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Versions: (draft-gont-tcpm-tcp-soft-errors) 00 01 02 03 04 05 06 07 08 09 RFC 5461

TCP Maintenance and Minor                                        F. Gont
Extensions (tcpm)                                                UTN/FRH
Internet-Draft                                          January 25, 2007
Intended status: Informational
Expires: July 29, 2007


                     TCP's Reaction to Soft Errors
                 draft-ietf-tcpm-tcp-soft-errors-03.txt

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   This Internet-Draft will expire on July 29, 2007.

Copyright Notice

   Copyright (C) The IETF Trust (2007).

Abstract

   This document discusses the problem of long delays between connection
   establishment attempts that may arise in a number of scenarios,
   including one in which dual stack nodes that have IPv6 enabled by
   default are deployed in IPv4 or mixed IPv4 and IPv6 environments.
   Additionally, this document describes a non-standard, but widely
   implemented modification to TCP's reaction to ICMP "soft errors" that
   can help overcome this problem.



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Table of Contents

   1.  Introduction . . . . . . . . . . . . . . . . . . . . . . . . .  3
   2.  Error Handling in TCP  . . . . . . . . . . . . . . . . . . . .  3
     2.1.  Reaction to ICMP error messages that indicate hard
           errors . . . . . . . . . . . . . . . . . . . . . . . . . .  4
     2.2.  Reaction to ICMP error messages that indicate soft
           errors . . . . . . . . . . . . . . . . . . . . . . . . . .  4
   3.  Problems that may arise from TCP's reaction to soft errors . .  5
     3.1.  General Discussion . . . . . . . . . . . . . . . . . . . .  5
     3.2.  Problems that may arise with Dual Stack IPv6 on by
           Default  . . . . . . . . . . . . . . . . . . . . . . . . .  5
   4.  A workaround for long delays between
       connection-establishment attempts  . . . . . . . . . . . . . .  6
   5.  A more conservative approach . . . . . . . . . . . . . . . . .  7
   6.  Possible drawbacks . . . . . . . . . . . . . . . . . . . . . .  8
     6.1.  Non-deterministic transient network failures . . . . . . .  8
     6.2.  Deterministic transient network failures . . . . . . . . .  8
   7.  Security Considerations  . . . . . . . . . . . . . . . . . . .  8
   8.  Acknowledgements . . . . . . . . . . . . . . . . . . . . . . .  9
   9.  Contributors . . . . . . . . . . . . . . . . . . . . . . . . .  9
   10. References . . . . . . . . . . . . . . . . . . . . . . . . . .  9
     10.1. Normative References . . . . . . . . . . . . . . . . . . .  9
     10.2. Informative References . . . . . . . . . . . . . . . . . . 10
   Appendix A.  Change log (to be removed before publication of
                the document as an RFC) . . . . . . . . . . . . . . . 10
     A.1.  Changes from draft-ietf-tcpm-tcp-soft-errors-02  . . . . . 10
     A.2.  Changes from draft-ietf-tcpm-tcp-soft-errors-01  . . . . . 11
     A.3.  Changes from draft-ietf-tcpm-tcp-soft-errors-00  . . . . . 11
     A.4.  Changes from draft-gont-tcpm-tcp-soft-errors-02  . . . . . 11
     A.5.  Changes from draft-gont-tcpm-tcp-soft-errors-01  . . . . . 11
     A.6.  Changes from draft-gont-tcpm-tcp-soft-errors-00  . . . . . 11
   Author's Address . . . . . . . . . . . . . . . . . . . . . . . . . 11
   Intellectual Property and Copyright Statements . . . . . . . . . . 13

















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1.  Introduction

   The handling of network failures can be separated into two different
   actions: fault isolation and fault recovery.  Fault isolation
   consists of the actions that hosts and routers take to determine that
   there is a network failure.  Fault recovery, on the other hand,
   consists of the actions that hosts and routers perform in an attempt
   to survive a network failure.[RFC0816]

   In the Internet architecture, the Internet Control Message Protocol
   (ICMP) [RFC0792] is one fault isolation technique to report network
   error conditions to the hosts sending datagrams over the network.

