[Docs] [txt|pdf] [Tracker] [WG] [Email] [Diff1] [Diff2] [Nits]

Versions: (draft-gont-tcpm-tcp-timestamps) 00 01 02 03 04 RFC 6191

TCP Maintenance and Minor                                        F. Gont
Extensions (tcpm)                                                UK CPNI
Internet-Draft                                          February 4, 2011
Intended status: BCP
Expires: August 8, 2011


           Reducing the TIME-WAIT state using TCP timestamps
                 draft-ietf-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.  This document only modifies processing of SYN
   segments received for connections in the TIME-WAIT state; processing
   in all other states is unchanged.

Status of this Memo

   This Internet-Draft is submitted in full conformance with the
   provisions of BCP 78 and BCP 79.

   Internet-Drafts are working documents of the Internet Engineering
   Task Force (IETF).  Note that other groups may also distribute
   working documents as Internet-Drafts.  The list of current Internet-
   Drafts is at http://datatracker.ietf.org/drafts/current/.

   Internet-Drafts are draft documents valid for a maximum of six months
   and may be updated, replaced, or obsoleted by other documents at any
   time.  It is inappropriate to use Internet-Drafts as reference
   material or to cite them other than as "work in progress."

   This Internet-Draft will expire on August 8, 2011.

Copyright Notice

   Copyright (c) 2011 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
   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
   to this document.  Code Components extracted from this document must
   include Simplified BSD License text as described in Section 4.e of



Gont                     Expires August 8, 2011                 [Page 1]

Internet-Draft  Reducing TIME-WAIT state with timestamps   February 2011


   the Trust Legal Provisions and are provided without warranty as
   described in the Simplified BSD License.

   This document may contain material from IETF Documents or IETF
   Contributions published or made publicly available before November
   10, 2008.  The person(s) controlling the copyright in some of this
   material may not have granted the IETF Trust the right to allow
   modifications of such material outside the IETF Standards Process.
   Without obtaining an adequate license from the person(s) controlling
   the copyright in such materials, this document may not be modified
   outside the IETF Standards Process, and derivative works of it may
   not be created outside the IETF Standards Process, except to format
   it for publication as an RFC or to translate it into languages other
   than English.


Table of Contents

   1.  Introduction . . . . . . . . . . . . . . . . . . . . . . . . .  3
   2.  Improved processing of incoming connection requests  . . . . .  3
   3.  Interaction with various timestamps generation algorithms  . .  6
   4.  Interaction with various ISN generation algorithms . . . . . .  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.  Behavior of the proposed mechanism in specific
                scenarios . . . . . . . . . . . . . . . . . . . . . . 10
     A.1.  Connection request after system reboot . . . . . . . . . . 10
   Appendix B.  Changes from previous versions of the draft (to
                be removed by the RFC Editor before publishing
                this document as an RFC)  . . . . . . . . . . . . . . 10
     B.1.  Changes from draft-ietf-tcpm-tcp-timestamps-03 . . . . . . 10
     B.2.  Changes from draft-ietf-tcpm-tcp-timestamps-02 . . . . . . 10
     B.3.  Changes from draft-ietf-tcpm-tcp-timestamps-01 . . . . . . 10
     B.4.  Changes from draft-ietf-tcpm-tcp-timestamps-00 . . . . . . 11
     B.5.  Changes from draft-gont-tcpm-tcp-timestamps-04 . . . . . . 11
     B.6.  Changes from draft-gont-tcpm-tcp-timestamps-03 . . . . . . 11
     B.7.  Changes from draft-gont-tcpm-tcp-timestamps-02 . . . . . . 11
     B.8.  Changes from draft-gont-tcpm-tcp-timestamps-01 . . . . . . 11
     B.9.  Changes from draft-gont-tcpm-tcp-timestamps-00 . . . . . . 11
   Author's Address . . . . . . . . . . . . . . . . . . . . . . . . . 11







Gont                     Expires August 8, 2011                 [Page 2]

Internet-Draft  Reducing TIME-WAIT state with timestamps   February 2011


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 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.  This problem is discussed in detail
   in [INFOCOM-99].

