TCP Maintenance and Minor                                      L. Eggert
Extensions  (tcpm)                                                   NEC
Internet-Draft                                                   F. Gont
Intended status: Standards Track                                 UTN/FRH
Expires: April 25, 2007                                 October 22, 2006

                        TCP User Timeout Option

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Copyright Notice

   Copyright (C) The Internet Society (2006).


   This document specifies a new TCP option - the TCP User Timeout
   Option - that allows a TCP to advertise its current user timeout for
   a connection.  Thus, the remote TCP may modify its local user timeout
   based on knowledge of the peer's user timeout.  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.  Increasing the user timeouts allows established TCP

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

Table of Contents

   1.  Introduction . . . . . . . . . . . . . . . . . . . . . . . . .  3
   2.  Conventions  . . . . . . . . . . . . . . . . . . . . . . . . .  4
   3.  Operation  . . . . . . . . . . . . . . . . . . . . . . . . . .  4
     3.1.  Changing the Local User Timeout  . . . . . . . . . . . . .  6
     3.2.  Reliability Considerations . . . . . . . . . . . . . . . .  8
     3.3.  Option Format  . . . . . . . . . . . . . . . . . . . . . .  9
     3.4.  Special Option Values  . . . . . . . . . . . . . . . . . .  9
   4.  Interoperability Issues  . . . . . . . . . . . . . . . . . . . 10
     4.1.  Middleboxes  . . . . . . . . . . . . . . . . . . . . . . . 10
     4.2.  TCP Keep-Alives  . . . . . . . . . . . . . . . . . . . . . 10
   5.  Security Considerations  . . . . . . . . . . . . . . . . . . . 11
   6.  IANA Considerations  . . . . . . . . . . . . . . . . . . . . . 12
   7.  Acknowledgments  . . . . . . . . . . . . . . . . . . . . . . . 12
   8.  References . . . . . . . . . . . . . . . . . . . . . . . . . . 13
     8.1.  Normative References . . . . . . . . . . . . . . . . . . . 13
     8.2.  Informative References . . . . . . . . . . . . . . . . . . 13
   Editorial Comments . . . . . . . . . . . . . . . . . . . . . . . .
   Appendix A.  Alternative solutions . . . . . . . . . . . . . . . . 14
   Appendix B.  Document Revision History . . . . . . . . . . . . . . 14
   Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 15
   Intellectual Property and Copyright Statements . . . . . . . . . . 17

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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 should
   notify applications when R1 is reached for a segment, and close the
   connection once R2 is reached.  [RFC1122] also defines 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 some TCP implementations allow applications to 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 the peer will still close the
   connection after its user timeout expires, even when the host has
   raised its local user timeout.  The ability to suggest the remote
   peer a user timeout to be used for the connection can improve TCP's
   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 [RFC4423] 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
   is available.  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

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   scenarios, the length and time of a network disconnection may even be
   predictable.  For example, an orbiting node on a non-geostationary
   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 a TCP to advertise its current local user timeout
   parameter.  Thus, based on the information advertised by the remote
   TCP peer, a TCP may modify its own user timeout accordingly.  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 disconnection.

   Use of the TCP User Timeout Option could be triggered either by an
   API call or by a system-wide toggle.  The API could be, for example,
   a Socket option that would need to be explicitly set by the
   corresponding application.  This option would default to "off".  A
   system-wide toggle would allow a system administrator to enable the
   use of the TCP User Timeout Option on a system-wide basis, and set
   the option a desired value.  This system-wide toggle would allow the
   use of the option by application programs that have not been
   explicitly coded to do so.  If such a system-wide toggle were
   provided, it would default to "off".

   In all cases, use of the TCP User Timeout Option would depend on an
   active decision, either by the application programmer (by means of an
   API call), or by a system administrator (by means of a system-wide

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 informs the remote peer of the
   current local user timeout for the connection, and suggests the TCP
   peer to adapt its user timeout accordingly.  The user timeout value
   included in a TCP User Timeout Option specifies the requested user

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   timeout during the synchronized states of a connection (ESTABLISHED,
   Connections in other states MUST the default timeout values defined
   in [RFC0793] [RFC1122].

   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 adopt a different user timeout,
   or to forcefully close or abort connections at any time for any
   reason, whether or not they use custom user timeouts or have
   suggested the peer to use them.

