NAT resource optimizing extension
draft-naito-nat-resource-optimizing-extension-01

Network Working Group                                           K. Naito
Internet-Draft                                              A. Matsumoto
Intended status: Informational                                       NTT
Expires: September 27, 2012                               March 26, 2012

                   NAT resource optimizing extension
            draft-naito-nat-resource-optimizing-extension-01

Abstract

   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 two extensions for
   optimizing NAT IP address and port resources in address resource
   restricted environments.  One extension enables simultaneous use of a
   NAT external port for different transport sessions, and the other
   makes use of a TCP timestamp for TIME_WAIT assassination.

Status of this Memo

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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
   limited.

   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 behaviors.

   1: In a lot of NAT implementations, a port that is available in NAT
      is allocated for a transport session.That is, a NAT does not use a
      port for multiple sessions simultaneously.

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

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

1.1.   TCP TIME_WAIT

   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.   Port overloading mechanism

   If destination addresses and ports are different at the outgoing
   sessions started by local clients, NAT MAY assign the same external
   port as the source ports at the sessions.  Port overlapping mechanism

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   manages mappings between external packets and internal packets by
   looking at and storing the 5-tuple (protocol, source address, source
   port, destination address, destination port) of them.  Thus, enables
   concurrent use of single port for multiple transport sessions, which
   enables NAT to work correctly in IP address resource limited network.

   Discussions:

   RFC4787 [RFC4787] and RFC5382 [RFC5382] requires "endpoint-
   independent mapping" at NAT, and port overlapping NAT cannot meet the
   requirement.  This mechanism can degrade the transparency of NAT in
   that its mapping mechanism is endpoint-dependent and makes NAT
   traversal harder.  However, if a NAT adopts endpoint-independent
   mapping together with endpoint-dependent filtering, then the actual
   behavior of the NAT will be the same as port overlapping NAT.  It
   should also be noted that a lot of existing NAT devices adopted this
   port overlapping mechanism.

2.2.   Apply RFC6191 to NAT

   RFC 6191 [RFC6191] defines a mechanism for reducing the TIME_WAIT
   state using TCP timestamps.  This document proposes to apply this
   RFC6191 mechanism at NAT.  By tracing timestamp values in NAT that
   manages states of traversing TCP sessions, a TIME_WAIT remaining
   wait-time can be reduced to zero, when a TCP-SYN packet carrying a
   larger timestamp value arrives.  In this case, PAWS (Protect Against
   Wrapped Sequence Numbers) works to discard old duplicate packets at
   NAT.  A packet can be discarded as an old duplicate if it is received
   with a timestamp value less than a timestamp recently received on the
   connection.  When there are several clients with nonsuccessive
   timestamp values are connected to a NAT device (i.e. not
   monotonically increasing among clients), it prevents some clients
   from getting a port to start a connection for a long time because
   other clients with larger timestamp values are preferred.Two
   workarounds for this issue are described below.

2.2.1.   Rewrite timestamp values at NAT

   Rewrite timestamp values of outgoings packets at NAT to be
   monotonically increasing.

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

   Set some rules among clients connecting to NAT, e.g., split
   assignable ports between clients.  This MAY be done by distributing
   rules to clients via NAT equipment.

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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.

   [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.

   [RFC4787]  Audet, F. and C. Jennings, "Network Address Translation
              (NAT) Behavioral Requirements for Unicast UDP", BCP 127,
              RFC 4787, January 2007.

   [RFC5382]  Guha, S., Biswas, K., Ford, B., Sivakumar, S., and P.
              Srisuresh, "NAT Behavioral Requirements for TCP", BCP 142,
              RFC 5382, October 2008.

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

Authors' Addresses

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

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

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

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

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