[Model-t] Definition of an intermediary

[Martin Thomson <[e-mail]>]

One of the pieces of feedback on the -tmi draft was related to how it defines intermediary.

I want to explore that a little more, because I think that this is important.

The definition I have now concentrates on entities whose participation in the protocol is primarily the receipt and forwarding of messages from others, rather than the origination or termination of messages.

This was always my intent, but it's hard to get this right.  The more you dig into this, the harder it gets.  

Firstly, there are functions that require that an intermediary terminate or originate communication in ways that are more sophisticated than mere forwarding.  And these are, often, the most useful and interesting functions.

Does a video conference bridge that drops video from someone to save bandwidth act as an endpoint when it throws that data away?  Is a DNS recursive resolver or CDN node that serves from cache doing more than intermediate?  Maybe those can be viewed as simple time-shifting, so they still fit the basic definition.  However, is my mail provider doing more than intermediate when it provides me with a search function over my inbox?

Second, the potential for an intermediary to modify messages is often key to the operation of a protocol.  When does that modification turn the intermediary into a full participant?  A SIP B2BUA is often cast as a full protocol participant, which is certainly true from the perspective of how the protocol is engineered, but in the abstract it is indistinguishable from an intermediary, even if it doesn't strictly adhere to the SIP definition of a proxy.

Third, the nature of some protocols is that they mix communication with intermediaries with end-to-end messages.  SIP, SMTP, and HTTP share lineage and so all suffer from this to varying degrees.  My sense is that this is largely OK and that we can rely on some interpretation of intent to clear things up, though we might need to recognize that we might end up disagreeing on the specifics.

Are there other wrinkles that I've failed to identify?

I've started iterating on a definition here: https://github.com/martinthomson/tmi/pull/5