[Docs] [txt|pdf] [Tracker] [WG] [Email] [Diff1] [Diff2] [Nits]

Versions: (draft-gont-tcpm-icmp-attacks) 00 01 02 03 04 05 06 07 08 09 10 11 12 RFC 5927

TCP Maintenance and Minor                                        F. Gont
Extensions (tcpm)                                                UTN/FRH
Internet-Draft                                          January 19, 2010
Intended status: Informational
Expires: July 23, 2010


                        ICMP attacks against TCP
                  draft-ietf-tcpm-icmp-attacks-09.txt

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) and other similar protocols.
   Additionally, 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 Internet-Draft is submitted to IETF in full conformance with the
   provisions of BCP 78 and BCP 79.

   Internet-Drafts are working documents of the Internet Engineering
   Task Force (IETF), its areas, and its working groups.  Note that
   other groups may also distribute working documents as Internet-
   Drafts.

   Internet-Drafts are draft documents valid for a maximum of six months
   and may be updated, replaced, or obsoleted by other documents at any
   time.  It is inappropriate to use Internet-Drafts as reference
   material or to cite them other than as "work in progress."

   The list of current Internet-Drafts can be accessed at
   http://www.ietf.org/ietf/1id-abstracts.txt.

   The list of Internet-Draft Shadow Directories can be accessed at
   http://www.ietf.org/shadow.html.

   This Internet-Draft will expire on July 23, 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



Gont                      Expires July 23, 2010                 [Page 1]

Internet-Draft          ICMP attacks against TCP            January 2010


   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 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                      Expires July 23, 2010                 [Page 2]

Internet-Draft          ICMP attacks against TCP            January 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 (ICMP) . . . . . . . . . . . . .  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  . . . . . . . .  9
     4.1.  TCP sequence number checking . . . . . . . . . . . . . . .  9
     4.2.  Port randomization . . . . . . . . . . . . . . . . . . . . 10
     4.3.  Filtering ICMP error messages based on the ICMP payload  . 10
   5.  Blind connection-reset attack  . . . . . . . . . . . . . . . . 11
     5.1.  Description  . . . . . . . . . . . . . . . . . . . . . . . 11
     5.2.  Attack-specific counter-measures . . . . . . . . . . . . . 12
   6.  Blind throughput-reduction attack  . . . . . . . . . . . . . . 15
     6.1.  Description  . . . . . . . . . . . . . . . . . . . . . . . 15
     6.2.  Attack-specific counter-measures . . . . . . . . . . . . . 15
   7.  Blind performance-degrading attack . . . . . . . . . . . . . . 16
     7.1.  Description  . . . . . . . . . . . . . . . . . . . . . . . 16
     7.2.  Attack-specific counter-measures . . . . . . . . . . . . . 17
     7.3.  The counter-measure for the PMTUD attack in action . . . . 21
       7.3.1.  Normal operation for bulk transfers  . . . . . . . . . 21
       7.3.2.  Operation during Path-MTU changes  . . . . . . . . . . 23
       7.3.3.  Idle connection being attacked . . . . . . . . . . . . 24
       7.3.4.  Active connection being attacked after discovery
               of the Path-MTU  . . . . . . . . . . . . . . . . . . . 25
       7.3.5.  TCP peer attacked when sending small packets just
               after the three-way handshake  . . . . . . . . . . . . 25
     7.4.  Pseudo-code for the counter-measure for the blind
           performance-degrading attack . . . . . . . . . . . . . . . 26
   8.  Security Considerations  . . . . . . . . . . . . . . . . . . . 30
   9.  IANA Considerations  . . . . . . . . . . . . . . . . . . . . . 31
   10. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 31
   11. References . . . . . . . . . . . . . . . . . . . . . . . . . . 32
     11.1. Normative References . . . . . . . . . . . . . . . . . . . 32
     11.2. Informative References . . . . . . . . . . . . . . . . . . 32
   Appendix A.  Changes from previous versions of the draft (to
                be removed by the RFC Editor before publishing
                this document as an RFC)  . . . . . . . . . . . . . . 35
     A.1.  Changes from draft-ietf-tcpm-icmp-attacks-08 . . . . . . . 35
     A.2.  Changes from draft-ietf-tcpm-icmp-attacks-07 . . . . . . . 35
     A.3.  Changes from draft-ietf-tcpm-icmp-attacks-06 . . . . . . . 35
     A.4.  Changes from draft-ietf-tcpm-icmp-attacks-05 . . . . . . . 35
     A.5.  Changes from draft-ietf-tcpm-icmp-attacks-04 . . . . . . . 35
     A.6.  Changes from draft-ietf-tcpm-icmp-attacks-03 . . . . . . . 36