   When a host is signaled of a network error, there is still the issue
   of what to do to let communication survive, if possible, the network
   failure.  The fault recovery strategy may depend on the type of
   network failure taking place, and the time the error condition is
   detected.

   This document analyzes the fault recovery strategy of TCP [RFC0793],
   and the problems that may arise due to TCP's reaction to ICMP soft
   errors.  Among others, it analyzes the problems that may arise in
   scenarios where dual stack nodes that have IPv6 enabled by default
   are deployed in IPv4 or mixed IPv4 and IPv6 environments.

   Additionally, we document a modification to TCP's reaction to ICMP
   messages indicating "soft errors" during connection startup, that has
   been implemented in a variety of TCP/IP stacks to help overcome the
   problems outlined below.  We stress that this modification runs
   contrary to the standard behavior and this document unambiguously
   does not change the standard reaction.


2.  Error Handling in TCP

   Network errors can be divided into soft and hard errors.  Soft errors
   are considered to be transient network failures, which are likely to
   be solved in the near term.  Hard errors, on the other hand, are
   considered to reflect network error conditions which are unlikely to
   be solved in the near future.

   The "Requirements for Internet Hosts -- Communication Layers" RFC
   [RFC1122] states, in section 4.2.3.9., that the ICMP "Destination
   Unreachable" messages that indicate soft errors are ICMP codes 0
   (network unreachable), 1 (host unreachable), and 5 (source route
   failed).  Even though ICMPv6 didn't exist when [RFC1122] was written,
   one could extrapolate the concept of soft errors to ICMPv6 Type 1
   Codes 0 (no route to destination) and 3 (address unreachable).



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   When there is a network failure that's not signaled to the sending
   host, such as a gateway corrupting packets, TCP's fault recovery
   action is to repeatedly retransmit the segment until either it gets
   acknowledged, or the connection times out.

   In case a host does receive an ICMP error message referring to an
   ongoing TCP connection, the IP layer will pass this message up to
   corresponding TCP instance to raise awareness of the network failure.
   [RFC1122]

   TCP's reaction to ICMP messages will depend on the type of error
   being signaled.

2.1.  Reaction to ICMP error messages that indicate hard errors

   When receiving an ICMP error message that indicates a hard error
   condition, TCP will simply abort the corresponding connection,
   regardless of the state the connection is in.

   The "Requirements for Internet Hosts -- Communication Layers" RFC
   [RFC1122] states, in section 4.2.3.9, that TCP SHOULD abort
   connections when receiving ICMP error messages that indicate hard
   errors.  This policy is based on the premise that, as hard errors
   indicate network error conditions that won't change in the near term,
   it will not be possible for TCP to usefully recover from this type of
   network failure.

2.2.  Reaction to ICMP error messages that indicate soft errors

   If an ICMP error message is received that indicates a soft error, TCP
   will repeatedly retransmit the packet until it either gets
   acknowledged or the connection times out.  In addition, the TCP may
   record the information for possible later use [Stevens].

   The "Requirements for Internet Hosts -- Communication Layers" RFC
   [RFC1122] states, in section 4.2.3.9, that TCP MUST NOT abort
   connections when receiving ICMP error messages that indicate soft
   errors.  This policy is based on the premise that, as soft errors are
   transient network failures that will hopefully be solved in the near
   term, one of the retransmissions will succeed.

   In case the connection timer expires, and an ICMP soft error message
   has been received before the timeout, TCP can use this information to
   provide the user with a more specific error message.  [Stevens]

   This reaction to soft errors exploits the valuable feature of the
   Internet that for many network failures, the network can be
   dynamically reconstructed without any disruption of the endpoints.



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3.  Problems that may arise from TCP's reaction to soft errors

3.1.  General Discussion

   Even though TCP's fault recovery strategy in the presence of soft
   errors allows for TCP connections to survive transient network
   failures, there are scenarios in which this policy may cause
   undesirable effects.

   For example, consider the case in which an application on a local
   host is trying to communicate with a destination whose name resolves
   to several IP addresses.  The application on the local host will try
   to establish a connection with the destination host, cycling through
   the list of IP addresses, until one succeeds [RFC1123].  Suppose that
   some (but not all) of the addresses in the returned list are
   permanently unreachable.  If such a permanently unreachable address
   is the first in the list, the application will likely try to use the
   permanently unreachable address first and block waiting for a timeout
   before trying alternate addresses.

