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Internet Engineering Task Force (IETF)                           F. GontRequest for Comments: 5927                                       UTN/FRHCategory: Informational                                        July 2010ISSN: 2070-1721ICMP Attacks against TCPAbstract   This document discusses the use of the Internet Control Message   Protocol (ICMP) to perform a variety of attacks against the   Transmission Control Protocol (TCP).  Additionally, this document   describes a number of widely implemented modifications to TCP's   handling of ICMP error messages that help to mitigate these issues.Status of This Memo   This document is not an Internet Standards Track specification; it is   published for informational purposes.   This document is a product of the Internet Engineering Task Force   (IETF).  It represents the consensus of the IETF community.  It has   received public review and has been approved for publication by the   Internet Engineering Steering Group (IESG).  Not all documents   approved by the IESG are a candidate for any level of Internet   Standard; seeSection 2 of RFC 5741.   Information about the current status of this document, any errata,   and how to provide feedback on it may be obtained athttp://www.rfc-editor.org/info/rfc5927.Gont                          Informational                     [Page 1]

RFC 5927                ICMP Attacks against TCP               July 2010Copyright Notice   Copyright (c) 2010 IETF Trust and the persons identified as the   document authors.  All rights reserved.   This document is subject toBCP 78 and the IETF Trust's Legal   Provisions Relating to IETF Documents   (http://trustee.ietf.org/license-info) in effect on the date of   publication of this document.  Please review these documents   carefully, as they describe your rights and restrictions with respect   to this document.  Code Components extracted from this document must   include Simplified BSD License text as described in Section 4.e of   the Trust Legal Provisions and are provided without warranty as   described in the Simplified BSD License.   This document may contain material from IETF Documents or IETF   Contributions published or made publicly available before November   10, 2008.  The person(s) controlling the copyright in some of this   material may not have granted the IETF Trust the right to allow   modifications of such material outside the IETF Standards Process.   Without obtaining an adequate license from the person(s) controlling   the copyright in such materials, this document may not be modified   outside the IETF Standards Process, and derivative works of it may   not be created outside the IETF Standards Process, except to format   it for publication as an RFC or to translate it into languages other   than English.Gont                          Informational                     [Page 2]

RFC 5927                ICMP Attacks against TCP               July 2010Table of Contents1.  Introduction . . . . . . . . . . . . . . . . . . . . . . . . .42.  Background . . . . . . . . . . . . . . . . . . . . . . . . . .52.1.  The Internet Control Message Protocol (ICMP) . . . . . . .52.1.1.  ICMP for IP version 4 (ICMPv4) . . . . . . . . . . . .52.1.2.  ICMP for IP version 6 (ICMPv6) . . . . . . . . . . . .62.2.  Handling of ICMP Error Messages  . . . . . . . . . . . . .6     2.3.  Handling of ICMP Error Messages in the Context of IPsec  .  73.  Constraints in the Possible Solutions  . . . . . . . . . . . .84.  General Counter-Measures against ICMP Attacks  . . . . . . . .104.1.  TCP Sequence Number Checking . . . . . . . . . . . . . . .104.2.  Port Randomization . . . . . . . . . . . . . . . . . . . .11     4.3.  Filtering ICMP Error Messages Based on the ICMP Payload  . 115.  Blind Connection-Reset Attack  . . . . . . . . . . . . . . . .125.1.  Description  . . . . . . . . . . . . . . . . . . . . . . .125.2.  Attack-Specific Counter-Measures . . . . . . . . . . . . .136.  Blind Throughput-Reduction Attack  . . . . . . . . . . . . . .166.1.  Description  . . . . . . . . . . . . . . . . . . . . . . .166.2.  Attack-Specific Counter-Measures . . . . . . . . . . . . .167.  Blind Performance-Degrading Attack . . . . . . . . . . . . . .167.1.  Description  . . . . . . . . . . . . . . . . . . . . . . .167.2.  Attack-Specific Counter-Measures . . . . . . . . . . . . .187.3.  The Counter-Measure for the PMTUD Attack in Action . . . .227.3.1.  Normal Operation for Bulk Transfers  . . . . . . . . .227.3.2.  Operation during Path-MTU Changes  . . . . . . . . . .247.3.3.  Idle Connection Being Attacked . . . . . . . . . . . .25       7.3.4.  Active Connection Being Attacked after Discovery               of the Path-MTU  . . . . . . . . . . . . . . . . . . .26       7.3.5.  TCP Peer Attacked when Sending Small Packets Just               after the Three-Way Handshake  . . . . . . . . . . . .26     7.4.  Pseudo-Code for the Counter-Measure for the Blind           Performance-Degrading Attack . . . . . . . . . . . . . . .278.  Security Considerations  . . . . . . . . . . . . . . . . . . .309.  Acknowledgements . . . . . . . . . . . . . . . . . . . . . . .3210. References . . . . . . . . . . . . . . . . . . . . . . . . . .3210.1. Normative References . . . . . . . . . . . . . . . . . . .3210.2. Informative References . . . . . . . . . . . . . . . . . .33Gont                          Informational                     [Page 3]

RFC 5927                ICMP Attacks against TCP               July 20101.  Introduction   ICMP [RFC0792] [RFC4443] is a fundamental part of the TCP/IP protocol   suite, and is used mainly for reporting network error conditions.   However, the current specifications do not recommend any kind of   validation checks on the received ICMP error messages, thus allowing   a variety of attacks against TCP [RFC0793] by means of ICMP, which   include blind connection-reset, blind throughput-reduction, and blind   performance-degrading attacks.  All of these attacks can be performed   even when the attacker is off-path, without the need to sniff the   packets that correspond to the attacked TCP connection.   While the possible security implications of ICMP have been known in   the research community for a long time, there has never been an   official proposal on how to deal with these vulnerabilities.  In   2005, a disclosure process was carried out by the UK's National   Infrastructure Security Co-ordination Centre (NISCC) (now CPNI,   Centre for the Protection of National Infrastructure), with the   collaboration of other computer emergency response teams.  A large   number of implementations were found vulnerable to either all or a   subset of the attacks discussed in this document [NISCC][US-CERT].   The affected systems ranged from TCP/IP implementations meant for   desktop computers, to TCP/IP implementations meant for core Internet   routers.   It is clear that implementations should be more cautious when   processing ICMP error messages, to eliminate or mitigate the use of   ICMP to perform attacks against TCP [RFC4907].   This document aims to raise awareness of the use of ICMP to perform a   variety of attacks against TCP, and discusses several counter-   measures that eliminate or minimize the impact of these attacks.   Most of the these counter-measures can be implemented while still   remaining compliant with the current specifications, as they simply   describe reasons for not taking the advice provided in the   specifications in terms of "SHOULDs", but still comply with the   requirements stated as "MUSTs".   We note that the counter-measures discussed in this document are not   part of standard TCP behavior, and this document does not change that   state of affairs.  The consensus of the TCPM WG (TCP Maintenance and   Minor Extensions Working Group) was to document this widespread   implementation of nonstandard TCP behavior but to not change the TCP   standard.Section 2 provides background information on ICMP.Section 3   discusses the constraints in the general counter-measures that can be   implemented against the attacks described in this document.Gont                          Informational                     [Page 4]

RFC 5927                ICMP Attacks against TCP               July 2010Section 4 describes several general validation checks that can be   implemented to mitigate any ICMP-based attack.  Finally,Section 5,Section 6, andSection 7, discuss a variety of ICMP attacks that can   be performed against TCP, and describe attack-specific counter-   measures that eliminate or greatly mitigate their impact.   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",   "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this   document are to be interpreted as described inRFC 2119 [RFC2119].2.  Background2.1.  The Internet Control Message Protocol (ICMP)   The Internet Control Message Protocol (ICMP) is used in the Internet   architecture mainly to perform the fault-isolation function, that is,   the group of actions that hosts and routers take to determine that   there is some network failure [RFC0816].   When an intermediate router detects a network problem while trying to   forward an IP packet, it will usually send an ICMP error message to   the source system, to inform the source system of the network problem   taking place.  In the same way, there are a number of scenarios in   which an end-system may generate an ICMP error message if it finds a   problem while processing a datagram.  The received ICMP errors are   handed to the corresponding transport-protocol instance, which will   usually perform a fault recovery function.   It is important to note that ICMP error messages are transmitted   unreliably and may be discarded due to data corruption, network   congestion, or rate-limiting.  Thus, while they provide useful   information, upper-layer protocols cannot depend on ICMP for correct   operation.   It should be noted that there are no timeliness requirements for ICMP   error messages.  ICMP error messages could be delayed for various   reasons, and at least in theory could be received with an arbitrarily   long delay.  For example, there are no existing requirements that a   router flush any queued ICMP error messages when it is rebooted.2.1.1.  ICMP for IP version 4 (ICMPv4)   [RFC0792] specifies the Internet Control Message Protocol (ICMP) to   be used with the Internet Protocol version 4 (IPv4) -- henceforth   "ICMPv4".  It defines, among other things, a number of error messages   that can be used by end-systems and intermediate systems to report   errors to the sending system.  The Host Requirements RFC [RFC1122]Gont                          Informational                     [Page 5]

