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BEST CURRENT PRACTICE

Network Working Group                                           S. Floyd
Request for Comments: 3360                                          ICIR
BCP: 60                                                      August 2002
Category: Best Current Practice


              Inappropriate TCP Resets Considered Harmful

Status 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.



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2.  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, in Section 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 in RFC 793 or RFC 1122 that suggests that it
   is acceptable to send a reset simply because a SYN packet uses
   Reserved flags in the TCP header, and RFC 793 explicitly forbids
   sending a reset for this reason.

   RFC 793 and RFC 1122 both include Jon Postel's famous robustness
   principle, also from RFC 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 in RFC 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.








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2.1.  The TCP Reserved Field

   RFC 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 in RFC 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 Firewalls

   RFC 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 that RFC 2979 is an Informational RFC.  RFC 2026 on
   Internet Standards Process says the following in Section 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 as





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   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 Field

   RFC 793 includes the following in Section 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 in RFC 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 Lengths

   RFC 1122 says the following in Section 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 defined





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   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.








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        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, from RFC 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 in RFC
   3168 as a procedure that may be included in TCP implementations.  We
   describe this work-around briefly below.




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   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 of



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   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 in Section 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 in Section 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.



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5.  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 in Section 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 in Section 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].





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   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 TCP



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   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.



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6.2.  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 available



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   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", RFC
              1812, June 1995.





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   [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", RFC
              3168, 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".






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   [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 middleboxes



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   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.











































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12.  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.  RFC
   3168 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.

   Following RFC 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.







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      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/







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15.  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.



















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