Gont                      Expires July 23, 2010                 [Page 3]

Internet-Draft          ICMP attacks against TCP            January 2010


     A.7.  Changes from draft-ietf-tcpm-icmp-attacks-02 . . . . . . . 36
     A.8.  Changes from draft-ietf-tcpm-icmp-attacks-01 . . . . . . . 36
     A.9.  Changes from draft-ietf-tcpm-icmp-attacks-00 . . . . . . . 36
     A.10. Changes from draft-gont-tcpm-icmp-attacks-05 . . . . . . . 37
     A.11. Changes from draft-gont-tcpm-icmp-attacks-04 . . . . . . . 37
     A.12. Changes from draft-gont-tcpm-icmp-attacks-03 . . . . . . . 37
     A.13. Changes from draft-gont-tcpm-icmp-attacks-02 . . . . . . . 38
     A.14. Changes from draft-gont-tcpm-icmp-attacks-01 . . . . . . . 38
     A.15. Changes from draft-gont-tcpm-icmp-attacks-00 . . . . . . . 38
   Author's Address . . . . . . . . . . . . . . . . . . . . . . . . . 39









































Gont                      Expires July 23, 2010                 [Page 4]

Internet-Draft          ICMP attacks against TCP            January 2010


1.  Introduction

   ICMP [RFC0792] 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 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 being
   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 vulnerabiliies.  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".

   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.
   Section 4 proposes 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 propose attack-specific counter-
   measures that eliminate or greatly mitigate their impact.

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",



Gont                      Expires July 23, 2010                 [Page 5]

Internet-Draft          ICMP attacks against TCP            January 2010


   "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 raise awareness 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 unreliable, 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 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
   flushes any queued ICMP error messages when it is rebooted.

2.1.1.  ICMP for IP version 4 (ICMP)

   [RFC0792] specifies the Internet Control Message Protocol (ICMP) to
   be used with the Internet Protocol version 4 (IPv4).  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] classifies ICMP error
   messages into those that indicate "soft errors", and those that
   indicate "hard errors", thus roughly defining the semantics of them.

   The ICMP specification [RFC0792] also defines the ICMP 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),



Gont                      Expires July 23, 2010                 [Page 6]

Internet-Draft          ICMP attacks against TCP            January 2010


   which makes use of ICMP 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.

   Appendix D of [RFC4301] provides information about which ICMP 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, that is analogous to the ICMP "fragmentation needed and DF
   bit set" (type 3, code 4) error message.  [RFC1981] defines the Path
   MTU Discovery mechanism for IP Version 6, that makes use of these
   messages to determine the MTU of an arbitrary internet path.

   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 a
   TCP MUST act on an ICMP error message passed up from the IP layer,
   directing it to the connection that elicited the error.

   In order to allow ICMP messages to be demultiplexed by the receiving
   system, part of the original packet that elicited 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 elicited it.

   Neither the Host Requirements RFC [RFC1122] nor the original TCP
   specification [RFC0793] recommend 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



Gont                      Expires July 23, 2010                 [Page 7]

Internet-Draft          ICMP attacks against TCP            January 2010


   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 document 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
   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 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 ICMP "fragmentation
   needed and DF bit set" (type 3, code 3) 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



Gont                      Expires July 23, 2010                 [Page 8]

Internet-Draft          ICMP attacks against TCP            January 2010


   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

   For ICMPv4, [RFC0792] states that the IP header plus the first 64
   bits of the packet that elicited the ICMP message are to be included
   in the payload of the ICMP error message.  Thus, it is assumed that
   all data needed to identify a transport protocol instance and process
   the ICMP 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 included in the payload
   of ICMP 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 ICMP specification.  The
   Requirements for IP Version 4 Routers RFC [RFC1812] states that ICMP
   error messages "SHOULD contain as much of the original datagram as
   possible without the length of the ICMP datagram exceeding 576
   bytes".

   Thus, for ICMP 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 ICMP 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 avalable 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 elicited 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.



Gont                      Expires July 23, 2010                 [Page 9]

Internet-Draft          ICMP attacks against TCP            January 2010


   For IPv6, the payload of ICMPv6 error messages includes as many
   octets from the IPv6 packet that elicited 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
   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 elicited 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.




