 |--------| R3 |--------| H2 |
+----+ +----+ +----+ +----+ +----+
MTU=4464 MTU=2048 MTU=1500 MTU=4464

Figure 1: Hypothetical Scenario

7.3.1. Normal Operation for Bulk Transfers

This subsection shows the counter-measure in normal operation, when a
TCP connection is used for bulk transfers. That is, it shows how the
counter-measure works when there is no attack taking place and a TCP
connection is used for transferring large amounts of data. This
section assumes that just after the connection is established, one of
the TCP endpoints begins to transfer data in packets that are as
large as possible.

Gont Informational [Page 22]

RFC 5927 ICMP Attacks against TCP July 2010

Host 1 Host 2

1. --> <SEQ=100><CTL=SYN> -->
2. <-- <SEQ=X><ACK=101><CTL=SYN,ACK> <--
3. --> <SEQ=101><ACK=X+1><CTL=ACK> -->
4. --> <SEQ=101><ACK=X+1><CTL=ACK><DATA=4424> -->
5. <--- ICMP "Packet Too Big" MTU=2048, TCPseq#=101 <--- R1
6. --> <SEQ=101><ACK=X+1><CTL=ACK><DATA=2008> -->
7. <--- ICMP "Packet Too Big" MTU=1500, TCPseq#=101 <--- R2
8. --> <SEQ=101><ACK=X+1><CTL=ACK><DATA=1460> -->
9. <-- <SEQ=X+1><ACK=1561><CTL=ACK> <--

Figure 2: Normal Operation for Bulk Transfers

The nsegrto variable is initialized to zero. Both maxsizeacked and
maxsizesent are initialized to the minimum MTU for the Internet
Protocol being used (68 for IPv4, and 1280 for IPv6).

In lines 1 to 3, the three-way handshake takes place, and the
connection is established. In line 4, H1 tries to send a full-sized
TCP segment. As described by [RFC1191] and [RFC1981], in this case,
TCP will try to send a segment with 4424 bytes of data, which will
result in an IP packet of 4464 octets. Therefore, maxsizesent is set
to 4464. When the packet reaches R1, it elicits an ICMP "Packet Too
Big" error message.

In line 5, H1 receives the ICMP error message, which reports a Next-
Hop MTU of 2048 octets. After performing the TCP sequence number
check described in Section 4.1, the Next-Hop MTU reported by the ICMP
error message (claimedmtu) is compared with maxsizesent. As it is
smaller than maxsizesent, it passes the check, and thus is then
compared with maxsizeacked. As claimedmtu is larger than
maxsizeacked, TCP assumes that the corresponding TCP segment was
performing the Initial PMTU Discovery. Therefore, the TCP at H1
honors the ICMP message by updating the assumed Path-MTU. The
maxsizesent variable is reset to the minimum MTU of the Internet
Protocol in use (68 for IPv4, and 1280 for IPv6).

In line 6, the TCP at H1 sends a segment with 2008 bytes of data,
which results in an IP packet of 2048 octets. The maxsizesent
variable is thus set to 2008 bytes. When the packet reaches R2, it
elicits an ICMP "Packet Too Big" error message.

In line 7, H1 receives the ICMP error message, which reports a Next-
Hop MTU of 1500 octets. After performing the TCP sequence number
check, the Next-Hop MTU reported by the ICMP error message
(claimedmtu) is compared with maxsizesent. As it is smaller than
maxsizesent, it passes the check, and thus is then compared with

Gont Informational [Page 23]

RFC 5927 ICMP Attacks against TCP July 2010

maxsizeacked. As claimedmtu is larger than maxsizeacked, TCP assumes
that the corresponding TCP segment was performing the Initial PMTU
Discovery. Therefore, the TCP at H1 honors the ICMP message by
updating the assumed Path-MTU. The maxsizesent variable is reset to
the minimum MTU of the Internet Protocol in use.

