TCP Maintenance and Minor                                      L. Eggert
Extensions (tcpm)                                                    NEC
Internet-Draft                                                   F. Gont
Expires: April 22, 2005                                          UTN/FRH
                                                        October 22, 2004

                        TCP User Timeout Option

Status of this Memo

   This document is an Internet-Draft and is subject to all provisions
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Copyright Notice

   Copyright (C) The Internet Society (2004).


   The TCP user timeout controls how long transmitted data may remain
   unacknowledged before a connection is aborted.  TCP implementations
   typically use a single, system-wide user timeout value.  The TCP User
   Timeout Option allows conforming TCP implementations to exchange

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   requests for individual, per-connection user timeouts.  Lengthening
   the system-wide default user timeout allows established TCP
   connections to survive extended periods of disconnection.  On the
   other hand, shortening the default user timeout allows busy servers
   to explicitly notify their clients they will maintain the connection
   state information only accross short periods of disconnection.

1.  Introduction

   The Transmission Control Protocol (TCP) specification [1] defines a
   "user timeout" parameter that specifies the maximum amount of time
   that transmitted data may remain unacknowledged before TCP will abort
   the corresponding connection.  If a disconnection lasts longer than
   the user timeout, no acknowledgments will be received for any
   transmission attempt, including keep-alives [5], and the TCP
   connection will be aborted when the user timeout occurs.

   The TCP specification [1] does not constrain the permitted values for
   user timeouts.  However, the Host Requirements RFC [2] mandates a
   timeout of at least three minutes for the SYN-SENT case.  Many TCP
   implementations default to user timeout values of a few minutes [5].
   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 SYN-SENT,

   System-wide user timeouts are a useful basic policy.  However, the
   ability to selectively choose individual user timeout values for
   different connections 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 [7], HIP [8] or
   transport-layer mobility mechanisms [9], are only intermittently
   connected to the Internet.  In between connected periods, mobile
   hosts may experience periods of disconnection during which no network
   service is available [10][11][12].  Other factors that can cause
   transient periods of disconnection are high levels of congestion as
   well as link or routing failures inside the network.

   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.

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   In the examples above, as well as in other cases, established TCP
   connections between two peers may be aborted if a disconnection
   exceeds the system-wide default user timeout.  This document
   specifies a new TCP option - the User Timeout Option - that allows
   conforming hosts to exchange per-connection user timeout requests.
   This allows, for example, mobile hosts to maintain TCP connections
   across disconnected periods that are longer than their system'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 disconnection.

   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 host must maintain, because a longer global timeout
   value will apply to all its connections.  The proposed User Timeout
   Option, on the other hand, allows hosts to selectively manage the
   user timeouts of individual connections.  They must then only
   maintain the state associated with selected connections across
   disconnected periods.

   A second benefit of the TCP User Timeout Option is that it allows
   hosts to both request specific user timeouts for new connections and
   to request changes to the effective user timeouts of established
   connections.  The latter allows connections to start with short
   timeouts and only request longer timeouts when disconnection is
   imminent, and only for connections considered important.  The ability
   to request changes to user timeouts of established connections is
   also useful to raise the user timeout after in-band authentication
   has occurred.  For example, peers could request longer user timeouts
   for the TCP connections underlying two-way authenticated TLS
   connections [13] after their authentication handshakes have

2.  Conventions

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   document are to be interpreted as described in RFC 2119 [3].

3.  Operation

   Sending a TCP User Timeout Option suggests to the remote peer to use
   the indicated user timeout value for the corresponding connection.
   Section 3.4 discusses the effects of different timeout values.

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

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

   It is important to note that the TCP User Timeout Option does not
   change the semantics of the TCP protocol.  Hosts remain free to abort
   connections at any time for any reason, whether or not they use
   custom user timeouts or have suggested the peer to use them.

   Hosts SHOULD impose upper and lower limits on the user timeouts they
   use.  Section 3.4 discusses user timeout limits.  A TCP User Timeout
   Option with a value of zero (i.e., "now") is nonsensical and MUST NOT
   be sent.  If received, it MUST be ignored.  Section 3.4 discusses
   potentially problematic effects of other user timeout durations.

