TCP Maintenance and Minor                                      L. Eggert
Extensions (tcpm)                                                  Nokia
Internet-Draft                                                   F. Gont
Intended status: Standards Track                                 UTN/FRH
Expires: December 13, 2007                                 June 11, 2007


                        TCP User Timeout Option
                       draft-ietf-tcpm-tcp-uto-06

Status of this Memo

   By submitting this Internet-Draft, each author represents that any
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   This Internet-Draft will expire on December 13, 2007.

Copyright Notice

   Copyright (C) The IETF Trust (2007).

Abstract

   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.  This document specifies a new TCP
   option - the TCP User Timeout Option - that allows one end of a TCP
   connection to advertise its current user timeout value.  This
   information provides advice to the other end to adapt its user
   timeout accordingly.  Increasing the user timeouts on both ends of a



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   TCP connection allows it to survive extended periods without end-to-
   end connectivity.  Decreasing the user timeouts allows busy servers
   to explicitly notify their clients that they will maintain the
   connection state only for a short time without connectivity.


Table of Contents

   1.  Introduction . . . . . . . . . . . . . . . . . . . . . . . . .  3
   2.  Conventions  . . . . . . . . . . . . . . . . . . . . . . . . .  4
   3.  Operation  . . . . . . . . . . . . . . . . . . . . . . . . . .  4
     3.1.  Changing the Local User Timeout  . . . . . . . . . . . . .  6
     3.2.  UTO Option Reliability . . . . . . . . . . . . . . . . . .  8
     3.3.  Option Format  . . . . . . . . . . . . . . . . . . . . . .  8
     3.4.  Reserved Option Values . . . . . . . . . . . . . . . . . .  9
   4.  Interoperability Issues  . . . . . . . . . . . . . . . . . . . 10
     4.1.  Middleboxes  . . . . . . . . . . . . . . . . . . . . . . . 10
     4.2.  TCP Keep-Alives  . . . . . . . . . . . . . . . . . . . . . 10
   5.  Security Considerations  . . . . . . . . . . . . . . . . . . . 10
   6.  IANA Considerations  . . . . . . . . . . . . . . . . . . . . . 12
   7.  Acknowledgments  . . . . . . . . . . . . . . . . . . . . . . . 12
   8.  References . . . . . . . . . . . . . . . . . . . . . . . . . . 12
     8.1.  Normative References . . . . . . . . . . . . . . . . . . . 12
     8.2.  Informative References . . . . . . . . . . . . . . . . . . 13
   Appendix A.  Document Revision History . . . . . . . . . . . . . . 13
   Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 15
   Intellectual Property and Copyright Statements . . . . . . . . . . 16
























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.  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 an end-to-end connectivity disruption lasts
   longer than the user timeout, no acknowledgments will be received for
   any transmission attempt, including keep-alives, 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), that control the
   number of retransmission attempts for a single segment.  It suggests
   that TCP should notify applications when R1 is reached for a segment,
   and close the connection when 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
   SYN-SENT, SYN-RECEIVED, or ESTABLISHED state [SOLARIS-MANUAL].

   Although some TCP implementations allow applications to set their
   local user timeout, TCP has no in-protocol mechanism to signal
   changes to the local user timeout to the other end.  This causes
   local changes to be ineffective in allowing a connection to survive
   extended periods without connectivity, because the other end will
   still close the connection after its user timeout expires.

   The ability to inform the other end about the local user timeout for
   the connection 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 Mobile IP [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 without end-to-end
   connectivity.  Other factors that can cause transient connectivity
   disruptions are high levels of congestion or link or routing failures
   inside the network.  In these scenarios, a host may not know exactly
   when or for how long connectivity disruptions will occur, but it
   might be able to determine an increased likelihood for such events
   based on past mobility patterns and thus benefit from using longer
   user timeouts.  In other scenarios, the time and duration of a



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   connectivity disruption may even be predictable.  For example, an
   orbiting node on a non-geostationary satellite might experience
   connectivity disruptions due to line-of-sight blocking by other
   planetary bodies.  The timing of these events may be computable from
   orbital mechanics.

   This document specifies a new TCP option - the TCP User Timeout
   Option - that allows one end of a TCP connection to advertise its
   current user timeout value.  This information provides advice to the
   other end of the connection to adapt its user timeout accordingly.
   That is, TCP remains free to disregard the advice provided by the UTO
   option if local policies suggest it to be appropriate.

   Increasing the user timeouts on both ends of a TCP connection allows
   it to survive extended periods without end-to-end connectivity.
   Decreasing the user timeouts allows busy servers to explicitly notify
   their clients that they will maintain the connection state only for a
   short time without connectivity.


.  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 [RFC2119].


.  Operation

   Use of the TCP User Timeout Option can be enabled either on a per-
   application basis, e.g., through a socket option, or controlled by a
   system-wide setting.  TCP maintains four per-connection state
   variables to control the operation of the UTO option, three of which
   (LOCAL_UTO, ENABLED and CHANGEABLE) are new:

   USER_TIMEOUT
      TCP's USER TIMEOUT parameter, as specified in [RFC0793].

