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Versions: 00 01 draft-durand-softwire-dual-stack-lite

Internet Engineering Task Force                                 R. Droms
Internet-Draft                                                     Cisco
Intended status: Informational                               B. Haberman
Expires: January 13, 2009                                  July 12, 2008


              Softwires Network Address Translation (SNAT)
                   draft-droms-softwires-snat-01.txt

Status of this Memo

   By submitting this Internet-Draft, each author represents that any
   applicable patent or other IPR claims of which he or she is aware
   have been or will be disclosed, and any of which he or she becomes
   aware will be disclosed, in accordance with Section 6 of BCP 79.

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   This Internet-Draft will expire on January 13, 2009.

Abstract

   Service providers (ISPs) are interested in reducing the use of IPv4
   in their internal networks because of the anticipated exhaustion of
   the IPv4 address space.  Softwires Network Address Translation (SNAT)
   combines IPv4 NAT and IPv4-in-IPv6 softwires to carry IPv4 traffic
   through the ISP network that uses only IPv6 service.  Multiple
   subscribers are multiplexed through a single external global IPv4
   address, reducing the total number of IPv4 addresses in use by the
   ISP to support Internet traffic to those subscribers.







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Table of Contents

   1.  Introduction . . . . . . . . . . . . . . . . . . . . . . . . .  3
   2.  Requirements Language  . . . . . . . . . . . . . . . . . . . .  3
   3.  Terminology  . . . . . . . . . . . . . . . . . . . . . . . . .  4
   4.  Problem statement and requirements . . . . . . . . . . . . . .  4
   5.  SNAT Architecture  . . . . . . . . . . . . . . . . . . . . . .  4
   6.  Example message flow . . . . . . . . . . . . . . . . . . . . .  7
   7.  Translation details  . . . . . . . . . . . . . . . . . . . . . 11
   8.  Supporting multiple subscribers through one IPv4 address . . . 12
   9.  Setting up state . . . . . . . . . . . . . . . . . . . . . . . 12
   10. Analysis and Future Work . . . . . . . . . . . . . . . . . . . 13
   11. Change log . . . . . . . . . . . . . . . . . . . . . . . . . . 13
     11.1.  Revision -01  . . . . . . . . . . . . . . . . . . . . . . 13
   12. Contributors . . . . . . . . . . . . . . . . . . . . . . . . . 13
   13. IANA Considerations  . . . . . . . . . . . . . . . . . . . . . 13
   14. Security Considerations  . . . . . . . . . . . . . . . . . . . 14
   15. References . . . . . . . . . . . . . . . . . . . . . . . . . . 14
     15.1.  Normative References  . . . . . . . . . . . . . . . . . . 14
     15.2.  Informative References  . . . . . . . . . . . . . . . . . 14
   Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 14
   Intellectual Property and Copyright Statements . . . . . . . . . . 15





























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1.  Introduction

   Service providers (ISPs) are interested in reducing the use of IPv4
   in their internal networks because of the anticipated exhaustion of
   the IPv4 address space.  Reducing the use of IPv4 addresses will
   allow the conservation of addresses assigned to the ISP for use in
   specific places where IPv4 is required.  One way of reducing the use
   of IPv4 addresses to deploy IPv6 to replace IPv4 in internal
   networks.

   Softwires Network Address Translation (SNAT) combines IPv4 NAT
   [RFC3022] and IPv4-in-IPv6 softwire to carry IPv4 traffic through the
   ISP network where only IPv6 is deployed.  RFC 4925 [RFC4925]
   describes the two initial softwires WG problem statements, "Hubs and
   Spokes" (Section 2) and "Mesh" (Section 3).  The problem that this
   document addresses is more narrowly scoped than either of these two
   initial problem statements, focusing on IPv4 over IPv6 only and the
   specific needs of a large ISP network facing scaling issues from lack
   of RFC 1918 address space for all of its devices.  It most closely
   resembles the "converse case" of that depicted in Figure 2, Case 2,
   of RFC 4925 (except that the Host may be IPv4-only, rather than Dual
   Stack as depicted, and the portion of the network labeled "IPv4-only"
   is likely larger than that implied in the diagram).  SNAT multiplexes
   multiple subscribers through a single IPv4 address, reducing the
   total number of IPv4 addresses in use by the ISP to support Internet
   traffic to those subscribers.