Gont                      Expires July 23, 2010                [Page 10]

Internet-Draft          ICMP attacks against TCP            January 2010


   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.

   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 wrap 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 connections would make it harder for an attacker to perform any
   of the attacks discussed in this document.

   [I-D.ietf-tsvwg-port-randomization] discusses a number of algorithms
   to randomize the ephemeral ports used by clients.

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.



Gont                      Expires July 23, 2010                [Page 11]

Internet-Draft          ICMP attacks against TCP            January 2010


   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 IP address of the IP header contained in the payload of
   the ICMP error message.  As the source IP 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
   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 attacks
   performed 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
   ICMP error message that indicates a "hard error", and states that
   ICMP 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 to indicate hard errors.
   In the case of ICMP 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



Gont                      Expires July 23, 2010                [Page 12]

Internet-Draft          ICMP attacks against TCP            January 2010


   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.

   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
   [I-D.ietf-tcpm-tcpsecure], by means authentication at the network
   layer [RFC4301], by means of the TCP MD5 signature option [RFC2385],
   by means of the TCP-AO [I-D.ietf-tcpm-tcp-auth-opt], or by means of
   the mechanism proposed in [I-D.ietf-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 to mitigate the
   impact of ICMP-based blind connection-reset attacks.

   ICMP 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.






Gont                      Expires July 23, 2010                [Page 13]

Internet-Draft          ICMP attacks against TCP            January 2010


   ICMP 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 which have their own mechanisms for notifying
      this condition should not be receiving these error messages, as
      the protocol would signal the port unreachable condition by means
      of its own messages.  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.

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

      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 ICMP 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



Gont                      Expires July 23, 2010                [Page 14]

Internet-Draft          ICMP attacks against TCP            January 2010


   TCP SHOULD abort the corresponding connection in response to ICMP
   messages of type 3, 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 ICMP port unreachable
   (type 3, code 3) for the same purpose as an RST.  Therefore, for ICMP
   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
   ICMP 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.
   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 end points
   [RFC0816].

   It is interesting to note that, as ICMP error messages are
   unreliable, 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
   contition, 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



Gont                      Expires July 23, 2010                [Page 15]

Internet-Draft          ICMP attacks against TCP            January 2010


   that for many internal failures it reconstructs its function without
   any disruption of the end points [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].


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 ICMP Source Quench messages by slowing
   transmission on the connection.  Thus, an attacker could send ICMP
   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 end-point 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 ICMP Source Quench is an
   ineffective (and unfair) antidote for congestion.  [RFC1812] further
   states that routers SHOULD NOT send ICMP Source Quench messages in
   response to congestion.  Furthermore, TCP implements its own
   congestion control mechanisms [RFC5681] [RFC3168], that do not depend
   on ICMP Source Quench messages.

   Based on this reasoning, a large number of implementations completely
   ignore ICMP 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 behaviour violates
   the requirement in [RFC1122] to react to ICMP Source Quench messages



Gont                      Expires July 23, 2010                [Page 16]

Internet-Draft          ICMP attacks against TCP            January 2010


   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
   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
   ICMP "Destination Unreachable" (type 3) "fragmentation needed and DF
   set" (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 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 ICMP "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



Gont                      Expires July 23, 2010                [Page 17]

Internet-Draft          ICMP attacks against TCP            January 2010


   the headers/data ratio, thus increasing the overhead needed to send
   data to the remote TCP end-point.  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 increase in the IRQ
   (Interrrupt ReQuest) rate and protocol processing time, thus
   increasing processor utilization, and degrading the overall system
   performance.

   A particular scenario that may take place is that 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
   fragmentation.  This particular scenario might lead to unpredictable
   results.  Another posible scenario is that 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 low as 68 octets, as
   [RFC0791] requires every internet module to be able to forward a
   datagram of 68 octets without further fragmentation.  For IPv6, the
   reported Next-Hop MTU could be as low as 1280 octets (the minimum
   IPv6 MTU) [RFC2460].

7.2.  Attack-specific counter-measures

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

   This section describes a modification to the PMTUD mechanism
   specified in [RFC1191] and [RFC1981] that has been implemented in a
   variety of TCP implementations to improve TCP's resistance to the
   blind performance-degrading attack described in Section 7.1.  The
   described mechanism basically disregards ICMP messages when a
   connection makes progress.  This modification does not violate any of
   the requirements stated in [RFC1191] and [RFC1981].




