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Network Working Group                                            F. Gont
Internet-Draft                                                   UTN FRH
Expires: November 19, 2004                                  May 19, 2004


               TCP Adaptive User TimeOut (AUTO) Option
                draft-gont-tcpm-tcp-auto-option-00.txt

Status of this memo

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Copyright Notice

      Copyright (C) The Internet Society (2004). All Rights Reserved.


Abstract

   The original TCP specification (RFC 793) defines a "USER TIMEOUT"
   parameter that sets the policy as to when a user connection should be
   aborted. However, TCP provides no means of letting users suggest an
   abort policy to a remote peer dynamically. Even though a fixed policy
   may work well in many cases, there are a number of scenarios where a
   fixed USER TIMEOUT value may be inappropriate, and some means of
   setting the abort policy dynamically may be necessary for TCP to be
   used effectively in such scenarios. This document defines a new TCP
   option, which lets a TCP peer suggest a USER TIMEOUT value to a
   remote TCP during the connection-establishment phase, and modify it




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   during the life of a connection, thus adapting TCP's connection-abort
   policy as necessary.


1. Introduction

   The original TCP specification [1] defines a USER TIMEOUT parameter,
   which sets the policy as to when a connection should be aborted. This
   parameter is usually set on a per-system basis, and there is no way
   for a TCP to suggest a value of USER TIMEOUT to be used for a
   connection by a remote peer.

   Even though having such a fixed policy may work well in many cases,
   there are scenarios in which the default USER TIMEOUT may be
   inappropriate. For example, a mobile host connected to a network by
   means of a wireless link may experience transient periods of
   disconnection that may be longer than the USER TIMEOUT selected by
   the remote peer. Another possible scenario is the development of
   high levels of congestion during the life of a connection.
   In such cases, valid connections may be aborted due to an incorrect
   abort policy.

   This document defines a new TCP option that lets TCP implementations
   suggest a USER TIMEOUT value during the connection-establishment
   phase, and modify it during the life of a connection, thus adapting
   TCP's connection-abort policy as necessary.


2. Conventions

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL
   NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and
   "OPTIONAL" in this document are to be interpreted as described in
   RFC 2119 [2].




















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3. Option Format

    0                   1                   2                     3
    0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |     Kind = X  |   Length = 4  |G|        User Timeout         |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

        Note that one tick mark represents one bit position

           Figure 1: Adaptive User Timeout Option Format


   Each field is to be interpreted as follows:

   Kind: 8 bits
      This is the "Kind" field as specified in [1]. The "X" in Figure 1
      is an option number to be assigned by IANA upon publication of
      this document (see Section 7)

   Length: 8 bits
      This is the "Length" field as specified in [1]. Its value is 4
      (the option length).

   G: 1 bit
      This is the "Granularity" bit. It indicates the granularity of the
      "User Timeout" field. When set, the time interval in the "User
      Timeout" field MUST be interpreted as being specified in minutes.
      Otherwise, the time interval in the "User Timeout" field MUST be
      interpreted as being specified in seconds.

   User Timeout: 15 bits
      This field, together with the Granularity bit, specifies the USER
      TIMEOUT suggested by the remote peer for this connection. It MUST
      be interpreted as a 15-bit unsigned integer. The units of this
      field are specified by the "G" bit.


3. Operation

   TCP implementations supporting the Adaptive User TimeOut (AUTO)
   Option MUST set this option during the connection-establishment phase
   (in segments with the SYN control bit set) to indicate the suggested
   USER TIMEOUT value to be used for the connection.

   A TCP MAY also use this option during the life of a connection, to
   suggest an a new value for the USER TIMEOUT parameter, thus adapting
   it to the current network conditions. For example, this option could
   be set by a TCP peer that is notified of congestion by means of
   ECN [5].




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   The setting of this option means "I suggest we use a USER TIMEOUT
   of X". The value of "X" may be larger or smaller than the default
   USER TIMEOUT (see Section 4).

   Hosts SHOULD impose upper and lower limits on the USER TIMEOUT. A
   discussion of these limits can be found in Section 5.

   Each TCP will adopt a USER TIMEOUT as defined by equation (1):

   USER_TIMEOUT = min( ULimit, max(localAUTO, remoteAUTO, LLimit))   (1)

   USER_TIMEOUT:
      USER TIMEOUT value to be adopted by the local TCP for this
      connection.

