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rfc:bcp:bcp159

Internet Engineering Task Force (IETF) F. Gont Request for Comments: 6191 UK CPNI BCP: 159 April 2011 Category: Best Current Practice ISSN: 2070-1721

         Reducing the TIME-WAIT State Using TCP Timestamps

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 memo documents an Internet Best Current Practice.
 This document is a product of the Internet Engineering Task Force
 (IETF).  It represents the consensus of the IETF community.  It has
 received public review and has been approved for publication by the
 Internet Engineering Steering Group (IESG).  Further information on
 BCPs is available in Section 2 of RFC 5741.
 Information about the current status of this document, any errata,
 and how to provide feedback on it may be obtained at
 http://www.rfc-editor.org/info/rfc6191.

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

Gont Best Current Practice [Page 1] RFC 6191 Reducing TIME-WAIT State with Timestamps April 2011

 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 . . . . . . . . . . . . . . . . . . . . . . . . .  2
 2.  Improved Processing of Incoming Connection Requests  . . . . .  3
 3.  Interaction with Various Timestamp Generation Algorithms . . .  6
 4.  Interaction with Various ISN Generation Algorithms . . . . . .  7
 5.  Security Considerations  . . . . . . . . . . . . . . . . . . .  7
 6.  Acknowledgements . . . . . . . . . . . . . . . . . . . . . . .  7
 7.  References . . . . . . . . . . . . . . . . . . . . . . . . . .  8
   7.1.  Normative References . . . . . . . . . . . . . . . . . . .  8
   7.2.  Informative References . . . . . . . . . . . . . . . . . .  8
 Appendix A.  Behavior of the Proposed Mechanism in Specific
              Scenarios . . . . . . . . . . . . . . . . . . . . . . 10
   A.1.  Connection Request after System Reboot . . . . . . . . . . 10

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 Timestamps 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.

Gont Best Current Practice [Page 2] RFC 6191 Reducing TIME-WAIT State with Timestamps April 2011

 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, Section 4.2.2.13 of RFC 1122
 [RFC1122] states that when a connection request is received with a
 four-tuple that is in the TIME-WAIT state, the connection request may
 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).  The goal of this requirement is to prevent the overlap of
 the sequence number spaces of the old and new incarnations of the
 connection so that segments from the old incarnation are not accepted
 as valid by the new incarnation.
 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

Gont Best Current Practice [Page 3] RFC 6191 Reducing TIME-WAIT State with Timestamps April 2011

 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), honor 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), honor 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
       of the data transfer), honor 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, honor the incoming connection request (creating

       a connection in the SYN-RECEIVED state).

Gont Best Current Practice [Page 4] RFC 6191 Reducing TIME-WAIT State with Timestamps April 2011

  • 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), honor 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 Timestamps 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
 250,000 times per second).  As a result, any connection that
 transfers more than 250,000 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 ISN heuristics would fail, and therefore the
 connection request would usually time out.  By including the TCP
 Timestamps option in the heuristics described above, all these
 constraints are greatly relaxed.

Gont Best Current Practice [Page 5] RFC 6191 Reducing TIME-WAIT State with Timestamps April 2011

 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, [1323bis], recommends the
    selection of timestamps such that they are monotonically
    increasing across connections.  An example of such a timestamp
    generation scheme can be found in [TS-Generation].

3. Interaction with Various Timestamp Generation Algorithms

 The algorithm proposed in Section 2 clearly benefits from timestamps
 that are monotonically increasing across connections to the same
 endpoint.  In particular, generation of timestamps such that they are
 monotonically increasing is important for TCP instances 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 (by
 itself) does not imply a requirement on the timestamp generation
 algorithm of other TCP implementations.
 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
 SYN.
 Some stacks may implement timestamp 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], which allows the
 implementation of extended TCP SYN cookies.
 Note:
    It should be noted that the "extended TCP SYN cookies" could
    coexist with an algorithm for generating timestamps such that they
    are monotonically increasing.  Monotonically increasing timestamps
    could be generated for TCP instances that perform the active open,
    while timestamps for TCP instances that perform the passive open
    could be generated according to [Opperman].

Gont Best Current Practice [Page 6] RFC 6191 Reducing TIME-WAIT State with Timestamps April 2011

 Some stacks (notably OpenBSD) implement timestamp 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
 that 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 BSD's 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).

5. Security Considerations

 [TCP-Security] contains a detailed discussion of the security
 implications of TCP Timestamps and of different timestamp generation
 algorithms.

6. 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).

Gont Best Current Practice [Page 7] RFC 6191 Reducing TIME-WAIT State with Timestamps April 2011

 The author of this document would like to thank (in alphabetical
 order) Mark Allman, Francis Dupont, Wesley Eddy, Lars Eggert, John
 Heffner, Alfred Hoenes, 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.
 Fernando Gont's attendance to IETF meetings was supported by ISOC's
 "Fellowship to the IETF" program.

7. References

7.1. 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
                  Requirement Levels", BCP 14, RFC 2119, March 1997.

7.2. Informative References

 [1323bis]        Borman, D., Braden, R., and V. Jacobson, "TCP
                  Extensions for High Performance", Work in Progress,
                  March 2009.
 [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>.
 [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.

Gont Best Current Practice [Page 8] RFC 6191 Reducing TIME-WAIT State with Timestamps April 2011

 [Linux]          Linux Kernel Organization, "The Linux Kernel
                  Archives", <http://www.kernel.org>.
 [Opperman]       Oppermann, A., "FYI: Extended TCP syncookies in
                  FreeBSD-current", post to the tcpm mailing list,
                  September 2006, <http://www.ietf.org/mail-archive/
                  web/tcpm/current/msg02251.html>.
 [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
                  randomization without sacrificing interoperability",
                  EuroBSDCon 2005.
 [TCP-Security]   Gont, F., "Security Assessment of the Transmission
                  Control Protocol (TCP)", Work in Progress,
                  January 2011.
 [TS-Generation]  Gont, F. and A. Oppermann, "On the generation of TCP
                  timestamps", Work in Progress, June 2010.

Gont Best Current Practice [Page 9] RFC 6191 Reducing TIME-WAIT State with Timestamps April 2011

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, loses 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, in terms
 of specifications, 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 timestamps, 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.

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 Best Current Practice [Page 10]

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