In line 8, the TCP at H1 sends a segment with 1460 bytes of data,
which results in an IP packet of 1500 octets. Thus, maxsizesent is
set to 1500. This packet reaches H2, where it elicits an
acknowledgement (ACK) segment.

In line 9, H1 finally gets the acknowledgement for the data segment.
As the corresponding packet was larger than maxsizeacked, TCP updates
maxsizeacked, setting it to 1500. At this point, TCP has discovered
the Path-MTU for this TCP connection.

7.3.2. Operation during Path-MTU Changes

Let us suppose a TCP connection between H1 and H2 has already been
established, and that the PMTU for the connection has already been
discovered to be 1500. At this point, both maxsizesent and
maxsizeacked are equal to 1500, and nsegrto is equal to 0. Suppose
some time later the PMTU decreases to 1492. For simplicity, let us
suppose that the Path-MTU has decreased because the MTU of the link
between R2 and R3 has decreased from 1500 to 1492. Figure 3
illustrates how the counter-measure would work in this scenario.

Host 1 Host 2

1. (Path-MTU decreases)
2. --> <SEQ=100><ACK=X><CTL=ACK><DATA=1460> -->
3. <--- ICMP "Packet Too Big" MTU=1492, TCPseq#=100 <--- R2
4. (Segment times out)
5. --> <SEQ=100><ACK=X><CTL=ACK><DATA=1452> -->
6. <-- <SEQ=X><ACK=1552><CTL=ACK> <--

Figure 3: Operation during Path-MTU Changes

In line 1, the Path-MTU for this connection decreases from 1500 to
1492. In line 2, the TCP at H1, without being aware of the Path-MTU
change, sends a 1500-byte packet to H2. When the packet reaches R2,
it elicits an ICMP "Packet Too Big" error message.

In line 3, H1 receives the ICMP error message, which reports a Next-
Hop MTU of 1492 octets. After performing the TCP sequence number
check, the Next-Hop MTU reported by the ICMP error message
(claimedmtu) is compared with maxsizesent. As claimedmtu is smaller
than maxsizesent, it is then compared with maxsizeacked. As

Gont Informational [Page 24]

RFC 5927 ICMP Attacks against TCP July 2010

claimedmtu is smaller than maxsizeacked (full-sized packets were
getting to the remote endpoint), this packet is assumed to be
performing Path-MTU Update, and a "pending error" condition is
recorded.

In line 4, the segment times out. Thus, nsegrto is incremented by 1.
As nsegrto is greater than or equal to MAXSEGRTO, the assumed Path-
MTU is updated. The nsegrto variable is reset to 0, maxsizeacked is
set to claimedmtu, and maxsizesent is set to the minimum MTU of the
Internet Protocol in use.

In line 5, H1 retransmits the data using the updated PMTU, and thus
maxsizesent is set to 1492. The resulting packet reaches H2, where
it elicits an acknowledgement (ACK) segment.

In line 6, H1 finally gets the acknowledgement for the data segment.
At this point, TCP has discovered the new Path-MTU for this TCP
connection.

7.3.3. Idle Connection Being Attacked

Let us suppose a TCP connection between H1 and H2 has already been
established, and the PMTU for the connection has already been
discovered to be 1500. Figure 4 shows a sample time-line diagram
that illustrates an idle connection being attacked.

Host 1 Host 2

1. --> <SEQ=100><ACK=X><CTL=ACK><DATA=50> -->
2. <-- <SEQ=X><ACK=150><CTL=ACK> <--
3. <--- ICMP "Packet Too Big" MTU=68, TCPseq#=100 <---
4. <--- ICMP "Packet Too Big" MTU=68, TCPseq#=100 <---
5. <--- ICMP "Packet Too Big" MTU=68, TCPseq#=100 <---

Figure 4: Idle Connection Being Attacked

In line 1, H1 sends its last bunch of data. In line 2, H2
acknowledges the receipt of these data. Then the connection becomes
idle. In lines 3, 4, and 5, an attacker sends forged ICMP "Packet
Too Big" error messages to H1. Regardless of how many packets it
sends and of the TCP sequence number each ICMP packet includes, none
of these ICMP error messages will pass the TCP sequence number check
described in Section 4.1, as H1 has no unacknowledged data "in
flight" to H2. Therefore, the attack does not succeed.

