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Versions: 00 01 02 03 04 draft-ietf-tcpm-tcp-timestamps

TCP Maintenance and Minor                                        F. Gont
Extensions (tcpm)                                                UK CPNI
Internet-Draft                                            March 30, 2010
Intended status: BCP
Expires: October 1, 2010


           Reducing the TIME-WAIT state using TCP timestamps
                 draft-gont-tcpm-tcp-timestamps-04.txt

Abstract

   This document describes an algorithm for processing incoming SYN
   segments that allows higher connection-establishment rates between
   any two TCP endpoints when a TCP timestamps option is present in the
   incoming SYN segment.

Status of this Memo

   This Internet-Draft is submitted to IETF in full conformance with the
   provisions of BCP 78 and BCP 79.  This document may not be modified,
   and derivative works of it may not be created, and it may not be
   published except as an Internet-Draft.

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   This Internet-Draft will expire on October 1, 2010.

Copyright Notice

   Copyright (c) 2010 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

   This document is subject to BCP 78 and the IETF Trust's Legal



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   Provisions Relating to IETF Documents
   (http://trustee.ietf.org/license-info) in effect on the date of
   publication of this document.  Please review these documents
   carefully, as they describe your rights and restrictions with respect
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   include Simplified BSD License text as described in Section 4.e of
   the Trust Legal Provisions and are provided without warranty as
   described in the BSD License.


Table of Contents

   1.  Introduction . . . . . . . . . . . . . . . . . . . . . . . . .  3
   2.  Improved processing of incoming connection requests  . . . . .  3
   3.  Interaction with various timestamps generation algorithms  . .  6
   4.  Corner-cases . . . . . . . . . . . . . . . . . . . . . . . . .  7
     4.1.  Connection request after system reboot . . . . . . . . . .  7
   5.  Security Considerations  . . . . . . . . . . . . . . . . . . .  8
   6.  IANA Considerations  . . . . . . . . . . . . . . . . . . . . .  8
   7.  Acknowledgements . . . . . . . . . . . . . . . . . . . . . . .  8
   8.  References . . . . . . . . . . . . . . . . . . . . . . . . . .  8
     8.1.  Normative References . . . . . . . . . . . . . . . . . . .  8
     8.2.  Informative References . . . . . . . . . . . . . . . . . .  9
   Appendix A.  Changes from previous versions of the draft (to
                be removed by the RFC Editor before publishing
                this document as an RFC)  . . . . . . . . . . . . . .  9
     A.1.  Changes from draft-gont-tcpm-tcp-timestamps-03 . . . . . .  9
     A.2.  Changes from draft-gont-tcpm-tcp-timestamps-02 . . . . . .  9
     A.3.  Changes from draft-gont-tcpm-tcp-timestamps-01 . . . . . .  9
     A.4.  Changes from draft-gont-tcpm-tcp-timestamps-00 . . . . . . 10
   Author's Address . . . . . . . . . . . . . . . . . . . . . . . . . 10




















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1.  Introduction

   The Timestamps option, specified in RFC 1323 [RFC1323], allows a TCP
   to include a timestamp value in its segments, that can be used used
   to perform two functions: Round-Trip Time Measurement (RTTM), and
   Protection Against Wrapped Sequences (PAWS).

   For the purpose of PAWS, the timestamps sent on a connection are
   required to be monotonically increasing.  While there is no
   requirement that timestamps are monotonically increasing across TCP
   connections, the generation of timestamps such that they are
   monotonically increasing across connections between the same two
   endpoints allows the use of timestamps for improving the handling of
   SYN segments that are received while the corresponding four-tuple is
   in the TIME-WAIT state.  That is, the timestamp option could be used
   to perform heuristics to determine whether to allow the creation of a
   new incarnation of a connection that is in the TIME-WAIT state.

   This use of TCP timestamps is simply an extrapolation of the use of
   Initial Sequence Numbers (ISNs) for the same purpose, as allowed by
   RFC 1122 [RFC1122], and it has been incorporated in a number of TCP
   implementations, such as that included in the Linux kernel [Linux].