   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



Gont                     Expires August 8, 2011                 [Page 3]

Internet-Draft  Reducing TIME-WAIT state with timestamps   February 2011


   incarnation of the connection (for that direction of the data
   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.  The processing of
   SYN segments received for connections in all other states is
   unchanged.  Both the ISN (Initial Sequence Number) and the timestamps
   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 TIME-WAIT
   state SHOULD occur as follows:

   o  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 greater than the last sequence number 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 not be enabled for the new incarnation
         of the connection, but the Sequence Number of the incoming SYN
         segment is greater than the last sequence number seen on the
         previous incarnation of the connection (for the same direction



Gont                     Expires August 8, 2011                 [Page 4]

Internet-Draft  Reducing TIME-WAIT state with timestamps   February 2011


         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.

   o  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
         (creating a connection in the SYN-RECEIVED state).

      *  If TCP timestamps would not be enabled for the new incarnation
         of the connection, but the Sequence Number of the incoming SYN
         segment is greater than the last sequence number seen on the
         previous incarnation of the connection (for the same direction
         of the data transfer), honour the incoming 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.

   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
      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



Gont                     Expires August 8, 2011                 [Page 5]

Internet-Draft  Reducing TIME-WAIT state with timestamps   February 2011


   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 kilobytes/second
   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.

   Note:
      The upcoming revision of RFC 1323, [I-D.ietf-tcpm-1323bis],
      recommends the selection of timestamps such that they are
      monotonically-increasing across connections.  An example of such a
      Timestamps generation scheme can be found in
      [I-D.gont-timestamps-generation].


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



Gont                     Expires August 8, 2011                 [Page 6]

Internet-Draft  Reducing TIME-WAIT state with timestamps   February 2011


   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 the "extended TCP SYN cookies" 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].

   Some stacks (notably OpenBSD) implement timestamps randomization
   algorithms which do not result in monotonically-increasing ISNs
   across connections.  As noted in [Silbersack], such randomization
   schemes may prevent the mechanism proposed in this document from
   recycling connections that are in the TIME-WAIT state.  However, as
   noted earlier in this section, in the worst-case scenario the
   heuristics fail, and the result is no worse than the current state-
   of-affairs.


4.  Interaction with various ISN generation algorithms

   [RFC0793] suggests that the ISNs of TCP connections be generated from
   a global timer, such that they are monotonically-increasing across
   connections.  However, this ISN-generation scheme leads to
   predictable ISNs, which have well-known security implications
   [CPNI-TCP].  [RFC1948] proposes an alternative ISN-generation scheme
   which results in monotonically-increasing ISNs across connections
   that are not easily-predictable by an off-path attacker.

   Some stacks (notably OpenBSD) implement ISN randomization algorithms
   which do not result in monotonically-increasing ISNs across
   connections.  As noted in [Silbersack], such ISN randomization
   schemes break the BSD improved handling of SYN segments received for
   connections that are in the TIME-WAIT state.

   An implementation of the mechanism proposed in this document would
   enable recycling of the TIME-WAIT state even in the presence of ISNs
   that are not monotonically-increasing across connections, except when
   the timestamp contained in the incoming SYN is equal to the last
   timestamp seen on the connection in the TIME-WAIT state (for that
   direction of the data transfer).



Gont                     Expires August 8, 2011                 [Page 7]

Internet-Draft  Reducing TIME-WAIT state with timestamps   February 2011


5.  Security Considerations

   [I-D.ietf-tcpm-tcp-security] contains a detailed discussion of the
   security implications of TCP timestamps and of different Timestamps
   generation algorithms.


6.  IANA Considerations

   This document has no actions for IANA.