   Elements of SNAT are inspired by the proposal from NTT to deploy dual
   IPv4 NAT and the proposal from Comcast to use IPv4-IPv6-IPv4
   translation.  SNAT retains the characteristics of IPv4-IPV4 NAT,
   rather than introducing IPv4-IPV6 translation, while saving IPv4
   addresses in the ISP core network.  This document has been submitted
   to foster discussion about these mechanisms for IPv4 address space
   conservation.

   SNAT requires one IPv4-in-IPv6 softwire per subscriber.  These
   softwires will require configuration and special effort for
   reliability, as well as resources for scaling at the ISP endpoint for
   potentially thousands or even millions of softwires.  SNAT also
   requires additional functions in subscriber CPEs.


2.  Requirements Language

   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].




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3.  Terminology

   This document uses softwires terminology described in Section 1.1 of
   RFC 4925 throughout.  The reader is expected to refer to this
   document for a number of terms and abbreviations.  In addition to the
   terminology defined in RFC 4925, this document defines the following
   terms:

   HGW:  Home gateway; the gateway between the subscriber network and
      the ISP network

   Subscriber host or host:  A host attached to a subscriber network

   SPSWE:  Service provider softwire endpoint; the endpoint of the
      softwires in the ISP network


4.  Problem statement and requirements

   The motivation for SNAT is to reduce the number of IPv4 addresses in
   use in an ISP network.  The reduction is achieved in two ways:

   o  Use NAT to multiplex subscribers through a single global IPv4
      address

   o  Use softwires to provide IPv4 service through an ISP core network
      that uses only IPv6 addresses

   The following requirements were considered in the design of SNAT:

   o  Provide IPv4 service to CPE through NAT similar to familiar NAT in
      use today

   o  Minimize the use of global IPv4 addresses for subscriber IPv4
      service

   o  Eliminate the use of IPv4 addresses (global or RFC 1918) within
      the ISP as much as possible

   o  No changes to subscriber CPEs (hosts attached to the subscribe
      network)


5.  SNAT Architecture

   As illustrated in Figure 1, SNAT consists of three components: the
   subscriber home gateway (HGW), the service provider softwire endpoint
   (SPSWE) and a softwire between the SI in the HGW and the SC in the



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   SPSWE.  The SPSWE performs IPv4-IPv4 NAT translations to multiplex
   multiple subscribers through a single globl IPv4 address.
   Overlapping address spaces used by subscribers are disambiguated
   through the identification of tunnel endpoints.















































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                   +-----------+
                   |    Host   |
                   +-----+-----+
                         |10.0.0.1
                         |
                         |
                         |10.0.0.2
               +---------|---------+
               |         |         |
               |HGW      |         |
               |+--------+--------+|
               ||     SNAT SI     ||
               |+--------+--------+|
               +--------|||--------+
                        |||2001:0:0:1::1
                        |||
                        |||<-IPv4-in-IPv6 softwire
                        |||
                 -------|||-------
               /        |||        \
              |   ISP core network  |
               \        |||        /
                 -------|||-------
                        |||
                        |||2001:0:0:2::1
               +--------|||--------+
               |SPSWE   |||        |
               |+--------+--------+|
               ||     SNAT SC     ||
               |+--------+--------+|
               |       |NAT|       |
               |       +-+-+       |
               +---------|---------+
                         |129.0.0.1
                         |
                 --------|--------
               /         |         \
              |       Internet      |
               \         |         /
                 --------|--------
                         |
                         |128.0.0.1
                   +-----+-----+
                   | IPv4 Host |
                   +-----------+

                        Figure 1: SNAT Architecture




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   The resulting solution accepts an IPv4 datagram that is translated
   into an IPv4-in-IPv6 softwire datagram for transmission across the
   softwire.  At the corresponding endpoint, the IPv4 datagram is
   decapsulated, and the translated IPv4 address is inserted based on a
   translation from the softwire.