   As discussed in Section 2, this unreachability condition may or may
   not be signaled to the sending host.  If the local TCP is not
   signaled concerning the error condition, there is very little that
   can be done other than repeatedly retransmit the SYN segment, and
   wait for the existing timeout mechanism in TCP, or an application
   timeout, to be triggered.  However, even if unreachability is
   signaled by some intermediate router to the local TCP by means of an
   ICMP soft error message, the local TCP will still repeatedly
   retransmit the SYN segment until the connection timer expires (in the
   hopes that the error is transient).  The "Requirements For Internet
   Hosts -- Communication Layers" RFC [RFC1122] states that this timer
   MUST be large enough to provide retransmission of the SYN segment for
   at least 3 minutes.  This would mean that the application on the
   local host would spend several minutes for each unreachable address
   it uses for trying to establish a TCP connection.  These long delays
   between connection establishment attempts would be inappropriate for
   many interactive applications such as the web.  ([Shneiderman] and
   [Thadani] offer some insight into the interactive systems.)  This
   highlights that there is no one definition of a "transient error" and
   that the level of persistence in the face of failure represents a
   tradeoff.

3.2.  Problems that may arise with Dual Stack IPv6 on by Default

   A particular scenario in which the above sketched type of problem may
   occur regularly is that where dual stack nodes that have IPv6 enabled
   by default are deployed in IPv4 or mixed IPv4 and IPv6 environments,
   and the IPv6 connectivity is non-existent



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   [I-D.ietf-v6ops-v6onbydefault].

   As discussed in [I-D.ietf-v6ops-v6onbydefault], there are two
   possible variants of this scenario, which differ in whether the lack
   of connectivity is signaled to the sending node, or not.

   In cases where packets sent to a destination are silently dropped and
   no ICMPv6 [RFC4443] errors are generated, there is little that can be
   done other than waiting for the existing connection timeout mechanism
   in TCP, or an application timeout to be triggered.

   In cases where a node has no default routers and Neighbor
   Unreachability Detection (NUD) fails for destinations assumed to be
   on-link, or where firewalls or other systems that enforce scope
   boundaries send ICMPv6 errors, the sending node will be signaled of
   the unreachability problem.  However, as discussed in Section 2.2,
   standard TCP implementations will not abort connections when
   receiving ICMP error messages that indicate soft errors.


4.  A workaround for long delays between connection-establishment
    attempts

   As discussed in Section 1, it may make sense for the fault recovery
   action to depend not only on the type of error being reported, but
   also on the state of the connection against which the error is
   reported.  For example, one could infer that when an error arrives in
   response to opening a new connection, it is probably caused by
   opening the connection improperly, rather than by a transient network
   failure.  [RFC0816]

   A number of TCP implementations have modified their reaction to soft
   errors, to treat the errors as hard errors in the SYN-SENT or SYN-
   RECEIVED states.  It must be noted that this change violates section
   4.2.3.9 of [RFC1122], which states that these Unreachable messages
   indicate soft error conditions and TCP MUST NOT abort the
   corresponding connection.

   This workaround has been implemented, for example, in the Linux
   kernel since version 2.0.0 (released in 1996) [Linux].  Section 5
   discusses a more conservative approach than that sketched above, that
   is implemented in FreeBSD.

   We note that the TCPM WG could not arrive at consensus on allowing
   the above described behavior as part of the standard.  Therefore,
   treating soft errors as hard errors during connection establishment,
   while widespread, is not part of standard TCP behavior and this
   document does not change that state of affairs.



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5.  A more conservative approach

   A more conservative approach than simply treating soft errors as hard
   errors as described above would be to abort a connection in the SYN-
   SENT or SYN-RECEIVED states only after an ICMP Destination
   Unreachable has been received a specified number of times, and the
   SYN segment has been retransmitted more than some specified number of
   times.

   Two new parameters would have to be introduced to TCP, to be used
   only during the connection-establishment phase: MAXSYNREXMIT and
   MAXSOFTERROR.  MAXSYNREXMIT would specify the number of times the SYN
   segment would have to be retransmitted before a connection is
   aborted.  MAXSOFTERROR would specify the number of ICMP messages
   indicating soft errors that would have to be received before a
   connection is aborted.

