Subject: Re: [RRG] Renumbering...

From: Christian Vogt <[email-masked]>
Date: Sat, 16 Aug 2008 19:04:03 -0700

> In Six/One Router, when a host in a non-upgraded network gets a
> packet from a host in an upgraded network the packet's source
> address has already been translated (by the translation router at
> the border of the source network) to a transit address.  Transit
> addresses are locator addresses.  So this fails your test.
>
> It is also possible for a host in an upgraded network to receive
> such a packet using the transit address, but I don't understand why
> this is necessary: bilateral mode when both networks are upgraded.
>
> I asked Christian about this here:
>
>  http://psg.com/lists/rrg/2008/msg02149.html

Robin,

you are referring to a packet exchange between two upgraded edge
networks where the initiating host has used a transit address to reach
the responding host.  In this case, the operation of Six/One Router
causes both hosts to use a transit address for their respective peer *.
You are asking why.  (Tell me if I didn't get your question right.)

   [*] Noteworthy, as you are bringing up this question in the context
       of renumbering:  Renumbering is not necessary nevertheless.
       Hosts in upgraded edge networks never see their transit
       addresses themselves; only local edge addresses are used within
       an edge network.  The transit addresses that may appear within
       an edge network are exclusively remote.

The reason why the initiating host sees its peer's transit address is
simple:  The initiating host has chosen the peer's transit address as
a destination address.  So in order not to break the connection, it
must see the peer's transit address also as a source address in
received packets.

The reason why the responding host sees its peer's (the initiating
host's) transit address is more subtle:  By enforcing that either both
or none of two peers see each other's transit addresses, Six/One
Router can ensure that hosts see the right type of source address
(edge or transit) in received packets.  Specifically, it enables a
Six/One router at the edge network originating a packet to tell, based
on the type of the packet's destination address, which type of source
address the packet should have when eventually delivered to the
recipient host.  The Six/One router can then accordingly set the
Bilateral/Unilateral bit in the packet, so that the Six/One router at
the recipient edge network knows whether it should rewrite the
packet's source address back to an edge address, or leave it as a
transit address.

Of course, since both edge networks are upgraded in the described
case, the initiating host should preferably use the responding host's
edge address.  And typically, it would do this because the standard
destination address selection algorithm prefers edge addresses over
transit addresses for hosts in upgraded edge networks.  This is due to
the use of edge address type prefixes in Six/One Router, and the
longest-prefix match in the address selection algorithm:  It makes a
host in an upgraded edge network prefer destination edge addresses as
destinations because the prefix of those addresses matches the prefix
of the host's own address.  However, the described case can still
occur when the initiating host doesn't know the peer's edge address,
e.g., when it is referred to a transit address by SIP signaling.

Does this clarify your question?

- Christian

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