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How does SMTP IPv4 and IPv6 work together

2008-04-05 07:49:03

Since John thinks we are confused, I felt I owe it to myself if I summarized and outline what I thought was the technical compatibility issues to see if I understood it and was not confused.

As far as I concern, there ought not be any confusion but common sense modeling here when considering IPv6 in 2821bis.

We have three (3) possible models for a SMTP system:

     - Current IPv4 only system
     - Dual Stack IPv4/v6 system
     - A pure Ipv6 only system

When combined in client/server transactions, the following chart shows the possibilities in terms of minimum requirement for an implicit MX and/or A or AAAA records to perform a connection (communication) between two similar or hybrid systems:

 |          |            SENDER              |
 |          |--------------------------------|
 |RECEIVER  |  IPv4  |   IPv4/6    |  IPv6   |
 |          |        |             |         |
 |  IPv4    |   A    |     A       |  ****   |
 |          |        |             |         |
 |          |        |             |         |
 |  IPv4/6  |   A    |  A or AAAA  |  AAAA   |
 |          |        |             |         |
 |          |        |             |         |
 |  IPv6    |  ****  |    AAAA     |  AAAA   |
 |          |        |             |         |

The **** means that it the client/server can not communicate simply because each are not compatible in terms of TCP protocol.

The basic principle above is that new technology work with each other in the same way old technology work with each other, and in a hybrid situations, the common denominator method is used.

I believe 2821BIS need not go into details but to illustrate the key points of maintaining maximum compatibility.


IPv4 to IPv4

  Status Quo - what we have today using the current IPv4 TCP layer.

IPv6 to IPv6

  Status Quo - what we have possible today and more tomorrow using
  a new pure IPv6 TCP layer.

IPv4 to IPv4/6

  Obviously the IPv4 system knows nothign about AAAA records or
  IPv6 TCP socket connections.  The A record can only be obtained
  via explicit MX or implicit MX.

IPv4 to IPv6
IPv6 to IPv4

  It is not possible for the IPv4 only system to talk to the IPv6
  system because each are using different TCP socket technology.

IPv4/6 to IPv4

  In this case, the IPv4/6 dual stack system performing a
  discovery on a IPv4 only system will find only A records.  With
  the IPv4/6 dual system having A records, it will allow the IPv4
  to return responses.

IPv4/6 to IPv4/6

  I presume these systems will use the maximum method (IPv6)
  available. What might be interesting here is a fall back to
  IPv4 if the IPv6 method fails due to some middle router
  compliancy issue or something else.

IPv4/6 to IPv6

  In the case, the sender will only see AAAA records and can only
  make a IPv6 socket connection.

IPv6 to IPv4/6

  In the case, the sender will only EXPECT AAAA records and can
  only make a IPv6 socket connection.

To me, this all makes common sense. I think the text in 2821bis should simply lay down the possibilities as done above. But I do think the above can be folded to make it into smaller text.


Hector Santos, CTO

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