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RFC 6191

Reducing the TIME-WAIT State Using TCP Timestamps

Pages: 10
Best Current Practice: 159

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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.
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   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.
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   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
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   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).
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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 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.
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   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].
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   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).
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   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.
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   [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.
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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