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Versions: 00

Network Working Group                                         M. Rosenau
Internet-Draft                                         February 10, 2018
Intended status: Experimental
Expires: August 14, 2018


                  TCP option "request IPv6 connection"
                   draft-rosenau-request-v6-option-00

Abstract

   This document describes an idea for a header "option" for the
   Transmission Control Protocol (TCP).

   The extension is used by dual-stack nodes to force the use of the
   IPv6 protocol instead of the IPv4 protocol when exchanging data over
   TCP.

   Especially if one of the two nodes is a NAT64 or NAT46 router it
   makes sense not to use IPv4 but IPv6 if both nodes are dual-stack
   nodes.

   This document also suggests to declare the usage of such a method
   mandatory for all nodes that use IPv4 addresses that will be assigned
   by RIRs in the future.

   Doing so will make it impossible to (mis-) use IPv4 addresses
   assigned by RIRs to ISPs for IPv4-only nodes that do not support
   IPv6.

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 https://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 August 14, 2018.





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Copyright Notice

   Copyright (c) 2018 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
   (https://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.

1.  Introduction

   Because of the IPv4 address shortage the IPv6 protocol has been
   developed.  Unfortunately many nodes in the internet are still
   IPv4-only and many ISPs use NATs to establish connections to servers.

   Some ISPs use NAT46 to allow IPv4-only clients to connect to an
   IPv6-only server.  In many cases both nodes would be able to
   establish an IPv6 connection instead of an IPv4 connection because
   the use of the NAT can be avoided in this case.

   This document describes a TCP [RFC0793] header "option" which allows
   both nodes to negotiate the use of the IPv6 protocol instead of the
   IPv4 protocol.

2.  Terminology

2.1.  Keywords in capital letters

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and
   "OPTIONAL" in this document are to be interpreted as described in BCP
   14, RFC 2119 [RFC2119] and indicate requirement levels for compliant
   implementations.

2.2.  client

   In the context of this document a "client" is a node which is
   establishing a TCP connection to another node by sending the first
   TCP packet.






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2.3.  server

   In the context of this document a "server" is a node which is waiting
   for a client to establish a TCP connection with that node.

2.4.  NAT64, NAT46

   A NAT64 is a node which allows an IPv6-only client to connect to an
   IPv4-only server by translating IPv4 to IPv6 packets and vice versa
   or by acting as IPv6 server and IPv4 client the same time forwarding
   the data from one TCP connection to the other one.

   A NAT46 is a node allowing an IPv4-only client to connect to an
   IPv6-only server.

   Note that according to the definitions above NAT64 and NAT46 nodes
   are both "clients" and "servers" the same time.

3.  Theory of operation

3.1.  Connection to an IPv4-only server

   A connection to a server not supporting this extension looks the
   following way:

      client                                  server
         |                                       |
         |    TCP/IPv4, SYN, V6REQ(00)           |
         +-------------------------------------->|
         |                                       |
         |    TCP/IPv4, SYN, ACK                 |
         |<--------------------------------------+
         |                                       |
         |             ...                       |

                Figure 1: Request not understood by server

   The client adds an option described in this document to the "options"
   field of the TCP header.

   The server does not understand the option described in this document
   and it will probably ignore it.  The TCP connection will be
   established "normally"; the option described in this document is not
   used.







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3.2.  IPv4 negotiated

   If both the client and the server support this extension but both
   nodes decide to use IPv4 for the connection the connection looks the
   following way:

      client                                  server
         |                                       |
         |    TCP/IPv4, SYN, V6REQ(00)           |
         +-------------------------------------->|
         |                                       |
         |    TCP/IPv4, SYN, ACK, V6REQ(10)      |
         |<--------------------------------------+
         |                                       |
         |             ...                       |

                         Figure 2: IPv4 negotiated

   The server will send the "normal" (SYN/ACK) packet back to the client
   - as it is done for a normal TCP connection.  It will add a certain
   option to the "options" field of the TCP header indicating that the
   option in the first packet is understood and supported by the server
   but it is not used for the current connection.

3.3.  IPv6 negotiated

   If the client and the server decide to use IPv6 for the connection
   the connection looks like this:

      client                                  server
         |                                       |
         |    TCP/IPv4, SYN, V6REQ(00)           |
         +-------------------------------------->|
         |                                       |
         |    TCP/IPv4, RST, V6REQ(11)           |
         |<--------------------------------------+
         |                                       |
         |    TCP/IPv6, SYN                      |
         +-------------------------------------->|
         |                                       |
         |             ...                       |

                         Figure 3: IPv6 negotiated

   The server sends back a TCP packet containing the IPv6 address of the
   server and indicating that the IPv4 TCP connection is rejected.





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   The client will re-send an initial TCP packet (SYN) to the server
   using IPv6.

