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Network Working Group                                           S. FloydRequest for Comments: 3360                                          ICIRBCP: 60                                                      August 2002Category: Best Current PracticeInappropriate TCP Resets Considered HarmfulStatus of this Memo   This document specifies an Internet Best Current Practices for the   Internet Community, and requests discussion and suggestions for   improvements.  Distribution of this memo is unlimited.Copyright Notice   Copyright (C) The Internet Society (2002).  All Rights Reserved.Abstract   This document is being written because there are a number of   firewalls in the Internet that inappropriately reset a TCP connection   upon receiving certain TCP SYN packets, in particular, packets with   flags set in the Reserved field of the TCP header.  In this document   we argue that this practice is not conformant with TCP standards, and   is an inappropriate overloading of the semantics of the TCP reset.   We also consider the longer-term consequences of this and similar   actions as obstacles to the evolution of the Internet infrastructure.1.  Introduction   TCP uses the RST (Reset) bit in the TCP header to reset a TCP   connection.  Resets are appropriately sent in response to a   connection request to a nonexistent connection, for example.  The TCP   receiver of the reset aborts the TCP connection, and notifies the   application [RFC793,RFC1122, Ste94].   Unfortunately, a number of firewalls and load-balancers in the   current Internet send a reset in response to a TCP SYN packet that   use flags from the Reserved field in the TCP header.Section 3 below   discusses the specific example of firewalls that send resets in   response to TCP SYN packets from ECN-capable hosts.   This document is being written to inform administrators of web   servers and firewalls of this problem, in an effort to encourage the   deployment of bug-fixes [FIXES].  A second purpose of this document   is to consider the longer-term consequences of such middlebox   behavior on the more general evolution of protocols in the Internet.Floyd                    Best Current Practice                  [Page 1]

RFC 3360                Inappropriate TCP Resets             August 20022.  The history of TCP resets.   This section gives a brief history of the use of the TCP reset in the   TCP standards, and argues that sending a reset in response to a SYN   packet that uses bits from the Reserved field of the TCP header is   non-compliant behavior.RFC 793 contained the original specification of TCP in September,   1981 [RFC793].  This document defined the RST bit in the TCP header,   and explained that reset was devised to prevent old duplicate   connection initiations from causing confusion in TCP's three-way   handshake.  The reset is also used when a host receives data for a   TCP connection that no longer exists.RFC 793 states the following, inSection 5:   "As a general rule, reset (RST) must be sent whenever a segment   arrives which apparently is not intended for the current connection.   A reset must not be sent if it is not clear that this is the case."RFC 1122 "amends, corrects, and supplements"RFC 793.RFC 1122 says   nothing specific about sending resets, or not sending resets, in   response to flags in the TCP Reserved field.   Thus, there is nothing inRFC 793 orRFC 1122 that suggests that it   is acceptable to send a reset simply because a SYN packet uses   Reserved flags in the TCP header, andRFC 793 explicitly forbids   sending a reset for this reason.RFC 793 andRFC 1122 both include Jon Postel's famous robustness   principle, also fromRFC 791: "Be liberal in what you accept, and   conservative in what you send."RFC 1122 reiterates that this   robustness principle "is particularly important in the Internet   layer, where one misbehaving host can deny Internet service to many   other hosts."  The discussion of the robustness principle inRFC 1122   also states that "adaptability to change must be designed into all   levels of Internet host software".  The principle "be liberal in what   you accept" doesn't carry over in a clear way (if at all) to the   world of firewalls, but the issue of "adaptability to change" is   crucial nevertheless.  The challenge is to protect legitimate   security interests without completely blocking the ability of the   Internet to evolve to support new applications, protocols, and   functionality.Floyd                    Best Current Practice                  [Page 2]

RFC 3360                Inappropriate TCP Resets             August 20022.1.  The TCP Reserved FieldRFC 793 says that the Reserved field in the TCP header is reserved   for future use, and must be zero.  A rephrasing more consistent with   the rest of the document would have been to say that the Reserved   field should be zero when sent and ignored when received, unless   specified otherwise by future standards actions.  However, the   phrasing inRFC 793 does not permit sending resets in response to TCP   packets with a non-zero Reserved field, as is explained in the   section above.2.2.  Behavior of and Requirements for Internet FirewallsRFC 2979 on the Behavior of and Requirements for Internet Firewalls   [RFC2979], an Informational RFC, contains the following:   "Applications have to continue to work properly in the presence of   firewalls.  This translates into the following transparency rule: The   introduction of a firewall and any associated tunneling or access   negotiation facilities MUST NOT cause unintended failures of   legitimate and standards-compliant usage that would work were the   firewall not present."   "A necessary corollary to this requirement is that when such failures   do occur it is incumbent on the firewall and associated software to   address the problem: Changes to either implementations of existing   standard protocols or the protocols themselves MUST NOT be   necessary."   "Note that this requirement only applies to legitimate protocol usage   and gratuitous failures -- a firewall is entitled to block any sort   of access that a site deems illegitimate, regardless of whether or   not the attempted access is standards-compliant."   We would note thatRFC 2979 is an Informational RFC.RFC 2026 on   Internet Standards Process says the following inSection 4.2.2: "An   `Informational' specification is published for the general   information of the Internet community, and does not represent an   Internet community consensus or recommendation" [RFC2026].2.3.  Sending Resets as a Congestion Control Mechanism   Some firewalls and hosts send resets in response to SYN packets as a   congestion control mechanism, for example, when their listen queues   are full.  These resets are sent without regard to the contents of   the TCP Reserved field.  Possibly in response to the use of resets asFloyd                    Best Current Practice                  [Page 3]

