Tony Finch wrote:
On Fri, 4 Jan 2008, John C Klensin wrote:
On Friday, 04 January, 2008 12:01 -0800 Bill Manning
<bmanning(_at_)ISI(_dot_)EDU> wrote:
The general answer when needing to communicate between similar
applications that run on different address families has traditionally
been the application layer gateway (ALG) ...
MTAs *are* ALGs.
(Uh Oh.)
Nope.
MTAs are email relays. ALGs can perform syntactic and semantic
transformations. A simple relay can't. ALGs connect similar applications that
entail heterogeneous technologies, conventions, whatever.
Now it happens that email gateways have to include MTA functionality, since
they all operate as relays, also. And lots of software the is email
gateway-capable are often used merely as MTAs, nicely confusing the heck out
of us.
But architecturally, ALGs are seriously different. Stated simplistically, an
MTA performs very few functions and makes minimal changes to a message, where
the 'changes' are mostly some additions. An email gateway can massively alter
the object. It has permission to do whatever is necessary to make
d/
ps. No one will be surprised that I've documented this issue in the email
--
Dave Crocker
Brandenburg InternetWorking
bbiw.net
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