   A TCP implementation that does not support the TCP User Timeout
   Option SHOULD silently ignore it [2], thus ensuring interoperability.

3.1  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 [1] to be assigned by IANA upon publication of
      this document (see Section 6.)

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   Length (8 bits)
      Length of the TCP option in octets [1]; its value MUST be 4.

   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.

3.2  Operation During the SYN Handshake

   A host that supports the TCP User Timeout Option MUST include an
   appropriate TCP User Timeout Option in its initial SYN segment to
   indicate that it supports the option and to suggest an initial user
   timeout for the connection.  [Comment.2]

   A host that supports the TCP User Timeout Option and receives a SYN
   segment that includes one MUST respond with an appropriate TCP User
   Timeout Option in its SYN-ACK segment.  If an incoming SYN segment
   does not include a TCP User Timeout Option, a host MUST NOT include
   one in the SYN-ACK segment nor in any other segment, and it MUST
   ignore the contents of any other received TCP User Timeout Option.

3.3  Operation During the Synchronized States

   Unless both the SYN and SYN-ACK of a connection contained TCP User
   Timeout Options, both hosts participating in the connection MUST NOT
   send TCP User Timeout Options in any other segment.  Additionally,
   they both MUST ignore the contents of any received TCP User Timeout

   If, however, both the SYN and SYN-ACK contained TCP User Timeout
   Options, hosts MAY choose to include additional TCP User Timeout
   Options in segments sent during the synchronized states (ESTABLISHED,

   Dynamically adapting the user timeout of a connection during its
   lifetime could be useful in a number of scenarios, for example:

   o  TCP may adapt the user timeout based on observed network
      characteristics.  [Comment.3]

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   o  TCP may use short timeouts when connections start and only suggest
      longer timeouts when disconnection was imminent.

   o  TCP may use short user timeouts when connections start and only
      raise them once in-band authentication has occurred, for example,
      once a TLS handshake across the connection has succeeded [13].

   Generally, whenever a host decides to change the local user timeout
   of a connection, it SHOULD include a TCP User Timeout Option
   indicating the new user timeout in its next segment to the peer.
   This allows the peer to adapt its local user timeout for the
   connection accordingly.

   TCP's SYN handshake has specific retransmission rules to guarantee
   reliability.  These mechanisms also guarantee that the exchange of
   TCP User Timeout Options during the SYN handshake is reliable.  This
   is not the case for TCP User Timeout Option exchanges during the
   synchronized states.  When a segment carrying a TCP User Timeout
   Option is lost, the peer will not update its local user timeout
   accordingly.  This draft does not currently describe mechanisms to
   ensure the reliability of the option exchange in the synchronized
   states, other than noting that periodic inclusion of the option may
   be an appropriate interim mechanism for implementations concerned
   with reliability.

3.4  Duration of the User Timeout

   The TCP User Timeout Option allows hosts to exchange user timeout
   values from zero seconds to over 9 hours at a granularity of seconds
   and from zero minutes to over 22 days at a granularity of minutes.

   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 aborts.  Many TCP implementations default
   to user timeout values of a few minutes [5].  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

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   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:

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

      Current upper limit imposed on the connection's user timeout by
      the local host.

      Current lower limit imposed on the connection's user timeout 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 transient 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, abort the connection whenever it wants.

   Enforcing a lower limit (L_LIMIT) protects against connection aborts
   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.

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   The Host Requirements RFC [2] 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 (LLIMIT)
   SHOULD be set to at least 100 seconds when following the RECOMMENDED
   scheme described in this section.

4.  Interoperability Issues

   This section discusses interoperability issues related to introducing
   the UTO option.

   One meta-issue of introducing new TCP options is that header space
   available for TCP options is currently limited to 40 bytes.  All
   negotiable options are exchanged during the SYN/SYN-ACK handshake,
   where option space is becoming limited.  Current proposals to extend
   the available option space may mitigate this issue [14].

