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Re: Guidance needed on well known ports

2006-03-23 21:57:20
PS...

Joe Touch wrote:

Noel Chiappa wrote:
    > From: Keith Moore <moore(_at_)cs(_dot_)utk(_dot_)edu>

    > Regarding SRV, it's not acceptable to expect that as a condition of
    > deploying a new application, every user who wishes to run that
    > application be able to write to a DNS zone. Most users do not have DNS
    > zones that they can write to.

Yes. Architecturally speaking, it's somewhat dubious that information which
really only needs to be localized to the host (application<->port binding)
has to be sent to the DNS.

It would be easy to run a tiny little USP "binding" server that took in an
application name (yes, we'd have to register those, but string-space is
infinite), and returned the port.

Only if it asked a well-known server ON THAT MACHINE. I.e., pick a port,
reserve it for resolution (e.g., like the RPC portmapper works).

But we cannot assume a hosts' DNS is available for that purpose. For
most of us, the DNS entry isn't under our control, nor is it likely to
be for the forseeable future.

About the only reason I can see that that would not be desirable would be to
avoid an extra RTT to the do that binding lookup. (DNS/SRV solutions might
avoid this RTT too, but in that case that benefit doesn't outweigh the
costs.) The "obvious" way to do it, which is have the ICP use the strings
directly (as the CHAOS protocols did) is not really feasible now - it would
require a change to TCP.

That could be done using a late-binding trick like we used for
string-based source routing (www.isi.edu/datarouter); we could have a
TCP option where the service name occurs (as below), and send it at
first to the 'portmapper' port, which would demux it and return it. That
does require a mod to TCP to allow the dest port to be unbound (e.g.,
'0') if the option is present, and enable the return SYN-ACK to update
the TCB on arrival.

Since it seems like this might be useful, I'll pull a draft together on
how to do this without 1078's extra connection, more like the
late-binding we do in datarouter, very shortly...

Joe

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