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
   document are to be interpreted as described in RFC 2119 [RFC2119].


2.  Improved processing of incoming connection requests

   In a number of scenarios a socket pair may need to be reused while
   the corresponding four-tuple is still in the TIME-WAIT state in a
   remote TCP peer.  For example, a client accessing some service on a
   host may try to create a new incarnation of a previous connection,
   while the corresponding four-tuple is still in the TIME-WAIT state at
   the remote TCP peer (the server).  This may happen if the ephemeral
   port numbers are being reused too quickly, either because of a bad
   policy of selection of ephemeral ports, or simply because of a high
   connection rate to the corresponding service.  In such scenarios, the
   establishment of new connections that reuse a four-tuple that is in
   the TIME-WAIT state would fail.

   In order to avoid this problem, RFC 1122 [RFC1122] (in Section
   4.2.2.13) states that when a connection request is received with a
   four-tuple that is in the TIME-WAIT state, the connection request
   could be accepted if the sequence number of the incoming SYN segment
   is greater than the last sequence number seen on the previous
   incarnation of the connection (for that direction of the data



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   transfer).  This requirement aims at avoiding the sequence number
   space of the new and old incarnations of the connection to overlap,
   thus avoiding old segments from the previous incarnation of the
   connection to be accepted as valid by the new connection.

   The same policy may be extrapolated to TCP timestamps.  That is, when
   a connection request is received with a four-tuple that is in the
   TIME-WAIT state, the connection request could be accepted if the
   timestamp of the incoming SYN segment is greater than the last
   timestamp seen on the previous incarnation of the connection (for
   that direction of the data transfer).

   The following paragraphs summarize the processing of SYN segments
   received for connections in the TIME-WAIT state.  Both the ISN
   (Initial Sequence Number) and the timestamp option (if present) of
   the incoming SYN segment are included in the heuristics performed for
   allowing a high connection-establishment rate.

   Processing of SYN segments received for connections in the
   synchronized states should occur as follows:

   o  If a SYN segment is received for a connection in any synchronized
      state other than TIME-WAIT, respond with an ACK, applying rate-
      throttling.

   o  If the corresponding connection is in the TIME-WAIT state, then,

      *  If the previous incarnation of the connection used timestamps,
         then,

         +  If TCP timestamps would be enabled for the new incarnation
            of the connection, and the timestamp contained in the
            incoming SYN segment is greater than the last timestamp seen
            on the previous incarnation of the connection (for that
            direction of the data transfer), honour the connection
            request (creating a connection in the SYN-RECEIVED state).

         +  If TCP timestamps would be enabled for the new incarnation
            of the connection, the timestamp contained in the incoming
            SYN segment is equal to the last timestamp seen on the
            previous incarnation of the connection (for that direction
            of the data transfer), and the Sequence Number of the
            incoming SYN segment is larger than the last sequence number
            seen on the previous incarnation of the connection (for that
            direction of the data transfer), then honour the connection
            request (creating a connection in the SYN-RECEIVED state).





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         +  If TCP timestamps would not be enabled for the new
            incarnation of the connection, but the Sequence Number of
            the incoming SYN segment is larger than the last sequence
            number seen on the previous incarnation of the connection
            (for the same direction of the data transfer), honour the
            connection request (creating a connection in the SYN-
            RECEIVED state).

         +  Otherwise, silently drop the incoming SYN segment, thus
            leaving the previous incarnation of the connection in the
            TIME-WAIT state.

      *  If the previous incarnation of the connection did not use
         timestamps, then,

         +  If TCP timestamps would be enabled for the new incarnation
            of the connection, honour the incoming connection request.

         +  If TCP timestamps would not be enabled for the new
            incarnation of the connection, but the Sequence Number of
            the incoming SYN segment is larger than the last sequence
            number seen on the previous incarnation of the connection
            (for the same direction of the data transfer), then honour
            the incoming connection request (even if the sequence number
            of the incoming SYN segment falls within the receive window
            of the previous incarnation of the connection).