6.  Example message flow

   In the example shown in Figure 2, the translation tables in the SPSWE
   is configured to forward between IP/TCP (10.0.0.1/10000) and IP/TCP
   (129.0.0.1/5000).  That is, a datagram received by the HGW from the
   CPE at address 10.0.0.1, using TCP DST port 10000 will be translated
   a datagram with IP SRC address 129.0.0.1 and TCP SRC port 5000 in the
   Internet.




































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                   +-----------+
                   |    CPE    |
                   +-----+-----+
                      |  |10.0.0.1
      IPv4 datagram 1 |  |
                      |  |
                      v  |10.0.0.2
               +---------|---------+
               |         |         |
               |HGW      |         |
               |+--------+--------+|
               ||     SNAT SI     ||
               |+--------+--------+|
               +--------|||--------+
                      | |||2001:0:0:1::1
       IPv6 datagram 2| |||
                      | |||<-IPv4-in-IPv6 softwire
                      | |||
                 -----|-|||-------
               /      | |||        \
              |   ISP core network  |
               \      | |||        /
                 -----|-|||-------
                      | |||
                      | |||2001:0:0:2::1
               +------|-|||--------+
               |SPSWE v |||        |
               |+--------+--------+|
               ||     SNAT SC     ||
               |+--------+--------+|
               |       |NAT|       |
               |       +-+-+       |
               +---------|---------+
                      |  |129.0.0.1
      IPv4 datagram 3 |  |
                 -----|--|--------
               /      |  |         \
              |       Internet      |
               \      |  |         /
                 -----|--|--------
                      |  |
                      v  |128.0.0.1
                   +-----+-----+
                   | IPv4 Host |
                   +-----------+

                        Figure 2: Outbound Datagram




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            +-----------------+--------------+---------------+
            |        Datagram | Header field | Contents      |
            +-----------------+--------------+---------------+
            | IPv4 datagram 1 |     IPv4 Dst | 128.0.0.1     |
            |                 |     IPv4 Src | 10.0.0.1      |
            |                 |      TCP Dst | 80            |
            |                 |      TCP Src | 10000         |
            | --------------- | ------------ | ------------- |
            | IPv6 Datagram 2 |     IPv6 Dst | 2001:0:0:2::2 |
            |                 |     IPv6 Src | 2001:0:0:1::1 |
            |                 |     IPv4 Dst | 128.0.0.1     |
            |                 |     IPv4 Src | 10.0.0.1      |
            |                 |      TCP Dst | 80            |
            |                 |      TCP Src | 10000         |
            | --------------- | ------------ | ------------- |
            | IPv4 datagram 3 |     IPv4 Dst | 128.0.0.1     |
            |                 |     IPv4 Src | 129.0.0.1     |
            |                 |      TCP Dst | 80            |
            |                 |      TCP Src | 5000          |
            +-----------------+--------------+---------------+

                         Datagram header contents

   When datagram 1 is received by the HGW, the SI function encapsulates
   the datagram in datagram 2 and forwards it to the SPSWE over the
   softwire.

   When it receives datagram 2, the SC in the SPSWE hands the IPv4
   datagram to the NAT, which determines from its translation table that
   the datagram received on Softwire_1 with TCP SRC port 10000 should be
   translated to datagram 3 with IP SRC address 129.0.0.1 and TCP SRC
   port 5000.