Gont Informational [Page 25]

RFC 5927 ICMP Attacks against TCP July 2010

7.3.4. Active Connection Being Attacked after Discovery of the Path-MTU

Let us suppose an attacker attacks a TCP connection for which the
PMTU has already been discovered. In this case, as illustrated in
Figure 1, the PMTU would be found to be 1500 bytes. Figure 5 shows a
possible packet exchange.

Host 1 Host 2

1. --> <SEQ=100><ACK=X><CTL=ACK><DATA=1460> -->
2. --> <SEQ=1560><ACK=X><CTL=ACK><DATA=1460> -->
3. --> <SEQ=3020><ACK=X><CTL=ACK><DATA=1460> -->
4. --> <SEQ=4480><ACK=X><CTL=ACK><DATA=1460> -->
5. <--- ICMP "Packet Too Big" MTU=68, TCPseq#=100 <---
6. <-- <SEQ=X><CTL=ACK><ACK=1560> <--

Figure 5: Active Connection Being Attacked after Discovery of PMTU

As we assume the PMTU has already been discovered, we also assume
both maxsizesent and maxsizeacked are equal to 1500. We assume
nsegrto is equal to zero, as there have been no segment timeouts.

In lines 1, 2, 3, and 4, H1 sends four data segments to H2. In
line 5, an attacker sends a forged ICMP error message to H1. We
assume the attacker is lucky enough to guess both the four-tuple that
identifies the connection and a valid TCP sequence number. As the
Next-Hop MTU claimed in the ICMP "Packet Too Big" message
(claimedmtu) is smaller than maxsizeacked, this packet is assumed to
be performing Path-MTU Update. Thus, the error message is recorded.

In line 6, H1 receives an acknowledgement for the segment sent in
line 1, before it times out. At this point, the "pending error"
condition is cleared, and the recorded ICMP "Packet Too Big" error
message is ignored. Therefore, the attack does not succeed.

7.3.5. TCP Peer Attacked when Sending Small Packets Just after the
Three-Way Handshake

This section analyzes a scenario in which a TCP peer that is sending
small segments just after the connection has been established is
attacked. The connection could be in use by protocols such as SMTP
[RFC5321] and HTTP [RFC2616], for example, which usually behave like
this.

Figure 6 shows a possible packet exchange for such a scenario.

Gont Informational [Page 26]

RFC 5927 ICMP Attacks against TCP July 2010

Host 1 Host 2

1. --> <SEQ=100><CTL=SYN> -->
2. <-- <SEQ=X><ACK=101><CTL=SYN,ACK> <--
3. --> <SEQ=101><ACK=X+1><CTL=ACK> -->
4. --> <SEQ=101><ACK=X+1><CTL=ACK><DATA=100> -->
5. <-- <SEQ=X+1><ACK=201><CTL=ACK> <--
6. --> <SEQ=201><ACK=X+1><CTL=ACK><DATA=100> -->
7. --> <SEQ=301><ACK=X+1><CTL=ACK><DATA=100> -->
8. <--- ICMP "Packet Too Big" MTU=150, TCPseq#=201 <---

Figure 6: TCP Peer Attacked when Sending Small Packets
Just after the Three-Way Handshake

The nsegrto variable is initialized to zero. Both maxsizesent and
maxsizeacked are initialized to the minimum MTU for the Internet
Protocol being used (68 for IPv4, and 1280 for IPv6).

In lines 1 to 3, the three-way handshake takes place, and the
connection is established. At this point, the assumed Path-MTU for
this connection is 4464. In line 4, H1 sends a small segment (which
results in a 140-byte packet) to H2. Therefore, maxsizesent is set
to 140. In line 5, this segment is acknowledged, and thus
maxsizeacked is set to 140.

