> On 7 Sep 2016, at 00:05, Glyph Lefkowitz <gl...@twistedmatrix.com> wrote:
>
>
>> On Sep 6, 2016, at 11:55 AM, Tristan Seligmann <mithra...@mithrandi.net
>> <mailto:mithra...@mithrandi.net>> wrote:
>>
>> Currently there is no way to explicitly compose Twisted endpoints, but
>> several endpoint implementations have arisen that explicitly wrap another
>> endpoint, and so have needed a way to do this. So far, this has been
>> implementing by passing in an endpoint description, and then calling
>> serverFromString/clientFromString internally in the endpoint to construct
>> the wrapped endpoint. I've seen two different ways of encoding the "inner"
>> endpoint:
>>
>
> Some other points to keep in mind for this discussion:
>
> We may want a syntax that supports composing multiple endpoints, not just 2.
> The existing syntax is kind of crummy; ":" as a separator has serious
> problems, considering its presence in both URLs and IPv6 literals. I
> wouldn't say we should necessarily re-design the whole syntax to accommodate
> this, but just having a whole new syntax might not be a bad thing either.
I mentioned this casually to Tristan in IRC, but the current syntax and use of
endpoints in Twisted gets close to a quite profound idea about protocol nesting
that is lurking in the space of convention. Extending the endpoint syntax to
have a blessed way of essentially composing endpoints together gives the
potential of using the endpoint syntax to design and implement various
“tunneling” features that are very useful.
If we take the arrow syntax, for a moment, you could conceive a truly insane
client wanting to write an endpoint to run FTP over that’s a bit like:
tcp:host=someftp.server:port=21->http:verb=connect->tcp:->socks5:targetname=mypersonalhttpproxy.server->tcp:host=mycorporatesocksproxy.server:port=2121
This would represent tunneling FTP over TCP over HTTP over TCP over SOCKS over
TCP. For extra fun you can throw in some Tor.
Alternatively, and quite a bit more realistically, you could have a userspace
SCTP implementation that supports being tunnelled over UDP. In this instance,
rather than write a single “sctp-over-udp” endpoint, you could write a generic
sctp endpoint that, if it is composed with another endpoint, expects that
endpoint to provide a datagram-style transport to it.
All of this is rather pie in the sky, and potentially the purest example of
YAGNI that it’s possible to imagine. However, it’s worth highlighting that
endpoints are inches away from being a really powerful composable tool for
saying “tunnel this protocol over this other protocol”. We may not *want* that,
but it’s an interesting thought.
Cory
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