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Versions: 00 01 02                                                      
                                                                K. Naito
Internet-Draft                                              A. Matsumoto
Intended status: Informational                                       NTT
Expires: September 4, 2012                                    March 2012

                    NAT resource optimizing extension


   When NAT is used in address resource restricted environment, or when
   a lot of users are located under a NAT device, IP address and port
   resources may be eaten up, and it brings severe bad effects on user
   experiences.  This situation can be greatly mitigated by tweaking
   mapping behavior and session timer handling at NAT function.  This
   document proposes to NAT IP address and port resource optimizing
   extension for address resource restricted environment.  One extension
   is to enable simultaneous use of a port for different transport
   sessions, and the other is to make use of TCP timestamp for TIME_WAIT

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
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   Internet-Drafts are draft documents valid for a maximum of six months
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   This Internet-Draft will expire on August 4, 2012.

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   publication of this document.  Please review these documents

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   carefully, as they describe your rights and restrictions with respect
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   than English.

1.  Introduction

   The internet has run out of IPv4 addresses, and after that IPv4
   addresses that can be allocated to a user will be much more
   restricted.  NAT is a tool that is widely used for this problem of
   IPv4 address shortage.  However, there will be more and more demands
   for resources to provide the Internet access to the users and
   devices.  IPv6 is a fundamental solution for this problem, but the
   deployment of IPv6 takes 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 short time.  What is worse is that when a NAT is in
   between, the NAT keeps track of each session for long time, typically
   for four minutes, even if the session is terminated by both ends
   within a few seconds.

   This problem is caused or worsened by the following behaviors of NAT.

   1.  In a lot of NAT implementation, a port that is available at NAT
   is allocated or a transport session. i.e. a NAT does not use a port
   for multiple sessions simultaneously.

   2.TCP TIME_WAIT state requires 2*MSL wait before a session is closed,
   So at a NAT device many session states are kept, even if both ends of
   a session have closed and deleted the state for the session.

   We propose two mechanisms to change the above behaviors of NAT that
   enables to save addresses and ports resources.

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   TCP TIME_WAIT mechanism is written in RFC793.  TCP TIME_WAIT status
   requires 2*MSL to wait before connection to be CLOSED, in case:

   1.Packet in previous session be transfered later
   2.Retransmission of ACK packet(which is respond to FIN) is needed.

   In case 1, the old packet should be discarded, not to harm new session.
   In case 2, retransmission should be done to close destination port.
   If TIME_WAIT mechanism do not work, TCP transmission problem may happen.

1.2.  TIME_WAIT Assassination

   The server may accept the TCP SYN from the client change the state of
   the port from TCP TIME-WAIT to TCP Established.  This is known as
   TIME-WAIT assassination.

1.3.  PAWS(Protect Against Wrapped Sequence numbers)

   PAWS(Protect Against Wrapped Sequence numbers) is written in RFC1323.
   PAWS is used to prevent packet in previous session be transfered to
   the new session.

2.  Proposal

2.1.  Adopt Intended Port over-use mechanism

   To use single port for several connections, over-use port.  For
   example, if destination address differ in two connections, use single
   port for NAT port assignment.  In such case, this mechanism SHOULD
   meet the requirement written in RFC5382. i.e.  REQ-1:A NAT MUST have
   an "Endpoint-Independent Mapping" behavior for TCP.
   (There may be some other violations to adopt this, which I'll consider
   and write by IETF83 Paris)

2.2.  Adopt RFC6191 to NAT

   To make connection re-usable for TCP transmission(e.g. http
   transmission) in address resource restricted network, force port
   state to be CLOSED, as soon as a session have finished(i.e. port is
   in TCP TIME_WAIT state), if there are any SYN-WAIT state for new
   session, waiting for port to be released.
   This mechanism is written in RFC6191.
   To make this mechanism work in network that several PCs connected to
   the NAT equipment, value of timestamp SHOULD be successive.
   (timestamps MAY be overwritten in NAT equipment, so that timestamp
   in each packet that go through NAT equipment be successive.)

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   Also, consideration of some transmission pattern and effect is
   (I'll consider patterns, and write by IETF83 Paris)

3.  Security Considerations
(I'll write some by IETF83, Paris.)

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.

   [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

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

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

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

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