TCP Maintenance and Minor                                      L. Eggert
Extensions (tcpm)                                                    NEC
Internet-Draft                                                   F. Gont
Expires: January 16, 2006                                        UTN/FRH
                                                           July 15, 2005

                        TCP User Timeout Option

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Copyright Notice

   Copyright (C) The Internet Society (2005).


   The TCP user timeout controls how long transmitted data may remain
   unacknowledged before a connection is forcefully closed.  It is a
   local, per-connection parameter.  The advisory TCP User Timeout
   Option allows conforming TCP implementations to exchange their local
   user timeouts.  This exchange provides an in-protocol mechanism to
   coordinate raising or lowering the two user timeouts of a connection.
   Increase the user timeouts allows established TCP connections to

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   survive extended periods of disconnection.  Decreasing user timeouts
   allows busy servers to explicitly notify their clients that they will
   maintain the connection state only across short periods of

1.  Introduction

   The Transmission Control Protocol (TCP) specification [RFC0793]
   defines a local, per-connection "user timeout" parameter that
   specifies the maximum amount of time that transmitted data may remain
   unacknowledged before TCP will forcefully close the corresponding
   connection.  Applications can set and change this parameter with OPEN
   and SEND calls.  If a network disconnection lasts longer than the
   user timeout, no acknowledgments will be received for any
   transmission attempt, including keep-alives [TCP-ILLU], and the TCP
   connection will close when the user timeout occurs.  In the absence
   of an application-specified user timeout, the TCP specification
   [RFC0793] defines a default user timeout of 5 minutes.

   The Host Requirements RFC [RFC1122] refines this definition by
   introducing two thresholds, R1 and R2 (R2 > R1), on the number of
   retransmissions of a single segment.  It suggests that TCP notify
   applications when R1 is reached for a segment, and close the
   connection once R2 is reached.  [RFC1122] also refines the
   recommended values for R1 (three retransmissions) and R2 (100
   seconds), noting that R2 for SYN segments should be at least 3
   minutes.  Instead of a single user timeout, some TCP implementations
   offer finer-grained policies.  For example, Solaris supports
   different timeouts depending on whether a TCP connection is in the

   Although applications may set their local user timeout, there is no
   in-protocol mechanism to signal changes in the local user timeout to
   remote peers.  This causes local changes to be ineffective, because,
   for example, the peer will still close the connection after its user
   timeout expires, even when a host has raised its local user timeout.
   The ability to modify the two user timeouts associated with a
   connection in a coordinated manner can improve TCP operation in
   scenarios that are currently not well supported.  One example of such
   scenarios are mobile hosts that change network attachment points
   based on current location.  Such hosts, maybe using MobileIP
   [RFC3344], HIP [I-D.ietf-hip-arch] or transport-layer mobility
   mechanisms [I-D.eddy-tcp-mobility], are only intermittently connected
   to the Internet.  In between connected periods, mobile hosts may
   experience periods of disconnection during which no network service
   factors that can cause transient periods of disconnection are high
   levels of congestion as well as link or routing failures inside the

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   In scenarios similar to the ones described above, a host may not know
   exactly when or for how long it will be disconnected from the
   network, but it might expect such events due to past mobility
   patterns and thus benefit from using longer user timeouts.  In other
   scenarios, the length and time of a network disconnection may even be
   predictable.  For example, an orbiting node on a satellite might
   experience disconnections due to line-of-sight blocking by other
   planetary bodies.  The disconnection periods of such a node may be
   easily computable from orbital mechanics.

   This document specifies a new TCP option - the User Timeout Option
   (UTO) - that allows conforming hosts to exchange their local, per-
   connection user timeout information.  This allows, for example,
   mobile hosts to maintain TCP connections across disconnected periods
   that are longer than their peer's default user timeout.  A second use
   of the TCP User Timeout Option is advertisement of shorter-than-
   default user timeouts.  This can allow busy servers to explicitly
   notify their clients that they will maintain the state associated
   with established connections only across short periods of

   The same benefits can be obtained through an application-layer
   mechanism, i.e., coordinating changes to the user timeout values of a
   connection through application messages.  This approach does not
   require a new TCP option, but requires application changes.