In lines 6 and 7, H1 sends two small segments to H2. In line 8,
while the segments from lines 6 and 7 are still "in flight" to H2, an
attacker sends a forged ICMP "Packet Too Big" error message to H1.
Assuming the attacker is lucky enough to guess a valid TCP sequence
number, this ICMP message will pass the TCP sequence number check.
The Next-Hop MTU reported by the ICMP error message (claimedmtu) is
then compared with maxsizesent. As claimedmtu is larger than
maxsizesent, the ICMP error message is silently discarded.
Therefore, the attack does not succeed.

7.4. Pseudo-Code for the Counter-Measure for the Blind Performance-
Degrading Attack

This section contains a pseudo-code version of the counter-measure
described in Section 7.2 for the blind performance-degrading attack
described in Section 7. It is meant as guidance for developers on
how to implement this counter-measure.

The pseudo-code makes use of the following variables, constants, and
functions:

Gont Informational [Page 27]

RFC 5927 ICMP Attacks against TCP July 2010

ack
Variable holding the acknowledgement number contained in the TCP
segment that has just been received.

acked_packet_size
Variable holding the packet size (data, plus headers) that the ACK
that has just been received is acknowledging.

adjust_mtu()
Function that adjusts the MTU for this connection, according to
the ICMP "Packet Too Big" that was last received.

claimedmtu
Variable holding the Next-Hop MTU advertised by the ICMP "Packet
Too Big" error message.

claimedtcpseq
Variable holding the TCP sequence number contained in the payload
of the ICMP "Packet Too Big" message that has just been received
or was last recorded.

current_mtu
Variable holding the assumed Path-MTU for this connection.

drop_message()
Function that performs the necessary actions to drop the ICMP
message being processed.

initial_mtu
Variable holding the MTU for new connections, as explained in
[RFC1191] and [RFC1981].

maxsizeacked
Variable holding the largest packet size (data, plus headers) that
has so far been acked for this connection, as explained in
Section 7.2.

maxsizesent
Variable holding the largest packet size (data, plus headers) that
has so far been sent for this connection, as explained in
Section 7.2.

nsegrto
Variable holding the number of times this segment has timed out,
as explained in Section 7.2.

packet_size
Variable holding the size of the IP datagram being sent.

Gont Informational [Page 28]

RFC 5927 ICMP Attacks against TCP July 2010

pending_message
Variable (flag) that indicates whether there is a pending ICMP
"Packet Too Big" message to be processed.

save_message()
Function that records the ICMP "Packet Too Big" message that has
just been received.

MINIMUM_MTU
Constant holding the minimum MTU for the Internet Protocol in use
(68 for IPv4, and 1280 for IPv6).

MAXSEGRTO
Constant holding the number of times a given segment must time out
before an ICMP "Packet Too Big" error message can be honored.

EVENT: New TCP connection

current_mtu = initial_mtu;
maxsizesent = MINIMUM_MTU;
maxsizeacked = MINIMUM_MTU;
nsegrto = 0;
pending_message = 0;

EVENT: Segment is sent

if (packet_size > maxsizesent)
maxsizesent = packet_size;

EVENT: Segment is received

if (acked_packet_size > maxsizeacked)
maxsizeacked = acked_packet_size;

if (pending_message)
if (ack > claimedtcpseq){
pending_message = 0;
nsegrto = 0;
}

EVENT: ICMP "Packet Too Big" message is received

if (claimedmtu <= MINIMUM_MTU)
drop_message();

if (claimedtcpseq < SND.UNA || claimedtcpseq >= SND.NXT)
drop_message();

Gont Informational [Page 29]

RFC 5927 ICMP Attacks against TCP July 2010

else {
if (claimedmtu > maxsizesent || claimedmtu >= current_mtu)
drop_message();

else {
if (claimedmtu > maxsizeacked){
adjust_mtu();
current_mtu = claimedmtu;
maxsizesent = MINIMUM_MTU;
}

else {
pending_message = 1;
save_message();
}
}
}

EVENT: Segment times out

nsegrto++;

if (pending_message && nsegrto >= MAXSEGRTO){
adjust_mtu();
nsegrto = 0;
pending_message = 0;
maxsizeacked = claimedmtu;
maxsizesent = MINIMUM_MTU;
current_mtu = claimedmtu;
}

Notes:
All comparisons between sequence numbers must be performed using
sequence number arithmetic.