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 the UTO 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 [15].

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

4.2  TCP Keep-Alives

   Some TCP implementations, such as the one in BSD systems, use a
   different abort policy for TCP keepalives 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 UTO Option, then the keep-alive timer MUST be set
   to a value larger than that of the adopted USER TIMEOUT (specified by
   Equation 1).

5.  Security Considerations

   Lengthening user timeouts has obvious security implications.

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   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.  [Comment.5]

   Several approaches can help mitigate this issue.  First,
   implementations can require prior peer authentication, e.g., using
   IPsec [16], 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 [13].  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.

   Per-peer limits cannot protect against distributed denial of service
   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.

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   Finally, upper and lower limits on user timeouts, discussed in
   Section 3.4, 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 [4].

   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, Phil Karn,
   Michael Kerrisk, Kostas Pentikousis, Juergen Quittek, Joe Touch,
   Stefan Schmid, Simon Schuetz and Martin Stiemerling.

   Part of this work is a byproduct of the Ambient Networks project,
   partially supported by the European Commission under its Sixth
   Framework Programme.  It is provided "as is" and without any express
   or implied warranties, including, without limitation, the implied
   warranties of fitness for a particular purpose.  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

8.1  Normative References

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

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

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

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

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8.2  Informative References

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

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

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

   [8]   Moskowitz, R., "Host Identity Protocol Architecture",
         draft-moskowitz-hip-arch-06 (work in progress), June 2004.

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

   [10]  Schuetz, S., "Network Support for Intermittently Connected
         Mobile Nodes", M.S. Thesis, University of Mannheim, Germany,
         June 2004.

   [11]  Schuetz, S., Eggert, L., Schmid, S. and M. Brunner, "Protocol
         Enhancements for Intermittently Connected Hosts", under
         submission (work in progress), July 2004.

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

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

   [14]  Eddy, W., "Extending the Space Available for TCP Options",
         draft-eddy-tcp-loo-01 (work in progress), September 2004.

   [15]  Medina, A., Allman, M. and S. Floyd, "Measuring Interactions
         Between Transport Protocols and Middleboxes", To appear: Proc.
         ACM SIGCOMM/USENIX Internet Measurement Conference , October

   [16]  Kent, S. and R. Atkinson, "Security Architecture for the
         Internet Protocol", RFC 2401, November 1998.

Editorial Comments

   [Comment.1]  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 timeouts.

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   [Comment.2]  LE: My original proposal was to allow hosts to choose
                whether or not to include the option. It's open for
                discussion whether this flexibility is worth the
                additional complexity. This is the corresponding text:
                "A host that supports the TCP User Timeout Option MAY
                omit the TCP User Timeout Option from the initial SYN if
                it will not permit custom user timeouts for the specific
                connection. It SHOULD omit the TCP User Timeout Option
                from the initial SYN if there is evidence that the peer
                does not support the TCP User Timeout Option, for
                example, if a prior connection attempt including a TCP
                User Timeout Option has failed. If a host does not
                include a TCP User Timeout Option in its initial SYN, it
                MUST NOT include it in any other segment either and MUST
                ignore the contents of any received TCP User Timeout

   [Comment.3]  FG: My original proposal suggested that TCP might adapt
                the user timeout when signalled of congestion by means
                of ECN.

   [Comment.4]  LE: This formula takes the maximum of the two announced
                values. I'd use USER_TIMEOUT = max(L_LIMIT,
                min(LOCAL_UTO, REMOTE_UTO, U_LIMIT)), instead. This
                version takes the minimum. My rationale is that the
                party announcing the lower value probably had a reason
                for it and may hence not be prepared to handle a longer
                value that it originally indicated.

   [Comment.5]  FG: IMO, in practice the TCP User Timeout option does
                not make the situation worse: the same type of attack
                can be performed even if the default "USER TIMEOUT" is
                used, since TCP requires no message exchange in order to
                keep a connection open.

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

   | Revision  | Comments                                              |
   | 00        | Initial version.                                      |
   | 01        | Merged the ATO and AUTO drafts.                       |

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