RFC 3360                Inappropriate TCP Resets             August 2002   a congestion control mechanism, several popular TCP implementations   immediately resend a SYN packet in response to a reset, up to four   times.   We would recommend that the TCP reset not be used as a congestion   control mechanism, because this overloads the semantics of the reset   message, and inevitably leads to more aggressive behavior from TCP   implementations in response to a reset.  We would suggest that simply   dropping the SYN packet is the most effective response to congestion.   The TCP sender will retransmit the SYN packet, using the default   value for the Retransmission Timeout (RTO), backing-off the   retransmit timer after each retransmit.2.4. Resets in Response to Changes in the Precedence FieldRFC 793 includes the following inSection 5:   "If an incoming segment has a security level, or compartment, or   precedence which does not exactly match the level, and compartment,   and precedence requested for the connection, a reset is sent and   connection goes to the CLOSED state."   The "precedence" refers to the (old) Precedence field in the (old)   ToS field in the IP header.  The "security" and "compartment" refer   to the obsolete IP Security option.  When it was written, this was   consistent with the guideline elsewhere inRFC 793 that resets should   only be sent when a segment arrives which apparently is not intended   for the current connection.RFC 2873 on "TCP Processing of the IPv4 Precedence Field" discusses   specific problems raised by the sending of resets when the precedence   field has changed [RFC2873].RFC 2873, currently a Proposed   Standard, specifies that TCP must ignore the precedence of all   received segments, and must not send a reset in response to changes   in the precedence field.  We discuss this here to clarify that this   issue never permitted the sending of a reset in response to a segment   with a non-zero TCP Reserved field.2.5. Resets in Response to Illegal Option LengthsRFC 1122 says the following inSection 4.2.2.5 about TCP options   [RFC1122]:   "A TCP MUST be able to receive a TCP option in any segment.  A TCP   MUST ignore without error any TCP option it does not implement,   assuming that the option has a length field (all TCP options definedFloyd                    Best Current Practice                  [Page 4]

RFC 3360                Inappropriate TCP Resets             August 2002   in the future will have length fields).  TCP MUST be prepared to   handle an illegal option length (e.g., zero) without crashing; a   suggested procedure is to reset the connection and log the reason."   This makes sense, as a TCP receiver is unable to interpret the rest   of the data on a segment that has a TCP option with an illegal option   length.  Again, we discuss this here to clarify that this issue never   permitted the sending of a reset in response to a segment with a   non-zero TCP Reserved field.3.  The Specific Example of ECN   This section has a brief explanation of ECN (Explicit Congestion   Notification) in general, and the ECN-setup SYN packet in particular.   The Internet is based on end-to-end congestion control, and   historically the Internet has used packet drops as the only method   for routers to indicate congestion to the end nodes.  ECN is a recent   addition to the IP architecture to allow routers to set a bit in the   IP packet header to inform end-nodes of congestion, instead of   dropping the packet.  ECN requires the cooperation of the transport   end-nodes.   The ECN specification,RFC 2481, was an Experimental RFC from January   1999 until June 2001, when a revised document [RFC3168] was approved   as Proposed Standard.  More information about ECN is available from   the ECN Web Page [ECN].   The use of ECN with TCP requires that both TCP end-nodes have been   upgraded to support the use of ECN, and that both end-nodes agree to   use ECN with this particular TCP connection.  This negotiation of ECN   support between the two TCP end-nodes uses two flags that have been   allocated from the Reserved field in the TCP header [RFC2481].        0   1   2   3   4   5   6   7   8   9  10  11  12  13  14  15      +---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+      |               |                       | U | A | P | R | S | F |      | Header Length |        Reserved       | R | C | S | S | Y | I |      |               |                       | G | K | H | T | N | N |      +---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+      Figure 1: The previous definition of bytes 13 and 14 of the TCP                header.Floyd                    Best Current Practice                  [Page 5]