Gont                      Expires July 23, 2010                [Page 18]

Internet-Draft          ICMP attacks against TCP            January 2010


   Henceforth, we will refer to both ICMP "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"
   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: maxsizeacked and
   maxsizesent.



Gont                      Expires July 23, 2010                [Page 19]

Internet-Draft          ICMP attacks against TCP            January 2010


   maxsizesent 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.

   On the other hand, maxsizeacked holds the size (in octets) of the
   largest packet 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.

   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 elicited 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 is
   introduced to TCP: nsegrto and MAXSEGRTO. nsegrto 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.  MAXSEGRRTO specifies the number of times a given segment must
   timeout 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.

   Thus, if nsegrto is greater than or equal to MAXSEGRTO, and there's a



Gont                      Expires July 23, 2010                [Page 20]

Internet-Draft          ICMP attacks against TCP            January 2010


   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 elicited 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.

   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 the proposed counter-measure in action.
   Section 7.4 shows the proposed counter-measure in pseudo-code.

   This behavior has been implemented in NetBSD [NetBSD] and OpenBSD
   [OpenBSD] since 2005.

   It is important to note that the mechanism proposed 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                      Expires July 23, 2010                [Page 21]

Internet-Draft          ICMP attacks against TCP            January 2010


7.3.  The counter-measure for the PMTUD attack in action

   This SECTION shows the proposed counter-measure for the ICMP attack
   against the PMTUD mechanism in action.  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 an 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 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 sake, all the following subsections assume the TCP
   implementation at Host 1 has chosen a 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 proposed counter-measure in normal
   operation, when a TCP connection is used for bulk transfers.  That
   is, it shows how the proposed 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                      Expires July 23, 2010                [Page 22]

Internet-Draft          ICMP attacks against TCP            January 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

   nsegrto 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. maxsizesent
   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. maxsizesent 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



Gont                      Expires July 23, 2010                [Page 23]

Internet-Draft          ICMP attacks against TCP            January 2010


   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. maxsizesent 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. maxsizesent is thus 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 proposed counter-measure would work in this
   scenario.


        Host 1                                         Host 2

   1.                     (Path-MTU decreases)
   2.      -->  <SEQ=100><ACK=X><CTL=ACK><DATA=1500>   -->
   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-



Gont                      Expires July 23, 2010                [Page 24]

Internet-Draft          ICMP attacks against TCP            January 2010


   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
   claimedmtu is smaller than maxsizeacked (full-sized packets were
   getting to the remote end-point), 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. nsegrto is reset to 0, and 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.  At 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 the TCP sequence number each ICMP packet includes, none of
   these ICMP error messages will pass the TCP sequence number check



Gont                      Expires July 23, 2010                [Page 25]

Internet-Draft          ICMP attacks against TCP            January 2010


   described in Section 4.1, as H1 has no unacknowledged data in flight
   to H2.  Therefore, the attack does not succeed.

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




Gont                      Expires July 23, 2010                [Page 26]

Internet-Draft          ICMP attacks against TCP            January 2010


   Figure 6 shows a possible packet exchange for such scenario.


      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#=101 <---


   Figure 6: TCP peer attacked when sending small packets just after the
                            three-way handshake

   nsegrto 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. maxsizesent is thus 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



Gont                      Expires July 23, 2010                [Page 27]

Internet-Draft          ICMP attacks against TCP            January 2010


   functions:

   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) 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.




Gont                      Expires July 23, 2010                [Page 28]

Internet-Draft          ICMP attacks against TCP            January 2010


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

   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 timeout
      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




Gont                      Expires July 23, 2010                [Page 29]

Internet-Draft          ICMP attacks against TCP            January 2010


        if (claimedmtu <= MINIMUM_MTU)
             drop_message();

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

        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 comparisions between sequence numbers must be performed using
      sequence number arithmethic.
      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].




Gont                      Expires July 23, 2010                [Page 30]

Internet-Draft          ICMP attacks against TCP            January 2010


8.  Security Considerations

   This document describes the use of ICMP error messages to perform a
   number of attacks against the TCP protocol, 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.

   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 obfuscation of TCP ephemeral ports require
   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 countermeasure eliminates
   the aforementioned vulnerability in synchronized connections at the
   expense of a 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 ICMP
   Source Quench messages meant for TCP connections.  This is in
   accordance with research results that indicate that ICMP Source
   Quench messages are ineffective and unfair antidote for congestion.