   A different approach to tolerate longer periods of disconnection is
   simply increasing the system-wide user timeout on both peers.  This
   approach has the benefit of not requiring a new TCP option.  However,
   it can also significantly increase the amount of connection state
   information a busy server must maintain, because a longer global
   timeout value will apply to all its connections.  The proposed TCP
   User Timeout Option, on the other hand, allows hosts to selectively
   manage the user timeouts of individual connections, reducing the
   amount of state they must maintain across disconnected periods.

2.  Conventions

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   document are to be interpreted as described in [RFC2119].

3.  Operation

   Sending a TCP User Timeout Option suggests that the remote peer
   SHOULD start using the indicated user timeout value for the

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   corresponding connection.  The user timeout value included in a TCP
   User Timeout Option specifies the requested user timeout during the
   synchronized states of a connection (ESTABLISHED, FIN-WAIT-1, FIN-
   WAIT-2, CLOSE-WAIT, CLOSING, or LAST-ACK).  Connections in other
   states MUST use standard timeout values [RFC0793][RFC1122]. [anchor4]

   Note that an exchange of TCP User Timeout Options between peers is
   not a binding negotiation.  Transmission of a TCP User Timeout Option
   is an advisory suggestion that the peer consider adapting its local
   user timeout.  Hosts remain free to forcefully close or abort
   connections at any time for any reason, whether or not they use
   custom user timeouts or have suggested to the peer to use them.

   A host that supports the TCP User Timeout Option SHOULD include it in
   the next possible segment to its peer whenever it starts using a new
   user timeout for the connection.  This allows the peer to adapt its
   local user timeout for the connection accordingly.

   When a host that supports the TCP User Timeout Option receives one,
   it decides whether to change its local user timeout of the connection
   based on the received value.  Generally, hosts should honor requests
   for changes to the user timeout (see Section 3.3), unless security
   concerns, resource constraints or external policies indicate
   otherwise (see Section 5).  If so, hosts may ignore incoming TCP User
   Timeout Options and use a different user timeout for the connection.

   When a host receives a TCP User Timeout Option, it first decides
   whether to change its local user timeout for the connection (see
   Section 3.3) and then decides whether to send a TCP User Timeout
   Option to its peer in response.  If it has never sent a TCP User
   Timeout Option to its peer during the lifetime of the connection or
   if it has changed its local user timeout, it SHOULD send TCP User
   Timeout Option with its current local user timeout to its peer.

   A host that supports the TCP User Timeout Option SHOULD include one
   in each packet that carries a SYN flag, but need not.  [MEDINA] has
   shown that unknown options are correctly handled by the vast majority
   of modern TCP stacks.  It is thus not necessary to require
   negotiation use of the TCP User Timeout Option for a connection.

   A TCP implementation that does not support the TCP User Timeout
   Option MUST silently ignore it [RFC1122], thus ensuring

   Hosts SHOULD impose upper and lower limits on the user timeouts they
   use.  Section 3.3 discusses user timeout limits.  A TCP User Timeout
   Option with a value of zero (i.e., "now") is nonsensical and is used

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   for a special purpose, see Section 3.4.  Section 3.3 discusses
   potentially problematic effects of other user timeout durations.

3.1  Reliability Considerations

   The TCP User Timeout Option is an advisory TCP option that does not
   change processing for subsequent segments.  Unlike other TCP options,
   it need not be exchanged reliably.  Consequently, the specification
   in this section does not define a reliability handshake for TCP User
   Timeout Option exchanges.  When a segment that carries a TCP User
   Timeout Option is lost, the option may never reach the intended peer.

   Implementations MAY implement local mechanisms to improve delivery
   reliability, such as retransmitting the TCP User Timeout Option when
   they retransmit the segment that originally carried it or "attaching"
   the option to a byte in the stream and retransmitting the option
   whenever that byte or its ACK are retransmitted.

   It is important to note that although these mechanisms can improve
   transmission reliability for the TCP User Timeout Option, they do not
   guarantee delivery (a three-way handshake would be required for
   this).  Consequently, implementations MUST NOT assume that a TCP User
   Timeout Option is reliably transmitted.

3.2  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         |

   (One tick mark represents one bit.)

              Figure 1: Format of the TCP User Timeout Option

   Figure 1 shows the format of the TCP User Timeout Option.  It
   contains these fields:

   Kind (8 bits)
      A TCP option number [RFC0793] to be assigned by IANA upon
      publication of this document (see Section 6).

   Length (8 bits)
      Length of the TCP option in octets [RFC0793]; its value MUST be 4.

