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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                                          February 2, 2009
Intended status: BCP
Expires: August 6, 2009


                  On the generation of TCP timestamps
                 draft-gont-tcpm-tcp-timestamps-01.txt

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   This Internet-Draft will expire on August 6, 2009.

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Abstract

   This document describes an algorithm for selecting the timestamps (TS
   value) used for TCP connections that use the TCP timestamp option,
   such that the resulting timestamps are monotonically-increasing
   across connections that involve the same four-tuple {local IP
   address, local TCP port, remote IP address, remote TCP port}.  The
   properties of the algorithm are such that it reduces the possibility
   of an attacker of guessing the exact value.  Additionally, it
   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.


Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . . . 3
   2.  Proposed algorithm  . . . . . . . . . . . . . . . . . . . . . . 3
   3.  Improved processing of incoming connection requests . . . . . . 4
   4.  Security Considerations . . . . . . . . . . . . . . . . . . . . 7
   5.  IANA Considerations . . . . . . . . . . . . . . . . . . . . . . 7
   6.  Acknowledgements  . . . . . . . . . . . . . . . . . . . . . . . 7
   7.  References  . . . . . . . . . . . . . . . . . . . . . . . . . . 7
     7.1.  Normative References  . . . . . . . . . . . . . . . . . . . 7
     7.2.  Informative References  . . . . . . . . . . . . . . . . . . 8
   Appendix A.  Changes from previous versions of the draft (to
                be removed by the RFC Editor before publishing
                this document as an RFC) . . . . . . . . . . . . . . . 8
     A.1.  Changes from draft-gont-tcpm-tcp-timestamps-00  . . . . . . 8
   Author's Address  . . . . . . . . . . . . . . . . . . . . . . . . . 8




















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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].

   In order to avoid the security implications of predictable
   timestamps, the proposed algorithm generates timestamps such that the
   possibility of an attacker guessing the exact value is reduced.

   Section 2 proposes the aforementioned algorithm for generating TCP
   timestamps.  Section 3 describes an improved processing of incoming
   connection requests, that may allow higher connection-establishment
   rates to any TCP end-point.

   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.  Proposed algorithm

   It is RECOMMENDED that timestamps are generated with a similar
   algorithm to that introduced by RFC 1948 [RFC1948] for the generation
   of Initial Sequence Numbers (ISNs).  That is,

   timestamp = T() + F(localhost, localport, remotehost, remoteport,
   secret_key)

   where the result of T() is a global system clock that complies with
   the requirements of Section 4.2.2 of RFC 1323 [RFC1323], and F() is a



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   function that should not be computable from the outside without
   knowledge of the secret key (secret_key).  Therefore, we suggest F()
   to be a cryptographic hash function of the connection-id and some
   secret data (which could be chosen randomly).

   F() provides an offset that will be the same for all incarnations of
   a connection between the same two endpoints, while T() provides the
   monotonically increasing values that are needed for PAWS.


3.  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 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 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.




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   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).

         +  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).



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         +  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
   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 depends on the timestamps to be monotonically
   increasing across connections between the same two TCP endpoints.
   Therefore, we strongly advice to generate timestamps as described in
   Section 2.





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4.  Security Considerations

   This document describes an algorithm that can be used to obfuscate
   the timestamp value used for new connections, such that the
   possibility of an attacker guessing the exact value is reduced.

   Some implementations are known to maintain a global timestamp clock,
   which is used for all connections.  This is undesirable, as an
   attacker that can establish a connection with a host would learn the
   timestamp used for all the other connections maintained by that host,
   which could be useful for performing any attacks that require the
   attacker to forge TCP segments.  Some implementations are known to
   initialize their global timestamp clock to zero when the system is
   bootstrapped.  This is undesirable, as the timestamp clock would
   disclose the system uptime.

   The algorithm discussed in this document for generating the TCP
   timestamps avoids these problems by generating timestamps as
   monotonically-increasing function with a per-connection-id random
   offset.  [CPNI-TCP]


5.  IANA Considerations

   This document has no actions for IANA.


6.  Acknowledgements

   The author of this document would like to thank (in alphabetical
   order) Alfred Hoenes and Eric Rescorla 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.


7.  References

7.1.  Normative References

   [RFC0793]  Postel, J., "Transmission Control Protocol", STD 7,
              RFC 793, September 1981.




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

7.2.  Informative References

   [CPNI-TCP]
              CPNI, "Security Assessment of the Transmission Control
              Protocol (TCP)", (to be published) .

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

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


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-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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