RFC 3360                Inappropriate TCP Resets             August 2002        0   1   2   3   4   5   6   7   8   9  10  11  12  13  14  15      +---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+      |               |               | C | E | U | A | P | R | S | F |      | Header Length |    Reserved   | W | C | R | C | S | S | Y | I |      |               |               | R | E | G | K | H | T | N | N |      +---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+      Figure 2: The current definition of bytes 13 and 14 of the TCP                Header, fromRFC 3168.   The two ECN flags in the TCP header are defined from the last two   bits in the Reserved field of the TCP header.  Bit 9 in the Reserved   field of the TCP header is designated as the ECN-Echo flag (ECE), and   Bit 8 is designated as the Congestion Window Reduced (CWR) flag.  To   negotiate ECN usage, the TCP sender sends an "ECN-setup SYN packet",   a TCP SYN packet with the ECE and CWR flags set.  If the TCP host at   the other end wishes to use ECN for this connection, then it sends an   "ECN-setup SYN-ACK packet", a TCP SYN packet with the ECE flag set   and the CWR flag not set.  Otherwise, the TCP host at the other end   returns a SYN-ACK packet with neither the ECE nor the CWR flag set.   So now back to TCP resets.  When a TCP host negotiating ECN sends an   ECN-setup SYN packet, an old TCP implementation is expected to ignore   those flags in the Reserved field, and to send a plain SYN-ACK packet   in response.  However, there are some broken firewalls and load-   balancers in the Internet that instead respond to an ECN-setup SYN   packet with a reset.  Following the deployment of ECN-enabled end   nodes, there were widespread complaints that ECN-capable hosts could   not access a number of websites [Kelson00].  This has been   investigated by the Linux community, and by the TBIT project [TBIT]   in data taken from September, 2000, up to March, 2002, and has been   discussed in an article in Enterprise Linux Today [Cou01].  Some of   the offending equipment has been identified, and a web page [FIXES]   contains a list of non-compliant products and the fixes posted by the   vendors.  In March 2002, six months after ECN was approved as   Proposed Standard, ECN-setup SYN packets were answered by a reset   from 203 of the 12,364 web sites tested, and ECN-setup SYN packets   were dropped for 420 of the web sites.  Installing software that   blocks packets using flags in TCP's Reserved field is considerably   easier than uninstalling that software later on.3.1.  ECN: The Work-Around.   A work-around for maintaining connectivity in the face of the broken   equipment was described in [Floyd00], and has been specified inRFC3168 as a procedure that may be included in TCP implementations.  We   describe this work-around briefly below.Floyd                    Best Current Practice                  [Page 6]

RFC 3360                Inappropriate TCP Resets             August 2002   To provide robust connectivity even in the presence of faulty   equipment, a TCP host that receives a reset in response to the   transmission of an ECN-setup SYN packet may resend the SYN with CWR   and ECE cleared.  This would result in a TCP connection being   established without using ECN.  This also has the unfortunate result   of the ECN-capable TCP host not responding properly to the first   valid reset.  If a second reset is sent in response to the second   SYN, which had CWR and ECE cleared, then the TCP host should respond   properly by aborting the connection.   Similarly, a host that receives no reply to an ECN-setup SYN within   the normal SYN retransmission timeout interval may resend the SYN and   any subsequent SYN retransmissions with CWR and ECE cleared.  To   overcome normal packet loss that results in the original SYN being   lost, the originating host may retransmit one or more ECN-setup SYN   packets before giving up and retransmitting the SYN with the CWR and   ECE bits cleared.   Some TCP implementors have so far decided not to deploy these   workarounds, for the following reasons:   * The work-arounds would result in ECN-capable hosts not responding     properly to the first valid reset received in response to a SYN     packet.   * The work-arounds would limit ECN functionality in environments     without broken equipment, by disabling ECN where the first SYN or     SYN-ACK packet was dropped in the network.   * The work-arounds in many cases would involve a delay of six seconds     or more before connectivity is established with the remote server,     in the case of broken equipment that drops ECN-setup SYN packets.     By accommodating this broken equipment, the work-arounds have been     judged as implicitly accepting both this delay and the broken     equipment that would be causing this delay.   One possibility would be for such work-arounds to be configurable by   the user.   One unavoidable consequence of the work-around of resending a   modified SYN packet in response to a reset is to further erode the   semantics of the TCP reset.  Thus, when a box sends a reset, the TCP   host receiving that reset does not know if the reset was sent simply   because of the ECN-related flags in the TCP header, or because of   some more fundamental problem.  Therefore, the TCP host resends the   TCP SYN packet without the ECN-related flags in the TCP header.  The   ultimate consequence of this absence of clear communications from the   middlebox to the end-nodes could be an extended spiral ofFloyd                    Best Current Practice                  [Page 7]