   Figure 3 shows an inbound message received at the SPSWE.  When the
   NAT function in the SPSWE receives datagram 1, it looks up the IP/TCP
   DST in its translation table.  In the example in Figure 3, the NAT
   translates the TCP DST port to 10000, sets the IP DST address to
   10.0.0.1 and hands the datagram to the SC for transmission over
   Softwire_1.  The SI in the HGW decapsulates IPv4 datagram from the
   inbound softwire datagram, and forwards it to the host.











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                   +-----------+
                   |    Host   |
                   +-----+-----+
                      ^  |10.0.0.1
      IPv4 datagram 3 |  |
                      |  |
                      |  |10.0.0.2
               +---------|---------+
               |       +-+-+       |
               |HGW    |NAT|       |
               |+--------+--------+|
               ||     SNAT SI     ||
               |+--------+--------+|
               +--------|||--------+
                      ^ |||2001:0:0:1::1
      IPv6 datagram 2 | |||
                      | |||<-IPv4-in-IPv6 softwire
                      | |||
                 -----|-|||-------
               /      | |||        \
              |   ISP core network  |
               \      | |||        /
                 -----|-|||-------
                      | |||
                      | |||2001:0:0:2::1
               +------|-|||--------+
               |SPSWE | |||        |
               |+--------+--------+|
               ||     SNAT SC     ||
               |+--------+--------+|
               |       |NAT|       |
               |       +-+-+       |
               +---------|---------+
                      ^  |129.0.0.1
      IPv4 datagram 1 |  |
                 -----|--|--------
               /      |  |         \
              |       Internet      |
               \      |  |         /
                 -----|--|--------
                      |  |
                      |  |128.0.0.1
                   +-----+-----+
                   | IPv4 Host |
                   +-----------+

                              The postamble.




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                        Figure 3: Inbound Datagram

            +-----------------+--------------+---------------+
            |        Datagram | Header field | Contents      |
            +-----------------+--------------+---------------+
            | IPv4 datagram 1 |     IPv4 Dst | 129.0.0.1     |
            |                 |     IPv4 Src | 128.0.0.1     |
            |                 |      TCP Dst | 5000          |
            |                 |      TCP Src | 80            |
            | --------------- | ------------ | ------------- |
            | IPv6 Datagram 2 |     IPv6 Dst | 2001:0:0:1::1 |
            |                 |     IPv6 Src | 2001:0:0:2::2 |
            |                 |     IPv4 Dst | 10.0.0.1      |
            |                 |       IP Src | 128.0.0.1     |
            |                 |      TCP Dst | 10000         |
            |                 |      TCP Src | 80            |
            | --------------- | ------------ | ------------- |
            | IPv4 datagram 3 |     IPv4 Dst | 10.0.0.1      |
            |                 |     IPv4 Src | 128.0.0.1     |
            |                 |      TCP Dst | 10000         |
            |                 |      TCP Src | 80            |
            +-----------------+--------------+---------------+

                         Datagram header contents


7.  Translation details

   The SPSWE has a NAT that translates between softwire/port pairs and
   IPv4-address/port pairs.  The same translation is applied to IPv4
   datagrams received on the device's external interface and from the
   softwire endpoint in the device.

   In Figure 2, the translator network interface in the SPSWE is on the
   Internet, and the softwire interface connects to the HGW.  The SPSWE
   translator is configured as follows:

   Network interface:  Translate IPv4 destination address and TCP
      destination port to the softwire identifier and TCP destination
      port

   Softwire interface:  Translate softwire identifier and TCP source
      port to IPv4 source address and TCP source port

   Here is how the translation in Figure 3 works:

   o  Datagram 1 is received on the SPSWE translator network interface.
      The translator looks up the IPv4-address/port pair in its



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      translator table, rewrites the IPv4 destination address to
      10.0.0.1 and the TCP source port to 10000, and hands the datagram
      to the SE to be forwarded over the softwire.

   o  The IPv4 datagram is received on the HGW SI.  The SI function
      extracts the IPv4 datagram and the HGW forwards datagram 3 to the
      host.