   LOCAL_UTO
      UTO option advertised to the remote TCP peer.  This is an
      application-specified value, and may be specified on a system-wide
      basis.  If unspecified, it default to the default system-wide USER
      TIMEOUT.

   ENABLED (Boolean)
      Flag that controls whether the UTO option is enabled for a
      connection.  Defaults to false.




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   CHANGEABLE (Boolean)
      Flag that controls whether USER_TIMEOUT (TCP's USER_TIMEOUT
      parameter) may be changed based on an UTO option received from the
      other end.  Defaults to true and becomes false when an application
      explicitly sets USER_TIMEOUT.

   Note that an exchange of UTO options between both ends of a
   connection is not a binding negotiation.  Transmission of a UTO
   option is a suggestion that the other end consider adapting its user
   timeout.  This adaptation only happens if the the other end has
   explicitly allowed it (both ENABLED and CHANGEABLE are true).

   Before opening a connection, an application that wishes to use the
   UTO option SHOULD enable its use by setting ENABLED to true.  It MAY
   pick an appropriate local UTO by setting LOCAL_UTO, which is
   otherwise set to the default USER TIMEOUT value.  Finally, the
   application should determine whether it will allow the local USER
   TIMEOUT to change based on received UTO options from the other end.
   The default is to allow this for connections that do not have a
   specific user timeout concerns.  If an application explicitly sets
   the USER TIMEOUT, CHANGEABLE MUST become false, to prevent UTO
   options from the other end to override local application requests.
   Alternatively, applications MAY set or clear CHANGEABLE directly.

   Performing these steps before an active or passive open causes UTO
   options to be exchanged in the SYN and SYN-ACK packets and is a
   reliable way to initially exchange and potentially adapt to UTO
   values.  Systems MAY provide system-wide default settings for the
   ENABLED, LOCAL_UTO and CHANGEABLE connection parameters.

   In addition to exchanging UTO options in the SYN segments, a
   connection that has enabled UTO options SHOULD include a UTO option
   in the first packet that does not have the SYN flag set.  This helps
   to minimize the amount of state information 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 UTO option SHOULD include one in the next
   possible outgoing segment whenever it starts using a new user timeout
   for the connection.  This allows the other end to adapt its local
   user timeout for the connection accordingly.  A TCP implementation
   that does not support the UTO option MUST silently ignore it
   [RFC1122], thus ensuring interoperability.

   Hosts MUST impose upper and lower limits on the user timeouts they



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   use for a connection.  Section 3.1 discusses user timeout limits and
   discusses potentially problematic effects of some user timeout
   settings.

   Finally, it is worth noting that TCP's option space is limited to 40
   bytes.  As a result, if other TCP options are in use, they may
   already consume all the available TCP option space, thus preventing
   the use of the UTO option specified in this document.  Therefore, TCP
   option space issues should be considered before enabling the UTO
   option.

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.  If the CHANGEABLE flag is false, USER_TIMEOUT MUST NOT
   be changed, regardless of the received UTO option.  Without this
   restriction, the UTO option would modify TCP semantics, because an
   application-requested USER TIMEOUT could be overridden by peer
   requests.  In this case, they SHOULD, however, notify the application
   about the user timeout value received from the other end.

   In general, unless the application on the local host has requested a
   specific USER TIMEOUT for the connection, CHANGEABLE will be true and
   hosts SHOULD adjust the local TCP USER TIMEOUT (USER_TIMEOUT) in
   response to receiving a UTO option, as described in the remainder of
   this section.

   The UTO option specifies the user timeout in seconds or minutes,
   rather than in number of retransmissions or round-trip times (RTTs).
   Thus, the UTO 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.

   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.  Although the UTO option
   allows suggestion of short timeouts, applications advertising them
   should consider these effects.

   Long USER TIMEOUT values allow hosts to tolerate extended periods
   without end-to-end connectivity.  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 values.




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   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 TCP's USER
   TIMEOUT based on upper and lower limits.

   Under the RECOMMENDED scheme, and when CHANGEABLE is true, each end
   SHOULD compute the local USER TIMEOUT for a connection according to
   this formula:

   USER_TIMEOUT = min(U_LIMIT, max(LOCAL_UTO, REMOTE_UTO, L_LIMIT))

   Each field is to be interpreted as follows:

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

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

   LOCAL_UTO
      User timeout advertised to the remote TCP peer in a TCP User
      Timeout Option.

   REMOTE_UTO
      Last "user timeout" value received from the other end in a TCP
      User Timeout Option.

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

   This means that, provided they are within the upper and lower limits,
   the maximum of the two advertised 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 without end-to-end connectivity.  If the end 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.