   A host that supports the TCP User Timeout Option SHOULD include one
   in each packet that carries a SYN flag.  The presence of this option
   is not a negotiation of the capability, but simply an advisory
   message specifying the currently preferred user timeout value.  This
   allows TCP to adopt a user timeout with knowledge of that used by the
   peer TCP from the very beginning of the data transfer phase.
   Additionally, a TCP that supports the User Timeout Option and has
   sent a SYN segment as a result of an active OPEN SHOULD include an
   UTO in the first packet that does not have the SYN flag set.  This
   helps to minimize the amount of state information a TCP must keep for
   connections in non-synchronized states, and is particularly useful
   when mechanisms such as "SYN cookies" [I-D.ietf-tcpm-syn-flood] are
   implemented, allowing a newly-established TCP connection to benefit
   from the information advertised by the UTO option, even if the UTO
   contained in the initial SYN segment was not recorded.

   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 will use the received value to compute the local user timeout for
   the connection.  Generally, hosts should honor requests for changes
   to the user timeout (see Section 3.1), unless security concerns,
   resource constraints or external policies indicate otherwise (see
   Section 5).  If so, hosts may use a different user timeout for the

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

   Hosts MUST impose upper and lower limits on the user timeouts they
   use.  Section 3.1 discusses user timeout limits, and describes a

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   RECOMMENDED scheme to apply them.  A TCP User Timeout Option with a
   value of zero (i.e., "now") is nonsensical and is used for a special
   purpose, see Section 3.4.  Section 3.1 discusses potentially
   problematic effects of other user timeout durations.

3.1.  Changing the Local User Timeout

   When a host receives a TCP User Timeout Option, it must decide
   whether to change the local user timeout of the corresponding
   connection.  Application-requested user timeout values always take
   precedence over timeout values received from the peer in a TCP User
   Timeout Option. [anchor3] Consequently, unless the application on the
   local host has requested a specific user timeout for the connection,
   e.g., through the OPEN or SEND calls, hosts SHOULD adjust their local
   user timeout in response to receiving a TCP User Timeout Option, as
   described in the remainder of this section.  If the local application
   has requested a specific local user timeout, TCP implementations MUST
   NOT change it in response to receiving a TCP User Timeout Option.  In
   this case, they SHOULD, however, notify the application about the
   user timeout value received from the peer.

   The User Timeout Option specifies the user timeout in terms of time
   units, rather than in terms of number of retransmissions or round-
   trip times (RTTs), as in most cases the periods of disconnection have
   to do with operation and mobility patterns, rather than with the
   current network conditions.  Thus, 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 MUST 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 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 this specific connection.

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

   This means that, provided they are within the upper and lower limits,
   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.

   It must be noted that the two endpoints of the connection will not
   necessarily adopt the same user timeout.

   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 5discusses 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-

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   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.  Adopting a user timeout smaller
   than the current retransmission timeout (RTO) for the connection
   would likely cause the connection to be aborted unnecessarily.
   Therefore, the lower limit (L_LIMIT) MUST be larger than the current
   retransmission timeout (RTO) for the connection.

3.2.  Reliability Considerations

   The TCP User Timeout Option is an advisory TCP option that does not
   change processing of 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 should not assume that a TCP
   User Timeout Option is reliably transmitted.

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3.3.  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         |


   (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.

   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.

3.4.  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

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   "don't care" value.

   Thus, the sender SHOULD adapt its local user timeout according to the
   peer's UTO, and the receiver SHOULD continue using its local user
   timeout.  In order to achieve this, 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 3.1, with a
   numeric value of zero seconds for REMOTE_UTO.  The sender SHOULD
   perform the same calculation as described in Section 3.1, with a
   numeric value of zero seconds for 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 send 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

   A TCP implementation that does not support the TCP User Timeout
   Option MUST silently ignore it [RFC1122], thus ensuring
   interoperability.  In a study of the effects of middleboxes on
   transport protocols, Medina et al. have shown that unknown TCP
   options are correctly handled by the vast majority of modern TCP
   stacks [MEDINA].  In this study, 3% of connections failed when an
   unknown TCP option appeared in the middle of a connection.  Because
   these failures violate existing requirements to ignore unknown
   options, they do not warrant taking special measures to handle these
   cases.  In particular, we do not define a separate mechanism to
   negotiate support of the TCP User Timeout Option on the three-way