   Finally, Section 7.2 describes an attack-specific countermeasure 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
   countermeasures parallels the Packetization Layer Path MTU Discovery
   (PLPMTUD) mechanism [RFC4821].




Gont                      Expires July 23, 2010                [Page 31]

Internet-Draft          ICMP attacks against TCP            January 2010


   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 [I-D.ietf-tcpm-tcp-security].


9.  IANA Considerations

   This document has no actions for IANA.  The RFC-Editor can remove
   this section before publication of this document as an RFC.


10.  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, 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.  This 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 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.


11.  References



Gont                      Expires July 23, 2010                [Page 32]

Internet-Draft          ICMP attacks against TCP            January 2010


11.1.  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.

   [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.

11.2.  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".



Gont                      Expires July 23, 2010                [Page 33]

Internet-Draft          ICMP attacks against TCP            January 2010


   [I-D.ietf-tcpm-tcp-auth-opt]
              Touch, J., Mankin, A., and R. Bonica, "The TCP
              Authentication Option", draft-ietf-tcpm-tcp-auth-opt-08
              (work in progress), October 2009.

   [I-D.ietf-tcpm-tcp-security]
              Gont, F., "Security Assessment of the Transmission Control
              Protocol (TCP)", draft-ietf-tcpm-tcp-security-00 (work in
              progress), August 2009.

   [I-D.ietf-tcpm-tcpsecure]
              Ramaiah, A., Stewart, R., and M. Dalal, "Improving TCP's
              Robustness to Blind In-Window Attacks",
              draft-ietf-tcpm-tcpsecure-12 (work in progress),
              September 2009.

   [I-D.ietf-tsvwg-port-randomization]
              Larsen, M. and F. Gont, "Port Randomization",
              draft-ietf-tsvwg-port-randomization-05 (work in progress),
              November 2009.

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

   [IP-filtering]
              NISCC, "NISCC Technical Note 01/2006: Egress and Ingress
              Filtering",  http://www.niscc.gov.uk/niscc/docs/
              re-20060420-00294.pdf?lang=en, 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 4.4BSD Operating
              System", Addison-Wesley , 1996.

   [NISCC]    NISCC, "NISCC Vulnerability Advisory 532967/NISCC/ICMP:
              Vulnerability Issues in ICMP packets with TCP payloads",
              http://www.niscc.gov.uk/niscc/docs/
              al-20050412-00308.html?lang=en, 2005.

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

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

   [OpenBSD-PF]



Gont                      Expires July 23, 2010                [Page 34]

Internet-Draft          ICMP attacks against TCP            January 2010


              The OpenBSD Packet Filter,
              "http://www.openbsd.org/faq/pf/".

   [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.

   [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



Gont                      Expires July 23, 2010                [Page 35]

Internet-Draft          ICMP attacks against TCP            January 2010


              Control", RFC 5681, September 2009.

   [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",
              2004 CanSecWest Conference , 2004.

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


Appendix A.  Changes from previous versions of the draft (to be removed
             by the RFC Editor before publishing this document as an
             RFC)

A.1.  Changes from draft-ietf-tcpm-icmp-attacks-08

   o  Fixes a couple of nits found by...  Alfred!.  Thanks! (again, and
      again, and again....).

A.2.  Changes from draft-ietf-tcpm-icmp-attacks-07

   o  Addresses some remaining WGLC feedback sent off-list by Donald
      Smith and Guillermo Gont.

A.3.  Changes from draft-ietf-tcpm-icmp-attacks-06

   o  Addresses WGLC feedback by Joe Touch and Donald Smith.

A.4.  Changes from draft-ietf-tcpm-icmp-attacks-05

   o  Addresses feedback submitted by Wes Eddy
      (http://www.ietf.org/mail-archive/web/tcpm/current/msg04573.html
      and
      http://www.ietf.org/mail-archive/web/tcpm/current/msg04574.html)
      and Joe Touch (on June 8th... couldn't find online ref, sorry) on
      the TCPM WG mailing-list.

A.5.  Changes from draft-ietf-tcpm-icmp-attacks-04

   o  The draft had expired and thus is resubmitted with no further
      changes.  Currently working on a rev of the document (Please send
      feedback!).






Gont                      Expires July 23, 2010                [Page 36]

Internet-Draft          ICMP attacks against TCP            January 2010


A.6.  Changes from draft-ietf-tcpm-icmp-attacks-03

   o  The draft had expired and thus is resubmitted with no further
      changes.