RFC 3360                Inappropriate TCP Resets             August 2002   communications specified for transport protocols, as end nodes   attempt to sacrifice as little functionality as possible in the   process of determining which packets will and will not be forwarded   to the other end.  This is discussed in more detail inSection 6.1   below.4.  On Combating Obstacles to the Proper Evolution of the Internet    Infrastructure   One of the reasons that this issue of inappropriate resets is   important (to me) is that it has complicated the deployment of ECN in   the Internet (though it has fortunately not blocked the deployment   completely).  It has also added an unnecessary obstacle to the future   effectiveness of ECN.   However, a second, more general reason why this issue is important is   that the presence of equipment in the Internet that rejects valid TCP   packets limits the future evolution of TCP, completely aside from the   issue of ECN.  That is, the widespread deployment of equipment that   rejects TCP packets that use Reserved flags in the TCP header could   effectively prevent the deployment of new mechanisms that use any of   these Reserved flags.  It doesn't matter if these new mechanisms have   the protection of Experimental or Proposed Standard status from the   IETF, because the broken equipment in the Internet does not stop to   look up the current status of the protocols before rejecting the   packets.  TCP is good, and useful, but it would be a pity for the   deployment of broken equipment in the Internet to result in the   "freezing" of TCP in its current state, without the ability to use   the Reserved flags in the future evolution of TCP.   In the specific case of middleboxes that block TCP SYN packets   attempting to negotiate ECN, the work-around described inSection 3.1   is sufficient to ensure that end-nodes could still establish   connectivity.  However, there are likely to be additional uses of the   TCP Reserved Field standardized in the next year or two, and these   additional uses might not coexist quite as successfully with   middleboxes that send resets.  Consider the difficulties that could   result if a path changes in the middle of a connection's lifetime,   and the middleboxes on the old and new paths have different policies   about exactly which flags in the TCP Reserved field they would and   would not block.   Taking the wider view, the existence of web servers or firewalls that   send inappropriate resets is only one example of functionality in the   Internet that restricts the future evolution of the Internet.  The   impact of all of these small restrictions taken together presents a   considerable obstacle to the development of the Internet   architecture.Floyd                    Best Current Practice                  [Page 8]

RFC 3360                Inappropriate TCP Resets             August 20025.  Issues for Transport Protocols   One lesson for designers of transport protocols is that transport   protocols will have to protect themselves from the unknown and   seemingly arbitrary actions of firewalls, normalizers, and other   middleboxes in the network.  For the moment, for TCP, this means   sending a non-ECN-setup SYN when a reset is received in response to   an ECN-setup SYN packet.  Defensive actions on the side of transport   protocols could include using Reserved flags in the SYN packet before   using them in data traffic, to protect against middleboxes that block   packets using those flags.  It is possible that transport protocols   will also have to add additional checks during the course of the   connection lifetime to check for interference from middleboxes along   the path.   The ECN standards document,RFC 3168, contains an extensive   discussion inSection 18 on "Possible Changes to the ECN Field in the   Network", but includes the following about possible changes to the   TCP header:   "This document does not consider potential dangers introduced by   changes in the transport header within the network.  We note that   when IPsec is used, the transport header is protected both in tunnel   and transport modes [ESP, AH]."   With the current modification of transport-level headers in the   network by firewalls (as discussed below inSection 6.2), future   protocol designers might no longer have the luxury of ignoring the   possible impact of changes to the transport header within the   network.   Transport protocols will also have to respond in some fashion to an   ICMP code of "Communication Administratively Prohibited" if   middleboxes start to use this form of the ICMP Destination   Unreachable message to indicate that the packet is using   functionality not allowed [RFC1812].6.  Issues for Middleboxes   Given that some middleboxes are going to drop some packets because   they use functionality not allowed by the middlebox, the larger issue   remains of how middleboxes should communicate the reason for this   action to the end-nodes, if at all.  One suggestion, for   consideration in more depth in a separate document, would be that   firewalls send an ICMP Destination Unreachable message with the code   "Communication Administratively Prohibited" [B01].Floyd                    Best Current Practice                  [Page 9]

