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Versions: 00 01 02 03                                    Standards Track
                                                        IPR declarations
Network Working Group                                           K. Naito
Internet-Draft                                              A. Matsumoto
Intended status: Informational                                       NTT
Expires: March 28, 2013                               September 24, 2012

                        NAT TIME_WAIT reduction


   When network address translation (NAT) is used in an address resource
   restricted environment, or when a lot of users are located under a
   NAT device, IP addresses and port resources may be eaten up, and this
   affects user experiences very negatively.  This situation can be
   greatly mitigated by tweaking mapping behavior and session timer
   handling in NAT functions.  This document proposes extension for
   optimizing NAT IP address and port resources in address resource
   restricted environments.  The extension makes use of TCP timestamps
   and sequence numbers for TIME_WAIT assassination.

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 March 28, 2013.

Copyright Notice

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   document authors.  All rights reserved.

   This document is subject to BCP 78 and the IETF Trust's Legal
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   publication of this document.  Please review these documents
   carefully, as they describe your rights and restrictions with respect

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   to this document.  Code Components extracted from this document must
   include Simplified BSD License text as described in Section 4.e of
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   This document may contain material from IETF Documents or IETF
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   than English.

1.  Introduction

   After IPv4 addresses run out, IPv4 address resources will be further
   restricted site-by-site.  If global IPv4 address are shared between
   several clients, assignable port resources at each client will be

   NAT is a tool that is widely used to deal with this IPv4 address
   shortage problem.  However, the demand for resources to provide
   Internet access to users and devices will continue to increase.  IPv6
   is a fundamental solution to this problem, but the deployment of IPv6
   will take time.

   In some cases, e.g. browsing a dynamic web page for a map service, a
   lot of sessions are used by the browser, and a number of ports are
   eaten up in a short time.  What is worse is that when a NAT is
   between a PC and a server, TIME_WAIT state of each TCP connection is
   kept for certain period, typically for four minutes, which consumes
   port resources.  Therefore, new connections cannot be established.

   This problem is caused or worsened by the following behavior.

      TIME_WAIT state assigned for a TCP connection remains active for
      2MSL after the last ACK to the last FIN is transferred.

   We propose mechanisms to change the above behavior that make it
   possible to save addresses and ports resources.

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   The TCP TIME_WAIT state is described in RFC793 [RFC0793].  The TCP
   TIME_WAIT state needs to be kept for 2MSL before a connection is
   CLOSED, for the reasons below.

   1: In the event that packets from a session are delayed in the in-
      between network, and delivered to the end relatively later, we
      should prevent the packets from being transferred and interpreted
      as a packet that belongs to a new session.
   2: If the remote TCP has not received the acknowledgment of its
      connection termination request, it will re-send the FIN packet
      several times.

   These points are important for the TCP to work without problems.

1.2.   TIME_WAIT Assassination

   A TCP server MAY accept a TCP SYN for the 5-tuple session that is
   just finished and marked as TIME_WAIT state,as far as the TCP
   sequence number is increased.  This is known as TIME-WAIT
   assassination.  It should also be noted that some assassination
   hazards are described in RFC1337 [RFC1337].

1.3.   Protect Against Wrapped Sequence numbers (PAWS)

   The TCP sequence number wraps frequently especially in a high
   bandwidth session.  PAWS is used to prevent old duplicate packets
   that occurred in a previous session from being transferred to the new
   session whose valid TCP sequence numbers happen to overlap with the
   old duplicate packets.  This is implemented by introducing TCP
   timestamp option, and checking the timestamp option value of each
   packet.  PAWS is described in RFC1323 [RFC1323].

2.   NAT resource optimizing extension proposal

2.1.   Apply RFC6191 to NAT

   RFC 6191 [RFC6191] defines a mechanism for reducing the TIME_WAIT
   state using TCP timestamps and sequence numbers.  This document
   proposes to apply this RFC6191 [RFC6191]mechanism at NAT.  In case
   there are several clients with nonsuccessive timestamp or sequence
   number values are connected to a NAT device (i.e. not monotonically
   increasing among clients), two mechanisms for applying RFC6191 to NAT
   are described below.  Also, PAWS works to discard old duplicate
   packets at NAT.  A packet can be discarded as an old duplicate if it
   is received with a timestamp or sequence number value less than a

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   value recently received on the connection.

2.2.   Rewrite timestamp and sequence number values at NAT

   Rewrite timestamp and sequence number values of outgoings packets at
   NAT to be monotonically increasing.  This can be done by adopting
   following mechanisms at NAT.

   A: Store the newest rewritten value of timestamp and sequence number
      as the 'max value at the time'
   B: NAT rewrite timestamp and sequence number values of incoming
      packets to be monotonically increasing.

   When packets come back as replies from remote hosts, NAT rewrite
   again the packets' timestamp and sequence number values to be the
   original values.  This can be done by adopting following mechanisms
   at NAT.

   C: Store the values of original timestamp and sequence number of
      packets, and rewritten values of those.

2.3.   Split an assignable number of port space to each client

   Adopt following mechanisms at NAT.

   A: Choose clients that can be assigned ports.
   B: Split assignable port numbers between clients.

   Packets from other clients which are not chosen by these mechanisms
   are rejected at NAT, unless there is unassigned port left.

2.4.   Resend the last ACK to the resended FIN

   In case the remote TCP could not receive the acknowledgment of its
   connection termination request, NAT, on behalf of clients. resends
   the last ACK packet when it recieves an FIN packet of the previous
   connection, and when the state of the previous connection is deleted
   from the NAT.  This mechanism should be used when clients starts
   closing process, and the remote host could not receive the last ACK.

2.5.   Remote host behavior of several implementations

   To solve the port shortage problem on the client side, the behavior
   of remote host should be compliant to RFC 6191 [RFC6191] or the
   mechanism written in of RFC1122 [RFC1122], since NAT may
   reuse the same 5 tuple for a new connection.We have investigated
   behaviors of OSes (e.g., Linux, FreeBSD, Windows, MacOS), and found

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   that they implemented the server side behavior of the above two.

3.  Security Considerations

   Security issues are not discussed in this memo.

4.  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.

   [RFC6191]  Gont, F., "Reducing the TIME-WAIT State Using TCP
              Timestamps", BCP 159, RFC 6191, April 2011.

Appendix A.  Revision History

   02: Changed intended status to "informational".

   01: 'draft-naito-nat-resource-optimizing-extension-01' was divided
   into two drafts after IETF83 meeting.
   'draft-naito-nat-resource-optimizing-extension-01' containes two
   mechanisms.  One mechanism, TIME_WAIT reduction is written in this
   draft, and the other is written in

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Authors' Addresses

   Kengo Naito
   NTT NT Lab
   3-9-11 Midori-Cho
   Musashino-shi, Tokyo  180-8585

   Phone: +81 422 59 4949
   Email: naito.kengo@lab.ntt.co.jp

   Arifumi Matsumoto
   NTT NT Lab
   3-9-11 Midori-Cho
   Musashino-shi, Tokyo  180-8585

   Phone: +81 422 59 3334
   Email: arifumi@nttv6.net

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