   Two additional state variables would need to be introduced to store
   additional state information during the connection-establishment
   phase: "nsynrexmit" and "nsofterror".  Both would be initialized to
   zero when a connection attempt is initiated, with "nsynrexmit" being
   incremented by one every time the SYN segment is retransmitted and
   "nsofterror" being incremented by one every time an ICMP message that
   indicates a soft error is received.

   A connection in the SYN-SENT or SYN-RECEIVED states would be aborted
   if nsynrexmit was greater than MAXSYNREXMIT and "nsofterror" was
   simultaneously greater than MAXSOFTERROR.

   This approach would give the network more time to solve the
   connectivity problem than simply aborting a connection attempt upon
   reception of the first soft error.  However, it should be noted that
   depending on the values chosen for the MAXSYNREXMIT and MAXSOFTERROR
   parameters, this approach could still lead to long delays between
   connection establishment attempts, thus not solving the problem.  For
   example, BSD systems abort connections in the SYN-SENT or the SYN-
   RECEIVED state when a second ICMP error is received, and the SYN
   segment has been retransmitted more than three times.  They also set
   up a "connection-establishment timer" that imposes an upper limit on
   the time the connection establishment attempt has to succeed, which
   expires after 75 seconds [Stevens2].  Even when this policy may be
   better than the three-minutes timeout policy specified in [RFC1122],
   it may still be inappropriate for handling the potential problems
   described in this document.  This more conservative approach has been
   implemented in BSD systems since, at least, 1994 [Stevens2].

   We also note that the approach given in this section is a generalized
   version of the approach sketched in the previous section.  In



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   particular, with MAXSOFTERROR set to 1 and MAXSYNREXMIT set to zero
   the schemes are identical.


6.  Possible drawbacks

   The following subsections discuss some of the possible drawbacks
   arising from the use of the non-standard modifications to TCP's
   reaction to soft errors described in Section 4 and Section 5.

6.1.  Non-deterministic transient network failures

   In case a transient network failure affects all of the addresses
   returned by the name-to-address translation function, all
   destinations could be unreachable for some short period of time.  In
   such a scenario, the application could quickly cycle through all the
   IP addresses in the list and return an error, when it could have let
   TCP retry a destination a few seconds later, when the transient
   problem could have disappeared.

6.2.  Deterministic transient network failures

   There are some scenarios in which transient network failures could be
   deterministic.  For example, consider the case in which upstream
   network connectivity is triggered by network use.  That is, network
   connectivity is instantiated only on an "as needed" basis.  In this
   scenario, the connection triggering the upstream connectivity would
   deterministically receive ICMP Destination Unreachables while the
   upstream connectivity is being activated, and thus would be aborted.


7.  Security Considerations

   This document describes a non-standard modification to TCP's reaction
   to soft errors that has been implemented in a variety of TCP
   implementations.  This modification makes TCP abort a connection in
   the SYN-SENT or the SYN-RECEIVED states when it receives an ICMP
   "Destination Unreachable" message that indicates a "soft error".
   Therefore, the modification could be exploited to reset valid
   connections during the connection-establishment phase.

   The non-standard workaround described in this document makes TCP more
   vulnerable to attack---even if only slightly.  However, we note that
   an attacker wishing to reset ongoing TCP connections could send any
   of the ICMP hard error messages in any connection state.

   A discussion of the use of ICMP to perform a variety of attacks
   against TCP, and a number of counter-measures that minimize the



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   impact of these attacks can be found in [I-D.ietf-tcpm-icmp-attacks].

   A discussion of the security issues arising from the use of ICMPv6
   can be found in [RFC4443].


8.  Acknowledgements

   The author wishes to thank Mark Allman, Ron Bonica, Sally Floyd,
   Guillermo Gont, Michael Kerrisk, Eddie Kohler, Mika Liljeberg, Pasi
   Sarolahti, Pekka Savola, and Joe Touch, for contributing many
   valuable comments on earlier versions of this document.


9.  Contributors

   Mika Liljeberg was the first to describe how their implementation
   treated soft errors.  Based on that, the solution discussed in
   Section 4 was documented in [I-D.ietf-v6ops-v6onbydefault] by
   Sebastien Roy, Alain Durand and James Paugh.


