Re: TRT bidirectional

[itojun@xxxxxxxxxx (Jun-ichiro itojun Hagino)]

> After reading RFC 3142 I am not totally clarified due to TRT mechanism.
> On abstract it is referred:
> 
> $BEe(B an IPv6-to-IPv4 transport relay translator. It enables IPv6-only hosts to
> exchange (TCP,UDP) traffic with IPv4-only hosts. A TRT system, which locates
> in the middle, translates (TCP,UDP)/IPv6 to (TCP/UDP)/IPv4, or vice-versa\x94.
> 
> I am a little confused. 
> 
> The TRT permits only that IPv6 hosts initiate a session with IPv4 hosts?
> IPv4 hosts cannot initiate a session with IPv6 hosts?

	the problem is in how you map IPv4 address within IPv6 address, or vice
	versa.

	when translate IPv6 -> IPv4, we can get a /64 (or /96) subnet for the
	mapping and embed IPv4 address at the bottom (like
	2001:240:ffff:ffff::133.138.1.1, which will get translated to
	133.138.1.1).  specially-crufted nameserver implementation which does
	this trick is freely available (called "totd").

	when translate IPv4 -> IPv6, it gets very tricky.  how would you map
	IPv4 address and IPv6?  you can't embed address like above.
	normally, you need to define static mapping between IPv4 and IPv6.
	(for instance, "if 192.168.1.1 is accessed, it will go to
	2001:240:ffff:ffff::1234")

	to summarize,
	- IPv6 -> IPv4 case is easy.
	- IPv4 -> IPv6 case is tough.
	- theoretically TRT works for both cases, so the RFC states it in
	  the abstract, and describe IPv6 -> IPv4 case in the later part of the
	  RFC.

itojun

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