The pseudo-code implements the mechanism described in Section 7.2,
the TCP sequence number checking described in Section 4.1, and the
validation check on the advertised Next-Hop MTU described in
[RFC1191] and [RFC1981].

8. Security Considerations

This document describes the use of ICMP error messages to perform a
number of attacks against TCP, and describes a number of widely
implemented counter-measures that either eliminate or reduce the
impact of these attacks when they are performed by off-path
attackers.

Gont Informational [Page 30]

RFC 5927 ICMP Attacks against TCP July 2010

Section 4.1 describes a validation check that could be enforced on
ICMP error messages, such that TCP reacts only to those ICMP error
messages that appear to relate to segments currently "in flight" to
the destination system. This requires more effort on the side of an
off-path attacker at the expense of possible reduced responsiveness
to network errors.

Section 4.2 describes how randomization of TCP ephemeral ports
requires more effort on the side of the attacker to successfully
exploit any of the attacks described in this document.

Section 4.3 describes how ICMP error messages could possibly be
filtered based on their payload, to prevent users of the local
network from successfully performing attacks against third-party
connections. This is analogous to ingress filtering and egress
filtering of IP packets [IP-filtering].

Section 5.2 describes an attack-specific counter-measure for the
blind connection-reset attack. It describes the processing of ICMP
"hard errors" as "soft errors" when they are received for connections
in any of the synchronized states. This counter-measure eliminates
the aforementioned vulnerability in synchronized connections at the
expense of possible reduced responsiveness in some network scenarios.

Section 6.2 describes an attack-specific counter-measure for the
blind throughput-reduction attack. It suggests that the
aforementioned vulnerability can be eliminated by ignoring ICMPv4
Source Quench messages meant for TCP connections. This is in
accordance with research results that indicate that ICMPv4 Source
Quench messages are ineffective and are an unfair antidote for
congestion.

Finally, Section 7.2 describes an attack-specific counter-measure for
the blind performance-degrading attack. It consists of the
validation check described in Section 4.1, with a modification that
makes TCP react to ICMP "Packet Too Big" error messages such that
they are processed when an outstanding TCP segment times out. This
counter-measure parallels the Packetization Layer Path MTU Discovery
(PLPMTUD) mechanism [RFC4821]. It should be noted that if this
counter-measure is implemented, in some scenarios TCP may respond
more slowly to valid ICMP "Packet Too Big" error messages.

A discussion of these and other attack vectors for performing similar
attacks against TCP (along with possible counter-measures) can be
found in [CPNI-TCP] and [TCP-SECURITY].

Gont Informational [Page 31]

RFC 5927 ICMP Attacks against TCP July 2010

9. Acknowledgements

This document was inspired by Mika Liljeberg, while discussing some
issues related to [RFC5461] by private e-mail. The author would like
to thank (in alphabetical order): Bora Akyol, Mark Allman, Ran
Atkinson, James Carlson, Alan Cox, Theo de Raadt, Wesley Eddy, Lars
Eggert, Ted Faber, Juan Fraschini, Markus Friedl, Guillermo Gont,
John Heffner, Alfred Hoenes, Vivek Kakkar, Michael Kerrisk, Mika
Liljeberg, Matt Mathis, David Miller, Toby Moncaster, Miles Nordin,
Eloy Paris, Kacheong Poon, Andrew Powell, Pekka Savola, Donald Smith,
Pyda Srisuresh, Fred Templin, and Joe Touch for contributing many
valuable comments.