3.4.  NAT scenario

   There is even a situation where using this option in an IPv6 packet
   makes sense: When using with NAT64.  Such a connection looks like
   this:

 client                                 NAT                                 server
    |                                    |                                     |
    |    TCP/IPv6, SYN, V6REQ(00)        |                                     |
    +----------------------------------->|    TCP/IP(v4 or v6), SYN, V6REQ(00) |
    |                                    +------------------------------------>|
    |                                    |                                     |
    |                                    |    TCP/IP, RST, V6REQ(11)           |
    |    TCP/IPv6, RST, V6REQ(11)        |<------------------------------------+
    |<-----------------------------------+                                     |
    |                                    |                                     |
    |    TCP/IPv6, SYN                                                         |
    +------------------------------------------------------------------------->|
    |                                                                          |
    |             ...                                                          |

                         Figure 4: Using with NATs

   The NAT64 sends back a TCP packet containing the IPv6 address of the
   server and indicating that the TCP connection to the NAT's address is
   rejected.

   The client will re-send an initial TCP packet (SYN) directly to the
   server using IPv6 not using the NAT64.

4.  Option formats

4.1.  Request packet

   In the initial TCP packet (the packet sent by the client to the
   server that has the SYN field set and the ACK field clear) the
   following TCP header option will be added by the client:

     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     | TYPE=V6REQ    | LENGTH=3      |0 0|   CODE    |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

                      Figure 5: Request option format





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   The first octet of the option is the code "V6REQ" to be assigned by
   the IANA

   The second octet of the option is the length of the option (as
   described in RFC 793 [RFC0793]).  All codes described in this
   document use a length of 3 octets.

   The high two bits of the third octet are zero and the low six bits
   are the code described below.

   A TCP implementation MUST ignore this option (with the two upper bits
   in the third octet being zero) when it is found in any other packet
   but the initial TCP packet (SYN) sent from the client to the server.

   If a TCP implementation does not understand the "CODE" (or the
   "LENGTH" field does not match the "CODE" field) it MUST ignore the
   option but it SHOULD add a "request not understood" option to the
   packet sent back to the client.

4.2.  Informational response

   If client and server decide to use IPv4 for the TCP connection the
   server answers as if the option was not present (SYN, ACK) but it
   adds the following option to the "options" field of the TCP header:

     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+--//---+
     | TYPE=V6REQ    | LENGTH        |1 0|   CODE    | DATA  |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+--//---+

              Figure 6: Informational response option format

   The highest bit of the third octet is one and the bit below is set to
   zero.  The low six bits are the code described below.

   A TCP implementation MUST ignore this option (with the two upper bits
   in the third octet being one and zero) when it is found in any other
   packet but the TCP packet (SYN, ACK) sent as response to the first
   packet.

   DATA is additional data depending on the "CODE" field.

   If a TCP implementation does not understand the "CODE" (or the
   "LENGTH" field does not match the "CODE" field) it MUST ignore the
   option.







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4.3.  Force restart response

   If client and server decide to use IPv6 for the TCP connection the
   server denies the IPv4-based connection and the client shall re-
   establish a connection via IPv6.

   The server sends a packet with a special form that any TCP
   implementation that supports this option MUST understand:

   The "source port" field contains the "destination port" of the
   initial packet and vice versa (just like in the case of a "normal"
   TCP response).

   The "acknowledgement number" field contains the value of the
   "sequence number" field in the initial packet and vice versa.  (Note
   that none of the two numbers is incremented.)

   The "RST" bit MUST be set and "ACK" and "SYN" MUST be zero.  The
   other control bits SHALL be zero.

   The "options" field in the TCP header contains the following option:

     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+--//---+
     | TYPE=V6REQ    | LENGTH        |1 1|   CODE    | DATA  |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+--//---+

              Figure 7: Force restart response option format

   The highest two bits of the third octet are one.  The low six bits
   are the code described below.

   A TCP implementation MUST ignore this option (with the two upper bits
   in the third octet being one) when it is found in any other packet
   but the TCP packet sent as response to the first packet.

   No TCP connection has been established between client and server when
   the server answered with this packet.

   Typically the "V6REQ" option will be the only option in the TCP
   header in this case because more options make no sense in this case.

4.4.  Reserved combination

   The combination "zero-one" for the highest two bits in the third
   octet is reserved for future use:






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     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+--//---+
     | TYPE=V6REQ    | LENGTH        |0 1|                   |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+--//---+

                     Figure 8: Reserved option format

   A TCP implementation MUST ignore this option (with the two upper bits
   in the third octet being zero and one) when it is found in any TCP
   packet.