   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
   different abort policy for TCP keep-alives than for user data.  Thus,

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   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.
   However, TCP implementations do not become more vulnerable to simple
   SYN flooding by implementing the TCP User Timeout Option, because
   user timeouts exchanged during the handshake only affect the
   synchronized states (ESTABLISHED, FIN-WAIT-1, FIN-WAIT-2, CLOSE-WAIT,
   CLOSING, LAST-ACK), 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 [RFC4301], before accepting long user timeouts for the peer's
   connections.  Similarly, a host can start to accept long user
   timeouts for an established connection only 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 3.1, 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 Allman, Caitlin Bestler, David Borman, Bob Braden,
   Marcus Brunner, Wesley Eddy, Gorry Fairhurst, Abolade Gbadegesin, Ted
   Faber, Guillermo Gont, Tom Henderson, Joseph Ishac, Jeremy Harris,
   Phil Karn, Michael Kerrisk, Dan Krejsa, Jamshid Mahdavi, Kostas
   Pentikousis, Juergen Quittek, Joe Touch, Stefan Schmid, Simon
   Schuetz, Tim Shepard and Martin Stiemerling.

   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.

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8.  References

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

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

              Eddy, W., "TCP SYN Flooding Attacks and Common
              Mitigations", draft-ietf-tcpm-syn-flood-00 (work in
              progress), July 2006.

   [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.

   [RFC4301]  Kent, S. and K. Seo, "Security Architecture for the
              Internet Protocol", RFC 4301, December 2005.

   [RFC4423]  Moskowitz, R. and P. Nikander, "Host Identity Protocol
              (HIP) Architecture", RFC 4423, May 2006.

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

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

Editorial Comments

   [anchor3]  Without this, UTO would modify TCP semantics, because
              application-requested UTOs could be overridden by peer

Appendix A.  Alternative solutions

   The same benefits could be obtained through an application-layer
   mechanism, i.e., exchanging user timeout information via application
   messages and having the application adjust the user timeouts through
   the TCP API on both sides of a connection.  This approach would not
   require a new TCP option, but would require changing all application
   implementations that desire to tolerate extended periods of
   disconnection, and in most cases would also require a modification to
   the corresponding application layer protocol.  With the proposed TCP
   option, application changes may not be necessary at all, or may be
   restricted to sender- or receiver-side only, and there is no need to
   modify the corresponding application protocol.

   A different approach to tolerate longer periods of disconnection
   would be to simply increase the system-wide user timeout on both
   peers.  This approach has the benefit of not requiring a new TCP
   option or application changes.  However, it can also significantly
   increase the amount of connection state a busy server must maintain,
   because a longer global timeout value would apply to all its

   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

Appendix B.  Document Revision History

   To be removed upon publication

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   | Revision | Comments                                               |
   | 04       | Clarified the results obtained by Medina et al.  Added |
   |          | text to suggest inclusion of the UTO in the first      |
   |          | non-SYN segment by the TCP that sent a SYN in response |
   |          | to an active OPEN.                                     |
   | 03       | Corrected use of RFC2119 terminology.  Clarified how   |
   |          | use of the TCP UTO is triggered.  Clarified reason for |
   |          | sending a UTO in the SYN and SYN/ACK segments.         |
   |          | Removed discussion of the SO_SNDTIMEO and SO_RCVTIMEO  |
   |          | options.  Removed text that suggested that a UTO       |
   |          | should be sent upon receipt of an UTO from the remote  |
   |          | peer.  Required minimum value for the lower limit of   |
   |          | the user timeout.  Moved alternative solutions to      |
   |          | appendix.  Miscellaneous editorial changes.            |
   | 02       | Corrected terminology by replacing terms like          |
   |          | "negotiate", "coordinate", etc. that were left from    |
   |          | pre-WG-document times when the UTO was a more          |
   |          | formalized exchange instead of the advisory one it is  |
   |          | now.  Application-requested UTOs take precedence over  |
   |          | ones received from the peer (pointed out by Ted        |
   |          | Faber).  Added a brief mention of SO_SNDTIMEO and a    |
   |          | slightly longer discussion of SO_RCVTIMEO.             |
   | 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.                                 |
   | 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.                                     |

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Authors' Addresses

   Lars Eggert
   NEC Network  Laboratories
   Kurfuerstenanlage 36
   Heidelberg  69115

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

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

   Phone: +54 11 4650 8472

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