   Ulimit:
      The upper limit imposed by this host for the USER TIMEOUT.

   Llimit:
      The lower limit imposed by this host for the USER TIMEOUT.

   localAUTO:
      The "USER TIMEOUT" value suggested by the local TCP by means of
      the AUTO Option.

   remoteAUTO:
      The "USER TIMEOUT" value suggested by the remote TCP peer by means
      of the AUTO Option.


   The adopted USER TIMEOUT SHOULD be used only for connections that are
   in one of the synchronized states (ESTABLISHED, FIN-WAIT-1,
   FIN-WAIT-2, CLOSE-WAIT, CLOSING, LAST-ACK or TIME-WAIT).

   Note that the USER TIMEOUT is not negotiated in any way. Each peer
   just "suggests" what USER TIMEOUT should be adopted for the
   connection. As can be inferred from the equation above, each peer may
   end up adopting a different timeout value.


4. Range of valid values

   The User Timeout Option allows a TCP peer to suggest USER TIMEOUT
   values ranging, in principle, from 0 seconds to about 22.76 days.
   However, implementations SHOULD impose limits on the USER TIMEOUT
   values actually adopted. A discussion of these limits can be found
   in Section 5.







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5. System limits on the USER TIMEOUT

   Implementations SHOULD impose an upper limit (Ulimit) and a lower
   limit (Llimit) on the value of the USER TIMEOUT. These limits could,
   for example, be set on a per-host or per-user basis.

   Furthermore, these limits need not be fixed. For example, they MAY
   be a function of the system resources that are available when the
   USER TIMEOUT is to be selected for a connection.

   The Host Requirements RFC [3] does not impose any limits for the USER
   TIMEOUT. However, a time interval of at least 100 seconds is
   RECOMMENDED. Thus, the lower limit (LLimit) should be set to at least
   100 seconds. As for the upper limit (ULimit), note that setting it to
   low values may reduce the functionality of the AUTO Option.


6. Interoperability issues

6.1 Firewalls

   Stateful firewalls are known to reset connections after some fixed
   period of inactivity is detected. In case there is such a firewall
   between the TCP peers, then, regardless of the use of the AUTO
   Option, connections may be lost due to the firewall policy.


6.2 TCP Keep-alive mechanism

   In case a TCP peer enables the TCP Keep-alive mechanism for a
   connection that is using the AUTO Option, then the Keep-alive timer
   MUST be set to a value larger than that of the adopted USER
   TIMEOUT (specified by Equation 1).


7. IANA Considerations

   This section is to be interpreted according to [4].

   This document does not define any new namespaces. It uses an 8-bit
   TCP option number maintained by IANA at
   http://www.iana.org/assignments/tcp-parameters.












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8. Security Implications

   Use of the AUTO Option implies that the adopted USER TIMEOUT be
   larger than the default USER TIMEOUT. This could cause a host to
   maintain state for a connection for a longer period of time than if
   the default USER TIMEOUT were used. An attacker could try to exhaust
   resources on the target host by establishing lots of connections
   and aborting them without signalling this to the attacked host's TCP.
   However, it must be noted that the same type of attack could be
   performed even if the default "USER TIMEOUT" is being used, since TCP
   requires no message exchange in order to keep a connection open.
   In any case, the system limits discussed in Section 5 would serve
   as a counter-measure against attackers trying to exploit the AUTO
   option for this type of attack.


9. Author's address

   Fernando Gont
   Evaristo Carriego 2644
   1706, Haedo
   Provincia de Buenos Aires
   ARGENTINA

   Phone:  +54 011 4650 8472
   E-Mail: fernando@gont.com.ar


10. Acknowledgements

   The author wishes to thank Michael Kerrisk for contributing many
   valuable comments.


11. References

11.1 Normative References

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

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

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

   [4] Narten, T., Alvestrand, H., "Guidelines for Writing an IANA
       Considerations Section in RFCs", BCP 26, RFC 2434, October 1998.





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11.2 Informative References

   [5] Ramakrishnan, K., Floyd, S., Black, D., "The Addition of Explicit
       Network Notification (ECN) to IP", RFC 3168, September 2001.



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Copyright Statement

   Copyright (C) The Internet Society (2004). This document is
   subject to the rights, licenses and restrictions contained in BCP
   78, and except as set forth therein, the authors retain all their
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