5.  Option codes

5.1.  List of option codes

   The following option codes are defined:

    Type* Code   Length   Code
     00     1       3     Strong IPv6 request
     00     2       3     Weak IPv6 request
     10     3       3     Supported for this port
     10     4       3     Not supported for this port
     10     5       4     Request not supported
     11     6      21     Address to be used

    (*) 00 = valid in request
        10 = valid in an informational response
        11 = valid in a force restart response

                      Figure 9: List of option codes

5.2.  Strong IPv6 request

   A client sends this option in the initial TCP packet whenever it
   desires to use IPv6 for the connection as far as it is possible.

   If the server understands this option but there is no TCP/IPv6 port
   which is equivalent to the TCP/IPv4 port the client wants to connect
   to it MUST answer with "not supported for this port".

   If the server understands this option AND there is a TCP/IPv6 port
   which is equivalent to the TCP/IPv4 port the client wants to connect
   to the server MUST answer with "address to be used".

5.3.  Weak IPv6 request

   This option is similar to the "strong IPv6 request" however it is
   used if the client does not necessarily prefer IPv6 over IPv4.




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   If the server understands the option and there is a TCP/IPv6
   equivalent for the TCP/IPv4 port but the server also does not prefer
   using IPv6 over IPv4 the server MUST answer with "supported for this
   port".

   The client may later send a "strong IPv6 request" to get information
   about the TCP/IPv6 port that can be used instead of the TCP/IPv4
   port.

   In all other cases the server MUST react on this option the same way
   as it reacts on a "strong IPv6 request".  Especially in the case that
   the server wants to communicate with the client over IPv6 instead of
   IPv4 it will answer with "address to be used".

5.4.  Supported for this port

   A server sends this option in its first packet (in the answer to the
   first packet of the TCP connection) as a response to a "weak IPv6
   request".

   Using this option it indicates that it would support a "strong IPv6
   request" for this TCP/IPv4 port.

5.5.  Not supported for this port

   A server sends this option in its first packet as a response to a
   "strong" or "weak IPv6 request".

   Using this option it indicates that the it is not possible to
   establish the desired TCP connection via IPv6.

5.6.  Request not supported

   A server sends this option in its first packet as a response to any
   "request" option that was not understood by the server.

   This is the case when the initial packet from the client contained a
   "V6REQ" option whose topmost two bits of the third octet were zero
   but the server did not understand the meaning of that option.  (For
   example if the "CODE" field did not have a value understood by the
   server.)

   The server indicates that it ignored the option it did not
   understand.

   Note that the "request not supported" option is only of informational
   kind; the client SHOULD NOT treat this as error message.




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   If the server did not understand the value of the "CODE" field the
   "request not supported" option has the following form:

     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     | TYPE=V6REQ    | LENGTH=4      |1 0| CODE=5    |L|0| ORG. CODE |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

              Figure 10: Request not supported option format

   "ORG.  CODE" is the value of the "CODE" field in the request not
   understood by the server.

   "L" is zero if the code is not understood by the server at all.  "L"
   is one if the server understands the code but the length of the
   option is not correct (currently: 3).

   Note: If there will be "requests" (type="00") in the future being
   longer than three octets a "response" indicating "invalid arguments"
   should be defined.  "Request not supported" should not be used in
   this case.

5.7.  Address to be used

   A server sends this option in a "force restart response"
   (Section 4.3) described above.

   Doing so the server instructs the client to use IPv6 instead of IPv4
   to connect to the server.

   The option has the following form:

     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     | TYPE=V6REQ    | LENGTH=21     |1 1| CODE=6    |               |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+               |
     |                                                               |
     |                       IPv6 ADDRESS                            |
     |                                               +-+-+-+-+-+-+-+-+
     |                                               |   TCP ...     |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |   ... PORT    |
     +-+-+-+-+-+-+-+-+

                Figure 11: Address to be used option format

   The "IPv6 ADDRESS" and "TCP PORT" fields are the IPv6 address and the
   TCP/IPv6 port number of a TCP port which is equivalent to the port
   the client wanted to connect to using IPv4.




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   Note that especially in the case of NAT46 the IPv6 address may be
   different for different TCP ports.  The TCP port number to be used
   with IPv6 may also differ from the TCP port number to be used with
   IPv4 - especially in a NAT46 scenario.

   The TCP/IPv6 port returned MUST be absolutely equivalent to the TCP/
   IPv4 port the client wanted to connect to.  This means that when
   establishing a connection from an IPv6 client to the server there
   must be no difference in functionality when directly connecting to
   the TCP/IPv6 port returned by the server compared to connecting to
   the TCP/IPv4 port via a NAT64.

6.  Using the option with IPv6

   As described above the option may also used in TCP/IPv6 packets.
   This typically only makes sense if the server is a NAT.

   If the server understands this option, it is a NAT and the client can
   connect to the actual server (behind the NAT) directly the NAT will
   respond to a "strong IPv6 request" with an "address to be used"
   answer.