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   Granularity (1 bit)
      Granularity bit, indicating the granularity of the "User Timeout"
      field.  When set (G = 1), the time interval in the "User Timeout"
      field MUST be interpreted as minutes.  Otherwise (G = 0), the time
      interval in the "User Timeout" field MUST be interpreted as

   User Timeout (15 bits)
      Specifies the user timeout suggestion for this connection.  It
      MUST be interpreted as a 15-bit unsigned integer.  The granularity
      of the timeout (minutes or seconds) depends on the "G" field.

3.3  Duration of the User Timeout

   The TCP User Timeout Option allows hosts to exchange user timeout
   values from 1 second to over 9 hours at a granularity of seconds and
   from 1 minute to over 22 days at a granularity of minutes.  (An
   option value of zero is reserved for a special purpose, see
   Section 3.4.)

   Very short user timeout values can affect TCP transmissions over
   high-delay paths.  If the user timeout occurs before an
   acknowledgment for an outstanding segment arrives, possibly due to
   packet loss, the connection closes.  Many TCP implementations default
   to user timeout values of a few minutes [TCP-ILLU].  Although the TCP
   User Timeout Option allows suggestion of short timeouts, applications
   advertising them SHOULD consider these effects.

   Long user timeout values allow hosts to tolerate extended periods of
   disconnection.  However, they also require hosts to maintain the TCP
   state information associated with connections for long periods of
   time.  Section 5 discusses the security implications of long timeout

   To protect against these effects, implementations SHOULD impose
   limits on the user timeout values they accept and use.  The remainder
   of this section describes a RECOMMENDED scheme to limit user timeouts
   based on upper and lower limits.  Under the RECOMMENDED scheme, each
   TCP SHOULD compute the user timeout (USER_TIMEOUT) for a connection
   according to this formula:


   Each field is to be interpreted as follows:

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      Resulting user timeout value to be adopted by the local TCP for a

      Current upper limit imposed on the user timeout of a connection by
      the local host.

      Current lower limit imposed on the user timeout of a connection by
      the local host.

      Current local user timeout of the specific connection.

      Last "user timeout" value suggested by the remote peer by means of
      the TCP User Timeout Option.

   This means that the maximum of the two announced values will be
   adopted for the user timeout of the connection.  The rationale is
   that choosing the maximum of the two values will let the connection
   survive longer periods of disconnection.  If the TCP that announced
   the lower of the two user timeout values did so in order to reduce
   the amount of TCP state information that must be kept on the host, it
   can, nevertheless, close or abort the connection whenever it wants.

   Enforcing a lower limit (L_LIMIT) prevents connections from closing
   due to transient network conditions, including temporary congestion,
   mobility hand-offs and routing instabilities.

   An upper limit (U_LIMIT) can reduce the effect of resource exhaustion
   attacks.  Section 5 discusses the details of these attacks.

   Note that these limits MAY be specified as system-wide constants or
   at other granularities, such as on per-host, per-user or even per-
   connection basis.  Furthermore, these limits need not be static.  For
   example, they MAY be a function of system resource utilization or
   attack status and could be dynamically adapted.

   The Host Requirements RFC [RFC1122] does not impose any limits on the
   length of the user timeout.  However, a time interval of at least 100
   seconds is RECOMMENDED.  Consequently, the lower limit (L_LIMIT)
   SHOULD be set to at least 100 seconds when following the RECOMMENDED
   scheme described in this section.

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3.4  Special Option Values

   Whenever it is legal to do so according to the specification in the
   previous sections, TCP implementations MAY send a zero-second TCP
   User Timeout Option, i.e, with a "User Timeout" field of zero and a
   "Granularity" of zero.  This signals their peers that they support
   the option, but do not suggest a specific user timeout value at that
   time.  Essentially, a zero-second TCP User Timeout Option acts as a
   "don't care" value.

   The receiver of a zero-second TCP User Timeout Option SHOULD perform
   the RECOMMENDED strategy for calculating a new local USER_TIMEOUT
   described in Section 3.3 with a numeric value of zero seconds for
   REMOTE_UTO.  The sender SHOULD perform the calculation as described
   in Section 3.3.  Essentially, the sender SHOULD adapt the peer's UTO
   and the receiver SHOULD continue using its local UTO.