RFC 3360                Inappropriate TCP Resets             August 2002   We acknowledge that this is not an ideal solution, for several   reasons.  First, middleboxes along the reverse path might block these   ICMP messages.  Second, some firewall operators object to explicit   communication because it reveals too much information about security   policies.  And third, the response of transport protocols to such an   ICMP message is not yet specified.   However, an ICMP "Administratively Prohibited" message could be a   reasonable addition, for firewalls willing to use explicit   communication.  One possibility, again to be explored in a separate   document, would be for the ICMP "Administratively Prohibited" message   to be modified to convey additional information to the end host.   We would note that this document does not consider middleboxes that   block complete transport protocols.  We also note that this document   is not addressing firewalls that send resets in response to a TCP SYN   packet to a firewalled-off TCP port.  Such a use of resets seems   consistent with the semantics of TCP reset.  This document is only   considering the problems caused by middleboxes that block specific   packets within a transport protocol when other packets from that   transport protocol are forwarded by the middlebox unaltered.   One complication is that once a mechanism is installed in a firewall   to block a particular functionality, it can take considerable effort   for network administrators to "un-install" that block.  It has been   suggested that tweakable settings on firewalls could make recovery   from future incidents less painful all around.  Again, because this   document does not address more general issues about firewalls, the   issue of greater firewall flexibility, and the attendant possible   security risks, belongs in a separate document.6.1.  Current Choices for Firewalls   Given a firewall that has decided to drop TCP packets that use   reserved bits in the TCP header, one question is whether the firewall   should also send a Reset, in order to prevent the TCP connection from   consuming unnecessary resources at the TCP sender waiting for the   retransmit timeout.  We would argue that whether or not the firewall   feels compelled to drop the TCP packet, it is not appropriate to send   a TCP reset.  Sending a TCP reset in response to prohibited   functionality would continue the current overloading of the semantics   of the TCP reset in a way that could be counterproductive all around.   As an example,Section 2.3 has already observed that some firewalls   send resets in response to TCP SYN packets as a congestion control   mechanism.  Possibly in response to this (or perhaps in response to   something else), some popular TCP implementations immediately resend   a SYN packet in response to a reset, up to four times.  Other TCPFloyd                    Best Current Practice                 [Page 10]

RFC 3360                Inappropriate TCP Resets             August 2002   implementations, in conformance to the standards, don't resend SYN   packets after receiving a reset.  The more aggressive TCP   implementations increase congestion for others, but also increase   their own chances of eventually getting through.  Giving these fluid   semantics for the TCP reset, one might expect more TCP   implementations to start resending SYN packets in response to a   reset, completely apart from any issues having to do with ECN.   Obviously, this weakens the effectiveness of the reset when used for   its original purpose, of responding to TCP packets that apparently   are not intended for the current connection.   If we add to this mix the use of the TCP reset by firewalls in   response to TCP packets using reserved bits in the TCP header, this   muddies the waters further.  Because TCP resets could be sent due to   congestion, or to prohibited functionality, or because a packet was   received from a previous TCP connection, TCP implementations (or,   more properly, TCP implementors) would now have an incentive to be   even more persistent in resending SYN packets in response to TCP   resets.  In addition to the incentive mentioned above of resending   TCP SYN packets to increase one's odds of eventually getting through   in a time of congestion, the TCP reset might have been due to   prohibited functionality instead of congestion, so the TCP   implementation might resend SYN packets in different forms to   determine exactly which functionality is being prohibited.  Such a   continual changing of the semantics of the TCP reset could be   expected to lead to a continued escalation of measures and   countermeasures between firewalls and end-hosts, with little   productive benefit to either side.   It could be argued that *dropping* the TCP SYN packet due to the use   of prohibited functionality leads to overloading of the semantics of   a packet drop, in the same way that the reset leads to overloading   the semantics of a reset.  This is true; from the viewpoint of end-   system response to messages with overloaded semantics, it would be   preferable to have an explicit indication about prohibited   functionality (for those firewalls for some reason willing to use   explicit indications).  But given a firewall's choice between sending   a reset or just dropping the packet, we would argue that just   dropping the packet does less damage, in terms of giving an incentive   to end-hosts to adopt counter-measures.  It is true that just   dropping the packet, without sending a reset, results in delay for   the TCP connection in resending the SYN packet without the prohibited   functionality.  However, sending a reset has the undesirable longer-   term effect of giving an incentive to future TCP implementations to   add more baroque combinations of resending SYN packets in response to   a reset, because the TCP sender can't tell if the reset is for a   standard reason, for congestion, or for the prohibited functionality   of option X or reserved bit Y in the TCP header.Floyd                    Best Current Practice                 [Page 11]

