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Re: OPES and Email

2005-07-08 10:37:39

Tony Hansen <tony(_at_)att(_dot_)com> wrote:
OPES SMTP Use Cases (draft-ietf-opes-smtp-use-cases-02.txt).
   The Open Pluggable Edge Services (OPES) framework is application
   agnostic.  Application specific adaptations extend that framework.
   This document describes OPES SMTP use cases and deployment scenarios
   in preparation for SMTP adaptation with OPES.

   We should be very careful here. "Open Pluggable Edge Services" suggests
we're talking only about "edge services"; and IMHO we should be sure to
explain any situations where the OPES WG might have a different opinion
than others as to what an "edge service" is.

] In SMTP the OPES processor may be any agent participating in SMTP
] exhanges, including MSA, MTA, MDA, and MUA.  This document focues on
] use cases in which the OPES processor is a mail transfer agent (MTA).

   I'm afraid we start out with different opinions. :^(

   MTAs have a number of functions; some of which we consider to be
"edge" functions, and some which we don't. (YMMV.)

] This work focuses on SMTP based services that want to modify command
] values and those that want to block commands by defining an error
] response that the MTA should send in response to the response it
] received.  OPES MTA will be involved in SMTP command modification and
] command satisfaction, analog to request modification and request
] satisfaction from HTTP [9].

   I'm afraid I don't understand why this describes an "edge service".

]  +-------+  +---------+      +---------+      +--------+  +-------+
]  |mail  M|  |M mail  M| SMTP |M mail  M| SMTP |M mail M|  |M mail |
]  |clnt  U|--|S srvr  T|------|T gway  T|------|T srvr D|--|U clnt |
]  |      A|  |A       A|      |A       A|      |A      A|  |A      |
]  +-------+  +---------+      +---------+      +--------+  +-------+
]                  |                |                |
]                  | OCP            | OCP            | OCP
]                  |                |                |
]             +----------+     +----------+     +----------+
]             |  callout |     |  callout |     |  callout |
]             |  server  |     |  server  |     |  server  |
]             +----------+     +----------+     +----------+
]                        Figure 2: OPES SMTP Flow

   This describes three situations. I think most of us would agree
that the middle (gateway) function is an edge function. I'm less sure
about the other two. (In fact, I'm fresh out of any "the" answers,

   There's also the very common situation:
"                -------+      +---------+      +------
"                mail  M| SMTP |M mail  M| SMTP |M mail
"                srvr  T|------|T xfr   T|------|T srvr
"                      A|      |A       A|      |A     
"                -------+      +---------+      +------

wherein the middle box is considered "an intermediate MTA", and does
no gateway functions. It's not sufficiently clear, IMHO, whether OPES
intends for it to be appropriate for it to link to an OCP. (I think
most of us would say it shouldn't.)

   There are, of course, cases where outgoing email is filtered on a
separate machine from the MSA which initially receives it. I honestly
don't know whether most of us would consider that an "edge" process.

   So, bottom line, I don't have good answers, but I'm pretty sure we
should clarify whether there's agreement as to what SMTP-related
functions deserve to interact with "edge services".

John Leslie <john(_at_)jlc(_dot_)net>

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