   A zero-minute TCP User Timeout Option, i.e., with a "User Timeout"
   field of zero and a "Granularity" bit of one, is reserved for future
   use.  TCP implementations MUST NOT sent it and MUST ignore it upon

4.  Interoperability Issues

   This section discusses interoperability issues related to introducing
   the TCP User Timeout Option.

4.1  Middleboxes

   The large number of middleboxes (firewalls, proxies, protocol
   scrubbers, etc.) currently present in the Internet pose some
   difficulty for deploying new TCP options.  Some firewalls may block
   segments that carry unknown options, preventing connection
   establishment when the SYN or SYN-ACK contains a TCP User Timeout
   Option.  Some recent results, however, indicate that for new TCP
   options, this may not be a significant threat, with only 0.2% of web
   requests failing when carrying an unknown option [MEDINA].

   Stateful firewalls usually reset connections after a period of
   inactivity.  If such a firewall exists along the path between two
   peers, it may close or abort connections regardless of the use of the
   TCP User Timeout Option.  In the future, such firewalls may learn to
   parse the TCP User Timeout Option and modify their behavior or the
   option accordingly.

4.2  TCP Keep-Alives

   Some TCP implementations, such as the one in BSD systems, use a

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   different abort policy for TCP keep-alives than for user data.  Thus,
   the TCP keep-alive mechanism might abort a connection that would
   otherwise have survived the transient period of disconnection.
   Therefore, if a TCP peer enables TCP keep-alives for a connection
   that is using the TCP User Timeout Option, then the keep-alive timer
   MUST be set to a value larger than that of the adopted USER TIMEOUT.

5.  Security Considerations

   Lengthening user timeouts has obvious security implications.
   Flooding attacks cause denial of service by forcing servers to commit
   resources for maintaining the state of throw-away connections.  TCP
   implementations do not become more vulnerable to simple SYN flooding
   by implementing the TCP User Timeout Option, because user timeouts
   negotiated during the handshake only affect the synchronized states
   which simple SYN floods never reach.

   However, when an attacker completes the three-way handshakes of its
   throw-away connections it can amplify the effects of resource
   exhaustion attacks, because the attacked server must maintain the
   connection state associated with the throw-away connections for
   longer durations.  Because connection state is kept longer, lower-
   frequency attack traffic, which may be more difficult to detect, can
   already cause resource exhaustion.

   Several approaches can help mitigate this issue.  First,
   implementations can require prior peer authentication, e.g., using
   IPsec [I-D.ietf-ipsec-rfc2401bis], before accepting long user
   timeouts for the peer's connections.  Similarly, a host can only
   start to accept long user timeouts for an established connection
   after in-band authentication has occurred, for example, after a TLS
   handshake across the connection has succeeded [RFC2246].  Although
   these are arguably the most complete solutions, they depend on
   external mechanisms to establish a trust relationship.

   A second alternative that does not depend on external mechanisms
   would introduce a per-peer limit on the number of connections that
   may use increased user timeouts.  Several variants of this approach
   are possible, such as fixed limits or shortening accepted user
   timeouts with a rising number of connections.  Although this
   alternative does not eliminate resource exhaustion attacks from a
   single peer, it can limit their effects.  Reducing the number of
   high-UTO connections a server supports in the face of an attack turns
   that attack into a denial-of-service attack against the service of
   high-UTO connections.

   Per-peer limits cannot protect against distributed denial of service

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   attacks, where multiple clients coordinate a resource exhaustion
   attack that uses long user timeouts.  To protect against such
   attacks, TCP implementations could reduce the duration of accepted
   user timeouts with increasing resource utilization.

   TCP implementations under attack may be forced to shed load by
   resetting established connections.  Some load-shedding heuristics,
   such as resetting connections with long idle times first, can
   negatively affect service for intermittently connected, trusted peers
   that have suggested long user timeouts.  On the other hand, resetting
   connections to untrusted peers that use long user timeouts may be
   effective.  In general, using the peers' level of trust as a
   parameter during the load-shedding decision process may be useful.
   Note that if TCP needs to close or abort connections with a long TCP
   User Timeout Option to shed load, these connections are still no
   worse off than without the option.

   Finally, upper and lower limits on user timeouts, discussed in
   Section 3.3, can be an effective tool to limit the impact of these
   sorts of attacks.

6.  IANA Considerations

   This section is to be interpreted according to [RFC2434].