RFC 3360                Inappropriate TCP Resets             August 20026.2.  The Complications of Modifying Packet Headers in the Network   In addition to firewalls that send resets in response to ECN-setup   SYN packets and firewalls that drop ECN-setup SYN packets, there also   exist firewalls that by default zero the flags in the TCP Reserved   field, including the two flags used for ECN.  We note that in some   cases this could have unintended and undesirable consequences.   If a firewall zeros the ECN-related flags in the TCP header in the   initial SYN packet, then the TCP connection will be set up without   using ECN, and the ECN-related flags in the TCP header will be sent   zeroed-out in all of the subsequent packets in this connection.  This   will accomplish the firewall's purpose of blocking ECN, while   allowing the TCP connection to proceed efficiently and smoothly   without using ECN.   If for some reason the ECN-related flags in the TCP header aren't   zeroed in the initial SYN packet from host A to host B, but the   firewall does zero those flags in the responding SYN/ACK packet from   host B to host A, the consequence could be to subvert end-to-end   congestion control for this connection.  The ECN specifications were   not written to ensure robust operation in the presence of the   arbitrary zeroing of TCP header fields within the network, because it   didn't occur to the authors of the protocol at the time that this was   a requirement in protocol design.   Similarly, if the ECN-related flags in the TCP header are not zeroed   in either the SYN or the SYN/ACK packet, but the firewall does zero   these flags in later packets in that TCP connection, this could also   have the unintended consequence of subverting end-to-end congestion   control for this connection.  The details of these possible   interactions are not crucial for this document, and are described in   the appendix.  However, our conclusion, both for the ECN-related   flags in the TCP header and for future uses of the four other bits in   the TCP Reserved field, would be that if it is required for firewalls   to be able to block the use of a new function being added to a   protocol, this is best addressed in the initial design phase by joint   cooperation between the firewall community and the protocol   designers.7.  Conclusions   Our conclusion is that it is not conformant with current standards   for a firewall, load-balancer, or web-server to respond with a reset   to a TCP SYN packet simply because the packet uses flags in the TCP   Reserved field.  More specifically, it is not conformant to respond   with a reset to a TCP SYN packet simply because the ECE and CWR flags   are set in the IP header.  We would urge vendors to make availableFloyd                    Best Current Practice                 [Page 12]

RFC 3360                Inappropriate TCP Resets             August 2002   fixes for any nonconformant code, and we could urge ISPs and system   administrators to deploy these fixes in their web servers and   firewalls.   We don't claim that it violates any standard for middleboxes to   arbitrarily drop packets that use flags in the TCP Reserved field,   but we would argue that behavior of this kind, without a clear method   for informing the end-nodes of the reasons for these actions, could   present a significant obstacle to the development of TCP.  More work   is clearly needed to reconcile the conflicting interests of providing   security while at the same time allowing the careful evolution of   Internet protocols.8.  Acknowledgements   This document results from discussions and activity by many people,   so I will refrain from trying to acknowledge all of them here.  My   specific thanks go to Ran Atkinson, Steve Bellovin, Alex Cannara,   Dennis Ferguson, Ned Freed, Mark Handley, John Klensin, Allison   Mankin, Jitendra Padhye, Vern Paxson, K. K. Ramakrishnan, Jamal Hadi   Salim, Pekka Savola, Alex Snoeren, and Dan Wing for feedback on this   document, and to the End-to-End Research Group, the IAB, and the IESG   for discussion of these issues.  I thank Mikael Olsson for numerous   rounds of feedback.  I also thank the members of the Firewall Wizards   mailing list for feedback (generally of disagreement) on an earlier   draft of this document.   Email discussions with a number of people, including Dax Kelson,   Alexey Kuznetsov, Kacheong Poon, David Reed, Jamal Hadi-Salim, and   Venkat Venkatsubra, have addressed the issues raised by non-   conformant equipment in the Internet that does not respond to TCP SYN   packets with the ECE and CWR flags set.  We thank Mark Handley,   Jitentra Padhye, and others for discussions on the TCP initialization   procedures.9.  Normative References   [RFC793]   Postel, J., "Transmission Control Protocol - DARPA              Internet Program Protocol Specification", STD 7,RFC 793,              September 1981.   [RFC1122]  Braden, R., "Requirements for Internet Hosts --              Communication Layers", STD 3,RFC 1122, October 1989.   [RFC1812]  Baker, F., "Requirements for IP Version 4 Routers",RFC1812, June 1995.Floyd                    Best Current Practice                 [Page 13]