          +-------------------------------+--------------------+
          |            Softwire/IPv4/Port | IPv4/Port          |
          +-------------------------------+--------------------+
          | Softwire_1/10.0.0.1/TCP 10000 | 129.0.0.1/TCP 5000 |
          +-------------------------------+--------------------+

                          SPSWE translation table


8.  Supporting multiple subscribers through one IPv4 address

   One key advantage of SNAT is the ability to provide Internet access
   for multiple subscribers through a single global IPv4 address.  The
   SPTE table can be configured to translate traffic from multiple
   customers through one global IPv4 address.  Even a small degree of
   multiplexing, as few as five subscribers through each global IPv4
   address, would give ISPs sufficient IPv4 address space to continue
   and grow operations until IPv6 is more fully deployed.


9.  Setting up state

   The translation tables in the SPSWE can be set up dynamically by
   outbound traffic from a CPE.  When the SPSWE receives the initial
   datagram in a new flow, there will be no corresponding IPv4-address/
   port pair for that flow in the SPSWE NAT translation table.  The NAT
   selects an unused outbound TCP port, adds the resulting mapping to
   the NAT translation table, performs the appropriate translation and
   forwards it to the destination.

   The resulting table entry is now in place for translation of
   returning inbound traffic.

   The translation table can also be configured manually, which would
   allow, for example, traffic to be forwarded to servers on subscriber
   networks.  However, because multiple subscribers may be supported
   through a single IPv4 address, only one of those subscribers would be
   able to have statically assigned external server address through the
   NAT/softwire.




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10.  Analysis and Future Work

   There are several opportunities for future work on SNAT:

   o  SNAT requires provisioning of a softwire from each HGW to an
      SPSWE.  This document should include a description of at least one
      provisioning mechanism.  Candidates include a new DHCP option and
      anycast.

   o  SNAT requires an IPv4-in-IPv6 softwire for each subscriber, and
      NAT for each flow from the subscriber.  What are the effects of
      scaling this architecture to millions of subscribers?

   o  Security issues have not been considered

   o  How can the configuration of the IPv4-in-IPv6 softwire be
      automated?

   o  What is the interaction between SNAT and native IPv6 service to
      the subscriber?


11.  Change log

   This section shall be removed prior to publication of this document
   as an RFC.

11.1.  Revision -01

   o  Eliminated the NAT function in the HGW, which simplifies
      forwarding IPv4 datagrams over the IPv6 softwire between the HGW
      and the SPSWE.

   o  Added DHCP as a mechanism for tunnel endpoint discovery.


12.  Contributors

   Mark Townsley suggested elimination of a NAT function in the HGW.

   Bernie Volz and Carlos Pignataro provided substantive and editorial
   review of draft-droms-softwires-snat-00.


13.  IANA Considerations

   This memo includes no requests to IANA.




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14.  Security Considerations

   Security considerations must be developed.


15.  References

15.1.  Normative References

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119, March 1997.

   [RFC3775]  Johnson, D., Perkins, C., and J. Arkko, "Mobility Support
              in IPv6", RFC 3775, June 2004.

15.2.  Informative References

   [RFC3022]  Srisuresh, P. and K. Egevang, "Traditional IP Network
              Address Translator (Traditional NAT)", RFC 3022,
              January 2001.

   [RFC4925]  Li, X., Dawkins, S., Ward, D., and A. Durand, "Softwire
              Problem Statement", RFC 4925, July 2007.


Authors' Addresses

   Ralph Droms
   Cisco
   1414 Massachusetts Avenue
   Boxborough, MA  01714
   US

   Phone: +1 978.936.1674
   Email: rdroms@cisco.com


   Brian Haberman
   11100 Johns Hopkins Road
   Laurel, MD  20723-6099
   US

   Phone: +1 443 778 1319
   Email: brian@innovationslab.net







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Full Copyright Statement

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   This document is subject to the rights, licenses and restrictions
   contained in BCP 78, and except as set forth therein, the authors
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