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   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, per-outgoing-
   interface 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.  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.  It is worth noting
   that an upper limit may be imposed on the RTO, provided it is at
   least 60 seconds [RFC2988].

3.2.  UTO Option Reliability

   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
   does not define a reliability handshake for UTO option exchanges.
   When a segment that carries a UTO option is lost, the other end will
   simply not have the opportunity to update its local UTO.

   Implementations MAY implement local mechanisms to improve delivery
   reliability, such as retransmitting a UTO option when they retransmit
   a 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 UTO option, they do not guarantee
   delivery (a three-way handshake would be required for this).
   Consequently, implementations MUST NOT assume that UTO options are
   transmitted reliably.

3.3.  Option Format

   Sending a TCP User Timeout Option informs the other end of the
   current local user timeout for the connection and suggests that the
   other end adapt its user timeout accordingly.  The user timeout value



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   included in a UTO option contains the LOCAL_UTO value, that is
   expected to be adopted for the TCP's USER TIMEOUT parameter 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 the default timeout values defined in [RFC0793] and
   [RFC1122].

      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
      seconds.

   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.  Reserved Option Values

   An TCP User Timeout Option with a "User Timeout" field of zero and a
   "Granularity" bit of either minutes (1) or seconds (0) is reserved
   for future use.  TCP implementations MUST NOT send it and MUST ignore
   it upon reception.






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.  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 the vast majority
   of modern TCP stacks correctly handle unknown TCP options [MEDINA].
   In this study, 3% of connections failed when an unknown TCP option
   appeared in the middle of a connection.  Because the number of
   failures caused by unknown options is small and they are a result of
   incorrectly implemented TCP stacks that violate existing requirements
   to ignore unknown options, they do not warrant special measures.
   Thus, this document does not define a mechanism to negotiate support
   of the TCP User Timeout Option during the three-way handshake.

   Stateful firewalls usually time out connection state after a period
   of inactivity.  If such a firewall exists along the path, 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 adapt connection state management
   accordingly.

4.2.  TCP Keep-Alives

   Some TCP implementations, such as those in BSD systems, use a
   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 without connectivity.
   Therefore, if a connection enables keep-alives that is also 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.


.  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.



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   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 exacerbate 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 [RFC4346].  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
   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



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   Section 3.1, can be an effective tool to limit the impact of these
   sorts of attacks.


.  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
   http://www.iana.org/assignments/tcp-parameters.


.  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,
   Alfred Hoenes, Phil Karn, Michael Kerrisk, Dan Krejsa, Jamshid
   Mahdavi, Kostas Pentikousis, Juergen Quittek, Anantha Ramaiah, Joe
   Touch, Stefan Schmid, Simon Schuetz, Tim Shepard and Martin
   Stiemerling.

   Lars Eggert has been partly funded by Ambient Networks, a research
   project supported by the European Commission under its Sixth
   Framework Program.


.  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.






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8.2.  Informative References

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

   [I-D.ietf-tcpm-syn-flood]
              Eddy, W., "TCP SYN Flooding Attacks and Common
              Mitigations", draft-ietf-tcpm-syn-flood-05 (work in
              progress), May 2007.

   [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.

   [RFC2988]  Paxson, V. and M. Allman, "Computing TCP's Retransmission
              Timer", RFC 2988, November 2000.

   [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.

   [RFC4346]  Dierks, T. and E. Rescorla, "The Transport Layer Security
              (TLS) Protocol Version 1.1", RFC 4346, April 2006.

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

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


Appendix A.  Document Revision History

   To be removed upon publication












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   +----------+--------------------------------------------------------+
   | Revision | Comments                                               |
   +----------+--------------------------------------------------------+
   | 06       | Includes a note on the limited space for TCP options   |
   |          | and miscelaneous editorial changes(suggested by        |
   |          | Anantha Ramaiah).  Includes possible enforcement of    |
   |          | per-outgoing-interface limits for the UTO, and         |
   |          | miscellaneous editorial changes (suggested by Alfred   |
   |          | Hoenes).  Includes relevant changes to reflect WG      |
   |          | consesus how the local user timeout should be selected |
   |          | (i.e., record both the current user timeout, and the   |
   |          | advertised UTO).                                       |
   | 05       | Made behavior on when to change/not change the local   |
   |          | UTO in response to incoming options consistent through |
   |          | the document.  This required some reshuffling of text  |
   |          | and also removed the need for the special "don't care" |
   |          | option value.                                          |
   | 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  |
   |          | socket options.  Removed text that suggested that a    |
   |          | UTO should be sent upon receipt of an UTO from the     |
   |          | other end.  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.                                 |






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   | 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.                                     |
   +----------+--------------------------------------------------------+


Authors' Addresses

   Lars Eggert
   Nokia Research Center
   P.O. Box 407
   Nokia Group  00045
   Finland

   Phone: +358 50 48 24461
   Email: lars.eggert@nokia.com
   URI:   http://research.nokia.com/people/lars_eggert/


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

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




















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