RFC 5927                ICMP Attacks against TCP               July 2010   classifies ICMPv4 error messages into those that indicate "soft   errors", and those that indicate "hard errors", thus roughly defining   the semantics of them.   The ICMPv4 specification [RFC0792] also defines the ICMPv4 Source   Quench message (type 4, code 0), which is meant to provide a   mechanism for flow control and congestion control.   [RFC1191] defines a mechanism called "Path MTU Discovery" (PMTUD),   which makes use of ICMPv4 error messages of type 3 (Destination   Unreachable), code 4 (fragmentation needed and DF bit set) to allow   systems to determine the MTU of an arbitrary internet path.   Finally, [RFC4884] redefines selected ICMPv4 messages to include an   extension structure and a length attribute, such that those ICMPv4   messages can carry additional information by encoding that   information in the extension structure.Appendix D of [RFC4301] provides information about which ICMPv4 error   messages are produced by hosts, intermediate routers, or both.2.1.2.  ICMP for IP version 6 (ICMPv6)   [RFC4443] specifies the Internet Control Message Protocol (ICMPv6) to   be used with the Internet Protocol version 6 (IPv6) [RFC2460].   [RFC4443] defines the "Packet Too Big" (type 2, code 0) error   message, which is analogous to the ICMPv4 "fragmentation needed and   DF bit set" (type 3, code 4) error message.  [RFC1981] defines the   Path MTU Discovery mechanism for IP version 6, which makes use of   these messages to determine the MTU of an arbitrary internet path.   Finally, [RFC4884] redefines selected ICMPv6 messages to include an   extension structure and a length attribute, such that those ICMPv6   messages can carry additional information by encoding that   information in the extension structure.Appendix D of [RFC4301] provides information about which ICMPv6 error   messages are produced by hosts, intermediate routers, or both.2.2.  Handling of ICMP Error Messages   The Host Requirements RFC [RFC1122] states inSection 4.2.3.9 that   TCP MUST act on an ICMP error message passed up from the IP layer,   directing it to the connection that triggered the error.Gont                          Informational                     [Page 6]

RFC 5927                ICMP Attacks against TCP               July 2010   In order to allow ICMP messages to be demultiplexed by the receiving   system, part of the original packet that triggered the message is   included in the payload of the ICMP error message.  Thus, the   receiving system can use that information to match the ICMP error to   the transport protocol instance that triggered it.   Neither the Host Requirements RFC [RFC1122] nor the original TCP   specification [RFC0793] recommends any validation checks on the   received ICMP messages.  Thus, as long as the ICMP payload contains   the information that identifies an existing communication instance,   it will be processed by the corresponding transport-protocol   instance, and the corresponding action will be performed.   Therefore, in the case of TCP, an attacker could send a crafted ICMP   error message to the attacked system, and, as long as he is able to   guess the four-tuple (i.e., Source IP Address, Source TCP port,   Destination IP Address, and Destination TCP port) that identifies the   communication instance to be attacked, he will be able to use ICMP to   perform a variety of attacks.   Generally, the four-tuple required to perform these attacks is not   known.  However, as discussed in [Watson] and [RFC4953], there are a   number of scenarios (notably that of TCP connections established   between two BGP routers [RFC4271]) in which an attacker may be able   to know or guess the four-tuple that identifies a TCP connection.  In   such a case, if we assume the attacker knows the two systems involved   in the TCP connection to be attacked, both the client-side and the   server-side IP addresses could be known or be within a reasonable   number of possibilities.  Furthermore, as most Internet services use   the so-called "well-known" ports, only the client port number might   need to be guessed.  In such a scenario, an attacker would need to   send, in principle, at most 65536 packets to perform any of the   attacks described in this document.  These issues are exacerbated by   the fact that most systems choose the port numbers they use for   outgoing connections from a subset of the whole port number space,   thus reducing the amount of work needed to successfully perform these   attacks.   The need to be more cautious when processing received ICMP error   messages in order to mitigate or eliminate the impact of the attacks   described in this RFC has been documented by the Internet   Architecture Board (IAB) in [RFC4907].2.3.  Handling of ICMP Error Messages in the Context of IPsecSection 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 isGont                          Informational                     [Page 7]

RFC 5927                ICMP Attacks against TCP               July 2010   matched to the corresponding security association by means of the SPI   (Security Parameters Index) included in the payload of the ICMP error   message.  Then, local policy is applied to determine whether to   accept or reject the message and, if accepted, what action to take as   a result.  For example, if an ICMP Destination Unreachable message is   received, the implementation must decide whether to act on it, reject   it, or act on it with constraints.Section 8 ("Path MTU/DF   Processing") discusses the processing of unauthenticated ICMPv4   "fragmentation needed and DF bit set" (type 3, code 4) and ICMPv6   "Packet Too Big" (type 2, code 0) messages when an IPsec   implementation is configured to process (vs. ignore) such messages.Section 6.1.1 of [RFC4301] notes that processing of unauthenticated   ICMP error messages may result in denial or degradation of service,   and therefore it would be desirable to ignore such messages.   However, it also notes that in many cases, ignoring these ICMP   messages can degrade service, e.g., because of a failure to process   PMTUD and redirection messages, and therefore there is also a   motivation for accepting and acting upon them.  It finally states   that to accommodate both ends of this spectrum, a compliant IPsec   implementation MUST permit a local administrator to configure an   IPsec implementation to accept or reject unauthenticated ICMP   traffic, and that this control MUST be at the granularity of ICMP   type and MAY be at the granularity of ICMP type and code.   Additionally, an implementation SHOULD incorporate mechanisms and   parameters for dealing with such traffic.   Thus, the policy to apply for the processing of unprotected ICMP   error messages is left up to the implementation and administrator.3.  Constraints in the Possible Solutions   If a host wants to perform validation checks on the received ICMP   error messages before acting on them, it is limited by the piece of   the packet that triggered the error that the sender of the ICMP error   message chose to include in the ICMP payload.  This constrains the   possible validation checks, as the number of bytes of the packet that   triggered the error message that is included in the ICMP payload is   limited.   For ICMPv4, [RFC0792] states that the IP header plus the first   64 bits of the packet that triggered the ICMPv4 message are to be   included in the payload of the ICMPv4 error message.  Thus, it is   assumed that all data needed to identify a transport protocol   instance and process the ICMPv4 error message is contained in the   first 64 bits of the transport protocol header.Section 3.2.2 of   [RFC1122] states that "the Internet header and at least the first 8   data octets of the datagram that triggered the error" are to beGont                          Informational                     [Page 8]

RFC 5927                ICMP Attacks against TCP               July 2010   included in the payload of ICMPv4 error messages, and that "more than   8 octets MAY be sent", thus allowing implementations to include more   data from the original packet than those required by the original   ICMPv4 specification.  The "Requirements for IP Version 4 Routers"   RFC [RFC1812] states that ICMPv4 error messages "SHOULD contain as   much of the original datagram as possible without the length of the   ICMP datagram exceeding 576 bytes".   Thus, for ICMPv4 messages generated by hosts, we can only expect to   get the entire IP header of the original packet, plus the first   64 bits of its payload.  For TCP, this means that the only fields   that will be included in the ICMPv4 payload are the source port   number, the destination port number, and the 32-bit TCP sequence   number.  This clearly imposes a constraint on the possible validation   checks that can be performed, as there is not much information   available on which to perform them.   This means, for example, that even if TCP were signing its segments   by means of the TCP MD5 signature option [RFC2385], this mechanism   could not be used as a counter-measure against ICMP-based attacks,   because, as ICMP messages include only a piece of the TCP segment   that triggered the error, the MD5 [RFC1321] signature could not be   recalculated.  In the same way, even if the attacked peer were   authenticating its packets at the IP layer [RFC4301], because only a   part of the original IP packet would be available, the signature used   for authentication could not be recalculated, and thus the   authentication header in the original packet could not be used as a   counter-measure for ICMP-based attacks against TCP.   [RFC4884] updated [RFC0792] and specified that ICMPv4 Destination   Unreachable (type 3), Time Exceeded (type 11), and Parameter Problem   (type 12) messages that have an ICMP Extension Structure appended   include at least 128 octets in the "original datagram" field.  This   would improve the situation, but at the time of this writing,   [RFC4884] is not yet widely deployed for end-systems.   For IPv6, the payload of ICMPv6 error messages includes as many   octets from the IPv6 packet that triggered the ICMPv6 error message   as will fit without making the resulting ICMPv6 error message exceed   the minimum IPv6 MTU (1280 octets) [RFC4443].  Thus, more information   is available than in the IPv4 case.   Hosts could require ICMP error messages to be authenticated   [RFC4301], in order to act upon them.  However, while this   requirement could make sense for those ICMP error messages sent by   hosts, it would not be feasible for those ICMP error messages   generated by routers, as this would imply either that the attacked   system should have a security association [RFC4301] with everyGont                          Informational                     [Page 9]