         +  Otherwise, silently drop the incoming SYN segment, thus
            leaving the previous incarnation of the connection in the
            TIME-WAIT state.

   Note:

      In the above explanation, the phrase "TCP timestamps would be
      enabled for the new incarnation for the connection" means that the
      incoming SYN segment contains a TCP Timestamps option (i.e., the
      client has enabled TCP timestamps), and that the SYN/ACK segment
      that would be sent in response to it would also contain a
      Timestamps option (i.e., the server has enabled TCP timestamps).
      In such a scenario, TCP timestamps would be enabled for the new
      incarnation of the connection.

      The "last sequence number seen on the previous incarnation of the
      connection (for the same direction of the data transfer)" refers
      to the last sequence number used by the previous incarnation of
      the connection (for the same direction of the data transfer), and
      not to the last value seen in the Sequence Number field of the
      corresponding segments.  That is, it refers to the sequence number



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      corresponding to the FIN flag of the previous incarnation of the
      connection, for that direction of the data transfer.

   Many implementations do not include the TCP timestamp option when
   performing the above heuristics, thus imposing stricter constraints
   on the generation of Initial Sequence Numbers, the average data
   transfer rate of the connections, and the amount of data transferred
   with them.  RFC 793 [RFC0793] states that the ISN generator should be
   incremented roughly once every four microseconds (i.e., roughly
   250000 times per second).  As a result, any connection that transfers
   more than 250000 bytes of data at more than 250 KB/s could lead to
   scenarios in which the last sequence number seen on a connection that
   moves into the TIME-WAIT state is still greater than the sequence
   number of an incoming SYN segment that aims at creating a new
   incarnation of the same connection.  In those scenarios, the 4.4BSD
   heuristics would fail, and therefore the connection request would
   usually time out.  By including the TCP timestamp option in the
   heuristics described above, all these constraints are greatly
   relaxed.

   It is clear that the use of TCP timestamps for the heuristics
   described above benefit from timestamps that are monotonically
   increasing across connections between the same two TCP endpoints.


3.  Interaction with various timestamps generation algorithms

   The algorithm proposed in Section 2 clearly benefits of timestamps
   that are monotonically-increasing across connections to the same end-
   point.  In particular, generation of timestamps such that they are
   monotonically-increasing timestamps are important for TCPs that
   perform the active open, as those are the timestamps that will be
   used for the proposed algorithm.

   While monotonically-increasing timestamps ensure that the proposed
   algorithm will be able to reduce the TIME-WAIT state of a previous
   incarnation of a connection, implementation of the algorithm does not
   imply by itself a requirement on the timestamps generation algorithm
   of other TCPs.

   In the worst-case scenario, an incoming SYN corresponding to a new
   incarnation of a connection in the TIME-WAIT contains a timestamp
   that is smaller than the last timestamp seen on the previous
   incarnation of the connection, the heuristics fail, and the result is
   no worse than the current state-of-affairs.  That is,






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   o  The TIME_WAIT state is assassinated, with the connection request
      being rejected (as specified in [RFC0793]), or,

   o  The SYN segment is ignored (as specified in [RFC1337]), and thus
      the connection request times out, or is accepted after future
      retransmissions of the SYN

   Some stacks may implement timestamps generation algorithms that do
   not lead to monotonically-increasing timestamps across connections
   with the same remote endpoint.  An example of such algorithms is the
   one described in [RFC4987] and [Opperman], that allows the
   implementation of extended TCP SYN cookies.

   Note:
      It should be noted that this algorithm could co-exist with an
      algorithm for generating timestamps such that they are
      monotonically-increasing.  Monotonically increasing timestamps
      could be generated for TCPs that perform the active open, while
      timestamps for TCPs that perform the passive open could be
      generated according to [Opperman].


4.  Corner-cases

4.1.  Connection request after system reboot

   The question was raised on the tcpm mailing-list as to how this
   algorithm would operate in case a computer reboots, keeps the same IP
   address, looses memory of the previous time stamps, and then tries to
   reestablish a previous connection.