RFC 3360                Inappropriate TCP Resets             August 2002   [RFC2026]  Bradner, S., "The Internet Standards Process -- Revision              3",BCP 9,RFC 2026, October 1996.   [RFC2481]  Ramakrishnan, K. and S. Floyd, "A Proposal to add Explicit              Congestion Notification (ECN) to IP",RFC 2481, January              1999.   [RFC2873]  Xiao, X., Hannan, A., Paxson, V., and E. Crabbe, "TCP              Processing of the IPv4 Precedence Field,RFC 2873, June              2000.   [RFC2979]  Freed, N., " Behavior of and Requirements for Internet              Firewalls",RFC 2979, October 2000.   [RFC3168]  Ramakrishnan, K., Floyd, S.  and D. Black, "The Addition              of Explicit Congestion Notification (ECN) to IP",RFC3168, September 2001.10.  Informative References   [B01]      Bellovin, S., "A "Reason" Field for ICMP "Administratively              Prohibited" Messages", Work in Progress.   [Cou01]    Scott Courtney, Why Can't My 2.4 Kernel See Some Web              Sites?, Enterprise Linux Today, Apr 17, 2001.  URL              "http://eltoday.com/article.php3?ltsn=2001-04-17-001-14-PS".   [ECN]      "The ECN Web Page", URL              "http://www.icir.org/floyd/ecn.html".   [FIXES]    ECN-under-Linux Unofficial Vendor Support Page, URL              "http://gtf.org/garzik/ecn/".   [Floyd00]  Sally Floyd, Negotiating ECN-Capability in a TCP              connection, October 2, 2000, email to the end2end-interest              mailing list.  URL              "http://www.icir.org/floyd/papers/ECN.Oct2000.txt".   [Kelson00] Dax Kelson, note sent to the Linux kernel mailing list,              September 10, 2000.   [QUESO]    Toby Miller, Intrusion Detection Level Analysis of Nmap              and Queso, August 30, 2000.  URL              "http://www.securityfocus.com/infocus/1225".Floyd                    Best Current Practice                 [Page 14]

RFC 3360                Inappropriate TCP Resets             August 2002   [Ste94]    Stevens, W., "TCP/IP Illustrated, Volume 1: The              Protocols", Addison-Wesley, 1994.   [SFO01]    FreeBSD ipfw Filtering Evasion Vulnerability, Security              Focus Online, January 23, 2001.  URL              "http://www.securityfocus.com/bid/2293".   [TBIT]     Jitendra Padhye and Sally Floyd, Identifying the TCP              Behavior of Web Servers, SIGCOMM, August 2001.  URL              "http://www.icir.org/tbit/".11.  Security Considerations   One general risk of using Reserved flags in TCP is the risk of   providing additional information about the configuration of the host   in question.   However, TCP is sufficiently loosely specified as it   is, with sufficiently many variants and options, that port-scanning   tools such as Nmap and Queso do rather well in identifying the   configuration of hosts even without the use of Reserved flags.   The security considerations and all other considerations of a   possible ICMP Destination Unreachable message with the code   "Communication Administratively Prohibited" will be discussed in a   separate document.   The traditional concern of firewalls is to prevent unauthorized   access to systems, to prevent DoS attacks and other attacks from   subverting the end-user terminal, and to protect end systems from   buggy code.  We are aware of one security vulnerability reported from   the use of the Reserved flags in the TCP header [SFO01].  A packet   filter intended only to let through packets in established   connections can let pass a packet not in an established connection if   the packet has the ECE flag set in the reserved field.  "Exploitation   of this vulnerability may allow for unauthorized remote access to   otherwise protected services." It is also possible that an   implementation of TCP could appear that has buggy code associated   with the use of Reserved flags in the TCP header, but we are not   aware of any such implementation at the moment.   Unfortunately, misconceived security concerns are one of the reasons   for the problems described in this document in the first place.  An   August, 2000, article on "Intrusion Detection Level Analysis of Nmap   and Queso" described the port-scanning tool Queso as sending SYN   packets with the last two Reserved bits in the TCP header set, and   said the following:  "[QUESO] is easy to identify, if you see [these   two Reserved bits and the SYN bit] set in the 13th byte of the TCP   header, you know that someone has malicious intentions for your   network."  As is documented on the TBIT Web Page, the middleboxesFloyd                    Best Current Practice                 [Page 15]

RFC 3360                Inappropriate TCP Resets             August 2002   that block SYNs using the two ECN-related Reserved flags in the TCP   header do not block SYNs using other Reserved flags in the TCP   header.   One lesson appears to be that anyone can effectively "attack" a new   TCP function simply by using that function in their publicly-   available port-scanning tool, thus causing middleboxes of all kinds   to block the use of that function.Floyd                    Best Current Practice                 [Page 16]