10.  References

10.1.  Normative References

   [RFC0792]  Postel, J., "Internet Control Message Protocol", STD 5,
              RFC 792, September 1981.

   [RFC0793]  Postel, J., "Transmission Control Protocol", STD 7,
              RFC 793, September 1981.

   [RFC1122]  Braden, R., "Requirements for Internet Hosts -
              Communication Layers", STD 3, RFC 1122, October 1989.

   [RFC1123]  Braden, R., "Requirements for Internet Hosts - Application
              and Support", STD 3, RFC 1123, October 1989.

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119, March 1997.

   [RFC4443]  Conta, A., Deering, S., and M. Gupta, "Internet Control
              Message Protocol (ICMPv6) for the Internet Protocol
              Version 6 (IPv6) Specification", RFC 4443, March 2006.







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10.2.  Informative References

   [Guynes]   Guynes, J., "Impact of System Response Time on State
              Anxiety", Communications of the ACM , 1988.

   [I-D.ietf-tcpm-icmp-attacks]
              Gont, F., "ICMP attacks against TCP",
              draft-ietf-tcpm-icmp-attacks-01 (work in progress),
              October 2006.

   [I-D.ietf-v6ops-v6onbydefault]
              Roy, S., Durand, A., and J. Paugh, "Issues with Dual Stack
              IPv6 on by Default", draft-ietf-v6ops-v6onbydefault-03
              (work in progress), July 2004.

   [Linux]    The Linux Project, "http://www.kernel.org".

   [RFC0816]  Clark, D., "Fault isolation and recovery", RFC 816,
              July 1982.

   [Shneiderman]
              Shneiderman, B., "Response Time and Display Rate in Human
              Performance with Computers", ACM Computing Surveys , 1984.

   [Stevens]  "TCP/IP Illustrated, Volume 1: The Protocols", Addison-
              Wesley , 1994.

   [Stevens2]
              Wright, G. and W. Stevens, "TCP/IP Illustrated, Volume 2:
              The Implementation", Addison-Wesley , 1994.

   [Thadani]  Thadani, A., "Interactive User Productivity", IBM Systems
              Journal No. 1, 1981.


Appendix A.  Change log (to be removed before publication of the
             document as an RFC)

A.1.  Changes from draft-ietf-tcpm-tcp-soft-errors-02

   o  Moved appendix on FreeBSD's approach to the body of the draft.

   o  Removed rest of the appendix, as suggested by Ron Bonica and Mark
      Allman.

   o  Reworded some parts of the document to make the text more neutral.





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   o  Miscellaneous editorial changes.

A.2.  Changes from draft-ietf-tcpm-tcp-soft-errors-01

   o  Addressed feedback posted by Sally Floyd (remove sentence in
      Section 2.1 regarding processing of RST segments)

A.3.  Changes from draft-ietf-tcpm-tcp-soft-errors-00

   o  Miscellaneous editorial changes

A.4.  Changes from draft-gont-tcpm-tcp-soft-errors-02

   o  Draft resubmitted as draft-ietf.

   o  Miscellaneous editorial changes

A.5.  Changes from draft-gont-tcpm-tcp-soft-errors-01

   o  Changed wording to describe the mechanism, rather than proposing
      it

   o  Miscellaneous editorial changes

A.6.  Changes from draft-gont-tcpm-tcp-soft-errors-00

   o  Added reference to the Linux implementation in Section 4

   o  Added Section 6

   o  Added section on Higher-Level API

   o  Added Section 5

   o  Moved section "Asynchronous Application Notification" to Appendix

   o  Added section on parallel connection requests

   o  Miscellaneous editorial changes












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Author's Address

   Fernando Gont
   Universidad Tecnologica Nacional / Facultad Regional Haedo
   Evaristo Carriego 2644
   Haedo, Provincia de Buenos Aires  1706
   Argentina

   Phone: +54 11 4650 8472
   Email: fernando@gont.com.ar
   URI:   http://www.gont.com.ar








































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Full Copyright Statement

   Copyright (C) The IETF Trust (2007).

   This document is subject to the rights, licenses and restrictions
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