Juan Fraschini and the author of this document implemented freely
available audit tools to help vendors audit their systems by
reproducing the attacks discussed in this document. These tools are
available at http://www.gont.com.ar/tools/index.html.

Markus Friedl, Chad Loder, and the author of this document produced
and tested in OpenBSD [OpenBSD] the first implementation of the
counter-measure described in Section 7.2. This first implementation
helped to test the effectiveness of the ideas introduced in this
document, and has served as a reference implementation for other
operating systems.

The author would like to thank the UK's Centre for the Protection of
National Infrastructure (CPNI) -- formerly the National
Infrastructure Security Co-ordination Centre (NISCC) -- for
coordinating the disclosure of these issues with a large number of
vendors and CSIRTs (Computer Security Incident Response Teams).

The author wishes to express deep and heartfelt gratitude to Jorge
Oscar Gont and Nelida Garcia, for their precious motivation and
guidance.

10. References

10.1. Normative References

[RFC0791] Postel, J., "Internet Protocol", STD 5, RFC 791,
September 1981.

[RFC0792] Postel, J., "Internet Control Message Protocol",
STD 5, RFC 792, September 1981.

[RFC0793] Postel, J., "Transmission Control Protocol", STD 7,
RFC 793, September 1981.

Gont Informational [Page 32]

RFC 5927 ICMP Attacks against TCP July 2010

[RFC1122] Braden, R., "Requirements for Internet Hosts -
Communication Layers", STD 3, RFC 1122,
October 1989.

[RFC1191] Mogul, J. and S. Deering, "Path MTU discovery",
RFC 1191, November 1990.

[RFC1812] Baker, F., "Requirements for IP Version 4 Routers",
RFC 1812, June 1995.

[RFC1981] McCann, J., Deering, S., and J. Mogul, "Path MTU
Discovery for IP version 6", RFC 1981, August 1996.

[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.

[RFC2460] Deering, S. and R. Hinden, "Internet Protocol,
Version 6 (IPv6) Specification", RFC 2460,
December 1998.

[RFC4301] Kent, S. and K. Seo, "Security Architecture for the
Internet Protocol", RFC 4301, December 2005.

[RFC4443] Conta, A., Deering, S., and M. Gupta, "Internet
Control Message Protocol (ICMPv6) for the Internet
Protocol Version 6 (IPv6) Specification", RFC 4443,
March 2006.

[RFC4884] Bonica, R., Gan, D., Tappan, D., and C. Pignataro,
"Extended ICMP to Support Multi-Part Messages",
RFC 4884, April 2007.

10.2. Informative References

[CPNI-TCP] CPNI, "Security Assessment of the Transmission
Control Protocol (TCP)", http://www.cpni.gov.uk/
Docs/tn-03-09-security-assessment-TCP.pdf, 2009.

[DClark] Clark, D., "The Design Philosophy of the DARPA
Internet Protocols", Computer Communication
Review Vol. 18, No. 4, 1988.

[FreeBSD] The FreeBSD Project, http://www.freebsd.org.

[ICMP-Filtering] Gont, F., "Filtering of ICMP error messages", http
://www.gont.com.ar/papers/
filtering-of-icmp-error-messages.pdf.

Gont Informational [Page 33]

RFC 5927 ICMP Attacks against TCP July 2010

[IP-filtering] NISCC, "NISCC Technical Note 01/2006: Egress and
Ingress Filtering",
http://www.cpni.gov.uk/Docs/re-20060420-00294.pdf,
2006.

[Linux] The Linux Project, "http://www.kernel.org".

[McKusick] McKusick, M., Bostic, K., Karels, M., and J.
Quarterman, "The Design and Implementation of the
4.4 BSD Operating System", Addison-Wesley, 1996.

[NISCC] NISCC, "NISCC Vulnerability Advisory 532967/NISCC/
ICMP: Vulnerability Issues in ICMP packets with TCP
payloads", http://www.cpni.gov.uk/docs/
re-20050412-00303.pdf?lang=en, 2005.