RFC 5927                ICMP Attacks against TCP               July 2010   existing intermediate system, or that it should be able to establish   one dynamically.  Current levels of deployment of protocols for   dynamic establishment of security associations makes this unfeasible.   Additionally, this would require routers to use certificates with   paths compatible for all hosts on the network.  Finally, there may be   some scenarios, such as embedded devices, in which the processing   power requirements of authentication might not allow IPsec   authentication to be implemented effectively.4.  General Counter-Measures against ICMP Attacks   The following subsections describe a number of mitigation techniques   that help to eliminate or mitigate the impact of the attacks   discussed in this document.  Rather than being alternative counter-   measures, they can be implemented together to increase the protection   against these attacks.4.1.  TCP Sequence Number Checking   The current specifications do not impose any validity checks on the   TCP segment that is contained in the ICMP payload.  For instance, no   checks are performed to verify that a received ICMP error message has   been triggered by a segment that was "in flight" to the destination.   Thus, even stale ICMP error messages will be acted upon.   Many TCP implementations have incorporated a validation check such   that they react only to those ICMP error messages that appear to   relate to segments currently "in flight" to the destination system.   These implementations check that the TCP sequence number contained in   the payload of the ICMP error message is within the range   SND.UNA =< SEG.SEQ < SND.NXT.  This means that they require that the   sequence number be within the range of the data already sent but not   yet acknowledged.  If an ICMP error message does not pass this check,   it is discarded.   Even if an attacker were able to guess the four-tuple that identifies   the TCP connection, this additional check would reduce the   possibility of considering a spoofed ICMP packet as valid to   Flight_Size/2^^32 (where Flight_Size is the number of data bytes   already sent to the remote peer, but not yet acknowledged [RFC5681]).   For connections in the SYN-SENT or SYN-RECEIVED states, this would   reduce the possibility of considering a spoofed ICMP packet as valid   to 1/2^^32.  For a TCP endpoint with no data "in flight", this would   completely eliminate the possibility of success of these attacks.   This validation check has been implemented in Linux [Linux] for many   years, in OpenBSD [OpenBSD] since 2004, and in FreeBSD [FreeBSD] and   NetBSD [NetBSD] since 2005.Gont                          Informational                    [Page 10]

RFC 5927                ICMP Attacks against TCP               July 2010   It is important to note that while this check greatly increases the   number of packets required to perform any of the attacks discussed in   this document, this may not be enough in those scenarios in which   bandwidth is easily available and/or large TCP windows [RFC1323] are   in use.  Additionally, this validation check does not help to prevent   on-path attacks, that is, attacks performed in scenarios in which the   attacker can sniff the packets that correspond to the target TCP   connection.   It should be noted that, as there are no timeliness requirements for   ICMP error messages, the TCP Sequence Number check described in this   section might cause legitimate ICMP error messages to be discarded.   Also, even if this check is enforced, TCP might end up responding to   stale ICMP error messages (e.g., if the Sequence Number for the   corresponding direction of the data transfer wraps around).4.2.  Port Randomization   As discussed in the previous sections, in order to perform any of the   attacks described in this document, an attacker would need to guess   (or know) the four-tuple that identifies the connection to be   attacked.  Increasing the port number range used for outgoing TCP   connections, and randomizing the port number chosen for each outgoing   TCP connection, would make it harder for an attacker to perform any   of the attacks discussed in this document.   [PORT-RANDOM] recommends that transport protocols randomize the   ephemeral ports used by clients, and proposes a number of   randomization algorithms.4.3.  Filtering ICMP Error Messages Based on the ICMP Payload   The source address of ICMP error messages does not need to be spoofed   to perform the attacks described in this document, as the ICMP error   messages might legitimately come from an intermediate system.   Therefore, simple filtering based on the source address of ICMP error   messages does not serve as a counter-measure against these attacks.   However, a more advanced packet filtering can be implemented in   middlebox devices such as firewalls and NATs.  Middleboxes   implementing such advanced filtering look at the payload of the ICMP   error messages, and perform ingress and egress packet filtering based   on the source address of the IP header contained in the payload of   the ICMP error message.  As the source address contained in the   payload of the ICMP error message does need to be spoofed to perform   the attacks described in this document, this kind of advanced   filtering serves as a counter-measure against these attacks.  As with   traditional egress filtering [IP-filtering], egress filtering based   on the ICMP payload can help to prevent users of the network beingGont                          Informational                    [Page 11]

RFC 5927                ICMP Attacks against TCP               July 2010   protected by the firewall from successfully performing ICMP attacks   against TCP connections established between external systems.   Additionally, ingress filtering based on the ICMP payload can prevent   TCP connections established between internal systems from being   attacked by external systems.  [ICMP-Filtering] provides examples of   ICMP filtering based on the ICMP payload.   This filtering technique has been implemented in OpenBSD's Packet   Filter [OpenBSD-PF], which has in turn been ported to a number of   systems, including FreeBSD [FreeBSD].5.  Blind Connection-Reset Attack5.1.  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 (inSection 4.2.3.9) that   a host SHOULD abort the corresponding connection when receiving an   ICMPv4 error message that indicates a "hard error", and states that   ICMPv4 error messages of type 3 (Destination Unreachable), codes 2   (protocol unreachable), 3 (port unreachable), and 4 (fragmentation   needed and DF bit set) should be considered as indicating "hard   errors".  In the case of ICMPv4 port unreachables, the specifications   are ambiguous, asSection 4.2.3.9 of [RFC1122] states that TCP SHOULD   abort the corresponding connection in response to them, butSection 3.2.2.1 of the same RFC ([RFC1122]) states that TCP MUST   abort the connection in response to them.   While [RFC4443] did not exist when [RFC1122] was published, one could   extrapolate the concept of "hard errors" to ICMPv6 error messages of   type 1 (Destination Unreachable), codes 1 (communication with   destination administratively prohibited), and 4 (port unreachable).   Thus, an attacker could use ICMP to perform a blind connection-reset   attack by sending any ICMP error message that indicates a "hard   error" to either of the two TCP endpoints of the connection.  Because   of TCP's fault recovery policy, the connection would be immediately   aborted.Gont                          Informational                    [Page 12]

RFC 5927                ICMP Attacks against TCP               July 2010   Some stacks are known to extrapolate ICMP "hard errors" across TCP   connections, increasing the impact of this attack, as a single ICMP   packet could bring down all the TCP connections between the   corresponding peers.   It is important to note that even if TCP itself were protected   against the blind connection-reset attack described in [Watson] and   [TCPM-TCPSECURE] by means of authentication at the network layer   [RFC4301], by means of the TCP MD5 signature option [RFC2385], by   means of the TCP-AO [RFC5925], or by means of the mechanism specified   in [TCPM-TCPSECURE], the blind connection-reset attack described in   this document would still succeed.5.2.  Attack-Specific Counter-Measures   An analysis of the circumstances in which ICMP messages that indicate   "hard errors" may be received can shed some light on opportunities to   mitigate the impact of ICMP-based blind connection-reset attacks.   ICMPv4 type 3 (Destination Unreachable), code 2 (protocol      unreachable)      This ICMP error message indicates that the host sending the ICMP      error message received a packet meant for a transport protocol it      does not support.  For connection-oriented protocols such as TCP,      one could expect to receive such an error as the result of a      connection-establishment attempt.  However, it would be strange to      get such an error during the life of a connection, as this would      indicate that support for that transport protocol has been removed      from the system sending the error message during the life of the      corresponding connection.   ICMPv4 type 3 (Destination Unreachable), code 3 (port unreachable)      This error message indicates that the system sending the ICMP      error message received a packet meant for a socket (IP address,      port number) on which there is no process listening.  Those      transport protocols that have their own mechanisms for signaling      this condition should not be receiving these error messages, as      the protocol would signal the port unreachable condition by means      of its own mechanisms.  Assuming that once a connection is      established it is not usual for the transport protocol to change      (or be reloaded), it should be unusual to get these error      messages.   ICMPv4 type 3 (Destination Unreachable), code 4 (fragmentation needed      and DF bit set)Gont                          Informational                    [Page 13]

RFC 5927                ICMP Attacks against TCP               July 2010      This error message indicates that an intermediate node needed to      fragment a datagram, but the DF (Don't Fragment) bit in the IP      header was set.  It is considered a "soft error" when TCP      implements PMTUD, and a "hard error" if TCP does not implement      PMTUD.  Those TCP/IP stacks that do not implement PMTUD (or have      disabled it) but support IP fragmentation/reassembly should not be      sending their IP packets with the DF bit set, and thus should not      be receiving these ICMP error messages.  Some TCP/IP stacks that      do not implement PMTUD and that do not support IP fragmentation/      reassembly are known to send their packets with the DF bit set,      and thus could legitimately receive these ICMP error messages.   ICMPv6 type 1 (Destination Unreachable), code 1 (communication with      destination administratively prohibited)      This error message indicates that the destination is unreachable      because of an administrative policy.  For connection-oriented      protocols such as TCP, one could expect to receive such an error      as the result of a connection-establishment attempt.  Receiving      such an error for a connection in any of the synchronized states      would mean that the administrative policy changed during the life      of the connection.  However, in the same way this error condition      (which was not present when the connection was established)      appeared, it could get solved in the near term.   ICMPv6 type 1 (Destination Unreachable), code 4 (port unreachable)      This error message is analogous to the ICMPv4 type 3 (Destination      Unreachable), code 3 (port unreachable) error message discussed      above.  Therefore, the same considerations apply.   The Host Requirements RFC [RFC1122] states inSection 4.2.3.9 that   TCP SHOULD abort the corresponding connection in response to ICMPv4   messages of type 3 (Destination Unreachable), codes 2 (protocol   unreachable), 3 (port unreachable), and 4 (fragmentation needed and   DF bit set).  However,Section 3.2.2.1 states that TCP MUST accept an   ICMPv4 port unreachable (type 3, code 3) for the same purpose as a   RST.  Therefore, for ICMPv4 messages of type 3, codes 2 and 4, there   is room to go against the advice provided in the existing   specifications, while in the case of ICMPv4 messages of type 3,   code 3, there is ambiguity in the specifications that may or may not   provide some room to go against that advice.   Based on this analysis, most popular TCP implementations treat all   ICMP "hard errors" received for connections in any of the   synchronized states (ESTABLISHED, FIN-WAIT-1, FIN-WAIT-2, CLOSE-WAIT,   CLOSING, LAST-ACK, or TIME-WAIT) as "soft errors".  That is, they do   not abort the corresponding connection upon receipt of them.Gont                          Informational                    [Page 14]

