[Standards] Connection Type in XEP-0198 'location' (Direct-TLS vs STARTTLS)

Georg Lukas georg at op-co.de
Wed Dec 20 11:07:28 UTC 2017


when reading the Instant-Stream-Resumption Proto-XEP, I encountered
this piece, which shows a deeper problem in 0198/6120:

| The <isr-enabled/> element can optionally also contain a 'location'
| attribute which specifies the preferred IP address or hostname, and a
| TCP port number of the host which should be used for instant stream
| resumption.

| Note that the hosts announced by the 'location' attribute qualified by
| the [ISR] namespace MUST be connected to using TLS from the beginning

I see how this attempts to create a fast-path to the Direct-TLS port, as
opposed to the STARTTLS 'location' of 0198 (which again is based on
RFC6120 § see-other-host).

I don't particularly like the 'location' mechanism in 0198, and by
extension the one proposed in ISR, as they add tremendous complexity to
the client due to breaking through multiple abstraction layers and
introducing very interesting security challenges.

However, I accept that there might be cluster deployments where such a
feature is required, and I see the need for specifying the connection
type, be it STARTTLS, Direct-TLS, BOSH or others for reconnecting.
Nevertheless, the ISR-proposed way of implicitly encoding the connection
type by the choice of which XEP carries the 'location' payload is not
quite optimal.

This problem needs to be fixed either in RFC6120 or in 0198, not worked
around from ISR. My strawman proposals for this would be:

a) the server inspects the client connection type and returns the port
for the same connection type in 0198/location, i.e. if the client
connected via Direct-TLS to 5223, the server will return server:5223.
This will obviously break clients that support Direct-TLS but fall back
to STARTTLS in 0198. As Direct-TLS is rather fresh, the impact of this
might be bearable, despite both specs being 'Draft'.

b) the server only returns the hostname, omitting the port, and the
client uses the original (cached) list of ports to connect via its
preferred method, allowing Direct-TLS resumption.

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