RFC 3360                Inappropriate TCP Resets             August 200212.  Appendix: The Complications of Modifying Packet Headers   In this section we first show that if the ECN-related flags in the   TCP header aren't zeroed in the initial SYN packet from Host A to   Host B, but are zeroed in the responding SYN/ACK packet from Host B   to Host A, the consequence could be to subvert end-to-end congestion   control for this connection.   Assume that the ECN-setup SYN packet from Host A is received by Host   B, but the ECN-setup SYN/ACK from Host B is modified by a firewall in   the network to a non-ECN-setup SYN/ACK, as in Figure 3 below.RFC3168 does not specify that the ACK packet in any way should echo the   TCP flags received in the SYN/ACK packet, because it had not occurred   to the designers that these flags would be modified within the   network.      Host A                    Firewall or router             Host B      -----------------------------------------------------------------      Sends ECN-setup SYN     ---------------->  Receives ECN-setup SYN                                             <- Sends ECN-setup SYN/ACK                   <- Firewall zeros flags      Receives non-ECN-setup SYN/ACK      Sends ACK and data      ---------------->   Receives ACK and data                                          <- Sends data packet with ECT                         <- Router sets CE      Receives data packet with ECT and CE      Figure 3: ECN-related flags in SYN/ACK packet cleared in network.   FollowingRFC 3168, Host A has received a non-ECN-setup SYN/ACK   packet, and must not set ECT on data packets.  Host B, however, does   not know that Host A has received a non-ECN-setup SYN/ACK packet, and   Host B may set ECT on data packets.RFC 3168 does not require Host A   to respond properly to data packets received from Host B with the ECT   and CE codepoints set in the IP header.  Thus, the data sender, Host   B, might never be informed about the congestion encountered in the   network, thus violating end-to-end congestion control.   Next we show that if the ECN-related flags in the TCP header are not   zeroed in either the SYN or the SYN/ACK packet, but the firewall does   zero these flags in later packets in that TCP connection, this could   also have the unintended consequence of subverting end-to-end   congestion control for this connection.  Figure 4 shows this   scenario.Floyd                    Best Current Practice                 [Page 17]

RFC 3360                Inappropriate TCP Resets             August 2002      Host A                    Firewall or router             Host B      -----------------------------------------------------------------      Sends ECN-setup SYN     ---------------->  Receives ECN-setup SYN      Receives ECN-setup SYN/ACK <------------  Sends ECN-setup SYN/ACK      Sends ACK and data      ---------------->   Receives ACK and data                                          <- Sends data packet with ECT                         <- Router sets CE      Receives data packet with ECT and CE      Sends ACK with ECE ->                            Firewall resets ECE ->                                                     Receives plain ACK      Figure 4: ECN-related flags in ACK packet cleared in network.   The ECN-related flags are not changed by the network in the ECN-setup   SYN and SYN/ACK packets for the scenario in Figure 4, and both end   nodes are free to use ECN, and to set the ECT flag in the ECN field   in the IP header.  However, if the firewall clears the ECE flag in   the TCP header in ACK packets from Node A to Node B, then Node B will   never hear about the congestion that its earlier data packets   encountered in the network, thus subverting end-to-end congestion   control for this connection.   Additional complications will arise when/if the use of the ECN nonce   in TCP becomes standardized in the IETF [RFC3168], as this could   involve the specification of an additional flag from the TCP Reserved   field for feedback from the TCP data receiver to the TCP data sender.   The primary motivation for the ECN nonce is to allow mechanisms for   the data sender to verify that network elements are not erasing the   CE codepoint, and that data receivers are properly reporting to the   sender the receipt of packets with the CE codepoint set.13.  IANA Considerations   There are no IANA considerations in this document.14. Author's Address   Sally Floyd   ICIR (ICSI Center for Internet Research)   Phone: +1 (510) 666-2989   EMail: floyd@icir.org   URL:http://www.icir.org/floyd/Floyd                    Best Current Practice                 [Page 18]

RFC 3360                Inappropriate TCP Resets             August 200215.  Full Copyright Statement   Copyright (C) The Internet Society (2002).  All Rights Reserved.   This document and translations of it may be copied and furnished to   others, and derivative works that comment on or otherwise explain it   or assist in its implementation may be prepared, copied, published   and distributed, in whole or in part, without restriction of any   kind, provided that the above copyright notice and this paragraph are   included on all such copies and derivative works.  However, this   document itself may not be modified in any way, such as by removing   the copyright notice or references to the Internet Society or other   Internet organizations, except as needed for the purpose of   developing Internet standards in which case the procedures for   copyrights defined in the Internet Standards process must be   followed, or as required to translate it into languages other than   English.   The limited permissions granted above are perpetual and will not be   revoked by the Internet Society or its successors or assigns.   This document and the information contained herein is provided on an   "AS IS" basis and THE INTERNET SOCIETY AND THE INTERNET ENGINEERING   TASK FORCE DISCLAIMS ALL WARRANTIES, EXPRESS OR IMPLIED, INCLUDING   BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF THE INFORMATION   HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED WARRANTIES OF   MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE.Acknowledgement   Funding for the RFC Editor function is currently provided by the   Internet Society.Floyd                    Best Current Practice                 [Page 19]