RFC 5927                ICMP Attacks against TCP               July 2010   Additionally, they do not extrapolate ICMP errors across TCP   connections.  This policy is based on the premise that TCP should be   as robust as possible.  Aborting the connection would be to ignore   the valuable feature of the Internet -- that for many internal   failures, it reconstructs its function without any disruption of the   endpoints [RFC0816].   It should be noted that treating ICMP "hard errors" as "soft errors"   for connections in any of the synchronized states may prevent TCP   from responding quickly to a legitimate ICMP error message.   It is interesting to note that, as ICMP error messages are   transmitted unreliably, transport protocols should not depend on them   for correct functioning.  In the event one of these messages were   legitimate, the corresponding connection would eventually time out.   Also, applications may still be notified asynchronously about the   error condition, and thus may still abort their connections on their   own if they consider it appropriate.   In scenarios such as that in which an intermediate system sets the DF   bit in the segments transmitted by a TCP that does not implement   PMTUD, or the TCP at one of the endpoints of the connection is   dynamically disabled, TCP would only abort the connection after a   USER TIMEOUT [RFC0793], losing responsiveness.  However, these   scenarios are very unlikely in production environments, and it is   probably preferable to potentially lose responsiveness for the sake   of robustness.  It should also be noted that applications may still   be notified asynchronously about the error condition, and thus may   still abort their connections on their own if they consider it   appropriate.   In scenarios of multipath routing or route changes, failures in some   (but not all) of the paths may elicit ICMP error messages that would   likely not cause a connection abort if any of the counter-measures   described in this section were implemented.  However, aborting the   connection would be to ignore the valuable feature of the Internet --   that for many internal failures, it reconstructs its function without   any disruption of the endpoints [RFC0816].  That is, communication   should survive if there is still a working path to the destination   system [DClark].  Additionally, applications may still be notified   asynchronously about the error condition, and thus may still abort   their connections on their own if they consider it appropriate.   This counter-measure has been implemented in BSD-derived TCP/IP   implementations (e.g., [FreeBSD], [NetBSD], and [OpenBSD]) for more   than ten years [Wright][McKusick].  The Linux kernel has also   implemented this policy for more than ten years [Linux].Gont                          Informational                    [Page 15]

RFC 5927                ICMP Attacks against TCP               July 20106.  Blind Throughput-Reduction Attack6.1.  Description   The Host Requirements RFC [RFC1122] states inSection 4.2.3.9 that   hosts MUST react to ICMPv4 Source Quench messages by slowing   transmission on the connection.  Thus, an attacker could send ICMPv4   Source Quench (type 4, code 0) messages to a TCP endpoint to make it   reduce the rate at which it sends data to the other endpoint of the   connection.  [RFC1122] further adds that the RECOMMENDED procedure is   to put the corresponding connection in the slow-start phase of TCP's   congestion control algorithm [RFC5681].  In the case of those   implementations that use an initial congestion window of one segment,   a sustained attack would reduce the throughput of the attacked   connection to about SMSS (Sender Maximum Segment Size) [RFC5681]   bytes per RTT (round-trip time).  The throughput achieved during an   attack might be a little higher if a larger initial congestion window   is in use [RFC3390].6.2.  Attack-Specific Counter-Measures   As discussed in the "Requirements for IP Version 4 Routers" RFC   [RFC1812], research seems to suggest that ICMPv4 Source Quench   messages are an ineffective (and unfair) antidote for congestion.   [RFC1812] further states that routers SHOULD NOT send ICMPv4 Source   Quench messages in response to congestion.  Furthermore, TCP   implements its own congestion control mechanisms ([RFC5681]   [RFC3168]) that do not depend on ICMPv4 Source Quench messages.   Based on this reasoning, a large number of implementations completely   ignore ICMPv4 Source Quench messages meant for TCP connections.  This   behavior has been implemented in, at least, Linux [Linux] since 2004,   and in FreeBSD [FreeBSD], NetBSD [NetBSD], and OpenBSD [OpenBSD]   since 2005.  However, it must be noted that this behavior violates   the requirement in [RFC1122] to react to ICMPv4 Source Quench   messages by slowing transmission on the connection.7.  Blind Performance-Degrading Attack7.1.  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 IPGont                          Informational                    [Page 16]

RFC 5927                ICMP Attacks against TCP               July 2010   systems determine the Path MTU of an arbitrary internet path.   [RFC1191] and [RFC1981] specify the PMTUD mechanism for IPv4 and   IPv6, respectively.   The PMTUD mechanism for IPv4 uses the Don't Fragment (DF) bit in the   IP header to dynamically discover the Path MTU.  The basic idea   behind the PMTUD mechanism is that a source system assumes that the   MTU of the path is that of the first hop, and sends all its datagrams   with the DF bit set.  If any of the datagrams is too large to be   forwarded without fragmentation by some intermediate router, the   router will discard the corresponding datagram and will return an   ICMPv4 "Destination Unreachable, fragmentation needed and DF set"   (type 3, code 4) error message to the sending system.  This message   will report the MTU of the constricting hop, so that the sending   system can reduce the assumed Path-MTU accordingly.   For IPv6, intermediate systems do not fragment packets.  Thus,   there's an "implicit" DF bit set in every packet sent on a network.   If any of the datagrams is too large to be forwarded without   fragmentation by some intermediate router, the router will discard   the corresponding datagram, and will return an ICMPv6 "Packet Too   Big" (type 2, code 0) error message to the sending system.  This   message will report the MTU of the constricting hop, so that the   sending system can reduce the assumed Path-MTU accordingly.   As discussed in both [RFC1191] and [RFC1981], the Path-MTU Discovery   mechanism can be used to attack TCP.  An attacker could send a   crafted ICMPv4 "Destination Unreachable, fragmentation needed and DF   set" packet (or their ICMPv6 counterpart) to the sending system,   advertising a small Next-Hop MTU.  As a result, the attacked system   would reduce the size of the packets it sends for the corresponding   connection accordingly.   The effect of this attack is two-fold.  On one hand, it will increase   the headers/data ratio, thus increasing the overhead needed to send   data to the remote TCP endpoint.  On the other hand, if the attacked   system wanted to keep the same throughput it was achieving before   being attacked, it would have to increase the packet rate.  On   virtually all systems, this will lead to an increased processing   overhead, thus degrading the overall system performance.   A particular scenario that may take place is one in which an attacker   reports a Next-Hop MTU smaller than or equal to the amount of bytes   needed for headers (IP header, plus TCP header).  For example, if the   attacker reports a Next-Hop MTU of 68 bytes, and the amount of bytes   used for headers (IP header, plus TCP header) is larger than   68 bytes, the assumed Path-MTU will not even allow the attacked   system to send a single byte of application data withoutGont                          Informational                    [Page 17]

RFC 5927                ICMP Attacks against TCP               July 2010   fragmentation.  This particular scenario might lead to unpredictable   results.  Another possible scenario is one in which a TCP connection   is being secured by means of IPsec.  If the Next-Hop MTU reported by   the attacker is smaller than the amount of bytes needed for headers   (IP and IPsec, in this case), the assumed Path-MTU will not even   allow the attacked system to send a single byte of the TCP header   without fragmentation.  This is another scenario that may lead to   unpredictable results.   For IPv4, the reported Next-Hop MTU could be as small as 68 octets,   as [RFC0791] requires every internet module to be able to forward a   datagram of 68 octets without further fragmentation.  For IPv6, while   the required minimum IPv6 MTU is 1280, the reported Next-Hop MTU can   be smaller than 1280 octets [RFC2460].  If the reported Next-Hop MTU   is smaller than the minimum IPv6 MTU, the receiving host is not   required to reduce the Path-MTU to a value smaller than 1280, but is   required to include a fragmentation header in the outgoing packets to   that destination from that moment on.7.2.  Attack-Specific Counter-Measures   The IETF has standardized a Path-MTU Discovery mechanism called   "Packetization Layer Path MTU Discovery" (PLPMTUD) that does not   depend on ICMP error messages.  Implementation of the aforementioned   mechanism in replacement of the traditional PMTUD (specified in   [RFC1191] and [RFC1981]) eliminates this vulnerability.  However, it   can also lead to an increase in PMTUD convergence time.   This section describes a modification to the PMTUD mechanism   specified in [RFC1191] and [RFC1981] that has been incorporated in   OpenBSD and NetBSD (since 2005) to improve TCP's resistance to the   blind performance-degrading attack described inSection 7.1.  The   described counter-measure basically disregards ICMP messages when a   connection makes progress, without violating any of the requirements   stated in [RFC1191] and [RFC1981].   Henceforth, we will refer to both ICMPv4 "fragmentation needed and DF   bit set" and ICMPv6 "Packet Too Big" messages as "ICMP Packet Too   Big" messages.   In addition to the general validation check described inSection 4.1,   these implementations include a modification to TCP's reaction to   ICMP "Packet Too Big" error messages that disregards them when a   connection makes progress, and honors them only after the   corresponding data have been retransmitted a specified number of   times.  This means that upon receipt of an ICMP "Packet Too Big"Gont                          Informational                    [Page 18]