   Firstly, as specified in [RFC0793], hosts must not establish new
   connections for a period of 2*MSL after they boot (this is the "quiet
   time" concept).  As a result, specs-wise, this scenario should never
   occur.

   If a host does not comply with the "quiet time concept", then the
   possible scenarios are:

   o  If the selected timestamp for the new connection is monotonically-
      increasing with respect to the last timestamp seen on the previous
      incarnation of the connection, the TIME-WAIT state is tossed, and
      the new connection request succeeds.

   o  Otherwise, the connection request may time out or be rejected
      (depending on whether the workaround described in [RFC1337] is
      implemented or not).  This case corresponds to the current state-
      of-affairs without the algorithm proposed in this document.



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5.  Security Considerations

   While the algorithm described in this document for processing
   incoming SYN segments would benefit from TCP timestamps that are
   monotonically-increasing across connections, this document does not
   propose any specific algorithm for generating timestamps, nor does it
   require monotonically-increasing timestamps across conenctions.

   [CPNI-TCP] contains a detailed discussion of the security
   implications of TCP timestamps.


6.  IANA Considerations

   This document has no actions for IANA.


7.  Acknowledgements

   The author of this document would like to thank (in alphabetical
   order) Mark Allman, Christian huitema, Alfred Hoenes, Eric Rescorla,
   Joe Touch, and Alexander Zimmermann for providing valuable feedback
   on an earlier version of this document.

   Additionally, the author would like to thank David Borman for a
   fruitful discussion on TCP timestamps at IETF 73.

   Finally, the author would like to thank the United Kingdom's Centre
   for the Protection of National Infrastructure (UK CPNI) for their
   continued support.


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.

   [RFC1323]  Jacobson, V., Braden, B., and D. Borman, "TCP Extensions
              for High Performance", RFC 1323, May 1992.

   [RFC1337]  Braden, B., "TIME-WAIT Assassination Hazards in TCP",
              RFC 1337, May 1992.




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

8.2.  Informative References

   [CPNI-TCP]
              CPNI, "Security Assessment of the Transmission Control
              Protocol (TCP)",  http://www.cpni.gov.uk/Docs/
              tn-03-09-security-assessment-TCP.pdf, 2009.

   [Linux]    The Linux Project, "http://www.kernel.org".

   [Opperman]
              Oppermann, A., "FYI: Extended TCP syncookies in FreeBSD-
              current",  Post to the tcpm mailing-list. Available at: ht
              tp://www.ietf.org/mail-archive/web/tcpm/current/
              msg02251.html, 2006.

   [RFC4987]  Eddy, W., "TCP SYN Flooding Attacks and Common
              Mitigations", RFC 4987, August 2007.


Appendix A.  Changes from previous versions of the draft (to be removed
             by the RFC Editor before publishing this document as an
             RFC)

A.1.  Changes from draft-gont-tcpm-tcp-timestamps-03

   o  Changed the document title

   o  Removed all the text related to the algorithm earlier proposed for
      timestamps generation.

   o  Addresses comments received from Alexander Zimmermann, Christian
      Huitema, Joe Touch, and others.

A.2.  Changes from draft-gont-tcpm-tcp-timestamps-02

   o  Minor edits (the I-D was just about to expire, so it was
      resubmitted with almost no changes).

A.3.  Changes from draft-gont-tcpm-tcp-timestamps-01

   o  Version -01 of the draft had expired, and hence the I-D is
      resubmitted to make it available again (no changes).






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A.4.  Changes from draft-gont-tcpm-tcp-timestamps-00

   o  Fixed author's affiliation.

   o  Addressed feedback submitted by Alfred Hoenes (see:
      http://www.ietf.org/mail-archive/web/tcpm/current/msg04281.html),
      plus nits sent by Alfred off-list.


Author's Address

   Fernando Gont
   UK Centre for the Protection of National Infrastructure

   Email: fernando@gont.com.ar
   URI:   http://www.cpni.gov.uk



































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