Eric Burger (eburger@snowshore.com)
Sun, 4 Feb 2001 21:36:04 -0500
It sounds like you will have to scan the entire message irrespective of the
message context.
One example: unless your user agent supports ALL codecs, even if the message
context is voice-message, you will have to scan the message to see if you
support the particular codec.
I suppose it depends on what your "limited user agent" is supposed to be.
If your audio-only user agent is a "thing that plays audio content in a
message", you must scan all messages in the event there is an audio
attachment. On the other hand, consider the audio-only user agent as a
"thing that plays voice mail". In this case, if the sending user agent went
out of its way to mark the message as "not voice mail", your user agent
would presumably respect that and not present the message, even if it
happened to contain audio.
The rationale for specifying loose receiving action (MUST NOT) is the
guiding Internet principle of "strict conformance on send, loose conformance
on receive." It is quite conceivable for a multimedia user agent to send a
well-formed message in a particular context that does not contain the media
type one would usually associate with the context. For example, consider a
voice messaging system that records a voice message and also performs ASR on
the message (IBM's got a patent on this). The message then passes through a
content gateway, such as a firewall, that removes non-critical body parts
over a certain length. The receiving user agent will see a message in the
voice-message context that has only a text part (no audio). Even though the
message does not have audio, it is still a voice message.
For that matter, in the case of a multimedia user agent, specifying MUST
here would result in rejection of a message even though the receiving user
agent could render the message. I doubt such action would make the keepers
of Internet Mail purity happy.
Remember, the receiving user agent can always issue a MDN indicating media
not supported if it does not support a given media type.
Given this, I should re-word the limited user agent bullet.
Thanks.
-- - Eric> -----Original Message----- > From: owner-vpim@lists.neystadt.org > [mailto:owner-vpim@lists.neystadt.org]On Behalf Of Caleb Clausen > Sent: Monday, January 29, 2001 3:03 PM > To: vpim@lists.neystadt.org > Subject: Re: [VPIM] I-D ACTION:draft-ietf-vpim-hint-02.txt > > > one would like to be able to use message-context in a limited user > agent (for instance, one which can only present audio to the user) > to provide the equivalent of multipart/voice-message from vpim v2. > that is, it is an easy-to-evaluate indicator to the receiving user > agent that the message contains data that it can present. > > the easy-to-evaluate part is important for performance. the > alternative is to scan through the entire message looking for body > parts with particular content-types. > > this type of use seems to be implied by the draft in, for instance, > the following paragraphs: > > The uses of this kind of presentation characteristic for each > > message is multi-fold: > [snip] > > o Filter the message list for presentation via > > limited-capability > > user interfaces (e.g., there is no point in offering images > > when the user is connected by a voice-only telephone user > > interface). > > however, there are some statements later on which make this use > impossible: > > > o Incorrect or invalid message classification must not result in > > failure to transfer or inability to present a message. > > > A receiving user agent MUST NOT depend on the indicated message- > > context value in a way that prevents proper presentation of the > > message. If the value is incorrect or does not match the message > > content, the receiving user agent MUST still be capable of > > displaying the message content at least as meaningfully as it would > > if no Message-Context value were present. > > > Message-Context is only an indicator. We do not intend for it to > > convey information that is critical for presentation of the > > message. One can conceive of goofy situations, such as a message > > marked "voice-message" but without an audio body part. In this > > case, the fact that the contents of a message don't match its > > context does not mean the receiving system should generate an error > > report or fail to deliver or process the message. > > the draft is saying, in other words, that one cannot depend on the > message-context being correct. if you aren't allowed to depend on > it, why should you even use it at all? > > let me give an example to illustrate what i mean. suppose an audio- > only user agent gets a message with a message-context of text- > message but which actually contains audio data. the receiving ua > sees the text message-context, but it cannot use that to filter the > message out of the set of messages it presents to the user since it > "MUST NOT depend on the indicated message-context value in a > way that prevents proper presentation of the message." in other > words, it must still go through the entire message looking at mime > headers for a compatible content-type. if the ua has to look at the > mime headers anyway, then what's the point in consulting the > message-context in the first place? > > why can't we just say that the message-context MUST indicate a > certain type of content? > > as i see it, either the message-context must be made reliable (if > it's present at all) or we must remove the paragraph about limited > user agents using the message-context to filter messages out. > > "For, I say fortunately, I always carry a spare set > of feathers." > --Foghorn Leghorn > >
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