RFC 5927                ICMP Attacks against TCP               July 2010   error message, TCP just records this information, and honors it only   when the corresponding data have already been retransmitted a   specified number of times.   While this basic policy would greatly mitigate the impact of the   attack against the PMTUD mechanism, it would also mean that it might   take TCP more time to discover the Path-MTU for a TCP connection.   This would be particularly annoying for connections that have just   been established, as it might take TCP several transmission attempts   (and the corresponding timeouts) before it discovers the PMTU for the   corresponding connection.  Thus, this policy would increase the time   it takes for data to begin to be received at the destination host.   In order to protect TCP from the attack against the PMTUD mechanism,   while still allowing TCP to quickly determine the initial Path-MTU   for a connection, the aforementioned implementations have divided the   traditional PMTUD mechanism into two stages: Initial Path-MTU   Discovery and Path-MTU Update.   The Initial Path-MTU Discovery stage is when TCP tries to send   segments that are larger than the ones that have so far been sent and   acknowledged for this connection.  That is, in the Initial Path-MTU   Discovery stage, TCP has no record of these large segments getting to   the destination host, and thus these implementations believe the   network when it reports that these packets are too large to reach the   destination host without being fragmented.   The Path-MTU Update stage is when TCP tries to send segments that are   equal to or smaller than the ones that have already been sent and   acknowledged for this connection.  During the Path-MTU Update stage,   TCP already has knowledge of the estimated Path-MTU for the given   connection.  Thus, in this case, these implementations are more   cautious with the errors being reported by the network.   In order to allow TCP to distinguish segments between those   performing Initial Path-MTU Discovery and those performing Path-MTU   Update, two new variables are introduced to TCP: maxsizesent and   maxsizeacked.   The maxsizesent variable holds the size (in octets) of the largest   packet that has so far been sent for this connection.  It is   initialized to 68 (the minimum IPv4 MTU) when the underlying Internet   Protocol is IPv4, and is initialized to 1280 (the minimum IPv6 MTU)   when the underlying Internet Protocol is IPv6.  Whenever a packet   larger than maxsizesent octets is sent, maxsizesent is set to that   value.Gont                          Informational                    [Page 19]

RFC 5927                ICMP Attacks against TCP               July 2010   On the other hand, maxsizeacked holds the size (in octets) of the   largest packet (data, plus headers) that has so far been acknowledged   for this connection.  It is initialized to 68 (the minimum IPv4 MTU)   when the underlying Internet Protocol is IPv4, and is initialized to   1280 (the minimum IPv6 MTU) when the underlying Internet Protocol is   IPv6.  Whenever an acknowledgement for a packet larger than   maxsizeacked octets is received, maxsizeacked is set to the size of   that acknowledged packet.  Note that because of TCP's cumulative   acknowledgement, a single ACK may acknowledge the receipt of more   than one packet.  When that happens, the algorithm may "incorrectly"   assume it is in the "Path-MTU Update" stage, rather than the "Initial   Path-MTU Discovery" stage (as described below).   Upon receipt of an ICMP "Packet Too Big" error message, the Next-Hop   MTU claimed by the ICMP message (henceforth "claimedmtu") is compared   with maxsizesent.  If claimedmtu is larger than maxsizesent, then the   ICMP error message is silently discarded.  The rationale for this is   that the ICMP error message cannot be legitimate if it claims to have   been triggered by a packet larger than the largest packet we have so   far sent for this connection.   If this check is passed, claimedmtu is compared with maxsizeacked.   If claimedmtu is equal to or larger than maxsizeacked, TCP is   supposed to be at the Initial Path-MTU Discovery stage, and thus the   ICMP "Packet Too Big" error message is honored immediately.  That is,   the assumed Path-MTU is updated according to the Next-Hop MTU claimed   in the ICMP error message.  Also, maxsizesent is reset to the minimum   MTU of the Internet Protocol in use (68 for IPv4, and 1280 for IPv6).   On the other hand, if claimedmtu is smaller than maxsizeacked, TCP is   supposed to be in the Path-MTU Update stage.  At this stage, these   implementations are more cautious with the errors being reported by   the network, and therefore just record the received error message,   and delay the update of the assumed Path-MTU.   To perform this delay, one new variable and one new parameter are   introduced to TCP: nsegrto and MAXSEGRTO.  The nsegrto variable holds   the number of times a specified segment has timed out.  It is   initialized to zero, and is incremented by one every time the   corresponding segment times out.  MAXSEGRTO specifies the number of   times a given segment must time out before an ICMP "Packet Too Big"   error message can be honored, and can be set, in principle, to any   value greater than or equal to 0.Gont                          Informational                    [Page 20]

RFC 5927                ICMP Attacks against TCP               July 2010   Thus, if nsegrto is greater than or equal to MAXSEGRTO, and there's a   pending ICMP "Packet Too Big" error message, the corresponding error   message is processed.  At that point, maxsizeacked is set to   claimedmtu, and maxsizesent is set to 68 (for IPv4) or 1280 (for   IPv6).   If, while there is a pending ICMP "Packet Too Big" error message, the   TCP SEQ claimed by the pending message is acknowledged (i.e., an ACK   that acknowledges that sequence number is received), then the   "pending error" condition is cleared.   The rationale behind performing this delayed processing of ICMP   "Packet Too Big" messages is that if there is progress on the   connection, the ICMP "Packet Too Big" errors must be a false claim.   By checking for progress on the connection, rather than just for   staleness of the received ICMP messages, TCP is protected from attack   even if the offending ICMP messages are "in window", and as a   corollary, is made more robust to spurious ICMP messages triggered   by, for example, corrupted TCP segments.   MAXSEGRTO can be set, in principle, to any value greater than or   equal to 0.  Setting MAXSEGRTO to 0 would make TCP perform the   traditional PMTUD mechanism defined in [RFC1191] and [RFC1981].  A   MAXSEGRTO of 1 provides enough protection for most cases.  In any   case, implementations are free to choose higher values for this   constant.  MAXSEGRTO could be a function of the Next-Hop MTU claimed   in the received ICMP "Packet Too Big" message.  That is, higher   values for MAXSEGRTO could be imposed when the received ICMP "Packet   Too Big" message claims a Next-Hop MTU that is smaller than some   specified value.  Both OpenBSD and NetBSD set MAXSEGRTO to 1.   In the event a higher level of protection is desired at the expense   of a higher delay in the discovery of the Path-MTU, an implementation   could consider TCP to always be in the Path-MTU Update stage, thus   always delaying the update of the assumed Path-MTU.Section 7.3 shows this counter-measure in action.Section 7.4 shows   this counter-measure in pseudo-code.   It is important to note that the mechanism described in this section   is an improvement to the current Path-MTU discovery mechanism, to   mitigate its security implications.  The current PMTUD mechanism, as   specified by [RFC1191] and [RFC1981], still suffers from some   functionality problems [RFC2923] that this document does not aim to   address.  A mechanism that addresses those issues is described in   [RFC4821].Gont                          Informational                    [Page 21]

RFC 5927                ICMP Attacks against TCP               July 20107.3.  The Counter-Measure for the PMTUD Attack in Action   This section illustrates the operation of the counter-measure for the   ICMP attack against the PMTUD mechanism that has been implemented in   OpenBSD and NetBSD.  It shows both how the fix protects TCP from   being attacked and how the counter-measure works in normal scenarios.   As discussed inSection 7.2, this section assumes the PMTUD-specific   counter-measure is implemented in addition to the TCP sequence number   checking described inSection 4.1.   Figure 1 illustrates a hypothetical scenario in which two hosts are   connected by means of three intermediate routers.  It also shows the   MTU of each hypothetical hop.  All the following subsections assume   the network setup of this figure.   Also, for simplicity's sake, all subsections assume an IP header of   20 octets and a TCP header of 20 octets.  Thus, for example, when the   PMTU is assumed to be 1500 octets, TCP will send segments that   contain, at most, 1460 octets of data.   For simplicity's sake, all the following subsections assume the TCP   implementation at Host 1 (H1) has chosen a MAXSEGRTO of 1.   +----+        +----+        +----+        +----+        +----+   | H1 |--------| R1 |--------| R2 |--------| R3 |--------| H2 |   +----+        +----+        +----+        +----+        +----+         MTU=4464      MTU=2048      MTU=1500      MTU=4464                      Figure 1: Hypothetical Scenario7.3.1.  Normal Operation for Bulk Transfers   This subsection shows the counter-measure in normal operation, when a   TCP connection is used for bulk transfers.  That is, it shows how the   counter-measure works when there is no attack taking place and a TCP   connection is used for transferring large amounts of data.  This   section assumes that just after the connection is established, one of   the TCP endpoints begins to transfer data in packets that are as   large as possible.Gont                          Informational                    [Page 22]