   If the server understands this option but it is not a NAT or the
   client cannot connect to the actual server directly it will respond
   with "not supported for this port".

   A NAT may behave differently for different ports - e.g. if one port
   is forwarded to a server which can be accessed directly and another
   port is forwarded to a server which cannot be accessed directly.

   The same is true when a "weak IPv6 request" is received; in this case
   the NAT may decide if it answers with a "supported for this port"
   option or with "address to be used".

7.  Suggestion for IPv4 addresses assigned by the RIRs

   IPv4 addresses are very rare.

   Unfortunately there still seem to be ISPs who receive IPv4 addresses
   from the RIRs using these addresses for IPv4-only nodes.

   To force these ISPs to use the IPv4 addresses they receive from the
   RIRs for dual-stack nodes and translation mechanisms only the author
   of the document has the following suggestion:

      - RIRs are only allowed to assign IPv4 addresses if the receiver
      guarantees that the receiver will observe some rules for ALL IPv4
      addresses used by the receiver.



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      - "ALL IPv4 addresses" means:

         - An organisation already having IPv4 addresses may only
         receive new IPv4 addresses from a RIR if it already observes
         the rules for all IPv4 addresses in use by this organisation.
         As soon as it receives the new IPv4 addresses observing the
         rules will be mandatory for the "old" IPv4 addresses!

         - In a situation where an organisation using IPv4 addresses
         being subject to these rules and one organisation using IPv4
         addresses not being subject to these rules merge (e.g. in a
         company takeover) the merged organisation MUST decide if ALL
         IPv4 used by the organisation will be subject to these rules or
         if the IPv4 addresses being subject to these rules before the
         merge are returned to the RIR.

      - The RIR should revoke the IPv4 addresses when the rules are
      violated.

   Possible rules could be (note that most of these rules are already
   mandatory for "new implementations" according to RFC 6540 [RFC6540]):

      - All functionality a node supports via IPv4 MUST also be
      supported via IPv6 - regardless if the functionality uses TCP, UDP
      or other protocols.

      - This also applies to the DNS entry: There MUST be an "AAAA"
      record for each host name having an "A" record using such an
      address.

      - The only exception is the use of a secondary host name for
      forcing an IPv4 connection like "www.ipv4.example.com" (force
      IPv4) instead of "www.example.com" (use the default protocol); the
      operator of the host MUST ensure that the secondary host name is
      only used in case of problems with the IPv6 connection.
      Especially it is not allowed to publish statements like: "The
      address of our home page is www.ipv4.example.com".

      - On the other hand such a node MAY support functionality which is
      only accessible via IPv6 but not via IPv4.

      - Nodes using these IPv4 addresses MUST prefer IPv6 over IPv4
      (Example: If a node wants to establish a connection to a host
      having both an "AAAA" and an "A" DNS record these nodes MUST first
      try to use the "AAAA" record; only if this fails they MAY use the
      "A" entry.)





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      - All clients using these IPv4 addresses MUST use the "strong IPv6
      request" option in outgoing TCP connections over IPv4.  This will
      force the clients to use the direct IPv6 connection in the case of
      a NAT46.  It will also guarantee that it is verifiable if the
      client using this IPv4 address supports IPv6.

      - All servers using these IPv4 addresses MUST respond to a
      "strong" and/or "weak IPv6 request" using "address to be used"
      (and refuse a TCP connection via IPv4).  This will reduce the load
      of NAT64 servers.  It will also guarantee that it is verifiable if
      the server using this IPv4 address supports IPv6.

8.  References

8.1.  Normative References

   [RFC0793]  Postel, J., "Transmission Control Protocol", STD 7,
              RFC 793, DOI 10.17487/RFC0793, September 1981,
              <https://www.rfc-editor.org/info/rfc793>.

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119,
              DOI 10.17487/RFC2119, March 1997,
              <https://www.rfc-editor.org/info/rfc2119>.

8.2.  Informational References

   [RFC0792]  Postel, J., "Internet Control Message Protocol", STD 5,
              RFC 792, DOI 10.17487/RFC0792, September 1981,
              <https://www.rfc-editor.org/info/rfc792>.

   [RFC6146]  Bagnulo, M., Matthews, P., and I. van Beijnum, "Stateful
              NAT64: Network Address and Protocol Translation from IPv6
              Clients to IPv4 Servers", RFC 6146, DOI 10.17487/RFC6146,
              April 2011, <https://www.rfc-editor.org/info/rfc6146>.

   [RFC6540]  George, W., Donley, C., Liljenstolpe, C., and L. Howard,
              "IPv6 Support Required for All IP-Capable Nodes", BCP 177,
              RFC 6540, DOI 10.17487/RFC6540, April 2012,
              <https://www.rfc-editor.org/info/rfc6540>.

Author's Address

   Martin D. J. Rosenau

   Email: martin@rosenau-ka.de





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