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rfc:rfc3360

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.

Floyd Best Current Practice [Page 1] RFC 3360 Inappropriate TCP Resets August 2002

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.

Floyd Best Current Practice [Page 2] RFC 3360 Inappropriate TCP Resets August 2002

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

Floyd 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 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

Floyd 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, 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.

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 of

Floyd 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 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.

Floyd Best Current Practice [Page 8] RFC 3360 Inappropriate TCP Resets August 2002

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].

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 TCP

Floyd 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 2002

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

Floyd 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", RFC
            1812, 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", 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".

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 middleboxes

Floyd 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 2002

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.

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 2002

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.

Floyd Best Current Practice [Page 19]

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