RFC 5927                ICMP Attacks against TCP               July 2010       Host 1                                       Host 2   1.    -->            <SEQ=100><CTL=SYN>           -->   2.    <--      <SEQ=X><ACK=101><CTL=SYN,ACK>      <--   3.    -->       <SEQ=101><ACK=X+1><CTL=ACK>       -->   4.    --> <SEQ=101><ACK=X+1><CTL=ACK><DATA=4424>  -->   5.       <--- ICMP "Packet Too Big" MTU=2048, TCPseq#=101 <--- R1   6.    --> <SEQ=101><ACK=X+1><CTL=ACK><DATA=2008>  -->   7.       <--- ICMP "Packet Too Big" MTU=1500, TCPseq#=101 <--- R2   8.    --> <SEQ=101><ACK=X+1><CTL=ACK><DATA=1460>  -->   9.    <--      <SEQ=X+1><ACK=1561><CTL=ACK>       <--               Figure 2: Normal Operation for Bulk Transfers   The nsegrto variable is initialized to zero.  Both maxsizeacked and   maxsizesent are initialized to the minimum MTU for the Internet   Protocol being used (68 for IPv4, and 1280 for IPv6).   In lines 1 to 3, the three-way handshake takes place, and the   connection is established.  In line 4, H1 tries to send a full-sized   TCP segment.  As described by [RFC1191] and [RFC1981], in this case,   TCP will try to send a segment with 4424 bytes of data, which will   result in an IP packet of 4464 octets.  Therefore, maxsizesent is set   to 4464.  When the packet reaches R1, it elicits an ICMP "Packet Too   Big" error message.   In line 5, H1 receives the ICMP error message, which reports a Next-   Hop MTU of 2048 octets.  After performing the TCP sequence number   check described inSection 4.1, the Next-Hop MTU reported by the ICMP   error message (claimedmtu) is compared with maxsizesent.  As it is   smaller than maxsizesent, it passes the check, and thus is then   compared with maxsizeacked.  As claimedmtu is larger than   maxsizeacked, TCP assumes that the corresponding TCP segment was   performing the Initial PMTU Discovery.  Therefore, the TCP at H1   honors the ICMP message by updating the assumed Path-MTU.  The   maxsizesent variable is reset to the minimum MTU of the Internet   Protocol in use (68 for IPv4, and 1280 for IPv6).   In line 6, the TCP at H1 sends a segment with 2008 bytes of data,   which results in an IP packet of 2048 octets.  The maxsizesent   variable is thus set to 2008 bytes.  When the packet reaches R2, it   elicits an ICMP "Packet Too Big" error message.   In line 7, H1 receives the ICMP error message, which reports a Next-   Hop MTU of 1500 octets.  After performing the TCP sequence number   check, the Next-Hop MTU reported by the ICMP error message   (claimedmtu) is compared with maxsizesent.  As it is smaller than   maxsizesent, it passes the check, and thus is then compared withGont                          Informational                    [Page 23]

RFC 5927                ICMP Attacks against TCP               July 2010   maxsizeacked.  As claimedmtu is larger than maxsizeacked, TCP assumes   that the corresponding TCP segment was performing the Initial PMTU   Discovery.  Therefore, the TCP at H1 honors the ICMP message by   updating the assumed Path-MTU.  The maxsizesent variable is reset to   the minimum MTU of the Internet Protocol in use.   In line 8, the TCP at H1 sends a segment with 1460 bytes of data,   which results in an IP packet of 1500 octets.  Thus, maxsizesent is   set to 1500.  This packet reaches H2, where it elicits an   acknowledgement (ACK) segment.   In line 9, H1 finally gets the acknowledgement for the data segment.   As the corresponding packet was larger than maxsizeacked, TCP updates   maxsizeacked, setting it to 1500.  At this point, TCP has discovered   the Path-MTU for this TCP connection.7.3.2.  Operation during Path-MTU Changes   Let us suppose a TCP connection between H1 and H2 has already been   established, and that the PMTU for the connection has already been   discovered to be 1500.  At this point, both maxsizesent and   maxsizeacked are equal to 1500, and nsegrto is equal to 0.  Suppose   some time later the PMTU decreases to 1492.  For simplicity, let us   suppose that the Path-MTU has decreased because the MTU of the link   between R2 and R3 has decreased from 1500 to 1492.  Figure 3   illustrates how the counter-measure would work in this scenario.       Host 1                                       Host 2   1.                   (Path-MTU decreases)   2.    -->  <SEQ=100><ACK=X><CTL=ACK><DATA=1460>   -->   3.       <--- ICMP "Packet Too Big" MTU=1492, TCPseq#=100 <--- R2   4.                   (Segment times out)   5.    -->  <SEQ=100><ACK=X><CTL=ACK><DATA=1452>   -->   6.    <--        <SEQ=X><ACK=1552><CTL=ACK>       <--                Figure 3: Operation during Path-MTU Changes   In line 1, the Path-MTU for this connection decreases from 1500 to   1492.  In line 2, the TCP at H1, without being aware of the Path-MTU   change, sends a 1500-byte packet to H2.  When the packet reaches R2,   it elicits an ICMP "Packet Too Big" error message.   In line 3, H1 receives the ICMP error message, which reports a Next-   Hop MTU of 1492 octets.  After performing the TCP sequence number   check, the Next-Hop MTU reported by the ICMP error message   (claimedmtu) is compared with maxsizesent.  As claimedmtu is smaller   than maxsizesent, it is then compared with maxsizeacked.  AsGont                          Informational                    [Page 24]

RFC 5927                ICMP Attacks against TCP               July 2010   claimedmtu is smaller than maxsizeacked (full-sized packets were   getting to the remote endpoint), this packet is assumed to be   performing Path-MTU Update, and a "pending error" condition is   recorded.   In line 4, the segment times out.  Thus, nsegrto is incremented by 1.   As nsegrto is greater than or equal to MAXSEGRTO, the assumed Path-   MTU is updated.  The nsegrto variable is reset to 0, maxsizeacked is   set to claimedmtu, and maxsizesent is set to the minimum MTU of the   Internet Protocol in use.   In line 5, H1 retransmits the data using the updated PMTU, and thus   maxsizesent is set to 1492.  The resulting packet reaches H2, where   it elicits an acknowledgement (ACK) segment.   In line 6, H1 finally gets the acknowledgement for the data segment.   At this point, TCP has discovered the new Path-MTU for this TCP   connection.7.3.3.  Idle Connection Being Attacked   Let us suppose a TCP connection between H1 and H2 has already been   established, and the PMTU for the connection has already been   discovered to be 1500.  Figure 4 shows a sample time-line diagram   that illustrates an idle connection being attacked.       Host 1                                       Host 2   1.    -->   <SEQ=100><ACK=X><CTL=ACK><DATA=50>    -->   2.    <--        <SEQ=X><ACK=150><CTL=ACK>        <--   3.       <--- ICMP "Packet Too Big" MTU=68, TCPseq#=100 <---   4.       <--- ICMP "Packet Too Big" MTU=68, TCPseq#=100 <---   5.       <--- ICMP "Packet Too Big" MTU=68, TCPseq#=100 <---                 Figure 4: Idle Connection Being Attacked   In line 1, H1 sends its last bunch of data.  In line 2, H2   acknowledges the receipt of these data.  Then the connection becomes   idle.  In lines 3, 4, and 5, an attacker sends forged ICMP "Packet   Too Big" error messages to H1.  Regardless of how many packets it   sends and of the TCP sequence number each ICMP packet includes, none   of these ICMP error messages will pass the TCP sequence number check   described inSection 4.1, as H1 has no unacknowledged data "in   flight" to H2.  Therefore, the attack does not succeed.Gont                          Informational                    [Page 25]

RFC 5927                ICMP Attacks against TCP               July 20107.3.4.  Active Connection Being Attacked after Discovery of the Path-MTU   Let us suppose an attacker attacks a TCP connection for which the   PMTU has already been discovered.  In this case, as illustrated in   Figure 1, the PMTU would be found to be 1500 bytes.  Figure 5 shows a   possible packet exchange.       Host 1                                       Host 2   1.    -->  <SEQ=100><ACK=X><CTL=ACK><DATA=1460>   -->   2.    -->  <SEQ=1560><ACK=X><CTL=ACK><DATA=1460>  -->   3.    -->  <SEQ=3020><ACK=X><CTL=ACK><DATA=1460>  -->   4.    -->  <SEQ=4480><ACK=X><CTL=ACK><DATA=1460>  -->   5.       <--- ICMP "Packet Too Big" MTU=68, TCPseq#=100 <---   6.    <--       <SEQ=X><CTL=ACK><ACK=1560>        <--    Figure 5: Active Connection Being Attacked after Discovery of PMTU   As we assume the PMTU has already been discovered, we also assume   both maxsizesent and maxsizeacked are equal to 1500.  We assume   nsegrto is equal to zero, as there have been no segment timeouts.   In lines 1, 2, 3, and 4, H1 sends four data segments to H2.  In   line 5, an attacker sends a forged ICMP error message to H1.  We   assume the attacker is lucky enough to guess both the four-tuple that   identifies the connection and a valid TCP sequence number.  As the   Next-Hop MTU claimed in the ICMP "Packet Too Big" message   (claimedmtu) is smaller than maxsizeacked, this packet is assumed to   be performing Path-MTU Update.  Thus, the error message is recorded.   In line 6, H1 receives an acknowledgement for the segment sent in   line 1, before it times out.  At this point, the "pending error"   condition is cleared, and the recorded ICMP "Packet Too Big" error   message is ignored.  Therefore, the attack does not succeed.7.3.5.  TCP Peer Attacked when Sending Small Packets Just after the        Three-Way Handshake   This section analyzes a scenario in which a TCP peer that is sending   small segments just after the connection has been established is   attacked.  The connection could be in use by protocols such as SMTP   [RFC5321] and HTTP [RFC2616], for example, which usually behave like   this.   Figure 6 shows a possible packet exchange for such a scenario.Gont                          Informational                    [Page 26]