A.7.  Changes from draft-ietf-tcpm-icmp-attacks-02

   o  Added a disclaimer to indicate that this document does not update
      the current specifications.

   o  Addresses feedback sent off-list by Alfred Hoenes.

   o  The text (particulary that which describes the counter-measures)
      was reworded to document what current implementations are doing,
      rather than "proposing" the implementation of the counter-
      measures.

   o  Some text has been removed: we're just documenting the problem,
      and what existing implementations have done.

   o  Miscelaneous editorial changes.

A.8.  Changes from draft-ietf-tcpm-icmp-attacks-01

   o  Fixed references to the antispoof documents (were hardcoded and
      missing in the References Section).

   o  The draft had expired and thus is resubmitted with only a minor
      editorial change.

A.9.  Changes from draft-ietf-tcpm-icmp-attacks-00

   o  Added references to the specific sections of each of the
      referenced specifications

   o  Corrected the threat analysys

   o  Added clarification about whether the counter-measures violate the
      current specifications or not.

   o  Changed text so that the document fits better in the Informational
      path

   o  Added a specific section on IPsec (Section 2.3)

   o  Added clarification and references on the use of ICMP filtering
      based on the ICMP payload




Gont                      Expires July 23, 2010                [Page 37]

Internet-Draft          ICMP attacks against TCP            January 2010


   o  Updated references to obsoleted RFCs

   o  Added a discussion of multipath scenarios, and possible lose in
      responsiveness resulting from the reaction to hard errors as soft
      errors

   o  Miscellaneous editorial changes

A.10.  Changes from draft-gont-tcpm-icmp-attacks-05

   o  Removed RFC 2119 wording to make the draft suitable for
      publication as an Informational RFC.

   o  Added additional checks that should be performed on ICMP error
      messages (checksum of the IP header in the ICMP payload, and
      others).

   o  Added clarification of the rationale behind each the TCP SEQ check

   o  Miscellaneous editorial changes

A.11.  Changes from draft-gont-tcpm-icmp-attacks-04

   o  Added section on additional considerations for validating ICMP
      error messages

   o  Added reference to (draft) [RFC4907]

   o  Added stress on the fact that ICMP error messages are unreliable

   o  Miscellaneous editorial changes

A.12.  Changes from draft-gont-tcpm-icmp-attacks-03

   o  Added references to existing implementations of the proposed
      counter-measures

   o  The discussion in Section 4 was improved

   o  The discussion of the blind connection-reset vulnerability was
      expanded and improved

   o  The proposed counter-measure for the attack against the PMTUD was
      improved and simplified

   o  Section 7.4 was added





Gont                      Expires July 23, 2010                [Page 38]

Internet-Draft          ICMP attacks against TCP            January 2010


   o  Miscellaneous editorial changes

A.13.  Changes from draft-gont-tcpm-icmp-attacks-02

   o  Fixed errors in in the discussion of the blind connection-reset
      attack

   o  The proposed counter-measure for the attack against the PMTUD
      mechanism was refined to allow quick discovery of the Path-MTU

   o  Section 7.3 was added so as to clarify the operation of the
      counter-measure for the attack against the PMTUD mechanism

   o  Added CPNI contact information.

   o  Miscellaneous editorial changes

A.14.  Changes from draft-gont-tcpm-icmp-attacks-01

   o  The document was restructured for easier reading

   o  A discussion of ICMPv6 was added in several sections of the
      document

   o  Added Section on Acknowledgement number checking

   o  Added Section 4.3

   o  Added Section 7

   o  Fixed typo in the ICMP types, in several places

   o  Fixed typo in the TCP sequence number check formula

   o  Miscellaneous editorial changes

A.15.  Changes from draft-gont-tcpm-icmp-attacks-00

   o  Added a proposal to change the handling of the so-called ICMP hard
      errors during the synchronized states

   o  Added a summary of the relevant RFCs in several sections

   o  Miscellaneous editorial changes







Gont                      Expires July 23, 2010                [Page 39]

Internet-Draft          ICMP attacks against TCP            January 2010


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: fernando@gont.com.ar
   URI:   http://www.gont.com.ar








































Gont                      Expires July 23, 2010                [Page 40]


Html markup produced by rfcmarkup 1.108, available from http://tools.ietf.org/tools/rfcmarkup/