[NetBSD] The NetBSD Project, "http://www.netbsd.org".

[OpenBSD] The OpenBSD Project, "http://www.openbsd.org".

[OpenBSD-PF] The OpenBSD Packet Filter,
"http://www.openbsd.org/faq/pf/".

[PORT-RANDOM] Larsen, M. and F. Gont, "Transport Protocol Port
Randomization Recommendations", Work in Progress,
April 2010.

[RFC0816] Clark, D., "Fault isolation and recovery", RFC 816,
July 1982.

[RFC1321] Rivest, R., "The MD5 Message-Digest Algorithm",
RFC 1321, April 1992.

[RFC1323] Jacobson, V., Braden, B., and D. Borman, "TCP
Extensions for High Performance", RFC 1323,
May 1992.

[RFC2385] Heffernan, A., "Protection of BGP Sessions via the
TCP MD5 Signature Option", RFC 2385, August 1998.

[RFC2616] Fielding, R., Gettys, J., Mogul, J., Frystyk, H.,
Masinter, L., Leach, P., and T. Berners-Lee,
"Hypertext Transfer Protocol -- HTTP/1.1",
RFC 2616, June 1999.

[RFC2923] Lahey, K., "TCP Problems with Path MTU Discovery",
RFC 2923, September 2000.

Gont Informational [Page 34]

RFC 5927 ICMP Attacks against TCP July 2010

[RFC3168] Ramakrishnan, K., Floyd, S., and D. Black, "The
Addition of Explicit Congestion Notification (ECN)
to IP", RFC 3168, September 2001.

[RFC3390] Allman, M., Floyd, S., and C. Partridge,
"Increasing TCP's Initial Window", RFC 3390,
October 2002.

[RFC4271] Rekhter, Y., Li, T., and S. Hares, "A Border
Gateway Protocol 4 (BGP-4)", RFC 4271,
January 2006.

[RFC4821] Mathis, M. and J. Heffner, "Packetization Layer
Path MTU Discovery", RFC 4821, March 2007.

[RFC4907] Aboba, B., "Architectural Implications of Link
Indications", RFC 4907, June 2007.

[RFC4953] Touch, J., "Defending TCP Against Spoofing
Attacks", RFC 4953, July 2007.

[RFC5321] Klensin, J., "Simple Mail Transfer Protocol",
RFC 5321, October 2008.

[RFC5461] Gont, F., "TCP's Reaction to Soft Errors",
RFC 5461, February 2009.

[RFC5681] Allman, M., Paxson, V., and E. Blanton, "TCP
Congestion Control", RFC 5681, September 2009.

[RFC5925] Touch, J., Mankin, A., and R. Bonica, "The TCP
Authentication Option", RFC 5925, June 2010.

[TCP-SECURITY] Gont, F., "Security Assessment of the Transmission
Control Protocol (TCP)", Work in Progress,
February 2010.

[TCPM-TCPSECURE] Ramaiah, A., Stewart, R., and M. Dalal, "Improving
TCP's Robustness to Blind In-Window Attacks", Work
in Progress, May 2010.

[US-CERT] US-CERT, "US-CERT Vulnerability Note VU#222750:
TCP/IP Implementations do not adequately validate
ICMP error messages",
http://www.kb.cert.org/vuls/id/222750, 2005.

[Watson] Watson, P., "Slipping in the Window: TCP Reset
Attacks", CanSecWest Conference, 2004.

Gont Informational [Page 35]

RFC 5927 ICMP Attacks against TCP July 2010

[Wright] Wright, G. and W. Stevens, "TCP/IP Illustrated,
Volume 2: The Implementation", Addison-
Wesley, 1994.

Author's Address

Fernando Gont
Universidad Tecnologica Nacional / Facultad Regional Haedo
Evaristo Carriego 2644
Haedo, Provincia de Buenos Aires 1706
Argentina

Phone: +54 11 4650 8472
EMail: [email protected]
URI: http://www.gont.com.ar

Gont Informational [Page 36]