RFC 5927                ICMP Attacks against TCP               July 2010       Host 1                                       Host 2   1.    -->           <SEQ=100><CTL=SYN>            -->   2.    <--      <SEQ=X><ACK=101><CTL=SYN,ACK>      <--   3.    -->       <SEQ=101><ACK=X+1><CTL=ACK>       -->   4.    -->  <SEQ=101><ACK=X+1><CTL=ACK><DATA=100>  -->   5.    <--       <SEQ=X+1><ACK=201><CTL=ACK>       <--   6.    -->  <SEQ=201><ACK=X+1><CTL=ACK><DATA=100>  -->   7.    -->  <SEQ=301><ACK=X+1><CTL=ACK><DATA=100>  -->   8.       <--- ICMP "Packet Too Big" MTU=150, TCPseq#=201 <---          Figure 6: TCP Peer Attacked when Sending Small Packets                    Just after the Three-Way Handshake   The nsegrto variable is initialized to zero.  Both maxsizesent and   maxsizeacked are initialized to the minimum MTU for the Internet   Protocol being used (68 for IPv4, and 1280 for IPv6).   In lines 1 to 3, the three-way handshake takes place, and the   connection is established.  At this point, the assumed Path-MTU for   this connection is 4464.  In line 4, H1 sends a small segment (which   results in a 140-byte packet) to H2.  Therefore, maxsizesent is set   to 140.  In line 5, this segment is acknowledged, and thus   maxsizeacked is set to 140.   In lines 6 and 7, H1 sends two small segments to H2.  In line 8,   while the segments from lines 6 and 7 are still "in flight" to H2, an   attacker sends a forged ICMP "Packet Too Big" error message to H1.   Assuming the attacker is lucky enough to guess a valid TCP sequence   number, this ICMP message will pass the TCP sequence number check.   The Next-Hop MTU reported by the ICMP error message (claimedmtu) is   then compared with maxsizesent.  As claimedmtu is larger than   maxsizesent, the ICMP error message is silently discarded.   Therefore, the attack does not succeed.7.4.  Pseudo-Code for the Counter-Measure for the Blind Performance-      Degrading Attack   This section contains a pseudo-code version of the counter-measure   described inSection 7.2 for the blind performance-degrading attack   described inSection 7.  It is meant as guidance for developers on   how to implement this counter-measure.   The pseudo-code makes use of the following variables, constants, and   functions:Gont                          Informational                    [Page 27]

RFC 5927                ICMP Attacks against TCP               July 2010   ack      Variable holding the acknowledgement number contained in the TCP      segment that has just been received.   acked_packet_size      Variable holding the packet size (data, plus headers) that the ACK      that has just been received is acknowledging.   adjust_mtu()      Function that adjusts the MTU for this connection, according to      the ICMP "Packet Too Big" that was last received.   claimedmtu      Variable holding the Next-Hop MTU advertised by the ICMP "Packet      Too Big" error message.   claimedtcpseq      Variable holding the TCP sequence number contained in the payload      of the ICMP "Packet Too Big" message that has just been received      or was last recorded.   current_mtu      Variable holding the assumed Path-MTU for this connection.   drop_message()      Function that performs the necessary actions to drop the ICMP      message being processed.   initial_mtu      Variable holding the MTU for new connections, as explained in      [RFC1191] and [RFC1981].   maxsizeacked      Variable holding the largest packet size (data, plus headers) that      has so far been acked for this connection, as explained inSection 7.2.   maxsizesent      Variable holding the largest packet size (data, plus headers) that      has so far been sent for this connection, as explained inSection 7.2.   nsegrto      Variable holding the number of times this segment has timed out,      as explained inSection 7.2.   packet_size      Variable holding the size of the IP datagram being sent.Gont                          Informational                    [Page 28]

RFC 5927                ICMP Attacks against TCP               July 2010   pending_message      Variable (flag) that indicates whether there is a pending ICMP      "Packet Too Big" message to be processed.   save_message()      Function that records the ICMP "Packet Too Big" message that has      just been received.   MINIMUM_MTU      Constant holding the minimum MTU for the Internet Protocol in use      (68 for IPv4, and 1280 for IPv6).   MAXSEGRTO      Constant holding the number of times a given segment must time out      before an ICMP "Packet Too Big" error message can be honored.   EVENT: New TCP connection    current_mtu = initial_mtu;    maxsizesent = MINIMUM_MTU;    maxsizeacked = MINIMUM_MTU;    nsegrto = 0;    pending_message = 0;   EVENT: Segment is sent    if (packet_size > maxsizesent)         maxsizesent = packet_size;   EVENT: Segment is received    if (acked_packet_size > maxsizeacked)         maxsizeacked = acked_packet_size;    if (pending_message)         if (ack > claimedtcpseq){              pending_message = 0;              nsegrto = 0;         }   EVENT: ICMP "Packet Too Big" message is received    if (claimedmtu <= MINIMUM_MTU)         drop_message();    if (claimedtcpseq < SND.UNA || claimedtcpseq >= SND.NXT)         drop_message();Gont                          Informational                    [Page 29]

RFC 5927                ICMP Attacks against TCP               July 2010    else {         if (claimedmtu > maxsizesent || claimedmtu >= current_mtu)              drop_message();         else {              if (claimedmtu > maxsizeacked){                   adjust_mtu();                   current_mtu = claimedmtu;                   maxsizesent = MINIMUM_MTU;              }              else {                   pending_message = 1;                   save_message();              }         }    }   EVENT: Segment times out    nsegrto++;    if (pending_message && nsegrto >= MAXSEGRTO){         adjust_mtu();         nsegrto = 0;         pending_message = 0;         maxsizeacked = claimedmtu;         maxsizesent = MINIMUM_MTU;         current_mtu = claimedmtu;    }   Notes:      All comparisons between sequence numbers must be performed using      sequence number arithmetic.      The pseudo-code implements the mechanism described inSection 7.2,      the TCP sequence number checking described inSection 4.1, and the      validation check on the advertised Next-Hop MTU described in      [RFC1191] and [RFC1981].8.  Security Considerations   This document describes the use of ICMP error messages to perform a   number of attacks against TCP, and describes a number of widely   implemented counter-measures that either eliminate or reduce the   impact of these attacks when they are performed by off-path   attackers.Gont                          Informational                    [Page 30]

RFC 5927                ICMP Attacks against TCP               July 2010Section 4.1 describes a validation check that could be enforced on   ICMP error messages, such that TCP reacts only to those ICMP error   messages that appear to relate to segments currently "in flight" to   the destination system.  This requires more effort on the side of an   off-path attacker at the expense of possible reduced responsiveness   to network errors.Section 4.2 describes how randomization of TCP ephemeral ports   requires more effort on the side of the attacker to successfully   exploit any of the attacks described in this document.Section 4.3 describes how ICMP error messages could possibly be   filtered based on their payload, to prevent users of the local   network from successfully performing attacks against third-party   connections.  This is analogous to ingress filtering and egress   filtering of IP packets [IP-filtering].Section 5.2 describes an attack-specific counter-measure for the   blind connection-reset attack.  It describes the processing of ICMP   "hard errors" as "soft errors" when they are received for connections   in any of the synchronized states.  This counter-measure eliminates   the aforementioned vulnerability in synchronized connections at the   expense of possible reduced responsiveness in some network scenarios.Section 6.2 describes an attack-specific counter-measure for the   blind throughput-reduction attack.  It suggests that the   aforementioned vulnerability can be eliminated by ignoring ICMPv4   Source Quench messages meant for TCP connections.  This is in   accordance with research results that indicate that ICMPv4 Source   Quench messages are ineffective and are an unfair antidote for   congestion.   Finally,Section 7.2 describes an attack-specific counter-measure for   the blind performance-degrading attack.  It consists of the   validation check described inSection 4.1, with a modification that   makes TCP react to ICMP "Packet Too Big" error messages such that   they are processed when an outstanding TCP segment times out.  This   counter-measure parallels the Packetization Layer Path MTU Discovery   (PLPMTUD) mechanism [RFC4821].  It should be noted that if this   counter-measure is implemented, in some scenarios TCP may respond   more slowly to valid ICMP "Packet Too Big" error messages.   A discussion of these and other attack vectors for performing similar   attacks against TCP (along with possible counter-measures) can be   found in [CPNI-TCP] and [TCP-SECURITY].Gont                          Informational                    [Page 31]