   This document does not define any new namespaces.  It uses an 8-bit
   TCP option number maintained by IANA at

7.  Acknowledgments

   The following people have improved this document through thoughtful
   suggestions: Mark Allmann, David Borman, Marcus Brunner, Wesley Eddy,
   Ted Faber, Guillermo Gont, Tom Henderson, Joseph Ishac, Jeremy
   Harris, Phil Karn, Michael Kerrisk, Dan Krejsa, Kostas Pentikousis,
   Juergen Quittek, Joe Touch, Stefan Schmid, Simon Schuetz and Martin

   Lars Eggert is partly funded by Ambient Networks, a research project
   supported by the European Commission under its Sixth Framework
   Program.  The views and conclusions contained herein are those of the
   authors and should not be interpreted as necessarily representing the
   official policies or endorsements, either expressed or implied, of
   the Ambient Networks project or the European Commission.

8.  References

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8.1  Normative References

   [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.

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

   [RFC2434]  Narten, T. and H. Alvestrand, "Guidelines for Writing an
              IANA Considerations Section in RFCs", BCP 26, RFC 2434,
              October 1998.

8.2  Informative References

              Ott, J. and D. Kutscher, "Drive-Thru Internet: IEEE
              802.11b for Automobile Users", Proc. Infocom , March 2004.

              Eddy, W., "Mobility Support For TCP",
              draft-eddy-tcp-mobility-00 (work in progress), April 2004.

              Moskowitz, R., "Host Identity Protocol Architecture",
              draft-ietf-hip-arch-02 (work in progress), January 2005.

              Kent, S. and K. Seo, "Security Architecture for the
              Internet Protocol", draft-ietf-ipsec-rfc2401bis-06 (work
              in progress), April 2005.

   [MEDINA]   Medina, A., Allman, M., and S. Floyd, "Measuring
              Interactions Between Transport Protocols and Middleboxes",
              Proc. 4th ACM SIGCOMM/USENIX Conference on Internet
              Measurement , October 2004.

   [RFC2246]  Dierks, T. and C. Allen, "The TLS Protocol Version 1.0",
              RFC 2246, January 1999.

   [RFC3344]  Perkins, C., "IP Mobility Support for IPv4", RFC 3344,
              August 2002.

              Schuetz, S., Eggert, L., Schmid, S., and M. Brunner,
              "Protocol Enhancements for Intermittently Connected

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              Hosts", To appear: ACM Computer Communication Review, Vol.
              35, No. 3, July 2005.

              Schuetz, S., "Network Support for Intermittently Connected
              Mobile Nodes", Diploma Thesis, University of Mannheim,
              Germany, June 2004.

              Sun Microsystems, "Solaris Tunable Parameters Reference
              Manual", Part No. 806-7009-10, 2002.

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

Editorial Comments

   [anchor4]  LE: A future version of this document may extend per-
              connection user timeouts to the SYN-SENT and SYN-RECEIVED
              states in a way that conforms to the required minimum

   [anchor5]  LE: Should it really always send UTO when it changes the
              local timeout? I can imagine some ping-pong effect when
              two hosts user different UTO adoption strategies. But
              maybe that's OK?

Authors' Addresses

   Lars Eggert
   NEC Network Laboratories
   Kurfuerstenanlage 36
   Heidelberg  69115

   Phone: +49 6221 90511 43
   Fax:   +49 6221 90511 55

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   Fernando Gont
   Universidad Tecnologica Nacional
   Evaristo Carriego 2644
   Haedo, Provincia de Buenos Aires  1706

   Phone: +54 11 4650 8472

Appendix A.  Document Revision History

   To be removed upon publication

   | Revision  | Comments                                              |
   | 00        | Resubmission of                                       |
   |           | draft-eggert-gont-tcpm-tcp-uto-option-01.txt to the   |
   |           | secretariat after WG adoption. Thus, permit           |
   |           | derivative works. Updated Lars Eggert's funding       |
   |           | attribution. Updated several references. No technical |
   |           | changes.                                              |
   | 01        | Clarified and corrected the description of the        |
   |           | existing user timeout in RFC793 and RFC1122. Removed  |
   |           | distinction between operating during the 3WHS and the |
   |           | established states and introduced zero-second "don't  |
   |           | care" UTOs in response to mailing list feedback.      |
   |           | Updated references and addressed many other comments  |
   |           | from the mailing list.                                |

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