7.  Acknowledgements

   This document is based on part of the contents of the technical
   report "Security Assessment of the Transmission Control Protocol
   (TCP)" [CPNI-TCP] written by Fernando Gont on behalf of the United
   Kingdom's Centre for the Protection of National Infrastructure (UK
   CPNI).

   The author of this document would like to thank (in alphabetical
   order) Mark Allman, Francis Dupont, Wesley Eddy, Lars Eggert, Alfred
   Hoenes, John Heffner, Christian Huitema, 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.

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate



Gont                     Expires August 8, 2011                 [Page 8]

Internet-Draft  Reducing TIME-WAIT state with timestamps   February 2011


              Requirement Levels", BCP 14, RFC 2119, March 1997.

8.2.  Informative References

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

   [I-D.gont-timestamps-generation]
              Gont, F. and A. Oppermann, "On the generation of TCP
              timestamps", draft-gont-timestamps-generation-00 (work in
              progress), June 2010.

   [I-D.ietf-tcpm-1323bis]
              Borman, D., Braden, R., and V. Jacobson, "TCP Extensions
              for High Performance", draft-ietf-tcpm-1323bis-01 (work in
              progress), March 2009.

   [I-D.ietf-tcpm-tcp-security]
              Gont, F., "Security Assessment of the Transmission Control
              Protocol (TCP)", draft-ietf-tcpm-tcp-security-02 (work in
              progress), January 2011.

   [INFOCOM-99]
              Faber, T., Touch, J., and W. Yue, "The TIME-WAIT state in
              TCP and Its Effect on Busy Servers", Proc. IEEE Infocom,
              1999, pp. 1573-1583 .

   [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.

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

   [RFC1948]  Bellovin, S., "Defending Against Sequence Number Attacks",
              RFC 1948, May 1996.

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

   [Silbersack]
              Silbersack, M., "Improving TCP/IP security through



Gont                     Expires August 8, 2011                 [Page 9]

Internet-Draft  Reducing TIME-WAIT state with timestamps   February 2011


              randomization without sacrificing interoperability",
              EuroBSDCon 2005 Conference .


Appendix A.  Behavior of the proposed mechanism in specific scenarios

A.1.  Connection request after system reboot

   This section clarifies how this algorithm would operate in case a
   computer reboots, keeps the same IP address, looses memory of the
   previous timestamps, 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 (Maximum Segment Lifetime) 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", a connection
   request might be sent to a remote host while there is a previous
   incarnation of the same connection in the TIME-WAIT state at the
   remote host.  In such a scenario, as a result of having lost memory
   of previous time stamps, the resulting timestamps might not be
   monotonically-increasing, and hence the proposed algorithm might be
   unable to recycle the previous incarnation of the connection that is
   in the TIME-WAIT state.  This case corresponds to the current state-
   of-affairs without the algorithm proposed in this document.


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

B.1.  Changes from draft-ietf-tcpm-tcp-timestamps-03

   o  Addresses Tim Polk's DISCUSS.

B.2.  Changes from draft-ietf-tcpm-tcp-timestamps-02

   o  Addresses COMMENTs received during IESG review, and maybe Tim
      Polk's DISCUSS.

B.3.  Changes from draft-ietf-tcpm-tcp-timestamps-01

   o  Addresses AD-review comments by Lars Eggert.






Gont                     Expires August 8, 2011                [Page 10]

Internet-Draft  Reducing TIME-WAIT state with timestamps   February 2011


B.4.  Changes from draft-ietf-tcpm-tcp-timestamps-00

   o  Addresses WG Last call comments received from Wesley Eddy, John
      Heffner and Joe Touch.

   o  Minor editorial fix (reported by Wes Eddy).

B.5.  Changes from draft-gont-tcpm-tcp-timestamps-04

   o  Draft resubmitted as draft-ietf.

B.6.  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.

B.7.  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).

B.8.  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).

B.9.  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




Gont                     Expires August 8, 2011                [Page 11]


Html markup produced by rfcmarkup 1.108, available from http://tools.ietf.org/tools/rfcmarkup/