RFC 5927                ICMP Attacks against TCP               July 20109.  Acknowledgements   This document was inspired by Mika Liljeberg, while discussing some   issues related to [RFC5461] by private e-mail.  The author would like   to thank (in alphabetical order): Bora Akyol, Mark Allman, Ran   Atkinson, James Carlson, Alan Cox, Theo de Raadt, Wesley Eddy, Lars   Eggert, Ted Faber, Juan Fraschini, Markus Friedl, Guillermo Gont,   John Heffner, Alfred Hoenes, Vivek Kakkar, Michael Kerrisk, Mika   Liljeberg, Matt Mathis, David Miller, Toby Moncaster, Miles Nordin,   Eloy Paris, Kacheong Poon, Andrew Powell, Pekka Savola, Donald Smith,   Pyda Srisuresh, Fred Templin, and Joe Touch for contributing many   valuable comments.   Juan Fraschini and the author of this document implemented freely   available audit tools to help vendors audit their systems by   reproducing the attacks discussed in this document.  These tools are   available athttp://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 inSection 7.2.  This first implementation   helped to test the effectiveness of the ideas introduced in this   document, and has served as a reference implementation for other   operating systems.   The author would like to thank the UK's Centre for the Protection of   National Infrastructure (CPNI) -- formerly the National   Infrastructure Security Co-ordination Centre (NISCC) -- for   coordinating the disclosure of these issues with a large number of   vendors and CSIRTs (Computer Security Incident Response Teams).   The author wishes to express deep and heartfelt gratitude to Jorge   Oscar Gont and Nelida Garcia, for their precious motivation and   guidance.10.  References10.1.  Normative References   [RFC0791]         Postel, J., "Internet Protocol", STD 5,RFC 791,                     September 1981.   [RFC0792]         Postel, J., "Internet Control Message Protocol",                     STD 5,RFC 792, September 1981.   [RFC0793]         Postel, J., "Transmission Control Protocol", STD 7,RFC 793, September 1981.Gont                          Informational                    [Page 32]

RFC 5927                ICMP Attacks against TCP               July 2010   [RFC1122]         Braden, R., "Requirements for Internet Hosts -                     Communication Layers", STD 3,RFC 1122,                     October 1989.   [RFC1191]         Mogul, J. and S. Deering, "Path MTU discovery",RFC 1191, November 1990.   [RFC1812]         Baker, F., "Requirements for IP Version 4 Routers",RFC 1812, June 1995.   [RFC1981]         McCann, J., Deering, S., and J. Mogul, "Path MTU                     Discovery for IP version 6",RFC 1981, August 1996.   [RFC2119]         Bradner, S., "Key words for use in RFCs to Indicate                     Requirement Levels",BCP 14,RFC 2119, March 1997.   [RFC2460]         Deering, S. and R. Hinden, "Internet Protocol,                     Version 6 (IPv6) Specification",RFC 2460,                     December 1998.   [RFC4301]         Kent, S. and K. Seo, "Security Architecture for the                     Internet Protocol",RFC 4301, December 2005.   [RFC4443]         Conta, A., Deering, S., and M. Gupta, "Internet                     Control Message Protocol (ICMPv6) for the Internet                     Protocol Version 6 (IPv6) Specification",RFC 4443,                     March 2006.   [RFC4884]         Bonica, R., Gan, D., Tappan, D., and C. Pignataro,                     "Extended ICMP to Support Multi-Part Messages",RFC 4884, April 2007.10.2.  Informative References   [CPNI-TCP]        CPNI, "Security Assessment of the Transmission                     Control Protocol (TCP)",http://www.cpni.gov.uk/Docs/tn-03-09-security-assessment-TCP.pdf, 2009.   [DClark]          Clark, D., "The Design Philosophy of the DARPA                     Internet Protocols", Computer Communication                     Review Vol. 18, No. 4, 1988.   [FreeBSD]         The FreeBSD Project,http://www.freebsd.org.   [ICMP-Filtering]  Gont, F., "Filtering of ICMP error messages",  http                     ://www.gont.com.ar/papers/                     filtering-of-icmp-error-messages.pdf.Gont                          Informational                    [Page 33]

RFC 5927                ICMP Attacks against TCP               July 2010   [IP-filtering]    NISCC, "NISCC Technical Note 01/2006: Egress and                     Ingress Filtering",http://www.cpni.gov.uk/Docs/re-20060420-00294.pdf,                     2006.   [Linux]           The Linux Project, "http://www.kernel.org".   [McKusick]        McKusick, M., Bostic, K., Karels, M., and J.                     Quarterman, "The Design and Implementation of the                     4.4 BSD Operating System", Addison-Wesley, 1996.   [NISCC]           NISCC, "NISCC Vulnerability Advisory 532967/NISCC/                     ICMP: Vulnerability Issues in ICMP packets with TCP                     payloads",http://www.cpni.gov.uk/docs/re-20050412-00303.pdf?lang=en, 2005.   [NetBSD]          The NetBSD Project, "http://www.netbsd.org".   [OpenBSD]         The OpenBSD Project, "http://www.openbsd.org".   [OpenBSD-PF]      The OpenBSD Packet Filter,                     "http://www.openbsd.org/faq/pf/".   [PORT-RANDOM]     Larsen, M. and F. Gont, "Transport Protocol Port                     Randomization Recommendations", Work in Progress,                     April 2010.   [RFC0816]         Clark, D., "Fault isolation and recovery",RFC 816,                     July 1982.   [RFC1321]         Rivest, R., "The MD5 Message-Digest Algorithm",RFC 1321, April 1992.   [RFC1323]         Jacobson, V., Braden, B., and D. Borman, "TCP                     Extensions for High Performance",RFC 1323,                     May 1992.   [RFC2385]         Heffernan, A., "Protection of BGP Sessions via the                     TCP MD5 Signature Option",RFC 2385, August 1998.   [RFC2616]         Fielding, R., Gettys, J., Mogul, J., Frystyk, H.,                     Masinter, L., Leach, P., and T. Berners-Lee,                     "Hypertext Transfer Protocol -- HTTP/1.1",RFC 2616, June 1999.   [RFC2923]         Lahey, K., "TCP Problems with Path MTU Discovery",RFC 2923, September 2000.Gont                          Informational                    [Page 34]

RFC 5927                ICMP Attacks against TCP               July 2010   [RFC3168]         Ramakrishnan, K., Floyd, S., and D. Black, "The                     Addition of Explicit Congestion Notification (ECN)                     to IP",RFC 3168, September 2001.   [RFC3390]         Allman, M., Floyd, S., and C. Partridge,                     "Increasing TCP's Initial Window",RFC 3390,                     October 2002.   [RFC4271]         Rekhter, Y., Li, T., and S. Hares, "A Border                     Gateway Protocol 4 (BGP-4)",RFC 4271,                     January 2006.   [RFC4821]         Mathis, M. and J. Heffner, "Packetization Layer                     Path MTU Discovery",RFC 4821, March 2007.   [RFC4907]         Aboba, B., "Architectural Implications of Link                     Indications",RFC 4907, June 2007.   [RFC4953]         Touch, J., "Defending TCP Against Spoofing                     Attacks",RFC 4953, July 2007.   [RFC5321]         Klensin, J., "Simple Mail Transfer Protocol",RFC 5321, October 2008.   [RFC5461]         Gont, F., "TCP's Reaction to Soft Errors",RFC 5461, February 2009.   [RFC5681]         Allman, M., Paxson, V., and E. Blanton, "TCP                     Congestion Control",RFC 5681, September 2009.   [RFC5925]         Touch, J., Mankin, A., and R. Bonica, "The TCP                     Authentication Option",RFC 5925, June 2010.   [TCP-SECURITY]    Gont, F., "Security Assessment of the Transmission                     Control Protocol (TCP)", Work in Progress,                     February 2010.   [TCPM-TCPSECURE]  Ramaiah, A., Stewart, R., and M. Dalal, "Improving                     TCP's Robustness to Blind In-Window Attacks", Work                     in Progress, May 2010.   [US-CERT]         US-CERT, "US-CERT Vulnerability Note VU#222750:                     TCP/IP Implementations do not adequately validate                     ICMP error messages",http://www.kb.cert.org/vuls/id/222750, 2005.   [Watson]          Watson, P., "Slipping in the Window: TCP Reset                     Attacks", CanSecWest Conference, 2004.Gont                          Informational                    [Page 35]

RFC 5927                ICMP Attacks against TCP               July 2010   [Wright]          Wright, G. and W. Stevens, "TCP/IP Illustrated,                     Volume 2: The Implementation", Addison-                     Wesley, 1994.Author's Address   Fernando Gont   Universidad Tecnologica Nacional / Facultad Regional Haedo   Evaristo Carriego 2644   Haedo, Provincia de Buenos Aires  1706   Argentina   Phone: +54 11 4650 8472   EMail: fernando@gont.com.ar   URI:http://www.gont.com.arGont                          Informational                    [Page 36]